Method of forming an isolation structure

The present invention relates to a method of forming an isolation structure, in which, a trench is formed in a substrate through a hard mask, and deposition, etch back, deposition, planarization, and etch back are performed in the order to form an isolation material layer of the isolation structure after the hard mask is removed. A silicon layer may be formed to cover the trench and original surface of the substrate before the former deposition, or to cover a part of the trench and original surface of the substrate after the former etch back and before the later deposition, to serve as a stop layer for the planarization process. Voids existing within the isolation material layer can be exposed or removed by partially etching the isolation material layer by the former etch back. The later deposition can be performed with a less aspect ratio to avoid forming voids.