  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 (fp) of 17.9 GHz.
These resulting figures of merit are ((FoM)_1=(k_t^2 Q)_p and
(FoM_2=f_p(FoM)_1x10^-9) ) 27.9 and 500 respectively. These and the k_t^2 are
significantly higher than previously reported An/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 dB
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 ~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 dBm and +40 dBm, respectively,
which are significantly higher than previously reported acoustic filters
operating at similar frequencies.
