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

Abstract

We present a modelling study of pulsed H2/CH4 microwaveplasmas obtained under moderate pressure discharge conditions in a tubularquartz reactor. The transport in the reactor was described using a Nusseltmodel for a radially quasi-homogeneous plasma. The thermal behaviour of theplasma was modelled by distinguishing a single heavy species energy mode andthe electron translation mode. The chemistry was described using a 30species-130 reaction model. The time variations of the electron energydistribution function, the species concentrations and the gas temperature weredetermined by solving the coupled set of the electron Boltzmann equation,species kinetics equations and a total energy equation. Some of the resultsobtained from the present model were compared to measurements previouslycarried out on the plasmas considered. Good agreement was obtained for thetime variations of the gas temperature, the relative concentration of theH-atom and the intensities of the Hα and the argon 750 nm emissionlines. The effect of the duty cycle on the time-averaged composition andtemperatures of the discharge was also studied. Results showed that moderatepressure H2/CH4 pulsed discharges obtained at duty cycles of less than20% show different behaviour than those obtained at higher duty cycles. Inparticular, while the plasma reaches the permanent periodic regime in lessthan 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 electronsare obtained when this parameter is greater than 20%. The model was also usedto determine if the use of a pulsed regime may bring some improvements inplasma-assisted diamond deposition processes. For this purpose we analysed thevariation with duty cycle of the time-averaged populations of the H-atom andCH3 that represent the key species for diamond deposition. Results showedthat pulsed discharges with small duty cycle, of typically less than 20%,lead to a substantial enhancement of the time-averaged dissociation yield. Onthe other hand, the CH3 concentration exhibits a strong decrease with theduty cycle. The methyl concentration in the investigated pulsed discharge isgenerally smaller than in continuous wave discharges obtained in the samereactor. These results indicate that short-pulse discharges would favour theformation of films with higher Raman quality, while long duty cycle pulseddischarges would enable deposition at higher growth rates.