Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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The Solvent Distribution Effect on the Self-Assembly of Symmetric Triblock Copolymers during Solvent Vapor Annealing Shisheng Xiong,*,† Dongxue Li,‡ Su-Mi Hur,∥ Gordon S. W. Craig,§ Christopher G. Arges,⊥ Xin-Ping Qu,‡ and Paul F. Nealey*,§
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†
School of Information Science and Technology and ‡State Key Lab of ASIC and System, School of Microelectronics, Fudan University, Shanghai 200433, China § Institute for Molecular Engineering, University of Chicago, Chicago, Illinois 60637, United States ∥ School of Polymer Science and Engineering, Chonnam National University, Gwangju, 61186 Korea ⊥ Cain Department of Chemical Engineering, Louisiana State University, Baton Rouge, Louisiana 70803, United States S Supporting Information *
ABSTRACT: Using a combination of systematic experiments and Monte Carlo simulations, this report demonstrates that the distribution of neutral solvent has a strong impact on the quality and kinetics of the self-assembly of block copolymers in thin films. Both methyl ethyl ketone (MEK, a good solvent) and acetone (a relatively poor solvent) were used for the solvent vapor annealing (SVA) of thin films of poly(2vinylpyridine)-block-polystyrene-block-poly(2-vinylpyridine) (VSV) triblock copolymer. Acetone, the poorer solvent, accumulated at the interface of the VSV domains, while MEK was distributed more uniformly throughout the VSV. As a result, acetone screened the interactions between the blocks of the copolymer more than MEK. Because MEK afforded less screening of the different blocks, solvent annealing with MEK led to self-assembly of lower molecular weight VSV triblock copolymers than was possible with acetone. Solvent annealing with MEK also led to slower self-assembly kinetics and smaller correlation lengths in the assembled pattern compared to solvent annealing with acetone. Finally, long-range ordered structures of low molecular weight VSV triblock copolymer on a chemical pattern via directed self-assembly was demonstrated with 6× density multiplication by annealing in MEK.
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INTRODUCTION Block copolymers (BCPs), with covalently bonded but distinctive blocks, are versatile nanomaterials because they can self-assemble into periodic two- or three-dimensional morphologies with feature sizes ranging from 3 to 50 nm. One of the major applications for this bottom-up process is to provide an alternative to conventional lithography for the patterning of small, periodic feature sizes on substrates.1 Block copolymer thin films are generally spin-coated onto a substrate and then directed to assemble into desired morphologies by either thermal heating or solvent vapor annealing (SVA). After annealing, the BCPs form ordered nanostructures with morphologies that are near or at equilibrium. The surface patterns are then transferred into the underlying substrate using conventional pattern transfer techniques (e.g., wet- or dry-etching processes). Although thermal annealing is commonly considered to be straightforward and can be easily integrated with on-track fabrication,2 the technique is fairly limited to a certain range of block copolymer chemistries that can give perpendicular orientations without application of topcoat layers. These layers typically require additional processing and introduce more complexity to the system. © XXXX American Chemical Society
SVA is a versatile method for facilitating the self-assembly of high χ or multiblock copolymers, where the blocks tend to have dissimilar surface energies.3 Adoption of SVA mitigates the degradation of polymer materials because it is operated at ambient conditions and well below the thermal decomposition temperature of the polymer. SVA can also ameliorate the surface energy differences between the domains of BCPs, thereby enabling the formation of through-film perpendicular domains over a range of film thicknesses.4 Through-film perpendicular domains simplify the pattern transfer step. Successful demonstrations of directed self-assembly (DSA) of BCPs via SVA is the result of engineered flow systems that control solvent flow rate, partial pressure, temperature, and quenching rate (i.e., solvent evaporation ratea key parameter for minimizing defects from the solvent−block copolymer equilibrium state).5−11 In many reports, the flow chambers are outfitted with in-line spectra reflectance tools, quartz crystal microbalances, or other in situ characterization techniques to Received: June 14, 2018 Revised: August 21, 2018
