Wetting Behavior of Block Copolymers on Self-Assembled Films of

The laterally alternating domains of PS and PMMA confirm a perpendicular .... Russell, T. P.; Coulon, G.; Deline, V. R.; Miller, D. C. Macromolecules ...
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Wetting Behavior of Block Copolymers on Self-Assembled Films of Alkylchlorosiloxanes: Effect of Grafting Density Richard D. Peters, Xiao M. Yang, Tae K. Kim, and Paul F. Nealey* Department of Chemical Engineering and Center for Nanotechnology, University of Wisconsin, Madison, WI 53706 Received June 14, 2000. In Final Form: September 5, 2000

The wetting behavior of thin films of symmetric poly(styrene-b-methyl methacrylate) was investigated on self-assembled (SA) films of octadecyltrichlorosilane on Si/SiOx with varying grafting density. The different types of SA organic films were produced by changing the deposition times (t) in solution. Two types of wetting behavior were observed on partial self-assembled monolayers (SAMs); 8 Å < thickness < 23 Å): (1) for short deposition times with t ) 1-10 h, asymmetric wetting with the poly(methyl methacrylate) block wetting the substrate, and (2) for t ) 12 h, neutral wetting with lamellae oriented perpendicular to the substrate. For t ) 21-24 h, the polymer films dewet complete SAMs (thickness ) 26 Å (the theoretical thickness for a complete SAM)). A metastable morphology of symmetric wetting with the polystyrene block wetting the substrate was observed on SA films with long deposition times, greater than 30 h (28 Å < thickness < 36 Å). The different wetting behavior of the block copolymer films on the different types of SA films was determined to depend on the chemistry and structure of the surface. These results have important implications regarding the use of SAMs of alkylsiloxanes to control the wetting behavior of block copolymers for patterning applications.

Introduction The wetting behavior of polymers on self-assembled monolayers (SAMs) is of technological interest for a number of micro- and nanofabrication strategies involving the self-assembly of block copolymers and polymer blends. Surfaces are typically patterned with SAMs of different chemical functionality, and transfer of the pattern into a polymer layer occurs by controlled wetting. In this paper, we demonstrate that the structure of SAMs of alkylsiloxanes on Si/SiOx, in addition to chemical functionality, plays an important role in determining the wetting behavior of polymer films. Understanding these effects is critical for exploiting SAMs of alkylsiloxanes for applications involving controlled wetting of surfaces. The wetting behavior of block copolymer films has been shown previously to depend on the chemistry of the substrate.1,2 To tune the wetting behavior of block copolymers of poly(styrene-b-methyl methacrylate) (P(Sb-MMA)) to achieve symmetric, neutral, and asymmetric wetting, Mansky et al. used random copolymer brushes of poly(styrene-r-methyl methacrylate), with varying chemical compositions that were grafted to the substrate.3 We have previously investigated the use of SAMs of alkylsiloxanes on Si/SiOx to regulate the wetting behavior of thin films of diblock copolymers.4 The surface chemistry of the SAMs was modified by exposure to X-rays in air to incorporate oxygen into polar functional groups on the surface of the SAMs.5 Thin films of P(S-b-MMA) on these * To whom correspondence should be addressed. E-mail: nealey@ engr.wisc.edu. (1) Russell, T. P.; Coulon, G.; Deline, V. R.; Miller, D. C. Macromolecules 1989, 22, 4600. (2) Coulon, G.; Russell, T. P.; Deline, V. R.; Green, P. F. Macromolecules 1989, 22, 2581. (3) Mansky, P.; Russell, T. P.; Hawker, C. J.; Pitsikalis, M.; Mays, J. Macromolecules 1997, 30, 6810. (4) Peters, R. D.; Yang, X. M.; Kim, T. K.; Sohn, B. H.; Nealey, P. F. Langmuir 2000, 16, 4625. (5) Kim, T. K.; Yang, X. M.; Peters, R. D.; Sohn, B. H.; Nealey, P. F. J. Phys. Chem. B 2000, 104, 7403.

