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Sol–gel derived mesoporous silica films using amphiphilic triblock copolymers
Journal of Non-Crystalline Solids 332 (2003) 199–206 www.elsevier.com/locate/jnoncrysol
Sol–gel derived mesoporous silica films using amphiphilic triblock copolymers Nobuaki Kitazawa *, Hideyoshi Namba, Masami Aono, Yoshihisa Watanabe Department of Materials Science and Engineering, National Defense Academy, 1-10-20 Hashirimizu, Yokosuka, Kanagawa 239-8686, Japan Received 25 February 2003
Abstract Mesoporous silica films have been synthesized by modifying the sol–gel method in the presence of poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) block copolymers as structure-directing agents. Depending on the types of block copolymers, mesoporous silica films with different mesostructures, pore sizes and porosities have been formed. When block copolymers having relatively low average molecular weights were used, evaporation induced self-assembly was observed and then calcination induced structural transformation occurred. While only evaporation induced self-assembly was observed when block copolymers with larger average molecular weights were used. Pore sizes of the films increased with increasing the average molecular weight of block copolymers. Chemical modification using hexamethyldisilazane vapor made it possible to obtain the hydrophobic pore surface of the films. The dielectric constant of the films fairly depended on the porosity of the films. The dielectric constants of about 1.9–2.1 were obtained for the films with the porosity of about 65 vol.%. Ó 2003 Elsevier B.V. All rights reserved. PACS: 81.20.F; 68.55.J; 83.70; 77.55
1. Introduction Ever since the discovery of the MCM family of mesoporous silicates by researchers at Mobil Oil Corporation [1,2], self-assembly of amphiphilic molecules has played a prominent role in sol–gel chemistry for creating interesting materials with ordered nanostructures. Self-assembly is defined as
*
Corresponding author. Tel.: +81-46 841 3810x3667; fax: +81-46 844 5910. E-mail address: [email protected] (N. Kitazawa).
the spontaneous organization of amphiphilic molecules via non-covalent interactions, such as hydrogen bonding, Van der Waals forces, electrostatic forces and p–p interactions, without an external intervention [3]. A variety of mesoporous materials with different compositions and mesostructures has been synthesized in the form of powders through a co-condensation of inorganic species in conjunction of surfactants [4–8]. A reliable processing of mesoporous materials as thin films, therefore, has been a subject of considerable interests because thin film materials open a wide range of opportunities for use in electrical, chemical, optical and mechanical
0022-3093/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2003.09.007
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applications [9–12]. Recently, evaporation-induced self-assembly of surfactants during sol–gel thin film deposition makes it possible to create mesoporous thin film materials [13–15]. Mesoporous silica (SiO2 ) films with high porosities, well-ordered and controlled pore sizes are potentially useful as an interlayer dielectric material for advanced semiconductor devices that require a low dielectric constant [12,14,16–19]. It is generally accepted that dielectric constants of mesoporous silica films are considerably dependent on the porosity and pore surface state of the films [14,17]. The specific mechanism for the formation of mesostructures is the current subject of discussions, but different mesostructures can be obtained by varying the synthetic conditions. Therefore it is possible to control the mesostructure, pore size and porosity of the films. In this article, mesoporous silica films have been synthesized by modifying sol– gel method in the presence of block copolymer surfactants as structure-directing agents, and their dielectric properties have been investigated.
2. Experimental procedure Mesoporous silica films were prepared by modifying sol–gel method in the presence of block copolymer surfactants under acidic conditions. Noted that the sols and as-deposited films were prepared under an nitrogen atomosphere. Reagent grade tetramethyl orthosilicate (Si(OCH3 )4 , abbreviated as TMOS hereafter), methyl alcohol (CH3 OH) hydrochlolic acid (35.5wt% HCl-aq) and distilled water were used. As structure-directing agents, poly(ethylene oxide)-blockpoly(propylene oxide)-block-poly(ethylene oxide) block copolymers (PEO-PPO-PEO), were used as follows; Pluronic P65 ðMav ¼ 3400Þ, EO20 PO30 EO20 ; Pluronic P123 ðMav ¼ 5800Þ, EO20 PO70 EO20 ; Pluronic F68 ðMav ¼ 8400Þ, EO80 PO30 EO80 ; Pluronic F127 ðMav ¼ 12 600Þ, EO106 PO70 EO106 . TMOS (2.28 g) was added to the mixture of water (0.27 g), HCl-aq (0.75 g) and CH3 OH (4.80 g). After being hydrolyzed for 60 min, the mixture of distilled water (1.90 g) and CH3 OH (4.80 g), and the surfactants (0.285–2.28 g) were added to the sols. The weight ratio of surfactant/TMOS varied
from 0.125 to 1.00. Clear and stable sols without any precipitation were obtained. The favorable molar ratio of TMOS:CH3 OH:HCl:H2 O was found to be 1:20:0.5:8. The sols were then aged under stirring for 24 h and finally used for the deposition of films on various substrates (vitreous silica, ITO-coated corningâ 7059 glass and Si(1 0 0) wafer) by a spin-coating method at 2500 rpm for 1 min. The as-deposited samples were dried at 100 °C for 12 h and then heated slowly (1 °C/min) to 500 °C in air to burn out the surfactants and stabilize the structure. The samples were kept for 4 h and then cooled slowly (1 °C/min) to room temperature. After heat-treatment, samples were subsequently exposed with hexamethyldisilazane ((CH3 )3 –Si–NH–Si–(CH3 )3 , abbreviated as HMDS hereafter, Aldrich) vapors in sealed glass containers for 24 h at room temperature. The calcined samples before and after HMDS treatment were stored in a dried box at room temperature. The mesostructure of the films were characterized by using X-ray diffraction (XRD, Rigaku, RINT2000 diffractometer) with Cr Ka radiation (40 kV, 60 mA). Field emission transmission electron microscopy observation was made for the calcined films operating at 200 kV. The pieces of the films were scratched from Si(1 0 0) substrates and were deposited on cupper grids. Fourier transformed infrared spectra of the as-dried and calcined films on Si(1 0 0) substrates were measured. The film thickness and refractive index of the calcined films on Si(1 0 0) substrates were measured by using an ellipsometry at 632.8 nm in wavelength and at an incident angle of 70°. The capacitance of the films deposited on ITO-coated glass substrates was measured using an LCZ meter. The frequency and oscillation level in this study were 1 MHz and 100 mV, respectively. The dielectric constant was calculated from the capacitance, the film thickness and the area of the metal electrode.