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DOI: 10.1021/acs.macromol.8b01275 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules monitor the film swelling and evaporation rate and morphology in real time.12−17 By measuring these properties, it is possible to correlate the system parameters to the BCPs’ assembled structures. The use of finely controlled flow chambers has led to nanopatterns of 8 nm features with precisely controllable variation in pattern dimensions (±5%). These high fidelity nanostructures have been important for technical applications such as bit patterned media using circular tracks.3 Other methods of solvent annealing, such as thermo-solvent annealing,18,19 raster-solvent annealing,20 or even microwave-solvent annealing,21−24 are also emerging, and they are playing a pivotal role in the assembly of more complex block copolymer systems and forming nanostructures at faster rates. While the demonstrated successes of SVA have been impressive, the effect of solvent distribution in the BCP has been unexplored, especially in the case of neutral solvents. The use of a neutral solvent in SVA is beneficial in that it retains the domain structure that the BCP would naturally form during thermal annealing. Previous research assumed that a neutral solvent was distributed uniformly along the block copolymer chain, implying that distribution effects across the different blocks in the film are not important. To better understand the role of solvent distribution along the BCP on SVA, we studied a model poly(2-vinylpyridine)block-polystyrene-block-poly(2-vinylpyridine) (VSV) triblock copolymer with two different neutral solvents, acetone and methyl ethyl ketone (MEK), at different swelling ratios (SRs) during the annealing process. VSV was selected because of its relatively high Flory−Huggins interaction parameter χ (0.18 at room temperature) that enables microphase separation with sub-10 nm periodic features and its relatively high order− disorder transition (ODT) temperature. The assembly kinetics were also investigated as a function of annealing processing parameters by analyzing the correlation length values of the fingerprint lamellae as well as the size of vitrified pitch from the solvated state. Additionally, the P2VP domains of the VSV were infiltrated with alumina using sequential infiltration synthesis (SIS),25,26 which enabled the formation of a hard mask helpful for pattern transfer.
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patterned with extreme ultraviolet (EUV) lithography to make the chemical contrast pattern. The period Lsof the patterns (100 nm) was set to match with 6 times that of the period L0,s of a self-assembled BCP when solvated with 16.7% MEK (L0,s= 16.7 nm, swelling ratio = 20%). The BCP film was spin-coated onto either a nonpreferentialbrush grafted substrate or chemical pattern. Solvent (acetone or MEK) vapor was fed into the annealing chamber to anneal the thin films of block copolymers. The swelling ratio used was in the range 10%−60%. After DSA of the BCP, alumina infiltrated the P2VP domains via SIS. The formed hard mask of AlOx was used to enhance the pattern transfer into the underlayer. The detailed presentation and information about processes including the preparation of chemical pattern, SVA, and pattern transfer can be found in the Supporting Information. Analysis. The computation of the orientational correlation length was done with a program following the method developed by Harrison et al. and Yokojima et al.28,29 The periodicity of the lamellae pattern was measured by taking a fast Fourier transform of the highresolution SEM image. The power spectral density was obtained and we calculated the L0 from the position of the first-order peak: L0 = 2π/k0. Numerical Simulation. The coarse-grained Monte Carlo simulation relied on a molecular representation of the polymer, while intermolecular interactions were represented by functions of the local densities of the different chemical species.30,31 The AB diblock copolymer was modeled with discrete Gaussian chains with coarsegrained beads and harmonic springs. The nonbonded Hamiltonian (Hnon‑bond) was expressed by a spatial integral of the quadratic form of density fields and was proportional to the segregation strength represented by Flory−Huggins parameter χAB. It also contained the penalty of density deviation from the average value proportional to the inverse of the incompressibility, κ. The solvent was represented by the same-sized beads as the polymer beads, and the solvent quality was controlled by the Flory−Huggins parameter, χA(B)S, which is the incompatibility parameter, between the solvent and polymer blocks.32 Hence, Hnon‑bond for the BCP in the presence of the solvent was easily obtained by extending the pure BCP model with additional terms of solvent density field, such that
Hnon‐bond = kBT +
∫V
{ }
N dr χAB NϕAϕB + χAS NϕAϕS + χBS NϕBϕS R e3
κN (1 − ϕA − ϕB)2 2
(1)
where ϕi = ρi/ρ0 is the normalized density field of specie i, ρ0 is the averaged number density of the system, N is the degree of polymerization of the BCP, kB is the Boltzmann constant, T is the absolute temperature in kelvin, and Re is the root-mean-square end-toend distance of the polymer chain. The optimal pitch size of the symmetric, lamellae-forming diblock copolymer, with N̅ = 112, and χABN= 30, was determined by varying the simulation box sizes to minimize the excess stress caused by chain compression or stretching.
