A new angle on the plasma discharge physics in magnetron sputtering

International audienceMagnetron sputtering is a wide-spread plasma-based thin film deposition technique. Over the last two decades, extensions of conventional magnetron sputtering, such as high-power impulse magnetron sputtering (HiPIMS)[1] ,have appeared that can achieve a high degree of ionization of the sputtered atoms. This opens new perspectives in the engineering and design of thin film materials. At the same time, these new techniques have also forced us to re-evaluate established descriptions of the plasma discharge physics in magnetron sputtering. In this contribution the mechanism for energizing the electrons will be expanded to not only include plasma sheath energization, but also direct Ohmic heating of the plasma electrons. Using a novel model benchmarked with experimental results it is found that Ohmic heating dominates over sheath energization by typically an order of magnitude[2]. This holds from low power densities, as typical for DC magnetron sputtering (DCMS), to high power densities used in HiPIMS. In addition, the ionization mechanism in the magnetron plasma has been investigated. It is found that although stepwise ionization of the argon process gas can be important in DCMSdischarges it is negligible during the breakdown phase of the HiPIMS pulse and becomes significant (but never dominating) only later in the pulse. For the sputtered species Penning ionization can be a significant ionization mechanism in DCMS, while in HiPIMS, Penning ionization is always negligible as compared to electron impact ionization. The main reasons for these differences are a higher plasma density and a higher electron temperature in HiPIMS