NOVEL ELECTROSTATIC ACTUATORS WITH APPLICATIONS

by

Faez Saleh Ahmed Ba Tis

A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy

Mechanical and Industrial Engineering Department University of Toronto

© Copyright by Faez Saleh Ahmed Ba Tis, 2016

ii

NOVEL ELECTROSTATIC ACTUATORS WITH APPLICATIONS

Faez Saleh Ahmed Ba Tis

Doctor of Philosophy

Mechanical and Industrial Engineering Department

University of Toronto

2016

iii

NOVEL ELECTROSTATIC ACTUATORS WITH APPLICATIONS

Faez Saleh Ahmed Ba Tis

Doctor of Philosophy

Mechanical and Industrial Engineering Department

University of Toronto

2016

Abstract

High-end smartphone cameras suffer from large size, high power consumption, and slow

performance. These issues are mainly due to the poor performance of the voice coil motors

(electromagnetic actuators) used to achieve autofocus (AF) and Optical Image Stabilization

(OIS) features in these cameras. Due the superior performance of Micro-Electro-Mechanical-

Systems (MEMS) electrostatic actuators over that of other actuation technologies in terms of

achieving low power consumption and fast response, micro-electrostatic actuators are being

pursued to achieve AF and OIS in smartphone cameras. The maximum mass load displaced by

MEMS electrostatic actuators reported in the literature has been limited to 2 mg (corresponds to

the mass of a single lens). However, the required mass load to be displaced to achieve AF and

OIS is in the order of 62 mg mass which represents the mass of a typical lens barrel containing 5

lenses.

In this thesis, a novel design of a MEMS piston-tube electrostatic actuator was developed to meet

the actuation requirements for AF and OIS in smartphone cameras. The new design overcomes

the limitations of the initial design of the piston-tube electrostatic actuator, previously developed

by the author. These limitations include the generation of an insufficient out-of-plane translation

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stroke which is limited to only 24 µm and a low output force also limited to displacing only a 1

mg mass.

Two versions of the new design were developed, fabricated, and tested. The latest version was

specifically developed to meet the actuation requirements for AF and OIS in smartphone

cameras. A new fabrication process, i.e. the MMDL fabrication process, was developed at the

university of Toronto cleanrooms to meet the fabrication requirements for this version. The

MMDL-fabricated actuator provides for 3 degrees of freedom motion and was able to translate

and rotate a 62-mg lens barrel a stroke of 65.5 µm and an angle of rotation of ±0.4°, respectively.

The actuator was integrated within a camera module to evaluate how well the actuator meets the

requirements of the AF. The actuator achieved autofocus form 15 cm to infinity within 0.5 s,

whereas high-end smartphone cameras achieve autofocus within 0.68 s.

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Acknowledgments

I would like to express my sincere appreciation to my supervisor Prof. Ridha Ben-Mrad for his

continuous support, motivation, and guidance throughout the course of my PHD program. I

would like to thank him being a tremendous mentor for me. His inspiring advice and

encouragement allowed me to further sharpen my research skills and make this project

successful.

I would like to extend my thanks to other members of my PHD committee, Prof. Pierre E.

Sullivan and Prof. Tobin Filleter, for their valuable suggestions and constructive feedback.

I would like to thank the staff of Toronto Nano Fabrication Center (TNFC) including Dr. Edward

Huaping Xu, Harlan Kuntz, Dr. Henry Lee, Dr. Lindsey Fiddes, and Dr. Dan Voicu for their

assistance and technical support throughout the development of the fabrication process of my

actuator chips.

My thanks to my fellow labmates at the Mechatronics and Microsystems Design Laboratory

(MMDL) Ahmed Galaom, Alaeddin Bani Milhin, Amro Ibrahim, Vainatey Kulkarni, Steffen

Blume, Donn Pasiliao, and Ali Baans for their technical support and help when it is needed.

I would also like to express my gratitude to my beloved parents and family for their kindness,

patience, and support throughout my PHD program.

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Table of Contents

Acknowledgments ......................................................................................................................... v

Table of Contents ......................................................................................................................... vi

List of Tables .............................................................................................................................. viii

List of Figures ............................................................................................................................... ix

Chapter 1 Introduction ........................................................................................................... 1

1.1. Motivation ........................................................................................................................... 1

1.2. Literature Review ................................................................................................................ 2

1.2.1. MEMS Actuators for AF and OIS .......................................................................... 2

1.3. Objectives of the Thesis .................................................................................................... 12

1.4. Structure of the Thesis ...................................................................................................... 12

Chapter 2 Conceptual Design .............................................................................................. 14

2.1. Introduction ....................................................................................................................... 14

2.2. Analytical Model of the Design ........................................................................................ 16

2.2.1. Out-of-plane Translation Stroke ........................................................................... 17

2.2.2. In-plane Pull-in Instability .................................................................................... 18

Chapter 3 Large Force and Large Stroke Actuator Design .............................................. 20

3.1. New Design Platform ........................................................................................................ 20

3.2. MicraGEM-Fabricated Actuator ....................................................................................... 22

3.2.1. Design Parameters ................................................................................................ 22

3.2.2. Analytical Model of the Actuator ......................................................................... 25

3.2.3. Finite Element Analysis ........................................................................................ 28

3.3. MMDL-Fabricated Actuator ............................................................................................. 31

3.3.1. Design Parameters ................................................................................................ 31

3.3.2. Main Features of the MMDL-Fabricated Actuator ............................................... 37

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3.3.3. Analytical Model of the Actuator ......................................................................... 38

3.3.4. Finite Element Analysis ........................................................................................ 40

3.4. Summary ........................................................................................................................... 43

Chapter 4 Fabrication Process ............................................................................................. 44

4.1. Background ....................................................................................................................... 44

4.2. The MicraGEM-Si™ Process ........................................................................................... 45

4.3. MMDL Fabrication Process .............................................................................................. 47

4.3.1. Flow Chart of the MMDL Fabrication Process .................................................... 48

4.3.2. Fabrication Results of the MMDL-Fabricated Prototypes .................................... 53

4.4. Summary ........................................................................................................................... 58

Chapter 5 Experimental Assessment ................................................................................... 60

5.1. MicraGEM-Fabricated Actuator ....................................................................................... 60

5.1.1. Static Performance ................................................................................................ 60

5.1.2. Dynamic Performance .......................................................................................... 61

5.1.3. Output Force Test ................................................................................................. 63

5.2. MMDL-Fabricated Actuator ............................................................................................. 65

5.2.1. Unloaded Actuator Performance ........................................................................... 65

5.2.2. Loaded Actuator Performance .............................................................................. 73

5.1.4. Integration of the Actuator into a Camera Module ............................................... 79

5.3. Summary of Experimental Results ................................................................................... 81

Chapter 6 Conclusions and Contributions ......................................................................... 82

6.1. Conclusions ....................................................................................................................... 82

6.2. Major Contributions .......................................................................................................... 85

6.3. Future Work and Recommendations ................................................................................ 86

References ............................................................................................................................... 88

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List of Tables

Table 3-1. Key design parameters of the MicraGEM-fabricated actuator. ................................... 25

Table 3-2. Key design specifications of the MMDL-fabricated actuator. .................................... 36

Table 6-1. Comparison between the MMDL-fabricated actuator and other AF technologies. .... 84

ix

List of Figures

Figure 1-1. Structure of a conventional VCM for AF in phone cameras [11]. ............................... 2

Figure 1-2. Teardown of the front camera of Samsung Galaxy S7 Edge [13]. .............................. 3

Figure 1-3. PoLight piezo AF technology [16]. .............................................................................. 5

Figure 1-4. Cambridge Mechatronics SMA-based AF and OIS technology [20]. ......................... 6

Figure 1-5. 3D schematics of the initial design of the MEMS piston-tube electrostatic actuator.

