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Using Self-Assembled Monolayers Exposed to X-rays To Control the Wetting Behavior of Thin Films of Diblock Copolymers Richard D. Peters, Xiao M. Yang, Tae K. Kim, B. H. Sohn, and Paul F. Nealey* Department of Chemical Engineering and Center for Nanotechnology, University of Wisconsin, Madison, Wisconsin 53706 Received November 16, 1999. In Final Form: February 18, 2000 The wetting behavior of thin films of symmetric poly(styrene-block-methyl methacrylate) was investigated on self-assembled monolayers (SAMs) of octadecyltrichlorosilane (OTS) that were chemically modified by exposure to X-rays in the presence of air. The concentration of aldehyde and hydroxyl groups on the surface of exposed OTS increases with increasing dose. The polarity and surface tension of the SAMs were tuned to control the wetting behavior of the polymer films. Symmetric, neutral, and asymmetric wetting of the films were observed for doses of 400-1000, 1200, and 1400-2000 mJ/cm2, respectively. The wetting behavior was qualitatively described by estimating the interfacial energies between each block of the copolymer and exposed SAMs and the surface tensions of both blocks using the Fowkes-van Oss-Chaudhury-Good model of surface tension. These results lay the foundation for controlling the morphology of thin films of block copolymers over macroscopic dimensions by nanopatterning substrates with regions of different wetting behavior using advanced lithography.
Introduction Block copolymers have tremendous potential for applications in nanofabrication because these systems generally microphase separate to form periodic structures with domains with length scales of 10-100 nm. The size and shape of the microphase-separated domains can be manipulated easily by controlling the molecular weight and composition of the polymer. For patterning applications, thin films of block copolymers with ordered structures typically are used as templates. Chaikin and Register, for example, have used spherical and cylindrical domain structures formed in thin films of block copolymers as templates for the fabrication of periodic arrays of dots or holes with dimensions as small as 20 nm.1,2 This parallel processing technique creates nanopatterned areas as large as a few centimeters squared, but the perfection in the periodic arrays is limited to length scales corresponding to the grain size. Many applications of thin films of block copolymers are possible if the orientation and perfection of ordering of the microphase-separated domains can be controlled. A significant body of literature has been directed at understanding the morphology of thin films of symmetric diblock copolymers. For unconfined films of symmetric diblock copolymers, Russell et al. have demonstrated that the orientation of the lamellar morphology depends on the relative interfacial energies between each block and the substrate and the relative surface energy of each block.3 Due to the specific interactions of the blocks with the air and substrate interfaces, the lamellar structure of these materials usually is oriented parallel to the plane of the film. For unconfined films, the film thickness is quantized in terms of the bulk lamellar period, Lo. The preferred * To whom correspondence may be addressed. E-mail: nealey@ engr.wisc.edu. (1) Park, M.; Harrison, C.; Chaikin, P. M.; Register, R. A.; Adamson, D. H. Science 1997, 276, 1401. (2) Harrison, C.; Park, M.; Chaikin, P. M.; Register, R. A.; Adamson, D. H. J. Vac. Sci. Technol., B 1998, 16, 544. (3) Mansky, P.; Liu, Y.; Huang, E.; Russell, T. P.; Hawker, C. Science 1997, 275, 1458.
