Enhancing film bulk acoustic resonators performance by optimizing AlN seed layer crystallinity and polarity alignment

The demand for high-performance filters in next-generation wireless communication systems underscores the limited electromechanical coupling of aluminum nitride-based film bulk acoustic resonators. Scandium incorporation enhances piezoelectricity but is hindered by crystallinity degradation and polarity inversion. Theoretical modeling reveals the polarity inversion interface between aluminum nitride and scandium-doped aluminum nitride as a key cause of piezoelectric degradation. This work proposes a dual-optimization strategy that improves crystalline alignment and eliminates polarity mismatch. A single-crystalline aluminum nitride seed layer promotes high c-axis oriented scandium-doped aluminum nitride films, yielding resonators with a maximum quality factor of 736. Subsequent seed layer removal eliminates the polarity inversion interface, raising the effective electromechanical coupling coefficient of resonators from 6.0% to 13.2%. Filters fabricated with this strategy achieve a center frequency of 6.4 GHz, a 3 dB bandwidth of 740 MHz, and out-of-band rejection above 40 dB, indicating potential for Sub-7 GHz communication systems.