The residual impurity gases in the atmospheric pressure processing equipment are becoming an important factor in the submicron ultralarge scale integrated industry. The present article includes three parts: (1) Ti-coated wafer for monitoring of trace O2, (2) O2 sensor and residual gas analyzer experimental study for real time monitoring of trace O2 under various purging conditions, and (3) dynamic investigation of trace gaseous impurity dependence on purging gas flow rate and gas type in various rapid thermal processing (RTP) chambers. Ti-coated wafer discoloration is used widely for trace O2 monitoring in the RTP industry. A quantitative relationship between the discoloration of a Ti-coated wafer and O2 concentration was obtained in this study under the conditions of O2 contents of 0, 0.2, 1, 3, 5, 10, and 100 ppm in an N2 ambient, indicating that the discoloration sensitivity is about 3 ppm. This article presents a semiempirical purging equation, called the pseudo-PST (“perfectly stirred tank”) model, which gives the dependence of the trace gaseous impurity on purging time, process gas flow rate, and chamber volume in an atmospheric processing system. The model has been tested in five chamber configurations, including rectangular and cylindrical chambers, with and without inlet gas diffusers. A correction factor, γ, called perfectly stirring coefficient was used in the modeling equations. The γ values of 0.71–1.49 have been determined for different chamber configurations. The agreement between experimental and modeling results has been confirmed. The effect of wafer rotation, gas type, gas trap, and air leaks on impur- - ity purging are also discussed. © 2003 American Vacuum Society.