Adaptive Body Bias for Reducing the Impacts of NBTI and Process Variations on 6T SRAM Cells

Reliability and variability have become big design challenges facing submicrometer SRAM designers. A low area overhead adaptive body bias (ABB) circuit is proposed in this paper to compensate for NBTI aging and process variations to improve the SRAM reliability and yield. The proposed ABB circuit consists of a threshold voltage sensing circuit and an on-chip analog controller. Postlayout simulation results, referring to an industrial hardware-calibrated STMicroelectronics 65 nm CMOS technology transistor model, are presented. The transistor model contains process variations and NBTI aging model cards, which are declared by STMicroelectronics to be silicon verified. Cadence RelXpert, Virtuoso Spectre, and Virtuoso UltraSim tools are used to estimate the NBTI aging and process variations impacts on the SRAM array. These results show that the proposed ABB compensates effectively for NBTI aging and process variations. For example, the proposed ABB reduces the read failure probability from 0.32% to 0.05% and the SNM degradation from 10.9% to 2.6% at 10 years aging time. In addition, the proposed ABB enhances the soft errors immunity of the SRAM cell by reducing the critical charge degradation from 12.7% to 3.4% at 10 years aging time.