Elsevier

Acta Materialia

Volume 87, 1 April 2015, Pages 259-265
Acta Materialia

Highly stable perpendicular magnetic anisotropies of CoFeB/MgO frames employing W buffer and capping layers

https://doi.org/10.1016/j.actamat.2015.01.022Get rights and content

Abstract

Typical CoFeB/MgO frames ensuring interface perpendicular magnetic anisotropy (PMA) features are one of the most reliable building blocks to meet the demand of PMA-based memory devices. However, employing the CoFeB/MgO frame with a Ta buffer layer has been restricted by the rapid PMA degradation that occurs during annealing at temperatures greater than 300 °C and the need of an ultrathin CoFeB layer of approximately 1.3 nm. Thus, the ability to enhance thermally strong PMA characteristics is still a key step toward extending their use. Here, we examine the effect of W layers on PMA features through both W buffer/CoFeB/MgO and MgO/CeFeB/W capping frames at various annealing temperatures. Highly stable PMA was achieved up to 450 °C at a specific W thickness, along with the presence of the dominant PMA properties at a relatively thick CoFeB greater than 1.3 nm and the achievement of a high Keff of approximately 5 Merg/cc.

Introduction

Spin transfer torque magnetoresistive random access memories (STT-MRAMs) are rapidly becoming one of the most reliable candidates for next generation memory due to their outstanding features, i.e., non-volatility and high-speed performance [1]. In particular, perpendicularly magnetized tunnel junctions (p-MTJ) used for STT-MRAMs show promise because it suggests the possibility of achieving a high thermal stability factor (Δ) determining memory retention and low write current density (JC) [2], [3], [4], [5]. Given perpendicular magnetic anisotropy (PMA) advantages, various PMA materials such as rare-earth-transition metal films, [Co/(Pt, Pd)] and [Fe/(Pt, Pd)] multilayers, or their L10 ordered alloy films, and CoFeB/MgO frames, have been widely studied for the past decade [6], [7], [8] as they show large PMA characteristics. Among the PMA materials considered recently, the CoFeB/MgO free layer frames were widely investigated for the development of STT-MRAM devices due to their large tunneling magnetoresistance (TMR) ratio [8]. However, several key issues affecting CoFeB/MgO frame-based p-MTJ performance have remained challenging. One issue is the thermal stability of Δ = Keff V/kBT, where Keff, V, kB, and T represent the effective magnetic anisotropy energy density, volume of the magnetic layer, Boltzmann constant, and absolute temperature, respectively. For example, high performance 1 Gb memory devices require a Δ greater than 60 [2], [5]. In addition, since the MTJ cell size is determined by the storage size, the higher Keff and relatively thick CoFeB layer are important factors to enhance the Δ [9]. Sato et al. announced novel MgO/CoFeB/Ta/CoFeB/MgO frames to address this issue [10]. These stacks doubled the total thickness of CoFeB, providing Δ = 95. However, employing the double MgO barriers caused the TMR ratio to decrease, resulting from the increased resistance and area product (RA). The other issue of CoFeB/MgO frames with a typical Ta buffer layer is the rapid PMA degradation during annealing at temperatures greater than 300 °C [11], [12]. It is widely believed that enhancement of TMR demands the (0 0 1) crystalline orientation of CoFeB and MgO for coherent tunneling with Δ1 symmetry [13], [14], [15]. Thus, post annealing is a generic approach for the formation of well-aligned crystalline structures through a boron out-diffusion. In addition, suitable MTJ features at high annealing temperatures have been the focus of immense interest because of the requirement of high temperatures at the back end of the line at approximately 400 °C. However, a typical Ta/CoFeB/MgO frame permits thermally-activated Ta ions to diffuse toward CoFeB/MgO during annealing at temperatures greater than 300 °C, causing PMA deterioration [16], [17]. Therefore, much effort has been dedicated toward the development of substitution materials for the replacement of the Ta buffer layer [18], [19]. Recently, Liu et al. announced outstanding PMA features of Mo/CoFeB/MgO frames up to 425 °C [20].

Therefore, we address the effect of W layers on W/CoFeB/MgO and MgO/CeFeB/W frames as an alternative material, ensuring thermally strong annealing characteristics up to 450 °C. The dominant PMA properties in our work were maintained even at a relatively thick CoFeB layer greater than 1.3 nm and a large in-plane saturation field of CoFeB showing a high Keff value of approximately 5 Merg/cc was demonstrated at high annealing temperatures compared to that of a typical Ta/CoFeB/MgO frame.

