Patent ID: 7364972

Claim:
A manufacturing method of a first and a second field-effect transistors comprising: forming first and second well forming regions separated by a plurality of isolation regions in a semiconductor substrate; forming first and second well regions of a first conductivity type by implanting an impurity of a first conductivity type into the first and the second well forming regions; forming a first gate insulation film on the first and the second well regions; introducing nitrogen into the first gate insulation film by exposing the first gate insulation film to a nitrogen-containing atmosphere; selectively removing the first gate insulation film on the first well region; newly forming a second gate insulation film on the first well region; forming a polycrystalline silicon film on the first and the second well regions via the first and the second gate insulation films; forming first and second gate electrodes above the first and the second well regions by selectively and anisotropically etching the polycrystalline silicon film; and forming a first and a second pair of source/drain regions by implanting and thermally diffusing an impurity of a second conductivity type into the first and the second well regions, using the first and the second gate electrodes as masks, respectively.