Semiconductor device and method of manufacturing the same

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

forming a trench having an upper quadrangular section and a lower circular section which is formed through a hydrogen annealing process, to extend in a depth direction of a semiconductor substrate;

forming an insulating film on a surface of said trench and a surface of said semiconductor substrate; and

forming a conductive film to fill said trench whose surface is covered with said insulating film,

wherein a relation L>W is met, where L is a depth for said lower section and W is a width of said upper section of said trench.

2. The method according to claim 1 , further comprising:

forming as source/drain regions, diffusion regions in said semiconductor substrate on both sides of said trench,

wherein said conductive film is a gate electrode.

3. The method according to claim 1 , wherein a relation L>1.12W is met.

4. The method according to claim 1 , wherein said forming a trench comprises:

forming a mask on said semiconductor substrate through an oxide film;

forming said trench for said upper section by etching a part of said mask and a part of said semiconductor substrate;

forming a protection film along a side surface of said trench for said upper section;

forming said trench for said lower section by etching a part of said semiconductor substrate in the depth direction of said semiconductor substrate; and

performing said hydrogen annealing process to transform said lower section to have a circular section.

5. The method according to claim 4 , wherein said mask is a silicon nitride film, said protection film is a laminate layer of a lower protection film of silicon oxide and an upper protection film of silicon nitride, and the oxide film on said semiconductor substrate is a silicon oxide film,

said forming a trench further comprises:

forming a sacrificed silicon oxide film on a surface of said lower section;

removing said mask and said upper protection film; and

removing the silicon oxide film on said semiconductor substrate, said lower protection film and said sacrificed silicon oxide film.

6. The method according to claim 1 , wherein said forming an insulating film comprises:

forming said insulating film through thermal oxidization.

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

forming a hard mask on a semiconductor substrate through an oxide film;

forming a shallow trench by etching a part of said hard mask and said oxide film and a part of said semiconductor substrate;

forming a protection film along a surface of said shallow trench;

forming a deep trench in a depth direction from said shallow trench by etching said protection film in a bottom of said shallow trench and another part of said semiconductor substrate while keeping said protection film on a side wall of said shallow trench; and

processing said deep trench into a circular trench through a hydrogen annealing process,

wherein a depth L of a portion to be etched in said forming a deep trench meets a relation L>W,

where W is a width of said shallow trench.

8. The method according to claim 7 , further comprising:

forming a gate insulating film along a surface of said circular trench; and

forming a gate electrode by filling said circular trench whose surface is covered with said gate insulating film.

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

forming a trench having an upper quadrangular section and a lower circular section which is formed through a hydrogen annealing process, to extend in a depth direction of a semiconductor substrate;

forming an insulating film on a surface of said trench and a surface of said semiconductor substrate; and

forming a conductive film to fill said trench whose surface is covered with said insulating film,

wherein the lower circular section of the trench is formed by etching a shallow trench defining the upper quadrangular section to form a deep trench portion; and

after forming the deep trench portion, performing said hydrogen annealing to impart a circular section to the deep trench portion.

10. The method according to claim 9 , further comprising:

forming as source/drain regions, diffusion regions in said semiconductor substrate on both sides of said trench,

wherein said conductive film is a gate electrode.

11. The method according to claim 9 , wherein a relation L>W is met, where L is a depth for said lower section and W is a width of said upper section of said trench.

12. The method according to claim 11 , wherein a relation L>1.12W is met.

13. The method according to claim 9 , wherein said forming a trench comprises:

forming a mask on said semiconductor substrate through an oxide film;

forming said trench for said upper section by etching a part of said mask and a part of said semiconductor substrate;

forming a protection film along a side surface of said trench for said upper section;

forming said trench for said lower section by etching a part of said semiconductor substrate in the depth direction of said semiconductor substrate; and

performing said hydrogen annealing process to transform said lower section to have a circular section.

14. The method according to claim 13 , wherein said mask is a silicon nitride film, said protection film is a laminate layer of a lower protection film of silicon oxide and an upper protection film of silicon nitride, and the oxide film on said semiconductor substrate is a silicon oxide film,

said forming a trench further comprises:

forming a sacrificed silicon oxide film on a surface of said lower section;

removing said mask and said upper protection film; and

removing the silicon oxide film on said semiconductor substrate, said lower protection film and said sacrificed silicon oxide film.

15. The method according to claim 9 , wherein said forming an insulating film comprises:

forming said insulating film through thermal oxidization.