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Scanning Probe Microscopy in Industrial Applications

Scanning Probe Microscopy in Industrial ApplicationsNanomechanical Characterization

Edited by

Dalia G Yablon

Copyright copy 2014 by John Wiley amp Sons Inc All rights reserved

Published by John Wiley amp Sons Inc Hoboken New JerseyPublished simultaneously in Canada

No part of this publication may be reproduced stored in a retrieval system or transmitted in any form or by any means electronic mechanical photocopying recording scanning or otherwise except as permitted under Section 107 or 108 of the 1976 United Stated Copyright Act without either the prior written permission of the Published or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center Inc 222 Rosewood Drive Danvers MA 01923 (978) 750-8400 fax (978) 750-4470 or on the web at wwwcopyrightcom Requests to the Publisher for permission should be addressed to the Permissions Department John Wiley amp Sons Inc 111 River Street Hoboken NJ 07030 NJ 07030 (201) 748-6011 fax (201) 748-6008 or online at httpwwwwileycomgopermission

Limit of LiabilityDisclaimer of Warranty While the publisher and author have used their best efforts in preparing this book they make no representations or warranties with respect to the accuaracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose No warranty may be created or extended by sales representation or written sales materials The advice and strategies contained herein may not be suitable for your situation You should consult with a professional where appropriate Neither the publisher nor author shall be liable for any loss of profit or any other commercial damages including but not limited to special incidental consequential or other damages

For general information on our other products and services or for technical support Please contact our Customer Care Department within the United States at (800) 762-2974 outside the United States at (371) 572-3993 or fax (317) 572-4002

Wiley also publishes its books in variety of electronic formats Some content that appears in print may not be available in electronic formats For more information about Wiley products visit our web site at wwwwileycom

Library of Congress Cataloging-in-Publication Data

Scanning probe microscopy for industrial applications nanomechanical characterization edited by Dalia G Yablon pages cm Includes bibliographical references and index ISBN 978-1-118-28823-8 (hardback)1 MaterialsndashMicroscopy 2 Scanning probe microscopyndashIndustrial applications I Yablon Dalia G 1975ndash TA41723S33 2013 6201prime127ndashdc23

2013009638

Printed in the United States of America

10 9 8 7 6 5 4 3 2 1

Dedicated with love toAndrew

andAyelet Daniela Mati and Eli

vii

Contents

Contributors List xiii

Preface xv

Acknowledgments xix

1 Overview of Atomic Force Microscopy 1

Dalia G Yablon

11 A Word on Nomenclature 212 Atomic Force MicroscopymdashThe Appeal to Industrial RampD 213 Mechanical Properties 514 Overview of AFM Operation 615 Nanomechanical Methods Surveyed in Book 1116 Industries Represented 13Acknowledgments 14References 14

2 Understanding the TipndashSample Contact An Overview of Contact Mechanics from the Macro- to the Nanoscale 15

Tevis D B Jacobs C Mathew Mate Kevin T Turner and Robert W Carpick

21 Hertz Equations for Elastic Contact 1522 Adhesive Contacts 2223 Further Extensions of Continuum Contact Mechanics Models 2924 Thin Films 3425 Tangential Forces 3726 Application of Continuum Mechanics to Nanoscale Contacts 42Acknowledgments 44Appendix 2A Surface Energy and Work of Adhesion 44References 45

viii Contents

3 Understanding Surface Forces Using Static and Dynamic ApproachndashRetraction Curves 49

Sudharsan Balasubramaniam Daniel Kiracofe and Arvind Raman

31 TipndashSample Interaction Forces 5332 Static FndashZ Curves 5833 Dynamic AmplitudePhasendashDistance Curves 6934 Brief Guide to VEDA Simulations 7835 Conclusions 90Glossary 91References 93

4 Phase Imaging 95

Dalia G Yablon and Greg Haugstad

41 Introduction 9542 Bistability Attractive and Repulsive Mode 9743 Complications in Phase Quantification 107References 113

5 Dynamic Contact AFM Methods for Nanomechanical Properties 115

Donna C Hurley and Jason P Killgore

51 Introduction 11552 Force Modulation Microscopy (FMM) 12153 Contact Resonance (CR) Techniques 12554 Comparison of FMM and CR-FM 13655 Other Dynamic Contact Approaches 13856 Summary and Conclusions 140Acknowledgments 141Appendix 5A Data Analysis Procedure for Contact Resonance Spectroscopy Measurements 141References 145

6 Guide to Best Practices for AFM Users 150

Greg Haugstad

61 ForcendashDistance MeasurementsmdashInstrumental Sources of Nonideality 151

62 ForcendashDistance MeasurementsmdashPhysical Sources of Nonideality 157

References 161

Contents ix

7 Nanoindentation Measurements of Mechanical Properties of Very Thin Films and Nanostructured Materials at High Spatial Resolution 162

Steve J Bull

71 Introduction 16272 Bulk Materials 16373 Coatings 17674 Conclusions 188Acknowledgments 188References 188

8 Scanning Probe Microscopy for Critical Measurements in the Semiconductor Industry 190

Johann Foucher

81 Introduction 19082 Critical Dimension in the Semiconductor Industry 19183 CD Metrology Techniques for Production 19284 Obtaining Accurate CD in the Semiconductor Industry 19485 Hybrid Metrology as a Final Solution to Overcome

CD-AFM CD-SEM and Scatterometry Intrinsic Limitations 20386 Conclusion 208References 208

9 Atomic Force Microscopy of Polymers 210

Andy H Tsou and Dalia G Yablon

91 Introduction 21092 Tapping Phase AFM 21393 Nanoindentation 21794 Force Modulation 21895 Pulsed Force Imaging 21996 ForcendashVolume AFM 22097 HarmoniX and Peak Force QNM Imaging 22298 Summary 227References 229

10 Unraveling Links between Food Structure and Function with Probe Microscopy 232

A Patrick Gunning and Victor J Morris

101 Introduction 232102 Gels and Thickeners Molecular Networks 236

x Contents

103 Emulsions and Foams ProteinndashSurfactant Competition 238104 Interfacial Structure and Digestion Designer Interfaces 241105 Force Spectroscopy Model Emulsions 244106 Force Spectroscopy Origins of Bioactivity 247107 Conclusions 248References 249

11 Microcantilever Sensors for Petrochemical Applications 251

Alan M Schilowitz

111 Introduction 251112 Background 252113 Applications 257114 Conclusion 266References 267

12 Applications of Scanning Probe Methods in Cosmetic Science 270

Gustavo S Luengo and Anthony Galliano

121 Introduction 270122 Substrates of Cosmetics 271123 Mechanical Properties and Modifications by Cosmetic Products 274124 Scanning Probe Technologies Adapted to Cosmetic Science 275125 Conclusions 285References 285

13 Applications of Scanning Probe Microscopy and Nanomechanical Analysis in Pharmaceutical Development 287

Matthew S Lamm

131 Introduction 287132 Applications of SPM Imaging 288133 SPM as a Screening Tool 291134 Applications of Nanoindentation 293135 Conclusion 299Acknowledgments 299References 300

14 Comparative Nanomechanical Study of Multiharmonic Force Microscopy and Nanoindentation on Low Dielectric Constant Materials 302

Katharine Walz Robin King Willi Volksen Geraud Dubois Jane Frommer and Kumar Virwani

141 Introduction 302142 Experimental 308

Contents xi

143 Results and Discussions 311144 Conclusions 319Acknowledgments 320References 320

15 Nanomechanical Characterization of Biomaterial Surfaces Polymer Coatings That Elute Drugs 323

Klaus Wormuth and Greg Haugstad

151 Introduction 323152 Materials and Methods 325153 Dexamethasone in PBMA or PBMAndashPLMA Polymer Blends 327154 Simvastatin in PEOndashPBT Copolymers 337155 Concluding Comments 340Acknowledgments 341References 341

Index 342

xiii

Contributors List

Sudharsan BalasubramaniamDepartment of Mechanical EngineeringPurdue UniversityWest Lafayette IN

Steve J BullSchool of Chemical Engineering and Advanced MaterialsUniversity of NewcastleNewcastle upon Tyne United Kingdom

Robert W CarpickDepartment of Mechanical Engineering and Applied MechanicsUniversity of PennsylvaniaPhiladelphia PA

Geraud DuboisIBM Almaden Research CenterSan Jose CA

Johann FoucherLETIndashCEAGrenoble France

Jane FrommerIBM Almaden Research CenterSan Jose CA

Anthony GallianoLrsquoOreal Research and InnovationAulnay sous Bois France

A Patrick GunningInstitute of Food ResearchNorwich Research ParkNorwich United Kingdom

Greg HaugstadUniversity of MinnesotaMinneapolis MN

Donna C HurleyNational Institute of Standards and TechnologyBoulder CO

Tevis D B JacobsDepartment of Materials Science and EngineeringUniversity of PennsylvaniaPhiladelphia PA

Jason P KillgoreNational Institute of Standards and TechnologyBoulder CO

Robin KingIBM Almaden Research CenterSan Jose CA

Daniel KiracofeDepartment of Mechanical EngineeringPurdue UniversityWest Lafayette IN

Matthew S LammMerck Research LaboratoriesMerck amp CoSummit NJ

Gustavo S LuengoLrsquoOreal Research and InnovationAulnay sous Bois France

xiv Contributors List

C Mathew MateHGST A Western Digital CompanySan Jose CA

Victor J MorrisInstitute of Food ResearchNorwich Research ParkNorwich United Kingdom

Arvind RamanDepartment of Mechanical EngineeringPurdue UniversityWest Lafayette IN

Alan M SchilowitzExxonMobil Research and EngineeringAnnandale NJ

Andy H TsouExxonMobil Research and EngineeringAnnandale NJ

Kevin T TurnerDepartment of Mechanical Engineering and Applied MechanicsUniversity of PennsylvaniaPhiladelphia PA

Kumar VirwaniIBM Almaden Research CenterSan Jose CA

Willi VolksenIBM Almaden Research CenterSan Jose CA

Katharine WalzIBM Almaden Research CenterSan Jose CA

Klaus WormuthInstitute for Physical ChemistryUniversity of CologneCologneGermany

Dalia G YablonSurfaceChar LLCSharon MA

xv

Preface

The idea for this book came a couple of years ago after perusing atomic force microscopy (AFM) book collections at many professional society conferences over the years and finding that there was constantly a key topic missing how AFM was used practically in industrial RampD This topic is close to my heart as I have pursued a career developing novel methods and applying scanning probe microscopy (SPM) to a wide variety of industrial RampD problems at ExxonMobil Research and Engineeringrsquos Corporate Strategic Research in New Jersey for over a decade At ExxonMobil I have had the opportunity to explore application of AFM in the three vast sectors in the petroleum industry the downstream chemi-cals and upstream sector studying a wide variety of problems from corrosion to lubrication to geology to polymer materials All the AFM-focused books I found were research reviews on focused topical areas or textbooks on operating princi-ples and theory So though these books offered excellent technical treatments of a variety of topics the incredible utility applicability and flexibility of AFM to solve real-world everyday problems was lacking

My background in graduate school was actually in scanning tunneling micro-scopy (STM) as I studied self-assembled monolayers at the liquidndashsolid interface in George Flynnrsquos lab at Columbia Universityrsquos chemistry department not the most rel-evant ldquoreal-world industrialrdquo problem However being an STM lab in the 1990s we also had AFMs in the lab (since they shared the same ldquobrainrdquo the controller) so every once in a while we would help out a colleague and study their samples As I moved to ExxonMobil after graduate school in 2002 my intent was not necessarily to continue in scanning probe microscopy In fact it was serendipity that upon arriving at ExxonMobilrsquos Corporate Strategic Research labs as a postdoctoral fellow I found an unused Digital Instruments Nanoscope III controller with a fully accessorized Multimode The rest as they say is history

Over my time at ExxonMobil I ran into many like-minded colleagues in other industrial facilities using AFM and we would hold fascinating discussions of how the technology was used in onersquos particular field On the other side I could sense great interest from my academic colleagues that industrial researchers were using SPM to such a great extent as well as recent graduates trying to connect their hard-earned knowledge and expertise in the field as students to practical useful areas A book covering this material for the wider audience did indeed seem missing

And so I began to survey the industrial RampD communityndashndashboth members I had known for years and new onesndashndashto gauge interest in putting such a volume together I was inspired by the enthusiasm I received from my colleagues and decided to go forward Then came the question of what kind of AFM As surveyed in Chapter 1

xvi Preface

AFM is a tremendously broad field that characterizes a wide variety of material and surface properties However at its heart AFM is a high-resolution nanomechanical probe of the surface That probe can be coated with an electronically conducting layer or magnetic material or even replaced by an optical fiber tip in order to probe a variety of properties However a strong majority of the industrial RampD both in my own direct personal experience and those that I was familiar with was conducted with mechanical probing of properties Of course there are some key counterexamples that stand out such as electrostatic force microscopy to probe electrical properties of conducting and semiconducting materials and magnetic force microscopy to probe magnetic storage devices Perhaps those applications will be covered in a future volume But the overwhelming use of AFM in industrial RampD was nanomechanical characterization and thus it was quickly chosen as the focus for this book

It was important that this book be self-contained While the bookrsquos focus is on the applications it is intended to be accessible to a broad audience both from a professional and educational background so that everyone from an advanced undergraduate to a seasoned professional will benefit Various professional backgrounds should benefit as well from those in an academic or academic-like environment who wish to learn how AFM is used in an industrial environment to those in an industrial environment eager to learn what other applications are possible While a freshman chemistry and physics background is assumed no prior experience with AFM is assumed There is certainly advanced information in each chapter so that the content should appeal to a variety of backgrounds in the AFM community from novice to expert

With this aim of appealing to a broad audience I include a tutorial on nanomechani-cal methods in the first half of the book so that applications could be understood and appreciated with context without having to refer to other books or references I chose the topics that I felt were most relevant to studying nanomechanical characterization in the practical industrial environment contact mechanics force curves phase imaging dynamic contact methods and nanoindentation Note that this is not an exhaustive list of all the nanomechanical characterization methods available with AFM

I worked closely with my co-authors to make these overview chapters as practical as possible and not overwhelming in theoretical detail several chapters include worked examples of useful calculations (eg using Hertz mechanics with and without adhesion to model a contact in Chapter 2 or data analysis of contact resonance measurements in Chapter 5) These chapters are meant to be overviews to introduce the terminology and key concepts They are not meant to be an exhaustive step-by-step guide to the operation of various techniques those treatments are done elsewhere and are referenced in the book Chapter 6 is a somewhat unique chapter on ldquoBest Practices in AFM Imagingrdquo that I co-wrote with my colleague Greg Haugstad This chapter outlines some of the common pitfalls encountered by AFM users again with a practical approach to what the most common and most important problems are and how to avoid them

Chapters 7ndash15 then dive into how AFM has been used in a variety of industrial RampD areas to explore (a) phenomena and processes such as new formulation development in pharmaceuticals the effect of humidity and temperature on biomate-rials and nanostructure formation in food processing (b) characterization of various

Preface xvii

materials including polymer blends and composites in the chemicals sector skin and hair in personal care products coatings and thin dielectric films in the semiconductor industry (c) AFMrsquos function as a key quality control tool in semiconductor metrology and (d) a slightly different application of AFM microcantilevers as a physical and chemical sensors in the petroleum industry

My hope is that this book will show what an incredibly useful tool AFM has become to industrial RampD in its short 25 years It is currently an indispensable tool in any microscopy laboratory despite its commercial youth So while the AFM field con-tinues to grow and develop in terms of its capability and understanding spearheaded by many notable groups in academia and elsewhere it has already penetrated significantly into a wide variety of industrial labs This trend will only continue and grow As the penetration into industrial labs continues it is my hope that the reader understands not only how the AFM capability increasingly benefits industry but also that industrial labs are quickly becoming a partner in driving AFMrsquos innovation and research as its commercial and practical utility and importance continue to thrive

Dalia G YablonSharon MA

xix

Acknowledgments

I have been blessed to have wonderful colleagues to share ideas with and gain inspiration from during this process First and foremost I must thank my co-authors on this book Working with each and every author has been an absolute pleasure I had the distinct pleasure of working closely with Andy Tsou and Alan Schilowitz for many years at ExxonMobil Some of the other co-authors I have been fortunate to count as longstanding collaborators including Rob Carpick Greg Haugstad Jason Killgore and Donna Hurley I thank them for their support and enthusiastic partic-ipation for this particular project From all I learned so much technically and made many new friends in the process This book would not have been possible without everyonersquos collective hard work to share their insights and wisdom with us

I would like to thank my colleagues at ExxonMobil with whom I have had the pleasure of developing our AFM capabilities in our AFM lab Specifically Jean Grabowski deserves a special callout and has been a tremendous asset to our efforts as well as a genuine friend along the way I also would like to thank Rebecca Locker Ishita Chakraborty and Daniel Kiracofe I also would like to thank Steve Minne and Roger Proksch for help and guidance in the incipient stages of this project as well as Emily Rapalino and Susan Klein for advice and proofreading

Most of all I have to thank my husband Andrew for his absolute unwavering support and cheerleading throughout the past year and a half His confidence in me was a source of real motivation and inspiration to carry forward despite obstacles And logistically of course I have to thank him for providing childcare to our chil-dren in the evenings and weekends (covering for me on many many bath nights and storyreadingndashndashsuch as on this night) so that I could complete this book one which hopefully one day they might be interested in enough to read and understand

1

Scanning Probe Microscopy in Industrial Applications Nanomechanical Characterization First Edition Edited by Dalia G Yablon copy 2014 John Wiley amp Sons Inc Published 2014 by John Wiley amp Sons Inc

Overview of Atomic Force MicroscopyDalia G Yablon

SurfaceChar LLC Sharon MA

Atomic force microscopy (AFM) is a family of nanoscale characterization techniques that has exploded onto the overall characterization and nanotechnology field as its versatility and high resolution continue to feed a dizzying variety of disciplines from biology to physics and chemistry to engineering AFM entered the scientific arena in 1981 with the now famous invention of its older sibling in the scanning probe micro-scopy family the scanning tunneling microscope (STM) in the IBM Zurich labs of Gerd Binnig and Heinrich Rohrer for which they received the Nobel Prize in Physics in 1986 The AFM was then invented in 1986 by Gerd Binnig Cal Quate and Christoph Gerber [1] Together STM and AFM formed the scanning probe micros-copy (SPM) family which includes other methods such as near-field scanning optical microscopy (NSOM)

The STM spawned the next 25 years of the continuously developing field of scanning probe microscopy and specifically atomic force microscopy which now includes dozens of different methods under its name to probe various propertiesmdashincluding mechanical electrical magnetic chemical and opticalmdashof materials and surfaces Atomic force microscopy is a powerful tool in various research enterprises It is found in practically any university characterization facility alongside optical and electron microscopes and most undergraduates in science or engineering fields at this point are at least familiar with it if not have performed a laboratory experiment in their undergraduate curriculum

The focus of this book is to understand and appreciate the role this young technique has played in industrial research and development (RampD) AFM has penetrated into a variety of industrial research sectors as witnessed by the diverse set of applications described in this book Alongside electron and optical microscopy which have been

Chapter 1

2 Chapter 1 Overview of Atomic Force Microscopy

around for decades and have reached an impressive level of commercial maturity and ease of use AFM has become a vital characterization method despite its youth and continued technical evolution So though AFM is still an active area of academic research as its capabilities continue to develop and be better understood it has proven to be a useful microscopy to address industrial and commercial needs from quality control and assurance to product formulation and process monitoring

The goal of this introductory chapter is to provide an overview of AFM to nonspecialists and introduce the various topics that are the subject of subsequent individual chapters As such this chapter will provide a brief review of the begin-nings of AFM and special features that make it particularly suited for industrial research Then a brief overview of AFM operation will be presented including the hardware software calibrations involved and finally the different nanomechanical methods that will be described in detail both in theory and application Entire books are written on AFM operation and this chapter is not meant to be an exhaustive introduction to its operation merely serving to provide enough information for the rest of the book to be followed intelligently Readers interested in more detail about AFM operation can consult a number of excellent books on the topic [2ndash5]

11 A Word on nomenclAture

Before the rest of this chapter continues some definitions are in order Similar to many other surface science techniques AFM has succumbed to a somewhat unwieldy mess of abbreviations and jargon that has become a hard-to-navigate alphabet soup SPM refers to an umbrella of a variety of methods Methods that fall under SPM include per-haps its most famous member atomic force microscopy (AFM) in addition to others such as scanning tunneling microscopy (STM) near-field scanning optical microscopy (NSOM or SNOM depending on the continent) and other lithographic methods And then within AFM there are dozens of methods that rely on the AFM probendashsample interactions to provide a variety of material properties including electrical optical magnetic and mechanical properties To date there are dozens of SPMAFM-based methods It is beyond the scope of this book to list andor define all the related methods These methods characterize a huge variety of material properties Some of the more common methods have been included in Figure 11 with the category of properties that they measure The wide diversity of properties of materials that can be measured with AFM is clear The focus of this book is on nanomechanical characterization due to its broad appeal in industrial RampD Again Figure 11 is offered to demonstrate the variety and flavor of properties that can be probed with SPM

12 Atomic Force microscopymdashthe AppeAl to industriAl rampd

This book specifically focuses on AFM which is the method that has most penetrated the general characterization and industrial research fields AFM has key features that make it especially attractive to industrial research and development First its resolution

12 Atomic Force MicroscopymdashThe Appeal to Industrial RampD 3

the heart of its utility as a characterization tool is indeed impressive with 5- to 10-nm lateral resolution and angstrom vertical resolution achieved routinely on commercial instruments with commercial cantilevers The limits of lateral resolution continues to be pushed with specialized techniques and cantilevers and it has reached true ldquoatomicrdquo resolution where point defects of certain materials can now be imaged under certain operating parameters and imaging modes especially under liquid [6ndash8] True atomic resolution is still currently mostly achieved through STM which operates through an entirely different tipndashsample interaction mechanism based on quantum mechanical tunneling Many of these STM studies are conducted in ultrahigh vacuum (UHV) though some are performed in ambient and liquid conditions It is fair to say that AFM is catching up however with recent groundbreaking work imaging cyclic aromatic molecules on Cu[111] [10 11]

A second critical feature of AFM that makes it particularly amenable to industrial research is the flexibility of the environment in which it can operate Despite being a high-resolution microscopy that rivals the resolution of electron microscopy AFM can operate in an ambient or even liquid environment with minimal compromise to its resolution The ability to work in ldquoreal-worldrdquo environments makes it a critical tool for many industrial RampD applications where the research emphasis is consis-tently to understand mechanisms and materials in real-world situations as opposed to

Figure 11 Schematic of AFM as an umbrella technique that includes a wide variety of methods to probe various properties of materials Some of these methods are shown here

4 Chapter 1 Overview of Atomic Force Microscopy

idealized materials and conditions that typically exist in academic research endeavors In many cases the researcher may not want the sample to be forced into a pristine vacuum environment where perhaps key materials or components will be evacuated and thus missed in the characterization effort In addition the flexibility of an envi-ronment permits in situ characterization of processes For example processes such as corrosion lubrication catalyst dissolution crystal growth (for active pharmaceutical ingredient API) can all be studied in situ with AFM as a unique attribute of this microscopy The power of in situ measurements for biological applications is simple as many biological processes and materials simple cannot survive ex situ

A corollary of the utility of the AFMrsquos flexible environment to industrial RampD is the flexibility of type and shape of surface or material that AFM can image There are virtually no restrictions on the size of a material that can be probed with AFM Many commercial ldquolarge-samplerdquo AFMs currently exist that can image any design or shape of a surface from engine parts to thin films Indeed one of the AFMs built in the early 1990s (Topometrix Explorer) was advertised as being able to be placed on top of jet wings to image fractures In addition there are practically no mechanical or electrical restrictions on the type of sample an AFM can image Unlike electron microscopy or STM that require conducting or semiconducting surfaces to image or avoid artifacts [or in the case of insulating surface in scanning electron microscopy (SEM) that requires deposition of a thin conducting layer such as Cr] AFM can image insulating surfaces in addition to conducting or semiconducting surfaces with minimal sample prep Typically the most significant requirement on the sample for an AFM is that it be smooth Commercial instruments will have a maximum z (vertical) range that can be imaged dictated by the design and desired resolution of the particular instrument The maximum z range is typically from 1 microm (for high-resolution studies) to several microns (for lower resolution studies) meaning that the sample cannot have features a tilt or overall roughness that exceed that limit in order for the AFM to image effectively Smooth surfaces are often prepared with the use of an ultramicrotome An ultramicrotome is simply a very controlled and precise way of cutting of thin (down to 100 nm) sections at controlled speeds and in a controlled fashion to result in ultrasmooth surfaces of nanometer or less than nanometer rough-enss The ultramicrotome typically employs the use of a glass knife for a coarse cut and then a diamond knife for a fine cut Cryomicrotomy is simply ultramicrotomy at cold temperatures Cryomicrotomy is typically used to prepare polymer surfaces to cut below the glass transition temperature of the material depending on the instru-ment cryomicrotomes can cool down to -180degC

Finally AFM is fundamentally a mechanical probe of the surface through actual contact between the tip and sample So in addition to the ability to provide high resolution on surface features the goal of any form of microscopy is the unique tipndashsample interaction mechanism enabling the probing of many properties most obvious of which is mechanical properties Mechanical properties covers a wide range of materials and properties Entire textbooks and courses are devoted to the studies of mechanics for example by Callister and Rethwisch [12] We very briefly review some of the main concepts below to provide enough of a background for the rest of the chapter

13 Mechanical Properties 5

This edition focuses specifically on nanomechanical characterization Probing other material properties such as optical electrical or magnetic properties is not covered in this book The reason is that though these other properties have been successfully probed in various industrial RampD applications using AFM to probe mechanical properties of surfaces is more ubiquitous in industrial RampD and so was a logical focus for a first book on SPM in industrial applications

13 mechAnicAl properties

When one considers mechanical properties a range of descriptors come to mind including ldquostiffnessrdquo ldquotoughnessrdquo ldquohardnessrdquo ldquobrittlenessrdquo and ldquoductilityrdquo to name a few Each of these properties has their own mathematical expressions with associ-ated experimental methods of measuring them An interaction between two materials can be typically categorized into either elastic (or recoverable) or dissipative or plastic (permanent or nonrecoverable) interaction On the bulk scale most mechanical analysis of materials relies on measuring their strain (material displacement) as a function of stress (pressure exerted on the material) or vica versa The measurement and analysis of stress versus strain behavior of various materials is a rich and active field and there are many excellent textbooks that describe it [12] The description provided below is simplified to provide a context for these measurements ultimately described on the nanoscale

Elastic deformation of a material is typically nonpermanent or reversible and exists in a regime where the strain (material displacement) is proportional to the stress (pressure exerted on the material) by a proportionality factor known as a modulus (though in reality no material is truly only elastic) Elastic materials thus typically follow Hookersquos law Figure 12 shows a representative stress versus strain curve for materials highlighting the elastic region (blue) Additionally in elastic

Plastic

Fracture

Strain

Elastic

Str

ess

Figure 12 Basic stress vs strain curve showing three mechanical regimes of elastic deformation plastic deformation and fracture For color details please see color plate section

6 Chapter 1 Overview of Atomic Force Microscopy

materials the loading curve (as force is applied) is identical to the unloading curve (as the force is removed) Modulus is a key parameter used to describe elastic behavior Depending on the direction in which a material is being stressed the material will have different moduli (eg a tensile modulus a shear modulus etc)

Plastic deformation is on the other end permanent and nonrecoverable Plastic deformation does not follow Hookersquos law and so stress is not proportional to strain In Figure 12 the region of plastic deformation is shown in green In contrast to elastic deformation the loading curve is not identical to the unloading curve and in fact the unloading curve returns to a nonzero strain at a zero stress level There are many common properties associated with plastic deformation including yielding (onset of plastic deformation) ductility (plastic deformation at a fracture) and hardness (resistance to localized plastic deformation)

Finally there is another very important class of materials called viscoelastic materials where the elastic properties are frequency dependent For example when a stress such as pulling is exerted on the material the materialrsquos response is rate dependent Perhaps the most familiar viscoelastic material to most readers from childhood is Silly Putty where its response to pulling depends on how fast it is pulled If Silly Putty is pulled very slowly the material stretches out very nicely If Silly Putty is pulled very fast it will quickly break Polymers and rubbers two materials that play a significant role in consumer products and which are character-ized quite heavily with AFM are also viscoelastic materials

14 overvieW oF AFm operAtion

141 AFm hardware

A schematic of AFM hardware is shown in Figure 13 The main components of the AFM are (1) the cantilever (2) optical detection system (3) x-y-z scanner and (4) feedback loop

142 cantilevers and probes

The heart of the AFM lies in its cantileverprobe assembly that interacts or probes materials to provide the information of interest The technology to fabricate AFM can-tilevers takes advantage of technology developed for the semiconductor industry to make similar scale devices and features out of silicon Thus today AFM cantilevers are typically made by micromachining silicon (single-crystal Si) or silicon nitride (Si

3N

4) The dimensions of a cantilever vary and dictate the stiffness or spring

constant of the cantilever Depending on the material that needs to be probed and the mode of operation one might pick a lever with a stiffer (higher) or softer (lower) spring constant Cantilevers are generally either rectangular in geometry (Si or Si

3N

4)

or triangular (Si3N

4) with now hybrid cantilevers being manufactured (eg Si tips on

SiN cantilevers) Generally the Si3N

4 probes have lower force constants and might be

used for imaging in liquid among other applications whereas the Si probes can be brittle and tend to break or chip in contact when scanning a surface Typically the

14 Overview of AFM Operation 7

cantilevers are hundreds of microns in length tens of microns in width and a few microns in thickness for reference the diameter of a human hair is approximately 100 microm Cantilevers can be coated with gold or aluminum to provide high reflectivity Commercial cantilevers are also available with other thin metal coatings for magnetic or electrical imaging Cantilevers and probes can also be functionalized chemically or biologically for specific interactions

Probes are grown at the end of the lever and vary in shape and sharpness A sharper probe can often improve the lateral resolution of the AFM Probe design is a continuously evolving field as manufacturers strive for the most robust reproducible and sharp tips Cantileverprobe assemblies are mounted into a cantilever holder that typically includes a piezoceramic element (described below) that can vibrate the can-tilever at a given frequencymdasheither above at or below its resonancemdashdepending on the desired tipndashsample interaction or AFM mode

143 optical detection system

In order to track the motion of the cantileverprobe assembly as it scans a surface an AFM typically includes an optical detection system that consists of a laser reflected off the back side of the cantilever and directed toward a position sensitive detector

Figure 13 Schematic of basic operating principles of AFM A laser is focused and directed off the back end of a cantilever and directed toward a position-sensitive director (PSD) to monitor its vertical and lateral motion as it interacts with the surface Depending on the mode used to image the sample the cantilever can be vibrated with a shake piezo for dynamic imaging modes that involve the resonance of the cantilever

8 Chapter 1 Overview of Atomic Force Microscopy

(PSD) as seen in Figure 13 Optical detection systems are the most common way to track cantilever motion in commercial AFM systems However other methods exist to bypass optical detection systems (in eg a dark medium where the laser light would be absorbed) with self-actuated cantilevers where their motion is read by piezos integrated into the levers The laser is typically a visible photodiode though some commercial instruments have implemented superluminescent diode (SLD) laser The laser is detected by a four-segment position-sensitive detector (PSD) which can track the vertical and lateral motion of the cantilever accurately which can be converted to units of nanometers or displacement through careful calibration described below

144 x-y-z scanner

An AFM can operate either by scanning the sample relative to a stationary tip (sample scanning) or scanning the tip relative to a stationary sample (tip scanning) Each has its own advantages in terms of size of sample it can accommodate (tip scanning more flexible) ability to accessorize (tip scanning again more flexible) and stability and signal to noise (sample scanning easier to build with better specifications) Each shares the requirement to move the tip relative to the sample in a highly accurate way with minimal noise

Tipndashsample motion is often accomplished in commercial instruments by piezoelectric materials which are materials that respond either by expansion or con-traction in response to an applied voltage These kinds of materials provide the ability for very fine motion (nanometers to microns) Piezoelectric materials can be made in several shapes including tubes disks and bimorphs depending on the range and geom-etry of motion required Although piezoelectric materials are very effective at moving tip or sample they are plagued by nonlinear behavior such as hysteresis and creep that has serious consequences for accurate AFM imaging and interpretation of data Artifacts induced by piezoelectric materials are reviewed in detail in Chapter 6

Piezoelectric scanners can be programmed to move with either open-loop or closed-loop feedback circuits In open-loop scanners the piezo is moved by programs to ramp x and y bias voltages to move the prescribed amounts So in open-loop scan-ners if the piezo has to move a large distance or move around on the sample there can be considerable hysteresis of creep with the best recourse of patience or movingzooming in and out in small increments since hysteresiscreep is proportional to the displacement or motion the scanner is trying to execute

One solution to this problem is to use closed-loop scanners that use additional x-y-z sensors to independently measure and then correct for scanner movement In closed-loop operation feedback circuits are continually adjusted to the applied scanner biases to correct for the x-y motion and to achieve the desired motion It is important to note that correction of x-y motion in an AFM is slightly different than correction of z motion This is due to the fact that x-y motion can be predicted whereas z motion cannot as it depends on the surface topography of the sample being scanned Therefore typically there are active feedback circuits to control x and y

Scanning Probe Microscopy in Industrial Applications

Scanning Probe Microscopy in Industrial ApplicationsNanomechanical Characterization

Edited by

Dalia G Yablon

Copyright copy 2014 by John Wiley amp Sons Inc All rights reserved

Published by John Wiley amp Sons Inc Hoboken New JerseyPublished simultaneously in Canada

No part of this publication may be reproduced stored in a retrieval system or transmitted in any form or by any means electronic mechanical photocopying recording scanning or otherwise except as permitted under Section 107 or 108 of the 1976 United Stated Copyright Act without either the prior written permission of the Published or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center Inc 222 Rosewood Drive Danvers MA 01923 (978) 750-8400 fax (978) 750-4470 or on the web at wwwcopyrightcom Requests to the Publisher for permission should be addressed to the Permissions Department John Wiley amp Sons Inc 111 River Street Hoboken NJ 07030 NJ 07030 (201) 748-6011 fax (201) 748-6008 or online at httpwwwwileycomgopermission

Limit of LiabilityDisclaimer of Warranty While the publisher and author have used their best efforts in preparing this book they make no representations or warranties with respect to the accuaracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose No warranty may be created or extended by sales representation or written sales materials The advice and strategies contained herein may not be suitable for your situation You should consult with a professional where appropriate Neither the publisher nor author shall be liable for any loss of profit or any other commercial damages including but not limited to special incidental consequential or other damages

For general information on our other products and services or for technical support Please contact our Customer Care Department within the United States at (800) 762-2974 outside the United States at (371) 572-3993 or fax (317) 572-4002

Wiley also publishes its books in variety of electronic formats Some content that appears in print may not be available in electronic formats For more information about Wiley products visit our web site at wwwwileycom

Library of Congress Cataloging-in-Publication Data

Scanning probe microscopy for industrial applications nanomechanical characterization edited by Dalia G Yablon pages cm Includes bibliographical references and index ISBN 978-1-118-28823-8 (hardback)1 MaterialsndashMicroscopy 2 Scanning probe microscopyndashIndustrial applications I Yablon Dalia G 1975ndash TA41723S33 2013 6201prime127ndashdc23

2013009638

Printed in the United States of America

10 9 8 7 6 5 4 3 2 1

Dedicated with love toAndrew

andAyelet Daniela Mati and Eli

vii

Contents

Contributors List xiii

Preface xv

Acknowledgments xix

1 Overview of Atomic Force Microscopy 1

Dalia G Yablon

11 A Word on Nomenclature 212 Atomic Force MicroscopymdashThe Appeal to Industrial RampD 213 Mechanical Properties 514 Overview of AFM Operation 615 Nanomechanical Methods Surveyed in Book 1116 Industries Represented 13Acknowledgments 14References 14

2 Understanding the TipndashSample Contact An Overview of Contact Mechanics from the Macro- to the Nanoscale 15

Tevis D B Jacobs C Mathew Mate Kevin T Turner and Robert W Carpick

21 Hertz Equations for Elastic Contact 1522 Adhesive Contacts 2223 Further Extensions of Continuum Contact Mechanics Models 2924 Thin Films 3425 Tangential Forces 3726 Application of Continuum Mechanics to Nanoscale Contacts 42Acknowledgments 44Appendix 2A Surface Energy and Work of Adhesion 44References 45

viii Contents

3 Understanding Surface Forces Using Static and Dynamic ApproachndashRetraction Curves 49

Sudharsan Balasubramaniam Daniel Kiracofe and Arvind Raman

31 TipndashSample Interaction Forces 5332 Static FndashZ Curves 5833 Dynamic AmplitudePhasendashDistance Curves 6934 Brief Guide to VEDA Simulations 7835 Conclusions 90Glossary 91References 93

4 Phase Imaging 95

Dalia G Yablon and Greg Haugstad

41 Introduction 9542 Bistability Attractive and Repulsive Mode 9743 Complications in Phase Quantification 107References 113

5 Dynamic Contact AFM Methods for Nanomechanical Properties 115

Donna C Hurley and Jason P Killgore

51 Introduction 11552 Force Modulation Microscopy (FMM) 12153 Contact Resonance (CR) Techniques 12554 Comparison of FMM and CR-FM 13655 Other Dynamic Contact Approaches 13856 Summary and Conclusions 140Acknowledgments 141Appendix 5A Data Analysis Procedure for Contact Resonance Spectroscopy Measurements 141References 145

6 Guide to Best Practices for AFM Users 150

Greg Haugstad

61 ForcendashDistance MeasurementsmdashInstrumental Sources of Nonideality 151

62 ForcendashDistance MeasurementsmdashPhysical Sources of Nonideality 157

References 161

Contents ix

7 Nanoindentation Measurements of Mechanical Properties of Very Thin Films and Nanostructured Materials at High Spatial Resolution 162

Steve J Bull

71 Introduction 16272 Bulk Materials 16373 Coatings 17674 Conclusions 188Acknowledgments 188References 188

8 Scanning Probe Microscopy for Critical Measurements in the Semiconductor Industry 190

Johann Foucher

81 Introduction 19082 Critical Dimension in the Semiconductor Industry 19183 CD Metrology Techniques for Production 19284 Obtaining Accurate CD in the Semiconductor Industry 19485 Hybrid Metrology as a Final Solution to Overcome

CD-AFM CD-SEM and Scatterometry Intrinsic Limitations 20386 Conclusion 208References 208

9 Atomic Force Microscopy of Polymers 210

Andy H Tsou and Dalia G Yablon

91 Introduction 21092 Tapping Phase AFM 21393 Nanoindentation 21794 Force Modulation 21895 Pulsed Force Imaging 21996 ForcendashVolume AFM 22097 HarmoniX and Peak Force QNM Imaging 22298 Summary 227References 229

10 Unraveling Links between Food Structure and Function with Probe Microscopy 232

A Patrick Gunning and Victor J Morris

101 Introduction 232102 Gels and Thickeners Molecular Networks 236

x Contents

103 Emulsions and Foams ProteinndashSurfactant Competition 238104 Interfacial Structure and Digestion Designer Interfaces 241105 Force Spectroscopy Model Emulsions 244106 Force Spectroscopy Origins of Bioactivity 247107 Conclusions 248References 249

11 Microcantilever Sensors for Petrochemical Applications 251

Alan M Schilowitz

111 Introduction 251112 Background 252113 Applications 257114 Conclusion 266References 267

12 Applications of Scanning Probe Methods in Cosmetic Science 270

Gustavo S Luengo and Anthony Galliano

121 Introduction 270122 Substrates of Cosmetics 271123 Mechanical Properties and Modifications by Cosmetic Products 274124 Scanning Probe Technologies Adapted to Cosmetic Science 275125 Conclusions 285References 285

13 Applications of Scanning Probe Microscopy and Nanomechanical Analysis in Pharmaceutical Development 287

Matthew S Lamm

131 Introduction 287132 Applications of SPM Imaging 288133 SPM as a Screening Tool 291134 Applications of Nanoindentation 293135 Conclusion 299Acknowledgments 299References 300

14 Comparative Nanomechanical Study of Multiharmonic Force Microscopy and Nanoindentation on Low Dielectric Constant Materials 302

Katharine Walz Robin King Willi Volksen Geraud Dubois Jane Frommer and Kumar Virwani

141 Introduction 302142 Experimental 308

Contents xi

143 Results and Discussions 311144 Conclusions 319Acknowledgments 320References 320

15 Nanomechanical Characterization of Biomaterial Surfaces Polymer Coatings That Elute Drugs 323

Klaus Wormuth and Greg Haugstad

151 Introduction 323152 Materials and Methods 325153 Dexamethasone in PBMA or PBMAndashPLMA Polymer Blends 327154 Simvastatin in PEOndashPBT Copolymers 337155 Concluding Comments 340Acknowledgments 341References 341

Index 342

xiii

Contributors List

Sudharsan BalasubramaniamDepartment of Mechanical EngineeringPurdue UniversityWest Lafayette IN

Steve J BullSchool of Chemical Engineering and Advanced MaterialsUniversity of NewcastleNewcastle upon Tyne United Kingdom

Robert W CarpickDepartment of Mechanical Engineering and Applied MechanicsUniversity of PennsylvaniaPhiladelphia PA

Geraud DuboisIBM Almaden Research CenterSan Jose CA

Johann FoucherLETIndashCEAGrenoble France

Jane FrommerIBM Almaden Research CenterSan Jose CA

Anthony GallianoLrsquoOreal Research and InnovationAulnay sous Bois France

A Patrick GunningInstitute of Food ResearchNorwich Research ParkNorwich United Kingdom

Greg HaugstadUniversity of MinnesotaMinneapolis MN

Donna C HurleyNational Institute of Standards and TechnologyBoulder CO

Tevis D B JacobsDepartment of Materials Science and EngineeringUniversity of PennsylvaniaPhiladelphia PA

Jason P KillgoreNational Institute of Standards and TechnologyBoulder CO

Robin KingIBM Almaden Research CenterSan Jose CA

Daniel KiracofeDepartment of Mechanical EngineeringPurdue UniversityWest Lafayette IN

Matthew S LammMerck Research LaboratoriesMerck amp CoSummit NJ

Gustavo S LuengoLrsquoOreal Research and InnovationAulnay sous Bois France

xiv Contributors List

C Mathew MateHGST A Western Digital CompanySan Jose CA

Victor J MorrisInstitute of Food ResearchNorwich Research ParkNorwich United Kingdom

Arvind RamanDepartment of Mechanical EngineeringPurdue UniversityWest Lafayette IN

Alan M SchilowitzExxonMobil Research and EngineeringAnnandale NJ

Andy H TsouExxonMobil Research and EngineeringAnnandale NJ

Kevin T TurnerDepartment of Mechanical Engineering and Applied MechanicsUniversity of PennsylvaniaPhiladelphia PA

Kumar VirwaniIBM Almaden Research CenterSan Jose CA

Willi VolksenIBM Almaden Research CenterSan Jose CA

Katharine WalzIBM Almaden Research CenterSan Jose CA

Klaus WormuthInstitute for Physical ChemistryUniversity of CologneCologneGermany

Dalia G YablonSurfaceChar LLCSharon MA

xv

Preface

The idea for this book came a couple of years ago after perusing atomic force microscopy (AFM) book collections at many professional society conferences over the years and finding that there was constantly a key topic missing how AFM was used practically in industrial RampD This topic is close to my heart as I have pursued a career developing novel methods and applying scanning probe microscopy (SPM) to a wide variety of industrial RampD problems at ExxonMobil Research and Engineeringrsquos Corporate Strategic Research in New Jersey for over a decade At ExxonMobil I have had the opportunity to explore application of AFM in the three vast sectors in the petroleum industry the downstream chemi-cals and upstream sector studying a wide variety of problems from corrosion to lubrication to geology to polymer materials All the AFM-focused books I found were research reviews on focused topical areas or textbooks on operating princi-ples and theory So though these books offered excellent technical treatments of a variety of topics the incredible utility applicability and flexibility of AFM to solve real-world everyday problems was lacking

My background in graduate school was actually in scanning tunneling micro-scopy (STM) as I studied self-assembled monolayers at the liquidndashsolid interface in George Flynnrsquos lab at Columbia Universityrsquos chemistry department not the most rel-evant ldquoreal-world industrialrdquo problem However being an STM lab in the 1990s we also had AFMs in the lab (since they shared the same ldquobrainrdquo the controller) so every once in a while we would help out a colleague and study their samples As I moved to ExxonMobil after graduate school in 2002 my intent was not necessarily to continue in scanning probe microscopy In fact it was serendipity that upon arriving at ExxonMobilrsquos Corporate Strategic Research labs as a postdoctoral fellow I found an unused Digital Instruments Nanoscope III controller with a fully accessorized Multimode The rest as they say is history

Over my time at ExxonMobil I ran into many like-minded colleagues in other industrial facilities using AFM and we would hold fascinating discussions of how the technology was used in onersquos particular field On the other side I could sense great interest from my academic colleagues that industrial researchers were using SPM to such a great extent as well as recent graduates trying to connect their hard-earned knowledge and expertise in the field as students to practical useful areas A book covering this material for the wider audience did indeed seem missing

And so I began to survey the industrial RampD communityndashndashboth members I had known for years and new onesndashndashto gauge interest in putting such a volume together I was inspired by the enthusiasm I received from my colleagues and decided to go forward Then came the question of what kind of AFM As surveyed in Chapter 1

xvi Preface

AFM is a tremendously broad field that characterizes a wide variety of material and surface properties However at its heart AFM is a high-resolution nanomechanical probe of the surface That probe can be coated with an electronically conducting layer or magnetic material or even replaced by an optical fiber tip in order to probe a variety of properties However a strong majority of the industrial RampD both in my own direct personal experience and those that I was familiar with was conducted with mechanical probing of properties Of course there are some key counterexamples that stand out such as electrostatic force microscopy to probe electrical properties of conducting and semiconducting materials and magnetic force microscopy to probe magnetic storage devices Perhaps those applications will be covered in a future volume But the overwhelming use of AFM in industrial RampD was nanomechanical characterization and thus it was quickly chosen as the focus for this book

It was important that this book be self-contained While the bookrsquos focus is on the applications it is intended to be accessible to a broad audience both from a professional and educational background so that everyone from an advanced undergraduate to a seasoned professional will benefit Various professional backgrounds should benefit as well from those in an academic or academic-like environment who wish to learn how AFM is used in an industrial environment to those in an industrial environment eager to learn what other applications are possible While a freshman chemistry and physics background is assumed no prior experience with AFM is assumed There is certainly advanced information in each chapter so that the content should appeal to a variety of backgrounds in the AFM community from novice to expert

With this aim of appealing to a broad audience I include a tutorial on nanomechani-cal methods in the first half of the book so that applications could be understood and appreciated with context without having to refer to other books or references I chose the topics that I felt were most relevant to studying nanomechanical characterization in the practical industrial environment contact mechanics force curves phase imaging dynamic contact methods and nanoindentation Note that this is not an exhaustive list of all the nanomechanical characterization methods available with AFM

I worked closely with my co-authors to make these overview chapters as practical as possible and not overwhelming in theoretical detail several chapters include worked examples of useful calculations (eg using Hertz mechanics with and without adhesion to model a contact in Chapter 2 or data analysis of contact resonance measurements in Chapter 5) These chapters are meant to be overviews to introduce the terminology and key concepts They are not meant to be an exhaustive step-by-step guide to the operation of various techniques those treatments are done elsewhere and are referenced in the book Chapter 6 is a somewhat unique chapter on ldquoBest Practices in AFM Imagingrdquo that I co-wrote with my colleague Greg Haugstad This chapter outlines some of the common pitfalls encountered by AFM users again with a practical approach to what the most common and most important problems are and how to avoid them

Chapters 7ndash15 then dive into how AFM has been used in a variety of industrial RampD areas to explore (a) phenomena and processes such as new formulation development in pharmaceuticals the effect of humidity and temperature on biomate-rials and nanostructure formation in food processing (b) characterization of various

Preface xvii

materials including polymer blends and composites in the chemicals sector skin and hair in personal care products coatings and thin dielectric films in the semiconductor industry (c) AFMrsquos function as a key quality control tool in semiconductor metrology and (d) a slightly different application of AFM microcantilevers as a physical and chemical sensors in the petroleum industry

My hope is that this book will show what an incredibly useful tool AFM has become to industrial RampD in its short 25 years It is currently an indispensable tool in any microscopy laboratory despite its commercial youth So while the AFM field con-tinues to grow and develop in terms of its capability and understanding spearheaded by many notable groups in academia and elsewhere it has already penetrated significantly into a wide variety of industrial labs This trend will only continue and grow As the penetration into industrial labs continues it is my hope that the reader understands not only how the AFM capability increasingly benefits industry but also that industrial labs are quickly becoming a partner in driving AFMrsquos innovation and research as its commercial and practical utility and importance continue to thrive

Dalia G YablonSharon MA

xix

Acknowledgments

I have been blessed to have wonderful colleagues to share ideas with and gain inspiration from during this process First and foremost I must thank my co-authors on this book Working with each and every author has been an absolute pleasure I had the distinct pleasure of working closely with Andy Tsou and Alan Schilowitz for many years at ExxonMobil Some of the other co-authors I have been fortunate to count as longstanding collaborators including Rob Carpick Greg Haugstad Jason Killgore and Donna Hurley I thank them for their support and enthusiastic partic-ipation for this particular project From all I learned so much technically and made many new friends in the process This book would not have been possible without everyonersquos collective hard work to share their insights and wisdom with us

I would like to thank my colleagues at ExxonMobil with whom I have had the pleasure of developing our AFM capabilities in our AFM lab Specifically Jean Grabowski deserves a special callout and has been a tremendous asset to our efforts as well as a genuine friend along the way I also would like to thank Rebecca Locker Ishita Chakraborty and Daniel Kiracofe I also would like to thank Steve Minne and Roger Proksch for help and guidance in the incipient stages of this project as well as Emily Rapalino and Susan Klein for advice and proofreading

Most of all I have to thank my husband Andrew for his absolute unwavering support and cheerleading throughout the past year and a half His confidence in me was a source of real motivation and inspiration to carry forward despite obstacles And logistically of course I have to thank him for providing childcare to our chil-dren in the evenings and weekends (covering for me on many many bath nights and storyreadingndashndashsuch as on this night) so that I could complete this book one which hopefully one day they might be interested in enough to read and understand

1

Scanning Probe Microscopy in Industrial Applications Nanomechanical Characterization First Edition Edited by Dalia G Yablon copy 2014 John Wiley amp Sons Inc Published 2014 by John Wiley amp Sons Inc

Overview of Atomic Force MicroscopyDalia G Yablon

SurfaceChar LLC Sharon MA

Atomic force microscopy (AFM) is a family of nanoscale characterization techniques that has exploded onto the overall characterization and nanotechnology field as its versatility and high resolution continue to feed a dizzying variety of disciplines from biology to physics and chemistry to engineering AFM entered the scientific arena in 1981 with the now famous invention of its older sibling in the scanning probe micro-scopy family the scanning tunneling microscope (STM) in the IBM Zurich labs of Gerd Binnig and Heinrich Rohrer for which they received the Nobel Prize in Physics in 1986 The AFM was then invented in 1986 by Gerd Binnig Cal Quate and Christoph Gerber [1] Together STM and AFM formed the scanning probe micros-copy (SPM) family which includes other methods such as near-field scanning optical microscopy (NSOM)

The STM spawned the next 25 years of the continuously developing field of scanning probe microscopy and specifically atomic force microscopy which now includes dozens of different methods under its name to probe various propertiesmdashincluding mechanical electrical magnetic chemical and opticalmdashof materials and surfaces Atomic force microscopy is a powerful tool in various research enterprises It is found in practically any university characterization facility alongside optical and electron microscopes and most undergraduates in science or engineering fields at this point are at least familiar with it if not have performed a laboratory experiment in their undergraduate curriculum

The focus of this book is to understand and appreciate the role this young technique has played in industrial research and development (RampD) AFM has penetrated into a variety of industrial research sectors as witnessed by the diverse set of applications described in this book Alongside electron and optical microscopy which have been

Chapter 1

2 Chapter 1 Overview of Atomic Force Microscopy

around for decades and have reached an impressive level of commercial maturity and ease of use AFM has become a vital characterization method despite its youth and continued technical evolution So though AFM is still an active area of academic research as its capabilities continue to develop and be better understood it has proven to be a useful microscopy to address industrial and commercial needs from quality control and assurance to product formulation and process monitoring

The goal of this introductory chapter is to provide an overview of AFM to nonspecialists and introduce the various topics that are the subject of subsequent individual chapters As such this chapter will provide a brief review of the begin-nings of AFM and special features that make it particularly suited for industrial research Then a brief overview of AFM operation will be presented including the hardware software calibrations involved and finally the different nanomechanical methods that will be described in detail both in theory and application Entire books are written on AFM operation and this chapter is not meant to be an exhaustive introduction to its operation merely serving to provide enough information for the rest of the book to be followed intelligently Readers interested in more detail about AFM operation can consult a number of excellent books on the topic [2ndash5]

11 A Word on nomenclAture

Before the rest of this chapter continues some definitions are in order Similar to many other surface science techniques AFM has succumbed to a somewhat unwieldy mess of abbreviations and jargon that has become a hard-to-navigate alphabet soup SPM refers to an umbrella of a variety of methods Methods that fall under SPM include per-haps its most famous member atomic force microscopy (AFM) in addition to others such as scanning tunneling microscopy (STM) near-field scanning optical microscopy (NSOM or SNOM depending on the continent) and other lithographic methods And then within AFM there are dozens of methods that rely on the AFM probendashsample interactions to provide a variety of material properties including electrical optical magnetic and mechanical properties To date there are dozens of SPMAFM-based methods It is beyond the scope of this book to list andor define all the related methods These methods characterize a huge variety of material properties Some of the more common methods have been included in Figure 11 with the category of properties that they measure The wide diversity of properties of materials that can be measured with AFM is clear The focus of this book is on nanomechanical characterization due to its broad appeal in industrial RampD Again Figure 11 is offered to demonstrate the variety and flavor of properties that can be probed with SPM

12 Atomic Force microscopymdashthe AppeAl to industriAl rampd

This book specifically focuses on AFM which is the method that has most penetrated the general characterization and industrial research fields AFM has key features that make it especially attractive to industrial research and development First its resolution

12 Atomic Force MicroscopymdashThe Appeal to Industrial RampD 3

the heart of its utility as a characterization tool is indeed impressive with 5- to 10-nm lateral resolution and angstrom vertical resolution achieved routinely on commercial instruments with commercial cantilevers The limits of lateral resolution continues to be pushed with specialized techniques and cantilevers and it has reached true ldquoatomicrdquo resolution where point defects of certain materials can now be imaged under certain operating parameters and imaging modes especially under liquid [6ndash8] True atomic resolution is still currently mostly achieved through STM which operates through an entirely different tipndashsample interaction mechanism based on quantum mechanical tunneling Many of these STM studies are conducted in ultrahigh vacuum (UHV) though some are performed in ambient and liquid conditions It is fair to say that AFM is catching up however with recent groundbreaking work imaging cyclic aromatic molecules on Cu[111] [10 11]

A second critical feature of AFM that makes it particularly amenable to industrial research is the flexibility of the environment in which it can operate Despite being a high-resolution microscopy that rivals the resolution of electron microscopy AFM can operate in an ambient or even liquid environment with minimal compromise to its resolution The ability to work in ldquoreal-worldrdquo environments makes it a critical tool for many industrial RampD applications where the research emphasis is consis-tently to understand mechanisms and materials in real-world situations as opposed to

Figure 11 Schematic of AFM as an umbrella technique that includes a wide variety of methods to probe various properties of materials Some of these methods are shown here

4 Chapter 1 Overview of Atomic Force Microscopy

idealized materials and conditions that typically exist in academic research endeavors In many cases the researcher may not want the sample to be forced into a pristine vacuum environment where perhaps key materials or components will be evacuated and thus missed in the characterization effort In addition the flexibility of an envi-ronment permits in situ characterization of processes For example processes such as corrosion lubrication catalyst dissolution crystal growth (for active pharmaceutical ingredient API) can all be studied in situ with AFM as a unique attribute of this microscopy The power of in situ measurements for biological applications is simple as many biological processes and materials simple cannot survive ex situ

A corollary of the utility of the AFMrsquos flexible environment to industrial RampD is the flexibility of type and shape of surface or material that AFM can image There are virtually no restrictions on the size of a material that can be probed with AFM Many commercial ldquolarge-samplerdquo AFMs currently exist that can image any design or shape of a surface from engine parts to thin films Indeed one of the AFMs built in the early 1990s (Topometrix Explorer) was advertised as being able to be placed on top of jet wings to image fractures In addition there are practically no mechanical or electrical restrictions on the type of sample an AFM can image Unlike electron microscopy or STM that require conducting or semiconducting surfaces to image or avoid artifacts [or in the case of insulating surface in scanning electron microscopy (SEM) that requires deposition of a thin conducting layer such as Cr] AFM can image insulating surfaces in addition to conducting or semiconducting surfaces with minimal sample prep Typically the most significant requirement on the sample for an AFM is that it be smooth Commercial instruments will have a maximum z (vertical) range that can be imaged dictated by the design and desired resolution of the particular instrument The maximum z range is typically from 1 microm (for high-resolution studies) to several microns (for lower resolution studies) meaning that the sample cannot have features a tilt or overall roughness that exceed that limit in order for the AFM to image effectively Smooth surfaces are often prepared with the use of an ultramicrotome An ultramicrotome is simply a very controlled and precise way of cutting of thin (down to 100 nm) sections at controlled speeds and in a controlled fashion to result in ultrasmooth surfaces of nanometer or less than nanometer rough-enss The ultramicrotome typically employs the use of a glass knife for a coarse cut and then a diamond knife for a fine cut Cryomicrotomy is simply ultramicrotomy at cold temperatures Cryomicrotomy is typically used to prepare polymer surfaces to cut below the glass transition temperature of the material depending on the instru-ment cryomicrotomes can cool down to -180degC

Finally AFM is fundamentally a mechanical probe of the surface through actual contact between the tip and sample So in addition to the ability to provide high resolution on surface features the goal of any form of microscopy is the unique tipndashsample interaction mechanism enabling the probing of many properties most obvious of which is mechanical properties Mechanical properties covers a wide range of materials and properties Entire textbooks and courses are devoted to the studies of mechanics for example by Callister and Rethwisch [12] We very briefly review some of the main concepts below to provide enough of a background for the rest of the chapter

13 Mechanical Properties 5

This edition focuses specifically on nanomechanical characterization Probing other material properties such as optical electrical or magnetic properties is not covered in this book The reason is that though these other properties have been successfully probed in various industrial RampD applications using AFM to probe mechanical properties of surfaces is more ubiquitous in industrial RampD and so was a logical focus for a first book on SPM in industrial applications

13 mechAnicAl properties

When one considers mechanical properties a range of descriptors come to mind including ldquostiffnessrdquo ldquotoughnessrdquo ldquohardnessrdquo ldquobrittlenessrdquo and ldquoductilityrdquo to name a few Each of these properties has their own mathematical expressions with associ-ated experimental methods of measuring them An interaction between two materials can be typically categorized into either elastic (or recoverable) or dissipative or plastic (permanent or nonrecoverable) interaction On the bulk scale most mechanical analysis of materials relies on measuring their strain (material displacement) as a function of stress (pressure exerted on the material) or vica versa The measurement and analysis of stress versus strain behavior of various materials is a rich and active field and there are many excellent textbooks that describe it [12] The description provided below is simplified to provide a context for these measurements ultimately described on the nanoscale

Elastic deformation of a material is typically nonpermanent or reversible and exists in a regime where the strain (material displacement) is proportional to the stress (pressure exerted on the material) by a proportionality factor known as a modulus (though in reality no material is truly only elastic) Elastic materials thus typically follow Hookersquos law Figure 12 shows a representative stress versus strain curve for materials highlighting the elastic region (blue) Additionally in elastic

Plastic

Fracture

Strain

Elastic

Str

ess

Figure 12 Basic stress vs strain curve showing three mechanical regimes of elastic deformation plastic deformation and fracture For color details please see color plate section

6 Chapter 1 Overview of Atomic Force Microscopy

materials the loading curve (as force is applied) is identical to the unloading curve (as the force is removed) Modulus is a key parameter used to describe elastic behavior Depending on the direction in which a material is being stressed the material will have different moduli (eg a tensile modulus a shear modulus etc)

Plastic deformation is on the other end permanent and nonrecoverable Plastic deformation does not follow Hookersquos law and so stress is not proportional to strain In Figure 12 the region of plastic deformation is shown in green In contrast to elastic deformation the loading curve is not identical to the unloading curve and in fact the unloading curve returns to a nonzero strain at a zero stress level There are many common properties associated with plastic deformation including yielding (onset of plastic deformation) ductility (plastic deformation at a fracture) and hardness (resistance to localized plastic deformation)

Finally there is another very important class of materials called viscoelastic materials where the elastic properties are frequency dependent For example when a stress such as pulling is exerted on the material the materialrsquos response is rate dependent Perhaps the most familiar viscoelastic material to most readers from childhood is Silly Putty where its response to pulling depends on how fast it is pulled If Silly Putty is pulled very slowly the material stretches out very nicely If Silly Putty is pulled very fast it will quickly break Polymers and rubbers two materials that play a significant role in consumer products and which are character-ized quite heavily with AFM are also viscoelastic materials

14 overvieW oF AFm operAtion

141 AFm hardware

A schematic of AFM hardware is shown in Figure 13 The main components of the AFM are (1) the cantilever (2) optical detection system (3) x-y-z scanner and (4) feedback loop

142 cantilevers and probes

The heart of the AFM lies in its cantileverprobe assembly that interacts or probes materials to provide the information of interest The technology to fabricate AFM can-tilevers takes advantage of technology developed for the semiconductor industry to make similar scale devices and features out of silicon Thus today AFM cantilevers are typically made by micromachining silicon (single-crystal Si) or silicon nitride (Si

3N

4) The dimensions of a cantilever vary and dictate the stiffness or spring

constant of the cantilever Depending on the material that needs to be probed and the mode of operation one might pick a lever with a stiffer (higher) or softer (lower) spring constant Cantilevers are generally either rectangular in geometry (Si or Si

3N

4)

or triangular (Si3N

4) with now hybrid cantilevers being manufactured (eg Si tips on

SiN cantilevers) Generally the Si3N

4 probes have lower force constants and might be

used for imaging in liquid among other applications whereas the Si probes can be brittle and tend to break or chip in contact when scanning a surface Typically the

14 Overview of AFM Operation 7

cantilevers are hundreds of microns in length tens of microns in width and a few microns in thickness for reference the diameter of a human hair is approximately 100 microm Cantilevers can be coated with gold or aluminum to provide high reflectivity Commercial cantilevers are also available with other thin metal coatings for magnetic or electrical imaging Cantilevers and probes can also be functionalized chemically or biologically for specific interactions

Probes are grown at the end of the lever and vary in shape and sharpness A sharper probe can often improve the lateral resolution of the AFM Probe design is a continuously evolving field as manufacturers strive for the most robust reproducible and sharp tips Cantileverprobe assemblies are mounted into a cantilever holder that typically includes a piezoceramic element (described below) that can vibrate the can-tilever at a given frequencymdasheither above at or below its resonancemdashdepending on the desired tipndashsample interaction or AFM mode

143 optical detection system

In order to track the motion of the cantileverprobe assembly as it scans a surface an AFM typically includes an optical detection system that consists of a laser reflected off the back side of the cantilever and directed toward a position sensitive detector

