e-, hv, ione-, hv, ion tip

Characterization Techniques

Primary Secondary

Investigate:• Structure/morphology

• Surface analysis

• Compositional/elemental and molecular

•Physical properties

•?

•?

Microscopy

Scanning ProbeOptical Electron

▪ uses visible light and

system of lenses to

magnify

▪ oldest and simplest

design

▪ new digital

microscopes use

CCD camera

▪ magnification up to

2000 times

▪ uses a particle beam

of electrons to

illuminate a

specimen

▪ create a highly-

magnified image

▪ uses electrostatic

and electromagnetic

lenses

▪ magnification up to

2 million times

▪ forms images of surfaces

using a physical probe that

scans the specimen

▪ surface image produced by

mechanically moving probe

in a raster scan of the

specimen and recording

probe-surface interaction

as a function of position

▪ atomic resolution

▪ was founded in 1981

Scanning probe microscopy has been essential in the development

of nanotechnology:

Characterisation/visualisation tool at nanoscale.

Nanomanipulator and modificator.

… a family of tools for the nanotechnology world..

Scanning Probe Microscopy (SPM)

Operates by measuring the interaction force between the tip

and sample

STM Modes of Operation• Constant Current

• Constant Height

AFM Operation Modes• Contact Mode

• Tapping Mode

• Non-Contact Mode

Wide range of applications

Topography/Atomic Structure

Magnetic/Electric fields

Surface temperatures

Scanning Tunneling Microscopy (STM)

• Electrons are transferred

between the tip and the sample

due to overlapping orbitals

– A net transfer can be

sustained by applying a

voltage across the gap

• Change in current is a result of

a change in the tip-sample

separation

E

Classical

mechanics

Impenetrable

barrier

Quantum

mechanicsTunneling

Quantum Tunnelling

➢ In classical physics e flows are not possible without a direct connection by a wire between two surfaces

➢ On an atomic scale a quantum mechanical particle behaves in its wave function.

➢ There is a finite probability that an electron will “jump” from one surface to the other of lower potential.

➢ A sharp conductive tip is brought to within a few Angstroms of the surface of a conductor (sample).

➢ Bias voltage is applied, Fermi levels shift

➢ The wave functions of the electrons in the tip overlap those of the sample surface

➢ Electrons tunnel from one surface to the other of lower potential.

Tunneling Current

The direction of current flow is determined by the polarity of the bias. If the sample is biased -Ve with respect to the tip, then electrons will flow from the surface to the tip, whilst if the sample is biased +Ve with respect to the tip, then electrons will flow from the tip to the surface.

)exp( cdVI −=

The probability of tunnelling is exponentially-dependent upon the distance

of separation between the tip and surface : the tunnelling current is

therefore a very sensitive probe of this separation

Ideally a STM probe tip is very pointed (1-2

atoms at the end) and has a relatively low work

function. Etched tungsten crystals are ideal and

are nearly identical to field emitters

➢Tunneling current exhibits an

exponentially decay with an

increase of the separation

distance!

➢Exponential dependence leads to fantastic resolutions. Order of 10-12 m in the perpendicular direction and ~10-10 m in the parallel directions

Resolution

Constant Height

Constant Current

If the tunneling current is kept

constant the Z position of the tip

must be moved up and down. If

this movement is recorded then

the topography of the specimen

can be inferred.

Alternatively if the Z position of

the tip is kept constant the

tunneling current will change as

it moves across the surface. If

the changes in current are

recorded the then the

topography of the specimen can

be inferred

STM Modes of Operation

STM tip:

atomically sharp needle and terminates in a single atom

➢Pure metals (W, Au)

➢Alloys (Pt-Rh, Pt-Ir)

➢Chemically modified conductor (W/S, Pt-Rh/S, W/C…)

➢Preparation of tips: cut by a wire cutter and used as is

cut followed by electrochemical etching

➢Electrochemical etching of tungsten tips. A tungsten wire, typically 0.25 mm in diameter, is vertically inserted in a solution of 2M NaOH. A counter electrode, usually a piece of platinum or stainless steel, is kept at a negative potential relative to the tungsten wire.

➢The etching takes a few minutes. When the neck of the wire near the interface becomes thin enough, the weight of the wire in electrolyte fractures the neck. The lower half of the wire drops off.

The two most commonly used tips are made from either a Pt-Ir (80/20) alloy or tungsten wire.

➢ Control of environment vibration:

➢ building the instrument with sufficient mechanical

rigidity;

➢ hung on a double bungee cord sling to manage

vibration;

➢ vibration isolation systems have also been made with

springs and frames;

➢ operate at night with everything silent.

➢ Ultrahigh vacuum (UHV): to avoid contamination of the

samples from the surrounding medium. (The STM itself

does not need vacuum to operate; it works in air as well as

under liquids.)

