An inventive method for forming a thin film comprises the steps of preparing a sputter-target of a material which is fully oxidized and crystallized to a perovskite structure, sputter-depositing a thin film on top of a sample with the target in an inert gas atmosphere, and annealing the thin film in non-oxygen ambient. With the use of such a target, it is possible to reduce the negative ion effect during the sputter deposition and to eliminate the presence of oxygen during the annealing process.