The system design and mode of operation for a dedicated low energy focused ion beam system based on immersion optics is presented. The ion beam is accelerated using a gun lens and the intermediate lens space is set on high potential. Beam retarding to the landing energy is accomplished within the objective lens. With this design the target is field free and can be kept on ground potential. As all voltage levels are comparatively low, both gun as well as objective lens can be operated in accel mode, thus improving system performance. The influence of Coulomb interactions is also taken into account. Calculations predict a spot size of approximately 80 nm and a corresponding probe current density of I A/cm(2) at a working distance of 15 mm and a final beam energy of 3 keV, respectively. (C) 2004 American Vacuum Society.