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Jul 10, 2012 - Photochemically Grafted Polystyrene Layer Assisting Selective Au. Electrodeposition. Koichi Nagase, Shoichi Kubo, and Masaru Nakagawa*...
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Photochemically Grafted Polystyrene Layer Assisting Selective Au Electrodeposition Koichi Nagase, Shoichi Kubo, and Masaru Nakagawa* Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, 2-1-1 Katahira, Aoba, Sendai 980-8577, Japan ABSTRACT: We describe the selective electrodeposition of submicrometer gold (Au) patterns achieved by a thin film resist layer of polystyrene (PS) that was exposed to ultraviolet (UV) light on a photoreactive monolayer of a benzophenone-containing alkylthiol formed on a Au-plated substrate and patterned by thermal nanoimprint lithography. The presence of a PS graft layer caused by the benzophenone monolayer photochemistry at an interface between the PS resist layer and photoreactive monolayer played the important role of suppressing the unfavorable growth of tiny Au grains in regions masked with the PS resist layer, resulting in the selective Au electrodeposition in aperture regions of PS resist patterns. The suppressive effect on selective Au electrodeposition depended on the molecular weight of PS used as a resist material. Among unimodal PSs having weight-average molecular weights (Mw's) of 2100, 10 900, and 106 000 g mol−1, the PS of Mw = 10 900 g mol−1 functioned most effectively as the resist layer. Au electrodeposition at a low current density allowed the preparation of Au lines having widths of submicrometers and a uniform height independent of line widths in resist aperture regions. Submicrometer bump structures of Au lines could be fabricated on transparent silica substrates by the subsequent wet etching of a Au electrode layer and then a chromium adhesive layer.

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Thermal nanoimprint lithography (TNIL)15 and UV nanoimprint lithography (UV-NIL)16 are considered to be powerful nanofabrication tools, which are capable of compensating for the throughput of EB lithography. A nanoimprint master mold is usually made with silicon and silica by EB lithography. Using the mold, many nanostructures can be replicated in a contact manner at high throughput. Although there are commercially available UV-curable resins for UV-NIL, the chemical formulations have not been made public, and this situation often prevents a scientific understanding of the defect generation of resist and metal nanopatterns. In contrast, a well-known thermoplastic polymer of poly(methyl methacrylate) (PMMA) is widely used as a resist material for TNIL since the first report.15 The thermal transformation of the resist layer on a substrate using a nanoimprint mold, the removal of the residual layer by oxygen dry etching, and then selective metallization are carried out for the fabrication of metal nanostructures on the substrate. Selective metallization on substrates masked with nanoimprinted resist patterns is performed by subtractive methods of dry and wet etching or by additive methods of physical vacuum sputtering deposition and chemical electrodeposition. Among them, metal electrodeposition is an approachable vacuum-free process that easily enables us to tune the height of metal nanostructures by controlling the number of coulombs applied. The facile controllability of the height and aspect ratio will allow the precise tuning of optical and electrical properties of assembled

INTRODUCTION Noble metals of gold (Au) and silver (Ag) show attractive characteristic properties of electronic conduction, optical reflectance, plasmon resonance, and so on. In particular, nanostructures smaller than a micrometer on substrates have recently attracted attention because they show unique scientific phenomena in the fields of photonics and electronics. For example, Ag and Au nanorods on substrates function as reaction fields of ultraviolet-curable resins by exposure to nearinfrared light owing to localized surface plasmon resonance.1,2 Assemblies of the ring-shaped,3 U-shaped,4,5 and net6−8 nanostructures behave as metamaterials like negative index materials. Wire grid polarizers comprising metal line-and-space patterns selectively pass a vector of linearly polarized ultraviolet and visible light.9−11 Metal mesh structures on transparent substrates show both high optical transmittance and low electrical surface resistance, which are promising for nextgeneration transparent conductive substrates for photovoltaic devices and display devices instead of indium tin oxide substrates containing rare metals.12−14 To date, the fundamental properties have been studied mainly by the fabrication of metal nanostructures by means of semiconductor nanofabrication techniques based on advanced photolithography and electron beam (EB) lithography involving a lift-off process. Advanced photolithography needs the specific skill to correct pattern shapes, and EB lithography usually needs a relatively long portrayal period to fabricate resist patterns. These situations limit the progress of nanomaterial science and technology and arrest the growth of innovative applications. Easy, approachable, sophisticated mass-production methods to fabricate noble metal nanostructures on substrates are desired. © 2012 American Chemical Society

