Dual Mode Patterning of Fluorine-Containing Block Copolymers

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Dual Mode Patterning of Fluorine-Containing Block Copolymers through Combined Top-down and Bottom-up Lithography Rina Maeda,† Teruaki Hayakawa,† and Christopher K. Ober*,‡ †

Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1-S8-36 Ookayama, Meguro-ku, Tokyo 152-8552, Japan ‡ Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853-1501, United States S Supporting Information *

ABSTRACT: Integrated nanopatterns were successfully obtained from a combination of high lateral ordering of newly designed self-assembling fluorine-containing block copolymers and degradation of the fluorine-containing polymer nanodomains in e-beam irradiated areas. The fluorine-containing block copolymers of poly(styrene-block-2,2,2-trifluoroethyl methacrylate) (PS-b-PTFEMA) and poly[styrene-block-(methyl methacrylate-co-2,2,2trifluoroethyl methacrylate)] (PS-b-(PMMA-co-PTFEMA)), which are capable of both top-down and bottom-up lithography, were developed. The reported block copolymers were synthesized by either anionic polymerization or atom transfer radical polymerization (ATRP). Characterization of bulk and thin films were carried out using differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and small-angle X-ray scattering (SAXS), and these studies revealed the formation of highly ordered self-assembled structures. Lateral ordering of arrays of PS dots was observed in the thin film of PS-b-PTFEMS, in which PS was the minor block, and the thin film of PS-b-(PMMA-co-PTFEMA), in which PS was the major block. These thin films were subjected to conventional lithographic processing using e-beam and deep-UV radiation to create integrated patterns such as “dots in lines”. KEYWORDS: self-assembly, block copolymer, fluorinated polymer, resist, lithography, nanopattern

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INTRODUCTION Block copolymer templating and lithography are attracting more attention as they have the potential to offer both ease of processing and high resolution. With this approach to create nanopatterns, the microphase separation of the block copolymer forms densely packed arrays of spheres, cylinders, and lamellae with fine-tunability of nanodomain size (3−50 nm) in thin films upon various substrates, and patterns have been transferred to the substrate or used as templates and scaffolds for magnetic materials or metals using the differences between phases.1−5 Unlike conventional photoresists, block copolymers can autonomously form regular patterns at dimensions not achievable by conventional lithographic means. Furthermore, progress in thin film fabrication of block copolymers enabled both lateral and vertical ordering of microdomains over an arbitrary large area with almost defectfree results,6−11 which allowed block copolymer templating and lithography to be more advantageous for many applications such as the manufacture of integrated circuits, patterned media, etc.12 Several routes to generate nanostructured films via block copolymer self-assembly have been reported since Nakahama and co-workers first demonstrated the formation of nanoporous polymer films from a siloxane-functionalized poly(styreneblock-isoprene) system.13 To date, several studies have reported block copolymer lithography by starting with a film of selfassembled PS-b-PMMA. One of the most common strategies to fabricate nanostructures in PS-b-PMMA thin films by selective © 2012 American Chemical Society

deep-UV degradation and removal of PMMA microdomains along with cross-linking of the PS block.1 However, the χ value of this block copolymer is not high enough to enable the formation of microdomains of the smallest desirable size and to generate nanostructures with a high degree of lateral ordering. Currently, only pore diameters ranging from 14 to 50 nm have been produced from the self-assembly of PS-b-PMMA.14 To overcome these drawbacks of PS-b-PMMA, Russell and Hawker et al. proposed using poly(ethylene oxide-block-methyl methacrylate-block-styrene) (PEO-b-PMMA-b-PS) triblock copolymers.15 In this system, phase separation occurs to create well-ordered arrays of PMMA/poly(ethylene oxide) (PEO) cylinders in a PS matrix due to the large χ value of PS and PEO. However, in a low molecular weight system, photochemical degradation of the PMMA chains was retarded, and no template was formed because PMMA and PEO are miscible. Thus, it still remains an issue for reducing the domain size and, in the two-dimensional fabrication of nanostructured templates, using block copolymer self-assembly techniques with selective degradation. In this study, we present a new strategy introducing a polymer that has a segment with the characteristics of being both “degradable” and “strongly immiscible” with other blocks in a block copolymer by introducing PTFEMA alone or as a Received: January 10, 2012 Revised: March 2, 2012 Published: March 5, 2012 1454

