NOVEL MEMS GRIPPERS FOR PICK-PLACE OF MICRO · PDF filemicrosystems. Despite the significant...

NOVEL MEMS GRIPPERS FOR PICK-PLACE OF MICRO

AND NANO OBJECTS

by

Ko Lun (Brandon) Chen

A thesis submitted in conformity with the requirements

for the degree of Master of Applied Science

Graduate Department of Mechanical and Industrial Engineering

University of Toronto

© Copyright by by Ko Lun (Brandon) Chen 2009

Abstract

NOVEL MEMS GRIPPERS FOR PICK-PLACE OF MICRO AND NANO OBJECTS

Ko Lun (Brandon) Chen

Master of Applied Science

Graduate Department of Mechanical and Industrial Engineering

University of Toronto

2009

Physical pick-and-place promises specificity, precision, and programmed motion, a feature

making microrobotic manipulation amenable to automation for the construction of

microsystems. Despite the significant progress made, a long-standing difficulty is the release

of micro objects from the end effector due to strong adhesion forces at the micro scale.

This research focuses on the development of microelectromechanical systems (MEMS)

based microgrippers that integrate an active release mechanism for pick-and-release

micromanipulation. The performance was experimentally quantified through the manipulation

of 7.5-10.9µm glass spheres, and for the first time, achieves a 100% success rate in release

(based on 700 trials) and a release accuracy of 0.45±0.24µm. Example patterns were then

constructed through automated microrobotic pick-and-place of microspheres, achieving a

speed of 6sec/sphere.

To further miniaturize the devices for nanomanipulation, a novel fabrication process was

developed. Through the manipulation of 100nm gold nano-particles inside a scanning electron

microscope (SEM), preliminary demonstrations were made.

ii

Acknowledgements

I am sincerely grateful to my advisor, Professor Yu Sun, for his support, advice, and guidance

throughout my Master’s studies at the University of Toronto. His enthusiasms for research

and constant encouragement have inspired me to overcome every challenge that I encountered

during my research. His insightful direction to achieve our goal has always been helpful and

led me to find the right path. I am delighted to continue further studies under his supervision.

I would like to give special thanks to Jian Chen, Keekyoung Kim, Jianhua Tong and Xinyu

Liu for sharing their valuable knowledge with me; Yong Zhang for working closely with me

for endless hours, and all past and present members of the Advanced Micro and Nanosystems

Laboratory for all their helpful discussion and encouragement.

I would also like to thank Yimin Zhou, Dr. Henry Lee, Dr. Edward Xu, and Dr. Aju

Jugessur of the Emerging Communication Technology Institute at U of Toronto, and Marie-

Hélène Bernier, Dominic Cappe, Philippe Vasseur, and Olivier Grenier of the Laboratory of

Microfabrication at Ecole Polytechnique in Montreal, for their cleanroom assistance that led

to successful fabrication every time. I also greatly appreciate the generous assistance from Sal

Boccia and Dr. David Hoyle with the operation of SEM.

I want to express my special thanks to my wonderful girlfriend, Cynthia, for accompanying

me through this long journey and for her love and care during the tough times.

I would like to express my deep appreciation to my parents, who always believed in me,

and Cynthia’s Parents, for their care and support.

iii

Contents

1. Introduction.....................................................................................................................1

1.1 Background.................................................................................................................1

1.1.1 Passive Release Methods......................................................................................2

1.1.2 Active Release ......................................................................................................3

1.2 Motivation...................................................................................................................5

1.3 Dissertation Outline ....................................................................................................6

2. Three-Pronged Microgrippers ......................................................................................7

2.1 Introduction.................................................................................................................7

2.1.1 Technical University of Denmark.........................................................................8

2.1.2 ETH-Zürich and University of Toronto..............................................................14

2.1.3 Summary of Microgrippers.................................................................................16

2.2 Proposed Design .......................................................................................................17

2.2.1 Structural Designs...............................................................................................27

2.2.2 Final Device Specifications ................................................................................32

2.3 Microfabrication .......................................................................................................34

2.4 Adhesion Force Analysis ..........................................................................................37

3. Experiments...................................................................................................................41

3.1 Experimental Setup...................................................................................................41

iv

3.2 Repeatability of Active Release................................................................................42

3.3 Quantification of Release Performance ....................................................................44

3.4 Understanding the Curved Trajectory ......................................................................49

4. Micromanipulation Automation..................................................................................51

4.1 Introduction...............................................................................................................51

4.2 Microrobotic Pick-Place of Microspheres ................................................................51

4.2.1 Recognition of Microgrippers and Spheres ........................................................51

4.2.2 Contact Detection and Micromanipulator Control .............................................53

4.2.3 Automated Pick-Place of Microspheres .............................................................55

5. From Microgripping to Nanogripping........................................................................57

5.1 Introduction...............................................................................................................57

5.2 Proposed Fabrication Process ...................................................................................58

5.3 Nanogrippers Post Processing ..................................................................................63

5.4 SEM Manipulation....................................................................................................66

5.4.1 Introduction.........................................................................................................66

5.4.2 SEM Manipulation Difficulties ..........................................................................66

5.4.3 Experimental Setup.............................................................................................67

5.4.4 Pick and Release of Nanospheres .......................................................................69

6. Conclusion .....................................................................................................................71

6.1 Contributions ............................................................................................................73

6.2 Future Directions ......................................................................................................73

v

List of Tables

2.1 Summary of important design features of existing designs…… 16

2.2 Design tradeoffs for electrostatic comb-drive microactuator….. 26

2.3 Final actuator specifications…………………………………… 32

3.1 Summary of release accuracies in ambient environment……… 45

vi

List of Figures

2.1 Denmark microgrippers and manipulation methods…………... 9

2.2 University of Tokyo microtweezers for DNA manipulation and

characterization………………………………………………...

13

2.3 ETH-Zürich and University of Toronto microgrippers………... 16

2.4 Solid model of the proposed microgrippers design with two

active arms and an active release plunger……………………...

18

2.5 Proposed manipulation sequence for pick-and-place of a

microsphere…………………………………………………….

19

2.6 Schematic of parallel-plate electrostatic actuator……………… 21

2.7 Schematic of a lateral comb-drive microactuator……………... 22

2.8 Device schematic. Colours indicate parts at different electric

potential………………………………………………………...

23

2.9 Packaging options for microgrippers. (Top) wire bonding.

(Bottom) rapid, exchangeable clamping……………………….

28

2.10 The effect of adding U-shaped impact absorber………………. 30

2.11 SEM image of the MEMS microgrippers with integrated active

release mechanism……………………………………………..

33

2.12 Microgrippers fabrication process using an SOI wafer,

vii

showing both front and back side for each fabrication step…… 35

2.13 Device performance. Experimental calibration data and

comparisons with FEA simulation results. (a) gripping arms.

(b) plunger……………………………………………………...

36

2.14 Adhesion forces acting on a microsphere on a rough surface…. 38

3.1 Experimental setup for micrograsping and active release tests.

Inset shows a wire-bonded microgrippers……………………

42

3.2 Representative microsphere landing locations after release

from different heights, on (a) glass substrate, (b) steel

substrate………………………………………………………...

45

3.3 Representative release accuracy quantification on glass

substrate………………………………………………………...

45

3.4 Pick-place to align microspheres (7.5µm to 10.9µm). Note that

the microgrippers was titled 25°……………………………...

48

3.5 High speed video camera (13,000 frames per second) mounted

on top of the microscope……………………………………….

48

3.6 High-speed videography (13,000 frames/second) quantifying

microsphere trajectories upon release from a height of 20µm

above the substrate……………………………………………..

50

3.7 Microsphere reveals a curved trajectory during active release.

(a) The plunger thrusts the microshphere that reaches the

roundish corner of the gripping arm. (b) The microsphere

escapes from the effective range of the adhesion forces. The

viii

trajectory is drawn under the assumption that there are no

disturbances when the microsphere is in the air……………….

50

4.1 (a) Recognized gripping arms and plunger. (b) Sidewall of a

gripping arm for determining the secured grasping position, C.

(c) 3D schematic showing the grasping………………………..

52

4.2 Visually determine which gripping arm the microsphere

adheres to after the gripping arms open………………………..

54

4.3 Contact detection by monitoring x coordinate of a gripping

arm in the image while lowering the microgrippers at a speed

of 20µm/sec…………………………………………………….

54

4.4 Pattern formation by autonomous pick-place. (a) Microspheres

before pick-place. (b) A circular pattern with circularity of

0.52µm…………………………………………………………

56

4.5 “U of T” pattern formed by autonomous microrobotic pick-

place of 7.5–10.9µm microspheres…………………………….

56

5.1 Proposed fabrication process capable of patterning two

material layers from a single side………………………………

59

5.2 Combined fabrication process capable of producing ultra thin

tip down to nanometre thickness……………………………….

61

5.3 SEM image of the nanogrippers tips fabricated using the new

process………………………………………………………….

62

5.4 Dual beam system used for reshaping nanogrippers…………... 64

5.5 Before and after FIB milling of (a) nanogrippers tips, (b)

ix

microgrippers tips…………………………………………..…. 65

5.6 SEM-based nanomanipulation system………………………… 68

5.7 Pick up of 100nm gold particle. (a) Lateral pushing to break initial bonding. (b) Before grasping. (c) After grasping. (d) Lift and release the grasp, nano particle remain adhered to one gripping tip……………………………………………………..

70

x

Chapter 1

1. Introduction

1.1 Background

Micromanipulation provides a bridge between human and a world only visible under a

microscope. It has the potential to extend the dexterity of a human hand to the micro scale,

allowing physical interactions with micro/nano objects while providing multiple real-time

sensory feedbacks. This capability is invaluable in terms of material handling and

characterization as well as the assembly of complex micro and nanosystems. For instance,

micromanipulation has been used to build a diamond-shaped structure by assembling

microspheres into a lattice [1]. Based on a combination of microfabrication and

micromanipulation [2], novel photonic crystals were also demonstrated.

Analogous to manipulation in the macroworld, manipulating micrometer-sized objects

necessitates gripping devices with end structures comparable in size to objects to be

manipulated. Enabled by MEMS (microelectromechanical systems) technologies, many

microgripping devices have been reported, including two-fingered devices such as [3-13] and

multi-fingered devices [14-19].

