Control of the Microdomain Orientation in Block Copolymer Thin Films

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Control of the Microdomain Orientation in Block Copolymer Thin Films with Homopolymers for Lithographic Application Hirofumi Kitano,† Satoshi Akasaka,† Tomohiro Inoue,† Feng Chen,† Mikihito Takenaka,† Hirokazu Hasegawa,*,† Hiroshi Yoshida,‡ and Hideki Nagano§ Department of Polymer Chemistry, Graduate School of Engineering, Kyoto UniVersity, Nishikyo-ku, Kyoto 615-8510, Materials Research Laboratory, Hitachi Ltd., Hitachi, Ibaraki 319-1292, and DeVelopment & Technology Center, Hitachi Maxell Ltd., Tsukubamirai, Ibaraki 300-2496, Japan ReceiVed December 21, 2006. In Final Form: March 10, 2007 Block copolymer lithography is a promising method for fabricating periodical nanopatterns of less than 20 nm by self-assembly and can be applicable for fabricating patterned magnetic media with a recording density over 1 Tb/in.2. We found a simple technique to control the orientation of cylindrical microdomains in thin films. Simply by mixing polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymers with the homopolymer (PS or PMMA) of the major component, we could align the cylindrical microdomains perpendicular to the film surface. The added homopolymer induces conformational entropic relaxation of the block chains in microdomain space and stabilizes the perpendicular orientation of hexagonally packed cylindrical microdomains. Thus formed perpendicular cylinders can be readily aligned in a regular array with a grating substrate.

Introduction Block copolymers consisting of immiscible pairs of polymers self-assemble to form periodic microphase-separated structures on the order of the radii of gyration of the block copolymer chains. A variety of equilibrium and nonequilibrium microdomain morphologies can be obtained by controlling the molecular architecture, molecular weight, and copolymer composition. Blending block copolymers with homopolymers or other block copolymers or manipulating the processing conditions is also an effective way to control the microdomain morphology.1-3 Possibilities to utilize 2D and 3D highly regular nanopatterns of block copolymers for industrial applications have been sought for many years, but no sophisticated product is on the market yet.3-5 Nanolithography may be the most pronounced technology utilizing 2D nanopatterns of block copolymers, and extensive studies have been reported in this field.4-10 Naito et al.7 employed the spherical microdomain structure of polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymer to form a self-assembled 2D nanopattern and successfully used it to pattern the underlying magnetic film and to prepare magnetic patterned media for high-density hard disk * To whom correspondence should be addressed. Phone: +81-75-3832620. Fax: +81-75-383-2623. E-mail: [email protected]. † Kyoto University. ‡ Hitachi Ltd. § Hitachi Maxell Ltd. (1) Hashimoto, T. In Thermoplastic Elastomers, A ComprehensiVe ReView; Legge, N. R., Holden, G., Schroeder, H. E., Eds.; Hanser: Munich, Germany, 1996. (2) Hasegawa, H.; Hashimoto, T. In ComprehensiVe Polymer Science, Second Supplement; Aggarwal, S. L., Russo, S., Eds.; Pergamon: New York, 1996. (3) Hamley, I. W. The Physics of Block Copolymers; Oxford University Press: Oxford, U.K., 1998. (4) Segalman, R. A. Mater. Sci. Eng. 2005, R48, 191. (5) Hawker, C. J.; Russell, T. P. MRS Bull. 2005, 30, 952. (6) Park, M.; Harrison, C. K.; Chaikin, P. M.; Register, R. A.; Adamson, D. H. Science 1997, 276, 1401. (7) Naito, K.; Hieda, H.; Sakurai, M.; Kamata, Y.; Asakawa, K. IEEE Trans. Magn. 2002, 38, 1949. (8) Black, C. T.; Guarini, K. W.; Milkove, K. R.; Baker, S. M.; Russell, T. P.; Tuominen, M. T. Appl. Phys. Lett. 2001, 79, 409. (9) Lazzari, M.; Lo´pez-Quintela, M. A. AdV. Mater. 2003, 15, 1583. (10) Chen, J. Y.; Ross, C. A.; Thomas, E. L.; Smith, H. I.; Vansco, G. J. Appl. Phys. Lett. 2002, 81, 3657.

