Templated Self-Assembly of Square Symmetry Arrays from an ABC

NANO LETTERS

Templated Self-Assembly of Square Symmetry Arrays from an ABC Triblock Terpolymer

2009 Vol. 9, No. 12 4364-4369

Vivian P. Chuang,†,§ Jessica Gwyther,‡,§ Rafal A. Mickiewicz,† Ian Manners,*,‡ and Caroline A. Ross*,† Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 and School of Chemistry, UniVersity of Bristol, Bristol, BS8 1TS U.K. Received August 13, 2009; Revised Manuscript Received October 12, 2009

ABSTRACT Self-assembly provides the ability to create well-controlled nanostructures with electronic or chemical functionality and enables the synthesis of a wide range of useful devices. Diblock copolymers self-assemble into periodic arrays of microdomains with feature sizes of typically 10-50 nm, and have been used to make a wide range of devices such as silicon capacitors and transistors, photonic crystals, and patterned magnetic media1-3. However, the cylindrical or spherical microdomains in diblock copolymers generally form close-packed structures with hexagonal symmetry, limiting their device applications. Here we demonstrate self-assembly of square-symmetry patterns from a triblock terpolymer in which one organometallic block imparts high etch selectivity and etch resistance. Long-range order is imposed on the microdomain arrays by self-assembly on topographical substrates, and the orientation of both square lattices and in-plane cylinders is controlled by the substrate chemistry. Pattern transfer is demonstrated by making an array of square-packed 30 nm tall, 20 nm diameter silica pillars. Templated selfassembly of triblock terpolymers can generate nanostructures with geometries that are unattainable from diblock copolymers, significantly enhancing the capabilities of block copolymer lithography.

The potential of diblock copolymer self-assembly for nanolithography and nanoscale device fabrication is now well established,1-4 but the study of triblock terpolymer thin films for this application is still in its infancy,5-10 despite their ability to self-assemble into patterns such as square symmetry arrays that cannot easily be formed from diblock copolymers. Furthermore, the templating of triblock terpolymer thin films by topographical or chemical substrate patterns, which is essential for their use in nanolithography, has not been demonstrated. The square-symmetry array is one of the device geometries considered essential for future lithography needs11 and is useful for structures such as via arrays in integrated circuits. A square symmetry cannot be achieved by the self-assembly of a coil-coil diblock copolymer, but has been found in certain polymer systems such as A-b-Bb-C triblock terpolymers in which the B block forms a matrix separating A and C cylindrical microdomains,12-14 combcoil diblock copolymers in which the cylindrical domains are surrounded by a liquid crystalline matrix,15 T-shaped liquid crystals with nonpolar lateral side chains16 or DNA/ * To whom correspondence should be addressed. E-mail: (C.A.R) [email protected]; (I.M.) [email protected]. † Massachusetts Institute of Technology. ‡ University of Bristol. § These authors contributed equally to this work. 10.1021/nl902646e CCC: $40.75 Published on Web 11/11/2009

 2009 American Chemical Society

dendrimer complexes in which charge matching is optimized.17 However, most of these studies focused on bulk morphologies. Self-assembled block polymer films with square symmetry microdomains were produced by templating of a cylindrical-morphology diblock copolymer on a chemically nanopatterned substrate with the same periodicity as the copolymer18 and by imposing fcc packing of a sphericalmorphology diblock copolymer19 or bcc packing of a coreshell spherical-morphology triblock terpolymer,10 both of which showed a square arrangement at the top surface. Recently, Tang et al. blended A-b-B and B′-b-C diblock copolymers in which the B and B′ blocks interact by hydrogen bonding;20 self-assembly of the blend led to a thin film with a square symmetry cylindrical microdomain arrangement. In this work, we show that square-symmetry patterns can be formed by the self-assembly of an A-b-Bb-C triblock terpolymer thin film, and that the self-assembly can be templated by topographical substrate features. The orientation of both the square out-of-plane cylinder lattice and the in-plane cylinder array is controlled by a brush layer on the substrate, enabling complex patterns to be created by a combination of topographical and chemical substrate modifications. Square-packed A and C cylinders in a B matrix (Figure 1a) are expected to form in an A-b-B-b-C triblock terpolymer

tion of the polymer are described in the Supporting Information, Figures S1 and S2.