Received: April 22, 2012 Revised: July 5, 2012 Published: July 10, 2012 11646

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Figure 1. Schematic illustration of the method for preparing a patterned Au thin film by reactive-monolayer-assisted thermal nanoimprint lithography (R-TNIL) involving Au electrodeposition.

photochemically grafted PS layer was formed at the interface. The presence of the PS graft layer in the PS resist layer suppressed the thermal dewetting of the PS resist layer during thermal nanoimprinting and improved the lateral resolution of a plated Au layer patterned by Au wet etching using patterned PS resist masks. We thought that the presence of PrM and PS graft layers might improve selective Au electrodeposition by suppressing the unfavorable growth of tiny Au grains in regions masked by the patterned PS resist layer. In this article, we disclosed the effect of the photoreactive monolayer and the PS graft layer on selective Au electrodeposition. It was found that the presence of the PS graft layer caused by the benzophenone monolayer photochemistry played a significant role of Au selective electrodeposition without unfavorable tiny Au grains. Interestingly, it was found that the suppressive effect on selective Au electrodeposition was markedly affected by the molecular weight of PS used for the resist layer. As a result of choosing a suitable molecular weight, Au line patterns having line widths of submicrometers and a uniform height could be prepared. Individual submicrometer Au bump structures on transparent silica substrates could be fabricated by the subsequent wet etching of a Au electrode layer and an adhesive chromium layer.

metal nanostructures. The selective metallization by TNIL involving metal electrodeposition has been demonstrated for applications to Cu field emitters,17 information storage patterned media comprising Co,18 Ni,19 and Fe20 magnetic dots, optical measurement substrates with Ag nanopatterns for surface-enhanced Raman spectroscopy,21 and structural control of Cu22 and Ag23 nanostructure arrays. However, there are almost no reports on the controllability of the width and height of metal nanostructures fabricated by TNIL involving metal electrodeposition. It is very important to fabricate precisely the size and shape of noble metal nanostructures designed theoretically and to study structure−property relationships for progress in nanomaterial science. We wondered why PMMA is used mostly as a resist material for TNIL among thermoplastic polymers, although PMMA is inferior to resistivity toward oxygen dry etching in comparison to polystyrene (PS) of a similar amorphous polymer.24,25 Actually, we compared the resist property of aqueous Au electrodeposition between PMMA and PS thin films patterned by TNIL.26 In the case of using a patterned PMMA resist layer on a Au-plated substrate, the surfaces of electrodeposited Au line patterns become very rough and the line-edge roughness of the Au lines markedly increased in comparison to a patterned PS resist layer. It was considered that oxygen dry etching deteriorated the resist functionality of PMMA. In contrast, shapes of electrodeposited Au line patterns almost agreed with those of resist aperture regions in the case of using a patterned PS resist layer. The PS resist layer had superior resist properties in terms of size fidelity for the line-edge roughness and line width. However, it was found that there was a fatal problem when the PS resist layer was removed. Tiny Au grains having a diameter of approximately 0.1 μm grew on a Au electrode surface, although the Au surface had been masked with the PS resist layer. It was considered that the poor physical adhesion of PS to the Au electrode surface probably brought about the interpenetration of an aqueous electrolyte solution at the interface. We thought that the chemical adhesion of PS to the Au electrode surface might overcome the problem. We noticed that there is a method for grafting PS chemically to a solid surface using a photoreactive monolayer. To carry out the wet etching of a metal thin layer, we have reported reactivemonolayer-assisted thermal nanoimprint lithography (R-TNIL) as an advanced technique.27,28 The photochemical attachment of polymers to solid surfaces using a photoreactive monolayer formed from the benzophenone-containing trimethoxysilane derivative demonstrated by Prucker et al.29 inspired us with respect to R-TNIL. In R-TNIL, a Au-plated substrate surface was modified with a photoreactive monolayer of a benzophenone-containing alkylthiol (PrM). A PS resist layer prepared by spin coating onto the PrM-modified Au-plated substrate was exposed to UV light to cause photochemical graft reactions between PS molecules and PrM at the interface. As a result, a

