Acoustic dispersion of solidly mounted resonators with an optimized reflector stack for dual wave reflection

Abstract

Spurious resonance suppression in Bulk Acoustic Wave (BAW) resonators is essential for achieving excellent filter characteristics. These spurious resonances are undesired resonances caused by the finite device edges. The type I mode is the preferred dispersion response for "spurious modes" suppression, since it allows for the direct use of the frame region as the edge termination of the device. In this work we present a novel analytical approach to adjust the dispersion response in a desired way for various material combinations. By employing the diffraction grating method (DGM), which provides dual wave reflection after tailoring the layer stack thickness analytically, we obtain a type I dispersion curve despite the use of a type II piezoelectric material for the resonator. The dispersion response was verified from analytical impedance curves and FEM simulations. The reason for this type of response can be the inherent thicker top-oxide layer. A transmission of −37dB is maintained for longitudinal waves whereas shear waves reach value below −22 dB for a five layer SiO2/W stack, while for a three layer SiOC/Pt (935/555nm) stack −36dB and −24dB is obtained, respectively.