7E-1 An Air-Gap Type FBAR Filter Fabricated Using a Thin Sacrificed Layer on a Flat Substrate

We describe the development of an air-gap type film bulk acoustic resonator (FBAR) filter fabricated using a thin sacrificed layer on a flat substrate. In the development process we focused on reducing the cost of the wafer process. Results from both simulations and experiments demonstrate that a dome- shaped air gap was formed between the substrate surface and the bottom electrode and that an air-gap type FBAR structure was possible. Even if a thin sacrificed layer is used, the air gap can be formed on the flat substrate using stress control. Consequently, the Q-factor at resonance and anti-resonance and effective k² were found to be 1500, 1100, and 7.2% in the 2-GHz range, respectively. The wideband code division multiple access (W- CDMA) band I duplexer was designed using the proposed air- gap type FBAR filters in a 3.0 times 2.5 times 0.7-mm ceramic package. The insertion losses were as small as 1.2 dB and 1.5 dB in the Tx and Rx bands respectively.