NANO LETTERS
Soft Graphoepitaxy of Block Copolymer Assembly with Disposable Photoresist Confinement
2009 Vol. 9, No. 6 2300-2305
Seong-Jun Jeong,† Ji Eun Kim,† Hyoung-Seok Moon,† Bong Hoon Kim,† Su Min Kim,‡ Jin Baek Kim,‡ and Sang Ouk Kim*,† Department of Materials Science and Engineering, KAIST, Daejeon, 305-701, Republic of Korea, and Department of Chemistry and School of Molecular Science (BK21), KAIST, Daejeon, 305-701, Republic of Korea Received February 14, 2009
ABSTRACT We demonstrate soft graphoepitaxy of block copolymer assembly as a facile, scalable nanolithography for highly ordered sub-30-nm scale features. Various morphologies of hierarchical block copolymer assembly were achieved by means of disposable topographic confinement of photoresist pattern. Unlike usual graphoepitaxy, soft graphoepitaxy generates the functional nanostructures of metal and semiconductor nanowire arrays without any trace of structure-directing topographic pattern. Our novel approach is potentially advantageous for multilayer overlay processing required for complex device architectures.
Various nanolithographic technologies such as e-beam lithography, scanning probe lithography, and nanoimprinting have been explored to overcome the resolution limit of conventional photolithography. Nonetheless, the low throughput of the serial writing process or technical barriers arising from mechanical contact obstructs a large-area production of densely packed nanostructures.1-3 Directed self-assembly of block copolymers is considered a promising alternative that may overcome those limitations.4-18 The spontaneous and parallel assembly of block copolymers generates thermodynamically stable, densely packed periodic nanodomains (5-50 nm) upon various substrates over a large area.19,20 Furthermore, laterally ordered, device-oriented nanostructures, which are potentially useful for diverse advanced applications, are attainable by the directed assembly of block copolymers exploiting chemically/topographically prepatterned substrates, application of external fields, and so on.7-17,21-24 Graphoepitaxy is a widely used directed assembly method for block copolymers.12-17 Unlike ordinary epitaxy, where the target ordering dimension is generally commensurate with the structure-directing dimension, surface relief structure, whose structural dimension is much larger than the target ordering dimension, may induce a highly ordered morphology. In the graphoepitaxy of block copolymer assembly, a * Corresponding author,
[email protected]. † Department of Materials Science and Engineering, KAIST. ‡ Department of Chemistry and School of Molecular Science (BK21), KAIST. 10.1021/nl9004833 CCC: $40.75 Published on Web 05/07/2009
2009 American Chemical Society
block copolymer thin film is assembled within the microscale confinement imposed by a topographically patterned inorganic substrate that can endure the high temperature annealing required to accomplish directed block copolymer assembly. Upon the deposition of block copolymer thin film over a prepatterned substrate and subsequent high-temperature annealing, the physical confinement imposed by the side wall of the topographic pattern enforces lateral ordering of selfassembled nanodomains along the pattern edge. The spontaneous close packing of neighboring nanodomains raises the propagation of laterally ordered morphology throughout the film plane such that a highly ordered nanopattern can be generated within the topographic confinement.25 In this approach, the pattern density of a lithographic process used to prepare a topographically prepatterned substrate is remarkably enhanced by the directed assembly of block copolymers. However, a multistep process consisting of photopatterning and the subsequent pattern transfer is required to prepare a prepatterned substrate. Furthermore, the structure-directing topographic pattern permanently remains at the substrate surface and, thus, may limit any further overlay process for multilayered device architectures. Herein, we introduce soft graphoepitaxy using disposable photoresist confinement as an alternative approach to overcome the aforementioned limitations. Soft graphoepitaxy utilizes the topographic pattern of an organic negative-tone photoresist prepared by conventional deep ultraviolet (DUV) projection photolithography to direct block copolymer assembly. Consequently, the number of process steps for
Figure 1. (A) Schematic representation of the soft graphoepitaxy process. (a) Photopatterning of negative tone photoresist by a projection photolithography. The substrate surface can be neutrally modified by organic deposition. (b) Spin-coating of PS-b-PMMA and subsequent thermal annealing for directed assembly within photoresist confinement. After assembly, the PMMA nanodomains were selectively removed. (c and d) Pattern transfer by selective deposition or selective etching. After pattern transfer, both photoresist layer and the PS nanotemplate were completely disposed of to retain highly ordered functional nanopatterns at desired locations without any trace of the structure-directing photoresist pattern. (B) Optical microphotograph of a soft graphoepitaxial pattern over a broad area (blue region, 80 nm thick PS-b-PMMA film layer; red region, 150 nm thick photoresist layer). (C) Cross-sectional SEM image of the PS-b-PMMA lamellar morphology (L0, 48 nm), aligned along the photoresist trench walls. (D and E) Plane view and 45° tilted view (insets) of SEM images of Al and Si nanowire arrays replicatng lamellar template morphology, prepared by soft graphoepitaxy.
