NEGATIVE-IONS IN A CCL2F2 RADIO-FREQUENCY DISCHARGE

The electron and negative ion densities in rf etching plasmas in pure CCl2F2 and its mixtures with argon have been measured using a microwav e resonance technique in combination with photodetachment. It has been found that the dominant negative ion is Cl-. These ions are about 100 times more abundant than electrons and other negative ions (most like ly F- and Cl-2(-),). Typical densities reach 10(17) m(-3) for Cl- and 10(15) m(-3) for the electrons in a 30 mTorr discharge with 50 W rf in put power. The decay time of the photodetached electrons towards the s teady state gives an estimate for the effective electron attachment ra te in the plasma. This rate (typically 2-5 x 10(-15) m(3) s(-1)) appea rs to be substantially higher than the attachment rate to pure CCl2F2: Moreover it depends on plasma conditions, e.g., it increases with pow er and decreasing pressure. This is attributed to dissociative attachm ent to active species, such as radicals and vibrationally excited mole cules, formed in the plasma. The total charge density in Ar/CCl2F2 mix tures is about ten times higher than that in a pure argon discharge, w hereas the electron density is ten times lower. This effect is due to the presence of negative ions. A transition between an electropositive and an electronegative discharge occurs when a small amount of CCl2F2 is added to a pure Ar plasma. Measurements of energy resolved positiv e ion fluxes to the grounded electrode show that in an electronegative discharge the positive ion loss rate to the wall is about 10 times lo wer than that in an Ar plasma. This proves that the Bohm velocity in a n electronegative plasma is much lower than that in an electropositive discharge. (C) 1995 American Vacuum Society.