Piezoelectric Micromachined Ultrasound Transducers Research Articles

Thin-Film Piezoelectric Micromachined Ultrasound Transducers in Biomedical Applications: A Review.

Thin-film piezoelectric micromachined ultrasound transducers (PMUTs) are an increasingly relevant and well-researched field, and their biomedical importance has been growing as the technology continues to mature. This review paper briefly discusses their history in biomedical use, provides a simple explanation of their principles for newer readers, and sheds light on the materials selection for these devices. Primarily, it discusses the significant applications of PMUTs in the biomedical industry and showcases recent progress that has been made in each application. The biomedical applications covered include common historical uses of ultrasound such as ultrasound imaging, ultrasound therapy, and fluid sensing, but additionally new and upcoming applications such as drug delivery, photoacoustic imaging, thermoacoustic imaging, biometrics, and intrabody communication. By including a device comparison chart for different applications, this review aims to assist MEMS designers that work with PMUTs by providing a benchmark for recent research works. Furthermore, it puts forth a discussion on the current challenges being faced by PMUTs in the biomedical field, current and likely future research trends, and opportunities for PMUT development areas, as well as sharing the opinions and predictions of the authors on the state of this technology as a whole. The review aims to be a comprehensive introduction to these topics without diving excessively deep into existing literature.

Piezoelectric Micromachined Ultrasonic Transducers with Micro-Hole Inter-Etch and Sealing Process on (111) Silicon Wafer.

Piezoelectric micromachined ultrasound transducers (PMUTs) have gained significant popularity in the field of ultrasound ranging and medical imaging owing to their small size, low power consumption, and affordability. The scar-free "MIS" (micro-hole inter-etch and sealing) process, a novel bulk-silicon manufacturing technique, has been successfully developed for the fabrication of pressure sensors, flow sensors, and accelerometers. In this study, we utilize the MIS process to fabricate cavity diaphragm structures for PMUTs, resulting in the formation of a flat cavity diaphragm structure through anisotropic etching of (111) wafers in a 70 °C tetramethylammonium hydroxide (TMAH) solution. This study investigates the corrosion characteristics of the MIS technology on (111) silicon wafers, arranges micro-pores etched on bulk silicon around the desired cavity structure in a regular pattern, and takes into consideration the distance compensation for lateral corrosion, resulting in a fully connected cavity structure closely approximating an ortho-hexagonal shape. By utilizing a sputtering process to deposit metallic molybdenum as upper and lower electrodes, as well as piezoelectric materials above the cavity structure, we have successfully fabricated aluminum nitride (AlN) piezoelectric ultrasonic transducer arrays of various sizes and structures. The final hexagonal PMUT cells of various sizes that were fabricated achieved a maximum quality factor (Q) of 251 and a displacement sensitivity of 18.49 nm/V across a range of resonant frequencies from 6.28 MHz to 11.99 MHz. This fabrication design facilitates the achievement of IC-compatible and cost-effective mass production of PMUT array devices with high resonance frequencies.

Demonstration of MHz piezoelectric micromachined ultrasonic transducers array and its potential application for biomedical treatment

In this paper, we successfully developed the 24 × 31 piezoelectric micromachined ultrasound transducers (PMUTs) array with 8 inch complementary metal oxide semiconductor (CMOS) compatible processes. Scandium aluminum nitride (ScxAl1−xN) thin film (x = 20%) was used as the piezoelectric material. The PMUTs array was fabricated on 8 inch cavity silicon-on-insulator wafers with the device silicon layer thickness of only 3 μm and a cavity pitch of 59 μm. The wafer-level electrical test was conducted using inline auto-prober and the yield rate >90% was achieved across the 8 inch wafer. The following acoustic test on our PMUTs array (1.5 mm × 2.0 mm) indicated the transmit sensitivity of 18.4 kPa V−1 @ 10 mm @ 11.6 MHz in oil with −3 dB bandwidth of 2.3 MHz. In order to validate the potential of using PMUTs array for biomedical treatments, subsequent ultrasonic heating experiments were carried out on a silicone block. A temperature rise of 4 °C was achievable in about 50 s. These results demonstrate that the potential of using PMUTs array as a disposable biomedical device in many therapeutic applications, for example, a treatment for dry eye disease.

