SEMICONDUCTOR DEVICE AND MANUFACTURING METHOD THEREOF

The contact resistance between an oxide semiconductor film and a metal film is reduced. A transistor that uses an oxide semiconductor film and has excellent on-state characteristics is provided. A semiconductor device capable of high-speed operation is provided. In a transistor that uses an oxide semiconductor film, the oxide semiconductor film is subjected to nitrogen plasma treatment. Thus, part of oxygen included in the oxide semiconductor film is replaced with nitrogen, so that an oxynitride region is formed. A metal film is formed in contact with the oxynitride region. The oxynitride region has lower resistance than the other region of the oxide semiconductor film. In addition, the oxynitride region is unlikely to form high-resistance metal oxide at the interface with the contacting metal film.