Article pubs.acs.org/Macromolecules
Fabrication of Sub‑3 nm Feature Size Based on Block Copolymer SelfAssembly for Next-Generation Nanolithography Jongheon Kwak, Avnish Kumar Mishra, Jaeyong Lee, Kyu Seong Lee, Chungryong Choi, Sandip Maiti, Mooseong Kim, and Jin Kon Kim* National Creative Research Initiative Center for Smart Block Copolymers, Department of Chemical Engineering, Pohang University of Science and Technology, Pohang, Republic of Korea S Supporting Information *
ABSTRACT: For ultrahigh-density storage media and D-RAM, the feature size of lithography should be much reduced (say less than 10 nm). Though some research groups reported feature size of 5−6 nm, further reduced feature size is needed for nextgeneration lithography. We synthesized, via a reversible addition−fragmentation chain-transfer polymerization, polydihydroxystyrene-block-polystyrene (PDHS-b-PS) copolymers showing lamellar and cylindrical microdomains by adjusting the volume fraction of PS block (f PS). We found that the Flory−Huggins interaction parameter (χ) between PDHS and PS was very large, 0.7 at 170 °C. Because of the huge χ, the lamellar domain spacing (L) of PDHS-b-PS with a total molecular weight of 2.1 kg mol−1 and f PS = 0.5 was only 5.9 nm; thus, a sub-3 nm feature size (half-pitch) was successfully obtained. Furthermore, PDHS-bPS with a molecular weight of 4.2 kg mol−1 and f PS = 0.79 showed hexagonally packed cylinders with 4 nm diameter. We also obtained thin films of PDHS-b-PS with cylindrical microdomains, showing 8.8 nm center-to-center spacing. Furthermore, we fabricated ultrahigh-density ZrO2 nanowire arrays from the cylindrical monolayer thin films via atomic layer deposition, indicating an applicability of PDHS-b-PS for next-generation lithography.
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INTRODUCTION In semiconducting and microelectronics industry, reducing feature size has been a challenging issue in manufacturing ultrahigh-density and miniaturized devices. E-beam lithography techniques have been developed to be able to reach the 10 nm length scale.1 However, the fabrication of even smaller (sub-10 nm) feature size remains still a great challenge. Meanwhile, block copolymers which self-assemble to form various microdomains have been extensively investigated due to easy fabrication of nanopatterns in large areas.2−22 Also, because the block copolymer nanopatterns are easily aligned and transferred to the underlying substrate, those could be used for next-generation lithography.23−41 The formation of nanostructures resulting from the segregation of two (or more) chemically different polymer blocks in a block copolymer is governed by the thermodynamic principle. At the equilibrium state, if constituent blocks have sufficient segregation power, the shapes and sizes of selfassembled nanostructures are determined, depending on the volume fraction ( f) of each block and the degree of polymerization (N).42,43 AB diblock copolymers with symmetric volume fraction become microphase-separated, when χN is larger than the critical value (χN)c = 10.5, in which χ is the Flory−Huggins segmental interaction parameter.42−44 Because the size of lamellar microdomains (L0 = 2π/q*, where q* is the © XXXX American Chemical Society
principal scattering peak in the small-angle X-ray scattering (SAXS) profile) is proportional to χ1/6N2/3 (> (χN)c2/3/χ1/2), a large χ is required to reduce L0 at a given N. Recently, Hillmyer and co-workers reported various block copolymers having high χ values and small domain spacings.45 Yue et al. reported the formation of lamellar microdomains with 8.5 nm domain spacing (L0) and cylindrical microdomains with 7.6 nm domain spacing (d = 2π/q*, which is the (100) interplanar distance of the hexagonal lattice) using a functionalized polyhedral oligomeric silsesquioxane-block-polystyrene (XPOSS-b-PS).46 