Patent attributes
A first oxide insulating film is formed over a substrate. After a first oxide semiconductor film is formed over the first oxide insulating film, heat treatment is performed, so that hydrogen contained in the first oxide semiconductor film is released and part of oxygen contained in the first oxide insulating film is diffused into the first oxide semiconductor film. Thus, a second oxide semiconductor film with reduced hydrogen concentration and reduced oxygen defect is formed. Then, the second oxide semiconductor film is selectively etched to form a third oxide semiconductor film, and a second oxide insulating film is formed. The second oxide insulating film is selectively etched and a protective film covering an end portion of the third oxide semiconductor film is formed. Then, a pair of electrodes, a gate insulating film, and a gate electrode are formed over the third oxide semiconductor film and the protective film.