1

CHAPTER 1, INTRODUCTION 2

Despite the availability of MEMS gripping tools and the significant progress made in

automation techniques for ultimately autonomous operation, micromanipulation is still largely

skill dependent and entails repeated trial-and-error efforts. Among the challenges, a long-

standing difficulty is the release of micro objects from the end effector due to strong adhesion

forces at the microscale. Force scaling causes surface forces (i.e., adhesion forces) including

the capillary force, electrostatic force, and van der Waals force to dominate volumetric forces

(e.g., gravity) [20]. Hence, objects below micro scale tend to adhere strongly to the end

effector during release process. In pursuit of rapid, accurate release methods, several

strategies have been proposed in the past decade. These methods can be group into two main

categories: passive release and active release.

1.1.1 Passive Release Methods

Passive release techniques control the adhesion forces between the tool-object interface (T-O)

and object-substrate (O-S) interface to favor either pick or release operation. Pick up is done

when T-O adhesion force is relatively larger than O-S adhesion force, and the opposite is true

for releasing micro object.

Using a single needle probe combined with sophisticated motion control, microspheres

were successfully pick and released on Au-coated substrate inside an SEM [21;22]. This was

made possible by altering the tool loading angle to selectively control the fracture of either the

tool-sphere or substrate-sphere interfaces. This manipulation technique was used to assemble

three dimensional diamond-type microporous structure for photonics application [2].

Different types of adhesives were also used to assist in object release. Under ambient

environment, ultraviolet cure adhesive was applied locally on the substrate to enable release


of 20µm glass spheres [23]. Inside the SEM, the electron beam induced deposition can be

used to solder objects onto the substrate [1;24], or commercially available electron beam

cured adhesives can be applied to form a bond between object-substrate interface.[25].

1.1.2 Active Release

Active release methods involve applying an external force to overcome the T-O surface

adhesion forces to allow release, and without making contact with the substrate. The source of

the external force includes electric field, vibration, vacuum suction, and freezing/thawing of

ice droplet.

Electric field

By applying a voltage between the probe and the substrate [26-29], an electric field was

created to detach the object from the probe. Nevertheless, this method requires the micro

object, the probe, and the substrate all to be conductive. More importantly, the released micro

objects landed at random locations on the substrate, resulting in a poor release accuracy.

Vibration

Requiring a large bandwidth of the manipulator, the vibration-based method takes advantage

of inertial effects of both the end-effector and the micro object to overcome adhesion forces

[30;31]. The release process has been modeled and simulated to predict the landing radius of

the released object [32]; however, the accuracy has not been experimentally quantified.


Vacuum-based tools

Vacuum-based tools [33-35] create a pressure difference for pick and release. However,

miniaturization and accurate control of vacuum-based tools can be difficult, and its use in a

vacuum environment can be limited.

Freezing/thawing

Micro peltier coolers were used to form ice droplets instantaneously for pick-place of micro

objects [36-38]. Thawing of the ice droplets was used to release objects. The freezing-heating

approach requires a bulky, complex end-effector and is limited to micromanipulation in an

aqueous environment.


1.2 Motivation

The lack of a highly repeatable and accurate release method limits efficient, automated

micromanipulation, which is important for in situ sample preparation and handling as well as

for the construction of micro and nano structures/devices. What is needed is an end-effector

design that permits (1) easy, secured grasping of micro-sized objects; and (2) rapid, highly

reproducible, accurate release of the objects.

The objectives of this research are:

• To design and microfabricate MEMS-based gripping tools capable of efficient and

repeatable pick-and-release operation.

• To manipulate microspheres under optical microscope and quantify system

performance.

• To automate the micromanipulation sequence to realize high-speed assembly.

• To develop a new fabrication process that allows further miniaturization of the

microgrippers to enable nanomanipulation inside a scanning electron microscope.


1.3 Dissertation Outline

An overview of the ensuing chapters is as follows: Chapter 2 describes the design, finite

element analysis, fabrication, and calibration of microgrippers. Chapter 3 describes the pick

and release manipulation result with the microgrippers. Chapter 4 presents the automation of

the manipulation process, including assembly of pre-defined patterns from microspheres.

Chapter 5 describes a novel fabrication process that allows further down scaling of

microgrippers to nanogrippers, as well as discussions of the difficulties in nanomanipulation.

The thesis is concluded in Chapter 6, with a summary and contributions of this research and

suggested future research directions.

Chapter 2

2. Three-Pronged Microgrippers

2.1 Introduction

Most past research focused on using a single needle-like probe combined with surface

adhesion forces to execute the operation of pick-and-place micro objects, which is poor in

reproducibility. The methods either rely on sophisticated robotic automation to perform

complex motions [21;22] or are only effective under predefined conditions [21-23;26-29;33-

38]. Microgrippers have also been demonstrated to ease the pick-up operation by applying

gripping force with two gripping arms. However, no active release mechanism was developed

to allow release-on-demand [9;11;24;26;39-43].

In this research, the focus was to develop a double-ended microgrippers with an integrated

active release mechanism for realizing efficient and reliable pick-and-place operation. This

configuration retains the advantage of double-ended gripping tool for easy pick up of micro

objects, while the integrated active release mechanism allows release on demand. Currently,

there are a limited number of major research groups that have been sustaining continuous

microgrippers development efforts.

7

CHAPTER 2, THREE-PRONGED MICROGRIPPERS

8

2.1.1 Technical University of Denmark

This group has published several papers on the design of microgrippers and applications

inside the scanning electron microscope (SEM). The progression of their designs can be

divided into three generations.

First Generation [39]

First generation was an electrostatically actuated microgrippers, shown in Figure 2.1(a). In

this design, electric potential difference is applied between the grounded gripping arms and

the stationary electrodes. The generated electrostatic force controls the motion of the gripping

arms. The movable structures were fabricated from 1µm thick silicon dioxide covered with a

layer of metal (100A Ti/1000A Au) [44]. The minimum feature dimension of the device is

2µm. To reduce the gripper tip dimension, the group used electron beam induced deposition

(EBID) to form tweezers-liked structure (~100nm) at the tip of the grippers, and was able to

reduce the grippers opening gap down to 25nm. In terms microgrippers testing, the original

microgrippers was used to detach a silicon nanowire from the edge of a substrate through

active gripping, while the microgrippers with add-on tweezers showed no active gripping.

The main design drawback is the low aspect ratio (i.e., ratio between device thickness and

device width) of the movable structures, making it unsuitable for manipulation due to large

undesired out-of-plane deflections during actuation.


9

(a) (b)

(c) (d)

Figure 2.1: Denmark microgrippers and manipulation methods [24;46;45]. Permissions to reproduce these figures are included in Appendix.


10

Second-generation [45]

The second-generation was an electrothermally actuated microgrippers, named symmetric

three-beams, which used unequal heating in three parallel beams (Figure 2.1(b)) to produce

bi-directional motion depending on the applied voltage configurations. Each gripping arm can

also be connected to a Wheatstone bridge for force sensing, where the difference in

piezoresistive changes between the bent beams can be used to measure actuator deflections

and hence, the applied force with a known spring constant of the structure.

The grippers from first two generations were used to investigate different methods for

pick-and-place of nanowires [46]. The methods are illustrated in Figure 2.1(c). For the case of

nanowire manipulation, from Figure 2.1(c), picking using method “h” and placing using

method “j” was found to be the most effective.

There are a number of problems associated with this design:

1. Structural: using the same fabrication process as the first generation, these devices still

suffer from undesired out-of-plane deflections due to low aspect ratio in structure.

2. Thermal:

a) The electrothermal actuation of the grippers results in a high temperature at the

gripping tips (90% of maximum temperature), which limits the materials suitable

for manipulation.

b) The vacuum environment inside SEM limits heat transfer to conduction through

the substrate, and the dedicated design of heat sink was not implemented to reduce

the temperature of the gripping tips.

c) Release of nanowire uses electron beam induced deposition (EBID), which is a

time consuming, irreversible, and inconsistent process that varies according to


11

vacuum quality inside the SEM, imaging parameters, and image drift.

3. Electrical:

a) Ideally, the gripping tips should have no potential difference if the same actuation

voltage was applied on both gripping arms, assuming both arms have identical

dimensions and material properties. However, it was reported that a variation in

maximum actuation motion of 10-30% between devices exists due to lithography

and etching uncertainties that produced slight variations in dimensions across

devices. A similar uncertainty also caused the two microactuators within a

microgrippers to behave differently. The problem appears more apparent when

manipulating conductive materials, where the gripper tips with slight differences

in electric potential would produce a sudden jerking motion when they close

around a conductive object.

b) The gripping tips of the microgrippers have a different electrical potential from

the earth ground during actuation. Therefore, the sample substrate needs to be

electrically insulated to avoid current flowing from the actuated microgrippers to

the substrate upon contact.

Third-generation [24]

The third-generation consists of two electrothermal microgrippers fabricated on a silicon-on-

insulator (SOI) wafer (Figure 2.1(d)). The first microgrippers was called asymmetrical rib-

cage, and it had one movable arm that uses a V-beam thermal actuator to generate large

gripping force. The second microgrippers was identical to the second-generation

microgrippers with unequal heating in three beams.


12

These microgrippers were made using a different fabrication process on SOI wafer,

producing a structural aspect ratio of 2.5. The higher aspect ratio has the advantage of reduced

out-of-plane motion and generating larger gripping force (25µN and 1µN, respectively for the

two designs). These microgrippers were used to transfer a catalytically grown multi-walled

carbon nanofibre from a fixed substrate to the tip of an atomic force microscope cantilever,

improving trench profile imaging of AFM compared with conventional silicon pyramidal tips.

Design limitations of the third-generation:

1. Structural: The increase in aspect ratio from 0.5 to 2.5 increased forces generated by

the gripper. However, it was reported that the symmetric three-beam design often

experienced difficulties detaching the nanowire from the substrate due to low gripping

forces.

2. Thermal/Electrical: The gripper tip varies in temperature and voltage during

manipulation. The lack of thermal and electrical insulation of the tip limits the

application of the microgrippers.