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Differential scanning calorimetry (DSC) was performed on a SII NanoTechnolgy Inc. DSC7020. Measurements were taken within the temperature range from −50 to 210 at 10 °C/min. Ellipsometry was performed on a Lastek Nanofilm EP3 Imaging Ellipsometer. Small angle X-ray scattering (SAXS) data were taken at the G1 station at the Cornell High Energy Synchrotron Source (CHESS) with typical beam energy of 9.5 keV and flight path length of 2.2 m. Two-dimensional (2D) scattering patterns acquired from a Flicam CCD detector were azimuthally integrated around the beam center. For transmission electron microscopy (TEM), samples were sectioned with a Leica Ultracut UCT ultramicrotome at room temperature. Sample slices were collected on water and transferred to copper TEM grids. Bright field (BF-TEM) images were taken with a FEI Tecnai T12 Spirit electron microscope equipped with a SIS Megaview III CCD camera, operated at an acceleration voltage of 120 kV. E-beam lithography was written using JEOL 9300 with a 0.5 nA beam current, 100 kV accelerating voltage, and 5 nm pixel size. In case of deep-UV exposed films, JBA 1000 was used for blanket-expose films; output was measured to be ∼14 mW/cm2 at 250 nm. Synthesis of PS-b-PTFEMA via Anionic Polymerization. PS-bPTFEMA was synthesized by sequential anionic polymerization as

block in a copolymer poly(methyl methacrylate-co-2,2,2trifluoroethyl methacrylate) (PMMA-co-PTFEMA). It is envisaged that these fluorine-containing polymers perform two roles; one is to provide a strong degree of segregation between the constituent blocks, as reported previously,16−20 while the other is degradability of PTFEMA under either deepUV or e-beam radiation (PTFEMA is known as one of the most sensitive positive-tone resist degradable under deep-UV, ebeam and X-ray).21,22 By introducing the property of sensitive resist to block copolymer as one component, we propose new dual mode patternable materials, in which integrated patterns can be made by conventional lithography (i.e., top-down methods) combined with self-assembly techniques (i.e., bottom-up methods). The convergence of top-down and bottom-up fabrication in the same block copolymer architecture has been demonstrated by several research groups.23−29 Russell and co-workers created cylindrical nanochannels at defined locations from PS-b-PMMA on the substrate, and thereby demonstrated its potential to generate integrated magnetoelectronic devices.23 Hillmyer and collaborators have used poly(lactic acid) as the cleavable block to create nanochannels.4 Ober and co-workers have successfully developed a novel block copolymer system poly(α-methylstyrene-b-4-hydroxystyrene) (PαMS-b-PHOST) to achieve spatial control through highresolution deep UV and e-beam lithographic processes taking advantage of chemically amplified negative-tone resist materials, PHOST.26−28 However, there have been fewer reports of block copolymers from positive-tone resist materials with high sensitivity to the radiative method. Herein, we report the synthesis of a new series of block copolymers, PS-b-PTFEMA and PS-b-(PMMA-co-PTFEMA), via either anionic polymerization or ATRP and the morphological study of these block copolymers in the bulk and thin films. Lithographic approaches using e-beam and deep-UV radiation were demonstrated to investigate their use in the fabrication of nanopatterns.

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Scheme 1. Synthesis of a Series of PS-b-PTFEMA and PS-b(PMMA-co-PTFEMA) via Anionic Polymerization

EXPERIMENTAL SECTION

seen in Scheme 1 or ATRP. The following is a typical synthetic procedure for PS-b-PTFEMA block copolymers via anionic polymerization. A 40 mL of freshly distilled THF was transferred to a glass reactor containing 6 mg of dry LiCl, and the glass reactor was cooled to −78 °C. Then, 86 μL of sec-BuLi (1.3 M sec-butyllithium in cyclohexane) was injected, before 3 mL of styrene was added. After 10 min of polymerization, an aliquot of PS was isolated for analysis after termination with degassed methanol and styryl anions were capped with 59 μL of DPE, which results in a deep red color of the reaction mixture. Half an hour later, the polymerization was resumed by injection of 1 mL of TFEMA with vigorous stirring, which leads to immediate disappearance of the red color. After an additional 30 min, the reaction was terminated with 1 mL of degassed methanol. The polymer was precipitated in 1 L of methanol, and the product was redissolved in THF and re-precipitated twice more into methanol and dried under vacuum at 60 °C for 8 h. Yield: 87%. Mn = 30700 g mol−1; Mw/Mn = 1.07. IR (NaCl): ν (cm−1) =3102, 3083, 3061, 3027, 3002, 2926, 2850, 1747, 1602, 1583, 1493, 1451, 1415, 1283, 1230, 1170, 1137, 1071, 1030, 968, 906, 843, 758, 748, 699, 656. 1H NMR (CDCl3): δ (ppm) = 0.80−1.19 (br, CH3), 1.18−2.28 (br, CH, CH2), 3.60 (s, −COO−CH3), 5.68−7.50 (br, aromatic); 13C NMR (CDCl3): δ (ppm) = 16.8, 17.6, 18.5, 40.3, 44.8, 45.1, 53.5, 60.9, 61.4, 117.4, 125.7, 128.3, 145.3, 174.6, 175.3, 175.6. Synthesis of PS-b-PTFEMA via ATRP. The following is a typical synthetic procedure for PS-b-PTFEMA block copolymers via ATRP. In a 50 mL round-bottom flask, 0.29 g (2.0 mmol) of CuBr was degassed by vacuum followed by argon backfill three times. Styrene, 21 g (0.20 mol), ethyl 2-bromoisobutylate, 0.29 mL (2.0 mmol), and

