To provide a method for forming a semiconductor device having a trench device isolation film.

The method includes a step of sequentially forming a buffer insulation film and a hard mask film on a semiconductor substrate. The hard mask and the buffer insulation film are sequentially patterned, to form an opening for exposing a predetermined region of the semiconductor substrate. A trench is formed by selectively etching the exposed semiconductor substrate, and a lower device isolation film having a groove in the upper sidewall is formed in the trench. An upper device isolation film filling the groove and the trench is formed on the lower device isolation film, and the hard mask film and the buffer insulation film are removed by etching, until the semiconductor substrate is exposed. At this time, the groove has a predetermined depth from the surface of the semiconductor substrate.