film thickness, t*, is quantized in values of nLo for symmetric wetting, i.e., the same block is present at both interfaces, and is quantized in values of (n + 1/2)Lo for asymmetric wetting, i.e., one block is present at the free interface and the other block is present at the substrate interface, where n is a positive integer. If the average film thickness, t* + , is incommensurate with this constraint, islands (for 0 < < 1/2Lo) or holes (for 1/2Lo < < Lo) form on the free surface with a step height of Lo.4-7 If is zero, the average film thickness equals the preferred thickness, and the surface of the film is smooth and featureless. The lateral dimensions of the holes or islands are typically a few micrometers and are easily imaged using optical or atomic force microscopy.7-9 If the interfacial energies between both blocks and the substrate are similar, then the lamellae form perpendicular to the substrate and persist to some extent through the thickness of the film.10,11 In a set of elegant experiments, Russell and Hawker et al. tuned interfacial energies between poly(d-styreneblock-methyl methacrylate) films and substrates with endgrafted random copolymer brushes of varying styrene/ methyl methacrylate content.12,13 They observed symmetric, neutral, and asymmetric wetting of the block copolymer film with increasing methyl methacrylate (4) Coulon, G.; Russell, T. P.; Deline, V. R.; Green, P. F. Macromolecules 1989, 22, 2581. (5) Anastasiadis, S. H.; Russell, T. P.; Satija, S. K.; Majkrzak, C. F. Phys. Rev. Lett. 1989, 62, 1852. (6) Russell, T. P.; Coulon, G.; Deline, V. R.; Miller, D. C. Macromolecules 1989, 22, 4600. (7) Coulon, G.; Collin, B.; Ausserre, D.; Chatenay, D.; Russell, T. P. J. Phys. (Paris) 1990, 51, 2801. (8) Collin, B.; Chatenay, D.; Coulon, G.; Ausserre, D.; Gallot, Y. Macromolecules 1992, 25, 1621. (9) Coulon, G.; Ausserre, D.; Russell, T. P. J. Phys. (Paris) 1990, 51, 777. (10) Huang, E.; Russell, T. P.; Harrison, C.; Chaikin, P. M.; Register, R. A.; Hawker, C. J.; Mays, J. Macromolecules 1998, 31, 7641. (11) Huang, E.; Mansky, P.; Russell, T. P.; Harrison, C.; Chaikin, P. M.; Register, R. A.; Hawker, C. J.; Mays, J. 1999. (12) Mansky, P.; Russell, T. P.; Hawker, C. J.; Pitsikalis, M.; Mays, J. Macromolecules 1997, 30, 6810. (13) Mansky, P.; Russell, T. P.; Hawker, C. J.; Mays, J.; Cook, D. C.; Satija, S. K. Phys. Rev. Lett. 1997, 79, 237.
10.1021/la991500c CCC: $19.00 © 2000 American Chemical Society Published on Web 04/07/2000
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content of the brush. Other groups have used functionalized SAMs of alkanethiols on gold to control the wetting behavior of block copolymer films14-16 and films of polymer blends.17-20 One strategy to induce lamellae to form perpendicular to the substrate with controlled orientation in the plane of the film is to nanopattern substrates with alternating regions that are wet by the different blocks of the copolymer. The relationships between the equilibrium morphology of the block copolymer film as a function of the film thickness, the period of the pattern on the substrate, the bulk lamellar period of the polymer, and the interaction potentials of the individual blocks of the polymer with the bounding surfaces have been the focus of numerous molecular modeling studies.21-27 We have recently employed a phenomenological model28 and Monte Carlo simulations29 to quantitatively calculate phase diagrams for this type of system. If the periodicity of the pattern on the substrate is commensurate with the bulk lamellar spacing of the polymer, and the top confining surface is neutral or is weakly preferential to one of the blocks, then lamellae form perpendicular to the substrate and amplify the pattern on the substrate into the film and over macroscopic dimensions in the plane of the film. Russell et al. demonstrated experimentally this behavior using ultrathin films (thickness less than Lo) of poly(styrene-block-methyl methacrylate) on nanopatterned surfaces (alternating gold and silicon