Floated Lamella Films of Styrenic Block Copolymers: Local Shearing

Article pubs.acs.org/Macromolecules

Floated Lamella Films of Styrenic Block Copolymers: Local Shearing Deformations and Heterogeneous Layer at the Substrate Eva Max,† Markus Hund,† Igor I. Potemkin,‡,§ and Larisa Tsarkova*,§ †

Physikalische Chemie II, Universität Bayreuth, Universitätsstraße 30, D-95440 Bayreuth, Germany Physics Department, Lomonosov Moscow State University, Moscow 119991, Russian Federation § DWI - Leibniz Institute for Interactive Materials, Forckenbeckstraße 50, 52056, Aachen, Germany ‡

S Supporting Information *

ABSTRACT: We examined the free surface as well as the substrate−film interface of thermally equilibrated block copolymer films forming stacks of glassy−rubbery lamellae and having quantized film thickness due to terrace formation. Upon reversing onto a new substrate, ∼100 nm thick floated films deform without loss of the film integrity, so that the characteristic macroscale topographic pattern is transmitted to the newly formed free surface. The adhesion-driven deformation is attributed to a local shear of the step regions in order to adjust the surface relief structures to the flat substrate geometry. Further, the polystyrene sheet at the film− substrate interface exhibits a heterogeneous phase structure which we assign to a partial autophobic dewetting. Stepwise erosion and reconstruction of the inner structure of the film disclosed a precise connection of the alternating wetting conditions at the substrate with the surface topography of the top layer. Apart from unveiling an adaptive mechanical behavior of nanostructured polymer films under confinement, observations reported here can be used for designing superimposed topographic structures by controlling the wetting at the substrate as well as allowing better prediction of possible mechanisms of structure formation and pattern transfer on chemically patterned surfaces.

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INTRODUCTION Thin polymer films are increasingly used in nanofabrication of coatings, membranes, lithographic resist, optical filters, sensors, etc.1 Most of these functional films possess chemical and/or structural heterogeneity on a nanoscale in a form of polymer blends, supramolecular assembles, inorganic nanocomposites, or microphase-separated structures in the case of block copolymer materials. Mechanical properties, stability, and processability of such submicrometer-thick films with extended internal interphase are of utmost importance for their effective functioning and usage.2−7 The particular technological potential of block copolymer films is based on their intrinsic ability to phase separate and self-assemble in well-defined microdomains which chemical heterogeneity and functionality can be controlled by synthetic procedures as well as precisely tailored by postmodification for designing of nanopatterned surfaces, 8,9 quantum dots/ arrays,10,11 functional membranes,12 organic electronics,13 photonic crystals,14 etc. The choice of the supporting substrate for the preparation and processing of thin block copolymer films in most cases is a decisive factor for achieving a desired pattern. Selective or neutral interactions with homogeneous substrates determine inplane or perpendicular orientation of cylindrical and lamella domains, respectively.15,16 In many research examples chemically patterned or topographically structured (graphoepitaxy) © 2013 American Chemical Society

substrates in many research examples have been shown to guide nanopatterns to a perfect order on a macroscale.17−21 Technological needs for cost-effective topographic templates initiated research approaches toward multiscale patterning which utilize the features of the microphase separation of block copolymers both at microscale (terrace formation) and at nanoscale (phase behavior),22,23 also in a combination with a controlled dewetting and conventional lithographic techniques.24,25 The efficiency of the thermodynamics-based “bottom up” approaches is evaluated mainly by the analysis of the structure of the top layer using imaging techniques such as scanning force microscopy (SFM) and scanning electron microscopy (SEM).26 Additionally, the inner structure of the film might be accessed by cross-sectional transmission electron microscopy (TEM)/SEM, by grazing-incidence small-angle X-ray scattering (GISAX),27−29 neutron-reflectivity,30 and secondary ion mass spectroscopy (SIMS) measurements,31−33 or much rarely by laborious tomographic protocols.34−36 The lateral structure at the film−substrate interfaces is more difficult to characterize. However, resolving the details of the microphase separation at the substrate is essential for Received: October 9, 2013 Revised: December 13, 2013 Published: December 20, 2013 316

