Thickness Limit for Alignment of Block Copolymer Films Using Solvent

May 9, 2018 - thylsiloxane (PDMS) pad adhered to a block copolymer (BCP) film ... films. Here, we examine how far this alignment can propagate through...
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Thickness Limit for Alignment of Block Copolymer Films Using Solvent Vapor Annealing with Shear Chao Zhang,† Kevin A. Cavicchi,† Ruipeng Li,‡ Kevin G. Yager,§ Masafumi Fukuto,‡ and Bryan D. Vogt*,† †

Department of Polymer Engineering, University of Akron, Akron, Ohio 44325, United States National Synchrotron Light Source II and §Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States

‡

S Supporting Information *

ABSTRACT: The swelling and deswelling of a cross-linked polydimethylsiloxane (PDMS) pad adhered to a block copolymer (BCP) film during solvent vapor annealing (SVA) provides sufficient shear force to produce highly aligned domains over macroscopic dimensions in thin films. Here, we examine how far this alignment can propagate through the thickness of a BCP film to understand the limits for efficacy of the SVA-S (SVA with shear) process. Films of cylinder-forming polystyrene-blockpolyisoprene-block-polystyrene (SIS) ranging from 100 nm to more than 100 μm are examined using the same processing conditions. The SIS surface in contact with the PDMS is always well-aligned, with Herman’s orientation parameter (S) exceeding 0.9 as determined from AFM micrographs, but the bottom surface in contact with the silicon wafer is not aligned for the thickest films. The average orientation through the film thickness was determined by transmission small-angle X-ray scattering (SAXS), with S decreasing gradually with increasing thickness for SIS films thinner than 24 μm, but S remains >0.8. S precipitously decreases for thicker films. A stop-etch-image approach allows the gradient in orientation through the thickness to be elucidated. The integration of this local orientation profile agrees with the average S obtained from SAXS. These results demonstrate the effective alignment of supported thick BCP films of order 10 μm, which could be useful for BCP coatings for optical applications.

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structures18 at the length scale of the BCP. In thin films, the self-assembly of the BCPs is impacted by the interfaces,19,20 where wetting at the substrate due to favorable interactions21 or the free surface22 due to surface tension differences can act to orient the domains in the plane of the film. Additionally, the ordering and coarsening kinetics are thickness-dependent.23 By chemically patterning the substrate to promote selective wetting of areas by the different segments of the BCP, the BCP can be oriented and directed to assemble into desired patterns in thin films.24 Similarly, BCP thin films can be oriented and aligned by the substrate topology.25,26 However, these prepatterning routes to generate highly aligned BCP films are in general expensive due to requirements for lithographic patterning. There are some thin film applications, such as polarizers, where a high degree of orientation is required, but not perfection in the ordered structure formed by the BCP.27 In these cases, the potential low cost associated with the selfassembly process drives the commercial potential, and thus low-cost routes to aligning BCP thin films are important. A variety of techniques that rely on shear,28,29 thermal gradients,30,31 or solidification fronts32,33 to align BCP thin

INTRODUCTION Block copolymers (BCPs) provide a facile and potentially lowcost route to generate controllable nanostructures.1,2 The periodic ordering of BCPs ideally leads to well-defined morphologies,3 but the nanostructure is strongly dependent on processing.4 The long-range ordering of BCPs is generally limited by the slow kinetics of defect annihilation associated with the necessity of pulling segments through an immiscible phase and its associated large energy penalty.5−7 Despite this limitation, highly ordered BCP structures have been generated since the pioneering work of Keller in 1970,8 where imposed shear during extrusion was shown to promote the ordering and alignment of BCPs. For bulk BCP materials, this shear alignment has been effective at generating well-aligned morphologies,9−11 although the details of the applied shear can impact the phase observed.12,13 Other external forces, such as electric14 and magnetic15 fields, can also align BCP materials by exploiting differences in the dielectric and magnetic properties of the segments that comprise the BCP. Recent work from Osuji and co-workers has used orthogonal fields to significantly improve the translational and orientational alignment of BCPs to fabricate near single crystals of BCPs.16 This near perfect registration of the BCP nanostructure is a requirement for directed self-assembly to generate patterns for microelectronics,17 but in this case thin films of BCPs are used to generate bends, jogs, and other lithographically important © XXXX American Chemical Society