➢ Using an atomically sharp tip.

Forces applied to a segment of

material lead to the appearance of

electrical charge on the surfaces of

the segment. The specific distribution

of electric charges in the unit cell of a

crystal is the source of this

phenomenon.

The Piezoelectric Effect:

Piezoelectric Effect

• Piezoelectricity is the ability of some materials (notably

crystals and certain ceramics) to generate an electric field in

response to applied mechanical stress.

• If the material is not short-circuited, the induced charge

generates a voltage across the material.

• The piezoelectric effect is reversible, that is, the piezo

materials exhibit:

• the direct piezoelectric effect – the production of electricity

when stress is applied,

• the inverse piezoelectric effect – the production of stress

and/or strain when an electric field is applied.

•PZT (Lead zirconium

titanate) ceramics must

be poled at an elevated

temperature.

•The ceramic now

exhibits piezoelectric

properties and will

change dimensions

when an electric

potential is applied.

Tube

Piezoelectric scanners

+y

-x +x

-y

Piezo

tube

A(111) S(√3x√3)R30o S8

Ru(0001)

Grafit

Ge (100)

Concluding remarks

➢Disadvantage of STM:

1.Making atomically sharp tips remains something of a dark art!

2.External and internal vibrations from fans, pumps, machinery, building movements, etc. are big problems.

3.UHV-STM is not easy to built and handle.

4.The STM can only scan conductive surfaces or thin nonconductive films and small objects deposited on conductive substrates. It does not work with nonconductive materials, such as glass, rock, etc.

5.The spatial resolution of STM is fantastic, but the temporal resolution is typically on the order of seconds, which prevents STM from imaging fast kinetics of electrochemical process.

➢STM is one the most powerful imaging tools with an unprecedented precision.

ATOMIC FORCE MICROSCOPY

Tips are typically made from Si3N4 or Si, and extended down from the end of a

cantilever. A diode laser is focused onto the back of the reflective cantilever. As

the tip scans the surface of the sample, moving up and down with the contour

of the surface, the laser beam is deflected into a dual element photodiode which

measures the difference in light intensities between the upper and lower

photodetectors, and then converts to voltage.

1. Laser

2. Mirror

3. Photodetector

4. Amplifier

6. Sample

7. Probe

8. Cantilever

Force Sensors - Cantilevers

Si , Si3N4,

Diomand

There are some simple criteria to be considered, when cantilevers are fabricated:

• resonance frequency fR > 100Hz (building vibrations), > 10kHz (sound waves)

• high force sensitivity requires low spring constants (MFM:, 0.1 N/m, mRFM: 0.001

N/m)

• atomic resolution requires spring constant to be in range of atomic spring constants

> 10N/m

• thermal vibrations of the cantilever < 0.1nm, i.e. k>0.4N/m @ 300K

It can be shown that only cantilevers of dimensions in the micrometer range fulfil

these design criteria.

Generally, higher resonance frequencies require smaller cantilevers.

P. Grutter, McGill University

Spring constants k and resonant frequency f of cantilevers

Spring constant k :

typical values: 0.01 - 100 N/m

Young’s modulus EY ~ 1012

N/m2

Resonant frequency fo:

typical values: 7 - 500 kHz

W

L

t

3

3

4 L

wtEk Y=

YE

L

tf

20 162.0=

Basics

• most widely used sensor.

• adjustment of Laser or Super-LED

beam to the cantilever (2 directions)

• adjustment of 4Q-diode to reflected

beam

Advantages:

• vertical & lateral deflection

• easy to use

• Robust and cheap

Disadvantages

• not intrisically quantitative, requires

calibration

• requires large volume

Deflection Sensors

A laser beam is reflected off the rear side of the cantilever. Angular deflections of the

laser beam are measured with a position sensitive detector (4-quadrant photo diode).

The A-B-signal is proportional to the normal force or topography and the C-D-

signal is proportional to the torsional force.

Electron Tunneling: original concept, potentially sensitive, practically problematic

Beam Deflection: most widely used, robust, high sensitivity, not directly quantitative,

requires calibration

Interferometry: best sensitivity, quantitative, uses limited space, complicated

Capacitance: sensor can be microfabricated, strong force from sensor, limited

sensitivity

Piezoresistance: ideal for microfabrication & integration, limited sensitivity, heating of

cantilever

Piezoelectric: mostly quartz tuning forks, good for true atomic resolution, limited

sensitivity

STM

AFM

Relevant Forces

typical long-range forces (> 1nm):

• van der Waals

• electrostatic, magnetic, …

forces in liquids:

• hydrophobic / hydrophilic forces

• steric forces

• solvation forces

typical short-range forces

(contact / near contact):

• short-range repulsive forces

(Pauli exclusion) or ionic repulsion forces

• short-range chemical binding forces

Refs.:

J. Israelachvili

Intermolecular and Surface Forces with Applications to Colloidal and

Biological Systems, Academic Press (1985)

D. Tabor

Gases, liquids and solids, Cambridge University Press (1979)

P. Grutter, McGill University

FvdW = AR/6z2

A…Hamaker const.

