Kirkendall void formation in thin-film diffusion couples

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Abstract
Transmission electron microscope studies of Sn/Au thin-film couples reveal complexities associated with the Kirkendall effect in thin films. The use of an offset film fabrication technique permitted the identification of two room-temperature interdiffusion mechanisms in Sn/Au: grain-boundary diffusion of Sn into Au, which causes film thinning, and bulk diffusion of Au into Sn, which results in the formation of microscopic Kirkendall voids. The ranges of importance of the two mechanisms were explored by varying the Sn and Au film thicknesses and the grain size. It is pointed out that interdiffusion studies in thin films required careful attention to relative film thicknesses, film geometry, and grain sizes in order to determine the fast-diffusing species and the operative diffusion mechanisms.