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Effect of solvent on the preparation of ambient pressure-dried SiO2 aerogel films
Microelectronic Engineering 65 (2003) 113–122 www.elsevier.com / locate / mee
Effect of solvent on the preparation of ambient pressure-dried SiO 2 aerogel films Sang-Bae Jung, Jung-Ho Kim, Hong-Ryul Kim, Hyung-Ho Park* Department of Ceramic Engineering, Yonsei University, 134 Shinchon-Dong, Seodaemun-Ku, Seoul 120 -749, South Korea Received 27 March 2002
Abstract SiO 2 aerogel film has a promising property as intermetal dielectrics (IMD) for its low dielectric constant. However, a stable and porous SiO 2 aerogel film was not properly synthesized due to the rapid evaporation of solvent during spin coating even in a solvent saturated atmosphere. So less evaporative solvent, 2-methoxyethanol (2MeEtOH), was introduced and the properties of film were compared with films using conventional solvent, ethanol (EtOH). It was found that gelation was faster with 2MeEtOH than EtOH. Highly porous and three-dimensionally well microstructured SiO 2 aerogel film could be fabricated with 2MeEtOH. The maximum porosity of the films using 2MeEtOH and EtOH was 85 and 71%, respectively. The amount of residual –OR, and –OH groups was smaller in the former. The compositional ratios of Si:O:C in the films were 1:2.1:0.2 for the former and 1:2.4:1.1 for the latter. And corresponded dielectric constants were 1.6 and 2.2, respectively. 2003 Elsevier Science B.V. All rights reserved. Keywords: SiO 2 aerogel film; Ambient pressure drying; Spin coating; Solvent
1. Introduction In deep submicron devices, circuit performance efficiency will be increased by the reduction in transistor gate length. However, a scaling down of feature device size results in the delay of signal runtime, called RC time delay. This delay can be reduced by using a higher electro-conductive material for interconnect and / or a lower dielectric constant-material for intermetal dielectric (IMD). Various materials have been focused for the application of that purpose [1,2]. In candidate materials for IMD application, aerogel is one of promising low-k materials because of its inherent porous structure. Aerogels are prepared through sol–gel and subsequent supercritical * Corresponding author. E-mail address: [email protected] (H.-H. Park). 0167-9317 / 03 / $ – see front matter PII: S0167-9317( 02 )00734-7
2003 Elsevier Science B.V. All rights reserved.
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drying procedure. Supercritical drying is a conventional method to form porous materials, but it is hazardous and discontinuous. So many attempts have been made to obtain porous materials without supercritical drying. Ambient pressure drying, one of non-supercritical drying procedure, is to modify a wet–gel surface with non-reactive chemical species. The surface modification obstructs additional condensation reaction and densification induced by capillary pressure during drying, and permits aerogel film to expand back to its original volume through low temperature anneal, i.e., springback effect [3]. Lots of factors such as starting alkoxide, solvent, pH and so on affect on the porosity, pore size, pore size distribution and other physico-chemical properties of films. Many works have been carried out to evaluate the relationship but mainly on bulky SiO 2 aerogel [4–6]. However, for applying it to electronic devices, it should be prepared in film shape. SiO 2 aerogel films are usually formed by spin coating. Spin coating process can be divided into four stages such as deposition, spin-up, spin-off, and evaporation [7]. Among these steps, fluid film thinning mainly happens in the last two stages. Although viscous force and solvent evaporation should be considered together, solvent evaporation is dominant during spin coating. Because of a simultaneous occurrence of gelation and drying during spin coating, film coating is usually accomplished in a solvent-saturated atmosphere to suppress the rapid evaporation of solvent. But one of most widely used solvents, ethanol (EtOH), rapidly evaporates even with the solvent-saturated atmosphere, due to its high volatility [8]. A rapid solvent evaporation during spin coating induces a sharp increase in sol viscosity resulting in a rapid gelation and high capillary pressure resulting in a collapse of network structure of film. Due to the above reasons, it is difficult to obtain a reproducible, porous SiO 2 aerogel film. So it is inferred that a stable, porous SiO 2 aerogel film can be made using a less volatile solvent. To that purpose, 2-methoxyethanol (2MeEtOH) was introduced in this experiment. The criteria for choosing a solvent is its vapor pressure (7.87 kPa for EtOH and 1.3 kPa for 2MeEtOH at 25 8C) and solubility with precursors [9].
