Largely due to the slit shape, and the presence of residual aberrations, the effectiveness of SMO varies across slit position. At each slit position, there are different aberrations and different azimuthal angles of incidence leading to different shadowing. Consequently, there could be uncorrected variations across slit for aberration-sensitive features, which may not be obviously seen with regular line-space patterns. At each slit position, although optical proximity correction (OPC), including the assist features mentioned above, may also be applied to address the aberrations, they also feedback into the illumination specification, since the benefits differ for different illumination conditions. This would necessitate the use of different source-mask combinations at each slit position, i.e., multiple mask exposures per layer.