Poly(dimethylsiloxane-b-methyl methacrylate) - ACS Publications

May 19, 2015 - PMMA and poly(dimethylsiloxane-block-styrene) (PDMS-b-PS) diblock copolymers. Flory−Huggins ... strong-segregation limit, by reducing...
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Poly(dimethylsiloxane‑b‑methyl methacrylate): A Promising Candidate for Sub-10 nm Patterning Yingdong Luo,†,‡ Damien Montarnal,† Sangwon Kim,†,⊥ Weichao Shi,† Katherine P. Barteau,†,∥ Christian W. Pester,† Phillip D. Hustad,# Matthew D. Christianson,% Glenn H. Fredrickson,*,†,§,∥ Edward J. Kramer,†,§,∥ and Craig J. Hawker*,†,‡,§ †

Materials Research Laboratory, ‡Department of Chemistry and Biochemistry, §Materials Department, and ∥Department of Chemical Engineering, University of California, Santa Barbara, Santa Barbara, California 93106, United States ⊥ Department of Polymer Science and Engineering, Inha University, Incheon 402-751, Republic of Korea # Dow Electronic Materials, Marlborough, Massachusetts 01752, United States % The Dow Chemical Company, Midland, Michigan 48674, United States S Supporting Information *

ABSTRACT: We report herein the modular synthesis and nanolithographic potential of poly(dimethylsiloxane-block-methyl methacrylate) (PDMS-b-PMMA) with self-assembled domains approaching sub-10 nm periods. A straightforward and modular coupling strategy, optimized for low molecular weight diblocks and using copper-catalyzed azide−alkyne “click” cycloaddition, was employed to obtain a library of PDMS-bPMMA and poly(dimethylsiloxane-block-styrene) (PDMS-b-PS) diblock copolymers. Flory−Huggins interaction parameters, determined from small-angle X-ray scattering experiments, were high for PDMS-b-PMMA (χ ∼ 0.2 at 150 °C), suggesting this diblock copolymer system has promise for sub-10 nm lithographic applications when compared to the corresponding PDMS-b-PS diblock copolymers (χ ∼ 0.1 at 150 °C). Performance evaluation in thin film self-assembly experiments allowed domain periods as small as 12.1 nm to be obtained, which is among the smallest highly ordered nanoscale patterns reported hitherto for thermally annealed materials.

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assembly is χN > 10.5.8 If one seeks to decrease the domain period of nanostructures, which scale as d ∼ N2/3χ1/6 in the strong-segregation limit, by reducing the size of polymers N while maintaining the segregation strength χN, the Flory− Huggins interaction parameter χ must be increased. Historically, poly(styrene-block-methyl methacrylate) (PS-bPMMA) has been the material of choice for DSA, owing to a high etching selectivity of PMMA vs PS under UV irradiation, as well as nearly equal surface energies between the two components that facilitate a vertical alignment of lamellae or cylinders on properly modified substrates.9 A challenge for this system is that the Flory−Huggins interaction parameter of PSb-PMMA at the typical melt annealing temperature of 170 °C is low (χ = 0.04), and therefore the possibility of reaching sub-20 nm features with PS-b-PMMA is limited.10 The development and facile synthesis of BCPs with increased χ have therefore attracted considerable interest in recent years.7,11−14 One major route to increasing χ relies on incorporating inorganic blocks, most of which are highly incompatible with organic blocks.15 Silicon-based BCPs are widely studied as the inorganic domain in high-χ BCPs with PDMS,16−24 polytrimethylsilylstyrene

INTRODUCTION Progress in semiconductor fabrication relies on shrinking the features of integrated circuits, enabling faster, cheaper, and more energy-efficient microchips. However, smaller features have traditionally relied on the development of new or improved lithographic techniques. State-of-the-art 193 nm immersion lithography encounters substantial difficulties in accessing sub-30 nm features, and although extreme ultraviolet (EUV) and electron-beam lithographies have demonstrated promising results, the cost and throughput of these methods remain problematic for industrial implementation. Block copolymer (BCP) nanolithography, also known as directed self-assembly (DSA), is a high-throughput and lowcost complementary strategy to traditional top-down photolithography,1−7 driven by thermodynamically immiscible polymer blocks self-assembling into ordered nanostructures with varying morphologies. In principle, the self-assembly behavior of BCPs is determined by two factors: χN, the product of the degree of polymerization (N) and the Flory−Huggins interaction parameter (χ), and the volume fraction of each block ( f). χN represents the segregation strength and must overcome a certain threshold for the BCP to self-assemble, while the equilibrium morphology (lamellar, hexagonal, cubic, gyroid, etc.) is mainly determined by the volume fraction f. For a symmetric diblock copolymer ( f = 0.5), the criterion for self© XXXX American Chemical Society