Figure 13 Schematic of basic operating principles of AFM A laser is focused and directed off the back end of a cantilever and directed toward a position-sensitive director (PSD) to monitor its vertical and lateral motion as it interacts with the surface Depending on the mode used to image the sample the cantilever can be vibrated with a shake piezo for dynamic imaging modes that involve the resonance of the cantilever

8 Chapter 1 Overview of Atomic Force Microscopy

(PSD) as seen in Figure 13 Optical detection systems are the most common way to track cantilever motion in commercial AFM systems However other methods exist to bypass optical detection systems (in eg a dark medium where the laser light would be absorbed) with self-actuated cantilevers where their motion is read by piezos integrated into the levers The laser is typically a visible photodiode though some commercial instruments have implemented superluminescent diode (SLD) laser The laser is detected by a four-segment position-sensitive detector (PSD) which can track the vertical and lateral motion of the cantilever accurately which can be converted to units of nanometers or displacement through careful calibration described below

144 x-y-z scanner

An AFM can operate either by scanning the sample relative to a stationary tip (sample scanning) or scanning the tip relative to a stationary sample (tip scanning) Each has its own advantages in terms of size of sample it can accommodate (tip scanning more flexible) ability to accessorize (tip scanning again more flexible) and stability and signal to noise (sample scanning easier to build with better specifications) Each shares the requirement to move the tip relative to the sample in a highly accurate way with minimal noise

Tipndashsample motion is often accomplished in commercial instruments by piezoelectric materials which are materials that respond either by expansion or con-traction in response to an applied voltage These kinds of materials provide the ability for very fine motion (nanometers to microns) Piezoelectric materials can be made in several shapes including tubes disks and bimorphs depending on the range and geom-etry of motion required Although piezoelectric materials are very effective at moving tip or sample they are plagued by nonlinear behavior such as hysteresis and creep that has serious consequences for accurate AFM imaging and interpretation of data Artifacts induced by piezoelectric materials are reviewed in detail in Chapter 6

Piezoelectric scanners can be programmed to move with either open-loop or closed-loop feedback circuits In open-loop scanners the piezo is moved by programs to ramp x and y bias voltages to move the prescribed amounts So in open-loop scan-ners if the piezo has to move a large distance or move around on the sample there can be considerable hysteresis of creep with the best recourse of patience or movingzooming in and out in small increments since hysteresiscreep is proportional to the displacement or motion the scanner is trying to execute

One solution to this problem is to use closed-loop scanners that use additional x-y-z sensors to independently measure and then correct for scanner movement In closed-loop operation feedback circuits are continually adjusted to the applied scanner biases to correct for the x-y motion and to achieve the desired motion It is important to note that correction of x-y motion in an AFM is slightly different than correction of z motion This is due to the fact that x-y motion can be predicted whereas z motion cannot as it depends on the surface topography of the sample being scanned Therefore typically there are active feedback circuits to control x and y

Scanning Probe Microscopy in Industrial ApplicationsNanomechanical Characterization

Edited by

Dalia G Yablon

Copyright copy 2014 by John Wiley amp Sons Inc All rights reserved

Published by John Wiley amp Sons Inc Hoboken New JerseyPublished simultaneously in Canada

No part of this publication may be reproduced stored in a retrieval system or transmitted in any form or by any means electronic mechanical photocopying recording scanning or otherwise except as permitted under Section 107 or 108 of the 1976 United Stated Copyright Act without either the prior written permission of the Published or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center Inc 222 Rosewood Drive Danvers MA 01923 (978) 750-8400 fax (978) 750-4470 or on the web at wwwcopyrightcom Requests to the Publisher for permission should be addressed to the Permissions Department John Wiley amp Sons Inc 111 River Street Hoboken NJ 07030 NJ 07030 (201) 748-6011 fax (201) 748-6008 or online at httpwwwwileycomgopermission

Limit of LiabilityDisclaimer of Warranty While the publisher and author have used their best efforts in preparing this book they make no representations or warranties with respect to the accuaracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose No warranty may be created or extended by sales representation or written sales materials The advice and strategies contained herein may not be suitable for your situation You should consult with a professional where appropriate Neither the publisher nor author shall be liable for any loss of profit or any other commercial damages including but not limited to special incidental consequential or other damages

For general information on our other products and services or for technical support Please contact our Customer Care Department within the United States at (800) 762-2974 outside the United States at (371) 572-3993 or fax (317) 572-4002

Wiley also publishes its books in variety of electronic formats Some content that appears in print may not be available in electronic formats For more information about Wiley products visit our web site at wwwwileycom

Library of Congress Cataloging-in-Publication Data

Scanning probe microscopy for industrial applications nanomechanical characterization edited by Dalia G Yablon pages cm Includes bibliographical references and index ISBN 978-1-118-28823-8 (hardback)1 MaterialsndashMicroscopy 2 Scanning probe microscopyndashIndustrial applications I Yablon Dalia G 1975ndash TA41723S33 2013 6201prime127ndashdc23

2013009638

Printed in the United States of America

10 9 8 7 6 5 4 3 2 1

Dedicated with love toAndrew

andAyelet Daniela Mati and Eli

vii

Contents

Contributors List xiii

Preface xv

Acknowledgments xix

1 Overview of Atomic Force Microscopy 1

Dalia G Yablon

11 A Word on Nomenclature 212 Atomic Force MicroscopymdashThe Appeal to Industrial RampD 213 Mechanical Properties 514 Overview of AFM Operation 615 Nanomechanical Methods Surveyed in Book 1116 Industries Represented 13Acknowledgments 14References 14

2 Understanding the TipndashSample Contact An Overview of Contact Mechanics from the Macro- to the Nanoscale 15

Tevis D B Jacobs C Mathew Mate Kevin T Turner and Robert W Carpick

21 Hertz Equations for Elastic Contact 1522 Adhesive Contacts 2223 Further Extensions of Continuum Contact Mechanics Models 2924 Thin Films 3425 Tangential Forces 3726 Application of Continuum Mechanics to Nanoscale Contacts 42Acknowledgments 44Appendix 2A Surface Energy and Work of Adhesion 44References 45

viii Contents

3 Understanding Surface Forces Using Static and Dynamic ApproachndashRetraction Curves 49

Sudharsan Balasubramaniam Daniel Kiracofe and Arvind Raman

31 TipndashSample Interaction Forces 5332 Static FndashZ Curves 5833 Dynamic AmplitudePhasendashDistance Curves 6934 Brief Guide to VEDA Simulations 7835 Conclusions 90Glossary 91References 93

4 Phase Imaging 95

Dalia G Yablon and Greg Haugstad

41 Introduction 9542 Bistability Attractive and Repulsive Mode 9743 Complications in Phase Quantification 107References 113

5 Dynamic Contact AFM Methods for Nanomechanical Properties 115

Donna C Hurley and Jason P Killgore

51 Introduction 11552 Force Modulation Microscopy (FMM) 12153 Contact Resonance (CR) Techniques 12554 Comparison of FMM and CR-FM 13655 Other Dynamic Contact Approaches 13856 Summary and Conclusions 140Acknowledgments 141Appendix 5A Data Analysis Procedure for Contact Resonance Spectroscopy Measurements 141References 145

6 Guide to Best Practices for AFM Users 150

Greg Haugstad

61 ForcendashDistance MeasurementsmdashInstrumental Sources of Nonideality 151

62 ForcendashDistance MeasurementsmdashPhysical Sources of Nonideality 157

References 161

Contents ix

7 Nanoindentation Measurements of Mechanical Properties of Very Thin Films and Nanostructured Materials at High Spatial Resolution 162

Steve J Bull

71 Introduction 16272 Bulk Materials 16373 Coatings 17674 Conclusions 188Acknowledgments 188References 188

8 Scanning Probe Microscopy for Critical Measurements in the Semiconductor Industry 190

Johann Foucher

81 Introduction 19082 Critical Dimension in the Semiconductor Industry 19183 CD Metrology Techniques for Production 19284 Obtaining Accurate CD in the Semiconductor Industry 19485 Hybrid Metrology as a Final Solution to Overcome

CD-AFM CD-SEM and Scatterometry Intrinsic Limitations 20386 Conclusion 208References 208

9 Atomic Force Microscopy of Polymers 210

Andy H Tsou and Dalia G Yablon

91 Introduction 21092 Tapping Phase AFM 21393 Nanoindentation 21794 Force Modulation 21895 Pulsed Force Imaging 21996 ForcendashVolume AFM 22097 HarmoniX and Peak Force QNM Imaging 22298 Summary 227References 229

10 Unraveling Links between Food Structure and Function with Probe Microscopy 232

A Patrick Gunning and Victor J Morris

101 Introduction 232102 Gels and Thickeners Molecular Networks 236

x Contents

103 Emulsions and Foams ProteinndashSurfactant Competition 238104 Interfacial Structure and Digestion Designer Interfaces 241105 Force Spectroscopy Model Emulsions 244106 Force Spectroscopy Origins of Bioactivity 247107 Conclusions 248References 249

11 Microcantilever Sensors for Petrochemical Applications 251

Alan M Schilowitz

111 Introduction 251112 Background 252113 Applications 257114 Conclusion 266References 267

12 Applications of Scanning Probe Methods in Cosmetic Science 270

Gustavo S Luengo and Anthony Galliano

121 Introduction 270122 Substrates of Cosmetics 271123 Mechanical Properties and Modifications by Cosmetic Products 274124 Scanning Probe Technologies Adapted to Cosmetic Science 275125 Conclusions 285References 285

13 Applications of Scanning Probe Microscopy and Nanomechanical Analysis in Pharmaceutical Development 287

Matthew S Lamm

131 Introduction 287132 Applications of SPM Imaging 288133 SPM as a Screening Tool 291134 Applications of Nanoindentation 293135 Conclusion 299Acknowledgments 299References 300

14 Comparative Nanomechanical Study of Multiharmonic Force Microscopy and Nanoindentation on Low Dielectric Constant Materials 302

Katharine Walz Robin King Willi Volksen Geraud Dubois Jane Frommer and Kumar Virwani

141 Introduction 302142 Experimental 308

Contents xi

143 Results and Discussions 311144 Conclusions 319Acknowledgments 320References 320

15 Nanomechanical Characterization of Biomaterial Surfaces Polymer Coatings That Elute Drugs 323

Klaus Wormuth and Greg Haugstad

151 Introduction 323152 Materials and Methods 325153 Dexamethasone in PBMA or PBMAndashPLMA Polymer Blends 327154 Simvastatin in PEOndashPBT Copolymers 337155 Concluding Comments 340Acknowledgments 341References 341

Index 342

xiii

Contributors List

Sudharsan BalasubramaniamDepartment of Mechanical EngineeringPurdue UniversityWest Lafayette IN

Steve J BullSchool of Chemical Engineering and Advanced MaterialsUniversity of NewcastleNewcastle upon Tyne United Kingdom

Robert W CarpickDepartment of Mechanical Engineering and Applied MechanicsUniversity of PennsylvaniaPhiladelphia PA

Geraud DuboisIBM Almaden Research CenterSan Jose CA

Johann FoucherLETIndashCEAGrenoble France

Jane FrommerIBM Almaden Research CenterSan Jose CA

Anthony GallianoLrsquoOreal Research and InnovationAulnay sous Bois France

A Patrick GunningInstitute of Food ResearchNorwich Research ParkNorwich United Kingdom

Greg HaugstadUniversity of MinnesotaMinneapolis MN

Donna C HurleyNational Institute of Standards and TechnologyBoulder CO

Tevis D B JacobsDepartment of Materials Science and EngineeringUniversity of PennsylvaniaPhiladelphia PA

Jason P KillgoreNational Institute of Standards and TechnologyBoulder CO

Robin KingIBM Almaden Research CenterSan Jose CA

Daniel KiracofeDepartment of Mechanical EngineeringPurdue UniversityWest Lafayette IN

Matthew S LammMerck Research LaboratoriesMerck amp CoSummit NJ

Gustavo S LuengoLrsquoOreal Research and InnovationAulnay sous Bois France

xiv Contributors List

C Mathew MateHGST A Western Digital CompanySan Jose CA

Victor J MorrisInstitute of Food ResearchNorwich Research ParkNorwich United Kingdom

Arvind RamanDepartment of Mechanical EngineeringPurdue UniversityWest Lafayette IN

Alan M SchilowitzExxonMobil Research and EngineeringAnnandale NJ

Andy H TsouExxonMobil Research and EngineeringAnnandale NJ

Kevin T TurnerDepartment of Mechanical Engineering and Applied MechanicsUniversity of PennsylvaniaPhiladelphia PA

Kumar VirwaniIBM Almaden Research CenterSan Jose CA

Willi VolksenIBM Almaden Research CenterSan Jose CA

Katharine WalzIBM Almaden Research CenterSan Jose CA

Klaus WormuthInstitute for Physical ChemistryUniversity of CologneCologneGermany

Dalia G YablonSurfaceChar LLCSharon MA

xv

Preface

The idea for this book came a couple of years ago after perusing atomic force microscopy (AFM) book collections at many professional society conferences over the years and finding that there was constantly a key topic missing how AFM was used practically in industrial RampD This topic is close to my heart as I have pursued a career developing novel methods and applying scanning probe microscopy (SPM) to a wide variety of industrial RampD problems at ExxonMobil Research and Engineeringrsquos Corporate Strategic Research in New Jersey for over a decade At ExxonMobil I have had the opportunity to explore application of AFM in the three vast sectors in the petroleum industry the downstream chemi-cals and upstream sector studying a wide variety of problems from corrosion to lubrication to geology to polymer materials All the AFM-focused books I found were research reviews on focused topical areas or textbooks on operating princi-ples and theory So though these books offered excellent technical treatments of a variety of topics the incredible utility applicability and flexibility of AFM to solve real-world everyday problems was lacking

My background in graduate school was actually in scanning tunneling micro-scopy (STM) as I studied self-assembled monolayers at the liquidndashsolid interface in George Flynnrsquos lab at Columbia Universityrsquos chemistry department not the most rel-evant ldquoreal-world industrialrdquo problem However being an STM lab in the 1990s we also had AFMs in the lab (since they shared the same ldquobrainrdquo the controller) so every once in a while we would help out a colleague and study their samples As I moved to ExxonMobil after graduate school in 2002 my intent was not necessarily to continue in scanning probe microscopy In fact it was serendipity that upon arriving at ExxonMobilrsquos Corporate Strategic Research labs as a postdoctoral fellow I found an unused Digital Instruments Nanoscope III controller with a fully accessorized Multimode The rest as they say is history

Over my time at ExxonMobil I ran into many like-minded colleagues in other industrial facilities using AFM and we would hold fascinating discussions of how the technology was used in onersquos particular field On the other side I could sense great interest from my academic colleagues that industrial researchers were using SPM to such a great extent as well as recent graduates trying to connect their hard-earned knowledge and expertise in the field as students to practical useful areas A book covering this material for the wider audience did indeed seem missing

And so I began to survey the industrial RampD communityndashndashboth members I had known for years and new onesndashndashto gauge interest in putting such a volume together I was inspired by the enthusiasm I received from my colleagues and decided to go forward Then came the question of what kind of AFM As surveyed in Chapter 1

xvi Preface

AFM is a tremendously broad field that characterizes a wide variety of material and surface properties However at its heart AFM is a high-resolution nanomechanical probe of the surface That probe can be coated with an electronically conducting layer or magnetic material or even replaced by an optical fiber tip in order to probe a variety of properties However a strong majority of the industrial RampD both in my own direct personal experience and those that I was familiar with was conducted with mechanical probing of properties Of course there are some key counterexamples that stand out such as electrostatic force microscopy to probe electrical properties of conducting and semiconducting materials and magnetic force microscopy to probe magnetic storage devices Perhaps those applications will be covered in a future volume But the overwhelming use of AFM in industrial RampD was nanomechanical characterization and thus it was quickly chosen as the focus for this book

It was important that this book be self-contained While the bookrsquos focus is on the applications it is intended to be accessible to a broad audience both from a professional and educational background so that everyone from an advanced undergraduate to a seasoned professional will benefit Various professional backgrounds should benefit as well from those in an academic or academic-like environment who wish to learn how AFM is used in an industrial environment to those in an industrial environment eager to learn what other applications are possible While a freshman chemistry and physics background is assumed no prior experience with AFM is assumed There is certainly advanced information in each chapter so that the content should appeal to a variety of backgrounds in the AFM community from novice to expert

With this aim of appealing to a broad audience I include a tutorial on nanomechani-cal methods in the first half of the book so that applications could be understood and appreciated with context without having to refer to other books or references I chose the topics that I felt were most relevant to studying nanomechanical characterization in the practical industrial environment contact mechanics force curves phase imaging dynamic contact methods and nanoindentation Note that this is not an exhaustive list of all the nanomechanical characterization methods available with AFM

I worked closely with my co-authors to make these overview chapters as practical as possible and not overwhelming in theoretical detail several chapters include worked examples of useful calculations (eg using Hertz mechanics with and without adhesion to model a contact in Chapter 2 or data analysis of contact resonance measurements in Chapter 5) These chapters are meant to be overviews to introduce the terminology and key concepts They are not meant to be an exhaustive step-by-step guide to the operation of various techniques those treatments are done elsewhere and are referenced in the book Chapter 6 is a somewhat unique chapter on ldquoBest Practices in AFM Imagingrdquo that I co-wrote with my colleague Greg Haugstad This chapter outlines some of the common pitfalls encountered by AFM users again with a practical approach to what the most common and most important problems are and how to avoid them

Chapters 7ndash15 then dive into how AFM has been used in a variety of industrial RampD areas to explore (a) phenomena and processes such as new formulation development in pharmaceuticals the effect of humidity and temperature on biomate-rials and nanostructure formation in food processing (b) characterization of various

Preface xvii

materials including polymer blends and composites in the chemicals sector skin and hair in personal care products coatings and thin dielectric films in the semiconductor industry (c) AFMrsquos function as a key quality control tool in semiconductor metrology and (d) a slightly different application of AFM microcantilevers as a physical and chemical sensors in the petroleum industry

My hope is that this book will show what an incredibly useful tool AFM has become to industrial RampD in its short 25 years It is currently an indispensable tool in any microscopy laboratory despite its commercial youth So while the AFM field con-tinues to grow and develop in terms of its capability and understanding spearheaded by many notable groups in academia and elsewhere it has already penetrated significantly into a wide variety of industrial labs This trend will only continue and grow As the penetration into industrial labs continues it is my hope that the reader understands not only how the AFM capability increasingly benefits industry but also that industrial labs are quickly becoming a partner in driving AFMrsquos innovation and research as its commercial and practical utility and importance continue to thrive

Dalia G YablonSharon MA

xix

Acknowledgments

I have been blessed to have wonderful colleagues to share ideas with and gain inspiration from during this process First and foremost I must thank my co-authors on this book Working with each and every author has been an absolute pleasure I had the distinct pleasure of working closely with Andy Tsou and Alan Schilowitz for many years at ExxonMobil Some of the other co-authors I have been fortunate to count as longstanding collaborators including Rob Carpick Greg Haugstad Jason Killgore and Donna Hurley I thank them for their support and enthusiastic partic-ipation for this particular project From all I learned so much technically and made many new friends in the process This book would not have been possible without everyonersquos collective hard work to share their insights and wisdom with us

I would like to thank my colleagues at ExxonMobil with whom I have had the pleasure of developing our AFM capabilities in our AFM lab Specifically Jean Grabowski deserves a special callout and has been a tremendous asset to our efforts as well as a genuine friend along the way I also would like to thank Rebecca Locker Ishita Chakraborty and Daniel Kiracofe I also would like to thank Steve Minne and Roger Proksch for help and guidance in the incipient stages of this project as well as Emily Rapalino and Susan Klein for advice and proofreading

Most of all I have to thank my husband Andrew for his absolute unwavering support and cheerleading throughout the past year and a half His confidence in me was a source of real motivation and inspiration to carry forward despite obstacles And logistically of course I have to thank him for providing childcare to our chil-dren in the evenings and weekends (covering for me on many many bath nights and storyreadingndashndashsuch as on this night) so that I could complete this book one which hopefully one day they might be interested in enough to read and understand

1

Scanning Probe Microscopy in Industrial Applications Nanomechanical Characterization First Edition Edited by Dalia G Yablon copy 2014 John Wiley amp Sons Inc Published 2014 by John Wiley amp Sons Inc

Overview of Atomic Force MicroscopyDalia G Yablon

SurfaceChar LLC Sharon MA

Atomic force microscopy (AFM) is a family of nanoscale characterization techniques that has exploded onto the overall characterization and nanotechnology field as its versatility and high resolution continue to feed a dizzying variety of disciplines from biology to physics and chemistry to engineering AFM entered the scientific arena in 1981 with the now famous invention of its older sibling in the scanning probe micro-scopy family the scanning tunneling microscope (STM) in the IBM Zurich labs of Gerd Binnig and Heinrich Rohrer for which they received the Nobel Prize in Physics in 1986 The AFM was then invented in 1986 by Gerd Binnig Cal Quate and Christoph Gerber [1] Together STM and AFM formed the scanning probe micros-copy (SPM) family which includes other methods such as near-field scanning optical microscopy (NSOM)

The STM spawned the next 25 years of the continuously developing field of scanning probe microscopy and specifically atomic force microscopy which now includes dozens of different methods under its name to probe various propertiesmdashincluding mechanical electrical magnetic chemical and opticalmdashof materials and surfaces Atomic force microscopy is a powerful tool in various research enterprises It is found in practically any university characterization facility alongside optical and electron microscopes and most undergraduates in science or engineering fields at this point are at least familiar with it if not have performed a laboratory experiment in their undergraduate curriculum

The focus of this book is to understand and appreciate the role this young technique has played in industrial research and development (RampD) AFM has penetrated into a variety of industrial research sectors as witnessed by the diverse set of applications described in this book Alongside electron and optical microscopy which have been

Chapter 1

2 Chapter 1 Overview of Atomic Force Microscopy

around for decades and have reached an impressive level of commercial maturity and ease of use AFM has become a vital characterization method despite its youth and continued technical evolution So though AFM is still an active area of academic research as its capabilities continue to develop and be better understood it has proven to be a useful microscopy to address industrial and commercial needs from quality control and assurance to product formulation and process monitoring

The goal of this introductory chapter is to provide an overview of AFM to nonspecialists and introduce the various topics that are the subject of subsequent individual chapters As such this chapter will provide a brief review of the begin-nings of AFM and special features that make it particularly suited for industrial research Then a brief overview of AFM operation will be presented including the hardware software calibrations involved and finally the different nanomechanical methods that will be described in detail both in theory and application Entire books are written on AFM operation and this chapter is not meant to be an exhaustive introduction to its operation merely serving to provide enough information for the rest of the book to be followed intelligently Readers interested in more detail about AFM operation can consult a number of excellent books on the topic [2ndash5]

11 A Word on nomenclAture

Before the rest of this chapter continues some definitions are in order Similar to many other surface science techniques AFM has succumbed to a somewhat unwieldy mess of abbreviations and jargon that has become a hard-to-navigate alphabet soup SPM refers to an umbrella of a variety of methods Methods that fall under SPM include per-haps its most famous member atomic force microscopy (AFM) in addition to others such as scanning tunneling microscopy (STM) near-field scanning optical microscopy (NSOM or SNOM depending on the continent) and other lithographic methods And then within AFM there are dozens of methods that rely on the AFM probendashsample interactions to provide a variety of material properties including electrical optical magnetic and mechanical properties To date there are dozens of SPMAFM-based methods It is beyond the scope of this book to list andor define all the related methods These methods characterize a huge variety of material properties Some of the more common methods have been included in Figure 11 with the category of properties that they measure The wide diversity of properties of materials that can be measured with AFM is clear The focus of this book is on nanomechanical characterization due to its broad appeal in industrial RampD Again Figure 11 is offered to demonstrate the variety and flavor of properties that can be probed with SPM

12 Atomic Force microscopymdashthe AppeAl to industriAl rampd

This book specifically focuses on AFM which is the method that has most penetrated the general characterization and industrial research fields AFM has key features that make it especially attractive to industrial research and development First its resolution

12 Atomic Force MicroscopymdashThe Appeal to Industrial RampD 3

the heart of its utility as a characterization tool is indeed impressive with 5- to 10-nm lateral resolution and angstrom vertical resolution achieved routinely on commercial instruments with commercial cantilevers The limits of lateral resolution continues to be pushed with specialized techniques and cantilevers and it has reached true ldquoatomicrdquo resolution where point defects of certain materials can now be imaged under certain operating parameters and imaging modes especially under liquid [6ndash8] True atomic resolution is still currently mostly achieved through STM which operates through an entirely different tipndashsample interaction mechanism based on quantum mechanical tunneling Many of these STM studies are conducted in ultrahigh vacuum (UHV) though some are performed in ambient and liquid conditions It is fair to say that AFM is catching up however with recent groundbreaking work imaging cyclic aromatic molecules on Cu[111] [10 11]

A second critical feature of AFM that makes it particularly amenable to industrial research is the flexibility of the environment in which it can operate Despite being a high-resolution microscopy that rivals the resolution of electron microscopy AFM can operate in an ambient or even liquid environment with minimal compromise to its resolution The ability to work in ldquoreal-worldrdquo environments makes it a critical tool for many industrial RampD applications where the research emphasis is consis-tently to understand mechanisms and materials in real-world situations as opposed to

Figure 11 Schematic of AFM as an umbrella technique that includes a wide variety of methods to probe various properties of materials Some of these methods are shown here

4 Chapter 1 Overview of Atomic Force Microscopy

idealized materials and conditions that typically exist in academic research endeavors In many cases the researcher may not want the sample to be forced into a pristine vacuum environment where perhaps key materials or components will be evacuated and thus missed in the characterization effort In addition the flexibility of an envi-ronment permits in situ characterization of processes For example processes such as corrosion lubrication catalyst dissolution crystal growth (for active pharmaceutical ingredient API) can all be studied in situ with AFM as a unique attribute of this microscopy The power of in situ measurements for biological applications is simple as many biological processes and materials simple cannot survive ex situ

A corollary of the utility of the AFMrsquos flexible environment to industrial RampD is the flexibility of type and shape of surface or material that AFM can image There are virtually no restrictions on the size of a material that can be probed with AFM Many commercial ldquolarge-samplerdquo AFMs currently exist that can image any design or shape of a surface from engine parts to thin films Indeed one of the AFMs built in the early 1990s (Topometrix Explorer) was advertised as being able to be placed on top of jet wings to image fractures In addition there are practically no mechanical or electrical restrictions on the type of sample an AFM can image Unlike electron microscopy or STM that require conducting or semiconducting surfaces to image or avoid artifacts [or in the case of insulating surface in scanning electron microscopy (SEM) that requires deposition of a thin conducting layer such as Cr] AFM can image insulating surfaces in addition to conducting or semiconducting surfaces with minimal sample prep Typically the most significant requirement on the sample for an AFM is that it be smooth Commercial instruments will have a maximum z (vertical) range that can be imaged dictated by the design and desired resolution of the particular instrument The maximum z range is typically from 1 microm (for high-resolution studies) to several microns (for lower resolution studies) meaning that the sample cannot have features a tilt or overall roughness that exceed that limit in order for the AFM to image effectively Smooth surfaces are often prepared with the use of an ultramicrotome An ultramicrotome is simply a very controlled and precise way of cutting of thin (down to 100 nm) sections at controlled speeds and in a controlled fashion to result in ultrasmooth surfaces of nanometer or less than nanometer rough-enss The ultramicrotome typically employs the use of a glass knife for a coarse cut and then a diamond knife for a fine cut Cryomicrotomy is simply ultramicrotomy at cold temperatures Cryomicrotomy is typically used to prepare polymer surfaces to cut below the glass transition temperature of the material depending on the instru-ment cryomicrotomes can cool down to -180degC

Finally AFM is fundamentally a mechanical probe of the surface through actual contact between the tip and sample So in addition to the ability to provide high resolution on surface features the goal of any form of microscopy is the unique tipndashsample interaction mechanism enabling the probing of many properties most obvious of which is mechanical properties Mechanical properties covers a wide range of materials and properties Entire textbooks and courses are devoted to the studies of mechanics for example by Callister and Rethwisch [12] We very briefly review some of the main concepts below to provide enough of a background for the rest of the chapter

13 Mechanical Properties 5

This edition focuses specifically on nanomechanical characterization Probing other material properties such as optical electrical or magnetic properties is not covered in this book The reason is that though these other properties have been successfully probed in various industrial RampD applications using AFM to probe mechanical properties of surfaces is more ubiquitous in industrial RampD and so was a logical focus for a first book on SPM in industrial applications

13 mechAnicAl properties

When one considers mechanical properties a range of descriptors come to mind including ldquostiffnessrdquo ldquotoughnessrdquo ldquohardnessrdquo ldquobrittlenessrdquo and ldquoductilityrdquo to name a few Each of these properties has their own mathematical expressions with associ-ated experimental methods of measuring them An interaction between two materials can be typically categorized into either elastic (or recoverable) or dissipative or plastic (permanent or nonrecoverable) interaction On the bulk scale most mechanical analysis of materials relies on measuring their strain (material displacement) as a function of stress (pressure exerted on the material) or vica versa The measurement and analysis of stress versus strain behavior of various materials is a rich and active field and there are many excellent textbooks that describe it [12] The description provided below is simplified to provide a context for these measurements ultimately described on the nanoscale

Elastic deformation of a material is typically nonpermanent or reversible and exists in a regime where the strain (material displacement) is proportional to the stress (pressure exerted on the material) by a proportionality factor known as a modulus (though in reality no material is truly only elastic) Elastic materials thus typically follow Hookersquos law Figure 12 shows a representative stress versus strain curve for materials highlighting the elastic region (blue) Additionally in elastic

Plastic

Fracture

Strain

Elastic

Str

ess

Figure 12 Basic stress vs strain curve showing three mechanical regimes of elastic deformation plastic deformation and fracture For color details please see color plate section

6 Chapter 1 Overview of Atomic Force Microscopy

materials the loading curve (as force is applied) is identical to the unloading curve (as the force is removed) Modulus is a key parameter used to describe elastic behavior Depending on the direction in which a material is being stressed the material will have different moduli (eg a tensile modulus a shear modulus etc)

Plastic deformation is on the other end permanent and nonrecoverable Plastic deformation does not follow Hookersquos law and so stress is not proportional to strain In Figure 12 the region of plastic deformation is shown in green In contrast to elastic deformation the loading curve is not identical to the unloading curve and in fact the unloading curve returns to a nonzero strain at a zero stress level There are many common properties associated with plastic deformation including yielding (onset of plastic deformation) ductility (plastic deformation at a fracture) and hardness (resistance to localized plastic deformation)

Finally there is another very important class of materials called viscoelastic materials where the elastic properties are frequency dependent For example when a stress such as pulling is exerted on the material the materialrsquos response is rate dependent Perhaps the most familiar viscoelastic material to most readers from childhood is Silly Putty where its response to pulling depends on how fast it is pulled If Silly Putty is pulled very slowly the material stretches out very nicely If Silly Putty is pulled very fast it will quickly break Polymers and rubbers two materials that play a significant role in consumer products and which are character-ized quite heavily with AFM are also viscoelastic materials

14 overvieW oF AFm operAtion

141 AFm hardware

A schematic of AFM hardware is shown in Figure 13 The main components of the AFM are (1) the cantilever (2) optical detection system (3) x-y-z scanner and (4) feedback loop

142 cantilevers and probes

The heart of the AFM lies in its cantileverprobe assembly that interacts or probes materials to provide the information of interest The technology to fabricate AFM can-tilevers takes advantage of technology developed for the semiconductor industry to make similar scale devices and features out of silicon Thus today AFM cantilevers are typically made by micromachining silicon (single-crystal Si) or silicon nitride (Si

3N

4) The dimensions of a cantilever vary and dictate the stiffness or spring

constant of the cantilever Depending on the material that needs to be probed and the mode of operation one might pick a lever with a stiffer (higher) or softer (lower) spring constant Cantilevers are generally either rectangular in geometry (Si or Si

3N

4)

or triangular (Si3N

4) with now hybrid cantilevers being manufactured (eg Si tips on

SiN cantilevers) Generally the Si3N

4 probes have lower force constants and might be

used for imaging in liquid among other applications whereas the Si probes can be brittle and tend to break or chip in contact when scanning a surface Typically the

14 Overview of AFM Operation 7

cantilevers are hundreds of microns in length tens of microns in width and a few microns in thickness for reference the diameter of a human hair is approximately 100 microm Cantilevers can be coated with gold or aluminum to provide high reflectivity Commercial cantilevers are also available with other thin metal coatings for magnetic or electrical imaging Cantilevers and probes can also be functionalized chemically or biologically for specific interactions

Probes are grown at the end of the lever and vary in shape and sharpness A sharper probe can often improve the lateral resolution of the AFM Probe design is a continuously evolving field as manufacturers strive for the most robust reproducible and sharp tips Cantileverprobe assemblies are mounted into a cantilever holder that typically includes a piezoceramic element (described below) that can vibrate the can-tilever at a given frequencymdasheither above at or below its resonancemdashdepending on the desired tipndashsample interaction or AFM mode

143 optical detection system

In order to track the motion of the cantileverprobe assembly as it scans a surface an AFM typically includes an optical detection system that consists of a laser reflected off the back side of the cantilever and directed toward a position sensitive detector

Figure 13 Schematic of basic operating principles of AFM A laser is focused and directed off the back end of a cantilever and directed toward a position-sensitive director (PSD) to monitor its vertical and lateral motion as it interacts with the surface Depending on the mode used to image the sample the cantilever can be vibrated with a shake piezo for dynamic imaging modes that involve the resonance of the cantilever

8 Chapter 1 Overview of Atomic Force Microscopy

(PSD) as seen in Figure 13 Optical detection systems are the most common way to track cantilever motion in commercial AFM systems However other methods exist to bypass optical detection systems (in eg a dark medium where the laser light would be absorbed) with self-actuated cantilevers where their motion is read by piezos integrated into the levers The laser is typically a visible photodiode though some commercial instruments have implemented superluminescent diode (SLD) laser The laser is detected by a four-segment position-sensitive detector (PSD) which can track the vertical and lateral motion of the cantilever accurately which can be converted to units of nanometers or displacement through careful calibration described below

144 x-y-z scanner

An AFM can operate either by scanning the sample relative to a stationary tip (sample scanning) or scanning the tip relative to a stationary sample (tip scanning) Each has its own advantages in terms of size of sample it can accommodate (tip scanning more flexible) ability to accessorize (tip scanning again more flexible) and stability and signal to noise (sample scanning easier to build with better specifications) Each shares the requirement to move the tip relative to the sample in a highly accurate way with minimal noise

Tipndashsample motion is often accomplished in commercial instruments by piezoelectric materials which are materials that respond either by expansion or con-traction in response to an applied voltage These kinds of materials provide the ability for very fine motion (nanometers to microns) Piezoelectric materials can be made in several shapes including tubes disks and bimorphs depending on the range and geom-etry of motion required Although piezoelectric materials are very effective at moving tip or sample they are plagued by nonlinear behavior such as hysteresis and creep that has serious consequences for accurate AFM imaging and interpretation of data Artifacts induced by piezoelectric materials are reviewed in detail in Chapter 6

Piezoelectric scanners can be programmed to move with either open-loop or closed-loop feedback circuits In open-loop scanners the piezo is moved by programs to ramp x and y bias voltages to move the prescribed amounts So in open-loop scan-ners if the piezo has to move a large distance or move around on the sample there can be considerable hysteresis of creep with the best recourse of patience or movingzooming in and out in small increments since hysteresiscreep is proportional to the displacement or motion the scanner is trying to execute

One solution to this problem is to use closed-loop scanners that use additional x-y-z sensors to independently measure and then correct for scanner movement In closed-loop operation feedback circuits are continually adjusted to the applied scanner biases to correct for the x-y motion and to achieve the desired motion It is important to note that correction of x-y motion in an AFM is slightly different than correction of z motion This is due to the fact that x-y motion can be predicted whereas z motion cannot as it depends on the surface topography of the sample being scanned Therefore typically there are active feedback circuits to control x and y

Copyright copy 2014 by John Wiley amp Sons Inc All rights reserved

Published by John Wiley amp Sons Inc Hoboken New JerseyPublished simultaneously in Canada

No part of this publication may be reproduced stored in a retrieval system or transmitted in any form or by any means electronic mechanical photocopying recording scanning or otherwise except as permitted under Section 107 or 108 of the 1976 United Stated Copyright Act without either the prior written permission of the Published or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center Inc 222 Rosewood Drive Danvers MA 01923 (978) 750-8400 fax (978) 750-4470 or on the web at wwwcopyrightcom Requests to the Publisher for permission should be addressed to the Permissions Department John Wiley amp Sons Inc 111 River Street Hoboken NJ 07030 NJ 07030 (201) 748-6011 fax (201) 748-6008 or online at httpwwwwileycomgopermission

Limit of LiabilityDisclaimer of Warranty While the publisher and author have used their best efforts in preparing this book they make no representations or warranties with respect to the accuaracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose No warranty may be created or extended by sales representation or written sales materials The advice and strategies contained herein may not be suitable for your situation You should consult with a professional where appropriate Neither the publisher nor author shall be liable for any loss of profit or any other commercial damages including but not limited to special incidental consequential or other damages

For general information on our other products and services or for technical support Please contact our Customer Care Department within the United States at (800) 762-2974 outside the United States at (371) 572-3993 or fax (317) 572-4002

Wiley also publishes its books in variety of electronic formats Some content that appears in print may not be available in electronic formats For more information about Wiley products visit our web site at wwwwileycom

Library of Congress Cataloging-in-Publication Data

Scanning probe microscopy for industrial applications nanomechanical characterization edited by Dalia G Yablon pages cm Includes bibliographical references and index ISBN 978-1-118-28823-8 (hardback)1 MaterialsndashMicroscopy 2 Scanning probe microscopyndashIndustrial applications I Yablon Dalia G 1975ndash TA41723S33 2013 6201prime127ndashdc23

2013009638

Printed in the United States of America

10 9 8 7 6 5 4 3 2 1

Dedicated with love toAndrew

andAyelet Daniela Mati and Eli

vii

Contents

Contributors List xiii

Preface xv

Acknowledgments xix

1 Overview of Atomic Force Microscopy 1

Dalia G Yablon

11 A Word on Nomenclature 212 Atomic Force MicroscopymdashThe Appeal to Industrial RampD 213 Mechanical Properties 514 Overview of AFM Operation 615 Nanomechanical Methods Surveyed in Book 1116 Industries Represented 13Acknowledgments 14References 14

2 Understanding the TipndashSample Contact An Overview of Contact Mechanics from the Macro- to the Nanoscale 15

Tevis D B Jacobs C Mathew Mate Kevin T Turner and Robert W Carpick

21 Hertz Equations for Elastic Contact 1522 Adhesive Contacts 2223 Further Extensions of Continuum Contact Mechanics Models 2924 Thin Films 3425 Tangential Forces 3726 Application of Continuum Mechanics to Nanoscale Contacts 42Acknowledgments 44Appendix 2A Surface Energy and Work of Adhesion 44References 45

viii Contents

3 Understanding Surface Forces Using Static and Dynamic ApproachndashRetraction Curves 49

Sudharsan Balasubramaniam Daniel Kiracofe and Arvind Raman

31 TipndashSample Interaction Forces 5332 Static FndashZ Curves 5833 Dynamic AmplitudePhasendashDistance Curves 6934 Brief Guide to VEDA Simulations 7835 Conclusions 90Glossary 91References 93

4 Phase Imaging 95

Dalia G Yablon and Greg Haugstad

41 Introduction 9542 Bistability Attractive and Repulsive Mode 9743 Complications in Phase Quantification 107References 113

5 Dynamic Contact AFM Methods for Nanomechanical Properties 115

Donna C Hurley and Jason P Killgore

51 Introduction 11552 Force Modulation Microscopy (FMM) 12153 Contact Resonance (CR) Techniques 12554 Comparison of FMM and CR-FM 13655 Other Dynamic Contact Approaches 13856 Summary and Conclusions 140Acknowledgments 141Appendix 5A Data Analysis Procedure for Contact Resonance Spectroscopy Measurements 141References 145

6 Guide to Best Practices for AFM Users 150

Greg Haugstad

61 ForcendashDistance MeasurementsmdashInstrumental Sources of Nonideality 151

62 ForcendashDistance MeasurementsmdashPhysical Sources of Nonideality 157

References 161

Contents ix

7 Nanoindentation Measurements of Mechanical Properties of Very Thin Films and Nanostructured Materials at High Spatial Resolution 162

Steve J Bull

71 Introduction 16272 Bulk Materials 16373 Coatings 17674 Conclusions 188Acknowledgments 188References 188

8 Scanning Probe Microscopy for Critical Measurements in the Semiconductor Industry 190

Johann Foucher

81 Introduction 19082 Critical Dimension in the Semiconductor Industry 19183 CD Metrology Techniques for Production 19284 Obtaining Accurate CD in the Semiconductor Industry 19485 Hybrid Metrology as a Final Solution to Overcome

CD-AFM CD-SEM and Scatterometry Intrinsic Limitations 20386 Conclusion 208References 208

9 Atomic Force Microscopy of Polymers 210

Andy H Tsou and Dalia G Yablon

91 Introduction 21092 Tapping Phase AFM 21393 Nanoindentation 21794 Force Modulation 21895 Pulsed Force Imaging 21996 ForcendashVolume AFM 22097 HarmoniX and Peak Force QNM Imaging 22298 Summary 227References 229

10 Unraveling Links between Food Structure and Function with Probe Microscopy 232

A Patrick Gunning and Victor J Morris

101 Introduction 232102 Gels and Thickeners Molecular Networks 236

x Contents

103 Emulsions and Foams ProteinndashSurfactant Competition 238104 Interfacial Structure and Digestion Designer Interfaces 241105 Force Spectroscopy Model Emulsions 244106 Force Spectroscopy Origins of Bioactivity 247107 Conclusions 248References 249

11 Microcantilever Sensors for Petrochemical Applications 251

Alan M Schilowitz

111 Introduction 251112 Background 252113 Applications 257114 Conclusion 266References 267

12 Applications of Scanning Probe Methods in Cosmetic Science 270

Gustavo S Luengo and Anthony Galliano

121 Introduction 270122 Substrates of Cosmetics 271123 Mechanical Properties and Modifications by Cosmetic Products 274124 Scanning Probe Technologies Adapted to Cosmetic Science 275125 Conclusions 285References 285

13 Applications of Scanning Probe Microscopy and Nanomechanical Analysis in Pharmaceutical Development 287

Matthew S Lamm

131 Introduction 287132 Applications of SPM Imaging 288133 SPM as a Screening Tool 291134 Applications of Nanoindentation 293135 Conclusion 299Acknowledgments 299References 300

14 Comparative Nanomechanical Study of Multiharmonic Force Microscopy and Nanoindentation on Low Dielectric Constant Materials 302

Katharine Walz Robin King Willi Volksen Geraud Dubois Jane Frommer and Kumar Virwani

141 Introduction 302142 Experimental 308

Contents xi

143 Results and Discussions 311144 Conclusions 319Acknowledgments 320References 320

15 Nanomechanical Characterization of Biomaterial Surfaces Polymer Coatings That Elute Drugs 323

Klaus Wormuth and Greg Haugstad

151 Introduction 323152 Materials and Methods 325153 Dexamethasone in PBMA or PBMAndashPLMA Polymer Blends 327154 Simvastatin in PEOndashPBT Copolymers 337155 Concluding Comments 340Acknowledgments 341References 341

Index 342

xiii

Contributors List

Sudharsan BalasubramaniamDepartment of Mechanical EngineeringPurdue UniversityWest Lafayette IN

Steve J BullSchool of Chemical Engineering and Advanced MaterialsUniversity of NewcastleNewcastle upon Tyne United Kingdom

Robert W CarpickDepartment of Mechanical Engineering and Applied MechanicsUniversity of PennsylvaniaPhiladelphia PA

Geraud DuboisIBM Almaden Research CenterSan Jose CA

Johann FoucherLETIndashCEAGrenoble France

Jane FrommerIBM Almaden Research CenterSan Jose CA

Anthony GallianoLrsquoOreal Research and InnovationAulnay sous Bois France

A Patrick GunningInstitute of Food ResearchNorwich Research ParkNorwich United Kingdom

Greg HaugstadUniversity of MinnesotaMinneapolis MN

Donna C HurleyNational Institute of Standards and TechnologyBoulder CO

Tevis D B JacobsDepartment of Materials Science and EngineeringUniversity of PennsylvaniaPhiladelphia PA

Jason P KillgoreNational Institute of Standards and TechnologyBoulder CO

Robin KingIBM Almaden Research CenterSan Jose CA

Daniel KiracofeDepartment of Mechanical EngineeringPurdue UniversityWest Lafayette IN

Matthew S LammMerck Research LaboratoriesMerck amp CoSummit NJ

Gustavo S LuengoLrsquoOreal Research and InnovationAulnay sous Bois France

xiv Contributors List

C Mathew MateHGST A Western Digital CompanySan Jose CA

Victor J MorrisInstitute of Food ResearchNorwich Research ParkNorwich United Kingdom

Arvind RamanDepartment of Mechanical EngineeringPurdue UniversityWest Lafayette IN

Alan M SchilowitzExxonMobil Research and EngineeringAnnandale NJ

Andy H TsouExxonMobil Research and EngineeringAnnandale NJ

Kevin T TurnerDepartment of Mechanical Engineering and Applied MechanicsUniversity of PennsylvaniaPhiladelphia PA

Kumar VirwaniIBM Almaden Research CenterSan Jose CA

Willi VolksenIBM Almaden Research CenterSan Jose CA

Katharine WalzIBM Almaden Research CenterSan Jose CA

Klaus WormuthInstitute for Physical ChemistryUniversity of CologneCologneGermany

Dalia G YablonSurfaceChar LLCSharon MA

xv

Preface

The idea for this book came a couple of years ago after perusing atomic force microscopy (AFM) book collections at many professional society conferences over the years and finding that there was constantly a key topic missing how AFM was used practically in industrial RampD This topic is close to my heart as I have pursued a career developing novel methods and applying scanning probe microscopy (SPM) to a wide variety of industrial RampD problems at ExxonMobil Research and Engineeringrsquos Corporate Strategic Research in New Jersey for over a decade At ExxonMobil I have had the opportunity to explore application of AFM in the three vast sectors in the petroleum industry the downstream chemi-cals and upstream sector studying a wide variety of problems from corrosion to lubrication to geology to polymer materials All the AFM-focused books I found were research reviews on focused topical areas or textbooks on operating princi-ples and theory So though these books offered excellent technical treatments of a variety of topics the incredible utility applicability and flexibility of AFM to solve real-world everyday problems was lacking

My background in graduate school was actually in scanning tunneling micro-scopy (STM) as I studied self-assembled monolayers at the liquidndashsolid interface in George Flynnrsquos lab at Columbia Universityrsquos chemistry department not the most rel-evant ldquoreal-world industrialrdquo problem However being an STM lab in the 1990s we also had AFMs in the lab (since they shared the same ldquobrainrdquo the controller) so every once in a while we would help out a colleague and study their samples As I moved to ExxonMobil after graduate school in 2002 my intent was not necessarily to continue in scanning probe microscopy In fact it was serendipity that upon arriving at ExxonMobilrsquos Corporate Strategic Research labs as a postdoctoral fellow I found an unused Digital Instruments Nanoscope III controller with a fully accessorized Multimode The rest as they say is history

Over my time at ExxonMobil I ran into many like-minded colleagues in other industrial facilities using AFM and we would hold fascinating discussions of how the technology was used in onersquos particular field On the other side I could sense great interest from my academic colleagues that industrial researchers were using SPM to such a great extent as well as recent graduates trying to connect their hard-earned knowledge and expertise in the field as students to practical useful areas A book covering this material for the wider audience did indeed seem missing

And so I began to survey the industrial RampD communityndashndashboth members I had known for years and new onesndashndashto gauge interest in putting such a volume together I was inspired by the enthusiasm I received from my colleagues and decided to go forward Then came the question of what kind of AFM As surveyed in Chapter 1

xvi Preface

AFM is a tremendously broad field that characterizes a wide variety of material and surface properties However at its heart AFM is a high-resolution nanomechanical probe of the surface That probe can be coated with an electronically conducting layer or magnetic material or even replaced by an optical fiber tip in order to probe a variety of properties However a strong majority of the industrial RampD both in my own direct personal experience and those that I was familiar with was conducted with mechanical probing of properties Of course there are some key counterexamples that stand out such as electrostatic force microscopy to probe electrical properties of conducting and semiconducting materials and magnetic force microscopy to probe magnetic storage devices Perhaps those applications will be covered in a future volume But the overwhelming use of AFM in industrial RampD was nanomechanical characterization and thus it was quickly chosen as the focus for this book

It was important that this book be self-contained While the bookrsquos focus is on the applications it is intended to be accessible to a broad audience both from a professional and educational background so that everyone from an advanced undergraduate to a seasoned professional will benefit Various professional backgrounds should benefit as well from those in an academic or academic-like environment who wish to learn how AFM is used in an industrial environment to those in an industrial environment eager to learn what other applications are possible While a freshman chemistry and physics background is assumed no prior experience with AFM is assumed There is certainly advanced information in each chapter so that the content should appeal to a variety of backgrounds in the AFM community from novice to expert

With this aim of appealing to a broad audience I include a tutorial on nanomechani-cal methods in the first half of the book so that applications could be understood and appreciated with context without having to refer to other books or references I chose the topics that I felt were most relevant to studying nanomechanical characterization in the practical industrial environment contact mechanics force curves phase imaging dynamic contact methods and nanoindentation Note that this is not an exhaustive list of all the nanomechanical characterization methods available with AFM

I worked closely with my co-authors to make these overview chapters as practical as possible and not overwhelming in theoretical detail several chapters include worked examples of useful calculations (eg using Hertz mechanics with and without adhesion to model a contact in Chapter 2 or data analysis of contact resonance measurements in Chapter 5) These chapters are meant to be overviews to introduce the terminology and key concepts They are not meant to be an exhaustive step-by-step guide to the operation of various techniques those treatments are done elsewhere and are referenced in the book Chapter 6 is a somewhat unique chapter on ldquoBest Practices in AFM Imagingrdquo that I co-wrote with my colleague Greg Haugstad This chapter outlines some of the common pitfalls encountered by AFM users again with a practical approach to what the most common and most important problems are and how to avoid them

Chapters 7ndash15 then dive into how AFM has been used in a variety of industrial RampD areas to explore (a) phenomena and processes such as new formulation development in pharmaceuticals the effect of humidity and temperature on biomate-rials and nanostructure formation in food processing (b) characterization of various

Preface xvii

materials including polymer blends and composites in the chemicals sector skin and hair in personal care products coatings and thin dielectric films in the semiconductor industry (c) AFMrsquos function as a key quality control tool in semiconductor metrology and (d) a slightly different application of AFM microcantilevers as a physical and chemical sensors in the petroleum industry

My hope is that this book will show what an incredibly useful tool AFM has become to industrial RampD in its short 25 years It is currently an indispensable tool in any microscopy laboratory despite its commercial youth So while the AFM field con-tinues to grow and develop in terms of its capability and understanding spearheaded by many notable groups in academia and elsewhere it has already penetrated significantly into a wide variety of industrial labs This trend will only continue and grow As the penetration into industrial labs continues it is my hope that the reader understands not only how the AFM capability increasingly benefits industry but also that industrial labs are quickly becoming a partner in driving AFMrsquos innovation and research as its commercial and practical utility and importance continue to thrive

Dalia G YablonSharon MA

xix

Acknowledgments

I have been blessed to have wonderful colleagues to share ideas with and gain inspiration from during this process First and foremost I must thank my co-authors on this book Working with each and every author has been an absolute pleasure I had the distinct pleasure of working closely with Andy Tsou and Alan Schilowitz for many years at ExxonMobil Some of the other co-authors I have been fortunate to count as longstanding collaborators including Rob Carpick Greg Haugstad Jason Killgore and Donna Hurley I thank them for their support and enthusiastic partic-ipation for this particular project From all I learned so much technically and made many new friends in the process This book would not have been possible without everyonersquos collective hard work to share their insights and wisdom with us

I would like to thank my colleagues at ExxonMobil with whom I have had the pleasure of developing our AFM capabilities in our AFM lab Specifically Jean Grabowski deserves a special callout and has been a tremendous asset to our efforts as well as a genuine friend along the way I also would like to thank Rebecca Locker Ishita Chakraborty and Daniel Kiracofe I also would like to thank Steve Minne and Roger Proksch for help and guidance in the incipient stages of this project as well as Emily Rapalino and Susan Klein for advice and proofreading

Most of all I have to thank my husband Andrew for his absolute unwavering support and cheerleading throughout the past year and a half His confidence in me was a source of real motivation and inspiration to carry forward despite obstacles And logistically of course I have to thank him for providing childcare to our chil-dren in the evenings and weekends (covering for me on many many bath nights and storyreadingndashndashsuch as on this night) so that I could complete this book one which hopefully one day they might be interested in enough to read and understand

1

Scanning Probe Microscopy in Industrial Applications Nanomechanical Characterization First Edition Edited by Dalia G Yablon copy 2014 John Wiley amp Sons Inc Published 2014 by John Wiley amp Sons Inc

Overview of Atomic Force MicroscopyDalia G Yablon

SurfaceChar LLC Sharon MA

Atomic force microscopy (AFM) is a family of nanoscale characterization techniques that has exploded onto the overall characterization and nanotechnology field as its versatility and high resolution continue to feed a dizzying variety of disciplines from biology to physics and chemistry to engineering AFM entered the scientific arena in 1981 with the now famous invention of its older sibling in the scanning probe micro-scopy family the scanning tunneling microscope (STM) in the IBM Zurich labs of Gerd Binnig and Heinrich Rohrer for which they received the Nobel Prize in Physics in 1986 The AFM was then invented in 1986 by Gerd Binnig Cal Quate and Christoph Gerber [1] Together STM and AFM formed the scanning probe micros-copy (SPM) family which includes other methods such as near-field scanning optical microscopy (NSOM)

The STM spawned the next 25 years of the continuously developing field of scanning probe microscopy and specifically atomic force microscopy which now includes dozens of different methods under its name to probe various propertiesmdashincluding mechanical electrical magnetic chemical and opticalmdashof materials and surfaces Atomic force microscopy is a powerful tool in various research enterprises It is found in practically any university characterization facility alongside optical and electron microscopes and most undergraduates in science or engineering fields at this point are at least familiar with it if not have performed a laboratory experiment in their undergraduate curriculum

The focus of this book is to understand and appreciate the role this young technique has played in industrial research and development (RampD) AFM has penetrated into a variety of industrial research sectors as witnessed by the diverse set of applications described in this book Alongside electron and optical microscopy which have been

Chapter 1

2 Chapter 1 Overview of Atomic Force Microscopy

around for decades and have reached an impressive level of commercial maturity and ease of use AFM has become a vital characterization method despite its youth and continued technical evolution So though AFM is still an active area of academic research as its capabilities continue to develop and be better understood it has proven to be a useful microscopy to address industrial and commercial needs from quality control and assurance to product formulation and process monitoring

The goal of this introductory chapter is to provide an overview of AFM to nonspecialists and introduce the various topics that are the subject of subsequent individual chapters As such this chapter will provide a brief review of the begin-nings of AFM and special features that make it particularly suited for industrial research Then a brief overview of AFM operation will be presented including the hardware software calibrations involved and finally the different nanomechanical methods that will be described in detail both in theory and application Entire books are written on AFM operation and this chapter is not meant to be an exhaustive introduction to its operation merely serving to provide enough information for the rest of the book to be followed intelligently Readers interested in more detail about AFM operation can consult a number of excellent books on the topic [2ndash5]

11 A Word on nomenclAture

Before the rest of this chapter continues some definitions are in order Similar to many other surface science techniques AFM has succumbed to a somewhat unwieldy mess of abbreviations and jargon that has become a hard-to-navigate alphabet soup SPM refers to an umbrella of a variety of methods Methods that fall under SPM include per-haps its most famous member atomic force microscopy (AFM) in addition to others such as scanning tunneling microscopy (STM) near-field scanning optical microscopy (NSOM or SNOM depending on the continent) and other lithographic methods And then within AFM there are dozens of methods that rely on the AFM probendashsample interactions to provide a variety of material properties including electrical optical magnetic and mechanical properties To date there are dozens of SPMAFM-based methods It is beyond the scope of this book to list andor define all the related methods These methods characterize a huge variety of material properties Some of the more common methods have been included in Figure 11 with the category of properties that they measure The wide diversity of properties of materials that can be measured with AFM is clear The focus of this book is on nanomechanical characterization due to its broad appeal in industrial RampD Again Figure 11 is offered to demonstrate the variety and flavor of properties that can be probed with SPM

12 Atomic Force microscopymdashthe AppeAl to industriAl rampd

This book specifically focuses on AFM which is the method that has most penetrated the general characterization and industrial research fields AFM has key features that make it especially attractive to industrial research and development First its resolution

12 Atomic Force MicroscopymdashThe Appeal to Industrial RampD 3

the heart of its utility as a characterization tool is indeed impressive with 5- to 10-nm lateral resolution and angstrom vertical resolution achieved routinely on commercial instruments with commercial cantilevers The limits of lateral resolution continues to be pushed with specialized techniques and cantilevers and it has reached true ldquoatomicrdquo resolution where point defects of certain materials can now be imaged under certain operating parameters and imaging modes especially under liquid [6ndash8] True atomic resolution is still currently mostly achieved through STM which operates through an entirely different tipndashsample interaction mechanism based on quantum mechanical tunneling Many of these STM studies are conducted in ultrahigh vacuum (UHV) though some are performed in ambient and liquid conditions It is fair to say that AFM is catching up however with recent groundbreaking work imaging cyclic aromatic molecules on Cu[111] [10 11]

A second critical feature of AFM that makes it particularly amenable to industrial research is the flexibility of the environment in which it can operate Despite being a high-resolution microscopy that rivals the resolution of electron microscopy AFM can operate in an ambient or even liquid environment with minimal compromise to its resolution The ability to work in ldquoreal-worldrdquo environments makes it a critical tool for many industrial RampD applications where the research emphasis is consis-tently to understand mechanisms and materials in real-world situations as opposed to

Figure 11 Schematic of AFM as an umbrella technique that includes a wide variety of methods to probe various properties of materials Some of these methods are shown here

4 Chapter 1 Overview of Atomic Force Microscopy

idealized materials and conditions that typically exist in academic research endeavors In many cases the researcher may not want the sample to be forced into a pristine vacuum environment where perhaps key materials or components will be evacuated and thus missed in the characterization effort In addition the flexibility of an envi-ronment permits in situ characterization of processes For example processes such as corrosion lubrication catalyst dissolution crystal growth (for active pharmaceutical ingredient API) can all be studied in situ with AFM as a unique attribute of this microscopy The power of in situ measurements for biological applications is simple as many biological processes and materials simple cannot survive ex situ

A corollary of the utility of the AFMrsquos flexible environment to industrial RampD is the flexibility of type and shape of surface or material that AFM can image There are virtually no restrictions on the size of a material that can be probed with AFM Many commercial ldquolarge-samplerdquo AFMs currently exist that can image any design or shape of a surface from engine parts to thin films Indeed one of the AFMs built in the early 1990s (Topometrix Explorer) was advertised as being able to be placed on top of jet wings to image fractures In addition there are practically no mechanical or electrical restrictions on the type of sample an AFM can image Unlike electron microscopy or STM that require conducting or semiconducting surfaces to image or avoid artifacts [or in the case of insulating surface in scanning electron microscopy (SEM) that requires deposition of a thin conducting layer such as Cr] AFM can image insulating surfaces in addition to conducting or semiconducting surfaces with minimal sample prep Typically the most significant requirement on the sample for an AFM is that it be smooth Commercial instruments will have a maximum z (vertical) range that can be imaged dictated by the design and desired resolution of the particular instrument The maximum z range is typically from 1 microm (for high-resolution studies) to several microns (for lower resolution studies) meaning that the sample cannot have features a tilt or overall roughness that exceed that limit in order for the AFM to image effectively Smooth surfaces are often prepared with the use of an ultramicrotome An ultramicrotome is simply a very controlled and precise way of cutting of thin (down to 100 nm) sections at controlled speeds and in a controlled fashion to result in ultrasmooth surfaces of nanometer or less than nanometer rough-enss The ultramicrotome typically employs the use of a glass knife for a coarse cut and then a diamond knife for a fine cut Cryomicrotomy is simply ultramicrotomy at cold temperatures Cryomicrotomy is typically used to prepare polymer surfaces to cut below the glass transition temperature of the material depending on the instru-ment cryomicrotomes can cool down to -180degC

Finally AFM is fundamentally a mechanical probe of the surface through actual contact between the tip and sample So in addition to the ability to provide high resolution on surface features the goal of any form of microscopy is the unique tipndashsample interaction mechanism enabling the probing of many properties most obvious of which is mechanical properties Mechanical properties covers a wide range of materials and properties Entire textbooks and courses are devoted to the studies of mechanics for example by Callister and Rethwisch [12] We very briefly review some of the main concepts below to provide enough of a background for the rest of the chapter

13 Mechanical Properties 5

This edition focuses specifically on nanomechanical characterization Probing other material properties such as optical electrical or magnetic properties is not covered in this book The reason is that though these other properties have been successfully probed in various industrial RampD applications using AFM to probe mechanical properties of surfaces is more ubiquitous in industrial RampD and so was a logical focus for a first book on SPM in industrial applications

13 mechAnicAl properties

When one considers mechanical properties a range of descriptors come to mind including ldquostiffnessrdquo ldquotoughnessrdquo ldquohardnessrdquo ldquobrittlenessrdquo and ldquoductilityrdquo to name a few Each of these properties has their own mathematical expressions with associ-ated experimental methods of measuring them An interaction between two materials can be typically categorized into either elastic (or recoverable) or dissipative or plastic (permanent or nonrecoverable) interaction On the bulk scale most mechanical analysis of materials relies on measuring their strain (material displacement) as a function of stress (pressure exerted on the material) or vica versa The measurement and analysis of stress versus strain behavior of various materials is a rich and active field and there are many excellent textbooks that describe it [12] The description provided below is simplified to provide a context for these measurements ultimately described on the nanoscale

Elastic deformation of a material is typically nonpermanent or reversible and exists in a regime where the strain (material displacement) is proportional to the stress (pressure exerted on the material) by a proportionality factor known as a modulus (though in reality no material is truly only elastic) Elastic materials thus typically follow Hookersquos law Figure 12 shows a representative stress versus strain curve for materials highlighting the elastic region (blue) Additionally in elastic

Plastic

Fracture

Strain

Elastic

Str

ess

Figure 12 Basic stress vs strain curve showing three mechanical regimes of elastic deformation plastic deformation and fracture For color details please see color plate section

6 Chapter 1 Overview of Atomic Force Microscopy

materials the loading curve (as force is applied) is identical to the unloading curve (as the force is removed) Modulus is a key parameter used to describe elastic behavior Depending on the direction in which a material is being stressed the material will have different moduli (eg a tensile modulus a shear modulus etc)

Plastic deformation is on the other end permanent and nonrecoverable Plastic deformation does not follow Hookersquos law and so stress is not proportional to strain In Figure 12 the region of plastic deformation is shown in green In contrast to elastic deformation the loading curve is not identical to the unloading curve and in fact the unloading curve returns to a nonzero strain at a zero stress level There are many common properties associated with plastic deformation including yielding (onset of plastic deformation) ductility (plastic deformation at a fracture) and hardness (resistance to localized plastic deformation)

Finally there is another very important class of materials called viscoelastic materials where the elastic properties are frequency dependent For example when a stress such as pulling is exerted on the material the materialrsquos response is rate dependent Perhaps the most familiar viscoelastic material to most readers from childhood is Silly Putty where its response to pulling depends on how fast it is pulled If Silly Putty is pulled very slowly the material stretches out very nicely If Silly Putty is pulled very fast it will quickly break Polymers and rubbers two materials that play a significant role in consumer products and which are character-ized quite heavily with AFM are also viscoelastic materials

