Behaviors of small molten metal islands on several substrates†

Abstract

Electro- and thermomigration of metallic islands of µm size produced by vapor deposition on the Si(100)2×1 surface were investigated by ultra-high-vacuum scanning electron microscopy at substrate temperatures higher than the melting-points of islands. The direction of electromigration due to electric current passing through the Si substrate depends on the type of metal, whereas the direction of thermomigration caused by a temperature gradient on the substrate surface is always from low to high temperature, independent of the type of metal. The speeds are 0.1-10 µm s^–1 and are approximately proportional to the island radius and increase exponentially with temperature in both cases. The driving forces of the island migrations are explained by the diffusion theory of metals in Si due to the electric field or the thermal gradient. Furthermore, it was found that the melting-points of metal islands on the carbon substrate are lower than those of the bulk and, moreover, the wettability (the contact angle) of the molten Cu or Ag islands changes in an oscillatory manner on the SiO₂ substrate with periods of 100-0.1 s depending on the island diameter and substrate temperature. The origin of the contact angle oscillation is explained by the periodic change of the interface profile between island and substrate and by the failure of Young's equation due to the change of orientations of surface and interface tensions.

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