regions) prepared using annealed miscut silicon substrates and obliquely deposited gold.30 We are developing techniques to nanopattern substrates with regions of different wetting behavior using advanced lithography and to use these substrates to control the morphology of thin films of block copolymers over macroscopic dimensions. The choice of exposure technology must meet the following requirements: (1) patterns must be generated at the scale of tens of nanometers such that reasonable molecular weight block copolymers can be used, and (2) large areas must be patterned at one time (parallel processing). Proximity X-ray lithography and extreme ultraviolet (EUV) lithography are promising candidates to meet these criteria. SAMs of alkylsiloxanes may be suitable as imaging layers because (1) the surface chemistry of SAMs of alkylsiloxanes on SiOx can be modified by exposure to X-rays in the presence of air,31 (2) SAMs (14) Heier, J.; Kramer, E. J.; Walheim, S.; Krausch, G. Macromolecules 1997, 30, 6610. (15) Heier, J.; Genzer, J.; Kramer, E. J.; Bates, F. S.; Walheim, S.; Krausch, G. J. Chem. Phys. 1999, 111, 11101. (16) Heier, J.; Sivaniah, E.; Kramer, E. J. Macromolecules 1999, 32, 9007. (17) Genzer, J.; Kramer, E. J. Phys. Rev. Lett. 1997, 78, 4946. (18) Boltau, M.; Walheim, S.; Mlynek, J.; Krausch, G.; Steiner, U. Nature 1998, 391, 877. (19) 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. (20) Nisato, G.; Ermi, B.; Douglas, J. F.; Karim, A. Macromolecules 1999, 32, 2356. (21) Chen, H.; Chakrabarti, A. J. Chem. Phys. 1998, 108, 6897. (22) Chakrabarti, A.; Chen, H. J. Polym. Sci., Part B 1998, 36, 3127. (23) Petera, D.; Muthukumar, M. J. Chem. Phys. 1997, 107, 9640. (24) Petera, D.; Muthukumar, M. J. Chem. Phys. 1998, 109, 5101. (25) Pereira, G. G.; Williams, D. R. M. Phys. Rev. Lett. 1998, 80, 2849. (26) Pereira, G. G.; Williams, D. R. M. Macromolecules 1998, 31, 5904. (27) Pereira, G. G.; Williams, D. R. M. Macromolecules 1999, 32, 758. (28) Wang, Q.; Nath, S. K.; Nealey, P. F.; de Pablo, J. J. Submitted for publication in J. Chem. Phys. (29) Wang, Q.; Yan, Q.; Nealey, P. F.; de Pablo, J. J. Submitted for publication in Macromolecules. (30) Rockford, L.; Liu, Y.; Mansky, P.; Russell, T. P.; Yoon, M.; Mochrie, S. G. J. Phys. Rev. Lett. 1999, 82, 2602.
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are ultrathin layers that eliminate opacity issues (for EUV lithography), and (3) SAMs of alkylsiloxanes are thermally stable under vacuum up to temperatures of 740 K.32 The concentration of surface-grafted hydroxyl and aldehyde groups on exposed SAMs increases with increasing dose. In this paper we demonstrate that the wetting behavior of block copolymer films can be controlled by tuning the polarity of the SAM-covered surface. We observed symmetric, neutral, and asymmetric wetting of thin films of poly(styrene-block-methyl methacrylate) on exposed SAMs of octadecyltrichlorosilane as a function of dose. The wetting behavior was correlated with interfacial and surface energies estimated using contact angles of organic liquids and the Fowkes-van Oss-Chaudhury-Good model of surface tension. These results lay the foundation for nanopatterning substrates with alternating regions of different chemical functionality and with dimensions commensurate with the size of diblock copolymer molecules using X-ray or extreme ultraviolet lithography.33 Experimental Section Materials. Polished test grade silicon 〈100〉 wafers were purchased from Tygh Silicon. Octadecyltrichlorosilane (OTS, CH3(CH2)17SiCl3, 95%) was purchased from Gelest and was used as received. Symmetric poly(styrene-block-methyl methacrylate) (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, and the styrene volume fraction was 0.48. Toluene (99.8%, anhydrous), chloroform (99+%, anhydrous), poly(acrylic acid) (25 wt % solution in water), glycerol (99+%), methylene iodide (99%), and ethylene glycol (99+%) 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 Monolayers. 