Neutral and ion beam SIMS of non-conducting materials

Abstract

Energetic neutral beams are increasingly being used as an alternative to ion beams for the analysis of low conductivity materials in secondary ion mass spectrometry, since their use greatly alleviates the problems caused by sample charging. After a brief review of the history of this development, this paper reviews various methods for producing inert gas atom beams of energies in the 0.5–15 keV range and a number of neutral gun designs based on these methods are discussed. They include the saddle-field atom gun, capillaritron, a conventional electron impact source based ion/atom gun and a novel gun producing a raster-scanned micro focused (⩾ 5 μm) beam of either atoms or ions. The effects of sample charging are discussed with special reference to a low extraction field configuration. A careful control of the surface potential, which is possible under either ion or atom bombardment in conjunction with charge compensating electron beams, can provide positive and negative spectra with a substantial molecular cluster ion contribution. Secondary ion mass spectra of insulating targets under ion (+electron) or atom bombardment are qualitatively similar in that the same characteristics molecular fragment ions occur. Differences in peak height distribution are at least partially explainable in terms of a less precise control of the surface potential in the case of ion and electron bombardment. On the other hand, essential differences between the two types of bombardment are observed in the damage rate in some polymer materials and the sputter rate in a range of low conductivity materials. These experiments, carried out after careful calibration procedures, provide evidence for the existence of a charge state induced damage mechanism and a sunstantial electronic sputtering contribution in these materials under ion bombardment.