Piezoelectric Micromachined Ultrasonic Transducer Research Articles

Fabrication Method for Spin‐Coating ITO‐Competitive PEDOT:PSS onto PVDF Film

This paper shows a four‐order reduction of magnitude in sheet resistance for spin‐coating poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) onto polyvinylidene fluoride (PVDF) film for a final sheet resistance comparable to commercially available indium tin oxide‐ (ITO‐) coated PVDF film. The resulting PEDOT:PSS transparency is 10% less than the transparency for ITO:PVDF. This is done by the combination of four techniques from the literature including (i) varying layers, (ii) plasma etching, (iii) post‐treatment submersion in an ethylene glycol (EG) bath, and (iv) adding EG and a surfactant to the PEDOT:PSS solution. The PEDOT:PSS sheet resistance falls from over 3 megaohms/sq to between 600 and 150 ohms/sq depending on the number of layers. The one‐layer sample has a sheet resistance of 600 ohms/sq and an optical transparency of about 76%, and the five‐layer sample has a sheet resistance of 150 ohms/sq and an optical transparency of about 50%. These results reported here make PEDOT:PSS an attractive option for transparent piezoelectric micromachined ultrasonic transducer (PMUT) array applications.

Stepped-Tube Backside Cavity Piezoelectric Ultrasound Transducer Based on Sc0.2AI0.8N Thin Films.

This paper presents a novel piezoelectric micromachined ultrasonic transducer (PMUT) with theoretical simulation, fabrication, and testing. Conventional methods using a PCB or an external horn to adjust the PMUT acoustic field angle are limited by the need for transducer size. To address this limitation, the stepped-tube (expanded tube) backside cavity PMUT has been proposed. The stepped-tube PMUT and the tube PMUT devices have the same membrane structure, and the acoustic impedance matching of the PMUT is optimized by modifying the boundary conditions of the back cavity structure. The acoustic comparison experiments show that the average output sound pressure of the stepped-tube backside cavity PMUT has increased by 17%, the half-power-beam-width (θ-3db) has been reduced from 55° to 30° with a reduction of 45%, and the side lobe level signal is reduced from 147 mV to 66 mV. In addition, this work is fabricated on an eight-inch wafer. The process is compatible with standard complementary metal oxide semiconductor (CMOS), conditions are stable, and the cost is controllable, plus it facilitates the batch process. These conclusions suggest that the stepped-tube backside cavity PMUT will bring new, effective, and reliable solutions to ranging applications.

A Gas Flow Measurement System Based on Lead Zirconate Titanate Piezoelectric Micromachined Ultrasonic Transducer.

Ultrasonic flowmeter is one of the most widely used devices in flow measurement. Traditional bulk piezoelectric ceramic transducers restrict their application to small pipe diameters. In this paper, we propose an ultrasonic gas flowmeter based on a PZT piezoelectric micromachined ultrasonic transducer (PMUT) array. Two PMUT arrays with a resonant frequency of 125 kHz are used as the sensitive elements of the ultrasonic gas flowmeter to realize alternate transmission and reception of ultrasonic signals. The sensor contains 5 × 5 circular elements with a size of 3.7 × 3.7 mm2. An FPGA with a resolution of ns is used to process the received signal, and a flow system with overlapping acoustic paths and flow paths is designed. Compared with traditional measurement methods, the sensitivity is greatly improved. The flow system achieves high-precision measurement of gas flow in a 20 mm pipe diameter. The flow measurement range is 0.5-7 m/s and the relative error of correction is within 4%.

Continuously monitoring of muscle fatigue based on a wearable micromachined ultrasonic transducer probe

Ultrasound based methods can assess muscle fatigue by penetrating deep muscles and are not susceptible to electrical noise, but traditional ultrasound probes have bulky size that hinders integration and wearability, making them impractical to continuously monitor muscle status. This paper proposed a wearable probe based on piezoelectric micromachined ultrasonic transducers (PMUTs), where a polymer acoustic lens was incorporated to focus ultrasonic beam and enhance the emitted acoustic pressure (increased by >20%). The fatigue dynamics of upper arm muscles during sustained exertion was investigated by continuously monitoring thickness changes with the PMUTs-based probe, revealing a temporal pattern characterized by swift and subsequent gradual thickening. Several muscle fatigue metrics were derived by piecewise linear fitting, including the initial and secondary deformation rate and the transition time, offering insight into muscle status and fatigue resistance. Additionally, an outlier elimination method for thickness profiles was utilized, mitigating the influence of uncontrollable motion on findings. Leveraging PMUTs technology and acoustic lens integration, this proposed wearable approach holds promise for real-time assessment of muscle fatigue.

