[Magic-dev] RE: cap values in magic vs. MOSIS process data

[R. Timothy Edwards]

Dear Wei,

>From Jeff Sondeen:

> area = 5.28e-03/.4 ff/um**2 = 0.0132 ff/um**2
> fringe = 2.94e-02 ff/um

Okay, let's translate everything to aF and um.  Now
we have for m2->substrate:

1) magic's lambda=0.15 extraction:
area cap = (0.354 / (0.15 * 0.15)) = 15.7 aF/um^2
perim cap = (0.6 / 0.15) = 4 aF/um

2) MOSIS test data:
area cap = 19 aF/um^2
perim cap = 60 aF/um

3) vendor data from Jeff:
area cap = 13.2 aF/um^2
perim cap = 29.4 aF/um


Some of this can be accounted for by the rather large
difference in fringe capacitance as seen in the
vendor data posted by Jeff according to the difference
in spacing of structures.  Nevertheless, there is
a factor of 15 difference between magic's value of
perimeter capactince and the MOSIS test data.  Don't
you agree that's a rather HUGE discrepancy?  The
value of 4 aF/um doesn't come close to matching any
vendor data for any process that I can find.  These
in turn are roughly backed up by calculating fringe
from a half-cylinder model, knowing the thickness of
the metal2 layer and the height above the substrate.

(See equation from Yuan and Trick, on page 17 of
http://www.stanford.edu/class/ee371/lectures/lect_02_updated.pdf)

Ultimately, magic should either attempt to incorporate
a more detailed model of capacitance extraction than
it currently has, but even with a simple model, one
would hope that the values are "ballpark".

				---Tim