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EXPERIMENTAL SECTION

Materials. A silicon mold (NTT-AT 100 L RESO) having 0.1−1.0 μm line-and-space patterns with a height of 0.2 μm was cleaned by exposure to UV/ozone. The mold surface was modified with a fluorinated release layer by immersion in a solution (HARVES DURASURF HD1100z) and multiple rinses with a solvent (HARVES DURASURF HD-ZV). Au-plated silicon substrates (15 × 15 mm2, 25 nm Au/20 nm Cr/0.5 mm Si) were purchased from NTT-AT. Four kinds of PS were used as PS resist materials: mixed 35k-PS and unimodal 2k-PS, 11k-PS, and 106k-PS. Mixed 35k-PS with a weightaverage molecular weight (Mw) of 35 000 g mol−1 was purchased from Sigma-Aldrich. Unimodal 2k-PS (Mw = 2100 g mol−1, number-average molecular weight (Mn) = 1800 g mol−1, and Mw/Mn = 1.15), 11k-PS (Mw = 10 900 g mol−1, Mn = 10 000 g mol−1, and Mw/Mn = 1.09), and 106k-PS (Mw = 106 000 g mol−1, Mn = 100 000 g mol−1, and Mw/Mn = 1.06) were purchased from Polymer Source, Inc. 4-((10Mercaptodecyl)oxy)benzophenone used as an adsorbate to form a photoreactive monolayer was synthesized by the reaction of 4hydroxybenzophene with 1,10-dibromodecane, followed by the reaction of 4-((10-bromodecyl)oxy)benzophenone with thiourea and hydrolysis with sodium hydroxide.27 An aqueous Au electrolyte solution (Tanaka Kikinzoku Microfab-Au640, pH 7.7) containing Na3[Au(SO3)2] as a Au source was used for Au electrodeposition. An iodine-type aqueous Au etchant (Kanto Chemical AURUM302) and a nitric acid aqueous Cr etchant (Hayashi Pure Chemical) were used as received. Thermal Nanoimprint Lithography and Au Electrodeposition. Figure 1 shows the successive steps in fabricating Au patterns on a Au electrode layer by TNIL and Au electrodeposition carried out in this study. The surfaces of a Au-plated substrate were cleaned by exposure to UV/ozone for 15 min, immersed in an ethanol solution containing 0.1 mmol dm−3 4-((10-mercaptodecyl)oxy)benzophenone 11647

