LETTER pubs.acs.org/NanoLett
Highly Tunable Self-Assembled Nanostructures from a Poly(2-vinylpyridine-b-dimethylsiloxane) Block Copolymer Jae Won Jeong,† Woon Ik Park,† Mi-Jeong Kim,‡ C. A. Ross,§ and Yeon Sik Jung*,† †
Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea ‡ Samsung Advanced Institute of Technology, Mt. 14-1, Nongseo-dong, Giheung-gu, Yongin-si, Gyeonggi-do, 446-712, Republic of Korea § Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States
bS Supporting Information ABSTRACT: An extraordinarily large degree of tunability in geometry and dimension is demonstrated in films of a self-assembled block copolymer. A poly(2-vinylpyridine-b-dimethylsiloxane) block copolymer with highly incompatible blocks was spun-cast on patterned substrates and treated with various solvent vapors. The degree of selective swelling in the poly(2-vinylpyridine) matrix block could be controlled over an extensive range, leading to the formation of various microdomain morphologies such as spheres, cylinders, hexagonally perforated lamellae, and lamellae from the same block copolymer. The systematic control of swelling ratio and the choice of solvent vapors offer the unusual ability to control the width of very well-ordered linear features within a range between 6 and 31 nm. This methodology is particularly useful for nanolithography based on directed self-assembly in that a single block copolymer film can form microdomains with a broad range of geometries and sizes without the need to change molecular weight or volume fraction. KEYWORDS: Block copolymers, self-assembly, nanolithography, polyvinylpyridine, polydimethylsiloxane
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elf-assembly of organic materials has become an essential process in the fabrication of nanostructures, with particular application to pattern generation for next-generation nanoscale lithography. The microelectronics industry is facing challenges arising from the increasing cost and limited resolution of deepUV optical lithography, and it is expected that bottom-up approaches using organic molecules will provide a strategy for next-generation nanolithography. Directed self-assembly (DSA) based on polymers or nanoparticles promises very high resolution and outstanding manufacturability of sub-20-nm structures.114 In particular, DSA of block copolymers (BCPs) has shown great advantages such as feature sizes below ∼10 nm and good compatibility with other planar processes and thus has been a topic of enormous research activity for the past decade.16,813,1526 The microphase separation of diblock copolymers is thermodynamically driven by the segregation strength χN, which is the product of the FloryHuggins interaction parameter χ and the degree of polymerization N.27,28 The χ parameter is quadratically proportional to the chemical incompatibility between the two blocks and governs many important parameters related to BCP self-assembly such as the order-to-disorder transition temperature, interfacial width, periodicity, and chain mobility.27,28 Moreover, for a BCP with a very large χ, the response of each block to a given solvent is significantly different. The selective swelling of r 2011 American Chemical Society
one block by the solvent from which the polymer is cast and subsequent morphological transitions in the BCP have been reported.29,30 We previously demonstrated the systematic tunability of a poly(styrene-b-dimethylsiloxane) (PS-PDMS) block copolymer thin film with a bulk cylindrical morphology, using controlled solvent vapors.31 The line width of the resulting patterns increased by up to 127% as the solvent became more preferential to PDMS, and eventually a transition to perforated lamellae was obtained. In this Letter, we report a significantly higher degree of tunability of pattern geometry and dimensions using a poly(2-vinylpyridine-b-dimethylsiloxane) (P2VP-PDMS) BCP with an extremely large χ parameter. The solubility parameters of P2VP, PS, and PDMS are 20.6, 18.5, and 15.5, respectively,32,33 which suggests that the χ parameter of P2VP-PDMS should be at least a few times larger than that of PS-PDMS, which was estimated be about 0.27 at room temperature.34 We report morphological transitions between spheres, cylinders, hexagonally perforated lamellae (HPL), and lamellae, which were induced by choosing different solvent vapors affecting the degree of swelling of the Received: May 14, 2011 Revised: July 12, 2011 Published: September 27, 2011 4095
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Figure 1. Schematic showing the highly tunable self-assembly process.