Materials. Chemicals were obtained from Aldrich, Oakwood Products Inc., or Alfa Aesar and used without further purification unless otherwise noted. For anionic polymerization, tetrahydrofuran (THF) was freshly distilled from sodium/benzophenone (deep purple color) under nitrogen and titrated with sec-BuLi before polymerization. Monomers of styrene, methyl methacrylate (MMA), and 2,2,2-trifluoroethyl methacrylate (TFEMA) were successively stirred and distilled with calcium hydride. These monomers were distilled again under vacuum over small amount of di-n-butylmagnesium (styrene) and trinoctylalminium (MMA and TFEMA) just prior to use. 1,1-Diphenylethylene (DPE) was distilled over 1,1-diphenyl-3-methylpentyllithium. For ATRP, monomers of styrene and TFEMA were passed through inhibitor remover (purchased from Aldrich) just prior to use. Instrumental. Nuclear magnetic resonance (NMR) (1H, 300 MHz; 13C, 75 MHz) spectra were acquired on a Varian MERCURY 300 NMR spectrometer using CDCl3 as a solvent and tetramethylsilane (TMS) as an internal standard. Size exclusion chromatography (SEC) of THF solutions of polymers (1−3 mg/mL) was carried out using four Waters Styragel HT columns operating at 40 °C and Waters 490 ultraviolet (254 nm wavelength) and Waters 410 refractive index detectors. Atomic force microscopy (AFM) was performed on a Veeco Dimension 3100 operated in tapping mode with Olympus tapping mode etched silicon probes (resonant frequency = 300 kHz, force constant = 42 N/m) under ambient conditions. Thermogravimetric analysis (TGA) was performed on a TA Instruments Q500. Measurements were taken by heating from 20 to 550 at 10 °C/min. 1455

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Table 1. Molecular Characteristics of Block Copolymers PS-b-PTFEMA and PS-b-(PMMA-co-PTFEMA) sample name

Mn (PS/second block)a

DPn ratio (PS/second block)b

f PSc

Mn (SEC)d

PDIe

morphologyf

PS-b-PTFEMA1 PS-b-PTFEMA2 PS-b-PTFEMA3 PS-b-PTFEMA4 PS-b-PTFEMA5 PS-b-(PMMA-co-PTFEMA)1 PS-b-(PMMA-co-PTFEMA)2 PS-b-(PMMA-co-PTFEMA)3

12.6k/48.0k 17.3k/10.7k 21.6k/11.2 24.3k/8.3k 29.4k/4.3k 30.1k/25.0k 34.2k/17.0k 34.9k/8.2k

121/285 166/64 207/67 233/50 282/26 289/167 328/130 335/57

0.27 0.69 0.72 0.80 0.90 0.62 0.72 0.85

44.3k 26.2k 30.7k 30.7k 32.3k 44.1k 44.6k 40.1k

1.29 1.36 1.07 1.07 1.08 1.16 1.11 1.06

HEX LAM HEX HEX −g HEX BCC −g

a

The Mns of PS (first block) were measured by SEC using PS standards for calibration. The Mns of the second block were calculated from the ratio of integrated intensity for the protons of PS and second block in the 1H NMR spectrum. Second block is PTFEMA in PS-b-PTFEMA and PMMAco-PTFEMA in PS-b-(PMMA-co-PTFEMA). bThe DPns (degree of polymerizations) of PS (first block) were calculated from the Mn. The DPns of the second block were calculated from the ratio of integrated intensity for the protons of PS and second block in the 1H NMR spectrum. cThe volume fractions of the PS block were calculated using the molecular weights and densities of homopolymers at room temperature: PS (1.05 g/cm3), PTFEMA (1.45 g/cm3), and PMMA (1.19 g/cm3). dRelative Mns measured by SEC using polystyrene standards for calibration. eThe polydispersity indices (PDIs) defined as the ratio of weight-average of molecular weight (Mw) to number-average of molecular weight (Mn). fBulk morphologies at room temperature, defined by SAXS measurement and TEM observation. HEX, hexagonally packed cylinders; LAM, lamellar. gMorphology unknown.

