designing, fabrication and post- fabrication

DESIGNING, FABRICATION AND POST-

FABRICATION CHARACTERIZATION OF

HALF-FREQUENCY DRIVEN 16 X 16

WATERBORNE TRANSMIT CMUT ARRAY

A THESIS SUBMITTED TO

THE GRADUATE SCHOOL OF ENGINEERING AND SCIENCE

OF BILKENT UNIVERSITY

IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR

THE DEGREE OF

MASTER OF SCIENCE

IN

ELECTRICAL AND ELECTRONICS ENGINEERING

By

Yusuph Abubakar Abhoo

February 2021

Designing, Fabrication and Post-fabrication Characterization

of Half-frequency driven 16 x 16 Waterborne Transmit CMUT Array

By Yusuph Abubakar Abhoo

February 2021

We certify that we have read this thesis and that in our opinion it is fully adequate, in scope

and in quality, as a thesis for the degree of Master of Science.

Hayrettin Köymen (Advisor)

Abdullah Atalar

Itır Köymen

Approved for the Graduate School of Engineering and Science:

Ezhan Karaşan

Director of the Graduate School

ii

ABSTRACT

DESIGNING, FABRICATION AND POST-FABRICATION

CHARACTERIZATION OF HALF-FREQUENCY DRIVEN

16 X 16 WATERBORNE TRANSMIT CMUT ARRAY


M.S. in Electrical and Electronics Engineering

Advisor: Hayrettin Köymen

February 2021

Capacitive Micromachined Ultrasonic Transducers (CMUT) are micro-

scaled electromechanical devices which are used to either transmit or receive

pressure signals and applicable for various purposes such as ultrasonic sensor,

medical imaging, accurate biometric sensing and parametric speakers. For

transmitting CMUT transducer, different sizes and array configurations are used

to intensify the transmission power depending on the application. The half-

frequency driven waterborne transmitting CMUT array designed in this work is

to be used for high resolution volumetric medical imaging purpose. This was

accomplished by a design which prioritizes maximizing the power output,

achieving a directive radiation pattern with low sidelobes which maximizes the

beamformable region. In this work, the issues with steering of the focused beam

are also resolved to achieve a focused steerable beam. This work is an

advancement from the earlier designed half-frequency driven airborne transmit

CMUT to improve power output, introduce the beamforming and focused

transmission capabilities and be applicable for high resolution volumetric

medical imaging purpose.

To improve the power output, the design was made to compensate for the

static depression. Compensating for static depression was achieved by designing

to operate the CMUT without DC bias voltage which allows for full-gap swing

and giving output signal of twice the input frequency. This property allows the

cell to produce high power output with low voltage levels but also brings the

advantage of operating the cell with very high voltages without collapsing.

iii

The CMUT was chosen to be operating at 7.5 MHz and be driven by Digital

Phased Array System (DiPhAS) which allowed to have maximum of 256

channels which for volumetric transmission meant a maximum of 16 x 16 array.

Since the radiation pattern and Rayleigh distance are both the functions of

radius, frequency and the pitch, the design optimization was found while

considering all the above preferences simultaneously. The cells’ radii were

determined to be 80 µm, the plate thickness was 15 µm, the gap height was

found to be 117 nm and the pitch was 192 µm. The array designing was carried

out using the large-signal equivalent circuit model and the radiation impedance

matrix phenomenon.

The simulations showed that with this design, the maximized Rayleigh

distance was 45.3 mm and the sidelobe of -17.4 dB. In simulations, very high

pressure outputs were achievable with individual cells up to 425 kPa per cell

with 150 VPP input while up to 1.5 MPa was emitted by the array plane wave

transmission with only 10 VPP input and almost doubles when the transmitted

beam was focused at zero degrees. Fabrication was done by the wafer boding

and flip-chip bonding techniques where the whole process required only two

lithography masks.

After fabrication, the tests were performed to identify the yield of the

transducer was 18.75% of the array then impedance analysis was done to

characterize the functional cells and resonance frequency drift. The transducer

was cased in a water-tight manner and the waterborne transmission were done

with individual cells to characterize and compare the performance with the

design simulations which were in the range of agreement achieving an average

of 1625 Pa per cell. The functional cells were then used for plane wave

transmission with 10 VPP and the output pressure of 397 kPa was achieved at

resonance frequency. The measurement results showed that the design could

further be improved by compensating the active area to improve the yield for

better results and be able to use it for high resolution 3D medical imaging.

Keywords: CMUT, Array, Half Frequency Operation, Unbiased mode operation,

Radiation Impedance, Waterborne transmission, Volumetric Imaging technique,

CMUT Lumped-element equivalent circuit, Microfabrication, MEMS

iv

ÖZET

YARI FREKANSTA SÜRÜLEN SUALTI 16 X 16 ELEMANLI

CMUT DIZININ DIZAYNI, FABRIKASYONU VE

FABRIKASYON SONRASI KARAKTERIZASYONU


Elektrik ve Elektronik Mühendisliği, Yüksek Lisans

Tez Danışmanı: Hayrettin Köymen

Şubat 2021

Kapasitif Mikro İşlenmiş Ultrasonik Dönüştürücüler (CMUT), basınç

sinyallerini iletmek veya almak için kullanılan ve ultrasonik sensör, tıbbi

görüntüleme, doğru biyometrik algılama, parametrik hoparlörler ve daha birçok

alan gibi çeşitli amaçlara uygulanabilen mikro ölçekli elektromekanik

cihazlardır. İletim CMUT dönüştürücüleri için, uygulamaya bağlı olarak iletim

gücünü yoğunlaştırmak için farklı boyutlar ve dizi konfigürasyonları kullanılır.

Bu çalışmada tasarlanan yarı frekansla çalışan su bazlı iletici CMUT dizisi,

yüksek çözünürlüklü volümetrik tıbbi görüntüleme amacıyla kullanılacak. Bu,

güç çıkışını en üst düzeye çıkarmaya öncelik veren, hüzme biçimlendirilebilir

bölgeyi en üst düzeye çıkaran düşük yan çubuklarla yönlendirici bir radyasyon

modeli elde eden bir tasarımla gerçekleştirildi. Bu çalışmada, odaklanmış

yönlendirilebilir bir hüzme elde etmek için odaklanmış hüzmenin

yönlendirilmesi ile ilgili sorunlar da çözülmüştür. Bu çalışma, güç çıkışını

iyileştirmek, hüzmeleme ve odaklanmış iletim yeteneklerini tanıtmak ve yüksek

çözünürlüklü volümetrik tıbbi görüntüleme amacına uygun olmak için daha önce

tasarlanmış yarı frekans tahrikli havadan iletim CMUT'tan bir ilerlemedir.

Güç çıkışını iyileştirmek için, statik çöküntüyü telafi eden bir tasarım

yapıldı. Statik çöküntüyü dengeleme, CMUT'u DC ön gerilim voltajı olmadan

çalıştıracak şekilde tasarlayarak, tam aralık salınımına izin vererek ve giriş

frekansının iki katı çıkış sinyali vererek sağlandı. Bu özellik, hücrenin düşük

voltaj seviyelerinde yüksek güç çıkışı üretmesini sağlarken aynı zamanda

hücreyi çökmeden çok yüksek voltajlarla çalıştırma avantajını da beraberinde

getirir.

v

CMUT, 7.5 MHz'de çalışacak ve hacimsel aktarım için maksimum 16 x 16

dizi anlamına gelen maksimum 256 kanala izin veren DiPhAS tarafından

çalıştırılacak şekilde seçildi. Işıma örüntüsü ve Rayleigh mesafesi hem yarıçap,

frekans hem de aralık mesafesinin fonksiyonları olduğundan, tasarım

optimizasyonu, yukarıdaki tüm tercihler aynı anda dikkate alınarak bulunmuştur.

Hücrelerin yarıçapları 80 µm, plaka kalınlığı 15 µm, boşluk yüksekliği 117 nm

ve aralık 192 µm olarak belirlendi. Dizi tasarımı, büyük sinyal eşdeğer devre

modeli ve radyasyon empedans matrisi fenomeni kullanılarak gerçekleştirildi.

Simülasyonlar, bu tasarımla, maksimize edilen Rayleigh mesafesinin 45,3

mm ve yan lobun -17,4 dB olduğunu gösterdi. Simülasyonlarda, 150 VPP girişli

hücre başına 425 kPa'ya kadar tekli hücrelerle çok yüksek basınç çıktıları elde

edilebilirken, yalnızca 10 VPP girişli dizi düzlem dalga iletimi tarafından

1.5MPa’a kadar iletim yapıldığı ve iletilen hüzme 0 dereceye odaklandığında bu

değerin 2 katına kadar çıktığı görülmüştür. Üretim, tüm işlemin sadece iki

litografi maskesi gerektirdiği gofret kaplama ve flip-chip bağlama teknikleriyle

yapıldı.

Üretimden sonra, dönüştürücünün veriminin, dizinin % 8.75'i olduğunu

belirlemek için testler yapıldı, ardından fonksiyonel hücreleri ve rezonans

frekans kaymasını karakterize etmek için empedans analizi yapıldı.

Dönüştürücü, su geçirmez bir şekilde muhafaza edildi ve su bazlı iletim, hücre

başına ortalama 1625 Pa'ya ulaşan mutabakat aralığındaki tasarım

simülasyonları ile performansı karakterize etmek ve karşılaştırmak için ayrı

hücrelerle yapıldı. Fonksiyonel hücreler daha sonra 10 VPP ile düzlem dalga

iletimi için kullanıldı ve rezonans frekansında 397 kPa çıkış basıncı elde edildi.

Ölçüm sonuçları, daha iyi sonuçlar için verimi iyileştirmek ve yüksek

çözünürlüklü 3D tıbbi görüntüleme için kullanabilmek için aktif alanı telafi

ederek tasarımın daha da geliştirilebileceğini gösterdi.

Anahtar Kelimeler: CMUT, CMUT dizi, Yarı frekansta operasyonu, Yüklemesiz

operasyon, radyasyon empedansı, Sualtı operasyon, hacimsel görüntüleme

tekniği, Büyük ışaret eşdeger devre modeli, Mikrofabrikasyon, MEMS

vi

Acknowledgements

Special thanks and highest appreciation to my advisor Prof. Dr. Hayrettin

Köymen who made me into what I am today with his incredible technical and

moral support throughout my master’s program at Bilkent University. Prof.

Köymen has been more than just my mentor in the academic work, always had

time to meet up for discussions, always gave his professional and friendly

opinion a matter based on his vast experience and was always clear on what

needs to be done. I have learnt a lot more than just the technical knowledge from

him.

I would like to extend my sincere gratitude to Akif Sinan Taşdelen and

Asst. Prof. Mehmet Yılmaz for their valuable time, technical mentoring and

support they have given me throughout my thesis work with a lot of patience

and care.

I would like to acknowledge and thank Prof. Dr. Abdullah Atalar for the

knowledge I have gathered as member of our research group, instructor of one

of my courses and for being part of the Jury for my defense.

This work would not have been completed without Kerem Enhoş who

initiated this work to whom a lot of thanks and appreciation go. The support

from my colleagues and other members of our research group was very

significant and cannot go without acknowledging Asst. Prof. Itir Köymen, Dr.

Fikret Yıldız, Giray İlhan, Murat Güngen, Abdulmalik Madigawa, Abdallah

Alkilani, Yasin Kumru and Doğu Kaan Özyiğit.

