A mechanism of surface micro-roughening by ion bombardment

Abstract

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.