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N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA), 0.42 mL (2.0 mL), which had been degassed with bubbling argon for at least 20 min, were added via cannula and placed in a thermostatted oil bath preheated at 100 °C. After 2 h, the flask was withdrawn and cooled to room temperature. The reaction mixture was diluted with THF. After passing the solution through a column with activated Al2O3 to remove the catalyst and precipitating into methanol, the obtained PS macroinitiator was dried in a vacuum oven overnight at room temperature. Yield: 67%. Mn = 12600 g mol−1 ; Mw/Mn = 1.09. In a 20 mL round-bottom flask, 0.31 g of PS (Mn = 12600 g mol−1, Mw/Mn = 1.09) and 4.9 mg (0.050 mmol) of CuCl were degassed by vacuum followed by argon backfill three times. 3.1 mL of α,α,αtrifluorotoluene, 20 μL (0.050 mmol) of PMDETA, and 3.7 g (22 mmol) of TFEMA were degassed with bubbling argon for at least 20 min and added via cannula and placed in a thermostatted oil bath preheated at 95 °C. After 30 min, the flask was withdrawn and cooled to room temperature. The reaction mixture was diluted with THF. After passing the solution through a column with activated Al2O3 to remove the catalyst and precipitating into methanol, the obtained compound was washed with hot cyclohexane to remove unreacted polystyrene macroinitiator then dried in a vacuum oven overnight at room temperature. Yield: 32%. Mn = 44300 g mol−1; Mw/Mn = 1.29. Synthesis of PS-b-(PMMA-co-PTFEMA) via Anionic Polymerization. A similar procedure was chosen for the preparation of the PSb-(PTFEMA-co-PMMA) block copolymers, as seen in Scheme 1. After capping the styryl anions with DPE, 1 mL of a mixture of MMA and TFEMA (50/50 mol/mol) was added and reacted for 1 h. Yield: 84%. Mn = 40100 g mol−1 ; Mw/Mn = 1.06. IR (NaCl): ν (cm−1) = 3101, 3083, 3060, 3026, 3002, 2925, 2850, 1747, 1602, 1493, 1453, 1416, 1284, 1172, 1137, 1072, 1031, 968, 906, 842, 751, 698, 656. 1H NMR (CDCl3): δ (ppm) = 0.70−1.18 (br, CH3), 1.18−2.28 (br, CH, CH2), 3.60 (s, −COO−CH3), 4.02−4.46 (br, −CH2−CF3), 5.68−7.50 (br, aromatic). 13C NMR (CDCl3): δ (ppm) = 16.7, 17.0, 40.4, 44.8, 46.0, 51.9, 53.4, 60.5, 61.1, 61.5, 117.3, 125.7, 127.7, 128.0, 145.3. Film Preparation. Samples for TEM observation were prepared by casting ∼5 wt % solution of the polymers in THF, for a week at room temperature. After casting, the polymer films were approximately 0.7 mm thick. They were then dried completely in vacuum condition at room temperature and subsequently annealed in a vacuum oven at 200 °C for 3 days. Samples for AFM observation were prepared as the following: Solutions of 0.6−1.0 wt % PS-b-PTFEMA and PS-b(PMMA-co-PTFEMA) in α,α,α-trifluorotoluene were spin-coated onto precleaned silicon wafers with 3000−6000 rpm. After spin-coating, the films were dried under vacuum at room temperature to remove the residual solvent and annealed in an appropriate solvent atmosphere.

