12E-2 X-Band Filters Utilizing AlN Thin Film Bulk Acoustic Resonators

A filter with aluminum nitride thin-film bulk acoustic resonators (FBAR) for the X-band was developed. The FBAR has an air gap beneath the resonator to isolate acoustically from a substrate, and is extremely thin to operate in the X-band. The FBAR has two structural features - a sacrificial layer to make the air gap was very thin in order to prevent the resonator from cracking on the edge of the air gap, and a resonator was deformed to dome shape by stresses of the films to keep the air gap. The fabricated FBAR operated successfully with a k
_eff

²
of 6.40%, a resonance Q of 246, and anti-resonance Q of 462. The fabricated filter had a center frequency of 9.07 GHz, a fractional bandwidth of 3.1%, a minimum insertion loss of -1.7 dB, and an out-of-band rejection of -21 dB.