Modeling of the sputtering process of cubic silver halide microcrystals and its relevance in depth profiling by secondary-ion mass spectrometry (SIMS).

Abstract

Secondary-ion mass spectrometry is frequently used for concentration-depth profiling of macroscopic samples, but it is certainly not a common analytical technique for the analysis of sub-micrometer-size particles. This is because of the additional ion-bombardment-induced artifacts which can occur when a three-dimensional microvolume is sputtered, instead of a flat surface. This paper presents a model of how small cubic photographic Ag(Cl,Br) crystals are eroded under primary-ion bombardment, and the extent to which secondary ions generated at different faces are extracted. The latter is studied by means of the program SIMION, which simulates ion trajectories in complex electrical field systems. It is shown that up to 90% of the secondary ions originating from the side face of a cubic crystal are unable to reach the detector, in contrast with most secondary ions originating from the top face. The angular dependence of the sputtering yield and the elemental ratio of Br/Cl sputtered particles have been calculated by using the well-known computer code TRIM (transport of ions in matter) under some limiting assumptions (possible preferential sputtering is disregarded and a steady-state sputtering process is assumed). The validity of the theoretical model and the calculated results were checked with experimental data. On the basis of the depth profiles presented it is explained why it is still possible to measure an interface inside a cubic volume, even though a group of several hundred crystals is sputtered simultaneously, and even though the orientations of the distinct faces of the cubes relative to the angle of incidence of the primary-ion beam are different.