Dual reference sensing scheme with triple steady states for deeply scaled STT-MRAM

Spin transfer torque magnetic random access memory (STT-MRAM) has emerged as a promising candidate for the next-generation nonvolatile memory. However, because of the increased process variations and reduced critical switching current of the magnetic tunnel junction (MTJ), the readability has become a new obstacle for STT-MRAM in scaled technology nodes. Thermal fluctuations further aggravates this phenomenon. There are multifarious sensing schemes and circuits have been proposed recently to bate this problem. However the technical advancement is incremental and the performance improvement is limited as technology continuously scales. In this paper, we propose a novel sensing circuit to achieve constant-current sensing, named triple steady states sensing circuit (TSSSC), which directly compares the P/AP state with the P/AP state of the memory cell to improve the sensing margin (SM). Using a physics-based MTJ model and the STMicroelectronics process design-kit, Monte-Carlo simulations were carried out at 40 nm technology node. The results validate the effectiveness of the proposed sensing scheme.