Design And Optimisation Of Microelectroelectromechanical System (MEMS) Cochlear Biomodel.

Faculty of Electronic and Computer Engineering

DESIGN AND OPTIMISATION OF AN ADAPTIVE

MICROELECTROMECHANICAL SYSTEM (MEMS) COCHLEAR

BIOMODEL

Thailis Bounya Anak Ngelayang

Master of Science in Electronic Engineering

DESIGN AND OPTIMISATION OF AN ADAPTIVE

MICROELECTROMECHANICAL SYSTEM (MEMS) COCHLEAR BIOMODEL

THAILIS BOUNYA ANAK NGELAYANG

A thesis submitted

in fulfillment of the requirements for the degree of Master of Science in Electronic Engineering

Faculty of Electronic and Computer Engineering

UNIVERSITI TEKNIKAL MALAYSIA MELAKA

DECLARATION

I declare that this thesis entitle “Design and Optimisation of an Adaptive Microelectromechanical System (MEMS) Cochlear Biomodel” is the result of my own research except as cited in the references. The thesis has not been accepted for any degree and is not concurrently submitted in candidature of any other degree.

Signature : .

Name : THAILIS BOUNYA ANAK NGELAYANG

APPROVAL

I hereby declare that I have read this thesis and in my opinion this thesis is sufficient in terms of scope and quality for the award of Master of Science in Electronic Engineering.

Signature : . Name : . Date : 16TH _{JUNE 2016 .}

DEDICATION

To my beloved family especially my parents; Mr Ngelayang Anak Asun and Mrs Ladai Anak Anchali, and my siblings; Jenet Enthie Anak Ngelayang, Frank Cylrai Anak

i

ABSTRACT

The research and development of cochlear biomodelling has nowadays become one of the common interests in the biomedical research field. The main criterion of the developed cochlear biomodel is to have the ability to work within an audible range of human ear that is between 20 Hz to 20000 Hz. Microelectromechanical system (MEMS) is seen to have the potential to be utilised in mimicking the tonotopic organisation behavior of human ear. The developed MEMS cochlear biomodel is designed and simulated by using Comsol Multiphysics software to have the dimension of 0.5 μm thickness, 30 μm wide and length varying from 280 μm to 1000 μm. Five MEMS cochlear biomodel designs which are the Straight Bridge Beam (SBB), Straight Bridge Beam with Centered Diaphragm (SBBCD), Straight Bridge Beam with Centered Mass (SBBCM), Crab Legged and Serpentine, have been suggested in order to examine their resonant frequency performances. Four different materials have been considered which are Aluminium (Al), Copper (Cu), Tantalum (Ta) and Platinum (Pt). The design performance has been further tested in terms of its total surface displacement and capacitive ability. SBBCD MEMS cochlear biomodel that was developed with platinum as its base structure material and tantalum as the added mass material gives the highest resonant frequency performance of 92.87 % operating within the desired audible range. The design provides the total surface displacement ranging from 1.4370 nm to 0.0125 µm. The capacitance reading was also recorded to be 14.875 fF at the shortest beam structure and then increased to 53.125 fF towards the longest beam structure. In order to test its adaptivity, the structure was also tested with a voltage ranges from 0.1 V to 0.5 V. The resonant frequency tuning has been found to decrease in the range of 0.57 % to 4.65 % and the surface displacement has been amplified by ~4 to ~25 times bigger as the voltage increases. Relevant microfabrication steps have been suggested to fabricate SBBCM MEMS cochlear biomodel.

