- Email: [email protected]
CoNiP electroless deposition process for fabricating ferromagnetic nanodot arrays
Electrochimica Acta 56 (2011) 9575–9580
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
CoNiP electroless deposition process for fabricating ferromagnetic nanodot arrays Takanari Ouchi, Naofumi Shimano, Takayuki Homma ∗ Department of Applied Chemistry, Waseda University, 3-4-1, Okubo, Shinjuku, Tokyo 169-8555, Japan
a r t i c l e
i n f o
Article history: Received 1 January 2011 Received in revised form 20 April 2011 Accepted 21 April 2011 Available online 3 May 2011 Keywords: Electroless deposition Nanodot arrays Nanoimprint lithography CoNiP alloy
a b s t r a c t An electroless deposition process for fabricating CoNiP nanodot arrays (less than 50 nm in height) with high magnetic coercivities was investigated. To fabricate such nanostructures, we improved the crystallinity of the CoNiP deposits in the initial deposition stage by applying an fcc-Cu(1 1 1) underlayer with low lattice mismatch to hcp-Co(0 0 0 2), and an autocatalytic electroless deposition process at the Cu surface was carried out by using dual reducing agents, H2 PO2 − and N2 H4 . CoNiP films demonstrated high perpendicular magnetic coercivities in the initial deposition stage since the highly crystalline hcp(0 0 0 2) CoNiP layers were grown parallel to the Cu underlayers. Nanopatterned substrates were formed by UVnanoimprint lithography. CoNiP was electroless deposited on the nanopatterned substrates. As a result, CoNiP was deposited selectively from the bottom of the nanopores with few defects in a large area. Perpendicular coercivities higher than 3000 Oe were obtained for nanodots even with heights of 50 nm. Thus, an electroless deposition process that can be used to form nanostructures with high crystallinities in the initial stage without any anomalous deposition was demonstrated. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Magnetic nanostructures with high magnetic coercivities are required for various advanced applications such as microelectromechanical systems or nano-electromechanical systems, sensing devices, and magnetic recording devices [1–5]. Physical processes are generally employed to fabricate such nanostructures; however, it is difficult to form various nanoscale shapes on the selected area of devices using physical processes. In contrast, electrochemical deposition processes can be used for area selective formation of nanostructures or microstructures with various patterns. In our previous study, we investigated the fabrication of CoPt nanodot arrays with high magnetic coercivities using the electrodeposition method [6–8]. We also demonstrated the fabrication of CoNiP nanodot arrays using electroless deposition, as an alternative technique [9]. Electroless deposition results in conformal depositions of metals with uniform structures over a large area. In the electroless deposition process, metals can only be deposited at the place where catalytic activities of the oxidation reactions of reducing agents occur [10]. Thus, in order to deposit metals only at the selected areas, it is important to control the catalytic activities of the oxidation reaction on the samples by changing the reducing agents and the surface reactivities of the samples. In our previous study, CoNiP nanodot arrays were fabricated on SiO2 /Si substrates by electroless deposition; the catalytic activities of the substrates
∗ Corresponding author. E-mail address: [email protected] (T. Homma). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.04.085
were controlled by a Pd activation process. However, the perpendicular magnetic coercivities of the CoNiP nanodot arrays were less than 800 Oe. This paper discusses the use of the electroless deposition process for fabricating CoNiP nanodot arrays less than 50 nm in height and with high magnetic coercivities. In order to achieve CoNiP nanodot arrays with high magnetic properties, the deposits should have high crystallinity during the initial deposition stage. Therefore, an fccCu(1 1 1) underlayer, with low lattice mismatch (2.3%) compared to that of hcp-Co(0 0 0 2), was used. In fact, in our previous study, highly crystalline hcp(0 0 0 2) CoPt nanodots were grown parallel to fcc-Cu(1 1 1) by using electrodeposition process [7]. The problem with the application of the fcc-Cu(1 1 1) underlayer in electroless deposition of CoNiP is that H2 PO2 − , which is used as a reducing agent in the general electroless deposition of CoNiP, is not oxidized to work as a reducing agent on the Cu surface. Therefore, catalytic activation processes such as immersion of the substrates into diluted PdCl2 solution were required. However, it is difficult to obtain area selective depositions on nanopatterns or micropatterns by using such activation processes, because anomalous depositions occur on the resists or insulators [11]. Thus, in this study, CoNiP nanodot arrays were fabricated using an autocatalytic system with dual reducing agents, H2 PO2 − and N2 H4 . In our previous study, we investigated whether CoNiP films could be deposited directly on Cu underlayers with high perpendicular magnetic coercivities at the initial deposition stage by the electroless deposition process [12]. The perpendicular magnetic coercivities of the films were more than 3000 Oe at the initial deposition stage and the thicknesses were lower than 80 nm. In particular, high perpendicular
9576
T. Ouchi et al. / Electrochimica Acta 56 (2011) 9575–9580
Table 1 Condition and bath compositions for electroless deposition and measurements of polarization curves. Concentration (mmol L−1 )
Chemicals
NaH2 PO2 ·H2 O N2 H4 (NH4 )2 SO4 NiSO4 ·6H2 O CoSO4 ·7H2 O pH (adjusted with NH4 OH) Temperature (◦ C)
A
B
C
D
E
100 50 250 16.3 8.7 8.6 50
100 – 250 – – 8.6 50
– 50 250 – – 8.6 50
100 50 250 – – 8.6 50
– – 250 16.3 8.7 8.6 50