Received: March 13, 2018 Revised: May 9, 2018

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DOI: 10.1021/acs.macromol.8b00539 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules films have been developed. Work from Karim and co-workers combined shear and zone annealing to improve the orientation of BCP thin films where the differential thermal expansion between the BCP and an elastomer placed on top of the film provides the shear force.34 This combination approach can be used to extend the common solvent vapor annealing (SVA) technique35,36 for improving the ordering of block copolymers to generating highly aligned structures with a similar approach where the differential swelling of the BCP and elastomer by the solvent provides a shear force on drying.37 This technique is applicable to a wide number of BCP systems,38 and the local orientation of the BCP film can be “written” by controlling the local solvent application.39 Careful in situ neutron scattering experiments from Epps and co-workers gleaned that the mechanism for alignment for this SVA with shear (SVA-S) approach goes through an intermediary disordered regime when the system is swollen due to the lateral expansion of the elastomer and pinning of the BCP film by the substrate.40 However, it is not clear how far the shear field can propagate through the BCP to effectively align the domains with prior studies demonstrating efficacy up to 1 μm thick films. Here, we explore the thickness limitations for the alignment of supported BCP films using SVA-S to better understand the underlying physics responsible for the orientation in these films. Surprisingly, the overall orientation of cylinders in polystyreneblock-polyisoprene-block-polystyrene (SIS) as quantified by Herman’s orientation factor can exceed 0.85 for films as thick as 24 μm. Examination of the free surface of the SIS films after SVA-S demonstrates near invariance of the orientation with film thickness, but the orientation near the substrate is lost for the thicker films. Through an etch-stop approach, a gradient in orientation that extends nearly a micrometer is identified in the thicker films. Integration of this gradient agrees quantitatively with the average orientation determined from scattering.

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Figure 1. Schematic of the setup used for solvent vapor annealing. Two mass flow controllers direct dry air through liquid toluene (channel 1) to generate a near saturated toluene vapor stream to swell the PDMS and SIS and then neat dry air (channel 2) for drying. SIS film on silicon was immersed in deionized water to obtain freestanding films. The PSS swells/dissolves in water, which delaminates the SIS from the silicon substrate. For convention, we defined the surface of BCP film in contact with the PDMS pad as the top surface, while the SIS that was in contact with the PSS at the silicon substrate was labeled as the bottom surface. Characterization. Variable angle spectroscopic ellipsometry (VASE, J.A. Woollam Co., M-2000) operating from 246 to 1689 nm with angles of incidence of 65°, 70°, and 75° was used to determine the thickness of the BCP films that were 99.5%), isopropyl alcohol (ACS grade), and poly(4styrenesulfonic acid) (PSS, 18 wt % in H2O; M = 75 000 g/mol) were obtained from Sigma-Aldrich. A polystyrene-block-polyisoprene-blockpolystyrene (SIS) triblock copolymer (Kraton 1164P, f PS = 0.29) was obtained from Kraton Corporation. Polydimethylsiloxane (PDMS, Sylgard 184, Dow Corning) was prepared by physically mixing 20:1 silicone elastomer base to curing agent and subsequent degassing in a vacuum oven at room temperature for 15 min. This mixture was carefully cast on a glass plate to approximately 0.5 mm thick and allowed to cure at room temperature for 1 h and then cured at 120 °C for 2 h. The cured PDMS was cut in 1.5 cm × 1.5 cm pieces for the SVA-S process. Preparation and Processing. (100)-oriented silicon wafers were received from University Wafer. The silicon wafers were cleaned in piranha (7:3 H2SO4:H2O2) solution at 90 °C for 30 min and then rinsed with deionized water and dried by a stream of nitrogen. For a release layer,41 a ∼15 nm PSS film was flow-coated onto the clean silicon wafer from 0.2 wt % isopropanol solution. The PSS film was dried at 130 °C for 24 h, and the PSS-coated silicon wafer was cut into approximately 2.5 cm × 4 cm pieces. SIS films were flow-coated42 onto the silicon wafers from 2 to 30 wt % toluene solutions to generate films from 1 to 120 μm thick. For the alignment of the BCP domains, the SVA-S process was used where the PDMS pieces were physically adhered to the BCP films and then exposed to toluene vapor at 800 mL min−1 for 2 h. This flow was controlled by mass flow controllers (MKS-146C-FF000-1) as shown in Figure 1. To produce the shear stress for alignment, the toluene was removed from the swollen SIS film and PDMS film by flowing dry air at 25 mL min−1 for 30 min. After the alignment process, the PDMS on