Finally, I would like to deeply thank my wife, Ronak for being here with me

away from home to give me the support I needed to keep going with the work. I

would also like to thank my father, Abubakar and mother, Masad for always

giving me wise words and advices whenever I needed them. I hope this work

will be the beginning of more to come and positively affect all those close to me.

vii

Contents

1 Introduction 1

2 The Design of the CMUT Array 3

2.1 Background on CMUT Cell and Array Designing ………………. 3

2.2 Designing of the CMUT and the Array……………………………7

2.2.1 CMUT cell radius, the pitch and the Rayleigh distance …….7

2.2.2 CMUT cell thickness membrane ……………………….…...9

2.2.3 Cell gap height, insulator thickness and collapse voltage …10

3 Simulation Results 12

3.1 Harmonic Balance Analysis ……………………………………..13

3.2 Admittance Simulations …………………………………………16

3.3 Transient response ………………………………………………18

3.4 Tone Burst Signal transmission …………………..……………...22

3.5 Pulse Width Modulation Transmission ………………………… 25

3.6 Radiation Pattern Simulation ……...……………………………. 28

3.7 Beamforming and Pressure field Patterns…………..…………... 29

4 Fabrication 32

4.1 Mask Design....…………………………………………………. 32

4.2 Cavity Etching …………………………………………………...33

4.3 Bottom Electrode Deposition ……………………………………35

viii

4.4 Wafer Bonding Process …………………………………………37

4.5 Flip-Chip Bonding and Ground Wire Bonding ………….………40

4.6 Parylene C Coating and Epoxy Coating ………...……………… 41

4.7 Vertical PCBs Mounting and Casing of the Device ……………. 42

5 Measurements and Transmission 44

5.1 Fabrication Yield Test …………………………………..………44

5.2 Resonance Frequency Shift…………………………………...…46

5.3 Impedance Measurement……………………………………….46

5.3.1 Loss tangents and parallel effective dielectric losses……...49

5.3.2 Measurement and simulation comparison ………………...51

5.4 Individual Cells Transmission……..…………………………...52

5.4.1 Individual cells transmission and simulation comparison…57

5.5 Plane Wave Transmission……………………………………….58

5.5.1 Plane wave transmission and simulation comparison……..60

6 Conclusion 61

A More Simulation Results 68

B More Impedance Analysis Results 79

C The Loss Tangents 84

D Transmission Oscilloscope Screenshots 87

D.1 Single CMUT Transmission Results ………………………………87

D.2 CMUT Array Transmission Results …………………………….....91

E CMUT Array and Pads Layout 98

F Hydrophone and Pre-Amp Calibration 100

ix

List of Figures

2.1 The cross-section of a depressed membrane CMUT with geometrical

illustration…………………………………………………….……………..3

2.2 The equivalent circuit of transmit CMUT in large signal. ............................ 4

2.3 The CMUT array equivalent circuit with the impedance matrix, Z…..……….6

3.1 The location of the cells to be analyzed………………………….………...12

3.2 Simulation flow diagram………………………………………………… . .13

3.3 Pressure output frequency response between 1 MHz – 20 MHz with 150 VPP

and unbiased……………………………………………………………………………..14

3.4 Particle velocity frequency domain analysis between 1 MHz – 20 MHz at 150

VPP…..…………………………………………………………………………………….15

3.5 Admittance values at 10 VPP………………………………………………………...16

3.6 Admittance values at 150 VPP……………………………………………………..17

3.7 Admittance response to change in input voltage at 7.5………………………….17

3.8 Transient analysis at 10 VPP and 3.75 MHz input signal……………………18

3.9 Transient analysis at 150 VPP and 3.75 MHz input………………………………19

3.10 Normalized steady state membrane displacement at 7.5 MHz with 150 VPP

input voltage at 3.75 MHz………………………………….…………………………19

3.11 The steady state normalized membrane displacement at 298.4 VPP and 3.75

MHz input signal……………………………………………………………………….20

3.12 The steady state output pressure from each CMUT cell excited at MVM,

298.4 VPP and at resonance frequency…………………………………….21

x

3.13 Particle velocity profile observed with one cycle signal of 300 VPP and 3.75

MHz ............................................................................................................ 21

3.14 Transient response of 5-Cycle Gaussian-enveloped tone burst signal of 10

VPP at 3.75 MHz……………………………………………………………………….22

3.15 Transient response of 5-Cycle Gaussian-enveloped tone burst signal of 150

VPP at 3.75 MHz……………………………………………………………23

3.16 Transient response of 5-Cycle Gaussian-enveloped tone burst signal of 372.3

VPP at 3.75 MHz……………………………………………………………23

3.17 Normalized membrane displacement analyzed at 372.3 VPP of 5-cycle

Gaussian-enveloped tone burst signal…………………………………….…24

3.18 Particle velocity profile analysis at 372.3 VPP of 5-cycle Gaussian-enveloped

tone burst signal. Peak observed at 1.362 m/s…………………………….. 24

3.19 The CMUT array as designed in the k-wave space……...…………………25

3.20 Simulation space created in k-wave with the transducer located at X=0 and

the sensor voxels as well as PML voxels placed radial to the transducer at

15.744 mm…………………………………………………………………26

3.21 Radiation Patterns formed in k-wave Vs. the computational radiation pattern

……………………………………………………………………………..27

3.22 The beamforming at points of interest, 0o, 15o, 30o, and 60o from the normal

along with the plane wave transmission…………………………………..29

3.23 Power of Individual CMUTs analyzed from the corner, middle and side

Elements………………………………………………………………….30

3.24 Power radiated into the medium by the Transducer………………………30

4.1 The full CMUT transducer mask with wires and electrical pads………..…33

xi

4.2 The Microscope images of the completely etched Epoxy wafer……………...34

4.3 Image from Microscope after deposition of TiPtAu stack……………….....36

4.4 Before (left) and after (right) Alumina etching……………………………..36

4.5 The microscope image of the CMUT cell right before wafer bonding……..37

4.6 The full wafer after modified bonding recipe (left) and damaged electrical

pads after wafer bonding (right)…………………………………………..37

4.7 Metal deposition on the top membrane with the shadow mask (left) and the

wafer after complete removal of Silicon layer on electrical pads (right)…....38

4.8 Photoresist stripped, and the chrome etched………………………………….39

4.9 A finalized single diced CMUT transducer chip……………………………………39

4.10 Flip-chip bonded Chip/PCB pair and ground pads connected………………….40

4.11 Parylene C coated Chip/PCB pair on the transmitting side (left) and Epoxy

coated on the backside (right)………………………………………………………...41

4.12 Epoxy coated at the rim of the Chip/PCB bond at the transmitting side……..42

4.13 Mounting of the vertical PCBs socketing on the connectors………………...43

4.14 Casing of the finalized transducer with connection coaxial cables…………. 43

5.1 Transducer array with labeled 48 functional cells; 26-Green colored cells from

first group, 18-purple colored cells from second group and 4-red colored

cells from third group…………………………………………………………...46

5.2 Impedance measurement setup with Impedance Analyzer and probestation...46

5.3 Admittance of the 255th cell excited at 1 VPP AC and 40 VDC bias in air……47

5.4 Admittance of the 227th cell excited at 1 VPP AC and 40 VDC bias in air...….47

5.5 Admittance of the 208th cell excited at 1 VPP AC and 40 VDC bias in air……48

xii

5.6 The modified large signal equivalent circuit……………….………………50

5.7 The block diagram of the SR844 Lock-in Amplifier……...……...…………..53

5.8 The inside of the DSP of the SR844 Lock-in Amplifier………….…………..54

5.9 Visualized signal (left) and the X and Y values from the amplifier (right)…..54

5.10 Transducer array with labeled 48 functional cells; 26-Green colored cells

from first group and 22-purple colored cells from second group …..........…55

5.11 The setup for transmission measurements with signal generator and reception

with hydrophone……………………………………………………………56

5.12 The transducer array with functional connected cells colored in red………58

5.13 Pressure emitted by 48 functional cells of the transducer with a frequency

sweep from 1 MHz to 12 MHz……………………………………………...59

A.1 Admittance for 15 Vac input with a frequency sweep 2.5 MHz – 20 MHz…68



A.4 Transient analysis with a continuous PWM signal of 10 VPP at 3.75 MHz…69

A.5 Transient analysis with a continuous PWM signal of 150 VPP at 3.75 MHz..70

A.6 Transient analysis with a continuous PWM signal of 262 VPP (MVM) at

3.75 MHz…………………………………………………………………..70

A.7 Transient analysis of a half-cycle PWM signal of 5 Vac at 3.75 MHz …...71

A.8 Transient analysis of a one-cycle PWM signal of 5 Vac at 3.75 MHz ..….71

A.9 Transient analysis of a four-cycle PWM signal of 5 Vac at 3.75 MHz .......72

A.10 Normalized membrane displacement of a half-cycle PWM signal input

of 5 Vac at 3.75 MHz …………………………………………………....72

xiii

A.11 Normalized membrane displacement of a one-cycle PWM signal input

of 5 Vac at 3.75 MHz ……..…………………………………………..…73

A.12 Normalized membrane displacement of a four-cycle PWM signal input of

5 Vac at 3.75 MHz ………...………….………………………………...73

A.13 Normalized membrane displacement analysis with a continuous PWM

signal of 10 VPP at 3.75 MHz ……………………………………………74


signal of 150 VPP at 3.75 MHz…………...………………………………74


signal of 262 VPP (MVM) at 3.75 MHz ...…………………………….....75

A.16 Particle velocity of a half-cycle PWM signal input of 75 Vac at 3.75

MHz………………………………………………………………………75

A.17 Particle velocity of a four-cycle PWM signal input of 75 Vac at 3.75

MHz………………………………………………………………………76

A.18 Particle velocity of a half-cycle PWM signal input of MVM at 3.75

MHz………………………………………………………………………76

A.19 Particle velocity of a four-cycle PWM signal input of MVM at 3.75

MHz……………………………………………………………………....77

A.20 Field Pattern of Plane wave transmission at 7.5 MHz and at 15.744 mm..77

A.21 Field Pattern of 0o focused transmission at 7.5 MHz and at 15.744 mm...78

A.22 Field Pattern of 30o focused transmission at 7.5 MHz and at 15.744 mm.78

B.1 Admittance of 132nd element………………………………………………79

B.2 Admittance of 145th element……………………………………………….79

B.3 Admittance of 2nd element…………………………………………………80

B.4 Admittance of 4th element……………………………………………….....80

B.5 Admittance of 7th element………………………………………………….81

xiv

B.6 Admittance of 223rd element……………………………………………….81




C.1 Loss tangents of the cells in group 2 changing with frequency……………84

C.2 Loss tangents of the cells in groups 1, 2 and 3 colored in green, yellow

and red respectively changing with frequency ………………………….…84

C.3 Loss tangents of the cells in groups 1 colored in green changing with

Frequency…………………………………………………………………..85

C.4 Loss tangents of the cells in groups 1, 2 and 3 colored in green, yellow

and red respectively changing with frequency………………………………85

C.5 Loss tangents of the cells in groups 1 and 3 colored in green and red

respectively changing with frequency……………………………………….86

D.1 Received voltage signal at 8.94 MHz from the cell 206 through

Lock-in Amplifier with a 1000-cycle tone burst input of 10 VPP at 4.47

MHz……………………………………………………………………….87

D.2 Received voltage signal at 8.94 MHz from the cell 198 through

Lock-in Amplifier with a 1000-cycle tone burst input of 10 VPP at 4.47

MHz……………………………………………………………………….87

D.3 Received voltage signal at 8.94 MHz from the cell 61 through Lock-in

Amplifier with a 1000-cycle tone burst input of 10 VPP at 4.47 MHz…...88





xv


Amplifier with a 1000-cycle tone burst input of 10 VPP at 4.47 MHz……89



D.8 Received voltage signal at 1 MHz from the transducer through Lock-in

Amplifier with a 1000-cycle tone burst input of 10 VPP at 0.5 MHz…….91


Amplifier with a 1000-cycle tone burst input of 10 VPP at 1 MHz………91




Amplifier with a 1000-cycle tone burst input of 10 VPP at 2 MHz……..92


Amplifier with a 1000-cycle tone burst input of 10 VPP at 2.5 MHz…..93




Amplifier with a 1000-cycle tone burst input of 10 VPP at 3.5 MHz…..94

D.15 Received voltage signal at 7.5 MHz from the transducer through Lock-in

Amplifier with a 1000-cycle tone burst input of 10 VPP at 3.75 MHz…94


Amplifier with a 1000-cycle tone burst input of 10 VPP at 4 MHz…….95

D.17 Received voltage signal at 8.94 MHz from the transducer through Lock-in

Amplifier with a 1000-cycle tone burst input of 10 VPP at 4.47 MHz….95



xvi





E.1 The CMUT Array and CMUT elements layout…………………………...98

E.2 The electrical pads configuration on the Pyrex wafer……………………..98

E.3 The electrical pads configuration on the PCB……………………………..99

E.4 The electrical pads configuration on the Vertical connecting PCBs………99

F.1 ONDA Hydrophone sensitivity calibration sensitivity certificate ….……..100

F.2 ONDA Pre-Amplifier gain calibration certificate………………………...101

xvii

List of Tables

2.1 The lumped element parameters of large signal equivalent CMUT circuit

Model…………………………………………………………………………………………5

2.2 The Parameters and specs of the designed 7.5 MHz centered CMUT…………11

3.1 Power and Intensity of the transducer transmitting at 150 VPP and 7.5 MH…31

5.1 Summary of the parallel effective resistors and capacitors for cells 255th, 227th

and 208th……………………………………………………………………...50

5.2 Summary of the received voltage and pressure signals of the transducer at

difference frequencies …………………………………………………………………..58

xviii

1

Chapter 1

Introduction

Capacitive Micromachined Ultrasonic Transducers (CMUTs) have ever

been advancing and broadening their application spectrum since first introduced

in 1994 [1][2] from sensing to medical imaging to gaming industries. The

advancements of the fabrication process and techniques have further expediated

their making and applications in arrays as they are commonly used now [3]. The

work that went in on this thesis focused on using the CMUT array for medical

imaging purposes. CMUTs are applicable in reception and transmission modes

and have been widely used for reception due to their high Signal to Noise Ratio,

SNR [2][3] as the noise is only introduced by the integrated electronics [2].