2. Experimental procedure EtOH-based and 2MeEtOH-based sols were prepared as follows. Tetraethoxyorthosilicate (TEOS; Fluka Chemica, Switzerland) and each solvent (2-methoxyethanol; Junsei and ethanol; Duk San) in a molar ratio of 1:4 were mixed and then two-step acid–base catalyzed process was adopted. First, 1.8 3 10 23 mol of HCl in H 2 O was added to a stock solution. Next, 3.6 3 10 23 mol of NH 4 OH in H2O was added successively. Final composition of the sol containing TEOS:solvent:H 2 O:HCl:NH 4 OH is 1:4:3:1.8 3 10 23 :3.6 3 10 23 . Each 2 ml sol was spin-deposited on 2 3 2 cm P-type Si (100) wafer using a commercial photo-resist spinner in an optimized viscosity range of each solvent in the solvent-saturated atmosphere. For EtOH-based sol it was 10–20 cP and for 2MeEtOH-based sol, it was 40–50 cP. Spin coating time was varied from 20 to 60 s with a step of 10 s. Then each spun-on film was aged in each mother solvent for 24 h at 70 8C. The aged film was immersed in acetone and subsequently in n-hexane in order to exchange each mother solvent in gel network structure with n-hexane. Surface modification with 6 vol% TMCS in n-hexane was conducted for 24 h at 30 8C. The morphology and thickness of films were investigated by field emission scanning electron microscopy (SEM, Hitachi S 4200) with 10 kV of electron beam energy. Refractive indices of films were measured using ellipsometer (Gaertner Scientific, L117C). From the refractive index, porosity of
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film could be calculated [10]. Viscosity was measured using viscometer (Brookfield, LVT B). Fourier transform infrared spectroscopic (FT-IR, Jasco 300Z) analyses were performed to compare chemical species for each aerogel film spin-coated for 20 s. Chemical composition was determined using Rutherford backscattering spectrometer (RBS) of a 2-MeV He 21 Pelletron accelerator. The dielectric constant in metal–insulator–semiconductor (MIS) structure was directly calculated using HP 4298A Impedance / Gain-Phase Analyzer at 1 MHz. Aluminum was deposited on film as upper and lower electrodes with thermal evaporator.
3. Results and discussion Fig. 1 shows the viscosity change of 2MeEtOH-based and EtOH-based sols with time at 2 20 8C. Gelling point was determined at a viscosity of 100 cP. In general, gelation-rate is dependent on steric and inductive effects of solvent [7]. Steric effect is a retardation of gelation by bulky group and inductive effect refers to the change of gelation-rate according to the chemical species around central metal ion. The above two effects determine the overall reaction kinetic. When we consider steric effect only, it could be estimated that gelation of more branched 2MeEtOH-based sol occurs slower than EtOH-based sol. However, according to Luk et al. [11], the methoxyethoxy group is more negatively charged than ethoxy group. So when the transesterification reaction occurs from ethoxy group to methoxyethoxy group, the electron density of central Si decreases. Then in the acid-catalyzed step, H 1 ion attack on the methoxyethoxy group is faster than that on the ethoxy group, while in the base-catalyzed step, OH 2 ion attack on Si bonding to the methoxyethoxy group is faster than that on the Si bonding to ethoxy group. In this experimental case, the inductive effect seemed to determine a gelation rate because the gelation of 2MeEtOH-based sol was faster than that of EtOH-based sol. Fig. 2 corresponds to the SEM images of 2MeEtOH-based aerogel films with respect to coating sol
Fig. 1. Viscosity changes of 2MeEtOH-based and EtOH-based sols with time at 2 20 8C.
