Kirkendall Voids Nucleation and Growth in the Polycrystalline Au-Al Wire Bond: An Atomic Phase-Field Study

Abstract

In this paper, the evolution of Kirkendall voids under external mechanical loading in the nano-polycrystalline Au-Al wire bond is studied at the atomic scale by modeling a polycrystalline diffusion couple with the binary phase field crystal (BPFC) method. For polycrystalline structures, the effects of multiple diffusion mechanisms on the evolution of voids are included simultaneously. The morphology of the Au-Al wire bond with the initiation and development of Kirkendall voids under different loadings are simulated in detail and the mechanism behind is discussed. We find that the voids initiate from the dislocations not only at the Au-Al interface but also at the inner grain boundaries in the Au phase, and the following development of these voids is significantly influenced by the loading directions. Besides, the amplitude of the cyclic load also exhibits considerable impact. The mechanism of the coupling between mechanical loading and solid diffusion is analyzed by revealing the free energy and porosity of the system. This work sheds light on evaluating the reliability of the metal interconnect system subjected to long-term mechanical loading based on physics at the atomic scale.

Keywords: Kirkendall voids, BPFC model, Polycrystalline structure, Grain boundary diffusion, Mass migration, Au-Al wire bond