H _α emission as a plasma vapor deposition control sensor for tribological, diamond-like carbon coatings [1]

To improve the reproducibility of metal-containing, hydrogenated diamond-like carbon (Me-DLC:H) coatings deposited by physical vapor deposition (PVD) techniques such as reactive sputtering, various plasma probes have been studied as potential in-situ sensors integrated with a closed-loop deposition control system [2, 3]. In a series of Design-of-Experiment (DoE) test runs on an unbalanced magnetron sputtering setup, the plasma probes were tested for their sensitivity to process input factors and as candidates for a feedback control system. These include optical emission and absorption spectroscopy of metal atoms, conventional and electron emissive Langmuir probes, and hydrogen atomic line emission at 656.3nm (H□). An important result of these studies was that Ha is potentially an ideal control sensor, because of the apparent high sensitivity of the total emission to changes in the film deposition chemistry that tend to occur naturally during the course of deposition of the functional coatings. A more detailed analysis of the spectral profile of H _α for the various regimes of the deposition have shown that the emission is dominated by a "hot" component (∼ 20 eV), which can be attributed to dissociative excitation of molecular hydrogen. Since electron collisional dissociation of hydrocarbons in the plasma can only produce atomic hydrogen (the observed "cold" component), it is inferred that the Ha is a measure of hydrogen generation in the film, thus confirming the concept that it can be used as a deposition chemistry probe. As such, it can take the form of a compact photo-detector with an H _α interference filter, providing economy, as well as fast time response. Another benefit of H _α emission that it can be used as a sensor for any type of DLC:H, including nano-structured (carbide-containing) Me:DLC:H coatings, produced for tribological applications.