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Investigation of the properties of organically modified ordered mesoporous silica films
Journal of Colloid and Interface Science 320 (2008) 527–534 www.elsevier.com/locate/jcis
Investigation of the properties of organically modified 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, Republic of Korea Received 11 October 2007; accepted 3 January 2008 Available online 8 February 2008
Abstract Organically modified, ordered mesoporous silica films, which can provide hydrophobicity and low polarizability to the framework, were prepared using Brij-76 block copolymer as a template. Due to a fast condensation reaction of the silica precursor, mesostructured silica films were not properly synthesized. To circumvent this problem, a synthesis procedure was modified to provide an enhancement of pore periodicity through the incorporation of methyl ligands on the framework. The micropore volume was reduced, and the pore size was enlarged, as the concentration of the methyl ligands on the framework was increased. A mesophase transition from a two-dimensional hexagonal structure to a body-centered cubic (BCC) structure was observed according to the concentration of incorporated methyl ligands. The mechanical properties of the fabricated films were investigated according to the pore ordering and film density. The mechanical properties of the films with random pore geometry show a positive correlation between film density and elastic modulus. Meanwhile, the mechanical behavior of organically modified mesoporous silica films with periodic pore distribution represents a negative correlation within a certain density range, which is advantageous to the low-k materials. Especially, film with a low micropore volume fraction and BCC pore ordering is more applicable to a low-k material due to low dielectric constant and high mechanical strength. © 2008 Elsevier Inc. All rights reserved. Keywords: Ordered mesoporous silica film; Organic modification; Brij-76; Micropore; Mechanical properties; Low-k
1. Introduction The development of materials of a low dielectric constant is necessary to enhance the device performance for ULSI devices [1,2]. Low dielectric constant films are required to reduce power dissipation, propagation delay, and cross-talk noise of the interconnect structure. It is generally accepted that extendibility to the ultralow dielectric region is not attainable without introducing the concept of porosity because air possesses the lowest dielectric constant of 1 [3]. Among the organic or inorganic porous materials, porous silica films are most promising due to their superior mechanical strength, thermal stability, and most importantly, compatibility with silicon wafers and related materials that are used in existing IC technology. A very attractive synthetic route for producing porous silica films is the sol–gel process, which easily enables control of the dielectric * Corresponding author. Fax: +82 2 365 5882.
E-mail address: [email protected] (H.-H. Park). 0021-9797/$ – see front matter © 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2008.01.003
constant. Among the sol–gel-derived porous silica films, ordered mesoporous materials have been widely investigated for low dielectric film applications [4,5]. From a homogeneous solution of silica and surfactant dissolved in alcohol below the critical micelle concentration, the preferential evaporation of solvent during dip or spin coating drives a cooperative selfassembly of silica and surfactant. In this process, evaporationinduced self-assembly (EISA) enables the formation of highly ordered mesoporous silica films of various pore structures [6]. Our group reported that an ordered mesoporous silica film with a Brij-76 (C18 H37 (OCH2 CH2 )10 OH) block copolymer shows appropriate mechanical and electrical properties [7,8]. A framework modification through the incorporation of an organic functional group can lower the dielectric constant of the film due to low polarizability. Furthermore, it can reduce water absorption due to the hydrophobicity of the organic group [9]. As a result, the introduction of organic groups on the framework of mesoporous materials has attracted considerable attention [10]. Generally, organically modified, ordered meso-
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porous silica films have been prepared by using RSi(OEt)3 (R = CH3 , C2 H5 , C8 H7 ) and tetraethylorthosilane (TEOS) as a hybrid framework source and surfactant as template, where the mesopores are distributed uniformly with some degree of order. In this work, methyltriethoxysilane (MTES) was introduced as a silica precursor and the physiochemical properties of the films were investigated according to the mixing ratio with TEOS. The ordered pore structure of the films was investigated regarding the sol synthesis procedure and sol aging time and as a result, a film synthesis procedure could be optimized. Furthermore, the film’s mechanical and electrical properties were investigated for low-k applications. The benefits of pore periodicity of the film are discussed from the viewpoint of mechanical stability. 2. Materials and methods Two kinds of sol synthesis procedures were performed. The first one was a prehydrolysis of a precursor (PHP) method [7]. Solution A was prepared by dissolving a Brij-76 block copolymer (C18 H37 (OCH2 CH2 )10 OH, Aldrich) in an ethyl alcohol (EtOH, Duksan, 99.9%)–H2 O–HCl solution with a molar ratio of 0.05 Brij-76:15 EtOH:1 H2 O:0.0028 HCl. Solution B was prepared by mixing TEOS (Fluka, 98%), MTES (Aldrich, 99%), and EtOH followed by the addition of an acid catalyst in H2 O. After stirring solution B for 10 min, solution B was added to solution A. The final composition of MTES:TEOS:EtOH:H2 O:HCl:Brij-76 was x:(1 − x):20:5: 0.01:0.05. The molar ratio, x, was varied from 0 to 0.2. Generally, the aging time of the sol prior to spin coating was 1 day. For the second synthesis procedure, the cohydrolysis of the precursor and cosolvent (CHPC) method was used. Brij-76 block copolymer was dissolved in EtOH and then acidified H2 O was added. After stirring for 2 h, TEOS, MTES, EtOH, acetone (Duksan, 99.5%), and acidified H2 O were separately added to the previous solution. Acetone was added because it can reduce the condensation rate due to its aprotic property and dilution effect [8]. The final composition of MTES:TEOS:EtOH:acetone:H2 O:HCl:Brij-76 was x:(1 − x):20:5:5:0.01:0.05. The molar ratio, x, was varied from 0 to 0.3. The aging time of each sol before spin coating was between 30 and 150 min. Each silica sol was prepared by two kinds of synthesis procedures and spin-deposited at 3000 rpm for 30 s with different sol aging times. Organically modified, ordered mesoporous silica films could be fabricated by removing the block copolymer at 400 ◦ C with a heating rate of 1 ◦ C/min. For the investigation of pore structure, X-ray powder diffraction (XRD) patterns were collected using FeKα radiation with a wavelength of 1.9373 Å. The film porosity was calculated by measuring the critical angle of the film using specular X-ray reflectivity at the 3C2 beamline of Pohang Light Source (PLS) in Korea and ellipsometry with a He–Ne laser source of 632 nm wavelength [11]. Fourier transform infrared (FTIR; Jasco 300Z) spectroscopic analysis was performed to reveal the chemical species of the silica wall. The mesostructure of the film was investigated using grazing incidence small-angle X-ray scattering (GISAXS) measurements at a 4C2 beamline (λ = 1.54 Å
Fig. 1. XRD patterns of as-prepared silica films prepared by the PHP procedure according to the (a) MTES/TEOS molar ratio at 1 day sol aging and (b) sol aging time at a MTES/TEOS molar ratio of 0.2.
