Formation of Nanogrooves with Sub-5 nm Periodicity Using Local

May 8, 2017 - Bottom-up fabrication of nanopatterns with single nanometer-scale periodicity is quite challenging. In this study, we have focused on th...
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Formation of Nanogrooves with Sub‑5 nm Periodicity Using Local Silicification at the Interspace between a Si Substrate and Lyotropic Liquid Crystals Shintaro Hara,† Hiroaki Wada,‡ Atsushi Shimojima,‡ and Kazuyuki Kuroda*,‡,§ †

Department of Advanced Science and Engineering, Faculty of Science and Engineering, Waseda University, 3-4-1 Ohkubo, Shinjuku-ku, Tokyo 169-8555, Japan ‡ Department of Applied Chemistry, Faculty of Science and Engineering, Waseda University, 3-4-1 Ohkubo, Shinjuku-ku, Tokyo 169-8555, Japan § Kagami Memorial Research Institute for Materials Science and Technology, Waseda University, 2-8-26 Nishiwaseda, Shinjuku-ku, Tokyo 169-0051, Japan S Supporting Information *

ABSTRACT: Bottom-up fabrication of nanopatterns with single nanometer-scale periodicity is quite challenging. In this study, we have focused on the use of the outermost convex surfaces of lyotropic liquid crystals (LLCs) as a template. Periodically arrayed single nanometer-scale nanogrooves consisting of silica are successfully formed on a Si substrate covered with LLCs composed of cylindrical micelles of cetyltrimethylammonium chloride. Soluble silicate species are generated from the Si substrate by a treatment with an NH3−water vapor mixture, infilling the interspaces between the Si substrate and the LLCs. The cross section of the nanogrooves has a symmetrical sawtooth-like profile with a periodicity of 4.7 nm, and the depth of each nanogroove is around 2 nm. Uniaxial alignment of the nanogrooves can be achieved using micrometer-scale grooves fabricated by a focused ion beam technique. Although formed nanogrooves contain defects, such as bends and discontinuities, this successful concept provides a novel fabrication method of arrayed concave patterns with sub-5 nm periodicity on the surfaces of Si substrates. KEYWORDS: liquid crystals, template, nanopattern, silicification, nanoimprint mold

S

nanoimprint mold. The periodicities of most LLCs of low molecular weight surfactants are usually around the single nanometer scale. The use of the outermost convex surface of LLCs as nanoimprint molds for ultrafine pitch resolution less than 5 nm would be greatly beneficial for further miniaturization of devices. LLCs can be prepared from typical surfactants and solvents and readily removed from substrates by washing, which facilitates the fabrication and removal of nanoimprint molds. The formation of nanopattern with sub-5 nm periodicity was recently demonstrated using a well-designed liquid crystals consisting of two oligo(dimethylsiloxane) units linked with an organic mesogenic core.10 Nonetheless, the process requires the preparation of monodisperse oligo(dimethylsiloxanes), selective etching of organic cores, and

urfactant micelles have been widely used as templates for the formation of nanostructured inorganic materials, including mesoporous materials. The size and array of micelles play a major role in defining the nanostructures. Various types of structure and morphology of mesoporous materials have been tailored for specific purposes by controlling the shape and size of the assembled micelles. Conventional mesostructured materials are prepared mainly from homogeneously mixed precursors composed of surfactants and inorganic sources. Thus, the entire surface of micelles is covered with inorganic sources typically through the cooperative self-assembly of surfactants and inorganic sources1−3 or through the deposition of inorganic sources in the lyotropic liquid crystals (LLCs)4,5 to form mesostructured materials. Such templating with assembled micelles normally uses all the surfaces of the surfactant micelles. Some studies on monolayer or single-layer mesoporous thin films are also categorized as the same concept,6−9 and the half-cup shape of the outermost surface of LLCs has never been used as a © 2017 American Chemical Society

