Polymeric Cross-Linked Surface Treatments for Controlling Block

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Polymeric Cross-Linked Surface Treatments for Controlling Block Copolymer Orientation in Thin Films Christopher M. Bates,† Jeffrey R. Strahan,† Logan J. Santos,‡ Brennen K. Mueller,‡ Benjamin O. Bamgbade,‡ Jonathan A. Lee,‡ Joshua M. Katzenstein,‡ Christopher J. Ellison,*,‡ and C. Grant Willson*,†,‡ †

Department of Chemistry and ‡Department of Chemical Engineering, The University of Texas at Austin, 1 University Station C0400, Austin, Texas 78712, United States Received October 26, 2010. Revised Manuscript Received December 8, 2010

The orientation of cylinder-forming poly(styrene-block-methyl methacrylate) [P(S-b-MMA)] was investigated on two sets of polymeric surface treatments: 10 para-substituted polystyrene derivatives with 10 nm were spin-coated, heated to crosslink via thermolysis of the azide functionality, and thoroughly rinsed to remove any non-cross-linked materials. Static contact angles were measured with H2O, diiodomethane, and glycerol. Using literature values for the dispersion and acid-base components of the surface tension and the actual surface tensions of H2O, diiodomethane, and glycerol, a system of equations involving the Young-Dupre equation and the acid-base surface tension model was solved to obtain the surface tensions of the XST films.23 Spin-Coating and Annealing. A clean, surface-treated wafer was spin-coated with a film of P(S-b-MMA) from toluene at various speeds and concentrations to give 20-130 nm block copolymer films as determined by ellipsometry. Once cast, the films were annealed at 170 °C under reduced pressure for 12-18 h.

Results and Discussion XST Synthesis. In a manner similar to that described by Hawker et al.,15 para-substituted styrenes were copolymerized with 4-vinylbenzyl chloride using AIBN as initiator. Nucleophilic substitution on the product with sodium azide led to a series of XSTs (Scheme 1). Each white powdery polymer was characterized by GPC and 1H NMR (Table 1), and the presence of the benzyl azide was confirmed by IR. XST Surface Tensions. Figure 1 displays the effect of the para substituent on the surface tension of the XST. Ten different surface tension measurements were made for each XST sample, yielding standard deviations between 0.6 and 1 dyn/cm. The error bars in Figure 1 represent (1 dyn/cm as a worst-case estimate of the error associated with each surface tension measurement. While some of the measured surface tension values overlap within error of each other, there are multiple regions of surface tensions that are distinguishable outside of error. The silicon-containing XSTs are lower surface tension (21-30 dyn/cm) than all the other XSTs, and the alkyl substituents are generally lower surface tension (34-40 dyn/cm) than the more polar halogen and acetoxy XSTs (22) (a) Busscher, H. J.; van Pelt, A. W. J.; de Boer, P.; de Jong, H. P.; Arends, J. The effect of surface roughening of polymers on measured contact angles of liquids. Colloids Surf. 1984, 9(4), 319–331. (b) Extrand, C. W.; Kumagai, Y. An Experimental Study of Contact Angle Hysteresis. J. Colloid Interface Sci. 1997, 191 (2), 378–383. (c) Lam, C. N. C.; Ko, R. H. Y.; Yu, L. M. Y.; Ng, A.; Li, D.; Hair, M. L.; Neumann, A. W. Dynamic Cycling Contact Angle Measurements: Study of Advancing and Receding Contact Angles. J. Colloid Interface Sci. 2001, 243(1), 208–218. (23) (a) Contact Angle, Wettability & Adhesion; VSP: Boston: 2002.(b) Van Oss, C. J.; Good, R. J.; Chaudhury, M. K. Additive and nonadditive surface tension components and the interpretation of contact angles. Langmuir 1988, 4(4), 884–91.

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Figure 1. Surface tensions of XST-R.

Figure 3. AFM image of P(S-b-MMA) cylinders on XST-Cl.

Figure 2. AFM images of P(S-b-MMA) cylinders on XST-Br. Film thickness of the P(S-b-MMA) film is indicated in the lower right of the images.

