Article pubs.acs.org/Macromolecules
Dewetting-Induced Hierarchical Patterns in Block Copolymer Films Su Yeon Choi,† Chansub Lee,† Jin Wook Lee,† Cheolmin Park,‡ and Seung Hyun Kim*,† †
Division of Nano-Systems Engineering, Inha University, Incheon 402-751, South Korea Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, South Korea
‡
S Supporting Information *
ABSTRACT: Thin film stability should be carefully monitored in many practical applications and at the same time can be effectively utilized to produce the surface pattern. In this work, it is demonstrated that block copolymer thin films can generate highly ordered, hierarchical surface pattern via combined process of self-assembly and film destabilization. The dewetting process generates the surface pattern of hole or island on the micrometer scale while the self-assembly produces well-ordered pattern on the nanometer scale. Highly ordered nanostructure is obtained by solvent annealing within the remaining part of dewetted film, and its orientation is systematically controlled by adjusting the relative humidity, ultimately leading to fine-tuned hierarchical surface pattern with high degree of lateral order. As a simple example, the formation of hierarchical structure containing Au nanowire along the block copolymer domains on the dewetted films illustrates that our simple approach can offer great opportunity to generate true hierarchical, complex patterns.
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INTRODUCTION The self-assembly of block copolymer (BCP) has emerged as a simple yet powerful tool to generate well-defined, wellorganized nanostructures that could be utilized in a variety of applications such as nanolithography and nanoscale device fabrication.1−5 Various shapes including spheres, cylinders, gyroids, and lamellae are generated on the nanometer scale due to the connectivity between phase-separating blocks, and their characteristic size can easily be controlled by the molecular weight of BCPs. In thin films, the situation becomes more complicated due to additional parameters and constraints that should be taken into account to control their structure and final properties. Confinement effects and surface energetics with substrate and air surfaces can lead to the formation of BCP nanostructure different from that in the bulk system and can also affect the orientation of the microdomains with asymmetric shape such as cylinders and lamellae, producing different surface patterns in thin film despite of the same components and composition. Moreover, island and hole structure can be formed at the film surface when the film thickness is not commensurate with BCP periodicity in the case of parallel orientation of microdomains. Consequently, the ability to control their phase separation behavior and nanostructure in thin films should be highly demanded in order to meet the requirements for practical applications of BCP self-assembly.6−19 In thin film geometry, the destabilization of films is another important technical issue that should be carefully considered for pattern formation.20−26 Under adequate condition, the local fluctuation of film thickness is developed, and the film is retracted from the substrate to create the fragmentation of continuous films into isolated droplets. Although there are © 2012 American Chemical Society
exceptional cases where dewetting process can be a route to generate the regular pattern, in most cases, the film destabilization causes detrimental problems because practical applications require stable films with uniform thickness. In contrast to homopolymer films, BCP films are known to exhibit strong stability against dewetting over wide range of film thickness due to the wetting layer by preferential interaction and the internal order within the film by phase behavior.27,28 Nonetheless, BCP films can become unstable and dewet from the substrate below critical film thickness. In dewetting case, rather interesting behavior may be observed due to two factors that have suppressed the destabilization in BCP films. Preferential interaction of block component with the substrate forces dewetting of BCP films to proceed in an autophobic way that is different from that in homopolymer films. On the other hand, the internal order in BCP films provides an opportunity to produce hierarchical structure because film dewetting generates the patterns on the micrometer length scale while BCP self-assembly produces well-defined structure on the nanometer scale. Consequently, dewetting of BCP films can produce the surface pattern with true hierarchical structure ranging from the micrometer to nanometer scale. In spite of such expectation, the results exhibiting the hierarchical structure established by destabilization and self-assembly of BCP films in the literature are rather rare. In many cases, the film instability of BCPs has been investigated in disordered state above the order−disorder transition (ODT).29−33 As a result, large size of droplets have been formed after dewetting, Received: August 27, 2011 Revised: January 4, 2012 Published: January 20, 2012 1492
