Directed Assembly of Cylinder-Forming Block Copolymer Films and

Directed Assembly of Cylinder-Forming Block Copolymer Films and Thermochemically Induced Cylinder to Sphere Transition: A Hierarchical Route to Linear...
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Directed Assembly of Cylinder-Forming Block Copolymer Films and Thermochemically Induced Cylinder to Sphere Transition: A Hierarchical Route to Linear Arrays of Nanodots

2005 Vol. 5, No. 7 1379-1384

Young-Hye La,† Erik W. Edwards,† Sang-Min Park,‡ and Paul F. Nealey*,† Department of Chemical and Biological Engineering and Material Science Program, Center for NanoTechnology, UniVersity of Wisconsin-Madison, Madison, Wisconsin 53706 Received April 14, 2005; Revised Manuscript Received May 18, 2005

ABSTRACT A morphological transition from cylinders to spheres was induced in an asymmetric diblock copolymer, poly(styrene)-block-poly(tert-butyl acrylate) (PS-b-PtBA). The periodic arrays of the poly(tert-butyl acrylate) (PtBA) domains were transformed to the ordered poly(acrylic anhydride) (PAA) spheres via the thermal deprotection of tert-butyl acrylate linkages and the subsequent volume change of the minority block. Coupled with techniques to direct the assembly of cylinder-forming block copolymers, this finding provides new routes to fabricate ordered geometries of nanodot arrays.

Nanofabrication, the generation of ultrafine structures, is central to modern technology and is key to producing powerful electronic/optical devices, miniaturized sensors, and other advanced technological devices. Recently, various molecular self-assembling systems have attracted immense interest for nanofabrication because they spontaneously generate highly ordered structures with sub-nanometer precision, and they provide simple, parallel, and cost-effective processes for fabrication.1 Block copolymers are exemplary self-assembling systems which, in thin films, have shown great potential as templates to fabricate quantum dot arrays,2,3 nanowires,4,5 and magnetic storage media.6 Block copolymers tend to form ordered, periodic microdomains including spheres, cylinders, and lamellae with typical dimensions of 5-50 nm. The size and shape of the microdomains can be controlled by manipulating the chain length (molecular weight), chemical functionality, volume fraction of each block, and temperature.7,8 A number of previous investigations have observed phase transitions of block copolymers between cylinders and spheres. Most of these transitions have been induced via temperature control near the phase boundary at a given volume fraction of each block (thermoreversible order-order transition: OOT),9-11 and some cylinder-to-sphere transitions have been caused * Corresponding author. E-mail: [email protected]. † Department Chemical and Biological Engineering. ‡ Material Science Program. 10.1021/nl0506913 CCC: $30.25 Published on Web 05/28/2005

© 2005 American Chemical Society

by changing the volume fraction through blending with the corresponding homopolymers.12,13 Recently, several reports have demonstrated that spherical domains can interconnect to form cylindrical domains under the application of electric field14,15 and shear.16,17 Detailed studies on phase transitions of block copolymers caused by these varied mechanisms are important not only for understanding their molecular assemblies and physical properties but also for the design and fabrication of new self-assembled nanostructures. Here, we demonstrate a new type of cylinder-to-sphere transition that is induced by the thermal deprotection and the subsequent volume change of the minority block in an asymmetric diblock copolymer. The cylindrical domains are transformed into a series of spheres via thermolysis and arranged along the original cylindrical structure in thin films. Such a hierarchical cylinder-to-sphere transition may be very useful for fabricating new geometries of long-range ordered spherical arrays from well-aligned cylindrical microdomains. Long-range ordered cylindrical domains can be achieved using the methods of graphoepitaxy18,19 and expitaxial assembly20 on chemically patterned substrates. By combining these techniques and the hierarchical cylinder-to-sphere transition, well-ordered linear arrays of spherical domains were observed over large areas. The geometry of the spherical arrays obtained using this strategy was quite different from the close-packed hexagonal arrays generally observed in thin films of sphere-forming diblock copolymers.

Scheme 1. Chemical Transformation from Poly(tert-butyl acrylate) to Poly(acrylic anhydride) by Thermal Deprotection of the tert-Butyl Acrylate Linkage

Figure 1. Thermal behavior of PS-b-PtBA diblock copolymer. (a) Isothermal TG curves obtained at 130 °C (O), and 160 °C (b). (b) FTIR spectra obtained before annealing (I), after annealing at 130 °C for 2 days (II), and after annealing at 160 °C for 1 day (III). (IV: reference spectrum of polystyrene-block-poly(acrylic acid) to confirm the peak position of carbonyl (CdO) region). Spectra were recorded using a Nicolet 860 FT-IR spectrometer equipped with an MCT detector and PEM-IRRAS accessory (scan numbers:1024, resolution: 4 cm-1).

