Control of wall thickness in the formation of ordered mesoporous silica films

Control of wall thickness in the formation of ordered mesoporous silica films

Thin Solid Films 515 (2007) 6521 – 6525 www.elsevier.com/locate/tsf Control of wall thickness in the formation of ordered mesoporous silica films San...

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Thin Solid Films 515 (2007) 6521 – 6525 www.elsevier.com/locate/tsf

Control of wall thickness in the formation of ordered mesoporous silica films Sang-Bae Jung, Tae-Jung Ha, Hyung-Ho Park ⁎ Department of Ceramic Engineering, Yonsei University, 134 Shinchon-Dong, Seodaemun-Ku, Seoul, 120-749, Korea Available online 10 January 2007

Abstract Ordered mesoporous silica thin films using block copolymer have drawn an attention for low-k application due to its ordered pore structure. From the respect of dielectric and mechanical properties of the film, there is trade-off between pore size and wall thickness. In this work, factors for increase of wall thickness were investigated. It was found that body-centered cubic structure was maintained irrespective of the concentration of catalytic acid. The catalytic acid thickens the framework wall because counterion reduces the repulsion force between silicic acids. The highly ordered mesoporous silica films were obtained although high concentration of acid was added to the silica sol. However, wormlike micelle exists more with high HCl concentration due to fast gellation rate. And excess water, which has the role similar to the humid atmosphere, also increases the thickness of silica wall. However, large amount of excess water at the micelle interface disrupts organic-inorganic electrostatic interaction. As a conclusion, optimization of HCl concentration in the silica sol and control of humidity during spin coating can simultaneously increase the framework thickness while maintaining the pore periodicity. © 2006 Elsevier B.V. All rights reserved. Keywords: Low-k; Ordered mesoporous silica films; Block copolymer; Silica wall

1. Introduction There has been an increasing interest on the ordered mesoporous material for the application of separation, catalysis, chemical sensing and optical coating since the discovery of MCM-41 mesoporous material [1]. Especially, ordered mesoporous material can be applied to low-k film in ultra large scaled integration devices in order to reduce interconnect RC time delay [2]. Synthesis procedure of mesoporous material involves the formation of organic-inorganic composites by self-assembly process, where organic surfactant forms a micelle surrounded by inorganic species on a mesoscopic scale and serves as a sacrificial template for the generation of mesopores. Especially, mesoporous material with thin film shape can be fabricated by evaporation-induced self-assembly (EISA) process. From the homogeneous solution of silica and surfactant dissolved in alcohol below critical micelle concentration, preferential evaporation of solvent during dip or spin coating drives cooperative self-assembly of silica and surfactant [3]. Structural characteristic of ordered mesoporous silica thin film can be described by mesostructure, pore size and wall thickness. Depending on the nature of templates, mesoporous ⁎ Corresponding author. Tel.: +82 2 2123 2853; fax: +82 2 365 5882. E-mail address: [email protected] (H.-H. Park). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.11.062

material can exhibit 2-dimensional hexagonal, 3-dimensional hexagonal or cubic mesostructure [4]. However, 3-dimensional hexagonal or cubic structure is more applicable to low-k dielectric than 2-dimensional structure. And pore size is the principal factor for lowering the dielectric constant of the film. Mechanical properties of the low-k film are also important because it should endure severe chemical mechanical polishing (CMP) process during planarization. In this respect, thickness of silica wall should be increased. So, there is trade-off between pore size and wall thickness for low-k application. Previously, our group reported ordered mesoporous silica thin film with body-centered cubic mesophase prepared using Brij-76 (C18H37 (OCH2CH2)10OH) block copolymer [5]. Generally, increase of block copolymer concentration can expand the pore diameter. However, the framework thickness reduces as the concentration of block copolymer increases. In the counterion-mediated selfassembly mechanism (S+ X– I+ ), catalytic acid affects the micelle aggregation behavior by means of charge contribution to the silica framework and block copolymer. So, it is anticipated that catalytic acid governs the polymerization behavior of the framework at the micelle interface. On the other hand, the catalytic activity is dependent on the amount of H2O. However, unintentional H2O from the atmosphere may diffuse into the film during EISA, which can involve the silica polymerization process [6]. So, the role of excess water during

