COLLISION CASCADE LIMIT TO SPATIAL-RESOLUTION IN FOCUSED ION-BEAM PROCESSES

Spatial resolution in ion microprobe chemical microanalysis relies on the capability to localize the emission sites of secondary ions emitted from a surface under energetic primary ion impact. The distribution of emitted ions around the primary impact point is obtained by a full 3D Monte-Carlo-type simulation of the sputtering cascade developing inside an ion-bombarded solid. Systems studied are 5-60 kV Ga and In on Si. The resulting emission distribution is folded with the ion optical beam size (assuming chromatically limited beams from liquid metal ion sources) to obtain the effective diameter of the emission intensity on emission spot size. Also, simple power-law approximations are given for the relationships between emission diameter and optimized ion current and between emission diameter and optimized beam voltage (for optimized secondary spot current). These relations allow an estimation of the optimized operating conditions for emission spot sizes in the 10-100 angstrom range under the assumptions of chromatically limited primary beams and negligible source size contribution.