A Compact Low-loss BAW Filter for Mobile Communication

This paper reports the design, fabrication and characterization of a radio-frequency (RF) band-pass filter based on bulk-acoustic wave (BAW) resonators. Fabricated using in-house microfabrication processes, the BAW resonator consists of two layers of molybdenum (Mo) electrodes and a layer of aluminum nitride (AlN) film sandwiched in between. Measurement results show that the resonator operating at 2.9GHz achieves an electromechanical coupling coefficient $\left( {k_t^2} \right)$ of 7% and a Q of over 1000, indicating a Figure-of-Merit (FoM) higher than 75. The fabricated ladder-type BAW filter shows a minimum insertion loss of 0.9 dB and a fractional 3dB bandwidth of 3.3% at 2.9GHz.