Electronic Materials Letters, Vol. 8, No. 5 (2012), pp. 485-489 DOI: 10.1007/s13391-012-2053-7
Various Metallic Nano-Sized Patterns Fabricated Using an Ag Ink Printing Technique Sang-Chul Oh,1 Ki-Yeon Yang,2 Kyeong-Jae Byeon,3 Ju-Hyeon Shin,3 Yang-Doo Kim,4 Lee-Mi Do,5 Kyung-Woo Choi,6,* and Heon Lee1,3,4,* 1
2
Department of Nano-Semiconductor, Korea University, Seoul, Korea Nano Fabrication Group, Samsung Advanced Institute of Technology, Gyeonggi-do, Korea 3 Department of Material Science and Engineering, Korea University, Seoul, Korea 4 Department of Biomicrosystem Technology, Korea University, Seoul, Korea 5 Electronics and Telecommunications Research Institute, Daejeon, Korea 6 Korea Institute of Nuclear Safety, Daejeon 305-338, Korea (received date: 4 April 2012 / accepted date: 30 April 2012 / published date: October 2012)
This paper presents a new simple metal patterning technique, which is based on soft nanoimprint lithography. By using this method with a commercial Ag nano particle ink, a nano-sized metal pattern was successfully fabricated. The problem of the residual layer of patterned Ag layer was minimized by controlling the concentration of the solution and the process conditions. By using this method, we could easily fabricate various patterns without reference to any shape. Furthermore, we fabricated an Ag mesh type pattern for the application of conducting transparent glass. Keywords: Ag nano particle solution, soft-nano imprint lithography, nano-sized metal pattern, mesh, No residual layer
1. INTRODUCTION Nanotechnology, by using nano-structures, has been recently highlighted as a way to improve the performance of various devices.[1-10] However, in the case of metal patterning, a general method such as the combination of conventional lithography and etching processes has some disadvantages, such as complexity and high processing cost.[11] General metal patterning methods by inkjet printing were widely used to fabricate the metal wiring due to their simple and cheap processes. However, they place a limit on the minimum linewidth of a few micrometers. Therefore, new methods to fabricate metallic nano-structures have been required to avoid the problems of general methods.[12-16] Various methods such as nanoimprint lithography, nano transfer printing and micro-contact printing for nano-sized metal patterning have been recently presented.[17-19] Nanoimprint lithography, which is one of the next generation of patterning techniques, presents the possibility of the fabrication of sub 10 nm patterns.[20,21] Moreover, it can be applied to mass pro*Corresponding author:
[email protected] *Corresponding author:
[email protected] ©KIM and Springer
duction because nano to micron-scale patterns can be fabricated through simple and cheap processes.[22,23] In particular, functional materials of nanometer scale can be transferred to the substrate by simple soft nanoimprint lithography.[24,25] In this study, an Ag nano-sized pattern was successfully fabricated by using an Ag ink printing technique, which is a type of soft nanoimprint lithography using an Ag nano particle ink and a polymeric mold.
2. EXPERIMENTAL The Ag ink (DGH 55-LT25C, made by Advanced Nano Products) had a specific resistivity of 2.4 ~ 3.0 µΩ·cm and contained 5 ~ 7 nm sized Ag particles. The Ag ink solution was diluted (5%) in toluene. In this study, a polydimethylsiloxane (PDMS) mold was duplicated from the Si master template with nano-sized pattern and was used as an imprint stamp. Since the PDMS mold can be replicated from the master with high fidelity with low cost and its high durability, it has been widely used in conventional soft nanoimprint lithography. Furthermore, flexible polymer mold can selectively absorb the solvent of Ag nano-ink during imprinting and can form the conformal contact to substrates which are not perfectly flat and smooth.
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Fig. 1. Schematic diagram of the Ag ink solution printing process.
The Ag ink solution printing process is shown in Fig. 1. As shown in Fig. 1, the Ag ink solution was dropped onto the PDMS mold, not onto the substrate, to enhance the solution filling of the patterns in the PDMS mold. In order to prevent the deformation of the Ag ink pattern in the PDMS mold during the detachment from the Si substrate, polyethylene terephthalate (PET) film was attached between the Si substrate and the PDMS mold. Then, the spin-coating process of the Ag ink solution was progressed under the condition of 3000 rpm for 30 s. The residual layer of patterned Ag layer was controlled by the spin-coating condition and the concentration of the diluted solution. After the spin-coating process of the Ag ink solution, various substrates such as Si wafers or glass were placed on the PDMS mold. Subsequently, 10 bar of pressure was applied for 5 min. The substrate was heated to 100°C and held there for 10 min as the pressure was applied. Then, the patterned substrate, which was attached to the PDMS mold and the PET film, was detached from the Si substrate. Then, the patterned substrate was separated from the PDMS mold and the PET film, and it was annealed at 250°C for 30 min in the air for the vaporization of the remaining solvents. Finally, the Ag nano pattern was
successfully fabricated on the various substrates. The morphology of the Ag nano pattern was investigated by SEM (Scanning Electron Microscopy). A mesh pattern is generally used to fabricate transparent conductive glass. Therefore, we fabricated an Ag mesh pattern to apply to conducting transparent glass due to the nonexistence of the residual layer. Furthermore, an Ag mesh pattern was fabricated on Corning Eagle XG glass, which has similar transmittance such as ordinary glass and high melting temperature as 700°C, by this printing technique for the application of conducting transparent glass. The control of the residual layer is very important in applying the Ag mesh pattern on the conducting transparent glass. Therefore, we optimized the minimization of the residual layer. The fabrication process of the mesh pattern was performed as shown in Fig. 2. Using this Ag ink printing technique, a line and space shaped Ag nano pattern was fabricated as a first layer. During the annealing process, the remaining solvents were vaporized and Ag nano particles were aggregated with other particles. The patterned first layer became tolerant by the annealing process. The tolerance of first layer was important to prevent the deformation of the Ag mesh pattern.
