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 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 .