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RESULTS AND DISCUSSION Figure 2 schematically illustrates the SVA-S process used to align the SIS films. A rectangular PDMS pad is adhered to a BCP film and allowed to wet until no bubbles are visible. Exposure to toluene vapor swells both the PDMS and SIS film. Because of the directional evaporation at the edge close to the dry air inlet, a directional drying front develops that generates a shear field parallel to diagonal of the rectangular PDMS pad. This process has been shown to effectively align BCP films up to 1 μm. The SVA-S process parameters selected for the data reported here are those optimized previously for aligning B

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Figure 2. Schematic of the SVA-S process where PDMS is physically adhered to a cast SIS film on a silicon wafer that includes a PSS release layer. A flow system (Figure 1) introduces toluene vapor to swell the PDMS and SIS and then dry air to directionally dry the stack to induce orientation.

Figure 3. Morphology of thick SIS films after SVA-S process for 5 μm thick film characterized by (A) AFM images (2 μm × 2 μm) and (B) SAXS with the (C) azimuthal dependent intensity determined from both SAXS and using the FFT of the AFM images (inset). A similar surface morphology for 61 μm thick SIS film is determined from (D) AFM, but the (E) azimuthal SAXS scattering is broader as quantified by (F) the normalized azimuthal dependent intensity of the primary peak.

thinner (ca. 200 nm) films of SIS.38 In this case, toluene is an ideal solvent for this system as it is a relatively neutral, good solvent for SIS and readily swells the PDMS. The primary question addressed here is can these optimized conditions for SVA-S be applied to much thicker films, and is there a limit for the efficacy of alignment? The SIS self-assembles into a cylindrical (p6mm) nanostructure, and this structure is aligned in the SVA-S process as shown in Figure 3. The unidirectional orientation of the SIS domains for a 5 μm thick film is clearly evident in the AFM micrographs (Figure 3A) with a domain spacing (d0) of approximately 35 nm. This preferential orientation can be confirmed by the Fourier transform of the image as shown in the inset with two well-defined spots at the poles in the FFT corresponding to the orientation of the cylinders. The alignment angle for the cylinders is determined by the orientation of the rectangular PDMS pad used for the SVA-S process. The diagonal from corner to opposite corner of the rectangle of the PDMS (in the direction of the dry air flow) defines the orientation for alignment.38 The alignment of the cylinders propagates into the film as preferential orientation is evident from the transmission SAXS pattern (Figure 3B) where similar diffraction spots are present. The azimuthal dependence of the intensity at the correlation length scale from both the FFT of the AFM micrographs and SAXS is shown in Figure 3C. The width of the peak is inversely related to the degree of orientation of the cylinders. For the 5 μm thick SIS film, relatively sharp peaks are found from both AFM and SAXS,

