Estimating electron drift velocities in magnetron discharges

Electron motion in magnetron discharges is complicated. In a first approximation, single particle motion can be considered in given electric and magnetic fields to estimate drifts. Based on magnetic and electric field measurements for discharges in an unbalanced magnetron with a strong magnet it is shown that, for the most energetic electrons, the ∇B and curvature drift velocities can be comparable to or even larger than the commonly mentioned E × B drift velocity. In the fluid approximation, the electron pressure gradient adds yet another drift component. Since all of those drifts are generally additive, the term “E × B drift” can be generically used but should be understood to include other drifts. Strong velocity gradients and direction reversal can be found, which suggest velocity shear as a source of waves and instabilities, likely creating the density-fluctuation “seeds” for ionization zones seen in high power impulse magnetron sputtering.

► The E × B drift velocity field was determined based on measured magnetic and electric fields.
► The gradient B and curvature drift velocities were estimated for the same magnetron.
► For energetic electrons, the gradient B and curvature drift may exceed the E × B drift.
► Strong velocity shear and direction reversal suggest seeding of instabilities.