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Radical nitridation of Al films for the barrier formation in ferromagnetic tunnel junctions
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 286 (2005) 162–166 www.elsevier.com/locate/jmmm
Radical nitridation of Al films for the barrier formation in ferromagnetic tunnel junctions Masakiyo Tsunodaa,, Toshihiro Shoyamaa, Satoru Yoshimuraa, Migaku Takahashia,b a
Department of Electronic Engineering, Tohoku University, Aobayama 05, Sendai 980-8579, Japan b New Industry Creation Hatchery Center, Tohoku University, Sendai 980-8579, Japan Available online 8 December 2004
Abstract Nitridation process of Al films using microwave-excited plasma was investigated to realize highly qualified Al–N barrier for magnetic tunnel junctions (MTJs), in comparison with reactive deposition process of Al–N films. The MTJs fabricated by the plasma nitridation method with Kr+N2 or Ar+N2 mixed gas showed larger values of tunnel magnetoresistance (TMR) ratio and resistance–area product (R A) than those of the MTJs fabricated with He+N2 plasma. The MTJs with reactive-deposited Al–N showed relatively small values of TMR ratio and R A. From the optical emission spectroscopy of the respective plasma, we concluded that N2 ion in the plasma is a responsible factor for lowering the barrier height of Al–N and that radical nitridation process is suitable to form the barriers without inducing defects. r 2004 Elsevier B.V. All rights reserved. PACS: 75.47.m; 73.40.Gk Keywords: Magnetic tunnel junction; Tunnel magnetoresistance; Nitridation; Al–N; Barrier formation
1. Introduction To reduce resistance–area product (R A) of magnetic tunnel junctions (MTJs) by maintaining large tunnel magnetoresistance (TMR) ratio is one of the most important technical issues to realize Corresponding
author. Tel.: +81 22 263 9402; fax: +81 22 263 9416. E-mail address: [email protected] (M. Tsunoda).
high-density and high-speed operating magnetic random access memories (MRAMs) [1]. AlN is a promising barrier material to provide low R A, because of its smaller band gap than that of normally used Al2O3 [2,3]. However, it has not been studied enough, because of the relatively low TMR ratio of MTJs using Al–N. The authors have demonstrated a large TMR ratio of 49%, in the MTJs with 1 nm-thick Al–N barrier fabricated by the microwave-excited plasma nitridation method
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.10.098
ARTICLE IN PRESS M. Tsunoda et al. / Journal of Magnetism and Magnetic Materials 286 (2005) 162–166
[4], while the conventional maximum of TMR ratio is 33% in MTJs having Al–N barrier [5]. This gives a technical breakthrough to utilize ultra-thin AlN films as tunnel barriers for the high-performance MRAMs. On the other hand, by using the same microwave-excited plasma source for oxidization of metal Al films, the authors have also succeeded in inducing relatively large TMR ratio of 59% in MTJs with Al–O barrier [6]. Thus the microwave-excited plasma oxidization/nitridation method seems to be a good fabrication process for the tunnel barriers of MTJs. Regarding the plasma oxidization process, the low electron temperature of the microwave-excited plasma and atomic oxygen radical, O(2p)1D, which is efficiently generated in Kr+O2 and He+O2 mixed gas plasma, are found to be responsible for the uniform oxidization of Al films and the resultant large TMR ratio [7]. In the present study, in order to clarify the physical factors that have induced large TMR ratio in MTJs having Al–N barrier, we investigated the plasma nitridation process of Al films in comparison with reactive deposition process of Al–N films, focusing on nitriding species in the respective plasma.
