Piezoelectric Micromachined Ultrasonic Transducer Research Articles

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.

Mass Sensors Based on Capacitive and Piezoelectric Micromachined Ultrasonic Transducers-CMUT and PMUT.

Microelectromechanical system (MEMS)-based mass sensors are proposed as potential candidates for highly sensitive chemical and gas detection applications owing to their miniaturized structure, low power consumption, and ease of integration with readout circuits. This paper presents a new approach in developing micromachined mass sensors based on capacitive and piezoelectric transducer configurations for use in low concentration level gas detection in a complex environment. These micromachined sensors operate based on a shift in their center resonant frequencies. This shift is caused by a change in the sensor’s effective mass when exposed to the target gas molecules, which is then correlated to the gas concentration level. In this work, capacitive and piezoelectric-based micromachined sensors are investigated and their principle of operation, device structures and configurations, critical design parameters and their candidate fabrication techniques are discussed in detail.

On the Effects of Package on the PMUTs Performances-Multiphysics Model and Frequency Analyses.

This paper deals with a multiphysics numerical modelling via finite element method (FEM) of an air-coupled array piezoelectric micromachined ultrasonic transducers (PMUTs). The proposed numerical model is fully 3D with the following features: the presence of the fabrication induced residual stresses, which determine a geometrically non-linear initial deformed configuration of the diaphragms and a remarkable shift of the fundamental frequency; the multiple coupling between different physics, namely electro-mechanical-coupling for the piezo-electric model, acoustic-structure interaction at the acoustic-structure interface and pressure acoustics in the surrounding air. The model takes into account the complete set of PMUTs belonging to the silicon die in a 4 × 4 array configuration and the protective package, as well. The results have been validated by experimental data, in terms of initial static pre-deflected configuration of the diaphragms and frequency response function of the PMUT. The numerical procedure was applied, to analyze different package configurations of the device, to study the influence of the holes on the acoustic transmission in terms of SPL and propagation pattern and consequently extract a set of design guidelines.

Vibration amplitude range enhancement method for a heterodyne interferometer

Scanning heterodyne interferometers are widely used in micro-vibration measurements due to its high sensitivity and high resistivity to environment change. However, the amplitude range of scanning heterodyne interferometers is limited by the approximation in the theory to several nanometers. To solve this problem, a method for vibration amplitude range enhancement is proposed. By exploiting the Bessel functions, the interferometer is able to measure vibration amplitude ranging up to several hundred nanometers. The method was validated by measuring a micro-cantilever and the results were desirable. To further demonstrate the method, measurements were performed on a piezoelectric micromachined ultrasonic transducer (PMUT) with the maximum amplitude up to ∼300 nm.

음향 매질의 추가질량 효과를 고려한 광음향 영상용 초소형 압전 기반 초음파 트랜스듀서의 개발

Typically, photoacoustic images are obtained in water or gelatin because the impedance of these mediums is similar to that of the human body . However, these acoustic mediums can have an additional mass effect that changes the resonance frequency of the transducer. The acoustic radiation impedance in air is negligible because it is very small compared to that of the transducer. However, the high acoustic impedance of mediums such as the human body and water is quite large compared to that of air, making it difficult to ignore. Specifically, in a case where the equivalent mass is very small, such as with a micro-machined ultrasound transducer, the additional mass effects of the acoustic medium should be considered for an accurate resonance frequency design. In this study , a piezoelectric micro-machined ultrasonic transducer (pMUT) was designed to have a resonance frequency of 10 MHz in the acoustic medium of water, which has similar impedance as the human body . At that time , the resonance frequency of the pMUT in air was calculated at 15.2 MHz. When measuring the center displacement of the manufactured pMUT using a laser vibrometer, the resonance frequencies were measured as 14.3-15.1 MHz, which is consistent with the finite element method (FEM) simulation results. Finally, photoacoustic images of human hair samples were successfully obtained using the fabricated pMUT.

