The Impact of Random Device Variation on SRAM Cell Stability in Sub-90-nm CMOS Technologies

The impact of process variation on SRAM yield has become a serious concern in scaled technologies. In this paper, we propose a methodology to analyze the stability of an SRAM cell in the presence of random fluctuations in the device parameters. First, we develop a theoretical framework for characterizing the dc noise margin of a memory cell. The framework is based on the concept that an SRAM cell is on the verge of instability when the gain across the loop formed by the cross-coupled inverters in the cell is unity. The noise margin criteria developed in this manner can be used to verify a cell stability in the presence of arbitrary DC noise offsets at the two storage nodes in the cell. We also develop metrics for estimating the cell stability during read and write operations and verify these models by extensive Monte Carlo simulations in a 65-nm CMOS process. Our results show that the proposed robustness metrics can be used to estimate cell failure probabilities in an efficient and accurate manner.