Modeling a Fluid-Coupled Single Piezoelectric Micromachined Ultrasonic Transducer Using the Finite Difference Method.

A complete model was developed to simulate the behavior of a circular clamped axisymmetric fluid-coupled Piezoelectric Micromachined Ultrasonic Transducer (PMUT). Combining Finite Difference and Boundary Element Matrix (FD-BEM), this model is based on the discretization of the partial differential equation used to translate the mechanical behavior of a PMUT. In the model, both the axial and the transverse displacements are preserved in the equation of motion and used to properly define the neutral line position. To introduce fluid coupling, a Green's function dedicated to axisymmetric circular radiating sources is employed. The resolution of the behavioral equations is used to establish the equivalent electroacoustic circuit of a PMUT that preserves the average particular velocity, the mechanical power, and the acoustic power. Particular consideration is given to verifying the validity of certain assumptions that are usually made across various steps of previously reported analytical models. In this framework, the advantages of the membrane discretization performed in the FD-BEM model are highlighted through accurate simulations of the first vibration mode and especially the cutoff frequency that many other models do not predict. This high cutoff frequency corresponds to cases where the spatial average velocity of the plate is null and is of great importance for PMUT design because it defines the upper limit above which the device is considered to be mechanically blocked. These modeling results are compared with electrical and dynamic membrane displacement measurements of AlN-based (500 nm thick) PMUTs in air and fluid. The first resonance frequency confrontation showed a maximum relative error of 1.13% between the FD model and Finite Element Method (FEM). Moreover, the model perfectly predicts displacement amplitudes when PMUT vibrates in a fluid, with less than 5% relative error. Displacement amplitudes of 16 nm and 20 nm were measured for PMUT with 340 µm and 275 µm diameters, respectively. This complete PMUT model using the FD-BEM approach is shown to be very efficient in terms of computation time and accuracy.

A two-channel ultrasonic flowmeter based on AlN piezoelectric micromachined ultrasonic transducers arrays with improved cross-correlation method

This paper presents a highly accurate two-channel ultrasonic flowmeter based on AlN piezoelectric micromachined ultrasonic transducers (PMUTs) to measure flow rate in small-diameter pipes (9.6 mm). The ultrasonic transducers consist of four 10 by 10 PMUTs arrays with resonant frequency of 1 MHz in air. The ultrasonic transducers are excited by continuous sine voltage, and the transmitted and received signals are subjected to cross-correlation operation to obtain the time delay of the ultrasonic wave in the liquid. A dual-channel design of the flowmeter can reduce measurement errors by taking the average value. To reduce errors in the cross-correlation operation, an iterative algorithm is proposed, which effectively improves the measurement accuracy. The flowmeter is evaluated in flow range of 3.5–10 l min−1, and has a small relative error of 0.7%.

Design and Performance Enhancement of a Piezoelectric Micromachined Ultrasonic Transducer Based on NBBT Lead-Free Piezoelectric Single-Crystal Thin Film