The polymer used in these experiments is an asymmetric poly(styrene)-block-poly(tert-butyl acrylate) (PS-b-PtBA) diblock copolymer (Mw: 66.2 kg mol-1 for PS and 32.0 kg mol-1 for PtBA blocks, PDI: 1.05, Polymer Source Inc.), which has a thermally labile ester linkage in the minority block. To determine the optimum annealing conditions for this block copolymer, the thermal behavior and stability were examined by isothermal TGA (thermal gravimetric analysis) and FT-IRRAS (Fourier transform infrared reflection absorption spectroscopy). Figure 1a shows isothermal TGA curves for PS-b-PtBA diblock copolymer obtained at temperatures of 130 °C and 160 °C. At 130 °C, there is no significant mass change up to 20 h. However, at 160 °C, the mass slightly decreases within 4 h, and significantly drops between 4 and 6 h, resulting in approximately 17% loss of total mass. The character of this TGA curve is reminiscent of earlier results reported by Wallraff et al., where they studied thermal and acid-catalyzed deprotection kinetics in a poly(tert-butyl methacrylate) polymer system and demonstrated that the thermal cleavage of the tert-butyl ester linkage follows two reaction steps of slow unimolecular thermolysis followed by a fast auto-acceleration step in which the deprotected groups catalyze further deprotection.21 1380

Detailed information about the structural changes of PSb-PtBA diblock copolymer induced by thermal cleavage was obtained by FTIR spectra as shown in Figure 1b. Thin films of PS-b-PtBA (thickness: ca. 145 nm) on gold substrates were used for surface IR analysis, and the spectra were recorded before annealing (I) and after annealing at two different conditions of 130 °C for 2 days (II), and of 160 °C for 1 day (III) (Nicolet 860 FT-IR spectrometer equipped with an MCT detector and PEM-IRRAS accessory). Only the spectral range of interest is shown in the figure. Before annealing, the spectrum shows absorption bands associated with the tert-butyl acrylate functionality, that is, CdO (ester linkage) stretching, CH3 bending, C-C-O stretching, and C-O stretching modes located at 1730, 1394/1368, 1277/ 1258, and 1160 cm-1, respectively.22,23 The characteristic peaks originating from the aromatic functionality in poly(styrene) also appeared between 1600 and 1400 cm-1. After annealing at 130 °C for 2 days, the shape and position of the peaks remained intact. However, after annealing at 160 °C for 1 day, the peaks attributed to the t-butoxy group, including CH3 bending, C-C-O stretching, and C-O stretching modes disappeared entirely, and the sharp peak due to the carbonyl group (CdO) of the ester linkage was shifted to a higher frequency by about 30 cm-1, and broadened. This change, caused by the thermal cleavage of the tert-butyl ester linkages at 160 °C, is in good agreement with the isothermal TGA analysis. To verify the final structure of the PS-b-PtBA block copolymer after thermolysis, we compared the IR spectrum obtained after annealing at 160 °C (III) with the spectrum of poly(styrene-b-acrylic acid) (IV), the expected polymer after cleavage of the tertbutyl ester linkage. However, the shape and position of peaks attributed to the characteristic carbonyl group (CdO, at 1806 and 1762 cm-1) in spectrum III were quite different from the peak corresponding to carbonyl group of carboxylic acid (at 1719 cm-1) in spectrum IV. This is likely due to the formation of anhydride linkages via a condensation reaction between carboxylic acid functionalities generated during thermal cleavage. As the tert-butyl ester linkage decomposes, a carboxylic acid group is generated releasing a hexadiene molecule, and two carboxylic acid groups quickly react with each other within the same polymer chain (intramolecular reaction) or between two different chains (intermolecular reaction) to form an anhydride linkage by removing a water molecule (Scheme 1). Intermolecular reactions of two carboxylic acid groups result in chemical cross-links between chains. The peaks at 1806 and 1762 cm-1 in spectrum III represent asymmetric and symmetric stretching modes of Nano Lett., Vol. 5, No. 7, 2005

