Method of optical proximity correction according to complexity of mask pattern

A method of optical proximity correction (OPC), comprising:

• providing a layout pattern to a computer system, wherein the layout pattern comprises a plurality of geometric patterns, each of the geometric patterns has a judgment value, and the judgment value is positively corresponding to a value of mask error enhancement factor (MEEF), negatively corresponding to a value of normalized image log slope (NILS), and negatively corresponding to a value of contrast;
• classifying the layout pattern into at least a first region and at least a second region, wherein each of the first region and the second region comprises a plurality of patterns, each pattern comprises at least one line segment, and a complexity of the layout pattern of the first region is substantially higher than a complexity of the layout pattern of the second region;
• performing several iterations of OPC calculations to modify the original layout pattern, wherein a total number of OPC calculations performed in the first region is substantially larger than a total number of OPC calculations performed in the second region;
• and
• outputting a corrected layout pattern through the computer system onto a mask.