Area Optimization Techniques for High-Density Spin-Orbit Torque MRAMs

This paper presents area optimization techniques for high-density spin-orbit torque magnetic random-access memories (SOT-MRAMs). Although SOT-MRAM has many desirable features of nonvolatility, high reliability and low write energy, it poses challenges to high-density memory implementation because of the use of two access transistors per cell. We first analyze the layout of the conventional SOT-MRAM bit-cell that includes two vertical metal lines, a bit-line and a source-line, limiting the horizontal dimension. We further propose two design techniques to reduce the horizontal dimension by decreasing the number of metal lines per cell without any performance overhead. Based on the fact that adjacent columns in a bit-interleaved array are not simultaneously accessed, the proposed techniques share a single source-line between two consecutive bit-cells in the same row. The simulation result shows that proposed techniques can achieve a bit-cell area reduction of 10–25% compared to the conventional SOT-MRAM. The comparison of our proposed designs with the standard spin-transfer torque MRAM shows 45% lower write energy, 84% lower read energy, and 2.3 × higher read-disturb margin.