The effects of oxygen vacancy on the electrical properties of (Ba,Sr)$TiO_3$ thin films have been investigated. To know the effects of oxygen vacancy, Rapid Thermal Annealing conditions were changed variously. As the RTA temperature increases, the leakage current densities were reduced. But dielectric constants were reduced as the RTA temperature increases. The decrease of leakage current densities is caused by the improvements of the interfacial properties of the Pt/BST and the decrease of dielectric constant is caused by the increase of the oxygen vacancy concentrations. In $O_2$ plasma and $O_2$ (not reactive) ambient, we annealed the Pt/BST/Pt MIM capacitor at the same temperature, time and pressure. And the leakage current densities were drastically reduced about 70% without decreasing dielectric constant for $O_2$ plasma annealed BST capacitor compared with as-deposited. The leakage current densities of $O_2$ annealed BST capacitor were reduced about 25% and the dielectric constants were reduced about 14% compared with as-deposited. This means that reactive oxygen atom in $O_2$ plasma filled the oxygen vacancies in BST/Pt interfacial layer. And these facts confirm that oxygen vacancy affects the leakage currents and the dielectric constants of BST capacitor. We can conclude that $O_2$ Plasma annealing is very effective method that can obtain BST capacitors with high dielectric constant and low leakage current densities.
