Piezoelectric Micromachined Ultrasonic Transducer Research Articles

Modeling and Characterization of AlN-Based Piezoelectric Micromachined Ultrasonic Transducers

Abstract In this paper, we present the modeling and characterization of AlN-based Piezoelectric Micromachined Ultrasonic Transducers (PMUTs) dedicated to biomedical applications. Our study encompasses detailed analytical modeling of these transducers in both air and water environments. The model is confronted with finite element simulations that account for realistic issues such as imperfect clamping. To validate the model's relevance, we compare its predictions with results from optical, electrical, and acoustical characterization. This comprehensive approach provides a robust framework for understanding PMUT behavior and facilitates efficient modeling of future devices.

Simulation and Investigation of Si-Based Piezoelectric Micromachined Ultrasonic Transducer (PMUT) Performances

Micro-electromechanical system (MEMS) based piezoelectric ultrasonic transducers for acoustic imaging of the surroundings are known as piezoelectric micromachined ultrasonic transducers (PMUTs). This research proposes a structural design of the PMUT with four fixed-guided beams. The beam is subjected to lateral loads, with vectors that are perpendicular to the longitudinal axis. This project simulated Piezoelectric Micromachined Ultrasonic Transducer (PMUT) with three different material properties i.e. Aluminium Nitride (AlN), Lead zirconate titanate (PZT) and Zinc Oxide (ZnO). Based on the study, it was found that reducing the beam dimensions and increasing the plate size will result in the first mode frequency reduction from 1.33x107 Hz to 3.74x106 Hz. Other than that, it was found that AlN PMUT experienced the maximum deflection of 6.3413 to 6.3478 μm when the loads applied in the range of 50 to 200 μN/m2. When the piezoelectric material changed to PZT, we obtained the maximum deflections of 0.3771 to 0.3786 μm when the same loads range applied to the PMUT. As for the ZnO PMUT, the maximum deflections obtained were in between 0.1702 μm to 0.1772 μm with the loads are maintained as in the loads applied to the AlN and PZT. This study proved the significant impact of altering the structural dimensions and material properties of PMUTs on their operational characteristics, specifically the first mode frequency and deflection behavior.

Robotically steerable intraluminal imaging probe with side-viewing 3D photoacoustic computed tomography.

This Letter presents, for the first time to our knowledge, a continuum robotic-steered endoluminal volumetric photoacoustic tomography probe. With regard to the design of the probe, a coaxial optical and acoustic imaging system has been developed that employs a custom designed piezoelectric micromachined ultrasonic transducer (PMUT). The dimensions of the system are constrained within a cylindrical housing with a diameter of 12 mm and a length of 30 mm. In regard to the robot design, the motion capabilities of the continuum robot have been successfully modeled and characterized. Based on the proposed system, the potential for early detection and screening of gastrointestinal cancers through phantoms has been validated. The experimental results demonstrate that the robotic-assisted photoacoustic system performs well in localizing unknown lesions and characterizing deep-seated abnormalities, exhibiting promising clinical applications.

Development of PMUT based ultrasonic gas flowmeter with conical transducer cavities for the validation of industrial standards

The recessed installation of transducers in ultrasonic gas flowmeters (USFMs) creates cylindrical transducer cavities that induce local flow distortions and vortex formations, resulting in significant measurement uncertainty. Currently, USFMs with small diameters predominantly rely on conventional bulk-PZT ultrasonic transducers, which are relatively large and hinder the optimization of vortices. Miniaturized piezoelectric micromachined ultrasonic transducers (PMUTs) present an effective solution for mitigating vortex effects; however, their acoustic performance is constrained by the sensor area, resulting in PMUT-based USFMs that fail to meet industrial measurement requirements. This article introduces a small-diameter USFM utilizing a single PMUT element integrated with the specially designed conical transducer cavity. This design effectively reduces the half-power beamwidth (HPBW) of the single PMUT element to one-fifth of its original size and increases the transmit signal amplitude by 7.2 dB. Computational fluid dynamics (CFD) simulations demonstrate that the conical transducer cavity optimizes vortex distribution within the cavity, reducing vortex intensity. The experimental result demonstrates that the proposed USFM achieves an indicated error of less than ±1 % and repeatability of less than 0.5 %, which meets the class 1.5 accuracy level. This design fully complies with the accuracy standards set in GB/T 39841–2021, validating the PMUT-based USFM for industrial flow measurement.

