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Electroless Nickel-Plated UV-Embossed Microstructured Surface with Very High Aspect Ratio Channels Yehai Yan,†,‡ Mary B. Chan-Park,*,†,‡ Jianxia Gao,† and C. Y. Yue‡ The Biological and Chemical Processing Laboratory, School of Mechanical and Production Engineering, and The Singapore-MIT Alliance, Innovation in Manufacturing Systems and Technology Program, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore Received August 8, 2003. In Final Form: December 13, 2003 The very high aspect ratio (14) and deep (132 µm) channels of an UV-embossed polymeric microstructured surface were successfully metallized by electroless nickel plating. A submicron conformal and highly adherent nickel coating was uniformly deposited on all the hydrophobic silicone-enriched surfaces of the channels. The nickel surface layer acts as a diffusion barrier and also allows for easy subsequent chemical modification by chemical grafting, electroplating, or electropolymerization for diverse applications in micromolding, microfluidics, sensors, microreactors, biomedical devices, and so forth. Hence, electroless nickel plating combined with UV embossing allows easy fabrication of high aspect ratio metallized polymeric microstructures with tailored surfaces.
High aspect ratio microstructures are of great interest due to their increased sensitivity or throughput.1 They are important for diverse applications such as sensors, flat panel multicolor displays, microfluidics, microarrays, tissue engineering, and micro-optical elements.2 Deep reactive ion etching (DRIE)3 or ultrathick photolithography using the epoxy-based NANO SU-8 photoresist4 can be used to create high aspect ratio microstructures. However, these processes are typically tedious and not easily accessible since they require specialized equipment. Replication from a microstructured mold is a more convenient, accessible, and economical route for the mass production of high aspect ratio microstructures.5 UV embossing, a fast microreplication method, can be used for the fabrication of high aspect ratio polymeric microstructures.6 UV embossing does not require high temperature or pressure, which can potentially damage the master mold. However, in many applications, the microstructures should have inert or metallic or chemically tailored surfaces. For example, the surface of a mold for liquid molding (such as by soft lithography5 or UV embossing6) should have low interaction with the molding resin to ensure its durability. UV embossing can be used for rapid replication of polymeric molds from either a DRIE or SU-8 master mold. However, we have found that residual molding resin gradually accumulates on an UVembossed polymeric mold with high aspect ratio channels after only several uses even though its surface has been made hydrophobic. It is known that UV curing of mul* Corresponding author. Tel: (65) 6790 6064. Fax: (65) 6792 4062. E-mail:
[email protected]. † The Biological and Chemical Processing Laboratory, School of Mechanical and Production Engineering. ‡ The Singapore-MIT Alliance, Innovation in Manufacturing Systems and Technology Program. (1) Becker, H.; Gartner, C. Electrophoresis 2000, 21, 12. (2) Madou, M. J. Fundamentals of Microfabrication: The Science of Miniaturization; CRC Press: Boca Raton, FL, 2002. (3) Chung, C. K.; Lu, H. C.; Jaw, T. H. Microsyst. Technol. 2000, 6, 106. (4) Lorenz, H.; Despont, M.; Fahrni, N.; Brugger, J.; Vettiger, P.; Renaud, P. Sens. Actuators, A 1998, 64, 33. (5) Ng, H. T.; Koehne, J. E.; Stevens, R. M.; Li, J.; Meyyappan, M.; Han, J. Nano Lett. 2002, 2, 961. (6) Chan-Park, M. B.; Yan, Y. H.; Neo, W. K.; Zhou, W. X.; Zhan, J.; Yue, C. Y. Langmuir 2003, 19, 4371.
Figure 1. Scheme of the procedure for rapid fabrication of a metallized UV-embossed microstructure (aspect ratio ) h/d): (a) master fabricated using deep reactive ion etching, (b,c) PDMS mold replication from the master, (d,e) UV embossing of polyester microstructures, and (f) metallization of polyester embossing via electroless nickel plating after argon plasma treatment, UV grafting, and Pd2+ activation.
tifunctional monomers used for the UV-embossed mold generally does not result in complete conversion.7 The liquid molding resin molecules gradually diffuse into the incompletely UV cured polymeric mold with repeated use. This problem is aggravated with high aspect ratio molding due to the higher mold surface area. To increase the durability of a polymeric mold, a nickel barrier surface coating may be applied. Nickel has the desired high hardness, wear resistance, and corrosion resistance.8 Nickel coating is also widely used for electronics compo(7) Andrzejewska, E. Prog. Polym. Sci. 2001, 26, 605.
