Nghiên cứu nâng cao độ nhạy của cảm biến đo vận tốc góc kiểu tuning fork

The basic architecture of a vibratory gyroscope is comprised of a drive-mode oscillator that generates and maintains a constant linear or angular momentum, coupled to a sense-mode Coriol

HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY INTERNATIONAL TRAINING INSTITUTE FOR MATERIALS SCIENCE

Thesis of graduation

IMPROVING THE SENSITIVITY OF MEMS

TUNING FORK GYROSCOPE

STUDENT: HA SINH NHAT ADVISORS: Dr CHU MANH HOANG

Assoc Prof Dr VU NGOC HUNG

Hanoi, April 2015

Improving the sensitivity of MEMS tuning fork gyroscopes

DEDICATION

This thesis is submitted for International Training Institute for Material Science in

Ha Noi University of Science and Technology The work has been carried out in International Training Institute for Material Science, Number 1, Dai Co Viet, Ha Noi, VietNam since October 2012 Except where specific references are made, this thesis is entirely the result of my own work and includes nothing that is the outcome of work done in collaboration No part of this work has been or being submitted for any other degree, diploma or qualification at this or any other university

Author

Ha Sinh Nhat

ACKNOWLEDGEMENT

This thesis is the result of my two years study in ITIMS where I am trained in the

field of materials science with the best study conditions, taken part in exciting and

interesting scientific seminars of both MEMS group and study class

I would like to say that the obtained results are due to the help of all the members

in ITIMS without this help I think that it would be difficult for me to reach this final

Foremost among them are my supervisors Dr Chu Manh Hoang and Assoc Prof

Dr Vu Ngoc Hung I would like to express my special thanks to them for many things

they had done for me, including their personal and professional guidance, encouraging

support, and for creating very good research environment

I wish thank Assoc Prof Dr Chu Duc Trinh, Msc Dang Van Hieu for his friendly

guidance and for co-examining this thesis

I would like to thank all members of MEMS group, Msc Nguyen Quang Long,

and the other members who created friendly research environment and shared

experiences in practical work

I would also like to thank sincerely to all the teachers who teach me during time I

