NANO LETTERS
Ordered Block-Copolymer Assembly Using Nanoimprint Lithography
2004 Vol. 4, No. 9 1633-1636
Hong-Wei Li and Wilhelm T. S. Huck* The Nanoscience Centre, Interdisciplinary Research Collaboration in Nanotechnology, UniVersity of Cambridge, 11 J. J. Thomson AVenue, CB3 0FF, UK, and MelVille Laboratory for Polymer Synthesis, Department of Chemistry, UniVersity of Cambridge, Lensfield Road, Cambridge, CB2 1EW, UK Received May 26, 2004; Revised Manuscript Received July 8, 2004
ABSTRACT Nanoimprint lithography and self-assembly of poly(styrene)-block-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymer are combined to induce order in the phase-separated domains. Tailored periodic arrays of poly(methyl methacrylate) (PMMA) cylinders normal or parallel to neutralized silicon surfaces can be formed inside the gap of imprint molds. This method opens up a new route to the controlled phase separation of block copolymers with precise placement of the phase-separated domains.
The formation of nanostructures using self-assembly of block copolymers has recently gained a great amount of attention.1,2 As the block lengths can be tuned via controlled polymerization methods, the morphology and size3 of the nanoscale phase-separated domains can be varied from spheres, to hexagonally close-packed cylinders, and to lamellar structures. In particular, diblock copolymer thin films have significant potential for use as templates and scaffolds for the fabrication of arrays of nanometer scale structures over large areas.4-8 Key to many future applications is the longrange ordering and precise placement of the phase-separated nanoscale domains.9,10 To achieve the ordering of the hexagonally close-packed vertical standing cylindrical morphology, strong electric alignment fields4,11 and balanced surface chemistry to avoid preferential wetting of either block12 have been successfully applied. A further alignment, e.g., long-range and positional ordering of these cylinders in the x,y dimensions is much harder to achieve, since there are no driving forces for the domains of close-packed columns to align macroscopically.13 Recently, several groups have shown how the ordering of self-assembled block copolymer nanostructures can be influenced by using physically9,14 or chemically15,16 templated surfaces. However, for all those physical and chemical patterns, complicated fabrication techniques are required every time before the selfassembly of block copolymer can be applied onto a surface. The possibility of locally controlling the positional order of the microphase separated morphology would open up applications where the block copolymers can be used as * Corresponding author. E-mail:
[email protected]. 10.1021/nl049209r CCC: $27.50 Published on Web 08/13/2004
© 2004 American Chemical Society
sacrificial evaporation masks to precisely position metallic films on a surface (e.g., a string of dots between two electrodes). In the graphoepitaxy experiments mentioned in refs 9 and 14, the ordering is based on the confinement of the block copolymer between two vertical walls. We are therefore interested in investigating the use of nanoimprint lithography (NIL) as a tool for locally controlling the selfassembly process of diblock copolymers and determining the precise positioning of the phase-separated domains via the topography of mold, rather than the substrate. NIL has attracted significant attention since its working principle is fundamentally different from conventional lithography. NIL creates features by a mechanical deformation of a polymer film by pressing a hard mold into the film at temperatures higher than the glass transition temperature of the polymer. This high-throughput, low cost process is not diffraction limited, and sub-10 nm resolution has been reported.17 In the case of controlling the morphology of block copolymer films, it would obviate the need for costly surface preparation and patterning and can be applied directly on spin-coated films. The polymer used in our experiments is poly(styrene)block-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymer with molecular weight of 46.1 k for poly(styrene) (PS) block and 21.0 k for poly(methyl methacrylate (PMMA) block (Polymer Source Inc.). This polymer forms cylindrical PMMA phases in a PS matrix. The orientation of PMMA cylinders can be aligned normal to the surface by controlling the film thickness and the surface energy.4,18 The PMMA blocks can be selectively removed with UV exposure and acetic acid etching.5 The porous PS matrix can then be used
Figure 1. Schematic diagram of procedures for controlled selfassembly nanolithography with block copolymers.
