Side-Chain-Grafted Random Copolymer Brushes as Neutral

After three freeze/thaw cycles, the mixture was sealed under nitrogen and heated ..... Mansky, P.; Liu, Y.; Huang, E.; Russell, T. P.; Hawker, C. Scie...
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Langmuir 2006, 22, 7855-7860

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Side-Chain-Grafted Random Copolymer Brushes as Neutral Surfaces for Controlling the Orientation of Block Copolymer Microdomains in Thin Films Insik In,† Young-Hye La,‡ Sang-Min Park,† Paul F. Nealey,‡ and Padma Gopalan*,† Departments of Materials Science and Engineering and Chemical and Biological Engineering, UniVersity of Wisconsin-Madison, Madison, Wisconsin 53706 ReceiVed March 20, 2006. In Final Form: June 8, 2006 Random copolymers of P(S-r-MMA-r-HEMA)s with a distribution of surface reactive hydroxyl groups were synthesized to formulate neutral surface layers on a SiO2 substrate. The layers were designed to drive vertical orientation of lamellar microdomains in a top P(S-b-MMA) thin film. Copolymers with a styrene weight fraction (fSt) of 0.58 and a HEMA fraction (fHEMA) ranging from 0.01 to 0.03, with a corresponding MMA fraction (fMMA) ranging from 0.41 to 0.39, in the P(S-r-MMA-r-HEMA) copolymer showed neutral surface characteristics. The morphology of block copolymer thin films was studied by scanning electron microscopy (SEM). P(S-r-MMA-r-HEMA) copolymers prepared by both living and classical free-radical polymerizations were equally effective in demonstrating the neutrality of the surface. These side-chain-grafted random copolymer brushes showed faster grafting kinetics than the end-chaingrafted P(S-r-MMA) because of multipoint attachment to the surface. The modified surfaces had a very thin layer of random copolymer brush (5-7 nm), which is desirable for effective pattern transfer. Furthermore, neutral surfaces could be obtained even when the grafting time was reduced to 3 h. These results indicate that the composition of the random copolymer brush, rather than its PDI or molecular weights, is the most important factor in controlling the neutrality of the surface. These results also demonstrate the feasibility of using a third comonomer (C) in the random copolymer brush P(A-r-B-r-C) to alter the interfacial and surface energies of a diblock copolymer (A-b-B).

Introduction Recently, block copolymer (BCP) thin films have received considerable attention because of their ability to form predetermined periodic structures via self-assembly with a high degree of registry and regularity at the sub-100-nm regime. Traditional photolithographic processes do not allow easy accessibility at these small length scales.1,2 Of particular interest are BCP thin films that produce the vertical orientation of microdomains such as cylinder or lamella phases. One of the two vertically oriented BCP microdomains can be easily removed, resulting in a patterned structure such as nanopores or nanochannels. These patterns can be efficiently utilized as nanomaterials themselves or as nanotemplates for fabricating other interesting nanostructures by pattern-transfer processes.3,4 For example, dense arrays of GaAs nanocrystals have been fabricated using sphere-forming diblock copolymers by selective etching, and cylinder-forming block copolymer films have been used to fabricate nanoporous membranes by selective etching.5,6 Although the phase behavior of a BCP is dictated by factors such as the degree of polymerization, composition, and FloryHuggins interaction parameter, the orientation of BCP thin-film morphologies is controlled by additional factors such as wetting behavior at the interfaces. Usually, strong preferential wetting of one or the other block of the BCP to the substrate drives the lamellae or cylinders to orient parallel to the plane of the film. On a neutral surface or a surface with nonpreferential wetting * Corresponding author. E-mail: [email protected]. † Department of Materials Science and Engineering. ‡ Department of Chemical and Biological Engineering. (1) Park, C.; Yoon, J.; Thomas, E. L. Polymer 2003, 44, 6725-6760. (2) Lazzari, M.; Lo´pez-Quintela, M. A. AdV. Mater. 2003, 15, 1583-1594. (3) Fasolka, M. J.; Mayes, A. M. Annu. ReV. Mater. Res. 2001, 31, 323-355. (4) Segalman, R. A. Mater. Sci. Eng. R 2005, 48, 191-226. (5) Li, R. R.; Dapkus, P. D.; Thompson, M. E. Appl. Phys. Lett. 2000, 76, 1689-1691. (6) Liu, G. J.; Ding, J. F.; Hashimoto, T. Chem. Mater. 1999, 11, 2233-2240.

