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The group "Experimental Physics II - Reactive Plasmas" at the faculty of physics and astronomy at Ruhr University Bochum.

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Fast charge exchange ions in high power impulse magnetron sputtering of titanium as probes for the electrical potential

High power impulse magnetron sputtering (HiPIMS) plasmas exhibit a high ionization fraction of the sputtered material and ions with high kinetic energies, which produce thin films with superior quality. These ion energy distribution functions (IEDF) contain energetic peaks, which are believed to be linked to a distinct electrical potential hump (ionization zone) inside rotating localized ionization zones, so called spokes, at target power densities above 1kW/cm^−2. Any direct measurement of this electrical potential structure is, however, very difficult due to the dynamic nature of the spokes and the very high local power density, which hampers the use of conventional emissive probes. Instead, we use a careful analysis of the IEDFs for singly and doubly charged titanium ions from a HiPIMS plasma at varying target power density. The energy peaks in the IEDFs measured at the substrate depend on the point of ionization and any charge exchange collisions on the path between ionization and impact at the substrate. Thereby, the IEDFs contain a convoluted information about the electrical potential structure inside the plasma. The analysis of these IEDFs reveal that higher ionization states originate at high target power densities from the central part of the plasma spoke, whereas singly charged ions originate from the perimeter of the plasma spoke. Consequently, we observe different absolute ion energies with the energy of Ti2+ being slightly higher than two times the energy of Ti+. Additional peaks are observed in the IEDFs of Ti+ originating from charge exchange reactions from Ti2+ and Ti3+ with titanium neutrals. Based on this analysis of the IEDFs, the structure of the electrical potential inside a spoke is inferred yielding Delta Phi_(ionization zone)= 25 V above the plasma potential, irrespective of target power density.

Release Date
Permanent Identifier (URI)
Is supplementing
Plasma Source Name
Plasma Source Application
Plasma Source Specification
Plasma Source Properties
Melec power supply, 50µs, 300Hz. Peak power densities range from 1100 W/cm² to 3200 W/cm².
Plasma Medium Name
Plasma Medium Properties
Ar 5.0, 15sccm. Pressure regulated via butterfly valve, pressure: 0.5 Pa
Plasma Target Name
Contact Name
Breilmann, Wolfgang
Plasma Target Properties
2 inch titanium target
Contact Email
Plasma Diagnostic Properties
Mass spectroscopy, MS looking at race track position at 8cm distance
Public Access Level
Plasma Diagnostic Name
Funding Agency

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