Monolithic Single PMUT-on-CMOS Ultrasound System With +17 dB SNR for Imaging Applications

This article presents a fully integrated ultrasound system based on a single piezoelectric micromachined ultrasonic transducer (PMUT) monolithically fabricated with a $0.13~\mu \text{m}$ complementary metal oxide semiconductor (CMOS) process analog front-end circuitry. The PMUT consists of an aluminum nitride, AlN, squared device with $80~\mu \text{m}$ side that resonates at 2.4 MHz in liquid environment. The monolithic integration of the PMUT with the CMOS circuitry allows a reduction of the parasitic capacitance, a reduction of the electronic noise contribution and a clear improvement in the Signal-to Noise ratio (SNR ~ 27 dB better) compared to a non-integrated equivalent system. A pulse-echo experiment with the single PMUT-on-CMOS for transmitting and sensing simultaneously is demonstrated, ensuring a 17.3 dB SNR, higher than the minimal necessary for accurate fingerprint images, paving the way towards a pixel sized imaging system with no need of multiple simultaneous PMUTs transmitters. Consuming only 0.3 mW and getting an input-referred noise of 3.26 mPa/ $\surd $ Hz at 2.4 MHz, the proposed pulse-echo system achieves a competitive noise efficient factor in comparison with the state-of-the-art.

Development Trends and Perspectives of Future Sensors and MEMS/NEMS.

With the fast development of the fifth-generation cellular network technology (5G), the future sensors and microelectromechanical systems (MEMS)/nanoelectromechanical systems (NEMS) are presenting a more and more critical role to provide information in our daily life. This review paper introduces the development trends and perspectives of the future sensors and MEMS/NEMS. Starting from the issues of the MEMS fabrication, we introduced typical MEMS sensors for their applications in the Internet of Things (IoTs), such as MEMS physical sensor, MEMS acoustic sensor, and MEMS gas sensor. Toward the trends in intelligence and less power consumption, MEMS components including MEMS/NEMS switch, piezoelectric micromachined ultrasonic transducer (PMUT), and MEMS energy harvesting were investigated to assist the future sensors, such as event-based or almost zero-power. Furthermore, MEMS rigid substrate toward NEMS flexible-based for flexibility and interface was discussed as another important development trend for next-generation wearable or multi-functional sensors. Around the issues about the big data and human-machine realization for human beings’ manipulation, artificial intelligence (AI) and virtual reality (VR) technologies were finally realized using sensor nodes and its wave identification as future trends for various scenarios.

Array Design of Piezoelectric Micromachined Ultrasonic Transducers With Low-Crosstalk and High-Emission Performance.

This article presents a resonant cavity-based array design for piezoelectric micromachined ultrasonic transducers (PMUTs). The cavity depth is designed to ensure that its open end achieves a considerably smaller acoustic impedance than the surrounding PMUT cells. The interference acoustic wave generated between every two adjacent PMUT cells at the near surface of the array will take an easy path down to the cavity bottom. As such, the crosstalk effect among different adjacent cells in the array can be largely reduced. An equivalent circuit model of the proposed array is established for its design and optimization. In addition, the solutions for circuit parameters in the electromechanical domain are analytically derived and verified via FEM simulations. Given the low crosstalk effect achieved by the proposed array design, the output sensitivity of the proposed PMUTs can be improved by 259% compared with the traditional PMUTs with a high distribution density of the same size. The cavity-based array design and its model can be used for further advanced PMUT cell structures in other arrays to improve their performance.

Piezoelectric Micromachined Ultrasonic Transducers with a Cost-Effective Bottom-Up Fabrication Scheme for Millimeter-Scale Range Finding

This study proposes a novel piezoelectric micromachined ultrasonic transducer (PMUT), fabricated on a metal foil. Using a bottom-up, cost-effective micromachining technique, the PMUTs made of electrodes, a piezoelectric film, or electrode-sandwiched structures with versatile patterns were implemented on a large-area foil thinner rather than regular paper. The proposed microfabrication facilitated the PMUT to be able to generate ultrasonic waves with fundamental and harmonic resonances. The fourth-order resonances of the fabricated PMUT functionally operated at an ultrasonic spectrum of approximately 30 kHz as an ultrasonic emitter. The developed PMUT was paired with a microelectromechanical system (MEMS) microphone module for range-finding applications in the range of several tens of millimeters. A signal-processing scheme was developed to extract the representative pattern from the acquired signals that were emitted and received. The pattern enabled finding the distance between the PMUT and the microphone using time-of-flight and strength-variation technology. The developed PMUT-microphone pair demonstrated its range-finding performance, displaying an error of less than 0.7% using the time-of-flight method.

