This document is the Accepted Manuscript version of a Published Work that appeared in final form in ACS Applied Materials and Interfaces, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see: https://dx.doi.org/10.1021/acsami.8b19062. Subscriber access provided by UNIV AUTONOMA DE BARCELONA Surfaces, Interfaces, and Applications Role of Penetrability into a Brush-coated Surface in Directed Self-Assembly of Block Copolymers Laura Evangelio, Marta Fernandez-Regulez, Jordi Fraxedas, Marcus Mueller, and Francesc Perez-Murano ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19062 • Publication Date (Web): 28 Dec 2018 Downloaded from http://pubs.acs.org on January 8, 2019 Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. 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ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts. is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties. Page 1 of 38 ACS Applied Materials & Interfaces 1 2 3 4 5 Role of Penetrability into a Brush-coated Surface in 6 7 8 9 Directed Self-Assembly of Block Copolymers 10 11 12 13 Laura Evangelio1,2, Marta Fernández-Regúlez1, Jordi Fraxedas2, Marcus Müller3, Francesc Pérez- 14 15 16 Murano1, * 17 18 1Institute of Microelectronics of Barcelona (IMB-CNM, CSIC). Campus de la UAB, 08193, Bellaterra, 19 20 21 Spain 22 23 24 2 Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, 25 26 27 Bellaterra, 08193 Barcelona, Spain 28 29 3Institute 30 for Theoretical Physics, Georg-August University, Friedrich-Hund-Platz 1, 37977 Göttingen, 31 32 Germany 33 34 35 *francesc.perez@imb-cnm.csic.es 36 37 38 39 40 41 KEYWORDS 42 43 44 Block copolymer, directed self-assembly, chemical epitaxy, interface energy, wide stripes 45 46 47 48 49 50 51 52 53 54 55 56 57 58 1 59 60 ACS Paragon Plus Environment ACS Applied Materials & Interfaces Page 2 of 38 1 2 3 4 5 6 7 8 ABSTRACT 9 10 11 12 High density and high resolution line and space patterns on surfaces are obtained by directed self- 13 14 assembly of lamella-forming block copolymers using wide-stripes chemical guiding patterns. When the 15 16 width of the chemical pattern is larger than the half-pitch of the block copolymer, the interaction energy 17 18 19 between each block copolymer domain and the surface is crucial to obtain the desired segregated film 20 21 morphology. We investigate how the intermixing between block copolymers and polymer brush 22 23 molecules at the surface influences the optimal surface and interface free energies to obtain a proper block 24 25 26 copolymer alignment. We have found that computational models successfully predict the experimentally 27 28 obtained guided patterns if the penetrability of the brush layer is taken into account instead of a hard, 29 30 impenetrable surface. Experiments on directed self-assembly of lamella-forming PS-b-PMMA using 31 32 chemical guiding patterns corroborate the models used in the simulations, where the values of the surface 33 34 35 free energy between block copolymer and guiding and background stripes are accurately determined using 36 37 an experimental method based on the characterization of contact angles in droplets formed after de- 38 39 wetting of homopolymer blends. 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 2 59 60 ACS Paragon Plus Environment Page 3 of 38 ACS Applied Materials & Interfaces INTRODUCTION 1 2 3 For many decades, miniaturization as defined by Moore’s Law has dictated the evolution of the 4 5 microelectronic industry bound to provide integrated circuits of enhanced functionality and performance 6 7 8 while decreasing production costs.1 At present, the critical dimensions required for the new generation of 9 10 integrated circuits forcedly implies new challenges; hence new fabrication strategies need to be 11 12 considered. 13 14 Directed self-assembly (DSA) of block copolymers (BCPs) is becoming a very promising alternative 15 16 17 lithographic technique for the creation of nanometer-scale patterns due to its high throughput and process 18 19 simplification compared to other approaches.2–5 Several recent studies have shown that DSA can improve 20 21 the line-width roughness (LWR)6 and line-edge roughness (LER)7 at very high patterning resolutions. 22 23 24 Block copolymers are macromolecules composed of two or more than two kinds of chemically distinct 25 26 homopolymers linked end-to-end by covalent bonds. When the different blocks are chemically 27 28 immiscible, the balance between entropically and enthalpically driven phase separation and the chemical 29 30 31 bond constraints between the blocks drives the formation of ordered micro-domains. The phase behavior 32 33 is determined by three factors: the overall degree of polymerization, N, the volume fraction of one of the 34 35 blocks in a diblock copolymer, f, and the Flory-Huggins interaction parameter, , which quantifies the 36 37 38 repulsive interaction between the blocks. The strength of the segregation power is dictated by the reduced 39 40 parameter N, which should exceed a critical value in order to have micro-phase segregation, i.e., 41 42 N=10.5 according to mean field theory for a symmetric (f=0.5) block copolymer. Block copolymers of 43 44 45 low molecular weight have a low value of N, which induces the decline of the value of N below the 46 47 minimum value to micro-phase segregate.8 48 49 50 Lamella-forming BCPs are one of the most interesting morphologies from a technological point of view. 51 52 53 After self-assembly, lamellae can be oriented perpendicular or parallel to the substrate, depending on 54 55 whether the interaction strength between the substrate and the BCP is similar for both blocks or stronger 56 57 58 3 59 60 ACS Paragon Plus Environment ACS Applied Materials & Interfaces Page 4 of 38 to one of them, respectively. Therefore, a proper wetting balance of the polymer blocks to the substrate is 1 2 3 needed to promote lamellae oriented perpendicular to the substrate. The most common way to control the 4 5 orientation of the BCP micro-domains consists in using neutral surfaces to control the surface free energy, 6 7 so that the difference in the surface interaction of the two blocks is small..9 8 9 10 Thin films of vertical lamellae form patterns whose morphology is randomly oriented; the width of the 11 12 lines being defined by the molecular weight of the BCP. Nevertheless, ordered arrays of lines with a pre- 13 14 defined orientation and registered with respect to device boundaries are required for most of technological 15 16 applications, not only for high volume manufacturing, but also as a general method to realize templates 17 18 19 for anisotropic metal nanostructures10. This can be achieved by pre-structuring the substrate with guiding 20 21 patterns of appropriate dimensions and shape. These guiding patterns are realized by grapho- or chemo- 22 23 epitaxy methods. In graphoepitaxy,11–13 the guiding patterns are made of topographical features. Usually, 24 25 26 the width of the trench becomes a limitation for achieving high density of integration. 