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Controlled fabrication of nano-scale double barrier magnetic tunnel junctions using focused ion beam milling method
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 303 (2006) e208–e211 www.elsevier.com/locate/jmmm
Controlled fabrication of nano-scale double barrier magnetic tunnel junctions using focused ion beam milling method H.X. Wei, T.X. Wang, Z.M. Zeng, X.Q. Zhang, J. Zhao, X.F. Han State Key Laboratory of Magnetism, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, PR China Available online 28 February 2006
Abstract The controlled fabrication method for nano-scale double barrier magnetic tunnel junctions (DBMTJs) with the layer structure of Ta(5)/Cu(10)/Ni79Fe21(5)/Ir22Mn78(12)/Co60Fe20B20(4)/Al(1)–oxide/Co60Fe20B20(6)/Al(1)–oxide/Co60Fe20B20(4)/Ir22Mn78(12)/Ni79Fe21 (5)/Ta(5) (thickness unit: nm) was used. This method involved depositing thin multi-layer stacks by sputtering system, and depositing a Pt nano-pillar using a focused ion beam which acted both as a top contact and as an etching mask. The advantages of this process over the traditional process using e-beam and optical lithography in that it involve only few processing steps, e.g. it does not involve any liftoff steps. In order to evaluate the nanofabrication techniques, the DBMTJs with the dimensions of 200 nm 400 nm, 200 nm 200 nm nano-scale were prepared and their R–H, I–V characteristics were measured. r 2006 Elsevier B.V. All rights reserved. PACS: 73.40.Gk; 75.60.d; 75.70.Ak; 75.70.i Keywords: Nano-scale; Double barrier magnetic tunnel junction (DNMTJ); Focused ion beam etching; Tunneling magnetoresistance (TMR)
1. Introduction Magnetic tunnel junctions [1–2] (MTJs) have attracted considerable attention in the last 10 years owing to their potential in commercial applications such as magnetic random access memory [3–4] (MRAM), magnetic read head in hard driving disk (HDD) and highly sensitive magnetic sensors [5–6]. During the last 2 years, much more advances focused on MTJ materials to increase the tunnel magnetoresistance (TMR) ratio have been enhanced [7–8]. A requirement for high-density MRAM is higher TMR and smaller junction size. On the nanometer scale, the uniformity of the resistance, switching field, and TMR are increasingly affected by the junction size, shape, and other geometrical factors [9]. The standard fabrication process for MTJ involves both e-beam and optical lithography. The top and bottom junction electrodes are defined using contact Corresponding author. Tel.: +86 10 8264 9268; fax: +86 10 8264 9485.
E-mail address: [email protected] (X.F. Han). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.01.072
optical lithography whereas the junction area using e-beam patterning. For large scale production of relatively large devices this process has many advantages. However, on the nanometer scale, difficulties are encountered with the lift-off step of both the insulator and the removing of the resist. In this work, the controlled fabrication method for nano-scale double barrier magnetic tunnel junctions (DBMTJs) is reported. The DBMTJs is preferable spin electronic device due to a higher V1/2 value [10–11] and also suitable for investigating the spin-polarized electron coherent tunneling [12]. The MTJ film with double barriers was deposited on Si/SiO2 wafer using magnetron sputtering system. A platinum nanoscale pillar was deposited by focused ion beam (FIB) on the metal stack to act as a patterning mask. UV lithography with Ar-ion etching was used to pattern the top and bottom electrodes of the DBMTJs. The magnetic transport properties of the nano-scale DBMTJs were measured at room temperature (RT) and 4.2 K with Physical Properties Measurement System (PPMS) using the DC four-probe method.
