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Design and Analysis of Robust Variability-Aware SRAM to Predict Optimum Access-Time to Achieve Yield Enhancement in Future Nano-Scaled CMOS.

  • Author(s): Samandari-Rad, Jeren
  • Advisor(s): Hughey, Richard
  • et al.

Design variability due to inter-die (D2D) and intra-die (WID) process variations has the potential to significantly reduce the maximum operating frequency and the effective yield of high-performance chips in future process technology generations. This variability manifests itself by increasing the access-time variance and mean of fabricated chips.

This thesis proposes a new hybrid analytical-empirical model, called VAR-TX, that exhaustively computes and compares all feasible architectures subject to D2D and WID process variations (PV). Based on its computation, VAR-TX predicts the optimal architecture that provides minimum access-time and minimum access-time variation for yield enhancement in future 16-nm on-chip conventional six-transistor static random access memories (6T-SRAMs) of given input specifications and given area and power constraints. The given specifications include SRAM size and shape, number of columns, and word-size.

In addition, this thesis reviews 6T-cell design challenges and the main causes for failure. Also provided are several newly designed or modified circuits that are crucial for SRAM stability, reliability, robustness, speed, and reduced power consumption. This thesis also compares the impact of D2D and WID variations on access-time for 16-nm SRAM with the 45-nm and 180-nm nodes and demonstrates that the drastic increase in the 1- and 3-sigma of the smaller nodes is mainly due to the increase in the WID variations. A considerable number of simulation results regarding access-time, leakage current, and dynamic power are presented and analyzed throughout this thesis to help predict the impact of process, operation, and temperature variations on SRAM variability, as well. Finally, the VAR-TX model argues previously published works that suggest that square SRAM always produces minimum delays and it significantly extends and enhances the older models by adding both an extra dimension of architectural consideration and additional device parameter fluctuation to the analysis, while producing delay estimates within 8\% of Hspice results.

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