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Metal patterning on silicon surface by site-selective electroless deposition through colloidal crystal templating
Journal of Colloid and Interface Science 316 (2007) 547–552 www.elsevier.com/locate/jcis
Metal patterning on silicon surface by site-selective electroless deposition through colloidal crystal templating Hidetaka Asoh ∗ , Seiji Sakamoto, Sachiko Ono Department of Applied Chemistry, Faculty of Engineering, Kogakuin University, 1-24-2 Nishi-shinjuku, Shinjuku-ku, Tokyo 163-8677, Japan Received 10 July 2007; accepted 3 September 2007
Abstract Site-selective Cu deposition on a Si substrate was achieved by a combination of colloidal crystal templating, hydrophobic treatment, and electroless plating. Uniformly sized nano/microstructures were produced on the substrate using a monolayer coating of colloidal spheres instead of a conventional resist. The Cu patterns obtained were of two different types: networklike honeycomb and isolated-island patterns with a minimum period of 200 nm. Each ordered pattern with the desired intervals was composed of clusters of Cu nanoparticles with a size range of 50–100 nm. By the present method, it is possible to control the periodicity of metal arrays by changing the diameter of the colloidal spheres used as an initial mask and to adjust the shape of the metal patterns by changing the mask structure for electroless plating. © 2007 Elsevier Inc. All rights reserved. Keywords: Metal pattern; Electroless plating; Colloidal crystal templating; Hydrophobic treatment
1. Introduction Two-dimensional (2D) patterned metals such as wire, tube, and dot arrays that are formed on semiconductors have attracted considerable attention because of their potential applications in magnetic, electronic, and optoelectronic devices. The common methods used in patterning metals are based on deposition processes using a physical template as a mask. However, patterning methods based on the dry process such as vapor deposition and sputtering are costly and not suited to the production of an ordered pattern with feature sizes ranging from micrometers to nanometers over a large area for industrial applications. In terms of fabrication simplicity and efficiency, electrochemical methods are economical, e.g., electrodeposition and electroless plating. For example, noble metals (i.e., Ag, Pt, and Au), which were deposited on a Si substrate, have recently received attention as a catalyst for producing textured Si surfaces for applications in Si-based optoelectronics such as solar cells [1–3]. We also reported that Si convex arrays and Si hole arrays with ordered periodicities have been successfully fabri* Corresponding author. Fax: +81 42 628 5647.
E-mail address: [email protected] (H. Asoh). 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.09.001
cated by the site-selective chemical etching of a Si substrate using patterned Ag nanoparticles as a catalyst [4,5]. Based on our previous work, the closely packed structure of microspheres formed on a Si substrate could act as a mask during electroless plating. The Ag pattern, which could be produced only in interspaces among spheres on a substrate, has a structure opposite to that of the 2D hexagonal arrays of microspheres used as a mask. Although there have been numerous reports on the fabrication of an inverse opal structure from a wide variety of materials, including polymers, ceramic materials, inorganic semiconductors, and metals, reports on the applications using 2D crystalline arrays of microspheres as a mask have been relatively few, particularly on metal deposition by the electrochemical process [6,7]. For details of the applications of colloidal crystals including 2D crystalline arrays, see separate review papers [8,9]. Electrochemical processes, however, have advanced features: for example, the dimensions of the desired structures can be adjusted easily by controlling electrolytic conditions, in comparison with the conventional dry process. In particular, a thin uniform metal layer can be precisely controlled, because the metal layer grows directly from the bottom of colloidal crystals on the underlying conductive substrate.
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Previously, we proposed the micropatterning of a solid substrate based on the electrochemical process using colloidal crystals as a mask [10–13]. We continued our preliminary work to control the fabrication of Si microstructures using colloidal crystals as a mask and examined the metal deposition behavior on a Si substrate during electroless plating. In particular, we focused on the controllability of the periodicity of metal pattern and tried to downsize the resultant metal pattern from micrometer scale to nanoscale. Here, we describe the fabrication of an ordered Cu pattern on Si surfaces by site-selective electroless plating using colloidal crystals as a mask. Moreover, the adjustment of the geometric pattern of metals is also investigated from the viewpoint of the application to various scientific fields, such as those related to chemical sensors and electronic devices. 2. Experimental 2.1. Fabrication of networklike honeycomb Cu pattern Patterned metal arrays were fabricated on a Si substrate by a combination of colloidal crystal templating, hydrophobic treatment, and electroless plating. By colloidal crystal templating, it is possible to fabricate negative and positive patterns by changing the mask structure for electroless plating. The principle of pattern transfer, which is similar to that in our previous studies [4,5], is schematically shown in Fig. 1. Si wafers (p type, 0.001 cm, (100) crystal orientation) were cut into the desired size, e.g., 1 × 1 cm, and used as the substrate. The Si wafers were precleaned in 1 wt% HF for 5 min to remove organic contaminants and native oxide on their surface. A monodisperse suspension of polystyrene (PS) microspheres (Polysciences, Inc.) was dropped into the region surrounded by the silicon rubber O-ring of 5.5 mm diameter on the substrate. The suspension on the substrate was dried in air, and spheres self-assembled into a close-packed structure with three-dimensional ordered lattices via attractive capillary forces were observed [14]. After the complete evaporation of the water solvent, the Si substrate with colloidal crystals was heated at 100 ◦ C for 2 h, which is higher than the glass transition point (Tg ∼ 93 ◦ C) of polystyrene, to fix the colloidal crystal mask to the underlying substrate (Fig. 1a) [7,15]. In this process, the binding force between the Si substrate and the monolayer of polystyrene spheres used as a mask is essential. If no heat treatment is applied, the contact area between the spheres and the substrate is expected to serve as a point contact. After heating, the Si substrate with colloidal crystals was immersed in an aqueous solution containing 1% HF with a Cu concentration of 0.01 wt% to fabricate a Cu honeycomb pattern. In this case, colloidal crystals act as a direct mask for electroless plating (Fig. 1b). After the deposition of Cu nanoparticles, the colloidal crystals used as a mask were selectively removed by immersing the specimens in 97% toluene. 2.2. Fabrication of isolated-island Cu pattern To fabricate a metal pattern opposite to the Cu honeycomb pattern, a two-step replication was applied. First, the
Fig. 1. Schematic model of site-selective metal deposition on Si. (a) Colloidal crystals on Si substrate, (b) electroless plating through a colloidal crystal mask, (c) HMDS coating, and (d) electroless plating through an HMDS mask. The bottom schematic view shows the top view after removal of each mask.