Received: March 11, 2015 Revised: April 30, 2015

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a 50 μm microfocus, a Cu target X-ray source with parallel beam multilayer optics and monochromator (Genix from XENOCS SA, France), high efficiency scatterless hybrid slit collimator developed in house, and a Mar345 image plate detector (Mar Research, Germany). Typical q range for the measurement spans 0.01−0.25 Å−1. Samples for SAXS were melt-pressed inside a metal washer and annealed in a high-vacuum oven (10−7 mbar, 120 °C) for 12 h. The metal washer was then carefully sealed with a Kapton film on both sides with epoxy glue. Thin Film Preparation and Characterization. Silicon wafers with a 100 nm SiO2 surface layer were cleaned by successive sonication steps in acetone, isopropanol, and DI water for 10 min each. Wafers were then immersed in piranha solution (H2SO4/H2O2 = 3:1 (v:v)) (caution: piranha solution is highly corrosive and reacts violently with organic matter!) at 80 °C for 20 min. PDMS-b-PMMA thin films were prepared by spin-coating from a 2 wt % benzene solution at 3000 rpm for 1 min onto a cleaned wafer. The thermal annealing process was performed in a high-vacuum oven at 200 °C for 1 h, cooled to 150 °C over 5 h, and then held at 150 °C for another 5 h. The solvent annealing was carried out in a 4 in. Petri dish charged with 10 mL of specified solvent. Reactive ionic etching (RIE) was performed with a Panasonic E626I dry etch tool using optimized reactive ion etching (RIE) conditions: CF4 20 sccm, 0.3 pa, 50 W BIAS, 20 W power for 5, 10, or 15 s. Film thickness was measured by a Woolam M2000DI variable angle spectroscopic ellipsometer, and tapping mode AFM experiments performed using a MFP-3D system (Asylum Research, Santa Barbara, CA). The measurements were conducted using commercial Si cantilevers. The height and phase images were acquired simultaneously at the set point ratio A/A0 = 0.8, where A and A0 are the “tapping” and “free” cantilever amplitudes, respectively. Preparation of ω-Chloro-PDMS. D3 monomer (24.0 g, 108 mmol) was sublimated into a three-necked 1000 mL flask cooled with liquid nitrogen. 400 mL of anhydrous THF was added, and the resulting D3 THF solution was warmed to room temperature. s-BuLi (11.6 mL, 1.4 M, 16.2 mmol) was added to initiate the polymerization, and after 1.5 h, the living anionic chain end was quenched by addition of chloro(3-chloropropyl)dimethylsilane (6.0 g, 35 mmol); the solution was left at room temperature for 12 h, concentrated to 30 mL, and then redissolved in 200 mL of hexane. The hexane solution was washed twice with 100 mL of H2O, concentrated to 40 mL, and then precipitated into 1 L of cold methanol (twice). The methanol was decanted and the PDMS was dried under high vacuum for 12 h. Representative 1H NMR (500 MHz, CDCl3) δ: 3.51 (t, J = 7.0 Hz, 2H, Cl−CH2), 1.85−1.75 (m, 2H, Cl−CH2−CH2), 1.60−1.51 (m, 1H, CH3−CH(PDMS)−CH(H)−CH3), 1.14 (ddq, J = 14.2, 9.4, 7.3 Hz, 1H, CH3−CH(PDMS)−CH(H)−CH3), 0.93 (t, J = 7.4 Hz, 3H, CH3−CH(PDMS)−CH(H)−CH3), 0.92 (d, J = 7.4 Hz, 3H, CH3− CH(PDMS)−CH(H)−CH3), 0.68−0.61 (m, 2H, Cl−CH2−CH2− CH2), 0.58−0.49 (m, 1H, CH3−CH(PDMS)−CH(H)−CH3), 0.21 to −0.05 (m, PDMS backbone). Preparation of ω-Azido-PDMS. A 5 mL microwave reaction tube was charged with a suspension of ω-chloro-PDMS (Mn = 1.7K, 500 mg, 0.32 mmol) and NaN3 (180 mg, 2.8 mmol) in a mixture of 5 mL of N,N-dimethylformamide (DMF) and 5 mL of dimethoxyethane (DME). The reaction was then heated at 110 °C in a Biotage microwave reactor for 5 h, cooled to room temperature, and diluted with 100 mL of hexane and washed with 100 mL of H2O three times. The organic phase was concentrated and dried under high-vacuum and used without further purification. Representative 1H NMR (600 MHz, CDCl3) δ: 3.24 (t, J = 7.0 Hz, 2H, N3−CH2), 1.68−1.59 (m, 2H, N3− CH2−CH2), 1.59−1.50 (m, 1H, CH3−CH(PDMS)−CH(H)−CH3), 1.19−1.08 (m, 1H, CH3−CH(PDMS)−CH(H)−CH3), 0.93 (t, J = 7.3 Hz, 3H, CH3−CH(PDMS)−CH(H)−CH3), 0.92 (d, J = 7.4 Hz, 3H, CH3−CH(PDMS)−CH(H)−CH3), 0.61−0.56 (m, 2H, N3− CH2−CH2−CH2), 0.56−0.49 (m, 1H, CH3−CH(PDMS)−CH(H)− CH3), 0.18 to −0.04 (m, PDMS backbone). General Procedure for Preparing PDMS Diblock Copolymers by Copper Nanoparticle31 Catalyzed “Click” Reaction. A microwave reaction tube was charged with copper nanoparticles (2.0 mg/50 mg ω-azido-PDMS), a toluene solution of ω-azido-PDMS (1.1