magnetic coercivity was obtained even in films with a thickness of 25 nm. In the present study, we analyzed the deposition mechanism of the electroless deposition process on Cu underlayers with dual reducing agents, H2 PO2 − and N2 H4 , and fabricated CoNiP nanodot arrays with high magnetic coercivities, when the heights of the nanodot arrays were less than 50 nm. 2. Experimental A Cr adhesion layer and a Cu underlayer were sputter-deposited on an n-type Si(1 0 0) substrate. The substrate was constructed of Cu (20 nm)/Cr (5 nm)/Si. A UV-curable resin (TR-21, Toyo Gosei Co., Ltd.) was coated on the substrate. Next, patterns were formed by pressing a quartz mold with pillar-arrayed structures (Toppan Printing Co., Ltd.) onto the substrate. The diameters of the pillars and pitches between the pillars were 150 nm and 300 nm, respectively. The resin was cured using ultraviolet irradiation. After the quartz mold was removed from the cured resin, the residual layers at the bottom of the pores were removed using O2 plasma. After the nanopatterned substrate was cleaned by 10 wt% H2 SO4 solution, CoNiP was electroless deposited from the bath optimized in our previous study [12]. The typical electroless deposition condition and bath composition are listed in Table 1 (concentration of column A). The pH value of the bath was adjusted to 8.6 using NH4 OH. The bath temperature was 50 ◦ C. The schematic model of the procedure for the fabrication of CoNiP nanodot arrays is shown in Fig. 1. Polarization curves of the baths listed in Table 1 (concentrations of columns B–E) were measured by an electrochemical measurement system (HZ5000, Hokuto Denko) with a scan rate of 10 mV s−1 . Bath B is an electroless deposition bath without metal ions and N2 H4 . Bath C is an electroless deposition bath without metal ions and NaH2 PO2 ·H2 O. Bath D is an electroless deposition bath without metal ions. Bath E is an electroless deposition bath without reducing agents. The pH and temperature of the baths were the same as those used for electroless deposition. The working electrodes were Cu (100 nm)/Cr (5 nm)/Si substrates with 0 s or 180 s of the CoNiP electroless deposition. The thickness of the CoNiP film with 180 s deposition was approximately 30 nm. The surface areas of the working electrodes were 0.071 cm2 . A Ag/AgCl reference electrode was used. In order to exclude the influence of the O2 reduction reaction, N2 gas was bubbled through the solution until just before the measurements.
The surface morphology was observed using a field emission scanning electron microscope (FE-SEM, S-4800, Hitachi). The microstructure of the sample was observed using a field emission transmission electron microscope (FE-TEM, JEM-2100F, JEOL). The crystal orientation of the CoNiP film was measured using an X-ray diffractometer (XRD, RINT-TTR III, Rigaku) with Cu-K␣ radiation. The magnetic properties of the CoNiP films and nanodot arrays were evaluated by using a superconducting quantum interference device (SQUID). 3. Results and discussion In our previous study, we optimized the electroless deposition condition and bath composition by changing the pH of the bath and the concentrations of the chemicals [12]. As a consequence of these optimizations, an autocatalytic electroless deposition process with N2 H4 and H2 PO2 − as the one listed in Table 1 (column A) was developed. Fig. 2 shows the polarization curves of various baths listed in Table 1 (columns B–E) on Cu (100 nm)/Cr (5 nm)/Si electrodes with 0 s (a) and 180 s (b) CoNiP deposition. The polarization curves (i), (ii), (iii), and (iv) were obtained in the baths listed in Table 1—columns B–E, respectively. In Fig. 2a, the rest potentials on the Cu surfaces in baths B, C, D, and E were −0.29, −0.41, −0.46, and −0.33 V (vs. Ag/AgCl), respectively. The anodic polarization curve of H2 PO2 − oxidation on the Cu surface in bath B did not intersect with the cathodic polarization curve of CoNi reduction on the Cu surface in bath E. Ohno explained that the anodic current with the Cu electrode surface at the potential around −0.2 V (vs. Ag/AgCl) is originated from the dissolution of Cu, rather than oxidation of H2 PO2 − [10]. This suggests that the metal ions will not be reduced electrolessly to deposit on the Cu surface, only with H2 PO2 − as reducing agent. On the other hand, the anodic polarization curve of N2 H4 oxidation on the Cu surface in bath C and the anodic polarization curve of oxidation of the dual reducing agents on the Cu surface in bath D intersected with the cathodic polarization curve of CoNi reduction on the Cu surface in bath E. That is, the metals were deposited directly on the Cu surface by the oxidation of N2 H4 . In Fig. 2b, the rest potentials of the electrodes in baths B–E were −0.81, −0.71, −0.83, and −0.55 V (vs. Ag/AgCl), respectively. The polarization curve of H2 PO2 − oxidation on the CoNiP surface intersected with the curve of the reduction reaction of metal ions on CoNiP surface. The polarization curve of N2 H4 oxidation on the CoNiP surface
Fig. 1. Schematic model of procedure for fabricating CoNiP nanodot arrays using electroless deposition and UV-nanoimprint lithography.
T. Ouchi et al. / Electrochimica Acta 56 (2011) 9575–9580
9577
Fig. 3. Magnetic coercivities of CoNiP films with various thicknesses: perpendicular direction (circle plots) and in-plane direction (square plots).
Fig. 2. Polarization curves on Cu (100 nm)/Cr (5 nm)/Si substrates with (a) 0 s electroless deposition and (b) 180 s electroless deposition in various baths: Table 1 – concentration B, i; Table 1 – concentration C, ii; Table 1 – concentration D, iii; Table 1 – concentration E, iv.
also intersected with the curve of the reduction reaction of metal ions on the CoNiP surface. These results indicated that CoNiP was deposited on the CoNiP surface by the oxidation of H2 PO2 − and N2 H4 .