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Fig. 2. Effect of coating sol viscosity on the morphology of 2MeEtOH-based aerogel films; (a) 20–30 and (b) . 50 cP.
viscosity. Our previous work showed that optimum coating viscosity of EtOH-based sol was 10–20 cP [12]. However, with 2MeEtOH-based sol, a stable film was not obtained at such a low coating sol viscosity due to the dissolution of the coated film during subsequent aging solution procedure. It was due to the low evaporation of 2MeEtOH, i.e., the solvent did not rapidly evaporate and the viscosity of the coated sol was insufficient for the occurrence of subsequent gelation. Even with coating sol-viscosity of 20–30 cP, surface morphology of the film was too porous with large pore to serve for microelectronics purposes. However, coating sol viscosity was above 50 cP, as shown in Fig. 2b; particles were over-grown and dense network formation of SiO 2 aerogel was obtained. When 2MeEtOH-based film was spin-coated at a viscosity of 40–50 cP, as shown in Fig. 3, an optimum film microstructure was obtained for IMD application. A difference in the appropriate range of coating sol viscosity between the solvents is mainly due to their evaporation rate. The more volatile the solvent is, a lower coating sol viscosity is required. Figs. 3 and 4 show surface morphology of SiO 2 aerogel films according to spin coating time for 2MeEtOH-based and EtOH-based sols, respectively. Even though they were spin coated at their appropriate coating sol viscosity, the images showed how the evaporation of solvent during spin coating could affect the microstructure of film. For 2MeEtOH-based films, porous and highly cross-linked microstructure was obtained until 30 s, while for EtOH-based films, it was limited within 20 s. Also with 2MeEtOH-based films, it was easily shown that the films spin-coated over 40 s showed dense microstructure containing distinguishable
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Fig. 3. Surface morphology of 2MeEtOH-based aerogel films after spin coating for (a) 20, (b) 30, (c) 40, (d) 50, and (e) 60 s.
particles. This was because the coating sol viscosity of 2MeEtOH-based sol, 40–50 cP, was high enough to form an appropriate size of primary particles. For the solvents with low evaporation rate, such as 2MeEtOH, the increase in the viscosity of coated sol was smaller than that of EtOH during spin coating. This permitted somewhat steady gelation for spin-coated 2MeEtOH-based sol, even with longer spin coating time. During spin coating, formation and growth of particles proceeded. However, with spin coating above 40 s the coated sol became so viscous that the gelation reaction was quickly terminated, and then a weak and incomplete three-dimensional network structure was generated. This
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Fig. 4. Surface morphology of EtOH-based aerogel films after spin coating for (a) 20, (b) 30, (c) 40, (d) 50, and (e) 60 s.
kind of incomplete network structure might collapse during spin coating due to a capillary pressure resulted from the solvent evaporation or during the drying even after surface modification with TMCS. Because of the above reasons, a collapsed and dense microstructure was obtained with 2MeEtOH-based sol after spin coating above 40 s. However in case of EtOH-based sol, a sharp increase in coated sol viscosity during spin coating due to high evaporation rate of EtOH induced a fast termination of gelation reaction. Particle growth and network formation was proceeded
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simultaneously and none of them was successfully accomplished. As a result, a badly constructed three-dimensional network composed of fine particle agglomeration was obtained. Furthermore with the increase in spin coating time, network structure could not be maintained and a highly dense microstructure with fine particle agglomeration was induced. Fig. 5a,b shows a variation in thickness and porosity of films with respect to spin coating time, respectively. They showed a similar behavior as surface microstructural change as shown in Figs. 3 and 4. The thickness of 2MeEtOH-based and EtOH-based films obtained after spin coating for 20 s was 3 and 1 mm, respectively. And the porosity of the films was 85 and 71%, respectively. For 2MeEtOH-based films, thickness and porosity were negligibly changed until 30 s of spin coating because of the low evaporation rate of 2MeEtOH. Within 30 s of spin coating, the coated sol viscosity increased but gelation was not completed. However, through the aging, network structure could be developed and strengthened. When we spin coated 2MeEtOH-based sol over 40 s or EtOH-based sol
Fig. 5. (a) Thickness ratio and (b) porosity variation of 2MeEtOH-based and EtOH-based aerogel films with spin coating time.
