Electron work function decrease in SIMS analysis induced by neutral cesium deposition

The negative secondary ion yields in secondary ion mass spectrometry (SIMS) increase when electropositive elements, especially alkali metals, are used as primary ions. In previous papers by the same authors, useful yield variations of several elements have been studied experimentally with respect to the neutral cesium deposition conditions. Besides, the Cs surface concentrations have been simulated using the TRIDYN code. The determined secondary ion sensitivities have been discussed with respect to the experimental conditions and they have been compared to the electron-tunneling model describing ion emission from metallic and semi-conducting samples. In this paper, the variations of the electron work function of the sample will be studied with respect to the experimental conditions used in the previous papers. The energy distributions of negative secondary ions will be recorded for the different experimental conditions. The electron work function shift, on which is based the electron-tunneling model and which thus gives evidence for the influence of cesium on ion emission, is extracted from these distributions. The variations of the electron work function are discussed with respect to the experimental conditions as well as the simulated cesium surface concentration. Besides, the secondary ion sensitivities are plotted with respect to the electron work function, giving a direct comparison with the electron-tunneling model.