ARTICLE IN PRESS H.X. Wei et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e208–e211
2. Experimental methods The DBMTJ films with the layer structures of Ta(5)/ Cu(10)/Ni79Fe21(5)/Ir22Mn78(12) /Co60Fe20B20(4)/ Al(1)– oxide/Co60Fe20B20(6) /Al(1)–oxide /Co60Fe20B20(4) /Ir22 Mn78 (12)/Ni79Fe21(5)/Ta(5) (thickness unit: nm) was first deposited on Si(100)/SiO2 substrate using an ULVAC TMR R&D magnetron sputtering system (MPS-4000-HC7). All the deposition processes were done at a base pressure of below 1 106 Pa and an Ar plasma sputtering pressure of 0.07 Pa without breaking vacuum at any point. An in-plane magnetic field of about 100 Oe was applied to define the uniaxial magnetic anisotropy of the magnetic layers. The Al-oxide barrier was formed by oxidizing a 1 nm Al-layer by inductively coupled plasma (ICP) with an oxidation time of 70 s in a mixture of oxygen and argon at a pressure of 1.0 Pa in a separate chamber. The bottom electrode was defined by contact optical lithography combined with Ar ion-beam etching techniques, as shown in Fig. 1(a). Using the FIB system (FEI Dual Beam 235 system), a 200 nm 200 nm platinum pillar was deposited in the middle of the bottom junction electrode, as shown in Fig. 1(b). For the deposition, a beam current of 10 pA, a dwell time of 30 ms, a pixel spacing of 5 nm and an emission current of 2.2 mA were used. The ion-beam-assisted Pt deposition will also implant Ga into magnetic films which has been shown to affect the magnetic properties, changing their grain size and increasing their coercivity [13] etc. There are two ways to solve this problem, one is to deposit a protective layer on the top of MTJ film stack; another way is to deposit a Pt mask using e-beam mode. Fig. 2 shows a secondary electron image of a Pt pillar (200 nm 200 nm) observed by scanning electron microscope (SEM). The metal stack was then etched using Ar ion milling until the Al-O layer has been etched completely, as shown in Fig. 1(c).Then the 100 nm SiO2 and 20 nm Cu (or Au on Fig) were deposited using magnetron sputtering as an insulating layer and the top electrode respectively, as shown in Fig. 1(d).
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In order to make contact with the DBMTJ a small hole was made, so that connection could be made though the Pt Pillar. On the other hand, the hole was successfully milled both using FIB milling and reactive ion etching (RIE) method. However, the later method was found more suitable to make the extremely small size of the contacting hole as shown in Fig. 1(e). The final step was to deposit 100 nm Cu and 10 nm Au on the top of the junction as the top electrode, as shown in Fig. 1(f). The advantages of this process over the traditional process using e-beam and optical lithography in that it involves only few processing steps, e.g. it does not involve any lift-off steps.
Fig. 2. SEM image of a 200 nm 200 nm DBMTJ topography.
Fig. 1. The procedure of nano-scale DBMTJ’s fabrication.
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H.X. Wei et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e208–e211
3. Results and discussion Fig. 3 shows the TMR curve measured at RT for a 200 nm 200 nm DBMTJ. The TMR ratio was up to 27% in the as-deposited state. TMR ratio is defined as TMR ¼ DR/Rp ¼ (RAPRp)/Rp, where RAP and Rp denote the tunnel resistance when the magnetizations of the free layer vs. two pinned magnetic electrodes are aligned antiparallel (AP) and parallel (P) configurations, respectively. The resistance-area product (RA) was 250 O mm2. After annealing at 275 1C for an hour, the TMR ratio increased to 51%. This value agrees well with those fabricated by a traditional process [14]. The increase of the TMR ratio after annealing may be attributed to an improvement of the smoothness of the interface between the ferromagnetic/ insulator (FM/I) layers and barrier homogenization as the result of the reduction in the defect density in the Al-oxide barrier. These characteristics are suitable for MRAM, magnetic read heads and other high-sensitivity field sensors. The coercivity of the DBMTJ (200 nm 200 nm) is 150 Oe, which is almost ten times that of a micro junction (8 mm 8 mm), whose TMR curve is shown in Fig. 4. This
Fig. 5. The bias voltage dependence of the normalized TMR ratio for DBMTJ.
increase may come from the shape anisotropy because with the reduction in dimension, the interaction between the magnetic films, and its demagnetized field increased significantly. Thus it becomes more difficult for the magnetization reversal of the free layer. Such coercivity is too high for the requirements in device applications, such as MRAM, etc. Fig. 5 shows the bias voltage dependence of the normalized TMR ratio for DBMTJ structure, which are slightly asymmetric with respect to zero bias voltage, measured at RT for the same junction as that shown in Fig. 3. The same results were also observed by other researchers [10,14], which is likely due to subtle differences in the upper and lower metal/oxide interfaces corresponding at each AlOx barrier. It can be clearly seen that the voltage value for which the TMR decreases to half of its maximum value measured at low bias V1/2 is about 1.06 V, which is almost two times that of the single barrier MTJs. 4. Conclusions
Fig. 3. TMR vs. field curves measured at RT for a 200 nm 200 nm DBMTJ.