2. Experimental procedure MTJs with the structure of substrate/Ta 5 nm/ Cu 20 nm/Ta 5 nm/Ni–Fe 2 nm/Cu 5 nm/Mn75Ir25 10 nm/Co70Fe30 4 nm/Al–N/Co70Fe30 4 nm/Ni–Fe 20 nm/Ta 5 nm were prepared on thermally oxidized Si wafers, using a UHV-compatible cluster tool without breaking vacuum. All the metallic films were deposited by a DC magnetron sputtering method. Only the barrier formation process was changed in the following two different ways. (1) The plasma nitridation was performed by depositing a metal Al film with thickness, dAl ¼ 0.8–1.5 nm and subsequently nitriding it in the chamber having a microwave antenna [8]. He, Ar, and Kr were used as the inert gases mixed with N2 gas for the plasma nitridation, respectively. The operating pressure of the mixed gas and N2 content in it, defined as the mass flow ratio, F N2 =F total ; were 1 Torr and 5%, respectively. The applied microwave power density to discharge the
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mixed gas was 1.1 W/cm2. The nitriding duration was varied from 10 to 150 s, to obtain the best TMR performance for the respective dAl. (2) The reactive deposition was performed by adding N2 gas into inert Ar gas for sputtering of a metal Al target. The applied DC power to the Al target was 0.64 W/cm2. N2 content ðF N2 =F total Þ was varied from 20% to 100%. Stoichiometric nitrogen content of 50 at% was obtained in the deposited Al–N films when the F N2 =F total was greater than 38%, similar to the previous report [9]. The deposited Al–N layer thickness, dAl–N, was varied from 1.5 to 5.5 nm, by changing the deposition time. The junctions were patterned by photolithography and ion milling techniques in normal area of 25–2500 mm2. The TMR measurements were performed with a four-point probe method at room temperature with a bias voltage of 5 mV. The thermal treatment consisted of 60 min of consecutive vacuum annealing at each temperature, followed by furnace field cooling (1 kOe).
3. Results and discussion Fig. 1 shows the resistance–area product (R A) of as-prepared MTJs as a function of the barrier layer thickness (dAl or dAl–N). Data from the literatures for the MTJs fabricated with both plasma nitridation method [5,10] and reactive deposition method [9] are also plotted for comparison. Compared to the reactive deposition method, the MTJs fabricated with plasma nitridation show higher R A values under a certain barrier layer thickness. The R A monotonously increases with increasing dAl in both cases of Kr+N2 and Ar+N2 plasma nitridation method, and no significant differences are observed between them. However, in the case of He+N2, the R A shows a value smaller than the above two cases and remains almost constant at 104 O mm2 against the increasing dAl. This fact implies that the inert gas plays a significant role in the plasma nitridation process of Al films, while it is chemically inactive. On the other hand, in the cases of reactive sputtering process, the R A also increases against dAl–N. It, however, needs more barrier layer
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FN2/Ftotal = 40%
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80%
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2
3
4
5
He+N2
6
dAl, dAl-N (nm) Fig. 1. The resistance–area product of as-prepared MTJs as a function of barrier layer thickness. The Al–N barrier was formed by plasma nitridation method with Kr+N2 (filled circle), Ar+N2 (open circle), He+N2 (doubled circle), and reactive sputtering method with N2 mixing ratio, F N2 =F total ¼ 40% (open triangle), 80% (filled triangle). The data from literatures for plasma nitridation method (Ref. [5]: diamond, Ref. [10]: square) and reactive sputtering method (Ref. [9]: reversed triangle) are also shown as references.
