NHK Laboratories Note No. 489


Fabrication of Magnetic Tunnel Junction Pillars with
Sub-Micron-Dimension by Dual Ion Beam Sputtering and
Electron Beam Lithography


Yasuyoshi MIYAMOTO, Hirotaka SHIINO and Kiyoshi KUGA
Materials Science

Abstract
We have started the research on ‘spin-filter’devices applicable for future storage or switching devices. Using these devices, we will be able to utilize not only electron charges but electron spin states. The adoption of half-metallic materials, such as Fe3O4, as the ferromagnetic layers in magnetic tunneling junctions is an attractive approach for realizing ‘spin-filter’ devices with large spin polarization.
[Fe3O4/ Ru] films were deposited using a dual ion beam sputtering method at room temperature. Highly crystalline [Fe3O4/ Ru] films could be obtained by tuning the Kr ion beam energy to exactly 160 eV during the film deposition. We have succeeded in fabricating 100-nm-dimension [Co-Fe/Al-O/Fe3O4] pillar structures using electron beam lithography and ion beam milling processes. Barrier heights for these pillars of approximately 2.6 eV are attained when the pillar size is larger than 120 nm.
1.  INTRODUCTION

Recently, the amount of information storage becomes extremely large day by day, due to the innovative change of information environment; i.e. large numbers of personal computers are directly connected to the Internet via Asymmetric Digital Subscriber Line (ADSL) and optical fibers, while the satellite and terrestrial digital broadcasting with high definition and multi-media services are started in Japan. In order to overcome this storage issue, many researchers are studying about spin-electronics field, which attempts to use and manipulate the state of electron spin. For example, recent developments, which utilized the spin-electronics, are the giant magnetoresistive (GMR) heads for hard disk drives [1]] and the magnetic random access memories (MRAM) with non-volatility [2]. We have started the research on ‘spin-filter’ devices, since it is necessary to distinguish up-spin electrons from down-spin ones if the spin-electronic technologies will be applied for future storage or switching devices.

The adoption of half-metallic materials, such as magnetite (Fe3O4), as the ferromagnetic layer in magnetic tunneling junctions (MTJ) is an attractive approach for achieving large magnetoresistance (MR) ratios [3] and for realizing ‘spin-filter’ devices, due to their high spin polarization of up to 100 % in the ideal case [4]. However, high substrate temperatures of approximately 300 °C are required to deposit stoichiometric Fe3O4films by conventional methods. In order to reduce the intermixing of atoms at the interfaces of each layer in MTJ, an effective deposition method at room temperature has been established in this study using a dual ion beam sputtering apparatus.

It is also important to reduce the size of magnetic thin-film devices below 100 nm in order to obtain single domain structures. Because conventional MTJ devices possess multi-domain structures, their true characteristics, such as spin-polarized conduction and scattering phenomena at the interface of each layer, are often not apparent. In addition, the etching process is especially difficult in the micro- or nano-fabrication of magnetic devices. It is therefore necessary to establish a new fabrication technique for future ultra-small spin-electronic devices, including ‘spin-filter’ devices. In this study, we have focused on MTJ devices with a dimension of 100 nm and we have developed a tool to test their conduction characteristics.
2.Experimental Procedure
Specimens were prepared by depositing [Fe3O4 / underlayer] films on MgO (100) and (110) substrates using a dual ion beam sputtering (DIBS) apparatus. DIBS is an effective ‘plasma-free’ method for constructing artificial lattices, since the sputtering conditions can be controlled precisely and independently [5]. Figure 1shows the configuration of the DIBS apparatus. The samples were introduced into the deposition chamber through a load-lock system.


The deposition chamber was equipped with two hollow-cathode-type ion sources with 5 cm-diameter grids, where one source is for sputtering targets and the other is for bombarding the depositing film surfaces. The acceleration voltage of the sputtering ion source Vmgwas set at 850 eV, and that of the bombarding ion source Vsgwas set at values in the range 0 to 200 eV. The deposition chamber was evacuated to a pressure of less than 5 x 10-7 Pa and then an operating krypton gas pressure of approximately 6 x 10-2 Pa was used. It is well known that krypton is the sputtering gas suitable for reducing the energy of recoiled particles, while krypton ions possess a heavier atomic mass compared to argon ions [6]. Sintered Fe3O4 targets and pure Ru targets (4N) of diameter 6-inches were used for the experiments.