(a) lower structure (contains the pistons). (b) upper structure (contains the tubes). (c) The bonded

structure [26]. ................................................................................................................................ 11

Figure 2-1. 3D sechmatics of the piston-tube actuator. (a) Arc shaped piston-tube. (b)

Rectangular shaped piston-tube [28]. ........................................................................................... 15

Figure 2-2. An illustration of two unit cells of the piston-tube actuator. (a) Top view of the

rectangle shaped actuator (b) Top view of the two unit cells along each in-plane axis(x and y) (c)

3D illustration of a unit cell of the actuator. ................................................................................. 16

Figure 3-1. 3D schematics of the new design of the piston-tube electrostatic actuator. (a) Moving

part (upper structure). (b) Fixed part (lower structure). (c) The final bonded structure of the

actutaor. (d) Unit cell. ................................................................................................................... 21

Figure 3-2. 3D schematics of the lower structure of the MicraGEM-fabricated actuator. ........... 22

Figure 3-3. 3D schematics of the upper structure of the MicraGEM-fabricated actuator. ........... 23

Figure 3-4. 3D schematics of the complete bonded structure of the MicraGEM-fabricated

actuator. (a) Top and cross-sectional views. (b) Isometeric view. ................................................ 24

Figure 3-5. 3D schematic of the crab-leg springs used in the MicraGEM-fabricated actuator. ... 26

Figure 3-6. Analitical translation strokes of the MicraGEM-fabricated actuator at different

voltages. ........................................................................................................................................ 27

x

Figure 3-7. 3D CoventorWare model of the upper structure of the MicraGEM-fabricated

actuator showing the first mode shape of the actuator. ................................................................. 29

Figure 3-8. Estimated translation stroke of the MicraGEM-fabricated actuator versus voltage

based on numerical simualtions. ................................................................................................... 29

Figure 3-9. Schematics of the lower structure of the MMDL-fabricated actuator. ...................... 31

Figure 3-10. 3D schematics of the lower structure of the MMDL-fabricated actuator. ............... 32

Figure 3-11. Cross-section views of the pistons. (a) ideal sidewall angle of 90°. (b) Expected

sidewall angle < 90°. ..................................................................................................................... 32

Figure 3-12. 3D schematics of the upper structure of the MMDL-fabricated actuator. ............... 34

Figure 3-13. Top views of the upper structure of the MMDL-fabricated actuator. ...................... 34

Figure 3-14. 3D schematics of the final structure of the MMDL-fabricated actuator after

bonding.(a) Isometric backside view. (b) Isometric frontside view. ............................................ 35

Figure 3-15. 3D schematics of the lens barrel to be integrated within the MMDL-fabricated

actuator. (a) Cross-section view. (b) Frontside view. (c) Isometric view. .................................... 35

Figure 3-16. 3D schematics of the attachement process of the lens barrel into the MMDL-

fabricated actuator. (a) During attachment. (b) After attachment. ................................................ 36

Figure 3-17. 3D schematic of the crab-leg springs of the MMDL-fabricated actuator. ............... 38

Figure 3-18. Analytical solution for the translation stroke of the MMDL-fabricated actuator

versus applied voltages . ............................................................................................................... 39

Figure 3-19. 3D CoventorWare model of the upper structure (rotor and springs) of the MMDL-

fabricated actuator showing the first mode shape of the actuator. ................................................ 41

Figure 3-20. Estimated translation stroke of the MMDL-fabricated actuator versus applied

voltage based on numrical simualtions and the analytical model. ................................................ 42

xi

Figure 3-21. First rotational mode shape and its corresponding natural frequency of the MMDL-

fabricated actuator. ........................................................................................................................ 42

Figure 3-22. Estimated angles of rotation of the rotor of the MMDL-fabricated actuator based on

numerical simuation results. ......................................................................................................... 43

Figure 4-1. Fabrication flow of the MICRALYNE MICRAGEM-Si™ MEMS process. (a)

Fabrication of a Base SOI wafer. (b) Fabrication of the Mirror SOI wafer. (c) Bonding of both

SOI wafers. (d) Final structure of the actuator. ............................................................................ 46

Figure 4-2. SEM micrograph of a prototype of the MicraGEM-fabricated actuator. ................... 46

Figure 4-3. SEM micrograph of the gap between adjacent pistons and tubes of a prototype

actuator. ......................................................................................................................................... 47

Figure 4-4. SEM micrographs showing the top view of an MMDL-fabricated actuator. ............ 53

Figure 4-5. SEM micrographs of a cross section of the MMDL-fabricated actuator. .................. 54

Figure 4-6. SEM micrograph showing a spring cross-section in the MMDL-fabricated actuator.

....................................................................................................................................................... 55

Figure 4-7. SEM micrographs of an actuator prototype cross section showing the sidewall angles

and the depth of the pistons of the MMDL-fabricated actuator. ................................................... 56

Figure 4-8. SEM micrographs illustrating the notching effect phenomenon of the occurred at the

bottom side of the tube walls. ....................................................................................................... 57

Figure 4-9. SEM micrographs of the topside of the MMDL-fabricated actuator showing: (a) the

alignemnt measurements between the pistons and tubes along the x-axis. (b) The alignemnt

measurements between the pistons and tubes along the y-axis. ................................................... 58

Figure 5-1. Test apparatus used for experimental assessment of the actuators. ........................... 60

Figure 5-2. Measured translation strokes versus voltage of the MicraGEM fabricated actuator in

the translation mode. Numerical simulations data is also shown for comparison. ....................... 61

xii

Figure 5-3. Dynamic performance of the actuator. (a) Step input rising from 0 to 20V. (b) Step

input decreasing from 20 V to 0 V. ............................................................................................... 62

Figure 5-4. SEM micrograph of the MicraGEM-fabricated actuator showing regions in the rotor

(defined by white circles) that contribute to thin-film damping with the lower structure. ........... 62

Figure 5-5. Dynamic performance of the unloaded MMDL-fabricated actuator. (a) The velocity

of the rotor when it is released. (b) FFT plot of the veloctiy signal. ............................................ 63

Figure 5-6. Snapshot of the actuator loaded with a lens barrel of 62 mg during testing. ............. 64

Figure 5-7. Measured translation versus applied voltage of the MMDL-fabricated actuator while

unloaded. Analytical and simulation results are also shown for comparison. .............................. 65

Figure 5-8. SEM micrographs illustrating the notching effect occurring at the bottom side of the

tube walls. ..................................................................................................................................... 66

Figure 5-9. SEM micrographs showing top and cross section views of the rotor parts that form

parallel plate capacitances with the lower structure...................................................................... 67

Figure 5-10. Dynamic performance of the unloaded MMDL-fabricated actuator in translation. (a)

Step input from 0 to 60V. (b) Step input from 60 V to 0 V. ......................................................... 69

Figure 5-11. A snapshot of the unloaded MMDL-fabricated actuator while in the actuation state.

....................................................................................................................................................... 70

Figure 5-12. Dynamic performance of the unloaded MMDL-fabricated actuator. (a) The velocity

of the rotor when it is released. (b) FFT plot of the veloctiy signal. ............................................ 70

Figure 5-13. Measured angles of rotation of the rotor of the unloaded MMDL-fabricated actuator.

Simulation data is also shown for comparison. ............................................................................ 71

Figure 5-14. Dynamic performance of the unloaded MMDL-fabricated actuator in the rotation

mode. (a) Step input going from 0 to 80V. (b) Step input from 80 V to 0 V. .............................. 72

Figure 5-15. A snapshot of the unloaded MMDL-fabricated actuator while operating in the

rotation mode. ............................................................................................................................... 72

xiii

Figure 5-16. Measured translation versus applied voltage of the 62 mg loaded MMDL-fabricated

actuator. ......................................................................................................................................... 74

Figure 5-17. A snapshot of the MMDL-fabricated actuator while in the actuation state in the

translation mode with a lens barrel attached to it. ........................................................................ 74

Figure 5-18. Dynamic performance of the unloaded MMDL-fabricated actuator in the translation

mode. (a) Step input from 0 to 30V. (b) Step input from 30 V to 0 V. ........................................ 75

Figure 5-19. Dynamic performance of the loaded MMDL-fabricated actuator. (a) The velocity of

the rotor when it is released in the time domain. (b) the peak amplitude of veloctiy of the rotor in

the frequency domain. ................................................................................................................... 76

Figure 5-20. The measured angles of rotation versus voltage of the rotor of the 62 mg loaded

MMDL-fabricated actuator. .......................................................................................................... 76

Figure 5-21. Dynamic performance of the loaded MMDL-fabricated actuator in the rotation

mode. (a) Step input from 0 to 30V. (b) Step input from 30 V to 0 V. ........................................ 77

Figure 5-22. A snapshot of the MMDL-fabricated actuator while in the actuation state in the

rotation mode with a lens barrel being attached to it. ................................................................... 77

Figure 5-23. A 3D exploded view showing the components of the prototype miniature camera

used to investigate the autofocus performance of the MMDL-fabricated actuator. ..................... 79

Figure 5-24. Miniature camera prototype used to investigate the autofocus performance of the

MMDL-fabricated actuator. (a) 3D schematic. (b) Actual camera module. ................................. 80

1

Chapter 1 Introduction

1.1. Motivation

High power consumption, large size, and slow speed are issues that need to be addressed in

current smartphone cameras. These issues are mainly related to the actuation methods used,

which are largely using Voice Coil Motor (VCM) technology, to achieve Autofocus (AF) and

Optical Image Stabilization (OIS).