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over 30 s, the coated sol viscosity passed the gelation point during spin coating. In this case, a dense microstructure with collapsed network was obtained and as a result, reduction in the thickness and porosity of film was induced. Fig. 6 shows the FT-IR spectra of surface-modified EtOH-based and 2MeEtOH-based aerogel films with TMCS. They were spin coated for 20 s. The peaks at 1080, 800, and 460 cm 21 corresponded to Si–O–Si asymmetric stretching, symmetric stretching, and bending vibration, respectively [7]. This observation showed that the films were constituted with siloxane network. In the EtOH-based aerogel film, due to incomplete hydrolysis, residual organic groups were easily observed, i.e., the peaks at 2885 and 2975 cm 21 from –CH 2 and –CH 3 , respectively. However in the 2MeEtOH-based aerogel film, residual organic –CH 2 and –CH 3 groups were negligible but absorption peak of –CH 3 group from surface modification was found at 2965 cm 21 [12]. Especially the relative intensities of Si–OH peak at 960 cm 21 and SiO–H related peaks at 3400–3750 cm 21 were more intensified in the EtOH-based aerogel film than the 2MeEtOH-based aerogel film. The FT-IR results showed that hydrolysis was incomplete for the EtOH-based aerogel film due to the fast evaporation of solvent during spin coating. The composition of each aerogel film is given in Fig. 7. The atomic ratio of Si:O:C was 1:2.1:0.2 for the 2MeEtOH-based aerogel film and 1:2.4:1.1 for the EtOH-based aerogel film. The ratio was calculated by considering the yield height of each element. The atomic contents of O and C were much higher in the EtOH-based aerogel film than the 2MeEtOH-based aerogel film. This result agreed well with the results observed from FT-IR analyses.
Fig. 6. FT-IR spectra of surface modified 2MeEtOH-based and EtOH-based aerogel films with TMCS; a magnified spectrum is given.
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Fig. 7. RBS spectra of (a) 2MeEtOH-based and (b) EtOH-based aerogel films.
To investigate the applicability of both films to IMD, dielectric constant was measured with MIS structure. The measured values were 1.6 and 2.2 for the 2MeEtOH-based and the EtOH-based aerogel films, respectively. The calculated ones from the consideration of porosity values were 1.53 and 2.04, respectively. A larger difference was found with the EtOH-based aerogel film due to the existence of polar groups such as –OR and –OH [13]. From the above results, it could be said that 2MeEtOH-
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based aerogel film of high porosity was more appropriate than EtOH-based aerogel film when applying them to IMD.
4. Conclusions Surface microstructure of SiO 2 aerogel film was found to be strongly dependent on coating sol viscosity and spin coating time. Due to relatively low evaporation of 2MeEtOH, optimum coating sol viscosity for 2MeEtOH-based sol was 40–50 cP, higher than that of EtOH, 10–20 cP. For 2MeEtOH-based film, porous and highly cross-linked microstructure was obtained up to 30 s of spin coating, while for EtOH-based films, it was limited within 20 s. When the viscosity of coated sol passed gelation point during spin coating due to the evaporation of solvent, the resultant microstructure of SiO 2 film always turned to be dense. The thickness of 2MeEtOH-based and EtOH-based aerogel films after 20 s of spin coating was found to be 3 and 1 mm, respectively, and their porosity was calculated as 85 and 71%, respectively. Sinusoidal change of thickness according to spin coating time was observed for 2MeEtOH-based aerogel film, while that of EtOH-based aerogel film gradually decreased within 60 s. The lowest dielectric constant was obtained as 1.6 and 2.2 for 2MeEtOH-based and EtOH-based aerogel films, respectively. To conclude, relatively less volatile solvent, 2MeEtOH was proved more adoptable than EtOH to the preparation of SiO 2 aerogel film for use in low dielectric application.
Acknowledgements The authors wish to acknowledge the financial support of Brain Korea 21 project.
