Lex wrote on Jul 6th, 2010, 1:54am:
I've always been skeptical about resistance calculations of extractors. Usually it checks the wires with an internal database of patterns, so it's not actually field solving. If not too cumbersome, you can check some critical paths by hand to see whether it did a good calculation.
Internal database for patterns is used only for capacitance calculations, not for resistance (as far as I know).
Various extraction tools handle resistance extraction differently, but most of them use one-dimensional extraction algorithm, in various approximations, where a metal line is considered to be one-dimensional, and is fractured along the length. Then resistors of known length and (estimated) width are created and added to the R network (then, parasitic coupling capacitances are added to fracturing points).
This approach is quite accurate and works really well when metal shapes are really one-dimensional - i.e. narrow and long, and when current flow in metal lines is really one-dimensional. This is the case for standard cells, digital designs, etc.
In case of analog circuits, and especially - power ICs (PMICs), metals are often wide, connected to each other through large 2D arrays of vias, and current flow may not be one-dimensional. In this case, 1D R extraction will have large inaccuracies.
(Note that not all 1D R extraction tools are equal - they are using quite different algorithms, and some are more accurate than others.)
That's why analog designers are often suspicious about the accuracy of R extraction. But, if you know the limitations of the extraction tools, and if you have some other tools at hand, situation may not be that bad.
Some extraction tools (like Quantus QRC) have an option to do 2D R extraction on wide metal layers.
There are also several other layout analysis tools that perform more accurate and more physical discretization (meshing) of complex metal / via shapes, to get very accurate Resistance and current density simulations. Please refer to this thread on EDABoard web site:http://www.edaboard.com/showthread.php?t=304145&highlight=r3d