Would the capacitance of a long `poly` line over s...
# sky130t
tnt
09/18/2024, 4:29 PMWould the capacitance of a long
polyndiffm
Mitch Bailey
09/18/2024, 4:49 PMpoly over ndiff is an nmos.
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tnt
09/18/2024, 4:51 PMYes I know 🙂
tnt
09/18/2024, 4:52 PMBut I'm working on an implant ROM compiler, so sometime there is diffusion, something there is not, and I'm trying to estimate what range of capacitance I can expect for the word line, trying to use the google docs with parasitics data.
d
David Lindley
09/18/2024, 7:35 PMGate oxide is much thinner than field oxide, so the capacitance will be higher over diff. That's one reason decap is made using long and wide fets vs. field poly.
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tnt
09/18/2024, 9:08 PMOk, thanks. I guess I need to dig into the model to find the capacitance estimation for gate ? And the part of the poly that's not right over the diffusion, will that be over thicker oxide ? So it "changes height" when crossing over ?
tnt
09/18/2024, 9:40 PMAnswering my own question, after re-reading parts of Weste/Harris, I think the poly is flat, it's the underlying substrate that was etched during the STI process which leads to thicker oxide between field poly and substrate vs the gate poly one.
tnt
09/19/2024, 11:36 AMI'm looking at the google docs with the parasitics and the fringe capacitance look quite significant. Am I correct that if you have a wire (so much longer than wide), you need to count twice the length when applying the fringe capacitance numbers ? In which case the fringe capacitance is like 10 times larger than the parallel plate capacitance ... (for a 0.15u field poly).
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