Received: April 5, 2017 Accepted: May 8, 2017 Published: May 8, 2017 5160

DOI: 10.1021/acsnano.7b02357 ACS Nano 2017, 11, 5160−5166

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pattern (Figure S1). By adding ammonia−water into a flask, the substrate was exposed to an NH3−water vapor mixture at a temperature of 58−59 °C in the flask while the surface temperature of the substrate was kept at 60 °C (Scheme 1b). During the contact of LLC with the mixture, the lamellar LLC phase of CTMACl was transformed to a 2D-hexagonal phase, which should induce the permeation of aqueous ammonia through the hydrophilic parts of the LLC. Then, aqueous ammonia was in contact with the surface of the Si substrate, and soluble silicate species were generated from the Si substrate. The interspace between the LLC and the substrate was infilled with soluble silicate species, whereas the diffusion of the soluble silicate species was suppressed up to half of the first layer of LLC. After the treatment, the substrate was washed with deionized water and ethanol to remove all the LLC phase for the formation of nanogrooves (Scheme 1c). The experimental details are described in the Materials and Methods that includes materials used, preparation of CTMACl thin films, preparation of nanogrooves, and characterization. The cross-sectional scanning electron microscopy (SEM) images (Figure 1a,b) of the sample after the removal of

careful preparation of a single-layer film, which makes the nanopatterning complicated. On the other hand, the conventional top-down11−16 and bottom-up processes,16−19 using complex instruments and procedures, have their own limits in resolution mostly larger than 10 nm. We have recently reported the usefulness of the surface of mesoporous silica thin films with open pores as a hard template for the fabrication of copper nanopatterns at the single nanometer scale.20 However, the surface is not sufficiently flat as a mold of nanopattern. If we were able to use only the outermost surfaces of LLCs, a groundbreaking method to fabricate nanopatterns with a single nanometer-scale periodicity would be possible. In order to realize the concept, the inorganic species forming the nanopatterns (nanogrooves in this case) must not permeate into assembled micelles. It is known that ionic species cannot permeate through cross-linked LLCs because of the large radii of hydrated ionic species.21,22 This suggests that the permeation of ionic inorganic sources into the molecular assemblies can be inhibited when such ionic species are supplied from the outside of the assembled micelles. Here, we report the formation of nanogrooves with a periodicity of 4.7 nm on a Si substrate by infilling the interspaces between the Si substrate and the 2D-hexagonal LLCs with soluble silicate species generated from the Si substrate during the treatment with an NH3−water vapor mixture (Scheme 1). The outermost surfaces of cylindrical Scheme 1. Idealized Formation Process of Nanogrooves between Lyotropic Liquid Crystals and a Si Substrate

micelles on the substrate act as a template. The treatment with the NH3−water vapor mixture can generate soluble silicate species from the Si substrate.23,24 The mixture was also used as a source of water, which causes the conversion of the lamellar phase (lyotropic liquid crystal of hygroscopic surfactants composed of cetyltrimethylammonium chloride (CTMACl)) to the 2D-hexagonal phase. The formation of nanogrooves with smaller periodicity was also confirmed by the use of surfactants with a shorter alkyl chain. The alignment of nanogrooves at the bottom face of micrometer-scale grooves was also achieved.

Figure 1. SEM images of the sample after washing. Cross-sectional observation at tilted angles of (a) 8° and (b) −2°, and (c) surface observation.

RESULTS AND DISCUSSION The overall strategy of the concept and the experimental key steps are described in this paragraph. A Si(100) substrate was coated with a solution containing CTMACl by spin-coating (Scheme 1a). The LLC phase of CTMACl was horizontally aligned as a lamellar phase after spin-coating, as proven by the grazing-incidence small-angle X-ray scattering (GISAXS)

CTMACl by washing indicate the formation of nanogrooves. The cross-sectional images observed from a viewpoint parallel to the nanogrooves clearly show the symmetrical sawtooth-like profile. The SEM image of the surface of the sample (Figure 1c) indicates a striped pattern, the spacing of which is approximately 4.7−4.8 nm. A low-magnification image (Figure 5161

