Electron density, temperature and the potential structure of spokes in HiPIMS

Abstract

In high power impulse magnetron sputtering (HiPIMS) bright plasma spots are observed during the discharge pulses that rotate with velocities in the order of 10 km s−1 in front of the target surface. It has proven very difficult to perform any quantitative measurements on these so-called spokes, which emerge stochastically during the build-up of each plasma pulse. In this paper, we propose a new time shift averaging method to perform measurements integrating over many discharge pulses, but without phase averaging of the spoke location, thus preserving the information of the spoke structure. This method is then applied to perform Langmuir probe measurements, employing magnetized probe theory to determine the plasma parameters inside the magnetic trap region of the discharge. Spokes are found to have a higher plasma density, electron temperature and plasma potential than the surrounding plasma. The electron density slowly rises at the leading edge of the spoke to a maximum value of about 1 × 1020 m−3 and then drops sharply at the trailing edge to 4 × 1019 m−3. The electron temperature rises from 2.1 eV outside the spoke to 3.4 eV at the trailing end of the spoke. A reversal of the plasma potential from about −7 V outside the spoke to values just above 0 V in a spoke is observed, as has been proposed in the literature.

Influence of the magnetic field on the discharge physics of a high power impulse magnetron sputtering discharge

“…The resemblance between the time-evolution of n e (t) and I D (t) during the HiPIMS pulse has been observed before experimentally by Held et al [27]. It can be understood by equalizing the discharge current with the ion current to the target.…”

Physics of E × B discharges relevant to plasma propulsion and similar technologies

“…The measurements reveal the existence of a double layer structure with large electric field at the leading edge of the ionization zone and that this double layer plays a crucial role in the energization of electrons [103]. References [101], [102] discuss the experimentally determined potential structures of spokes in HIPIMS. The presence of a double layer and enhanced ionization at the spoke front is reminiscent of the Critical Ionization Velocity model of [105] and of the PIC simulations of [63], [106], although the velocity of the spoke measured in HiPIMS and magnetron discharges does not necessarily match the theoretical critical ionization velocity.…”

Ionized particle transport in reactive HiPIMS discharge: correlation between the energy distribution functions of neutral and ionized atoms

“…As mentioned above, the dense HiPIMS plasma near the target is not homogeneous, and can exhibit one or more spokes with a rotational velocity of about 10 km s −1 in the E ⃗ ⃗ × B ⃗ ⃗ azimuthal direction. Recently, Held et al [38] characterized the plasma potential inside those structures using time-shift averaging Langmuir probe measurements. They reported that the plasma potential becomes higher inside a spoke than in the surrounding region, with the measured potential in those structures being positive and having a weak gradient.…”