Improved SHSAW transduction efficiency using grating and uniform electrode guiding

Pure shear-horizontal surface acoustic wave (SHSAW) devices have been increasingly considered for liquidphase and biosensing applications because of their ability to operate under liquid-loaded conditions and intrinsic sensitivity to mass, stiffness, viscosity, and electrical perturbations occurring at the device/fluid interface. Typically, the SHSAW is weakly guided by a free surface boundary condition (BC) or may not even exist for some materials and orientations, such as the quartz ST-90° orientation considered in this work. For a surrounding free surface BC, the interdigital transducer (IDT) typically generates strong shear-horizontal bulk acoustic waves (SHBAWs) relative to SHSAW. For that reason, guiding structures, e.g., dense and/or thick electrodes in periodic or uniform configurations, are incorporated into the design and placed between IDTs in delay-line devices to increase the ratio of transduced SHSAW power to IDT input power, ηSHSAW. The degree of ηSHSAW improvement depends on the thickness, composition, and geometry of the guiding structure. In previous work, the authors evaluated ηSHSAW using hybrid finite and boundary element method (FEM/BEM) models, but were limited to cases of stress-free or finite-thickness-grating surrounding surfaces. This work extends the analysis to the important boundary condition case of uniform finite-thickness electrode guiding, which is typically employed in liquid-phase and biosensor applications. To integrate the uniform electrode guiding structure with the SHSAW device analysis, a combined finite-length uniform electrode structure followed by an additional quarter-wavelength electrode was considered. In this work, it is shown that adjusting the quarter-wavelength electrode's film thickness and length allows cancellation of the SHSAW reflection from the edge discontinuity. As a result, the finite-length uniform guiding electrode can be treated as if it extends to infinity, and ηSHSAW can be easily obtained. In addition, the finite thickness of all electrodes is considered in the calculations. To verify the model, an IDT with uniform guiding electrodes was simulated and compared with the experimental results of a fabricated and tested device. The simulations predict SHSAW excitation directivity of 9 dB by the IDT, which is experimentally confirmed to within 0.8 dB.