Semiconductor device and method of manufacturing semiconductor device

1. A semiconductor device, comprising:

a semiconductor substrate of a first conductivity type, having a front surface and a rear surface opposite to the front surface;

a first semiconductor layer of the first conductivity type, provided on the front surface of the semiconductor substrate, the first semiconductor layer having an impurity concentration lower than an impurity concentration of the semiconductor substrate, and having a first side and a second side opposite to the first side and facing toward the front surface of the semiconductor substrate;

a first base region of a second conductivity type, selectively provided in a surface layer at the first side of the first semiconductor layer;

second base regions of the second conductivity type, each selectively provided within the first semiconductor layer;

a second semiconductor layer of the second conductivity type, provided on the first semiconductor layer at the first side thereof;

a first semiconductor region of the first conductivity type, selectively provided in a surface layer of the second semiconductor layer;

a plurality of trenches each penetrating the first semiconductor region and the second semiconductor layer, and reaching the first semiconductor layer;

a plurality of gate electrodes, each gate electrodes being provided in a corresponding one of the trenches, via a gate insulating film;

an interlayer insulating film provided on each of the gate electrodes;

a first electrode in contact with the second semiconductor layer and the first semiconductor region; and

a second electrode provided at a rear surface opposite to the front surface of the semiconductor substrate, wherein

the first base region is provided between adjacent two of the trenches, and each of the second base regions is provided at a bottom of a corresponding one of the trenches,

the first base region includes a lower region having a thickness equal to a thickness of one of the second base regions, and an upper region provided on the lower region, the lower region being disposed closer to the front surface of the semiconductor substrate than is the upper region, in a thickness direction and

the first base region has a plurality of peaks of impurity concentration that are local maximum values in the thickness direction, and among the plurality of peaks, a peak that is nearest an interface between the upper region and the lower region is located at a position furthest from any other peak among the plurality of peaks in the thickness direction.

2. The semiconductor device according to claim 1 , wherein among the plurality of peaks of the first base region, a peak nearest the bottom of the trenches in the thickness direction is located at a positon furthest from any other peak among the plurality of peaks.

3. The semiconductor device according to claim 1 , wherein

the second semiconductor layer has a first side and a second side opposite to the first side and facing toward the first side of the first semiconductor layer, and

among the plurality of peaks of the first base region, a peak nearest a surface of the second base regions, at one side closer to the second side of the second semiconductor layer than is another side opposite the one side of the second base regions, is located at a position furthest from any other peak among the plurality of peaks.

4. A method of manufacturing a semiconductor device, the method comprising:

forming, on a front surface of a semiconductor substrate of a first conductivity type, a first semiconductor layer of a first conductivity type, having an impurity concentration lower than an impurity concentration of the semiconductor substrate;

selectively forming, in the first semiconductor layer, a lower region of a first base region a second conductivity type and a second base region of the second conductivity type, the lower region and the second base having respective thicknesses that are the same as each other;

forming, on the lower region, an upper region of the first base region;

forming, on the first semiconductor layer at a first side thereof, a second semiconductor layer of the second conductivity type, a second side of the first semiconductor layer being opposite to the first side and facing the front surface of the semiconductor substrate;

selectively forming in a surface layer of the second semiconductor layer, a first semiconductor region of the first conductivity type;

forming a trench that penetrates the first semiconductor region and the second semiconductor layer and reaches the first semiconductor layer;

forming, in the trench, a gate electrode via a gate insulating film;

forming on the gate electrode, an interlayer insulating film;

forming a first electrode in contact with the second semiconductor layer and the first semiconductor region; and

forming, at a rear surface of the semiconductor substrate, a second electrode, wherein

forming the upper region includes forming the upper region by three or fewer ion implantation sessions, and setting an acceleration energy of an initial ion implantation session of the three or fewer ion implantation sessions to a value such that implanted ions reach an interface between the upper region and the lower region.

5. The method according to claim 4 , wherein forming the upper region includes forming a peak of the implanted ions by the initial ion implantation session at a depth in a range of 0.42 μm to 0.53 μm, from a first side of the second semiconductor layer, opposite to a second side that faces toward the first side of the first semiconductor layer.

6. The method according to claim 4 , wherein forming the upper region includes setting the acceleration energy of the initial ion implantation session to be in a range of 410 keV to 580 keV.