LOW PRIMARY ION FLUENCE DEPENDENCE OF SINGLE-CRYSTAL SPUTTERING - A MOLECULAR-DYNAMICS STUDY

In recent experiments, the fluence dependence of the sputtering yield for single crystals was investigated in the submonolayer range. Surprisingly, a 50% decrease of the yield was observed in the case of Ar ion bombardment of a ruthenium single crystal. We have investigated this behaviour by means of a molecular dynamics simulation for 100 to 500 eV Ne, Ar and Xe ion bombardment of a Cu(100) surface. The simulations indicate that changes in the surface structure on an atomic scale can be responsible for this observed decrease in the sputtering yield. Removal of a surface atom due to sputtering will leave a 'hole' at the surface. We have calculated how the sputtering yield changes in the neighbourhood of such a missing surface atom. For example, in the case of 500 eV Ar on Cu the yield drops from 3.3 to 2.3 for primary ion impact parameters within half of the lattice constant of the missing atom position and is only slightly above normal outside this area, reaching the value for the undamaged surface (3.3) within a lattice constant. The effect is even more pronounced for lower energies and higher ion masses (Xe). Thus the yield of a virgin surface will decrease with fluence until an equilibrium surface topography has developed. This effect could also explain why molecular dynamics calculations typically give sputtering yields higher than experimentally observed values.