3D FEM Analysis of High-Frequency AlN-Based PMUT Arrays on Cavity SOI.

This paper presents three-dimensional (3D) models of high-frequency piezoelectric micromachined ultrasonic transducers (PMUTs) based on the finite element method (FEM). These models are verified with fabricated aluminum nitride (AlN)-based PMUT arrays. The 3D numerical model consists of a sandwiched piezoelectric structure, a silicon passive layer, and a silicon substrate with a cavity. Two types of parameters are simulated with periodic boundary conditions: (1) the resonant frequencies and mode shapes of PMUT, and (2) the electrical impedance and acoustic field of PMUT loaded with air and water. The resonant frequencies and mode shapes of an electrically connected PMUT array are obtained with a laser Doppler vibrometer (LDV). The first resonant frequency difference between 3D FEM simulation and the measurement for a 16-MHz PMUT is reasonably within 6%, which is just one-third of that between the analytical method and the measurement. The electrical impedance of the PMUT array measured in air and water is consistent with the simulation results. The 3D model is suitable for predicting electrical and acoustic performance and, thus, optimizing the structure of high-frequency PMUTs. It also has good potential to analyze the transmission and reception performances of a PMUT array for future compact ultrasonic systems.

AlN/SiC MEMS for High-Temperature Applications

The creation of microelectromechanical systems (MEMS) that can operate through elevated temperatures would enable systems diagnostics and controls that are not possible with conventional-off-the-shelf components. The integration of silicon carbide (SiC) with aluminum nitride (AlN) has led to the fabrication of devices that can withstand elevated temperature anneals >935 °C. The results from a piezoelectric micromachined ultrasonic transducer (PMUT) and a microresonator are reported as demonstrations of the fabrication process. Testing the PMUT response before and after annealing at 935 °C led to a change in resonant frequency of less than 1%, which is attributable to a shift in film stress. The response of the microresonator was RF tested $in~ situ$ up to 500 °C and showed no degradation in its electromechanical coupling coefficient. The resonant frequency decreased with temperature due to the temperature coefficient of Young’s modulus, and the quality factor decreased with temperature and remained unrecoverable upon cooling. The degradation in the quality factor is suspected to be a result of oxidation of the titatium nitride (TiN) top electrode, which increases the resistivity and leads to an unrecoverable reduction in the quality factor. The robust piezoelectric response of AlN at these temperatures show that AlN is a very promising candidate for elevated temperature applications. [2019-0019]

Dynamics of piezoelectric micro-machined ultrasonic transducers for contact and non-contact resonant sensors

The dynamics of piezoelectric micromachined ultrasonic transducers (pMUTs) are studied, and an envelope method is proposed for measuring the frequency shift when they operate in pulse–echo mode. To understand pMUTs comprehensively, a theoretical model is established and the dynamic equation is solved, and the experimental results match well with the theoretical ones. A pMUT is measured for each stage of its off-resonance dynamic response with pulse excitation. Through the proposed method, the pMUT can work as either (i) a contact resonant sensor to measure the resonance frequency shift induced by an external mass or pressure or (ii) a noncontact ultrasonic transceiver to detect velocity by the Doppler effect. Tests with both electrical and acoustic signal excitation validate the ability to detect a frequency shift through a predictable transient response. In addition, such an envelope-curve measurement can significantly reduce the sampling rate and lower the hardware cost, especially for high-frequency applications.

Multi-Channel Signal-Generator ASIC for Acoustic Holograms.