and λ/λ = 5 × 10−4 ) of PLS [12]. The sample-to-detector distance was 75 cm. The mechanical properties of ordered or disordered mesoporous silica films were investigated by continuous stiffness measurements using a MTS nanoindentor XP [13]. In this technique, a small oscillating force is applied to the Berkovich indentor, and the amplitude and phase shift of the oscillations are measured. This allows for a continuous measurement of hardness and modulus even for extremely thin layered materials. For electrical property measurements, circular Al dots were deposited on ordered mesoporous film using an ebeam evaporator. Aluminum was also deposited on the backside of the Si for ohmic contact. After deposition, postmetal annealing was performed by rapid thermal annealing at 300 ◦ C for 1 min. Capacitance–voltage (C–V ) characteristics in the metal– insulator–semiconductor structure were obtained using an HP 4284A impedance/gain-phase analyzer at 1 MHz. 3. Results and discussion Figs. 1a and 1b show XRD patterns of as-prepared silica films with different MTES/TEOS molar ratios for a fixed sol aging time of 1 day [7] and with different sol aging times for a fixed MTES/TEOS molar ratio of 0.2. When MTES was not added to the sol, it was found that a highly ordered mesostructured film could be fabricated, as shown in Fig. 1a. Our previous work revealed that the mesophase by the PHP procedure was a
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Fig. 3. Optimized aging time variations of silica sol for the synthesis of mesostructured silica films by the CHPC procedure as a function of the MTES/ TEOS molar ratio.
Fig. 2. GISAXS patterns of as-prepared silica films prepared by the PHP procedure with MTES/TEOS molar ratios of (a) 0.05 and (b) 0.2 at 1 day sol aging.
body-centered cubic (BCC) structure with a Brij-76/TEOS molar ratio of 0.05 and an EtOH/TEOS molar ratio of 20 [14]. The diffraction peak in Fig. 1a comes from (011) of the BCC structure. The as-prepared silica film showed a highly textured structure up to an MTES/TEOS molar ratio of 0.05. Figs. 2a and 2b show GISAXS patterns of as-prepared silica structure films using the sol aged for 1 day with two different MTES/TEOS molar ratios: 0.05 of bcc and 0.2 of random micelle structure. The periodicity of the film was destroyed over a MTES/TEOS molar ratio of 0.1. It is generally recognized that the condensation rate of MTES is 7 times faster than TEOS [15]. The disordering of the films with a high MTES/TEOS molar ratio results from a long sol aging time, because as shown in Fig. 1b, the pore periodicity is slightly enhanced by a shortening of sol aging time. However, a broadening of the X-ray diffraction peak signifies that highly ordered mesoporous silica film cannot be obtained even by shortening the aging time. One possible assumption is that hydrophobic Si–C bonds in the silica precursor may impede the electrostatic interactions between inorganic and organic species [16]. However, the concentration of Si–C bonds in the film with a MTES/TEOS molar ratio of 0.05–0.2 is not considered to be significantly high. As a result, it can be concluded that the condensation rate of the silica precursor before mixing the two kinds of solutions is not properly controlled due to the fast condensation rate of MTES. So, the synthesis procedure had to be modified in order to control the condensation rate of MTES. Acetone as a cosolvent was added to the sol to reduce the condensation rate and enhance the surface properties [8]. Fig. 3 shows a variation of optimized mesostructure formation time of as-prepared silica film with a MTES/TEOS molar ratio when the CHPC procedure was adopted. The optimized time was the duration required to obtain the most highly intensified peaks in the XRD pattern. Due to the fast condensation re-
Fig. 4. XRD patterns of the (a) as-prepared and (b) ordered mesoporous silica films prepared by the CHPC procedure according to the MTES/TEOS molar ratio.
action of MTES, the duration was significantly shortened when compared with the PHP procedure. The duration also decreased with increasing MTES/TEOS molar ratios. However, when the averaged sol was used, a disordered mesoporous silica film was obtained. Based on these results, all subsequent mesostructured silica films were prepared by the CHPC procedure. Fig. 4 shows XRD patterns of as-prepared and ordered mesoporous silica films with different MTES/TEOS molar ratios.
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Fig. 6. Porosity variations for the ordered mesoporous silica film as a function of the MTES/TEOS molar ratio.
Fig. 5. (a) The intensity ratio of first to second peak in the XRD pattern for ordered mesoporous silica films and (b) the shrinkage ratio of major diffraction peaks during calcination as a function of the MTES/TEOS molar ratio.