RESULTS AND DISCUSSION Synthesis of Block Copolymers. A series of block copolymers that consist of a PS block and either a PTFEMA or a PTFEMA-co-PMMA block designed as the positive-tone resist were synthesized by either anionic polymerization or ATRP. Not only PS-b-PTFEMA but also PS-b-(PTFEMA-coPMMA) were prepared to obtain templates that are potentially useful for various applications (see following section). Synthesis of PS-b-PTFEMA by controlled radical polymerization has been reported,30 but neither morphological study nor lithographic performance has been previously described. In this study, anionic polymerization was adopted because molecular weight, polydispersity, and composition of each block can be accurately controlled. ATRP was also used for synthesis of block copolymers with large volume fraction of PTFEMA (PSb-PTFEMA1 and PS-b-PTFEMA2, listed in Table 1) because polydispersity (Mw/Mn) was broadened during anionic polymerization when a feed ratio of TFEMA to initiator increased, as previously reported by Nakahama and coworkers.31 The synthetic schemes for PS-b-PTFEMA and PSb-(PTFEMA-co-PMMA) via anionic polymerization are illustrated in Scheme 1. All reactions were carried out in 1.5-fold excess of LiCl and 4-fold excess of DPE against initiator as both PTFEMA and PMMA have carbonyl groups.32 Results of polymerization are summarized in Table 1. PS-b-PTFEMA with a total molecular weight of 26.2k∼44.3k g/mol (PS-bPTFEMA1∼PS-b-PTFEMA5, listed in Table 1) and PS-b(PMMA-co-PTFEMA) with a total molecular weight of 40.1k∼44.6k (PS-b-(PMMA-co-PTFEMA)1∼PS-b-(PMMA-coPTFEMA)3, listed in Table 1) were obtained with narrow polydispersity indices. Figure S1 in the Supporting Information shows a representative SEC trace of the PS homopolymer isolated after finishing the polymerization of PS (first block), and block copolymer PS-b-PTFEMA3 in THF. After the injection of second monomer TFEMA to the polymerization flask, the SEC peak shifts to smaller elution volume region, as expected. Relative molecular weights of the resulting block copolymers measured by SEC calibrated against PS standard. The volume fractions of PS were calculated from integrated intensity of 1H NMR spectrum and density of PS (1.05 g/ cm3),33 PMMA (1.19 g/cm3)33 and PTFEMA (1.45 g/cm3).34 The primary structure of the resulting block copolymers was 1456

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characterized by FT-IR, 1H NMR, and 13C NMR spectroscopies. The 1H NMR spectrum of PS-b-(PMMA-co-PTFEMA)2 is shown in Figure 1. The aromatic proton signals of PS were

(DMSO), N,N-dimethylformamide (DMF), and hexamethylphosphoramide (HMPA).16 From these results, it is conceivable that the χ values of PS-b-PTFEMA and PS-b-(PMMA-coPTFEMA) are higher than that of PS-b-PMMA, although χ values of the block copolymers obtained are not explored in this study. Bulk Morphological Study. Thermal analyses were carried out to prove microphase separation of block copolymers of PSb-PTFEMA and PS-b-(PMMA-co-PTFEMA) before attempting to fabricate the block copolymer thin films. We first confirmed that no significant weight loss was observed below 300 °C by TGA analysis (typical TGA data is shown in Figure 2a). Typical

Figure 1. 1H NMR spectrum of PS-b-(PMMA-co-PTFEMA)2 (listed in Table 1) in CDCl3.

observed between 6.3 to 7.5 ppm. The adjacent protons to trifluoromethyl group in PTFEMA appeared at 4.5 ppm. The signal around 3.6 ppm was attributed to the methyl group of PMMA. The other proton signals, carbon signals in the 13C NMR spectra, and peaks in IR spectra were also successfully assigned with the final products. SEC data and analysis of the FT-IR and NMRspectra confirmed the desired structure of diblock copolymers PS-b-PTFEMA and PS-b-(PMMA-coPTFEMA). Comparison of Solubility Data for the Block Copolymers’ Components. As a simple check of miscibility of the block copolymers’ components, we compared the solubilities of homopolymers of PS, PMMA, PTFEMA, and copolymers of PMMA and PTFEMA in a range of common solvents. Table 2 Table 2. Solubility Data for PS, PMMA, PMMA-coPTFEMA, and PTFEMAa solvent hexane cyclohexane toluene chloroform tetrahydrofuran methylene chloride dimethyl sulfoxide methanol

δ (MPa)1/2

PS

PMMA

PMMA-coPTFEMA

PTFEMA

14.9 16.8 18.8 19.0 19.4 20.3

□ ● ■ ■ ■ ■

□ □ ■ ■ ■ ■

□ □ ■ ■ ■ ■

□ □ ● ■ ■ ■

24.6

□

■

■

■

29.7

□

□

●

■

■ = soluble at room temperature; ● = soluble when heated; and □ = insoluble.

