Variation-aware static and dynamic writability analysis for voltage-scaled bit-interleaved 8-T SRAMs

Abstract

As process technology scales, SRAM robustness is compromised. In addition, lowering the supply voltage to reduce power consumption further reduces the read and write margins. To maintain robustness, a new bitcell topology, 8-T bitcell, has been proposed and read where write operation can be separately optimized. However, it can aggravate the half select disturb when write word-line boosting is applied or the bitcell sizing is done to enable robust writability. The half select disturb issue limits the use of a bit-interleaved array configuration required for immunity to soft errors. The opposing characteristic between write operation and half select disturb generates a new constraint which should be carefully considered for robust operation of voltage-scaled bit-interleaved 8-T SRAMs. In this paper, we propose bit-interleaved writability analysis that captures the double-sided constraints placed on the word-line pulse width and voltage level to ensure writability while avoiding half select disturb issue. Using the proposed analysis, we investigate the effectiveness of word-line boosting and device sizing optimization on improving bitcell robustness in low voltage region. With 57.7% of area overhead and 0.1V of word-line boosting, we can achieve 4.6σ of VTH mismatch tolerance at 0.6V and it shows 41% of energy saving.