as a physical mask for etching and metal evaporation.19,20 Here we show that within the confinements of the nanostructured molds the PMMA columns can be oriented perpendicular or parallel to the surface, while at the same time, lateral ordering can be realized. The experimental process is shown schematically in Figure 1. The process includes four main steps: (1) neutralize the native silicon surface with PS/PMMA random copolymer, (2) spin coat PS-b-PMMA polymer to a specific thickness, (3) anneal the film with guiding patterns on the top, and (4) selectively remove PMMA columns through UV exposure and acetic acid washing. A neutralized silicon surface, which has balanced interfacial interactions to PS and PMMA, was prepared by anchoring a random copolymer of styrene and methyl methacrylate (58% PS) to the native silicon oxide layer.12,21 The anchoring reaction is carried out by annealing the spincoated random copolymer layer in a vacuum oven at 180 °C for 20 h. After rinsing away the residual random copolymer with toluene, the thickness of the anchored layer was confirmed by ellipsometry to be about 5 nm. The PSb-PMMA diblock copolymers were dissolved in toluene and then spin coated on those neutralized silicon surfaces. The specific film thickness was reached by controlling the solution concentration (typically 1-2 wt %) and the spin speed (1500-6000 rpm) of the spin coater. The film thicknesses were measured by ellipsometry and confirmed by atomic force microscopy (AFM). For nanoimprint lithography, the molds are imprinted under pressure (up to 50 bar) and the samples are subsequently annealed at 180 °C for 20 h, with the molds in place. On an unpatterned, “neutral” surface, a spin-coated film of the block copolymer shows the cylindrical nanodomains oriented normal to the surface, when the thickness of the film is around 40-45 nm and after annealing in a vacuum.8 Figure 2 shows an AFM image of a phase-separated film after removal of the PMMA block, illustrating that the cylinders are oriented perpendicular to the surface and 1634
Figure 2. AFM image of PS matrix after removing PMMA columns on neutralized silicon surface with a total film thickness of 42 nm. The inset shows the center-to-center distance d and the distance D required to fit two rows of PMMA columns.
showing short-range hexagonal ordering. However, the adjacent domains are not aligned, hence making it impossible to predict the precise placement of one specific column. Following the procedure of Figure 1, we imprinted nanopatterned molds (InP gratings with different line widths, total area 1 cm2) into the polymer films before annealing. After annealing, the InP molds were removed carefully and PMMA blocks were removed as before. Figures 3a and 3b show two SEM images of imprinted block copolymer patterns. In Figure 3a, a 200 nm periodicity with about 100 nm spacing grating and 135 nm depth (see Supporting Information for SEM image) is used, while Figure 3b shows the results from molds with 210 nm periodicity and approximately 120 nm spacing. The thickness of the spin-coated block copolymer films was 27.2(0.2 nm in Figure 3a, and 25.7(0.4 nm in Figure 3b. For Figure 3a, oxygen plasma etching was carried for about 20 s at 100 W to reveal the structure more clearly. As can be measured from Figure 2 and similar SEM images, the center-to-center distance d between the PMMA columns (inset of Figure 2) in the free hexagonal array is 41.7(3.0 nm. It can thus be calculated that the optimum gap to fit exactly two rows of PMMA column, a distance of D ) 2 × (dx3/2) ) 72.2 nm (inset of Figure 2) is required. For three columns, 108.3 nm is required. In our experiments, the molds do not contain gaps with sizes matching an integer number of row spacings (dx3/2). Therefore, the PMMA columns will deform (compress or expand) to adapt to the gaps. A fine-tuning of the gap distances, with interfacial interactions between the molds and the polymer blocks to be considered at the same time, is required to get the polymer columns fit exactly. For 100 nm gaps in Figure 3a, we only have two rows of PMMA cylinders. The center-to-center distance of PMMA columns inside the same row d1 is 40.3(2.6 nm, while the centerto-center distance across the two rows d2 is 41.5(2.3 nm. While d2 is very similar to the free-space distance d, d1 has Nano Lett., Vol. 4, No. 9, 2004
Figure 3. SEM images of controlled self-assembled PS-b-PMMA diblock copolymer structures with imprinted InP grating of (a) 200 nm periodicity with about 100 nm gap and (b) 210 nm periodicity with about 120 nm gap. The PMMA columns are etched away for both samples and 20 s oxygen plasma etching are employed for sample (a).