Quasi-closed diaphragm based piezoelectric micromachined ultrasonic transducer with reduced Q and stress sensitivity for in-air rangefinding

To improve the performance of the Piezoelectric Micromachined Ultrasonic Transducer (PMUT) based rangefinder and decrease its stress sensitivity, a novel design with quasi-closed structure is proposed. It adopts a circular piezoelectric composite diaphragm structure with clamped boundary, in which all the deposited stack layers in its central region are intentionally removed and additional cross-slits are created into the remaining silicon device layer. Due to the reduced mass and the enhanced thermal-viscous damping at slits, a 35.2 % decrease in quality factor Q has been achieved in the proposed PMUT when compared with the conventional design, resulting in a distinctly reduced blind area from 231.3 mm to 170.7 mm. At the same time, the proposed quasi-closed PMUT facilitates the release of accumulated stress in the device structure during fabrication and operation. As a result, an approximate 50 % reduction in frequency deviation between different as-fabricated PMUTs across the same wafer has been successfully obtained. Moreover, due to the increased linear operation range, the developed bare PMUT chip demonstrates a maximum detection distance of 3 m at the operation frequency of 71.5 kHz under 40 Vpp driving voltage. Given the advantages of lower Q, insensitivity to stress, good fabrication consistency and large linear operation range, the proposed quasi-closed PMUT design can well address the requirements on small blind area and large detection range for distance sensing applications.

Close Range and High-Resolution Detection of Vibration by Ultrasonic Wave Using Silicon-on-Nothing PMUTs.

This article presents a novel method for close-range, high-resolution ultrasonic time-of-flight (ToF) ranging using piezoelectric micromachined ultrasonic transducers (PMUTs) operating below the device resonance in air. The proposed method involves cross correlation techniques to accurately detect the reflected echo signals despite the presence of ringdown signal interference. For the experiments, a high fill-factor array of silicon-on-nothing (SON) PMUTs was used to enhance the signal-to-noise ratio (SNR). A thorough investigation was conducted to determine the optimal driving frequency for below-resonance ToF ranging, to improve resolution and minimize detection errors. The results of the experiments showed that the system was able to accurately measure sub- μ m vibrations of a metal plate placed 13 mm away from the PMUT array. The system exhibited the ability to detect target object vibrations with a peak-to-peak displacement under 6 μ m and sub- μ m floor noise. Moreover, the maximum detectable vibration frequency reached up to 1 kHz. This study highlights the potential of the proposed ToF ranging method in noncontact vibration monitoring applications across various fields, such as robotics and predictive maintenance.

Readout circuit noise analysis and low-noise design for piezoelectric micro-machined ultrasonic transducers with large parasitic capacitance.

Readout circuits are fundamental components in many application systems that utilize piezoelectric micro-machined ultrasonic transducers (pMUTs). This study models the noise and signal transfer functions of trans-impedance amplifiers (TIAs), charge-sensitive circuits, and voltage-mode readout circuits in detail. A series of simulations and experiments were conducted to elucidate the advantages and disadvantages of these circuit types. Both theoretical and experimental results indicate that the intrinsic capacitance of large pMUTs can significantly degrade the quality of the readout signal. Furthermore, while the TIA-based readout circuit demonstrates clear gain advantages, it is also susceptible to considerable noise interference. This work proposes an improved readout circuit design that effectively mitigates noise interference while preserving the gain advantages of the TIA architecture. The implemented prototype circuit successfully reduces the noise from 73 mVp-p to 13 mVp-p.

On the Dynamics of a Novel Liquid-Coupled Piezoelectric Micromachined Ultrasonic Transducer Designed to Have a Reduced Resonant Frequency and Enhanced Ultrasonic Reception Capabilities.

