Effect of Mesa-Shaping on Spurious Modes in ZnO/Si Bulk-Wave Composite Resonators
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Bulk-wave composite resonators (BWCRs) employing thin piezo-electric films on silicon membranes have received considerable attention because of potential for integrated selectivity at VHF and UHF. The TE-mode zinc oxide on silicon configuration has received the most attention . A major design problem is the presence of close-in spurious modes on both low and high frequency sides of the fundamental. Mesa-shaping of the BWCR by etching the zinc oxide film in the region surrounding the top electrode has been shown experimentally to reduce the spurii , although this improvement has not been adequately explained. Here we describe further developments to a two-dimensional model which now includes acoustic loss into the membrane, and which predicts admittance at all frequencies, contrasting with earlier models which find solutions only at the resonant frequencies. This model is compared to experiment and is shown to explain the effect of mesa-shaping. Computer simulations illustrate the relationship between geometry and performance.