Method of manufacturing semiconductor device

A method of manufacturing a semiconductor device, comprising the steps of:

• 1) providing a substrate and forming a pad oxide over the substrate;
• 2) etching the pad oxide and the substrate to form two shallow trenches and filling the two shallow trenches with an oxide to form two shallow trench isolation structures, a high-voltage device region being provided between the two shallow trench isolation structures;
• 3) forming a hard mask layer over the substrate;
• 4) performing a dry etching process to remove a portion of the hard mask layer in the high-voltage device region to expose both a portion of the pad oxide in the high-voltage device region and adjacent portions of the respective shallow trench isolation structures, wherein hard mask residues are left along boundaries between the high-voltage device region and the shallow trench isolation structures after the dry etching process;
• 5) reducing a thickness of each of the exposed portions of the respective shallow trench isolation structures using a first rinsing liquid to expose the hard mask residues;
• and
• 6) removing the hard mask residues using a second rinsing liquid.