Rodwogin et al. showed that polylactideblock-poly(dimethylsiloxane)-block-polylactide (PLA-b-PDMSb-PLA) triblock copolymers had χ = 1.1 at 150 °C estimated by strong segregation theory, that is, L0 = 1.1bN2/3χ1/6 where b is the statistical segment length.47 Genabeek et al. reported that PDMS-b-PLA with an ultralow molar mass dispersity (Mw/Mn ≤ 1.000 02) showed L0 = 6.8 nm for a lamellar sample with 2.5 kg mol−1 and d = 6.5 nm for a cylindrical sample with 2.6 kg mol−1, although a very long time aging (6 months) at room temperature is required for obtaining high-order peaks in SAXS.48 Sweat et al. showed that poly(4-hydroxystyrene)-blockReceived: May 8, 2017 Revised: August 4, 2017
A
DOI: 10.1021/acs.macromol.7b00945 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules polystyrene (PHS-b-PS) had χ = 0.12 at 150 °C, where the hydroxyl group of 4-hydroxystyrene triggered a high incompatibility between two blocks, which formed lamellar microdomains with L0 = 11.8 nm.49 It is quite expected that when one adds two hydroxyl groups in PS (namely, poly(3,4-dihydroxystyrene) [PDHS]), this would have more incompatibility than PHS. In this study, we synthesized, via reversible addition−fragmentation chain-transfer (RAFT) polymerization, PDHS-b-PSs with various volume fractions and total molecular weights. We characterized the block copolymers by size exclusion chromatography (SEC) and 1 H nuclear magnetic resonance (1H NMR) spectra. The molecular characteristics of PDHS-b-PSs employed in this study are summarized in Table 1. Bulk morphologies of PDHSb-PSs were studied by SAXS and transmission electron microscopy (TEM).
The smallest domain spacings of lamellae (L0) and cylinders (d) for PDHS-b-PS were 5.9 and 7.6 nm; thus, sub-3 nm size of half-pitch line and 4 nm of cylinder diameter (D) were successfully obtained. We estimated temperature dependence of χ between PS and PDHS from the SAXS profiles fitted by the Leibler theory by using a low molecular weight and symmetric PDHS-b-PS showing the disordered state in the entire temperature range.42 Furthermore, by using cylindrical microdomains parallel oriented to a solid substrate, an ultrahigh density of ZrO2 nanowires array was prepared by atomic layer deposition (ALD) because dihydroxyl group in PDHS blocks selectively interacted with metal precursors.50−53
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RESULTS AND DISCUSSION Figure 1 shows SAXS profiles and TEM images for various PDHS-b-PSs. The SAXS profile of DHS8.4-S10 (total numberaverage molecular weight (Mn) = 2.1 kg mol−1 and f PS = 0.5) shows scattering peaks at the position of 1:2:3 relative to q* (1.059 nm−1), indicating that DHS8.4-S10 has lamellar microdomains (Figure 1a). The domain spacing (L0) obtained from SAXS profile was 5.9 nm, which is the smallest domain spacing for lamellar structure in the pristine diblock copolymer reported so far.18,22,45,46,48,49,54−58 This indicates that the width of each lamellar microdomain becomes sub-3 nm. This result is consistent with TEM image given in Figure 1e. In TEM images (Figures 1e−h), the bright and dark parts correspond to PDHS and PS microdomains, respectively, due to the selective staining of PS by RuO4 vapor. In Figures 1b, 1c, and 1d, other PDHS-bPSs have SAXS profiles of scattering peaks at the position of 1:√3:√7 relative to q* (0.824, 0.768, and 0.644 nm−1, respectively) exhibiting hexagonal cylindrical microdomains. Each domain spacing (d) is 7.6, 8.1, and 9.7 nm, respectively, which has ∼4 nm of cylinder diameter, when considering the volume fraction of PDHS block. From the SAXS and TEM results, we confirm that PDHS-b-PS with small N had extremely small feature sized nanostructure. According to the mean-field theory, PDHS-b-PS should have a very large χ judged from the criterion of χN > 10.5 for ordered lamellar microdomain42,43 because of smaller N (18.4)
