s. kawata 1,3,4 microstructures by site-selective metal

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1 Appl. Phys. A 90, (2008) DOI: /s Applied Physics A Materials Science & Processing t. tanaka 1,2 s. kawata 1,3,4 microstructures by site-selective metal n. takeyasu 1, Fabrication of 3D metal/polymer coating 1 Nanophotonics Laboratory, RIKEN (The Institute of Physical and Chemical Research), Hirosawa 2-1, Wako, Saitama , Japan 2 PRESTO, JST, Honcho Kawaguchi, Saitama , Japan 3 Department of Applied Physics, Osaka University, Suita, Osaka , Japan 4 CREST, JST, Honcho Kawaguchi, Saitama , Japan Received: 2 May 2007/Accepted: 21 September 2007 Published online: 26 October 2007 Springer-Verlag 2007 ABSTRACT Three-dimensional silver/polymer conjugated microstructures were fabricated by site-selective metal deposition on photopolymer structures in the sub-micrometer scale. Photopolymerizable resins with and without an amide group were independently prepared, and a three-dimensional polymer structure was fabricated with those resins by means of the twophoton-induced photopolymerization technique to confine the photopolymerization to a sub-micrometer volume. Silver was selectively deposited on the surface of the amide-containing polymer parts by electroless plating. This method can provide 3D arbitrary silver/polymer composite microstructures with sub-micrometer resolution. PACS b; c; Pr 1 Introduction Interactions between light and metallic nano- and micro-structures have been extensively studied recently. In particular, there has been a great deal of attention on the coupling of electromagnetic fields with mass-electrons on such scales in plasmonics, because the kinds of metals and geometrical structures that can be used exhibit a wide variety of interesting phenomena, such as enhancement of the electric field; these features are expected to be valuable in many applications. One area that is enthusiastically studied in this field is meta-materials with unusual electromagnetic properties, for example, a negative refractive index in the optical regime. A negative value of the refractive index can be achieved when both the permittivity and the permeability are negative [1]. Split-ring resonators have been proposed to realize negative permeability [2, 3]. So far, a resonant frequency in the near-infrared region has been obtained experimentally in such devices by using e-beam lithography [4], steadily approaching the visible region [5]. Another interest in this field is fabrication of three-dimensional (3D) meta-materials, which are volumetric medium. The split-ring resonator mentioned above is a planar ring structure, and each resonator can interact only with the electromagnetic waves whose mag- Fax: , ntakeyasu@postman.riken.jp netic components penetrate the plane of the ring structure. In practice, therefore, those resonators should be aligned threedimensionally in order to interact with incident light at any incident angle. Fabrication techniques for 3D metallic nanoand micro-structures are desired not only for practical applications in plasmonics and photonics, but also in other fields such as MEMS. One promising candidate for fabricating 3D meta-materials is two-photon induced photopolymerization (TPIP) [6, 7]. In this method, high-power femtosecond laser pulses are tightly focused in a photopolymerizable resin, and polymerization is induced through the two-photon process within the focal volume. This results in the creation of polymer spots that are smaller than the diffraction limit owing to the nonlinear nature of the two-photon absorption process. We have previously reported the fabrication of an entire silver-coated 3D structure using a combination of TPIP and electroless silver deposition [8 10]. It has also been reported that 3D metal microstructures are fabricated by reducing metal ions directly through the two-photon process [11, 12]. To advance these works, we have developed a site-selective metal deposition technique capable of fabricating more complex structures. Use of resin monomers or oligomers coupled with suitable functional groups, which are receptive to metal deposition, enables site-selective metal coating at the surface of a polymer structure. Furthermore, the combination of activated and non-activated resins allows more complex metal/polymer composite structures. Control of the geometry and chemical properties at the surface of the polymer structures extends the possibility of designing 3D metal structures. Methacrylamide was added to a resin in order to act as an activator for metal deposition, and a 3D metal/polymer composite structure was fabricated with sub-micrometer resolution using TPIP and site-selective metal coating. 2 Metal deposition onto activated polymer Deionized water (Milli-Q water, > 18.2MΩ cm) was used in all experiments. We used a photopolymerizable resin (KC1102, JSR Co., Ltd.), methacrylamide (CH 2 C(CH 3 )CONH 2, 98%, Aldrich), N,N-dimetyl formamide (HCON(CH 3 ) 2, > 99.5%, Junsei Chemical Co., Ltd.), and acetone (CH 3 COCH 3, > 99.5%, Wako Pure Chemical). For the silver deposition, we used silver nitrate (AgNO 3, > 95%,

206 Applied Physics A Materials Science & Processing Junsei Chemical Co., Ltd.), ammonia water (NH 3, 28%,Wako Pure Chemical), and glucose (C 6 H 12 O 6, > 98%, WakoPure Chemical).

