Piezoelectric Micromachined Ultrasound Transducer (PMUT) Arrays for Integrated Sensing, Actuation and Imaging

Many applications of ultrasound for sensing, actuation and imaging require miniaturized and low power transducers and transducer arrays integrated with electronic systems. Piezoelectric micromachined ultrasound transducers (PMUTs), diaphragm-like thin film flexural transducers typically formed on silicon substrates, are a potential solution for integrated transducer arrays. This paper presents an overview of the current development status of PMUTs and a discussion of their suitability for miniaturized and integrated devices. The thin film piezoelectric materials required to functionalize these devices are discussed, followed by the microfabrication techniques used to create PMUT elements and the constraints the fabrication imposes on device design. Approaches for electrical interconnection and integration with on-chip electronics are discussed. Electrical and acoustic measurements from fabricated PMUT arrays with up to 320 diaphragm elements are presented. The PMUTs are shown to be broadband devices with an operating frequency which is tunable by tailoring the lateral dimensions of the flexural membrane or the thicknesses of the constituent layers. Finally, the outlook for future development of PMUT technology and the potential applications made feasible by integrated PMUT devices are discussed

DEVELOPMENT OF PIEZOELECTRIC MEMS DEVICES

Ph.DDOCTOR OF PHILOSOPH

Mixed Nonlinear Response and Transition of Nonlinearity in a Piezoelectric Membrane

Nonlinearities play a critical role in the dynamics of mechanical resonators, enhancing sensitivity and enabling signal manipulation. Understanding the parameters affecting nonlinearities is crucial for developing strategies to counteract their effects or manipulate them for improved device performance. This study investigates the impact of fabrication-induced curvature on the dynamics of zinc oxide-based piezoelectric micromachined ultrasonic transducers (PMUTs). Our experiments reveal that these devices exhibit hardening, softening, and mixed nonlinear responses, with varying initial static displacements. Notably, PMUTs with almost flat initial static displacement exhibit hardening nonlinearity, while those with a curved initial static displacement show softening nonlinearity. An exotic mixed nonlinear response is observed for intermediate static displacement. We attribute the observed nonlinear response to the interplay between static displacement-induced quadratic nonlinearity and midplane stretching-induced cubic nonlinearity. We provide a theoretical formulation for the dynamics of the devices, which explains the experimental results and highlights the nonlinear responses and their dependence on the initial static displacement. Our findings underscore the significance of nonlinearities in the dynamics of mechanical resonators and suggest ways to optimize device performance.Comment: 19 pages, 7 figure

Xenon difluoride etching of amorphous silicon for release of piezoelectric micromachined ultrasonic transducer structures

Piezoelectric micromachined ultrasonic transducers (PMUT) are devices, which are based on the piezoelectric effect and are used for sensing applications. A typical PMUT structure has diaphragm with a piezoelectric material between thin high conductivity electrode layers. There are several methods which can be used for PMUT structure fabrication, including back- and front-side etching, wafer bonding, and sacrificial layer release. The state-of-the-art methods used currently for PMUT structure fabrication still face several problems. Xenon difluoride (XeF2) etching is a fluorine-based dry vapour etch method that provides highly selective isotropic etch. It is an ideal solution for the release of self-supporting layers within MEMS devices. In this work, XeF2 etching of amorphous silicon (a-Si) for the release of PMUT structures was investigated. Different designs with varying dimensions were tested and characterized. The XeF2 etching process demonstrated to be efficient and very fast compared to other methods used for PMUT/MEMS release etching. Results from the optimization tests on the XeF2 process demonstrated total etching of 2 µm thick a-Si. Structures with sizes from 50 to 500 µm diameter were completely released after only 20 minutes of etching. Additionally, this work demonstrates that the etching rate of XeF2 is also influenced by the size, shape and location of the via openings. Furthermore, sputtered aluminium nitride AlN piezo layer process optimization and residual stress control contributed to the fabrication of suspended structures. All observed structures from 50 to 500 µm diameter which used AlN in the structural layer were suspended after release