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freshly cleaved mica substrates from PS−PEP solutions in toluene with concentrations of 1.4 and 1.8 wt % under rotation speeds of 2000 and 3500 rpm, followed by thermal annealing in vacuum at 160 °C (below the order−disorder transition temperature (ODT) of PS−PEP) for 16 h. Mica-supported films were floated onto water subphase and then deposited on silicon substrates in two ways: with the former mica− polymer interface (bottom sample) or former free surface (top sample) facing the air. The transferred on silicon substrates films have been characterized with optical microscopy (Axiotech microscope from Carl Zeiss AG), spectroscopic ellipsometry (Sentech SE 850), and scanning force microscopy (SFM) in Tapping Mode (Dimension 3100 from VEECO Instrument Inc.) using silicon tips with a spring constant of ∼40 N/m, with a typical resonance frequency of ca. 300 kHz (OMCL-AC160TS from Olympus, Japan). The measurements of the surface morphology (phase images) were performed at free amplitudes of about 30−50 nm and a relative set point of ∼0.95. Quasi-In-Situ Etching of Polymer Films. The SFM-based setup with quasi-in-situ sample treatment capabilities (QIS-SFM) and its application to depth profiling of block copolymer films has been recently described in great detail.38,42,43 For the quasi-in-situ plasma etching the sample is placed in the chamber, and the desired spot on the sample is imaged as a starting point. After the tip retraction, the chamber is closed and the sample is treated by the air plasma. Typically the power used to sustain the plasma is ∼2 W, although ∼8 W power pulse is required to initiate the discharge at a process pressure of about 1.5 mbar. The etching time for the particular system was 20−30 s. Films of PS−PEP as well as films from respective PS and PEP have been subjected to the etching procedure to determine etching rates (Figure 6b (for PS−PEP) and Figure 1S (for PEP and PS in Supporting Information)). Since an accurate thickness determination using SFM requires light tapping conditions which do not always allow a detailed phase resolution, morphological studies (Figure 7a and Figure 2S) have been performed in a separate etching experiment on a similar piece of the bottom sample. We note that with this technique the etching rate of the polymer material (height signal) as well as local changes in the lateral structure (phase signal) can be measured with a high precision within reasonable acquisition times.

understanding the connection between the bottom and top layers which might be different even for one-layer-thick films.37,38 Apart from the registration of the pattern at the substrate and its transformation through the entire film, the bottom layer determines the adhesion to the substrate as well as the functionality of the resulting nanostructure, e.g., in a form of a through porosity of block copolymer-based membranes. Also, the information about the structure at film−substrate interface is important for preventing block copolymer films from dewetting,6,39 a dynamic process which competes with the ordering of microdomains during equilibration procedures. Further, the real lateral structure of the film next to the substrate may not satisfy the assumptions that are made to model data from scattering space-averaging methods such as GISAX as well as neutron and X-ray reflectivity and thus affect the interpretation of the measurements. Techniques used to access the structure at the film−substrate interface include peeling off or floating off the film from glass or mica substrate onto a water surface as well as dissolution of a sacrificial coating at the substrate (which however can affect the microphase separation). The floating technique is used when there is a need to sequentially stack layers of different polymer materials, to transport thin membrane layer to a support membrane, or when specific requirements to the substrate to be used (hydrophilic, rough, ultrahydrophobic, conductive, etc.) do not match the surface field to guide the microphase separation into a desired shape/orientation of microdomains. In all these cases, the transfer of a thin film without damage to its integrity and to the internal microphase separated structure is of significant importance. Here we examine thin films of a styrenic diblock copolymer, which form stacks of glassy−rubbery lamellae oriented parallel to the substrate. Thermally annealed lamella films with a quantized thickness and characteristic macroscaled topographic features (terraces with an integer/half integer number of layers)40,41 have been floated and transferred to a flat substrate so that the topographic features are transmitted to the newly formed film−air surface. We analyze the possible deformation mechanisms and suggest that it is adhesion-driven and proceeds via local shearing of the parts of the film between adjacent terraces. Further, we disclose a heterogeneous structure of the bottom lamella sheet and reveal its connection to the surface topography of the top layer by performing a reconstruction of the inner structure of the film. Reported here observations unveil novel possibilities to design hierarchical superimposed topographic structures by controlling the microphase separation at the substrate as well as deliver knowledge to better predict possible mechanisms of structure formation and pattern transfer on chemically patterned surfaces.

