Harnessing Mode Conversion for Spurious Mode Suppression in AlN Laterally Vibrating Resonators

This paper reports on a spurious mode suppression technique for S0 mode aluminum nitride (AlN) laterally vibrating resonators. The technique utilizes a notch in the resonator body to convert spurious asymmetrical (A0) modes into the intended S0 mode vibration. Finite-element analyses were employed to theoretically investigate the mode conversion from A0 mode to S0 mode, and identify the optimal notch configuration. The technique has been experimentally validated to simultaneously eradicate the A0 spurious mode and enhance the S0 mode electromechanical coupling (kt2) for the monolithically integrated resonators over a wide frequency range (100~600 MHz). It is a crucial technology development, because the spurious modes are a major bottleneck obstructing the deployment of single-chip multi-frequency AlN resonators as a commercially viable solution for radio frequency front-end filtering.