Up to a MTES/TEOS molar ratio of 0.25, highly ordered, mesostructured, silica films could be obtained. The (100) diffraction of the two-dimensional hexagonal structure was observed up to a MTES/TEOS molar ratio of 0.2 and (011) diffraction of BCC was observed with a MTES/TEOS molar ratio of 0.25. However, ordered mesostructured silica films were not obtained over a molar ratio of 0.3 MTES/TEOS due to the uncontrolled condensation reaction when the molar ratio of acetone to the total moles of silica precursor was only 5. Furthermore, the MTES molecules condensed at the exterior of the silica oligomer, which impeded the EISA process due to the hydrophobicity of Si–C bonds as the MTES molar ratio was increased. The interplanar spacing increased with the MTES/TEOS molar ratio irrespective of calcination. Because it is the sum of the wall thickness and pore diameter, each contribution to the variation, according to MTES/TEOS molar ratio, could not be clearly distinguished in the XRD pattern. Fig. 5a shows an intensity ratio of the first peak (100 or 011) and the second peak (200 or 022) in Fig. 4b, even though the second peaks could not be observed in the figure due to their small intensity. The intensity ratio of first to second peak is related to the relative length of wall thickness and pore diameter [17, 18]. The abrupt increase at a 0.25 MTES/TEOS molar ratio in Fig. 5a was evidence of a structural transition. Fig. 5b shows the calculated shrinkage ratio from the peak position as it changes between Figs. 4a and 4b. It was observed that the shrinkage
ratio gradually decreased with the MTES/TEOS molar ratio as shown in Fig. 5b. A contraction of cell parameters during calcination was determined by wall thickness, pore diameter, and the micropores. However, Fig. 6 shows that film porosity was increased with the MTES/TEOS molar ratio, which contradicted the general notion that high porosity results in high shrinkage during calcinations. It was concluded that the reduction of the shrinkage ratio in Fig. 5b be can related to the reduction of the micropores in the film, and that an increase of pore diameter is mainly associated with the interplanar spacing variation according to the MTES/TEOS molar ratio, as given in Fig. 4b. The increase of pore diameter and a reduction of the micropores with respect to the MTES/TEOS molar ratio are attributed to the existence of hydrophobic Si–C bonds [19]. Si–C bonds can reduce the electrostatic repulsion forces between the charged silica oligomers. As a result, g factor and micelle diameter are increased with the MTES/TEOS molar ratio. Furthermore, the Si–C bond obstructs the penetration of the polyethylene chain in the silica framework due to the lack of solubility [20]. Fig. 7 shows GISAXS patterns of as-prepared silica films with different MTES/TEOS molar ratios. Interestingly, the mesostructured silica films with MTES/TEOS molar ratios under 0.2 (including 0 molar ratio, not shown in the figure) show a two-dimensional hexagonal structure, as shown in Figs. 7a, 7b, and 7c [21]. The formation of a two-dimensional hexagonal structure by the CHPC procedure may be ascribed to a high MTES molar ratio or the addition of a cosolvent. However, the as-prepared silica film by the CHPC procedure has a two-dimensional hexagonal structure, even if MTES was not added (not shown). Therefore, the structural transition was attributed to the degree of condensation reaction in the silica sol. It means that silica oligomers were too large to form the twodimensional hexagonal structure when the PHP procedure was adopted. A structural transition to BCC with a MTES/TEOS molar ratio of 0.25 was attributed to the degree of the condensation reaction in the silica sol, as shown in Fig. 7d. A high MTES concentration accelerates the condensation reaction. As a result, the aging period for the formation of a two-dimensional hexagonal structure passed over after the addition of silica precursor with a 0.25 MTES/TEOS molar ratio. Similar results are
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Fig. 7. GISAXS patterns of the as-prepared silica films with MTES/TEOS molar ratios of (a) 0.1, (b) 0.15, (c) 0.2, and (d) 0.25.
Fig. 8. FTIR spectra of ordered mesoporous silica films with MTES/TEOS molar ratios of (a) 0.1, (b) 0.15, (c) 0.2, and (d) 0.25.
reported in the case of a CTAB-templated silica film, where the mesophase transition from a two-dimensional hexagonal to three-dimensional structure occurs according to aging time [22]. From observations of the interplanar spacing along the out-of-plane direction, the interplanar spacing was very similar, even though a phase transition is seen in Fig. 7. In fact, an epitaxial relationship between the (011) plane of the BCC structure and the (100) plane of two-dimensional hexagonal structure is well documented [23–25]. The {111} direction of the BCC phase is related to the cylinder direction of the two-dimensional hexagonal phase, and the (100) plane of the two-dimensional hexagonal phase is related to the (110) plane of the BCC phase. BCC pore structure of ordered mesoporous silica film had been confirmed through TEM observations in our previous works [14,26]. Fig. 8 shows the FTIR spectra of ordered mesoporous silica films for various MTES/TEOS molar ratios. The peaks at 1067 and 1109 cm−1 correspond to the TO3 and cyclic Si–O vibra-
tions, respectively [27]. The peak corresponding to the cyclic Si–O bond originates from the Si–O bonds at the pore surface, which signify the porous nature of the film, and the peak at 1279 cm−1 corresponds to the Si–C vibrations. Si–C bonds were not dissociated during calcination at 400 ◦ C. It was observed that the intensity of the Si–C bond increased with respect to the MTES molar ratio. However, an absorption appeared at 960 cm−1 due to the residual Si–OH group, even though there were framework modifications with hydrophobic Si–C bonds. The peak intensity of the cyclic Si–O bonds increased with increasing MTES/TEOS molar ratios. This is related to the fact that the terminal Si–C bonds in the framework make a short chain Si–O–Si ring structure rather than a 6-member ring structure. Synthesis of ordered mesoporous silica films by cocondensation of TEOS and MTES was reported by some authors [5,15]. However, investigations on the mechanical properties of the ordered mesoporous silica film by organic modification have been scarce. In this work, enhancement of the mechanical properties by MTES addition was focused on. Fig. 9 shows the elastic modulus and hardness of the ordered mesoporous silica films with respect to MTES/TEOS molar ratio. Due to the hard substrate effect, the elastic modulus and hardness steadily rose as the indentation depth increased. Because of this, mechanical properties were extracted from the flat region in Fig. 9 [28]. Surprisingly, it was observed that the elastic modulus and hardness of the film were increased with all MTES/TEOS molar ratios except 0.2. In order to verify the benefits of pore structure ordering, disordered mesoporous silica films were prepared at each MTES/TEOS molar ratio. Each silica sol with different MTES/TEOS molar ratios was spin-deposited after 30 min plus the optimized aging time according to Fig. 3. XRD analyses (not given) showed the absence of diffraction peaks, and this indicated that the films were disordered. Fig. 10 shows the elastic modulus and hardness of the disordered mesoporous silica films
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Fig. 9. (a) Elastic modulus and (b) hardness of ordered mesoporous silica films according to the MTES/TEOS molar ratio.