References [1] H. Treichel, G. Ruhl, P. Ansmann, R. Wurl, Ch. Muller, M. Dietlmeier and G. Majer, Dielectrics for ULSI Multilevel Interconnection Conference (DUMIC), The Institute for Microelectronics Interconnection (IMIC), Santa Clara, 16–17 February, 333D (1998) 201–210. [2] S.P. Murarka, Solid State Tech. 39 (3) (1996) 83–87. [3] R.A. Cairncross, P.R. Schunk, K.S. Chen, S.S. Prakash, J. Samuel, A.J. Hurd, C.J. Brinker, Drying Tech. 15 (6–8) (1997) 1815–1915. [4] J. Fricke, J. Non-Cryst. Solids 100 (1988) 169–173. [5] A.V. Rao, P.B. Wagh, G.M. Pajonk, N.N. Parvathy, J. Mater. Res. 29 (1994) 1807–1817. [6] Y. Cao, Z. Xia, Q. Li, J. Shen, L. Chen, B. Zhou, IEEE Trans. 5 (1) (1998) 58–62. [7] C.J. Brinker, G.W. Scherer, in: Sol–Gel Science, Academic Press, San Diego, CA, 1990. [8] D.M. Smith, W.C. Ackerman, US Patent 5736425 (1998). [9] J.A. Riddick, W.B. Bunger, T.K. Sakano, in: Organic Solvents: Physical Properties and Methods of Purification, Wiley-Interscience, New York, 1986. [10] J.K. Hong, H.S. Yang, M.H. Jo, H.H. Park, S.Y. Choi, Thin Solid Films 308–309 (1997) 495–500. [11] C.H. Luk, C.L. Mak, K.H. Wong, Thin Solid Films 298 (1997) 57–61. [12] J.K. Hong, H.R. Kim, H.H. Park, Thin Solids Films 332 (1998) 449–454. [13] L.W. Hrubesh, L.E. Keene, V.R. Latorre, J. Mater. Res. 8 (7) (1993) 1736–1741.
Effect of solvent on the preparation of ambient pressure-dried SiO 2 aerogel films Sang-Bae Jung, Jung-Ho Kim, Hong-Ryul Kim, Hyung-Ho Park* Department of Ceramic Engineering, Yonsei University, 134 Shinchon-Dong, Seodaemun-Ku, Seoul 120 -749, South Korea Received 27 March 2002
Abstract SiO 2 aerogel film has a promising property as intermetal dielectrics (IMD) for its low dielectric constant. However, a stable and porous SiO 2 aerogel film was not properly synthesized due to the rapid evaporation of solvent during spin coating even in a solvent saturated atmosphere. So less evaporative solvent, 2-methoxyethanol (2MeEtOH), was introduced and the properties of film were compared with films using conventional solvent, ethanol (EtOH). It was found that gelation was faster with 2MeEtOH than EtOH. Highly porous and three-dimensionally well microstructured SiO 2 aerogel film could be fabricated with 2MeEtOH. The maximum porosity of the films using 2MeEtOH and EtOH was 85 and 71%, respectively. The amount of residual –OR, and –OH groups was smaller in the former. The compositional ratios of Si:O:C in the films were 1:2.1:0.2 for the former and 1:2.4:1.1 for the latter. And corresponded dielectric constants were 1.6 and 2.2, respectively. 2003 Elsevier Science B.V. All rights reserved. Keywords: SiO 2 aerogel film; Ambient pressure drying; Spin coating; Solvent
1. Introduction In deep submicron devices, circuit performance efficiency will be increased by the reduction in transistor gate length. However, a scaling down of feature device size results in the delay of signal runtime, called RC time delay. This delay can be reduced by using a higher electro-conductive material for interconnect and / or a lower dielectric constant-material for intermetal dielectric (IMD). Various materials have been focused for the application of that purpose [1,2]. In candidate materials for IMD application, aerogel is one of promising low-k materials because of its inherent porous structure. Aerogels are prepared through sol–gel and subsequent supercritical * Corresponding author. E-mail address: [email protected] (H.-H. Park). 0167-9317 / 03 / $ – see front matter PII: S0167-9317( 02 )00734-7
