Lateral-Field-Excited PMUTs Based on Bilayer X-Cut Lithium Niobate

Abstract

This work demonstrates the design, fabrication, and characterization of the first piezoelectric micromachined ultrasonic transducers (PMUTs) based on bilayer X-cut lithium niobate (LiNbO₃). A comparison of PMUT materials based on different figures of merit (FoMs) is presented, highlighting LiNbO₃ as a promising and well-balanced alternative to more conventional materials. To leverage its superior material properties, PMUTs were designed based on bilayer X-cut LiNbO₃ to fully harnesses the in-plane stress associated with the bending of the structure, thereby enhancing transduction. The fabricated devices show high electromechanical coupling (k_t²) of 4.6 %, albeit significantly lower than the simulated value due to parasitic effects. Mechanical vibration characterization shows a high static displacement of 0.88 nm/V and excellent linear dynamic range. Based on this design, an 8 x 1 array is demonstrated showing excellent consistency among the elements, with a frequency spread of 0.006 MHz and a displacement sensitivity spread of 0.15 nm/V. Our devices show comparable performance to monocrystalline PZT-based PMUTs, and substantially outperform ScAlN-based PMUTs in terms of static displacement sensitivity by a factor of 5. These results underscore the strong potential of LiNbO₃ for high-performance PMUTs.