Industries as diverse as semiconductor fabrication and aluminum smelting can benefit from the simplicity and ppt sensitivity of Wavelength-Scanned Cavity Ring Down Spectroscopy.

In pristine fabrication processes such as front-end semi, several trace gas species must be completely purged from the ambient air. This includes airborne molecular contaminants like NH₃ that can impact yields, as well as toxic species such as H₂S, HCl, and HF, which can cause safety issues and potentially damage expensive optics. Scrubbers are effectively used to eliminate several of these species, but the purity of the filtered air must then be verified and monitored by trace gas analysis instruments. A new tool —WS-CRDS (wavelengthscanned cavity ring down spectroscopy) — is the first technology to combine high speed data sampling with sensitivity to the parts per trillion level, in a hands-free platform that does not require frequent calibration. This article reviews this new technology, compares it to traditional methods, and presents data for NH₃, HF, and H₂S.

WS-CRDS BACKGROUND
Nearly every small molecule (e.g., H₂O, H₂S, NH₃) has a unique near-infrared absorption spectrum. At sub-atmospheric pressure, this consists of a series of narrow, well-resolved sharp lines, each at a characteristic wavelength. Because these lines are well-spaced and their wavelength is wellknown, the concentration of any species can be determined by measuring the strength of this absorption, i.e. the height of a specific absorption peak.

However, in conventional infrared spectrometers, trace gases provide far too little absorption to measure, typically limiting sensitivity to the parts per million at best. And in NDIR (nondispersive infrared) instruments, the measurements often have the additional problem of cross-talk between certain gas species. Alternatively, more sensitive methods based on ions (mass spectrometry), wet chemistry, and/or separation science (gas chromatography) are slow, require frequent calibration, and a skilled operator. Furthermore, the instruments are often bulky and expensive (up to several hundred thousand dollars) limiting them to lab-based measurements rather than real-time monitoring applications. WS-CRDS combines the simplicity and real-time speed of infrared spectroscopy with the sensitivity and precision of these other technologies, offering a LDL (lower detection limit) and sensitivity at the parts per trillion level.

WS-CRDS utilizes infrared absorption but avoids the low signal/noise limitation of traditional IR instrumentation in several ways. First, it creates a long (up to tens of kilometers) effective path in the compact sample chamber, so that the light absorption is much more detectable. Second, it performs absorption measurements in a way that is independent of any fluctuations in the light source (laser) intensity. And third, WS-CRDS scans the entire absorption line with a narrow-line laser, so even if there is another gas component present with a close overlapping line, the target molecule measurement is unaffected.