Core–Shell and Zigzag Nanostructures from a Thin Film Silicon

Jun 25, 2019 - The self-assembly of multiblock copolymers generates diverse hierarchical nanostructures and greatly extends the range of microdomain ...
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Letter Cite This: ACS Macro Lett. 2019, 8, 852−858

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Core−Shell and Zigzag Nanostructures from a Thin Film SiliconContaining Conformationally Asymmetric Triblock Terpolymer Ling-Ying Shi,*,†,‡ Fen Liao,† Li-Chen Cheng,‡ Sangho Lee,‡ Rong Ran,† Zhihao Shen,§ and Caroline A. Ross*,‡

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College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China ‡ Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States § Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, Center of Soft Matter Science and Engineering, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China S Supporting Information *

ABSTRACT: The self-assembly of multiblock copolymers generates diverse hierarchical nanostructures and greatly extends the range of microdomain geometries beyond those produced by diblock copolymers. We report the synthesis of a conformationally asymmetric ABC triblock terpolymer in which the end blocks are a mesogen-jacketed liquid crystalline polymer and poly(dimethylsiloxane), respectively, and its selfassembly under mixed solvent vapor annealing forms a range of sphere, cylinder, and perforated lamellar core−shell morphologies, as well as stacked multilevel structures. Sub-7 nm diameter SiOx nanopatterns were generated by selective plasma etching of the small volume fraction Si-containing core block giving a line/space ratio of ∼1:4. Moreover, the conformational asymmetry of this terpolymer leads to zigzag cylinders on a flat substrate and stable cylinder alignment transverse to template sidewalls within lithographically patterned trenches. he thin film self-assembly of block copolymers (BCPs) is an important method to generate various highly ordered nanostructures for a wide range of applications such as templates for nanolithography,1−4 porous membranes for filtration,5,6 and high surface area support for catalysis and energy storage.7−9 In microelectronics, the reduction in device dimensions drives the development of lithographic methods for making smaller feature sizes and for producing three-dimensional multilevel structures.10,11 BCPs with Si-containing blocks possess a high interaction parameter and a natural etch contrast for facile pattern transfer, making them attractive for nanofabrication.12−16 However, coil−coil BCPs typically generate line or dot patterns in which the microdomain width is approximately half the period, yielding patterns with line/space ratios of around 1:1.17 In contrast, rod−coil BCPs can present lower symmetry, anisotropic morphologies such as zigzag lamellae, wavy lamellae, and tetragonally perforated layers driven by the contribution of the geometrical asymmetry and the liquid crystalline alignment,18−20 but there has been little work on the formation and application of these morphologies in thin films. Further innovation in nanofabrication requires new molecular architectures to extend the available microdomain geometries and line/space ratios, as well as strategies to

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© 2019 American Chemical Society

produce three-dimensional (3D) multilevel patterns from selfassembly. Synthetic techniques now enable the creation of diverse block architectures including cyclic blocks,21 multiblocks,22−24 bottlebrushes,25,26 and giant surfactants.27 Linear ABC triblock terpolymers are particularly attractive because they can produce a much wider spectrum of accessible morphologies than diBCPs,28−30 with expanded opportunities for functionalization,31,32 and their cost of synthesis is similar to that of AB or ABA BCPs. By designing combinations of chain architecture, block sequence, degree of polymerization, and volume fraction, the interaction parameters and morphologies can be selected.33,34 Multiphase and core−shell morphologies can be formed analogous to the well-studied diBCPs morphologies (lamellae, perforated lamellae, gyroids, cylinders, and spheres),31,33,35,36 and nanoscale patterns can be produced by removing one or two blocks. In particular, the selective removal of the shell and matrix of core−shell structures is expected to yield asymmetric line-space patterns. Moreover, the inherent Received: April 16, 2019 Accepted: June 17, 2019 Published: June 25, 2019 852