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annealing, the vapor molecules of solvent are absorbed into BCP films so that they impart BCP chains with sufficient mobility to repair the structural defects and improve the lateral order in thin films. In addition, solvent molecules can mediate the interactions at substrate interface, at air surface, and between block components, which enables to manipulate the microdomain orientation by appropriate choice of solvent. Such effects appear more prominent for BCPs with stronger segregation tendency such as polystyrene-block-poly(ethylene oxide) (PS-b-PEO) and polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP). Figure 1a exhibits solvent annealing effects where cylinderforming PS-b-PEO diblock copolymer film with 30 nm thick
but no lateral order resulting from BCP self-assembly has been observed. On the other hand, BCP order has been detected after film dewetting in very limited systems where grazing incidence small-angle X-ray scattering (GISAXS) or grazing incidence small-angle neutron scattering (GISANS) were used to examine the structure of dewetted films.28,34,35 The scattering patterns revealed the internal structure of BCP films after dewetting, but weak signals indicated ill-defined nanostructure with poor order within dewetted droplets. In other approach reported recently, the substrate was prepatterned by combining lithographic methods with chemical modification of surface to guide the dewetting process of BCP films.36 Dewetting was controlled to occur by following the surface pattern so that it produced highly ordered pattern in the dewetted film. However, the order by BCP self-assembly was still fully not developed in such case and controlling BCP selfassembly within dewetted patterns needs to be further improved in order to realize hierarchical structure ranging from nanometer to micrometer length scale. In this work, it is demonstrated that ideal hierarchical pattern with highly ordered lateral order can be produced by combining the dewetting process with self-assembly of BCPs in their films. The dewetting behavior of BCP films with different thickness was systematically investigated where the dewetting was triggered by solvent vapor. The order and orientation of BCP microdomains within the patterns resulting from dewetting in thin films were manipulated to realize true hierarchical structure. Moreover, the complexation with inorganic salts was utilized together with dewetting behavior to deposit inorganic materials along dewetted pattern on the nanometer scale.
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EXPERIMENTAL SECTION
Cylinder-forming polystyrene-block-poly(ethylene oxide) (PS-b-PEO) diblock copolymer with a molecular weight Mn = 25.3 kg/mol and a polydispersity Mw/Mn = 1.04 was purchased from Polymer Source, Inc. Benzene was used as a solvent without further purification. BCP films were deposited on Si wafers with a native oxide layer by spincoating, and before use, the substrates were cleaned in a piranha solution (H2SO4/H2O2 = 3/7). Film thickness was controlled by solution concentration/spinning rate and measured by using a Filmetrics interferometer Model F20-UV. Dewetting of BCP films in our work was induced by solvent annealing where the films were put in the chamber filled with solvent vapor. Relative humidity with the chamber was controlled during solvent annealing to see its effects. After solvent annealing, the films were taken out from the chamber and quenched quickly in the air. The surface structure of films was examined with tapping-mode atomic force microscopy (AFM, MultiMode V scanning probe microscope (Digital Instruments/Veeco)) and field emission scanning electron microscope (FESEM, Hitach S-4300). For resolution, BCP films were treated with oxygen plasma for 10 s before SEM observation. For complexation with BCP, the gold precursor, HAuCl4·3H2O, was purchased from Sigma-Aldrich and preloaded by dissolving in THF and adding to BCP solution. The precursor concentration was set to be 16:1 molar ratio of [PEO]/[Au]. Film formation and dewetting proceeded in the same way as before without gold precursor. After solvent annealing, BCP film complexed with gold precursor was exposed to oxygen plasma for about 20 s at 20 W and a pressure of 5 × 10−1 Torr to remove BCP films and reduce the gold ions.
Figure 1. AFM height and SEM images of solvent-annealed PS-b-PEO films with different thickness: (a) 30, (b) 25, (c) 19, (d) 16, (e) and (f) 13 nm. All films were annealed under benzene vapor for 12 h at RH = 80%.
was annealed under benzene vapor for 12 h. Highly ordered array of PEO cylinders that are oriented perpendicular to the film surface is generated over large area, as seen in the previous work.8 However, it should be borne in mind that solvent annealing can occasionally give rise to film instability, leading to catastrophic failure in controlling nanopatterns of BCP thin films. Under fully saturated vapor, copolymer films tend to be completely dewetted from the substrate and be left featureless and useless. The vapor pressure of solvents should be adjusted on annealing to prevent BCP films from dewetting and at the same time to provide sufficient chain mobility so that BCP films can take full advantage of solvent annealing. As a result, PS-b-PEO films thicker than 30 nm can remain stable during solvent annealing and finally produce highly ordered nanostructure after solvent annealing.
