Theoretical consideration of electromigration damage around a right-angled corner in a passivated line composed of dissimilar metals

Abstract

Electromigration (EM) evaluation near contact corners in interconnection structures composed of dissimilar materials in particular is becoming increasingly important. A theoretical analysis of the atomic density distribution around a right-angled corner in a passivated line composed of dissimilar metals is performed. The 2D structure considered, including the corner, is assumed to consist of dissimilar single-crystal metals with uniform passivation and line widths. While the atomic density distribution in the passivated 1D straight line has been reported previously, the atomic density distribution in a passivated 2D structure that contains a right-angled corner composed of two dissimilar metals has not been studied. This work clarifies the atomic density distribution in the 2D structure under equilibrium conditions for the two fluxes, which are the atomic flux due to the electron wind force and the flux due to the back flow caused by the atomic density gradient in EM, and presents the predicted locations of EM damage in the form of voids and hillocks. In addition, this paper proposes countermeasures to increase the threshold current density for EM, which contributes to enhancement of the reliability of the metal line against EM. The selection of suitable values for several material properties are discussed from the viewpoint of preventing initiation of EM damage by increasing the EM threshold current density. Suitable material combinations to increase the threshold current density are also discussed.