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Citation: Jia, L.; Liang, Y.; Meng, F.;
Zhang, G.; Wang, R.; He, C.; Yang, Y.;
Cui, J.; Zhang, W.; Wu, G.
Performance-Enhanced Piezoelectric
Micromachined Ultrasonic Transducers
by PDMS Acoustic Lens Design.
Micromachines 2024,15, 795. https://
doi.org/10.3390/mi15060795
Academic Editors: Klaus Stefan Drese
and Huikai Xie
Received: 20 April 2024
Revised: 5 June 2024
Accepted: 7 June 2024
Published: 17 June 2024
Copyright: © 2024 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
micromachines
Article
Performance-Enhanced Piezoelectric Micromachined Ultrasonic
Transducers by PDMS Acoustic Lens Design
Licheng Jia 1,* , Yong Liang 1, Fansheng Meng 1, Guojun Zhang 1, Renxin Wang 1, Changde He 1, Yuhua Yang 1,
Jiangong Cui 1, Wendong Zhang 1and Guoqiang Wu 2
1Key Laboratory of Instrumentation Science and Dynamic Measurement, North University of China,
Taiyuan 030051, China; s202106112@st.nuc.edu.cn (Y.L.); sz202206139@st.nuc.edu.cn (F.M.);
zhangguojun1977@nuc.edu.cn (G.Z.); wangrenxin@nuc.edu.cn (R.W.); hechangde@nuc.edu.cn (C.H.);
13513641974@163.com (Y.Y.); cuijiangong99999@163.com (J.C.); wdzhang@nuc.edu.cn (W.Z.)
2Hubei Key Laboratory of Electronic Manufacturing and Packaging Integration, Institute of Technological
Sciences and the School of Microelectronics, Wuhan University, Wuhan 430072, China;
wuguoqiang@whu.edu.cn
*Correspondence: jialicheng@nuc.edu.cn
Abstract: This paper delves into enhancing the performance of ScAlN-based Piezoelectric Micro-
machined Ultrasonic Transducers (PMUTs) through the implementation of Polydimethylsiloxane
(PDMS) acoustic lenses. The PMUT, encapsulated in PDMS, underwent thorough characterization
through the utilization of an industry-standard hydrophone calibration instrument. The experimental
results showed that the ScAlN-based PMUT with the PDMS lenses achieved an impressive sensitivity
of
−
160 dB (re: 1 V/
µ
Pa), an improvement of more than 8 dB compared to the PMUT with an
equivalent PDMS film. There was a noticeable improvement in the
−
3 dB main lobe width within the
frequency response when comparing the PMUT with PDMS encapsulation, both with and without
lenses. The successful fabrication of high-performance PDMS lenses proved instrumental in signifi-
cantly boosting the sensitivity of the PMUT. Comprehensive performance evaluations underscored
that the designed PMUT in this investigation surpassed its counterparts reported in the literature and
commercially available transducers. This encouraging outcome emphasizes its substantial potential
for commercial applications.
Keywords: PMUT array; PDMS; acoustic lenses; sensitivity
1. Introduction
The field of underwater acoustics continually strives for advancements in sensor tech-
nologies to meet the demands of diverse applications, such as environmental monitoring,
marine research, and underwater communication systems [
1
–
6
]. In the field of underwater
acoustics, ultrasonic transducers play a crucial role. Ultrasonic transducers exhibit excep-
tional performance in underwater environments, providing essential support for various
key applications. Whether in underwater communication systems, marine research, or
environmental monitoring, the role of ultrasonic transducers is paramount. They have
the ability to convert acoustic signals into electrical signals, facilitating the transmission,
reception, and interpretation of these signals underwater.
To date, the most advanced transducer available in the market are constructed using
bulky piezoceramic materials through traditional precision manufacturing technologies
[7–10].
However, the emergence of microelectromechanical system (MEMS) technology has sparked
interest in aluminum nitride (AlN)-based PMUTs due to their compatibility with CMOS
processes [
11
–
13
]. In comparison to other ultrasonic transducer technologies, AlN-based
PMUTs often require lower bias voltages, enhancing their energy efficiency
[14,15].