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RESULTS AND DISCUSSION Solvent annealing, when applied for BCP thin films, has been shown to be able to improve the degree of lateral order of microdomains and to control their orientation.8,37 On solvent 1493
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Figure 2. AFM height images and height profiles of 13 nm thick PS-b-PEO films annealed at RH = 43% and taken at different annealing times: (a) 30 min, (b) 1 h, and (c) 12 h. AFM images in the middle correspond to those scanned at higher magnification for PS-b-PEO film in the left, and each height profile corresponds to the line in the respective AFM image.
When film thickness decreases, however, BCP films suffer from instability and start to dewet from the substrate, as shown in Figure 1b−f. Figure 1b shows the surface morphology of 25 nm thick BCP film which was solvent-annealed for 12 h under the same conditions as in Figure 1a. It can be observed that BCP film destabilization is developed so that the dewetting proceeds in BCP films with thickness less than BCP periodicity. After solvent annealing, a number of holes are created at the film surface due to dewetting, and their diameter and depth are measured to be 2.7 ± 0.9 μm and 23.7 ± 2.3 nm, respectively. Closer examination of the remaining part of film reveals that highly ordered array of PEO microdomains are formed with orientation normal to the film surface, in spite of hole formation. Such highly ordered nanostructure in dewetted film is very exceptional and hard to be found in the literature. In most cases, no regular pattern or internal structure with poor order within dewetted film has been reported.29−35 BCP film with 19 nm thickness exhibits the same hole structure, as shown in Figure 1c, except the size and number of holes in the film. With film thickness decreasing, the size of the holes increases while their number decreases. However, as thickness decreases further, dewetting in BCP films proceeds in a different way as can be seen in Figure 1d. This film with 16 nm thickness was solvent-annealed for 12 h under the same conditions, but in this case, island-like droplets with the micrometer length scale, instead of holes, are formed by dewetting during solvent annealing. In the island region, highly ordered arrays of PEO cylinders are observed while the self-assembled structure of BCP is not observed in the other region. The height of island is measured to be 28.6 ± 3.2 nm by AFM. Similar dewetting behavior is observed for thin film with thickness of 13 nm, as shown in Figure 1e. The SEM image in Figure 1f clearly shows that highly ordered PEO microdomains with perpendicular orientation are developed in the droplet region. Therefore, our results clearly indicate that dewetting process can be combined with BCP self-assembly to construct true hierarchical patterns ranging from nanometer to micrometer scales. The orientation of PEO cylinders can be controlled by the relative humidity (RH) inside chamber during solvent annealing. When BCP film is annealed at RH less than about 60%, the orientation of PEO cylinders changes into the
direction parallel to the film surface. (See Figure S1 in the Supporting Information.) This behavior can be explained by detailed balance of interaction energies between components at the interfaces. Benzene used for solvent annealing, although dissolving well both PS and PEO blocks, is a solvent slightly selective for PS blocks. Moreover, PS blocks have lower surface tension than PEO blocks and so exhibit tendency to be segregated to the films surface. However, under humid conditions, water molecules, good solvent for minor PEO component, are introduced together with benzene vapor to balance the surface interactions of PS and PEO components, which results in the neutral condition for both blocks and thus the perpendicular orientation of PEO microdomains. This balance for neutral condition will be broken at relatively low RH to produce preferential interaction at the interface, and consequently, the orientation of PEO cylinders changes from perpendicular to parallel. The same experiments were conducted on the substrate coated with hexamethyldisilazane (HMDS) which induced change in the substrate property from hydrophilic to hydrophobic. In spite of surface modification, PEO cylinders in the film are still oriented normal to the surface at high RH while parallel to the surface at lower RH, as seen in Figure S2 of the Supporting Information. These results support the explanation for the importance of balance of surface energy in controlling the microdomain orientation during solvent annealing. The same RH effects are observed in the dewetted PS-b-PEO films. PEO cylinders are found to be oriented parallel to the surface in the remaining part of dewetted films with hole or island patterns when annealed at RH below 60%. (See Figures 2 and 4.) It can be concluded that, in addition to the formation of hierarchical structure, the microstructure within the dewetted region can be adequately fine-tuned in dewetting process. It may be possible that the mismatch between the film thickness and BCP periodicity causes the formation of hole or island patterns in thin film. The incommensurability of the film thickness will bring about large entropic penalty due to chain extension or contraction for compensation of mismatch between the thickness and natural periodicity and can ultimately force BCP films to be destabilized, producing holes or islands depending on the film thickness. However, such 1494
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Figure 3. AFM height images and height profiles of 13 nm thick PS-b-PEO films annealed at RH = 80% and taken at different annealing times: (a) 30 min and (b) 12 h. AFM images in the middle correspond to those scanned at higher magnification for PS-b-PEO film in the left, and each height profile corresponds to the line in the respective AFM image.
