2023 AIChE Annual Meeting

Ellipsometry of Ultra Thin Polymer Films

The usage of ellipsometry to characterize thin film properties such as refractive profile, film thickness and more has been an extremely potent tool for scientific research. Ellipsometry leverages the polarization of light being changed as a function of light-matter interactions to acquire a signal that can be decoded into material properties. As incident light moves through the thin film, the axes of polarizations deviate into an elliptical shape, the difference of which can be calculated into an ellipsometric ratio ρ through data collection of the amplitude, Ψ, and angle difference, Δ. However, the lower parameters to which ellipsometry can accurately characterize a thin film are not fully understood. Previous studies have reported anomalous increases in material density at lower thicknesses without change to chemical structure, resulting in large increases in refractive index as measured by the ellipsometer.

Here we report on our ongoing study investigating the the refractive profiles of characterized thin films as a function of their thickness. To diversify the collected data, the combinations of thin films and substrates are expanded to include three different substrates as the basis for the ellipsometry. We utilize spin-coating of varied concentrations of thin-film solutions to deposit films with a range of thicknesses onto optical glass substrates. Importantly, the usage of high-speed spin coating allows depositions of uniform thickness. The coated substrates are then analyzed using rotating compensator ellipsometry to characterize their refractive profiles. The aim of this research is to develop straightforward analytical models to predict the refractive index of different thin films as a function of their thickness. Next steps of the research include the expansion of the thin film formulation set to include chemically unique thin films in the research.