Water vapor concentration measurements in high purity gases by means of comb assisted cavity ring down spectroscopy

In manufacturing processes of semiconductor industry accurate detection and monitoring of water vapor concentration in trace amount is of great importance. The ability to perform reliable measurements in ultra-pure gases, with a wide dynamic range and low uncertainty, can have a substantial impact on product quality and process performances. Here, we report on the development of a second-generation comb-assisted cavity ring-down spectrometer and present H2O mole fraction measurements in high-purity N2 gas. Based on the use of a pair of phase-locked lasers and referenced to an optical frequency comb synthesizer, the spectrometer allowed to record high-quality absorption spectra in coincidence with the 32,2→22,1 H2O transition at 1.3946μm. Retrieval of water mole fractions, at levels as low as 380 part per billion, was accomplished through a careful spectra analysis procedure based on the use of refined line shape models which include speed-dependent effects. Measurements were performed with a statistical reproducibility of 5 parts per billion, for an integration time of about 0.2 s. The noise equivalent and minimum detectable absorption coefficients were found to be 3.1 × 10−11 cm−1/Hz and 6.5 × 10−12 cm−1, respectively. This latter allowed for a minimum detectable water mole fraction (limit of detection) of 160 parts per trillion. Finally, the main sources of systematic uncertainty have been discussed and quantified.