drives. Their procedure is schematically illustrated in Figure 1a: (i) They formed a spiral groove and land on a glass plate or a silicon plate coated with a magnetic film by using a master disk and resist film. (ii) A PS-b-PMMA diblock copolymer possessing the microdomain morphology of PMMA spheres in a PS matrix was spin-coated to fill the groove and annealed to form selfassembled arrays of PMMA spheres. (iii) The PMMA spheres were selectively removed by the oxygen plasma treatment to produce holes. (iv) The holes were filled with spin-on-glass. (v) The underlying magnetic film was patterned by ion milling using the spin-on-glass pattern as the mask. (vi) A magnetic patterned medium was produced. It is a good idea to use spherical microdomains of block copolymers because it is relatively easy to align the spherical microdomains in a groove if the thickness of the film is just enough to contain one layer of spherical microdomains. However, as long as spherical microdomains are employed, the thickness of the spin-on-glass mask is limited by the size of the spherical microdomains as illustrated by the etching process of the spherical microdomains, steps i-iii in Figure 1b. Therefore, to secure the sufficient mask thickness for the substrate etching, relatively large spherical microdomains of large molecular weight block copolymers must be used, which obstructs the reduction of the size of the nanopattern. One way to avoid this problem is to utilize cylindrical microdomains of block copolymers, which have a much higher aspect ratio than spheres. However, for this purpose, the cylinders must be aligned with their axes normal to the media substrates (hereafter referred as “perpendicular cylinders”, while those with their axes parallel to the substrates are referred as “parallel cylinders”). Due to the difference in affinities of two block components of a diblock copolymer for a substrate surface, parallel cylinders are favored over perpendicular cylinders.11 In this paper we report an extremely simple and effective method to form perpendicular cylinders in thin films. Many attempts were made to control the orientation of cylindrical microdomains of block copolymers in thin films.12-23 (11) Knoll, A.; Horvat, A.; Lyakhova, K. S.; Krausch, G.; Sevink, G. J. A.; Zvelindovsky, A. V.; Magerle, R. Phys. ReV. Lett. 2004, 89, 035501. (12) Angelescu, D. E.; Waller, J. H.; Adamson, D. H.; Deshpande, P.; Chou, S. Y.; Register, R. A.; Chaikin, P. M. AdV. Mater. 2004, 16, 1736.

10.1021/la0637014 CCC: $37.00 © 2007 American Chemical Society Published on Web 04/28/2007

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stabilizes hexagonal packing of the cylinders. Consequently, in the thin films of the blends the driving force for the cylinders to form a hexagonal lattice tends to overcome the requirement to achieve the minimum interfacial energy by other morphologies. Thus, the cylinders must be perpendicular to the substrate to form the hexagonal lattice when the thickness of the thin film is not enough for the parallel cylinders to form the hexagonal lattice. We also examined the effect of a grating substrate on the alignment of perpendicular cylinders. Experiment

Figure 1. Schematic illustration of the etching process utilizing spherical microdomains.

Mechanical methods12 only orient cylinder axes parallel to film surfaces or to extrusion directions. An electric field13,14 is effective to produce perpendicular cylinders, but the procedure is too complicated. Chemical modification of the substrate surface with a self-assembled monolayer (SAM) neutral to both components of a diblock copolymer is another way to promote perpendicular cylinders.15-17 Jeong et al.17 proposed a technique to blend PMMA homopolymer having a relatively high molecular weight with asymmetric PS-b-PMMA rich in PS content to obtain perpendicular cylinders with a high aspect ratio. The dissolved PMMA homopolymer chains in the PMMA cylinders are confined and stretched in the center of the cylindrical microdomains and guide the oriented cylinders to propagate into the interior of the film. Peng et al.21 reported the preparation of perpendicular cylinders with a blend of symmetric PS-b-PMMA and PMMA homopolymer. To obtain PMMA cylinders, they exposed the cast film to saturated acetone (selective for the PMMA component) vapor. The reason why they mixed the homopolymer into the cylindrical microdomains is that they intended to remove the homopolymer subsequently to obtain cylindrical nanopores. Our approach to obtain perpendicular cylinders might look similar to that of Jeong et al.17 since they also used PS-b-PMMA/ homopolymer blends. However, the principle or driving force that we used to orient the cylinders is entirely different from the reported ones. We utilized the thermodynamic driving force to realize perpendicular cylinders. Addition of homopolymer to the matrix of a cylindrical microdomain structure induces conformational entropic relaxation of the block chains in the matrix and (13) Thurn-Albrecht, T.; Schotter, J.; Kastle, G. A.; Emley, N.; Shibauchi, T.; Krusin-Elbaum, L.; Guarini, K.; Black, C. T.; Tuominen, M. T.; Russell, T. P. Science 2000, 290, 2126. (14) Xu, T.; Zvelindovsky, A. V.; Sevink, G. J. A.; Lyakhova, K. S.; Jinnai, H.; Russell, T. P. Macromolecules 2005, 38, 10788. (15) Xu, T.; Stevens, J.; Villa, J. A.; Goldbach, J. T.; Guarini, K. W.; Black, C. T.; Hawker, C. J.; Russell, T. R. AdV. Funct. Mater. 2003, 13, 698. (16) Yang, X.; Xiao, S.; Liu, C.; Pelhos, K.; Minor, K. J. Vac. Sci. Technol. 2004, B22, 3331. (17) Jeong, U.; Ryu, D. Y.; Kho, D. H.; Kim, J. K.; Goldbach, J. T.; Kim, D. H.; Russell, T. P. AdV. Mater. 2004, 16, 533. (18) Kim, S. H.; Misner, M. J.; Xu, T.; Kimura, M.; Russell, T. P. AdV. Mater. 2004, 16, 226. (19) De Rosa, C.; Park, C.; Thomas, E. L.; Lotz, B. Nature 2000, 405, 433. (20) Morkved, T. L.; Lu, M.; Urbas, A. M.; Ehrichs, E. E.; Jaeger, H. M.; Mansky, P.; Russell, T. P. Science 1996, 273, 931. (21) Peng, J.; Gao, X.; Wei, Y.; Wang, H.; Li, B.; Han, Y. J. Chem. Phys. 2005, 122, 114706. (22) Segalman, R. A.; Hexemer, A.; Kramer, E. J. Phys. ReV. Lett. 2003, 91, 196101. (23) Hexemer, A.; Stein, G. E.; Kramer, E. J.; Magonov, S. Macromolecules 2005, 38, 7083.

