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Power/GND metal layers

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AllenD

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Hi Team
I have 2 rookie level questions about power mesh and ground mesh.
1. According to online resources, I found that a lot of people use M1 as gnd and M2 as vdd. Is this choice have any benefit over M1 as vdd and M2 as gnd?
2. My pdk have 9 metal layers with M8,M9 as ultra thick metal layers. And if I choose M1 as vdd, M2 as gnd and up to M4 to layout the interconnection within local cells. And M6-M7 to layout the interconnection on the top level. I learned that all the even/odd number layers are supposed to connect to the same power net(in my case, M1/3/5/7/9 as vdd and M2/4/6/8 as gnd). But the problem I am facing is that my M6 are consist of only 1 metal trace and a HUGE ground mash because there is not much to connect there. Is this sounds right? If it is mornal, what is the "metal filler" tool, which is to fill the empty space on chip with metal traces to meet up with density requirement, used for?

Thanks very much!
Allen
 

Hi AllenD

Generally In Schematic, current values mentioned for nets(VDD & GND). According to current values we can consider metals.These current values are mentioned to overcome of IR drop.

M1 metal- High resistance.Less current flow through this metal.

when will go for higher metals, resistance less and high current flow these metals.
 

Thanks for the reply. So should I use m1 and m2 for local vdd and gnd and the global vdd and gnd should be top 2 metals? Can anyone please let me know how do I connect the top global power net to bottom local power net. I know I should via down. But in what form? Assuming I only have a local circuit for simplicity. I mand a M1 and M2 mesh for power. How should I connect them up?
 

Thanks for the reply. So should I use m1 and m2 for local vdd and gnd and the global vdd and gnd should be top 2 metals? Can anyone please let me know how do I connect the top global power net to bottom local power net. I know I should via down. But in what form? Assuming I only have a local circuit for simplicity. I mand a M1 and M2 mesh for power. How should I connect them up?

You need a local grid and a global grid. Local is M1 only in old technologies, but in recent high performance digital ASICs it is M1/M2/M3
Your top grid is M8/M9 or whatever the top two metals are. You connect them often and regularly with the use of stripes.
 

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