A method for forming a semiconductor device is disclosed. A substrate having at least two fins thereon and an isolation trench between the at least two fins is provided. A liner layer is then deposited on the substrate. The liner layer conformally covers the two fins and interior surface of the isolation trench. A stress-buffer film is then deposited on the liner layer. The stress-buffer film completely fills a lower portion that is located at least below half of a trench depth of the isolation trench. A trench-fill oxide layer is then deposited to completely fill an upper portion of the isolation trench.