Fig. 2. Fabrication of Ag mesh type pattern using Ag ink printing process.
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Then, the second line was fabricated in the perpendicular direction to that of the first layer. Finally, the Ag mesh pattern layer was fabricated on the glass. The presence of a mesh type pattern on the glass was confirmed by SEM images. Then, the electrical conductivity and the transmittance were measured by using a 4 point probe (CMT-SERIES, made by Advanced Instrument Technology) and a UV-visible spectrometer (V650 Spectro-photometer, made by JASCO), respectively.
3. RESULTS AND DISCUSSION Figure 3 shows the SEM images of the results produced by the Ag ink printing technique. The 200 nm-sized line and space pattern was successfully fabricated as shown in Fig. 3(a). Then, two kinds of nano-sized pillars were successfully
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fabricated as shown in Fig. 3(b) and (c). Usually, it is hard to fabricate high fidelity patterns with low concentration resist. However, nano-patterns in the master stamp were successfully duplicated and fabricated on the substrate without reference to any shape by this printing technique. Thus, it was confirmed that this printing technique is the method to fabricate fine patterns with low concentration resist. In Figure 3(d), the residual layer, which causes problems in electronic devices, on the region without the nano-pattern was nonexistent. Therefore, we confirmed that the metallic nano-pattern was successfully fabricated by using a simple Ag printing technique without an etching process. The shape of Ag nano-pattern was slightly different to the imprint stamp, since the Ag nano-pattern was formed by agglomerating Ag nano-particles. The mesh pattern was deposited by the method shown in
Fig. 3. SEM Images of Ag nano patterns: (a) line and space pattern, (b,c) pillar pattern, and (d) cross sectional view of pillar pattern.
Fig. 4. SEM images of Ag mesh type nano-pattern on glass. (a) low-magnification SEM image of Ag mesh type nano-pattern, and (b) high magnification SEM image of Ag mesh type nano-pattern.
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Fig. 5. Fabricated Ag mesh pattern on glass and transmittance graph with average sheet resistances of bare glass and patterned glass.
Fig. 2. As shown in Fig. 4(a) and (b), an Ag mesh type pattern was successfully fabricated by using the crossed deposition of a 200 nm-sized line & space pattern. In Figure 4, we confirmed that the Ag mesh pattern was uniformly fabricated on the whole substrate. Then, in spite of the crossed deposition, the configuration of the mesh pattern was retained without any damage. An optical image of glass with an Ag mesh pattern is shown in Fig. 5(a). As shown in Fig. 5(a), the transparency of the glass was almost maintained after the fabrication of the Ag mesh pattern. The transmittance of the glass with and without the Ag mesh pattern was measured by using a UVvisible spectrometer and the results are shown in Fig. 5(a). The average transmittance values of bare glass and patterned glass from 250 nm to 850 nm were 87% and 28%, respectively. The average transmittance of the patterned glass was decreased by 59% as compared with the bare glass due to the decreased penetration area of light in the patterned glass. Furthermore, we confirmed the improvement of the electrical conduction caused by the Ag mesh pattern. In the case of the bare glass, electrical conduction was absent. However, after the fabrication of the Ag mesh pattern, the sheet resistance of glass was measured as 23 Ω/□. Therefore, conductive glass was fabricated by this Ag ink printing technique.
4. CONCLUSIONS In this study, metal patterns were successfully fabricated by an Ag ink printing technique based on soft nanoimprint lithography. Nano-sized Ag patterns were fabricated by using this simple and cheap method, and the problem of the residual layer was minimized. Then, an Ag mesh pattern for application to conducting transparent glass was successfully fabricated by the crossed deposition of line & space patterns. Consequently, the possibility of producing a conducting transparent glass with an Ag mesh pattern was confirmed by the maintenance of transparency and the improvement of electrical conductivity, in spite of the decrease in transmittance.
ACKNOWLEDGEMENTS This work was supported by the Technology Innovation Program R1107151 funded by the Ministry of Knowledge Economy in Korea and the R&D program for Industrial Core Technology through the Korea Evaluation Institute of Industrial Technology supported by the Ministry of Knowledge Economy in Korea (Grant No. 10040225).
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