14 overvieW oF AFm operAtion

141 AFm hardware

A schematic of AFM hardware is shown in Figure 13 The main components of the AFM are (1) the cantilever (2) optical detection system (3) x-y-z scanner and (4) feedback loop

142 cantilevers and probes

The heart of the AFM lies in its cantileverprobe assembly that interacts or probes materials to provide the information of interest The technology to fabricate AFM can-tilevers takes advantage of technology developed for the semiconductor industry to make similar scale devices and features out of silicon Thus today AFM cantilevers are typically made by micromachining silicon (single-crystal Si) or silicon nitride (Si

3N

4) The dimensions of a cantilever vary and dictate the stiffness or spring

constant of the cantilever Depending on the material that needs to be probed and the mode of operation one might pick a lever with a stiffer (higher) or softer (lower) spring constant Cantilevers are generally either rectangular in geometry (Si or Si

3N

4)

or triangular (Si3N

4) with now hybrid cantilevers being manufactured (eg Si tips on

SiN cantilevers) Generally the Si3N

4 probes have lower force constants and might be

used for imaging in liquid among other applications whereas the Si probes can be brittle and tend to break or chip in contact when scanning a surface Typically the

14 Overview of AFM Operation 7

cantilevers are hundreds of microns in length tens of microns in width and a few microns in thickness for reference the diameter of a human hair is approximately 100 microm Cantilevers can be coated with gold or aluminum to provide high reflectivity Commercial cantilevers are also available with other thin metal coatings for magnetic or electrical imaging Cantilevers and probes can also be functionalized chemically or biologically for specific interactions

Probes are grown at the end of the lever and vary in shape and sharpness A sharper probe can often improve the lateral resolution of the AFM Probe design is a continuously evolving field as manufacturers strive for the most robust reproducible and sharp tips Cantileverprobe assemblies are mounted into a cantilever holder that typically includes a piezoceramic element (described below) that can vibrate the can-tilever at a given frequencymdasheither above at or below its resonancemdashdepending on the desired tipndashsample interaction or AFM mode

143 optical detection system

In order to track the motion of the cantileverprobe assembly as it scans a surface an AFM typically includes an optical detection system that consists of a laser reflected off the back side of the cantilever and directed toward a position sensitive detector

Figure 13 Schematic of basic operating principles of AFM A laser is focused and directed off the back end of a cantilever and directed toward a position-sensitive director (PSD) to monitor its vertical and lateral motion as it interacts with the surface Depending on the mode used to image the sample the cantilever can be vibrated with a shake piezo for dynamic imaging modes that involve the resonance of the cantilever

8 Chapter 1 Overview of Atomic Force Microscopy

(PSD) as seen in Figure 13 Optical detection systems are the most common way to track cantilever motion in commercial AFM systems However other methods exist to bypass optical detection systems (in eg a dark medium where the laser light would be absorbed) with self-actuated cantilevers where their motion is read by piezos integrated into the levers The laser is typically a visible photodiode though some commercial instruments have implemented superluminescent diode (SLD) laser The laser is detected by a four-segment position-sensitive detector (PSD) which can track the vertical and lateral motion of the cantilever accurately which can be converted to units of nanometers or displacement through careful calibration described below

144 x-y-z scanner

An AFM can operate either by scanning the sample relative to a stationary tip (sample scanning) or scanning the tip relative to a stationary sample (tip scanning) Each has its own advantages in terms of size of sample it can accommodate (tip scanning more flexible) ability to accessorize (tip scanning again more flexible) and stability and signal to noise (sample scanning easier to build with better specifications) Each shares the requirement to move the tip relative to the sample in a highly accurate way with minimal noise

Tipndashsample motion is often accomplished in commercial instruments by piezoelectric materials which are materials that respond either by expansion or con-traction in response to an applied voltage These kinds of materials provide the ability for very fine motion (nanometers to microns) Piezoelectric materials can be made in several shapes including tubes disks and bimorphs depending on the range and geom-etry of motion required Although piezoelectric materials are very effective at moving tip or sample they are plagued by nonlinear behavior such as hysteresis and creep that has serious consequences for accurate AFM imaging and interpretation of data Artifacts induced by piezoelectric materials are reviewed in detail in Chapter 6

Piezoelectric scanners can be programmed to move with either open-loop or closed-loop feedback circuits In open-loop scanners the piezo is moved by programs to ramp x and y bias voltages to move the prescribed amounts So in open-loop scan-ners if the piezo has to move a large distance or move around on the sample there can be considerable hysteresis of creep with the best recourse of patience or movingzooming in and out in small increments since hysteresiscreep is proportional to the displacement or motion the scanner is trying to execute

One solution to this problem is to use closed-loop scanners that use additional x-y-z sensors to independently measure and then correct for scanner movement In closed-loop operation feedback circuits are continually adjusted to the applied scanner biases to correct for the x-y motion and to achieve the desired motion It is important to note that correction of x-y motion in an AFM is slightly different than correction of z motion This is due to the fact that x-y motion can be predicted whereas z motion cannot as it depends on the surface topography of the sample being scanned Therefore typically there are active feedback circuits to control x and y

Dedicated with love toAndrew

andAyelet Daniela Mati and Eli

vii

Contents

Contributors List xiii

Preface xv

Acknowledgments xix

1 Overview of Atomic Force Microscopy 1

Dalia G Yablon

11 A Word on Nomenclature 212 Atomic Force MicroscopymdashThe Appeal to Industrial RampD 213 Mechanical Properties 514 Overview of AFM Operation 615 Nanomechanical Methods Surveyed in Book 1116 Industries Represented 13Acknowledgments 14References 14

2 Understanding the TipndashSample Contact An Overview of Contact Mechanics from the Macro- to the Nanoscale 15

Tevis D B Jacobs C Mathew Mate Kevin T Turner and Robert W Carpick

21 Hertz Equations for Elastic Contact 1522 Adhesive Contacts 2223 Further Extensions of Continuum Contact Mechanics Models 2924 Thin Films 3425 Tangential Forces 3726 Application of Continuum Mechanics to Nanoscale Contacts 42Acknowledgments 44Appendix 2A Surface Energy and Work of Adhesion 44References 45

viii Contents

3 Understanding Surface Forces Using Static and Dynamic ApproachndashRetraction Curves 49

Sudharsan Balasubramaniam Daniel Kiracofe and Arvind Raman

31 TipndashSample Interaction Forces 5332 Static FndashZ Curves 5833 Dynamic AmplitudePhasendashDistance Curves 6934 Brief Guide to VEDA Simulations 7835 Conclusions 90Glossary 91References 93

4 Phase Imaging 95

Dalia G Yablon and Greg Haugstad

41 Introduction 9542 Bistability Attractive and Repulsive Mode 9743 Complications in Phase Quantification 107References 113

5 Dynamic Contact AFM Methods for Nanomechanical Properties 115

Donna C Hurley and Jason P Killgore

51 Introduction 11552 Force Modulation Microscopy (FMM) 12153 Contact Resonance (CR) Techniques 12554 Comparison of FMM and CR-FM 13655 Other Dynamic Contact Approaches 13856 Summary and Conclusions 140Acknowledgments 141Appendix 5A Data Analysis Procedure for Contact Resonance Spectroscopy Measurements 141References 145

6 Guide to Best Practices for AFM Users 150

Greg Haugstad

61 ForcendashDistance MeasurementsmdashInstrumental Sources of Nonideality 151

62 ForcendashDistance MeasurementsmdashPhysical Sources of Nonideality 157

References 161

Contents ix

7 Nanoindentation Measurements of Mechanical Properties of Very Thin Films and Nanostructured Materials at High Spatial Resolution 162

Steve J Bull

71 Introduction 16272 Bulk Materials 16373 Coatings 17674 Conclusions 188Acknowledgments 188References 188

8 Scanning Probe Microscopy for Critical Measurements in the Semiconductor Industry 190

Johann Foucher

81 Introduction 19082 Critical Dimension in the Semiconductor Industry 19183 CD Metrology Techniques for Production 19284 Obtaining Accurate CD in the Semiconductor Industry 19485 Hybrid Metrology as a Final Solution to Overcome

CD-AFM CD-SEM and Scatterometry Intrinsic Limitations 20386 Conclusion 208References 208

9 Atomic Force Microscopy of Polymers 210

Andy H Tsou and Dalia G Yablon

91 Introduction 21092 Tapping Phase AFM 21393 Nanoindentation 21794 Force Modulation 21895 Pulsed Force Imaging 21996 ForcendashVolume AFM 22097 HarmoniX and Peak Force QNM Imaging 22298 Summary 227References 229

10 Unraveling Links between Food Structure and Function with Probe Microscopy 232

A Patrick Gunning and Victor J Morris

101 Introduction 232102 Gels and Thickeners Molecular Networks 236

x Contents

103 Emulsions and Foams ProteinndashSurfactant Competition 238104 Interfacial Structure and Digestion Designer Interfaces 241105 Force Spectroscopy Model Emulsions 244106 Force Spectroscopy Origins of Bioactivity 247107 Conclusions 248References 249

11 Microcantilever Sensors for Petrochemical Applications 251

Alan M Schilowitz

111 Introduction 251112 Background 252113 Applications 257114 Conclusion 266References 267

12 Applications of Scanning Probe Methods in Cosmetic Science 270

Gustavo S Luengo and Anthony Galliano

121 Introduction 270122 Substrates of Cosmetics 271123 Mechanical Properties and Modifications by Cosmetic Products 274124 Scanning Probe Technologies Adapted to Cosmetic Science 275125 Conclusions 285References 285

13 Applications of Scanning Probe Microscopy and Nanomechanical Analysis in Pharmaceutical Development 287

Matthew S Lamm

131 Introduction 287132 Applications of SPM Imaging 288133 SPM as a Screening Tool 291134 Applications of Nanoindentation 293135 Conclusion 299Acknowledgments 299References 300

14 Comparative Nanomechanical Study of Multiharmonic Force Microscopy and Nanoindentation on Low Dielectric Constant Materials 302

Katharine Walz Robin King Willi Volksen Geraud Dubois Jane Frommer and Kumar Virwani

141 Introduction 302142 Experimental 308

Contents xi

143 Results and Discussions 311144 Conclusions 319Acknowledgments 320References 320

15 Nanomechanical Characterization of Biomaterial Surfaces Polymer Coatings That Elute Drugs 323

Klaus Wormuth and Greg Haugstad

151 Introduction 323152 Materials and Methods 325153 Dexamethasone in PBMA or PBMAndashPLMA Polymer Blends 327154 Simvastatin in PEOndashPBT Copolymers 337155 Concluding Comments 340Acknowledgments 341References 341

Index 342

xiii

Contributors List

Sudharsan BalasubramaniamDepartment of Mechanical EngineeringPurdue UniversityWest Lafayette IN

Steve J BullSchool of Chemical Engineering and Advanced MaterialsUniversity of NewcastleNewcastle upon Tyne United Kingdom

Robert W CarpickDepartment of Mechanical Engineering and Applied MechanicsUniversity of PennsylvaniaPhiladelphia PA

Geraud DuboisIBM Almaden Research CenterSan Jose CA

Johann FoucherLETIndashCEAGrenoble France

Jane FrommerIBM Almaden Research CenterSan Jose CA

Anthony GallianoLrsquoOreal Research and InnovationAulnay sous Bois France

A Patrick GunningInstitute of Food ResearchNorwich Research ParkNorwich United Kingdom

Greg HaugstadUniversity of MinnesotaMinneapolis MN

Donna C HurleyNational Institute of Standards and TechnologyBoulder CO

Tevis D B JacobsDepartment of Materials Science and EngineeringUniversity of PennsylvaniaPhiladelphia PA

Jason P KillgoreNational Institute of Standards and TechnologyBoulder CO

Robin KingIBM Almaden Research CenterSan Jose CA

Daniel KiracofeDepartment of Mechanical EngineeringPurdue UniversityWest Lafayette IN

Matthew S LammMerck Research LaboratoriesMerck amp CoSummit NJ

Gustavo S LuengoLrsquoOreal Research and InnovationAulnay sous Bois France

xiv Contributors List

C Mathew MateHGST A Western Digital CompanySan Jose CA

Victor J MorrisInstitute of Food ResearchNorwich Research ParkNorwich United Kingdom

Arvind RamanDepartment of Mechanical EngineeringPurdue UniversityWest Lafayette IN

Alan M SchilowitzExxonMobil Research and EngineeringAnnandale NJ

Andy H TsouExxonMobil Research and EngineeringAnnandale NJ

Kevin T TurnerDepartment of Mechanical Engineering and Applied MechanicsUniversity of PennsylvaniaPhiladelphia PA

Kumar VirwaniIBM Almaden Research CenterSan Jose CA

Willi VolksenIBM Almaden Research CenterSan Jose CA

Katharine WalzIBM Almaden Research CenterSan Jose CA

Klaus WormuthInstitute for Physical ChemistryUniversity of CologneCologneGermany

Dalia G YablonSurfaceChar LLCSharon MA

xv

Preface

The idea for this book came a couple of years ago after perusing atomic force microscopy (AFM) book collections at many professional society conferences over the years and finding that there was constantly a key topic missing how AFM was used practically in industrial RampD This topic is close to my heart as I have pursued a career developing novel methods and applying scanning probe microscopy (SPM) to a wide variety of industrial RampD problems at ExxonMobil Research and Engineeringrsquos Corporate Strategic Research in New Jersey for over a decade At ExxonMobil I have had the opportunity to explore application of AFM in the three vast sectors in the petroleum industry the downstream chemi-cals and upstream sector studying a wide variety of problems from corrosion to lubrication to geology to polymer materials All the AFM-focused books I found were research reviews on focused topical areas or textbooks on operating princi-ples and theory So though these books offered excellent technical treatments of a variety of topics the incredible utility applicability and flexibility of AFM to solve real-world everyday problems was lacking

My background in graduate school was actually in scanning tunneling micro-scopy (STM) as I studied self-assembled monolayers at the liquidndashsolid interface in George Flynnrsquos lab at Columbia Universityrsquos chemistry department not the most rel-evant ldquoreal-world industrialrdquo problem However being an STM lab in the 1990s we also had AFMs in the lab (since they shared the same ldquobrainrdquo the controller) so every once in a while we would help out a colleague and study their samples As I moved to ExxonMobil after graduate school in 2002 my intent was not necessarily to continue in scanning probe microscopy In fact it was serendipity that upon arriving at ExxonMobilrsquos Corporate Strategic Research labs as a postdoctoral fellow I found an unused Digital Instruments Nanoscope III controller with a fully accessorized Multimode The rest as they say is history

Over my time at ExxonMobil I ran into many like-minded colleagues in other industrial facilities using AFM and we would hold fascinating discussions of how the technology was used in onersquos particular field On the other side I could sense great interest from my academic colleagues that industrial researchers were using SPM to such a great extent as well as recent graduates trying to connect their hard-earned knowledge and expertise in the field as students to practical useful areas A book covering this material for the wider audience did indeed seem missing

And so I began to survey the industrial RampD communityndashndashboth members I had known for years and new onesndashndashto gauge interest in putting such a volume together I was inspired by the enthusiasm I received from my colleagues and decided to go forward Then came the question of what kind of AFM As surveyed in Chapter 1

xvi Preface

AFM is a tremendously broad field that characterizes a wide variety of material and surface properties However at its heart AFM is a high-resolution nanomechanical probe of the surface That probe can be coated with an electronically conducting layer or magnetic material or even replaced by an optical fiber tip in order to probe a variety of properties However a strong majority of the industrial RampD both in my own direct personal experience and those that I was familiar with was conducted with mechanical probing of properties Of course there are some key counterexamples that stand out such as electrostatic force microscopy to probe electrical properties of conducting and semiconducting materials and magnetic force microscopy to probe magnetic storage devices Perhaps those applications will be covered in a future volume But the overwhelming use of AFM in industrial RampD was nanomechanical characterization and thus it was quickly chosen as the focus for this book

It was important that this book be self-contained While the bookrsquos focus is on the applications it is intended to be accessible to a broad audience both from a professional and educational background so that everyone from an advanced undergraduate to a seasoned professional will benefit Various professional backgrounds should benefit as well from those in an academic or academic-like environment who wish to learn how AFM is used in an industrial environment to those in an industrial environment eager to learn what other applications are possible While a freshman chemistry and physics background is assumed no prior experience with AFM is assumed There is certainly advanced information in each chapter so that the content should appeal to a variety of backgrounds in the AFM community from novice to expert

With this aim of appealing to a broad audience I include a tutorial on nanomechani-cal methods in the first half of the book so that applications could be understood and appreciated with context without having to refer to other books or references I chose the topics that I felt were most relevant to studying nanomechanical characterization in the practical industrial environment contact mechanics force curves phase imaging dynamic contact methods and nanoindentation Note that this is not an exhaustive list of all the nanomechanical characterization methods available with AFM

I worked closely with my co-authors to make these overview chapters as practical as possible and not overwhelming in theoretical detail several chapters include worked examples of useful calculations (eg using Hertz mechanics with and without adhesion to model a contact in Chapter 2 or data analysis of contact resonance measurements in Chapter 5) These chapters are meant to be overviews to introduce the terminology and key concepts They are not meant to be an exhaustive step-by-step guide to the operation of various techniques those treatments are done elsewhere and are referenced in the book Chapter 6 is a somewhat unique chapter on ldquoBest Practices in AFM Imagingrdquo that I co-wrote with my colleague Greg Haugstad This chapter outlines some of the common pitfalls encountered by AFM users again with a practical approach to what the most common and most important problems are and how to avoid them

Chapters 7ndash15 then dive into how AFM has been used in a variety of industrial RampD areas to explore (a) phenomena and processes such as new formulation development in pharmaceuticals the effect of humidity and temperature on biomate-rials and nanostructure formation in food processing (b) characterization of various

Preface xvii

materials including polymer blends and composites in the chemicals sector skin and hair in personal care products coatings and thin dielectric films in the semiconductor industry (c) AFMrsquos function as a key quality control tool in semiconductor metrology and (d) a slightly different application of AFM microcantilevers as a physical and chemical sensors in the petroleum industry

My hope is that this book will show what an incredibly useful tool AFM has become to industrial RampD in its short 25 years It is currently an indispensable tool in any microscopy laboratory despite its commercial youth So while the AFM field con-tinues to grow and develop in terms of its capability and understanding spearheaded by many notable groups in academia and elsewhere it has already penetrated significantly into a wide variety of industrial labs This trend will only continue and grow As the penetration into industrial labs continues it is my hope that the reader understands not only how the AFM capability increasingly benefits industry but also that industrial labs are quickly becoming a partner in driving AFMrsquos innovation and research as its commercial and practical utility and importance continue to thrive

Dalia G YablonSharon MA

xix

Acknowledgments

I have been blessed to have wonderful colleagues to share ideas with and gain inspiration from during this process First and foremost I must thank my co-authors on this book Working with each and every author has been an absolute pleasure I had the distinct pleasure of working closely with Andy Tsou and Alan Schilowitz for many years at ExxonMobil Some of the other co-authors I have been fortunate to count as longstanding collaborators including Rob Carpick Greg Haugstad Jason Killgore and Donna Hurley I thank them for their support and enthusiastic partic-ipation for this particular project From all I learned so much technically and made many new friends in the process This book would not have been possible without everyonersquos collective hard work to share their insights and wisdom with us

I would like to thank my colleagues at ExxonMobil with whom I have had the pleasure of developing our AFM capabilities in our AFM lab Specifically Jean Grabowski deserves a special callout and has been a tremendous asset to our efforts as well as a genuine friend along the way I also would like to thank Rebecca Locker Ishita Chakraborty and Daniel Kiracofe I also would like to thank Steve Minne and Roger Proksch for help and guidance in the incipient stages of this project as well as Emily Rapalino and Susan Klein for advice and proofreading

Most of all I have to thank my husband Andrew for his absolute unwavering support and cheerleading throughout the past year and a half His confidence in me was a source of real motivation and inspiration to carry forward despite obstacles And logistically of course I have to thank him for providing childcare to our chil-dren in the evenings and weekends (covering for me on many many bath nights and storyreadingndashndashsuch as on this night) so that I could complete this book one which hopefully one day they might be interested in enough to read and understand

1

Scanning Probe Microscopy in Industrial Applications Nanomechanical Characterization First Edition Edited by Dalia G Yablon copy 2014 John Wiley amp Sons Inc Published 2014 by John Wiley amp Sons Inc

Overview of Atomic Force MicroscopyDalia G Yablon

SurfaceChar LLC Sharon MA

Atomic force microscopy (AFM) is a family of nanoscale characterization techniques that has exploded onto the overall characterization and nanotechnology field as its versatility and high resolution continue to feed a dizzying variety of disciplines from biology to physics and chemistry to engineering AFM entered the scientific arena in 1981 with the now famous invention of its older sibling in the scanning probe micro-scopy family the scanning tunneling microscope (STM) in the IBM Zurich labs of Gerd Binnig and Heinrich Rohrer for which they received the Nobel Prize in Physics in 1986 The AFM was then invented in 1986 by Gerd Binnig Cal Quate and Christoph Gerber [1] Together STM and AFM formed the scanning probe micros-copy (SPM) family which includes other methods such as near-field scanning optical microscopy (NSOM)

The STM spawned the next 25 years of the continuously developing field of scanning probe microscopy and specifically atomic force microscopy which now includes dozens of different methods under its name to probe various propertiesmdashincluding mechanical electrical magnetic chemical and opticalmdashof materials and surfaces Atomic force microscopy is a powerful tool in various research enterprises It is found in practically any university characterization facility alongside optical and electron microscopes and most undergraduates in science or engineering fields at this point are at least familiar with it if not have performed a laboratory experiment in their undergraduate curriculum

The focus of this book is to understand and appreciate the role this young technique has played in industrial research and development (RampD) AFM has penetrated into a variety of industrial research sectors as witnessed by the diverse set of applications described in this book Alongside electron and optical microscopy which have been

Chapter 1

2 Chapter 1 Overview of Atomic Force Microscopy

around for decades and have reached an impressive level of commercial maturity and ease of use AFM has become a vital characterization method despite its youth and continued technical evolution So though AFM is still an active area of academic research as its capabilities continue to develop and be better understood it has proven to be a useful microscopy to address industrial and commercial needs from quality control and assurance to product formulation and process monitoring

The goal of this introductory chapter is to provide an overview of AFM to nonspecialists and introduce the various topics that are the subject of subsequent individual chapters As such this chapter will provide a brief review of the begin-nings of AFM and special features that make it particularly suited for industrial research Then a brief overview of AFM operation will be presented including the hardware software calibrations involved and finally the different nanomechanical methods that will be described in detail both in theory and application Entire books are written on AFM operation and this chapter is not meant to be an exhaustive introduction to its operation merely serving to provide enough information for the rest of the book to be followed intelligently Readers interested in more detail about AFM operation can consult a number of excellent books on the topic [2ndash5]

11 A Word on nomenclAture

Before the rest of this chapter continues some definitions are in order Similar to many other surface science techniques AFM has succumbed to a somewhat unwieldy mess of abbreviations and jargon that has become a hard-to-navigate alphabet soup SPM refers to an umbrella of a variety of methods Methods that fall under SPM include per-haps its most famous member atomic force microscopy (AFM) in addition to others such as scanning tunneling microscopy (STM) near-field scanning optical microscopy (NSOM or SNOM depending on the continent) and other lithographic methods And then within AFM there are dozens of methods that rely on the AFM probendashsample interactions to provide a variety of material properties including electrical optical magnetic and mechanical properties To date there are dozens of SPMAFM-based methods It is beyond the scope of this book to list andor define all the related methods These methods characterize a huge variety of material properties Some of the more common methods have been included in Figure 11 with the category of properties that they measure The wide diversity of properties of materials that can be measured with AFM is clear The focus of this book is on nanomechanical characterization due to its broad appeal in industrial RampD Again Figure 11 is offered to demonstrate the variety and flavor of properties that can be probed with SPM

12 Atomic Force microscopymdashthe AppeAl to industriAl rampd

This book specifically focuses on AFM which is the method that has most penetrated the general characterization and industrial research fields AFM has key features that make it especially attractive to industrial research and development First its resolution

12 Atomic Force MicroscopymdashThe Appeal to Industrial RampD 3

the heart of its utility as a characterization tool is indeed impressive with 5- to 10-nm lateral resolution and angstrom vertical resolution achieved routinely on commercial instruments with commercial cantilevers The limits of lateral resolution continues to be pushed with specialized techniques and cantilevers and it has reached true ldquoatomicrdquo resolution where point defects of certain materials can now be imaged under certain operating parameters and imaging modes especially under liquid [6ndash8] True atomic resolution is still currently mostly achieved through STM which operates through an entirely different tipndashsample interaction mechanism based on quantum mechanical tunneling Many of these STM studies are conducted in ultrahigh vacuum (UHV) though some are performed in ambient and liquid conditions It is fair to say that AFM is catching up however with recent groundbreaking work imaging cyclic aromatic molecules on Cu[111] [10 11]

A second critical feature of AFM that makes it particularly amenable to industrial research is the flexibility of the environment in which it can operate Despite being a high-resolution microscopy that rivals the resolution of electron microscopy AFM can operate in an ambient or even liquid environment with minimal compromise to its resolution The ability to work in ldquoreal-worldrdquo environments makes it a critical tool for many industrial RampD applications where the research emphasis is consis-tently to understand mechanisms and materials in real-world situations as opposed to

Figure 11 Schematic of AFM as an umbrella technique that includes a wide variety of methods to probe various properties of materials Some of these methods are shown here

4 Chapter 1 Overview of Atomic Force Microscopy

idealized materials and conditions that typically exist in academic research endeavors In many cases the researcher may not want the sample to be forced into a pristine vacuum environment where perhaps key materials or components will be evacuated and thus missed in the characterization effort In addition the flexibility of an envi-ronment permits in situ characterization of processes For example processes such as corrosion lubrication catalyst dissolution crystal growth (for active pharmaceutical ingredient API) can all be studied in situ with AFM as a unique attribute of this microscopy The power of in situ measurements for biological applications is simple as many biological processes and materials simple cannot survive ex situ

A corollary of the utility of the AFMrsquos flexible environment to industrial RampD is the flexibility of type and shape of surface or material that AFM can image There are virtually no restrictions on the size of a material that can be probed with AFM Many commercial ldquolarge-samplerdquo AFMs currently exist that can image any design or shape of a surface from engine parts to thin films Indeed one of the AFMs built in the early 1990s (Topometrix Explorer) was advertised as being able to be placed on top of jet wings to image fractures In addition there are practically no mechanical or electrical restrictions on the type of sample an AFM can image Unlike electron microscopy or STM that require conducting or semiconducting surfaces to image or avoid artifacts [or in the case of insulating surface in scanning electron microscopy (SEM) that requires deposition of a thin conducting layer such as Cr] AFM can image insulating surfaces in addition to conducting or semiconducting surfaces with minimal sample prep Typically the most significant requirement on the sample for an AFM is that it be smooth Commercial instruments will have a maximum z (vertical) range that can be imaged dictated by the design and desired resolution of the particular instrument The maximum z range is typically from 1 microm (for high-resolution studies) to several microns (for lower resolution studies) meaning that the sample cannot have features a tilt or overall roughness that exceed that limit in order for the AFM to image effectively Smooth surfaces are often prepared with the use of an ultramicrotome An ultramicrotome is simply a very controlled and precise way of cutting of thin (down to 100 nm) sections at controlled speeds and in a controlled fashion to result in ultrasmooth surfaces of nanometer or less than nanometer rough-enss The ultramicrotome typically employs the use of a glass knife for a coarse cut and then a diamond knife for a fine cut Cryomicrotomy is simply ultramicrotomy at cold temperatures Cryomicrotomy is typically used to prepare polymer surfaces to cut below the glass transition temperature of the material depending on the instru-ment cryomicrotomes can cool down to -180degC

Finally AFM is fundamentally a mechanical probe of the surface through actual contact between the tip and sample So in addition to the ability to provide high resolution on surface features the goal of any form of microscopy is the unique tipndashsample interaction mechanism enabling the probing of many properties most obvious of which is mechanical properties Mechanical properties covers a wide range of materials and properties Entire textbooks and courses are devoted to the studies of mechanics for example by Callister and Rethwisch [12] We very briefly review some of the main concepts below to provide enough of a background for the rest of the chapter

13 Mechanical Properties 5

This edition focuses specifically on nanomechanical characterization Probing other material properties such as optical electrical or magnetic properties is not covered in this book The reason is that though these other properties have been successfully probed in various industrial RampD applications using AFM to probe mechanical properties of surfaces is more ubiquitous in industrial RampD and so was a logical focus for a first book on SPM in industrial applications

13 mechAnicAl properties

When one considers mechanical properties a range of descriptors come to mind including ldquostiffnessrdquo ldquotoughnessrdquo ldquohardnessrdquo ldquobrittlenessrdquo and ldquoductilityrdquo to name a few Each of these properties has their own mathematical expressions with associ-ated experimental methods of measuring them An interaction between two materials can be typically categorized into either elastic (or recoverable) or dissipative or plastic (permanent or nonrecoverable) interaction On the bulk scale most mechanical analysis of materials relies on measuring their strain (material displacement) as a function of stress (pressure exerted on the material) or vica versa The measurement and analysis of stress versus strain behavior of various materials is a rich and active field and there are many excellent textbooks that describe it [12] The description provided below is simplified to provide a context for these measurements ultimately described on the nanoscale

Elastic deformation of a material is typically nonpermanent or reversible and exists in a regime where the strain (material displacement) is proportional to the stress (pressure exerted on the material) by a proportionality factor known as a modulus (though in reality no material is truly only elastic) Elastic materials thus typically follow Hookersquos law Figure 12 shows a representative stress versus strain curve for materials highlighting the elastic region (blue) Additionally in elastic

Plastic

Fracture

Strain

Elastic

Str

ess

Figure 12 Basic stress vs strain curve showing three mechanical regimes of elastic deformation plastic deformation and fracture For color details please see color plate section

6 Chapter 1 Overview of Atomic Force Microscopy

materials the loading curve (as force is applied) is identical to the unloading curve (as the force is removed) Modulus is a key parameter used to describe elastic behavior Depending on the direction in which a material is being stressed the material will have different moduli (eg a tensile modulus a shear modulus etc)

Plastic deformation is on the other end permanent and nonrecoverable Plastic deformation does not follow Hookersquos law and so stress is not proportional to strain In Figure 12 the region of plastic deformation is shown in green In contrast to elastic deformation the loading curve is not identical to the unloading curve and in fact the unloading curve returns to a nonzero strain at a zero stress level There are many common properties associated with plastic deformation including yielding (onset of plastic deformation) ductility (plastic deformation at a fracture) and hardness (resistance to localized plastic deformation)

Finally there is another very important class of materials called viscoelastic materials where the elastic properties are frequency dependent For example when a stress such as pulling is exerted on the material the materialrsquos response is rate dependent Perhaps the most familiar viscoelastic material to most readers from childhood is Silly Putty where its response to pulling depends on how fast it is pulled If Silly Putty is pulled very slowly the material stretches out very nicely If Silly Putty is pulled very fast it will quickly break Polymers and rubbers two materials that play a significant role in consumer products and which are character-ized quite heavily with AFM are also viscoelastic materials

14 overvieW oF AFm operAtion

141 AFm hardware

A schematic of AFM hardware is shown in Figure 13 The main components of the AFM are (1) the cantilever (2) optical detection system (3) x-y-z scanner and (4) feedback loop

142 cantilevers and probes

The heart of the AFM lies in its cantileverprobe assembly that interacts or probes materials to provide the information of interest The technology to fabricate AFM can-tilevers takes advantage of technology developed for the semiconductor industry to make similar scale devices and features out of silicon Thus today AFM cantilevers are typically made by micromachining silicon (single-crystal Si) or silicon nitride (Si

3N

4) The dimensions of a cantilever vary and dictate the stiffness or spring

constant of the cantilever Depending on the material that needs to be probed and the mode of operation one might pick a lever with a stiffer (higher) or softer (lower) spring constant Cantilevers are generally either rectangular in geometry (Si or Si

3N

4)

or triangular (Si3N

4) with now hybrid cantilevers being manufactured (eg Si tips on

SiN cantilevers) Generally the Si3N

4 probes have lower force constants and might be

used for imaging in liquid among other applications whereas the Si probes can be brittle and tend to break or chip in contact when scanning a surface Typically the

14 Overview of AFM Operation 7

cantilevers are hundreds of microns in length tens of microns in width and a few microns in thickness for reference the diameter of a human hair is approximately 100 microm Cantilevers can be coated with gold or aluminum to provide high reflectivity Commercial cantilevers are also available with other thin metal coatings for magnetic or electrical imaging Cantilevers and probes can also be functionalized chemically or biologically for specific interactions

Probes are grown at the end of the lever and vary in shape and sharpness A sharper probe can often improve the lateral resolution of the AFM Probe design is a continuously evolving field as manufacturers strive for the most robust reproducible and sharp tips Cantileverprobe assemblies are mounted into a cantilever holder that typically includes a piezoceramic element (described below) that can vibrate the can-tilever at a given frequencymdasheither above at or below its resonancemdashdepending on the desired tipndashsample interaction or AFM mode

143 optical detection system

In order to track the motion of the cantileverprobe assembly as it scans a surface an AFM typically includes an optical detection system that consists of a laser reflected off the back side of the cantilever and directed toward a position sensitive detector

Figure 13 Schematic of basic operating principles of AFM A laser is focused and directed off the back end of a cantilever and directed toward a position-sensitive director (PSD) to monitor its vertical and lateral motion as it interacts with the surface Depending on the mode used to image the sample the cantilever can be vibrated with a shake piezo for dynamic imaging modes that involve the resonance of the cantilever

8 Chapter 1 Overview of Atomic Force Microscopy

(PSD) as seen in Figure 13 Optical detection systems are the most common way to track cantilever motion in commercial AFM systems However other methods exist to bypass optical detection systems (in eg a dark medium where the laser light would be absorbed) with self-actuated cantilevers where their motion is read by piezos integrated into the levers The laser is typically a visible photodiode though some commercial instruments have implemented superluminescent diode (SLD) laser The laser is detected by a four-segment position-sensitive detector (PSD) which can track the vertical and lateral motion of the cantilever accurately which can be converted to units of nanometers or displacement through careful calibration described below

144 x-y-z scanner

An AFM can operate either by scanning the sample relative to a stationary tip (sample scanning) or scanning the tip relative to a stationary sample (tip scanning) Each has its own advantages in terms of size of sample it can accommodate (tip scanning more flexible) ability to accessorize (tip scanning again more flexible) and stability and signal to noise (sample scanning easier to build with better specifications) Each shares the requirement to move the tip relative to the sample in a highly accurate way with minimal noise

Tipndashsample motion is often accomplished in commercial instruments by piezoelectric materials which are materials that respond either by expansion or con-traction in response to an applied voltage These kinds of materials provide the ability for very fine motion (nanometers to microns) Piezoelectric materials can be made in several shapes including tubes disks and bimorphs depending on the range and geom-etry of motion required Although piezoelectric materials are very effective at moving tip or sample they are plagued by nonlinear behavior such as hysteresis and creep that has serious consequences for accurate AFM imaging and interpretation of data Artifacts induced by piezoelectric materials are reviewed in detail in Chapter 6

Piezoelectric scanners can be programmed to move with either open-loop or closed-loop feedback circuits In open-loop scanners the piezo is moved by programs to ramp x and y bias voltages to move the prescribed amounts So in open-loop scan-ners if the piezo has to move a large distance or move around on the sample there can be considerable hysteresis of creep with the best recourse of patience or movingzooming in and out in small increments since hysteresiscreep is proportional to the displacement or motion the scanner is trying to execute

One solution to this problem is to use closed-loop scanners that use additional x-y-z sensors to independently measure and then correct for scanner movement In closed-loop operation feedback circuits are continually adjusted to the applied scanner biases to correct for the x-y motion and to achieve the desired motion It is important to note that correction of x-y motion in an AFM is slightly different than correction of z motion This is due to the fact that x-y motion can be predicted whereas z motion cannot as it depends on the surface topography of the sample being scanned Therefore typically there are active feedback circuits to control x and y

vii

Contents

Contributors List xiii

Preface xv

Acknowledgments xix

1 Overview of Atomic Force Microscopy 1

Dalia G Yablon

11 A Word on Nomenclature 212 Atomic Force MicroscopymdashThe Appeal to Industrial RampD 213 Mechanical Properties 514 Overview of AFM Operation 615 Nanomechanical Methods Surveyed in Book 1116 Industries Represented 13Acknowledgments 14References 14

2 Understanding the TipndashSample Contact An Overview of Contact Mechanics from the Macro- to the Nanoscale 15

Tevis D B Jacobs C Mathew Mate Kevin T Turner and Robert W Carpick

21 Hertz Equations for Elastic Contact 1522 Adhesive Contacts 2223 Further Extensions of Continuum Contact Mechanics Models 2924 Thin Films 3425 Tangential Forces 3726 Application of Continuum Mechanics to Nanoscale Contacts 42Acknowledgments 44Appendix 2A Surface Energy and Work of Adhesion 44References 45

viii Contents

3 Understanding Surface Forces Using Static and Dynamic ApproachndashRetraction Curves 49

Sudharsan Balasubramaniam Daniel Kiracofe and Arvind Raman

31 TipndashSample Interaction Forces 5332 Static FndashZ Curves 5833 Dynamic AmplitudePhasendashDistance Curves 6934 Brief Guide to VEDA Simulations 7835 Conclusions 90Glossary 91References 93

4 Phase Imaging 95

Dalia G Yablon and Greg Haugstad

41 Introduction 9542 Bistability Attractive and Repulsive Mode 9743 Complications in Phase Quantification 107References 113

5 Dynamic Contact AFM Methods for Nanomechanical Properties 115

Donna C Hurley and Jason P Killgore

51 Introduction 11552 Force Modulation Microscopy (FMM) 12153 Contact Resonance (CR) Techniques 12554 Comparison of FMM and CR-FM 13655 Other Dynamic Contact Approaches 13856 Summary and Conclusions 140Acknowledgments 141Appendix 5A Data Analysis Procedure for Contact Resonance Spectroscopy Measurements 141References 145

6 Guide to Best Practices for AFM Users 150

Greg Haugstad

61 ForcendashDistance MeasurementsmdashInstrumental Sources of Nonideality 151

62 ForcendashDistance MeasurementsmdashPhysical Sources of Nonideality 157

References 161

Contents ix

7 Nanoindentation Measurements of Mechanical Properties of Very Thin Films and Nanostructured Materials at High Spatial Resolution 162

Steve J Bull

71 Introduction 16272 Bulk Materials 16373 Coatings 17674 Conclusions 188Acknowledgments 188References 188

8 Scanning Probe Microscopy for Critical Measurements in the Semiconductor Industry 190

Johann Foucher

81 Introduction 19082 Critical Dimension in the Semiconductor Industry 19183 CD Metrology Techniques for Production 19284 Obtaining Accurate CD in the Semiconductor Industry 19485 Hybrid Metrology as a Final Solution to Overcome

CD-AFM CD-SEM and Scatterometry Intrinsic Limitations 20386 Conclusion 208References 208

9 Atomic Force Microscopy of Polymers 210

Andy H Tsou and Dalia G Yablon

91 Introduction 21092 Tapping Phase AFM 21393 Nanoindentation 21794 Force Modulation 21895 Pulsed Force Imaging 21996 ForcendashVolume AFM 22097 HarmoniX and Peak Force QNM Imaging 22298 Summary 227References 229

10 Unraveling Links between Food Structure and Function with Probe Microscopy 232

A Patrick Gunning and Victor J Morris

101 Introduction 232102 Gels and Thickeners Molecular Networks 236

x Contents

103 Emulsions and Foams ProteinndashSurfactant Competition 238104 Interfacial Structure and Digestion Designer Interfaces 241105 Force Spectroscopy Model Emulsions 244106 Force Spectroscopy Origins of Bioactivity 247107 Conclusions 248References 249

11 Microcantilever Sensors for Petrochemical Applications 251

Alan M Schilowitz

111 Introduction 251112 Background 252113 Applications 257114 Conclusion 266References 267

12 Applications of Scanning Probe Methods in Cosmetic Science 270

Gustavo S Luengo and Anthony Galliano

121 Introduction 270122 Substrates of Cosmetics 271123 Mechanical Properties and Modifications by Cosmetic Products 274124 Scanning Probe Technologies Adapted to Cosmetic Science 275125 Conclusions 285References 285

13 Applications of Scanning Probe Microscopy and Nanomechanical Analysis in Pharmaceutical Development 287

Matthew S Lamm

131 Introduction 287132 Applications of SPM Imaging 288133 SPM as a Screening Tool 291134 Applications of Nanoindentation 293135 Conclusion 299Acknowledgments 299References 300

14 Comparative Nanomechanical Study of Multiharmonic Force Microscopy and Nanoindentation on Low Dielectric Constant Materials 302

Katharine Walz Robin King Willi Volksen Geraud Dubois Jane Frommer and Kumar Virwani

141 Introduction 302142 Experimental 308

Contents xi

143 Results and Discussions 311144 Conclusions 319Acknowledgments 320References 320

15 Nanomechanical Characterization of Biomaterial Surfaces Polymer Coatings That Elute Drugs 323

Klaus Wormuth and Greg Haugstad

151 Introduction 323152 Materials and Methods 325153 Dexamethasone in PBMA or PBMAndashPLMA Polymer Blends 327154 Simvastatin in PEOndashPBT Copolymers 337155 Concluding Comments 340Acknowledgments 341References 341

Index 342

xiii

Contributors List

Sudharsan BalasubramaniamDepartment of Mechanical EngineeringPurdue UniversityWest Lafayette IN

Steve J BullSchool of Chemical Engineering and Advanced MaterialsUniversity of NewcastleNewcastle upon Tyne United Kingdom

Robert W CarpickDepartment of Mechanical Engineering and Applied MechanicsUniversity of PennsylvaniaPhiladelphia PA

Geraud DuboisIBM Almaden Research CenterSan Jose CA

Johann FoucherLETIndashCEAGrenoble France

Jane FrommerIBM Almaden Research CenterSan Jose CA

Anthony GallianoLrsquoOreal Research and InnovationAulnay sous Bois France

A Patrick GunningInstitute of Food ResearchNorwich Research ParkNorwich United Kingdom

Greg HaugstadUniversity of MinnesotaMinneapolis MN

Donna C HurleyNational Institute of Standards and TechnologyBoulder CO

Tevis D B JacobsDepartment of Materials Science and EngineeringUniversity of PennsylvaniaPhiladelphia PA

Jason P KillgoreNational Institute of Standards and TechnologyBoulder CO

Robin KingIBM Almaden Research CenterSan Jose CA

Daniel KiracofeDepartment of Mechanical EngineeringPurdue UniversityWest Lafayette IN

Matthew S LammMerck Research LaboratoriesMerck amp CoSummit NJ

Gustavo S LuengoLrsquoOreal Research and InnovationAulnay sous Bois France

xiv Contributors List

C Mathew MateHGST A Western Digital CompanySan Jose CA

Victor J MorrisInstitute of Food ResearchNorwich Research ParkNorwich United Kingdom

Arvind RamanDepartment of Mechanical EngineeringPurdue UniversityWest Lafayette IN

Alan M SchilowitzExxonMobil Research and EngineeringAnnandale NJ

Andy H TsouExxonMobil Research and EngineeringAnnandale NJ

Kevin T TurnerDepartment of Mechanical Engineering and Applied MechanicsUniversity of PennsylvaniaPhiladelphia PA

Kumar VirwaniIBM Almaden Research CenterSan Jose CA

Willi VolksenIBM Almaden Research CenterSan Jose CA

Katharine WalzIBM Almaden Research CenterSan Jose CA

Klaus WormuthInstitute for Physical ChemistryUniversity of CologneCologneGermany

Dalia G YablonSurfaceChar LLCSharon MA

xv

Preface

The idea for this book came a couple of years ago after perusing atomic force microscopy (AFM) book collections at many professional society conferences over the years and finding that there was constantly a key topic missing how AFM was used practically in industrial RampD This topic is close to my heart as I have pursued a career developing novel methods and applying scanning probe microscopy (SPM) to a wide variety of industrial RampD problems at ExxonMobil Research and Engineeringrsquos Corporate Strategic Research in New Jersey for over a decade At ExxonMobil I have had the opportunity to explore application of AFM in the three vast sectors in the petroleum industry the downstream chemi-cals and upstream sector studying a wide variety of problems from corrosion to lubrication to geology to polymer materials All the AFM-focused books I found were research reviews on focused topical areas or textbooks on operating princi-ples and theory So though these books offered excellent technical treatments of a variety of topics the incredible utility applicability and flexibility of AFM to solve real-world everyday problems was lacking

My background in graduate school was actually in scanning tunneling micro-scopy (STM) as I studied self-assembled monolayers at the liquidndashsolid interface in George Flynnrsquos lab at Columbia Universityrsquos chemistry department not the most rel-evant ldquoreal-world industrialrdquo problem However being an STM lab in the 1990s we also had AFMs in the lab (since they shared the same ldquobrainrdquo the controller) so every once in a while we would help out a colleague and study their samples As I moved to ExxonMobil after graduate school in 2002 my intent was not necessarily to continue in scanning probe microscopy In fact it was serendipity that upon arriving at ExxonMobilrsquos Corporate Strategic Research labs as a postdoctoral fellow I found an unused Digital Instruments Nanoscope III controller with a fully accessorized Multimode The rest as they say is history

Over my time at ExxonMobil I ran into many like-minded colleagues in other industrial facilities using AFM and we would hold fascinating discussions of how the technology was used in onersquos particular field On the other side I could sense great interest from my academic colleagues that industrial researchers were using SPM to such a great extent as well as recent graduates trying to connect their hard-earned knowledge and expertise in the field as students to practical useful areas A book covering this material for the wider audience did indeed seem missing

And so I began to survey the industrial RampD communityndashndashboth members I had known for years and new onesndashndashto gauge interest in putting such a volume together I was inspired by the enthusiasm I received from my colleagues and decided to go forward Then came the question of what kind of AFM As surveyed in Chapter 1

xvi Preface

AFM is a tremendously broad field that characterizes a wide variety of material and surface properties However at its heart AFM is a high-resolution nanomechanical probe of the surface That probe can be coated with an electronically conducting layer or magnetic material or even replaced by an optical fiber tip in order to probe a variety of properties However a strong majority of the industrial RampD both in my own direct personal experience and those that I was familiar with was conducted with mechanical probing of properties Of course there are some key counterexamples that stand out such as electrostatic force microscopy to probe electrical properties of conducting and semiconducting materials and magnetic force microscopy to probe magnetic storage devices Perhaps those applications will be covered in a future volume But the overwhelming use of AFM in industrial RampD was nanomechanical characterization and thus it was quickly chosen as the focus for this book

It was important that this book be self-contained While the bookrsquos focus is on the applications it is intended to be accessible to a broad audience both from a professional and educational background so that everyone from an advanced undergraduate to a seasoned professional will benefit Various professional backgrounds should benefit as well from those in an academic or academic-like environment who wish to learn how AFM is used in an industrial environment to those in an industrial environment eager to learn what other applications are possible While a freshman chemistry and physics background is assumed no prior experience with AFM is assumed There is certainly advanced information in each chapter so that the content should appeal to a variety of backgrounds in the AFM community from novice to expert

With this aim of appealing to a broad audience I include a tutorial on nanomechani-cal methods in the first half of the book so that applications could be understood and appreciated with context without having to refer to other books or references I chose the topics that I felt were most relevant to studying nanomechanical characterization in the practical industrial environment contact mechanics force curves phase imaging dynamic contact methods and nanoindentation Note that this is not an exhaustive list of all the nanomechanical characterization methods available with AFM

I worked closely with my co-authors to make these overview chapters as practical as possible and not overwhelming in theoretical detail several chapters include worked examples of useful calculations (eg using Hertz mechanics with and without adhesion to model a contact in Chapter 2 or data analysis of contact resonance measurements in Chapter 5) These chapters are meant to be overviews to introduce the terminology and key concepts They are not meant to be an exhaustive step-by-step guide to the operation of various techniques those treatments are done elsewhere and are referenced in the book Chapter 6 is a somewhat unique chapter on ldquoBest Practices in AFM Imagingrdquo that I co-wrote with my colleague Greg Haugstad This chapter outlines some of the common pitfalls encountered by AFM users again with a practical approach to what the most common and most important problems are and how to avoid them

Chapters 7ndash15 then dive into how AFM has been used in a variety of industrial RampD areas to explore (a) phenomena and processes such as new formulation development in pharmaceuticals the effect of humidity and temperature on biomate-rials and nanostructure formation in food processing (b) characterization of various

Preface xvii

materials including polymer blends and composites in the chemicals sector skin and hair in personal care products coatings and thin dielectric films in the semiconductor industry (c) AFMrsquos function as a key quality control tool in semiconductor metrology and (d) a slightly different application of AFM microcantilevers as a physical and chemical sensors in the petroleum industry

My hope is that this book will show what an incredibly useful tool AFM has become to industrial RampD in its short 25 years It is currently an indispensable tool in any microscopy laboratory despite its commercial youth So while the AFM field con-tinues to grow and develop in terms of its capability and understanding spearheaded by many notable groups in academia and elsewhere it has already penetrated significantly into a wide variety of industrial labs This trend will only continue and grow As the penetration into industrial labs continues it is my hope that the reader understands not only how the AFM capability increasingly benefits industry but also that industrial labs are quickly becoming a partner in driving AFMrsquos innovation and research as its commercial and practical utility and importance continue to thrive

Dalia G YablonSharon MA

xix

Acknowledgments

I have been blessed to have wonderful colleagues to share ideas with and gain inspiration from during this process First and foremost I must thank my co-authors on this book Working with each and every author has been an absolute pleasure I had the distinct pleasure of working closely with Andy Tsou and Alan Schilowitz for many years at ExxonMobil Some of the other co-authors I have been fortunate to count as longstanding collaborators including Rob Carpick Greg Haugstad Jason Killgore and Donna Hurley I thank them for their support and enthusiastic partic-ipation for this particular project From all I learned so much technically and made many new friends in the process This book would not have been possible without everyonersquos collective hard work to share their insights and wisdom with us

I would like to thank my colleagues at ExxonMobil with whom I have had the pleasure of developing our AFM capabilities in our AFM lab Specifically Jean Grabowski deserves a special callout and has been a tremendous asset to our efforts as well as a genuine friend along the way I also would like to thank Rebecca Locker Ishita Chakraborty and Daniel Kiracofe I also would like to thank Steve Minne and Roger Proksch for help and guidance in the incipient stages of this project as well as Emily Rapalino and Susan Klein for advice and proofreading

Most of all I have to thank my husband Andrew for his absolute unwavering support and cheerleading throughout the past year and a half His confidence in me was a source of real motivation and inspiration to carry forward despite obstacles And logistically of course I have to thank him for providing childcare to our chil-dren in the evenings and weekends (covering for me on many many bath nights and storyreadingndashndashsuch as on this night) so that I could complete this book one which hopefully one day they might be interested in enough to read and understand

1

Scanning Probe Microscopy in Industrial Applications Nanomechanical Characterization First Edition Edited by Dalia G Yablon copy 2014 John Wiley amp Sons Inc Published 2014 by John Wiley amp Sons Inc

Overview of Atomic Force MicroscopyDalia G Yablon

SurfaceChar LLC Sharon MA

Atomic force microscopy (AFM) is a family of nanoscale characterization techniques that has exploded onto the overall characterization and nanotechnology field as its versatility and high resolution continue to feed a dizzying variety of disciplines from biology to physics and chemistry to engineering AFM entered the scientific arena in 1981 with the now famous invention of its older sibling in the scanning probe micro-scopy family the scanning tunneling microscope (STM) in the IBM Zurich labs of Gerd Binnig and Heinrich Rohrer for which they received the Nobel Prize in Physics in 1986 The AFM was then invented in 1986 by Gerd Binnig Cal Quate and Christoph Gerber [1] Together STM and AFM formed the scanning probe micros-copy (SPM) family which includes other methods such as near-field scanning optical microscopy (NSOM)

The STM spawned the next 25 years of the continuously developing field of scanning probe microscopy and specifically atomic force microscopy which now includes dozens of different methods under its name to probe various propertiesmdashincluding mechanical electrical magnetic chemical and opticalmdashof materials and surfaces Atomic force microscopy is a powerful tool in various research enterprises It is found in practically any university characterization facility alongside optical and electron microscopes and most undergraduates in science or engineering fields at this point are at least familiar with it if not have performed a laboratory experiment in their undergraduate curriculum

The focus of this book is to understand and appreciate the role this young technique has played in industrial research and development (RampD) AFM has penetrated into a variety of industrial research sectors as witnessed by the diverse set of applications described in this book Alongside electron and optical microscopy which have been

Chapter 1

2 Chapter 1 Overview of Atomic Force Microscopy

around for decades and have reached an impressive level of commercial maturity and ease of use AFM has become a vital characterization method despite its youth and continued technical evolution So though AFM is still an active area of academic research as its capabilities continue to develop and be better understood it has proven to be a useful microscopy to address industrial and commercial needs from quality control and assurance to product formulation and process monitoring

The goal of this introductory chapter is to provide an overview of AFM to nonspecialists and introduce the various topics that are the subject of subsequent individual chapters As such this chapter will provide a brief review of the begin-nings of AFM and special features that make it particularly suited for industrial research Then a brief overview of AFM operation will be presented including the hardware software calibrations involved and finally the different nanomechanical methods that will be described in detail both in theory and application Entire books are written on AFM operation and this chapter is not meant to be an exhaustive introduction to its operation merely serving to provide enough information for the rest of the book to be followed intelligently Readers interested in more detail about AFM operation can consult a number of excellent books on the topic [2ndash5]

11 A Word on nomenclAture

Before the rest of this chapter continues some definitions are in order Similar to many other surface science techniques AFM has succumbed to a somewhat unwieldy mess of abbreviations and jargon that has become a hard-to-navigate alphabet soup SPM refers to an umbrella of a variety of methods Methods that fall under SPM include per-haps its most famous member atomic force microscopy (AFM) in addition to others such as scanning tunneling microscopy (STM) near-field scanning optical microscopy (NSOM or SNOM depending on the continent) and other lithographic methods And then within AFM there are dozens of methods that rely on the AFM probendashsample interactions to provide a variety of material properties including electrical optical magnetic and mechanical properties To date there are dozens of SPMAFM-based methods It is beyond the scope of this book to list andor define all the related methods These methods characterize a huge variety of material properties Some of the more common methods have been included in Figure 11 with the category of properties that they measure The wide diversity of properties of materials that can be measured with AFM is clear The focus of this book is on nanomechanical characterization due to its broad appeal in industrial RampD Again Figure 11 is offered to demonstrate the variety and flavor of properties that can be probed with SPM

12 Atomic Force microscopymdashthe AppeAl to industriAl rampd

This book specifically focuses on AFM which is the method that has most penetrated the general characterization and industrial research fields AFM has key features that make it especially attractive to industrial research and development First its resolution

12 Atomic Force MicroscopymdashThe Appeal to Industrial RampD 3

the heart of its utility as a characterization tool is indeed impressive with 5- to 10-nm lateral resolution and angstrom vertical resolution achieved routinely on commercial instruments with commercial cantilevers The limits of lateral resolution continues to be pushed with specialized techniques and cantilevers and it has reached true ldquoatomicrdquo resolution where point defects of certain materials can now be imaged under certain operating parameters and imaging modes especially under liquid [6ndash8] True atomic resolution is still currently mostly achieved through STM which operates through an entirely different tipndashsample interaction mechanism based on quantum mechanical tunneling Many of these STM studies are conducted in ultrahigh vacuum (UHV) though some are performed in ambient and liquid conditions It is fair to say that AFM is catching up however with recent groundbreaking work imaging cyclic aromatic molecules on Cu[111] [10 11]

A second critical feature of AFM that makes it particularly amenable to industrial research is the flexibility of the environment in which it can operate Despite being a high-resolution microscopy that rivals the resolution of electron microscopy AFM can operate in an ambient or even liquid environment with minimal compromise to its resolution The ability to work in ldquoreal-worldrdquo environments makes it a critical tool for many industrial RampD applications where the research emphasis is consis-tently to understand mechanisms and materials in real-world situations as opposed to

Figure 11 Schematic of AFM as an umbrella technique that includes a wide variety of methods to probe various properties of materials Some of these methods are shown here

4 Chapter 1 Overview of Atomic Force Microscopy

idealized materials and conditions that typically exist in academic research endeavors In many cases the researcher may not want the sample to be forced into a pristine vacuum environment where perhaps key materials or components will be evacuated and thus missed in the characterization effort In addition the flexibility of an envi-ronment permits in situ characterization of processes For example processes such as corrosion lubrication catalyst dissolution crystal growth (for active pharmaceutical ingredient API) can all be studied in situ with AFM as a unique attribute of this microscopy The power of in situ measurements for biological applications is simple as many biological processes and materials simple cannot survive ex situ

A corollary of the utility of the AFMrsquos flexible environment to industrial RampD is the flexibility of type and shape of surface or material that AFM can image There are virtually no restrictions on the size of a material that can be probed with AFM Many commercial ldquolarge-samplerdquo AFMs currently exist that can image any design or shape of a surface from engine parts to thin films Indeed one of the AFMs built in the early 1990s (Topometrix Explorer) was advertised as being able to be placed on top of jet wings to image fractures In addition there are practically no mechanical or electrical restrictions on the type of sample an AFM can image Unlike electron microscopy or STM that require conducting or semiconducting surfaces to image or avoid artifacts [or in the case of insulating surface in scanning electron microscopy (SEM) that requires deposition of a thin conducting layer such as Cr] AFM can image insulating surfaces in addition to conducting or semiconducting surfaces with minimal sample prep Typically the most significant requirement on the sample for an AFM is that it be smooth Commercial instruments will have a maximum z (vertical) range that can be imaged dictated by the design and desired resolution of the particular instrument The maximum z range is typically from 1 microm (for high-resolution studies) to several microns (for lower resolution studies) meaning that the sample cannot have features a tilt or overall roughness that exceed that limit in order for the AFM to image effectively Smooth surfaces are often prepared with the use of an ultramicrotome An ultramicrotome is simply a very controlled and precise way of cutting of thin (down to 100 nm) sections at controlled speeds and in a controlled fashion to result in ultrasmooth surfaces of nanometer or less than nanometer rough-enss The ultramicrotome typically employs the use of a glass knife for a coarse cut and then a diamond knife for a fine cut Cryomicrotomy is simply ultramicrotomy at cold temperatures Cryomicrotomy is typically used to prepare polymer surfaces to cut below the glass transition temperature of the material depending on the instru-ment cryomicrotomes can cool down to -180degC

Finally AFM is fundamentally a mechanical probe of the surface through actual contact between the tip and sample So in addition to the ability to provide high resolution on surface features the goal of any form of microscopy is the unique tipndashsample interaction mechanism enabling the probing of many properties most obvious of which is mechanical properties Mechanical properties covers a wide range of materials and properties Entire textbooks and courses are devoted to the studies of mechanics for example by Callister and Rethwisch [12] We very briefly review some of the main concepts below to provide enough of a background for the rest of the chapter

13 Mechanical Properties 5

This edition focuses specifically on nanomechanical characterization Probing other material properties such as optical electrical or magnetic properties is not covered in this book The reason is that though these other properties have been successfully probed in various industrial RampD applications using AFM to probe mechanical properties of surfaces is more ubiquitous in industrial RampD and so was a logical focus for a first book on SPM in industrial applications

13 mechAnicAl properties

When one considers mechanical properties a range of descriptors come to mind including ldquostiffnessrdquo ldquotoughnessrdquo ldquohardnessrdquo ldquobrittlenessrdquo and ldquoductilityrdquo to name a few Each of these properties has their own mathematical expressions with associ-ated experimental methods of measuring them An interaction between two materials can be typically categorized into either elastic (or recoverable) or dissipative or plastic (permanent or nonrecoverable) interaction On the bulk scale most mechanical analysis of materials relies on measuring their strain (material displacement) as a function of stress (pressure exerted on the material) or vica versa The measurement and analysis of stress versus strain behavior of various materials is a rich and active field and there are many excellent textbooks that describe it [12] The description provided below is simplified to provide a context for these measurements ultimately described on the nanoscale

Elastic deformation of a material is typically nonpermanent or reversible and exists in a regime where the strain (material displacement) is proportional to the stress (pressure exerted on the material) by a proportionality factor known as a modulus (though in reality no material is truly only elastic) Elastic materials thus typically follow Hookersquos law Figure 12 shows a representative stress versus strain curve for materials highlighting the elastic region (blue) Additionally in elastic

Plastic

Fracture

Strain

Elastic

Str

ess

Figure 12 Basic stress vs strain curve showing three mechanical regimes of elastic deformation plastic deformation and fracture For color details please see color plate section

6 Chapter 1 Overview of Atomic Force Microscopy

materials the loading curve (as force is applied) is identical to the unloading curve (as the force is removed) Modulus is a key parameter used to describe elastic behavior Depending on the direction in which a material is being stressed the material will have different moduli (eg a tensile modulus a shear modulus etc)

Plastic deformation is on the other end permanent and nonrecoverable Plastic deformation does not follow Hookersquos law and so stress is not proportional to strain In Figure 12 the region of plastic deformation is shown in green In contrast to elastic deformation the loading curve is not identical to the unloading curve and in fact the unloading curve returns to a nonzero strain at a zero stress level There are many common properties associated with plastic deformation including yielding (onset of plastic deformation) ductility (plastic deformation at a fracture) and hardness (resistance to localized plastic deformation)

Finally there is another very important class of materials called viscoelastic materials where the elastic properties are frequency dependent For example when a stress such as pulling is exerted on the material the materialrsquos response is rate dependent Perhaps the most familiar viscoelastic material to most readers from childhood is Silly Putty where its response to pulling depends on how fast it is pulled If Silly Putty is pulled very slowly the material stretches out very nicely If Silly Putty is pulled very fast it will quickly break Polymers and rubbers two materials that play a significant role in consumer products and which are character-ized quite heavily with AFM are also viscoelastic materials

14 overvieW oF AFm operAtion

141 AFm hardware

A schematic of AFM hardware is shown in Figure 13 The main components of the AFM are (1) the cantilever (2) optical detection system (3) x-y-z scanner and (4) feedback loop

142 cantilevers and probes

The heart of the AFM lies in its cantileverprobe assembly that interacts or probes materials to provide the information of interest The technology to fabricate AFM can-tilevers takes advantage of technology developed for the semiconductor industry to make similar scale devices and features out of silicon Thus today AFM cantilevers are typically made by micromachining silicon (single-crystal Si) or silicon nitride (Si

3N

4) The dimensions of a cantilever vary and dictate the stiffness or spring

constant of the cantilever Depending on the material that needs to be probed and the mode of operation one might pick a lever with a stiffer (higher) or softer (lower) spring constant Cantilevers are generally either rectangular in geometry (Si or Si

3N

4)

or triangular (Si3N

4) with now hybrid cantilevers being manufactured (eg Si tips on

SiN cantilevers) Generally the Si3N

4 probes have lower force constants and might be

used for imaging in liquid among other applications whereas the Si probes can be brittle and tend to break or chip in contact when scanning a surface Typically the

14 Overview of AFM Operation 7

cantilevers are hundreds of microns in length tens of microns in width and a few microns in thickness for reference the diameter of a human hair is approximately 100 microm Cantilevers can be coated with gold or aluminum to provide high reflectivity Commercial cantilevers are also available with other thin metal coatings for magnetic or electrical imaging Cantilevers and probes can also be functionalized chemically or biologically for specific interactions

