EVALUATION OF DEPTH-OF-FOCUS IN PHOTOLITHOGRAPHY AT 193 AND 248 NM FOR FEATURE SIZES OF 0.25 MU-M AND BELOW

The aerial image of alternative exposure systems was calculated in ord er to determine process latitude in photolithography at 0.25 mum and b elow. A fast software package was used for both simple and complex mas k patterns. The depth-of-focus (DOF) was obtained with the aid of expo sure-defocus plots at 193 and 248 nm. The numerical aperture and degre e of spatial coherence were varied over a wide range. The effects of a nnular illumination were also studied. It was shown that for most geom etries the DOF at 193 nm exceeds that of 248-nm systems. Only for equa l lines and spaces does a 248-nm tool with annular illumination posses s a DOF comparable to that of a conventionally illuminated 193-nm syst em.