PURPOSE: To make even a superthin silicon film into a large-grain polycrystalline silicon thin film by casting the laser on the superthin silicon film, by etching an organic thin film and a diamond thin film to flatten the surface of the diamond thin film and by forming a silicon thin film on the diamond thin film.

CONSTITUTION: A fluid resist film is applied onto a polycrystalline diamond thin film to flatten the surface of the diamond thin film. Next the polycrystalline diamond film is etched from the surface of the resist by reactive ion etching and then the flat-surface diamond thin film is obtained. After that, a silicon thin film is formed on the diamond thin film and the energy beam is cast on the silicon thin film to crystallize the silicon thin film. By this laser casting, a polycrystalline silicon thin film PCS is formed. By this method, a large crystal grain silicon thin film can be formed in a very wide range of the laser casting intensity.