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 Ba
_0.25
Sr
_0.75
TiO
₃
films separated by a 50-nm-thick SrRuO
₃
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 (f
₂
/f
₁
≥ 2) depends on the thickness of the electrodes. Some adjustment of f
₂
/f
₁
is possible by changing the applied dc bias.