Information Resources

The spectroscopic database for the properties and interactions of many of the gases used in plasma processing is quite extensive. Ultraviolet or infrared absorption strengths and fluorescence quenching or pressure broadening cross sections are the parameters needed to predict or interpret the results of absorption, emission, and fluorescence diagnostic experiments, and while not every parameter is available for every molecule, information on related species can often provide a basis for estimation. Data may be sparser for more specialized techniques, but the coverage for systems of the greatest interest may be adequate. On the other hand, much of this information is dispersed in many journals. Although some reviews have been written, as diagnostic fields mature there is less interest in continuing to produce comprehensive review articles. In addition, it is rare that either reviews or molecular property measurements are focused on the needs of plasma processing diagnostics. Areas receiving focused attention, such as combustion or atmospheric chemistry, sometimes produce results that also serve the plasma processing community. However, rapidly growing areas involving, for example, heavy metal halogens and metal-organics must be supported entirely within the community. Typically, this means that the research group developing the diagnostic application must also carry out any basic property measurements, the result being that those data may not even appear as the subject of that group's own publication in the primary literature, let alone in one of the centralized repositories briefly reviewed below.

Workers at the National Institute of Standards and Technology (NIST) have produced a number of compilations of atomic line positions and strengths,2 some of which are now available as electronic databases.3 However, as was pointed out recently, this is still an active area.4 The compilation of Huber and Herzberg5 occupies a similar position for diatomic molecules, and has also been compiled into a NIST database.6 In general the classic volumes of Herzberg7 still provide an excellent entry point to the older spectroscopic literature, while some newer spectroscopy texts provide some useful examples.8 Books edited by Suchard9 and Boyko,10 the book by Pearse and Gaydon,11 two reviews in the 1992 Royal Society annual reports volume12 and one in the 1990 Annual Review of Physical Chemistry,13 several reviews in the book series edited by Rao,14 15 and the ongoing compilation of Jacox, again available in both print16 and electronic17 versions, are other useful sources. The review article by Smith and co-workers18 deserves special mention as a source of collision broadening parameters as well as of infrared band intensities.

The HITRAN absorption line listing19 and an associated table of unresolved band absorption coefficients,20 together with software allowing prediction of spectra, form a valuable resource for the set of atmospheric molecules. A NIST electronic database21 provides moderate-resolution infrared spectra for a selection of 5244 molecules, while a number of commercial databases22 provide higher resolution and additional species, in both print and electronic forms. One drawback of almost all these infrared spectral compilations, however, is that they are not quantified (path length or concentration are unavailable, or were initially unknown). For this reason, much earlier compilations of quantitative infrared spectra23 are still useful.

A number of sources provide compiled data or useful keys to the primary literature even though their focus is not on plasma processing. The book by Okabe24 lists a variety of ultraviolet absorption cross section spectra. Several review articles by Hirota and co-workers25 provide good introductions to the work of this prolific group and supplement early comprehensive review articles on high-resolution infrared spectroscopy.26 Volumes whose purpose is to list infrared band positions27 also provide primary literature references, often leading to full, quantitative spectra. Finally, the JANAF thermochemical tables28 can perform the same function as part of their documentation of entropy calculations, thus centralizing references to infrared spectra for a large number of molecular species.

Examples of what is involved in the collection and application of primary data to estimate the feasibility of particular diagnostic experiments are given in a 1983 SPIE paper by Wormhoudt, Stanton, and Silver.29 Focusing on the techniques of laser-induced fluorescence and infrared absorption, it attempted to tabulate the parameters needed to estimate minimum detectable densities of a wide variety of stable and



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