(42-45 dyn/cm). The range of surface tensions observed (21-45 dyn/cm) is much larger than what is available by cross-linking P(S-r-MMA).4a,12a This approach of changing para substituents on styrene to control surface tension not only provides a wider range of surface tensions than conventional random copolymers including only the block monomers but also is far more convenient since it utilizes one less monomer. Cylinder Forming P(S-b-MMA) on XSTs. Films of various thicknesses of P(S-b-MMA) containing PMMA cylinders were coated on the XSTs, thermally annealed, and investigated by AFM. XST-Br provided perpendicular cylinder formation with various feature densities on 23-41 nm thick BC films. This process window is similar to other terpolymer surface treatments reported by Nealey4a and Hawker and Russell.4c,12 Representative AFM images are shown in Figure 2. The center-to-center cylinder distance is 3540 nm in films with dense perpendicular cylinders, which is consistent with the SAXS data from bulk samples (see Supporting Langmuir 2011, 27(5), 2000–2006

Information). Outside of the perpendicular film thickness range, either mixed morphologies (coexisting parallel and perpendicular surface features) of varying densities or no surface features were observed. Cylinder forming P(S-b-MMA) films on XST-Cl consistently displayed mixed morphologies with BC films 25-40 nm thick. A representative AFM image is shown in Figure 3. Outside of this thickness range, sparse or no surface features were observed. P(S-b-MMA) films coated onto cross-linked XST-Me and XSTtBu were reproducibly rough (∼(3 nm or more). Since the crosslinked XSTs themselves had low roughness (Ra ∼ 0.277 nm), this result is likely an effect of the alkyl substituents, the cause of which was not investigated in detail. Likewise, XST-tBoc resulted in poor films that suffered from dewetting. This may be due to thermally induced deprotection of the tBoc group during annealing which created a rough surface not suitable for controlled BC orientation. XST-OMe and XST-OAc led to smooth XSTs, and the P(S-bMMA) films were optically smooth after annealing; however, no surface features were observed by AFM despite varying the film thickness from 15 to 60 nm in 5 nm increments. The most interesting result was observed with the XST-H. With films around 20 nm thick, parallel features were observed (Figure 4A). As the film thickness was increased (Figure 4B), AFM images showed fairly rough surfaces ((5 nm) with both orientation DOI: 10.1021/la1042958

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Figure 4. AFM images of P(S-b-MMA) cylinders on XST-H with P(S-b-MMA) film thicknesses of (A) 19, (B) 25, (C) 33, (D) 52, (E) 93, and (F) 128 nm.

and feature density dependence on the BC film thickness. Surprisingly, perpendicular cylinders were observed in films from 33 to 128 nm (Figure 4C-F) with various feature and defect densities. While many studies4a,12a,24 have established that BC orientation is dependent on both BC film thickness and the composition of random copolymer surface treatments, the BC film thickness tolerance of XST-H reported herein occurs with a surprisingly large styrene molar composition (92 mol % polystyrene and 8 mol % poly(4-vinylbenzyl azide)) in the surface treatment. One such study showed a surface treatment composed of 85 mol % polystyrene produces perpendicular cylinders of P(S-b-MMA) within only a 3 nm thickness window.12b Furthermore, polystyrene homopolymer surface treatments prefer to contact polystyrene BC domains because of a minimization of interfacial energy,10 which resulted in parallel BC orientation25 and/or a wetting layer of the polystyrene block on the substrate.26 Like XST-Br, XST-H represents a two-component surface treatment that produces perpendicular orientation of cylinder-forming P(S-b-MMA). The serendipitous discovery of perpendicular cylinders observed on XST-H prompted a set of experiments probing the effect of the cross-linked poly(4-vinylbenzyl azide) content on the orientation of the BC. A series of five random copolymers of polystyrene and poly(4-vinylbenzyl azide) were synthesized with compositions between 5 and 100 mol % PVBzAz (Table 2). Five different thicknesses of P(S-b-MMA) films were spin-coated onto each of these random copolymers, and the BC orientation was (24) (a) Han, E.; Stuen, K. O.; La, Y.-H.; Nealey, P. F.; Gopalan, P. Effect of Composition of Substrate-Modifying Random Copolymers on the Orientation of Symmetric and Asymmetric Diblock Copolymer Domains. Macromolecules 2008, 41(23), 9090–9097. (b) In, I.; La, Y.-H.; Park, S.-M.; Nealey, P. F.; Gopalan, P. Sidechain-grafted random copolymer brushes as neutral surfaces for controlling the orientation of block copolymer microdomains in thin films. Langmuir 2006, 22(18), 7855–7860. (25) Mansky, P.; Russell, T. P.; Hawker, C. J.; Pitsikalis, M.; Mays, J. Ordered Diblock Copolymer Films on Random Copolymer Brushes. Macromolecules 1997, 30(22), 6810–6813. (26) Liu, G.; Kang, H.; Craig, G. S. W.; Detcheverry, F.; de Pablo, J. J.; Nealey, P. F.; Tada, Y.; Yoshida, H. Cross-sectional imaging of block copolymer thin films on chemically patterned surfaces. J. Photopolym. Sci. Technol. 2010, 23(2), 149–154.

