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The structure and the stability of Cu-In alloy films have been studied especially at their growth stage by using XRD, XPS and AFM. The growth mode was found to vary near the composition of intermetallic compounds [1]. As an extremal phase of alloy films the nature of pure indium films is studied in the present experiment.
Indium films of 10-200 nm mass thickness were deposited on a silicon wafer. The morphology was studied with AFM. Films of this thickness range consisted of cap-shaped islands.
Compositional depth profiles could be obtained by a standard technique combining surface analysis of XPS with ion beam sputter etching. The surface composition of indium decreased with sputter-etching time as a quadratic curve. Furthermore, the curve was found to be common almost perfectly for all films thicker than 30 nm when the horizontal axis of the etching time was normalized by the deposited mass.
The universal quadratic curve can be explained by assuming that the silicon surface is covered just completely with indium islands of the same size and shape and the shape similarity is maintained during the sputter etching. That is, each island of indium shrinks its size in the same manner as a water drop on a sheet glass evaporates. The depth profile curve can be described only by the sputtering rate of island material. By fitting the theoretical curve to the experimental data, we could obtain a value of 0.33 nm/s for the sputter-etching rate of indium. The current density of an Ar+ ion beam of 30 µA/cm2 in the present experiment leads a value of 6.7 for the sputtering yield of indium by an Ar+ ion at 3.0 kV.
Besides the determination of the sputtering yield of indium, we can summarize this study that structural parameters such as island size and roughness can be estimated uniquely by the film thickness for indium films thicker than 30 nm.
[1] T.Nakano, H.Mizuhashi, S.Baba: Jpn.J.Appl.Phys., vol.44, pp.1932-1938 (2005). |