Process instability in plasma etching of organic bottom-antireflection coating (BARC) using the plasma of O-2 mixed with halogen (Cl-2) or hydrogen halide (HBr, HCl) has been studied. From a series of process tests performed on a high density plasma etcher, the process instability has been found to be chamber-wall related as the BARC etch rate and critical-dimension bias shift when chamber coating changes. The process sensitivity to the chamber wall condition depends on the type of halogen-containing additive used and the O-2 percentage in the total gas feed. Through plasma diagnostics, the root cause of the process shift has been identified as the variation in the surface recombination rate of reactive free radicals with chamber wall condition. The recombination of O radicals to form O-2 is faster on alumina/anodized aluminum than on silicon oxide so that the O radical density becomes lower in the bulk plasma. The enhanced competing surface recombination of O and H radicals when HBr or HCl is used further lowers the O density, leading to a more significant process shift. Approaches to improve the process stability and performance consistency have been discussed.