cells are temporary capacitors added in the design between power and ground rails to counter functional failures due to dynamic IR drop.Dynamic I.R. drop happens at the active edge of the clock at which a high percentage of Sequential and Digital elements switch.Due to this simultaneous switching a high current is drawn from the power grid for a small duration.If the power source is far away from a flop the chances are that this flop can go into a metastable state due to IR Drop.To overcome this decaps are added. At an active edge of clock when the current requirement is high , these decaps discharge and provide boost to the power grid. One caveat in usage of decaps is that these add to leakage current. De caps are placed as fillers. The closer they are to the flop’s sequential elements, the better it is.
Decap cells are typically poly gate transistors where source and drain are connected to the ground rail, and the gate is connected to the power rail. when there is an instantaneous switching activity the charge required moves from intrinsic and extrinsic local charge reservoirs as oppose to voltage sources. Extrinsic capacitances are decap cells placed in the design. Intrinsic capacitances are those present naturally in the circuit, such as the grid capacitance, the variable capacitance inside nearby logic, and the neighborhood loading capacitance exposed when the P or N channel are open.One drawback of decap cells is that they are very leaky, so the more decap cells the more leakage. Another drawback, which many designers ignore, is the interaction of the decap cells with the package RLC network. Since the die is essentially a capacitor with very small R and L, and the package is a hug RL network, the more decap cells placed the more chance of tuning the circuit into its resonance frequency. That would be trouble, since both VDD and GND will be oscillating. I have seen designs fail because of thisDesigners typically place decap cells near high activity clock buffers, but I recommend a decap optimization flow where tools study charge requirements at every moment in time and figure out how much decap to place at any node. This should be done while taking package models into account to ensure resonance frequency is not hit.