3. Results 3.1. Thin film formation The structural characterization of the samples was estimated using X-ray diffraction. Fig. 1 shows
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Fig. 1. X-ray diffraction patterns of the as-deposited, as-dried and calcined films on silica glass substrates using (a) Pluronic P65, (b) P123, (c) F68 and (d) F127 as the structure-directing agents. The weight ratio of the block copolymers/TMOS fixed at 0.5.
the typical XRD patterns of the as-deposited, asdried and calcined films on silica glass substrates using (a) Pluronic P65, (b) P123, (c) F68 and (d) F127 as the structure-directing agents. Noted that the weight ratio of the block copolymers/TMOS fixed at 0.5. In Fig. 1(a), strong and narrow (1 0 0) Bragg diffraction peak associated with an onedimensional (1D) hexagonal mesostructure (unit ) was clearly observed for cell parameter, a ¼ 77 A the as-deposited film. On the contrary, four diffraction peaks were observed for the calcined film in the 2h range of 1–5°, which can be indexed as (2 0 0), (2 1 0), (2 2 0) and (2 2 2) reflections of a . When Plucubic mesostructure with a ¼ 139 A
ronic P123 was used shown in Fig. 1(b), sharp (1 0 0), (2 0 0) and (3 0 0) diffraction peaks associated with the formation of a two-dimensional (2D) ) were observed for hexagonal structure (a ¼ 113 A the as-deposited film. After calcination at 500 °C, the XRD pattern showed five reflections in the 2h range of 1–5°. These peaks can also be indexed as (2 0 0), (2 1 0), (2 1 1), (2 2 0) and (2 2 2) reflections . Comof a cubic mesostructure with a ¼ 127 A pared with the as-deposited and calcined films using Pluronic P65 and P123, these films exhibited different mesostructures. The specific mechanism observed here is unknown at this point, but this phenomenon can probably be attributed to
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calcination-induced structural transformation [15]. When block copolymers with a relatively larger average molecular weight such as Pluronic F68 ðMav ¼ 8400Þ and F127 ðMav ¼ 12; 600Þ were used, the as-deposited films showed no typical X-ray Bragg diffraction (see Fig. 1(c) and (d)). Mesostructured films were obtained after drying. In Fig. 1(c), hexagonal structured thin films were obtained after drying, and the (1 0 0) reflection of the film shifted continuously towards high diffraction angle with increasing heat-treatment temperature. A cubic mesostructured thin film was obtained when Pluronic F127 was used, shown in Fig. 1(d). The as-dried film showed (1 1 0), (2 0 0), (2 1 1) and (2 2 2) reflections of a cubic mesostructure (a ¼ 147 ). Strong (2 0 0) Bragg reflection associated with A ) was observed after cala cubic phase (a ¼ 138 A cination. The above estimation is supported by FE-TEM observation. Fig. 2 shows the FE-TEM images of the calcined films on Si(1 0 0) substrates using (a) Pluronic P65, (b) P123, (c) F68 and (d) F127 as the structure-directing agents. In these pictures, white part shows the pores and black part represents the skeleton of silica matrices. The TEM images of the calcined films confirm small pore sizes less than 10 nm and an excellent uniformity. Pore sizes of the films increased with increasing the average molecular weight of the surfactants. In the pictures (a), (b) and (d), the pores are randomly and continuously arranged, suggesting the formation of a cubic mesostructure. When Pluronic F68 was used, on the contrary, a highly oriented 1D hexagonal structure was clearly observed. Thus, TEM observation for the calcined films showed good
agreement with the XRD experiment as shown in Fig. 1. However, it was impossible to observe the as-deposited films because the films were easily dissolved into solvents used for TEM observation. The effect of the surfactant content on the mesostructure formation was examined. Fig. 3 shows the X-ray diffraction patterns of the calcined films prepared with different surfactant concentrations. In each case, the X-ray diffraction intensity in the 2h range of 1–5° increased with increasing the surfactant content and showed maximum at surfactant/TMOS ¼ 0.50 for Pluronic P65, P123 and F68, and at 0.75 for F127 (weight ratio), then decreased. From these results, the surfactant concentration in the sols appreciably affected the mesostructure formation. 3.2. Pore surface treatment using HMDS Generally, a pore surface of calcined films is covered with polar hydroxyl (–OH) groups. Apparently, such hydrophilic surface is not suitable for low-k dielectric materials. Calcined films were then subjected to dehydroxylation treatment using HMDS vapor. Fig. 4 shows the typical Fourier transformed infrared (FT-IR) spectra for the films before and after HMDS treatment. The absorption band at around 1100 cm1 can be assigned to the stretching vibration of Si–O–Si framework. The as-dried film shown in (a), the absorption bands due to the stretching vibration of hydroxyl and alkyl (–Cn H2nþ1 ) groups were observed at around 3500 and 2900 cm1 , respectively. For the film after heat-treatment shown in (b), the absorption band of alkyl groups disappeared due to
Fig. 2. FE-TEM pictures of the calcined films on Si(1 0 0) substrates using (a) Pluronic P65, (b) P123, (c) F68 and (d) F127 as the structure-directing agents.