The crystal structure was analyzed using X-ray diffractometry (XRD) with Co-Kα radiation. In this study, we should note that all the peaks at a 2θ value of approximately 45° in the XRD patterns correspond to the Al (111) lattice plane, and originate from the sample holder. Surface morphology was observed by atomic force microscopy (AFM) in contact mode and/or torsional resonance mode, in order to obtain both AFM images and current conduction images simultaneously. Magnetic properties were evaluated using a vibrating sample magnetometer (VSM). Resistivity of the samples was measured by the conventional d.c. 4-probe method.
3.Results and Discussion

3.1. Deposition of Fe3O4films at room temperature
Figures 2 and 3show the XRD patterns for [Fe3O4 (50 nm) / Ru (5 nm)] films deposited on Figure 2. XRD patterns for [Fe3O4/Ru] films deposited on MgO (100) at various Vsg. MgO (100) and (110) substrates by DIBS for various bombarding ion source accelerating voltages Vsg. In Figure 2, only weak peaks of Fe3O4 crystallites can be detected for the samples deposited on the MgO (100) substrate. On the contrary, in Figure 3 clear peaks can be seen at 2θ values of approximately 35°, corresponding to the (220) peak of Fe3O4, for samples deposited on MgO (110). These results may be due to the difference in lattice spacing of the MgO (100) and (110) planes. Because the atomic distances for MgO (110) plane and Fe3O4(220) plane have almost the same value (misfit ~3 %), it appears that Fe3O4crystallites were constructed and rearranged by the hetero-epitaxial effect throughout the thin Ru underlayer. The maximum intensity of the Fe3O4 (220) peak (in the absence of other Fe3O4peaks) reached a maximum value at Vsg= 160 eV, signifying that the deposited atoms were aligned to form a (110) lattice plane through diffusion and reaction processes on the substrate. In addition, the resistivity of the specimen was lowest at Vsg= 160 eV, reaching a value of approximately 3 x 10-2Ω -cm. The coercivity for this specimen is 550 Oe. We adopted this condition as the deposition parameter for Fe3O4thin films in order to fabricate small pillar structures.

3.2. Fabrication of [Co-Fe / Al-O / Fe3O4] pillars
The process used to fabricate [Co-Fe / Al-O / Fe3O4] pillars is shown in Figure 4, as follows: (1) A Ru film is deposited on the substrate using a metal mask. (2) A [pure Al (1 nm) / Fe3O4(20 nm)] thin film is deposited on the Ru underlayer using a metal mask with different shape. Before this deposition, the Ru surface is bombarded and pre-cleaned by krypton ions at a Vsgof 200 eV. The specimen is then loaded into the oxidation chamber, which is equipped with a radial line slot antenna (RLSA) type plasma source [7]. The Al films are oxidized for 10 seconds in the plasma, comprised of a mixture of xenon and oxygen excited by microwaves at an optimized low energy (~ 1 eV). The specimen is then loaded back into the deposition chamber, and [Ta (5 nm) cap layer / Co-Fe (10 nm)] films are successively deposited on it. (3) Electron beam resist (negative-type) is spin-coated onto the specimen. The pillar pattern is transcribed using electron beam lithography. (4) The exposed electron beam resist is developed, rinsed and dried. (5) The specimen is milled twice along the electron beam resist template using an argon ion beam at a fixed angles of incidence, with the substrate cooled to -10 °C. The process is controlled by secondary ion mass spectroscopy. (6) The milling process is completed. (7) The pillar fabrication is complete once the electron beam resist has been removed and the specimen has been cleaned using acetone and 2-propanol.