(PTMSS),25−27 and polyhedral oligomeric silsesquioxanes (POSS)28 being prime examples. Among these, the most widely studied DSA candidate is PDMS-b-PS16−19 due to its ease of synthesis via anionic procedures, relatively large χ parameter (χ ∼ 0.1 at 150 °C), and high etching contrast under oxygen reactive ion etching conditions (RIE). Other high-χ candidates with more polar organic blocks such as poly(dimethylsiloxane-b-lactide) (PDMS-b-PLA)21−23 or poly(dimethylsiloxane-b-2-vinylpyridine) (PDMS-b-P2VP)24 have been investigated; however, these PDMS-based systems are typically challenging to synthesize and therefore have not been examined. Although PMMA has shown numerous applications in lithography due to its UV degradability, the self-assembly of PDMS-b-PMMA diblocks has not been investigated. One of the major roadblocks is the lack of an efficient synthetic pathway, especially to access low molecular weight BCPs with narrow polydispersity.29 To avoid the inherent difficulties in preparing PDMS-b-PMMA diblocks by polymerization from macroinitiators and to take advantage of the synthetic accessibility of functionalized homopolymers, we have employed azide− alkyne “click” cycloaddition30−34 as a modular strategy for preparing a diverse library of PDMS-b-PS and PDMS-b-PMMA BCPs in high yields and with facile purification procedures. Determination of bulk and thin film morphologies demonstrates that PDMS-b-PMMA has a significantly higher Flory−Huggins interaction parameter than PDMS-b-PS and leads to smaller feature sizes. Since the synthetic approach to PDMS-b-PMMA is simple and can be applied to other systems, this strategy is a good candidate for extending BCP nanolithography toward ultrasmall domain sizes.

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EXPERIMENTAL SECTION

Materials and Methods. All reagents were used as received from Sigma-Aldrich unless otherwise noted. THF was dried with a solvent purification system. Hexamethyltrisiloxane (D3) and chloropropyldimethylchlorosilane were purchased from Gelest. Hexamethyltrisiloxane was dried over CaH2 at 80 °C overnight and sublimated before use. Chloropropyldimethylchlorosilane was distilled before use. Prop2-ynyl α-bromoisobutyrate (ATRP initiator) was synthesized and purified according to literature procedures.35 Preparation details of αalkynyl-PMMA and α-alkynyl-PS are summarized in the Supporting Information. Silicon wafers with a 100 nm SiO2 surface layer were purchased from Silicon Quest. Nuclear magnetic resonance (NMR) spectra were recorded on either a Varian 500 MHz or a Varian 600 MHz instrument. All 1H NMR experiments are reported in δ units, parts per million (ppm), and were measured relative to the signals for residual chloroform (7.26 ppm) in CDCl3, unless otherwise stated. Size exclusion chromatography (SEC) was performed on a Waters 2695 separation module with a Waters 2414 refractive index detector in chloroform with 0.25% triethylamine. Number-average molecular weights (Mn) and weightaverage molecular weights (Mw) were calculated relative to linear polystyrene standards for calculation of Mw/Mn. The volume fraction of PDMS was estimated from the mass compositions measured by 1H NMR spectroscopy and the bulk densities of the respective homopolymers at ambient temperature: ρ(PDMS) = 0.97 g/cm3; ρ(PS) = 1.05 g/cm3; ρ(PMMA)= 1.18 g/cm3.36 Microwave reactions were performed in an Initiator Eight Microwave System (Biotage). Small-Angle X-ray Scattering (SAXS). Room temperature SAXS measurements of bulk samples were performed at beamline 8-ID-E at the Advanced Photon Source (APS) located at Argonne National Laboratories. Variable temperature SAXS measurements of bulk samples were conducted using a custom constructed small-angle Xray scattering (SAXS) instrument in the X-ray diffraction facility in the Materials Research Laboratory (MRL) at UCSB. The instrument used B

