Variable angle spectroscopic ellipsometry of fluorocarbon films from hot filament chemical vapor deposition

Abstract

Hot filament chemical vapor deposition using hexafluoropropylene oxide as the precursor gas yielded two sets of fluorocarbon films, one with varying OH/COOH content and the other with varying grain aspect ratio, as revealed by Fourier transform infrared spectroscopy and atomic force microscopy, respectively. Variable angle spectroscopic ellipsometry was performed to derive film thickness and film optical constants. A uniaxial Cauchy–Urbach dispersion layer, with separate in-plane and out-of-plane dispersion parameters, was found to realistically describe the films. Derived film thickness agreed well with profilometry measurements. Anisotropy in index of refraction n and extinction coefficient k was on the order of ${10}^{\mathrm{-2}}$ and ${10}^{\mathrm{-5}}$ to ${10}^{\mathrm{-3}},$ respectively. The relationship between the complex index of refraction and the dielectric function allowed the optical dielectric constant ${\epsilon }_1$ to be calculated. The presence of OH did not affect the film optical dielectric constant significantly. Even though OH/COOH groups are considered to contribute adversely by increasing the dielectric constant, their effect may have been masked by the dominance of F atoms. The optical dielectric constant decreased linearly with increasing grain aspect ratio. The presence of voids was thought to lower the effective index of refraction and optical dielectric constant, and the increase in grain asymmetry created more of these voids due to less efficient packing. By assuming a linear contribution from bulk grains and voids to the effective optical dielectric constant, film porosity was derived. A maximum film porosity of 33%, corresponding to an optical dielectric constant of 1.74, was observed.