Fundamental questions in the theory of electromigration

The theory of electromigration is focused on the force acting on a lattice defect in a metallic sample that carries an electric current. Much work has been done to obtain a better understanding of the underlying physical mechanisms. The force has been calculated numerically for defects in several metals, and a qualitative agreement with the experiments has often been found. There are, however, still discussions about the relevance of certain contributions to the force. These originate from conceptual difficulties related to 1) the nature of the screening of the electric field at the site of an impurity by the conduction electrons and 2) the existence and significance of inhomogeneities in the electric field and current flow near an impurity. This paper provides a review of the basic models and of questions which still exist in the theory of electromigration. The relevance of these questions is illustrated by results of experimental work on the electromigration of H in V, Nb, and Ta.