for either block, the BCP thin-film morphologies can be easily driven to either vertical lamellae or vertical cylinders over large areas depending on the composition. Vertical alignment of BCP films can be accomplished by several methods such as solvent annealing, application of electric field, and surface modification to tune the interaction between the BCP and substrate.7-10 Among these approaches, the concept of surface modification is rather promising as it does not require the control of any external factors such as electric fields or the rate of evaporation of the solvent.7,11-13 Homogeneous or mixed self-assembled monolayers (SAMs), end-grafted random copolymer brushes, and cross-linked random copolymer brushes have been used for surface modification. For example, SAMs of alkylsiloxane on a Si/SiO2 substrate were used to control the wetting behavior of BCP thin films.14,15 Varying the grafting density of the SAMs resulted in variation of the surface from asymmetric to neutral to dewetting. Subsequently, a combination of both top-down and bottom-up approaches was reported. A 60-nm period was used to guide microphase separation. SAMs of alkyl siloxane were employed as imaging layers and chemically patterned by EUV lithography to give the same period as the (7) Mansky, P.; Liu, Y.; Huang, E.; Russell, T. P.; Hawker, C. Science 1997, 275, 1458-1460. (8) Lin, Z.; Kim, D. H.; Wu, X.; Boosahda, L.; Stone, D.; Larose, L.; Russell, T. P. AdV. Mater. 2002, 14, 1373-1376. (9) Morkved, T. L.; Lu, M.; Urbas, A. M.; Ehrichs, E. E.; Jaeger, H. M.; Mansky, P. Russell, T. P. Science 1996, 273, 931-933. (10) Park, C.; De Rosa, C.; Lotz, B,; Fetters, L. J.; Thomas, E. L. AdV. Mater. 2001, 13, 724-728. (11) Mansky, P.; Russel, T. P.; Hawker, C. J.; Pitsikalis, M.; Mays, J. Macromolecules 1997, 30, 6810-6813. (12) Huang, E.; Russell, T. P.; Harrison, C.; Chaikin, P. M.; Register, R. A.; Hawker, C. J.; Mays, J. Macromolecules 1998, 31, 7641-7650. (13) Ryu, D. Y.; Shin, K.; Drockenmuller, E.; Hawker, C. J.; Russell, T. P. Science 2005, 308, 236-239. (14) Peters R. D.; Yang, X. M.; Kim, T. K.; Nealey, P. F. Langmuir 2000, 16, 9620-9626. (15) Peters, R. D.; Yang, X. M.; Kim, T. K.; Nealey, P. F. Langmuir 2000, 16, 4625-4631.

10.1021/la060748g CCC: $33.50 © 2006 American Chemical Society Published on Web 08/01/2006

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Figure 1. PS/PMMA random copolymer brushes: (a) end-hydroxy-functionalized random copolymer, (b) side-chain hydroxy-containing random copolymer.

