Paper TF-FrM9
Towards Achieving a High Degree of Carbon Ionization in Magnetron Sputtering Discharges

Friday, November 4, 2011, 11:00 am, Room 110

Physical vapour deposition (PVD) methods, which are characterized by highly ionized deposition fluxes of the film forming species, provide added means for the synthesis of tailor-made materials. They can, for instance, facilitate the growth of meta-stable phases, nanostructures as well as selective deposition on complex-shaped substrates. In such methods, the generation of highly ionized deposition fluxes stems from high electron (plasma) densities. Cathodic arc and pulsed laser deposition are examples of such discharges where electron densities in the order of 10²¹ m^-3 can be obtained. These techniques, while providing as high as 100% degree of ionization of the deposition flux, exhibit several drawbacks, such as macroparticle ejection from the target, lack of lateral film uniformity, and in some cases are difficult to scale up. Magnetron sputtering based techniques are technologically more relevant, owing to their inherent advantages of conceptual simplicity, upscalability, and film uniformity. However, electron densities in magnetron discharges are significantly smaller, in the range of 10¹⁴-10¹⁶ m^-3 and therefore generation of a highly ionized deposition flux is often difficult. This difficulty is overcome by high power impulse magnetron sputtering (HiPIMS), where plasma densities on the order of 10¹⁹ m^-3 are achieved. HiPIMS has been successful in enhancing the ionization for most common metals (Cu, Al, Ta, Ti), but it is challenged when non-metals such as carbon is considered. Previous investigations have shown that C⁺/C ratio in HiPIMS does not exceed 5%, which does not provide efficient control over the physical properties and synthesis of carbon in various technologically relevant forms, e.g. tetrahedral amorphous carbon. In the present study we address the low degree of ionization of carbon in magnetron discharges. We have developed a new HiPIMS based process, which provides a plasma characterized by high electron temperature and plasma density as determined by time-resolved Langmuir probe measurements. The C⁺ ion energy distribution functions (IEDFs) determined by time-averaged energy resolved mass spectrometry demonstrate an energetic C⁺ ion population and an overall five-fold increase of the C⁺ ion fraction as compared to standard HiPIMS methods. The enhanced ionized fraction of carbon facilitates the growth of carbon films with mass densities as high as approx. 2.8 g/cm³ as determined by high resolution x-ray reflectively measurements. Determination of the D-peak to G-peak ratio (I(D)/I(G)) and full width at half maximum of the G-peak in Raman spectra indicate that the films contain a large fraction of diamond-like bonded (sp³) carbon.