Analysis of MTJ Edge Deformation Influence on Switching Current Distribution for Next-Generation High-Speed MRAMs

We have demonstrated 500-MHz operation with our high-speed magnetoresistive random access memory (MRAM). For the next-generation MRAM, reducing magnetization switching current and its distribution is very important for decreasing power consumption. We found that some cells showed both large switching current and large switching current distribution under repeated writing tests. Although some failure modes related to the switching current distribution among multiple cells caused by rotated or shifted magnetic properties have been reported, they do not explain the extraordinary properties of each cell. To determine the origin of these properties, we investigated the influence of magnetic tunnel junction (MTJ) edge concave and convex deformation observed in MRAM fabrication. We found that the edge deformation and changed switching process generated a magnetization vortex, causing large switching current in a simulation. The switching characteristics of MTJs with some MTJ edge structures were fabricated and their characteristics were evaluated. We revealed that the edge deformation causes both large switching current and large switching current distribution of each cell. These characteristics correspond with those observed in the MRAM evaluation. Because cell sizes will become smaller and deformation size will become relatively large in the future, controlling MTJ shapes will become increasingly important for developing next-generation MRAMs.