Methods for forming trench isolation structure

A method for forming a trench isolation structure is provided. The method includes providing a substrate and forming a patterned mask layer on the substrate. A first etching step is performed to the substrate by using the patterned mask layer to form a trench in the substrate. A dielectric material is formed in the trench and on the patterned mask layer, wherein the dielectric material on the patterned mask layer has a first height. A etch back step is performed to decrease the dielectric material on the patterned mask layer from the first height to a second height. A planarization process is performed to remove the dielectric material on the patterned mask layer, wherein a polishing pad is used during the planarization process, and a first pressure is applied on a central portion of the polishing pad, a second pressure is applied on a peripheral portion of the polishing pad, wherein the second pressure is larger than the first pressure.