Topographically Uniform but Chemically Heterogeneous

Aug 26, 2010 - Nanoimprint lithography is applied to fabricate topographically uniform patterns onto demixed polymer blend films. The high fidelity of...
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Topographically Uniform but Chemically Heterogeneous Nanostructures by Nanoimprinting Demixed Polymer Blends Zhen Wang, Dae Up Ahn, and Yifu Ding* Department of Mechanical Engineering, University of Colorado, Boulder, Colorado 80309-0427 Received June 21, 2010. Revised Manuscript Received August 3, 2010 Nanoimprint lithography is applied to fabricate topographically uniform patterns onto demixed polymer blend films. The high fidelity of pattern replications is achieved for 150 nm polystyrene (PS)/polymethylmethacrylate (PMMA) blend films with varying compositions. When imprinted at 150 °C, the morphology of the blend across the patterned structures is similar to that in the as-casted films. Significant morphological evolutions occur for patterns imprinted at 180 and 210 °C. For all the patterns, PMMA is found to segregate into the residual layer, driven by the preferential wetting of PMMA onto the SiOx surfaces. The combined domain coarsening and preferential wetting of PMMA leads to the formations of unique encapsulated structures within the topographically uniform features, ranging from blocks to threads.

Introduction Nanoimprinting lithography (NIL) is a low-cost, accessible lithographic technique with sub-10 nm patterning resolution.1-5 By utilizing the viscoelastic deformations of a polymer resist, features on a mold can be faithfully replicated onto the polymer film. This offers unique capability and potential to directly pattern a range of functional materials.3,6-12 Up to now, neat polymers have been widely used in NIL fabrication, where their viscoelastic properties dictate both the fidelity of the pattern replication and the stress state of the obtained structures.13-16 However, neat polymers are intrinsically limited by their chemical and physical characteristics. Multifunctional polymer nanostructures with a combination of topographic features and diverse chemical functionalities are critical to many emerging technologies. Mixing or *To whom correspondence should be addressed. E-mail: yifu.ding@ colorado.edu. (1) Austin, M. D.; Ge, H. X.; Wu, W.; Li, M. T.; Yu, Z. N.; Wasserman, D.; Lyon, S. A.; Chou, S. Y. Appl. Phys. Lett. 2004, 84, 5299. (2) Chou, S. Y.; Krauss, P. R.; Renstrom, P. J. Science 1996, 272, 85. (3) Guo, J. L. Adv. Mater. 2007, 19, 1. (4) Hua, F.; Sun, Y. G.; Gaur, A.; Meitl, M. A.; Bilhaut, L.; Rotkina, L.; Wang, J. F.; Geil, P.; Shim, M.; Rogers, J. A.; Shim, A. Nano Lett. 2004, 4, 2467. (5) Gates, B. D.; Xu, Q. B.; Stewart, M.; Ryan, D.; Willson, C. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1171. (6) Chao, C. Y.; Guo, L. J. J. Vac. Sci. Technol. B 2002, 20, 2862. (7) Choi, H. G.; Choi, D. S.; Kim, E. W.; Jung, G. Y.; Choi, J. W.; Oh, B. K. BioChip J. 2009, 3, 76. (8) Hong, S. H.; Han, K. S.; Lee, H.; Cho, J. U.; Kim, Y. K. Jpn. J. Appl. Phys. Part 1 2007, 46, 6375. (9) Kim, M. S.; Kim, J. S.; Cho, J. C.; Shtein, M.; Guo, L. J.; Kim, J. Appl. Phys. Lett. 2007, 90, 123113. (10) Ahn, S. H.; Guo, L. J. Adv. Mater. 2008, 20, 2044. (11) Cheng, X.; Guo, L. J.; Fu, P. F. Adv. Mater. 2005, 17, 1419. (12) Holdcroft, S. Adv. Mater. 2001, 13, 1753. (13) Ding, Y. F.; Ro, H. W.; Alvine, K. J.; Okerberg, B. C.; Zhou, J.; Douglas, J. F.; Karim, A.; Soles, C. L. Adv. Funct. Mater. 2008, 18, 1854. (14) Ding, Y. F.; Ro, H. W.; Douglas, J. F.; Jones, R. L.; Hine, D. R.; Karim, A.; Soles, C. L. Adv. Mater. 2007, 19, 1377. (15) Ding, Y. F.; Ro, H. W.; Germer, T. A.; Douglas, J. F.; Okerberg, B. C.; Karim, A.; Soles, C. L. Acs Nano 2007, 1, 84. (16) Schulz, H.; Wissen, M.; Bogdanski, N.; Scheer, H. C.; Mattes, K.; Friedrich, C. Microelectron. Eng. 2006, 83, 259. (17) Bates, F. S. Science 1991, 251, 898. (18) Han, C. D. Rheology and Processing of Polymeric Materials; Oxford University Press: New York, 2007; Vol. 2. (19) Mark, J. E. Physical Properties of Polymers Handbook; AIP Press: New York, 1996.