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Figure 2. SEM images of (a−c) mixed 35k-PS resist layers transformed thermally to 0.5 μm line-and-space patterns and (d−f) Au surfaces after residual layer removal, Au electrodeposition, and resist layer removal. (a, d) A PS film on a bare Au substrate, (b, e) an unexposed PS film on a PrMmodified Au substrate, and (c, f) a UV-exposed PS film on a PrM-modified Au substrate were used as resist layers. for 12 h, rinsed with pure ethanol, and dried in flowing nitrogen gas. As a result, a photoreactive monolayer (PrM) was formed from the adsorbate on the Au surface.27 Respective PS resist layers were prepared on the PrM-modified substrate by spin coating a toluene solution of PS. The PS resist layer on the PrM-modified substrate was exposed to UV light at an exposure energy of 2.0 J cm−2 monitored at 254 nm.27,28 As a result, interfacial PS molecules on the PrM-modified substrate were grafted photochemically on the photoreactive monolayer. The UV-exposed PS thin film was annealed at 190 °C for 10 min. For comparison, a PS thin film on an unmodified Auplated substrate and an unexposed PS thin film on a PrM-modified Auplated substrate were also prepared. Considering the height of the mold pattern, the thickness of the PS resist layers was adjusted to approximately 0.12 μm. The PS resist layer was transformed with the fluorinated silicon mold by thermal nanoimprinting. The process of thermal nanoimprinting comprised heating to the desired temperature of 200 °C for 60 s, steadily increasing applied pressure up to 7.5 MPa in 60 s, maintaining the temperature and pressure for 300 s, cooling to 30 °C in 60 s, and demolding for 60 s. The patterned PS layer was subjected to exposure to UV/ozone to remove residual layers formed at the concave parts, and a Au-plated substrate masked with patterned PS resist layers was prepared for Au electrodeposition. Au electrodeposition was performed at 30 °C using the aqueous Au electrolyte solution. The current density was adjusted in the range of 1.61−6.44 mA cm−2 using a galvanostat. A masked Au-plated substrate, a platinum mesh, and a Ag/AgCl electrode were used as the working, counter, and reference electrodes, respectively. After electrodeposition, the PS resist layer was removed by multiple rinses with chloroform, and the Au surface was cleaned by exposure to vacuum ultraviolet (VUV) light at a wavelength of 172 nm under a reduced pressure of 1.0 kPa for 15 min to remove the PS graft layer and photoreactive monolayer. Underlying Au and Cr layers except electrodeposited regions were finally removed by immersing the substrate in an aqueous Au wet etching solution for 15 s and then an aqueous acidic Cr wet etching solution for 5 s. Instruments. The surface of the Au-plated substrate was cleaned using a UV/ozone cleaner (Sen-Lights PL-16-110). The photochemical graft reaction was caused by exposure to UV light emitted from a 200 W Hg−Xe lamp (San-ei Electric Supercure 203S). The light intensity was monitored at 254 nm using an optical power meter (Custom UVC-254). The thickness of a PS resist layer was measured using a surface profiler (Veeco Dektak3ST). Thermal nanoimprinting

was carried out using a nanoimprinter (Meisyo Kiko NM-400). Au electrodeposition was performed by using a galvanostat (Hokuto Denko HA-151). The removal of the PS graft layer and photoreactive monolayer after Au electrodeposition was carried out using a Xe excimer lamp (UHIO UER20-172VA). The morphology of PS and Au patterns was observed using scanning electron microscopes (SEM, Hitachi S-3000N and SU-6000) and an atomic force microscope (AFM, SII S-image) with a cantilever (Olympus AC200TS).

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RESULTS AND DISCUSSION PS Graft Layer Suppressing Au Electrodeposition in Masked Regions of the Working Electrode. We first investigated whether the PS graft layer, resulting from the photochemical graft reaction of the benzophenone-containing monolayer with interfacial PS molecules in the PS resist layer, had any effect on Au electrodeposition. Three kinds of resist layers of mixed 35k-PS with a thickness of approximately 0.12 μm were prepared as on Au-plated substrates: the PS layer on a bare Au-plated substrate, an unexposed PS layer on a PrMmodified Au-plated substrate, and a UV-exposed PS layer on a PrM-modified Au-plated substrate causing the photoinduced graft reaction. Figure 2a−c shows SEM images of 35k-PS resist layers transformed thermally to 0.5 μm line-and-space patterns. The dark and bright regions correspond to convex thick and concave thin PS layers, respectively. Unshaped bright regions were observed here and there owing to PS dewetting and/or capillary bridging after thermal nanoimprinting in the case of the PS layers on the bare substrate (Figure 2a) and the PrMmodified substrate (Figure 2b). No obvious pattern defects occurred after thermal nanoimprinting in the case of the UVexposed PS layer on the PrM-modified substrate (Figure 2c). These results indicate that the presence of the PS graft layer improves the thermal stability of the morphology of PS resist layers, which agreed with our previous report.27,28 The concave thin PS layers, named residual layers, were removed by exposure to UV/ozone, and Au electrodeposition was carried out at a current density of 3.22 mA cm−2 for 50 s. Electrodeposited Au surfaces after the removal of the PS resist layers with solvent were observed using SEM and AFM. The root-mean-square surface roughness of the outermost surfaces 11648