The
ease of fabrication and compatibility with mainstream system-in-packaging (SiP) tech-
nologies further solidify PMUTs as a promising choice for next-generation ultrasonic
Micromachines 2024,15, 795. https://doi.org/10.3390/mi15060795 https://www.mdpi.com/journal/micromachines
Micromachines 2024,15, 795 2 of 10
applications
[16–18].
The sensitivity and directivity of AlN-based PMUTs are paramount
for accurately detecting and analyzing underwater sounds, ranging from marine life com-
munication to detecting potential threats.
One way to enhance the performance of AlN-based PMUTs is improving the piezo-
electric coefficient of the materials and innovative structure designs, such as inducing a
change in the mode shape from Gaussian-like to piston-like [
19
–
21
]. Other approaches to
enhance AlN-based performance include manipulating the material properties through
the utilization of dimpled piezoelectric elements, implementing a dual-electrode bimorph
design [
22
], and adopting a dual-electrode design [
23
]. Methods of sound focusing in-
volve controlling the propagation and concentration of sound waves to achieve precise
manipulation of specific areas. Common techniques for sound focusing include acoustic
lense focusing and phased array technology. In recent years, the integration of acoustic
lens technology has emerged as a promising avenue for advancing transducer capabilities.
Acoustic lenses, drawing inspiration from their optical counterparts, offer the potential
to focus, steer, and enhance acoustic signals. This technology presents a transformative
opportunity to elevate PMUT sensitivity, improve noise resolution, and extend detection
ranges. PDMS, known for its biocompatibility, flexibility, and ease of fabrication, presents
itself as an ideal material for constructing acoustic lenses tailored to PMUTs. Acoustic
lenses play a pivotal role in focusing and directing incoming acoustic waves onto PMUTs,
optimizing sensitivity, and improving the overall functionality of the transducer.
This paper describes the innovative design and implementation of PDMS acoustic
lenses to improve the performance of ScAlN-based PMUTs in underwater environments.
The underwater domain presents unique challenges for PMUTs, requiring specialized en-
hancements to overcome sensitivity and directivity issues. By focusing on the incorporation
of PDMS acoustic lenses, this study aims to address these challenges and open up new
possibilities for PMUT applications in underwater scenarios.
2. PMUT Array Design
The cross-sectional view of the discussed PMUT, crafted using a piezoelectric-on-
cavity silicon-on-insulator (CSOI) platform, is showcased in Figure 1a. It comprises a
1
µ
m-thick ScAlN piezoelectric thin film, positioned between a 0.15
µ
m thick molybdenum
layer serving as the top electrode and a 5.2
µ
m thick highly doped silicon (HDS) device
layer acting as the bottom electrode. The cross-sectional SEM image of the sputtered ScAlN
piezoelectric thin-film is illustrated in Figure 1b. Figure 1c shows an optical microscope
image of the PMUT. The PMUT, with dimensions of 4 mm
×
4 mm, is configured in
a honeycomb architecture, as depicted in Figure 1c. The key design parameters of the
reported PMUT array are listed in Table 1.
Table 1. Detailed design parameters of PMUT array.
Parameter Value
Array length 4 mm
Array width 4 mm
Piezoelectric layer thickness 1 µm
Diaphragm characteristic size 500 µm
Electrode thickness 0.3 µm
Top Oxide layer thickness 0.2 µm
Bottom Oxide layer thickness 1 µm
Gap height 300 µm
Characteristic size 500 µm
Number of cells per array 56
Micromachines 2024,15, 795 3 of 10
Figure 1. ScAlN-thin-film-based PMUT. (a) Cross-sectional view of the PMUT structure. (b) Cross-sectional
SEM image of a deposited ScAlN thin-film. (c) Optical microscope image of a fabricated PMUT.
3. Lens Design
As acoustic waves propagate, the lens converges or diverges them to achieve a desired
focal point. This focusing mechanism enhances the sensitivity and resolution of PMUTs,
enabling precise detection and characterization of targets in the acoustic field. The effective-
ness of the acoustic lens in shaping and concentrating sound waves contributes to improved
performance in applications such as medical imaging, underwater communication, and
industrial sensing. The process of acoustic lens focusing involves the precise manipulation
of sound waves through a carefully designed lens structure. The acoustic lens-focusing
process is shown in Figure 2a. Figure 2b illustrates the packaging structure of the PMUT
acoustic lens, encompassing the PDMS acoustic convex lens, PMUT, preamplifier circuit,
and packaging tube.
