Method of manufacturing semiconductor device, and semiconductor device

1. A method of manufacturing a semiconductor device, comprising:

simultaneously implanting an equal amount of an element selected from the group consisting of fluorine and nitrogen into a first transistor-forming region and a second transistor-forming region of a semiconductor substrate;

oxidizing said semiconductor substrate to thereby form a first oxide film over said first transistor-forming region and said second transistor-forming region;

selectively removing said first oxide film formed over said second transistor-forming region;

oxidizing said semiconductor substrate to thereby form a second oxide film at least over said second transistor-forming region, a thickness of said second oxide film being less than a thickness of said first oxide film;

forming a first transistor including said first oxide film as a gate insulating film in said first transistor-forming region, and forming a second transistor including said second oxide film as a gate insulating film in said second transistor-forming region;

acquiring relations between a dose of said element and threshold voltage values of said first transistor and said second transistor said semiconductor device comprising a plurality of semiconductor devices said plurality of semiconductor devices being manufactured while varying said dose of said element in said implanting said element; and

determining said dose of said element based on said relations and depending on target threshold voltage values expected for said semiconductor device to be manufactured.

2. The method of manufacturing a semiconductor device as claimed in claim 1 , wherein said first oxide film and said second gate oxide film are formed such that a concentration of said element in said first oxide film is greater than a concentration of said element in said second oxide film.

3. The method of manufacturing a semiconductor device as claimed in claim 1 , wherein said element comprises fluorine.

4. The method of manufacturing a semiconductor device as claimed in claim 1 , wherein said implanting the element further comprises implanting said element at said dose determined in said determining said dose.

5. The method of manufacturing a semiconductor device as claimed in claim 1 , wherein said first and second oxide films are formed over an entire surface of the semiconductor substrate.

6. The method of manufacturing a semiconductor device as claimed in claim 1 , further comprising:

before forming said first oxide film, forming channels in said first transistor-forming region and said second transistor-forming region by introducing an impurity by ion implantation commonly into said first transistor-forming region and said second transistor-forming region.

7. The method of manufacturing a semiconductor device as claimed in claim 6 , wherein an equal amount of said impurity is implanted simultaneously over an entire surface of the semiconductor substrate.

8. The method of manufacturing a semiconductor device as claimed in claim 6 , further comprising:

acquiring second relations between a dose of said impurity and threshold voltage values of said first transistor and said second transistor, said plurality of semiconductor devices being manufactured while varying said dose of said impurity in said forming the channels; and

determining said doses of said element and said impurity based respectively on said first relations and said second relations and depending on said target threshold voltage values expected for said semiconductor device to be manufactured.

9. The method of manufacturing a semiconductor device as claimed in claim 8 , wherein said implanting said element further comprises implanting said element at said dose determined in said determining said dose of said element, and

wherein said forming said channels further comprises implanting said impurity at said dose determined in said determining said dose of said impurity.

10. A semiconductor device, comprising:

a semiconductor substrate having a first transistor-forming region and a second transistor-forming region that respectively comprise an element selected from the group consisting of fluorine and nitrogen;

a first transistor formed in said first transistor-forming region of said semiconductor substrate and having a first gate oxide film that comprises said element; and

a second transistor formed in said second transistor-forming region of said semiconductor substrate and having a second gate oxide film that comprises said element, said second gate oxide film having a thickness that is less than a thickness of said first gate oxide film,

wherein a concentration of said element in said first gate oxide film is greater than a concentration of said element in said second gate oxide film,

wherein the concentration of said element is uniform in an in-plane direction in each of said first gate oxide film and said second gate oxide film,

wherein a difference in the concentration of said element in said first gate oxide film and the concentration of said element in said second gate oxide film, as is observed at a same level of height, is greater than a difference in a concentration of said element in said first transistor-forming region of said semiconductor substrate and a concentration of said element in said second transistor-forming region of said semiconductor substrate, as is observed at a same level of depth,

wherein relations between a dose of said element and threshold voltage values of said first transistor and said second transistor are predetermined, said semiconductor device comprising a plurality of semiconductor devices, said plurality of semiconductor devices being manufactured while varying said dose of said element in implanting said element; and

wherein said dose of said element is determined based on said relations and depending on target threshold voltage values expected for said semiconductor device.

11. The semiconductor device as claimed in claim 10 , wherein a conductivity type of said first transistor is equivalent to a conductivity type of said second transistor.

12. The semiconductor device as claimed in claim 10 , wherein said element comprises fluorine.

13. The method of manufacturing a semiconductor device as claimed in claim 1 , wherein a thickness of said first oxide film formed over said first transistor-forming region is equal to a thickness of said first oxide film formed over said second transistor-forming region.

14. The method of manufacturing a semiconductor device as claimed in claim 1 , further comprising:

before said implanting said element, forming an isolation region in said semiconductor substrate between said first transistor-forming region and said second transistor-forming region,

wherein an edge of said isolation region is coplanar to an edge of said semiconductor substrate.

15. The method of manufacturing a semiconductor device as claimed in claim 1 , wherein a concentration of said element in said second oxide film is less than a concentration of said element in said first oxide film.

16. The method of manufacturing a semiconductor device as claimed in claim 1 , wherein a conductivity type of said first transistor is equivalent to a conductivity type of said second transistor.