Modeling of carbon plasma discharges in high-power impulse magnetron sputtering

Abstract: Diamond like carbon (DLC) is a metastable state of amorphous carbon that has very important and wide-ranging thin film applications. DLC has a strong resemblance to pure diamond and exhibits many traits of real diamond, like mechanical hardness and chemical inertness, but with a drastically lower deposition cost. DLC is characterized by a high fraction of sp3 hybridization. To reach a high fraction of sp3 bonding by sputtering of a graphite target, an energetic ion population and a high ionized flux fraction (Fflux) is beneficial. High-power impulse magnetron sputtering (HiPIMS), an ionized physical vapour deposition technique (iPVD) based on magnetron sputtering, has been shown to produce significantly higher ionized fluxes and more energetic ions compared to the industry standard technique of direct current magnetron sputtering (dcMS). For carbon however, the ionized flux fraction is significantly lower than that of common metal targets like titanium and aluminium, even with HiPIMS. In this thesis the ionization region model is applied to experimental carbon-argon 50 µs HiPIMS discharges at peak current densities of 1, 2 and 3 A/cm2 to investigate why the fraction of sputtered carbon reaching the substrate as ions is so low. The ionized flux fraction of the experimental discharges was measured by an ion meter to be lower than 5 %. From the computational modeling we find that the ionization probability of a carbon neutral (α) increases with increased peak discharge current densities from 40 % at 1 A/cm2 to over 60 % at 3 A/cm2. However, the back attraction probability of carbon ions (β) is high or above 90 %. The model predicts a higher Fflux than measured for all cases. The modeled Fflux values were 6-8 %, 10-13 % and 13-15 % for peak discharge current densities of 1, 2 and 3 A/cm2, respectively. By the time evolution of the particle densities, it is clear that most of the ionization takes place at the end of the pulse and thus the afterglow plays a significant role, especially for shorter pulses. The main conclusion is that the HiPIMS carbon discharge is mainly governed by the argon working gas and shares many traits with a typical working gas recycling process