Isomeric Effect Enabled Thermally Driven Self-Assembly of

Jun 28, 2016 - We demonstrate through isomeric effect the modulation of thermal properties of poly(hydroxystyrene) (PHS)-based block copolymers (BCPs)...
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Isomeric Effect Enabled Thermally Driven Self-Assembly of Hydroxystyrene-Based Block Copolymers Catherine Kanimozhi,†,‡ Myungwoong Kim,‡,∥ Steven R. Larson,§ Jonathan W. Choi,† Youngwoo Choo,⊥ Daniel P. Sweat,§ Chinedum O. Osuji,⊥ and Padma Gopalan*,†,§ †

Department of Materials Science and Engineering and §Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706, United States ∥ Department of Chemistry, Inha University, Incheon 22212, Korea ⊥ Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06511, United States S Supporting Information *

ABSTRACT: We demonstrate through isomeric effect the modulation of thermal properties of poly(hydroxystyrene) (PHS)based block copolymers (BCPs). A minimal structural change of substituting 3HS for 4HS in the BCP results in a drastic decrease in Tg, which in turn enables the thin film assembly of the BCP via thermal annealing. We synthesized a series of poly(3-hydroxystyrene-b-tert-butylstyrene) [P(3HS-b-tBuSt)] and poly(4-hydroxystyrene-b-tert-butylstyrene) [P(4HS-b-tBuSt)] BCPs by sequential anionic polymerization of protected 3HS/4HS monomer and tBuSt followed by deprotection. Measured Tg of P(3HS) was ∼20−30 °C lower than P(4HS) of comparable molecular weights. As a result, thermally driven self-assembly of P(3HS-b-tBuSt) BCPs in both bulk and thin film is demonstrated. For P(4HS-btBuSt) thermal annealing in thin-film at high temperatures results in poorly developed morphology due to cross-linking reaction of the 4HS block. The smallest periodicity observed for P(3HS-b-tBuSt) was 8.8 nm in lamellar and 11.5 nm in cylindrical morphologies. The functionality of the 3HS block was exploited to incorporate vapor phase metal oxide precursors to generate sub-10 nm alumina nanowires.

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molecular weights and narrow dispersity afforded through high-yielding polymerization method from readily accessible monomers. Poly(hydroxystyrene) (PHS)-based polymers are widely used resist materials and an easily accessible class of polymers.20,21 It is a highly polar material relative to polystyrene, for example, as reflected in its high solubility parameter of 24.55 (J/cm3)1/2.22 A variety of postpolymerization modifications have been developed to alter the chemical and physical properties of P(4HS),20,21,23−26 making a number of applications such as, chemically amplified resists for deep UV or extreme UV lithography24,27 and cross-linkable polymers possible.28 The strong polarity mismatch between PHS and nonpolar chains such as PS leads to a high χeff, thereby enabling microphase separation in the relevant BCPs at relatively low molecular weights. For example, χeff of P(4HS-b-S) is ∼0.12 at 150 °C,29 which is approximately 4× higher than P(S-bMMA),30 the current canonical material for BCP lithography. As a result, P(4HS-b-S) can form self-assembled lamellae with periodicities (L0) as small as 12 nm, which is about a factor of 2

irecting microphase separation in block copolymer (BCP) thin films to yield highly ordered nanostructures has attracted scientific and technological interest due to its potential to enable rapid, low-cost large-area fabrication of nanoscale features.1−7 A significant driver, for example, has been in the use of block copolymer lithography for producing bit-patterned media.8 The minimum feature size (or domain size) is governed principally by the product of the degree of polymerization (N) and the effective interaction parameter (χeff) which is a measure of segregation strength between the polymer blocks, with microphase separation occurring for χeffN > 10.5.9 In principle, the self-assembly of BCPs can be controlled by adjusting these parameters, resulting in the fabrication of dense arrays of sub-50 nm nanostructures such as spheres, cylinders, and lamellae.1,3 To achieve the single digit nanometer feature sizes desired for emerging applications, BCPs should be designed with10 (i) thermodynamically incompatible blocks to yield a large χeff for phase separation at lower degree of polymerization,11−13 (ii) thermal properties such as glass transition temperatures suited for low cost industrial processing (e.g., thermal annealing),14 (iii) large intrinsic etch selectivity between the blocks for pattern transfer,15,16 or with functionalities that can provide etch contrast by enabling selective sequestration of etch-resistant materials, or precursors thereof,17−19 and (iv) controlled © XXXX American Chemical Society