University of Tokyo [40;41;43]

The group has developed microtweezers for the manipulation and characterization of DNA

molecules (Figure 2.2). During operation, the tips were dipped inside a droplet containing

DNA molecules. Dielectrophoresis was used to capture DNA between the tips to form a DNA

bundle. To conduct mechanical testing on the DNA bundle, one tip was kept fixed while the

other was actuated electrostatically to stretch the bundle. Differential capacitive sensor

attached to one arm was used to measure the force needed to stretch the bundle, and the

number of DNA in the bundle was estimated from the increase in stiffness of the mechanical


13

structure. Although not explicitly stated, both tips were electrically insulated from the actuator

and sensor using the same technique as Sun et al. [47]. This allows the two tweezers tips to be

electrically connected to an outside circuit for DNA electrical property characterization.

For manipulating nanometre-sized objects, it is important for the gripping tips to have

comparable size as the object to be manipulated. Unlike most proposed microgrippers design

where the thickness of the gripper/tweezers tips is the same as the rest of the grippers

structure, this group uses KOH anisotropic silicon wet etching locally to produce sharp

tweezers tips with tip dimension smaller than 10nm while maintaining high aspect ratio for

the rest of the device. This has the advantage of high actuation and sensing capability due to

the increase in overlapping area, while the tweezers dimensions were reduced to the

dimensions of targeted object. Although the design was not used for active grasping, with

slight modification on the actuation range and gripper tips dimensions, this design would be

capable of nanomanipulation.

Figure 2.2: University of Tokyo microtweezers for DNA manipulation and characterization [40]. Permission to reproduce this figure is included in Appendix.


14

This device design had a complex fabrication process combining multiple steps of wet

etching, dry etching, and metal/non-metal deposition and removal steps, resulting in a low

fabrication yield. Additionally, no release mechanism was integrated into the device, and the

quality of the gripping tips (e.g., dimension and surface roughness) is difficult to control

during fabrication.

2.1.2 ETH-Zürich and University of Toronto

The research groups from ETH-Zürich and the University of Toronto have published several

papers on microgrippers designs and the manipulation of biomaterials. The manipulation of

cells and other biomaterials requires the microgrippers to work in an aqueous environment,

produce low heat, and have gripping force feedback to avoid material damage. Additionally,

the integration of contact sensors is desirable to avoid microgrippers breakage when

interacting with the substrate.

First Generation [4]

The first generation devices used electrostatic comb-drives for actuation and differential

capacitive comb-drives for gripping force sensing (Figure 2.3(a)). The gripper tips were

electrically insulated using the fabrication process developed by Sun [42] for capacitive force

sensors, enabling 2mm long gripper arms to submerged into fluid. The electrostatic comb-

drive actuation enables continuous and easily predictable gripping motion. The microgrippers

had one active gripping arm and one sensing arm, making gripping force measurement

independent of the size or mechanical properties of the grasped object. The parallel-plate


15

differential capacitive sensor offers high sensitivity measurements of gripping forces. The

gripper configuration also allows the measurement of gripper-object adhesion force. In terms

of experiments, the manipulation of glass spheres (20-90µm) and adhesion force

measurements in air were demonstrated.

These devices were well designed for picking up objects and measuring gripper-object

adhesion; however, the devices required high voltages for actuation (tens of volts). The

structures of the two gripping arms are not exactly identical, and the differences in out-of-

plane structural stiffness would cause the gripper tips to misalign in the out-of-plane axis.

This small misalignment would limit the minimum size of objects the device can grasp.

Second-generation [11]

The second-generation of microgrippers (Figure 2.3(b)) was actuated by electrothermal

actuators, and was capable of sensing both gripping forces and grippers tip-substrate contact

forces. The addition of contact force sensor was essential for automated micromanipulation

since it provided quantitative contact force information to prevent device breakage. The active

gripping arm used electrothermal actuation to provide large gripping forces. The devices also

contained heat sinks for both conduction and convection heat transfer. Using a fine-gauge

thermocouple, the temperature of the gripper tip at working voltages was found to be 29oC in

air. The second-generation devices would not be capable of manipulating nanometer-sized

objects due to out-of-plane misalignment of the two gripping tips.


16

(a) (b)

Figure 2.3, ETH-Zürich and University of Toronto microgrippers [4;11]. Permissions to reproduce these figures are included in Appendix.

2.1.3 Summary of Microgrippers

The goal for a microgrippers to achieve is automated pick-and-place of micro-nanometer-

sized objects and elimination of human skill dependence and errors. The microgrippers

discussed in this chapter have demonstrated some aspects of an ideal end-effector. However,

all existing designs encounter difficulties in releasing an object. Table 2.1 summarizes

important design features of the microgrippers discussed in this chapter.

Table 2.1: Summary of important features of existing designs.

Denmark Tokyo Zürich/Toronto

Structure thickness* (Micron) 5 25 50

Tip thickness (Micron) 5 0.01 50

Tip operating Temperature

90% of max. actuation temp. N/A <29oC in air

Tip electrical insulation no yes yes

Release method EBID N/A N/A**

Relative fabrication difficulty easy difficult medium

* Structure thickness refers to all locations excluding the gripper tip. This parameter is reported instead of the aspect ratio because some papers did not mention the minimum feature size of their device. ** The manipulation was done in fluids. Objects were usually released without external assistance.


17

2.2 Proposed Design

No techniques exist for easy, rapid, accurate, and highly repeatable release of micro objects in

micromanipulation. In this research, an active release strategy by using a MEMS

microgrippers that integrates a plunging structure between two gripping arms was developed,

as shown in Figure 2.4. While the method retains the advantage of double-ended tools for

picking up micro objects, the plunger is capable of thrusting a micro object adhered to a

gripping arm to a desired destination on a substrate, enabling highly repeatable release with a

high accuracy of 0.45±0.24µm. The manipulation experiments were conducted with

microspheres that are widely used for demonstrating manipulation capabilities [1;21;22].

Figure 2.5 illustrates the proposed manipulation sequence of microspheres. (a) The

microgrippers approaches a microsphere and uses one gripping arm to laterally push it to

break the initial adhesion bond between the microsphere and the substrate. (b) Two gripping

arms are closed, grasping the microsphere and lifting it. (c) The microsphere is transported to

a target area and positioned at a small distance above the substrate. (d) The gripping arms are

opened, and the microsphere remains adhered to one of the gripping arm, randomly. (e)

Microsphere is properly aligned to the plunger. (f) The plunger thrusts out the microsphere

that lands accurately on the substrate.


18

spring flexures

Figure 2.4: Solid model of the proposed microgrippers design with two active arms and an active release plunger.

motion limiter

gripping arms

plunger

gripping arm comb-drive actuators

electrodes

z x

y

plunger comb-drive actuators

1mm


19

(a) (b) (c)

Based on the manipulation sequence, the following design requirements were determined:

1. Gripping arms:

a) Gripping arms should be independently controlled to allow object alignment with

plunger before release.

b) Linear, high resolution motion.

c) Electrically/thermally grounded/insulated gripper tips.

2. Active release plunger

a) Variable plunging speed control.

b) Consistent plunging speed.

c) Relatively high actuation bandwidth.

d) Electrically/thermally grounded/insulated plunger tip.

(d) (e) (f)

break adhesion bond

move to destination grasp and pick up

release grasp align microsphere active release

Figure 2.5: Proposed manipulation sequence for pick-and-place of a microsphere.


20

Choosing Microactuators

Possible microactuator choices include electrostatic, thermoelectric, piezoelectric, magnetic,

shape memory alloy, and fluidic. In this research, silicon-based MEMS microactuators were

chosen since they can be more readily integrated and can be batch microfabricated on a

silicon wafer along with the rest of the device structures. The two most common types of

microactuators are electrostatic and thermoelectric. Brief comparisons are described below.

Thermoelectric microactuators

Thermal expansion from Ohmic heating produces large forces and large motion ranges when

combined with motion amplification mechanism [48;49]. The actuation performance depends

upon the heating and cooling rate of the actuator material, where the dominating heat transfer

mechanisms at the micro scale are conduction through the substrate and convection through

air.

Due to Ohmic heating, thermoelectric actuators feature high actuation temperature, low

bandwidth, and actuation speed dependent upon the frequency of actuation and environmental

conditions. Additional fabrication process can be included to provide thermal and electrical

insulation [11;42]; however, the above mentioned issues cannot be overcome easily. Based on

the design requirements, thermoelectric microactuator was determined unsuitable for this

application.


21

flexure spring

V

Figure 2.6: Schematic of parallel-plate electrostatic actuator.

Electrostatic microactuators

The two common configurations of an electrostatic actuator are lateral comb-drives and

transverse parallel-plates. Figure 2.6 shows a simple parallel-plate actuator, consisting of one

fixed plate and one movable plate connected to spring flexures. By applying a potential

difference between the two plates, electrostatic force is generated according to equation (2.1).

The magnitude of the generated force depends upon the plate overlapping area A and spacing

d, actuation voltage V, and electric permittivity ε. This type of actuator features non-linear

large force output and a stable motion range limited by the snap-in or pull-in effect. Snap-in

occurs when the increase in restoring spring force from the flexure can no longer balance

electrostatic force, causing the two capacitor plate to snap together.

222

1 VgAF ε

−= (2.1)

Figure 2.7 shows a schematic of comb-drive actuators. It uses the fringing electric field to

generate attraction forces according to equation (2.2). The magnitude of the generated force

depends upon actuation voltage V, number of comb finger pairs N, thickness of the comb


22

fingers h, gap between comb fingers g, electric permittivity ε. This type of actuator features

linear force output, smaller force output, and no snap-in effect. The motion range depends

upon the structural stabilities of the comb fingers and flexures.

ghNVF ⋅⋅

=ε2

21

(2.2)

Both types of electrostatic actuators can be connected in parallel to provide larger force

output. Based on the design requirements, both kinds of actuators can satisfy all conditions;

however, comb-drive electrostatic actuator was chosen due to its linear force output and no

snap-in effect.

flexure spring

V

Figure 2.7: Schematic of a lateral comb-drive microactuator.


23

x C D

y

F2 E1

E2

gripper and plunger tips

E3

E4 F1

B F3

Figure 2.8: Device schematic. Colours indicate parts at different electric potential.