R…Tip radius

z…Tip - sample separation

A depends on type of materials (polarizability). For most materials and vacuum A~1eV

Krupp, Advances Colloidal Interface Sci. 1, 113 (1967)

R~100nm typical effective radius

-> FvdW ~ 10 nN at z~0.5 nm

Dipole-dipole forces between non-permanent dipoles (dispersion forces). These act

between dipoles that arise from fluctuations and dipoles induced in their electric field.

They are always present and attract even chemically inert noble gas atoms. The

range of dispersion forces is limited. When the distance between molecules is larger

than the way light can travel during the characteristic lifetime of the fluctuations, the

dispersion forces are weakened. This effect is called retardation. The van der Waals

force at short distances decays like F = 1/r6, beyond r > 5 nm this power law reduces

to F = 1/r7.

Because the range of van der Waals forces is limited, the tip-sample geometry of the

force microscope can be well approximated as a sphere approaching a semi-infinite

body.

van der Waal's Forces

Short-Range Forces

Due to overlap of electron wave functions and from the repulsion of the ion cores.

can be both attractive:

• attractive when the overlap of electron waves reduces the total energy. These

situations are comparable to molecular binding.

• around 0.5 - 1 nN per interacting atom at tip-sample distances typical for STM

operation.

• decay length of the order of atomic units, i.e. 0.05 nm for metallic adhesion, but

around 0.2 nm for covalent bonding.

true atomic resolution AFM operates with these forces

or repulsive:

• repulsive when strong electron wave overlap (Pauli exclusion principle). These

forces are directly connected to the total electron density. The ionic repulsion acts

for small distances, where the screening of the ion cores by the electrons falls

away.

• usual contact AFM operates with these forces.

• Model potentials like the Lennard-Jones or the Morse potential are used to describe

shortrange forces.

Electrostatic forces

Felectrostatic = p e0 RU2/ z

U…Potential difference

R…Tip radius

z…Tip - sample separation

R~100nm typical effective radius

U=1V

-> Felectrostatic ~ 5 nN at z~0.5 nm

•The strength and distance dependence of

electrostatic forces obey Coulombs law.

Chemical forces

FMorse = Ebond / z • (2e-k(z-s) - e-2k(z s))

Ebond …Bond energy

k …decay length radius

s…equilibrium distance

Electrostatic interaction: Caused by both

the localized charges and the polarization of

the substrate due to the potential difference

between the tip and the sample. It has been

used to study the electrostatic properties of

samples such as microelectronic structures,

charges on insulator surfaces, or

ferroelectric domains.

Solvation Forces

Magnetic Forces

Fmagntic = mtip • Hsample

Magnetic interaction: Caused by magnetic dipoles both on the tip and the sample.

This interaction is used for Magnetic Force Microscopy to study magnetic domains

on the sample surface.

P. Grutter, McGill University

Capillary forces (water layer)

There is always a water layer on a surface in air!

Fcapillary = 4p R g cos

g …surface tension, ~10-50 mJ/m2

…contact angle Surface

Water

Tip

Can be LARGE (several 1-10 nN)

Total force on cantilever = sum of ALL forces

http://www.google.com.tr/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&docid=7pX9XnhJ--GRYM&tbnid=Jv4nsX1cem8HfM:&ved=0CAUQjRw&url=http://www.cmbi.ru.nl/redock/Glossary.php&ei=B5V0U_SzBcauO5WIgZgJ&bvm=bv.66699033,d.bGQ&psig=AFQjCNGL1OmriMIsclyStIKMZioS78sYsw&ust=1400235586659937

At very small tip-sample distances (a few angstroms) a very strong repulsive force

appears between the tip and sample atoms. Its origin is the so-called exchange

interactions due to the overlap of the electronic orbitals at atomic distances. When this

repulsive force is predominant, the tip and sample are considered to be in “contact”.

Noncontact (NC) AFM

Both are based on a Feedback Mechanism

of constant oscillation amplitude.

•Non-contact mode: amplitude set as ~

100% of “Free” amplitude;

•Tapping mode: amplitude set as ~ 50 -

60% of “Free” amplitude.

•Tapping mode provides higher resolution

with minimum sample damage.

•Most of times, non-contact mode is

operated as tapping mode.

