Recovery Boosting: A Technique to Enhance NBTI Recovery in SRAM Arrays

Negative Bias Temperature Instability (NBTI) is an important lifetime reliability problem in microprocessors. SRAM-based structures within the processor are especially susceptible to NBTI since one of the PMOS devices in the memory cell always has an input of `0´. Previously proposed recovery techniques for SRAM cells aim to balance the degradation of the two PMOS devices by attempting to keep their inputs at a logic `0´ exactly 50% of the time. However, one of the devices is always in the negative bias condition at any given time. In this paper, we propose a technique called Recovery Boosting that allows both PMOS devices in the memory cell to be put into the recovery mode by slightly modifying the design of conventional SRAM cells. We present the circuit-level design of an issue queue that uses such cells and perform SPICE-level simulations to verify its functionality and quantify area and power consumption. We then conduct an architecture-level evaluation of the performance and reliability of using an area-neutral design of such an issue queue using the M5 simulator and the SPEC CPU2000 benchmark suite. We show that recovery boosting provides a 56% improvement in the static noise margin for the issue queue while having very little impact on power consumption and a negligible loss in performance.