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Laterally Ordered Sub-10 nm Features Obtained From Directed Self-Assembly of Si-Containing Block Copolymer Thin Films

2015; Wiley; Volume: 11; Issue: 48 Linguagem: Inglês

10.1002/smll.201500439

1613-6829

Yecheol Rho, Karim Aissou, M. Mumtaz, Wonsang Kwon, Gilles Pécastaings, Cristian Mocuta, Stefan Stanecu, Éric Cloutet, Cyril Brochon, Guillaume Fleury, Georges Hadziioannou,

Advanced Polymer Synthesis and Characterization

Laterally ordered sub-10 nm features are produced from the directed self-assembly of poly(1,1-dimethyl silacyclo­butane)-block-poly(methyl methacrylate) (PDMSB-b-PMMA) thin films on sinusoidal azobenzene-containing patterns. The use of sinusoidal surface relief grating enables the formation of very large grain areas (over several µm2) consisting of out-of-plane PMMA cylinders. Directed self-assembly (DSA) of block copolymers (BCPs) has received great research attention as a promising candidate for next-generation lithography since it offers a new route to fabricate laterally ordered sub-10 nm features.1-3 By combining the bottom-up self-assembly of BCP thin films into various nanoscale arrays (i.e., spheres, cylinders, or lamellae) with top–down lithography to create guiding patterns, DSA generates laterally ordered periodic nanopatterns with dimensions smaller than those that can be created by conventional photolithography.4, 5 To control defect density within BCP thin films, guiding patterns are generally classified into chemical epitaxy (or chemoepitaxy) and graphoepitaxy. The former consists of the introduction of periodic chemical heterogeneities on the substrate surface, which directs the alignment of BCP nanodomains by minimizing the polymer–surface interaction energy. The use of periodic nanoscale chemical patterns to control BCP domain alignment and orientation was first demonstrated by Nealey and co-workers.6 Graphoepitaxy consists in the use of a topographical substrate for which the selective wetting of a particular BCP nanodomain at the trench side walls enforces the lateral ordering of features along grooves. Graphoepitaxy, first introduced by Kramer and co-workers,7 has been systematically investigated by Ross and co-workers8 who determine how elastic strain in BCP arrays governs their local arrangement within grooves. More recently, Russell and co-workers reported on DSA of cylindrical BCP domains on a sawtoothed sapphire substrate.1 In spite of the presence of defects (such as dislocations) on the substrate surface, they found that partial confinement of BCP thin films deposited on the faceted substrate provided directional guidance for self-assembled nanodomains. Hadziioannou and co-workers9 demonstrated a very simple and fast route to produce laterally ordered BCP domains using defect-free sinusoidal surface-relief gratings interferometrically inscribed onto an azobenzene containing polymer layer. Domain registration is also required for lithographic application. Generally, BCP domain registration is achieved by minimizing the polymer-surface interaction energy. Indeed, optimization of interfacial interactions with pre-patterned chemical heterogeneity (chemoepitaxy) or the edges of the guiding pattern (graphoepitaxy) self-registers features. For a sinusoidal surface-relief grating, curving a hexagonal microdomain lattice with a sine waveform introduces stresses which force features to be precisely located on top of the wave. In this case, domain registration is due to the geometrical constraints. In this work, we report on DSA of poly(1,1-dimethyl silacyclobutane)-block-poly(methyl methacrylate) (PDMSB-b-PMMA) thin films on sinusoidal azobenzene-containing patterns obtained from the cooperative motion of azo-chromophores under UV-light illumination. Periodic nanostructures consist of out-of-plane PMMA cylinders separated by a Si-containing matrix for which the etching selectivity under oxygen plasma is higher than PMMA:polystyrene one (PMMA:PS, 2:1).10 The combination of high etching selectivity (PMMA:PDMSB, 15:1)11 with sub-10 nm feature sizes makes these BCPs attractive for next-generation lithography. Due to the excellent directional guidance of sinusoidal substrates, the average grain area within templated PDMSB-b-PMMA thin films is found to be increased by an order of magnitude in comparison with BCP thin films deposited on smooth azobenzene-containing polymer layer. Moreover, partial confinement of templated PDMSB-b-PMMA domains preferentially orients grains along the groove direction, while random grain orientation is observed from untemplated BCP thin films. The control of grain orientation within BCP thin films enables the production of very large grains, which results in a defect density 10 times smaller than that of an untemplated one. Semicrystalline PDMSB-b-PMMA chains with a low molecular weight (6.6 kg mol−1) and a PMMA volume fraction, ΦPMMA, of 0.26 have been used to fabricate 2D arrays of out-of-plane cylinders with an areal density of 2.9 teradots per square inch (Td in.