However, the transmission mode has also attracted scientist’s interests after

CMUT array’s evident incredible performance when designed correctly for high

power transmission[3][5], beamforming, increasing the Directivity and with a

wide bandwidth [3] while having it fit it in a desired package size. Under proper

optimization, multiple CMUT array chips can be relatively easily fabricated

from one full wafer with the Wafer bonding process and with flip chip bonding

process [4], CMUTs can be easily integrable with electronics hence making

them very compatible in various applications.

Motivation of the work in this thesis was to design a waterborne CMUT

array of small ka value which stands for radial wave number – less than 3 [4],

higher directivity and SNR, higher sensitivity and operating at 7.5 MHz. Having

such a CMUT would produce clearer images with larger depth of focus for

medical application but also with low excitation voltages and achieve larger

swing before collapsing [4]. In this thesis, single CMUT was designed then a 16

x 16 array according to the preferences, constraints and accessibility, the

fabrication process and later testing and application were done.

The design started with fixing the parameters such as the desired operating

frequency of 7.5 MHz and the phased array of size 16 x 16 since our Digital

Phased Array System (DiPhAS) has a total of 256 outlet channels for maximum

utilization. Then other parameters were determined initially by considering the

2

radiation pattern, and the Rayleigh distance and with the help of lumped-element

circuit for array parameters, the rest of the parameters were determined, and

performance was validated on k-wave MATLAB tool for Finite Element

Analysis (FEA) and Advanced Design System (ADS) for array performance.

The designed CMUT arrays were then fabricated using bottom up layer

deposition for the epoxy substrate cavity and wafer bonding technique with

silicon wafer as the membrane. The fabricated CMUTs were then diced and

integrated with the PCB using the flip-chip bonding process and sealed with

Parylene C. The fabrication process was finalized by the encasing the CMUT

chips with the PCBs in the water-isolating material for safe application in the

water environment. The whole process required a total of two photolithography

masks and one shadow mask with the self-alignment process.

The finalized devices were then put through electrical tests to identify the

functional cells. The impedance measurements were performed on the functional

cells to analyze the electrical conductivity of individual CMUTs, the resonance

frequencies of the cells and the charging up of the CMUT cells. The

compensation to center all the CMUTs around the operational frequency was

done and made ready for transmission.

The work was concluded with firstly the waterborne transmission from

individual cells at half the operating acoustic frequency and then plane wave

transmission from all the functional cells. Even though, the yield was lower than

required for optimum performance and imaging purpose, the half-frequency

driven transmitting waterborne array was achieved and found to behave as

intended for most part of the aspects providing the confidence that if the yield is

improved, then it will be perfectly applicable for volumetric medical imaging

purpose with all the desirable and intended characteristics.

This work was split in two parts, the first part involving designing,

simulations and fabrication which was initiated by Kerem Enhoş and the second

part of the work including electrical testing, packaging and characterization of

the transducer was completed by the author of this thesis.

3

Chapter 2

The Design of the CMUT Array

2.1 Background on CMUT Cell and Array

Designing

When designing CMUT, a few geometrical parameters have to be defined

depending on the preferences of performance individually and as an array. A

background on the geometry of a typical CMUT will be necessary to understand

the concepts in this part. For this purpose, figure 2.1 is attached below showing

the cross-section of a CMUT with description of the parameters.

D

e

s

i

g

n

Figure 2.1: The cross-section of a depressed membrane CMUT with geometrical

illustration [6]

As seen in figure 2.1 above, in the longitudinal cross-section of circular CMUT,

𝑎 is the radius of the CMUT, 𝑡𝑔 is the gap height, 𝑡𝑖 is the thickness of the

insulator and 𝑡𝑚 is the thickness of membrane. These physical geometries

constitute of the CMUT design which are decided on by considering the desired

features of the individual CMUT and CMUT array performance. The 𝑥(𝑟) is the

depression behavior of the membrane at a distance 𝑟 from the center of the

CMUT when operated at the first resonance frequency and can be defined using

plate theory as [7].

𝑥(𝑟, 𝑡) = 𝑥𝑝(𝑡) (1 −𝑟2

𝑎2) 𝑓𝑜𝑟 𝑟 ≤ 𝑎 (2.1)

4

Even though the membrane mechanical swing is nonlinear in nature, it is safe to

assume linearity if Xp, the peak displacement is less than 20% of membrane

thickness [4]

Designing CMUTs, like other MEMS devices, the FEA tools like COMSOL

are very useful and powerful to provide accurate response to a design as it

considers far more factors such as minute effects of thermoviscous acoustics loss

[8], effect of using more than one material for a membrane or plate, etc.

However, since the simulations take a lot of time and processing to generate

results, it is only convenient for a few cells array and not large size arrays as it

takes forever to simulate. Thanks to Prof. Koymen and Prof. Atalar with their

work in 2012 [7] which led to presentation of the CMUT cell with an electric

circuit making designing easier with electric circuit simulation tools which are

much faster and still accurate enough. The introduction of equivalent electric

circuit enabled designing of CMUT arrays while considering the spurious effects

and the radiation impedance which is the main difference and fundamental

concept when designing arrays [9]. The presentation of the equivalent circuit of

a CMUT in receiver mode is given in small signal while for the transmit CMUT

it is given in large signal as seen in figure 2.2 due to their mode of operation

respectively.

F

i

g

u

r

e

2.2: The equivalent circuit of transmit CMUT in large signal [7]

Each of the circuit parameters in rms, average and peak expressed in terms of

CMUT physical geometry and properties were defined as seen in the table 2.1

below

5

Table 2.1: The lumped element parameters of large signal equivalent CMUT

circuit model [7]

However, when talking of array designing, a very important and fundamental

difference we see from a single CMUT design in the radiation impedance

parameter denoted as ZR. The total radiation impedance experienced by any

operational CMUT becomes a contribution of each cell in the array

configuration which is determined by the position of the adjacent and diagonal

cells from reference, termed as Mutual impedance and the self-impedance,

which is the impedance by the reference CMUT cell. Therefore, the total

impedance by a single CMUT can be calculated as follows [10].

𝑍𝑖 = 𝑍𝑖𝑖 + ∑𝑣𝑗

𝑣𝑖𝑍𝑖𝑗

𝑁𝑖=1, 𝑖≠𝑗 (2.2)

Where 𝑍𝑖 is the Total impedance experienced by a reference cell, 𝑍𝑖𝑖 is the

mutual impedance, N is the total number of CMUT in the array and 𝑍𝑖𝑗 is the

mutual impedance.

6

Therefore, when dealing with arrays, the total impedance experienced by each

cell is calculated and put in a matrix called the Impedance matrix [6] as seen

below.

Once the Impedance matrix above has been created, the force provided by each

CMUT cell can be calculated by matrix multiplication of the impedance matrix

by the particle velocity on the membrane of each cell. The equivalent circuit of

the CMUT array would then be in such a scheme as seen in figure 2.3 below.

Figure 2.3: The CMUT array equivalent circuit with the impedance matrix, Z [6]

The equivalent circuit in figure 2.3 shows that the cells are connected to the

same impedance matrix from left to right and below, the cell blocks 1, 2, to N

are the equivalent CMUT circuits for the CMUT cell and in the case of

transmitter, then the large signal equivalent circuits are used. The

𝑉1(𝑡), 𝑉2(𝑡) 𝑡𝑜 𝑉𝑁(𝑡) are the total input voltages to each CMUT, that is the ac

and DC voltages. The 𝑓𝐼 are the forces due to incident acoustics signals and the

𝐹𝑏 are the incident static forces such as forces due to atmospheric pressure.

7

2.2 Designing of the CMUT and the Array

Designing of this transmitting CMUT was done in two main steps and

simultaneously: the geometrical design of the CMUT and then the Array

geometry while keeping in mind the considerations and constraints of the

performance and design. The preferences and constraints for designed CMUT

were as follows; the CMUT was preferred to have the operation frequency at 7.5

MHz in water for optimum operation in medical applications and ka value of

less than 𝜋 [4] to eliminated other modes of operation. These preferences were

also constrained within the limitations of the DiPhAS we have in our lab which

could transmit at center frequencies of 1 – 20 MHz with a total of 256 channels.

The DiPhAS used has the transmit voltage limit of 150 VPP with transmit pulse

of standard gaussian enveloped tone burst customized with frequency, cycle

count and polarity.

Therefore, for a 2D array design, to maximize the utilization of the channels,

a 16 x 16 array was decided on for reasons of generating higher directivity but

also for reasons of having symmetry in elevation and azimuth planes. The design

was made to have ka value of less than 3 for another reason of having a smaller

radius to compensate for the static pressure effect in predepression of the

membrane for sensitivity improvement [7].

2.2.1 CMUT cell radius, the pitch and the Rayleigh

distance

The CMUT parameters were determined while considering the design

preferences, DiPhAS limitations and fabrication limitations. The hierarchy of the

designing procedure will be briefly outlined here.

(a) The radiation pattern and Rayleigh distance criteria

The very first consideration was the directivity of the cell and the entire

array which are dependent on the ka and the pitch, d respectively.

8

Determining these values, will tell us about the radius of the cell and the

center-to-center distance between successive cells as we already know our

operating frequency.

Another very important parameter considered while designing was the

Rayleigh distance – the maximum beamformable region which is supposed

to be large but still maintaining low sidelobes by keeping the kd value low.

The Rayleigh distance, 𝑅0 =𝑆

𝜆 being a function of ka and kd, was

maximized while keeping the pitch, d lower than 187.5 𝜇𝑚 as the limit to

avoid sidelobes in the radiation pattern found as seen below [4];

𝑑 <𝑁 − 1

𝑁𝜆 => 𝑑 <

15

16(2 𝑥 10−4) => 𝑑 < 187.5 𝜇𝑚

The above inequality determines the maximum pitch to avoid the sidelobes

in an array configuration as a function of N, the number of the columns in

the array and wavelength. These three features, the cell directivity, the array

directivity and the Rayleigh distance all as functions of either a, 𝑑, ka or and

kd were simultaneously solved on MATLAB with below equations to find

the optimum Rayleigh distance, radius and the pitch of the array.

𝑅0 =𝑆

𝜆 (2.3)

𝐷𝑝(𝜃, 𝑘𝑎) = 48𝐽3(𝑘𝑎𝑠𝑖𝑛(𝜃))

(𝑘𝑎𝑠𝑖𝑛(𝜃))3 (2.4)

𝐷𝑎(𝜃) =1

𝑛

𝑠𝑖𝑛 (𝑛𝜋𝑑

𝜆 𝑠𝑖𝑛(𝜃))

𝑠𝑖𝑛 (𝜋𝑑

𝜆 𝑠𝑖𝑛(𝜃))

(2.5)

The equation (2.3) [11] is the Rayleigh distance equation as the function of

surface area of the array and the wavelength of the signal. The equation

(2.4) [11] is the radiation pattern of a cell as a third-order Bessel’s function

of angle of interest from the center of reference cell and the ka value. The

equation (2.5) [11] is the radiation pattern of the array as the function of the

pitch and the angle of interest from center of the array. Then the radiation

pattern of the array as a function of radius and pitch is given in the equation

(2.6) [11]

𝐷(𝜃) = 𝐷𝑎(𝜃) 𝑥 𝐷𝑝(𝜃, 𝑘𝑎) (2.6)

9

(b) Wiring and fabrication criterion

While deciding on the pitch, the fanning out of the electrical connections

from the CMUTs to the electrical pads on the chip was considered to ensure

there was enough space for all the required connections to be made. Since

the array was a total of 256 cells, the outermost cells were a total of 60 cells

while the inner cells were a total of 196 meaning there were 196 wires to be

passed through 60 outer openings to the connection pads. Arbitrarily, it was

decided that 16 openings accommodate four wires while 44 openings

accommodate three wires to accommodate all the 196 wires. Considering

the fabrication limitations in our UNAM facility, the minimum wire width

was 3 𝜇𝑚 while the minimum spacing between wires was also 3 𝜇𝑚 .

Therefore, the minimum safe pitch was to be 36 𝜇𝑚.

With all the considerations discussed above, the MATLAB script to utilize

the above equations and constraints was created to calculate the optimum radius,

pitch and Rayleigh distance. The calculations yielded a radius of 78.61 𝜇𝑚,

pitch of 190.24 𝜇𝑚 and with the Rayleigh distance of 45.3 mm which gives the

Sidelobe Level of -26.38 dB. However, for the fabrication purpose, the numbers

were lightly modified for ease and accuracy but still without harm to the design

where the new radius was concluded to be 80 𝜇𝑚, the pitch to be 192um which

produced the Sidelobe Level of -17.4 dB.

2.2.2 CMUT cell thickness membrane

Membrane thickness was the next cell parameter to be considered after

deciding on the optimum radius as by this point, we had all we need to

determine the membrane thickness of silicon as can be seen from the equation

(2.5) [12] below;

𝑓 =

1

√𝐿𝐴𝑚𝐶𝐴𝑚

2𝜋 =

𝑡𝑚

𝑎2

√80

9

𝑌0𝜌𝑚(1−𝜎2)

2𝜋 (2.7)

10

where the frequency is 7.5 MHz, the Young’s Modulus, 𝑌0 for silicon was used

as 149 GPa, the density of silicon, 𝜌𝑚 was used as 2370 kg/m3, the Poisson’s

ration, 𝜎2 was used as 0.17.

