Investigation and optimization of the emission parameters of alloy liquid metal ion sources

Liquid metal ion sources (LMIS) are one of the key elements in focused ion beam (FIB) technology. For special tasks, alloy LMIS are needed which have to be operated at elevated temperatures. For the adjustment of an optimal beam performance in a FIB system the source parameters, temperature of the ion emitter and the energy spread of the different used ion species are of fundamental importance. Emitters wetted with Au73Ge27, Au77Ge14Si9 and Co36Nd64 alloys were investigated with respect to the influence of the source temperature on the emission current - extraction voltage characteristics, the mass distribution, as well as the energy spread of the different emitted ions using an equipment containing of an ExB mass filter and a retarding field energy analyser. The energy spread significantly determines the available FIB spot size and depends also on the emission current, the charge state, and the mass of ions or clusters. The axial angular intensity and the resulting target current of the FIB were measured as a function of the source parameters. Two operating regimes were found. Whether one works in a high stable target current mode and lower resolution (emission current ~ 10 µA), or in the high resolution, high chromatic angular intensity mode (emission current a few µA), at the risk of lower target current including stability, both at source temperatures some ten degrees above the melting point.