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RESULTS AND DISCUSSION Mechanism of the Topographical Structuring of the Free Surface upon Reversion of Lamella Films. Films of PS−PEP block copolymer have been spin-coated and thermally annealed on freshly cleaved mica. The annealing results in the alignment of the lamella parallel to the film plane and in the structuring of the film surface (terrace formation, Figure 1b) as long as the film thickness does not commensurate with the lamella period L0 in the case of symmetric wetting when the same block is segregated to both film interfaces and with (n + 1 /2)L0 in the case of asymmetric wetting, where n is an integer. Floated films (Figure 1c) have been deposited on silicon wafers so that the former bottom layer was exposed to the free surface (Figure 1d). For comparison, a part of each floated film was deposited in the original position with the bottom layer down to the silicon substrate. Throughout the paper, the samples that were prepared according to the procedure in Figure 1 are referred to as “bottom” and “top” layers/samples, respectively. The procedure has been applied to the films with the thicknesses between ∼70 and 110 nm (from ∼1L0 to ∼1.5L0) to cover the thickness range which satisfies symmetric or asymmetric wetting conditions. We note that L0 in thin films might differ from that in bulk due to confinement effects, and these deviations have been shown to be more pronounced for one-layer-thick films,44,45 resulting in a change of material properties such as ODT.29 Further, the lamella structures in confined films can adjust to the film thickness by changing the

EXPERIMENTAL SECTION

Polymer. Polystyrene-block-polyethylenepropylene (Kraton, 1701E), designed as PS−PEP, with a molecular weight of Mw = 110 kg/mol and a volume fraction of the polystyrene block of 37% has been used as received. In thin films PS−PEP forms lamella structures with a period L0 of ∼60 ± 3 nm. The glass transition temperatures of the PS−PEP determined with differential scanning calorimetry (Mettler Toledo Star) are Tg(PEP) = −55.2 °C and Tg(PS) = 106.8 °C. According to the thermogravimetric analysis, the degradation of PS−PEP starts at ∼269.1 °C, and at 384.2 °C more than 99% of the sample is degraded. In films the PEP component segregates to the free surface due to its lower surface tension as compared to the PS block. Sample Preparation and Characterization. Thin films with the thicknesses in the range of ∼70−110 nm have been spin-coated on 317

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Figure 1. Schematic of the sample preparation: (a) spin-coating on freshly cleaved mica surface, (b) thermal annealing resulting in topographic structuring of the free surface (terrace formation), (c) floating to the water surface, and (d) transfer to silicon wafer upsidedown (“bottom”) as well as in the original position (“top”). In (a−c), the block copolymer film is sketched without details of the microphase separation to emphasize the topographic structuring of the free surface (not drawn to scale).

domain period46 as well as by changing the lamella orientation perpendicular to the film plane even on selectively attractive substrates.47 Finally, stabilization of coexisting in-plane and vertically oriented domains and transitions to nonbulk morphologies are established mechanisms of polymorphism which block copolymers exhibit under confinement.48 Figure 2 presents an overview of the SFM measurements of the mesoscale surface topography of three samples with indicated starting thicknesses. The original surface patterns of the annealed PS−PEP films in Figures 2a and 2c are represented by terraces in a form of holes and bicontinuous structures, respectively. PS−PEP film with a starting thickness of ∼70 nm (Figure 2e) has a smoothest surface topography, indicating that the film thickness matches the best out of the three samples the equilibrium thickness of the layered PS−PEP lamella. The newly formed free surfaces of the floated samples reveal a close similarity with the original films in the averaged amplitude and in the period of the topographic corrugations (Figure 2b,d,f). Shown in Figure 2a,b are the topographic images of PS−PEP film with an initial thickness of ∼100 nm. Both the top and the bottom samples disclose terraces with extended flat regions (see the cross-section plot) and a regular height difference between the terraces of ∼60 ± 2 nm, in agreement with L0. Figures 2c and 2d present parts of the floated PS−PEP film with an initial thickness of ∼80 nm, which upon annealing exhibits coexisting terraces with step heights of 54 ± 5 nm. The sinusoidal-like profile of the terraces (see the respective crosssection plot) and a significantly smaller as compared to L0 stepheight values indicate that the redistribution of the materials between the lamella layers, i.e., the process of terrace formation, is incomplete. Still this particular topography is precisely replicated upon reversing of the film. As seen in Figure 2e,f, the top and the bottom surfaces of the film with a starting thickness of ∼70 nm are undulated with ∼8 nm large corrugations. The respective bottom layer is additionally characterized by 1−2 nm large subroughness (Figure 2f). We come later to the possible origin of this surface heterogeneity. The presence of the surface corrugations at newly formed film−air interfaces (Figure 2) suggests that upon flipping of the film the glassy−rubbery lamella stacks undergo local shearing