DOI: 10.1021/acsmacrolett.9b00283 ACS Macro Lett. 2019, 8, 852−858

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ACS Macro Letters

Figure 1. (a) Chemical structure of the PDMS-b-PS-b-PMPCS triblock terpolymer (the numbers are the repeat units of corresponding blocks) and a representation of the coil−coil−rod shape of the terpolymer. (b) SAXS profile of the terpolymer after thermal annealing and (c−e) TEM images of core−shell spherical microdomain array with the beam perpendicular to the (100), (111), and (110) planes. The insets in the TEM images are schematic illustrations of the corresponding arrangements of core−shell spheres.

eter between PDMS and PMPCS (χDM), which scales with the square of the solubility parameter difference, is about 4× that between PDMS and PS (χDS). The chemical structure and molecular weight of the synthesized D58S70M100 triblock terpolymer were confirmed by 1H NMR spectroscopy (Supporting Information, Figure S1) and gel permeation chromatography (GPC, Figure S2 and Table S1). The volume fractions of PDMS (f D), PS (f S), and PMPCS (f M) are calculated to be 11%, 16%, and 73%, respectively. After thermal annealing at 120 °C, the small-angle X-ray scattering (SAXS) profile (Figure 1b) displays peaks with a scattering vector ratio of 1:√2:√3:√7, representative of a spherical morphology with the primary peak corresponding to the (110) reflection of the body centered cubic (BCC) cell with a = 28 nm. The transmission electron microscopy (TEM) images in Figure 1c−e illustrate the spherical microdomains in images of the (100), (111), and (110) planes of the BCC array. PDMS and PS form core−shell spheres embedded in the PMPCS matrix, and the PDMS with the highest electron density forms the cores of spheres. The insets of Figure 1c−e schematically illustrate the sphere patterns that can be observed by TEM, where the bright spheres represent the spheres in a lattice plane and the gray ones are the orthographic projection of the spheres in the neighboring plane.45 The annealing temperature here (120 °C) is lower than the LC ordering temperature of PMPCS (150 °C), and therefore, the PMPCS block is in an amorphous state, as indicated by the wide-angle X-ray scattering (WAXS) profiles in Figure S3. The PMPCS is a thermotropic LC polymer, and the room temperature solvent vapor annealing (SVA) method is also expected to result in a disordered phase of the PMPCS block in bulk or thin films, indicated by the amorphous halo in the WAXS profiles of the bulk films annealed under different SVA conditions (Figure S3). In SVA, the selectivity of the solvent vapor toward different polymer blocks influences the chain conformation and the effective volume fraction in the swelled state and provides a convenient method to obtain a range of morphologies from a single BCP.46,47 The use of SVA to tune the morphologies of rod−coil BCP thin films has received little attention compared to coil−coil BCPs. We chose acetone (δace = 19.8 MPa1/2) with δ close to that of PMPCS and added heptane (δhep = 15.2

hierarchical 3D nature of terpolymers suggests their possible use in fabricating 3D devices in a single step, rather than by using several processing steps to build up the structure sequentially.11 In this study, we designed a silicon-containing conformationally asymmetric ABC triblock terpolymer which has the following useful characteristics: (i) the interaction strength sequence, χAC > χAB > χBC promotes the formation of core− shell phase-separated nanostructures; (ii) a silicon-containing block, poly(dimethylsiloxane) (PDMS) constitutes the minor end block that forms the cores in the core−shell structures and produces narrow SiOx patterns after etching; (iii) the large differences in solubility parameters and conformations between the blocks promotes morphology tunability via solvent vapor annealing in mixed solvents; (iv) the semirigid block leads to anisotropic properties and morphologies distinct from those seen in coil BCPs; (v) the carbonyl groups of the semirigid block could enable its possible application in sequential infiltration synthesis (SIS)37 to fabricate for example SiOx− MOx nanocomposites. The poly{2,5-bis[(4-methoxyphenyl)-oxycarbonyl]styrene} (PMPCS), a typical mesogen-jacketed liquid crystalline polymer (MJLCP; in which the linkage of the side chains is located at the center of gravity of the mesogens with short spacers or without spacers),38 was chosen as a semirigid block. The PMPCS usually adopts an extended-chain conformation to form a columnar nematic liquid crystalline (LC) phase when thermally annealed above an appropriate temperature in bulk.38 The polymer chain of PMPCS was also shown to be extended to some extent in solutions but less extended than in the LC phase.39−41 The triblock terpolymer, poly(dimethylsiloxane)-bpolystyrene-b-poly{2,5-bis[(4-methoxyphenyl)-oxycarbonyl]styrene} (PDMS-b-PS-b-PMPCS, DSM; Figure 1a), was synthesized by sequential atom transfer radical polymerization from a PDMS macroinitiator (Scheme S1). PMPCS was the major block resulting in the coil−coil−long-rod terpolymer molecules. The three blocks are incompatible with one another, confirmed by the strong microphase separation observed in the PDMS-b-PS, PS- b-PMPCS, and PDMS-b- P M P C S diBCPs.20,41−43 The Hildebrand solubility parameters of the PDMS, PS, and PMPCS are 15.3, 18.5, and 20.7 MP1/2,43,44 respectively, and thus, the Flory−Huggins interaction param853

