Defect-Free Nickel Micropillars Fabricated at a High Current Density

May 23, 2011 - Precision and Intelligence Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan. Ind. Eng. Ch...
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Defect-Free Nickel Micropillars Fabricated at a High Current Density by Application of a Supercritical Carbon Dioxide Emulsion Tso-Fu Mark Chang,* Toshikazu Tasaki, Chiemi Ishiyama, and Masato Sone Precision and Intelligence Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan ABSTRACT: Fast transport properties, extremely low surface tension, and nonpolar property of supercritical carbon dioxide (Sc-CO2) are the key advantages of applying Sc-CO2 in heterogeneous reaction like electroplating. Hydrogen gas bubbles have been found to be a cause of defects found in materials fabricated by electroplating. The physical limitation at the bottom of the photoresist patterns increases the difficulty for removal of hydrogen gas bubbles and diffusion of reactant species to the reaction site. Therefore, low current density is usually applied to prevent formation of defects, but it also leads to low growth rate. Fabrication of defect-free Ni micropillars at a high current density has been demonstrated by application of Sc-CO2 emulsified electrolyte in this work. The average growth rate of the Ni micropillars in this study was about 4.7 μm/min. Diameters of the electroplated Ni micropillars ranged from 50 to 125 μm.

1. INTRODUCTION Electroplating is a key technology for fabricating microcomponents used in micro-electro-mechanical systems (MEMS).1,2 Transport properties of materials in supercritical CO2 (Sc-CO2) are enhanced because of the low density, low viscosity, high selfdiffusivity, and low surface tension of Sc-CO2.3,4 The extremely low surface tension allows Sc-CO2 to be applied in drying and extraction of solvents out of nanopores and nanoarrays.5,6 Application of Sc-CO2 in heterogeneous reaction like electroplating can enhance transfer of reactant species to the reaction site and product (byproduct) away from the reaction site. Removal of byproduct such as hydrogen gas bubbles away from the reaction site is particularly important. Evolution of hydrogen is an inevitable side reaction when performing electrochemical reaction using aqueous electrolyte. Hydrogen gas bubbles that adsorb on the surface of the cathode can cause defects and pinholes in electroplated materials and structures.7 Desorption of hydrogen gas bubbles from the surface of the cathode is enhanced by addition of a surface brightener into the electrolyte, such as saccharine for electroplating of Ni.8 Desorption of hydrogen gas bubbles can be further promoted by application of Sc-CO2,9 because the solubility and diffusivity of hydrogen are both higher in Sc-CO2 when compared to those in aqueous solution.10 In addition, the surface tension of Sc-CO2 is extremely low, which further promotes desorption of hydrogen gas bubbles. Therefore, Sc-CO2 is promising in solving problems found in miniaturization of electronic devices. However, electrical conductivity and metal salts solubility are both very low in Sc-CO2, which limit its application in electrochemical reaction. The limitations can be overcome by addition of a cosolvent3 or usage of a surfactant to form an emulsified electrolyte composed of aqueous electrolyte and Sc-CO2.11,12 Electrolyte used in ScCO2 emulsion (Sc-CO2-E) can be commercially available electrolyte, which simplifies application of Sc-CO2 in electrochemical reaction and practical use in the industries. Two types of the emulsion can be formed depend on the type and concentration of surfactant used and concentration of CO2 r 2011 American Chemical Society

