Addition of CF4 into HBr-based plasma for polycrystalline-silicon gate etching reduces the deposition of an etch byproduct, silicon oxide, onto the chamber wall but tends to generate organic polymer. In this work, a detailed study has been carried out to analyze the mechanism of polymerization and to characterize the polymer composition and quantity. The study has shown that the polymer formation is due to the F-radical depletion by H atoms dissociated from HBr. The composition of the polymer changes significantly with CF4 concentration in the gas feed, and the polymer deposition rate depends on CF4% and other process conditions such as source power, bias power, and pressure. Surface temperature also affects the polymer deposition rate. Adding O-2 into the plasma can clean the organic polymer, but the O-2 amount has to be well controlled in order to prevent the formation of silicon oxide. Based on a series of tests to evaluate polymer deposition and oxide cleaning with O-2 addition, an optimized process regime in terms of O-2-to-CF4 ratio has been identified to simultaneously suppress the polymer and oxide deposition so that the etch process becomes self-cleaning.