Received: May 13, 2016 Accepted: June 24, 2016

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DOI: 10.1021/acsmacrolett.6b00376 ACS Macro Lett. 2016, 5, 833−838

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ACS Macro Letters

Tg[P(2HS)] ∼ 172 °C, likely attributed to differences in hydrogen bonding abilities, which is influenced by steric hindrance to phenol groups.40−42 By changing the position of hydroxyl group on phenyl ring from para to meta to ortho position, the hydrophilicity of the block is still preserved; hence, similar phase separation tendency is expected. With this rationale, in this work we have designed a new class of P(3HS-b-tBuSt) BCPs to not only utilize the isomeric effect to thermally process the assembly while minimizing any possible side reactions including self-cross-linking during phase separation, but also introduce tBuSt instead of styrene as the second block for higher thermodynamic incompatibility.12,43 Ultimately, the stronger phase segregation strength leads to phase separation at low N and hence, sub-10 nm L0, which was not achievable with P(4HS-b-S).29 For the synthesis for PHS-based BCPs by living anionic polymerization, several protected monomers have been developed.44−46 However, after chain extension with a second monomer the chemistry used to deprotect the PHS block should be mild enough to prevent the degradation of the second block. We recently demonstrated that the anionic polymerization of 2-tetrahydropyran (THP) protected 4HS offered an ideal route to fully satisfy the above criteria.38 With this approach we expanded the family of 4HS BCPs to include PDMS, POSS, and other acid-sensitive second blocks. Using a similar synthetic protocol here we report the first anionic polymerization of THP-protected 3-hydroxystyrene (3-(2tetrahydropyranyloxy)styrene, 3OTHPSt). The monomer 3OTHPSt was successfully synthesized in large quantities through THP protection and subsequent Wittig methenylation reaction of 3-hydroxybenzaldehyde (Figures S1−S3).38 The anionic polymerization of 3OTHPSt was conducted in THF at −78 °C using sec-butyl lithium (s-BuLi) as an initiator under identical conditions to P(4OTHPSt) synthesis (Figure 2a).29 As a result, controlled molecular weights (Mn = 2.5−206 kg/mol) as well as low Đ (180 °C for Mn > 7.5 kg/ mol).29 In order to make the copolymer chains mobile to rapidly drive the assembly process within several minutes, typically annealing at temperatures far above Tg is required. For example, annealing P(S-b-MMA) thin film at temperatures higher than Tg + 120 °C is required for complete assembly within 5 min.34 However, thermal annealing at 220 °C (∼Tg + 40 °C) for a P(4HS)-based BCP is difficult,35 as the highly reactive phenol group undergoes side reactions such as selfcross-linking or degradation of second block before complete phase separation can occur.29,36,37 We recently published the synthesis and chemical characterization of a 4HS-based BCP, P(4HS-b-tert-butylstyrene) [P(4HS-b-tBuSt)].38 Upon thermal annealing of this BCP thin-film at 220 °C, ill-defined microphase-separated morphology resulted due to self-crosslinking reaction in the P(4HS) block (Figure 1a). However,

Figure 1. Top-down SEM images of cylinder forming P(4HS-b-tBuSt) (4A3 in Table S2) thin film on a silicon substrate prepared by (a) thermal annealing at 220 °C for 12 h and (b) THF vapor annealing for 21 h. Both films were stained with RuO4 prior to imaging.

