Quote:The sf and fs corners are very rare in production for most processes.
I think this can be very misleading. It would be physically impossible to have two different thicknesses of Tox for the PMOS and NMOS. Because of that, the fs and sf corners are always pessimistic. However, since the N's and P's have two different V
t adjust implant steps, the corners will be at least somewhat uncorrelated. There still is some level of correlation since the V
t does depend on the thickness of the oxide during the implant step, however, for modeling purposes, they are always assumed to be fully uncorrelated, i.e. correlation coefficient of 0.
Assuming that the V
t's are uncorrelated, then the sf and fs corners are just as likely as the ss and ff corners in terms of V
t. As mentioned above, though, the models present a capacitive loading situation that is not physically possible.
At the heart of all of this is what parameter is most important to your particular circuit. Some combinations for some parameters are unrealizable, but that does not discount those corners since an s or f corner does not mean one thing.