Nanoscale Subsurface Morphologies in Block Copolymer Thin Films

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Nanoscale Subsurface Morphologies in Block Copolymer Thin Films Revealed by Combined Near-Field Infrared Microscopy and Mechanical Mapping Kevin Ho, Kris S Kim, Leonid Gilburd, Ruben Mirzoyan, Sissi de Beer, and Gilbert C Walker ACS Appl. Polym. Mater., Just Accepted Manuscript • DOI: 10.1021/acsapm.9b00189 • Publication Date (Web): 16 Apr 2019 Downloaded from http://pubs.acs.org on April 16, 2019

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ACS Applied Polymer Materials

Nanoscale Subsurface Morphologies in Block Copolymer Thin Films Revealed by Combined Near-Field Infrared Microscopy and Mechanical Mapping Kevin Ho†, Kris S. Kim†, Leonid Gilburd†, Ruben Mirzoyan†, Sissi de Beer‡, and Gilbert C. Walker*,† Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada Materials Science and Technology of Polymers, MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, the Netherlands † ‡

ABSTRACT: Block copolymer (BCP) thin films are commonly characterized by techniques that either lack nanometric spatial resolution or the ability for subsurface characterization. In this work, we combine scanning near-field optical microscopy and atomic force microscopy mechanical mapping to probe the subsurface composition of poly(styrene-block-tert-butyl acrylate) thin film with nanometric spatial resolution and compare our results to a theoretical description. Our work demonstrates a novel imaging approach for interrogating the internal morphology of BCP thin films. A better understanding of subsurface morphologies will enable better design principles for nanolithography and templating thin films for photonics, photovoltaics, and tissue engineering. Keywords: near-field optical microscopy, subsurface imaging, atomic force microscopy, mechanical properties, block copolymer, thin film

Block copolymers (BCPs) are intensively studied for their ability to self-assemble, as a route to cost-effective, “bottomup” fabrication of structured materials. BCPs have potential applications in nanopatterning1,2 and organic photovoltaics.3 BCP patterns can be controlled rationally by varying parameters such as the polymer composition f, chain length N, and Flory–Huggins interaction parameter χ, resulting in the formation of a wide variety of structures such as lamellae, cylinders, and spheres.4 In contrast to bulk BCPs, the morphology of BCP thin films is highly dependent on how the copolymer interacts with both the substrate and the free surface. Only certain thicknesses t are energetically and entropically favorable, depending on how they compare with the characteristic period L0. Otherwise, holes and islands develop to compensate for this entropic penalty.5,6 In addition, for a very thin film with t < L0, more complicated structures can form due to an interplay between entropic considerations caused by film restriction, as well as surface tension or adhesion effects at the different interfaces.6 When designing for applications that make use of BCP thin films, understanding both the surface and subsurface structures is crucial. For example, knowledge of the subsurface morphology would allow for better design of sacrificial BCP nanotemplates for nanopatterning applications.7 Surface and subsurface structures also influence the performance of materials used in photovoltaics, where an exact understanding of the nanometric dimensions and

orientations of charge transport blocks,3 BCP coatings that limit cell and tissue adhesion,8 3D BCP-based nanoscale devices,9 and energy storage.10

Figure 1. (a) AFM height image of the polymer thin film, and (b) SNOM image taken at 1728 cm–1. The cross-section line profile obtained at the white line reveals the spatial resolution of SNOM to be at least 22 ± 2 nm.

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Figure 2. (a) AFM height map of the polymer thin film, showing different morphologies with different thicknesses. The arrows in the height map indicate the points at which the spectra in (d) and (e) were collected. (b) The height profile of the specimen, taken along the line in (a). The profile is partitioned by height into four regions. (c) SNOM images taken at wavenumbers in the range 1720–1750 cm–1. The signal intensity is maximized at 1728 cm–1 due to absorption by the carbonyl bond in the PtBuA block. (d) SNOM spectra on Region C on the thicker and thinner parts. (e) SNOM spectra on Region D. (f) FTIR spectrum of a PS-b-PtBuA thin ﬁlm on a silicon substrate. Inset: expanded view of the band at 1728 cm–1.