thickness to obtain same R A value with the plasma-nitrided ones. This becomes more remarkable when the F N2 =F total of the reactive deposition process increases from 40% to 80%. Fig. 2 shows the annealing temperature dependence of TMR properties for the plasma-nitrided MTJs with dAl ¼ 1.5 nm. For comparison, the data for the reactive-deposited MTJ under F N2 =F total ¼ 80% with dAlN ¼ 1.9 nm are also shown. All the other reactive-deposited MTJs showed the TMR ratio of nearly zero. The barrier height and the barrier width were obtained by fitting the current–voltage (I– V) curves with Simmons’ formula [11]. The data plotted at 100 1C correspond to those for the as-prepared MTJs. The TMR ratio of the plasma-nitrided MTJ with Kr+N2 shows relatively high value in excess of 35% even at the as-prepared state and gradually increases with increasing the annealing temperature to reach 40% at 250 1C then decreases. The corresponding R A does not change significantly at this time. For the plasma-nitrided MTJ with Ar+N2, similar trends
105 104 103 102
Barrier height, φ (eV)
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Kr+N2
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R ×A (Ωµm2)
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TMR ratio (%)
109
(c)
1
0
(d) 100
200 300 Annealing temperature (°C)
Fig. 2. Annealing temperature dependence of (a) TMR ratio, (b) resistance–area product, (c) barrier height, and (d) barrier width for the MTJs with Al–N barrier fabricated by plasma nitridation method with Kr+N2 (filled circle), Ar+N2 (open circle), He+N2 (doubled circle), and reactive sputtering method with N2 mixing ratio, F N2 =F total ¼ 80% (filled triangle). The thickness of plasma-nitrided Al films is 1.5 nm, and that of sputter-deposited Al–N is 1.9 nm.
are observed in the TMR ratio and in the R A, except for their absolute values. On the other hand, the plasma-nitrided MTJ with He+N2 shows smaller values in the TMR ratio and in the R A, through the whole annealing procedure. In the case of the reactive-deposited MTJ, the TMR ratio shows only 5% and the R A remains small value of 103 O mm2 in spite of its larger Al–N layer thickness. Corresponding to these differences in the TMR ratio and the R A value,
ARTICLE IN PRESS M. Tsunoda et al. / Journal of Magnetism and Magnetic Materials 286 (2005) 162–166
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Intensity (a.u.)
Log intensity (a.u.)
Plasma ü nitridation Kr+N2 Ar+N2 He+N2
Reactive ü sputtering
400
500 600 Wavelength, λ (nm)
700
when the inert gas kind is changed from Kr or Ar to He. Contrary to this, the emission peak intensities from N2 radicals do not change significantly at the same time. It means that the nitriding species in the plasma were changed on changing the kind of inert gas used. In order to compare the amount of N2 ions among the respective microwave-excited plasma, we plotted the emission intensity from the ion normalized by that from the radical in Fig. 4 as functions of N2 content in the discharging gas. The data for the reactive sputtering plasma were also shown. The emission intensity from ion, Iion, is defined as the peak intensity around l ¼ 391:5 nm; and that from radical, Iradical, is defined as the averaged intensity of spectrum within the range of l ¼ 6472665 nm; i.e., Z I radical Dl ¼ IðlÞdl; (1) where I(l) and Dl are the emission intensity at l and the integral region width, respectively. The normalized ion intensity is larger in the cases of
Plasma nitridation Intensity ratio, Iion / Iradical
the barrier height significantly differs among these MTJs. The plasma-nitrided MTJ with He+N2 and the reactive-deposited MTJ show smaller barrier height than the MTJs nitrided with Kr+N2 or Ar+N2. Taking into account that defects and impurities in tunnel barriers form trap sites for tunneling electrons and lower the barrier height and resultant TMR ratio via a two-step tunneling mechanism [12,13], one may say that both the plasma nitridation process with He+N2 and the reactive deposition process tend to induce defects in the Al–N layers. In order to identify the physical factors in the plasmas responsible for the inducing defect in the barriers, optical emission spectroscopy was performed on both the process plasmas. Fig. 3 shows the optical emission spectra from the microwaveexcited Kr+N2, Ar+N2, and He+N2 plasmas and DC discharged Ar+N2 (reactive sputtering) plasma. Except for the arrow-marked peaks, which correspond to the optical transitions of the respective inert gas radical and atomic aluminum, all the emission peaks are assigned to those of nitrogen molecular radicals and nitrogen molecular ion. One can roughly state that the emission peaks observed in the wavelength range less and greater than l ¼ 500 nm corresponds with the transition of N2 radical, C2Pu-B3Pg and B3Pg-A3S+ u , respectively [14]. The emission observed at l ¼ 391:5 nm corresponds with the 2 + transition of N2 ion, B2S+ u -X Sg [14]. In cases of the microwave-excited plasma, the emission peak intensity from N2 ion drastically changes
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Reactive sputtering
101
He+N2
100
Ar+N2 Kr+N2 100 101 N2 mixing ratio, FN2/Ftotal (%)
102
390 392 394
Fig. 3. Optical emission spectrum from Kr+N2, Ar+N2, He+N2 microwave-excited plasma with 5% nitrogen, and reactive sputtering Ar+N2 plasma with 80% nitrogen.
