Recently, the semiconductor memory device has faced the difficulties of scaling down, low power consumption and high retention time. In particular, dynamic random access memory (DRAM) has been struggling with scaling limit because of reducing capacitor area. In addition, alternative memories have been studied intensively to replace DRAM. Among them, perpendicular spin transfer torque magnetic random access memory (STT-MRAM) is considered as a strong candidate because of the small scaling down, low power consumption, high read and write speed and non-volatility.

STT-MRAM has the structure of 1 transistor and 1 resistance (1T-1R) called magnetic tunnel junction (MTJ). Resistance difference between parallel and antiparallel states in MTJ indicates memory sensing margin. The large difference of tunneling magneto-resistance (TMR) is required for a good memory operation. To obtain the high TMR ratio, not only perfect switching of the free layer but also hard pinning of the pinned layer is required. It is known that pinning the pinned layer is due to the exchange coupling induced by a synthetic anti-ferro-magnetic (SyAF) layer based on Ruderman Kittel Kasuya Yosida (RKKY) interaction.

In our study, therefore, we investigated how to optimize the SyAF layer structure for the proper exchange coupling. MgO tunnel barrier should be b.c.c (100) crystallinity because it acts as an electrons filter which determined whether spin direction between free and pinned layers is aligned parallel or not. In particular, we propose double MgO based perpendicular magnetic-tunnel-junction spin-valve without upper synthetic-anti-ferromagnetic Multi-layers in order to obtain high TMR ratio. In addition, we investigated the different characteristics of the p-MTJ spin-valve with and without upper SyAF multi-layers.

For the experiment, two structures, double MgO based p-MTJ spin-valve with and without upper SyAF multi-layers, were fabricated on a 12-inch-diameter wafer deposited with SiO₂/W/TiN films in a 12-inch multi-chamber sputtering system under a high vacuum of less than 1ⅹ10^-8 torr (without breaking the vacuum; see Figs. 1a and b). The thicknesses of the tungsten (W) were varied ranging from 0.18 to 0.48 nm. All samples were subject to ex-situ annealing at 350^oC for 30 min under a perpendicular magnetic field of 3 Tesla.

 The dependence of the TMR ratio on the thickness of W bridge layer was estimated by using current-in-plane tunneling (CIPT) at room temperature as shown in Fig.1c. It shows that a TMR ratio of p-MTJ spin-valve with upper SyAF multi-layers slightly increases at the bridge layer thickness, t_w up to around 0.3 nm. Then it abruptly decreases when the thickness was increased from 0.36 to 0.5 nm. On the other hand, the TMR ratio of p-MTJ spin-valve without upper SyAF multi-layer increases at the t_w up to around 0.24 m. Then, the TMR ratio abruptly decreases when the thickness was increased from 0.36 to 0.4 nm. And maximum TMR ratio of p-MTJ spin-valve with upper SyAF multi-layers is 156 % while, that of p-MTJ spin-valve without upper SyAF multi-layers is 180 %. Obviously, the TMR ratio of p-MTJ spin-valve without upper SyAF multi-layers is higher 24 % than p-MTJ spin-valve wih upper SyAF multi-layers.

In our presentation, we report the different characteristics of the p-MTJ spin-valve with and without upper SyAF multi-layers by examining the static magnetization behavior, MgO crystallinity, and I-V characteristic curve. In addition, we present how these properties influence the TMR ratio and mechanism of p-MTJ spin-valve.

* 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.