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blending polymers has been a traditional processing strategy to improve the properties of the neat polymer materials in bulk.17-19 It remains unclear whether it is a viable approach to fabricate chemically heterogeneous nanostructures using NIL. Besides the implication to the practical applications, the morphological evolution of multicomponent polymers under the NIL process is a fundamentally intriguing problem. At the length scale relevant to the NIL, surface and interfacial interactions between the constituent polymers and the confining environments may be dominant.20-23 In addition, the physical confinement of the cavity walls is stronger than that in the planar thin films, which will significantly influence the phase separation and morphological evolutions such as domain breakup or coarsening.24,25 Under these motivations, this article presents the fabrication of polymer blend patterns, focusing on the morphological evolutions of the blend under the NIL conditions. The results clearly demonstrate the significant influence of preferential wetting and physical confinement on the morphological evolutions of the confined polymer blend, which leads to a range of unique topographically uniform and chemically heterogeneous patterns.

Experimental Sections Polystyrene (PS)/polymethylmethacrylate (PMMA) blends were selected for this study because their morphological behaviors have been extensively studied in both bulk18,19 and thin films.26-32 PS and PMMA with corresponding molecular (20) Sung, L.; Karim, A.; Douglas, J. F.; Han, C. C. Phys. Rev. Lett. 1996, 76, 4368. (21) Tanaka, H. Phys. Rev. Lett. 1993, 70, 53. (22) Tanaka, H. Phys. Rev. Lett. 1993, 70, 2770. (23) Tanaka, K.; Yoon, J. S.; Takahara, A.; Kajiyama, T. Macromolecules 1995, 28, 934. (24) Chen, D.; Chen, J. T.; Glogowski, E.; Emrick, T.; Russell, T. P. Macromol. Rapid Commun. 2009, 30, 377. (25) Chen, J. T.; Zhang, M. F.; Russell, T. P. Nano Lett. 2007, 7, 183. (26) Cui, L.; Ding, Y.; Li, X.; Wang, Z.; Han, Y. C. Thin Solid Films 2006, 515, 2038. (27) Walheim, S.; Boltau, M.; Mlynek, J.; Krausch, G.; Steiner, U. Macromolecules 1997, 30, 4995. (28) Ton-That, C.; Shard, A. G.; Teare, D. O. H.; Bradley, R. H. Polymer 2001, 42, 1121. (29) Tanaka, K.; Takahara, A.; Kajiyama, T. Macromolecules 1996, 29, 3232. (30) Ton-That, C.; Shard, A. G.; Daley, R.; Bradley, R. H. Macromolecules 2000, 33, 8453.