The above equation yielded the membrane thickness of 12.572 𝜇𝑚, however,

this figure was slightly modified to achieve the preferred resonance frequency

with the array configuration which was found to be 15 𝜇𝑚.

2.2.3 CMUT cell gap height, insulator thickness and

Collapse voltage

To finalize our design, the gap height and the insulator thickness were left to

be decided on, and to approach this, the breakdown voltage of Alumina which

was used as the insulator was 620kV/mm according to our records in previous

work. Since the DiPhAS is limited to 150 V Peak to Peak and considering the

safety region, the dielectric breakdown voltage was set at 195 V which means

the alumina thickness should be 300 nm.

The effective gap height was to be established next after sorting the

insulator thickness. To figure out the gap height, the collapse voltage at vacuum

was fixed to be 100 V and the effective gap height was calculated with the

equation (2.8) [7]

𝑉𝑟 = 8𝑡𝑚

32

𝑎2 𝑡𝑔𝑒

2

3 √𝑌0

27 0(1−𝜎2) (2.8)

Using 100 V for the collapse voltage at vacuum, 𝑉𝑟 in equation (2.8), the 𝑡𝑔𝑒 was

found to be 171.2 nm and the gap height was calculated using the equation (2.9)

[7] below to be 137.87 nm.

𝑡𝑔 = 𝑡𝑔𝑒 −𝑡𝑖

𝑟 (2.9)

Where 휀𝑟 for alumina was used as 9.

11

To calculate the collapse voltage, 𝑉𝑐, we need to calculate the 𝐹𝑏/𝐹𝑔 term

which stands for the normalized static depression due to the static pressure such

as ambient pressure, 𝑃0 which is taken as 1 atm for a typical shallow depth

waterborne design. The 𝐹𝑏/𝐹𝑔 was calculated using the equation (2.8) [7] and

found to be 0.0149 which suits our design by being very close to zero so as to

compensate the static depression as much as possible and leaving more gap for

the dynamic depression.

𝐹𝑏

𝐹𝑔=

3 𝑎4 𝑃0 (1−𝜎2)

16 𝑡𝑔𝑒 𝑌0 𝑡𝑚3 (2.10)

The Collapse voltage was then calculated using the equation (2.11) [7] to be

98.48 V

𝑉𝑐

𝑉𝑟= 0.9961 − 1.0468

𝐹𝑏

𝐹𝑔+ 0.06972 (

𝐹𝑏

𝐹𝑔− 0.25)

2

+ 0.01148 (𝐹𝑏

𝐹𝑔)

6

(2.11)

where 𝑉𝑟 is the collapse voltage at vacuum set as 100 V as seen in an earlier

paragraph.

Table 2.2: The Parameters and specs of the designed 7.5 MHz centered CMUT

Parameter Description Value

f Resonance frequency (MHz) 7.5

a Plate radius (µm) 80

d Element pitch (µm) 192

SLL Sidelobe level (dB) -17.4

R0 Rayleigh Distance (mm) 46.2

tm Plate thickness (µm) 15

tge Effective gap height (nm) 171.2

ti Insulator thickness (nm) 300

tg Gap height (nm) 137.8

Fpb/Fpg Normalized exerted pressure 0.015 @SAP

12

Chapter 3

Simulations Results

The simulations done for this work were mainly done in Advanced Design

Systems (ADS) and k-wave where the specification and limitations were

according to DiPhAS such as the clock rate, voltage levels and other

specifications. In ADS, the transient analyses were done with the clock rate of

480 MHz which is the clock rate for transmission in DiPhAS, therefore the time

domain resolution was set to 2.083 ns. The excitation voltages used for the

simulations were 10 VPP and 150 VPP to assess the difference in response and all

the time at half the frequency of the desired acoustic frequency. Considering the

fabrication limitations, for the k-wave simulations, the grid sizes were set as 32

µm while the gap height was set to 138 nm. As in [13], the Rayleigh Bloch

waves have an effect to the array since the array size is much larger than the

operating wavelength therefore this phenomenon was also considered in the

simulations [14].

For simulation resulting purposes, three cells were considered in the array

to reflect the major operational differences between the cells of different

regions as presenting results of all 256 cells would be impossible. Therefore,

one cell from outermost corner cells, one cell from center and one cell from the

side were considered for analysis. The operational differences were mostly due

to the mutual radiation impedance and effect of the rigid baffle around the cells

[11].

Figure 3.1: The location of the cells to be analyzed

13

The cells’ responses from the electrical input such as Harmonic balance

and Transient analysis were done in ADS and the responses were then fed into

the k-wave to analyze the focused transmission, beam forming and radiation

pattern. The workflow shown in figure 3.2 better illustrates the work going

into the simulation section.

Figure 3.2: Simulation flow diagram

3.1 Harmonic Balance Analysis

This simulation was done to analyze the frequency response of the CMUTs

in the array in an unbiased mode of operation with excitation of 75 Vac. The

analysis was done from the three cells mentioned above even though all the cells

in the array were all excited in phase. The use of lumped element model enabled

us to perform this analysis on ADS which allowed us to simulate the non-linear

array device without having to use nonlinear simulation tools which would take

a long time.

The output pressure of the CMUT array was observed with a second

harmonic in the range of 1 MHz to 15 MHz with a step size of 50 kHz and

according to these results, the peak pressure of 240.6 dB re 1µPa was observed

at 6.36 MHz while at 7.5 MHz, a pressure of 232.8 dB re 1µPa was observed as

seen in figure 3.3. The operation frequency was decided not to be at peak

because of the narrow bandwidth and undesired transient response at this

frequency, therefore it was decided to be at 7.5 MHz – at the right of the peak

frequency.

14

Figure 3.3: Pressure output frequency response between 1 MHz – 20 MHz

with 150 VPP and unbiased

The presence of Rayleigh Bloch waves on the surface of the transducer,

introduce the difference in the velocities and hence pressure levels produced by

individual cells [14][15] and this is due to the difference in radiation impedance

observed by individual cells in the array. Therefore, the cells in the array each

produced a slightly different pressure level as observed in the 1st cell where the

pressure emitted was 440 kPa, 120th cell was 420 kPa while the 128th cell was

435 kPa all with excitation of 150 VPP. The maximum pressure difference at the

operation frequency was calculated to be 0.7 dB re 1µPa. The spurious

resonances due to the Rayleigh Bloch waves were visible with this analysis

method, unlike in the biased mode it was evident that these resonances were at

the right side of the operation frequency at around 11.5 MHz.

The frequency response of the particle velocity of the cells’ membranes was

also analyzed between 1 MHz to 15 MHz at 150 VPP and the same peak

frequency was observed. The difference in particle velocity between the cells in

the array was however found to be more significant than the pressure levels

between the cells which mostly explains the difference in power between the

CMUT cells.

15

Figure 3.4: Particle velocity frequency domain analysis between 1 MHz – 20

MHz at 150 VPP

The difference between the particle velocities of cells at the peak frequency

of 6.36 MHz was higher than the resonance frequency of 7.5 MHz just like in

Pressure emitted. The particle velocities of cells at 6.36 MHz varied between 0.4

m/s to 0.8 m/s while at 7.5 MHz particle velocities varied between 1.11 m/s to

0.22 m/s with outer cells having the higher velocities than the inner cells.

The simulations for operating the array with a biasing voltage for the

purpose of comparing the biased and unbiased operations were done. The VDC

was calculated as 52.5 VDC, the calculated value enough to pre-deflect the

membrane as much as ambient pressure deflects the unbiased cell membrane

while the ac voltage to produce the same power was found to be 28.8 Vac. The

harmonic balance simulations for the three dedicated cells were repeated for

biased mode of operation and the results were exactly the same when plotted on

top of the unbiased mode of operation frequency response results

16

3.2 Admittance Simulations

Conductance and Susceptance are another important characterizing entity

telling how conductive the CMUT is at a specific frequency and voltage level.

The conductance analysis also tells us more about the resonant frequency and

charging up of the CMUT cell [16][4]. The conductance and susceptance are

the reciprocal of the impedance and are calculated as follows [4];

𝐺 = 𝑅 𝐼𝑖𝑛𝑝𝑢𝑡

𝑉𝑖𝑛𝑝𝑢𝑡 (3.1)

𝐵 = 𝐼 𝐼𝑖𝑛𝑝𝑢𝑡

𝑉𝑖𝑛𝑝𝑢𝑡 (3.2)

Operating a CMUT in an unbiased mode, introduces more nonlinearity

nature which leads to changing of admittance values with change in input

voltage. Therefore, the analysis was done with a voltage sweep from 10 VPP –

150 VPP with 20 and 40 VPP step size as seen on figure 3.7 and Appendix A.

Figure 3.5: Admittance values at 10 VPP

17

Figure 3.6: Admittance values at 150 VPP

More admittance plots for this section are attached in the Appendix A

which were all used to plot the admittance response to the change in the input

voltage at 7.5 MHz with unbiased mode of operation as shown in the next plot.

Figure 3.7: Admittance response to change in input voltage at 7.5 MHz

18

3.3 Transient response

Transient analysis with a sinusoidal signal was done for the three dedicated

cells to analyze the magnitude and behavior of pressure transmitted and well as

the membrane displacement behavior at acoustic resonance frequency, 7.5 MHz

and electrical resonance of 3.75 MHz with two different voltage levels, 10 VPP

and 150 VPP. These excitation voltages were considered for simulation as they are

the minimum and maximum output voltages of DiPhAS, and the transmission

length was specified at 4 𝜇𝑠 which is the maximum transmit phase length for

DiPhAS [4].

Figure 3.8: Transient analysis at 10 VPP and 3.75 MHz input signal

As the above plot shows, three cells were simulated with voltage input of 10

VPP at 3.5 MHz and the pressure output of three cells were observed at 7.5 MHz

almost the same seen with different color codes peaking at 1933 Pa at steady

state.

19

Figure 3.9: Transient analysis at 150 VPP and 3.75 MHz input signal

After observing the individual CMUT output pressure, the steady state

membrane displacement was observed with 75 VPP sinusoidal input signal and the

normalized steady state membrane displacement was found to be 0.1072.

Figure 3.10: Normalized steady state membrane displacement at 7.5 MHz with

150 VPP input voltage at 3.75 MHz

20

Since with the unbiased mode of operation the entire gap height was

available for maximum swing gap, this meant that the maximum normalized

membrane displacement is 0.806 as seen in the calculation below, the

displacement achieved by 150 VPP was nowhere near maximum displacement.

max 𝑛𝑜𝑟𝑚𝑎𝑙𝑖𝑧𝑒𝑑 𝑑𝑖𝑠𝑝𝑙𝑎𝑐𝑒𝑚𝑒𝑛𝑡 =𝑡𝑔

𝑡𝑔𝑒= 0.806 (3.3)

To achieve the maximum allowable displacement, which is the gap height,

the CMUT must be excited with the MVM [17][4] voltage which is 300 VPP

calculated in chapter 2. Driving the CMUT cell beyond this voltage, the CMUT

membrane will be touching the substrate which would increase the backing loss

so better to be avoided [4].

Figure 3.11: The steady state normalized membrane displacement at 298.4

VPP and 3.75 MHz input signal

The maximum normalized membrane displacement was found to be

achieved only in the first cycle of the input while it dropped to about 0.4 for the

following cycles. To maintain the maximum displacement, it would take giving

a ramp input at 376 VPP but then that would allow a tapping motion of the

membrane - bouncing back from the substrate periodically which is not desired.

21

Figure 3.12: The steady state output pressure from each CMUT cell excited at

MVM, 298.4 VPP and at resonance frequency

The pressure emitted by each CMUT cell was observed for when the

CMUT cells were sinusoidally excited with at the MVM, 300 VPP at the

resonance frequency, 3.75 MHz and the steady state maximum pressure was

observed at 1.673 MPa with 7.5 MHz acoustic signal as seen in the figure 3.12.

The particle velocity was also analyzed from the three dedicated cells at the

MVM with only one cycle of a sinusoidal signal of 3.75 MHz and the velocity

profile observed was of 2 cycles at 7.5 MHz with peak velocity of 1.231 m/s.

Figure 3.13: Particle velocity profile observed with one cycle signal of 300 VPP

and 3.75 MHz

22

3.4 Tone Burst Signal Transmission

Transmission with ultrasonic transducers for biomedical imaging is mostly

done a with tone burst signal [18] and for that reason it was necessary to perform

simulations of transmission with a gaussian-enveloped tone burst signal of a few

cycles to inspect the array response and behavior when transmitted with

different voltages. The tone burst response was analyzed at three different

voltage levels, 10 VPP, 150 VPP and 372.3 VPP (MVM), for Pressure emitted,

particle velocity and membrane displacement.

The gaussian-enveloped sinusoidal tone burst signal of 5 cycles with

amplitude of 10 VPP was generated at 3.75 MHz and from the three cells

analyzed the transient response is shown below with an acoustic signal of 10

cycles resulting twice the input frequency, 7.5 MHz and peak pressure of 2255

Pa.