Probes are grown at the end of the lever and vary in shape and sharpness A sharper probe can often improve the lateral resolution of the AFM Probe design is a continuously evolving field as manufacturers strive for the most robust reproducible and sharp tips Cantileverprobe assemblies are mounted into a cantilever holder that typically includes a piezoceramic element (described below) that can vibrate the can-tilever at a given frequencymdasheither above at or below its resonancemdashdepending on the desired tipndashsample interaction or AFM mode

143 optical detection system

In order to track the motion of the cantileverprobe assembly as it scans a surface an AFM typically includes an optical detection system that consists of a laser reflected off the back side of the cantilever and directed toward a position sensitive detector

Figure 13 Schematic of basic operating principles of AFM A laser is focused and directed off the back end of a cantilever and directed toward a position-sensitive director (PSD) to monitor its vertical and lateral motion as it interacts with the surface Depending on the mode used to image the sample the cantilever can be vibrated with a shake piezo for dynamic imaging modes that involve the resonance of the cantilever

8 Chapter 1 Overview of Atomic Force Microscopy

(PSD) as seen in Figure 13 Optical detection systems are the most common way to track cantilever motion in commercial AFM systems However other methods exist to bypass optical detection systems (in eg a dark medium where the laser light would be absorbed) with self-actuated cantilevers where their motion is read by piezos integrated into the levers The laser is typically a visible photodiode though some commercial instruments have implemented superluminescent diode (SLD) laser The laser is detected by a four-segment position-sensitive detector (PSD) which can track the vertical and lateral motion of the cantilever accurately which can be converted to units of nanometers or displacement through careful calibration described below

144 x-y-z scanner

An AFM can operate either by scanning the sample relative to a stationary tip (sample scanning) or scanning the tip relative to a stationary sample (tip scanning) Each has its own advantages in terms of size of sample it can accommodate (tip scanning more flexible) ability to accessorize (tip scanning again more flexible) and stability and signal to noise (sample scanning easier to build with better specifications) Each shares the requirement to move the tip relative to the sample in a highly accurate way with minimal noise

Tipndashsample motion is often accomplished in commercial instruments by piezoelectric materials which are materials that respond either by expansion or con-traction in response to an applied voltage These kinds of materials provide the ability for very fine motion (nanometers to microns) Piezoelectric materials can be made in several shapes including tubes disks and bimorphs depending on the range and geom-etry of motion required Although piezoelectric materials are very effective at moving tip or sample they are plagued by nonlinear behavior such as hysteresis and creep that has serious consequences for accurate AFM imaging and interpretation of data Artifacts induced by piezoelectric materials are reviewed in detail in Chapter 6

Piezoelectric scanners can be programmed to move with either open-loop or closed-loop feedback circuits In open-loop scanners the piezo is moved by programs to ramp x and y bias voltages to move the prescribed amounts So in open-loop scan-ners if the piezo has to move a large distance or move around on the sample there can be considerable hysteresis of creep with the best recourse of patience or movingzooming in and out in small increments since hysteresiscreep is proportional to the displacement or motion the scanner is trying to execute

One solution to this problem is to use closed-loop scanners that use additional x-y-z sensors to independently measure and then correct for scanner movement In closed-loop operation feedback circuits are continually adjusted to the applied scanner biases to correct for the x-y motion and to achieve the desired motion It is important to note that correction of x-y motion in an AFM is slightly different than correction of z motion This is due to the fact that x-y motion can be predicted whereas z motion cannot as it depends on the surface topography of the sample being scanned Therefore typically there are active feedback circuits to control x and y

viii Contents

3 Understanding Surface Forces Using Static and Dynamic ApproachndashRetraction Curves 49

Sudharsan Balasubramaniam Daniel Kiracofe and Arvind Raman

31 TipndashSample Interaction Forces 5332 Static FndashZ Curves 5833 Dynamic AmplitudePhasendashDistance Curves 6934 Brief Guide to VEDA Simulations 7835 Conclusions 90Glossary 91References 93

4 Phase Imaging 95

Dalia G Yablon and Greg Haugstad

41 Introduction 9542 Bistability Attractive and Repulsive Mode 9743 Complications in Phase Quantification 107References 113

5 Dynamic Contact AFM Methods for Nanomechanical Properties 115

Donna C Hurley and Jason P Killgore

51 Introduction 11552 Force Modulation Microscopy (FMM) 12153 Contact Resonance (CR) Techniques 12554 Comparison of FMM and CR-FM 13655 Other Dynamic Contact Approaches 13856 Summary and Conclusions 140Acknowledgments 141Appendix 5A Data Analysis Procedure for Contact Resonance Spectroscopy Measurements 141References 145

6 Guide to Best Practices for AFM Users 150

Greg Haugstad

61 ForcendashDistance MeasurementsmdashInstrumental Sources of Nonideality 151

62 ForcendashDistance MeasurementsmdashPhysical Sources of Nonideality 157

References 161

Contents ix

7 Nanoindentation Measurements of Mechanical Properties of Very Thin Films and Nanostructured Materials at High Spatial Resolution 162

Steve J Bull

71 Introduction 16272 Bulk Materials 16373 Coatings 17674 Conclusions 188Acknowledgments 188References 188

8 Scanning Probe Microscopy for Critical Measurements in the Semiconductor Industry 190

Johann Foucher

81 Introduction 19082 Critical Dimension in the Semiconductor Industry 19183 CD Metrology Techniques for Production 19284 Obtaining Accurate CD in the Semiconductor Industry 19485 Hybrid Metrology as a Final Solution to Overcome

CD-AFM CD-SEM and Scatterometry Intrinsic Limitations 20386 Conclusion 208References 208

9 Atomic Force Microscopy of Polymers 210

Andy H Tsou and Dalia G Yablon

91 Introduction 21092 Tapping Phase AFM 21393 Nanoindentation 21794 Force Modulation 21895 Pulsed Force Imaging 21996 ForcendashVolume AFM 22097 HarmoniX and Peak Force QNM Imaging 22298 Summary 227References 229

10 Unraveling Links between Food Structure and Function with Probe Microscopy 232

A Patrick Gunning and Victor J Morris

101 Introduction 232102 Gels and Thickeners Molecular Networks 236

x Contents

103 Emulsions and Foams ProteinndashSurfactant Competition 238104 Interfacial Structure and Digestion Designer Interfaces 241105 Force Spectroscopy Model Emulsions 244106 Force Spectroscopy Origins of Bioactivity 247107 Conclusions 248References 249

11 Microcantilever Sensors for Petrochemical Applications 251

Alan M Schilowitz

111 Introduction 251112 Background 252113 Applications 257114 Conclusion 266References 267

12 Applications of Scanning Probe Methods in Cosmetic Science 270

Gustavo S Luengo and Anthony Galliano

121 Introduction 270122 Substrates of Cosmetics 271123 Mechanical Properties and Modifications by Cosmetic Products 274124 Scanning Probe Technologies Adapted to Cosmetic Science 275125 Conclusions 285References 285

13 Applications of Scanning Probe Microscopy and Nanomechanical Analysis in Pharmaceutical Development 287

Matthew S Lamm

131 Introduction 287132 Applications of SPM Imaging 288133 SPM as a Screening Tool 291134 Applications of Nanoindentation 293135 Conclusion 299Acknowledgments 299References 300

14 Comparative Nanomechanical Study of Multiharmonic Force Microscopy and Nanoindentation on Low Dielectric Constant Materials 302

Katharine Walz Robin King Willi Volksen Geraud Dubois Jane Frommer and Kumar Virwani

141 Introduction 302142 Experimental 308

Contents xi

143 Results and Discussions 311144 Conclusions 319Acknowledgments 320References 320

15 Nanomechanical Characterization of Biomaterial Surfaces Polymer Coatings That Elute Drugs 323

Klaus Wormuth and Greg Haugstad

151 Introduction 323152 Materials and Methods 325153 Dexamethasone in PBMA or PBMAndashPLMA Polymer Blends 327154 Simvastatin in PEOndashPBT Copolymers 337155 Concluding Comments 340Acknowledgments 341References 341

Index 342

xiii

Contributors List

Sudharsan BalasubramaniamDepartment of Mechanical EngineeringPurdue UniversityWest Lafayette IN

Steve J BullSchool of Chemical Engineering and Advanced MaterialsUniversity of NewcastleNewcastle upon Tyne United Kingdom

Robert W CarpickDepartment of Mechanical Engineering and Applied MechanicsUniversity of PennsylvaniaPhiladelphia PA

Geraud DuboisIBM Almaden Research CenterSan Jose CA

Johann FoucherLETIndashCEAGrenoble France

Jane FrommerIBM Almaden Research CenterSan Jose CA

Anthony GallianoLrsquoOreal Research and InnovationAulnay sous Bois France

A Patrick GunningInstitute of Food ResearchNorwich Research ParkNorwich United Kingdom

Greg HaugstadUniversity of MinnesotaMinneapolis MN

Donna C HurleyNational Institute of Standards and TechnologyBoulder CO

Tevis D B JacobsDepartment of Materials Science and EngineeringUniversity of PennsylvaniaPhiladelphia PA

Jason P KillgoreNational Institute of Standards and TechnologyBoulder CO

Robin KingIBM Almaden Research CenterSan Jose CA

Daniel KiracofeDepartment of Mechanical EngineeringPurdue UniversityWest Lafayette IN

Matthew S LammMerck Research LaboratoriesMerck amp CoSummit NJ

Gustavo S LuengoLrsquoOreal Research and InnovationAulnay sous Bois France

xiv Contributors List

C Mathew MateHGST A Western Digital CompanySan Jose CA

Victor J MorrisInstitute of Food ResearchNorwich Research ParkNorwich United Kingdom

Arvind RamanDepartment of Mechanical EngineeringPurdue UniversityWest Lafayette IN

Alan M SchilowitzExxonMobil Research and EngineeringAnnandale NJ

Andy H TsouExxonMobil Research and EngineeringAnnandale NJ

Kevin T TurnerDepartment of Mechanical Engineering and Applied MechanicsUniversity of PennsylvaniaPhiladelphia PA

Kumar VirwaniIBM Almaden Research CenterSan Jose CA

Willi VolksenIBM Almaden Research CenterSan Jose CA

Katharine WalzIBM Almaden Research CenterSan Jose CA

Klaus WormuthInstitute for Physical ChemistryUniversity of CologneCologneGermany

Dalia G YablonSurfaceChar LLCSharon MA

xv

Preface

The idea for this book came a couple of years ago after perusing atomic force microscopy (AFM) book collections at many professional society conferences over the years and finding that there was constantly a key topic missing how AFM was used practically in industrial RampD This topic is close to my heart as I have pursued a career developing novel methods and applying scanning probe microscopy (SPM) to a wide variety of industrial RampD problems at ExxonMobil Research and Engineeringrsquos Corporate Strategic Research in New Jersey for over a decade At ExxonMobil I have had the opportunity to explore application of AFM in the three vast sectors in the petroleum industry the downstream chemi-cals and upstream sector studying a wide variety of problems from corrosion to lubrication to geology to polymer materials All the AFM-focused books I found were research reviews on focused topical areas or textbooks on operating princi-ples and theory So though these books offered excellent technical treatments of a variety of topics the incredible utility applicability and flexibility of AFM to solve real-world everyday problems was lacking

My background in graduate school was actually in scanning tunneling micro-scopy (STM) as I studied self-assembled monolayers at the liquidndashsolid interface in George Flynnrsquos lab at Columbia Universityrsquos chemistry department not the most rel-evant ldquoreal-world industrialrdquo problem However being an STM lab in the 1990s we also had AFMs in the lab (since they shared the same ldquobrainrdquo the controller) so every once in a while we would help out a colleague and study their samples As I moved to ExxonMobil after graduate school in 2002 my intent was not necessarily to continue in scanning probe microscopy In fact it was serendipity that upon arriving at ExxonMobilrsquos Corporate Strategic Research labs as a postdoctoral fellow I found an unused Digital Instruments Nanoscope III controller with a fully accessorized Multimode The rest as they say is history

Over my time at ExxonMobil I ran into many like-minded colleagues in other industrial facilities using AFM and we would hold fascinating discussions of how the technology was used in onersquos particular field On the other side I could sense great interest from my academic colleagues that industrial researchers were using SPM to such a great extent as well as recent graduates trying to connect their hard-earned knowledge and expertise in the field as students to practical useful areas A book covering this material for the wider audience did indeed seem missing

And so I began to survey the industrial RampD communityndashndashboth members I had known for years and new onesndashndashto gauge interest in putting such a volume together I was inspired by the enthusiasm I received from my colleagues and decided to go forward Then came the question of what kind of AFM As surveyed in Chapter 1

xvi Preface

AFM is a tremendously broad field that characterizes a wide variety of material and surface properties However at its heart AFM is a high-resolution nanomechanical probe of the surface That probe can be coated with an electronically conducting layer or magnetic material or even replaced by an optical fiber tip in order to probe a variety of properties However a strong majority of the industrial RampD both in my own direct personal experience and those that I was familiar with was conducted with mechanical probing of properties Of course there are some key counterexamples that stand out such as electrostatic force microscopy to probe electrical properties of conducting and semiconducting materials and magnetic force microscopy to probe magnetic storage devices Perhaps those applications will be covered in a future volume But the overwhelming use of AFM in industrial RampD was nanomechanical characterization and thus it was quickly chosen as the focus for this book

It was important that this book be self-contained While the bookrsquos focus is on the applications it is intended to be accessible to a broad audience both from a professional and educational background so that everyone from an advanced undergraduate to a seasoned professional will benefit Various professional backgrounds should benefit as well from those in an academic or academic-like environment who wish to learn how AFM is used in an industrial environment to those in an industrial environment eager to learn what other applications are possible While a freshman chemistry and physics background is assumed no prior experience with AFM is assumed There is certainly advanced information in each chapter so that the content should appeal to a variety of backgrounds in the AFM community from novice to expert

With this aim of appealing to a broad audience I include a tutorial on nanomechani-cal methods in the first half of the book so that applications could be understood and appreciated with context without having to refer to other books or references I chose the topics that I felt were most relevant to studying nanomechanical characterization in the practical industrial environment contact mechanics force curves phase imaging dynamic contact methods and nanoindentation Note that this is not an exhaustive list of all the nanomechanical characterization methods available with AFM

I worked closely with my co-authors to make these overview chapters as practical as possible and not overwhelming in theoretical detail several chapters include worked examples of useful calculations (eg using Hertz mechanics with and without adhesion to model a contact in Chapter 2 or data analysis of contact resonance measurements in Chapter 5) These chapters are meant to be overviews to introduce the terminology and key concepts They are not meant to be an exhaustive step-by-step guide to the operation of various techniques those treatments are done elsewhere and are referenced in the book Chapter 6 is a somewhat unique chapter on ldquoBest Practices in AFM Imagingrdquo that I co-wrote with my colleague Greg Haugstad This chapter outlines some of the common pitfalls encountered by AFM users again with a practical approach to what the most common and most important problems are and how to avoid them

Chapters 7ndash15 then dive into how AFM has been used in a variety of industrial RampD areas to explore (a) phenomena and processes such as new formulation development in pharmaceuticals the effect of humidity and temperature on biomate-rials and nanostructure formation in food processing (b) characterization of various

Preface xvii

materials including polymer blends and composites in the chemicals sector skin and hair in personal care products coatings and thin dielectric films in the semiconductor industry (c) AFMrsquos function as a key quality control tool in semiconductor metrology and (d) a slightly different application of AFM microcantilevers as a physical and chemical sensors in the petroleum industry

My hope is that this book will show what an incredibly useful tool AFM has become to industrial RampD in its short 25 years It is currently an indispensable tool in any microscopy laboratory despite its commercial youth So while the AFM field con-tinues to grow and develop in terms of its capability and understanding spearheaded by many notable groups in academia and elsewhere it has already penetrated significantly into a wide variety of industrial labs This trend will only continue and grow As the penetration into industrial labs continues it is my hope that the reader understands not only how the AFM capability increasingly benefits industry but also that industrial labs are quickly becoming a partner in driving AFMrsquos innovation and research as its commercial and practical utility and importance continue to thrive

Dalia G YablonSharon MA

xix

Acknowledgments

I have been blessed to have wonderful colleagues to share ideas with and gain inspiration from during this process First and foremost I must thank my co-authors on this book Working with each and every author has been an absolute pleasure I had the distinct pleasure of working closely with Andy Tsou and Alan Schilowitz for many years at ExxonMobil Some of the other co-authors I have been fortunate to count as longstanding collaborators including Rob Carpick Greg Haugstad Jason Killgore and Donna Hurley I thank them for their support and enthusiastic partic-ipation for this particular project From all I learned so much technically and made many new friends in the process This book would not have been possible without everyonersquos collective hard work to share their insights and wisdom with us

I would like to thank my colleagues at ExxonMobil with whom I have had the pleasure of developing our AFM capabilities in our AFM lab Specifically Jean Grabowski deserves a special callout and has been a tremendous asset to our efforts as well as a genuine friend along the way I also would like to thank Rebecca Locker Ishita Chakraborty and Daniel Kiracofe I also would like to thank Steve Minne and Roger Proksch for help and guidance in the incipient stages of this project as well as Emily Rapalino and Susan Klein for advice and proofreading

Most of all I have to thank my husband Andrew for his absolute unwavering support and cheerleading throughout the past year and a half His confidence in me was a source of real motivation and inspiration to carry forward despite obstacles And logistically of course I have to thank him for providing childcare to our chil-dren in the evenings and weekends (covering for me on many many bath nights and storyreadingndashndashsuch as on this night) so that I could complete this book one which hopefully one day they might be interested in enough to read and understand

1

Scanning Probe Microscopy in Industrial Applications Nanomechanical Characterization First Edition Edited by Dalia G Yablon copy 2014 John Wiley amp Sons Inc Published 2014 by John Wiley amp Sons Inc

Overview of Atomic Force MicroscopyDalia G Yablon

SurfaceChar LLC Sharon MA

Atomic force microscopy (AFM) is a family of nanoscale characterization techniques that has exploded onto the overall characterization and nanotechnology field as its versatility and high resolution continue to feed a dizzying variety of disciplines from biology to physics and chemistry to engineering AFM entered the scientific arena in 1981 with the now famous invention of its older sibling in the scanning probe micro-scopy family the scanning tunneling microscope (STM) in the IBM Zurich labs of Gerd Binnig and Heinrich Rohrer for which they received the Nobel Prize in Physics in 1986 The AFM was then invented in 1986 by Gerd Binnig Cal Quate and Christoph Gerber [1] Together STM and AFM formed the scanning probe micros-copy (SPM) family which includes other methods such as near-field scanning optical microscopy (NSOM)

The STM spawned the next 25 years of the continuously developing field of scanning probe microscopy and specifically atomic force microscopy which now includes dozens of different methods under its name to probe various propertiesmdashincluding mechanical electrical magnetic chemical and opticalmdashof materials and surfaces Atomic force microscopy is a powerful tool in various research enterprises It is found in practically any university characterization facility alongside optical and electron microscopes and most undergraduates in science or engineering fields at this point are at least familiar with it if not have performed a laboratory experiment in their undergraduate curriculum

The focus of this book is to understand and appreciate the role this young technique has played in industrial research and development (RampD) AFM has penetrated into a variety of industrial research sectors as witnessed by the diverse set of applications described in this book Alongside electron and optical microscopy which have been

Chapter 1

2 Chapter 1 Overview of Atomic Force Microscopy

around for decades and have reached an impressive level of commercial maturity and ease of use AFM has become a vital characterization method despite its youth and continued technical evolution So though AFM is still an active area of academic research as its capabilities continue to develop and be better understood it has proven to be a useful microscopy to address industrial and commercial needs from quality control and assurance to product formulation and process monitoring

The goal of this introductory chapter is to provide an overview of AFM to nonspecialists and introduce the various topics that are the subject of subsequent individual chapters As such this chapter will provide a brief review of the begin-nings of AFM and special features that make it particularly suited for industrial research Then a brief overview of AFM operation will be presented including the hardware software calibrations involved and finally the different nanomechanical methods that will be described in detail both in theory and application Entire books are written on AFM operation and this chapter is not meant to be an exhaustive introduction to its operation merely serving to provide enough information for the rest of the book to be followed intelligently Readers interested in more detail about AFM operation can consult a number of excellent books on the topic [2ndash5]

11 A Word on nomenclAture

Before the rest of this chapter continues some definitions are in order Similar to many other surface science techniques AFM has succumbed to a somewhat unwieldy mess of abbreviations and jargon that has become a hard-to-navigate alphabet soup SPM refers to an umbrella of a variety of methods Methods that fall under SPM include per-haps its most famous member atomic force microscopy (AFM) in addition to others such as scanning tunneling microscopy (STM) near-field scanning optical microscopy (NSOM or SNOM depending on the continent) and other lithographic methods And then within AFM there are dozens of methods that rely on the AFM probendashsample interactions to provide a variety of material properties including electrical optical magnetic and mechanical properties To date there are dozens of SPMAFM-based methods It is beyond the scope of this book to list andor define all the related methods These methods characterize a huge variety of material properties Some of the more common methods have been included in Figure 11 with the category of properties that they measure The wide diversity of properties of materials that can be measured with AFM is clear The focus of this book is on nanomechanical characterization due to its broad appeal in industrial RampD Again Figure 11 is offered to demonstrate the variety and flavor of properties that can be probed with SPM

12 Atomic Force microscopymdashthe AppeAl to industriAl rampd

This book specifically focuses on AFM which is the method that has most penetrated the general characterization and industrial research fields AFM has key features that make it especially attractive to industrial research and development First its resolution

12 Atomic Force MicroscopymdashThe Appeal to Industrial RampD 3

the heart of its utility as a characterization tool is indeed impressive with 5- to 10-nm lateral resolution and angstrom vertical resolution achieved routinely on commercial instruments with commercial cantilevers The limits of lateral resolution continues to be pushed with specialized techniques and cantilevers and it has reached true ldquoatomicrdquo resolution where point defects of certain materials can now be imaged under certain operating parameters and imaging modes especially under liquid [6ndash8] True atomic resolution is still currently mostly achieved through STM which operates through an entirely different tipndashsample interaction mechanism based on quantum mechanical tunneling Many of these STM studies are conducted in ultrahigh vacuum (UHV) though some are performed in ambient and liquid conditions It is fair to say that AFM is catching up however with recent groundbreaking work imaging cyclic aromatic molecules on Cu[111] [10 11]

A second critical feature of AFM that makes it particularly amenable to industrial research is the flexibility of the environment in which it can operate Despite being a high-resolution microscopy that rivals the resolution of electron microscopy AFM can operate in an ambient or even liquid environment with minimal compromise to its resolution The ability to work in ldquoreal-worldrdquo environments makes it a critical tool for many industrial RampD applications where the research emphasis is consis-tently to understand mechanisms and materials in real-world situations as opposed to

Figure 11 Schematic of AFM as an umbrella technique that includes a wide variety of methods to probe various properties of materials Some of these methods are shown here

4 Chapter 1 Overview of Atomic Force Microscopy

idealized materials and conditions that typically exist in academic research endeavors In many cases the researcher may not want the sample to be forced into a pristine vacuum environment where perhaps key materials or components will be evacuated and thus missed in the characterization effort In addition the flexibility of an envi-ronment permits in situ characterization of processes For example processes such as corrosion lubrication catalyst dissolution crystal growth (for active pharmaceutical ingredient API) can all be studied in situ with AFM as a unique attribute of this microscopy The power of in situ measurements for biological applications is simple as many biological processes and materials simple cannot survive ex situ

A corollary of the utility of the AFMrsquos flexible environment to industrial RampD is the flexibility of type and shape of surface or material that AFM can image There are virtually no restrictions on the size of a material that can be probed with AFM Many commercial ldquolarge-samplerdquo AFMs currently exist that can image any design or shape of a surface from engine parts to thin films Indeed one of the AFMs built in the early 1990s (Topometrix Explorer) was advertised as being able to be placed on top of jet wings to image fractures In addition there are practically no mechanical or electrical restrictions on the type of sample an AFM can image Unlike electron microscopy or STM that require conducting or semiconducting surfaces to image or avoid artifacts [or in the case of insulating surface in scanning electron microscopy (SEM) that requires deposition of a thin conducting layer such as Cr] AFM can image insulating surfaces in addition to conducting or semiconducting surfaces with minimal sample prep Typically the most significant requirement on the sample for an AFM is that it be smooth Commercial instruments will have a maximum z (vertical) range that can be imaged dictated by the design and desired resolution of the particular instrument The maximum z range is typically from 1 microm (for high-resolution studies) to several microns (for lower resolution studies) meaning that the sample cannot have features a tilt or overall roughness that exceed that limit in order for the AFM to image effectively Smooth surfaces are often prepared with the use of an ultramicrotome An ultramicrotome is simply a very controlled and precise way of cutting of thin (down to 100 nm) sections at controlled speeds and in a controlled fashion to result in ultrasmooth surfaces of nanometer or less than nanometer rough-enss The ultramicrotome typically employs the use of a glass knife for a coarse cut and then a diamond knife for a fine cut Cryomicrotomy is simply ultramicrotomy at cold temperatures Cryomicrotomy is typically used to prepare polymer surfaces to cut below the glass transition temperature of the material depending on the instru-ment cryomicrotomes can cool down to -180degC

Finally AFM is fundamentally a mechanical probe of the surface through actual contact between the tip and sample So in addition to the ability to provide high resolution on surface features the goal of any form of microscopy is the unique tipndashsample interaction mechanism enabling the probing of many properties most obvious of which is mechanical properties Mechanical properties covers a wide range of materials and properties Entire textbooks and courses are devoted to the studies of mechanics for example by Callister and Rethwisch [12] We very briefly review some of the main concepts below to provide enough of a background for the rest of the chapter

13 Mechanical Properties 5

This edition focuses specifically on nanomechanical characterization Probing other material properties such as optical electrical or magnetic properties is not covered in this book The reason is that though these other properties have been successfully probed in various industrial RampD applications using AFM to probe mechanical properties of surfaces is more ubiquitous in industrial RampD and so was a logical focus for a first book on SPM in industrial applications

13 mechAnicAl properties

When one considers mechanical properties a range of descriptors come to mind including ldquostiffnessrdquo ldquotoughnessrdquo ldquohardnessrdquo ldquobrittlenessrdquo and ldquoductilityrdquo to name a few Each of these properties has their own mathematical expressions with associ-ated experimental methods of measuring them An interaction between two materials can be typically categorized into either elastic (or recoverable) or dissipative or plastic (permanent or nonrecoverable) interaction On the bulk scale most mechanical analysis of materials relies on measuring their strain (material displacement) as a function of stress (pressure exerted on the material) or vica versa The measurement and analysis of stress versus strain behavior of various materials is a rich and active field and there are many excellent textbooks that describe it [12] The description provided below is simplified to provide a context for these measurements ultimately described on the nanoscale

Elastic deformation of a material is typically nonpermanent or reversible and exists in a regime where the strain (material displacement) is proportional to the stress (pressure exerted on the material) by a proportionality factor known as a modulus (though in reality no material is truly only elastic) Elastic materials thus typically follow Hookersquos law Figure 12 shows a representative stress versus strain curve for materials highlighting the elastic region (blue) Additionally in elastic

Plastic

Fracture

Strain

Elastic

Str

ess

Figure 12 Basic stress vs strain curve showing three mechanical regimes of elastic deformation plastic deformation and fracture For color details please see color plate section

6 Chapter 1 Overview of Atomic Force Microscopy

materials the loading curve (as force is applied) is identical to the unloading curve (as the force is removed) Modulus is a key parameter used to describe elastic behavior Depending on the direction in which a material is being stressed the material will have different moduli (eg a tensile modulus a shear modulus etc)

Plastic deformation is on the other end permanent and nonrecoverable Plastic deformation does not follow Hookersquos law and so stress is not proportional to strain In Figure 12 the region of plastic deformation is shown in green In contrast to elastic deformation the loading curve is not identical to the unloading curve and in fact the unloading curve returns to a nonzero strain at a zero stress level There are many common properties associated with plastic deformation including yielding (onset of plastic deformation) ductility (plastic deformation at a fracture) and hardness (resistance to localized plastic deformation)

Finally there is another very important class of materials called viscoelastic materials where the elastic properties are frequency dependent For example when a stress such as pulling is exerted on the material the materialrsquos response is rate dependent Perhaps the most familiar viscoelastic material to most readers from childhood is Silly Putty where its response to pulling depends on how fast it is pulled If Silly Putty is pulled very slowly the material stretches out very nicely If Silly Putty is pulled very fast it will quickly break Polymers and rubbers two materials that play a significant role in consumer products and which are character-ized quite heavily with AFM are also viscoelastic materials

14 overvieW oF AFm operAtion

141 AFm hardware

A schematic of AFM hardware is shown in Figure 13 The main components of the AFM are (1) the cantilever (2) optical detection system (3) x-y-z scanner and (4) feedback loop

142 cantilevers and probes

The heart of the AFM lies in its cantileverprobe assembly that interacts or probes materials to provide the information of interest The technology to fabricate AFM can-tilevers takes advantage of technology developed for the semiconductor industry to make similar scale devices and features out of silicon Thus today AFM cantilevers are typically made by micromachining silicon (single-crystal Si) or silicon nitride (Si

3N

4) The dimensions of a cantilever vary and dictate the stiffness or spring

constant of the cantilever Depending on the material that needs to be probed and the mode of operation one might pick a lever with a stiffer (higher) or softer (lower) spring constant Cantilevers are generally either rectangular in geometry (Si or Si

3N

4)

or triangular (Si3N

4) with now hybrid cantilevers being manufactured (eg Si tips on

SiN cantilevers) Generally the Si3N

4 probes have lower force constants and might be

used for imaging in liquid among other applications whereas the Si probes can be brittle and tend to break or chip in contact when scanning a surface Typically the

14 Overview of AFM Operation 7

cantilevers are hundreds of microns in length tens of microns in width and a few microns in thickness for reference the diameter of a human hair is approximately 100 microm Cantilevers can be coated with gold or aluminum to provide high reflectivity Commercial cantilevers are also available with other thin metal coatings for magnetic or electrical imaging Cantilevers and probes can also be functionalized chemically or biologically for specific interactions

Probes are grown at the end of the lever and vary in shape and sharpness A sharper probe can often improve the lateral resolution of the AFM Probe design is a continuously evolving field as manufacturers strive for the most robust reproducible and sharp tips Cantileverprobe assemblies are mounted into a cantilever holder that typically includes a piezoceramic element (described below) that can vibrate the can-tilever at a given frequencymdasheither above at or below its resonancemdashdepending on the desired tipndashsample interaction or AFM mode

143 optical detection system

In order to track the motion of the cantileverprobe assembly as it scans a surface an AFM typically includes an optical detection system that consists of a laser reflected off the back side of the cantilever and directed toward a position sensitive detector

Figure 13 Schematic of basic operating principles of AFM A laser is focused and directed off the back end of a cantilever and directed toward a position-sensitive director (PSD) to monitor its vertical and lateral motion as it interacts with the surface Depending on the mode used to image the sample the cantilever can be vibrated with a shake piezo for dynamic imaging modes that involve the resonance of the cantilever

8 Chapter 1 Overview of Atomic Force Microscopy

(PSD) as seen in Figure 13 Optical detection systems are the most common way to track cantilever motion in commercial AFM systems However other methods exist to bypass optical detection systems (in eg a dark medium where the laser light would be absorbed) with self-actuated cantilevers where their motion is read by piezos integrated into the levers The laser is typically a visible photodiode though some commercial instruments have implemented superluminescent diode (SLD) laser The laser is detected by a four-segment position-sensitive detector (PSD) which can track the vertical and lateral motion of the cantilever accurately which can be converted to units of nanometers or displacement through careful calibration described below

144 x-y-z scanner

An AFM can operate either by scanning the sample relative to a stationary tip (sample scanning) or scanning the tip relative to a stationary sample (tip scanning) Each has its own advantages in terms of size of sample it can accommodate (tip scanning more flexible) ability to accessorize (tip scanning again more flexible) and stability and signal to noise (sample scanning easier to build with better specifications) Each shares the requirement to move the tip relative to the sample in a highly accurate way with minimal noise

Tipndashsample motion is often accomplished in commercial instruments by piezoelectric materials which are materials that respond either by expansion or con-traction in response to an applied voltage These kinds of materials provide the ability for very fine motion (nanometers to microns) Piezoelectric materials can be made in several shapes including tubes disks and bimorphs depending on the range and geom-etry of motion required Although piezoelectric materials are very effective at moving tip or sample they are plagued by nonlinear behavior such as hysteresis and creep that has serious consequences for accurate AFM imaging and interpretation of data Artifacts induced by piezoelectric materials are reviewed in detail in Chapter 6

Piezoelectric scanners can be programmed to move with either open-loop or closed-loop feedback circuits In open-loop scanners the piezo is moved by programs to ramp x and y bias voltages to move the prescribed amounts So in open-loop scan-ners if the piezo has to move a large distance or move around on the sample there can be considerable hysteresis of creep with the best recourse of patience or movingzooming in and out in small increments since hysteresiscreep is proportional to the displacement or motion the scanner is trying to execute

One solution to this problem is to use closed-loop scanners that use additional x-y-z sensors to independently measure and then correct for scanner movement In closed-loop operation feedback circuits are continually adjusted to the applied scanner biases to correct for the x-y motion and to achieve the desired motion It is important to note that correction of x-y motion in an AFM is slightly different than correction of z motion This is due to the fact that x-y motion can be predicted whereas z motion cannot as it depends on the surface topography of the sample being scanned Therefore typically there are active feedback circuits to control x and y

Contents ix

7 Nanoindentation Measurements of Mechanical Properties of Very Thin Films and Nanostructured Materials at High Spatial Resolution 162

Steve J Bull

71 Introduction 16272 Bulk Materials 16373 Coatings 17674 Conclusions 188Acknowledgments 188References 188

8 Scanning Probe Microscopy for Critical Measurements in the Semiconductor Industry 190

Johann Foucher

81 Introduction 19082 Critical Dimension in the Semiconductor Industry 19183 CD Metrology Techniques for Production 19284 Obtaining Accurate CD in the Semiconductor Industry 19485 Hybrid Metrology as a Final Solution to Overcome

CD-AFM CD-SEM and Scatterometry Intrinsic Limitations 20386 Conclusion 208References 208

9 Atomic Force Microscopy of Polymers 210

Andy H Tsou and Dalia G Yablon

91 Introduction 21092 Tapping Phase AFM 21393 Nanoindentation 21794 Force Modulation 21895 Pulsed Force Imaging 21996 ForcendashVolume AFM 22097 HarmoniX and Peak Force QNM Imaging 22298 Summary 227References 229

10 Unraveling Links between Food Structure and Function with Probe Microscopy 232

A Patrick Gunning and Victor J Morris

101 Introduction 232102 Gels and Thickeners Molecular Networks 236

x Contents

103 Emulsions and Foams ProteinndashSurfactant Competition 238104 Interfacial Structure and Digestion Designer Interfaces 241105 Force Spectroscopy Model Emulsions 244106 Force Spectroscopy Origins of Bioactivity 247107 Conclusions 248References 249

11 Microcantilever Sensors for Petrochemical Applications 251

Alan M Schilowitz

111 Introduction 251112 Background 252113 Applications 257114 Conclusion 266References 267

12 Applications of Scanning Probe Methods in Cosmetic Science 270

Gustavo S Luengo and Anthony Galliano

121 Introduction 270122 Substrates of Cosmetics 271123 Mechanical Properties and Modifications by Cosmetic Products 274124 Scanning Probe Technologies Adapted to Cosmetic Science 275125 Conclusions 285References 285

13 Applications of Scanning Probe Microscopy and Nanomechanical Analysis in Pharmaceutical Development 287

Matthew S Lamm

131 Introduction 287132 Applications of SPM Imaging 288133 SPM as a Screening Tool 291134 Applications of Nanoindentation 293135 Conclusion 299Acknowledgments 299References 300

14 Comparative Nanomechanical Study of Multiharmonic Force Microscopy and Nanoindentation on Low Dielectric Constant Materials 302

Katharine Walz Robin King Willi Volksen Geraud Dubois Jane Frommer and Kumar Virwani

141 Introduction 302142 Experimental 308

Contents xi

143 Results and Discussions 311144 Conclusions 319Acknowledgments 320References 320

15 Nanomechanical Characterization of Biomaterial Surfaces Polymer Coatings That Elute Drugs 323

Klaus Wormuth and Greg Haugstad

151 Introduction 323152 Materials and Methods 325153 Dexamethasone in PBMA or PBMAndashPLMA Polymer Blends 327154 Simvastatin in PEOndashPBT Copolymers 337155 Concluding Comments 340Acknowledgments 341References 341

Index 342

xiii

Contributors List

Sudharsan BalasubramaniamDepartment of Mechanical EngineeringPurdue UniversityWest Lafayette IN

Steve J BullSchool of Chemical Engineering and Advanced MaterialsUniversity of NewcastleNewcastle upon Tyne United Kingdom

Robert W CarpickDepartment of Mechanical Engineering and Applied MechanicsUniversity of PennsylvaniaPhiladelphia PA

Geraud DuboisIBM Almaden Research CenterSan Jose CA

Johann FoucherLETIndashCEAGrenoble France

Jane FrommerIBM Almaden Research CenterSan Jose CA

Anthony GallianoLrsquoOreal Research and InnovationAulnay sous Bois France

A Patrick GunningInstitute of Food ResearchNorwich Research ParkNorwich United Kingdom

Greg HaugstadUniversity of MinnesotaMinneapolis MN

Donna C HurleyNational Institute of Standards and TechnologyBoulder CO

Tevis D B JacobsDepartment of Materials Science and EngineeringUniversity of PennsylvaniaPhiladelphia PA

Jason P KillgoreNational Institute of Standards and TechnologyBoulder CO

Robin KingIBM Almaden Research CenterSan Jose CA

Daniel KiracofeDepartment of Mechanical EngineeringPurdue UniversityWest Lafayette IN

Matthew S LammMerck Research LaboratoriesMerck amp CoSummit NJ

Gustavo S LuengoLrsquoOreal Research and InnovationAulnay sous Bois France

xiv Contributors List

C Mathew MateHGST A Western Digital CompanySan Jose CA

Victor J MorrisInstitute of Food ResearchNorwich Research ParkNorwich United Kingdom

Arvind RamanDepartment of Mechanical EngineeringPurdue UniversityWest Lafayette IN

Alan M SchilowitzExxonMobil Research and EngineeringAnnandale NJ

Andy H TsouExxonMobil Research and EngineeringAnnandale NJ

Kevin T TurnerDepartment of Mechanical Engineering and Applied MechanicsUniversity of PennsylvaniaPhiladelphia PA

Kumar VirwaniIBM Almaden Research CenterSan Jose CA

Willi VolksenIBM Almaden Research CenterSan Jose CA

Katharine WalzIBM Almaden Research CenterSan Jose CA

Klaus WormuthInstitute for Physical ChemistryUniversity of CologneCologneGermany

Dalia G YablonSurfaceChar LLCSharon MA

xv

Preface

The idea for this book came a couple of years ago after perusing atomic force microscopy (AFM) book collections at many professional society conferences over the years and finding that there was constantly a key topic missing how AFM was used practically in industrial RampD This topic is close to my heart as I have pursued a career developing novel methods and applying scanning probe microscopy (SPM) to a wide variety of industrial RampD problems at ExxonMobil Research and Engineeringrsquos Corporate Strategic Research in New Jersey for over a decade At ExxonMobil I have had the opportunity to explore application of AFM in the three vast sectors in the petroleum industry the downstream chemi-cals and upstream sector studying a wide variety of problems from corrosion to lubrication to geology to polymer materials All the AFM-focused books I found were research reviews on focused topical areas or textbooks on operating princi-ples and theory So though these books offered excellent technical treatments of a variety of topics the incredible utility applicability and flexibility of AFM to solve real-world everyday problems was lacking

My background in graduate school was actually in scanning tunneling micro-scopy (STM) as I studied self-assembled monolayers at the liquidndashsolid interface in George Flynnrsquos lab at Columbia Universityrsquos chemistry department not the most rel-evant ldquoreal-world industrialrdquo problem However being an STM lab in the 1990s we also had AFMs in the lab (since they shared the same ldquobrainrdquo the controller) so every once in a while we would help out a colleague and study their samples As I moved to ExxonMobil after graduate school in 2002 my intent was not necessarily to continue in scanning probe microscopy In fact it was serendipity that upon arriving at ExxonMobilrsquos Corporate Strategic Research labs as a postdoctoral fellow I found an unused Digital Instruments Nanoscope III controller with a fully accessorized Multimode The rest as they say is history

Over my time at ExxonMobil I ran into many like-minded colleagues in other industrial facilities using AFM and we would hold fascinating discussions of how the technology was used in onersquos particular field On the other side I could sense great interest from my academic colleagues that industrial researchers were using SPM to such a great extent as well as recent graduates trying to connect their hard-earned knowledge and expertise in the field as students to practical useful areas A book covering this material for the wider audience did indeed seem missing

And so I began to survey the industrial RampD communityndashndashboth members I had known for years and new onesndashndashto gauge interest in putting such a volume together I was inspired by the enthusiasm I received from my colleagues and decided to go forward Then came the question of what kind of AFM As surveyed in Chapter 1

xvi Preface

AFM is a tremendously broad field that characterizes a wide variety of material and surface properties However at its heart AFM is a high-resolution nanomechanical probe of the surface That probe can be coated with an electronically conducting layer or magnetic material or even replaced by an optical fiber tip in order to probe a variety of properties However a strong majority of the industrial RampD both in my own direct personal experience and those that I was familiar with was conducted with mechanical probing of properties Of course there are some key counterexamples that stand out such as electrostatic force microscopy to probe electrical properties of conducting and semiconducting materials and magnetic force microscopy to probe magnetic storage devices Perhaps those applications will be covered in a future volume But the overwhelming use of AFM in industrial RampD was nanomechanical characterization and thus it was quickly chosen as the focus for this book

It was important that this book be self-contained While the bookrsquos focus is on the applications it is intended to be accessible to a broad audience both from a professional and educational background so that everyone from an advanced undergraduate to a seasoned professional will benefit Various professional backgrounds should benefit as well from those in an academic or academic-like environment who wish to learn how AFM is used in an industrial environment to those in an industrial environment eager to learn what other applications are possible While a freshman chemistry and physics background is assumed no prior experience with AFM is assumed There is certainly advanced information in each chapter so that the content should appeal to a variety of backgrounds in the AFM community from novice to expert

With this aim of appealing to a broad audience I include a tutorial on nanomechani-cal methods in the first half of the book so that applications could be understood and appreciated with context without having to refer to other books or references I chose the topics that I felt were most relevant to studying nanomechanical characterization in the practical industrial environment contact mechanics force curves phase imaging dynamic contact methods and nanoindentation Note that this is not an exhaustive list of all the nanomechanical characterization methods available with AFM

I worked closely with my co-authors to make these overview chapters as practical as possible and not overwhelming in theoretical detail several chapters include worked examples of useful calculations (eg using Hertz mechanics with and without adhesion to model a contact in Chapter 2 or data analysis of contact resonance measurements in Chapter 5) These chapters are meant to be overviews to introduce the terminology and key concepts They are not meant to be an exhaustive step-by-step guide to the operation of various techniques those treatments are done elsewhere and are referenced in the book Chapter 6 is a somewhat unique chapter on ldquoBest Practices in AFM Imagingrdquo that I co-wrote with my colleague Greg Haugstad This chapter outlines some of the common pitfalls encountered by AFM users again with a practical approach to what the most common and most important problems are and how to avoid them

Chapters 7ndash15 then dive into how AFM has been used in a variety of industrial RampD areas to explore (a) phenomena and processes such as new formulation development in pharmaceuticals the effect of humidity and temperature on biomate-rials and nanostructure formation in food processing (b) characterization of various

Preface xvii

materials including polymer blends and composites in the chemicals sector skin and hair in personal care products coatings and thin dielectric films in the semiconductor industry (c) AFMrsquos function as a key quality control tool in semiconductor metrology and (d) a slightly different application of AFM microcantilevers as a physical and chemical sensors in the petroleum industry

My hope is that this book will show what an incredibly useful tool AFM has become to industrial RampD in its short 25 years It is currently an indispensable tool in any microscopy laboratory despite its commercial youth So while the AFM field con-tinues to grow and develop in terms of its capability and understanding spearheaded by many notable groups in academia and elsewhere it has already penetrated significantly into a wide variety of industrial labs This trend will only continue and grow As the penetration into industrial labs continues it is my hope that the reader understands not only how the AFM capability increasingly benefits industry but also that industrial labs are quickly becoming a partner in driving AFMrsquos innovation and research as its commercial and practical utility and importance continue to thrive

Dalia G YablonSharon MA

xix

Acknowledgments

I have been blessed to have wonderful colleagues to share ideas with and gain inspiration from during this process First and foremost I must thank my co-authors on this book Working with each and every author has been an absolute pleasure I had the distinct pleasure of working closely with Andy Tsou and Alan Schilowitz for many years at ExxonMobil Some of the other co-authors I have been fortunate to count as longstanding collaborators including Rob Carpick Greg Haugstad Jason Killgore and Donna Hurley I thank them for their support and enthusiastic partic-ipation for this particular project From all I learned so much technically and made many new friends in the process This book would not have been possible without everyonersquos collective hard work to share their insights and wisdom with us

I would like to thank my colleagues at ExxonMobil with whom I have had the pleasure of developing our AFM capabilities in our AFM lab Specifically Jean Grabowski deserves a special callout and has been a tremendous asset to our efforts as well as a genuine friend along the way I also would like to thank Rebecca Locker Ishita Chakraborty and Daniel Kiracofe I also would like to thank Steve Minne and Roger Proksch for help and guidance in the incipient stages of this project as well as Emily Rapalino and Susan Klein for advice and proofreading

Most of all I have to thank my husband Andrew for his absolute unwavering support and cheerleading throughout the past year and a half His confidence in me was a source of real motivation and inspiration to carry forward despite obstacles And logistically of course I have to thank him for providing childcare to our chil-dren in the evenings and weekends (covering for me on many many bath nights and storyreadingndashndashsuch as on this night) so that I could complete this book one which hopefully one day they might be interested in enough to read and understand

1

Scanning Probe Microscopy in Industrial Applications Nanomechanical Characterization First Edition Edited by Dalia G Yablon copy 2014 John Wiley amp Sons Inc Published 2014 by John Wiley amp Sons Inc

Overview of Atomic Force MicroscopyDalia G Yablon

SurfaceChar LLC Sharon MA

Atomic force microscopy (AFM) is a family of nanoscale characterization techniques that has exploded onto the overall characterization and nanotechnology field as its versatility and high resolution continue to feed a dizzying variety of disciplines from biology to physics and chemistry to engineering AFM entered the scientific arena in 1981 with the now famous invention of its older sibling in the scanning probe micro-scopy family the scanning tunneling microscope (STM) in the IBM Zurich labs of Gerd Binnig and Heinrich Rohrer for which they received the Nobel Prize in Physics in 1986 The AFM was then invented in 1986 by Gerd Binnig Cal Quate and Christoph Gerber [1] Together STM and AFM formed the scanning probe micros-copy (SPM) family which includes other methods such as near-field scanning optical microscopy (NSOM)

The STM spawned the next 25 years of the continuously developing field of scanning probe microscopy and specifically atomic force microscopy which now includes dozens of different methods under its name to probe various propertiesmdashincluding mechanical electrical magnetic chemical and opticalmdashof materials and surfaces Atomic force microscopy is a powerful tool in various research enterprises It is found in practically any university characterization facility alongside optical and electron microscopes and most undergraduates in science or engineering fields at this point are at least familiar with it if not have performed a laboratory experiment in their undergraduate curriculum

The focus of this book is to understand and appreciate the role this young technique has played in industrial research and development (RampD) AFM has penetrated into a variety of industrial research sectors as witnessed by the diverse set of applications described in this book Alongside electron and optical microscopy which have been

Chapter 1

2 Chapter 1 Overview of Atomic Force Microscopy

around for decades and have reached an impressive level of commercial maturity and ease of use AFM has become a vital characterization method despite its youth and continued technical evolution So though AFM is still an active area of academic research as its capabilities continue to develop and be better understood it has proven to be a useful microscopy to address industrial and commercial needs from quality control and assurance to product formulation and process monitoring

The goal of this introductory chapter is to provide an overview of AFM to nonspecialists and introduce the various topics that are the subject of subsequent individual chapters As such this chapter will provide a brief review of the begin-nings of AFM and special features that make it particularly suited for industrial research Then a brief overview of AFM operation will be presented including the hardware software calibrations involved and finally the different nanomechanical methods that will be described in detail both in theory and application Entire books are written on AFM operation and this chapter is not meant to be an exhaustive introduction to its operation merely serving to provide enough information for the rest of the book to be followed intelligently Readers interested in more detail about AFM operation can consult a number of excellent books on the topic [2ndash5]

11 A Word on nomenclAture

Before the rest of this chapter continues some definitions are in order Similar to many other surface science techniques AFM has succumbed to a somewhat unwieldy mess of abbreviations and jargon that has become a hard-to-navigate alphabet soup SPM refers to an umbrella of a variety of methods Methods that fall under SPM include per-haps its most famous member atomic force microscopy (AFM) in addition to others such as scanning tunneling microscopy (STM) near-field scanning optical microscopy (NSOM or SNOM depending on the continent) and other lithographic methods And then within AFM there are dozens of methods that rely on the AFM probendashsample interactions to provide a variety of material properties including electrical optical magnetic and mechanical properties To date there are dozens of SPMAFM-based methods It is beyond the scope of this book to list andor define all the related methods These methods characterize a huge variety of material properties Some of the more common methods have been included in Figure 11 with the category of properties that they measure The wide diversity of properties of materials that can be measured with AFM is clear The focus of this book is on nanomechanical characterization due to its broad appeal in industrial RampD Again Figure 11 is offered to demonstrate the variety and flavor of properties that can be probed with SPM

12 Atomic Force microscopymdashthe AppeAl to industriAl rampd

This book specifically focuses on AFM which is the method that has most penetrated the general characterization and industrial research fields AFM has key features that make it especially attractive to industrial research and development First its resolution

12 Atomic Force MicroscopymdashThe Appeal to Industrial RampD 3

the heart of its utility as a characterization tool is indeed impressive with 5- to 10-nm lateral resolution and angstrom vertical resolution achieved routinely on commercial instruments with commercial cantilevers The limits of lateral resolution continues to be pushed with specialized techniques and cantilevers and it has reached true ldquoatomicrdquo resolution where point defects of certain materials can now be imaged under certain operating parameters and imaging modes especially under liquid [6ndash8] True atomic resolution is still currently mostly achieved through STM which operates through an entirely different tipndashsample interaction mechanism based on quantum mechanical tunneling Many of these STM studies are conducted in ultrahigh vacuum (UHV) though some are performed in ambient and liquid conditions It is fair to say that AFM is catching up however with recent groundbreaking work imaging cyclic aromatic molecules on Cu[111] [10 11]

A second critical feature of AFM that makes it particularly amenable to industrial research is the flexibility of the environment in which it can operate Despite being a high-resolution microscopy that rivals the resolution of electron microscopy AFM can operate in an ambient or even liquid environment with minimal compromise to its resolution The ability to work in ldquoreal-worldrdquo environments makes it a critical tool for many industrial RampD applications where the research emphasis is consis-tently to understand mechanisms and materials in real-world situations as opposed to

Figure 11 Schematic of AFM as an umbrella technique that includes a wide variety of methods to probe various properties of materials Some of these methods are shown here

4 Chapter 1 Overview of Atomic Force Microscopy

idealized materials and conditions that typically exist in academic research endeavors In many cases the researcher may not want the sample to be forced into a pristine vacuum environment where perhaps key materials or components will be evacuated and thus missed in the characterization effort In addition the flexibility of an envi-ronment permits in situ characterization of processes For example processes such as corrosion lubrication catalyst dissolution crystal growth (for active pharmaceutical ingredient API) can all be studied in situ with AFM as a unique attribute of this microscopy The power of in situ measurements for biological applications is simple as many biological processes and materials simple cannot survive ex situ

A corollary of the utility of the AFMrsquos flexible environment to industrial RampD is the flexibility of type and shape of surface or material that AFM can image There are virtually no restrictions on the size of a material that can be probed with AFM Many commercial ldquolarge-samplerdquo AFMs currently exist that can image any design or shape of a surface from engine parts to thin films Indeed one of the AFMs built in the early 1990s (Topometrix Explorer) was advertised as being able to be placed on top of jet wings to image fractures In addition there are practically no mechanical or electrical restrictions on the type of sample an AFM can image Unlike electron microscopy or STM that require conducting or semiconducting surfaces to image or avoid artifacts [or in the case of insulating surface in scanning electron microscopy (SEM) that requires deposition of a thin conducting layer such as Cr] AFM can image insulating surfaces in addition to conducting or semiconducting surfaces with minimal sample prep Typically the most significant requirement on the sample for an AFM is that it be smooth Commercial instruments will have a maximum z (vertical) range that can be imaged dictated by the design and desired resolution of the particular instrument The maximum z range is typically from 1 microm (for high-resolution studies) to several microns (for lower resolution studies) meaning that the sample cannot have features a tilt or overall roughness that exceed that limit in order for the AFM to image effectively Smooth surfaces are often prepared with the use of an ultramicrotome An ultramicrotome is simply a very controlled and precise way of cutting of thin (down to 100 nm) sections at controlled speeds and in a controlled fashion to result in ultrasmooth surfaces of nanometer or less than nanometer rough-enss The ultramicrotome typically employs the use of a glass knife for a coarse cut and then a diamond knife for a fine cut Cryomicrotomy is simply ultramicrotomy at cold temperatures Cryomicrotomy is typically used to prepare polymer surfaces to cut below the glass transition temperature of the material depending on the instru-ment cryomicrotomes can cool down to -180degC

Finally AFM is fundamentally a mechanical probe of the surface through actual contact between the tip and sample So in addition to the ability to provide high resolution on surface features the goal of any form of microscopy is the unique tipndashsample interaction mechanism enabling the probing of many properties most obvious of which is mechanical properties Mechanical properties covers a wide range of materials and properties Entire textbooks and courses are devoted to the studies of mechanics for example by Callister and Rethwisch [12] We very briefly review some of the main concepts below to provide enough of a background for the rest of the chapter

13 Mechanical Properties 5

This edition focuses specifically on nanomechanical characterization Probing other material properties such as optical electrical or magnetic properties is not covered in this book The reason is that though these other properties have been successfully probed in various industrial RampD applications using AFM to probe mechanical properties of surfaces is more ubiquitous in industrial RampD and so was a logical focus for a first book on SPM in industrial applications

13 mechAnicAl properties

When one considers mechanical properties a range of descriptors come to mind including ldquostiffnessrdquo ldquotoughnessrdquo ldquohardnessrdquo ldquobrittlenessrdquo and ldquoductilityrdquo to name a few Each of these properties has their own mathematical expressions with associ-ated experimental methods of measuring them An interaction between two materials can be typically categorized into either elastic (or recoverable) or dissipative or plastic (permanent or nonrecoverable) interaction On the bulk scale most mechanical analysis of materials relies on measuring their strain (material displacement) as a function of stress (pressure exerted on the material) or vica versa The measurement and analysis of stress versus strain behavior of various materials is a rich and active field and there are many excellent textbooks that describe it [12] The description provided below is simplified to provide a context for these measurements ultimately described on the nanoscale

Elastic deformation of a material is typically nonpermanent or reversible and exists in a regime where the strain (material displacement) is proportional to the stress (pressure exerted on the material) by a proportionality factor known as a modulus (though in reality no material is truly only elastic) Elastic materials thus typically follow Hookersquos law Figure 12 shows a representative stress versus strain curve for materials highlighting the elastic region (blue) Additionally in elastic

Plastic

Fracture

Strain

Elastic

Str

ess

Figure 12 Basic stress vs strain curve showing three mechanical regimes of elastic deformation plastic deformation and fracture For color details please see color plate section

6 Chapter 1 Overview of Atomic Force Microscopy

materials the loading curve (as force is applied) is identical to the unloading curve (as the force is removed) Modulus is a key parameter used to describe elastic behavior Depending on the direction in which a material is being stressed the material will have different moduli (eg a tensile modulus a shear modulus etc)

Plastic deformation is on the other end permanent and nonrecoverable Plastic deformation does not follow Hookersquos law and so stress is not proportional to strain In Figure 12 the region of plastic deformation is shown in green In contrast to elastic deformation the loading curve is not identical to the unloading curve and in fact the unloading curve returns to a nonzero strain at a zero stress level There are many common properties associated with plastic deformation including yielding (onset of plastic deformation) ductility (plastic deformation at a fracture) and hardness (resistance to localized plastic deformation)

Finally there is another very important class of materials called viscoelastic materials where the elastic properties are frequency dependent For example when a stress such as pulling is exerted on the material the materialrsquos response is rate dependent Perhaps the most familiar viscoelastic material to most readers from childhood is Silly Putty where its response to pulling depends on how fast it is pulled If Silly Putty is pulled very slowly the material stretches out very nicely If Silly Putty is pulled very fast it will quickly break Polymers and rubbers two materials that play a significant role in consumer products and which are character-ized quite heavily with AFM are also viscoelastic materials

14 overvieW oF AFm operAtion

141 AFm hardware

A schematic of AFM hardware is shown in Figure 13 The main components of the AFM are (1) the cantilever (2) optical detection system (3) x-y-z scanner and (4) feedback loop

142 cantilevers and probes

The heart of the AFM lies in its cantileverprobe assembly that interacts or probes materials to provide the information of interest The technology to fabricate AFM can-tilevers takes advantage of technology developed for the semiconductor industry to make similar scale devices and features out of silicon Thus today AFM cantilevers are typically made by micromachining silicon (single-crystal Si) or silicon nitride (Si

3N

4) The dimensions of a cantilever vary and dictate the stiffness or spring

constant of the cantilever Depending on the material that needs to be probed and the mode of operation one might pick a lever with a stiffer (higher) or softer (lower) spring constant Cantilevers are generally either rectangular in geometry (Si or Si

3N

4)

or triangular (Si3N

4) with now hybrid cantilevers being manufactured (eg Si tips on

SiN cantilevers) Generally the Si3N

4 probes have lower force constants and might be

used for imaging in liquid among other applications whereas the Si probes can be brittle and tend to break or chip in contact when scanning a surface Typically the

14 Overview of AFM Operation 7

cantilevers are hundreds of microns in length tens of microns in width and a few microns in thickness for reference the diameter of a human hair is approximately 100 microm Cantilevers can be coated with gold or aluminum to provide high reflectivity Commercial cantilevers are also available with other thin metal coatings for magnetic or electrical imaging Cantilevers and probes can also be functionalized chemically or biologically for specific interactions

Probes are grown at the end of the lever and vary in shape and sharpness A sharper probe can often improve the lateral resolution of the AFM Probe design is a continuously evolving field as manufacturers strive for the most robust reproducible and sharp tips Cantileverprobe assemblies are mounted into a cantilever holder that typically includes a piezoceramic element (described below) that can vibrate the can-tilever at a given frequencymdasheither above at or below its resonancemdashdepending on the desired tipndashsample interaction or AFM mode

143 optical detection system

In order to track the motion of the cantileverprobe assembly as it scans a surface an AFM typically includes an optical detection system that consists of a laser reflected off the back side of the cantilever and directed toward a position sensitive detector

Figure 13 Schematic of basic operating principles of AFM A laser is focused and directed off the back end of a cantilever and directed toward a position-sensitive director (PSD) to monitor its vertical and lateral motion as it interacts with the surface Depending on the mode used to image the sample the cantilever can be vibrated with a shake piezo for dynamic imaging modes that involve the resonance of the cantilever

8 Chapter 1 Overview of Atomic Force Microscopy

(PSD) as seen in Figure 13 Optical detection systems are the most common way to track cantilever motion in commercial AFM systems However other methods exist to bypass optical detection systems (in eg a dark medium where the laser light would be absorbed) with self-actuated cantilevers where their motion is read by piezos integrated into the levers The laser is typically a visible photodiode though some commercial instruments have implemented superluminescent diode (SLD) laser The laser is detected by a four-segment position-sensitive detector (PSD) which can track the vertical and lateral motion of the cantilever accurately which can be converted to units of nanometers or displacement through careful calibration described below

144 x-y-z scanner

An AFM can operate either by scanning the sample relative to a stationary tip (sample scanning) or scanning the tip relative to a stationary sample (tip scanning) Each has its own advantages in terms of size of sample it can accommodate (tip scanning more flexible) ability to accessorize (tip scanning again more flexible) and stability and signal to noise (sample scanning easier to build with better specifications) Each shares the requirement to move the tip relative to the sample in a highly accurate way with minimal noise

Tipndashsample motion is often accomplished in commercial instruments by piezoelectric materials which are materials that respond either by expansion or con-traction in response to an applied voltage These kinds of materials provide the ability for very fine motion (nanometers to microns) Piezoelectric materials can be made in several shapes including tubes disks and bimorphs depending on the range and geom-etry of motion required Although piezoelectric materials are very effective at moving tip or sample they are plagued by nonlinear behavior such as hysteresis and creep that has serious consequences for accurate AFM imaging and interpretation of data Artifacts induced by piezoelectric materials are reviewed in detail in Chapter 6

Piezoelectric scanners can be programmed to move with either open-loop or closed-loop feedback circuits In open-loop scanners the piezo is moved by programs to ramp x and y bias voltages to move the prescribed amounts So in open-loop scan-ners if the piezo has to move a large distance or move around on the sample there can be considerable hysteresis of creep with the best recourse of patience or movingzooming in and out in small increments since hysteresiscreep is proportional to the displacement or motion the scanner is trying to execute

One solution to this problem is to use closed-loop scanners that use additional x-y-z sensors to independently measure and then correct for scanner movement In closed-loop operation feedback circuits are continually adjusted to the applied scanner biases to correct for the x-y motion and to achieve the desired motion It is important to note that correction of x-y motion in an AFM is slightly different than correction of z motion This is due to the fact that x-y motion can be predicted whereas z motion cannot as it depends on the surface topography of the sample being scanned Therefore typically there are active feedback circuits to control x and y

x Contents

103 Emulsions and Foams ProteinndashSurfactant Competition 238104 Interfacial Structure and Digestion Designer Interfaces 241105 Force Spectroscopy Model Emulsions 244106 Force Spectroscopy Origins of Bioactivity 247107 Conclusions 248References 249

11 Microcantilever Sensors for Petrochemical Applications 251

Alan M Schilowitz

111 Introduction 251112 Background 252113 Applications 257114 Conclusion 266References 267

12 Applications of Scanning Probe Methods in Cosmetic Science 270

Gustavo S Luengo and Anthony Galliano

121 Introduction 270122 Substrates of Cosmetics 271123 Mechanical Properties and Modifications by Cosmetic Products 274124 Scanning Probe Technologies Adapted to Cosmetic Science 275125 Conclusions 285References 285

13 Applications of Scanning Probe Microscopy and Nanomechanical Analysis in Pharmaceutical Development 287

Matthew S Lamm

131 Introduction 287132 Applications of SPM Imaging 288133 SPM as a Screening Tool 291134 Applications of Nanoindentation 293135 Conclusion 299Acknowledgments 299References 300

14 Comparative Nanomechanical Study of Multiharmonic Force Microscopy and Nanoindentation on Low Dielectric Constant Materials 302

Katharine Walz Robin King Willi Volksen Geraud Dubois Jane Frommer and Kumar Virwani

141 Introduction 302142 Experimental 308

Contents xi

143 Results and Discussions 311144 Conclusions 319Acknowledgments 320References 320

15 Nanomechanical Characterization of Biomaterial Surfaces Polymer Coatings That Elute Drugs 323

Klaus Wormuth and Greg Haugstad

151 Introduction 323152 Materials and Methods 325153 Dexamethasone in PBMA or PBMAndashPLMA Polymer Blends 327154 Simvastatin in PEOndashPBT Copolymers 337155 Concluding Comments 340Acknowledgments 341References 341

