novel mems grippers for pick-place of micro · pdf filemicrosystems. despite the significant...
TRANSCRIPT
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
CHAPTER 1, INTRODUCTION 3
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.
CHAPTER 1, INTRODUCTION 4
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.
CHAPTER 1, INTRODUCTION 5
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.
CHAPTER 1, INTRODUCTION 6
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.
CHAPTER 2, THREE-PRONGED MICROGRIPPERS
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.
CHAPTER 2, THREE-PRONGED MICROGRIPPERS
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
CHAPTER 2, THREE-PRONGED MICROGRIPPERS
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.
CHAPTER 2, THREE-PRONGED MICROGRIPPERS
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
CHAPTER 2, THREE-PRONGED MICROGRIPPERS
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.
CHAPTER 2, THREE-PRONGED MICROGRIPPERS
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
CHAPTER 2, THREE-PRONGED MICROGRIPPERS
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.
CHAPTER 2, THREE-PRONGED MICROGRIPPERS
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.
CHAPTER 2, THREE-PRONGED MICROGRIPPERS
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.
CHAPTER 2, THREE-PRONGED MICROGRIPPERS
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
CHAPTER 2, THREE-PRONGED MICROGRIPPERS
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.
CHAPTER 2, THREE-PRONGED MICROGRIPPERS
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.
CHAPTER 2, THREE-PRONGED MICROGRIPPERS
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
CHAPTER 2, THREE-PRONGED MICROGRIPPERS
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.
CHAPTER 2, THREE-PRONGED MICROGRIPPERS
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).
CHAPTER 2, THREE-PRONGED MICROGRIPPERS
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].
CHAPTER 2, THREE-PRONGED MICROGRIPPERS
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
CHAPTER 2, THREE-PRONGED MICROGRIPPERS
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
CHAPTER 2, THREE-PRONGED MICROGRIPPERS
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
CHAPTER 2, THREE-PRONGED MICROGRIPPERS
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)
CHAPTER 2, THREE-PRONGED MICROGRIPPERS
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.
CHAPTER 2, THREE-PRONGED MICROGRIPPERS
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
CHAPTER 2, THREE-PRONGED MICROGRIPPERS
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.
CHAPTER 2, THREE-PRONGED MICROGRIPPERS
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
CHAPTER 2, THREE-PRONGED MICROGRIPPERS
33
Figure 2.11: SEM image of the MEMS microgrippers with integrated active release mechanism.
CHAPTER 2, THREE-PRONGED MICROGRIPPERS
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.
CHAPTER 2, THREE-PRONGED MICROGRIPPERS
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.
CHAPTER 2, THREE-PRONGED MICROGRIPPERS
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
CHAPTER 2, THREE-PRONGED MICROGRIPPERS
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.
CHAPTER 2, THREE-PRONGED MICROGRIPPERS
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.
CHAPTER 2, THREE-PRONGED MICROGRIPPERS
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,
CHAPTER 2, THREE-PRONGED MICROGRIPPERS
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
CHAPTER 3, EXPERIMENTS 43
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.
CHAPTER 3, EXPERIMENTS 44
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
CHAPTER 3, EXPERIMENTS 45
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)
CHAPTER 3, EXPERIMENTS 46
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
CHAPTER 3, EXPERIMENTS 47
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.
CHAPTER 3, EXPERIMENTS 49
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
CHAPTER 4, MICROMANIPULATION AUTOMATION
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.
CHAPTER 4, MICROMANIPULATION AUTOMATION
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.
CHAPTER 4, MICROMANIPULATION AUTOMATION
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
CHAPTER 4, MICROMANIPULATION AUTOMATION
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.
CHAPTER 5, FROM MICROGRIPPING TO NANOGRIPPING 60
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.
CHAPTER 5, FROM MICROGRIPPING TO NANOGRIPPING
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.
CHAPTER 5, FROM MICROGRIPPING TO NANOGRIPPING 62
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.
CHAPTER 5, FROM MICROGRIPPING TO NANOGRIPPING 63
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
CHAPTER 5, FROM MICROGRIPPING TO NANOGRIPPING 64
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.
CHAPTER 5, FROM MICROGRIPPING TO NANOGRIPPING
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.
CHAPTER 5, FROM MICROGRIPPING TO NANOGRIPPING 66
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,
CHAPTER 5, FROM MICROGRIPPING TO NANOGRIPPING 67
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.
CHAPTER 5, FROM MICROGRIPPING TO NANOGRIPPING 68
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,
CHAPTER 5, FROM MICROGRIPPING TO NANOGRIPPING 69
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.
CHAPTER 5, FROM MICROGRIPPING TO NANOGRIPPING 70
(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.