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 cleaned substrates were immersed in a 0.05% (v/v) solution of OTS in toluene in a glovebox with a nitrogen atmosphere.34 The optimal immersion time for formation of a complete monolayer was determined from kinetic studies of contact angle and thickness and was found to be 24 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 monolayers (CH3(CH2)17SiO-/SiOx) were baked at 120 °C for 5 min and then were removed from the glovebox. The monolayers were rinsed with absolute ethanol and were dried under a stream of nitrogen. Exposure of SAMs to X-rays. The monolayers were irradiated with soft X-rays in the ES-1 beamline at the Center for Nanotechnology (CNT). CNT facilities are located at the Synchrotron Radiation Center at the University of Wisconsin. The wavelength (λ) of the broadband radiation was centered at 1.1 nm with ∆λ/λ ≈ 3. The intensity of the incident radiation was 17-38 mW/cm2 and varied with the synchrotron ring current. Samples were irradiated at an incident angle of 90°. The exposures were carried out in a chamber with a pressure of 1 Torr of air with relative humidity of ∼20%. The intensity of the X-ray beam was attenuated when passing through the atmosphere of the chamber. The reported doses from 0 to 2000 mJ/ (31) Kim, T. K.; Sohn, B. H.; Yang, X. M.; Peters, R. D.; Nealey, P. F. Submitted for publication in J. Phys. Chem. B. (32) Kluth, G. J.; Sung, M. M.; Maboudian, R. Langmuir 1997, 13, 3775. (33) The chemical modification of SAMs and the effect on the wetting behavior of films of block copolymers are similar for X-ray radiation and EUV radiation. The EUV results will be published separately. (34) Wasserman, S. R.; Tao, Y. T.; Whitesides, G. M. Langmuir 1989, 5, 1074.
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Table 1. Advancing Contact Angles (deg) of Water (W), Glycerol (G), and Methylene Iodide (MI) on SAMs Exposed to X-rays at Doses from 0 to 2000 mJ/cm2 surface/dose (mJ/cm2)
θaW
θaG
θaMI
OTS/0 OTS/400 OTS/800 OTS/1200 OTS/1600 OTS/2000
110 90 81.5 76 69 53
94 79 74.5 69 63 49
68.5 51.5 50 42 44.5 34
cm2 refer to the dose that is delivered to the chamber. The effective dose that reaches the surface of the monolayers was approximately 93% of the reported dose. Contact angles were measured or polymer thin films were deposited on the exposed monolayers less than 2-3 h after irradiation. Measurement of Contact Angles. Advancing contact angles (θa) of deionized water, glycerol, methylene iodide, and ethylene glycol were measured on the monolayers and films of PS and PMMA homopolymers at ambient temperature using a Rame´Hart model 100-00 goniometer. Contact angles were measured on the opposite edges of at least three drops and averaged. The values were reproducible to within 1.3°. Sample Preparation. Thin films of P(S-b-MMA) were deposited onto clean or SAM-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. 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 mode. A triangular cantilever with an integral pyramidal Si3N4 tip was used. The typical imaging force was on the order of 10-9 N. Lateral Force Microscopy. Lateral force microscopy of the surfaces of some of the films was performed under similar conditions as AFM with the Nanoscope III Multimode AFM. A similar Si3N4 tip was used with a scan angle of 90°. Transmission Electron Microscopy. The internal structure of the films was studied using transmission electron microscopy. 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.35 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 the sample was dried in air overnight, the P(S-b-MMA)-carbon-PAA 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 The advancing contact angles of deionized water (W), glycerol (G), and methylene iodide (MI) on SAMs of OTS exposed to X-rays at doses from 0 to 2000 mJ/cm2 are shown in Table 1. The advancing contact angles of all three test liquids decreased monotonically with increasing (35) Fasolka, M. J.; Harris, D. J.; Mayes, A. M.; Yoon, M.; Mochrie, S. G. J. Phys. Rev. Lett. 1997, 79, 3018.