14–16 27 28 Therefore, in order to avoid this limitation, chemical epitaxy methods have been intensively explored 29 30 to produce highly dense patterns.3,17–19 Moreover, in chemical epitaxy there is a reduction of the edge 31 32 33 roughness due to the self-healing of the BCP, which means that the irregularities of the guiding patterns 34 35 are not transferred to the BCP pattern.20 Chemical guiding patterns are fabricated by conventional top- 36 37 down lithographic methods like optical, extreme ultraviolet (EUV) projection or electron beam 38 39 lithography (EBL), in combination with a selective, local chemical modification of the brush surface layer. 40 41 42 Usually, chemical guiding patterns are formed by the combination of guiding stripes that have strong 43 44 affinity for one domain, and background stripes which present neutral or almost neutral affinity for both 45 46 BCP domains. When the surface has strong affinity for one of the domains, the width of guiding stripes 47 48 49 should be approximately half of the pitch of the BCP, if one wants to avoid a parallel, lying lamellar 50 51 morphology. In consequence, high accuracy and high resolution lithography methods are needed for the 52 53 definition of guiding patterns, increasing thus the process complexity and cost. 3,21,22 54 55 56 57 58 4 59 60 ACS Paragon Plus Environment Page 5 of 38 ACS Applied Materials & Interfaces An alternative patterning strategy is based on using wider guiding stripes, which present slightly affinity 1 2 3 for one block. In this way, large density multiplication factors (defined as the relation between the pitch 4 5 of the guiding pattern and the pitch of the DSA pattern) can be obtained. Here, we investigate under which 6 7 conditions guiding stripes with width equal to (n+0.5)L0 (with n≥1 and L0 being the lamellar periodicity) 8 9 10 can efficiently guide the self-assembly when the interface energies are tuned adequately. There are several 11 12 works which employ this guiding pattern strategy.22–29 13 14 Previous investigations have shown that the optimal guiding pattern conditions depends on both the 15 16 dimensions of the guiding patterns and the chemistry of interactions with the BCP domains occurring at 17 18 19 the surface of the chemical guiding patterns.19 In consequence, the achievement of the suitable block 20 21 copolymer alignment with high-density multiplication factors by chemical epitaxy using wide guiding 22 23 stripes requires considering multiple parameters. The prediction of their optimal value is complex due to 24 25 26 the involvement of many inter-related phenomena. The most significant parameters are of dimensional 27 28 nature (BCP film thickness, D, and guiding pattern dimensions), materials related (number of statistical 29 30 segments per diblock copolymer, N, the statistical segment length, b, and the Flory-Huggins parameter, 31 32 33 ) and process related (annealing temperature, solvent material, solution concentration and interface 34 35 boundary energies). In addition to the here-referred parameters, the chemical interactions that take place 36 37 between each domain of the BCP and the patterned surface (guiding and background stripes) is of prime 38 39 40 importance as the main driving force in DSA by chemical epitaxy30. These interactions, in turn, are related 41 42 to the difference of surface free energies of each copolymer domain and the confining boundary, Δ = SA 43 44 - SB. 45 46 47 On the other hand, computational, top-down models of DSA have proven useful to explore the large 48 49 parameter space spanned by thermodynamic conditions, like incompatibility and surface affinity, as well 50 51 as process conditions. The use of these coarse-grained models relies on the identification of a small 52 53 54 number of coarse-grained parameters, i.e., the chains’ end-to-end distance, Re, the composition of the 55 56 block copolymer, and the incompatibility, χN, between blocks. The physical behavior depends on the 57 58 5 59 60 ACS Paragon Plus Environment ACS Applied Materials & Interfaces Page 6 of 38 detailed material properties only via these coarse-grained parameters, which, in turn, can be obtained from 1 2 3 independent experimental measurements. These top-down models can successfully and quantitatively 4 5 predict the kinetics of DSA.31–34 In the spirit of a top-down strategy, the surface affinities of the guiding 6 7 stripes and neutral background are often modeled by an impenetrable surface with short-range attraction. 8 9 10 The strength of the interaction is chosen as to reproduce the coarse-grained parameter, Δ, but the detailed 11 12 molecular mechanism that gives rise to a surface free-energy difference is often assumed not to be 13 14 relevant. Density multiplication with wide guiding stripes is a demanding test of modeling because 15 16 17 successful DSA only occurs in a narrow parameter window of surface affinities. On the one hand, the 18 19 wide guiding stripe must present sufficiently strong affinity for one block in order to direct the structure 20 21 formation. On the other hand, a too strong affinity will result in lying lamellae on top of the guiding 22 23 stripes.29 24 25 26 27 Knowing the properties of the brush layer is key to investigate the characteristic of the self-assembly 28 29 process. Previous works include the influence of the grafting temperature of the brush layer35, its 30 31 thickness36 and the mechanical properties37. Grafting density has been found to improve the thickness 32 33 34 process window for perpendicular orientation of lamellar on free surfaces. 35 36 37 Here, we introduce the effect of brush penetrability on the alignment of PS-b-PMMA lamellae forming 38 39 block copolymers by chemical guiding patterns with wide guiding stripes. The presence of block 40 41 42 copolymer / brush intermixing has being pointed out previously9. We include this effect in computer 43 44 simulations to obtain the conditions that sustain standing lamella in chemical epitaxy. The experimental 45 46 values of the free surface energies of the chemical guiding patterns are included in the model and they 47 48 need to be determined with high accuracy. Because of this, we have employed an experimental method 49 50 51 based on the characterization of the contact angle in droplets formed by de-wetting of thin layers of 52 53 homopolymer blends forming a droplet-in-droplet structure. 