Spurious-Free Lithium Niobate Bulk Acoustic Wave Resonator with Grounded-Ring Electrode
This work addresses the need for mechanically resilient and thermally stable PMUTs for applications requiring high performance in extreme conditions, 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 PMUT using a 20 um thick periodically poled lithium niobate active layer, achieving a 775 kHz device with a quality factor of 200, electromechanical coupling of 6.4%, and stable operation up to 600°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 (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 um 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 k^2. We demonstrate a 775 kHz flexural mode device with a quality factor (Q) of 200 and an extracted 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 degrees C and survival up to 900 degrees C, highlighting LN's potential as a resilient PMUT platform.