- Email: [email protected]
Co alloys and multilayers by organic additives
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 304 (2006) 60–63 www.elsevier.com/locate/jmmm
Property changes of electroplated Cu/Co alloys and multilayers by organic additives Kimin Honga,, Jungju Leea, Jinhan Leea, Young-Dong Kob, Jin-Seok Chungb, Jin-Gyu Kimc a
Department of Physics, Chungnam National University, 220 Gung Dong, Daejeon, 305-764, Korea b Department of Physics, Soongsil University, Seoul, 156-743, Korea c Division of Electron Microscopic Research, Korea Basic Science Institute, 52 Yeoeun-dong, Daejeon, 305-333, Korea Available online 28 February 2006
Abstract We investigated the change of magnetic properties of the electroplated Cu/Co alloys and multilayers caused by organic additives and high temperature annealing. When plated with a pure Cu/Co electrolyte, the alloy contained 25% of Cu and 75% of Co. The alloy was made of hcp-Co, fcc-Co and Cu(1 1 1) and was super-paramagnetic at room temperature. As we add a few organic additives in the plating electrolyte, the hcp-Co of the films disappeared. The organic additives contained in the electrolytes changed paramagnetic Cu/Co multilayers to ferromagnets. High-temperature thermal annealing increased coercivity due to the growth of the Co grains. r 2006 Elsevier B.V. All rights reserved. PACS: 75.50. y; 36.40.Cg; 75.60.Ej; 72.15.Gd; 81.15.Pq; 74.43.Qt Keywords: Magnetic alloys; Copper; Cobalt; Magnetic multilayers; Organic additives; Electroplating; Magnetism; Annealing
1. Introduction For decades, electroplating has been in use to fabricate various magnetic materials and devices. It has the advantage of changing material properties by using various plating chemistry and by adjusting the plating conditions. There were many attempts to apply the plating technique to the fabrication of new devices [1–4]. For many years, copper electroplating has been used in the semiconductor industry for fabrication of metallic interconnects. Gap-fill of dual damascene copper interconnects has successfully been obtained with the technique. One of the interesting features of the Cu interconnect process is that the plating chemistry utilizes organic materials or, so-called organic additives, in addition to the inorganic electrolytes. They are known to play an important role in achieving the super-fill process, where plating is faster at the recess and is slower at flat surfaces. Depending on the roles in baths, organic additives are categorized as accelerators and suppressors. An accelerator Corresponding author. Tel.: +82 42 821 5456; fax: +82 42 822 8011
E-mail address: [email protected] (K. Hong). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.02.008
is a catalyst having a large diffusion length and enhances the plating current density. A suppressor is a surfactant having a short diffusion length and prohibits plating [4]. Void-free super-fill and quality copper films are obtained by using the proper compositions and concentrations for the accelerators and the suppressors. In addition to enabling the super-fill process, the organic additives also alter the properties of the plated copper. Copper films plated with the inorganic electrolyte only usually have large grains. Additives alter the grain growth mechanism and change microscopic structures such as crystalline orientation, surface smoothness, and grain size [5]. In addition, additives are known to cause the so-called ‘‘self-annealing’’ effect where the Cu grains grow slowly even at room temperature after the deposition. It takes place only when the electrolytes contain specific additives. Later, it was discovered that rapid thermal annealing (RTA) had same effect but at higher growth rates [6–11]. Since the magnetic properties are closely related to the microscopic structures, we expect the organic additives would play a role in the determination of magnetic properties. In this work, we studied the change of magnetic properties caused by a few organic additives and the thermal annealing.