Implementation of Microactuators

The electrostatically actuated microgrippers comprises of three parts, as illustrated in Figure

2.8: (i) two electrostatic comb-drive microactuators (B, C) each controlling one of the two

gripping arms for grasping and gripper-plunger alignment; (ii) electrostatic comb-drive

actuator D for controlling active release plunger; and (iii) Linear beam flexures used to

transform actuated forces into displacements (F1,F2,F3).


24

Lateral comb-drive microactuators are ideal for micro-nanomanipulation due to their high

bandwidth, high motion resolution, no temperature gradient, ease to implement, and adequate

force output to overcome surface adhesion forces. By changing the dimensions of the flexures

connected to it, the motion range and resolution of the actuator can be adjusted.

In the proposed design, comb-drive microactuator B produces forces to deflect flexures F1,

and the linear motion is directly transferred to the gripping arm. The second gripping arm

connected to microactuator C through flexure F2 has a symmetrical configuration. The

gripping arms are individually controlled by applying voltage between electrode E2 and E1,

or E4 and E1. The grippers tip separation determines the largest size of objects to be grasped.

The active release plunger is controlled by applying a voltage between electrode E3 and E1,

where forces produced by the comb-drive microactuator deflects flexures F3 and produces

linear motion. The four tethered flexures F3 minimize out-of-plane motion in the x-y plane,

relative to the gripping tips.

The active release plunger can be used in different ways. To achieve a substrate

independent release, a sharp increase in the actuation voltage would allow the plunger to

move at a high speed and collide with the object adhered to one of the gripping arms. The

impact allows the adhered object to gain sufficient momentum to overcome the adhesion

forces between the object and a gripping arm, resulting in release. In the case when the

plunger moves at a relatively low speed, the adhered object can be pushed off from the

gripping arm and directly into the substrate; however, the success would depend on adhesion

force differences between the plunger-object and the object-substrate contact surfaces. When

a plunger is extended beyond the gripping arm tip, it can also function as a needle probe for

manipulation, enabling single-probe manipulation as in [21;22].


25

All three actuators share one common electrode, E1, which is directly linked to the

grippers and plunger tips. By altering the electric potential at E1, the electrostatic force can be

changed accordingly for different environments. In the case of micromanipulation in the

ambient environment, E1 is preferably connected to the electrical ground to reduce

electrostatic forces at the object-gripper interface. For SEM manipulation where electron

irradiation can produce charging, E1 can be controlled to adjust the electrostatic force at the

object-tool interface to favor either the pick or release process.

Determination of Microactuator Specifications

The specifications of a microactuator (comb-drive dimensions and actuator flexures)

determine force and displacement output as well as its fabrication yield and device robustness.

The force and displacement output of a microactuator can be easily varied by changing the

actuation voltage; however, the fabrication yield and device robustness require careful

considerations during the design stage.

It is desirable for a microactuator to have high force output and large motion range,

allowing it to manipulate objects of different sizes under different environments. However,

the tradeoff is often the reduction in fabrication yield and device robustness. For example, a

smaller comb finger gap and larger number of comb fingers produce larger force output

according to equation (2.2); however, photolithography becomes more difficult, and structural

integrity of the device would be sacrificed. The microfabrication yield cannot be

quantitatively estimated because it depends upon fabrication skills and prior fabrication yield

statistics. This creates much difficulty in determining an optimal specification for the


26

actuators because the trade-offs cannot be quantitatively compared. Table 2.2 summarizes the

trade-offs in comb actuator design.

The design and fabrication of MEMS devices have a lengthy turnaround time. To ensure

the research to be completed in a two-year time frame, the highest priority was to maximize

the device yield while maintaining an adequate level of performance. Based on the design

tradeoffs, it is not possible to design a microgrippers suitable for a wide range of applications

while maintaining a high fabrication yield and device robustness. Therefore, the

microgrippers design was narrowed down to applications involving the manipulation of micro

spherical objects ranging from 1µm to 15µm in diameter, and gripping force output less than

50µN. The maximum actuation voltage was chosen to be 80V, which can be easily obtained

using the available power supply in the lab.

Table 2.2: Design tradeoffs for electrostatic comb-drive microactuator.

Device Modifications Force Output Displacement

Output Device Yield Device Robustness

Decreasing Comb Gap increased - decreased decreased

Increasing Finger Numbers increased - decreased decreased

Increasing Actuation Voltage increased - - -

Increasing comb finger overlapping

areas increased - decreased increased

Increasing finger length - increased decreased decreased


27

2.2.1 Structural Designs

Device Packaging

The fabricated microgrippers were individually attached to a custom made circuit board with

adhesives, and then wire bonded to establish electrical connections, as shown in Figure 2.9(a).

The wire bonder used was a manual ultrasonic wedge wire bonder (K&S 4129). Wire bonding

quality was highly skill dependent, and the high frequency vibration could cause damage to

the microgripper. Hence, an alternative packaging method was pursued and illustrated in

Figure 2.9(b). By modifying off-the-shelf connectors, metal electrodes can be clamped

directly onto the electrodes of the microgrippers using a paper clip. The setup can be

assembled in a fraction of time needed for wire bonding a device. This method worked well

with electrostatic actuators despite of the large contact resistance at the clamped interface,

since almost no current flow exists between capacitor plates.

Motion Limiter

Under normal circumstances, no current can flow between the comb-drive pairs due to the

separation gap. However, in the case when the two comb-drive pairs snap together due to

unexpected external influence, large current will flow through the device creating Ohmic

heating and subsequent device melt down.

Two preventive methods were implemented into the design. The first method involved

mechanical motion limiters to prevent combs drive pairs from making contacts, as shown in

Figure 2.4. The second method involved limiting the current flow to the microgrippers by

setting a current limit on the power supply, or adding a large resistor in series to dissipate the


28

power in the case of snap-in. Adding a large resistor would ensure the safety of the

microgrippers during snap-in; however, it also slowed down the charging of the capacitor

plates and produced slow actuation speed. Through experiments, a 10kΩ resistor was found

suitable for our application.

Figure 2.9: Packaging options for microgrippers. (Top) wire bonding. (Bottom) rapid, exchangeable clamping.

paper clip microgrippers

clamped electrodes

(a)

(b)


29

Impact Absorber

Micromanipulation under an optical microscope provides only two-dimensional visual

feedback, where the height above the substrate cannot be accurately estimated. As a result, the

gripper tips should be flexible enough to absorb the impact from contacting the substrate.

According to Figure 2.8, the gripping-arm actuators require a high stiffness in the Y axis to

avoid comb fingers collapsing onto each other, yet a low stiffness in the same axis to reduce

the impact of the substrate. To locally reduce the stiffness of the gripper tips, U-shaped

structure was implemented on the gripping arms. Figure 2.10 shows the deflection and stress

distribution from gripper-substrate impact obtained from finite element analysis (FEA), where

displacement loadings were applied at the tip the gripper. In the case without the added

flexure, the comb-drive actuator produces 66% larger rotational motion that would increase

the chance of short circuit, and the gripper tips would experience approximately 42% larger

stress.

Minimization of Out-of-Plane Motion

To effectively grasp and actively release micro objects, the gripping arms and the plunger

should remain in the same plane during operation. Three design factors were considered:

1. All flexure thickness should be at least 5 times larger than the width, providing a

good aspect ratio to prevent out-of-plane bending.

2. Structures for gripping arms should be identical to provide the same out-of-plane

stiffness.

3. The plunging arm length should be minimized to reduce the out-of-plane bending

due to gravity.


30

Flexure Added No Flexure Added

displacement distribution

stress distribution

minimum maximum

Figure 2.10: The effect of adding U-shaped impact absorber.

From FEA, a maximum out-of-plane deflection of 23nm within the device structures was

found due to gravity. This is an insignificant amount compare to the size of the micro object

this grippers was design to handle. For the actual fabricated microgripper, an out-of-plane

difference of 3µm between the plunger and the gripping arm was measured. The source of

this large out-of-plane deflection was when a photolithography masking layer is deposited

onto a wafer, stresses in the masking layer would cause slight deformation in the wafer. This

deformed wafer would then go through deep etching processes that permanently engrave the

stresses into the wafer. Since different structures would experience a different amount of


31

stress from masking layer, it is expected that they would not remain in the same plane after

released.

The plunger and the gripping arms have different out-of-plane stiffness. Thus, they deflect

by a different amount in the presence of stress. By controllably depositing a thin layer of

material over the microgrippers to apply either compressive or tensile stress, the plunger and

the gripping arm would deflect less. Through trial and error, it is possible to apply a precise

amount of stress that deflects the structures until they are in the same plane. In this research,

0.7µm of silicon dioxide (SiO2) was deposited onto the microgrippers using plasma enhance

chemical vapour deposition (PECVD) to produce in-plane structures.

Device Handling

To allow easy handling of the devices during fabrication and packaging, large unused area

between the actuators and the electrodes serves many purposes, including a firm grasping

location for tweezers, a large surface area for applying adhesives, a large surface to press

down against, and a place to label each grippers type.


32

2.2.2 Final Device Specifications

Table 2.3 summarizes the final design specifications, and Figure 2.11 shows the image of the

microgrippers taken inside the SEM.

Table 2.3: Final actuator specifications.

Component Dimension Value (µm) gripping-arm actuator out of plane thickness 25

comb finger length 20

comb finger gap 4.5

comb finger overlap 5

# of comb finger pairs 280

gripping-arm flexures X2 out of plane thickness 25

length 580

width 6

plunger actuator out of plane thickness 25

comb finger length 20

comb finger gap 4

comb finger overlap 5

# of comb finger pairs 540

plunger flexures X4 out of plane thickness 25

length 628

width 5


33

Figure 2.11: SEM image of the MEMS microgrippers with integrated active release mechanism.


34

2.3 Microfabrication

To fabricate the microgrippers with active release plunger, the DRIE-SOI process proposed

by Sun et al. [47] was modified. The simplified process eliminates the step formation process

on the handle silicon layer, which increases fabrication yield and shortens microfabrication

time. For this application, the chosen SOI wafer included 25µm thick device layer, 1µm thick

buried oxide (BOX layer), 300µm thick handle silicon layer, and 1µm thermally grown oxide

on the handle layer. These thicknesses were chosen to provide a structural aspect ratio of 5 to

minimize out-of-plane motion and for the manipulation of objects larger than 1µm. The

microfabrication process is illustrated in Figure 2.12 and summarized below.

