Hα emission as a feedback control sensor for reactive sputter deposition of nano-structured, diamond-like carbon coatings

This paper discusses the potential use of the hydrogen atomic line emission at 656.3 nm (H_α) as an effective in-situ sensor for a closed-loop control system to improve the reproducibility of reactive sputter deposition of nano-structured, metal-containing, hydrogenated, diamond-like carbon (Me:DLC:H) coatings. The paper includes experimental results showing a good correlation between H_α emission in the process plasma and the formation of metal-carbide, an important component of these coatings. The first attempts at actual feedback-control of the process showed that this sensor can be effective, at least over the stage of the deposition when mostly carbides are formed in the coating. A spectrally resolved analysis of the H_α emission for the various stages of the deposition have shown that the emission profile is dominated by a "hot" component (∼10-20 eV), which can be attributed to dissociative excitation of molecular hydrogen (H₂). The molecular hydrogen is understood to evolve from the coating as a result of carbon incorporation from the reactive gas (C₂H₂) and is particularly sensitive to metal carbide formation in the film, when most of the hydrogen is released from the surface in molecular form.