Modeling and diagnostic of an SF6 RF plasma at low pressure

This research concerns the experimental and theoretical analysis of an SF₆ radio-frequency (RF) discharge at low pressure in a small reactor for industrial applications. The plasma is produced in the pressure range of 0.05-1 mbar by a 13.56-MHz RF supplier at moderate power, up to 25 W. The pumping system sustains a flowrate of about 50 cm/s, with residence time in the discharge of about 0.2 s. The discharge parameters have been measured and lie in the range n_e≃10 ⁷-10⁹ cm^-3, n_i≃10¹⁰-10¹² cm^-3, T _e≃5-10 eV. Measurements were performed by means of movable electrostatic probes and of a photodiode. 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 and neutral 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 us to pin down the relative importance of transport and bulk processes in the discharge state