DRC ERROR "minimum density"

Can anyone explain me what's the meaning of the error: "Minimum MET1
density = 30%" and how can I solve this problem?

 

Dear Sykab,

This means that the amount of metal1 shapes in your design is less
that 30% of the chip area. 30% is the minimum required.
Depending on the process you're working on, the min MET1 density could
be required at both chip level or 'window' level. Let me explain.
Suppose your die size is 1000um*1000um. In order to achieve a good
planarity and to make the metal etching as required, your foundry will
ask for a minimum of 30% of metal1 over the entire die. That's a
global density rule. Let's suppose you've got a very dense digital
bloc as the half of your die. I guess this would be enough to achieve
the required density even the other half of the die is fairly 'empty'.
But some processes don't support big metal density gradients between
different areas. In this case, foundries put in place 'Local' density
rules as well. An example is a min MEt1 density is required by windows
of 100um*100um. In this case, your DRC engine will parse your die as a
matrix of 100*100 squares and make the density check in there. Hope it
is clear for you. As I said earlier, density is very important for
planarity and pattern etching. It is important to avoid the dishing/
erosion in the modern nanotechnologies with Copper BE.
Many IP designers are complaining about the density rules being
checked at IP level. Many of them ask to check the densities at chip
TOP level only. I've seen many TOP assembly engineers struggling to
reach the density because the IPs haven't been thought with regards to
this problem. So my point of view is to take this into account at the
early stages.
So how to solve this problem ? you just need to add 'dummy' MEt1
shapes into your layout. These are floating bits of metals without any
electrical functionality. Well, you've to be careful though because
those pieces of metals are adding some parasitic to your true design.
Many 'modern' PDKs come with 'dummy filling' procedures that saves you
from a hand-made filling. Please ask your PDK provider.

Hope this help !

Riad.

 

This means over a defined window (say 20ux20u) sum of area of
MET1polygons should be >= 0.3 of the area of the window. Routing tools
usually can add dummy metal to fix this issue or your foundry adds the
dummy metal.