Nano Patterning

  • View
    49

  • Download
    0

Embed Size (px)

Text of Nano Patterning

ARTICLESPUBLISHED ONLINE: 26 SEPTEMBER 2010 | DOI: 10.1038/NNANO.2010.175

Direct nanoprinting by liquid-bridge-mediated nanotransfer mouldingJae K. Hwang1, Sangho Cho1, Jeong M. Dang1, Eun B. Kwak1, Keunkyu Song2, Jooho Moon2 and Myung M. Sung1 *Several techniques for the direct printing of functional materials have been developed to fabricate micro- and nanoscale structures and devices. We report a new direct patterning method, liquid-bridge-mediated nanotransfer moulding, for the formation of two- or three-dimensional structures with feature sizes as small as tens of nanometres over large areas up to 4 inches across. Liquid-bridge-mediated nanotransfer moulding is based on the direct transfer of various materials from a mould to a substrate through a liquid bridge between them. We demonstrate its usefulness by fabricating nanowire eldeffect transistors and arrays of pentacene thin-lm transistors.

T

he fabrication of micro- and nanoscale structures is essential for electronics1, micro/nanoelectromechanical systems24, biological and chemical sensors58, microuidics912, display units, and optoelectronic devices13. Of existing patterning methods, the direct printing of functional materials is the most efcient method for the fabrication of new types of structures and devices at low cost and low environmental impact. Direct printing includes a number of non-photolithographic techniques that directly transfer the functional materials to the substrates: ink-jet printing14, screen printing15, exographic printing16, gravure printing17,18, offset printing1921, and microtransfer moulding2227. Microtransfer moulding is the most versatile and cost-effective method for the fabrication of functional microstructures over a large area, but it suffers from problems such as poor edge resolution (due to the lateral diffusion of the liquid inks), residues between patterns, and difculty in multi-alignment. Several alternative residue-free direct printing methods have been developed for patterning at the nanoscale, such as nanoimprint lithography2832, capillary force lithography33,34, and nanotransfer printing28,3539. Recently, nanoimprint lithography and capillary force lithography have been used with selective dewetting to fabricate residue-free patterns of functional polymers. However, imprinting methods suffer from residues and difculty in multi-alignment. Nanotransfer printing is based on the adhesive transfer of a patterned metal thin lm from a stamp to a substrate with tailored surface chemistries3537, but it also suffers from problems. For instance, it only works with a limited number of materials (mainly metals), it only works in a small range of processing conditions, and continuous operation can be difcult because vacuum conditions are required. We have developed a direct printing technique that is based on a liquid-bridge-mediated transfer moulding process. The polar liquid layer serves as an adhesion layer that provides good conformal contact between the functional materials and the substrate38,39. Unlike microtransfer moulding, our technique is not subject to surface diffusion and can generate complex nanostructures with minimum feature sizes below 60 nm with an edge resolution of 26 nm. The new technique allows two- or three-dimensional complex nanostructures to be directly fabricated over a large area using many types of inks.1

Liquid-bridge-mediated transfer mouldingFigure 1 illustrates the procedure for patterning functional materials using liquid-bridge-mediated nanotransfer moulding (LB-nTM). In a rst step, patterned hard and soft moulds were fabricated by using polyurethane acrylate (PUA) and polydimethylsiloxane (PDMS), respectively. These two materials have very low surface free energies (PUA, 25 mJ m22, PDMS, 20 mJ m22). The patterned mould was then lled with an ink solution using selective inking. Discontinuous dewetting40 was used to ll only the recessed areas of the mould with the ink solution. By dragging a deposited ink solution over the patterned mould with a glass stick or a needle, the meniscus of the ink solution moves over the surface of the mould to ll inside the features without leaving any residues on the raised surface (Fig. 1b). Discontinuous dewetting takes advantage of the interfacial free energy between the mould and the ink solution, and the ink solution must have a surface free energy (between 30 mJ m22 and 70 mJ m22) appropriate to the PDMS and PUA moulds. The rate of dragging the solution, the aspect ratio of the features in the mould (depth/width 1/20), and the viscosity of the ink solution (,500 cP) also determine the success of the discontinuous dewetting process40. The lled ink is next solidied by drying it at mild temperatures (,100 8C). Almost no residue remains on the protruding surfaces of the mould as a result of the selective inking (Fig. 1c). The very small amount of excess residue can be removed by application of a brisk stream of nitrogen, because the mould has a very low surface free energy. The absence of residue was conrmed by analysis of the patterns using energy dispersive X-ray analysis and a cross-sectional view obtained by means of scanning electron microscopy (SEM; Supplementary Fig. S1). Because of the solidication of the ink solution, LB-nTM does not suffer from surface diffusion and can generate nanostructures on a scale well below 100 nm. The mould with the solidied ink was then brought into contact with a substrate surface covered by a thin polar liquid layer. A substrate of area 1 1 cm2 can be uniformly covered with a 100-mmthick liquid layer by using 10 ml of a polar liquid. The polar liquid layer on the substrate forms a liquid bridge (a capillary bridge) between the substrate and a mould containing recessed patterns (Fig. 1d). The liquid bridge allows good conformal contact between the solidied ink and the substrate38. The substrate must

