The present invention relates to an in-line detector system (200) and method for detecting impurity concentration in a flow gas stream in real time. In certain aspects, the system includes an inline monitoring system that determines the calibration concentration of impurities in the flow gas stream in a low concentration area below a predetermined concentration value and a high concentration area above a predetermined concentration value. The sensor coupling 214 interconnects the sensing means 412 and the purifier 208 to monitor the impurity level of the gas stream. The switching valve 230 is interposed in the flow loop between the purifier 208 and the sensor 412 to selectively send purified or uncleaned gas from the gas stream to the impurity sensing means. The system can use a hygrometer-type sensor when a significant impurity is water, or a surface acoustic wave (SAW) device coated with an appropriate film having affinity for an impurity. The system is particularly useful for monitoring low impurity concentration levels (e.g., from 0.1 ppm to about 100 ppm) in the gas stream employed in vapor state fixation, such as chemical vapor deposition, in the manufacture of semiconductor devices.
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