surfaces exhibited dewetting and symmetric, neutral, and asymmetric wetting with increasing exposure dose. The interfacial energies between the two blocks of the copolymer and the SAM-covered surfaces were estimated using the Fowkes-van Oss-Chaudhury-Good (FOCG) model of surface tension, and the wetting behavior was directly correlated to the interfacial energy. This system was used to create chemically nanopatterned surfaces to guide the orientation and macroscopic order of the self-assembled structure in thin films of P(S-b-MMA).6,7 A number of groups have used SAMs of alkanethiols on gold to control polymer wetting. Genzer et al.8 investigated the wetting of binary polymer blends on mixed SAMs of CH3- and COOH-terminated alkanethiols on gold and observed a wetting reversal transition as the composition of the SAM changed. Nisato et al.9 and Karim et al.10 patterned striped SAMs of CH3- and COOH-terminated alkanethiols on gold, and Boltau et al.11 patterned stripes of gold and SAMs of CH3-terminated alkanethiols on gold to direct the phase separation of polymer blends on the patterned surfaces. In these experiments, the wetting behavior of polymer was controlled by the functionality, or surface chemistry, of the SAMs. Heier et al. used striped SAMs of CH3- and HO-terminated alkanethiols on gold and demonstrated that the surface pattern could be transferred to films of poly(styrene-b-2-vinylpyridine) (P(S-b-2VP)).12-15 The pattern of the copolymer film (6) Yang, X. M.; Peters, R. D.; Nealey, P. F. Macromolecules, submitted for publication, 2000. (7) Peters, R. D.; Yang, X. M.; Wang, Q.; de Pablo, J. J.; Nealey, P. F. J. Vac. Sci. Technol. B, in press. (8) Genzer, J.; Kramer, E. J. Phys. Rev. Lett. 1997, 78, 4946. (9) Nisato, G.; Ermi, B.; Douglas, J. F.; Karim, A. Macromolecules 1999, 32, 2356. (10) Karim, A.; Douglas, J. F.; Lee, B. P.; Glotzer, S. C.; Rogers, J. A.; Jackman, R. J.; Amis, E. J.; Whitesides, G. M. Phys. Rev. E 1998, 57, R6273. (11) Boltau, M.; Walheim, S.; Mlynek, J.; Krausch, G.; Steiner, U. Nature 1998, 391, 877. (12) Heier, J.; Kramer, E. J.; Walheim, S.; Krausch, G. Macromolecules 1997, 30, 6610.

10.1021/la000822+ CCC: $19.00 © 2000 American Chemical Society Published on Web 10/21/2000

Wetting Behavior of Block Copolymers

consisted of different surface topography over adjacent regions of the patterned SAM. On top of the CH3terminated SAMs, the lamellae were oriented primarily perpendicular to the substrate, indicative of a neutral substrate with respect to the diblock copolymer. This neutral wetting behavior was different than the expected parallel orientation of lamellae with the polystyrene (PS) block wetting the substrate. Heier et al. concluded that the neutral wetting was due to the structure of the CH3terminated SAMs and that defects in the monolayer film allowed the poly(2-vinylpyridine) (PVP) block to interact with the Au layer.13 Disadvantages of using SAMs of alkanethiols on Au for patterning applications include thermal instability and incompatibility with silicon processing. Thermal instability may be problematic because the polymer systems must often be annealed at high temperatures to increase mobility and induce self-assembly. Delamarche et al.16 reported that SAMs of alkanethiols on Au had limited thermal stability at 100 °C. During annealing of block copolymer films, however, Heier et al.13 reported that desorption of alkanethiols on Au at high temperatures occurs after the establishment of well-ordered structures. SAMs of alkylsiloxanes on Si/SiOx have two major advantages over SAMs of alkanethiols on Au: (1) they are thermally stable under vacuum up to temperatures of 740 K17 and are thermally stable when annealed under a polymeric confining layer,18 and (2) they are compatible with standard silicon processing in the microelectronics industry. The disadvantage of using SAMs of alkylsiloxanes compared to alkanethiols on Au is the difficulty in reproducibly depositing the organic films.19-22 Small changes in humidity and temperature, for example, can result in deposition of films with completely different structures. The general objective of this paper was to quantify the difference in wetting behavior of block copolymers on selfassembled (SA) films of alkylsiloxanes with varying grafting density (structure). A specific objective related to patterning applications was to identify deposition conditions of SA films of alkylsiloxanes that result in the desired wetting behavior of a block copolymer film. For patterning applications using SAMs of octadecyltrichlorosilane (OTS) exposed to X-rays and P(S-b-MMA), for example, the desired wetting behavior is preferential wetting of the PS block on unexposed regions of SAMs of OTS and preferential wetting of the poly(methyl methacrylate) (PMMA) block on exposed regions of SAMs of OTS. In this paper, we demonstrate that the PMMA block preferentially wets the substrate on partial SAMs with thicknesses < 17 Å (deposition time (t) < 12 h), neutral wetting with perpendicular lamellae occurs on partial (13) Heier, J.; Genzer, J.; Kramer, E. J.; Bates, F. S.; Walheim, S.; Krausch, G. J. Chem. Phys. 1999, 111, 11101. (14) Heier, J.; Sivaniah, E.; Kramer, E. J. Macromolecules 1999, 32, 9007. (15) Heier, J.; Kramer, E. J.; Groenewold, J.; Fredrickson, G. H. Macromolecules 2000, 33, 6060. (16) Delamarche, E.; Michel, B.; Kang, H.; Gerber, C. Langmuir 1994, 10, 4103. (17) Kluth, G. J.; Sung, M. M.; Maboudian, R. Langmuir 1997, 13, 3775. (18) Calistri-Yeh, M.; Kramer, E. J.; Sharma, R.; Zhao, W.; Rafailovich, M. H.; Sokolov, J.; Brock, J. D. Langmuir 1996, 12, 2747. (19) Silberzan, P.; Leger, L.; Aussere, D.; Benattar, J. J. Langmuir 1991, 7, 1647. (20) Brzoska, J. B.; Shahidzadeh, N.; Rondelez, F. Nature 1992, 360, 719. (21) Brzoska, J. B.; Ben Azouz, I.; Rondelez, F. Langmuir 1994, 10, 4367. (22) Parikh, A. N.; Allara, D. L.; Ben Azouz, I.; Rondelez, F. J. Phys. Chem. 1994, 98, 7577.