ARTICLE IN PRESS K. Hong et al. / Journal of Magnetism and Magnetic Materials 304 (2006) 60–63
2. Experiments Electrochemical properties of a few organic materials were investigated with a cyclic voltammetry (CV) system consisting of a potentiostat (273, EG&G), a rotating disk electrode (RDE), a counter electrode, and an Ag/AgCl reference electrode. In this system, an alternating voltage is applied on the RDE and metallic ions are alternately deposited on and stripped off the surface of the RDE. Plating current densities can be obtained from the resulting voltage–current characteristics or a voltammogram [12,13]. A kind of additives causing the increase of the current density are called as accelerators while other additives reducing the current density are called suppressors. Amongst a few organic additives we investigated in the Cu- and Co-electrolytes, Sulfopropyl disulfide sodium salt (SPSA) and Dimethyldithiocarbamic acid (DPSA) had significant effects and we were focussed on these two additives. Electrodeposition was carried out using a conventional three terminal method, i.e., with a working electrode, a Ag/ AgCl reference electrode, and a flat platinum plate as a counter electrode [12]. The working electrode was fabricated by physical vapor deposition of 150 nm thick Cu on Si(1 0 0). Root-mean-square (rms) surface roughness measured with an AFM (atomic force microscope) was 3 nm. A potentiostat (SI 1286, Solartron) was used as a power supply for the deposition. Plating voltage was 0.3 V for Cu and 0.95 V for Co, respectively. The plating electrolyte used in this experiment was made of 120 g/L of CoSO4 7H2O, 1.7 g/L of CuSO4 5H2O, and 50 ppm of chloride ions [1,3]. Alloys of Cu/Co were plated by applying 0.95 V and the multilayers were plated by alternately applying 0.3 V and 0.95 V. The thickness of each layer was adjusted by controlling the pulse widths.
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different from previous experiments on Cu/Co alloys prepared with vacuum deposition techniques, where only hcp-Co was observed at temperatures above 470 1C and fcc-Co was observed at room temperature [15,16]. It indicates that either electroplating in itself or the electrolyte composition we used altered the structure of Cu/Co alloy. Another interesting feature is that DPSA (center of Fig. 2) and SPSA (top of Fig. 2) removed hcp-Co and the alloys have only fcc-Co. Since fcc-Co is a stable structure at room temperature, we think that SPSA and DPSA enabled growth of Cu/Co alloys in a thermodynamically stable manner than with the pure electrolyte. Cu/Co multilayers were electroplated at various thicknesses and number of layers. Fig. 3(a) shows TEM pictures of [Cu(10 nm)/Co(42 nm)] 40 made with the electrolyte only. It shows varying layer thicknesses both in Cu- and Co-layers and the interfaces are not clear. However, as we add 0.5 mmol/L of SPSA, we could obtain improved thickness uniformity and better interfaces, as shown in Fig. 3(b). Similar results were obtained using DPSA. Since SPSA and DPSA reduce the grain size and improve the
3. Results and discussions Fig. 1 shows the change of current density by the two additives. In proportion to the concentration of each additive, the current density increases both in the Cu- and the Co-electrolytes. Note that DPSA in the Co-electrolyte has the largest effect. When we carried out similar tests with other polymers, we observed various behaviors including suppressing effects [14]. When we apply 0.95 V to the working electrode, we obtain Cu/Co alloys. EDX analysis of the alloys has shown that they had 75.2% of Co and 24.8% of Cu. As we add 0.5 mmol/L of SPSA, the Co content increased to 76.7%. When we add same amount of DPSA, the Co content was 78.3%. The changes are due to the accelerating effct of SPSA and DPSA on Co deposition. Since DPSA works as a stronger accelerator, the increase of Co content is larger. Fig. 2 is the X-ray diffraction (XRD) result showing the change of crystalline structures in 1 mm thick Cu/Co alloy films. Thin films plated with the pure electrolyte (bottom of Fig. 2) had hcp-Co and fcc-Co as well as Cu(1 1 1). This is
Fig. 1. Increase of current density by SPSA and DPSA in the electrolytes of Cu and Co, respectively.
SPSA
DPSA
Electrolyte Only
θ
Fig. 2. XRD results of Cu/Co alloy thin films.
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K. Hong et al. / Journal of Magnetism and Magnetic Materials 304 (2006) 60–63
Fig. 3. TEM pictures of Cu(bright)/Co(dark) multilayers: (a) plated with the pure electrolyte, and (b) with 5 mmol/L of SPSA added in the electrolyte.
Fig. 4. Effect of SPSA and DPSA on magnetic behavior of [Cu(1 nm)/ Co(1 nm)] 100 multilayers.
