In this paper we report on the method of measuring the magnitude of scattered light, flare in a stepper lens and its effect on the overall process latitude. By measuring the required energy to clear large square patterns, it was possible to calculate the amount of scattered light in a lens system. When the size of a square pattern is larger than 10 micrometers , the magnitude of scattered light saturates to be independent on the size. But it gradually increases as the clear to dark area ratio of the mask increases. Simulation result shows that scattered light degrade the MTF of an aerial image and when it becomes larger than 2%, the process latitude start to decrease. Measuring scattered light with different mask opening ratio reveal that it exceed 2% when the mask opening ratio is around 70%. There was no significant difference in scattered light between different DUV scanner tools when the patterns were larger than 10 micrometers , but some difference was observed at the smaller patterns. Overall process latitude were measured with masks with different opening ratio. The exposure latitude decreased as mask opening ratio increase for positive photoresist, depth of focus was not significantly affected at increase of 20 to 50%. But we can observe 0.2 micrometers of focus margin decrease for mask open ratio change from 50 to 80% for both 260 and 240 nm patterns. So if we assume that there is no difference in process performances between positive and negative photoresists, using a negative photoresist will give more process latitude.