Materials. Polystyrene-block-poly(methyl methacrylate) diblock copolymers (PS-b-PMMA1, Mn,PS ) 4.61 × 104, Mn,PMMA ) 2.10 × 104, Mw/Mn ) 1.09, volume fraction of PMMA fPMMA ) 0.31; PS-b-PMMA2, Mn,PS ) 2.02 × 104, Mn,PMMA ) 5.05 × 104, Mw/Mn ) 1.07, volume fraction of PS fPS ) 0.286) and poly(methyl methacrylate) homopolymers (hPMMA1, Mn ) 1 × 103, Mw/Mn )1.04; hPMMA2, Mn ) 6.54 × 103, Mw/Mn ) 1.09) were purchased from Polymer Source Inc. (Dorval, QC, Canada). Polystyrene homopolymer (hPS; Mn ) 7.5 × 103, Mw/Mn ) 1.04) was prepared by living anionic polymerization in the lab by the Kyoto University group. The neat diblock copolymers and the diblock copolymer/ homopolymer blends were dissolved in toluene to form a 1-3% solution and spin-cast on the substrates at room temperature in the ambient atmosphere using a Kyowariken K-359SD-1 spinner. As the substrates, indium tin oxide (ITO) glass, silicon wafers, and patterned silicon wafers were used. The ITO glass was purchased from Furuuchi Chemical Co., Ltd. and used as received, while the P-type silicon wafers manufactured by Shin-etsu Semiconductor Co., Ltd. and the patterned silicon wafers supplied by Hitachi Co., Ltd. were soaked in piranha solution and rinsed with purified water prior to the spin-casting. The thin-film-coated substrates were annealed at various temperatures in vacuo for different periods of time. The annealing temperature was 170 °C for the PS-rich samples and 180 and 230 °C, respectively, for the PMMA-rich samples on a flat surface and a grating substrate. However, we did not observe a marked change of the morphology in the annealed thin films by changing the annealing temperature between 170 and 210 °C for the PS-rich samples and between 170 and 230 °C for the PMMA-rich samples. Atomic Force Microscopy (AFM) Observation. A Nanoscope III (Digital Instruments, Inc.) was used for tapping-mode AFM to observe surface morphologies of thin films. Prior to AFM observations, the surfaces of the thin films were etched by UV irradiation (peak wavelength 254 nm, 18 mW/cm2) for 6 min at room temperature under a nitrogen atmosphere. Under this condition, the thickness of the PS component is reduced by ca. 5 nm while the PMMA component is etched much faster. Differences in the etching rate against UV between PS and PMMA creates topographical profiles corresponding to microdomain structures at film surfaces. Phase images as well as height images were obtained by AFM. Transmission Electron Microscopy (TEM) Observation. A JEOL JEM-2000FX was used for TEM of thin films, which were spin-cast and annealed on ITO substrates. Thin films cut in small pieces were removed from the ITO substrate using acidic solution, picked up with copper grids, and stained with ruthenium tetroxide (RuO4) vapor prior to TEM observation. For the 3D observation of the thin films, electron tomography24 was performed. Single-axis tilt-series TEM images were corrected for the tilt angles between -60° and +60° with a 2° increment. Homemade software was used for the 3D reconstruction. Scanning Electron Microscopy (SEM) Observation. A Hitachi S-4000 field-emission SEM instrument operated at an acceleration voltage of 5 kV was used for SEM observation. Sample specimens were first subjected to reactive ion etching with O2 using a SAMCO, Inc. RIE-10NR and then sputtered with Pt-Pd with a thickness of 1.8 nm prior to SEM observations. (24) Frank, J. Electron Tomography; Plenum: New York, 1992.