bulk lamellar domain period of the BCP, resulting in perfect long-range alignment of lamellar domains.16 A SAM-based approach allows further modification of the treated substrate by exposure to X-ray or UV radiation to alter the wettability and hence create a chemical pattern. The morphology of the SAM and hence the conditions for SAM formation dictate the reproducibility of this method. Random copolymer brushes can also be used for surface modification to control the wetting behavior of BCP films. The interfacial energies of the poly(styrene) (PS) and poly(methyl methacrylate) (PMMA) blocks of a BCP with the substrate can be carefully balanced by end grafting a random copolymer of P(S-r-MMA) onto the substrate. The composition of the copolymer and hence the interfacial energies can be easily tuned by synthesis. Previous studies have showed that a P(S-r-MMA) having a styrene volume fraction, fSt, of 0.58 results in the necessary neutrality for the vertical orientation of lamellar or cylindrical domains.7 End grafting or covalent grafting of the random copolymer brush to the substrate leads to a kinetically stable neutral surface, as it prevents diffusion of the brush layer onto the top BCP film during the spin coating or thermal annealing process. Covalent grafting is usually achieved through the dehydration reaction between end-hydroxy-functionalized P(Sr-MMA) and the native oxide layer of silicon wafers under vacuum at 140 °C for more than 48 h. The end-hydroxyfunctionalized P(S-r-MMA) is synthesized by the nitroxidemediated controlled radical polymerization of styrene (St) and methyl methacrylate (MMA) employing a hydroxy-functionalized unimer-type initiator.17 Recently, Hawker et al. reported an elegant chemistry-based approach to a substrate-independent neutral surface.13 Random copolymer brushes containing a thermally cross-linkable group placed along the backbone were used to obtain an insoluble thin film constituting the neutral surface. The random copolymer consisted of styrene, methyl methacrylate, and benzocyclobutene (BCB). The BCB units along the backbone undergo a cross-linking reaction at 200 °C to create an insoluble film. The lowest temperature at which BCB units can be crosslinked was found to be 150 °C. Surfaces modified with the right composition of P(S-r-MMA-r-BCB) were shown to be effective neutral surfaces for P(S-b-MMA) block copolymers. Here, we present a simple strategy for formulating a neutral surface to obtain vertically oriented domains in a symmetric P(S-b-MMA) thin film. Instead of end grafting, we explored side-chain grafting of the random copolymer brush to the substrate, (16) Kim, S. O.; Solak, H. H.; Stoykovich, M. P.; Nealey, P. F. Nature 2003, 424, 411-414. (17) Hawker, C. J.; Barclay, G. G.; Orellana, A.; Dao, J.; Devonport, W. Macromolecules 1996, 29, 5245-5254.

which is silicon in this study (Figure 1). In addition to ease of synthesis, we also anticipated faster binding kinetics because of multipoint attachment of the brushes to the substrate. Hydroxy groups were distributed along the polymer side chain by incorporating a small amount of the hydroxy-containing comonomer 2-hydroxyethyl methacrylate (HEMA) during the polymerization to synthesize P(S-r-MMA-r-HEMA) brushes. We first explored the use of controlled radical polymerization (CRP) with an alkoxyamine-based unimer to synthesize these P(S-r-MMAr-HEMA) random copolymer brushes for creating neutral surfaces and then examined the effect of the polydispersity of the copolymers on the neutrality of the surface. For the latter investigation, we synthesized a series of equivalent random copolymer brushes by conventional polymerization using R,R′azoisobutyronitile (AIBN) as the initiator. Our studies so far confirm faster binding kinetics of P(S-r-MMA-r-HEMA) to the SiO2 substrate compared to the end-hydroxyl-containing P(Sr-MMA) copolymer. Highly polydisperse samples synthesized by conventional polymerization were also effective in generating neutral surfaces. These results open up a versatile synthetic route via conventional polymerization to the formation of neutral surfaces that can be easily extended to block copolymer systems with a wide range of functionality and chemistry. Experimental Section Materials. Styrene (St), methyl methacrylate (MMA), and 2-hydroxyethyl methacrylate (HEMA) were purified by column chromatography using basic alumina under a nitrogen atmosphere. Alkoxyamine-based unimer (1) was synthesized according to published literature reports.18 R,R′-Azoisobutyronitile (AIBN) was purified by recrystallization in benzene. A symmetric poly(styrene)block-poly(methyl methacrylate) [P(S-b-MMA)] diblock copolymer (Mw, 104 kg/mol; fSt, ∼0.5; PDI, 1.05) was purchased from Polymer Source Incorporation and used without further purification. Synthesis of P(S-r-MMA-r-HEMA) by Nitroxide-Mediated Controlled Radical Polymerization. All random copolymer samples were synthesized by CRP following similar procedures, and one such representative polymerization procedure is detailed below for sample PH1. A mixture of 1 (0.0545 g, 0.167 mmol), St (3.350 g, 32.16 mmol), MMA (2.396 g, 21.76 mmol), and HEMA (0.0611 g, 0.470 mmol) was degassed by three freeze/thaw cycles and sealed under nitrogen. The polymerization mixture was heated at 120 °C for 32 h. The resulting highly viscous mixture was diluted with 25 mL of dichloromethane and precipitated into 500 mL of methanol. The precipitated solid was filtered and dried under reduced pressure to give the random copolymer PH1 as a white solid (4.76 g, 81.0% yield), Mn ) 33 000, PDI ) 1.29. The fraction of styrene determined by NMR spectroscopy was 0.58. (18) Benoit, D.; Chaplinski, V.; Braslau, R.; Hawker, C. J. J. Am. Chem. Soc. 1999, 121, 3904-3920.

