A Drift Tube for Monitoring ppb Trace Water

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Abstract
This paper demonstrates the possibility of using a drift tube to monitor trace water in nitrogen gases for semiconductor fabrication processes. In the drift tube, ions are produced by corona discharge and drift at atmospheric pressure. The arrival-time spectra are observed for nitrogen gases with water concentrations at several ppb levels. The spectra show an obvious peak, which disappears when the water concentration increases to several tens of ppb. This suggests that the peak results from nitrogen cluster ions not reacting with water molecules. This is supported by the ion mobility calculated for the peak in the spectra, and also by related ion-molecule reaction kinetics. It is possible that the developed drift tube determines the water concentration at several tens of ppb levels or below from the intensity change in the arrival-time spectra.