I want implement some 20pF capacitors in a chip which will operate at about 5MHz. What kind of capacitor should I use? Can anyone give me some references which talk about it.
It depends on what is important to you. You could pick the one with the smallest area, but that may have the highest stray capacitance to substrate. I assume you want floating capacitors? If so, you need to find out how much capacitance to substrate you can tolerate. Sandwich capacitors have a reasonable ratio of capacitance to stray - e.g. poly/Met1/Met2 sandwich with poly & Met2 connected.