Electromigration and failure in electronics: An introduction

Some fundamental aspects of electromigration phenomena as they have been studied in "bulk" metallic conductors are reviewed. In an electric field atoms are subjected to a force due to the field, and to a force which results from the motion of electrical carriers, electrons, or holes. In bulk samples, and at high temperatures, these forces cause the displacement of atoms by a lattice mechanism which is also responsible for the diffusion of atoms in a concentration gradient. In thin films, electromigration has been found to occur at lower temperatures (and higher current densities) by a grain boundary diffusion mechanism. Electromigration may cause failures at material discontinuities, such as found at terminals, at temperature gradients, or at structural inhomogeneities. The process of crack formation, as observed in aluminum thin films, is described. Failure times are a function of the activation energy for diffusion and of exponents of the current density which vary for different failure modes. The effects of film purity, orientation, grain size, glass overcoat, and solute additions on lifetime are reviewed. Practical guidelines for the design of thin-film interconnections, and for the interpretation of accelerated test data are given.