Work function shifts and variations of ionization probabilities occurring during SIMS analyses using an in situ deposition of Cs0

To optimize the advantageous quantification technique consisting in analyzing MCsx+ clusters, the Laboratoire dʹAnalyse des Matériaux (LAM) has developed the Cation Mass Spectrometer (CMS), a new instrument specially dedicated to performing this kind of analysis. To further enhance the potential of this instrument, we have developed a column that delivers a collimated and adjustable stream of neutral Cs atoms to be deposited on the surface of the sample while this one is being analyzed by SIMS. As this configuration permits a successful decoupling of the sputtering and Cs introduction processes by avoiding the constraints imposed by an energetic Cs+ ion bombardment, it becomes possible to optimize simultaneously the sensitivity of the analysis, by carefully adjusting the Cs concentration to its optimum value, and the depth resolution of the analysis, by choosing adequate primary bombardment conditions. For the present paper, we have performed in situ measurements of the produced variations of the sampleʹs work function, which are a typical concomitant of alkali metal deposition, by recording successive energy distributions of secondary Cs+ ions sputtered from three different samples (Al, Si, Ni). The work function shifts detected by using this method can subsequently be used to explain the variations of the ionization probability of secondary Cs found in a previous study of MCsx+ cluster analysis based upon the described technique. In this respect, we find curves which are typical for what one would expect considering the electron-tunneling model and this theoretical prediction is thus corroborated by our experimental results.