The autofocus feature is used to achieve an improved sharpness of the image by translating a lens

barrel of typically 62 mg a stroke of 90 µm to keep the image in focus on the image sensor plane.

The OIS feature is used to cancel any hand-shake effects by rotating the lens barrel an angle of

±0.5º in a direction opposite to that of the hand-shake, or by translating it along the in-plane axes

(x and y) where the x-y plane is parallel to the image sensor plane[1][2][3].

A number of actuation methods are used to achieve AF and OIS in phone cameras. These include

Voice Coil Motors (VCM), piezoelectric actuators, liquid lens actuators, and MEMS electrostatic

actuators [4][5]. VCM actuators are the most commonly used in commercial phone cameras.

VCMs work on the principle of electromagnetic actuation. They are known to have a number of

issues when used in smartphones such as high power consumption of the order of 100 mW, large

size, and slow response of around 30 ms [6][7]. Piezoelectric actuation is also used to achieve

autofocus; nevertheless, it leads to motion control degradation which leads to the need for a

position sensor; and piezoelectric based actuation is sensitive to temperature [7]. Liquid lens

technology is under development, and is known to require high power, offers a slow response

speed and sets stringent demands on packaging [8].

An emerging technology to achieve AF in phone cameras is MEMS electrostatic actuation. This

actuation method offers low power consumption of less than 1 mW, fast response with a settling

time of less than 10 ms, and small size, especially in terms of the thickness which is very

important for smartphones. Currently, two types of electrostatic actuators for AF have been

reported and are still under development: one is being developed by Digital Optics Corporation

(DOC) [9], and the other being developed by Apple Inc.[10]. DOC’s electrostatic actuator has a

number of drawbacks such as failure to survive the standard drop test, insufficient force to actuate

a lens barrel of tens of milligrams in mass, and complexity in its assembly process which makes it

expensive to produce. Apple’s electrostatic actuator provides limited displacement below what is

2

required to achieve autofocus from 10 cm to infinity; it also requires a high driving voltage which

is not safe for use in handheld devices; and it generates a low force that is not sufficiently large to

actuate a lens barrel of tens of milligrams in mass.

The actuation requirements needed for AF and OIS for high performance smartphone cameras can

be summarized as follows:

1- The ability to translate a 62 mg lens barrel a stroke of 90 µm.

2- The ability to rotate the lens barrel an angle of ±0.5º.

3- A settling time of the actuator of less than 30 ms.

4- An input voltage limited to 40 V.

1.2. Literature Review

1.2.1. MEMS Actuators for AF and OIS

1.2.1.1. Electromagnetic Actuators

Electromagnetic actuators or Voice Coil Motors (VCMs) are the most dominant and

commercially available technology to achieve AF and OIS in smartphone cameras. A

conventional VCM consists of two parts: fixed and moving parts. The fixed part, as shown in

Figure 1-1, includes two permanent magnets, a yoke, and a fixed base. The moving part includes

an optical lens module, a lens-holder, and a coil. The Lorentz force FVCM acts in an opposite

direction to the restoring force of two spring plates that are mounted on the top and bottom faces

of the actuator body as shown in Figure 1-1.

Figure 1-1. Structure of a conventional VCM for AF in phone cameras [11].

3

The maximum operational load occurs when the actuator is in a vertical position as shown in

Figure 1-1(b). At this position, the force due to the weight of the moving part Fw also comes into

play and therefore, FVCM interacts with both forces Fw and the spring restoring force Fs. As the

current flow through the coil increases, FVCM increases to move the lens barrel to achieve AF

[11][12].

VCMs have been only used extensively solely to achieve AF in smartphone cameras. However,

in recent years, high-end smartphones such as the iPhone 6s Plus and the Samsung Galaxy S7

Edge have incorporated two decoupled VCMs to achieve both AF and OIS. One is used to

translate the lens barrel along the optical axis (z-axis) to achieve AF, while the other VCM is

used to translate the AF VCM, with the lens barrel attached, along the x-y plane to achieve OIS,

refer to Figure 1-2.

Although VCMs are easy to fabricate and simple to integrate, they are associated with a number

of disadvantages. These disadvantages include high power consumption in the order of 100 mW,

large size, and slow speed as they lead to a settling time of the order of 30 ms.

Figure 1-2. Teardown of the front camera of Samsung Galaxy S7 Edge [13].

4

1.2.1.2. Piezoelectric Actuators

A number of technologies based on piezoelectric actuation were developed to achieve AF in

smartphone cameras. These technologies include the UTAF and Tlens actuators which were

developed by New Scale Technology Inc. and PoLight Technologies Inc., respectively. These

two technologies are further described next.

UTAF is an ultrasonic piezoelectric actuator. It comprises a piezo motor, a drive IC, a digital hall

position sensor, a low-friction preload mechanism, a precise lens guide, and a Smart Step

algorithm [14]. The piezo motor is a beam that vibrates causing a threaded screw to rotate

generating a linear motion, thus driving the optical module within the camera. UTAF actuator

provides large stroke and consumes low power; however, it requires a position sensor to operate

and it is sensitive to temperature[8][15].

The Tlens actuator is another piezoelectric-based AF technology that achieves autofocus by

modifying the shape of the lens instead of translating it. It consists of a ring-shaped piezo thin-

film deposited on the top of a glass membrane. A soft polymer lens is sandwiched between this

glass membrane and another glass support as shown in Figure 1-3 (a). By applying a varying

voltage of up to 40 V, the actuator applies radial forces on the glass membrane causing the

deformation of the polymer lens, thus varying the optical focus; see Figure 1-3 (b).

(a)

5

(b)

Figure 1-3. PoLight piezo AF technology [16].

The PoLight AF actuation technology offers a number of advantages including low power

consumption of less than 5mW and a high AF speed leading to a settling time of less than1ms.

However, changing the AF of a single lens within a train of optical lenses (the lens barrel of the

smartphone camera contains typically 5-6 lenses) might deteriorate the image quality by

generating low Modulation Transfer Function MTF (i.e. image quality measure) images. In

addition, the TLens actuator is a piezoelectric-based which is sensitive to temperature changes,

and the technology cannot achieve OIS [17].

1.2.1.3. Shape-Memory-Alloy (SMA) Actuators

Cambridge Mechatronics Ltd. has developed an actuation technology to meet the requirements

of AF and OIS in smartphone cameras. See Figure 1-4. It is an SMA-based technology in which

the actuator material changes its shape in the presence of heat. Four actuator wires are positioned

around the mass load, which could be a lens barrel when used to achieve AF or the entire camera

module when used to achieve OIS, see Figure 1-4. In the autofocus mode, the current flows

through all wires causing them to heat up and shrink, thus displacing the lens barrel vertically up

and down. While to achieve OIS, only one or two wires are heated up by the current flow, which

results in the rotation of the camera module to counteract any handshake effect. The SMA-based

OIS actuator achieves OIS using the best technique to cancel the handshake, i.e. rotation of the

6

entire camera module. However, SMA actuators consume high power as heat is generated during

the actuation and are difficult to manufacture [18][19].

Figure 1-4. Cambridge Mechatronics SMA-based AF and OIS technology [20].

1.2.1.4. Liquid Lens Actuation

Liquid lens technology for AF in smartphone cameras was developed by VariOptics Inc. The

technology works on the principle of electrowetting. The lens is a sealed cell that contains two

transparent liquids (water and oil) having the same density, but different refractive indices. The

lens cell is surrounded by two electrodes. When a voltage is applied across both surfaces

(electrodes) of the lens, an electric field is generated forcing the liquids to be redistributed

according to the applied voltage. Such redistribution of liquids changes the lens shape, resulting

in a variation of the focal length of the lens. This technology has been shown to be reliable up

to 1 million cycles. However, it has a slow response time leading to a settling time of the order

of 200 ms. It also has a relatively high power consumption and high cost [8][15][21].