2003 Elsevier Science B.V. All rights reserved.
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drying procedure. Supercritical drying is a conventional method to form porous materials, but it is hazardous and discontinuous. So many attempts have been made to obtain porous materials without supercritical drying. Ambient pressure drying, one of non-supercritical drying procedure, is to modify a wet–gel surface with non-reactive chemical species. The surface modification obstructs additional condensation reaction and densification induced by capillary pressure during drying, and permits aerogel film to expand back to its original volume through low temperature anneal, i.e., springback effect [3]. Lots of factors such as starting alkoxide, solvent, pH and so on affect on the porosity, pore size, pore size distribution and other physico-chemical properties of films. Many works have been carried out to evaluate the relationship but mainly on bulky SiO 2 aerogel [4–6]. However, for applying it to electronic devices, it should be prepared in film shape. SiO 2 aerogel films are usually formed by spin coating. Spin coating process can be divided into four stages such as deposition, spin-up, spin-off, and evaporation [7]. Among these steps, fluid film thinning mainly happens in the last two stages. Although viscous force and solvent evaporation should be considered together, solvent evaporation is dominant during spin coating. Because of a simultaneous occurrence of gelation and drying during spin coating, film coating is usually accomplished in a solvent-saturated atmosphere to suppress the rapid evaporation of solvent. But one of most widely used solvents, ethanol (EtOH), rapidly evaporates even with the solvent-saturated atmosphere, due to its high volatility [8]. A rapid solvent evaporation during spin coating induces a sharp increase in sol viscosity resulting in a rapid gelation and high capillary pressure resulting in a collapse of network structure of film. Due to the above reasons, it is difficult to obtain a reproducible, porous SiO 2 aerogel film. So it is inferred that a stable, porous SiO 2 aerogel film can be made using a less volatile solvent. To that purpose, 2-methoxyethanol (2MeEtOH) was introduced in this experiment. The criteria for choosing a solvent is its vapor pressure (7.87 kPa for EtOH and 1.3 kPa for 2MeEtOH at 25 8C) and solubility with precursors [9].
2. Experimental procedure EtOH-based and 2MeEtOH-based sols were prepared as follows. Tetraethoxyorthosilicate (TEOS; Fluka Chemica, Switzerland) and each solvent (2-methoxyethanol; Junsei and ethanol; Duk San) in a molar ratio of 1:4 were mixed and then two-step acid–base catalyzed process was adopted. First, 1.8 3 10 23 mol of HCl in H 2 O was added to a stock solution. Next, 3.6 3 10 23 mol of NH 4 OH in H2O was added successively. Final composition of the sol containing TEOS:solvent:H 2 O:HCl:NH 4 OH is 1:4:3:1.8 3 10 23 :3.6 3 10 23 . Each 2 ml sol was spin-deposited on 2 3 2 cm P-type Si (100) wafer using a commercial photo-resist spinner in an optimized viscosity range of each solvent in the solvent-saturated atmosphere. For EtOH-based sol it was 10–20 cP and for 2MeEtOH-based sol, it was 40–50 cP. Spin coating time was varied from 20 to 60 s with a step of 10 s. Then each spun-on film was aged in each mother solvent for 24 h at 70 8C. The aged film was immersed in acetone and subsequently in n-hexane in order to exchange each mother solvent in gel network structure with n-hexane. Surface modification with 6 vol% TMCS in n-hexane was conducted for 24 h at 30 8C. The morphology and thickness of films were investigated by field emission scanning electron microscopy (SEM, Hitachi S 4200) with 10 kV of electron beam energy. Refractive indices of films were measured using ellipsometer (Gaertner Scientific, L117C). From the refractive index, porosity of
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film could be calculated [10]. Viscosity was measured using viscometer (Brookfield, LVT B). Fourier transform infrared spectroscopic (FT-IR, Jasco 300Z) analyses were performed to compare chemical species for each aerogel film spin-coated for 20 s. Chemical composition was determined using Rutherford backscattering spectrometer (RBS) of a 2-MeV He 21 Pelletron accelerator. The dielectric constant in metal–insulator–semiconductor (MIS) structure was directly calculated using HP 4298A Impedance / Gain-Phase Analyzer at 1 MHz. Aluminum was deposited on film as upper and lower electrodes with thermal evaporator.
3. Results and discussion Fig. 1 shows the viscosity change of 2MeEtOH-based and EtOH-based sols with time at 2 20 8C. Gelling point was determined at a viscosity of 100 cP. In general, gelation-rate is dependent on steric and inductive effects of solvent [7]. Steric effect is a retardation of gelation by bulky group and inductive effect refers to the change of gelation-rate according to the chemical species around central metal ion. The above two effects determine the overall reaction kinetic. When we consider steric effect only, it could be estimated that gelation of more branched 2MeEtOH-based sol occurs slower than EtOH-based sol. However, according to Luk et al. [11], the methoxyethoxy group is more negatively charged than ethoxy group. So when the transesterification reaction occurs from ethoxy group to methoxyethoxy group, the electron density of central Si decreases. Then in the acid-catalyzed step, H 1 ion attack on the methoxyethoxy group is faster than that on the ethoxy group, while in the base-catalyzed step, OH 2 ion attack on Si bonding to the methoxyethoxy group is faster than that on the Si bonding to ethoxy group. In this experimental case, the inductive effect seemed to determine a gelation rate because the gelation of 2MeEtOH-based sol was faster than that of EtOH-based sol. Fig. 2 corresponds to the SEM images of 2MeEtOH-based aerogel films with respect to coating sol
Fig. 1. Viscosity changes of 2MeEtOH-based and EtOH-based sols with time at 2 20 8C.