DOI: 10.1021/acsmacrolett.9b00283 ACS Macro Lett. 2019, 8, 852−858

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Figure 2. SEM images of oxidized PDMS nanopatterns formed in DSM terpolymer thin films with initial thickness 30 nm (first row) and 65 nm (second row) as a function of the mixed acetone/heptane ratio during SVA: (a, b) 5:0, (c, d) 5:1, (e, f) 4:1, (g, h) 3.5:1. The scale bars are the same as in (a). (i) Schematic illustration of HCP, BCC, and IP-C on S, IP-C, double-layer HPL, and HPL on IP-C, where the orange color represents the patterns in the top layer, and the yellow represents those in the lower layer. (j−l) Low-magnification SEM images that illustrate large-scale grain sizes of (j) BCC, (k) IP-C, and (l) HPL morphologies. In (j, l), the grain orientations are shown in different colors. In (k), the lighter colored band is an inset from a single grain with size >6 μm.

MPa1/2), with δ approaching that of PDMS (Table S2) for SVA.44 DSM thin films were spin-coated with an initial thickness of 25−70 nm on PS-brush functionalized silicon substrates and SVA was performed by placing the film sample in a chamber supported above 5 mL of a liquid acetone/heptane mixture. The film preparation, solvent annealing, and etching are described in the Supporting Information and Figures S4− S7. The swelling ratio (SR) of the film varied from 2.2 to 1.8 (Figures S6 and S7). After deswelling, the film was etched to remove the PS and PMPCS blocks and convert the PDMS into oxidized silica-rich patterns that were characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). We now describe the dependence of the DSM self-assembly on film thickness and the acetone/heptane (A/H) volumetric liquid ratio in the mixed solvent. The morphologies of films with initial thicknesses ∼30 and 65 nm were observed by SEM (Figure 2a−h) and AFM (Figure 3a) after SVA from a solvent mixture with A/H = 5:0, 5:1, 4:1, and 3.5:1. In top-view SEM images, the upper layer of the oxidized PDMS features appears brighter than the lower layer.1 When pure acetone was used (A/ H = 5:0), the SR was 2.15 (Figure S6), and spherical morphologies were obtained. Figure 2a shows double-layer hexagonally close-packed (HCP) spheres in the film with a 30 nm initial thickness. The sphere-to-sphere distance (L0,s) is 28 nm, while the diameter of the spheres (ds) is only 7 nm, and the

