DC bias-dependent shift of the resonance frequencies in BST thin film membranes

Direct current (DC) bias-dependent acoustic resonance phenomena have been observed in microma-chined tunable thin film capacitors based on Ba_0.3Sr_0.7TiO₃ (BST) thin films. The antiresonance frequency is only weakly DC bias dependent, and the resonance frequency exhibits a much stronger dependence on the applied DC bias. The resonance frequency shifted by 1.2% for a frequency of about 6.7 GHz and an applied field of 667 KV/cm. At the same time the effective electromechanical coupling constant k² _t,eff increased to 2.0%. The tuning of the resonance frequency depends on the tunability of the film permittivity and on the mechanical load on the piezoactive layer. The experimental observations correlate well with the theoretical predictions derived from the free energy P expansion using Landau theory.