Semiconductor device including insulated gate bipolar transistor with contact hole in interlayer insulating film and method of manufacturing the same

A method of manufacturing a semiconductor device comprising the steps of:

• (a) preparing a semiconductor substrate having a semiconductor layer of a first conductivity type;
• (b) forming a trench in the semiconductor layer;
• (c) forming a gate insulating film inside the trench;
• (d) forming a gate electrode on the gate insulating film so as to fill an inside of the trench;
• (e) forming a first insulating film on the semiconductor layer;
• (f) forming a first impurity region of a second conductivity type opposite to the first conductivity type in the semiconductor layer;
• (g) forming a second impurity region of the first conductivity type in the first impurity region;
• (h) forming a second insulating film having a thickness larger than a thickness of the first insulating film on the first insulating film, thereby forming an interlayer insulating film including the second insulating film and the first insulating film on the semiconductor layer;
• (i) forming a contact hole penetrating the interlayer insulating film and the second impurity region and reaching the first impurity region;
• and
• (j) forming a plug electrically connected to the first impurity region and the second impurity region so as to fill an inside of the contact hole,
• wherein the first insulating film is a silicon oxide film,
• wherein the second insulating film is a BPSG film,
• wherein the step of (i) includes:
• (i1) forming a first contact hole in the second insulating film, the first insulating film, the second impurity region, and the first impurity region;
• and
• (i2) after the step of (i1), performing isotropic etching process to the second insulating film and the first insulating film, thereby forming, in the second insulating film and the first insulating film, a second contact hole having an opening width larger than an opening width of the first contact hole and communicating with the first contact hole,
• wherein in the step of (c), the gate insulating film is formed also on the semiconductor layer outside the trench,
• wherein in the step of (e), the first insulating film is formed by performing isotropic etching process to the gate insulating film formed on the semiconductor layer outside the trench, and
• wherein the thickness of the first insulating film formed on the semiconductor layer outside the trench is smaller than a thickness of the gate insulating film formed inside the trench.