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Figure 2. AFM height images of PS-b-PMMA1/hPS blends on ITO substrates after annealing at 170 °C for 24 h. The volume fraction of the PMMA component is (a) 31% (neat PS-b-PMMA1), (b) 22%, (c) 19%, and (d) 16%. Each scale bar is 100 nm.

Results PMMA Cylinder System. Figure 2 shows AFM height images of the thin films of neat PS-b-PMMA1 (Figure 2a) and a part of the PS-b-PMMA1/hPS blends (Figure 2b-d) cast on ITO substrates. The volume fractions of PMMA component in the specimens are (a) 31%, (b) 22%, (c) 19%, and (d) 16%. The average thickness of the thin films was 40 nm after annealing at 170 °C for 24 h. Selective etching of the film surfaces by UV irradiation gives the contrast in the height images. The PMMA microdomains were etched by UV and appear dark, while unchanged PS microdomains appear bright. It was confirmed that the as-spun films had a disordered structure25 due to the fast solvent-evaporation rate and the incomplete microphase separation. The ordering or completion of microphase separation occurred during the annealing process in vacuo. Parts a and b of Figure 2 (neat PS-b-PMMA1 and the blend with 22% PMMA component, respectively) exhibit mixed patterns of dark dots and curved lines. The former suggests cylindrical PMMA microdomains oriented perpendicular to the film surface (perpendicular cylinders), while the latter suggests the parallel orientation of cylinders (parallel cylinders). The average domain spacing estimated from FFT power spectra of the AFM image was 35 nm and the average diameter of the cylinders was 20 nm for both samples. On the other hand, parts c and d of Figure 2 (the blend with 19% and 16% PMMA component, respectively) exhibit almost only dot patterns, suggesting the existence of perpendicular cylinders or spherical microdomains. The average domain spacing was 34 nm and the average diameter of cylinders or spheres was 20 nm for both samples. We also examined thin films cast on silicon wafers by AFM and obtained the same results. We observed the entire area of these samples by optical microscopy and a 10 µm × 10 µm area by AFM and found that the thin films were macroscopically uniform. Neither macrophase separation nor “terrace” formation was observed. Surface morphologies observed by AFM do not necessarily represent interior microdomain morphologies even for thin films. (25) Russell, T. P.; Mayes, A. M.; Kunz, M. Z. In Ordering in Macromolecular Systems; Teramoto, A., Kobayashi, M., Norisue, T., Eds.; Springer-Verlag: Berlin, 1994.

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Figure 3. TEM images of PS-b-PMMA1/hPS blends spin-cast on ITO substrates, annealed at 170 °C for 24 h, removed from the substrates, and stained with RuO4. The volume fraction of the PMMA component is (a) 31% (neat PS-b-PMMA1), (b) 22%, (c) 19%, and (d) 16%. Each scale bar is 100 nm.

Therefore, we performed TEM utilizing thin films cast on ITO substrates. The thin films were removed from ITO using an aqueous solution of HCl and picked up with TEM grids. The thin films were then stained with RuO4 and served for TEM observation. Figure 3 shows the TEM images of the same samples as observed by AFM in Figure 2, i.e., Figure 3a for neat PSb-PMMA1 and Figure 3b-d for the PS-b-PMMA1/hPS blends with 22%, 19%, and 16% PMMA component, respectively. Since RuO4 stains only PS component, PMMA microdomains appear bright in the TEM images. Figure 3a (neat PS-b-PMMA1) exhibits mixed patterns of bright dots and curved lines similar to those of Figure 2a, indicating perpendicular and parallel cylinders, respectively. Therefore, the surface morphology observed by AFM represents the interior morphology as well in this case. The TEM image of the blend with 22% PMMA appears differently as shown in Figure 3b. Bright dots with a high contrast observed in the TEM image imply the perpendicular cylinder across the film thickness. The gray area also exhibits complex patterns with low contrast. The microdomain morphology is not clear for this sample. A hexagonal array of bright dot pattern is observed in the TEM image of the blend with 19% PMMA (Figure 3c), implying perpendicular cylinders piercing the film thickness. Although not shown, a similar result was obtained for the blend with 18% PMMA. In contrast, the contrast of the TEM image was very low for the blend with 16% PMMA as shown in Figure 3d. In this specimen, PMMA microdomains must have a spherical morphology. Overlapping of the projections of spherical microdomains in the thickness direction smears out the contrast in the TEM image. The morphological transition from cylinders to spheres on addition of homopolymer is a well-known phenomenon for a bulk mixture of block copolymer and homopolymer.1-3 The added homopolymer having a lower molecular weight than that of the corresponding block chains selectively swells the block chains. The increase in the asymmetry of the molecular volume for the two components causes an increase in the

