Thin film Ba0.25Sr0.75TiO3 varactors on Au bottom electrode for microwave applications

Thin film Au/Pt/Ba0.25Sr0.75TiO3/Au/Ti varactors were fabricated on fused silica substrates using pulsed laser deposition of the ferroelectric films. Deposition temperature was optimized to improve varactor tuneability and loss tangent. The varactors were characterized in the frequency range 1-25 GHz at different dc bias voltage. The resonant absorption of the microwave power observed below 12 GHz, under dc field, is due to electrostriction and field induced piezoelectric effects. Above 12 GHz varactors on Au/Ti bottom electrode reveal loss tangent less than 0.02 and tuneability up to 35%. These parameters indicate the potential of Ba0.25Sr0.75TiO3 varactors for applications in tuneable microwave devices. The estimated figure of merit of the phase shifters utilizing these varactors is more than 200 degree/dB at frequencies up to 20 GHz. A resonator may be tuned at more than 10 unloaded bandwidths at frequencies up to 25 GHz.