Isotopic fractionation in sputtering

[摘要] Isotopic fractionation due to sputtering has been investigated via a collectortype experiment in which targets of known isotopic composition have been bombardedwith several keV Ar⁺ and Xe⁺ ions with fluences down to 3.0x10¹⁴ ions/cm² ,believed to be the lowest fluences for which such detailed measurements have everbeen made. The isotopes were sputtered onto carbon collectors and analyzed withSecondary Ion Mass Spectroscopy (SIMS.) There is clear indication of preferentialeffects several times that predicted by the dominant analytical theory. Results alsoshow a fairly strong angular variation in the fractionation. The maximum effect isusually seen in the near normal direction, measured from the target surface, fallingcontinuously, by a few percent in some cases, to a minimum in the oblique direction.Measurements have been made using Mo isotopes: ¹⁰⁰Mo and ⁹²Mo and aliquid metal system of In:Ga eutectic. The light isotope of Mo is found to suffer a53 ± 5‰ (note: 1.0‰ ≡ 0.1%) enrichment in the sputtered flux in the near normaldirection, compared to the steady state near normal sputtered composition, under5.0 keV Xe⁺ bombardment of 3.0 x 10¹⁴ ions/cm². In the liquid metal study only theangular dependence of the fractionation could be measured due to the lack of a welldefined reference and the nature of the liquid surface, which is able to 'repair' itselfduring the course of a bombardment. The results show that ¹¹³In is preferentiallysputtered over ¹¹⁵In in the near normal direction by about 8.7 ± 2.7‰ comparedto the oblique direction. ⁶⁹Ga, on the other hand, is sputtered preferentially over⁷¹Ga in the oblique direction by about 13 ± 4.4‰ with respect to the near normaldirection.

[发布机构] University:California Institute of Technology;Department:Physics, Mathematics and Astronomy

[关键词] Physics