Piezoelectric Micromachined Ultrasound Transducer Research Articles

Synchronous gesture recognition and muscle force estimation based on piezoelectric micromachined ultrasound transducer

Early prosthetic control research mainly focused on motion intention recognition, but in recent years, study on control force has emerged. The simultaneous integration of motion intention recognition and control force estimation has significant practical value. This paper proposes an ultrasound control scheme using wearable device that enables synchronous intention recognition and accurate control force estimation. Eleven gestures abstracted from daily life are selected, with a force range of 0–100 % maximum voluntary contraction (MVC). The wearable device employed for muscle morphology monitoring consists of a piezoelectric micromachined ultrasound transducer (PMUT) and polymer encapsulation. It is small, lightweight, and eliminates the need for ultrasound gel, enhancing its practicality. Ultrasound echogenicity of two forearm muscles and corresponding output forces are simultaneously collected. We propose peak and location (PKL) feature and combine it with mean and standard deviation (MSD) feature to improve the accuracy of pattern recognition and force estimation. Experimental results show that during dynamic muscle contraction forces, the classification accuracy achieves 95.84 ± 1.48 %, while the synchronous estimation of forces yields a R2 correlation coefficient of 0.919 and a normalized root-mean-square deviation of 0.127. This scheme demonstrates the feasibility of PMUT-based ultrasound control for supporting synchronous gesture recognition and force estimation, and is expected to contribute to the practicality of prosthesis control.

Sensing and Controlling Strategy for Upper Extremity Prosthetics Based on Piezoelectric Micromachined Ultrasound Transducer.

Surface electromyography (sEMG) is currently the primary method for user control of prosthetic manipulation. Its inherent limitations of low signal-to-noise ratio, limited specificity and susceptibility to noise, however, hinder successful implementation. Ultrasound provides a possible alternative, but current systems with medical probes are expense, bulky and non-wearable. This work proposes an innovative prosthetic control strategy based on a piezoelectric micromachined ultrasound transducer (PMUT) hardware system. Two PMUT-based probes were developed, comprising a 23×26 PMUT array and encapsulated in Ecoflex material. These compact and wearable probes represent a significant improvement over traditional ultrasound probes as they weigh only 1.8 grams and eliminate the need for ultrasound gel. A preliminary test of the probes was performed in non-disabled subjects performing 12 different hand gestures. The two probes were placed perpendicular to the flexor digitorum superficialis and brachioradialis muscles, respectively, to transmit/receive pulse-echo signals reflecting muscle activities. Hand gesture was correctly predicted 96% of the time with only these two probes. The adoption of the PMUT-based strategy greatly reduced the required number of channels, amount of processing circuit and subsequent analysis. The probes show promise for making prosthesis control more practical and economical.

A piezoelectric micromachined ultrasound transducer combined with recording electrodes for acute brain preparations in vitro

BackgroundUltrasound stimulation is used to noninvasively stimulate the local and deep areas of the brain. However, the detailed cellular mechanisms of neural activation are still unclear because studies on micro-stimulation at the cellular level are lacking. New methodTo modulate neural activity at the cellular level, we developed a piezoelectric micromachined ultrasound transducer (PMUT), having circular diaphragms for application on acute brain slice preparations. To monitor neural activities, additionally, we fabricated recording microelectrodes onto the same PMUT device for closed-loop application. ResultsTo examine the PMUT-driven cellular responses of a brain slice, intracellular calcium signals in individual cells were measured using two calcium indicators. We successfully observed the intracellular responses triggered by the ultrasound of our novel PMUT. In addition, we performed recordings of local field potentials in a brain slice, demonstrating its usefulness as a simultaneous recording interface. Comparison with existing method(s)Conventional ultrasound stimulators are open-loop systems that risk inducing excessive neural activity because of the absence of neural activity monitoring. In contrast, our PMUT is packaged in a single device with both stimulation and sensor interface for neuromodulation. Further, there are no published reports on in vitro microdevices that can be used for ultrasound stimulation in rodent cortical slices that are several hundred micrometers thick, which maintain the cortical laminar structure and intrinsic neural networks. ConclusionsOur findings suggest that this novel PMUT device has the potential for being a powerful tool for in vitro brain slice applications and effective closed loop ultrasound stimulation.

