Azimuthal ion movement in HiPIMS plasmas—part I: velocity distribution function

Abstract

Magnetron sputtering discharges feature complex magnetic field configurations to confine the electrons close to the cathode surface. This magnetic field configuration gives rise to a strong electron drift in azimuthal direction, with typical drift velocities on the order of 100 km s−1. In high power impulse magnetron sputtering plasmas, the ions have also been observed to follow the movement of electrons with velocities of a few km s−1, despite being not magnetized. In this work, we report on measurements of the azimuthal ion velocity using spatially resolved optical emission spectroscopy, allowing for a more direct measurement compared to experiments performed using mass spectrometry. The azimuthal ion velocities increase with target distance, peaking at about 1.55 km s−1 for argon ions and 1.25 km s−1 for titanium ions. Titanium neutrals are also found to follow the azimuthal ion movement which is explained with resonant charge exchange collisions. The experiments are then compared to a simple test-particle simulation of the titanium ion movement, yielding good agreement to the experiments when only considering the momentum transfer from electrons to ions via Coulomb collisions as the only source of acceleration in azimuthal direction. Based on these results, we propose this momentum transfer as the primary source for ion acceleration in azimuthal direction.

Plasma dynamics of individual HiPIMS pulses: imaging study using high-frame-rate camera

“…In such a region, the E × B drift will be opposite to the drift from the target-facing electric field. However, other drifts associated with curvature and gradient of the magnetic field also contribute to the electron drift [58,60]. Depending on the axial distance these drifts are similar or even larger in magnitude than the E × B drift (see figure 7 in [60]), therefore this should not significantly alter the propagation of electrons.…”

“…However, other drifts associated with curvature and gradient of the magnetic field also contribute to the electron drift [58,60]. Depending on the axial distance these drifts are similar or even larger in magnitude than the E × B drift (see figure 7 in [60]), therefore this should not significantly alter the propagation of electrons. Furthermore, the drift velocity associated with the electric field pointing axially away from the cathode should be low due to weak electric field strength.…”

Azimuthal ion movement in HiPIMS plasmas—Part II: lateral growth fluxes

“…This azimuthal movement of ions in magnetron plasmas is addressed in a two-part series with part I addressing the velocity distribution functions of the ions inside the plasma [46]. This is part II focusing on the lateral deposition of species leaving the magnetic trap region.…”