Asymmetric particle fluxes from drifting ionization zones in sputtering magnetrons

Abstract

Electron and ion fluxes from direct current and high-power impulse magnetron sputtering (dcMS and HiPIMS) plasmas were measured in the plane of the target surface. Biased collector probes and a particle energy and mass analyzer showed asymmetric emission of electrons and of singly and doubly charged ions. For both HiPIMS and dcMS discharges, higher fluxes of all types of particles were observed in the direction of the electrons' E × B drift. These results are put in the context with ionization zones that drift over the magnetron's racetrack. The measured currents of time-resolving collector probes suggest that a large fraction of the ion flux originates from drifting ionization zones, while energy-resolving mass spectrometry indicates that a large fraction of the ion energy is due to acceleration by an electric field. This supports the recently proposed hypothesis that each ionization zone is associated with a negative-positive-negative space charge structure, thereby producing an electric field that accelerates ions from the location where they were formed.

Plasma potential of a moving ionization zone in DC magnetron sputtering

“…Such an observation was in fact made in Ref. 56 where we measured the same ion flux asymmetry for DCMS and HiPIMS discharges despite different directions of ionization zone motion for the two discharge modes.…”

“…46 More specifically, it was suggested that electrons gain energy when crossing from the lowpotential side of the ionization zone to the high-potential side. These concepts have been so far supported by particle energy measurements 56 and by spectroscopically resolved snapshot-images of ionization zones. 57 Electron energization ("heating") is important since the energetic electrons produce ion-electron pairs and are therefore critical to sustaining the discharge.…”

“…Indeed, such asymmetry was observed in our earlier experiments. 56 The asymmetry was thought to be associated with a moving potential hump. 39 Data presented here (e.g., Fig.…”

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

“…18 Fast imaging of HiPIMS [2][3][4]11,19 and direct current magnetron sputtering 20 (dcMS) plasmas indicated that ionization rate and emission of light and particles are usually not uniformly distributed but concentrated in bright zones that move along the racetrack. Additionally, measurements of charged particle fluxes have revealed important features: (a) ion and electron fluxes appear in short pulses (jets), 21 (b) doubly charged ions have approximately twice the energy of singly charged ions, 18,21 and (c) ions emitted in the E× B direction have several 10 eV higher energy than ions going in the opposite direction, i.e., leaving the magnetron in the −E× B direction. 18 All of these features can be explained 18 assuming that an ionization zone is a locations of a potential hump of several 10 V. Consistent with Maxwell's equations, each potential hump is surrounded by an electric double layer.…”

Phase tailoring of tantalum thin films deposited in deep oscillation magnetron sputtering mode

“…The time-averaged ion energy distributions (IEDFs) for 181 Ta + , 181 Ta 2+ , 40 Ar + and 40 Ar 2+ ion species measured from the DOMS plasma obtained at different peak powers for a MPPMS discharge. The Ar 2+ IEDs (figure 2b) exhibit a low energy peak centred close to 9 eV for both processes although the number of ions is almost one order of magnitude higher in DOMS than in DCMS.…”