  Focused ion beams are indispensable tools in the semiconductor industry
because of their ability to image and modify structures at the nanometer length
scale. Here we report on performance predictions of a new type of focused ion
beam based on photo-ionization of a laser cooled and compressed atomic beam.
Particle tracing simulations are performed to investigate the effects of
disorder-induced heating after ionization in a large electric field. They lead
to a constraint on this electric field strength which is used as input for an
analytical model which predicts the minimum attainable spot size as a function
of amongst others the flux density of the atomic beam, the temperature of this
beam and the total current. At low currents (I<10 pA) the spot size will be
limited by a combination of spherical aberration and brightness, while at
higher currents this is a combination of chromatic aberration and brightness.
It is expected that a nanometer size spot is possible at a current of 1 pA. The
analytical model was verified with particle tracing simulations of a complete
focused ion beam setup. A genetic algorithm was used to find the optimum
acceleration electric field as a function of the current. At low currents the
result agrees well with the analytical model while at higher currents the spot
sizes found are even lower due to effects that are not taken into account in
the analytical model.