Fig. 4. Intensity ratio of the emission from N2 ions to that from N2 radicals, as a function of the nitrogen content in the microwave-excited plasma with Kr+N2 (filled circle), Ar+N2 (open circle), He+N2 (doubled circle), and the reactive sputtering Ar+N2 plasma (filled triangle).
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He+N2 microwave-excited plasma and reactive deposition plasma with large N2 mixing ratio (410%) than the cases of Kr+N2 and Ar+N2. This is due to the difference of energy transfer mechanism in the plasma. In the reactive sputtering plasma, high-energy electrons, accelerated by the large potential gradient at a target sheath, easily discharge nitrogen molecules by collision. On the other hand, owing to the low electron temperature of the microwave-excited plasma [7], nitrogen molecules are not directly ionized by electrons. The energy transfer from inert gas radicals to nitrogen molecules might generate N2 ions. Taking into account the energy level of metastable state of the respective inert gas radical (9.9–10.6 eV for Kr, 11.6–11.7 eV for Ar, and 19.8–20.6 eV for He [15]) and ionizing energy of 2 + nitrogen molecules (18.7 eV; X1S+ g -B Su [14]), one can explain the larger amount of ions observed in He+N2 microwave-excited plasma than in Kr+N2 or Ar+N2 within the framework of the above energy transfer mechanism. Comparing the data in Fig. 4 with TMR properties in Fig. 2, we safely state that the N2 ion in the process plasma, which is accelerated to the substrate by the plasma potential, is one of the factors responsible for lowering the Al–N barrier height. Though it is not a proven fact, the highenergy ions bombard the tunnel barrier during its formation and might cause damage to it. In summary, we found the correlation between the nitriding species in the process plasma and deposited Al–N barrier properties. The barrier height of plasma-nitrided MTJs with Kr+N2 or Ar+N2 showed a relatively high value even in asprepared samples and did not change significantly through thermal annealing up to 300 1C. We conclude that the Kr+N2 or Ar+N2 plasma nitridation method with low electron temperature
is suitable to form highly qualified Al–N barriers in MTJs, because it generates less high-energy ions.