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spin coating should be identified in order to maximize the merits of catalyst on the framework thickness. In this study, ordered mesoporous silica films using Brij-76 block copolymer were fabricated at the various HCl molar ratio and structural characteristic of the mesostructured silica film was elucidated. And excess H2O was intentionally added to the preaged silica sol in order to investigate the effect of water contents during EISA process. And then, structural characteristic of the mesostructured silica film was discussed in terms of humidity. 2. Experimental procedure Precursor solutions were prepared as follows. For changing the HCl concentration, solution A was prepared by dissolving Brij-76 block copolymer in ethyl alcohol (EtOH)–H2O–HCl solution. The concentration of acid was changed in solution A. And then, solution B was prepared by mixing tetraethoxyorthosilicate (TEOS) and EtOH followed by the addition of acid catalyst in H2O. After stirring solution B for a while, solution B was added to solution A. The final composition of TEOS:EtOH: H2O:HCl:Brij-76 was 1:20:5:0.01∼0.15:0.05. Silica sol was spin-deposited at 3000 rpm for 30 s after sol aging for one day at 30% humidity. In order to show the exact concentration of excess water that affects the mesophase formation, following procedure was performed instead of control of humidity. First, solution C was prepared by dissolving Brij-76 block copolymer in ethyl alcohol (EtOH)–H2O–HCl solution with the molar ratio of 0.05Brij76:15EtOH:1H2O:0.0028HCl. And then, the solution B was prepared in the same way. After stirring solution B for a while, solution B was added to solution C. Composition of TEOS: EtOH:H2O:HCl:Brij-76 was 1:20:5:0.01:0.05. This silica sol serves as basic sol. This sol was aged for one day, which is the optimized sol aging time in case of Brij-76/TEOS molar ratio of 0.05 [5]. After sol aging, excess water was added to the aged sol and the spin coating was immediately performed. The spin speed was 3000 rpm and spin coating time was 30 s at 30% humidity. The molar ratio of excess H2O/TEOS was changed from 0 to 10. Namely, maximum 15 mol of H2O per TEOS mole exist in the coating sol because of pre-existing 5 mol of H2O. Ordered mesoporous silica film could be fabricated by removing the block copolymer at 400 °C with a heating rate of 1 °C/min. For the investigation of interplanar spacing and structural ordering, X-ray powder diffraction (XRD) patterns were collected using Fe Kα radiation with wavelength of 1.9373 Å. The porosity of the films was calculated by measuring the critical angle of film using specular X-ray reflectivity at 3C2 beamline of Pohang Light Source (PLS) in Korea [7]. And glazing incidence small angle X-ray scattering (GISAXS) measurements were also carried out at 4C2 beamline of PLS for the identification of mesophase [8]. 3. Results and discussion Fig. 1 shows interplanar spacing variation of (011) plane (d011) in bcc structure for as-prepared and ordered mesoporous silica film as a function of HCl/TEOS molar ratio. It was

Fig. 1. d011 variation obtained from XRD patterns of as-prepared and ordered mesoporous silica films with various HCl/TEOS molar ratios.

