Insulated gate semiconductor device and method for manufacturing the insulated gate semiconductor device

1. An insulated gate semiconductor device, comprising:

a semiconductor substrate;

a front electrode provided on a front surface of the semiconductor substrate; and

a rear electrode provided on a rear surface of the semiconductor substrate,

wherein

the insulated gate semiconductor device is configured to switch a conducting path between the front electrode and the rear electrode, and

the semiconductor substrate comprises:

a first region of a first conductivity type being in contact with the front electrode;

a second region of a second conductivity type being in contact with the front electrode and the first region;

a third region of the first conductivity type separated from the first region by the second region;

a plurality of gate trenches provided in the front surface and penetrating the second region so as to reach the third region;

a plurality of fourth regions of the second conductivity type each of which is exposed on a bottom surface of the corresponding gate trench;

a plurality of first circumferential trenches and a plurality of the second circumferential trenches which are provided in the front surface in a range outside the second region, the first circumferential trenches and the second circumferential trenches being repeatedly and alternately provided in the range outside the second region, and the second circumferential trenches having a depth deeper than a depth of the first circumferential trenches;

fifth regions of the second conductivity type each of which is exposed on a bottom surface of the corresponding first circumferential trench;

sixth regions of the second conductivity type each of which is exposed on a bottom surface of the corresponding second circumferential trench, front side end portions of the sixth regions being located on a rear side with respect to rear side end portions of the fifth regions; and

a seventh region of the first conductivity type connected to the third region and separating the fifth regions from the sixth regions.

2. The insulated gate semiconductor device of claim 1 , wherein each of the fifth regions is configured to partially overlap with the corresponding sixth region in a plane view of the semiconductor substrate from a front side.

3. The insulated gate semiconductor device of claim 1 , wherein thicknesses of the sixth regions are thicker than thicknesses of the fifth regions.

4. The insulated gate semiconductor device of claim 3 , wherein

a first kind of second conductivity type impurities are included in the fifth regions, and

a second kind of second conductivity type impurities are included in the sixth regions, the second kind of second conductivity type impurities having a diffusion coefficient in the semiconductor substrate higher than a diffusion coefficient of the first kind of second conductivity type impurities in the semiconductor substrate.

5. The insulated gate semiconductor device of claim 3 , wherein

the semiconductor substrate is constituted of SiC,

carbon and boron are included in the fifth regions and the sixth regions, and

a concentration of carbon in the fifth regions is higher than a concentration of carbon in the sixth regions.

6. The insulated gate semiconductor device of claim 1 ,

wherein the fifth regions and the sixth regions are provided in a range within which a depletion layer spreading from a border of the second region and the third region reaches when the insulated gate semiconductor device turns off.

7. A method for manufacturing the insulated gate semiconductor device of claim 1 , the method comprising:

forming the first circumferential trenches;

forming the fifth regions by implanting second conductivity type impurities into the bottom surfaces of the first circumferential trenches;

forming the second circumferential trenches; and

forming the sixth regions by implanting second conductivity type impurities into the bottom surfaces of the second circumferential trenches.

8. The method of claim 7 , wherein

either of the first or second circumferential trenches are formed,

either of the fifth or sixth regions which are exposed on bottom surfaces of the either trenches are formed after the formation of the either trenches,

insulating layers are formed in the either trenches after the formation of the either regions exposed on the bottom surfaces of the either trenches,

the other of the first and second circumferential trenches are formed after the formation of the insulating layers,

the other of the fifth and sixth regions which are exposed on bottom surfaces of the other trenches are formed after the formation of the other trenches, and

insulating layers are formed in the other trenches after the formation of the other regions exposed on the bottom surfaces of the other trenches.

9. The method of claim 7 , wherein

the formation of the sixth regions is performed before the formation of the fifth regions,

in the formation of the sixth regions, the semiconductor substrate is annealed after the implantation of the second conductivity type impurities into the bottom surfaces of the second circumferential trenches, and

in the formation of the fifth regions, the semiconductor substrate is annealed after the implantation of the second conductivity type impurities into the bottom surfaces of the first circumferential trenches.

10. The method of claim 9 , wherein an annealing temperature in the formation of the sixth regions is higher than an annealing temperature in the formation of the fifth regions.

11. The method of claim 7 , wherein

a first kind of second conductivity type impurities are implanted into the bottom surfaces of the first circumferential trenches in the formation of the fifth regions; and

a second kind of second conductivity type impurities are implanted into the bottom surfaces of the second circumferential trenches in the formation of the sixth regions, the second kind of second conductivity type impurities having a diffusion coefficient in the semiconductor substrate higher than a diffusion coefficient of the first kind of second conductivity type impurities in the semiconductor substrate.

12. The method of claim 7 , wherein

the semiconductor substrate is constituted of SiC,

carbon and boron are implanted into the bottom surfaces of the first circumferential trenches in the formation of the fifth regions, and

boron is implanted into the bottom surfaces of the second circumferential trenches in the formation of the sixth regions.

13. The method of claim 7 , wherein the gate trenches are formed at the same time as the formation of the first circumferential trenches.