Acknowledgements This study was supported by the IT-program of Research Revolution 2002 (RR2002) ‘‘Development of Universal Low-Power Spin Memory’’ from the Ministry of Education, Culture, Sports, Science and Technology of Japan. References [1] K. Inomata, J. Magn. Soc. Jpn. 23 (1999) 1826. [2] J. Robertson, J. Vac. Sci. Technol. B 18 (3) (2000) 1785. [3] Z. Wang, H. Terai, A. Kakami, Y. Uzawa, Appl. Phys. Lett. 75 (1999) 701. [4] S. Yoshimura, T. Shoyama, T. Nozawa, M. Tsunoda, M. Takahashi, IEEE Trans. Magn. 40 (2004) in press. [5] J. Wang, S. Cardoso, P.P. Freitas, P. Wei, N.P. Barradas, J.C. Soares, J. Appl. Phys. 89 (2001) 6868. [6] M. Tsunoda, K. Nishikawa, S. Ogata, M. Takahashi, Appl. Phys. Lett. 80 (2002) 3135. [7] S. Yoshimura, M. Tsunoda, S. Ogata, M. Takahashi, J. Magn. Soc. Jpn. 27 (2003) 1130. [8] K. Nishikawa, M. Tsunoda, S. Ogata, M. Takahashi, IEEE Trans. Magn. 38 (2002) 2718. [9] M.M. Schwickert, J.R. Childress, R.E. Fontana, A.J. Kellock, P.M. Rice, M.K. Ho, T.J. Thompson, B.A. Gurney, J. Appl. Phys. 89 (2001) 6871. [10] M. Sharma, J.H. Nickel, T.C. Anthony, S.X. Wang, Appl. Phys. Lett. 77 (2000) 2219. [11] J.G. Simmons, J. Appl. Phys. 34 (1963) 1793. [12] E.L. Wolf, Principles of Electron Tunneling Spectroscopy, Oxford University Press, Oxford, 1985. [13] J. Zhang, R.M. White, J. Appl. Phys. 83 (1998) 6512. [14] R.W.B. Pearse, A.G. Gaydon, The Identification of Molecular Spectra, Chapman & Hall, London, 1976. [15] J.S. Chang, R.M. Hobson, Y. Ichikawa, T. Kaneda, Atomic and Molecular Process in Ionization Gases, Tokyo Denki University Press, Tokyo, 1982.
Journal of Magnetism and Magnetic Materials 286 (2005) 162–166 www.elsevier.com/locate/jmmm
Radical nitridation of Al films for the barrier formation in ferromagnetic tunnel junctions Masakiyo Tsunodaa,, Toshihiro Shoyamaa, Satoru Yoshimuraa, Migaku Takahashia,b a
Department of Electronic Engineering, Tohoku University, Aobayama 05, Sendai 980-8579, Japan b New Industry Creation Hatchery Center, Tohoku University, Sendai 980-8579, Japan Available online 8 December 2004
Abstract Nitridation process of Al films using microwave-excited plasma was investigated to realize highly qualified Al–N barrier for magnetic tunnel junctions (MTJs), in comparison with reactive deposition process of Al–N films. The MTJs fabricated by the plasma nitridation method with Kr+N2 or Ar+N2 mixed gas showed larger values of tunnel magnetoresistance (TMR) ratio and resistance–area product (R A) than those of the MTJs fabricated with He+N2 plasma. The MTJs with reactive-deposited Al–N showed relatively small values of TMR ratio and R A. From the optical emission spectroscopy of the respective plasma, we concluded that N2 ion in the plasma is a responsible factor for lowering the barrier height of Al–N and that radical nitridation process is suitable to form the barriers without inducing defects. r 2004 Elsevier B.V. All rights reserved. PACS: 75.47.m; 73.40.Gk Keywords: Magnetic tunnel junction; Tunnel magnetoresistance; Nitridation; Al–N; Barrier formation
1. Introduction To reduce resistance–area product (R A) of magnetic tunnel junctions (MTJs) by maintaining large tunnel magnetoresistance (TMR) ratio is one of the most important technical issues to realize Corresponding
author. Tel.: +81 22 263 9402; fax: +81 22 263 9416. E-mail address: [email protected] (M. Tsunoda).
high-density and high-speed operating magnetic random access memories (MRAMs) [1]. AlN is a promising barrier material to provide low R A, because of its smaller band gap than that of normally used Al2O3 [2,3]. However, it has not been studied enough, because of the relatively low TMR ratio of MTJs using Al–N. The authors have demonstrated a large TMR ratio of 49%, in the MTJs with 1 nm-thick Al–N barrier fabricated by the microwave-excited plasma nitridation method
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.10.098
ARTICLE IN PRESS M. Tsunoda et al. / Journal of Magnetism and Magnetic Materials 286 (2005) 162–166
[4], while the conventional maximum of TMR ratio is 33% in MTJs having Al–N barrier [5]. This gives a technical breakthrough to utilize ultra-thin AlN films as tunnel barriers for the high-performance MRAMs. On the other hand, by using the same microwave-excited plasma source for oxidization of metal Al films, the authors have also succeeded in inducing relatively large TMR ratio of 59% in MTJs with Al–O barrier [6]. Thus the microwave-excited plasma oxidization/nitridation method seems to be a good fabrication process for the tunnel barriers of MTJs. Regarding the plasma oxidization process, the low electron temperature of the microwave-excited plasma and atomic oxygen radical, O(2p)1D, which is efficiently generated in Kr+O2 and He+O2 mixed gas plasma, are found to be responsible for the uniform oxidization of Al films and the resultant large TMR ratio [7]. In the present study, in order to clarify the physical factors that have induced large TMR ratio in MTJs having Al–N barrier, we investigated the plasma nitridation process of Al films in comparison with reactive deposition process of Al–N films, focusing on nitriding species in the respective plasma.
