The National Metrology Institute of Japan (NMIJ) has developed a “Multi-gas Trace-moisture Generator” that can generate trace-moisture standards for various types of gas used in semiconductor industry. This system mainly consists of two parts: a generation chamber based on the diffusion tube method and a two-stage dilution system. In the generation chamber, water vapor that evaporates from the diffusion tube is mixed with dry nitrogen gas to obtain 100 μmol/mol (ppm). This gas mixture is further diluted at two stages. In the first stage, it is mixed with dry nitrogen gas. Then, this mixed gas is partially exhausted. The remaining gas is mixed with dry target gas in the second stage, to obtain expected moisture concentration.
In this study, we introduced dry argon gas in the second stage and generated trace moisture for argon in the range of 10 nmol/mol (ppb) to 1 ppm. A commercial moisture analyzer based on cavity ring-down spectroscopy (CRDS) was connected to the system as a device under test (DUT), and uncertainty associated with this CRDS-based DUT was evaluated. The readings of CRDS moisture analyzer were fluctuating in the range of ±10 % from the average, even though such fluctuation was not observed in the measurement of moisture in nitrogen. We identified the cause of this fluctuation and improved the stability of the readings.
Minami Amano has been a researcher at the NMIJ/AIST since 2010, and responsible for the development of trace-moisture standards for nitrogen, argon, oxygen and helium using Multi-gas Trace-moisture Generator. In 2015, she established a trace-moisture standard in argon gas in the range between 10 nmol/mol and 1 μmol/mol. She received the degree of Ph.D. in Engineering from the University of Tsukuba in 2017.