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Ambient-dried low dielectric SiO2 aerogel thin film
Journal of Non-Crystalline Solids 221 Ž1997. 151–156
Ambient-dried low dielectric SiO 2 aerogel thin film Hee-Sun Yang, Se-Young Choi ) , Sang-Hoon Hyun, Hyung-Ho Park, Jung-Kyun Hong Department of Ceramic Engineering, Yonsei UniÕersity, 134, Shinchon-dong, Sudaemoon-ku, Seoul 120-749, South Korea Received 11 March 1997; revised 10 June 1997
Abstract Silica sol synthesized by two steps was deposited on SiŽ100. substrate by spin coating. After aging and washing the wet gel film was modified with TMCS Žtrimethylchlorosilane. to prevent additional condensation during drying. By detecting –CH 3ŽC–H. substituted for –OH species, surface modification was identified by DTA and FTIR. Films dried were heated at different temperatures to investigate heating temperature effects on silica thin film properties. When heated in the range of temperatures Ž200–5008C., crack-free silica thin films with refractive indices of 1.23–1.15 and dielectric constant 2.2–2.8 were synthesized. q 1997 Elsevier Science B.V.
1. Introduction Silica aerogels have unusual properties such as high porosity, large surface area, low density, low thermal conductivity and low dielectric constant. Therefore a number of applications have been proposed. Among applications proposed, silica aerogel as a dielectrics has been investigated. In multilevel metallization, dielectrics play an important role in providing the isolation between metal lines and layers. The capacitance of dielectrics, which is a very significant contributor to interconnection delays, is related to the cross-sectional area and length of the capacitor and to the dielectric constant of the dielectric layer. Lowering of capacitance can be achieved by using materials of low dielectric constants w1x. Commonly used dielectrics have a dielectric constant in the range of 3.7–4.1 )
Corresponding author. Tel.: q82-2 361 2851; fax: q82-2 365 0821; e-mail: [email protected].
while silica aerogel has the ultralow dielectric constant of less than 1.7. Supercritical drying, a conventional method to synthesize aerogel, costs much energy and does not permit continuous process; hence a new method of aerogel synthesis has been attempted w2x. Since the thickness uniformity and planarization of the film across the wafer is required in the microelectric manufacturing processes, spin coating, which is used to deposit photoresist in lithographic stages of microelectric manufacturing, was adopted as a deposition technique w3,4x. During silica sol–gel transition, condensation occurs continuously. The capillary force, resulting from pore fluid evaporation during drying, promotes shrinkage and additional condensation between –OH species. Si–O–Si bonds formed make shrinkage irreversible. Additional condensations in the wet gel can be prevented by developing non-reactive –CH 3 species by means of surface modification with silane compound such as TMCS Žtrimethylchlorosilane..
0022-3093r97r$17.00 q 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 0 9 3 Ž 9 7 . 0 0 3 3 5 - 9
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H.-S. Yang et al.r Journal of Non-Crystalline Solids 221 (1997) 151–156
Substitution of –CH 3 species for –OH also contributes to gel porosity. Brinker et al. prepared porous silica thin film by redispersing modified wet gel and dipping the substrate into redispersed sol w5x. In this work we prepared aerogel-like silica thin film with the refractive indices in the range of 1.23–1.15 Žequivalent porosity 50–67%. by spin coating with two-step synthesized silica sol.
2. Experimental procedure Silica sols were prepared by two-step acidrbase catalysis. In the first step, TEOS Žtetraethoxysilane, Aldrich, USA., ethanol Žethyl alcohol, Duksan, Korea., H 2 O and HCl were combined in the molar ratio 1:3:1:1.4 = 10y3 and mixed at room temperature for 90 min. In the second step, a mixture of 14 ml of ethanol and 2 ml of 0.05 M NH 4 OH was added into 10 ml of stock solution and mixed for 30 min. Viscosities in the range of 8–14 cP were chosen for spin coating. Spin coating conditions were optimized 2000 rpm and 10 s. Spin coating was conducted within an atmosphere saturated with ethanol vapor to limit the rapid solvent evaporation. Immediately after spinning, the coated substrate was moved into a container saturated with ethanol atmosphere and the network was strengthened through aging. Pore fluids in the wet gel film were alternately exchanged with ethanol and n-hexane ŽDuksan, Korea. to facilitate the following surface modification ŽTMCS reacts with H 2 O and ethanol but does not react with n-hexane although it has good miscibility with n-hexane.. Subsequent modification proceeded by immersing coated substrate in 6 vol.% TMCS Žtrimethylchlorosilane, Lancaster, England.rn-hexane solution at 608C for 12 h. Modified wet gel film was washed in n-hexane for 12 h, dried at 608C and heated to temperatures in the range of 200–5008C Žheating rate; 18Crmin, soaking time; 2 h, cooling rate; 28Crmin.. Fourier-transform infrared spectroscopy ŽFTIR; Jasco, FTIR-300Z, France. was employed to distinguish the unmodified and modified silica thin film and observe the compositional change of films upon heating. Differential thermal analysis ŽDTA; TGrDTA-92, Setaram, France., the result of which presented silica gel monolith prepared under the
same conditions as thin film, was used to supplement FTIR. Ellipsometry ŽL117, Gaertner, USA. was used to determine the refractive index and thickness of the films ŽSEM was used to supplement ellipsometric data for the film thickness.. The morphology of films was characterized by scanning electron microscopy ŽSEM; Hitachi, H600, Japan.. The filmrsubstrate coated with a Au electrode was made to form MIS Žmetal–insulator–semiconductor. structure, and an impedencergain-phase analyzer ŽHP, 4194 A, USA. was used to measure film dielectric constant.
