A method of forming a shallow trench isolation in a semiconductor substrate. First, a hard mask consisting of a pad nitride and a pad oxide is formed on the semiconductor substrate. The semiconductor substrate is anisotrpically etched to form a trench while the hard mask is used as the etching mask. A thermal oxide film is grown on the trench. Then, a nitride liner is formed on the thermal oxide film. Next, a silicon rich oxide layer is conformally deposited on the nitride liner by high density plasma chemical vapor deposition without a bias voltage applied to the semiconductor substrate. Then, a silicon oxide is deposited to fill the trench by high density plasma chemical vapor deposition while a bias voltage is applied to the semiconductor substrate.