EXPERIMENTAL SECTION
Materials. All the solvents used in this work such as methyl ethyl ketone (MEK), acetone, 1-butyl-3-methylimidazolium hexafluorophosphate, toluene, potassium naphthalenide, and so on were used as received from Aldrich. ZEP Photoresist (Zeon Chemicals) was diluted before spin-coating. AZ electronic materials provided the crosslinkable PS. The cross-linking agent was glycidyl methacrylate (GMA). Triblock copolymers with three different molecular weights were used in the study: P2VP-b-PS-b-P2VP (VSV-47, Mn = 47 kg/ mol, 12K-23K-12K), P2VP-b-PS-b-P2VP (VSV-33, Mn = 32.8 kg/mol, 7.9K-17K-7.9K), and P2VP-b-PS-b-P2VP (VSV-26, Mn = 26.4 kg/ mol, 6.7K-13K-6.7K). VSV-26 and VSV-47 were from Polymer Source, Inc. VSV-33 was synthesized by living anionic polymerization as reported previously.3 Styrene, 2-vinylpyridene, and PS-OH brush from Polymer Source, Inc., were used for the synthesis of the VSV-33 triblock copolymer. Nonpreferential random copolymer brushes containing styrene and 2-vinylpyridine (P(S-r-2VP)) were synthesized as before27 and contained ∼2% hydroxyethyl methacrylate for the purpose of bonding the brush to the silicon substrate. The styrene content in the PS-r-P2VP brush ranged from 40% to 100% (OHterminated PS homopolymer). PS-r-P2VP with a 60% styrene fraction was used in self-assembly studies. Fabrication. The general fabrication protocol has been reported previously.3 Briefly, the cross-linked polystyrene (XPS) mat was
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RESULTS AND DISCUSSION To make the lamellar domains perpendicularly oriented, a neutral solvent was needed for the two different polymers in the BCP. Neutral solvents can be identified via experimental means or by applying theory and performing calculations. Experimental screening enabled us to select acetone, a nearly neutral solvent for polystyrene (PS) and P2VP.3,10 This screening process included the preparation of a substrate grafted with a nonpreferential brush, self-assembly of the BCP with SVA, and postmicroscopic inspection. The nonpreferential brush window for SVA of thin film and thick film of PSb-P2VP BCP are shown in the SEM images in the Supporting Information (Figure S1). The observation of fingerprint lamellae verified the solvent used as a functional neutral solvent. Alternatively, neutral solvents can be found by B
DOI: 10.1021/acs.macromol.8b01275 Macromolecules XXXX, XXX, XXX−XXX
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are stable when swollen with solvent.14,35 These groups also showed that the thin films only experience contraction in the direction of the film thickness during the rapid solvent evaporation process. With this knowledge, it was suitable to use the observed structures of a quenched dry sample to infer what morphologies were formed in the thin films when solvated. Well-defined perpendicular lamellae microstructures were obtained for the high molecular weight VSV with acetone or MEK during SVA. The effective χN of the thin films of VSV in solvated state was well above the ODT limit.36 In Figure 1, we find all the lamellae in acetone look swollen adequately (good connectivity in the lamellae) at the three swelling ratios. On the other hand, the lamellae in MEK only show good connectivity at the swelling ratio of 30% and 40%, but very limited connectivity at the swelling ratio of 20%. Around the threshold swelling ratio of 20% with MEK, the morphology of the VSV was partially trapped in a nonfully self-assembled morphology set by the fast drying of the spin-coating process. These results highlight that the critical swelling ratio for the polymer chains to gain sufficient mobility is higher in the MEK case. To make a detailed analysis, the period of the lamellae and their orientational correlation length were extracted with fast Fourier transforms (FFTs) on the SEM images. We assumed that the SIS process did not change the vitrified pitch size (denoted as L0,s) in the quasi-equilibrated, solvated state. The pitch sizes were consistently larger (Figure 2a), and the orientational correlation lengths were consistently shorter, at all three swelling ratios in MEK vapor (Figure 2b). At the solvated state, the interaction between the composing blocks is weakened by the addition of solvent to both blocks. Here we define the effective interaction parameter, χeff, which is substituted for χ, to describe the enthalpic interaction between the two solvated polymer blocks. χeff is a function of the original χ of the block copolymer (no solvent addition), the volume fraction of BCP in the mixture, φ, and the power exponent β:34