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Scheme 1. Synthesis of Random Copolymer Brushes: (a) Nitroxide-Mediated Living Free-Radical Polymerization and (b) Classical Free-Radical Polymerization

Synthesis of P(S-r-MMA-r-HEMA) by Classical Free-Radical Polymerization. All random copolymer samples were synthesized by classical free-radical polymerization following similar procedures, and one such representative polymerization procedure is detailed below for sample PH′1. A mixture of AIBN (0.1237 g, 0.7531 mmol), St (1.4295 g, 13.73 mmol), MMA (1.0184 g, 10.17 mmol), and HEMA (0.0255 g, 0.196 mmol) was dissolved in 10 mL of toluene. After three freeze/thaw cycles, the mixture was sealed under nitrogen and heated at 80 °C for 72 h. The resulting viscous polymerization mixture was diluted with 20 mL of dichloromethane and precipitated into 300 mL of methanol. The precipitated solid was filtered and dried under reduced pressure to give the random copolymer PH′1 as a white solid (1.51 g, 61%), Mn ) 20 000, Mw/ Mn ) 3.72. The fraction of styrene determined by NMR spectroscopy was 0.58. Substrate Preparation. Silicon(100) wafers were cut into 1.5cm2 pieces (or used without cutting) and cleaned in a toluene bath under sonication for 3 min. The wafer pieces were then washed with toluene, acetone, and ethanol and dried in a stream of nitrogen. After these precleaning steps, two procedures were used for cleaning the silicon substrate. In piranha cleaning, the substrates were treated with a mixture of H2O2 (30%) and H2SO4 (70%) (v/v) at 80 °C for 30 min (CAUTION! Piranha solution can cause explosion upon contact with organic material!) and then rinsed with distilled water followed by ethanol and dried in a stream of nitrogen. In UV/O3 cleaning, after the precleaning steps, the samples were oxidized in a UV/O3 chamber for 15 min and used immediately. Both cleaning methods produced native oxide layers having a thickness of about 1.6 nm. Covalent Grafting of the Brush and Block Copolymers for Film Preparation. Random copolymer brush layers were generated by spin-coating of films of P(S-r-MMA-r-HEMA) solution (PH1PH5 or PH′1/PH′2) onto clean silicon wafers. Spin-coating of 1.0 wt % solution in toluene at 4000 rpm gave 35-40-nm-thick films. The samples were then annealed in a vacuum oven at 140 °C for various time intervals and subsequently quenched to room temperature. During this annealing process, the hydroxy groups of the random copolymer brush covalently bound to the native oxide layer of the silicon wafer through a dehydration reaction. The unbound random copolymer molecules were thoroughly removed by repeated (three times) sonication in hot toluene for 3 min. The thicknesses of the remaining brush layers were measured by ellipsometry. On these covalently anchored brush layers, P(S-b-MMA) was cast from 1.5 wt % toluene solution at 4000 rpm, giving 40-nm-thick BCP films. These samples were then annealed in a vacuum oven at 190 °C for 72 h and subsequently quenched to room temperature. Characterization. 1H NMR spectra were recorded in solution with a Bruker AC+ 300 (300-MHz) spectrometer, with the tetramethyl silane (TMS) proton signal as an internal standard. The molecular weights of the polymers were measured by size-exclusion chromatography, which was carried out on a Waters chromatograph connected to a Waters 410 differential refractometer using tetrahydrofuran (THF) as the eluent with poly(styrene) standards. The film thicknesses of the brush layers and block copolymer layers were measured with a Rudolph Research ellipsometer using a helium-