A method for the controlled fabrication of nano-scale DBMTJs has been outlined. The method presented requires few processing steps as compared to the standard fabrication process. Such nano-scale DBMTJs show good characteristic properties with TMR ratio, resistance-area product and switching field. Furthermore, the values of the switching field of free layer in such nano-scale DBMTJs can be modulated further by changing the free layer’s material, structure, shape or thickness etc. for device applications. Acknowledgements
Fig. 4. TMR vs. field curves measured at RT for a 8 mm 8 mm DBMTJ.
The project was supported by the State Key Project of Fundamental Research of Ministry of Science and Technology Grant no. 2001CB610601 and the Knowledge Innovation Program project of Chinese Academy of Science in 2002. X.F. Han gratefully thanks the partial
ARTICLE IN PRESS H.X. Wei et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e208–e211
support of Distinct Young Researcher Foundation (50325104) and Chinese National Natural Science Foundation (10274103, 50271081). References [1] T. Miyazaki, N. Tezuka, J. Magn. Magn. Mater. 139 (1995) L231. [2] J.S. Moodera, L.R. Kinder, T.M. Wong, R. Meservery, Phys. Rev. Lett. 74 (1995) 3273. [3] S.S.P. Parkin, K.P. Roche, M.G. Samant, P.M. Rice, R.B. Beyers, R.E. Scheuerlein, E.J. O’Sullivan, S.L. Brwon, J. Bucchigano, D.W. Abraham, Yu. Lu, M. Rooks, P.L. Trouilloud, R.A. Wanner, W.J. Gallagher, J. Appl. Phys. 85 (1999) 5828. [4] S. Tehrani, B. Engel, J.M. Slaughter, E. Chen, M. DeHerrera, M. Derlam, P. Naji, R. Whig, J. Janesky, J. Calder, IEEE Trans. Magn. 36 (2000) 2752. [5] S. Araki, K. Sato, T. Kagami, S. Sakurai, T. Uesugi, N. Kasahara, T. Kuwashima, N. Ohta, J. Sun, K. Nagai, S. Li, N. Hachisuka, H.
[6] [7] [8] [9] [10] [11] [12] [13] [14]
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Hatae, T. Kagotani, N. Takahashi, K. Ueda, M. Matsuzaki, IEEE Trans. Magn. 38 (2002) 72. E. Nakashio, J. Sugawara, S. Onoe, S. Kumagai, J. Appl. Phys. 89 (2001) 7356. S.S.P. Parkin, C. Kaiser, A. Panchula, Nat. Mater. 3 (2004) 862. S. Yuasa, T. Nagahama, A. Fukushima, Y. Suzuki, K. Ando, Nat. Mater. 3 (2004) 868. W.J. Gallagher, S.S. Parkin, X.P. Bian, A. Marley, T.M. Shaw, G. Xiao, J. Appl. Phys. 81 (1997) 3741. S. Colis, G. Gieres, L. Bar, J. Wecker, Appl. Phys. Lett. 83 (2003) 948. M.-S. Song, B.S. Chun, Y. Kim, I. Hwang, T. Kim, J. Appl. Phys. 97 (2005) 10C917. S. Stein, R. Schmitz, H. Kohlstedt, Solid State Comm. 11 (2001) 599. D. Ozkaya, R.M. Langford, L. Wai, A.K. Petford-Long, J. Appl. Phys. 91 (2002) 9937. Z.M. Zeng, H.X. Wei, L.X. Jiang, G.X. Du, W.S. Zhan, X.F. Han, J. Magn. Magn. Mater, unpublished.
