Study on the Annealing Stability of Perpendicular-Magnetic Tunnel Junctions with Heavy-Metal Interlayers

Abstract

The perpendicular-spin-transfer-torque magnetic random-access memory (p-STT MRAM) has been intensively researched because of the possible applications in the variety of memory fields and its numerous advantages such as a non-volatile characteristic, a fast read/write speed (~ 10 ns), an extremely low power consumption (< 1 pJ/bit), and a high write endurance (> 1012), among others. Additionally, the ability to use an embedded system on chip (SoC) memory for the p-STT MRAM is enabled for mobile applications, while a terabit-integrated p-STT MRAM cell is now being used to overcome the scaling limitations of the current dynamic random-access memory (DRAM) below the 10 nm design rule. Moreover, many researchers have studied the p-STT MRAM with respect to the cross-point memory of the memory-mapped storage-class memory (SCM). In particular, to realize the mass production of the p-STT MRAM, the perpendicular-magnetic tunnel junction (p-MTJ) should be fabricated on 12-inch titanium-nitride (TiN)-electrode wafers depending on the selective device; that is, either the n-type metal-oxide-semiconductor field-effect transistor (n-MOSFET), the mixed ionic-electronic conductor (MIEC), or the ovonic-threshold-switch (OTS) diode. Recent studies have provided evidence that the Co2Fe6B2/MgO-based p-MTJ with a [Co/Pt]n-SyAF layer, which indicate a high TMR ratio and low resistance-area (RA) , is suitable for the realization of p-STT MRAM devices. In this dissertation, the CoFeB/MgO-based p-MTJ, for which a heavy metal (i.e., tungsten) was used with [Co/Pt]n-SyAF layer on 12-inch TiN-electrode wafer at a back-end of line (BEOL) temperature of 400° C, was studied for the realization of a mass-produced of p-STT MRAM. First, the effects of the tantalum (Ta) and tungsten (W) seed-layer thicknesses on the CFB/MgO-based PMA stacks were investigated. In addition, the seed layer in the PMA stacks was studied to confirm the advantage of W material, which can be used to avoid the diffusion of the W atoms from the seed layer into the CoFeB layer, and to improve the i-PMA characteristics of the CoFeB layer at an annealing temperature of 400° C; for this reason, the tunnel-magnetoresistance (TMR) ratio is extremely sensitive to the seed/cap layer in terms of the p-MTJs that were annealed at 400° C ex situ, as follow: the TMR ratio for the W-seed/cap layer (~ 160 %) is much higher than that for Ta-seed/cap layer (~ 1 %). Further, three types of the p-MTJ structure were investigated, as follows: The first types is the bottom-CoFeB-free-layer p-MTJ with a [Co/Pt]n-SyAF layer, the second type is the top-CoFeB-free-layer p-MTJ with a [Co/Pt]n-SyAF-layer, and the final structure is the double PMA-based top-CoFeB-free-layer p-MTJ with a [Co/Pt]n-SyAF layer. First, the method for the improvement of the TMR ratio and crystallinity of the MgO tunneling barrier that is achieved through the modulation of the RF-sputtering-power condition of MgO target in the bottom-CoFeB free-layer p-MTJ was studied. The TMR ratio strongly depends on the RF-sputtering power in an MgO tunneling barrier with a thickness from 0.65 to 1.15 nm, whereby a TMR ratio of 168 % is achieved at 300 W. The TMR ratio linearly increased with the decreasing of the RF-sputtering power between 300 W and 500 W; then, it abruptly decreased at 250 W, since the f.c.c crystallinity of the tunneling barrier improved with the decreasing of the RF-sputtering power between 300 W and 500 W, and this was followed by an abrupt degradation at 250 W. The second study is regarding the structural design of p-MTJs; that is, the p-MTJ with a bottom free layer or the p-MTJ with a top free layer at an annealing temperature of 400° C ex situ. For the p-MTJ with the bottom CFB free layer was annealed at 400° C ex situ, the platinum (Pt) atoms of the Pt-buffer layer diffused into the MgO tunneling barrier; alternatively, the p-MTJ with a top CFB free layer could prevent the diffusion of the Pt atoms into the MgO tunneling barrier during the 400° C ex situ annealing because of non-necessity of a Pt-buffer layer, thereby demonstrating a TMR ratio of ~ 143 %. The dependency of the TMR ratio on the Pt seed-layer thickness for the double PMA-based p-MTJs with the top CoFeB free layer at the annealing temperature of 400° C was studied next to increase the TMR ratio. For the double PMA-based p-MTJ with a top CFB free layer that was annealed at 400° C ex situ, the TMR ratio strongly depends on the Pt seed-layer thickness (tPt), as follows: It peaked (∼ 134 %) at a specific tPt of 3.3 nm. Lastly, the spacer in the double PMA-based p-MTJ was studied to confirm the advantage of the W spacer. In comparison with the Ta spacer, the TMR ratio for the W spacer (~ 134 %) is higher than that for the Ta spacer (~ 98 %); this dependency on the spacer material is associated with the b.c.c (100)-textured crystallinity of the MgO layer. In particular, the strain-enhanced diffusion length in the MgO layers of the W atoms (~ 1.40 nm) is much shorter than that of the Ta atoms (~ 2.85 nm) and this shorter diffusion length led to a more-effective b.c.c (100)-textured crystallinity regarding the MgO layer. As a result, a high TMR ratio of 143 % was obtained for the single PMA-based p-MTJ ex situ, while a TMR ratio of 134 % was obtained for the double PMA-based p-MTJ ex situ, on a 12-inch TiN-electrode wafer at the annealing temperature of 400° C. In addition, the mechanisms of the magnetic properties were clearly verified through the conduction of analyses such as that regarding the crystallinity, the atomic compositional profile, the surface-roughness observation, and an energy-band-gap estimation. The results of this dissertation are especially important, as they are related to the research fields of electrical engineering, material science, and physics with respect to development of terabit-level p-STT MRAMs.