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

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Velocity distribution of metal ions in the target region of HiPIMS: the role of Coulomb collisions

High power impulse magnetron sputtering (HiPIMS) discharges have become an important tool for the deposition of thin, hard coatings. Such discharges are operated at a very low working gas pressure in the order of 1 Pa. Therefore, elastic collisions between ions and other heavy particles are often calculated to occur with low frequency, using the hard sphere approximation. However, inside the magnetic trap region of the discharge, a very dense plasma is created and Coulomb collisions become the dominant collision process for ions. In this article, we show that Coulomb collisions are a necessary part of a complete description of ion movement in the magnetic trap region of HiPIMS. To this end, the velocity distribution function (VDF) of chromium and titanium ions is measured using high-resolution optical emission spectroscopy. The VDF of those ions is then described using a simple simulation which employs a direct simulation Monte Carlo scheme. The simulation describes the self-relaxation of the VDF that is initially a Thompson distribution as being created during the sputtering process. Measurement positions inside the discharge are matched to the simulation results choosing an appropriate relaxation time. In this fashion, excellent agreement between simulation and measurement is obtained. We find, that the distribution quickly becomes mostly Maxwellian with a temperature of 9 eV for titanium ions and 4.5 eV in the case of chromium ions. Only the high energy tail of the VDF retains the shape of the initial Thompson distribution. The observed high temperature is explained with an energy redistribution from the highly energetic Thompson distribution into an partly-thermalized Maxwell-like distribution. Finally, the temperature resulting from this energy redistribution is calculated using a simple analytical model which shows good agreement with the measurements.

Release Date
Permanent Identifier (DOI)
Permanent Identifier (URI)
Is supplementing
Plasma Source Name
Plasma Source Application
Plasma Source Specification
Plasma Source Properties
Power Supply: TRUMPF Hüttinger TruPlasma Highpulse 4002 in voltage control mode. Connected via an inductance between power supply and magnetron. Magnetron: Thin Film Consulting IX2U for round 50 mm targets Titanium discharge: voltage: 590 V, peak current: 50 A, frequency: 40 Hz, pulse length: 100 µs Chromium discharge: voltage: 800 V, peak current: 36 A, frequency: 60 Hz, pulse length: 150 µs
Plasma Medium Name
Plasma Medium Properties
gas flow rate: 40 sccm, pressure: 0.5 Pa
Plasma Target Name
Plasma Target Properties
round sputter target: 50 mm diameter, 3 mm thickness
Plasma Diagnostic Properties
OES Measurement: Zeiss PGS 2, 1300 lines/mm grating, third diffraction order, camera as detector (Andor iStar DH320T-25U-A3), measuring Ti II 453.396 nm or Cr II 455.864 nm Measurements performed parallel to the target surface for different distances between z=1 mm and z=18 mm. Data acquired from the last 10 µs of the discharge pulse. Emission lines were deconvolved to remove the instrumental profile and obtain the VDF. DSMC model: Particles were created following a Thompson distribution. The relaxation of this velocity distribution is then followed in time as particles collide with each other via Coulomb collisions. The ion density is assumed to be 1e20 m^-3.
Plasma Diagnostic Name
English (United States)
Public Access Level
Funding Agency
Contact Name
Held, Julian
Contact Email

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