54 55 56 57 58 6 59 60 ACS Paragon Plus Environment Page 7 of 38 ACS Applied Materials & Interfaces RESULTS AND DISCUSSION 1 2 3 a) Chemical epitaxy DSA using wide guiding stripes 4 5 6 Figure 1.a illustrates the process to direct the self-assembly of poly(styrene-block-methyl methacrylate) 7 8 9 (PS-b-PMMA) of 22 nm pitch using wide guiding stripes. Chemical guiding patterns are created on silicon 10 11 substrates by first depositing a 8.6 nm thick layer of 70% rich poly(styrene-random-methyl methacrylate) 12 13 (PS70%-r-PMMA) brush. Then a layer of 80 nm of PMMA is patterned by EBL, and after the development, 14 15 the sample is exposed to oxygen plasma to chemically modify the defined regions. The conditions under 16 17 18 which the brush is exposed to oxygen plasma are critical to obtain efficient guiding patterns. We have 19 20 investigated two different conditions, which are described in the materials and methods sections; they are 21 22 referred to as mild (150 W, 10 sccm O2 for 5 seconds) and strong (300 W, 50 sccm O2 for 10 seconds) 23 24 25 oxygen plasma conditions. The resulting chemical guiding patterns, after removing the electron beam 26 27 resist, have a guiding stripe width of 1.5L0 and a period equal to nL0. AFM characterization before BCP 28 29 deposition reveals a chemical contrast without almost any change in topography for the mild oxygen 30 31 32 plasma conditions (the oxygen plasma exposure removes less than 0.5 nm of the brush layer). (See 33 34 supplemental information, figure S1). Surface characterization of the brush layer reveals an increased 35 36 affinity of the brush layer with respect to PMMA when it is exposed to oxygen plasma 30 and, under mild 37 38 oxygen plasma conditions, a very low damage (See the AFM characterization of the chemically 39 40 41 functionalized guiding patterns in the supplemental information, figure S1). 42 43 44 Then, PS-b-PMMA is deposited and annealed at 230ºC for 10 min with a continuous nitrogen flow. The 45 46 typical thickness, D, of the PS-b-PMMA layer is 3L0/4. The dependence of block copolymer orientation 47 48 49 on the layer thickness is complex and related to the surface wetting behavior38. This thickness maximizes 50 51 the geometric frustration of lying lamellae,39,40 and thereby allows to make compatible a strong enough 52 53 surface affinity for one block while keeping a standing lamellae morphology. 54 55 56 57 58 7 59 60 ACS Paragon Plus Environment ACS Applied Materials & Interfaces Page 8 of 38 Figures 1.b-d show SEM images of aligned PS-b-PMMA of 22 nm pitch with density multiplication 1 2 3 factors of 3, 4 and 5, respectively. In all cases, the guiding stripe width is fixed to 1.5L0. For guiding 4 5 patterns with pitch larger than 5L0, the BCP alignment is lost on the background stripes. The same method 6 7 for guiding pattern generation has also been applied to align PS-b-PMMA BCP of 28 nm and 38 nm pitch. 8 9 10 A good alignment using wide stripes is also achieved, adapting the processing conditions and brush layers 11 12 to each BCP material (see supplemental information, figure S2). In the following, we center our analysis 13 14 to the case of alignment of PS-b-PMMA of 11 nm half-pitch, as this is the material with higher 15 16 technological interest because it delivers higher resolution features. 17 18 19 20 b) Experimental determination of surface free energies 21 22 23 The understanding of the guiding mechanism requires a precise knowledge of multiple parameters, among 24 25 which the difference of surface free energies between the substrate and each BCP domain, Δ, plays a 26 27 28 dominant role. Next, we show that an accurate value of the surface free energy difference can be obtained 29 30 by the contact angle, θA-B that the AB interface of an homopolymer blend makes with the brush-coated 31 32 surface. We use this value to quantitatively match the simulation model to the experimental system. 33 34 35 Additionally, and independently, the contact angle can be employed in conjunction with a simple 36 37 geometrical model to start analyzing the conditions to obtain free standing lamellae. 38 39 40 The experimental method to determine the value of the interfacial energies is based on performing 41 42 homopolymer blend de-wetting experiments,41,42 and measuring the contact angle, θA-B, of the internal 43 44 45 interface between both homopolymers at the brush surface.43 This method is an extension of the method 46 47 proposed by Mansky et al.9 It allows to directly determining Δ from the experimental measure of one 48 49 single angle, and, as it is shown below, it is more accurate. 50 51 52 53 The de-wetting experiment is performed as follows: a 50:50 PS/PMMA homopolymer blend is spin- 54 55 coated on the top of the brush, and it is subsequently annealed under the same conditions as for the DSA 56 57 process. After the annealing, if the substrate is not attractive enough to at least one of the homopolymers, 58 8 59 60 ACS Paragon Plus Environment Page 9 of 38 ACS Applied Materials & Interfaces a de-wetting phenomenon occurs, forming a drop-in-drop structure, which contains both homopolymers 1 2 3 (figure 2.a). The angle θA-B, that the internal interface between both homopolymers forms at the contact 4 5 line with the brush provides direct evidence of the difference of surface free energy, Δ. If θA-B < 90º, then 6 7 the surface has stronger affinity for homopolymer B, and if θA-B = 90º, then the surface presents equal 8 9 10 affinity for both homopolymers. SEM images of PS/PMMA polymer blend droplets on top of un-modified 11 12 and modified (mild) PS70%-r-PMMA brush layers after removing PMMA domains are shown in figure 13 14 2.b. and 2.c, respectively. The change of the affinity character of the surface is made evident by the change 15 16 17 of the morphology of the droplets and by the change of contact angle. To get a representative value of the 18 19 contact angle we have measured and averaged the values from five different droplet structures for each 20 21 material combination and processing conditions. 22 23 24 By applying Young’s equation to the specific case of PS/PMMA blends, we directly relate the surface 25 26 27 affinity to the observed contact angle and the interface tension between the blocks: 28 29 30 Δ=γBrush-PS - γBrush-PMMA = γPS/PMMA·cos θPS/PMMA, [1] 31 32 33 where γBrush-PS and γBrush-PMMA are the surface tensions between each homopolymer and the brush-coated 34 35 surface, and γPS/PMMA is the interface tension between PS and PMMA, which slightly depends on the 36 37 38 annealing temperature and can be obtained from literature:44 39 40 41 γPS/PMMA [mN/m] = 3.6 - 0.013T [ºC], [2] 42 43 44 Equations [1] and [2] show that the difference of surface tension of PS and PMMA with the brush is 45 46 directly obtained from the measurement of the contact angle of the PS/PMMA interface. 47 48 49 Previous works have used de-wetting experiments to determine surface affinity, but not using 50 51 52 homopolymer blends as we propose here.45–48 The advantage of the homopolymer blend method is that it 53 54 provides an information (contact angle between the homopolymers at the brush surface) which can be 55 56 directly fed into the simulations. Compared to the method proposed by Mansky et al9 in which, the surface 57 58 9 59 60 ACS Paragon Plus Environment ACS Applied Materials & Interfaces Page 10 of 38 energies are obtained from independent dewetting experiments of homopolymers, the present method is 1 2 3 about an order of magnitude more accurate because the relevant difference of surface energies is not 4 5 calculated as the difference between two large numbers but directly, as the PS-PMMA interface tension 6 7 is about one order of magnitude smaller than the surface tensions of both polymers (as it is presented 8 9 10 below) . Additionally, we have to point out that the present method works even if one of the both 11 12 components wet the brush, while Mansky’s method requires that the homopolymer films dewet. 