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Figure 1. Optical and topographic AFM images of the PS/PMMA films with varying compositions (three rows, as marked on the left) and thicknesses around 150 nm. From left to right: columns one and two correspond to the optical and AFM images of as-casted films; column three and four show the AFM images of the 150 nm films after they were etched with cyclohexane (CLE) and acidic acid (AA), respectively. All AFM are 10 μm  10 μm in size. weight of 190,000 g mol-1 and 94,000 g mol-1 were purchased from Scientific Polymers and used as received. The glass transition temperature (Tg) for PS and PMMA were determined to be 100 °C ( 2 and 126 °C ( 2 °C, respectively, by differential scanning calorimetry (DSC) with a scanning rate of 10 °C min-1. PS/PMMA films on silicon wafers (with a native oxide layer) were prepared by spin-coating, at 2000 rpm for 1 min, from their toluene solutions. From a predetermined spin-coating curve, the PS/PMMA film obtained was ∼150 nm. The films were subsequently annealed at 50 °C under vacuum for 2 h to remove the residual toluene. Three different compositions of the PS/PMMA films, 30/70, 50/50, and 70/30 in weight ratio, were prepared. NIL processes on the PS/PMMA films were carried out on a nanoimprinter (Eitrie 3, Obducat, Inc.). The mold applied is a parallel line-and-space grating consisting of SiOx with a periodicity of 834 nm, a line-to-space ratio of 1:1, and a cavity depth of ∼195 nm. The mold was treated with piranha solution prior to use. To facilitate the mold separation after imprinting, a low surface energy self-assembly monolayer of CF3(CF2)5(CH2)2SiCl3 (tridecafluoro-1,1,2,2-tetrahydrooctyltrichlorosilane, Aldrich, Inc.) was deposited onto the mold surface through a vapor deposition process.33 For each PS/PMMA composition, NIL was carried out at three temperatures, 150, 180, and 210 °C, under a pressure of 4 MPa for 30 min. The mold was separated from the PS/PMMA replica after the system was cooled down to 50 °C. To distinguish different components within the films and patterns, cyclohexane (CLE) and acetic acid (AA) were used to selectively dissolve PS and PMMA, respectively.34-36 AFM (DI3100, Vecco) and a high resolution optical microscope (Nikon LV 150) were (31) Saunders, A. E.; Dickson, J. L.; Shah, P. S.; Lee, M. Y.; Lim, K. T.; Johnston, K. P.; Korgel, B. A. Phys. Rev. E 2006, 73, 7. (32) Sugihara, H.; Oya, K.; Murase, H.; Akabori, K.; Tanaka, K.; Kajiyama, T.; Takahara, A. Appl. Surf. Sci. 2008, 254, 3180. (33) Jung, G. Y.; Li, Z. Y.; Wu, W.; Chen, Y.; Olynick, D. L.; Wang, S. Y.; Tong, W. M.; Williams, R. S. Langmuir 2005, 21, 1158. (34) Chavez, K. L.; Hess, D. W. J. Electrochem. Soc. 2003, 150, G284. (35) Harton, S. E.; Luning, J.; Betz, H.; Ade, H. Macromolecules 2006, 39, 7729. (36) Otsuka, T.; Taki, K.; Ohshima, M. Macromol. Mater. Eng. 2008, 293, 78.

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used to examine the morphologies of the films or patterns obtained.

