EM

Electromigration: 

Electromigration is the gradual displacement of metal atoms in a semiconductor. It occurs when the current density is high enough to cause the drift of metal ions in the direction of the electron flow, and is characterized by the ion flux density. 

This density depends on the magnitude of forces that tend to hold the ions in place, i.e., the nature of the conductor, crystal size, interface and grain-boundary chemistry, and the magnitude of forces that tend to dislodge them, including the current density, temperature and mechanical stresses. 

Failure Mechanisms

There are two different EM failure mechanisms that occur due to asymmetry in the ion flow. The below shows a void where the outgoing ion flux exceeds the incoming ion flux, resulting in an open circuit. The second example shows a hillock where the incoming ion flux exceeds the outgoing ion flux, resulting in a short circuit.

Fig: Open circuit and Short circuit

Electromigration Dependency on Physical Effects:

1) Temperature:

Electromigration is dependent on temperature, however, there is a more sinister dependence on temperature that accelerates failures due to voids. Below figure highlights a cyclical positive feedback loop that ultimately ends in failure.

Once a void begins to develop in a metal wire, the wire itself becomes narrower at that point. Due to the reduction in width, the current density increases and, therefore, the interconnect temperature increases due to Joule heating. Joule heating is a result of root-mean square (RMS) current. As the temperature of the wire increases, the growth of the void accelerates, and eventually an open circuit occurs.

2) Wire Width:

Current density is the primary factor influencing electromigration. By increasing the wire width, current density is reduced and susceptibility to EM is reduced.  

3) Wire Length:

There is a lower limit for the length of interconnect that will be subjected to the effects of electromigration. It is known as “Blech length”, and any wire that has a length below this limit will not fail by electromigration.