Characterization of ferroelectric BaSrTiO₃ thin films using a flip-chip technique at microwave frequency ranges

Microwave properties of ferroelectric Ba_0.5Sr_0.5TiO₃ (BST) thin films were investigated utilizing a gold resonator combined with a planar flip-chip structure. The resonator was designed to have bias circuits at the zero field points in the microstrip line. The dielectric properties, capacitance (C) and dielectric loss (tan δ) of the BST film are characterized under the influence of an applied dc voltage, V_b) at room temperature. The BST ferroelectric capacitor with a capacitance of about 0.6 pF successfully controlled the tunability of the resonance frequency in the resonator with a factor of ∼2. In addition, dependence of the loss tangent and the capacitance on the electrode were determined for both of the SrRuO₃/BST/LaAlO₃ (SRO/BST/LAO) and BST/LAO structures. The use of a SRO buffer layer in the multilayer provided a low loss tangent of about 3 × 10^-3 at about 2 GHz and 80 V bias voltage. This low loss seems to be related to the reduction of defect density in the interfacial layer between the electrode and the ferroelectric thin film.