Patent ID: 7998806

Claim:
A method of manufacturing a semiconductor device comprising a first transistor formed over a first device region surrounded by a first device separating area in a first area of a substrate, and a second transistor formed over a second device region surrounded by a second device separating area in a second area of the substrate, comprising: forming an oxidation film over the first and the second device region; forming an first etching preventing film extending over the first and the second area; removing the first etching preventing film over the first area; removing the oxidation film over the first device region and exposing a surface of the first device region; forming a first gate insulating film having a first thickness over the first device region while the second device region is covered by the oxidation film; removing the oxidation film over the second device region; forming a second gate insulating film over the second device region, the second gate insulating film having a second thickness less than the first thickness of the first gate insulating film; forming a first gate electrode over the first gate insulating film, and forming a second gate electrode over the second gate insulating film; and forming first source and drain regions in the first device region at both sides of the first gate electrode and forming second source and drain regions in the second device region at both sides of the second gate electrode.