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EM is Electromigration. which determines the maximum current that can carried by a metal line for a period of time. Usually there are three important currents that we calculate for EM, peak current, RMS current and the DC current. Almost all the relations between current and metal width is given by the foundry and you can find them in your Design Rule Manual. RMS current normally decides the metal width that you require in your design. RMS currents are usually applied to signals that are charging and discharging . DC currents for signals which have steady state values. Peak current to complement the other two. RMS current can be calculated by putting a resistance at the output of the signal driver. With the RMS current value, apply it in the equation provided by the foundry and calculate the required metal width as well as no of vias that you need for your signal. EM is directly responsible for reliability of your design. Usually a chip has a reliability of 20yrs...But due to improper EM or not doing EM analysis can lead to shortening of the chips life...To sum it up, its about Heating effect and the current density in a metal line...another way is to find the average current by CVF formula...
The above explaination is a very good one about Electron Migration.
Electronmigration ,is the mass transport of a metal due to momentum transfer between conducting electrons and diffusing metal atoms ,exists when ever current flows through metal wires.
The conditions necesary for the EM to be a significant problem are :
1).high curent Densities
2).Long Narrow wires
3). Logic Hazards
4).High Operating frequencies.
design issues that cause EM are :
1).power Grids
2).signal lines
How to solve EM?
Transition from aluminium to copper for wire purpose was suppose to solve EM,
but usuage of copper has made analysis of chip more complex.
To find EM problems you need to extract and analyze full_chip data.Cadence verifcation tools such as VSTROM & ELECTRON STROM ,extract and analyze power grid and signal EM.
Finding signal problems reqire two analyses:
a).EM and b).Joule Heating.
Em requires average current data on signal lines.
Joule Heating analysis requires RMS current information.
signal analysis :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 ,current density is computed and failure models are applied to each wire segment .
Power Grid EM Issues can be solved with place and Route Tools ,which provide crtical info such as :
1.Average switching info for each cells within the block being routed .The switching info would be used to estimate the avg current dissipated by each cell in the design.
2.Loading info for each cell that drives an output of the block.
3.Flow of power current through the blockdue to how block is oriented with respect to power pins and other blocks on global grid.
Though we cannot entirely eliminate EM,we need to understand what cause EM so to esure that your chip do not suffer from EM.
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