preparing a topographical confinement can be considerably reduced, and the overall nanolithographic process ensures scalable and parallel processing, highly compatible to conventional device fabrication process. More significantly, any trace of the topographic pattern that directs self-assembly can be completely eliminated by a mild cleaning process after pattern transfer, which is potentially advantageous for further overlay processes. In this work, we successfully created metal and semiconductor nanowire arrays as well as various morphologies of block copolymer nanotemplates by means of soft graphoepiatxy to demonstrate its versatility as a nanolithographic process. The overall process of soft graphoepitaxy is briefly described in Figure 1A. A negative tone photoresist was uniformly spin-coated on a substrate and topographically patterned by conventional DUV projection photolithography (I-line source; wavelength 365 nm; 9.5 mW/cm2). Note that the substrate surface can be organically modified to control the surface tension or surface mobility before photoresist deposition.17,26 A thin film of polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) copolymer was spin-coated on Nano Lett., Vol. 9, No. 6, 2009
the topographic pattern of the photoresist and annealed at a high temperature (250 °C) to reach a highly ordered equilibrium morphology. After thermal annealing, the PMMA nanodomains were selectively dry-etched from the block copolymer thin film by oxygen plasma, leaving a cross-linked PS nanotemplate within the photoresist pattern trench. In the following pattern transfer by selective deposition or selective etching, both the photoresist layer and the PS nanotemplate remaining upon the substrate acted as a disposable mask. After pattern transfer, both organic templates could be completely disposed of by a mild cleaning process. On the remaining substrate surface, highly ordered nanopatterned functional materials having a desired pattern shape and orientation could be fabricated at desired locations without any trace of the structure-directing substrate pattern. For successful soft graphoepitaxy, the use of an organic negative tone photoresist is crucial. First, the negative tone photoresist layer cross-linked by UV photopatterning maintains its structural integrity as topographic confinement during the high-temperature annealing required for the directed assembly of block copolymers. Second, owing to beam 2301
blocking by a photomask, the trench region of the photoresist pattern is not exposed to high-energy UV radiation during photopatterning. Accordingly, the organic polymer brush or self-assembled monolayer deposited on the bottom surface of the trench is not damaged and effectively directs the orientation (surface perpendicular or surface parallel) of anisotropic (cylinder or lamellar) self-assembled nanodomains. Third, as mentioned above, the organic photoresist layer can be completely disposed of by a mild cleaning process, and this removal is a key feature for compatibility with conventional semiconductor fabrication processes requiring multilayer overlay processing. In this work, SU8 was used as an organic negative tone photoresist. The photosensitive range of SU-8 is 300-400 nm, a region accessible with conventional I-line photolithography. Its high transparency in the near-UV range allows the fabrication of high aspect ratio topographic patterns with vertical side wall profiles. Furthermore, SU8 has high heat/chemical resistance and low line-edge roughness that are greatly advantageous for soft graphoepitaxy (Figure S1 in Supporting Information; photoresist pattern). Soft graphoepitaxy may produce a highly uniform hierarchical nanopattern over a broad area. Figure 1B shows an optical microphotograph of a soft graphoepitaxial pattern, where lamellar forming symmetric block copolymers (PSb-PMMA; Mn 104 kg mol-1; lamellar period 48 nm) were assembled within the parallel trenches of a photoresist pattern (trench width 800 nm). The characteristic reflective colors of blue and red correspond to the block copolymer film layer (film thickness ∼80 nm) and the photoresist layer (layer thickness ∼150 nm), respectively. The highly regular alternation of the reflective color demonstrates the periodic thickness variation of the hierarchically patterned morphology. A close examination of the nanoscale morphology by scanning electron microscopy (SEM) reveals that selfassembled lamellae are highly aligned along photoresist trenches over a broad area (see Figure S2 in Supporting Information). Figure 1C shows a cross-sectional SEM image of the PS-b-PMMA lamellar domains, aligned along the trench walls of the photoresist pattern. Despite hightemperature annealing at 250 °C, the photoresist pattern maintained the vertical profile of its side walls. Owing to the neutral modification of the bottom surface with a brush layer (poly(styrene-random-methyl methacylate); P(S-rMMA)), lamellae were vertically oriented throughout the film thickness of 80 nm. As noted above, the organic neutral brush layer at the bottom of the trench region was not exposed to UV radiation during the photopatterning of the negativetone SU8 photoresist and, thus, successfully developed surface perpendicular lamellar orientation. The trench side wall of the SU8 photoresist was preferentially wet by PS blocks, inducing parallel alignment of neighboring lamellar domains along the trench walls. This vertical lamellar array offers significant advantages for pattern transfer due to its vertical side wall profile. Panels D and E of Figure 1 present SEM images of metal and silicon nanowire arrays prepared from soft graphoepitaxy. In order to produce the aluminum (Al) nanowire array 2302