Si substrate was cleaned in dilute HF as described above. At this point, Si surfaces were covered by hydrogen termination. Secondly, the specimens were immersed in 98% H2 SO4 overnight. Hydrophilic surfaces were formed by the terminal silanol (SiOH) groups. After this pretreatment, a colloidal crystal mask was formed on the Si substrate, as described above. To modify the exposed Si surfaces among the spheres, the specimens were placed in hexamethyldisilazane (HMDS) vapor at room temperature for more than one day (Fig. 1c). HMDS is a popular reagent for forming hydrophobic surfaces based on the immobilization of trimethylsilyl ((CH3 )3 Si–) groups on a silanol surface [16], namely, HMDS-coated Si parts show localized hydrophobicity and are expected to inhibit Cu deposition. Finally, electroless plating was conducted as described above (Fig. 1d). 2.3. Characterization of obtained pattern The ordered geometric pattern formed on the Si substrate was evaluated by scanning electron microscopy (SEM, Hitachi S-4200 and JEOL JSM-6360LA), energy dispersive X-ray spectrometry (EDS, JEOL JED-2300), and atomic force mi-
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tern, EDS analysis was performed. Figs. 2b and 2c show EDS mapping images (Cu Lα and Si Kα mappings) of the same area shown in Fig. 2a. It was ascertained from these images that the small particles were metallic copper, and that the other flat parts were silicon. The distribution of copper and silicon was in good accordance with their morphology in Fig. 2a. The center-to-center distance between the holes of the Cu honeycomb pattern, which was basically determined by the diameter of the PS spheres, was approximately 3 µm. The framework of this Cu pattern was composed of clusters of Cu nanoparticles with a size range of 50–100 nm. No Cu deposition was found inside the center circle surrounded by the frame of honeycomb pattern. In the case without a mask, fine Cu particles randomly spread out on the entire Si surface. These results indicate that colloidal crystals can act as a mask for electroless plating in CuSO4 /HF solution. That is, selective metal deposition can proceed in only the exposed parts of the Si surface, which are located in the interspaces among the spheres on the Si substrate, and consequently the contact area between the PS spheres and the underlying Si substrate remains like a circular Si surface without Cu particles. In this study, the electroless plating of Cu particles was thought to be based on displacement plating: (I) In the initial stage, Si surfaces are covered with thin native oxide layers consisting of silicon oxide. (II) When the Si substrate with colloidal crystals is immersed in HF-containing solutions, the dissolution of silicon oxide proceeds. (III) Bare Si parts are instantly oxidized, yielding electrons required for reduction of metals. (IV) Finally, Cu2+ is reduced to Cu as reduction proceeds in only the exposed parts of the Si surface. The mechanism of metal deposition is expressed by the following equations: SiO2 + 6HF → H2 SiF6 + 2H2 O,
(1)
Si + 2H2 O → SiO2 + 4H+ + 4e − ,
(2)
Cu2+
+
2e −
→ Cu.
