Piezoelectric Micromachined Ultrasonic Transducer Research Articles

A dual-frequency piezoelectric micromachined ultrasound transducer array with low inter-element coupling effects

This paper presents a dual-frequency piezoelectric micromachined ultrasonic transducer (PMUT) line array with low crosstalk level, which was fabricated on silicon-on-insulator (SOI) wafers with sputtered piezoelectric thin film (PZT) and Si diaphragm structure. The obtained array consists of 120 of 0.77 MHz and 192 of 2.30 MHz PMUT units in total with minimum interspace of 50 μm. Due to the high piezoelectric coefficient of PZT, the PMUT shows high transmitting sensitivity in air and good effective electromechanical coupling factor. The displacement sensitivities are assessed to be 595 nm V−1 and 112 nm V−1 at the resonant frequencies of 0.77 MHz and 2.30 MHz respectively in air. To reduce the vibration coupling, rectangular grooves in the bottom silicon are designed between the adjacent line elements, and the PMUT units in the array are arranged in a mis-aligned style. Modal analysis for the 0.77 MHz units indicates neighbouring coupling-vibration decreases greatly from 44.5% to 14.8% of the excited vibration when the excited line is driven at 4Vpp, which proves both the grooves and the mis-aligned ranking are effective for coupling effect reduction. Moreover, results indicate the coupling effect between different frequencies can be ignored due to their inherent resonance characteristics. The sound pressures for a single 0.77 MHz line element and 2.30 MHz line element are evaluated to be 53 kPa and 73 kPa at a distance of 1 cm in water. This high performance dual-frequency PMUT line array makes some high resolution imaging methods possible based on PMUT technology, such as those of dual-frequency, total focus.

A High Frame Rate Analog Front-End IC With Piezoelectric Micromachined Ultrasound Transducers Using Analog Multi-Line Acquisition for Ultrasound Imaging Systems

This paper proposes a high frame rate analog front-end (AFE) IC combined with a piezoelectric micromachined ultrasonic transducer (PMUT) for ultrasound imaging systems (UISs). The proposed AFE IC, which consists of 8-channel transceivers and a peripheral circuit, employs analog multi-line acquisition (AMLA) based on sub-array beamforming. This AMLA generates multiple scanlines during a scan time by simultaneously and precisely controlling the delay of the AFE IC, thus achieving the high frame rate of the AFE IC. The sensitivities of the PMUT for transmitting and receiving ultrasound waves are simulated to be 67.5 kPa/V and 1.2 mV/kPa, respectively, which are high enough to mitigate the needs of the high-voltage electronics for UISs. The proposed AFE IC was fabricated using a 0.18-μm silicon-on-insulator process and occupies an active area of 2.56 mm × 1.68 mm. The measurement results using the simulated PMUT parameters show that the proposed AFE IC with the AMLA precisely controls its delay up to 1 μs at a clock frequency of 40 MHz. It also acquires more than 9,000 scan/s even at a focal depth of 150 mm, which is at least twice greater than those of prior works, achieving a high frame rate of 93 Hz when 100 scanlines are used for a frame. In addition, a figure-of-merit, which considers the energy consumption per channel/scan and the energy loss, is achieved to be 20 nJ/scan/dB at a focal depth of 150 mm and a center frequency of 3.6 MHz; this is the best result compared to previously reported works. Therefore, the proposed AFE IC is suitable for UISs requiring a high frame rate.

Airborne Piezoelectric Micromachined Ultrasonic Transducers for Long-Range Detection

This paper presents an airborne piezoelectric micromachined ultrasonic transducers (PMUTs) operated at low frequency (40-50 kHz) for long-range detection, where the acoustic absorption loss in air is relatively low (0.8-1 dB/m). The PMUTs made with single-crystal Lead Zirconate Titanate (PZT) enables a high piezoelectric coefficient (e <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">31,</sub> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</sub> ≈ 16-24 C/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ), and a low dielectric constant (ε <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> ≈ 308), achieving high PMUT transceiver efficiency. The 2 × 2 PMUT array achieves a very high sound pressure level (SPL) output of 109.4 dB at 26 cm distance. Different from conventional PZT PMUTs, this study utilized single-crystal PZT with a low permittivity to achieve a good acoustic reception, demonstrating the sensitivity of 2 mV/Pa. This work reports the PMUT design, modeling, fabrication, characterization, enabling a long-range detection of 4.8 meters in a pulse-echo experiment, which was conducted by a pair of 2 × 2 PMUT arrays with the matched resonances.

