Cesium/xenon co-sputtering at different energies during ToF-SIMS depth profiling

In this paper, ToF-SIMS dual beam depth profiles of H-terminated silicon wafers were performed with cesium primary ions and for different beam energies. The aim of this study was to investigate the influence of the cesium beam energy on the secondary ion yields during ToF-SIMS dual beam depth profiling. For this purpose, both the cesium beam energy and the cesium surface concentration were varied but the analysis conditions were kept identical for all depth profiles (i.e. Ga⁺ at 25 keV, 45°). For each sputter beam energy (i.e. 250 eV, 750 eV and 2000 eV), the cesium surface concentration was varied by diluting the cesium sputtering beam by xenon ions. This technique allows performing ToF-SIMS depth profiles with cesium surface concentration varying from zero (for pure xenon beam) to a maximum value (for pure Cs beam), depending on the bombardment conditions. For all the beam energies, the Si⁺ signals were found to decrease with the increasing cesium coverage and the lower the energy, the faster the decrease. The Cs⁺, the SiCs⁺ and the signals were found to exhibit a maximum for well defined Cs/Xe mixtures, which were found to depend on the secondary ion species and on the beam energy. Moreover, the maxima were found to shift to higher Cs beam content with the increasing energy. This effect is due to the variation of the cesium surface concentration with the varying beam energy. XPS analysis of the Cs/Xe craters and DYNTRIM computer simulations allowed us to convert the cesium beam scale to a cesium surface concentration scale and to interpret our results.