with respect to the MTES/TEOS molar ratio. The mechanical properties of the film deteriorated as the MTES/TEOS molar ratio increased. This mechanical behavior is opposite from that observed in ordered mesoporous films. In fact, pore collapse and densification during calcination occurred in the case of a 0.3 MTES/TEOS molar ratio. The mechanical properties of a porous film are largely dependent on its density. Generally, the elastic modulus and density of a film have a linear relationship in log-scale with a positive slope of 2 to 4 [29]. The value without an underline in Fig. 11 represents the MTES/TEOS molar ratio of ordered mesoporous silica films, but the underlined value belongs to the disordered mesoporous silica film. As shown in Fig. 6, the density of the film decreased with an increase in the MTES/TEOS molar ratio. This result could also be applied to the disordered mesoporous silica films because the density of the film decreases with an increase in aging time [30]. The disordered mesoporous silica films showed normal density-dependent mechanical properties. However, the mechanical properties were especially enhanced, even though there was a reduction of the density in the case of the ordered mesoporous silica films. As discussed in Fig. 5b, the films with higher MTES/TEOS molar ratios have a more rigid wall structure. Although terminal Si–C bonds may make the silica framework weaker, the mechanical properties of the film are more strongly controlled by the mi-
Fig. 10. (a) Elastic modulus and (b) hardness of disordered mesoporous silica films according to the MTES/TEOS molar ratio.
Fig. 11. Relationship between density and elastic modulus in log–log scale for ordered and disordered mesoporous silica films. The numerical number around each data represents the MTES/TEOS molar ratio.
cropore volume in the film. The ordered mesoporous silica film with a 0.25 MTES/TEOS molar ratio has a BCC structure. An abrupt increase in the elastic modulus for the ordered mesoporous silica film with a 0.25 MTES/TEOS molar ratio could be attributed to the fact that cubic structures with spherical pores are more mechanically robust than hexagonally packed structures [31]. This result highlights the benefits of BCC geometry and pore structure ordering for the application of ordered mesoporous silica films, such as low-k dielectrics.
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provide mechanical stability and a low dielectric property to an ordered mesoporous silica film for low-k applications due to reduction of micropore volume and BCC pore ordering. Acknowledgment This work was supported by Grant R01-2005-000-10058-0 from the Basic Research Program of the Korea Science & Engineering Foundation. References
Fig. 12. The dielectric constant of ordered mesoporous silica films according to the MTES/TEOS molar ratio.
Fig. 12 shows the variations in the dielectric constant of the ordered mesoporous silica films with respect to the MTES/TEOS molar ratio. The dielectric constant gradually decreased due to a reduction of the density. The dielectric constant is a little smaller than the calculated one by using the Bruggeman effective medium approximation. This is attributed to the existence of Si–C bonds with low molar polarizability [32]. In particular, the ordered mesoporous silica film with a 0.25 MTES/TEOS molar ratio has the lowest dielectric constant. Namely, the film with BCC pore structure shows the lowest dielectric constant and highest mechanical strength, as shown in Figs. 11 and 12. So it can be concluded that film with low micropore volume fraction and BCC pore ordering is more applicable to a low-k material due to low dielectric constant and high mechanical strength. 4. Summary A modification of a silica framework with Si–C bonds was successfully performed using TEOS and MTES as silica precursors. By controlling the rate of the condensation reaction, ordered mesoporous silica films could be prepared up to a 0.25 MTES/TEOS molar ratio. The mesophase of the films with 0.1–0.2 MTES/TEOS molar ratios showed a two-dimensional hexagonal structure while the mesophase of the film with a 0.25 MTES/TEOS molar ratio showed a bcc structure. A disordered mesoporous silica film was obtained when the silica sol aging time was long or the MTES/TEOS molar ratio was over 0.3. A reduction of the micropore volume and an increase of the pore size were observed with the increase of the MTES/TEOS molar ratio. The mechanical properties of the ordered mesoporous silica films represented an exceptional increase with the increase of the MTES/TEOS molar ratio or porosity. This was due to the reduction of micropore volume and pore ordering in the case of organically modified mesoporous silica films. The dielectric constant was lowered with an increase of the MTES/TEOS molar ratio due to a reduction of film density and polarizability of the framework. Especially, the film with a 0.25 MTES/TEOS molar ratio has the lowest dielectric constant and highest mechanical strength. In conclusion, the organic modification of the framework by methyl ligands could
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Investigation of the properties of organically modified 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, Republic of Korea Received 11 October 2007; accepted 3 January 2008 Available online 8 February 2008
Abstract Organically modified, ordered mesoporous silica films, which can provide hydrophobicity and low polarizability to the framework, were prepared using Brij-76 block copolymer as a template. Due to a fast condensation reaction of the silica precursor, mesostructured silica films were not properly synthesized. To circumvent this problem, a synthesis procedure was modified to provide an enhancement of pore periodicity through the incorporation of methyl ligands on the framework. The micropore volume was reduced, and the pore size was enlarged, as the concentration of the methyl ligands on the framework was increased. A mesophase transition from a two-dimensional hexagonal structure to a body-centered cubic (BCC) structure was observed according to the concentration of incorporated methyl ligands. The mechanical properties of the fabricated films were investigated according to the pore ordering and film density. The mechanical properties of the films with random pore geometry show a positive correlation between film density and elastic modulus. Meanwhile, the mechanical behavior of organically modified mesoporous silica films with periodic pore distribution represents a negative correlation within a certain density range, which is advantageous to the low-k materials. Especially, film with a low micropore volume fraction and BCC pore ordering is more applicable to a low-k material due to low dielectric constant and high mechanical strength. © 2008 Elsevier Inc. All rights reserved. Keywords: Ordered mesoporous silica film; Organic modification; Brij-76; Micropore; Mechanical properties; Low-k
