Photomask and method for forming dual STI structure by using the same

A method for manufacturing a dual shallow trench isolation structure, the method comprising:

• forming a photomask with a variable transparency stacking substrate, wherein the variable transparency stacking substrate comprises at least one first transparent region and at least one second transparent region, and transparency through a recess portion of the at least one first transparent region is different from transparency through a recess portion of the at least one second transparent region;
• providing a substrate;
• forming a hard mask layer on the substrate;
• forming a photoresist layer on the hard mask layer;
• performing an exposure and development process on the photoresist layer by using the photomask to form at least one first hole and at least one second hole in the photoresist layer, the at least one first hole and the at least one second hole corresponding to the at least one first transparent region and the at least one second transparent region respectively, wherein a depth of the at least one first hole is different from a depth of the at least one second hole;
• etching the photoresist layer, the hard mask layer and the substrate to form at least one first trench having a first depth and at least one second trench having a second depth, wherein the first depth is different from the second depth;
• removing the remaining photoresist layer;
• forming an isolating material layer to cover the hard mask layer and fill the at least one first trench and the at least one second trench;
• and
• removing the isolating material layer on the hard mask layer and the hard mask layer.