This electroless deposition process was analyzed in our previous study by characterization of the deposited CoNiP films with various deposition durations using X-ray photoelectron spectroscopy (XPS) [12]. In the depth profiles of the CoNiP films, the peaks of the Co–P and Ni–P alloys were observed in the P2p3/2 spectra as a result of codeposition reactions with. The peaks of the Co–P and Ni–P alloys were results of the codeposition reactions with H2 PO2 − . However, those peaks were not seen at the initial deposition stage. From this result, it was suggested that, at the initial deposition stage, the Co and Ni were reduced mainly by the oxidation of N2 H4 . These results suggest that the electroless deposition process proceeds in the following two steps: initially, the oxidation reaction of N2 H4 occurred on the Cu surface, which deposited CoNi; next the oxidation of H2 PO2 − occurred increasingly on the initial CoNi deposits, which formed the CoNiP deposit. Such a deposition process using N2 H4 with other reducing agent was reported by some researchers [13,14]. Next, we investigated the origins of high perpendicular magnetic coercivities of the films in the initial deposition stage by characterizing the CoNiP films with various thicknesses.
Fig. 4. (a) Cross-sectional bright field TEM image of CoNiP film, (b) composition profiles of each element measured by EDX on the black line in the bright field TEM image, (c) [Co]/[Ni] of the film as the function of the thickness. The thickness and perpendicular coercivity of the CoNiP film were 30 nm and 3040 Oe.
9578
T. Ouchi et al. / Electrochimica Acta 56 (2011) 9575–9580
Fig. 5. (a) Highly magnified cross-sectional TEM image and (b) electron diffraction image of CoNiP film of thickness 30 nm. The perpendicular coercivity was 3040 Oe. The electron diffraction image was observed from the [2–1–10] direction.
Fig. 3 shows the thickness dependence of perpendicular and in-plane magnetic coercivities of CoNiP films. The perpendicular magnetic coercivities of the films increased with an increase in the thickness from 5 nm to 30 nm, and a perpendicular magnetic coercivity as high as 3000 Oe was obtained even with a thickness of 30 nm. The perpendicular magnetic coercivities of the films thicker than 30 nm were steady. On the other hand, the in-plane magnetic coercivities of the films were lower than their corresponding perpendicular magnetic coercivities, for each thickness. The in-plane magnetic coercivity for a thickness of 30 nm was 700 Oe. These results indicate that electroless deposited CoNiP films showed perpendicular magnetic anisotropy from the initial deposition stage. Fig. 4 shows the cross-sectional TEM image of the CoNiP film at the deposition duration of 180 s with a magnetic coercivity of 3040 Oe, a, the composition profiles measured by energy dispersive X-ray spectrometry (EDX), b, and the ratio of [Co]/[Ni] as a function of thickness, c. In Fig. 4b, the composition of the CoNiP film on the black line in Fig. 4a from the Cr adhesion layer to the CoNiP layer is shown. Fig. 4c shows the ratio of [Co]/[Ni] calculated from the data in Fig. 4b. From Fig. 4, in the initial deposition stage, the Co composition was high, and it then decreased with an increase in the thickness. Additionally, we also observed the preferential deposition of Co to Ni at the initial deposition stage by using XPS and Auger electron spectroscopy in our previous study [12]. In the case of electroless deposited CoNiP films, Co was the origin of the magnetic properties [15,16]. Thus, it was suggested that preferential deposition of Co onto Ni in the initial deposition stage improved the magnetic properties of the films. Fig. 5 shows the structure of a CoNiP film with a thickness of 30 nm: (a) a high-resolution cross-sectional TEM image and (b) electron diffraction. The crystal lattice of CoNiP grown from the Cu underlayer was clearly observed in Fig. 5a. This indicated that CoNiP was deposited on the Cu underlayer with high crystallinity. The lattice spacing of the Cu and CoNiP layers were 0.208 and 0.202 nm, respectively. The lattice spacing of the CoNiP was almost the same as that of hcp-Co(0 0 0 2). Electron diffraction at the initial deposition of CoNiP on the Cu underlayer showed hcp-CoNiP(0 0 2) to be highly oriented parallel to the fcc-Cu(1 1 1) orientation. In order to confirm the crystal orientation of the CoNiP films deposited with various durations, crystal orientations of the films with various thicknesses were examined by XRD, as shown in Fig. 6. The peaks at 2Â = 43.3◦ and at 2Â = 74.2◦ were fcc(1 1 1) and fcc(2 2 0) of the Cu underlayers, respectively. The peaks at 2Â = 41.8◦ and at
2Â = 76.0◦ were hcp(1 0 0) and hcp(1 1 0) of the CoNiP films, respectively. The intensities of the Cu peaks decreased with an increase in the thickness of the CoNiP films. However, high intensities of the CoNiP peaks were observed from a thickness of 10 nm (Fig. 6d). These peaks appear at a thickness of 5 nm (Fig. 6e) as well. These results indicate that hcp-CoNiP(0 0 2) was highly oriented in the perpendicular direction from the initial deposition stage. From these results, the origin of the high perpendicular magnetic coercivities of the CoNiP films in the initial deposition stage could be explained as follows. In the initial deposition stage, the oxidation reaction of N2 H4 occurred primarily on the Cu surface to form CoNi with high purity and crystallinity. At this initial deposition, the Co composition was high. Since Co dominates the overall magnetic properties of the film, the Co rich condition at the initial deposition with high crystal orientation on the Cu underlayer induced perpendicular magnetic coercivity. Next, the oxidation reaction of H2 PO2 − proceeds at the surface of the deposited CoNi, which induces magnetic isolation between magnetic domains by the formation of a Ni–P non-magnetic region [15,16]. Consequently, this process forms CoNiP films with high perpendicular magnetic coercivities in the initial deposition stage.
Fig. 6. XRD spectra of CoNiP films with various thicknesses: (a) 25 nm, (b) 20 nm, (c) 15 nm, (d) 10 nm, (e) 5 nm and (f) 0 nm. The angle of X-ray incidence was 0.4◦ .