1. Introduction The development of materials of a low dielectric constant is necessary to enhance the device performance for ULSI devices [1,2]. Low dielectric constant films are required to reduce power dissipation, propagation delay, and cross-talk noise of the interconnect structure. It is generally accepted that extendibility to the ultralow dielectric region is not attainable without introducing the concept of porosity because air possesses the lowest dielectric constant of 1 [3]. Among the organic or inorganic porous materials, porous silica films are most promising due to their superior mechanical strength, thermal stability, and most importantly, compatibility with silicon wafers and related materials that are used in existing IC technology. A very attractive synthetic route for producing porous silica films is the sol–gel process, which easily enables control of the dielectric * Corresponding author. Fax: +82 2 365 5882.
E-mail address: [email protected] (H.-H. Park). 0021-9797/$ – see front matter © 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2008.01.003
constant. Among the sol–gel-derived porous silica films, ordered mesoporous materials have been widely investigated for low dielectric film applications [4,5]. From a homogeneous solution of silica and surfactant dissolved in alcohol below the critical micelle concentration, the preferential evaporation of solvent during dip or spin coating drives a cooperative selfassembly of silica and surfactant. In this process, evaporationinduced self-assembly (EISA) enables the formation of highly ordered mesoporous silica films of various pore structures [6]. Our group reported that an ordered mesoporous silica film with a Brij-76 (C18 H37 (OCH2 CH2 )10 OH) block copolymer shows appropriate mechanical and electrical properties [7,8]. A framework modification through the incorporation of an organic functional group can lower the dielectric constant of the film due to low polarizability. Furthermore, it can reduce water absorption due to the hydrophobicity of the organic group [9]. As a result, the introduction of organic groups on the framework of mesoporous materials has attracted considerable attention [10]. Generally, organically modified, ordered meso-
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porous silica films have been prepared by using RSi(OEt)3 (R = CH3 , C2 H5 , C8 H7 ) and tetraethylorthosilane (TEOS) as a hybrid framework source and surfactant as template, where the mesopores are distributed uniformly with some degree of order. In this work, methyltriethoxysilane (MTES) was introduced as a silica precursor and the physiochemical properties of the films were investigated according to the mixing ratio with TEOS. The ordered pore structure of the films was investigated regarding the sol synthesis procedure and sol aging time and as a result, a film synthesis procedure could be optimized. Furthermore, the film’s mechanical and electrical properties were investigated for low-k applications. The benefits of pore periodicity of the film are discussed from the viewpoint of mechanical stability. 2. Materials and methods Two kinds of sol synthesis procedures were performed. The first one was a prehydrolysis of a precursor (PHP) method [7]. Solution A was prepared by dissolving a Brij-76 block copolymer (C18 H37 (OCH2 CH2 )10 OH, Aldrich) in an ethyl alcohol (EtOH, Duksan, 99.9%)–H2 O–HCl solution with a molar ratio of 0.05 Brij-76:15 EtOH:1 H2 O:0.0028 HCl. Solution B was prepared by mixing TEOS (Fluka, 98%), MTES (Aldrich, 99%), and EtOH followed by the addition of an acid catalyst in H2 O. After stirring solution B for 10 min, solution B was added to solution A. The final composition of MTES:TEOS:EtOH:H2 O:HCl:Brij-76 was x:(1 − x):20:5: 0.01:0.05. The molar ratio, x, was varied from 0 to 0.2. Generally, the aging time of the sol prior to spin coating was 1 day. For the second synthesis procedure, the cohydrolysis of the precursor and cosolvent (CHPC) method was used. Brij-76 block copolymer was dissolved in EtOH and then acidified H2 O was added. After stirring for 2 h, TEOS, MTES, EtOH, acetone (Duksan, 99.5%), and acidified H2 O were separately added to the previous solution. Acetone was added because it can reduce the condensation rate due to its aprotic property and dilution effect [8]. The final composition of MTES:TEOS:EtOH:acetone:H2 O:HCl:Brij-76 was x:(1 − x):20:5:5:0.01:0.05. The molar ratio, x, was varied from 0 to 0.3. The aging time of each sol before spin coating was between 30 and 150 min. Each silica sol was prepared by two kinds of synthesis procedures and spin-deposited at 3000 rpm for 30 s with different sol aging times. Organically modified, ordered mesoporous silica films could be fabricated by removing the block copolymer at 400 ◦ C with a heating rate of 1 ◦ C/min. For the investigation of pore structure, X-ray powder diffraction (XRD) patterns were collected using FeKα radiation with a wavelength of 1.9373 Å. The film porosity was calculated by measuring the critical angle of the film using specular X-ray reflectivity at the 3C2 beamline of Pohang Light Source (PLS) in Korea and ellipsometry with a He–Ne laser source of 632 nm wavelength [11]. Fourier transform infrared (FTIR; Jasco 300Z) spectroscopic analysis was performed to reveal the chemical species of the silica wall. The mesostructure of the film was investigated using grazing incidence small-angle X-ray scattering (GISAXS) measurements at a 4C2 beamline (λ = 1.54 Å
Fig. 1. XRD patterns of as-prepared silica films prepared by the PHP procedure according to the (a) MTES/TEOS molar ratio at 1 day sol aging and (b) sol aging time at a MTES/TEOS molar ratio of 0.2.