Figure 2. SFM topography images of the top layers (left column) and of the bottom layers (right column) of PS−PEP films with initial thicknesses of 100 ± 5 nm (a, b), 80 ± 5 nm (c, d), and ∼70 ± 5 nm (e, f). Dashed lines in (a, c, and e) indicate the place in the image which corresponds to the respective cross-sectional plot. (g) Sketch of the film reversion (not to the scale) illustrating the processes of the film adhesion via local shearing of the parts of the film along the dashed lines (step regions). The microphase-separated structures are not resolved in the sketch.

on a scale of tens of nanometers in order to adjust the surface relief structures to the flat geometry of the substrate (schematically presented in Figure 3j). To confirm this deformation pathway, we compare the microphase separation behavior in the top and in the bottom layers. 318

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Figures 3 displays detailed topography and phase information on the films shown in Figure 2a−d. In contrast to the similarity of the mesoscale topography, the microphase separation behavior exhibits certain differences between the top and the bottom layers. A general observation is that the phase images of the top samples (Figure 3c,g) reveal a soft wetting layer and a poor phase contrast suggesting an existence of a continues sheet of the PEP block at the free surface, in agreement with its lower surface tension as compared to the PS block. Upon harder tapping, the underlying microphase-separated structures can be slightly resolved (Figure 3c). In the bottom layer (Figure 3d,h) the phase contrast is well resolved and, unexpectedly, reveals a heterogeneous phase pattern which points to a vicinity of both blocks to the former film−substrate interface (we come later to a possible origin of this heterogeneity). Regarding the morphological behavior, Figure 3 discloses featureless areas which are characteristic of in-plane lamella phase as well as other types of the microphase-separated structures. The buried structures in the lower terrace in Figure 3c appear as stripes formed by up-standing lamella phase which coexists with featureless areas on in-plane lamella. Similar stripped features are visible in the bottom layer in Figure 3d, suggesting that the standing lamella propagates through the terrace. The step regions between the terraces in the top layer (Figure 3c) are marked by a pattern of dark dots which we attribute to a perforated lamella (PL) phase. This assignment is based on the strong phase contrast between the glassy PS (bright color in the phase image) and the soft PEP blocks. Since PS block is a minority phase, the layer of the PS is perforated by the majority PEP block, resulting in the dark dots within the white (PS) matrix. PL phase is a deviation from the bulk lamella morphology, and it develops locally at the film areas where the film thickness is slightly smaller than the equilibrium one.45,49 Note that the PL phase is not present in the bottom layer (Figure 3d). The steps between adjacent terraces in 80 nm thick film (Figure 3g) are characterized by few stripes of up-standing lamella. In the bottom layer these stripes are included it the matrix of the soft PEP block (dark color). Generally, the step regions of the PS−PEP films contain other than in-plane lamella morphologies and thus represent mechanically weakest part of the film. The presence of the soft PEP block (Figure 3h) in the steps of the bottom samples and of the mixed phase (an intermediate color in Figure 3d) suggests that the continuous PS sheet at the former mica−film interface is raptured upon shearing of the parts of the film between adjacent terraces in the direction normal to the surface. Afterward, the deformation is likely proceeds due to the viscous flow of the PEP block in the sheared region, so that the air voids at the substrate interface are substituted by the adhered thinner parts of the film (Figure 2g). The driving force for the local shearing is attributed to the adhesion force between the polymer film and the native silicon oxide layer covering the silicon wafer. The existence of the adhesion force between brush melt film and mica surface in air has been demonstrated earlier using surface force apparatus (SFA).50 We assume that the surface properties of dry mica are very similar to that of the silicon oxide layer and that low-Tg melt brushes model to a good approximation the chain conformation in microphase-separated block copolymers. We now consider two adhering surfaces, PS and silica, at a distance y ∼ L0 apart. We assume that the PS−PEP film first

