Spectroscopic studies of conventional and active screen N2–H2 plasma nitriding processes with admixtures of CH4 or CO2

Abstract

Low-pressure pulsed dc H2–N2 plasmas with admixtures of CH4 or CO2 used for conventional (CPN) and active screen plasma nitriding (ASPN) were studied by infrared absorption and optical emission spectroscopy (OES) techniques. The experiments were performed in two modes: (i) the plasma at an internal model probe, driven by a bias voltage operated only, representing a CPN approach, and (ii) the screen plasma operated only, which corresponds to an ASPN experiment. Combining in situ tunable diode laser absorption spectroscopy with ex situ Fourier transform infrared spectroscopy the evolution of the concentrations of the methyl radical and of eight stable molecules, C2H2, CH4, C2H4, CO, CO2, NH3, HCN and H2O was monitored. The degree of dissociation of the carbon-containing precursor molecules varied between 40% and 98%. The methyl radical concentration was found to be in the range 1011–1012 molecules cm−3. By analysing the development of molecular concentrations with changes in gas mixtures and plasma power values, it was found that (i) C2H2, HCN and NH3 were the main products of plasma conversion in the case of methane admixture and (ii) CO, HCN and NH3 in the carbon dioxide case. The fragmentation efficiencies of methane and carbon dioxide (RF (CH4) ≈ (0.5–4) × 1016 molecules J−1, RF (CO2) ≈ (0.5–3.4) × 1016 molecules J−1) and the respective conversion efficiencies to the product molecules (RC (product) ≈ 1013–1015 molecules J−1) were determined for different gas mixtures and plasma power values. With the help of OES the rotational temperature of the screen plasma could be determined, which increased with power from 600 to 850 K. Also with power the ionic component of nitrogen molecules, i.e. the intensity of the -(0–0)-band, increased strongly in relation to the intensity of the neutral component, represented by the N2-(0–2)-band.