Photoacoustic Imaging Based on Broadened Bandwidth Aluminum Nitride Piezoelectric Micromachined Ultrasound Transducers

This letter reports broadened bandwidth aluminum nitride (AlN) piezoelectric micromachined ultrasound transducers (BB-PMUT) for photoacoustic imaging (PAI). PMUT based on microelectromechanical systems (MEMS) technology has gained significant attention for PA endoscopic imaging inside colorectal and even blood vessels due to its miniature size, high sensitivity, and versatility. Broadened bandwidth, resulting from variable membrane cavity size during the fabrication process, provides benefits to imaging spatial resolution improvement. Moreover, AlN thin film is a nontoxic material, which makes it harmless for biomedical applications with potential of complementary metal-oxide semiconductor (CMOS) MEMS integration. In this letter, a BB-PMUT is designed and fabricated. The vibration modes, impedance, and bandwidth are analyzed. The BB-PMUT provides a wider fractional bandwidth (65%) for signal detection, which significantly increases the resolution compared with traditional single frequency PMUT design. We have conducted experimental validation on an agar sample to present the sensor's performance in improving axial resolution.

Improving PMUT Receive Sensitivity via DC Bias and Piezoelectric Composition.

The receive sensitivity of lead zirconate titanate (PZT) piezoelectric micromachined ultrasound transducers (PMUTs) was improved by applying a DC bias during operation. The PMUT receive sensitivity is governed by the voltage piezoelectric coefficient, h31,f. With applied DC biases (up to 15 V) on a 2 μm PbZr0.52Ti0.48O3 film, e31,f increased 1.6 times, permittivity decreased by a factor of 0.6, and the voltage coefficient increased by ~2.5 times. For released PMUT devices, the ultrasound receive sensitivity improved by 2.5 times and the photoacoustic signal improved 1.9 times with 15 V applied DC bias. B-mode photoacoustic imaging experiments showed that with DC bias, the PMUT received clearer photoacoustic signals from pencil leads at 4.3 cm, compared to 3.7 cm without DC bias.

Phased Array Based on AlScN Piezoelectric Micromachined Ultrasound Transducers Monolithically Integrated on CMOS

In this work we present an array of piezoelectric micromachined ultrasound transducer (PMUT’s) based on AlScN piezoelectric layer, monolithically integrated on top of a 130 nm CMOS circuit. The <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${7}\times {7}$ </tex-math></inline-formula> array along with the analog-front-end circuit has been configured to work as a linear phased array. Experiments carried out have demonstrated the homogeneous PMUT’s behavior, showing a small dispersion in terms of frequency, bandwidth, and sensitivity. The feasibility of easily performing a transmission beam steering and focusing has been proven. In particular, the beam directivity has been characterized in terms of the width of the main lobe. The integrated CMOS analog-front-end allows the reduction of parasitic elements and noise, improving the signal to noise ratio (SNR). As a proof of concept, a B-mode image, with a 16 dB dynamic range, using a single <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${7}\times {1}$ </tex-math></inline-formula> AlScN PMUT-on-CMOS channel as a receiver has been obtained.

Data-Over-Sound With PMUTs

Data-over-sound is an emerging technology for digital communication which uses frequencies at the upper bounds of human hearing, usually between 15 kHz to 25 kHz. We report a successful development of Piezoelectric Micromachined Ultrasound Transducers (PMUTs) for low-power data-over-sound applications. Piezoelectric thin films used in PMUTs can have high residual tensile stresses ranging from 300MPa to 1.5GPa. These stresses have the effect of increasing the resonant frequencies of the transducers, making it a challenge to fabricate low frequency devices. Using the optimum dimensions by estimating the net residual stress in the fabricated diaphragms, transducers suitable for a frequency range of 17 kHz to 21 kHz were fabricated, capable of generating as much as 83 dB of sound pressure level at a distance of 5 cm in continuous operation.