carbonyl groups in the anhydride linkages, respectively, and the 17% weight loss shown in the isothermal TGA curves is also consistent with the calculated value (16.5%), taking into account the loss of hexadiene and water. Similar results observing the formation of anhydride linkages during the thermal decomposition of tertiary ester linkages have been previously reported, in poly(tert-butyl methacrylate),21 as well as in 2,5-dimethyl-2,5-hexanediol dimethacrylate (DHDMA) films.24 This chemical transformation from poly(styrene-b-t-butyl acrylate) (PS-b-PtBA) to poly(styrene-b-acrylic anhydride) (PS-b-PAA), induced by thermal deprotection of the tertbutyl ester linkage is accompanied by a change in the volume fraction of each of the blocks. The weight ratio between PS and PtBA is 68:32 before thermolysis, changing to 81:19 (PS:PAA) after thermal deprotection. Such a large change causes a morphological transformation of the PS-b-PtBA diblock copolymer from cylinder to sphere before and after thermolysis. To study the morphological change on chemically homogeneous substrates, we treated a cleaned silicon wafer with a hydroxy-terminated poly(methyl methacrylate) homopolymer (HO-PMMA, Mw ) 9.2 kg mol-1) through a procedure reported elsewhere in the literature;25 the cleaned silicon wafers contain surface OH-functionalities that can act as an acid catalyst to cleave tert-butyl acrylate linkages even though their acidity is very low. To prevent deprotection of the tert-butyl acrylate linkage by OH functionalities, the substrates were covered with brush layers. The PS-b-PtBA diblock copolymer was then spin-coated from a 1% toluene solution at 4000 rpm on a PMMA polymer brush, yielding a thin film approximately 40 nm thick. The coated samples were annealed under vacuum at 130 °C for 2 days, and part of the annealed sample was then further annealed under vacuum at 160 °C for 1 day. Figures 2a and 2b show planview field-emission scanning electron micrographs (SEM) of PS-b-PtBA diblock copolymers annealed at 130 °C for 2 days, and further annealed at 160 °C for 1 day, respectively. To improve the contrast in the SEM images, the PtBA-related blocks were selectively removed with UV exposure (1.7 mW/ cm2, 5 min) and acetic acid etching.26 When the film was annealed at 130 °C for 2 days, which results in no chemical transformation, the PtBA block (dark region) forms cylinders parallel to the substrate in a matrix of PS (bright region) (spacing between the cylinders: ca. 45 nm as determined by fast Fourier transform analysis). (Figure 2a). When this sample was further annealed at 160 °C for 1 day, resulting in thermal deprotection, the cylindrical morphology transformed to PAA spheres (diameter average: 20 nm) as shown in Figure 2b. This result clearly confirms that the thermolysis of asymmetric PS-b-PtBA diblock copolymers causes a phase transition due to changes in the polymer structure and the relative volume fraction of two blocks. The PAA spheres do not, however, form an ordered domain structure such as close-packed hexagonal (hcp) arrays generally observed in thin films of spherical diblock copolymers (body-centered cubic lattice in bulk). Instead, the spherical domains appear to be spatially located along the contours of the original cylindrical domains. Nano Lett., Vol. 5, No. 7, 2005

Figure 2. Plan-view SEM images of asymmetric PS-b-PtBA diblock copolymer on the HO-PMMA treated silicon substrates. (a) Cylindrical morphology: annealed at 130 °C for 2 days. (b) Spherical morphology: further annealed at 160 °C for 1 day. To improve the contrast in SEM images, the PtBA-related blocks were selectively removed with UV exposure (1.7 mW/cm2, 5 min) and acetic acid rinsing.

Coupled with techniques to induce long-range order in cylinder forming block copolymers, a hierarchical approach taking advantage of the cylinder-to-sphere transition may provide a simple and effective route to fabricate varied geometries of spherical arrays that are difficult to form using sphere-forming block copolymers. Various strategies such as electric fields,27 directional crystallization,28 and shearing29 have been reported for alignment of cylindrical microdomains. Recently, graphoepitaxy, the use of topographic substrate features to guide the formation of a single crystal domain structure, has been successfully applied to achieve long-range order of cylindrical domains.18,19,30 In the topographic pattern, the confined volume between two vertical sidewalls and the preferential affinity of the sidewalls to one block of the block copolymer promotes the alignment of the block copolymer microdomains parallel to the features. The topographic pattern used in this experiment (trough width ) 200 nm, depth ) 75 nm) was made by photolithography and reactive ion etching (IBM, Watson Research Center), and the patterned silicon oxide substrates were treated with hydroxy-terminated polystyrene (HO-PS, Mw ) 9.9 kg mol-1)25 to form 5-nm thick PS brushes. The PS brushes are preferentially wet by the PS block at the PS-bPtBA diblock copolymer. The hierarchical process to create 1381

