Increased piezoelectric coupling factor in temperature-compensated film bulk acoustic resonators

Deterioration of the effective electromechanical coupling factor (K
_eff
²
) upon improving the temperature coefficient of frequency (TCF) is a serious issue for temperature-compensated bulk acoustic wave (TC-BAW) resonators, because TC-BAW filter applications are restricted to bandpass filters with narrow relative bandwidths under 1.5% in wireless communication systems. Our objective is to therefore enhance the K
_eff
²
of TC-BAW resonators for radio frequency filters and duplexers with wide relative bandwidths. In this work, the two improvements were investigated to enhance the K
_eff
²
of the temperature-compensated thin film bulk acoustic wave resonator (TC-FBAR). First, we applied the fluorine-doped silicon oxide (SiOF) film with a large positive temperature coefficient of velocity (TCV) to a TC-FBAR. The TCV of the SiOF film (8.8 at.% F) was 164 ppm/°C, which was much larger than 70 ppm/°C of the SiO
₂
film. Second, we optimized the layer structures of the TC-FBAR. TCF of -11.1 ppm/°C and K
_eff
²
of 6.26% were achieved on the TC-FBAR with the structure of Ru/AlN/Ru/SiOF/Ru. We believe that the SiOF-based TC-FBAR thus enables the application to bandpass filters with wide relative bandwidths greater than 2.5%.