10.1021/la035454z CCC: $27.50 © 2004 American Chemical Society Published on Web 01/17/2004
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Table 1. Comparison of the Dimensions of Microstructures (Microchannels for the Silicon Mold, Polyester Embossing, and Nickel-Plated Polyester Mold; Microwalls for the PDMS Mold and PEGDA Embossing)a microstructure
width (µm)
depth or height (µm)
silicon mold PDMS mold polyester embossing nickel-plated polyester mold PEGDA embossing
9.4 ( 0.3 9.4 ( 0.2 9.0 ( 0.8 8.3 ( 0.6 8.1 ( 0.3
131.6 ( 0.1 131.5 ( 0.7 130.2 ( 1.0 130.0 ( 0.8 126.2 ( 0.5
a Each datum reported here is an average of at least five measurements.
nents and electromagnetic interference (EMI) shielding because of its solderability, high diffusion barrier resistance, and low electrical resistance.8-10 Many microsys-
tems are highly fragile and need EMI shielding.11 Electroless nickel coating containing low phosphorus content possesses ferromagnetism and high wear resistance, making it a suitable choice for thin film micromagnets and microactuators.12 The nickel surface can also be easily modified by silanes, self-assembled monolayers of nalkanethiols,13 conducting polymers,14 or other electroplated metals for use in corrosion resistant coating, sensors, lubrication, interfacial reactivity, microreactors, biomedical devices, and so forth. Electroless nickel plating is ideal for nickel deposition on nonconductive polymeric surfaces. However, electroless nickel plating over a three-dimensional (3D) polymeric microstructured surface with deep, narrow, and nearly vertical (that is, “high aspect ratio”) channels has not been
Figure 2. (A) Silicon master, (B) PDMS microstructure replicated from the silicon master, (C) polyester microstructure replicated from the PDMS microstructure, and (D) metallized polyester microstructure: (a,b) SEM images; (c) z-profile along the X-X′ line.
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Figure 3. Cross-section of metallized polyester microstructure: (a) overview; (b) enlarged horizontal top; (c) enlarged vertical section; (d) enlarged bottom section.
Figure 4. SEM image of the cross-section of a tension-failed nickel-plated flat polymer sample.
previously reported. It has been shown that UV grafting of nitrogen-containing monomer enables successful electroless nickel plating on flat polymeric surfaces.15,16 It is not obvious that the vertical walls of high aspect ratio channels can be sufficiently UV-grafted to enable uniform and conformal deposition of nickel. Further, the surface of our UV-embossed polymeric microstructure is typically silicone-enriched, making adhesion of nickel to it even more difficult without proper surface treatment. We have found that the silicone mold release additive, added in small amounts (less than 5%) to facilitate demolding of the UV embossing, typically saturates at the surface. Hence, the surface of the UV-embossed microstructure is typically hydrophobic and silicone-enriched, both of which inhibit the adhesion of metallic nickel. Electroless nickel plating on a silicone-enriched surface has also not previously been demonstrated. Using appropriate surface
modification and processing conditions, we show that highly adherent electroless nickel plating on the surface of deep (132 µm), very high aspect ratio (14), and nearly vertical channels of an UV-embossed microstructure can be successfully achieved. A schematic of the process is shown in Figure 1. A silicon master was fabricated using DRIE and passivated with octafluorocyclobutane (C4F8) in accordance with a published procedure.3 The passivated silicon master had microchannels measuring 1 mm long, 9.4 µm wide, and 131.6 µm deep (i.e., aspect ratio of 14) separated by 80.0 µm wide silicon walls (Figure 2Aa-c and Table 1). Silastic J RTV (Dow Corning), processed according to the supplier’s recommendations,17 was used for the replication of the DRIE silicon master. Then, an oligomeric polyester tetraacrylate (EB830 from UCB Chemicals) with fairly high cured rigidity (measured storage modulus of 1.83GPa) was used for UV embossing from the poly(dimethylsiloxane) (PDMS) mold according to a published procedure.6 The UV embossing resin consisted of 68/20/10/1.8/0.2 w/w EB830, dipropylene glygol diacrylate, trimethylolpropane triacrylate, EB350, and Irgacure 651. EB350 is a silicone diacrylate release additive obtained from UCB Chemicals, (8) Mallory, G. O.; Hajdu, J. B. Electroless Plating: Fundamentals and Applications; America Electroplaters and Surface Finishers Society: Orlando, FL, 1990. (9) Huang, C. Y.; Pai, J. F. Eur. Polym. J. 1998, 34, 261. (10) Shinagawa, S.; Kumagai, Y; Urabe, K. J. Porous Mater. 