Electromigration & Self Heating

Electromigration 

Electromigration (EM) is the movement of material that results from the transfer of momentum  between electrons and metal atoms under an applied electric field. This momentum transfer causes  the metal atoms to be displaced from their original positions. This effect increases with increasing current density in a wire, and at higher temperatures the momentum transfer becomes more severe. Thus in sub-100nm designs, with higher device currents, narrower wires, and increasing on-die temperatures, the reliability of interconnects and their possible degradation from EM is a serious concern

The transfer of metal ions over time from EM can lead to either narrowing or hillocks (bumps) in the wires. Narrowing of the wire can result in degradation of performance, or in extreme cases can result in the complete opening of the conduction path. Widening and bumps in the wire can result in shorts to neighboring wires, especially if they are routed at the minimum pitch in the newer technologies.
Foundries typically specify the maximum amount pitch in the newer technologies.

Foundries typically specify the maximum amount of current that can flow through a wire under varying conditions. These EM limits depend on several design parameters, such as wire topology, width, and metal density. EM degradation and EM limits depend on the temperature at which interconnects operate, as well as on the material properties of the wires and vias, on the direction of current flow in the wire, and on the distance of the wire segment from the driver(s).

One common EM check employed is to measure the average or DC current density flowing through a wire and compare it against foundry-specified limits.

1) EM is a Physical Design issue

ii) Due to high electric fields, fast moving electrons knock off ions forming the metallic interconnect thereby eroding the interconnect
iii) EM takes place over a long period of time and becomes a reliability issue, becoming a factor in determining the lifetime of an IC
iv) Usually occurs in interconnects where electron movement is continuous and unidirectional (Power Network in an IC)
v) As technology nodes keep getting smaller, EM is becoming a physical roadblock in scaling of interconnects

Fix methods:

1. increasing the metal width to reduce the current density is a typical solution.
2. for a via EM violation, you can increase the number of vias to fix potential EM issues
3. additional straps for the current supply
4. Layer switching is another option; typically, upper metal layers in the technology have higher current driving capability (due to greater thickness).

Self heating

i) It is a physical design issue
ii) Takes place in the output nodes/interconnects of circuits that charge and discharge frequently
iii) Leads to other problems caused by heating, like increase in resistance of the interconnect and hence increase in charging time of the node
iv) Also cause thermal reliability issues