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Figure 1. From top to bottom: synthetic pathway for copper-catalyzed azide−alkyne cycloaddition (CuAAC) between PDMS and PMMA/PS; 1H NMR of ω-chloro-PDMS; ω-azido-PDMS; α-alkynyl-PMMA, and PDMS-b-PMMA by CuAAC. The numbers above peaks correspond to the peak integration values. Peak assignments of PDMS-b-PMMA were determined by 2D COSY as shown in Figure S17. All spectra are referenced to residual solvent CHCl3 at 7.26 ppm. NMR of purified PDMS-b-PMMA confirmed the successful synthesis of BCPs. equiv), and its counterpart, α-alkynyl-PMMA or α-alkynyl-PS (1.0 equiv). The mixture was heated at 150 °C for 1 h in a Biotage microwave reactor, followed by filtration through a 0.45 μm PTFE syringe filter. The filtrate was then centrifuged at 3000 rpm for 5 min and a clear top layer carefully decanted and concentrated. Flash chromatography was performed in order to remove the excess PDMS. ω-Azido-PDMS was readily removed by passing hexanes through a silica plug. The diblock copolymers were obtained after further elution with a more polar eluent: ethyl acetate for PDMS-b-PMMA; 20% ethyl acetate in hexane for PDMS-b-PS. The fractions were combined and dried under high vacuum to yield pure diblock copolymer with a typical yield of 50−80%. Representative 1H NMR (500 MHz, CDCl3) δ: 7.57 (s, 1H, triazole), 5.22−5.12 (m, 2H, −O−CH2−triazole), 4.31

(s, 2H, PDMS−(CH2)2−CH2−triazole), 3.59 (s, PMMA), 2.23 to −0.05 (m, PMMA and PDMS).

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RESULTS AND DISCUSSION Synthesis of PDMS-Based Block Copolymers. To enable a facile and modular strategy for the preparation of a library of PDMS-based block copolymers, the direct coupling of ω-azido-PDMS derivatives with the corresponding α-alkynylPMMA or α-alkynyl-PS blocks by Cu nanoparticle catalyzed “click” reaction was developed (Figure 1). The starting αalkynyl-PMMA and α-alkynyl-PS blocks were synthesized by ATRP from alkyne-functionalized initiators with the corre-

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Macromolecules sponding ω-azido-PDMS prepared by living anionic polymerization of cyclic D3 monomer followed by chain end functionalization with chloro(3-chloropropyl)dimethylsilane. It should be noted that to maintain good end group control, the anionic polymerization of D3 should be quenched at low conversion to prevent undesired side reactions37 and to maintain a low polydispersity (1.1−1.2). Analysis of the chain end fidelity was accomplished by NMR spectroscopy with the integration values for the unique H-resonances for the sec-Bu-Li initiator at 0.53 ppm agreeing well with the integration values for the (3-chloropropyl)dimethylsilane group at 3.51 ppm and 0.64 pm. Significantly, the subsequent SN2 reaction to convert the chloride chain end to the desired azido unit was challenging due to the limited cosolubility of NaN3 and PDMS. In a variety of different solvents, including DMF, toluene, or THF, either no conversion or incomplete conversion was observed in each case. An appropriate mixed solvent system1:1 volume ratio of DMF (N,N-dimethylformamide) and DME (1,2-dimethoxyethane)was eventually found effective, and after heating at 90 °C in a microwave for 5 h, the reaction reached 100% conversion without any residual chloride, as confirmed by NMR (Figure 1). Quantitative large-scale reactions (>20 g) could also be performed in this mixed solvent system using a Schlenk flask with heating at 90 °C in an oil bath for 2 days. As a modular approach to block copolymers, copper nanoparticle catalyzed “click” coupling proved to be extremely efficient with full conversion being observed within 1 h for a variety of molecular weights. The small excess (10 mol %) of ωazido-PDMS used to drive the coupling to completion could be easily removed by eluting the reaction mixture through a plug of silica with hexanes. The desired BCPs could subsequently be obtained by eluting with a more polar solvent, the nature of which depends on the polarity of the non-PDMS block. As shown in Figures 1 and 2, NMR and GPC traces show that no residual homopolymer could be observed after “click” reaction and purification. Detailed characterization data, including molecular weight, polydispersity, and volume fraction of all

polymers made by “click” chemistry, are summarized in Table 1 and the Supporting Information. The ease of synthesis and Table 1. Summary of PDMS-Based Block Copolymer Characterization samplea

Mn (kDa)

Đb

f Dc (%)

Mord

d0e (nm)

