Method of manufacturing semiconductor device

A method of manufacturing a semiconductor device, comprising:

• forming a precursor structure, wherein the precursor structure comprises:
• a patterned substrate having at least one trench therein;
• an oxide layer covering the patterned substrate;
• and
• a nitride layer on the oxide layer and exposing a portion of the oxide layer in the trench;
• forming a first barrier layer on the oxide layer;
• forming a first gate structure on the first barrier layer;
• removing a portion of the first barrier layer with an etchant including CF4, C2F6, C3F8, C4F8, F2, NF3, SF6, CHF3, HF, COF2, ClF3 or H2O2 to expose a sidewall of the oxide layer;
• forming a second barrier layer on the first gate structure and the oxide layer;
• removing a portion of the second barrier layer with the etchant including CF4, C2F6, C3F8, C4F8, F2, NF3, SF6, CHF3, HF, COF2, ClF3 or H2O2, wherein after removing the portion of the second barrier layer, the second barrier layer remaining on the first gate structure has a thickness of about 2-4 nm;
• and
• forming a second gate structure on the second barrier layer.