Estimation of temperature dependence of C44 elastic constant in LiTaO3 single crystals

In this work, electromechanical coupling factor, K², and Temperature Coefficient of Frequency (TCF) of Bulk Acoustic Wave (BAW) resonators based on high aspect ratio ridge structures in 42°Y-X LiTaO₃ (LT) and Z-cut LT single crystals with different Li stoichiometries were studied experimentally, analytically and by combined finite element / boundary element analysis. Initially, analytical analysis was carried out to determine the contribution of different elastic constants in thickness field excitation of shear modes in high aspect ratio ridge structures. The analysis shows shear modes that are dependent on C₄₄ in ridge resonators based on 42°Y-X and Z-cut LT crystals. However, the measured frequency response of such structures exhibited many spurious resonances, making difficult the direct extraction of acoustical parameters. A fitting procedure that allows taking into account several resonances occurring simultaneously in a restricted frequency range was used. Higher K² and more positive temperature dependence of frequency were determined for ridge structures in nearly stoichiometric crystals compared to those of equivalent structures in congruent ones.