DESIGNING, FABRICATION AND POST- FABRICATION ...
Transcript of 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
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ABSTRACT
DESIGNING, FABRICATION AND POST-FABRICATION
CHARACTERIZATION OF HALF-FREQUENCY DRIVEN
16 X 16 WATERBORNE TRANSMIT CMUT ARRAY
Yusuph Abubakar Abhoo
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.
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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
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ÖZET
YARI FREKANSTA SÜRÜLEN SUALTI 16 X 16 ELEMANLI
CMUT DIZININ DIZAYNI, FABRIKASYONU VE
FABRIKASYON SONRASI KARAKTERIZASYONU
Yusuph Abubakar Abhoo
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.
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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
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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.
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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
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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
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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
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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
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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
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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.2 Admittance for 35 Vac input with a frequency sweep 2.5 MHz – 20 MHz…68
A.3 Admittance for 55 Vac input with a frequency sweep 2.5 MHz – 20 MHz…69
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
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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
A.14 Normalized membrane displacement analysis with a continuous PWM
signal of 150 VPP at 3.75 MHz…………...………………………………74
A.15 Normalized membrane displacement analysis with a continuous PWM
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
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B.6 Admittance of 223rd element……………………………………………….81
B.7 Admittance of 178th element……………………………………………….82
B.8 Admittance of 255th element……………………………………………….82
B.9 Admittance of 237th element……………………………………………….83
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
D.4 Received voltage signal at 8.94 MHz from the cell 227 through Lock-in
Amplifier with a 1000-cycle tone burst input of 10 VPP at 4.47 MHz…...88
D.5 Received voltage signal at 8.94 MHz from the cell 127 through Lock-in
Amplifier with a 1000-cycle tone burst input of 10 VPP at 4.47 MHz…...89
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D.6 Received voltage signal at 8.94 MHz from the cell 221 through Lock-in
Amplifier with a 1000-cycle tone burst input of 10 VPP at 4.47 MHz……89
D.7 Received voltage signal at 8.94 MHz from the cell 221 through Lock-in
Amplifier with a 1000-cycle tone burst input of 10 VPP at 4.47 MHz…...90
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
D.9 Received voltage signal at 2 MHz from the transducer through Lock-in
Amplifier with a 1000-cycle tone burst input of 10 VPP at 1 MHz………91
D.10 Received voltage signal at 3 MHz from the transducer through Lock-in
Amplifier with a 1000-cycle tone burst input of 10 VPP at 1.5 MHz…...92
D.11 Received voltage signal at 4 MHz from the transducer through Lock-in
Amplifier with a 1000-cycle tone burst input of 10 VPP at 2 MHz……..92
D.12 Received voltage signal at 5 MHz from the transducer through Lock-in
Amplifier with a 1000-cycle tone burst input of 10 VPP at 2.5 MHz…..93
D.13 Received voltage signal at 6 MHz from the transducer through Lock-in
Amplifier with a 1000-cycle tone burst input of 10 VPP at 3 MHz……..93
D.14 Received voltage signal at 7 MHz from the transducer through Lock-in
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
D.16 Received voltage signal at 8 MHz from the transducer through Lock-in
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
D.18 Received voltage signal at 10 MHz from the transducer through Lock-in
Amplifier with a 1000-cycle tone burst input of 10 VPP at 5 MHz……..96
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D.19 Received voltage signal at 11 MHz from the transducer through Lock-in
Amplifier with a 1000-cycle tone burst input of 10 VPP at 5.5 MHz…...96
D.20 Received voltage signal at 12 MHz from the transducer through Lock-in
Amplifier with a 1000-cycle tone burst input of 10 VPP at 6 MHz……..97
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
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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
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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
Figure 3.15: Transient response of 5-Cycle Gaussian-enveloped tone burst signal
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.
Figure 3.16: Transient response of 5-Cycle Gaussian-enveloped tone burst signal
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.
Figure 5.4: Admittance of the 227th cell excited at 1 VPP AC and 40 VDC bias in
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.
