WO2004109820A1

WO2004109820A1 - MAGNETIC TUNNEL JUNCTIONS INCORPORATING AMORPHOUS CoNbZr ALLOYS AND NANO-OXIDE LAYERS

Info

Publication number: WO2004109820A1
Application number: PCT/KR2004/000909
Authority: WO
Inventors: Young-Geun Kim; Byung-Sun Jeon; In-Chang Chu; Seong-Rae Lee
Original assignee: Korea University Foundation
Priority date: 2003-06-05
Filing date: 2004-04-21
Publication date: 2004-12-16
Also published as: KR20040105187A

Abstract

Magnetic tunnel junctions (MTJs) comprising amorphous CoNbZr underlayer are proposed. Co85.5Nb8Zr6.5 (in at. %) layers are employed to substitute traditionally used Ta layers with an emphasis given on understanding under layer effect. The typical junction structure is SiO2/CoNbZr or Ta 2/CoFe 8/IrMn 7.5/CoFe 3/Al 1.6 + oxidation/CoFe 3/CoNbZr or Ta 2 (nm). To prevent Mn interdiffusion toward the bottom ferromagnetic electrode, nano oxide layer (NOL) is formed on a underlayer or a buffer layer. The NOL in the MTJ is thermally stable up to the 350 °C.

Description

Description MAGNETIC TUNNEL JUNCTIONS INCORPORATING AMORPHOUS CoNbZr ALLOYS AND NANO-OXIDE

LAYERS

Technical Field

[1] The present invention relates to magnetic tunnel junctions, and particularly, to a magnetic tunnel junction with improved materials properties as well as improved resistance to interdiffusion . Background Art

[2] Magnetic tunnel junctions(MTJs) comprise sandwich type structure of two ferromagnetic layers separated by an insulating layer(generally, Al O ) which acts as a

2 3 tunneling barrier. And the current is passed perpendicular to each layer of the MTJs structures. The resistance of the MTJs is lowest and therefore the tunneling of the current is highest when spin direction of the two ferromagnetic layers is aligned paralled. On the other hand, the resistance of the MTJs is highest and the tunneling of the current is lowest when spin direction of the two ferromagnetic layers is aligned an- tiparalled. The tunneling current in the MTJs depends on the relative magnetization orientation of the two ferromagnetic layers. This phenomenon was first demonstrated by Julliere on 1975, and it is called Tunneling Magnetoresistance (TMR).

[3] The MTJs have a large potential for use in high density head and magnetic random access memory(MRAM) applications because they exhibit large TMR ratios [See, J. S. Moodera, L. R. Kinder, T. M. Wong, and R. Meservey, Phys. Rev. Lett. 74 , 3273 (1995), W. J. Gallagher, S. S. P. Parkin, Yu Lu, X. P. Han, A. Marley, K. P. Roche, R. A. Altman, S. A. Rishton, C. Jahnes, T. M. Shaw, and Gang Xiao, J. Appl. Phys. 81 , 3741 (1997)]. In the implementation of the MRAM, the core technology is a technology for developing thin film materials exhibiting superior and stable magnetoresistance and an integrated process technology using the semiconductor circuit and the process. At this point, it has been recognized that magnetoresistance thin film showing the TRM phenomenon i.e., MTJ is the most compatible as thin film materials in developing non volatile MRAM elements having superior properties .

[4] In a MTJ, it has been known that the junction resistance and TMR ratio are very sensitive to surface smoothness of both the bottom electrode and the insulating layer [See, J. J. Sun, K. Shimazawa, N. Kasahara, K. Sato, S. Saruki, T. Kagami, O. Redon, S. Araki, H. Morita, and M. Matsuzaki, Appl. Phys. Lett. 76, 2424 (2000)].

