Two-dimensional laser-induced fluorescence imaging of metastable density in low-pressure radio frequency argon plasmas with added O2, Cl2, and CF4

The effect of minor additions of O2, Cl2, and CF4 on the argon metastable relative density and spatial distribution in low-pressure, radio-frequency argon plasmas, generated within a parallel-plate Gaseous Electronics Conference reference reactor, has been investigated using planar laser-induced fluorescence imaging. For the conditions examined (33.3 Pa, 75–300 V, <10 W), the addition of only a few percent of these electron attaching gases was found to decrease the metastable density by as much as an order of magnitude, despite the fact that the excited-state argon emission indicated an increase in the metastable production rate. In the dilute O2/Ar discharges examined here, the spatial distribution of metastables was similar to that of the electropositive, pure argon cases, exhibiting a strong axial peak near the interface between the plasma bulk and the sheath at the powered electrode. In contrast, the addition of either Cl2 or CF4 was found to significantly modify the spatial distribution of the emission intensity and metastable density, resulting in a more symmetric and uniform axial metastable distribution. This change in metastable distribution for these mixtures was particularly apparent at lower powers and/or higher Cl2/CF4 concentrations, and suggests a transition from an electropositive to a somewhat electronegative discharge.