Introduction

Surface topographic changes occur as a result of all particle–solid interactions. For large doses > 1017 particles/cm2 of energetic (tens of kilo-electronvolts) particles, these changes are often visible with the naked eye. Many mechanisms give rise to these topographic features, for example bombardment-induced defects in a solid or differential sputtering yields across a surface, due to grain boundaries or impurity inclusions. Electron micrographs of such surfaces reveal a wide variety of features such as etch pits, ridges, facets, ripples, cones and pyramids. The article by Carter, Navinšek and Whitton (1983) illustrates some of the features that have been observed, but many examples can be found also in most recent issues of the Journal of Vacuum Science and Technology. Features that develop can be an unwanted artefact of the ion bombardment technique, for example, they can be responsible for considerable uncertainty in the depth resolution of surface analytical techniques such as dynamic SIMS (secondary ion mass spectroscopy).

On the other hand, surface engineering seeks to etch well-defined patterns on surfaces. Ion beam lithography uses focused beams to etch patterns directly into a substrate but for most technological applications broad beams with masks are used. The modelling of the development of surface shape as a result of particle bombardment is therefore important from the point of view of understanding the basic physical processes involved. In addition it also has important engineering applications.