Monolithic ultrasound fingerprint sensor.

This paper presents a 591×438-DPI ultrasonic fingerprint sensor. The sensor is based on a piezoelectric micromachined ultrasonic transducer (PMUT) array that is bonded at wafer-level to complementary metal oxide semiconductor (CMOS) signal processing electronics to produce a pulse-echo ultrasonic imager on a chip. To meet the 500-DPI standard for consumer fingerprint sensors, the PMUT pitch was reduced by approximately a factor of two relative to an earlier design. We conducted a systematic design study of the individual PMUT and array to achieve this scaling while maintaining a high fill-factor. The resulting 110×56-PMUT array, composed of 30×43-μm2 rectangular PMUTs, achieved a 51.7% fill-factor, three times greater than that of the previous design. Together with the custom CMOS ASIC, the sensor achieves 2 mV kPa-1 sensitivity, 15 kPa pressure output, 75 μm lateral resolution, and 150 μm axial resolution in a 4.6 mm×3.2 mm image. To the best of our knowledge, we have demonstrated the first MEMS ultrasonic fingerprint sensor capable of imaging epidermis and sub-surface layer fingerprints

Development of a high-density piezoelectric micromachined ultrasonic transducer array based on patterned aluminum nitride thin film

This study presents the fabrication and characterization of a piezoelectric micromachined ultrasonic transducer (pMUT; radius: 40 μm) using a patterned aluminum nitride (AlN) thin film as the active piezoelectric material. A 20 x 20 array of pMUTs using a 1 μm thick AlN thin film was designed and fabricated on a 2 x 2 mm2 footprint for a high fill factor. Based on the electrical impedance and phase of the pMUT array, the electromechanical coefficient was ~1.7% at the average resonant frequency of 2.82 MHz in air. Dynamic displacement of the pMUT surface was characterized by scanning laser Doppler vibrometry. The pressure output while immersed in water was 19.79 kPa when calculated based on the peak displacement at the resonant frequency. The proposed AlN pMUT array has potential applications in biomedical sensing for healthcare, medical imaging, and biometrics. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.1

DEVELOPMENT OF PIEZOELECTRIC MEMS DEVICES

Ph.DDOCTOR OF PHILOSOPH

Air-coupled PMUT at 100 kHz with PZT active layer and residual stresses: Multiphysics model and experimental validation

In this paper a complete Multiphysics modelling via the Finite Element Method (FEM) of an air-coupled Piezoelectric Micromachined Ultrasonic Transducer (PMUT) is described, with its experimental validation related to the mechanical and acoustic responses

MEMS Technology for Biomedical Imaging Applications

Biomedical imaging is the key technique and process to create informative images of the human body or other organic structures for clinical purposes or medical science. Micro-electro-mechanical systems (MEMS) technology has demonstrated enormous potential in biomedical imaging applications due to its outstanding advantages of, for instance, miniaturization, high speed, higher resolution, and convenience of batch fabrication. There are many advancements and breakthroughs developing in the academic community, and there are a few challenges raised accordingly upon the designs, structures, fabrication, integration, and applications of MEMS for all kinds of biomedical imaging. This Special Issue aims to collate and showcase research papers, short commutations, perspectives, and insightful review articles from esteemed colleagues that demonstrate: (1) original works on the topic of MEMS components or devices based on various kinds of mechanisms for biomedical imaging; and (2) new developments and potentials of applying MEMS technology of any kind in biomedical imaging. The objective of this special session is to provide insightful information regarding the technological advancements for the researchers in the community

Design and Modeling of Piezoelectric Micromachined Ultrasonic Transducer (PMUT) using a Multi-User MEMS Process for Medical Imaging

According to the Canadian Cancer Society, 2020, “1 in 8 women will be affected by breast cancer and 1 in 33 will die from it.” There has been a decline in breast cancer causalities due to the early detection using advanced imaging technologies. This signifies the importance of early detection of breast cancer that increases the survival rate and treatment options for the patients. One of the platforms which are aiding the early detection is Microelectromechanical Systems (MEMS)-base imaging system. In this thesis, a Piezoelectric Micromachined Ultrasonic Transducer (PMUT) is proposed to work at lower frequency ranges for higher penetration aiding imaging applications while operating at a lower voltage. In this work, a comprehensive study based on the Multi-User MEMS Process (MUMPs) has been conducted to investigate the effect of critical design parameters on output performance. Three sets of PMUTs are fabricated based on the investigated parameters. The resonant frequency and acoustic output pressure of these fabricated devices are evaluated and compared based on their respective areas of the piezo layer using COMSOL Multiphysics. The resonant frequency of the fabricated PMUT ranges from 0.5 MHz to 2 MHz. Keysight Impedance Analyzer E4990A has been utilized for the electrical characterization of the fabricated PMUT devices to determine their respective resonant frequencies and validate the COMSOL simulation results. It is shown that the fabricated individual circular PMUT achieves a high acoustic output pressure of 39 kPa at 1.3 MHz and the rectangular PMUT provides 4.7 kPa of acoustic pressure at 1.4 MHz. The results indicate that the proposed PMUT design can deliver acoustic pressure at a lower frequenc