although the SAXS peak is slightly broader. This difference between AFM and SAXS is larger than has previously been reported for thinner films that have been aligned by SVA-S,37 but the larger volume probed by the X-ray measurement (averaged over the film thickness and the beam size of 200 μm × 200 μm) tends to lead to a slightly broader azimuthal peak in the scattered intensity in comparison to the FFT of the AFM image. Similar to the 5 μm thick SIS film, the surface morphology of a 61 μm thick film appears equally well aligned by the SVA-S process (Figure 3D). This similarity in the surface alignment of the SIS is found for all of the film thicknesses examined as shown in Figure S2. However, there is a significant difference in the SAXS patterns with the diffraction spots (Figure 3B) for the 5 μm film broadening into arcs (Figure 3E) for the 61 μm film. These broader arcs are indicative of a decreased orientation of the nanostructure in the thicker film, but the surface morphology is consistent with excellent orientation of the cylindrical nanostructure. Figure 3F better illustrates this difference with the azimuthal intensity peak much broader from SAXS than AFM. This difference is suggestive of differences in the orientation of the nanostructure for the bulk film and top surface. In order to more quantitatively describe the orientation of the SIS nanostructure in the films, Hermans orientation factor (S)45 is calculated from azimuthal dependence of the intensity of the primary diffraction peak associated with the ordered nanostructure as C

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Macromolecules 3⟨cos2 ϕ⟩ − 1 (1) 2 2 where ϕ is the azimuthal angle and ⟨cos ϕ⟩ is the Legendre polynomial of the azimuthal intensity, I(ϕ), calculated as

probed by GISAXS or that the orientation decreases through the thickness as the backside of the film appeared to be well aligned.37 For the 5 μm film, the surface orientation is nearly 0.95, while the average S determined from SAXS is only 0.89. This difference could be associated with intrinsic fluctuations as this orientation extent is similar to that obtained for thinner films when using GISAXS. For the thicker films examined here, there is a clear decrease in the orientation efficacy as the film thickness is increased with a precipitous drop around 30 μm. S determined from SAXS decreases monotonically as the thickness of the SIS film increases. At 32 μm, which is nearly 1000d0, S decreases to 0.83 from SAXS, while S from AFM (surface) remains at approximately 0.95. Although the surface remains oriented, the bulk orientation is degraded to the point where the orientation is not clearly evident from visual inspection of the SAXS profiles when the film thickness is 99 μm. Examination of the thickness dependence of the average orientation of the films (as determined from GISAXS and SAXS shown in Figure 4) suggests that the orientation exhibits a power law decay as the thickness increases to about 20 μm (neglecting the thinnest film where the no slip boundary condition at the substrate will be important). There is a precipitous drop in the orientation for thicker films, which suggests that there is some critical thickness for effective alignment of BCP nanostructures with SVA-S. To explain the decreased orientation of the nanostructure, the mechanism for alignment can be examined. Shear from the deswelling of the PDMS is responsible for the orientation, but the effectiveness of this shear stress apparently decreases through the thickness of the film. A gradient in the shear through the thickness of the film is likely responsible for the decreasing average orientation of the nanostructure through the film as the thickness is increased; as the thickness increases, the gradient in shear must decrease. If there is a critical shear stress requirement for alignment as suggested from the dependence of the orientation on the swelling of the PDMS,38 this could provide some explanation for the decreased alignment in the thicker films. Additionally, there will be a gradient in solvent concentration through the SIS film during drying where the contraction of the PDMS provides the force for alignment. This concentration gradient will impact the local mobility and rheological properties of the SIS through the film thickness. To address the decreased orientation factor from SVA-S as the film thickness increases, the AFM micrographs of the backside of the SIS films after SVA-S process are shown in Figure 5. As there should ideally be no slip at the substrate, this defines the lowest shear stress that will be applied to the SIS films. From visual examination of Figure 5, the orientation of

S=

π

⟨cos2 ϕ⟩ =

∑ϕ = 0 I(ϕ) sin ϕ cos2 ϕ π

∑ϕ = 0 I(ϕ) sin ϕ

(2)

This formulizm for S is the average over three dimensions, thus allowing for the possibility of not all cylinders being aligned in the plane of the film. Hermans orientation parameter has been extensively used to characterize orientation in crystals,46 composites,47 and block copolymers48 and thus represents an ideal route for quantification of the orientation obtained from SVA-S. Figure 4 illustrates how S depends on film thickness for

Figure 4. Hermans orientation factor determined from (■) SAXS and (●) AFM. For comparison, the orientation of thin SIS films aligned by SVA-S with 10:1 PDMS is included for both (□) GISAXS and (○) AFM as reported previously.37 The dashed line corresponds to S = 0.95, which is the average surface orientation.