CHAPTER 6, CONCLUSION 73
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,
CHAPTER 6, CONCLUSION 74
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
APPENDIX 77
Source: Institute of Physics (IOP)
BIBLIOGRAPHY 78
Bibliography
[1] F.Garcia-Santamara, H.T.Miyazaki, A.Urquia, M.Ibisate, M.Belmonte, N.Shinya, F.Meseguer, and C.Lopez, "Nanorobotic manipulation of microscopheres for on-chip diamond architectures," Adv. Mater., vol. 16, pp. 1144-1147, 2002.
[2] K.Aoki, H.T.Miyazaki, H.Hirayama, K.Inoshita, T.Baba, K.Sakoda, N.Shinya, and Y.Aoyagi, "Microassembly of semiconductor three-dimensional photonic crystals," Nat. Mater., vol. 2, pp. 117-121, 2003.
[3] D.H.Kim, M.G.Lee, B.Kim, and Y.Sun, "A superelastic alloy microgripper with embedded electromagnetic actuators and piezoelectric force sensors: a numerical and experimental study," Smart Mater. Struct., vol. 14, pp. 1265-1272, 2005.
[4] 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.
[5] N.Chronis and L.P.Lee, "Electrothermally activated SU-8 microgripper for single cell manipulation in solution," J. Microelectromech. Syst., vol. 14, pp. 857-863, 2005.
[6] T.C.Duc, G.K.Lau, and P.Sarro, "Polymeric thermal microactuator with embedded silicon skeleton: Part II-fabrication, characterization, and application for 2-DOF microgripper," J. Microelectromech. Syst. , vol. 17, no. 4, pp. 823-831, 2008.
[7] T.C.Duc, G.K.Lau, J.F.Greemer, and P.M.Sarro, "Electrothermal microgripper with large jaw displacement and integrated force sensors," J. Microelectromech. Syst., vol. 17, no. 6, pp. 1546-1555, 2008.
[8] Y.Choi, J.Ross, B.Wester, and M.G.Allen, "Mechanically driven microtweezers with integrated microelectrodes," J. Micromech. Microeng., vol. 18, 065004, 2008.
[9] C.Yamahata, D.Collard, B.Legrand, T.T.M.Kumemura, G.Hashiguchi, and H.Fujita, "Silicon nanotweezers with subnanometer resolution for the micromanipulation of biomolecules," J. Microelectromech. Syst. , vol. 17, no. 3, pp. 623-631, 2008.
BIBLIOGRAPHY 79
[10] K.N.Andersen, D.H.Petersen, K.Carlson, K.Molhave, O.Sardan, A.Horsewell, V.Eichhorn, S.Fatikow, and P.Boggild, "Multimodal electrothermal silicon microgrippers for nanotube manipulation," IEEE Trans. Nanotechnology, vol. 8, no. 1, pp. 76-85, 2009.
[11] K.Y.Kim, X.Y.Liu, Y.Zhang, and Y.Sun, "Nanonewton force-controlled manipulation of biological cells using a monolithic MEMS microgripper with two-axis force feedback," J. Micromech. Microeng., vol. 18, 055013, 2008.
[12] G.K.Lau, J.F.L.Goosen, F.v.Keulen, T.C.Duc, and P.M.Sarro, "Polymeric thermal microactuator with embedded silicon skeleton: Part I-design and analysis," J. Microelectromech. Syst., vol. 17, no. 4, pp. 809-822, 2008.
[13] O.Sardan, V.Eichhorn, D.H.Petersen, S.Fatikow, O.Sigmund, and P.Boggild, "Rapid prototyping of nanotube-based devices using topology-optimized microgrippers," Nanotechnology, vol. 19, 49550, 2008.
[14] H.Y.Chan and W.J.Li, "A thermally actuated polymer micro robotic gripper for manipulation of biological cells," in Proc. IEEE/RSJ Int. Conf. Intell. Robot. Syst., Taipei, Taiwan: 2003, pp. 288-293.
[15] Y.W.Lu and C.J.Kim, "Microhand for biological applications," Appl. Phys. Lett., vol. 89, 164101, 2006.
[16] S.Krishnan and L.Saggere, "A multi-fingered micromechanism for coordinated micro/nano manipulation," J. Micromech. Microeng., vol. 17, pp. 576-585, 2007.
[17] J.K.Luo, J.H.He, Y.Q.Fu, A.J.Flewitt, S.M.Spearing, N.A.Fleck, and W.I.Milne, "Fabrication and characterization of diamond like carbon/Ni bimorph normally closed microcages," J. Micromech. Microeng., vol. 15, pp. 1406-1413, 2005.
[18] T.G.Leong, C.L.Randall, B.R.Benson, N.B.G.M.Stern, and D.H.Gracias, "Tetherless thermobiochemically actuated microgrippers," Proc. Nat. Acad. Sci. U. S. A, vol. 106, no. 3, pp. 703-708, 2009.
