Change in piezoelectric boundary acoustic wave characteristics with overlay and metal grating materials

This paper describes how the characteristics of shear-horizontal type piezoelectric boundary acoustic waves (PBAWs) change with combination of different overlay and metal grating materials. It is shown that PBAWs are supported in various structures provided that highly piezoelectric material(s) are employed as structural member(s). For verification, numerical simulation of different material combinations is done. The results are in good agreement with the qualitative prediction. That is, large electromechanical coupling factor K² is obtainable when materials having small mass densities shear modulus c₄₄ and shear velocity VBS; and materials having extremely large shear modulus c₄₄ are chosen, respectively, for overlay and metallic grating. When YX-LiNbO₃ is assumed as a substrate, for example, the best choice seems to be SiO₂ and Au for overlay and metallic grating, respectively. Although metals with extremely large ?? and c₄₄ such as W and Ta offer large K², they may not be acceptable for practical PBAW applications because of their large electric resistivity.