[5] In the MTJ, an insulating layer acts as a tunneling barrier of spin-polarized tunneling electrons between two ferromagnetic electrodes. The spin-dependent tunneling behavior is strongly affected not only by the spin-polarization of both ferromagnetic electrodes but by the ferromagnet/insulator interface properties [See, J. C. Slonczewski, Phys. Rev. B 39, 6995 (1989)]. In order to deposit a smooth nanometer- thick tunneling barrier, it is necessary to have a smooth surface for the bottom electrode before and after annealing.

[6] As a way to achieve smooth interfaces, an amorphous film is employed instead of traditionally used Ta layers with an emphasis given on understanding underlayer effects. Amorphous materials in absence of grain boundaries, in principle, could offer better surface smoothness as well as interdiffusion resistance. Among many candidates, a GbNbZr is employed because it has demonstrated good thermal stability and smooth surface structure. In addition, a GbNbZr exhibits lower electrical resistivity, which can be beneficial for reducing the total resistance of an MTJ. Nano oxide layer on buffer layer or underlayer is also employed in order to avoid interdiffusion of Mn of IrMn layer into tunneling barrier layer. Disclosure of Invention Technical Problem

[7] Accordingly, it is an object of the present invention to provide a MTJ having better surface smoothness.

[8] It is another object of the present invention to provide a MTJ having small temperature dependence of the magnetoresistance(MR) and a high MR ratio.

[9] It is still another object of the present invention to provide a MTJ to avoid interdiffusion of Mn of IrMn layer into tunneling barrier layer.

[10] To achieve the above objects, the present invention is characterized by employing a GbNbZr alloy(particularly Gb Nb Zr ) as an underlayer of the bottom electrode in

85 5 8 6 5 a MTJ which has an insulating layer overlaying between the ferromagnetic layers. [11] Also, the present invention is characterized that nano oxide layer on buffer layer or underlayer is employed in order to avoid interdiffusion of Mn of IrMn layer(pinning layer) into tunneling barrier layer.

Technical Solution [12] Reference will now be made in detail to the preferred embodiment of the present invention, examples of which are illustrated in the accompanying drawings. [13] To achieve these objects of the present invention as mentioned above, the present invention employs a GbNbZr among amorphous materials as an underlayer in a MTJ because it has demonstrated good thermal stability and smooth surface structure [See, E.-H. Kim, Y. K. Kim, and S.-R. Lee, J. of Magn. Magn. Mater. 233 , L142 (2001), H. G. Cho, Y. K. Kim, and S. -R. Lee, J. Appl. Phys. 91 , 8581 (2002)]. In addition, GbNbZr exhibits lower electrical resistivity than Ta (80 vs. 270 ?Ωcm at 10 nm thickness), which can be beneficial for reducing the total resistance of an MTJ. The purpose of the present invention is to investigate the MR, bias voltage and thermal behaviors of new MTJ structures comprising GbNbZr layers. Annealing after deposition was performed to understand the thermal stability of the structures. And nano oxide deposited on GbNbZr underlayer or GbFe buffer layer is employed to avoid interdiffusion of Mn of IrMn into tunneling barrier layer(Al O layer).

2 3

[14] A MTJ of the present invention consists of SiO / GbNbZr (2nm) / GbFe (8 nm) /

2

IrMn (7.5 nm) / GbFe (3 nm) / Al (1.6nm) + oxidation/ GbFe (3 nm) / GbNbZr (2 nm), wherein Al + oxidation means Al O layer, and GbFe in top and bottom of this layer is

2 3 a ferromagnet. NiFe or GbFeB may be used as the ferromagnet in the MTJ. A ferromagnet GbFe in bottom of the Al O layer is a pinned layer and antiferromagnet

2 3

IrMn in the bottom layer is a pinning layer. A GbFe in bottom of the IrMn is a buffer layer and the GbNbZr acts as an underlayer. A GbFe layer in the upper of the Al O

2 3 layer is a free layer. [15] In the experimental procedure of the present invention, both tunnel junctions consisting of SiO / GbNbZr (2 nm) / GbFe(8 nm) / IrMn (7.5 nm) / GbFe(3 nm) /