32 Element Piezoelectric Micromachined Ultrasound Transducer (PMUT) Phased Array for Neuromodulation

Interest in utilizing ultrasound (US) transducers for non-invasive neuromodulation treatment, including for low intensity transcranial focused ultrasound stimulation (tFUS), has grown rapidly. The most widely demonstrated US transducers for tFUS are either bulk piezoelectric transducers or capacitive micromachine transducers (CMUT) which require high voltage excitation to operate. In order to advance the development of the US transducers towards small, portable devices for safe tFUS at large scale, a low voltage array of US transducers with beam focusing and steering capability is of interest. This work presents the design methodology, fabrication, and characterization of 32-element phased array piezoelectric micromachined ultrasound transducers (PMUT) using 1.5 μm thick Pb(Zr0.52 Ti0.48)O3 films doped with 2 mol% Nb. The electrode/piezoelectric/electrode stack was deposited on a silicon on insulator (SOI) wafer with a 2 μm silicon device layer that serves as the passive elastic layer for bending-mode vibration. The fabricated 32-element PMUT has a central frequency at 1.4 MHz. Ultrasound beam focusing and steering (through beamforming) was demonstrated where the array was driven with 14.6 V square unipolar pulses. The PMUT generated a maximum peak-to-peak focused acoustic pressure output of 0.44 MPa at a focal distance of 20 mm with a 9.2 mm and 1 mm axial and lateral resolution, respectively. The maximum pressure is equivalent to a spatial-peak pulse-average intensity of 1.29 W/cm2, which is suitable for tFUS application.

Aluminum nitride piezoelectric micromachined ultrasound transducers with applications in sonic anemometry

Recent results demonstrate the promise of sonic anemometry for accurate, three dimensional, high update relative wind measurements in high altitude (stratospheric) balloons and for planetary science missions on Mars, Venus, and other environments (Banfield, JASA 2016, White, ASA 2020, White, AIAA 2020). For these applications, low weight and small size is desirable. We propose an aluminum nitride piezoelectric micromachined ultrasound transducer (PMUT) as a candidate for a small (∼1 cm3) low weight (∼10 g) sonic anemometry system. Two PMUT designs were fabricated using Vesper Technologies MEMS microphone process by Vanguard Semiconductor International on 1.4 × 1.4 mm2 die. The first device has 270 μm diameter diaphragms and operates at 610 kHz, the second 190 μm diameter diaphragms and operates at 890 kHz. By operating at these high frequencies, the acoustic path length can notionally be reduced to 1 cm, while still retaining sufficient phase resolution to measure wind with 1 to 10 cm/s accuracy. The substantial challenge is producing sufficient signal to overcome acoustic absorption at these elevated frequencies. This design is aggressively exploratory in nature. The PMUT devices have been fabricated and operate, but the sonic anemometry system has not yet been demonstrated. [Work supported by NASA PICASSO NNX16AJ24G, NSF 1934553 and NSF 2110083.]

10 MHz Thin-Film PZT-Based Flexible PMUT Array: Finite Element Design and Characterization.

Piezoelectric micromachined ultrasound transducers (PMUT) incorporating lead zirconate titanate PbZr0.52Ti0.48O3 (PZT) thin films were investigated for miniaturized high-frequency ultrasound systems. A recently developed process to remove a PMUT from an underlying silicon (Si) substrate has enabled curved arrays to be readily formed. This research aimed to improve the design of flexible PMUT arrays using PZFlex, a finite element method software package. A 10 MHz PMUT 2D array working in 3-1 mode was designed. A circular unit-cell was structured from the top, with concentric layers of platinum (Pt)/PZT/Pt/titanium (Ti) on a polyimide (PI) substrate. Pulse-echo and spectral response analyses predicted a center frequency of 10 MHz and bandwidth of 87% under water load and air backing. A 2D array, consisting of the 256 (16 × 16) unit-cells, was created and characterized in terms of pulse-echo and spectral responses, surface displacement profiles, crosstalk, and beam profiles. The 2D array showed: decreased bandwidth due to protracted oscillation decay and guided wave effects; mechanical focal length at 2.9 mm; 3.7 mm depth of field for -6 dB; and -55.6 dB crosstalk. Finite element-based virtual prototyping identified figures of merit—center frequency, bandwidth, depth of field, and crosstalk—that could be optimized to design robust, flexible PMUT arrays.

Fluid Density Sensing Using Piezoelectric Micromachined Ultrasound Transducers

We report on the suitability of a Piezoelectric Micromachined Ultrasound Transducer (PMUT) based system for fluid density measurement in both static and dynamic conditions. We exploit the well-known phenomenon of the virtual added mass induced changes in the resonant frequency of a structure vibrating in a fluid medium to design, fabricate and test a unique PMUT-fluid-PMUT (PFP) system as a fluid density sensor. The proposed system uses through-transmission of ultrasound waves from one PMUT transducer to another identical transducer with the test fluid in between and records the shift in the resonant peak of the receiver PMUT. We also study the sensitivity of the system using different sized PMUTs with resonant frequencies ranging from 140 kHz to 1.8 MHz and show how the system can be designed for a desired sensitivity. Further, we carry out experiments with dynamic density changes by mixing fluids to study how the system could be used to track changes in the density of a flowing liquid in real-time. We also study the effect of the distance separating the transmitter and the receiver to establish the utility of the system in both micro and macro scales in static as well as dynamic conditions.

