Gradual degradation of etch rate uniformity and etched shape was found in the etching of polysilicon using chlorine gas, where the stainless steel gas cylinder and gas tubes were electroytically polished, and the chlorine was highly purified. A large quantity of aluminum was detected from the sidewalls of the degraded polysilicon patterns by using the microprobe atomic electron microscopy. It is supposed that this degradation is due to the contamination by aluminum and that the origin of this contamination is the reaction between pure chlorine and anodized aluminum on the chamber wall. By mixing a very small amount of oxygen into the etching gas, aluminum contamination was suppressed and etching uniformity and etched shape were improved. From a thermodynamics calculation, it is revealed that, when chlorine gas or a gas containing chlorine atoms is used as an etching gas, if oxygen atoms are not present in the plasma, anodized aluminum is etched by the chlorine atoms. However, if a small amount of oxygen atoms or OH radicals exist, the surface of the anodized aluminum is protected from the chlorine atom attack.