2 206 Applied Physics A Materials Science & Processing Junsei Chemical Co., Ltd.), ammonia water (NH 3, 28%,Wako Pure Chemical), and glucose (C 6 H 12 O 6, > 98%, WakoPure Chemical). The methacrylamide was dissolved in the N,Ndimetyl formamide to prepare a 15 wt. % solution. This solution was mixed with the KC1102 to form a 5wt. % solution of methacrylamide. We call this solution the activated resin. We prepared a sheet of unmodified KC1102 by sandwiching liquid resin between a pair of glass slides with 50-µm-thick polyethylene sheet (LC522, Japan Polyethylene Co.) used as spacers. The resin was polymerized by ultraviolet irradiation (UI-100, Ushio) for 1min. After polymerization, the glass slides were removed and the polymer sheets were washed with acetone. Each polymer sheet was more than 10 mm in diameter. The transmission spectra of the samples were measured with a spectrophotometer (UV-2550, Shimadzu) from near-infrared to the UV. Photographs of the samples prepared for the measurement of transmission spectra are shown in Fig. 1. We measured the transmittance of polymerized films with the spectrophotometer over a wide wavelength range from 350 to 800 nm and observed a transmittance of about 90% from 400 to 800 nm, as shown in Fig. 2. The inset of Fig. 1a shows a photograph of a polymer film made of the activated resin (γ ), which is transparent. Although not shown here, absorption spec- tra of the non-activated and activated polymers were also measured in the infrared region using a Fourier-transform infrared spectrometer (IR Prestige-21, Shimadzu), and the spectra were compared to confirm incorporation of the amide group. A noticeable difference in absorption between the two samples was found at 1670 cm 1. Weak and broad absorption was also observed from 3400 cm 1 to 3500 cm 1 in the activated polymer. These absorption bands arise from the primary amide group (R CO NH 2 ) of the additive chemicals. Subsequently, the two samples were immersed in 0.05 mol/l AgNO 3 (aq) solutions at 310 K for 24 h, and were then washed with water and desiccated. The activated polymer appeared brown (α), whereas the non-activated polymer remained no color (β), as shown in Fig. 1a. The brown color of the activated polymer was caused from the deposition of the silver nano-particles. Those transmission spectra are shown in Fig. 2. This contrast is the key for site-selective deposition, because it allows localized metal deposition. Figure 2b also shows measured transmission spectra of activated polymer samples for different soaking times up to 72 h. The initially transparent film started to turn yellow after 3h. Subsequently, an absorption band at 445 nm became more pronounced as the soaking time increased. These results suggest that the additive methacrylamide caused adsorption of silver grains at the surface of the polymer film. These grains served as seeds FIGURE 1 (a) Polymer films from the activated (α) and non-activated (β) resin after soaking for 24 h in the Ag solution; the inset (γ ) shows polymerized film made from activated resin by UV irradiation; (b) the activated polymer film after silver deposition by electroless plating FIGURE 2 Transmission spectra of (a) non-activated and (b) activated polymer films with different soaking times, measured from 350 nm to 800 nm

TAKEYASU et al. Fabrication of 3D metal/polymer microstructures by site-selective metal coating 207 FIGURE 3 Schematic diagram of the fabrication process for metal/ polymer 3D microstructures.

3 TAKEYASU et al. Fabrication of 3D metal/polymer microstructures by site-selective metal coating 207 FIGURE 3 Schematic diagram of the fabrication process for metal/ polymer 3D microstructures. The process can be divided into 3 steps: (a) fabrication of non-activated polymer structures, (b) fabrication of activated polymer structures, and (c) pretreatment and silver coating by electroless plating for the subsequent electroless plating because the adsorbed silver acted as a catalyst for further metal reduction [13]. 0.2mol/L silver nitrate solutions, 5.6% ammonium water, and 1.9mol/L glucose in water were prepared for silver plating. The samples were immersed in a mixture of the 0.2mol/L AgNO 3 (aq) solution, diluted NH 3 water, and glucose solution with a volumetric ratio of 5 : 3 : 8. The temperature was kept at 310 K. The samples were removed from the plating solution after 5minand rinsed with acetone and water. The temperature was kept at 310 K. The samples were removed from the plating solution after 5minand washed with acetone and water. As shown in Fig. 1b, the brown-colored activated polymer sheet was readily coated with silver by electroless plating and exhibited a metallic luster. The transmission spectrum of the activated resin sheet after electroless plating was also measured and is shown in Fig. 2b. From this spectrum, almost no transmission was observed in the visible region. The plated sample was conductive, whereas the brown film shown in Fig. 1a indicated no conductivity. The resistivity of the silver film was measured to be 12 µω cm. On the other hand, no silver was deposited on the original polymer film even after electroless plating was completed. It is crucial to avoid absorption (more precisely, single-photon absorption) and scattering in the resin itself at the laser wavelength during 3D fabrication because they inevitably lead to a loss of the laser power inside the resin. Additionally, the reactivity between the base monomers and the additive monomers should be considered in order to produce smooth polymer structures. If the reactivity is poor, more laser power is required, which causes bubbles inside the resin during fabrication. These bubbles lead to scattering of the incident light and voids in the polymer structure, resulting in poor resolution. Monomers suitable for radical polymerization are restricted to vinyl monomers with functional groups such as acrylate, stylene, telephtalate. Acry-

