Semiconductor device and method for forming device isolation film of semiconductor device

A device isolation film in a semiconductor device and a method for forming the same are provided. The method includes etching a middle portion of a device isolation film having a deposition structure including a Spin-On-Dielectric (SOD) oxide film and a High Density Plasma (HDP) oxide film to form a hole and filling an upper portion of the hole with an oxide film having poor step coverage characteristics to form a second hole extending along the middle portion of the device isolation film. The second hole serves as a buffer for stress generated at the interface between an oxide film, which can be a device isolation film, and a silicon layer, which can be a semiconductor substrate, thereby increasing the operating current of a transistor and improving the electrical characteristics of the resulting device.