A Circular Crested Lamb Wave Resonator with Spurious Mode Suppression and Quality Factor Enhancement

Abstract

To date, nearly all the reported Lamb wave resonators (LWRs) are straight crested LWRs, which suffer from inherent spurious modes and low quality factors (Q). For the first time, this work demonstrates a circular crested LWR. Its advantages over the straight crested LWR are presented comprehensively by studying their fundamental symmetric (S0) mode, which is the simplest and most representative Lamb wave mode. Utilizing circular crested Lamb waves, the proposed resonator avoids only utilizing waves propagating in the lateral direction in the straight crested LWRs, thus eliminating the transverse spurious modes as no transverse direction exists. Besides, different from straight crested Lamb waves maintaining the same displacement amplitude along the propagation direction, circular crested Lamb waves exhibit displacement attenuation toward device edges, which effectively concentrates energy in the device center, assists in reducing energy loss through anchors, and improves Q. Based on 20 at.% scandium-doped aluminum nitride (Al0.8Sc0.2N) thin films, the microfabricated circular crested LWR effectively suppresses transverse spurious modes and achieves a 40.7% Q improvement in experiments with no degradation in effective electromechanical coupling coefficients (keff2) compared with the conventional straight crested LWR when working in S0 mode in experiments, in contrast with the conventional straight crested LWR which shows inherent experimental transverse spurious modes. Moreover, the free edges covered by top electrodes enhance device robustness against misalignment and over-etching in the fabrication process. With the advantages of spurious mode suppression, Q enhancement, and fabrication robustness, the circular crested LWR is a promising candidate for next-generation filters and oscillators.