An offset-tolerant dual-reference-voltage sensing scheme for deep submicrometer STT-RAM

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_ref sensing scheme (DVSS) that selectively uses an optimal V_ref between V_ref+ and V_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_ref is selected after fabrication, and the calibrated switch control bit, which contains V_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× faster sensing speed and 1.5× lower read energy per bit compared with the state-of-the-art offset-tolerant sensing scheme.