This paper discusses piezoelectric acoustic devices based on widely used piezoelectric materials. Commonly used piezoelectric thin film deposition techniques and the influence of process parameters on the growth, texture and orientation of the films are discussed. Etching techniques are also outlined. A comparative study of different devices developed previously is given. Also, applications of developed devices in aero-acoustic and medical fields have been briefly discussed. Flow charts of various techniques of deposition along with a combined one for full acoustic device fabrication are given. Various techniques, frequently used for thin film characterization have been discussed. The testing and measurement techniques to determine the responses of acoustic devices such as sensitivity, resonance frequency, frequency response, piezoelectric co-efficient etc. have been briefly illustrated. This paper discusses common failure modes with respect to the field of use of acoustic devices. It also concisely discusses various reliability tests done in industries to assess the quality of the developed devices. This review has also suggested directions for future development of thin film acoustic sensors.

Piezoelectric MEMS based acoustic sensors: A review

Flexible resonant acoustic sensors have attracted substantial attention as an essential component for intuitive human-machine interaction (HMI) in the future voice user interface (VUI). Several researches have been reported by mimicking the basilar membrane but still have dimensional drawback due to limitation of controlling a multifrequency band and broadening resonant spectrum for full-cover phonetic frequencies. Here, highly sensitive piezoelectric mobile acoustic sensor (PMAS) is demonstrated by exploiting an ultrathin membrane for biomimetic frequency band control. Simulation results prove that resonant bandwidth of a piezoelectric film can be broadened by adopting a lead-zirconate-titanate (PZT) membrane on the ultrathin polymer to cover the entire voice spectrum. Machine learning-based biometric authentication is demonstrated by the integrated acoustic sensor module with an algorithm processor and customized Android app. Last, exceptional error rate reduction in speaker identification is achieved by a PMAS module with a small amount of training data, compared to a conventional microelectromechanical system microphone.

https://advances.sciencemag.org/content/advances/7/7/eabe5683.full.pdf

Biomimetic and flexible piezoelectric mobile acoustic sensors with multiresonant ultrathin structures for machine learning biometrics.

Handbook of thin film technology

Anomalies in Nanostructure Size Measurements by AFM

Photoacoustic imaging (PAI) is drawing extensive attention and gaining rapid development as an emerging biomedical imaging technology because of its high spatial resolution, large imaging depth, and rich optical contrast. PAI has great potential applications in endoscopy, but the progress of endoscopic PAI was hindered by the challenges of manufacturing and assembling miniature imaging components. Over the last decade, microelectromechanical systems (MEMS) technology has greatly facilitated the development of photoacoustic endoscopes and extended the realm of applicability of the PAI. As the key component of photoacoustic endoscopes, micromachined ultrasound transducers (MUTs), including piezoelectric MUTs (pMUTs) and capacitive MUTs (cMUTs), have been developed and explored for endoscopic PAI applications. In this article, the recent progress of pMUTs (thickness extension mode and flexural vibration mode) and cMUTs are reviewed and discussed with their applications in endoscopic PAI. Current PAI endoscopes based on pMUTs and cMUTs are also introduced and compared. Finally, the remaining challenges and future directions of MEMS ultrasound transducers for endoscopic PAI applications are given.

MEMS Ultrasound Transducers for Endoscopic Photoacoustic Imaging Applications.

The purpose of the paper is to design and fabricate a ZnO-based MEMS acoustic sensor for higher sound pressure level (SPL) measurement in the range of 120---200 dB and low frequency infrasonic wave detection. The thickness of silicon diaphragm was optimized for higher SPL using MEMS-CAD-Tool COVENTORWARE. The microtunnel which relates the cavity to the atmosphere was designed and simulated analytically for low cut-off frequency of the sensor in infrasonic band. The resonance frequency of the sensor was obtained using modal analysis. The sensitivity of the sensor was also estimated using COVENTORWARE. The optimized Si-diaphragm thickness for the intended SPL range was determined and found to be 50 μm. The lower cut-off frequency of the sensor for a 10 μm-deep microtunnel was found to be 0.094 Hz. The resonance frequency of the sensor was obtained using modal analysis and found to be 78.9 kHz. Based on simulation results, the MEMS acoustic sensor with 10 μm-deep microtunnel was fabricated. The optimum sensitivity of sensor was calculated using simulated results and found to be 116.4 μVolt/Pa. The lower cut-off frequency of the sensor can be utilized to detect low frequency sounds. The high SPL sensing capability of the device up to 200 dB facilitates detection of high sound pressure level in launch vehicles, rocket motors and weapons' discharge applications.

Washim Reza Ali

Papers

Piezoelectric MEMS based acoustic sensors: A review

Design and fabrication of microtunnel and Si-diaphragm for ZnO based MEMS acoustic sensor for high SPL and low frequency application

Deposition and optimization of piezoelectric zinc oxide-layer using sol-gel technique for MEMS acoustic sensor

Study of the effects of annealing temperature on the properties of piezoelectric ZnO thin film for the development of MEMS acoustic sensor

Platform Development for CMOS-MEMS Multi-Gap Capacitive Transducers