Electromigration (EM) is
the movement of material that results from the transfer of momentum between
electrons and metal atoms under the influence of 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.
Narrowing of the wire can result in degradation of performance, or in extreme cases can result in the complete opening of the conduction path as shown in the picture below.
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 of pitch
in the newer technologies.
Broadly EM is classified as cell EM and Wire EM.
Cell EM
Cell EM rules address the EM caused by current
within a cell. Cell EM rules operate on the principle that, although the
currents within a cell cannot be calculated due to a lack of physical layout
information, they can be controlled based on external physical entities. The tool estimates the detrimental effects of currents within a cell as a function
of its,
- Output load
- Input slew
- Switching frequency
There are two types of wire EM:
Signal EM – It is performed net by net, simulating the charging and discharging for all possible paths to determine the worst-case average and RMS current for each wire segment. Once currents are determined, the current density is computed.
Self-Heating: It is a physical design issue that takes place in the output
nodes/interconnects of circuits that charge and discharge frequently, Leads to other problems caused by heating, like an increase in resistance of the interconnect and hence an increase in charging time of the node. Also, it causes thermal reliability issues.
Techniques to solve EM:
- Increasing the metal width to reduce the current density is a typical solution
- For a via EM violation, you can increase the number of vias to fix potential EM issues
- Additional straps for the current supply
- Layer switching is another option; typically, upper metal layers in the technology have higher current
- driving capability (due to greater thickness)
- Reduce the cell size driving the signal net if we have positive slack on that path
Source: From a Friend in PDFundamentals Group---Thanking him for sharing his knowledge
