Film curvature effects are characterized for positive resist films patterned on topography. The radius of curvature (ROC) of resist contours is measured for different film thicknesses and step heights. Estimates of exposure dose and linewidth variations are obtained from this data. On reflective substrates the reflected light ray contributes to longer effective path lengths and greater linewidth changes for a given percent dose change. From film curvature effects alone linewidth deviations can be as large as 0.4 to 0.5 microns. The presence of these effects accounts for the reduction in reflective notching observed with dyed resists. An experimental verification of this result is also presented using water soluble, transparent overcoats. These overcoats effectively eliminate film curvature effects without introducing any exposure dose, wall profile, or development time penalities.
L. K. White,
"Film Curvature Effects for Optical Resists Patterned on Topography", Proc. SPIE 1086, Advances in Resist Technology and Processing VI, (30 January 1989); doi: 10.1117/12.953065; https://doi.org/10.1117/12.953065