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Figure 3. SEM images of (a−c) PS resist layers nanoimprinted to produce 0.5 μm line-and-space patterns and (d−h) Au surfaces after residual layer removal, Au electrodeposition, and resist layer removal. The PS resist layer comprised (a, d) 2k-PS, (b, e, g, h) 11k-PS, and (c, f) 106k-PS. Au electrodeposition was carried out at an identical applied electric flux density of 0.322 C cm−2, which corresponded to current densities of (d−f) 3.22 mA cm−2 for 100 s, (g) 6.44 mA cm−2 for 50 s, and (h) 1.61 mA cm−2 for 200 s.

106k-PS. In the same manner as for the preparation of a mixed 35k-PS resist layer, each resist layer of 2k-PS, 11k-PS, and 106K-PS with a thickness of approximately 0.12 μm was prepared on a PrM-modified Au substrate by spin coating, exposure to UV light causing the photochemical graft reaction, and annealing at 190 °C for 10 min. Figure 3a−c shows SEM images of nanoimprinted 0.5 μm line-and-space patterns of 2k-PS, 11k-PS, and 106k-PS, respectively. Figure 3d−f shows SEM images of Au surfaces after the removal of residual layers, Au electrodeposition, and the removal of the resist layers. The morphology of the Au surfaces after Au electrodeposition was compared among different PS molecular weights under an identical current density of 3.22 mA cm−2 for 100 s. As shown in Figure 3e, Au patterns could be fabricated by electrodeposition in accordance with aperture shapes of the PS resist pattern in the case of 11kPS. However, the resist property of Au electrodeposition deteriorated in the cases of 2k-PS and 106k-PS, although the film thicknesses were almost constant. The growth of tiny Au grains occurred in regions masked with the PS resist layer. It was confirmed that the suppressive effect of the electrochemical formation of Au grains in regions masked with the PS graft and resist layers was dependent on the molecular weight of PS. It was found that 11k-PS had the most suppressive effect of the electrochemical growth of undesirable Au grains. Norton et al. reported the electrochemical shield effect of a hexadecanethiol self-assembled monolayer formed on a Au surface on silver electrodeposition.30 It is considered that silver

of electrodeposited Au lines was approximately 3 nm and independent of the presence of the PrM and photochemically grafted PS layers. Although Au electrodeposition was brought about at unmasked Au surfaces almost in accordance with the shapes of aperture regions of the patterned PS resist layer, it was found that a lot of tiny, bright Au grains with a size of approximately 0.03−0.1 μm were formed even at masked Au surfaces in the case of the PS resist layers on the bare Au substrate (Figure 2d) and the PrM-modified Au substrate (Figure 2e). In contrast, such tiny Au grains hardly grew in the case of the UV-exposed PS layer on the PrM-modified substrate, as shown in Figure 2f. It was obvious that the PS graft layer resulting from the benzophenone monolayer photochemistry contributed to fine Au electrodeposition by suppressing the growth of unfavorable Au grains on Au surfaces masked with the PS resist layer. These results imply that the restricted molecular motion of PS molecules anchored covalently to the benzophenone monolayer resulted in a suppressive effect on the growth of Au grains. PS Molecular Weight Suitable for the Suppression of the Formation of Au Grains. Because the mixed 35k-PS comprises polystyrenes having weight-average molecular weights of 4000 and 200 000 g mol−1, we investigated whether the suppression of tiny Au grains growing under a PS resist layer was influenced by the molecular weight of PS used as a resist material in this section. We chose three kinds of unimodal polystyrene: 2k-PS, 11k-PS, and 106k-PS. Respective values of Tg were 50 °C for 2k-PS, 90 °C for 11k-PS, and 105 °C for 11649

