Tunable high-quality-factor interdigitated (Ba, Sr)TiO₃ capacitors fabricated on low-cost substrates with copper metallization

Interdigitated capacitors containing the field-tunable ferroelectric Ba_0.75Sr_0.25TiO₃, polycrystalline alumina substrates, and copper metallization have been fabricated. Dielectric layers were prepared by magnetron sputtering, while the Cu metallization was evaporated. The dielectric tunability of the Ba_0.75Sr _0.25TiO₃ was 40% at an applied electric field of 12 V/μm. This corresponds to a 3-μm electrode gap width and a 35 V dc bias. Low-frequency (1 MHz) loss tangent measurements indicate a dielectric Q (quality factor) of ∼100 while microwave measurements reveal a zero bias device Q of ∼30 at 26 GHz. These values are comparable or superior to numerous reports of barium strontium titanate interdigitated capacitors prepared using single crystalline substrates and noble metallization. As such, this technology is significantly less expensive and more amenable to large-volume manufacturing.