Section snippets

Experimental

Two sets of sample series were prepared: substrate/W (tW)/CoFeB (tCFB)/MgO (3)/Ta (3) and substrate/Ta (3)/MgO (3)/CoFeB (tCFB)/W (tW) on the thermally-oxidized Si wafers at room temperature by dual DC & RF magnetron sputtering with a Co20Fe60B20 target, where the numbers in parentheses represent the nominal thickness of each layer in nanometers. The former is termed the W buffer series and the latter is termed the W capping series. A base pressure of the sputtering system was better than 2 × 10−8

Results and discussion

Fig. 1a and b shows the schematics of stack architectures for both the W buffer and capping series, respectively. Also, Fig. 1c and d shows the representative magnetization curves for the W buffer and W capping with various W thicknesses and fixed 1.4 nm CoFeB annealed at 350 °C. In the W buffer case, the PMA properties were clearly observed when the W thickness was in the range of 1.8–7.4 nm. The transition to IMA appeared for a relatively thick W buffer layer. The maximum Hk was approximately 5.9

Conclusion

In conclusion, we report that a W layer could be an attractive species to achieve thermally strong PMA features up to 450 °C, even though the deterministic origin of our findings is still a challenge due to the presence of various unknown sources. A significantly large Hk value of 10 kOe was obtained and even a relatively thick CoFeB also demonstrated dominant PMA features at higher annealing temperatures. In addition, in a W/CoFeB/MgO frame, the PMA features are highly dependent on W thickness,

Acknowledgement

This research was supported by a Grant from the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2013R1A1A2060350).

References (28)

  • J.C. Slonczewski

    Current-driven excitation of magnetic multilayers

    J. Magn. Magn. Mater.

    (1996)
  • T. Kawahara et al.

    Spin-transfer torque RAM technology: review and prospect

    Microelectron. Reliab.

    (2012)
  • S.A. Wolf et al.

    Spintronics: a spin-based electronics vision for the future

    Science

    (2001)
  • S. Mangin et al.

    Current-induced magnetization reversal in nanopillars with perpendicular anisotropy

    Nat. Mater.

    (2006)
  • Ki Chul Chun et al.

    A Saling roadmap and performance evaluation of in-plane and perpendicular MTJ based STT-MRAMs for high-density cache memory

    IEEE J. Solid-State Circuits

    (2013)
  • K. Mibu et al.

    Magnetic anisotropy in Fe/rare-earth multilayers

    Hyperfine Interact.

    (1998)
  • J.B. Lee et al.

    Thermally stable perpendicular magnetic anisotropy features of [Co/Pd]m multilayer matrix integrated with [CoO/Pd]n bottom layer

    Appl. Phys. Lett.

    (2014)
  • S. Ikeda et al.

    A perpendicular-anisotropy CoFeB–MgO magnetic tunnel junction

    Nat. Mater.

    (2010)
  • H. Sato et al.

    CoFeB thickness dependence of thermal stability factor in CoFeB/MgO perpendicular magnetic tunnel junctions

    IEEE Magn. Lett.

    (2012)
  • H. Sato et al.

    Perpendicular-anisotropy CoFeB–MgO magnetic tunnel junctions with a MgO/CoFeB/Ta/CoFeB/MgO recording structure

    Appl. Phys. Lett.

    (2012)
  • M. Yamanouchi et al.

    Dependence of magnetic anisotropy on MgO thickness and buffer layer in Co20Fe60B20–MgO structure

    J. Appl. Phys.

    (2011)
  • S. Ikeda et al.

    Tunnel magnetoresistance of 604% at 300 K by suppression of Ta diffusion in CoFeB/MgO/CoFeB pseudo-spin-valves annealed at high temperature

    Appl. Phys. Lett.

    (2008)
  • W.H. Butler et al.

    Spin-dependent tunneling conductance of Fe|MgO|Fe sandwiches

    Phys. Rev. B

    (2001)
  • S.P. Stuart Parkin∗ et al.

    Giant tunneling magnetoresistance at room temperature with MgO (1 0 0) tunnel barriers

    Nat. Mater.

    (2004)
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