Pure-shear mode BAW resonators consisting of (11-20)textured ZnO films

This paper presents pure-shear mode film bulk acoustic wave resonators (FBARs) based on the (112̄0) textured ZnO and AlN films. We have also introduced FBAR structure consisting of two layers of the (112̄0) textured ZnO film with opposite polarization directions. This FBAR structure operated in second overtone pureshear mode, and allowed shear-mode FBARs at higher frequency. For ZnO films, the effective electromechanical coupling coefficients κ _eff² of pure-shear mode FBAR and second overtone pure-shear mode FBAR in this study were found to be 3.3% and 0.8%, respectively. The temperature coefficient of frequency (TCF) for three types of FBARs were measured in the temperature range of 10-60 °C. TCF values of -63.1 ppm/°C, -34.7 ppm/°C, and -35.6 ppm/°C were found for the thickness extensional mode FBAR, the pure-shear mode FBAR, and the second overtone pure-shear mode FBAR, respectively. These results demonstrated that pure-shear mode FBARs have more stable temperature characteristics than the conventional thickness extensional mode FBARs. For AlN films, we report the first synthesis of the (112̄0) textured AlN film on a silica glass substrate. Shear wave excitation in the GHz range was demonstrated using AlN high overtone bulk acoustic resonator (HBAR) structure.