13 14 15 According to Mansky et al., the contact angle 𝜃𝑖 between one homopolymer i and the brush is determined 16 17 by Young’s equation: 18 𝛾𝑏 = 𝛾ℎ𝑖 cos(𝜃𝑖 ) + 𝛾𝑏ℎ𝑖 [3] 19 20 where: 21 22 𝛾𝑏 is the surface tension of the brush; 23 24 𝛾ℎ𝑖 is the surface tension of homopolymer i; 25 26 𝛾𝑏ℎ𝑖 is the surface tension between brush and homopolymer i; 27 28 29 30 The difference of surface tension (∆ 𝛾) between the brush and two different pure homopolymers (h1 and 31 32 h2) can be obtained by performing two different de-wetting experiments, and then calculated as follows: 33 34 35 ∆ 𝛾 = 𝛾𝑏ℎ1 − 𝛾𝑏ℎ2 = −𝛾ℎ1 cos(𝜃1 ) + 𝛾ℎ2 cos(𝜃2 ) ≈ 𝛾ℎ𝑖 (cos(𝜃2 ) − cos(𝜃1 )) [4] 36 37 Typical values 𝛾ℎ𝑖 of PS and PMMA are very similar and the resulting difference of surface tension is 38 39 usually below 1 mN/m9. This makes that any inaccuracy on the determination of (cos(𝜃2 ) − cos(𝜃1 )) is 40 41 amplified in the calculation of ∆ 𝛾. 42 43 44 On the contrary, when ∆ 𝛾 is calculated from the contact angle between the two homopolymers in blend 45 dewetting experiments, the uncertainty is lower, as it is calculated from: 46 47 48 ∆ 𝛾 = 𝛾12 cos(𝜃12 ) [5] 49 50 where 𝛾12 is the interface tension between the coexisting phases of the binary homopolymer system, and, 51 52 so, it is of the same order of magnitude of ∆ 𝛾. 53 54 55 The uncertainty in the calculation of ∆ 𝛾 , 𝜀𝑖 (∆ 𝛾), can be calculated, assuming that the uncertainty is only 56 57 present in the determination of the angles, as follows: 58 10 59 60 ACS Paragon Plus Environment Page 11 of 38 ACS Applied Materials & Interfaces 1 i) 𝜀𝑖 (∆ 𝛾) = 𝛾ℎ𝑖 √sin2 (𝜃1 ) + sin2 (𝜃2 ) 𝜀(𝜃𝑖 ) for the case of droplets of homopolymers 2 ii) 𝜀𝑖𝑖 (∆ 𝛾) = 𝛾ℎ12 sin(𝜃12 ) 𝜀(𝜃12 ) for the case of the droplet of a blend of homopolymers 3 4 5 which clearly shows that the uncertainty using the methods of homopolymers blends is more accurate: 6 7 8 𝜀𝑖 (∆ 𝛾) 𝛾ℎ𝑖 √sin2 (𝜃1 )+sin2 (𝜃2 ) 𝛾ℎ𝑖 = ≈ 0.6 [6] 9 𝜀𝑖𝑖 (∆ 𝛾) 𝛾ℎ12 sin(𝜃12 ) 𝛾ℎ12 10 11 12 Taking a specific quantitative situation for one of the systems investigated here (brush PS70%-r-PMMA 13 exposed to an oxygen plasma using mild conditions): 14 15 16 𝛾ℎ𝑖 = 25.3 mN/m (for both, PS and PMMA); 𝛾12=0.61 mN/m; 𝜃1 = 12.5; 𝜃2 = 5 º; 𝜃12 = 146.4 º 17 18 yields to the following results (assuming un uncertainty of 5º in the determination of the angles): 19 20 i) ∆ 𝛾 = -0.5 mN/m ± 0.52 for the case of droplets of homopolymers 21 22 ii) ∆ 𝛾 = -0.5 mN/m ± 0.03 for the case of the droplet of a blend of homopolymers 23 24 25 Tables 1 summarize the results of contact angles for the different material systems and processing 26 27 conditions that have been investigated here (the values depict the average and standard deviation of the 28 29 five images analyzed for each situation). Table 1 shows the values of the contact angle θA-B for the 30 31 32 processing conditions used to align PS-b-PMMA of 22 nm pitch (i.e., (T=230ºC, which implies that the 33 34 value of γPS/PMMA is 0.61 mN/m as stated in equation 2). It shows that a PS percentage higher than 60% in 35 36 the PS-r-PMMA brush is required to have an almost neutral surface (∆ 𝛾 ≈ 0), as corroborated 37 38 experimentally by the fact that this condition has to be fulfilled to obtain perpendicular lamellae when no 39 40 41 chemical guiding patterns are present (see supplemental information, figure S3). Table S2 in the 42 43 supplemental information provides complementary data of the contact angle determination for other 44 45 combinations of brush layer and annealing conditions. Figure S4 in the supplemental information provides 46 47 48 the value of water contact angle for the investigated brush layers. 49 50 51 Another method to obtain information from the interfacial energies is by measuring the contact angle with 52 53 the sessile drop method49. It is interesting to compare the results obtained from this method with the ones 54 55 obtained from the de-wetting experiments in order to check their consistency. In order to determine the 56 57 58 11 59 60 ACS Paragon Plus Environment ACS Applied Materials & Interfaces Page 12 of 38 surface free energy from the contact angle, at least two liquids with known disperse and polar components 1 2 3 of surface tensions are required. Nevertheless, the sessile drop method provides the surface free energy 4 5 of the brush, but not directly the difference of surface free energies between the BCP domains. Therefore, 6 7 we use the value of the difference of interface energies, obtained from homopolymer blends de-wetting 8 9 10 experiments, in the simulations. Figure S4 and table S2, in the supplemental information, show that the 11 12 water-contact-angle dependence on surface functionalization follows the same trend than the contact 13 14 angles determined from the de-wetting experiments of homopolymer blends. 15 16 17 c) Geometrical model to relate contact angle with surface free energies 18 19 20 In the case of thin films of block copolymers, neglecting any effect from the arrangement of the 21 22 23 copolymers as they uniform fill space, we assume that the internal AB interfaces of perpendicular lamellae 24 25 are characterized by the homopolymer contact angle, θA-B, with the substrate. The contact angle arises 26 27 from a force balance, i.e., Young’s law, and in the ultimate vicinity of the three-phase contact line (A- 28 29 30 domain, B-domain, brush) the forces in homopolymer blend and diblock copolymer are similar. 31 32 Moreover, previous work on thin films indicated that, in the case of pinned three-phase contacts, the 33 34 internal AB interfaces of perpendicular lamellae are only slightly curved and the domains adopt a shape 35 36 37 between ideal wedges and domains with vertical, internal AB interfaces.50 For simplicity, we assume an 38 39 ideal-wedge geometry in the following, and this simple geometric construction results in the condition 40 41 42 tan θA-B > 2D/L0 [7] 43 44 45 for vertical lamellae, where L0 is the lamellar period and D the thickness of the copolymer layer. Given 46 47 48 that the ideal-wedge geometry is only an approximation, this relation tends to slightly underestimate the 49 50 maximally permissible surface preference. Using the optimal value, D=3L0/4, which maximally frustrates 51 52 lying lamellae, we obtain the condition tan θA-B > 3/2 or 123.7º> θA-B > 56.3°. 53 54 55 56 57 58 12 59 60 ACS Paragon Plus Environment Page 13 of 38 ACS Applied Materials & Interfaces Specifically, for PS-b-PMMA of 22 nm pitch and PS70%-r-PMMA, it is observed that the background 1 2 3 regions have stronger affinity for PS domains. After the oxygen plasma functionalization (i.e. guiding 4 5 stripes) the surface presents higher affinity for PMMA. However, the contact angle observed is (see Table 6 7 1) above the value that guarantees standing lamellae according to the geometrical model mentioned above 8 9 10 (see Eq. (7)). 