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Fig. 2. Effect of coating sol viscosity on the morphology of 2MeEtOH-based aerogel films; (a) 20–30 and (b) . 50 cP.
viscosity. Our previous work showed that optimum coating viscosity of EtOH-based sol was 10–20 cP [12]. However, with 2MeEtOH-based sol, a stable film was not obtained at such a low coating sol viscosity due to the dissolution of the coated film during subsequent aging solution procedure. It was due to the low evaporation of 2MeEtOH, i.e., the solvent did not rapidly evaporate and the viscosity of the coated sol was insufficient for the occurrence of subsequent gelation. Even with coating sol-viscosity of 20–30 cP, surface morphology of the film was too porous with large pore to serve for microelectronics purposes. However, coating sol viscosity was above 50 cP, as shown in Fig. 2b; particles were over-grown and dense network formation of SiO 2 aerogel was obtained. When 2MeEtOH-based film was spin-coated at a viscosity of 40–50 cP, as shown in Fig. 3, an optimum film microstructure was obtained for IMD application. A difference in the appropriate range of coating sol viscosity between the solvents is mainly due to their evaporation rate. The more volatile the solvent is, a lower coating sol viscosity is required. Figs. 3 and 4 show surface morphology of SiO 2 aerogel films according to spin coating time for 2MeEtOH-based and EtOH-based sols, respectively. Even though they were spin coated at their appropriate coating sol viscosity, the images showed how the evaporation of solvent during spin coating could affect the microstructure of film. For 2MeEtOH-based films, porous and highly cross-linked microstructure was obtained until 30 s, while for EtOH-based films, it was limited within 20 s. Also with 2MeEtOH-based films, it was easily shown that the films spin-coated over 40 s showed dense microstructure containing distinguishable
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Fig. 3. Surface morphology of 2MeEtOH-based aerogel films after spin coating for (a) 20, (b) 30, (c) 40, (d) 50, and (e) 60 s.
particles. This was because the coating sol viscosity of 2MeEtOH-based sol, 40–50 cP, was high enough to form an appropriate size of primary particles. For the solvents with low evaporation rate, such as 2MeEtOH, the increase in the viscosity of coated sol was smaller than that of EtOH during spin coating. This permitted somewhat steady gelation for spin-coated 2MeEtOH-based sol, even with longer spin coating time. During spin coating, formation and growth of particles proceeded. However, with spin coating above 40 s the coated sol became so viscous that the gelation reaction was quickly terminated, and then a weak and incomplete three-dimensional network structure was generated. This
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Fig. 4. Surface morphology of EtOH-based aerogel films after spin coating for (a) 20, (b) 30, (c) 40, (d) 50, and (e) 60 s.
kind of incomplete network structure might collapse during spin coating due to a capillary pressure resulted from the solvent evaporation or during the drying even after surface modification with TMCS. Because of the above reasons, a collapsed and dense microstructure was obtained with 2MeEtOH-based sol after spin coating above 40 s. However in case of EtOH-based sol, a sharp increase in coated sol viscosity during spin coating due to high evaporation rate of EtOH induced a fast termination of gelation reaction. Particle growth and network formation was proceeded
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simultaneously and none of them was successfully accomplished. As a result, a badly constructed three-dimensional network composed of fine particle agglomeration was obtained. Furthermore with the increase in spin coating time, network structure could not be maintained and a highly dense microstructure with fine particle agglomeration was induced. Fig. 5a,b shows a variation in thickness and porosity of films with respect to spin coating time, respectively. They showed a similar behavior as surface microstructural change as shown in Figs. 3 and 4. The thickness of 2MeEtOH-based and EtOH-based films obtained after spin coating for 20 s was 3 and 1 mm, respectively. And the porosity of the films was 85 and 71%, respectively. For 2MeEtOH-based films, thickness and porosity were negligibly changed until 30 s of spin coating because of the low evaporation rate of 2MeEtOH. Within 30 s of spin coating, the coated sol viscosity increased but gelation was not completed. However, through the aging, network structure could be developed and strengthened. When we spin coated 2MeEtOH-based sol over 40 s or EtOH-based sol
Fig. 5. (a) Thickness ratio and (b) porosity variation of 2MeEtOH-based and EtOH-based aerogel films with spin coating time.