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SAMs with thickness ) 17.4 Å (t ) 12 h), dewetting occurs on complete SAMs with thickness ) 26 Å (t ) 21-24 h), and the PS block preferentially wets the substrate for SAMs with thicknesses > 26 Å (t > 30 h) for annealing times of ∼ 24 h. Experimental Section Materials. Polished test-grade silicon 〈100〉 wafers were purchased from Tygh Silicon. Octadecyltrichlorosilane (CH3(CH2)17SiCl3, 95%) was purchased from Gelest and was used as received. Symmetric P(S-b-MMA) was purchased from Polymer Source, Inc. The number average molar mass was 51,200 g/mol, the polydispersity was 1.06, the styrene volume fraction was 0.48, and the bulk lamellar period (Lo) was 30 nm. Toluene (99.8%, anhydrous), chloroform (99+%, anhydrous), and poly(acrylic acid) (25 wt % solution in water) were purchased from Aldrich and were used without further purification. Ethanol (dehydrated, 200 proof) was purchased from Aaper Alcohol and Chemical Co. and was used as received. Ruthenium tetroxide (0.5% aqueous solution) was purchased from Polysciences, Inc. Deposition of Self-Assembled Organic Films. Silicon wafers were cleaved into pieces approximately 2 cm × 2 cm and were cleaned by immersion in a piranha solution (7/3 (v/v) of 98% H2SO4/30% H2O2; Caution! Piranha solution reacts violently with organic compounds and should not be stored in closed containers) at 90 °C for 30 min. The silicon pieces were immediately rinsed with deionized water (resistivity g 18 MΩ/ cm) several times and were blown dry with nitrogen. The cleansed substrates were immersed in a 0.25% (v/v) solution of OTS in toluene in a glovebox with a nitrogen atmosphere. The substrates were removed from the OTS solutions after deposition times between 1 and 48 h. After the substrates were removed from the silane solution, they were rinsed with chloroform for approximately 30 s, and excess chloroform was allowed to evaporate. The SA organic films (CH3(CH2)17SiO/SiOx) were baked at 120 °C for 5 min, then were removed from the glovebox. The SA films were rinsed with absolute ethanol and were dried under a stream of nitrogen. Measurement of Contact Angles. Advancing and receding contact angles of deionized water were measured on the SA films at ambient temperature using a Rame´-Hart model 100 goniometer. Contact angles were measured on the opposite edges of at least three drops and averaged. The values were reproducible to within 1.3°. Ellipsometry Measurements. Ellipsometry measurements were made on a Rudolf Research/Auto EL II ellipsometer using a He-Ne Laser (λ ) 632.8 nm) at an incident angle of 70° relative to the surface normal of the substrates. The thicknesses of the SA films and the oxide layer cannot be measured simultaneously, and at least three separate spots were measured on each substrate before and after deposition of the SA film to determine the thickness of the oxide layer and the thickness of the SA film plus the oxide layer. The thickness of the oxide layer of silicon wafers was typically 12-18 Å. A refractive index of 1.45 was used for calculation of the thicknesses of the SA films and the oxide. Sample Preparation. Thin films of P(S-b-MMA) were deposited onto clean or SA film-covered Si/SiOx substrates by spin coating (2500 rpm) from a dilute solution (2% w/w) of the copolymer in toluene. The initial thicknesses of the films were determined by scraping some of the polymer away from the surface with a razor blade and by measuring the difference in height between the substrate and the surface of the film using an Alpha Step 200 profilometer (0.5-nm resolution). The polymer films were annealed at 180 °C in a vacuum oven for 24 h. After annealing, the morphology of the films was investigated using optical microscopy, atomic force microscopy (AFM), and transmission electron microscopy (TEM). Optical Microscopy. Polymer films were imaged using an Olympus model BX600 optical microscope in reflection mode. Island and hole topography resulted in contrast (different interference colors) due to differences in film thickness. Atomic Force Microscopy. The surface topography of the polymer films was imaged using a Nanoscope III MultiMode atomic force microscope from Digital Instruments in contact

