PRIMARY-ION CHARGE COMPENSATION IN SIMS ANALYSIS OF INSULATORS

Primary-ion charge compensation by means of simultaneous bombardment of the insulating specimen with a focused beam of low-energy electrons has been investigated. The measurements were performed in a quadrupole-type SIMS instrument which allowed the amount of charging to be measured in situ. The effectiveness of charge neutralization was investigated by monitoring the intensity, the mass resolution, and the line shape of both (positive) atomic and molecular secondary ions. It was found that in the range of electron energies investigated (100-500 eV) the energy has little effect on the amount of charge neutralization (for bombardment of a gadolinium iron garnet sample with 12-keV argon ions). A well-defined minimum electron current was required to provide compensation of the primary ion charge. Supersaturation" with electrons caused only a negligible reduction in signal height. Studies with focused ion beams showed that effective neutralization requires compensation of the local primary ion current density. In case of current density compensation the mass resolution observed with insulating specimens is as good as for metals or semiconductors. Possible mechanisms of charge compensation are discussed briefly."