A Photoacoustic Imaging Device Using Piezoelectric Micromachined Ultrasound Transducers (PMUTs).

A linear piezoelectric micromachined ultrasound transducer (PMUT) array was fabricated and integrated into a device for photoacoustic imaging (PAI) of tissue phantoms. The PMUT contained 65 array elements, with each element having 60 diaphragms of [Formula: see text] diameter and [Formula: see text] pitch. A lead zirconate titanate (PZT) thin film was used as the piezoelectric layer. The in-air vibration response of the PMUT array elements showed a first mode resonance between 6 and 8 MHz. Hydrophone measurements showed 16.2 kPa average peak ultrasound pressure output at 7.5 mm from one element excited with 5 Vpp input. A receive sensitivity of ~0.48 mV/kPa was observed for a PMUT array element with 0 dB gain. The PMUT array was bonded to a custom-printed circuit board to enable compact integration with an optical fiber bundle for PAI. A broad photoacoustic bandwidth of ~89% was observed for the photoacoustic response captured from absorbing pencil lead targets. Linear scanning of a single element of a PMUT array was performed on different tissue phantoms embedded with light-absorbing targets to successfully demonstrate B-mode PAI using PMUTs.

Evaluation of High Frequency Piezoelectric Micromachined Ultrasound Transducers for Photoacoustic Imaging.

In this work, the design, fabrication, and characterization of piezoelectric micromachined ultrasound transducer (PMUT) arrays for photoacoustic imaging applications are reported. An 80-element linear PMUT array with each element having 53 PMUT cells of 125 μm cell diameter were fabricated using 650 nm thick lead zirconate titanate (PZT) as the active piezoelectric layer. The PMUTs are designed to operate at ~10 MHz resonant frequency. The PMUT elements are validated for photoacoustic imaging using an agar gel phantom with embedded pencil leads as the imaging target. Photoacoustic A-line response of the targets captured by single PMUT element shows ~7 MHz center frequency with ~4.8 MHz bandwidth. B-mode images reconstructed from A-lines recorded during the linear scanning of a single element clearly imaged all the targets, thus validating the potential of the fabricated PMUTs for photoacoustic imaging.

Ultrasound Transducers for Biomedical Imaging and Therapy

Ultrasound Transducers for Biomedical Imaging and Therapy

(Invited) Fluid Spectroscopy with Piezoelectric Ultrasound MEMS Transducers

Piezoelectric micromachined ultrasound transducers (PMUTs) are making rapid strides in terms of application development in varied areas such as proximity sensing, gesture recognition, medical imaging, photoacoustics, etc. Our ability to fabricate these devices more reliably and integrate them in suitable form factors is paving the way for new and promising applications. PMUTs are now expanding in terms of their fabrication technology as well, with the introduction of new active materials such as AlN and ZnO. In particular, AlN based PMUTs hold great promise because of their CMOS compatibility. We have been exploring PMUT fabrication using three different materials — PZT, AlN and ZnO — and integrating them in fluidic environments for developing fluid spectroscopy. Probing fluids in very small volumes for detecting changes in their properties such as density, viscosity, and homogeneity can pave the way for online and in-situ monitoring of various fluids in applications as wide ranging as petrochemical industries to medical emergency care. In most health monitoring applications, whether for machines or humans, the emphasis is on early detection of onset of undesirable conditions. For fluids, minute but persistent changes in their density and viscosity can be used as signatures for such onsets. PMUTs hold promise for such detection because of the ease with which they can scan for these changes. We have carried out experiments with PMUTs in different fluids to find spectral shifts with change in their density and viscosity. In this paper, we show how these shifts are quantitatively related to density variation. We report a density sensitivity of 145 Hz/(kg/m3) with a 500 kHz PMUT. We also show some initial results with an array of PMUTs of different frequencies — a new technique we are developing for fluid spectroscopy. PMUTs spanning a range of frequencies and embedded in microfluidic channels are intended for probing fluid inhomogeneity due to particulate suspensions or variations in discrete constituents such as cells in biofluids. Our PMUTs in these studies employ the same material stack for the individual PMUT-cell structures with thin film PZT as the active material, and the frequency variation is achieved simply from geometric scaling. This approach provides great flexibility in terms of building frequency-pixelation of desired resolution. The study of the relationship between frequency-pixelation and the resolution of inhomogeneity in fluids does not exist and is currently underway in our research group.