1.2.1.5. Electrostatic actuators

When a voltage is applied across two conductive plates placed closely to each other, an electric

field is generated between them, thus generating an attractive force. This force is called an

electrostatic force. A basic design of an electrostatic actuator comprises of moving (rotor) and

fixed (stator) electrodes. An attractive electrostatic force develops between the electrodes in

response to difference in potential, causing the moving electrode to move towards the fixed one,

SMA wire actuator

7

hence motion is generated. Electrostatic actuation is the most commonly used in MEMS devices

due to its simple design, ease of fabrication, low power consumption and fast response.

However, electrostatic actuators suffer from a number of drawbacks such as limited

displacement, low output force and high driving voltage [22][23][24].

Electrostatic actuators can be classified into two categories: in-plane and out-of-plane actuators.

In-plane electrostatic actuators provide motion parallel to the substrate, while out-of-plane

actuator provides motion along the plane normal to the substrate. For achieving AF and OIS in

smartphones, the out-of-plane electrostatic actuator is the best candidate as the required motion

of the lens barrel is along the direction normal to the actuator substrate.

1.2.1.5.1. Out-of-plane Electrostatic Actuators

The electrostatic actuators that provide an out-of-plane stroke include parallel plate and Vertical

Comb-Drive (VCD) actuators. The former suffers from the pull-in effect which limits the vertical

stroke of the actuator to one third of the initial gap between the plates [9]. The latter could be

divided into two types: rotational and translational (piston-style) VCD actuators.

In rotational comb-drive actuators (including staggered and angular VCD actuators), the original

motion of the rotor is a rotation, thus providing a rotational out-of-plane stroke; however, these

actuators often utilize mechanical amplification mechanisms, such as levers, to enlarge the stroke

as well as to transform the original rotary motion of the rotor into a translation of the load. Due

to the motion amplification and transformation, the output torque of the actuator acting on the

load is generally lower than the one generated. Different designs of rotational VCD actuators

have been developed. For example, a rotational VCD actuator developed by V. Milanovic et al.

[11] achieved a vertical deflection of 60 μm (corresponding to an angle of rotation of 20°) at 150

V. It utilizes a cantilever for mechanical amplification. Li et al. [12] developed a rotational

(staggered) VCD actuator that achieved a vertical deflection of 180 μm at 35 V. A plate was

attached to the free comb of the actuator to achieve the large rotational stroke while reducing the

output torque.

U.S. Pat. No. 8,269,395 B2 [14] discloses a large stroke rotational comb-drive actuator. It works

on the repulsive force principle, and the rotor of the actuator achieves an 86 μm vertical

deflection at 200 V based on a rotational stroke at each of the four edges of the actuator which is

8

then amplified using a cantilever beam; however, it provides a low output torque due to use of an

amplification mechanism and to the small area of the fingers used to generate the force.

In translational VCDs, the original motion of the rotor is a translation, and the total electrostatic

force that is developed between the electrodes is directly applied to the load attached to the rotor

without the use of any stroke amplification or transformation mechanism. A number of

translational VCD actuators were developed. A translational VCD actuator, developed by V.

Milanovic et al., achieved a translation stroke of 15 μm at 140 V. The actuator is fabricated using

a Deep Reactive Ion Etching (DRIE) of an SOI wafer which enables the fabrication of large

height electrodes; however, it provides a low output force as the comb electrode configuration is

not area-efficient in terms of overall electrode capacitance. That is because the rotor of the

actuator consists of two arrays of fingers (each array is formed along one side of the rotor). The

number of fingers in these two arrays can be increased only along one direction, i.e. the lateral

direction of comb fingers [11]. A self-aligned translational VCD actuator [15], developed by E.

Carr et al., was able to achieve only a stroke of only 1.4 μm due to the high stiffness of the

supporting beams along the z-axis (out-of plane axis) and due to the low output force that can be

generated by the actuator which is attributed to the area-inefficient configuration as is the case in

the previous translational VCD actuators [11].

U.S. Pat. No. 6,384,952 B1 [16] discloses a translational vertical comb-drive actuator used to

actuate a deformable mirror. The actuator has a cavity-tooth configuration which enables

achieving a wide area for the electrodes, and it provides an out-of-plane translation of 20 μm at

100 V; however, the actuator provides only 1 degree of freedom (DOF) motion, i.e. vertical

translation. The differences in the translations of the VCD actuators beneath the mirror surface

result in a bi-axial rotation of the mirror surface. In other words, the comb-drive actuators have

only 1-DOF motion which is a translation along the z-axis, whereas the mirror surface itself has

a 3-DOF motion, i.e. translation along the z-axis, and bi-axial rotation about the in-plane axes (x

and y). A limitation of this actuator is that the tooth-cavity configuration requires the rotor and

the stator of the actuator to be fabricated separately. The fabricated rotor and stator wafers are

then bonded together which may lead to a misalignment of sub-microns size between the upper

and lower electrodes. This misalignment limits the stroke of the actuator.

9

U.S. Pat. No. 7,538,471 B2 [17] discloses a vertical comb-drive actuator configuration that

provides an increased rigidity to the optical surface. The goal of the invention is to overcome the

problem of optical surface deformation that ensues from the deposition of a reflective metal such

as gold or aluminum on the optical surface to enhance its reflectivity. The invention eliminates

this problem by reinforcing the comb electrodes beneath the reflecting surface in more than one

direction. The actuator provides 3-DOF motion, i.e. translation along the z-axis and bi-axial

rotation about the in-plane axes (x and y), without the use of any stroke amplification

mechanism. It also provides a considerable large output force due to the ability of the electrode

configuration to be expanded in more than one dimension. The actuator is fabricated using a

surface micromachining process in which the height of the comb electrodes is limited due to the

nature of the layer deposition process. These layers cannot be of a large height (thickness), which

leads to a limitation on the out-of-plane translation of the actuator.

1.2.1.5.2. VCD AF Actuator

Two AF technologies utilizing out-of-plane electrostatic actuators to achieve the AF feature in

smartphone cameras have been reported. The first electrostatic-based autofocus technology was

developed by Digital Optics Inc. and is disclosed in U.S. Pat. No. 8,358,925 B2 [13]. It utilizes a

rotational comb-drive actuator to translate a lens along the optical axis (z) to achieve AF. The

original motion of the rotor is an out-of-plane rotation which is transformed (with the assistance

of similar actuators surrounding the lens) into a vertical deflection of the lens using a complex

transformation mechanism. A significant amount of the rotor torque is dissipated during the

transformation of the motion. Due to the complexity of the structure of the rotor of this actuator,

an undesirable tilt occurs during the translation of the lens when it is actuated by a number of

similar actuators. This tilt needs a motion controller to be eliminated. In addition, the actuator is

limited to move a 2 mg mass load and thus it cannot be used to achieve AF by displacing the

entire lens barrel.

The second electrostatic-based autofocus technology was developed by Apple Inc. and is

disclosed in U.S. Pat. No. 8,711,495[25]. It utilizes three or more translation vertical comb-drive

actuators to achieve autofocus by translating a lens. The goal of this technology is to increase the

resistance of the electrostatic actuator structure to shocks that occur during the drop test of the

mobile phone. The drawbacks of this actuation mechanism include an inefficient area-electrode

10

layout, as it utilizes single array comb-drive actuators distributed around the lens, meaning a

higher driving voltage is required; a limited out-of-plane translational stroke, as the maximum

height (thickness) of the electrodes is 20 µm; and a low resonant frequency, as the supporting

beams have to buckle during the loading of the lens to the central ring to provide an offset

between the comb fingers. Besides, it is limited to translate only a single lens of 2 mg in mass.

1.2.1.6. Limitations of the Current VCD Actuators

Based on the review of the literature, current VCD Actuators have limited capabilities in terms of

achieving simultaneously a large output force and a large stroke due to the fact that: (1) The

fabrication of well-defined supporting beams of the rotor with low transverse stiffness as

compared to the lateral stiffness (the height of the beam is less than the width) is challenging. (2)

The configurations of the VCD actuators used so far are not area-efficient because the comb

fingers have an array-style structure, meaning that the output force can only be increased by

increasing the number of fingers in one dimension which is along the lateral axis of the comb

fingers. In other words, the comb fingers are essentially free-end cantilevers; hence they cannot

be largely elongated along the longitudinal axis to increase the output force. Therefore, the output

force can only be increased by multiplying the number of comb fingers along one dimension

which is the lateral axis of the comb fingers.

