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Co magnetic tunnel junctions
Materials Science and Engineering B83 (2001) 192– 197 www.elsevier.com/locate/mseb
Oxidation mechanism of the insulation layer in NiFe/Co/Al(Ta)-oxide /Co magnetic tunnel junctions H. Kyung *, C.S. Yoon, C.K. Kim Hanyang Uni6ersity, Department of Materials Science and Engineering, Seoul 133 791, South Korea Received 22 September 2000; received in revised form 18 January 2001; accepted 25 January 2001
Abstract Ferromagnetic tunneling junctions with Al-oxide and Ta-oxide as the insulating layer were fabricated using metal mask and Inductively Coupled Plasma (ICP) sputtering system. To interpret growth mechanism of the insulating layer during plasma oxidation process, the microstructure of the oxide layers was investigated with cross-sectional TEM. TEM analysis showed Al-oxide had different microstructures depending on the thickness of the layer. At 13 A, , the Al-oxide layer was flat while the Al-oxide layer became progressively wavy with regular periodicity of 20 nm at increasing oxide thickness. Ta-oxide layer was partially oxidized under equal oxidizing conditions, but remained flat regardless of the thickness. Growth mechanism for the two different oxide layers is proposed in terms of oxidation kinetics and oxygen plasma. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Magnetic tunnel junction; Insulation layer; Tunneling; Plasma oxidation
1. Introduction Tunneling magnetoresistive (TMR) junctions consist of ferromagnet (FM)/insulator/ferromagnet structure in which TMR ratio changes as a function of an applied magnetic field. The resistance of the junction depends upon spin arrangement of the FM layers separated by an insulating layer. When the magnetization orientations of the two FM layers are parallel, conductance perpendicular to the junction is enhanced. Since a number of authors have reported the TMR effect over 20% at room temperature [1 – 3], a great amount of research has been carried out to characterize the TMR junction in order to commercialize the effect in such potential applications as novel high density read-head for HDD and MRAM (magnetic random access memory) [4,5]. As the magnetoresistive properties of the TMR junctions is dictated by the interface quality and nature of the insulating oxide [6], in order to utilize the junction commercially, the microstructure of the insulating layer needs to be controlled within a few angstrom ranges to * Corresponding author.
ensure reliable operation of the device. In addition, sharp interfaces of insulating layer between two FM layers are thought to be the limiting factor in achieving the theoretical TMR effect [7]. Thus, a microstructural evaluation of the interface is of crucial importance in studying the TMR junctions. In this work, we have investigated microstructural characteristics of Al and Ta oxide layer to establish reliable and reproducible processing conditions for TMR junction fabrication.
2. Experimental details The magnetic tunnel junctions were deposited by dc magnetron sputtering at room temperature on Si(100) wafers which were cleaned with ethanol for 5 min to eliminate H2O and oxidized thermally prior to the deposition. The following layered structure was sequentially deposited in the following order and the thickness: Si wafer/SiO2/NiFe(170 A, )/Co(48 A, )/Al(x)-oxide/ Co(750 A, ), Si wafer/SiO2/NiFe(170 A, )/Co(48 A, )/Ta(x)-oxide/ Co(750 A, )
0921-5107/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 1 ) 0 0 5 2 6 - 8
H. Kyung et al. / Materials Science and Engineering B83 (2001) 192–197
The base pressure was B 3 ×10 − 6 Pa and the sputtering of NiFe and Co were done at 0.074 Pa, 2.3 sccm Ar at the deposition rate of 0.7 A, s − 1. The NiFe layers were deposited using alloying target. The Al-oxide was formed by first depositing Al film with thickness ranging from 13 to 63 A, at 0.7 sccm Ar followed by exposure to an oxygen plasma. For the plasma oxidation, 3.0 sccm Ar at 0.3 Pa and 9.1 sccm O2 at 0.7 Pa was maintained for 5 min. The cross pattern for the junction was formed using metal mask and the chamber was vented to change the metal mask. The total area of the TMR junction was 100 × 100 m. The chemical composition of the oxide layers was examined by AES (PHI 650, Physical Electronics, USA) with sputter etch depth profiling. Interface study of TMR junctions was performed using cross sectional TEM (JEM2010, 200 kV, JEOL, Japan).
