Method for making high-voltage thick gate oxide

A method for making a high-voltage thick gate oxide, wherein the method comprises the following steps:

• step 1:
• depositing a pad silicon oxide on a silicon substrate and depositing a pad silicon nitride on the pad silicon oxide;
• then performing photolithography and etching to form trenches on the silicon substrate;
• then performing silicon oxide filling in the trenches and chemical mechanical polishing to form shallow trench isolations;
• step 2:
• depositing a mask silicon nitride on a silicon wafer and depositing a mask silicon oxide on the mask silicon nitride;
• step 3:
• performing photolithography and etching by using the mask silicon oxide as a hard mask, removing the mask silicon oxide and the mask silicon nitride in a high-voltage thick gate oxide region, and remaining the pad silicon nitride between two shallow trench isolations in the high-voltage thick gate oxide region;
• step 4:
• performing first thermal oxidation growth to passivate shallow trench isolation corners in the high-voltage gate oxide region;
• step 5:
• removing the pad silicon nitride between the two shallow trench isolations in the high-voltage thick gate oxide region;
• step 6:
• performing second thermal oxidation growth to produce a high-voltage thick gate oxide;
• and
• step 7:
• removing all of the mask silicon nitride and pad silicon nitride.