Importance of C₃H_y and C₃H_y⁺ in Modeling of Radio Frequency Methane Plasma

Methane is widely used in not only semiconductor industry but also diamond-like carbon and carbon nanotubes production. Radio frequency methane plasma was studied by Tachibana in 1984 [1], and some other researchers presented different models. In their models, however, species and related reaction steps were limited up to C₂H_y, C₂H_y⁺, and C₃H₈, although mass spectroscopy analysis showed that various C₃H_y/C₃H₅⁺ species were detected in the plasma. In the present study, methane plasma was studied by a capacitively coupled plasma modeling with 21 neutrals and 19 ions in the gas phase reaction, including some C₃H_y and C₃H_y⁺. One- and two-dimensional (1-D and 2-D respectively) simulation models were prepared to calculate two different gases: CH₄ and CH₄/H₂ (8/92 %). In the case of pure CH₄, CH₅⁺, C₂H₅⁺, and C₃H₅⁺ were abundant ions; among them, C₃H₅⁺ was the most major ion which was omitted in the past reports. It was proved that including C₃H_y and C₃H_y⁺ into the reaction set was essentially important to reproduce the experimental results. Time evolution of major ions and neutrals showed similar trends in both the 1-D and 2-D models, and continuous decrease of CH₄ in the plasma for a longer process time was similar to the experiment results by [1].