vacant space between neighbor spheres is 21 nm, resulting in an in-plane filling factor (sphere occupied area/total area, [1/ 2πds2]/[√3L0,s2]) of 6%. The 65 nm thick film forms a multilayer BCC array with the (110) closest-packed plane parallel to the substrate surface (Figure 2b). The rectangular net has dimensions of 41 and 28 nm, with the expected ratio of √2. AFM confirms the HCP and BCC ordering, Figure 3a. Both spherical morphologies exhibit domain sizes exceeding 10 μm in low-magnification SEM images (Figures 2j and S8a). We expect that the addition of heptane will increase the volume fraction of the minority PDMS blocks in the swollen state and thereby change the morphology of the thin film. When the acetone:heptane ratio was in the range of 9:1−5:1, the SR remained in the range of 2.0−2.1, and a core−shell cylindrical morphology was obtained. Notably, the 30 nm thick films present a double-layer structure composed of spheres (S) in the bottom layer and in-plane cylinders (IP-C) in the top layer (IPC on S), and the spheres in the bottom layer were located beneath the gap between adjacent cylinders (Figures 2c and 3a). An IP-C on S pattern has been produced by multistep fabrication48 but not previously in a single step process. The 65 nm thick film presents a IP-C morphology with five layers of cylinders (Figures 2d and 3a,b). Cross-section SEM images (Figure 3b) demonstrate that a 45 nm thick film produced three layers of IP-C. The IP-C on S and the multilayer IP-C structures exhibit good long-range order with grain sizes exceeding several 854

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Figure 3. (a) AFM height images of various oxidized PDMS nanopatterns. (b) Cross-section SEM images of the three layer and five layer IP-C in films with initial thickness of 45 nm (left) and 65 nm (right) after annealing with vapor from an acetone/heptane = 5:1 mixture. (c) Experimental morphologies of commensurate thickness DSM films as a function of the initial film thickness (30, 45, and 65 nm) and the acetone/heptane ratio. The calculated volume fraction of the PMPCS in the swollen state (f SM) is indicated. The blue line shows the SR of the film.

μm. The inset of Figure 2k illustrates a section of IP-C morphology ordered over at least 6 μm. The film thickness is incommensurate with the layer spacing for a 35 nm thick film, and terracing occurred. The film exhibited a double-layer IP-C morphology in the thicker regions and IP-C on S in thinner regions. For a 20 nm thick film, IP-C on S formed in the thicker regions and HCP spheres in thinner regions, Figure S9. The SEM image in the inset of Figure 2c demonstrates uniform thin grating patterns of IP-C where the diameter of the oxidized PDMS cylinders (dc) is only 6 nm and the cylinder− cylinder distance (L0,c) is 31 nm. The fill factor (line width/ period length, dc/L0,c) is only 19%, that is, the line/space ratio is 1:4.2. Both the line width and the fill factor are smaller than the lowest value reported for a PS-b-PDMS diBCP49 and result from the PDMS core of the core−shell cylindrical morphology, stabilizing narrow cylinders with a low (11%) PDMS fraction. When the acetone/heptane ratio was changed from 4.0:1 to 3.0:1, the SR was 1.85 to 1.95 (Figure S6) and the morphology transformed to hexagonally coordinated perforated lamellae (HPL). SEM (Figure 2e,f) and AFM images (Figure 3a, IP-C +HPL) show the mixed morphology of IP-C coexisting with the HPL for both the 30 and 65 nm thick film annealed in A/H = 4:1. The cylinders connect with the junctions of the HPL in the same layer. Figure 2g shows the well-ordered double-layer HPL morphology of a 30 nm thick film annealed at A/H = 3.5:1, where the holes in one layer overlay the junctions of the other layer (Figure 2g). An analogous double layer HPL structure has been reported in a ABA triBCP.50 The center-to-center distance L0,PL of the pores in the HPL is 37 nm, and the thin skeleton is only 6 nm wide. Figure 2h shows HPL on top of IP-C for a 65 nm thick thin film, where the cylinders lie under rows of pores in the HPL so that the cylinder-to-cylinder distance (∼31 nm) equals √3/2 times the center-to-center distance between the HPL pores (∼37 nm). The grain sizes of the HPL and HPL on IP-C also exceed micrometers (Figures 2l and S8c,d). Both AFM images of 30 and 65 nm thick films confirmed the HPL

