According to my knowledge there are only two types of delay cells are availabe, 1)inverter chain 2)current starved inverter. In this current starved inverter consumes less area but highly unstable across corners, and the other way for inverter chain. And because of this i gues mostly inverter-chain(or bit modification of that) is used. Is it correct?

Anybody knows about other types of delay cells?  Because i could not get anything on this. Please give me reference if anyone finds.