D1.7S3.3 D2.8S4.1 D1.0M2.3 D1.1M2.3 D1.7M2.2 D3.7M2.2 D1.7M5.1 D3.7M5.1 D2.8M6.0

5.0 6.9 3.3 3.4 3.9 5.9 6.8 8.8 8.8

1.16 1.15 1.14 1.19 1.14 1.12 1.09 1.18 1.10

36 42 35 37 49 67 29 47 36

dis lam dis dis lam lam hex lam lam

(11.5) 11.4 (7.5) (8.7) 8.7 11.3 10.8 13.4 13.4

a

Sample name refers to the components (D = PDMS; S = PS; M = PMMA), and subscripts refer to the number-average molecular weight (in thousand scale) of each block. bĐ is polydispersity, Mw/Mn measured with SEC. cf D is volume fraction of PDMS. dMor refers to morphology, which was determined by the ratio of scattering peak positions in the SAXS spectra. Lam, hex, and dis indicate lamella, hexagonal cylinder, and disordered morphologies, respectively. e Domain periods were calculated by d0 = 2π/q*, where q* is the position of the principal peak in the SAXS pattern. The correlation lengths of disordered samples are given in parentheses.

associated long-term stability of the respective starting homopolymers illustrates the power of this strategy for the preparation of PDMS-based block copolymer libraries. Morphology and Domain Periods. Small-angle X-ray scattering (SAXS) measurements were carried out to determine the morphology and domain period for each BCP. Transmission SAXS data are summarized in Figure 3 with the domain period determined for each sample through the expression d0 = 2π/q*, where q* is the position of the principal (first-order) SAXS peak. For D1.7S3.3 (ca. 5K combined MW), the principal scattering peak in SAXS pattern was weak and broad, and no secondary peaks could be identified. This is clear evidence that D1.7S3.3 is disordered at the annealing temperature 120 °C. As the molecular weight is increased, D2.8S4.1 (ca. 7K combined MW) presented a sharp principal peak at q* = 0.551 nm−1 and a secondary peak at 2q* = 1.099 nm−1, indicative of a wellordered lamellar morphology. This allows a domain period of 11.4 nm to be calculated. All PDMS-b-PMMA samples were characterized by the same procedure with the two lowest molecular weight BCPs, D1.0M2.3 and D1.1M2.3, showing very similar disordered SAXS spectra. Significantly, a much lower molecular weight threshold was required for these PMMA derivatives to undergo phase separation with D1.7M2.2, D3.7M2.2, D3.7M5.1, and D2.8M6.0 showing well-ordered lamella morphologies with their peak positions following a 1:2:3 ratio. An important requirement for DSA materials is the ability to access different morphologies. As a result, it was interesting to observe a 1:√3:√7 set of peaks for the SAXS pattern of D1.7M5.1, which is consistent with a hexagonal cylinder morphology. The combination of well-defined morphologies and strong phase separation allowed all PDMS-b-PMMA diblock copolymers to have d0 < 13.5 nm with D1.7M2.2 leading to domain periods as small as 8.7 nm, among the smallest domain periods reported.7 Order−Disorder Transition Temperature (TODT) and Flory−Huggins Interaction Parameter χ. The TODT values were then measured by variable temperature SAXS experiments for the lowest molecular weight, ordered BCPs (D2.8S4.1 and

Figure 2. Overlaid SEC traces (CHCl3, 25 °C) of representative BCP, D3.7M5.1, synthesized by “click” chemistry in comparison with the starting homopolymers. Purified PDMS-b-PMMA is shown as a solid blue curve; PMMA homopolymer is shown as red dashed line; PDMS homopolymer is shown in black dots. PDMS traces show negative RI values in CHCl3 and have been reversed for clarity. The SEC trace of PDMS-b-PMMA is shifted to higher molecular weight, indicating a successful “click” reaction. D

