Li-doped (Ba,Sr)TiO3 thick film interdigital capacitors for microwave applications

Microwave properties of Li-doped (Ba,Sr)TiO₃ thick film interdigital capacitors have been investigated. According to the reported papers, BaSrTiO₃ materials, paraelectric state at the room temperature, have high dielectric permittivity (>500 @ 1 MHz) and low loss tangent (<0.01 @ 1 MHz) in epitaxial thin film form; however, the sintering temperature of BaSrTiO₃ is over 1350 °C. In order to reduce the sintering temperature, Li (3 wt%) was added to the BaSrTiO₃ materials, and 10 μm thick Li-doped (Ba,Sr)TiO₃ films were screen printed on the alumina (Al₂O₃) substrate and sintered at 900 °C. Interdigital capacitor patterns with five fingers of 200 μm gap and 250 μm length were also designed and fabricated by employing the screen printing method with Ag electrode. The structural feature was analyzed with X-ray diffraction method. Frequency and temperature-dependent dielectric properties were characterized from 1 kHz to 1 MHz and 303-403 K, respectively. Also, current-voltage characteristics were investigated with an elevated temperature. Microwave transmission and reflectance properties of thick film interdigital capacitors will be discussed, and frequency dispersion of dielectric properties will be presented. Specially, designed Au/Li-doped (Ba,Sr)TiO₃/Ag-Pd/Al₂O₃ vertical structure was prepared to measure the tunability. In this sandwich type structure, Li-doped (Ba,Sr)TiO₃ films showed tenability of 7.15% at a bias electric field of 20 kV/cm.