AlN ja Sc0.2Al0.8N ohutkalvojen märkäkemiallinen etsaus

Aluminium nitride is a piezoelectric material commonly used in piezoelectric microelectromechanical systems (MEMS) in the form of thin films deposited by sputtering. AlN-based devices are found in wireless electronics in the form of acoustic filters, but they also have prospective applications in a wide variety of sensor systems. To enhance the piezoelectric properties of AlN, some of the Al can be replaced with scandium, which is required for next-generation devices. However, addition of Sc makes both the deposition and patterning of the film more difficult. This work focuses on patterning of AlN and Sc0.2Al0.8N thin films with wet etching. Both materials are etched anisotropically, which in theory enables etching the materials with little deviation from the mask dimensions. However, in practise, undercutting at the mask edges occurs easily making the structures narrower compared to the etch mask. This work investigates and compares the mechanisms and etch rates of AlN and Sc0.2Al0.8N. Tetramethyl ammonium hydroxide was mostly used for etching, but also H3PO4 and H2SO4 were tested. Addition of 20 atom-% Sc lowered the etch rate of the material and resulted in more undercutting. The causes behind mask undercutting were examined by using 11 differently deposited etch masks, and the undercutting was minimized by optimizing the mask deposition, using thermal annealing, and optimizing the etching temperature. Finally, the work identifies and discusses the relevant factors in depositing and patterning the AlN, ScxAl1-xN and mask films

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 fabrication of a flexible membrane ultrasound transducer

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2018.Cataloged from PDF version of thesis.Includes bibliographical references (pages 143-152).Wearable ultrasound sensing could enable novel medical diagnostics by facilitating continuous, real-time, and direct measurement of physiological phenomena, such as blood pressure. Currently, ultrasound is not used in wearable health sensing applications because clinical ultrasound systems are expensive, bulky, and require high operating power. Realizing wearable ultrasound therefore requires significant reductions in cost, size, and power consumption. Manufacturing cost is of particular concern because sensors are frequently incorporated into consumer goods, where cost is a key driver of technology adoption. Toward that goal, this thesis explored the first steps toward the opportunity to fabricate low-cost ultrasound transducers by contact printing. Contact printing was selected because it could be scaled for high-throughput manufacturing, and it could be performed at ambient temperature and pressure. For this thesis, a capacitive microscale ultrasound transducer was fabricated by contact printing a gold-parylene composite flexible membrane onto a silicon chip substrate. Significant challenges with the adhesion between the membrane and the chip were overcome during fabrication process development and a high yield process for the contact printing step was developed. The transducer was characterized for electromechanical performance. The first mode resonant frequency of the transducer was 2.2MHz, with a 2MHz bandwidth, placing it in the range of interest for medical ultrasound applications (typically 1-15MHz). These results demonstrate that flexible membrane ultrasound transducers can be fabricated. Furthermore, they illuminate a path toward wearable ultrasound sensing and more broadly, flexible medical devices.by Megan Johnson Roberts.Ph. D

Design, Analysis and Fabrication of Capacitive Micromachined Resonator – based Mass Sensors

A challenge in greenhouses is the presence of various pests, virus, and bacteria. Although many pest management strategies are available, however, they all depend on visually identifying these invasive forces when they have eradicated the crop. To avoid the impacts on the agricultural sector due to such pests, early detection is required. Therefore, in this thesis MEMS-based capacitive mass resonators are proposed for early detection of such invasive forces through identifying their released volatile organic compounds (VOCs). In this work, multiple moving membrane capacitive micromachined ultrasonic transducer (M3-CMUT) as a mass sensor is proposed due to its advantages shared with and beyond capacitive micromachined ultrasonic transducer (CMUT) mass sensor such as reversibility, high sensitivity, low limit of detection and selectivity. Analytical modeling is done to identify critical design parameters of the proposed M3-CMUT sensor. This is followed by FEA simulations to analyze the effect of the critical design parameters on the operating resonant frequency of the sensor. Further FEA simulations are done to conduct a sensitivity analysis on the proposed sensor for PolyMUMPs parameters. Moreover, several M3-CMUT-based and CMUT-based mass sensors are fabricated using PolyMUMPs, after which measurements are done on the sensors and a comparative analysis done between the experimental and simulated results. The experimental results are in agreement with the simulated results, wherein, it has been found that larger anchor width devices have a higher collapse voltage than the small anchor width devices. Therefore, the larger anchor width mass sensors can be operated at a higher voltage to get a higher resonant frequency shift and hence a higher sensitivity. Furthermore, the larger anchor width mass sensors can also be run at lower voltages for low level mass detection, hence expanding the limit of detection. Therefore, in an unconventional manner, the detection range of capacitive micromachined resonator mass – based sensors has been increased by increasing the width of the anchors supporting the top flexible membrane

