The aerial image of alternative exposure systems was calculated in order to determine process latitude in photolithography at 0.25 mum and below. A fast software package was used for both simple and complex mask patterns. The depth-of-focus (DOF) was obtained with the aid of exposure-defocus plots at 193 and 248 nm. The numerical aperture and degree of spatial coherence were varied over a wide range. The effects of annular illumination were also studied. It was shown that for most geometries the DOF at 193 nm exceeds that of 248-nm systems. Only for equal lines and spaces does a 248-nm tool with annular illumination possess a DOF comparable to that of a conventionally illuminated 193-nm system.