In a method of fabricating a trench isolation structure of a semiconductor device, excellent gap filling properties are attained, without the generation of defects. In one aspect, the method comprises: loading a substrate with a trench formed therein into a high-density plasma (HDP) chemical vapor deposition apparatus; primarily heating the substrate; applying a first bias power to the apparatus so as to form an HDP oxide liner on side wall and bottom surfaces of the trench, a gap remaining in the trench following formation of the HDP oxide liner; removing the application of the first bias power and secondarily heating the substrate; applying a second bias power at a power level that is greater than that of the first bias power to the substrate so as to form an HDP oxide film to fill the gap in the trench; and unloading the substrate from the apparatus. |