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replication of the outermost convex surfaces, and the depth of each nanogroove is approximately 2 nm. The GISAXS pattern of the uncalcined sample after washing (Figure 3a) shows a streak at 2θf = 1.87°, corresponding to the

S2 of the Supporting Information (SI)) indicates that the size of the domain consisting of nanogrooves oriented in one direction exceeds the sub-micrometer size range. This fact is appropriate because the domain sizes of surfactant-templated mesoporous thin films are known to be about tens of micrometer.25 Nanogrooves in multiple domains are randomly aligned with each other, as shown in the orientation histogram (Figure S3a). This also means that nanogrooves were formed on the entire surface of the substrate. The measurement of the orientation angles is described in the SI containing the related information (Figure S3b−d). Some discontinuity and bends of nanogrooves were observed, as shown in Figure 1a,c. The shape of nanogrooves is not perfectly linear, being different from the ideal straight shape, as shown in Scheme 1. The imperfectness of nanogrooves is explained later. The scanning transmission electron microscopy (STEM) images of the cross section of the sample (Figure 2a,b) can be

Figure 3. GISAXS patterns of (a) sample after washing and (b) CTMACl thin film recorded in a 60 °C, ca. 90% RH atmosphere. Inset: Primitive unit of 2D-hexgonal structure. The simulated diffraction patterns due to reflected and transmitted beam are overlaid as green and red squares, respectively.

d spacing of 4.7 nm.27 The d spacing value is consistent with the periodicity of the nanogrooves observed by SEM (Figure 1). Such streak patterns are observed in the GISAXS patterns of perpendicularly aligned lamellar structures of block copolymers whose configuration is similar to the nanogrooves in terms of the line patterns on the substrates.28,29 Similar GISAXS patterns were observed even at other points on the sample. The GISAXS pattern of the CTMACl thin film (thickness: 170 nm) in an atmospheric environment at 60 °C, with ca. 90% relative humidity (RH), similar to the conditions during the treatment with the NH3−water vapor mixture, is shown in Figure 3b. A GISAXS simulation software, NANOCELL,30 was used to explain the structure. The simulated diffraction patterns due to reflected and transmitted beam are overlaid as green and red squares, respectively, in Figure 3b. The space group of simulated pattern was supposed to be p6mm with a = 4.7 nm, b = 4.7 nm, and γ = 120° on the condition that the [01] is perpendicular to the substrate. The GISAXS pattern was almost coincident with the simulated one, which indicates that micelle configuration is a 2D-hexagonal (p6mm) structure with cylindrical micelles arrayed parallel to the substrate surface. The quaternary ammonium groups of the cylindrical micelles of CTMACl were expected to contact the wettable surface of the hydrophilic Si substrate. This was confirmed by the contact angle of water (ca. 20°). The spacing between the rod-like micelles in the in-plane direction was 4.7 nm, which was determined by the 2θf value of the (10) spot. This spacing is consistent with the periodicity of nanogrooves described above. Assuming that the LLC phase of CTMACl under the conditions mentioned above is unchanged even in the presence of NH3, the shape, size, and periodicity of the concavities imply that almost half of the outermost cylindrical micelles facing the substrate act as a template during the treatment. The amount of water absorbed in CTMACl also supports the formation of a 2D-hexagonal structure under water vapor treatment, as described below. After the exposure of powdery CTMACl to a humid condition (60 °C, ca. 90% RH), the weight concentration of CTMACl in the gel-like solid, as calculated from the weight gain, was 70 wt %. The experimental details are described in SI. According to the LLC diagram of CTMACl

Figure 2. (a,b) STEM images of the sample after washing, EDX elemental mappings of (c) O and (d) Si, and (e) STEM image of a sample calcined after washing.

