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Pressure Broadening of H2O Absorption Lines in the 1.3 m Region Measured by Continuous Wave-Cavity Ring-Down Spectroscopy: Application in the Trace Detection of Water Vapor in N2, SiH4, CF4, and PH3
Volume 62, Number 12 (Dec. 2008) Page 1354-1362
Fiadzomor, Phyllis A.Y.; Baker, Derek M.; Keen, Anthony M.; Grant, Robert B.; Orr-Ewing, Andrew J.
A continuous wave cavity ring-down (cw-CRD) spectrometer has been developed for the measurement of trace levels of water vapor by absorption spectroscopy at wavelengths in the vicinity of 1358 nm and 1392 nm. The speed of data acquisition and selectivity make cavity ringdown spectroscopy potentially more useful than current techniques for measurement of trace water in process gases and vacuum environments used for semiconductor manufacture where water vapor contamination has a detrimental effect on the final product. The pressure broadening coefficients (γ) for bath gases N2, air, and Ar and semiconductor process gases SiH4, PH3, and CF4 were determined for a range of absorption lines in the 2ν1 and ν1 + ν3 bands of H2O. For the transitions investigated, the concentration of water vapor in the sample gas varied from 1.7 × 1012 to 2.9 × 1015 molecule cm−3 in N2 at a total pressure of <10 mbar and was mixed with the bath gas of increasing pressure up to ∼200 mbar. The values of γ quantify the reduction in peak absorption cross-sections with bath gas pressure and, thus, their effects on the detection limit of water vapor. For a CRD spectrometer with a ring-down time of τ = 12.0 μs measured with a precision of 0.6%, detection limits for the measurement of water vapor in 1 atm N2 and of CF4 were estimated to be 18 and 14 ppbv, respectively. Competing absorption by SiH4 and PH3 in the 1.3 μm wavelength region results in respective detection limits for water vapor of 98 and 319 ppbv (relative to 1 atm) in 0.2 atm SiH4 and 0.37 atm PH3.