11 12 13 d) Simulations 14 15 16 To understand the origin of this discrepancy, we investigate by means of computer simulations the 17 18 formation of standing lamellae in chemical epitaxy when using wide guiding stripes. First, we consider 19 20 the substrate as an impenetrable surface (hard-wall model) with short-ranged interactions , whose strength 21 22 23 is tailored to yield the same contact angle as the de-wetting experiment. In a coarse-grained model, the 24 25 strength of the affinity for wetting is characterized by the parameter N. In the strong-segregation regime, 26 27 χN>>10,51 and assuming a uniform density profile at the wall and an identical entropy loss of the A and 28 29 30 B chains as they pack against the surface, we can approximately relate the affinity parameter ΛN to the 31 32 contact angle, θA-B 33 34 35 Δ𝛾 12𝜋 4𝑙𝑛2 −1 36 𝑐𝑜𝑠 (𝜃𝐴𝐵 ) = 𝛾 ≈ √ 𝜒𝑁 (1 − ) Λ𝑁 = 𝑐 Λ𝑁 [8] 𝐴𝐵 𝜒𝑁 37 38 39 40 In order to validate the linear relation between the cosine of the contact angle and the surface preference, 41 42 cos 𝜃𝐴𝐵 = c N, and accurately determine the constant of proportionality, c, simulations have been 43 44 performed for a symmetric homopolymer blend (A and B homopolymers). (See methods). The 45 46 47 antisymmetric set- up gives rise to a (nearly) straight AB interface that runs from the bottom to the top 48 49 surface (see figure 3.a). The minimization of interface curvature allows us to accurately measure the 50 51 contact angle between the AB interface and the surface – in contrast to a microscopic droplet set-up, the 52 53 54 line tension does not influence the contact angle and the AB interface is not curved. Figure 3.b presents 55 56 cos 𝜃𝐴𝐵 as a function of the strength of the surface interaction Λ𝑁 as obtained from the simulation data 57 58 13 59 60 ACS Paragon Plus Environment ACS Applied Materials & Interfaces Page 14 of 38 (and compares the data with the analytical prediction, (dashed line)). Simple linear fits (solid lines) yield 1 2 3 cos 𝜃𝐴𝐵 = c Λ𝑁 with c = 1.58; 1.4 and 1.15 for N = 16; 20 and 30, respectively. Deviations from the 4 5 analytical prediction arise from discretization effects of the chain contour and space, and from the fact 6 7 that the analytical formula is obtained in the strong segregation limit N >>1. Figure 3.c shows the result 8 9 10 of the simulations by introducing the experimental conditions (materials and dimensions) corresponding 11 12 to the system shown in figure 1.d (guiding stripe = 1.5L0; background stripe 3.5L0). Simulations predict 13 14 the formation of parallel, lying lamellae on top of the guiding stripes and vertical lamella on top of the 15 16 17 background stripe. This finding is in agreement with the simple geometric estimate, Eq. (7), i.e., the 18 19 surface preference of the guiding stripes is too strong. The vertical lamellae exhibit a quasi-orthogonal 20 21 orientation with respect to the guiding pattern orientation because this orientation minimizes the grain- 22 23 24 boundary free energy of these orthogonal lamellar structures. 25 26 27 To rationalize the discrepancy between the experimental results and the simulations, three effects may be 28 29 considered: (1) a decrease of the actual value of the difference of surface free energies, (2) the role of 30 31 surface topography, and (3) considering that the brush layer provides a soft surface, whose properties 32 33 34 cannot be entirely quantified by the coarse-grained parameter, Δγ, alone. 35 36 37 (1) A reduction of the surface affinity compared to the de-wetting experiments (without altering the 38 39 incompatibility inside the film) could possibly result from a temporary enrichment of solvent between 40 41 substrate and copolymer material in the early stages of the DSA process. There is, however, no direct 42 43 44 quantitative experimental evidence for this hypothesis. (2) We have also discarded the effect of surface 45 46 topography, because the AFM measurements for the guiding patterns do not show appreciable topography 47 48 for the mild oxygen plasma condition. (3) Recent studies have indicated that (i) a random copolymer brush 49 50 51 responds to a contacting liquid by exposing favorable moieties to the interface and thereby lowering the 52 53 surface tension52,53 and (ii) the interpenetration of copolymers into a soft surface tend to stabilize 54 55 56 57 58 14 59 60 ACS Paragon Plus Environment Page 15 of 38 ACS Applied Materials & Interfaces perpendicular lamellae.54 Therefore, we have explicitly modeled the surface by a random copolymer brush 1 2 3 tailoring the brush composition as to match the experimental values of the contact angle. 4 5 6 To provide a more detailed representation of the soft experimental brush surface, we explicitly consider 7 8 a random copolymer brush in the simulations. In accordance with the experiment, the length of the brush 9 10 chains is a quarter of length of the copolymers. The random copolymer brush is characterized by two 11 12 13 parameters: its composition, fb, and its grafting density, . The composition, fb, mostly controls the 14 15 difference, , of surface tensions and the grafting density, , the penetrability. An extremely high 16 17 18 grafting density corresponds to an impenetrable hard surface, whereas intermediate and small grafting 19 20 densities allow for inter-digitation. Extremely low grafting densities even allow the polymer melt to 21 22 interact with the non-preferential, impenetrable grafting surface. The effective brush height, h, is related 23 24 to the grafting density, σ, of the random copolymers via 25 26 27 28 29 [9] 30 31 32 Specifically, one end of the random AB copolymers that form the brush is immobilized at a random lateral 33 34 position (y,z) on top of a hard, non-preferential grafting surface. The sequence of segments is 35 36 37 uncorrelated, and the average fraction of A content of the brush is denoted as fb. For simplicity, the A 38 39 segments of the random copolymer interact like A segments of the copolymer or homopolymer and have 40 41 identical statistical segment length; likewise, the B segments of the random copolymer are structurally 42 43 44 identical to the corresponding copolymer or homopolymer segments. 45 46 47 A typical two-dimensional composition profile of a homopolymer blend between two antisymmetric 48 49 random-copolymer brushes is presented in figure 4.a. The A and B fractions of the random copolymer 50 51 brush is switched between the top and bottom, fb,top=1-fb,bottom. The antisymmetric setup allows for an 52 53 54 accurate determination of the contact angle in a film with rather modest thickness, D. 55 56 57 58 15 59 60 ACS Paragon Plus Environment ACS Applied Materials & Interfaces Page 16 of 38 The dependence of the contact angle θAB on the average composition of the random copolymer brush, fb, 1 2 3 is presented in Figure 4.b for various grafting densities. It is found that (i) cos θAB depends roughly linearly 4 5 on the composition, and (ii) at a fixed composition, the contact angle decreases with grafting density. 