(3)
The overall reaction through redox reactions is Si + 2Cu2+ + 6HF → H2 SiF6 + 2Cu + 4H+ . Fig. 2. (a) SEM images of Cu particles deposited on Si. (b) EDS mapping result of Cu Lα corresponding to (a). (c) EDS mapping of Si Kα. Electroless plating was conducted in CuSO4 /HF for 1 min through a 3-µm colloidal crystal mask.
croscopy (AFM, Digital Instrument Nano Scope IIIa) using Si conical tips with a typical radius of curvature of 10 nm. 3. Results and discussion 3.1. Networklike honeycomb Cu pattern Fig. 2a shows a typical SEM image of a honeycomb metal pattern formed on the Si substrate. When a two-dimensional hexagonal array of PS spheres was used as a direct mask during electroless plating in a CuSO4 /HF solution, metal particles of uniform sizes were preferentially deposited at the triple points of PS spheres. To identify the chemical composition of this pat-
(4)
In fact, the self-assembled PS spheres, which were used as a mask, were arrayed in a hexagonally close packed or facecentered cubic lattice with a large number of layers on the Si substrate. In this process, however, the arrangement of the PS spheres of first layer formed on the Si substrate is essential because redox reactions proceed at only the liquid/Si interface. Fig. 3 shows a Cu honeycomb pattern with a different periodicity. The magnifications of these images were the same. It was found from these images that the periodicity of the honeycomb pattern is basically determined by the diameter of the PS spheres used as a mask. To reduce the periodicity of the center-to-center distance among holes, spheres of 1 and 0.5 µm diameter were used. In each case, Cu honeycomb patterns were obtained over the entire area of the specimen corresponding to a 2D hexagonal array of PS spheres used as a physical mask. This indicates that the formation of an ordered honeycomb metal pattern could be achieved by this process even with a short periodicity less than 1 µm.
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Fig. 4. Isolated pattern of Cu particles with 1 µm periodicity formed on Si. Electroless plating was conducted in CuSO4 /HF for 2 min through an HMDS honeycomb mask.
Fig. 3. SEM images of Cu honeycomb pattern with different intervals: (a) 3 µm, (b) 1 µm, and (c) 500 nm. Electroless plating was conducted in CuSO4 /HF for (a) 1 min, (b) 15 min, and (c) 2 min through a colloidal crystal mask.
Intrinsic defects (vacancies and dislocations) are often observed as shown in Fig. 3c. These disordered patterns are thought to be the cause of inappropriate conditions for the evaporation of solvent during the formation of 2D arrays of PS spheres. For the general background of the formation mechanism of 2D colloidal crystals, see Refs. [9,14]. 3.2. Isolated-island Cu pattern Concerning the application using 2D colloidal crystals, “natural lithography,” which has been proposed by Deckman and
Dunsmuir, is well known [17]. Based on such a process, uniformly sized microstructures could be produced on a substrate using a monolayer coating of colloidal spheres instead of a conventional resist. Recently, many methods, which are often called “colloidal lithography” or “nanosphere lithography,” have been reported on the nano/micropatterning on a wide variety of solid substrates. In the case of the use of 2D colloidal crystals as a physical mask, however, the shape of the obtained pattern is restricted to only a honeycomb pattern corresponding to interspaces among spheres. To expand the application field of fabricated patterns, it is necessary to modulate the morphology of patterned surfaces. By colloidal crystal templating, it is possible to fabricate negative and positive patterns by changing the conformation of the mask. To fabricate a Cu pattern opposite to the Cu honeycomb pattern shown in Fig. 2a, a two-step replication using hydrophobic treatment was applied. Fig. 4 shows the isolated pattern of the Cu particles deposited by selective electroless plating using HMDS-coated Si. The isolated dot pattern was composed of clusters of Cu nanoparticles with a size range of 50–100 nm, which is similar to the deposited Cu particles shown in Fig. 2. It is confirmed from this image that the deposition was restricted to well-defined bare Si surfaces, and did not occur on the HMDS-coated Si area. This result indicates that the HMDScoated parts, which are located in the interspaces among the spheres on the Si substrate, possess effective hydrophobicity and can act as a mask for localized electroless plating in aqueous solutions. In this case, heat treatment was carried out to fix the colloidal crystal mask to the underlying Si substrate. The diameter of the spheres used as a mask was 1 µm. The contact area between the PS spheres and the underlying Si substrate was expected to expand through heat treatment. From this image, the diameter of the circular contact area was estimated to be approximately 0.7 µm, which was nearly equivalent to 70% of the diameter of the PS spheres used as a mask. Because the periodicity of the isolated dot patterns was basically determined by the diameter of the polystyrene spheres, the formation of the Cu dot pattern with a further reduced peri-
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Fig. 6. AFM tapping-mode image of isolated Cu patterns with 500-nm intervals shown in Fig. 5b.
Fig. 5. SEM images of isolated Cu patterns with different intervals: (a) 1 µm, (b) 500 nm, and (c) 200 nm. Electroless plating was conducted in CuSO4 /HF for (a and b) 2 min and (c) 1 min through an HMDS honeycomb mask. In this case, no heat treatment to fix the colloidal crystal to the Si substrate was applied.
