Thin Solid Films 446 (2004)

Transcription

1 Thin Solid Films 446 (004) New electroplating method of nickel in emulsion of supercritical carbon dioxide and electroplating solution to enhance uniformity and hardness of plated film Hideo Yoshida, Masato Sone*, Hiroaki Wakabayashi, Hao Yan, Kentaro Abe, Xu Tang Tao, Aya Mizushima, Shoji Ichihara, Seizo Miyata Department of Organic and Polymer Materials Chemistry, Faculty of Technology, Tokyo University of Agriculture and Technology, Naka-Cho, Koganei-shi, Tokyo , Japan Received 5 November 00; received in revised form 8 July 003; accepted 6 September 003 Abstract High quality nickel films having fine grains have been developed using a new electroplating method involving the emulsion of a supercritical carbon dioxide, an electroplating solution and a surfactant. The plated films using this method have a uniformity of the surface and Vickers hardness, which is better than the results obtained by using conventional electroplating methods. 003 Elsevier B.V. All rights reserved. Keywords: Carbon dioxide; Nickel; Electrochemistry; Surface roughness 1. Introduction Electroplating is a widely used industrial technique that has a long history w1x. Recently, electrochemical techniques attracted much interest in the preparation of nanometer metals, alloys, and semiconductors that have different forms w 5x. A common problem exists during electroplating, which is that the electric current also causes the dissociation of water in addition to the electrolysis of metal ions, resulting in hydrogen to be released at the cathode. The formation of hydrogen may create several pinholes on the film. This is an important problem that has recently been confirmed by Tsai et al. w6x, by using synchrotron X-ray. The problem of pinholes in plated films has remained unsolved w1x. Supercritical fluids have attracted extensive attention as an alternative to conventional solvents for a variety of applications w7 16x. Supercritical carbon dioxide (Sc- CO ) is generally viewed as a replacement for harmful organic solvents used for extraction, separation, reaction, and for many other processes. Recently, plating technology with sc-co has attracted special attention because sc-co, in particular, can transport the solute into fine nano-space of the materials and clean even integrated *Corresponding author. Tel.yfax: q circuits without shrinking or causing other harm due to the interface tension that exists between liquids and gases. However, there have been no reports in the literature of sc-co being applied to the electroplating method because metal salts are generally not soluble in sc-co. In previous studies, we reported a new electroplating in emulsion that was formed by surfactant, sc- CO, and an electroplating solution w15x. In this paper, we demonstrate that the electroplated films in an emulsion of sc-co and an electroplating solution have a uniformity and Vickers hardness, which is better than the results obtained by using conventional electroplating methods.. Experimental details.1. Materials Carbon dioxide with a minimum purity of 99.9% was purchased from Toei Kagaku Co. Ltd. In our experiments, we employed a non-ionic block co-polymer poly(ethylene oxide)-b-poly(propylene oxide) (PEO- PPO) (Lion Co. Ltd.) as surfactants. The electroplating solution, which is usually referred to as a modified Watt bath, consisted of nickel sulfate (37 gyl), nickel chloride (88 gyl), boric acid (95 gyl), saccharin (3 gyl) /04/$ - see front matter 003 Elsevier B.V. All rights reserved. doi: /j.tsf

