Dependency of Multi-Level-Cell Behavior on Thickness of Top MgO Tunneling Barrier in Double Pinned Structure Perpendicular Spin-Torque-Transfer Magnetic Random Access Memory

Abstract

These days dynamic-random-access-memory (DRAM) fabrication to scale down under 10-nm is complex which leads to high cost [1]. On the other hand, perpendicular spin-transfer-torque magnetic random access memory (p-STT MRAM) cell has a possibility of high-density integration with feature size of 4F2. Therefore, p-STT MRAM is the most powerful candidate of new memory instead of DRAM. In hence, p-STT MRAM requires high tunneling magnetoresistance (TMR) ratio greater than 150%, enough thermal stability (Δ=KuV/kBT) above 75 for a ten-year retention-time, and low switching current (J C0 ~ 1 MA/cm2) must be achieved for low power consumption [2-4]. However, the thermal stability is hard to maintain when the cell size is scaled down to 10 nm because the small volume of free layer leads to the changes of direction of spin by thermal agitation [5]. In this study, we designed a p-MTJ spin-valve with a double pinned structure to realize two-bit operation to overcome a larger scaling limit. We calculated four resistance states using relationship between the two different TMR of each MTJs. In particular, if the TMR of MTJ1 is half of MTJ2 and ratio of parallel resistance(Rp2/Rp1) is 0.8-0.9, the difference of each resistance state is equal. (see Fig. 1(b)) The TMR is mainly dependent on thickness of MgO tunnel barrier; therefore, we fabricated the p-MTJ spin-valve with a different thickness of MgO tunnel barrier (0.90 nm-1.35 nm) of MTJ1. Finally, we confirmed that the R-H curves match the four resistance states of p-MTJ with double pinned structure with our predicted results, and interval of resistance state is uniform when the thickness of top and bottom MgO are 1.36 nm and 1.10 nm as shown in Fig. 1(c). This implies that the double pinned p-MTJ spin-valve may be suitable for terabit integration compared to that of the single-bit operating p-STT MRAM memory cells. In the end, we report dependency of MLC behavior on thickness of top MgO tunneling barrier in double pinned structure pSTT MRAM. In addition, we present the MLC behavior of pSTT-MRAM with double pinned structure and show the thickness of top MgO for uniform resistance state. Acknowledgements This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No.2017R1A2A1A05001285) and Brain Korea 21 PLUS Program in 2014.