Table 1. Properties of Random Copolymer Brushes polym polymer methoda fStb fMMAb fHEMAb PH1 PH2 PH3 PH4 PH5 PH′1 PH′2

living living living living living classical classical

0.58 0.58 0.58 0.58 0.58 0.58 0.58

0.41 0.40 0.39 0.38 0.37 0.41 0.40

0.01 0.02 0.03 0.04 0.05 0.01 0.02

Mn

BCP Mw/Mn morphologyc

32 900 28 000 30 000 30 000 33 400 20 000 18 000

1.29 1.21 1.20 1.23 1.25 3.72 1.90

vertical vertical vertical mixed parallel vertical vertical

a Method of polymerization of random copolymer brushes (living ) nitroxide-mediated living free-radical polymerization, classical ) classical free-radical polymerization by AIBN). b Mole fraction of each monomer in the random copolymer brushes. c From plan-view SEM images of lamella forming symmetric P(S-b-MMA) over silicon substrates modified with random copolymer brushes.

neon laser (λ ) 633 nm). Scanning electron microscope (SEM) images of the block copolymer thin films were obtained with a LEO 1550 VP field-emission SEM. The surface topography of the covalently attached brush thin films was imaged using a Nanoscope III Multimode atomic force microscope (Digital Instruments) in contact mode. A triangular cantilever with an integral pyramidal Si3N4 tip was used. The typical imaging force was on the order of 10-9 N.

Result and Discussion Synthesis of Hydroxy-Containing Random Copolymer Brushes. P(S-r-MMA-r-HEMA) copolymers with a distribution of hydroxyl groups along the chain were first synthesized by nitroxide-mediated living free-radical polymerization (NMP) (Scheme 1). In this system, the covalent binding of the random copolymers to the substrate is through grafting of the hydroxyl side chains rather than end grafting via the hydroxyl-functionalized initiator as reported earlier. We synthesized a series of P(S-r-MMA-r-HEMA) copolymers(PH1-PH5) through the NMP of St, MMA, and HEMA as monomers at 120 °C. It is well-known that the actual composition of a P(S-r-MMA) copolymer closely matches the feed ratio of the monomers, hence giving greater control over the copolymer composition.7 Previous studies have confirmed that end-hydroxyfunctionalized P(S-r-MMA) with fSt ) 0.58 is known to show nonpreferential or neutral wetting behavior to P(S-b-MMA). This neutral wetting results in vertically oriented lamellar or cylindrical morphologies depending on the block copolymer composition. Hence, we fixed fSt at 0.58 in the P(S-r-MMA-r-HEMA) copolymer and varied the HEMA fraction (fHEMA) from 0.01 (PH1) to 0.05 (PH5); accordingly, the MMA fraction (fMMA) was varied from 0.41 (PH1) to 0.37 (PH5) (Table 1). The alkoxyamine-based unimer 1 was used as an initiator.18 The number-average molecular weights of the resulting random

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Figure 2. Remaining thicknesses of random copolymer thin-film brushes after thermal binding for 48 h and subsequent washing in toluene.

