Comparative study on excitation characteristics of surface acoustic wave and acoustic plate mode wave

On the basis of the partial wave theory, the surface effective permittivity method was introduced to analyze the excitation characteristics of surface acoustic wave (SAW) and acoustic plate mode wave (APM), and the experiments verified the effectivity of the theoretical study. The results show that only one SAW mode can be excited on the piezoelectric substrate under certain cut orientation and propagation direction, while the APMs have multiple modes.. Furthermore, the high-level modes can be excited only when the ratio of the substrate thickness to the acoustic wavelength is bigger than a certain value. With the increase of the substrate thickness, the propagation velocities (v) and electro-mechanical coupling coefficients (ks2) of the APM high-level modes both decrease. Both propagation velocities of the two APM low-level modes approximate to that of SAW when the substrate thickness increases. As to the electro-mechanical coupling coefficients, one is close to that of SAW, but the other approximates to zero. Additionally, the electrical boundary conditions of the substrate's back side will influence the APM low-level modes' tendency to SAW. From these studies we get a conclusion about how to excite SAW or APM by IDT and the corresponding relationship of the two acoustic waves.