Mass effect in the physical sputtering of multicomponent materials

Abstract

Bombardment of solid surfaces by energetic ions produces a layer of altered chemical composition at the surface. This is a consequence of the fact that the sputtering yields for the various components are unequal. There has been much discussion in the literature as to whether the observed compositional changes correlate more successfully with mass or with chemical bonding. Definitive evidence on this subject is lacking because it is difficult to conduct experiments where only one of the important parameters is changed. The sputtering of chemisorbed nitrogen from W(100) and Mo(100) sheds light on this problem. The lattice constants, the binding energy of nitrogen to the surface, and the adsorption sites are quite similar for these two systems. This should be a clear cut example of where the difference in sputtering yields depends only upon the mass of the surrounding atoms; i.e., whether they are adsorbed upon molybdenum or tungsten. Experiments on these systems using argon and xenon ions in the energy range between 300–5000 eV show that the mass effect is large. The ratio of the sputtering cross sections σ(N–Mo)/σ(N–W) varies from 0.46 at 500 eV to 0.65 at 5000 eV for xenon. The corresponding numbers for argon are 0.73 and 0.82. It is concluded that a similar mass effect is likely to occur in alloy systems. The experiments also demonstrated the absence of significant sputtering due to thermal spikes. Moreover, cascade mixing and recoil implantation were not detected in these experiments even though these phenomena were originally expected. A small amount of chemical sputtering was observed.