Similarities and Differences in the Mechanisms of High and Low Energy Ion Mixing
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SIMILARITIES AND DIFFERENCES IN THE MECHANISMS OF HIGH AND LOW ENERGY ION MIXING Yang-Tse Cheng, Steven J. Simko, Maria C. Militello, and Audrey A. Dow, General Motors Research Laboratories, Warren, Michigan 48090-9055; Gregory W. Auner, M. H. Alkaisi, and K. R. Padmanabhan, Wayne State University, Detroit, Michigan 48202. ABSTRACT High energy ion mixing occurs when an ion beam of a few hundred keV bombards an interface under the surface. Low energy ion mixing arises when an ion beam of a few keV bombards an interface near the surface during, for example, sputter depth profiling and low energy ion assisted deposition. At low temperatures, the rate of both high and low energy ion mixing can be influenced by thermodynamic parameters, such as the heat of mixing and the cohesive energy of solids. These effects are demonstrated by ion mixing experiments using metallic bilayers consisting of high atomic number elements. A model of diffusion in thermal spikes is used to explain this similarity. Low energy ion mixing can also be strongly affected by surface diffusion and the morphological stability of thin films. These effects are illustrated using results obtained from sputter depth profiling of Ag/Ni bilayers at elevated temperatures. High energy ion mixing at low temperatures can be influenced by the anisotropic momentum distribution in a collision cascade as seen from a set of marker experiments to determine the dominant moving species in high energy ion mixing. These similarities and differences between high and low energy ion mixing illustrate the diversity of ion-solid interactions. INTRODUCTION High energy ion mixing was first observed in 1972 by Lee et al., 1 who observed the intermixing of Pd and Si at the interface of a Pd thin film and a single crystal Si substrate during phosphorus ion implantation. Subsequently, it was found that many 2 5 metastable as well as equilibrium phases could be obtained by ion mixing. - These new phases offered possibilities for industrial applications. The mechanisms of ion mixing became an important issue in high energy ion mixing. Low energy ion mixing has been associated with sputter depth profiling and, more recently, with low energy ion assisted deposition. Ever since the introduction of the sputter depth profiling technique by Sizmann and co-workers, 6 it has been known that the low energy ion beam of a few keV, intended for the sputter removal of materials from surfaces, can also cause ion mixing that inevitably distorts the structure to be examined. Ion mixing can, therefore, limit the depth resolution of the sputter depth profiling technique. In addition, low energy ion beams are now used for ion beam assisted deposition of new thin film structures with applications in optical, wear and corrosion resistant coatings. (For recent reviews, see: Rossnagel and Cuomo,' Mattox,s Wolf et al.9 ) An understanding of the mechanisms of low energy ion mixing is, therefore, also important. In practice, ion mixing experiments are carried out using various ion energies and at different am
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