Simulation of isotopic mass effects in sputtering

Abstract

The multiple interaction, molecular dynamics code SPUT1 has been used to simulate the effects of isotopic mass differences on atoms sputtered from single crystal Cu targets by normally incident Ar ions. Calculations were carried out for 1 keV and 5 keV ions incident on natural Cu targets (69.1% ^(63)Cu, 30.9% ^(65)Cu). and for 5 keV ions incident on pseudo-Cu targets composed of mixtures of natural Cu (63.546 amu) and "very light" Cu (50.837 amu) in the abundance ratios 1:3, 1:1, and 3:1. In all cases the sputtered ejecta showed an overall enrichment in the light isotope relative to the isotopic composition of the target. Preferential enrichment of the light isotope in the normal direction was pronounced. Material ejected at oblique angles was either depleted in the light isotope or had a much lower enrichment of the light isotope compared to material ejected normal to the target. Studies with the pseudo-Cu targets showed that smaller enrichments were obtained when the incident ion recoiled immediately back through the first layer of the target, while larger enrichments were associated with deeper penetration of the incident ion into the target crystallite. In both cases, the average energy of the light atoms in the collision cascade was found to be higher than that of the heavy atoms. However, this effect was enhanced with deeper penetration of the incident ion into the target. The preferential enrichment of the light ejected atoms normal to the target is largely the result of a strong momentum asymmetry in the collision cascades. Light atoms in the cascades, on average, carry far greater momentum towards the surface of the target than do the heavy atoms. A limited number of simulation runs also were carried out with heavy ions (74 amu) incident on pseudo-Cu targets. Overall enrichment of the light atoms in the sputtered material was reduced, but the angular variation of the isotopic yields persisted.

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