COULOMB INTERACTIONS IN A SHAPED ION-BEAM PATTERN GENERATOR

The statistical Coulomb interactions between the particles of a focused ion beam can severely reduce the quality of a system. The interactions in the source region of a liquid metal ion source have been studied extensively and lead to the well-known virtual source diameter of 50 nm and energy width of 5 eV in a Ga source. In this report, the interactions are described in the rest of the column of the Delft ion beam pattern generator, a system using a shaped beam with Kohler illumination. The simulations have been done with the Interac and Montec programs developed by Jansen [G. H. Jansen, Coulomb Interactions in Particle Beams (Academic, San Diego, 1990), Suppl. 22; G. H. Jansen, Montec, and Interac Programs, information obtainable from, Particle Optics Foundation, Lorentzweg 1, 2628 CJ Delft, The Netherlands]. The results show that the Coulomb interactions are a limiting factor for the beam quality, not only in the source region but also in the rest of the column. In particular, long segments with small opening angles cause deterioration of the beam. In these segments, the beam can even tend to thermodynamic relaxation. For the case of a square shape with size 100 nm and edge sharpness 25 nm, for which the normal probe current should be about 1 nA, interactions cause the edge sharpness to deteriorate by a factor of two at 100 pA while for more than 200 pA the square shape is not recognizable anymore.