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Miroslav Valtr, Ivan Ohlídal, Daniel Franta

Optical characterization of carbon films prepared by PECVD using ellipsometry and reflectometry

Czechoslovak Journal of Physics 56 (2006) B1103–B1109

Carbon polymer-like films were prepared using plasma enhanced chemical vapor deposition in pulsed regime in Ar-acetylene gas mixture. The optical characterization of these films was performed by variable angle spectroscopic ellipsometry and spectroscopic reflectometry applied at near-normal incidence within the spectral region 190 divided by 1000 nm. The ellipsometric and reflectometric data were treated separately for the individual samples of the films mentioned above. For treating these data the structural model of the films containing roughness of the boundaries was used. Boundary roughness was included into the formulae for the ellipsometric quantities and reflectance of the films by means of the scalar diffraction theory. Furthermore, the dispersion model based on parameterization of the density of electronic states was employed for expressing the spectral dependences of the optical constants describing the films studied. Within the optical characterization the thicknesses, spectral dependences of the refractive index and extinction coefficient, dispersion parameters and the root-mean-square values of the heights of boundary roughness were determined for all the films investigated. Non-negligible differences between the dependences of the film thickness on the deposition time determined using the ellipsometric data on the one hand and reflectometric data on the other hand were observed. These differences were explained by the fact that the films exhibited the other defects that were not included in their structural model. It. is assumed that the defect's consisting in refracting index profile, non-uniformity in thickness and refractive index and transition interlayers can influence the treatment of the experimental data. A surprising fact consisting in sensitivity of the optical parameters of the films on UV irradiation was also observed. In the paper this fact will be illustrated through quantitative dependences.

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