Figure 3. Hierarchical transition from cylinder to spheres on topographically patterned substrates. (a) Schematic of the phase transition from PS-b-PtBA cylindrical morphology to PS-b-PAA spherical morphology on a topographic pattern. (b) Plan-view SEM Image of PSb-PtBA thin film obtained after initially annealing at 130 °C for 2 days. (c) Plan-view SEM Image of PS-b-PAA thin film obtained after further annealing at 135 °C for 5 days (PS: bright region, PtBA and PAA: dark region).

linear arrays of ordered spheres using topographic substrates is depicted schematically in Figure 3a. First, to align the PtBA cylinders on the topographic pattern, PS-b-PtBA diblock copolymer was spin-coated from a 1% toluene solution (at 4000 rpm, thickness measured on flat substrate ) ca. 40) and annealed at 130 °C for 2 days. After this first annealing process, half of the sample was used for SEM analysis (see Figure 3b), and the other half was further annealed to effect the transition from cylinders to spheres (see Figure 3c). In this second annealing, a lower temperature and longer annealing time (135 °C, 5 days) was applied instead of a high-temperature annealing condition (160 °C, 1 day) used on flat substrates (Figure 2) to minimize any thickness change of the films in troughs due to polymer flow from the crests to the troughs of the pattern. Figure 3b shows an SEM image of a PS-b-PtBA thin film on the topographic pattern treated with polystyrene brushes. The preferential interaction of the PS block with the trough sidewalls coated with PS brushes drives the alignment of PtBA cylinders parallel to the edge of the troughs. Since the molecular weight of PS-b-PtBA (98.2 kg mol-1) used in this experiment is too high to have enough mobility to achieve complete ordering on the topographic patterns, the alignment was not perfect, and many defects were observed. Figure 3c shows the other half of the sample after it has been further annealed at 135 °C for 5 days. The PtBA cylinders are completely transformed into 5-lines of PAA spheres. The PAA spheres are created along the original path of the cylindrical domains, even at defect sites. (Defect sites in the cylindrical and spherical arrays, respectively in Figures 3b and 3c, are marked with red arrows.) 1382

Defect-free and registered alignment of both cylindrical and spherical microdomains was achieved using epitaxial self-assembly on chemically patterned substrates as developed by our group. Previously, we have demonstrated that perpendicularly oriented lamellar domains in PS-b-PMMA block copolymer films could be aligned to perfection over arbitrarily large areas on chemically patterned substrates and registered with the underlying chemical pattern.20,31 The chemical patterns used in this work were made by procedures described previously in the literature.31 Polystyrene brushes were coated with photoresist, which was subsequently patterned with arrays of lines and spaces having a period of 55 nm using extreme ultraviolet interference lithography (EUV-IL).32 The PS polymer brushes were removed from areas of the substrate not protected by overlying photoresist with an oxygen plasma etch, leaving behind a hydrophilic (polar) surface. Finally, the remaining photoresist was removed in a chlorobenzene solution to yield chemically patterned PS brushes. For epitaxial self-assembly of the block copolymer films, these chemical patterns were spin-coated with a thin film (ca. 40 nm) of PS-b-PtBA (98.2 kg mol-1), and annealed at 130 °C for 2 days to initially form a cylindrical morphology. Then, half of the sample was used for SEM analysis, and the other half was further annealed at 160 °C for 1 day to form corresponding PAA spheres (Figure 4a). In the pattern, the stripes that are treated with oxygen plasma have hydrophilic (polar) moieties and are preferentially wet by the PtBA block of the PS-b-PtBA block copolymer. The stripes of unmodified PS brushes are preferentially wet by PS block. This wetting layer directs the formation of a single layer of well-ordered PtBA Nano Lett., Vol. 5, No. 7, 2005

Figure 4. Hierarchical transition from cylinder to spheres on chemically patterned substrates. (a) Schematic of the phase transition from PS-b-PtBA cylindrical morphology to PS-b-PAA spherical morphology on a chemical patterns. (b) Plan-view SEM Image of PS-b-PtBA thin film obtained after first annealing at 130 °C for 2 days. (c) Plan-view SEM Image of PS-b-PAA thin film obtained after further annealing at 160 °C for 1 day. Insets: 2-D fast Fourier transform images. (d) Magnified image of (b). (e) Magnified image of (c) (pattern period ) 55 nm, PS: bright region, PtBA and PAA: dark region).

