Experiments by radioactive tracer methods on sputtering by rare-gas ions

Abstract

By incorporating the radioactive isotope 198 Au either homogeneously or in specific layers parallel to the surface in plane, polycrystalline gold targets, it has been possible to investigate the angular distributions of particles ejected in the sputtering process induced by bombardment with A + and Kr + ions in the energy range 5 to 10 keV, and also to study the ejection of particles from different depths below the surface of the target. Compared with a cosine-law representation, the angular distributions were found to be more strongly peaked in the direction of the target normal. This departure from the cosine law was found: ( a ) to decrease with decreasing ion energy, ( b ) to be smaller for Kr + ions than for A + ions of the same energy, and ( c ) to decrease when the 198 Au was restricted to thin surface layers. These results show systematic behaviour with respect to the penetration range of the incidentions and are consistent with the view that the sputtering process includes the ejection of atoms from finite depths, resulting in some degree of collimation along the target normal. Direct experimental verification of this conclusion was sought by the use of subsurface layers of 198 Au. With A + ions of 8 keV energy a significantly positive result was obtained, but the depth involved with Kr + ions of the same energy was much smaller. The maximum depths from which Au atoms were ejected by A + and Kr + ions of this energy were found to correspond reasonably with calculated ion ranges. The experiments have shown that in the sputtering process, atoms are ejected instantaneously from depths of several atomic diameters.