Structural Design for Tunneling-Magneto-Resistance and Antiferromagnetic Coupling Strength in Perpendicular Spin-Transfer-Torque Magnetic-Random-Access-Memory

Abstract

Recently, p-STT MRAM has been intensively researched as a promising alternative to conventional Si-based DRAM, which has a physical scaling limit of less than 20 nm, because it can achieve a fast write time of ~10 ns, non-volatile memory operation, and extremely low power consumption. However, for use in terra-bit-level nonvolatile memory, p-STT MRAM needs to achieve the critical device performances of a p-MTJ, including a superior thermal stability (△=E/KBT) of 74, a high tunneling magneto-resistance (TMR) ratio of 150%, and a low critical current density (Jc) of 13.4 MA/cm2. In particular, anti-ferro-magnetic coupling strength (Jex) in SyAF layers of > 0.7 erg/cm2 is essential to ensure the read/write failure margin of p-STT MRAM. Furthermore, in order to realize mass-production of p-STT MRAM, p-MTJ should be fabricated on TiN electrodes sputtered on 12-inch Si wafers due to being connected with a selective n-MOSFET cell-transistor. Recent studies have provided evidence that Co2Fe6B2/MgO or Co2FeAl/MgO based p-MTJ stacks with a SyAF layer based on [Co/Pd]n- MLs, which have a high TMR ratio and low resistance-area (RA) for realizing p-STT MRAM devices. This work aims to research and develop the key parameters for realizing mass production of p-STT MRAM such as a high TMR ratio, high Jex and low Jc through 12-inch TiN-WF process. At the former part of this dissertation, we adopted to insert a noble Fe layer between Co2Fe6B2 pinned layer and MgO tunneling barrier for achieving required high TMR ratio, which can avoid the out-diffusion of Pd atoms from a [Co/Pd]n-SyAF layer into Co2Fe6B2/MgO interface and improve i-PMA characteristics of the Co2Fe6B2 pinned layer via improving the crystalline linearity of the MgO tunneling barrier and enhancing the Fe3d-O2p hybridization. In this work, we investigated the dependency of a TMR ratio on various Fe insertion layer thicknesses in Co2Fe6B2/MgO-based p-MTJ spin valves with a [Co/Pd]n-SyAF layer on 12-inch TiN electrode wafers. The TMR ratio strongly depended on nanoscale Fe insertion-layer thickness (tFe) between the Co2Fe6B2 pinned layer and MgO tunneling barrier. The TMR ratio rapidly increased with tFe up to 0.4 nm by improving the crystalline linearity of a MgO tunneling barrier and suppressing the diffusion from of Pd atoms from a [Co/Pd]n-SyAF while it abruptly decreased with increasing further tFe by transferring from i-PMA into IMA characteristic of the Co2Fe6B2 pinned layer. Thus, the TMR ratio peaked at tFe=0.4 nm: i.e., 120% at 29 Ωμm2. Then, for obtaining required higher TMR ratio, we investigated the TMR dependency on various Co2Fe6B2 free-layer thicknesses using optimized the Fe/Co2Fe6B2 pinned-layer thickness of 0.4/1.0 nm. The TMR ratio rapidly increased with tCFB_FL up to 0.6 nm by improving the i-PMA characteristics while it abruptly decreased with increasing further tCFB_FL by transferring from i-PMA into IMA characteristic of the Co2Fe6B2 free layer. Thus, the TMR ratio peaked at tCFB_FL =1.1 nm: i.e., 123% at 20 Ωμm2. As a result, TMR ratio strongly depended on nanoscale thickness of Co2Fe6B2 free-layer. In order to achieve a high Jex in the SyAF layer of > 0.7 erg/cm2, we investigated the Ru spacer-thickness effect on the Jex of a [Co/Pd]n-SyAF layer fabricated with Co2Fe6B2/MgO based p-MTJ spin-valves on 12-inch TiN electrode wafers. Jex peaked at a certain Ru spacer-thickness: specifically, a Jex of 0.78 erg/cm2 at 0.6 nm. Otherwise, Jex rapidly degraded when the Ru spacer-thickness was less than or higher than 0.6 nm. As a result, the allowable Ru thickness variation to satisfy the Jex criteria (> 0.7 erg/cm2) should be controlled less than 0.12 nm. Finally, we suggest the way to simultaneously improve TMR ratio and reduce Jc by using a Co2FeAl-MgO p-MTJs which have high spin polarization (SP=1) and a lower damping constant (α=0.001). In this work, we investigated the dependence of the i-PMA features of full-Heusler based Co2FeAl/MgO/ Co2Fe6B2 MTJs on the Pt seed layer thickness and ex-situ annealing temperature, where all of the samples were processed on 12-inch silicon substrates. The experimental observations demonstrate the preference of a suitable thin thickness of the Pt seed layer and high ex-situ annealing temperature due to the enhanced surface roughness of the seed layer, comparable to those of the Cr seed layer prepared at a much higher in-situ annealing temperature. The HR-TEM measurements demonstrated the improved surface features of the thinner Pt seed layer, corresponding to the enhanced PMA performance. As a result, we simultaneously obtained a high TMR ratio of 123%, a low RA value of 20 Ωμm2, and a novel Jex value of 0.78 erg/cm2 on a 12-inch TiN electrode wafer at a Co2Fe6B2/MgO-based p-MTJ spin-valves with a [Co/Pd]n-SyAF layer. In addition, we clearly obtained i-PMA features by suppressing the surface roughness with the optimized Pt seed-layer thickness and ex-situ annealing temperature at a Co2FeAl/MgO-based p-MTJ spin-valve