am studying at ITIMS such as Assoc Prof Dr Nguyen Van Hieu, Assoc Prof Dr

Nguyen Phuc Duong, Assoc Prof Dr Nguyen Anh Tuan, Dr Nguyen Van Quy…,

kind librarian Nguyen Phuong Loan and other ITIMS staffs

Finally, at home, I am indebted with my family for their love, and continuous

encouragement over the past several years

Ha Noi, April 2015

Ha Sinh Nhat

CONTENTS

DEDICATION ii

ACKNOWLEDGEMENT iii

CONTENTS iv

LIST OF FIGURES vi

LIST OF TABLES x

Chapter 1 INTRODUCTION 1

1.1 The MEMS technology 1

1.2 MEMS tuning fork gyroscopes 3

1.2.1 The operation principle of gyroscope 3

1.2.2 The dynamic characteristics of gyroscope 5

1.2.3 Tuning fork gyroscopes 11

1.3 Recent developments in improving the sensitivity of tuning fork gyroscope 13

1.3.1 Operation matching of sense and drive modes 13

1.3.2 Improving structure 17

1.3.3 Decreasing damping 20

1.4 Purpose of this thesis 24

Chapter 2 DESIGNS OPTIMIZATION OF TUNING FORK GYROSCOPE 27

2.1 Tuning Fork Gyroscope structure analysis methods 27

2.2 The 10 kHz resonant operation Tuning Fork Gyroscope 28

2.2.1 Structural construction and modal analysis 28

2.2.2 Sense displacement simulation 33

2.3 The 10 kHz version advantage and disadvantage points 35

2.4 The focus point in Tuning Fork Gyroscope optimization: 36

2.4.1 Frequency reduction and proof masses selection 38

2.4.2 The sensing energy arrangement 39

2.4.3 The device thickness selection 40

2.5 Simulation result comparison of two sensor versions 41

2.5.1 Structural properties comparison 43

2.5.2 Angular rate sensitivity 45

2.5.3 Slide damping reduction 49

Chapter 3 FABRICATION PROCESS AND MEASUREMENT 55

3.1 Mask design 55

3.2 Fabrication flow 59

3.3 Packaging 62

3.4 Experimental measurement 64

3.4.1 Frequency response measurement block diagram 64

3.4.2 Characteristic measurement block diagram 66

Chapter 4 RESULTS AND DISCUSSION 69

4.1 Results of gyroscope fabrication 69

4.2 Results of package 71

4.3 Results of characteristics measurement 72

CONCLUSION 75

FUTURE WORK 78

LIST OF THESIS PUBLICATION 79

REFERENCES 80

LIST OF FIGURES

Figure 1.1: MEMS devices application in automobile industry 2Figure 1.2: Coriolis acceleration concept 4Figure 1.3: A generic MEMS implementation of a linear vibratory rate gyroscope [1] 5Figure 1.4: A mass-spring-damper gyroscope lumped model 6Figure 1.5: Lumped mass-spring-damper resonator model 7Figure 1.6: The scanning electron micrograph images of the first working prototype tuning fork gyroscope from the Draper Laboratory (a), The high-Q in-plane SOI tuning fork gyroscope from Georgia Institute of Technology (b), and the micromachined gyroscope with high shock resistance from Tongji University (c) 12Figure 1.7: Amplitude-frequency response of a 1-DOF drive-mode oscillator and a 1-DOF sense-mode oscillator 16Figure 1.8: Design model of lateral micromachined tuning fork gyroscope: (1) outer mass frame, (2) inner mass frame, (3) drive comb electrodes, (4) sense electrodes, (5) folded beam, (6) anchor, (7) lozenge-shaped coupling spring, and (8) self-rotation ring [8] 19Figure 1.9: Optical photograph of a vacuum packaged tuning fork gyroscope, showing the die, the ceramic package, the glass lid with getter material and metal sealing ring, and structural schematic of the gyroscope architecture [11] 22Figure 2.1:MEMS TFG 10 kHz version without drive and sense combs (1-Drive comb frame; 2-Sense beam, 3-Drive beam; 4-Sense comb frame; 5-Lozenge shape’s beam; 6-Self-rotate ring; 7-Constrain beam; 8-Connecting beam; 9-Anchor; 10-Drive outer frame; 11-Sense frame) 29Figure 2.2: Deformation of the anti-phase drive and sensing modes of 10 kHz TFG (a) sense mode (b) drive mode 31

Figure 2.3:The parasitic modes of 10 kHz TFG version: mode 3rd to mode 6th (a) anti phase x-axis outer frame warp mode; (b) in phase x-axis outer frame warp mode; (c) in phase drive mode; (d) in phase x-axis twist mode 32Figure 2.4: The parasitic modes of 10 kHz TFG version: mode 7th to mode 10th(a) anti phase x-axis twist mode; (b) anti phase y-axis inner frame twist mode; (c) in phase y-axis inner frame twist mode; (d) in phase sense mode 33Figure 2.5: The sense displacement versus rotation velocity 34Figure 2.6: The self-rotate ring increases its radius and changes from 4 constrain beams (a-10 kHz TFG) into 3 beams (b-4.5 kHz TFG) 40Figure 2.7: The dependence of 10 first order natural frequencies on the thickness of 10 kHz TFG 41Figure 2.8: Drive modes of 10 kHz TFG version (a) and 4.5 kHz TFG version (b) 44Figure 2.9: Sense modes of 10 kHz TFG version (a) and 4.5 kHz TFG version (b) 44Figure 2.10: The dependence of TFG’s resonant drive amplitude on the drive force frequency with Ω=10rad/s (a) 10 kHz TFG and (b) 4.5 kHz TFG 46Figure 2.11: The dependence of TFG’s resonant drive amplitude on the drive force frequency with Ω=10rad/s (a) 10 kHz TFG and (b) 4.5 kHz TFG 47Figure 2.12: The sense displacement of 10 kHz TFG and 4.5 kHz TFG calculated as function of the input angular velocity (Ω) 47Figure 2.13: Capacitance change versus introduced angular velocity 48Figure 2.14: Schematic of freestanding MEMS tuning-fork gyroscope 50Figure 2.15: The dependence of sense displacement versus drive force frequency of (a)

10 kHz TFG and (b) 4.5 kHz TFG for non-freestanding and freestanding versions with angular rate of 10 rad/s 53Figure 2.16: The dependence of sense displacement versus angular rate of (a) 10 kHz TFG and (b) 4.5 kHz TFG for non-freestanding and freestanding versions 54Figure 3.1: Positive mask design of complete gyroscope with Clewin software 55

Figure 3.2: TFG 10 kHz sensor mask design with drive comb and sense comb structures (Positive mask on Clewin software) 58Figure 3.3: The 4.5 kHz TFG mask design with holes, drive combs and sense combs structure (Negative mask on AutoCAD software) 58Figure 3.4: The 4.5 kHz TFG Chromium on glass mask for photolithography technique 59Figure 3.5: Silicon etching profile with Bosch process 60Figure 3.6: Fabrication process for TFG non-free standing (a) and freestanding (b) TFG sensor 61Figure 3.7: Westbond Ultrasonic weld wire bonding machine (a) and tip motions in welded process (b) 63Figure 3.8: The diagram of actuation and sense electrodes for wire circuitry interconnection between gyroscope and PCB (1-Proof mass to VDC connecting terminal, 2-Proof mass to VAC connecting terminal, 3-Sensing capacitor Cs+ terminal, 4-Proof mass for sensing capacitor terminal; 5- Sensing capacitor Cs- terminal) 63Figure 3.9: Two-port actuation and detection scheme for measuring the natural frequency of gyroscope 65Figure 3.10: One-port actuation and detection scheme for measuring the natural frequency of gyroscope 66Figure 3.11: The angular rate detection scheme for gyroscope with MS3110-All capacitance read out IC 67Figure 3.12: The block diagram of gyroscope characterize system 68Figure 4.1: SEM images of fabricated 10 kHz and 4.5 kHz TFG sensor: entire of structure (a), (b); self-rotation ring and Lozenge beams (c), (d), folded beam connecting driving and sensing mass frame (e), and zoom-in sense comb-fingers (f) 70Figure 4.2: Back side image of (a) 10 kHz and (b) 4.5 kHz version with unreleased oxide layer 71