Dual-frequency piezoelectric micromachined ultrasound transducer based on polarization switching in ferroelectric thin films

Due to its additional frequency response, dual-frequency ultrasound has advantages over conventional ultrasound, which operates at a specific frequency band. Moreover, a tunable frequency from a single transducer enables sonographers to achieve ultrasound images with a large detection area and high resolution. This facilitates the availability of more advanced techniques that simultaneously require low- and high-frequency ultrasounds, such as harmonic imaging and image-guided therapy. In this study, we present a novel method for dual-frequency ultrasound generation from a ferroelectric piezoelectric micromachined ultrasound transducer (PMUT). Uniformly designed transducer arrays can be used for both deep low-resolution imaging and shallow high-resolution imaging. To switch the ultrasound frequency, the only requirement is to tune a DC bias to control the polarization state of the ferroelectric film. Flextensional vibration of the PMUT membrane strongly depends on the polarization state, producing low- and high-frequency ultrasounds from a single excitation frequency. This strategy for dual-frequency ultrasounds meets the requirement for either multielectrode configurations or heterodesigned elements, which are integrated into an array. Consequently, this technique significantly reduces the design complexity of transducer arrays and their associated driving circuits.

A transceiver integrated piezoelectric micromachined ultrasound transducer array for underwater imaging

In this article, a scandium-doped aluminum nitride (Sc0.2Al0.8N) piezoelectric micromachined ultrasound transducer (PMUT) array for underwater imaging is presented. The reported PMUT array integrates a transmitter and a receiver in one transduction cell, in which the outer top electrode is used for transmitting while the inner top electrode is employed for simultaneously receiving ultrasonic waves, respectively. A 7 × 8 transduction cell array with characteristic size of 500μm is designed in a bioinspired honeycomb architecture and developed based on a piezoelectric on silicon-on-insulator (SOI) platform. The top electrode of the transduction cell is split into the outer and inner sections with the optimized size ratio for achieving transceiver integration in one cell. The fabricated PMUT array exhibits a transmit sensitivity of 146.59 dB (re: 1 μPa/V), and a receive sensitivity of −173.70 dB (re: 1 V/μPa) at its resonant frequency of around 150 kHz in water. In order to demonstrate the feasibility of underwater imaging, the PMUT array and its signal conditional circuits are packaged using an acoustically transparent material. Ultrasonic pulse-echo imaging with the developed PMUT array is demonstrated without implementation of a switching circuit to separate the transmit and reflected signals, due to the transceiver integration approach proposed in our work. This work demonstrates the feasibility of underwater imaging for the transceiver integrated PMUT array, which has great commercial value for minimized and portable underwater imaging.

Cost-Effective Strategy for Developing Small Sized High Frequency PMUTs Toward Phased Array Imaging Applications

This article presents the development of small sized piezoelectric micromachined ultrasound transducer (PMUT) with cost effective fabrication process. Improved ultrasound imaging quality requires small sized, high frequency and high performance PMUT cell and further high density arrays. Fabrication with currently well developed and cost effective sacrificial process benefits their mass production and application. To determine small sized AlN PMUT cell, commonly reported elastic layer materials including Si, Si3N4, SiO2 are compared by mathematic derivation under the same stack thickness and device frequency. SiO2 has the smallest Young modulus and it serving as elastic layer can yield the expected one. Arranging SiO2 layer at the top of the device is compatible with phosphor silicate glass (PSG) sacrificial manufacturing process for cost effective fabrication. This fabrication strategy further downsizes cell size since extra device protection layer is omissible. We make 12MHz PMUT linear arrays with cell diameter, thickness and element pitch to be <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$40~\mu \text{m}$ </tex-math></inline-formula> , <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.9~\mu \text{m}$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$120~\mu \text{m}$ </tex-math></inline-formula> . Their electrical and ultrasound results are quite uniform at both device and wafer levels. One PMUT line with 20 parallelly connected units can produce pulse-echo signal with over 25dB SNR at 10mm distance. Lateral and axial ultrasound imaging resolutions of ~1 mm and ~0.3 mm are obtained in practical verification. These indicate the good ultrasound performance of our developed PMUT array. Plus the advantages of small size and cost effective fabrication, using SiO2 elastic layer and arranging it at the top of the device is a competitive strategy to develop phased array PMUTs. [2022-0184]