in the system. One is water in CO2 (W/C, where the continuous phase is CO2 and the dispersed phase is water) and the other one is CO2 in water (C/W, where the continuous phase is water and the dispersed phase is CO2).13 The structure of the dispersed phase in C/W emulsion is similar to micelles in mixtures of oil and water.14 C/W emulsion is usually used for application in electrochemical reaction because solubility of metal salts and electrical conductivity are both higher in C/W emulsion when compared to W/C emulsion. Emulsion particles dispersed in the solution can also help transfer of reactants to the reaction site and product away from the reaction site based on the particle film model proposed by Roha.15 Properties of Ni electroplated can influence its application in MEMS devices, especially for the mechanical properties.16 Surface roughness,17 grain size,18 hardness,19 and wear properties20 of Ni film have been reported to improve significantly when electroplated with Sc-CO2-E. A template made up of photoresist patterns on top of a conductive substrate is often used for fabrication of micropillars by the electroplating process. The photoresist patterns are used to confine dimensions of the structures electroplated on the conductive substrate. However, transport of hydrogen gas bubbles away from the reaction site is expected to be less efficient at the bottom/holes of the photoresist patterns, and hydrogen gas bubbles remaining at the bottom of the photoresist patterns will cause defects in the micropillars fabricated. There are several methods to eliminate the problems caused by the evolution of hydrogen. The most often applied method is to reduce the evolution rate of hydrogen using a lower current density, but the growth rate of the micropillars is also decreased with low current density. In this study, we want to demonstrate that defect-free metallic micropillars can be fabricated by electroplating with ScCO2-E (ESCE) at a high current density. In addition, nonpolar Received: March 9, 2011 Accepted: May 23, 2011 Revised: May 17, 2011 Published: May 23, 2011 8080

dx.doi.org/10.1021/ie200469e | Ind. Eng. Chem. Res. 2011, 50, 8080–8085

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Figure 1. Template preparation process: (a) sticking SU-8 film onto surface of copper substrate, (b) patterned SU-8 film on copper substrate (the diameter of the holes range from 50 to 125 μm), and (c) Ni micropillars with diameters from ca. 50 to 125 μm on a copper substrate, and height of the micropillars can be controlled by current applied, deposition time, and thickness of SU-8 film.

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Figure 2. OM micrographs: Ni micropillars fabricated by (a) CONV at 2 A/dm2 for 15 min using 15-μm-thick SU-8 film and (b) CONV at 10 A/dm2 for 3 min using 15-μm-thick SU-8 film.

property of Sc-CO2 can weaken adhesion strength of the photoresist patterns on metallic substrate. By controlling the experimental conditions, this property can be utilized to simplify removal of photoresist patterns after the electroplating process.

S-4300SE, Hitachi High technologies Co., Ltd.). The diameter and height of the Ni micropillars were measured by a scanning laser microscope (SLM, 1LM21W, Lasertec Inc.).

2. EXPERIMENTAL SECTION The photoresist used in this study was film-type SU-8 from Kayaku MicroChem. Two film thicknesses of the SU-8 film were used, which were 15 and 50 μm. The template used for electroplating of Ni micropillars was prepared by sticking SU-8 film onto a 1.0  2.0 cm2 copper substrate according to the instruction given by Kayaku MicroChem and patterns of cylindrical-shaped holes were prepared through lithography and development process as reported in previous study.21 Diameters of the cylindrical-shaped holes ranged from 50 to 125 μm. Process flow for preparation of the template is shown in Figure 1. Ni micropillars were fabricated by conventional electroplating (CONV) at ambient pressure and 323 K to have a comparison with Ni micropillars fabricated by ESCE. The Ni electrolyte used was a commercially available modified Watts bath supplied by AIKOH Chemicals. The composition of the modified Watts bath was nickel sulfate (372 g/L), nickel chloride (88 g/L), boric acid (95 g/L), saccharine (10 mg/L), and 1,4-butynediol (5 mg/L). The pH of the electrolyte is 3.49. For ESCE, the high-pressure experimental apparatus was purchased from Japan Spectra Company.18 The volume fraction of CO2 was 10 vol % with respect to the overall volume of the reaction chamber. A nonionic surfactant, polyoxyethylene lauryl ether (C12H25(OCH2CH2)15OH) supplied by Toshin Yuka Kogyo was used for formation of a C/W type emulsion. The volume fraction of surfactant was 0.2 vol % with respect to volume of the aqueous electrolyte. Experimental temperature was 323 K for both CONV and ESCE, and pressure was 15 MPa for ESCE. Ni micropillars were also fabricated by changing either pressure to 6 MPa, volume fraction of CO2 to 0 vol %, or volume fraction of surfactant to 0 vol % while leaving the other experimental parameters constant to study the effects of Sc-CO2-E. For CONV, Ni micropillars were fabricated at two different current densities, which were 2 and 10 A/dm2, to demonstrate the effect of high current density. For comparison between CONV and ESCE, samples were electroplated with constant current at 250 mA. Ni micropillars with different aspect ratio were fabricated by controlling the deposition time. Removal of the SU-8 film after the electroplating process was done by immersing the template in Remover PG Solution supplied by Kayaku MicroChem at 353 K for 10 min followed by ultrasonic agitation in acetone solution at room temperature for 10 min. Morphology of the Ni micropillars was examined by a digital optical microscope (OM, VHX-100F, Keyence) and a scanning electron microscope (Fe-SEM,

