Multi-Level Cell Spin Transfer Torque MRAM Based on Stochastic Switching

Spin Transfer Torque Magnetic Random Access Memory (STT-MRAM) provides a promising pathway forthe next generation of non-volatile memory and logic chips. The perpendicular magnetic anisotropy (PMA) in CoFeB/MgO/CoFeB magnetic tunnel junction (MTJ) nanopillar provides high thermal stability and low critical current. However, the STT switching mechanism of MTJ has been revealed intrinsically stochastic, which results from the unavoidable thermal fluctuations of magnetization. This phenomenon affects deeply the reliability of hybrid CMOS/MTJ interface circuits and drives important power overhead. In this paper, we present a multilevel cell (MLC) STT-MRAM benefiting from the stochastic behaviors. It allows not only higher storage density, but also reduces the programming power and delay. This new cell can be also used as electrical synapse to build up neuromorphic computing systems or other biological networks. Monte-Carlo statistical simulations based on a 40 nm technology node have been carried out to validate its functionality and demonstrate its performance.