Investigating different carbon-based target materials: Can we improve ionization in HiPIMS for the deposition of diamondlike carbon films?

Abstract

Diamondlike carbon (DLC) thin films have attracted growing interest due to their extraordinary properties, which occur if the fraction of sp3 C-bonds in the amorphous carbon films is high. This high fraction of sp3 C-bonds requires a high ionization rate of the sputtered carbon and a high kinetic energy of the carbon species. The first part of this article provides a detailed overview of the possibilities to increase the ionized fraction of the sputtered carbon and a brief description of the DLC growth models. The overview will include previously unpublished calculations by our group that include the ionization rate of carbon compared to some metals, the mean ionization path length of carbon, and the carbon ion flux at the substrate. In addition, the problem of simultaneous deposition of sp2- and sp3-bonded carbon during a HiPIMS pulse is explained for the first time. In the second part, we will present the influence of different carbon-based target materials on ionization, arcing, and deposition rates. Therefore, three different carbon-based target materials were investigated for high-power impulse magnetron sputtering (HiPIMS) depositions of a-C films: (a) graphite target, (b) fine-grained graphite target, and (c) glassy carbon target. The acquired data were compared to dc magnetron sputtering (dcMS). For HiPIMS, the pulse parameters and the total argon gas pressure were varied. The deposition process was characterized by the acquisition of the target currents and voltages, the arcing rate, optical emission spectroscopy (OES), and monitoring the deposition rate using a quartz crystal microbalance. The studies revealed that with HiPIMS, arcing was increased strongly with the peak current density for the graphite target. With the glassy carbon target, arcing was low at the beginning but increased with the duration of the tests. This target had a polished surface in the as-delivered state, which became rougher during sputtering. Similar deposition rates have been measured for dcMS and HiPIMS. With OES, only a low ionization of carbon was identified. The deposition of a-C coatings produced films with a low hardness of about 1200 HV (about 12 GPa) for both sputtering methods (dcMS and HiPIMS), as no substrate bias was applied. It can be concluded that arcing was lowest with the glassy carbon target and that the ionization rate was not significantly influenced by the change in the target material.