3. Result and discussion
3.1. Al-oxide microstructure Fig. 1 shows the TEM cross sectional view of NiFe/ Co/Al-oxide/Co junction with different oxide thickness. The Al-oxide layer in Fig. 1(a) was flat at 13 A, . As the oxide layer became thicker, although the NiFe, Co/Aloxide interface (bottom electrode) remained flat, the Al-oxide/Co interface on top became increasingly wavy with periodicity of 20 nm as can be seen in Fig. 1(b– c). As the thickness of the insulation layer was increased, the amplitude of the thickness fluctuation increased while maintaining the periodicity of 20 nm. To determine the cause of the wavy interface, the multi-layer junction was created without the plasma oxidation process. Shown in Fig. 2(a) is the ferromagnet/Al-metal/ferromagnet junction. The interfaces with the Al metal layer remained flat at 63 A, thickness, which indicates that the deposition of Al layer did not cause such undulating microstructure. Also shown in Fig. 2(b) is the Al-oxide layer created by exposing the Al metal layer to air in absence of oxygen plasma. In both cases, the oxide layer did not have any perturbations at the interfaces from which it can be concluded that the plasma oxidation process led to the interface instability when the oxide layer thickness exceeded a critical point. Since the conductance of the TMR junctions varies exponentially with the barrier thickness, most of the tunnel current flows through the thinnest part of the Al-oxide [6]. Therefore, it is critical that such roughness seen in Fig. 1 needs to be strictly controlled.
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3.2. Ta-oxide microstructure Also investigated is NiFe/Co/Ta-oxide/Co junction shown in Fig. 3. Compare to the Al-oxide, Ta is much slower to oxidize. Due to the slow oxidation rate, the Ta metal was not fully oxidized in the oxygen plasma and the residual Ta metal layer can be seen at the interface. Unlike Al-insulation layer, no significant morphological distortion was found at the Ta-oxide layer during oxidation process at the comparable deposited metal layer thickness. The insulation layer remained straight regardless of the metal layer thickness.
3.3. Oxidation mechanism We have shown that the plasma oxidation process can produce different oxide microstructures at the junction interface depending on the oxide thickness and the host material. Fig. 4 illustrates the elementary processes that govern the plasma oxidation process. In the magnetron sputtering system, the plasma is tightly confined near the cathode area. Oxygen species such as O2 + , O−, O created in the plasma have to diffuse out of the plasma and the confining magnetic field [8]. The oxygen species then needs to be transported across the chamber to reach the deposited aluminum. The ensuing diffusion rate and gas transport rate could determine the oxidation behavior. At the substrate, the oxidation involves the gas adsorption to the surface, diffusion of metal atoms to the surface and finally reaction between Al and O. As discussed above, Ta partially oxidized due to the slow oxidation rate. The governing process is either the oxygen/metal interface reaction or diffusion of migrating species which should generate a flat interface at a microscopic level because the amorphous structure of the Ta-oxide is essentially isotropic without any preferred orientation or grain boundaries. In contrast, in case of Al, its reaction with the oxygen species should proceed fast compared to the other elementary processes due to the large thermodynamic driving force (DfG° (Al2O3)= − 1580 KJ mol − 1 at 298 K as compared to DfG° (Ta2O5)= − 880 KJ mol − 1 at 298 K [9]). It can be also assumed that the Al diffusion rate through loose network of amorphous oxide layer is relatively high since unreacted metallic residue was not found at the interface (Auger Analysis). Hence, the Al-oxide morphology will be dictated by the plasma diffusion process and the gas transport. Fig. 5 shows the HRTEM images of the junctions oxidized at different oxygen partial pressures with the constant total pressure to keep the oxygen plasma nearly constant during the oxidation experiment. As the oxygen partial pressure is increased, the population of the ions should increase which, in turn, should enhance the transport probability of the oxygen species to the
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substrate. As can be seen, no microstructural changes were observed as the partial pressure was varied. The periodicity of 20 nm and roughness of the Co/Al-oxide interfaces are nearly equal in all three micrographs. The micrographs suggest that the oxidation behavior is likely to be controlled by an inherent ion/atom movement in the oxygen plasma.
