Secondary ion mass spectrometry

Secondary-ion mass spectrometry (SIMS) is a technique used to analyze the composition of solid surfaces and thin films by sputtering the surface of the specimen with a focused primary ion beam and collecting and analyzing ejected secondary ions. The mass/charge ratios of these secondary ions are measured with a mass spectrometer to determine the elemental, isotopic, or molecular composition of the surface to a depth of 1 to 2 nm. Due to the large variation in ionization probabilities among elements sputtered from different materials, comparison against well-calibrated standards is necessary to achieve accurate quantitative results. SIMS is the most sensitive surface analysis technique, with elemental detection limits ranging from parts per million to parts per billion. In 1910 British physicist J. J. Thomson observed a release of positive ions and neutral atoms from a solid surface induced by ion bombardment. Improved vacuum pump technology in the 1940s enabled the first prototype experiments on SIMS by Herzog and Viehböck in 1949, at the University of Vienna, Austria. In the mid-1950s Honig constructed a SIMS instrument at RCA Laboratories in Princeton, New Jersey. Then in the early 1960s two SIMS instruments were developed independently. One was an American project, led by Liebel and Herzog, which was sponsored by NASA at GCA Corp, Massachusetts, for analyzing Moon rocks, the other at the University of Paris-Sud in Orsay by R. Castaing for the PhD thesis of G. Slodzian. These first instruments were based on a magnetic double focusing sector field mass spectrometer and used argon for the primary beam ions. In the 1970s, K. Wittmaack and C. Magee developed SIMS instruments equipped with quadrupole mass analyzers. Around the same time, A. Benninghoven introduced the method of static SIMS, where the primary ion current density is so small that only a negligible fraction (typically 1%) of the first surface layer is necessary for surface analysis.