odicity was investigated. In addition, to reduce the contact area between the PS spheres and the Si substrate, no heat treatment was applied. Fig. 5 shows Cu dot arrays with a different periodicity. The magnifications of these images were the same. Regardless of the difference in periodicity, isolated Cu dot patterns with long range ordering were obtained. Fig. 6 shows an AFM image of the deposited Cu arrays on the Si surface. From the cross-sectional analysis of the AFM image, it was revealed that the height of the deposited Cu particles was within 20–30 nm, and the diameter of the circular clusters of Cu par-
ticles was within 200–250 nm, which was equivalent to less than half that of the PS spheres used as a mask. The diameter of the deposited area was considerably smaller than that shown in Fig. 4. Although each dot pattern has a different periodicity, the sizes of the deposited Cu nanoparticles were almost the same in the range of 50–100 nm. These deposited particle sizes were in good agreement with that of the case shown in Fig. 2. This result indicates that eventual patterns could be consistently compounded from uniformly sized Cu particles regardless of the interval and arrangement of patterns. We previously reported that the fabrication of Cu dot patterns with a 100 nm periodicity could be achieved using selfordered porous alumina as a mask for electroless plating [4]. In our previous work, Cu dots on a Si substrate are thought to be deposited in CuSO4 /HF solution by the selective dissolution of silicon oxide, which is produced by the localized anodization of the Si substrate underneath the barrier layer of the upper alumina mask. The periodicity of the Cu dot arrays obtained on the Si substrate could be basically determined by the pore interval of the upper anodic porous alumina. There is a similarity between the method in our previous work and the present method; that is, the periodicity of the eventual patterns reflects the interval of the apertural area of the mask structure. Using a colloidal crystal mask, further study to fabricate 2D controlled metal patterns with a short periodicity less than 100 nm is underway. The proposed templating process using an electrochemical reaction is somewhat similar to the electrodeposition technique for colloidal assemblies on a conductive glass substrate reported by Braun and Wiltzius [6]. However, our approach has some characteristic points of interest: (I) selective metal deposition can be achieved based on displacement plating in only the exposed parts of the Si surface; (II) reversal structures can be formed by changing the shape of the mask; and (III) the present method makes it possible to modulate Si surfaces by an addi-
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tional chemical reaction, such as metal-assisted chemical etching [5]. In addition, metal patterning based on this process is applicable to different types of material including other metals and semiconductors. 4. Conclusions We have described the fabrication of ordered metal patterns on Si surfaces by a combination of colloidal crystal templating, hydrophobic treatment, and electroless plating. The process presented is suitable for the large-scale production of ordered metal honeycomb and isolated metal patterns on a Si substrate that is not achievable by conventional lithographic techniques, because the patterning process consists of colloidal crystal templating based on a relatively easy chemical treatment. On the basis of the present method, it is possible to control the periodicity of metal arrays by changing the diameter of PS spheres used as an initial mask, and to adjust the shape of the metal patterns by modifying the mask structure for electroless plating using a hydrophobic Si treatment. Patterned metals formed on a Si substrate have potential technological and scientific applications in electronics, optics, chemical sensors, and biochemical sensors that require ordered patterns composed of metal nanoparticles in the range from submicrometer to several 10 µm. In addition, the preparation of ordered nano/micropatterns based on this process is applicable to different types of material including other metals and functional semiconductor materials.
Acknowledgments Parts of this work was financially supported by a Grant-inAid for Scientific Research from the Japan Society for the Promotion of Science. Thanks are also due to the “High-Tech Research Center” Project for Private Universities: matching fund subsidy from the Ministry of Education, Culture, Sports, Science and Technology. References [1] S. Yae, Y. Kawamoto, H. Tanaka, N. Fukumuro, H. Matsuda, Electrochem. Commun. 5 (2003) 632. [2] T. Qiu, X.L. Wu, X. Yang, G.S. Huang, Z.Y. Zhang, Appl. Phys. Lett. 84 (2004) 3867. [3] K. Tsujino, M. Matsumura, Adv. Mater. 17 (2005) 1045. [4] S. Ono, A. Oide, H. Asoh, Electrochim. Acta 52 (2007) 2898. [5] H. Asoh, F. Arai, S. Ono, Electrochem. Commun. 9 (2007) 535. [6] P.V. Braun, P. Wiltzius, Nature 402 (1999) 603. [7] F. Sun, W. Cai, Y. Li, B. Cao, F. Lu, G. Duan, L. Zhang, Adv. Mater. 16 (2004) 1116. [8] O.D. Velev, E.W. Kaler, Adv. Mater. 12 (2000) 531. [9] Y. Xia, B. Gates, Y. Yin, Y. Lu, Adv. Mater. 12 (2000) 693. [10] H. Asoh, A. Uehara, S. Ono, Jpn. J. Appl. Phys. 43 (2004) 5667. [11] H. Asoh, S. Ono, Appl. Phys. Lett. 87 (2005) 103102. [12] H. Asoh, A. Oide, S. Ono, Electrochem. Commun. 8 (2006) 1817. [13] H. Asoh, K. Nakamura, S. Ono, Electrochim. Acta, in press. [14] N.D. Denkov, O.D. Velev, P.A. Kralchevsky, I.B. Ivanov, H. Yoshimura, K. Nagayama, Langmuir 8 (1992) 3183. [15] B. Gates, S.H. Park, Y. Xia, Adv. Mater. 12 (2000) 653. [16] A. Ivanisevic, C.A. Mirkin, J. Am. Chem. Soc. 123 (2001) 7887. [17] H.W. Deckman, J.H. Dunsmuir, Appl. Phys. Lett. 41 (1982) 377.