ii

ABSTRAK

Pembangunan dan penyelidikan tentang koklea biomodel telah menjadi salah satu bidang penting dalam dunia penyelidikan biomedikal pada masa ini. Kriteria utama untuk menghasilkan koklea biomodel adalah kebolehan model tersebut untuk berfungsi di dalam julat pendengaran telinga manusia dalam lingkungan 20 Hz sehingga 20000 Hz. Sistem mikroelektromekanikal (MEMS) dilihat mempunyai potensi yang boleh digunakan untuk meniru sifat organisasi tonotopik telinga manusia. MEMS koklea biomodel yang dibina telah direka bentuk dan disimulasi menggunakan perisian Comsol Multiphysics untuk mempunyai dimensi ketebalan 0.5 μm, kelebaran 30 μm dan kepanjangan yang bervariasi dari 280 μm ke 1000 μm. Pada asalnya, lima reka bentuk MEMS koklea biomodel yang dikenali sebagai rusuk jambatan lurus (SBB), rusuk jambatan lurus dengan diafragma di tengah (SBBCD), rusuk jambatan lurus dengan bebanan tengah (SBBCM), kaki ketam dan serpentin telah dicadangkan untuk diuji prestasi frekuensi alunannya. Setiap reka bentuk juga turut diuji dengan empat bahan berbeza di mana aluminium (Al), tembaga (Cu), tantalum (Ta) dan platinum (Pt) telah digunakan. Prestasi reka bentuk tersebut akan selanjutnya diuji dari aspek jumlah anjakan permukaan dan keupayaan kapasitif. SBBCM MEMS koklea biomodel yang dihasilkan dengan menggunakan platinum sebagai bahan asas rusuk jambatan dan tantalum sebagai bahan asas bebanan tengah telah mencatatkan prestasi frekuensi alunan tertinggi dengan 92.87 % keupayaan untuk berfungsi di dalam julat pendengaran yang dikehendaki. MEMS koklea biomodel yang dipilih mencatatkan jumlah anjakan permukaan dalam julat 1.4370 nm ke 0.0125 µm. Bacaan kapasitif yang

telah direkod turut mencatatkan 14.875 fF pada struktur terpendek dan meningkat kepada

53.125 fF pada struktur terpanjang. Untuk menguji kebolehan pengadaptasian, struktur

reka bentuk tersebut juga diuji dengan sambungan voltan sebanyak 0.1 V ke 0.5 V. Penalaan frekuensi alunan didapati semakin menurun dalam julat 0.57 % ke 4.65 % dan jumlah anjakan permukaan telah menjadi ~4 ke ~25 kali lebih besar apabila jumlah voltan semakin meningkat. Langkah-langkah mikrofabrikasi yang sesuai telah dicadangkan untuk membantu proses mikrofabrikasi reka bentuk SBBCM MEMS koklea biomodel yang dipilih.

iii

ACKNOWLEDGEMENTS

First and foremost, I would like to give my highest gratitude to the Almighty God for His blessings that I have now completed my Master of Science in Electronic Engineering. Special thanks also dedicated to my supervisors Dr Low Yin Fen and Dr Rhonira Binti Latif for their supervisions during the duration of my research. They have helped and guided me very well regarding useful informations and research techniques in order for me to complete this research project.

Special dedications also to the Ministry of Science, Technology and Innovation (MOSTI), Malaysia for funding my research project under ScienceFund research grant and the authority of University Teknikal Malaysia Melaka, especially to the Faculty of Electronic and Computer Engineering for the university short term research grant. The faculty had also provided useful and conductive facilities for me to conduct my research works.

At last, I would like to extend my gratitudes to my parents; Mr. Ngelayang Anak Asun and Mrs. Ladai Anak Anchali, my siblings, and my friends for their encouragement, love and motivations throughout my whole journey. Once again, thank you so much.