dose. These results indicated the SAMs became more hydrophilic, more polar, and more energetic with increasing dose. The wetting behavior and surface topography of thin films of symmetric P(S-b-MMA) on SAMs of OTS irradiated with X-rays were dependent on the exposure dose. Optical micrographs and AFM images of annealed films with initial thickness of 67 ( 2 nm (2.2Lo) are shown in Figure 1 and Figure 2, respectively. Films of P(S-b-MMA) could not be deposited by spin coating on SAMs exposed with doses below 100 mJ/cm2. Films could be deposited on SAMs exposed to doses from 100 to 300 mJ/cm2, but the polymer dewet upon annealing (Figure 1a). Island formation and partial dewetting were observed on the surface of films deposited on SAMs exposed to X-rays between 400 and 1000 mJ/cm2 (Figure 1b,c). Islands appear darker than the surrounding areas of the films in the optical micrographs in parts b and c of Figure 1. The AFM image in Figure 2a (corresponding to Figure 1c) confirmed the existence of islands with a step height (Lo) of approximately 30.5 nm. On the basis of the initial film thickness and Lo, island formation was characteristic of symmetric wetting, with the PS block present at both the substrate and free interfaces. Films of P(S-b-MMA) on SAMs exposed to 1200 mJ/cm2 did not exhibit surface topography (Figure 1d). On the basis of the initial thickness and Lo, a featureless surface was indicative of an orientation of lamellae perpendicular to the surface. To reveal the internal structure of the annealed thin film from Figure 1d, a planview image of the film was obtained using TEM (Figure 3). After staining with RuO4, the PS domains appeared as dark regions, and the PMMA domains appeared as light regions. Laterally alternating domains of PS and PMMA blocks confirmed an orientation of lamellae perpendicular to the substrate. An LFM image (Figure 4) of a similarly prepared film also exhibited contrast corresponding to perpendicularly oriented lamellae at the surface of the film. The light areas in which the block copolymer morphology is not observed are believed to be covered with a thin layer of PS. The TEM and LFM data suggest that the orientation of the lamellae perpendicular to the substrate persists throughout the film thickness. Hole formation was observed on the surfaces of films deposited on SAMs exposed above 1600 mJ/cm2 (Figure 1e) and on the native oxide layer of a Si wafer (Figure 1f). Holes appear as light areas in the optical micrographs in parts e and f of Figure 1. The AFM image in Figure 2b (corresponding to Figure 1e) confirmed the existence of holes also with a step height of approximately 30.5 nm. On the basis of the initial film thickness and Lo, hole formation was characteristic of asymmetric wetting, with the PMMA block present at the substrate interface and the PS block present at the free interface. Discussion Chemical Modification and Surface Tensions of SAMs Exposed to X-rays. We have reported elsewhere that hydroxyl (C-OH) and aldehyde (CdOH) functional groups are incorporated onto the surface of OTS when the monolayers are exposed to X-rays in the presence of oxygen.31 The decrease in the advancing contact angles of water, glycerol, and methylene iodide with increasing dose was consistent with the presence of an increasing number of hydroxyl and aldehyde groups on the surface of the monolayers. The surface tensions of SAMs exposed to X-rays were estimated as a function of dose from the contact angle data in Table 1 using the Fowkes-van Oss-Chaudhury-
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Figure 1. Optical micrographs of thin films (thickness ) 67 ( 2 nm) of symmetric P(S-b-MMA) on SAMs exposed to X-rays and annealed at 180 °C for 24 h: (a) dose ) 300 mJ/cm2, dewetting; (b) dose ) 400 mJ/cm2, island formation and partial dewetting; (c) dose ) 800 mJ/cm2, island formation; (d) dose ) 1200 mJ/cm2, featureless free surface; (e) dose ) 1600 mJ/cm2, hole formation; (f) hole formation on SiOx/Si substrate. Lighter regions in the images of the films are thinner than darker regions.
Good (FOCG) surface tension model.36 This model predicts the solid surface tension based on the combination of attractive van der Waals forces (or dispersive interactions) and Lewis acid-base polar interactions (or hydrogen bonding). In the FOCG model, the total surface tension is additive and is given by
γtotal ) γLW + γAB
(1)
where γLW is the Lifshitz-van der Waals (or dispersive) component. γAB is the polar interaction term and is given by
γAB ) 2(γ+γ-)1/2
(2)
where γ+ is the electron-acceptor component and γ- is the electron-donor component. The three solid surface tension (36) van Oss, C. J.; Chaudhury, M. K.; Good, R. J. Chem. Rev. 1988, 88, 927.