6 7 Moreover, we note that the homopolymer blend makes a finite contact angle even on a homopolymer 8 9 10 brush, fb=0 or 1, because the long homopolymer chains cannot penetrate into the brush, giving rise to a 11 12 finite surface tension of the brush-homopolymer interface (autophobicity).55,56 The contact angle on a 13 14 pure homopolymer brush, fb=0 or 1, increases with grafting density. For the grafting densities that we 15 16 have considered, however, |cos θAB| remains smaller than 0.9. Therefore, using the same segment types, 17 18 19 A and B, for the random copolymer brush as for the copolymer we cannot represent the strong plasma 20 21 condition of the experiment. 22 23 24 The cosines of the experimental contact angles for the soft plasma condition from table I are indicated by 25 26 the horizontal lines in figure 4.b. Note that due to the structural symmetry between A and B segments, the 27 28 29 contact angle, θAB (fb), as a function of brush composition, fb, obeys the symmetry relation, cos θAB (1-fb)= 30 31 - cos θAB (fb). 32 33 34 For the DSA simulation, presented in figure 4.c, we have chosen an intermediate value of the grafting 35 36 37 density (aka brush height, cf. Eq. 9) that is compatible with the experimental data and have tailored the 38 39 composition, fb, as to match the experimental contact angle values in table I. As it is shown in figure 4.c, 40 41 modelling the brush as a soft, penetrable layer of finite thickness and using a composition that results in 42 43 the experimental contact angles allows that the simulations to reproduce the experimental successful DSA 44 45 46 conditions on wide guiding stripes without any adjustment of parameters. 47 48 49 e) Effect of oxygen plasma exposure of the brush layer and critical discussion. 50 51 52 The affinity of the guiding pattern for the PMMA block can be increased by increasing the exposure to 53 54 the oxygen plasma. Figure 5 shows a SEM image of a pattern created under strong plasma conditions 55 56 57 (guiding stripe width: 1.5L0; background stripe width: 3.5L0). In this case, on top of the guiding stripe the 58 16 59 60 ACS Paragon Plus Environment Page 17 of 38 ACS Applied Materials & Interfaces block copolymer presents parallel oriented configuration, and on top of the background stripes, the 1 2 3 polymer is oriented orthogonal to the direction of the guiding pattern. Similar patterns have been shown 4 5 before.19,22,57–59 This morphology resembles the simulation result in Figure 3.a, indicating that it 6 7 corresponds to a too strong preference of the guiding pattern. In our model of a random copolymer brush, 8 9 10 the segments of the brush and the copolymer are structurally identical and Figure 3.b already employs the 11 12 maximal preference of the guiding stripes within the random copolymer brush model. In Figure S5 we 13 14 verify that using an even stronger preference of the guiding stripes for an impenetrable surface with short- 15 16 range interaction, which matches the experimental contact angle, does not alter the morphology on top of 17 18 19 the guiding stripes. 20 21 22 EXPERIMENTAL 23 24 25 Materials 26 27 28 Lamellae-forming poly(styrene-block-methyl methacrylate) (PS-b-PMMA) (Mn = 42.3 kg·mol-1 and PDI 29 30 = 1.10) has been used as a BCP material, by using a poly(styrene-random-methyl methacrylate) (PS-r- 31 32 PMMA) brush with PS:PMMA volume ratio of 70:30 (Mn = 9.9 kg·mol-1 and PDI = 1.34). These 33 34 35 materials have been supplied by ARKEMA under the tradename Nanostrenght® in powder, and then dis- 36 37 solved in propylene glycol methyl ether acetate (PGMEA). 38 39 40 For e-beam definition of the chemical guiding patterns, PMMA (polymethyl methacylate) (950 kg/mol, 41 42 dissolved at 2% in anisole) has been used as a resists. The developer solution are prepared by mixing 43 44 45 MIBK (methyl isobutyl ketone): IPA (Isopropanol) ratio 1:3. These products, the e-beam resist and 46 47 developer, are purchased form MicroChem. 48 49 50 Samples preparation 51 52 53 The starting substrates are 0.9 x 0.9 cm2 chips bearing a native silicon oxide layer ((100), p-type silicon 54 55 wafers of 4-40 Ω·cm resistivity). Previously to the grafting process, the samples are cleaned and activated 56 57 58 17 59 60 ACS Paragon Plus Environment ACS Applied Materials & Interfaces Page 18 of 38 by oxygen plasma for 10 min at 500 W and 50 sccm oxygen flow. This step allows to create OH- groups 1 2 3 on the surface that will facilitate the grafting of the brush layer . Oxygen plasma exposure of silicon 4 5 substrate efficiently remove surface contamination from species containing carbon and oxygen 6 7 components and reduce the concentration of Si-O bonds typically presents on non-stoichiometric SiOx 8 9 10 natural silicon60. On the other hand, the plasma also increases the amount of –OH groups61 improved on 11 12 the surface reaction for polymer grafting of random co-polymers. 13 14 15 In DSA of block copolymers the surface preparation by using an oxygen plasma pre-cleaning before co- 16 17 polymer grafting is a standard procedure4,62 18 19 20 Then a 2% PGMEA brush solution is spin-coated at 5000 rpm on the top of the surface, and annealed at 21 22 23 230ºC for 5 min in a nitrogen environment. Consequently, the non-grafted brush is removed by dipping 24 25 the sample in PGMEA for 5 min in an ultrasonic bath at 40ºC. The brush film thickness grafted after 26 27 PGMEA rinsing is 8.6 nm as determined by ellipsometer. Then a 2% 950k PMMA anisole solution is 28 29 30 spin-coated on the top of the brush at 2000 rpm giving rise to an 80 nm film thickness. 31 32 33 Once the resist has been deposited, the guiding patterns are defined by electron beam lithography (EBL) 34 35 in a RAITH150Two equipment. Guiding patterns for line/space applications with guiding stripes widths 36 37 of 1.5L0 and density multiplication factors of 3 to 5 have been designed. The desired guiding patterns 38 39 40 have been defined by using an acceleration voltage of 20 kV and 110 pA of beam current with a nominal 41 42 beam diameter of 2 nm. The exposure has been defined as a set of single exposures, with an e-beam dose 43 44 of 225 μC/cm2 and line step size of 10 nm. 45 46 47 After the exposure, the resist is developed and the sample is exposed to oxygen plasma to chemically 48 49 50 modify the lithographically defined regions. The functionalization is performed in an Alcatel AMS 110 51 52 DE reactive ion etching (RIE) equipment by using two different conditions, referred in the text as mild 53 54 (150 W, 10 sccm O2 for 5 seconds) and strong (300 W, 50 sccm O2 for 10 seconds). 55 56 57 58 18 59 60 ACS Paragon Plus Environment Page 19 of 38 ACS Applied Materials & Interfaces After performing the brush functionalization, the resist is removed. As revealed in the AFM images 1 2 3 (topography and phase signals) of figure S1, when mild conditions are used, only some chemical contrast 4 5 is observed (no topographical changes are seen), whereas when functionalization conditions are stronger, 6 7 there are both topographical and chemical changes on the surface, thus promoting different orientations 8 9 10 of the BCP. 11 12 13 Finally, the BCP is deposited from a 1.5% PGMEA solution at 2500 rpm, and annealed at 230ºC for 10 14 15 min with a continuous nitrogen flow, in order to direct the self-assemble with respect to the previous 16 17 chemical pattern. 