Metal patterning on silicon surface by site-selective electroless deposition through colloidal crystal templating Hidetaka Asoh ∗ , Seiji Sakamoto, Sachiko Ono Department of Applied Chemistry, Faculty of Engineering, Kogakuin University, 1-24-2 Nishi-shinjuku, Shinjuku-ku, Tokyo 163-8677, Japan Received 10 July 2007; accepted 3 September 2007
Abstract Site-selective Cu deposition on a Si substrate was achieved by a combination of colloidal crystal templating, hydrophobic treatment, and electroless plating. Uniformly sized nano/microstructures were produced on the substrate using a monolayer coating of colloidal spheres instead of a conventional resist. The Cu patterns obtained were of two different types: networklike honeycomb and isolated-island patterns with a minimum period of 200 nm. Each ordered pattern with the desired intervals was composed of clusters of Cu nanoparticles with a size range of 50–100 nm. By the present method, it is possible to control the periodicity of metal arrays by changing the diameter of the colloidal spheres used as an initial mask and to adjust the shape of the metal patterns by changing the mask structure for electroless plating. © 2007 Elsevier Inc. All rights reserved. Keywords: Metal pattern; Electroless plating; Colloidal crystal templating; Hydrophobic treatment
1. Introduction Two-dimensional (2D) patterned metals such as wire, tube, and dot arrays that are formed on semiconductors have attracted considerable attention because of their potential applications in magnetic, electronic, and optoelectronic devices. The common methods used in patterning metals are based on deposition processes using a physical template as a mask. However, patterning methods based on the dry process such as vapor deposition and sputtering are costly and not suited to the production of an ordered pattern with feature sizes ranging from micrometers to nanometers over a large area for industrial applications. In terms of fabrication simplicity and efficiency, electrochemical methods are economical, e.g., electrodeposition and electroless plating. For example, noble metals (i.e., Ag, Pt, and Au), which were deposited on a Si substrate, have recently received attention as a catalyst for producing textured Si surfaces for applications in Si-based optoelectronics such as solar cells [1–3]. We also reported that Si convex arrays and Si hole arrays with ordered periodicities have been successfully fabri* Corresponding author. Fax: +81 42 628 5647.
E-mail address: [email protected] (H. Asoh). 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.09.001
cated by the site-selective chemical etching of a Si substrate using patterned Ag nanoparticles as a catalyst [4,5]. Based on our previous work, the closely packed structure of microspheres formed on a Si substrate could act as a mask during electroless plating. The Ag pattern, which could be produced only in interspaces among spheres on a substrate, has a structure opposite to that of the 2D hexagonal arrays of microspheres used as a mask. Although there have been numerous reports on the fabrication of an inverse opal structure from a wide variety of materials, including polymers, ceramic materials, inorganic semiconductors, and metals, reports on the applications using 2D crystalline arrays of microspheres as a mask have been relatively few, particularly on metal deposition by the electrochemical process [6,7]. For details of the applications of colloidal crystals including 2D crystalline arrays, see separate review papers [8,9]. Electrochemical processes, however, have advanced features: for example, the dimensions of the desired structures can be adjusted easily by controlling electrolytic conditions, in comparison with the conventional dry process. In particular, a thin uniform metal layer can be precisely controlled, because the metal layer grows directly from the bottom of colloidal crystals on the underlying conductive substrate.
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Previously, we proposed the micropatterning of a solid substrate based on the electrochemical process using colloidal crystals as a mask [10–13]. We continued our preliminary work to control the fabrication of Si microstructures using colloidal crystals as a mask and examined the metal deposition behavior on a Si substrate during electroless plating. In particular, we focused on the controllability of the periodicity of metal pattern and tried to downsize the resultant metal pattern from micrometer scale to nanoscale. Here, we describe the fabrication of an ordered Cu pattern on Si surfaces by site-selective electroless plating using colloidal crystals as a mask. Moreover, the adjustment of the geometric pattern of metals is also investigated from the viewpoint of the application to various scientific fields, such as those related to chemical sensors and electronic devices. 2. Experimental 2.1. Fabrication of networklike honeycomb Cu pattern Patterned metal arrays were fabricated on a Si substrate by a combination of colloidal crystal templating, hydrophobic treatment, and electroless plating. By colloidal crystal templating, it is possible to fabricate negative and positive patterns by changing the mask structure for electroless plating. The principle of pattern transfer, which is similar to that in our previous studies [4,5], is schematically shown in Fig. 1. Si wafers (p type, 0.001 cm, (100) crystal orientation) were cut into the desired size, e.g., 1 × 1 cm, and used as the substrate. The Si wafers were precleaned in 1 wt% HF for 5 min to remove organic contaminants and native oxide on their surface. A monodisperse suspension of polystyrene (PS) microspheres (Polysciences, Inc.) was dropped into the region surrounded by the silicon rubber O-ring of 5.5 mm diameter on the substrate. The suspension on the substrate was dried in air, and spheres self-assembled into a close-packed structure with three-dimensional ordered lattices via attractive capillary forces were observed [14]. After the complete evaporation of the water solvent, the Si substrate with colloidal crystals was heated at 100 ◦ C for 2 h, which is higher than the glass transition point (Tg ∼ 93 ◦ C) of polystyrene, to fix the colloidal crystal mask to the underlying substrate (Fig. 1a) [7,15]. In this process, the binding force between the Si substrate and the monolayer of polystyrene spheres used as a mask is essential. If no heat treatment is applied, the contact area between the spheres and the substrate is expected to serve as a point contact. After heating, the Si substrate with colloidal crystals was immersed in an aqueous solution containing 1% HF with a Cu concentration of 0.01 wt% to fabricate a Cu honeycomb pattern. In this case, colloidal crystals act as a direct mask for electroless plating (Fig. 1b). After the deposition of Cu nanoparticles, the colloidal crystals used as a mask were selectively removed by immersing the specimens in 97% toluene. 2.2. Fabrication of isolated-island Cu pattern To fabricate a metal pattern opposite to the Cu honeycomb pattern, a two-step replication was applied. First, the
Fig. 1. Schematic model of site-selective metal deposition on Si. (a) Colloidal crystals on Si substrate, (b) electroless plating through a colloidal crystal mask, (c) HMDS coating, and (d) electroless plating through an HMDS mask. The bottom schematic view shows the top view after removal of each mask.