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Fig. 3. XRD patterns of the calcined films prepared with different block copolymer/TMOS ratios.
burning out of the block copolymer. However, the absorption band of hydroxyl groups still remained, indicating the presence of hydroxyl groups in the film. After HMDS treatment, the absorption intensity of hydroxyl groups decreased drastically. Also, the absorption band of alkyl groups observed slightly at around 2950 cm1 . Therefore, the hydrophobic surface can be obtained by exposing the films to HMDS vapors. 3.3. Dielectric properties of the mesoporous SiO2 films Fig. 4. FT-IR spectra of the (a) as-dried and calcined films on Si(1 0 0) substrates (b) before and (c) after HMDS treatment. Noted that Pluronic P65 was used as the structure-directing agent.
Capacitance measurements for the calcined samples before and after HMDS treatment were performed. Prior to the measurements, the refractive
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index and porosity of the films were estimated using an ellipsometry. The thickness, refractive index and porosity of the calcined films before HMDS treatment are listed in Table 1. The film thickness chan (Pluronic P65) to 4500 A (Pluronic ged from 2200 A F127) with errors of about ±100 A depending on the average molecular weight of the surfactants. The refractive index of the corresponding films varied from 1.152 to 1.293 with errors of ±0.005–0.01. The porosity, P (vol.%), of the films was estimated using the Lorentz–Lorentz equation: ðn2c 1Þ=ðn2c þ 2Þ ¼ ð100 P Þðn20 1Þ=ðn20 þ 2Þ; where n0 is the refractive index of a vitreous silica ðn0 ¼ 1:4585Þ and nc is the measured values of the films. Using the equation, porosities of about 65 vol.% (P65), 34 vol.% (P123), 47 vol.% (F68) and 42 vol.% (F127) were obtained for the films after calcination. Table 2 shows the dielectric constant of the calcined films before and after HMDS treatment. The dielectric constant of the as-calcined films showed 2.19–2.49 (Pluronic P65), 4.31–4.49 (Pluronic P123), 2.96–3.06 (Pluronic F68) and 2.70– 2.90 (Pluronic F127), respectively. When Pluronic P123 was used, the estimeted values were larger Table 1 The thickness, refractive index and porosity of the calcined films Template
Film ) thicknessa (A
Refractive index, n
Porosity, P (vol.%)
Pluronic Pluronic Pluronic Pluronic
2200 ± 100 2500 ± 50 3000 ± 100 4500 ± 100
1.152 ± 0.01 1.293 ± 0.005 1.226 ± 0.01 1.248 ± 0.005
65 34 47 42
a
P65 P123 F68 F127
Estimated using a surface profilometer.
Table 2 The dielectric constant of the calcined films before and after HMDS treatment Template
Dielectric constant at 1 MHz As-calcined
Pluronic Pluronic Pluronic Pluronic
P65 P123 F68 F127
2.34 ± 0.15 4.40 ± 0.09 3.01 ± 0.05 2.80 ± 0.10
After HMDS treatment 2.00 ± 0.10 3.28 ± 0.30 2.45 ± 0.20 2.19 ± 0.10
Fig. 5. Change in the dielectric constant of the calcined films after HMDS treatment as a function of porosity.