Figure 2. The acoustic lenses’ structure. (a) The acoustic lenses’ focusing process. (b) The packaging
structure of the PMUT acoustic lenses.
The amplitude of the transmitted wave is subject to variations induced by both the
attenuation occurring within the lens and the refraction at its boundary. These factors play
a crucial role in shaping the characteristics of the transmitted wave, impacting its overall
behavior and signal integrity. Understanding and managing these effects is essential for
optimizing the performance of the acoustic lens and ensuring accurate and reliable signal
transmission in applications such as underwater communication systems.
The transmission coefficient, Tl, through the lens material can be articulated as:
Tl=10−α(f)∗H/10 (1)
where
α(f) = α(0) + αf∗(f−f0)(2)
where H and
f
are the thickness of the material and the frequency, respectively.
α(f)
is the
frequency-dependent attenuation coefficient of the material. The attenuation coefficient of
the material at reference frequency f0is represented by α(0).
It is essential to emphasize that
Tl
exponentially depends on frequency, influencing
the shape of the frequency impulse response. This leads to a shift in the center frequency
Micromachines 2024,15, 795 4 of 10
and a reduction in bandwidth. The attenuation coefficient is particularly crucial for high-
frequency transducers, as signals at these frequencies can be completely attenuated.
The transmission coefficient (
Tr
) for transmitted power after partial refraction at the
boundary is given by:
Tr=4ZlZm
(Zm+Zl)2(3)
where
Zm
and
Zl
are the acoustic impedance of the imaging medium and lens material,
respectively. The transmitted wave power increases when the ratio
Zm
/
Zl
is close to 1,
which is a constraint on the density and speed of sound in the selected material.
The total transmitted power (
Ttot
) is then given by the product of the two transmission
coefficients:
Ttot =Tl∗Tr(4)
To optimize the transmitted wave amplitude, Equation
(6)
suggests that the lens
should be minimized in thickness and the material should possess a low attenuation
coefficient. In general, the formula commonly used to determine the required radius of
curvature (Rc) for an acoustic lens is:
RC=Fe f f ×VH−VL
VL
(5)
where
Fe f f
is the desired effective geometric focal distance,
VH
is the sound speed of human
tissue, and VLis the sound speed of the acoustic lens material (VL<VHin this study).
Furthermore, the thickness of the acoustic lens (TL) is expressed by the following
equation:
TL=RC−qR2
C−(D/2)2(6)
where
RC
is the radius of curvature and
D
is the aperture length. The characteristics of
the acoustic lens can be summarized in two aspects. Firstly, the radius of curvature of the
acoustic lens increases as the sound speed of the acoustic lens decreases. Secondly, the
thickness of the acoustic lens decreases as the radius of the lens increases, considering the
same aperture size.
Achieving a suitable impedance match between the acoustic lens and the working
environment medium is imperative to prevent the occurrence of imaging artifacts. This
meticulous alignment ensures optimal signal transmission and reception, contributing
to the overall quality and accuracy of the imaging process. Undoubtedly, if the wave
undergoes substantial reflection within the lens, it leads to the generation of secondary
echoes, appearing as reverberations in the ultrasound image. To mitigate this phenomenon,
minimizing the power ratio of the reflected to transmitted wave (υ) is crucial.
υ=(Zm−Zl)2
4ZmZl
(7)
The optimal acoustic transmission and minimal impedance mismatches in ultrasound
imaging are influenced by specific parameters, especially the impedance matching between
the lens material and imaging medium. Figure 3visually demonstrates the interplay among
frequency, thickness, and the transmission coefficient of the acoustic lens. Additionally,
Figure 3a outlines the critical relationship between the transmission coefficient and fre-
quencies. High-frequency transducers, crucial for ultrasound imaging, are significantly
impacted by the attenuation coefficient, which can potentially completely dampen signals
at these frequencies. Moreover, the thickness of the acoustic lens plays a substantial role in
reducing both total acoustic attenuation and the acoustic attenuation of the lens material,
as illustrated in Figure 3b.