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Figure 4. Electron tomography 3D reconstructed images of PSb-PMMA1/hPS blends spin-cast on ITO substrates, annealed at 170 °C for 24 h, removed from the substrates, and stained with RuO4. The volume fraction of the PMMA component is (a) 31% (neat PS-b-PMMA1) and (b) 19%. The reconstructed images have a volume of (a) 270 × 270 × 41 nm3 and (b) 270 × 270 × 45 nm3.

interfacial curvature, resulting in the transition from cylinders to spheres. A similar morphological transition in thin films has been observed for polystyrene-block-polyisoprene-block-polystyrene triblock copolymer/homopolymer blends.26 These observations suggest that the addition of a certain amount of homopolymer in the matrix phase of a cylinder-forming diblock copolymer promotes a perpendicular orientation of cylindrical microdomains in thin films. TEM images give only 2D information on the projection of the 3D structures. To confirm our speculation on the orientation of the cylindrical microdomains, we performed 3D reconstruction of two samples, neat PS-b-PMMA1 and the blend with 19% PMMA, by electron tomography.24 Parts a and b of Figure 4 are surface-rendering 3D images of neat PS-b-PMMA1 and the blend with 19% PMMA, respectively. The interfaces between PS and PMMA microphases are indicated by the solid surfaces. Figure 4 definitely confirms that the thin film of neat PS-b-PMMA1 contains both parallel and perpendicular cylinders whereas the blend with 19% PMMA contains only perpendicular cylinders piercing the film thickness. For the next step, we investigated the effect of grooves formed on the silicon wafers on the arrangement of cylinders. We used the grating substrate with different pitch lengths varying from 200 nm (crest) + 200 nm (groove) to 500 nm + 500 nm. Figure 5 shows the AFM images of the blend with 25% PMMA, which forms a mixed morphology of parallel and perpendicular cylinders in the thin films on ITO substrates. The grating substrate of a 400 nm + 400 nm pitch was used. Parts a and c of Figure 5 are height images, and parts b and d of Figure 5 are phase images. The scanning direction is horizontal for each image and, therefore, normal to the grooves for Figure 5a,b and along the grooves for Figure 5c,d. In Figure 5a, only the grating shows the contrast because of its large height difference. The height difference due to the microdomain structure is too small to give the contrast in the same image. However, the phase image in Figure 5b reveals the mixed morphology of parallel and perpendicular cylinders, in which the PMMA microphase appears dark. On the other hand, the height image in Figure 5c clearly shows the mixed morphology of parallel and perpendicular cylinders, which coincides with the phase image in Figure 5d. The result of this observation suggests that the grooves do not improve the orientation of the cylinders for the samples exhibiting mixed parallel and perpendicular cylinders on a flat substrate. Figure 6 shows the results for the blend with 19% PMMA on (26) Mykhaylyk, T. A.; Mykhaylyk, O. O.; Collins, S.; Hamley, I. W. Macromolecules 2004, 37, 3369.

Figure 5. AFM images of a PS-b-PMMA1/hPS blend (PMMA volume fraction 25%) on a grating substrate with a 400 nm + 400 nm pitch: (a) height and (b) phase images scanned along the direction normal to the grooves, (c) height and (d) phase images scanned along the grooves. Each scale bar is 100 nm.

