Comparison between different imaging modes in focussed ion beam instruments

In a two-lens focussed ion beam instrument, one can choose to either have a crossover in between the lenses or not. For a representative instrument, it is shown that the optimized probe size of the non-crossover mode is smaller than that of the crossover mode. Both the lens aberrations and the statistical Coulomb effects are responsible for this effect. We analyzed the differences for a large range of beam currents and found that in the absence of statistical Coulomb interactions, the difference can be as much as a factor 1.6. In the presence of Coulomb interactions, the difference increases to a factor 1.8.