Study of the early stage growth of carbon-based films and application to multi-nanolayers plasma synthesis

The plasma deposition processes and solid-state growth mechanisms of two kinds of amorphous carbon films, one harder than the other, and nano-layered hard/soft nanocomposite carbon films will be presented. Two plasma processes were used for the film synthesis: rf sputtering and plasma assisted chemical vapor deposition. The mechanical properties and gas barrier functionality of such materials will also be discussed. By means of residual stress measurements and angle resolved x-ray photoelectron spectroscopy applied to the films in their early growth stages, the film growth mechanism could be identified in order to better understand the single film growth itself and, further, to control the internal stress of composite structures for protective coatings. The sputtered film growth occurred via the high mobility Wolmer-Veber mechanism. The soft film growth was found to start by a layer-by-layer mechanism, inducing a compression stress building in the very first growth stages, which tends to relax when going to higher thicknesses through a change of the growth mechanism from a layer-by-layer to a columnar growth one. The gas barrier properties of the hard films on polyethylene terephtalate (PET) were studied. The film intrinsic permeabilities to He, CO2, O2, N2 gases and H2O vapor were determined and found to be orders of magnitude lower than that of PET. The permeation mechanism was found to be based more likely on a solubility-diffusion process than on a gas flow through microdefects or gas transport through nanodefects in the films. However, these films suffer from adhesion weakness, which limits their application as protective coatings. By combining the two plasma processes in sequential film depositions, composite films consisting of periodically stacked hard and soft amorphous carbon layers were produced. They were depth-profiled by means of Auger electron spectroscopy and analysed by transmission electron microscopy. Positive effects of nanolayering on film mechanical stability are evidenced. The role of interfaces and the stored stress at these interfaces is discussed.