Figure 2. TGA curve of PS-b-PTFEMA2 in nitrogen (a) and DSC traces of the second heating cycle for homopolymer of PTFEMA and block copolymers of PS-b-PTFEMA1∼PS-b-PTFEMA 3 (b).

shows the results of the tests. Known data of PS, PMMA, and PTFEMA were added for convenience. Only PS was soluble in moderately nonpolar cyclohexane (δ = 16.8 MPa1/2) when it was heated, indicating that PS is less polar than other polymers. PS and PMMA were insoluble in methanol (δ = 29.7 MPa1/2), whereas PMMA-co-PTFEMA and PTFEMA were soluble. Although qualitative, these results imply that the cohesive energy density or the solubility parameter of PTFEMA exceeds that of PMMA. This is plausible, given the polar nature of the −CH2−CF3 moiety in the PTFEMA repeat unit. Indeed, polyvinylidene fluoride is quite polar (δ = 24.5−25.2 MPa1/2) and is soluble in polar solvents such as dimethyl sulfoxide

DSC curves of the synthesized block copolymers and homopolymer of PTFEMA (Mn = 18000; Mw/Mn = 1.05; synthesized by ATRP) with temperature range from −50 to 210 °C are shown in Figure 2b. All DSC curves showed distinct baseline shifts corresponding to the glass transition temperatures (Tg) of both blocks. PTFEMA homopolymer showed the Tg at 75 °C, while PS-bPTFEMA1∼PS-b-PTFEMA3 showed the Tgs of the PTFEMA block and the PS block around 72 °C and 100 °C, respectively. This result is consistent with the formation of microphaseseparated structures composed of PS and PTFEMA. Microphase-separated morphologies were investigated using SAXS in

a

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explanations: (1) large χ value of PS-b-PTFEMA,17,18 (the result of the solubility test also suggested the large χ value of PS-b-PTFEMA) and/or (2) conformationally asymmetric segmental length of PS and PTFEMA by virtue of the bulky side group of trifluoroethyl group.35 Spherical nanodomain structure with a degradable block is attractive because its monolayer films can be transformed into nanoporous films;34 however, spherical nanodomain structure of PTFEMA block was not observed in PS-b-PTFEMAs. Thus, block copolymers, in which a PTFEMA minor block was replaced by the copolymer of PMMA and PTFEMA in order to slightly reduce χ value and/or asymmetry of segmental length, were synthesized, and their microphase-separated structures were investigated. TEM and SAXS data of the resulting PS-b(PTFEMA-co-PMMA)s with low PTFEMA-co-PMMA volume content showed a morphological difference from that of PS-bPTFEMAs. PS-b-(PTFEMA-co-PMMA)1, which has 62 vol % of PS showed hexagonally packed cylindrical structures of PTFEMA-co-PMMA block within PS matrix although lamellar morphology was observed in PS-b-PTFEMA2 with 69 vol % of PS. PS-b-(PTFEMA-co-PMMA)2, which has 72 vol % of PS, showed BCC spherical structure of PTFEMA-co-PMMA block, which was not observed in PS-b-PTFEMA5 with 90 vol % of PS. These results indicate that we are able to manipulate the micellar cubic strucure of degradable block by adding the MMA unit into PTFEMA, decreasing the repulsive interaction and/or reduce the segmental length asymmetry. Although the present investigation does not calculate the segregation strength (χN) of block copolymers, it will be valuable to make a correlation between the composition of the MMA unit in TFEMA unit profile, χN and the phase diagram of the self-assembled structures. This study is currently in progress. Microphase-Separated Structures in Thin Films. To investigate the microphase separation behavior of the polymers in thin films, films of PS-b-PTFEMA1, PS-b-PTFEMA4, and PS-b-(PTFEMA-co-PMMA)3 with thickness of ∼30 nm were prepared on a precleaned Si wafer and subsequently annealed in the appropriate solvent vapor. Morphologies in the resulting films were investigated using AFM. Figure 4a shows AFM height and phase images of the PS-b-PTFEMA1 film with a thickness of 39 nm. PS-b-PTFEMA1, which contains a small volume fraction of PS (27%) block film annealed in a α,α,αtrifluorotoluene atmosphere for 2 h, showed the formation of well-ordered hexagonally packed spherical structures. However, PS-b-PTFEMA4, which contains a large volume fraction of PS (80%), showed poorly ordered microstructures even after annealing in a variety of solvent atmospheres such as benzene, toluene, and acetone (Figure 4b). Either slight volume change of PTFEMA (PS-b-PTFEMA2∼PS-b-PTFEMA5) or other annealing conditions such as thermal annealing did not result in any improvement on the ordering of a microphase-separated structure. In contrast, well-ordered hexagonally packed dot arrays were observed in the thin film of PS-b-(PTFEMA-coPMMA)3 which contains a large volume fraction of PS (85%) after annealing in a toluene atmosphere for 2 h (Figure 5a). This result is consistent with the morphological study of the bulk films. Lithographic Characteristics of the Block Copolymers. In this patternable block copolymer system, PS was designed as a negative-tone segment, and PTFEMA and PTFEMA-coPMMA were designed as positive-tone segment for e-beam and deep-UV lithography. Exposure to deep-UV (∼250 nm) and ebeam irradiation triggers a cross-linking of PS and degradation

