Electromechanical Coupling to Lamb and SH Modes in Piezoelectric Plates

Recent theoretical studies arid experiments have shown that interdigital transducers (IDTs) can couple strongly to plate modes in piezoelectric materials and in piezoelectric-on-nonpiezoelectic composite membranes. It has been demonstrated that silicon can support zinc oxide films to form thin composite membranes with large electromechanical coupling factors, a technology which allows integration of plate mode devices with ICs. In this paper the calculated velocity dispersion and electromechanical coupling factors for plate modes in representative piezoelectric materials are described. The frequency dependence of velocity and electromechanical coupling factors are given, under different metallization conditions, for: generalized stiffened-Lamb, pure stiffenedLamb, and stiffened- shear (SH) modes, for various plate orientations in lithium niobate, lithium tantalate, quartz, bismuth germanium oxide and zinc oxide. For lithium niobate electromechanical coupling (AVIV) values as high as 15% are found under narrow-band band-pass conditions arid 5% under wide-band low-pass conditions. For lithium tantalate, bismuth germanium oxide, and zinc oxide, coupling values of 0.5%, 2%, and 4% are obtained. For quartz, with its weaker piezoelectricity, the coupling is still smaller. Details of the frequency dependence of coupling factors, wave velocities, and electric and mechanical field profiles which characterize the various mode types are presented.