than that of a dense SiO2 . This can be explained by the significant number of polar –OH groups. After HMDS treatment, the dielectric constant of each film decreased. Especially, dielectric constants about 1.90–2.10 were obtained for the mesoporous SiO2 films after removal of Pluronic P65 surfactant. Change in the dielectric constant of the calcined films as a function of porosity is shown in Fig. 5. As seen in the figure, the dielectric constant of the films fairly correlated with the porosity. In the films using Pluronic P65, P123 and F68, the estimated values agreed with the correlation for SiO2 aerogels, k ¼ 1 þ 7:1ðnc 1Þ [20]. Despite the low porosity (40 vol.%) of the films using Pluronic F127, the films showed low dielectric constants. From the TEM observation shown in Fig. 2, relatively larger pore sizes about 10 nm were observed than those of other films. The pore size of the film is, therefore, one of the possible factors of which affect the dielectric properties. 4. Discussion There is general consensus that evaporation-induced self-assembly of surfactants stimulates both the formation and ordering of micelles [3]. In
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aqueous solution, surfactants usually form into micelles with a spherical or cylindrical shape at above their critical micelle concentration (cmc). These micelles assemble spontaneously into a hexagonal, cubic or lamellar phases further increase in the surfactant concentration [21]. In sol–gel templating process using PEO–PPO–PEO block copolymers as structure-directing agents, homogeneous precursor solutions consisting of inorganic species, organic solvents, water, acid catalyst and a block copolymer surfactant are used. In this case surfactant concentrations in precursor solutions, c, are quite below cmc ðc cmcÞ. Therefore, surfactants are present as free-surfactants. During thin film processing such as dipping, spinning and casting, the preferential evaporation of organic solvents occurs, which increases the surfactant concentration as well as water, acid catalysts and silicate spices. Silicate networks are also formed simultaneously. Since PEO–PPO–PEO block copolymers consist of the hydrophilic PEO block and the hydrophobic PPO block under acidic conditions, positively charged silicate species preferentially interact with the hydrophilic PEO part of the block copolymer via both the electrostatic and hydrogen bonding interactions [5,22]. At some critical point corresponding to the cmc, both the block copolymers and silicate networks start to coassemble. As one can see, the preferential evaporation of solvents occurs at the air/film interface within a short period of time. Assuming that the degree of condensation is not enough after drying, a structural transformation of silicate networks induced by the re-arrangement of block copolymers is probably possible because of the low film viscosity. Indeed, calcination induced structural transformation was also observed. Thus, the degree of interactions between the PEO blocks and silicate species changes, which results in calcination induced structural transformation as well as evaporation induced self-assembly. However, owing to the complexity as described above, it is difficult to fully understand the templating mechanism at this stage during sol–gel thin film deposition. According to references, the EO/PO ratio in the PEO–PPO–PEO block copolymers has a large effect on the formation of the mesostructure [4,5,14]. Cubic mesostructured silica films have been syn-
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thesized using PEO–PPO–PEO block copolymers with higher EO/PO ratios above 1.5, e.g. Pluronic F127 [14]. Hexagonal mesostructured silica films have been formed when the EO/PO ratios are 0.2– 1.5, e.g. Pluronc P65 and P123 [14]. As shown in Fig. 1, cubic mesostructured silica films were obtained after calcination using Pluronic P65, P123 and F127. When Pluronic F68 was used, in contrast, a hexagonal mesostructured silica film was formed. In this study, TMOS and CH3 OH were used as raw materials, respectively. On the contrary, the authors in Ref. [14] have been prepared the films using Si(OC2 H5 )4 and C2 H5 OH. Therefore, not only the EO/PO ratios of PEO-PPO-PEO block copolymers but also the raw materials can be affected the mesostructure formation. Only qualitative information is available at this point, when PEO–PPO–PEO block copolymers with the EO/PO ratios below 1.5 are used, hexagonal mesostructured silica films can be prepared using Si(OC2 H5 )4 and C2 H5 OH as raw materials. Cubic mesostructured silica films have also been formed using Pluronic F68 and F127. There are many synthetic parameters of which affect the mesostructure formation; e.g., compositions, surrounding mediums, types of surfactants, raw materials used, preparation conditions such as aging time of sols, coating and drying atmosphere, etc. And ideally, mesoporous SiO2 films with a high porosity and closed pore should be prepared for improved reliability. More detailed studies on the synthesis of mesoporous silica films and their dielectric properties are being carried out and will be described in a separate publication.
5. Conclusions In this study, mesoporous silica films have been synthesized by modifying sol–gel method in the presence of poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) block copolymers as structure-directing agents. Depending on the types of block copolymers, mesoporous silica films with different mesostructures, pore sizes and porosities have been formed. Pore sizes of the films increased with increasing the average
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molecular weight of block copolymers. Cubic structured mesoporous silica films have been synthesized using Pluronic P65, P123 and F127. On the contrary, hexagonal structured mesoporous films have been obtained using Pluronic F68. Both evaporation induced self-assembly and calcination induced structural transformation were observed when block copolymers having relatively low average molecular weights, such as Pluronic P65 and P123, were used. While only evaporation induced self-assembly was observed when block copolymers with larger average molecular weights were used. By exposing the films to hexamethyldisilazane vapors, the hydrophobic pore surface can be obtained. The dielectric constant of the films strongly depended on the porosity of the films. The dielectric constants of about 1.9–2.1 were obtained for the films with the porosity of about 65 vol.%, after dehydroxylation treatment.
