Pressure Broadening of H 2 O Absorption Lines in the 1.3 μm Region Measured by Continuous Wave-Cavity Ring-Down Spectroscopy: Application in the Trace Detection of Water Vapor in N 2,SiH 4,CF 4,and PH 3

Abstract

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 ring-down 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 2v₁ and v₁ + v₃ 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.