Collector-type experiments have been conducted to investigate two different aspectsof sputtering induced by keV ions. The first study looked for possible ejectionmechanisms related to the primary charge state of the projectile. Targets of CsI andLiNbO_3 were bombarded with 48 keV Ar^(q+), and a Au target was bombarded with60 keV Ar^(q+), for q = 4, 8, and 11. The collectors were analyzed using heavy-ionRutherford backscattering spectroscopy to determine the differential angular sputtering yields; these and the corresponding total yields were examined for variations as a function of projectile charge state. For the Au target, no significant changes were seen, but for the insulating targets slight (~10%) enhancements were observed in the total yields as the projectile charge state was increased from 4+ to 11+.
In the second investigation, artificial ^(92)Mo/^(100)Mo targets were bombarded with5 and 10 keV beams of Ar^+ and Xe^+ to study the isotopic fractionation of sputteredneutrals as a function of emission angle and projectile fluence. Using secondary ionmass spectroscopy to measure the isotope ratio on the collectors, material ejectedinto normal directions at low bombarding fluences (~ 10^(15) ions cm^(-2)) was foundto be enriched in the light isotope by as much as ~70‰ compared to steady state.Similar results were found for secondary Mo ions sputtered by 14.5 keV O^-. For low-fluence5 keV Xe^+ bombardment, the light-isotope enrichment at oblique angles was~20‰ less than the corresponding enrichment in the normal direction. No angulardependence could be resolved for 5 keV Ar^+ projectiles at the lowest fluence. Theabove fractionation decreased to steady-state values after bombarding fluences of afew times 10^(16) ions cm^(-2) , with the angular dependence becoming more pronounced.The fractionation and total sputtering yield were found to be strongly correlated,indicating that the above effects may have been related to the presence of a modifiedtarget surface layer. The observed effects are consistent with other secondary ionmeasurements and multiple-interaction computer simulations, and are considerablylarger than predicted by existing analytic theory.
