PROJECT SUMMARY

        Sol-gel chemistry is a popular technique for making new materials and high-technology coatings of a wide variety of types. Because of the ever-expanding importance of sol-gel synthesis, and because of the need to make high quality coatings with a high-throughput industrial procedure, it is important to understand the fundamentals of spin coating when performed using sol-gel solutions. Therefore, work has been planned to monitor and understand, in real time, the coating formation and drying of sol-gel solutions during spin coating. Dye molecules will be incorporated into the sol-gel solutions to help sense the chemical and physical changes taking place during spin coating.
        Two fundamental processes are critical in controlling coating formation during spin coating in general: 1) viscous flow and 2) evaporation. Early stages are dominated by the viscous behavior, i.e. relatively smooth fluid flow radially outward on the substrate. As the fluid layer gets thinner the outward flow rate reduces until the second process (evaporation) takes over as the dominant process. This transition corresponds to a "setting" point for coating formation. When spin coating is performed on sol-gel solutions then extra complexities arise including accelerated chemical reaction caused by solvent evaporation and viscosity changes being driven by the hydrolysis and condensation reactions in solution. Thus, sol-gel solutions may be particularly sensitive to the conditions that are imposed by the spin coating process.
        Carefully designed optical measurements will be carried out which would allow in-depth study of the dynamical and chemical effects that are occurring in sol-gel solutions during spin coating. Small concentrations of dye molecules would be incorporated into the sol-gel solutions for use as structure/chemistry probes during the spinning process. A thorough literature review has identified several known dye molecules that can be used to measure water/alcohol mixture or to probe viscosity increases as gelation occurs within the sol-gel solution. The primary focus will be on molecules that change their absorption spectra depending on the characteristics of the local environment. Therefore, equipment would be set up to allow absorption characteristics to be measured during spin coating processing. In addition, laser interferometry would be used to dynamically monitor the rapid fluid thinning process occurring during spinning.
        Throughout the work, the emphasis will remain focused at learning more about coating setting, drying, and gelation during the spin coating process. It is anticipated that increased understanding of this important technological process would lead to new methods for improving the quality of spin coated sol-gel layers, thus allowing the production of materials with higher reliability and increased manufacturing yield. For example, since the evaporation of the most volatile solution component will dominate during coating formation, then sol-gel scientists would learn to focus particularly close attention to the chemical effects caused by early and rapid removal of this component, especially with respect to premature gelation. And conversely, since the chemical condensation reactions can cause profound viscosity changes, then coating developers would seek to understand more about the rheological characteristics of their sol-gel solutions, even before coating quality issues arise.