calculations, although the solubility parameter values vary between references. The Flory−Huggins interaction parameter χ can be calculated based on Hansen’s solubility theory.33 ,34 χ12 between solvent and copolymer can be written as χ12 = α
ν1 ((δ1, d − δ2, d)2 + 0.25(δ1, p − δ2, p)2 RT
+ 0.25(δ1, hb − δ2, hb)2 )
(2)
where δi,d, δi,p, and δi,hb are the dispersion parameter, polar parameter, and hydrogen bonding parameter, respectively, and subscripts 1 and 2 denote the solvent and the polymer, respectively. ν1 is molar volume of solvent. R and T are the ideal gas constant and temperature in kelvin, respectively. αis a correction factor ranging from 0.5 to 1 and determines the weight of the formula in the interaction parameter. We used eq 2 to evaluate the quality and selectivity of a solvent. We first calculated χ12 between the solvent and PS and then repeated the calculation of χ13 for the solvent and P2VP. We defined the solvent as a neutral solvent if Δχ = χ12 − χ13 was close to 0. The absolute value of the solvent−polymer χis an indicator of the quality of the specific solvent to the corresponding homopolymer, while Δχ relates to the selectivity of solvent to the two homopolymers. We determined Δχ, χ12, and χ13 for a number of common solvents with PS and P2VP and presented the results in a χ12 −Δχplot (Figure S2). For VSV, MEK (Δχ= 0.05) stands out as a neutral solvent, along with acetone (Δχ =0.06). The results of the solvent sorption curve of acetone with PS, P2VP, and PS-b-P2VP (Figure S3) were consistent with the results based on the Hansen approach. The vapor pressure of acetone and MEK is about 200 and 80 mmHg at room temperature, respectively. Thus, both MEK and acetone are sufficiently volatile and suitable for operation in the flow chamber system for SVA. Experiments on SVA of VSV with a relatively high molecular weight (VSV-47, 12K-23K-12K) were conducted in acetone vapor and in MEK vapor, at swelling ratios of 20%, 30%, and 40%. The swelling ratio is defined as the mass of solvent divided by the mass of polymer in the solvated BCP film. Scanning electron microscopy (SEM) was used to inspect the self-assembled films, as shown in Figure 1. Researchers have used in situ grazing-incidence small X-ray scattering (GISAXS) to demonstrate that the morphologies of lamellae or cylinders
χeff = φ β χ
(3)
β has usually been determined from the measured value of L0,s and the relationship of L0,s∝ χeff1/6 ∝ φβ/6, which is applicable in the strong segregation regime. 3,37 However, it is inappropriate to use the dilution approximation theory to explain the assembly kinetics of BCP in two different solvents because they may correspond to distinct segregation regimes. Instead, we refer to the distribution effect of the solvent as a result of its solvent quality, appreciating that a good solvent will have better uniformity of distribution. From Figure S2, both MEK and acetone are neutral solvents (a prerequisite to gain the desired and perpendicular, through-film morphology), but MEK is the better solvent in terms of solvent quality (with a smaller solvent−polymer interaction parameter). According to mean-field theory, the poorer solvent (i.e., acetone) will aggregate at the interface of the two copolymer domains, leading to a stronger screening effect, such that the χeff of the copolymer in the poorer solvent will be lower than in the good solvent (i.e., MEK herein).38 Coarse-grained Monte Carlo simulations were used to investigate how the solvent density distribution and L0,s varied with solvent quality. For simplicity, a symmetric diblock copolymer in a neutral solvent was simulated because the
Figure 1. SEM images of self-assembled thin films of VSV-47 (12K23K-12K) triblock copolymer annealed in acetone and methyl ethyl ketone (MEK) at different swelling ratios (SR). The insets are FFTs of the corresponding SEM images. The scale bar applies to all of the SEM images. C
DOI: 10.1021/acs.macromol.8b01275 Macromolecules XXXX, XXX, XXX−XXX
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Figure 2. Relationship between swelling ratio and pitch size or orientational correlation length of VSV-47 triblock copolymer annealed with acetone or methyl ethyl ketone (MEK). (a) Dependence of pitch size of fingerprint lamellae on the swelling ratio. (b) Dependence of orientational correlation length of fingerprint lamellae on the swelling ratio.
