Variation of etch rates and linewidths due to various seasoning methods after plasma cleaning of an etch chamber in a poly-Si/oxide (SiO2) etching process are studied. An HBr/O-2 based inductively coupled plasma (ICP) was investigated to determine the root cause and mechanisms of the variation. Poly-Si main-etch rates remain nearly constant after seasonings with bare-Si etching (hereafter, we denote the seasoning as Si seasoning) succeeding plasma cleaning of the chamber wall with an SF6 based ICP. Poly-Si over-etch rates increased and leveled off as the Si-seasoning time increased, whereas SiO2 etch rates drastically varied with increased Si-seasoning time. The optically estimated density of SiBr in the SiO2 etching plasma was found to increase with increased Si-seasoning time. SiO2 etch rates also varied as a function of O-2 flow. rate and bias power to the bottom. electrode of the ICP etcher. These results suggest that etch rates of poly-Si and SiO2 are influenced by reaction products such as SiBrx (x = 1,2,3) in the plasma. Critical-dimension (CD) shifts of poly-Si linewidths increased after Si seasonings. On the other hand, when short-time (about 10 s) plasma cleaning of the chamber was performed between etchings in the Si seasoning, CD shifts hardly increased or slightly decreased. Poly-Si over-etch rates also hardly increased with the short-time plasma cleaning. The results suggest that reaction products derived from deposition on the chamber wall would lead to the increase of CD shifts of poly-Si linewidths and poly-Si over-etch rates.