The effects of sputtering temperature on the dielectric properties and the microstructure of barium strontium titanate thin films have been investigated. (Ba0.6Sr0.4)TiO3 films were deposited on Ni foils by radio frequency magnetron sputtering at temperatures between 100??450 ?°C. Pt top electrodes were sputtered and the metal-insulator-metal configuration was obtained. The capacitors were subsequently co-fired at 900 ?°C for crystallization and densification in a reducing atmosphere to avoid oxidation of Ni. The permittivity and the tunability of the films were observed to increase with the sputtering temperature, reaching a maximum at 400 ?°C with a permittivity of 1800 and a tunability ratio of 10:1 at an applied electric field of 45 V/??m. Loss tangents were less than 1.5% at 10 kHz. The formation of NiO was observed in samples sputtered at temperatures above 450 ?°C and resulted in degraded dielectric properties. Atomic force microscopy images revealed increasing grain size with the increasing sputtering temperature. This microstructural trend accounts for the increasing permittivity values and sharper ferroelectric anomalies found in films processed with increasing sputter temperature.
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