Piezoelectric micromachined ultrasonic transducers (PMUTs) have attracted widespread attention due to their high performance, miniaturization, and easy integration with semiconductor processes. In this paper, a PMUT device based on high-performance and lead-free Na0.5Bi0.5TiO3-BaTiO3 (NBBT) piezoelectric single-crystal thin films was designed for the application of medical high-frequency ultrasonics. Three-dimensional modeling and analysis of PMUT elements on the proposed structure were performed via the finite element method. The relationship between structure configuration in terms of the top electrode and the cavity shape of the bottom was studied. The PMUT properties and its device performance, including resonant frequency, effective electromechanical coupling factor (keff2), and the static sensitivity of different device structures, were analyzed. In addition, by rotating the Euler Angle γ of the NBBT piezoelectric single-crystal film, the static sensitivity and keff2 of the model are improved to 1.34 when γ is rotated to 45 ± 5°. It was shown that the PMUT using rotated NBBT demonstrated an enhanced relative pulse-echo sensitivity of −46 dB and a bandwidth of 35% when the reflective surface was 200 μm. We conclude that the PMUT in accordance with an NBBT piezoelectric single-crystal film designed by simulation offers a high frequency, larger keff2, and high sensitivity, which provides application prospects in high-resolution and high-frequency medical ultrasonic imaging.

Low frequency Piezoelectric Micromachined Ultrasonic Transducers optimized for concrete structures.

AbstractEmbedded sensors operating within a reinforced concrete structure enable the timely detection of structural degradation causes, such as chloride ions present in the pore solution. The best communication protocol to use for inter‐device communication within the concrete structure would ideally be wireless such as an ultrasonic system. Concrete is a composite material which imposes constraints on ultrasonic transmission, especially when operating at frequencies above 100kHz where incident radiation is scattered by the aggregate. Furthermore, for an effective coupling mechanism, a liquid with high acoustic impedance is required to reduce energy reflection at the interface. This paper outlines the design, construction, and characterisation of a range of Piezoelectric Micromachined Ultrasonic Transducers (PMUTs) which are specifically designed to operate in liquids, at frequencies near or below 100kHz. The PMUT devices described in this paper have diaphragm diameters ranging between 550μm and 2,000μms which sizes are needed for PMUTs to resonate at the required low frequency. This paper outlines the device geometry calculations and the design and fabrication of the prototype devices. The resonant frequencies were determined using a laser vibrometer.

Enhancing the Performance of Piezoelectric Micromachined Ultrasonic Transducers Using C Slots

This paper aims to improve the performance of piezoelectric micromachined ultrasonic transducers (PMUT) in the transmission and reception phase including sensitivity, quality factor, and electromechanical coupling coefficient using a slot-based design. The slots help to increase the vibration amplitude and reduce the stiffness and create a membrane with piston-like movements. Also, the thermoelastic loss as one of the fundamental loss parameters in PMUT is reduced and conditions for increasing the quality factor are provided. The proposed structure is simulated in Comsol Multiphysics. The results expose that the quality factor of the new structure is equal to 13069, which is increased by 122.5% compared to the simple clamped PMUT. Also, at the resonance frequency of 98.02 kHz, displacement and voltage sensitivity are equal to 929.22 nm/V and -106.88 dB, improved by 7.6 and 12.49 times. The designed transducer is used in many applications such as sonar, hydrophones, and fish finders as underwater communication receivers.

78‐2: A flat‐panel‐display compatible ultrasound platform

We present a thin‐film piezoelectric micromachined ultrasonic transducer (PMUT) technology compatible with flat‐panel manufacturing methods. Using the developed flow which is based on a low temperature AlScN piezoelectric layer, we fabricate large area 48x48 element PMUT arrays on glass. Beam steering and ultrasound medical imaging are demonstrated. The developed transducer technology can be combined with a TFT backplane and has the potential of direct integration on top of display size glass sheets, expanding the boundaries of ultrasound application domains.

Polyimide-On-Silicon 2D Piezoelectric Micromachined Ultrasound Transducer (PMUT) Array

This paper presents a fully addressable 8 × 8 two-dimensional (2D) rigid piezoelectric micromachined ultrasonic transducer (PMUT) array. The PMUTs were fabricated on a standard silicon wafer, resulting in a low-cost solution for ultrasound imaging. A polyimide layer is used as the passive layer in the PMUT membranes on top of the active piezoelectric layer. The PMUT membranes are realized by backside deep reactive ion etching (DRIE) with an oxide etch stop. The polyimide passive layer enables high resonance frequencies that can be easily tuned by controlling the thickness of the polyimide. The fabricated PMUT with 6 µm polyimide thickness showed a 3.2 MHz in-air frequency with a 3 nm/V sensitivity. The PMUT has shown an effective coupling coefficient of 14% as calculated from the impedance analysis. An approximately 1% interelement crosstalk between the PMUT elements in one array is observed, which is at least a five-fold reduction compared to the state of the art. A pressure response of 40 Pa/V at 5 mm was measured underwater using a hydrophone while exciting a single PMUT element. A single-pulse response captured using the hydrophone suggested a 70% −6 dB fractional bandwidth for the 1.7 MHz center frequency. The demonstrated results have the potential to enable imaging and sensing applications in shallow-depth regions, subject to some optimization.

