Effect of solvent evaporation rate on "Skin" formation during spin coating of complex solutions

Spin coating can often be used to create highly uniform coatings on flat substrates using sol-gels and other precursor solutions. Typically, fluid flow considerations dominate the early part of spinning while solvent evaporation controls the behavior at later stages. However, even though evaporation does not control the early stages, it is still occurring throughout the entire process and evaporation-related issues can arise during the flow-dominated stage that may ultimately become coating thickness non-uniformities in the final product Striation defect formation is a good example of this. During the spin coating process the evaporation of solvent takes place at the top surface of the flowing fluid, thus a concentration profile for the solute species will exist within the flowing solution. Since the solute species in sol-gel solutions are often prone to condensation or cross-linking reactions, it is possible for this top surface to experience a rapid increase in viscosity and potentially to act as a barrier to further evaporation This top layer could be equivalent to the skin layer on drying paint In this paper we explore the effect of solvent depletion in the near-surface-region by closely analyzing the flow and evaporation rates for real solutions, both sol-gel and polymer, during spinning. We report interferometrically measured fluid thickness data that are analyzed to directly extract the net material evaporation rate. Data are compared for different spin speeds, thus directly adjusting the evaporation rate and potential solvent depletion effect at the surface. It is found that the evaporation rate applicable through the entire first stage of spin coating is nearly the same as that for pure solvent. Thus the top surface composition does not deviate greatly from its starting value until very near the end of coating formation process, when flow has effectively ceased and the coating drying stage begins.