The exposure and development of PMMA sublayers used in multilayer resist processing schemes have been simulated. For thick planarization sublayers (>2 μm) diffraction of the deep-UV radiation produces significant exposure variations across the resist mask apertures. This effect leads to an edge-sharpening phenomena. The Fresnel diffraction approximation is used to calculate exposure variations within the sublayer and a modified SAMPLE program to obtain the sublayer image. Estimates of the wall profiles and dimensional deviations have been obtained for various isolated space feature sizes.