iv

TABLE OF CONTENTS

PAGE DECLARATION

APPROVAL DEDICATION

ABSTRACT i

ABSTRAK ii

ACKNOWLEDGEMENTS iii

TABLE OF CONTENTS iv

LIST OF TABLES vi

LIST OF FIGURES viii

LIST OF ABBREVIATIONS xiii

LIST OF PUBLICATIONS xv

ACHIEVEMENT xvi

CHAPTER

1. INTRODUCTION 1

1.0 Research Background 1

1.1 Problem Statement 3

1.2 Aim and Objectives 4

1.3 Scope of Work 4

1.4 Thesis Organisation 5

2. MICROELECTROMECHANICAL SYSTEM (MEMS) AND

MICROFABRICATION 8

2.0 Introduction 8

2.1 Microelectromechanical System (MEMS) 8

2.2 The Biomimetical Study 10

2.2.1 Human Auditory System 10

2.2.2 The Cochlea 12

2.3 Microbridge Resonator Beams 14

2.3.1 Microbridge Resonator Beam Design 14

2.3.2 Resonant Frequency 16

2.3.3 Parallel Plate Capacitor 17

2.4 Introduction to Microfabrication 18

2.4.1 Material Selection 19

2.4.2 Lithography 20

2.4.3 Etching 22

2.4.4 Bulk Micromachining 23

2.4.4.1 Wet Etching 24

2.4.4.2 Dry Etching 26

2.4.5 Surface Micromachining 27

2.5 Summary 29

3. LITERATURE REVIEW 30

3.0 Introduction 30

v

3.2 Mechanical Cochlear Biomodel 30

3.2.1 Mechanical Cochlear Implementation in Air

Surrounding 31

3.2.2 Mechanical Cochlear Implementation in Fluidic

Surrounding 36

3.3 Microelectromechanical System (MEMS) Cochlear Biomodel 40 3.3.1 MEMS Cochlear Implementation in Air

Surrounding 41

3.3.2 MEMS Cochlear Implementation in Fluidic

Surrounding 48

3.4 Summary 53

4. METHODOLOGY 54

4.0 Introduction 54

4.1 Project Overview 55

4.2 MEMS Cochlear Biomodel Design Process 56

4.3 Analysis of SBBCM MEMS Cochlear Biomodel Design 66

4.4 Microfabrication Process Steps 69

4.5 Summary 70

5. RESULTS, ANALYSIS AND DISCUSSION 71

5.0 Introduction 71

5.1 MEMS Cochlear Biomodel Design 72

5.1.1 Straight Bridge Beam (SBB) 73

5.1.2 Straight Bridge Beam with Centered Diaphragm (SBBCD) 82 5.1.3 Straight Bridge Beam with Centered Mass (SBBCM) 84

5.1.4 Crab Legged 91

5.1.5 Serpentine 94

5.2 The Selected MEMS Cochlear Biomodel Design 96

5.3 Performance Analysis of SBBCM MEMS Cochlear Biomodel Design 97

5.4 Suggestion for Microfabrication Process 103

5.5 Summary 109

6. CONCLUSION AND RECOMMENDATIONS FOR FUTURE

WORK 110

6.0 Conclusion 110

6.1 Recommendations for Future Work 113

vi

LIST OF TABLES

TABLE TITLE PAGE

4.1 Properties of the materials employed as the micro resonator beams

57

4.2 Meshing used during software simulations 64

5.1 The beam length of micro bridge resonator beams 72

5.2 The simulated FEM resonant frequency values for SBB

MEMS cochlear biomodel design 75

5.3 Comparison between the LEM and FEM for aluminium SBB MEMS cochlear biomodel design and the percentage of difference for each entry

76

5.4 Comparison between the LEM and FEM for copper SBB MEMS cochlear biomodel design and the percentage of difference for each entry

78

5.5 Comparison between the LEM and FEM for tantalum SBB MEMS cochlear biomodel design and the percentage of difference for each entry

79

5.6 Comparison between the LEM and FEM for platinum SBB MEMS cochlear biomodel design and the percentage of difference for each entry

81

5.7 The simulated resonant frequency values for SBBCD MEMS

cochlear biomodel design 83

5.8 The simulated resonant frequency values for SBBCM MEMS

cochlear biomodel design with aluminium as the base structure 86 5.9 The simulated resonant frequency values for SBBCM MEMS

cochlear biomodel design with copper as the base structure

vii

5.10 The simulated resonant frequency values for SBBCM MEMS

cochlear biomodel design with tantalum as the base structure 89 5.11 The simulated resonant frequency values for SBBCM MEMS

cochlear biomodel design with platinum as the base structure

90 5.12 The simulated resonant frequency values for crab legged

MEMS cochlear biomodel design 93

5.13 The simulated resonant frequency values for serpentine MEMS

cochlear biomodel design 95

5.14 The performance details for the best designs of MEMS

cochlear biomodel 97

5.15 The recorded capacitance value for every beam entry when

there is no voltage applied 99

5.16 The suggested microfabrication steps for SBBCM MEMS

viii

LIST OF FIGURES

FIGURE TITLE PAGE

1.1 The cross sectional view of human auditory system 2

2.1 The feedback mechanisms of the incoming sound wave into

the auditory system 11

2.2 The tonotopic organisation characteristics of human cochlear 13 2.3 MEMS resonator design that can be employed in cochlear

biomodelling, (a) cantilever design, (b) fixed-fixed design, (c) crab legged design, (d) serpentine design and (e) diaphragm design