A Piezoelectric Micromachined Ultrasonic Transducer Using Thin-Film Lithium Niobate

This letter presents the first piezoelectric micromachined ultrasonic transducer (PMUT) based on thin-film lithium niobate (LiNbO3). The figures of merit (FoMs) of LiNbO3 as ultrasound sensors and transducers are first studied, showing great prospective as a balanced transceiver platform. Efficient flexural mode excitation is achieved using a proposed lateral-field-excitation (LFE) structure. The implemented device shows a flexural mode at 7.6 MHz, with a high electromechanical coupling ( $k^{2}$ ) of 4.2%. Measured quality factor ( $Q$ ) in vacuum is 2605, indicating the low structural loss, while measured $Q$ in air is 264, suggesting the ultrasound radiation. A dynamic displacement sensitivity of 20.2 nm/V is measured. Upon further optimizations, LiNbO3-based PMUTs are promising candidates for miniature ultrasound applications. [2020-0287]

Design and Ultrasonic Characterization of a Thin-Film, Flexible, PMUT Array

Abstract Ultrasound is widely used for nondestructive evaluation, structural health monitoring, acoustic emission, sound navigation ranging, and in sensors for automobiles, medicine, and many other applications. Next-generation, small-form-factor sensors have been achieved through advances in piezoelectric micromachined ultrasonic transducers (PMUTs) that can be positioned on either a flexible polymer or a silicon substrate to form an array. This article describes a materials and design optimization study that used finite element analyses to improve designs for robust and practical PMUT sensor arrays.

Tent-Plate AlN PMUT With a Piston-Like Shape Under Liquid Operation

In this work, a waterproof tent-plate piezoelectric micromachined ultrasonic transducer (PMUT) with enhanced performance as actuator and sensor in comparison with standard clamped PMUT is presented. The squared AlN PMUT has four linear holes that are sealed by the passive layer allowing to increase the movement and giving the capability to work in liquid environment. The dimension of the holes was optimized to increase the PMUT displacement at least twice in relation with the regular clamped device. The acoustic performance of the PMUT was simulated in COMSOL Multiphysics and the results were experimentally verified under liquid operation. The experimental transmitting sensitivity, 3.9 kPa/V, as well as the sensor sensitivity, 13.4 V/MPa, provide a 2x factor improvement in contrast with the clamped PMUT with competitive values. Additionally, the presented tent-plate PMUT provides advantages in relation with the area, cost, power consumption and imaging quality thanks to the capability to be monolithically integrated over CMOS substrates.

Air-coupled PMUTs array with residual stresses: experimental tests in the linear and non-linear dynamic regime

ABSTRACT The mechanical characterization of a 4 × 4 air-coupled array of Piezoelectric Micromachined Ultrasonic Transducers (PMUTs) is presented. The experimental campaign consists of three set of experimental tests, namely: topography measurements, small signal dynamic measurements, and vibrometry in the non-linear dynamic regime. The behavior of three different kinds of PMUT are reported. They differ according to the thermo-electrical treatment that has been applied to the piezoelectric material. The presence of the fabrication induced residual stresses is investigated and the treatment effect is evaluated in terms of the initial deflected configuration. The results reported in this paper represent an experimental mechanical investigation useful for the design of PMUT structures with advanced functionalities in the linear and non-linear regime.

