Radio-frequency glow discharges in methane gas: modelling of the gas-phase physics and chemistry

Abstract

A methane discharge fluid model is developed, and subsequently combined with a simple gas-phase chemical kinetics model. The aim is to provide better understanding of the charged species dynamics, and their interaction with the gas-phase kinetics in a CH4 plasma. Swarm data are used as input in the fluid model, which predicts the ion and electron densities, electric fields and ionization rates as a function of space and time in the radio-frequency period. Results show that, due to detachment, the negative ion density in CH4 is of order 10-2 that of electrons; a capacitive discharge behaviour is observed analogous to that of an electropositive gas. The effects of electrode spacing (2-6 cm), gas pressure (80 mTorr to 1 Torr) and radio-frequency current (2.2-3.4 mA cm-2 0.06-0.15 W cm-2) are studied and compared successfully with experimental data. The time-averaged, spatially-resolved electron density and energy, the set of cross sections for CH2 and CH3 dissociation, together with an assumption about the form of the electron energy distribution function, are subsequently used as input in a simplified one-dimensional gas-phase kinetic model. The model predicts the CH2 and CH3 spatial profiles, which compare well with experimental data.

Fluid Modeling of the Conversion of Methane into Higher Hydrocarbons in an Atmospheric Pressure Dielectric Barrier Discharge

“…A methane plasma has a very strong electropositive character. The negative ion densities are about one order of magnitude lower than the electron and the positive ion densities40, 41 and therefore, negative ions are not incorporated in the model. Although some vibrational excitation reactions are included in the model, vibrationally excited species are not taken into account separately in order to limit the number of species and reactions.…”

“…Modeling results on gas phase physics and chemistry in different kind of methane containing discharges have been investigated extensively during the past thirty years 39–57. A minority of them concerns the modeling of the plasma as a gas conversion reactor 45, 48, 51, 53, 56.…”

Predicting the amount of carbon in carbon nanotubes grown by CH4 rf plasmas

“…The detail of the numerical scheme is based on earlier works. [18][19][20][21] The number densities n j of electrons, neutrals and ions ͑subscript j represents species͒ are calculated spatiotemporally by the following continuity equations:…”

“…22 The reaction rate coefficients k for H 2 , CH 2 , CH 3 , C 2 H 2 , C 2 H 4 , C 2 H 5 , C 2 H 6 were estimated from their reaction cross sections. 10,23,24 The mobilities and diffusion coefficients of charged species, except electron mobility, the secondary electron emission coefficient ͑␥ ion = 0.01͒, the initial energy for secondary electrons ͑=1.0 eV͒, the detachment coefficient ͑=3.69ϫ 10 −11 cm 3 /s͒ and the recombination coefficient ͑=0.0 cm 3 /s͒ are cited from Gogolides et al 20 D j of neutral species in the CH 4 plasma were calculated as referred to in Herrebout et al 25 An overview of the species ͑nonradi-cal neutrals, ions, radical neutrals͒ taken into account in the present model is given in Table II. The reaction processes among electrons, ions, excited atoms and molecules taken here and their reaction rate coefficients are listed in Tables III-V: 28 for electron-neutral, 15 for ion-neutral, and 25 for neutral-neutral reactions.…”

Deposition of a-Si:H and a-C:H using an expanding thermal arc plasma

“…Hence, we have to conclude that the surface is hydrogenated, leading to a smaller effective sticking probability (a similar reason would hold for the radical CH [37]) which depends on the exact surface state. Since the refractive index is almost independent of the substrate temperature in the range 50-150…”