become somewhat smaller. For Figure 3b, the values of d1 and d2, which are averaged among all three rows, are 41.3(3.1 nm and 36.5(2.5 nm, respectively. In this case, d2 is much smaller than the free-space distance d, while d1 remains roughly the same. It is interesting to note that the gap spacing of 120 nm, which is larger than the required distance for three PMMA rows, still yields significantly compressed rows (small d2). When we examine the image carefully, we can see that the distance between the holes left over by the PMMA cylinder and the edge of the PS frame is larger than normally required in free space. It is about 40 nm and very similar to the free-space center-to-center distance d of PMMA columns. The reason for this may lie in the different interfacial interaction between the surface of the InP mold and PS and PMMA blocks.22 Our InP molds are coated with a monolayer of CF3(CF2)9(CH2)11SH to avoid adhesion of the mold to the imprinted polymer film. We have not investigated the use of other thiolate monolayers. In Figure 3, the PMMA columns are not perfectly two or three rows along the gaps, but develop some defects. Two causes can be suggested for this observation: (1) the gap width of the molds is not uniform along the line direction, which would cause distortion of the regularity in phaseseparation (an SEM image of the mold is presented in the Nano Lett., Vol. 4, No. 9, 2004
Supporting Information); and (2) the thickness of the polymer layer, once the polymer film is squeezed into the gap of the molds, is not ideal to form vertical standing PMMA arrays. The determination of the thickness of the polymer films inside the mold is quite difficult. When we measured the height difference between embossed and raised areas with AFM, the thickness of the features was approximately 8-10 nm higher than the original film thickness. This does not account for all the spin-coated material and seems to indicate that the mold did not push all the way down to the silicon wafer. We believe that with improved molds (precise matching of periodicities and smoother edges) as well as a fine adjustment of the polymer thickness (by controlling imprinting times, temperature and pressure), both of the two imperfections can be remediated and ideally aligned arrays of PMMA columns can be produced. Previous investigations showed that the orientation of cylinder blocks can change from normal to the surface to a parallel structure with a small change in film thickness.23 In our experiments the thickness of the polymer film inside the gaps of the molds increases during imprinting (as explained above). By imprinting thicker spin-coated films, we can tip the balance toward an in-plane orientation of the PMMA blocks, instead of the perpendicular columnar phase. The SEM images in Figure 4a show an example of aligned structures resulting from films with a thickness of 45.7(0.5 nm (approximately 20 nm thicker than the films in Figure 3). After imprint annealing, the PMMA block has been removed as before. The remaining PS matrix shows very well aligned lines (although some PS lines seem to have collapsed). The width of these thin lines is about 26 nm with the distance between them roughly the same. For PS-bPMMA films on a free-space surface (Figure 4b), we observed that, with the increase of the film thickness, the morphology changes from the hexagonal arrays to a mixture of the array and lines, and then to curled lines. At the same time, the width of the lines increased from 20 nm to about 25 nm. Figure 4b shows an SEM image of PS frame left over on free-space surface with a film thickness of 55.6 nm. The lines in Figure 4a are approximately the same width as those in Figure 4b. From the enlarged image (Figure 4c), it can be seen clearly that the middle PS line is brighter than the other two, which is due to different heights of the lines. Figure 4d shows an AFM cross-section scanning line from a very similar sample. The height of those PS lines is about 40 nm and the middle PS line reaches 50 nm, which is very consistent with the thickness of the free-space films. This height difference of the middle lines might be due to the thickness of the film not being perfectly suitable for the formation of uniform horizontally aligned cylinders, which also causes some defects among the lines. It could also reflect the presence of a more parabolic shape of the film prior to etching the PMMA domains. In summary, we have shown that NIL can provide a driving force to order phase-separated block copolymer films in lateral dimensions. The packing of vertically aligned PMMA columns seems flexible to conform to the width of the gaps. Different topographies of PMMA columns, either 1635
the experimental simplicity of this method makes this a strategy with great potential for precise positioning of nanoscale features, which will be very useful in future nanodevice fabrication. Acknowledgment. The authors thank the EPSRC (Grant GR/R15092/01) and the IRC in Nanotechnology for financial support. We gratefully acknowledge Dr. Wilfred Booij from Agilent Technologies, Ipswich, UK, for providing us with the InP gratings and Dr. Craig J. Hawker (IBM, Almaden) for providing PS/PMMA random copolymer. Supporting Information Available: SEM images of sample at 200 nm periodicity and the mold used in this study. This material is available free of charge via the Internet at http://pubs.acs.org. References
Figure 4. (a) SEM image of lines structures resulted from controlled self-assembly of diblock copolymer with a large film thickness. (b) SEM image of remaining PS structure on free-space surface at a film thickness of 55.6 nm. (c) Zoom-in image of (a). (d) AFM cross-section profile from another imprint-annealing sample of similar thickness.
