Strip Bulk Acoustic Wave Resonators (SBAR) for Boosting Quality Factor

This work proposes a novel bulk acoustic wave (BAW) resonator structure composed of a series of strip-shaped film bulk acoustic resonator (FBAR) unit cells. Each strip is only a few micrometers wide (with length on the order of hundreds of micrometers), dramatically increasing the proportion of free (air) boundaries relative to anchored edges. Implemented on a Mo/Sc0.2Al0.8N/Mo stack, these strip bulk acoustic wave resonators (SBARs) demonstrate significantly higher quality factors (Q) than traditional FBAR devices on the same wafer. The lateral acoustic waves are effectively confined by the air interfaces, yielding the Q-factor improvement and simultaneously reducing a series of lateral spurious modes. Furthermore, an SBAR variant employing round strips is introduced to eliminate standing-wave resonances between parallel strip edges, achieving a spurious-free impedance response with only minimal compromise in Q. The presented SBAR designs offer a path toward high-Q resonators for RF filters without sacrificing electromechanical coupling or introducing unwanted modes.