Analysis and Suppression of Spurious In-Plane Lamé-Like Modes in Lamb Wave Resonators

Spurious-free Lamb wave resonators (LWRs) offer an effective solution for constructing multiband filters on a single chip; however, various spurious modes are commonly observed, which needs modal analysis of these LWRs to eliminate them. In this study, we conduct transverse dispersion analysis of aluminum nitride (AlN) LWRs operated at the lowest order symmetric (S0) mode utilizing theoretical, simulation, and experimental approaches. Our analysis reveals that in-plane Lamé-like modes exhibit the strongest coupling with S0 mode among various contour modes. This coupling in the frequency spectrum occurs periodically, with the period determined by the lateral overtone order of the S0 mode. Through theoretical analysis of resonant frequency and the period of weakly coupled regions between Lamé-like modes and S0 modes, we demonstrate that such Lamé-like modes can be suppressed by modulating the length-to-width ratio to target weakly coupled regions in the frequency spectrum. Our findings offer a critical guidance for the development of high-performance and spurious-free LWR devices, filling crucial gaps in the modal analysis of LWRs.