Scaling effect of interfacial reaction on intermetallic compound formation in Sn/Cu pillar down to 1 μm diameter

Different diameter Sn/Cu pillars varying from 30 to 1 mu m have been made by focus ion beam, and the state reaction to form Cu6Sn5 in these pillars has been studied. An increased Cu-Sn reaction rate to form Cu6Sn5 was observed as the pillar diameter decrease from 20 to 1 mu m when annealed at 185 and 195 degrees C. With the shrinking of pillar size, surface diffusion becomes important as compared to grain boundary and lattice diffusion in the Cu-Sn interfacial reaction. Also, surface diffusion induced Kirkendall was observed to locate at the peripheral area just above Cu of the Cu-Sn pillars. A simple kinetic model of surface diffusion controlled intermetallic compound growth of Cu6Sn5 is proposed for pillars with diameter below 5 mu m. What is essential in the model is that we assume a rapid interstitial diffusion of Cu in Sn, and we are able to calculated surface diffusivity of Cu on Cu6Sn5. At 185 degrees C, the surface diffusivity is about 3.1 x 10(-7) cm(2)/s and the activation energy is about 0.2 +/- 0.1 eV/atom. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.