combination with TEM. Samples for both experiments were cast from THF solution and annealed at 200 °C for 24 h under vacuum conditions and then cooled slowly to room temperature. For TEM observation, the samples were sectioned to give thin specimens, which, after embedding in epoxy resin, were exposed to RuO4 vapor to stain aromatic group in PS domain preferentially. TEM images and SAXS profiles of PS-bPTFEMA1∼PS-b-PTFEMA3 and PS-b-PTFEMA5 shown in Figure 3 provided clear evidence of the formation of well-

Figure 3. TEM images and one-dimensional SAXS patterns for the bulk samples of PS-b-PTFEMA1 (a, b), PS-b-PTFEMA2 (c, d), PS-bPTFEMA3 (e, f) and PS-b-PTFEMA5 (g, h) at room temperature.

defined microphase-separated structures. It was also found that nanodomain structures of low interfacial curvature (lamellar and hexagonal) are favored over the geometrically expected ones of high interfacial curvature (micellar cubic) in the PS-bPTFEMA with low PTFEMA volume content. PS-b-PTFEMA1 with a small volume fraction of PS (27 vol %) block showed hexagonally packed cylindrical PS domains. A lamellar phase occurred in PS-b-PTFEMA2, although it contains a moderately large volume fraction of PS (69 vol %). PS-b-PTFEMA3 (72 vol % of PS), PS-b-PTFEMA4 (80 vol % of PS), and PS-bPTFEMA5 (90 vol % of PS) showed hexagonally packed PTFEMA cylinders. It should be noted that a micellar cubic structure was not formed, even in PS-b-PTFEMA5 (90 vol % of PS). These results can be attributed to the following 1458

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thickness of 39 nm after selective rinsing of degraded PTFEMA. Integrated structures of “dots in lines” were successfully obtained from the resist, which has “dual patternability” using top-down process and bottom-up method. One undesirable aspect of the present system as employed here is low contrast between exposed and unexposed areas due to the moderately low dose of 65 μC/cm2. On the other hand, a higher e-beam dose of 413 μC/cm2 resulted in a much higher contrast between exposed and unexposed area, but cross-linked PS dot arrays were not clear in the exposed area (Figure S2, Supporting Information). This result can be attributed to the degradation rate of PTFEMA, which is much higher than the cross-linking rate of PS, which results in disordered cross-linked PS arrays. There are several alternative approaches. These include increasing the sensitivity of the negative-tone block and/or decreasing the sensitivity of the positive-tone block. Detailed studies are under investigation and will be reported elsewhere.

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Figure 4. AFM height and phase images of PS-b-PTFEMA1 (a) thin film after annealing in α,α,α-trifluotoluene for 4 h and PS-b-PTFEMA4 (b) thin film after annealing in benzene for 2 h. The z scale is 5 nm for height and 5° for phase.

