Ultrawideband BAW filters with high Sc doped AlScN

Abstract This work reports on a dual optimization of process and device to extend the bandwidth of bulk acoustic wave (BAW) filters based on Al 0.7 Sc 0.3 N piezoelectric film. The thicknesses of the AlN seed layer and Al 0.7 Sc 0.3 N film were adjusted to obtain high-quality Al 0.7 Sc 0.3 N film while maintain high effective electromechanical coupling coefficient ( k eff 2 ) of the BAW resonators. X-ray diffraction of Al 0.7 Sc 0.3 N film show the full-width at half-maximum of the (002) orientation as 1.67° with 0.91 nm surface roughness (Rq). The BAW resonator structure was also optimized based on above piezoelectric film, and a modified Mason model considering the area effect of frequency is proposed. The results show that the BAW resonator has a k eff 2 up to 22% at the operating frequency 3.5 GHz and a Bode quality factor of 992. The filter with a 3 dB bandwidth of 371 MHz was designed and fabricated on 8 inch wafers. The k eff 2 difference of the resonators across the entire 8 inch wafer is no more than 1.6%. The center frequency of the developed BAW filter is 3.35 GHz and the insertion loss is only 1.28 dB with an out-of-band rejection more than −38 dB.