2 H. Yoshida et al. / Thin Solid Films 446 (004) each ternary system were simultaneously measured. Moreover, the current efficiencies were evaluated by measuring the weight difference before and after the electroplating reaction..3. Analysis Fig. 1. Experimental apparatus used for batch reaction in the electroplating experiments beyond the critical point of CO ; (a) CO cylinder; (b) cooler; (c) high pressure pump; (d) temperature controlled air bath; (e) reactor with magnetic stirrer; (f) programmable power supply; and (g) trap; BPR: back-pressure regulator; PI: pressure indicator; TI: temperature indicator; V: valves. and 1,4-butynediol (0.5 gyl). Nickel sulfate, nickel chloride and boric acid with a minimum purity of 99.9% were purchased from Toshin Yuka Kogyo Co. Ltd. The anode was a 99.99% purity nickel plate with a size of 10=0 mm and the cathode was a brass substrate of the same size. Brass substrate, which was composed of 65.4% of Cu and 34.6% of Zn has a Vickers hardness value of 11.7 Hv. Before a reaction, both the brass substrate and the nickel plate were degreased by dipping successively in a 10 wt.% NaOH and a 0 wt.% HCl and rinsing in deionized water... Experimental apparatus A high-pressure experimental apparatus (Japan Spectra Company), shown in Fig. 1, was used for electroplating. The temperature variation of each run was observed to be less than 1.0 K. The maximum working temperature and the maximum pressure were 44 K and 50 MPa, respectively. The reaction cell that had a volume of 50 ml was a stainless steel 316 vessel in a temperature-controlled air bath with a magnetic agitator. An agitation was performed using a cross-magnetic stirrer bar. Both the anode and the cathode were attached using platinum wires to the reactor and were connected to a programmable power-supply, model YPP15030, manufactured by Yamamoto-ms Co. Ltd. A typical electroplating reaction was performed in a constantly agitated ternary system of dense CO, the electroplating solution and a surfactant. The nickel electroplating solution and the surfactant were both put in a high-pressure cell. Liquid CO was introduced to the high-pressure cell using a HPLC pump and pressurized to a predetermined pressure. The ternary system was then constantly agitated at a speed of 400 rev.ymin under a desirable constant temperature. After 0 min the electroplating reaction started, and the resistances in The optical microscopic images were taken using a KEYENCE Digital HF Microscope, model number VH The surface features and average surface roughness values (Ra) were measured using a KEYENCE color laser three-dimensional profile microscope, model number VK-8510 in which the minimum resolutions both of height and for horizontal measurement are 10 nm. Vickers hardness values were measured using an AKASHI Vickers hardness Machine, model number AVK-C with a weight of either 0 or 50 g. For each sample 10 impressions were made, from which an average hardness value was calculated. Actually, considering that the nickel films from both the front and back of the brass substrate were also given each hardness value is actually based on 15 measurements. The electron microscopy images are cross-sections of plated nickel films (15 000= magnification) of the electroplated nickel films produced from the emulsion were measured using a JEOL JSM-5600LV. The X-ray diffraction spectra were measured with a Rigaku RINT- RAPID using Cu Ka radiation. 3. Results and discussion Metal salts are generally soluble in water, but water and CO are almost not miscible. As a result, it was believed that sc-co was not suitable for electrolysis. However, surfactants could overcome this problem. Anionic, cationic, and non-ionic surfactants have been found to form water in CO (wyc) or CO in water (cy w) emulsions w13x. At first the dispersion behavior of a ternary system of water, surfactant and CO was observed through a view cell. A picture of phase separation and the formation of an emulsion formation are shown pictorially in Fig.. At 33 K, under a pressure of 10 MPa and without stirring two separated phases were observed (a). Under the same temperature and pressure but with agitation, a cyw emulsion emerged (b). After decreasing the pressure to 6 MPa while maintaining the same agitation, an incomplete dispersion state was observed (c). This result indicates that the formation of a cyw emulsion by the surfactant is not only affected by temperature and pressure, it is also affected by agitation. Electroplating was done at a current density of.0 Aydm at a temperature of 33 K and a pressure of 10 MPa in a constantly agitated ternary system of sc-co (15 ml), and the electroplating solution (30 ml) and the surfactant (6 wt.% to the electroplating solution). After

3 196 H. Yoshida et al. / Thin Solid Films 446 (004) Fig.. Photographs of the sc-co -water-surfactant systems: (a) two phases of water and CO at a temperature of 33 K and a pressure of 10 MPa without agitation, (b) emulsion at the same temperature and pressure with agitation, and (c) an incomplete dispersion state at 33 K, 6 MPa and under agitation. 15 min of plating, we obtained a thickness of 1.8 mm on the front surface and 11.8 mm on the reverse surface. The current efficiency was 88.5%. In our experiments, the emulsion may have a lower viscosity than those of the electroplating solution only which has a viscosity of 1717 mpaøs, because one component of emulsion was sc-co that has a viscosity of 30.1 mpaøs at a temper- ature of 33 K and a pressure of 10 MPa. These factors enable free movement of the ions in the bath and the agitation of the emulsion increases the throwing power higher. Fig. 3 shows optical microscopy images of nickel deposits from (a) the emulsion containing sc-co, (b) the electroplating solution only at atmospheric pressure, and (c) an electroplating solution at a pressure of 4 MPa. The surface of the plated nickel film used in our method is clearly smoother than of the films prepared using the electroplating solution only at atmospheric pressure. The film has some pinholes with a diameter of approximately 5 mm. The films plated using the electroplating solution only at a pressure of 4 MPa have several large pinholes with a diameter of approximately 500 mm. High pressure electroplating did not give good results because the buoyancy of the hydrogen bubbles produced by the electrolysis of water in the high pressure system becomes smaller than that at atmospheric pressure. Hence the larger bubbles prevent the metal from covering the plate. It is interesting to note that the pinholes did not appear in the nickel film plated using our method although the high pressure conditions used during electroplating are not good for the uniformity, since sc-co and hydrogen are miscible. The hydrogen bubbles produced by the electrolysis of water could dissolve in the sc-co parts of the emulsion and therefore the pinholes would not appear in the plated film using our method. Fig. 3. Optical microscopy images of nickel film that were plated from (a) the emulsion, (b) electroplating solution only at atmospheric pressure and (c) electroplating solution only at a pressure of 4 MPa.

