Measurement of the plasma potential in a magnetron discharge and the prediction of the electron drift speeds

Abstract

Using an emissive probe the distribution of plasma potential Vp in the bulk plasma of a dc Magnetron discharge has been determined for a range of argon pressures (0.26, 0.53 and 0.78 Pa) and cathode voltages (between -236 and -338 V). The results reveal a large axial variation in the space potential in the confined plasma, with ΔVp~25 V over a distance of 5 cm, from plasma to sheath-edge. By combining the derived electric field with the modelled magnetic field, the distribution of single-particle drifts have been found, namely the electron E∧B, ∇B and curvature drift speeds. The predicted E∧B drift speeds (with values up to about 1.5×105 m s-1) are typically two to three times higher than the ∇B and curvature drifts. The Hall current channel is a broad region extending from above the `racetrack' down to a position close to the axis, 6 cm from the cathode. The calculated total Hall current is approximately five times the discharge current. Using a simple model of the discharge, in which there is no spatial variation in electron current density Je, the gyrofrequency to collision frequency ratio averaged over the plasma bulk is found to be ω/ν≈7.7±4.2. In an extension to the model, a possible distribution of electron current throughout the plasma is considered, which allows the determination of ω/ν locally in the bulk. Using this method, the maximum value of ω/ν is found to be about 25, however both models indicate that cross-field electron transport occurs more rapidly than from a classical prediction.