copolymers were about 30 000 g/mol, and the polydispersity indexes (PDIs) of the polymers were around 1.2. All polymerization produced random copolymer brushes with yields of more than 80%. To examine the effect of the polydispersity of the random copolymer brushes on the neutrality of the surfaces, we synthesized a comparable series (PH′ series) of copolymers through classical free-radical polymerization using a traditional initiator, AIBN. The yields of these classical radical polymerizations were less than those of the living radical polymerizations, possibly because of termination reactions occurring before full conversion of the monomers could be achieved. Here again, the copolymer compositions were very close to the feed composition. PH′1 with fHEMA ) 0.01 and PH′2 with fHEMA ) 0.02 were synthesized at a constant styrene fraction of 0.58. The numberaverage molecular weights of both polymers were more than 18 000 g/mol, and the PDIs were higher than 1.9. Covalent Grafting over the Silicon Wafer. The hydroxycontaining random copolymer brushes were spin-cast over clean silicon wafers. Covalent grafting of these brushes over the native oxide layers of silicon wafers was achieved by annealing these samples at 140 °C under reduced pressure for various times. The grafting behavior of these hydroxy-containing random copolymer brushes was monitored by measuring the residual brush thickness after successive washings. The thicknesses of the grafted brush layers were between 5 and 7 nm. Among random copolymer brushes PH1-PH5, PH1, with the lowest amount of hydroxy groups (fHEMA ) 0.01), had the highest brush thickness (Figure 2). Increasing fHEMA strongly decreased the residual brush thickness, indicating multivalent grafting of the copolymer on the substrate. As the molecular weights of PH1-PH5 are in the same range, the only difference is the fraction of HEMA, i.e., the number of hydroxy groups in one polymer brush. The number of hydroxy groups in PH5 (fHEMA ) 0.05), which has the highest value of fHEMA, is 13, and therefore, multiple grafting reactions are anticipated in this case in contrast to PH1 (fHEMA ) 0.01), which has three hydroxy groups, resulting in the thinnest grafted brush layer for PH5. However, within experimental error, the resulting thicknesses for PH4 and PH5 were similar. The modified surfaces have a very thin layer (5-7 nm) of random copolymer brush, which is desirable for effective pattern transfer. The surface roughnesses for both the side-chain-grafted and end-hydroxygrafted systems determined by AFM were around 0.19 nm. These data are included in the Supporting Information. To compare the grafting kinetics of the random copolymer brushes as a function of composition, we normalized the brush thicknesses after various annealing times to the brush thickness

Figure 3. Normalized thicknesses of random copolymer brushes as a function of time. (t24 ) the remaining thickness of each random copolymer brush after thermal binding for 24 h and subsequent washing.)

obtained after annealing each sample for 24 h (Figure 3). It is clear from Figure 3 that PH1 shows the fastest binding tendency. In just 3 h, the brush thickness of PH1 is about 6 nm, which is about 77% of the brush thickness after 24 h of annealing, i.e., more than 77% binding is achieved in less than 3 h. The endhydroxy-terminated random copolymer, with a much lower molecular weight of 10 000, showed comparable binding (74%) after 24 h at 140 °C.7 As PH1 has a larger number of hydroxy groups than the end-hydroxy-terminated random copolymer, the covalent bonding between the substrate and the brush is faster and shows neutrality after a shorter binding time. However, a further increase in fHEMA (PH2-PH5) decreases the grafting rate of these brushes. At first, this seems counterintuitive; however, given the fact that PH5, having the highest value of fHEMA, is most likely to form the greatest number of covalent anchors (multiple grafting) to the surface, it is likely to screen further covalent binding of additional brush chains to the surface. Neutral Surface Formation. To check the neutral or nonpreferential wetting of these hydroxy-containing random copolymer brush layers, a symmetric P(S-b-MMA) copolymer was spin-coated onto the grafted brush layers. P(S-b-MMA) [104 kg mol-1; L0 (bulk), ∼48 nm; fSt, ∼0.5] was spin-coated on PH1-PH5 grafted (140 °C, 48 h) brush layers and annealed under reduced pressure at 190 °C for 72 h. The thickness of the P(S-b-MMA) thin films was about 40 nm. The resulting domain structures of the P(S-b-MMA) thin films were imaged using scanning electron microscopy (SEM). BCP thin films annealed on PH1, PH2, and PH3 brush layers showed perfect lamellar structures oriented perpendicularly to the substrate as observed in the SEM images (Figure 4). One concern in designing these random copolymer brushes was the high polarity of HEMA compared to MMA, which could essentially alter the interfacial energy between the native oxide substrate and the BCP film and hence the expected neutrality of the surface. However, covalent grafting of the brush through a dehydration reaction between the silanol and HEMA hydroxyl groups should result in an ether bond with a polarity close to that of MMA units in the copolymer. Therefore, PH1-PH3 showed neutrality to P(S-b-MMA) BCP thin films. As fHEMA increased further, PH4 showed partial neutrality (mixed parallel and perpendicular orientations), and PH5 showed preferential wetting behavior (hole morphology) of one of the two blocks. Although the explanation is speculative

