Impact of Process Variability on Write Error Rate and Read Disturbance in STT-MRAM Devices

Abstract

Spin-transfer torque magnetic random-access memory (STT-MRAM) is the most promising next-generation memory technology that combines the advantages of mainstream memory technologies, such as SRAM, DRAM, and flash. In the STT-MRAM, a magnetic tunnel junction (MTJ) is used as a bit-cell to store the data, and its magnetic properties have a critical role in thermal noise-aware STT-switching operations. This work analyzes the impact of MTJ material and geometric parameter variations, such as saturation magnetization (MS), magnetic anisotropy (HK), damping factor (α), spin polarization efficiency factor (η), oxide thickness (tOX), free layer thickness (tF), tunnel magnetoresistance (TMR), and cross-sectional area of free layer (AF) variations on write error rate (WER) and read disturbance rate (RDR) for reliable write and read operations, respectively. To evaluate the scalability of MRAM devices, we investigate both WER and RDR with a wide range of MTJ diameters between 90 and 30 nm, which corresponds to mainstream technology nodes from 40 up to 14 nm advance node. In our work, the Fokker-Planck (FP) numerical approach is mainly utilized for an efficient analysis, which allows for parametric variation and evaluates its impact on switching. Although the impact of material and geometric parameter variations on WER is decreased as MTJ scales down from 90 to 30 nm, the variation effect can be still critical with small MTJ diameter, and the most significant influential variation is η, MS, HK, and α in that order. By contrast, the impact of material and geometric parameter variation on RDR increases in MTJ scaling, and we show that negative variations of HK and MS could be a critical bottleneck in 30 and 40 nm MTJ diameters. Our work finally emphasizes the necessity of the WER and RDR analysis by considering the parameter variation in MTJ scaling for practical STT-MRAM development.