Figure 4.3: Gyroscope interconnection circuit and its package 72Figure 4.4: The dependence of sensor output voltage versus applied VDC with a constant angular rate 73Figure 4.5: Output voltage of the 10 kHz tuning fork gyroscope measured as a function

of input angular rate 74

LIST OF TABLES

Table 2.1: The structural parameters of 10 kHz gyroscope 30Table 2.2: First 10 natural mode frequencies order of 10 kHz TFG version 30Table 2.3: The structural parameters of 4.5 kHz TFG and 10 kHz TFG 42Table 2.4: First 10 ascending ordered natural frequencies of 10 kHz and 4.5 kHz TFGs 43Table 2.5: The Damping ratio and Quality factor of 10 kHz TFG and 4.5 kHz TFG for non-freestanding and freestanding version 52Table 3.1 The 10 kHz and 4.5 kHz TFGs drive and sense comb parameters 57

Chapter 1 INTRODUCTION

1.1 The MEMS technology

The acronym MEMS stands for Micro Electro Mechanical System that is a technology of the combination of electronic and mechanical elements forming a complete system at micro scale In practice, MEMS could include devices fabricated using photo-lithography based technique with micrometer scale features that utilizes both electrical and mechanical function In general, a MEMS device might comprise the following:

- A sensor that inputs information into the system

- An electronic circuit that conditions the sensor signal

- An actuator that responds to the electrical signals generated within the circuit Silicon with special semiconductor, physical and commercial properties has been the most popular material used for MEMS technologies Single crystal silicon is elastic, is lighter than aluminum, and has a modulus of elasticity that is similar to stainless steel In the other hand, silicon is wide use within the microelectronic integrated circuit industry and has been got well understood and controllable electrical properties Other materials that are often used within the MEMS fabrication process include glasses, quartz, ceramics, silicon nitride and carbide, alloys of various metals, and a variety of special materials that are used for very specific purposes

Evolved from the semiconductor fabrication technologies, the most striking feature

of the MEMS technology is that it allows building moving micro-structures on a substrate With this capability, extremely complex mechanical and electrical systems can be created Masses, flexures, actuators, detectors, levers, linkages, gears, dampers and many other functional building blocks can be combined to build completely sophisticated systems on a chip Inertial sensors such as accelerometers and gyroscopes utilize this capability to its fullest [1]

Photolithography based pattern transfer methods and successive patterning of thin structural layers adapted from standard IC fabrication processes are the enabling technologies behind micromachining By dramatically miniaturizing and batch processing complete electro-mechanical systems (MEMS), substantial reductions in device size, weight, and cost are achieved Many products exist today that use MEMS technology, such as micro heat exchangers, ink jet printer heads, micro-mirror arrays for high-definition projectors, pressure sensors, infrared detectors and many more Figure 1.1 shows an impressive example of widely MEMS devices application in automotive industry

Figure 1.1: MEMS devices application in automobile industry

MEMS devices perform many of the same tasks as macroscopic devices while also offering many advantages The first and most obvious of these is miniaturization MEMS devices are small enough to be manufactured in a batch fabrication process, similar to the ICs of today And as with the IC industry, batch fabrication can significantly reduce the costs of mass production MEMS also, in general, require a much smaller amount of material to produce, which can further reduce costs In

addition to the prospect of being cheaper, MEMS devices can also be more applicable than their much larger equivalents functional devices Designing metal ball and spring accelerometers into smartphones, cameras, airbag control units, or similarly small sized devices would be impractical at best; by reducing device size by several orders of magnitude, MEMS can be used in applications where a conventional sensor would be far too large

One main branch of MEMS device that strongly researched is about inertial sensors MEMS inertial sensors, consisting of accelerometers and gyroscopes, are one of the most important types of silicon-based sensors The accelerometer is used to measure acceleration and gyroscope that used to measure angular changes The combination of

an accelerometer and a gyroscope forms an Inertial Measurement Unit (IMU), a unit that can measure the rates of motion and displacements of an object in action From output signals, IMU is especially useful in developing virtual timing tracking systems for measuring position and navigation Some of IMU’s well known automotive applications such as vehicle stability control, navigation assisting, and roll-over detection are only used in high-end cars, where cost is not a major factor Examples of IMU’s consumer applications are 3D input devices, robotics, platform stability, camcorder stabilization, virtual reality, and more Primarily due to cost and the size most of these applications have not reached abruptly significant volume

1.2 MEMS tuning fork gyroscopes

1.2.1 The operation principle of gyroscope

The operating principle of micromachined gyroscope is based on Coriolis Effect The Coriolis Effect is deflection of moving object when they are viewed in a rotating reference frame Coriolis acceleration is a result of Newton’s law of motion applied to rotating frames Almost all reported MEMS gyroscopes use vibrating mechanical elements (proof-mass) to sense rotation They have no rotating parts that require

bearings, and hence they can be easily miniaturized and batch fabricated using micromachining techniques The basic architecture of a vibratory gyroscope is comprised of a drive-mode oscillator that generates and maintains a constant linear or angular momentum, coupled to a sense-mode Coriolis accelerometer that measures the sinusoidal Coriolis force induced due to the combination of the drive vibration and an angular rate input In other words, vibratory gyroscopes are based on the transfer of energy between two vibration modes of a structure

Figure 1.2: Coriolis acceleration concept

Most of reported micromachined rate gyroscope utilizes a vibratory proof mass suspended by flexible system above a substrate This flexible system is anchored on the substrate, making the mass free to oscillate in two orthogonal directions – the drive and the sense direction The primary objective of the dynamical system is to form a vibratory drive oscillator, coupled to an orthogonal sense accelerometer by the Coriolis force [1]