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Figure 5. Plan-view SEM images of symmetric P(S-b-MMA) diblock copolymer on silicon substrates modified with (a) PH1 and (b) PH2 (brush binding time ) 3 h at 140 °C).

Figure 4. Plan-view SEM images of symmetric P(S-b-MMA) diblock copolymer on silicon substrates modified with (a) PH1, (b) PH2, (c) PH3, and (d) PH5 (brush binding time ) 48 h at 140 °C).

at this point, this is most likely due to the residual hydroxy groups in these random copolymer brushes. Quantification of these residual hydroxy groups is experimentally challenging, as there are very few of them in the copolymer. AFM study of the BCP thin film (∼40 nm) over the PH5 grafted brush layer clearly showed the depth profile (∼24 nm) of hole regions, confirming the hole morphology and the preferential wetting to the PS block. The BCP thin films annealed on the PH4 grafted brush layer showed both vertical and parallel morphologies because of the intermediate wetting behavior of PH4 for P(S-b-MMA). These results also open up the possibility of using a third monomer C (HEMA) to alter the interfacial and surface energies of a diblock copolymer P(A-b-B) [P(S-b-MMA)]. At this stage, it is clear from these results that multivalent attachment via the P(S-r-MMA-r-HEMA) copolymers to the silicon surface shows faster kinetics than that of the end-hydroxyterminated P(S-r-MMA) system reported earlier. However, the role of the polydispersity of the grafted brushes on the neutrality of the surface has not yet been addressed in the literature. Hence, we synthesized two hydroxy-containing random copolymer brush layers by covalent anchoring of copolymers synthesized by classical free-radical polymerization, PH′1 and PH′2. BCP thin films annealed over both PH′1 and PH′2 grafted brush layers showed vertical lamella morphologies. These results for PH′1 and PH′2 suggest that the polydispersity of hydroxy-containing random copolymer brushes is not a critical parameter in the formation of neutral surfaces. We are currently examining the density of defect structures, such as block copolymer pinning to the substrate, to evaluate the detailed effect of PDI. From these results, it appears that the most important parameter for creating neutral surfaces is the control of interfacial energy by fine-tuning of the copolymer composition. In living free-radical polymerizations such as ATRP (atom-transfer radical polymerization) and NMP, control of the composition of the copolymer is possible only through the control of initial comonomer feed ratio. The only difference is that the propagating radicals are not “associated” but are “free” in classical free-radical polymerization; however, the composition is still dictated by the feed ratio of the monomer. Therefore, both copolymers from either one of the two polymerization methods should have the same composition as long as the feed ratio is the same.19 The lower polymerization temperature in classical free-radical polymerization might induce a change in the composition of the resulting block copolymer. Because the relative reactivity ratio of different monomers is not temperature-sensitive, the resulting composition of the random copolymers is usually unaffected. (19) Hawker, C. J.; Bosman, A. W.; Harth, E. Chem. ReV. 2001, 101, 36613688.

Figure 6. Plan-view SEM images of the arranged lamella morphologies of symmetric P(S-b-MMA) around the defect sites having (a) C4, (b) C3, and (c) C2 symmetries over silicon substrate modified with PH2 (brush binding time ) 48 h at 140 °C).