(Invited) Fluid Spectroscopy with Piezoelectric Ultrasound MEMS Transducers

In this work, Piezoelectric Micromachined Ultrasound Transducers (PMUTs) are explored as a potential MEMS solution for fluid property monitoring. PZT based PMUTs fabricated with three different diameters—1000 μm, 500 μm, 250 μm—showing in-air resonant frequencies between 100 kHz to 2 MHz are tested in water-glycerol mixtures with glycerol concentrations varying between 0% to 80% in both static and dynamic conditions. Spectral shifts of PMUT vibrations with changing fluid environment are quantitatively related to the density and viscosity of the fluid. The density sensitivity of the PMUTs is found to increase from 27 Hz/kg.m-3 for 1000 μm to 365 Hz/kg.m-3 for 250 µm, which is in close agreement with theoretical expectations. Similarly, viscosity sensing is also explored using PMUT arrays. A through-the-fluid ultrasound transmission architecture is proposed for a potential fluid property sensor that can fill the gap of miniature sensors for sensing physical properties of various fluids.

Design, Fabrication, and Characterization of Scandium Aluminum Nitride-Based Piezoelectric Micromachined Ultrasonic Transducers

This paper presents the design, fabrication, and characterization of piezoelectric micromachined ultrasound transducers (PMUTs) based on scandium aluminum nitride (ScxAl1–xN) thin films (x = 15%). ScAlN thin film was prepared with a dual magnetron system and patterned by a reactive ion etching system utilizing chlorine-based chemistry with an etching rate of 160 nm/min. The film was characterized by X-ray diffraction, which indicated a crystalline structure expansion compared with pure AlN and a well-aligned ScAlN film. ScAlN PMUTs were fabricated by a two-mask process based on cavity SOI wafers. ScAlN PMUTs with 50- and 40- $\mu \text{m}$ diameter had a large dynamic displacement sensitivity measured in air of 25 nm/V at 17 MHz and 10 nm/V at 25 MHz, twice that of AlN PMUTs with the same dimensions. The peak displacement as a function of electrode coverage was characterized, with maximum displacement achieved with an electrode radius equal to 70% of the PMUT radius. Electrical impedance measurements indicated that the ScAlN PMUTs had 36% greater electromechanical coupling coefficient ( $\text{k}_{\mathrm {t}}^{2})$ compared with AlN PMUTs. The output pressure of a $7\times7$ ScAlN PMUT array was 0.7 kPa/V at ~1.7 mm away from the array, which is approximately three times greater that of an $8\times8$ AlN PMUT array with the same element geometry and fill factor measured at the same distance. Acoustic spreading loss and PMUT insertion loss from mechanical transmit to receive were characterized with a 15 $\times $ 15 ScAlN PMUT array via hydrophone and laser Doppler vibrometer. [17509-2017]

Piezoelectric micromachined ultrasound transducer (PMUT) arrays for integrated sensing, actuation and imaging.

Many applications of ultrasound for sensing, actuation and imaging require miniaturized and low power transducers and transducer arrays integrated with electronic systems. Piezoelectric micromachined ultrasound transducers (PMUTs), diaphragm-like thin film flexural transducers typically formed on silicon substrates, are a potential solution for integrated transducer arrays. This paper presents an overview of the current development status of PMUTs and a discussion of their suitability for miniaturized and integrated devices. The thin film piezoelectric materials required to functionalize these devices are discussed, followed by the microfabrication techniques used to create PMUT elements and the constraints the fabrication imposes on device design. Approaches for electrical interconnection and integration with on-chip electronics are discussed. Electrical and acoustic measurements from fabricated PMUT arrays with up to 320 diaphragm elements are presented. The PMUTs are shown to be broadband devices with an operating frequency which is tunable by tailoring the lateral dimensions of the flexural membrane or the thicknesses of the constituent layers. Finally, the outlook for future development of PMUT technology and the potential applications made feasible by integrated PMUT devices are discussed.