Table 1. Molecular Characteristics of PDHS-b-PSs Employed in This Study sample
Mna (g/mol)
Mw/Mnb
f PSc
morphologyd
DHS6-S7.8 DHS8.4-S10
1500 2150
1.06 1.05
0.52 0.5
DHS7-S31
4200
1.09
0.79
DHS7.8-S38
4900
1.08
0.8
DHS8.4-S54
6750
1.09
0.84
disorder LAM (TODT = 230 °C) HEX CYL (TODT = 280 °C) HEX CYL (TODT = 300 °C) HEX CYL (TODT > 310 °C)
d-spacingd L0 (LAM) d (CYL) (nm) 5.9 7.6 8.1 9.7
a
Determined by 1H NMR. bMeasured by GPC using PS standards. Calculated by 1H NMR and density at room temperature (PS = 1.05 g/cm3 and PDHS = 1.14 g/cm3 which was measured by density gradient column method). dDetermined by SAXS and TEM images. The ordered-to-disordered transition temperature (TODT) was measured by depolarized light scattering. c
Figure 1. SAXS profiles (top panels) and TEM images (bottom panels) for DHS8.4-S10 (a, e), DHS7-S31 (b, f), DHS7.8-S38 (c, g), and DHS8.4S54 (d, h). Insets in TEM images (f, g, h) are obtained when each sample was cut perpendicularly to the cylindrical axis. B
DOI: 10.1021/acs.macromol.7b00945 Macromolecules XXXX, XXX, XXX−XXX
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Figure 2. (a) Structural factor profiles of disordered DHS6-S7.8 at various temperatures: 170 °C (□), 200 °C (△), 220 °C (◇), 230 °C (tilted △), and 240 °C (○) where the solid lines are obtained from the fitting by the Leibler theory. (b) Temperature dependence of χ between PDHS and PS.
Figure 3. Tapping mode AFM phase images of cylindrical PDHS-b-PS thin films with near 1.5L0 thickness on UV/O3-treated Si substrate. (a) DHS7-S31, (b) DHS7.8-S38, and (c) DHS8.4-S54.
Figure 4. FE-SEM images of ZrO2 nanowire on Si substrate from cylinder monolayer of (a) DHS7-S31, (b) DHS7.8-S38, and (c) DHS8.4-S54. PDHS-b-PS thin film was completely removed by O2 plasma.
for DHS8.4-S10. To estimate the χ between PDHS and PS experimentally, we measured the SAXS profiles of fully disordered DHS6-S7.8 (Mn = 1.5 kg mol−1 and f PS = 0.52) at various temperatures and fitted the results with Leibler theory.59−62 Figure 2 shows the structure factor (S(q)) as a function of q at five representative temperatures, and the temperature dependence of χ between PDHS and PS above 170 °C is expressed by χ=
184.07 + 0.2845 T
The obtained χ at 170 °C is 0.7, which is large enough to induce ordered microdomains for a small molecular weight DHS8.4-S10 (χN = 12.88 at 170 °C). This χ is very large compared with that of other reported block copolymers having large incompatibility between two components (0.41 for PTMSS-b-PLA,54 0.11 for polystyrene-block-polydimethylsiloxane (PS-b-PDMS),57 0.18 for PCHE-b-PMMA,57 0.11 for poly(4-tert-butylstyrene)-block-poly(2-vinylpyridine) (PtBS-bP2VP),56 and 0.4 for polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP)59). It is very interesting that one hydroxyl group difference between 3,4-dihydroxystyrene and 4-hydroxystyrene
(1) C
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generates great difference in incompatibility with styrene monomer, considering the χ value of PS-b-PHS (0.12 at 150 °C).49 The higher incompatibility between hydrophobic PS and PDHS than that between PS and PHS is ascribed to the increased hydrophilicity of PDHS with two hydroxyl groups compared to PHS with a single hydroxyl group. For lithographic application, we studied the self-assembly of cylindrical PDHS-b-PS in thin films. We prepared thin films of cylindrical PDHS-b-PS with near 1.5d thickness (11 nm for DHS7-S31, 12.5 nm for DHS7.8-S38, and 14.4 nm for DHS8.4S54) to satisfy the commensurability condition for monolayer nanostructure on UV/O3 treated silicon substrate which has good wetting for hydrophilic PDHS block. Figure 3 shows AFM phase images of cylindrical fingerprint nanopatterns parallel oriented to the substrate. In Figures 3a and 3b, each cylindrical nanostructure has sub-10 nm of center-to-center spacing (8.8 