T. Ouchi et al. / Electrochimica Acta 56 (2011) 9575–9580
9579
Fig. 7. SEM images and perpendicular M–H loop of CoNiP nanodot arrays with diameters of 150 nm, pitches between dots of 300 nm, and heights of 50 nm: (a) top view of SEM image, (b) cross-sectional view of SEM image, (c) highly magnified cross-sectional view of SEM image, and (d) M–H loops.
Using this electroless deposition condition, CoNiP nanodot arrays were fabricated. Nanopatterns were formed on the substrates by UV-nanoimprint lithography, and CoNiP was electroless deposited into these pores. Using a Pd activation process, CoNiP was deposited not only on the bottom of the pores but also on the UV-curable resin. However, by using the autocatalytic electroless deposition process applied in this study, the oxidation reaction of N2 H4 was expected to occur only on the Cu surface that forms the initial layers; subsequently, CoNiP was expected to be deposited only on the bottom of the pores. Fig. 7 shows representative SEM images (a–c) and a perpendicular M–H loop (d) of CoNiP nanodot arrays with diameters of 150 nm, pitches between dots of 300 nm, and heights of 50 nm fabricated by the autocatalytic electroless deposition process. In the top view (Fig. 7a), the CoNiP nanodot arrays were clearly observed with no defect. Furthermore, it was confirmed that CoNiP was deposited only on the Cu surface as shown in the cross-sectional SEM image of the CoNiP nanodots (Fig. 7b). In addition, a high perpendicular magnetic coercivity of 3100 Oe was observed in Fig. 7c. These results indicated that this autocatalytic electroless deposition process is applicable for area selective formation of magnetic nanostructures with high perpendicular magnetic coercivities without any anomalous deposition.
4. Conclusions The use of electroless deposition for the fabrication of CoNiP nanodot arrays shorter than 50 nm and with high perpendicular magnetic coercivities was investigated. In order to achieve high magnetic coercivities for CoNiP alloys in the initial stage, we applied an fcc-Cu(1 1 1) underlayer and autocatalytic electroless deposition
process with dual reducing agents, H2 PO2 − and N2 H4 . Owing to the optimization of the electroless deposition condition and bath composition, we obtained CoNiP films which had perpendicular magnetic coercivities of more than 3000 Oe with a thickness of less than 30 nm. In order to elucidate the origin of high magnetic coercivity from the initial deposition stage, we analyzed CoNiP electroless deposition. It was suggested that initial deposition on the Cu underlayer by the oxidation reaction of N2 H4 induced high crystallinity of CoNiP film, which improved perpendicular magnetic coercivities. CoNiP nanodot arrays with high magnetic properties in large areas without any anomalous deposition were fabricated by this electroless deposition process. These results demonstrate that electroless deposition can be applied to area selective formation of nano-structures with high crystallinities. Acknowledgments This work was carried out at the Center for Practical Chemical Wisdom in the Global-COE Program, MEXT, Japan, and was financially supported in part by the Storage Research Consortium (SRC) Japan, Grant-in-Aid for Scientific Research (B), and by Waseda University Grant for Special Research Projects. References [1] D.L. Graham, H.A. Ferreira, P.P. Freitas, Trends Biotechnol. 22 (2004) 455. [2] N.V. Myunga, D.Y. Parkb, B.Y. Yoob, P.T.A. Sumodjo, J. Magn. Magn. Mater. 265 (2003) 189. [3] X.T. Tang, G.C. Wang, M. Shima, J. Magn. Magn. Mater. 309 (2007) 188. [4] L. Gao, L.P. Yue, T. Yokota, R. Skomski, S.H. Liou, H. Takahoshi, H. Saito, S. Ishio, IEEE Trans. Magn. 16 (2004) 2194. [5] H.J. Richter, A.Y. Dobin, O. Heinonen, K.Z. Gao, R.J.M.v.d. Veerdonk, R.T. Lynch, J. Xue, D. Weller, P. Asselin, M.F. Erden, R.M. Brockie, IEEE Trans. Magn. 42 (2006) 2255. [6] T. Ouchi, Y. Arikawa, T. Homma, J. Magn. Magn. Mater. 320 (2008) 3104.
9580
T. Ouchi et al. / Electrochimica Acta 56 (2011) 9575–9580
[7] T. Ouchi, Y. Arikawa, J. Mizuno, S. Shoji, T. Homma, IEEE Trans. Magn. 46 (2010) 2224. [8] T. Ouchi, Y. Arikawa, Y. Konishi, T. Homma, Electrochim. Acta 55 (2010) 8081. [9] J. Kawaji, F. Kitaizumi, H. Oikawa, D. Niwa, T. Homma, T. Osaka, J. Magn. Magn. Mater. 287 (2005) 245. [10] I. Ohno, Mater. Sci. Eng. A – Struct. Mater. Prop. Microstruct. Process. 146 (1991) 33. [11] T. Yokoshima, Y. Yamaji, K. Kikuchi, H. Nakagawa, M. Aoyagi, J. Electrochem. Soc. 157 (2010) D65.
