Relationship between the optical and mechanical properties of fluorinated amorphous carbon thin films

Fluorinated amorphous carbon (a-C:H:F) thin films were grown from CH4/CF4/Ar gas mixtures using a 13.56 MHz radiofrequency (RF) plasma source. The effect of fluorine incorporation on the mechanical properties of the films was investigated by depth sensing indentation, while Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray reflectivity (XRR) and ellipsometry measurements were exploited to study their vibrational, structural and optical properties. While raising fluorine content in the film, visible Raman results show that the sp2 carbon sites undergo a gradual ordering into graphitic clusters, which are responsible for a higher optical absorption. Nanoindentation measurements showed decreased elasticity of the film, as well as decreased hardness upon fluorine incorporation. A clear correlation between the mechanical properties and the optical constants was found as a function of fluorine content in the film.