A complementary metal-oxide-semiconductor (CMOS) application-specific integrated circuit (ASIC) has been developed to generate arbitrary, dynamic phase patterns for acoustic hologram applications. An experimental prototype has been fabricated to demonstrate phase shaping. It comprises a cascadable 1 ×9 array of identical, independently controlled signal generators implemented in a 0.35- [Formula: see text] minimum-feature-size process. It can individually control the phase of a square wave on each of the nine output pads. The footprint of the integrated circuit is [Formula: see text]. A 128-MHz clock frequency is used to produce outputs at 8 MHz with a phase resolution of 16 levels (4 bits) per channel. A 6 ×6 air-coupled matrix array ultrasonic transducer was built and driven by four ASICs, with the help of commercial buffer amplifiers, for the application demonstration. Acoustic pressure mapping and particle manipulation were performed. In addition, a 2 ×2 array piezoelectric micromachined ultrasonic transducer (PMUT) was connected and driven by four output channels of a single ASIC, demonstrating the flexibility of the ASIC to work with different transducers and the potential for direct integration of CMOS and PMUTs.

Thin Film PZT-Based PMUT Arrays for Deterministic Particle Manipulation

Lead zirconate titanate (PZT)-based piezoelectric micromachined ultrasonic transducers (PMUTs) for particle manipulation applications were designed, fabricated, characterized, and tested. The PMUTs had a diaphragm diameter of 60 [Formula: see text], a resonant frequency of ~8 MHz, and an operational bandwidth (BW) of 62.5%. Acoustic pressure output in water was 9.5 kPa at 7.5 mm distance from a PMUT element excited with a unipolar waveform at 5 Vpp . The element consisted of 20 diaphragms connected electrically in parallel. Particle trapping of 4 [Formula: see text] silica beads was shown to be possible with 5 Vpp unipolar excitation. Trapping of multiple beads by a single element and deterministic control of particles via acoustophoresis without the assistance of microfluidic flow were demonstrated. It was found that the particles move toward diaphragm areas of highest pressure, in agreement with literature and simulations. Unique bead patterns were generated at different driving frequencies and were formed at frequencies up to 60 MHz, much higher than the operational BW. Levitation planes were generated above the 30 MHz driving frequency.

The Application of Adaptive Time Gain Compensation in an Improved Breast Ultrasound Tomography Algorithm

In order to better detect information about a mass in breast tissue, an ultrasound tomography algorithm based on adaptive time gain compensation (TGC) was designed. Field II was utilized to automatically evaluate the phantom attenuation coefficient and compensate for the attenuated image. The image reconstruction algorithm process is presented here. Furthermore, the experimental setup with the cylindrical motion of a piezoelectric micromachined ultrasonic transducer (PMUT) linear array was used to detect the mass in the breast model. The attenuation coefficient was evaluated by using the spectral cross-correlation method. According to the acquired attenuation coefficients, TGC compensates for the pulse-echo signal, and the horizontal slice image was reconstructed using the tomography algorithm. The experimental results show that this algorithm can evaluate the attenuation coefficient of the breast model and improve the ability to detect an internal mass. At the same time, the realization of attenuation compensation with software is beneficial to the development of portable medical equipment.

High-SPL Air-Coupled Piezoelectric Micromachined Ultrasonic Transducers Based on 36% ScAlN Thin-Film

In this paper, air-coupled piezoelectric micromachined ultrasonic transducers (PMUTs) using 36% scandium-doped aluminum nitride (ScAlN) thin-film are presented. ScAlN is known to exhibit higher piezoelectric properties compared to pure AlN leading to significant performance improvements in various piezoelectric micro-electromechanical systems (MEMS) applications including PMUTs. Here, the concentration of Sc in the actual sputtered 1- [Formula: see text]-thick ScAlN film was 36%, which is slightly below the maximum at the phase boundary. The ScAlN PMUTs were fabricated from an SOI wafer, where the dry etching of ScAlN film was optimized. The frequency response and displacement sensitivity of the PMUTs were characterized in the air using laser Doppler vibrometry confirming 2× higher transverse piezoelectric coefficient than AlN. The acoustic transmission and reception of the PMUTs were evaluated from a high-sensitivity microphone and pulse-echo measurements. The PMUTs were designed to operate below 100 kHz in order to mitigate the absorption loss, which resulted in a high transmit pressure of 105-dB sound pressure level (SPL) at 10 cm and only 30-dB attenuation at the 2-m range. Through implementing 36% ScAlN film, the presented PMUTs exhibited a large displacement and consequently, a high SPL compared to the state-of-the-art PMUTs and the bulk transducer considering the size and the excitation voltage.

