Bimorph Lithium Niobate Piezoelectric Micromachined Ultrasonic Transducers

TL;DR

This paper presents a bimorph lithium niobate PMUT with high electromechanical coupling, thermal resilience up to 600°C, and optimized membrane geometry for improved performance.

Contribution

It introduces a mechanically robust, high-performance bimorph LN PMUT with enhanced k2 and temperature stability, leveraging advanced film transfer technology.

Findings

Achieved a 775 kHz flexural mode with Q of 200.

Demonstrated stable operation up to 600°C and survival up to 900°C.

Realized a transmit efficiency of 65 nm/V.

Abstract

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 $\mu$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 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.