The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> Analog Design >> Estimating bottom plate capacitance
https://designers-guide.org/forum/YaBB.pl?num=1121407689

Message started by vivkr on Jul 14^th, 2005, 11:08pm

Title: Estimating bottom plate capacitance
Post by vivkr on Jul 14^th, 2005, 11:08pm

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

Title: Re: Estimating bottom plate capacitance
Post by rf-design on Jul 18^th, 2005, 12:23am

Modern process have about eps=4.2 between thin film layers (metal&silicon). Lateral between the fill glass have about eps=4.1. Field oxide possible under doublepoly cap have eps=3.9. The thickness of the thinoxide is difficult to measure and then calculate. So use instead the direct cap measurement. Be aware that the silicon undernead the field oxide have also a series resistance. So it could lower significant Q of the cap.

Title: Re: Estimating bottom plate capacitance
Post by DReynolds on Jul 20^th, 2005, 6:41am

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