1. RIE (reactive ion etching) to pattern the handle layer SiO2.

2. Patterns device layer silicon and create chromium/gold electrode through lift-off

process.

3. DRIE (deep reactive ion etching) handle layer to remove 300µm of silicon and

expose buried oxide layer.

4. HF wet etch to remove exposed SiO2.

5. DRIE etch through the top device layer to release individual device.


35

Front Side Back Side

(a)

(b)

(c)

(d)

(e)

SOI handle layer

SOI device layer

SiO2

gold electrode

buried oxide

Figure 2.12: Microgrippers fabrication process using an SOI wafer, showing both front and back side for each fabrication step.


36

Device Calibration

Batch microfabrication allows a large number of devices to be fabricated from a single wafer.

However, the performance of each device should still be calibrated individually. During

microfabrication, it is not feasible to ensure all areas on a wafer would experience the same

deposition and etch rate, which creates slight variations in structural dimensions across

devices.

Motion calibration of the gripping arms and plunger were conducted under an optical

microscope. Actuation voltages were applied by a DC power supply. The resulting

displacements were recorded using a digital camera. With a 50× microscope objective, 1 pixel

corresponds to 0.11µm physically. Figure 2.13(a) and 2.13(b) show experimental calibration

results for both gripping arms and plunging arms as well as comparisons with FEA simulation

results. FEA in ANSYS was done by first calculating electrostatic forces using equation (2.2),

and then applying the forces as input for structural analyses to determine actuator

displacements.

(a) (b)

0.00

2.00

4.00

6.00

8.00

0 1000 2000 3000 4000 5000 6000

disp

lace

men

t (µm

).

FEAExperiment

0.001.002.003.004.005.006.007.00

0 500 1000 1500 2000 2500 3000

disp

lace

men

t (µm

).

FEAExperiment

actuation voltage squared (V2) actuation voltage squared (V2)

Figure 2.13: Device performance. Experimental calibration data and comparisons with FEA simulation results. (a) gripping arms. (b) plunge


37

The differences between the simulation results and experimental results can be due to: (1)

material properties assumed in FEA analysis were not accurate, and (2) slight variations in

dimensions due to fabrication imperfection were not taken into account in design simulation.

2.4 Adhesion Force Analysis

The device permits experimentally estimating adhesion forces between the gripping arms and

a grasped microsphere. After the microsphere is gently but securely grasped, the actuation

voltages for the gripping arms are released in a continuous and synchronous manner until the

voltage, V2 at which the gripping arms are opened is obtained. The adhesion forces can then

be estimated as,

)(21 2

22

1 VVb

tNF −=ε

(2.3)

where ε is the permittivity of air, N is the number of comb finger pairs, t is the thickness of

the comb fingers, b is the gap between opposing comb fingers, and V1 is the voltage applied to

both of the gripping arms to create a gap of the size of the microsphere. Despite

microfabrication imperfections, the adhesion forces obtained through this electrostatic force

estimation are deemed valid for understanding purposes.

Adhesion forces in an ambient environment include three types of attractive forces, namely,

the van der Waals force, the electrostatic force, and the capillary force, all of which depend on

the separation distance, δ, between a microsphere and a flat surface it adheres to. Figure 2.14

shows a microsphere adhered to a flat surface with surface roughness exaggerated.


38

Figure 2.14: Adhesion forces acting on a microsphere on a rough surface.

The van der Waals force [50] is

⎟⎟⎠

⎞⎜⎜⎝

⎛+⎟

⎠⎞

⎜⎝⎛

+=

3

2

2

2

8162/ πδρ

πδδδ HHdr

Fvdw (2.4)

where r is the roughness of the flat surface, H is the Lifshitzvan der Waals constant which

ranges from 0.6eV for polymers to 9.0eV for metals, d is the microsphere diameter, and ρ is

the radius of the adhesion surface area.

To estimate the van der Waals force between a 10µm borosilicate microsphere and the

sidewall of a gripping arm, δ is assumed to be 0.35nm [51], ρ is assumed to be 0.65% of the

radius of the microsphere [51], H is assumed to be 7.5eV [51], and r is assumed to be 100nm.

Thus, the van der Waals force is calculated to be 1.51× 10−4µN.

The electrostatic force [52] is

δπε

2

2dUFelec = (2.5)

where ε is the permittivity of air, and U is the voltage difference between the microsphere and

the flat surface. When U is assumed to be 0.40V [51], the electrostatic force between a 10µm

microsphere and the sidewall of a gripping arm is calculated to be 6.36×10−2µN.


39

The third type of attractive force is the capillary force [53],

)cos2/(1cos2

δθδθγπ−+

=K

cap rdF (2.6)

where γ is the liquid surface tension, which is 0.073Nm−1 for water at 22oC, θ is the contact

angle of the meniscus with the microsphere, and rK is the Kelvin radius, which is defined as

the mean radius of the curvature of the liquid-vapour interface.

For estimating the capillary force exerted on a 10µm microsphere by a water meniscus at

room temperature, θ is assumed to be 10o, δ is still assumed to be 0.35nm as for the

calculation of the van der Waals force, and rK is assumed to be 1nm. The capillary force is

calculated to be 3.71µN.

For comparison purposes, the gravity of the 10µm microsphere is calculated to be

1.31×10−5µN, using the density of borosilicate glass, 2.55g/cm3. In summary, the pecking

order is

gravvdweleccap FFFF >>>>>>

It can be seen that the van der Waals force is the smallest among the three attractive forces.

The van der Waals force heavily depends on the roughness of the surface. Since devices were

formed through deep reactive ion etching, which produces scallop structures on the sidewalls

of the gripping arms, the rough surface makes the van der Waals force negligible. The

electrostatic force depends on voltage differences, which are difficult to accurately estimate

when the microsphere is nonconductive. Unlike the van der Waals force and electrostatic

force, neither of which requires physical contact, the capillary force in the air results from a

phenomenon called capillary condensation [52]. Liquid from the vapour phase condenses

between sufficiently close asperities and forms menisci that cause the capillary force. Thus,


40

there exists a working range, beyond which the capillary force as well as the liquid menisci

disappears.

Chapter 3

3. Experiments

3.1 Experimental Setup

The experimental setup (Figure 3.1) consists of an optical microscope (Motic PSM-1000)

with a CMOS camera (Sony XCD710). A custom made circuit board with a wire bonded

microgrippers was mounted on a 3-DOF micromanipulator (Sutter MP285) at a tilting angle

of 25o. The angle can vary greatly depending on the setup of the experiment, and is chosen to

avoid the circuit board from contacting the substrate while ensuring a clear view of the

gripping arm tip.

Borosilicate glass microspheres (diameters: 7.5-10.9µm) were manipulated at room

temperature of 22oC with relative humidity of 50±5%. A droplet of microspheres in

isopropanol was micropipetted onto the substrate and let dry in air. The surface tension of

isopropanol (0.021N/m at room temperature) is smaller than that of water. However, due to

the volatility of isopropanol and because the microspheres were let dry in air for a prolonged

period, water was assumed to constitute most of the liquid menisci between the microspheres

and the substrate. Therefore, the surface tension of water was used in equation (2.6) in Section

2.5 for estimating the capillary force.

41

CHAPTER 3, EXPERIMENTS 42

Figure 3.1: Experimental setup for micrograsping and active release tests. Inset shows a wire-bonded microgripper.

Two types of substrates, wipe cleaned with isopropanol and let dry in air, were used in the

experiments, including an electrically conductive substrate (steel) and a non-conductive

substrate (glass). These two substrates were expected to exert different electrostatic forces and

van der Waals forces on the microsphere while it is traveling in air during release, which

might affect the release accuracy.

Two types of substrates, wipe cleaned with isopropanol and let dry in air, were used in the

experiments, including an electrically conductive substrate (steel) and a non-conductive

substrate (glass). These two substrates were expected to exert different electrostatic forces and

van der Waals forces on the microsphere while it is traveling in air during release, which

might affect the release accuracy.

3.2 Repeatability of Active Release 3.2 Repeatability of Active Release

After the gripping arms opened, the microsphere randomly adhered to a gripping arm in all

cases. The overall adhesion forces between the gripping arms and microsphere were estimated

to be 3.6µN to 5.8µN through measuring actuation voltages required to open the gripping

arms after a gentle yet secured grasping of a microsphere, as described by equation (2.3) in

Section 2.5.

After the gripping arms opened, the microsphere randomly adhered to a gripping arm in all

cases. The overall adhesion forces between the gripping arms and microsphere were estimated

to be 3.6µN to 5.8µN through measuring actuation voltages required to open the gripping

arms after a gentle yet secured grasping of a microsphere, as described by equation (2.3) in

Section 2.5.

For successful release, the microsphere must gain a sufficient amount of momentum from

the collision with the plunger in order to overcome the adhesion forces. The speed of the

For successful release, the microsphere must gain a sufficient amount of momentum from

the collision with the plunger in order to overcome the adhesion forces. The speed of the


plunger can be varied by controlling the rising time of the actuation voltage, which in turn

controls the momentum transfer. Meanwhile, the adhesion force between the microsphere and

the gripping arm is more difficult to estimate, because it can vary depending upon factors

such as microsphere size/material, contact interfaces, sample preparation, testing environment,

etc. Many of these factors are difficult to estimate and control, and some factors vary over

time.

To identify the optimal conditions for maximizing release repeatability, these parameters

are systematically tested. These experiments involve varying one parameter while keeping

other controllable parameters fix, and the release success rate was recorded and analyzed. In

the case when plunger fails to overcome the adhesion force, the microsphere will either

adhere to one of the gripping arm or the plunger tip. To reset the experiment, a needle probe is

often used to carefully remove the adhered microsphere, which is a time consuming process

that relies on trial and error.

Through hundreds of trial, two key influencing parameters were identified that guaranteed

the release. The first is to apply high plunging speed (e.g., from 0V to 50V within 0.1sec), and

the second is to pre-bake the sample prior to the experiment to reduce the moisture content on

the sample and reduces the capillary force. It was difficult to quantify the baking requirements

for the sample, since it depends upon environmental factors such as room temperature and

humidity. In general, baking is done on a hotplate at 90 degree Celsius for 5-10 minutes. In

the case of over baking, it was observed that the microsphere might fall off from the gripping

arm from vibration due to reduced adhesion force, in that case, letting the sample sit in

ambient environment for a few hours generally restore the adhesion force.


