6A-3 Pure Shear Horizontal SAW Network Model for Periodic Structures Including Bulk Scattering

Abstract

Rigorous simulations using combined finite/boundary element methods (FEM/BEM) are often too time consuming for the rapid evaluation of long surface-acoustic wave (SAW) structures, and soon become impractical as device length increases. This situation is complicated by the need of subsequent design modifications and analyses. Scalar models, and in particular, transmission line (TL) based network descriptions, enable fast evaluation of arbitrary device topologies comprised of inter digital-transducers (IDTs) and reflector/grating structures. For example, simulating a resonator structure using a network model reduces computation time from days or hours to minutes, allowing the identification of optimal designs, which may then be further analyzed using rigorous FEM/BEM techniques. The traditional network model (NM) consists of mismatched TLs, shunt susceptances, and transformers, and has been successfully used to simulate transduction and propagation of pure Rayleigh type and generalized SAWs in periodic structures. In these cases, it is often acceptable to neglect bulk acoustic wave (BAW) radiation, since the operational frequencies of the SAW devices are far away from the BAW radiation frequency. However, in the case of pure shear horizontal (SH) SAW orientations significant BAW scattering is frequently observed because the BAW radiation frequency takes place near the SAW bandwidth, and thus the traditional NM fails to account for the energy radiated to the bulk. In this work a modification of the traditional NM is introduced that accounts for energy loss near the upper stopband edge for a long periodic structure, thus allowing the use of the NM to adequately simulate structures along pure SH SAW orientations. The BAW radiation is modeled by a frequency-dependent shunt conductance located at electrode edge discontinuities. A parameter extraction technique is presented that allows the rapid determination of frequency-dependent parameters. In particular, the parameters are fit to FEM/BEM computed dispersion and harmonic admittance relations. The response of a SH SAW resonator was computed using the NM and compared to measurements of devices fabricated on langasite, Euler Angles (0deg, 22deg, 90deg). Good agreement is observed with a slight variation in center frequency ~0.25%, thus verifying the modified model and its applicability to rapid design of pure shear horizontal SAW devices