COLLISIONAL ELECTRON HEATING IN A RF DISCHARGE

The question of how far the concept of reflection of electrons at the oscillating sheath can be applied to the collisional situation is investigated. A radiofrequency discharge between two parallel plates is considered. Model calculations on the basis of the transport equations result in two contributions to the collisional sheath oscillation heating. The first contribution represents heating in an expanding sheath and cooling in a contracting sheath due to reflection at the sheath edge. The second contribution describes Joule heating within the sheath range and is of the same order of magnitude as the corresponding bulk heating. In the experiment a 300 kHz discharge is operated by a non-sinusoidal voltage with a steep rise and fall to a plateau. Optical emission spectroscopy resolved in time and space is applied. Owing to the special voltage shape, emission caused by radiofrequency heating can be separated from emission caused by secondary electrons from the cathode. The experimental results confirm the predictions of the model.