The impact of residual stress on resonating piezoelectric devices

Resonating piezoelectric devices, such as aluminum nitride (AlN) piezoelectric micromachined ultrasonic transducers (PMUTs), display superior performance to previous generations of resonating microelectromechanical systems (MEMS). However, the quality of the piezoelectric thin film can greatly impact operating characteristics, such as the resonant frequency. Several AlN PMUT devices fabricated on the same silicon wafer exhibited a range of resonance frequencies (400-600 kHz), indicating that there is nonuniformity across the processed wafer. AlN film nonuniformity is likely introduced during the reactive sputtering process. Two key parameters identified as influencing the resonance frequency include: (i) the membrane diameter and (ii) residual stress. This work focuses on the residual stress, and uses X-ray diffraction technique (XRD) to determine the in-plane biaxial residual stress values, as a function of die position on the wafer. Results show that there is a compressive stress gradient along the wafer, ranging from -357 MPa to -56 MPa. A plot of in-plane biaxial residual stress as a function of resonance frequency shows a relation between the measured stress and frequency. As resonating piezoelectric devices require well defined operating frequencies and bandwidths, this work demonstrates the importance of studying not only global stresses, but also local residual stresses.

Piezo-micro-ultrasound-transducers for air-coupled arrays: Modeling and experiments in the linear and non-linear regimes

The paper is focused on the multi-physics modeling, via the finite element method (FEM), of a 4 × 4 air-coupled array of Piezoelectric Micromachined Ultrasonic Transducers (PMUTs), and on its mechanical and acoustic validation by means of the comparison, in the time domain, with experimental results. A two-stage numerical procedure is employed, in order to evaluate the complete performance of the device, by means of two FEM models. In the first stage, the electro-mechanical-acoustic (EMA) problem is solved for the stand-alone transducer, taking into account the fabrication induced residual stresses and the multiple couplings between different physics. The numerical results are compared with the experimental ones in terms of initial deflection and time histories of pressure, in the linear and non-linear regime: the proposed model correctly captures the reported phenomena and perfectly matches the experimental trends. The second stage is devoted to the simulation of the 4 × 4 PMUTs array performance, belonging to the silicon die. The vibrating plates are modeled as equivalent rigid pistons. The acceleration histories, computed in the first stage, are imposed on the pistons, while a rigid baffle condition is enforced on the remaining part of the die surface. The model is adopted to predict the pressure field for different patterns of the array’s activation.

Miniaturized PMUT-Based Receiver for Underwater Acoustic Networking

The present work reports on the novel implementation of a miniaturized receiver for underwater networking merging a Piezoelectric Micromachined Ultrasonic Transducer (PMUT) array and signal conditioning circuitry in a single, packaged device. Tests in both a large water tank and a pool demonstrated that the system can attain large enough Signal-to-Noise Ratio (SNR) for communication at distances beyond two meters. An actual communication test, implementing an Orthogonal Frequency Division Multiplexing (OFDM) scheme, was used to characterize the performance of the link in terms of Bit Error Rate (BER) vs SNR. In comparison to previous work demonstrating high-data rate communication for intra-body links and acoustic duplexing, this implementation allows for significantly larger distances of transmission, while addressing the signal conditioning and submersible packaging needs for underwater conditions, thus enabling PMUT arrays for operating as complete underwater communication receivers. [2020-0185]

Piezoelectric Micromachined Ultrasonic Transducer With Superior Acoustic Outputs for Pulse-Echo Imaging Application

This article proposes a novel structured aluminum nitrate (AlN) thin film based piezoelectric micromachined ultrasonic transducer (PMUT) with superior ultrasonic performances that can directly be applied for pulse-echo imaging. The bottom electrode of the PMUT is thickened and serves both as the electrical layer and supporting layer. Large numbers of elements are parallelly connected to form square arrays. This method can not only improve the acoustic output but also reduce the total impedance. The ultrasonic transmitting, receiving behaviors of our developed 6.37 MHz (in air) PMUT array containing vibration elements of horizontal x vertical = 20 × 20 are investigated. It can generate superior acoustic output with a normalized value of 1.71 kPa·V <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> ·mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> at 3.6 mm away in deionized water media. Thanks to the lager acoustic output and reduced electrical impedance, ultrasonic pulse-echo imaging with our developed PMUT array is realized only with the assistance of a switch to separate the ultrasound transmission and reception process. We have mechanically scanned a mark in steel plate and generate a B-mode image with great accuracy using the recorded pulse-echo signals. These demonstrate the superior acoustic behavior of our developed PMUT array and open an avenue to the development of portable ultrasonic imaging devices.

