Secondary ionisation
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The process known as secondary ion mass spectrometry (SIMS) involves bombarding the surface to be tested with a stream of ions. The test piece then emits particles, some of which are themselves ions. These secondary ions are measured with a mass spectrometer to determine the quantitative elemental or isotopic composition of the surface. SIMS is the most sensitive surface analysis technique, but is more difficult to accurately quantify than some other techniques.
The first SIMS instrument was constructed under a NASA contract in the early 1960's to analyze moon rocks. The process was developed in the late 1960s by Alfred Benninghoven at the University of Munster, Germany.
Detection limits for most trace elements are between 1e12 and 1e16 atoms/cc. Because the primary beam erodes the surface, a depth profile (e.g., 1 micrometer deep) may be obtained from a time trace. The lateral resolution is determined by the width of the primary beam and can be better than 50 nanometers.
Charles Evans & Associates (http://www.cea.com/evanscea/evanscea.htm) has excellent tutorial pages for SIMS theory (http://www.cea.com/cai/simstheo/caistheo.htm) and instrumentation (http://www.cea.com/cai/simsinst/caisinst.htm), where the book Secondary Ion Mass Spectrometry: Basic Concepts, Instrumental Aspects, Applications, and Trends, by A. Benninghoven, F. G. Rüdenauer, and H. W. Werner, Wiley, New York, 1987 (1227 pages), is cited as "the best SIMS reference".