About Power routing being done on top metal layers..

[Simranjeet Singh]

I am new in ASIC design and had a question..
I read that power straps are routed on top metal layers because the top layers are more conductive because of their greater thickness than bottom layers.
now my question is...Is this arrangement done deliberately and if it is... why and what if the bottom layers had more thickness and conductivity?

Plz reply!!!
Thanks in advance!!

 

[nisshith]

i think the the lower metal layers are thinner than that of topmist layers so that we get more routing tracks for signal routing.
As we increase the width the min spacing requirement also increases and hence the pitch. so the number of tracks decreases resulting in decrease of routing resources available. Also signal pins do not have to withstand that much amount of current as of the power rails so there is no reqirement for them to be wider.

Hence for power routing topmost layers are reserved which have more width and conductivity.

 

[Simranjeet Singh]

Thanks...that makes sense. It was really helpful.