Figure 1. Triblock terpolymer bulk and thin film morphologies. (a) Sketch of the bulk morphology showing PFS (red) and PI (light gray) parallel cylinders forming a checkerboard pattern in a matrix of PS. (b) Bright field TEM image of the bulk morphology of the pure ISF82 triblock terpolymer. The dark spots represent sections through the PFS microdomains. (c-f) SEM images of thin films of (c) pure ISF82 and (d-f) ISF82/PS blend on oxidized Si after spin-coating and annealing in chloroform vapor for 2.5 h at room temperature, followed by etching with oxygen RIE to remove both PI and PS blocks. (e) This panel includes an additional step of OsO4 staining for 4 h before oxygen RIE, giving the PI higher resistance to an oxygen etch. The PFS shows as bright features with the PI, a lighter gray, in between.

when the Flory-Huggins interaction parameter χAC between the A and C blocks is greater than χAB and χBC, and the volume fraction of the B block is 60-70%.21 We designed a new triblock terpolymer that satisfies these criteria, polyisoprene-b-polystyrene-b-polyferrocenylsilane (PI-b-PSb-PFS) with volume fractions of 25, 65, and 10%, respectively. Of the three interaction parameters, χPI-PFS is the largest, based on the solubility parameters of PI, PS, and PFS, which are 17.0 (MPa)1/2, 18.5 (MPa)1/2, and 18.7 (MPa)1/2, respectively.22,23 The incorporation of both organic (PI and PS) and organometallic (PFS) segments, which contain iron and silicon, imparts high etch selectivity between the blocks, and the organometallic block has a high etch resistance to an oxygen plasma.24,25 These are desirable characteristics for pattern transfer compared with all-organic triblock terpolymers.8 The total molecular weight of the triblock terpolymer, denoted ISF82, is 82 kg/mol. The synthesis and characterizaNano Lett., Vol. 9, No. 12, 2009

Figure 1b shows a TEM image of the bulk morphology of unetched pure ISF82 after thermal annealing at 150 °C under vacuum for four days followed by quenching in liquid nitrogen. Without staining, there is very little contrast between PI and PS and thus the dark dots in the image represent cross sections through PFS cylinders with an average center-to-center distance between PFS domains of 41.1 nm. Thin films of the ISF82 were formed on oxidized silicon substrates by spin-coating from a solution in toluene and were annealed in saturated chloroform vapor for 2-5 h at room temperature to induce microphase separation, followed by quenching in ambient air. The effectiveness of chloroform for solvent annealing is attributed to its good match in solubility parameter with PS and PFS and its high vapor pressure at room temperature. The PI and PS blocks were removed simultaneously by etching in an oxygen plasma to leave oxidized PFS cylinders. Figure 1c-f shows SEM images of thin film morphologies of ISF82 in which the cylinders orient perpendicular to the plane. (Results for other film thicknesses, for different solvents, and for thermally annealed films are discussed in Supporting Information, Figure S3 and the accompanying text.) For the pure triblock terpolymer, a film with average thickness of 29.7 nm showed regions of both hexagonal and square-packed PFS cylinders (Figure 1c). However, by blending the ISF82 with 17.9 wt % of PS homopolymer (Mn ) 27 kg/mol), square-packed patterns were formed over the entire sample area studied (Figure 1d). To confirm the locations of the PI microdomains, Figure 1e shows a sample of the ISF82/PS blend film that was stained with osmium tetraoxide (OsO4) before etching in oxygen plasma. OsO4 is a preferential staining agent for the PI domains and increases their etch resistance. The alternating PI (gray) and PFS (white) domains form the expected checkerboard arrangement, analogous to the result of Tang et al. for blended H-bonded diblock copolymers.20 Figure 1f shows an unstained sample at the same magnification in which only PFS domains remain after oxygen etching. The period of the PFS microdomains measured from the thin film of the ISF82/PS blend is 40.5 nm, which is larger than that measured from the unblended ISF82 film (39.1 nm) due to the presence of the homopolymer. It is likely that the PS homopolymer stabilizes the squarepacked morphology in the thin film by enabling the highly stretched triblock terpolymer chains to relax as the homopolymer occupies the volume between the cylinders,26 and by increasing the volume fraction of the PS matrix. TEM analysis of thin films of the blend shows that the hexagonally packed structure has a lower volume fraction of PS than the square-packed structure (see Supporting Information, Table S1). The hexagonally packed structure found in the unblended ISF82 film consists of core-shell cylinders with a PFS core and PS shell in a PI matrix (Supporting Information, Figure S4). The formation of this structure is attributed to the difference between the volume fractions of PI and PFS and the asymmetry between χPI-PS and χPFS-PS. The 4365