[12] T. Ouchi, Y. Arikawa, T. Kuno, T. Homma, Electrochem. Soc. Trans. 25 (2010) 125. [13] C.D. Iacovangelo, J. Electrochem. Soc. 138 (1991) 976. [14] Y. Saito, K. Tashiro, I. Koiwa, H. Honma, J. Jpn. Inst. Electron. Packaging 10 (2007) 62. [15] T. Homma, T. Osaka, Y. Yamazaki, T. Namikawa, Scripta Metall. Mater. 33 (1995) 1569. [16] T. Homma, Y. Sezai, T. Osaka, Y. Maeda, D.M. Donnet, J. Magn. Magn. Mater. 173 (1997) 314.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
CoNiP electroless deposition process for fabricating ferromagnetic nanodot arrays Takanari Ouchi, Naofumi Shimano, Takayuki Homma ∗ Department of Applied Chemistry, Waseda University, 3-4-1, Okubo, Shinjuku, Tokyo 169-8555, Japan
a r t i c l e
i n f o
Article history: Received 1 January 2011 Received in revised form 20 April 2011 Accepted 21 April 2011 Available online 3 May 2011 Keywords: Electroless deposition Nanodot arrays Nanoimprint lithography CoNiP alloy
a b s t r a c t An electroless deposition process for fabricating CoNiP nanodot arrays (less than 50 nm in height) with high magnetic coercivities was investigated. To fabricate such nanostructures, we improved the crystallinity of the CoNiP deposits in the initial deposition stage by applying an fcc-Cu(1 1 1) underlayer with low lattice mismatch to hcp-Co(0 0 0 2), and an autocatalytic electroless deposition process at the Cu surface was carried out by using dual reducing agents, H2 PO2 − and N2 H4 . CoNiP films demonstrated high perpendicular magnetic coercivities in the initial deposition stage since the highly crystalline hcp(0 0 0 2) CoNiP layers were grown parallel to the Cu underlayers. Nanopatterned substrates were formed by UVnanoimprint lithography. CoNiP was electroless deposited on the nanopatterned substrates. As a result, CoNiP was deposited selectively from the bottom of the nanopores with few defects in a large area. Perpendicular coercivities higher than 3000 Oe were obtained for nanodots even with heights of 50 nm. Thus, an electroless deposition process that can be used to form nanostructures with high crystallinities in the initial stage without any anomalous deposition was demonstrated. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Magnetic nanostructures with high magnetic coercivities are required for various advanced applications such as microelectromechanical systems or nano-electromechanical systems, sensing devices, and magnetic recording devices [1–5]. Physical processes are generally employed to fabricate such nanostructures; however, it is difficult to form various nanoscale shapes on the selected area of devices using physical processes. In contrast, electrochemical deposition processes can be used for area selective formation of nanostructures or microstructures with various patterns. In our previous study, we investigated the fabrication of CoPt nanodot arrays with high magnetic coercivities using the electrodeposition method [6–8]. We also demonstrated the fabrication of CoNiP nanodot arrays using electroless deposition, as an alternative technique [9]. Electroless deposition results in conformal depositions of metals with uniform structures over a large area. In the electroless deposition process, metals can only be deposited at the place where catalytic activities of the oxidation reactions of reducing agents occur [10]. Thus, in order to deposit metals only at the selected areas, it is important to control the catalytic activities of the oxidation reaction on the samples by changing the reducing agents and the surface reactivities of the samples. In our previous study, CoNiP nanodot arrays were fabricated on SiO2 /Si substrates by electroless deposition; the catalytic activities of the substrates
∗ Corresponding author. E-mail address: [email protected] (T. Homma). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.04.085
were controlled by a Pd activation process. However, the perpendicular magnetic coercivities of the CoNiP nanodot arrays were less than 800 Oe. This paper discusses the use of the electroless deposition process for fabricating CoNiP nanodot arrays less than 50 nm in height and with high magnetic coercivities. In order to achieve CoNiP nanodot arrays with high magnetic properties, the deposits should have high crystallinity during the initial deposition stage. Therefore, an fccCu(1 1 1) underlayer, with low lattice mismatch (2.3%) compared to that of hcp-Co(0 0 0 2), was used. In fact, in our previous study, highly crystalline hcp(0 0 0 2) CoPt nanodots were grown parallel to fcc-Cu(1 1 1) by using electrodeposition process [7]. The problem with the application of the fcc-Cu(1 1 1) underlayer in electroless deposition of CoNiP is that H2 PO2 − , which is used as a reducing agent in the general electroless deposition of CoNiP, is not oxidized to work as a reducing agent on the Cu surface. Therefore, catalytic activation processes such as immersion of the substrates into diluted PdCl2 solution were required. However, it is difficult to obtain area selective depositions on nanopatterns or micropatterns by using such activation processes, because anomalous depositions occur on the resists or insulators [11]. Thus, in this study, CoNiP nanodot arrays were fabricated using an autocatalytic system with dual reducing agents, H2 PO2 − and N2 H4 . In our previous study, we investigated whether CoNiP films could be deposited directly on Cu underlayers with high perpendicular magnetic coercivities at the initial deposition stage by the electroless deposition process [12]. The perpendicular magnetic coercivities of the films were more than 3000 Oe at the initial deposition stage and the thicknesses were lower than 80 nm. In particular, high perpendicular
9576
T. Ouchi et al. / Electrochimica Acta 56 (2011) 9575–9580
Table 1 Condition and bath compositions for electroless deposition and measurements of polarization curves. Concentration (mmol L−1 )
Chemicals
NaH2 PO2 ·H2 O N2 H4 (NH4 )2 SO4 NiSO4 ·6H2 O CoSO4 ·7H2 O pH (adjusted with NH4 OH) Temperature (◦ C)