Figure 3.14: Transient response of 5-Cycle Gaussian-enveloped tone burst signal

of 10 VPP at 3.75 MHz

Then the gaussian-enveloped sinusoidal tone burst signal of 5 cycles with

amplitude of 150 VPP was generated at 3.75 MHz and a similar response was

recorded with an increased output peak pressure 2.3 kPa with 10 VPP to 498.1

kPa.

23


of 150 VPP at 3.75 MHz

Finally, the gaussian-enveloped sinusoidal tone burst signal of 5 cycles with

amplitude of 372.3 VPP which is the MVM was generated at 3.75 MHz and peak

output pressure was observed to have marginally increased to 2.764 MPa. The

normalized membrane displacement was also analyzed for this input voltage and

noted to be 0.8 very close to the maximum achievable displacement and similar

to the one found in chapter 3.4. The particle velocity at MVM was analyzed as

well and found to peak at 1.362 m/s.


of 372.3 VPP at 3.75 MHz

24

Figure 3.17: Normalized membrane displacement analyzed at 372.3 VPP of 5-

cycle Gaussian-enveloped tone burst signal

Figure 3.18: Particle velocity profile analysis at 372.3 VPP of 5-cycle Gaussian-

enveloped tone burst signal. Peak observed at 1.362 m/s

25

3.5 Radiation Pattern Simulations

Radiation pattern is a very important property of a transmitter as it describes

the pattern of the pressure radiated into the medium and was among the starting

point in the designing of this transducer array. So far, we have been analyzing

performances of individual cells in the array which even though were affected

by other cells in the array but could not tell us much on the array’s performance.

The radiation pattern simulation tells us how the transducer radiates in the

medium and this had to be done on a FEM simulation tool which in this case, the

k-wave MATLAB toolbox was used [19]. For the k-wave simulation, a very

accurate environment had to be created in a k-wave workspace for the accuracy

of the simulation results; that included designing the CMUT array, the medium,

the receiver and the simulation space. However, since FEM simulations usually

take long, an optimization was to be found between the accuracy and simulation

time [20].

The CMUTs was designed using the grid points (voxel) of size 160 𝜇𝑚 x

160 𝜇𝑚. This voxel dimension was chosen based on the diameter of the CMUT

which is 160 𝜇𝑚 and in each voxel there were smaller 5x5 cube grids of 32 𝜇𝑚

x 32 𝜇𝑚 . Having smaller grids improves the accuracy and this 32 𝜇𝑚 was

chosen from the minimum feature size in the array which was the spacing

between successive CMUT cells. The transducer was then a 16 x 16 of the 160

𝜇𝑚 x 160 𝜇𝑚 voxels.

Figure 3.19: The CMUT array as designed in the k-wave space [4]

26

The radius of the 3-D simulation space was then designed based on the

Rayleigh distance in such a way the simulation space should be smaller than the

Rayleigh distance to make sure the sensors were placed in the near field. Since

each sensor cell was one voxel defined above, the enough sensors to be placed

radially around the transducer at the Rayleigh distance were 1406 and to be sure

we were well within the beamformable region, 1024 sensors were used. The

symmetry of the Azimuth plane was used to reduce the simulation time, hence

only 512 sensor voxels (equal to 16.384 mm) were used to provide results of

1024 sensors with short time. 10 voxels from each plane were set as Perfectly

Matched Layers (PML) to avoid scattering of the acoustic waves from the

boundary due to impedance difference between layers of simulation and far field

[4]. The medium was set as water, by matching the properties such as density,

speed of sound, nonlinearity coefficient and attenuation.

The ultrasonic source was set as dipole since the exert force on the

surrounding medium causing about the physical membrane displacement.

Figure 3.20: Simulation space created in k-wave with the transducer located at

X=0 and the sensor voxels as well as PML voxels placed radial to the transducer

at 15.744 mm [4]

27

For transmission, the time-domain results obtained from 1 cycle of tone

burst sinusoidal excitation with 10 VPP at 3.75 MHz in ADS for the individual

cells were fed into the corresponding CMUT cells respectively for a volumetric

transmission analysis. The transmission was done with 1 ns step sizes for 32 𝜇𝑠

and in order to use 1 ns step sizes, the results from ADS had to be interpolated.

Two operational modes were simulated, as a piston membrane and as a clamped

plate which best emulates the CMUT operation. For the piston membrane, the

pressure distribution across the voxel was uniform while for the clamped plate,

the pressure distribution through the membrane was not uniform but obeyed

membrane deflection profile seen in chapter 2. The pressure values received

from each of the 81 sensors from each transmission mode were used to plot the

radiation patterns and compared with the computational radiation pattern with

MATLAB formed by using the maximum pressure values obtained directly from

the ADS simulation.

Figure 3.21: Radiation Patterns formed in k-wave Vs. the computational

radiation pattern (dB re 1Pa)

28

3.6 Beamforming and Focusing

Beamforming is another property of CMUT arrays which allows the user to

focus the pressure and a specific pressure and steer it as per the need.

Beamforming is created by introducing specific transmission time delays to

specific CMUT cells depending on the focus point location from the transmitting

array and the center of the transducer. Focusing the beam at four angle points

normal to the center of the transducer were considered; 0o, 15o, 30o, and 60o and

each channel’s respective time delays for all those angle points were calculated

using the equation (3.6) [21].

𝜏𝑛 = 𝑟 −√(𝑥𝑟−𝑥𝑛)2+𝑧𝑟

2

𝑐0+ 𝑡0 (3.4)

where 𝑟 is the distance from the center of the transducer to the point of focus, 𝑥𝑛

is the distance between the center of transducer to the center of CMUT of

interest and 𝑡0 being the fictitious value to avoid negative values or very large

delay values. The points of focus were only chosen to be in the elevation plane,

which is the plane for the sensors, the points in azimuth plane were not

considered to avoid complex computations.

After calculating the transmission delay times of each cell for a specific

point, the pressure outputs obtained from 1 cycle of tone burst sinusoidal signal

of 10 VPP at 3.75 MHz in ADS were fed in the k-wave space with their

transmission respective time delays. The beamforming analysis was performed

four times with different focusing locations mentioned above and the same 81

sensors were used to record the pressures.

It was observed that with the increase in angle of focus from the normal

(0o), the peak pressure decreased and this is due to the diffraction limited focus

[22], a phenomenon which implies that as the angle of focus increases, the total

area of transmission increases hence sparse energy distribution.

29

Figure 3.22: The beamforming at points of interest, 0o, 15o, 30o, and 60o from the

normal along with the plane wave transmission

3.7 Power and Intensity

Mechanical Power and Intensity of the transducer are other important

characterization for a transducer which in this work, the calculation for Power of

the emitted pressure were done with equation (3.7) [23]. Since mechanical

power is the radiated power by the CMUT into the medium, the power of the

whole transducer was calculated by superposing the powers by individual

CMUTs.

𝑊𝑚𝑒𝑐ℎ = 𝑅𝑒𝑎𝑙𝑓 ∙ 𝑢∗ (3.5)

where 𝑓 is the force exerted on the receiver by the transducer and 𝑢∗ is the

complex conjugate of the particle velocity. The power of pressure emitted by

individual cells were calculated for the three dedicated cells from the values of

pressure obtained in frequency domain analysis in ADS simulations. The power

values recorded from the corner, side and middle cells at 7.5 MHz were 0.997

mW, 0.557 mW and 0.777 𝜇𝑊 respectively. The difference of power between

the cells was huge and this was the result of the huge difference between particle

velocities of CMUT cells.

30

Figure 3.23: Power of Individual CMUTs analyzed from the corner, middle and

side elements

The total power radiated into the medium by the transducer was calculated

by adding all the powers from the individual cells and the resulting plot was

found to show that the total power at 7.5 MHz was 86.9 mW while at the peak

frequency, the radiated power was 1.157 Watt.

Figure 3.24: Power radiated into the medium by the Transducer

31

To calculate the Intensity of the transducer, the time domain analysis will be

used. In contrast to the calculation of power, the intensity is calculated by both

the real and imaginary parts of the pressure and conjugate of particle velocity as

in (3.8) [23].

𝐼 =∑

1

𝑡𝑎𝑣∫ |𝑓𝑖 × 𝑢𝑖

∗|𝑑𝑡256𝑖=1

𝑆 (3.6)

Where 𝑖 is the number of CMUT cells in the array, S is the total surface area

of the 16 x 16 array transducer, 𝑓𝑖 being the force of individual CMUT cell and

𝑢𝑖∗ the complex conjugate of particle velocity of individual CMUT. The

intensities obtained from the time domain analysis discussed earlier are

summarized in the table 3.3 below

Table 3.1: Power and Intensity of the transducer transmitting at 150 VPP and 7.5

MHz

150 VPP at 7.5 MHz MVM

Power Intensity Power Intensity

(W) (W/cm2) (W) (W/cm

2)

Sinusoidal 1 0.0140 0.2952 0.3768 7.5010

Cycle

Sinusoidal 4 0.0250 0.5567 0.4739 9.7263

Cycle

Gaussian

Enveloped Tone 0.0146 0.3032 0.5856 10.8678

Burst

PWM 1 Cycle 0.0213 0.4472 0.3947 7.3303

PWM 4 Cycle 0.0390 0.7743 0.5201 9.0862

32

Chapter 4

Fabrication

The fabrication of the CMUT array was performed a wafer scale and later on

diced into separate chips of arrays. In this section, the fabrication processes will

be discussed with the sequence of process flow. Mask designing will be discussed

first, then the etching of the cavity will come next followed by deposition of

bottom electrode and insulation layers. The wafer bonding will be discussed next

before Flip-Chip bonding and sealing processes.

4.1 Mask Design

The designing of the mask was done after considering a few things discussed

in the designing of the CMUT array itself and the fabrication facility limitations.

Considering the pitch was designed to be 192 𝜇𝑚, the 256 cells were arranged in

square manner of size 16 x 16 making sure the designed pitch is constant

throughout. Since shadow mask was going to be used to this purpose and not the

chrome masks, the electrical pads were determined to be 500 x 500 𝜇𝑚 which is a

feasible dimension for a shadow mask. These pads were ideally located and in a

square manner with gaps of 250 𝜇𝑚 in such a way they would allow enough space

for fanning out the wires to their respective pads by keeping the wires as short as

possible. Keeping the wires short was to avoid higher resistances due to long

wires which would result to power losses [4].

Since the minimum fabricable wire width in our cleanroom facilities is 3 𝜇𝑚

and spacing of 3 𝜇𝑚 [4] and as discussed in designing chapter earlier, that 27 𝜇𝑚

gap would be needed to accommodate maximum of 4 wires while fanning out the

wires to the pads, the pitch of 192 𝜇𝑚 of the CMUTs would leave 32 𝜇𝑚 gap

which would allow that. The designed mask is in figure 4.1 below.

33

Figure 4.1: The full CMUT transducer mask with wires and electrical pads

4.2 Cavity Etching

The epoxy wafer was used as the bottom wafer which contained the bottom

electrode, the insulator and the gap. The cavity of approximately 800nm was to be

etched by first cleaning the wafer with acetone, propanol and DI and depositing a

layer of 35 nm thick chrome as a hard mask for high temperature processes

during lithography and metal depositions. The photoresist AZ4562 was the spin-

coated at 6000 rpm on the chrome as the layer of 5 𝜇𝑚 which depended on the

smallest resolvable feature being 3 𝜇𝑚 . The UV exposure through vacuum

contact was then done on the designed mask in figure 4.1 with dosage of 50

mJ/cm2 using the EVG 620 mask aligner system. Then the development was

performed using the AZ400K solution for 8 minutes after which the wafer was

cleaned with DI water and vacuumed to kept dry for next process.

Etching of the chrome in the exposed regions was the next step which was

done by anisotropic Inductively Coupled Plasma (ICP) with Argon plasma to

avoid undercuts since the thickness of thinnest wires was 4 𝜇𝑚. The process was

performed in a total of 11 cycles where the first seven cycles were one minute

long and the last four of one and a half minutes for fully etching the chrome.

34

However, the Pyrex was also etched in the process by about 250 nm due to the

uniformity of the deposited chrome. After successful etching of chrome layer, the

wafer was hard baked at 110 0C for 90 minutes and then at 150 0C for three hours

to stabilize and increase the durability of the photoresist for the metal layers

deposition under very high temperature.

To have the cavity of 800 nm in the Pyrex, we need to etch 550 nm more into

the Pyrex since about 250 nm was already etched while etching the chrome.

Therefore, for the Pyrex etching, both anisotropic and isotropic etching were

employed; for the anisotropic etching, the DRIE process with ICP using SF6 and

argon plasma used while the BOE was used was used for the isotropic etching.