18 19 20 21 22 23 24 Simulations 25 26 27 In our top-down coarse-grained model63 we describe the symmetric copolymers be a discretized Edwards- 28 29 Hamiltonian using N=32 beads per chain molecule. The molecules of the random copolymer brush are 30 31 composed of Nrc=8 coarse-grained segments. This chain-length ratio between the copolymers and the 32 33 34 brush molecules mimics the experimental system. Bonded beads interact via harmonic springs and the 35 36 mean-squared end-to-end distance, Re0, of the copolymer (in the absence of non-bonded interactions) sets 37 38 the unit of length of the computational model. Non-bonded interactions are catered for by the Hamiltonian: 39 40 41 42 43 [2] 44 45 46 where denotes the invariant degree of polymerization, and ϕA and ϕB are the local, dimensionless 47 48 densities of the A and B segments, respectively. The first term in Eq. [2] accounts for the small 49 50 compressibility of a melt, and we use κN=50 to set the inverse isothermal compressibility. The second 51 52 53 term quantifies the repulsion between the different blocks of the copolymer, whose strength is set by the 54 55 product χN of the Flory-Huggins parameter and the number of segments per copolymer. The third term 56 57 58 19 59 60 ACS Paragon Plus Environment ACS Applied Materials & Interfaces Page 20 of 38 represents the short-range interaction between the components of the copolymer and the impenetrable 1 2 3 substrate. For positive values of ΛN, the surface attracts the A component and repels the B segments of 4 5 the copolymer. The interaction extends a distance ε=0.15Re away from the surface into the film. f(y,z) 6 7 encodes the guiding pattern as a function of the two lateral coordinates, y and z. For the DSA simulation 8 9 10 the top surface of the film is hard and non-preferential. 11 12 13 In single-chain-mean-field (SCMF) simulations the non-bonded interactions are temporarily replaced by 14 15 fluctuating fields, which are frequently recomputed from the density distribution. The fields are computed 16 17 by a linear assignment scheme on a collocation grid with spacing, ΔL≈ Re/6. The molecular conformations 18 19 20 are updated by a Smart-Monte-Carlo algorithm, which employs the strong bonded force to propose a trial 21 22 displacement, resulting in a realistic Rouse-like dynamics. The “time” is takes a polymer to diffuse a 23 24 distance Re is τ=8900 SMC-steps. We use an invariant degree of polymerization ≈902 for the 25 26 copolymers and χN=17 if not stated otherwise. 27 28 29 30 In order to calculate the macroscopic contact angle θA-B of a homopolymer blend, we simulate a symmetric 31 32 homopolymer blend, NA=NB=32, in an anti-symmetric thin-film geometry, i.e., the top confining surface 33 34 attracts the A segments with exactly the same strength as the bottom substrate attracts the B segments, i.e. 35 36 37 ΛNtop=-ΛNbottom for the hard surface, and the A and B fractions of the random copolymer brush is switched, 38 39 fb,top=1-fb,bottom, between the top and bottom surface in case of brush-covered surfaces. If 0°<θA-B<180°, 40 41 i.e., homopolymers do not wet the respective surface, the internal AB interface will run straight across the 42 43 film (as shown in figure 4.a), and we can read off the macroscopic contact angle from the orientation of 44 45 46 the AB interface in the middle of the film. 64 47 48 49 In order to study DSA we start the simulation from a disordered configuration, χNinitial=0, instantaneously 50 51 quench to χN=17, and follow the kinetics of DSA. We use L0=1.67Re0. This value is slightly larger than 52 53 the approximate estimate of the bulk lamellar spacing obtained from the isotropy of the virial of bonded 54 55 56 forces in the bulk lamellae but it yields a broader parameter window for defect-free DSA. The snapshots 57 58 20 59 60 ACS Paragon Plus Environment Page 21 of 38 ACS Applied Materials & Interfaces depict the final, metastable structure at the end of the simulation that lasted longer than 400 000 SMC 1 2 3 steps ≈ 45τ. 4 5 6 The simulated system for determination of the contact angle on hard surfaces is defined by: N = 32, N 7 8 ̅ = 𝑛𝑅𝑒0 = 16; 20 and 30, and invariant degree of polymerization √𝑁 3 /𝑉 = 90 and 128. The geometry of 9 10 3 11 the simulated system is defined by V = 2.56x10x2 𝑅𝑒0 , discretization of collocation grid is 25 x 6 x 12 12 13 with linear assignment between particle positions and grid points, hard walls on top and bottom, and 14 15 16 antisymmetric surface fields of strength Λ𝑁. 17 18 19 In order to calculate the macroscopic contact angle θAB of a homopolymer blend, we simulate a symmetric 20 21 homopolymer blend, NA=NB=32, in an anti-symmetric thin-film geometry, where the A and B fractions 22 23 of the random copolymer brush is switched, fb,top=1-fb,bottom, between the top and bottom surface in case of 24 25 26 brush-covered surfaces. If 0°<θAB<180°, i.e., homopolymers do not wet the respective surface, the internal 27 28 AB interface will run straight across the film (as shown in Figure 4.a), and we can read off the macroscopic 29 30 contact angle from the orientation of the AB interface in the middle of the film. 64 31 32 33 34 CONCLUSIONS 35 36 In summary, we have shown that the interpenetration between the copolymer brush on the brush substrate 37 38 influences the final morphology of the block copolymer patterns in directed self-assembly by chemical 39 40 41 epitaxy when using wide guiding stripes. Experimental results have been quantitatively compared to 42 43 computer simulations using the experimentally determined contact angle of the interface of the polymer 44 45 components with the substrate surface to characterize the surface affinity. Simulations and experiments 46 47 48 only agree if the responsive, penetrable structure of the copolymer brush on the substrate is accounted for. 49 50 This observation indicates that the surface free energy difference between the components is not the only 51 52 quantity that the top-down coarse-grained model requires in order to quantitatively describe the 53 54 experimental system. In consequence, not only the composition of the brush layer dictates the surface 55 56 57 58 21 59 60 ACS Paragon Plus Environment ACS Applied Materials & Interfaces Page 22 of 38 affinity, but also structural parameters like its grafting density must be considered to achieve an optimal 1 2 3 DSA performance. 