A
B
C
D
E
100 50 250 16.3 8.7 8.6 50
100 – 250 – – 8.6 50
– 50 250 – – 8.6 50
100 50 250 – – 8.6 50
– – 250 16.3 8.7 8.6 50
magnetic coercivity was obtained even in films with a thickness of 25 nm. In the present study, we analyzed the deposition mechanism of the electroless deposition process on Cu underlayers with dual reducing agents, H2 PO2 − and N2 H4 , and fabricated CoNiP nanodot arrays with high magnetic coercivities, when the heights of the nanodot arrays were less than 50 nm. 2. Experimental A Cr adhesion layer and a Cu underlayer were sputter-deposited on an n-type Si(1 0 0) substrate. The substrate was constructed of Cu (20 nm)/Cr (5 nm)/Si. A UV-curable resin (TR-21, Toyo Gosei Co., Ltd.) was coated on the substrate. Next, patterns were formed by pressing a quartz mold with pillar-arrayed structures (Toppan Printing Co., Ltd.) onto the substrate. The diameters of the pillars and pitches between the pillars were 150 nm and 300 nm, respectively. The resin was cured using ultraviolet irradiation. After the quartz mold was removed from the cured resin, the residual layers at the bottom of the pores were removed using O2 plasma. After the nanopatterned substrate was cleaned by 10 wt% H2 SO4 solution, CoNiP was electroless deposited from the bath optimized in our previous study [12]. The typical electroless deposition condition and bath composition are listed in Table 1 (concentration of column A). The pH value of the bath was adjusted to 8.6 using NH4 OH. The bath temperature was 50 ◦ C. The schematic model of the procedure for the fabrication of CoNiP nanodot arrays is shown in Fig. 1. Polarization curves of the baths listed in Table 1 (concentrations of columns B–E) were measured by an electrochemical measurement system (HZ5000, Hokuto Denko) with a scan rate of 10 mV s−1 . Bath B is an electroless deposition bath without metal ions and N2 H4 . Bath C is an electroless deposition bath without metal ions and NaH2 PO2 ·H2 O. Bath D is an electroless deposition bath without metal ions. Bath E is an electroless deposition bath without reducing agents. The pH and temperature of the baths were the same as those used for electroless deposition. The working electrodes were Cu (100 nm)/Cr (5 nm)/Si substrates with 0 s or 180 s of the CoNiP electroless deposition. The thickness of the CoNiP film with 180 s deposition was approximately 30 nm. The surface areas of the working electrodes were 0.071 cm2 . A Ag/AgCl reference electrode was used. In order to exclude the influence of the O2 reduction reaction, N2 gas was bubbled through the solution until just before the measurements.
The surface morphology was observed using a field emission scanning electron microscope (FE-SEM, S-4800, Hitachi). The microstructure of the sample was observed using a field emission transmission electron microscope (FE-TEM, JEM-2100F, JEOL). The crystal orientation of the CoNiP film was measured using an X-ray diffractometer (XRD, RINT-TTR III, Rigaku) with Cu-K␣ radiation. The magnetic properties of the CoNiP films and nanodot arrays were evaluated by using a superconducting quantum interference device (SQUID). 3. Results and discussion In our previous study, we optimized the electroless deposition condition and bath composition by changing the pH of the bath and the concentrations of the chemicals [12]. As a consequence of these optimizations, an autocatalytic electroless deposition process with N2 H4 and H2 PO2 − as the one listed in Table 1 (column A) was developed. Fig. 2 shows the polarization curves of various baths listed in Table 1 (columns B–E) on Cu (100 nm)/Cr (5 nm)/Si electrodes with 0 s (a) and 180 s (b) CoNiP deposition. The polarization curves (i), (ii), (iii), and (iv) were obtained in the baths listed in Table 1—columns B–E, respectively. In Fig. 2a, the rest potentials on the Cu surfaces in baths B, C, D, and E were −0.29, −0.41, −0.46, and −0.33 V (vs. Ag/AgCl), respectively. The anodic polarization curve of H2 PO2 − oxidation on the Cu surface in bath B did not intersect with the cathodic polarization curve of CoNi reduction on the Cu surface in bath E. Ohno explained that the anodic current with the Cu electrode surface at the potential around −0.2 V (vs. Ag/AgCl) is originated from the dissolution of Cu, rather than oxidation of H2 PO2 − [10]. This suggests that the metal ions will not be reduced electrolessly to deposit on the Cu surface, only with H2 PO2 − as reducing agent. On the other hand, the anodic polarization curve of N2 H4 oxidation on the Cu surface in bath C and the anodic polarization curve of oxidation of the dual reducing agents on the Cu surface in bath D intersected with the cathodic polarization curve of CoNi reduction on the Cu surface in bath E. That is, the metals were deposited directly on the Cu surface by the oxidation of N2 H4 . In Fig. 2b, the rest potentials of the electrodes in baths B–E were −0.81, −0.71, −0.83, and −0.55 V (vs. Ag/AgCl), respectively. The polarization curve of H2 PO2 − oxidation on the CoNiP surface intersected with the curve of the reduction reaction of metal ions on CoNiP surface. The polarization curve of N2 H4 oxidation on the CoNiP surface
Fig. 1. Schematic model of procedure for fabricating CoNiP nanodot arrays using electroless deposition and UV-nanoimprint lithography.
T. Ouchi et al. / Electrochimica Acta 56 (2011) 9575–9580
9577
Fig. 3. Magnetic coercivities of CoNiP films with various thicknesses: perpendicular direction (circle plots) and in-plane direction (square plots).
Fig. 2. Polarization curves on Cu (100 nm)/Cr (5 nm)/Si substrates with (a) 0 s electroless deposition and (b) 180 s electroless deposition in various baths: Table 1 – concentration B, i; Table 1 – concentration C, ii; Table 1 – concentration D, iii; Table 1 – concentration E, iv.
also intersected with the curve of the reduction reaction of metal ions on the CoNiP surface. These results indicated that CoNiP was deposited on the CoNiP surface by the oxidation of H2 PO2 − and N2 H4 .
