Electrostatic probe measurement of the negative ion fraction in an SF6 helicon discharge

The ratio of the negative ion density to the electron density has been dete rmined using a novel two-probe technique in the diffusion chamber of an SF6 helicon reactor. The Bohm flux (as modified by negative ions) was measured using a guarded planar probe, while the electron thermal current was obtai ned using a small cylindrical probe. The negative ion concentration was the n determined from the ratio of these two currents. Results obtained with th is simple technique show that the plasma in the diffusion chamber is divide d radially into three regions. The central region contains hot electrons fr om the source that are confined by the magnetic field, positive ions being created through impact ionization, and a large proportion of negative ions (n(-)/n(e) approximate to 5). The edge region is a positive ion-negative io n plasma having a negligible electron density. These two regions are separa ted by a transition layer with a potential drop of approximate to 3 V. This layer performs some of the functions of a sheath. Consequently, the sheath at the chamber walls may be quite small since the thermal fluxes of positi ve and negative ions are nearly equal. The negative ion temperature is foun d to be approximate to 0.5 eV, which is much higher than the neutral gas te mperature.