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library metal width requirement

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edagops

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Hi,
In the technology library, why in most scenarios the lower metal layers(M1) have lesser width(eg. Max width for M1 is 1u) compared with higher metal layers(M7, M8 etc) have wider width preference(eg. Max width for M8 is 4u).

Thanks,
Gopi
 

I guess, maybe that is because the differences between each metal layers' resistivity.
 
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    hawker

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Actually, there is one more reason. Higher level metals are used to route across long distances. And the longer you want a metal line to be, the thicker it has to be. This is because of process limitations. (Otherwise even a small variation on the line can cause shorts/open ckts)

Also, vias decrease in size as you go to lower levels, basically from pad size to just a contact to the substrate. So obviously the metals follow the same rules.
 
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    hawker

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Coz in most of the designs higher metals are used for power routing. Since resistance is inversely proportional to width,higher metals have less resistance and hence less IR Drop.
 

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