SECONDARY-ION MASS-SPECTROSCOPY RESOLUTION WITH ULTRA-LOW BEAM ENERGIES

The depth resolution attainable in secondary ion mass spectroscopy (SI MS) depth profiling is shown to be greatly enhanced by the use of a fl oating ion gun, which is capable of delivering high primary ion curren ts (ca. 100 nA) at impact energies down to 250 eV. With specially grow n, genuinely abrupt GaAs (Si,Al) delta layers and Si0.8Ge0.2Si structu res we have achieved SIMS profile full width at half maximum data as l ow as 1.3-2.2 nm (species dependent). Approximating the leading edges by exp(z/lambda(L)), where z is depth, we achieve lambda(L) (growth le ngth) values approximate to 0.2 nm (i.e., 0.5 nm per decade of concent ration). The trailing edge data, exp(-z/lambda(T)), exhibit the expect ed elemental dependence due to differences in preferential sputtering and more complex effects, with lambda(T) (decay length) ranging from 0 .6 nm for Si to 1.8 nm for Al. This analytical performance, which can be obtained in combination with at least 1 part per million atomic ele mental sensitivity, represents a major improvement in SIMS depth profi ling capability. (C) 1996 American Vacuum Society.