Thermal Stability Enhancement of Magnetic Perpendicular-Magnetic Tunnel Junctions Using Double MgO Interface Structure

Perpendicular spin-transfer-torque magnetic-random-access-memory (p-STT MRAM) has been investigated widely as a next generation memory, due to their capability of non-volatility, low power consumption, write endurance and high speed operation. Many efforts have been done to develop of p-STT MRAM, especially CoFeB/MgO-based perpendicular magnetic tunnel junction (p-MTJ) have widely studied because it shows high tunneling magnetoresistance (TMR) ratio caused by Δ₁coherent tunneling effect and relatively low current density required than that of in-plane easy axis MTJ. To accommodate the p-MTJ to p-STT MRAM, p-MTJ should be increased the thermal stability (Δ=K_uV/k_BT where K_u is the anisotropy energy density, V is the volume of recording layer, k_B is Boltzmann constant and T is absolute temperature respectively) of 74 to assure the data retention for 10 years.

Thus, in this work, we investigated the effect of double MgO interface CoFeB-MgO based p-MTJ on the thermal stability. In CoFeB-MgO based p-MTJs, thermal stability is improve with increasing the thickness of CoFeB free (recording) layer [1]. However, magnetic anisotropy become in plane when CoFeB thickness over 1.3 nm [2]. With double MgO interface, CoFeB free layer maintain perpendicular magnetic anisotropy even the thickness is over 1.3 nm and lead to higher thermal stability.

We prepared CoFeB-MgO based p-MTJs utilizing magnetron sputtering on TiN electrode constructed 12-inch Si wafers. Afterword p-MTJs were annealed at several ex-situ annealing temperatures (T_ex) for 1 hour under perpendicular magnetic field of 3T. And we analyzed p-MTJ magnetic properties by using Vibrating-Sample-Magnetometer (VSM), crystallinity by observing high resolution cross-sectional TEM (x-TEM) images and TMR ratio by using a current-in-plane tunnelling (CIPT) technique. Fig. 1 shows the anisotropy energy density (Kut) of p-MTJs as a function of CoFeB thickness and spacer thickness. Single MgO interface p-MTJ with Ta seed layer exhibited Δ=21  at T_ex=275°C as shown in Fig. 1 (A). On the other hand, single MgO interface p-MTJ with bcc seed layer showed Δ=26 even after it was annealed at 400°C, as shown in Fig. 1 (B). We could improve D and annealing temperature at onetime using bcc seed layer, actually improving of annealing temperature is also required to accommodate p-STT-MRAM. Kut of Double MgO interface p-MTJ with bcc seed layer are shown in Fig. 1 (C). It showed almost two times higher Δ (61) than that of single MgO interface at Tex=400°C, and almost independent of spacer thickness. Fig. 2 shows the x-TEM images of single and double MgO interface p-MTJ spin-valves. We could obtain well crystallinity MgO barrier layer at both of single and double MgO interface p-MTJs, and those TMR ratio were 163 and 141% respectively. TMR ratio of double interface p-MTJ was decreased because of increasing of resistance area product (RA), which caused by bottom MgO layer.

In the conference, we will present how thermal stability of p-MTJ using MgO double interface structures was improved and review those mechanisms by analyzing magnetic property, crystallinity of MgO and TMR ratio using VSM, x-TEM and CIPT respectively.

 * This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) grant funded by the Korea government (No. 2014R1A2A1A01006474 & No. 1004608) and Brain Korea 21 PLUS Program in 2014.

References

[1] H. Sato, M. Yamanouchi, K. Miura, S. Ikeda, R. Koizumi, F. Matsukura, and H. Ohno, IEEE MAGNETICS LETTERS, 3, 3000204 (2012)

[2] J. G. Park, T. H. Shim, K. S. Chae, D. Y. Lee, Y. Takemura, S. E. Lee, M. S. Jeon, J. U. Baek, S. O. Park, and J.P. Hong, in IEEE International Electron Devices Meeting (IEDM) IEEE, 2014, pp. 19.2.1.