Thermal-desorption x-ray photoelectron spectroscopy (TDXPS) is a technique in which the thermal aspect of thermogravimetric analysis (TGA) is combined with non-angle-resolved x-ray photoelectron spectroscopy (XPS). TDXPS was developed to enhance the physical and chemical characterization of contaminants on surfaces. A combination of TDXPS and conventional XPS [including angle-resolved XPS (ARXPS)] should prove useful in industries in which surface contamination can adversely affect the results of plating, coating, and bonding processes.

XPS and TGA have different strengths and weaknesses: XPS provides both qualitative and quantitative information about chemical species (including physisorbed and chemisorbed contaminants) on solid specimens, to depths that range between 10 and 100 Å below specimen surfaces. TGA provides information on the degrees of bonding and chemical activity of those chemical species that can be desorbed from the surfaces and/or the depths of specimens. TGA is practical only for specimens of materials with high surface-area/weight ratios; e.g.,materials with fine pores. While XPS is not restricted to any particular range of surface-area/weight ratios, it yields no direct information on adsorption/desorption characteristics.

These X-Ray Photoelectron Spectra were obtained at the two indicated temperatures during heating of a nickel-alloy specimen that had previously been cleaned by use of methyl ethyl ketone (MEK). These spectra show a decrease in the C(1s) peak, indicative of desorption of carbon in MEK residue.

The prototype TDXPS apparatus was constructed by modifying an XPS system to incorporate a specimen-heating stage and a digital subsystem for feedback control of experiments and acquisition of data. In TDXPS, one acquires x-ray photoelectron spectra at various temperatures as the temperature of the specimen is increased.

TDXPS provides information that cannot be obtained through either technique alone. A succession of two or more spectra at increasing temperatures can be interpreted in terms of a decrease in the concentration of a species of interest (see figure) and/or changes in interactions with other species. Stated somewhat differently, TDXPS yields information about the energy level, reached during the increase in thermal energy of the surface, where each species attains sufficient energy (activation energy) to become desorbed. From activation energies of desorption as determined by TDXPS, one can gain understanding of how surfaces of specific materials become contaminated in various processes. This understanding can be fed back into the designs of processes to decrease or at least beneficially modify contamination.

This work was done by John D. Olivas of Caltech and Enrique Barrera of Rice University for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at under the Physical Sciences category, or circle no. 108 on the TSP Order Card in this issue to receive a copy by mail ($5 charge). NPO-20149

Photonics Tech Briefs Magazine

This article first appeared in the April, 1998 issue of Photonics Tech Briefs Magazine.

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