Semiconductor device and method for manufacturing semiconductor device

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

• forming a first layer including impurities on a semiconductor substrate;
• forming a second layer on the first layer;
• forming a first trench in the second layer, the first trench including an inner surface having a corner;
• forming a first thermal oxidation film on the inner surface of the first trench by performing a first thermal oxidation process;
• forming a first embedment layer in the first trench;
• forming a second trench in the first trench, the second trench including an inner surface having a corner and being deeper than the first trench;
• forming a second thermal oxidation film on the inner surface of the second trench by performing a second thermal oxidation process;
• and
• forming a second embedment film in the second trench;
• wherein the step of forming a first thermal oxidation film includes rounding the corner of the first trench;
• and
• the step of forming a second thermal oxidation film includes rounding the corner of the second trench and forming the second thermal oxidation film so as to be thinner than the first thermal oxidation film.