2. Experimental procedure MTJs with the structure of substrate/Ta 5 nm/ Cu 20 nm/Ta 5 nm/Ni–Fe 2 nm/Cu 5 nm/Mn75Ir25 10 nm/Co70Fe30 4 nm/Al–N/Co70Fe30 4 nm/Ni–Fe 20 nm/Ta 5 nm were prepared on thermally oxidized Si wafers, using a UHV-compatible cluster tool without breaking vacuum. All the metallic films were deposited by a DC magnetron sputtering method. Only the barrier formation process was changed in the following two different ways. (1) The plasma nitridation was performed by depositing a metal Al film with thickness, dAl ¼ 0.8–1.5 nm and subsequently nitriding it in the chamber having a microwave antenna [8]. He, Ar, and Kr were used as the inert gases mixed with N2 gas for the plasma nitridation, respectively. The operating pressure of the mixed gas and N2 content in it, defined as the mass flow ratio, F N2 =F total ; were 1 Torr and 5%, respectively. The applied microwave power density to discharge the
163
mixed gas was 1.1 W/cm2. The nitriding duration was varied from 10 to 150 s, to obtain the best TMR performance for the respective dAl. (2) The reactive deposition was performed by adding N2 gas into inert Ar gas for sputtering of a metal Al target. The applied DC power to the Al target was 0.64 W/cm2. N2 content ðF N2 =F total Þ was varied from 20% to 100%. Stoichiometric nitrogen content of 50 at% was obtained in the deposited Al–N films when the F N2 =F total was greater than 38%, similar to the previous report [9]. The deposited Al–N layer thickness, dAl–N, was varied from 1.5 to 5.5 nm, by changing the deposition time. The junctions were patterned by photolithography and ion milling techniques in normal area of 25–2500 mm2. The TMR measurements were performed with a four-point probe method at room temperature with a bias voltage of 5 mV. The thermal treatment consisted of 60 min of consecutive vacuum annealing at each temperature, followed by furnace field cooling (1 kOe).
3. Results and discussion Fig. 1 shows the resistance–area product (R A) of as-prepared MTJs as a function of the barrier layer thickness (dAl or dAl–N). Data from the literatures for the MTJs fabricated with both plasma nitridation method [5,10] and reactive deposition method [9] are also plotted for comparison. Compared to the reactive deposition method, the MTJs fabricated with plasma nitridation show higher R A values under a certain barrier layer thickness. The R A monotonously increases with increasing dAl in both cases of Kr+N2 and Ar+N2 plasma nitridation method, and no significant differences are observed between them. However, in the case of He+N2, the R A shows a value smaller than the above two cases and remains almost constant at 104 O mm2 against the increasing dAl. This fact implies that the inert gas plays a significant role in the plasma nitridation process of Al films, while it is chemically inactive. On the other hand, in the cases of reactive sputtering process, the R A also increases against dAl–N. It, however, needs more barrier layer
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FN2/Ftotal = 40%
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2
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dAl, dAl-N (nm) Fig. 1. The resistance–area product of as-prepared MTJs as a function of barrier layer thickness. The Al–N barrier was formed by plasma nitridation method with Kr+N2 (filled circle), Ar+N2 (open circle), He+N2 (doubled circle), and reactive sputtering method with N2 mixing ratio, F N2 =F total ¼ 40% (open triangle), 80% (filled triangle). The data from literatures for plasma nitridation method (Ref. [5]: diamond, Ref. [10]: square) and reactive sputtering method (Ref. [9]: reversed triangle) are also shown as references.
