Method for manufacturing an isolation trench filled with a high-density plasma-chemical vapor deposition oxide

A method for filling an isolation trench in a semiconductor substrate includes the steps of forming a first silicon oxide layer on sidewalls and the floor of each trench by an oxidation step, forming a second silicon oxide layer on the sidewalls and floor of the trench by a first high-density plasma-chemical vapor deposition process without applying an RF voltage to a wafer so that the ratio of depositing to etching is extremely high and then forming a third silicon oxide layer by a second high-density plasma-chemical vapor deposition process having an RF voltage applied to the wafer so that the ratio of depositing to etching is much lower than in the first-mentioned process.