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Table 1. Advancing and Receding Contact Angles of Deionized Water and Thicknesses of SA Films of OTS with Different Deposition Times deposition advancing contact receding contact thickness time (hrs) angle of water (deg) angle of water (deg) (Å) 1 4 6 12 18 21 24 30 48

70 ( 2 65 ( 2 89 ( 2 89 ( 2 93 ( 2 105 ( 2 103 ( 2 105 ( 2 110 ( 2

53 ( 2 45 ( 2 64 ( 2 62 ( 2 65 ( 2 80 ( 2 86 ( 2 82 ( 2 85 ( 2

8.7 12.5 18 17.4 22.7 27.5 25.6 28.9 36

mode. A triangular cantilever with an integral pyramidal Si3N4 tip was used. The typical imaging force was on the order of 10-9 N. Transmission Electron Microscopy. The internal structure of the polymer films was studied using TEM. TEM was performed on a JEOL 200CX at 200 kV in the bright field mode at the Materials Science Center at the University of Wisconsin. Samples were imaged in plan-view. Samples were prepared for TEM analysis as described by Fasolka et al.23 A layer of carbon (ca. 20-nm thick) was evaporated onto the surface of films and then covered with a 25% aqueous solution of poly(acrylic acid) (PAA). After drying the sample in air overnight, the P(S-b-MMA)-carbonPAA composite was peeled off the substrate and floated on deionized water with the PAA side down. After the PAA layer dissolved, the floating film was collected onto TEM grids. The films were then exposed to the vapor of the RuO4 solution for 15 min. The RuO4 selectively stains the PS block and provides contrast in electron density.

Results and Discussion Characterization of SA Films of OTS for Different Deposition Times. The advancing and receding contact angles of deionized water and the thicknesses of the SA films of OTS are shown in Table 1 as a function of deposition time. A bare Si/SiOx substrate was completely wet by water, and contact angles were too low to measure. After an hour of deposition, both the advancing and receding contact angles increased significantly, and a partial layer of OTS, 8.7 Å in thickness, was measured using ellipsometry. The advancing and receding contact angles and the thickness increased with increasing deposition times. For deposition times between 21 and 30 h, the advancing and receding contact angles plateaued near 104° and 83°, respectively, and the thickness plateaued near 27.3 ( 1.6 Å. After 48 h of deposition, the contact angles changed only slightly, but the thickness of the OTS layer grew to 36 Å. We have categorized our SA films into three regimes: complete SAMs, partial SAMs, and “thick” SAMs. Complete SAMs have been reported to be highly ordered, quasicrystalline films in which the hydrocarbon chains are oriented nearly perpendicular to the surface.22,24,25 The SA films described in Table 1 with deposition times between 21 and 24 h are considered to be complete SAMs based on the agreement between our thickness measurements and the reported thickness of 26 Å for SAMs of OTS,26 and this categorization refers primarily to the density of the films. The contact angles of these films were slightly lower than the reported value of 110°,26 and the (23) Fasolka, M. J.; Harris, D. J.; Mayes, A. M.; Yoon, M.; Mochrie, S. G. J. Phys. Rev. Lett. 1997, 79, 3018. (24) Tillman, N.; Ulman, A.; Schildkraut, J. S.; Penner, T. L. J. Am. Chem. Soc. 1988, 110, 6136. (25) Maoz, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 100, 465. (26) Wasserman, S. R.; Tao, Y. T.; Whitesides, G. M. Langmuir 1989, 5, 1074.

