Barium strontium titanate (BST) thin-films deposited on a SiO2/Si substrate by argon ion-beam sputtering technique were annealed at 400, 500 and 600 degrees C in oxygen for 30 min, respectively, and were used to fabricate integrated parallel-plate capacitors by standard integrated-circuit technology. These capacitors can achieve tunability greater than 60% at an applied dc voltage of 2 V and a frequency of 100 kHz at room temperature. Considering tunability, loss factor and hysteresis effect, the BST thin-film annealed at 500 degrees C is superior for making tunable microwave integrated capacitors. The effects of annealing treatment in oxygen on the tuning properties of the thin-film capacitors are analyzed, and the results indicate that the tunability is strongly dependent on both oxygen vacancies and negatively charged oxygen, trapped at the grain boundary and/or at the electrode/dielectric interface. (c) 2005 Elsevier B.V. All rights reserved.