Electromigration is a term most often used in the context of the semiconductor
technology of integrated circuits
. It refers to the phenomenon
of mass transportation of material within the thin film metal
tracks used to carry signals within IC
When a metal is used to conduct electricity, this entails the movement of electrons through the material. As electrons move through the conductor their progress is impeded as they collide with the crystal lattice of the conducting material and momentum is exchanged. This is electrical resistance.
The number of collisions between electron and lattice at any point in the conduting material is dependant upon current density and the physical integrity of the lattice itself at that point. What this boils down to is that the amount of momentum transfer is not uniform throughout the conductor. Since such collisions generate heat, temperature gradients will be set up within the conductor and material will begin to move if the current density is sufficient. As conducting material moves away, voids are formed and the local current density increases. This, in turn, reinforces the movement of material until such time as an open-circuit in the conducting material brings the electrical current to a halt. The IC has now failed.
Electromigration is one of the biggest reliability issues faced by IC manufacturers but it is a well understood failure mechanism and steps can be taken to reduce its effect. These include the introduction of a small percentage of copper into the mostly aluminium material used for the metal layers.