Low Energy Ion Scattering and Secondary Ion Mass Spectrometry

Abstract

When a beam of accelerated ions impinges on the surface of a solid, a number of phenomena occur. Interacting with the solid, the ions experience elastic or inelastic scattering from the surface atoms. Momentum transfer gives rise to the ejection (or sputtering) of atoms and molecules from this solid as charged and neutral species. The dissipation of the incident energy also brings about the emission of electrons and photons. Finally, the primary ions penetrate into the solid, thus producing implantation and radiation defects profiles. All these emission phenomena can be used for the purposes of materials characterization; however, the analysis of the charged components of the scattered and sputtered particles has turned out most promising in the development of powerful spectroscopic techniques known as low-energy ion scattering (LEIS) [or ion scattering spectroscopy (ISS)] and secondary ion mass spectrometry (SIMS). From the viewpoint of detection sensitivity, LEIS and SIMS are undoubtedly the champions in the “surface analysis league”: SIMS has an extremely high elemental sensitivity (10 –10 7 at.%) and LEIS demonstrates exclusively firstatomic layer sensitivity (below 10 4 monolayer). Sputtering and scattering appear as a matter of course in present-day materials analyses. However, similarly to other particle–surface interactions, a long history has paved the way to the status quo. In 1910, Thomson suggested a method to identify chemically ions produced in a discharge tube using superimposed electric and magnetic fields for mass separation [1]. He noticed that the accelerated positive ions caused “disintegration” of the metal cathode – a process that has later come to be known as sputtering. The phenomenon of secondary ion emission seems to have been discovered by Campbell, who reported in 1915 [2] on the observation of a charged component in ion-induced “disintegration” of a solid. First experiments which may be regarded as a kind of SIMS precursor were performed in 1938 by Sloane and Press [3], who observedmass spectrometrically the emission of sputtered negative ions and even measured their energy distribution. The history of ion scattering spectroscopy seems to have started with the work of Klein, who reported