the grating substrates with pitch lengths of 500 nm + 500 nm (Figure 6a,b,e) and 200 nm + 200 nm (Figure 6c,d,f). The depth of the grooves was 50 nm. Parts a and c of Figure 6 are AFM height images, in which only the contrast given by the crests and the grooves can be seen. Parts b and d of Figure 6 are phase images of PMMA perpendicular cylinders (dark dots) in the grooves. Parts e and f of Figure 6 are 2D FFT patterns of the AFM phase images as shown in parts b and d, respectively, of Figure 6. The sharp line on the equator is an array of the bright spots due to the slitlike grating patterns. The FFT pattern in Figure 6e shows some spotlike peaks characteristic of a hexagonal array of the perpendicular cylinders, but the second-order peaks are smeared in agreement with the polygrain structure as observed in Figure 6b. In contrast, the FFT pattern in Figure 6f exhibits discrete spots up to the higher order typical for single crystals with a hexagonal lattice. An insertion in Figure 6d shows a magnified view of the phase image, which also exhibits a hexagonal array of the perpendicular cylinders with its (110) plane parallel to the groove walls. The results suggest that the use of grating substrates promotes alignment of the perpendicular cylinders along the grooves. Perpendicular cylinders are aligned more effectively in a narrow groove than a wide groove. PS Cylinder System. Experiments similar to those of the PS-b-PMMA1/hPS blends were performed for PS-b-PMMA2/ hPMMA2 blend systems to examine the generality of the phenomena. If addition of homopolymer to a matrix phase of a cylinder-forming diblock copolymer results in perpendicularly oriented cylinders in general, similar results should be obtained for reversed systems such as PS-b-PMMA2/hPMMA2 blends, in which PS-b-PMMA2 has an equilibrium morphology of PS cylindrical microdomains in a PMMA matrix. Figure 7 shows AFM height images of thin films of the PSb-PMMA2/hPMMA2 blends cast on ITO substrates. The volume fractions of the PS component are (a) 25%, (b) 22%, (c) 19%, and (d) 16%. The average thickness of the thin films was 50 nm after annealing at 180 °C for 24 h. The AFM height images show patterns exhibiting contrast reversed from that of the patterns in Figure 2; i.e., bright PS microdomains are dispersed in dark

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Figure 7. AFM height images of PS-b-PMMA2/hPMMA2 blends on ITO glass substrates after annealing at 180 °C for 24 h. The volume fraction of the PS component is (a) 25%, (b) 22%, (c) 19%, and (d) 16%. Each scale bar is 100 nm.

Figure 6. AFM (a, c) height and (b, d) phase images of a PS-bPMMA1/hPS blend (PMMA volume fraction 19%) on grating substrates with a 500 nm + 500 nm pitch (a, b) and a 200 nm + 200 nm pitch (c, d) and FFT patterns obtained from (e) the image in (b) and (f) the image in (e). Each scale bar is 500 nm. The inset in (d) shows the magnified view.

PMMA matrixes. The AFM image of the blend with 25% PS component in Figure 7a exhibits bright curved lines, suggesting PS parallel cylinders. The AFM image has changed dramatically by increasing the hPMMA2 content to have 22% PS component. The image exhibits mostly bright circles in a dark matrix, implying PS perpendicular cylinders as shown in Figure 7b. By further increasing the hPMMA2 content to have PS components of 19% and 16%, only bright circles are observed in the AFM images as shown in parts c and d, respectively, of Figure 7, suggesting that all the cylindrical PS microdomains are oriented perpendicular to the film surface. The average domain spacing and the diameter of the cylinders of these samples are 29-30 and 20-22 nm, respectively. TEM observation of the thin films removed from the ITO substrates after annealing at 180 °C for 24 h was performed for the neat PS-b-PMMA2 and the PS-b-PMMA2/hPMMA2 blends. Figure 8a is the TEM image of the neat PS-b-PMMA2 (28.6% PMMA content). It shows irregularly arranged dark dots and short curved lines, which indicate perpendicular and parallel cylinders of PS microdomains, respectively. No region of hexagonally packed perpendicular cylinders, i.e., a hexagonal array of dots, can be seen. The TEM image of the blend with

Figure 8. TEM images of PS-b-PMMA2/hPMMA2 blends spincast on ITO substrates, annealed at 180 °C for 24 h, removed from the substrates, and stained with RuO4. The volume fraction of the PS component is (a) 28.6% (neat PS-b-PMMA2), (b) 22%, (c) 19%, and (d) 16%. Each scale bar is 100 nm.

22% PS (Figure 8b) resembles that of the neat PS-b-PMMA2 in the sense that dark dots and short curved lines are both observed. However, it can be noticed by careful observation that regions of a hexagonal array of dark dots appear. The area of the hexagonal array of dark dots increases with increasing hPMMA2 content, and finally in the TEM image of the blend with 19% PS (Figure 8c), the hexagonal arrays of dark dots occupy the most area. A further increase in hPMMA2 may result in the morphological transition from PS cylinders to PS spheres. Figure 8d shows the TEM image of the blend with 16% PS, in which overlapping of

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Figure 9. SEM images of a PS-b-PMMA2/hPMMA1 blend (volume fraction of PS component 22%) spin-coated on a grating substrate with a 200 nm pitch and a 100 nm groove depth after annealing at 230 °C for 4 h and subsequent RIE etching with O2. Each scale bar is 300 nm.

