Application and limitations of inverted fireballs in a magnetron sputter device

In this paper the results of a first attempt to ignite an inverted fireball with a magnetron sputter source are presented. The conducted measurements show that there is no inverted fireball visible to the naked eye present during the experiments. However, some features are observed that indicate at least an initial onset of such an inverted fireball with an interruption before it is fully developed. In the course of the experiments magnetron sputtered molybdenum films have been deposited inside a highly transparent gridded anode and the obtained films have been analysed. It turns out that even without a visible inverted fireball being present, the film morphology and mechanical properties such as elastic modulus, hardness and surface roughness have changed significantly compared to a setup without inverted fireball anode. Additionally, the deposition rate is elevated by up to 20% inside the inverted fireball anode compared to the reference setup, although with a nearly threefold increase in total input power (cathode power + grid anode power). This increase in deposition rate was achieved despite the fact that a mesh electrode with only about 37% transparency was used. On the other hand, it was demonstrated that the use of additional gridded electrodes in such a device triggers instabilities and non-linearities in the plasma, which might be of interest for further fundamental research.