divided into two regions (X and Y) with different contrasts perpendicular to the substrate. In a layer X, the symmetrical sawtooth-like profile was similarly observed on the surface, as found for the SEM images. The thickness of the layer X periodically varied from ca. 4 to 2 nm. The elemental mappings of Si and O atoms obtained by energy-dispersive X-ray spectroscopy (EDX) (Figure 2c,d) indicate that the layer X with the concave surface consists of Si and O atoms. Region Y comprises the Si substrate. Consequently, nanogrooves were formed on the ultrathin silicate layer. The silicate layer was thicker than the native oxide layer (ca. 1 nm) formed in air,26 which means that the adsorbed ammonia−water etched both the native oxide layer as well as silicon. Concavities were more clearly observed in the STEM image of another sample calcined after washing (Figure 2e). This was prepared in a different batch with almost identical experimental conditions, which indicates some tolerance of the conditions (e.g., the exposure time). The cross section of each nanogroove has an almost semicircular profile, strongly suggesting the 5162

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Nanogrooves are better aligned when the micrometer-scale grooves are used as a directional guide. The experimental details are described in the SI. This includes the preparation of nanogrooves at the bottom faces of 36 lines of micrometer-scale grooves (Figure S7a). One of the alignments is shown in the magnified SEM image of nanogrooves (Figure S7b). Figure 4

and water, the LLC phase adopts a 2D-hexagonal structure at a temperature of 60 °C and concentration of 70 wt %.31 Accordingly, the controlled amount of water in LLCs under the conditions facilitates the formation of stable LLC phases. A plausible mechanism of the formation of the silica nanogrooves is qualitatively understood as follows. First, water is absorbed into the CTMACl thin film due to its hygroscopic nature, and NH3 is dissolved in the absorbed water. Then, the LLC phase of CTMACl transforms from a lamellar phase into a 2D-hexagonal phase. Simultaneously, aqueous NH3 reaches the Si substrate through the intermicellar spaces. Then the Si substrate, including the native oxide layer, is etched under the basic condition.23 Silicon atoms with hydroxy groups were produced by hydrolysis of Si−H and Si−Si bonds, and consequently, soluble silicate species are formed and dissolved in the basic aqueous solution.32 The soluble silicate species generated from the Si substrate diffuse within the interface, where an aqueous phase is present between the Si substrate and the CTMACl micelles. Soluble silicate species may be trapped in a confined space between the micelles. Nanogrooves are then formed due to local silicification at the interspace with the increase in the concentration of the soluble silicate species. The reasons for the trapping of the silicate species are probably the strong interactions between the quaternary ammonium cations and the anionic silicate species and/or too narrow intermicellar spaces into which the hydrated silicate species would permeate.33 This mechanism is completely different from the vapor-phase synthesis of mesoporous silica thin films, which were fabricated by transporting inorganic sources such as tetraethoxysilane.34,35 There are a couple of possibilities that the formation of nanogrooves is not due to local silicification at the interspaces between the Si substrate and 2D-hexagonal LLCs. One possibility is that nanogrooves might be formed by washing of a multilayer assembly of silicate species and micelles formed supposedly after the treatment with the NH3−water vapor mixture. In order to exclude the possibility of the formation of multilayer assemblies, the sample after the treatment with the NH3−water vapor mixture (denoted as sample before washing) and the calcined one without washing (denoted as sample without washing) were analyzed. The GISAXS pattern of the sample before washing was recorded at room temperature and humidity and is shown in Figure S4. A streak pattern was observed at 2θf = 1.87°, which is the same 2θf value of the streak in Figure 3a, which suggests the formation of the nanogrooves at the step after the treatment with the NH3− water vapor mixture. A spot observed at αf = 2.9° and 2θf = 0° in Figure S4 indicates the structure of CTMACl thin film returns to lamellar after the treatment. This fact means that the 2D-hexagonal structure of CTMACl was not solidified with soluble silicate species during the treatment. The SEM image of the cross section of the calcined sample without washing indicates arrayed concave surface (Figure S5), which also means a multilayer mesostructure is not formed during the treatment. The second possibility is that silicate species might permeate into micelle assembly to form multilayer assembly of silicate species and micelles if the treatment time with the NH3−water vapor mixture was extended. However, the formed nanogroove structure and thickness of the silicate layer (Figure S6) are almost the same as those observed in Figures 1 and 2 when the treatment time was extended to 75 h. Therefore, it is appropriate that the formation of nanogrooves should be due to local silicification.