The ion/atom diffusion out of the plasma should be slow as the magnetron deposition system is designed to confine the plasma near the cathode. The density of the oxidizing agent available to the substrate will be a small fraction of the gas phase oxygen during the relatively short period of 5 min exposure to the plasma. Therefore, it is likely that the molecular diffusivity of the
Fig. 1. Cross-sectional TEM images of: (a) SiO2/NiFe/Co/Al(13 A, )-oxide/Co; (b) SiO2/NiFe/Co/Al(43 A, )-oxide/Co; and (c) SiO2/NiFe/Co/Al(63 A, )-oxide/Co.
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Fig. 4. Schematic illustration of the elementary kinetic processes involved in the plasma oxidation.
Fig. 2. Cross sectional TEM images of: (a) NiFe/Co/Al-metal/Co junction; and (b) NiFe/Co/Al-oxide (oxidized in air)/Co junction.
oxygen species across the plasma sheath will control the oxide growth [10]. Then, the periodic structure of the Co/Al-oxide interface could have arisen from the perturbations in the plasma.
In fact, many different types of waves can propagate in a plasma to cause oscillations of ions and electrons. All plasma has harmonic frequencies with which electrons and ions are supposed to oscillate. In addition to the natural harmonic wave, acoustic, electromagnetic, hydromagnetic (Alfve´ n), magnetoacoustic, and ion waves can also propagate in the plasma depending on the conditions in the plasma [11]. Although it is difficult to pinpoint the exact nature of the oscillation due to the complex behavior of the magnetized plasma, considering that all the wavy junctions exhibited 20 nm periodicity, one of the characteristic waves formed within the plasma could have led to the periodic local variation of the ion transport probability.
Fig. 3. Cross-sectional TEM image of SiO2/NiFe/Co/Ta (63 A, )-oxide/Co/Pt junction.
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Other possible causes of such non-uniformity of the oxidized layer could be the volumetric strain arising from the oxidation or the textured growth of the metallic film whose roughness has been accentuated during the oxidation process. However, the oxidized film was amorphous so that the interface mismatch strain should be readily accommodated. As for the textured growth, the Al metal film in Fig. 2(a) does not show a textured columnar growth. In addition, if the aforementioned causes led to the non-uniformity, the same microstructure should have been observed in the Ta-oxide junc-
tion; thus, we believe that the periodicity is due to the inherent nature of the plasma at the given condition. As ions/atoms arrive at the sample at different densities (in regular pattern), when the Al layer is thin, there is a sufficient amount of oxygen radicals present to uniformly oxidize the Al layer. However, as the insulation layer becomes thicker, at locations where the oxygen species are deficient, the oxide growth is eventually inhibited whereas at locations the oxygen density is high, Al atoms are quickly depleted. Such depletion of the cation sets up a local concentration gradient for Al
Fig. 5. Cross-sectional TEM images of SiO2/NiFe/Co/Al(43 A, )-oxide/Co with the Al metal layer oxidized at different oxygen partial pressures in the plasma: (a) Ar at 5.7 sccm, O2 at 6.0 sccm; (b) Ar at 3.0 sccm, O2 at 9.1 sccm atm; (c) Ar at 2.1 sccm, O2 at 10.0 sccm. Total pressure was maintained at 1 Pa.