[19] J.Ok, Y.W.Lu, and C.J.Kim, "Pneumatically driven microcage for microbe manipulation in a biological liquid environment," J. Microelectromech. Syst., vol. 15, no. 6, pp. 1499-1505, 2006.
[20] R.S.Fearing, "Survey of sticking effects for micro-parts," in in Proc. IEEE/RSJ Int. Conf. Intell. Robot. Syst., Pittsburgh, PA: 1995, pp. 212-217.
[21] H.T.Miyazaki, Y.Tomizawa, S.Saito, T.Sato, and N.Shinya, "Adhesion of micrometer-size polymer particles under a scanning electron microscope," J. Appl. Phys., vol. 88, no. 6, pp. 3330-3340, 2000.
BIBLIOGRAPHY 80
[22] S.Saito, H.T.Miyazaki, T.Sato, and K.Takahashi, "Kinematics of mechanical and adhesional micromanipulation under a scanning electron microscope," J. Appl. Phys., vol. 92, no. 9, pp. 5140-5149, 2002.
[23] O.Fuchiwaki, A.Ito, D.Misaki, and H.Aoyama, "Multi-axial micromanipulation organized by veratile micro robots and micro tweezers," in Proc. IEEE Int. Conf. Robot. Autom., Pasadena, CA, 2008, pp. 893-898.
[24] K.Carlson, K.N.Andersen, V.Eichhorn, D.H.Petersen, K.Molhave, I.Y.Y.Bu, K.B.K.Teo, W.I.Milne, S.Fatikow, and P.Boggild, "A carbon nanofibre scanning probe assembled using an electrothermal microgripper," Nanotechnology, vol. 18, 345501, 2007.
[25] Kleindiek, "SEM-compatible glue," 2009.
[26] K.Takahashi, H.Kajihara, M.Urago, S.Saito, Y.Mochimaru, and T.Onzawa, "Voltage required to detach an adhered particle by Coulomb interaction for micromanipulation," J. Appl. Phys., vol. 90, no. 1, pp. 432-437, 2001.
[27] S.Saito, H.Himeno, and K.Takahashi, "Electrostatic detachment of an adhering particle from a micromanipulated probe," J. Appl. Phys., vol. 93, no. 4, pp. 2219-2224, 2003.
[28] S.Saito, H.Himeno, K.Takahashi, and M.Urago, "Kinetic control of a particle by voltage sequence for a nonimpact electrostatic micromanipulation," Appl. Phys. Lett., vol. 83, no. 10, pp. 2076-2078, 2003.
[29] S.Saito and M.Sonoda, "Non-impact deposition for electrostatic micromanipulation of a conductive particle by a single probe," J. Micromech. Microeng, vol. 18, no. 107001 2008.
[30] D.S.Haliyo, S.Regnier, and J.C.Guinot, "[mu]mad, the adhesion based dynamic micro-manipulator," Eur. J. Mech. A, vol. 22, no. 6, pp. 903-916, 2003.
[31] D.S.Haliyo, Y.Rollot, and S.Regnier, "Manipulation of micro objects using adhesion forces and dynamical effects," in Proc. IEEE Int. Conf. Robot. Autom., Washington, DC, 2002, pp. 1949-1954.
[32] Y.Fang and X.Tan, "A dynamic JKR model with application to vibration release in micromanipulation," in Proc. IEEE/RSJ Int. Conf. Intell. Robot. Syst. Beijing, China, 2006, pp. 1341-1345.
[33] W.Zesch, M.Bmnner, and A.Weber, "Vacuum tool for handling microobjects with a nanorobot," in Proc. IEEE Int. Conf. Robot. Autom., Albuquerque, NM: 1997, pp. 1761-1766.
BIBLIOGRAPHY 81
[34] F.Arai and T.Fukuda, "A new pick up and release method by heating for micromanipulation," in Int. Workshop on Micro Electro Mech. Syst., Nagoya, Japan, 1997, pp. 383-388.
[35] B.Vikramaditya and B.J.Nelson, "Modeling microassembly tasks with interactive forces," in Proc. Int. Symp. Assembly and Task Planning, Fukuoka, Japan: 2001, pp. 482-487.
[36] J.Liu, Y.X.Zhou, and T.H.Yu, "Freeze tweezer to manipulate mini/micro objects," J. Micromech. Microeng., vol. 14, pp. 269-276, 2004.
[37] Y.Yang, J.Liu, and Y.X.Zhou, "A convective cooling enabled freeze tweezer for manipulating micro-scale objects," J. Micromech. Microeng, vol. 18, 095008, 2008.
[38] B.Lopez-Walle, M.Gauthier, and N.Chaillet, "Principle of a submerged freeze gripper for microassembly," IEEE Trans. Robot. , vol. 24, no. 4, pp. 897-902, 2008.