nm for DHS7-S31 and 9.3 nm for DHS7.8-S38). Moreover, the diameters of all samples are near 4 nm, which is the smallest size reported in the literature so far. Furthermore, PDHS-b-PS has a potential for generating metal oxide nanowires because dihydroxyl group in PDHS can be exploited to selectively incorporate vapor phase metal oxide precursors via the atomic layer deposition method.50−53 Figure 4 shows field emission scanning electron microscopy (FESEM) images of ZrO2 nanowire arrays fabricated from each cylindrical monolayer sample (Figure 4a for DHS7-S31, Figure 4b for DHS7.8-S38, and Figure 4c for DHS8.4-S54) subjected to tetrakis(ethylmethylamino)zirconium (TEMAZr) vapor exposure and subsequent O2 plasma etching. The line widths and center-to-center distances of ZrO2 nanowires are well matched to the original PDHS cylinder diameter sizes and center-to-center distances of cylinder nanopatterns, indicating that PDHS-b-PS nanostructure can be used to fabricate metal oxide nanoarrays as a template. We found via AFM height image that ZrO2 nanowires were uniformly formed through the entire substrate. For instance, the height, diameter, and centerto-center spacing of ZrO2 nanowires prepared by using DHS8.4-S54 were 0.97 ± 0.16 nm, 5.8 ± 1.0 nm, and 10.6 ± 1.5 nm, respectively, as shown in Figure S4. In addition, metal oxide can be reduced at the condition of high temperature and hydrogen atmosphere; it would be possible to fabricate ultrahigh-density array of metal nanowires using PDHS-b-PS.
Article
EXPERIMENTAL SECTION
Materials. Cyanomethyldodecyl trithiocarbonate (CMDTTC) (Aldrich, 98%), anhydrous 1,4-dioxane (Aldrich, 98%), boron tribromide 1 M in methylene chloride solution (BBr3) (Aldrich), 1 M hydrochloric acid (Aldrich), hexane (Samchun pure Chemical, Korea), methanol (Samchun pure Chemical, Korea), and dichloromethane (Samchun pure Chemical, Korea) were used as received. 3,4Dimethoxystyrene (DMOS) technical grade, containing 1% hydroquinone as inhibitor (Aldrich, ≥99%), and styrene (Aldrich, ≥99%) were dried over CaH2 and then distilled under reduced pressure. 2,2′Azobis(isobutyronitrile) (AIBN) was recrystallized from methanol. Synthesis of PDHS-b-PS. Various molecular weights and volume fractions of PDHS-b-PS were synthesized via reversible addition− fragmentation chain-transfer (RAFT) polymerization and hydrolysis reaction (see section 1 in the Supporting Information). Briefly, a targeted low molecular weight of poly(3,4-dimethoxystyrene) (PDMOS) was synthesized by RAFT polymerization of DMOS in 1,4-dioxane at 68 °C using cyanomethyldodecyl trithiocarbonate RAFT agent. Then, PDMOS was used as a macro-chain-transfer agent for the RAFT polymerization of styrene in 1,4-dioxane at 68 °C. PDMOS-b-PS was then reacted with BBr3 to convert PDMOS methoxy group into the corresponding PDHS hydroxyl group. Self-Assembly in Bulk. All PDHS-b-PSs were annealed at 170 °C, a temperature above the glass transition temperature (Tg), for 24 h under vacuum, followed by quenching at room temperature. SAXS profiles (I(q) vs q (= (4π/λ) sin θ), where q and 2θ are the scattering vector and scattering angle, respectively) were obtained at the invacuum Undulator 20 beamline (4C SAXS II) of the Pohang Accelerator Laboratory (PAL), Korea. The wavelength and beam size were 0.675 Å and 0.2 (H) × 0.6 (W) mm2, respectively. A twodimensional charge-coupled detector (Mar USA, Inc.) was employed. The sample-to-detector distance was 1 m. The thickness of the samples was 1.0 mm, and the exposure time was 10 s. The TODT of the samples was measured by the depolarized light scattering using a polarized beam from a He−Ne laser at a wavelength of 632.8 nm, where the intensity detected at photodiode through an A/D converter was recorded as a function of temperature at the heating rate of 1.0 °C/min from 170 to 310 °C under nitrogen flow (see section