A new in-line sensor for trace moisture monitoring has been developed based on a piezoelectric quartz crystal microbalance (QCM) coated with a reactive metal thin film. The properties of the metal coating and QCM allow the sensor to have fast response and fast recovery to moisture in the process gas stream. The properties of the metal-coated QCM also allow the sensor to be compact so it can be in-line in the gas distribution system and readily integrated into the process tools. This paper presents the principle of moisture detection using this technology. The experimental results of accuracy, limit of detection (< 1 ppb), and error analysis of this sensor in parallel with atmospheric pressure ionization mass spectrometry (APIMS) are presented. The experimental setup and procedures are described. The effect of an upstream gas purifier on its rate of response was also studied and the results are presented and discussed. The applications of this technology in semiconductor processing are briefly mentioned.

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