Publication details
On spokes in reactive Ar/N2 atmosphere using HiPIMS
| Authors | |
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| Year of publication | 2023 |
| Type | Conference abstract |
| MU Faculty or unit | |
| Citation | |
| Description | Although the first observations of spokes, to which plasma self-organises, in HiPIMS were made a decade ago, a complete description of this phenomenon is still lacking. To this day overwhelming majority of spoke studies were conducted in a non-reactive pure Ar atmosphere. In this work, the effect of nitrogen addition was studied on 7.6 cm in diameter Nb, NbN and Ti targets. Power was supplied in 100 µs pulses with a repetition rate of 5 Hz. Plasma emission was simultaneously captured by PI-MAX 4 camera and Horiba FHR 1000 spectrometer. In the experiments, the flow of Ar was adjusted to the change of N2 content in order to keep the overall pressure constant. The effect of the target poisoning on spoke appearance was investigated using Nb and NbN targets. The stochiometric NbN target mimicked the fully poisoned Nb target regardless of the N2 content. In both cases, the same trends in spoke number, spoke rotation velocity and spoke shape were observed with increasing N2 content. Generally, triangular spokes transformed towards stochastic. The results suggest that the target poisoning has no effect on spoke appearance for the Nb target and any changes affecting spokes are induced within the plasma. Similarly, the spoke appearance was studied in Ar/N2 atmosphere using a titanium target. The spoke shape was observed to change from triangular to diffusive. The spectral line intensity evolutions were recorded over the whole spoke and the results for chosen spectral lines are shown in Figure 1. Ionic species of Ar and N2 show peak intensity coinciding with the trailing edge of the spoke, while metallic species show their maximum in the middle of the spoke. As the structure of the emission does not change with the N2 content, it can be assumed that the inner structure of the spoke is conserved. The overall change in emission pattern captured by the camera is thus given by the dominant species emission, which changes from Ar dominated for low N2 content towards metal dominated for high N2 content. The relative decrease of the peak Ar and N2 ion intensity with an increase of the N2 content is ascribed to the transition from the discharge supported by the secondary electrons towards the discharge where the homogeneous Ohmic heating is more prominent. In contrast, the secondary electron production and energisation are non-uniform, the highest in the potential hump region, which coincides with the spoke. |