Index 342

xiii

Contributors List

Sudharsan BalasubramaniamDepartment of Mechanical EngineeringPurdue UniversityWest Lafayette IN

Steve J BullSchool of Chemical Engineering and Advanced MaterialsUniversity of NewcastleNewcastle upon Tyne United Kingdom

Robert W CarpickDepartment of Mechanical Engineering and Applied MechanicsUniversity of PennsylvaniaPhiladelphia PA

Geraud DuboisIBM Almaden Research CenterSan Jose CA

Johann FoucherLETIndashCEAGrenoble France

Jane FrommerIBM Almaden Research CenterSan Jose CA

Anthony GallianoLrsquoOreal Research and InnovationAulnay sous Bois France

A Patrick GunningInstitute of Food ResearchNorwich Research ParkNorwich United Kingdom

Greg HaugstadUniversity of MinnesotaMinneapolis MN

Donna C HurleyNational Institute of Standards and TechnologyBoulder CO

Tevis D B JacobsDepartment of Materials Science and EngineeringUniversity of PennsylvaniaPhiladelphia PA

Jason P KillgoreNational Institute of Standards and TechnologyBoulder CO

Robin KingIBM Almaden Research CenterSan Jose CA

Daniel KiracofeDepartment of Mechanical EngineeringPurdue UniversityWest Lafayette IN

Matthew S LammMerck Research LaboratoriesMerck amp CoSummit NJ

Gustavo S LuengoLrsquoOreal Research and InnovationAulnay sous Bois France

xiv Contributors List

C Mathew MateHGST A Western Digital CompanySan Jose CA

Victor J MorrisInstitute of Food ResearchNorwich Research ParkNorwich United Kingdom

Arvind RamanDepartment of Mechanical EngineeringPurdue UniversityWest Lafayette IN

Alan M SchilowitzExxonMobil Research and EngineeringAnnandale NJ

Andy H TsouExxonMobil Research and EngineeringAnnandale NJ

Kevin T TurnerDepartment of Mechanical Engineering and Applied MechanicsUniversity of PennsylvaniaPhiladelphia PA

Kumar VirwaniIBM Almaden Research CenterSan Jose CA

Willi VolksenIBM Almaden Research CenterSan Jose CA

Katharine WalzIBM Almaden Research CenterSan Jose CA

Klaus WormuthInstitute for Physical ChemistryUniversity of CologneCologneGermany

Dalia G YablonSurfaceChar LLCSharon MA

xv

Preface

The idea for this book came a couple of years ago after perusing atomic force microscopy (AFM) book collections at many professional society conferences over the years and finding that there was constantly a key topic missing how AFM was used practically in industrial RampD This topic is close to my heart as I have pursued a career developing novel methods and applying scanning probe microscopy (SPM) to a wide variety of industrial RampD problems at ExxonMobil Research and Engineeringrsquos Corporate Strategic Research in New Jersey for over a decade At ExxonMobil I have had the opportunity to explore application of AFM in the three vast sectors in the petroleum industry the downstream chemi-cals and upstream sector studying a wide variety of problems from corrosion to lubrication to geology to polymer materials All the AFM-focused books I found were research reviews on focused topical areas or textbooks on operating princi-ples and theory So though these books offered excellent technical treatments of a variety of topics the incredible utility applicability and flexibility of AFM to solve real-world everyday problems was lacking

My background in graduate school was actually in scanning tunneling micro-scopy (STM) as I studied self-assembled monolayers at the liquidndashsolid interface in George Flynnrsquos lab at Columbia Universityrsquos chemistry department not the most rel-evant ldquoreal-world industrialrdquo problem However being an STM lab in the 1990s we also had AFMs in the lab (since they shared the same ldquobrainrdquo the controller) so every once in a while we would help out a colleague and study their samples As I moved to ExxonMobil after graduate school in 2002 my intent was not necessarily to continue in scanning probe microscopy In fact it was serendipity that upon arriving at ExxonMobilrsquos Corporate Strategic Research labs as a postdoctoral fellow I found an unused Digital Instruments Nanoscope III controller with a fully accessorized Multimode The rest as they say is history

Over my time at ExxonMobil I ran into many like-minded colleagues in other industrial facilities using AFM and we would hold fascinating discussions of how the technology was used in onersquos particular field On the other side I could sense great interest from my academic colleagues that industrial researchers were using SPM to such a great extent as well as recent graduates trying to connect their hard-earned knowledge and expertise in the field as students to practical useful areas A book covering this material for the wider audience did indeed seem missing

And so I began to survey the industrial RampD communityndashndashboth members I had known for years and new onesndashndashto gauge interest in putting such a volume together I was inspired by the enthusiasm I received from my colleagues and decided to go forward Then came the question of what kind of AFM As surveyed in Chapter 1

xvi Preface

AFM is a tremendously broad field that characterizes a wide variety of material and surface properties However at its heart AFM is a high-resolution nanomechanical probe of the surface That probe can be coated with an electronically conducting layer or magnetic material or even replaced by an optical fiber tip in order to probe a variety of properties However a strong majority of the industrial RampD both in my own direct personal experience and those that I was familiar with was conducted with mechanical probing of properties Of course there are some key counterexamples that stand out such as electrostatic force microscopy to probe electrical properties of conducting and semiconducting materials and magnetic force microscopy to probe magnetic storage devices Perhaps those applications will be covered in a future volume But the overwhelming use of AFM in industrial RampD was nanomechanical characterization and thus it was quickly chosen as the focus for this book

It was important that this book be self-contained While the bookrsquos focus is on the applications it is intended to be accessible to a broad audience both from a professional and educational background so that everyone from an advanced undergraduate to a seasoned professional will benefit Various professional backgrounds should benefit as well from those in an academic or academic-like environment who wish to learn how AFM is used in an industrial environment to those in an industrial environment eager to learn what other applications are possible While a freshman chemistry and physics background is assumed no prior experience with AFM is assumed There is certainly advanced information in each chapter so that the content should appeal to a variety of backgrounds in the AFM community from novice to expert

With this aim of appealing to a broad audience I include a tutorial on nanomechani-cal methods in the first half of the book so that applications could be understood and appreciated with context without having to refer to other books or references I chose the topics that I felt were most relevant to studying nanomechanical characterization in the practical industrial environment contact mechanics force curves phase imaging dynamic contact methods and nanoindentation Note that this is not an exhaustive list of all the nanomechanical characterization methods available with AFM

I worked closely with my co-authors to make these overview chapters as practical as possible and not overwhelming in theoretical detail several chapters include worked examples of useful calculations (eg using Hertz mechanics with and without adhesion to model a contact in Chapter 2 or data analysis of contact resonance measurements in Chapter 5) These chapters are meant to be overviews to introduce the terminology and key concepts They are not meant to be an exhaustive step-by-step guide to the operation of various techniques those treatments are done elsewhere and are referenced in the book Chapter 6 is a somewhat unique chapter on ldquoBest Practices in AFM Imagingrdquo that I co-wrote with my colleague Greg Haugstad This chapter outlines some of the common pitfalls encountered by AFM users again with a practical approach to what the most common and most important problems are and how to avoid them

Chapters 7ndash15 then dive into how AFM has been used in a variety of industrial RampD areas to explore (a) phenomena and processes such as new formulation development in pharmaceuticals the effect of humidity and temperature on biomate-rials and nanostructure formation in food processing (b) characterization of various

Preface xvii

materials including polymer blends and composites in the chemicals sector skin and hair in personal care products coatings and thin dielectric films in the semiconductor industry (c) AFMrsquos function as a key quality control tool in semiconductor metrology and (d) a slightly different application of AFM microcantilevers as a physical and chemical sensors in the petroleum industry

My hope is that this book will show what an incredibly useful tool AFM has become to industrial RampD in its short 25 years It is currently an indispensable tool in any microscopy laboratory despite its commercial youth So while the AFM field con-tinues to grow and develop in terms of its capability and understanding spearheaded by many notable groups in academia and elsewhere it has already penetrated significantly into a wide variety of industrial labs This trend will only continue and grow As the penetration into industrial labs continues it is my hope that the reader understands not only how the AFM capability increasingly benefits industry but also that industrial labs are quickly becoming a partner in driving AFMrsquos innovation and research as its commercial and practical utility and importance continue to thrive

Dalia G YablonSharon MA

xix

Acknowledgments

I have been blessed to have wonderful colleagues to share ideas with and gain inspiration from during this process First and foremost I must thank my co-authors on this book Working with each and every author has been an absolute pleasure I had the distinct pleasure of working closely with Andy Tsou and Alan Schilowitz for many years at ExxonMobil Some of the other co-authors I have been fortunate to count as longstanding collaborators including Rob Carpick Greg Haugstad Jason Killgore and Donna Hurley I thank them for their support and enthusiastic partic-ipation for this particular project From all I learned so much technically and made many new friends in the process This book would not have been possible without everyonersquos collective hard work to share their insights and wisdom with us

I would like to thank my colleagues at ExxonMobil with whom I have had the pleasure of developing our AFM capabilities in our AFM lab Specifically Jean Grabowski deserves a special callout and has been a tremendous asset to our efforts as well as a genuine friend along the way I also would like to thank Rebecca Locker Ishita Chakraborty and Daniel Kiracofe I also would like to thank Steve Minne and Roger Proksch for help and guidance in the incipient stages of this project as well as Emily Rapalino and Susan Klein for advice and proofreading

Most of all I have to thank my husband Andrew for his absolute unwavering support and cheerleading throughout the past year and a half His confidence in me was a source of real motivation and inspiration to carry forward despite obstacles And logistically of course I have to thank him for providing childcare to our chil-dren in the evenings and weekends (covering for me on many many bath nights and storyreadingndashndashsuch as on this night) so that I could complete this book one which hopefully one day they might be interested in enough to read and understand

1

Scanning Probe Microscopy in Industrial Applications Nanomechanical Characterization First Edition Edited by Dalia G Yablon copy 2014 John Wiley amp Sons Inc Published 2014 by John Wiley amp Sons Inc

Overview of Atomic Force MicroscopyDalia G Yablon

SurfaceChar LLC Sharon MA

Atomic force microscopy (AFM) is a family of nanoscale characterization techniques that has exploded onto the overall characterization and nanotechnology field as its versatility and high resolution continue to feed a dizzying variety of disciplines from biology to physics and chemistry to engineering AFM entered the scientific arena in 1981 with the now famous invention of its older sibling in the scanning probe micro-scopy family the scanning tunneling microscope (STM) in the IBM Zurich labs of Gerd Binnig and Heinrich Rohrer for which they received the Nobel Prize in Physics in 1986 The AFM was then invented in 1986 by Gerd Binnig Cal Quate and Christoph Gerber [1] Together STM and AFM formed the scanning probe micros-copy (SPM) family which includes other methods such as near-field scanning optical microscopy (NSOM)

The STM spawned the next 25 years of the continuously developing field of scanning probe microscopy and specifically atomic force microscopy which now includes dozens of different methods under its name to probe various propertiesmdashincluding mechanical electrical magnetic chemical and opticalmdashof materials and surfaces Atomic force microscopy is a powerful tool in various research enterprises It is found in practically any university characterization facility alongside optical and electron microscopes and most undergraduates in science or engineering fields at this point are at least familiar with it if not have performed a laboratory experiment in their undergraduate curriculum

The focus of this book is to understand and appreciate the role this young technique has played in industrial research and development (RampD) AFM has penetrated into a variety of industrial research sectors as witnessed by the diverse set of applications described in this book Alongside electron and optical microscopy which have been

Chapter 1

2 Chapter 1 Overview of Atomic Force Microscopy

around for decades and have reached an impressive level of commercial maturity and ease of use AFM has become a vital characterization method despite its youth and continued technical evolution So though AFM is still an active area of academic research as its capabilities continue to develop and be better understood it has proven to be a useful microscopy to address industrial and commercial needs from quality control and assurance to product formulation and process monitoring

The goal of this introductory chapter is to provide an overview of AFM to nonspecialists and introduce the various topics that are the subject of subsequent individual chapters As such this chapter will provide a brief review of the begin-nings of AFM and special features that make it particularly suited for industrial research Then a brief overview of AFM operation will be presented including the hardware software calibrations involved and finally the different nanomechanical methods that will be described in detail both in theory and application Entire books are written on AFM operation and this chapter is not meant to be an exhaustive introduction to its operation merely serving to provide enough information for the rest of the book to be followed intelligently Readers interested in more detail about AFM operation can consult a number of excellent books on the topic [2ndash5]

11 A Word on nomenclAture

Before the rest of this chapter continues some definitions are in order Similar to many other surface science techniques AFM has succumbed to a somewhat unwieldy mess of abbreviations and jargon that has become a hard-to-navigate alphabet soup SPM refers to an umbrella of a variety of methods Methods that fall under SPM include per-haps its most famous member atomic force microscopy (AFM) in addition to others such as scanning tunneling microscopy (STM) near-field scanning optical microscopy (NSOM or SNOM depending on the continent) and other lithographic methods And then within AFM there are dozens of methods that rely on the AFM probendashsample interactions to provide a variety of material properties including electrical optical magnetic and mechanical properties To date there are dozens of SPMAFM-based methods It is beyond the scope of this book to list andor define all the related methods These methods characterize a huge variety of material properties Some of the more common methods have been included in Figure 11 with the category of properties that they measure The wide diversity of properties of materials that can be measured with AFM is clear The focus of this book is on nanomechanical characterization due to its broad appeal in industrial RampD Again Figure 11 is offered to demonstrate the variety and flavor of properties that can be probed with SPM

12 Atomic Force microscopymdashthe AppeAl to industriAl rampd

This book specifically focuses on AFM which is the method that has most penetrated the general characterization and industrial research fields AFM has key features that make it especially attractive to industrial research and development First its resolution

12 Atomic Force MicroscopymdashThe Appeal to Industrial RampD 3

the heart of its utility as a characterization tool is indeed impressive with 5- to 10-nm lateral resolution and angstrom vertical resolution achieved routinely on commercial instruments with commercial cantilevers The limits of lateral resolution continues to be pushed with specialized techniques and cantilevers and it has reached true ldquoatomicrdquo resolution where point defects of certain materials can now be imaged under certain operating parameters and imaging modes especially under liquid [6ndash8] True atomic resolution is still currently mostly achieved through STM which operates through an entirely different tipndashsample interaction mechanism based on quantum mechanical tunneling Many of these STM studies are conducted in ultrahigh vacuum (UHV) though some are performed in ambient and liquid conditions It is fair to say that AFM is catching up however with recent groundbreaking work imaging cyclic aromatic molecules on Cu[111] [10 11]

A second critical feature of AFM that makes it particularly amenable to industrial research is the flexibility of the environment in which it can operate Despite being a high-resolution microscopy that rivals the resolution of electron microscopy AFM can operate in an ambient or even liquid environment with minimal compromise to its resolution The ability to work in ldquoreal-worldrdquo environments makes it a critical tool for many industrial RampD applications where the research emphasis is consis-tently to understand mechanisms and materials in real-world situations as opposed to

Figure 11 Schematic of AFM as an umbrella technique that includes a wide variety of methods to probe various properties of materials Some of these methods are shown here

4 Chapter 1 Overview of Atomic Force Microscopy

idealized materials and conditions that typically exist in academic research endeavors In many cases the researcher may not want the sample to be forced into a pristine vacuum environment where perhaps key materials or components will be evacuated and thus missed in the characterization effort In addition the flexibility of an envi-ronment permits in situ characterization of processes For example processes such as corrosion lubrication catalyst dissolution crystal growth (for active pharmaceutical ingredient API) can all be studied in situ with AFM as a unique attribute of this microscopy The power of in situ measurements for biological applications is simple as many biological processes and materials simple cannot survive ex situ

A corollary of the utility of the AFMrsquos flexible environment to industrial RampD is the flexibility of type and shape of surface or material that AFM can image There are virtually no restrictions on the size of a material that can be probed with AFM Many commercial ldquolarge-samplerdquo AFMs currently exist that can image any design or shape of a surface from engine parts to thin films Indeed one of the AFMs built in the early 1990s (Topometrix Explorer) was advertised as being able to be placed on top of jet wings to image fractures In addition there are practically no mechanical or electrical restrictions on the type of sample an AFM can image Unlike electron microscopy or STM that require conducting or semiconducting surfaces to image or avoid artifacts [or in the case of insulating surface in scanning electron microscopy (SEM) that requires deposition of a thin conducting layer such as Cr] AFM can image insulating surfaces in addition to conducting or semiconducting surfaces with minimal sample prep Typically the most significant requirement on the sample for an AFM is that it be smooth Commercial instruments will have a maximum z (vertical) range that can be imaged dictated by the design and desired resolution of the particular instrument The maximum z range is typically from 1 microm (for high-resolution studies) to several microns (for lower resolution studies) meaning that the sample cannot have features a tilt or overall roughness that exceed that limit in order for the AFM to image effectively Smooth surfaces are often prepared with the use of an ultramicrotome An ultramicrotome is simply a very controlled and precise way of cutting of thin (down to 100 nm) sections at controlled speeds and in a controlled fashion to result in ultrasmooth surfaces of nanometer or less than nanometer rough-enss The ultramicrotome typically employs the use of a glass knife for a coarse cut and then a diamond knife for a fine cut Cryomicrotomy is simply ultramicrotomy at cold temperatures Cryomicrotomy is typically used to prepare polymer surfaces to cut below the glass transition temperature of the material depending on the instru-ment cryomicrotomes can cool down to -180degC

Finally AFM is fundamentally a mechanical probe of the surface through actual contact between the tip and sample So in addition to the ability to provide high resolution on surface features the goal of any form of microscopy is the unique tipndashsample interaction mechanism enabling the probing of many properties most obvious of which is mechanical properties Mechanical properties covers a wide range of materials and properties Entire textbooks and courses are devoted to the studies of mechanics for example by Callister and Rethwisch [12] We very briefly review some of the main concepts below to provide enough of a background for the rest of the chapter

13 Mechanical Properties 5

This edition focuses specifically on nanomechanical characterization Probing other material properties such as optical electrical or magnetic properties is not covered in this book The reason is that though these other properties have been successfully probed in various industrial RampD applications using AFM to probe mechanical properties of surfaces is more ubiquitous in industrial RampD and so was a logical focus for a first book on SPM in industrial applications

13 mechAnicAl properties

When one considers mechanical properties a range of descriptors come to mind including ldquostiffnessrdquo ldquotoughnessrdquo ldquohardnessrdquo ldquobrittlenessrdquo and ldquoductilityrdquo to name a few Each of these properties has their own mathematical expressions with associ-ated experimental methods of measuring them An interaction between two materials can be typically categorized into either elastic (or recoverable) or dissipative or plastic (permanent or nonrecoverable) interaction On the bulk scale most mechanical analysis of materials relies on measuring their strain (material displacement) as a function of stress (pressure exerted on the material) or vica versa The measurement and analysis of stress versus strain behavior of various materials is a rich and active field and there are many excellent textbooks that describe it [12] The description provided below is simplified to provide a context for these measurements ultimately described on the nanoscale

Elastic deformation of a material is typically nonpermanent or reversible and exists in a regime where the strain (material displacement) is proportional to the stress (pressure exerted on the material) by a proportionality factor known as a modulus (though in reality no material is truly only elastic) Elastic materials thus typically follow Hookersquos law Figure 12 shows a representative stress versus strain curve for materials highlighting the elastic region (blue) Additionally in elastic

Plastic

Fracture

Strain

Elastic

Str

ess

Figure 12 Basic stress vs strain curve showing three mechanical regimes of elastic deformation plastic deformation and fracture For color details please see color plate section

6 Chapter 1 Overview of Atomic Force Microscopy

materials the loading curve (as force is applied) is identical to the unloading curve (as the force is removed) Modulus is a key parameter used to describe elastic behavior Depending on the direction in which a material is being stressed the material will have different moduli (eg a tensile modulus a shear modulus etc)

Plastic deformation is on the other end permanent and nonrecoverable Plastic deformation does not follow Hookersquos law and so stress is not proportional to strain In Figure 12 the region of plastic deformation is shown in green In contrast to elastic deformation the loading curve is not identical to the unloading curve and in fact the unloading curve returns to a nonzero strain at a zero stress level There are many common properties associated with plastic deformation including yielding (onset of plastic deformation) ductility (plastic deformation at a fracture) and hardness (resistance to localized plastic deformation)

Finally there is another very important class of materials called viscoelastic materials where the elastic properties are frequency dependent For example when a stress such as pulling is exerted on the material the materialrsquos response is rate dependent Perhaps the most familiar viscoelastic material to most readers from childhood is Silly Putty where its response to pulling depends on how fast it is pulled If Silly Putty is pulled very slowly the material stretches out very nicely If Silly Putty is pulled very fast it will quickly break Polymers and rubbers two materials that play a significant role in consumer products and which are character-ized quite heavily with AFM are also viscoelastic materials

14 overvieW oF AFm operAtion

141 AFm hardware

A schematic of AFM hardware is shown in Figure 13 The main components of the AFM are (1) the cantilever (2) optical detection system (3) x-y-z scanner and (4) feedback loop

142 cantilevers and probes

The heart of the AFM lies in its cantileverprobe assembly that interacts or probes materials to provide the information of interest The technology to fabricate AFM can-tilevers takes advantage of technology developed for the semiconductor industry to make similar scale devices and features out of silicon Thus today AFM cantilevers are typically made by micromachining silicon (single-crystal Si) or silicon nitride (Si

3N

4) The dimensions of a cantilever vary and dictate the stiffness or spring

constant of the cantilever Depending on the material that needs to be probed and the mode of operation one might pick a lever with a stiffer (higher) or softer (lower) spring constant Cantilevers are generally either rectangular in geometry (Si or Si

3N

4)

or triangular (Si3N

4) with now hybrid cantilevers being manufactured (eg Si tips on

SiN cantilevers) Generally the Si3N

4 probes have lower force constants and might be

used for imaging in liquid among other applications whereas the Si probes can be brittle and tend to break or chip in contact when scanning a surface Typically the

14 Overview of AFM Operation 7

cantilevers are hundreds of microns in length tens of microns in width and a few microns in thickness for reference the diameter of a human hair is approximately 100 microm Cantilevers can be coated with gold or aluminum to provide high reflectivity Commercial cantilevers are also available with other thin metal coatings for magnetic or electrical imaging Cantilevers and probes can also be functionalized chemically or biologically for specific interactions

Probes are grown at the end of the lever and vary in shape and sharpness A sharper probe can often improve the lateral resolution of the AFM Probe design is a continuously evolving field as manufacturers strive for the most robust reproducible and sharp tips Cantileverprobe assemblies are mounted into a cantilever holder that typically includes a piezoceramic element (described below) that can vibrate the can-tilever at a given frequencymdasheither above at or below its resonancemdashdepending on the desired tipndashsample interaction or AFM mode

143 optical detection system

In order to track the motion of the cantileverprobe assembly as it scans a surface an AFM typically includes an optical detection system that consists of a laser reflected off the back side of the cantilever and directed toward a position sensitive detector

Figure 13 Schematic of basic operating principles of AFM A laser is focused and directed off the back end of a cantilever and directed toward a position-sensitive director (PSD) to monitor its vertical and lateral motion as it interacts with the surface Depending on the mode used to image the sample the cantilever can be vibrated with a shake piezo for dynamic imaging modes that involve the resonance of the cantilever

8 Chapter 1 Overview of Atomic Force Microscopy

(PSD) as seen in Figure 13 Optical detection systems are the most common way to track cantilever motion in commercial AFM systems However other methods exist to bypass optical detection systems (in eg a dark medium where the laser light would be absorbed) with self-actuated cantilevers where their motion is read by piezos integrated into the levers The laser is typically a visible photodiode though some commercial instruments have implemented superluminescent diode (SLD) laser The laser is detected by a four-segment position-sensitive detector (PSD) which can track the vertical and lateral motion of the cantilever accurately which can be converted to units of nanometers or displacement through careful calibration described below

144 x-y-z scanner

An AFM can operate either by scanning the sample relative to a stationary tip (sample scanning) or scanning the tip relative to a stationary sample (tip scanning) Each has its own advantages in terms of size of sample it can accommodate (tip scanning more flexible) ability to accessorize (tip scanning again more flexible) and stability and signal to noise (sample scanning easier to build with better specifications) Each shares the requirement to move the tip relative to the sample in a highly accurate way with minimal noise

Tipndashsample motion is often accomplished in commercial instruments by piezoelectric materials which are materials that respond either by expansion or con-traction in response to an applied voltage These kinds of materials provide the ability for very fine motion (nanometers to microns) Piezoelectric materials can be made in several shapes including tubes disks and bimorphs depending on the range and geom-etry of motion required Although piezoelectric materials are very effective at moving tip or sample they are plagued by nonlinear behavior such as hysteresis and creep that has serious consequences for accurate AFM imaging and interpretation of data Artifacts induced by piezoelectric materials are reviewed in detail in Chapter 6

Piezoelectric scanners can be programmed to move with either open-loop or closed-loop feedback circuits In open-loop scanners the piezo is moved by programs to ramp x and y bias voltages to move the prescribed amounts So in open-loop scan-ners if the piezo has to move a large distance or move around on the sample there can be considerable hysteresis of creep with the best recourse of patience or movingzooming in and out in small increments since hysteresiscreep is proportional to the displacement or motion the scanner is trying to execute

One solution to this problem is to use closed-loop scanners that use additional x-y-z sensors to independently measure and then correct for scanner movement In closed-loop operation feedback circuits are continually adjusted to the applied scanner biases to correct for the x-y motion and to achieve the desired motion It is important to note that correction of x-y motion in an AFM is slightly different than correction of z motion This is due to the fact that x-y motion can be predicted whereas z motion cannot as it depends on the surface topography of the sample being scanned Therefore typically there are active feedback circuits to control x and y

Contents xi

143 Results and Discussions 311144 Conclusions 319Acknowledgments 320References 320

15 Nanomechanical Characterization of Biomaterial Surfaces Polymer Coatings That Elute Drugs 323

Klaus Wormuth and Greg Haugstad

151 Introduction 323152 Materials and Methods 325153 Dexamethasone in PBMA or PBMAndashPLMA Polymer Blends 327154 Simvastatin in PEOndashPBT Copolymers 337155 Concluding Comments 340Acknowledgments 341References 341

Index 342

xiii

Contributors List

Sudharsan BalasubramaniamDepartment of Mechanical EngineeringPurdue UniversityWest Lafayette IN

Steve J BullSchool of Chemical Engineering and Advanced MaterialsUniversity of NewcastleNewcastle upon Tyne United Kingdom

Robert W CarpickDepartment of Mechanical Engineering and Applied MechanicsUniversity of PennsylvaniaPhiladelphia PA

Geraud DuboisIBM Almaden Research CenterSan Jose CA

Johann FoucherLETIndashCEAGrenoble France

Jane FrommerIBM Almaden Research CenterSan Jose CA

Anthony GallianoLrsquoOreal Research and InnovationAulnay sous Bois France

A Patrick GunningInstitute of Food ResearchNorwich Research ParkNorwich United Kingdom

Greg HaugstadUniversity of MinnesotaMinneapolis MN

Donna C HurleyNational Institute of Standards and TechnologyBoulder CO

Tevis D B JacobsDepartment of Materials Science and EngineeringUniversity of PennsylvaniaPhiladelphia PA

Jason P KillgoreNational Institute of Standards and TechnologyBoulder CO

Robin KingIBM Almaden Research CenterSan Jose CA

Daniel KiracofeDepartment of Mechanical EngineeringPurdue UniversityWest Lafayette IN

Matthew S LammMerck Research LaboratoriesMerck amp CoSummit NJ

Gustavo S LuengoLrsquoOreal Research and InnovationAulnay sous Bois France

xiv Contributors List

C Mathew MateHGST A Western Digital CompanySan Jose CA

Victor J MorrisInstitute of Food ResearchNorwich Research ParkNorwich United Kingdom

Arvind RamanDepartment of Mechanical EngineeringPurdue UniversityWest Lafayette IN

Alan M SchilowitzExxonMobil Research and EngineeringAnnandale NJ

Andy H TsouExxonMobil Research and EngineeringAnnandale NJ

Kevin T TurnerDepartment of Mechanical Engineering and Applied MechanicsUniversity of PennsylvaniaPhiladelphia PA

Kumar VirwaniIBM Almaden Research CenterSan Jose CA

Willi VolksenIBM Almaden Research CenterSan Jose CA

Katharine WalzIBM Almaden Research CenterSan Jose CA

Klaus WormuthInstitute for Physical ChemistryUniversity of CologneCologneGermany

Dalia G YablonSurfaceChar LLCSharon MA

xv

Preface

The idea for this book came a couple of years ago after perusing atomic force microscopy (AFM) book collections at many professional society conferences over the years and finding that there was constantly a key topic missing how AFM was used practically in industrial RampD This topic is close to my heart as I have pursued a career developing novel methods and applying scanning probe microscopy (SPM) to a wide variety of industrial RampD problems at ExxonMobil Research and Engineeringrsquos Corporate Strategic Research in New Jersey for over a decade At ExxonMobil I have had the opportunity to explore application of AFM in the three vast sectors in the petroleum industry the downstream chemi-cals and upstream sector studying a wide variety of problems from corrosion to lubrication to geology to polymer materials All the AFM-focused books I found were research reviews on focused topical areas or textbooks on operating princi-ples and theory So though these books offered excellent technical treatments of a variety of topics the incredible utility applicability and flexibility of AFM to solve real-world everyday problems was lacking

My background in graduate school was actually in scanning tunneling micro-scopy (STM) as I studied self-assembled monolayers at the liquidndashsolid interface in George Flynnrsquos lab at Columbia Universityrsquos chemistry department not the most rel-evant ldquoreal-world industrialrdquo problem However being an STM lab in the 1990s we also had AFMs in the lab (since they shared the same ldquobrainrdquo the controller) so every once in a while we would help out a colleague and study their samples As I moved to ExxonMobil after graduate school in 2002 my intent was not necessarily to continue in scanning probe microscopy In fact it was serendipity that upon arriving at ExxonMobilrsquos Corporate Strategic Research labs as a postdoctoral fellow I found an unused Digital Instruments Nanoscope III controller with a fully accessorized Multimode The rest as they say is history

Over my time at ExxonMobil I ran into many like-minded colleagues in other industrial facilities using AFM and we would hold fascinating discussions of how the technology was used in onersquos particular field On the other side I could sense great interest from my academic colleagues that industrial researchers were using SPM to such a great extent as well as recent graduates trying to connect their hard-earned knowledge and expertise in the field as students to practical useful areas A book covering this material for the wider audience did indeed seem missing

And so I began to survey the industrial RampD communityndashndashboth members I had known for years and new onesndashndashto gauge interest in putting such a volume together I was inspired by the enthusiasm I received from my colleagues and decided to go forward Then came the question of what kind of AFM As surveyed in Chapter 1

xvi Preface

AFM is a tremendously broad field that characterizes a wide variety of material and surface properties However at its heart AFM is a high-resolution nanomechanical probe of the surface That probe can be coated with an electronically conducting layer or magnetic material or even replaced by an optical fiber tip in order to probe a variety of properties However a strong majority of the industrial RampD both in my own direct personal experience and those that I was familiar with was conducted with mechanical probing of properties Of course there are some key counterexamples that stand out such as electrostatic force microscopy to probe electrical properties of conducting and semiconducting materials and magnetic force microscopy to probe magnetic storage devices Perhaps those applications will be covered in a future volume But the overwhelming use of AFM in industrial RampD was nanomechanical characterization and thus it was quickly chosen as the focus for this book

It was important that this book be self-contained While the bookrsquos focus is on the applications it is intended to be accessible to a broad audience both from a professional and educational background so that everyone from an advanced undergraduate to a seasoned professional will benefit Various professional backgrounds should benefit as well from those in an academic or academic-like environment who wish to learn how AFM is used in an industrial environment to those in an industrial environment eager to learn what other applications are possible While a freshman chemistry and physics background is assumed no prior experience with AFM is assumed There is certainly advanced information in each chapter so that the content should appeal to a variety of backgrounds in the AFM community from novice to expert

With this aim of appealing to a broad audience I include a tutorial on nanomechani-cal methods in the first half of the book so that applications could be understood and appreciated with context without having to refer to other books or references I chose the topics that I felt were most relevant to studying nanomechanical characterization in the practical industrial environment contact mechanics force curves phase imaging dynamic contact methods and nanoindentation Note that this is not an exhaustive list of all the nanomechanical characterization methods available with AFM

I worked closely with my co-authors to make these overview chapters as practical as possible and not overwhelming in theoretical detail several chapters include worked examples of useful calculations (eg using Hertz mechanics with and without adhesion to model a contact in Chapter 2 or data analysis of contact resonance measurements in Chapter 5) These chapters are meant to be overviews to introduce the terminology and key concepts They are not meant to be an exhaustive step-by-step guide to the operation of various techniques those treatments are done elsewhere and are referenced in the book Chapter 6 is a somewhat unique chapter on ldquoBest Practices in AFM Imagingrdquo that I co-wrote with my colleague Greg Haugstad This chapter outlines some of the common pitfalls encountered by AFM users again with a practical approach to what the most common and most important problems are and how to avoid them

Chapters 7ndash15 then dive into how AFM has been used in a variety of industrial RampD areas to explore (a) phenomena and processes such as new formulation development in pharmaceuticals the effect of humidity and temperature on biomate-rials and nanostructure formation in food processing (b) characterization of various

Preface xvii

materials including polymer blends and composites in the chemicals sector skin and hair in personal care products coatings and thin dielectric films in the semiconductor industry (c) AFMrsquos function as a key quality control tool in semiconductor metrology and (d) a slightly different application of AFM microcantilevers as a physical and chemical sensors in the petroleum industry

My hope is that this book will show what an incredibly useful tool AFM has become to industrial RampD in its short 25 years It is currently an indispensable tool in any microscopy laboratory despite its commercial youth So while the AFM field con-tinues to grow and develop in terms of its capability and understanding spearheaded by many notable groups in academia and elsewhere it has already penetrated significantly into a wide variety of industrial labs This trend will only continue and grow As the penetration into industrial labs continues it is my hope that the reader understands not only how the AFM capability increasingly benefits industry but also that industrial labs are quickly becoming a partner in driving AFMrsquos innovation and research as its commercial and practical utility and importance continue to thrive

Dalia G YablonSharon MA

xix

Acknowledgments

I have been blessed to have wonderful colleagues to share ideas with and gain inspiration from during this process First and foremost I must thank my co-authors on this book Working with each and every author has been an absolute pleasure I had the distinct pleasure of working closely with Andy Tsou and Alan Schilowitz for many years at ExxonMobil Some of the other co-authors I have been fortunate to count as longstanding collaborators including Rob Carpick Greg Haugstad Jason Killgore and Donna Hurley I thank them for their support and enthusiastic partic-ipation for this particular project From all I learned so much technically and made many new friends in the process This book would not have been possible without everyonersquos collective hard work to share their insights and wisdom with us

I would like to thank my colleagues at ExxonMobil with whom I have had the pleasure of developing our AFM capabilities in our AFM lab Specifically Jean Grabowski deserves a special callout and has been a tremendous asset to our efforts as well as a genuine friend along the way I also would like to thank Rebecca Locker Ishita Chakraborty and Daniel Kiracofe I also would like to thank Steve Minne and Roger Proksch for help and guidance in the incipient stages of this project as well as Emily Rapalino and Susan Klein for advice and proofreading

Most of all I have to thank my husband Andrew for his absolute unwavering support and cheerleading throughout the past year and a half His confidence in me was a source of real motivation and inspiration to carry forward despite obstacles And logistically of course I have to thank him for providing childcare to our chil-dren in the evenings and weekends (covering for me on many many bath nights and storyreadingndashndashsuch as on this night) so that I could complete this book one which hopefully one day they might be interested in enough to read and understand

1

Scanning Probe Microscopy in Industrial Applications Nanomechanical Characterization First Edition Edited by Dalia G Yablon copy 2014 John Wiley amp Sons Inc Published 2014 by John Wiley amp Sons Inc

Overview of Atomic Force MicroscopyDalia G Yablon

SurfaceChar LLC Sharon MA

Atomic force microscopy (AFM) is a family of nanoscale characterization techniques that has exploded onto the overall characterization and nanotechnology field as its versatility and high resolution continue to feed a dizzying variety of disciplines from biology to physics and chemistry to engineering AFM entered the scientific arena in 1981 with the now famous invention of its older sibling in the scanning probe micro-scopy family the scanning tunneling microscope (STM) in the IBM Zurich labs of Gerd Binnig and Heinrich Rohrer for which they received the Nobel Prize in Physics in 1986 The AFM was then invented in 1986 by Gerd Binnig Cal Quate and Christoph Gerber [1] Together STM and AFM formed the scanning probe micros-copy (SPM) family which includes other methods such as near-field scanning optical microscopy (NSOM)

The STM spawned the next 25 years of the continuously developing field of scanning probe microscopy and specifically atomic force microscopy which now includes dozens of different methods under its name to probe various propertiesmdashincluding mechanical electrical magnetic chemical and opticalmdashof materials and surfaces Atomic force microscopy is a powerful tool in various research enterprises It is found in practically any university characterization facility alongside optical and electron microscopes and most undergraduates in science or engineering fields at this point are at least familiar with it if not have performed a laboratory experiment in their undergraduate curriculum

The focus of this book is to understand and appreciate the role this young technique has played in industrial research and development (RampD) AFM has penetrated into a variety of industrial research sectors as witnessed by the diverse set of applications described in this book Alongside electron and optical microscopy which have been

Chapter 1

2 Chapter 1 Overview of Atomic Force Microscopy

around for decades and have reached an impressive level of commercial maturity and ease of use AFM has become a vital characterization method despite its youth and continued technical evolution So though AFM is still an active area of academic research as its capabilities continue to develop and be better understood it has proven to be a useful microscopy to address industrial and commercial needs from quality control and assurance to product formulation and process monitoring

The goal of this introductory chapter is to provide an overview of AFM to nonspecialists and introduce the various topics that are the subject of subsequent individual chapters As such this chapter will provide a brief review of the begin-nings of AFM and special features that make it particularly suited for industrial research Then a brief overview of AFM operation will be presented including the hardware software calibrations involved and finally the different nanomechanical methods that will be described in detail both in theory and application Entire books are written on AFM operation and this chapter is not meant to be an exhaustive introduction to its operation merely serving to provide enough information for the rest of the book to be followed intelligently Readers interested in more detail about AFM operation can consult a number of excellent books on the topic [2ndash5]

11 A Word on nomenclAture

Before the rest of this chapter continues some definitions are in order Similar to many other surface science techniques AFM has succumbed to a somewhat unwieldy mess of abbreviations and jargon that has become a hard-to-navigate alphabet soup SPM refers to an umbrella of a variety of methods Methods that fall under SPM include per-haps its most famous member atomic force microscopy (AFM) in addition to others such as scanning tunneling microscopy (STM) near-field scanning optical microscopy (NSOM or SNOM depending on the continent) and other lithographic methods And then within AFM there are dozens of methods that rely on the AFM probendashsample interactions to provide a variety of material properties including electrical optical magnetic and mechanical properties To date there are dozens of SPMAFM-based methods It is beyond the scope of this book to list andor define all the related methods These methods characterize a huge variety of material properties Some of the more common methods have been included in Figure 11 with the category of properties that they measure The wide diversity of properties of materials that can be measured with AFM is clear The focus of this book is on nanomechanical characterization due to its broad appeal in industrial RampD Again Figure 11 is offered to demonstrate the variety and flavor of properties that can be probed with SPM

12 Atomic Force microscopymdashthe AppeAl to industriAl rampd

This book specifically focuses on AFM which is the method that has most penetrated the general characterization and industrial research fields AFM has key features that make it especially attractive to industrial research and development First its resolution

12 Atomic Force MicroscopymdashThe Appeal to Industrial RampD 3

the heart of its utility as a characterization tool is indeed impressive with 5- to 10-nm lateral resolution and angstrom vertical resolution achieved routinely on commercial instruments with commercial cantilevers The limits of lateral resolution continues to be pushed with specialized techniques and cantilevers and it has reached true ldquoatomicrdquo resolution where point defects of certain materials can now be imaged under certain operating parameters and imaging modes especially under liquid [6ndash8] True atomic resolution is still currently mostly achieved through STM which operates through an entirely different tipndashsample interaction mechanism based on quantum mechanical tunneling Many of these STM studies are conducted in ultrahigh vacuum (UHV) though some are performed in ambient and liquid conditions It is fair to say that AFM is catching up however with recent groundbreaking work imaging cyclic aromatic molecules on Cu[111] [10 11]

A second critical feature of AFM that makes it particularly amenable to industrial research is the flexibility of the environment in which it can operate Despite being a high-resolution microscopy that rivals the resolution of electron microscopy AFM can operate in an ambient or even liquid environment with minimal compromise to its resolution The ability to work in ldquoreal-worldrdquo environments makes it a critical tool for many industrial RampD applications where the research emphasis is consis-tently to understand mechanisms and materials in real-world situations as opposed to

Figure 11 Schematic of AFM as an umbrella technique that includes a wide variety of methods to probe various properties of materials Some of these methods are shown here

4 Chapter 1 Overview of Atomic Force Microscopy

idealized materials and conditions that typically exist in academic research endeavors In many cases the researcher may not want the sample to be forced into a pristine vacuum environment where perhaps key materials or components will be evacuated and thus missed in the characterization effort In addition the flexibility of an envi-ronment permits in situ characterization of processes For example processes such as corrosion lubrication catalyst dissolution crystal growth (for active pharmaceutical ingredient API) can all be studied in situ with AFM as a unique attribute of this microscopy The power of in situ measurements for biological applications is simple as many biological processes and materials simple cannot survive ex situ

A corollary of the utility of the AFMrsquos flexible environment to industrial RampD is the flexibility of type and shape of surface or material that AFM can image There are virtually no restrictions on the size of a material that can be probed with AFM Many commercial ldquolarge-samplerdquo AFMs currently exist that can image any design or shape of a surface from engine parts to thin films Indeed one of the AFMs built in the early 1990s (Topometrix Explorer) was advertised as being able to be placed on top of jet wings to image fractures In addition there are practically no mechanical or electrical restrictions on the type of sample an AFM can image Unlike electron microscopy or STM that require conducting or semiconducting surfaces to image or avoid artifacts [or in the case of insulating surface in scanning electron microscopy (SEM) that requires deposition of a thin conducting layer such as Cr] AFM can image insulating surfaces in addition to conducting or semiconducting surfaces with minimal sample prep Typically the most significant requirement on the sample for an AFM is that it be smooth Commercial instruments will have a maximum z (vertical) range that can be imaged dictated by the design and desired resolution of the particular instrument The maximum z range is typically from 1 microm (for high-resolution studies) to several microns (for lower resolution studies) meaning that the sample cannot have features a tilt or overall roughness that exceed that limit in order for the AFM to image effectively Smooth surfaces are often prepared with the use of an ultramicrotome An ultramicrotome is simply a very controlled and precise way of cutting of thin (down to 100 nm) sections at controlled speeds and in a controlled fashion to result in ultrasmooth surfaces of nanometer or less than nanometer rough-enss The ultramicrotome typically employs the use of a glass knife for a coarse cut and then a diamond knife for a fine cut Cryomicrotomy is simply ultramicrotomy at cold temperatures Cryomicrotomy is typically used to prepare polymer surfaces to cut below the glass transition temperature of the material depending on the instru-ment cryomicrotomes can cool down to -180degC

Finally AFM is fundamentally a mechanical probe of the surface through actual contact between the tip and sample So in addition to the ability to provide high resolution on surface features the goal of any form of microscopy is the unique tipndashsample interaction mechanism enabling the probing of many properties most obvious of which is mechanical properties Mechanical properties covers a wide range of materials and properties Entire textbooks and courses are devoted to the studies of mechanics for example by Callister and Rethwisch [12] We very briefly review some of the main concepts below to provide enough of a background for the rest of the chapter

13 Mechanical Properties 5

This edition focuses specifically on nanomechanical characterization Probing other material properties such as optical electrical or magnetic properties is not covered in this book The reason is that though these other properties have been successfully probed in various industrial RampD applications using AFM to probe mechanical properties of surfaces is more ubiquitous in industrial RampD and so was a logical focus for a first book on SPM in industrial applications

13 mechAnicAl properties

When one considers mechanical properties a range of descriptors come to mind including ldquostiffnessrdquo ldquotoughnessrdquo ldquohardnessrdquo ldquobrittlenessrdquo and ldquoductilityrdquo to name a few Each of these properties has their own mathematical expressions with associ-ated experimental methods of measuring them An interaction between two materials can be typically categorized into either elastic (or recoverable) or dissipative or plastic (permanent or nonrecoverable) interaction On the bulk scale most mechanical analysis of materials relies on measuring their strain (material displacement) as a function of stress (pressure exerted on the material) or vica versa The measurement and analysis of stress versus strain behavior of various materials is a rich and active field and there are many excellent textbooks that describe it [12] The description provided below is simplified to provide a context for these measurements ultimately described on the nanoscale

Elastic deformation of a material is typically nonpermanent or reversible and exists in a regime where the strain (material displacement) is proportional to the stress (pressure exerted on the material) by a proportionality factor known as a modulus (though in reality no material is truly only elastic) Elastic materials thus typically follow Hookersquos law Figure 12 shows a representative stress versus strain curve for materials highlighting the elastic region (blue) Additionally in elastic

Plastic

Fracture

Strain

Elastic

Str

ess

Figure 12 Basic stress vs strain curve showing three mechanical regimes of elastic deformation plastic deformation and fracture For color details please see color plate section

6 Chapter 1 Overview of Atomic Force Microscopy

materials the loading curve (as force is applied) is identical to the unloading curve (as the force is removed) Modulus is a key parameter used to describe elastic behavior Depending on the direction in which a material is being stressed the material will have different moduli (eg a tensile modulus a shear modulus etc)

Plastic deformation is on the other end permanent and nonrecoverable Plastic deformation does not follow Hookersquos law and so stress is not proportional to strain In Figure 12 the region of plastic deformation is shown in green In contrast to elastic deformation the loading curve is not identical to the unloading curve and in fact the unloading curve returns to a nonzero strain at a zero stress level There are many common properties associated with plastic deformation including yielding (onset of plastic deformation) ductility (plastic deformation at a fracture) and hardness (resistance to localized plastic deformation)

Finally there is another very important class of materials called viscoelastic materials where the elastic properties are frequency dependent For example when a stress such as pulling is exerted on the material the materialrsquos response is rate dependent Perhaps the most familiar viscoelastic material to most readers from childhood is Silly Putty where its response to pulling depends on how fast it is pulled If Silly Putty is pulled very slowly the material stretches out very nicely If Silly Putty is pulled very fast it will quickly break Polymers and rubbers two materials that play a significant role in consumer products and which are character-ized quite heavily with AFM are also viscoelastic materials

14 overvieW oF AFm operAtion

141 AFm hardware

A schematic of AFM hardware is shown in Figure 13 The main components of the AFM are (1) the cantilever (2) optical detection system (3) x-y-z scanner and (4) feedback loop

142 cantilevers and probes

The heart of the AFM lies in its cantileverprobe assembly that interacts or probes materials to provide the information of interest The technology to fabricate AFM can-tilevers takes advantage of technology developed for the semiconductor industry to make similar scale devices and features out of silicon Thus today AFM cantilevers are typically made by micromachining silicon (single-crystal Si) or silicon nitride (Si

3N

4) The dimensions of a cantilever vary and dictate the stiffness or spring

constant of the cantilever Depending on the material that needs to be probed and the mode of operation one might pick a lever with a stiffer (higher) or softer (lower) spring constant Cantilevers are generally either rectangular in geometry (Si or Si

3N

4)

or triangular (Si3N

4) with now hybrid cantilevers being manufactured (eg Si tips on

SiN cantilevers) Generally the Si3N

4 probes have lower force constants and might be

used for imaging in liquid among other applications whereas the Si probes can be brittle and tend to break or chip in contact when scanning a surface Typically the

14 Overview of AFM Operation 7

cantilevers are hundreds of microns in length tens of microns in width and a few microns in thickness for reference the diameter of a human hair is approximately 100 microm Cantilevers can be coated with gold or aluminum to provide high reflectivity Commercial cantilevers are also available with other thin metal coatings for magnetic or electrical imaging Cantilevers and probes can also be functionalized chemically or biologically for specific interactions

Probes are grown at the end of the lever and vary in shape and sharpness A sharper probe can often improve the lateral resolution of the AFM Probe design is a continuously evolving field as manufacturers strive for the most robust reproducible and sharp tips Cantileverprobe assemblies are mounted into a cantilever holder that typically includes a piezoceramic element (described below) that can vibrate the can-tilever at a given frequencymdasheither above at or below its resonancemdashdepending on the desired tipndashsample interaction or AFM mode

143 optical detection system

In order to track the motion of the cantileverprobe assembly as it scans a surface an AFM typically includes an optical detection system that consists of a laser reflected off the back side of the cantilever and directed toward a position sensitive detector

Figure 13 Schematic of basic operating principles of AFM A laser is focused and directed off the back end of a cantilever and directed toward a position-sensitive director (PSD) to monitor its vertical and lateral motion as it interacts with the surface Depending on the mode used to image the sample the cantilever can be vibrated with a shake piezo for dynamic imaging modes that involve the resonance of the cantilever

8 Chapter 1 Overview of Atomic Force Microscopy

(PSD) as seen in Figure 13 Optical detection systems are the most common way to track cantilever motion in commercial AFM systems However other methods exist to bypass optical detection systems (in eg a dark medium where the laser light would be absorbed) with self-actuated cantilevers where their motion is read by piezos integrated into the levers The laser is typically a visible photodiode though some commercial instruments have implemented superluminescent diode (SLD) laser The laser is detected by a four-segment position-sensitive detector (PSD) which can track the vertical and lateral motion of the cantilever accurately which can be converted to units of nanometers or displacement through careful calibration described below

144 x-y-z scanner

An AFM can operate either by scanning the sample relative to a stationary tip (sample scanning) or scanning the tip relative to a stationary sample (tip scanning) Each has its own advantages in terms of size of sample it can accommodate (tip scanning more flexible) ability to accessorize (tip scanning again more flexible) and stability and signal to noise (sample scanning easier to build with better specifications) Each shares the requirement to move the tip relative to the sample in a highly accurate way with minimal noise

Tipndashsample motion is often accomplished in commercial instruments by piezoelectric materials which are materials that respond either by expansion or con-traction in response to an applied voltage These kinds of materials provide the ability for very fine motion (nanometers to microns) Piezoelectric materials can be made in several shapes including tubes disks and bimorphs depending on the range and geom-etry of motion required Although piezoelectric materials are very effective at moving tip or sample they are plagued by nonlinear behavior such as hysteresis and creep that has serious consequences for accurate AFM imaging and interpretation of data Artifacts induced by piezoelectric materials are reviewed in detail in Chapter 6

Piezoelectric scanners can be programmed to move with either open-loop or closed-loop feedback circuits In open-loop scanners the piezo is moved by programs to ramp x and y bias voltages to move the prescribed amounts So in open-loop scan-ners if the piezo has to move a large distance or move around on the sample there can be considerable hysteresis of creep with the best recourse of patience or movingzooming in and out in small increments since hysteresiscreep is proportional to the displacement or motion the scanner is trying to execute

One solution to this problem is to use closed-loop scanners that use additional x-y-z sensors to independently measure and then correct for scanner movement In closed-loop operation feedback circuits are continually adjusted to the applied scanner biases to correct for the x-y motion and to achieve the desired motion It is important to note that correction of x-y motion in an AFM is slightly different than correction of z motion This is due to the fact that x-y motion can be predicted whereas z motion cannot as it depends on the surface topography of the sample being scanned Therefore typically there are active feedback circuits to control x and y

xiii

Contributors List

Sudharsan BalasubramaniamDepartment of Mechanical EngineeringPurdue UniversityWest Lafayette IN

Steve J BullSchool of Chemical Engineering and Advanced MaterialsUniversity of NewcastleNewcastle upon Tyne United Kingdom

Robert W CarpickDepartment of Mechanical Engineering and Applied MechanicsUniversity of PennsylvaniaPhiladelphia PA

Geraud DuboisIBM Almaden Research CenterSan Jose CA

Johann FoucherLETIndashCEAGrenoble France

Jane FrommerIBM Almaden Research CenterSan Jose CA

Anthony GallianoLrsquoOreal Research and InnovationAulnay sous Bois France

A Patrick GunningInstitute of Food ResearchNorwich Research ParkNorwich United Kingdom

Greg HaugstadUniversity of MinnesotaMinneapolis MN

Donna C HurleyNational Institute of Standards and TechnologyBoulder CO

Tevis D B JacobsDepartment of Materials Science and EngineeringUniversity of PennsylvaniaPhiladelphia PA

Jason P KillgoreNational Institute of Standards and TechnologyBoulder CO

Robin KingIBM Almaden Research CenterSan Jose CA

Daniel KiracofeDepartment of Mechanical EngineeringPurdue UniversityWest Lafayette IN

Matthew S LammMerck Research LaboratoriesMerck amp CoSummit NJ

Gustavo S LuengoLrsquoOreal Research and InnovationAulnay sous Bois France

xiv Contributors List

C Mathew MateHGST A Western Digital CompanySan Jose CA

Victor J MorrisInstitute of Food ResearchNorwich Research ParkNorwich United Kingdom

Arvind RamanDepartment of Mechanical EngineeringPurdue UniversityWest Lafayette IN

Alan M SchilowitzExxonMobil Research and EngineeringAnnandale NJ

Andy H TsouExxonMobil Research and EngineeringAnnandale NJ

Kevin T TurnerDepartment of Mechanical Engineering and Applied MechanicsUniversity of PennsylvaniaPhiladelphia PA

Kumar VirwaniIBM Almaden Research CenterSan Jose CA

Willi VolksenIBM Almaden Research CenterSan Jose CA

Katharine WalzIBM Almaden Research CenterSan Jose CA

Klaus WormuthInstitute for Physical ChemistryUniversity of CologneCologneGermany

Dalia G YablonSurfaceChar LLCSharon MA

xv

Preface

The idea for this book came a couple of years ago after perusing atomic force microscopy (AFM) book collections at many professional society conferences over the years and finding that there was constantly a key topic missing how AFM was used practically in industrial RampD This topic is close to my heart as I have pursued a career developing novel methods and applying scanning probe microscopy (SPM) to a wide variety of industrial RampD problems at ExxonMobil Research and Engineeringrsquos Corporate Strategic Research in New Jersey for over a decade At ExxonMobil I have had the opportunity to explore application of AFM in the three vast sectors in the petroleum industry the downstream chemi-cals and upstream sector studying a wide variety of problems from corrosion to lubrication to geology to polymer materials All the AFM-focused books I found were research reviews on focused topical areas or textbooks on operating princi-ples and theory So though these books offered excellent technical treatments of a variety of topics the incredible utility applicability and flexibility of AFM to solve real-world everyday problems was lacking

My background in graduate school was actually in scanning tunneling micro-scopy (STM) as I studied self-assembled monolayers at the liquidndashsolid interface in George Flynnrsquos lab at Columbia Universityrsquos chemistry department not the most rel-evant ldquoreal-world industrialrdquo problem However being an STM lab in the 1990s we also had AFMs in the lab (since they shared the same ldquobrainrdquo the controller) so every once in a while we would help out a colleague and study their samples As I moved to ExxonMobil after graduate school in 2002 my intent was not necessarily to continue in scanning probe microscopy In fact it was serendipity that upon arriving at ExxonMobilrsquos Corporate Strategic Research labs as a postdoctoral fellow I found an unused Digital Instruments Nanoscope III controller with a fully accessorized Multimode The rest as they say is history

Over my time at ExxonMobil I ran into many like-minded colleagues in other industrial facilities using AFM and we would hold fascinating discussions of how the technology was used in onersquos particular field On the other side I could sense great interest from my academic colleagues that industrial researchers were using SPM to such a great extent as well as recent graduates trying to connect their hard-earned knowledge and expertise in the field as students to practical useful areas A book covering this material for the wider audience did indeed seem missing

And so I began to survey the industrial RampD communityndashndashboth members I had known for years and new onesndashndashto gauge interest in putting such a volume together I was inspired by the enthusiasm I received from my colleagues and decided to go forward Then came the question of what kind of AFM As surveyed in Chapter 1

xvi Preface

AFM is a tremendously broad field that characterizes a wide variety of material and surface properties However at its heart AFM is a high-resolution nanomechanical probe of the surface That probe can be coated with an electronically conducting layer or magnetic material or even replaced by an optical fiber tip in order to probe a variety of properties However a strong majority of the industrial RampD both in my own direct personal experience and those that I was familiar with was conducted with mechanical probing of properties Of course there are some key counterexamples that stand out such as electrostatic force microscopy to probe electrical properties of conducting and semiconducting materials and magnetic force microscopy to probe magnetic storage devices Perhaps those applications will be covered in a future volume But the overwhelming use of AFM in industrial RampD was nanomechanical characterization and thus it was quickly chosen as the focus for this book

It was important that this book be self-contained While the bookrsquos focus is on the applications it is intended to be accessible to a broad audience both from a professional and educational background so that everyone from an advanced undergraduate to a seasoned professional will benefit Various professional backgrounds should benefit as well from those in an academic or academic-like environment who wish to learn how AFM is used in an industrial environment to those in an industrial environment eager to learn what other applications are possible While a freshman chemistry and physics background is assumed no prior experience with AFM is assumed There is certainly advanced information in each chapter so that the content should appeal to a variety of backgrounds in the AFM community from novice to expert

With this aim of appealing to a broad audience I include a tutorial on nanomechani-cal methods in the first half of the book so that applications could be understood and appreciated with context without having to refer to other books or references I chose the topics that I felt were most relevant to studying nanomechanical characterization in the practical industrial environment contact mechanics force curves phase imaging dynamic contact methods and nanoindentation Note that this is not an exhaustive list of all the nanomechanical characterization methods available with AFM

I worked closely with my co-authors to make these overview chapters as practical as possible and not overwhelming in theoretical detail several chapters include worked examples of useful calculations (eg using Hertz mechanics with and without adhesion to model a contact in Chapter 2 or data analysis of contact resonance measurements in Chapter 5) These chapters are meant to be overviews to introduce the terminology and key concepts They are not meant to be an exhaustive step-by-step guide to the operation of various techniques those treatments are done elsewhere and are referenced in the book Chapter 6 is a somewhat unique chapter on ldquoBest Practices in AFM Imagingrdquo that I co-wrote with my colleague Greg Haugstad This chapter outlines some of the common pitfalls encountered by AFM users again with a practical approach to what the most common and most important problems are and how to avoid them

Chapters 7ndash15 then dive into how AFM has been used in a variety of industrial RampD areas to explore (a) phenomena and processes such as new formulation development in pharmaceuticals the effect of humidity and temperature on biomate-rials and nanostructure formation in food processing (b) characterization of various

Preface xvii

materials including polymer blends and composites in the chemicals sector skin and hair in personal care products coatings and thin dielectric films in the semiconductor industry (c) AFMrsquos function as a key quality control tool in semiconductor metrology and (d) a slightly different application of AFM microcantilevers as a physical and chemical sensors in the petroleum industry

My hope is that this book will show what an incredibly useful tool AFM has become to industrial RampD in its short 25 years It is currently an indispensable tool in any microscopy laboratory despite its commercial youth So while the AFM field con-tinues to grow and develop in terms of its capability and understanding spearheaded by many notable groups in academia and elsewhere it has already penetrated significantly into a wide variety of industrial labs This trend will only continue and grow As the penetration into industrial labs continues it is my hope that the reader understands not only how the AFM capability increasingly benefits industry but also that industrial labs are quickly becoming a partner in driving AFMrsquos innovation and research as its commercial and practical utility and importance continue to thrive

Dalia G YablonSharon MA

xix

Acknowledgments

I have been blessed to have wonderful colleagues to share ideas with and gain inspiration from during this process First and foremost I must thank my co-authors on this book Working with each and every author has been an absolute pleasure I had the distinct pleasure of working closely with Andy Tsou and Alan Schilowitz for many years at ExxonMobil Some of the other co-authors I have been fortunate to count as longstanding collaborators including Rob Carpick Greg Haugstad Jason Killgore and Donna Hurley I thank them for their support and enthusiastic partic-ipation for this particular project From all I learned so much technically and made many new friends in the process This book would not have been possible without everyonersquos collective hard work to share their insights and wisdom with us

I would like to thank my colleagues at ExxonMobil with whom I have had the pleasure of developing our AFM capabilities in our AFM lab Specifically Jean Grabowski deserves a special callout and has been a tremendous asset to our efforts as well as a genuine friend along the way I also would like to thank Rebecca Locker Ishita Chakraborty and Daniel Kiracofe I also would like to thank Steve Minne and Roger Proksch for help and guidance in the incipient stages of this project as well as Emily Rapalino and Susan Klein for advice and proofreading

Most of all I have to thank my husband Andrew for his absolute unwavering support and cheerleading throughout the past year and a half His confidence in me was a source of real motivation and inspiration to carry forward despite obstacles And logistically of course I have to thank him for providing childcare to our chil-dren in the evenings and weekends (covering for me on many many bath nights and storyreadingndashndashsuch as on this night) so that I could complete this book one which hopefully one day they might be interested in enough to read and understand

1

Scanning Probe Microscopy in Industrial Applications Nanomechanical Characterization First Edition Edited by Dalia G Yablon copy 2014 John Wiley amp Sons Inc Published 2014 by John Wiley amp Sons Inc

Overview of Atomic Force MicroscopyDalia G Yablon

SurfaceChar LLC Sharon MA

Atomic force microscopy (AFM) is a family of nanoscale characterization techniques that has exploded onto the overall characterization and nanotechnology field as its versatility and high resolution continue to feed a dizzying variety of disciplines from biology to physics and chemistry to engineering AFM entered the scientific arena in 1981 with the now famous invention of its older sibling in the scanning probe micro-scopy family the scanning tunneling microscope (STM) in the IBM Zurich labs of Gerd Binnig and Heinrich Rohrer for which they received the Nobel Prize in Physics in 1986 The AFM was then invented in 1986 by Gerd Binnig Cal Quate and Christoph Gerber [1] Together STM and AFM formed the scanning probe micros-copy (SPM) family which includes other methods such as near-field scanning optical microscopy (NSOM)

The STM spawned the next 25 years of the continuously developing field of scanning probe microscopy and specifically atomic force microscopy which now includes dozens of different methods under its name to probe various propertiesmdashincluding mechanical electrical magnetic chemical and opticalmdashof materials and surfaces Atomic force microscopy is a powerful tool in various research enterprises It is found in practically any university characterization facility alongside optical and electron microscopes and most undergraduates in science or engineering fields at this point are at least familiar with it if not have performed a laboratory experiment in their undergraduate curriculum

The focus of this book is to understand and appreciate the role this young technique has played in industrial research and development (RampD) AFM has penetrated into a variety of industrial research sectors as witnessed by the diverse set of applications described in this book Alongside electron and optical microscopy which have been

Chapter 1

2 Chapter 1 Overview of Atomic Force Microscopy

around for decades and have reached an impressive level of commercial maturity and ease of use AFM has become a vital characterization method despite its youth and continued technical evolution So though AFM is still an active area of academic research as its capabilities continue to develop and be better understood it has proven to be a useful microscopy to address industrial and commercial needs from quality control and assurance to product formulation and process monitoring

