Semiconductor device and method for fabricating the same

A semiconductor device comprising:

• a first MOS field effect transistor of an n-type including a first oxynitride film as a first gate insulator film;
• and
• a second MOS field effect transistor of a p-type including a second oxynitride film as a second gate insulator film, said second MOS field effect transistor being disposed adjacent to said first MOS field effect transistor;
• wherein a difference in thicknesses between said first and second gate insulator films is in proportion to a difference between concentrations of nitrogen in said first and second gate insulator films as a result of introduction of different concentrations of nitrogen into oxide films having had the same thickness.

A semiconductor device comprising:

• a first MOS field effect transistor of an n-type including a first oxynitride film as a first gate insulator film;
• and
• a second MOS field effect transistor of a p-type including a second oxynitride film as a second gate insulator film, said second MOS field effect transistor being disposed adjacent to said first MOS field effect transistor;
• wherein said first and second gate insulator films have different concentrations of nitrogen and different thickness values as a result of introduction of nitrogen at the same surface density into oxide films having had different thickness values.

A method for fabricating a semiconductor device, said method comprising:

• oxidizing a surface of a semiconductor substrate in first and second MOS field effect transistor regions, which are isolated from each other by an element isolating insulator film of the surface portion of said semiconductor substrate, to form a first oxide film;
• introducing nitrogen into said first oxide film to form a first oxynitride film having a first concentration of nitrogen;
• depositing a first polycrystalline silicon film on a surface of said first oxynitride film;
• forming a first mask on said first polycrystalline silicon film in one of said first and second MOS field effect transistor regions;
• removing said first polycrystalline silicon film in the other region of said first and second MOS field effect transistor regions by using said first mask;
• removing said first mask to use said first polycrystalline silicon film as a mask, which remains in the one of said first and second MOS field effect transistor regions, to remove said first oxynitride film in the other region of said first and second MOS field effect transistor regions;
• oxidizing the surface of said semiconductor substrate in the other region of said first and second MOS field effect transistor regions and a surface of said first polycrystalline silicon film remaining in the one of said first and second MOS field effect transistor regions to form a second oxide film on the surface of said semiconductor substrate in the other region of said first and second MOS field effect transistor regions and the surface of said first polycrystalline silicon film remaining in the one of said first and second MOS field effect transistor regions;
• and
• introducing nitrogen into said second oxide film to form a second oxynitride film having a second concentration of nitrogen.

A method for fabricating a semiconductor device, said method comprising:

• oxidizing a surface of a semiconductor substrate in first and second MOS field effect transistor regions, which are isolated from each other by an element isolating insulator film of the surface portion of the semiconductor substrate, to form a first oxide film having a first thickness;
• forming a first mask on said first oxide film in one of said first and second MOS field effect transistor regions;
• removing said first oxide film in the other region of said first and second MOS field effect transistor regions by using said first mask;
• removing said first mask to oxidize the surface of the semiconductor substrate in the other region of said first and second MOS field effect transistor regions to form a second oxide film having a second thickness, which is smaller than said first thickness, on the surface of said semiconductor substrate in the other region of said first and second MOS field effect transistor regions;
• and
• introducing nitrogen into said first and second oxide films to form first and second oxynitride films.