Development of PVDF-TrFE/SiO₂ composite film bulk acoustic resonator

This paper reports a film bulk acoustic resonator (FBAR) using polyvinylidene fluoride- trifluoroethylene (PVDF-TrFE)/SiO₂ composite. The PVDF-TrFE/SiO₂ composite FBAR was designed, simulated and developed. A Micro Electro Mechanical Systems (MEMS) process including xenon difluoride (XeF₂) release etching was adopted with some tuning for PVDF-TrFE. The influences of the process on the surface morphology and crystal orientation of PVDF-TrFE were evaluated for RIE, gold sputtering and XeF₂ etching. After poling PVDF-TrFE, the thickness expansion mode was observed at 387–398 MHz. Impedance characteristics was evaluated using a modified Butterworth-Van Dyke (MBVD) model for a lossy piezoelectric transducer. Impedance ratio, coupling coefficient (k²), mechanical quality factor (Q_m) and figure of merit increase with the radius of the device. The maximum Q_m is 33 for 80 μm radius of the device. The residual film stress of PVDF-TrFE was estimated as 34.8 MPa in tensile mean stress and −79.7 kPa in gradient stress. Elastic stiffness constants c₃₃ ^D and c₃₃ ^E at electrical displacement D = 0 and electric field E = 0 were obtained as 4.45 GPa and 4.46 GPa, respectively.