Micromachines 2024,15, 795 5 of 10
Figure 3. Relationship among frequency, thickness, and transmission coefficient of acoustic lenses.
(a) Calculated transmission coefficients at various frequencies for an acoustic lens thickness of
5 mm
.
(b) Calculated transmission coefficients at 220 kHz for various thicknesses.
4. Lens Fabrication
The stepwise process for encapsulating the PMUT with an acoustic lens is depicted in
Figure 4. Initially, the required amount of PDMS is measured and thoroughly mixed in a
beaker to eliminate any entrapped air bubbles (step 1). Subsequently, the PDMS is subjected
to degassing in a vacuum drying chamber oven (step 2), with steps 1 and 2 repeated until
complete removal of the bubbles. Moving forward, the PMUT and preamplifier circuitry
are assembled within a custom packaging shell (step 3). The PDMS is carefully poured into
the tube shell, positioned in the oven, and heated at 90
◦
C for 120 min (step 4). Following
this, the mold is removed, allowing the material to cool, and the solidified PMUT acoustic
lens is released from the cavity (step 5). The accomplished PMUT acoustic lens is visually
presented in step 6.
PDMS lenses with a carefully selected radius of curvature have been successfully
manufactured. Full details of these lenses can be found in Table 2.
Table 2. Lens properties.
Property No Lens (Water) PDMS Film PDMS Lens
Lens Material
vl(m/s) 1480 930 930
ρ(kg/m−3)1.0 ×1030.97 ×1030.97 ×103
Z(MRayl) 1.48 0.9 0.9
α0at 6 MHz (dB/cm) 0.0022 31.0 31.0
αf(dB/cm/MHz) −7.6 7.6
Micromachines 2024,15, 795 6 of 10
Table 2. Cont.
Property No Lens (Water) PDMS Film PDMS Lens
Lens Geometry
Shape −No lens Convex
Radius (mm) −42.64 42.64
Thickness (mm) −4 4
Response
Resonance frequency
(kHz) 221 183 183
Sensitivity (re: 1 V/µPa) −163 −168 −160
Figure 4. The PMUT acoustic lenses’ fabrication process.
5. Performance Characterization
Utilizing the advanced capabilities of the PolyTec MSA-600 LDV ensures accurate
characterization, facilitating a comprehensive analysis of the PMUT’s frequency response.
This approach is essential to evaluate and optimize the performance of the PMUT for its
intended applications. Figure 5shows a comparison of the frequency responses of the
PMUT without a lens and with a PDMS lens, obtained using a Polytec MSA-600 LDV. The
resonant frequencies of the PMUT are measured at 221 kHz without a lens and 183 kHz
with the PDMS lens.
The measurement of acoustic pressure sensitivity comprises two steps. As depicted
in Figure 6, initially, a pair of standard piezoelectric transducers with a resonance fre-
quency of 180 kHz (this transducer is made from piezoelectric ceramic wafers and is a
type HPCTB-180-20-II standard piezoelectric transducer, calibrated and certified by the
first-level metrological station for underwater acoustics of China’s defense science and
technology industry) is fixed at the transmitting and receiving ends, facing each other with
a distance of 10 cm. The transmitting end is driven by an AFG31000 continuous signal gen-
erator, powering an ATA-4315 high-voltage power amplifier to apply a 30 Vpp sinusoidal
pulse AC signal with five cycles from 100 kHz to 300 kHz. The voltage amplitude at the
receiving end is captured using a DSOX3014G digital storage oscilloscope, which is then
converted to determine the acoustic pressure (P) on the surface of the standard piezoelectric
transducer (PZT-180 kHz). Subsequently, while maintaining the transmitting end and its
parameter settings unchanged, the standard piezoelectric transducer (PZT-180 kHz) at the
receiving end is replaced with the PMUT equipped with a PDMS lens/film. The voltage
signal amplitude (U) at the receiving end is then re-collected, and the acoustic pressure
sensitivity of the PMUT with the PDMS lens/film is calculated using the values of P and U.
