Optical Emission from C₂^– Anions in Microwave-Activated CH₄/H₂ Plasmas for Chemical Vapor Deposition of Diamond

Visible emission from C₂⁻(B₂Σ_u⁺) anions has been identified underlying the much strongerSwan band emission from neutral C₂(d^₃Π_g) radicals (henceforth C₂⁻* and C₂*, respectively) in MW-activated C/H/(Ar) plasmas operating under conditions appropriate for the chemical vapour deposition (CVD) of diamond. Spatially resolved measurements of the C₂⁻* and C₂* emissions as functions of the C/H/(Ar) ratio in the input gas mixture, the total pressure, and the applied MW power, together with complementary 2-D(r, z) plasma modelling, identifies dissociative electron attachment (DEA) to C₂H radicals in the hot plasma as the dominant source of the observed C₂⁻* emission. Modelling indicates substantially higher concentrations of C₂H⁻ anions (from analogous DEA to C₂H₂) in the near-substrate region, but also suggests that the anion number densities will typically be 3–4 orders of magnitude lower than those of the electrons and partner cations, i.e. mainly C₂H₂⁺ and C₂H₃⁺. The identification of negatively charged carbon-containing species in diamond CVD plasmas offers a possible rationale for previous reports that nucleation densities and growth rates can be enhanced by applying a positive bias to the substrate.