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

Abstract

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.

Time-resolved ion energy distribution functions during a HiPIMS discharge with cathode voltage reversal

“…This is expected due to the increased collisionality. At 0.67 Pa, the energy of the ions reaching the substrate is like the expected distribution which is a partially thermalized Maxwellian [39].…”

Control of spoke movement in DCMS plasmas

“…(ii) Change in spoke frequency. We have previously proposed that the ion transport should depend strongly on the absolute spoke frequency [14,16]: if the spoke frequency is high, the inertia of ions is too large for them to follow the change in electric field. The ions will mostly move according to the average electric field strength, just as they would if no spokes existed in the plasma.…”

“…In short, such ionization zones may resembles drift waves that are driven by the strong gradients in these magnetized plasmas [11,12]. The velocity of these ionization zone is typically at 10 km s −1 along the racetrack [13,14]. The electrons move much faster at typically 100 km s −1 , whereas the ions move only at 1 km s −1 [15,16].…”

Space-resolved line shape model for sputtered atoms of finite-size targets

“…At linear plasma devices, such as PSI-2, or in magnetron sputtering discharges, high-resolution emission spectroscopy, laser absorption spectroscopy and laser-induced fluorescence resolve the shape of the radiative transitions by sputtered atoms [26][27][28][29]. Also, the measurements of polarization properties of metallic mirrors using emission properties of the backscattered atoms rely on the theoretical description of the measured line shapes [30].…”