lower values are probably due to decreased in-plane order of the alkylsiloxane film compared to other OTS films reported in the literature.22,26,27 This decrease in in-plane order is likely due to the deposition conditions used with extremely low levels of water as discussed below. We define partial SAMs as those films with density less than complete SAMs. Decreased density results in more disorder and increased defect density than in complete SAMs. Organic films with deposition times less than 21 h are considered to be partial SAMs. Thick SAMs are films with thicknesses typically 1.1-1.4 times the thickness of a complete SAM. The molecular configuration of the excess alkylsilane ligands is unclear, but the excess material cannot be removed by washing with solvent or wiping with a cotton swab. The films with deposition times greater than 30 h are considered to be thick SAMs. The formation and structure of self-assembled monolayers of alkylsiloxanes, specifically SAMs of OTS, have been studied extensively on various substrates and under different deposition conditions.22,27-34 Two mechanisms of growth are supported in the literature. In the mechanism known as the “uniform” model, silane molecules are distributed homogeneously over the substrate in a disordered, liquidlike manner.33,34 With increasing coverage, the disordered film is transformed into a densely packed, quasi-crystalline structure. In the mechanism known as the “island” model, domains of well-ordered, fully extended OTS molecules are heterogeneously grown on the substrate, and these domains are separated by uncovered regions of the substrate.27,30,31 These islands typically have dimensions on the order of hundreds of nanometers.30,31 With increasing deposition time, more domains are adsorbed until complete coverage of the substrate is achieved. Brunner et al.29 and Vallant et al.28 found that the dominant mechanism of growth of SAMs of OTS is determined by two major factors: (1) concentration of OH groups on the surface of the substrate and (2) concentration of water in the silane solutions. The concentration of OH groups on the surface affects the mobility of the adsorbed OTS molecules.29 The OH groups act as nucleation and bonding sites for the formation of the monolayers. On mica substrates with very few OH groups, the OTS molecules are very mobile and can aggregate easily into ordered domains or “islands” before bonding to the substrate. On silicon substrates with many silanol groups, the OTS molecules have limited mobility and bond more quickly to the substrate. On these substrates, the OTS molecules adsorb more uniformly over the surface of the substrate and, initially, in a more disordered configuration. The presence of water in the OTS solutions causes the OTS molecules to oligomerize in solution. These oligomers have been shown to adsorb to surfaces more quickly than single molecules,31 and the adsorption of these oligomers gives rise to the formation of well-ordered “islands” on surfaces in fractal patterns.31 With increasing deposition (27) Tidswell, I. M.; Rabedeau, T. A.; Pershan, P. S.; Kosowsky, S. D.; Folkers, J. P.; Whitesides, G. M. J. Chem. Phys. 1991, 95, 2854. (28) Vallant, T.; Brunner, H.; Mayer, U.; Hoffmann, H.; Leitner, T.; Resch, R.; Friedbacher, G. J. Phys. Chem. B 1998, 102, 7190. (29) Brunner, H.; Vallant, T.; Mayer, U.; Hoffmann, H.; Basnar, B.; Vallant, M.; Friedbacher, G. Langmuir 1999, 15, 1899. (30) Bierbaum, K.; Grunze, M.; Baski, A. A.; Chi, L. F.; Schrepp, W.; Fuchs, H. Langmuir 1995, 11, 2143. (31) Schwartz, D. K.; Steinberg, S.; Israelachvili, J.; Zasadzinski, J. A. N. Phys. Rev. Lett. 1992, 69, 3354. (32) Allara, D. L.; Parikh, A. N.; Rondelez, F. Langmuir 1995, 11, 2357. (33) Wasserman, S. R.; Whitesides, G. M.; Tidswell, I. M.; Ocko, B. M.; Pershan, P. S.; Axe, J. D. J. Am. Chem. Soc. 1989, 111, 5852. (34) Banga, R.; Yarwood, J.; Morgan, A. M. Langmuir 1995, 11, 618.

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Figure 1. AFM images of thin films of symmetric P(S-b-MMA) on a Si/SiOx substrate (a) and SA films of OTS with deposition times of (b) 4 h, hole formation characteristic of asymmetric wetting on a partial SAM; (c) 12 h, featureless surface characteristic of neutral wetting on a partial SAM; and (d) 48 h, island formation characteristic of symmetric wetting on a thick SAM. In parts a, b, and d, the step heights of the terraces equal approximately 30 nm (Lo) in each case.

time, the gaps between islands are filled by other adsorbed islands and single molecules. Vallant et al. have shown that with decreasing concentrations of water, the growth of islands decreases and the formation of monolayers becomes more uniform.28 In addition, Angst et al. showed that films of OTS on nonhydrated SiO2 were less ordered than films of OTS deposited on well-hydrated SiO2 and that both contact angles of water and thicknesses of the monolayers were lower on the nonhydrated surfaces.35 We have examined the surface topography of SA films of OTS using AFM. In all cases, we have found uniform coverage of OTS molecules on the surface with root-meansquare (rms) roughness less than 1 nm. Our experimental method of depositing monolayers in a N2 atmosphere in a glovebox (dew point less than -60 °C (10 ppm)) on (35) Angst, D. L.; Simmons, G. W. Langmuir 1991, 7, 2236.

