Abstract
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.
Highlights
The magnetron sputtering method was developed during the sixties and seventies of the last century [1]
The pulses are of high instantaneous power, whereas the average power remains comparable with the power used for direct current magnetron sputtering, because a duty cycle is in percent range to protect the target from overheating
The transition of the plasma pattern from spokes into the homogeneously distributed plasma in an argon/chromium high power impulse magnetron sputtering (HiPIMS) discharge was studied by a flat probe, optical emission spectroscopy and ion mass spectrometry
Summary
The magnetron sputtering method was developed during the sixties and seventies of the last century [1]. A source of these ions is a plasma discharge, which is confined close to the cathode target by an external magnetic field. In HiPIMS, the power is applied to the cathode target in pulses with a duration from several tens to hundreds of microseconds. The pulses are of high instantaneous power, whereas the average power remains comparable with the power used for direct current magnetron sputtering (dcMS), because a duty cycle is in percent range to protect the target from overheating. The discharge current densities in the pulse are usually several A·cm−2. This leads to a high ionization degree of the working gas atoms, intense sputtering and high ionization degree of the target atoms [5, 6, 7]
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