15

2.4 Parallel plate capacitor structure 17

2.5 The sequential steps of microfabrication process 19

2.6 The typical bulk micromachining structures. (a) membranes

and beams, (b) wafer through holes, (c) microwells 24 2.7 Processing steps of surface micromachining technology, (a)

the deposition of mechanical layer and (b) the sacrificial layer on the wafer

28

3.1 Chan Keen Leong cochlear biomodels, (a) The single

chamber model and (b) the double chamber model 31 3.2 The frequency transfer function on the single chamber and

double chamber model 33

3.3 The cochlear biomodel proposed by Bryan S Joyce and Pablo A Tarazaga

34

3.4 The simulated (a) velocity and (c) gain from the Simulink model at different excitation levels. The measured (b) velocity and (d) gain using laser vibrometer

ix

3.5 The mechanical cochlear biomodel designed by Georg 37 3.6 The measurement of basilar membrane displacement as a

function of position along the length of the membrane at different input frequency

38

3.7 The dimension of the cochlear duct and its base made by

Shuangqin Liu et al 39

3.8 The influence of 20cSt and 500 cSt silicon oil viscosity on

the measured velocity of the basilar membrane 40

3.9 MEMBAC model with resonator beam length varying from

400 µm to 7000 µm 41

3.10 Resonator beams sensitivity measurement from MEMBAC 42 3.11 Both sides of the transverse beam in the fishbone structure

are supported by a diaphragm 43

3.12 The frequency response of the resonator beams after being

stimulated to vibrate at the same time 44

3.13 An array of RGT devices that represent the basilar

membrane of human cochlear 45

3.14 The natural frequency measurement of an aluminium bridge 46 3.15 The artificial basilar membrane design as reported by Harto

Tanujaya 47

3.16 Contour map of the vibrating amplitude of the artificial basilar membrane prototype at a) 6 kHz, b) 9 kHz, and c) 12.8 kHz

48

3.17 The designed two ducts model by Robert D. White and Karl

Grosh 49

3.18 Data from Laser Doppler Velocimetry (LDV) showing the

magnitude of the membrane displacement 50

3.19 The artificial basilar membrane with 32 beams. (a) The schematic layout showing the copper beams are arrayed on the substrate. (b) The zoom-in view of the beam structure. (c) A cross sectional view of a beam section where 9 µm piezo membrane is located in between 12.5 µm copper beams on both sides

x

3.20 The sensitivity response of beam #32 (a) in frequency

domain and (b) in time domain 52

3.21 The amplitude displacement response in a particular cochlear

partition 53

4.1 Flow chart for the project overview 56

4.2 Flow chart of the design process of MEMS cochlear

biomodel 58

4.3 Flow chart of simulation design in Comsol Multiphysics

software 61

4.4 Dimensional design of the suggested MEMS micro bridge beams. (a) SBB micro bridge structure, (b) SBBCD micro bridge structure, (c) SBBCM micro bridge structure, (d) Crab legged micro bridge structure and (e) Serpentine micro bridge structure

63

4.5 Flow chart of the chosen MEMS cochlear biomodel design

analysis 67

4.6 Flow chart of the basic MEMS microfabrication steps 70

5.1 SBB MEMS cochlear biomodel design. (a) with mesh, (b)

simulated 73

5.2 Graph of Resonant Frequency (Hz) against Beam Length (µm) for Straight Bridge Beam (SBB) MEMS Cochlear Biomodel

75

5.3 Graph of the Comparison Between FEM and LEM for Straight Bridge Beam (SBB) MEMS Cochlear Biomodel together with the percentage of difference for every entry (Material: Aluminium)

77

5.4 Graph of the Comparison Between FEM and LEM for Straight Bridge Beam (SBB) MEMS Cochlear Biomodel together with the percentage of difference for every entry (Material: Copper)

78

80 5.5 Graph of the Comparison Between FEM and LEM for

Straight Bridge Beam (SBB) MEMS Cochlear Biomodel together with the percentage of difference for every entry (Material: Tantalum)

xi

5.6 Graph of the Comparison Between FEM and LEM for Straight Bridge Beam (SBB) MEMS Cochlear Biomodel together with the percentage of difference for every entry (Material: Platinum)

81

5.7 SBBCD MEMS cochlear biomodel design. (a) with mesh,

(b) simulated 82

5.8 Graph of Resonant Frequency (Hz) against Beam Length (µm) for Straight Bridge Beam with Centered Diaphragm (SBBCD) MEMS Cochlear Biomodel

84

5.9 SBBCM MEMS cochlear biomodel design. (a) with mesh,

(b) simulated 85

5.10 Graph of Resonant Frequency (Hz) against Beam Length (µm) for Straight Bridge Beam with Centered Mass (SBBCM) MEMS Cochlear Biomodel (Base: Aluminium (Al))

