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PVS QRC / Calibre PEX both extracting 3x lower Resistance (Read 533 times)
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PVS QRC / Calibre PEX both extracting 3x lower Resistance
Dec 30th, 2016, 7:25am
Hello all,

I am trying to extract one half of an IC using CALIBRE PEX. We are most interested in R on our PWR/GND Rails. We have a really wide ground rail running horizontally (around 40 squares. So knowing this we expected a certain R to be calculated. But on probing using Calibre probes the resistance is calculated as roughly being 3x lower.

At this point I tried a few different things:
1) Tried a simple test case with the same metal layer and the tool seems to extract correct R when probed
2) Took the same exact layout, kept only the metal layers (and deleted all other layers) and did extraction with the same settings as before and got the correct R Value!
3) Also tried QRC with PVS, where the R being extracted is also incorrect.

If anyone has any other ideas or inputs then please let me know..  There is parallel metal to this thick ground layer of 40squares but in thicker layers and based on experiment 2) above this is not the issue.
I know these tools do not extract substrate resistances, so I am at a loss as to what could be going on. Also the design has 8 instances of the same subcell tapping off this ground rails fairly evenly all the way across. I have also checked if there are multiple pins or labels floating around on the GND rails, and that is not the case. Could it be that the tool does not care as to how accurate the R is if it is lets say less than an ohm..
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Re: PVS QRC / Calibre PEX both extracting 3x lower Resistance
Reply #1 - Apr 14th, 2017, 4:42pm
The extraction tool fractures the metal polygons into pieces, and assigns resistance values to each of them, based on the fracture length, width, and sheet rho of the metal layer. This is 1D R extraction. (QRC can also do 2D R extraction, but it's not the baseline algorithm used for R extraction).

If metal layout is more or less one dimensional (40 squares is pretty much 1D), this approach should work well, in a sense it should be giving accurate resistance value (say within a few percents).

However, if individual fracture resistance is less than a critical/threshold value (I think, typically 0.001 Ohm - i.e. 1 mOhm) - these resistance may be "reduced" by simply shorting (ignoring them). If you have many less than 1 mOhm resistors in series - here is one possible root cause for R accuracy loss.

Second (after fixing the first issue described above), you can look at the post-layout netlist (you need to generate DSPF file - which is a straightforward text file format - rather than extracted view or calibre view), and look at individual resistances, their L, W, and R values - and check if that is correct. Calibre XRC should have a pretty good facilities for visualizing extracted resistors. QRC's one is not that good (in extracted view, you can hardly see parasitic resistors).

What's the sheet resistance of the metal layer that you have problem with?

In general, if layout shapes are not simple 1D polygons, R extraction as done by most of the standard extraction tools is not very accurate, especially at low resistance values - typical examples are power transistors with trapezoidal or other complex shapes in upper metal layers.

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