Characterization of AlN thin film piezoelectric coefficient d31 using Piezoelectric Micromachined Ultrasonic Transducers (PMUTs)

The transverse piezoelectric coefficients d 31 of piezoelectric membranes is fundamental to the design, simulation, optimization, and applications of piezoelectric devices using Aluminum Nitride (AlN). In this work, a method based on the inverse piezoelectric effect is proposed to characterize d 31. Based on the piezoelectric equation and the thin film vibration equation, the central displacement of the Piezoelectric Micromachined Ultrasonic Transducers (PMUTs) is linearly related to the DC voltages when the PMUTs are in the linear operating region. The central displacement of PMUTs is measured by 3D Measuring Laser Microscope at various DC voltages. The finite element method (FEM) is combined with change of d 31 to approximate the relationship between displacement and voltage. Then, The proposed method is verified for its accuracy through the use of the cantilever technique where the cantilever is fabricated on the same wafer as the above PMUTs. The d 31 of the AlN thin film obtained by the proposed method and the cantilever method are -1.58pC/N and -1.60pC/N. The error of the two methods is within 2%. Ultimately, the extraction and measurement of d 31 can be achieved by the proposed method.

An Ultrasonic Target Identification System Based on Piezoelectric Micromachined Ultrasonic Transducers

In this paper, an ultrasonic target identification system based on Piezoelectric Micromachined Ultrasonic Transducers (PMUTs) is proposed to improve the productivity efficiency of industrial production. The proposed system can accurately identify targets of different shapes. Two pieces of 3 × 3 PMUTs arrays with a resonant frequency of 115 kHz are used as the transmitter and receiver of the PMUTs-based ultrasonic sensor. The sensor is small in size and has high instability. In addition, the PMUTs based ultrasonic sensors have lower environmental requirements than vision sensors and are less expensive than laser sensors. Therefore, the sensors are suitable for large-scale use in industry. The characteristics of the proposed system are investigated in the shape experiments on the square, circle, frame, and ring targets. The four kinds of targets are acrylic plates with a thickness of 5mm. The experimental results show that the system can accurately identify the shapes of the four targets through the grid-based scheme. Besides, the identification process is automatically completed by the 3D mobile platform. The proposed system has demonstrated a cost-effective method to identify targets of different shapes, which can more efficiently guide the robotic manipulator’s grasping and other behaviors.

A miniaturized transit-time ultrasonic flowmeter based on ScAlN piezoelectric micromachined ultrasonic transducers for small-diameter applications

Transit-time ultrasonic flowmeters (TTUFs) are among the most widely used devices for flow measurements. However, traditional TTUFs are usually based on a bulk piezoelectric transducer, which limits their application in small-diameter channels. In this paper, we developed a miniaturized TTUF based on scandium-doped aluminum nitride (ScAlN) piezoelectric micromachined ultrasonic transducers (PMUTs). The proposed TTUF contains two PMUT-based transceivers and a π-type channel. The PMUTs contain 13 × 13 square cells with dimensions of 2.8 × 2.8 mm2. To compensate for the acoustic impedance mismatch with liquid, a layer of polyurethane is added to the surface of the PMUTs as a matching layer. The PMUT-based transceivers show good transmitting sensitivity (with 0.94 MPa/V surface pressure) and receiving sensitivity (1.79 mV/kPa) at a frequency of 1 MHz in water. Moreover, the dimensions of the π-type channel are optimized to achieve a measurement sensitivity of 82 ns/(m/s) and a signal-to-noise ratio (SNR) better than 15 dB. Finally, we integrate the fabricated PMUTs into the TDC-GP30 platform. The experimental results show that the developed TTUF provides a wide range of flow measurements from 2 to 300 L/h in a channel of 4 mm diameter, which is smaller than most reported channels. The accuracy and repeatability of the TTUF are within 0.2% and 1%, respectively. The proposed TTUF shows great application potential in industrial applications such as medical and chemical applications.

