The 2D plasma potential distribution in a HiPIMS discharge

Abstract

Using an emissive probe the spatial-temporal distribution of the plasma potential Vp in a high power impulse magnetron sputtering discharge has been measured. The magnetron (with a planar circular titanium target) was operated in argon gas at a fixed pressure of 0.54 Pa, a pulse frequency of 100 Hz with a 100 µs on-time and average power of 650 W. In the early part of the voltage pulse (∼6–8 µs), Vp attains deep negative values (∼−150 V) at positions close to the target (10 mm) and above the racetrack, diminishing with distance, but never reaching ground potential, even at excursions of 80 mm. In the confined plasma region, extraordinarily high axial and radial electric field components, up to several kV m−1, are calculated from the plasma potential measurements.As the plasma develops and the discharge current reaches a maximum (at ∼40 µs), Vp is elevated everywhere in the plasma, however, still with deep negative values (down to −40 V) at positions closest to the target. From the derived electric fields and modelled (vacuum) magnetic field, the 2D distribution of E × B electron drift velocities has been determined. During the early discharge phase, a broad drift channel is predicted above the racetrack, with drift speeds up to ∼3 × 105 ms−1 centred ∼30 mm above the target racetrack. As the discharge develops, these speeds reduce by about a factor 3 and the centre of the velocity distribution moves further away from the target and inwards towards the discharge axis, resembling that observed in dc and pulsed-dc magnetron operation.