4 5 6 7 8 9 SUPPORTING INFORMATION 10 11 12 AFM images of chemical guiding patterns; Values of the difference of surface free-energies for various 13 14 brush-block copolymer systems; SEM images of directed self-assembly of block copolymers of different 15 16 17 pitch by using chemical wide patterns; Water contact angle values on different modified and un-modified 18 19 brush layers; SEM images of PS-b-PMMA (L0 = 22 nm) on top of various brush layers; Values of 20 21 experimental surface free energies obtained by using the sessile drop method for different brush layer 22 23 24 materials; Simulations of the same system as in Figure 4.c but with an increased strength of the short- 25 26 range potential of the guiding pattern; 27 28 29 30 31 32 ACKNOWLEDGMENTS 33 34 35 This work was partially funded by the projects CoLiSA.MMP (ICT Project no. 619793) and 36 37 38 NANOINTEGRA (TEC2015-69864-R, MINECO/FEDER, UE). Laura Evangelio acknowledges the 39 40 Ministry of Education of Spain for her FPU grant (FPU 13/03746). Marcus Müller thanks the Deutsche 41 42 Forschungsgemeinschaft for financial support under grant Mu1674/16. The ICN2 is funded by the 43 44 CERCA programme / Generalitat de Catalunya. The ICN2 is supported by the Severo Ochoa programme 45 46 47 of the Spanish Ministry of Economy, Industry and Competitiveness (MINECO, grant no. SEV-2013- 48 49 0295). 50 51 52 REFERENCES 53 54 55 (1) Moore, G. E. Craming More Components onto Integrated Circuits. Electronics 1965, 38 (8), 114– 56 57 117. 58 22 59 60 ACS Paragon Plus Environment Page 23 of 38 ACS Applied Materials & Interfaces (2) Mansky, P.; haikin, P.; Thomas, E. L. Monolayer Films of Diblock Copolymer Microdomains for 1 2 3 Nanolithographic Applications. J. Mater. Sci. 1995, 30 (8), 1987–1992. 4 5 6 (3) Ruiz, R.; Kang, H.; Detcheverry, F. A.; Dobisz, E.; Kercher, D. S.; Albrecht, T. R.; De Pablo, J. J.; 7 8 Nealey, P. F. 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Rev. 56 57 58 30 59 60 ACS Paragon Plus Environment Page 31 of 38 ACS Applied Materials & Interfaces E 2000, 62 (4), 5281–5295. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 31 59 60 ACS Paragon Plus Environment ACS Applied Materials & Interfaces Page 32 of 38 1 2 FIGURES 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 Figure 1. (a) Schematic representation of DSA on chemical wide patterns using EBL and oxygen plasma 34 35 functionalization. SEM images of PS-b-PMMA DSA on 1.5L0 width guiding stripes and density 36 37 multiplication factors of (b) 3L0, (c) 4L0 and (d) 5L0 (red stripes represent the guiding stripe width). 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 32 59 60 ACS Paragon Plus Environment Page 33 of 38 ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 Figure 2. (a): Scheme of the method to determine the contact angle between homopolymers in un- 40 41 modified (top) and modified (bottom) brush surfaces. Droplets are obtained by dewetting of 42 43 homopolymer blends under the same annealing conditions than the ones used for DSA. (b-g): SEM 44 45 46 images showing the droplets of polymer blends after dewetting and after being removed the PMMA 47 48 domains by O2 plasma (260 W, 50 sccm O2 for 18 seconds); SEM images in (b-d) corresponds to a 49 50 dewetting experiment on an un-modified brush; SEM images in (e-g) corresponds to the modified 51 52 53 brush. In both cased the brush is PS70%-r-PMMA 54 55 56 57 58 33 59 60 ACS Paragon Plus Environment ACS Applied Materials & Interfaces Page 34 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Figure 3. (a) Examples of simulation of symmetric homopolymer blends for two values of 25 26 N as a function of the surface strength, ΛN. (b) Relation between contact angle 𝜃𝐴𝐵 and 27 28 surface strength ΛN from the analytical prediction (dashed lines) and from simulations (c) 29 30 Front and top-down views of SCMF simulation for a thin symmetric BCP film. Results for a 31 32 hard film surface with a short-range attraction, ΛN=0.538 for the narrow guiding stripe with 33 34 width 1.5L0 and ΛN=-0.135 for the background with width 3.5L0. The film thickness is 35 D=1.13Re0. 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 34 59 60 ACS Paragon Plus Environment Page 35 of 38 ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 Figure 4. a) Two-dimensional composition profile of a homopolymer blend, N=32, 28 29 confined between two antisymmetric random-copolymer brushes with A fractions, 30 31 fb=0.1 and 0.9. The system geometry is 3.56Re0×10Re0×2Re0 and the system is comprised 32 33 of 2304 A homopolymers, 2304 B homopolymers and 7200 grafted random copolymers 34 that are a factor 4 shorter. The white wedge indicates the contact angle that the internal 35 36 AB interface makes with the confining surfaces in the middle of the film. (b) Contact 37 38 angle, θA-B, of a symmetric homopolymer blend with N=32 on a random-copolymer 39 brush with Nrc=8 as a function of the composition, fb. χN=17 and ≈902. The horizontal 40 41 lines mark the contact angle on the wide background stripe and the symmetry-related 42 43 value of the narrow guiding stripe corresponding to the experimental sample PS70%-r- 44 45 PMMA. The arrows on the abscissa indicate the compositions of the random copolymer 46 brush, fb=0.37 (background) and 1-fb=0 (guiding stripe). (c) Front and top-down views 47 48 of SCMF simulation for a thin symmetric BCP film for a brush-coated surface with 49 50 effective brush thickness h=0.5Re0 and total film thickness D=1.64Re0. The composition 51 of the random copolymer brush fb is 1 (i.e., A-homopolymer brush) and 0.37 on the 52 53 54 narrow guiding stripes and the wide background, respectively. 55 56 57 58 35 59 60 ACS Paragon Plus Environment ACS Applied Materials & Interfaces Page 36 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 Figure 5. SEM images of the result of directed self-assembly when the chemical guiding patterns are 29 30 31 formed using strong oxygen plasma conditions (300W, 50sccm O2 for 10s). The block copolymer is 32 33 PS-b-PMMA that provides a pitch L0 = 22 nm. The width of the chemical guiding stripes dimensions 34 35 is 1.5 L0 and the period is 5L0 . The right image is a zoom-in on the right image to better appreciate the 36 37 parallel alignment of the block copolymer on the chemically modified stripes. 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 36 59 60 ACS Paragon Plus Environment Page 37 of 38 ACS Applied Materials & Interfaces 1 2 3 4 5 6 PS60%-r-PMMA PS70%-r-PMMA 7 Strong Strong System Un- Mild O2 Un- Mild O2 8 O2 O2 modified plasma modified plasma 9 plasma plasma 10  107.7° 154.5° 159.7° 77.7° 145° 155.8° 11 12 cos  -0.30 -0.90 -0.94 0.21 -0.82 -0.91 13 ∆ 𝛾(mN/m) -0.18 -0.55 -0.57 0.13 -0.50 -0.56 14 PS80%-r-PMMA PS-OH 15 Strong Strong 16 System Un- Mild O2 Un- Mild O2 O2 O2 17 modified plasma modified plasma plasma plasma 18 19  64.9° 141.6° 155.3° 55° 128° 147.1° 20 cos  0.42 -0.78 -0.91 0.57 -0.62 -0.84 21 ∆ 𝛾(mN/m) 0.26 -0.48 -0.56 0.35 -0.38 -0.51 22 23 24 25 Table 1. Values of experimental difference of surface free energies obtained from homopolymer blend 26 27 28 contact angle measurements for different brush layer materials. The values are calculated for the 29 30 conditions to process PS-b-PMMA (T=230ºC, i.e., γPS/PMMA=0.61 mN/m). For each brush layer (i.e., 31 32 different fraction of PS), the contact angles are determined for the as grafted surface, and for two different 33 34 35 conditions of oxygen plasma exposure to chemically modidy its character. 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 37 59 60 ACS Paragon Plus Environment ACS Applied Materials & Interfaces Page 38 of 38 TABLE OF CONTENTS GRAPHIC 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 38 59 60 ACS Paragon Plus Environment