Micromachines 2024,15, 795 7 of 10
Figure 5. Comparison frequency responses of the PMUT with no lenses and PDMS lenses, obtained
by a Polytec MSA-600 LDV.
Figure 6. The PMUT sensitivity test diagram with PDMS lens (step 2).
A detailed comparison of the acoustic pressure sensitivity between the PMUT with
a convex lens and the PMUT without a lens reveals a significant difference, as shown
in Figure 7. The PMUT with a convex lens exhibits a measured acoustic pressure sen-
sitivity exceeding
−
160 dB (re: 1 V/
µ
Pa) at 200 kHz. The PMUT with the convex lens
exhibits approximately 10 dB higher acoustic pressure sensitivity compared to the PMUT
without a lens. The measured acoustic pressure sensitivity is in good agreement with the
theoretical values.
In a carefully designed directivity experiment, we employed a standard piezoelectric
transducer (PZT-180 kHz) as the receiving end, positioned within a water tank. Meanwhile,
a PMUT equipped with the PDMS lens/film served as the transmitting end, precisely
situated below a precision graduated turntable, maintaining a 10 cm linear distance and
facing the receiving end. To drive the PMUT, we utilized an AFG31000 continuous signal
generator, coupled with an ATA-4315 high-voltage power amplifier, to apply a sinusoidal
pulse AC signal with a frequency of 200 kHz, spanning five cycles, and an amplitude
of 30 Vpp. At the receiving end, we relied on a DSOX3014G digital storage oscilloscope
for precise data acquisition. Initially, we adjusted the signal to its maximum intensity in
Micromachines 2024,15, 795 8 of 10
the horizontal direction. Subsequently, by rotating the precision graduated turntable, we
identified the specific graduation at which the signal reached its maximum and marked
it as the 0
◦
position. Immediately following that, we rotated the turntable from
−
100
◦
to
100
◦
in 1
◦
increments, capturing the amplitude data of the received signals sequentially.
Ultimately, we normalized the collected voltage amplitude data.
Figure 7. Comparison of acoustic pressure sensitivity of PMUTs with convex lenses and PMUTs
without lenses. The PMUTs with convex lenses had approximately 8 dB more acoustic pressure
sensitivity than the PMUTs without lenses.
The results of directional testing revealed variations in sensitivity at different azimuthal
angles for the PMUTs, both with and without the convex lens, as illustrated in Figure 8. The
presence of the convex lens enhances the directivity of the PMUT, showing a more focused
and directional response compared to the configuration without a lens. These findings
underscore the importance of the lens in shaping and optimizing the directional sensitivity
of the PMUT for specific applications.
Figure 8. The normalized directivity at various azimuthal angles is obtained from measurements
with both a convex lens and no lens for the PMUTs.
Table 3provides a comprehensive illustration of the performance comparison between
the reported PMUT with a PDMS lens and those documented in the literature, as well
as with advanced commercially available transducer. The PMUT with a PDMS lens, as
Micromachines 2024,15, 795 9 of 10
investigated in this study, shows exceptional performance, characterized in particular by
its comparatively high sensitivity. This promising result suggests significant potential for
commercial applications.
Table 3. Comparative analysis of the performance between the developed PMUT with a PDMS lens
and advanced commercially available transducers.
Hydrophone Technology Encapsulation Lens Size Sensitivity (dB, re: 1 V/µPa)
DophinEar DE200 [8] Piezoceramic Polyurethane No cm level −209 ±1.5
Aquarian H2a [9] Piezoceramic Polyurethane No cm level −180 ±4
Brüel&Kjær 8103 [10] Piezoceramic Polyurethane No cm level −211 ±2
Ref. [17] AlN Polyurethane No 3.5 mm ×3.5 mm −182 ±0.3
This work ScAlN PDMS No 4 mm ×4 mm −168
This work ScAlN PDMS Yes 4 mm ×4 mm −160
6. Conclusions
This paper presents a successful exploration of performance enhancement of Piezoelec-
tric Micromachined Ultrasonic Transducers (PMUTs) through the incorporation of PDMS
acoustic lenses. The fabrication of high-performance PDMS lenses has proven to be a
key factor in significantly improving the sensitivity of the PMUTs, as evidenced by the
achieved sensitivity of
−
167.5 dB (re: 1 V/
µ
Pa). The observed enhancement in the
−
3 dB
main lobe width within the frequency response further supports the efficacy of the PDMS
lens design. Through detailed performance comparisons, it has been established that the
designed PMUT in this study surpasses its counterparts documented in the literature and
commercially available transducers. The promising outcomes obtained underscore the
considerable potential of the designed PMUT for diverse commercial applications in the
field of ultrasonic transduction.