This paper introduces a novel design for a liquid-deployed Piezoelectric Micromachined Ultrasonic Transducer (PMUT). This design was specifically developed to resonate at a lower ultrasonic frequency than a PMUT with a circular, fully clamped diaphragm with the same diameter. Furthermore, the novel design was also optimised to enhance its ultrasonic radiation reception capabilities. These parametric enhancements were necessary to develop a PMUT device that could form part of an eventual microscale sensory device used for the Structural Health Monitoring (SHM) of reinforced concrete (RC) structures. Through these two enhancements, an eventual microscale sensor can be made smaller, thus taking up a smaller die footprint and also be able to be deployed further apart from each other. Eventually, this would reduce the developed distributed sensor system's cost. The innovative design employed a configuration where the diaphragm was only pinned at particular points along its circumference. This paper presents results from Finite Element Modelling (FEM), as well as experimental work that was conducted to develop and test this novel PMUT. The experimental work presented involved both laser vibrometry and ultrasonic radiation lab work. The results show that when compared to a clamped diaphragm design, the novel device managed to achieve the required reduction in resonant frequency and presented an enhanced sensitivity to incoming ultrasonic radiation.

High Sensitivity Performance of Piezoelectric Micromachined Ultrasonic Transducer Employing Sc-Doped AlN Thin Film

Abstract Aluminum Nitride (AlN) based piezoelectric micromachined ultrasonic transducers (PMUTs) have been the prime choice in acoustic imaging, because it has advantages of high frequency and thus high resolution. However, low piezoelectric constants of AlN gives rises to limitation in transmitting sensitivity. Scandium (Sc) doping method for AlN can be used to improve its piezoelectricity. In this paper, single element PMUT based on ScAlN thin film was proposed and modeled by performing finite element method (FEM). Influence of different electrode configurations on PMUT performance were studied. The results show that, compared with other electrode configurations, PMUT has the best overall performance when the upper electrode is platinum (Pt) and the lower electrode is molybdenum (Mo). In addition, the acoustic field characteristics and its dynamic analysis of PMUT based on ScAlN in water were studied. The investigation results demonstrated that the relative pulse-echo sensitivity level M of PMUT is significantly improved with the increase of Sc doping concentration in AlN thin film. When the Sc concentration is 40%, the M reaches -44 dB, which is 20 dB higher than that without Sc (-64 dB). The PMUT based on the high Sc-doped AlN thin film has a larger effective electromechanical coupling coefficient and sensitivity, which is beneficial to the application in high sensitivity ultrasound imaging.

Design and Fabrication of High-Performance Piezoelectric Micromachined Ultrasonic Transducers Based on Aluminum Nitride Thin Films.

Ultrasound is widely applied in diverse domains, such as medical imaging, non-destructive evaluation, and acoustic communication. Piezoelectric micromachined ultrasonic transducers (PMUTs) capable of generating and receiving ultrasonic signals at the micrometer level have become a prominent technology in the field of ultrasound. It is important to enrich the models of the PMUTs to meet the varied applications. In this study, a series of PMUT devices featured with various top electrode configurations, square, circular, and doughnut, were designed to assess the influence of shape on the emission efficacy. It was demonstrated that the PMUTs with a circular top electrode were outperformed, which was calculated from the external acoustic pressure produced by the PMUTs operating in the fundamental resonant mode at a specified distance. Furthermore, the superior performance of PMUT arrays were exhibited through computational simulations for the circular top electrode geometries. Conventional microelectromechanical systems (MEMS) techniques were used to fabricate an array of PMUTs based on aluminum nitride (AlN) films. These findings make great contributions for enhancing the signal transmission sensitivity and bandwidth of PMUTs, which have significant potential in non-destructive testing and medical imaging applications.