Faster Neutral Surface Formation. As discussed earlier, the hydroxy-containing random copolymer brushes showed relatively faster binding kinetics, which effectively decreases the grafting time needed to create these neutral surfaces. Both PH1 and PH2 showed “neutrality” after just 3 h of annealing at 140 °C (Figure 5). This is rather promising if we consider the fact that both PH1 and PH2 have much higher molecular weights (Mn ≈ 30 000) than the previously reported end-hydroxy-functionalized random copolymer brushes (Mn ≈ 8000-10 000), which require longer grafting times (>40 h) at the same temperature.7 Recently, one report on optimization of the BCP thin-film annealing time over random copolymer-based neutral brushes has been published in the literature.20 Combined with our strategy of creating neutral surfaces, the overall time to obtain a vertical orientation of P(Sb-MMA) thin films can be shortened to 6 h. Arrangement of Vertical Lamellae. In the course of this study, we observed some interesting defect patterns in some of the SEM images, as shown in Figure 6. These structures typically resulted when the wafer cleaning process was altered. For these samples, instead of the piranha cleaning, we used a UV/ozone treatment. The UV/ozone treatment was not as effective in removing some of the silicon wafer particles resulting from the cutting process. When the random copolymer brush PH2 was spin-cast for grafting on a silicon wafer (containing these small pieces of wafer particles), the SEM image of the annealed P(Sb-MMA) thin film on this brush layer showed several defect sites along with the vertically aligned lamellar region. The size of these defect regions varied from 100 to 500 nm. Interestingly, near the defect boundary, we also observed nearly perfect alignment of the vertical lamellar domains. As random copolymer brushes are not bound to the regions where the wafer dust remains, these spots contain native oxide, which shows preferential wetting for the MMA block. These bare regions promote parallel alignment of the lamellar morphology. These defect regions with parallel lamellae or alternation of PS and PMMA layers also present a second neutral surface to the adjacent vertically oriented lamellae. The resulting neutrality over two surfaces most likely induces the observed additional alignment of vertically orientated sheets of lamellae near the defect sites. (20) Guarini, K. W.; Black, C. T.; Yeung, S. H. I. AdV. Mater. 2002, 14, 1290-1294.

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The symmetry of the orientation pattern of the lamellae near the defect boundary follows the symmetry of the defect site itself. Defect sites having C2, C3, and C4 symmetries induced lamellar arrangements having the corresponding symmetries (Figure 6). In addition to the control of the vertical orientation of the BCP thin films, this type of symmetry control might be very useful for developing specific nanodevices or nanomaterials based on BCP thin films. Patterned neutral surfaces having specific symmetries can be used to make vertically oriented and aligned BCP thin-film microphases.21

Conclusion We have demonstrated a new type of polymer brush containing a distribution of hydroxy groups on the side chain, for covalent anchoring on native silicon oxide to create neutral surfaces. Random copolymer brushes, P(S-r-MMA-r-HEMA), were synthesized to create neutral surfaces for achieving vertically oriented lamellar microdomains of P(S-b-MMA) BCP thin films. Copolymers with styrene fractions, fSt, of 0.58 and HEMA fractions (fHEMA) ranging from 0.01 to 0.03, with a corresponding (21) Bockstaller, M. R.; Mickiewicz, R. A.; Thomas, E. L. AdV. Mater. 2005, 17, 1331-1349.

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range of MMA fractions (fMMA) from 0.41 to 0.39, in the P(Sr-MMA-r-HEMA) copolymer showed neutral surface characteristics. An equivalent series of copolymers synthesized by conventional free-radical polymerization were equally effective in generating neutral surface layers. The main advantages of side-chain anchoring of random copolymer brushes to alter the wetting behavior of the surface are (a) ease of synthesis; (b) faster binding kinetics; (c) ability to use a third monomer (C) to alter the interfacial and surface energies of a diblock copolymer (A-b-B); and (d) multipoint attachment chemistry leading to very thin (5-7 nm) modified surfaces, which are desirable for effective pattern transfer. Acknowledgment. This work was supported by the Semiconductor Research Corporation and the UW-NSF Nanoscale Science and Engineering Center (DMR-0425880). Supporting Information Available: Large-area plan-view SEM image of Figure 6 and AFM images of silicon substrates modified with a end-hydroxy-terminated random copolymer brush or side-chain-grafted random copolymers. This material is available free of charge via the Internet at http://pubs.acs.org. LA060748G