Method for forming shallow trenches of the dual active regions

A method for forming shallow trenches of the dual active regions, comprising the steps of

• Step S01:
• providing a semiconductor substrate having first shallow trench regions and second shallow trench regions, on which forming an etch stop layer;
• Step S02:
• using a first accurate photomask to expose and develop a first photoresist covering the semiconductor substrate, until the etch stop layer has been exposed on the top of the first shallow trench regions and the second shallow trench regions;
• Step S03:
• etching the etch stop layer entirely in the exposed regions;
• Step S04:
• using a second low-cost photomask to expose and develop a second photoresist to expose the first shallow trench regions;
• Step S05:
• etching and forming the first shallow trenches with a preliminary depth in the first shallow trench regions, and then removing the second low-cost photomask, wherein an in-situ stripping process is utilized to remove polymer residues generated by the etching reaction;
• Step S06:
• taking the etch stop layer as a mask, and simultaneously etching the first shallow trench regions and the second shallow trench regions to form the first shallow trenches and second shallow trenches having different depths, wherein the depth of the first shallow trenches are deeper than that of the second shallow trenches.