observed that d011 was increased up to 0.05 HCl/TEOS molar ratio and saturated over 0.05 HCl/TEOS molar ratio. Up to 0.05 HCl/TEOS molar ratio, interplanar spacing is dependent on the degree of condensation reaction induced by the different sol composition during aging and concentration of H+ and Cl− ions. As H+ concentration increases, protonation of EO chains increases, resulting in an increase of hydrophilic volume and therefore decrease of g factor and micelle diameter [9]. On the other side, increase of counterion concentration reduces the repulsion force between silicic acids and finally, makes the framework thicker due to enhanced polymerization reaction at the interface. Furthermore, counterion contributes to the partial reduction in the electrostatic repulsion between the protonated EO chains. This increases the g factor and micelle diameter [10,11]. So, variation behavior of micelle diameter is unclear as HCl/TEOS molar ratio increases. However, it manifests that increase of acid concentration thickens the silica wall. Fig. 2 shows the porosity variations of ordered mesoporous silica film as a function of HCl/TEOS molar ratio. It can be observed that the porosity gradually decreases up to 0.05 HCl/TEOS molar ratio and then the porosity of the film is almost constant up to 0.15 HCl/TEOS molar ratio. This confirms the increase of silica wall thickness as discussed in Fig. 1. And over 0.05 HCl/TEOS molar ratio, the addition of HCl does not contribute to expand wall thickness. Fig. 3 shows GISAXS patterns of ordered mesoporous silica film with different HCl/TEOS molar ratio. Irrespective of molar ratio, the patterns are indexed with bcc structure. Existence of second or third order peaks means that the film has the highly ordered mesostructure even though HCl/TEOS molar ratio is extremely high such as 0.15 as shown in Fig. 3(c). The exact indexing of each spot is given elsewhere [5]. However, an extra diffused Debye ring pattern is more clearly found in Fig. 3(c) than Fig. 3(a) or (b). It is due to the fact that gellation rate of silica framework is fast when HCl concentration is high and as a result, kinetically trapped mesophase can exist in the interior of the film [12]. This result means that highly ordered mesoporous silica film with different wall thickness can be prepared using the sol with HCl/TEOS molar ratio between 0.01 and 0.05 while maintaining the mesostructure.

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decreases the g factor, which means that the mesostructured film prepared using the sol with high excess water has the more curved micelle interface [16]. In case of spherical micelle, the increase of curvature means the decrease of pore size. So, it is considered that the increase of silica wall thickness leads to the variation in d011. However, saturation with d011 over excess H2O/TEOS molar ratio of 6 means that there is a certain amount of water to contribute on the expansion of wall thickness. From the consideration of humidity during EISA process, disordered mesoporous silica film may be obtained if spin coating is performed under heavily wet atmosphere. However, it could be derived that HCl molar ratio of 0.01∼0.05 and humid atmosphere, which corresponds to the condition similar to the Fig. 2. Porosity variation of ordered mesoporous silica films with various HCl/ TEOS molar ratios.

Fig. 4 shows GISAXS patterns of as-prepared silica film with different excess H2O/TEOS molar ratio. Two diffraction spots with vertical alignment can be shown and these correspond to diffraction spots induced by transmitted and reflected beam [13]. As shown in Fig. 4(a), excess H2O/TEOS molar ratio of 2 does not modify the bcc mesophase of asprepared silica film. However with excess H2O/TEOS molar ratio of 4, a diffused Debye ring pattern was observed instead of clear spot pattern as shown in Fig. 4(b). Fig. 5 shows d011 variation and FWHM of the corresponding XRD peak as a function of excess H2O/TEOS molar ratio for as-prepared and ordered mesoporous silica film. It was observed that FWHM was greatly increased with excess H2O/TEOS molar ratio as shown in Fig. 5(a) and (b). From the GISAXS pattern and FWHM values of the XRD peak, it can be concluded that the films using the sol over excess H2O/TEOS molar ratio of 4 are not highly ordered. The degree of condensation reaction is not different because sol composition is the same during sol aging. On the other hand, it is generally accepted that excess water, which does not participate in the sol-gel reaction, resides at the micelle interface between organic and inorganic species during structuration [14]. The spun-on film using the sol with large amount of water has the residual water more than that using the sol with small amount of water due to the low vapor pressure of water. Fast evaporation of all volatile constituents during EISA is important for the synthesis of highly ordered mesophase. And the addition of excess water in the pre-aged sol has the role of diluting the concentration of the organic and inorganic species. Furthermore, pH of the sol with excess H2O/TEOS molar ratio of over 4 is larger than 2, which is the isoelectric point of silica. Due to these reasons, structuration is impeded for the sol with excess H2O/TEOS molar ratio of over 4. It was also observed that d011 gradually increased and saturated over excess H2O/ TEOS molar ratio of 6 for as-prepared and ordered mesoporous silica films as shown in Fig. 5. Mesostructured silica film in acidic condition could be synthesized through I+X–(S0H+) mechanism and interplanar spacing is the sum of corona and silica wall thickness and pore diameter [15]. Existence of water with high polar characteristic at the micelle interface increases the distance between headgroup by steric hindrance and

Fig. 3. GISAXS patterns of ordered mesoporous silica films with HCl/TEOS molar ratios of (a) 0.015, and (b) 0.05, and (c) 0.15.

