Abstract: In a series of recent studies, we evaluated the effects of statistical Coulomb interactions on the obtainable probe sizes and probe currents in Focussed Ion Beam (FIB) systems. In order to find the fundamental limits, we assumed that the final probe current was selected by an aperture very close to the source. However, in practical FIB's, the aperture is somewhere halfway the column, so the current is much higher in the first part of the column, with associated stronger Coulomb interactions. We have now analyzed the influence of the aperture position on the FIB performance and found that the position is very important, but only in the range of small probe sizes or low probe currents. Many FIB's have a fixed first aperture close to the source to limit the beam current early on and then a second aperture to select the final probe current. We have also analyzed the effect of the fixed aperture and found that it controls the statistical Coulomb interactions only if the current is limited very substantially, which then makes it impossible to operate in high current mode.!12
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