Method for fabricating device isolation structure

A method of fabricating a semiconductor device comprising a device isolation structure, the method comprising:

• providing a substrate having a first region and a second region;
• forming a pad layer over the substrate;
• forming a mask layer over the pad layer;
• patterning the mask layer, the pad layer and the substrate to form a plurality of first trenches in the first region and a plurality of second trenches in the second region;
• performing a trimming process to remove at least a portion of an upper part of the mask layer to form a trimmed mask layer;
• forming an insulation layer over the substrate and completely filling the plurality of the first trenches and the plurality of the second trenches;
• and
• performing a planarization process on the insulation layer to form a plurality of first isolation structures and a plurality of second isolation structures in the first region and the second region respectively, wherein a top surface of each first isolation structure of the plurality of the first isolation structures is lower than a top surface of each second isolation structure of the plurality of the second isolation structures.