1. A method of generating a photo mask layout, the method comprising: providing a first photo mask layout including main patterns and sub-resolution assist features (SRAF) patterns; defining a plurality of mesh cells by dividing the first photo mask layout into regions; generating a rule based table including correction information for correcting defects in the SRAF patterns for at least one of the plurality of mesh cells; and correcting the SRAF patterns by applying values of the correction information to the SRAF patterns corresponding to each mesh cell.