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Figure 4. SEM images of (a, d, g, j, m) nanoimprinted PS resist layers and (b, e, h, k, n) plain-view and (c, f, i, l, o) tilted-view electrodeposited Au lines on Au surfaces after resist removal. Au electrodeposition was performed at 1.61 mA cm−2 for 100 s. The line-and-space patterns were (a−c) 1.0 μm, (d−f) 0.5 μm, (g−i) 0.3 μm, (j−l) 0.2 μm, and (m−o) 0.1 μm.

layer as shown in Figure 2d. Therefore, it was considered that the PS graft layer caused by the benzophenone monolayer photochemistry compensates for and reinforces the electrochemical shield effect. Actually, the PS graft layer formed from mixed 35k-PS and unimodal 11k-PS showed a suppressive effect on the growth of Au grains, whereas such an electrochemical shield effect was weakened in the case of 2kPS and 106k-PS. If the photochemical graft reactions with PS molecules having different molecular weight were caused in the same

grains are formed electrochemically at pinholes and orientational defects of the self-assembled monolayer. In this study, the photoreactive monolayer is formed by a shorter decanethiol derivative containing a bulky benzophenone moiety. It is anticipated that the photoreactive monolayer could scarcely show an electrochemical shield effect on Au electrodeposition owing to loose packing. As shown in Figure 2e, a lot of Au grains were formed in masked regions in the case of using the unexposed PS resist layer on the PrM-modified Au substrate. In addition, only the PS resist layer did not function well as a resist 11650

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identical current density of 1.61 mA cm−2 for different deposition periods of 30, 50, 75, and 100 s, and the respective heights of Au lines were measured by AFM. Figure 5 shows