2

Al(1.6 nm) + oxidation / GbFe(3 nm) / GbNbZr(2 nm) in the MTJ of this invention and tunnel junctions SiO / Ta (2 nm) / GbFe(8 nm) / IrMn (7.5 nm) / GbFe(3 nm) / Al(1.6

2 nm) + oxidation / GbFe(3 nm) / Ta(2 nm) in the traditional MTJ are produced by a

-7 four-target rf magnetron sputtering system under typical base pressure below 5 x 10 Torr. The thickness of SiO is about 200 A . Wherein the composition of GbNbZr layer

2 is Gb Nb Zr Gb-Fe layer is Gb Fe , and Ir-Mn layer is Ir Mn .

85 5 8 65, 90 10 20 80

[16] A Gb target with small Nb and Zr chips added are used to get proper composition of the amorphous GbNbZr films. The film composition is occasionally confirmed by energy dispersive x-ray spectroscopy. Junctions are patterned by a set of metal shadow mask with an opening area of 200 x 200 ?m². Tunnel barriers are formed by oxidizing 1.6 nm thick aluminim (Al) layers under rf plasma environment: flowing pure oxygen at 40 seem, 100 mTorr, and at the power density of 3.44 watt/cm². Nano-oxide layers (NOL) are formed by oxidizing GbNbZr or GbFe layer in 5 mTorr oxygen partial pressure. Annealing is done in situ at 300°C in 5 x 10 Torr vacu n under applied field of 500 Oe. The ramp up and cool down rates are 2.5°C/s and l°C/s, respectively. The temperature dependence is measured using a cryogenic dewar in the temperature range of 10 K < T < 300 K. The interface roughness and crystalline texture of the film are characterized by atomic force microscopy (AFM) and x-ray diffraction (XRD), respectively. To uncover interdiffusion behaviors, Auger electron spectroscopy(AES) is used. [17] FIG. 1 shows the MR ratio, and the variations of the resistance and bias voltage of

MTJ in accordance with underlayer materials and annealing conditions. As mentioned above, the MTJ of the present invention consists of SiO / GbNbZr (2 nm) / GbFe(8

2 nm) / IrMn (7.5 nm) / GbFe(3 nm) / Al(1.6 nm) + oxidation/ GbFe(3 nm) / GbNbZr(2 nm), wherein Al + oxidation is Al O , the bottom ferromagnetic GbFe is a pinned

2 3 layer and antiferromagnet IrMn in its bottom is a pinning layer, GbFe of the IrMn bottom is a buffer layer and GbNbZr in its bottom is an underlayer. [18] Here, the bias voltage V is defined as a voltage where a MR ratio becomes half of h its unbiased value. The GbNbZr-based MTJ showed a lower MR ratio (11%) than the Ta-based MTJ (15%) measured at room temperature. It seems that exhibiting lower ratio was due to a poorly developed crystal structure. When annealed at 300°C for 10 min, the GbNbZr-based MTJs exhibit higher ration than the Ta-based one.

[19] To compare two MTJ structures in view of the crystallinity, XRD patterns are shown in Fig. 2. Fig. 2 shows XRD patterns of GbNbZr and Ta-based MTJs. As clearly seen in Fig. 2, the development of crystallinity was limited for the GbNbZr- based MTJ due to the presence of an amorphous underlayer.

[20] As shown in Fig. 1, when annealed at 300°C for 10 min, MR ratio of the GbNbZr- based MTJ was increased from 11% to 17% while resistance was decreased from 115Ω to 80Ω. Unlike the GbNbZr-based MTJ, the Ta-based MTJ structure exhibited short-circuiting probably due to barrier discontinuity after the same annealing condition, resulting in unmeaningful results . The increase in MR can be attributed to an improved interface with the bottom portion of ferromagnetic electrode [See, S. S. P. Parkin, K. P. Roche, M. G. Samant, P. M. Rice, R. B. Beyers, R. E. Scheuerlein, E. J. OSullivan, S. L. Brown, J. Bucchigano, D. W. Abraham, Y. Lu, M. Rooks, P. L. Trouilloud, R. A. Wanner, and W. J. Gallagher, J. Appl. Phys. 85 , 5828 (1999)]. The same GbNbZr sample that underwent 300°C, 30 min annealing displayed a reduction in the TMR ratio compared to the 10 min annealing one (see Fig. 1).