Journal of Magnetism and Magnetic Materials 303 (2006) e208–e211 www.elsevier.com/locate/jmmm
Controlled fabrication of nano-scale double barrier magnetic tunnel junctions using focused ion beam milling method H.X. Wei, T.X. Wang, Z.M. Zeng, X.Q. Zhang, J. Zhao, X.F. Han State Key Laboratory of Magnetism, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, PR China Available online 28 February 2006
Abstract The controlled fabrication method for nano-scale double barrier magnetic tunnel junctions (DBMTJs) with the layer structure of Ta(5)/Cu(10)/Ni79Fe21(5)/Ir22Mn78(12)/Co60Fe20B20(4)/Al(1)–oxide/Co60Fe20B20(6)/Al(1)–oxide/Co60Fe20B20(4)/Ir22Mn78(12)/Ni79Fe21 (5)/Ta(5) (thickness unit: nm) was used. This method involved depositing thin multi-layer stacks by sputtering system, and depositing a Pt nano-pillar using a focused ion beam which acted both as a top contact and as an etching mask. The advantages of this process over the traditional process using e-beam and optical lithography in that it involve only few processing steps, e.g. it does not involve any liftoff steps. In order to evaluate the nanofabrication techniques, the DBMTJs with the dimensions of 200 nm 400 nm, 200 nm 200 nm nano-scale were prepared and their R–H, I–V characteristics were measured. r 2006 Elsevier B.V. All rights reserved. PACS: 73.40.Gk; 75.60.d; 75.70.Ak; 75.70.i Keywords: Nano-scale; Double barrier magnetic tunnel junction (DNMTJ); Focused ion beam etching; Tunneling magnetoresistance (TMR)
1. Introduction Magnetic tunnel junctions [1–2] (MTJs) have attracted considerable attention in the last 10 years owing to their potential in commercial applications such as magnetic random access memory [3–4] (MRAM), magnetic read head in hard driving disk (HDD) and highly sensitive magnetic sensors [5–6]. During the last 2 years, much more advances focused on MTJ materials to increase the tunnel magnetoresistance (TMR) ratio have been enhanced [7–8]. A requirement for high-density MRAM is higher TMR and smaller junction size. On the nanometer scale, the uniformity of the resistance, switching field, and TMR are increasingly affected by the junction size, shape, and other geometrical factors [9]. The standard fabrication process for MTJ involves both e-beam and optical lithography. The top and bottom junction electrodes are defined using contact Corresponding author. Tel.: +86 10 8264 9268; fax: +86 10 8264 9485.
E-mail address: [email protected] (X.F. Han). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.01.072
optical lithography whereas the junction area using e-beam patterning. For large scale production of relatively large devices this process has many advantages. However, on the nanometer scale, difficulties are encountered with the lift-off step of both the insulator and the removing of the resist. In this work, the controlled fabrication method for nano-scale double barrier magnetic tunnel junctions (DBMTJs) is reported. The DBMTJs is preferable spin electronic device due to a higher V1/2 value [10–11] and also suitable for investigating the spin-polarized electron coherent tunneling [12]. The MTJ film with double barriers was deposited on Si/SiO2 wafer using magnetron sputtering system. A platinum nanoscale pillar was deposited by focused ion beam (FIB) on the metal stack to act as a patterning mask. UV lithography with Ar-ion etching was used to pattern the top and bottom electrodes of the DBMTJs. The magnetic transport properties of the nano-scale DBMTJs were measured at room temperature (RT) and 4.2 K with Physical Properties Measurement System (PPMS) using the DC four-probe method.
