Diagnostic of a pulsed CH4-H-2 plasma to improve microwave plasma assistedchemical vapour deposition process for diamond synthesis

A microwave plasma assisted chemical vapour deposition (MWPACVD) process us ed for diamond growth was studied under continuous wave (CW) and pulsed mod e. Depending on the plasma conditions, it is shown that the discharge exhib its two different regimes. One is characteristic of a resonant cavity and a llows a stationary wave to be created in the centre part of the reactor. Th us the discharge can be placed in the middle of the tubular reactor, just a bove the treated substrate. Within the second regime, the microwave power i s absorbed as soon as it enters the reactor. thus implying a displacement o f the plasma ball on the side wall of the reactor. These two regimes are ex plained by considering the wave propagation according to the plasma paramet ers, especially the electron density and the collisions through the gas pre ssure. It is pointed out that the pulsed mode allows the input peak power t o be higher than in CW operation and consequently to increase the H-atom de nsity in the close vicinity of the substrate. Experiments with specific con ditions have shed light on contradictory results dealing with pulsed MWPACV D processes. (C) 2001 Elsevier Science B.V. All rights reserved.