Diagnostic of a pulsed CH4–H2 plasma to improve microwave plasma assisted chemical vapour deposition process for diamond synthesis

A microwave plasma assisted chemical vapour deposition (MWPACVD) process used for diamond growth was studied under continuous wave (CW) and pulsed mode. Depending on the plasma conditions, it is shown that the discharge exhibits two different regimes. One is characteristic of a resonant cavity and allows a stationary wave to be created in the centre part of the reactor. Thus the discharge can be placed in the middle of the tubular reactor, just above the treated substrate. Within the second regime, the microwave power is absorbed as soon as it enters the reactor, thus implying a displacement of the plasma ball on the side wall of the reactor. These two regimes are explained by considering the wave propagation according to the plasma parameters, especially the electron density and the collisions through the gas pressure. It is pointed out that the pulsed mode allows the input peak power to be higher than in CW operation and consequently to increase the H-atom density in the close vicinity of the substrate. Experiments with specific conditions have shed light on contradictory results dealing with pulsed MWPACVD processes.