The reason for using both dry and wet etch is that as the dry etch is employed first

to etch about 480 nm of Pyrex anisotropically, it leaves some residues behind

which will be removed by the wet etch when etching the remaining part of the

Pyrex. The other reason is that the undercuts which will result from the wet etch

will be useful during the liftoff process with ease and accuracy after deposition of

the metal layers [4].

After a few trials of the dry etch with DRIE using SF6 and Argon plasma on

the dummy wafer, the etch rate was found to be 6.44nm/min, therefore, to

accomplish the etching of about 480nm of epoxy, 75 minutes of etching with

same recipe was done.

Figure 4.2: The Microscope images of the completely etched Epoxy wafer

35

After the completion of process, the thickness was measured on Stylus

profilometer and found that the total cavity formed in the Pyrex was about

670nm, therefore, 130nm was remaining to be etched by wet etch. For BOE etch

rate, after an inspection from four different dummy wafers, the etch rate on epoxy

was found to be 21nm/min. For etching of about 130nm, six minutes of etching

was done to result in 126nm etching. After the process, the cavity was once again

measured by the stylus profilometer and found it to have increased to 978nm

including the thickness of chrome which meant the BOE had etched the epoxy by

127nm as very close to expectation.

4.3 Bottom Electrode Deposition

The bottom electrode was made up of the layers of 100 nm of Titanium as an

adhesion layer, 100 nm of Platinum as barrier layer and 500 nm of Gold for low-

impedance electrical connection layer. The deposition of the metals was done by

MiDAS PVD 1eB e-beam evaporator and the total of these layers will leave 100

nm air gap as supposed. The depositions were done every time after the chamber

reached the pressure of 2.0 × 10−6 𝑇𝑜𝑟𝑟.

Titanium was deposited first at the rate of 1 𝐴/𝑠𝑒𝑐 [4] then followed by the

Platinum which was deposited at the rate of 1.5 𝐴/𝑠𝑒𝑐 and finally Gold was

deposited was deposited and that at the rate of 2.5 𝐴/𝑠𝑒𝑐. The depositions were

done without breaking the vacuum but leaving the chamber for 30 minutes for the

pressure to lower down to 0.4 𝜇𝑇𝑜𝑟𝑟 and prevent the deformation on the

photoresist. Lift-off of the chrome mask was performed by the Piranha wet etch

after the completion of deposition of metal layers – the piranha solution is a

solution made of Sulphuric acid and Hydrogen peroxide at the ratio of 3:1 [4].

Piranha solution tends to slowly etch titanium, therefore, prolonged exposure of

the wafer in the solution was avoided but still some of the wires (3 𝜇𝑚) were also

lifted off. The chrome wet etch after the complete removal of the TiPtAu stack

was done by CR-7 solution (Sigma-Aldrich) with short exposure due to its high

etch rate on chrome. Therefore, at this point, the epoxy and gold layer are visible

from top view.

36

Figure 4.3: Image from Microscope after deposition of TiPtAu stack

Finally, the insulator layer was to be deposited before the gap height to

prevent shorting in the case of a CMUT collapsing. The alumina was deposited

by Atomic Layer Deposition (ALD) using Savannah Thermal ALD with a

recommended recipe which deposits 1.005 A per cycle. With that recipe, to

deposit 300 nm layer of Alumina it took 3000 cycles which lasted about 14 hours.

The next step was the removal of alumina on the electrical connections and

pads which was done on conventional chrome lithography mask for better

alignment. The development of photoresist was done using AZ4562 for five

minutes then chrome was etched by CR-7 in the regions of pads until alumina

was visible. Then the exposed alumina was etched by the solution AZ4562 for

three hours and since the exposure time was long enough to also etch the

photoresist, chrome mask was necessary to prevent etching the unwanted areas.

Acetone was used at this stage to clean any stained photoresists and the stained

chrome was cleaned by CR-7. The DC resistance on gold was measured with the

probe station to make sure of complete alumina etching. Below are the photos of

the before and after etching of alumina on the connection pads as well as the

photo of CMUT cells awaiting wafer bonding.

Figure 4.4: Before (left) and after (right) Alumina etching

37

Figure 4.5: The microscope image of the CMUT cell right before wafer bonding

4.4 Wafer Bonding Process

The wafer bonding was performed to introduce the top electrode for the

CMUTs which was done with SOI wafers bonded on top of the Pyrex wafers

covering the gaps. The process was performed in EVG Austria facilities by EVG

520IS 200mm semi-automated system with SOI wafer of 15 𝜇𝑚 silicon thickness.

After a few trials on recipe optimization, the appropriate recipe was boding with

1000 volts and at 4500 C which was more intensive and extensive exposure than

the default recipe to ensure the wafer was bonded well throughout the area. This

recipe however, introduced defects to some of the metal wire connections while

other were undamaged.

Figure 4.6: The full wafer after modified bonding recipe (left) and damaged

electrical pads after wafer bonding (right)

38

The Silicon handle layer of 350 𝜇𝑚 thickness was etched by DRIE with SF6

at our facility upon receiving the wafers back from EVG Austria done with the

recipe of 13 cycles of 20 minutes each in order to reach the silicon membrane.

Then the stack of 30 nm Cr, 40 nm Au and 30 nm Cr was deposited on the entire

silicon wafer to lower the resistance between the ground pads and silicon

followed by depositing of ground pads by shadow mask where starting with 30

nm of Cr and followed by 500 nm of Au squares of 1 mm2.

Figure 4.7: Metal deposition on the top membrane with the shadow mask (left)

and the wafer after complete removal of Silicon layer on electrical pads (right)

The next step after the depositions on the silicon wafer was to etch the Cr-

Au-Cr stack and the silicon through to expose the electrical pads. The Cr-Au-Cr

stack was etched first by wet etch on the 2 𝜇𝑚 thick AZ5214E photoresist; the Cr

layers were etched using CR-07 solution and gold layer was wet-etched by

Diluted Aqua Regia solution (3:1:2 of HCl, HNO3 and DI water) [4]. The Silicon

layer was etched by DRIE Bosch process with ICP for 16 minutes for complete

removal of the silicon layer on top of the electrical pads.

The remaining photoresist was etched by O2 plasma in ICP for approximately

17 mins until the entire photoresist was dry etched away and since the O2 plasma

also etched the chrome layer under the photoresist, Gold layer was remaining

exposed in the end as the top layer.

39

Figure 4.8: Photoresist stripped, and the chrome etched

Finally, the dicing of the devices on the wafers was performed using UV

curable dicing tape. Since this process involved spraying DI water on the surface

of the wafer, the wafer was placed upside down over the dicing tape considering

that at this stage the gaps were not sealed to prevent water leakage into the gaps.

Figure 4.9: A finalized single diced CMUT transducer chip

40

4.5 Flip-Chip Bonding and Ground Wire Bonding

After dicing the chips, the flip-chip bonding was done using the Atom

Adhesives to mechanically bond the CMUT array chip with the PCB as an

alternative approach of the original plan which was to use the Finetech

FINEPLACER sigma with its ultrasonic arm which failed due to its insufficient

power of 20W to bond 256 gold ball bumps. Therefore, for the use of Atom

Adhesives, a stencil for the pads was designed for the purpose of applying the

adhesive on the pads at the correct place and amount without causing any short

during the bonding. The stencil was designed to have an opening of 250 x 250𝜇𝑚

at the center of each pad as the pad size on the PCB was 500 x 500 𝜇𝑚 and the

thickness of the stencil was 100 𝜇𝑚 to avoid possibility of shorting between pads

during bonding.

The stencil was properly aligned on the PCB and Atom Adhesives AA-

DUCT 916LC was applied on the pads through the stencil then the CMUT array

chip was aligned on the PCB and pressed against the PCB to mechanically bond it

to the PCB and left for the 24 hours in the RTP for curing.

After flip-chip bonding, wire bonding for the ground pad connections

between the chip ground pads and PCB ground pads were done using the Wire

bonder. After wire bonding, the wires were physically protected by the EPO-TEK

310M flexible optical epoxy to avoid any physical damage to the connections.

Figure 4.10: Flip-chip bonded Chip/PCB pair and ground pads connected

41

4.6 Parylene C Coating and Epoxy Coating

The gaps of the Chip were sealed with Parylene C by coating both the front

side of the chip and the back side of the chip with about 4 𝜇𝑚 thick Parylene C in

vacuum. The Parylene C coating process of five chips was done by Bogazici

University’s Parylene-C coater tool.

The PCB/Chip bond were prepared for the coating process by covering the

places where Parylene C was not to reach such as the electrical pads by Kapton

Tape and the back and front leaving only the regions to be coated exposed. The

prepared chips were encased properly to avoid any physical damage during

shipment.

Upon receiving the coated PCB/Chips bonds, the Kapton tapes were removed

and the chips were physically coated with EPO-TEK 310 M flexible optical

epoxy at the back side of the chip to increase physical strength of the attachment

between the PCB and the Chip but to also seal the openings around the pillars of

the conductive paste to restrict passage of water between the chip and the PCB.

After applying enough flexible epoxy on the back side and spacing between of the

Chip-PCB mount, the epoxy was left to cure in RTP for 24 hours.

Figure 4.11: Parylene C coated Chip/PCB pair on the transmitting side (left) and

Epoxy coated on the backside (right)

42

Figure 4.12: Epoxy coated at the rim of the Chip/PCB bond at the transmitting

side

4.7 Vertical PCBs Mounting and Casing of the

Device

The total of four vertical PCBs were mounted on the chip PCB extending the

electrical connections from the pads to the coaxial cables. These vertical PCBs

were mounted on the main PCB via connectors which were soldered on the pads

from the main PCBs using the solder paste and heat gun. The vertical PCBs were

designed to accommodate for 64 connections with 32 connections on each side

from which the cable extend to the power cards.

The device was the placed in the 3D-printed case and properly sealed at the

openings avoid water flowing into the device. The 3D case was printed from the

PETG material with high infill ratio of 98% to ensure water tightness. The case

was designed to have a circular opening at the transmitting side to expose the chip

for transmission. The PCB was kept and help in place in the case surface by the

Silicon gel to ensure strong adhesion between the case and the PCB equivalently

through the surface and yet allow for flexibility to prevent the PCB from bending.

The gaps between the PCB and the case through the transmission opening were

sealed by silicone gel carefully not to reach the CMUT cells area.

43

Figure 4.13: Mounting of the vertical PCBs socketing on the connectors

Figure 4.14: Casing of the finalized transducer with connection coaxial cables

44

Chapter 5

Measurements and Transmission

5.1 Fabrication Yield Test

Before going into any measurements, impedance tests with DVM were done

to all the cells in the transducer array to identify which cells were shorted to

ground, shorted to other cells, the leaking cells and the fully capacitive cells. For

the functional cells, the frequency response impedance analysis to determine the

actual acoustic resonance frequency after the fabrication were done.

The DVM was set at the highest resistance scale (20 MΩ) to identify the

fully capacitive cells which appeared as open circuit with this scale and were

grouped as group 1. The DVM was scaled down to 1 MΩ or 200 kΩ for the cells

which appeared short in the 20 MΩ scale and the open circuit cells in this scale

were classified as highly resistive cells and put in the group 2. The DVM was

further scaled down to 2 kΩ or 200 Ω for the short cells in the previous scale

and the ones that appeared open in this scale were classified as leaking cells and

were grouped in group 3.

There were a total 48 functional cells which were grouped in three; The first

group of 26 perfectly capacitive cells colored in green, a second group of 18

cross-shorted leaking cells colored in purple, and a group of 4 leaking cells

colored in red. These 48 cells were considered as the functional cells in the array

of 256 cells making a fabrication yield of 18.75%. This low yield resulted in a

very scattered array which led to losing the array effects to the transducer such

as array resonance, directivity, and beamforming abilities due to their

dependence on array element pitch which is undefined. The three possible

reasons for low yield include the designing of the element pitch which left a

marginal gap between adjacent elements – 32 µm, just enough to accommodate

the passage of four wires between them. Considering the fabrication limitations

45

in our UNAM cleanroom facilities of 3 µm minimum wire thickness and 3 µm

minimum spacing between each wire, accommodating 4 wires is quite risky and

shorting between some wires was inevitable. Another reason could be the faults

during the Anodic wafer bonding which was done at EVG facilities in Austria as

there was a difficulty optimizing the recipe for perfect wafer bonding. In the

process, some faults were inevitable and could be seen on the pads such as in

figure 4.6 which are much larger than the smallest feature which is the wire

thickness of 3 µm. The last source of shorting and especially cross-shorting was

during the flip-chip bonding process which done mechanically using the

conductive paste and stencil. Besides choosing stencil thickness of 100 µm to

avoid overspreading of the conductive paste between two pads, applying more

pressure or shear force could still lead to overspreading of the conductive paste

and shorting the adjacent pads.

Figure 5.1: Transducer array with labeled 48 functional cells; 26-Green colored

cells from first group, 18-purple colored cells from second group and 4-red

colored cells from third group

46

5.2 Resonance Frequency Shift

In the chapter 3, we saw that the peak frequency of the transducer array was

6.3 MHz while the designed array resonance frequency was 7.5 MHz. However,

due to the low fabrication yield, after the experimental verification discussed in

this chapter, the peak frequency of the array was found to have right-shifted to

8.9 MHz closing on the designed resonance and peak frequency of individual

CMUTs of 8.95 MHz as seen in the calculations below from the equation (2.7).