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Table 2. Characterization of Poly[styrene-r-(4-vinylbenzyl azide)] mol % PVBzAz

Mw (kDa)a

Mn (kDa)a

5 21.6 28 35.4 49 35.2 56 40.1 100 49.9 a Measured relative to polystyrene standards.

11.6 13.9 13.0 17.3 34.2

PDIa 1.85 2.54 2.71 2.32 1.46

studied by AFM (Figure 5). Some P(S-b-MMA) films were rough ((5 nm) and displayed regions of mixed morphology or differing feature density as a function of the local thickness, e.g., the P(S-bMMA) film 25 nm in thickness prepared on 5 mol % PVBzAz. In these cases, the AFM images shown in Figure 5 were selected to show the boundary between distinct regions of different orientation so that all were faithfully represented in one image. Full AFM images are provided in the Supporting Information (Figures S2-S6). Many P(S-b-MMA) films, such as the 25 nm thick film on 28 mol % PVBzAz, were homogeneous and displayed the same orientation and feature density over the entire film. Generally, 28 mol % poly(4-vinylbenzyl azide) provided the widest range of BC film thicknesses that produced dense perpendicular cylinders, but dense perpendicular cylinders were also achieved for other selected cases such as the 75 nm thick film on 56 mol % poly(4-vinylbenzyl azide). It is especially interesting to note that even the cross-linked homopolymer of poly(4-vinylbenzyl azide) induced dense perpendicular cylinders at 75 nm BC film thickness. To the authors’ knowledge, this is the first example of a single component surface treatment that can produce dense perpendicular cylinders of P(S-b-MMA) after thermal annealing without also prepatterning the substrate either physically or chemically. It should be noted that there are examples of singlecomponent neutralization layers for block copolymers such as poly(styrene-block-ethylene oxide) with organosilicate additives27 (27) Cheng, J. Y.; Pitera, J.; Park, O.-H.; Flickner, M.; Ruiz, R.; Black, C. T.; Kim, H.-C. Rapid directed self assembly of lamellar microdomains from a block copolymer containing hybrid. Appl. Phys. Lett. 2007, 91(14), 143106/1–143106/3.

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Figure 5. AFM images of five P(S-b-MMA) film thicknesses on five different P(S-r-VBzAz) random copolymers of various compositions. The AFM images were chosen to be representative of the entire sample. Full AFM images are provided in the Supporting Information (Figures S2-S6). L0 is defined as the row-to-row distance (d100) of the cylinders as measured by SAXS (see Supporting Information).