Design and characterization of a high performance transceiver integrated PMUTs array for breast cancer imaging

This paper introduces an innovative transceiver integrated with high-performance piezoelectric micromachined ultrasonic transducers (PMUTs) array, specifically designed for breast cancer imaging. The PMUTs array is meticulously partitioned into drive and detection sections, optimizing the size ratios of the top electrodes within a transduction cell. In this configuration, the outer drive electrode facilitates ultrasonic wave transmission, while the detection electrode concurrently receives signals, enhancing both sensitivity and reducing the overall size of the breast cancer ultrasound imaging system. The sensor dimensions of the scandium-doped aluminum nitride (ScxAl1−xN, x=9.5%)-based PMUTs array are compact, measuring 3.2 mm × 3.2 mm. Thorough characterization of the packaged PMUTs array is conducted using an industry-standard calibration instrument. Notably, the reported PMUTs array achieves an impressive receiving sensitivity of −162 dB (re: 1 V/μPa) with a 40 dB gain, a transmitting sensitivity of 156 dB (re: 1 μPa/V), and a maximum drive nonlinearity of 0.21% at the resonant frequency. Comparative analysis with state-of-the-art commercially available transducers underscores the excellence of the transceiver integrated PMUTs array for breast cancer imaging applications. The outstanding performance and the integration of the transceiver design contribute to a significantly reduced footprint, rendering the reported integrated transceiver PMUTs array highly valuable for portable breast cancer imaging devices with promising commercial prospects.

An acoustic pressure-gradient MEMS vector hydrophone system based on piezoelectric micromachined ultrasonic transducer array

This article presents an acoustic pressure-gradient MEMS vector hydrophone system, which consists of a 2 × 2 piezoelectric scalar hydrophone array and a microcontroller unit (MCU)-based signal-conditioning circuit. The reported scalar hydrophone is fabricated based on a scandium-doped aluminum nitride (Sc0.2Al0.8N) piezoelectric micromachined ultrasonic transducer (PMUT) array. Four identical scalar hydrophones are adopted and arranged in a 2 × 2 array for detecting the acoustic pressure gradient along different incident angles. The received electrical signals in each scalar hydrophone are collected and processed by the MCU directly. Experimental results show that the vector hydrophone system has a receive sensitivity of −168 dB (re: 1 V/μPa) in the omnidirectional receive mode, and a receive sensitivity of −198 dB (re: 1 V/μPa) at 10 kHz in the dipole receive mode, respectively. With the aid of an advanced data preprocessing approach, the reported vector hydrophone system achieves the direction-of-arrival (DOA) estimations of underwater acoustic signals with an error of less than 2°. The measurement results show that the reported MEMS vector hydrophone system has the potential for the detection of underwater sound sources.

PMUT-Based System for Continuous Monitoring of Bolted Joints Preload.

In this paper, we present a bolt preload monitoring system, including the system architecture and algorithms. We show how Finite Element Method (FEM) simulations aided the design and how we processed signals to achieve experimental validation. The preload is measured using a Piezoelectric Micromachined Ultrasonic Transducer (PMUT) in pulse-echo mode, by detecting the Change in Time-of-Flight (CTOF) of the acoustic wave generated by the PMUT, between no-load and load conditions. We performed FEM simulations to analyze the wave propagation inside the bolt and understand the effect of different configurations and parameters, such as transducer bandwidth, transducer position (head/tip), presence or absence of threads, as well as the frequency of the acoustic waves. In order to couple the PMUT to the bolt, a novel assembly process involving the deposition of an elastomeric acoustic impedance matching layer was developed. We achieved, for the first time with PMUTs, an experimental measure of bolt preload from the CTOF, with a good signal-to-noise ratio. Due to its low cost and small size, this system has great potential for use in the field for continuous monitoring throughout the operative life of the bolt.

Performance-Enhanced Piezoelectric Micromachined Ultrasonic Transducers by PDMS Acoustic Lens Design.

This paper delves into enhancing the performance of ScAlN-based Piezoelectric Micromachined 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/μPa), 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 significantly 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.