3.3 Quantification of Release Performance

To quantitatively characterize release performance, single microspheres were repeatedly

picked and released from different heights (2–30µm) above the substrate. Figure 3.2(a) shows

representative data of landing positions on a glass substrate. The results show a fairly linear

and predictable relationship between landing positions and heights from the substrate,

indicating that forces including the van der Waals forces and the electrostatic forces from both

the substrate and the microgripper, as well as the gravitational force, do not have a significant

effect on the high-speed microsphere that travels a short distance in air.

Figure 3.2(a) also shows that the precision of landing is inversely proportional to the height

from the substrate. When the height was over 20µm, random landing locations were observed,

which should be partly due to the more pronounced air flow effect. The landing locations of

the microspheres released from both gripping arms are not identical, which should be caused

by the slight difference in surface roughness on each gripping arm due to fabrication

processes that alters the magnitude of adhesion forces. To investigate the influence of

substrate differences on release performance, experiments were also repeated using a steel

substrate. Compared to data in Figure 3.2(a), results shown in Figure 3.2(b) confirm that the

active release approach does not have observable substrate dependence.

Given the above findings, the release height was set to 2µm above the substrate for

quantifying release accuracy. The small distance of 2µm from the substrate reduces the

distance/time the microsphere travels in air, making the landing location less sensitive to

environmental disturbances. The height was chosen to ensure the plunger does not push the

microsphere into the substrate and established plunger-microsphere-substrate contact, which

will make the release dependent upon the adhesion property. Figure 3.3 shows the recorded


0

40

80

-60 -40 -20 0 20 40x (µm)

y (µ

m)

2µm5µm10µm

0

40

80

-60 -40 -20 0 20 40x (µm)

y (µ

m)

2µm5µm10µm

(a) (b)

Figure 3.2: Representative microsphere landing locations after release from different heights, on (a) glass substrate, (b) steel substrate.

-1

-0.5

0

0.5

1

-0.5 0 0.5

x (µm)

y(µm

)

Figure 3.3: Representative release accuracy quantification on glass substrate.

Table 3.1: Summary of release accuracies in ambient environment.

released from left arm released from right arm

glass substrate 0.70±0.46µm (n=18) 0.67±0.55µm (n=18)

steel substrate 0.64±0.46µm (n=18) 0.67±0.55µm (n=20)


landing positions of the microsphere on glass substrate, proving an accuracy of 0.70±0.46µm.

A summary of the characterized release accuracy is given in Table 3.1. The 0.55µm standard

deviation of landing positions can be due to either (1) slight variations of initially adhered

lateral and/or vertical positions of the microsphere on the gripping arm or (2) imperfect

control of the microgrippers height above the substrate.

To further improved upon the height control above the substrate, substrate contact

detection was done for each trial, which further improved the release accuracy to

0.45±0.24µm (700 trials) and will be discussed in more detail in Chapter 4.

Besides a high accuracy, the active release technique enables easy, fast pick-place

operation in micromanipulation. Figure 3.4 shows the result of a series of pick and release of

microspheres. While grasping was manually conducted, which is skill dependent, positioning

the microsphere properly for plunging was rapid and took less than 1sec with the use of

calibration results shown in Figure 2.12.

To better quantify the release, high speed videography (13,000 frames/sec) was used to

record the release process, and the experimental set up is shown in Figure 3.5. The recorded

release process revealed two insights. Upon actuation, the plunger extends beyond the

equilibrium position of the flexure spring force and electrostatic force, and then follows a

damped-spring harmonic motion. High-speed videography also demonstrated that a

microsphere was separated from the plunger upon impact. For example, a plunging speed of

65.24mm/sec produced a microsphere speed of 105.01mm/sec.

It is of interest to determine a threshold plunging speed, where any plunging speeds above

this threshold will guarantee the release. However, due to the lack of understanding in many

of the parameters that might influence the release performance, it was not possible at the


moment to determine this threshold speed. For example, we will need to determine the

strength of adhesion force for every single microsphere at the moment of release to determine

a suitable plunging speed. Instead, a high plunging speed alleviates careful sample preparation

requirements (e.g., baking) or environmental control requirements (e.g., humidity).

CHAPTER 3, EXPERIMENTS

48

Figure 3.4: Pick-place to align microspheres (7.5µm to 10.9µm). Note that the microgrippers was titled 25°. (a) The microgrippers approaches a microsphere, and uses one gripping arm to laterally push it to break the initial adhesion bond between the microsphere and the substrate. (b) Two gripping arms are closed, grasping the microsphere and lifting it up. (c) The microsphere is transported to the target area where some microspheres have already been aligned. (d) The gripping arms are opened and the gripping arm that the microsphere adheres to positions the microsphere properly to the right position in relation to the plunger. (e) The plunger thrusts out the microsphere that land accurately on the substrate. (f) Microgrippers retracts and repeat the pick-place process.

high speed video camera

MEMS probing station

Figure 3.5: High speed video camera (13,000 frames per second) mounted on top of the microscope.


3.4 Understanding the Curved Trajectory

Interestingly, it can be seen from Figure 3.2 that the microspheres all landed to the right/left

side of the plunger (plunger was along the y axis) depending on which gripping arm they

adhered to. High-speed imaging verified that the flying path of the microsphere was indeed

curved. Images shown in Figure 3.6 were taken through high-speed videography

(13,000frames/second) when the gripping arms were 20µm above the substrate before the

release of the microsphere.

According to the brief force analysis in Section 2.5, the van der Waals force and

electrostatic force decrease with increased distances between the microsphere and gripping

arm. Additionally, the capillary force vanishes beyond a certain distance. Thus, it is assumed

that the gripping arm has an adhesion force effective region around it, as indicated by dashed

lines in Figure 3.7(a).

During release, the plunger first impacts the microsphere along the sidewall of the gripping

arm at a high speed as shown in Figure 3.7(a). When the traveling microsphere approaches

the gripping arm corner, which was rounded by deep reactive ion etching, the adhesion forces

create a radial acceleration towards the corner, which curves its travel direction. Eventually,

the microsphere leaves the gripping arm tip and hence the adhesion force effective region. It

then travels straightly and lands on the substrate, as depicted in Figure 3.7(b).

CHAPTER 3, EXPERIMENTS

50

Figure 3.6: High-speed videography (13,000 frames/second) quantifying microsphere trajectories upon release from a height of 20µm above the substrate.

Figure 3.7: Microsphere reveals a curved trajectory during active release. (a) The plunger thrusts the microshphere that reaches the roundish corner of the gripping arm. (b) The microsphere escapes from the effective range of the adhesion forces. The trajectory is drawn under the assumption that there are no disturbances when the microsphere is in the air.

Chapter 4

4. Micromanipulation Automation

4.1 Introduction

Through manual operation, the microgrippers have demonstrated pick-and-place of

microspheres with a high repeatability and accuracy. To eliminate human skill dependency

and allow high-speed micromanipulation, automated pick-place was realized.

4.2 Microrobotic Pick-Place of Microspheres

4.2.1 Recognition of Microgrippers and Spheres

The microspheres on the substrate were recognized using a Hough transform to determine

their centers and radii. Contours formed from Canny edge detection readily recognize the

gripping arms and the plunger. As shown in Figure 4.1(a), M1, M2, and M3 denote the

centroids of the two gripping arms and the plunger. By comparing the y coordinates of their

centroids, the left gripping arm, right gripping arm, and plunger were distinguished.

51

CHAPTER 4, MICROMANIPULATION AUTOMATION

52

Figure 4.1: (a) Recognized gripping arms and plunger. (b) Sidewall of a gripping arm for determining the secured grasping position, C. (c) 3D schematic showing the grasping.

Minimum bound rectangles (MBRs) were used to further define the positions of the two

gripping arms, as shown in Figure 4.1(a). Point D was then taken as the overall position of the

microgripper, which is the intersection of the horizontal line going through the plunger

centroid, M3, and the line connecting the left adjacent corners of the top and bottom MBRs.

To attain secured grasping, the system aligns the grasping position of the gripping arms

with respect to a microsphere, as illustrated in Figure 4.1(b) where g is the width of the

gripping arm (denoted by k in Figure 4.1(a)). r is the radius of the microsphere. The contact

position of the gripping arm with the microsphere is on the segment AB. In particular, the

middle position C provides the most security for grasping when microspheres slide during


53

grasping (Figure 4.1(b)). According to geometry, the distance from the microgrippers position

D to the optimal grasping position C is ααα cotcos2

sin rgtl −+= , which is a function of

the size of the microsphere to be grasped

When the gripping arms open during the release process, the microsphere randomly

adheres to one of the two gripping arms. As shown in Figure 4.2, the boundary of the gripping

arm to which the microsphere adheres is connected with that of the plunger. Thus, only two

contours are detected with the larger contour containing the microsphere. By comparing the y

coordinates of the centroids of the contours (M1 and M2 in Figure 4.2), the system determines

to which gripping arm the microsphere adheres.

4.2.2 Contact Detection and Micromanipulator Control

Knowledge of relative depth positions of the gripping arms and microsphere is gained through

the detection of the contact between the gripper tips and the surface of the substrate.

Obviating the need for additional force/touch sensors, the system employs a vision-based

contact detection algorithm [54] that provides a detection accuracy of 0.2µm. The contact

detection process completes within 5–8 seconds.

The microgrippers was controlled to move downward at a constant speed (e.g., 20µm/sec)

to establish contact with the substrate while the algorithm ran in real time. Since further

lowering the gripping arms after contact is established causes the arms to slide on the

substrate, the x coordinates of the gripping arms result in a V-shaped curve, as shown in

Figure 4.3. The global minimum represents the initial contact of the microgrippers with the

substrate.


54

Figure 4.2: Visually determine which gripping arm the microsphere adheres to after the gripping arms open.

38

40

42

44

46

48

50

0 10 20 30 40 50 60 70 80image number

x (p

ixel

) in

imag

e fra

m

contact point

Figure 4.3: Contact detection by monitoring x coordinate of a gripping arm in the image while lowering the microgrippers at a speed of 20µm/sec.


