US Patent 9117662 Method for manufacturing semiconductor device

To improve productivity of a transistor that includes an oxide semiconductor and has good electrical characteristics. In a top-gate transistor including a gate insulating film and a gate electrode over an oxide semiconductor film, a metal film is formed over the oxide semiconductor film, oxygen is added to the metal film to form a metal oxide film, and the metal oxide film is used as a gate insulating film. After an oxide insulating film is formed over the oxide semiconductor film, a metal film may be formed over the oxide insulating film. Oxygen is added to the metal film to form a metal oxide film and added also to the oxide semiconductor film or the oxide insulating film.