A generic z-axis sensing linear vibratory rate gyroscope is shown in Fig 1.3, with drive-mode and sense-mode vibration along x-axis, y-axis respectively The drive-mode oscillator is comprised of the proof mass, its suspension system and the drive-

Figure 1.3: A generic MEMS implementation of a linear vibratory rate gyroscope [1] mode actuator The drive-mode actuator drives the proof mass into resonance vibratory motion along the drive-direction by external sinusoidal force at the drive-mode resonant frequency The sense-mode oscillator is formed by proof-mass, the suspension system that allows the proof mass to oscillate in the sense direction, and the sense-mode detector system In the present of angular rate on device substrate, the proof mass vibration will be deflected orthogonal in a secondary direction that calls sensing vibration by Coriolis force By tracing the proof mass deflection, the angular rate is detected

1.2.2 The dynamic characteristics of gyroscope

As seen in Fig 1.3, the gyroscope is comprised of the proof mass, its suspension system, the drive-mode actuator and sensing comb electrodes This architecture is a basically gyroscope model with completely functional of rate measurement The lumped model of a generic z-axis gyroscope is shown in the Fig 1.4 In this model, the proof mass had been driven into a vibration along linear x-orientation Whenever the z-axis angular rate is introduced to device substrate (x-y plane), the Coriolis acceleration

causes the proof mass deviated into y-direction This deflection is sensed by different signal transfer functional mechanisms

Figure 1.4: A mass-spring-damper gyroscope lumped model

The motion of the proof masses along each axis can be considered separately because there is a small cross coupling terms can be neglected in the computation of the normal modes The reduced equations of motion along the drive and sense axes for vibratory gyroscope had been calculated in some literature and are shown in equations below (Eq 1.1-1.2) See [1] for excellent explaination

The is the external force in the drive direction, which is usually a sinusoidal drive excitation force, and is the total external force in the sense direction, comprised of parasitic and external inertial forces And is external z axis rotation

z

velocity applied on device The drive and sense resonance vibration characteristics will

be discussed below

First of all, we investigate the dynamic of a typical 1-DOF resonator that similar to gyroscope drive and sense operations The model of proof mass (mass m), spring system (stiffness k) and damping condition (damping coefficient c) is lumped in Fig 1.5 below

Figure 1.5: Lumped mass-spring-damper resonator model

The equation of motion for this resonator model is:

2 2

Assume the drive force is a harmonic form of F  F0sin  tat the frequency ω, the steady-state component of proof mass is oscillated of the form

12

n

m Q

d

F x

1

d d

d

k m

d d d

d

m Q

For these reasons, almost all reported gyroscopes operate exactly at the drive mode resonant frequency in practical implementations At resonance, the drive mode phase

0 2

d res d

C d z

s

k m

s s s

s

m Q

res z

s s

Q x m y

m

1.2.3 Tuning fork gyroscopes

Since the first demonstration of a micromachined gyroscope by the Draper Laboratory in 1991 [2], various micromachined gyroscope designs fabricated in a variety of processes including surface, bulk and hybrid surface-bulk micromachining technologies or alternative fabrication techniques have been reported in the literature Design types of those devices include ring gyros, vibrating plate gyroscope and vibrating beam gyroscope, and tuning fork shaped gyroscope Mostly, these gyroscope have the vibrating structures that need to be suspended on the silicon substrate The vibratory gyroscope has been the main subject in the micromachining technology because they are easier with well-designed flexures Among them, the tuning folk gyroscope (TFG) with electrostatic drive and capacitive-type sense is most preferred due to potential capabilities and advantages such as common-mode rejection and low power consumption

Figure 1.6: The scanning electron micrograph images of the first working prototype tuning fork gyroscope from the Draper Laboratory (a), The high-Q in-plane SOI tuning fork gyroscope from Georgia Institute of Technology (b), and the micromachined

gyroscope with high shock resistance from Tongji University (c)

In 1993, Draper reported an improved 1 mm silicon-on-glass tuning fork gyroscope fabricated through the dissolved wafer process [3] The silicon-on-glass technology used in this device has the advantage of low stray capacitance This gyroscope was electrostatically driven and capacitively monitored Any rotation in the plane of the substrate perpendicular to the drive mode will then excite the out-of-plane rocking mode of the structure Figure 1.6 shows an SEM image of the device with a perforated mass to minimize damping

In 2004, Georgia Tech presented a high-Q in-plane SOI tuning fork gyroscope that has the potential of attaining sub deg/h rate resolutions Fig 1.6(b) shows an SEM image of the device The proof masses are driven at resonance along the x-axis, and the Coriolis acceleration induced by rotation around the z-axis is sensed capacitively along the y-axis To achieve sub-deg/h rate resolutions, a vibratory gyroscope must attain very high Q, large sense capacitances, large proof mass and large drive amplitude The two-mask process is very simple and precludes the requirement of any perforations in the proof mass, resulting in a larger mass per unit area The drive and sense resonant

modes were balanced electrostatically to within 0.07% of each other and the measured rate results show a sensitivity of 1.25 mV/◦/s in a bandwidth of 12 Hz