Results and Discussion For all the PS/PMMA films, phase separations were observed after the spin-coating, as expected from the phase diagram of PS/ PMMA with the given molecular weights.37 We note that all the temperatures that the films and patterns were exposed to in this study are within the two-phase region of the PS/PMMA phase diagram. The phase separation causes the film surfaces to be rather rough, and the exact film thicknesses were not determined, but estimations from the solvent treated patterns show that the thickness is close to the targeted value. Figure 1 presents the optical and topographic AFM images of the as-casted PS/PMMA with the three designated compositions, revealing two-phase morphologies for all the films. Each of the two phases was identified by the AFM measurements on the films after selective dissolutions of either PMMA (by acetic acid, or AA) or PS (by cyclohexane, or CLE). Islands or cylindrical domains were observed after the PS dissolution (Figure 1A3, B3, C3), while holes were found after corresponding PMMA removal (Figure 1A4, B4, and C4). This clearly shows that PS forms the continual phase for all the films even in the 30/70 composition where PS is the minor phase. It is known that bulk morphology for a pair of immiscible polymer blends is determined by both the composition and the viscosity ratio between the two components.18 According to the minimum energy dissipation mechanism, the lower (higher) viscosity component tends to form the continuous (dispersed) phase, as frequently witnessed by melt processing of immiscible polymer blends.18 However, a major component of the blend is more likely to form the continuous phase. The interplay between these two factors will determine the (37) Ruzette, A. V. G.; Mayes, A. M. Macromolecules 2001, 34, 1894.

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Figure 2. Images of the 30/70 PS/PMMA patterns imprinted at three different temperatures, as marked on the left. From left to right, different columns correspond to optical images of films after imprinting, corresponding topographic AFM images of the as-imprinted films, after the removal of PS with CLE, and after the removal of PMMA with AA. All AFM images are 10 μm  10 μm. Insets are the FFT images of the optical images.

final morphology of the blend. In the present case, PMMA is the more viscous component and will tend to form the dispersed phase, according to the minimum energy dissipation mechanism. The observation that the major component PMMA forms the dispersed phase in the 30/70 PS/PMMA sample suggests that the viscosity ratio might be the dominating factor. In addition, in blend thin films, the morphology is also strongly affected by the surface and interfacial energies of the components. In this regard, PS has slightly lower surface tension than that of PMMA, which could also favor higher surface coverage of PS.38 Further, studies have shown that autophobic dewetting of PMMA due to the favorable interactions of PMMA with the surface hydroxyl groups at the substrate surface could also lead to similar morphologies.39,40 Regardless, PMMA domains are elevated over the PS domains during spin-coating due to the difference in critical surface tension of PS and PMMA: lower surface tension PS tends to spread over larger area, while the PMMA phase tends to contract to form protrusions.29 All of the morphologies are consistent with the previous reports.41 Figure 1 also shows that the size of the PMMA domains progressively decreases with its concentrations. For the 30/70 PS/ PMMA films, the spatial distributions of the PMMA islands clearly indicate a bicontinuous morphology resulting from the spinodal decomposition during the spin-coating process.17 The wavelength (λf) or the correlation length, from the FFT (inset) of Figure 1A1, was determined to be 3.3 ( 0.3 μm. For the 50/50 and 70/30 films, the PMMA rich phases have evolved into cylindrical shape. For the 50/50 PS/PMMA film, the diameter (dPMMA) of the PMMA domain was estimated, from the holes left after the AA dissolution, as up to 0.6 μm. Similarly, dPMMA was estimated to be ∼0.35 μm for the 70/30 PS/PMMA film. Consistent with the (38) Wu, S. J. Phys. Chem. 1970, 74, 632. (39) Chen, X. C.; Anthamatten, M. Langmuir 2009, 25, 11555. (40) Xue, L. J.; Han, Y. C. Langmuir 2009, 25, 5135. (41) Morin, C.; Ikeura-Sekiguchi, H.; Tyliszczak, T.; Cornelius, R.; Brash, J. L.; Hitchcock, A. P.; Scholl, A.; Nolting, F.; Appel, G.; Winesett, D. A.; Kaznacheyev, K.; Ade, H. J. Electron Spectrosc. Relat. Phenom. 2001, 121, 203.