which have been successfully combined with directed self-assembly to yield excellent control of lamellar features within surface topographic features.28 Much effort was exerted in an attempt to quantify the surface tension of the different P(S-r-VBzAz) copolymer surface treatments. Unfortunately, the relatively low precision ((1 dyn/cm) of the calculated surface tensions did not allow identification of a trend in the values as a function of molar composition. The authors’ current hypothesis is that the surface tension of the cross-linked homopolymer of poly(4-vinylbenzyl azide) is similar enough to polystyrene that it cannot be distinguished with a contact angle measurement within the error of the measurement. Many of the AFM images in Figure 5 contain areas where there appear to be no apparent features on the surface. This phenomenon has been observed with silicon-containing block copolymers which can form a wetting layer of the silicon-rich block at the polymer-air interface due to a minimization of the interfacial energy.18,19 A recent etch study by Sperschneider et al.29 has demonstrated that triblock copolymer thin films with no obvious (28) Park, S.-M.; Park, O.-H.; Cheng, J. Y.; Rettner, C. T.; Kim, H.-C. Patterning sub-10 nm line patterns from a block copolymer hybrid. Nanotechnology 2008, 19(45), 455304/1–455304/6. (29) Sperschneider, A.; Hund, M.; Schoberth, H. G.; Schacher, F. H.; Tsarkova, L.; M€uller, A. H. E.; B€oker, A. Going beyond the Surface: Revealing Complex Block Copolymer Morphologies with 3D Scanning Force Microscopy. ACS Nano 2010, in press.

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features at the polymer-air interface have rich phase behavior just a few nanometers under the surface. First, perpendicular core-shell structures appear, and upon further etching, parallel structures or connected perpendicular structures become apparent. “Sparse” regions of block copolymer features have also been observed with P(S-b-MMA) on surfaces with surface tensions that are insufficient for producing defect-free orientation of the block copolymer.5,24,30 Furthermore, a combined experimental and theoretical study26 of P(S-b-MMA) has demonstrated it is possible for perpendicular cylinders on prepatterned substrates to “connect” to each other at the polymer-substrate interface, causing imperfections normal to the substrate surface. Grazingincidence small-angle neutron scattering has also been used to observe up to 8% stretching deviations in bulk morphology at a block copolymer-silicon substrate interface, which is attributed to an energy balance of conformational entropy and polymer-surface interactions.31 In the case of P(S-r-VBzAz) surface (30) (a) Ji, S.; Liao, W.; Nealey, P. F. Block Cooligomers A Generalized Approach to Controlling the Wetting Behavior of Block Copolymer Thin Films. Macromolecules 2010, 43(16), 6919–6922.(b) Suh, H. S.; Kang, H.; Nealey, P. F.; Char, K. Thickness Dependence of Neutral Parameter Windows for Perpendicularly Oriented Block Copolymer Thin Films. Macromolecules 2010, in press.(c) Suh, H.-S.; Kang, H.; Liu, C.-C.; Nealey, P. F.; Char, K. Orientation of Block Copolymer Resists on Interlayer Dielectrics with Tunable Surface Energy. Macromolecules 2010, 43(1), 461–466. (31) M€uller-Buschbaum, P.; Schulz, L.; Metwalli, E.; Moulin, J. F.; Cubitt, R. Lateral Structures of Buried Interfaces in ABA-Type Triblock Copolymer Films. Langmuir 2008, 24(15), 7639–7644.

