In this article, we analyzed in-field uniformity (IFU) on wafer considering exposure margin [linewidth variation (nm) per % exposure dose variation (%)] and the MEEF (mask error enhancement factor). As gate linewidth becomes smaller, the controllability of in-field uniformity (IFU) plays a key role in wafer manufacturing yield. IFU depends on various lithography parameters including mask CD (critical dimension) uniformity, MEEF, exposure margin, focus margin, transmittance, flare and illumination uniformity. In real world, the combination of wafer exposure machine and mask characteristics should be carefully considered to achieve better IFU on wafer. This presentation discusses the various experimental works including CD uniformity on mask, IFU on wafer, MEEF, exposure margin and wafer exposure machine. CD uniformity data on mask and IFU data on wafer is obtained from optical measurement tool to reduce measurement error disregarding local CD variation. Even though one handles a unit pattern, various MEEF exists in a unit pattern in case of complex pattern. In addition, the MEEF varying with the area across one mask degrades IFU on a wafer. IFU on a wafer is predictable using mask CD uniformity and exposure margin mean. Variation of exposure margin is the measure of stability of photo process. In manufacturing devices, mask and Litho. Tool should be well harmonized to achieve better IFU and a higher manufacturing yield. The photo process including resist process should be well controlled to get stability, as well as mask CD uniformity.