Development of a trace-moisture analyzer based on rapid-scan cavity ring-down spectroscopy

In the manufacturing processes of semiconductor devices, the requirements for gas purity control are becoming increasingly strict. Measurement and control of trace water vapor (trace moisture) remaining in the high purity gases is a challenging issue. The National Metrology Institute of Japan (NMIJ) has developed a simple technique for measuring trace water vapor based on cavity ring-down spectroscopy (CRDS) named “rapid-scan CRDS”, where the laser frequency is continuously varied by rapidly sweeping the drive current to record the absorption spectra of H2O. The rapid-scan CRDS has made it possible to determine moisture concentration at the nmol mol−1 level from the absorption spectra in 1 s. This paper outlines the rapid-scan CRDS and evaluates its performance by comparison with the trace-moisture standard in N2.