Figure 3. SFM topography images (a, b, e, f) and phase images (c, d, g, h) of the top and bottom layers (as indicated) of PS−PEP films with an initial thickness (a−d) of ∼100 nm (11/2L0) and (e−h) of ∼80 nm. The dashed and solid squares in (e−h) highlight the same spots in the lower and in the higher terrace, respectively. The sketch in (i) depicts the asymmetric wetting conditions in original films with the gray color (PS block) at the mica substrate and the dark color (PEP block) at the film−air surface. In the bottom sample (j) the wetting is also asymmetric but reversed. L0 (∼60 nm) and lterrace (an averaged lateral radius of terraces ∼1 μm) represent characteristic dimensions of the topographic structures at the free surface of annealed PS−PEP films (not to scale). 319

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reversing of 70 nm thick film with the smoothest surface topography (Figure 2e,f and Figure 5a−d). Figure 5c displays a phase image of the top sample which has a soft wetting layer of PEP block and an underlying dot-like pattern of the PL phase.

deforms under the attractive adhesion force, so that the starting geometry of the interaction is sphere of radius R near a flat surface (Figure 4). The radius of curvature R can be roughly

Figure 4. Schematic of the part of the film to be adhered to the silicon substrate with characteristic dimensions which are used for the evaluation of the adhesion force and of shear force. The dashed line represents the form of the stress distributions σ(r) according to JKR model, indicating a compressive tensile stress.

estimated using Chord theorem r2 = (2R − y)y ≈ 2Ry which gives for R ∼ 10−5 m. According to Johnson−Kendall−Roberts (JKR) model,51 adhering surfaces deform by flattening the contact zone even under no external load50 with a nonuniform pressure distribution σ(r) across the contact area (Figure 4). The van der Waals adhesion force is given as Fad = AR/6y2, where A is a complex Hamaker constant for the PS/SiOx system, and r is an averaged radius of terraces rterrace (∼μm) which define the area of the contact.52 The constant A can be estimated from the known values of individual Hamaker constants using a relation described by Israelachvili A ≈ (APS2ASi2)1/2 ≈ 6.4 × 10−20 J, where APS ∼ 6.5 × 10−20 J and ASi ∼ 6.3 × 10−20 J.52 Substituting all estimated values we get for Fad ∼ 1−10 mN. We now evaluate whether this compressive force due to adhesion is sufficient to cause a rapture of the glassy PS layer in step regions. The shear strain γ in the sheared region can be approximated as 1/2L0/lstep giving for γ ∼ 10−1−10−2. Assuming that a mean shear response is provided by the PS sheet (with a shear modulus G of ∼3.4 GPa),7 the shear strain τ is given as τ ∼ Gγ ∼ F0/A, where A is the force-resisting area 2πrterraceL0 ∼ 10−13 m. Evaluating in this way an applied shear force F0 which PS can sheet sustain without rupture, we get 1−10 μN, significantly smaller than the evaluated above adhesion force Fad. Further, we assume that upon rupture of the glassy PS the shear deformation proceeds via viscous flow of the PEP block. Here, we can refer to experimental studies in which shear forces in the same rangeof order of several tens of μNhave been measured while shearing past each other weakly penetrating melt brushes.53 Both the estimates and the experimental evidence support adhesion-driven shearing deformation of the step areas of the film. It should be noted that concepts of contact mechanics and JKR adhesion theory are too simplistic for most polymer systems where nonequilibrium, timedependent, and confinement effects occur at the contacts. Currently there are now applicable theories that can account for adhesion dynamics, and therefore the above analysis represents very approximate estimations. Heterogeneous Layer at the Substrate. As noted earlier, the phase image of the bottom layer in Figure 3d revealed phase heterogeneity in a form of darker (softer) protrusions in the glassy PS sheet. Similar features have been observed upon

