A method for fabricating a semiconductor device is disclosed. The method includes forming a first oxide film, a nitride film, and a second oxide film on a semiconductor substrate in succession, etching the second oxide film and the nitride film to form a second oxide film pattern and a nitride film pattern, exposing a portion of the first oxide film, performing at least one nitrogen implantation into the semiconductor substrate to form a nitrogen injection region under the exposed portion of the first oxide film, forming a third oxide film over the second oxide film pattern, the nitride film pattern, and the semiconductor substrate, forming a trench that is deeper than the nitrogen ion injection region by etching the semiconductor substrate using the second oxide film pattern as a mask, and filling the trench with an oxide film to form a device isolating film.