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

This paper describes how the propagation characteristics of shear-horizontal (SH) type piezoelectric boundary acoustic waves (PBAWs) change with overlay and metallic grating materials. We theoretically investigated the PBAW characteristics for various combinations of the overlay and metallic grating materials, and attempted to validate the qualitative prediction previously discussed and find the possible best material combination. The results are in agreement with the qualitative prediction. That is, large electromechanical coupling factor K² is obtained when materials having small mass densities rho and materials having extremely large rho 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 SiO₂ and Au for overlay and metallic grating, respectively. Although metals with extremely large rho and c₄₄ such as W and Ta offer large K², they may not be acceptable for practical applications for their large electric resistivity.