EFFECT OF LENS DISTORTION IN OPTICAL STEP-AND-SCAN LITHOGRAPHY

A model is presented with which the effect of lens distortion on the o ptical transfer function is calculated for a step-and-scan lithographi c system. The spatially varying lateral image shift and the relative l oss in modulation depth are derived hom the exposure pattern that is b uilt up during the image scanning. Other important phenomena such as l ens aberrations, the effect of finite image aperture, and polarization are deliberately left out of the model; from the simplified model, an alytic expressions can he obtained that relate the distortion coeffici ents of the projection system to the local shift and the relative loss in modulation depth of the exposed pattern that is built up during sc anning. As a scanning aperture, we have taken either part of an annulu s or a rectangular section in the image field. (C) 1996 Optical Societ y of America