S4 in the Supporting Information). To observe the morphologies of PDHS-b-PS with TEM, the samples were ultrasectioned by using a Leica Ultracut Microtome (EM UC6 Leica Ltd.) at room temperature with a thickness of ∼40 nm. Then, they were stained by exposure to RuO4 vapor for 5 min at room temperature. The PS microdomains look dark in TEM images. The micrographs were taken at room temperature by bright-field TEM (S-7600 Hitachi Ltd.) at 80 kV. Thin Film Preparation and Analysis. Thin film samples were prepared by spin-coating of tetrahydrofuran (THF) solutions of cylindrical PDHS-b-PS samples with 1.5L0 thickness for each domain spacing on UV/O3-treated Si substrate. All film samples were thermally annealed at 170 °C for 12 h under vacuum. For fabricating ZrO2 nanowire films, ZrO2 was deposited inside PDHS cylinders of PDHS-b-PS monolayer films in flow-type ALD reactor using tetrakis(ethylmethylamino)zirconium (TEMAZr) and O3 as a zirconium source and reactant gas. Argon (Ultra high purity, 99.99995%) was used as a carrier and purge gas. The ALD was performed with alternating exposure to TEMAZr/O3 with cyclic evacuate−fill−purge−evacuate process in a semistatic mode as described.50−53 The deposition temperature was 200 °C. The exposure times to the Zr precursor, O3, and purging gas were 60, 60, and 300 s, respectively, and four cycles of the evacuate−fill−purge−evacuate process were performed. The PDHS-b-PS in the film was completely removed by O2 plasma etching at 250 W for 1 min. The surface morphologies of PDHS-b-PS thin films and ZrO3 nanowire films were observed by atomic force microscopy in the tapping mode (AFM, Veeco DI dimension 3100 with Nanoscope V) and field-emission scanning electron microscopy (FE-SEM, Hitachi, S4800) operating at 3 kV.
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CONCLUSIONS We investigated the self-assembly of PDHS-b-PSs with very small molecular weights exhibiting lamellar and cylindrical microdomains. PDHS-b-PS was prepared via RAFT polymerization and hydrolysis reaction and characterized by SEC and 1 H NMR. The DHS showed a very large incompatibility with styrene. The Flory−Huggins interaction parameter between PDHS and PS estimated by the Leibler theory was 0.7 at 170 °C, which was almost 6 times larger than that of PS-b-PHS, where PHS has only one hydroxyl group per monomer. Because of the huge incompatibility between PDHS and PS, the symmetric volume fraction of PDHS-b-PS showed well-ordered lamellar nanostructure at a molecular weight as small as 2.1 kg mol−1, which enables to form the sub-3 nm lamellar feature size. In addition, cylindrical microdomains of PDHS-b-PS with low molecular weights showed ∼4 nm of cylinder diameter. The monolayer thin films of cylindrical microdomains could achieve sub-10 nm of center-to-center spacing, which could be used to fabricate ultrahigh density of ZrO2 nanowire arrays with the ALD method. D
DOI: 10.1021/acs.macromol.7b00945 Macromolecules XXXX, XXX, XXX−XXX
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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/acs.macromol.7b00945. Text giving detailed synthetic route and molecular characterization of PDHS-b-PS block copolymers (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (J.K.K.). ORCID
Jin Kon Kim: 0000-0002-3872-2004 Author Contributions
J.K. and A.K.M. contributed equally to this work. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Creative Research Initiative Program supported by the National Research Foundation of Korea (2013R1A3A2042196). SAXS experiment was done at 4C beamline of PAL (Korea).
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REFERENCES
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DOI: 10.1021/acs.macromol.7b00945 Macromolecules XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.macromol.7b00945 Macromolecules XXXX, XXX, XXX−XXX