𝑓𝑜 = 𝑡𝑚

𝑎2

√809

𝑌0

𝜌𝑚(1 − 𝜎2)

2𝜋

𝑓𝑜 = (15 ∗ 10−6)

(80 ∗ 10−6)2

√809

149 ∗ 109

2370(1 − 0.172)

2𝜋

𝑓𝑜 = 8.948 𝑀𝐻𝑧

Therefore, for the impedance analysis it was made sure that the frequency

domain covers a wide range of frequencies including the designed and the new

resonance frequencies for a detailed inspection of the resonance frequency.

5.3 Impedance Measurements

The impedance analysis also tells us about the charging of the CMUT which can

be seen by looking at the behavior of conductance and susceptance plots with

change in input voltages. The conductance and susceptance were measured with

the HP4194A Impedance Analyzer and the Probe station as were one of the

reasons for taking the measurements in air and not water. The other reason for

taking the measurements in air was that the conductance values in water were

too low with 1 VPP excitation and conducting it in air provide a better visibility

of the resonant frequencies and the peak conductance.

47

The impedance analyzer used has a maximum of 1 VPP of AC output which

was extremely low for unbiased operation, therefore the measurements were

done in a biased mode. The measurements were done with the bias voltage of 40

VDC which is the maximum output bias voltage for the used impedance analyzer

and 1 VPP AC voltage for the sinusoidal excitation voltage with frequency swing

between 5 MHz – 12 MHz. The probe station was used to directly tap on the

pads from the PCB with the needle probes to avoid using cables as they

introduced reflection from the capacitances between adjacent cables. The

Impedance analyzer was connected to the PC with the GPIB interface to control

it with LabVIEW so that the plots could be saved.

Figure 5.2: Impedance measurement setup with Impedance Analyzer and probe

station

The admittance plot below is of the cell 255 and a typical plot from all the

cells which belongs to the first group of cells. The plot shows that the peak

frequency was 8.89 MHz which was very close to the new resonance frequency,

8.95 MHz and the peak conductance was 60.37 µS.

48

Figure 5.3: Admittance of the 255th cell excited at 1 VPP AC and 40 VDC bias in

air

The admittance plot from the 227th cell is plotted below as is a typical

admittance plot from the cells in the second group. The peak frequency was 8.94

MHz with conductance of 321.2 µS.


air

49

The admittance from 208th cell is plotted below as is a typical conductance

plot from the cells in the third group. The plots show that the peak frequency is

at 8.94 MHz and the peak conductance is 1.5 mS. More admittance plots for

more cells are attached in the Appendix B.


air

It is clear that the admittance plots from all the groups had the same peak

frequency which was also the resonance frequency and even though the peak

conductance values between the three groups were different, they were all lossy

differing by the extent.

5.3.1 Loss tangents and parallel effective dielectric

loss resistors and capacitors

The loss tangents for each cell were calculated in Appendix C, from which

the parallel effective dielectric loss resistances [17] were calculated, and the

actual conductance was determined.

tan 𝛿 =𝐺

𝐶𝑇 (5.1)

𝐶𝑇 =𝐵

(5.2)

50

The tan 𝛿 is the loss tangent, 𝐺 is the conductance at resonance frequency, 𝐵 is

the susceptance at resonance frequency, is the angular resonance frequency

and 𝐶𝑇 is the total capacitance.

Then the parallel effective dielectric loss resistance, 𝑅𝑃 [17] and the parallel

effective capacitance can be calculated using the below equations respectively.

𝑅𝑃 =1

𝐶𝑇 tan 𝛿 (5.3)

𝐶𝑃 = 𝐶𝑇 − 𝐶𝑜 (5.4)

Therefore, for the three cells demonstrated above; the 255th, 227th and the

208th, had the 𝑅𝑃 and 𝐶𝑃 values summarized in the table below.

Table 5.1: Summary of the parallel effective resistors and capacitors for cells

255th, 227th and 208th

After determining the 𝑅𝑃 and 𝐶𝑃 values of every cell, the large signal

equivalent circuit is modified as in the figure 5.6 below by introducing the

unique values of 𝑅𝑃 and 𝐶𝑃 in parallel to the C0 for every cell to best represent

the performance of the CMUT with the dielectric losses due to the insulator.

Figure 5.6: The modified large signal equivalent circuit [17]

CMUT Cell

tan 𝛿

𝑅𝑃(𝑘𝑂ℎ𝑚)

𝐶𝑇(𝑝𝐹)

𝐶𝑜(𝑝𝐹)

𝐶𝑃 (𝑝𝐹)

255th

0.0153

119

9.8

1.3

8.5

227th

0.2463

3.5

20.6

1.3

19.3

208th

0.4437

1.2

57.8

1.3

56.5

51

5.3.2 Measurement and Simulation Comparison

Compared to the admittance plots from chapter 3.2, all the conductance

plots from the measurements have a sloppy baseline which is due to the

dielectric loss of insulating layer. The steeper the baseline, the more lossy the

CMUT cell and the less clear the peak. The simulation results did not have any

losses as their baseline could be seen to be horizontally straight.

Focusing on the peak frequency, the simulations had resonance frequency at

7.5 MHz as designed while the measurement results show the peak and

resonance frequency to be 8.94 MHz which is the designed resonance frequency

of a single CMUT cell in the array. And this shift is due to loss in array effect

due to the low yield and scattered array as discussed above.

The peak from the admittance simulations in figure A.2 is 0.14 µS while

from the measurement results, we saw a peak conductance of 60 µS in figure

5.3. This difference is explained by two reasons, one is the fact that the

admittance simulated in water while was measured in air which would yield a

sharper conductance peak due to the radiation impedance the CMUT would

experience in air compared to the radiation impedance experienced in water.

Another reason is the dielectric losses experienced in the cells; the loss tangents

should be considered to determine the actual peak conductance. The cells in the

simulations were completely lossless as for the reason of lower conductance

peak. The parallel effective dielectric losses and loss tangents were discussed in

the previous subsection.

52

5.4 Individual Cells Transmission

The transducer was connected to the socketing vertical PCBs extending the

connections to the CMUTs after completing the impedance measurements and

placed in the casing for in-water transmission. The transducer was placed in the

water-tight case and sealed with Silicone gel through all the openings to make

sure there was no opening for water to pass through. In this part, individual

CMUTs were used for transmission to detect the received signal and be

compared with the simulation results for individual CMUT transmitted signals.

The transmission was done in the phantom, driving the CMUTs with the

signal generator Tektronix AFG 3101 and the acoustic signal was received by

the HGL-0200 ONDA hydrophone - 15 mm from the transducer to make sure

the reception was done in the nearfield region.

The individual CMUTs among the functional CMUTs were half-frequency

driven and in an unbiased mode with a 10 VPP sinusoidal tone burst signal of

1000 cycles and 800 µs cycle duration at 4.47 MHz (half of 8.94 MHz). Even

though a signal of 10 VPP was quite small for unbiased mode of operation, it was

still the largest output signal of the signal generator used. The hydrophone was

connected to its pre-amplifier and then through the SR844 RF Lock-in amplifier

to the oscilloscope.

The need to use the Lock-in amplifier came after having a difficulty in

observing the received signal with a bare oscilloscope as it was very low in the

range of ones of millivolts overlapping with the noise and impossible to detect

the signal with the frequency of 8.94 MHz. Therefore, the received signal was

passed through a Lock-in amplifier which was referenced at 8.94 MHz with

another signal generator but time-phased with the CMUT-driving signal

generator. The block diagram of the Lock-in amplifier is shown in the figure 5.7

below.

53

Figure 5.7: The block diagram of the SR844 Lock-in Amplifier [24]

The Lock-in amplifier locks itself to the frequency of the reference signal –

which is the signal of the target frequency and detects the signal with the

frequency of the reference signal. So, the output signal received from the lock-in

amplifier at oscilloscope was an envelope of the output signal displaying in

terms of magnitude, X and the quadrature component, Y of the signal at 8.94

MHz. The cycle count of the driving tone-burst signal was initially kept low in

the range of tens of cycles but was eventually increased to thousand cycles for

the signal to be detected with better accuracy by the Lock-in amplifier. For the

entire measurement, the sensitivity of the lock-in amplifier was set as 30 mV,

the Xoffset was set at 0 and the Expand was set as 1. The amplitude of the

received signal was calculated using the equation (5.5) [24] below

𝐴𝑚𝑝𝑙𝑖𝑡𝑢𝑑𝑒 = (𝑋

𝑆𝑒𝑛𝑠𝑖𝑡𝑖𝑣𝑖𝑡𝑦− 𝑋𝑜𝑓𝑓𝑠𝑒𝑡) × 𝐸𝑥𝑝𝑎𝑛𝑑 × 10𝑉 (5.5)

54

Majority of the work in the Lock-in Amplifier happens in the digital signal

processor shown in the figure 5.8 below. The DSP takes in the digitized X-IF

and Y-IF signals from the IF section with the IF chop signal allowing the DSP to

demodulate the X-IF and Y-IF signals at the correct IF frequency. The X-IF and

Y-IF signals are converted back to DC through multiplication by a digital IF

chop waveform. The demodulation in the DSP eliminate the DC output errors of

analog mixers.

Figure 5.8: The inside of the DSP of the SR844 Lock-in Amplifier [24]

The output received from the Lock-in amplifier can be described as in the

figure 5.9 with a frequency of the reference signal which is fed into the reference

input terminal of the lock-in amplifier and the magnitude and quadrature

components are provided as X and Y respectively. To calculate the magnitude of

the enveloped signal, the equation (5.5) above is used.

Figure 5.9: visualized signal (left) and the X and Y values from the amplifier (right)

[24]

55

For the CMUTs transmitted with, the received signals were compared to the

simulation results in figure 3.27 in chapter 3.4 which was a steady state pressure

of 1933 Pa and peak pressure of 2500 Pa. Some of the cells were found to be

emitting less pressure than the simulation steady state while majority were found

to be emitting more or less than but around the steady states pressure but less

that simulation peak pressure. Therefore, they were grouped into two; the group

of cells emitting less than 1933 Pa and the cells that emitted pressures between

1933 Pa and 2500 Pa. The cells in the first group were the 26 cells that fell in

first group among the groups from the impedance analysis – they had

conductance values around the expected range and with low loss tangent values.

The cells in the second group were the 22 cells of the second and third group

defined in the impedance analysis – they had high conductance values due to

either insulation leakage losses or shorting between the two cells.

Figure 5.10: Transducer array with labeled 48 functional cells; 26-Green colored

cells from first group and 22-purple colored cells from second group

56

Transmission measurements results from three CMUTs are presented in this

thesis report; from cell 208, cell 227 and cell 255. The 208th cell belong to the

second group which had the cells with high conductance, 227th cell and 255th

cells belonged to the first group of cells which had conductance values around

the expected values.

Figure 5.11: The setup for transmission measurements with signal generator and

reception with hydrophone

The acoustic signal received from cell 208 was the lowest observed on the

oscilloscope as 936 𝜇𝑉 while the signal received from cell 227 was around the

simulation steady state pressure recorded on the oscilloscope, 1.25 𝑚𝑉 and the

signal from cell 255 was 1.33 𝑚𝑉 which was over the steady state but not more

than the maximum peak pressure from simulation results. After converting these

voltage signals to pressure signal using the Hydrophone’s sensitivity from

equation (5.6), the cell 208 emitted 1483 Pa, the cell 227 emitted 1980 Pa, the

cell 255 emitted 2059.2 Pa and the calculated average pressure of the 48 cells

was 1625.2 Pa.

𝐻𝑦𝑑𝑟𝑜𝑝ℎ𝑜𝑛𝑒′𝑠𝑒𝑛𝑠𝑖𝑡𝑖𝑣𝑖𝑡𝑦 + 𝑃𝑟𝑒𝑎𝑚𝑝𝑙𝑖𝑓𝑖𝑒𝑟 = −244 𝑑𝐵 𝑟𝑒.1𝑉

𝜇𝑃𝑎

1 𝑚𝑉 = 1584 𝑃𝑎 (5.6)

57

5.4.1 Individual cells transmission measurement

and simulation comparison

The waterborne single-cell transmission was done at the new acoustic

resonance frequency of 8.9 MHz while the simulations were done at a

theoretical resonance frequency of 7.5 MHz. Therefore, the comparison between

the transmission measurement and simulation will be done at their respective

resonance frequencies and at 10 VPP excitation. The results observed in this

section are not far off from the simulation results when the average emitted

pressure from all the 48 cells is considered. The calculated emitted pressure

average of 48 cells was 1625.2 Pa while the emitted pressure average of the 256

cells of the array was 1900 Pa. The average pressure levels are in the agreement

range however, the variation in pressure levels emitted by individual levels is

huge in the measurement compared to the variation in the simulations. This

variation in pressure levels is caused by the variation in particle velocities of the

cells which is the result of variation in radiation impedance experienced by cells

at different locations in the array. But again, the losses in the cells is then reason

for the lower average pressure compared to the lossless emissions in the

simulations.

