The present disclosure describes a method of forming a pattern by an electron beam lithography system. The method includes receiving an integrated circuit (IC) design layout data having a polygon and a forbidden pattern, modifying the polygon and the forbidden pattern using an electron proximity correction (EPC) technique, stripping the modified polygon into subfields, converting the stripped polygon to an electron beam writer format data, and writing the electron beam writer formatted polygon onto a substrate by an electron beam writer. Stripping the modified polygon includes finding the modified forbidden pattern as a reference layer, and stitching the modified polygon to avoid stitching the modified forbidden pattern.