An Offset-Tolerant Dual-Reference-Voltage Sensing Scheme for Deep Submicrometer STT-RAM

Abstract

Due to the increased process variation and reduced supply voltage in deep submicrometer technology nodes, an offset-tolerant sensing scheme has become essential. However, most offset-tolerant sensing schemes suffer from inherent performance degradation owing to multiple-stage sensing. In this paper, a dual $V_{\mathrm {ref}}$ sensing scheme (DVSS) that selectively uses an optimal $V_{\rm {ref}}$ between $V_{\rm {ref+}}$ and $V_{\rm {ref-}}$ is proposed. This scheme is tolerant to process variations, and can be used as a spin-transfer-torque random access memory. Because of no additional sensing stage, the offset-tolerant sensing is achieved without sacrificing the performance. The optimal $V_{\rm {ref}}$ is selected after fabrication, and the calibrated switch control bit, which contains $V_{{\rm {ref}}}$ selection information, is stored permanently in an on-chip nonvolatile latch. Monte Carlo HSPICE simulation results, using an industry-compatible 45-nm model parameters, show that the proposed DVSS achieves a read yield of 98.24% for 32 Mb (6.1 sigma) with $2\times $ faster sensing speed and $1.5\times $ lower read energy per bit compared with the state-of-the-art offset-tolerant sensing scheme.