why sandwich cap is preferred?
from this paper:
Capacity Limits and Matching Properties of Integrated Capacitors
By Roberto Aparicio, Ali Hajimiri,
The sandwich caps made by interconnect metals don't match well.
In the fab's matching report, only finger(comb like) caps are used. and the matching is not good.
2% for a 20-finger unit.
We need a 0.1% matching cap, if I put 20 units together, the total cap is too big (4pF).
My another question is, since I can put some unit caps in series and some in parallel to get the capacitance desired, how do i calculate the matching of such combination. is there a formula i can follow?
Thanks!