86

5.11 Graph of Resonant Frequency (Hz) against Beam Length (µm) for Straight Bridge Beam with Centered Mass (SBBCM) MEMS Cochlear Biomodel (Base: Copper (Cu))

88

5.12 Graph of Resonant Frequency (Hz) against Beam Length (µm) for Straight Bridge Beam with Centered Mass (SBBCM) MEMS Cochlear Biomodel (Base: Tantalum (Ta))

89

5.13 Graph of Resonant Frequency (Hz) against Beam Length (µm) for Straight Bridge Beam with Centered Mass (SBBCM) MEMS Cochlear Biomodel (Base: Platinum (Pt))

91

5.14 Crab legged MEMS cochlear biomodel design. (a) with

mesh, (b) simulated 92

5.15 Graph of Resonant Frequency (Hz) against Beam Length

(µm) for Crab Legged MEMS Cochlear Biomodel 93

5.16 Serpentine MEMS cochlear biomodel design. (a) with mesh,

(b) simulated 94

5.17 Graph of Resonant Frequency (Hz) against Beam Length

(µm) for Serpentine MEMS Cochlear Biomodel 95

5.18 Graph of the Total Surface Displacement (µm) against

Frequency (Hz) 98

5.19 Graph of the Static Capacitance (fF) against the Beam

xii

5.20 Graph of the Normalised Surface Displacement against

Normalised Frequency with respect to 0.1 V 102

5.21 Graph of the Static Capacitance (fF) against the Applied

Voltage (V) 103

5.22 The illustration of SBBCM cochlear biomodel where (a) top

view, (b) isometric view, and (c) front view 104

5.23 The suggested photolithography mask for SBBCM MEMS cochlear biomodel, (a) electrode pad, (b) circuitry base layer, (c) anchors, (d) circuitry and bridge structure, and (e) added mass structure

xiii

LIST OF ABBREVIATIONS

Al - Aluminium

ABM - Artificial Basilar Membrane

CAD - Computer Aided Design

CF4 - Tetrafluoromethane

Cl2 - Chlorine Gas

CNT - Carbon Nano Tubes

Cu - Copper

DBM - Discrete Basilar Membrane

DCS - Dichlorosilane

EDP - Ethylene Diamine Pyrocatecol

F2 - Fluorine

FEM - Finite Element Model

H4N2 - Hydrazine

HF - Hydrogen Fluoride

HNO3 - Nitric Acid

IC - Integrated Circuit

KOH - Potassium Hydroxide

LDV - Laser Doppler Velocimetry

LEM - Lumped Element Model

LPCVD - Low Pressure Chemical Vapour Deposition

MEMBAC - Microelectromechanics Based Artificial Cochlear

MEMS - Microelectromechanical System

MOSFET - Metal Oxide Semiconductor Field Effect Transistor

NF3 - Nitrogen Trifluorine

NH3 - Ammonia

PSG - Phosphorus Doped Silicon Dioxide

Pt - Platinum

PVDF - Polyvinylidene Fluoride

RF - Radio Frequency

RGT - Resonant Gate Transistor

RIE - Reactive Ion Etching

SBB - Straight Bridge Beam

SBBCD - Straight Bridge Beam With Centered Diaphragm

xiv

SCS - Single-Crystal Silicon

SF6 - Hexafluoride

Si3N4 - Silicon Nitride

SiO2 - Silicon Dioxide

SPL - Sound Pressure Level

Ta - Tantalum

TMAH - Tetramethylammonium Hydroxide

UV - Ultra-Violate

xv

LIST OF PUBLICATIONS

The research papers produced and published during the course of this research are as follows:

1. Ngelayang, T., Majlis, B., Azam, M., Arith, F. and Latif, R., 2015. Platinum and Aluminium Microresonator Bridges for Artificial Basilar Membrane. Applied

Mechanics and Materials, 761, pp.462-467. (Scopus)

2. Ngelayang, T. and Latif, R., 2015. Development of Micro-electromechanical System (MEMS) Cochlear Biomodel. International Conference on Mathematics,

Engineering and Industrial Applications 2014 (ICoMEIA 2014), 1660. (Scopus)

3. Ngelayang, T., Low, Y. F., Latif, R. and Majlis, B., 2016. The Evolution of Research and Development on Cochlear Biomodel. Jurnal Teknologi, 8(6-8), pp.83-92. (Scopus)

