Investigation of Ion Beam Sputter Mechanisms by Electron Beam Techniques

Abstract

The mechanisms of unisotropic atomic emission from single crystals under ion bombardment (Ar+, E < 25keV) have been investigated by depositing the sputtered atoms in a semi-spherical collector. A subsequent determination of the thickness distributions of the deposited films by electron backscattering gives the total angular atomic emission distribution, showing maxima in certain crystallographic directions of the sputtered target crystal: Wehner spots.Several mechanisms for the appearance of spot patterns are discussed in the literature:1.For normal incidence of the ions collision cascades are assumed with finally isotropic momentum distributions within the target crystal. Then a momentum transfer to atoms near the target surface may be possible by the following models (fig. 1):1a.Collision sequences along close packed atomic chains (e.g. the <110> direction in fcc crystals). If the residual collision energy exceeds the surface binding energy the atom at the surface end of the chain is emitted into the chain direction: Focusson model (fig. 1a).1b.Emission of subsurface atoms through potential minima between surface atoms (fig. 1b), and emission of surface atoms by central collision of subsurface atoms (fig. 1c). Both processes lead to emission into preferred directions: Surface model.