Atomic force microscopy (AFM), scanning electron microscopy (SEM), and grazing-incidence small-angle X-ray scattering (GISAXS) are techniques commonly used to characterize BCP thin films.11,12 Independently, these methods cannot probe the chemical composition of the subsurface with nanometric spatial resolution. For example, AFM provides the necessary spatial resolution but is mainly surfacesensitive.11 Similarly, SEM possesses nanometric spatial resolution but often requires additional sample preparation steps to enhance the image contrast.12 On the other hand, GISAXS excels in the determination of the internal structures of BCP thin films, but its spatial resolution is limited to the micron scale.13,14 To fully understand the layered composition of BCP thin films, both nanometric spatial resolution and subsurface chemical and structural information are needed. Scanning near-field optical microscopy (SNOM) allows for chemical characterization on the nanoscale. This technique couples infrared (IR) spectroscopy with AFM to directly visualize different blocks by selecting a wavelength at which one block absorbs but the other does not.15 Since the

spatial resolution of SNOM exceeds the diffraction limit,15 it has an edge over other optical techniques. Figure 1 shows the AFM height map (1a) and SNOM image at 1728 cm–1 (1b) of the lamella-forming poly(styrene-block-tert-butyl acrylate) (PS-b-PtBuA) BCP, depicting how SNOM can differentiate between the two blocks with a spatial resolution of at least 22 ± 2 nm (see SNOM – Limit of Spatial Resolution in the Supporting Information). In addition, SNOM has been shown to be sensitive to subsurface material changes, suggesting that it may be used for subsurface imaging.16,17 Therefore, SNOM could possibly be used to study the subsurface lamellar structure of BCP thin films. We note that AFM has been shown to be capable of subsurface imaging.18–20 However, these techniques rely principally on differences in mechanical18 or electrical properties19 in the different materials or incorporate ultrasound holography.20 In contrast, SNOM depends on the materials’ vibrational absorption, so it is more suitable for chemical characterization. Herein, we demonstrate the use of SNOM in tandem with AFM mechanical imaging to chemically identify and


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ACS Applied Polymer Materials characterize the subsurface structure of BCP thin films. As a model system, we used a PS-b-PtBuA thin film on a silicon substrate. (see Materials and Methods in the Supporting Information). A height image and line profile are provided in Figure 2, obtained using AFM, of a representative region of the sample. Region A (0–20 nm) consists of the silicon substrate. As t increases, the morphology evolves from lamellar in Region B (20–60 nm), to holes in Region C (60–85 nm), and to planar again in Region D (>85 nm). We collected SNOM spectra and compared them with Fourier-transform infrared (FTIR) spectra (Figure 2d-f)— see Comparison of SNOM and FTIR in the Supporting Information for additional comments. We also obtained SNOM images of Regions B–D at different wavenumbers (Figure 2c). The SNOM signal intensity increases from 1720 to 1728 cm–1, where it is maximized, and decreases up to 1750 cm– 1. To investigate the polymer morphology within the film, we examined how the SNOM signal intensity changes as a function of distance from the air–polymer interface to the polymer–substrate interface. In the PS-b-PtBuA system, there are two possible contributions to the SNOM signal intensity, which may or may not apply concurrently. (1) A thin layer of PS above the PtBuA block acts as a spacer that increases the distance between the AFM tip and the absorbing sample (the PtBuA block). It is known that the SNOM signal intensity decays rapidly with increasing tip–sample distance.21 Therefore, this property can be used to determine the thickness of the PS film. (2) The SNOM signal intensity depends on the thickness tPtBuA below a certain thickness threshold. To identify the description that better represents our system, we first need to determine whether it is the PS or PtBuA block that is at the free surface. To obtain this information, we employed AFM mechanical mapping to measure the elastic modulus E, deformation δ, and adhesion Fa, along with the height, for Regions B–D (Figure 3). The elastic modulus was calculated using the Derjaguin–Muller–Toporov (DMT) model with the relative calibration method employing bulk PS properties. The literature value of the bulk Young’s modulus is 2.7 GPa for PS22,23 and ~2 GPa for acrylate polymers with similar molecular weight and structure.24 In Regions B and C, the thinner part of the film has E = 3.14 ± 0.08 GPa and δ = 4.8 ± 0.3 nm, whereas the thicker part has E = 3.45 ± 0.09 GPa and δ = 3.9 ± 0.2 nm. The larger values of E and δ at the thicker part suggest that, in those areas, the PS block dominates at the surface. In Region D, we measured E = 3.27 ± 0.08 GPa and δ = 4.5 ± 0.3 nm. These values closely match those of the thinner parts in Regions B and C, suggesting that the upper layer at Region D consists of PtBuA. The overestimation of these properties may be due to the multilayer nature of the film25 or the substrate.26 The adhesion force, on the other hand, is dependent solely on the interaction between the tip and the sample surface and is largely insensitive to t even for these thin films.27 Thus, measuring Fa can provide more reliable information about the film’s surface. In Regions B and C, the thinner part of the film has Fa = 5.0 ± 0.2 nN, while the thicker part yields Fa = 3.1 ± 0.5 nN. Because a silicon tip was used

