Laterally coupled BAW filters with 5% bandwidth

Lateral coupling between thin-film bulk acoustic wave resonators (LBAW) is a promising technology for fabrication of bandpass filters. Previously, narrow bandwidth of realized filters has been the main problem for the applicability of this technology. In this paper, solidly-mounted LBAW filters with different number of electrodes and passband width up to 5% are presented. Thin-film AlN is used as the piezoelectric material with a Mo bottom electrode and an Al top electrode. Filters consist of interdigital electrodes with varied electrode number, electrode width and gap between the electrodes. Acoustic coupling strength obtained at 2 GHz varies from 1%...1.5% for 2-electrode devices to 5% given by 31-electrode filters. With post-measurement matching to 120 Ω ∥ 5 nH, electrical frequency response obtained with 31-electrode LBAW shows a minimum insertion loss of 2...3 dB and a relative 3-dB bandwidth of 5% at 2 GHz. Effect of electrode number to filter passband is studied.