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the SAXS profiles in the linear range were analyzed to obtain temperature-dependent χ values. While the Flory−Huggins interaction parameter χ can be obtained by a variety of methods, such as small-angle X-ray scattering,10 small-angle neutron scattering,40 rheology,41 and homopolymer blending,42 different methods often lead to a high variability for the same χ parameter measured.10,43 Furthermore, calculation of χ significantly depends on a reference volume v0 used to normalize the size of the different monomers; as different v0 values have been employed in the literature, great care should be taken when comparing χ parameter values from different sources. We chose here to determine χPDMS−PS and χPDMS−PMMA parameters from the random phase approximation (RPA) analysis of SAXS spectra of disordered diblocks.10,44,45 Measurements were taken on the same instrument and analyzed following the same process to ensure consistency. Disordered SAXS profiles at different temperatures for D2.8S4.1, D1.0M2.3, and D1.1M2.3 were then analyzed following Hashimoto theory, accounting for both the asymmetry and polydispersity of the diblock copolymers. A detailed description of the RPA method and data analysis is summarized in the Supporting Information. Four parameters, including the statistical segment lengths of both polymers (bX, bDMS) and fitting constants K and χ, were optimized to fit the SAXS pattern at a certain temperature. The resulting fitting curves to the SAXS spectra of D1.0M2.3 are presented in Figure 5a. The optimized χ of each sample at various temperatures is shown in Figure 5b, with PDMS-b-PMMA clearly having a higher χ than PDMS-b-PS which has been previously calculated and reported in several different papers.42,46,47 Because of the discrepancy among these values, χ of PDMS-b-PS was revisited recently by Kennemur and co-workers (reporting χ of 0.11 at 150 °C, v0 = 0.118 nm3).12 Our data indicate that χ of PDMS-bPS has a similar magnitude (χ D2.8S4.1 = 6.85/T + 0.072, 0.088 at 150 °C, v0 = 0.1 nm3), although we find a weaker temperature dependence. We believe our expression to be more accurate since it was obtained by fitting a continuous data set outside the fluctuation region, rather than relying on a limited number of data points (TODT values) from different samples. The χ values calculated for the two PDMS-b-PMMA samples (χ D1.0M2.3 = 31.5/T + 0.13; χ D1.1M2.3 = 24.5/T + 0.13, ca. 0.2 at 150 °C, v0 = 0.1 nm3) agree well with each other and are roughly twice as large as the χ of PDMS-b-PS. By using the same instrument, method, and analysis approach, a quantitative comparison of Flory−Huggins interaction parameters between the two BCP systems can be made. These results add further support to the promise of PDMS-b-PMMA as a platform for reducing feature sizes in nanopatterning beyond those attainable with PDMS-b-PS. Thin Film Studies of PDMS-b-PMMA. The self-assembly of block copolymers in thin films can be significantly different from the bulk. Changes in order−disorder transition temperature,48 morphology, and nanostructure orientation are often observed, with the final morphology of a thin film depending greatly on the film thickness, surface energies, and, in some cases, the annealing process.2 Herein, we studied both the thermal and solvent annealing of PDMS-b-PMMA thin films for the fabrication of highly ordered domain periods for 16 nm and less, values which are crucial for DSA-based, nanopatterning applications. As a cylindrical morphology assembled parallel to the substrate (lying down) is commonly used as a template pattern for line structures, initial studies were performed with D1.7M5.1,

Figure 3. SAXS 1-D profiles for samples at 25 °C ordered by increasing domain size. D = PDMS; S = PS; M = PMMA. The numbers are the number-average molecular weight (in thousand scale) of each blocks. Lam/Hex/dis refer to lamella/hexagonal cylinder/ disordered. The minimum domain period accessible for PDMS-bPMMA (d0 = 2π/q* = 8.7 nm) is smaller than that of PDMS-b-PS (11.4 nm).

D1.7M2.2) (Figure 4a,c). Upon increasing the temperature, the intensity of the SAXS patterns decreased and the width of peaks increased monotonically. A more accurate determination of TODT was carried out by monitoring the maximum intensity (Im) and the half-width at half-maximum (σ) of the main scattering peak at different temperatures. Im−1 and σ2 are plotted as a function of T−1 for both samples and are shown in Figure 4b,d. TODT was characterized as the sharp discontinuity, occurring at the same temperature in Im−1(1/T) and σ2(1/T), which was observed at 130−140 ± 5 °C for D1.7M2.2 and 175− 180 ± 5 °C for D2.8S4.1. Both TODTs are close to the annealing temperature (120 °C) and to the Tg of PMMA and PS, which means the sample domain periods are near the minimum limit for well-ordered domains (d0,min) for both types of BCPs. Comparing d0,min between materials is a direct way to investigate which BCP should perform better in terms of accessing smaller nanostructures. Since we estimate d0,min of PDMS-b-PMMA as 8.7 nm and d0,min of PDMS-b-PS as 11.4 nm, the promise of PDMS-b-PMMA as a nanolithographic material for ultrasmall features is significant. According to Fredrickson and Helfand, the behavior of a BCP in the weak segregation regime is controlled by composition fluctuations.38,39 Consistently with their predictions, we observed that the temperature dependence of Im−1 and σ2 becomes nonlinear in the 30 °C temperature window above TODT. As temperature is increased above this range, the curves gradually return to a linear trend, which indicates the decreased influence of composition fluctuations. Subsequently, E

