Improved SHSAW transduction efficiency using gratings and uniform electrode guiding

Pure shear horizontal surface acoustic wave (SHSAW) devices have been increasingly considered for liquid-phase and biosensing applications due to their ability to operate under liquid-loaded conditions and intrinsic sensitivity to mass, stiffness, viscosity, and electrical surface perturbations. Typically, the SHSAW is weakly guided by a free surface boundary condition, or in certain orientations may not even exist, e.g. ST-90° orientation of quartz. As a result strong excitation of spurious bulk acoustic wave (BAW) by the interdigital transducer (IDT) is typical. For that reason dense or thick electrodes in periodic or uniform configurations must be placed between IDTs in delay-line devices in order to increase the ratio of excited SHSAW power to IDT input power, ¿_SHSAW. The degree of ¿_SHSAW improvement depends on the structure metallization thickness, composition, and geometry. In previous work the authors evaluated ¿_SHSAW using hybrid finite and boundary element method (FEM/BEM) models, but were limited to cases of free or finite thickness periodic electrode surrounding surfaces. This work extends the analysis to the important case of uniform electrode guiding, which is used in many liquid-phase and biosensor applications. The reported analysis depends on the simulation of a modified structure which behaves as if the uniform guiding electrodes extend infinitely and allows estimation of ¿_SHSAW for the actual infinite-length structure. To verify the model, an IDT with uniform guiding electrodes was simulated and compared to a fabricated device. The simulations predict SHSAW excitation directivity of 9 dB by the IDT, which is experimentally confirmed to within 1 dB.