interfacial energy can be lowered if the two interfaces have different curvatures, stabilizing core-shell structures at the expense of alternating structures.21,27 We now discuss templating of the self-assembly of the square-packed triblock terpolymer microdomains. Although there are many reports of the topographical templating (graphoepitaxy) of diblock copolymer films,28-31 there has been no report of templating applied to a triblock terpolymer. Figure 2 shows the ISF82/PS blend self-assembled in templates consisting of shallow grooves of different widths etched into Si. Both the periodicity of the microdomains and their orientation within the trench can vary in response to the trench width and surface chemistry. In templates without chemical functionalization, square arrays of PFS cylinders formed with the principal axis primarily at 90° to the walls of the trench, Figure 2a. However, when the templates were coated with a PS-brush, Figure 2b, the PFS microdomain array was oriented primarily at 45° to the trench wall. In addition, the PS-brushed substrates contained an increasing minority population of 90° and other orientations of arrays as the groove width increases, for example ∼16% of the trench length was occupied by 90°-oriented arrays at a trench width of 235 nm. The differences in orientation of the microdomain arrays may be explained by considering the preferential wetting of the substrate by the blocks, shown schematically in Figure 2c,e. The uncoated template made of silicon with native oxide attracts both PI and PFS similarly,32 and preferentially compared to PS, favoring an arrangement with one of the end blocks at the walls. The adjacent row of cylinders is therefore composed entirely of the other end block, and a 90° orientation of the PFS microdomains is promoted. In contrast, the PS-coated substrate favors an arrangement with the PS blocks present at the walls, and the adjacent row of cylinders consists of alternating PI and PFS, giving a 45° orientation of the PFS microdomains. As the trench width increases, the number of rows of PFS microdomains increases in a stepwise manner, as seen in diblock copolymers self-assembled in trenches.29,31 The number of PFS rows is plotted in Figure 2d,f vs confinement width W divided by P0, where P0 represents the equilibrium (untemplated) spacing between the rows of PFS posts measured perpendicular to the trench edge. This spacing is P0,90 ) 40.5 nm for the ISF82/PS blend in the 90° orientation and P0,45 ) 40.5/2 ) 28.6 nm in the 45° orientation. Figure 2d gives data for trenches with a PS brush for both the 45° (common) and 90° (rare) orientations. Both sets of data fit to a slope of 1, as expected. The intercept on the horizontal axis is indicative of the morphology of the microdomains and brush layers present at the trench edge. The majority 45° array has an intercept of approximately 1, showing that there is a separation of ∼P0,45 (28.6 nm) between the centers of the outer row of PFS microdomains and the trench edge. For the 90° orientation, the intercept is zero indicating a separation of P0,90/2 ) 20.3 nm. Given that only the PS segments of the triblock chains are expected to be in contact with the PS brush, one might expect the separations to be smaller than these values to avoid the presence of PI or PFS 4366

