Optical computerized tomography of the E-H transition in inductively coupled plasmas in Ar and Ar-CF₄ mixtures

We have applied optical emission spectroscopy together with computerized tomography (CT) to study the transition between E and H modes in inductively coupled plasma. Measurements were performed in pure Ar and in CF₄(5%)/Ar mixture in the pressure range between 5 and 300 m Torr. It was found that the radial and azimuthal CT images of the E (capacitively coupled) mode are quite different from those of the H (inductively coupled) mode. The E mode structures show clearly the presence of capacitive coupling, while the structure of the H mode corresponds to that of the predicted induced fields. The differences diminish at low pressures due to the non-local transport of electrons. The contributions of direct and stepwise excitation are separated by using two different energy levels, 2p₁ and 2p₉, of Ar. In addition, the measurements obtained in electronegative gas mixture with large losses of electrons of moderate and small energy show more clearly the kinetics of excitation in the two modes and the regions where high-energy electrons are produced.