what is Electromigration
Electromigration is defined as the molecular displacement of atoms caused by the
flow of electrons over extended periods of time. What this really means is that
metal lines will eventually break and create an open circuit by this effect.
Poorly designed parts will fail in the field after a period of flawless
operation because of a high degree of susceptibility to electromigration. With
decreasing scales of integrated circuits, concern has grown over the susceptibility
of metal lines, operating with high current densities and elevated temperature, to
degradation due to electromigration.
Electromigration is most prevalent in aluminum and aluminum-alloy interconnect
layers. This is one reason why new metals are being introduced into the
manufacturing process that will have no electromigration limitations (i.e., tungsten,
copper, etc.) and very low resistance.
Electromigration failures are avoided simply by increasing the width of
current-carrying metal lines so that an open circuit is avoided. As electromigration
is dependent on the amount of current flowing within a conductor, a set of
guidelines is used to guide layout designers to address this issue effectively
without incurring significant area overhead.