Aluminum Nitride Piezoelectric Micromachined Ultrasound Transducer Arrays for Non-Invasive Monitoring of Radial Artery Stiffness

An aluminum nitride (AlN) piezoelectric micromachined ultrasound transducer (PMUT) array was proposed and fabricated for non-invasive radial artery stiffness monitoring, which could be employed in human vascular health monitoring applications. Using surface micromachining techniques, four hexagonal PMUT arrays were fabricated within a chip area of 3 × 3 mm2. The mechanical displacement sensitivity and quality factor of a single PMUT were tested and found to be 24.47 nm/V at 5.94 MHz and 278 (in air), respectively. Underwater pulse-echo tests for the array demonstrated a -3 dB bandwidth of 0.76 MHz at 3.75 MHz and distance detection limit of approximately 25 mm. Using the PMUT array as an ultrasonic probe, the depth and diameter changes over cardiac cycles of the radial artery were measured to be approximately 3.8 mm and 0.23 mm, respectively. Combined with blood pressure calibration, the biomechanical parameters of the radial artery vessel were extracted using a one-dimensional vascular model. The cross-sectional distensibility, compliance, and stiffness index were determined to be 4.03 × 10-3/mmHg, 1.87 × 10-2 mm2/mmHg, and 5.25, respectively, consistent with the newest medical research. The continuous beat-to-beat blood pressure was also estimated using this model. This work demonstrated the potential of miniaturized PMUT devices for human vascular medical ultrasound applications.

Piezoelectric Micromachined Ultrasound Transducer Technology: Recent Advances and Applications

The objective of this article is to review the recent advancement in piezoelectric micromachined ultrasound transducer (PMUT) technology and the associated piezoelectric materials, device fabrication and characterization, as well as applications. PMUT has been an active research topic since the late 1990s because of the ultrasound application needs of low cost large 2D arrays, and the promising progresses on piezoelectric thin films, semiconductors, and micro/nano-electromechanical system technology. However, the industrial and medical applications of PMUTs have not been very significant until the recent success of PMUT based fingerprint sensing, which inspired growing interests in PMUT research and development. In this paper, recent advances of piezoelectric materials for PMUTs are reviewed first by analyzing the material properties and their suitability for PMUTs. PMUT structures and the associated micromachining processes are next reviewed with a focus on the complementary metal oxide semiconductor compatibility. PMUT prototypes and their applications over the last decade are then summarized to show the development trend of PMUTs. Finally, the prospective future of PMUTs is discussed as well as the challenges on piezoelectric materials, micro/nanofabrication and device integration.

Beyond fundamental resonance mode: high-order multi-band ALN PMUT for in vivo photoacoustic imaging

This paper reports on an aluminum nitride (AlN) piezoelectric micromachined ultrasound transducer (PMUT) array for photoacoustic (PA) imaging, where the high-order resonance modes of the PMUT are utilized to improve imaging resolution. A flexural vibration mode (FVM) PMUT is fabricated and applied in a photoacoustic imaging (PAI) system. Specifically, the microelectromechanical system (MEMS)-based PMUT is suitable for PA endoscopic imaging of blood vessels and bronchi due to its miniature size and high sensitivity. More importantly, AlN is a nontoxic material, which makes it harmless for biomedical applications. In the PAI system, the AlN PMUT array is used to detect PA signals, and the acousto–mechanical response is designed and optimized at the PMUT’s fundamental resonance. In this work, we focus on the high-order resonance performance of the PMUT PAI beyond the fundamental resonance. The acoustic and electrical responses of the PMUT’s high-order resonance modes are characterized and analyzed. The fundamental and three high-order resonance bandwidths are 2.2, 8.8, 18.5, and 48.2 kHz. Compared with the resolution at the fundamental resonance mode, the resolutions at third- and fourth-order resonance modes increase by 38.7% and 76.9% in a phantom experiment. The high-order resonance modes of the AlN PMUT sensor array provide higher central frequency and wider bandwidth for PA signal detection, which increase the resolution of PAI compared to the PMUT working at the fundamental resonance mode.

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.

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.]