Figure 10. Chain-packing model in cylindrical microdomain space for (a, left) the neat diblock copolymer and (b, right) the diblock copolymer/homopolymer blend. The dented-triangular interstitial regions must be filled by the elongated block chains in (a), while they can be filled by the homopolymer chains in (b).

the dark dots can be observed, suggesting the spherical microdomains of the PS component. Thus, we could confirm that addition of a certain amount of PMMA homopolymer into the PMMA matrix promotes perpendicular orientation of PS cylindrical microdomains in thin films. The amount of homopolymer necessary to promote perpendicular cylinders seems to depend on the molecular weight of the homopolymer. In the case of PS-b-PMMA2/hPMMA1 blends, the blend with 22% PS component already shows the morphology of mostly perpendicular cylinders. Therefore, we applied this blend to a grating substrate with a 200 nm + 200 nm pitch and a 100 nm groove depth. After the blend film was spin-coated onto the grating substrate, it was annealed at 230 °C for 4 h, then subjected to reactive ion etching (RIE) with O2 gas to remove the PMMA matrix, and served for SEM observation. Figure 9 shows the SEM images thus obtained. In Figure 9a, arrays of the perpendicular cylinders of PS microdomains aligning along the grooves are seen. Some irregularity of the arrays could be due to collapse of the cylinders during the RIE etching process. The side view SEM image in Figure 9b shows a reasonable aspect ratio (>2) of the PS cylinders.

component polymers stimulates reorganization of the microdomain structures. In the bulk, cylindrical microdomains spontaneously form a hexagonal lattice (close packing of cylinders) because energetically it is the most stable structure as compared with any other form of packing. But how stable is the hexagonal lattice of cylinders? Let us consider the chain conformation of an AB diblock copolymer in a hexagonally packed cylindrical microdomain structure. In Figure 10a, a projection of cylindrical microdomains onto a plane perpendicular to the cylinder axis is schematically illustrated. All cylinder-forming A block chains are under the same conditions in terms of the given space. However, the space given to B block chains, which form the matrix phase, probably depends on the position of the A-B junction points on the interface. For example, if the block chain designated as B-2 is forced to fill the dented-triangular interstitial regions in Figure 10a, it must take a more extended conformation than the block chain designated as B-1, which causes a significant loss of conformational entropy for B-2. Such a penalty for the microdomain structure of hexagonally packed cylinders can be easily understood by comparing it with lamellar microdomains where all block chains are under the same conditions in terms of the given space. Thus, the free energy level of the hexagonally packed cylindrical morphology formed by a neat block copolymer is kept relatively high. Addition of a small amount of homopolymer to the matrix phase may change this situation dramatically. If the added homopolymer selectively fills the interstitial regions as illustrated in Figure 10b, the block chain B-2 does not have to extend, resulting in an increase in the conformational entropy. Indeed, such a phenomenon has been observed by the computer simulation with self-consistent field theory.27 Therefore, the hexagonally packed cylindrical structure formed by the diblock copolymer/ homopolymer blend is energetically more stable than that formed by a pure diblock copolymer. In other words, the added homopolymer gives a stronger driving force for the diblock copolymer to form a hexagonal lattice. A similar phenomenon was observed for a three-miktoarm terpolymer system consisting of polystyrene, polyisoprene, and poly(1,4-dimethylsiloxane). Addition of a small amount (5 wt %) of poly(1,4-dimethylsiloxane) homopolymer dramatically increased the regularity of the tetragonally packed cylinders and increased the grain size.28 Another factor to be considered for the structure formation of diblock copolymers in thin films is the interfacial energy of polymer/air and polymer/substrate interfaces. If two components

Discussion Both of the PS-b-PMMA diblock copolymers, PS-b-PMMA1 and PS-b-PMMA2, employed in this study have a cylindrical microdomain morphology in their equilibrium state with their compositions reversed. In thin films with a thickness of less than 100 nm, they both have a mixed morphology of parallel and perpendicular cylinders as observed by AFM (Figure 2) and TEM (Figures 3 and 8). Blending homopolymers having a molecular weight less than 1/5 that of the corresponding block chains into a matrix phase of diblock copolymers increased the ratio of perpendicular cylinders. Eventually almost all of the cylinders became perpendicular to the thin film surface, but excess addition of homopolymer resulted in a morphological transition from cylinders to spheres. Since this phenomenon is common to both PMMA cylinder/PS matrix systems and PS cylinder/ PMMA matrix systems, there must be a physical principle which can be utilized to control the orientation of cylindrical microdomains in thin films. In thin films with a thickness of less than a few repeat units of microdomains, structure formation of block copolymers is quite different from that in the bulk. In as-spun thin films, block copolymers usually form a spongelike irregular network structure induced by quick solvent evaporation.25 Microdomain sizes of such network structures are much smaller than the intrinsic microdomain sizes of the block copolymers. Subsequent annealing at a temperature above the glass-transition temperatures of the