4. Ngelayang, T., Latif, R. and Majlis, B., 2016. Straight Bridge Beams with Centered Diaphragm (SBBCD) Design for MEMS Cochlear Biomodel. The 2016

IEEE International Conference on Semiconductor Electronics (ICSE2016).

xvi

ACHIEVEMENT

2nd _{Runner Up in 3 MINUTES THESIS COMPETITION 2016, organised by Centre for}

CHAPTER 1

INTRODUCTION

1.0 Research Background

Microelectromechanical system (MEMS) is a technology that is mostly defined as miniaturized mechanical and electro-mechanical elements. All MEMS devices consist of a combination of mechanical and electrical components working together to execute certain functions (Korvink and Paul, 2006). MEMS have been implemented as the key components in automotive, industrial, aerospace, and medical applications. The technique of micro fabrication is well known in the application of MEMS.

Since centuries ago, many researchers had attempted to study the behaviour of human auditory system. The cross sectional view of a human auditory system is shown in Figure 1.1. Human ear has a complex mechanism of sensing the incoming sound waves through the auditory canal which will then be transmitted into the brain to be processed so that human brain can detect and reflect to the incoming messages. Human ear has the ability to convert mechanical energy into electrical energy. When the incoming sound waves strikes the hair cell structure in the organ of Corti which is located on the basilar membrane, the vibration of the hair cells produces mechanical energy that is responding to the strength of the waves. Thus, an electrical signal is produced. These electrical signals will then be transmitted into human brain to be perceived and detected as sound (Sininger, 2009). The natural behaviour of human auditory system especially the cochlea itself is studied in order to construct the cochlear design that will perform closely replicating its

2

natural performance. Once the characteristic of a human cochlear is understood, the potential of MEMS technology to be used in this project is studied.

Figure 1.1: The cross sectional view of human auditory system (Kaskel, Hummer and Daniel, 1999).

In this project, an adaptive MEMS microresonator design is proposed to be employed in the cochlear biomodel so that it can mimic the active human cochlear response. It has the capability of self-tuning its sensitivity through an electrostatic effect that is closely imitating the actual principle of the basilar membrane in an active cochlear mechanism (Latif, 2012). Besides that, the proposed cochlear biomodel must be able to replicate the tonotopic organisation of the basilar membrane that resides within the cochlea. Briefly, tonotopic organisation is the ability of every partition in the basilar membrane to respond to certain frequency of the incoming sound signals. MEMS microresonator bridge has the ability to converts the incoming sound input into mechanical vibrations. Various shapes of MEMS micro resonator bridge will be designed using Comsol Multiphysics software. The design evaluation will be based on the efficiency of

3

each shape to operate closely to the range of human audible frequency which is in between 20 Hz up to 20000 Hz (Dallos, Popper and Fay, 1996).

1.1 Problem Statement

Cochlear biomodels are studied in order to imitate the complex human cochlear response. Many researches were conducted to understand the unique characteristics of cochlear structure and its function. Current existing cochlear biomodels have been reported to be developed by implementing mechanical structures and MEMS. Both types of cochlear biomodels were studied in air and fluidic surrounding. However, the present types of cochlear biomodel could not imitate closely the behavior of the cochlea (Lyon et al., 2010), (Kolston, 1998). It did not operate within the entire audible frequency range of a normal human ear. The development of an electrical sensing in the existence cochlear biomodel is still lacking. Current microphones for the hearing aids devices also did not have the capability to adapt the human cochlear sensitive and adaptive characteristics. These circumstances have made the existing cochlear biomodel to become a passive model. Thus, an adaptive microelectromechanical systems (MEMS) micro resonator is suggested to be applied in human cochlear biomodel in order to obtain good imitation of the active human cochlear response (Jang et al., 2015), (Bachman et al., 2006).

xiv SCS - Single-Crystal Silicon

SF6 - Hexafluoride

Si3N4 - Silicon Nitride

SiO2 - Silicon Dioxide

SPL - Sound Pressure Level

Ta - Tantalum

TMAH - Tetramethylammonium Hydroxide

UV - Ultra-Violate

LIST OF PUBLICATIONS

The research papers produced and published during the course of this research are as follows:

1. Ngelayang, T., Majlis, B., Azam, M., Arith, F. and Latif, R., 2015. Platinum and Aluminium Microresonator Bridges for Artificial Basilar Membrane. Applied Mechanics and Materials, 761, pp.462-467. (Scopus)

