Understanding the Effect of Process Variations on the Delay of Static and Domino Logic

Abstract

In this paper, the effect of process variations on delay is analyzed in depth for both static and dynamic CMOS logic styles. Analysis allows for gaining an insight into the delay dependence on fan-in, fan-out, and sizing in sub-100-nm technologies. Simple but reasonably accurate models are derived to capture the basic dependences. The effect of process variations in transistor stacks is analytically modeled and analyzed in detail. The impact of both interdie and intradie variations is evaluated and discussed. Interestingly, the input capacitance of static and dynamic logic is shown to be rather insensitive to variations. The delay variability was also shown to be a weak function of the input rise/fall time and load. Analysis shows that domino logic circuits suffer from a doubled variability as compared to the static CMOS logic style. The positive feedback associated with the keeper transistor is shown to be responsible for the variability increase, which, in turn, limits the speed performance. This adds to the well-known speed degradation due to the current contention associated with the keeper transistor. Monte Carlo simulations on a 90-nm technology, including layout parasitics, are performed to validate the results.