ARTICLE IN PRESS H.X. Wei et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e208–e211
2. Experimental methods The DBMTJ films with the layer structures of Ta(5)/ Cu(10)/Ni79Fe21(5)/Ir22Mn78(12) /Co60Fe20B20(4)/ Al(1)– oxide/Co60Fe20B20(6) /Al(1)–oxide /Co60Fe20B20(4) /Ir22 Mn78 (12)/Ni79Fe21(5)/Ta(5) (thickness unit: nm) was first deposited on Si(100)/SiO2 substrate using an ULVAC TMR R&D magnetron sputtering system (MPS-4000-HC7). All the deposition processes were done at a base pressure of below 1 106 Pa and an Ar plasma sputtering pressure of 0.07 Pa without breaking vacuum at any point. An in-plane magnetic field of about 100 Oe was applied to define the uniaxial magnetic anisotropy of the magnetic layers. The Al-oxide barrier was formed by oxidizing a 1 nm Al-layer by inductively coupled plasma (ICP) with an oxidation time of 70 s in a mixture of oxygen and argon at a pressure of 1.0 Pa in a separate chamber. The bottom electrode was defined by contact optical lithography combined with Ar ion-beam etching techniques, as shown in Fig. 1(a). Using the FIB system (FEI Dual Beam 235 system), a 200 nm 200 nm platinum pillar was deposited in the middle of the bottom junction electrode, as shown in Fig. 1(b). For the deposition, a beam current of 10 pA, a dwell time of 30 ms, a pixel spacing of 5 nm and an emission current of 2.2 mA were used. The ion-beam-assisted Pt deposition will also implant Ga into magnetic films which has been shown to affect the magnetic properties, changing their grain size and increasing their coercivity [13] etc. There are two ways to solve this problem, one is to deposit a protective layer on the top of MTJ film stack; another way is to deposit a Pt mask using e-beam mode. Fig. 2 shows a secondary electron image of a Pt pillar (200 nm 200 nm) observed by scanning electron microscope (SEM). The metal stack was then etched using Ar ion milling until the Al-O layer has been etched completely, as shown in Fig. 1(c).Then the 100 nm SiO2 and 20 nm Cu (or Au on Fig) were deposited using magnetron sputtering as an insulating layer and the top electrode respectively, as shown in Fig. 1(d).
e209
In order to make contact with the DBMTJ a small hole was made, so that connection could be made though the Pt Pillar. On the other hand, the hole was successfully milled both using FIB milling and reactive ion etching (RIE) method. However, the later method was found more suitable to make the extremely small size of the contacting hole as shown in Fig. 1(e). The final step was to deposit 100 nm Cu and 10 nm Au on the top of the junction as the top electrode, as shown in Fig. 1(f). The advantages of this process over the traditional process using e-beam and optical lithography in that it involves only few processing steps, e.g. it does not involve any lift-off steps.
Fig. 2. SEM image of a 200 nm 200 nm DBMTJ topography.
Fig. 1. The procedure of nano-scale DBMTJ’s fabrication.
ARTICLE IN PRESS e210
H.X. Wei et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e208–e211
3. Results and discussion Fig. 3 shows the TMR curve measured at RT for a 200 nm 200 nm DBMTJ. The TMR ratio was up to 27% in the as-deposited state. TMR ratio is defined as TMR ¼ DR/Rp ¼ (RAPRp)/Rp, where RAP and Rp denote the tunnel resistance when the magnetizations of the free layer vs. two pinned magnetic electrodes are aligned antiparallel (AP) and parallel (P) configurations, respectively. The resistance-area product (RA) was 250 O mm2. After annealing at 275 1C for an hour, the TMR ratio increased to 51%. This value agrees well with those fabricated by a traditional process [14]. The increase of the TMR ratio after annealing may be attributed to an improvement of the smoothness of the interface between the ferromagnetic/ insulator (FM/I) layers and barrier homogenization as the result of the reduction in the defect density in the Al-oxide barrier. These characteristics are suitable for MRAM, magnetic read heads and other high-sensitivity field sensors. The coercivity of the DBMTJ (200 nm 200 nm) is 150 Oe, which is almost ten times that of a micro junction (8 mm 8 mm), whose TMR curve is shown in Fig. 4. This
Fig. 5. The bias voltage dependence of the normalized TMR ratio for DBMTJ.
increase may come from the shape anisotropy because with the reduction in dimension, the interaction between the magnetic films, and its demagnetized field increased significantly. Thus it becomes more difficult for the magnetization reversal of the free layer. Such coercivity is too high for the requirements in device applications, such as MRAM, etc. Fig. 5 shows the bias voltage dependence of the normalized TMR ratio for DBMTJ structure, which are slightly asymmetric with respect to zero bias voltage, measured at RT for the same junction as that shown in Fig. 3. The same results were also observed by other researchers [10,14], which is likely due to subtle differences in the upper and lower metal/oxide interfaces corresponding at each AlOx barrier. It can be clearly seen that the voltage value for which the TMR decreases to half of its maximum value measured at low bias V1/2 is about 1.06 V, which is almost two times that of the single barrier MTJs. 4. Conclusions
Fig. 3. TMR vs. field curves measured at RT for a 200 nm 200 nm DBMTJ.