Acknowledgement The authors would like to thank BASF (Mt. Olive, NJ) for providing the block copolymer surfactants.
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Sol–gel derived mesoporous silica films using amphiphilic triblock copolymers Nobuaki Kitazawa *, Hideyoshi Namba, Masami Aono, Yoshihisa Watanabe Department of Materials Science and Engineering, National Defense Academy, 1-10-20 Hashirimizu, Yokosuka, Kanagawa 239-8686, Japan Received 25 February 2003
Abstract Mesoporous silica films have been synthesized by modifying the sol–gel method in the presence of poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) block copolymers as structure-directing agents. Depending on the types of block copolymers, mesoporous silica films with different mesostructures, pore sizes and porosities have been formed. When block copolymers having relatively low average molecular weights were used, evaporation induced self-assembly was observed and then calcination induced structural transformation occurred. While only evaporation induced self-assembly was observed when block copolymers with larger average molecular weights were used. Pore sizes of the films increased with increasing the average molecular weight of block copolymers. Chemical modification using hexamethyldisilazane vapor made it possible to obtain the hydrophobic pore surface of the films. The dielectric constant of the films fairly depended on the porosity of the films. The dielectric constants of about 1.9–2.1 were obtained for the films with the porosity of about 65 vol.%. Ó 2003 Elsevier B.V. All rights reserved. PACS: 81.20.F; 68.55.J; 83.70; 77.55
1. Introduction Ever since the discovery of the MCM family of mesoporous silicates by researchers at Mobil Oil Corporation [1,2], self-assembly of amphiphilic molecules has played a prominent role in sol–gel chemistry for creating interesting materials with ordered nanostructures. Self-assembly is defined as
*
Corresponding author. Tel.: +81-46 841 3810x3667; fax: +81-46 844 5910. E-mail address: [email protected] (N. Kitazawa).
the spontaneous organization of amphiphilic molecules via non-covalent interactions, such as hydrogen bonding, Van der Waals forces, electrostatic forces and p–p interactions, without an external intervention [3]. A variety of mesoporous materials with different compositions and mesostructures has been synthesized in the form of powders through a co-condensation of inorganic species in conjunction of surfactants [4–8]. A reliable processing of mesoporous materials as thin films, therefore, has been a subject of considerable interests because thin film materials open a wide range of opportunities for use in electrical, chemical, optical and mechanical
0022-3093/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2003.09.007
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applications [9–12]. Recently, evaporation-induced self-assembly of surfactants during sol–gel thin film deposition makes it possible to create mesoporous thin film materials [13–15]. Mesoporous silica (SiO2 ) films with high porosities, well-ordered and controlled pore sizes are potentially useful as an interlayer dielectric material for advanced semiconductor devices that require a low dielectric constant [12,14,16–19]. It is generally accepted that dielectric constants of mesoporous silica films are considerably dependent on the porosity and pore surface state of the films [14,17]. The specific mechanism for the formation of mesostructures is the current subject of discussions, but different mesostructures can be obtained by varying the synthetic conditions. Therefore it is possible to control the mesostructure, pore size and porosity of the films. In this article, mesoporous silica films have been synthesized by modifying sol– gel method in the presence of block copolymer surfactants as structure-directing agents, and their dielectric properties have been investigated.
2. Experimental procedure Mesoporous silica films were prepared by modifying sol–gel method in the presence of block copolymer surfactants under acidic conditions. Noted that the sols and as-deposited films were prepared under an nitrogen atomosphere. Reagent grade tetramethyl orthosilicate (Si(OCH3 )4 , abbreviated as TMOS hereafter), methyl alcohol (CH3 OH) hydrochlolic acid (35.5wt% HCl-aq) and distilled water were used. As structure-directing agents, poly(ethylene oxide)-blockpoly(propylene oxide)-block-poly(ethylene oxide) block copolymers (PEO-PPO-PEO), were used as follows; Pluronic P65 ðMav ¼ 3400Þ, EO20 PO30 EO20 ; Pluronic P123 ðMav ¼ 5800Þ, EO20 PO70 EO20 ; Pluronic F68 ðMav ¼ 8400Þ, EO80 PO30 EO80 ; Pluronic F127 ðMav ¼ 12 600Þ, EO106 PO70 EO106 . TMOS (2.28 g) was added to the mixture of water (0.27 g), HCl-aq (0.75 g) and CH3 OH (4.80 g). After being hydrolyzed for 60 min, the mixture of distilled water (1.90 g) and CH3 OH (4.80 g), and the surfactants (0.285–2.28 g) were added to the sols. The weight ratio of surfactant/TMOS varied
from 0.125 to 1.00. Clear and stable sols without any precipitation were obtained. The favorable molar ratio of TMOS:CH3 OH:HCl:H2 O was found to be 1:20:0.5:8. The sols were then aged under stirring for 24 h and finally used for the deposition of films on various substrates (vitreous silica, ITO-coated corningâ 7059 glass and Si(1 0 0) wafer) by a spin-coating method at 2500 rpm for 1 min. The as-deposited samples were dried at 100 °C for 12 h and then heated slowly (1 °C/min) to 500 °C in air to burn out the surfactants and stabilize the structure. The samples were kept for 4 h and then cooled slowly (1 °C/min) to room temperature. After heat-treatment, samples were subsequently exposed with hexamethyldisilazane ((CH3 )3 –Si–NH–Si–(CH3 )3 , abbreviated as HMDS hereafter, Aldrich) vapors in sealed glass containers for 24 h at room temperature. The calcined samples before and after HMDS treatment were stored in a dried box at room temperature. The mesostructure of the films were characterized by using X-ray diffraction (XRD, Rigaku, RINT2000 diffractometer) with Cr Ka radiation (40 kV, 60 mA). Field emission transmission electron microscopy observation was made for the calcined films operating at 200 kV. The pieces of the films were scratched from Si(1 0 0) substrates and were deposited on cupper grids. Fourier transformed infrared spectra of the as-dried and calcined films on Si(1 0 0) substrates were measured. The film thickness and refractive index of the calcined films on Si(1 0 0) substrates were measured by using an ellipsometry at 632.8 nm in wavelength and at an incident angle of 70°. The capacitance of the films deposited on ITO-coated glass substrates was measured using an LCZ meter. The frequency and oscillation level in this study were 1 MHz and 100 mV, respectively. The dielectric constant was calculated from the capacitance, the film thickness and the area of the metal electrode.
