Periodically poled aluminum scandium nitride bulk acoustic wave resonators and filters for communications in the 6G era.

Abstract

Bulk Acoustic Wave (BAW) filters find applications in radio frequency (RF) communication systems for Wi-Fi, 3G, 4G, and 5G networks. In the beyond-5G (potential 6G) era, high-frequency bands (>8 GHz) are expected to require resonators with high-quality factor (Q) and electromechanical coupling ( $k_t^2$ ) to form filters with low insertion loss and high selectivity. However, both the Q and $k_t^2$ of resonator devices formed in traditional uniform polarization piezoelectric films of aluminum nitride (AlN) and aluminum scandium nitride (AlScN) decrease when scaled beyond 8 GHz. In this work, we utilized 4-layer AlScN periodically poled piezoelectric films (P3F) to construct high-frequency (~17-18 GHz) resonators and filters. The resonator performance is studied over a range of device geometries, with the best resonator achieving a $k_t^2$ of 11.8% and a $Q_p$ of 236.6 at the parallel resonance frequency ( $f_p$ ) of 17.9 GHz. These resulting figures-of-merit are ( ${\mathrm{FoM}}_1={k_t^{2}Q}_p$ and ${\mathrm{FoM}}_2=f_p{\mathrm{FoM}}_1\times {10}^{-9}$ ) 27.9 and 500, respectively. These and the $k_t^2$ are significantly higher than previously reported AlN/AlScN-based resonators operating at similar frequencies. Fabricated 3-element and 6-element filters formed from these resonators demonstrated low insertion losses (IL) of 1.86 and 3.25 dB, and -3 dB bandwidths (BW) of 680 MHz (fractional BW of 3.9%) and 590 MHz (fractional BW of 3.3%) at a ~17.4 GHz center frequency. The 3-element and 6-element filters achieved excellent linearity with in-band input third-order intercept point (IIP3) values of +36 and +40 dBm, respectively, which are significantly higher than previously reported acoustic filters operating at similar frequencies.