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

Abstract

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.