An insertable electron beam ionizer into a quadrupole-based secondary ion mass
spectrometer instrument has been designed and installed to analyze sputtered neutrals. The
optimum design conditions of the ionizer have been obtained by modeling various configurations
of the system using a simulation program developed by us. The program has allowed us to
compute the potentials and ion trajectories inside the system to test the performance of the ion
optics design. We have investigated the advantages of using a large ionization volume with low
electron current to minimize the space charge effect in the ionizer, as this is the major
problem in this type of instrument. In addition, we have used the simulations to obtain all
electrodes voltages which provide an efficient suppression of residual gas and secondary
ions. A good useful yield was obtained, even with low electron densities, thanks to the high
geometrical acceptance of the ionizer and its large active volume. This configuration
implies less thermal radiation in the ionizer and, in addition, a longer life time of the
filament. Although the signal of residual gas not suppressed (i.e. in residual gas
mode) is two orders of magnitude higher than the signal of sputtered neutrals, we have
achieved a good background suppression.