Department of Chemistry, Hanyang University, Seoul 133-791, Korea, 2 Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Korea. *e-mail: smm@hanyang.ac.krNATURE NANOTECHNOLOGY | VOL 5 | OCTOBER 2010 | www.nature.com/naturenanotechnology

742

2010 Macmillan Publishers Limited. All rights reserved.

NATURE NANOTECHNOLOGYa b

DOI: 10.1038/NNANO.2010.175

ARTICLES

Mould

500 nm

Fill channels with an ink solution by selective inking

InkMould

Solidify the ink Solidied ink

c

Mould

500 nm

Contact

range of materials, and we have made various functional structures using many types of inks (liquid prepolymers, metal particle solutions, molecular precursors, and so on). It can also be used to fabricate nanometre-sized structures without leaving any residue on the regions of the substrates not to be coated. In contrast to other direct patterning methods using liquid inks, such as microtransfer moulding and gravure printing, here, the lled inks are solidied before transfer onto the substrate to prevent lateral diffusion. The nanometre-sized patterns can be made on diverse substrates as long as their surface free energies are high enough to exhibit strong capillary action with a polar liquid layer. In fact, by using LB-nTM with UV activation of the substrates, complex structures can be patterned on various substrates including silicon, TiO2 , polyethersulphone (PES) and gold (Supplementary Fig. S2). LB-nTM can be used to create complex two- or three-dimensional nanostructures over a large area in a repetitive, continuous process. The mould can be aligned easily on complex structures because, before the polar liquid layer is dried, it acts as an adhesive lubricant, enabling the mould to be moved over the substrate. Furthermore, deformation and distortion of the polymer mould can result in errors in the replicated patterns, as well as misalignment of the patterns. Such problems are difcult to correct in direct printing methods because the pattern transfer occurs immediately at the time of contact. In the LB-nTM method, however, the position of the mould can be adjusted even after contact with the substrate, because the pattern is not transferred to the substrate before drying of the liquid layer.

dMould Solidied inkSubst rate

Nanoscale patternsNanometre-scale patterns of various materials were made on silicon substrates using the LB-nTM method with hard moulds (PUA). The masters used for fabrication of the moulds were silicon wafers with dense nanoscale patterns, which were made by laser interference lithography and subsequent dry etching steps, as described previously43. The moulds were fabricated by casting PUA on them. After UV curing, the PUA moulds were peeled away from the masters. To pattern an array of zinctin oxide (ZTO) on a nanometre scale, the recessed spaces of the patterned PUA moulds were lled with a 2-methoxyethanol solution of ZTO ink. The ZTO ink solution in the mould was solidied at 80 8C for 10 min. The mould was then placed in contact with an oxidized Si(100) substrate covered by a thin ethanol layer. Following drying of the ethanol layer between the mould and the substrate at 70 8C for 10 min, the mould was peeled away, leaving the ZTO nanopatterns on the substrate. Scanning electron microscopy (SEM) images of the representative structures formed in this manner are shown in Fig. 1, including the PUA mould (Fig. 1a), the mould lled with ZTO ink (Fig. 1c) and the ZTO patterns fabricated on the substrate (Fig. 1e). SEM images of the ZTO patterns fabricated using the PUA mould (140-nm-wide parallel lines, 60-nm-wide spaces) clearly show that the ZTO patterns retain the x and y dimensions of the mould, as shown in Fig. 2a. The height of each ZTO pattern, however, is 54 nm, which means that it is reduced by 46% in the z-direction when compared to the depth of the mould (100 nm). Figure 2b shows an SEM image of ZTO dots