4 H. Yoshida et al. / Thin Solid Films 446 (004) Fig. 4. Three-dimensional images of the surface of (a) the brass substrate and nickel films that were plated from (b) the emulsion, (c) electroplating solution only at atmospheric pressure. The surface uniformities of the bare substrates and of the films deposited by electroplating and by the method described herein have been measured using three-dimensional laser microscopy. The representation of the surface of the substrate (a) and of the film plated using our process (b) and using only the solution (c) are shown in Fig. 4. These images clearly show that the surface of the film plated by our method is smoother than those of the brass substrate and the surface of the film plated by the solution only. For each sample, the average surface roughness values (Ra) for three 150=10 mm regions of 15-mm thick films have been measured. The Ra was 0 nm for the film plated by our method and 31 nm for the film plated using the electroplating solution only while that of brass substrate is 33 nm. Hence, the uniformity of the film deposited by our method was better than that by electroplating solution only. Fig. 5 shows a cross-section of scanning electron microscopy images of the nickel films plated with same current density of.0 Aydm (a) from the emulsion, (b) from the electroplating solution only, and (c) from Fig. 5. Scanning electron microscopy photographs of cross-sections of nickel film that were plated from (a) the emulsion, (b) electroplating solution only at atmospheric pressure and (c) electroplating solution only at a pressure of 4 MPa.

5 198 H. Yoshida et al. / Thin Solid Films 446 (004) Fig. 6. X-Ray diffraction spectra of as-deposited 15 mm thick coating made from (a) the emulsion of supercritical CO and the electroplating solution, (b) the solution only at atmospheric pressure and (c) the brass substrate. The blue line is a profile of pure nickel in the reference data and the pink lines are from the brass. the electroplating solution only at 4 MPa with the same thickness of 15 mm. The grain size in the deposited film from the emulsion of sc-co and electroplating solution is within the range of sub-100 nm. However, the grain size of the nickel films plated using the electroplating solution only at both atmospheric pressure and at a pressure of 4 MPa is within the range of approximately 1.0 mm. Moreover, the surface hardness was measured using standard Vickers hardness equipment. As previously stated, the hardness value of the nickel films from both the front and the back of the brass substrate is actually based on 15 measurements. The average hardness of these nickel films was 705 Hv, which is much higher than the hardness 550 Hv of the bright nickel produced from the electroplating solution only. The deposited films produced by our method have a significantly smaller grain size, as shown in Fig. 3. According to the Hall Petch law, the hardness of metals, alloys, and ceramic materials increases as the grain size decreases within certain ranges w17x. However, since the hardness value of this plated film in the emulsion is very high, there is a possibility that an impurity could have been included in the nickel metal and influence the metal structure. In order to confirm that the plated film is composed of pure nickel, we measured the X-ray diffraction spectra, in Fig. 6, of as-deposited 15 mm thick coating made from the emulsion of supercritical CO and the electroplating solution, the solution only at atmospheric pressure and the brass substrate. The XRD spectra show that both the plated nickel films produced using our method and those produced by the electroplating solution only are composed of a pure nickel with the same crystalline lattice on the brass substrata. Thus, we could conclude that the high hardness of the plated film produced by our method originated from the small grains of the nickel and the small grain strengthening that occurs in our system. Several factors exist that influence the electroplating in the emulsion of sc-co and the electroplating solution only. The first factor is that the formation of hydrogen does not cause any problems to the uniformity of the film anymore since hydrogen gas is miscible with sc- CO. The second factor is the periodic plating character of the emulsion as stated at above. Emulsion plating, which is electroplating in an emulsion of the electroplating solution and emulsifier, has often been used to plate the rough surface coating using the static emulsion in order to lose the brightness of the plated metal w1x. In our experiment, the plating solution is a dynamic emulsion formed by sc-co, surfactant and an electroplating solution. Johnston et al. reported that the dynamic emulsion of a saline solution with sc-co contains numerous micelles with radii in the range of several micrometers w18x. Thus, numerous micelles could depart within in our system. When the electroplating solution comes in contact with the cathode, the nucleation and the crystal growth can occur. However, when the sc- CO comes in contact with the cathode, the nucleation and the crystal growth cannot occur. Therefore, the plating in the emulsion is similar to pulse plating, and the grain size might be limited. To confirm that the quality of the film has not originated from high pressure we performed an electroplating without sc-co at the same pressure and temperature. The qualities of the films that we obtained are poor qualities with rough surfaces and larger pinholes. This result indicates that sc-co plays an important role in increasing the quality of plated film higher. When we have also plated Ni using an electroplating solution only under surfactants doping, the deposits have no improvement in quality, either. Other factors may include a higher diffusion coefficient, lower viscosity and the surface tension of the sc-co -electroplating solution emulsion, which makes the adopted atoms on the cathode move with greater ease into the depressions of place than that using only electroplating solution, and so the uniform films with smooth surfaces were obtained. 4. Conclusion In summary, we developed a high quality film using a new technology that combines the merits of traditional