A method for the controlled fabrication of nano-scale DBMTJs has been outlined. The method presented requires few processing steps as compared to the standard fabrication process. Such nano-scale DBMTJs show good characteristic properties with TMR ratio, resistance-area product and switching field. Furthermore, the values of the switching field of free layer in such nano-scale DBMTJs can be modulated further by changing the free layer’s material, structure, shape or thickness etc. for device applications. Acknowledgements
Fig. 4. TMR vs. field curves measured at RT for a 8 mm 8 mm DBMTJ.
The project was supported by the State Key Project of Fundamental Research of Ministry of Science and Technology Grant no. 2001CB610601 and the Knowledge Innovation Program project of Chinese Academy of Science in 2002. X.F. Han gratefully thanks the partial
ARTICLE IN PRESS H.X. Wei et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e208–e211
support of Distinct Young Researcher Foundation (50325104) and Chinese National Natural Science Foundation (10274103, 50271081). References [1] T. Miyazaki, N. Tezuka, J. Magn. Magn. Mater. 139 (1995) L231. [2] J.S. Moodera, L.R. Kinder, T.M. Wong, R. Meservery, Phys. Rev. Lett. 74 (1995) 3273. [3] S.S.P. Parkin, K.P. Roche, M.G. Samant, P.M. Rice, R.B. Beyers, R.E. Scheuerlein, E.J. O’Sullivan, S.L. Brwon, J. Bucchigano, D.W. Abraham, Yu. Lu, M. Rooks, P.L. Trouilloud, R.A. Wanner, W.J. Gallagher, J. Appl. Phys. 85 (1999) 5828. [4] S. Tehrani, B. Engel, J.M. Slaughter, E. Chen, M. DeHerrera, M. Derlam, P. Naji, R. Whig, J. Janesky, J. Calder, IEEE Trans. Magn. 36 (2000) 2752. [5] S. Araki, K. Sato, T. Kagami, S. Sakurai, T. Uesugi, N. Kasahara, T. Kuwashima, N. Ohta, J. Sun, K. Nagai, S. Li, N. Hachisuka, H.
[6] [7] [8] [9] [10] [11] [12] [13] [14]
e211
Hatae, T. Kagotani, N. Takahashi, K. Ueda, M. Matsuzaki, IEEE Trans. Magn. 38 (2002) 72. E. Nakashio, J. Sugawara, S. Onoe, S. Kumagai, J. Appl. Phys. 89 (2001) 7356. S.S.P. Parkin, C. Kaiser, A. Panchula, Nat. Mater. 3 (2004) 862. S. Yuasa, T. Nagahama, A. Fukushima, Y. Suzuki, K. Ando, Nat. Mater. 3 (2004) 868. W.J. Gallagher, S.S. Parkin, X.P. Bian, A. Marley, T.M. Shaw, G. Xiao, J. Appl. Phys. 81 (1997) 3741. S. Colis, G. Gieres, L. Bar, J. Wecker, Appl. Phys. Lett. 83 (2003) 948. M.-S. Song, B.S. Chun, Y. Kim, I. Hwang, T. Kim, J. Appl. Phys. 97 (2005) 10C917. S. Stein, R. Schmitz, H. Kohlstedt, Solid State Comm. 11 (2001) 599. D. Ozkaya, R.M. Langford, L. Wai, A.K. Petford-Long, J. Appl. Phys. 91 (2002) 9937. Z.M. Zeng, H.X. Wei, L.X. Jiang, G.X. Du, W.S. Zhan, X.F. Han, J. Magn. Magn. Mater, unpublished.