Fast mass migration in SnBi deposits enhanced by electric current

Electromigration in SnBi thin film was investigated experimentally and analyzed with the finite element simulation and the first-principles method. After current stressing, both Sn and Bi atoms migrated from the cathode to the anode and a higher migration rate of Bi than Sn was observed due to its lower diffusion activation energy, which resulted in forming the layer-like structure in the film. At the anode, besides Sn and Bi hillocks, unexpected voids were found which were associated with the current density distribution there. It was also interesting to find a much faster diffusion of Bi in the SnBi thin film than in the bulk sample. And this enhanced mass migration was considered to result from the small grain size in the thin film which could provide more fast diffusion paths for Bi atomic diffusion.