Establiment d'un canal de comunicacions en líquid mitjançant PMUTs

Aquest treball busca estudiar les possibilitats de formar un canal de comunicacions inalàmbriques mitjançant ultrasons, amb dispositius del tipus PMUT. En aquest sentit, s'implementen simulacions realitzades conjuntament en els programes Matlab i LTspice. Amb Matlab s'usen llibreries com el FIELD II per simular camps ultrasònics originats per transductors, i s'estableix un enllaç amb LTspice per poder simular la resposta obtinguda al passar pels filtres freqüencials necessaris per una comunicació Full-Duplex. Finalment, s'utilitzarà el protocol BPSK per establir el sistema de comunicacions.Este trabajo busca estudiar las posibilidades de formar un canal de comunicaciones inalámbricas mediante ultrasonidos, con dispositivos del tipo PMUT. En este sentido, se implementan simulaciones realizadas conjuntamente en los programas Matlab y LTspice. Con Matlab se usan librerías como FIELD II para simular campos ultrasónicos originados por transductores, y se establece un enlace con LTspice para poder simular la respuesta obtenida al pasar por los filtros frecuenciales necesarios para una comunicación Full-Duplex. Finalmente, se usará el sistema BPSK para establecer el sistema de comunicaciones.This dissertation looks for studying the possibilities of forming a wireless communication channel through ultrasounds use, with PMUT devices. To do that, simulations are made jointly in Matlab and LTspice programs. With Matlab libraries like FIELD II are used to simulate ultrasonic fields originated by transducers, and a link is established with LTspice in order to be able to simulate the response obtained when passing through frequency filters needed for a Full-Duplex communication. Finally, a BPSK system will be used to form the communication system

Design Modelling and Mechanical/Acoustic Characterization of Piezoelectric Micro Ultrasound Transducers

This master thesis is realized in STMicroelectronics thanks to a collaboration between the company and Politecnico di Milano. The activity is aimed at modelling and experimentally characterizing a Piezoelectric Micro-machined Ultrasound Transducer (PMUT). This is a new generation MEMS (Micro Electro Mechanical System) able to send and receive ultrasound waves by exploiting the piezoelectric effect: here the attention will be focused on the sending mode only. Throughout all the activity a continuous comparison between numerical simulation and experimental results is proposed. This approach is the typical work flow to launch a product into the market. The design modelling is done by using the finite element software COMSOL Multiphysics 5.6 while the laboratory campaign is carried out through Polytec MSA500 and other electronic equipment. The analysis performed are mainly focused on investigating the static and dynamic mechanical behavior of the device and only as a closing section the emitted acoustic field has been considered. Concerning the mechanical characterization, the main fields of investigation are the deformed configurations both of the single membrane and the whole device, the resonance frequencies of the membranes, the dynamic oscillation and the cross-talk phenomena through which the membranes within the same die can interact. Starting from the distorted geometry of the membranes, this is due to the fabrication process which introduces residual stresses and in turn causes a non flat configuration of the membrane: it is noted that the initial upward deformation flattens by applying an increasing DC voltage. Similarly, even the deformation of the die is caused by the presence of residual stresses. Going on, the modal analysis is performed: the first six modes are evaluated to have a complete characterization but the only one exploited in applications is the first one, having a frequency equal to 140kHz. Once the frequency is known, a dynamic analysis is carried out. The membranes are activated by means of a single sinusoidal voltage signal at the resonance frequency and the oscillation ring down is analyzed. Thanks to these measurements, it has been possible to measure the damping of the device by computing the Q factor. This is carried out in presence of Air and Vacuum and the values obtained are respectively 22 and 182: in this way the fluid and mechanic contributions to the damping are divided. Furthermore, by studying the oscillation ring down it appears the need to develop a non linear hysterical piezoelectric model to simulate the dynamic behavior of PZT layer: it will be part of the future activity. Subsequently, the presence of the undesired phenomena of cross-talk has been experimentally investigated. Because of this effect, the membranes can interact each other and the oscillation of one membrane can put in motion the close ones. The analysis has been performed in vacuum and air: it is noted that the acoustic contribution to the cross-talk has a higher influence and in particular the communication occurs through the back cavities. The last part of the thesis is devoted to the acoustic measurements of the emitted field in terms of directionality and sound pressure level. The radiation pattern of the emitted acoustic field by the membrane is simulated by means of a 2D axysimmetric model. Moreover, the pressure intensity has been evaluated at 2cm over the membrane both through simulation and experimentally: a mismatch is noted and it is due to the inability of the model to consider the oscillation cross-talk of the other membranes. From here comes the second future development to be investigated: the emitted acoustic field considering the oscillation of the other membranes or avoid the cross-talk by changing the design of the device, i.e closing the cavity at the bottom of the membranes. This thesis is the starting point of future activities aimed at modelling the non linear hysterical piezoelectric behavior to better match the dynamic response and studying the cross-talk effect in the acoustic emission.Universidad de Sevilla. Máster en Ingeniería Industria