shown in Figure 1D, a 15 nm thick Al layer was thermally evaporated over a soft graphoepitaxially assembled lamellar template where PMMA nanodomains had been selectively dry-etched. Since the remaining organic templates consisting of SU8 and PS could be easily lifted-off after thermal deposition, no trace of the structure-directing pattern remained on the substrate surface. The produced Al nanowire array had both a wire width and spacing of 24 nm, exactly replicating the lamellar template morphology. The SEM image in Figure 1D shows a polycrystalline Al nanowire with a typical grain size of 10-15 nm. The cross-sectional image (inset in Figure 1D) reveals the highly smooth surface morphology of the nanowires. This highly aligned metal nanowire array with a low line-edge roughness has potential applications to electronics27,28 and photonics.29 The semiconductor nanowire array shown in Figure 1E was prepared by reactive ion etching (RIE). After the selective removal of PMMA nanodomains in a lamellar template by oxygen RIE, the remaining PS lamellar morphology was exactly replicated into the underlying silicon substrate by RIE with Br and CF4 mixture gas. This highly aligned silicon nanowire array can be used as a channel array element for a multinanowire field effect transistor.30 As shown in panels D and E of Figure 1, soft graphoepitaxy can be employed to generate highly aligned functional nanowires having a low edge roughness and accurate registration of their location without any trace of the structure-directing large-scale pattern. For numerous applications of nanolithography in sensors,31 electronics,26-28 photonics,29 energy devices,32,33 and so on, the tunability of the pattern size and shape is crucial. In soft graphoepitaxy, the tunability can be fulfilled by adjusting the processing parameters, such as the chemical structures of the block copolymers, surface energy modification, and photoresist confinement shape. Panels A and B of Figure 2 show the well-aligned lamellar domains of two PS-b-PMMA copolymers having different average molecular weights (Mn) of 51 kg mol-1 (lamellar period 32 nm) and 68.5 kg mol-1 (lamellar period 39 nm). The sub-20-nm (16 and 19 nm) thick lamellae were highly aligned in the wide parallel trenches of photoresist patterns having widths of 760 and 860 nm. Panels C and D of Figure 2 demonstrate the influence of bottom substrate modification on the soft graphoepitaxy of block copolymer assembly. Figure 2C presents a vertical hexagonal cylinder array of an asymmetric PS-b-PMMA block copolymer (Mn 67 kg mol-1; cylinder diameter 20 nm; cylinder center to center distance 43 nm) assembled within a parallel trench. Since the underlying substrate was neutrally modified, cylinders were oriented perpendicular to the substrate surface. Owing to the physical confinement from the vertical side walls of the photoresist trench, the hexagonal close packing of the cylinders was perfectly ordered across the trench width of 900 nm. A well-ordered nanocylinder array can serve as a useful nanotemplate in a range of applications, such as patterned recording media,34 nanopatterned catalyst arrays,35 nanoporous membrane,36 and so on. When the bottom substrate surface was not neutrally modiNano Lett., Vol. 9, No. 6, 2009
Figure 2. The tunability of pattern shape and size for soft graphoepitaxy. (A and B) SEM images of highly aligned lamellar morphologies of two PS-b-PMMA copolymers having different Mn of 51 kg mol-1 (L0 32 nm) and 68.5 kg mol-1 (L0 39 nm). (C) The plane view SEM image of a vertical hexagonal clinder array of an asymmetric PS-b-PMMA (Mn 67 kg mol-1; cylinder diameter 20 nm; cylinder center to center distance 43 nm) assembled within a parallel photoresist trench formed on the neutrally modified substrate. The 2D FFT pattern (inset) demonstrates a perfect hexagonal ordering across the trench width of 900 nm. (D) A 45° tilted SEM image of highly aligned surface parallel cylinders of the same asymmetric PS-b-PMMA block copolymer assembled within a photoresist trench formed on a bare silicon substrate. (E and F) SEM images of concentric cylinders of a lamellar block copolymer (PS-b-PMMA; Mn 104 kg mol-1; L0: 48 nm) assembled within the cylinder-shaped hole confinement.
