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 in the most cases 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 ion species used are of fundamental importance. Emitters wetted with Au 73Ge 27, Au 77Ge 14Si 9 and Co 36Nd 64 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 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. Either one works in a high stable target current mode with lower resolution (emission current ∼10 μA), or in the high resolution, high chromatic angular intensity mode (emission current a few μA), with a reduced target current and less current stability, both at source temperatures some 10 K above the melting point.