with film thickness. As a result, the height of islands formed at early time of dewetting remains almost constant during solvent annealing, and most of the region of isolated pattern is flat except the boundary region. In this work, it appears more appropriate to express those structures as islands rather than as droplets at the final stage of dewetting. The results for BCP films with the same thickness but annealed at higher RH are presented in Figure 3 where a number of islands are observed to be produced during dewetting. In this case, PEO cylinders with high degree of lateral order are oriented normal to the surface in the island region. In contrast to parallel orientation, the islands that have the microdomains oriented normal to the surface show height variation with time as seen in the section analysis of Figure 3. This can be expected since there are no constraints on thickness variation in the island regions during dewetting for perpendicular orientation. Without such constraints, BCP molecules are much freer to move and dewet from the substrate. As a result, the height of islands is higher than that with parallel orientation, and their shape is even closer to droplets. Moreover, the dewetting rate seems to be faster in the perpendicular orientation, and such spinodal-like patterns as seen in Figure 2a cannot be observed even at very early annealing time. The variation of diameter and height of droplets with time is presented in Figure S4 of the Supporting Information. Figure 4 includes AFM height images taken at different annealing times for BCP films with 22 nm thickness where the holes are produced via destabilization process. For comparison, the corresponding phase images taken at low magnification are included in Figure S5 of the Supporting Information. Here, the dewetting seems to follow the nucleation and growth dewetting mechanism. The holes form and grow with time, but their growth stops after some annealing time. Neither formation of networks of the holes nor eventual evolvement to the isolated droplets is observed. The results for longer annealing times are included in Figure S3 of the Supporting Information where the films were annealed for 18 and 100 h, respectively. Even after 100 h, the holes are still observed, and their size shows little change over annealing time. As in the case of thin films seen in Figures 2 and 3, the dewetting in the film with perpendicular orientation of microdomains (Figure 4c,d) is faster than with parallel orientation (Figure 4a,b). The holes in the films with parallel orientation exhibit no appreciable increase in their depth after formation of holes at short annealing time while small yet continuous increase in the hole depth is observed in the film with perpendicular orientation up to 12 h, and then
behavior is usually observed in parallel orientation not perpendicular one. In the case of perpendicular orientation, the mismatch of the film thickness with the natural periodicity of BCPs is not a problem so serious as to cause the film destabilization. Therefore, the fact that the dewetting and the formation of holes or islands were observed irrespective of microdomain orientation in our work suggests that the hole or island formation is caused by different contribution from the incommensurability of thickness. On the other hand, it has been very recently reported that the asymmetry in chain dynamics due to crystallization of one component in BCP induce dewetting of the films to produce hierarchical structure.38 In this case, the hybrid annealing that corresponds to sequential combination of thermal and solvent annealing and the presence of both components of BCP are the prerequisites for the formation of various hierarchical patterns. However, in our case, PEO blocks do not crystallize in thin films, which was confirmed by GIWAXD and DSC data.40 This suggests that dewetting/crystallization is not the mechanism possible for production of hierarchical morphologies in our work. Several mechanisms for formation of hierarchical meso/nanoscale morphologies in polymer thin films including dewetting were discussed in the recent review paper.39 Detailed information and related references can be found there. The dewetting process in BCP films has been monitored as a function of