The objective of this paper is to study the effect of magnetic field strength on spoke behaviour in a high power impulse magnetron sputtering discharge. In three magnetic field configurations, a broad range of experimental conditions was investigated by high-speed camera imaging. The dual image feature of the employed camera enabled us to determine the shape and spoke mode number, as well as the velocity of the spokes. Five distinct spoke shapes were detected. For each magnetic field, a map was created relating the occurrence of the spoke shape with the discharge conditions. Similarly, the spoke mode number, as well as the spoke velocity, was found to be strongly dependent on the discharge conditions. Based on the optical emission results, it was proposed that the trailing edge of the triangular spoke is related to the production of secondary electrons created by argon ions on the spoke edges, while in the case of a round spoke shape, the secondary electrons are mainly created by multiple charged titanium ions in the centre of the spoke.

Thispaper reports on the oscillations both on the cathode voltage and on the discharge current which emerged suddenly and simultaneously during ahigh-power impulse magnetron sputtering pulse. Detailedexperiments identified the pressure and the discharge current ranges to detect these oscillations. The oscillationsoriginatedfrom the magnetized plasma and their frequency ranged from 225 kHz to 400 kHz depending on the experimental conditions. Simultaneous measurement of both oscillations and spokes was conducted to find mutual correlations. The oscillations always accompanied the spokes and no particular conditions to detect oscillations in theabsence ofspokes were found. The oscillations were not caused by the rotational motion of the spokes butemerged when acertain threshold amount of spokes was overreached.

Plasma in high-power impulse magnetron sputtering discharge, similarly to other discharges utilising E × B field (Hall thrusters, homopolar devices), undergoes self-organisation into the ionisation zones predominantly rotating in the E × B direction, called spokes. Many studies were conducted focussing on the characterisation of their appearance, mode number, rotational velocity, merging and splitting events in different experimental conditions. Nevertheless, only very little research has been conducted in the case of reactive sputtering, where only the general spoke characteristics were evaluated. A dual-image fast camera screening was utilised to capture plasma emission on 3 Nb target in a reactive mixture of nitrogen and argon. Spoke characteristics were evaluated while overall pressure and supplied power was kept constant and the content of nitrogen in N 2 /Ar mixture was varied. The shape, velocity and spoke mode number were significantly affected by the higher content of N 2 in the mixture. To distinguish between the effects of the modified target surface state and reactive gas present in the plasma volume on spokes experiments with compound NbN target were also performed. Surprisingly, no real differences of spoke behaviour between Nb and NbN targets were observed.

The rotating plasma patterns, also known as ionisation zones or spokes, observed, among other discharges, in high power impulse magnetron sputtering discharge (HiPIMS) require non-invasive diagnostics favourable for a precise characterisation of their properties. In this contribution, the single-shot spatial-resolved optical emission spectroscopy of the spoke was conducted in non-reactive HiPIMS discharge using a titanium target. Investigated working pressures cover the conditions with the presence of localised, well-defined spokes. A fast photodiode and a cylindrical Langmuir probe were utilised to capture and determine the passing spoke position. These signals were synchronised with the acquisition of the optical emission spectrum by the intensified charge-coupled device detector. A large amount of single-shot data enabled the statistical analysis of the spoke. The optical emissions of argon atoms and ions and titanium atoms and ions were investigated in the passing spoke. It was found that the intensities of the spectral lines of the Ar and Ti species have the characteristic evolution for all studied spectral lines of this specific species within the spoke. The intensity evolutions are independent of the applied pressure. The evolution of the excitation temperatures determined by the Boltzmann plot method using the Ar II and Ti I and Ti II spectral lines remains constant within the spoke in the margin of standard error for all investigated pressures.

The self-organised plasma patterns, known as spokes or ionisation zones in magnetron sputtering discharges, were observed in a wide range of power densities, from low power direct current magnetron sputtering (dcMS) discharge to high power impulse magnetron sputtering (HiPIMS) discharge. For some target materials and non-reactive gases, it was observed that at very high power densities (> ) the plasma exhibits a transition from a regime where spokes are observed to a homogeneous plasma regime. In this contribution, we present a comparison of plasma properties: plasma emission (optical emission spectroscopy) and flux of argon and chromium ions (mass spectrometry), measured both in the spoke regime and in the homogeneous plasma regime, aimed to expand the understanding of the plasma transition between the two modes. A simple biased flat probe was used to distinguish between the spoke regime and the homogeneous plasma regime. It was found that the flux of multiply charged ions (Ar2+ , Cr2+ , Cr3+ , Cr4+) increases abruptly at the transition between the spoke regime and the homogeneous plasma regime. Similarly, the emission from Cr+ ions exhibits a strong increase of about 50% when the plasma torus becomes homogeneous. These observations are interpreted as an increase in electron temperature and a change in the electron heating mode, from a combination of secondary electron heating and Ohmic heating towards pure Ohmic heating. The transition to the homogeneous plasma regime and pure Ohmic heating is only observed in non-reactive HiPIMS discharges for target atoms with the second ionisation potential higher than the first ionisation potential of Ar (15.76 eV), and a self-sputter yield larger than 1.