Figure 5 shows typical AFM images for the fabricated arrays of [Co-Fe/Al-O/Fe3O4] pillar structures with pillar sizes of (I) 300 nm x 300 nm, (II) 150 nm x 150 nm and (III) 80 nm x 80 nm, respectively. We could successfully fabricate well-defined pillar structures with dimensions greater than 120 nm x 120 nm, which denotes the transition size between multi-domain and single-domain structures. In particular, we could obtain a rectangular shape with a size greater than 200 nm, even using the ion-milling process. However, it is difficult to achieve high-quality pillar structures with dimensions of less than 100 nm, as shown in Figure 5(III). Using AFM at high magnification, it was found that etched materials, for example electron beam resist and constituent metals, were re-deposited on the side wall and on the top region of the pillars by the ion-milling process.

Figure 6 shows a schematic illustration of the method used to test the conduction characteristics of small-size pillar devices. A metal-coated AFM cantilever is used as the top electrode, while the ruthenium underlayer is connected to the ground state [8] (bottom electrode). Because the position of the AFM cantilever is controlled by an AFM piezo-scanner, we could successively perform measurements of each pillar device on the same substrate. A DC sample bias Vappis applied to the specimen pillar through the AFM cantilever and bottom electrode. The output current Itunais detected using a picoampere amplifier.

Figure 7
shows typical dependences of Itunaon Vappfor various pillar sizes of (I) 300 nm x 300 nm, (II) 150 nm x 150 nm and (III) 80 nm x 80 nm, respectively. As the pillar size is decreased, the threshold Vappfor the detection of Itunais dramatically reduced from approximately -4.5 V to -1.5 V. Here, Simmons’ equation describes a smooth and average barrier characteristics with no temperature dependence for the [Ferromagnetic / Insulator / Ferromagnetic] sandwiched structure, as follows [9] : ,


where Jis the tunneling current density, Vis the applied voltage, m and eare electron mass and charge, V0and a are the barrier height and width for the insulation layer, respectively. By considering the series contact resistance between the AFM cantilever and tantalum cap layer, the barrier height for the aluminum oxide layer is estimated using Simmons’ equation to be (I) 2.6 eV, (II) 2.55 eV and (III) 0.32 eV, respectively. The aluminum oxide layer appeared to separate neighbor ferromagnetic layers completely in specimens (I) and (II). However, for specimen (III) etched debris was re-deposited on the side wall of the pillar by the ion-milling process, essentially degrading the barrier properties. Therefore, we need to develop a ‘less-debris’

etching process, for example a reactive ion etching method using sublimation gas coupled with metal materials, by which single-domain small pillars with dimensions less than 100 nm could be fabricated. In addition, argon ions seemed to be easily incorporated into the atomic lattices of specimens during the ion milling process. With decrease of the pillar size less than 100 nm, these incorporated argon ions could not be negligible and they became the ‘scattering center,’ and then, degrade the conduction characteristics.

Due to limitations of the system configuration, we could not evaluate the MR curves in detail. We intend in the near future to report the relationship between applied magnetic field and MR ratio for the pillar structure discussed above.

4.Conclusions
The use of half-metallic Fe3O4 films in the fabrication of tunneling magnetoresistance devices with large magnetoresistance ratios has great potential due to the attainment of high spin-polarization. It is necessary to establish the deposition conditions of stoichiometric and half-metallic Fe3O4thin-films for realizing the future ‘spin-filter’ devices. By using a dual ion beam sputtering method, the crystal structures of [Fe3O4/ Ru] films could be optimized by precisely controlling the energy of Kr ion bombardment at 160 eV during film deposition. In order to develop future spin electronic devices, we have focused on the 100 nm-scale fabrication process of ferromagnetic pillar structures using electron beam lithography. An excellent barrier height of approximately 2.6 eV could be attained for [Co-Fe/Al-O/Fe3O4] pillars with sizes above 120 nm.