1.2.1.7. Previous Work Done by the Author

A novel electrostatic actuator that utilizes a piston-tube configuration has been developed by the

author during his master’s degree program [26]. This piston-tube configuration allows for the use

of a wide area of electrode capacitances as it uses a matrix-style structure that can be expanded

in two dimensions, refer to Figure 1-5. Therefore, a high output force could be generated. The

actuator also utilizes a flexure mechanism that has a low out-of-plane stiffness (Kmz) as opposed

to the in-plane stiffnesses (Kmx, Kmy). This mechanism enables the actuator to achieve a large

translation stroke (piston-style motion) before experiencing lateral pull-in instability. The piston-

tube actuator also provides 3-DOF motion, i.e. translation along the z-axis, and rotations about

the x and y axes. The 3-DOF motion is an important characteristic of the design in view of the

fact that other electrostatic comb-drive actuators have a maximum of 2-DOF motion of the rotor.

11

Figure 1-5. 3D schematics of the initial design of the MEMS piston-tube electrostatic actuator. (a)

lower structure (contains the pistons). (b) upper structure (contains the tubes). (c) The bonded

structure [26].

The initial design of the 3 DOF piston-tube electrostatic actuator was associated with a number

of limitations. These limitations include the generation of insufficient force, i.e. 59 µN, and a

limited translation stroke, i.e. 28 µm, which results in the ability of the actuator to drive only a

limited mass load, i.e. 1 mg. These performance characteristics do not meet the requirements for

AF and OIS in smartphone cameras. These limitations are mainly due to two reasons: the spring

configuration used in the initial design of the piston-tube actuator and the design restrictions

imposed by the standard fabrication process parameters as defined by the MicraGEM-Si

process[27]. The springs in this design of the actuator extend radially inward towards the center

of the actuator plate. This extension has restricted the area of the electrode capacitance of the

rotor; hence low force is generated and a high driving voltage is required. The parameters of the

standard fabrication process, i.e. MicraGEM-Si, have restricted the design dimensions of the

actuator which led to the limitation of the stroke, i.e. 28 µm. In particular, the thickness of the

mirror wafer layer (rotor) in this process is 30 µm which means that in the best circumstances,

the stroke of the rotor of the actuator will not exceed 30 µm as the pistons and tubes reach their

maximum engagement (overlapping area).

12

1.3. Objectives of the Thesis

The main objective of the thesis is to develop a large stroke and large force MEMS electrostatic

actuator platform. The actuator should address the key challenges of the MEMS electrostatic

actuators when used for large stroke and large force applications. The first challenge is the

generation of a large force that is able to translate/rotate a mass load in the range of tens of

milligrams at an input voltage of less than 40 V. The second challenge is the fabrication of the

actuator such that the actuator is able to translate the mass load a stroke of up to 90 µm. The

specifications of this actuator platform are set to meet the requirements of AF and OIS in

smartphone cameras. Therefore, the goals of the project are as follows:

1- Development of a 3 DOF MEMS electrostatic actuator platform that is able to translate a

lens barrel of 62 mg a distance of 90 µm and to rotate the lens barrel an angle of ±0.5º at

an input voltage of less than 40 V, while offering a response time of less than 10 ms;

2- Development of the microfabrication process to fabricate the actuator;

3- Experimental assessment of the performance of the actuator;

4- Integration of a 62 mg lens barrel within the actuator central opening; and

5- Integration of the actuator, to which the barrel being attached, into a camera module to

assess the performance of the actuator in achieving the AF requirements.

1.4. Structure of the Thesis

In Chapter 2, the analytical model of the piston-tube electrostatic actuator is developed. The

analytical solution for translation stroke and the pull-in instability are presented.

Chapter 3 describes a new design of the piton-tube electrostatic actuator that is able to generate

large force and large stroke. Two versions of this design will be developed and fabricated. The

analytical model and the Finite Element Analysis (FEA) results for both versions are also

provided.

In Chapter 4, the fabrication process of the first version is briefly discussed as it is a standard

fabrication process; followed by a detailed description of the new fabrication process developed

for the second version of the actuator that meets the actuation requirements for AF and OIS in

smartphone cameras.

13

In Chapter 5, an extensive experimental assessment of both versions of the new design is

presented. Static and dynamic performance test results in translation and rotation modes of the

unloaded and loaded actuators are discussed. Other test results such as power consumption and

life cycle are also presented.

Chapter 6 summarizes the conclusions, contributions, and recommendations for future work.

14

Chapter 2 Conceptual Design

This chapter presents the mathematical modelling of the piston-tube electrostatic actuator. This

analytical modelling provides the solution for the expected translation strokes and lateral

instability limit of the actuator. In previous work, the analytical modelling was not particularly

developed for piston-tube electrode configuration-based actuators as it was mainly based on the

analysis of the comb-drive electrode configuration. The analytical model presented in this thesis

would provide MEMS actuator designers with the required mathematical formulation to predict

the performance of piston-tube based electrostatic actuators.

2.1. Introduction

The basic design of the 3-DOF electrostatic piston-tube actuator consists of two structures: lower

and upper. The lower structure consists of pistons (teeth) which could have a square, rectangular,

circular, or arc shaped cross-sections. The pistons are attached to the base and they form the

stator of the actuator, refer to Figure 2-1. These pistons could be grouped in multiple arrays of

pistons, each of which forms a single stator.

The upper structure contains tubes (through holes) that have cross-sections similar to the cross-

sections of the pistons, but larger in size as shown in Figure 2-1. The moving tubes surround the

actuator plate, and they are suspended by mechanical springs that provide high in-plane stiffness

while allowing the rotor to achieve translation along the z-axis. The pistons could be divided into

three or more groups that are electrically separated from each other to allow bi-axial rotation of

the rotor about the x and y axes.

15

Figure 2-1. 3D sechmatics of the piston-tube actuator. (a) Arc shaped piston-tube. (b)

Rectangular shaped piston-tube [28].

When a voltage is applied across the rotor and all of the stators, an electrostatic force develops

between the pistons and the tubes in the z direction. This electrostatic force moves the rotor (the

free upper structure) from its static position towards the stators (fixed lower structure) in a pure

translation (piston-style) motion along the z-axis. When the voltage drops, the rotor moves back

to its equilibrium position due to the restoring mechanical forces in the supporting beams. To

rotate the rotor of the actuator about the x and/or y axes, a voltage is applied across the rotor and

only one or two stators.

The advantages of the piston-tube actuator can be summarized as follows:

1- The piston-tube actuator enables the use of wide area of the electrodes and can be

expanded in two dimensions along the x-y plane to enable large force generation.

2- The force generated by the actuator is directly applied to any load attached to the

actuator’s central plate without the use of a mechanical amplification mechanism.

3- The actuator provides 3-DOF motion, i.e. a translation along the z-axis and rotations

about the x and y axes.

4- The actuator exhibits fairly high tolerance against misalignments between the electrodes

in the translation mode. This is due to the high in-plane stiffness of the rotor that resists

any in-plane electrostatic forces that may ensue from misalignments.

16

The 3-DOF piston-tube actuator may experience two types of instability. The first instability is

the lateral pull-in instability which may occur during the translation of the rotor and it is related

to the in-plane (x and y) stiffnesses of the rotor. And the second one is the angular pull-in

instability which may take place during the rotation of the actuator about the x or the y axes, and

it is related to the non-linear change in the electrode capacitance between the walls of the pistons

and the tubes during rotation. Due to the significant effect of the lateral pull-in instability on the

translational stroke, it is discussed in detail in the next section.

2.2. Analytical Model of the Design

Due to the complexity of the electrode structure in the arc shaped actuator as compared to that of

the rectangle shaped actuator, the instability analysis is investigated on the latter. The initial

engagement between the pistons and the tubes is assumed to be zero (z0 = 0 where z0 refers to the

initial value of z). It’s also assumed that all unit cells that extend along each in-plane axis have

the same dimensions, so that the total capacitance can be calculated based on two unit cells: each

cell extending along one of the in-plane axes (x and y), see Figure 2-2(b).

Figure 2-2. An illustration of two unit cells of the piston-tube actuator. (a) Top view of the

rectangle shaped actuator (b) Top view of the two unit cells along each in-plane axis(x and

y) (c) 3D illustration of a unit cell of the actuator.