Si substrate was cleaned in dilute HF as described above. At this point, Si surfaces were covered by hydrogen termination. Secondly, the specimens were immersed in 98% H2 SO4 overnight. Hydrophilic surfaces were formed by the terminal silanol (SiOH) groups. After this pretreatment, a colloidal crystal mask was formed on the Si substrate, as described above. To modify the exposed Si surfaces among the spheres, the specimens were placed in hexamethyldisilazane (HMDS) vapor at room temperature for more than one day (Fig. 1c). HMDS is a popular reagent for forming hydrophobic surfaces based on the immobilization of trimethylsilyl ((CH3 )3 Si–) groups on a silanol surface [16], namely, HMDS-coated Si parts show localized hydrophobicity and are expected to inhibit Cu deposition. Finally, electroless plating was conducted as described above (Fig. 1d). 2.3. Characterization of obtained pattern The ordered geometric pattern formed on the Si substrate was evaluated by scanning electron microscopy (SEM, Hitachi S-4200 and JEOL JSM-6360LA), energy dispersive X-ray spectrometry (EDS, JEOL JED-2300), and atomic force mi-
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tern, EDS analysis was performed. Figs. 2b and 2c show EDS mapping images (Cu Lα and Si Kα mappings) of the same area shown in Fig. 2a. It was ascertained from these images that the small particles were metallic copper, and that the other flat parts were silicon. The distribution of copper and silicon was in good accordance with their morphology in Fig. 2a. The center-to-center distance between the holes of the Cu honeycomb pattern, which was basically determined by the diameter of the PS spheres, was approximately 3 µm. The framework of this Cu pattern was composed of clusters of Cu nanoparticles with a size range of 50–100 nm. No Cu deposition was found inside the center circle surrounded by the frame of honeycomb pattern. In the case without a mask, fine Cu particles randomly spread out on the entire Si surface. These results indicate that colloidal crystals can act as a mask for electroless plating in CuSO4 /HF solution. That is, selective metal deposition can proceed in only the exposed parts of the Si surface, which are located in the interspaces among the spheres on the Si substrate, and consequently the contact area between the PS spheres and the underlying Si substrate remains like a circular Si surface without Cu particles. In this study, the electroless plating of Cu particles was thought to be based on displacement plating: (I) In the initial stage, Si surfaces are covered with thin native oxide layers consisting of silicon oxide. (II) When the Si substrate with colloidal crystals is immersed in HF-containing solutions, the dissolution of silicon oxide proceeds. (III) Bare Si parts are instantly oxidized, yielding electrons required for reduction of metals. (IV) Finally, Cu2+ is reduced to Cu as reduction proceeds in only the exposed parts of the Si surface. The mechanism of metal deposition is expressed by the following equations: SiO2 + 6HF → H2 SiF6 + 2H2 O,
(1)
Si + 2H2 O → SiO2 + 4H+ + 4e − ,
(2)
Cu2+
+
2e −
→ Cu.
