Using emission spectroscopy to perform impurity analyses in UHP gases

Mark L. Malczewski, Hollis C. Demmin, James D. Borkman, William A. Kilinskas, and Joseph Wegrzyn, The last decade of the 19th century saw the discovery of all the noble gases. The ability to liquefy air in large quantities made it possible to concentrate measurable quantities of these rare gases, but of equal importance was the use of a new technique: emission spectroscopy. When excited in an electric discharge, the gases dissipated some of the energy as electromagnetic radiation at wavelengths that were specific to each gas. The spectral lines were intense enough to be easily observable by the simple spectroscopes of the time and even by the human eye. The appearance of unfamiliar spectral lines in a gas sample indicated the presence of new elements, which greatly facilitated the discovery of previously unknown gases. Early on, the sensitivity of the technique for certain combinations of gases was noted, leading to the use of emission spectroscopy for quantitative analysis as well as qualitative identification. Equipped with sophisticated electronics and detectors, today's spectroscopic analyzers can perform continuous, multicomponent analyses with limits of detection (LODs) in the single-digit parts-per-billion range for a number of impurities. As a result, these instruments have applications in the control of the ultra-high-purity (UHP) process gases used in the semiconductor industry. In addition to monitoring the purity of delivered gases, they can be used for point-of-use purity monitoring and for the certification of newly installed gas distribution systems. After discussing the operating principles of spectroscopic systems, this article presents some examples of these applications. The heart of a spectroscopic analyzer is a pair of analytical tubes that contain the plasma undergoing excitation. In contrast to conventional low-pressure gas-discharge tubes, those using the silent electric discharge (SED) excitation technique operate at atmospheric pressure with high-voltage ac power supplied by two