Process for producing a semiconductor device having a single thermal oxidizing step

A process for producing a vertical MOSFET using silicon carbide, comprising the steps of:

• forming a trench in a silicon carbide substrate having rounded corners and a flat bottom face with a (0001) carbon face;
• and
• forming a gate oxide layer in the trench so as to form
• a thinner oxide layer on a trench side face in which a channel region is formed upon operation of the vertical MOSFET and
• a thicker oxide layer having a greater thickness than the thinner oxide layer on a trench bottom face,
• wherein the thinner oxide layer and the thicker oxide layer are formed in a single step.

A process for producing a semiconductor device, comprising:

• laminating a first conductive type high-resistance layer formed of a single crystal silicon carbide of a hexagonal system on a first conductive type low-resistance layer formed of a single crystal silicon carbide of a hexagonal system to form a semiconductor substrate;
• forming, on the surface of said semiconductor substrate, a second conductive type semiconductor layer of a single crystal silicon carbide of a hexagonal system, and forming a first conductive type semiconductor region in a predetermined region of said semiconductor layer;
• forming a trench extending through said semiconductor region and said semiconductor layer into said semiconductor substrate, said trench having a side face and a bottom face, said bottom face having a surface with a first inclination of 30.degree. or less to a (0001) carbon face and said side face of said trench having a second inclination greater than 45.degree. and less than 90.degree. with respect to said (0001) carbon face;
• thermally oxidizing an inner wall of said trench to form a gate oxide layer as a gate insulating layer in a single step, wherein a first oxidation speed of said side face is smaller than a second oxidation speed of said bottom face, the thickness of said gate oxide layer on the bottom face of said trench being larger than the thickness of said gate oxide layer on the side face of said trench, the difference between the thickness of said gate oxide layer on said bottom face of said trench to the thickness of said gate oxide layer on said side face of said trench varying with said second inclination, and the thicker gate oxide layer on the bottom face being formed in the same oxidizing step as the thinner gate oxide layer on the side face;
• and
• forming a gate electrode layer on the surface of said gate oxide layer, formed by thermal oxidation, within said trench, forming a first electrode layer on the surface of said semiconductor layer and on the surface of said semiconductor region, and forming a second electrode layer on the back surface of said semiconductor substrate.