Figure 5.5: Admittance of the 208th cell excited at 1 VPP AC and 40 VDC bias in
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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68
Appendix A
More Simulations Results
Figure A.1: Admittance for 15 Vac input with a frequency sweep 2.5 MHz – 20
MHz [4]
Figure A.2: Admittance for 35 Vac input with a frequency sweep 2.5 MHz – 20
MHz [4]
69
Figure A.3: Admittance for 55 Vac input with a frequency sweep 2.5 MHz – 20
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
input of 5 Vac at 3.75 MHz [4]
Figure A.12: Normalized membrane displacement of a four-cycle PWM signal
input of 5 Vac at 3.75 MHz [4]
74
Figure A.13: Normalized membrane displacement analysis with a continuous
PWM signal of 10 VPP at 3.75 MHz
Figure A.14: Normalized membrane displacement analysis with a continuous
PWM signal of 150 VPP at 3.75 MHz
75
Figure A.15: Normalized membrane displacement analysis with a continuous
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
Figure B.4: Admittance of 4th element
81
Figure B.5: Admittance of 7th element
Figure B.6: Admittance of 223rd element
82
Figure B.7: Admittance of 178th element
Figure B.8: Admittance of 255th element
83
Figure B.9: Admittance of 237th element
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
Figure 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
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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
Figure D.4: Received voltage signal at 8.94 MHz from the cell 227 through
Lock-in Amplifier with a 1000-cycle tone burst input of 10 VPP at 4.47 MHz
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Figure D.5: Received voltage signal at 8.94 MHz from the cell 127 through
Lock-in Amplifier with a 1000-cycle tone burst input of 10 VPP at 4.47 MHz
Figure D.6: Received voltage signal at 8.94 MHz from the cell 221 through
Lock-in Amplifier with a 1000-cycle tone burst input of 10 VPP at 4.47 MHz
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Figure D.7: Received voltage signal at 8.94 MHz from the cell 221 through
Lock-in Amplifier with a 1000-cycle tone burst input of 10 VPP at 4.47 MHz
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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 0.5 MHz
Figure D.9: Received voltage signal at 2 MHz from the transducer through
Lock-in Amplifier with a 1000-cycle tone burst input of 10 VPP at 1 MHz
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Figure D.10: Received voltage signal at 3 MHz from the transducer through
Lock-in Amplifier with a 1000-cycle tone burst input of 10 VPP at 1.5 MHz
Figure D.11: Received voltage signal at 4 MHz from the transducer through
Lock-in Amplifier with a 1000-cycle tone burst input of 10 VPP at 2 MHz
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Figure D.12: Received voltage signal at 5 MHz from the transducer through
Lock-in Amplifier with a 1000-cycle tone burst input of 10 VPP at 2.5 MHz
Figure D.13: Received voltage signal at 6 MHz from the transducer through
Lock-in Amplifier with a 1000-cycle tone burst input of 10 VPP at 3 MHz
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Figure D.14: Received voltage signal at 7 MHz from the transducer through
Lock-in Amplifier with a 1000-cycle tone burst input of 10 VPP at 3.5 MHz
Figure 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
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Figure D.16: Received voltage signal at 8 MHz from the transducer through
Lock-in Amplifier with a 1000-cycle tone burst input of 10 VPP at 4 MHz
Figure 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
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Figure D.18: Received voltage signal at 10 MHz from the transducer through
Lock-in Amplifier with a 1000-cycle tone burst input of 10 VPP at 5 MHz
Figure D.19: Received voltage signal at 11 MHz from the transducer through
Lock-in Amplifier with a 1000-cycle tone burst input of 10 VPP at 5.5 MHz
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Figure D.20: Received voltage signal at 12 MHz from the transducer through
Lock-in Amplifier with a 1000-cycle tone burst input of 10 VPP at 6 MHz
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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
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Figure E.3: The electrical pads configuration on the PCB
Figure E.4: The electrical pads configuration on the Vertical connecting PCBs
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Appendix F
Hydrophone and Pre-Amplifier Calibration
Figure F.1: ONDA Hydrophone sensitivity calibration sensitivity certificate
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Figure F.2: ONDA Pre-Amplifier gain calibration certificate