H. Kyung et al. / Materials Science and Engineering B83 (2001) 192–197
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4. Conclusion It was shown that the microstructure of NiFe–Co/Alor Ta-oxide/Co junction can be different depending on the oxide thickness and the host material. Although we were not able to pinpoint the exact mechanism in the plasma processing that created the initial periodic fluctuation of the oxygen density, it was proven that the plasma process was responsible for the wavy interface at the Co/Al-oxide interface.As it appears that the wavy interface was due to the oxygen diffusion from the tightly confined magnetron plasma, the oxygen leakage from the plasma can be improved by deliberately degrading the plasma confinement as done in the unbalanced magnetron system used to produce nitride films [13]. The enhanced oxygen diffusion should provide fast uniform oxidation of the pre-deposited metallic layer. Acknowledgements Fig. 6. Plasma oxidation mechanism of Al-oxide insulation layer. (a) When the metal layer is thin, the oxygen supply is sufficient to uniformly oxidize the metal. (b) When the metal layer is thicker, localized oxygen starvation leads to the cation diffusion and wavy oxide layer.
atoms so that Al atoms will migrate to the oxygen-rich region to locally enhance the oxide growth. The migration of Al atoms responding to the local fluctuation of the oxygen density eventually results in growth of the wavy oxide. The Al-oxidation process is schematically illustrated in Fig. 6. The wavy oxide layer could have been created by massive movement of the reactants through the surface migration driven by the forces arising from the surface energy, interface energy and strain. However, since the oxidation was carried out at room temperature for a relatively short period (5 min), we expect the surface migration of the reactants to be localized. In addition, thermodynamic driving forces for such surface migration is rather small for our case as the oxide layer is amorphous, which could readily relieve the stresses arising from the surface energy, interface energy and strain. The local movement of the Al atoms is evidenced by the fact that the thickness of the Al-oxide at the thinnest part of the oxide layer is actually thinner than the initial Al metal thickness prior to the plasma oxidation (Fig. 1(c)). Since the plasma oxidation has been known to be free of the metal loss by forming volatile metal oxides [12], the formation of oxide region thinner than the initial metal layer indicates the Al atom movement. .
This work was supported by Korea Research Foundation Grant (KRF-2000-041-E00536). The authors would also like to gratefully thank Dr Y. Ando and Professor T. Miyazaki at Tohoku University, Sendai, Japan for kindly preparing the junction samples. References [1] S.S.P. Parkin, Spin Dependent Tunneling and Its Application to On-volatile Magnetic Random Access Memory, The 43rd Annual Conference of MMM Proceeding, GA-03, 255 (1998). [2] P. Seneor, A. Fert, J.-L. Maurice, F. Montaigne, F. Petroff, A. Vaures, Appl. Phys. Lett. 74 (1999) 4017. [3] N. Tezuka, M. Oohane, T. Miyazaki, J. Magn. Magn. Mater. 198-199 (1999) 149. [4] T. Miyazaki, N. Tezuka, J. Magn. Mater. 139 (1995) L231. [5] J.S. Moodera, L.R. Kinder, J. Appl. Phys. 79 (1996) 4724. [6] C.L. Platt, B. Dieny, A.E. Berkowitz, J. Appl. Phys. 81 (8) (1997) 5523. [7] J.S. Moodera, L.R. Kinder, T.M. Wong, R. Meservey, Phys. Rev. Lett. 74 (1995) 3273. [8] M.A. Lieberman, A.J. Lichtenberg, Principles of Plasma Discharges and Materials Processing, Wiley, New York, 1994, p. 129. [9] Handbook of Physics and Chemistry, 73rd ed., D.R. Lide (Ed.), CRC Press, Boca Raton, FL, 1992. [10] J.V. Cole, H.H. Lee, J. Electrochem. Soc. 138 (2) (1991) 567. [11] M. Sedlacek, Electron Physics of Vacuum and Gaseous Devices, Wiley, New York, 1996, p. 408. [12] R. Go´ mez-San Roma´ n, R. Pe´ rez-Casero, J. Perrie`re, J.P. Enrard, J.M. Martı´nez-Duart, Appl. Suf. Sci. 70/71 (1993) 479. [13] S.M. Rossnagel, Sputtering for semiconductor applications, in: K. Upadhya (Ed.), Plasma Synthesis and Processing of Materials, The Minerals, Metals & Materials Society, 1993, p. 47.
