A2 Degradation by Electromigration

Topic

When this metallic interconnect carries a very high current density, it generates an internal "electron wind" force acting upon its own metal atoms: as conduction electrons traverse the interconnect at this high density, they collide with the metal atoms along their path. The momentum transferred during these collisions pushes the metal atoms in the direction of electron flow—contrary to the direction an isolated positive ion would move under the influence of an electric field. Consequently, a higher current density results in a stronger electron wind force acting on the metal atoms.

This same electron wind force induces an atomic flux within the metallic interconnect: pushed by the force in the direction of electron flow, the metal atoms physically migrate—atom by atom—along the interconnect. This results in a net displacement that ordinary thermal agitation alone would not produce in any preferred direction. Consequently, a stronger electron wind force acting on these atoms leads to a greater induced atomic flux. Consequently, in this model—initiated by the force of the electron wind and the resulting induced atomic flux—that same induced atomic flux creates voids wherever it accumulates unevenly along the metal interconnect: in sections where more metal atoms leave (dragged by the flux) than enter, a deficit of metal atoms builds up, eventually opening a void; once this void grows large enough to completely sever the interconnect's cross-section, it causes an open circuit. The fact that the current density—which initiated the process in this model—remains higher for a longer period across the metal interconnect causes the induced atomic flux to build up this void more rapidly; consequently, the mean time to this open-circuit failure—which Black's equation predicts based precisely on this current density and the interconnect's temperature—is reduced.

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