• Frictional Force

Lateral Force Microscopy

Low

Friction

Low

Friction

High

Friction

Topography Profile

Friction Profile

Non-ContactTappingContact

•Polymer

latex

•particle on

•mica

smallsmalllarge•Damage

>10 nm~1 nm<0.2 nmDistance

smallsmalllargeFriction

0.1-0.01nN0.1-0.01nN1-10 nNForce

•hard•hard•softCantilever

•High resolution topography (top)

and Lateral Force mode (bottom)

images of a commercially avail-

able PET film. The silicate fillers

show increased friction in the

lateral force image

Laterally twisted due to friction

www.tmmicro.com/tech/modes/lfm.htm

Lateral Force Microscope

Force Modulation Microscopy

•Contact AFM mode

•Periodic signal is applied to either to

tip or sample

•Simultaneous imaging of

topography and material properties

•Amplitude change due to elastic

properties of the sample

Magnetic Force Microscopy

•Ferromagnetic tip: Co, Cr

•Noncontact mode

•vdW force: short range force

•Magnetic force: long range force; small

force gradient

•Close imaging: topography

•Distant imaging: magnetic properties

http://www.tmmicro.com/tech/index.htm

Glass Hard Disk Sample

• The tip is magnetized (a soft-magnet

coating tip is used)

• Cantilever deflects when it scans over

magnetized domains of sample

• MFM images locally magnetized domains.

• Magnitude of deflection (up/down) is

proportional to sample magnetization

• MFM is used to determine the local

magnetization variation.

•• Ferroelectric

materials

•• Charge

distribution on

surfaces

•• Failure analysis

on the device

Electrostatic Force Microscopy(EFM)

• Voltage is applied between tip and sample while

cantilevers hovers above sample.

• Cantilever deflects when it scans over static charges

• EFM plots locally charged domains similar to MFM plots

of magnetic domains.

• Magnitude of deflection is proportional to the charge

density.

• EFM is used to determine the local charge density

variation.

Metallic tip

Bias Voltage

Chemical Force Microscopy

A. Noy et al, Ann. Rev. Mater. Sci. 27, 381 (1997)

• (A) topography

• (B) friction force using a tip modified with a COOH-terminated SAM,

• (C) friction force using a tip modified with a methyl-terminated SAM.Light

regions in (B) and (C) indicate high friction; dark regions indicate low

friction.

-COOH-CH3

Force / Distance Curves in AFM using adhesion forces to obtain chemical specificity

COOH CH3 scanned in air CH3 tip air H2O COOH CH3 hydrophobic tip [CH3 covered]

Contrast Reversal

Electrochemistry

Surface structure

Electrochemical deposition

Corrosion

Scanning Electrochemical Microscopy

Nearfield Scanning Optical Microscopy

hv

Al

Cam

f=60-200nm

Detektör

Topography and Optical properties

d

Fluorescence image of

a single DiIC molecule.

Scanning Thermal Microscopy

• Subsurface defect review

•Semiconductor failure analysis

•Measure conductivity differences in copolymers,

•surface coatings etc

P. Grutter, McGill University

Blunt tip :

Imaging Artifacts

‘High’ resolution and double tip:

Image Artifacts

•Tip imaging

•Feedback artifacts

•Convolution with other

physics

•Imaging processing

How to check ?

•Repeat the scan

•Change the direction

•Change the scan size

•Rotate the sample

•Change the scan speed

www.psia.co.kr/appnotes/apps.htm

AFM image of random noise. Left

image is raw data. Right image is a

false image, produced by applying a

narrow bandpass filter to the raw data

SPM Advantages

• resolution not limited by diffraction, but only by the size of the probe-sample interaction volume ( few picometers)

• ability to measure small local differences in object height (like that of 135 picometre steps on <100> silicon)

• probe-sample interaction extends only across the tip atom or atoms involved in the interaction

• interaction can be used to modify the sample to create small structures (nanolithography)

• do not require a partial vacuum but can be observed in air at standard temperature and pressure or while submerged in a liquid reaction vessel.

SPM Disadvantages

• detailed shape of the scanning tip difficult to determine (effect particularly noticeable if the specimen varies greatly in height over lateral distances of 10 nm or less)

• generally slower in acquiring images due to the scanning process

• embedding of spatial information into a time sequence leads to uncertainties in metrology (lateral spacings and angles) which arise due to time-domain effects like specimen drift, feedback loop oscillation, and mechanical vibration

• The maximum image size is generally smaller

• not useful for examining buried solid-solid or liquid-liquid interfaces

SPM has eyes to see the geometry and properties of

nanostructures and fingers to manipulate and build

nanostructures

There is a great tendency to “see what you want” to in

AFM images, although multimode operation helps to

reduce interpretations. This technique has not-so-

obvious limitations:

• Tip contamination

• Piezo non-linearities and drift

• Tips are rarely characterized for spring constant, geometry

• Artifacts (double tip)

• topography / phase convolution in lateral force

• “near sightedness”