The current-density distribution in a pulsed dc magnetron deposition discharge

Abstract

Using a carefully constructed magnetic probe (a B-dot probe) the spatial and temporal evolution of the perturbation in the magnetic field ?B in an unbalanced pulsed dc magnetron has been determined. The plasma was run in argon at a pressure of 0.74?Pa and the plasma ions sputtered a pure graphite target. The pulse frequency and duty were set at 100?kHz and 55%, respectively. From the ?B measurements (measured with magnitudes up to about 0.01?mT) the axial, azimuthal and radial components of the total current density j in the plasma bulk were determined. In the plasma 'on' phase, the axial current density jz has a maximum value of approximately 200?A?m?2 above the racetrack region, while high values in the azimuthal current density j? are distributed in a region from 1 to 3?cm into the bulk plasma with j? exceeding 350?A?m?2.In the 'off' phase of the plasma, jz decays almost instantaneously (at least within the 100?ns time-resolution of the ?B measurements) as the electric field collapses; however, j? decays with a characteristic time constant of about 1??s. This slow decay can be attributed to the presence of decaying Grad-B and curvature drifts, with their rates controlled by the decay in the plasma density. A comparison between axial and azimuthal current densities in the plasma 'on' time, when the plasma is being driven, strongly indicates that classical transport does not operate in the magnetron discharge.