structure (Figure 3a, HPL or HPL on IP-C). From the above results, the IP-C structure gradually transforms to HPL structure as the heptane ratio increased, and the mixed morphology of the IP-C and the HPL further help us to confirm the complex structure of HPL on IP-C. The alignment direction of cylinders under the HPL can be clearly distinguished in the SEM images (Figure S8e,f). These morphologies possessing high surface area and hierarchical porosity may be applicable as catalyst supports, and chemical separation and filtration membranes. When A/H was decreased from 3:1 to 2:1, the morphology of the thin film became increasingly disordered (Figure S10). The dominant morphologies are illustrated in Figure 2i and summarized as a function of as-cast film thickness, solvent ratio, and estimated volume fraction of PMPCS block in Figure 3c. The A/H ratio governs the effective volume fractions, the segregation strength χeffN, the chain conformation, and the morphologies. The effective volume fraction of the PMPCS in the swollen state (f sM) was estimated from the measured swelling ratios of the DSM terpolymer and PDMS and PS homopolymer films. It decreased from 75% to 68% as A/H changed from 5:0 to 3.5:1 (Figure 3c). Moreover, the high-χ values in the DSM enable microphase separation under saturated SVA conditions even though the effective interaction parameter χeff is reduced by almost half compared to χ, according to χeff = χ(1 − ϕsol), where the fraction of solvent ϕsol is 0.44−0.54 based on the SR of 2.15−1.85. From the homopolymer swelling ratios (Figure S7), heptane swells the PDMS, increasing the dimensions of the microdomains (radius of gyration Rg,D), whereas the swelling of the PS block (radius of gyration Rg,S) generally decreases for larger fractions of heptane. The solubility parameter of the mixed solvent decreases with increasing heptane fraction: δmixture = δ1ϕ1 + δ2ϕ2, where δi and ϕi are the solubility parameter and mole fraction of solvent i, respectively. Since heptane is a poor solvent and acetone is a good solvent for PMPCS, the PMPCS is expected to swell more and become more isotropic as the 855

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Figure 4. (a−d) Representative bends or zigzag morphologies in cylindrical oxidized PDMS nanopatterns formed in 65 nm thick DSM films after SVA with acetone/heptane ratios of (a, b) 7:1 and (c, d) 5:1. Green lines indicate some of the bends in (a, c). An arrow is outlined in (b). (e) AFM height image of the zigzag cylindrical morphology. (f) Schematic illustration of the proposed chain conformation, interdigitated packing and tilting of the rods in the vicinity of the bends. (g) Templated self-assembly of DSM terpolymer in 100 nm deep trenches with trench width (W) of 60, 130, 135, 230, and 760 nm.

BCPs,19,20 but this is the first report of a zigzag cylindrical morphology in thin film. The bends provide continuity between regions of different cylinder orientation, and were observed in both the top and bottom layers of cylinders (Figure 4d). The formation of these bends and zig-zags is expected to be related to the semirigid chain conformation of the PMPCS block, the conformational asymmetry and the packing frustration at the domain interfaces. As schematically illustrated in Figure 4f, the coil−coil−long-rod conformational shape of the terpolymer promotes interdigitation of the PMPCS rods in the majority block.51 Although the PMPCS block was amorphous under SVA conditions, the semirigid rods still preferentially orient perpendicular to the intermaterial dividing surface (IMDS).20,51 Moreover, in order to maintain a uniform density and orientation, tilting can occur to accommodate the localized bends and zig-zags. These results suggest an influence of the rigid block orientation on ordering of the triblock terpolymer, in which improved ordering and orientation may reduce the energy penalty of the unfavorable rod packing. The directed self-assembly of the triblock terpolymer was investigated within 100 nm deep lithographically defined trenches. After SVA with A/H from 9:1 to 5:1, the DSM forms uniform IP-C microdomains aligned transverse to the trench walls. The SEM images in Figure 4g demonstrate

