Ultra-Low Acoustic Loss Micro-Machined Butterfly Lamb Wave Resonators on AlN Plates.

This study reports the design of a novel Butterfly Lamb wave resonator (LWR) employing the $S_{0}$ mode in the AlN plate, and for the first time, its ultrahigh parallel-resonance quality factor ( $Q_{p}$ ) of 4,021 is demonstrated, indicating an ultralow acoustic loss. Although the series resonance quality factor ( $Q_{s}$ ) is widely used for various loss comparisons, it is inconclusive since $Q_{s}$ is always dominated by the routing resistance ( $R_{s}$ ), which is normally huge without the thick metal rewiring. Instead, $Q_{p}$ is a precise representation of the acoustic loss level for its independence of $R_{s}$ and as it is closer to $Q_{\max }$ of the Bode $Q$ -curve in the IDT-excited devices. A butterfly-shaped resonance cavity, theoretically predicted to reduce the anchor loss and suppress the transverse spurious mode, has been applied to the AlN LWR and experimentally shown to boost the $Q_{p}$ by 2.3 times. In addition, a directly measured Bode- $Q$ curve for the LWR is reported for the first time, showing superior $Q$ profile for the Butterfly-LWR than the conventional-LWR and good agreement with the 3 dB- $Q_{p}$ ’s.