Electromigration - Page 1 of 2

Electromigration

Electromigration refers to the gradual displacement of the metal atoms of a conductor as a result of the current flowing through that conductor. The process of electromigration is analogous to the movement of small pebbles in a stream from one point to another as a result of the water gushing through the pebbles.

Because of the mass transport of metal atoms from one point to another during electromigration, this mechanism leads to the formation of voids at some points in the metal line and hillocks or extrusions at other points. It can therefore result in either: 1) an open circuit if the void(s) formed in the metal line become big enough to sever it; or 2) a short circuit if the extrusions become long enough to serve as a bridge between the affected metal and another one adjacent to it.

Electromigration is actually not a function of current, but a function of current density. It is also accelerated by elevated temperature. Thus, electromigration is easily observed in Al metal lines that are subjected to high current densities at high temperature over time.

Figure 1. Photo of a metal line that has developed

voids due to electromigration; source: www.nd.edu

Electromigration is widely believed to be the effect of momentum transfer from the electrons of the metal, which move according to the applied electric field, to the ions that constitute the lattice of the metal.

There are two major driving factors that make electromigration happen: 1) the direct action of the electric field on the charged atoms or ions of the metal; and the 2) frictional force or momentum exchange between the flowing electrons and these ions. The total driving force is the sum of the effects of these two factors.

All metal films have imperfections or microstructural variations that cause the atomic flow rates through them to be non-uniformly distributed. This non-uniform atomic flow rates (or flux divergence) through different sections of the conductor result in mass depletion (which causes voids) and mass accumulation (which causes hillocks) as the mass transport mechanism occurs during electromigration.

Figure 2. Photos of metal lines that have developed

hillocks due to electromigration; source: www.nd.edu

In Al films, the dominant mechanism of atomic migration is along grain boundaries and surfaces. Lattice mismatches (such as those between adjacent large and small grains or when three grain boundaries meet) can create grain boundary interconnections that provide shorter paths for the atoms, enabling the latter to move faster through the film.

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