High-velocity non-attenuated acoustic waves in LiTaO₃/quartz layered substrates for high frequency resonators

Multilayered substrates for Surface Acoustic Wave (SAW) devices are able to combine SAW characteristics that cannot coexist in a single crystal substrate and, thus, meet the strong requirements of the new class of SAW devices developed for the next generations of communication systems. Recently, high performance resonators arranged on LiTaO₃/quartz bonded wafers and utilizing shear horizontally polarized acoustic waves were reported. Leaky SAWs with quasi-longitudinal polarization propagate faster and can facilitate fabrication of high frequency SAW devices but generally leak strongly into the substrate. This paper describes how the LiTaO₃/quartz structure can be optimized to allow longitudinal SAWs to propagate without attenuation. Due to the symmetry consideration, which is supplemented by a rigorous numerical simulation of the admittance functions of SAW resonators and an accurate extraction of the propagation losses, the found optimal LiTaO₃ and quartz orientations with the optimized LiTaO₃ thickness ensure the propagation of acoustic waves with a velocity exceeding 5400 m/s and an electromechanical coupling of 6.8% in resonators with Q factors up to 10,000. The optimal LT/quartz structures with plate thicknesses varying between 0.32 and 0.68 wavelengths can be employed in SAW resonators operating at high frequencies, up to 5 GHz. The existence of numerous orientations in quartz supporting the propagation of non-attenuated longitudinal SAWs is explained based on the concept of exceptional bulk waves, which is a part of SAW theory.