Figure 5. SFM topography images (a, c, e, g) and phase images (b, d, f, h) of the top layers (left column) and of the bottom layers (right column) of PS−PEP films with an initial thickness of ∼70 nm (as in Figure 2e,f). Before floating and transfer, the film in (a−d) has been annealed in vacuum at 160 °C for 16 h, and the film in (e−h) has been subjected to a short 15 min drying on a hot plate at 150 °C to remove the residual solvent. In images (c) and (d) the ∼80 nm-large perforations and the phase features with dimensions of 100 s of nm are exemplary marked with circles and arrows, respectively. 320

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The bottom layer (Figure 5d) is characterized by a large phase heterogeneity which also appears as 1−2 nm large subroughness in the topography image (Figure 5b). Three distinct colors, i.e. three polymer phases with different material properties, are assigned to the softest PEP block (the darkest color), to the glassy PS block (white color), and to a mixed phase of the two blocks (intermediate color). We note that soft protrusions seen in Figure 5d have a bimodal size distribution: a monodisperse one with a regular diameter of 80 nm, which is a characteristic spacing of the PL phase,54 and the other one in the range of hundreds of nanometers. Interestingly, depressions with similar dimension are noticeable in the top layer in Figure 5c, suggesting a structural correlation between the pattern in the top and in the bottom layers. A possible origin of the heterogeneous composition of the bottom layer is supposed to be a dewetting instability of the PS thin sheets on polar substrates. To get more insights into this process, we have analyzed the top and bottom layers of 70 nm thick film which prior to floating was subjected to a short thermal treatment (for 15 min) to remove the residual solvent. We believe that the structures shown in Figure 5e,h disclose the microphase separation behavior directly after the removal of the solvent by spin-coating, and they resemble an interfacial instability of a wetting layer at the substrate upon spin-coating of polymer blends.55 Topography and phase images of the top and bottom layers show heterogeneous structures which can be interpreted as dewetted sheets of the PS block. The dewetting proceeds differently at the free surface and at the solid substrate (bottom layer). In Figure 5g, surprisingly enough, the PS sheet appears not stable even beneath the PEP wetting layer and shows a beginning of the rearrangement into the PL phase. We note that the height undulations in Figure 5e correlate with the phase pattern in Figure 5f. We believe that the phase transition in the top layer is driven by the phase separation and dewetting of the PS sheet at the bottom layer (Figure 5h). We note that this bottom layer after spin-coating shows very similar features, just with less regular dimensions of the dark-colored protrusions, as the bottom layer after long-term annealing (Figure 5d). This observation confirms that once formed during spin-coating the instability patterns in the PS sheet at the mica−film interface affect and guide the microphase separation through the film in a form of alternating asymmetric/symmetric wetting conditions. Depth Profiling of the Lamella Films by Stepwise Plasma Etching. The free surface of the bottom samples in Figure 3c and in Figure 5c appear to be locally substructured, as has been pointed earlier. This local substructuring is even more pronounced in the case of ∼105 nm thick thermally annealed and floated PS−PEP film shown in Figure 6a. Three areas I, II, and III with distinct film thicknesses are presumably assigned to ∼1L0, 11/2L0, and 1/2L0 thick films, respectively (Figure 6b), although the step height between terraces I and II (∼24 nm) is somewhat smaller than 1/2L0. The microphase-separated structures are represented by in-plane lamella in flat areas and by narrow regions of the PL phase at transition thicknesses between the terraces. To follow the structural connection between the top layer and heterogeneous layer at former mica−film interface, we performed consecutive plasma etching with SFM imaging of the same spot which gave access to the inner structure of the film. Details on the etching and imaging of the top sample are presented in Figure 2S (Supporting Information). Phase images after consecutive etching steps (Figure 6b) emphasize the

Figure 6. (a) SFM topography (left) and phase (right) images of PS− PEP film (top sample) with an initial thickness of ∼105 nm. Areas with distinct film thickness I, II, and III are marked in the height image. The dashed line in the topography image indicates the position of the respective cross-sectional plot. (b) Schematic reconstruction of inner layers of the film. The dashed lines are evaluated from the topography images after selected etching steps. Phase images A and B display the structure after respective etching steps.

