The Influence of Inert Gases on the a-C and CNx Thin Film Deposition: A Comparison between DCMS and HiPIMS Processes

• DCMS and HiPIMS discharges of C in Ne, Ar, and Kr as well as their reactive counterparts (N2/Ne, N2/Ar, and N2/Kr) were investigated for the growth of carbon and carbon-nitride (CNx) thin films. The thin films were synthesized in an industrial deposition chamber from a pure graphite target. Time averaged plasma mass spectroscopy showed that the energies of the most abundant plasma cations depend on the inert gas and the amount of N2 in the sputter gas rather than the sputter modes. The ion species population in the plasma, on the other hand, was found to depend heavily on the sputter mode; HiPIMS processes yield approximately ten times higher flux ratios of ions originating from the target to ions originating from the process gas. Exceptional cases are the discharges in Ne or N2/Ne mixtures containing up to 20% N2. Here, no influence of the sputter mode on cation energies and population was found. CNx and a-C thin films deposited in 14% N2/inert gas mixture and pure inert gas, respectively, were characterized regarding the chemical composition, chemical bonding and microstructure as well as their mechanical properties using elastic recoil detection analysis, X-ray photoelectron spectroscopy, transmission electron microscopy in combination with selected area electron diffraction, and nanoindentation, respectively. The thin film characteristics showed strong correlations to the energies of abundant plasma cations (namely C+, Ar+, Ar++, Ne+,22Ne+, Ne++, 82Kr+, 84Kr+, 86Kr+, Kr++, N+, N2+, CN+ as well as C2N2+)and cation population of the corresponding deposition process. High amounts of C bond in sp3 hybridization state were found for thin films sputtered in Ne, accounting for their elevated hardness and amorphous microstructure. With increasing inert gas atomic number the a-C and CNx thin films show an increasingly distinct near range ordered microstructural evolution. This effect is more pronounced for HiPIMS processes and accompanied by a lowered hardness, but elevated elastic properties.

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