Ring formation of Kirkendall voids in Pb-alloy thin-film diffusion couples

Abstract

Ring formation has been observed in thin-film diffusion couples consisting of a 400-nm-thick Pb0.71Bi0.29 (in weight fraction) or Pb layer above a 200-nm-thick Pb0.84In0.12Au0.04 layer with a very thin (∼6 nm) rf-grown oxide diffusion barrier in between. When interdiffusion was initiated by a rupture in the oxide barrier, concentric rings were observed to form in Pb-Bi or Pb layer centering at the rupture site as interdiffusion proceeded. Rings were found to be a string of voids resulting from Kirkendall interdiffusion. In addition, relatively high concentrations of indium and oxygen were found at rings. Although the mechanism for ring formation is not clear at present, factors which influence ring formation are discussed.