Investigation of chemical kinetics and energy transfer in a pulsed microwave H-2/CH4 plasma

We present a modelling study of pulsed H-2/CH4 microwave plasmas obtained u nder moderate pressure discharge conditions in a tubular quartz reactor. Th e transport in the reactor was described using a Nusselt model for a radial ly quasi-homogeneous plasma. The thermal behaviour of the plasma was modell ed by distinguishing a single heavy species energy mode and the electron tr anslation mode. The chemistry was described using a 30 species-130 reaction model. The time variations of the electron energy distribution function, t he species concentrations and the gas temperature were determined by solvin g the coupled set of the electron Boltzmann equation, species kinetics equa tions and a total energy equation. Some of the results obtained from the pr esent model were compared to measurements previously carried out on the pla smas considered. Good agreement was obtained for the time variations of the gas temperature, the relative concentration of the H-atom and the intensit ies of the H-alpha and the argon 750 nm emission lines. The effect of the d uty cycle on the time-averaged composition and temperatures of the discharg e was also studied. Results showed that moderate pressure H-2/CH4 pulsed di scharges 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 variat ions of hydrocarbon species, ions and electrons are obtained when this para meter 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 d eposition processes. For this purpose we analysed the variation with duty c ycle of the time-averaged populations of the H-atom and CH3 that represent the key species for diamond deposition. Results showed that pulsed discharg es with small duty cycle, of typically less than 20%, lead to a substantial enhancement of the time-averaged dissociation yield. On the other hand, th e CH3 concentration exhibits a strong decrease with the duty cycle. The met hyl concentration in the investigated pulsed discharge is generally smaller than in continuous wave discharges obtained in the same reactor. These res ults indicate that short-pulse discharges would favour the formation of fil ms with higher Raman quality, while long duty cycle pulsed discharges would enable deposition at higher growth rates.