which metal layer to use for GND and POWER LAYER

layout, route the power track

hi there,
which metal layer do you use for GND and VDD track, some say metal 1 for GND and metal 2 for VDD, some say metal 2 for both. since i am new on analog layout, i really do not know which layer to use in terms of ease of placement and route.When i finish my cell ,the power lines just make my entire layout look messy. I need your advice on how to place and route the power track( GND and VDD). thanks!
regards,
henry

gnd of power

Hi Henry

Generally you should use one of your top metals for power routing at CHIP level in order to minimise IR drops. What metal you choose depends on the metal stack available to you, should you have 7 layers with three thick layers you may choose to route VDD in Metal 7 from pad to cells and route GND underneath it in metal 6, saving space for routing channels and giving yourself a little decoupling. If you only have three layers then that is unlikely to be an option.
At cell level I generally route both VDD and GND in Metal 1 horizonally, this means I can tap directly into my current mirrors, guard rings etc. I then use metal 2 exclusively for vertical routing and metal 3 for horizontal. It would be nice to use metal 1 exclusively for horizontal, but the prevelance of guardrings etc generally makes that an awkward strategy.
It is worth remembering these are only guidelines, ultimately the decision will come down to

- block function (high currents / high frequency will require thick high stack metals)
- Metalisation options
- Top level requirements (what is the power strategy for the chip top level?)

how to route vdd and gnd

Which job should be done from beginning of layout? route the draft VDD and GND paths in top and its underneath layer or route the detail connections of circuit?

Re: GND and POWER LAYER

I will generally try to route the power lines first and try and minimize its length, while roughly making sections for the main layout

Re: GND and POWER LAYER

In some process m1 sheet resistance is more than all other metals because of its lesser thickness. Hence avoid POWER/GND routing in m1.
-Dinesh

Re: GND and POWER LAYER

you should do power routing first .. than signal routing...... but keep an ideal of space you might require for signal routing while doing power routing after placing the block......

Added after 9 minutes:

Pat_Mustard said:

Hi Henry


At cell level I generally route both VDD and GND in Metal 1 horizonally, this means I can tap directly into my current mirrors, guard rings etc. I then use metal 2 exclusively for vertical routing and metal 3 for horizontal. It would be nice to use metal 1 exclusively for horizontal, but the prevelance of guardrings etc generally makes that an awkward strategy.


Pat, what i m getting from this is that you are using metal 1 as main branch and metal 2 and 3 as subbrances for power routing...
I think this is not the best way..... both current handling and parasitic of higher metal(2 and 3 ) is better.... so i think we should use metal 3 and metal 2 as main supply.....

Let me know your views.

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