3. RESULTS AND DISCUSSION The suggested upper limit of current density for using Watts bath is 11 A/dm2.22 Current density near the upper limit is usually not used, because mass transfer of reactants and products become the limiting step when current density is too high. Current density used for ESCE in this study was 10 A/dm2, which is within the suggested range, but it is still considered to be high in conventional electroplating of Ni. However, transfer efficiency can be improved by application of Sc-CO2-E to allow usage of high current density. Hence high growth rate of defectfree Ni micropillars can be achieved because of transport properties of Sc-CO2, periodic-plating-characteristic (PPC),18,23 and the particle film model.15 3.1. Defect-Free Micropillars. Defect-free Ni microsctructures were fabricated by CONV with current density at 2 A/dm2 as shown in the OM micrograph in Figure 2a. As current density was increased to 10 A/dm2, a defect was found as shown in the OM micrograph in Figure 2b and SEM micrographs in Figure 3a and b. The defects are believed to be caused by hydrogen gas bubbles remaining at the bottom of the SU-8 pattern during the electroplating process. Although, saccharine should be effective in removal of hydrogen gas bubbles in Ni electroplating,8 the result showed that removal of hydrogen gas bubbles from the bottom of the SU-8 pattern was not efficient enough with only saccharine present in the electrolyte. On the other hand, defectfree Ni micropillars were obtained when ESCE was applied as shown in Figure 3c. Desorption and removal of hydrogen gas bubbles are not only promoted by physical and transport properties of Sc-CO2 but also contributed by the PPC of ESCE.18 PPC is similar to pulse plating to some extent, but the overall current is never completely turned-off in a time scale. Electrochemical reaction at a particular region on surface of cathode is turned off when it has contact with the dispersed phase, because the dispersed phase is mainly composed of Sc-CO2, where both electrical conductivity and metal salts solubility are low in ScCO2. The dispersed phase is continuously moving in the system; thus, electrochemical reaction at the particular region on surface of cathode is periodically turned on and off in the system throughout the entire reaction. Desorption of hydrogen gas bubbles is promoted in pulse plating during the off time period;24 therefore, desorption of hydrogen gas bubbles can be also promoted during “off-time” of PPC. A particle film formed around the dispersed phase is another mechanism that contributes in improving transfer efficiency according to Roha’s 8081

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Figure 3. SEM micrographs: Ni micropillars fabricated by (a) CONV at 10 A/dm2 for 3 min using 15-μm-thick SU-8 film and (b) CONV and (c) ESCE at 10 A/dm2 for 5 min using 50-μm-thick SU-8 film.

