ISOTOPIC FRACTIONATION IN THE SPUTTERING OF MO-92-MO-100 TARGETS

Collector-type experiments have been conducted using isotopically enriched Mo targets to investigate nonstoichiometric effects in sputtering induced by 5 and 10 keV Ar+ and Xe+ ions. Sputtered Mo was collected on carbon foils and subsequently analyzed using SIMS to quantify variations in the Mo-92: Mo-100 ratio as a function of emission angle in the sputtering geometry and projectile fluence. In general, the sputtered material was found to be enriched in the light isotope, with the enrichment being most pronounced in near-normal emission directions. This angular dependence was clearly resolved at the lowest bombarding fluences, although it was also seen under steady-state conditions. The most extreme low-fluence angular variation was seen for 5 keV Xe+ bombardment at a fluence of 3.0 x 10(14) ions cm-2, for WhiCh the sputtered light-isotope enrichment was approximately 30 parts per thousand larger in near-normal emission directions than in oblique directions. The low-fluence angular variation was observed to have a projectile dependence. In the near-normal direction, light-isotope enrichments as large as 53+/-5 parts per thousand with respect to the steady-state ratio at the same angle were measured. Similar results were found for secondary Mo+ ions sputtered by 14.5 keV O-. The observed effects are generally consistent with other low-fluence measurements of sputtered neutrals and secondary ions from solid targets, and with the results of multiple-interaction computer simulations, but do not agree well with the predictions of analytic theory.