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Figure 4. (a, c) SAXS profile of D1.7M2.2 in the temperature range from 110 to 195 °C and D2.8S4.1 in the temperature range from 130 to 245 °C. Color gradient from dark to light blue corresponds to decreasing temperature. (b, d) Inverse intensity (Im−1) of q* and square of half-width of halfmaximum (σ2) of D1.7M2.2 and D2.8S4.1 are plotted as a function of inverse temperature (K−1). Sharp discontinuities of Im−1 and σ2 indicate TODT. Solid lines are guides to eye. The shaded blue region marks the order disorder transition temperature (TODT) window.

an unconfined parallel cylindrical morphology when approaching ultrasmall features.48 In contrast, 60 nm multilayered films from the same D1.7M5.1 copolymer show well-ordered line features in both AFM phase and height images (Figure 6a and Figure S10). The center-tocenter distance between cylinders (full pitch) was Lc = 12.1 nm according to fast Fourier transform (FFT) analysis of the AFM image (Figure 6a, inset). Terracing was observed in larger 20 × 20 μm2 images (Figure S13), confirming that the initial morphology was a multilayer of parallel cylinders. Grazing incidence small-angle X-ray scattering (GISAXS) was carried out to study the in-depth structure of this film with the pattern being shown in Figure S16a. The presence of discrete scattering peaks along the vertical line at qy* = 0.052 nm−1 confirms the presence of hexagonally packed cylinders oriented parallel to the substrate with a secondary peak at 2qy*, indicating a degree of lateral order. Although smaller domain periods have been achieved with other BCPs by solvent annealing,28,49 the lateral center to center distance of Lc(GISAXS) = 2π/qy* = 12.1 nm is close to the smallest interdomain period realized by thermal annealing.50 Encouraged by these thermal annealing results, we further studied different solvent annealing conditions for PDMS-bPMMA. Because of the solubility incompatibilities of PDMS

which forms cylinders in bulk. A solution of D1.7M5.1 was therefore spin-coated on a Si wafer, thermally annealed, and etched successively as described in the Experimental Section. Before etching, 0.5 and 2 wt % benzene solutions resulted in 17 and 60 nm thick films, respectively, according to ellipsometry. Since the energy of PDMS is much lower than the surface energy of PMMA (γPDMS = 20.5 mN/m, γPMMA = 41.0 mN/ m),36 the PDMS block was expected to wet the air interface. Therefore, the 17 nm thick film is assumed to be a monolayer of D1.7M5.1 (d0 = 10.8 nm in bulk), with a surface brush layer of PDMS. Similarly, the 60 nm thick film is expected to be a multilayer of D1.7M5.1 cylinders with a similar surface layer of PDMS. As anticipated, no structures could be identified by AFM for either thickness, which is fully consistent with a continuous PDMS surface layer. CF4-based RIE was therefore employed to remove the PDMS top surface,16 and after a systematic optimization, RIE conditions (CF4 at 20 sccm, 0.3 pa, 50 W BIAS, 20 W power) corresponding to an etching rate of 1 nm/s resulted in reproducible film thickness and etching profiles (Table S1). After RIE, the 17 nm thick films of D1.7M5.1 thin films were characterized by AFM, as shown in Figure S9. In a 1 μm2 image, the expected monolayer revealed isotropically oriented irregular features that could be explained by the inevitable increase in defect density for monolayer thick films in F

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and PMMA, both polymers tend to swell differently in a given solvent with the effective volume fraction between the PMMA and PDMS phases varying. This swelling allows the selfassembled morphology to be tuned. D2.8M6.0 in the bulk has a PDMS volume fraction of 36% and self-assembles in a lamellar morphology. On the classical phase diagram of χN vs f for diblocks copolymers, this material is close to the hexagonal phase and therefore constitutes an ideal candidate for morphology tuning experiments. Solvent vapor annealing was performed with four different solvents: tetrahydrofuran (THF), toluene, N-methyl-2-pyrrolidone (NMP), and acetonitrile. THF and toluene are expected to swell both PDMS and PMMA domains. Unfortunately, the D2.8M6.0 monolayer dewetted in THF or toluene before ordering was observed. In contrast, NMP and acetonitrile are expected to swell only the PMMA domains due to the low solubility of PDMS in polar solvents.51 After solvent annealing by NMP vapor for 2 h followed by 10 s of CF4 RIE, the D2.8M6.0 monolayer was imaged by AFM. Relatively well-ordered but isotropically oriented line features were observed in phase and height images (Figure 6b and Figure S11) with the center-to-center distance between cylinders measuring Lc = 15.7 nm according to FFT analysis (Figure 6b, inset). GISAXS data for this sample are shown in Figure S16b, with the principal peak at qy* = 0.040 nm−1 confirming a lateral center-to-center distance of Lc(GISAXS) = 2π/qy* = 15.7 nm. No secondary peak was observed along the in-plane direction, indicating that line features were not well ordered over a long-range. Because NMP selectively swells PMMA, the volume fraction of PDMS is expected to decrease. Indeed, upon NMP annealing the morphology shifted from lamellae to cylinders with the existence of terraces (Figure S14), indicating that the thin film morphology consists of parallel cylinders rather than perpendicular lamellae. D2.8M6.0 monolayer thin films were then solvent annealed with acetonitrile for 2 h followed by 10 s of CF4 RIE. In a 1 μm2 AFM image shown in Figure 6c, well-ordered hexagonally aligned dot features were observed. Terracing observed in a 20 × 20 μm2 image (Figure S15) suggests layers of spheres with the in-plane center-to-center distance of spheres found to be Ls = 16.0 nm, according to FFT analysis (Figure 6c inset; GISAXS of this film being shown in Figure S16c). The principal peak at qy* = 0.041 nm−1 corresponded to a center-to-center distance of spheres of Ls(GISAXS) = 2π/qy* = 15.3 nm. Two secondary peaks at √3qy* and √4qy* along the in-plane directions