Piezoelectric Micromachined Ultrasonic Transducers With Pinned Boundary Structure

This paper reports the approach to boost the acoustic performances of PMUTs (Piezoelectric Micromachined Ultrasonic Transducers) including vibrational amplitude, acoustic pressure and electromechanical coupling by using the pinning boundary structure. An analytical model is developed based on an assumed mode shape and is validated with matching results from numerical simulations. Prototyped devices are fabricated and tested with a measured 2.5X improvement in displacement and 3.5X higher pressure output per volt at resonance as compared to those of PMUTs with clamped boundary. As a demonstration example, a PMUT-based ultrasonic tilt sensor is investigated with measured result using the receiving PMUT’s pressure amplitude for a tilting range of ±8 degrees and an error of ±0.7 degrees. [2020-0152]

Equivalent Circuit Model for a Large Array of Coupled Piezoelectric Micromachined Ultrasonic Transducers With High Emission Performance.

In this article, an analytical equivalent circuit model is established for the piezoelectric micromachined ultrasonic transducer (PMUT) cell and array with a combination of the annular and circular diaphragms used for structural optimization and complex array design. Based on this model, a comprehensive analysis is conducted on the acoustic-structural coupling of an annular and circular diaphragm-coupled PMUT (AC-PMUT) with a new excitation method. The model-derived results are in good agreement with the simulation and experimental results. Then, an optimized design has been presented to achieve high-output pressure and a good array working performance. In summary, a comparison of the array working performance is conducted between the arrays that consist of AC-PMUTs and traditional circular diaphragm PMUTs (C-PMUTs). The results indicate that the AC-PMUT array has a much lower crosstalk effect than that of the traditional C-PMUT array. By this means, the AC-PMUT array can fully use the high vibration amplitude achieved by each AC-PMUT cell to improve its output ability. As a result, the highest ultrasonic output pressure generated by the AC-PMUT array in its resonant condition can achieve an increase of 155%, compared with that generated by the C-PMUT array.

Electromechanical Tuning of Piecewise Stiffness and Damping for Long-Range and High-Precision Piezoelectric Ultrasonic Transducers

This paper presents a novel technique for quickly deactivating piezo-electric micromachined ultrasonic transducers (PMUT) in order to increase their potential for acoustic imaging. The proposed PMUT is anchored by four arms and surrounded by two mechanical dampers. The dampers are moved using electrostatic actuators in order to come into contact with the PMUT. By doing so, the resonant frequency of the PMUT can be shifted and the amplitude at the working frequency can be reduced. Measurement results show that this technique can effectively dampen the displacement of the PMUT. [2019-0123]

Bi-Stable Aluminum Nitride-Based Piezoelectric Micromachined Ultrasonic Transducer (PMUT)

This work presents a new type of piezoelectric micromachined ultrasonic transducer (PMUT) based on a bi-stable MEMS membrane structure with integrated aluminum nitride (AlN) actuators. By switching the membrane between both stable ground states, large, design-dependent membrane center displacements in the range of $10~\mu \text{m}$ can be achieved, which result in sound pressure levels of up to 96.8 dB in the ultrasonic range at frequencies between 30 and 60 kHz at a distance of 10.5 mm. This bi-stable operation mode can be triggered by moderate voltage levels of up to 40 V, which is not uncommon for PMUT devices. The usage of AlN compared to the more commonly utilized lead zirconate titanates (PZT) significantly simplifies the device integration process and ensures CMOS compatibility. A comprehensive comparison with similar devices in literature shows that despite the lower piezoelectric coefficients of AlN compared to PZT, this bi-stable actuation approach features exceptionally high stroke levels with a correspondingly high overall sound pressure level. [2020-0104]

Low Temperature Adhesive Bonding-Based Fabrication of an Air-Borne Flexible Piezoelectric Micromachined Ultrasonic Transducer.