time during solvent annealing, and the results are presented in Figure 2. The film with thickness of 13 nm was solvent-annealed at 43% RH, in which case island structure with parallel orientation was created after solvent annealing. As seen in Figure 2, the islands with nonspherical shape are produced at very early time of solvent annealing, but the pattern eventually evolves to form spherical islands, suggesting that BCP film dewet via the spinodal dewetting mechanism. Magnified images clearly show that highly ordered PEO cylinders with parallel orientation are produced in the island regions. In the boundary region of islands, the dot patterns, instead of lying-down cylinders, are observed due to abrupt change of film thickness, and hence the incommensurability with BCP periodicity that may lead to orientation changes of PEO cylinders or order− order transition of BCPs from cylinders to sphere.41 On the other hand, the island height exhibits little change over the annealing time although its diameter continues to increase, as seen in the height profiles of islands obtained from AFM images. In the parallel orientation, it is important to match the film thickness with the periodicity of BCP; otherwise, BCP chains might be subject to severe entropic penalty by chain expansion or compression in the process of trying to match 1495
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interaction can be described by the van der Waals interaction, the strength of which is parametrized by the Hamaker constant. The interplay of short- and long-range interactions determines the shape of the effective interface potential as a function of film thickness and utimately the dewetting process and its mechanism. The films are stable when both short- and longrange interactions are repulsive, but in the other cases, they become unstable and dewet. For PS-b-PEO BCP, the effects of long-range interaction cannot be precisely determined since the Hamaker constant may be positive or negative depending on what data for the interfacial tension of copolymer components will be chosen in the literature.36 On the other hand, shortrange interaction becomes dominant as the film thickness decreases. Moreover, on solvent annealing, the short-range interaction is main factor in the film destabilization while the long-range interaction causes the film dewetting on thermal annealing.25 In our work, no dewetting was observed to occur in PS-b-PEO BCP films even with thickness less than 30 nm when thermally annealed at 140 °C. The result indicates that the contribution of long-range interactions to the BCP dewetting is negligibly small. Dense packing of polymer chains at the substrate surface is also known to be able to bring about the film destabilization that is called as autophobic dewetting. Dense brush layer formed at substrate surface prevents chain molecules from penetrating into that layer, which leads to the formation of new interface between the brush layer and upper part of the film. The entropic barrier established at new interface induces the destabilization of the film, and as a result, droplets formed by dewetting reside on the remaining brush layer. In order to check whether our BCP films have such brush layer at the substrate, the region near the boundary of holes or droplets was scratched out by razor blade and the images were taken by the AFM. As shown in Figure 5, the films are not completely dewetted, but a very thin flat layer is present without any surface feature, which means that the holes or droplets are sitting on very thin brush-like layer. In both cases of holes and droplets, the thickness of such thin layer is estimated to be about 10.5 ± 0.8 nm, which exactly corresponds to half-period of characteristic length of BCPs. This layer is believed to naturally form at the substrate due to selective interaction between PEO blocks and substrate, and consequently, the dewetting of BCPs during solvent annealing proceeds in a self-phobic way, as in the case of other BCP
Figure 4. AFM height images of 22 nm thick PS-b-PEO films annealed at RH = 43% (a, b) and 80% (c, d) after annealing times of 30 min (a, c) and 12 h (b, d). AFM images in the right (a′−d′) correspond to those scanned at higher magnification for PS-b-PEO film in the left (a−d).
