Patent ID: 7422944

Claim:
A manufacturing method of a first and a second field-effect transistors comprising: forming a first and a second well forming region separated by a plurality of isolation regions in a semiconductor substrate; forming a first and a 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; selectively forming a nitrogen diffusion preventive film above the surface of the first well region; removing the first gate insulation film on the second well region by using the nitrogen diffusion preventive film as a mask; newly forming a second gate insulation film on the first and the second well regions; introducing nitrogen into the second gate insulation film; removing the second gate insulation film and the nitrogen diffusion preventive film on the first well region; after removing the nitrogen diffusion preventive film, forming a polycrystalline silicon film on the first and the second well regions via the first and the second gate insulation films; forming a first and a 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.