solvent vapor annealing worked quite well (Figure 1b), leading to well-defined morphology. From this example and others,31,35,39 it is hence not surprising that solvent annealing has been predominantly employed for most of P(4HS)-based BCPs. To overcome the barrier to thermal processing, here we show through a simple and minimal structural change in the repeat unit a drastic decrease in Tg of PHS block. This structural change which reduces the Tg is essentially using an isomeric HS monomer, which in turn affects its thin-film assembly and processing. In an early work, Nakamura et al. reported that the position of the phenol group on the phenyl ring in PHS repeat unit dramatically changes the glass transition behavior; Tg[P(4HS)] ∼ 182 °C, Tg[P(3HS)] ∼ 167 °C and 834

DOI: 10.1021/acsmacrolett.6b00376 ACS Macro Lett. 2016, 5, 833−838

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Figure 2. Synthesis of P(3OTHPSt) and P(3OTHPSt-b-tBuSt). (a) General synthetic scheme for the anionic polymerization of 3OTHPSt and acidcatalyzed hydrolysis of OTHP group, (b) Mn and Đ values as a function of monomer to initiator ratio ([M]0/[I]0), (c) Tg as a function of Mn for protected and deprotected P(3OTHPSt) and P(4OTHPSt) (the lines are guides to the eye), and (d) SEC profiles showing P(3OTHPSt) (Mn = 13.5 kg/mol, Đ = 1.03) obtained from an aliquot prior to tBuSt addition (black) and P(3OTHPSt-b-tBuSt) (S1, Mn = 22.2 kg/mol, Đ = 1.04) (red). The given values of Mn and Đ were determined by SEC.

Figure 3. Deprotection of THP through thermal annealing under vacuum at 160 °C. (a) 1H NMR spectra of S2 (Mn = 11.5 kg/mol) sample showing time-dependent thermal deprotection, (b) in situ SAXS profiles showing thermal generation of lamellar morphology, and (c) [3OTHPSt]t/ [3OTHPSt]0 (measured by comparing acetal peak to aryl peak in 1H NMR spectra) and L0 (measured from the principal scattering peak, q*) as a function of annealing time (the lines are guides to the eye).

comparison to those for P(4HS) [137−186 °C (Mn = 1−15 kg/mol)]. For block copolymerization, sequential anionic polymerization of tBuSt was carried out with 3OTHPSt as the first block (Figure 2a). A series of BCPs with Mn = 3.2−32.5 kg/mol (Đ < 1.13) with symmetric (0.47 < f 3OTHPSt < 0.62) and asymmetric (0.19 < f 3OTHPSt < 0.43) compositions were

synthesis of P(3HS) homopolymers. Deprotected homopolymers showed only one degradation at ∼350 °C, which is comparable to Td2 of P(3OTHPSt) homopolymers (Figure S4). More importantly, Tg of P(3HS) is drastically reduced by this simple change in the substitution from the para to the meta position (Figure 2c). Measured Tg for P(3HS) were on an average 20 °C lower [118−162 °C (Mn = 1−15 kg/mol)] in 835

DOI: 10.1021/acsmacrolett.6b00376 ACS Macro Lett. 2016, 5, 833−838

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Figure 4. SAXS profiles for a series of chemically deprotected (a) symmetric BCPs and (b) asymmetric BCPs after annealing at 160 °C in bulk and top-down SEM images of (c) chemically deprotected and (d) protected A2 (Mn = 15.5 kg/mol) thin films prepared by thermal annealing at 220 °C for 6 h and 160 °C for 24 h, respectively; (e) aluminum oxide line arrays fabricated by selective functionalization of TMA on P(3HS) block and subsequent O2 plasma treatment and (f) corresponding width distribution (scale bar = 200 nm).