Figure 3. Results of simulations of a symmetric diblock copolymer with χABN = 37 solvated in solvents such that χASN = χBSN = 0, 20, and 50. (a) Dependence of pitch size of lamellae on the swelling ratio. (b) Normalized solvent density distribution in block copolymer solution of 20% solvent volume fraction.
Solvents with different χA(B)S values were used and the simulation box was chosen to be commensurate with the optimal pitch size L0,s at each condition. In Figure 3b, the
solvent distribution in a microphase-separated, symmetric BCP should be similar to the symmetric triblock copolymer case. Optimal pitch sizes, corresponding to different values of L0,s, were calculated at several swelling ratios for a neutral solvent with different levels of solvent quality expressed by χA(B)SN = 0, 20, and 50. A larger value of χA(B)SN represents a poorer solvent, while χA(B)SN = 0 corresponds to a good solvent. The results of the simulation in terms of the effect of solvent quality and swelling ratio on L0,s are presented in Figure 3a. When more solvent was present in the system (larger swelling ratio), the pitch size decreased due to the larger screening effect of the solvent, which led to a smaller effective χABN. As shown in Figure 3a, L0,s decreases as the solvent quality becomes poorer at a fixed swelling ratio, suggesting that the screening effect of a poorer solvent is stronger. The role of solvent quality can be understood with the aid of Figure 3b, which shows the solvent density distribution across a lamellar domain, as determined by simulations, in a solvated diblock copolymer with φ = 0.8 (swelling ratio = 16.7%). Detailed relative distributions of copolymer and solvent are shown in Figure S4.
normalized solvent density field ϕS the rescaled domain,
x . L0,s
( )= x L0,s
ρS ρ0
is plotted over
Regardless of the solvent quality, the
solvent density field oscillates and its peaks are located at the interface of lamellae where x = 0, 0.5, and 1. The solvent L0,s
aggregates at the interface, against the inclination of entropy, which drives for a uniform distribution of solvents over the two domains, to reduce the unfavorable contact of the A and B blocks. For a poor solvent, the aggregation of solvent at the interface is enhanced to minimize the large energy difference between the solvent and polymer. As a result, the screening effect of poor solvent becomes stronger compared to that of good solvent. The greater screening reduces the effective χABN and the corresponding pitch size of the assembled domains. The simulation results are consistent with our experimental observations. D
DOI: 10.1021/acs.macromol.8b01275 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules For next-generation lithography, there is a great need to decrease the feature size of the pattern for fabricating small integrated circuit devices. The most common strategy to shrink the feature size generated by self-assembled BCPs is to reduce the degree of polymerization (N) of BCPs. However, this strategy can only work if the shorter BCP can still microphase separate. In other words, making N too small to achieve a small periodic structure may cause χN to fall below the order− disorder transition leading to a nonordered structure. The experiments revealed that the VSV triblock copolymers (VSV26, 6.7K-13K-6.7K) with a low molecular weight produced illdefined morphologies when annealed at a relatively low vapor pressure in acetone. The resulting perforated lamellae structure indicated that the solvated BCP was close to the disordered state (Figure S5b). Comparatively, the SAXS result of the bulk sample, thermally annealed at 150 °C, did show a scattering peak, indicating the formation of a lamellar morphology (Figure S6). We attribute this result to the decrease of χeff in acetone solvent vapor. This result highlights the importance for accounting of solvent composition and interactions when attempting to self-assemble BCPs because standard processing techniques and theories (self-consistent field theory, SCFT) may lead researchers astray. To exclude the bias of experimental equipment, we assembled VSV-47 at the same condition, and well-ordered fingerprint lamellae were observed. In addition, we mixed the two triblock copolymers together, and an ordered lamella microstructure, with a pitch size of ∼18.5 nm, was obtained, which suggests that the nearly disordered morphology of VSV triblock of low molecular weight was independent of sample preparation. From the T−φ phase diagram (Figure S7), two methods can be discerned to enlarge the process window of SVA: increasing the order−disorder transition temperature or suppressing Tg. Switching from a poorer solvent like acetone to