Measurement of the magnetic field change in a pulsed high current magnetron discharge

Abstract

In this paper we present a study of how the magnetic field of a circular planar magnetron is affected when it is exposed to a pulsed high current discharge. Spatially resolved magnetic field measurements are presented and the magnetic disturbance is quantified for different process parameters. The magnetic field is severely deformed by the discharge and we record changes of several millitesla, depending on the spatial location of the measurement. The shape of the deformation reveals the presence of azimuthally drifting electrons close to the target surface. Time resolved measurements show a transition between two types of magnetic perturbations. There is an early stage that is in phase with the axial discharge current and a late stage that is not in phase with the discharge current. The later part of the magnetic field deformation is seen as a travelling magnetic wave. We explain the magnetic perturbations by a combination of E × B drifting electrons and currents driven by plasma pressure gradients and the shape of the magnetic field. A plasma pressure wave is also recorded by a single tip Langmuir probe and the velocity (∼103 m s−1) of the expanding plasma agrees well with the observed velocity of the magnetic wave. We note that the axial (discharge) current density is much too high compared to the azimuthal current density to be explained by classical collision terms, and an anomalous charge transport mechanism is required.