Method of manufacturing a semiconductor device having different kinds of insulating films with different thicknesses

1. A method of manufacturing a semiconductor device comprising:

forming isolation regions in a semiconductor substrate to form a first area, a second area and third area which are different from each other;

oxidizing a surface of the semiconductor substrate to form a first insulating film covering the first area, the second area, and the third area of the semiconductor substrate;

removing portions of the first insulating film lying on the first area and the second area;

oxidizing the surface of the semiconductor substrate to form a second insulating film covering the first area and the second area;

oxidizing the third area covered with the first insulating film;

removing a portion of the second insulating film lying on the second area and the portion of the first insulating film lying on the third area;

oxidizing the surface of the semiconductor substrate to form a first gate insulating film covering the second area and the third area; and

oxidizing the first area covered with the second insulating film to form a second gate insulating film, the second gate insulating film covering the first area is thicker than the first gate insulating film.

2. The method of manufacturing a semiconductor device according to claim 1 , further comprising:

doping the second area with a first dopant having a first conductivity type and doping the third area with a second dopant having a second conductivity type.

3. The method of manufacturing a semiconductor device according to claim 1 , further comprising:

forming a conductive film covering the first gate insulating film and the second gate insulating film and patterning the conductive film to form a first gate electrode on a portion of the first gate insulating film lying on the second area, a second gate electrode on a portion of the first gate insulating film lying on the third area, and a third gate electrode on a portion of the second gate insulating film lying on the first area.

4. The method of manufacturing a semiconductor device according to claim 1 , wherein: the first insulating film is formed to additionally cover a fourth area of the semiconductor substrate;

oxidizing the fourth area covered with the first insulating film; and

oxidizing the fourth area covered with the first insulating film to be covered with a third gate insulating film, the third gate insulating film is thicker than the second gate insulating film.

5. The method of manufacturing a semiconductor device according to claim 1 , wherein the second area corresponds to a N-type low-voltage transistor formation area and the third area corresponds to a P-type low-voltage transistor formation area.

6. The method of manufacturing a semiconductor device according to claim 1 , wherein the first area corresponds to a medium-voltage transistor formation area.

7. The method of manufacturing a semiconductor device according to claim 2 , wherein: the amount of the first dopant is equal to or greater than 1×10 13 cm −2.

8. The method of manufacturing a semiconductor device according to claim 2 , wherein: the first dopant is a P-type dopant; and the second dopant is an N-type dopant.

9. The method of manufacturing a semiconductor device according to claim 2 , further comprising:

covering the semiconductor substrate with an element-partitioning film that defines the first area, the second area, and the third area.

10. A method of manufacturing a semiconductor device comprising:

oxidizing a surface of a semiconductor substrate to form a first insulating film covering a first area, a second area, and a third area of the semiconductor substrate;

removing portions of the first insulating film lying on the first area and the second area;

oxidizing the surface of the semiconductor substrate to form a second insulating film covering the first area and the second area;

oxidizing the third area covered with the first insulating film;

removing a portion of the second insulating film lying on the second area and the portion of the first insulating film lying on the third area;

oxidizing the surface of the semiconductor substrate to form a first gate insulating film covering the second area and the third area; and

oxidizing the first area covered with the second insulating film to form a second gate insulating film, the second gate insulating film covering the first area is thicker than the first gate insulating film,

wherein: the first insulating film is formed to additionally cover a fourth area of the semiconductor substrate;

oxidizing the fourth area covered with the first insulating film; and

oxidizing the fourth area covered with the first insulating film to be covered with a third gate insulating film, the third gate insulating film is thicker than the second gate insulating film.