Improvement of the crystallinity of ZnO thin films and frequency characteristics of a film bulk acoustic wave resonator by using an Ru buffer layer and annealing treatment

We deposited zinc oxide (ZnO) thin films on an Ru buffer layer in order to protect the amorphous layer at the ZnO and Al interface. Also, ZnO thin films grown by means of an annealing treatment were investigated as regards improvement of the cc-axis orientation and morphology properties. In the X-ray diffraction (XRD) pattern, it was observed that there was an improvement of the (002) orientation achieved by the variation of the annealing treatment temperature. Also, the surface roughness and specific resistance were increased by the annealing treatment but the full width at half-maximum (FWHM) was decreased. For film bulk acoustic resonators (FBARs) fabricated using these results, we finally confirmed that a resonant frequency of 0.79 GHz, without a shift, was measured. In addition, the values of the return loss were improved by the annealing treatment.