piranha-cleaned silicon wafers follows conditions that would favor the “uniform” model of growth that produces more disordered SA films. Wetting Behavior of P(S-b-MMA) on SA Organic Films. Thin films of symmetric P(S-b-MMA) were deposited on the SA films of OTS described in Table 1. All of the films had an initial thickness of 67 ( 2 nm (2.2Lo). SA films with different times of deposition produced different wetting behavior in the thin films of diblock copolymers after annealing. The formation of topography on the surface of the films and the type of topography were used to determine the wetting of the block copolymer at the substrate. For example, consider the film of P(Sb-MMA) deposited on Si/SiOx that is shown in part a of Figure 1. It is well-known that the PMMA block wets SiOx and that the PS block is segregated to the free interface leading to asymmetric wetting conditions. For the film

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thickness of 2.2Lo, holes form for asymmetric wetting and islands form for symmetric wetting. From part a of Figure 1, holes were observed to form in the film deposited on SiOx, indicating asymmetric wetting with the PMMA block wetting the substrate and the PS block wetting the free interface. AFM images of annealed films of P(S-b-MMA) on SA films of OTS with deposition times between 4 and 48 h are shown in parts b-d of Figure 1. Hole formation was observed in polymer films supported on partial SAMs with deposition times less than 12 h (part b of Figure 1). Hole formation indicated asymmetric wetting with the PMMA block at the substrate. This behavior is surprising in light of the structure of the partial SAMs. We observed uniform growth of these partial SAMs with no discernible domains of OTS and rms roughness < 1 nm. Only methyl and methylene groups should be presented at the surface. In the absence of polar interactions, dispersive forces are the only contributions to the interfacial energy between the polymer and the surface. If the surface is composed only of CH2 and CH3 groups, then the PS block should have a lower interfacial energy than the PMMA block on the partial SAMs because PS has a lower total surface tension than PMMA. However, for deposition times less than 12 h, the PMMA block wet the substrate. Miller et al.36 have shown that long-range van der Waals forces can act over distances larger than the thickness of SAMs of alkanethiols on gold to affect the wetting behavior of a liquid on the SAM-covered surface. These forces do not act over distances as large as 1 nm in our system because the substrate (Si/SiOx) is an insulator with a much lower dielectric constant than gold. In fact, for partial SAMs of OTS on Si/SiOx substrates, the van der Waals forces between Si/SiOx and the polymer film are negligible for separation distances greater than 5 Å when the gap is filled with a hydrocarbon. Therefore, for the PMMA block to wet substrates covered with partial SAMs, the PMMA chains must penetrate the alkylsiloxane layer to allow for short-range interactions between PMMA segments and SiOH groups or adsorbed water in the siloxane network. Even though the SAMs used in this study are believed to uniformly cover the substrates, the lateral density of the partial SAMs must be less than that of a complete SAM, and the PMMA blocks must be able to penetrate into the defects of the partial SAM. For a specific partial SAM with a deposition time of 12 h, the thin film (part c of Figure 1) was featureless on the free surface, which is characteristic of neutral wetting at the substrate with perpendicular orientation of the lamellae at the substrate. AFM measurements verified the smoothness of the surface of the annealed film (rms roughness < 2 nm). A plan-view TEM image of the film from part c of Figure 1 is shown in Figure 2. The contrast between the PS and PMMA domains was achieved by staining the PS domains with RuO4. The laterally alternating domains of PS and PMMA confirm a perpendicular orientation of the lamellae in the film. After 12 h of deposition, the concentration of OTS molecules on the surface increased such that the penetration of PMMA blocks into the partial monolayer likely decreased. The short-range interactions between PMMA and the substrate were balanced with the interfacial interactions between PS and the methyl and methylene groups on the surface of the partial SAM. This surface was neutral in interfacial energy with respect to both blocks of the copolymer. (36) Miller, W. J.; Abbott, N. L. Langmuir 1997, 13, 7106.

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Figure 2. Plan-view TEM image of thin film of P(S-b-MMA) on a partial SAM of OTS with a deposition time of 12 h. Dark regions correspond to PS due to staining with RuO4. The alternating light and dark regions confirm an orientation of the lamellae perpendicular to the substrate and the neutral wetting behavior of the polymer film.

