Homogeneity characterization of a large microwave plasma

Microwave plasma reactor without magnetic field has been designed for large surface treatment or deposition. Using the surfaguide principle, a 2.45 GHz wave is coupled to the plasma created in a 12 cm tube diameter surrounded by a metallic cylinder of 18.8 cm diameter. The aim of this article is to characterize this plasma, particularly the spatial homogeneity, along the tube and mainly over its section. Argon and oxygen gases have been used. Pressure and microwave power were in the range 0.01–5 Torr and 200–2000 W. Wave propagation study has shown that mainly a plasma mode (surface wave) of azimuthal hexapolar symmetry (⁠$π/3$ periodicity) propagates. In argon gas, electron and ion densities $(1011–1012 cm−3),$ and electron mean energy (1–4 eV) locally measured by probes, are quite homogeneous over the plasma section. Emission spectroscopy provides excited states repartition. These external diagnostics have been performed along several axes to obtain quasi local information. Azimuthally, the excited states are quite homogeneous at low pressure, and weakly modulated (⁠$π/3$ periodicity) at higher ones. Radially, the profile depends on the electric field repartition. Actinometry has shown that the atomic oxygen density is rather homogeneous over the section, promising for future surface treatment applications.