and λ/λ = 5 × 10−4 ) of PLS [12]. The sample-to-detector distance was 75 cm. The mechanical properties of ordered or disordered mesoporous silica films were investigated by continuous stiffness measurements using a MTS nanoindentor XP [13]. In this technique, a small oscillating force is applied to the Berkovich indentor, and the amplitude and phase shift of the oscillations are measured. This allows for a continuous measurement of hardness and modulus even for extremely thin layered materials. For electrical property measurements, circular Al dots were deposited on ordered mesoporous film using an ebeam evaporator. Aluminum was also deposited on the backside of the Si for ohmic contact. After deposition, postmetal annealing was performed by rapid thermal annealing at 300 ◦ C for 1 min. Capacitance–voltage (C–V ) characteristics in the metal– insulator–semiconductor structure were obtained using an HP 4284A impedance/gain-phase analyzer at 1 MHz. 3. Results and discussion Figs. 1a and 1b show XRD patterns of as-prepared silica films with different MTES/TEOS molar ratios for a fixed sol aging time of 1 day [7] and with different sol aging times for a fixed MTES/TEOS molar ratio of 0.2. When MTES was not added to the sol, it was found that a highly ordered mesostructured film could be fabricated, as shown in Fig. 1a. Our previous work revealed that the mesophase by the PHP procedure was a
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Fig. 3. Optimized aging time variations of silica sol for the synthesis of mesostructured silica films by the CHPC procedure as a function of the MTES/ TEOS molar ratio.
Fig. 2. GISAXS patterns of as-prepared silica films prepared by the PHP procedure with MTES/TEOS molar ratios of (a) 0.05 and (b) 0.2 at 1 day sol aging.
body-centered cubic (BCC) structure with a Brij-76/TEOS molar ratio of 0.05 and an EtOH/TEOS molar ratio of 20 [14]. The diffraction peak in Fig. 1a comes from (011) of the BCC structure. The as-prepared silica film showed a highly textured structure up to an MTES/TEOS molar ratio of 0.05. Figs. 2a and 2b show GISAXS patterns of as-prepared silica structure films using the sol aged for 1 day with two different MTES/TEOS molar ratios: 0.05 of bcc and 0.2 of random micelle structure. The periodicity of the film was destroyed over a MTES/TEOS molar ratio of 0.1. It is generally recognized that the condensation rate of MTES is 7 times faster than TEOS [15]. The disordering of the films with a high MTES/TEOS molar ratio results from a long sol aging time, because as shown in Fig. 1b, the pore periodicity is slightly enhanced by a shortening of sol aging time. However, a broadening of the X-ray diffraction peak signifies that highly ordered mesoporous silica film cannot be obtained even by shortening the aging time. One possible assumption is that hydrophobic Si–C bonds in the silica precursor may impede the electrostatic interactions between inorganic and organic species [16]. However, the concentration of Si–C bonds in the film with a MTES/TEOS molar ratio of 0.05–0.2 is not considered to be significantly high. As a result, it can be concluded that the condensation rate of the silica precursor before mixing the two kinds of solutions is not properly controlled due to the fast condensation rate of MTES. So, the synthesis procedure had to be modified in order to control the condensation rate of MTES. Acetone as a cosolvent was added to the sol to reduce the condensation rate and enhance the surface properties [8]. Fig. 3 shows a variation of optimized mesostructure formation time of as-prepared silica film with a MTES/TEOS molar ratio when the CHPC procedure was adopted. The optimized time was the duration required to obtain the most highly intensified peaks in the XRD pattern. Due to the fast condensation re-
Fig. 4. XRD patterns of the (a) as-prepared and (b) ordered mesoporous silica films prepared by the CHPC procedure according to the MTES/TEOS molar ratio.
action of MTES, the duration was significantly shortened when compared with the PHP procedure. The duration also decreased with increasing MTES/TEOS molar ratios. However, when the averaged sol was used, a disordered mesoporous silica film was obtained. Based on these results, all subsequent mesostructured silica films were prepared by the CHPC procedure. Fig. 4 shows XRD patterns of as-prepared and ordered mesoporous silica films with different MTES/TEOS molar ratios.
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Fig. 6. Porosity variations for the ordered mesoporous silica film as a function of the MTES/TEOS molar ratio.
Fig. 5. (a) The intensity ratio of first to second peak in the XRD pattern for ordered mesoporous silica films and (b) the shrinkage ratio of major diffraction peaks during calcination as a function of the MTES/TEOS molar ratio.