[21] FIG. 3 shows an Auger electron spectroscopy(AES) depth profile of a GbNbZr- based MTJ. The composition-resolved AES depth profiling was conducted to study interdiffusion. However, for 0 - 30 min annealed samples, no appreciable changes within the resolution of the instrument were observed. Thus, the annealing time is intentionally increased to 360 min. As shown in Fig. 3, a Mn from the antiferromagnetic IrMn layer was interdiffused primarily toward the GbFe pinned layer and AlOx barrier [See, S. Cardoso, R. Ferreira, P. P. Freitas, P. Wei, and J. C. Soares, Appl. Phys. Lett. 76, 3792 (2000)] . A small amount of interdiffusion has deteriorated MR ratio of the 30 min annealed MTJ. [22] The GbNbZr-based MTJs exhibit better V characteristics (samples annealed at h

300°C for 10 min: 325 mV) and V gradually increased at higher temperatures in the GbNbZr-based MTJs.

[23] Another feature observed with the GbNbZr-based junction is that it displayed relatively less sensitive temperature-dependent MR and resistance changes. Because GbFe layers in pinned and free layers with the same thicknesses were used for both type of junctions, the same temperature dependence was expected, but the result was not. Therefore, it is speculated that the difference in temperature dependence might be attributed to the difference in interfacial roughness. To confirm the speculation, it is measured the layer surface roughness evolution at all layer construction stages up to the al nina barriers by AFM.

[24] [Table 1]

Surface roughness (rms) Seed layer only Up to CoFe Up to AlOx

As-dep./Annealed (300°C, 10 min) (nm) (nm) (nm)

Ta-based MTJ 0.12 / 0.15 0.18 / 0.19 0.26 / 0.34

CoNbZr-based MTJ 0.09 / 0.11 0.10 / 0.12 0.12 / 0.16

[25] As displayed in Table 1, the GbNbZr-based bottom structure, in comparison with the Ta-based one, offered smaller root-mean- square (rms) surface roughnesses (0.12 vs. 0.26 nm). Another encouraging feature is that the surface smoothness is nearly preserved before and after annealing for the GbNbZr-based structure (0.12 → 0.16 nm) while that of Ta-based one becomes worse (0.26 → 0.34 nm) after annealing (at 300°C, 10 min). Therefore, achieving very smooth interfaces, in particular, up to the barrier, is beneficial for avoiding large temperature dependence. Note that the interface structures of the top electrode including a GbNbZr capping layer appeared less significant. [26] FIG. 4 shows TMR ratio as a function temperature for various GbNbZr-based

MTJs. FIGs. 4(a), 4(b), 4(c) and 4(d) show a series of tunnel junctions consisting of GbNbZr(underlayer) /GbFe(buffer layer) y bottom electrodes: x/y being (a) 2/8, (b) 2/10, (c) 2/17, and (d) 4/10 (in nm). Even though the buffer GbFe layer thickness is increased, the development of crystal structure is still restricted (not shown here). After annealing, the MR ratio increased slightly as the buffer layer thickness is increased to 10 nm, but decreased when it became 17 nm. When the buffer layer is too thick, it is considered that the surface coverage became poor for the ultrathin tunnel barrier. When the layer thicknesses are properly adjusted! 4/10 (in nm)}, the TMR ratio of up to 32% is achieved as shown in FIG. 4(d).