Oxidation mechanism of the insulation layer in NiFe/Co/Al(Ta)-oxide /Co magnetic tunnel junctions H. Kyung *, C.S. Yoon, C.K. Kim Hanyang Uni6ersity, Department of Materials Science and Engineering, Seoul 133 791, South Korea Received 22 September 2000; received in revised form 18 January 2001; accepted 25 January 2001
Abstract Ferromagnetic tunneling junctions with Al-oxide and Ta-oxide as the insulating layer were fabricated using metal mask and Inductively Coupled Plasma (ICP) sputtering system. To interpret growth mechanism of the insulating layer during plasma oxidation process, the microstructure of the oxide layers was investigated with cross-sectional TEM. TEM analysis showed Al-oxide had different microstructures depending on the thickness of the layer. At 13 A, , the Al-oxide layer was flat while the Al-oxide layer became progressively wavy with regular periodicity of 20 nm at increasing oxide thickness. Ta-oxide layer was partially oxidized under equal oxidizing conditions, but remained flat regardless of the thickness. Growth mechanism for the two different oxide layers is proposed in terms of oxidation kinetics and oxygen plasma. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Magnetic tunnel junction; Insulation layer; Tunneling; Plasma oxidation
1. Introduction Tunneling magnetoresistive (TMR) junctions consist of ferromagnet (FM)/insulator/ferromagnet structure in which TMR ratio changes as a function of an applied magnetic field. The resistance of the junction depends upon spin arrangement of the FM layers separated by an insulating layer. When the magnetization orientations of the two FM layers are parallel, conductance perpendicular to the junction is enhanced. Since a number of authors have reported the TMR effect over 20% at room temperature [1 – 3], a great amount of research has been carried out to characterize the TMR junction in order to commercialize the effect in such potential applications as novel high density read-head for HDD and MRAM (magnetic random access memory) [4,5]. As the magnetoresistive properties of the TMR junctions is dictated by the interface quality and nature of the insulating oxide [6], in order to utilize the junction commercially, the microstructure of the insulating layer needs to be controlled within a few angstrom ranges to * Corresponding author.
ensure reliable operation of the device. In addition, sharp interfaces of insulating layer between two FM layers are thought to be the limiting factor in achieving the theoretical TMR effect [7]. Thus, a microstructural evaluation of the interface is of crucial importance in studying the TMR junctions. In this work, we have investigated microstructural characteristics of Al and Ta oxide layer to establish reliable and reproducible processing conditions for TMR junction fabrication.
2. Experimental details The magnetic tunnel junctions were deposited by dc magnetron sputtering at room temperature on Si(100) wafers which were cleaned with ethanol for 5 min to eliminate H2O and oxidized thermally prior to the deposition. The following layered structure was sequentially deposited in the following order and the thickness: Si wafer/SiO2/NiFe(170 A, )/Co(48 A, )/Al(x)-oxide/ Co(750 A, ), Si wafer/SiO2/NiFe(170 A, )/Co(48 A, )/Ta(x)-oxide/ Co(750 A, )
0921-5107/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 1 ) 0 0 5 2 6 - 8
H. Kyung et al. / Materials Science and Engineering B83 (2001) 192–197
The base pressure was B 3 ×10 − 6 Pa and the sputtering of NiFe and Co were done at 0.074 Pa, 2.3 sccm Ar at the deposition rate of 0.7 A, s − 1. The NiFe layers were deposited using alloying target. The Al-oxide was formed by first depositing Al film with thickness ranging from 13 to 63 A, at 0.7 sccm Ar followed by exposure to an oxygen plasma. For the plasma oxidation, 3.0 sccm Ar at 0.3 Pa and 9.1 sccm O2 at 0.7 Pa was maintained for 5 min. The cross pattern for the junction was formed using metal mask and the chamber was vented to change the metal mask. The total area of the TMR junction was 100 × 100 m. The chemical composition of the oxide layers was examined by AES (PHI 650, Physical Electronics, USA) with sputter etch depth profiling. Interface study of TMR junctions was performed using cross sectional TEM (JEM2010, 200 kV, JEOL, Japan).