structural and phase difference of the step areas as compared to the rest of the film, supporting our assumption on the mechanical weakness of the step regions. The results of the etching of the bottom sample are presented in Figure 7. Three different types of terraces seen in the phase image [1] in Figure 7a can be qualitatively assigned to the areas I−III in the top layer (Figure 6a). Important for the present discussion is that the PS block covers the terraces with thicknesses of 11/2L0 and 1/2L0 while the PEP block covers one lamella thick terrace as schematically depicted in Figure 7b. In order to get quantitative information on the inner structure of the film, we determined the etching rate of the lamella film, by measuring the film thickness in terrace II of the bottom sample with ∼nm resolution using the quasi-in-situ QIS-SFM setup. The plot in Figure 7a shows clear differences in the etching rates of the glassy PS and soft PEP phases, and the etching rates of the distinct layers are very close to those of the respective homopolymer films (Figure 1S in Supporting Information). We note that the vicinity of the silicon wafer 321

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by analysis of the phase images of the same spot after each etching step (Figure 7a). The relative heights of the terraces upon each erosion step are extracted from the SFM topography data and are shown as dashed lines in Figure 7b. The results in Figure 7 confirm the existence of structural connection between the top and bottom layers as well as the shearing of the step regions without loss of the overall film integrity. We note that the PL phase appears only after the third etching step (image [3] in Figure 7a) when 30−40 nm of the film are removed, and therefore the utilization of the potential porosity of this dot-like pattern requires some additional processing steps in a form, e.g., of initial adjustment of the surface properties of the substrate or of erosion protocols, as presented here. Interestingly, the narrow dark (soft) regions around terraces I (image [2] in Figure 7a) which develop next to the PL phase (see image [3] in Figure 7a) reveal additional phase heterogeneity and most probably represent the shearing plane.

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CONCLUSIONS Thermally equilibrated films from PS−PEP diblock elastomer with thicknesses in the range of ∼1L0−11/2L0 have been floated and transferred onto new supports to compare the details of the lateral microphase separation at the free surface and at the film−substrate interface. Specific features of the topographic structuring due to terrace formation at the free surface are retained upon reversing of the films as a result of the adhesiondriven local shear deformation of the step regions. SFM imaging of bottom layers of floated films revealed a nonuniform heterogeneous structure representing submicroscopic regions of phase-separated blocks as well as of the disordered (mixed) phase for each studied film thickness. The free surface is then correspondingly substructured into terraces with a height difference of (n + 1/2)L0 and of nL0 matching the wetting pattern at the substrate. The phase heterogeneity is attributed to a partial autophobic dewetting which is balanced by the lateral microphase separation in the layer next to the substrate. This observation is likely relevant to the issue of suppression of dewetting of thin polymer film and may contribute to the understanding of the influence of molecular architecture on the dewetting.56 Reconstruction of the inner structure of the film disclosed a precise connection of the alternating wetting conditions at the substrate with the surface topography of the top layer. Partial segregation of the PEP block directly to the film−substrate interface changes locally the wetting conditions from asymmetric to symmetric ones and thus leads to a hierarchical structuring of the free surface of the lamella film. Reported here novel insights into microphase separation in thin films of elastomers might be effectively combined with existing theoretically justified approaches of chemical or topographical guidance to achieve high degrees of precision in pattern transfer and in hierarchical (topographical and chemical) multiplication of the underlying patterns.21,57−59

Figure 7. (a) SFM phase images of the bottom sample upon depthprofiling measurements/analysis of the bottom sample of the film as in Figure 6a. Corresponding areas with distinct film thickness I, II, and III are marked in image [1]. Images display the lateral structure of the layers at the indicated steps of the etching procedure. The dashed squares highlight the area which is the same for all phase images. Plot representing kinetics of the stepwise erosion of PS−PEP films in area II. Areas with indicated etching rates are assigned to alternating PS and PEP sheets in accordance with the etching rates of the respective homopolymers (Figure 1S). (b) Schematic reconstruction of the lamella layers upon flipping of the film. The dashed lines are extracted from the height images (not shown) of the respective etching steps in (a).

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ASSOCIATED CONTENT

S Supporting Information *

Etching kintetics of PS and PEP homopolymers as well as the details on the stepwise errosion of the top sample as in Figure 6. This material is available free of charge via the Internet at http://pubs.acs.org.

leads to an effective decrease in the etching rate of the PEP block in step 6. The calibration curve allowed the reconstruction of the layered structure of the bottom sample 322

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (L.T.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support of the Ministry of Education and Science (Russian Federation) within the agreement N 8816 is gratefully acknowledged.

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