Pressure Broadening of H₂O 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 N₂, SiH₄, CF₄, and PH₃

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 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 (gamma) for bath gases N(2), air, and Ar and semiconductor process gases SiH(4), PH(3), and CF(4) were determined for a range of absorption lines in the 2nu(1) and nu(1) + nu(3) bands of H(2)O. For the transitions investigated, the concentration of water vapor in the sample gas varied from 1.7 x 10(12) to 2.9 x 10(15) molecule cm(-3) in N(2) at a total pressure of <10 mbar and was mixed with the bath gas of increasing pressure up to approximately 200 mbar. The values of gamma 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 tau = 12.0 mus measured with a precision of 0.6%, detection limits for the measurement of water vapor in 1 atm N(2) and of CF(4) were estimated to be 18 and 14 ppbv, respectively. Competing absorption by SiH(4) and PH(3) in the 1.3 mum wavelength region results in respective detection limits for water vapor of 98 and 319 ppbv (relative to 1 atm) in 0.2 atm SiH(4) and 0.37 atm PH(3).

Development of a primary measurement standard for trace moisture in Ar

“…where T is the gas temperature (296 K), S(T) is the line strength at T, γ(T) is the broadening coefficient (half width at half maximum, HWHM) of H 2 O by the target gas at T, and k B is the Boltzmann constant. γ(T) for Ar is 4.106 × 10 −7 cm −1 Pa −1 [16], and S(T) is 1.5048 × 10 −20 cm [17]. By using these values and equation ( 15), we obtained x w = 0.58 nmol mol −1 .…”

“…respectively, equation ( 20) is thus introduced. A and B can be calculated with the quantity values of T/K = 296.0 ± 0.1, x w /(nmol mol −1 ) = 905 ± 14, γ Ar /(cm −1 Pa −1 ) = (4.11 ± 0.18) × 10 −7 [16], γ N2 /(cm −1 Pa −1 ) = (1.1181 ± 0.0067) × 10 −6 [17], S(T)/cm = (1.505 ± 0.012) × 10 −20 at T = 296.0 K [17], and x N2 /(mol mol −1 ) = 0.00999 ± 0.00014, where the number following the symbol ± represents the expanded uncertainty (k = 2). The numerical values of A and B obtained from the calculations were (1.72 ± 0.12) and (2.58 ± 0.12) × 10 −6 , respectively.…”

Trace-moisture measurement using cavity ring-down spectroscopy at NMIJ

“…This is a distinct advantage over other spectroscopic methods because the atmospheric moisture in the optical path outside the sample cell is a major source of uncertainty in the measurement of trace moisture in gases. Owing to these advantages, CRDS has become a standard tool for trace-moisture measurement [13][14][15][16][17][18][19][20][21][22][23][24][25][26]. NMIJ also began to develop measurement systems for trace moisture based on CRDS in 2009.…”

Quantitative Moisture Measurement with a Cavity Ring-down Spectrometer using Telecom Diode Lasers

“…Tunable diode laser absorption spectroscopy (TDLAS) is relatively simple and robust, therefore has been widely applied [8][9][10]. Cavity ringdown spectroscopy (CRDS) using cw diode lasers provides great enhancement in sensitivity and receives increasing interests in recent years [11][12][13][14][15]. The reported detection limit of water vapor varies from a few ppbv to several hundred pptv (part per trillion by volume).…”