(3)
The overall reaction through redox reactions is Si + 2Cu2+ + 6HF → H2 SiF6 + 2Cu + 4H+ . Fig. 2. (a) SEM images of Cu particles deposited on Si. (b) EDS mapping result of Cu Lα corresponding to (a). (c) EDS mapping of Si Kα. Electroless plating was conducted in CuSO4 /HF for 1 min through a 3-µm colloidal crystal mask.
croscopy (AFM, Digital Instrument Nano Scope IIIa) using Si conical tips with a typical radius of curvature of 10 nm. 3. Results and discussion 3.1. Networklike honeycomb Cu pattern Fig. 2a shows a typical SEM image of a honeycomb metal pattern formed on the Si substrate. When a two-dimensional hexagonal array of PS spheres was used as a direct mask during electroless plating in a CuSO4 /HF solution, metal particles of uniform sizes were preferentially deposited at the triple points of PS spheres. To identify the chemical composition of this pat-
(4)
In fact, the self-assembled PS spheres, which were used as a mask, were arrayed in a hexagonally close packed or facecentered cubic lattice with a large number of layers on the Si substrate. In this process, however, the arrangement of the PS spheres of first layer formed on the Si substrate is essential because redox reactions proceed at only the liquid/Si interface. Fig. 3 shows a Cu honeycomb pattern with a different periodicity. The magnifications of these images were the same. It was found from these images that the periodicity of the honeycomb pattern is basically determined by the diameter of the PS spheres used as a mask. To reduce the periodicity of the center-to-center distance among holes, spheres of 1 and 0.5 µm diameter were used. In each case, Cu honeycomb patterns were obtained over the entire area of the specimen corresponding to a 2D hexagonal array of PS spheres used as a physical mask. This indicates that the formation of an ordered honeycomb metal pattern could be achieved by this process even with a short periodicity less than 1 µm.
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Fig. 4. Isolated pattern of Cu particles with 1 µm periodicity formed on Si. Electroless plating was conducted in CuSO4 /HF for 2 min through an HMDS honeycomb mask.
Fig. 3. SEM images of Cu honeycomb pattern with different intervals: (a) 3 µm, (b) 1 µm, and (c) 500 nm. Electroless plating was conducted in CuSO4 /HF for (a) 1 min, (b) 15 min, and (c) 2 min through a colloidal crystal mask.
Intrinsic defects (vacancies and dislocations) are often observed as shown in Fig. 3c. These disordered patterns are thought to be the cause of inappropriate conditions for the evaporation of solvent during the formation of 2D arrays of PS spheres. For the general background of the formation mechanism of 2D colloidal crystals, see Refs. [9,14]. 3.2. Isolated-island Cu pattern Concerning the application using 2D colloidal crystals, “natural lithography,” which has been proposed by Deckman and
Dunsmuir, is well known [17]. Based on such a process, uniformly sized microstructures could be produced on a substrate using a monolayer coating of colloidal spheres instead of a conventional resist. Recently, many methods, which are often called “colloidal lithography” or “nanosphere lithography,” have been reported on the nano/micropatterning on a wide variety of solid substrates. In the case of the use of 2D colloidal crystals as a physical mask, however, the shape of the obtained pattern is restricted to only a honeycomb pattern corresponding to interspaces among spheres. To expand the application field of fabricated patterns, it is necessary to modulate the morphology of patterned surfaces. By colloidal crystal templating, it is possible to fabricate negative and positive patterns by changing the conformation of the mask. To fabricate a Cu pattern opposite to the Cu honeycomb pattern shown in Fig. 2a, a two-step replication using hydrophobic treatment was applied. Fig. 4 shows the isolated pattern of the Cu particles deposited by selective electroless plating using HMDS-coated Si. The isolated dot pattern was composed of clusters of Cu nanoparticles with a size range of 50–100 nm, which is similar to the deposited Cu particles shown in Fig. 2. It is confirmed from this image that the deposition was restricted to well-defined bare Si surfaces, and did not occur on the HMDS-coated Si area. This result indicates that the HMDScoated parts, which are located in the interspaces among the spheres on the Si substrate, possess effective hydrophobicity and can act as a mask for localized electroless plating in aqueous solutions. In this case, heat treatment was carried out to fix the colloidal crystal mask to the underlying Si substrate. The diameter of the spheres used as a mask was 1 µm. The contact area between the PS spheres and the underlying Si substrate was expected to expand through heat treatment. From this image, the diameter of the circular contact area was estimated to be approximately 0.7 µm, which was nearly equivalent to 70% of the diameter of the PS spheres used as a mask. Because the periodicity of the isolated dot patterns was basically determined by the diameter of the polystyrene spheres, the formation of the Cu dot pattern with a further reduced peri-
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Fig. 6. AFM tapping-mode image of isolated Cu patterns with 500-nm intervals shown in Fig. 5b.
Fig. 5. SEM images of isolated Cu patterns with different intervals: (a) 1 µm, (b) 500 nm, and (c) 200 nm. Electroless plating was conducted in CuSO4 /HF for (a and b) 2 min and (c) 1 min through an HMDS honeycomb mask. In this case, no heat treatment to fix the colloidal crystal to the Si substrate was applied.
