Discharge dynamics, plasma kinetics and gas flow effect in argon–acetylene discharges

Abstract

We investigated capacitively coupled Ar/C 2 H 2 RF plasmas with a 1D fluid model that couples a 13.56 MHz discharge module, a long timescale chemical module and a flow transport module. A new solution procedure was developed in order to accurately describe the coupling between the short timescale discharge dynamics and the long characteristic time processes that play a major role in the molecular growth of reactive species. The plasma was simulated for different inlet gas configurations and flowrates. We showed that for a showerhead configuration one may distinguish two situations. For short residence time the plasma was strongly electronegative in the very center of the discharge gap and dominated by large hydrocarbon positive and negative ions. In this situation the acetylene conversion, although moderate, lead to a significant molecular growth. For long residence time, although C 2 H 2 underwent a total conversion, the products of the primary C 2 H 2 dissociation process were consumed by surface deposition which reduced drastically the molecular growth in the short gap discharge considered here. Whatever the conditions, we confirmed the key-role of Ar* in the acetylene conversion, ionization kinetics as well as the subsequent molecular growth for neutral and charged species. We also showed that remote feed gas and showerhead configurations predicted similar results at low flowrate. At larger flowrate the two configurations presented some discrepancy. Especially H 2 density was much larger for the remote feed gas configuration, which affected the overall plasma behavior. Our results highlight that realistic gas-flow models are essential for an accurate description of acetylene conversion in Ar/C 2 H 2 plasma.

Molecular growth paths and dust‐particles nucleation precursors in Ar/C2H2 low pressure discharges

“…The obtained plasma composition is used to simulate a new discharge dynamic and the procedure is iterated until stable plasma composition and electron dynamics are obtained. [24] This simulation procedure enables one overcoming the time stiffness that characteristises Ar/C 2 H 2 plasma. This stiffness is due to the coupling between fast electron processes and slow chemistry.…”

“…The term F s that appears when taking into account gas flow effects includes a convective flux divergence term and a source term corresponding to the radial convective losses that characterize stagnation line flows. [24] These equations are coupled with the following electron average-energy equation:…”

“…The considered equations are derived by time-averaging the species continuity equations over one RF period. [24] These may be expressed as:…”

“…Due to the resulting stagnation flow configuration, the flow exits radially outside the gap region. The flow regime has been discussed in Tetard et al [24] where we compared results assuming stagnation line theory flow and two continuously stirred tank reactor (CSTR) models corresponding to two asymptotic values of the discharge volume to the reactor volume ratios, that is…”

“…Negative species do not interact with the surface as they are trapped in the bulk of the discharge. Boundary conditions for electron density and temperature are detailed in Tetard et al [24] T A B L E 2 Molecular growth process considered in low-temperature argonacetylene plasmas Process Rate constant (m 3 mol −1 s −1 ) Number…”