a good solvent such as MEK met both criteria. As shown in the Monte Carlo simulations, the good solvent distributed more uniformly across the polymer blocks, and the accumulation at the interface was minimized. χeff of the solvated VSV had a greater value when annealed in MEK vapor at the same swelling ratio. However, on the basis of the calculations of Tg shown in the Supporting Information, Tg was also lower when VSV was mixed with MEK than with acetone. SVA results on the same triblock with three different molecular weights are compared in Figure 4. Each BCP was annealed at a series of swelling ratios and compared after rapid evaporation of the solvent. Fingerprint lamellae were observed after vitrification on the substrates grafted with the neutral brush. The orientational correlation length increased as the swelling ratio enlarged, until the polymer thin film reached the disordered state. Using MEK for SVA, DSA with 6× density multiplication was performed with VSV-26 on a chemical pattern with a guiding pitch of 100 nm. The assembly after SIS, shown in Figure 5, consisted of uniform, well-ordered lines with a sub-10 nm half-pitch. Aided by the implantation of AlOx by SIS, the pattern was successfully transferred into the Si substrate by plasma etching, demonstrating the through-film nature of the assembled domains. The pitch of the fingerprint lamellae was about 16.7 nm, greater than the bulk pitch determined by SAXS, but almost identical to the pitch size of VSV-33 (7.9K17K-7.9K) annealed in acetone vapor. In this work, the processing windows in terms of solvent vapor pressure/swelling ratio were investigated for VSV of
Figure 4. Top view SEM images of VSV triblock copolymer thin films of three different molecular weights, annealed in methyl ethyl ketone vapor at four different swelling ratios (15%, 25%, 35%, and 45%).
Figure 5. Directed self-assembly with 6-fold density multiplication. The top image is the top-down SEM image of the line-space pattern of the EUV resist pattern. The bottom image is the assembled VSV triblock copolymer VSV-26 after solvent vapor annealing in MEK vapor, implanting AlOx with the SIS process, and plasma etching to remove the polymer. The bright lines are AlOx. The guiding pitch was 100 nm, and the vitrified pitch size after SIS was about 16.7 nm.
three different molecular weights. Our results follow the same trends seen in previous reports documenting that the processing windows shift with molecular weight because χeffN is proportional to molecular weight. The behavior of the VSV when self-assembled via SVA was consistent with the simulation work of Hur et al.39 In the final set of experiments, VSV triblock copolymers with other molecular weights were annealed at different swelling ratio values as shown in Figure S5. We found that when we fixed χ by using a triblock copolymer with the same molecular architecture and composition, the processing window shifted to lower swelling ratios as the molecular weight decreased.40 The ability of the solvated BCP to microphase separate was shown by the uniform, well-defined AlOx lines that were present after SIS (Figure S8). We further transferred the pattern from the AlOx hard mask to the underlying Si. The ability of SVA to drive the uniform self-assembly of BCPs over large areas, as well as to enable DSA over a range of pattern Ls values, is shown in the SEMs in Figures S9 and S10, respectively. E
DOI: 10.1021/acs.macromol.8b01275 Macromolecules XXXX, XXX, XXX−XXX
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CONCLUSION In this report, we demonstrate that the solvent distribution profile in BCP films during SVA depended on the solvent quality, characterized by the solvent−polymer χ. A poor solvent such as acetone tends to accumulate at the interface between the domains. The solvent distribution in turn affects the self-assembly of a BCP. The nanostructured films annealed in acetone have smaller periods and larger grain sizes, whereas the opposite results occur in the MEK case, at the same swelling ratio. We further studied the process window in terms of solvent vapor pressure and found it shifted to the lower vapor pressures as the molecular weight decreased. Besides the fundamental investigation of the effect of solvent distribution on self-assembly, we also successfully achieved sub-10 nm through-film features via SVA in MEK vapor to direct the self-assembly of a VSV copolymer with a relatively low Mn. Because DSA with SVA shows great promise to generating line/space pattern (L0,s = 16.7 nm) with perfect orientation control, our experimental process is closely relevant to 1.6 and 2.0 Tb/in2 BPM fabrication. However, for DSA and pattern transfer at pitches