208 Applied Physics A Materials Science & Processing late is frequently used as a photopolymer because of its high transparency in the visible region.

3 Site-selective silver coating on 3D polymer nano-/micro-structures A schematic diagram of the fabrication process for metal/polymer 3D microstructures is shown in Fig. 3. A femtosecond Ti:sapphire laser with an emission wavelength of 796 nm, an output power of 1.

4 208 Applied Physics A Materials Science & Processing late is frequently used as a photopolymer because of its high transparency in the visible region. In fact, KC1102 that we used in our experiments is an acrylate. In order to satisfy the conditions mentioned above, methacrylamide was employed as the additive monomer. 3 Site-selective silver coating on 3D polymer nano-/micro-structures A schematic diagram of the fabrication process for metal/polymer 3D microstructures is shown in Fig. 3. A femtosecond Ti:sapphire laser with an emission wavelength of 796 nm, an output power of 1.6W, a pulse width of 138 fs,and a pulse repetition rate of 1kHzwas used for a light source of TPIP. The laser was focused into the non-activated photopolymerizable resin by an oil-immersion objective lens (60, NA = 1.4) after passing through a microlens array (50 50 lens elements, diameter = 300 µm) [14]. The microlens array was provided for simultaneously fabricating more than 750 structures over a wide area [8]. The laser pulses were attenuated with neutral density (ND) filters down to an average power of 300 mw. The exposure time required to create each polymer spot was 200 ms. The laser spots were scanned in the resin to fabricate polymer squares (size 2 µm 2 µm; center-to-center spacing 5.5 µm). The uncured resin was completely washed away with acetone. After desiccation, a small amount of the activated resin was dripped onto the fabricated structures, and the same procedure described above was repeated to form pairs of polymer layers, one composed of the non-activated resin and the other composed of the activated resin on a glass substrate. The fabricated polymer samples were soaked in 0.05 mol/l AgNO 3 (aq) solution at 310 K. After at least 6h, the samples were removed from the solution and washed with water. Subsequently, the silver plating was carried out at 310 K. The samples were removed from the plating solution after 5minand washed with acetone and water. Images of the samples were observed with an optical microscope (40, NA = 0.70). We prepared polymer micropatterns made from the two resins on a glass substrate, and the sample was immersed in the 0.05 mol/l AgNO 3 (aq) solution for 6h as a pretreatment. Figure 4a shows a transmission image of the sample after the pretreatment. All upper-left squares were composed of the non-activated resin, and all bottom-right squares were composed of the activated resin. These two squares before the pretreatment of AgNO 3 are both transparent, and they are not distinguished from each other through the optical microscope. After the pretreatment, we found that the activated resin parts were covered with Ag nano-particles and appeared slightly darker than the non-activated parts as shown in Fig. 4a. We attempted to deposit silver on the sample by electroless plating. A transmission image of the sample after this procedure is shown in Fig. 4b. From this image, we observed that all of the bottom-right squares appeared opaque, while all of the upper-left squares remained transparent, thus confirming the deposition of silver. The main advantage of the two-photon method is its ability to directly create a 3D nano/microstructure with an arbitrary shape. To demonstrate this, we attempted to fabricate a complex 3D structure, namely, silver ring structures, using the activated resin, on a polymer square made from the nonactivated resin. We fabricated polymer structures using an average laser power of 300 mw for both resins due to the multiple fabrications, and silver plating was carried out with the same procedure mentioned before. Then, the fabricated sample was observed with a scanning electron microscope (SEM) after washing and desiccation. Figure 5a shows an SEM image of the polymer structures before silver deposition. Each ring had an inner diameter of 0.7 µm, and each square was 2 µm 2 µm. The line width of each ring was about 200 nm. Figure 5b shows an SEM image of the sample after silver deposition. We observed that the glass substrate and the rings were selectively coated with silver, whereas no silver was found on the surface of the non-activated squares. These results indicate the viability of this technique for site-selective metal deposition with sub-micrometer resolution. By using a two-photon technique, it is feasible to fabricate an arbitrary 3D fine structure which FIGURE 4 Microscope images of pairs of polymer sheets made by TPIP with the non-activated resin and the activated resin after (a) soaking in Ag solution for 6 h; and (b) electroless plating. The size of each square sheet is 2 µm 2 µm FIGURE 5 SEM images of (a) polymer structures for site-selective silver deposition; (b) selectively silver coated microstructures; and (c) T -shaped polymer structures after electroless plating, whose structures were prepared with KC1102 and the modified resin used in [8, 9], respectively. The inset shows the magnified image of a pair of those structures

TAKEYASU et al. Fabrication of 3D metal/polymer microstructures by site-selective metal coating 209 FIGURE 6 SEM image of silver/polymer 3D microstructures.