1999, 6, 185. (11) Janting, J.; Branebjerg, J.; Rombach, P. Sens. Actuators, A 2001, 92, 229. (12) Bozzini, B.; Sidorov, V. E.; Dovgopol, A. S.; Birukov, J. P. Int. J. Inorg. Mater. 2000, 2, 437. (13) Mekhalif, Z.; Laffineur, F.; Couturier, N.; Delhalle, J. Langmuir 2003, 19, 637. (14) Labaye, D. E.; Jerome, C.; Geskin, V. M.; Louette, P.; Lazzaroni, R.; Martinot, L.; Jerome, R. Langmuir 2002, 18, 5222. (15) Yang, G. H.; Kang, E. T.; Neoh, K. G. Appl. Surf. Sci. 2001, 178, 165. (16) Zhang, Y.; Tan, K. L.; Yang, G. H.; Kang, E. T.; Neoh, K. G. J. Electrochem. Soc. 2001, 148, C574. (17) Product Information of Dow Corning from www.dowcorning.com.
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and Irgacure 651 is a photoinitiator of 2,2-dimethoxy-2phenylacetophenone supplied by Ciba Chemicals. Other monomers were obtained from Sartomer Chemicals. The UV resin was irradiated using a 400 W Hg lamp (areaaveraged UV intensity at 365 nm of 110 mW/cm2) for 50 s. After demolding, the UV embossing was further irradiated for another 20 s in an argon atmosphere to complete the curing. The UV-embossed polyester surface was then pretreated by argon plasma using a plasma discharge power of 100 W, treatment time of 30 s, and argon flow rate of 350 sccm in a March PX-500 cleaning system. After “aging” in air for at least 30 min, the plasma-pretreated polyester surface was UV-irradiated for 30 s in the presence of 1-vinylimidazole monomer (Aldrich Chemical) using an UV intensity of 110 mW/cm2 (365 nm). The UV-grafted polyester microstructure was thoroughly rinsed with deionized (DI) water to remove residual ungrafted monomer. It was then activated by immersion in Pd2+ catalyst solution containing 1.0 mg/mL PdCl2 and 10 mg/mL HCl (37 wt %) for 10 min. After the polyester surface was thoroughly rinsed with DI water, the electroless plating was carried out in a nickel solution, PEN-99 (Plaschem Specialty Products, Singapore), at 90 °C for 2 min. The C4F8 passivation on the silicon master mold produces a Teflon-like coating that enables the replicated PDMS mold to be simply peeled from the master without defect or cohesive failure; without this passivation, the rubber has been found to break within the channels of the silicon mold during demolding. Due to the low linear shrinkage of Silastic J RTV (0.1%),17 the PDMS mold was found by optical profilometry to accurately reproduce the microstructural dimensions of the silicon master (Figure 2Ba-c and Table 1). The polyester was successfully UVembossed from the silicone rubber, again with good dimensional fidelity (Figure 2Ca-c and Table 1). Another oligomeric epoxy diacrylate (EB600 from UCB Chemicals) was also investigated as the primary formulation constituent in place of EB830, but this adhered strongly to the PDMS leading to unsuccessful demolding. The highly cross-linked polyester has high thermal stability; this is essential for electroless nickel plating, which is done at 90 °C. No glass transition at any temperature below 200 °C was observed on scans using a differential scanning calorimeter and a dynamic mechanical analyzer. Figure 2Da,b shows scanning electron microscopy (SEM) images of the metallized polyester microstructure. There was no gold coating used for the SEM imaging of the nickelplated UV-embossed microstructure. The width of the microchannel decreased by approximately 0.7 µm due to the nickel-plated layer (Figure 2Dc and Table 1). Despite the narrowness and great depth of the channels, the