A highly reliable semiconductor device that controls both defects and impurity diffusion and a method for manufacturing such a semiconductor device. An N.sup.+ embedment layer and an N-type epitaxial layer are formed on a main surface region of a P-type silicon substrate. An STI trench is formed in the N-type epitaxial layer. A thermal oxidation film is formed on the inner surface of the STI trench. The STI trench is filled with an HDP-NSG film. A deep trench is formed in the STI trench with a depth reaching the silicon substrate. A further thermal oxidation film is formed on the inner surface of the deep trench. The thermal oxidation film of the deep trench is thinner than that of the STI trench. A silicon oxidation film is also formed in the deep trench and filled with a polysilicon film.