5 TAKEYASU et al. Fabrication of 3D metal/polymer microstructures by site-selective metal coating 209 FIGURE 6 SEM image of silver/polymer 3D microstructures. The size of the structures is 2 µm 2 µm 2 µm is not obtainable with lithography technique. Compared to the direct metal reduction with two-photon absorption, which can also provide 3D metallic fine structures, the fabrication process in this letter is more complicated because 3D polymer structures should be prepared before metal coating. However, in the case of the direct metal reduction, the laser should be irradiated from the same direction as the metal growth, because the reduced metal itself blocks the laser and interferes with the further fabrication in the longitudinal direction of the laser. On the other hand, in the polymer case, the laser can penetrate the solidified polymer structures which give also almost no change of the refractive index. Therefore, the configuration of the irradiation is not restricted, and then, this method gives a neat 3D metallic structure. Another SEM image of a sample fabricated with KC1102 and our previous resin used in [8, 9] is shown in Fig. 5c. From this image, it is obvious that the selectivity of silver coating was poor compared to Fig. 5b. Here, about one thousand pairs of T -shaped patterns were prepared; however, more than half of the structures with the conventional UV resin were partially or completely coated with silver, as shown in the inset. This poorer selectivity arises from the less silver-adhesive property of the previous resin. In [8, 9], SnCl 2 was additionally used to enhance the silver deposition on the polymer structure. The pre-treatment of SnCl 2 is useful for metal coating onto a polymer surface because Sn 2+ ions can combine with oxygen atoms at the surface of polymer and subsequently reduce metal ions. However, this means that Sn 2+ ions interfere the selective metal deposition due to the existence of the oxygen atoms in the conventional UV resin. Therefore, it was necessary to deposit metal onto the polymer structures without SnCl 2 in the case of the selective metal deposition. On the other hand, the UV resin with methacrylamide does not require SnCl 2. This is the reason why most of the rings were selectively coated with silver, as shown in Fig. 5b. The advantage of the two-photon method is fabrication of 3D structures. Here, we show an example of the fabricated 3D silver/polymer microstructures in Fig. 6. Pairs of cubic-frame structures with two kinds of resins are prepared as templates. The up-right structures indicated by white arrow are fabricated using the activated resin, and only these structures were coated with silver during electroless plating process. As we have shown in our previous work [8, 14], any 3D shapes, such as a coil can be fabricated with the two-photon method. In the case of 3D fabrication, it is important to fabricate self-standing rigid polymer templates before silver coating, although it strongly depends also on the material properties of the polymer structures. The polymer structures fabricated with the modified resin were not broken even when they were washed, which means the polymer structures are strong enough for 3D fabrication. 4 Conclusion We conclude that silver deposition was successfully controlled on the polymer nano/microstructures fabricated by TPIP with the chemical modification of the resin. The modified resin, prepared using methacrylamide, exhibited good silver deposition activity, whereas no silver was deposited on the conventional UV resin. Therefore, the active/non-active polymer structure was prepared with those two resins, and a metal/polymer complex structure was obtained after electroless plating. Three-dimensional complex metal structures can be fabricated by using the technique introduced in this letter, and it is expected to enable fabrication of 3D metallic meta-materials in the various frequency regime. ACKNOWLEDGEMENTS This work was supported in part by Japan Society for the Promotion of Science (JSPS) and Japan Science and Technology Agency (JST). REFERENCES 1 V.G. Veselago, Sov. Phys. Uspekhi 10, 509 (1968) 2 J.B. Pendry, A.J. Holden, D.J. Robbins, W.J. Stewart, IEEE Trans. Microw. Theor. Tech. 47, 2075 (1999) 3 R. Shelby, D.R. Smith, S. Schultz, Science 292, 77 (2001) 4 C. Enkrich, F. Perez-Willard, D. Gerthsen, J. Zhou, T. Koschny, C.M. Soukoulis, M. Wegener, S. Linden, Adv. Mater. 17, 2547 (2005) 5 A. Ishikawa, T. Tanaka, S. Kawata, Phys. Rev. Lett. 95, (2005) 6 S. Maruo, O. Nakamura, S. Kawata, Opt. 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