2. Ngelayang, T. and Latif, R., 2015. Development of Micro-electromechanical System (MEMS) Cochlear Biomodel. International Conference on Mathematics, Engineering and Industrial Applications 2014 (ICoMEIA 2014), 1660. (Scopus) 3. Ngelayang, T., Low, Y. F., Latif, R. and Majlis, B., 2016. The Evolution of

Research and Development on Cochlear Biomodel. Jurnal Teknologi, 8(6-8), pp.83-92. (Scopus)

4. Ngelayang, T., Latif, R. and Majlis, B., 2016. Straight Bridge Beams with Centered Diaphragm (SBBCD) Design for MEMS Cochlear Biomodel. The 2016 IEEE International Conference on Semiconductor Electronics (ICSE2016). (Scopus) (Accepted)

xvi

ACHIEVEMENT

2nd _{Runner Up in 3 MINUTES THESIS COMPETITION 2016, organised by Centre for}

CHAPTER 1

INTRODUCTION

1.0 Research Background

Microelectromechanical system (MEMS) is a technology that is mostly defined as miniaturized mechanical and electro-mechanical elements. All MEMS devices consist of a combination of mechanical and electrical components working together to execute certain functions (Korvink and Paul, 2006). MEMS have been implemented as the key components in automotive, industrial, aerospace, and medical applications. The technique of micro fabrication is well known in the application of MEMS.

Since centuries ago, many researchers had attempted to study the behaviour of human auditory system. The cross sectional view of a human auditory system is shown in Figure 1.1. Human ear has a complex mechanism of sensing the incoming sound waves through the auditory canal which will then be transmitted into the brain to be processed so that human brain can detect and reflect to the incoming messages. Human ear has the ability to convert mechanical energy into electrical energy. When the incoming sound waves strikes the hair cell structure in the organ of Corti which is located on the basilar membrane, the vibration of the hair cells produces mechanical energy that is responding to the strength of the waves. Thus, an electrical signal is produced. These electrical signals will then be transmitted into human brain to be perceived and detected as sound (Sininger, 2009). The natural behaviour of human auditory system especially the cochlea itself is studied in order to construct the cochlear design that will perform closely replicating its

2

natural performance. Once the characteristic of a human cochlear is understood, the potential of MEMS technology to be used in this project is studied.

Figure 1.1: The cross sectional view of human auditory system (Kaskel, Hummer and Daniel, 1999).

In this project, an adaptive MEMS microresonator design is proposed to be employed in the cochlear biomodel so that it can mimic the active human cochlear response. It has the capability of self-tuning its sensitivity through an electrostatic effect that is closely imitating the actual principle of the basilar membrane in an active cochlear mechanism (Latif, 2012). Besides that, the proposed cochlear biomodel must be able to replicate the tonotopic organisation of the basilar membrane that resides within the cochlea. Briefly, tonotopic organisation is the ability of every partition in the basilar membrane to respond to certain frequency of the incoming sound signals. MEMS microresonator bridge has the ability to converts the incoming sound input into mechanical vibrations. Various shapes of MEMS micro resonator bridge will be designed using Comsol Multiphysics software. The design evaluation will be based on the efficiency of

each shape to operate closely to the range of human audible frequency which is in between 20 Hz up to 20000 Hz (Dallos, Popper and Fay, 1996).

1.1 Problem Statement

Cochlear biomodels are studied in order to imitate the complex human cochlear response. Many researches were conducted to understand the unique characteristics of cochlear structure and its function. Current existing cochlear biomodels have been reported to be developed by implementing mechanical structures and MEMS. Both types of cochlear biomodels were studied in air and fluidic surrounding. However, the present types of cochlear biomodel could not imitate closely the behavior of the cochlea (Lyon et al., 2010), (Kolston, 1998). It did not operate within the entire audible frequency range of a normal human ear. The development of an electrical sensing in the existence cochlear biomodel is still lacking. Current microphones for the hearing aids devices also did not have the capability to adapt the human cochlear sensitive and adaptive characteristics. These circumstances have made the existing cochlear biomodel to become a passive model. Thus, an adaptive microelectromechanical systems (MEMS) micro resonator is suggested to be applied in human cochlear biomodel in order to obtain good imitation of the active human cochlear response (Jang et al., 2015), (Bachman et al., 2006).