Semiconductor device with electric field relaxation portion in insulating layer between lower and upper trench electrodes

1. A semiconductor device comprising:

a semiconductor layer having a trench;

a first insulating film formed along an inner surface of the trench;

an upper electrode and a lower electrode embedded in the trench via the first insulating film and disposed above and below a second insulating film; and

an electric field relaxation portion that relaxes an electric field arising between the upper electrode and the semiconductor layer, the electric field relaxation portion being provided between a side surface of the trench and a lower end portion of the upper electrode,

wherein the first insulating film includes a thick film portion contacting the lower electrode and a thin film portion having a thickness less than the thickness of the thick film portion and contacting the upper electrode, and

wherein the electric field relaxation portion includes an insulating film provided integral to and between the thick film portion and the thin film portion of the first insulating film and having a thickness greater than the thickness of the thin film portion.

2. The semiconductor device according to claim 1 , wherein the electric field relaxation portion has a thickness such that a ratio of the thickness of the electric field relaxation portion with respect to the thickness of the thin film portion of the first insulating film is greater than 1 and not more than 4.

3. The semiconductor device according to claim 1 , wherein the electric field relaxation portion has a thickness less than the thickness of the thick film portion of the first insulating film.

4. The semiconductor device according to claim 3 , wherein the electric field relaxation portion has a thickness such that a ratio of the thickness of the electric field relaxation portion with respect to the thickness of the thick film portion of the first insulating film is not less than 0.5 and less than 1.

5. The semiconductor device according to claim 1 , wherein the electric field relaxation portion has a thickness greater than the thickness of the second insulating film.

6. The semiconductor device according to claim 1 , wherein the second insulating film has a thickness greater than the thickness of the thin film portion of the first insulating film.

7. The semiconductor device according to claim 1 , wherein the electric field relaxation portion includes the same insulating material as the first insulating film.

8. The semiconductor device according to claim 1 , further comprising: a source layer, a body layer, and a drain layer formed in that order from a front surface side of the semiconductor layer at a side of the trench; and

wherein the electric field relaxation portion is in contact with the drain layer.

9. The semiconductor device according to claim 8 , wherein the electric field relaxation portion is formed further to a rear surface side of the semiconductor layer than a boundary portion between the body layer and the drain layer.

10. The semiconductor device according to claim 8 , wherein the electric field relaxation portion is formed so as to cross the boundary portion between the body layer and the drain layer.

11. The semiconductor device according to claim 1 , wherein an entirety of the upper electrode is positioned higher than an upper end portion of the lower electrode across the second insulating film.

12. The semiconductor device according to claim 1 , wherein a projecting portion, provided along the side surface of the trench and projecting toward the lower electrode side, is formed at the lower end portion of the upper electrode, and

the electric field relaxation portion is in contact with the projecting portion of the upper electrode.

13. The semiconductor device according to claim 12 , wherein an entirety of the projecting portion of the upper electrode is positioned higher than the upper end portion of the lower electrode across the second insulating film.

14. The semiconductor device according to claim 1 , wherein a plurality of the trenches, including a gate trench, are formed in the semiconductor layer, and

the upper electrode embedded in the gate trench serves in common as a gate electrode.

15. The semiconductor device according to claim 1 , wherein a plurality of the trenches, including a field plate trench, are formed in the semiconductor layer, and

the upper electrode embedded in the field plate trench serves in common as a field plate.

16. The semiconductor device according to claim 15 , wherein the upper electrode and the lower electrode include polysilicon.

17. The semiconductor device according to claim 15 , wherein the first insulating film includes silicon oxide.