Bimorph Lithium Niobate Piezoelectric Micromachined Ultrasonic Transducers

TL;DR

This paper evaluates and demonstrates a bimorph LFE PMUT based on a 20 μm P3F LN active layer, showing high transmit efficiency and robust high-temperature performance. Through material FoM comparisons, anisotropic design optimization, and rigorous fabrication/verification steps, it achieves a peak transduction efficiency of $\xi_{peak}=35.4\ \mathrm{nm/V}$ (predicted) and $k^2_{eff}=6.4\%$ (measured after post-processing) at 775 kHz, with excellent endurance up to 600 °C and survival to 900 °C. Post-measurement simulations reconcile discrepancies due to lateral overetch, refining predictions to $k^2_{eff}\approx6.5\%$ and $\xi_{norm}\approx0.345\ \mathrm{nm/V}$, and revealing an open-circuit sensitivity of $S_{OC}=2.46\ \mathrm{mV/Pa}$ at 775 kHz. Overall, LN PMUTs show promise as rugged, high-frequency transducers capable of bidirectional operation in harsh environments, motivating further optimization toward reduced parasitics and enhanced receiving performance.

Abstract

Piezoelectric micromachined ultrasonic transducers (PMUTs) are widely used in applications that demand mechanical resilience, thermal stability, and compact form factors. Lead zirconate titanate (PZT) and aluminum nitride (AlN) active layers are used in PMUTs to enable acoustic actuation, sensing, or bidirectional operation. These platforms rely on bimorph films to maximize electromechanical coupling ($k^2$) through thin-film deposition, which uses intermediate electrode layers to establish opposing electric fields. Consequently, incumbent PMUT platforms are limited in achievable film thickness and feature material interfaces that compromise mechanical integrity and thermal performance. Combined with the intrinsic limitations of PZT and AlN, these factors motivate exploration of alternative PMUT material platforms. Recent efforts have sought to demonstrate that single-crystal lithium niobate (LN) is a promising candidate, offering substantially higher $k^2$ and bidirectional performance. Advances in LN film transfer technology have enabled the formation of periodically poled piezoelectric (P3F) LN, facilitating a bimorph stack without intermediate electrodes. In this work, we showcase bimorph PMUTs incorporating a mechanically robust, 20 micron 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.

Figures (22)

• Figure 1: Cross-sectional views of a conventional thickness field excited PMUT in (a) unimorph configuration with a passive layer. (b) Bimorph configuration with three electrode layers. (c) Proposed P3F bimorph configuration excited with lateral electrical fields using a single electrode layer.
• Figure 2: (a) Cross-sectional schematic of a basic LFE LN PMUT and its design parameters. (b) Top-view schematic of the proposed LN PMUT design, highlighting the LFE excitation mechanism and an elliptical membrane shape.
• Figure 3: $g_\textit{11}$ electromechanical coupling constant in X-cut LN against wafer in-plane rotation ($\alpha$).
• Figure 4: Cross-sectional views of (a) the generated electrical field in a P3F LN membrane due to a unit pressure input. (b) The longitudinal stress induced by a pressure load in the same geometry, showing that the electrical field reflects the stress profile.
• Figure 5: Top-view perspective of the excited open-circuit voltage due to a pressure boundary load in a (a) square, (b) circular, (c) rectangular, and (d) elliptical membranes, showcasing how (b) and (d) reduce the excitation of the parasitic transverse fields.