CONCLUSIONS

Block copolymers of PS-b-PTFEMA and PS-b-(PMMA-coPTFEMA) with various volume fractions of each block were synthesized via anionic polymerization or ATRP with a goal of development of novel functional resist that has “dual mode patternability”. From DSC, TEM, and SAXS data, PS-bPTFEMAs and PS-b-(PMMA-co-PTFEMA)s were found to show well-defined microphase-separated structures in both bulk and thin films. The nanodomain structures in PS-b-PTFEMA are favored to form low interfacial curvature (lamellar and hexagonal) over the geometrically expected ones of high interfacial curvature (micellar cubic) in the PS-b-PTFEMAs with low PTFEMA volume content. In contrast, PS-b-(PMMAco-PTFEMA) with large volume fraction of PS showed BCC spherical structure of PMMA-co-PTFEMA block due to reduced repulsive interaction between PS block and PMMAco-PTFEMA block and/or reduced conformational asymmetry of segmental length. In thin film, well-ordered hexagonally packed arrays of dots of PS block were observed from PS-bPTFEMA with low PS volume content after annealing in a α,α,α-trifluorotoluene atomosphere, whereas PS-b-PTFEMAs with high volume content of PS did not show well-ordered dot structures of PTFEMA domains. For the thin film of PS-b(PMMA-co-PTFEMA) with high PS volume content, wellordered hexagonally packed arrays of dots of PMMA-coPTFEMA block were observed. Deep-UV irradiation applied to the obtained thin film with arrays of dots of PMMA-coPTFEMA block induced selective degradation of PMMA-coPTFEMA. Consequently, hexagonally ordered arrays of holes were obtained by selectively rinsing degraded PTFEMA-coPMMA block. Through a combination of block copolymer selfassembly techniques with e-beam lithography taking advantage of high e-beam sensitivity of PTFEMA, integrated structures of “dots in lines” were successfully obtained from the novel photoresist. These high-resolution hierarchical structures can function as nanodevices with integrated nanoscale pattern, which cannot be obtained by only conventional lithography or block copolymer lithography, either. Furthermore, they can also serve as patterned templates or scaffolds for the fabrication of various types of multifunctional nanomaterials.

Figure 5. AFM height and phase images of the PS-b-(PMMA-coPTFEMA)3 thin film (a) after annealing in toluene for 2 h, (b) subsequently exposed to deep-UV and (c) after rinsing with methanol to selectively remove the degraded PTFEMA. The z scales are indicated in each image.

of PTFEMA and PTFEMA-co-PMMA. In order to confirm a degradability of PTFEMA-co-PMMA block, we first studied the deep-UV lithographic characteristics of PS-b-(PMMA-coPTFEMA). AFM measurements were performed before and after every processing stage, and they show that the hexagonally ordered arrays of dots were maintained throughout the entire process. Figure 5b shows AFM height and phase images obtained from a PS-b-(PMMA-co-PTFEMA)3 thin film after 25 J/cm2 deep-UV irradiation. Figure 5c shows the AFM height and phase images of the films rinsed in methanol after deep-UV irradiation, indicating that hexagonally ordered arrays of holes were obtained by selectively rinsing degraded PMMA-coPTFEMA block. Figure 6a shows a schematic image of PS-b-PTFEMA1 thin film after the e-beam irradiation and the subsequent development by methyl isobutyl ketone/isopropanol (1/3 vol/vol) mixture (AFM images of an unexposed film are shown in Figure 4a). An e-beam dose of 65 μC/cm2 cross-links the dot arrays of PS and induces degradation of PTFEMA (Figure 6b). A film thickness decrease of ∼12 nm was observed in the exposed regions (Figure 6b inset) from an original film 1459

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Figure 6. Schematic image (a) and AFM height image (b) of PS-b-PTFEMA1 after line exposure by e-beam in a vertical direction of the image followed by development in methyl isobutyl ketone/isopropanol (1/3 vol/vol) mixture. The e-beam exposure dose used is 65 μC/cm2. The inset in (b) is height information for the region depicted in the AFM image with the green line. The z scale is 32 nm for the AFM image.

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ASSOCIATED CONTENT

S Supporting Information *

SEC traces of the PS homopolymer isolated after finishing the polymerization of PS (first block), and block copolymer PS-bPTFEMA3 in THF. AFM image of PS-b-PTFEMA1 after exposure by e-beam with exposure doses of 413 μC/cm2. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (607) 255-8417. Fax: (607) 255-2365. E-mail: cko3@ cornell.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Nissan Chemical Co., and R.M. was supported by fellowships from Japan Society for the Promotion of Science as well as Cornell University. This work was performed using facilities at CHESS, the Cornell Center for Materials Research (CCMR), and The Cornell NanoScale Science & Technology Facility (CNF). CHESS is supported by the National Science Foundation (NSF) and the National Institutes of Health/National Institute of General Medical Sciences under NSF award DMR-0225180. CCMR is supported by NSF award DMR 0520404, part of the NSF MRSEC Program. CNF, a member of the National Nanotechnology Infrastructure Network, is supported by NSF award ECS-0335765. We thank H. Sai and S. Wang of Cornell for assistance with SAXS measurements and analyses and H. Sai, E. L. Schwartz, and Y. Zhang of Cornell and R. Kikuchi of Tokyo Tech for assistance with TEM works.

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