Experimental study and simulations of electronegative discharges at low pressure

We have performed an experimental and theoretical analysis of the rf discharges at low pressure in a small reactor for industrial applications. In particular, we focused on the plasma produced from electronegative gases (air and SF6) in the low pressure range between 0.05 and 1 mbar. The device pumping system sustains a flow velocity of about 50 cm/s, with residence time in the discharge of about 0.2 s, at P=0.1 mbar. The discharge parameters have been measured by means of movable electrostatic probes and lies in the range ne≅107–109 cm−3, ni≅1010–1012 cm−3, and Te≅5–10 eV. Particular care in the analysis of the data proved necessary due to the presence of a substantial amount of negative ions. On the other hand, we have performed simulations of the discharge composition through the implementation of a suitable numerical model of the chemical kinetics in the device. On the basis of the experiments, a comparison of charged species production with those predicted by the theory was performed and a more realistic description of the involved phenomena was obtained. In particular, several assumptions concerning the ion diffusion coefficient were tested, allowing one to pin down the relative importance of transport and bulk processes in the discharge state.