New Collision Theory of Cathode Sputtering of Metals at Low Ion Energies

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Abstract
Cathode sputtering is conceived as being produced by purely mechanical collisions between the impinging ions and surface atoms of the target. Energy is lost during the collisions by excitation of Debye waves in the lattice. The collisions are assumed to occur as between perfectly elastic spheres with radii determined by the largest closed electronic shells of the ion and the target atom. Only upper surface atoms of the target can be sputtered; the ejection requires that the momentum transferred to an upper surface atom have a component in normal outward direction and the transferred energy be larger than its binding energy to the lattice, which is assumed to equal the heat of vaporization. With the introduction of a dissipation coefficient, which determines the energy loss during the collision, formulas for the threshold energies at oblique and normal ion incidence and a sputtering rate formula for normal ion incidence at low ion energy have been derived. Experimental data on sputtering are in agreement with the derived formulas. From the derivation of the sputtering rate formula it can be concluded that the threshold energies must be roughly proportional to the squares of the collision radii of the target atoms. This implies a periodicity of the threshold energies within the periods of the periodic system, which has been evidenced by plotting experimental data on threshold energies for 26 metals versus atomic number.