manner, then the thickness of a generated PS graft layer would decrease with a decrease in PS molecular weight. In the case of low-molecular-weight 2k-PS, the PS graft layer will show an insufficient shielding effect on Au electrodeposition. In contrast, in the case of high-molecular-weight 106k-PS, another factor will allow the growth of tiny Au grains. In general, a spin-coated PS layer on a substrate is in a metastable state. Subsequent annealing above its Tg shifts it to a thermodynamically stable state where the PS layer becomes dense so as to fill molecular cavities arising from solvent evaporation. The 106k-PS molecules were too large to fill the molecular cavities by annealing at 190 °C for 10 min because the thermally induced segmental motion was restricted. This situation might make pinholes remain in the PS graft layer, resulting in the growth of Au grains. At present, it was difficult to demonstrate the reason that the tiny Au grains grew on Au working electrode surfaces masked with the PS graft layer and the PS resist layer in the case of using 2k-PS and 106k-PS as resist materials. It was worthy to note that there was an optimal molecular weight for Au electrodeposition in R-TNIL using a PS resist mask. Figure 3e,g,h shows SEM images of electrodeposited Au patterns on PrM-modified Au substrates prepared using a UVexposed 11k-PS resist layer at an identical applied electric flux density of 0.322 C cm−2 under the conditions of a current density of 3.22 mA cm−2 for a deposition period of 100 s (Figure 3e), 6.44 mA cm−2 for 50 s (Figure 3g), and 1.61 mA cm−2 for 200 s (Figure 3h). Obviously, electrodeposited Au line widths were increased in the case of the highest applied current density of 6.44 mA cm−2. The average line width of concave PS lines (Figure 3b) was 0.49 μm, and the average line widths of convex electrodeposited Au lines were 0.56 μm for 1.61 mA cm−2, 0.64 μm for 3.22 mA cm−2, and 0.71 μm for 6.44 mA cm−2. This is probably because the rapid growth of Au grains occurred at edges in aperture regions of the PS resist pattern. Therefore, the walls of the PS resist pattern would be too mechanically weak to maintain the initial shapes of the PS resist pattern by stress generated by electrochemical Au growth. Leveling Heights of Electrodeposited Au Lines under a Small Current Density. Figure 4 shows SEM images of lineand-space patterns of 1 μm (Figure 4a−c), 0.5 μm (Figure 4d− f), 0.3 μm (Figure 4g−i), 0.2 μm (Figure 4j−l), and 0.1 μm (Figure 4m−o), with results for nanoimprinted 11k-PS resist layers (Figure 4a,d,g,j,m) and electrodeposited Au films on Au substrates in plain view (Figure 4b,e,h,k,n) and in tilted view (Figure 4c,f,i,l,o). Au electrodeposition was performed at a current density of 1.61 mA cm−2 for a deposition period of 100 s. According to the plain-view SEM images, the average line widths of aperture regions of the 11k-PS resist layers were 1.02 μm for 1.0 μm, 0.49 μm for 0.5 μm, 0.29 μm for 0.3 μm, 0.19 μm for 0.2 μm, and 0.10 μm for 0.1 μm. The corresponding average line widths of electrodeposited Au patterns were 1.06, 0.51, 0.33, 0.21, and 0.11 μm, respectively. Lateral expansion of the Au line widths could be suppressed within 0.04 μm by performing electrodeposition at a current density of 1.61 mA cm−2. The heights of the electrodeposited Au line patterns were measured by AFM. The 1.0 μm line-and-space Au patterns had a height of 0.18 μm. The 0.5 and 0.3 μm line-and-space Au patterns had an almost identical height of 0.17 μm; however, the 0.2 and 0.1 μm line-and-space Au patterns had obviously smaller heights of 0.15 and 0.12 μm, respectively. To investigate the dependence of the height of electrodeposited Au line patterns on the aperture line width of the 11k-PS resist pattern, Au electrodeposition was performed at an

Figure 5. Correlation of PS aperture line width to the height of electrodeposited Au lines. Au electrodeposition was carried out at 1.61 mA cm −2 for (○) 30 s, (◊) 50 s, (□) 75 s, and (△) 100 s and (■) at 0.805 mA cm−2 for 150 s. The identical applied electric flux density of 0.121 C cm−2 was used in the case of open and closed squares.

plots of the height of Au lines against the aperture line width of the PS resist pattern. The heights of Au lines markedly decreased at an aperture line width smaller than 0.2 μm. The results indicated that Au electrodeposition in this study was a diffusion-limited reaction. It was anticipated that, with a decrease in the aperture size of the PS resist layer, the diffusion of [Au(SO3)2]3− as a Au source would be suppressed. To level the heights of electrodeposited Au lines independently of the aperture sizes of the resist mask, a lower current density of 0.805 mA cm−2 was set for 150 s. The applied electric flux density of 0.121 C cm−2 was the same as that in the case of a current density of 1.61 mA cm−2 for 75 s. In the former case, the heights of Au line patterns are represented as closed squares in Figure 5, and in the latter case, these are represented as open squares. The heights of electrodeposited Au lines were made uniform at the lower current density. The fact tells us that, when Au electrodeposition is carried out using resist patterns with several aperture sizes, we should check the heights of electrodeposited Au patterns that hardly depend on the aperture sizes. The choice of a lower current density decreases the contribution of diffusion-limited reactions, which is one way to fabricate leveled Au patterns by Au electrodeposition. Submicrometer Au Bump Patterns Fabricated by Subsequent Wet Etching. Concave regions of Au patterns after Au electrodeposition comprise a Au electrode layer and a Cr adhesive layer on a substrate. Therefore, the underlying metal layers should be removed by etching to fabricate individual submicrometer Au bump structures. Figure 6a−c shows SEM images of 1.0, 0.5, and 0.2 μm line-and-space Au line patterns after two steps of Au and Cr wet etching. The height of the Au lines was measured by AFM. The average height of 0.22 μm before wet etching decreased to 0.19 μm after the wet etching of Au and Cr layers. To confirm the uniformity of the removal of the Au and Cr underlying layers, Au-plated fused silica substrates (15 × 15 mm2, 25 nm Au/20 nm Cr/0.5 mm SiO2) were used as substrates, and Au bump patterns were prepared on optically transparent silica substrates in the same manner as the preparation of Au bump structures on silicon substrates. Figure 6d,e shows transmission optical microscope images of the Au bump structures on a silica substrate prepared by Au and Cr wet etching. It was confirmed 11651

