Study of ionization processes during TOF-SIMS analysis by co-sputtering cesium and xenon

Because they are less sensitive to matrix effect than their homologues M+, the MCsn+ clusters are used for semi-quantitative depth profiling with ToF-SIMS. Unfortunately, their useful yields are low, particularly when the cesium surface concentration is high, leading to the drastic decrease of the positive ionization probability. In this paper, ToF-SIMS depth profiles of a bare silicon wafer were performed and the significant positive ions yields (e.g. Si+, SiCs+, Cs+) were monitored with respect to the varying cesium surface concentration. This analysis has been performed in the dual beam mode by diluting the cesium sputtering beam with xenon ions. This approach allows ToF-SIMS depth profiling with cesium beam concentrations varying from 0% (for pure xenon) to 100% (for pure cesium). At first, it was found that the Si+ and the Si2+ signals decrease exponentially with the increasing cesium beam concentration. The presence of optimums for the Cs+ and the MCsn+ signals was also observed. Finally, the tunneling model has been used to fit those maximums, which are a trade off between the increase of the cesium surface concentration and the decrease of the positive ionization probability.