This electroless deposition process was analyzed in our previous study by characterization of the deposited CoNiP films with various deposition durations using X-ray photoelectron spectroscopy (XPS) [12]. In the depth profiles of the CoNiP films, the peaks of the Co–P and Ni–P alloys were observed in the P2p3/2 spectra as a result of codeposition reactions with. The peaks of the Co–P and Ni–P alloys were results of the codeposition reactions with H2 PO2 − . However, those peaks were not seen at the initial deposition stage. From this result, it was suggested that, at the initial deposition stage, the Co and Ni were reduced mainly by the oxidation of N2 H4 . These results suggest that the electroless deposition process proceeds in the following two steps: initially, the oxidation reaction of N2 H4 occurred on the Cu surface, which deposited CoNi; next the oxidation of H2 PO2 − occurred increasingly on the initial CoNi deposits, which formed the CoNiP deposit. Such a deposition process using N2 H4 with other reducing agent was reported by some researchers [13,14]. Next, we investigated the origins of high perpendicular magnetic coercivities of the films in the initial deposition stage by characterizing the CoNiP films with various thicknesses.
Fig. 4. (a) Cross-sectional bright field TEM image of CoNiP film, (b) composition profiles of each element measured by EDX on the black line in the bright field TEM image, (c) [Co]/[Ni] of the film as the function of the thickness. The thickness and perpendicular coercivity of the CoNiP film were 30 nm and 3040 Oe.
9578
T. Ouchi et al. / Electrochimica Acta 56 (2011) 9575–9580
Fig. 5. (a) Highly magnified cross-sectional TEM image and (b) electron diffraction image of CoNiP film of thickness 30 nm. The perpendicular coercivity was 3040 Oe. The electron diffraction image was observed from the [2–1–10] direction.
Fig. 3 shows the thickness dependence of perpendicular and in-plane magnetic coercivities of CoNiP films. The perpendicular magnetic coercivities of the films increased with an increase in the thickness from 5 nm to 30 nm, and a perpendicular magnetic coercivity as high as 3000 Oe was obtained even with a thickness of 30 nm. The perpendicular magnetic coercivities of the films thicker than 30 nm were steady. On the other hand, the in-plane magnetic coercivities of the films were lower than their corresponding perpendicular magnetic coercivities, for each thickness. The in-plane magnetic coercivity for a thickness of 30 nm was 700 Oe. These results indicate that electroless deposited CoNiP films showed perpendicular magnetic anisotropy from the initial deposition stage. Fig. 4 shows the cross-sectional TEM image of the CoNiP film at the deposition duration of 180 s with a magnetic coercivity of 3040 Oe, a, the composition profiles measured by energy dispersive X-ray spectrometry (EDX), b, and the ratio of [Co]/[Ni] as a function of thickness, c. In Fig. 4b, the composition of the CoNiP film on the black line in Fig. 4a from the Cr adhesion layer to the CoNiP layer is shown. Fig. 4c shows the ratio of [Co]/[Ni] calculated from the data in Fig. 4b. From Fig. 4, in the initial deposition stage, the Co composition was high, and it then decreased with an increase in the thickness. Additionally, we also observed the preferential deposition of Co to Ni at the initial deposition stage by using XPS and Auger electron spectroscopy in our previous study [12]. In the case of electroless deposited CoNiP films, Co was the origin of the magnetic properties [15,16]. Thus, it was suggested that preferential deposition of Co onto Ni in the initial deposition stage improved the magnetic properties of the films. Fig. 5 shows the structure of a CoNiP film with a thickness of 30 nm: (a) a high-resolution cross-sectional TEM image and (b) electron diffraction. The crystal lattice of CoNiP grown from the Cu underlayer was clearly observed in Fig. 5a. This indicated that CoNiP was deposited on the Cu underlayer with high crystallinity. The lattice spacing of the Cu and CoNiP layers were 0.208 and 0.202 nm, respectively. The lattice spacing of the CoNiP was almost the same as that of hcp-Co(0 0 0 2). Electron diffraction at the initial deposition of CoNiP on the Cu underlayer showed hcp-CoNiP(0 0 2) to be highly oriented parallel to the fcc-Cu(1 1 1) orientation. In order to confirm the crystal orientation of the CoNiP films deposited with various durations, crystal orientations of the films with various thicknesses were examined by XRD, as shown in Fig. 6. The peaks at 2Â = 43.3◦ and at 2Â = 74.2◦ were fcc(1 1 1) and fcc(2 2 0) of the Cu underlayers, respectively. The peaks at 2Â = 41.8◦ and at
2Â = 76.0◦ were hcp(1 0 0) and hcp(1 1 0) of the CoNiP films, respectively. The intensities of the Cu peaks decreased with an increase in the thickness of the CoNiP films. However, high intensities of the CoNiP peaks were observed from a thickness of 10 nm (Fig. 6d). These peaks appear at a thickness of 5 nm (Fig. 6e) as well. These results indicate that hcp-CoNiP(0 0 2) was highly oriented in the perpendicular direction from the initial deposition stage. From these results, the origin of the high perpendicular magnetic coercivities of the CoNiP films in the initial deposition stage could be explained as follows. In the initial deposition stage, the oxidation reaction of N2 H4 occurred primarily on the Cu surface to form CoNi with high purity and crystallinity. At this initial deposition, the Co composition was high. Since Co dominates the overall magnetic properties of the film, the Co rich condition at the initial deposition with high crystal orientation on the Cu underlayer induced perpendicular magnetic coercivity. Next, the oxidation reaction of H2 PO2 − proceeds at the surface of the deposited CoNi, which induces magnetic isolation between magnetic domains by the formation of a Ni–P non-magnetic region [15,16]. Consequently, this process forms CoNiP films with high perpendicular magnetic coercivities in the initial deposition stage.
Fig. 6. XRD spectra of CoNiP films with various thicknesses: (a) 25 nm, (b) 20 nm, (c) 15 nm, (d) 10 nm, (e) 5 nm and (f) 0 nm. The angle of X-ray incidence was 0.4◦ .
