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Design >> Analog Design >> How to Calculate Metal Widths (Electromigration)
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Message started by nikogian on Jan 23^rd, 2007, 11:38am

Title: How to Calculate Metal Widths (Electromigration)
Post by nikogian on Jan 23^rd, 2007, 11:38am

Hi all,
I need some help with calculating the metal widths in my design in order to avoid electromigration.
My IC process provides me with the maximum allowed DC current density for each metal layer, VIA etc at 110 degrees Celsius. I have the following questions concerning the width calculation:

1) What about AC currents? the process gives me a rule about the allowed peak AC current density but what about steady sinusoidal currents? Should I calculate the width according to the effective current Iac/(2*pi) ? (Iac is the amplitude of the sinusoidal current)

2) When a wire transfers both AC and DC current, Should I just add the DC current and the effective AC current and then calculate the width?

3) What about the temperature. I don't think my circuit will ever reach 110 degrees while working. Is there a chance the the given DC current values are too pessimistic? If so, is there a way to calculate the maximum current density at a lower temperature?

4) I have read that electromigration is not a very big problem at high frequencies. If I calculate the width for sinusoidal currents using the effective current Iac/(2*pi), it seems like frequency is ignored. How can I account for the AC current frequency while calculating the metal width?

Thanks in advance,
John

Title: Re: How to Calculate Metal Widths (Electromigratio
Post by ACWWong on Jan 23^rd, 2007, 1:12pm

Electron migration rules addressing ac current typically use an rms limit value. The limit value is usually >10 times greater than the dc value, but depends on the metal processing in your process.
Temperature plays a huge part in the limits.... i have used kits previously where a exponential temperture adjustment calculation is given. If your kit is for 110 degrees then you say operate at only 55 degrees, the rules will relax by about 4 times (for the same power-on-lifetime).
Yes, high frequency doesn't cause additional problems.
I seem to recall some previous posts/paper links on electron migration on this forum. Try doing a search.

Cheers
aw

Title: Re: How to Calculate Metal Widths (Electromigratio
Post by bharat on Feb 7^th, 2007, 1:08am

I would like to add here:
EM is basically a DC current phenomenon though highly dependent on temprature. There is another term called SH (self heating). Here we calculate the Irms (RMS current) and power dissipation. This gives rise to increase in temprature and in turn giving reason for EM. My process provides both the EM and SH values in terms of metal widths.
Therefore EM is more relevant for power nets and SH is more for signals.
Now, there is something called SH for power nets which manifests the RMS value of current due to AC variation on power nets.
Similarly the EM of signal nets is leakage or short circuit DC durrent.
For example in a inverter circuit: The input and output is SH (where the current is flowing in both the directions) and flow of DC current from Vcc to Gnd is EM phenomenon.