Infrared spectroscopic ellipsometry study of molecular orientation induced anisotropy in polymer substrates

Many commercial products use thin films on polymer substrates. SE has strong potential for measuring films on polymer substrates; however, further research is needed to better characterize these optically complex substrates and develop simplified analysis methods. In this work, optical anisotropy and dichroism are studied in plastic substrates across the mid- to far-infrared spectral range. Preferential molecular orientation introduces directional-dependent vibrational absorption, thus molecular orientation can be studied by measuring in-plane and out-of-plane optical constants. Simulations are used to demonstrate how anisotropic absorption (different amplitude and/or vibrational frequency) affect ellipsometric data. Experimental measurements from three polymer substrates are presented: polyethylene terepthalate (PET), Poly-IR-2 (high density polyethylene), and Kapton polyimide. Vibrational absorption bands demonstrating predicted anisotropy results are assigned to their corresponding molecular bonds and molecular orientation is discussed