successive steps required for adherent metallization, namely, argon plasma pretreatment, UV grafting, and chemical nickel deposition, were all successful. Figure 3a-d shows the cross-section of fractured nickel-plated UV-embossed microstructure. A nickel layer (0.3-0.4 µm thick) has been uniformly deposited on the top, vertical, and bottom faces of the deep channels of the 3D polyester microstructure. The coating thickness was observed to be more or less uniform on all the surfaces. Figure 4 shows the cross-section of a metallized flat polyester sample broken in tension. No delamination between the nickel layer and the polyester was observed, indicating good adhesion between the two. As an illustration of the use of nickel-plated UVembossed microstructures, the plated polyester surface was further deposited with an antistick release coating and used as a mold for a further step of UV embossing.
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Figure 5. Embossed poly(ethylene glycol) diacrylate microstructures: (a,b) SEM images; (c) z-profile along the X-X′ line.
Figure 5 shows UV-embossed very high aspect ratio microstructures formed from a mixture containing primarily poly(ethylene glycol) diacrylate (PEGDA), which is a biocompatible material that has promising biological applications.18,19 Detailed descriptions of the composition and the procedure of UV embossing of PEGDA resin can be found in ref 6. PEGDA-containing microarrays can be used for protein encapsulation.6 The microwalls of the PEGDA embossing were 8.1 µm wide and 126.2 µm high (i.e., aspect ratio of 16) as summarized in Table 1. For comparison, the dimensions of the different generation microstructures are also summarized in Table 1. The reductions in dimensions (width and depth or height) of the polyester embossing and PEGDA molding are at(18) Bryant, S. J.; Nuttelman, C. R.; Anseth, K. S. J. Biomater. Sci., Polym. Ed. 2000, 5, 439. (19) Lee, K. Y.; Labianca, N.; Rishton, S. A.; Zolgharnain, S.; Gelome, J. D.; Shaw, J.; Chang, T. H.-P. J. Vac. Sci. Technol., B 1995, 13, 3012.
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tributed to modest shrinkage of the polyester and PEGDA formulations during UV curing. The small reduction in channel depth and width of the nickel-plated polyester mold (compared to the dimensions of the polyester embossing) is due to the nickel layer. Thus, within measurement error, good fidelity was achieved with UV embossing of PEGDA from the nickel-plated polyester mold, which itself was faithfully and quickly replicated from the silicon master. In our application example, the nickel acts as a diffusion barrier to promote the durability of the embossed polyester mold and allows the antistick release coating to adhere to it. The nickel surface can also be modified by other techniques such as chemical grafting, electroplating, or electropolymerization for diverse applications. It is envisaged that electroless plating of other metals, such as copper, gold, cobalt, silver, or platinum, can also be used for the metallization. In summary, electroless plating was successfully used to deposit a uniform, conformal, and adherent submicron thick nickel layer around the walls
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of very high aspect ratio (14) and deep (132 µm) channels in an UV-embossed microstructure. Electroless nickel plating combined with UV embossing can be used for rapid fabrication of polymeric microstructures with very high aspect ratio channels and tailored metallized surfaces. The technique does not require “clean-room” processing conditions or other expensive specialized facilities. Also, this technique is not restricted to a specific master fabrication method or polymer formulation; it can be easily extended to many other master fabrication techniques and rigid polymers with high Tg. Acknowledgment. This research was supported by an A-STAR (Singapore) grant (Project No. 022 107 0004). Y. H. Yan acknowledges the financial support of the Singapore-MIT Alliance Program (IMST) through a postdoctoral fellowship. LA035454Z
