Electromigration is a term applied to the transport of mass in both molten and solid metals when the materials are stressed at high current densities. Therefore, the atomic motion is not determined only by the electrostatic force imposed by the applied field but also by the direction of motion of the charge carriers.
In other words, electromigration is the transport of atoms induced by the gradual displacement of ions in a conductor or semiconductor due to the momentum transfer between conducting electrons and diffusing metal atoms. This effect occurs when the current density is high enough to cause the drift of ions in the direction of the electron flow, and is characterized by the ion flux density. The electromigration mechanisms are related to the nature of the conductor, crystal size, interface and grain-boundary chemistry, and the magnitude of forces such as the current density, temperature and mechanical stresses. As the structure size in electronics such as integrated circuits (ICs) decreases, the practical significance of the electromigration effect increases.
The driving forces for electromigration can be categorized by the two components,
i) purely electrostatic force,
ii) interaction with moving charge carriers, e.g. electron wind force.
The total driving force on a moving ion can be given by,
F = Fwl + Fel ---------------------- [2893a]
Fwl -- the electron wind force,
Fel -- the electrostatic force.
The electron wind force Fwl is normally larger than the electrostatic force Fel.
In theoretical treatment, the driving force is usually given in terms of an effective charge Z*,
Feff = |e|Z*ξ ---------------------- [2893b]
|e| -- the absolute value of the electronic charge,
ξ -- the electric field (= -∇ϕ).
The sign of Z* can be positive or negative, depending on the properties of the materials.