A PMUT Integrated Microfluidic System for Fluid Density Sensing

We demonstrate the design, fabrication and use of a dual electrode PMUT (Piezoelectric Micromachined Ultrasound Transducer) integrated with a microfluidic channel as a fluid density sensor in both static and dynamic density-change conditions. The dual electrode configuration makes the PMUT resonator a self-contained resonant peak-shift sensor and the microfluidic integration makes this system a versatile fluid density sensing platform that can be used with extremely low volumes of fluids in various industrial and healthcare applications. The density measurements carried out here under flowing fluid conditions demonstrate the potential of this system as a real-time fluid density monitoring system. We include results of density measurements in the range of 1020 - 1090 kg/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> that corresponds to the human blood density variation generally due to the change in its hemoglobin content. The sensitivity of the sensor-26.3 Hz/(kg/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> )-is good enough to reliably detect even 1% change in the hemoglobin content of the human blood. Thus, this system could potentially be used also as a hemoglobin measurement sensor in healthcare applications. [2021-0118]

Characterization of AlScN-Based Multilayer Systems for Piezoelectric Micromachined Ultrasound Transducer (pMUT) Fabrication

Scandium-alloyed aluminum nitride (AlScN) is a potential material for micro-electromechanical systems because of its unique advantages, such as strong piezoelectric effect and high thermal stability. However, issues related to its stability and interaction with other materials in multilayer systems require investigation. The formation of new phases at the interface between piezomaterial and electrode material can lead to the device failure. In this study, multilayer structures Si substrate/AlN/Ti-Mo/Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.8</sub> Sc <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.2</sub> N/top electrode (TE) were studied after annealing at a wide range of temperatures and durations. Four different TE materials (i.e. Al, AlSi (1%), Mo/Al, and Mo) were examined to determine the most reliable electrode material for the structure. The phase stability, interfacial quality, and piezoelectric response of the multilayer systems after thermal annealing were investigated. The structure with Mo TE layer was stable after annealing at 800 °C for 300 h and at 1000 °C for 100 h. None of the structures formed any new phases at the interface between the electrode layer and AlScN. The transverse piezoelectric coefficient (e <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">31,f</sub> ) was determined for Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.8</sub> Sc <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.2</sub> N before and after annealing. The absolute value of the e <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">31,f</sub> was -1.39 C/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for as-deposited structure and -1.67 C/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for the same structure annealed for 300 h at 800 °C. [2020-0361]

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.

Fabrication and Characterization of PZT Fibered-Epitaxial Thin Film on Si for Piezoelectric Micromachined Ultrasound Transducer.

This paper presents a fibered-epitaxial lead zirconate titanate (PZT) thin film with intermediate features between the monocrystalline and polycrystalline thin films for piezoelectric micromachined ultrasound transducer (pMUT). The grain boundaries confirmed by scanning electron microscopy, but it still maintained the in-plane epitaxial relationship found by X-ray diffraction analyses. The dielectric constant (εr33 = 500) was relatively high compared to those of the monocrystalline thin films, but was lower than those of conventional polycrystalline thin films near the morphotropic phase boundary composition. The fundamental characterizations were evaluated through the operation tests of the prototyped pMUT with the fibered-epitaxial thin film. As a result, its piezoelectric coefficient without poling treatment was estimated to be e31,f = −10–−11 C/m2, and thus reasonably high compared to polycrystalline thin films. An appropriate poling treatment increased e31,f and decreased εr33. In addition, this unique film was demonstrated to be mechanically tougher than the monocrystalline thin film. It has the potential ability to become a well-balanced piezoelectric film with both high signal-to-noise ratio and mechanical toughness for pMUT.

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.

A flexible ultrasound transducer array with micro-machined bulk PZT.

This paper proposes a novel flexible piezoelectric micro-machined ultrasound transducer, which is based on PZT and a polyimide substrate. The transducer is made on the polyimide substrate and packaged with medical polydimethylsiloxane. Instead of etching the PZT ceramic, this paper proposes a method of putting diced PZT blocks into holes on the polyimide which are pre-etched. The device works in d31 mode and the electromechanical coupling factor is 22.25%. Its flexibility, good conformal contacting with skin surfaces and proper resonant frequency make the device suitable for heart imaging. The flexible packaging ultrasound transducer also has a good waterproof performance after hundreds of ultrasonic electric tests in water. It is a promising ultrasound transducer and will be an effective supplementary ultrasound imaging method in the practical applications.