Microwave-induced thermoacoustic imaging with a multi-cell AlScN piezoelectric micromachined ultrasonic transducer

This Letter reports on microwave-induced thermoacoustic imaging utilizing a multi-cell aluminum scandium nitride (AlScN) piezoelectric micromachined ultrasonic transducer (PMUT). Thermoacoustic signals are induced by microwave pulses and then detected by the ultrasonic transducer. It carries biological information, thus enabling noninvasive clinical diagnosis. Our AlScN PMUT device was characterized and experimented to show that it could significantly improve the imaging of the biological sample, decreasing the mean square error by up to 72%, which benefits from a 2.96 times larger detection angle of thermoacoustic signals compared to commercial ultrasonic transducers. The miniaturized and high-performance PMUT devices have the potential to be applied to clinical or personal diagnosis and monitoring equipment.

An Ultrasonic Target Detection System Based on Piezoelectric Micromachined Ultrasonic Transducers

In this paper, an ultrasonic target detection system based on Piezoelectric Micromachined Ultrasonic Transducers (PMUTs) is proposed, which consists of the PMUTs based ultrasonic sensor and the sensor system. Two pieces of 3 × 3 PMUTs arrays with the resonant frequency of 115 kHz are used as transmitter and receiver of the PMUTs-based ultrasonic sensor. Then, the sensor system can calculate the target's position through the signal received by the above receiver. The static and dynamic performance of the proposed prototype system are characterized on black, white, and transparent targets. The experiment results demonstrated that the proposed system can detect targets of different colors, transparencies, and motion states. In the static experiments, the static location errors of the proposed system in the range of 200 mm to 320 mm are 0.51 mm, 0.50 mm and 0.53 mm, whereas the errors of a commercial laser sensor are 2.89 mm, 0.62 mm, and N\A. In the dynamic experiments, the experimental materials are the targets with thicknesses of 1 mm, 1.5 mm, 2 mm and 2.5 mm, respectively. The proposed system can detect the above targets with a maximum detection error of 4.00%. Meanwhile, the minimum resolution of the proposed system is about 0.5 mm. Finally, in the comprehensive experiments, the proposed system successfully guides a robotic manipulator to realize the detecting, grasping, and moving of a transparent target with 1 mm. This ultrasonic target detection system has demonstrated a cost-effective method to detect targets, especially transparent targets, which can be widely used in the detection and transfer of glass substrates in automated production lines.

A High Sensitivity AlN-Based MEMS Hydrophone for Pipeline Leak Monitoring

In this work, a miniaturized, low-cost, low-power and high-sensitivity AlN-based micro-electro-mechanical system (MEMS) hydrophone is proposed for monitoring water pipeline leaks. The proposed MEMS Hydrophone consists of a piezoelectric micromachined ultrasonic transducer (PMUT) array, an acoustic matching layer and a pre-amplifier amplifier circuit. The array has 4 (2 × 2) PMUT elements with a first-order resonant frequency of 41.58 kHz. Due to impedance matching of the acoustic matching layer and the 40 dB gain of the pre-amplifier amplifier circuit, the packaged MEMS Hydrophone has a high sound pressure sensitivity of -170 ± 2 dB (re: 1 V/μPa). The performance with respect to detecting pipeline leaks and locating leak points is demonstrated on a 31 m stainless leaking pipeline platform. The standard deviation (STD) of the hydroacoustic signal and Monitoring Index Efficiency (MIE) are extracted as features of the pipeline leak. A random forest model is trained for accurately classifying the leak and no-leak cases using the above features, and the accuracy of the model is about 97.69%. The cross-correlation method is used to locate the leak point, and the localization relative error is about 10.84% for a small leak of 12 L/min.