particle film models.15 In Roha’s particle film models, the particle film serves as a medium to transfer mass between the bulk solution and the boundary layer on top of the substrate. The dispersed phase is always rotating, and the particle film rotates with it. The particle film absorbs mass from the bulk solution and transfers the mass to the boundary layer when the film comes in contact with the boundary layer. On the basis of the same mechanism, mass within the boundary layer can be transferred to the bulk solution by the particle film. Combing all these effects, desorption and removal of hydrogen are significantly enhanced in Sc-CO2-E. Ni micropillars with higher aspect ratio were also fabricated by ESCE at 10 A/dm2 for 20 min using 50-μm-thick SU-8 film as shown in Figure 4, and the micropillars were still defect-free. The height and diameter of the Ni micropillars were both measured by SLM as shown in Figure 5. The average of five data points was taken each for values of height and diameter reported here. The average height was 22.7, 23.0, 23.3, and 24.0 μm for Ni micropillars fabricated using patterns with 50, 75, 100, and 125 μm in diameter on photomask, respectively. The electrical conductivity of Sc-CO2-E is much higher than pure Sc-CO, but it is still lower than the pure aqueous electrolyte;25 therefore, current efficiency is expected to be lower for ESCE. The average heights of all the Ni micropillars were 23.5 and 25.5 μm when they were electroplated at 250 mA for 5 min for ESCE and CONV, respectively. Average growth rates calculated were 4.7 and 5.1 μm/min for ESCE and CONV, respectively. The top (where it has contact with the electrolyte) and bottom (where it connects to the substrate) diameters of the Ni micropillars fabricated with ESCE at 250 mA for 5 and 20 min were measured. Figure 6 shows that the diameters of all Ni micropillars decreased from the bottom to the top. The reduction in diameter

Figure 4. SEM micrographs: Ni micropillars fabricated by ESCE at 10 A/dm2 for 20 min. 50-μm-thick SU-8 film was used.

from the bottom to the top of all Ni micropillars was calculated as shown in Figure 6. The reduction in diameter which increased with an increase in the diameter of patterns used was observed in both Ni micropillars electroplated with 5 and 20 min, and the reduction in diameter increased with an increase in deposition time. When the percent of reduction in diameter was considered, the percent of reduction in diameter decreased from 8.4 to 4.4% for the diameter of the patterns used from 50 to 125 μm when deposition time was 5 min. The percent reduction in diameter decreased from 10.7 to 6.0% when deposition time was 20 min as 8082

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Figure 5. SLM micrographs: Ni micropillars electroplated at 10 A/dm2 for 5 min by ESCE using patterns on photomask with (a) 50, (b) 75, (c) 100, and (d) 125 μm in diameter. A 50-μm-thick SU-8 film was used.

Figure 6. Relationship between diameter of the patterns on photomask with diameter of the Ni micropillars. The diameter of the Ni micropillars was measured from the top and the bottom of the Ni micropillars.

shown in Figure 7. This decrease in percent of reduction is believed to be caused by better transport efficiency for both reactants to travel to the surface of cathode and byproducts away from the surface of cathode when diameter of the patterns is increased. Defects or pinholes possibly existing inside the Ni micropillars were not examined in this study, because similar study was conducted before showing that defects and pinholes only formed at the interface between the electroplated film and the metallic substrate and on the surface of the electroplated film.26 Therefore, we believed defects and pinholes do not exist inside the Ni micropillars.

Figure 7. Relationship between diameter of the patterns on photomask with diameter reduced (primary y-axis) and percent of diameter reduced (secondary y-axis) of the Ni micropillars from the bottom to the top.

3.2. Effect of Sc-CO2-E. The homogeneity of Sc-CO2-E is an important factor for performing chemical reaction with Sc-CO2E.3 Homogeneity can be referred as uniformity and stability of the dispersed phase in Sc-CO2-E. The homogeneity of Sc-CO2-E is improved when physical properties of CO2 are adjusted to close to those of the aqueous electrolyte in the emulsion.27 Differences in physical properties between CO2 and the aqueous electrolyte increase with decrease in pressure.3 Also, hydrogen solubility in CO2 decreases with decrease in pressure. Therefore, more defects and pinholes were found on surface of the Ni micropillars when experimental pressure was decreased to 6 MPa as shown in Figure 8a. 8083

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Figure 8. SEM micrographs: Ni micropillars fabricated with either (a) pressure changed to 6 MPa, (b) volume fraction of CO2 changed to 0 vol %, or (c) volume fraction of surfactant changed to 0 vol %. Current density applied was 10 A/dm2 for 5 min and 50-μm-thick SU-8 film was used for all cases. Examples of pinholes are indicated by dotted circles; a defect is pointed out by an arrow.

