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SIMS Theory: Secondary Ion Yields - Elemental Effects
The SIMS ionization efficiency is called ion yield, defined as the fraction of sputtered atoms that become ionized. Ion yields vary over many orders of magnitude for the various elements. The most obvious influences on ion yield are ionization potential for positive ions and electron affinity for negative ions. For example, the following figure shows the logarithm of positive ion yields plotted as a function of ionization potential. The ion yields are relative to silicon in a silicon matrix with oxygen sputtering.
The correlations of ionization potential with secondary ion yields are not perfect. Variations depend both on the sample matrix and on the element itself. For example, the presence of oxygen in the sample enhances positive ion yields for most elements, but fluorine exhibits anomalously high positive ion yields in nearly all samples. Some elements, such as helium and neon fall outside the trend shown in the picture.
The next figure shows a similar treatment for negative ions where the logarithms of relative ion yields are plotted against electron affinities. The ion yields are relative to silicon for measurements in a silicon matrix with cesium ion sputtering. The four halides are the elements that deviate furthest from the trend line.
