poly poly capacitor
Well I'm not sure, but I think that your capacitor has 20V breakdown voltage between each node and well. This means that 20V is maxisimum that you can have between nodes and ground (or vdd).
My advice is don't use it. If in any moment you have voltage over 8V at capacitor nodes you broke it. And if your suply voltage is 20V (or let's say it 15) than it would be possible that in some time (at "power up" for example) voltage at capacitor's nodes exceeds 8V.