55.4 GHz Bulk Acoustic Resonator in Thin-Film Scandium Aluminum Nitride

This study describes a sputtered scandium aluminum nitride (ScAlN) thin-film bulk acoustic resonator (FBAR) at 55.4 GHz. The resonator leverages the third-order symmetric (S3) mode in 37 nm Al on 85 nm Sc0.3Al0.7N on a 37 nm Al film stack, enabled by a new fabrication procedure. The 55.4 GHz S3 tone achieves electromechanical coupling (k²) of 3.8% and a 3-dB quality factor (Q) of 30, surpassing the reported state-of-the-art figure of merits (FoM, k²∙Q). Besides, the first-order symmetric (S1) mode at 21.0 GHz achieves k² of 6.4% and Q of 62. Upon further optimization of the sputtering procedure, ScAlN/AlN-based mmWave acoustic resonators will continue to be improved.