3. Result and discussion
3.1. Al-oxide microstructure Fig. 1 shows the TEM cross sectional view of NiFe/ Co/Al-oxide/Co junction with different oxide thickness. The Al-oxide layer in Fig. 1(a) was flat at 13 A, . As the oxide layer became thicker, although the NiFe, Co/Aloxide interface (bottom electrode) remained flat, the Al-oxide/Co interface on top became increasingly wavy with periodicity of 20 nm as can be seen in Fig. 1(b– c). As the thickness of the insulation layer was increased, the amplitude of the thickness fluctuation increased while maintaining the periodicity of 20 nm. To determine the cause of the wavy interface, the multi-layer junction was created without the plasma oxidation process. Shown in Fig. 2(a) is the ferromagnet/Al-metal/ferromagnet junction. The interfaces with the Al metal layer remained flat at 63 A, thickness, which indicates that the deposition of Al layer did not cause such undulating microstructure. Also shown in Fig. 2(b) is the Al-oxide layer created by exposing the Al metal layer to air in absence of oxygen plasma. In both cases, the oxide layer did not have any perturbations at the interfaces from which it can be concluded that the plasma oxidation process led to the interface instability when the oxide layer thickness exceeded a critical point. Since the conductance of the TMR junctions varies exponentially with the barrier thickness, most of the tunnel current flows through the thinnest part of the Al-oxide [6]. Therefore, it is critical that such roughness seen in Fig. 1 needs to be strictly controlled.
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3.2. Ta-oxide microstructure Also investigated is NiFe/Co/Ta-oxide/Co junction shown in Fig. 3. Compare to the Al-oxide, Ta is much slower to oxidize. Due to the slow oxidation rate, the Ta metal was not fully oxidized in the oxygen plasma and the residual Ta metal layer can be seen at the interface. Unlike Al-insulation layer, no significant morphological distortion was found at the Ta-oxide layer during oxidation process at the comparable deposited metal layer thickness. The insulation layer remained straight regardless of the metal layer thickness.
3.3. Oxidation mechanism We have shown that the plasma oxidation process can produce different oxide microstructures at the junction interface depending on the oxide thickness and the host material. Fig. 4 illustrates the elementary processes that govern the plasma oxidation process. In the magnetron sputtering system, the plasma is tightly confined near the cathode area. Oxygen species such as O2 + , O−, O created in the plasma have to diffuse out of the plasma and the confining magnetic field [8]. The oxygen species then needs to be transported across the chamber to reach the deposited aluminum. The ensuing diffusion rate and gas transport rate could determine the oxidation behavior. At the substrate, the oxidation involves the gas adsorption to the surface, diffusion of metal atoms to the surface and finally reaction between Al and O. As discussed above, Ta partially oxidized due to the slow oxidation rate. The governing process is either the oxygen/metal interface reaction or diffusion of migrating species which should generate a flat interface at a microscopic level because the amorphous structure of the Ta-oxide is essentially isotropic without any preferred orientation or grain boundaries. In contrast, in case of Al, its reaction with the oxygen species should proceed fast compared to the other elementary processes due to the large thermodynamic driving force (DfG° (Al2O3)= − 1580 KJ mol − 1 at 298 K as compared to DfG° (Ta2O5)= − 880 KJ mol − 1 at 298 K [9]). It can be also assumed that the Al diffusion rate through loose network of amorphous oxide layer is relatively high since unreacted metallic residue was not found at the interface (Auger Analysis). Hence, the Al-oxide morphology will be dictated by the plasma diffusion process and the gas transport. Fig. 5 shows the HRTEM images of the junctions oxidized at different oxygen partial pressures with the constant total pressure to keep the oxygen plasma nearly constant during the oxidation experiment. As the oxygen partial pressure is increased, the population of the ions should increase which, in turn, should enhance the transport probability of the oxygen species to the
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substrate. As can be seen, no microstructural changes were observed as the partial pressure was varied. The periodicity of 20 nm and roughness of the Co/Al-oxide interfaces are nearly equal in all three micrographs. The micrographs suggest that the oxidation behavior is likely to be controlled by an inherent ion/atom movement in the oxygen plasma.