−2).11 This PDMSB-b-PMMA system was synthesized by sequential anionic polymerization of (1,1-dimethyl silacyclobutane) (DMSB) and (methyl methacrylate) (MMA) as described previously.11 Thin films of PDMSB-b-PMMA were spin-coated on smooth and on sinusoidal azobenzene-containing polymer layers from 2 wt% polymer solution in toluene/propylene glycol monomethyl ether acetate (PGMEA) (1/1) and the film thickness, h, was controlled by varying the spin coating speed (1.5–5 krpm). The self-assembly of PDMSB-b-PMMA thin films was promoted by thermal annealing at 80 °C for 10 min to 2 h. To avoid dewetting during the thermal annealing, the thin films consisting of low-molecular-weight PDMSB-b-PMMA chains were heated at low temperature (80 °C), which induces a long time process (2 h) to reach highly ordered 2D arrays in comparison with different high-molecular-weight systems heated at higher temperatures.11 As the film thickness roughly corresponds to a pentalayer (h = 3 / 2 p where p is the cylinder pitch), the formation of out-of-plane cylinders throughout the full film thickness was confirmed with in situ grazing-incidence small-angle scattering (GISAXS) measurements (Figure S1, Supporting Information). Figure 1 illustrates the different processing steps used to control the orientational order of dense 2D arrays by templating PDMSB-b-PMMA thin films on the surface-relief grating pattern. AFM phase images presented in Figure 2 show the morphology evolution over annealing time for different PDMSB-b-PMMA thin films deposited on flat silicon substrates and having a thickness, h, of about 70 nm. The as-cast film exhibits both in-plane and out-of-plane PMMA cylinders with a period, p, around 15 nm (Figure 2a) which are pre-ordered into a hexagonal close-packed (hcp) structure as revealed by the corresponding 2D GISAXS pattern (Figure S1, Supporting Information). This short-range ordered cylinder mixture is transformed over (annealing) time into out-of-plane cylinders with a p6mm symmetry. Importantly, a short annealing time treatment (80 °C, 10 min) is required to induce out-of-plane cylinder orientation (Figure 2b) while short in-plane cylinder number slowly decreases over time until the PDMSB-b-PMMA thin film consists of highly ordered cylinders oriented normal to the substrate in agreement with both AFM phase images (Figure 2c) and GISAXS data (Figure S1e, Supporting Information) (80 °C, 2 h). In situ GISAXS measurements confirm this mechanism since broad scattered signal corresponding to the as-cast phase are progressively replaced by a sharp Bragg rod scattering peak on GISAXS images, which indicates a structural reorganization from a short-range ordered cylinder mixture to a well-ordered out-of-plane cylindrical phase (Figure S1a–e, Supporting Information). The increase of this scattered intensity with the annealing time suggests that the formation of cylinders oriented normal to the substrate is incomplete at short annealing times (Figure S1f, Supporting Information). Controlling the lateral order of cylindrical domains is crucial to enable the use of these periodic arrays in nanotechnological applications. This point was accomplished by the use of photoinduced sinusoidal surface-relief gratings as a template. The sinusoidal pattern with a wavenumber of 313 nm and an amplitude of 22 nm was inscribed into a 200 nm thick azobenzene-containing polymer layer by using Lloyd interferometry where the pitch of the pattern as well as its amplitude have been tuned by varying the incident angle of the writing beams onto the film and the exposure time, respectively.9 The generation of topography is related to trans–cis photoisomerization of diazoic bonds leading the surface relief grating.13, 14 Importantly, this mechanism (responsible of lateral mass transport) is optimized when the azobenzene-containing polymer film thickness reaches several hundred nanometers and so, sinusoidal patterns with a sub-100 nm thick layer, desirable for lithographic application, are more challenging to process. AFM phase images presented in Figure 3 show thermally annealed (80 °C, 2 h) PDMSB-b-PMMA thin films on azobenzene containing polymer layers having either a smooth or a sinusoidal surface profile. Untemplated PDMSB-b-PMMA thin film is composed of well-ordered PMMA cylinders formed within randomly oriented grains as revealed by the corresponding 2D-fast Fourier transform (FFT), which mainly consists of an isotropic