peaks of THP and phenol groups (Figure 3a). Figure 3c shows time-dependent changes in the periodicity (L0) (blue trace) and mole fraction of protected THP group (red trace). It is worthwhile to note that P(3OTHPSt) shows faster deprotection rate (>90% after 2 h annealing) compared to P(4OTHPSt) (>90% deprotection after 24 h annealing).29 Higher chain mobility due to lower Tg combined with the larger relief in steric hindrance most likely facilitates faster deprotection kinetics. The L0 increases steadily from 12.5 to 15.3 nm as the extent of deprotection increases. The domain size for symmetric diblock copolymers scales with χeff as L0 ∝ χeff1/6Nsα (α = 2/3 or 1/2 for strongly or weakly segregated BCPs, respectively). Contributions to the increased periodicity may originate in changes in χeff and the number of statistical segments, Ns. We speculate the increase is principally driven by changes in χeff on thermal deprotection, though the exact nature of those changes cannot be inferred yet. Future work may better address this. Bulk self-assembly behavior of the chemically deprotected P(3HS-b-tBuSt) BCPs were studied using SAXS. Figure 4a,b shows SAXS profiles of symmetric and asymmetric BCPs after thermal annealing at 160 °C for 24 h. All symmetric BCPs exhibited sharp q* peak and higher order peaks confirming lamellar morphologies (L0 = 8.8−24.8 nm), except for the lowest molecular weight (S5), which showed correlation hole scattering indicating a disordered state. All the four asymmetric BCPs showed typical well-resolved cylindrical morphologies with L0 of 11.5−17.0 nm. The smallest N and L0 in ordered symmetric and asymmetric BCPs were 33 and 8.8 nm and 48 and 11.5 nm, respectively, which are remarkably low and small compared to symmetric P(4HS-b-S), where N and L0 values of 93 and 11.8 nm were observed.29 Figure S7 displays SAXS profiles of various P(4HS-b-tBuSt) in bulk; the similarities with P(3HS-b-tBuSt) suggest the segregation strength is not markedly changed, from which we surmise that the change from 4-isomer to 3-isomer does not significantly change the polarity of the system. Therefore, 3-isomer does not only make the thermal processing easier by reduction in Tg, but it can assemble into sub-5 nm critical dimension when paired with P(tBuSt) as the second block. Studies are currently ongoing to determine χeff in this series of BCPs.

synthesized (Table S1). SEC traces confirm the effective initiation of the second monomer by the living P(3OTHPSt) anion and successful chain extension (Figure 2d). Deprotection of these copolymers was carried out by the same acidolysis procedure described above for homopolymers and confirmed by changes in characteristic peaks in 1H NMR spectra (Figure S6). The protected symmetric BCPs typically exhibited two Tgs corresponding to the two blocks, whereas the asymmetric BCPs showed only one Tg around 110 °C. Upon deprotection for most compositions only one Tg was observed. Tg for P(tBuSt) ranges from 94 to 147 °C (Mn range 2−32 kg/mol)10,47 and for P(3HS) from 118 to 163 °C (Mn range 1.6−68 kg/mol, Table S3). Therefore, depending on the molecular weights of the blocks, the Tgs of the P(3HS) and P(tBuSt) blocks are expected to overlap between 140 and 150 °C, which is in good agreement with the observed results. In contrast, P(4HS-btBuSt) typically exhibited two Tg at ∼146 °C for P(tBuSt) and 166−183 °C for P(4HS) (Mn range 1.0−7.8 kg/mol), highlighting the dramatic decrease in Tg due to the isomeric effect (Tables S2 and S4). In addition to chemical deprotection of the THP group, thermal deprotection is also possible.29 Thermal deprotection can be advantageous as the protected HS block has lower Tg, hence simultaneous deprotection and deprotection driven phase-separation can be achieved in thin films. Therefore, we studied the morphology evolution of protected symmetric BCP (S2) during thermal annealing under vacuum at 160 °C using small-angle X-ray scattering (SAXS; Figure 3b). The asprepared solvent-cast protected BCP showed a disordered state; however, upon ramping up the temperature to 160 °C, a principal scattering peak emerges at q*, though no higher order peaks were observed. After 1 h of isothermal annealing at 160 °C, the q* peak became sharper and shifts to lower q, with the concurrent emergence of a series of higher order peaks. The appearance of higher order peaks confirms the formation of a well-ordered microphase separated system with a lamellar structure. After 4 h, no further change in the q* peak was observed, indicating that there were no further changes in the microphase separated structure. The annealed samples were further characterized with 1H NMR to measure the extent of THP deprotection P(3HS) by integrating the characteristic 836