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treatments presented herein (and the XSTs with low surface density of features), it is hypothesized that the phenomena described above occur and are driven by interactions of the block copolymer with the P(S-r-VBzAz) surface treatment. Thus, the regions of Figure 5 that do not display any obvious features of P(S-b-MMA) are attributed to block copolymer domains below the surface which are covered by a wetting layer of polystyrene due to its lower surface tension. Recently, there has been increasing interest in using perpendicular cylinders, as shown in Figure 4D, for bit-patterned data storage media. This is the next-generation hard disk drive technology in development for surpassing areal bit densities of 1 Tbit/ in.2, but the technology requires precise control of bit registration over entire wafers (long-range order).32 Both chemoepitaxy33 (chemically patterning a surface) and graphoepitaxy34 (physically patterning a surface) have been used to produce long-range ordering of block copolymer thin films,35 which is generally accomplished by having preferential interactions between each block with a different region of the pattern on the substrate. These preferential interactions reduce undesirable defects, such as grain (32) Ruiz, R.; Kang, H.; Detcheverry Francois, A.; Dobisz, E.; Kercher Dan, S.; Albrecht Thomas, R.; de Pablo Juan, J.; Nealey Paul, F. Density multiplication and improved lithography by directed block copolymer assembly. Science (Washington, DC, U.S.) 2008, 321(5891), 936–9. (33) (a) Edwards, E. W.; Montague, M. F.; Solak, H. H.; Hawker, C. J.; Nealey, P. F. Precise control over molecular dimensions of block-copolymer domains using the interfacial energy of chemically nanopatterned substrates. Adv. Mater. (Weinheim, Ger.) 2004, 16(15), 1315–1319. (b) Kim Sang, O.; Solak Harun, H.; Stoykovich Mark, P.; Ferrier Nicola, J.; De Pablo Juan, J.; Nealey Paul, F. Epitaxial selfassembly of block copolymers on lithographically defined nanopatterned substrates. Nature 2003, 424(6947), 411–4. (34) (a) Bita, I.; Yang, J. K. W.; Jung, Y. S.; Ross, C. A.; Thomas, E. L.; Berggren, K. K. Graphoepitaxy of Self-Assembled Block Copolymers on TwoDimensional Periodic Patterned Templates. Science (Washington, DC, U.S.) 2008, 321(5891), 939–943. (b) Cheng, J. Y.; Rettner, C. T.; Sanders, D. P.; Kim, H.-C.; Hinsberg, W. D. Dense self-assembly on sparse chemical patterns: rectifying and multiplying lithographic patterns using block copolymers. Adv. Mater. (Weinheim, Ger.) 2008, 20(16), 3155–3158. (35) Wan, L.; Yang, X. Directed Self-Assembly of Cylinder-Forming Block Copolymers: Prepatterning Effect on Pattern Quality and Density Multiplication Factor. Langmuir 2009, 25(21), 12408–12413. (36) Stein, G. E.; Liddle, J. A.; Aquila, A. L.; Gullikson, E. M. Measuring the Structure of Epitaxially Assembled Block Copolymer Domains with Soft X-ray Diffraction. Macromolecules 2010, 43(1), 433–441. (37) (a) Hadjichristidis, N.; Iatrou, H.; Pispas, S.; Pitsikalis, M. Anionic polymerization: high vacuum techniques. J. Polym. Sci., Part A: Polym. Chem. 2000, 38 (18), 3211–3234.(b) Hsieh, H. L.; Q, R. P. Anionic Polymerization: Principles and Practical Applications; Marcel Dekker Inc.: New York, 1996.(c) Ndoni, S.; Papadakis, C. M.; Bates, F. S.; Almdal, K. Laboratory-scale setup for anionic polymerization under inert atmosphere. Rev. Sci. Instrum. 1995, 66(2, Pt. 1), 1090–5. (d) Uhrig, D.; Mays, J. W. Experimental techniques in high-vacuum anionic polymerization. J. Polym. Sci., Part A: Polym. Chem. 2005, 43(24), 6179–6222. (38) (a) Allen, R. D.; Long, T. E.; McGrath, J. E. Preparation of high purity, anionic polymerization grade alkyl methacrylate monomers. Polym. Bull. (Heidelberg, Ger.) 1986, 15(2), 127–34. (b) Varshney, S. K.; Gao, Z.; Zhong, X. F.; Eisenberg, A. Effect of Lithium Chloride on the “Living” Polymerization of tert-Butyl Methacrylate and Polymer Microstructure Using Monofunctional Initiators. Macromolecules 1994, 27 (5), 1076–82. (39) Fetters, L. J.; Lohse, D. J.; Richter, D.; Witten, T. A.; Zirkel, A. Connection between Polymer Molecular Weight, Density, Chain Dimensions, and Melt Viscoelastic Properties. Macromolecules 1994, 27(17), 4639–47.

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boundaries, and improve the distribution of cylinder sizes observed in the block copolymer thin film by registering the position of the block copolymer cylinders with the pattern on the substrate. These methods have been applied for directing assembly of block copolymers on polystyrene surfaces36 and should be equally compatible in promoting long-range order and larger perpendicular process windows for the substituted polystyrene XSTs described herein without the need for additional modification. Furthermore, work is currently underway utilizing a combination of graphoepitaxy and P(S-r-VBzAz) surface treatments to reduce or entirely remove the defects observed at various random copolymer compositions and block copolymer thicknesses and produce long-range orientation control.

Conclusion Two sets of surface treatments were synthesized to influence the orientation of P(S-b-MMA): cross-linked surface treatments (XSTs) composed of para-substituted polystyrenes containing