High thermal tolerance synthetic ferrimagnetic reference layer with modified buffer layer by ion irradiation for perpendicular anisotropy magnetic tunnel junctions.

The development of the magnetic tunnel junctions with perpendicular easy axis using interfacial anisotropy [1–4] (p-MTJs) has been intensively advanced toward realization of high performance spin-transfer-torque magnetoresistive random access memories (STT-MRAMs). The p-MTJs used in the STT-MRAMs are required to withstand 400°C annealing to be compatible with CMOS back-end-of-line-process. Although we have demonstrated the thermal tolerance against 400°C annealing for the free layer in the CoFeB/MgO-based p-MTJs [5], we also found variation of properties in synthetic ferrimagnetic (SyF) reference layer. We revealed that the variation was caused by the diffusion of Fe in the CoFeB reference layer toward the bottom Co/Pt pinned layer after annealing [6]. We simultaneously observed degradation of perpendicular anisotropy of the Co/Pt multi layer. Perpendicular anisotropy of Co/Pt is influenced by the Co/Pt thickness ratio, buffer layer material, and roughness [7]. In this study, we focus on modification of the buffer layer and reveal that degradation can be suppressed by employing a surface modification treatment (SMT) of the Pt buffer layer. MTJ stacks from the substrate side, Ta /Pt buffer layer/with or without SMT/ [Co/Pt] 4 /Co /Ru/ [Co /Pt] 2 /Co /Ta /CoFeB /MgO barrier/free layer/top electrode, were deposited on a 300-mm $\Phi $ thermally oxidized Si wafer using a DC/RF magnetron sputtering system at room temperature (Fig.1(a)). In the SMT process, the Pt buffer layer was exposed to an ion plasma. The upper layers on the Pt buffer layer were deposited without breaking the vacuum. After deposition, samples were annealed at 350 °C for 2 h or 400 °C for 1 h in vacuum without applying magnetic field. The out-of-plane magnetic moment per unit area versus magnetic field (m-H) curves of the blanket films were measured by a vibrating sample magnetometer, and the film structures were investigated by energy dispersive x-ray spectroscopy (EDX) and high-resolution X-ray diffraction (HRXRD) using Cu Ka radiation. As shown in Fig. 1(b) and 1(c), perpendicular anisotropy of the SyF reference layer without SMT degraded after 400 °C annealing, while that of the SyF reference layer with SMT hardly changed. Interestingly, after annealing at 400 °C, magnetic moment variation in the top and bottom parts of the reference layer is observed in the stack without SMT due to Fe diffusion from the CoFeB reference layer toward the bottom Co/Pt layer, as we previously reported [6]. In contrast, the magnetic moments of the top and bottom parts of the reference layer in the stack with SMT are almost the same. This indicates that Fe diffusion in the reference layer is suppressed by the SMT of the Pt buffer layer. In order to verify this hypothesis, we performed cross-sectional EDX line analysis (Fig. 2). In the stack without SMT, Fe element in the CoFeB reference layer is observed in the other layers such as the top and bottom Co/Pt multilayers after annealing at 400 °C Fig. 2(a)), which indicates that Fe diffused toward the bottom Co/Pt layer. In contrast, in the stack with SMT, Fe element is observed only in the CoFeB layer in the as-deposited state and after annealing at 400 °C Fig. 2(b)). These results are consistent with the variation of magnetic properties shown in Fig. 1(b) and 1(c). In order to clarify the reason for the suppression of perpendicular anisotropy degradation of the Co/Pt multilayer after 400 °C annealing by SMT, we evaluated the crystal structure of the Co/Pt multilayer using HRXRD. X-ray diffraction analysis revealed that the ions penetrated the Pt buffer layer, expanding the Pt lattice constant and enhancing Co/Pt grain growth, which well oriented to the fcc (111) closed-packed plane parallel to the substrate surface. This caused larger perpendicular anisotropy in the Co/Pt multilayer and higher thermal tolerance. In addition, this may suppress Fe diffusion from the CoFeB reference layer toward the bottom Co/Pt layer. This work is supported by CIES’s Industrial Affiliation on STT MRAM program, and JST ACCEL Grant Number JPMJAC1301, Japan, and JST-OPERA.