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.

I. Introduction

Magnetoresistive random access memory (MRAM) with magnetic tunnel junctions (MTJs) has attracted a great deal of attention due to its nonvolatility, potential for high-speed operations, and unlimited endurance [1]–[5]. We have proposed a shape-varying MTJ (SVM) with inserted write line and have shown that the SVM structure leads to a low write current of 0.9 mA [6] [7]. Using MRAMs with SVM cells, we have demonstrated 500-MHz operation [2]. In the system of large-scale integrated circuits (LSIs) suitable for high-speed MRAM application, it will become more important to reduce writing current in order to decrease power consumption. To reduce writing current, both switching current distribution among multiple cells and switching current distribution of each cell should be minimized. Related to the distribution among multiple cells, some failure modes caused by rotated or shifted magnetic properties of MTJ have been reported in Toggle MRAMs [8]. However, switching current distribution of each cell in MRAM has not been investigated yet. In this paper, first we will describe the switching current distribution and switching probability of SVM-MRAM. These results show that there are several cells that exhibit a large switching current distribution for each cell. We postulate that these differences are caused by the deformation in the MTJ's shape. Next, we will present the simulated results of artificially deformed MTJ. We will compare the writing currents of the simulation with those of the fabricated MTJ with artificial deformation. Finally, we will show the switching current distribution of each cell of the fabricated MTJ with the artificial deformation and compare the results with those of typical MRAM cells.