Multi-frequency broadband piezoelectric micromachined ultrasonic transducer utilizing helmholtz resonance

Piezoelectric micromachined ultrasonic transducers (PMUTs) with multi-frequency are appealing for ultrasonic applications, which can meet the requirements of large detection depth and high resolution simultaneously. Hence, a novel structure with series Helmholtz resonators under the PMUT membrane term as dual-cavity serial PMUT is presented, which utilizes both the piezoelectric effect and Helmholtz resonant effect to achieve multi-frequency and further widens the bandwidth. Based on the mechanism of vibration coupling between the membrane and Helmholtz resonator, a lumped model and an equivalent circuit model are formulated. Meanwhile, the finite element method (FEM) is developed to investigate the multi-resonance and make comprehensive validations. The results demonstrate that the multi-frequency is contributed by the induced acoustic impedance of the Helmholtz resonator, causing a simulated average sound pressure of up to 260 Pa/V at the resonance frequency and a 77% -6 dB frequency bandwidth in air by theoretical analysis and parameter optimization. The proposed dual-cavity serial PMUT provides a new approach for the application of high transmission power and broad bandwidth transducers.

PMUTs Arrays for Structural Health Monitoring of Bolted-Joints.

Micro-electro-mechanical systems (MEMS) have enabled new techniques for the miniaturization of sensors suitable for Structural Health Monitoring (SHM) applications. In this study, MEMS-based sensors, specifically Piezoelectric Micromachined Ultrasonic Transducers (PMUT), are used to evaluate and monitor the pre-tensioning of a bolted joint structural system. For bolted joints to function properly, it is essential to maintain a suitable level of pre-tensioning. In this work, an array of PMUTs attached to the head and to the end of a bolt, serve as transmitter and receiver, respectively, in a pitch-catch Ultrasonic Testing (UT) scenario. The primary objective is to detect the Change in Time of Flight (CTOF) of the acoustic wave generated by the PMUT array and propagating along the bolt's axis between a non-loaded bolt and a bolt in service. To model the pre-tensioning of bolted joints and the transmission of the acoustic wave to and from a group of PMUTs through the bolt, a set of numerical models is created. The CTOF is found to be linearly related to the amount of pre-tensioning. The numerical model is validated through comparisons with the results of a preliminary experimental campaign.

An AlN Piezoelectric Micromachined Ultrasonic Transducer-Based Liquid Density Sensor

As an important physical parameter of liquids, the liquid density plays an important role in many applications. As a result, the development of liquid density sensors is critical. However, current liquid density sensors have drawbacks, such as large size of conventional liquid density sensors, low quality factor of microcantilever beam structures, low sensitivity of quartz crystal tuning forks, and high operating frequency requirement of surface-acoustic-wave Love-mode resonators. In this article, we report an aluminum nitride (AlN) piezoelectric micromachined ultrasonic transducer (PMUT)-based liquid density sensor to mitigate the aforementioned drawbacks of traditional liquid density sensors. The PMUT sensor utilizes CMOS-compatible AlN as the piezoelectric material. This sensor operates by measuring the change in resonant frequency of the PMUT to calculate the liquid density. In an experiment, the resonance of the PMUT sensor is measured by a digital holographic microscope (DHM), which allows direct measurement of the PMUT surface displacement, providing intuitive vibration amplitude and mode. Moreover, the DHM-based optical measurement approach has a higher sensitivity than the conventional electrical impedance-based measurement approach, allowing measurement of weak PMUT vibrations in dense liquids. The quality factor ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Q}$ </tex-math></inline-formula> ) value of the PMUT sensor is measured to be 49.38 in water. A high sensitivity of 12.71 kHz/( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{g}\cdot $ </tex-math></inline-formula> cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{-{3}}$ </tex-math></inline-formula> ) was determined for this sensor by measuring the resonant frequencies immersed in different glycerol aqueous solutions. We measured 12 PMUTs and the maximum deviation of resonant frequency between different PMUTs is 9.85%. For the first time, the resolution of the PMUT density sensor is determined 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">$5.9\times 10^{-{4}}\,\,\text{g}\cdot $ </tex-math></inline-formula> cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{-{3}}$ </tex-math></inline-formula> .