A Computational Piezoelectric Micro-Machined Ultrasonic Transducer Toward Acoustic Communication

Piezoelectric micro-machined ultrasonic transducers (pMUTs) are gaining popularity in integrated smart ultrasonic systems. In this letter, we report a pMUT toward acoustic communication with mechanical computation ability for the first time. The novel pMUT design can successfully operate 2-bit OR, XOR, XNOR, and NOT mechanical logic operations based on resonance frequency tuning. The proposed mechanical actuation can operate steady and reliable binary amplitude shift keying (BASK), binary frequency shift keying (BFSK) modulations with its naturally detached resonance modes in a single device. Such an idea can greatly reduce the complexity and cost of the acoustic communication system design in terms of hardware. A minimum response time of about 0.14 ms is achieved with the proposed device, equivalent to a bit rate over 3.5 kb/s. In addition, a higher data rate can be achieved since the operating speed can be greatly improved by impedance matching and shrinking the pMUT size.

Multi-Perspective Ultrasound Imaging Technology of the Breast with Cylindrical Motion of Linear Arrays

In this paper, we propose a multi-perspective ultrasound imaging technology with the cylindrical motion of four piezoelectric micromachined ultrasonic transducer (PMUT) rotatable linear arrays. The transducer is configured in a cross shape vertically on the circle with the length of the arrays parallel to the z axis, roughly perpendicular to the chest wall. The transducers surrounded the breast, which achieves non-invasive detection. The electric rotary table drives the PMUT to perform cylindrical scanning. A breast model with a 2 cm mass in the center and six 1-cm superficial masses were used for the experimental analysis. The detection was carried out in a water tank and the working temperature was constant at 32 °C. The breast volume data were acquired by rotating the probe 90° with a 2° interval, which were 256 × 180 A-scan lines. The optimized segmented dynamic focusing technology was used to improve the image quality and data reconstruction was performed. A total of 256 A-scan lines at a constant angle were recombined and 180 A-scan lines were recombined according to the nth element as a dataset, respectively. Combined with ultrasound imaging algorithms, multi-perspective ultrasound imaging was realized including vertical slices, horizontal slices and 3D imaging. The seven masses were detected and the absolute error of the size was approximately 1 mm where even the image of the injection pinhole could be seen. Furthermore, the breast boundary could be seen clearly from the chest wall to the nipple, so the location of the masses was easier to confirm. Therefore, the validity and feasibility of the data reconstruction method and imaging algorithm were verified. It will be beneficial for doctors to be able to comprehensively observe the pathological tissue.

An Improved Breast Ultrasound Tomography Algorithm Using Adaptive Time Gain Compensation

In order to solve the problem of ultrasonic transmission attenuation in breast soft tissue, an ultrasound tomography algorithm based on time gain compensation (TGC) is presented. It is imaging of the breast with cylindrical motion of Piezoelectric Micromachined Ultrasonic Transducer (PMUT) linear array. The attenuation coefficients of the water, breast and mass were automatically evaluated by the spectral cross-correlation. Field II is used for simulation analysis, plus the detailed extraction process of attenuation coefficients and the influencing factors are provided. According to the acquired attenuation coefficients, and the pulse echo signal is compensated using TGC. The horizontal slice image is obtained. Experimental results show that this algorithm can effectively evaluate the attenuation coefficient of soft tissue. Besides, the size, location and boundary of breast masses can be detected. At the same time, using software to realize attenuation compensation is beneficial to the development of portable medical equipment.