parameters (γSLW, γS+, and γS-) are related to the contact angle of a liquid on a surface through the modified YoungDupre´ equation
(1 + cosθ)γL ) 2((γSLWγLLW)1/2 + (γS+γL-)1/2 + (γS-γL+)1/2) (3) where the subscript L refers to liquid surface tension parameters and γL is the total liquid surface tension against air. To find the three parameters for the solid surface (γSLW, γS+, and γS-), three independent equations are obtained by measuring the contact angles of three liquids (two of which must be polar) with known values of γLLW, γL+, and γL- on the solid surface. The surface tension parameters of the four test liquids we used were taken from the literature.37 The estimated surface tensions of the SAMs based on the methodology discussed above are given in Table 2. The decrease in the advancing contact angles of the three
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Figure 2. AFM images of films of P(S-b-MMA) annealed at 180 °C for 24 h on exposed methyl-terminated monolayers: (a) islands (corresponding to the sample in Figure 1c), and (b) holes (corresponding to the sample in Figure 1e). Along the lines in (a) and (b), the depth of holes and the height of islands are 30.5 nm, revealing the quantization of film thickness in terms of Lo, the bulk lamellar period.
test liquids with increasing dose was evidence of an increase in the overall surface tension of the SAMs. From Table 2, the dispersive component of the surface tension (γLW) of the SAMs increased monotonically with dose and represented the major contribution to the total surface tension. The electron-donor component (γ-) also increased monotonically with dose. These observations were consistent with an increasing number of hydroxyl and aldehyde groups on the surface with increasing dose. For doses less than or equal to 1200 mJ/cm2, the electronacceptor component (γ+) and, therefore, the polar component (γAB) equaled zero. The electron-acceptor component became greater than zero and increased only for doses greater than 1200 mJ/cm2, and therefore, γAB contributed to the total surface tension only for doses above 1200 mJ/ cm2. Relationship between Surface Topography and Interfacial Energies of PS and PMMA on SAMs Exposed to X-rays. As mentioned in the Introduction, it is now well established that the topography of thin films of block copolymers depends on the wetting behavior of the polymer at the boundaries of the film. The information necessary to understand and predict the topography of an unconfined film on a particular surface is the interfacial energy between each block of the copolymer and the substrate and the surface tension of each block of the (37) Lee, L. H. Langmuir 1996, 12, 1681. The surface tension parameters of the four test liquids are as follows: for water, γLW ) 21.8 mJ/m2, γ+ ) 34.2 mJ/m2, and γ- ) 19 mJ/m2; for glycerol, γLW ) 34 mJ/m2, γ+ ) 5.3 mJ/m2, and γ- ) 42.5 mJ/m2; for methylene iodide, γLW ) 50.8 mJ/m2, γ+ ) 0 mJ/m2, and γ- ) 0 mJ/m2; for ethylene glycol, γLW ) 48 mJ/m2, γ+ ) 2.6 mJ/m2, and γ- ) 34.8 mJ/m2.