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studies by Morin et al., smaller domains of PS (or PMMA) within the PMMA (PS) rich domains are clearly observed in the AFM images in Figure 1.41 For each composition, three temperatures (Timp values) were applied, 150 °C, 180 °C, and 210 °C, to imprint the PS/PMMA films (see Experimental Section). Because the mobility of PS and PMMA strongly depends on the temperature, the selected Timp will provide a kinetic pathway of the morphological evolutions of the PS/PMMA during the NIL process. Specifically, the steadystate viscosity of the PS is estimated to be 2.7  106 Pa 3 s, 6.5  104 Pa 3 s, and 6.3  103 Pa 3 s at 150, 180, and 210 °C, respectively.42 In comparison, the viscosity of PMMA is roughly 3.1  109 Pa 3 s, 1.8  107 Pa 3 s, and 5.3  105 Pa 3 s at 150, 180, and 210 °C.43 In contrast to the dramatic changes in mobility, the surface tensions (γ) of both PS and PMMA only changes slightly: γPS is 31.3, 29.2, and 27 mN/m, while γPMMA is 30.8, 28.5, and 26.3 mN/m, at 150, 180 and 210 °C, respectively.19 Figures 2, 3, and 4 display the optical and AFM images of the patterned PS/PMMA with corresponding compositions of 30/70, 50/50, and 70/30. Similarly, to distinguish PS and PMMA within the structure, AFM images on the remaining patterns after selective dissolutions are shown in Figures 2-4. Determined by the mold geometry, the minimum thickness of the resist film to completely fill the cavities is ∼100 nm. Below this thickness, incomplete filling of the cavity will occur, which results in significant broken lines during imprinting (images not shown). This is due to the nucleated dewetting of the PS/PMMA within the cavities, which has been observed even in the neat polymer resist.44-46 Thicker films such as 150 nm will provide enough polymers to completely fill the cavities and leave no space for the (42) Majeste, J. C.; Montfort, J. P.; Allal, A.; Marin, G. Rheol. Acta 1998, 37, 486. (43) Oconnor, K. M.; Scholsky, K. M. Polymer 1989, 30, 461. (44) Zhang, H. L.; Bucknall, D. G.; Dupuis, A. Nano Lett. 2004, 4, 1513. (45) Suh, K. Y.; Lee, H. H. J. Chem. Phys. 2001, 115, 8204. (46) Bogdanski, N.; Wissen, M.; Mollenbeck, S.; Scheer, H. C. J. Vac. Sci. Technol. B 2006, 24, 2998.

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Figure 3. Images of the 50/50 PS/PMMA patterns imprinted at three different temperatures, as marked on the left. From left to right, different columns corresponding to optical images of films after imprinting, corresponding topographic AFM images of the as-imprinted film, after the removal of PS with CLE, and after the removal of PMMA with AA. All AFM images are 10 μm  10 μm. Insets are the FFT images of the optical images.

nucleated dewetting. This was indeed the case for all our blend films. High fidelity of pattern replications was achieved for all of the samples across the whole field of the patterns (2.5  2.5 cm) at all three Timp values, as confirmed by AFM mapping. Representative topographic AFM images are shown in A2, B2, and C2 of Figures 2-4. The as-imprinted PS/PMMA lines have heights of ∼195 nm, matching the depths of the mold cavities. In addition, the patterned lines display sharp corners, consistent with complete cavity filling. Such good pattern replication is determined by the viscoelastic properties of the PS/PMMA, i.e., both components are capable of large deformation under the imprinting conditions. In the following, detailed morphologies of PS/PMMA within the uniform patterns are described. For the 30/70 PS/PMMA patterns, the optical images (Figure 2A1, B1, and C1) clearly reveal the two distinct phases, in contrast to the topographic AFM images (Figure 2A2, B2, and C2). The dark and bright regions in Figures 2A1, B1, and C1 correspond to the PS and PMMA phases, which were further verified by the AFM measurements on the patterns after selective dissolution. Similar to the as-casted film (Figure 1A1), the PS/ PMMA patterns created at Timp=150 °C (Figure 2A1) display an inverted morphology across the patterned lines: correlated PMMA-rich domains dispersed in the PS-rich matrix. This unique structure is manifested in the FFT of the optical image (inset of Figure 2A1): the scattering ring corresponds to the spatial correlations of the PMMA domains, and the two symmetric spots are the first order diffraction peaks (corresponding to a pitch of ∼834 nm) of the periodic PS/PMMA pattern. The λf of the PMMA domains is determined to be 3.8 ( 0.3 μm, slightly larger than that obtained from the as-casted film (∼3.3 μm). The PMMA phase boundaries remain smooth and continuous across different pattern lines (Figure 2A1), which is consistent with the AFM images on the solvent treated patterns (Figure 2A3 and A4). Apparently, the flow of PS/PMMA during cavity filling does not generate appreciable domain breakups, neither does it involve considerable mass flow in the directions parallel to the lines. 14912 DOI: 10.1021/la102523t

