Sputtering metal targets at low ion energies (Hg or Ar at <300 eV) under normal ion incidence causes the lighter atoms (lighter isotopes or lighter elements of alloys) to be preferentially ejected in a direction normal to the target surface. Experimental results are shown for several elements and alloys at various bombardment energies. The amount of enrichment of the lighter species normal to the target surface decreases quite rapidly with increasing ion energy. The phenomenon is a result of reflective collisions because lighter atoms can be backscattered from heavier ones underneath but not vice versa. The effect provides an explanation why solar-wind-exposed lunar material is enriched in the heavier isotopes since sputtered lower-mass elements have a higher chance of achieving the lunar escape velocity.
Auger electron spectroscopy has been used to analyze the composition of sputtered deposits as a function of ejection angle from Fe–Ni and Ni–Cu alloy targets. The sputtering was performed with 1-keV Hg+ or Ar+ multibeamlet ion beam (from a NASA thruster) impinging on the target at normal incidence. It was found that the composition varies substantially with ejection angle. In Fe–Ni the material which is ejected normal to the target surface shows up to 30% enrichment in Ni. Under very oblique ejection angles, one finds the corresponding Ni deficiency. In Ni–Cu it is again the Ni which becomes enriched (up to 30%) normal to the target surface. The effect has far-reaching implications in film deposition as well as in SIMS analysis.
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