AlN/AlScN composite film bulk acoustic wave filters for 5G bands

Bulk acoustic wave (BAW) resonators with tailored effective electromechanical coupling coefficients (Keff2) are essential for advanced filter design. Although film stack design is a common method for Keff2 adjustment, its effectiveness diminishes above 5 GHz, where AlN/AlScN composite films present a promising alternative. In this work, we developed a composite film Mason model specifically for characterizing AlN/AlScN composite film resonators. This model not only accurately predicts Keff2 across varying thickness ratios but also simulates composite film resonator impedance, establishing a framework for quantitative analysis and filter design. Leveraging this model, we designed and fabricated high-performance composite film BAW resonators and filters. The optimized resonators achieved a center frequency of 5.3 GHz, a Qmax of 1277, and an f × Keff2 × Qmax of 815.9 GHz, higher than that of pure AlScN film resonators (Qmax = 471, f × Keff2 × Qmax = 275.6 GHz). Furthermore, a BAW filter with a center frequency of 5.3 GHz and a −3 dB bandwidth of 300 MHz was fabricated, exhibiting steep roll-off, high out-of-band rejection, and low insertion loss. These performance improvements originate from the synergistic optimization of Keff2 and resonator performance enabled by the composite film architecture, conclusively demonstrating its distinct advantages for high-frequency, custom-designed BAW filters.