Fundamental Thermal Limits on Data Retention in Low-Voltage CMOS Latches and SRAM

Abstract

Ultra-low-power systems with substantial computing capacity require latches and SRAMs to operate at extremely low supply voltages. However, with aggressive technology scaling, reliability becomes a major challenge due to unavoidable process variations and the presence of multiple noise sources, including intrinsic thermal noise. This paper provides a quantitative measure of reliability by calculating the probability distribution function (PDF) of errors induced by thermal noise in latches and SRAMs operating in subthreshold conditions. Implemented in a novel simulation tool for thermal-noise analysis of CMOS circuits (STTACC), our algorithm uses a stochastic differential equation circuit model that preserves the proper Poissonian statistics for thermal-noise-driven current fluctuations in MOSFETs. Our probabilistic error model can handle error rate analysis for arrays of latches or full SRAMs on time scales from seconds to years without excessive computational overhead. We demonstrate that the time-to-error (TTE) statistics of subthreshold SRAMs obey log-normal distributions that depend on parameters such as node and device capacitance, device threshold variations and operating conditions of supply voltage and temperature. This makes it possible to quantitatively evaluate the asymptotic behavior of extremely rare error events that are inaccessible to standard SPICE-based simulations.