for these measurements, we expected larger Fa values on the PtBuA block because it interacts preferentially with the silicon versus the PS block.28 These results also indicate that the thinner part of the film comprises PtBuA, which is consistent with the measured E and δ values. Region D presents Fa = 4.7 ± 0.5 nN, similar to the thinner part of Regions B and C, suggesting again that the surface at Region D is dominated by PtBuA. Overall, these mechanical measurements indicate that the thicker part of Regions B and C consist mainly of PS, whereas the thinner part comprises PtBuA. The surface of Region D also appears to consist of PtBuA. We provide additional comments on our mechanical measurements in the Supporting Information.

Figure 3. AFM height map and mechanical maps of the elastic modulus, deformation, and adhesion force in Regions B, C, and D. The scale bar in the height map corresponds to 1 μm and is the same for all maps.

Now that we have determined qualitatively the composition of the topmost layer in each region, we can choose the correct SNOM description to apply. In Regions B and C, the topmost layer consists of PS in the thicker part and PtBuA in the thinner part. In these regions, we can estimate the thickness tPS of the PS overlayer as follows. The SNOM signal can be modeled by the effective polarizability of the tip– sample system:21 𝛼𝛽

−1

𝛼eff = 𝛼(1 + 𝛽) [1 − 16𝜋(𝑟+𝑧)3 ] ,

(1)

where α = 4πr3 (εt – 1) / (εt + 2) and β = (εs – 1) / (εs + 1) are related to the optical properties of the tip and sample, respectively, through the respective dielectric functions ε. The tip radius is r and the distance from the sample surface to the tip apex is z. At a fixed wavenumber, αeff is a function of only z and we assume that z is equal to tPS. The actual SNOM signal intensity I that is measured can be related to αeff by the relation

𝐼 = 𝐴 ∙ Im{𝛼eff,𝑛 (𝑧)} + 𝐵,

(2)

where A and B are constants related to the reference and incident electric fields used in the experimental setup. In addition, the signal is demodulated at a higher harmonic n to suppress the far-field background signal.



To obtain the constants A and B, we prepared standard samples consisting of PtBuA homopolymer on top of a silicon substrate, and PS homopolymers with known thicknesses over the PtBuA films (Figure S3). SNOM

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measurements taken on these samples provided a set of known I and z values, which was least-squares fitted using Equation (2), yielding A = 3.19 × 10–7, B = 6.71 × 10–3, and r = 138 nm. Measurements were taken at

Figure 4. (a) SNOM image, showing where the profiles in (b), (c), and (d) were taken. (b–d) The total height of the polymer thin film and the predicted PtBuA thickness in (b) Region B, (c) Region C, and (d) Region D. (e) Plot from Ref. 30, showing distinct types of morphologies that are expected at different t / L0 ratios and values of R, which is a ratio of measures of the air–polymer and polymer– substrate interactions. Adapted with permission from Ref. 30. Copyright 2000 American Chemical Society.

1728 cm–1, as this is where the absorption of PtBuA is maximized, resulting in good signal-to-noise ratios in the SNOM images (viz. Figure 2). With A and B known, we may employ Equation (2) to compute tPS. The results of the calculations are provided in Figure 4. SNOM images are also displayed to show where the cross-section used for each SNOM analysis was taken. In Region B, which exhibits a lamellar structure, the thicker and thinner parts are dominated by PS and PtBuA, respectively. The fraction of PtBuA increases when moving from the thicker to the thinner part. To model this behavior, we can calculate the volume fraction of the blocks using ϕPS = NPS / (NPS + NPtBuA), where NPS and NPtBuA are the numbers of monomers in the respective blocks. Assuming that the monomer volumes are roughly equal (an assumption that we validate in the Supporting Information under Additional Details on the Polymer Structures), and using NPS = 3800 and

NPtBuA = 3276, we calculated theoretical volume fractions of ϕPS = 46.3% and ϕPtBuA = 53.7%. We then computed the volume fractions from the SNOM images by considering the area occupied by each block and assuming that this distribution does not change in the vertical direction. We found that ϕPS = 46.6% and ϕPtBuA = 53.4%, which is in agreement with the theoretical volume fractions. In Region C, the film thickness increases further relative to Region B, and holes are observed instead of lamellae, as indicated by the height images. SNOM shows that the thicker part contains a higher concentration of PS that gradually decreases going down toward the substrate (Figure 4c). At the polymer–substrate interface, the BCP film consists of only PtBuA. From the twodimensional representation given in Figure 4c, one may conclude that ϕPtBuA > ϕPS. However, inspection of the surface (Figure 4a) reveals that the surface coverage of PS is larger than that of PtBuA. Therefore, ϕPtBuA ≈ ϕPS overall.