thickness to obtain same R A value with the plasma-nitrided ones. This becomes more remarkable when the F N2 =F total of the reactive deposition process increases from 40% to 80%. Fig. 2 shows the annealing temperature dependence of TMR properties for the plasma-nitrided MTJs with dAl ¼ 1.5 nm. For comparison, the data for the reactive-deposited MTJ under F N2 =F total ¼ 80% with dAlN ¼ 1.9 nm are also shown. All the other reactive-deposited MTJs showed the TMR ratio of nearly zero. The barrier height and the barrier width were obtained by fitting the current–voltage (I– V) curves with Simmons’ formula [11]. The data plotted at 100 1C correspond to those for the as-prepared MTJs. The TMR ratio of the plasma-nitrided MTJ with Kr+N2 shows relatively high value in excess of 35% even at the as-prepared state and gradually increases with increasing the annealing temperature to reach 40% at 250 1C then decreases. The corresponding R A does not change significantly at this time. For the plasma-nitrided MTJ with Ar+N2, similar trends
105 104 103 102
Barrier height, φ (eV)
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Ar+N2
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Kr+N2
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104 103
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Barrier width, d (nm)
R ×A (Ωµm2)
107
TMR ratio (%)
109
(c)
1
0
(d) 100
200 300 Annealing temperature (°C)
Fig. 2. Annealing temperature dependence of (a) TMR ratio, (b) resistance–area product, (c) barrier height, and (d) barrier width for the MTJs with Al–N barrier fabricated by plasma nitridation method with Kr+N2 (filled circle), Ar+N2 (open circle), He+N2 (doubled circle), and reactive sputtering method with N2 mixing ratio, F N2 =F total ¼ 80% (filled triangle). The thickness of plasma-nitrided Al films is 1.5 nm, and that of sputter-deposited Al–N is 1.9 nm.
are observed in the TMR ratio and in the R A, except for their absolute values. On the other hand, the plasma-nitrided MTJ with He+N2 shows smaller values in the TMR ratio and in the R A, through the whole annealing procedure. In the case of the reactive-deposited MTJ, the TMR ratio shows only 5% and the R A remains small value of 103 O mm2 in spite of its larger Al–N layer thickness. Corresponding to these differences in the TMR ratio and the R A value,
ARTICLE IN PRESS M. Tsunoda et al. / Journal of Magnetism and Magnetic Materials 286 (2005) 162–166
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Intensity (a.u.)
Log intensity (a.u.)
