Drifting Ionization Zone in DC Magnetron Sputtering Discharges at Very Low Currents

Journal of Applied Physics

2017

Using movable emissive and floating probes, we determined the plasma and floating potentials of an ionization zone (spoke) in a direct current magnetron sputtering discharge. Measurements were recorded in a space and time resolved manner, which allowed us to make a three-dimensional representation of the plasma potential. From this information we could derive the related electric field, space charge, and the related spatial distribution of electron heating. The data reveal the existence of strong electric fields parallel and perpendicular to the target surface. The largest E-fields result from a double layer structure at the leading edge of the ionization zone. We suggest that the double layer plays a crucial role in the energization of electrons since electrons can gain several 10 eV of energy when crossing the double layer. We find sustained coupling between the potential structure, electron heating, and excitation and ionization processes as electrons drift over the magnetron target. The brightest region of an ionization zone is present right after the potential jump, where drifting electrons arrive and where most local electron heating occurs. The ionization zone intensity decays as electrons continue to drift in the Ez × B direction, losing energy by inelastic collisions; electrons become energized again as they cross the potential jump. This results in the elongated, arrowhead-like shape of the ionization zone. The ionization zone moves in the –Ez × B direction from which the to-be-heated electrons arrive and into which the heating region expands; the zone motion is dictated by the force of the local electric field on the ions at the leading edge of the ionization zone. We hypothesize that electron heating caused by the potential jump and physical processes associated with the double layer also apply to magnetrons at higher discharge power, including high power impulse magnetron sputtering.

Section: Introductionmentioning

confidence: 99%

Plasma potential of a moving ionization zone in DC magnetron sputtering

Panjan

Journal of Applied Physics

2017

“…Spokes can be found in high power impulse magnetron sputtering (HiPIMS), 1,2 as well as in the more traditional direct current magnetron sputtering (dcMS). 5 In HiPIMS, spokes drift in the E× B direction, i.e. the direction of the electron drift.…”

mentioning

confidence: 99%

“…As we know from probe measurements and fast frame camera imaging, 5,8,12 certain power and pressure conditions lead to one, two, or more spokes that are relatively stable as they move along the racetrack, while other parameter combinations exhibit less stability. Fig.…”

mentioning

confidence: 99%

Direct observation of spoke evolution in magnetron sputtering

Applied Physics Letters

Yang

2017

Ionization zones, also known as spokes, are plasma instabilities manifested as locations of intensified excitation and ionization over a sputtering magnetron's racetrack. Using a linear magnetron and a streak camera we were able to observe and quantify spoke dynamics. The technique allows us to image the onset and changes for both direct current magnetron sputtering (dcMS) and high power impulse magnetron sputtering (HiPIMS). Spokes in dcMS exhibit substructures. Spokes in HiPIMS are not stable as they shift along the racetrack, rather, they tend to grow or diminish, and they may split and merge. Their evolution can be interpreted in the context of localized electric fields and associated electron heating.

“…spokes 11,16,17 ), and associated potential humps. 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.…”

mentioning

confidence: 99%

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

Applied Physics Letters

2014

The fundamental question of how energy is supplied to a magnetron discharge is commonly answered by the Penning-Thornton paradigm invoking secondary electrons. Recently, Huo and coworkers (Plasma Sources Sci. Technol. 22, 045005 (2013)) used a global discharge model to show that electron heating in the electric field of the magnetic presheath is dominant over heating by secondary electrons. In this contribution, this concept is applied locally taking into account the electric potential structure of ionization zones. Images of ionization zones can and should be interpreted as diagrams of the localization of high electric potential and related electron energy.