Moreover, no significant morphological evolutions were evident after the cavity filling, which is probably due to the slow mobility of both polymers at 150 °C. At Timp = 180 and 210 °C, high fidelity of pattern replications was also achieved, as demonstrated by representative AFM images (Figure 2B2, C2). The as-imprinted patterns (Figure 2B1 and C1) show that significant morphological evolutions occurred during the NIL process at both Timp values. At 180 °C, PMMA domains are still highly correlated across different lines, but the phase boundaries become noticeably rough (Figure 2B1). This trend becomes more evident for PS/PMMA patterns created at 210 °C (Figure 2C1). From the FFT image (inset of Figure 2C1), the correlation for PMMA domains along the pattern line direction is completely lost, while noticeable correlations remained at the direction perpendicular to the lines. Such anisotropic disintegration of the PMMA domains indicates that the mass transport that contributed to the domain breakup mostly occurred along the line direction due to the confinement of cavity walls. Clearly, both PS and PMMA become increasingly mobile with the increase of Timp, which enables the domain breakup during the imprint process. Surprisingly, the remaining PS patterns after PMMA removal (Figure 2A4, B4, and C4) are ∼240 nm, which is ∼45 nm taller than the as-imprinted pattern (Figure 2A2, B2, and C2). In contrast, after the CLE dissolution, the maximum height of the remaining PMMA pattern was ∼195 nm (Figure 2A3, B3, and C3). The discrepancies indicate that the residual layers in the patterned 30/70 PS/PMMA were mostly PMMA. From the mass conservation consideration, a 45 nm residual layer thickness implies an initial film thickness of ∼145 nm, close to the targeted 150 nm. This suggests that PMMA may prefer the extrusion of the mold (SiOx surface) and the Si substrate,47,48 the two surfaces confining the residual layer, which will drive the PS out of the residual layer or isolate them into smaller domains. Upon AA (47) Heriot, S. Y.; Jones, R. A. L. Nat. Mater. 2005, 4, 782. (48) Harris, M.; Appel, G.; Ade, H. Macromolecules 2003, 36, 3307.

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Figure 4. Images of the 70/30 PS/PMMA patterns imprinted at three different temperatures, as marked on the left. From left to right, different columns corresponding to optical images of films after imprinting, corresponding topographic AFM images of the as-imprinted film, after the removal of PS with CLE, and after the removal of PMMA with AA. All AFM images are 10 μm  10 μm. Insets are the FFT images of the optical images. (D) The cross-sectional profiles of B2, B4, and C4, as marked in the AFM images.