surface smoothness, better separation between Cu- and Colayers could be obtained [17]. Fig. 4 shows the magnetization of [Cu(1 nm)/ Co(1 nm)]x100 using a VSM (vibrating sample magnetometer) at room temperature, as we applied the magnetic field in the plane of the multilayers. The solid line was obtained from a film plated with the electrolyte only. It is a paramagnet and we think this sample does not have separate Cu- and Co-layers. Instead, it seems to have granular structures. A strikingly different behavior was obtained when we added SPSA and DPSA. Closed squares were obtained from a same structure under identical process except that the electrolyte contained 0.5 mmol/L of SPSA. Open circle was obtained similarly with 0.5 mmol/L DPSA. Compared to that prepared with the pure electrolyte, they clearly show ferromagnetism. One might think that the additives increased Co content, which, in turn, changed the magnetic property since the magnetic properties change in proportion to the concentration of Cu in the alloys [16]. However, as we have shown above, the additives increased the content of Co by less than 3%. At the high concentration regime of Co, this change does not have such a large impact on the magnetic properties of the alloy [15]. On the other hand, taking the TEM results into
consideration where the layers have higher thickness uniformity, we think that the additives separated the Cu/ Co interfaces distinctly and, therefore, the Co layers could have higher coercivity. As we apply the fields perpendicular to the planes, the multilayers prepared with the pure electrolyte showed similar paramagnetic behavior. On the other hand, multilayers made with the additives showed saturation magnetization at 10 kOe, as shown in the inset of Fig. 4. Comparing the demagnetizing field of Co thin film, 4pM17 kOe, the saturating field is lower. However, considering the Cu content in Co layers, we expect lower magnetization and reduced demagnetizing field. It is known that the magnetization of Co/Cu alloy decreases linearly in proportion to the Cu concentration and the relation is given as dM/dC 1.6 per added Cu, where C is the Cu concentration [15,18]. Considering 25% of Cu concentration and the resulting 40% reduction of the magnetization, the demagnetizing field 10 kOe agrees well with the prediction. Cu and Co are immiscible below 420 1C and turn to an alloy above the temperature. It should be interesting to compare the effect of the Cu grain growth by annealing caused by the additives with the alloying effect of Cu/Co at elevated temperatures. In order to observe the temperature effects, we annealed the multilayered thin films at 200, 400, 500, and 600 1C for 30 min using an RTA. Fig. 5 shows the effect of annealing on multilayers made with DPSA. The solid line has a magnetization behavior without thermal treatment. Open circles were obtained after annealing at 400 1C and closed squares were obtained after annealing at 600 1C. As can be seen in the figure, annealing increased the coercivity and showed a multi-domain behavior. The inset shows the increase of coercivity as we vary the temperature: annealing below 400 1C did not have a significant impact on the coercivity. However, annealing at 600 1C resulted in a significant increase of coercivity. Similar behavior could be seen by addition of SPSA where the coercivity increased to 3 times, as shown in Fig. 6.
Fig. 5. Change of coercivity by annealing at various temperatures in the [Cu(1 nm)/Co(1 nm)] 100 multilayers plated with 0.5 mmol/l of DPSA.
ARTICLE IN PRESS K. Hong et al. / Journal of Magnetism and Magnetic Materials 304 (2006) 60–63
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Acknowledgments This work was supported by ReCAMM at Chungnam National University and by Grant no.R01-2004-00010104-0 from Ministry of Science and Technology/Korea Science and Engineering Foundation.
References
Fig. 6. Change of coercivity by annealing at various temperatures in the [Cu(1 nm)/Co(1 nm)] 100 multilayers plated with 0.5 mmol/l of SPSA.
4. Conclusions We have utilized organic additives in the electrodeposition of Cu/Co alloys and multilayers. DPSA and SPSA changed the properties of Cu/Co alloys and multilayered thin films. As we add DPSA and SPSA in the plating electrolyte of Cu/Co, Co content increased by 2–3%. In the Cu/Co alloys, the additives removed hcp-Co, thereby turning to an alloy of Cu(1 1 1) and fcc-Co. TEM analysis showed that DPSA and SPSA improved the thickness uniformity and the interface between Cu- and Co- layers. We think this results from the improved surface roughness and smaller grains caused by the additives. Thin multilayers of Cu/Co showing paramagnetic behavior turned to ferromagnetic by adding 5 mmol/L of each additive. High-temperature annealing resulted in the increase of coercivity by 3 times, both when we used SPSA and DPSA. We think this is due to the growth of Co-grains by annealing and the resulting multi-domain structure of Co.