Author Contributions: Conceptualization, L.J., G.W. and W.Z.; methodology, L.J., Y.L. and W.Z.;
simulation, Y.L. and L.J.; validation, L.J.; fabrication, L.J. and G.W.; writing—original draft preparation,
Y.L. and L.J.; writing—review and editing, F.M., Y.Y., J.C., C.H., R.W. and G.Z.; project administration,
W.Z. All authors have read and agreed to the published version of the manuscript.
Funding: This research was supported in part by the National Key Research and Development
Project of China under Grant 2023YFB3211205, in part by the National Natural Science Foundation of
China under Grants 61927807, 62320106011, and 62304208, in part by the China Postdoctoral Science
Foundation under Grant 2023M733277, in part by the Shanxi Province Science Foundation for Youths
under Grant 202203021222025, and in part by the Open Fund of Hubei Key Laboratory of Electronic
Manufacturing and Packaging Integration (Wuhan University) under Grant EMPI2023001.
Data Availability Statement: The data are available upon request from the authors.
Conflicts of Interest: The authors declare no conflicts of interest.
References
1.
Ingraham, J.M.; Deng, Z.D.; Martinez, J.J.; Trumbo, B.A.; Mueller, R.P.; Weiland, M.A. Feasibility of tracking fish with acoustic
transmitters in the ice harbor dam tailrace. Sci. Rep. 2014,4, 4090. [CrossRef] [PubMed]
2.
Herrera, B.; Pop, F.; Cassella, C.; Rinaldi, M. Miniaturized PMUT-based receiver for underwater acoustic networking. J.
Microelectromech. Syst. 2020,29, 832–838. [CrossRef]
3.
Yang, D.; Yang, L.; Chen, X.; Qu, M.; Zhu, K.; Ding, H.; Li, D.; Bai, Y.; Ling, J.; Xu, J.; et al. A piezoelectric AlN MEMS hydrophone
with high sensitivity and low noise density. Sens. Actuators A 2021,318, 112493. [CrossRef]
4.
Liu, X.; Chen, D.; Yang, D.; Chen, X.; Le, X.; Xie, J. A computational piezoelectric micro-machined ultrasonic transducer toward
acoustic communication. IEEE Electron Device Lett. 2019,40, 965–968. [CrossRef]
5.
Przybyla, R.; Flynn, A.; Jain, V.; Shelton, S.; Guedes, A.; Izyumin, I.; Horsley, D.; Boser, B. A micromechanical ultrasonic distance
sensor with >1 meter range. In Proceedings of the 2011 16th International Solid-State Sensors, Actuators and Microsystems
Conference, Beijing, China, 5–9 June 2011; pp. 2070–2073.
6.
Almeida, R.; Cruz, N.; Matos, A. Synchronized intelligent buoy network for underwater positioning. In Proceedings of the
OCEANS 2010 MTS/IEEE SEATTLE, Seattle, WA, USA, 20–23 September 2010; pp. 1–6.
Micromachines 2024,15, 795 10 of 10
7.
Benthowave Instrument Inc. Product Datasheet. Available online: https://www.benthowave.com/products/BII-7150
Hydrophone.html (accessed on 6 June 2024).
8. DolphinEar Hydrophones, Product Datasheet. Available online: http://www.dolphinear.com/de200.html (accessed on 6 June 2024).
9.
H2a Hydrophone User’s Guide; Aquarian Audio: Anacortes, WA, USA. Available online: https://www.aquarianaudio.com/
(accessed on 6 June 2024).
10.
