On sheath energization and Ohmic heating in sputtering magnetrons

Abstract

In most models of sputtering magnetrons, the mechanism for energizing the electrons in the discharge is assumed to be sheath energization. In this process, secondary emitted electrons from the cathode surface are accelerated across the cathode sheath into the plasma, where they either ionize directly or transfer energy to the local lower energy electron population that subsequently ionizes the gas. In this work, we present new modeling results in support of an alternative electron energization mechanism. A model is experimentally constrained, by a fitting procedure, to match a set of experimental data taken over a large range in discharge powers in a high-power impulse magnetron sputtering (HiPIMS) device. When the model is matched to real data in this way, one finding is that the discharge can run with high power and large gas rarefaction without involving the mechanism of secondary electron emission by twice-ionized sputtered metal. The reason for this is that direct Ohmic heating of the plasma electrons is found to dominate over sheath energization by typically an order of magnitude. This holds from low power densities, as typical for dc magnetrons, to so high powers that the discharge is close to self-sputtering, i.e. dominated by the ionized vapor of the sputtered gas. The location of Ohmic heating is identified as an extended presheath with a potential drop of typically 100-150 V. Such a feature, here indirectly derived from modeling, is in agreement with probe measurements of the potential profiles in other HiPIMS experiments, as well as in conventional dc magnetrons.

Spectroscopic imaging of self-organization in high power impulse magnetron sputtering plasmas

“…When interacting, energy is transferred from hot electrons, providing a positive feedback to cause even more ionization at that location. However, in very recent work by Hou et al 18 it was shown that heating by secondary electrons is not the dominant mechanism of power dissipation in magnetrons. Rather, plasma electrons are predominantly "energized" by Ohmic heating in the magnetic presheath, where the electron cross-field current density is high and the electric potential gradient is still large.…”

“…Additional local Ohmic heating, of roughly the same order of magnitude, is caused by electrons experiencing a potential drop in the z-direction, 18 i.e. when escaping the closed drift and forming plasma flares.…”

Localized heating of electrons in ionization zones: Going beyond the Penning-Thornton paradigm in magnetron sputtering

“…It was therefore proposed 9,11 that the generation of doubly and higher charged ions must be critical to HiPIMS operation. An new mechanism, recently presented by Huo et al, 12 greatly reduces or even eliminates the need for secondary electrons. Using a global model of the magnetron discharge, Huo et al 12 showed that the voltage drop in the magnetic presheath can lead to very significant energy dissipation and electron heating.…”

“…Electrons gain energy as they diffuse to magnetic field lines where the potential is higher. 12 In this contribution, the idea of presheath heating of electrons 12 is examined on a local level, and it is shown that there is a interdependence between presheath heating of electrons, the formation (and disappearance) of ionization zones 2-4 (a.k.a. spokes 11,16,17 ), and associated potential humps.…”

High-power sputtering employed for film deposition

“…Two areas of performance improvement are being developed in the MS technology. The first one is based on an increase in the degree of ionization of the sputtered flux [1][2][3][4][5][6][7], the second one is based on an increase in the temperature of the target surface up to the melting temperature [8][9][10][11][12][13]. The methods that ensure a high degree of ionization of sputtered atoms are called ionized physical vapor deposition (IPVD) [1].…”