The influence of film thickness on electrophysical properties of planar varactors based on ion-plasma deposited BaSrTiO 3 films is investigated. Planar capacitors with copper electrodes were used for tunability and loss factor investigation. Microwave parameters of varactors based on BaSrTiO 3 films were measured at (1 ÷ 8) GHz frequency range and (170 ÷ 370) K temperature range. The tunability and loss factor was found to be strongly depended on film thickness. The optimal thickness of BaSrTiO 3 film from high tunability and low loss factor point of view was estimated as 1 µm approximately. Dielectric losses at frequency range (1 ÷ 8) GHz (tan δ ∼ 0,01 at 1 GHz, tanδ ∼ 0,014 at 8 GHz and zero bias) and high tunability (Cmax/Cmin ∼ 2) at room temperature were observed.
The existence of an intersection point of electric field strength dependencies of permittivity of ferroelectrics at different temperatures is theoretically described by Ginzburg-Devonshire (GD) theory and experimentally confirmed by studying (Ba 0.3 Sr 0.7 )TiO 3 (BSTO) and SrTiO 3 (STO) thin film varactors. GD analysis enables one to propose a way to improve the thermal stability of ferroelectric microwave devices. In particular, parameters of a L-band digital (0 • /180 • ) phase-shifter fabricated from thick BSTO film varactors were stabilized in the temperature range T = (210-310) K by application of variable to DC control voltages.
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