acetone ratio increases. These trends explain the morphology transformation from spheres to cylinders and then to perforated lamellae as the heptane fraction increases. The differences in chain confinement and wetting behavior at the air and substrate interfaces and in the bulk of the film are assumed to lead to the different morphologies observed at the two interfaces. Solvent vapor mediated self-assembly of the ABC terpolymer thin film of various thickness therefore produces a diverse range of bilayer or multilayer core−shell nanostructures in a single-step process. Our previous work improved the ordering of PDMS-bPMPCS rod−coil diBCP thin films via thermal annealing.42 The different initial morphologies induced by SVA shown here are likely to influence the ordering process during subsequent thermal annealing and contribute to the final morphology of thermotropic LCBCPs. We now describe a cylindrical zigzag morphology observed in the DSM triblock terpolymer. Cylinders with sharp bends were observed in films annealed in vapor from a mixture with A/H ratio from 9:1 to 5:1 (Figure 4a−e). Examples of concentric sharp bends in Figure 4a,c are indicated by the green lines. Bends were seen with a range of angles, averaging ∼90°, and an arrow morphology occurs when the angle of the bend is less than 90° and especially less than 70° (Figures 4b and S11). Analogous zigzag features have been seen in lamellar rod−coil 856

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transverse cylinders formed in trenches of various width ∼60− 760 nm after annealing in A:H = 7:1. The periodicities of the cylinders in different width trenches (∼31 nm) are the same as that on the flat substrate. Well-ordered transverse cylinders can be achieved with 30 min annealing (Figure S12) and are stable for annealing times of at least 48 h. This alignment is different from the typical behavior of diBCPs, where cylinders are generally oriented parallel to the trench walls, as exemplified by PS-b-PDMS.43 Transverse orientations may be formed in templated BCPs by neutralizing the walls52 or metastably due to flow alignment,43 but for the DSM, we believe that the preferential attachment of the middle PS block to the PS-brush functionalized trench wall and the orientation of the rigid rods parallel to the trench wall53 drive the cylinders to be perpendicular to the walls. The transverse cylinders and the zig-zags formed by this triBCP constitute some of the essential geometric features for device layouts.54 In conclusion, we synthesized a high-χ coil−coil−long-rod ABC triblock terpolymer which self-assembles into core−shell nanostructures. SVA produced core−shell spheres, cylinders, and perforated lamellar structures by adjusting the ratio of the mixed solvents and the film thickness. Structures, including single, double, and multilayer spheres, cylinders, and HPL, as well as stacked multilevel structures, were produced from the same terpolymer. Etching removed the organic matrix and shell blocks and produced well-ordered oxidized SiOx nanopatterns with dimensions below 7 nm. The conformational asymmetry of the molecules promotes sharp bends or zigzag cylindrical features instead of the fingerprint patterns seen in coil diBCPs and a stable transverse alignment of the cylinders in topographic trench templates. The complex patterns produced by the terpolymer and its processing and templating strategies provide additional routes for two- and three-dimensional structural control.

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ACKNOWLEDGMENTS Financial support from NSF DMR-1606911 and the National Natural Science Foundation of China (Grants 51773124 and 51403132) are gratefully acknowledged. Shared experimental facilities of CMSE, an NSF MRSEC under Award DMR1419807, and the NanoStructures Laboratory were used.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.9b00283.

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Letter

Experimental method description of synthesis of DSM triblock terpolymer, characterization of the chemical structure, liquid crystalline behavior and morphology of the bulk sample, preparation and characterization of BCP films, estimation of volume faction in the swollen state, and additional images of thin film and template selfassembly (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Ling-Ying Shi: 0000-0002-6620-7878 Li-Chen Cheng: 0000-0001-9975-9903 Sangho Lee: 0000-0003-4164-1827 Zhihao Shen: 0000-0003-2858-555X Caroline A. Ross: 0000-0003-2262-1249 Notes

The authors declare no competing financial interest. 857

DOI: 10.1021/acsmacrolett.9b00283 ACS Macro Lett. 2019, 8, 852−858

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DOI: 10.1021/acsmacrolett.9b00283 ACS Macro Lett. 2019, 8, 852−858