A miniaturized sensing system for liquid level and concentration based on piezoelectric micromachined ultrasonic transducers

This paper presents a system utilizing scandium-doped aluminum nitride (ScAlN) piezoelectric micromachined ultrasonic transducers (PMUTs) for simultaneous detection of liquid level and concentration. The PMUTs comprise a grid of 13×13 square units, covering an area of 2.8×2.8 mm2 in total. The PMUTs exhibit a resonant frequency of approximately 1 MHz in water and a beam angle of 30.16°, providing excellent sound field directivity. Additionally, a custom package housing with a cantilever baffle is designed to measure sound velocity in real time without modifying the container's structure. Finally, the fabricated PMUTs transceiver is combined with the TDC1000 ultrasonic sensing analog front-end to produce a high-precision liquid level and concentration detector. Glycerol and NaCl solutions are used as test samples. The system demonstrates measurement errors of ±0.5 % / ±1.1 % and ±0.5 mm / ±0.5 mm for concentration and liquid level, respectively, with resolutions of 0.323 % / 0.914 % and 0.077 mm / 0.070 mm. Compared with the related work results, the system based on PMUTs demonstrates advantages in miniaturization and high precision, making it particularly suitable for small-volume fluid containers, where sensor size and detection precision are crucial.

Investigation of performance dependence on transduction diaphragm design for piezoelectric micromachined ultrasonic transducers

One of the most important performance metrics for piezoelectric micromachined ultrasonic transducers (PMUTs) is their transmitting sensitivity, which is significantly dependent on the design of the transduction diaphragm. In this work, a comprehensive comparison of transmission sensitivities for PMUTs with circular, square, and hexagonal transduction diaphragms are investigated through theoretical calculations, finite element analysis (FEA), as well as experimental verifications. Compared to the PMUTs with circular and square transduction diaphragms, PMUTs with hexagonal transduction diaphragm can achieve greater volume displacement and hence better transmitting sensitivity as a transmitter, due to their higher fill-factor and larger quality factor (Q). The measured average resonant volume displacement and Q of 15 fabricated PMUTs with hexagonal transduction diaphragm are 3.07 × 10−13 m3/V and 798, respectively. The transmitting sensitivity of the hexagonal PMUTs exceeds that of the circular PMUTs by over 15% and that of the square PMUTs by over 3.5%. This work highlights an effective and simple approach for PMUTs to achieve high transmitting sensitivity.

Highly-accurate and non-invasive flowrate monitoring for miniature pipelines using piezoelectric micromachined ultrasonic transducers

Noninvasive and real-time flowrate measurement for small pipes is crucial in medical diagnostics and aviation hydraulic system health monitoring. However, current bulk PZT-based ultrasonic flowmeters are unsuitable for these scenarios due to their large size, high power consumption, and poor integration with ICs. This paper proposes a highly accurate and universal ultrasonic flowrate measurement technique for pipes with small diameters (≤ 15 mm) using piezoelectric micromachined ultrasonic transducers (PMUTs) by taking advantage of their miniaturized size, low power consumption, and superior integration capability. A clamp-on testing method is proposed for ultrasound transmitting and receiving. The inside liquid flowrate is measured based on the propagation time difference between the upstream and downstream ultrasound waves. To enhance accuracy, a unique PMUTs chip with a center circular element and an outer annular element is developed to improve acoustic pressure. Additionally, the incident angle and working frequency of ultrasound waves are optimized to reduce the interference caused by ultrasound wave superposition at the PMUTs and pipe wall interface. This optimization also improves acoustic energy transmission and response. The flowrate of water in a steel pipe is successfully measured, demonstrating excellent linearity (99.69%) between flowrate and time difference, with a high accuracy of 2%. Furthermore, flowrate testing for pipes with different diameters and pipe wall materials is experimentally demonstrated, showing widespread usability. The developed PMUTs-based flowmeter is versatile and compatible with common pipes, allowing compact integration in tight spaces, making it highly promising for practical applications.

A Novel Nondestructive Testing Probe Using AlN-Based Piezoelectric Micromachined Ultrasonic Transducers (PMUTs).