treatment, the residual layer is thus completely removed. Note that such preferential segregation of PMMA will effectively cause the increase of PS composition within the patterned lines. However, a quantitative assessment is difficult from the AFM images shown in Figure 2. Additionally, one would also expect that the PMMA will segregate underneath the patterned lines, which could lead to the complete dissolution/removal of the patterns after acetic acid treatment. Indeed, further dissolution of the remaining pattern in Figure 2A4 reveals a layer of PMMA with thickness around 20-30 nm underneath the PS in the patterned lines (images not shown). The fact that these PMMA did not dissolve in acetic acid within the short time is likely attributed to both the protection from the PS layer and the strong interactions between the PMMA and the substrate. Interestingly, the AFM images of the AA dissolved pattern (Figure 2A4, B4, and C4) revealed that some thin polymer lines (marked by the arrows) left within the PMMA rich domains. These lines were 60-100 nm tall and 50-80 nm wide. These remaining lines were identified to be undissolved PMMA, through the following two experiments. First, these patterns were completely removed by using more rigorous dissolutions with AA (49) Zhou, J.; Berry, B.; Douglas, J. F.; Karim, A.; Snyder, C. R.; Soles, C. Nanotechnology 2008, 19, 9.

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at 50 °C. Second, we determined the Tg of these small lines to be close to the bulk value (126 °C) of the PMMA, by using the newly developed Nanothermal analysis (Nano-TA).49 Note that the dissolution of PMMA thin films supported on substrate is a complex phenomenon. The origin of the reduced solubility of PMMA in the 30/70 patterns remains unclear. Xue et al. have shown that favorable interactions between PMMA and hydroxyl groups on the substrate surface can be strong enough to prevent PMMA dissolution.40 However, similar reduction in the solubility of high molecular weight PS in toluene after being imprinted at high temperatures was also observed recently.50 Similar morphological analysis on the patterned 50/50 PS/ PMMA films is shown in Figure 3. Successful pattern replications were also achieved at all three Timp values (Figure 3A2, B2, and C2), forming topographically uniform patterns onto phase separated PS/PMMA (Figure 3A1, B1, and C1). The solvent dissolution experiments revealed that the residual layers (∼35 nm) of the 50/ 50 PS/PMMA with all three Timp mostly consisted of PMMA, similar to the 30/70 samples. Within the patterned lines, the morphological evolutions progressively increase with the increase of Timp. Specifically, PMMA domains are mostly isolated within the lines (Figure 3A3 and A4) at Timp = 150 °C, while growing (50) Soles, C. L. EIPBN, Baltimore, MD, 2006.

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large enough to bridge across the lines and interconnect with the residual layer at Timp = 180 °C (Figure 3B3 and B4). Such morphological change is more evident at Timp = 210 °C (Figure 3C3 and C4). Correspondingly, the average height of the remaining PMMA pattern, in regions where the PS was removed, decreased from ∼60-80 nm (Figure 3A3) and 4060 nm (Figure 3B3), to 10-20 nm (Figure 3C3) as Timp increased from 150 and 180 to 210 °C. These observations suggest that the PMMA domains within the lines gradually coalesce with the residual layer and drive the original continuous PS matrix into encapsulated blocks within the line. The surface coverage of the PS along the lines decreases simultaneously. Such a phase inversion in the patterned PS/ PMMA blends is driven by the stronger segregation tendency of PMMA into the residual layer and coarsening within the cavity walls. Intriguingly, the encapsulated block morphology formed via anisotropic growth of the PMMA domain in the 50/50 pattern (Figure 3C1, C3, and C4) is similar to that developed via the anisotropic breakup of the PMMA domain in the 30/70 pattern (Figure 2C1, C3, and C4). Such similarity maybe the result of the complex interfacial interactions among PS, PMMA, and the cavity walls. Excellent pattern replications were also achieved for the 70/30 PS/PMMA at all Timp values (Figure 4A2, B2, and C2). The optical images of the as-imprinted structures cannot distinguish the two-phase morphology, due to the insufficient resolution. With the selective dissolution experiments, dispersed PMMA domains were found distributed randomly across the patterned lines and the residual layers with Timp = 150 °C (Figure 4A4). Some of these domains were deformed by the squeeze flow during cavity filling (Figure 4A3). The PS/PMMA morphology is almost identical to that in the as-casted films, which is consistent with the 30/70 and 50/50 PS/PMMA films patterned with Timp = 150 °C. In particular, because the PMMA domain (e0. 35 μm, Figure 1C3 and C4) is smaller than the widths of the lines and residual layer (∼ 0.42 μm), they are completely isolated. Significant coarsening of the PMMA domains occurred with increase of Timp, within both the patterned lines and the residual layers. At Timp = 180 °C, after the PMMA removal, the remaining PS lines become noticeably thinner, indicating that the top of the lines are covered by a thin layer of PMMA (Figure 4D). This is consistent with the observations that most of the line surfaces are intact after the CLE dissolution (Figure 4B3). In addition, the remaining PS lines are significantly smoother than that in Figure 4A4. Moreover, PMMA domains start to coalesce within the residual layer, forming larger holes after the AA dissolutions (Figure 4B4). Such morphological evolutions become more dramatic in the patterns created at 210 °C. After the PMMA removal (Figure 4C4), the remaining PS pattern height is ∼245 nm, suggesting that almost all residual layers (∼50 nm) consisted of PMMA (Figure 4D). In addition, the remaining PS lines grow thicker than that observed in Figure 4C4, suggesting that more PS moved into the patterned lines due to volume conservation. Second, almost no lines were dissolved by CLE (Figure 4C3), indicating that the entire surfaces of the lines