In 2009, a tuning fork gyroscope designed for high-g shock environments is presented This micro gyroscope with 10 kHz designed working frequency is composed of double symmetrical frame structures that are connected by middle coupling beams (Fig 1.6(c)) Two level elastic stoppers were designed in order to improve the shock resistance of the gyroscope Slots were etched to reduce damping in the inner bonding regions By bulk silicon micromachining technology, the gyroscope was fabricated on a 300 m thickness silicon wafer The working frequencies of the gyroscope on the driving and sensing modes are 10240 and 11160 Hz, respectively Shock experiments to bare chips indicate that the shock resistance of the gyroscope along X-axis is 15000 g, Y-axis is 14000 g and Z-axis is 11000 g

In general, the output capacitance signal of the micromachined tuning fork gyroscope is small that is affected by noise and not easy for measurement In recent years, the different structure designs have been pursued to improve the performance of TFG including the sensitivity, low noise level, and robustness

1.3 Recent developments in improving the sensitivity of tuning fork gyroscope 1.3.1 Operation matching of sense and drive modes

In order to overcome disadvantages of rotating wheel gyroscopes concerning bearing friction and wear, vibrating gyroscopes such as Tuning-Fork Gyroscopes are proposed The design of Tuning-Fork gyroscopes has no rotating parts that require bearings This is the primary reason why vibratory gyroscopes have been successfully miniaturized by MEMS technology, and have become an attractive alternative to their macro-scale counterparts The device has been modeled as a lumped mass-spring system operating in its first two fundamental in-plane mode, where the rotation-induced Coriolis force causes energy transfer to the sense-mode proportional to the

angular rate input In most of the reported micromachined vibratory rate gyroscopes, the proof mass is driven into resonance in the drive direction by an external sinusoidal electrostatic or electromagnetic force When the gyroscope is subjected to an angular rotation, a sinusoidal Coriolis force at the driving frequency is induced in the direction orthogonal to both the drive-mode oscillation and the angular rate axis Under resonant operation, the amplitude of an oscillator is amplified by its quality factor Therefore, it

is desired to utilize resonance in both the drive and the sense modes in order to attain the maximum possible response gain and sensitivity This is typically achieved by designing the drive and sense resonant frequencies to match

To achieve the maximum possible gain in the sense-mode, it is generally desirable

to operate at or near the peak of the sense-mode response curve This is typically achieved by matching drive and sense resonant frequencies When operating at sense-mode resonance, i.e d  s, the sense-mode phase becomes -90˚ from the drive velocity, and the amplitude reduces to

0 0

2 s C

res z

s s

Q x m y

In the gyroscope design, the proper choose of the working frequency is crucial to its performance It’s clears from the Eq (1.27) that the lower natural frequency can increase sensitivity However the frequency cannot be too low because the lower frequency can also decrease the robustness of the vibration system The natural frequency decreasing makes damping ratio increasing also, because of the moving proof mass area has been increased

When the two modes are matched, the output signal is amplified by the quality factor of the sense mode, thereby increasing the sense displacements by orders of magnitude [1] Then, the amplitude along sensing direction achieves the maximum at matched frequency of driven force However, it leads to a problem that is the response time would be long The response of the gyroscope to time varying rotation rate gives

an indication of its bandwidth The larger the bandwidth, the quicker is the response of the sensor That is why there must be the difference between these frequencies causing

a width of the range (or bandwidth) of frequencies The frequency of sense-mode designed with slight shift from that of the drive-mode is also to improve robustness and thermal stability However, the larger bandwidth TFG is attained at the cost of gain and sensitivity The drive and sense mode frequencies often have a mismatch of about 100

Hz corresponding to the sensor bandwidth This result satisfies the requirement to optimize sensitivity and bandwidth [1] Fig.1.7 illustrates a combination of a 1-DOF drive-mode oscillator and a 1-DOF sense-mode oscillator and bandwidth of the complete gyroscope system The sense vibration amplitude is determined at the frequency of drive resonance in the mode-matched system

Figure 1.7: Amplitude-frequency response of a DOF drive-mode oscillator and a

1-DOF sense-mode oscillator

However, the driving and the sensing mode frequencies of the fabricated gyroscope are barely same due to fabrication error, even though the gyroscope has the exactly same frequencies in design stage Therefore, post-fabrication frequency tuning for the sense and drive modes matching is needed In ref.[8], the tuning mechanism based on the electrostatic effect is proposed On the other hand, the coupled gyroscope that used same spring in driving and sensing modes shows mode nonlinear behavior when the difference between driving and sensing mode frequencies is smaller than 100 Hz [9] Operating close to the sense resonant peak also makes the system very sensitive to variations in system parameters that cause a shift in the resonant frequencies or damping In the case that a sense-mode system has a low quality factor, a relative shift between the operating frequency and the sense-mode resonant frequency can cause less the gain drop Under higher quality factor conditions the gain is higher, however, the bandwidth becomes even narrower This means that for only slight mismatch in frequency, the gain significantly drops Thus, especially in vacuum-packaged devices,

the relative position of the sense-mode resonant frequency with respect to the operating frequency has to be controlled with extreme precision

