Ion Bombardment Effects on Material Compositions: Preferential Sputtering and Atomic Mixing

Author: Liau, Zong-Long

Year: 1979

Degree: Dissertation (Ph.D.)

Advisor: Mayer, James Walter

Committee Member: Unknown, Unknown

Option: Applied Physics

DOI: 10.7907/08ty-be56

Abstract

This thesis reports a study of the effects of ion bombardment on material composition and their implications in material modification and analysis. First, composition changes in binary alloys and compounds as a result of rare-gas sputtering were observed by using Rutherford backscattering techniques. The heavier components were generally found to become enriched in a surface layer whose thickness corresponded to the ion range. After an amount of material comparable to this thickness had been sputter-removed, the surface layer reached a steady-state. The steady-state surface composition was independent of the mass and energy of the sputtering ion. (Chapters 2 and 3)

The results were interpreted in terms of a preferential sputtering, which generated enrichment of the heavy species at surface, in combination with an ion-induced atomic mixing effect, which propagated the composition change over a depth comparable to the ion range. A model based on this interpretation seemed to combine all experimental results into a consistent picture. (Chapter 4)

The model was then extended to study the phenomena of high-dose ion implantation. The idea of preferential sputtering was used to predict the limits of compositions achievable by implanting ion species A into material B, or by implanting A+ into material AB. The formation kinetics of the implanted surface layer was determined by both sputtering and atomic mixing effects. The model yielded results in good agreement with preliminary experimental results. (Chapter 5)

One of the important implications of sputter-induced surface layer composition changes has been their effects on the use of sputtering in surface-cleaning and in depth-profiling techniques. In this respect, we also studied the effect of atomic mixing and preferential sputtering on the evolution of very thin surface layers during sputter-etching. We observed that, for low ion doses, the atomic mixing effect first produced a uniformly alloyed surface layer with a thickness comparable to the ion range. Then, during the successive steps of sputter-etching, the surface layer maintained a constant thickness, but with a decreasing alloy (or impurity) concentration. Again, the previously developed model was extended for the present case. It also combined the results into a consistent picture. Based on these studies, we then extended the model further to predict the effect of atomic mixing and preferential sputtering on the depth-profiling techniques. A simple equation was obtained, which related the "apparent" depth profiles to the true ones. (Chapter 6)

Finally, the effect of atomic mixing has been studied in the cases where the ion range penetrated through the interface between a surface metal film and an underlying Si-substrate. Silicide formation at the interface was observed for ion doses ≾ 1014cm-2. For higher doses, more Si-atoms were incorporated into the surface layer and the system appeared to be amorphized. After being thermally annealed, the samples showed formation of metastable phases which had not been reported previously. The present results suggest that the ion-induced atomic mixing effect has the potential of producing thin-film materials with any desirable compositions or with compositions and structures unachievable by conventional metallurgical means. (Chapter 7)

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