the SIS aligned by SVA-S. A comparison of S calculated from AFM and SAXS demonstrates an increasing divergence as the film thickness increases. The surface orientation (determined from AFM) is effectively independent of film thickness (S ∼ 0.95) even for the thickest films examined. This result is consistent with the expected mechanism for orientation by SVA-S where the shear applied from the PDMS to the SIS due to differential swelling drives the orientation.38,40 As the PDMS is in contact with the SIS at the surface, the same strain from the PDMS is applied, so the similarity in the surface orientation is not surprising. In previous studies with thinner (≤1 μm) SIS films,37 good agreement in S between AFM and GISAXS measurements has been found (Figure 4). For the thinnest and thickest films, there was some apparent degradation in the orientation, but it is not clear if these differences were due to the larger area

Figure 5. Morphology of the backside of the SIS films after SVA-S process for (A) 1 μm and (B) 16 μm thick films as characterized by AFM (2 μm × 2 μm). (C) Influence of film thickness on the orientation of the backside of the SIS films after SVA-S. D

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Figure 6. AFM micrographs of the etched backside of the 24 μm thick SIS film after SVA-S process for different distances from the substrate: (A) 0, (B) 42, (C) 146, (D) 209, and (E) 1245 nm. The insets illustrate the FFT of the AFM images. (F) Spatial dependence of the orientation factor through the thickness of the (red ●) 24 μm and (black ●) 5 μm SIS films. The horizontal dashed lines correspond to S = 0.85 and S = 0.87, which are the average surface orientation for the (red - -) 24 μm and (black - -) 5 μm films.

etching approximately 1 d0 into the film (40 nm), the orientation is only marginally increased (Figure 6B). This suggests that the poor alignment of the nanodomains at the substrate is not solely associated with interactions between the SIS and the substrate. Increasing the depth into the backside of the film leads to a rapid increase in S (Figure 6C,D). However, S is only 0.68 when the SIS film is etched 200 nm, and even more than 1 μm (1200 nm) into the film S is still only 0.8 (Figure 6E). However, as the orientation factor appears to be gradually decreasing with thickness even for films less than 1 μm (Figure 4), this local reduction in S nearly 23 μm from the surface is not surprising. Figure 6F illustrates how the orientation factor depends on the distance from the substrate for the 24 μm SIS film. There appear to be two regimes associated with the orientation. First far from the substrate, S likely gradually decreases from the best alignment at the surface over nearly 23 μm. Etching the free (top) surface approximately 1 μm into a 97 μm thick film leads to very limited change in the orientation (Figure S5), which is consistent with the assertion for a gradual decrease in S from the free surface. We assume that this decrease in S is linear from the orientation at the surface to the furthest etch position in the 24 μm thick SIS film. If this rate (∼0.007/μm, see Supporting Information for details) is applied along with the experimentally determined local orientation over the nearest micrometer to the substrate to estimate the average orientation for the 24 μm thick SIS film, the calculated S (0.87) agrees relatively well with the orientation from SAXS (0.86) as shown in Figure S6. This result suggests that there is always a gradient in the alignment through the thickness of BCP films when using SVA-S, but the extent of this gradient is small and thus difficult to determine that this is present for most film thicknesses previously examined (0.85 for films as thick as 24 μm, but there is a steady decrease in the average orientation as the film thickness increases above 1 μm. AFM images provide insight into this decrease with invariant orientation at the free surface of the film, but the orientation near the substrate decreases as the overall film thickness increases. Etching demonstrates a gradient in orientation through the film that depends on the overall thickness. For films thicker than 25 μm, there is a precipitous decrease in the overall orientation through the film, which signifies an upper limit for the efficacy of alignment with SVA-S.

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on orientation, thickness-dependent orientation at a faster drying rate (PDF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00539. Calibration curve for the etching of SIS with UVO, AFM images of the surface and backside of films of different thicknesses, transmission SAXS patterns after SVA-S as a function of film thickness, AFM images from stop-etch process, examination of influence of the PSS release layer F

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