fied, the same asymmetric PS-b-PMMA block copolymer formed a well-aligned surface parallel cylinder array (Figure 2D). Since both bottom bare silicon substrate and the side wall of the photoresist trench were preferential surfaces (the bottom substrate prefers the PMMA component, whereas the photoresist side wall prefers the PS component), the cylinder becomes aligned parallel to these surfaces, resulting in a highly aligned surface parallel cylinder array. Although the surface parallel cylinder array has a low pattern aspect ratio and a nonvertical pattern profile in the film thickness direction, it can be easily assembled on various substrates without surface modification. This morphology can be also exploited for the lithographic templates of multinanowires for electronics applications.13 Nano Lett., Vol. 9, No. 6, 2009
Panels E and F of Figure 2 show the lamellar PS-bPMMA copolymer (Mn 104 kg mol-1; lamellar period 48 nm) morphology assembled within the cylinder-shaped hole confinement of the photoresist. The low bending modulus of the block copolymer lamellae allowed for directed assembly into the concentric cylinders.37,38 The bottom substrate surface was neutrally modified by a polymer brush such that the concentric cylindrical morphology had a vertical side wall profile and a high aspect ratio along the cylinder axis. Note that the concentric cylinders demonstrate a highly regular morphology even in the central region, where the curvature of the morphology is remarkably high. The radius of hole confinement determined the chemical component of the central cylinder (PS or PMMA) as well as the number 2303
Figure 3. Electrical characterization of Al nanowire arrays. (A) SEM image of Al nanowire arrays located between circular-shaped Al electrodes. (B) SEM image of Al nanowires after breakdown. (C) Current vs voltage curve of Al nanowires. It shows an Ohmic behavior, confirming that the prepared nanowires were electrically continuoous over their entire length. (D) Current vs voltage curve of Al NWs up to a large current for breakdown.
of surrounding concentric cylinders. This concentric cylinder morphology with a high aspect ratio can potentially provide templates for magnetic39 and photonic nanostructures.40 We investigated the electrical conductivity of Al nanowires prepared by soft graphoepitaxy. As shown in Figure 3A, circular-shaped Al electrodes (diameter 142 µm) were thermally deposited over nanowire arrays. The current versus the voltage of the nanowire (I-V) was measured at room temperature using a probe station and an HP parameter analyzer. The typical width, thickness, and length of the nanowire were 24 nm, 15 nm, and 224 µm, respectively. The measured I-V curve exhibits a linear metallic Ohmic behavior (Figure 3C), confirming that the prepared nanowires were electrically continuous over their entire length. The 2304
room temperature resistance (R) measured from the slope of linear I-V curve was 1.197 × 103 Ω. The electrical resistivity of nanowire (F) defined by F ) RA/L was 17.6 µΩ cm, where L and A correspond to the length and crosssectional area of a nanowire. This value is larger than the usual resistivity value of bulk aluminum (FAl-bulk ) 2.65 µΩ cm at 293 K). The resistivity of polycrystalline nanowires is generally larger than that of a bulk aluminum due to electron scattering on the wire surface41,42 and electron reflection at grain boundaries.43 The resistivity increase of our nanowires having a highly smooth surface morphology may be attributed to the electron reflection at grain boundaries. The linear Ohmic behavior was continued up to the current value of 3.65 mA, where the Al nanowires broke down (Figure 3D). The breakdown occurs at the location of the highest electrical stress and depends on the current density of the metallic nanowire. The maximum current density (Jmax), given by the electric current at breakdown (I) per unit cross-sectional area of nanowire (A), was 4.69 × 108 A/cm2. In summary, we have developed block copolymer lithography via soft graphoepitaxy as a novel strategy for nanopatterning. Our approach provides an efficient and scalable route to various functional nanostructures such as metal and semiconductor nanowire arrays, replicating highly ordered block copolymer thin film morphology, whose nanoscale assembly is directed by a disposable topographic pattern of photoresist. Since the photoresist pattern directing nanoscale block copolymer assembly can be easily disposed of by a mild cleaning process, soft graphoepitaxy is anticipated to serve as a useful nanolithographic process for complex device architectures requiring multilayer overlay processing. Furthermore, taking advantage of its high compatibility with conventional microfabrication process, and wide tunability over pattern shape/size, and patternability of high aspect ratio vertical nanostructures, soft graphoepitaxialy offers an immediate opportunity for applications to various nanomaterials and nanodevices. Acknowledgment. We thank Dr. C. M. Koo for helpful discussions and Ji Young Park and Ju Young Kim for supporting photoreist patterning and Sung Soon Bae for assisting in SEM analysis. This work was supported by the second stage of the Brain Korea 21 Project, the National Research Laboratory Program (R0A-2008-000-20057-0), KAIST EEWS (Energy, Environment, Water, and Sustainability) Initiative (EEWS0903), the Korea Science and Engineering Foundatin (KOSEF) grant (R11-2008-05803002-0), and the Fundamental R&D Program for Core Technology of Materials funded by the Korean government (MEST & MKE). Supporting Information Available: Experimental methods, supporting results. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) (2) (3) (4)
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