Up to a MTES/TEOS molar ratio of 0.25, highly ordered, mesostructured, silica films could be obtained. The (100) diffraction of the two-dimensional hexagonal structure was observed up to a MTES/TEOS molar ratio of 0.2 and (011) diffraction of BCC was observed with a MTES/TEOS molar ratio of 0.25. However, ordered mesostructured silica films were not obtained over a molar ratio of 0.3 MTES/TEOS due to the uncontrolled condensation reaction when the molar ratio of acetone to the total moles of silica precursor was only 5. Furthermore, the MTES molecules condensed at the exterior of the silica oligomer, which impeded the EISA process due to the hydrophobicity of Si–C bonds as the MTES molar ratio was increased. The interplanar spacing increased with the MTES/TEOS molar ratio irrespective of calcination. Because it is the sum of the wall thickness and pore diameter, each contribution to the variation, according to MTES/TEOS molar ratio, could not be clearly distinguished in the XRD pattern. Fig. 5a shows an intensity ratio of the first peak (100 or 011) and the second peak (200 or 022) in Fig. 4b, even though the second peaks could not be observed in the figure due to their small intensity. The intensity ratio of first to second peak is related to the relative length of wall thickness and pore diameter [17, 18]. The abrupt increase at a 0.25 MTES/TEOS molar ratio in Fig. 5a was evidence of a structural transition. Fig. 5b shows the calculated shrinkage ratio from the peak position as it changes between Figs. 4a and 4b. It was observed that the shrinkage
ratio gradually decreased with the MTES/TEOS molar ratio as shown in Fig. 5b. A contraction of cell parameters during calcination was determined by wall thickness, pore diameter, and the micropores. However, Fig. 6 shows that film porosity was increased with the MTES/TEOS molar ratio, which contradicted the general notion that high porosity results in high shrinkage during calcinations. It was concluded that the reduction of the shrinkage ratio in Fig. 5b be can related to the reduction of the micropores in the film, and that an increase of pore diameter is mainly associated with the interplanar spacing variation according to the MTES/TEOS molar ratio, as given in Fig. 4b. The increase of pore diameter and a reduction of the micropores with respect to the MTES/TEOS molar ratio are attributed to the existence of hydrophobic Si–C bonds [19]. Si–C bonds can reduce the electrostatic repulsion forces between the charged silica oligomers. As a result, g factor and micelle diameter are increased with the MTES/TEOS molar ratio. Furthermore, the Si–C bond obstructs the penetration of the polyethylene chain in the silica framework due to the lack of solubility [20]. Fig. 7 shows GISAXS patterns of as-prepared silica films with different MTES/TEOS molar ratios. Interestingly, the mesostructured silica films with MTES/TEOS molar ratios under 0.2 (including 0 molar ratio, not shown in the figure) show a two-dimensional hexagonal structure, as shown in Figs. 7a, 7b, and 7c [21]. The formation of a two-dimensional hexagonal structure by the CHPC procedure may be ascribed to a high MTES molar ratio or the addition of a cosolvent. However, the as-prepared silica film by the CHPC procedure has a two-dimensional hexagonal structure, even if MTES was not added (not shown). Therefore, the structural transition was attributed to the degree of condensation reaction in the silica sol. It means that silica oligomers were too large to form the twodimensional hexagonal structure when the PHP procedure was adopted. A structural transition to BCC with a MTES/TEOS molar ratio of 0.25 was attributed to the degree of the condensation reaction in the silica sol, as shown in Fig. 7d. A high MTES concentration accelerates the condensation reaction. As a result, the aging period for the formation of a two-dimensional hexagonal structure passed over after the addition of silica precursor with a 0.25 MTES/TEOS molar ratio. Similar results are
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Fig. 7. GISAXS patterns of the as-prepared silica films with MTES/TEOS molar ratios of (a) 0.1, (b) 0.15, (c) 0.2, and (d) 0.25.
Fig. 8. FTIR spectra of ordered mesoporous silica films with MTES/TEOS molar ratios of (a) 0.1, (b) 0.15, (c) 0.2, and (d) 0.25.
reported in the case of a CTAB-templated silica film, where the mesophase transition from a two-dimensional hexagonal to three-dimensional structure occurs according to aging time [22]. From observations of the interplanar spacing along the out-of-plane direction, the interplanar spacing was very similar, even though a phase transition is seen in Fig. 7. In fact, an epitaxial relationship between the (011) plane of the BCC structure and the (100) plane of two-dimensional hexagonal structure is well documented [23–25]. The {111} direction of the BCC phase is related to the cylinder direction of the two-dimensional hexagonal phase, and the (100) plane of the two-dimensional hexagonal phase is related to the (110) plane of the BCC phase. BCC pore structure of ordered mesoporous silica film had been confirmed through TEM observations in our previous works [14,26]. Fig. 8 shows the FTIR spectra of ordered mesoporous silica films for various MTES/TEOS molar ratios. The peaks at 1067 and 1109 cm−1 correspond to the TO3 and cyclic Si–O vibra-
tions, respectively [27]. The peak corresponding to the cyclic Si–O bond originates from the Si–O bonds at the pore surface, which signify the porous nature of the film, and the peak at 1279 cm−1 corresponds to the Si–C vibrations. Si–C bonds were not dissociated during calcination at 400 ◦ C. It was observed that the intensity of the Si–C bond increased with respect to the MTES molar ratio. However, an absorption appeared at 960 cm−1 due to the residual Si–OH group, even though there were framework modifications with hydrophobic Si–C bonds. The peak intensity of the cyclic Si–O bonds increased with increasing MTES/TEOS molar ratios. This is related to the fact that the terminal Si–C bonds in the framework make a short chain Si–O–Si ring structure rather than a 6-member ring structure. Synthesis of ordered mesoporous silica films by cocondensation of TEOS and MTES was reported by some authors [5,15]. However, investigations on the mechanical properties of the ordered mesoporous silica film by organic modification have been scarce. In this work, enhancement of the mechanical properties by MTES addition was focused on. Fig. 9 shows the elastic modulus and hardness of the ordered mesoporous silica films with respect to MTES/TEOS molar ratio. Due to the hard substrate effect, the elastic modulus and hardness steadily rose as the indentation depth increased. Because of this, mechanical properties were extracted from the flat region in Fig. 9 [28]. Surprisingly, it was observed that the elastic modulus and hardness of the film were increased with all MTES/TEOS molar ratios except 0.2. In order to verify the benefits of pore structure ordering, disordered mesoporous silica films were prepared at each MTES/TEOS molar ratio. Each silica sol with different MTES/TEOS molar ratios was spin-deposited after 30 min plus the optimized aging time according to Fig. 3. XRD analyses (not given) showed the absence of diffraction peaks, and this indicated that the films were disordered. Fig. 10 shows the elastic modulus and hardness of the disordered mesoporous silica films
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Fig. 9. (a) Elastic modulus and (b) hardness of ordered mesoporous silica films according to the MTES/TEOS molar ratio.
