Effects of capillary forces on the global thinning of copper metallization under electromigration stress

The global thinning of copper interconnections is modeled via the shape evolution of voids located at the copper/dielectric interface under electromigration stress conditions. The model includes the instabilities driven by both capillary and electron wind forces, and employs an axisymetric 3D finite difference numerical method, combined with the boundary element method, for solving the electrostatic problem. With zero electric field or with small fields applied, a large void experiences a capillary instability which leads to open circuit failure. As the electric field becomes larger, the numerical solution predicts that the growth of this instability is suppressed and the void shape stabilizes. Thus, for a typical electromigration stress condition, a large copper void elongates its shape along the interface parallel to the electric field, suggesting a mechanism for the delayed open circuit failure observed in copper metallization systems.