Fabrication imperfections are inevitable, and affect material properties and geometry

of MEMS structures In surface micromachined gyroscopes, the thickness of the suspension elements is determined by deposition process, and the width is affected by etching process In addition, Young’s Modulus of the structure is affected by deposition conditions [9] In bulk-micromachined devices, the width and cross section

of the suspension beams often vary due to lateral over-etching Variations in these parameters cause the resonant frequencies to vary drastically from device to device Furthermore, fluctuations in the ambient temperature and stresses result in frequency fluctuations during operation Given the structural and environmental effects that result

in quite large variations in the resonant frequencies, it is extremely difficult to control the drive and sense frequencies with the precision required for mode-matched devices Thus, it is a common practice to operate away from the resonant frequency of the sense-mode, where the frequency variations have reduced effect on the output gain and phase This is achieved by setting the sense-mode frequency spaced by a certain percentage away from the drive-mode frequency

In order to reduce the mode coupling effect that results from interference between the driving and sensing modes, the newly designed gyroscope has independent springs for the driving and sensing modes The decoupled structure design is introduced for this requirement [9]

1.3.2 Improving structure

In order to improve the sensitivity of a vibratory gyroscope, the driving and sensing vibration modes of the gyroscope need to be designed and fabricated with the matched resonant frequencies [10] When operated at the resonant condition, the amplitude of sensing mode vibration is amplified by its mechanical quality factor In addition, the

use of twin masses in TFG operated in anti-phase manner helps reject linear accelerations in the sense direction using differential measurement The use of twin masses vibrating in anti-phase also causes moment to cancel locally and make the gyroscope less sensitive to mounting [7, 11] The twin masses may have coupled or separate suspensions [10-13] When the twin masses are not mechanically coupled to each other, it is difficult to achieve perfect anti-phase drive oscillations [14] Moreover, the mechanical coupling of the twin masses introduces an in-phase mode at a frequency lower than the operational anti-phase one and there is no much difference between higher modes with driving and sensing mode

In order to reduce error in angular rate measurement, the effect of parasitic modes to driving and sensing mode needs suppressed This was solved by designing mechanical structure that the discrepancy between driving and sensing modes and parasitic modes

is larger than 10% [8] There are several reports on designing the TFG which has mechanical coupling between the sensing and driving masses to improve mode order and anti-phase operation of driving or sensing mode [8, 12] However, noise sources causing by vibration have not yet taken into considering sensor designs In a latest report [13], the research showed that the tuning fork gyroscopes with decoupled sense and drive masses is less sensitive to vibration than tuning fork gyroscope designs featured in literature

In ref.[10], a new dual mass vibratory MEMS z-axis rate gyroscope architecture that prioritizes the sense-mode quality factor and provides improved ordering of the mechanical vibrational modes The proposed linearly coupled, dynamically balanced anti-phase sense-mode design minimizes substrate energy dissipation to maximize the quality factor The levered drive-mode mechanism structurally forces the anti-phase drive-mode motion of the symmetrically decoupled tines eliminating the lower frequency spurious mode and providing true mechanical rejection of external shocks and accelerations A single-mass gyroscope identical to one uncoupled tine was also

fabricated and characterized to analyze the advantages of the proposed tuning fork architecture Compared to the single-mass device, the momentum balance of the tuning fork drive-mode results in a 6.7 times improvement of the quality factor; the momentum and torque balance of the tuning fork sense-mode allows a 12.5 times improvement in quality factor and sensitivity

Figure 1.8: Design model of lateral micromachined tuning fork gyroscope: (1) outer mass frame, (2) inner mass frame, (3) drive comb electrodes, (4) sense electrodes, (5) folded beam, (6) anchor, (7) lozenge-shaped coupling spring, and (8) self-rotation ring

[8]

In the development of the tuning fork gyroscope, our research group has also established a compact tuning fork gyroscopes design The schematic of the designed tuning fork is shown in Fig 1.8 [15] The sensor is designed to reduce noises and improve the sensitivity by using a drive coupling spring in the lozenge shape The in-phase sensing mode is suppressed by using a self-rotation ring The designed sensor prioritizes anti-phase driving and sensing modes The frequencies of anti-phase driving and sensing modes are far from those of parasitic ones The design also enables the sensing mode to decouple from the driving one, which is considered to decrease vibration-induced error The simulated frequencies of the driving and sensing modes are 9.9 kHz and 10.0 kHz, respectively, which show the bandwidth of sensor of 100

Hz The frequency difference between the driving and sensing modes and the parasitic ones is obtained to be 50%

1.3.3 Decreasing damping

Damping in MEMS devices strongly affects their performance, design, and control Damping influences the behavior of MEMS in various ways, depending on their design criteria and operating conditions In resonant sensors, a high quality factor is required for achieving high sensitivity and resolution In vibrating gyroscopes, there exist damping mechanisms affecting their performance such as viscous damping and structural damping

In ref.[16], the proof masses are perforated to minimize damping At pressures of 100 mTorr, a quality factor (Q) of 40 000 was observed for the drive mode and 5000 for the sense mode Specially, a silicon bulk micromachined lateral axis tuning-fork gyroscope with a decoupled comb drive and torsional sensing comb capacitors is presented The torsional sensing combs are designed to differentially sense the out-of-plane rotational moment and are arranged centroidally to be immune to fabrication imperfections for good linearity and electrostatic force balancing The torsional sensing combs adopted

in the TFG help to lower the air damping of the sensing mode while the driving mode

of the gyroscope is dominated by slide-film air damping; hence, it can work even at atmospheric pressure [20]

