  This work describes sputtered scandium aluminum nitride (ScAlN) thin-film
bulk acoustic resonators (FBAR) at millimeter wave (mmWave) with high quality
factor (Q) using platinum (Pt) electrodes. FBARs with combinations of Pt and
aluminum (Al) electrodes, i.e., Al top Al bottom, Pt top Al bottom, Al top Pt
bottom, and Pt top Pt bottom, are built to study the impact of electrodes on
mmWave FBARs. The demonstrated FBAR with Pt top and bottom electrodes achieve
electromechanical coupling (k2) of 4.0% and Q of 116 for the first-order
symmetric (S1) mode at 13.7 GHz, and k2 of 1.8% and Q of 94 for third-order
symmetric (S3) mode at 61.6 GHz. Through these results, we confirmed that even
in the frequency band of approximately 60 GHz, ScAlN FBAR can achieve a Q
factor approaching 100 with optimized fabrication and acoustic/EM design.
Further development calls for stacks with better quality in piezoelectric and
metallic layers.