Figure 4. Low-magnification SEM image of a micrometer-scale groove and high-magnification SEM images of nanogrooves formed at the bottom face of the groove (images are 100 nm × 100 nm).

shows that nanogrooves formed on the entire bottom face of a micrometer-scale groove are highly aligned in the lengthwise direction. The orientation histogram (Figure S8) indicates that the nanogrooves are well-aligned parallel to the lengthwise direction of the large groove. Because the walls of the micrometer-scale grooves are wettable, micelles in LLCs are aligned parallel to the lengthwise direction to avoid contact between the alkyl chains and the walls. In addition to that, because the domain consisting of nanogrooves oriented in one direction is larger than sub-micrometer scale as mentioned before, a 5 μm wide groove could guide the alignment of nanogrooves. Therefore, well-aligned nanogrooves were formed with the aligned LLCs. Such an orientation control method was 5163

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MATERIALS AND METHODS

applied to the alignment of mesochannels of mesoporous thin films,36,37 and the driving force of alignment may be the same. For the strict alignment of nanogrooves, further study on the correlation between the feature size of micrometer-scale grooves and the orientation of nanogrooves is required. Because micelles are known to dynamically deform in an aqueous condition,38 the first layer of micelles dynamically bend or relocate during the vapor treatment until fixed with soluble silicate species. Inhomogeneous co-presence of dynamically deforming and fixed domains of micelles may cause the bended and disconnected nanogrooves at the boundary. The suppression of deformation of micelles is important for the improvement in the straightness of nanogrooves. The liquid crystals with rod-like shape confined in narrow grooves (e.g., 100 nm in width) are aligned parallel to the grooves, whereas such rods are bended under the conditions without confinement.10 Therefore, the fabrication of nanogrooves in such very narrow grooves will lead to much better uniaxial alignment and reduction of defects. The defectfree cylindrical nanopores are known to potentially attain a length more than 100 nm through the cooperative selfassembly of surfactant and silicate species.39 The combination of top-down and our bottom-up approaches may work to a much better nanopatterning. We have already reported that highly uniaxial orientation of rod-like micelles can be prepared using substrates with well-designed surfaces, like surfaces coated with rubbing-treated polyimide films.40,41 In addition, LLCs can be aligned by application of magnetic fields.42 Therefore, the alignment of nanogrooves on the entire substrate will be further developed by combining with other orientation techniques. When a surfactant with a shorter alkyl chain, lauryltrimethylammonium chloride, was used as a compound forming LLCs, the top-view SEM image and GISAXS patterns of the product suggest the formation of nanogrooves with a smaller periodicity of 3.7 nm (Figure S9a,b). Hygroscopicity of lauryltrimethylammonium chloride was also confirmed. It is expected that various hygroscopic quaternary ammonium surfactants can be applied as a template for the formation of nanogrooves with different patterns and periodicities. This newly developed concept will be further applied to the formation of nanopatterns. By etching the thinnest part of nanogrooves to expose a bare Si surface, ultrathin onedimensional silica lines will remain on a Si substrate, and those lines would be potentially used as a mask of the nanopattern. The idea on the formation of nanogrooves reported here will contribute to studies on surface function,43,44 including some hints toward one-dimensional assembly of nanoparticles and higher integration of electronic circuits.45

Materials. Cetyltrimethylammonium chloride (CTMACl) and lauryltrimethylammonium chloride (Wako Pure Chemical Industries, Ltd.) were used without further purification.46 Aqueous ammonia (Wako Pure Chemical Industries, Ltd.) was used as received. Si(100) substrates were purchased from Silicon Technology Co., Ltd. Semicoclean 23, used for cleaning and improvement in wettability of the surface of the Si substrates, was purchased from Furuuchi Chemical Corporation. Semicoclean 23 is an alkaline aqueous solution of