COULOMB INTERACTIONS IN MICROFOCUSED ELECTRON AND ION-BEAMS

Using established theories for calculating statistical Coulomb interac tions in electron and ion beams, the effect of these interactions on t he performance of practical instruments is estimated. The analysis con centrates on low voltage (1 kV) electron probe forming instruments and intermediate voltage (30 kV) focussed ion beam instruments. The appro ach is to first calculate the optimized probe size-probe current relat ions for these instruments in the absence of Coulomb interactions. As a function of probe size, it is then calculated how much current is re ally allowed in the probe before the Coulomb interactions significantl y contribute to the probe size. The result is a comprehensive represen tation of instrument parameter values at which Coulomb interactions be come important. The absolute maximum current ever obtainable in a give n probe size is derived.