Surface Transverse Wave Mode Analysis and Coupling to Interdigital Transducers

The acoustic modes propagating under a surface grating of metal strips are computed for arbitrary cuts, propagation directions, and choices of piezoelectric material. Surface Skimming Bulk Waves (SSBW) which exist in the absence of the metal grating are shown to be trapped at the surface when the grating is present, giving the Surface Transverse Wave (STW) solutions. The relevant physical quantities such as acoustic displacement, scalar potential, electric field, and power density are also computed for these modes. Excellent agreement with experimental results for STW resonators fabricated on rotated Y-cuts of Quartz is shown. Next, the coupling of an unapodized Interdigital Transducer (IDT) to STW’s is analyzed using normal mode theory. The real part of the IDT radiation impedance is calculated directly, and the imaginary part of the impedance is obtained from the real part through a Hilbert transform. Good agreement with experimental results for devices fabricated on rotated Y-cuts of Quartz is demonstrated.