Optimization of LiNbO₃/Quartz Substrate for High-Frequency Wideband SAW Devices Using Longitudinal Leaky Waves

Multilayered structures can successfully replace single crystal substrates in high-performance surface acoustic wave (SAW) devices. A combination of high propagation velocity and strong piezoelectric coupling is required for acoustic modes used in wideband high-frequency SAW devices. These characteristics can be achieved simultaneously in SAW devices arranged on a lithium niobate (LN) thin plate bonded to quartz and using longitudinal leaky SAW (LLSAW) if the orientations of both the crystals and the plate thickness are optimized. This article describes an optimization procedure developed to find low-attenuated LLSAWs in layered structures that can be used in LN/quartz applications. The symmetry consideration of two crystals was followed by the optimization of the LN orientation to achieve the largest electromechanical coupling and by an analysis of quartz anisotropy as a crucial factor of the existence of nonattenuated LLSAWs. Finally, the quartz orientation and LN thickness were optimized by a rigorous numerical simulation of resonator admittances and by the extraction of LLSAW attenuation at resonant and antiresonant frequencies. The discovered structures enable the propagation of LLSAWs with velocities in the range of 5400-6000 m/s, electromechanical coupling up to 18%, and negligible attenuation. High Q -factors can be achieved at resonance and antiresonance by the variation of the duty factor in the same layered structure. The specific behavior of the LLSAW attenuation with a variation of the LN thickness and quartz cut angle is illustrated by contour plots for each of the optimal structures discovered.