The goal of this introductory chapter is to provide an overview of AFM to nonspecialists and introduce the various topics that are the subject of subsequent individual chapters As such this chapter will provide a brief review of the begin-nings of AFM and special features that make it particularly suited for industrial research Then a brief overview of AFM operation will be presented including the hardware software calibrations involved and finally the different nanomechanical methods that will be described in detail both in theory and application Entire books are written on AFM operation and this chapter is not meant to be an exhaustive introduction to its operation merely serving to provide enough information for the rest of the book to be followed intelligently Readers interested in more detail about AFM operation can consult a number of excellent books on the topic [2ndash5]

11 A Word on nomenclAture

Before the rest of this chapter continues some definitions are in order Similar to many other surface science techniques AFM has succumbed to a somewhat unwieldy mess of abbreviations and jargon that has become a hard-to-navigate alphabet soup SPM refers to an umbrella of a variety of methods Methods that fall under SPM include per-haps its most famous member atomic force microscopy (AFM) in addition to others such as scanning tunneling microscopy (STM) near-field scanning optical microscopy (NSOM or SNOM depending on the continent) and other lithographic methods And then within AFM there are dozens of methods that rely on the AFM probendashsample interactions to provide a variety of material properties including electrical optical magnetic and mechanical properties To date there are dozens of SPMAFM-based methods It is beyond the scope of this book to list andor define all the related methods These methods characterize a huge variety of material properties Some of the more common methods have been included in Figure 11 with the category of properties that they measure The wide diversity of properties of materials that can be measured with AFM is clear The focus of this book is on nanomechanical characterization due to its broad appeal in industrial RampD Again Figure 11 is offered to demonstrate the variety and flavor of properties that can be probed with SPM

12 Atomic Force microscopymdashthe AppeAl to industriAl rampd

This book specifically focuses on AFM which is the method that has most penetrated the general characterization and industrial research fields AFM has key features that make it especially attractive to industrial research and development First its resolution

12 Atomic Force MicroscopymdashThe Appeal to Industrial RampD 3

the heart of its utility as a characterization tool is indeed impressive with 5- to 10-nm lateral resolution and angstrom vertical resolution achieved routinely on commercial instruments with commercial cantilevers The limits of lateral resolution continues to be pushed with specialized techniques and cantilevers and it has reached true ldquoatomicrdquo resolution where point defects of certain materials can now be imaged under certain operating parameters and imaging modes especially under liquid [6ndash8] True atomic resolution is still currently mostly achieved through STM which operates through an entirely different tipndashsample interaction mechanism based on quantum mechanical tunneling Many of these STM studies are conducted in ultrahigh vacuum (UHV) though some are performed in ambient and liquid conditions It is fair to say that AFM is catching up however with recent groundbreaking work imaging cyclic aromatic molecules on Cu[111] [10 11]

A second critical feature of AFM that makes it particularly amenable to industrial research is the flexibility of the environment in which it can operate Despite being a high-resolution microscopy that rivals the resolution of electron microscopy AFM can operate in an ambient or even liquid environment with minimal compromise to its resolution The ability to work in ldquoreal-worldrdquo environments makes it a critical tool for many industrial RampD applications where the research emphasis is consis-tently to understand mechanisms and materials in real-world situations as opposed to

Figure 11 Schematic of AFM as an umbrella technique that includes a wide variety of methods to probe various properties of materials Some of these methods are shown here

4 Chapter 1 Overview of Atomic Force Microscopy

idealized materials and conditions that typically exist in academic research endeavors In many cases the researcher may not want the sample to be forced into a pristine vacuum environment where perhaps key materials or components will be evacuated and thus missed in the characterization effort In addition the flexibility of an envi-ronment permits in situ characterization of processes For example processes such as corrosion lubrication catalyst dissolution crystal growth (for active pharmaceutical ingredient API) can all be studied in situ with AFM as a unique attribute of this microscopy The power of in situ measurements for biological applications is simple as many biological processes and materials simple cannot survive ex situ

A corollary of the utility of the AFMrsquos flexible environment to industrial RampD is the flexibility of type and shape of surface or material that AFM can image There are virtually no restrictions on the size of a material that can be probed with AFM Many commercial ldquolarge-samplerdquo AFMs currently exist that can image any design or shape of a surface from engine parts to thin films Indeed one of the AFMs built in the early 1990s (Topometrix Explorer) was advertised as being able to be placed on top of jet wings to image fractures In addition there are practically no mechanical or electrical restrictions on the type of sample an AFM can image Unlike electron microscopy or STM that require conducting or semiconducting surfaces to image or avoid artifacts [or in the case of insulating surface in scanning electron microscopy (SEM) that requires deposition of a thin conducting layer such as Cr] AFM can image insulating surfaces in addition to conducting or semiconducting surfaces with minimal sample prep Typically the most significant requirement on the sample for an AFM is that it be smooth Commercial instruments will have a maximum z (vertical) range that can be imaged dictated by the design and desired resolution of the particular instrument The maximum z range is typically from 1 microm (for high-resolution studies) to several microns (for lower resolution studies) meaning that the sample cannot have features a tilt or overall roughness that exceed that limit in order for the AFM to image effectively Smooth surfaces are often prepared with the use of an ultramicrotome An ultramicrotome is simply a very controlled and precise way of cutting of thin (down to 100 nm) sections at controlled speeds and in a controlled fashion to result in ultrasmooth surfaces of nanometer or less than nanometer rough-enss The ultramicrotome typically employs the use of a glass knife for a coarse cut and then a diamond knife for a fine cut Cryomicrotomy is simply ultramicrotomy at cold temperatures Cryomicrotomy is typically used to prepare polymer surfaces to cut below the glass transition temperature of the material depending on the instru-ment cryomicrotomes can cool down to -180degC

Finally AFM is fundamentally a mechanical probe of the surface through actual contact between the tip and sample So in addition to the ability to provide high resolution on surface features the goal of any form of microscopy is the unique tipndashsample interaction mechanism enabling the probing of many properties most obvious of which is mechanical properties Mechanical properties covers a wide range of materials and properties Entire textbooks and courses are devoted to the studies of mechanics for example by Callister and Rethwisch [12] We very briefly review some of the main concepts below to provide enough of a background for the rest of the chapter

13 Mechanical Properties 5

This edition focuses specifically on nanomechanical characterization Probing other material properties such as optical electrical or magnetic properties is not covered in this book The reason is that though these other properties have been successfully probed in various industrial RampD applications using AFM to probe mechanical properties of surfaces is more ubiquitous in industrial RampD and so was a logical focus for a first book on SPM in industrial applications

13 mechAnicAl properties

When one considers mechanical properties a range of descriptors come to mind including ldquostiffnessrdquo ldquotoughnessrdquo ldquohardnessrdquo ldquobrittlenessrdquo and ldquoductilityrdquo to name a few Each of these properties has their own mathematical expressions with associ-ated experimental methods of measuring them An interaction between two materials can be typically categorized into either elastic (or recoverable) or dissipative or plastic (permanent or nonrecoverable) interaction On the bulk scale most mechanical analysis of materials relies on measuring their strain (material displacement) as a function of stress (pressure exerted on the material) or vica versa The measurement and analysis of stress versus strain behavior of various materials is a rich and active field and there are many excellent textbooks that describe it [12] The description provided below is simplified to provide a context for these measurements ultimately described on the nanoscale

Elastic deformation of a material is typically nonpermanent or reversible and exists in a regime where the strain (material displacement) is proportional to the stress (pressure exerted on the material) by a proportionality factor known as a modulus (though in reality no material is truly only elastic) Elastic materials thus typically follow Hookersquos law Figure 12 shows a representative stress versus strain curve for materials highlighting the elastic region (blue) Additionally in elastic

Plastic

Fracture

Strain

Elastic

Str

ess

Figure 12 Basic stress vs strain curve showing three mechanical regimes of elastic deformation plastic deformation and fracture For color details please see color plate section

6 Chapter 1 Overview of Atomic Force Microscopy

materials the loading curve (as force is applied) is identical to the unloading curve (as the force is removed) Modulus is a key parameter used to describe elastic behavior Depending on the direction in which a material is being stressed the material will have different moduli (eg a tensile modulus a shear modulus etc)

Plastic deformation is on the other end permanent and nonrecoverable Plastic deformation does not follow Hookersquos law and so stress is not proportional to strain In Figure 12 the region of plastic deformation is shown in green In contrast to elastic deformation the loading curve is not identical to the unloading curve and in fact the unloading curve returns to a nonzero strain at a zero stress level There are many common properties associated with plastic deformation including yielding (onset of plastic deformation) ductility (plastic deformation at a fracture) and hardness (resistance to localized plastic deformation)

Finally there is another very important class of materials called viscoelastic materials where the elastic properties are frequency dependent For example when a stress such as pulling is exerted on the material the materialrsquos response is rate dependent Perhaps the most familiar viscoelastic material to most readers from childhood is Silly Putty where its response to pulling depends on how fast it is pulled If Silly Putty is pulled very slowly the material stretches out very nicely If Silly Putty is pulled very fast it will quickly break Polymers and rubbers two materials that play a significant role in consumer products and which are character-ized quite heavily with AFM are also viscoelastic materials

14 overvieW oF AFm operAtion

141 AFm hardware

A schematic of AFM hardware is shown in Figure 13 The main components of the AFM are (1) the cantilever (2) optical detection system (3) x-y-z scanner and (4) feedback loop

142 cantilevers and probes

The heart of the AFM lies in its cantileverprobe assembly that interacts or probes materials to provide the information of interest The technology to fabricate AFM can-tilevers takes advantage of technology developed for the semiconductor industry to make similar scale devices and features out of silicon Thus today AFM cantilevers are typically made by micromachining silicon (single-crystal Si) or silicon nitride (Si

3N

4) The dimensions of a cantilever vary and dictate the stiffness or spring

constant of the cantilever Depending on the material that needs to be probed and the mode of operation one might pick a lever with a stiffer (higher) or softer (lower) spring constant Cantilevers are generally either rectangular in geometry (Si or Si

3N

4)

or triangular (Si3N

4) with now hybrid cantilevers being manufactured (eg Si tips on

SiN cantilevers) Generally the Si3N

4 probes have lower force constants and might be

used for imaging in liquid among other applications whereas the Si probes can be brittle and tend to break or chip in contact when scanning a surface Typically the

14 Overview of AFM Operation 7

cantilevers are hundreds of microns in length tens of microns in width and a few microns in thickness for reference the diameter of a human hair is approximately 100 microm Cantilevers can be coated with gold or aluminum to provide high reflectivity Commercial cantilevers are also available with other thin metal coatings for magnetic or electrical imaging Cantilevers and probes can also be functionalized chemically or biologically for specific interactions

Probes are grown at the end of the lever and vary in shape and sharpness A sharper probe can often improve the lateral resolution of the AFM Probe design is a continuously evolving field as manufacturers strive for the most robust reproducible and sharp tips Cantileverprobe assemblies are mounted into a cantilever holder that typically includes a piezoceramic element (described below) that can vibrate the can-tilever at a given frequencymdasheither above at or below its resonancemdashdepending on the desired tipndashsample interaction or AFM mode

143 optical detection system

In order to track the motion of the cantileverprobe assembly as it scans a surface an AFM typically includes an optical detection system that consists of a laser reflected off the back side of the cantilever and directed toward a position sensitive detector

Figure 13 Schematic of basic operating principles of AFM A laser is focused and directed off the back end of a cantilever and directed toward a position-sensitive director (PSD) to monitor its vertical and lateral motion as it interacts with the surface Depending on the mode used to image the sample the cantilever can be vibrated with a shake piezo for dynamic imaging modes that involve the resonance of the cantilever

8 Chapter 1 Overview of Atomic Force Microscopy

(PSD) as seen in Figure 13 Optical detection systems are the most common way to track cantilever motion in commercial AFM systems However other methods exist to bypass optical detection systems (in eg a dark medium where the laser light would be absorbed) with self-actuated cantilevers where their motion is read by piezos integrated into the levers The laser is typically a visible photodiode though some commercial instruments have implemented superluminescent diode (SLD) laser The laser is detected by a four-segment position-sensitive detector (PSD) which can track the vertical and lateral motion of the cantilever accurately which can be converted to units of nanometers or displacement through careful calibration described below

144 x-y-z scanner

An AFM can operate either by scanning the sample relative to a stationary tip (sample scanning) or scanning the tip relative to a stationary sample (tip scanning) Each has its own advantages in terms of size of sample it can accommodate (tip scanning more flexible) ability to accessorize (tip scanning again more flexible) and stability and signal to noise (sample scanning easier to build with better specifications) Each shares the requirement to move the tip relative to the sample in a highly accurate way with minimal noise

Tipndashsample motion is often accomplished in commercial instruments by piezoelectric materials which are materials that respond either by expansion or con-traction in response to an applied voltage These kinds of materials provide the ability for very fine motion (nanometers to microns) Piezoelectric materials can be made in several shapes including tubes disks and bimorphs depending on the range and geom-etry of motion required Although piezoelectric materials are very effective at moving tip or sample they are plagued by nonlinear behavior such as hysteresis and creep that has serious consequences for accurate AFM imaging and interpretation of data Artifacts induced by piezoelectric materials are reviewed in detail in Chapter 6

Piezoelectric scanners can be programmed to move with either open-loop or closed-loop feedback circuits In open-loop scanners the piezo is moved by programs to ramp x and y bias voltages to move the prescribed amounts So in open-loop scan-ners if the piezo has to move a large distance or move around on the sample there can be considerable hysteresis of creep with the best recourse of patience or movingzooming in and out in small increments since hysteresiscreep is proportional to the displacement or motion the scanner is trying to execute

One solution to this problem is to use closed-loop scanners that use additional x-y-z sensors to independently measure and then correct for scanner movement In closed-loop operation feedback circuits are continually adjusted to the applied scanner biases to correct for the x-y motion and to achieve the desired motion It is important to note that correction of x-y motion in an AFM is slightly different than correction of z motion This is due to the fact that x-y motion can be predicted whereas z motion cannot as it depends on the surface topography of the sample being scanned Therefore typically there are active feedback circuits to control x and y

xiv Contributors List

C Mathew MateHGST A Western Digital CompanySan Jose CA

Victor J MorrisInstitute of Food ResearchNorwich Research ParkNorwich United Kingdom

Arvind RamanDepartment of Mechanical EngineeringPurdue UniversityWest Lafayette IN

Alan M SchilowitzExxonMobil Research and EngineeringAnnandale NJ

Andy H TsouExxonMobil Research and EngineeringAnnandale NJ

Kevin T TurnerDepartment of Mechanical Engineering and Applied MechanicsUniversity of PennsylvaniaPhiladelphia PA

Kumar VirwaniIBM Almaden Research CenterSan Jose CA

Willi VolksenIBM Almaden Research CenterSan Jose CA

Katharine WalzIBM Almaden Research CenterSan Jose CA

Klaus WormuthInstitute for Physical ChemistryUniversity of CologneCologneGermany

Dalia G YablonSurfaceChar LLCSharon MA

xv

Preface

The idea for this book came a couple of years ago after perusing atomic force microscopy (AFM) book collections at many professional society conferences over the years and finding that there was constantly a key topic missing how AFM was used practically in industrial RampD This topic is close to my heart as I have pursued a career developing novel methods and applying scanning probe microscopy (SPM) to a wide variety of industrial RampD problems at ExxonMobil Research and Engineeringrsquos Corporate Strategic Research in New Jersey for over a decade At ExxonMobil I have had the opportunity to explore application of AFM in the three vast sectors in the petroleum industry the downstream chemi-cals and upstream sector studying a wide variety of problems from corrosion to lubrication to geology to polymer materials All the AFM-focused books I found were research reviews on focused topical areas or textbooks on operating princi-ples and theory So though these books offered excellent technical treatments of a variety of topics the incredible utility applicability and flexibility of AFM to solve real-world everyday problems was lacking

My background in graduate school was actually in scanning tunneling micro-scopy (STM) as I studied self-assembled monolayers at the liquidndashsolid interface in George Flynnrsquos lab at Columbia Universityrsquos chemistry department not the most rel-evant ldquoreal-world industrialrdquo problem However being an STM lab in the 1990s we also had AFMs in the lab (since they shared the same ldquobrainrdquo the controller) so every once in a while we would help out a colleague and study their samples As I moved to ExxonMobil after graduate school in 2002 my intent was not necessarily to continue in scanning probe microscopy In fact it was serendipity that upon arriving at ExxonMobilrsquos Corporate Strategic Research labs as a postdoctoral fellow I found an unused Digital Instruments Nanoscope III controller with a fully accessorized Multimode The rest as they say is history

Over my time at ExxonMobil I ran into many like-minded colleagues in other industrial facilities using AFM and we would hold fascinating discussions of how the technology was used in onersquos particular field On the other side I could sense great interest from my academic colleagues that industrial researchers were using SPM to such a great extent as well as recent graduates trying to connect their hard-earned knowledge and expertise in the field as students to practical useful areas A book covering this material for the wider audience did indeed seem missing

And so I began to survey the industrial RampD communityndashndashboth members I had known for years and new onesndashndashto gauge interest in putting such a volume together I was inspired by the enthusiasm I received from my colleagues and decided to go forward Then came the question of what kind of AFM As surveyed in Chapter 1

xvi Preface

AFM is a tremendously broad field that characterizes a wide variety of material and surface properties However at its heart AFM is a high-resolution nanomechanical probe of the surface That probe can be coated with an electronically conducting layer or magnetic material or even replaced by an optical fiber tip in order to probe a variety of properties However a strong majority of the industrial RampD both in my own direct personal experience and those that I was familiar with was conducted with mechanical probing of properties Of course there are some key counterexamples that stand out such as electrostatic force microscopy to probe electrical properties of conducting and semiconducting materials and magnetic force microscopy to probe magnetic storage devices Perhaps those applications will be covered in a future volume But the overwhelming use of AFM in industrial RampD was nanomechanical characterization and thus it was quickly chosen as the focus for this book

It was important that this book be self-contained While the bookrsquos focus is on the applications it is intended to be accessible to a broad audience both from a professional and educational background so that everyone from an advanced undergraduate to a seasoned professional will benefit Various professional backgrounds should benefit as well from those in an academic or academic-like environment who wish to learn how AFM is used in an industrial environment to those in an industrial environment eager to learn what other applications are possible While a freshman chemistry and physics background is assumed no prior experience with AFM is assumed There is certainly advanced information in each chapter so that the content should appeal to a variety of backgrounds in the AFM community from novice to expert

With this aim of appealing to a broad audience I include a tutorial on nanomechani-cal methods in the first half of the book so that applications could be understood and appreciated with context without having to refer to other books or references I chose the topics that I felt were most relevant to studying nanomechanical characterization in the practical industrial environment contact mechanics force curves phase imaging dynamic contact methods and nanoindentation Note that this is not an exhaustive list of all the nanomechanical characterization methods available with AFM

I worked closely with my co-authors to make these overview chapters as practical as possible and not overwhelming in theoretical detail several chapters include worked examples of useful calculations (eg using Hertz mechanics with and without adhesion to model a contact in Chapter 2 or data analysis of contact resonance measurements in Chapter 5) These chapters are meant to be overviews to introduce the terminology and key concepts They are not meant to be an exhaustive step-by-step guide to the operation of various techniques those treatments are done elsewhere and are referenced in the book Chapter 6 is a somewhat unique chapter on ldquoBest Practices in AFM Imagingrdquo that I co-wrote with my colleague Greg Haugstad This chapter outlines some of the common pitfalls encountered by AFM users again with a practical approach to what the most common and most important problems are and how to avoid them

Chapters 7ndash15 then dive into how AFM has been used in a variety of industrial RampD areas to explore (a) phenomena and processes such as new formulation development in pharmaceuticals the effect of humidity and temperature on biomate-rials and nanostructure formation in food processing (b) characterization of various

Preface xvii

materials including polymer blends and composites in the chemicals sector skin and hair in personal care products coatings and thin dielectric films in the semiconductor industry (c) AFMrsquos function as a key quality control tool in semiconductor metrology and (d) a slightly different application of AFM microcantilevers as a physical and chemical sensors in the petroleum industry

My hope is that this book will show what an incredibly useful tool AFM has become to industrial RampD in its short 25 years It is currently an indispensable tool in any microscopy laboratory despite its commercial youth So while the AFM field con-tinues to grow and develop in terms of its capability and understanding spearheaded by many notable groups in academia and elsewhere it has already penetrated significantly into a wide variety of industrial labs This trend will only continue and grow As the penetration into industrial labs continues it is my hope that the reader understands not only how the AFM capability increasingly benefits industry but also that industrial labs are quickly becoming a partner in driving AFMrsquos innovation and research as its commercial and practical utility and importance continue to thrive

Dalia G YablonSharon MA

xix

Acknowledgments

I have been blessed to have wonderful colleagues to share ideas with and gain inspiration from during this process First and foremost I must thank my co-authors on this book Working with each and every author has been an absolute pleasure I had the distinct pleasure of working closely with Andy Tsou and Alan Schilowitz for many years at ExxonMobil Some of the other co-authors I have been fortunate to count as longstanding collaborators including Rob Carpick Greg Haugstad Jason Killgore and Donna Hurley I thank them for their support and enthusiastic partic-ipation for this particular project From all I learned so much technically and made many new friends in the process This book would not have been possible without everyonersquos collective hard work to share their insights and wisdom with us

I would like to thank my colleagues at ExxonMobil with whom I have had the pleasure of developing our AFM capabilities in our AFM lab Specifically Jean Grabowski deserves a special callout and has been a tremendous asset to our efforts as well as a genuine friend along the way I also would like to thank Rebecca Locker Ishita Chakraborty and Daniel Kiracofe I also would like to thank Steve Minne and Roger Proksch for help and guidance in the incipient stages of this project as well as Emily Rapalino and Susan Klein for advice and proofreading

Most of all I have to thank my husband Andrew for his absolute unwavering support and cheerleading throughout the past year and a half His confidence in me was a source of real motivation and inspiration to carry forward despite obstacles And logistically of course I have to thank him for providing childcare to our chil-dren in the evenings and weekends (covering for me on many many bath nights and storyreadingndashndashsuch as on this night) so that I could complete this book one which hopefully one day they might be interested in enough to read and understand

1

Scanning Probe Microscopy in Industrial Applications Nanomechanical Characterization First Edition Edited by Dalia G Yablon copy 2014 John Wiley amp Sons Inc Published 2014 by John Wiley amp Sons Inc

Overview of Atomic Force MicroscopyDalia G Yablon

SurfaceChar LLC Sharon MA

Atomic force microscopy (AFM) is a family of nanoscale characterization techniques that has exploded onto the overall characterization and nanotechnology field as its versatility and high resolution continue to feed a dizzying variety of disciplines from biology to physics and chemistry to engineering AFM entered the scientific arena in 1981 with the now famous invention of its older sibling in the scanning probe micro-scopy family the scanning tunneling microscope (STM) in the IBM Zurich labs of Gerd Binnig and Heinrich Rohrer for which they received the Nobel Prize in Physics in 1986 The AFM was then invented in 1986 by Gerd Binnig Cal Quate and Christoph Gerber [1] Together STM and AFM formed the scanning probe micros-copy (SPM) family which includes other methods such as near-field scanning optical microscopy (NSOM)

The STM spawned the next 25 years of the continuously developing field of scanning probe microscopy and specifically atomic force microscopy which now includes dozens of different methods under its name to probe various propertiesmdashincluding mechanical electrical magnetic chemical and opticalmdashof materials and surfaces Atomic force microscopy is a powerful tool in various research enterprises It is found in practically any university characterization facility alongside optical and electron microscopes and most undergraduates in science or engineering fields at this point are at least familiar with it if not have performed a laboratory experiment in their undergraduate curriculum

The focus of this book is to understand and appreciate the role this young technique has played in industrial research and development (RampD) AFM has penetrated into a variety of industrial research sectors as witnessed by the diverse set of applications described in this book Alongside electron and optical microscopy which have been

Chapter 1

2 Chapter 1 Overview of Atomic Force Microscopy

around for decades and have reached an impressive level of commercial maturity and ease of use AFM has become a vital characterization method despite its youth and continued technical evolution So though AFM is still an active area of academic research as its capabilities continue to develop and be better understood it has proven to be a useful microscopy to address industrial and commercial needs from quality control and assurance to product formulation and process monitoring

The goal of this introductory chapter is to provide an overview of AFM to nonspecialists and introduce the various topics that are the subject of subsequent individual chapters As such this chapter will provide a brief review of the begin-nings of AFM and special features that make it particularly suited for industrial research Then a brief overview of AFM operation will be presented including the hardware software calibrations involved and finally the different nanomechanical methods that will be described in detail both in theory and application Entire books are written on AFM operation and this chapter is not meant to be an exhaustive introduction to its operation merely serving to provide enough information for the rest of the book to be followed intelligently Readers interested in more detail about AFM operation can consult a number of excellent books on the topic [2ndash5]

11 A Word on nomenclAture

Before the rest of this chapter continues some definitions are in order Similar to many other surface science techniques AFM has succumbed to a somewhat unwieldy mess of abbreviations and jargon that has become a hard-to-navigate alphabet soup SPM refers to an umbrella of a variety of methods Methods that fall under SPM include per-haps its most famous member atomic force microscopy (AFM) in addition to others such as scanning tunneling microscopy (STM) near-field scanning optical microscopy (NSOM or SNOM depending on the continent) and other lithographic methods And then within AFM there are dozens of methods that rely on the AFM probendashsample interactions to provide a variety of material properties including electrical optical magnetic and mechanical properties To date there are dozens of SPMAFM-based methods It is beyond the scope of this book to list andor define all the related methods These methods characterize a huge variety of material properties Some of the more common methods have been included in Figure 11 with the category of properties that they measure The wide diversity of properties of materials that can be measured with AFM is clear The focus of this book is on nanomechanical characterization due to its broad appeal in industrial RampD Again Figure 11 is offered to demonstrate the variety and flavor of properties that can be probed with SPM

12 Atomic Force microscopymdashthe AppeAl to industriAl rampd

This book specifically focuses on AFM which is the method that has most penetrated the general characterization and industrial research fields AFM has key features that make it especially attractive to industrial research and development First its resolution

12 Atomic Force MicroscopymdashThe Appeal to Industrial RampD 3

the heart of its utility as a characterization tool is indeed impressive with 5- to 10-nm lateral resolution and angstrom vertical resolution achieved routinely on commercial instruments with commercial cantilevers The limits of lateral resolution continues to be pushed with specialized techniques and cantilevers and it has reached true ldquoatomicrdquo resolution where point defects of certain materials can now be imaged under certain operating parameters and imaging modes especially under liquid [6ndash8] True atomic resolution is still currently mostly achieved through STM which operates through an entirely different tipndashsample interaction mechanism based on quantum mechanical tunneling Many of these STM studies are conducted in ultrahigh vacuum (UHV) though some are performed in ambient and liquid conditions It is fair to say that AFM is catching up however with recent groundbreaking work imaging cyclic aromatic molecules on Cu[111] [10 11]

A second critical feature of AFM that makes it particularly amenable to industrial research is the flexibility of the environment in which it can operate Despite being a high-resolution microscopy that rivals the resolution of electron microscopy AFM can operate in an ambient or even liquid environment with minimal compromise to its resolution The ability to work in ldquoreal-worldrdquo environments makes it a critical tool for many industrial RampD applications where the research emphasis is consis-tently to understand mechanisms and materials in real-world situations as opposed to

Figure 11 Schematic of AFM as an umbrella technique that includes a wide variety of methods to probe various properties of materials Some of these methods are shown here

4 Chapter 1 Overview of Atomic Force Microscopy

idealized materials and conditions that typically exist in academic research endeavors In many cases the researcher may not want the sample to be forced into a pristine vacuum environment where perhaps key materials or components will be evacuated and thus missed in the characterization effort In addition the flexibility of an envi-ronment permits in situ characterization of processes For example processes such as corrosion lubrication catalyst dissolution crystal growth (for active pharmaceutical ingredient API) can all be studied in situ with AFM as a unique attribute of this microscopy The power of in situ measurements for biological applications is simple as many biological processes and materials simple cannot survive ex situ

A corollary of the utility of the AFMrsquos flexible environment to industrial RampD is the flexibility of type and shape of surface or material that AFM can image There are virtually no restrictions on the size of a material that can be probed with AFM Many commercial ldquolarge-samplerdquo AFMs currently exist that can image any design or shape of a surface from engine parts to thin films Indeed one of the AFMs built in the early 1990s (Topometrix Explorer) was advertised as being able to be placed on top of jet wings to image fractures In addition there are practically no mechanical or electrical restrictions on the type of sample an AFM can image Unlike electron microscopy or STM that require conducting or semiconducting surfaces to image or avoid artifacts [or in the case of insulating surface in scanning electron microscopy (SEM) that requires deposition of a thin conducting layer such as Cr] AFM can image insulating surfaces in addition to conducting or semiconducting surfaces with minimal sample prep Typically the most significant requirement on the sample for an AFM is that it be smooth Commercial instruments will have a maximum z (vertical) range that can be imaged dictated by the design and desired resolution of the particular instrument The maximum z range is typically from 1 microm (for high-resolution studies) to several microns (for lower resolution studies) meaning that the sample cannot have features a tilt or overall roughness that exceed that limit in order for the AFM to image effectively Smooth surfaces are often prepared with the use of an ultramicrotome An ultramicrotome is simply a very controlled and precise way of cutting of thin (down to 100 nm) sections at controlled speeds and in a controlled fashion to result in ultrasmooth surfaces of nanometer or less than nanometer rough-enss The ultramicrotome typically employs the use of a glass knife for a coarse cut and then a diamond knife for a fine cut Cryomicrotomy is simply ultramicrotomy at cold temperatures Cryomicrotomy is typically used to prepare polymer surfaces to cut below the glass transition temperature of the material depending on the instru-ment cryomicrotomes can cool down to -180degC

Finally AFM is fundamentally a mechanical probe of the surface through actual contact between the tip and sample So in addition to the ability to provide high resolution on surface features the goal of any form of microscopy is the unique tipndashsample interaction mechanism enabling the probing of many properties most obvious of which is mechanical properties Mechanical properties covers a wide range of materials and properties Entire textbooks and courses are devoted to the studies of mechanics for example by Callister and Rethwisch [12] We very briefly review some of the main concepts below to provide enough of a background for the rest of the chapter

13 Mechanical Properties 5

This edition focuses specifically on nanomechanical characterization Probing other material properties such as optical electrical or magnetic properties is not covered in this book The reason is that though these other properties have been successfully probed in various industrial RampD applications using AFM to probe mechanical properties of surfaces is more ubiquitous in industrial RampD and so was a logical focus for a first book on SPM in industrial applications

13 mechAnicAl properties

When one considers mechanical properties a range of descriptors come to mind including ldquostiffnessrdquo ldquotoughnessrdquo ldquohardnessrdquo ldquobrittlenessrdquo and ldquoductilityrdquo to name a few Each of these properties has their own mathematical expressions with associ-ated experimental methods of measuring them An interaction between two materials can be typically categorized into either elastic (or recoverable) or dissipative or plastic (permanent or nonrecoverable) interaction On the bulk scale most mechanical analysis of materials relies on measuring their strain (material displacement) as a function of stress (pressure exerted on the material) or vica versa The measurement and analysis of stress versus strain behavior of various materials is a rich and active field and there are many excellent textbooks that describe it [12] The description provided below is simplified to provide a context for these measurements ultimately described on the nanoscale

Elastic deformation of a material is typically nonpermanent or reversible and exists in a regime where the strain (material displacement) is proportional to the stress (pressure exerted on the material) by a proportionality factor known as a modulus (though in reality no material is truly only elastic) Elastic materials thus typically follow Hookersquos law Figure 12 shows a representative stress versus strain curve for materials highlighting the elastic region (blue) Additionally in elastic

Plastic

Fracture

Strain

Elastic

Str

ess

Figure 12 Basic stress vs strain curve showing three mechanical regimes of elastic deformation plastic deformation and fracture For color details please see color plate section

6 Chapter 1 Overview of Atomic Force Microscopy

materials the loading curve (as force is applied) is identical to the unloading curve (as the force is removed) Modulus is a key parameter used to describe elastic behavior Depending on the direction in which a material is being stressed the material will have different moduli (eg a tensile modulus a shear modulus etc)

Plastic deformation is on the other end permanent and nonrecoverable Plastic deformation does not follow Hookersquos law and so stress is not proportional to strain In Figure 12 the region of plastic deformation is shown in green In contrast to elastic deformation the loading curve is not identical to the unloading curve and in fact the unloading curve returns to a nonzero strain at a zero stress level There are many common properties associated with plastic deformation including yielding (onset of plastic deformation) ductility (plastic deformation at a fracture) and hardness (resistance to localized plastic deformation)

Finally there is another very important class of materials called viscoelastic materials where the elastic properties are frequency dependent For example when a stress such as pulling is exerted on the material the materialrsquos response is rate dependent Perhaps the most familiar viscoelastic material to most readers from childhood is Silly Putty where its response to pulling depends on how fast it is pulled If Silly Putty is pulled very slowly the material stretches out very nicely If Silly Putty is pulled very fast it will quickly break Polymers and rubbers two materials that play a significant role in consumer products and which are character-ized quite heavily with AFM are also viscoelastic materials

14 overvieW oF AFm operAtion

141 AFm hardware

A schematic of AFM hardware is shown in Figure 13 The main components of the AFM are (1) the cantilever (2) optical detection system (3) x-y-z scanner and (4) feedback loop

142 cantilevers and probes

The heart of the AFM lies in its cantileverprobe assembly that interacts or probes materials to provide the information of interest The technology to fabricate AFM can-tilevers takes advantage of technology developed for the semiconductor industry to make similar scale devices and features out of silicon Thus today AFM cantilevers are typically made by micromachining silicon (single-crystal Si) or silicon nitride (Si

3N

4) The dimensions of a cantilever vary and dictate the stiffness or spring

constant of the cantilever Depending on the material that needs to be probed and the mode of operation one might pick a lever with a stiffer (higher) or softer (lower) spring constant Cantilevers are generally either rectangular in geometry (Si or Si

3N

4)

or triangular (Si3N

4) with now hybrid cantilevers being manufactured (eg Si tips on

SiN cantilevers) Generally the Si3N

4 probes have lower force constants and might be

used for imaging in liquid among other applications whereas the Si probes can be brittle and tend to break or chip in contact when scanning a surface Typically the

14 Overview of AFM Operation 7

cantilevers are hundreds of microns in length tens of microns in width and a few microns in thickness for reference the diameter of a human hair is approximately 100 microm Cantilevers can be coated with gold or aluminum to provide high reflectivity Commercial cantilevers are also available with other thin metal coatings for magnetic or electrical imaging Cantilevers and probes can also be functionalized chemically or biologically for specific interactions

Probes are grown at the end of the lever and vary in shape and sharpness A sharper probe can often improve the lateral resolution of the AFM Probe design is a continuously evolving field as manufacturers strive for the most robust reproducible and sharp tips Cantileverprobe assemblies are mounted into a cantilever holder that typically includes a piezoceramic element (described below) that can vibrate the can-tilever at a given frequencymdasheither above at or below its resonancemdashdepending on the desired tipndashsample interaction or AFM mode

143 optical detection system

In order to track the motion of the cantileverprobe assembly as it scans a surface an AFM typically includes an optical detection system that consists of a laser reflected off the back side of the cantilever and directed toward a position sensitive detector

Figure 13 Schematic of basic operating principles of AFM A laser is focused and directed off the back end of a cantilever and directed toward a position-sensitive director (PSD) to monitor its vertical and lateral motion as it interacts with the surface Depending on the mode used to image the sample the cantilever can be vibrated with a shake piezo for dynamic imaging modes that involve the resonance of the cantilever

8 Chapter 1 Overview of Atomic Force Microscopy

(PSD) as seen in Figure 13 Optical detection systems are the most common way to track cantilever motion in commercial AFM systems However other methods exist to bypass optical detection systems (in eg a dark medium where the laser light would be absorbed) with self-actuated cantilevers where their motion is read by piezos integrated into the levers The laser is typically a visible photodiode though some commercial instruments have implemented superluminescent diode (SLD) laser The laser is detected by a four-segment position-sensitive detector (PSD) which can track the vertical and lateral motion of the cantilever accurately which can be converted to units of nanometers or displacement through careful calibration described below

144 x-y-z scanner

An AFM can operate either by scanning the sample relative to a stationary tip (sample scanning) or scanning the tip relative to a stationary sample (tip scanning) Each has its own advantages in terms of size of sample it can accommodate (tip scanning more flexible) ability to accessorize (tip scanning again more flexible) and stability and signal to noise (sample scanning easier to build with better specifications) Each shares the requirement to move the tip relative to the sample in a highly accurate way with minimal noise

Tipndashsample motion is often accomplished in commercial instruments by piezoelectric materials which are materials that respond either by expansion or con-traction in response to an applied voltage These kinds of materials provide the ability for very fine motion (nanometers to microns) Piezoelectric materials can be made in several shapes including tubes disks and bimorphs depending on the range and geom-etry of motion required Although piezoelectric materials are very effective at moving tip or sample they are plagued by nonlinear behavior such as hysteresis and creep that has serious consequences for accurate AFM imaging and interpretation of data Artifacts induced by piezoelectric materials are reviewed in detail in Chapter 6

Piezoelectric scanners can be programmed to move with either open-loop or closed-loop feedback circuits In open-loop scanners the piezo is moved by programs to ramp x and y bias voltages to move the prescribed amounts So in open-loop scan-ners if the piezo has to move a large distance or move around on the sample there can be considerable hysteresis of creep with the best recourse of patience or movingzooming in and out in small increments since hysteresiscreep is proportional to the displacement or motion the scanner is trying to execute

One solution to this problem is to use closed-loop scanners that use additional x-y-z sensors to independently measure and then correct for scanner movement In closed-loop operation feedback circuits are continually adjusted to the applied scanner biases to correct for the x-y motion and to achieve the desired motion It is important to note that correction of x-y motion in an AFM is slightly different than correction of z motion This is due to the fact that x-y motion can be predicted whereas z motion cannot as it depends on the surface topography of the sample being scanned Therefore typically there are active feedback circuits to control x and y

xv

Preface

The idea for this book came a couple of years ago after perusing atomic force microscopy (AFM) book collections at many professional society conferences over the years and finding that there was constantly a key topic missing how AFM was used practically in industrial RampD This topic is close to my heart as I have pursued a career developing novel methods and applying scanning probe microscopy (SPM) to a wide variety of industrial RampD problems at ExxonMobil Research and Engineeringrsquos Corporate Strategic Research in New Jersey for over a decade At ExxonMobil I have had the opportunity to explore application of AFM in the three vast sectors in the petroleum industry the downstream chemi-cals and upstream sector studying a wide variety of problems from corrosion to lubrication to geology to polymer materials All the AFM-focused books I found were research reviews on focused topical areas or textbooks on operating princi-ples and theory So though these books offered excellent technical treatments of a variety of topics the incredible utility applicability and flexibility of AFM to solve real-world everyday problems was lacking

My background in graduate school was actually in scanning tunneling micro-scopy (STM) as I studied self-assembled monolayers at the liquidndashsolid interface in George Flynnrsquos lab at Columbia Universityrsquos chemistry department not the most rel-evant ldquoreal-world industrialrdquo problem However being an STM lab in the 1990s we also had AFMs in the lab (since they shared the same ldquobrainrdquo the controller) so every once in a while we would help out a colleague and study their samples As I moved to ExxonMobil after graduate school in 2002 my intent was not necessarily to continue in scanning probe microscopy In fact it was serendipity that upon arriving at ExxonMobilrsquos Corporate Strategic Research labs as a postdoctoral fellow I found an unused Digital Instruments Nanoscope III controller with a fully accessorized Multimode The rest as they say is history

Over my time at ExxonMobil I ran into many like-minded colleagues in other industrial facilities using AFM and we would hold fascinating discussions of how the technology was used in onersquos particular field On the other side I could sense great interest from my academic colleagues that industrial researchers were using SPM to such a great extent as well as recent graduates trying to connect their hard-earned knowledge and expertise in the field as students to practical useful areas A book covering this material for the wider audience did indeed seem missing

And so I began to survey the industrial RampD communityndashndashboth members I had known for years and new onesndashndashto gauge interest in putting such a volume together I was inspired by the enthusiasm I received from my colleagues and decided to go forward Then came the question of what kind of AFM As surveyed in Chapter 1

xvi Preface

AFM is a tremendously broad field that characterizes a wide variety of material and surface properties However at its heart AFM is a high-resolution nanomechanical probe of the surface That probe can be coated with an electronically conducting layer or magnetic material or even replaced by an optical fiber tip in order to probe a variety of properties However a strong majority of the industrial RampD both in my own direct personal experience and those that I was familiar with was conducted with mechanical probing of properties Of course there are some key counterexamples that stand out such as electrostatic force microscopy to probe electrical properties of conducting and semiconducting materials and magnetic force microscopy to probe magnetic storage devices Perhaps those applications will be covered in a future volume But the overwhelming use of AFM in industrial RampD was nanomechanical characterization and thus it was quickly chosen as the focus for this book

It was important that this book be self-contained While the bookrsquos focus is on the applications it is intended to be accessible to a broad audience both from a professional and educational background so that everyone from an advanced undergraduate to a seasoned professional will benefit Various professional backgrounds should benefit as well from those in an academic or academic-like environment who wish to learn how AFM is used in an industrial environment to those in an industrial environment eager to learn what other applications are possible While a freshman chemistry and physics background is assumed no prior experience with AFM is assumed There is certainly advanced information in each chapter so that the content should appeal to a variety of backgrounds in the AFM community from novice to expert

With this aim of appealing to a broad audience I include a tutorial on nanomechani-cal methods in the first half of the book so that applications could be understood and appreciated with context without having to refer to other books or references I chose the topics that I felt were most relevant to studying nanomechanical characterization in the practical industrial environment contact mechanics force curves phase imaging dynamic contact methods and nanoindentation Note that this is not an exhaustive list of all the nanomechanical characterization methods available with AFM

I worked closely with my co-authors to make these overview chapters as practical as possible and not overwhelming in theoretical detail several chapters include worked examples of useful calculations (eg using Hertz mechanics with and without adhesion to model a contact in Chapter 2 or data analysis of contact resonance measurements in Chapter 5) These chapters are meant to be overviews to introduce the terminology and key concepts They are not meant to be an exhaustive step-by-step guide to the operation of various techniques those treatments are done elsewhere and are referenced in the book Chapter 6 is a somewhat unique chapter on ldquoBest Practices in AFM Imagingrdquo that I co-wrote with my colleague Greg Haugstad This chapter outlines some of the common pitfalls encountered by AFM users again with a practical approach to what the most common and most important problems are and how to avoid them

Chapters 7ndash15 then dive into how AFM has been used in a variety of industrial RampD areas to explore (a) phenomena and processes such as new formulation development in pharmaceuticals the effect of humidity and temperature on biomate-rials and nanostructure formation in food processing (b) characterization of various

Preface xvii

materials including polymer blends and composites in the chemicals sector skin and hair in personal care products coatings and thin dielectric films in the semiconductor industry (c) AFMrsquos function as a key quality control tool in semiconductor metrology and (d) a slightly different application of AFM microcantilevers as a physical and chemical sensors in the petroleum industry

My hope is that this book will show what an incredibly useful tool AFM has become to industrial RampD in its short 25 years It is currently an indispensable tool in any microscopy laboratory despite its commercial youth So while the AFM field con-tinues to grow and develop in terms of its capability and understanding spearheaded by many notable groups in academia and elsewhere it has already penetrated significantly into a wide variety of industrial labs This trend will only continue and grow As the penetration into industrial labs continues it is my hope that the reader understands not only how the AFM capability increasingly benefits industry but also that industrial labs are quickly becoming a partner in driving AFMrsquos innovation and research as its commercial and practical utility and importance continue to thrive

Dalia G YablonSharon MA

xix

Acknowledgments

I have been blessed to have wonderful colleagues to share ideas with and gain inspiration from during this process First and foremost I must thank my co-authors on this book Working with each and every author has been an absolute pleasure I had the distinct pleasure of working closely with Andy Tsou and Alan Schilowitz for many years at ExxonMobil Some of the other co-authors I have been fortunate to count as longstanding collaborators including Rob Carpick Greg Haugstad Jason Killgore and Donna Hurley I thank them for their support and enthusiastic partic-ipation for this particular project From all I learned so much technically and made many new friends in the process This book would not have been possible without everyonersquos collective hard work to share their insights and wisdom with us

I would like to thank my colleagues at ExxonMobil with whom I have had the pleasure of developing our AFM capabilities in our AFM lab Specifically Jean Grabowski deserves a special callout and has been a tremendous asset to our efforts as well as a genuine friend along the way I also would like to thank Rebecca Locker Ishita Chakraborty and Daniel Kiracofe I also would like to thank Steve Minne and Roger Proksch for help and guidance in the incipient stages of this project as well as Emily Rapalino and Susan Klein for advice and proofreading

Most of all I have to thank my husband Andrew for his absolute unwavering support and cheerleading throughout the past year and a half His confidence in me was a source of real motivation and inspiration to carry forward despite obstacles And logistically of course I have to thank him for providing childcare to our chil-dren in the evenings and weekends (covering for me on many many bath nights and storyreadingndashndashsuch as on this night) so that I could complete this book one which hopefully one day they might be interested in enough to read and understand

1

Scanning Probe Microscopy in Industrial Applications Nanomechanical Characterization First Edition Edited by Dalia G Yablon copy 2014 John Wiley amp Sons Inc Published 2014 by John Wiley amp Sons Inc

Overview of Atomic Force MicroscopyDalia G Yablon

SurfaceChar LLC Sharon MA

Atomic force microscopy (AFM) is a family of nanoscale characterization techniques that has exploded onto the overall characterization and nanotechnology field as its versatility and high resolution continue to feed a dizzying variety of disciplines from biology to physics and chemistry to engineering AFM entered the scientific arena in 1981 with the now famous invention of its older sibling in the scanning probe micro-scopy family the scanning tunneling microscope (STM) in the IBM Zurich labs of Gerd Binnig and Heinrich Rohrer for which they received the Nobel Prize in Physics in 1986 The AFM was then invented in 1986 by Gerd Binnig Cal Quate and Christoph Gerber [1] Together STM and AFM formed the scanning probe micros-copy (SPM) family which includes other methods such as near-field scanning optical microscopy (NSOM)

The STM spawned the next 25 years of the continuously developing field of scanning probe microscopy and specifically atomic force microscopy which now includes dozens of different methods under its name to probe various propertiesmdashincluding mechanical electrical magnetic chemical and opticalmdashof materials and surfaces Atomic force microscopy is a powerful tool in various research enterprises It is found in practically any university characterization facility alongside optical and electron microscopes and most undergraduates in science or engineering fields at this point are at least familiar with it if not have performed a laboratory experiment in their undergraduate curriculum

The focus of this book is to understand and appreciate the role this young technique has played in industrial research and development (RampD) AFM has penetrated into a variety of industrial research sectors as witnessed by the diverse set of applications described in this book Alongside electron and optical microscopy which have been

Chapter 1

2 Chapter 1 Overview of Atomic Force Microscopy

around for decades and have reached an impressive level of commercial maturity and ease of use AFM has become a vital characterization method despite its youth and continued technical evolution So though AFM is still an active area of academic research as its capabilities continue to develop and be better understood it has proven to be a useful microscopy to address industrial and commercial needs from quality control and assurance to product formulation and process monitoring

The goal of this introductory chapter is to provide an overview of AFM to nonspecialists and introduce the various topics that are the subject of subsequent individual chapters As such this chapter will provide a brief review of the begin-nings of AFM and special features that make it particularly suited for industrial research Then a brief overview of AFM operation will be presented including the hardware software calibrations involved and finally the different nanomechanical methods that will be described in detail both in theory and application Entire books are written on AFM operation and this chapter is not meant to be an exhaustive introduction to its operation merely serving to provide enough information for the rest of the book to be followed intelligently Readers interested in more detail about AFM operation can consult a number of excellent books on the topic [2ndash5]

11 A Word on nomenclAture

Before the rest of this chapter continues some definitions are in order Similar to many other surface science techniques AFM has succumbed to a somewhat unwieldy mess of abbreviations and jargon that has become a hard-to-navigate alphabet soup SPM refers to an umbrella of a variety of methods Methods that fall under SPM include per-haps its most famous member atomic force microscopy (AFM) in addition to others such as scanning tunneling microscopy (STM) near-field scanning optical microscopy (NSOM or SNOM depending on the continent) and other lithographic methods And then within AFM there are dozens of methods that rely on the AFM probendashsample interactions to provide a variety of material properties including electrical optical magnetic and mechanical properties To date there are dozens of SPMAFM-based methods It is beyond the scope of this book to list andor define all the related methods These methods characterize a huge variety of material properties Some of the more common methods have been included in Figure 11 with the category of properties that they measure The wide diversity of properties of materials that can be measured with AFM is clear The focus of this book is on nanomechanical characterization due to its broad appeal in industrial RampD Again Figure 11 is offered to demonstrate the variety and flavor of properties that can be probed with SPM

12 Atomic Force microscopymdashthe AppeAl to industriAl rampd

This book specifically focuses on AFM which is the method that has most penetrated the general characterization and industrial research fields AFM has key features that make it especially attractive to industrial research and development First its resolution

12 Atomic Force MicroscopymdashThe Appeal to Industrial RampD 3

the heart of its utility as a characterization tool is indeed impressive with 5- to 10-nm lateral resolution and angstrom vertical resolution achieved routinely on commercial instruments with commercial cantilevers The limits of lateral resolution continues to be pushed with specialized techniques and cantilevers and it has reached true ldquoatomicrdquo resolution where point defects of certain materials can now be imaged under certain operating parameters and imaging modes especially under liquid [6ndash8] True atomic resolution is still currently mostly achieved through STM which operates through an entirely different tipndashsample interaction mechanism based on quantum mechanical tunneling Many of these STM studies are conducted in ultrahigh vacuum (UHV) though some are performed in ambient and liquid conditions It is fair to say that AFM is catching up however with recent groundbreaking work imaging cyclic aromatic molecules on Cu[111] [10 11]

A second critical feature of AFM that makes it particularly amenable to industrial research is the flexibility of the environment in which it can operate Despite being a high-resolution microscopy that rivals the resolution of electron microscopy AFM can operate in an ambient or even liquid environment with minimal compromise to its resolution The ability to work in ldquoreal-worldrdquo environments makes it a critical tool for many industrial RampD applications where the research emphasis is consis-tently to understand mechanisms and materials in real-world situations as opposed to

Figure 11 Schematic of AFM as an umbrella technique that includes a wide variety of methods to probe various properties of materials Some of these methods are shown here

4 Chapter 1 Overview of Atomic Force Microscopy

idealized materials and conditions that typically exist in academic research endeavors In many cases the researcher may not want the sample to be forced into a pristine vacuum environment where perhaps key materials or components will be evacuated and thus missed in the characterization effort In addition the flexibility of an envi-ronment permits in situ characterization of processes For example processes such as corrosion lubrication catalyst dissolution crystal growth (for active pharmaceutical ingredient API) can all be studied in situ with AFM as a unique attribute of this microscopy The power of in situ measurements for biological applications is simple as many biological processes and materials simple cannot survive ex situ

A corollary of the utility of the AFMrsquos flexible environment to industrial RampD is the flexibility of type and shape of surface or material that AFM can image There are virtually no restrictions on the size of a material that can be probed with AFM Many commercial ldquolarge-samplerdquo AFMs currently exist that can image any design or shape of a surface from engine parts to thin films Indeed one of the AFMs built in the early 1990s (Topometrix Explorer) was advertised as being able to be placed on top of jet wings to image fractures In addition there are practically no mechanical or electrical restrictions on the type of sample an AFM can image Unlike electron microscopy or STM that require conducting or semiconducting surfaces to image or avoid artifacts [or in the case of insulating surface in scanning electron microscopy (SEM) that requires deposition of a thin conducting layer such as Cr] AFM can image insulating surfaces in addition to conducting or semiconducting surfaces with minimal sample prep Typically the most significant requirement on the sample for an AFM is that it be smooth Commercial instruments will have a maximum z (vertical) range that can be imaged dictated by the design and desired resolution of the particular instrument The maximum z range is typically from 1 microm (for high-resolution studies) to several microns (for lower resolution studies) meaning that the sample cannot have features a tilt or overall roughness that exceed that limit in order for the AFM to image effectively Smooth surfaces are often prepared with the use of an ultramicrotome An ultramicrotome is simply a very controlled and precise way of cutting of thin (down to 100 nm) sections at controlled speeds and in a controlled fashion to result in ultrasmooth surfaces of nanometer or less than nanometer rough-enss The ultramicrotome typically employs the use of a glass knife for a coarse cut and then a diamond knife for a fine cut Cryomicrotomy is simply ultramicrotomy at cold temperatures Cryomicrotomy is typically used to prepare polymer surfaces to cut below the glass transition temperature of the material depending on the instru-ment cryomicrotomes can cool down to -180degC

Finally AFM is fundamentally a mechanical probe of the surface through actual contact between the tip and sample So in addition to the ability to provide high resolution on surface features the goal of any form of microscopy is the unique tipndashsample interaction mechanism enabling the probing of many properties most obvious of which is mechanical properties Mechanical properties covers a wide range of materials and properties Entire textbooks and courses are devoted to the studies of mechanics for example by Callister and Rethwisch [12] We very briefly review some of the main concepts below to provide enough of a background for the rest of the chapter

13 Mechanical Properties 5

This edition focuses specifically on nanomechanical characterization Probing other material properties such as optical electrical or magnetic properties is not covered in this book The reason is that though these other properties have been successfully probed in various industrial RampD applications using AFM to probe mechanical properties of surfaces is more ubiquitous in industrial RampD and so was a logical focus for a first book on SPM in industrial applications

13 mechAnicAl properties

When one considers mechanical properties a range of descriptors come to mind including ldquostiffnessrdquo ldquotoughnessrdquo ldquohardnessrdquo ldquobrittlenessrdquo and ldquoductilityrdquo to name a few Each of these properties has their own mathematical expressions with associ-ated experimental methods of measuring them An interaction between two materials can be typically categorized into either elastic (or recoverable) or dissipative or plastic (permanent or nonrecoverable) interaction On the bulk scale most mechanical analysis of materials relies on measuring their strain (material displacement) as a function of stress (pressure exerted on the material) or vica versa The measurement and analysis of stress versus strain behavior of various materials is a rich and active field and there are many excellent textbooks that describe it [12] The description provided below is simplified to provide a context for these measurements ultimately described on the nanoscale

Elastic deformation of a material is typically nonpermanent or reversible and exists in a regime where the strain (material displacement) is proportional to the stress (pressure exerted on the material) by a proportionality factor known as a modulus (though in reality no material is truly only elastic) Elastic materials thus typically follow Hookersquos law Figure 12 shows a representative stress versus strain curve for materials highlighting the elastic region (blue) Additionally in elastic

Plastic

Fracture

Strain

Elastic

Str

ess

Figure 12 Basic stress vs strain curve showing three mechanical regimes of elastic deformation plastic deformation and fracture For color details please see color plate section

6 Chapter 1 Overview of Atomic Force Microscopy

materials the loading curve (as force is applied) is identical to the unloading curve (as the force is removed) Modulus is a key parameter used to describe elastic behavior Depending on the direction in which a material is being stressed the material will have different moduli (eg a tensile modulus a shear modulus etc)

Plastic deformation is on the other end permanent and nonrecoverable Plastic deformation does not follow Hookersquos law and so stress is not proportional to strain In Figure 12 the region of plastic deformation is shown in green In contrast to elastic deformation the loading curve is not identical to the unloading curve and in fact the unloading curve returns to a nonzero strain at a zero stress level There are many common properties associated with plastic deformation including yielding (onset of plastic deformation) ductility (plastic deformation at a fracture) and hardness (resistance to localized plastic deformation)

Finally there is another very important class of materials called viscoelastic materials where the elastic properties are frequency dependent For example when a stress such as pulling is exerted on the material the materialrsquos response is rate dependent Perhaps the most familiar viscoelastic material to most readers from childhood is Silly Putty where its response to pulling depends on how fast it is pulled If Silly Putty is pulled very slowly the material stretches out very nicely If Silly Putty is pulled very fast it will quickly break Polymers and rubbers two materials that play a significant role in consumer products and which are character-ized quite heavily with AFM are also viscoelastic materials

14 overvieW oF AFm operAtion

141 AFm hardware

A schematic of AFM hardware is shown in Figure 13 The main components of the AFM are (1) the cantilever (2) optical detection system (3) x-y-z scanner and (4) feedback loop

142 cantilevers and probes

The heart of the AFM lies in its cantileverprobe assembly that interacts or probes materials to provide the information of interest The technology to fabricate AFM can-tilevers takes advantage of technology developed for the semiconductor industry to make similar scale devices and features out of silicon Thus today AFM cantilevers are typically made by micromachining silicon (single-crystal Si) or silicon nitride (Si

3N

4) The dimensions of a cantilever vary and dictate the stiffness or spring

constant of the cantilever Depending on the material that needs to be probed and the mode of operation one might pick a lever with a stiffer (higher) or softer (lower) spring constant Cantilevers are generally either rectangular in geometry (Si or Si

3N

4)

or triangular (Si3N

4) with now hybrid cantilevers being manufactured (eg Si tips on

SiN cantilevers) Generally the Si3N

4 probes have lower force constants and might be

used for imaging in liquid among other applications whereas the Si probes can be brittle and tend to break or chip in contact when scanning a surface Typically the

14 Overview of AFM Operation 7

cantilevers are hundreds of microns in length tens of microns in width and a few microns in thickness for reference the diameter of a human hair is approximately 100 microm Cantilevers can be coated with gold or aluminum to provide high reflectivity Commercial cantilevers are also available with other thin metal coatings for magnetic or electrical imaging Cantilevers and probes can also be functionalized chemically or biologically for specific interactions

Probes are grown at the end of the lever and vary in shape and sharpness A sharper probe can often improve the lateral resolution of the AFM Probe design is a continuously evolving field as manufacturers strive for the most robust reproducible and sharp tips Cantileverprobe assemblies are mounted into a cantilever holder that typically includes a piezoceramic element (described below) that can vibrate the can-tilever at a given frequencymdasheither above at or below its resonancemdashdepending on the desired tipndashsample interaction or AFM mode

143 optical detection system

In order to track the motion of the cantileverprobe assembly as it scans a surface an AFM typically includes an optical detection system that consists of a laser reflected off the back side of the cantilever and directed toward a position sensitive detector

Figure 13 Schematic of basic operating principles of AFM A laser is focused and directed off the back end of a cantilever and directed toward a position-sensitive director (PSD) to monitor its vertical and lateral motion as it interacts with the surface Depending on the mode used to image the sample the cantilever can be vibrated with a shake piezo for dynamic imaging modes that involve the resonance of the cantilever

8 Chapter 1 Overview of Atomic Force Microscopy

(PSD) as seen in Figure 13 Optical detection systems are the most common way to track cantilever motion in commercial AFM systems However other methods exist to bypass optical detection systems (in eg a dark medium where the laser light would be absorbed) with self-actuated cantilevers where their motion is read by piezos integrated into the levers The laser is typically a visible photodiode though some commercial instruments have implemented superluminescent diode (SLD) laser The laser is detected by a four-segment position-sensitive detector (PSD) which can track the vertical and lateral motion of the cantilever accurately which can be converted to units of nanometers or displacement through careful calibration described below

144 x-y-z scanner

An AFM can operate either by scanning the sample relative to a stationary tip (sample scanning) or scanning the tip relative to a stationary sample (tip scanning) Each has its own advantages in terms of size of sample it can accommodate (tip scanning more flexible) ability to accessorize (tip scanning again more flexible) and stability and signal to noise (sample scanning easier to build with better specifications) Each shares the requirement to move the tip relative to the sample in a highly accurate way with minimal noise

Tipndashsample motion is often accomplished in commercial instruments by piezoelectric materials which are materials that respond either by expansion or con-traction in response to an applied voltage These kinds of materials provide the ability for very fine motion (nanometers to microns) Piezoelectric materials can be made in several shapes including tubes disks and bimorphs depending on the range and geom-etry of motion required Although piezoelectric materials are very effective at moving tip or sample they are plagued by nonlinear behavior such as hysteresis and creep that has serious consequences for accurate AFM imaging and interpretation of data Artifacts induced by piezoelectric materials are reviewed in detail in Chapter 6

Piezoelectric scanners can be programmed to move with either open-loop or closed-loop feedback circuits In open-loop scanners the piezo is moved by programs to ramp x and y bias voltages to move the prescribed amounts So in open-loop scan-ners if the piezo has to move a large distance or move around on the sample there can be considerable hysteresis of creep with the best recourse of patience or movingzooming in and out in small increments since hysteresiscreep is proportional to the displacement or motion the scanner is trying to execute

One solution to this problem is to use closed-loop scanners that use additional x-y-z sensors to independently measure and then correct for scanner movement In closed-loop operation feedback circuits are continually adjusted to the applied scanner biases to correct for the x-y motion and to achieve the desired motion It is important to note that correction of x-y motion in an AFM is slightly different than correction of z motion This is due to the fact that x-y motion can be predicted whereas z motion cannot as it depends on the surface topography of the sample being scanned Therefore typically there are active feedback circuits to control x and y

xvi Preface

AFM is a tremendously broad field that characterizes a wide variety of material and surface properties However at its heart AFM is a high-resolution nanomechanical probe of the surface That probe can be coated with an electronically conducting layer or magnetic material or even replaced by an optical fiber tip in order to probe a variety of properties However a strong majority of the industrial RampD both in my own direct personal experience and those that I was familiar with was conducted with mechanical probing of properties Of course there are some key counterexamples that stand out such as electrostatic force microscopy to probe electrical properties of conducting and semiconducting materials and magnetic force microscopy to probe magnetic storage devices Perhaps those applications will be covered in a future volume But the overwhelming use of AFM in industrial RampD was nanomechanical characterization and thus it was quickly chosen as the focus for this book

It was important that this book be self-contained While the bookrsquos focus is on the applications it is intended to be accessible to a broad audience both from a professional and educational background so that everyone from an advanced undergraduate to a seasoned professional will benefit Various professional backgrounds should benefit as well from those in an academic or academic-like environment who wish to learn how AFM is used in an industrial environment to those in an industrial environment eager to learn what other applications are possible While a freshman chemistry and physics background is assumed no prior experience with AFM is assumed There is certainly advanced information in each chapter so that the content should appeal to a variety of backgrounds in the AFM community from novice to expert

With this aim of appealing to a broad audience I include a tutorial on nanomechani-cal methods in the first half of the book so that applications could be understood and appreciated with context without having to refer to other books or references I chose the topics that I felt were most relevant to studying nanomechanical characterization in the practical industrial environment contact mechanics force curves phase imaging dynamic contact methods and nanoindentation Note that this is not an exhaustive list of all the nanomechanical characterization methods available with AFM

I worked closely with my co-authors to make these overview chapters as practical as possible and not overwhelming in theoretical detail several chapters include worked examples of useful calculations (eg using Hertz mechanics with and without adhesion to model a contact in Chapter 2 or data analysis of contact resonance measurements in Chapter 5) These chapters are meant to be overviews to introduce the terminology and key concepts They are not meant to be an exhaustive step-by-step guide to the operation of various techniques those treatments are done elsewhere and are referenced in the book Chapter 6 is a somewhat unique chapter on ldquoBest Practices in AFM Imagingrdquo that I co-wrote with my colleague Greg Haugstad This chapter outlines some of the common pitfalls encountered by AFM users again with a practical approach to what the most common and most important problems are and how to avoid them

Chapters 7ndash15 then dive into how AFM has been used in a variety of industrial RampD areas to explore (a) phenomena and processes such as new formulation development in pharmaceuticals the effect of humidity and temperature on biomate-rials and nanostructure formation in food processing (b) characterization of various