Figure 5. (a) Representative SAXS profiles and corresponding fitting curves of D1.0M2.3 according to the random phase approximation at different temperatures. (b) Flory−Huggins interaction parameter χ of D2.8S4.1, D1.0M2.3, and D1.1M2.3 as a function of inverse temperature. Linear dashed lines and equations were obtained by fitting the calculated χ values at each temperature to a linear trend. PDMS-bPMMA has significantly higher values of χ than PDMS-b-PS.

Figure 6. AFM phase images (1 μm2) of PDMS-b-PMMA thin film: (a) thermally annealed D1.7M5.1 multilayer after CF4 RIE; (b) D2.8M6.0 monolayer solvent annealed in NMP after CF4 RIE; (c) D2.8M6.0 monolayer solvent annealed in acetonitrile after CF4 RIE. Height images are shown in the Supporting Information. G

DOI: 10.1021/acs.macromol.5b00518 Macromolecules XXXX, XXX, XXX−XXX

Macromolecules

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indicate that the dot features are well-ordered laterally in a hexagonal lattice. The significance of these results is that by solvent annealing a single thin film systemD2.8M6.0 different morphology and domain transitions can be achieved. This interesting solvent-dependent morphology has also been previously observed in PDMS-b-PS thin film studies, albeit at significantly larger domain periods due to the system’s lower Flory−Huggins interaction parameter. The well-ordered, solvent-dependent nanostructures of PDMS-b-PMMA demonstrated above have domain periods around 15 nm, which will enhance the utility of these materials in DSA applications.

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REFERENCES

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In the past two decades, block copolymer nanolithography has been shown to be a very promising alternative to traditional photolithography for the fabrication of nanoscale features. Increased χ and high etching contrast are vital properties for block copolymers to succeed in enabling pattern transfer with sub-10 nm feature sizes. In this work, a modular strategy for the preparation of high etch contrast PDMS-b-PMMA diblock copolymers was developed using Cu-catalyzed “click” coupling of stable, homopolymer precursors. Significantly, the Flory− Huggins interaction parameter χ of PDMS-b-PMMA (ca. 0.2 at 150 °C) was shown to be much higher than that of the current leading DSA candidate, PDMS-b-PS (ca. 0.1 at 150 °C based on the same reference volume of 0.1 nm3) with thin film studies demonstrating well-ordered sub-10 nm line and sphere features under either thermal or solvent annealing conditions.

S Supporting Information *

Preparation details of α-alkynyl-PMMA, α-alkynyl-PS, AFM height and phase images, GISAXS images, etching rate test, and details of RPA theory. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b00518.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (C.J.H.). *E-mail [email protected] (G.H.F). Present Address

D.M.: CNRS-Polymer Materials Engineering Laboratory, Villeurbanne, 69621, France. Notes

The authors declare no competing financial interest. Edward J. Kramer passed away on Dec. 27, 2014.

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ACKNOWLEDGMENTS We thank The Dow Chemical Company for financial support of this research through the Dow Materials Institute at UCSB; we thank Dr. John W. Kramer and Dr. Bryan E. Barton (Dow Chemical Company) for insightful discussions and Dr. Joseph Strzalka at Sector 8 of the APS (Advanced Photon Source) and Dr. Youli Li (UCSB) for help with GISAXS/SAXS acquisition and analysis. Portions of this work were performed at the MRL Shared Experimental Facilities, which are supported by the MRSEC Program of the NSF under Award DMR 1121053; a member of the NSF-funded Materials Research Facilities Network. H

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DOI: 10.1021/acs.macromol.5b00518 Macromolecules XXXX, XXX, XXX−XXX