Electrical Frequency Tuning of Film Bulk Acoustic Resonator

This paper describes the design, fabrication, and measurement of an electrically tunable film bulk acoustic resonator (FBAR) that is formed by integrating FBAR with an electrostatic microelectromechanical systems actuator. Around 1.47% tuning of the series resonant frequency ( Deltaf cong 22.5 MHz) at 1.5 GHz is experimentally obtained with an electrostatic actuation voltage of 7 V. This is the highest frequency tuning reported for FBAR operating at above 1 GHz without any extra power consumption. Two integration approaches of FBAR and air-gap capacitor are presented and compared, in terms of fabrication process and Q factor. The approach that minimizes any possible energy loss in the acoustic wave propagation path shows a quality factor (160-304) significantly higher than the one having a capacitor right on top surface of the FBAR's piezoelectric film. Furthermore, we have characterized the electrical tuning of FBAR through piezoelectric stiffening due to an applied DC electric field and report a linear frequency shift of about -8 ppm/V at 3.4 GHz.