55

The microrobotic system is a “looking-and-moving” system. Transformation between the

image frame (x-y) and the microrobot frame (X-Y) was achieved with calibrated pixel sizes.

With the centroid and radius of a target microsphere recognized, the micromanipulator moves

the microgrippers to the target position via a PID controller.

4.2.3 Automated Pick-Place of Microspheres

To quantify the operation speed of the microrobotic system, microspheres were picked and

placed to form patterns. The system starts with contact detection to determine the depth

position of the gripping arms relative to the substrate surface. The microgrippers was then

moved 15µm above the substrate, ready for the pick-and-place operation.

Microspheres in the field of view were visually recognized. Their positions in the image

frame, sizes, and the optimal grasping positions were determined. Then, by using the contact

detection result and coordinate transformation, the X-Y-Z positions were determined for the

micromanipulator. The microspheres were picked up from the source area in the order of their

x coordinates in the image frame. According to the actuation calibration results (Figure 2.13),

the system determined actuation voltages to apply to the gripping arms for secured grasping

while ensuring no excessively large actuation voltage was applied.

The micromanipulator lifted the securely grasped microsphere to 15µm above the substrate.

When a pre-planned target position is reached, the micromanipulator moved downward and

stopped at 2µm above the substrate for release. The gripping arm to which the microsphere

adhered was first visually detected and then aligned the microsphere accurately in front of the

plunger based on the calibration results shown in Figure 2.13. The plunger was then actuated

to release the microsphere, after which the microgrippers was raised 15µm above the substrate


56

and return to the source area for picking up the next microsphere. Figure 4.4 shows that

microspheres were arranged into a circular pattern with a circularity of 0.52µm, which is

defined as the standard deviation of the distances from the microspheres to the center of the

circle. Figure 4.5 shows an assembled “U OF T” pattern. The average pick-place speed was

6sec/sphere.

Figure 4.4: Pattern formation by autonomous pick-place. (a) Microspheres before pick-place. (b) A circular pattern with circularity of 0.52µm.

Figure 4.5: “U of T” pattern formed by autonomous microrobotic pick-place of 7.5-10.9µm microspheres.

Chapter 5

5. From Microgripping to Nanogripping

5.1 Introduction

The manipulation of large nanofibres was previously demonstrated with the use of micro-

sized grippers [24]. However, to manipulate small nanometer-sized objects, the manipulation

tip ideally should be comparable in size to the object. This is difficult to accomplish in most

fabrication processes for MEMS-based microtools, where all structural features in the device

typically have the same thickness. By reducing the device thickness, the performance of the

microactuators is greatly reduced due to decreased overlapping areas or volume; and the poor

aspect ratio in flexures produces undesired out-of-plane motions during operation. As the

device thickness approaches sub micrometer, it also becomes difficult to manually handle the

device because of poor structural integrity.

What is needed is a reliable fabrication process that allows controllable thickness

variations within the device structure. For example, thick microactuators and flexures for

good performance, combined with thin device tips for manipulation use.

57

CHAPTER 5, FROM MICROGRIPPING TO NANOGRIPPING 58

5.2 Proposed Fabrication Process

For a wafer with more than two material layers (e.g., SOI wafer), the top-down patterning

approach of bulk micromachining method has difficulty in creating a different pattern on each

layer, forcing all structures to be made from the same layer of material. No reliable batch

fabrication method exists to locally reduce the device thickness with high precision.

In this research, a novel batch fabrication process using conventional bulk micromachining

methods was developed to overcome this limitation. The new process can be combined with

the original microgrippers microfabrication process described in section 2.3.3, allowing the

thick device silicon layer to be patterned into device structures (microactuators and flexures),

and the thin buried oxide layer (BOX layer) to be patterned to form the gripping tips and

plunger tip.

The following uses a general example to describe the new process. In this example, a wafer

with two material layers, layer A (top) and layer B (bottom), can both be patterned from a

single side of the wafer through the following steps:

1. Deposit a layer of material B onto layer A as etch mask.

2. Pattern the deposited layer into final desired pattern of layer B.

3. Pattern photoresist on deposited layer B into final desired pattern of layer A.

4. Etch exposed material A from top.

5. Etch exposed material B from top.

6. Etch exposed material A from top.

The working conditions for the new process include:

1. Suitable dry etch method available for materials A and B.

2. Material A and B have suitable dry etch selectivity, such as between Silicon and SiO2.

3. Photoresist can withstand dry etching of both material A and B.

CHAPTER 5, FROM MICROGRIPPING TO NANOGRIPPING

59

Figure 5.1: Proposed fabrication process capable of patterning two material layers from a single side.


Figure 5.1 illustrates the new fabrication process, where material A is device silicon layer,

and material B is silicon dioxide. This new process can be easily integrated into the original

fabrication process for microgrippers. The combined fabrication process is illustrated in

Figure 5.2 and described below:

1. SiO2 is thermally grown on both sides of an SOI wafer.

2. Chromium (Cr) is evaporated onto device layer, and patterned to define features such

as comb fingers and flexures (photolithographic mask 1).

3. Top SiO2 layer is etched with RIE (reactive ion etching) using photolithographic mask

2 and predefined Cr etch mask.

4. Ohmic contacts are formed by e-beam evaporation and patterned by lift-off (mask 3).

5. Bottom SiO2 layer is patterned to form DRIE (deep reactive ion etching) etch mask on

handle layer. (mask 4).

6. Handle layer is etched using DRIE until SiO2 BOX layer.

7. (Optional) Thin film of metal/non-metal is evaporated onto the handle layer.

8. Device layer is patterned using photolithographic mask 5. Then the exposed silicon is

etched using DRIE.

9. Exposed SiO2 from both top layer and BOX layer are etched from the top.

10. (Optional depending on Step 7) Exposed metal/non-metal thin film is etched using

RIE from the top.

11. Exposed device layer silicon is etched using DRIE from the top.

12. (optional depending on needs) Exposed SiO2 from top layer and BOX layer are etched

away from the top to expose metal/non-metal thin film at gripper tips.


61

(1)

silicon silicon dioxide chromium and gold chromium device layer = 25 micron buried oxide layer = 1 micron handle layer = 300 micron oxide layer around wafer = 1 micron minimum feature = 2 micron

SOI

ultra thin Cr tip

(7)

(8) (2)

(9) (3)

(10) (4)

silicon dioxide tip

(11) (5)

(12) (6)

Figure 5.2: Combined fabrication process capable of producing ultra thin tip down to nanometre thickness.


Due to the increased complexity in fabrication sequence, Step 2 was added to minimize

alignment issues with small features. Depending on the application requirement, new process

also allows the gripper tips to be made from a broad range of materials (determined by Step 7),

conductive or non-conductive. When the grippers is used upside down inside an SEM, the

deposited thin film (Step 7) can also prevent charging effect and provide clearer images.

Through the integration of the new process, it is now possible to selectively reduce the gripper

tips thickness to sub-micrometers for manipulating nanometre-sized objects. Figure 5.3 shows

an SEM image of the nanogrippers tip fabricated using the new process, where the device tip

is made from 1µm SiO2.

25µm thick Si

1µm thick SiO2

Figure 5.3: SEM image of the nanogripper tip fabricated using the new process.


5.3 Nanogrippers Post Processing

Due to the multiple masking layers used in this process (SiO2 and chromium layer), a

considerable amount of residual stress from thin film deposition exists on device structures.

This added surface stresses produce out-of-plane deflections, where a large out-of-plane

difference of approximately 14µm between the plunger and the gripper tips was measured. To

reduce this large deflection, a combination of two methods can be used:

1. Chromium has an as-deposited stress in the order of GPa [55-57], which is an

unsuitable masking layer for this application. In contrast, soft metals such as

aluminium have an as-deposited stress in the order of MPa, which is a possible

alternative masking layer.

2. A thin layer of insulator such as SiO2 can be deposited over the nanogrippers (with the

electrodes covered by stencil) to counteract with existing stresses in the device. In this

research, 1µm high-stress silicon dioxide was deposited onto the grippers using

plasma enhance chemical vapour deposition (PECVD) to produce in-plane structures.

Conventional silicon-based fabrication methods always produce slightly rounded edge and

corner from etching. When the gripping tips fully close, the rounded tips would produce a

large gap in between, preventing nanometer-sized objects from being picked up. To overcome

this difficulty, focus ion beam (FIB) milling was used in post processing to produce sharp tips.

An FEI dual beam system (Figure 5.4) was used to individually reshape and sharpen the

nanogrippers tips. Gripping tips, before and after FIB, are shown in Figure 5.5(a). The FIB

process typically takes ~10 minutes per nanogripper. This milling technique can also be used


to thin and reshape thick silicon of microgrippers (shown in Figure 2.11) directly into

nanogrippers. However, the FIB process took approximately 2 hours per device and always

caused damage to surrounding structures as shown in Figure 5.5(b).

SEM

FIB

Figure 5.4: Dual beam system used for reshaping nanogrippers.


65

(a) before etch after etch

(b) 2 minutes into etch after etch

damages caused by FIB

Figure 5.5: Before and after FIB milling of (a) nanogripper tips, (b) microgrippers tips.


5.4 SEM Manipulation

5.4.1 Introduction

Limited by the wavelength of light, an optical microscope has poor resolving power compared

to a scanning electron microscope (SEM). The SEM offers superior image resolution and

large depth of focus, making it an interesting platform for nanomanipulation. The high

vacuum inside the chamber also minimizes the moisture level, which greatly reduces the

capillary force.

5.4.2 SEM Manipulation Difficulties

During SEM imaging, electron-solid interactions are accumulative and potentially destructive,

including e-beam induced sample charging, temperature change, and contamination. In the

case of sample charging, insulating objects within the chamber could exert short/long-range

attractive/repulsive electrostatic forces on one another depending on the accumulation of

charges. The sign and magnitude of these charges are difficult to determine since they vary

with imagining parameters, sample properties, and imaging time [58]. These accumulated

charges on the sample can only be dissipated by increasing the air pressure around the sample,

which is done by either venting the chamber or in the case of environmental SEM, the

pressure is increased locally around the sample. Furthermore, prolonged exposure to electron

beam also increases the temperature of the sample, which can be potentially destructive and

also alter the surface adhesion forces [51]. When the electron beam is focused on a small area,


electron beam induced deposition (EBID) would occur, where the nearby impurities in the

chamber are decomposed by the electron beam and re-deposited onto the sample.

