Ultralow Acoustic Loss Micromachined Butterfly Lamb Wave Resonators on AlN Plates

This study reports the design of a novel Butterfly Lamb wave resonator (LWR) employing the S₀ 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.