Highly Enhanced TMR Ratio and Δ for Double MgO-Based p-Mtj Spin-Valves with Top Co2Fe6B2 Free Layer By Nanoscale-Thick Iron Diffusion-Barrier
Authors: Lee Seung-Eun , Baek Jong-Ung , Shim Tae-Hun , Hong Jin-Pyo , Park Jea-Gun 
Published: February 2020

Recently, perpendicular spin-transfer-torque magnetic-random-access-memory (p-STT MRAM) has intensively been researched as a promising memory cell of overcoming the physical scaling limit of less than 10 nm in current dynamic-random-access-memory (DRAM) because of its fast write time (~10 ns), non-volatile memory operation, and low power consumption. A p-STT MRAM cell consists of a selective device and a perpendicular-magnetic-tunneling-junction (p-MTJ) spin-valve. CoFeB/MgO based p-MTJ spin-valves have been widely researched. In particular, for tera-bit-level integration, they require a high tunneling-magneto-resistance (TMR) ratio (> 150%) for a sufficient write/erase margin, high thermal stability (Δ) (> 74) for a 10-year data retention-time, and a low switching current (J
c) (~1x102 MA/cm2) for low power consumption at the back end of line (BEOL) temperature of 400oC for p-STT-MRAM cells1. Recently, a Ta based double MgO-based p-MTJ spin-valve structure has been attractively proposed to achieve simultaneously a high TMR ratio and high thermal stability2. However, it has been extremely difficult to achieve a TMR ratio of >100% at 400oC (BEOL) because of Ta diffusion into the MgO tunneling barrier degrading the (100) crystallinity3.
Thus, in this study, we designed a novel double MgO-based p-MTJ spin-valve with a top CoFeB free layer using W as a nanoscale-thick capping layer, spacer layer, and bridging layer by inserting a nanoscale-thick b.c.c. crystallized Fe diffusion-barrier between the W capping layer and the MgO capping layer, as shown in Fig. 1(a). First, we investigated the effect of this Fe diffusion barrier on the TMR ratio in a double MgO-based p-MTJ spin-valve with a top Co2F6B2 free layer ex-situ annealed at 400oC. Then, we examined the mechanism by which the Fe diffusion barrier enhances the TMR ratio by using current-in-plane tunneling (CIPT) at room temperature, the anisotropy energy density and thermal stability calculation by estimating the PMA magnetic moment.
The TMR ratio was strongly dependent on the Fe diffusion-barrier thickness; i.e., it rapidly increased from ~131% to ~154% when the thickness increased from 0 to ~0.30 nm and it considerably decreased from ~154% to ~134% when the thickness increased from ~0.30 nm to ~0.66 nm. Thus, the TMR ratio peaked at a specific nanoscale Fe diffusion-barrier thickness, i.e., ~154% at ~0.30 nm, as shown in Fig. 1(b).
Two kinds of the PMA structure, i.e., the PMA structure without a Fe diffusion barrier and with 0.30-nm thick Fe diffusion-barrier, were prepared to estimate the anisotropy energy and thermal stability. The anisotropy energy density (~0.31 erg/cm2) peaked at a specific effective thickness of the upper Co2Fe6B2 free layer (~0.67 nm) for the PMA structure without a Fe diffusion barrier; otherwise, it (~0.35 erg/cm2) peaked at a specific effective thickness of the upper Co2Fe6B2 free layer (~0.83 nm) for the PMA structure with 0.30-nm thick Fe diffusion barrier, as shown in Fig. 1(c). These result indicate that the insertion of a 0.30-nm-thick Fe diffusion barrier between the W and MgO capping layer could enhance the anisotropy energy density at a higher effective thickness of the upper Co2Fe6B2 free layer. Furthermore, thermal stability (Δ) was calculated. The Δ (33.8)peaked at a specific effective thickness of the upper Co2Fe6B2 free layer (~0.67 nm) for the PMA structure without a Fe diffusion barrier; otherwise, it (38.0) peaked at a specific effective thickness of the upper Co2Fe6B2 free layer (~0.83 nm) for the PMA structure with 0.30-nm thick Fe diffusion barrier, as shown in Fig. 3(b). These result indicate that the insertion of a 0.30-nm-thick Fe diffusion barrier between the W and MgO capping layer would increase the of the upper Co2Fe6B2 free layer approximately 15% since the ~0.30-nm thick Fe diffusion-barrier plays an excellent a role of the diffusion barrier of the W atoms ex-situ annealing at 400oC.
In our presentation, we will report the mechanism by which thickness of the Fe diffusion barrier influence the TMR ratio was revealed by examining the static magnetization behavior, (100) bcc crystallinity, depth profile of the atomic composition of the spin-valves, the dead-layer thickness and thermal stability calculation, and the strain at the interface between the W capping and MgO tunneling barrier layer by theoretical calculation.
* This work was supported by a Basic Science Research Program grant from the National Research Foundation of Korea (NRF) funded by the Korean government (MSIP) (No. 2014R1A2A1A01006474) and the Brain Korea 21 PLUS Program in 2014.

Reference

[1] H. Honjo et al., Symp. VLSI Tech. T160-161 (2015)
[2] Y. Tomczak et al., Appl. Phys. Lett. 108, 042402 (2016)
[3] Y. Takemura et al., Nanotecknology. 26, 195702 (2015)

Figure 1