EBID was used by Carlson et al. [24] to solder a carbon nanofibre to an AFM tip and used

by Boggild et al. [39] to grow high-aspect-ratio nanotweezers tips. Due to its continuous

deposition in the field of view, the bonding force between objects increases over time [58].

All these factors alter the forces between objects within the SEM chamber, making

nanomanipulation inside the SEM difficult.

Electron-solid interactions also create many difficulties in terms of imagining. Positive

charging in a sample results in dimmer views while negative charging produces image

distortions or image intensity fluctuations. When MEMS actuators are used inside the SEM,

images shift according to the magnitude of applied voltages because the electrons are either

repelled or attracted to the actuator again depending on applied voltages. In the case of

electrostatic actuators where large actuation voltages are applied, image shift can be

significant at high imaging magnifications.

5.4.3 Experimental Setup

The experimental setup (Figure 5.6) consists of four piezoelectric nanomanipulators (Zyvex

Inc.) installed inside a scanning electron microscope (Hitachi S-4000). A custom made circuit

board with a wire bonded nanogrippers was attached to the nanomanipulator via an electrical

connector, where electrical connection is made with the manipulator control box outside of

the SEM chamber. The nanogrippers was tilted at an angle of 40o, an angle chosen to avoid

the circuit board from colliding with the substrate while ensuring a clear view of the gripping

tips.


SEM Zyvex control

box

Zyvex nanomanipulator installed nanogripper

Figure 5.6: SEM-based nanomanipulation system.

The manipulated samples were 100nm gold nano particles that were dispensed onto a gold-

sputtered silicon substrate. The substrate was attached to the SEM sample stage using vacuum

compatible adhesive and electrically grounded by applying conductive carbon paint.

The nanogrippers and samples were installed by opening the high-vacuum chamber of the

SEM one day before an experiment. After sealing the chamber and pumping down, aperture

heating and display power were turned on to reduce contamination and stabilize electronics,


respectively. Electron source flashing was done to ensure consistent electron emission. These

procedures were conducted for every experiment.

5.4.4 Pick and Release of Nanospheres

Preliminary trials of pick-up nanospheres were conducted. Figure 5.7 shows the grasping of a

100nm gold nano particle. Applying the same manipulation strategy in the ambient

environment to manipulation inside SEM revealed three major difficulties. These difficulties

prevented the completion of quantifying release repeatability and accuracy within the tenure

of this research.

1. The nanomanipulator used for nanopositioning was open loop controlled. Although

the in-plane (X-Y) positioning can be estimated visually, no reliable method enables

out-of-plane (Z) positioning with a nanometre resolution. For nanosphere pick-up, the

gripping tips must be positioned within 100nm above the substrate, presently relying

on trial and error.

2. Several components of the nanogrippers and packaging materials are

dielectric/insulating, causing charging problems that result in image distortions and

fluctuations.

3. SEM images shift resulting from applied voltages to actuate the nanogrippers. In the

case of electrostatic actuators where large actuation voltages are needed, image shift

can be significant at high imaging magnifications.


(a) (b)

(c) (d)

Figure 5.7: Pick up of 100nm gold particle. (a) Lateral pushing to break initial bonding. (b) Before grasping. (c) After grasping. (d) Lift and release the grasp, nano particle remain adhered to one gripping tip.

Chapter 6

6. Conclusion

This thesis presented new MEMS-based gripping devices that integrate both gripping and

release mechanisms. The devices were applied to the grasping and active release of

microspheres and nano particles.

Micromanipulation was conducted under an optical microscope. The plunger provided the

microsphere with sufficient momentum to overcome adhesion forces, resulting in highly

repeatable release (100% of 700 trials) and a release accuracy of 0.45±0.24µm. The tested

borosilicate microspheres varied from 7.5µm to 10.9µm in size. Within this size range, release

accuracy was found independent of microsphere sizes. Release performance was also found

independent of electrical conductivity of substrates (steel and glass). Considering structural

dimensions of the present device (e.g., thickness of gripping arms and plunger: 25µm and

initial gripping arm opening: 17µm), we speculate that the reported release accuracy should

be consistent for microspheres ranging from a few micrometers up to 17µm. This research

revealed that the most important operating parameters are plunging speed and the height from

the substrate. The highly controllable active release capability represents an important

progress for reliable pick-and-release micromanipulation. By virtue of its grasping

71

CHAPTER 6, CONCLUSION 72

and rapid, accurate release capabilities, automated micromanipulations were demonstrated.

The system demonstrated a pick-and-place speed of 6sec/microsphere, which is much faster

than a skilled operator and an order of magnitude faster than the highest speed reported in the

literature thus far.

This technique can be limited in the size, geometry, and material of micro objects that can

be manipulated. In consideration of the structural dimensions of the present device (e.g.,

thickness of the gripping arms and plunger: 25µm, and initial gripping arm opening: 17µm),

micro objects suitable in size can be up to 17µm. Regarding the geometry, it is speculated that

this technique is effective for symmetrical objects, such as micro cubes and triangular objects,

if the shape of the microgrippers tips are modified to conform to the object. For irregular

shaped micro objects, however, this technique might not be effective because the orientation

control of micro objects and the plunger alignment can be practically difficult. As for

materials with a higher surface energy than glass, it is believed that the object can still be

successfully released as long as it gains sufficient momentum from the plunging impact.

Within this research, a novel fabrication process was also developed to miniaturize the

gripping tips such that they are more comparable in size to nano objects for nanomanipulation

inside SEM. The fabrication process enables the patterning of the BOX layer of an SOI wafer.

The batch microfabrication process significantly reduces the time consumption in subsequent

FIB post processing, from hours to minutes as well as significantly reduce device damage

during FIB milling. Initial trials of picking up 100nm gold nano particles proved that the

nanogrippers were functional, paving the ground for using these nano devices for further

research in SEM-based nanomanipulation.


6.1 Contributions

• Designed and microfabricated new types of MEMS microgrippers with integrated active

release plunger, including structural and electrostatic analysis, structural FEA simulation,

microfabrication process development, packaging, circuit development, and calibration.

• For the first time, achieved 100% repeatability and high release accuracy capability with

pick-and-place manipulation of microspheres in the ambient environment. Conditions

for achieving great release performance were also given both quantitatively and

qualitatively.

• For the first time, demonstrated high-speed microrobotic pick-place assembly of

microspheres into predefined patterns.

• Developed a novel batch microfabrication process that allows localized scaling of

gripper tips to make them more suitable for nanomanipulation.

• Determined important considerations for SEM-based nanomanipulation through pick-

and-release 100nm gold nano particles.

6.2 Future Directions

• To integrate contact force sensors into the gripping devices to enable better control Z-

axis motion. Combined with the current closed-loop vision-based X-Y motion control,

the performance of pick-and-place automation would improve.

• To more systematically investigate the influence of different factors on the release

performance at the micro scale, including temperature, humidity, sample materials/size,


substrate materials, plunging speed, etc. Such investigations would enable the

optimization of the microgrippers performance under different conditions.

• To investigate the capability of active release mechanism for releasing objects of

different shapes, in order to extend the manipulation capability to a wide range of

applications, such as the construction of photonic crystals, handling biological cells, and

the construction of complex microsystems.

• To improve the nanogrippers fabrication process to minimize the relative out-of-plane

deflection of the device tips. This can be achieved by using different materials as etch

masks as well as optimizing mask deposition processes.

• To integrate visual tracking algorithms to allow closed-loop X-Y-Z motion control of

nanogrippers inside the SEM.

• To study electron-solid interactions to better understand force interactions inside the

SEM. This would allow the development of better manipulation strategies with the

nanogrippers inside SEM.

• To implement electrical shielding over the nanogrippers to minimize charging due to

dielectric materials and image shift due to actuation.

Appendix

Permissions to Reproduce

75

APPENDIX 76

Source: Institute of Electrical and Electronics Engineers (IEEE)

Comments/Response to Case ID: 0078583B ReplyTo: [email protected] From: Jacqueline Hansson Date: 09/04/2009 Subject: Re: permission to use figures Send To: Brandon Chen <[email protected]> Dear Brandon Chen: This is in response to your letter below, in which you have requested permission to reprint, in your upcoming thesis/dissertation, the described IEEE copyrighted figures. We are happy to grant this permission. Our only requirements are that you credit the original source (author, paper, and publication), and that the IEEE copyright line ( © [Year] IEEE) appears prominently with each reprinted figure. Sincerely yours, Jacqueline Hansson : © © © © © © © © © © © © © © © © © © IEEE Intellectual Property Rights Office 445 Hoes Lane Piscataway, NJ 08855-1331 USA +1 732 562 3966 (phone) +1 732 562 1746 (fax) IEEE-- Fostering technological innovation and excellence for the benefit of humanity. © © © © © © © © © © © © © © © © © © To whom it may concern I am preparing a master’s thesis in University of Toronto, entitled "NOVEL MEMS GRIPPERS FOR PICK-PLACE OF MICRO AND NANO OBJECTS" Below are the figure number used, and source information. 1. Figure 1, C.Yamahata, D.Collard, B.Legrand, T.Takekawa, M.Kumemura, G.Hashiguchi, G.Hashiguchi, and H.Fujita, "Silicon nanotweezers with sub nanometer resolution for the micromanipulation of biomolecules," J. Microelectromech. Syst., vol. 17, no. 3, pp. 623-631, 2008. 2. Figure 2, F.Beyeler, A.Neild, S.Oberti, D.J.Bell, Y.Sun, J.Dual, and B.J.Nelson, "Monolithically fabricated microgripper with integrated force sensor for manipulating microobjects and biological cells aligned in an ultrasonic field," J. Microelectromech. Syst., vol. 16, no. 1, pp. 7-15, 2007. Please grant me the premission to include these figures in my Thesis. Sincerely Brandon Chen

mailto:[email protected]

mailto:[email protected]

APPENDIX 77

Source: Institute of Physics (IOP)

BIBLIOGRAPHY 78

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