Composite ferroelectric FBARs that are switchable between the first and second harmonics: experimental demonstration

Digital switching between the first and second harmonics, in a composite thin-film bulk acoustic wave resonator (FBAR), is demonstrated experimentally. The FBAR consists of two 180-nm-thick paraelectric-phase Ba0.25Sr0.75TiO3 films separated by a 50-nm-thick SrRuO3 conducting layer. The resonant frequency of this composite resonator (with Pt bottom and Al top electrodes) is switched from 3.6 GHz to 7.6 GHz, where the polarity of the 5 V dc bias is reversed on one of the ferroelectric films. The frequency switching ratio (f2/f1 ≥ 2) depends on the thickness of the electrodes. Some adjustment of f2/f1 is possible by changing the applied dc bias.