1. A method of manufacturing a mask, the method comprising: designing a first mask data pattern; designing a second mask data pattern for forming the first mask data pattern; acquiring a first emulation pattern, which is predicted from the second mask data pattern, using layout-based Self-Aligning Double Patterning (SADP) emulation; comparing the first emulation pattern with the first mask data pattern, and modifying the second mask data pattern according to results of the comparison; performing Optical Proximity Correction (OPC) on the modified second mask data pattern; acquiring second emulation patterns, which are predicted from the second mask data pattern on which the OPC has been performed, using image-based SADP emulation; and comparing the second emulation patterns and the first mask data pattern, and manufacturing a first mask layer, which corresponds to the second mask data pattern on which the OPC has been performed, according to results of the comparison.