cylinders as shown in the schematic in Figure 4a (left). Figure 4b shows an SEM image of ordered PtBA cylinders over a 2.5 µm by 2.5 µm area. Due to the contrast in interfacial energy between adjacent stripes of chemically nanopatterned substrates, perfect ordering of PtBA cylinders (or PtBA wetting layers) could be achieved even though the pattern period (55 nm) was not commensurate with the spacing between cylinders (ca. 45 nm).31 The fast Fourier transform (FFT) image shown in the inset of Figure 4b exhibits narrow high intensity peaks at regular intervals, demonstrating that the cylinders are directed to assemble in registration with the chemically nanopatterned substrate. (A detailed study regarding the long-range order of cylindrical block copolymers on chemically patterned substrate will be discussed in a following paper).33 When the PS-b-PtBA thin film aligned in this manner was further annealed at 160 °C, rows of PAA spheres were successfully generated from the PtBA cylinders (Figure 4c). The average diameter of the spheres is ca. 20 nm as determined by SEM image analysis. In the FFT image shown in the inset of Figure 4c, the narrow high intensity peaks along the vertical central axis indicate that the rows of spheres have the same period (55 nm) as the cylinders in Nano Lett., Vol. 5, No. 7, 2005

Figure 4b. The broad character of the peaks parallel to the central vertical axis indicates that the spherical domains are regularly spaced (ca. 49 nm) within each row. The absence of narrow high-intensity off-central axis peaks indicates that the spheres are not hexagonally close packed, in contrast to the behavior of sphere-forming block copolymer films with long-range order.34-36 In all experiments in which arrays of spheres were created by the transformation of a cylindrical morphology via a volume change of the minority block, the spheres seem to form by pinching off the cylindrical structures at regular intervals along the contour of the cylinders with little interaction between adjacent cylinders. The driving force for the morphological transition is substantial and includes density gradients in addition to energetic contributions associated with interfacial energy and chain conformations. It is unclear why the spherical morphology does not subsequently adopt a hexagonal morphology to further (incrementally) reduce the free energy of the system. It is likely that the polymer is kinetically hindered from rearrangement by a combination of low annealing temperature and the high molecular weight of the material. In addition, 1383

it is possible that rearrangement was further hindered by chemical cross-links generated through the formation of intermolecular anhydride linkages. In summary, we have demonstrated that a phase transition from cylinders to spheres occurs in asymmetric PS-b-PtBA diblock copolymers through thermal deprotection of tertbutyl acrylate linkages and the subsequent volume change of the minority block. Combined with techniques to direct the assembly of the cylindrical morphology into ordered arrays, this phase transition mechanism provides the means to hierarchically generate linear arrays of spherical domains. Chemically nanopatterned substrates, for example, can be used to precisely control the spacing between rows of spheres and may be used to generate arrays in nonlinear geometries such as spheres along lines of concentric circles or along lines forming corners. Potential applications of these onedimensional dot arrays include high-density magnetic storage media and other advanced technological devices. Acknowledgment. This work was supported by the Semiconductor Research Corporation, the UW-NSF Nanoscale Science and Engineering Center (DMR-0425880), and the Camille Dreyfus Teacher-Scholar Award. Y.-H. La acknowledges a research fellowship from the Postdoctoral Fellowship Program of the Korea Science and Engineering Foundation (KOSEF). The Synchrotron Radiation Center is supported by the National Science Foundation. The authors thank C. T. Black for providing the topographically patterned substrates. References (1) Parviz, B. A.; Ryan, D.; Whitesides, G. M. IEEE Transactions On AdVanced Packaging 2003, 26(3), 233-241. (2) 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(13), 1689-1691. (3) Park, M.; Harrison, C.; Chaikin, P. M.; Register, R. A.; Adamson, D. H. Science 1997, 276(5317), 1401-1404. (4) Lopes, W. A.; Jaeger, H. M. Nature 2001, 414(6865), 735-738. (5) Thurn-Albrecht, T.; Schotter, J.; Kastle, C. A.; Emley, N.; Shibauchi, T.; Krusin-Elbaum, L.; Guarini, K.; Black, C. T.; Tuominen, M. T.; Russell, T. P. Science 2000, 290(5499), 2126-2129. (6) Cheng, J. Y.; Ross, C. A.; Chan, V. Z. H.; Thomas, E. L.; Lammertink, R. G. H.; Vancso, G. J. AdV. Mater. 2001, 13(15), 1174-1178. (7) Park, C.; Yoon, J.; Thomas, E. L. Polymer 2003, 44(25), 77797779. (8) Bates, F. S.; Fredrickson, G. H. Phys. Today 1999, 52(2), 32-38. (9) Sota, N.; Sakamoto, N.; Saijo, K.; Hashimoto, T. Macromolecules 2003, 36(12), 4534-4543. (10) Kimishima, K.; Koga, T.; Hashimoto, T. Macromolecules 2000, 33(3), 968-977.

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Nano Lett., Vol. 5, No. 7, 2005