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The photochemical graft reaction caused by the benzophenone-containing monolayer has the advantage of the facile chemical modification of substrate surfaces with widely used PS at room temperature under ambient conditions. The preparation of specific polymers having functional groups is not necessary for the formation of a PS graft layer on a substrate surface. In comparison with photo-cross-linkable perfluorinated azides, the benzophenone monolayer photochemistry is not accompanied by a release of outgas, which would deteriorate the adhesion at the PS−Au interface. Individual Au bump line patterns on a substrate demonstrated in this article will provide opportunities for fabricating, at high throughput and in a cost-effective way, optical materials such as wire grid polarizers, plasmonic materials, metamaterials, and electrode materials for tape automated bonding in future fine liquid-crystal displays (LCDs) and organic electroluminescence displays (OELDs).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel and Fax: +81-22217-5668. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by a KAKENHI (20350103) Grant-in-Aid for Scientific Research (B), a KAKENHI (22108502) Grant-in-Aid for Scientific Research of Innovative Areas, and Management Expenses Grants for National Universities Corporations from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT). We thank the NOF Corporation for financial support in joint research.

Figure 6. (a−c) SEM images of Au bump patterns on a silicon substrate and (d−e) transmission optical microscope images of Au bump patterns on a fused silica substrate fabricated by R-TNIL involving Au electrodeposition and subsequent Au and Cr wet etching. The Au line patterns had line widths of (a, d) 1.0 μm, (b, e) 0.5 μm, and (c) 0.2 μm.

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that submicrometer Au bump patterns could be prepared on a transparent silica substrate.

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REFERENCES

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CONCLUSIONS We demonstrated reactive-monolayer-assisted thermal nanoimprint lithography (R-TNIL) involving Au electrodeposition and subsequent wet etching to fabricate submicrometer Au patterns on substrates. Au-plated substrates were modified with a photoreactive monolayer of a benzophenone-containing alkylthiol and covered with a PS thin film by spin coating. Interfacial PS molecules were covalently anchored by a graft reaction of the photoreactive monolayer upon exposure to UV light. The presence of a PS graft layer allowed the preparation of nanoimprinted PS resist layers without dewetting and showed a suppressive effect on the growth of Au grains in undesired regions masked with a PS resist layer. It was revealed that the suppression of the electrochemical growth of defect Au grains was dependent on the molecular weight of PS used as the graft and resist layers. A molecular weight of approximately 100 000 g mol−1 was suitable for the resist materials for electrodeposition. A lower current density in Au electrodeposition was required to obtain Au line patterns with small size deviation from aperture sizes of PS resist patterns and with a uniform height independent of Au line width. As a result, Au line patterns having submicrometer line widths ranging from 0.1 to 1.0 μm could be successfully fabricated by R-TNIL involving Au electrodeposition. Subsequent wet etching of underlying metal layers allowed the fabrication of Au bump line patterns. 11652

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