Development of film bulk acoustic wave resonators based on piezoelectric aluminum nitride

Abstract

Film bulk acoustic wave resonators (FBARs) have high quality factors (Q) and are smaller than conventional surface acoustic wave devices, so they are particularly suitable for constructing the bandpass filters of mobile phones. This report covers the simulation and fabrication of FBARs consisting of an aluminum nitride (AlN) piezoelectric layer sandwiched between molybdenum (Mo) electrode layers. The simulation was based on Mason’s equivalent circuit model. Other than material Q, the AlN material constants for the FBAR model were those reported by Tsubouchi and Mikoshiba [K. Tsubouchi and N. Mikoshiba, IEEE Trans. Sonics Ultrason. 32, 634–644 (1985)]. The material Q of AlN was assumed to be 1000. Thickness values for the AlN and Mo films were determined by simulation, after which the FBARs were fabricated. Reactive radio-frequency sputtering and direct-current sputtering were adopted for deposition of the AlN and Mo films, respectively. In terms of frequency properties, our FBARs achieved Q of 800 and an effective electromechanical coupling coefficient of 5.7%. The transmission loss was greater than that calculated through simulation. This result suggests that the material Q of AlN is less than 1000. This may be due to suboptimal film quality.