Novel strategies for improving quantification in SIMS analysis involving sample loading with Cs.

In this lecture the essential results of a comprehensive recent review on Cs controlled secondary ion formation [1] are summarized and the consequences for future work in the field are discussed. Of key importance were the following findings. (i) In contrast to commonly employed theoretical models, the work function of the analysed sample is not the prime quantity dictating the charge state of a sputtered atom. Instead, the most adequate parameter for describing the status of the sample turned out to be the areal density of Cs available at the bombarded surface. This implies that the ability to generate secondary ions is controlled by the local chemistry. (ii) Detailed inspection of available experimental data provided convincing evidence that the ionisation probability of atoms does not depend on their emission velocity. Hence previously advocated theories of charge exchange between the departing atom and the electrons in the sample (‘tunnelling model’) cannot serve to explain the observed phenomena. Developing alternative, more adequate models of ion formation constitutes a real challenge to theoreticians. On the experimental side it is highly desirable to expand the currently available set of data to a larger variety of sample materials. There are good reasons to assume that Cs based SIMS can be developed further so that the semi-quantitative results currently achieved in bulk analysis and depth profiling can be improved significantly. Using advanced experimental strategies it appears possible to obtain truly quantitative information on sample composition. The instrumental requirements that need to be fulfilled in order to avoid artefacts are discussed