copolymer. Ideally the surface and interfacial energies should be determined at the annealing temperature, i.e., the temperature at which the ordering of the system occurs. We were unable to measure the surface and interfacial energies of our system at 180 °C. We were able to estimate surface tensions of PS and PMMA and interfacial energies of PS and PMMA on SAMs exposed to X-rays at room temperature. Due to the thermal stability of our surfaces and the apparent well-behaved manner in which the values of the surface tension parameters depend on temperature,38 our measurements at room temperature can be used to qualitatively understand and predict the observed behavior of P(S-b-MMA) films on our surfaces. The interfacial energy of PS and PMMA homopolymers on each type of surface was determined using the FOCG model. The interfacial energy between a solid and a liquid is comprised of a Lifshitz-van der Waals component and a polar interaction component
γSLtot ) γSLLW + γSLAB
(4)
Using the Good-Girifalco-Fowkes combining rule39 and expressions for the Lewis acid-base interactions across the interface,36 the interfacial energy between the solid and a liquid was calculated using
γSL ) ((γSLW)1/2 - (γLLW)1/2)2 + 2((γS+γS-)1/2 + (γL+γL-)1/2 - (γS+γL-)1/2 - (γS-γL+)1/2) (5) The surface tension parameters of PS and PMMA homopolymers were estimated by measuring advancing contact
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Figure 3. Transmission electron micrograph in plane view of a film of P(S-b-MMA) on a SAM of OTS exposed at 1200 mJ/cm2. The PS domains appear as darker regions after staining with RuO4. The alternating light and dark regions confirm an orienation of the lamellae perpendicular to the substrate. Table 2. Calculated Surface Tension Parameters in mJ/m2 for OTS Exposed to X-rays at Doses from 0 to 2000 mJ/cm2
Figure 4. Lateral force microscopy image of the surface of a film of P(S-b-MMA) on a SAM of OTS exposed near 1200 mJ/ cm2. The light areas correspond to PS domains, and the dark regions correspond to PMMA domains. The contrast in the LFM image suggests the presence of perpendicular lamellae of PS and PMMA at the free surface of the film. The film thickness was 2.2Lo.
angles of deionized water and glycerol on thin films of both homopolymers and advancing contact angles of ethylene glycol (EG) on PS and methylene iodide on PMMA.40 The contact angles were measured as follows:
solid surface/ dose (mJ/cm2)
γtot
γLW
γAB
OTS/0 OTS/400 OTS/800 OTS/1200 OTS/1600 OTS/2000
23.7 ( 0.7 33.4 ( 0.7 34.3 ( 0.7 38.6 ( 0.7 40.8 ( 1.3 49.4 ( 1.2
23.7 ( 0.7 33.4 ( 0.7 34.3 ( 0.7 38.6 ( 0.7 37.3 ( 0.7 42.5 ( 0.6
0 0 0 0 3.5 ( 0.6 6.9 ( 0.6
γ+
γ-
0 0 0 2.3 ( 0.4 0 5.6 ( 0.7 0 6.9 ( 0.7 0.3 ( 0.1 10.0 ( 0.8 0.7 ( 0.1 17.2 ( 0.9
for PS, θW ) 94°, θG ) 81°, and θEG ) 65°; for PMMA, θW ) 72°, θG ) 69°, and θMI ) 42°. Using eq 3, the surface tension parameters were calculated as follows: for PS, γLW ) 35.8 mJ/m2, γ+ ) 0 mJ/m2, and γ- ) 1.0 mJ/m2; for PMMA, γLW ) 38.6 mJ/m2, γ+ ) 0 mJ/m2, and γ- ) 10.1 mJ/m2. Figure 5 shows a plot of the estimated interfacial energies at room temperature of PS and PMMA on SAMs exposed to X-rays between 0 and 2000 mJ/cm2. The polymer with the lower interfacial energy is the preferred polymer at the SAM surface. The plot in Figure 5 qualitatively predicts the wetting behavior and, thus, the topography of the thin films in Figure 1. The surface interactions that govern the wetting behavior can be understood by examining the trends in interfacial energies (38) The experimentally determined change in surface tension of a mixed SAM of alkanethiols as a function of temperature is -dγ/dT ≈ 0.07 mJ/m2‚K (ref 17). The values of -dγ/dT for PS and PMMA are similar (Wu, S. J. Phys. Chem. 1970, 74, 632). (39) Good, R. J.; Girifalco, L. A. J. Phys. Chem. 1960, 64, 561. (40) Ethylene glycol was used as the third test liquid on PS in place of methylene iodide because of partial dissolution of the PS film in the methylene iodide drops. The surface tension components of PMMA were the same if either methylene iodide or ethylene glycol were used as the third test liquid.