In order to minimize damping and increase the Q factors in drive and sense modes

of the gyroscope, vacuum packaging is required Maintaining a high and stable Q facto rover the lifetime of the device is extremely critical Furthermore, higher vacuum levels help ensuring stability of Q factor with temperature, improving the bias stability over temperature Vacuum packaging can be implemented at the die level or at the wafer level Die-level vacuum packaging is commonly realized by sealing the die in a ceramic package in a high-vacuum environment Wafer-level vacuum packaging involves sealing the capping wafer in vacuum In both approaches, two primary factors have to be controlled to achieve vacuum: outgassing and leaking [17]

In 2000, Samsung demonstrated a wafer-level vacuum packaged 40µm thick bulk micromachined single crystal silicon sensor with mode decoupling The ambient pressure for the packaged gyroscope is 150 mTorr The Q factor of the driving and sensing modes was measured to about 2000 and 500, respectively The resolution in this work was reported to be 0.0130/s/√Hz [16]

Figure 1.9: Optical photograph of a vacuum packaged tuning fork gyroscope, showing the die, the ceramic package, the glass lid with getter material and metal sealing ring,

and structural schematic of the gyroscope architecture [11]

In 2011, Alexander A Trusov et al reported a vacuum packaged tuning fork gyroscope [10] Fig 1.9 shows optical photograph of a vacuum packaged tuning fork gyroscope The untrimmed sense-mode resonance was measured at 2538 Hz, which is

55 Hz above the drive-mode operational frequency The device can be operated in air without frequency tuning providing a practically feasible bandwidth on the order of 50

Hz For ultra-high sensitivity, mode-matched operation at reduced pressures, the mode resonance can be tuned down to 2483 Hz using the electrostatic negative spring effect A vacuum packaging technology was based on a ceramic package The sealed gyroscope with a quality factor on the order of 100.000, which is higher than 103 times before sealing The ultra-high quality factors enable an ultra-high mechanical scale factor of 0.4 nm of sense-mode displacement per 1◦h−1 of input angular rate This kind

sense-of tuning fork gyroscope architecture is especially suitable for applications, where ultra-high precision inertial measurements are required in a relatively low bandwidth

1.3.3.2 Intrinsic Structural Damping

Although viscous damping is the dominating damping mechanism in the presence of

a gas in the gyroscope ambient, the total damping in the gyroscope system is a combination of multiple effects The damping of the structural material is usually orders of magnitude lower than the viscous damping except under high-vacuum conditions The damping components other than viscous damping start limiting the quality factor as the pressure inside the gyroscope cavity approaches high vacuum Under vacuum conditions, thermoelastic damping is one of the primary damping mechanisms Thermoelastic damping is the intrinsic material damping that occurs as a result of thermal energy dissipation due to elastic deformation In a vibrating beam, alternating tensile and compressive strains across the width cause irreversible heat flow, which in turn results in an effective damping due to dissipation of vibrational energy [19] Thermoelastic damping has been reported to limit the Q factor of vacuum packaged gyroscopes to values from 100,000 to 200,000 [10]

Many other factors from anchor losses to die attach methods contribute to the total damping in vibratory gyroscopes The classic functional form for the total quality

factor Q total is as follows:

thermoelastic damping Q TED represents for intrinsic losses

It is obvious that Q total cannot exceed a smallest component Q i calculated for each such mechanism Qmaterial is due to material defects which is in the order of 1011, the other Q values capture remaining damping effects estimated around 250,000 [18]

Qanchor is due to the anchor losses which could be as low as 10,000 depending on the anchor type and material Anti-phase devices that locally cancel vibration injection into

the anchors provide much higher Qanchor values Usually viscous damping and other intrinsic damping components are very difficult to estimate theoretically for complicated gyroscope systems It is common to empirically measure the overall Q factor of drive and sense modes by a frequency response or ring-down test [1]

1.4 Purpose of this thesis

In order to improve the sensitivity of a vibratory gyroscope, the driving and sensing vibration modes of the gyroscope need to be designed and fabricated with the matched resonant frequencies When operated at the resonant condition, the amplitude of sensing mode vibration is amplified by its mechanical quality factor In addition, the use of twin masses in TFG operated in anti-phase manner helps reject linear accelerations in the sense direction using differential measurement The use of twin masses vibrating in anti-phase also causes momentum to cancel locally and make the gyroscope less sensitive to mounting In order to reduce error in angular rate measurement, the effect of parasitic modes to driving and sensing mode needs suppressed This is solved by designing mechanical structure that the discrepancy between driving and sensing modes and parasitic modes is larger than 10% Several reports on designing the TFG which has mechanical coupling between the sensing and driving masses are proposed to improve mode order and anti-phase operation of driving or sensing mode However, noise sources causing by vibration have not yet taken into considering sensor designs In a latest report, the research showed that the tuning fork gyroscopes with decoupled sense and drive masses is less sensitive to vibration than tuning fork gyroscope designs featured in literature Specially, a silicon bulk micromachined lateral axis tuning-fork gyroscope with a decoupled comb drive and torsional sensing comb capacitors is presented The torsional sensing combs are designed to lower the air damping of the sensing mode Hence, the device can work even at atmospheric pressure

Taking the above mentioned subjects into considering TFG design, we developed a capacitive-type tuning folk micro-gyroscope The anti-phase operation of driving and sensing modes is obtained by designing mechanical couple springs between proof masses The design also matched the resonant driving and sensing frequencies to the first and second mode The bandwidth between the resonant driving and sensing frequencies was designed to be 1%, which is considered to provide a modest mechanical gain without significant phase shift reduction

