Element isolation structure of semiconductor and method for forming the same

Disclosed are an element isolation structure of a semiconductor device and a method for forming the same, the method including preparing a semiconductor substrate having an inactive region and an active region defined thereon, forming a first hard mask on the semiconductor substrate, exposing the inactive region of the semiconductor substrate by patterning the first hard mask, forming a second hard mask on the entire surface of the semiconductor substrate including the first hard mask, forming a deep trench in the semiconductor substrate by patterning the second hard mask and the semiconductor substrate, removing the patterned second hard mask, forming a shallow trench overlapped with the deep trench by patterning the semiconductor substrate using the first hard mask as a mask, forming an insulation film on the entire surface of the substrate including the shallow trench and the deep trench, filling the shallow trench and the deep trench by forming an element isolation film on the insulation film, and forming an element isolation film pattern in the deep trench and the shallow trench by selectively removing the element isolation film.