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addition of excess H2O/TEOS molar ratio of 2, are the best conditions for increasing wall thickness with maintaining the mesostructure. The variation of pore diameter and wall thickness can be indirectly confirmed by the shrinkage ratio of d011 and porosity of the film. Fig. 6(a) shows the shrinkage ratio of d011 for the mesostructured silica film during calcination according to the excess H2O/TEOS molar ratio and Fig. 6(b) shows the porosity variation of ordered mesoporous silica film with excess H2O/ TEOS molar ratio. It can be observed that the shrinkage ratio and porosity variation according to the molar ratio show the same tendency. When an excess H2O/TEOS molar ratio increased up to 2, the porosity was decreased from 39.2 to 28.9% as shown in Fig. 6(b). This porosity reduction is related to the increase of pore wall thickness as discussed in Fig. 5. In case of pore arrangement with bcc symmetry, mesoporosity can be geometrically calculated as follows [17]. p ¼ 8=3  p  ðR=aÞ3

Fig. 4. GISAXS patterns of as-prepared silica film with excess H2O/TEOS molar ratio of (a) 2 and (b) 4.

Fig. 5. d011 and FWHM variation obtained from XRD patterns of (a) as-prepared and (b) ordered mesoporous silica films with various excess H2O/TEOS molar ratios.

ð1Þ

where p is the mesoporosity, R is the pore radius and a is the lattice parameter of bcc structure. From the result of Fig. 5(b) and Eq. (1), about 10% difference of porosity cannot simply be

Fig. 6. (a) Shrinkage ratio of d011 for mesostructured silica films during calcination and (b) porosity variation of ordered mesoporous silica films with various excess H2O/TEOS molar ratios.

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explained by the increase of wall thickness. Instead, decrease of pore diameter and increase of pore wall thickness is more plausible explanation from the observation of the shrinkage ratio and porosity variation. The thick-walled mesoporous silica film with excess H2O/TEOS molar ratio of 2 can maintain its porous nature during calcination. As a result, the shrinkage ratio reduced as shown in Fig. 6(a). However, it was observed that the porosity and shrinkage ratio gradually increased when the excess H2O/TEOS molar ratio was larger than 2. It was mentioned that the residual water at the micelle interface increases the d011 for as-prepared silica film. From this fact, the film using the sol with large excess water content does not have dense framework due to the existence of water at the micelle interface. As a result, the shrinkage ratio increases due to the less densely structured framework and the porosity increases due to the microporous nature of the silica framework after calcination.

synthesized and the porosity was increased due to the microporous nature of the silica framework. From the consideration of humidity during EISA process, disordered mesoporous silica film might be obtained if the spin coating was performed under wet atmosphere. As a conclusion, optimization of HCl concentration in the silica sol and control of humidity during EISA can simultaneously increase the framework thickness while maintaining the pore periodicity.

4. Conclusions

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In this work, mesostructured silica films with different excess water and catalytic acid concentration were prepared and the properties of the films were investigated. It was found that interplanar spacing increased and then saturated over 0.05 of HCl/TEOS molar ratio. The porosity was gradually decreased with increase of HCl concentration. As a result, it is concluded that catalytic acid thickens the framework because counterion reduces the repulsion force between silicic acids, resulting in increase of polymerization reaction. Interestingly, the highly ordered mesophase was obtained even though catalytic acid with high concentration was added to the silica sol. However, wormlike micelle exists more in case of high HCl concentration due to fast gellation rate. This result means that highly ordered mesoporous silica film with different wall thickness can be prepared using the sol with HCl/TEOS molar ratio between 0.01 and 0.05. And the interplanar spacing increased and saturated over excess H2O/TEOS molar ratio of 6. Up to the excess H2O/ TEOS molar ratio of 2, silica framework with thick wall was synthesized with structural ordering. However, the excess H2O/ TEOS molar ratio over 4 resides at the micelle interface and impedes the structuration process due to low evaporation rate of water, dilution effect and pH of the sol. The films using the sol over excess 4 H2O/TEOS molar ratio was not properly

Acknowledgements The authors would like to acknowledge the financial support from University Research Program supported by Ministry of Information and Communication in republic of Korea (B12200501-0077). References