Wetting Behavior of Thin Films
Figure 5. Interfacial energy between SAMs of OTS exposed at different doses and polystyrene (O) and poly(methyl methacrylate) (1). Approximate experimentally determined regimes of wetting behavior of PS and PMMA also are indicated on the plot.
in terms of the FOCG model. Thin films of P(S-b-MMA) on SAMs of OTS exposed below 300 mJ/cm2 did not coat the SAMs or dewet completely during annealing. At these doses, SAMs of OTS have a lower total surface tension than both PS and PMMA. The copolymer films dewet from the substrate to maximize the surface area of OTS in contact with air and minimize the interactions between OTS and the polymer film. For doses between 300 and 500 mJ/cm2, γSL decreased (see Figure 5), and the films partially dewet during annealing. For doses between 500 and 1200 mJ/cm2, γSL decreased further, and the films wet the substrate before, during, and after annealing. For SAMs exposed to doses between 400 and 1000 mJ/ cm2, symmetric wetting was observed for the thin films with PS at both the substrate and free interface. SAMs of OTS exposed to doses between 400 and 1000 mJ/cm2 have surface tensions with negligible contribution from the polar component, and therefore, there were no polar interactions between the SAMs and the polymer film. Only dispersive forces affected the polymer wetting. Since PS has a lower γLW than PMMA, γSL was lower for the PS block than for the PMMA block (see Figure 5), and the PS block preferentially was segregated to the substrate interface. Polystyrene has a lower surface tension (γtot) than PMMA and, therefore, was segregated to the free surface also. For SAMs exposed at 1200 mJ/cm2, neutral wetting was observed with the lamellae oriented perpendicularly to the plane of the film. The neutrality of the surface can be
Langmuir, Vol. 16, No. 10, 2000 4631
explained in the following way. As γtot and γLW of the SAMs increased with dose, both blocks became more attracted to the surface through increased dispersive forces, and γSL for both blocks decreased for doses less than 1200 mJ/ cm2 (see Figure 5). For a dose of 1200 mJ/cm2, there were still no polar interactions between the SAM and the polymer film because γ+ of the SAM equaled zero. At a dose of 1200 mJ/cm2, γLW of the SAM approximately equaled γLW of both blocks, and γSL was at a minimum near zero and was approximately equal for both blocks. This condition at 1200 mJ/cm2 corresponded to a neutral surface that had no strong affinity for either block, and the lamellae oriented perpendicularly to the plane of the film. Asymmetric wetting was observed for thin films deposited on SAMs exposed to doses higher than 1200 mJ/ cm2. Above doses of 1200 mJ/cm2, γLW of the SAMs continued to increase with dose such that γSL for both blocks increased from the minimum value. The consequences of the polar nature and nonzero electron-acceptor component of the surface tension of the SAMs exposed at doses above 1200 mJ/cm2 were hydrogen bonding and dipole-dipole interactions between PMMA and the surface. These interactions were between the surface and PMMA because γ- was larger for PMMA than for PS, and they resulted in a lower value of γSL for PMMA compared to PS (see Figure 5). Therefore, the PMMA block preferentially was segregated to the substrate interface. Conclusions We have demonstrated that the wetting behavior of block copolymers can be controlled using SAMs of OTS exposed to X-rays in the presence of air. Symmetric, neutral, and asymmetric wetting of P(S-b-MMA) films were observed as a function of exposure dose. The wetting behavior was understood by analyzing the interfacial energies between the blocks of the copolymer and the chemically modified surfaces (incorporation of aldehyde and hydroxyl groups) using the FOCG surface tension model and the Good-Girifalco-Fowkes combining rule. The transition from symmetric to neutral to asymmetric wetting resulted from increased hydrogen bonding and dipole-dipole interactions between the surface and PMMA with increasing dose. The ability to control the wetting behavior of films of block copolymers with exposure to synchrotron radiation represents an enabling technology for inducing macroscopic orientation of block copolymer domains using chemically heterogeneous substrates patterned with advanced lithography. Acknowledgment. Funding for this work was provided by the Semiconductor Research Corporation (Grant Number 98-LP-452), NSF Small Grant for Exploratory Research (Grant Number CTS-9708944), and NSF Career Award (Grant Number CTS-9703207). Facilities were supported by DARPA/ONR (Grant Number N00014-971-0460) and the NSF (Grant Number DMR-95-31009). LA991500C
