Cesium retention during sputtering with low energy Cs+ and oxygen flooding

Accurate knowledge of the stationary Cs retention during Ultra Low Energy (ULE; < 150 eV) Cs+ sputtering (of semiconductor substrates) is the first step towards understanding the basics of SIMS. We have investigated the effect of oxygen on the retention capacity for cesium on silicon and germanium targets. The amount of cesium that is retained was directly measured using Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Even a very small partial pressure of oxygen (in the 10(-9) to 10(-8) mbar range) will tremendously increase the dose of cesium that is retained at the sample surface. Concurrent with this increased retention (due to Cs-oxide formation) strong variations in the target sputtering yield and the ionization degree of silicon can be observed. These data combined support the model that the observed target sputtering yield is directly linked to the Cs-surface coverage and that a cap layer containing a very high amount of Cs can be formed by the ion beam. The sensitivity to oxygen becomes stronger as the primary ion energy decreases, and implies that SIMS measurements with Cs aimed at a very high depth resolution will suffer from drastically reduced erosion rates due to the deposition process.([1]) Copyright (C) 2010 John Wiley & Sons, Ltd.