X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) are two analytical techniques to identify trace amounts of surface contaminants. Last month, we introduced the theory. However, perhaps you don’t care how it works. You want to know if you should use one, the other, both, or neither to help you analyze the source of contamination.
Speciation. The most compelling reason to use XPS is to provide molecular speciation. Both techniques can identify the element from which the photoelectrons were emitted (except that neither is sensitive to either hydrogen or helium). Although both techniques can determine molecular structure, more “fingerprints” are available in databases for XPS and, therefore, AES is utilized less for chemical identity.1
Spatial resolution. AES has the better spatial resolution, allowing identification of small particles, because electrons can be focused to a spot size about an order of magnitude smaller than X-rays. The higher energy of incoming electrons in AES presents a larger potential for surface damage than XPS. However, AES has been a mainstay technique for analyzing semiconductor wafers.
Non-conductive surfaces. While charging effects can impact the sensitivity and precision of both, XPS is the choice for non-conducting surfaces. With XPS, the positive charge shifts the energies of the characteristic lines. Because all lines shift by the same amount, characteristic footprints can be identified by waiting until shifting slows or stops.1Without advanced analytical gymnastics, AES is restricted to conducting surfaces because the negative surface charge that builds up in non-conductors reduces and eventually stops the flux of emitted Auger electrons.
Surface carbon. AES has an advantage where adventitious carbon (AC) is expected. Any surface can acquire AC, organic chemicals deposited on surfaces during atmospheric exposure. AC tends to be comprised of volatile molecules; the heat generated by the incident electron flux in AES vaporizes the volatiles.1 With XPS, the problem of AC can be partially ameliorated by adding reference samples exposed to the same air and packaging as the test samples.2
Specific profiling. Scanning techniques extend the utility of AES for larger surface areas through high resolution microscopy with elemental identification. Combining scanning with surface specificity enables rapid depth profiling. Most AES information is obtained from a few nm into the surface region. To depth profile, Auger scanning analysis determines elemental composition at the existing surface and then argon ions scour away (ion etch) that surface, exposing the next underlying region for a subsequent AES scan.
Surface volatiles. Because both techniques require a high vacuum environment, neither are suitable for surfaces containing volatile, outgassing materials.
CHOOSE WISELY
Table 1 summarizes some features of AES and XPS. Often, either can provide your answers. The technique to use depends on your application, the available laboratory instrumentation, as well as the skill, experience, and creativity of the analyst. For the best return on your analytical investment, communicatewhat you hope to accomplish to your analytical chemist.
Acknowledgement: The authors thank Dr. Ben Schiefelbein, RJ Lee Group, for many helpful comments and suggestions.
References:
- B. Schiefelbein, personal communication.
- C. Geosling and J. Koran, “Contamination Control and Analytical Techniques”, Handbook for Critical Cleaning (HBCC), CRC Press (2001), pg 445.
Barbara Kanegsberg and Ed Kanegsberg are independent consultants in critical and precision cleaning, surface preparation, and contamination control. They are the editors of The Handbook for Critical Cleaning, CRC Press. Contact them at BFK Solutions LLC., 310-459-3614; [email protected]; www.bfksolutions.com.