3. Results 3.1. Thin film formation The structural characterization of the samples was estimated using X-ray diffraction. Fig. 1 shows
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Fig. 1. X-ray diffraction patterns of the as-deposited, as-dried and calcined films on silica glass substrates using (a) Pluronic P65, (b) P123, (c) F68 and (d) F127 as the structure-directing agents. The weight ratio of the block copolymers/TMOS fixed at 0.5.
the typical XRD patterns of the as-deposited, asdried and calcined films on silica glass substrates using (a) Pluronic P65, (b) P123, (c) F68 and (d) F127 as the structure-directing agents. Noted that the weight ratio of the block copolymers/TMOS fixed at 0.5. In Fig. 1(a), strong and narrow (1 0 0) Bragg diffraction peak associated with an onedimensional (1D) hexagonal mesostructure (unit ) was clearly observed for cell parameter, a ¼ 77 A the as-deposited film. On the contrary, four diffraction peaks were observed for the calcined film in the 2h range of 1–5°, which can be indexed as (2 0 0), (2 1 0), (2 2 0) and (2 2 2) reflections of a . When Plucubic mesostructure with a ¼ 139 A
ronic P123 was used shown in Fig. 1(b), sharp (1 0 0), (2 0 0) and (3 0 0) diffraction peaks associated with the formation of a two-dimensional (2D) ) were observed for hexagonal structure (a ¼ 113 A the as-deposited film. After calcination at 500 °C, the XRD pattern showed five reflections in the 2h range of 1–5°. These peaks can also be indexed as (2 0 0), (2 1 0), (2 1 1), (2 2 0) and (2 2 2) reflections . Comof a cubic mesostructure with a ¼ 127 A pared with the as-deposited and calcined films using Pluronic P65 and P123, these films exhibited different mesostructures. The specific mechanism observed here is unknown at this point, but this phenomenon can probably be attributed to
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calcination-induced structural transformation [15]. When block copolymers with a relatively larger average molecular weight such as Pluronic F68 ðMav ¼ 8400Þ and F127 ðMav ¼ 12; 600Þ were used, the as-deposited films showed no typical X-ray Bragg diffraction (see Fig. 1(c) and (d)). Mesostructured films were obtained after drying. In Fig. 1(c), hexagonal structured thin films were obtained after drying, and the (1 0 0) reflection of the film shifted continuously towards high diffraction angle with increasing heat-treatment temperature. A cubic mesostructured thin film was obtained when Pluronic F127 was used, shown in Fig. 1(d). The as-dried film showed (1 1 0), (2 0 0), (2 1 1) and (2 2 2) reflections of a cubic mesostructure (a ¼ 147 ). Strong (2 0 0) Bragg reflection associated with A ) was observed after cala cubic phase (a ¼ 138 A cination. The above estimation is supported by FE-TEM observation. Fig. 2 shows the FE-TEM images of the calcined films on Si(1 0 0) substrates using (a) Pluronic P65, (b) P123, (c) F68 and (d) F127 as the structure-directing agents. In these pictures, white part shows the pores and black part represents the skeleton of silica matrices. The TEM images of the calcined films confirm small pore sizes less than 10 nm and an excellent uniformity. Pore sizes of the films increased with increasing the average molecular weight of the surfactants. In the pictures (a), (b) and (d), the pores are randomly and continuously arranged, suggesting the formation of a cubic mesostructure. When Pluronic F68 was used, on the contrary, a highly oriented 1D hexagonal structure was clearly observed. Thus, TEM observation for the calcined films showed good
agreement with the XRD experiment as shown in Fig. 1. However, it was impossible to observe the as-deposited films because the films were easily dissolved into solvents used for TEM observation. The effect of the surfactant content on the mesostructure formation was examined. Fig. 3 shows the X-ray diffraction patterns of the calcined films prepared with different surfactant concentrations. In each case, the X-ray diffraction intensity in the 2h range of 1–5° increased with increasing the surfactant content and showed maximum at surfactant/TMOS ¼ 0.50 for Pluronic P65, P123 and F68, and at 0.75 for F127 (weight ratio), then decreased. From these results, the surfactant concentration in the sols appreciably affected the mesostructure formation. 3.2. Pore surface treatment using HMDS Generally, a pore surface of calcined films is covered with polar hydroxyl (–OH) groups. Apparently, such hydrophilic surface is not suitable for low-k dielectric materials. Calcined films were then subjected to dehydroxylation treatment using HMDS vapor. Fig. 4 shows the typical Fourier transformed infrared (FT-IR) spectra for the films before and after HMDS treatment. The absorption band at around 1100 cm1 can be assigned to the stretching vibration of Si–O–Si framework. The as-dried film shown in (a), the absorption bands due to the stretching vibration of hydroxyl and alkyl (–Cn H2nþ1 ) groups were observed at around 3500 and 2900 cm1 , respectively. For the film after heat-treatment shown in (b), the absorption band of alkyl groups disappeared due to
Fig. 2. FE-TEM pictures of the calcined films on Si(1 0 0) substrates using (a) Pluronic P65, (b) P123, (c) F68 and (d) F127 as the structure-directing agents.
