US Patent 7119412 Semiconductor device and method for manufacturing the same

After formation of a gate insulating film for a high voltage transistor on the entire surface, when removing the gate insulating film existing within a low voltage region, etching is not finished upon expose of an active region, but overetching is performed until the surface of an element isolation insulating film becomes lower by, for example, about 15 nm than the surface of the active region within the low voltage region. Then, a high-temperature rapid thermal hydrogen treatment is performed on the active region within the low voltage region. As a result of this, a natural oxide film is removed from the surface of the active region within the low voltage region, so that the flatness is increased and its corners are rounded.