This paper presents the development of a flexible piezoelectric micromachined ultrasonic transducer (PMUT) that can conform to flat, concave, and convex surfaces and work in air. The PMUT consists of an Ag-coated polyvinylidene fluoride (PVDF) film mounted onto a laser-manipulated polymer substrate. A low temperature (<100 °C) adhesive bonding technique is adopted in the fabrication process. Finite element analysis (FEA) is implemented to confirm the capability of predicting the resonant frequency of composite diaphragms and optimizing the device. The manufactured PMUT exhibits a center frequency of 198 kHz with a wide operational bandwidth. Its acoustic performance is demonstrated by transmitting and receiving ultrasound in air on curved surface. The conclusions from this study indicate the proposed PMUT has great potential in ultrasonic and wearable devices applications.

Design of piezoelectric micro-machined ultrasonic transducer for wideband ultasonic radiation in air

Design of piezoelectric micro-machined ultrasonic transducer for wideband ultasonic radiation in air

Design, Characterization, and Analysis of PZT Micromachined Piezoelectric Ultrasonic Transducers With Good Coupling to Solids.

This article presents the piezoelectric micromachined ultrasonic transducer (PMUT) and its arrays that were based on a sputtered PZT/Si diaphragm structure and prototyped from an SOI substrate. Due to the high piezoelectric coefficient of PZT, polarization tuning pretreatment, and membrane thickness optimization, the PMUT shows high transmitting sensitivity in air and good coupling capability to liquid and solid. The PMUT transmitter exhibited a high sensitivity of 809, 190, and 135 nm/V at a resonant frequency of 0.450, 0.887, and 1.689 MHz, respectively, in air. The 5 ×5 array of 0.5-MHz PMUTs' acoustic output in water was measured as 42.4 Pa at a distance of 3 cm with a 10.0- [Formula: see text] input. Thickness-measuring ability in solids was evaluated with an 8 ×8 array of 1-MHz PMUTs as transmitter providing 8.0- [Formula: see text] input and another single PMUT of identical frequency response as receiver showing 0.2 [Formula: see text] (after 20 times magnification) output when the acoustic wave was transmitted through a 5-cm-thick graphite plate. Meanwhile, the time response of the receiver through different thicknesses of graphite plates is in reasonable agreement with predication from the analytical calculation. This high-performance PMUT with good coupling to solids will be utilized in various applications for solid-state sensing and detecting or as an alternative to the bulk piezoelectric ceramic transducers in the near future.

Equivalent Circuit Models of Cell and Array for Resonant Cavity-Based Piezoelectric Micromachined Ultrasonic Transducer.

This article presents a design of resonant cavity-based piezoelectric micromachined ultrasonic transducers (PMUTs), including impedance matching tube-integrated (T) and Helmholtz resonant (HR) cavity-integrated PMUTs. In addition, equivalent circuit models for single PMUT cell and PMUT array are developed for structural optimization and complex array design. The model-derived results agree well with the FEM results. On the basis of the proposed models, an optimized design is established to achieve high output pressure and a good array working performance. The working performance of arrays that consist of HR-PMUTs and traditional circular diaphragm PMUTs (C-PMUTs) is compared. Results indicate that the HR-PMUT array has a lower crosstalk effect than the traditional C-PMUT array. Furthermore, the highest ultrasonic output pressure of HR-PMUT array at the resonant frequency can be achieved with an increase of up to 163% compared with that of the C-PMUT array because of the liquid amplification effect. Also, the cavity-based design and its model can be used for further advanced PMUT cell structures in other arrays to improve their performance.

Effects of flexural vibration and thickness vibration on receiving characteristics of a diaphragm-type PZT resonator

The piezoelectric micromachined ultrasonic transducer (PMUT) is increasingly being used in medical applications with the development of microelectromechanical system technology. The PMUT generates flexural vibration depending on its structure, but there is also thickness vibration depending on the film thickness of the resonator. This paper clarifies the effect of each vibration on the PMUT reception characteristics. Two approaches that may weaken flexural vibration are investigated in finite element simulation. One is thickening the support layer to increase the damping function. The other is increasing the thickness vibration by thickening the lead zirconate titanate (PZT) layer. Results reveal that flexural vibration is strongly related to ringing in the received waveform, owing to flexural vibration generating rapid phase changes in the frequency domain. Meanwhile, thickness vibration, the degree of which depends on the thickness of the PZT layer, is confirmed to affect the pulse width but be less relevant to ringing.