vdd , gnd power line layout

[EHY]

1) vdd, gnd power line : using upper(M6, Top metal) metal layer


2) vdd, gnd power line : using M1 M2(lower) metal layer



which one is correct??

 

[erikl]

I'd advice to use the top metal layer (wherever possible): in most processes this is a thicker layer, so has less resistance.

 

[AMS012]

It mainly depends on your application. If you are doing a standard cell layout that sits in another block, it is better to use lower metals and preserve higher metals for other blocks routing.
If you are laying out a single block and are sure that there is no top metal blockage, then it is good to go for higher metals for lower resistance.

A good layout should contain all the metal layers in a uniform density, which will avoid 'dummy metal filling' later.