film. We also report a variation in the line width of in-plane cylinders from 6 to 31 nm, a change of 417%. This provides a convenient method for obtaining a variety of morphologies from a single block copolymer film. Furthermore, the edge roughness and pattern quality of the P2VP-PDMS BCP are excellent. This system is particularly attractive for nanolithography because the oxidized PDMS block, which remains after the P2VP has been removed with an oxygen etch, forms a robust etch mask.35,36 Figure 1 schematically describes the procedure to fabricate various types of self-assembled patterns using a P2VP-PDMS BCP. First, topographic templates with 1.8 μm wide and 40 nm deep trench patterns were made using phase-shift optical lithography with an i-line mask aligner. The templates were coated with a hydroxy-terminated PDMS homopolymer (molecular weight = 5 kg/mol), which was spun-cast on the substrates and annealed at 150 °C for 3 h, and then washed with toluene to remove unattached polymer. After the brush-treated substrates were dried, a P2VP-PDMS BCP with a molecular weight of 26 kg/mol and a minority volume fraction (fPDMS) of 41.6% was spin-coated. The BCP thin film was solvent-annealed to form a monolayer of PDMS microdomains in a P2VP matrix, with a thin surface layer of PDMS which forms at the air interface because PDMS has a lower surface energy than P2VP. The BCP film thickness was optimized between 25 and 85 nm depending on solvent-annealing conditions. Vapors from a range of solvents including toluene, acetone, acetic acid, pyridine, pentanol, isopropyl alcohol (IPA), dimethylformamide (DMF), 1-propanol, ethanol, methanol, propylene glycol, ethylene glycol, and water were used, and the degree of swelling of the BCP was controlled by changing the empty volume of the solvent annealing system9,31 which changed the vapor pressure of the solvent. The Hildebrand solubility parameters and equilibrium vapor pressures of the solvents at room temperature are listed in Table S1 in the Supporting Information. The swelling ratio (SR = swollen thickness during solvent anneal/initial thickness) was measured in situ using an optical thickness measurement tool25 (Filmetrics, F20-UV) which was used to measure the reflectance spectra of dry and swollen BCP films in the wavelength range of 2001100 nm. The SR can be controlled between 1 and a maximum SR, SRmax, by changing vapor pressure. SRmax is determined by the compatibility between the BCP and a specific solvent vapor but did not correlate
with the equilibrium vapor pressure. For example, despite its much lower equilibrium vapor pressure, SRmax for pentanol was higher than those of methanol and water, which have higher equilibrium vapor pressures. However, the time taken to reach a saturated SR was less when the BCP films were treated with solvent vapors of higher equilibrium vapor pressure. After the completion of solvent annealing, the BCP samples were quickly exposed to dry air by lifting the lid of the chamber in order to quench the solvent-annealed morphology by allowing rapid evaporation of incorporated solvent molecules from the BCP. Depending on the initial film thickness and the degree of swelling, the BCP films showed different interference colors. Color changes indicated that the film deswelled within ∼1 s of being removed from the solvent vapor. When the film was dry, we assumed there was no volume shrinkage in the in-plane direction because the dried films completely filled the trenches and the SR returned to 1. A two-step reactive ion etching process composed of CF4 and O2 plasma treatments sequentially eliminated the PDMS surface layer and the P2VP matrix, leaving oxidized PDMS patterns. Thermal annealing of the P2VP-PDMS at 190 °C for 10 h resulted in a lamellar morphology aligned parallel to the substrate, as shown in Figure S1a of the Supporting Information. Although the equilibrium morphology of the thermally annealed dry BCP films was lamellar, the formation of various other morphologies could be induced using appropriate solvent vapors and swelling levels. First, we report the time evolution of morphology from a disordered pattern to lamellae to cylinders in a P2VPpreferential solvent. An as-spun-cast sample from a toluene solution showed a disorganized morphology (Figure 2a). With a treatment of pentanol vapor, a lamellar morphology was observed after a few hours (Figure 2b). This evolved into a mixed morphology of lamellae and in-plane cylinders after a few more hours. Cylinders formed first at the edges of trenches, and this structure propagated to the center of trenches. After 24 h, a uniform assembly of cylinders filling the entire trenches could be obtained. The saturation swelling ratio was 1.41 ( 0.02 in this case, and was constant throughout the annealing process after the initial transient, Figure 2e. Similar morphological transitions occurred for treatments with other P2VP-selective solvents such as isopropyl alcohol, ethanol, propylene glycol, and ethylene glycol, and the morphological transitions occurred sooner for a higher SR. The selective uptake of the solvents in this study into P2VP vs 4096
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Figure 2. Time evolution of morphology. SEM images were obtained for samples treated with pentanol vapor for (a) 0, (b) 3, (c), 12, and (d) 24 h. The scale bar represents 500 nm. (e) The swelling ratio of the BCP film vs treatment time.