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ACS Macro Letters We finally studied the self-assembled P(3HS) cylinder forming BCP in thin film to investigate the potential for P(3HS) based BCPs in lithographic applications. Chemically deprotected P(3HS-b-tBuSt) BCP (A2) was deposited onto a native silicon oxide surface, followed by thermal annealing at 220 °C (Tg + 72 °C) for 6 h under vacuum. Figure 4c displays a top-down SEM image of the resulting thin film stained with RuO4, showing well-ordered parallel cylinders over the entire film. The center-to-center distance (equal to L0) and cylinder diameter (the width of dark line) was measured to be 14.0 ± 1.2 and 6.5 ± 1.5 nm, respectively, which is in good agreement with bulk morphology by SAXS. A protected BCP sample was tested to investigate the possibility of in situ generation of the desired morphology during thermal deprotection in thin films. The protected sample was deposited and annealed at 160 °C (Tg + 46 °C) at 24 h. The SEM image (Figure 4d) shows the formation of parallel cylinders with similar L0 to chemically deprotected samples, with perpendicular cylinders present at the edges of holes formed inadvertently by dewetting. The thin film in Figure 4c was further subjected to trimethylaluminum (TMA) vapor exposure and subsequent O2 plasma etching to assess the functionalization potential.48 As a result, aluminum oxide line arrays with the average width of 6.2 ± 1.2 nm were well-defined (Figure 4e,f). Therefore, these results strongly suggest that PHS based BCPs can be assembled via thermal annealing in thin film to define nanostructure in sub-10 nm dimension for lithographic applications. In conclusion, we have shown that with P(3HS) as the first block instead of P(4HS), the thermal properties of the BCP can be largely modulated. We synthesized a series of P(3OTHPStb-tBuSt) BCPs by sequential anionic polymerization, with wellcontrolled molecular weight and low dispersity. P(3HS-btBuSt) BCPs were synthesized by acidolysis or thermal annealing to deprotect the P(3OTHPSt) block. In general, the Tg of the P(3HS) block was ∼20 °C lower for comparable molecular weights. This allowed the thermally driven selfassembly of the P(3HS-b-tBuSt) in bulk and in thin film while minimizing possible side reaction from the phenolic group during self-assembly at elevated temperatures. The lowest periodicity of the BCPs was 8.8 and 11.5 nm in lamellar and cylindrical morphologies, respectively. Incorporating metal oxide precursor into P(3HS) domain in thermally annealed thin film and subsequent O2 plasma treatment led to aluminum oxide nanowire arrays, highlighting its potential to achieve in single digit nanometer features.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The synthesis, characterization, and self-assembly studies were supported by the Division of Materials Sciences and Engineering, Office of Basic Energy Science, U.S. Department of Energy under Award No. ER46590. C.O. and Y.C. acknowledge support from the National Science Foundation (Grant No. DMR-1410568) for the temperature-dependent X-ray characterization of the samples. We thank Prof. Xudong Wang’s group for the help for TMA exposure experiments. We acknowledge support from the staff and the use of equipment at the Materials Science Center and Soft Materials Laboratory at the University of Wisconsin, Madison.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.6b00376. Experimental details, characteristics of synthesized homopolymers and BCPs, NMR spectra, TGA and DSC curves, and SAXS profiles of P(4HS-b-tBuSt) (PDF).

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

‡ These authors contributed equally to this work (C.K. and M.K.).

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