[27] FIG. 5 shows AES depth profile of the Mn in the GbNbZr-based MTJ with and without nano oxide layer(NOL). The position of the layer structures is qualitatively denoted on top of the figure. To prevent Mn interdiffusion toward the bottom GbFe pinned layer and tunnel barrier, thin NOL with a thickness of several A to several tens A is deposited on the buffer GbFe layer. Thin NOL with a thickness of several A to several tens A deposited on the GbNbZr under layer has a similar effect. This is because oxygen in the NOL provided high chemical potential for Mn diffusion. Thus, Mn from the antiferromagnet IrMn layer is interdiffused toward both buffer and pinned GbFe layer not the tunnel barrier. However, in the MTJs without NOL, Mn is interdiffused into pinned layer, tunnel barrier and the buffer layer as shown in FIG. 5. MTJs with NOL are thermally stable up to 350°C (TMR ratio are 20% and 8% at room temperature and 350°C, respectively).

[28] FIG. 6 shows V variation of various MTJ samples as a function of measured h temperature: (a) Ta-based MTJ, (b) GbNbZr-based MTJ, and (c) intentionally roughened GbNbZr-based MTJ. Has voltage dependence of the MR ratio in MTJs can be affected by several factors: metal particles, magnons, magnetic impurities, and so on [J. S. Moodera, and G. Mathon, J. Magn. Magn. Mater. 200, 248 (1999)]. In FIG. 6, it is attempted to uncover the temperature-dependent bias voltage V (defined as a h voltage where a MR ratio becomes half of its unbiased value) variation by surface roughness of the tunnel barrier. The GbNbZr-based MTJs exhibit better V charac- h teristics than the Ta-based one as depicted in Fig. 6. Moreover, V gradually increased h at higher temperatures in the GbNbZr-based MTJs whereas almost no change is observed for Ta-based one. To confirm the surface effect on bias voltage characteristics, it is prepared a sample where the GbNbZr underlayer alone is deposited intentionally at 10 mTorr. This particular sample shows a roughness of 0.18 nm when measured on the surface of the tunnel barrier. In the deposited state, the surface roughened GbNbZr-based MTJ shows 151 mV of V at 10 K, which gradually h increased up to 270 mV at 300 K. The MTJs with rougher surface exhibited lower and less rapid increase of V . h

[29] FIG. 7 shows the I-V curves of samples with different surface roughness values: (a)

GbNbZr-based MTJ, (b) intentionally roughened GbNbZr-based MTJ, and (c) Ta- based MTJ. They are either symmetric or asymmetric depending on the bias voltage direction, that is, forward (from top to bottom electrode) or reverse (from bottom to top electrode). As shown in FIGs. 7(a) and 7(b), the GbNbZr-based MTJ exhibits nearly symmetric I-V curves. However, the Ta-based MTJ shows an asymmetric curve. In rough junctions, there might be more unoxidized residual Al and/or magnetically dead zone in the valley and s rrnit portion of the bottom electrode, respectively (See, S. Zhang, P. M. Levy, A. C. Marley, and S. S. P. Parkin, Phys. Rev. Lett. 79, 3744). Thus, I-V curves became more asymmetric at higher voltage range for rough junctions. Considering forward biasing case, the breakdown voltages of the samples (a), (b), and (c) were 1.04 V, 0.92 V, and 0.87 V, respectively.

[30] Structure and operation according to the embodiment of the present invention as described above may be achieved. It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention. Thus, it is intended that the present invention covers the modifications and variations of this invention and they come within the scope of the appended claims and their equivalents.

[31] Magnetic tunnel junctions (MTJs) comprising amorphous GbNbZr underlayer have been investigated. Unlike Ta-based tunnel junctions, GbNbZr-based junctions do not possess crystalline structures. At elevated temperature (300°C), a short-time (10 min) annealing shows an increase in MR ratio and a decrease electrical resistance, respectively, for GbNbZr-based tunnel junctions. However, a longer annealing (30 min) deteriorated MR properties due to interlay er diffusion. By forming nano-oxide layer(NOL) on GbFe buffer layer or GbNbZr underlayer, the Mn's interdiffusion into tunneling barrier is restricted and this MTJs with NOL are thermally stable up to 350 °C. It can be confirmed that the bias voltage behavior is affected by the interfacial roughness. The use of an amorphous GbNbZr layer as an underlayer is effective in getting both thermal and bias voltage characteristics.

