The spatial distribution of sputter-etch effects is analysed theoretically under the assumption of random slowing-down of the bombarding ions. When a surface is bombarded at locally oblique incidence the most pronounced sputtering effect is likely to be observed not at the very point of impact, but further “downstream”. This effect may cause a significant reduction of the local sputtering yield on top of a spike or a ridge, and an increase on the bottom of a groove or a crater. As a consequence, small irregularities on a relatively smooth surface may be enhanced during bombardment. It is concluded that a microscopically flat surface is unstable under high-dose ion bombardment unless atom migration acts as a dominating smoothing effect. Conversely, sharp cones appear to be surprisingly stable under bombardment. Even on a clean surface, cones erode more slowly than a plane surface, provided that their dimensions are of the order of, or less than, the penetration depth of the ions.

A mechanism of surface micro-roughening by ion bombardment

After a brief outline of the present sputtering theory for a random solid, recent results of the sputtering yieldS for polycrystalline targets are discussed, in particular in view of the influence of the projectile mass and the bombarding angle. The angle dependence ofS at low bombarding energies, and results on the angular distribution of sputtered particles for oblique ion incidence point out necessary modifications of present sputtering theories with respect to the anisotropy of the collision cascades in the solid and the influence of the target surface. The energy distribution of the neutral particles ejected along the target normals is related to the theoretically predictedE
 −2-distribution of low energy recoils in the Recent mass spectrometric studies of postionized sputtered neutrals are discussed in view of the formation of sputtered molecules and the application of sputtered neutral mass spectroscopy for surface analysis. Finally, the paper deals with ion-induced surface effects on non-elementary sputtering targets, and the protracted removal of foreign atoms from a matrix.

Sputtering—a review of some recent experimental and theoretical aspects

The two kinematic theorems of crystal dissolution by Frank are applied to surfaces undergoing ion-bombardment in order to determine the topographies evolved in any given time. The predicted shapes show good agreement with experimental observations.

Prediction of ion-bombarded surface topographies using Frank's kinematic theory of crystal dissolution

http://arc.nucapt.northwestern.edu/refbase/files/MatChar44p059LEAPTFKelly.pdf

Local Electrode Atom Probes

The surfaces of semiconductor single crystals were observed using a scanning electron microscope after bombardment at normal incidence at room temperature with doses between 0.5 × 1018 and 3 × 1018 of 40 keV argon ions/cm2. The usual topography of the sputtered surface and the perturbing effects of dirt and contamination are reported, and observations of the variation of topography with dose are reported for GaAs. An explanation for the features observed is discussed with particular reference to current models of cone formation and secondary processes such as flux enhancement at the foot of steep slopes. The behaviour of Si, Ge, GaAs and InP was consistent with an amorphous surface. In the case of CdTe, CdS and GaP it appeared that the crystal structure was retained resulting in a topography of the sputtered surface that is sensitive to orientation. InSb appears to undergo a chemical change during ion bombardment.

The topography of sputtered semiconductors

Argon and xenon bombardment of gold surfaces at 5 to 20 keV resulted in considerable erosion and the formation of cones. A high cone density was found to be characteristic of amorphous (mechanically polished) surfaces whilst a low cone density with height contrast of the individual grains was discovered on polycrystalline (etched) surfaces. The cone angles were found to vary only slowly with energy, indicating an ion/atom potential that varied much more rapidly with separation than predicted by using a screened coulomb potential.

I. H. Wilson

Papers

A study of cones developed by ion-bombardment of gold