[39] P.Boggild, T M Hansen, C Tanasa, and F Grey, "Fabrication and actuation of customized nanotweezers with a 25 nm gap," Nanotechnology, vol. 12, pp. 331-335, 2001.
[40] 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.
[41] C.Yamahata, T.Takekawa, K.Ayano, M.Hosogi, M.Kumemura, B.Legrand, D.Collard, G.Hashiguchi, and H.Fujita, "Silicon nanotweezers with adjustable and controllable gap for the manipulation and characterization of DNA molecules," in International Conf. on Microtech. in Med. and Bio. 2006, pp. 123-126.
[42] Y.Sun, S.N.Fry, D.P.Potasek, D.J.Bell, and B.J.Nelson, "Characterizing fruit fly flight behavior using a microforce sensor with a new comb-drive configuration," J. Microelectromech. Syst., vol. 14, no. 1, pp. 4-11, 2005.
[43] C.Yamahata, D.Collard, T.Takekawa, M.Kumemura, G.Hashiguchi, and H.Fujita, "Humidity dependence of charge transport through DNA revealed by silicon-based nanotweezers manipulation," Biophys J, vol. 94, no. 1, pp. 63-70, 2008.
[44] R Lin, M Bammerlin, O Hansen, R R Schlittler, and P Boggild, "Micro-four-point-probe characterization of nanowires fabricated using the nanostencil technique," Nanotechnology, vol. 15, pp. 1363-1367, 2004.
[45] K.Molhave and O.Hansen, "Electro-thermally actuated microgrippers with integrated force-feedback," Journal of Micromechanics and Microengineering, vol. 15, pp. 1265-1270, 2005.
BIBLIOGRAPHY 82
[46] K.Molhave, T.Wich, A.Kortschack, and P.Boggild, "Pick-and-place nanomanipulation using microfabricated grippers," Nanotechnology, vol. 17, pp. 2434-2441, 2006.
[47] Y.Sun, B.J.Nelson, D.P.Potasek, and E.Enikov, "A bulk microfabricated multi-axis capacitive cellular force sensor using transverse comb drives," J. Micromech. Microeng., vol. 12, no. 6, pp. 832-840, 2002.
[48] L.L.Chu and Y.B.Gianchandani, "A micromachined 2D positioner with electrothermal actuation and sub-nanometer capacitive sensing," J. Micromech. Microeng., vol. 13, pp. 279-285, 2003.
[49] Y.S.Yang, Y.H.Lin, Y.C.Hu, and C.H.Liu, "A large-displacement thermal actuator designed for MEMS pitch-tunable grating," J. Micromech. Microeng., vol. 19, 015001, 2009.
[50] F.Arai, D.Ando, and T.Fukuda, "Adhesion forces reduction for micro manipulation based on microphysics," in Proc. Int. Workshop on Micro Electro Mech. Syst., San Diego, CA, 1996, pp. 354-359.
[51] Y.Zhou and B.J.Nelson, "Adhesion force modeling and measurement for micromanipulation," in Proc. SPIE Conf. Microrobot. Micromanipulation, Boston, MA, 1998, pp. 169-180.
[52] R.A.Bowling, "A theoretical review of particle adhesion," in Particles on Surfaces I: Detection, Adhesion and Removal, K. L. Mittal, Ed. New York: Plenum Press, 1988, pp. 129-155.
[53] J.N.Israelachvili, "Intermolecular and surface forces," 2nd ed. New York: Academic Press, 1992.
[54] W.H.Wang, X.Y.Liu, and Y.Sun, "Contact detection in microrobotic manipulation," Int. J. Robot. Res., vol. 26, pp. 821-828, 2007.
[55] K.Nakajima, K.Onisawa, K.Chahara, T.Minemura, M.Kamei, and E.Setoyama, "Stress reduction of chromium thin films deposited by cluster-type sputtering system for ultra-large-size (550X650 mm) substrates," Vacuum, vol. 51, no. 4, pp. 761-764, 1998.
[56] L.Shaginyan, J.G.Han, and H.M.Lee, "Structural nonuniformity and internal stress in chromium films deposited by magnetron sputtering," Japanese Journal of Applied Physics, vol. 43, no. 5A, pp. 2594-2601, 2004.
[57] J.A.Thornton and D.W.Hoffman, "Stress-related effects in thin films," Thin Solid Films, vol. 171, pp. 5-31, 1989.
BIBLIOGRAPHY 83
[58] H.T.Miyazaki, Y.Tomizawa, S.Saito, T.Sato, and N.Shinya, "Adhesion of micrometer-sized polymer particles under a scanning electron microscope," J. Appl. Phys., vol. 88, no. 6, pp. 3330-3340, 2000.