K-band ladder filters employing air-gap type thin film bulk acoustic resonators

We describe the fabrication and characteristics of a filter with aluminum nitride thin film bulk acoustic resonators (FBAR) for the K-band. The FBAR has an air-gap beneath the resonator to acoustically isolate itself from the substrate, and is has an extremely thin film thickness to operate in the K-band. The FBAR has two structural features, a sacrificial layer to make the air-gap very thin in order to prevent the resonator from cracking on the edge of the air-gap and a resonator that is deformed into the shape of a dome by the film stresses to maintain the air-gap. The fabricated FBAR operated successfully at a k_eff ² of 6.25%, a resonance Q of 189, and an anti-resonance Q of 171. The fabricated filter had a center frequency of 19.8 GHz, a minimum insertion loss of 4.1 dB and a fractional bandwidth of 2.0%.