It is unclear whether the lamellae were oriented perpendicular to the substrate throughout the thickness of the film or whether a mixed morphology occurred with perpendicular lamellae near the substrate and parallel lamellae near the free surface. Neutral surfaces have been shown to induce a perpendicular orientation of microdomains in thin films of block copolymers.3,4,37,38 For films with thicknesses greater than 3.0Lo, the lamellae formed a mixed morphology,3,38 except when the annealing temperature was raised such that the surface tensions of the two blocks were equal and perpendicular lamellae extended throughout the thickness of the film.37 On neutral surfaces under similar annealing conditions as employed for this paper, perpendicular lamellae were observed on the free surface of a 2.2Lo thick film (same thickness as used in this paper) using lateral force microscopy.4 Whether the perpendicular orientation of the lamellae extended throughout the film thickness was inconsequential for this paper, and we did not pursue this issue further. For complete SAMs of OTS, either we were unable to spin-coat thin films of P(S-b-MMA) or the films completely dewet from the substrate during annealing. For complete SAMs, the interactions between the polymer and the surface diminished as more OTS molecules chemisorbed onto the surface to complete the monolayer. Only CH3 groups are presented at the surface of complete SAMs of OTS, and the surface tension (γOTS) is approximately 23 mJ/m2.4 Nonpolar PS is more likely to wet complete SAMs of OTS than polar PMMA. The spreading coefficient for PS (S ) γOTS - γPS - γPS/OTS) was calculated from the surface tension of PS (γPS ) 35.8 mJ/m2) and the interfacial energy between PS and the surface (γPS/OTS ) 1.2 mJ/m2).4 The negative spreading coefficient predicted nonwetting conditions for P(S-b-MMA) on complete SAMs of OTS. For thick SAMs, island formation was observed on the free surface of the films (part d of Figure 1). Island formation was characteristic of symmetric wetting for the initial film thickness with the PS block wetting both the substrate and free interface. Partial dewetting of the films, however, was observed after annealing at 180 °C for several days (Figure 3). Thermodynamics dictates that the films dewet (S < 0), but the dewetting process is kinetically hindered. Island formation indicative of symmetric wetting is a metastable morphology. The configuration of the excess alkylsilane ligands on the surface of thick SAMs is unclear, but this material probably grows in a disordered manner, lending itself to more chain(37) Mansky, P.; Russell, T. P.; Hawker, C. J.; Mays, J.; Cook, D. C.; Satija, S. K. Phys. Rev. Lett. 1997, 79, 237. (38) Huang, E.; Russell, T. P.; Harrison, C.; Chaikin, P. M.; Register, R. A.; Hawker, C. J.; Mays, J. Macromolecules 1998, 31, 7641.

Wetting Behavior of Block Copolymers

Figure 3. Optical micrograph of film of P(S-b-MMA) on a thick SAM annealed at 180 °C for several days. The film exhibited island formation (dark areas) and partial dewetting.

chain contacts and interdigitation between the excess material and the polymer than with complete SAMs. These interactions decrease the mobility of the polymer in contact with the SA film, as compared to the mobility on complete SAMs of OTS. The wetting behavior of thin films of block copolymers has been determined to depend on the interfacial energy between the surfaces and each block of the copolymer.4,39 In a previous paper, interfacial energies were calculated from contact angle data of three test liquids using the FOCG model of surface tension.40 We attempted to use this approach to analyze the wetting behavior of P(S-bMMA) on partial and thick SAMs of OTS, but the results were unsatisfactory. Estimation of surface and interfacial energies using the FOCG model depends on contact angle measurements that require homogeneous surfaces. The FOCG model, therefore, is applicable when probing surface chemistry effects but not surface structural effects. The wetting of P(S-b-MMA) on partial and thick SAMs depends on both surface chemistry and structure. The wetting behavior of films of block copolymers on SAMs can be considered a sensitive characterization tool for probing the structure of SAMs. Sharp transitions in wetting behavior were observed on different SAMs that exhibited only small changes in both contact angle and thickness. The wetting behavior of the block copolymers amplifies small differences in the SAM structure that cannot be detected by contact angle and thickness measurements. As mentioned in the Introduction, Heier et al. observed neutral wetting behavior of P(S-b-2VP) films on CH3terminated SAMs of alkanethiols on Au after annealing at 176 °C.12-15 The wetting behavior was consistent with penetration of the PVP block into defects in the SAM structure and interaction of the PVP block with the underlying gold. They concluded that the defects in the SAMs were present in the initial monolayer structure and were not introduced during the annealing process. The conclusions were based on concentration versus depth profiles of sulfur in the samples obtained using dynamic secondary ion mass spectroscopy (dSIMS).13 Significant differences in the concentration of sulfur at the polymer/ substrate interface were not observed for annealing times up to 4 h. Based on the results presented in this paper and information in the literature, we believe that defects in the SAMs of alkanethiols on Au were not present after stamping but were related to thermal desorption during annealing. Several groups have shown that stamped SAMs of alkanethiols (contact times greater than 0.3 s) are extremely well-ordered, with few defects, and exhibit (39) Mansky, P.; Liu, Y.; Huang, E.; Russell, T. P.; Hawker, C. Science 1997, 275, 1458. (40) van Oss, C. J.; Chaudhury, M. K.; Good, R. J. Chem. Rev. 1988, 88, 927.

