Optimal design of immersed SAW devices for high- and low-frequency sensor applications

In many sensor applications surface acoustic wave (SAW) devices may have to be immersed in a liquid. Attenuation of the surface acoustic wave modes and leakage of energy into the liquid have to be considered in designing appropriate devices. To design an immersible SAW device, the attenuation of the SAW must be decreased to get the highest SAW generation efficiency. Through numerical simulation, the optimal geometry of underwater SAW devices is studied. Parameters such as optimal piezoelectric crystal cut, SAW propagation direction and non-dimensional wavenumber (ka), are determined for all the modes of surface wave propagation to get the maximum SAW excitation efficiency, the minimum attenuation in propagation and pure mode propagation. In this paper we have studied SAW propagation in a thin piezoelectric film on a semi-infinite substrate which is exposed to a liquid. The assumption of finite thickness for the piezoelectric film allows us to consider both low- (<50 MHz) and high-frequency devices.