Fez

17

During the translation of the rotor along the z-axis, two parallel plate capacitances in each unit

cell exist (fringe field capacitances, Cxf and Cyf, are neglected):

).

(2)( 0021g

zxNCCNC rxxxxx (2-1)

).

(2)( 0021g

zyNCCNC ryyyyy (2-2)

where Nx and Ny are the numbers of the unit cells that extend along the x and y axes,

respectively; Cx1, Cx2, Cy1, and Cy2 are parallel plate capacitances in the two unit cells along the x

and y axes, respectively; 0 is the permittivity of vacuum; r is the relative permittivity of the

medium; x0 and y0 are the lengths (longest sides) of the pistons along the x and y axes,

respectively; g is the gap between the parallel plates formed by the longest sides of an adjacent

piston and tube in the unit cell; and z is the displacement along the z-axis.

The total electrostatic energy [24] stored between the pistons and the tubes during the translation

of the rotor along the z-axis is:

yxtot CVCVU22

2

1

2

1 (2-3)

200

200

..V

g

zyNV

g

zxNU yrxrtot (2-4)

where V is the applied voltage; Cx and Cy are, as defined previously, the parallel plate

capacitances of the unit cells along the x and y axes, respectively.

2.2.1. Out-of-plane Translation Stroke

The electrostatic force along the z-axis causes translation of the rotor in the z direction. This

force is calculated by differentiating the total electrostatic energy equation with respect to the z-

axis:

200

0

)(V

g

yNxN

z

UFe

yx

rtot

z

(2-5)

An opposing mechanical force along the z-axis develops in the supporting beams:

18

zKmFm zz (2-6)

where Kmz is the mechanical spring constant along the z-axis, and z is the translation stroke

along the z-axis. Equilibrium along the z-axis is reached when the mechanical and electrostatic

forces are equal. Then, the critical stroke along the z-axis can be calculated based on setting:

zz FeFm (2-7)

Which leads to:

200

0

)(V

gKm

yNxNz

z

yx

r

(2-8)

2.2.2. In-plane Pull-in Instability

Along with the electrostatic force in the z direction, in-plane electrostatic forces also develop.

These forces include two opposing electrostatic forces in each unit cell along the x and y axes.

They are calculated by differentiating the electrostatic energy equation with respect to the x and y

axes:

22

2

00)(

1

)(

1

xgxgzVyN

x

UFe yr

totx (2-9)

22

2

00)(

1

)(

1

ygygzVxN

y

UFe xr

toty (2-10)

Normally, these opposing lateral in-plane electrostatic forces in each unit cell cancel each other,

but when the first derivative of the electrostatic forces Fex and Fey (at x = y = 0 ) along the x or

y axis exceeds the mechanical spring constant along that axis, lateral pull-in instability takes

place [29]. Therefore, to avoid this instability, the mechanical spring constant along the x or y

axis needs to be always greater than the first derivative of the electrostatic force along the

respective axis:

3

2

00

0

4

g

zVyN

x

FeKm

yr

x

xx

(2-11)

3

2

00

0

4

g

zVxN

y

FeKm xr

y

y

y

(2-12)

19

By solving Equations (2-11) and (2-12) for V to obtain the pull-in voltage values that lead to

lateral pull-in instability along the x and y axes and substituting it in Equation (2-8), the

maximum displacements along the z-axis that can be achieved before lateral instability occurs

would be limited by the following terms:

z

xxp

Km

Kmgz

2

1 (2-13)

z

y

ypKm

Kmgz

2

1 (2-14)

Based on Equations (2-13) and (2-14), in order to achieve a large out-of-plane translation along

the z-axis at low input voltage (i.e. selecting a small value of g), high ratios of z

x

Km

Kmand

z

y

Km

Kmare needed.

20

Chapter 3 Large Force and Large Stroke Actuator Design

This chapter describes a new design of the 3-DOF MEMS piston-tube electrostatic actuator that

provides a large stroke in the order of 90 µm and a large force able to move masses in the order

of 62 mg. Such performance characteristics meet the actuation requirements for autofocus and

optical image stabilization in smartphone cameras. Two versions of this new design were

developed. Design parameters and analytical and numerical simulation solutions for both

versions are also presented.

3.1. New Design Platform

The new design of the 3-DOF MEMS piton-tube electrostatic actuator is developed to achieve

the objectives of this thesis. The new design consists of two parts: moving and fixed. The fixed

part, i.e. the lower structure, consists of pistons which have rectangular cross-sections, refer to

Figure 3-1 (b). The pistons are vertically supported by the base, and they are divided into four

groups that are electrically isolated from each other to allow bi-axial rotation of the rotor about

the x and y axes. Each group of the electrically connected pistons forms a single stator. The

moving part, i.e. upper structure, contains tubes (through holes) that have cross-sections similar

in shape to the cross-sections of the pistons, but they are wider than the pistons to allow the

engagement of the pistons into the tubes during the actuation, refer to Figure 3-1 (a). The moving

tubes surround the central opening of the actuator, and they are attached to the substrate via four

crab-leg springs, which are extending along the outer periphery of the rotor. This spring

configuration and the piston-tube electrode layout eliminate the issues associated with the initial

design of the piston-tube actuator developed by the author in the master’s degree program and

previously discussed at the end of Chapter 1. The new configuration of springs has four

advantages. First, it allows springs to be largely long (leads to having a high ratio of the

longitudinal stiffness (Kmx or Kmy) to the transverse stiffness (Kmz) as the longitudinal

stiffness of the beams is inversely proportional to the length and the transverse stiffness is

inversely proportional to the length cube). This high value of the ratio pushes the lateral

instability limit further, so a large stroke could be achieved. Second, it provides a wide area of

the rotor to be filled with piston-tube pairs, thus generating a large electrostatic force. This is

advantageous over the spring configuration used in the initial design, in which the springs are

extending radially inwards leading to the shrinkage of the area of the rotor. Third, the springs

21

could have the same thickness as the rotor (tubes) layer, meaning the fabrication step needed for

etching the rotor layer to soften the spring could be eliminated, which simplifies the fabrication

process. Fourth, springs with large height (thickness), similar to the electrodes height, make the

MEMS electrostatic actuation more resistant to shock when loaded with a mass such as lens

barrels.

Figure 3-1. 3D schematics of the new design of the piston-tube electrostatic actuator. (a) Moving

part (upper structure). (b) Fixed part (lower structure). (c) The final bonded structure of the

actutaor. (d) Unit cell.

Two versions of the new design of the MEMS piston-tube electrostatic actuator were developed.

The first version, referred to as MicraGEM-fabricated actuator, was developed such that it meets

the design rules of the standard MicraGEM-Si fabrication process [27], by which the actuator is

able to achieve a maximum translation stroke of only 30 µm. This limitation is due to small

thickness of the rotor layer (30 µm) set by specifications of the MicraGEM-Si process. However,

this version provides significantly large electrostatic force as the spring configuration enables the

use of a large number of pistons and tubes in the design.

The second version was developed at the Mechatronics and Microsystems Design Laboratory

(MMDL) and fabricated at the Toronto Nano-Fabrication Center at the University of Toronto,

and it is referred to as the MMDL-fabricated actuator. It was designed to achieve a larger stroke

than that of the MicraGEM-fabricated actuator such that it meets the actuation requirements of

the AF and OIS features in smartphone cameras.

22

3.2. MicraGEM-Fabricated Actuator

3.2.1. Design Parameters

The MicraGEM-Si standard fabrication process produces MEMS chips with a standard

dimension of 8x8 mm. All of the heights (thicknesses) of the structural layers (tube, piston,

electrical routing, and BOX layers) are set according to the design rules of this standard process.

In this version of the actuator, the stator layer is segmented into four groups of pistons as shown

in Figure 3-2. Each group of pistons forms a single stator that is electrically addressable. Each

stator consists of multiple arrays of rectangular-shaped pistons protruding upward with height (h)

of 50 µm, width of 5 µm, and length of 190 µm. The height of the small thickness routing layer

is 15 µm (set by the design rules of the MicraGEM process) and width of 20 µm to ensure high

conductivity routings. This layer connects the pistons within each array to the electrical pads as

shown in Figure 3-2. The multiple arrays within each stator are connected to three separated

electrical pads at the outer periphery of the lower structure. These pads could be electrically

joined in the outside circuitry to have the same electrical polarity applied to all of them. The

reason for making multiple electrical pads within each stator is to avoid the electrical short

circuit problem that might occur between one array of pistons with the upper rotor due to

fabrication imperfections, so that it doesn’t lead to the entire stator not being functional. So in

this case, only the electrical pad with the short circuited array is not used. The layer beneath the

stator, shown in red, is the oxide layer (BOX layer of an SOI wafer) with a thickness of 5 µm,

and the layer beneath the BOX layer is the substrate of the actuator.