Preface xvii

materials including polymer blends and composites in the chemicals sector skin and hair in personal care products coatings and thin dielectric films in the semiconductor industry (c) AFMrsquos function as a key quality control tool in semiconductor metrology and (d) a slightly different application of AFM microcantilevers as a physical and chemical sensors in the petroleum industry

My hope is that this book will show what an incredibly useful tool AFM has become to industrial RampD in its short 25 years It is currently an indispensable tool in any microscopy laboratory despite its commercial youth So while the AFM field con-tinues to grow and develop in terms of its capability and understanding spearheaded by many notable groups in academia and elsewhere it has already penetrated significantly into a wide variety of industrial labs This trend will only continue and grow As the penetration into industrial labs continues it is my hope that the reader understands not only how the AFM capability increasingly benefits industry but also that industrial labs are quickly becoming a partner in driving AFMrsquos innovation and research as its commercial and practical utility and importance continue to thrive

Dalia G YablonSharon MA

xix

Acknowledgments

I have been blessed to have wonderful colleagues to share ideas with and gain inspiration from during this process First and foremost I must thank my co-authors on this book Working with each and every author has been an absolute pleasure I had the distinct pleasure of working closely with Andy Tsou and Alan Schilowitz for many years at ExxonMobil Some of the other co-authors I have been fortunate to count as longstanding collaborators including Rob Carpick Greg Haugstad Jason Killgore and Donna Hurley I thank them for their support and enthusiastic partic-ipation for this particular project From all I learned so much technically and made many new friends in the process This book would not have been possible without everyonersquos collective hard work to share their insights and wisdom with us

I would like to thank my colleagues at ExxonMobil with whom I have had the pleasure of developing our AFM capabilities in our AFM lab Specifically Jean Grabowski deserves a special callout and has been a tremendous asset to our efforts as well as a genuine friend along the way I also would like to thank Rebecca Locker Ishita Chakraborty and Daniel Kiracofe I also would like to thank Steve Minne and Roger Proksch for help and guidance in the incipient stages of this project as well as Emily Rapalino and Susan Klein for advice and proofreading

Most of all I have to thank my husband Andrew for his absolute unwavering support and cheerleading throughout the past year and a half His confidence in me was a source of real motivation and inspiration to carry forward despite obstacles And logistically of course I have to thank him for providing childcare to our chil-dren in the evenings and weekends (covering for me on many many bath nights and storyreadingndashndashsuch as on this night) so that I could complete this book one which hopefully one day they might be interested in enough to read and understand

1

Scanning Probe Microscopy in Industrial Applications Nanomechanical Characterization First Edition Edited by Dalia G Yablon copy 2014 John Wiley amp Sons Inc Published 2014 by John Wiley amp Sons Inc

Overview of Atomic Force MicroscopyDalia G Yablon

SurfaceChar LLC Sharon MA

Atomic force microscopy (AFM) is a family of nanoscale characterization techniques that has exploded onto the overall characterization and nanotechnology field as its versatility and high resolution continue to feed a dizzying variety of disciplines from biology to physics and chemistry to engineering AFM entered the scientific arena in 1981 with the now famous invention of its older sibling in the scanning probe micro-scopy family the scanning tunneling microscope (STM) in the IBM Zurich labs of Gerd Binnig and Heinrich Rohrer for which they received the Nobel Prize in Physics in 1986 The AFM was then invented in 1986 by Gerd Binnig Cal Quate and Christoph Gerber [1] Together STM and AFM formed the scanning probe micros-copy (SPM) family which includes other methods such as near-field scanning optical microscopy (NSOM)

The STM spawned the next 25 years of the continuously developing field of scanning probe microscopy and specifically atomic force microscopy which now includes dozens of different methods under its name to probe various propertiesmdashincluding mechanical electrical magnetic chemical and opticalmdashof materials and surfaces Atomic force microscopy is a powerful tool in various research enterprises It is found in practically any university characterization facility alongside optical and electron microscopes and most undergraduates in science or engineering fields at this point are at least familiar with it if not have performed a laboratory experiment in their undergraduate curriculum

The focus of this book is to understand and appreciate the role this young technique has played in industrial research and development (RampD) AFM has penetrated into a variety of industrial research sectors as witnessed by the diverse set of applications described in this book Alongside electron and optical microscopy which have been

Chapter 1

2 Chapter 1 Overview of Atomic Force Microscopy

around for decades and have reached an impressive level of commercial maturity and ease of use AFM has become a vital characterization method despite its youth and continued technical evolution So though AFM is still an active area of academic research as its capabilities continue to develop and be better understood it has proven to be a useful microscopy to address industrial and commercial needs from quality control and assurance to product formulation and process monitoring

The goal of this introductory chapter is to provide an overview of AFM to nonspecialists and introduce the various topics that are the subject of subsequent individual chapters As such this chapter will provide a brief review of the begin-nings of AFM and special features that make it particularly suited for industrial research Then a brief overview of AFM operation will be presented including the hardware software calibrations involved and finally the different nanomechanical methods that will be described in detail both in theory and application Entire books are written on AFM operation and this chapter is not meant to be an exhaustive introduction to its operation merely serving to provide enough information for the rest of the book to be followed intelligently Readers interested in more detail about AFM operation can consult a number of excellent books on the topic [2ndash5]

11 A Word on nomenclAture

Before the rest of this chapter continues some definitions are in order Similar to many other surface science techniques AFM has succumbed to a somewhat unwieldy mess of abbreviations and jargon that has become a hard-to-navigate alphabet soup SPM refers to an umbrella of a variety of methods Methods that fall under SPM include per-haps its most famous member atomic force microscopy (AFM) in addition to others such as scanning tunneling microscopy (STM) near-field scanning optical microscopy (NSOM or SNOM depending on the continent) and other lithographic methods And then within AFM there are dozens of methods that rely on the AFM probendashsample interactions to provide a variety of material properties including electrical optical magnetic and mechanical properties To date there are dozens of SPMAFM-based methods It is beyond the scope of this book to list andor define all the related methods These methods characterize a huge variety of material properties Some of the more common methods have been included in Figure 11 with the category of properties that they measure The wide diversity of properties of materials that can be measured with AFM is clear The focus of this book is on nanomechanical characterization due to its broad appeal in industrial RampD Again Figure 11 is offered to demonstrate the variety and flavor of properties that can be probed with SPM

12 Atomic Force microscopymdashthe AppeAl to industriAl rampd

This book specifically focuses on AFM which is the method that has most penetrated the general characterization and industrial research fields AFM has key features that make it especially attractive to industrial research and development First its resolution

12 Atomic Force MicroscopymdashThe Appeal to Industrial RampD 3

the heart of its utility as a characterization tool is indeed impressive with 5- to 10-nm lateral resolution and angstrom vertical resolution achieved routinely on commercial instruments with commercial cantilevers The limits of lateral resolution continues to be pushed with specialized techniques and cantilevers and it has reached true ldquoatomicrdquo resolution where point defects of certain materials can now be imaged under certain operating parameters and imaging modes especially under liquid [6ndash8] True atomic resolution is still currently mostly achieved through STM which operates through an entirely different tipndashsample interaction mechanism based on quantum mechanical tunneling Many of these STM studies are conducted in ultrahigh vacuum (UHV) though some are performed in ambient and liquid conditions It is fair to say that AFM is catching up however with recent groundbreaking work imaging cyclic aromatic molecules on Cu[111] [10 11]

A second critical feature of AFM that makes it particularly amenable to industrial research is the flexibility of the environment in which it can operate Despite being a high-resolution microscopy that rivals the resolution of electron microscopy AFM can operate in an ambient or even liquid environment with minimal compromise to its resolution The ability to work in ldquoreal-worldrdquo environments makes it a critical tool for many industrial RampD applications where the research emphasis is consis-tently to understand mechanisms and materials in real-world situations as opposed to

Figure 11 Schematic of AFM as an umbrella technique that includes a wide variety of methods to probe various properties of materials Some of these methods are shown here

4 Chapter 1 Overview of Atomic Force Microscopy

idealized materials and conditions that typically exist in academic research endeavors In many cases the researcher may not want the sample to be forced into a pristine vacuum environment where perhaps key materials or components will be evacuated and thus missed in the characterization effort In addition the flexibility of an envi-ronment permits in situ characterization of processes For example processes such as corrosion lubrication catalyst dissolution crystal growth (for active pharmaceutical ingredient API) can all be studied in situ with AFM as a unique attribute of this microscopy The power of in situ measurements for biological applications is simple as many biological processes and materials simple cannot survive ex situ

A corollary of the utility of the AFMrsquos flexible environment to industrial RampD is the flexibility of type and shape of surface or material that AFM can image There are virtually no restrictions on the size of a material that can be probed with AFM Many commercial ldquolarge-samplerdquo AFMs currently exist that can image any design or shape of a surface from engine parts to thin films Indeed one of the AFMs built in the early 1990s (Topometrix Explorer) was advertised as being able to be placed on top of jet wings to image fractures In addition there are practically no mechanical or electrical restrictions on the type of sample an AFM can image Unlike electron microscopy or STM that require conducting or semiconducting surfaces to image or avoid artifacts [or in the case of insulating surface in scanning electron microscopy (SEM) that requires deposition of a thin conducting layer such as Cr] AFM can image insulating surfaces in addition to conducting or semiconducting surfaces with minimal sample prep Typically the most significant requirement on the sample for an AFM is that it be smooth Commercial instruments will have a maximum z (vertical) range that can be imaged dictated by the design and desired resolution of the particular instrument The maximum z range is typically from 1 microm (for high-resolution studies) to several microns (for lower resolution studies) meaning that the sample cannot have features a tilt or overall roughness that exceed that limit in order for the AFM to image effectively Smooth surfaces are often prepared with the use of an ultramicrotome An ultramicrotome is simply a very controlled and precise way of cutting of thin (down to 100 nm) sections at controlled speeds and in a controlled fashion to result in ultrasmooth surfaces of nanometer or less than nanometer rough-enss The ultramicrotome typically employs the use of a glass knife for a coarse cut and then a diamond knife for a fine cut Cryomicrotomy is simply ultramicrotomy at cold temperatures Cryomicrotomy is typically used to prepare polymer surfaces to cut below the glass transition temperature of the material depending on the instru-ment cryomicrotomes can cool down to -180degC

Finally AFM is fundamentally a mechanical probe of the surface through actual contact between the tip and sample So in addition to the ability to provide high resolution on surface features the goal of any form of microscopy is the unique tipndashsample interaction mechanism enabling the probing of many properties most obvious of which is mechanical properties Mechanical properties covers a wide range of materials and properties Entire textbooks and courses are devoted to the studies of mechanics for example by Callister and Rethwisch [12] We very briefly review some of the main concepts below to provide enough of a background for the rest of the chapter

13 Mechanical Properties 5

This edition focuses specifically on nanomechanical characterization Probing other material properties such as optical electrical or magnetic properties is not covered in this book The reason is that though these other properties have been successfully probed in various industrial RampD applications using AFM to probe mechanical properties of surfaces is more ubiquitous in industrial RampD and so was a logical focus for a first book on SPM in industrial applications

13 mechAnicAl properties

When one considers mechanical properties a range of descriptors come to mind including ldquostiffnessrdquo ldquotoughnessrdquo ldquohardnessrdquo ldquobrittlenessrdquo and ldquoductilityrdquo to name a few Each of these properties has their own mathematical expressions with associ-ated experimental methods of measuring them An interaction between two materials can be typically categorized into either elastic (or recoverable) or dissipative or plastic (permanent or nonrecoverable) interaction On the bulk scale most mechanical analysis of materials relies on measuring their strain (material displacement) as a function of stress (pressure exerted on the material) or vica versa The measurement and analysis of stress versus strain behavior of various materials is a rich and active field and there are many excellent textbooks that describe it [12] The description provided below is simplified to provide a context for these measurements ultimately described on the nanoscale

Elastic deformation of a material is typically nonpermanent or reversible and exists in a regime where the strain (material displacement) is proportional to the stress (pressure exerted on the material) by a proportionality factor known as a modulus (though in reality no material is truly only elastic) Elastic materials thus typically follow Hookersquos law Figure 12 shows a representative stress versus strain curve for materials highlighting the elastic region (blue) Additionally in elastic

Plastic

Fracture

Strain

Elastic

Str

ess

Figure 12 Basic stress vs strain curve showing three mechanical regimes of elastic deformation plastic deformation and fracture For color details please see color plate section

6 Chapter 1 Overview of Atomic Force Microscopy

materials the loading curve (as force is applied) is identical to the unloading curve (as the force is removed) Modulus is a key parameter used to describe elastic behavior Depending on the direction in which a material is being stressed the material will have different moduli (eg a tensile modulus a shear modulus etc)

Plastic deformation is on the other end permanent and nonrecoverable Plastic deformation does not follow Hookersquos law and so stress is not proportional to strain In Figure 12 the region of plastic deformation is shown in green In contrast to elastic deformation the loading curve is not identical to the unloading curve and in fact the unloading curve returns to a nonzero strain at a zero stress level There are many common properties associated with plastic deformation including yielding (onset of plastic deformation) ductility (plastic deformation at a fracture) and hardness (resistance to localized plastic deformation)

Finally there is another very important class of materials called viscoelastic materials where the elastic properties are frequency dependent For example when a stress such as pulling is exerted on the material the materialrsquos response is rate dependent Perhaps the most familiar viscoelastic material to most readers from childhood is Silly Putty where its response to pulling depends on how fast it is pulled If Silly Putty is pulled very slowly the material stretches out very nicely If Silly Putty is pulled very fast it will quickly break Polymers and rubbers two materials that play a significant role in consumer products and which are character-ized quite heavily with AFM are also viscoelastic materials

14 overvieW oF AFm operAtion

141 AFm hardware

A schematic of AFM hardware is shown in Figure 13 The main components of the AFM are (1) the cantilever (2) optical detection system (3) x-y-z scanner and (4) feedback loop

142 cantilevers and probes

The heart of the AFM lies in its cantileverprobe assembly that interacts or probes materials to provide the information of interest The technology to fabricate AFM can-tilevers takes advantage of technology developed for the semiconductor industry to make similar scale devices and features out of silicon Thus today AFM cantilevers are typically made by micromachining silicon (single-crystal Si) or silicon nitride (Si

3N

4) The dimensions of a cantilever vary and dictate the stiffness or spring

constant of the cantilever Depending on the material that needs to be probed and the mode of operation one might pick a lever with a stiffer (higher) or softer (lower) spring constant Cantilevers are generally either rectangular in geometry (Si or Si

3N

4)

or triangular (Si3N

4) with now hybrid cantilevers being manufactured (eg Si tips on

SiN cantilevers) Generally the Si3N

4 probes have lower force constants and might be

used for imaging in liquid among other applications whereas the Si probes can be brittle and tend to break or chip in contact when scanning a surface Typically the

14 Overview of AFM Operation 7

cantilevers are hundreds of microns in length tens of microns in width and a few microns in thickness for reference the diameter of a human hair is approximately 100 microm Cantilevers can be coated with gold or aluminum to provide high reflectivity Commercial cantilevers are also available with other thin metal coatings for magnetic or electrical imaging Cantilevers and probes can also be functionalized chemically or biologically for specific interactions

Probes are grown at the end of the lever and vary in shape and sharpness A sharper probe can often improve the lateral resolution of the AFM Probe design is a continuously evolving field as manufacturers strive for the most robust reproducible and sharp tips Cantileverprobe assemblies are mounted into a cantilever holder that typically includes a piezoceramic element (described below) that can vibrate the can-tilever at a given frequencymdasheither above at or below its resonancemdashdepending on the desired tipndashsample interaction or AFM mode

143 optical detection system

In order to track the motion of the cantileverprobe assembly as it scans a surface an AFM typically includes an optical detection system that consists of a laser reflected off the back side of the cantilever and directed toward a position sensitive detector

Figure 13 Schematic of basic operating principles of AFM A laser is focused and directed off the back end of a cantilever and directed toward a position-sensitive director (PSD) to monitor its vertical and lateral motion as it interacts with the surface Depending on the mode used to image the sample the cantilever can be vibrated with a shake piezo for dynamic imaging modes that involve the resonance of the cantilever

8 Chapter 1 Overview of Atomic Force Microscopy

(PSD) as seen in Figure 13 Optical detection systems are the most common way to track cantilever motion in commercial AFM systems However other methods exist to bypass optical detection systems (in eg a dark medium where the laser light would be absorbed) with self-actuated cantilevers where their motion is read by piezos integrated into the levers The laser is typically a visible photodiode though some commercial instruments have implemented superluminescent diode (SLD) laser The laser is detected by a four-segment position-sensitive detector (PSD) which can track the vertical and lateral motion of the cantilever accurately which can be converted to units of nanometers or displacement through careful calibration described below

144 x-y-z scanner

An AFM can operate either by scanning the sample relative to a stationary tip (sample scanning) or scanning the tip relative to a stationary sample (tip scanning) Each has its own advantages in terms of size of sample it can accommodate (tip scanning more flexible) ability to accessorize (tip scanning again more flexible) and stability and signal to noise (sample scanning easier to build with better specifications) Each shares the requirement to move the tip relative to the sample in a highly accurate way with minimal noise

Tipndashsample motion is often accomplished in commercial instruments by piezoelectric materials which are materials that respond either by expansion or con-traction in response to an applied voltage These kinds of materials provide the ability for very fine motion (nanometers to microns) Piezoelectric materials can be made in several shapes including tubes disks and bimorphs depending on the range and geom-etry of motion required Although piezoelectric materials are very effective at moving tip or sample they are plagued by nonlinear behavior such as hysteresis and creep that has serious consequences for accurate AFM imaging and interpretation of data Artifacts induced by piezoelectric materials are reviewed in detail in Chapter 6

Piezoelectric scanners can be programmed to move with either open-loop or closed-loop feedback circuits In open-loop scanners the piezo is moved by programs to ramp x and y bias voltages to move the prescribed amounts So in open-loop scan-ners if the piezo has to move a large distance or move around on the sample there can be considerable hysteresis of creep with the best recourse of patience or movingzooming in and out in small increments since hysteresiscreep is proportional to the displacement or motion the scanner is trying to execute

One solution to this problem is to use closed-loop scanners that use additional x-y-z sensors to independently measure and then correct for scanner movement In closed-loop operation feedback circuits are continually adjusted to the applied scanner biases to correct for the x-y motion and to achieve the desired motion It is important to note that correction of x-y motion in an AFM is slightly different than correction of z motion This is due to the fact that x-y motion can be predicted whereas z motion cannot as it depends on the surface topography of the sample being scanned Therefore typically there are active feedback circuits to control x and y

Preface xvii

materials including polymer blends and composites in the chemicals sector skin and hair in personal care products coatings and thin dielectric films in the semiconductor industry (c) AFMrsquos function as a key quality control tool in semiconductor metrology and (d) a slightly different application of AFM microcantilevers as a physical and chemical sensors in the petroleum industry

My hope is that this book will show what an incredibly useful tool AFM has become to industrial RampD in its short 25 years It is currently an indispensable tool in any microscopy laboratory despite its commercial youth So while the AFM field con-tinues to grow and develop in terms of its capability and understanding spearheaded by many notable groups in academia and elsewhere it has already penetrated significantly into a wide variety of industrial labs This trend will only continue and grow As the penetration into industrial labs continues it is my hope that the reader understands not only how the AFM capability increasingly benefits industry but also that industrial labs are quickly becoming a partner in driving AFMrsquos innovation and research as its commercial and practical utility and importance continue to thrive

Dalia G YablonSharon MA

xix

Acknowledgments

I have been blessed to have wonderful colleagues to share ideas with and gain inspiration from during this process First and foremost I must thank my co-authors on this book Working with each and every author has been an absolute pleasure I had the distinct pleasure of working closely with Andy Tsou and Alan Schilowitz for many years at ExxonMobil Some of the other co-authors I have been fortunate to count as longstanding collaborators including Rob Carpick Greg Haugstad Jason Killgore and Donna Hurley I thank them for their support and enthusiastic partic-ipation for this particular project From all I learned so much technically and made many new friends in the process This book would not have been possible without everyonersquos collective hard work to share their insights and wisdom with us

I would like to thank my colleagues at ExxonMobil with whom I have had the pleasure of developing our AFM capabilities in our AFM lab Specifically Jean Grabowski deserves a special callout and has been a tremendous asset to our efforts as well as a genuine friend along the way I also would like to thank Rebecca Locker Ishita Chakraborty and Daniel Kiracofe I also would like to thank Steve Minne and Roger Proksch for help and guidance in the incipient stages of this project as well as Emily Rapalino and Susan Klein for advice and proofreading

Most of all I have to thank my husband Andrew for his absolute unwavering support and cheerleading throughout the past year and a half His confidence in me was a source of real motivation and inspiration to carry forward despite obstacles And logistically of course I have to thank him for providing childcare to our chil-dren in the evenings and weekends (covering for me on many many bath nights and storyreadingndashndashsuch as on this night) so that I could complete this book one which hopefully one day they might be interested in enough to read and understand

1

Scanning Probe Microscopy in Industrial Applications Nanomechanical Characterization First Edition Edited by Dalia G Yablon copy 2014 John Wiley amp Sons Inc Published 2014 by John Wiley amp Sons Inc

Overview of Atomic Force MicroscopyDalia G Yablon

SurfaceChar LLC Sharon MA

Atomic force microscopy (AFM) is a family of nanoscale characterization techniques that has exploded onto the overall characterization and nanotechnology field as its versatility and high resolution continue to feed a dizzying variety of disciplines from biology to physics and chemistry to engineering AFM entered the scientific arena in 1981 with the now famous invention of its older sibling in the scanning probe micro-scopy family the scanning tunneling microscope (STM) in the IBM Zurich labs of Gerd Binnig and Heinrich Rohrer for which they received the Nobel Prize in Physics in 1986 The AFM was then invented in 1986 by Gerd Binnig Cal Quate and Christoph Gerber [1] Together STM and AFM formed the scanning probe micros-copy (SPM) family which includes other methods such as near-field scanning optical microscopy (NSOM)

The STM spawned the next 25 years of the continuously developing field of scanning probe microscopy and specifically atomic force microscopy which now includes dozens of different methods under its name to probe various propertiesmdashincluding mechanical electrical magnetic chemical and opticalmdashof materials and surfaces Atomic force microscopy is a powerful tool in various research enterprises It is found in practically any university characterization facility alongside optical and electron microscopes and most undergraduates in science or engineering fields at this point are at least familiar with it if not have performed a laboratory experiment in their undergraduate curriculum

The focus of this book is to understand and appreciate the role this young technique has played in industrial research and development (RampD) AFM has penetrated into a variety of industrial research sectors as witnessed by the diverse set of applications described in this book Alongside electron and optical microscopy which have been

Chapter 1

2 Chapter 1 Overview of Atomic Force Microscopy

around for decades and have reached an impressive level of commercial maturity and ease of use AFM has become a vital characterization method despite its youth and continued technical evolution So though AFM is still an active area of academic research as its capabilities continue to develop and be better understood it has proven to be a useful microscopy to address industrial and commercial needs from quality control and assurance to product formulation and process monitoring

The goal of this introductory chapter is to provide an overview of AFM to nonspecialists and introduce the various topics that are the subject of subsequent individual chapters As such this chapter will provide a brief review of the begin-nings of AFM and special features that make it particularly suited for industrial research Then a brief overview of AFM operation will be presented including the hardware software calibrations involved and finally the different nanomechanical methods that will be described in detail both in theory and application Entire books are written on AFM operation and this chapter is not meant to be an exhaustive introduction to its operation merely serving to provide enough information for the rest of the book to be followed intelligently Readers interested in more detail about AFM operation can consult a number of excellent books on the topic [2ndash5]

11 A Word on nomenclAture

Before the rest of this chapter continues some definitions are in order Similar to many other surface science techniques AFM has succumbed to a somewhat unwieldy mess of abbreviations and jargon that has become a hard-to-navigate alphabet soup SPM refers to an umbrella of a variety of methods Methods that fall under SPM include per-haps its most famous member atomic force microscopy (AFM) in addition to others such as scanning tunneling microscopy (STM) near-field scanning optical microscopy (NSOM or SNOM depending on the continent) and other lithographic methods And then within AFM there are dozens of methods that rely on the AFM probendashsample interactions to provide a variety of material properties including electrical optical magnetic and mechanical properties To date there are dozens of SPMAFM-based methods It is beyond the scope of this book to list andor define all the related methods These methods characterize a huge variety of material properties Some of the more common methods have been included in Figure 11 with the category of properties that they measure The wide diversity of properties of materials that can be measured with AFM is clear The focus of this book is on nanomechanical characterization due to its broad appeal in industrial RampD Again Figure 11 is offered to demonstrate the variety and flavor of properties that can be probed with SPM

12 Atomic Force microscopymdashthe AppeAl to industriAl rampd

This book specifically focuses on AFM which is the method that has most penetrated the general characterization and industrial research fields AFM has key features that make it especially attractive to industrial research and development First its resolution

12 Atomic Force MicroscopymdashThe Appeal to Industrial RampD 3

the heart of its utility as a characterization tool is indeed impressive with 5- to 10-nm lateral resolution and angstrom vertical resolution achieved routinely on commercial instruments with commercial cantilevers The limits of lateral resolution continues to be pushed with specialized techniques and cantilevers and it has reached true ldquoatomicrdquo resolution where point defects of certain materials can now be imaged under certain operating parameters and imaging modes especially under liquid [6ndash8] True atomic resolution is still currently mostly achieved through STM which operates through an entirely different tipndashsample interaction mechanism based on quantum mechanical tunneling Many of these STM studies are conducted in ultrahigh vacuum (UHV) though some are performed in ambient and liquid conditions It is fair to say that AFM is catching up however with recent groundbreaking work imaging cyclic aromatic molecules on Cu[111] [10 11]

A second critical feature of AFM that makes it particularly amenable to industrial research is the flexibility of the environment in which it can operate Despite being a high-resolution microscopy that rivals the resolution of electron microscopy AFM can operate in an ambient or even liquid environment with minimal compromise to its resolution The ability to work in ldquoreal-worldrdquo environments makes it a critical tool for many industrial RampD applications where the research emphasis is consis-tently to understand mechanisms and materials in real-world situations as opposed to

Figure 11 Schematic of AFM as an umbrella technique that includes a wide variety of methods to probe various properties of materials Some of these methods are shown here

4 Chapter 1 Overview of Atomic Force Microscopy

idealized materials and conditions that typically exist in academic research endeavors In many cases the researcher may not want the sample to be forced into a pristine vacuum environment where perhaps key materials or components will be evacuated and thus missed in the characterization effort In addition the flexibility of an envi-ronment permits in situ characterization of processes For example processes such as corrosion lubrication catalyst dissolution crystal growth (for active pharmaceutical ingredient API) can all be studied in situ with AFM as a unique attribute of this microscopy The power of in situ measurements for biological applications is simple as many biological processes and materials simple cannot survive ex situ

A corollary of the utility of the AFMrsquos flexible environment to industrial RampD is the flexibility of type and shape of surface or material that AFM can image There are virtually no restrictions on the size of a material that can be probed with AFM Many commercial ldquolarge-samplerdquo AFMs currently exist that can image any design or shape of a surface from engine parts to thin films Indeed one of the AFMs built in the early 1990s (Topometrix Explorer) was advertised as being able to be placed on top of jet wings to image fractures In addition there are practically no mechanical or electrical restrictions on the type of sample an AFM can image Unlike electron microscopy or STM that require conducting or semiconducting surfaces to image or avoid artifacts [or in the case of insulating surface in scanning electron microscopy (SEM) that requires deposition of a thin conducting layer such as Cr] AFM can image insulating surfaces in addition to conducting or semiconducting surfaces with minimal sample prep Typically the most significant requirement on the sample for an AFM is that it be smooth Commercial instruments will have a maximum z (vertical) range that can be imaged dictated by the design and desired resolution of the particular instrument The maximum z range is typically from 1 microm (for high-resolution studies) to several microns (for lower resolution studies) meaning that the sample cannot have features a tilt or overall roughness that exceed that limit in order for the AFM to image effectively Smooth surfaces are often prepared with the use of an ultramicrotome An ultramicrotome is simply a very controlled and precise way of cutting of thin (down to 100 nm) sections at controlled speeds and in a controlled fashion to result in ultrasmooth surfaces of nanometer or less than nanometer rough-enss The ultramicrotome typically employs the use of a glass knife for a coarse cut and then a diamond knife for a fine cut Cryomicrotomy is simply ultramicrotomy at cold temperatures Cryomicrotomy is typically used to prepare polymer surfaces to cut below the glass transition temperature of the material depending on the instru-ment cryomicrotomes can cool down to -180degC

Finally AFM is fundamentally a mechanical probe of the surface through actual contact between the tip and sample So in addition to the ability to provide high resolution on surface features the goal of any form of microscopy is the unique tipndashsample interaction mechanism enabling the probing of many properties most obvious of which is mechanical properties Mechanical properties covers a wide range of materials and properties Entire textbooks and courses are devoted to the studies of mechanics for example by Callister and Rethwisch [12] We very briefly review some of the main concepts below to provide enough of a background for the rest of the chapter

13 Mechanical Properties 5

This edition focuses specifically on nanomechanical characterization Probing other material properties such as optical electrical or magnetic properties is not covered in this book The reason is that though these other properties have been successfully probed in various industrial RampD applications using AFM to probe mechanical properties of surfaces is more ubiquitous in industrial RampD and so was a logical focus for a first book on SPM in industrial applications

13 mechAnicAl properties

When one considers mechanical properties a range of descriptors come to mind including ldquostiffnessrdquo ldquotoughnessrdquo ldquohardnessrdquo ldquobrittlenessrdquo and ldquoductilityrdquo to name a few Each of these properties has their own mathematical expressions with associ-ated experimental methods of measuring them An interaction between two materials can be typically categorized into either elastic (or recoverable) or dissipative or plastic (permanent or nonrecoverable) interaction On the bulk scale most mechanical analysis of materials relies on measuring their strain (material displacement) as a function of stress (pressure exerted on the material) or vica versa The measurement and analysis of stress versus strain behavior of various materials is a rich and active field and there are many excellent textbooks that describe it [12] The description provided below is simplified to provide a context for these measurements ultimately described on the nanoscale

Elastic deformation of a material is typically nonpermanent or reversible and exists in a regime where the strain (material displacement) is proportional to the stress (pressure exerted on the material) by a proportionality factor known as a modulus (though in reality no material is truly only elastic) Elastic materials thus typically follow Hookersquos law Figure 12 shows a representative stress versus strain curve for materials highlighting the elastic region (blue) Additionally in elastic

Plastic

Fracture

Strain

Elastic

Str

ess

Figure 12 Basic stress vs strain curve showing three mechanical regimes of elastic deformation plastic deformation and fracture For color details please see color plate section

6 Chapter 1 Overview of Atomic Force Microscopy

materials the loading curve (as force is applied) is identical to the unloading curve (as the force is removed) Modulus is a key parameter used to describe elastic behavior Depending on the direction in which a material is being stressed the material will have different moduli (eg a tensile modulus a shear modulus etc)

Plastic deformation is on the other end permanent and nonrecoverable Plastic deformation does not follow Hookersquos law and so stress is not proportional to strain In Figure 12 the region of plastic deformation is shown in green In contrast to elastic deformation the loading curve is not identical to the unloading curve and in fact the unloading curve returns to a nonzero strain at a zero stress level There are many common properties associated with plastic deformation including yielding (onset of plastic deformation) ductility (plastic deformation at a fracture) and hardness (resistance to localized plastic deformation)

Finally there is another very important class of materials called viscoelastic materials where the elastic properties are frequency dependent For example when a stress such as pulling is exerted on the material the materialrsquos response is rate dependent Perhaps the most familiar viscoelastic material to most readers from childhood is Silly Putty where its response to pulling depends on how fast it is pulled If Silly Putty is pulled very slowly the material stretches out very nicely If Silly Putty is pulled very fast it will quickly break Polymers and rubbers two materials that play a significant role in consumer products and which are character-ized quite heavily with AFM are also viscoelastic materials

14 overvieW oF AFm operAtion

141 AFm hardware

A schematic of AFM hardware is shown in Figure 13 The main components of the AFM are (1) the cantilever (2) optical detection system (3) x-y-z scanner and (4) feedback loop

142 cantilevers and probes

The heart of the AFM lies in its cantileverprobe assembly that interacts or probes materials to provide the information of interest The technology to fabricate AFM can-tilevers takes advantage of technology developed for the semiconductor industry to make similar scale devices and features out of silicon Thus today AFM cantilevers are typically made by micromachining silicon (single-crystal Si) or silicon nitride (Si

3N

4) The dimensions of a cantilever vary and dictate the stiffness or spring

constant of the cantilever Depending on the material that needs to be probed and the mode of operation one might pick a lever with a stiffer (higher) or softer (lower) spring constant Cantilevers are generally either rectangular in geometry (Si or Si

3N

4)

or triangular (Si3N

4) with now hybrid cantilevers being manufactured (eg Si tips on

SiN cantilevers) Generally the Si3N

4 probes have lower force constants and might be

used for imaging in liquid among other applications whereas the Si probes can be brittle and tend to break or chip in contact when scanning a surface Typically the

14 Overview of AFM Operation 7

cantilevers are hundreds of microns in length tens of microns in width and a few microns in thickness for reference the diameter of a human hair is approximately 100 microm Cantilevers can be coated with gold or aluminum to provide high reflectivity Commercial cantilevers are also available with other thin metal coatings for magnetic or electrical imaging Cantilevers and probes can also be functionalized chemically or biologically for specific interactions

Probes are grown at the end of the lever and vary in shape and sharpness A sharper probe can often improve the lateral resolution of the AFM Probe design is a continuously evolving field as manufacturers strive for the most robust reproducible and sharp tips Cantileverprobe assemblies are mounted into a cantilever holder that typically includes a piezoceramic element (described below) that can vibrate the can-tilever at a given frequencymdasheither above at or below its resonancemdashdepending on the desired tipndashsample interaction or AFM mode

143 optical detection system

In order to track the motion of the cantileverprobe assembly as it scans a surface an AFM typically includes an optical detection system that consists of a laser reflected off the back side of the cantilever and directed toward a position sensitive detector

Figure 13 Schematic of basic operating principles of AFM A laser is focused and directed off the back end of a cantilever and directed toward a position-sensitive director (PSD) to monitor its vertical and lateral motion as it interacts with the surface Depending on the mode used to image the sample the cantilever can be vibrated with a shake piezo for dynamic imaging modes that involve the resonance of the cantilever

8 Chapter 1 Overview of Atomic Force Microscopy

(PSD) as seen in Figure 13 Optical detection systems are the most common way to track cantilever motion in commercial AFM systems However other methods exist to bypass optical detection systems (in eg a dark medium where the laser light would be absorbed) with self-actuated cantilevers where their motion is read by piezos integrated into the levers The laser is typically a visible photodiode though some commercial instruments have implemented superluminescent diode (SLD) laser The laser is detected by a four-segment position-sensitive detector (PSD) which can track the vertical and lateral motion of the cantilever accurately which can be converted to units of nanometers or displacement through careful calibration described below

144 x-y-z scanner

An AFM can operate either by scanning the sample relative to a stationary tip (sample scanning) or scanning the tip relative to a stationary sample (tip scanning) Each has its own advantages in terms of size of sample it can accommodate (tip scanning more flexible) ability to accessorize (tip scanning again more flexible) and stability and signal to noise (sample scanning easier to build with better specifications) Each shares the requirement to move the tip relative to the sample in a highly accurate way with minimal noise

Tipndashsample motion is often accomplished in commercial instruments by piezoelectric materials which are materials that respond either by expansion or con-traction in response to an applied voltage These kinds of materials provide the ability for very fine motion (nanometers to microns) Piezoelectric materials can be made in several shapes including tubes disks and bimorphs depending on the range and geom-etry of motion required Although piezoelectric materials are very effective at moving tip or sample they are plagued by nonlinear behavior such as hysteresis and creep that has serious consequences for accurate AFM imaging and interpretation of data Artifacts induced by piezoelectric materials are reviewed in detail in Chapter 6

Piezoelectric scanners can be programmed to move with either open-loop or closed-loop feedback circuits In open-loop scanners the piezo is moved by programs to ramp x and y bias voltages to move the prescribed amounts So in open-loop scan-ners if the piezo has to move a large distance or move around on the sample there can be considerable hysteresis of creep with the best recourse of patience or movingzooming in and out in small increments since hysteresiscreep is proportional to the displacement or motion the scanner is trying to execute

One solution to this problem is to use closed-loop scanners that use additional x-y-z sensors to independently measure and then correct for scanner movement In closed-loop operation feedback circuits are continually adjusted to the applied scanner biases to correct for the x-y motion and to achieve the desired motion It is important to note that correction of x-y motion in an AFM is slightly different than correction of z motion This is due to the fact that x-y motion can be predicted whereas z motion cannot as it depends on the surface topography of the sample being scanned Therefore typically there are active feedback circuits to control x and y

xix

Acknowledgments

I have been blessed to have wonderful colleagues to share ideas with and gain inspiration from during this process First and foremost I must thank my co-authors on this book Working with each and every author has been an absolute pleasure I had the distinct pleasure of working closely with Andy Tsou and Alan Schilowitz for many years at ExxonMobil Some of the other co-authors I have been fortunate to count as longstanding collaborators including Rob Carpick Greg Haugstad Jason Killgore and Donna Hurley I thank them for their support and enthusiastic partic-ipation for this particular project From all I learned so much technically and made many new friends in the process This book would not have been possible without everyonersquos collective hard work to share their insights and wisdom with us

I would like to thank my colleagues at ExxonMobil with whom I have had the pleasure of developing our AFM capabilities in our AFM lab Specifically Jean Grabowski deserves a special callout and has been a tremendous asset to our efforts as well as a genuine friend along the way I also would like to thank Rebecca Locker Ishita Chakraborty and Daniel Kiracofe I also would like to thank Steve Minne and Roger Proksch for help and guidance in the incipient stages of this project as well as Emily Rapalino and Susan Klein for advice and proofreading

Most of all I have to thank my husband Andrew for his absolute unwavering support and cheerleading throughout the past year and a half His confidence in me was a source of real motivation and inspiration to carry forward despite obstacles And logistically of course I have to thank him for providing childcare to our chil-dren in the evenings and weekends (covering for me on many many bath nights and storyreadingndashndashsuch as on this night) so that I could complete this book one which hopefully one day they might be interested in enough to read and understand

1

Scanning Probe Microscopy in Industrial Applications Nanomechanical Characterization First Edition Edited by Dalia G Yablon copy 2014 John Wiley amp Sons Inc Published 2014 by John Wiley amp Sons Inc

Overview of Atomic Force MicroscopyDalia G Yablon

SurfaceChar LLC Sharon MA

Atomic force microscopy (AFM) is a family of nanoscale characterization techniques that has exploded onto the overall characterization and nanotechnology field as its versatility and high resolution continue to feed a dizzying variety of disciplines from biology to physics and chemistry to engineering AFM entered the scientific arena in 1981 with the now famous invention of its older sibling in the scanning probe micro-scopy family the scanning tunneling microscope (STM) in the IBM Zurich labs of Gerd Binnig and Heinrich Rohrer for which they received the Nobel Prize in Physics in 1986 The AFM was then invented in 1986 by Gerd Binnig Cal Quate and Christoph Gerber [1] Together STM and AFM formed the scanning probe micros-copy (SPM) family which includes other methods such as near-field scanning optical microscopy (NSOM)

The STM spawned the next 25 years of the continuously developing field of scanning probe microscopy and specifically atomic force microscopy which now includes dozens of different methods under its name to probe various propertiesmdashincluding mechanical electrical magnetic chemical and opticalmdashof materials and surfaces Atomic force microscopy is a powerful tool in various research enterprises It is found in practically any university characterization facility alongside optical and electron microscopes and most undergraduates in science or engineering fields at this point are at least familiar with it if not have performed a laboratory experiment in their undergraduate curriculum

The focus of this book is to understand and appreciate the role this young technique has played in industrial research and development (RampD) AFM has penetrated into a variety of industrial research sectors as witnessed by the diverse set of applications described in this book Alongside electron and optical microscopy which have been

Chapter 1

2 Chapter 1 Overview of Atomic Force Microscopy

around for decades and have reached an impressive level of commercial maturity and ease of use AFM has become a vital characterization method despite its youth and continued technical evolution So though AFM is still an active area of academic research as its capabilities continue to develop and be better understood it has proven to be a useful microscopy to address industrial and commercial needs from quality control and assurance to product formulation and process monitoring

The goal of this introductory chapter is to provide an overview of AFM to nonspecialists and introduce the various topics that are the subject of subsequent individual chapters As such this chapter will provide a brief review of the begin-nings of AFM and special features that make it particularly suited for industrial research Then a brief overview of AFM operation will be presented including the hardware software calibrations involved and finally the different nanomechanical methods that will be described in detail both in theory and application Entire books are written on AFM operation and this chapter is not meant to be an exhaustive introduction to its operation merely serving to provide enough information for the rest of the book to be followed intelligently Readers interested in more detail about AFM operation can consult a number of excellent books on the topic [2ndash5]

11 A Word on nomenclAture

Before the rest of this chapter continues some definitions are in order Similar to many other surface science techniques AFM has succumbed to a somewhat unwieldy mess of abbreviations and jargon that has become a hard-to-navigate alphabet soup SPM refers to an umbrella of a variety of methods Methods that fall under SPM include per-haps its most famous member atomic force microscopy (AFM) in addition to others such as scanning tunneling microscopy (STM) near-field scanning optical microscopy (NSOM or SNOM depending on the continent) and other lithographic methods And then within AFM there are dozens of methods that rely on the AFM probendashsample interactions to provide a variety of material properties including electrical optical magnetic and mechanical properties To date there are dozens of SPMAFM-based methods It is beyond the scope of this book to list andor define all the related methods These methods characterize a huge variety of material properties Some of the more common methods have been included in Figure 11 with the category of properties that they measure The wide diversity of properties of materials that can be measured with AFM is clear The focus of this book is on nanomechanical characterization due to its broad appeal in industrial RampD Again Figure 11 is offered to demonstrate the variety and flavor of properties that can be probed with SPM

12 Atomic Force microscopymdashthe AppeAl to industriAl rampd

This book specifically focuses on AFM which is the method that has most penetrated the general characterization and industrial research fields AFM has key features that make it especially attractive to industrial research and development First its resolution

12 Atomic Force MicroscopymdashThe Appeal to Industrial RampD 3

the heart of its utility as a characterization tool is indeed impressive with 5- to 10-nm lateral resolution and angstrom vertical resolution achieved routinely on commercial instruments with commercial cantilevers The limits of lateral resolution continues to be pushed with specialized techniques and cantilevers and it has reached true ldquoatomicrdquo resolution where point defects of certain materials can now be imaged under certain operating parameters and imaging modes especially under liquid [6ndash8] True atomic resolution is still currently mostly achieved through STM which operates through an entirely different tipndashsample interaction mechanism based on quantum mechanical tunneling Many of these STM studies are conducted in ultrahigh vacuum (UHV) though some are performed in ambient and liquid conditions It is fair to say that AFM is catching up however with recent groundbreaking work imaging cyclic aromatic molecules on Cu[111] [10 11]

A second critical feature of AFM that makes it particularly amenable to industrial research is the flexibility of the environment in which it can operate Despite being a high-resolution microscopy that rivals the resolution of electron microscopy AFM can operate in an ambient or even liquid environment with minimal compromise to its resolution The ability to work in ldquoreal-worldrdquo environments makes it a critical tool for many industrial RampD applications where the research emphasis is consis-tently to understand mechanisms and materials in real-world situations as opposed to

Figure 11 Schematic of AFM as an umbrella technique that includes a wide variety of methods to probe various properties of materials Some of these methods are shown here

4 Chapter 1 Overview of Atomic Force Microscopy

idealized materials and conditions that typically exist in academic research endeavors In many cases the researcher may not want the sample to be forced into a pristine vacuum environment where perhaps key materials or components will be evacuated and thus missed in the characterization effort In addition the flexibility of an envi-ronment permits in situ characterization of processes For example processes such as corrosion lubrication catalyst dissolution crystal growth (for active pharmaceutical ingredient API) can all be studied in situ with AFM as a unique attribute of this microscopy The power of in situ measurements for biological applications is simple as many biological processes and materials simple cannot survive ex situ

A corollary of the utility of the AFMrsquos flexible environment to industrial RampD is the flexibility of type and shape of surface or material that AFM can image There are virtually no restrictions on the size of a material that can be probed with AFM Many commercial ldquolarge-samplerdquo AFMs currently exist that can image any design or shape of a surface from engine parts to thin films Indeed one of the AFMs built in the early 1990s (Topometrix Explorer) was advertised as being able to be placed on top of jet wings to image fractures In addition there are practically no mechanical or electrical restrictions on the type of sample an AFM can image Unlike electron microscopy or STM that require conducting or semiconducting surfaces to image or avoid artifacts [or in the case of insulating surface in scanning electron microscopy (SEM) that requires deposition of a thin conducting layer such as Cr] AFM can image insulating surfaces in addition to conducting or semiconducting surfaces with minimal sample prep Typically the most significant requirement on the sample for an AFM is that it be smooth Commercial instruments will have a maximum z (vertical) range that can be imaged dictated by the design and desired resolution of the particular instrument The maximum z range is typically from 1 microm (for high-resolution studies) to several microns (for lower resolution studies) meaning that the sample cannot have features a tilt or overall roughness that exceed that limit in order for the AFM to image effectively Smooth surfaces are often prepared with the use of an ultramicrotome An ultramicrotome is simply a very controlled and precise way of cutting of thin (down to 100 nm) sections at controlled speeds and in a controlled fashion to result in ultrasmooth surfaces of nanometer or less than nanometer rough-enss The ultramicrotome typically employs the use of a glass knife for a coarse cut and then a diamond knife for a fine cut Cryomicrotomy is simply ultramicrotomy at cold temperatures Cryomicrotomy is typically used to prepare polymer surfaces to cut below the glass transition temperature of the material depending on the instru-ment cryomicrotomes can cool down to -180degC

Finally AFM is fundamentally a mechanical probe of the surface through actual contact between the tip and sample So in addition to the ability to provide high resolution on surface features the goal of any form of microscopy is the unique tipndashsample interaction mechanism enabling the probing of many properties most obvious of which is mechanical properties Mechanical properties covers a wide range of materials and properties Entire textbooks and courses are devoted to the studies of mechanics for example by Callister and Rethwisch [12] We very briefly review some of the main concepts below to provide enough of a background for the rest of the chapter

13 Mechanical Properties 5

This edition focuses specifically on nanomechanical characterization Probing other material properties such as optical electrical or magnetic properties is not covered in this book The reason is that though these other properties have been successfully probed in various industrial RampD applications using AFM to probe mechanical properties of surfaces is more ubiquitous in industrial RampD and so was a logical focus for a first book on SPM in industrial applications

13 mechAnicAl properties

When one considers mechanical properties a range of descriptors come to mind including ldquostiffnessrdquo ldquotoughnessrdquo ldquohardnessrdquo ldquobrittlenessrdquo and ldquoductilityrdquo to name a few Each of these properties has their own mathematical expressions with associ-ated experimental methods of measuring them An interaction between two materials can be typically categorized into either elastic (or recoverable) or dissipative or plastic (permanent or nonrecoverable) interaction On the bulk scale most mechanical analysis of materials relies on measuring their strain (material displacement) as a function of stress (pressure exerted on the material) or vica versa The measurement and analysis of stress versus strain behavior of various materials is a rich and active field and there are many excellent textbooks that describe it [12] The description provided below is simplified to provide a context for these measurements ultimately described on the nanoscale

Elastic deformation of a material is typically nonpermanent or reversible and exists in a regime where the strain (material displacement) is proportional to the stress (pressure exerted on the material) by a proportionality factor known as a modulus (though in reality no material is truly only elastic) Elastic materials thus typically follow Hookersquos law Figure 12 shows a representative stress versus strain curve for materials highlighting the elastic region (blue) Additionally in elastic

Plastic

Fracture

Strain

Elastic

Str

ess

Figure 12 Basic stress vs strain curve showing three mechanical regimes of elastic deformation plastic deformation and fracture For color details please see color plate section

6 Chapter 1 Overview of Atomic Force Microscopy

materials the loading curve (as force is applied) is identical to the unloading curve (as the force is removed) Modulus is a key parameter used to describe elastic behavior Depending on the direction in which a material is being stressed the material will have different moduli (eg a tensile modulus a shear modulus etc)

Plastic deformation is on the other end permanent and nonrecoverable Plastic deformation does not follow Hookersquos law and so stress is not proportional to strain In Figure 12 the region of plastic deformation is shown in green In contrast to elastic deformation the loading curve is not identical to the unloading curve and in fact the unloading curve returns to a nonzero strain at a zero stress level There are many common properties associated with plastic deformation including yielding (onset of plastic deformation) ductility (plastic deformation at a fracture) and hardness (resistance to localized plastic deformation)

Finally there is another very important class of materials called viscoelastic materials where the elastic properties are frequency dependent For example when a stress such as pulling is exerted on the material the materialrsquos response is rate dependent Perhaps the most familiar viscoelastic material to most readers from childhood is Silly Putty where its response to pulling depends on how fast it is pulled If Silly Putty is pulled very slowly the material stretches out very nicely If Silly Putty is pulled very fast it will quickly break Polymers and rubbers two materials that play a significant role in consumer products and which are character-ized quite heavily with AFM are also viscoelastic materials

14 overvieW oF AFm operAtion

141 AFm hardware

A schematic of AFM hardware is shown in Figure 13 The main components of the AFM are (1) the cantilever (2) optical detection system (3) x-y-z scanner and (4) feedback loop

142 cantilevers and probes

The heart of the AFM lies in its cantileverprobe assembly that interacts or probes materials to provide the information of interest The technology to fabricate AFM can-tilevers takes advantage of technology developed for the semiconductor industry to make similar scale devices and features out of silicon Thus today AFM cantilevers are typically made by micromachining silicon (single-crystal Si) or silicon nitride (Si

3N

4) The dimensions of a cantilever vary and dictate the stiffness or spring

constant of the cantilever Depending on the material that needs to be probed and the mode of operation one might pick a lever with a stiffer (higher) or softer (lower) spring constant Cantilevers are generally either rectangular in geometry (Si or Si

3N

4)

or triangular (Si3N

4) with now hybrid cantilevers being manufactured (eg Si tips on

SiN cantilevers) Generally the Si3N

4 probes have lower force constants and might be

used for imaging in liquid among other applications whereas the Si probes can be brittle and tend to break or chip in contact when scanning a surface Typically the

14 Overview of AFM Operation 7

cantilevers are hundreds of microns in length tens of microns in width and a few microns in thickness for reference the diameter of a human hair is approximately 100 microm Cantilevers can be coated with gold or aluminum to provide high reflectivity Commercial cantilevers are also available with other thin metal coatings for magnetic or electrical imaging Cantilevers and probes can also be functionalized chemically or biologically for specific interactions

Probes are grown at the end of the lever and vary in shape and sharpness A sharper probe can often improve the lateral resolution of the AFM Probe design is a continuously evolving field as manufacturers strive for the most robust reproducible and sharp tips Cantileverprobe assemblies are mounted into a cantilever holder that typically includes a piezoceramic element (described below) that can vibrate the can-tilever at a given frequencymdasheither above at or below its resonancemdashdepending on the desired tipndashsample interaction or AFM mode

143 optical detection system

In order to track the motion of the cantileverprobe assembly as it scans a surface an AFM typically includes an optical detection system that consists of a laser reflected off the back side of the cantilever and directed toward a position sensitive detector

Figure 13 Schematic of basic operating principles of AFM A laser is focused and directed off the back end of a cantilever and directed toward a position-sensitive director (PSD) to monitor its vertical and lateral motion as it interacts with the surface Depending on the mode used to image the sample the cantilever can be vibrated with a shake piezo for dynamic imaging modes that involve the resonance of the cantilever

8 Chapter 1 Overview of Atomic Force Microscopy

(PSD) as seen in Figure 13 Optical detection systems are the most common way to track cantilever motion in commercial AFM systems However other methods exist to bypass optical detection systems (in eg a dark medium where the laser light would be absorbed) with self-actuated cantilevers where their motion is read by piezos integrated into the levers The laser is typically a visible photodiode though some commercial instruments have implemented superluminescent diode (SLD) laser The laser is detected by a four-segment position-sensitive detector (PSD) which can track the vertical and lateral motion of the cantilever accurately which can be converted to units of nanometers or displacement through careful calibration described below

144 x-y-z scanner

An AFM can operate either by scanning the sample relative to a stationary tip (sample scanning) or scanning the tip relative to a stationary sample (tip scanning) Each has its own advantages in terms of size of sample it can accommodate (tip scanning more flexible) ability to accessorize (tip scanning again more flexible) and stability and signal to noise (sample scanning easier to build with better specifications) Each shares the requirement to move the tip relative to the sample in a highly accurate way with minimal noise

Tipndashsample motion is often accomplished in commercial instruments by piezoelectric materials which are materials that respond either by expansion or con-traction in response to an applied voltage These kinds of materials provide the ability for very fine motion (nanometers to microns) Piezoelectric materials can be made in several shapes including tubes disks and bimorphs depending on the range and geom-etry of motion required Although piezoelectric materials are very effective at moving tip or sample they are plagued by nonlinear behavior such as hysteresis and creep that has serious consequences for accurate AFM imaging and interpretation of data Artifacts induced by piezoelectric materials are reviewed in detail in Chapter 6

Piezoelectric scanners can be programmed to move with either open-loop or closed-loop feedback circuits In open-loop scanners the piezo is moved by programs to ramp x and y bias voltages to move the prescribed amounts So in open-loop scan-ners if the piezo has to move a large distance or move around on the sample there can be considerable hysteresis of creep with the best recourse of patience or movingzooming in and out in small increments since hysteresiscreep is proportional to the displacement or motion the scanner is trying to execute

One solution to this problem is to use closed-loop scanners that use additional x-y-z sensors to independently measure and then correct for scanner movement In closed-loop operation feedback circuits are continually adjusted to the applied scanner biases to correct for the x-y motion and to achieve the desired motion It is important to note that correction of x-y motion in an AFM is slightly different than correction of z motion This is due to the fact that x-y motion can be predicted whereas z motion cannot as it depends on the surface topography of the sample being scanned Therefore typically there are active feedback circuits to control x and y

1

Scanning Probe Microscopy in Industrial Applications Nanomechanical Characterization First Edition Edited by Dalia G Yablon copy 2014 John Wiley amp Sons Inc Published 2014 by John Wiley amp Sons Inc

Overview of Atomic Force MicroscopyDalia G Yablon

SurfaceChar LLC Sharon MA

Atomic force microscopy (AFM) is a family of nanoscale characterization techniques that has exploded onto the overall characterization and nanotechnology field as its versatility and high resolution continue to feed a dizzying variety of disciplines from biology to physics and chemistry to engineering AFM entered the scientific arena in 1981 with the now famous invention of its older sibling in the scanning probe micro-scopy family the scanning tunneling microscope (STM) in the IBM Zurich labs of Gerd Binnig and Heinrich Rohrer for which they received the Nobel Prize in Physics in 1986 The AFM was then invented in 1986 by Gerd Binnig Cal Quate and Christoph Gerber [1] Together STM and AFM formed the scanning probe micros-copy (SPM) family which includes other methods such as near-field scanning optical microscopy (NSOM)

The STM spawned the next 25 years of the continuously developing field of scanning probe microscopy and specifically atomic force microscopy which now includes dozens of different methods under its name to probe various propertiesmdashincluding mechanical electrical magnetic chemical and opticalmdashof materials and surfaces Atomic force microscopy is a powerful tool in various research enterprises It is found in practically any university characterization facility alongside optical and electron microscopes and most undergraduates in science or engineering fields at this point are at least familiar with it if not have performed a laboratory experiment in their undergraduate curriculum

The focus of this book is to understand and appreciate the role this young technique has played in industrial research and development (RampD) AFM has penetrated into a variety of industrial research sectors as witnessed by the diverse set of applications described in this book Alongside electron and optical microscopy which have been

Chapter 1

2 Chapter 1 Overview of Atomic Force Microscopy

around for decades and have reached an impressive level of commercial maturity and ease of use AFM has become a vital characterization method despite its youth and continued technical evolution So though AFM is still an active area of academic research as its capabilities continue to develop and be better understood it has proven to be a useful microscopy to address industrial and commercial needs from quality control and assurance to product formulation and process monitoring

The goal of this introductory chapter is to provide an overview of AFM to nonspecialists and introduce the various topics that are the subject of subsequent individual chapters As such this chapter will provide a brief review of the begin-nings of AFM and special features that make it particularly suited for industrial research Then a brief overview of AFM operation will be presented including the hardware software calibrations involved and finally the different nanomechanical methods that will be described in detail both in theory and application Entire books are written on AFM operation and this chapter is not meant to be an exhaustive introduction to its operation merely serving to provide enough information for the rest of the book to be followed intelligently Readers interested in more detail about AFM operation can consult a number of excellent books on the topic [2ndash5]

11 A Word on nomenclAture

Before the rest of this chapter continues some definitions are in order Similar to many other surface science techniques AFM has succumbed to a somewhat unwieldy mess of abbreviations and jargon that has become a hard-to-navigate alphabet soup SPM refers to an umbrella of a variety of methods Methods that fall under SPM include per-haps its most famous member atomic force microscopy (AFM) in addition to others such as scanning tunneling microscopy (STM) near-field scanning optical microscopy (NSOM or SNOM depending on the continent) and other lithographic methods And then within AFM there are dozens of methods that rely on the AFM probendashsample interactions to provide a variety of material properties including electrical optical magnetic and mechanical properties To date there are dozens of SPMAFM-based methods It is beyond the scope of this book to list andor define all the related methods These methods characterize a huge variety of material properties Some of the more common methods have been included in Figure 11 with the category of properties that they measure The wide diversity of properties of materials that can be measured with AFM is clear The focus of this book is on nanomechanical characterization due to its broad appeal in industrial RampD Again Figure 11 is offered to demonstrate the variety and flavor of properties that can be probed with SPM

12 Atomic Force microscopymdashthe AppeAl to industriAl rampd

This book specifically focuses on AFM which is the method that has most penetrated the general characterization and industrial research fields AFM has key features that make it especially attractive to industrial research and development First its resolution

12 Atomic Force MicroscopymdashThe Appeal to Industrial RampD 3

the heart of its utility as a characterization tool is indeed impressive with 5- to 10-nm lateral resolution and angstrom vertical resolution achieved routinely on commercial instruments with commercial cantilevers The limits of lateral resolution continues to be pushed with specialized techniques and cantilevers and it has reached true ldquoatomicrdquo resolution where point defects of certain materials can now be imaged under certain operating parameters and imaging modes especially under liquid [6ndash8] True atomic resolution is still currently mostly achieved through STM which operates through an entirely different tipndashsample interaction mechanism based on quantum mechanical tunneling Many of these STM studies are conducted in ultrahigh vacuum (UHV) though some are performed in ambient and liquid conditions It is fair to say that AFM is catching up however with recent groundbreaking work imaging cyclic aromatic molecules on Cu[111] [10 11]

A second critical feature of AFM that makes it particularly amenable to industrial research is the flexibility of the environment in which it can operate Despite being a high-resolution microscopy that rivals the resolution of electron microscopy AFM can operate in an ambient or even liquid environment with minimal compromise to its resolution The ability to work in ldquoreal-worldrdquo environments makes it a critical tool for many industrial RampD applications where the research emphasis is consis-tently to understand mechanisms and materials in real-world situations as opposed to

Figure 11 Schematic of AFM as an umbrella technique that includes a wide variety of methods to probe various properties of materials Some of these methods are shown here

4 Chapter 1 Overview of Atomic Force Microscopy

idealized materials and conditions that typically exist in academic research endeavors In many cases the researcher may not want the sample to be forced into a pristine vacuum environment where perhaps key materials or components will be evacuated and thus missed in the characterization effort In addition the flexibility of an envi-ronment permits in situ characterization of processes For example processes such as corrosion lubrication catalyst dissolution crystal growth (for active pharmaceutical ingredient API) can all be studied in situ with AFM as a unique attribute of this microscopy The power of in situ measurements for biological applications is simple as many biological processes and materials simple cannot survive ex situ

A corollary of the utility of the AFMrsquos flexible environment to industrial RampD is the flexibility of type and shape of surface or material that AFM can image There are virtually no restrictions on the size of a material that can be probed with AFM Many commercial ldquolarge-samplerdquo AFMs currently exist that can image any design or shape of a surface from engine parts to thin films Indeed one of the AFMs built in the early 1990s (Topometrix Explorer) was advertised as being able to be placed on top of jet wings to image fractures In addition there are practically no mechanical or electrical restrictions on the type of sample an AFM can image Unlike electron microscopy or STM that require conducting or semiconducting surfaces to image or avoid artifacts [or in the case of insulating surface in scanning electron microscopy (SEM) that requires deposition of a thin conducting layer such as Cr] AFM can image insulating surfaces in addition to conducting or semiconducting surfaces with minimal sample prep Typically the most significant requirement on the sample for an AFM is that it be smooth Commercial instruments will have a maximum z (vertical) range that can be imaged dictated by the design and desired resolution of the particular instrument The maximum z range is typically from 1 microm (for high-resolution studies) to several microns (for lower resolution studies) meaning that the sample cannot have features a tilt or overall roughness that exceed that limit in order for the AFM to image effectively Smooth surfaces are often prepared with the use of an ultramicrotome An ultramicrotome is simply a very controlled and precise way of cutting of thin (down to 100 nm) sections at controlled speeds and in a controlled fashion to result in ultrasmooth surfaces of nanometer or less than nanometer rough-enss The ultramicrotome typically employs the use of a glass knife for a coarse cut and then a diamond knife for a fine cut Cryomicrotomy is simply ultramicrotomy at cold temperatures Cryomicrotomy is typically used to prepare polymer surfaces to cut below the glass transition temperature of the material depending on the instru-ment cryomicrotomes can cool down to -180degC

Finally AFM is fundamentally a mechanical probe of the surface through actual contact between the tip and sample So in addition to the ability to provide high resolution on surface features the goal of any form of microscopy is the unique tipndashsample interaction mechanism enabling the probing of many properties most obvious of which is mechanical properties Mechanical properties covers a wide range of materials and properties Entire textbooks and courses are devoted to the studies of mechanics for example by Callister and Rethwisch [12] We very briefly review some of the main concepts below to provide enough of a background for the rest of the chapter

13 Mechanical Properties 5

This edition focuses specifically on nanomechanical characterization Probing other material properties such as optical electrical or magnetic properties is not covered in this book The reason is that though these other properties have been successfully probed in various industrial RampD applications using AFM to probe mechanical properties of surfaces is more ubiquitous in industrial RampD and so was a logical focus for a first book on SPM in industrial applications

13 mechAnicAl properties

When one considers mechanical properties a range of descriptors come to mind including ldquostiffnessrdquo ldquotoughnessrdquo ldquohardnessrdquo ldquobrittlenessrdquo and ldquoductilityrdquo to name a few Each of these properties has their own mathematical expressions with associ-ated experimental methods of measuring them An interaction between two materials can be typically categorized into either elastic (or recoverable) or dissipative or plastic (permanent or nonrecoverable) interaction On the bulk scale most mechanical analysis of materials relies on measuring their strain (material displacement) as a function of stress (pressure exerted on the material) or vica versa The measurement and analysis of stress versus strain behavior of various materials is a rich and active field and there are many excellent textbooks that describe it [12] The description provided below is simplified to provide a context for these measurements ultimately described on the nanoscale

Elastic deformation of a material is typically nonpermanent or reversible and exists in a regime where the strain (material displacement) is proportional to the stress (pressure exerted on the material) by a proportionality factor known as a modulus (though in reality no material is truly only elastic) Elastic materials thus typically follow Hookersquos law Figure 12 shows a representative stress versus strain curve for materials highlighting the elastic region (blue) Additionally in elastic

