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
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
iv
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
2.3 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
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
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
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 velocity and (d) gain using laser vibrometer
ix
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 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
xi (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
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
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