Figure 3-2. 3D schematics of the lower structure of the MicraGEM-fabricated actuator.

(a) Top view (b) Isometeric and detailed views.

23

The thickness of the rotor layer (ht) is limited to 30 µm as set by the design rules of the

MicraGEM process which means that the maximum stroke that could be achieved is less than 30

µm.

The tubes in the rotor layer have a length (lt) of 200 µm and a width (wt) of 10 µm, refer to

Figure 3-3. The tubes are supported by 4 crab-leg springs that have a length (ls) of 2212 µm, a

width (ws) of 200 µm, and they have the same height as the rotor layer, i.e. 30 µm. The length of

the springs was selected to be relatively short because of the low thickness of the rotor layer (30

µm); which means that if the springs are extending along the entire edge of the periphery of the

rotor (i.e. 6950 µm), the actuator will have low stiffness springs. In other words, the actuator has

to have a high natural frequency such that it can actuate large loads. The twelve electrical pads

located on the periphery of the actuator are electrically isolated form the rotor layer, and they

provide the wire bonding platform from the outside circuitry to the four stators in the lower

structure. The periphery of the actuator (silicon regions that surround the pads) provides the

electrical routing to the rotor.

Figure 3-3. 3D schematics of the upper structure of the MicraGEM-fabricated actuator.

(a) Top and detailed views. (b) Isometeric view.

24

The overall structure of the actuator when the upper structure is attached to the lower one is

shown in Figure 3-4. The pistons and tubes are horizontally aligned with a gap selected to be 2.5

µm as the tolerance of the alignment between the upper and lower structures in the MicraGEM-

Si process is of the order of ±0.4 µm. Such gap compensates for any misalignment to avoid any

pull-in effect or short circuit between the electrodes.

Figure 3-4. 3D schematics of the complete bonded structure of the MicraGEM-fabricated

actuator. (a) Top and cross-sectional views. (b) Isometeric view.

25

The following table lists the key design parameters of the MicraGEM-fabricated actuator.

Design parameter Value Description

wp 5 µm Pistons’ width

hp 50 µm Pistons’ height

wt 5 µm Tubes’ width

ht 30 µm Tubes’ height

g 2.5 µm Gap between an adjacent piston and tube

N 10,928 Number of unit cells

D 2540 µm Central opening diameter

lxwxh 8000x8000x580 µm3 Actuator size

lsxwsxhs 2212x200x30 µm3 Spring dimensions

Table 3-1. Key design parameters of the MicraGEM-fabricated actuator.

3.2.2. Analytical Model of the Actuator

The analytical model solution for the actuator stroke was calculated based on the simplified

mathematical modeling developed in Chapter 2. The maximum translation stroke that the

actuator can provide before experiencing the lateral pull-in instability is calculated based on the

following equation:

z

x

xpKm

Kmgz

2

1 (3-1)

The horizontal gap (g) between the electrodes of the MicraGEM-fabricated actuator is 2.5 µm.

Due to the fact that the crab-leg spring design used in the MicraGEM-fabricated actuator has a

26

short length of the second part l2 as opposed to the first part l1, see Figure 3-5, the torsion within

this part l2 could be negligible. Therefore, the springs are considered as guided cantilever springs

with length (l1= ls) of 2212 µm and width (ws) of 200 µm and height (hs) of 30 µm.

Figure 3-5. 3D schematic of the crab-leg springs used in the MicraGEM-fabricated

actuator.

The stiffnesses of the springs along the x, y, and z axes are calculated as follows

mNl

hwEKmz

s

ssz /8.643

3

(3-2)

where Ez is Young’s modulus of a standard (100) Silicon wafer along the z-axis and is equal to

130 GPa [30].

mNl

hwEKmyKmx

s

ssx /408,458 (3-3)

where Ex is Young’s modulus of a standard (100) Silicon wafer along the in-plane axes (x and y)

and is equal to 169 GPa [30].

Therefore, the maximum translation stroke that could be achieved before experiencing lateral

pull-in instability, based on Equation (3-1), with such spring mechanism is

m 105 2

1

z

xxp

Km

Kmgz (3-4)

This implies that the stroke of the MicraGEM-fabricated actuator is not limited by the lateral

instability; it is rather limited by the thickness of the rotor layer, i.e. 30 µm.

The translation stroke of the actuator in response to an input voltage is calculated based on the

following equation:

27

200

0

)(V

gKm

yNxNz

z

yx

r

(3-5)

where Nx and Ny are the numbers of unit cells that extend along the x and y axes, and x0 and y0

are the lengths of the pistons along the x and y axes, respectively. Due to the fact that all of the

unit cells in the MicraGEM-fabricated actuator have the same dimensions along the x and y axes

(x0=y0=195 µm, and Nx=Ny= 5464), the previous equation could be simplified as follows:

200

2V

gKm

xNz

z

xr (3-6)

The stiffness of the springs along the z-axis as shown in Equation (3-2) represents the stiffness

for only one spring, which implies that the Kmz of all four springs is 4*64.8 N/m; the horizontal

gap g is 2.5 µm. Therefore, the translation strokes corresponding to input voltages of (10, 15, 20,

25V) are calculated using Equation (3-6).

umVVgKm

xNz

z

xr )029117.0(

2 2200 (3-7)

The calculated translation stroke of the actuator versus voltage applied is plotted in Figure 3-6.

Figure 3-6. Analytical translation strokes of the MicraGEM-fabricated actuator at

different voltages.

The analytical solution of the MicraGEM-fabricated actuator is estimated based on the

assumption that the rotor is a rigid body, and there is no deformation that takes place within its

structure. The displacement plotted in Figure 3-6 is calculated at the moving ends of the springs

28

where they connect to the rotor. However, the rotor layer is actually thin (30 µm) and is wide in

dimensions (6950x6950 µm) along the x-y plane, and therefore, there is a deflection that takes

place within the rotor layer itself. This deformation will be demonstrated and clearly observed

through the finite element analysis that follows.

3.2.3. Finite Element Analysis

The finite element analysis of the MicraGEM-fabricated actuator was conducted using the

CoventorWare™ software [31]. Since the actuator has a large number of small features (pistons

and tubes) and large dimensions of 8000x8000 µm, two 3D models of the actuator were

constructed. The first model represents one quarter of the actuator (one stator and a quarter of the

rotor containing one fourth of the total unit cells in the entire actuator), and the second model

represents only the upper structure (rotor and springs layer).

The first model is simulated using the MemElectro module to calculate the electrostatic forces

between the pistons and tubes in response to different input voltage values. These electrostatic

forces are then applied to the rotor of the actuator in the second model using the MemMech

module to calculate the translation strokes of the rotor. Modal analysis is also conducted on the

upper structure to calculate the undamped natural frequency of the actuator. It should be noted

that the circular central opening of the actuator was replaced with a squared one in the 3D

CoventorWare model to facilitate the meshing of the model. The mesh type used was Manhattan

bricks which is suitable for meshing large structures with fine features; however, the model to be

meshed should not contain curved structure. Hence, the circle was replaced with a square having

the same area to conserve the mass of the rotor.

The first mode shape of the actuator is translation along the z-axis as shown in Figure 3-7. The

simulation results show that the actuator has an undamped natural frequency of 1390.7 Hz. The

mode shape shows a deflection within the rotor of the actuator as the springs are significantly

stiff and the rotor layer is thin, and therefore it behaves as a membrane. The largest vertical

displacement (translation stroke) is at the center of the actuator, and it represents the area of the

structure where the load is to be placed.

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Figure 3-7. 3D CoventorWare model of the upper structure of the MicraGEM-fabricated

actuator showing the first mode shape of the actuator.

The electrostatic forces were calculated on the first model by applying a series of input voltages

across a single stator and a quarter of the rotor. These electrostatic forces are then applied on the

rotor of the actuator constructed in the second model. Figure 3-8 shows the translation stroke of

the center of the actuator at different values of the input voltage.