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Fig. 3. XRD patterns of the calcined films prepared with different block copolymer/TMOS ratios.
burning out of the block copolymer. However, the absorption band of hydroxyl groups still remained, indicating the presence of hydroxyl groups in the film. After HMDS treatment, the absorption intensity of hydroxyl groups decreased drastically. Also, the absorption band of alkyl groups observed slightly at around 2950 cm1 . Therefore, the hydrophobic surface can be obtained by exposing the films to HMDS vapors. 3.3. Dielectric properties of the mesoporous SiO2 films Fig. 4. FT-IR spectra of the (a) as-dried and calcined films on Si(1 0 0) substrates (b) before and (c) after HMDS treatment. Noted that Pluronic P65 was used as the structure-directing agent.
Capacitance measurements for the calcined samples before and after HMDS treatment were performed. Prior to the measurements, the refractive
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index and porosity of the films were estimated using an ellipsometry. The thickness, refractive index and porosity of the calcined films before HMDS treatment are listed in Table 1. The film thickness chan (Pluronic P65) to 4500 A (Pluronic ged from 2200 A F127) with errors of about ±100 A depending on the average molecular weight of the surfactants. The refractive index of the corresponding films varied from 1.152 to 1.293 with errors of ±0.005–0.01. The porosity, P (vol.%), of the films was estimated using the Lorentz–Lorentz equation: ðn2c 1Þ=ðn2c þ 2Þ ¼ ð100 P Þðn20 1Þ=ðn20 þ 2Þ; where n0 is the refractive index of a vitreous silica ðn0 ¼ 1:4585Þ and nc is the measured values of the films. Using the equation, porosities of about 65 vol.% (P65), 34 vol.% (P123), 47 vol.% (F68) and 42 vol.% (F127) were obtained for the films after calcination. Table 2 shows the dielectric constant of the calcined films before and after HMDS treatment. The dielectric constant of the as-calcined films showed 2.19–2.49 (Pluronic P65), 4.31–4.49 (Pluronic P123), 2.96–3.06 (Pluronic F68) and 2.70– 2.90 (Pluronic F127), respectively. When Pluronic P123 was used, the estimeted values were larger Table 1 The thickness, refractive index and porosity of the calcined films Template
Film ) thicknessa (A
Refractive index, n
Porosity, P (vol.%)
Pluronic Pluronic Pluronic Pluronic
2200 ± 100 2500 ± 50 3000 ± 100 4500 ± 100
1.152 ± 0.01 1.293 ± 0.005 1.226 ± 0.01 1.248 ± 0.005
65 34 47 42
a
P65 P123 F68 F127
Estimated using a surface profilometer.
Table 2 The dielectric constant of the calcined films before and after HMDS treatment Template
Dielectric constant at 1 MHz As-calcined
Pluronic Pluronic Pluronic Pluronic
P65 P123 F68 F127
2.34 ± 0.15 4.40 ± 0.09 3.01 ± 0.05 2.80 ± 0.10
After HMDS treatment 2.00 ± 0.10 3.28 ± 0.30 2.45 ± 0.20 2.19 ± 0.10
Fig. 5. Change in the dielectric constant of the calcined films after HMDS treatment as a function of porosity.
