Super-High-Frequency Band Filters Configured with Air-Gap-Type Thin-Film Bulk Acoustic Resonators

The first 24 and 30 GHz band thin-film bulk acoustic resonator (FBAR) filters are reported in this paper. The filters were configured with air-gap-type FBARs on a flat silicon substrate. They were designed using a Butterworth--Van-Dyke (BVD) equivalent circuit considering the linear relationship between resonant frequency, capacitance ratio, and loss in the FBAR. The measured characteristics corresponded with those of the simulation using the equivalent circuit in the pass-band. The center frequency, fractional bandwidth, minimum insertion loss, and out-of-band suppression were 23.8 GHz, 3.4%, $-3.8$ dB, and $-13$ dB in the 24 GHz band filter, and 29.2 GHz, 3.4%, $-3.8$ dB, and $-11$ dB in the 30 GHz band filter, respectively.