Reference

[1] B. Dieny, V. S. Speriosu, S. S. P. Parkin, B. A. Gurney, D. R. Wilhoit, and D. Mauri, “Giant magnetoresistive in soft ferromagnetic multilayers,” Physical Review B, 43, (1), pp. 1297?1300 (1991).
[2] J.M. Daughton, “Magnetic tunneling applied to memory,” Journal of Applied Physics , 81 , (8), pp. 3758-3763 (1997).
[3] K. Aoshima and S.X. Wang, “Fe3O4and its magnetic tunneling junctions grown by ion beam deposition,” Journal of Applied Physics, 93, (10), pp. 7954-7956 (2003).
[4] Z. Zhang and S. Satpathy,“Electron states, magnetism, and the Verwey transition in magnetite,” Physical Review B, 44, (24), pp. 13319-13331 (1991).
[5] Y. Miyamoto, K. Machida, N. Hayashi, T. Tamaki and H. Okuda, “Deposition of [Ni-Fe/Al-O/Co-Fe] Films with Tunneling Magnetoresistance Effect using The Interfacial Modulation Technique,” Journal of Applied Physics,89, (11), pp. 6647-6649 (2001).
[6] Y. Miyamoto, K. Machida, N. Hayashi, T. Tamaki, H. Okuda and M. Naoe, “Deposition of Stoichiometric Ta2O5Films for TMR Devices by Reactive Ion Beam Sputtering,” Ferrites: Proceedings of ICF8., pp. 728-730 (2001).
[7] K. Nishikawa, M. Tsunoda, S. Ogata and M. Takahashi, “New Plasma Source with Low Electron Temperature for Fabrication of an Insulating Barrier in Ferromagnetic Tunnel Junctions,” IEEE Transactions on Magnetics,35, (5), pp. 2718-2720 (2002).
[8] Y. Miyamoto, K. Kuga, N. Hayashi, K. Machida and K. Aoshima, “Influence of Magnetic Field on the Tunneling Current in Magnetic 10-nm-scale Point Contact Junctions using Tunneling Atomic Force Microscopy,” Journal of Applied Physics, 95, (11), pp. 7246-7248 (2004).
[9] J.G. Simmons, “Generalized Formula for the Electric Tunnel Effect between Similar Electrodes Separated by a Thin Insulating Film,” Journal of Applied Physics, 34, (6), pp. 1793-1803 (1963).



Dr. Yasuyoshi MIYAMOTO Dr. Yasuyoshi MIYAMOTO

He received the B.S., M.S. and Ph.D. degrees in physical electronics from Tokyo Institute of Technology in 1993, 1995 and 1998, respectively. He was a research fellow of the Japan Society for the Promotion of Science (JSPS) from 1996 to 1998. He joined NHK in 1998 and has been with NHK Science and Technical Research Laboratories. He was also a visiting scholar at Stanford University, California, from 2001 to 2002. He has been engaged in the magnetic materials and magnetic thin-film devices for recording and the fundamental research on spin-electronics. He is a member of the Institute of the Electrical and Electronics Engineers (IEEE) and the Magnetics Society of Japan (MSJ).
Mr. Hirotaka SHIINO

Mr. Hirotaka SHIINO

He received the B.S. and M.S. degrees in engineering science and fundamental energy science from the Kyoto University in 1998 and 2000, respectively. He joined NHK in 2000. Since 2003, he has been with NHK Science and Technical Research Laboratories. He has been engaged in the researching on spin-tunneling devices and related materials.
Mr. Kiyoshi KUGA Mr. Kiyoshi KUGA

He received the B.S. and M.S. degrees in electrical engineering from Waseda University, Tokyo, in 1981 and 1983, respectively. He joined NHK in 1983. Since 1986, he has been with NHK Science and Technical Research Laboratories. He has been engaged in the researching on the perpendicular magnetic recording media, and fundamental research on spin-electronics including ferromagnetic semiconductors and spin-filter devices. He is currently a senior research engineer of materials science division. He is a member of the Magnetics Society of Japan (MSJ), the Japan Society of Applied Physics (JSAP) and the Institute of Image Information and Television Engineers of Japan (ITE).
 



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