Measurement and modelling of the bulk plasma in magnetron sputtering sources

Langmuir probes have been used to diagnose the plasma above the "race-track" of a magnetron plasma source and the measurements compared with a simple, one-dimensional fluid model. The model is developed initially with two fluid components: plasma ions (created by electron-impact ionization), which fall to the; sheath edge, and trapped thermal electrons whose motion is mobility limited in the crossed E and B fields. Predictions show that with increasing magnetic field strength or decreasing ionization rate, a peak in density can occur close to the cathode, consistent with experiment. The model is then extended to allow for the possibility that energetic electrons may emerge from the sheath into the plasma. The results show that the magnitude of the sheath-edge potential rises with increasing "fast" electron concentration. Above a critical "fast"-electron-to-ion density ratio, no sheath is predicted to form. Good agreement between experiment and prediction for the plasma density variation demonstrates the usefulness of the one-dimensional treatment as a first step to more complex modelling of the region. (C) 1997 Elsevier Science S.A.