Quantitative Analysis of Trace Water in Highly Puified Nitrogen Gas by Atmospheric Pressure Ionization Mass Spectrometer

Quantitative analysis of trace water, at ppb level and below, in highly purified nitrogen gas used in semiconductor fabrication processes was investigated using an atmospheric pressure ionization mass spectrometer (APIMS). Until recently, the trace analysis of strongly adhesive water was difficult because of the lack of a standard gas for calibration and high background water levels. The standard gas was prepared by using a gas containing a ppm level of water evaporated from a water bottle, and diluting it 105 times using a two-step dilution system. To reduce the background, a simplified SUS316L electropolished gas line with no valves, mass flow controllers or connectors was assembled. Using this method, a calibration curve applicable to the low ppm to ppt range was obtained. The background obtained was 40 ppt. The curve was fitted with a theoretical curve calculated from ion-molecule reactions.