Plastic

Fracture

Strain

Elastic

Str

ess

Figure 12 Basic stress vs strain curve showing three mechanical regimes of elastic deformation plastic deformation and fracture For color details please see color plate section

6 Chapter 1 Overview of Atomic Force Microscopy

materials the loading curve (as force is applied) is identical to the unloading curve (as the force is removed) Modulus is a key parameter used to describe elastic behavior Depending on the direction in which a material is being stressed the material will have different moduli (eg a tensile modulus a shear modulus etc)

Plastic deformation is on the other end permanent and nonrecoverable Plastic deformation does not follow Hookersquos law and so stress is not proportional to strain In Figure 12 the region of plastic deformation is shown in green In contrast to elastic deformation the loading curve is not identical to the unloading curve and in fact the unloading curve returns to a nonzero strain at a zero stress level There are many common properties associated with plastic deformation including yielding (onset of plastic deformation) ductility (plastic deformation at a fracture) and hardness (resistance to localized plastic deformation)

Finally there is another very important class of materials called viscoelastic materials where the elastic properties are frequency dependent For example when a stress such as pulling is exerted on the material the materialrsquos response is rate dependent Perhaps the most familiar viscoelastic material to most readers from childhood is Silly Putty where its response to pulling depends on how fast it is pulled If Silly Putty is pulled very slowly the material stretches out very nicely If Silly Putty is pulled very fast it will quickly break Polymers and rubbers two materials that play a significant role in consumer products and which are character-ized quite heavily with AFM are also viscoelastic materials

14 overvieW oF AFm operAtion

141 AFm hardware

A schematic of AFM hardware is shown in Figure 13 The main components of the AFM are (1) the cantilever (2) optical detection system (3) x-y-z scanner and (4) feedback loop

142 cantilevers and probes

The heart of the AFM lies in its cantileverprobe assembly that interacts or probes materials to provide the information of interest The technology to fabricate AFM can-tilevers takes advantage of technology developed for the semiconductor industry to make similar scale devices and features out of silicon Thus today AFM cantilevers are typically made by micromachining silicon (single-crystal Si) or silicon nitride (Si

3N

4) The dimensions of a cantilever vary and dictate the stiffness or spring

constant of the cantilever Depending on the material that needs to be probed and the mode of operation one might pick a lever with a stiffer (higher) or softer (lower) spring constant Cantilevers are generally either rectangular in geometry (Si or Si

3N

4)

or triangular (Si3N

4) with now hybrid cantilevers being manufactured (eg Si tips on

SiN cantilevers) Generally the Si3N

4 probes have lower force constants and might be

used for imaging in liquid among other applications whereas the Si probes can be brittle and tend to break or chip in contact when scanning a surface Typically the

14 Overview of AFM Operation 7

cantilevers are hundreds of microns in length tens of microns in width and a few microns in thickness for reference the diameter of a human hair is approximately 100 microm Cantilevers can be coated with gold or aluminum to provide high reflectivity Commercial cantilevers are also available with other thin metal coatings for magnetic or electrical imaging Cantilevers and probes can also be functionalized chemically or biologically for specific interactions

Probes are grown at the end of the lever and vary in shape and sharpness A sharper probe can often improve the lateral resolution of the AFM Probe design is a continuously evolving field as manufacturers strive for the most robust reproducible and sharp tips Cantileverprobe assemblies are mounted into a cantilever holder that typically includes a piezoceramic element (described below) that can vibrate the can-tilever at a given frequencymdasheither above at or below its resonancemdashdepending on the desired tipndashsample interaction or AFM mode

143 optical detection system

In order to track the motion of the cantileverprobe assembly as it scans a surface an AFM typically includes an optical detection system that consists of a laser reflected off the back side of the cantilever and directed toward a position sensitive detector

Figure 13 Schematic of basic operating principles of AFM A laser is focused and directed off the back end of a cantilever and directed toward a position-sensitive director (PSD) to monitor its vertical and lateral motion as it interacts with the surface Depending on the mode used to image the sample the cantilever can be vibrated with a shake piezo for dynamic imaging modes that involve the resonance of the cantilever

8 Chapter 1 Overview of Atomic Force Microscopy

(PSD) as seen in Figure 13 Optical detection systems are the most common way to track cantilever motion in commercial AFM systems However other methods exist to bypass optical detection systems (in eg a dark medium where the laser light would be absorbed) with self-actuated cantilevers where their motion is read by piezos integrated into the levers The laser is typically a visible photodiode though some commercial instruments have implemented superluminescent diode (SLD) laser The laser is detected by a four-segment position-sensitive detector (PSD) which can track the vertical and lateral motion of the cantilever accurately which can be converted to units of nanometers or displacement through careful calibration described below

144 x-y-z scanner

An AFM can operate either by scanning the sample relative to a stationary tip (sample scanning) or scanning the tip relative to a stationary sample (tip scanning) Each has its own advantages in terms of size of sample it can accommodate (tip scanning more flexible) ability to accessorize (tip scanning again more flexible) and stability and signal to noise (sample scanning easier to build with better specifications) Each shares the requirement to move the tip relative to the sample in a highly accurate way with minimal noise

Tipndashsample motion is often accomplished in commercial instruments by piezoelectric materials which are materials that respond either by expansion or con-traction in response to an applied voltage These kinds of materials provide the ability for very fine motion (nanometers to microns) Piezoelectric materials can be made in several shapes including tubes disks and bimorphs depending on the range and geom-etry of motion required Although piezoelectric materials are very effective at moving tip or sample they are plagued by nonlinear behavior such as hysteresis and creep that has serious consequences for accurate AFM imaging and interpretation of data Artifacts induced by piezoelectric materials are reviewed in detail in Chapter 6

Piezoelectric scanners can be programmed to move with either open-loop or closed-loop feedback circuits In open-loop scanners the piezo is moved by programs to ramp x and y bias voltages to move the prescribed amounts So in open-loop scan-ners if the piezo has to move a large distance or move around on the sample there can be considerable hysteresis of creep with the best recourse of patience or movingzooming in and out in small increments since hysteresiscreep is proportional to the displacement or motion the scanner is trying to execute

One solution to this problem is to use closed-loop scanners that use additional x-y-z sensors to independently measure and then correct for scanner movement In closed-loop operation feedback circuits are continually adjusted to the applied scanner biases to correct for the x-y motion and to achieve the desired motion It is important to note that correction of x-y motion in an AFM is slightly different than correction of z motion This is due to the fact that x-y motion can be predicted whereas z motion cannot as it depends on the surface topography of the sample being scanned Therefore typically there are active feedback circuits to control x and y

2 Chapter 1 Overview of Atomic Force Microscopy

around for decades and have reached an impressive level of commercial maturity and ease of use AFM has become a vital characterization method despite its youth and continued technical evolution So though AFM is still an active area of academic research as its capabilities continue to develop and be better understood it has proven to be a useful microscopy to address industrial and commercial needs from quality control and assurance to product formulation and process monitoring

The goal of this introductory chapter is to provide an overview of AFM to nonspecialists and introduce the various topics that are the subject of subsequent individual chapters As such this chapter will provide a brief review of the begin-nings of AFM and special features that make it particularly suited for industrial research Then a brief overview of AFM operation will be presented including the hardware software calibrations involved and finally the different nanomechanical methods that will be described in detail both in theory and application Entire books are written on AFM operation and this chapter is not meant to be an exhaustive introduction to its operation merely serving to provide enough information for the rest of the book to be followed intelligently Readers interested in more detail about AFM operation can consult a number of excellent books on the topic [2ndash5]

11 A Word on nomenclAture

Before the rest of this chapter continues some definitions are in order Similar to many other surface science techniques AFM has succumbed to a somewhat unwieldy mess of abbreviations and jargon that has become a hard-to-navigate alphabet soup SPM refers to an umbrella of a variety of methods Methods that fall under SPM include per-haps its most famous member atomic force microscopy (AFM) in addition to others such as scanning tunneling microscopy (STM) near-field scanning optical microscopy (NSOM or SNOM depending on the continent) and other lithographic methods And then within AFM there are dozens of methods that rely on the AFM probendashsample interactions to provide a variety of material properties including electrical optical magnetic and mechanical properties To date there are dozens of SPMAFM-based methods It is beyond the scope of this book to list andor define all the related methods These methods characterize a huge variety of material properties Some of the more common methods have been included in Figure 11 with the category of properties that they measure The wide diversity of properties of materials that can be measured with AFM is clear The focus of this book is on nanomechanical characterization due to its broad appeal in industrial RampD Again Figure 11 is offered to demonstrate the variety and flavor of properties that can be probed with SPM

12 Atomic Force microscopymdashthe AppeAl to industriAl rampd

This book specifically focuses on AFM which is the method that has most penetrated the general characterization and industrial research fields AFM has key features that make it especially attractive to industrial research and development First its resolution

12 Atomic Force MicroscopymdashThe Appeal to Industrial RampD 3

the heart of its utility as a characterization tool is indeed impressive with 5- to 10-nm lateral resolution and angstrom vertical resolution achieved routinely on commercial instruments with commercial cantilevers The limits of lateral resolution continues to be pushed with specialized techniques and cantilevers and it has reached true ldquoatomicrdquo resolution where point defects of certain materials can now be imaged under certain operating parameters and imaging modes especially under liquid [6ndash8] True atomic resolution is still currently mostly achieved through STM which operates through an entirely different tipndashsample interaction mechanism based on quantum mechanical tunneling Many of these STM studies are conducted in ultrahigh vacuum (UHV) though some are performed in ambient and liquid conditions It is fair to say that AFM is catching up however with recent groundbreaking work imaging cyclic aromatic molecules on Cu[111] [10 11]

A second critical feature of AFM that makes it particularly amenable to industrial research is the flexibility of the environment in which it can operate Despite being a high-resolution microscopy that rivals the resolution of electron microscopy AFM can operate in an ambient or even liquid environment with minimal compromise to its resolution The ability to work in ldquoreal-worldrdquo environments makes it a critical tool for many industrial RampD applications where the research emphasis is consis-tently to understand mechanisms and materials in real-world situations as opposed to

Figure 11 Schematic of AFM as an umbrella technique that includes a wide variety of methods to probe various properties of materials Some of these methods are shown here

4 Chapter 1 Overview of Atomic Force Microscopy

idealized materials and conditions that typically exist in academic research endeavors In many cases the researcher may not want the sample to be forced into a pristine vacuum environment where perhaps key materials or components will be evacuated and thus missed in the characterization effort In addition the flexibility of an envi-ronment permits in situ characterization of processes For example processes such as corrosion lubrication catalyst dissolution crystal growth (for active pharmaceutical ingredient API) can all be studied in situ with AFM as a unique attribute of this microscopy The power of in situ measurements for biological applications is simple as many biological processes and materials simple cannot survive ex situ

A corollary of the utility of the AFMrsquos flexible environment to industrial RampD is the flexibility of type and shape of surface or material that AFM can image There are virtually no restrictions on the size of a material that can be probed with AFM Many commercial ldquolarge-samplerdquo AFMs currently exist that can image any design or shape of a surface from engine parts to thin films Indeed one of the AFMs built in the early 1990s (Topometrix Explorer) was advertised as being able to be placed on top of jet wings to image fractures In addition there are practically no mechanical or electrical restrictions on the type of sample an AFM can image Unlike electron microscopy or STM that require conducting or semiconducting surfaces to image or avoid artifacts [or in the case of insulating surface in scanning electron microscopy (SEM) that requires deposition of a thin conducting layer such as Cr] AFM can image insulating surfaces in addition to conducting or semiconducting surfaces with minimal sample prep Typically the most significant requirement on the sample for an AFM is that it be smooth Commercial instruments will have a maximum z (vertical) range that can be imaged dictated by the design and desired resolution of the particular instrument The maximum z range is typically from 1 microm (for high-resolution studies) to several microns (for lower resolution studies) meaning that the sample cannot have features a tilt or overall roughness that exceed that limit in order for the AFM to image effectively Smooth surfaces are often prepared with the use of an ultramicrotome An ultramicrotome is simply a very controlled and precise way of cutting of thin (down to 100 nm) sections at controlled speeds and in a controlled fashion to result in ultrasmooth surfaces of nanometer or less than nanometer rough-enss The ultramicrotome typically employs the use of a glass knife for a coarse cut and then a diamond knife for a fine cut Cryomicrotomy is simply ultramicrotomy at cold temperatures Cryomicrotomy is typically used to prepare polymer surfaces to cut below the glass transition temperature of the material depending on the instru-ment cryomicrotomes can cool down to -180degC

Finally AFM is fundamentally a mechanical probe of the surface through actual contact between the tip and sample So in addition to the ability to provide high resolution on surface features the goal of any form of microscopy is the unique tipndashsample interaction mechanism enabling the probing of many properties most obvious of which is mechanical properties Mechanical properties covers a wide range of materials and properties Entire textbooks and courses are devoted to the studies of mechanics for example by Callister and Rethwisch [12] We very briefly review some of the main concepts below to provide enough of a background for the rest of the chapter

13 Mechanical Properties 5

This edition focuses specifically on nanomechanical characterization Probing other material properties such as optical electrical or magnetic properties is not covered in this book The reason is that though these other properties have been successfully probed in various industrial RampD applications using AFM to probe mechanical properties of surfaces is more ubiquitous in industrial RampD and so was a logical focus for a first book on SPM in industrial applications

13 mechAnicAl properties

When one considers mechanical properties a range of descriptors come to mind including ldquostiffnessrdquo ldquotoughnessrdquo ldquohardnessrdquo ldquobrittlenessrdquo and ldquoductilityrdquo to name a few Each of these properties has their own mathematical expressions with associ-ated experimental methods of measuring them An interaction between two materials can be typically categorized into either elastic (or recoverable) or dissipative or plastic (permanent or nonrecoverable) interaction On the bulk scale most mechanical analysis of materials relies on measuring their strain (material displacement) as a function of stress (pressure exerted on the material) or vica versa The measurement and analysis of stress versus strain behavior of various materials is a rich and active field and there are many excellent textbooks that describe it [12] The description provided below is simplified to provide a context for these measurements ultimately described on the nanoscale

Elastic deformation of a material is typically nonpermanent or reversible and exists in a regime where the strain (material displacement) is proportional to the stress (pressure exerted on the material) by a proportionality factor known as a modulus (though in reality no material is truly only elastic) Elastic materials thus typically follow Hookersquos law Figure 12 shows a representative stress versus strain curve for materials highlighting the elastic region (blue) Additionally in elastic

Plastic

Fracture

Strain

Elastic

Str

ess

Figure 12 Basic stress vs strain curve showing three mechanical regimes of elastic deformation plastic deformation and fracture For color details please see color plate section

6 Chapter 1 Overview of Atomic Force Microscopy

materials the loading curve (as force is applied) is identical to the unloading curve (as the force is removed) Modulus is a key parameter used to describe elastic behavior Depending on the direction in which a material is being stressed the material will have different moduli (eg a tensile modulus a shear modulus etc)

Plastic deformation is on the other end permanent and nonrecoverable Plastic deformation does not follow Hookersquos law and so stress is not proportional to strain In Figure 12 the region of plastic deformation is shown in green In contrast to elastic deformation the loading curve is not identical to the unloading curve and in fact the unloading curve returns to a nonzero strain at a zero stress level There are many common properties associated with plastic deformation including yielding (onset of plastic deformation) ductility (plastic deformation at a fracture) and hardness (resistance to localized plastic deformation)

Finally there is another very important class of materials called viscoelastic materials where the elastic properties are frequency dependent For example when a stress such as pulling is exerted on the material the materialrsquos response is rate dependent Perhaps the most familiar viscoelastic material to most readers from childhood is Silly Putty where its response to pulling depends on how fast it is pulled If Silly Putty is pulled very slowly the material stretches out very nicely If Silly Putty is pulled very fast it will quickly break Polymers and rubbers two materials that play a significant role in consumer products and which are character-ized quite heavily with AFM are also viscoelastic materials

14 overvieW oF AFm operAtion

141 AFm hardware

A schematic of AFM hardware is shown in Figure 13 The main components of the AFM are (1) the cantilever (2) optical detection system (3) x-y-z scanner and (4) feedback loop

142 cantilevers and probes

The heart of the AFM lies in its cantileverprobe assembly that interacts or probes materials to provide the information of interest The technology to fabricate AFM can-tilevers takes advantage of technology developed for the semiconductor industry to make similar scale devices and features out of silicon Thus today AFM cantilevers are typically made by micromachining silicon (single-crystal Si) or silicon nitride (Si

3N

4) The dimensions of a cantilever vary and dictate the stiffness or spring

constant of the cantilever Depending on the material that needs to be probed and the mode of operation one might pick a lever with a stiffer (higher) or softer (lower) spring constant Cantilevers are generally either rectangular in geometry (Si or Si

3N

4)

or triangular (Si3N

4) with now hybrid cantilevers being manufactured (eg Si tips on

SiN cantilevers) Generally the Si3N

4 probes have lower force constants and might be

used for imaging in liquid among other applications whereas the Si probes can be brittle and tend to break or chip in contact when scanning a surface Typically the

14 Overview of AFM Operation 7

cantilevers are hundreds of microns in length tens of microns in width and a few microns in thickness for reference the diameter of a human hair is approximately 100 microm Cantilevers can be coated with gold or aluminum to provide high reflectivity Commercial cantilevers are also available with other thin metal coatings for magnetic or electrical imaging Cantilevers and probes can also be functionalized chemically or biologically for specific interactions

Probes are grown at the end of the lever and vary in shape and sharpness A sharper probe can often improve the lateral resolution of the AFM Probe design is a continuously evolving field as manufacturers strive for the most robust reproducible and sharp tips Cantileverprobe assemblies are mounted into a cantilever holder that typically includes a piezoceramic element (described below) that can vibrate the can-tilever at a given frequencymdasheither above at or below its resonancemdashdepending on the desired tipndashsample interaction or AFM mode

143 optical detection system

In order to track the motion of the cantileverprobe assembly as it scans a surface an AFM typically includes an optical detection system that consists of a laser reflected off the back side of the cantilever and directed toward a position sensitive detector

Figure 13 Schematic of basic operating principles of AFM A laser is focused and directed off the back end of a cantilever and directed toward a position-sensitive director (PSD) to monitor its vertical and lateral motion as it interacts with the surface Depending on the mode used to image the sample the cantilever can be vibrated with a shake piezo for dynamic imaging modes that involve the resonance of the cantilever

8 Chapter 1 Overview of Atomic Force Microscopy

(PSD) as seen in Figure 13 Optical detection systems are the most common way to track cantilever motion in commercial AFM systems However other methods exist to bypass optical detection systems (in eg a dark medium where the laser light would be absorbed) with self-actuated cantilevers where their motion is read by piezos integrated into the levers The laser is typically a visible photodiode though some commercial instruments have implemented superluminescent diode (SLD) laser The laser is detected by a four-segment position-sensitive detector (PSD) which can track the vertical and lateral motion of the cantilever accurately which can be converted to units of nanometers or displacement through careful calibration described below

144 x-y-z scanner

An AFM can operate either by scanning the sample relative to a stationary tip (sample scanning) or scanning the tip relative to a stationary sample (tip scanning) Each has its own advantages in terms of size of sample it can accommodate (tip scanning more flexible) ability to accessorize (tip scanning again more flexible) and stability and signal to noise (sample scanning easier to build with better specifications) Each shares the requirement to move the tip relative to the sample in a highly accurate way with minimal noise

Tipndashsample motion is often accomplished in commercial instruments by piezoelectric materials which are materials that respond either by expansion or con-traction in response to an applied voltage These kinds of materials provide the ability for very fine motion (nanometers to microns) Piezoelectric materials can be made in several shapes including tubes disks and bimorphs depending on the range and geom-etry of motion required Although piezoelectric materials are very effective at moving tip or sample they are plagued by nonlinear behavior such as hysteresis and creep that has serious consequences for accurate AFM imaging and interpretation of data Artifacts induced by piezoelectric materials are reviewed in detail in Chapter 6

Piezoelectric scanners can be programmed to move with either open-loop or closed-loop feedback circuits In open-loop scanners the piezo is moved by programs to ramp x and y bias voltages to move the prescribed amounts So in open-loop scan-ners if the piezo has to move a large distance or move around on the sample there can be considerable hysteresis of creep with the best recourse of patience or movingzooming in and out in small increments since hysteresiscreep is proportional to the displacement or motion the scanner is trying to execute

One solution to this problem is to use closed-loop scanners that use additional x-y-z sensors to independently measure and then correct for scanner movement In closed-loop operation feedback circuits are continually adjusted to the applied scanner biases to correct for the x-y motion and to achieve the desired motion It is important to note that correction of x-y motion in an AFM is slightly different than correction of z motion This is due to the fact that x-y motion can be predicted whereas z motion cannot as it depends on the surface topography of the sample being scanned Therefore typically there are active feedback circuits to control x and y

12 Atomic Force MicroscopymdashThe Appeal to Industrial RampD 3

the heart of its utility as a characterization tool is indeed impressive with 5- to 10-nm lateral resolution and angstrom vertical resolution achieved routinely on commercial instruments with commercial cantilevers The limits of lateral resolution continues to be pushed with specialized techniques and cantilevers and it has reached true ldquoatomicrdquo resolution where point defects of certain materials can now be imaged under certain operating parameters and imaging modes especially under liquid [6ndash8] True atomic resolution is still currently mostly achieved through STM which operates through an entirely different tipndashsample interaction mechanism based on quantum mechanical tunneling Many of these STM studies are conducted in ultrahigh vacuum (UHV) though some are performed in ambient and liquid conditions It is fair to say that AFM is catching up however with recent groundbreaking work imaging cyclic aromatic molecules on Cu[111] [10 11]

A second critical feature of AFM that makes it particularly amenable to industrial research is the flexibility of the environment in which it can operate Despite being a high-resolution microscopy that rivals the resolution of electron microscopy AFM can operate in an ambient or even liquid environment with minimal compromise to its resolution The ability to work in ldquoreal-worldrdquo environments makes it a critical tool for many industrial RampD applications where the research emphasis is consis-tently to understand mechanisms and materials in real-world situations as opposed to

Figure 11 Schematic of AFM as an umbrella technique that includes a wide variety of methods to probe various properties of materials Some of these methods are shown here

4 Chapter 1 Overview of Atomic Force Microscopy

idealized materials and conditions that typically exist in academic research endeavors In many cases the researcher may not want the sample to be forced into a pristine vacuum environment where perhaps key materials or components will be evacuated and thus missed in the characterization effort In addition the flexibility of an envi-ronment permits in situ characterization of processes For example processes such as corrosion lubrication catalyst dissolution crystal growth (for active pharmaceutical ingredient API) can all be studied in situ with AFM as a unique attribute of this microscopy The power of in situ measurements for biological applications is simple as many biological processes and materials simple cannot survive ex situ

A corollary of the utility of the AFMrsquos flexible environment to industrial RampD is the flexibility of type and shape of surface or material that AFM can image There are virtually no restrictions on the size of a material that can be probed with AFM Many commercial ldquolarge-samplerdquo AFMs currently exist that can image any design or shape of a surface from engine parts to thin films Indeed one of the AFMs built in the early 1990s (Topometrix Explorer) was advertised as being able to be placed on top of jet wings to image fractures In addition there are practically no mechanical or electrical restrictions on the type of sample an AFM can image Unlike electron microscopy or STM that require conducting or semiconducting surfaces to image or avoid artifacts [or in the case of insulating surface in scanning electron microscopy (SEM) that requires deposition of a thin conducting layer such as Cr] AFM can image insulating surfaces in addition to conducting or semiconducting surfaces with minimal sample prep Typically the most significant requirement on the sample for an AFM is that it be smooth Commercial instruments will have a maximum z (vertical) range that can be imaged dictated by the design and desired resolution of the particular instrument The maximum z range is typically from 1 microm (for high-resolution studies) to several microns (for lower resolution studies) meaning that the sample cannot have features a tilt or overall roughness that exceed that limit in order for the AFM to image effectively Smooth surfaces are often prepared with the use of an ultramicrotome An ultramicrotome is simply a very controlled and precise way of cutting of thin (down to 100 nm) sections at controlled speeds and in a controlled fashion to result in ultrasmooth surfaces of nanometer or less than nanometer rough-enss The ultramicrotome typically employs the use of a glass knife for a coarse cut and then a diamond knife for a fine cut Cryomicrotomy is simply ultramicrotomy at cold temperatures Cryomicrotomy is typically used to prepare polymer surfaces to cut below the glass transition temperature of the material depending on the instru-ment cryomicrotomes can cool down to -180degC

Finally AFM is fundamentally a mechanical probe of the surface through actual contact between the tip and sample So in addition to the ability to provide high resolution on surface features the goal of any form of microscopy is the unique tipndashsample interaction mechanism enabling the probing of many properties most obvious of which is mechanical properties Mechanical properties covers a wide range of materials and properties Entire textbooks and courses are devoted to the studies of mechanics for example by Callister and Rethwisch [12] We very briefly review some of the main concepts below to provide enough of a background for the rest of the chapter

13 Mechanical Properties 5

This edition focuses specifically on nanomechanical characterization Probing other material properties such as optical electrical or magnetic properties is not covered in this book The reason is that though these other properties have been successfully probed in various industrial RampD applications using AFM to probe mechanical properties of surfaces is more ubiquitous in industrial RampD and so was a logical focus for a first book on SPM in industrial applications

13 mechAnicAl properties

When one considers mechanical properties a range of descriptors come to mind including ldquostiffnessrdquo ldquotoughnessrdquo ldquohardnessrdquo ldquobrittlenessrdquo and ldquoductilityrdquo to name a few Each of these properties has their own mathematical expressions with associ-ated experimental methods of measuring them An interaction between two materials can be typically categorized into either elastic (or recoverable) or dissipative or plastic (permanent or nonrecoverable) interaction On the bulk scale most mechanical analysis of materials relies on measuring their strain (material displacement) as a function of stress (pressure exerted on the material) or vica versa The measurement and analysis of stress versus strain behavior of various materials is a rich and active field and there are many excellent textbooks that describe it [12] The description provided below is simplified to provide a context for these measurements ultimately described on the nanoscale

Elastic deformation of a material is typically nonpermanent or reversible and exists in a regime where the strain (material displacement) is proportional to the stress (pressure exerted on the material) by a proportionality factor known as a modulus (though in reality no material is truly only elastic) Elastic materials thus typically follow Hookersquos law Figure 12 shows a representative stress versus strain curve for materials highlighting the elastic region (blue) Additionally in elastic

Plastic

Fracture

Strain

Elastic

Str

ess

Figure 12 Basic stress vs strain curve showing three mechanical regimes of elastic deformation plastic deformation and fracture For color details please see color plate section

6 Chapter 1 Overview of Atomic Force Microscopy

materials the loading curve (as force is applied) is identical to the unloading curve (as the force is removed) Modulus is a key parameter used to describe elastic behavior Depending on the direction in which a material is being stressed the material will have different moduli (eg a tensile modulus a shear modulus etc)

Plastic deformation is on the other end permanent and nonrecoverable Plastic deformation does not follow Hookersquos law and so stress is not proportional to strain In Figure 12 the region of plastic deformation is shown in green In contrast to elastic deformation the loading curve is not identical to the unloading curve and in fact the unloading curve returns to a nonzero strain at a zero stress level There are many common properties associated with plastic deformation including yielding (onset of plastic deformation) ductility (plastic deformation at a fracture) and hardness (resistance to localized plastic deformation)

Finally there is another very important class of materials called viscoelastic materials where the elastic properties are frequency dependent For example when a stress such as pulling is exerted on the material the materialrsquos response is rate dependent Perhaps the most familiar viscoelastic material to most readers from childhood is Silly Putty where its response to pulling depends on how fast it is pulled If Silly Putty is pulled very slowly the material stretches out very nicely If Silly Putty is pulled very fast it will quickly break Polymers and rubbers two materials that play a significant role in consumer products and which are character-ized quite heavily with AFM are also viscoelastic materials

14 overvieW oF AFm operAtion

141 AFm hardware

A schematic of AFM hardware is shown in Figure 13 The main components of the AFM are (1) the cantilever (2) optical detection system (3) x-y-z scanner and (4) feedback loop

142 cantilevers and probes

The heart of the AFM lies in its cantileverprobe assembly that interacts or probes materials to provide the information of interest The technology to fabricate AFM can-tilevers takes advantage of technology developed for the semiconductor industry to make similar scale devices and features out of silicon Thus today AFM cantilevers are typically made by micromachining silicon (single-crystal Si) or silicon nitride (Si

3N

4) The dimensions of a cantilever vary and dictate the stiffness or spring

constant of the cantilever Depending on the material that needs to be probed and the mode of operation one might pick a lever with a stiffer (higher) or softer (lower) spring constant Cantilevers are generally either rectangular in geometry (Si or Si

3N

4)

or triangular (Si3N

4) with now hybrid cantilevers being manufactured (eg Si tips on

SiN cantilevers) Generally the Si3N

4 probes have lower force constants and might be

used for imaging in liquid among other applications whereas the Si probes can be brittle and tend to break or chip in contact when scanning a surface Typically the

14 Overview of AFM Operation 7

cantilevers are hundreds of microns in length tens of microns in width and a few microns in thickness for reference the diameter of a human hair is approximately 100 microm Cantilevers can be coated with gold or aluminum to provide high reflectivity Commercial cantilevers are also available with other thin metal coatings for magnetic or electrical imaging Cantilevers and probes can also be functionalized chemically or biologically for specific interactions

Probes are grown at the end of the lever and vary in shape and sharpness A sharper probe can often improve the lateral resolution of the AFM Probe design is a continuously evolving field as manufacturers strive for the most robust reproducible and sharp tips Cantileverprobe assemblies are mounted into a cantilever holder that typically includes a piezoceramic element (described below) that can vibrate the can-tilever at a given frequencymdasheither above at or below its resonancemdashdepending on the desired tipndashsample interaction or AFM mode

143 optical detection system

In order to track the motion of the cantileverprobe assembly as it scans a surface an AFM typically includes an optical detection system that consists of a laser reflected off the back side of the cantilever and directed toward a position sensitive detector

Figure 13 Schematic of basic operating principles of AFM A laser is focused and directed off the back end of a cantilever and directed toward a position-sensitive director (PSD) to monitor its vertical and lateral motion as it interacts with the surface Depending on the mode used to image the sample the cantilever can be vibrated with a shake piezo for dynamic imaging modes that involve the resonance of the cantilever

8 Chapter 1 Overview of Atomic Force Microscopy

(PSD) as seen in Figure 13 Optical detection systems are the most common way to track cantilever motion in commercial AFM systems However other methods exist to bypass optical detection systems (in eg a dark medium where the laser light would be absorbed) with self-actuated cantilevers where their motion is read by piezos integrated into the levers The laser is typically a visible photodiode though some commercial instruments have implemented superluminescent diode (SLD) laser The laser is detected by a four-segment position-sensitive detector (PSD) which can track the vertical and lateral motion of the cantilever accurately which can be converted to units of nanometers or displacement through careful calibration described below

144 x-y-z scanner

An AFM can operate either by scanning the sample relative to a stationary tip (sample scanning) or scanning the tip relative to a stationary sample (tip scanning) Each has its own advantages in terms of size of sample it can accommodate (tip scanning more flexible) ability to accessorize (tip scanning again more flexible) and stability and signal to noise (sample scanning easier to build with better specifications) Each shares the requirement to move the tip relative to the sample in a highly accurate way with minimal noise

Tipndashsample motion is often accomplished in commercial instruments by piezoelectric materials which are materials that respond either by expansion or con-traction in response to an applied voltage These kinds of materials provide the ability for very fine motion (nanometers to microns) Piezoelectric materials can be made in several shapes including tubes disks and bimorphs depending on the range and geom-etry of motion required Although piezoelectric materials are very effective at moving tip or sample they are plagued by nonlinear behavior such as hysteresis and creep that has serious consequences for accurate AFM imaging and interpretation of data Artifacts induced by piezoelectric materials are reviewed in detail in Chapter 6

Piezoelectric scanners can be programmed to move with either open-loop or closed-loop feedback circuits In open-loop scanners the piezo is moved by programs to ramp x and y bias voltages to move the prescribed amounts So in open-loop scan-ners if the piezo has to move a large distance or move around on the sample there can be considerable hysteresis of creep with the best recourse of patience or movingzooming in and out in small increments since hysteresiscreep is proportional to the displacement or motion the scanner is trying to execute

One solution to this problem is to use closed-loop scanners that use additional x-y-z sensors to independently measure and then correct for scanner movement In closed-loop operation feedback circuits are continually adjusted to the applied scanner biases to correct for the x-y motion and to achieve the desired motion It is important to note that correction of x-y motion in an AFM is slightly different than correction of z motion This is due to the fact that x-y motion can be predicted whereas z motion cannot as it depends on the surface topography of the sample being scanned Therefore typically there are active feedback circuits to control x and y

4 Chapter 1 Overview of Atomic Force Microscopy

idealized materials and conditions that typically exist in academic research endeavors In many cases the researcher may not want the sample to be forced into a pristine vacuum environment where perhaps key materials or components will be evacuated and thus missed in the characterization effort In addition the flexibility of an envi-ronment permits in situ characterization of processes For example processes such as corrosion lubrication catalyst dissolution crystal growth (for active pharmaceutical ingredient API) can all be studied in situ with AFM as a unique attribute of this microscopy The power of in situ measurements for biological applications is simple as many biological processes and materials simple cannot survive ex situ

A corollary of the utility of the AFMrsquos flexible environment to industrial RampD is the flexibility of type and shape of surface or material that AFM can image There are virtually no restrictions on the size of a material that can be probed with AFM Many commercial ldquolarge-samplerdquo AFMs currently exist that can image any design or shape of a surface from engine parts to thin films Indeed one of the AFMs built in the early 1990s (Topometrix Explorer) was advertised as being able to be placed on top of jet wings to image fractures In addition there are practically no mechanical or electrical restrictions on the type of sample an AFM can image Unlike electron microscopy or STM that require conducting or semiconducting surfaces to image or avoid artifacts [or in the case of insulating surface in scanning electron microscopy (SEM) that requires deposition of a thin conducting layer such as Cr] AFM can image insulating surfaces in addition to conducting or semiconducting surfaces with minimal sample prep Typically the most significant requirement on the sample for an AFM is that it be smooth Commercial instruments will have a maximum z (vertical) range that can be imaged dictated by the design and desired resolution of the particular instrument The maximum z range is typically from 1 microm (for high-resolution studies) to several microns (for lower resolution studies) meaning that the sample cannot have features a tilt or overall roughness that exceed that limit in order for the AFM to image effectively Smooth surfaces are often prepared with the use of an ultramicrotome An ultramicrotome is simply a very controlled and precise way of cutting of thin (down to 100 nm) sections at controlled speeds and in a controlled fashion to result in ultrasmooth surfaces of nanometer or less than nanometer rough-enss The ultramicrotome typically employs the use of a glass knife for a coarse cut and then a diamond knife for a fine cut Cryomicrotomy is simply ultramicrotomy at cold temperatures Cryomicrotomy is typically used to prepare polymer surfaces to cut below the glass transition temperature of the material depending on the instru-ment cryomicrotomes can cool down to -180degC

Finally AFM is fundamentally a mechanical probe of the surface through actual contact between the tip and sample So in addition to the ability to provide high resolution on surface features the goal of any form of microscopy is the unique tipndashsample interaction mechanism enabling the probing of many properties most obvious of which is mechanical properties Mechanical properties covers a wide range of materials and properties Entire textbooks and courses are devoted to the studies of mechanics for example by Callister and Rethwisch [12] We very briefly review some of the main concepts below to provide enough of a background for the rest of the chapter

13 Mechanical Properties 5

This edition focuses specifically on nanomechanical characterization Probing other material properties such as optical electrical or magnetic properties is not covered in this book The reason is that though these other properties have been successfully probed in various industrial RampD applications using AFM to probe mechanical properties of surfaces is more ubiquitous in industrial RampD and so was a logical focus for a first book on SPM in industrial applications

13 mechAnicAl properties

When one considers mechanical properties a range of descriptors come to mind including ldquostiffnessrdquo ldquotoughnessrdquo ldquohardnessrdquo ldquobrittlenessrdquo and ldquoductilityrdquo to name a few Each of these properties has their own mathematical expressions with associ-ated experimental methods of measuring them An interaction between two materials can be typically categorized into either elastic (or recoverable) or dissipative or plastic (permanent or nonrecoverable) interaction On the bulk scale most mechanical analysis of materials relies on measuring their strain (material displacement) as a function of stress (pressure exerted on the material) or vica versa The measurement and analysis of stress versus strain behavior of various materials is a rich and active field and there are many excellent textbooks that describe it [12] The description provided below is simplified to provide a context for these measurements ultimately described on the nanoscale

Elastic deformation of a material is typically nonpermanent or reversible and exists in a regime where the strain (material displacement) is proportional to the stress (pressure exerted on the material) by a proportionality factor known as a modulus (though in reality no material is truly only elastic) Elastic materials thus typically follow Hookersquos law Figure 12 shows a representative stress versus strain curve for materials highlighting the elastic region (blue) Additionally in elastic

Plastic

Fracture

Strain

Elastic

Str

ess

Figure 12 Basic stress vs strain curve showing three mechanical regimes of elastic deformation plastic deformation and fracture For color details please see color plate section

6 Chapter 1 Overview of Atomic Force Microscopy

materials the loading curve (as force is applied) is identical to the unloading curve (as the force is removed) Modulus is a key parameter used to describe elastic behavior Depending on the direction in which a material is being stressed the material will have different moduli (eg a tensile modulus a shear modulus etc)

Plastic deformation is on the other end permanent and nonrecoverable Plastic deformation does not follow Hookersquos law and so stress is not proportional to strain In Figure 12 the region of plastic deformation is shown in green In contrast to elastic deformation the loading curve is not identical to the unloading curve and in fact the unloading curve returns to a nonzero strain at a zero stress level There are many common properties associated with plastic deformation including yielding (onset of plastic deformation) ductility (plastic deformation at a fracture) and hardness (resistance to localized plastic deformation)

Finally there is another very important class of materials called viscoelastic materials where the elastic properties are frequency dependent For example when a stress such as pulling is exerted on the material the materialrsquos response is rate dependent Perhaps the most familiar viscoelastic material to most readers from childhood is Silly Putty where its response to pulling depends on how fast it is pulled If Silly Putty is pulled very slowly the material stretches out very nicely If Silly Putty is pulled very fast it will quickly break Polymers and rubbers two materials that play a significant role in consumer products and which are character-ized quite heavily with AFM are also viscoelastic materials

14 overvieW oF AFm operAtion

141 AFm hardware

A schematic of AFM hardware is shown in Figure 13 The main components of the AFM are (1) the cantilever (2) optical detection system (3) x-y-z scanner and (4) feedback loop

142 cantilevers and probes

The heart of the AFM lies in its cantileverprobe assembly that interacts or probes materials to provide the information of interest The technology to fabricate AFM can-tilevers takes advantage of technology developed for the semiconductor industry to make similar scale devices and features out of silicon Thus today AFM cantilevers are typically made by micromachining silicon (single-crystal Si) or silicon nitride (Si

3N

4) The dimensions of a cantilever vary and dictate the stiffness or spring

constant of the cantilever Depending on the material that needs to be probed and the mode of operation one might pick a lever with a stiffer (higher) or softer (lower) spring constant Cantilevers are generally either rectangular in geometry (Si or Si

3N

4)

or triangular (Si3N

4) with now hybrid cantilevers being manufactured (eg Si tips on

SiN cantilevers) Generally the Si3N

4 probes have lower force constants and might be

used for imaging in liquid among other applications whereas the Si probes can be brittle and tend to break or chip in contact when scanning a surface Typically the

14 Overview of AFM Operation 7

cantilevers are hundreds of microns in length tens of microns in width and a few microns in thickness for reference the diameter of a human hair is approximately 100 microm Cantilevers can be coated with gold or aluminum to provide high reflectivity Commercial cantilevers are also available with other thin metal coatings for magnetic or electrical imaging Cantilevers and probes can also be functionalized chemically or biologically for specific interactions

Probes are grown at the end of the lever and vary in shape and sharpness A sharper probe can often improve the lateral resolution of the AFM Probe design is a continuously evolving field as manufacturers strive for the most robust reproducible and sharp tips Cantileverprobe assemblies are mounted into a cantilever holder that typically includes a piezoceramic element (described below) that can vibrate the can-tilever at a given frequencymdasheither above at or below its resonancemdashdepending on the desired tipndashsample interaction or AFM mode

143 optical detection system

In order to track the motion of the cantileverprobe assembly as it scans a surface an AFM typically includes an optical detection system that consists of a laser reflected off the back side of the cantilever and directed toward a position sensitive detector

Figure 13 Schematic of basic operating principles of AFM A laser is focused and directed off the back end of a cantilever and directed toward a position-sensitive director (PSD) to monitor its vertical and lateral motion as it interacts with the surface Depending on the mode used to image the sample the cantilever can be vibrated with a shake piezo for dynamic imaging modes that involve the resonance of the cantilever

8 Chapter 1 Overview of Atomic Force Microscopy

(PSD) as seen in Figure 13 Optical detection systems are the most common way to track cantilever motion in commercial AFM systems However other methods exist to bypass optical detection systems (in eg a dark medium where the laser light would be absorbed) with self-actuated cantilevers where their motion is read by piezos integrated into the levers The laser is typically a visible photodiode though some commercial instruments have implemented superluminescent diode (SLD) laser The laser is detected by a four-segment position-sensitive detector (PSD) which can track the vertical and lateral motion of the cantilever accurately which can be converted to units of nanometers or displacement through careful calibration described below

144 x-y-z scanner

An AFM can operate either by scanning the sample relative to a stationary tip (sample scanning) or scanning the tip relative to a stationary sample (tip scanning) Each has its own advantages in terms of size of sample it can accommodate (tip scanning more flexible) ability to accessorize (tip scanning again more flexible) and stability and signal to noise (sample scanning easier to build with better specifications) Each shares the requirement to move the tip relative to the sample in a highly accurate way with minimal noise

Tipndashsample motion is often accomplished in commercial instruments by piezoelectric materials which are materials that respond either by expansion or con-traction in response to an applied voltage These kinds of materials provide the ability for very fine motion (nanometers to microns) Piezoelectric materials can be made in several shapes including tubes disks and bimorphs depending on the range and geom-etry of motion required Although piezoelectric materials are very effective at moving tip or sample they are plagued by nonlinear behavior such as hysteresis and creep that has serious consequences for accurate AFM imaging and interpretation of data Artifacts induced by piezoelectric materials are reviewed in detail in Chapter 6

Piezoelectric scanners can be programmed to move with either open-loop or closed-loop feedback circuits In open-loop scanners the piezo is moved by programs to ramp x and y bias voltages to move the prescribed amounts So in open-loop scan-ners if the piezo has to move a large distance or move around on the sample there can be considerable hysteresis of creep with the best recourse of patience or movingzooming in and out in small increments since hysteresiscreep is proportional to the displacement or motion the scanner is trying to execute

One solution to this problem is to use closed-loop scanners that use additional x-y-z sensors to independently measure and then correct for scanner movement In closed-loop operation feedback circuits are continually adjusted to the applied scanner biases to correct for the x-y motion and to achieve the desired motion It is important to note that correction of x-y motion in an AFM is slightly different than correction of z motion This is due to the fact that x-y motion can be predicted whereas z motion cannot as it depends on the surface topography of the sample being scanned Therefore typically there are active feedback circuits to control x and y

13 Mechanical Properties 5

This edition focuses specifically on nanomechanical characterization Probing other material properties such as optical electrical or magnetic properties is not covered in this book The reason is that though these other properties have been successfully probed in various industrial RampD applications using AFM to probe mechanical properties of surfaces is more ubiquitous in industrial RampD and so was a logical focus for a first book on SPM in industrial applications

13 mechAnicAl properties

When one considers mechanical properties a range of descriptors come to mind including ldquostiffnessrdquo ldquotoughnessrdquo ldquohardnessrdquo ldquobrittlenessrdquo and ldquoductilityrdquo to name a few Each of these properties has their own mathematical expressions with associ-ated experimental methods of measuring them An interaction between two materials can be typically categorized into either elastic (or recoverable) or dissipative or plastic (permanent or nonrecoverable) interaction On the bulk scale most mechanical analysis of materials relies on measuring their strain (material displacement) as a function of stress (pressure exerted on the material) or vica versa The measurement and analysis of stress versus strain behavior of various materials is a rich and active field and there are many excellent textbooks that describe it [12] The description provided below is simplified to provide a context for these measurements ultimately described on the nanoscale

Elastic deformation of a material is typically nonpermanent or reversible and exists in a regime where the strain (material displacement) is proportional to the stress (pressure exerted on the material) by a proportionality factor known as a modulus (though in reality no material is truly only elastic) Elastic materials thus typically follow Hookersquos law Figure 12 shows a representative stress versus strain curve for materials highlighting the elastic region (blue) Additionally in elastic

Plastic

Fracture

Strain

Elastic

Str

ess

Figure 12 Basic stress vs strain curve showing three mechanical regimes of elastic deformation plastic deformation and fracture For color details please see color plate section

6 Chapter 1 Overview of Atomic Force Microscopy

materials the loading curve (as force is applied) is identical to the unloading curve (as the force is removed) Modulus is a key parameter used to describe elastic behavior Depending on the direction in which a material is being stressed the material will have different moduli (eg a tensile modulus a shear modulus etc)

Plastic deformation is on the other end permanent and nonrecoverable Plastic deformation does not follow Hookersquos law and so stress is not proportional to strain In Figure 12 the region of plastic deformation is shown in green In contrast to elastic deformation the loading curve is not identical to the unloading curve and in fact the unloading curve returns to a nonzero strain at a zero stress level There are many common properties associated with plastic deformation including yielding (onset of plastic deformation) ductility (plastic deformation at a fracture) and hardness (resistance to localized plastic deformation)

Finally there is another very important class of materials called viscoelastic materials where the elastic properties are frequency dependent For example when a stress such as pulling is exerted on the material the materialrsquos response is rate dependent Perhaps the most familiar viscoelastic material to most readers from childhood is Silly Putty where its response to pulling depends on how fast it is pulled If Silly Putty is pulled very slowly the material stretches out very nicely If Silly Putty is pulled very fast it will quickly break Polymers and rubbers two materials that play a significant role in consumer products and which are character-ized quite heavily with AFM are also viscoelastic materials

14 overvieW oF AFm operAtion

141 AFm hardware

A schematic of AFM hardware is shown in Figure 13 The main components of the AFM are (1) the cantilever (2) optical detection system (3) x-y-z scanner and (4) feedback loop

142 cantilevers and probes

The heart of the AFM lies in its cantileverprobe assembly that interacts or probes materials to provide the information of interest The technology to fabricate AFM can-tilevers takes advantage of technology developed for the semiconductor industry to make similar scale devices and features out of silicon Thus today AFM cantilevers are typically made by micromachining silicon (single-crystal Si) or silicon nitride (Si

3N

4) The dimensions of a cantilever vary and dictate the stiffness or spring

constant of the cantilever Depending on the material that needs to be probed and the mode of operation one might pick a lever with a stiffer (higher) or softer (lower) spring constant Cantilevers are generally either rectangular in geometry (Si or Si

3N

4)

or triangular (Si3N

4) with now hybrid cantilevers being manufactured (eg Si tips on

SiN cantilevers) Generally the Si3N

4 probes have lower force constants and might be

used for imaging in liquid among other applications whereas the Si probes can be brittle and tend to break or chip in contact when scanning a surface Typically the

14 Overview of AFM Operation 7

cantilevers are hundreds of microns in length tens of microns in width and a few microns in thickness for reference the diameter of a human hair is approximately 100 microm Cantilevers can be coated with gold or aluminum to provide high reflectivity Commercial cantilevers are also available with other thin metal coatings for magnetic or electrical imaging Cantilevers and probes can also be functionalized chemically or biologically for specific interactions

Probes are grown at the end of the lever and vary in shape and sharpness A sharper probe can often improve the lateral resolution of the AFM Probe design is a continuously evolving field as manufacturers strive for the most robust reproducible and sharp tips Cantileverprobe assemblies are mounted into a cantilever holder that typically includes a piezoceramic element (described below) that can vibrate the can-tilever at a given frequencymdasheither above at or below its resonancemdashdepending on the desired tipndashsample interaction or AFM mode

143 optical detection system

In order to track the motion of the cantileverprobe assembly as it scans a surface an AFM typically includes an optical detection system that consists of a laser reflected off the back side of the cantilever and directed toward a position sensitive detector

Figure 13 Schematic of basic operating principles of AFM A laser is focused and directed off the back end of a cantilever and directed toward a position-sensitive director (PSD) to monitor its vertical and lateral motion as it interacts with the surface Depending on the mode used to image the sample the cantilever can be vibrated with a shake piezo for dynamic imaging modes that involve the resonance of the cantilever

8 Chapter 1 Overview of Atomic Force Microscopy

(PSD) as seen in Figure 13 Optical detection systems are the most common way to track cantilever motion in commercial AFM systems However other methods exist to bypass optical detection systems (in eg a dark medium where the laser light would be absorbed) with self-actuated cantilevers where their motion is read by piezos integrated into the levers The laser is typically a visible photodiode though some commercial instruments have implemented superluminescent diode (SLD) laser The laser is detected by a four-segment position-sensitive detector (PSD) which can track the vertical and lateral motion of the cantilever accurately which can be converted to units of nanometers or displacement through careful calibration described below

144 x-y-z scanner

An AFM can operate either by scanning the sample relative to a stationary tip (sample scanning) or scanning the tip relative to a stationary sample (tip scanning) Each has its own advantages in terms of size of sample it can accommodate (tip scanning more flexible) ability to accessorize (tip scanning again more flexible) and stability and signal to noise (sample scanning easier to build with better specifications) Each shares the requirement to move the tip relative to the sample in a highly accurate way with minimal noise

Tipndashsample motion is often accomplished in commercial instruments by piezoelectric materials which are materials that respond either by expansion or con-traction in response to an applied voltage These kinds of materials provide the ability for very fine motion (nanometers to microns) Piezoelectric materials can be made in several shapes including tubes disks and bimorphs depending on the range and geom-etry of motion required Although piezoelectric materials are very effective at moving tip or sample they are plagued by nonlinear behavior such as hysteresis and creep that has serious consequences for accurate AFM imaging and interpretation of data Artifacts induced by piezoelectric materials are reviewed in detail in Chapter 6

Piezoelectric scanners can be programmed to move with either open-loop or closed-loop feedback circuits In open-loop scanners the piezo is moved by programs to ramp x and y bias voltages to move the prescribed amounts So in open-loop scan-ners if the piezo has to move a large distance or move around on the sample there can be considerable hysteresis of creep with the best recourse of patience or movingzooming in and out in small increments since hysteresiscreep is proportional to the displacement or motion the scanner is trying to execute

One solution to this problem is to use closed-loop scanners that use additional x-y-z sensors to independently measure and then correct for scanner movement In closed-loop operation feedback circuits are continually adjusted to the applied scanner biases to correct for the x-y motion and to achieve the desired motion It is important to note that correction of x-y motion in an AFM is slightly different than correction of z motion This is due to the fact that x-y motion can be predicted whereas z motion cannot as it depends on the surface topography of the sample being scanned Therefore typically there are active feedback circuits to control x and y

6 Chapter 1 Overview of Atomic Force Microscopy

materials the loading curve (as force is applied) is identical to the unloading curve (as the force is removed) Modulus is a key parameter used to describe elastic behavior Depending on the direction in which a material is being stressed the material will have different moduli (eg a tensile modulus a shear modulus etc)

Plastic deformation is on the other end permanent and nonrecoverable Plastic deformation does not follow Hookersquos law and so stress is not proportional to strain In Figure 12 the region of plastic deformation is shown in green In contrast to elastic deformation the loading curve is not identical to the unloading curve and in fact the unloading curve returns to a nonzero strain at a zero stress level There are many common properties associated with plastic deformation including yielding (onset of plastic deformation) ductility (plastic deformation at a fracture) and hardness (resistance to localized plastic deformation)

Finally there is another very important class of materials called viscoelastic materials where the elastic properties are frequency dependent For example when a stress such as pulling is exerted on the material the materialrsquos response is rate dependent Perhaps the most familiar viscoelastic material to most readers from childhood is Silly Putty where its response to pulling depends on how fast it is pulled If Silly Putty is pulled very slowly the material stretches out very nicely If Silly Putty is pulled very fast it will quickly break Polymers and rubbers two materials that play a significant role in consumer products and which are character-ized quite heavily with AFM are also viscoelastic materials

14 overvieW oF AFm operAtion

141 AFm hardware

A schematic of AFM hardware is shown in Figure 13 The main components of the AFM are (1) the cantilever (2) optical detection system (3) x-y-z scanner and (4) feedback loop

142 cantilevers and probes

The heart of the AFM lies in its cantileverprobe assembly that interacts or probes materials to provide the information of interest The technology to fabricate AFM can-tilevers takes advantage of technology developed for the semiconductor industry to make similar scale devices and features out of silicon Thus today AFM cantilevers are typically made by micromachining silicon (single-crystal Si) or silicon nitride (Si

3N

4) The dimensions of a cantilever vary and dictate the stiffness or spring

constant of the cantilever Depending on the material that needs to be probed and the mode of operation one might pick a lever with a stiffer (higher) or softer (lower) spring constant Cantilevers are generally either rectangular in geometry (Si or Si

3N

4)

or triangular (Si3N

4) with now hybrid cantilevers being manufactured (eg Si tips on

SiN cantilevers) Generally the Si3N

4 probes have lower force constants and might be

used for imaging in liquid among other applications whereas the Si probes can be brittle and tend to break or chip in contact when scanning a surface Typically the

14 Overview of AFM Operation 7

cantilevers are hundreds of microns in length tens of microns in width and a few microns in thickness for reference the diameter of a human hair is approximately 100 microm Cantilevers can be coated with gold or aluminum to provide high reflectivity Commercial cantilevers are also available with other thin metal coatings for magnetic or electrical imaging Cantilevers and probes can also be functionalized chemically or biologically for specific interactions

Probes are grown at the end of the lever and vary in shape and sharpness A sharper probe can often improve the lateral resolution of the AFM Probe design is a continuously evolving field as manufacturers strive for the most robust reproducible and sharp tips Cantileverprobe assemblies are mounted into a cantilever holder that typically includes a piezoceramic element (described below) that can vibrate the can-tilever at a given frequencymdasheither above at or below its resonancemdashdepending on the desired tipndashsample interaction or AFM mode

143 optical detection system

In order to track the motion of the cantileverprobe assembly as it scans a surface an AFM typically includes an optical detection system that consists of a laser reflected off the back side of the cantilever and directed toward a position sensitive detector

Figure 13 Schematic of basic operating principles of AFM A laser is focused and directed off the back end of a cantilever and directed toward a position-sensitive director (PSD) to monitor its vertical and lateral motion as it interacts with the surface Depending on the mode used to image the sample the cantilever can be vibrated with a shake piezo for dynamic imaging modes that involve the resonance of the cantilever

8 Chapter 1 Overview of Atomic Force Microscopy

(PSD) as seen in Figure 13 Optical detection systems are the most common way to track cantilever motion in commercial AFM systems However other methods exist to bypass optical detection systems (in eg a dark medium where the laser light would be absorbed) with self-actuated cantilevers where their motion is read by piezos integrated into the levers The laser is typically a visible photodiode though some commercial instruments have implemented superluminescent diode (SLD) laser The laser is detected by a four-segment position-sensitive detector (PSD) which can track the vertical and lateral motion of the cantilever accurately which can be converted to units of nanometers or displacement through careful calibration described below

144 x-y-z scanner

An AFM can operate either by scanning the sample relative to a stationary tip (sample scanning) or scanning the tip relative to a stationary sample (tip scanning) Each has its own advantages in terms of size of sample it can accommodate (tip scanning more flexible) ability to accessorize (tip scanning again more flexible) and stability and signal to noise (sample scanning easier to build with better specifications) Each shares the requirement to move the tip relative to the sample in a highly accurate way with minimal noise

Tipndashsample motion is often accomplished in commercial instruments by piezoelectric materials which are materials that respond either by expansion or con-traction in response to an applied voltage These kinds of materials provide the ability for very fine motion (nanometers to microns) Piezoelectric materials can be made in several shapes including tubes disks and bimorphs depending on the range and geom-etry of motion required Although piezoelectric materials are very effective at moving tip or sample they are plagued by nonlinear behavior such as hysteresis and creep that has serious consequences for accurate AFM imaging and interpretation of data Artifacts induced by piezoelectric materials are reviewed in detail in Chapter 6

Piezoelectric scanners can be programmed to move with either open-loop or closed-loop feedback circuits In open-loop scanners the piezo is moved by programs to ramp x and y bias voltages to move the prescribed amounts So in open-loop scan-ners if the piezo has to move a large distance or move around on the sample there can be considerable hysteresis of creep with the best recourse of patience or movingzooming in and out in small increments since hysteresiscreep is proportional to the displacement or motion the scanner is trying to execute

One solution to this problem is to use closed-loop scanners that use additional x-y-z sensors to independently measure and then correct for scanner movement In closed-loop operation feedback circuits are continually adjusted to the applied scanner biases to correct for the x-y motion and to achieve the desired motion It is important to note that correction of x-y motion in an AFM is slightly different than correction of z motion This is due to the fact that x-y motion can be predicted whereas z motion cannot as it depends on the surface topography of the sample being scanned Therefore typically there are active feedback circuits to control x and y

14 Overview of AFM Operation 7

cantilevers are hundreds of microns in length tens of microns in width and a few microns in thickness for reference the diameter of a human hair is approximately 100 microm Cantilevers can be coated with gold or aluminum to provide high reflectivity Commercial cantilevers are also available with other thin metal coatings for magnetic or electrical imaging Cantilevers and probes can also be functionalized chemically or biologically for specific interactions

Probes are grown at the end of the lever and vary in shape and sharpness A sharper probe can often improve the lateral resolution of the AFM Probe design is a continuously evolving field as manufacturers strive for the most robust reproducible and sharp tips Cantileverprobe assemblies are mounted into a cantilever holder that typically includes a piezoceramic element (described below) that can vibrate the can-tilever at a given frequencymdasheither above at or below its resonancemdashdepending on the desired tipndashsample interaction or AFM mode

143 optical detection system

In order to track the motion of the cantileverprobe assembly as it scans a surface an AFM typically includes an optical detection system that consists of a laser reflected off the back side of the cantilever and directed toward a position sensitive detector

Figure 13 Schematic of basic operating principles of AFM A laser is focused and directed off the back end of a cantilever and directed toward a position-sensitive director (PSD) to monitor its vertical and lateral motion as it interacts with the surface Depending on the mode used to image the sample the cantilever can be vibrated with a shake piezo for dynamic imaging modes that involve the resonance of the cantilever

8 Chapter 1 Overview of Atomic Force Microscopy

(PSD) as seen in Figure 13 Optical detection systems are the most common way to track cantilever motion in commercial AFM systems However other methods exist to bypass optical detection systems (in eg a dark medium where the laser light would be absorbed) with self-actuated cantilevers where their motion is read by piezos integrated into the levers The laser is typically a visible photodiode though some commercial instruments have implemented superluminescent diode (SLD) laser The laser is detected by a four-segment position-sensitive detector (PSD) which can track the vertical and lateral motion of the cantilever accurately which can be converted to units of nanometers or displacement through careful calibration described below

144 x-y-z scanner

An AFM can operate either by scanning the sample relative to a stationary tip (sample scanning) or scanning the tip relative to a stationary sample (tip scanning) Each has its own advantages in terms of size of sample it can accommodate (tip scanning more flexible) ability to accessorize (tip scanning again more flexible) and stability and signal to noise (sample scanning easier to build with better specifications) Each shares the requirement to move the tip relative to the sample in a highly accurate way with minimal noise

Tipndashsample motion is often accomplished in commercial instruments by piezoelectric materials which are materials that respond either by expansion or con-traction in response to an applied voltage These kinds of materials provide the ability for very fine motion (nanometers to microns) Piezoelectric materials can be made in several shapes including tubes disks and bimorphs depending on the range and geom-etry of motion required Although piezoelectric materials are very effective at moving tip or sample they are plagued by nonlinear behavior such as hysteresis and creep that has serious consequences for accurate AFM imaging and interpretation of data Artifacts induced by piezoelectric materials are reviewed in detail in Chapter 6

Piezoelectric scanners can be programmed to move with either open-loop or closed-loop feedback circuits In open-loop scanners the piezo is moved by programs to ramp x and y bias voltages to move the prescribed amounts So in open-loop scan-ners if the piezo has to move a large distance or move around on the sample there can be considerable hysteresis of creep with the best recourse of patience or movingzooming in and out in small increments since hysteresiscreep is proportional to the displacement or motion the scanner is trying to execute

One solution to this problem is to use closed-loop scanners that use additional x-y-z sensors to independently measure and then correct for scanner movement In closed-loop operation feedback circuits are continually adjusted to the applied scanner biases to correct for the x-y motion and to achieve the desired motion It is important to note that correction of x-y motion in an AFM is slightly different than correction of z motion This is due to the fact that x-y motion can be predicted whereas z motion cannot as it depends on the surface topography of the sample being scanned Therefore typically there are active feedback circuits to control x and y

8 Chapter 1 Overview of Atomic Force Microscopy

(PSD) as seen in Figure 13 Optical detection systems are the most common way to track cantilever motion in commercial AFM systems However other methods exist to bypass optical detection systems (in eg a dark medium where the laser light would be absorbed) with self-actuated cantilevers where their motion is read by piezos integrated into the levers The laser is typically a visible photodiode though some commercial instruments have implemented superluminescent diode (SLD) laser The laser is detected by a four-segment position-sensitive detector (PSD) which can track the vertical and lateral motion of the cantilever accurately which can be converted to units of nanometers or displacement through careful calibration described below

144 x-y-z scanner

An AFM can operate either by scanning the sample relative to a stationary tip (sample scanning) or scanning the tip relative to a stationary sample (tip scanning) Each has its own advantages in terms of size of sample it can accommodate (tip scanning more flexible) ability to accessorize (tip scanning again more flexible) and stability and signal to noise (sample scanning easier to build with better specifications) Each shares the requirement to move the tip relative to the sample in a highly accurate way with minimal noise

Tipndashsample motion is often accomplished in commercial instruments by piezoelectric materials which are materials that respond either by expansion or con-traction in response to an applied voltage These kinds of materials provide the ability for very fine motion (nanometers to microns) Piezoelectric materials can be made in several shapes including tubes disks and bimorphs depending on the range and geom-etry of motion required Although piezoelectric materials are very effective at moving tip or sample they are plagued by nonlinear behavior such as hysteresis and creep that has serious consequences for accurate AFM imaging and interpretation of data Artifacts induced by piezoelectric materials are reviewed in detail in Chapter 6

Piezoelectric scanners can be programmed to move with either open-loop or closed-loop feedback circuits In open-loop scanners the piezo is moved by programs to ramp x and y bias voltages to move the prescribed amounts So in open-loop scan-ners if the piezo has to move a large distance or move around on the sample there can be considerable hysteresis of creep with the best recourse of patience or movingzooming in and out in small increments since hysteresiscreep is proportional to the displacement or motion the scanner is trying to execute

One solution to this problem is to use closed-loop scanners that use additional x-y-z sensors to independently measure and then correct for scanner movement In closed-loop operation feedback circuits are continually adjusted to the applied scanner biases to correct for the x-y motion and to achieve the desired motion It is important to note that correction of x-y motion in an AFM is slightly different than correction of z motion This is due to the fact that x-y motion can be predicted whereas z motion cannot as it depends on the surface topography of the sample being scanned Therefore typically there are active feedback circuits to control x and y