Using Cs- in the TOF-SIMS dual beam mode offers a semi-quantitative solution to depth profiling. Specifically, the use of these alkali ions strongly increases negative ion yields, decreases the positive ones and allows the formation of MCs+ and MCs2+ clusters. Recently. Niehuis and Grehl [Proceedings SIMS XII (2000) 49] developed a new approach consisting of co-sputtering Xe and Cs in order to control the Cs surface concentration, thus allowing the optimization of elemental and cluster ion yields. We applied that technique on different well-defined samples (e.g. Si, SiO, and Al2O3) and we monitored positive ions (e.g. Si+, Al-, CsSi+, CsAl+, CsO+, Cs2O+, Cs2Si+, etc.) as a function of the sputtering beam Cs concentration. First, we observed the decrease of the elemental ions due to the work function lowering, as is predicted by the tunneling model. We then studied the behavior of the MCs+ and the MCs2+ clusters. The MCs+ yield exhibits a maximum at a given Cs/Xe beam concentration ratio, depending on the studied element M and also on its chemical environment (e.g. Si and SiO2), and on the energy of the Cs beam. In other words, it is hypothesized that this yield maximum is a consequence of the competition between the varying surface Cs coverage (direct concentration effect) and the decreasing ionization probability due to that varying Cs [Phys. Rev. Lett. 50 (1983) 127; Phys. Rev. B 29 (1984) 2311; K. Wittmaack, Proceedings SIMS VHI, (1992) 91]. Simple models based on the tunneling model were applied to interpret our results. The MCs2+ signal behaves in a very different way. As shown by Gao [Y. Gao, Y Marie, F. Saldi, H.N. Migeon, Proc. SIMS IX, (1994) 382], these clusters are predominant for electronegative elements and increase in a monotonous way with Cs beam concentration. (C) 2004, Elsevier B.V. All rights reserved.
