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Although the use of cesium (or oxygen) primary ions in secondary ion mass spectrometry (SIMS) enabled its success in microelectronics, issues arise at ultra-low energies, as required for extreme depth resolution, as the sputter yield becomes very small such that one enters the regime of ion beam deposition instead of material erosion. A potential solution is then to lower the Cs supply by using a Cs/Xe co-sputtering as introduced by ION-TOF and explored extensively by J. Brison. In this work, we describe a somewhat similar implementation in a Cameca SC Ultra and assess its performance and impact on ion yields. Because of the specific Cameca instrumental configuration, one alternates with short time intervals between Cs+ and Xe+ primary ions in the same ion column. Depending on the time intervals used, this approach leads to either quasi-simultaneous sputtering (intervals ~80 ms) or sequential (intervals >1 s) sputtering. An exponential variation of the Si− yield is observed when the Cs beam fraction varies from 0 (Xe) to 1 (Cs) and is ascribed to the corresponding increase in the near surface Cs concentration, CCs. Moreover, we observed detailed timing effects of the beams implying that the same nominal CCs may lead to different secondary ion yields suggesting effectively a different CCs. These effects are further investigated by observing the finer details of Cs accumulation and migration mechanisms in situ. Finally, when analysing SiGe/Si layers, it is found that with increasing Cs/Xe ratio, the decay lengths tend to decrease whereas matrix effects at interfaces show an opposite trend. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: In this work, a caesium/xenon co-sputtering gun was used to perform depth profiles of a RhSi layer with varying caesium beam concentration. The positive ion yields were monitored with respect to the varying work function of the solid and the intensities of the ions were plotted with respect to the caesium surface concentration. As expected by the tunneling model, all the M+ signals decrease exponentially with the increasing caesium beam concentration. Moreover, the heaviest ion yields decrease faster than the lighter ion ones. This phenomenon can be explained by the different velocities of the departing atoms, which has an important impact on the ionization processes. We then studied the variations of the MCs+ yields with respect to the caesium surface concentration and with respect to the nature of the departing atom. Finally, we applied models based on the tunneling model in order to fit our results. [Copyright &y& Elsevier]

Abstract: In this paper, ionization processes of secondary ions during ToF-SIMS dual beam depth profiling were studied by co-sputtering with 500eV cesium and xenon ions and analyzing with 25keV Ga+ ions. The Cs/Xe technique consists in diluting the cesium sputtering/etching beam with xenon ions to control the cesium surface concentration during ToF-SIMS dual beam depth profiling. Several depth profiles of a H-terminated silicon wafer were performed with varying Cs beam concentration and the steady state Si, Xe and Cs surface concentrations were measured in situ by Auger electron spectroscopy. It was found that the implanted Cs surface concentration increases with the Cs fraction in the beam from 0% for the pure Xe beam to a maximum Cs surface concentration for the pure Cs beam. Secondly, the variation of the silicon work function, due to the Cs implantation, was measured in situ and during depth profiling as the shift of the secondary ion kinetic energy distributions. Finally, the positive and negative elemental ion yields generated by the Ga analysis beam were recorded and modeled with respect to varying Cs/Xe mixture. We found that the Si− and the Cs− yields increase exponentially with the decrease of the silicon’s work function while that of Cs+ and Si+ decrease exponentially, as expected by the electron tunneling model. [Copyright &y& Elsevier]

Abstract: In this paper, a H-terminated silicon wafer was bombarded by low energy cesium ions during ToF-SIMS analysis and work function variations of the target were measured for different analysis conditions. This measurement was performed by measuring the shift of the secondary ions energy distributions with a reflectron type analyzer. At first, the silicon’s work function change was found to be −2.3eV during 500eV Cs+ bombardment at 45°. This effect is due to the creation of a dipolar layer at the surface of the silicon by the implanted cesium. Then the work function variation was measured at 300eV for varying cesium surface concentrations. The work function was found to decrease monotonously with the increasing cesium surface concentration, as during cesium adsorption experiments. The results were modeled following three different approaches and the value of the effective polarizability α of cesium was found to be equal to 1.9×10−39 Cm2/V. Finally, the effect of the bombardment energy on the work function variation was studied for beams with energies ranging from 250 to 2000eV. The effective polarizability of cesium was found to increase with increasing Cs beam energy. [Copyright &y& Elsevier]