58

5.5 Plane Wave Transmission

After individual cell transmission, all the 48 cells were connected and

driven in phase with a 1000-cycle tone-burst signal of 10 VPP over acoustic

frequency range from 1 MHz to 12 MHz and the signal was received by

hydrophone 15 mm from deep from the center of the transducer in a water-filled

phantom.

Figure 5.12: The transducer array with functional connected cells colored in red

Table 5.2: Summary of the received voltage and pressure signals of the

transducer at different frequencies

Acoustic Frequency

(MHz)

Received voltage

(mV)

Corresponding

Pressure (kPa)

1 MHz 9.31 14.75

2 MHz 11 17.42

3 MHz 13 20.59

4 MHz 14 22.18

5 MHz 14.9 23.60

6 MHz 15.2 24.08

7 MHz 18.2 28.83

7.5 MHz 19.1 30.25

8 MHz 20 31.68

8.9 MHz 47 74.45

10 MHz 24.9 39.44

11 MHz 30.5 48.31

12 MHz 25.1 39.76

59

The signal from the transducer showed to have the peak pressure at 8.9 MHz

with the recorded voltage signal of 47 mV while at other frequencies the

received signal was lower as seen in the summarized table above with the

conversions to their corresponding pressure values. The oscilloscope graphics of

received signal for every frequency are attached in the Appendix D.2

The results obtained from this measurement was not far from the

expectation from the looks of the frequency response plot as the peak was at 8.9

MHz but also the emission at adjacent frequencies to the peak frequency had

about half of the peak pressure at resonance frequency. With considerations that

only 48 cells were used for transmission and the average pressure emitted by a

single cell for the 48 functional cells at resonance frequency was 1625 Pa as

calculated in chapter 5.3, the peak pressure of 74.45 kPa at resonance frequency

is in the range of expectation.

Figure 5.13: Pressure emitted by 48 functional cells of the transducer with a

frequency sweep from 1 MHz to 12 MHz

60

The necessary measurement to form the radiation pattern was not possible

due to the scattering of the matrix and the complexity of the setup however, it

was expected to be omnidirectional due to the scattering. For the same reason,

the radial pressure depreciation was found to obey the spherical model of 1/r2.

With the low yield of 20%, the transducer was not suitable to be used for

image formation with DiPhAS, however, for the functional cells, the

measurement for the half-frequency transmission in water was successful.

5.5.1 Plane wave transmission measurement and

simulation comparison

The measurement results from waterborne plane wave transmission can be

compared to the simulation results of the radiation pattern or beamforming in

chapter 3.5 and 3.6 respectively. The plane wave transmission simulations from

figures 3.21 and 3.22 at 0o show that the emitted pressure by the entire

transducer of 256 cells when excited by 10 VPP at resonance frequency was 708

kPa. On the measurement side, the emitted pressure by the transducer of 48 cells

excited by 10 VPP at the new resonance frequency as seen in figure 5.13 and

table 5.2 was 74.45 kPa.

To compare the simulation results and the measurement results, we have to

consider the number of emitting cells, the dominant compact array made of the

functional cells and the shape of the array – whether it is rectangular or clustered

determine the transmission directivity of the array. The larger the radius of the

array, the more directive the transmission, hence higher magnitude of pressure at

the center of the array and opposite is true for the array with smaller radius.

Considering the arrangement of the 48 functional cells, about 17 cells form a

dominant array with a radius of about 0.4 mm at the bottom right of the array

while the total array of 256 cells had a radius of 1.28 mm. Since the radiation

pattern is a linear function of area of the array, the difference in the areas of the

two arrays suggest that only about 10.2% of the simulated pressure emitted will

be expected. Therefore, comparing 10.2% of the 708 kPa which is 70 kPa and

the measured 74.45 kPa, they are in a good range of agreement.

61

Chapter 6

Conclusion

The work covered in this thesis, covers the designing, simulation,

fabrication and post-fabrication characterization and transmission of half-

frequency driven 16 x 16 waterborne transmit CMUT array. The designed

CMUT array is aimed for high-intensity volumetric medical imaging application

and the idea for the design came from the earlier designed half-frequency driven

single airborne CMUT. To achieve both, high intensity pressure emission and an

easily integrable device for medical imaging purposes, an array with maximized

number of cells had to be designed while considering other designing and

fabrication constraints. This array designing was possible by using the large

signal equivalent circuit model and the radiation impedance matrix for

computational and simulation convenience from having to deal with large array.

The initial considerations during the designing involved knowing the size of

the array which was determined by the maximum number of output channels of

DiPhAS which was to be used for transmission, and that was 16 x 16. The

second step was to set the operation frequency which was selected as 7.5 MHz

based on the optimal application requirements. From this starting point, the

design of the physical parameters proceeded by considering the constraints of

the preferred properties such as radiation pattern with low sidelobes and

maximized Rayleigh distance. After determining geometrical dimensions of the

design from the lumped element model equations, the fabrication limitations and

conveniences were considered for fine adjustments to the design and iteratively

optimized for an optimum design with all the requirements fulfilled.

Verification of the design performance and characterization before

fabrication was done through various simulations in ADS with the help of large

signal equivalent circuit and radiation impedance matrix to avoid using FEM

tools due to having a large matrix which would cost a lot of computational

power and time. Simulations such as impedance analysis, frequency domain

62

simulations and responses to transmissions with different waveforms were all

done in ADS except for radiation pattern and pressure field pattern which were

done in MATLAB with the k-wave tool.

Wafer scale production was done for the fabrication of the device upon

finalizing on the designing. Wafer bonding process was used with Pyrex as the

bottom electrode and SOI wafer as the top electrode. The fabrication was

optimized in such a way the layers were deposited in the bottom electrode in a

self-aligned manner with conventional bottom to top lithography processes and

the top membrane was the bonded SOI wafer. The fabrication of the devices cost

only two masks and one shadow mask with the employed fabrication technique.

After the core fabrication processes completion, the wafer was diced into

separate devices and flip-chip bonding was mechanically done between the

CMUT chip and the PCB with conductive paste using the metallic stencils for

alignment. The chip/PCB bond pair was then placed in the watertight 3-D

printed case and carefully sealed with silicone gel through all the opening ready

for water-borne application.

The transducer was put through short circuiting test with DVM to identify

the ground-shorted cells, cross-shorted cells and the open circuit cells. The

impedance analysis was done afterwards with the Impedance Analyzer to

measure the conductance and susceptance of each operational CMUT cell, the

resonance frequency of each cell, the charging, and losses in each CMUT cell.

Transmission from each CMUT cell individually was done to compare the

performance with the simulations and finally, the plane wave transmission from

all the functional cells was performed in water to assess the emitted pressure

from the transducer.

The transducer could not be used for imaging purpose due to the low yield

achieved after fabrication as only about 18.75% of the transducer was

functional, leading to a massive compromise on the emitted signal. The low

yield in this work as explained earlier is this work, could be due to the

technology used for wafer bonding coming short of accommodating the

resolution we used for connectivity wires, cross-shorting occurred during the

flip-chip bonding or soldering of the pins on the vertical cable-connecting PCBs.

63

Therefore, more work needs to be done on improving the yield by designing to

have a sparser array, which means smaller cells to allow more room between

adjacent wires passing between adjacent cells during fanning out. Performing

the Anodic wafer bonding in campus would have made the process more

controlled and easier to personally optimize the recipe for better yield.

64

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[3] Asli Unlugedik, Abdullah Atalar, and Hayrettin Koymen, "Designing an

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analysis of CMUT arrays with very large number of cells," in IEEE

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in CMUT elements or arrays," in IEEE International Ultrasonics Symposium,

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"Acoustical tuning of CMUT receiver arrays," in IEEE International

Ultrasonics Symposium (IUS), Tours, 2016.

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Ferroelectrics, and Frequency Control, vol. 61, no. 12, pp. 2139-2148,

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[15] Hayrettin Koymen, Abdullah Atalar, and A. Sinan Tasdelen, "Bilateral

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and Frequency Control, vol. 64, no. 2, pp. 414-423, February 2017.

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[22] Jian-Yu Lu, Hehong Zou, and James F. Greenleaf, "Biomedical ultrasound

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[23] David T. Blackstock, Fundamentals of Physical Acoustics. New York,

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[24] Model SR844 RF Lock-In Amplifier, 3rd ed. Sunnyvale: Stanford Research

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68

Appendix A

More Simulations Results

Figure A.1: Admittance for 15 Vac input with a frequency sweep 2.5 MHz – 20

MHz [4]


MHz [4]

69


MHz [4]

Figure A.4: Transient pressure analysis with a continuous PWM signal of 10 VPP

at 3.75 MHz

70

Figure A.5: Transient pressure analysis with a continuous PWM signal of 150

VPP at 3.75 MHz

Figure A.6: Transient pressure analysis with a continuous PWM signal of 262

VPP (MVM) at 3.75 MHz

71

Figure A.7: Transient analysis of a half-cycle PWM signal input of 5 Vac at 3.75

MHz [4]

Figure A.8: Transient analysis of a one-cycle PWM signal input of 5 Vac at 3.75

MHz [4]

72

Figure A.9: Transient analysis of a four-cycle PWM signal input of 5 Vac at

3.75 MHz [4]

Figure A.10: Normalized membrane displacement of a half-cycle PWM signal

input of 5 Vac at 3.75 MHz [4]

73

Figure A.11: Normalized membrane displacement of a one-cycle PWM signal


Figure A.12: Normalized membrane displacement of a four-cycle PWM signal


74

Figure A.13: Normalized membrane displacement analysis with a continuous

PWM signal of 10 VPP at 3.75 MHz


PWM signal of 150 VPP at 3.75 MHz

75


PWM signal of 262 VPP (MVM) at 3.75 MHz

Figure A.16: Particle velocity of a half-cycle PWM signal input of 75 Vac at

3.75 MHz [4]

76

Figure A.17: Particle velocity of a four-cycle PWM signal input of 75 Vac at

3.75 MHz [4]

Figure A.18: Particle velocity of a half-cycle PWM signal input of MVM at 3.75

MHz [4]

77

Figure A.19: Particle velocity of a four-cycle PWM signal input of MVM at 3.75

MHz [4]

Figure A.20: Field Pattern of Plane wave transmission at 7.5 MHz and at 15.744

mm

78

Figure A.21: Field Pattern of 0o focused transmission at 7.5 MHz and at 15.744

mm

Figure A.22: Field Pattern of 30o focused transmission at 7.5 MHz and at 15.744

mm

79

Appendix B

More Impedance Analysis Results

Figure B.1: Admittance of 132nd

element.

Figure B.2: Admittance of 145th element

80

Figure B.3: Admittance of 2nd element


81


Figure B.6: Admittance of 223rd element

82



83


84

Appendix C

The Loss Tangents

Figure C.1: Loss tangents of the cells in group 2 changing with frequency

Figure C.2: Loss tangents of the cells in groups 1, 2 and 3 colored in green, yellow

and red respectively changing with frequency

85

Figure C.3: Loss tangents of the cells in groups 1 colored in green changing with frequency

Figure C.4: Loss tangents of the cells in groups 1, 2 and 3 colored in green, yellow and red

respectively changing with frequency

86

Figure C.5: Loss tangents of the cells in groups 1 and 3 colored in green and red respectively

changing with frequency

87

Appendix D

Transmission Oscilloscope Screenshots

D.1 Single CMUT Transmission Results

Figure D.1: Received voltage signal at 8.94 MHz from the cell 206 through

Lock-in Amplifier with a 1000-cycle tone burst input of 10 VPP at 4.47 MHz



88

Figure D.3: Received voltage signal at 8.94 MHz from the cell 61 through Lock-

in Amplifier with a 1000-cycle tone burst input of 10 VPP at 4.47 MHz



89





90



91

D.2 CMUT Array Transmission Results

Figure D.8: Received voltage signal at 1 MHz from the transducer through



Lock-in Amplifier with a 1000-cycle tone burst input of 10 VPP at 1 MHz

92





93





94



Figure D.15: Received voltage signal at 7.5 MHz from the transducer through


95



Figure D.17: Received voltage signal at 8.94 MHz from the transducer through


96





97



98

Appendix E

CMUT Array and Pads Layout

Figure E.1: The CMUT Array and CMUT elements layout

Figure E.2: The electrical pads configuration on the Pyrex wafer

99

Figure E.3: The electrical pads configuration on the PCB

Figure E.4: The electrical pads configuration on the Vertical connecting PCBs

100

Appendix F

Hydrophone and Pre-Amplifier Calibration

Figure F.1: ONDA Hydrophone sensitivity calibration sensitivity certificate

101

Figure F.2: ONDA Pre-Amplifier gain calibration certificate

designing, fabrication and post- fabrication

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