Properties of the plasma containing Cl-2, HBr, and O-2 species used for polysilicon etching change when the chamber wall is coated with organic polymer, causing the etch performance to drift. In this work, the plasma instability due to surface polymerization has been characterized through the comparison of free radical densities in an etch chamber coated with fluorocarbon polymer and other materials such as silicon oxide and anodized aluminum. The results of plasma diagnostics show that chlorine, bromine, and oxygen densities decrease after a polymer film is deposited onto the chamber wall. The root cause for the decrease in halogen radical densities has been attributed to the higher surface recombination rate of these free radicals on polymer surface, while the decrease in oxygen radical density is mainly caused by the consumption of oxygen in the cleaning of the polymer deposition. The impact of the plasma property change on etch performance has also been analyzed through etch rate and critical dimension (CD) evaluation. The lower free radical density in polymer-coated chamber generally lowers the etch rate and generates CD gain but the sensitivity of the process drift to the plasma property variation depends on process conditions. Approaches to reduce the effect of surface polymerization have been discussed.