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over 30 s, the coated sol viscosity passed the gelation point during spin coating. In this case, a dense microstructure with collapsed network was obtained and as a result, reduction in the thickness and porosity of film was induced. Fig. 6 shows the FT-IR spectra of surface-modified EtOH-based and 2MeEtOH-based aerogel films with TMCS. They were spin coated for 20 s. The peaks at 1080, 800, and 460 cm 21 corresponded to Si–O–Si asymmetric stretching, symmetric stretching, and bending vibration, respectively [7]. This observation showed that the films were constituted with siloxane network. In the EtOH-based aerogel film, due to incomplete hydrolysis, residual organic groups were easily observed, i.e., the peaks at 2885 and 2975 cm 21 from –CH 2 and –CH 3 , respectively. However in the 2MeEtOH-based aerogel film, residual organic –CH 2 and –CH 3 groups were negligible but absorption peak of –CH 3 group from surface modification was found at 2965 cm 21 [12]. Especially the relative intensities of Si–OH peak at 960 cm 21 and SiO–H related peaks at 3400–3750 cm 21 were more intensified in the EtOH-based aerogel film than the 2MeEtOH-based aerogel film. The FT-IR results showed that hydrolysis was incomplete for the EtOH-based aerogel film due to the fast evaporation of solvent during spin coating. The composition of each aerogel film is given in Fig. 7. The atomic ratio of Si:O:C was 1:2.1:0.2 for the 2MeEtOH-based aerogel film and 1:2.4:1.1 for the EtOH-based aerogel film. The ratio was calculated by considering the yield height of each element. The atomic contents of O and C were much higher in the EtOH-based aerogel film than the 2MeEtOH-based aerogel film. This result agreed well with the results observed from FT-IR analyses.
Fig. 6. FT-IR spectra of surface modified 2MeEtOH-based and EtOH-based aerogel films with TMCS; a magnified spectrum is given.
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Fig. 7. RBS spectra of (a) 2MeEtOH-based and (b) EtOH-based aerogel films.
To investigate the applicability of both films to IMD, dielectric constant was measured with MIS structure. The measured values were 1.6 and 2.2 for the 2MeEtOH-based and the EtOH-based aerogel films, respectively. The calculated ones from the consideration of porosity values were 1.53 and 2.04, respectively. A larger difference was found with the EtOH-based aerogel film due to the existence of polar groups such as –OR and –OH [13]. From the above results, it could be said that 2MeEtOH-
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based aerogel film of high porosity was more appropriate than EtOH-based aerogel film when applying them to IMD.
4. Conclusions Surface microstructure of SiO 2 aerogel film was found to be strongly dependent on coating sol viscosity and spin coating time. Due to relatively low evaporation of 2MeEtOH, optimum coating sol viscosity for 2MeEtOH-based sol was 40–50 cP, higher than that of EtOH, 10–20 cP. For 2MeEtOH-based film, porous and highly cross-linked microstructure was obtained up to 30 s of spin coating, while for EtOH-based films, it was limited within 20 s. When the viscosity of coated sol passed gelation point during spin coating due to the evaporation of solvent, the resultant microstructure of SiO 2 film always turned to be dense. The thickness of 2MeEtOH-based and EtOH-based aerogel films after 20 s of spin coating was found to be 3 and 1 mm, respectively, and their porosity was calculated as 85 and 71%, respectively. Sinusoidal change of thickness according to spin coating time was observed for 2MeEtOH-based aerogel film, while that of EtOH-based aerogel film gradually decreased within 60 s. The lowest dielectric constant was obtained as 1.6 and 2.2 for 2MeEtOH-based and EtOH-based aerogel films, respectively. To conclude, relatively less volatile solvent, 2MeEtOH was proved more adoptable than EtOH to the preparation of SiO 2 aerogel film for use in low dielectric application.
Acknowledgements The authors wish to acknowledge the financial support of Brain Korea 21 project.
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