ring (Figure 3a). By templating this PDMSB-b-PMMA system, a single grain orientation (area of 2 μm2) with a long-range hexagonal order is produced as indicated by the 2D-FFT, which shows six sharp first-order spots concurrent with an increase of the orientational order (Figure 3b). Defect density of untemplated and templated PDMSB-b-PMMA thin films has been determined by using the Voronoi tessellation constructions in which it is possible to locate dislocations, disclinations, and other point defects. Voronoi construction generated from 0.75 × 0.75 μm2 AFM phase image corresponding to an untemplated PDMSB-b-PMMA film reveals that the 2D array contains free dislocations and dislocation lines delimiting grains in a density, ρd, of 0.049 where ρd is defined as the ratio of number of pentagons (or heptagons) on the total number of polygons (Figure 4a,b). Absence of isolated disclinations indicates that the 2D-lattice presents a hexatic order with quasi-long-range orientational and short-range translational correlations since isolated disclinations are only seen in 2D liquids.14 Single grain with a defect density decreased by an order of magnitude (ρd = 0.004) is produced when the PDMSB-b-PMMA layer is templated as shown on Figure 4c,d. Highly ordered arrays of features extending over lateral ranges is also confirmed, for surfaces of mm2, by corresponding GISAXS patterns where measured signals are very sharp (Figure S1e, Supporting Information). These results support that the use of sinusoidal pattern inscribed onto azobenzene containing layer enables the improvement of the 2D order. Importantly, the defect density is found to reach its minimum value when the templated PDMSB-b-PMMA layer tends towards a smooth free-surface since geometrical constraints, imposed by the relief (i.e., sinusoidal free surface), impinge defect diffusion (via glide process) within 2D arrays in accordance with the previous experimental and theoretical works.9, 16, 17 This result indicates the confinement induced by a square waveform (graphoepitaxy) on BCP features is more effective than a sine one to improve their long-range order when the BCP thin film is rough. In contrast, the ordering kinetics of out-of-plane cylinders formed within templated smooth BCP thin films seems to be improved when a sine waveform is used. To know more about the influence of the surface-relief grating on grain sizes and their orientation within PDMSB-b-PMMA layers, moiré analyses have been used. This methodology enables the analysis of the 2D order at relatively large area, 10 × 10 μm2, which is useful to image several large grains at the same time.18, 19 Moiré patterns are the product of interference between two overlapping gratings, which are the BCP pitch (p ≈13.6 nm) and the AFM grating pitch, pG, given by the ratio of the scan size, L, to the total of pixels (pG = 10 μm/768, or 13 nm). From the moiré pattern, the grain orientation angle, θ, is extracted by applying local FFTs (43 × 43 pixel square) at every image point. The inverse FFTs are colored according to θ value (the Fourier peak position) and superimposed over the original AFM image. From the untemplated PDMSB-b-PMMA thin film, high mismatch in orientation between two adjacent grains could be observed since hot and cold colored regions, corresponding to grain orientations, are found juxtaposed (Figure 5a). The high mismatch in orientation between two adjacent grains increases the dislocation density since a dislocation line, in general, arises to delimit high angle grain boundaries (HAGBs) (15°–30°).20, 21 This phenomenon is in accordance with results presented on Figure 4a,b. In contrast, as revealed by the color map, only low angle grain boundaries (LAGB) (0°–15°) are observed between two neighboring grains when the PDMSB-b-PMMA layer is self-assembled on the sinusoidal pattern (Figure 5b). The plot of the AFM FFT intensity profile along reciprocal lattice vectors, k → , of moiré fringes defined as[18,21] | k → | = L p sin 2 θ + ( cos θ − p P G ) 2 confirms that grains are randomly oriented within the untemplated PDMSB-b-PMMA film since their angles form a flat dis­tribution while the angle distribution generated from the templated PDMSB-b-PMMA film adopts a Gaussian shape (mean = 0.1°, σ = 4.1°) (Figure 6). Moreover, DSA of the PDMSB-b-PMMA thin film on sinusoidal pattern improves the average grain area by an order of magnitude since the number average grain area of the templated BCP layer is found to be 11.05 ± 10.34 μm2 while the number average grain area of the untemplated PDMSB-b-PMMA thin film is 1.19 ± 0.90 μm2 (see Figure 5a,b). Such behavior demonstrates that the use of sinusoidal template enables the control of grain orientation due to partial confinement of BCP features, which results in the improvement of the average grain size within the PDMSB-b-PMMA layer. In summary, highly ordered 2D arrays with an areal density of 2.9 td in.