Threshold Energies in Mechanical Collision Theories of Cathode Sputtering

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Abstract
It is shown that the very low threshold energies for sputtering, measured with highly sensitive methods (surface ionization, radioactive deposits, spectroscopic method), immediately after applying a ``clean‐up'' sputtering with higher ion energy, are due to the fact that the applied intermediate sputtering produces on the target upper‐most surface atoms with reduced binding energies ranging from a maximum to a minimum number of bonds. The lowest ion energy, at which a yield can be measured in the described manner, is just sufficient to remove the latter atoms and is defined as the ``minimum‐bond'' threshold. The thresholds, formerly determined for polycrystalline targets at normal ion incidence without intermediate sputtering for many metal‐ion combinations by ``detection of deposits'', correspond to the ``full‐plane'' thresholds, necessary for sputtering of atoms, having the maximum number of bonds, and represent the lowest ion energies at which continuous, i.e., plane by plane, sputtering of the target surface is feasible. The ratios of the ``full‐plane'' thresholds to the ``minimum‐bond'' thresholds, the latter measured at exclusively normal ion incidence with a sensitive method, after intermediate sputtering with a higher ion energy, are assumed to be the same (about 2:1) as calculated for a polycrystalline surface from the ratio of the binding energies of an atom in a full plane to the binding energies of an atom on top of a full‐plane area. The larger ratios (about 3:1) of the formerly measured ``full‐plane'' thresholds to the very low thresholds, as measured with highly sensitive methods, are shown to be probably due to boubly‐charged ions and to oblique ion incidence, which are exclusively active at the lowest ion energies.