Figure 3-8. Estimated translation stroke of the MicraGEM-fabricated actuator versus

voltage based on numerical simualtions.

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The numerical simulations results show that the actuator was able to achieve a translation stroke

of 25.4 µm at the center area of the actuator. The discrepancy between the analytical and

numerical simulations results is attributed to the fact that a deformation takes place across the

rotor structure as it is clearly shown in the first mode shape of the actuator. The analytical model

calculates the translation at the moving ends of the springs (shown in green color in Figure 3-7)

based on the assumption that the rotor is a rigid body, whereas the numerical simulations results

represent the translation at the center of the rotor.

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3.3. MMDL-Fabricated Actuator

Due to the limitations of the MicraGEM-fabricated actuator in terms of providing a limited

translation stroke, a new version of the pistons-tube electrostatic actuator, referred to as the

MMDL-fabricated actuator, was developed. The specifications of the MMDL-fabricated actuator

were set to meet the requirements of AF and OIS in phone cameras (translation of 62 mg lens

barrel a stroke of 90 µm and an angle of rotation around the two in-plane axes of ±0.5ᵒ at a low

input voltage).

3.3.1. Design Parameters

The lower structure of the MMDL-fabricated actuator contains four stators, which are

electrically insulated from each other, as shown in Figure 3-9, by means of deep etching during

the fabrication process. Those stators are mechanically separated throughout the lower structure

by insulating trenches and mechanically connected through the upper structure as the lower

structure is attached to the upper one at the periphery of the actuator. The bonding interface

between both structures is shown in blue color in Figure 3-9(a). The blue colored interface

represents a thermal oxide layer of 3 µm that is used to achieve the electrical insulation among

the individual stators as well as between the stators and the rotor layer. Subsequently, the

actuator would have five electrodes with different polarities (four stators and one rotor).

Figure 3-9. Schematics of the lower structure of the MMDL-fabricated actuator.

(a) Top view. (b) Bottom view.

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Each of the four segmented stators contains 1044 pistons that have a length lp of 320 µm, a width

wp of 6 µm, and a height hp of 160 µm as shown in Figure 3-10.

Figure 3-10. 3D schematics of the lower structure of the MMDL-fabricated actuator.

(a) Detailed view. (b) Isometric view.

The pistons could have each a smaller width such as 3 µm leading to the accommodation of a

large number of these pistons in the lower structure to generate a large electrostatic force.

However, due to the limitations of the fabrication equipment at the university-based cleanrooms

to fabricate vertical pistons with sidewall angle at 90°, the walls were selected to be wider at the

top such that they are not weaker at the base when they are etched deeper, refer to Figure 3-11.

Figure 3-11. Cross-section views of the pistons. (a) ideal sidewall angle of 90°. (b) Expected

sidewall angle < 90°.

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The upper structure of the MMDL-fabricated actuator comprises a rotor that contains the tubes

and is supported by four crab-leg springs that extend along the periphery of the actuator as

illustrated in Figure 3-12. The required translation stroke of the MMDL-fabricated actuator is 90

µm and, therefore, the rotor layer thickness should be slightly larger than that, e.g. 100 µm.

However, the ultrathin wafers provided by silicon wafers manufacturers usually have a thickness

tolerance of ±10 µm, thus the thickness of the rotor is finally chosen to be 110 µm.

A new feature added to the MMDL-fabricated actuator is a lens barrel holder, refer to Figure

3-12. This lens barrel holder is attached to the central opening of the actuator so that it absorbs

the pressing force during the attachment process of the lens barrel to the actuator. It consists of

two parts: upper and lower. The upper part is formed by the thickness of the rotor layer that is

110 µm, and the lower part is formed by the thickness of the stator layer that has a standard

thickness of 500 µm. The lens barrel holder is fabricated by a backside etching step during the

fabrication process of the actuator to separate the lower part from the stators as shown in Figure

3-12(b).

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Figure 3-12. 3D schematics of the upper structure of the MMDL-fabricated actuator.

(a) Front isometric view. (b) Backside isometric view.

The openings of the tubes were set larger in size than the width of the pistons to allow the pistons

to penetrate during actuation and, therefore, the tube openings are selected to have a width of 17

µm and a length (lt) of 350 µm. The width of the tube walls (wt) (the wall between two

neighboring tubes as shown Figure 3-13) is set to be 12 µm. This large width of tube walls is due

to the difficulty in fabricating vertical walls with sidewall angle of 90° during the fabrication

process, and therefore the walls were made larger to maintain a sufficient width at the bottom.

Figure 3-13. Top views of the upper structure of the MMDL-fabricated actuator.

(a) Detailed view showing tubes. (b) Top view of the entire upper stucture.

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The complete structure of the MMDL-fabricated actuator after bonding of the lower structure to

the upper structure is illustrated in Figure 3-14. The horizontal gap between the adjacent

sidewalls of the pistons and tubes is selected to be 5.5 µm to compensate for any misalignment

between the upper and lower structures. The misalignment is due to the fact that the mask

aligners available at the university-based clean rooms have alignment tolerances of ±1.5 µm.

Figure 3-14. 3D schematics of the final structure of the MMDL-fabricated actuator after

bonding.(a) Isometric backside view. (b) Isometric frontside view.

The ideal lens barrel to be used with the MMDL-fabricated actuator is selected to have a stepped

outer diameter as illustrated in Figure 3-15. The portion of the lens barrel with the smaller

diameter is to be inserted into the central opening of the rotor that is surrounded by the lens

barrel holder using a press-fit technique.

Figure 3-15. 3D schematics of the lens barrel to be integrated within the MMDL-fabricated

actuator. (a) Cross-section view. (b) Frontside view. (c) Isometric view.

(a) (b) (c)

(a) (b)

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The importance of the lens barrel holder can be realized during the attachment of the lens barrel

into the central opening of the actuator. When the lens barrel is press-fitted, it applies a

mechanical force on the rotor. However, during the attachment process, the lower part of the lens

barrel holder grounds the rotor, so that it absorbs these forces without the rotor being displaced,

see Figure 3-16.

Figure 3-16. 3D schematics of the attachement process of the lens barrel into the MMDL-

fabricated actuator. (a) During attachment. (b) After attachment.

The following table lists the key design specifications of the MMDL-fabricated actuator.

Design parameter Value Description

wp 6 µm Pistons’ width

hp 160 µm Pistons’ height

wt 12 µm Tubes’ width

ht 110 µm Tubes’ height

g 5.5 µm Gap between an adjacent piston and tube

N 4176 Number of unit cells (piston-tube pair)

D 5740 µm central opening Diameter

lxwxh 11800x11800x600 µm3 Actuator size

lsxwsxhs 9625x106x110 µm3 Spring dimensions (µm)

Table 3-2. Key design specifications of the MMDL-fabricated actuator.

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3.3.2. Main Features of the MMDL-Fabricated Actuator

The main features of the MMDL-fabricated actuators are as follows.

1- Electrical insulation between the stators

The fabrication process of the MMDL-fabricated actuator utilizes standard Si wafers instead of

Silicon on Insulator (SOI) wafers as the electrical insulation between the four stators is done by

mechanical means using backside etching. This is different form the MicraGEM-fabricated

actuator in which the buried oxide layer (BOX layer) of an SOI wafer is utilized to provide the

electrical insulation between the stators. Therefore, the MMDL-fabricated actuator is a cost-

effective design as the SOI wafers are much more expensive than the standard Si wafers.

2- Large translation stroke and large angle of rotation

The thickness of the upper structure (rotor layer) used in the MMDL-fabricated actuator is in the

order of 110 µm, which means that large translation stroke of up to 100 µm and large angle of

rotation of up to ±0.5° could be achieved. These performance characteristics could not be

achieved with the MicraGEM-fabricated actuator due to the limited thickness of the rotor layer,

i.e. 30 µm.

3- Manipulation of large masses

Due to the large thickness of the mechanical springs used in the MMDL-fabricated actuator, the

springs are stiff, and therefore the actuator can handle large masses. In addition, the springs

provide large resistance to sudden shocks and drops.

4- Ease of integration of the lens barrel into the actuator

The lens barrel holder facilitates the integration of the lens barrel into the central opening of the

actuator by protecting the rotor against any mechanical forces that arise during the attachment

process.

It also balances the load on the rotor as the inserted part of the lens barrel is usually smaller than

the part which is not inserted; refer to the structure of the lens barrel shown in Figure 3-15. In