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ACS Applied Polymer Materials In Region D, we showed using adhesion measurements that the topmost layer consists of PtBuA. Here, we may apply a linear approximation to compute tPtBuA for the block. We collected SNOM intensities from homopolymer PtBuA thin films with known values of tPtBuA (Figure S4). For tPtBuA < 24 nm, the SNOM intensity increases linearly with tPtBuA, whereas for tPtBuA > 24 nm, the SNOM intensity is roughly constant. Therefore, assuming that tPtBuA < 24 nm for Region D, we can compute tPtBuA ~ 15 nm for the upper layer. Note that, if tPtBuA > 24 nm, the SNOM intensity would not be reduced compared to other regions, so our assumption is justified. The PtBuA block in PS-b-PtBuA has previously been found to interact favorably with both the silicon substrate and free air surface,28 resulting in a case of approximately symmetric wetting, although other reports are of slightly favorable PtBuA–silicon interaction.29 Despite the fact that PtBuA adheres preferentially to both the air–polymer and substrate–polymer surfaces in Regions B and C, we observed morphologies where both blocks are in contact with both interfaces. This is because, in these regions, the film is extremely thin such that there would be a large entropic penalty for the PtBuA block to fully occupy both interfaces. In Region D, the film is thick enough that we expect PtBuA to be at both the air–polymer and polymer–substrate interface without a large entropic penalty, with all of the PS block occupying the interior of the thin film. Because the SNOM intensity decreases as the thickness of the PS overlayer increases (Figure S3) we were unable to detect PtBuA near the substrate due to the thick PS block at the interior. These morphologies have been predicted previously by Fasolka et al. for ultrathin BCP films with symmetric wetting behaviors.30 As the thickness of the polymer film increases, the morphology changes from perpendicular lamellar (PL), to a hybrid of perpendicular and parallel lamellar (HY), and finally to full parallel lamellar (FL). In our thin-film sample, the PL structure is observed at heights of 20 to 60 nm, the HY structure is observed at heights of 60 to 85 nm, and above 85 nm, the FL structure is present. Using a characteristic period of L0 ≈ 230 nm (determined in the Supporting Information under Additional Details on the Polymer Structures), we computed t / L0 values of 0.09–0.26 for the PL morphology, 0.26–0.37 for the HY morphology, and exceeding 0.37 for the FL morphology. Comparing this to the predictions of Fasolka et al. (Figure 4e), we see that this matches the predicted value of R in the range 0.7–1, where R is a measure of the ratio of interactions of the blocks with the air–polymer and polymer–substrate interfaces.30 In conclusion, we imaged subsurface structures of the diblock copolymer PS-b-PtBuA using a combination of SNOM and AFM nanomechanical imaging. In contrast to other techniques, this approach enables direct visualization of the chemical composition in real space with a spatial resolution of 22 ± 2 nm for chemical identification and at least 100 nm for subsurface imaging. Our method creates new opportunities to better understand the subsurface composition of BCP thin films and, therefore, to better design BCPs for nanopatterning applications. Broadly applicable, the ability to characterize subsurface features will provide insight into

BCP structures that are deliberately created both near to and far from thermodynamic equilibrium.

ASSOCIATED CONTENT Supporting Information. Materials and methods; comparison of SNOM and FTIR; further analysis of AFM mechanical measurements; SNOM – dependence of signal intensity on film thickness; and additional details on the polymer structures. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

ACKNOWLEDGMENT This work was supported by the Natural Sciences and Engineering Research Council of Canada (Grant No. RGPIN-2016O6448) and the Canada Foundation for Innovation (Grant No. 30516). S.d.B. acknowledges the Foundation for Fundamental Research on Matter (FOM), which is financially supported by The Netherlands Organization for Scientific Research (NWO) and the University of Twente Stimuleringsfonds, for financially supported her research stay at the University of Toronto. The authors gratefully acknowledge Professor Zoltan Zajacz at the Department of Earth Sciences, University of Toronto, for providing access to the Bruker HYPERION 3000 FTIR Microscope. Finally, the authors thank Hannah V. White for editorial comments.

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Nanoscale Subsurface Morphologies in Block Copolymer Thin Films

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