MEMS enabled miniaturisation of photoacoustic imaging and sensing systems

This work presents multiple advances toward miniaturised photoacoustic imaging systems. Miniaturising the system is done in two steps. Firstly, by using novel custom arrays of piezoelectric miniaturised ultrasound transducers. The arrays were fabricated using a cost-efficient multi-user process. The achievable upper frequency limits were restricted by the design limitations of the multi-user process. The designs comprised of a single frequency and two frequency staggered arrays. They were characterised using laser Doppler velocimetry, pitch and catch technique as well as photoacoustic excitation. Additionally, the arrays were compared to commercial bulk ultrasound transducers. The custom-made PMUT arrays perform well compared to commercial transducer, despite their significantly smaller (two orders of magnitude) detection area. Secondly, an optical resolution photoacoustic microscope consisting consisting of MEMS based excitation - using a fast-scanning micro-mirror for Q-switching - and detection schemes is built and used to image synthetic targets and phantoms. Furthermore, a simulation model of the system is developed to evaluate influences of the miniaturised elements on the photoacoustic signal generation and received spectra and signal strength. Finally, a novel photoacoustic excitation scheme based on CW - laser excitation and a MEMS based fast-scanning micro-mirror is presented and its performance relative to pulsed excitation photoacoustic imaging is evaluated. Here, the photoacoustic excitation is not due to fast pulsed laser excitation, but caused by scanning a focused CW - beam over a sample.This work presents multiple advances toward miniaturised photoacoustic imaging systems. Miniaturising the system is done in two steps. Firstly, by using novel custom arrays of piezoelectric miniaturised ultrasound transducers. The arrays were fabricated using a cost-efficient multi-user process. The achievable upper frequency limits were restricted by the design limitations of the multi-user process. The designs comprised of a single frequency and two frequency staggered arrays. They were characterised using laser Doppler velocimetry, pitch and catch technique as well as photoacoustic excitation. Additionally, the arrays were compared to commercial bulk ultrasound transducers. The custom-made PMUT arrays perform well compared to commercial transducer, despite their significantly smaller (two orders of magnitude) detection area. Secondly, an optical resolution photoacoustic microscope consisting consisting of MEMS based excitation - using a fast-scanning micro-mirror for Q-switching - and detection schemes is built and used to image synthetic targets and phantoms. Furthermore, a simulation model of the system is developed to evaluate influences of the miniaturised elements on the photoacoustic signal generation and received spectra and signal strength. Finally, a novel photoacoustic excitation scheme based on CW - laser excitation and a MEMS based fast-scanning micro-mirror is presented and its performance relative to pulsed excitation photoacoustic imaging is evaluated. Here, the photoacoustic excitation is not due to fast pulsed laser excitation, but caused by scanning a focused CW - beam over a sample

A Miniaturized Chemical Vapor Detector Using MEMS Flexible Platform

According to the Canadian Cancer Society, lung cancer is the one of the leading causes of cancer death. It has been shown that cancer survival chance depends on factors including the availability of early detection and diagnostic tools such as miniaturized and sensitive gas sensor. This can detect the released volatiles in addition to be implementable in portable electronics, which decisively improves the patient’s survival rate. Therefore, in this thesis and in an effort to develop high-sensitive and miniaturized gas sensor, a microelectromechanical systems (MEMS) platform is utilized. In this work, a sensitive gas sensor is proposed by employing capacitive micromachined ultrasonic transducer (CMUT) configuration due to its high sensitivity, low LOD and reversibility. The comprehensive analytical model is proposed for this circular bilayer CMUT-based gas sensor for the first time, which includes all the known critical design parameters of the sensor. The model also includes effects of membrane softening and residual stress of the top membrane and the sensing component. The model is further followed by conducting FEA simulations, to investigate eect of critical parameters on center resonant frequency of the device. The achieved results for FEA simulations are compared with the proposed model, which shows less than 5% average variation. Both model and simulations verify that maximum sensitivity occurs at smaller radius, thinner membrane and structural material with lower density. The simulations results are utilized to maximize the sensitivity of the gas sensor in a sample frequency range of 5MHz and 25MHz. The proposed device has a 500nm functionalized polysilicon membrane with 300nm polyisobutylene (PIB) while the cavity height is 500nm and 30V DC bias voltage is applied. The proposed and designed CMUT-based gas sensor offers a 222Hz/zg sensitivity (∆f /∆m) in the aforementioned frequency range

Wide Bandgap Based Devices: Design, Fabrication and Applications, Volume II

Wide bandgap (WBG) semiconductors are becoming a key enabling technology for several strategic fields, including power electronics, illumination, and sensors. This reprint collects the 23 papers covering the full spectrum of the above applications and providing contributions from the on-going research at different levels, from materials to devices and from circuits to systems

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.ASTAR (Agency for Sci., Tech. and Research, S’pore)Published versio