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 (∼10 3 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.

Internal current measurements in high power impulse magnetron sputtering

“…Following the time-evolution of the internal currents it is seen in the middle panel of times the discharge current (~300-350 A) compared to the present investigation (60 A) [11]. The left panel of Figure 3a shows slightly less than a quarter of that value, which is consistent with our lower discharge current.…”

“…It might be that both the azimuthal and the cross-B current are increasingly concentrated in the region above the race track because the plasma density still rises there. Similar double peak structures were observed by Bohlmark et al [11] when investigating time-dependent variations of the magnetic field on the same device. The authors ascribe this phenomenon as related to an interplay between the electric field (-z) and the plasma pressure (+z), where the electric field is strongly reduced in the decay phase of the HiPIMS pulse, and thereby causing an expansion of the dense plasma torus.…”

“…Large azimuthal currents, circulating in the magnetic field trap close to the target surface, also arise due to the presence of the magnetic field [10]. Measurements by Bohlmark et al [11] show that the ratio between the azimuthal Hall current density, H J , and the cross-B discharge current density, D J  , in a typical HiPIMS process is surprisingly low, meaning a high speed in the electron transport across the magnetic field lines during the HiPIMS discharge. This so-called anomalous transport is known to also affect the transport of ions in this type of discharge [4] and has recently been investigated in more detail using electric probes [10] to estimate the azimuthal current, J  .…”

An ionization region model for high-power impulse magnetron sputtering discharges

“…The analytical fits used in the model are shown in figure 4(a). As standard parameters in the model we use r 1 = 2 cm, r 2 = 7 cm (based on the racetrack erosion area), z 1 = 0.1 cm (based on estimates of the sheath thickness) and z 2 = 3 cm (based on plasma density measurements in the same device by Bohlmark et al [17]). Figure 4(b) shows that quite a good current fit is already obtained with a time independent F PWR = 0.273.…”

A phenomenological model for the description of rotating spokes in HiPIMS discharges

“…[4,16,17]), but for this rough estimate the classical formula is assumed to hold in the azimuthal direction. The ionization coefficient k ion is evaluated using the following expressions [18,19] for the rate coefficients As concerns the excitation de-excitation term neglected in (4), a crude estimate of the rate coefficient (−k exc + k dexc n Ar * /n Ar ) can be carried out using a simple steady state balance for Ar* dn Ar * dt = 0 = k exc n e n Ar − k iz,Ar * n e n Ar * − k dexc n e n Ar * − k p n e n Ar * n Ar * = k exc k dexc + k p + k ion,Ar * that plugged into the neglected term in the first line of (4) gives…”