segments at the walls and to minimize stretching of the PS blocks. We speculate that the PS-brush, 3-4 nm thick, and the presence of adjacent PS homopolymer, increase the spacing between the trench walls and the first row of cylindrical microdomains. In contrast, Figure 2f shows similar data for the uncoated substrates in which the 90° orientation dominates. The data fall onto two lines, one with an intercept of zero and the other with an intercept of 1. The data are generated from different samples, each of which produced data that fell on one or other line. These results can be explained by preferential wetting of some substrates by PI (intercept ) 0) and others by PFS (intercept ) 1) as shown schematically in Figure 2e, which may be due to differences in processing of the substrates which affect their preference for PI versus PFS. Measurements of the period of the PFS posts indicate a rectangular (for the 90°) or oblique (for the 45° orientation) lattice distortion of the 2D square pattern by expansion or contraction of the spacing perpendicular to the trench edge. The distortion, which changed the period perpendicular to the edge by up to 5-10%, is a response to the incommensurability between W and P0, analogous to the distortion of confined close-packed sphere arrays seen previously in templated diblock copolymers.29 We also demonstrate topographical templating of in-plane cylinders for the ISF82/PS blend for thicker (∼35 nm) polymer films. Figure 3 compares templated cylinders in trenches with a PS brush and with no brush. For the PSbrushed trenches the PFS (and PI) cylinders lie in plane perpendicular to the trench walls without contacting the walls (Figure 3a). Unlike previous examples of cylinders oriented perpendicular to trench walls, which were attributed to kinetic effects,30 the orientation in this case arises from the requirement for the middle PS block to contact the trench walls while maintaining its connectivity to the in-plane PI and PFS cylinders. In contrast, an untreated topographical substrate shows regions of mixed parallel and perpendicular cylinders (Figure 3b,c). Narrow trenches contain in-plane PFS cylinders parallel to the trench walls as well as rows of cylinders oriented perpendicular to the substrate (Figure 3b). Wider trenches show a variety of morphologies including branched cylinders and 90 and 45° oriented arrays (Figure 3c). On the basis of the separation between the in-plane cylinders and the trench edge, it appears that PI wets the trench walls in these samples. Finally, a PFS-brushed substrate was also examined (not shown). The thickness uniformity of the ISF82/ PS was poor, but in thin regions, in-plane hemicylinders of PFS were observed wetting the trench floors, oriented parallel to the trenches, while in thicker films either perpendicular cylinders above the hemicylinders or a second layer of inplane cylinders were seen. These results demonstrate that by tuning the film thickness and the relative affinity of the trench wall for each of the blocks, the cylindrical microdomains can be oriented in-plane or out-of-plane, and in both cases the angle between the trench wall and the principal axes of the cylinder lattice can be controlled. This gives considerNano Lett., Vol. 9, No. 12, 2009

Figure 3. Templated in-plane cylinders from the triblock terpolymer. SEM images of ∼35 nm thick films of ISF82/PS blend assembled in (a) a PS-brushed template, showing PFS cylinders perpendicular to the trenches but terminating before joining the trenches and (b,c) uncoated substrates with different trench widths, after annealing and etching with oxygen RIE. (b) Narrower trenches typically show in-plane cylinders parallel to the trench walls surrounding 90°-oriented out-of-plane cylinder arrays. (c) Wider trenches show in-plane cylinders parallel to the trench walls, often with branches, surrounding a mix of inplane cylinders and 45°-oriented and 90°-oriented out-of-plane cylinder arrays.

Figure 2. Templated triblock terpolymer. (a,b) SEM images of templated assembly of the ISF82/PS blend after annealing in chloroform vapor for 2.5 h at room temperature, followed by etching with oxygen RIE to remove the PI and PS domains, (a) with uncoated trenches and (b) with PS-brushed trenches. (c) Schematic of the options for packing the microdomains in a PS-brushed trench. For the 90° orientation a greater degree of stretching in the PS center block is required for it to contact the trench wall. The row spacing P0 is defined for each orientation, and the observed spacing between the outer PFS row and the trench wall is indicated. (d) A plot of the number of rows of PFS posts vs confinement width W normalized by row spacing P0,90 and P0,45 respectively for the 90 and the 45° orientations for arrays in PS-brushed trenches. Both data sets were fitted with a line of slope 1. (e) Schematic of the 90° orientations possible in an uncoated trench which can attract either the PI (left) or the PFS (right). The expected spacing between the outer PFS row and the trench wall is indicated. (f) A plot of number of rows of PFS posts vs confinement width W normalized by row spacing P0,90 for the 90° orientation self-assembled in an uncoated trench. Two different populations are observed with intercepts corresponding to the two cases shown in (e). Nano Lett., Vol. 9, No. 12, 2009