Brüel&Kjær, “Hydrophones-Types 8103, 8104, 8105 and 8106”. September 2017. Available online: https://www.bksv.com/zh/
transducers/acoustic/microphones/hydrophones/ (accessed on 6 June 2024).
11.
Jia, L.; Shi, L.; Sun, C.; Liu, S.; Wu, G. AlN based piezoelectric micromachined ultrasonic transducers for continuous monitoring
of the mechano-acoustic cardiopulmonary signals. In Proceedings of the 2021 IEEE 34th International Conference on Micro
Electro Mechanical Systems (MEMS), Gainesville, FL, USA, 25–29 January 2021; pp. 426–429.
12.
Jia, L.; Shi, L.; Liu, C.; Sun, C.; Wu, G. Enhancement of transmitting sensitivity of piezoelectric micromachined ultrasonic
transducers by electrode design. IEEE Trans. Ultrason. Ferroelectr. Freq. Control. 2021,68, 3371–3377. [CrossRef] [PubMed]
13.
Jiang, X.; Lu, Y.; Tang, H.Y.; Tsai, J.M.; Ng, E.J.; Daneman, M.J.; Boser, B.E.; Horsley, D.A. Monolithic ultrasound fingerprint sensor.
Microsyst. Nanoeng. 2017,3, 17059. [CrossRef] [PubMed]
14.
Wang, T.; Sawada, R.; Lee, C. A piezoelectric, micromachined ultrasonic transducer using piston-like membrane motion. IEEE
Electron Device Lett. 2015,36, 957–959. [CrossRef]
15.
Muralt, P. PZT thin films for microsensors and actuators: Where do we stand? IEEE Trans. Ultrason. Ferroelectr. Freq. Control. 2000,
47, 903–915. [CrossRef] [PubMed]
16.
Ledesma, E.; Zamora, I.; Uranga, A.; Barniol, N. 9.5% Scandium doped ALN PMUT compatible with pre-processed CMOS
substrates. In Proceedings of the 2021 IEEE 34th International Conference on Micro Electro Mechanical Systems (MEMS),
Gainesville, FL, USA, 25–29 January 2021; pp. 887–890.
17.
Xu, J.; Zhang, X.; Fernando, S.N.; Chai, K.T.; Gu, Y. AlN-on-SOI platform-based micro-machined hydrophone. Appl. Phys. Lett.
2016,109, 032902. [CrossRef]
18.
Xu, J.; Chai, K.T.; Wu, G.; Han, B.; Wai, E.L.; Li, W.; Yeo, J.; Nijhof, E.; Gu, Y. Low-cost, tiny-sized MEMS hydrophone sensor for
water pipeline leak detection. IEEE Trans. Ind. Electron. 2018,66, 6374–6382. [CrossRef]
19.
Wang, M.; Zhou, Y.; Randles, A. Enhancement of the transmission of piezoelectric micromachined ultrasonic transducer with an
isolation trench. J. Microelectromech. Syst. 2016,25, 691–700. [CrossRef]
20.
Chen, X.; Chen, D.; Liu, X.; Yang, D.; Pang, J.; Xie, J. Transmitting sensitivity enhancement of piezoelectric micromachined
ultrasonic trans-ducers via residual stress localization by stiffness modification. IEEE Electron Device Lett. 2019,40, 796–799.
[CrossRef]
21.
Liang, Y.; Eovino, B.; Lin, L. Piezoelectric micromachined ultrasonic transducers with pinned boundary structure. J. Microelec-
tromech. Syst. 2020,29, 585–591. [CrossRef]
22.
Akhbari, S.; Sammoura, F.; Eovino, B.; Yang, C.; Lin, L. Bimorph piezoelectric micromachined ultrasonic transducers. J.
Microelectromech. Syst. 2016,25, 326–336. [CrossRef]
23.
Akhbari, S.; Sammoura, F.; Yang, C.; Mahmoud, M.; Aqab, N.; Lin, L. Bimorph pMUT with dual electrodes. In Proceedings of
the 28th IEEE International Conference on Micro Electro Mechanical Systems (MEMS), Estoril, Portugal, 18–22 January 2015;
pp. 928–931.
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