The effect of electrodes on the surface acoustic wave propagation in elastic solids

Abstract

Electrodes on the surface of an elastic substrate have important effects on the surface acoustic wave (SAW) propagation, and related changes like wave velocity and mode shapes due to the presence of electrodes should be considered in the analysis so the information available to practical applications can be accurate and adequate. Because the essential characteristics of surface acoustic wave propagation is obtained from semi-infinite solids, practical applications related to device analysis and design are derived with assumptions and approximations through simplifying the electrodes and reduction of numbers of variables. In an attempt to improve the accuracy and complexity of SAW analysis, a simple electrode as a thin metal layer over an infinite isotropic elastic solid is considered for a demonstrative study. With precise solutions from three- dimensional equations of elasticity for surface acoustic wave propagation, we can extend the results to discontinuous periodic electrodes, or the interdigital transducers (IDTs), for better estimation of the effect on wave velocity and mode shapes. In a manner similar to our previous work on bulk acoustic wave resonators, simple but effective concepts like the thickness and mass ratios are introduced for a direct estimation and easy solution of wave properties. The method will be further extended to anisotropic materials that are widely used in surface acoustic wave devices as substrates for better and practical analytical approaches closer to demands of design process. Eventually, we want to further apply these methods to the two-dimensional theory we are working on so the analysis of surface acoustic wave propagation, both analytical and numerical, can be improved for the device design.