Aspects of quantitative secondary ion mass spectrometry

Abstract

The present state of quantitative secondary ion mass spectrometry (SIMS) is analysed critically. Because of the strong gain in sensitivity obtained by loading the instantaneous sample surface with either oxygen or cesium, chemically enhanced secondary ion emission is discussed almost exclusively. The implications brought about by a fixed experimental geometry and by the mass dependence of instrumental factors are discussed in some detail. Normal beam incidence is to be prefered because of the high steady state concentration of oxygen attainable. Positive secondary ion yields of elements emitted from a common matrix under conditions of complete saturation with oxygen are proportional to exp(− βE i), where E i is the respective ionization potential and β a parameter of unknown origin. Interpretation of β in terms of equilibrium thermodynamics (i.e. β −1 = kT) is shown to be unjustified and misleading. Enhanced positive ion emission in the presence of oxygen is likely to result from disintegration of metal oxygen complexes occurring at the final stage of sputter emission (bond breaking model). Similar processes may determine negative ion emission in the presence of cesium. However, a simple relation between negative ion yields and electron affinity does not seem to exist. Presently, the most accurate quantitative analyses are obtained by the use of experimentally determined relative sensitivity factors.