Patterning method

A patterning method, comprising:

• forming a second mask layer on a first mask layer;
• performing a patterning process to the first mask layer and the second mask layer, wherein the first mask layer is patterned to be a first mask pattern, and the second mask layer is patterned to be a second mask pattern formed on the first mask pattern;
• performing a first trim process to the second mask pattern, wherein a width of the second mask pattern is smaller than a width of the first mask pattern after the first trim process;
• forming a cover layer covering the first mask pattern and the second mask pattern after the first trim process;
• performing an etching process to the first mask pattern after the step of forming the cover layer;
• removing a part of the cover layer for exposing the second mask pattern before the etching process;
• removing the second mask pattern before the etching process;
• and
• performing a second trim process to the cover layer after the step of removing the second mask pattern and before the etching process.