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are covered by PMMA. However, the thickness of this PMMA layer seems to be thinner than that observed in the patterns with Timp = 180 °C. The observation that the PMMA surface coverage in the 70/30 system is the most dramatic among all three compositions is rather surprising because PMMA concentration is the lowest in this system. Such a counterintuitive phenomenon could be driven by multiple factors. First, the size of the PMMA domains in the 70/30 PS/PMMA films/patterns is the smallest, creating the largest interfacial tension that favors domain coarsening. Second, the average spacing between the PMMA domains is only ∼350 nm, which is significantly smaller than the other two compositions. This leads to the smaller diffusion length required for PMMA to bridge and cover the surfaces both in the residual layer and within the lines. In addition, PS is the less viscous component, and the higher composition of PS is likely to make the matrix more mobile, thus easier for PMMA to diffuse within the matrix. To conclude, we demonstrate the successful fabrication of topographically uniform patterns over phase separated polymer blend films, focusing on the morphological evolutions of the blend during the patterning process. The results demonstrate that for patterns created at relatively low temperature such as 150 °C, the PS/PMMA morphology within the patterns is similar to that in the as-cast films. With increase of imprinting temperature, the morphological evolutions become more dramatic. For all of the compositions, PMMA increasingly migrates to the residual layer (∼35-50 nm thick), driven by the wetting preference of PMMA on SiOx surfaces. The morphological evolutions at higher Timp (180 and 210 °C) vary with the compositions. In the 30/70 PS/PMMA patterns, the large PMMA domains start to break anistropically within the cavities during imprinting. This leads to randomly distributed blocks or plugs of PS (or PMMA) domains within the lines. For the 50/50 PS/ PMMA patterns, the anistropic coarsening and PMMA preferential wetting during imprinting cause a phase inversion: the originally dispersed PMMA domains evolve into the continuous phase across the residual layer and patterned lines which drives the PS into isolated blocks within the patterned lines. For the 70/30 PS/PMMA, PMMA migrates to cover the surface of the patterned lines in addition to occupying the residual layer. This also leads to encapsulated long PS threads within the lines, which is different from the block morphologies observed in the other two compositions. These results demonstrate that the complex morphological evolutions under the nanoimprint conditions are due to the interplay of domain coarsening and preferential wetting of different components. By controlling the processing parameters and surface energy of the substrates and superstrates, designing different polymer blends, and adjusting their compositions, one can achieve uniform topographic surface patterns with chemically highly heterogeneous multicomponent polymers. Acknowledgment. We acknowledge the partial funding support from National Science Foundation under Grant No. CMMI-0928067.

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