[1] K. Liu, K. Nagodawithana, P.C. Searson, C.L. Chien, Phys. Rev. B. 51 (1995) 7381. [2] L. Pirauxdel, et al., Appl. Phys. Lett. 65 (1994) 2484. [3] A. Blondel, et al., Appl. Phys. Lett. 65 (1994) 3019. [4] K. Hong, J.-K. Kim, S.-K. Lee, S. Park, B. Gyun, Y.-D. Ko, N.-J. Jeon, J.-S. Chung, Phys. Stat. Sol. (b) 241 (2004) 1681. [5] G.B. McFadden, S.R. Coriell, T.P. Moffat, D. Josell, D. Wheeler, W. Schwarzacher, J. Mallett, J. Electrochem. Soc. 150 (9) (2003) C591. [6] W.H. The, et al., Microelectron. J. 32 (2001) 579 and references therein. [7] P. Cao, P. Taephaisitphongse, R. Chalupa, A. West, J. Electrochem. Soc. 148 (2001) C466. [8] J. Reid, S. Mayer, Proc. Adv. Metallization Conf. 53 (2000). [9] T. Moffat, J. Bonevich, W. Huber, A. Stanishevsky, D. Kelly, G. Stafford, D. Josell, J. Electrochem. Soc. 147 (2000) 4524. [10] J. Reid, S. Mayer, E. Broadbent, E. Klawuhn, K. Ashtiani, Solid State Tech 43 (2000) 86. [11] G.B. McFadden, S.R. Coriell, T.P. Moffat, D. Josell, D. Wheeler, W. Schwarzacher, J. Mallett, J. Electrochem. Soc. 150 (9) (2003) C591. [12] A.J. Bard, L.R. Faulkner, Electrochemical Methods, Fundamentals and Applications, Wiley, New York, 2001. [13] P. Bratin, New developments in the use of cyclic voltammetric stripping for alaysis of plating solutions, in: Proceedings of AES Analytical Method Symposium, March Chicago, IL, 1985. [14] J. Lee, MS Thesis, Chungnam National University, 2005 [15] M.J. Besnus, A.J.P. Meyer, R. Berninger, Phys. Lett. 32A (1970) 192. [16] H.P.J. Wijn (Ed.), Magnetic Properties of Metals, Springer, New York, 1991. [17] V.M. Dubin, C.D. Thomas, N. Baxter, C. Block, V. Chikarmane, P. McGregor, D. Jentz, K. Hong, S. Hearne, C. Zhi, D. Zierath, B. Miner, M. Kuhn, A. Budrevich, H. Simka, S. Shankar, Proc. 2001 IEEE Int. Interconnect Technol. Conf. 1 (1) (2001) 271. [18] N.A. Tuan, et al., Physica B 327 (2003) 400.
Journal of Magnetism and Magnetic Materials 304 (2006) 60–63 www.elsevier.com/locate/jmmm
Property changes of electroplated Cu/Co alloys and multilayers by organic additives Kimin Honga,, Jungju Leea, Jinhan Leea, Young-Dong Kob, Jin-Seok Chungb, Jin-Gyu Kimc a
Department of Physics, Chungnam National University, 220 Gung Dong, Daejeon, 305-764, Korea b Department of Physics, Soongsil University, Seoul, 156-743, Korea c Division of Electron Microscopic Research, Korea Basic Science Institute, 52 Yeoeun-dong, Daejeon, 305-333, Korea Available online 28 February 2006
Abstract We investigated the change of magnetic properties of the electroplated Cu/Co alloys and multilayers caused by organic additives and high temperature annealing. When plated with a pure Cu/Co electrolyte, the alloy contained 25% of Cu and 75% of Co. The alloy was made of hcp-Co, fcc-Co and Cu(1 1 1) and was super-paramagnetic at room temperature. As we add a few organic additives in the plating electrolyte, the hcp-Co of the films disappeared. The organic additives contained in the electrolytes changed paramagnetic Cu/Co multilayers to ferromagnets. High-temperature thermal annealing increased coercivity due to the growth of the Co grains. r 2006 Elsevier B.V. All rights reserved. PACS: 75.50. y; 36.40.Cg; 75.60.Ej; 72.15.Gd; 81.15.Pq; 74.43.Qt Keywords: Magnetic alloys; Copper; Cobalt; Magnetic multilayers; Organic additives; Electroplating; Magnetism; Annealing
