Effect of the initial viscosity and substrate corner geometry on edge beading of photoresist films

This paper explores the effect of viscosity and corner geometry on the width of the edge bead region for axisymmetric substrates. Specifically we model the edge of the substrate as a part of a circle with different subtended angles and various radii, in combination with straight segments of given length. We employ the physiochemical properties of a typical SU-8 3000 photoresist with different concentrations of solvent, and therefor a large range of initial viscosities. Using the lubrication approximations, we derive the governing equations for a photoresist on such a substrate that includes rotation in the initial phase and the evaporation of solvent in the drying stage, with a subsequent increase in viscosity. The resulting equations are solved numerically using an efficient implicit finite difference algorithm. The results indicate that high initial viscosities lead to a more uniform film thickness near the edge of the substrate, but require a significantly greater rotation time. Larger corner radii reduce edge beading, but require a larger substrate, and consequently the absolute width of the bead region is actually larger for higher values of the corner radius. Using a substrate with a chamfered corner region can reduce edge beading on the horizontal region of the substrate. However, this leads to a larger substrate and consequently the width of the region affected by edge beading is actually greater. Consequently we conclude that a corner region with a small radius of curvature may produce the smallest edge bead width in industrial applications.