Secondary Ion Mass Spectrometry

Abstract

Secondary ion mass spectrometry (SIMS) is a technique for surface and thin-film analysis with a long history and a mature instrumental base dating from the early 1960s, which nevertheless has only recently achieved broad credibility as an analytical technique. This lack of acceptance stems from the complexity ofthe sputtering and ion emission processes, which are still imperfectly understood. Impact of a heavy ion (oxygen or heavier) on a surface at energies of a few ke V results in the ejection (sputtering) of surface material, with typical yields of one to several atoms per incident ion. A fraction of this sputtered flux is ionized during the ejection process; this fraction is an extremely sensitive function of the chemistry of the surface and of the sputtered species. Atoms, clusters of atoms, and intact molecular species can all be ejected and ionized, with varying efficiencies. Mass spectrometric analysis of the sputtered (secondary) ion flux allows chemical identification of the sputtered species. All elements can be detected, including hydrogen and helium, and recent developments in sampling methods have made the technique increasingly important in molecular mass spectrometry. SIMS has been extensively reviewed in recent years, from a variety of viewpoints. Various reviews emphasized instrumental aspects (1), analytical applications (2, 3), comparison with other surface analytical techniques (4--9), applications to surface studies (10, 1 1), and fundamental aspects of ion emission (12, 13). A series of biennial international conferences, beginning in 1977, has charted the development of the field (14--16); the proceedings of the most recent in this series (16) offers a useful snapshot of the field at the present time.