−2 have been produced from DSA of PDMSB-b-PMMA thin films. The use of sinusoidal surface relief grating enables the formation of very large grain areas (over several μm2) within the PDMSB-b-PMMA thin films in which the defect density is decreased by an order of magnitude in comparison with untemplated PDMSB-b-PMMA layers. This behavior is inherent in the fact that the sinusoidal pattern guides the orientation of grains, which concomitantly adopt a shape elongated along the groove direction. Independent of the pattern wavenumber, many defects were observed when the BCP film was not continuous (i.e., when valleys of pattern were partially filled). This phenomenon is due to the fact that the gradual variation of trench width imposed by sinusoidal pattern makes the PMMA feature pitch incommensurate with the trench width contrarily to the case where graphoepitaxial substrates are used to confine BCP films. Here, the long-range order can only be obtained when the BCP film is continuous so that the lateral ordering determines the exact placement of features in spite of the formation of a continuous film on sine wave pattern can destabilize the hexagonal close-packed (hcp) lattice. To regulate the formation of topological defects which relieve the stress introduced by curving the hcp lattice with the sinusoidal template, the BCP film thickness was increased until reaching a low free surface deformation (in this study, t roughly corresponds to a pentalayer). However, local film thickness variations (thickness inhomogeneity of few nm) inherent to the substrate roughness make that the average grain size is about 10 μm2 while their orientation does not match perfectly with the azobenzene-based polymer template. We believe that the high etch-resistant Si-containing PDMSB block in regard to the azobenzene-based guiding sublayer could be an excellent methodology to produce laterally ordered sub-10 nm features on flat substrates. The use of the sinusoidal azobenzene-based pattern as a sacrificial guiding layer (i.e., as a removable template) is still under evaluation. Topographical Grating Fabrication: The methodology to produce defect-free sinusoidal pattern consisting of azobenzene-containing polymer layer has been previously described in details.9 Briefly, azobenzene-containing polymer was first deposited on a silicon substrate from a tetrahydrofuran (THF) solution (3% w/w). Then, the photoinduced motion property of azobenzene moieties was used to inscribe sinusoidal pattern in the polymer layer using a Lloyd's mirror interferometer set up with a p-polarized laser beam (λw = 532 nm). A large range of pitches were obtained by rotating the sample holder/mirror with respect to the incident beam while the amplitude was tuned with the exposure time. Self-Assembly of PDMSB-b-PMMA Thin Films: Untemplated and templated PDMSB-b-PMMA thin films were spin-coated from toluene/PGMEA (1/1) solutions (2%, w/w) and the film thickness was controlled by varying the spin-coating speed (1.5–5 krpm). Samples were thermally annealed at 80 °C on a hot plate with a typical time range of 10 min to 2 h in order to promote the self-assembly of PDMSB-b-PMMA thin films. AFM Characterization: Atomic force microscopy (AFM Dimension FastScan, Bruker) was used in tapping mode at 22 °C to characterize the surface morphology of PDMSB-b-PMMA thin films. Silicon cantilevers (Fastscan-A) with a typical tip radius of ≈5 nm and a cantilever resonance frequency of about 1.25 kHz were used. GISAXS Measurement: GISAXS measurements were performed with X-ray energy of 7 keV (λ = 0.177 nm) at DiffAbs of synchrotron-SOLEIL (France) by using a six-circle diffractometer. For the experiments, the sample-to-detector distance was 648.05 mm and scattering signals were collected by a XPAD detector. Each measurement was performed with samples exposed during 10 s in open air condition (see Supporting Information for much details). The authors acknowledge financial support from Arkema and the Région Aquitaine as well as from the Industrial Chair (Arkema/ANR) within the grant agreement no. AC-2013-365. The research leading to these results also received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 619793 CoLiSA.MMP. The X-ray scattering experiments (GISAXS) were performed at synchrotron-SOLEIL (France) on the DiffAbs beamline. The authors acknowledge excellent technical support during the experimental session. The authors acknowledge Christophe Navarro, Xavier Chevalier and Celia Nicolet for helpful discussions. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.