than that of a dense SiO2 . This can be explained by the significant number of polar –OH groups. After HMDS treatment, the dielectric constant of each film decreased. Especially, dielectric constants about 1.90–2.10 were obtained for the mesoporous SiO2 films after removal of Pluronic P65 surfactant. Change in the dielectric constant of the calcined films as a function of porosity is shown in Fig. 5. As seen in the figure, the dielectric constant of the films fairly correlated with the porosity. In the films using Pluronic P65, P123 and F68, the estimated values agreed with the correlation for SiO2 aerogels, k ¼ 1 þ 7:1ðnc 1Þ [20]. Despite the low porosity (40 vol.%) of the films using Pluronic F127, the films showed low dielectric constants. From the TEM observation shown in Fig. 2, relatively larger pore sizes about 10 nm were observed than those of other films. The pore size of the film is, therefore, one of the possible factors of which affect the dielectric properties. 4. Discussion There is general consensus that evaporation-induced self-assembly of surfactants stimulates both the formation and ordering of micelles [3]. In
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aqueous solution, surfactants usually form into micelles with a spherical or cylindrical shape at above their critical micelle concentration (cmc). These micelles assemble spontaneously into a hexagonal, cubic or lamellar phases further increase in the surfactant concentration [21]. In sol–gel templating process using PEO–PPO–PEO block copolymers as structure-directing agents, homogeneous precursor solutions consisting of inorganic species, organic solvents, water, acid catalyst and a block copolymer surfactant are used. In this case surfactant concentrations in precursor solutions, c, are quite below cmc ðc cmcÞ. Therefore, surfactants are present as free-surfactants. During thin film processing such as dipping, spinning and casting, the preferential evaporation of organic solvents occurs, which increases the surfactant concentration as well as water, acid catalysts and silicate spices. Silicate networks are also formed simultaneously. Since PEO–PPO–PEO block copolymers consist of the hydrophilic PEO block and the hydrophobic PPO block under acidic conditions, positively charged silicate species preferentially interact with the hydrophilic PEO part of the block copolymer via both the electrostatic and hydrogen bonding interactions [5,22]. At some critical point corresponding to the cmc, both the block copolymers and silicate networks start to coassemble. As one can see, the preferential evaporation of solvents occurs at the air/film interface within a short period of time. Assuming that the degree of condensation is not enough after drying, a structural transformation of silicate networks induced by the re-arrangement of block copolymers is probably possible because of the low film viscosity. Indeed, calcination induced structural transformation was also observed. Thus, the degree of interactions between the PEO blocks and silicate species changes, which results in calcination induced structural transformation as well as evaporation induced self-assembly. However, owing to the complexity as described above, it is difficult to fully understand the templating mechanism at this stage during sol–gel thin film deposition. According to references, the EO/PO ratio in the PEO–PPO–PEO block copolymers has a large effect on the formation of the mesostructure [4,5,14]. Cubic mesostructured silica films have been syn-
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thesized using PEO–PPO–PEO block copolymers with higher EO/PO ratios above 1.5, e.g. Pluronic F127 [14]. Hexagonal mesostructured silica films have been formed when the EO/PO ratios are 0.2– 1.5, e.g. Pluronc P65 and P123 [14]. As shown in Fig. 1, cubic mesostructured silica films were obtained after calcination using Pluronic P65, P123 and F127. When Pluronic F68 was used, in contrast, a hexagonal mesostructured silica film was formed. In this study, TMOS and CH3 OH were used as raw materials, respectively. On the contrary, the authors in Ref. [14] have been prepared the films using Si(OC2 H5 )4 and C2 H5 OH. Therefore, not only the EO/PO ratios of PEO-PPO-PEO block copolymers but also the raw materials can be affected the mesostructure formation. Only qualitative information is available at this point, when PEO–PPO–PEO block copolymers with the EO/PO ratios below 1.5 are used, hexagonal mesostructured silica films can be prepared using Si(OC2 H5 )4 and C2 H5 OH as raw materials. Cubic mesostructured silica films have also been formed using Pluronic F68 and F127. There are many synthetic parameters of which affect the mesostructure formation; e.g., compositions, surrounding mediums, types of surfactants, raw materials used, preparation conditions such as aging time of sols, coating and drying atmosphere, etc. And ideally, mesoporous SiO2 films with a high porosity and closed pore should be prepared for improved reliability. More detailed studies on the synthesis of mesoporous silica films and their dielectric properties are being carried out and will be described in a separate publication.
5. Conclusions In this study, mesoporous silica films have been synthesized by modifying sol–gel method in the presence of poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) block copolymers as structure-directing agents. Depending on the types of block copolymers, mesoporous silica films with different mesostructures, pore sizes and porosities have been formed. Pore sizes of the films increased with increasing the average
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molecular weight of block copolymers. Cubic structured mesoporous silica films have been synthesized using Pluronic P65, P123 and F127. On the contrary, hexagonal structured mesoporous films have been obtained using Pluronic F68. Both evaporation induced self-assembly and calcination induced structural transformation were observed when block copolymers having relatively low average molecular weights, such as Pluronic P65 and P123, were used. While only evaporation induced self-assembly was observed when block copolymers with larger average molecular weights were used. By exposing the films to hexamethyldisilazane vapors, the hydrophobic pore surface can be obtained. The dielectric constant of the films strongly depended on the porosity of the films. The dielectric constants of about 1.9–2.1 were obtained for the films with the porosity of about 65 vol.%, after dehydroxylation treatment.
Acknowledgement The authors would like to thank BASF (Mt. Olive, NJ) for providing the block copolymer surfactants.
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