with respect to the MTES/TEOS molar ratio. The mechanical properties of the film deteriorated as the MTES/TEOS molar ratio increased. This mechanical behavior is opposite from that observed in ordered mesoporous films. In fact, pore collapse and densification during calcination occurred in the case of a 0.3 MTES/TEOS molar ratio. The mechanical properties of a porous film are largely dependent on its density. Generally, the elastic modulus and density of a film have a linear relationship in log-scale with a positive slope of 2 to 4 [29]. The value without an underline in Fig. 11 represents the MTES/TEOS molar ratio of ordered mesoporous silica films, but the underlined value belongs to the disordered mesoporous silica film. As shown in Fig. 6, the density of the film decreased with an increase in the MTES/TEOS molar ratio. This result could also be applied to the disordered mesoporous silica films because the density of the film decreases with an increase in aging time [30]. The disordered mesoporous silica films showed normal density-dependent mechanical properties. However, the mechanical properties were especially enhanced, even though there was a reduction of the density in the case of the ordered mesoporous silica films. As discussed in Fig. 5b, the films with higher MTES/TEOS molar ratios have a more rigid wall structure. Although terminal Si–C bonds may make the silica framework weaker, the mechanical properties of the film are more strongly controlled by the mi-
Fig. 10. (a) Elastic modulus and (b) hardness of disordered mesoporous silica films according to the MTES/TEOS molar ratio.
Fig. 11. Relationship between density and elastic modulus in log–log scale for ordered and disordered mesoporous silica films. The numerical number around each data represents the MTES/TEOS molar ratio.
cropore volume in the film. The ordered mesoporous silica film with a 0.25 MTES/TEOS molar ratio has a BCC structure. An abrupt increase in the elastic modulus for the ordered mesoporous silica film with a 0.25 MTES/TEOS molar ratio could be attributed to the fact that cubic structures with spherical pores are more mechanically robust than hexagonally packed structures [31]. This result highlights the benefits of BCC geometry and pore structure ordering for the application of ordered mesoporous silica films, such as low-k dielectrics.
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provide mechanical stability and a low dielectric property to an ordered mesoporous silica film for low-k applications due to reduction of micropore volume and BCC pore ordering. Acknowledgment This work was supported by Grant R01-2005-000-10058-0 from the Basic Research Program of the Korea Science & Engineering Foundation. References
Fig. 12. The dielectric constant of ordered mesoporous silica films according to the MTES/TEOS molar ratio.
Fig. 12 shows the variations in the dielectric constant of the ordered mesoporous silica films with respect to the MTES/TEOS molar ratio. The dielectric constant gradually decreased due to a reduction of the density. The dielectric constant is a little smaller than the calculated one by using the Bruggeman effective medium approximation. This is attributed to the existence of Si–C bonds with low molar polarizability [32]. In particular, the ordered mesoporous silica film with a 0.25 MTES/TEOS molar ratio has the lowest dielectric constant. Namely, the film with BCC pore structure shows the lowest dielectric constant and highest mechanical strength, as shown in Figs. 11 and 12. So it can be concluded that film with low micropore volume fraction and BCC pore ordering is more applicable to a low-k material due to low dielectric constant and high mechanical strength. 4. Summary A modification of a silica framework with Si–C bonds was successfully performed using TEOS and MTES as silica precursors. By controlling the rate of the condensation reaction, ordered mesoporous silica films could be prepared up to a 0.25 MTES/TEOS molar ratio. The mesophase of the films with 0.1–0.2 MTES/TEOS molar ratios showed a two-dimensional hexagonal structure while the mesophase of the film with a 0.25 MTES/TEOS molar ratio showed a bcc structure. A disordered mesoporous silica film was obtained when the silica sol aging time was long or the MTES/TEOS molar ratio was over 0.3. A reduction of the micropore volume and an increase of the pore size were observed with the increase of the MTES/TEOS molar ratio. The mechanical properties of the ordered mesoporous silica films represented an exceptional increase with the increase of the MTES/TEOS molar ratio or porosity. This was due to the reduction of micropore volume and pore ordering in the case of organically modified mesoporous silica films. The dielectric constant was lowered with an increase of the MTES/TEOS molar ratio due to a reduction of film density and polarizability of the framework. Especially, the film with a 0.25 MTES/TEOS molar ratio has the lowest dielectric constant and highest mechanical strength. In conclusion, the organic modification of the framework by methyl ligands could
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