6 H. Yoshida et al. / Thin Solid Films 446 (004) electroplating with sc-co techniques. The emulsion of surfactants-sc-co-electroplating solution has a significantly higher miscibility with hydrogen, a lower viscosity and higher diffusivities than the results obtained using only an electroplating solution. These properties made electroplating in such a system offer a promising method for high quality electroplating. The deposits with smaller grain size obtained by this technique indicate that it may be adaptable for more uniform as well as harder coating of very complex topographies than that by the electroplating solution only. Acknowledgments This work was supported by the New Energy and Industrial Technology Development Organization and the Kanto Bureau of Economy, Trade and Industry for the Regional Consortium Project in the New Sunshine Program, AIST, Japan. References w1x M. Schlesinger, M. Paunovic (Eds.), Modern Electroplating, 4th Ed, John-Wiley and Sons Inc, New York, 000. wx P.M. Penner, J. Phys. Chem. B. 106 (00) w3x R. Schuster, Chem. Phys. Phys. Chem. (001) 411. w4x F. Endres, S.Z.E. Abedin, Chem. Commun. (00) 89. w5x L. Xu, W. Zhou, M.E. Kozlov, I.I. Khayrullin, I. Udod, A.A. Zakhidov, R.H. Baughman, J.B. Wiley, J. Am. Chem. Soc. 13 (001) 763. w6x W.L. Tsai, P.C. Hsu, Y. Hwu, C.H. Chen, L.W. Chang, J.H. Je, H.M. Lin, A. Groso, G. Margaritondo, Nature 417 (00) 139. w7x A.I. Cooper, J. Mater. Chem. 10 (000) 07. w8x J.A. Darr, M. Poliakoff, Chem. Rev. 99 (1999) 495. w9x R.E. Sievers, B.N. Hansen, US Patent (1990). w10x J.J. Watkins, J.M. Blackburn, T. McCarthy, J. Chem. Mater. 11 (1999) 13. w11x H. Wakayama, Y. Fukushima, Chem. Commun. 4 (1999) 391. w1x I. Tabata, J. Lyu, S. Cho, T. Tominaga, T. Hori, Color. Technol. 117 (001) 346. w13x K.P. Johnston, K.L. Harrison, M.J. Clarke, S.M. Howdle, M.P. Heitz, F.V. Bright, C. Carlier, T.W. Randolph, Science 71 (1996) 64. w14x J.B. McClain, D.E. Betts, D.A. Canelas, E.T. Samulski, J.M. DeSimone, J.D. Londono, H.D. Cochran, G.D. Wignall, D. Chillura-Martio, R. Triolo, Science 74 (1996) 049. w15x H. Yoshida, M. Sone, A. Mizushima, K. Abe, X.T. Tao, S. Ichihara, S. Miyata, Chem. Lett. 11 (00) w16x G.B. Jacobson, C.T. Lee Jr, K.P. Johnston, W. Tumas, J. Am. Chem. Soc. 11 (1999) w17x E.O. Hall, Nature 173 (1954) 948. w18x S.R.P. da Rocha, P.A. Psathas, E. Klein, K.P. Johnston, J. Colloid Interface Sci. 39 (001) 41.