Plasma ü nitridation Kr+N2 Ar+N2 He+N2
Reactive ü sputtering
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500 600 Wavelength, λ (nm)
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when the inert gas kind is changed from Kr or Ar to He. Contrary to this, the emission peak intensities from N2 radicals do not change significantly at the same time. It means that the nitriding species in the plasma were changed on changing the kind of inert gas used. In order to compare the amount of N2 ions among the respective microwave-excited plasma, we plotted the emission intensity from the ion normalized by that from the radical in Fig. 4 as functions of N2 content in the discharging gas. The data for the reactive sputtering plasma were also shown. The emission intensity from ion, Iion, is defined as the peak intensity around l ¼ 391:5 nm; and that from radical, Iradical, is defined as the averaged intensity of spectrum within the range of l ¼ 6472665 nm; i.e., Z I radical Dl ¼ IðlÞdl; (1) where I(l) and Dl are the emission intensity at l and the integral region width, respectively. The normalized ion intensity is larger in the cases of
Plasma nitridation Intensity ratio, Iion / Iradical
the barrier height significantly differs among these MTJs. The plasma-nitrided MTJ with He+N2 and the reactive-deposited MTJ show smaller barrier height than the MTJs nitrided with Kr+N2 or Ar+N2. Taking into account that defects and impurities in tunnel barriers form trap sites for tunneling electrons and lower the barrier height and resultant TMR ratio via a two-step tunneling mechanism [12,13], one may say that both the plasma nitridation process with He+N2 and the reactive deposition process tend to induce defects in the Al–N layers. In order to identify the physical factors in the plasmas responsible for the inducing defect in the barriers, optical emission spectroscopy was performed on both the process plasmas. Fig. 3 shows the optical emission spectra from the microwaveexcited Kr+N2, Ar+N2, and He+N2 plasmas and DC discharged Ar+N2 (reactive sputtering) plasma. Except for the arrow-marked peaks, which correspond to the optical transitions of the respective inert gas radical and atomic aluminum, all the emission peaks are assigned to those of nitrogen molecular radicals and nitrogen molecular ion. One can roughly state that the emission peaks observed in the wavelength range less and greater than l ¼ 500 nm corresponds with the transition of N2 radical, C2Pu-B3Pg and B3Pg-A3S+ u , respectively [14]. The emission observed at l ¼ 391:5 nm corresponds with the 2 + transition of N2 ion, B2S+ u -X Sg [14]. In cases of the microwave-excited plasma, the emission peak intensity from N2 ion drastically changes
165
Reactive sputtering
101
He+N2
100
Ar+N2 Kr+N2 100 101 N2 mixing ratio, FN2/Ftotal (%)
102
390 392 394
Fig. 3. Optical emission spectrum from Kr+N2, Ar+N2, He+N2 microwave-excited plasma with 5% nitrogen, and reactive sputtering Ar+N2 plasma with 80% nitrogen.
Fig. 4. Intensity ratio of the emission from N2 ions to that from N2 radicals, as a function of the nitrogen content in the microwave-excited plasma with Kr+N2 (filled circle), Ar+N2 (open circle), He+N2 (doubled circle), and the reactive sputtering Ar+N2 plasma (filled triangle).
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He+N2 microwave-excited plasma and reactive deposition plasma with large N2 mixing ratio (410%) than the cases of Kr+N2 and Ar+N2. This is due to the difference of energy transfer mechanism in the plasma. In the reactive sputtering plasma, high-energy electrons, accelerated by the large potential gradient at a target sheath, easily discharge nitrogen molecules by collision. On the other hand, owing to the low electron temperature of the microwave-excited plasma [7], nitrogen molecules are not directly ionized by electrons. The energy transfer from inert gas radicals to nitrogen molecules might generate N2 ions. Taking into account the energy level of metastable state of the respective inert gas radical (9.9–10.6 eV for Kr, 11.6–11.7 eV for Ar, and 19.8–20.6 eV for He [15]) and ionizing energy of 2 + nitrogen molecules (18.7 eV; X1S+ g -B Su [14]), one can explain the larger amount of ions observed in He+N2 microwave-excited plasma than in Kr+N2 or Ar+N2 within the framework of the above energy transfer mechanism. Comparing the data in Fig. 4 with TMR properties in Fig. 2, we safely state that the N2 ion in the process plasma, which is accelerated to the substrate by the plasma potential, is one of the factors responsible for lowering the Al–N barrier height. Though it is not a proven fact, the highenergy ions bombard the tunnel barrier during its formation and might cause damage to it. In summary, we found the correlation between the nitriding species in the process plasma and deposited Al–N barrier properties. The barrier height of plasma-nitrided MTJs with Kr+N2 or Ar+N2 showed a relatively high value even in asprepared samples and did not change significantly through thermal annealing up to 300 1C. We conclude that the Kr+N2 or Ar+N2 plasma nitridation method with low electron temperature
is suitable to form highly qualified Al–N barriers in MTJs, because it generates less high-energy ions.
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