Mechanisms responsible for inducing and balancing the presence of Cs adatoms in dynamic Cs based SIMS

Abstract

Secondary ion mass spectrometry (SIMS) is an established technique for sensitive compositional analysis of solids. To achieve high sensitivity in the so-called dynamic SIMS mode, notably in the analysis of negative secondary ions, it is common practice to use Cs primary ions for sputter erosion as well as for loading the sample with Cs. In qualitative terms, the negative-ion yield enhancement has been attributed to a lowering of the sample's work function but, remarkably, the physical processes involved in producing the favorable conditions have not been clarified in any detail before. This study provides evidence that work function changes observed under Cs bombardment can be explained if the implanted ions are transiently converted to adatoms. Previously disregarded properties of Cs atoms include the huge size, the high mobility and bond formation with coverage dependent strength. It is shown that implanted Cs atoms are rapidly relocated towards the receding surface, presumably in response to the stress generated by the retained material. After passage through the solid–vacuum interface, Cs atoms loose their valance electron and become bound to the surface via dipole interaction. This way they become adatoms. Stationary conditions in terms of Cs surface coverage are established by the balance between adatom formation and sputter ejection at energies exceeding the bond strength. Combining calculated sputter cross sections for adatom removal with experimental data from various sources, the rates of Cs implantation and reemission are shown to be in balance with an uncertainty of only about±20%.