odicity was investigated. In addition, to reduce the contact area between the PS spheres and the Si substrate, no heat treatment was applied. Fig. 5 shows Cu dot arrays with a different periodicity. The magnifications of these images were the same. Regardless of the difference in periodicity, isolated Cu dot patterns with long range ordering were obtained. Fig. 6 shows an AFM image of the deposited Cu arrays on the Si surface. From the cross-sectional analysis of the AFM image, it was revealed that the height of the deposited Cu particles was within 20–30 nm, and the diameter of the circular clusters of Cu par-
ticles was within 200–250 nm, which was equivalent to less than half that of the PS spheres used as a mask. The diameter of the deposited area was considerably smaller than that shown in Fig. 4. Although each dot pattern has a different periodicity, the sizes of the deposited Cu nanoparticles were almost the same in the range of 50–100 nm. These deposited particle sizes were in good agreement with that of the case shown in Fig. 2. This result indicates that eventual patterns could be consistently compounded from uniformly sized Cu particles regardless of the interval and arrangement of patterns. We previously reported that the fabrication of Cu dot patterns with a 100 nm periodicity could be achieved using selfordered porous alumina as a mask for electroless plating [4]. In our previous work, Cu dots on a Si substrate are thought to be deposited in CuSO4 /HF solution by the selective dissolution of silicon oxide, which is produced by the localized anodization of the Si substrate underneath the barrier layer of the upper alumina mask. The periodicity of the Cu dot arrays obtained on the Si substrate could be basically determined by the pore interval of the upper anodic porous alumina. There is a similarity between the method in our previous work and the present method; that is, the periodicity of the eventual patterns reflects the interval of the apertural area of the mask structure. Using a colloidal crystal mask, further study to fabricate 2D controlled metal patterns with a short periodicity less than 100 nm is underway. The proposed templating process using an electrochemical reaction is somewhat similar to the electrodeposition technique for colloidal assemblies on a conductive glass substrate reported by Braun and Wiltzius [6]. However, our approach has some characteristic points of interest: (I) selective metal deposition can be achieved based on displacement plating in only the exposed parts of the Si surface; (II) reversal structures can be formed by changing the shape of the mask; and (III) the present method makes it possible to modulate Si surfaces by an addi-
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tional chemical reaction, such as metal-assisted chemical etching [5]. In addition, metal patterning based on this process is applicable to different types of material including other metals and semiconductors. 4. Conclusions We have described the fabrication of ordered metal patterns on Si surfaces by a combination of colloidal crystal templating, hydrophobic treatment, and electroless plating. The process presented is suitable for the large-scale production of ordered metal honeycomb and isolated metal patterns on a Si substrate that is not achievable by conventional lithographic techniques, because the patterning process consists of colloidal crystal templating based on a relatively easy chemical treatment. On the basis of the present method, it is possible to control the periodicity of metal arrays by changing the diameter of PS spheres used as an initial mask, and to adjust the shape of the metal patterns by modifying the mask structure for electroless plating using a hydrophobic Si treatment. Patterned metals formed on a Si substrate have potential technological and scientific applications in electronics, optics, chemical sensors, and biochemical sensors that require ordered patterns composed of metal nanoparticles in the range from submicrometer to several 10 µm. In addition, the preparation of ordered nano/micropatterns based on this process is applicable to different types of material including other metals and functional semiconductor materials.
Acknowledgments Parts of this work was financially supported by a Grant-inAid for Scientific Research from the Japan Society for the Promotion of Science. Thanks are also due to the “High-Tech Research Center” Project for Private Universities: matching fund subsidy from the Ministry of Education, Culture, Sports, Science and Technology. References [1] S. Yae, Y. Kawamoto, H. Tanaka, N. Fukumuro, H. Matsuda, Electrochem. Commun. 5 (2003) 632. [2] T. Qiu, X.L. Wu, X. Yang, G.S. Huang, Z.Y. Zhang, Appl. Phys. Lett. 84 (2004) 3867. [3] K. Tsujino, M. Matsumura, Adv. Mater. 17 (2005) 1045. [4] S. Ono, A. Oide, H. Asoh, Electrochim. Acta 52 (2007) 2898. [5] H. Asoh, F. Arai, S. Ono, Electrochem. Commun. 9 (2007) 535. [6] P.V. Braun, P. Wiltzius, Nature 402 (1999) 603. [7] F. Sun, W. Cai, Y. Li, B. Cao, F. Lu, G. Duan, L. Zhang, Adv. Mater. 16 (2004) 1116. [8] O.D. Velev, E.W. Kaler, Adv. Mater. 12 (2000) 531. [9] Y. Xia, B. Gates, Y. Yin, Y. Lu, Adv. Mater. 12 (2000) 693. [10] H. Asoh, A. Uehara, S. Ono, Jpn. J. Appl. Phys. 43 (2004) 5667. [11] H. Asoh, S. Ono, Appl. Phys. Lett. 87 (2005) 103102. [12] H. Asoh, A. Oide, S. Ono, Electrochem. Commun. 8 (2006) 1817. [13] H. Asoh, K. Nakamura, S. Ono, Electrochim. Acta, in press. [14] N.D. Denkov, O.D. Velev, P.A. Kralchevsky, I.B. Ivanov, H. Yoshimura, K. Nagayama, Langmuir 8 (1992) 3183. [15] B. Gates, S.H. Park, Y. Xia, Adv. Mater. 12 (2000) 653. [16] A. Ivanisevic, C.A. Mirkin, J. Am. Chem. Soc. 123 (2001) 7887. [17] H.W. Deckman, J.H. Dunsmuir, Appl. Phys. Lett. 41 (1982) 377.