T. Ouchi et al. / Electrochimica Acta 56 (2011) 9575–9580
9579
Fig. 7. SEM images and perpendicular M–H loop of CoNiP nanodot arrays with diameters of 150 nm, pitches between dots of 300 nm, and heights of 50 nm: (a) top view of SEM image, (b) cross-sectional view of SEM image, (c) highly magnified cross-sectional view of SEM image, and (d) M–H loops.
Using this electroless deposition condition, CoNiP nanodot arrays were fabricated. Nanopatterns were formed on the substrates by UV-nanoimprint lithography, and CoNiP was electroless deposited into these pores. Using a Pd activation process, CoNiP was deposited not only on the bottom of the pores but also on the UV-curable resin. However, by using the autocatalytic electroless deposition process applied in this study, the oxidation reaction of N2 H4 was expected to occur only on the Cu surface that forms the initial layers; subsequently, CoNiP was expected to be deposited only on the bottom of the pores. Fig. 7 shows representative SEM images (a–c) and a perpendicular M–H loop (d) of CoNiP nanodot arrays with diameters of 150 nm, pitches between dots of 300 nm, and heights of 50 nm fabricated by the autocatalytic electroless deposition process. In the top view (Fig. 7a), the CoNiP nanodot arrays were clearly observed with no defect. Furthermore, it was confirmed that CoNiP was deposited only on the Cu surface as shown in the cross-sectional SEM image of the CoNiP nanodots (Fig. 7b). In addition, a high perpendicular magnetic coercivity of 3100 Oe was observed in Fig. 7c. These results indicated that this autocatalytic electroless deposition process is applicable for area selective formation of magnetic nanostructures with high perpendicular magnetic coercivities without any anomalous deposition.
4. Conclusions The use of electroless deposition for the fabrication of CoNiP nanodot arrays shorter than 50 nm and with high perpendicular magnetic coercivities was investigated. In order to achieve high magnetic coercivities for CoNiP alloys in the initial stage, we applied an fcc-Cu(1 1 1) underlayer and autocatalytic electroless deposition
process with dual reducing agents, H2 PO2 − and N2 H4 . Owing to the optimization of the electroless deposition condition and bath composition, we obtained CoNiP films which had perpendicular magnetic coercivities of more than 3000 Oe with a thickness of less than 30 nm. In order to elucidate the origin of high magnetic coercivity from the initial deposition stage, we analyzed CoNiP electroless deposition. It was suggested that initial deposition on the Cu underlayer by the oxidation reaction of N2 H4 induced high crystallinity of CoNiP film, which improved perpendicular magnetic coercivities. CoNiP nanodot arrays with high magnetic properties in large areas without any anomalous deposition were fabricated by this electroless deposition process. These results demonstrate that electroless deposition can be applied to area selective formation of nano-structures with high crystallinities. Acknowledgments This work was carried out at the Center for Practical Chemical Wisdom in the Global-COE Program, MEXT, Japan, and was financially supported in part by the Storage Research Consortium (SRC) Japan, Grant-in-Aid for Scientific Research (B), and by Waseda University Grant for Special Research Projects. References [1] D.L. Graham, H.A. Ferreira, P.P. Freitas, Trends Biotechnol. 22 (2004) 455. [2] N.V. Myunga, D.Y. Parkb, B.Y. Yoob, P.T.A. Sumodjo, J. Magn. Magn. Mater. 265 (2003) 189. [3] X.T. Tang, G.C. Wang, M. Shima, J. Magn. Magn. Mater. 309 (2007) 188. [4] L. Gao, L.P. Yue, T. Yokota, R. Skomski, S.H. Liou, H. Takahoshi, H. Saito, S. Ishio, IEEE Trans. Magn. 16 (2004) 2194. [5] H.J. Richter, A.Y. Dobin, O. Heinonen, K.Z. Gao, R.J.M.v.d. Veerdonk, R.T. Lynch, J. Xue, D. Weller, P. Asselin, M.F. Erden, R.M. Brockie, IEEE Trans. Magn. 42 (2006) 2255. [6] T. Ouchi, Y. Arikawa, T. Homma, J. Magn. Magn. Mater. 320 (2008) 3104.
9580
T. Ouchi et al. / Electrochimica Acta 56 (2011) 9575–9580
[7] T. Ouchi, Y. Arikawa, J. Mizuno, S. Shoji, T. Homma, IEEE Trans. Magn. 46 (2010) 2224. [8] T. Ouchi, Y. Arikawa, Y. Konishi, T. Homma, Electrochim. Acta 55 (2010) 8081. [9] J. Kawaji, F. Kitaizumi, H. Oikawa, D. Niwa, T. Homma, T. Osaka, J. Magn. Magn. Mater. 287 (2005) 245. [10] I. Ohno, Mater. Sci. Eng. A – Struct. Mater. Prop. Microstruct. Process. 146 (1991) 33. [11] T. Yokoshima, Y. Yamaji, K. Kikuchi, H. Nakagawa, M. Aoyagi, J. Electrochem. Soc. 157 (2010) D65.
[12] T. Ouchi, Y. Arikawa, T. Kuno, T. Homma, Electrochem. Soc. Trans. 25 (2010) 125. [13] C.D. Iacovangelo, J. Electrochem. Soc. 138 (1991) 976. [14] Y. Saito, K. Tashiro, I. Koiwa, H. Honma, J. Jpn. Inst. Electron. Packaging 10 (2007) 62. [15] T. Homma, T. Osaka, Y. Yamazaki, T. Namikawa, Scripta Metall. Mater. 33 (1995) 1569. [16] T. Homma, Y. Sezai, T. Osaka, Y. Maeda, D.M. Donnet, J. Magn. Magn. Mater. 173 (1997) 314.