Design and Characterization of Novel Barium Strontium Titanate Thick Films for Sub-6 GHz RF Applications

In this article, a novel study of the design and characterization of barium strontium titanate (BST) thick films produced by electrophoretic deposition (EPD) on flexible platinum foils is reported. Intended for sub-6 GHz radio frequency (RF) applications, a study on film and electrode thickness is conducted. First, a comparison of tunability and quality factor between different film thicknesses regarding frequency is performed. The $Ba_{0.7}Sr_{0.3}TiO_{3}$ varactors demonstrate variable capacitance between 49% and 55% for frequencies from 500 MHz to 5 GHz. Afterward, the influence of electrode thickness on the capacitance and quality factor of the films are studied. The variable capacitance and the quality factor present a tradeoff regarding the parametric studies of film and electrode thicknesses. Lastly, a microstrip line is deposited over the BST thick film to simulate and measure the phase-shifting capabilities of the device. Using a one-tone measurement setup, the small-signal S-parameter measurements of the phase shifter are also presented, and the linearity is analyzed considering two different bias voltages, 0 and 10 V. For the first time, the BST thick films fabricated by EPD process are implemented and measured in the RF regime as a metal-insulator-metal capacitor and as a phase shifter circuit.