Variability-Aware Noise-Induced Dynamic Instability of Ultra-Low-Voltage SRAM Bitcells

Abstract: Stability of ultra-low-voltage SRAM bitcells in retention mode is threatened by two types of uncertainty: process variability and intrinsic noise. While variability dominates the failure probability, noise-induced bit flips in weakened bitcells lead to dynamic instability. We study both effects jointly in a unified SPICE simulation framework. Starting from a synthetic representation of process variations introduced in a previous work, we identify the cases of poor noise immunity that require thorough noise analyses. Relying on a rigorous and systematic methodology, we simulate them in the time domain so as to emulate a true data retention operation. Short times to failure, unacceptable for a practical ultra-low-power memory system application, are recorded. The transient bit-flip mechanism is analysed and a dynamic failure criterion involving the unstable point is established. We conclude that, beyond static variability, the dynamic noise inflates defectiveness among SRAM bitcells. We also discuss the limits of existing analytical formulas from the literature, which rely on a linear near-equilibrium approximation of the SRAM dynamics to, inaccurately, predict the mean time to failure.