what and why? question about layout construction ( IC layout, topology)

[vadim888]

Hi all,

I just found this construction (see below), I don't understand why this poly ring used there.



RED -diff
GREEN - poly

Can anyone explain this way of design?

Thanks.

 

[jkatic]

If you focus on the center ring only, you can see two identical "transistors" without diff layer on both sides. Those are so called dummy transistors and they are used for matching purposes only (two in the middle are probably differencial pair). During fabrication outer layers are etched/deposited with larger missmatch, so you put there dummy cells for which you dont care. Rest of the ring is interconnection of the dummy cells (they need to be connected to ground/vdd), and poly is used here because its high resistance/capacitance is not important and you save metal layers for other connections.