Electromigration, copper

A failure mechanism in semiconductor devices. As in the case of aluminum (Al), momentum exchange between current-carrying electrons and copper (Cu) ions in a Cu line cause Cu ions to drift in the direction of the electron current. In the presence of flux divergence sites where the flux of Cu atoms into the site is unequal to the flux leaving the site, a stress gradient is induced that is proportional to the current density, and can be tensile or compressive, depending on the sign of the divergence. In sufficiently long conductors with sufficiently high current densities, the tensile stress at a negative divergence site will increase to the point where voids will form due to vacancy coalescence, and will grow until they are large enough to cause failure. Compressive stress at sites of positive divergence will cause the formation of extrusions and hillocks that can cause cracking in the protective passivation, and short circuiting of neighboring conductors due to extruded Cu. (Reference JEP122F)