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similar properties as SAMs deposited from solution.41,42 It is also well-known that SAMs of alkanethiols on Au are more ordered than SAMs of alkylsiloxanes on Si/SiOx. We observed dewetting of P(S-b-MMA) block copolymers on CH3-terminated complete SAMs of alkylsiloxanes and did not observe neutral wetting behavior on partial SAMs unless the imperfections in the SAMs were extensive. The thickness of the partial SAMs that exhibited neutral wetting was ∼9 Å less than the thickness of the complete SAMs (26 Å), and the difference in contact angle between the partial and complete SAMs was ∼15°. It is possible that the results obtained with dSIMS by Heier et al. were misleading in that the data provided no information concerning the integrity of the SAM. Sulfur may remain at the interface between the polymer and the substrate (within the depth resolution of the experiment) for long periods of time but may not remain physisorbed to the gold. We are interested in the wetting behavior of block copolymer films on SA films of alkylsiloxanes because we are developing a technique to macroscopically orient block copolymer domains on chemically patterned surfaces where we use SA films of alkylsiloxanes as imaging layers to produce the chemically patterned surface.6,7,43,44 A key aspect of this technique is that the adjacent regions of the chemically patterned surface are preferentially wet by the different blocks of the copolymer. We have shown previously that SA films of alkylsiloxanes can be chemically modified by exposure to X-rays in air and that the wetting behavior of diblock copolymers on these surfaces can be controlled.4,5 As shown in our previous work, however, films of P(S-b-MMA) dewet from complete SAMs of OTS.4 Such dewetting is undesirable for this application. In this paper, we show that thick SAMs of OTS are preferentially wet by the PS block without dewetting within annealing times of interest (24 h). Therefore, when thick SAMs of OTS are patterned using X-ray lithography, the desired wetting behavior is achieved with the PS block preferentially wetting unexposed regions and the PMMA block preferentially wetting exposed regions of the SA film. Conclusions Partial, complete, and thick SAMs of OTS were produced by varying the deposition time of the SA films. Thin films of P(S-b-MMA) exhibited asymmetric wetting on partial SAMs with deposition times less than 12 h, indicating penetration of the PMMA blocks into defects of the SA film. Neutral wetting was observed on partial SAMs with a deposition time of 12 h, and dewetting was observed on complete SAMs. Symmetric wetting on thick SAMs was a metastable morphology due to kinetically hindered dewetting due to interdigitation of the polymer with excess material on the SA film. The wetting behavior of the polymer films depended on both chemical and structural properties of the SA films. The use of SA films of OTS as imaging layers to create chemically patterned surfaces to direct pattern formation in P(S-b-MMA) films requires the use of thick SAMs that are preferentially wet by the PS block and that are kinetically hindered to dewet. (41) Larsen, N. B.; Biebuyck, H.; Delamarche, E.; Michel, B. J. Am. Chem. Soc. 1997, 119, 3017. (42) Eberhardt, A. S.; Nyquist, R. M.; Parikh, A. N.; Zawodzinski, T.; Swanson, B. I. Langmuir 1999, 15, 1595. (43) Yang, X. M.; Peters, R. D.; Kim, T. K.; Nealey, P. F. J. Vac. Sci. Technol., B 1999, 17, 3203. (44) Yang, X. M.; Peters, R. D.; Kim, T. K.; Nealey, P. F.; Brandow, S. L.; Chen, M.-S.; Shirey, L. M.; Dressick, W. J. Langmuir, submitted for publication, 2000.

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Acknowledgment. Funding for this work was provided by the Semiconductor Research Corporation (Grant Number 98-LP-452), an NSF Small Grant for Exploratory Research (Grant Number CTS-9708944), and an NSF Career Award (Grant Number CTS-9703207). Fa-

Peters et al.

cilities were supported by DARPA/ONR (Grant Number N00014-97-1-0460) and the NSF (Grant Number DMR95-31009). LA000822+