Bimorph Lithium Niobate Piezoelectric Micromachined Ultrasonic Transducers
This work addresses the need for more resilient and efficient PMUTs in applications requiring mechanical robustness and thermal stability, representing an incremental advancement in material and design optimization.
The researchers tackled the challenge of improving piezoelectric micromachined ultrasonic transducers (PMUTs) by developing a bimorph device using a 20 μm thick periodically poled lithium niobate (LN) active layer, achieving a high electromechanical coupling (k²) of 6.4%, transmit efficiency of 65 nm/V, and stable operation up to 600°C with survival up to 900°C.
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 (k2) 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 $μ$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 k2. We demonstrate a 775 kHz flexural mode device with a quality factor (Q) of 200 and an extracted k2 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.