Based on the capacitive-type tuning folk micro-gyroscope developed recently in our research group [15], this thesis will suggest and investigate a complete TFG structure operating at atmospheric pressure The methods for improving the sensitivity of TFG gyroscopes are proposed, investigated, and compared with our previously developed sensor structure The thesis concentrates on the following studies

1 Optimizing a gyroscope structure

The structure of gyroscope is optimized by the finite analysis method in which structure parameters affecting the performance of the gyroscope such as springs and thickness are investigated From these investigated results, the changes in the sensor structure design have been done

2 Improving the sensitivity of gyroscope based on modifying the resonant frequencies of the driving and sensing modes

In conventional resonance rate gyroscope, the resonant frequencies of drive and sense modes are designed to match so that get maximum possible gain and hence the sensitivity The relationship between gyroscope operating modes and its resonance vibration amplitudes have been analyzed as a damped system From this result, gyroscope operation modes have been reduced and mode-matched at an optimal range

3 Improving the sensitivity by free-standing gyroscope structure

Viscous damping in the gyroscope is dominated by the internal friction of the gas between the proof masses and the substrate Therefore, in the current study a free-

standing gyroscope structure is proposed, in which the proof masses are free to vibrate without restricted by the substrate Through the simulation, it showed that the sensitivity of the gyroscope can be enhanced

Chapter 2 DESIGNS OPTIMIZATION OF TUNING FORK

GYROSCOPE

This chapter presents the design details of MEMS gyroscopes Section 2.1 gives the specifications of previously designed 10 kHz resonance frequency tuning fork gyroscope Section 2.2 gives the argument in tuning fork gyroscope sensitivity improvement via its structure optimization Finally, section 2.3 gives the comparison in the sensitivity between optimized and previous gyroscope version

2.1 Tuning Fork Gyroscope structure analysis methods

One of most important and effective part in MEMS design is simulation part In this thesis, the Finite Element Method (FEM) simulation is used to find out optimized design of the gyroscope Two FEM analysis techniques used in this thesis are modal and harmonic response analysis The TFG structures were simulated by ANSYS software and solved under a set of operating conditions such as drive force, angular rate, and damping effect which are modeled into parameters by mathematical calculation ANSYS software is most effective in this case to solve physically problems

The modal analysis is used to characterize the TFG structure’s static natural behavior This method uses to determine the natural frequencies and mode shapes of a structure The natural frequencies and mode shapes are important parameters in the design of a structure for dynamic loading conditions They are also required if a spectrum analysis or a mode superposition harmonic or transient analysis is needed Based on the modal analysis results, the structure parameters are optimized to ensure its good natural response

The harmonic response analysis is a dynamic simulation method that is used to calculate structure properties at harmonical state Because any sustained cyclic load will produce a sustained cyclic response (a harmonic response) in a structural system

From harmonic response analysis we can predict the sustained dynamic behavior of the TFG structure, thus enabling us to verify whether or not structure designs will successfully overcome resonance, fatigue, and other harmful effects of forced vibrations This simulation method is based on the sweeping all boundary conditions over frequency range and collect result into a set of overall structure’s responding parameters versus swept frequency For resonance gyroscope simulation, the input conditions are sinusoidal drive force, angular velocity and damping coefficient and the result from harmonic response analysis of gyroscope at resonant state are drive amplitude and Coriolis induced sense amplitude On the other hand, gyroscope’s structure information such as the structure stability, deformation shape, coupling effect also obtained from this simulation method

2.2 The 10 kHz resonant operation Tuning Fork Gyroscope

2.2.1 Structural construction and modal analysis

Based on the ideal gyroscope structure discussed above, a tuning fork gyroscope was constructed [15] This gyroscope version is designed to operate at resonance frequency around 10 kHz (TFG10K version), in which its drive and sense frequencies are mode-matched Mode-matching between drive and sense mode frequencies is 27

Hz for this gyroscope design The 10 kHz TFG version has the lateral dimension of 3935x4555 µm2 and 30 µm in thickness, which will be fabricated in a 30 µm SOI (Silicon on Insulator) wafer via bulk micromachining process Since, the device built-

in material is silicon with a density of 2330 kg/m3, Poisson ratio of 0.28 and Young's modulus of 170 GPa

The gyroscope design requires a good topology selection and well-arranged drive and sense mode resonance frequencies Because the TFG senses angular rate at its resonant state so that its topology requires the design of two different resonance modes(mode-matched) This complex topology also introduces some unwanted

Figure 2.1:MEMS TFG 10 kHz version without drive and sense combs (1-Drive comb frame; 2-Sense beam, 3-Drive beam; 4-Sense comb frame; 5-Lozenge shape’s beam; 6-Self-rotate ring; 7-Constrain beam; 8-Connecting beam; 9-Anchor; 10-Drive outer

frame; 11-Sense frame) resonance modes that causes cross axis sensitivity Hence the gyroscope structure should be carefully designed and should be verified with FEM simulation Selecting gyroscope topology is also correlated to selecting its resonance frequencies of drive and sense modes

In a common vibratory mechanical structure, the higher frequency mode is harder to

be excited Hence the most preferred drive and sense mode frequencies are set to in separated lower range in compare with the unwanted mode frequencies which are considered as structure parasitic modes