there is little change in the depth of the holes for longer annealing time. Consequently, the holes in BCP films with perpendicular orientation are wider and deeper than those with parallel orientation. Such difference results from the quantization of the film thickness due to matching with chain dimension in the case of parallel orientation. The stability of thin polymer films is theoretically characterized by the effective interface potential that corresponds to the excess free energy per unit area required to bring two interfaces from infinity to a certain distance. It consists of two major contributions: one from strong, short-range interaction and the other from weak but long-range interaction. Short-range interaction is phenomenologically treated by the spreading coefficient that is determined by the interfacial tensions between components and substrate. Long-range
Figure 5. AFM height images and height profiles of PS-b-PEO films with the thickness of (a) 22 nm (a) and (b) 13 nm. The films were annealed at RH = 80%, and the height profiles correspond to the line in the AFM images. The insets are magnified images of selected regions. 1496
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meter to micrometer scales. Such hierarchical structure looks very naive and simple compared to that observable in nature system. However, as already seen in several examples in the literature, combining with other techniques to control the structure can offer opportunity to generate more complicated hierarchical structure than can be very useful in wide range of applications. Our very simple yet robust route can be the first step toward such goals.
systems presented previously. Contrary to the previous works, however, our BCP films exhibit highly ordered regular pattern on the nanometer scale within the remaining regions of the films that have been formed on the micrometer length scale via dewetting process during solvent annealing. Ultimately, wellorganized hierarchical structure with high degree of lateral order which has hardly been observed before is produced in a very simple way. If the lithography techniques such as photolithography, e-beam lithography, and soft lithography could be employed to direct the dewetting process in very controlled manner, then highly ordered, well-controlled structure with hierarchical order would be obtained by combining with BCP self-assembly. We have already witnessed such possibility in a couple of remarkable examples in the literature.36,41,42 Therefore, our ability to control the nanopatterns in dewetting process offers great opportunity to generate well-organized multifunctional nanostructure with hierarchical order. As a simple illustrative example of practical applications, the formation of metallic nanowires on the dewetted film was demonstrated, as shown in Figure 6. BCPs are well-known to
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ASSOCIATED CONTENT
S Supporting Information *
Figure S1: AFM phase image of PS-b-PEO BCP film solventannealed at lower RH; Figure S2: AFM phase images of solvent-annealed PS-b-PEO films on HDMS-coated substrate; Figure S3: AFM height images showing BCP films solventannealed at longer times; Figure S4: diameter and height of droplets formed by dewetting in 13 nm thick film as a function of annealing time; Figure S5: AFM phase images of dewetted PS-b-PEO films with parallel and perpendicular orientations taken at low magnification. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel +82-32-860-7493, fax +82-32-873-0181, e-mail shk@inha. ac.kr.
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ACKNOWLEDGMENTS This work was financially supported by the National Research Foundation of Korea (NRF) grant (No. 2010-0015541) funded by the Ministry of Education, Science and Technology (MEST).
Figure 6. SEM images for Au nanowires generated by complexation with 13 nm thick PS-b-PEO film. The film was annealed at RH = 43% and then treated by O2 plasma. (b) A magnified image of selected region in (a).
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be able to act as a template for nanowire formation through selective interaction,43 and in this work, PEO block was selectively complexed with HAuCl4 in solution. PS-b-PEO complexed with gold precursor was deposited on the substrate and solvent-annealed at low humidity in the same way as described above. Finally, oxygen plasma was used to remove the BCP films and at the same time reduce the gold precursor to Au. As seen in Figure 6b, Au nanowires were generated along the PEO domains within the remaining BCP film after dewetting. The low-magnification image in Figure 6a exhibits that the Au nanowires are produced on the local area along the dewetted pattern, not on the entire substrate. These results indicate that the fabrication of functional nanostructures with hierarchical order is possible simply in terms of dewetting process of BCP films.
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CONCLUSIONS In this work, we demonstrate that the dewetting process of BCP films can be combined with BCP self-assembly to offer great opportunity for forming highly organized structure with hierarchical order. Controlling the film thickness and vapor pressure during solvent annealing enables to determine the hole or droplet patterns on the micrometer scale while the order and orientation of copolymer microdomains within the film can be controlled on the nanometer scale by adjusting the RH and vapor pressure on solvent annealing. As a result, fine-tuned surface patterns with highly ordered internal structure are produced that exhibit hierarchical order ranging from nano1497
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dx.doi.org/10.1021/ma2019655 | Macromolecules 2012, 45, 1492−1498