(27) Stoykovich, M. P.; Mu¨ller, M.; Kim, S. O.; Solak, H. H.; Edwards, E. W.; de Pablo, J. J.; Nealey, P. F. Science 2005, 308, 1442. (28) Yamauchi, K.; Akasaka, S.; Hasegawa, H.; Iatrou, H.; Hadjichristidis, N. Macromolecules 2005, 38, 8022.

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of a diblock copolymer have significantly different interfacial tensions against air and/or a substrate, the equilibrium structure of the thin film will be totally controlled by the interfacial energy. However, in the case of PS-b-PMMA diblock copolymer, this difference is not so large, especially at the temperatures employed in our annealing experiments.29 Thus, the contribution of the conformational entropy to the total free energy for a hexagonally packed cylindrical morphology discussed above becomes more important. Obviously, perpendicular cylinders are more favorable in thin films than parallel cylinders because the perpendicular cylinders can expand the hexagonal lattice laterally without limitation, while not enough space is available for the parallel cylinders to form the stable hexagonal lattice within the thickness of the thin films. However, this is not the case for a pure PSb-PMMA diblock copolymer because the difference in stability between hexagonal packing and other packing forms of cylinders is so small that a subtle difference between the interfacial energy for PS/substrate and PMMA/substrate interfaces controls the orientation of the cylinders. According to the principle of our method to form perpendicular cylinders, the molecular weight of added homopolymers and film thickness are important parameters to affect the orientation of the cylinders. We have investigated their effects also and will report the results in our subsequent paper.30

Conclusions We investigated the orientation of cylindrical microdomains in thin films of pure PS-b-PMMA diblock copolymers and their blends with homopolymers of the same component as the matrixforming block chains. Both PMMA cylinder/PS matrix and PS cylinder/PMMA matrix systems were studied. We found that the addition of homopolymers with a molecular weight of less than 1/5 that of the corresponding block chains promotes the (29) Ryu, D. Y.; Shin, K.; Drockenmuller, E.; Hawker, C. J.; Russell, T. P. Science 2005, 308, 236. (30) Inoue, T.; Kitano, H.; Akasaka, S.; Takenaka, M.; Chen, F.: Hasegawa, H.; Yoshida, H.; Nagano, H. Manuscript in preparation.

Kitano et al.

perpendicular orientation of the cylinders in thin films annealed on the substrate surfaces. The driving force of the preferred perpendicular orientation of the cylinders for the blend systems may be attributed to a large free energy difference between the hexagonal packing and other packing forms of cylinders for the blends. The added homopolymer may be localized to fill the dented-triangular interstitial regions within the matrix phase of the hexagonal lattice and reduces the conformational entropy loss of the matrix-forming block chains to stabilize the hexagonal packing of the cylinders. In thin films with a thickness of less than a few repeat distances of the domain spacing, only perpendicular orientation of the cylinders can realize the hexagonal packing. Thus, perpendicular orientation of the cylinders is preferred in the blend systems. On the other hand, for pure diblock copolymers a subtle difference in interfacial energy between PS/substrate and PMMA/substrate interfaces may be critical because of the significant conformational entropy loss for the hexagonal packing of cylinders, resulting in the morphology driven by the interfacial energy. Grating substrates are useful to align perpendicular cylinders for the PS-b-PMMA diblock copolymer/homopolymer blends. The cylinders were aligned with (110) planes of the hexagonal lattice parallel to the groove walls in the case of narrow grooves of 200 nm width. In the case of wider grooves, polygrain structures of perpendicular cylinders were observed. Addition of homopolymer to diblock copolymers is a simple and effective method to produce a hexagonal nanopattern of perpendicular cylinders in thin films and may be very useful in nanolithography. Acknowledgment. This work was supported in part by a Grant-in-Aid for Scientific Research (No. 17105004(S)) from the Japan Society for the Promotion of Science and in part by the 21st century COE program for a United Approach to New Materials Science. We thank Hitachi Ltd. for financial support. LA0637014