A preparation for forming a thin film capacitor includes forming an amorphous ferroelectric film, such as barium strontium titanate [(Ba,Sr)TiO3] film, for use as an interface between a metal electrode and a polycrystalline ferroelectric film, such as (Ba,Sr) TiO3 film. The polycrystalline ferroelectric film serves as a dielectric layer of the thin film capacitor in view of the fact that the polycrystalline ferroelectric film has a high dielectric constant. The amorphous ferroelectric film serves as a buffer layer for inhibiting the leakage current of the thin film capacitor. The amorphous ferroelectric film is grown by sputtering and by introducing a working gas, such as argon, and a reactive gas, such as oxygen, into a reaction chamber in which a plasma is generated at room temperature.