Bimorph Lithium Niobate Piezoelectric Micromachined Ultrasonic Transducers.

Piezoelectric micromachined ultrasonic transducers (PMUTs) are widely utilized in applications that demand mechanical resilience, thermal stability, and compact form factors. Recent efforts have sought to demonstrate that single-crystal lithium niobate (LN) is a promising PMUT material platform, offering high electromechanical coupling ( $\textit {k}^{{2}}$ ) and bidirectional performance. In addition, advances in LN film transfer technology have enabled high-quality periodically poled piezoelectric films (P3F), facilitating a bimorph piezoelectric stack without intermediate electrodes. In this work, we showcase a bimorph PMUT incorporating a mechanically robust, 20- $\mu $ m-thick P3F LN active layer. We establish the motivation for LN PMUTs through a material comparison, followed by extensive membrane geometry optimization and subsequent enhancement of the PMUT's $\textit {k}^{{2}}$ . We demonstrate a 775-kHz flexural mode device with a quality factor (Q) of 200 and an extracted $\textit {k}^{{2}}$ of 6.4%, yielding a high-transmit efficiency of 65 nm/V with a mechanically robust active layer. We leverage the high performance to demonstrate extreme-temperature resilience, showcasing stable device operation up to $600~^{\circ } $ C and survival up to $900~^{\circ } $ C, highlighting LN's potential as a resilient PMUT platform.