normal or parallel to surface, can be obtained inside the gap of imprinted molds. Although the alignment is not yet perfect,
1636
(1) Park, M.; Harrison, C. K.; Chaikin, P. M.; Register, R. A.; Adamson, D. H. Science 1997, 276, 1401. (2) Hamley, I. W. Nanotechnology 2003, 14, R39. (3) Jeong, U.; Ryu, D. Y.; Kim, J. K.; Kim, D. H.; Wu, X.; Russell, T. P. Macromolecules 2003, 36, 10126. (4) Morkved, T. L.; Lu, M.; Urbas, A. M.; Ehrichs, E. E.; Jaeger, H. M.; Mansky, P.; Russell, T. P. Science 1996, 273, 931. (5) Thurn-Albrecht, T.; Steiner, R.; DeRouchey, J.; Stafford, C. M.; Huang, E.; Bal, M.; Tuominen, M.; Hawker, C. J.; Russell, T. P. AdV. Mater. 2000, 12, 787. (6) Thurn-Albrecht, T.; Schotter, J.; Ka¨stle, G. A.; Emley, N.; Shibauchi, T.; Krusin-Elbaum, L.; Guarini, K.; Black, C. T.; Tuominen, M. T.; Russell, T. P. Science 2000, 290, 2126. (7) Segalman, R. A.; Yokoyama, H.; Kramer, E. J. AdV. Mater. 2001, 13, 1152. (8) Guarini, K. W.; Black, C. T.; Yeung, S. H. I. AdV. Mater. 2002, 14, 1290. (9) Cheng, J. Y.; Ross, C. A.; Thomas, E. L.; Smith, H. I.; Vancso, G. J. Appl. Phys. Lett. 2002, 81, 3657. (10) Sundrani, D.; Darling, S. B.; Sibener, S. J. Nano Lett. 2004, 4, 273. (11) Thurn-Albrecht, T.; DeRouchey, J.; Russell, T. P.; Kolb, R. Macromolecules 2002, 35, 8106. (12) Mansky, P.; Liu, Y.; Huang, E.; Russell, T. P.; Hawker, C. Science 1997, 275, 1458. (13) Kim, S. H.; Misner, M. J.; Xu, T.; Kimura, M.; Russell, T. P. AdV. Mater. 2004, 16, 226. (14) Segalman, R. A.; Hexemer, A.; Kramer, E. J. Macromolecules 2003, 36, 6831. (15) Yang, X. M.; Peters, R. D.; Nealey, P. F.; Solak, H. H.; Cerrina, F. Macromolecules 2000, 33, 9575. (16) Kim, S. O.; Solak, H. H.; Stoykovich, M. P.; Ferrier, N. J.; de Pablo, J. J.; Nealey, P. F. Nature 2003, 424, 411. (17) Chou, S. Y.; Krauss, P. R.; Zhang, W.; Guo, L.; Zhuang, L. J. Vac. Sci. Technol. B 1997, 15, 2897. (18) Guarini, K. W.; Black, C. T.; Milkove, K. R.; Sandstrom, R. L. J. Vac. Sci. Technol. B 2001, 19, 2784. (19) Li, R. R.; Dapkus, P. D.; Thompson, M. E.; Jeong W. G.; Harrison, C.; Chaikin, P. M.; Register, R. A.; Adamson, D. H. Appl. Phys. Lett. 2000, 76, 1689. (20) Shin, K.; Leach, K. A.; Goldbach, J. T.; Kim, D. H.; Jho, J. Y.; Tuominen, M.; Hawker, C. J.; Russell, T. P. Nano Lett. 2002, 2, 933. (21) Kindly provided by Dr. Craig J. Hawker, IBM Almaden Research Center. (22) Zhang, P.; Blum, F. D. Macromolecules 2003, 36, 8522. (23) Knoll, A.; Horvat, A.; Lyakhova, K. S.; Krausch, G.; Sevink, G. J. A.; Zvelindovsky, A. V.; Magerle, R. Phys. ReV. Lett. 2002, 89, 35501.
NL049209R
Nano Lett., Vol. 4, No. 9, 2004