The ion/atom diffusion out of the plasma should be slow as the magnetron deposition system is designed to confine the plasma near the cathode. The density of the oxidizing agent available to the substrate will be a small fraction of the gas phase oxygen during the relatively short period of 5 min exposure to the plasma. Therefore, it is likely that the molecular diffusivity of the
Fig. 1. Cross-sectional TEM images of: (a) SiO2/NiFe/Co/Al(13 A, )-oxide/Co; (b) SiO2/NiFe/Co/Al(43 A, )-oxide/Co; and (c) SiO2/NiFe/Co/Al(63 A, )-oxide/Co.
H. Kyung et al. / Materials Science and Engineering B83 (2001) 192–197
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Fig. 4. Schematic illustration of the elementary kinetic processes involved in the plasma oxidation.
Fig. 2. Cross sectional TEM images of: (a) NiFe/Co/Al-metal/Co junction; and (b) NiFe/Co/Al-oxide (oxidized in air)/Co junction.
oxygen species across the plasma sheath will control the oxide growth [10]. Then, the periodic structure of the Co/Al-oxide interface could have arisen from the perturbations in the plasma.
In fact, many different types of waves can propagate in a plasma to cause oscillations of ions and electrons. All plasma has harmonic frequencies with which electrons and ions are supposed to oscillate. In addition to the natural harmonic wave, acoustic, electromagnetic, hydromagnetic (Alfve´ n), magnetoacoustic, and ion waves can also propagate in the plasma depending on the conditions in the plasma [11]. Although it is difficult to pinpoint the exact nature of the oscillation due to the complex behavior of the magnetized plasma, considering that all the wavy junctions exhibited 20 nm periodicity, one of the characteristic waves formed within the plasma could have led to the periodic local variation of the ion transport probability.
Fig. 3. Cross-sectional TEM image of SiO2/NiFe/Co/Ta (63 A, )-oxide/Co/Pt junction.
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H. Kyung et al. / Materials Science and Engineering B83 (2001) 192–197
Other possible causes of such non-uniformity of the oxidized layer could be the volumetric strain arising from the oxidation or the textured growth of the metallic film whose roughness has been accentuated during the oxidation process. However, the oxidized film was amorphous so that the interface mismatch strain should be readily accommodated. As for the textured growth, the Al metal film in Fig. 2(a) does not show a textured columnar growth. In addition, if the aforementioned causes led to the non-uniformity, the same microstructure should have been observed in the Ta-oxide junc-
tion; thus, we believe that the periodicity is due to the inherent nature of the plasma at the given condition. As ions/atoms arrive at the sample at different densities (in regular pattern), when the Al layer is thin, there is a sufficient amount of oxygen radicals present to uniformly oxidize the Al layer. However, as the insulation layer becomes thicker, at locations where the oxygen species are deficient, the oxide growth is eventually inhibited whereas at locations the oxygen density is high, Al atoms are quickly depleted. Such depletion of the cation sets up a local concentration gradient for Al
Fig. 5. Cross-sectional TEM images of SiO2/NiFe/Co/Al(43 A, )-oxide/Co with the Al metal layer oxidized at different oxygen partial pressures in the plasma: (a) Ar at 5.7 sccm, O2 at 6.0 sccm; (b) Ar at 3.0 sccm, O2 at 9.1 sccm atm; (c) Ar at 2.1 sccm, O2 at 10.0 sccm. Total pressure was maintained at 1 Pa.