PDMS was confirmed by measurements of the SR of homopolymer films, as shown in Figure S4 of the Supporting Information. The small self-diffusivity of the BCP and disordered initial configuration of the chains may be responsible for the initial formation of lamellae during the solvent anneal. The highly asymmetric interfaces at the airpolymer and polymersubstrate boundaries, as a result of the very low surface energy of PDMS compared to that of P2VP and the existence of the PDMS brush on the substrates, caused preferential segregation of the PDMS block to both interfaces during spin-coating, which promoted a lamellar morphology when diffusion distances are short. However, for high SR annealing conditions (1.3 < SR < 1.7) in P2VP-selective solvent vapors, cylinders are expected to be the more stable morphology due to the effective increase in P2VP volume fraction. Cylinders nucleated heterogeneously near the trench walls as the lamellar structures reordered into cylinders. This reordering phenomenon can be distinguished from the previously reported initiation of ordering at step edges,5,37 in that the lamellarcylinder reordering seen here involves a morphological transition rather than just a change of the in-plane orientation of cylindrical microdomains. The slow conversion can be understood by considering the exponential decrease of self-diffusivity with the χ parameter.38 The transitions occurred faster for higher SR, which increased chain mobility and decreased the effective χ parameter. For example, the perfect ordering of cylinder patterns is achieved within a few hours when SR > 1.6. We next discuss the effect of different solvents and vapor pressures on the morphology of the P2VP-PDMS film. By variation of the solvent composition and the vapor pressure, control over a wide variety of morphologies was possible by controlling the
Figure 3. Demonstration of morphological tunability: (a, b) lamella, (c, d) HPL, (e, f) cylinder, (g, h) sphere. (i) Map of different morphologies depending on the swelling ratio (SR) of the BCP films and the solubility parameter of the solvent vapors.
degree of swelling of each block. For the lowest SR, in-plane lamellae were formed as shown in panels a and b of Figure 3. (This is the same morphology as that obtained by thermal annealing.) For example, lamellae were observed at very low vapor pressures of P2VP-selective solvents such as isopropyl alcohol at SR = 1.17, or by using an almost nonswelling solvent such as water at SR = 1.08. As the SR increased, using low vapor pressures of P2VPselective solvent vapors, a hexagonally perforated lamellar (HPL) morphology was obtained as shown in panels c and d of Figure 3. HPL consists of a PDMS lamella containing hexagonally packed P2VP-filled holes and is obtained when the volume fraction is in the range between lamellae and cylinders.3941 For even higher SR, cylinders were observed, e.g., using high vapor pressures of 4097
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Figure 4. Line width (w) change as a function of solvent vapor and swelling ratio (SR): (ac) isopropyl alcohol, (df) ethanol, (gi) ethylene glycol, and (j) methanol. The numbers indicate the average swelling ratio (SR) and line width (w) of each BCP sample.
isopropyl alcohol and ethylene glycol, where the volume fraction of PDMS is significantly decreased. In this regime the cylinder width varied over a wide range, as discussed below. At the highest
SR, even spherical morphologies were observed, for example in acetic acid or pyridine vapors. The solubility parameter of these solvents is almost the same as that of the P2VP block and 4098
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Figure 5. Measurements of (a) line width, (b) period, and (c) fill factor obtained for different solvent vapors and swelling ratio. (d) Plot of maximum and minimum line width vs the solubility parameters of solvent vapors.
produced extensive preferential swelling (SR > 1.8). This morphology was distinguished from perpendicular cylinders by the observation of multilayers of spheres in thicker films as shown in Figure S3 of the Supporting Information. A map of different morphologies obtained using solvents with various solubility parameters and swelling ratios is given in Figure 3i. It is interesting that the morphological transitions occur at higher swelling ratios for solvents with smaller solubility parameters, which may be due to some uptake of solvent molecules by the PDMS block, resulting in a higher effective volume fraction of PDMS at the same swelling ratio. This argument is confirmed by the slightly larger SR of PDMS homopolymers when treated with pentanol compared to the cases of solvents with higher solubility parameters (Figure S4 in the Supporting Information). We now demonstrate the very large tunability of line width for the linear patterns obtained from the in-plane cylindrical morphology. Figure 4 presents well-aligned linear patterns with a line width from 6 to 31 nm, a change of 417%. Even 34-nm-wide patterns were observed, although the ordering was relatively poor. When the BCP was treated with highly swelling solvents such as pentanol or IPA as shown in panels ac of Figure 4, narrow lines between 6 and 15 nm were obtained, with a period that varied gradually between 26 and 30 nm. SR ∼ 1.7 produced ultranarrow 6-nm-wide patterns, as shown in Figure 4a. With a
decrease of vapor pressure, the fill factor (=line width/period) increased from 0.23 to 0.50. Moderately swelling solvents such as ethanol produced patterns with a line width of 1318 nm, a period of 3440 nm, and a fill factor of 0.390.45. The swelling ratio was adjusted between 1.19 and 1.50 for ethylene glycol, a slightly swelling solvent, and the available pattern width, pitch, and fill factor were 2234 nm, 4858 nm, and 0.460.58, respectively. The cylinders were characterized by very sharp edges (3σ line edge roughness