1. Introduction For decades, electroplating has been in use to fabricate various magnetic materials and devices. It has the advantage of changing material properties by using various plating chemistry and by adjusting the plating conditions. There were many attempts to apply the plating technique to the fabrication of new devices [1–4]. For many years, copper electroplating has been used in the semiconductor industry for fabrication of metallic interconnects. Gap-fill of dual damascene copper interconnects has successfully been obtained with the technique. One of the interesting features of the Cu interconnect process is that the plating chemistry utilizes organic materials or, so-called organic additives, in addition to the inorganic electrolytes. They are known to play an important role in achieving the super-fill process, where plating is faster at the recess and is slower at flat surfaces. Depending on the roles in baths, organic additives are categorized as accelerators and suppressors. An accelerator Corresponding author. Tel.: +82 42 821 5456; fax: +82 42 822 8011
E-mail address: [email protected] (K. Hong). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.02.008
is a catalyst having a large diffusion length and enhances the plating current density. A suppressor is a surfactant having a short diffusion length and prohibits plating [4]. Void-free super-fill and quality copper films are obtained by using the proper compositions and concentrations for the accelerators and the suppressors. In addition to enabling the super-fill process, the organic additives also alter the properties of the plated copper. Copper films plated with the inorganic electrolyte only usually have large grains. Additives alter the grain growth mechanism and change microscopic structures such as crystalline orientation, surface smoothness, and grain size [5]. In addition, additives are known to cause the so-called ‘‘self-annealing’’ effect where the Cu grains grow slowly even at room temperature after the deposition. It takes place only when the electrolytes contain specific additives. Later, it was discovered that rapid thermal annealing (RTA) had same effect but at higher growth rates [6–11]. Since the magnetic properties are closely related to the microscopic structures, we expect the organic additives would play a role in the determination of magnetic properties. In this work, we studied the change of magnetic properties caused by a few organic additives and the thermal annealing.
ARTICLE IN PRESS K. Hong et al. / Journal of Magnetism and Magnetic Materials 304 (2006) 60–63
2. Experiments Electrochemical properties of a few organic materials were investigated with a cyclic voltammetry (CV) system consisting of a potentiostat (273, EG&G), a rotating disk electrode (RDE), a counter electrode, and an Ag/AgCl reference electrode. In this system, an alternating voltage is applied on the RDE and metallic ions are alternately deposited on and stripped off the surface of the RDE. Plating current densities can be obtained from the resulting voltage–current characteristics or a voltammogram [12,13]. A kind of additives causing the increase of the current density are called as accelerators while other additives reducing the current density are called suppressors. Amongst a few organic additives we investigated in the Cu- and Co-electrolytes, Sulfopropyl disulfide sodium salt (SPSA) and Dimethyldithiocarbamic acid (DPSA) had significant effects and we were focussed on these two additives. Electrodeposition was carried out using a conventional three terminal method, i.e., with a working electrode, a Ag/ AgCl reference electrode, and a flat platinum plate as a counter electrode [12]. The working electrode was fabricated by physical vapor deposition of 150 nm thick Cu on Si(1 0 0). Root-mean-square (rms) surface roughness measured with an AFM (atomic force microscope) was 3 nm. A potentiostat (SI 1286, Solartron) was used as a power supply for the deposition. Plating voltage was 0.3 V for Cu and 0.95 V for Co, respectively. The plating electrolyte used in this experiment was made of 120 g/L of CoSO4 7H2O, 1.7 g/L of CuSO4 5H2O, and 50 ppm of chloride ions [1,3]. Alloys of Cu/Co were plated by applying 0.95 V and the multilayers were plated by alternately applying 0.3 V and 0.95 V. The thickness of each layer was adjusted by controlling the pulse widths.
