New insight into the development of pyramidal structures on bombarded copper surfaces

Abstract

The morphological evolution of pyramids, developed on bombarded Cu surfaces, as a function of consecutive doses of normally incident mass-separated 12 keV Kr⁺ ions from 2.4 × 10¹⁶ up to 3 × 10¹⁹ ions/cm² has been studied. Three types of polycrystalline sample, with initial surface topography due to distinct polishing procedures, were used, Three main conclusions were reached. (a) The presence on the unbombarded surfaces of asperities is very important, there being a 1:1 correlation between pyramids and pre-existing asperities, at least for the surfaces as here prepared. (b) In agreement with the predictions of Carter, Colligon, and Nobes, all pyramids disappeared at high enough doses with all specimens. Pyramids are therefore not an equilibrium structure. (c) Features with a mushroom-like (i.e. undercut) shape were observed which can, under certain circumstances, be an intermediate stage towards the final pyramidal shapes but are, more generally, an indication of the existence of what we will call tertiary effects. The results, taken as a whole, suggest a model based on the evolution of asperities by a combination of primary sputtering with further shape modification due to secondary and teriary effects, i.e. due to particle trajectories as influenced by the pyramids (“secondary”) and the grooves underlying the pyramids (“tertiary”).