Impacts of NBTI/PBTI on Timing Control Circuits and Degradation Tolerant Design in Nanoscale CMOS SRAM

Abstract

Negative-bias temperature instability (NBTI) and positive-bias temperature instability (PBTI) weaken PFET and NFET over the lifetime of usage, leading to performance and reliability degradation of nanoscale CMOS SRAM. In addition, most of the state-of-the-art SRAM designs employ replica timing control circuit to mitigate the effects of leakage and process variation, optimize the performance, and reduce power consumption. NBTI and PBTI also degrade the timing control circuits and may render them ineffective. In this paper, we provide comprehensive analyses on the impacts of NBTI and PBTI on a two-port 8T SRAM design, including the stability and Write margin of the cell, Read/Write access paths, and replica timing control circuits. We show, for the first time, that because the Read/Write replica timing control circuits are activated in every Read/Write cycle, they exhibit distinctively different degradation behavior from the normal array access paths, resulting in degradation of timing control and performance. We also discuss degradation tolerant design techniques to mitigate the performance and reliability degradation induced by NBTI/PBTI.