able control over the geometry of the final self-assembled pattern, beyond what can be achieved in a diblock copolymer. The results show some analogies to predictions of a model33 for a bulk cylindrical A-b-B-b-C triblock terpolymer confined between two parallel planes. Structures which in cross-section resemble the 45 and 90° orientations of Figure 2a are predicted to form when the confining planes are neutral to both blocks, based on a selfconsistent field theory. However, if the planes are attractive to B, the 45° orientation becomes more favorable because it minimizes stretching of the B blocks. Additionally, a morphology resembling Figure 3a with A and C cylinders perpendicular to the planes is promoted in which the cylinders terminate without touching the planes. Although the modeling does not capture all the details of the experimental system, the similarities suggest that the results of this study can be generalized to other terpolymer systems, and perhaps to other microdomain morphologies such as spherical or lamellar. We finally discuss pattern transfer from the etched triblock terpolymer film. A good etch selectivity between the blocks is one of the essential requirements for block copolymer lithography, so that one or two of the blocks can be removed leaving the morphology of the remaining block(s) intact. As found earlier for PS-PFS34 and PI-PFS32 diblock copolymers and a PS-b-PFS-b-P2VP triblock 4367

creating a wide range of pattern geometries useful in selfassembled nanolithography. Acknowledgment. C.A.R. and V.P.C. acknowledge support from the Semiconductor Research Corporation and the Singapore-MIT Alliance. J.G. and I.M. thank EPSRC for funding. Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs. org. References Figure 4. Pattern transfer to form square arrays of posts. (a) Schematic of the pattern transfer process. (b-g) SEM images of (b-d) top view and (e-g) side views of (b,e) PFS posts after PI and PS removal from a thin film of an ISF82/PS blend on silica, (c,f) silica posts with PFS caps, and (d,g) silica posts.

terpolymer,9 the organic blocks can be removed using an oxygen reactive ion etch (RIE), leaving partly oxidized organometallic PFS posts. Figure 4 shows the transfer of a square-packed PFS microdomain array to make silica posts with height 30 nm and an aspect ratio (height/diameter) of ∼1.6, greater than that of the original PFS posts (aspect ratio ∼1). These cross-sectional images suggest that the vertically oriented PFS cylinders extend throughout all or most of the film thickness. For the uncoated substrates used in Figure 4, the PFS cylinders are expected to touch the silica substrate. For PS-coated substrates the PFS cylinders are expected to terminate just above the PS surface layer, and etching would leave a thin PS layer under the PFS microdomains. In summary, square-symmetry microdomain arrays were obtained in a simple process from thin films of a new PIb-PS-b-PFS triblock terpolymer, as well as from a PI-bPS-b-PFS/PS blend, which exhibits a wider process window for forming square patterns consisting of alternating PI and PFS cylinders oriented perpendicular to the substrate within a PS matrix. For the first time, templated self-assembly of a triblock terpolymer was demonstrated on topographical substrates. For templated square arrays, the 90 or 45° orientation of the lattice vector of the PFS array with respect to the trench edge and the spacing between the trench edge and the PFS microdomains was controlled by the surface chemistry of the trenches. In thicker films, which form in-plane cylinders, the orientation of the cylinders parallel or perpendicular to the trench walls was also controlled via the substrate chemistry. Robust PFS arrays, formed by simultaneous removal of the PI and PS blocks using an oxygen plasma, were transferred into a 30 nm thick silica film to make square arrays of silica post diameter of 20 nm, height 30 nm and period 40 nm. This work illustrates the extensive level of control over microdomain geometry and lattice orientation achievable from a designed triblock terpolymer, templated using chemically functionalized topographical substrates and confirms the promise of triblock terpolymers for 4368

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