The nonionic surfactant, polyoxyethylene lauryl ether, is used to form an emulsion composed of nonpolar substance and the aqueous electrolyte. When CO2 is not used in the system, the surfactant itself can still solubilize some amount of hydrogen gas bubbles produced during the electroplating reaction, but the affinity for hydrogen gas bubbles to desorb from surface of cathode to form the dispersed phase is decreased without presence of CO2 in the system. Therefore, there were still many pinholes, which is believed to be caused by adsorbed hydrogen gas bubbles on the surface of cathode. However, surface condition of Ni micropillars fabricated without CO2 (0 vol %) was better than the case when pressure was decreased to 6 MPa, as shown in Figure 8b. The emulsion cannot be formed without the addition of the surfactant, and properties of Sc-CO2 are too different with those of the aqueous electrolyte to form an uniform phase. In addition, desorption of hydrogen gas bubbles is demoted when pressure is increased from ambient pressure to 15 MPa. Therefore, both defect and pinholes were found in Ni micropillars fabricated without addition of surfactant as shown in Figure 8c. 3.3. Removal of SU-8 Film. Removal of SU-8 film is usually done by treatment with Remover PG Solution. Sc-CO2 is expected to contribute in removal of SU-8 film, because ScCO2 has high solvation power for both organic and nonpolar substances. Also, surface tension of Sc-CO2 is extremely low. It is easier for the emulsified electrolyte to penetrate into the gap between the polymeric film and the metallic surface to break the bonding between them. For ESCE, damage on the SU-8 film was observed when 15-μm-thick SU-8 film was used with deposition time more than 5 min. Therefore, deposition time was decreased to 3 min when 15-μm-thick SU-8 film was used. For 50-μm-thick

SU-8 film, no significant damage was observed after 20 min of immersion in Sc-CO2-E. When 15-μm-thick SU-8 film was used for ESCE, treatment with Remover PG Solution was not needed after electroplating process. The SU-8 film could be easily pilled off using tweezers. When 50-μm-thick SU-8 film was used, treatment with Remover PG Solution was still required. These results showed that adhesion strength of 50-μm-thick SU-8 film is higher, and this is because degree of cross-linking is increased and delamination is less likely to occur when thickness of SU-8 film is increased.28 Therefore, direct removal of SU-8 film by Sc-CO2-E is possible when thinner film thickness of SU-8 film is used.

4. CONCLUSIONS Fabrication of defect-free Ni micropillars at high current density is made possible by application of ESCE. Removal of the SU-8 film is simplified when Sc-CO2-E and 15-μm-thick SU8 film are used. A defect is found in Ni micropillars fabricated by CONV and when the surfactant is not used in the system. The size and total amount of pinholes found on the surface of the Ni micropillars increase when CO2 is not used in the system and when gaseous phase CO2 is used instead of supercritical state CO2, that is when experimental pressure is below the critical pressure of CO2. On the basis of the extraordinary properties of Sc-CO2 and the results obtained, a defect-free material and high growth rate can be acquired at the same time by application of ScCO2-E. ’ AUTHOR INFORMATION Corresponding Author

