Hi,

I am looking for a reliable way to estimate the bottom plate capacitance of poly-poly caps. Usually, I just assume a worst-case estimate of 10% which although conservative is not a problem as I never really have to deal with extremely large caps and this approach also accounts for extra caps from the layout.

However, I am currently working with very large on-chip caps (~1 nF) and I think it would be better if I could get a more realistic estimate. I have data about field oxide thickness vs. poly-poly oxide thickness. However, I dont have much information about the permittivity ratios. Is it wise to assume that the ratio of bottom plate cap to nominal cap would be in ratio of the oxide thicknesses?

Thanks
Vivek

Vivek, in the documentation for the process you are using should have capacitance tables that include poly_to_field which you can use to caluculate the capacitance exactly if you know the size of the poly-poly cap you are making.

David