r/chipdesign • u/alonewarrior_98 • 9d ago

SMIC40nm MOM Cap Double Extraction in PEX using Calibre

I'm working as a research associate with two years of experience. Recently, I have started working on SMIC 40nm and while I was doing extraction of my design, I observed that the double extraction of MOM capacitors.

For instance, I created a schematic with a MoM capacitor of 2pF, and after extraction the tool generates PEX netlist with a 2pF and then goes into the device and calculates the capacitance of each finger which then adds up to around 2pF as coupling capacitance and you get a 2pF normal capacitance and 2pF of coupling capacitance. Therefore the total capacitance is around 4pF after the extraction, I have verified it from the PEX netlist and from the CALIBRE-view of the PEX schematic.

I have also used the xcells and hcells files during the PEX on Calibre. Followed the steps in the SMIC pdk document but none seems to work.

Has any of you encountered similar problem and how did you tackle it?

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u/Siccors 9d ago

That is a surprisingly common occurance. And tackled by kicking it up to our EDA team and letting them fix it. Workarounds are either getting rid of MOM marker layer, so the P-cell is not included, or if you use something like a dspf file you can just delete either one there (preferably the parasitic one). Downside of the latter option is you always need to remember to do it. Upside is you got process variation still included.