*Tel.: þ81-45-924-5631. E-mail address: [email protected]. 8084

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’ ACKNOWLEDGMENT This work was supported by Funding Program for Next Generation World-leading Researchers (NEXT Program), Cabinet Office (CAO), Japan. ’ REFERENCES (1) Gad-el-Hak, M. The MEMS Handbook; CRC, Taylor & Francis: Boca Raton, Fla., 2006. (2) Schultze, J. W.; Bressel, A. Principles of electrochemical microand nano-system technologies. Electrochima Acta 2001, 47, 3. (3) Clifford, T. Fundamentals of Supercritical Fluids; Oxford University Press: U.K., 1999. (4) Vesovic, V.; Wakeham, W. A. The Transport Properties of Carbon Dioxide. J. Phys. Chem. Ref. Data 1990, 19, 763. (5) Pierre, A. C.; Pajonk, G. M. Chemistry of Aerogels and Their Applications. Chem. Rev. 2002, 102, 4243. (6) Liang, Y.; Zhen, C.; Zou, D.; Xu, D. Preparation of Free-Standing Nanowire Arrays on Conductive Substrates. J. Am. Chem. Soc. 2004, 126, 16338. (7) Tsai, W. L.; Hsu, P. C.; Hwu, Y.; Chen, C. H.; Chang, L. W.; Je, J. H.; Lin, M. H.; Groso, A.; Margaritondo, G. Building on Bubbles in Metal Electrodeposition. Nature 2002, 417, 139. (8) Rashidi, A. M.; Amadeh, A. The Effect of Saccharin Addition and Bath Temperature on the Grain Size of Nanocrystalline Nickel Coatings. Surf. Coat. Technol. 2009, 204, 353. (9) Ke, J.; Su, W.; Howdle, S. M.; George, M. W.; Cook, D.; PerdjonAbel, M.; Bartlett, P. N.; Zhang, W.; Cheng, F.; Levason, W.; Reid, G.; Hyde, J.; Wilson, J.; Smith, D. C.; Mallik, K.; Sazio, P. Electrodeposition of metals from supercritical fluids. Proc. Natl. Acad. Sci., U.S.A. 2009, 106, 14768. (10) Howdle, S. M.; Bagratshvili, V. N. The Effects of Fluid Density on the Rotational Raman Spectrum of Hydrogen Dissolved in Supercritical Carbon Dioxide. Chem. Phys. Lett. 1993, 214, 215. (11) Darr, J. A.; Poliakoff, M. New Directions in Inorganic and Metal-Organic Coordination Chemistry in Supercritical Fluids. Chem. Rev. 1999, 99, 495. (12) McClain, J. B.; Betts, D. E.; Canelas, D. A.; Samulski, E. T.; DeSimone, J. M.; Londono, J. D.; Cochran, H. D.; Wignall, G. D.; Chillura-Martino, D.; Triolo, R. Design of Nonionic Surfactants for Supercritical Carbon Dioxide. Science 1996, 274, 2049. (13) Johnston, K. P.; da Rocha, S. R. P. Colloids in Supercritical Fluids Over the Last 20 Years and Future Directions. J Supercrit. Fluid 2009, 47, 523. (14) Lee, C. T.; Ryoo, W.; Smith, P. G.; Arellano, J., Jr.; Mitchell, D. R.; Lagow, R. J.; Webber, S. E.; Johnston, K. P. Carbon Dioxide-inWater Microemulsions. J. Am. Chem. Soc. 2003, 125, 3181. (15) Roha, D. Electrochemistry in Colloids and Dispersions; Mackay, R. A., Texter, J., Eds.; VCH: New York, 1992; p 180. (16) Tang, J.; Wang, H.; Guo, X.; Liu, R.; Dai, X.; Ding, G.; Yang, C. An Investigation of Microstructure and Mechanical Properties of UVLIGA Nickel Thin Films Electroplated in Different Electrolytes. J. Micromech. Microeng. 2010, 20, 025033. (17) Chung, S. T.; Tsai, W. T. Nanocrystalline NiC Electrodeposits Prepared in Electrolytes Containing Supercritical Carbon Dioxide. J. Electrochem. Soc. 2009, 156, D457. (18) Chang, T. F. M.; Sone, M.; Shibata, A.; Ishiyama, C.; Higo, Y. Bright Nickel Film Deposited by Supercritical Carbon Dioxide Emulsion Using Additive-Free Watts Bath. Electrochima Acta 2010, 55, 6469. (19) Chung, S. T.; Huang, H. C.; Pan, S. J.; Tsai, W. T.; Lee, P. Y.; Yang, C. H.; We, M. B. Material Characterization and Corrosion Performance of Nickel Electroplated in Supercritical CO2 Fluid. Corros. Sci. 2008, 50, 2614. (20) Rahman, M. Z.; Sone, M.; Eguchi, M.; Ikeda, K.; Miyata, S.; Yamamoto, T. Wear Properties of Nickel Coating Film Plated From Emulsion With Dense Carbon Dioxide. Surf. Coat. Technol. 2006, 201, 606.

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