Ring-shaped and Dual-electrode Bimorph Piezoelectric Micromachined Ultrasonic Transducers

The potential widespread usages of ultrasonic transducer technology have inspired the development of microelectromechanical systems (MEMS)-based devices. The research presented in this dissertation focuses on enhancing the performance of piezoelectric micromachined ultrasonic transducers (PMUTs) by devising innovative design structures, analytically deriving models to understand and optimize performance, and using these findings to demonstrate PMUT-based systems with increased capabilities. The ring-shaped PMUT is firstly introduced as a high-performance PMUT architecture, which is demonstrated both analytically and experimentally to show several important advantages over traditional designs. Due to specific effects of acoustic interactions that have not been previously studied, an extremely wide bandwidth can be achieved with aptly defined ring-shaped PMUTs in liquid-coupled operation. Specifically, fabricated devices are measured to have a velocity bandwidth of up to 160%, which is more than 60% greater than the highest bandwidth of any reported standalone PMUT and is tunable by altering device dimensions. PMUT arrays are used to demonstrate a new type of ultrasonic flow sensor that offers a robust, simple, and low-power chip-scale solution. The sensor operates in the pulse-echo mode to detect changes in flow with a measured sensitivity that is 286% of that for previously reported MUT-based flow meters, all without commanding any voltage over 5 V. The enhancement is a result of four unique features reported herein: (1) the high-sensitivity bimorph structure of the fabricated PMUTs; (2) the spatial separation between the transmitter and receiver transducer elements; (3) the high directivity of the transmitted acoustic pulse; and (4) the differential readout. Finally, arrays of PMUTs are also proposed for high intensity applications, beginning with the development of an equivalent circuit network model that accounts for the inter-transducer acoustic coupling and leads to practical design equations with interesting implications for the design of high-power arrays. The findings are corroborated by experimental results from several fabricated prototypes, which also use electronic phased array focusing to achieve acoustic outputs that demonstrate feasibility for many high-intensity applications, especially for those requiring small transducer sizes. The prototype device exhibits an output pressure as high as 3.2 kPa/V in standard transmission, which can be boosted to 12.2 kPa/V by applying phased array focusing at a depth of 2 mm. Furthermore, a linear relationship between focal pressure and applied voltage is observed with a maximum measured pressure of 187 kPa peak-to-peak, demonstrating feasibility to reach the intensity ranges required for medical operations in the future.