Moving-coil waveguide discharge for inner coating of metal tubes

The moving-coil waveguide discharge is described as a technique for inner wall coating of metallic tubes. A magnetized microwave plasma, which can be shifted inside a tube by means of the surrounding magnetic coil, is employed for plasma-enhanced chemical vapor deposition of hydrogenated carbon films (C:H) from methane. Process parameter studies at varying microwave power, magnetic field, discharge pressure, and gas flow have been performed with a locally fixed magnetic coil. The deposition was found to be more efficient at a magnetic field which is lower than the electron cyclotron resonance (ECR) field. The power absorption remains nearly constant while decreasing the maximum magnetic field from ECR condition to half of that. This can be understood in terms of the microwave cutoff in the waveguide filled with a magnetized plasma. A simplified model of the plasma is suggested resulting from the power balance of a steady-state discharge. Electron temperatures and plasma densities measured in argon plasmas by means of a single Langmuir probe at varying pressure are in good agreement with the model predictions, while the averaged deposition from a methane plasma cannot be derived simply from the modeled methyl density.