One of the important issues for the deep-submicron shallow trench isolation process is profile control in the trench etch process for voidless gap filling with the chemical vapor deposition oxide. We examined the effects of some additive gases such as N-2 or He-O-2 (30% O-2) on the trench slope to obtain an optimized trench profile with a commercial magnetron enhanced reactive ion etching system. It was found that the thickness of deposited film on the trench sidewall was highly related to the trench slope. As the film thickness increases, the trench profile becomes more tapered. The thickness of the sidewall film was directly proportional to the amount of the additive gases, N-2 and He-O-2. As the amount of He-O-2 increases in the Cl-2+HBr+He-O-2 chemistry, the etch rate of silicon nitride decreases and the ratio of the silicon etch rate to the silicon nitride etch rate drastically increases. Scanning electron microscopy shows thick film deposits on the trench sidewall during the trench etch process. To know the chemical constituents in the deposited film on the sidewall, an angle-resolved x-ray photoelectron spectroscopy (XPS) was used. XPS results reveal that the deposited film on the trench sidewall is composed of SiOx in Cl-2+HBr+He-O-2 and SiOxNy in Cl-2+HBr+N-2. Cl or Br were not detected in our XPS measurements because the absorbed Cl or Br might react with water vapor to form HCl(g) or HBr(g) and grow SiOx when the samples are exposed to air. An aspect ratio dependence of the trench sidewall slope was observed. Usually the dense patterns are more vertical than the isolated patterns.