Investigation of E × B mass filters for focused ion beam systems

E × B mass filters have been investigated by 3-D simulations of electromagnetic field and ion trajectories, based on a 3-D boundary integral method. A new algorithm has been developed which enables the simulation of complex 3-D field configurations with less complicated data preparation. Five different shapes and arrangements of electrode/pole-piece, from parallel to non-parallel plates to curved inner surface, have been investigated in an attempt to find the optimum geometry of field configuration giving the least deterioration to the focused beam. The effect of beam energy spread has been studied by strictly 3-D simulation of ion trajectories under different focusing conditions. It has been found that the optimum geometry of an E × B mass filter is not the widely used parallel-plate configuration but the one with slightly curved inner surface in its electrode/pole-piece, which significantly reduces the astigmatic focusing effect. The beam dispersion caused by ion energy spread is the intrinsic feature of an E × B mass filter, which cannot be cancelled, as previously suggested, by arranging the beam crossover at the centre of an E × B mass filter.