61
different from previous experiments on Cu/Co alloys prepared with vacuum deposition techniques, where only hcp-Co was observed at temperatures above 470 1C and fcc-Co was observed at room temperature [15,16]. It indicates that either electroplating in itself or the electrolyte composition we used altered the structure of Cu/Co alloy. Another interesting feature is that DPSA (center of Fig. 2) and SPSA (top of Fig. 2) removed hcp-Co and the alloys have only fcc-Co. Since fcc-Co is a stable structure at room temperature, we think that SPSA and DPSA enabled growth of Cu/Co alloys in a thermodynamically stable manner than with the pure electrolyte. Cu/Co multilayers were electroplated at various thicknesses and number of layers. Fig. 3(a) shows TEM pictures of [Cu(10 nm)/Co(42 nm)] 40 made with the electrolyte only. It shows varying layer thicknesses both in Cu- and Co-layers and the interfaces are not clear. However, as we add 0.5 mmol/L of SPSA, we could obtain improved thickness uniformity and better interfaces, as shown in Fig. 3(b). Similar results were obtained using DPSA. Since SPSA and DPSA reduce the grain size and improve the
3. Results and discussions Fig. 1 shows the change of current density by the two additives. In proportion to the concentration of each additive, the current density increases both in the Cu- and the Co-electrolytes. Note that DPSA in the Co-electrolyte has the largest effect. When we carried out similar tests with other polymers, we observed various behaviors including suppressing effects [14]. When we apply 0.95 V to the working electrode, we obtain Cu/Co alloys. EDX analysis of the alloys has shown that they had 75.2% of Co and 24.8% of Cu. As we add 0.5 mmol/L of SPSA, the Co content increased to 76.7%. When we add same amount of DPSA, the Co content was 78.3%. The changes are due to the accelerating effct of SPSA and DPSA on Co deposition. Since DPSA works as a stronger accelerator, the increase of Co content is larger. Fig. 2 is the X-ray diffraction (XRD) result showing the change of crystalline structures in 1 mm thick Cu/Co alloy films. Thin films plated with the pure electrolyte (bottom of Fig. 2) had hcp-Co and fcc-Co as well as Cu(1 1 1). This is
Fig. 1. Increase of current density by SPSA and DPSA in the electrolytes of Cu and Co, respectively.
SPSA
DPSA
Electrolyte Only
θ
Fig. 2. XRD results of Cu/Co alloy thin films.
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Fig. 3. TEM pictures of Cu(bright)/Co(dark) multilayers: (a) plated with the pure electrolyte, and (b) with 5 mmol/L of SPSA added in the electrolyte.
Fig. 4. Effect of SPSA and DPSA on magnetic behavior of [Cu(1 nm)/ Co(1 nm)] 100 multilayers.
surface smoothness, better separation between Cu- and Colayers could be obtained [17]. Fig. 4 shows the magnetization of [Cu(1 nm)/ Co(1 nm)]x100 using a VSM (vibrating sample magnetometer) at room temperature, as we applied the magnetic field in the plane of the multilayers. The solid line was obtained from a film plated with the electrolyte only. It is a paramagnet and we think this sample does not have separate Cu- and Co-layers. Instead, it seems to have granular structures. A strikingly different behavior was obtained when we added SPSA and DPSA. Closed squares were obtained from a same structure under identical process except that the electrolyte contained 0.5 mmol/L of SPSA. Open circle was obtained similarly with 0.5 mmol/L DPSA. Compared to that prepared with the pure electrolyte, they clearly show ferromagnetism. One might think that the additives increased Co content, which, in turn, changed the magnetic property since the magnetic properties change in proportion to the concentration of Cu in the alloys [16]. However, as we have shown above, the additives increased the content of Co by less than 3%. At the high concentration regime of Co, this change does not have such a large impact on the magnetic properties of the alloy [15]. On the other hand, taking the TEM results into
consideration where the layers have higher thickness uniformity, we think that the additives separated the Cu/ Co interfaces distinctly and, therefore, the Co layers could have higher coercivity. As we apply the fields perpendicular to the planes, the multilayers prepared with the pure electrolyte showed similar paramagnetic behavior. On the other hand, multilayers made with the additives showed saturation magnetization at 10 kOe, as shown in the inset of Fig. 4. Comparing the demagnetizing field of Co thin film, 4pM17 kOe, the saturating field is lower. However, considering the Cu content in Co layers, we expect lower magnetization and reduced demagnetizing field. It is known that the magnetization of Co/Cu alloy decreases linearly in proportion to the Cu concentration and the relation is given as dM/dC 1.6 per added Cu, where C is the Cu concentration [15,18]. Considering 25% of Cu concentration and the resulting 40% reduction of the magnetization, the demagnetizing field 10 kOe agrees well with the prediction. Cu and Co are immiscible below 420 1C and turn to an alloy above the temperature. It should be interesting to compare the effect of the Cu grain growth by annealing caused by the additives with the alloying effect of Cu/Co at elevated temperatures. In order to observe the temperature effects, we annealed the multilayered thin films at 200, 400, 500, and 600 1C for 30 min using an RTA. Fig. 5 shows the effect of annealing on multilayers made with DPSA. The solid line has a magnetization behavior without thermal treatment. Open circles were obtained after annealing at 400 1C and closed squares were obtained after annealing at 600 1C. As can be seen in the figure, annealing increased the coercivity and showed a multi-domain behavior. The inset shows the increase of coercivity as we vary the temperature: annealing below 400 1C did not have a significant impact on the coercivity. However, annealing at 600 1C resulted in a significant increase of coercivity. Similar behavior could be seen by addition of SPSA where the coercivity increased to 3 times, as shown in Fig. 6.
