Negative ions in a CCl2F2 radio frequency discharge

Abstract

The electron and negative ion densities in rf etching plasmas in pure CCl2F2 and its mixtures with argon have been measured using a microwave 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 likely F− and Cl2−). Typical densities reach 1017 m−3 for Cl− and 1015 m−3 for the electrons in a 30 mTorr discharge with 50 W rf input power. The decay time of the photodetached electrons towards the steady state gives an estimate for the effective electron attachment rate in the plasma. This rate (typically 2–5×10−15 m3s−1) appears to be substantially higher than the attachment rate to pure CCl2F2. Moreover it depends on plasma conditions, e.g., it increases with power and decreasing pressure. This is attributed to dissociative attachment to active species, such as radicals and vibrationally excited molecules, formed in the plasma. The total charge density in Ar/CCl2F2 mixtures is about ten times higher than that in a pure argon discharge, whereas 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 positive ion fluxes to the grounded electrode show that in an electronegative discharge the positive ion loss rate to the wall is about 10 times lower than that in an Ar plasma. This proves that the Bohm velocity in an electronegative plasma is much lower than that in an electropositive discharge.