Investigation of chemical kinetics and energy transfer in a pulsed microwave H2/CH4 plasma

We present a modelling study of pulsed H-2/CH4 microwave plasmas obtained under moderate pressure discharge conditions in a tubular quartz reactor. The transport in the reactor was described using a Nusselt model for a radially quasi-homogeneous plasma. The thermal behaviour of the plasma was modelled by distinguishing a single heavy species energy mode and the electron translation mode. The chemistry was described using a 30 species-130 reaction model. The time variations of the electron energy distribution function, the species concentrations and the gas temperature were determined by solving the coupled set of the electron Boltzmann equation, species kinetics equations and a total energy equation. Some of the results obtained from the present model were compared to measurements previously carried out on the plasmas considered. Good agreement was obtained for the time variations of the gas temperature, the relative concentration of the H-atom and the intensities of the H-alpha and the argon 750 nm emission lines. The effect of the duty cycle on the time-averaged composition and temperatures of the discharge was also studied. Results showed that moderate pressure H-2/CH4 pulsed discharges obtained at duty cycles of less than 20% show different behaviour than those obtained at higher duty cycles. In particular, while the plasma reaches the permanent periodic regime in less than 2 pulse-periods, i.e. 60 ms, for duty cycle values of less than 20%, long-time-scale density variations of hydrocarbon species, ions and electrons are obtained when this parameter is greater than 20%. The model was also used to determine if the use of a pulsed regime may bring some improvements in plasma-assisted diamond deposition processes. For this purpose we analysed the variation with duty cycle of the time-averaged populations of the H-atom and CH3 that represent the key species for diamond deposition. Results showed that pulsed discharges with small duty cycle, of typically less than 20%, lead to a substantial enhancement of the time-averaged dissociation yield. On the other hand, the CH3 concentration exhibits a strong decrease with the duty cycle. The methyl concentration in the investigated pulsed discharge is generally smaller than in continuous wave discharges obtained in the same reactor. These results indicate that short-pulse discharges would favour the formation of films with higher Raman quality, while long duty cycle pulsed discharges would enable deposition at higher growth rates.