Ultrasonic nondestructive testing (NDT) usually utilizes conventional bulk piezoelectric transducers as transceivers. However, the complicated preparation and assembly process of bulk piezoelectric ceramics limits the development of NDT probes toward miniaturization and high frequency. In this paper, a 4.4 mm × 4.4 mm aluminum nitride (AlN) piezoelectric micromachined ultrasonic transducer (PMUT) array is designed, fabricated, characterized, and packaged for ultrasonic pulse-echo NDT of solids for the first time. The PMUT array is prepared based on the cavity silicon-on-insulator (CSOI) process and packaged using polyurethane (PU) material with acoustic properties similar to water. The fabricated PMUT array resonates at 2.183 MHz in air and at around 1.25 MHz after PU encapsulation. The bandwidth of the packaged PMUT receiver (244 kHz) is wider than that of a bulk piezoelectric transducer (179 kHz), which is good for axis resolution improvement. In this work, a hybrid ultrasonic NDT probe is designed using two packaged PMUT receivers and one 1.25 MHz bulk transmitter. The bulk transmitter radiates an ultrasonic wave into the sample, and the defect echo is received by two PMUT receivers. The 2D position of the defect could be figured out by time-of-flight (TOF) difference, and a 30 mm × 65 mm detection area is acquired. This work demonstrates the feasibility of applying AlN PMUTs to ultrasonic NDT of solids and paves the way toward a miniaturized NDT probe using AlN PMUT technology.

Fabrication of a piezoelectric micromachined ultrasonic transducer (PMUT) with dual heterogeneous piezoelectric thin film stacking

In this study, a piezoelectric micromachined ultrasonic transducer (pMUT) was developed using dual heterogeneous piezoelectric thin films stacked on top of each other. One piezoelectric layer is specialized for the actuation, while the other is specialized for the reception of the ultrasonic waves. This combined use of two materials promises to realize a pMUT transceiver array with an excellent transmitting and receiving performance and a high fill factor. Taking fabrication feasibility into consideration, AlN/Pb(Mg1/3, Nb2/3)O3-PbTiO3 (PMN-PT) and Pb(Zr, Ti)O3 (PZT)/AlN pMUTs were selected as two candidates for prototyping as the dual-layer pMUTs. The driving tests were performed by actuation of each piezoelectric layer and a resonance frequencies around 265 kHz and 203 kHz were confirmed for AlN/PMN-PT and PZT/AlN pMUT, respectively. The diaphragm of AlN/PMN-PT pMUT has a displacement sensitivity of 3538 nm V−1 and 306 nm V−1 when actuating PMN-PT layer and AlN layer at resonance frequency, respectively. While the diaphragm of PZT/AlN pMUT has a displacement sensitivity of 1036 nm V−1 and 744 nm V−1 when actuating the PZT layer and the AlN layer at the resonance frequency, respectively.

Effect of humid environment on electromechanical performance of piezoelectric micromachined ultrasonic transducers (PMUTs)

In recent years, piezoelectric micromechanical ultrasonic transducers (PMUTs) have received widespread attention. The deployment of PMUTs in humid environment exposes them to changes in relative humidity, which effects their electromechanical performance. This paper investigates the effect of humid environments (30 %−80 %) on the electromechanical performance of uncoated PMUTs from the perspective of electromechanical conversion model and equivalent circuit. Two types of transducers with different top layer materials were experimented to compare the effects of moisture absorption on the electromechanical performances of the transducers. The top layer material of Device A is SiO2 film, and the top layer material of Device B is Si3N4 film. The experimental results show that with the rise of relative humidity, the frequency of Device A decreases by −0.47 %, while the frequency of Device B increases by 0.13 %, and the phase resonance peaks of both increases by 0.441° and 0.285°, respectively. Equivalent circuit fitting was conducted on the impedance characteristics of Device A and Device B, and the fitting results were discussed based on acoustic impedance and moisture absorption theory. The electromechanical performances of the two devices show a different variation trend, with the potential reason being that Device A is mainly affected by the combined effects of moisture absorption leading to a decrease in Young’s modulus of the membrane and a decrease in the acoustic impedance of the humid air, while Device B is mainly affected by the acoustic impedance.