Piezoelectric Micromachined Ultrasound Transducers : From Design to Applications

Abstract

Ultrasonic sensors are well known for various applications such as NDT, ultrasound imaging, and proximity sensing. Conventional ultrasound transducers are bulky, work at notoriously high voltages, and consume significant power. Microfabrication techniques are leading to a paradigm shift in the field of ultrasonics by enabling development of low power - small footprint ultrasound transducers. This work focuses on the development of piezoelectric type flexural mode micromachined ultrasound transducer also known as PMUTs. We start by establishing a system level analytical model of a PMUT and use it to offer insights into scaling of the performance of the transducer with respect to various design parameters. In this analysis we give special attention to residual stresses thus establishing a contrast between membrane type and plate type PMUTs. After going through various steps of material development and microfabrication, we obtain arrays of PMUTs with different designs. PZT thin films deposited by sol-gel method are used as the piezoelectric layer in the multilayer stack. Further, we present a thorough characterization of fabricated PMUTs which includes measurement of the piezoelectric properties of the embedded PZT thin film, electrical impedance of the electromechanical transducer, its vibrational charac-teristics and acoustic radiation from a single PMUT cell. We also develop a pre-amplifier circuit for a PMUT receiver and present its working as a simple proximity sensor. After establishing the repeatability and predictability of our PMUT sensors we delve into application development beyond ultrasound imaging. Experiments and analysis of PMUTs submerged in water show strong structural-acoustic coupling between the PMUT membrane and the surrounding fluid. We hypothesize the applicability of this feature to sense changes in the acoustic environment of a PMUT. To this end, we integrate an array of PMUTs with a micro-fluidic chip and study the changes in the vibrational behaviour of the PMUT in response to change in the air-water ratio in a closed cell around a PMUT membrane. We also present our preliminary results on presence of micro-bubbles in the closed cell around the PMUT.