[32] The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this ap- plication, illustrate embodiments of the invention and together with the description serve to explain the principle of the invention.

[33] FIG. 1 shows a MR ratio, and the variations of resistance and bias voltage of MTJ in accordance with underlayer materials and annealing conditions.

[34] FIG. 2 is X-ray patterns of a GbNbZr-based MTJ and a Ta-based MTJ .

[35] FIG. 3 shows an Auger electron spectroscopy (AES) depth profile of a GbNbZr- based MTJ.

[36] FIG. 4 shows TMR ratio as a function temperature for various GbNbZr-based

MTJs.

[37] FIG. 5 shows an AES depth profile of the Mn in the GbNbZr-based MTJ with and without nano-oxide layers.

[38] FIG. 6 shows V variation of various MTJ samples as a function of measured h temperature: (a) Ta-based MTJ, (b) GbNbZr-based MTJ, and (c) intentionally roughened GbNbZr-based MTJ. [39] FIG. 7 shows the I-V curves of samples with different surface roughness values: (a)

GbNbZr-based MTJ, (b) intentionally roughened GbNbZr-based MTJ, and (c) Ta- based MTJ.

Claims

[1] A GbNbZr-based magnetic tunnel junction having ferromagnets formed on top and bottom of an insulating layer, comprising that a GbNbZr layer on which a nano-oxide layer with a thickness of several A to several tens A is formed is employed as an underlayer forming a part of bottom electrode of said magnetic tunnel junction.

[2] A GbNbZr-based magnetic tunnel junction as claimed in claim 1, wherein said

GbNbZr layer is Gb Nb Zr (in at. %) layer.

85 5 8 6 5

[3] A GbNbZr-based magnetic tunnel junction as claimed in claim 1, wherein said

GbNbZr-based magnetic tunnel junction is annealed at 300°Cfor 10 min after being formed.

[4] A GbNbZr-based magnetic tunnel junction as claimed in claim 1, wherein a

GbFe buffer layer is formed on said GbNbZr layer.

[5] A magnetic tunnel junction having ferromagnets formed on top and bottom of an insulating layer, comprising that a GbFe buffer layer on which a nano-oxide layer with a thickness of several A to several tens A is formed is formed on a GbNbZr underlayer forming a bottom electrode of said magnetic tunnel junction.

[6] A magnetic tunnel junction as claimed in claim 5, wherein the thicknesses of said GbNbZr underlayer and said GbFe buffer layer are 4 nm and 10 nm, respectively.

[7] A magnetic tunnel junction, comprising an insulating layer; ferromagnetic layers formed on top and bottom of said insulating layer; an IrMn layer formed as a pinning layer under said ferromagnetic layer; a GbFe layer formed as a buffer layer under said IrMn layer wherein said GbFe layer comprises an oxide layer with a thickness of several A to several tens A formed thereon; and a GbNbZr alloy layer formed under said GbFe layer as an underlayer of bottom ferromagnetic electrode.

[8] A magnetic tunnel junction as claimed in claim 7, wherein said insulating layer is Al O layer.

2 3

[9] A magnetic tunnel junction, comprising an insulating layer; ferromagnetic layers formed on top and bottom of said insulating layer; an IrMn layer formed as a pinning layer under said ferromagnetic layer; a GbFe layer formed as a buffer layer under said IrMn layer; and a GbNbZr alloy layer formed under said GbFe layer as an underlayer of bottom ferromagnetic electrode layer wherein said GbNbZr layer comprises an oxide layer with a thickness of several A to several tens A formed thereon. [10] A magnetic tunnel junction as claimed in claim 9, wherein said insulating layer is Al O layer.

2 3