Parasitic Effect of the (Ba,Sr)TiO3 Thin Film for Tunable Microwave Device Application

The (Ba-0.6,Sr-0.4)TiO3 (BST) thin films were deposited on the Pt bottom electrode (Pt/Ti/SiO2/Si) by radio frequency magnetron sputter. After that, a 100 nm-thick Pt thin film was deposited by electron beam evaporator as the top electrode to form metal/insulator/metal (MIM) structure. The capacitance-voltage characteristics were recorded to study the voltage-controlled capacitance. The frequency dispersion was found in the measurements of capacitance and loss tangent. The reason should be the parasitic effect of series resistance and parasitic inductance. The simulations of the four-element modified model were performed to evaluate the influences of the measurement frequency and parasitic resistance on the electrical measurement. The simulation results indicate that the capacitance and loss tangent increase with increasing the measurement frequency. The tunability slightly increases with increasing measurement frequency while the figure of merit seriously degrades with increasing measurement frequency and series resistance. The correct electrical properties of the tunable microwave device should be measured at low frequency (< 100 kHz) or extracted with the four-element modified model.