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4. Conclusion It was shown that the microstructure of NiFe–Co/Alor Ta-oxide/Co junction can be different depending on the oxide thickness and the host material. Although we were not able to pinpoint the exact mechanism in the plasma processing that created the initial periodic fluctuation of the oxygen density, it was proven that the plasma process was responsible for the wavy interface at the Co/Al-oxide interface.As it appears that the wavy interface was due to the oxygen diffusion from the tightly confined magnetron plasma, the oxygen leakage from the plasma can be improved by deliberately degrading the plasma confinement as done in the unbalanced magnetron system used to produce nitride films [13]. The enhanced oxygen diffusion should provide fast uniform oxidation of the pre-deposited metallic layer. Acknowledgements Fig. 6. Plasma oxidation mechanism of Al-oxide insulation layer. (a) When the metal layer is thin, the oxygen supply is sufficient to uniformly oxidize the metal. (b) When the metal layer is thicker, localized oxygen starvation leads to the cation diffusion and wavy oxide layer.
atoms so that Al atoms will migrate to the oxygen-rich region to locally enhance the oxide growth. The migration of Al atoms responding to the local fluctuation of the oxygen density eventually results in growth of the wavy oxide. The Al-oxidation process is schematically illustrated in Fig. 6. The wavy oxide layer could have been created by massive movement of the reactants through the surface migration driven by the forces arising from the surface energy, interface energy and strain. However, since the oxidation was carried out at room temperature for a relatively short period (5 min), we expect the surface migration of the reactants to be localized. In addition, thermodynamic driving forces for such surface migration is rather small for our case as the oxide layer is amorphous, which could readily relieve the stresses arising from the surface energy, interface energy and strain. The local movement of the Al atoms is evidenced by the fact that the thickness of the Al-oxide at the thinnest part of the oxide layer is actually thinner than the initial Al metal thickness prior to the plasma oxidation (Fig. 1(c)). Since the plasma oxidation has been known to be free of the metal loss by forming volatile metal oxides [12], the formation of oxide region thinner than the initial metal layer indicates the Al atom movement. .
This work was supported by Korea Research Foundation Grant (KRF-2000-041-E00536). The authors would also like to gratefully thank Dr Y. Ando and Professor T. Miyazaki at Tohoku University, Sendai, Japan for kindly preparing the junction samples. References [1] S.S.P. Parkin, Spin Dependent Tunneling and Its Application to On-volatile Magnetic Random Access Memory, The 43rd Annual Conference of MMM Proceeding, GA-03, 255 (1998). [2] P. Seneor, A. Fert, J.-L. Maurice, F. Montaigne, F. Petroff, A. Vaures, Appl. Phys. Lett. 74 (1999) 4017. [3] N. Tezuka, M. Oohane, T. Miyazaki, J. Magn. Magn. Mater. 198-199 (1999) 149. [4] T. Miyazaki, N. Tezuka, J. Magn. Mater. 139 (1995) L231. [5] J.S. Moodera, L.R. Kinder, J. Appl. Phys. 79 (1996) 4724. [6] C.L. Platt, B. Dieny, A.E. Berkowitz, J. Appl. Phys. 81 (8) (1997) 5523. [7] J.S. Moodera, L.R. Kinder, T.M. Wong, R. Meservey, Phys. Rev. Lett. 74 (1995) 3273. [8] M.A. Lieberman, A.J. Lichtenberg, Principles of Plasma Discharges and Materials Processing, Wiley, New York, 1994, p. 129. [9] Handbook of Physics and Chemistry, 73rd ed., D.R. Lide (Ed.), CRC Press, Boca Raton, FL, 1992. [10] J.V. Cole, H.H. Lee, J. Electrochem. Soc. 138 (2) (1991) 567. [11] M. Sedlacek, Electron Physics of Vacuum and Gaseous Devices, Wiley, New York, 1996, p. 408. [12] R. Go´ mez-San Roma´ n, R. Pe´ rez-Casero, J. Perrie`re, J.P. Enrard, J.M. Martı´nez-Duart, Appl. Suf. Sci. 70/71 (1993) 479. [13] S.M. Rossnagel, Sputtering for semiconductor applications, in: K. Upadhya (Ed.), Plasma Synthesis and Processing of Materials, The Minerals, Metals & Materials Society, 1993, p. 47.