Fig. 5. Change of coercivity by annealing at various temperatures in the [Cu(1 nm)/Co(1 nm)] 100 multilayers plated with 0.5 mmol/l of DPSA.
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Acknowledgments This work was supported by ReCAMM at Chungnam National University and by Grant no.R01-2004-00010104-0 from Ministry of Science and Technology/Korea Science and Engineering Foundation.
References
Fig. 6. Change of coercivity by annealing at various temperatures in the [Cu(1 nm)/Co(1 nm)] 100 multilayers plated with 0.5 mmol/l of SPSA.
4. Conclusions We have utilized organic additives in the electrodeposition of Cu/Co alloys and multilayers. DPSA and SPSA changed the properties of Cu/Co alloys and multilayered thin films. As we add DPSA and SPSA in the plating electrolyte of Cu/Co, Co content increased by 2–3%. In the Cu/Co alloys, the additives removed hcp-Co, thereby turning to an alloy of Cu(1 1 1) and fcc-Co. TEM analysis showed that DPSA and SPSA improved the thickness uniformity and the interface between Cu- and Co- layers. We think this results from the improved surface roughness and smaller grains caused by the additives. Thin multilayers of Cu/Co showing paramagnetic behavior turned to ferromagnetic by adding 5 mmol/L of each additive. High-temperature annealing resulted in the increase of coercivity by 3 times, both when we used SPSA and DPSA. We think this is due to the growth of Co-grains by annealing and the resulting multi-domain structure of Co.
[1] K. Liu, K. Nagodawithana, P.C. Searson, C.L. Chien, Phys. Rev. B. 51 (1995) 7381. [2] L. Pirauxdel, et al., Appl. Phys. Lett. 65 (1994) 2484. [3] A. Blondel, et al., Appl. Phys. Lett. 65 (1994) 3019. [4] K. Hong, J.-K. Kim, S.-K. Lee, S. Park, B. Gyun, Y.-D. Ko, N.-J. Jeon, J.-S. Chung, Phys. Stat. Sol. (b) 241 (2004) 1681. [5] G.B. McFadden, S.R. Coriell, T.P. Moffat, D. Josell, D. Wheeler, W. Schwarzacher, J. Mallett, J. Electrochem. Soc. 150 (9) (2003) C591. [6] W.H. The, et al., Microelectron. J. 32 (2001) 579 and references therein. [7] P. Cao, P. Taephaisitphongse, R. Chalupa, A. West, J. Electrochem. Soc. 148 (2001) C466. [8] J. Reid, S. Mayer, Proc. Adv. Metallization Conf. 53 (2000). [9] T. Moffat, J. Bonevich, W. Huber, A. Stanishevsky, D. Kelly, G. Stafford, D. Josell, J. Electrochem. Soc. 147 (2000) 4524. [10] J. Reid, S. Mayer, E. Broadbent, E. Klawuhn, K. Ashtiani, Solid State Tech 43 (2000) 86. [11] G.B. McFadden, S.R. Coriell, T.P. Moffat, D. Josell, D. Wheeler, W. Schwarzacher, J. Mallett, J. Electrochem. Soc. 150 (9) (2003) C591. [12] A.J. Bard, L.R. Faulkner, Electrochemical Methods, Fundamentals and Applications, Wiley, New York, 2001. [13] P. Bratin, New developments in the use of cyclic voltammetric stripping for alaysis of plating solutions, in: Proceedings of AES Analytical Method Symposium, March Chicago, IL, 1985. [14] J. Lee, MS Thesis, Chungnam National University, 2005 [15] M.J. Besnus, A.J.P. Meyer, R. Berninger, Phys. Lett. 32A (1970) 192. [16] H.P.J. Wijn (Ed.), Magnetic Properties of Metals, Springer, New York, 1991. [17] V.M. Dubin, C.D. Thomas, N. Baxter, C. Block, V. Chikarmane, P. McGregor, D. Jentz, K. Hong, S. Hearne, C. Zhi, D. Zierath, B. Miner, M. Kuhn, A. Budrevich, H. Simka, S. Shankar, Proc. 2001 IEEE Int. Interconnect Technol. Conf. 1 (1) (2001) 271. [18] N.A. Tuan, et al., Physica B 327 (2003) 400.