Methods for fabricating trench isolation structure

A method for fabricating a trench isolation structure, comprising:

• providing a substrate;
• forming a patterned mask layer on the substrate;
• performing a first etching step to the substrate by using the patterned mask layer to form a trench in the substrate;
• forming a dielectric material in the trench and on the patterned mask layer, wherein the dielectric material on the patterned mask layer has a first height;
• performing an etch back step to decrease the dielectric material on the patterned mask layer from the first height to a second height;
• performing a second etching step to the dielectric material on the patterned mask layer, wherein after the second etching step, the dielectric material on the patterned mask layer has a third height that is 2%-3.5% of the second height;
• and
• performing a planarization process to remove the dielectric material on the patterned mask layer, wherein a polishing pad is used during the planarization process, 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, and the second pressure is greater than the first pressure.