(Invited) New Design of Double MgO-Based p-Mtj Spin-Valves with Top Co2Fe6B2 Free Layer Performain Highly Increased TMR Ratio and Thermal Stability
Authors: Park Jea-Gun 
Published: February 2020

Perpendicular spin-transfer-torque magnetic-random-access-memory (p-STT MRAM) has intensively been researched as a promising memory cell to overcome a physical scaling limit of less than 20 nm in current dynamic-random-access-memory (DRAM) because of its fast write time (~10 ns), non-volatile memory operation, and extremely low power consumption. For realizing terra-bit-level integration of p-STT-MRAM cells fabricated with a selective transistor and a perpendicular-magnetic-tunnel-junction (p-MTJ) spin-valve, the p-MTJ spin-valves with a bottom CoFeB free layer using a Ta bridging and capping layer have been generally and widely researched. They need to achieve a high tunneling-magneto-resistance (TMR) ratio (>150%), thermal stability (Δ) (>74), and switching current (Jc) (~1x102 MA/cm2) at the back end of line (BEOL) temperature of 400oC. It has been reported that a Ta bridging and capping layer would be a good absorption layer for Boron (B) to enhance i-PMA characteristic but Ta atoms easily diffuse into the MgO tunneling barrier during ex-
situ annealing temperature (or a thermal budget at BEOL)1,2. Thus, the p-MTJ spin-valves with a bottom CoFeB free layer using a tantalum (Ta: body-centered-cubic structure) bridging and capping layer have extreme difficulty achieving the TMR ratio of > 100% at the BEOL of 400 oC since the TMR ratio decreases rapidly as ex-
situ annealing temperature (or a BEOL temperature) increases. Recently, the p-MTJ spin-valves with a top CoFeB free layer using a Ta bridging and capping layer have been intensively studied to obtain a higher TMR ratio at a higher ex-
situ annealing temperature (or a BEOL temperature) since it would minimize the diffusion of platinum (Pt) atoms into the MgO tunneling barrier3-5. However, they confront a significant diffusion of Ta atoms into the MgO tunneling barrier at ex-
situ annealing (or BEOL) of 400oC, so the b.c.c crystallinity of the MgO tunneling barrier is abruptly degraded, thereby rapidly decreasing the TMR ratio of the p-MTJ spin-valves. An innovative solution proposed in this paper is a new design of a p-MTJ spin-valve with a top Co2Fe6B2 free layer using a W bridging with sub-nanoscale thickness (~0.24 nm) and a tungsten (W: body-centered-cubic structure) capping layer with nanoscale thickness (~4.0 nm). The proposed design demonstrated no diffusion of Ta atoms into the MgO tunneling barrier and minimized the diffusion of Fe atoms at both Co2Fe6B2 free and pinned layer at an ex-situ annealing of 400oC. Therefore, b.c.c crystallinity degradation of the MgO tunneling barrier could be prevented, and high PMA magnetic moments at both Co2Fe6B2 free and pinned layers could be achieved. Thus, the p-MTJ spin-valve with a Co2Fe6B2 free layer using a W bridging and capping layer with nanoscale thickness (~4.0 nm) achieved the TMR ratio of ~143% at ex-
situ annealing of 400oC, as shown Fig. 1. Furthermore, as a way to achieve a TMR ratio of >150% at 400°C, in this paper, we present the design of a new double MgO-based p-MTJ spin-valve with a top Co2Fe6B2 free layer by inserting a nanoscale Fe diffusion barrier (~0.30 nm: only two Fe atomic layers thick) between the W capping layer (~ 4.0 nm in thickness) and MgO capping layer (~1.0 nm in thickness). This new p-MTJ spin-valve had a TMR ratio of ~154% and Δ of 90 in ex-situ magnetic annealing at 400°C, as shown Fig. 2.
* 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] J. Appl. Phys. 2009, 106, 023920
[2] Acta Materialia 2015, 87, 259
[3] IEDM 2015, pp. 26. 2
[4] IEDM 2015, pp. 26. 3
[5] IEDM 2015, pp. 26. 4
Fig. 1. (a) p-MTJ spin-valve structure using a Ta bridging and capping layer, (b) p-MTJ spin-valve structure using a W bridging and capping layer, and (c) dependency of the TMR ratio on bridging and capping layer thickness for both p-MTJ spin-valves using Ta and W bridging and capping layers.
Fig. 2. Dependence of TMR ratio on nanoscale thickness of Fe diffusion barrier for double MgO-based p-MTJ spin-valves with a top Co2Fe6B2 free layer.

Figure 1