3. Results Silica sol, which has a water:TEOS molar ratio of 1 and final pH range of 8.5–8.7, exhibits about 2 h gelation time right after two steps preparation. The prepared silica sol was allowed to stand at room temperature until it had the appropriate viscosity for spin coating. Fig. 1 shows the change of viscosity versus time measured with two-step synthesized sol. As shown in Fig. 1, the viscosity range for the coating lies in between near 6 and 17.5 cP. Therefore
Fig. 1. Viscosity changes of SiO 2 sol according to time Žat room temperature.. Each point represents the mean value from three times measurement.
H.-S. Yang et al.r Journal of Non-Crystalline Solids 221 (1997) 151–156
the viscosities in the range of 8–14 cP were established for the spin coating. Films coated below near 6 cP exhibited non-homogeneous quality and those above 17.5 cP exhibited a cracked surface. Immediately after spin coating under the saturated ethanol atmosphere aging process for the condensation reaction under ethanol atmosphere is needed to enhance films adhesion and strengthen the network. Films without this aging show poor adhesion and disappear during ethanol washing. Fig. 2 shows FTIR spectra of Ža. unmodified and Žb. modified films. The two insets indicate magnified regions of C–H. While the 2976, 2933 and 796 cmy1 peaks indicate C–H bonding due to ethoxy groups Ž –OC 2 H 5 ., the 2968 and 852 cmy1 peaks C–H bonding due to the presence of Si–CH 3 w6x.
153
Fig. 3 shows DTA curve of 608C dried silica gel monolith which experienced surface modification. It is believed that the exothermic peak in the vicinity of 3108C is a consequence of oxidation of –CH 3 which resulted from modification. In addition SEM photographs shown in Fig. 4 exhibit the significant morphological distinction between unmodified and modified films. The substrate coated with silica film was divided into two pieces for comparison. One does not undergo modification and the other undergoes it. This presents the effect of modification that prevents additional condensation between –OH species resulting from the capillary force when drying w7x. Whereas the unmodified film has densesurface morphology and thickness of 180 nm approximately. The modified film has porous-surface mor-
Fig. 2. Comparison of FTIR spectra of Ža. unmodified and Žb. modified 608C dried films.
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H.-S. Yang et al.r Journal of Non-Crystalline Solids 221 (1997) 151–156
Fig. 3. DTA curve for modified SiO 2 gel Žmonolith type..
Fig. 5. Changes of FTIR spectra of modified films according to temperature.
phology and thickness over twice that of the unmodified film. In a multilevel metallization scheme when alu-
minum is present as a metal layer on the wafer surface, the maximum temperature of the intermetal dielectric layer is limited to ; 4508C w3x. Therefore
Fig. 4. SEM photographs of planar and cross-sectional views of Ža. unmodified and Žb. modified 608C dried films, respectively.
H.-S. Yang et al.r Journal of Non-Crystalline Solids 221 (1997) 151–156
155
Fig. 6. Changes of refractive index of SiO 2 thin film according to temperature.
Fig. 7. Changes of dielectric constant of SiO 2 thin film according to temperature.
the influence of temperature was considered to ; 5008C. The FTIR spectra of films heated in the different range of temperatures are shown in Fig. 5. The remarkable change of peak lies in 2968 and 3736 cmy1 which correspond to C–H and Si–OH bonding, respectively w6,8x. It is observed that –CH 3 species has the decreasing tendency with temperature but do not disappear completely with heat treatment to 5008C. As shown in the DTA curve, the continuous exothermic reaction proceeds to ; 5008C. It is
clear from this result that oxidation of organic groups Ž –CH 3 , –OC 2 H 5 . does not cease to ; 5008C. Figs. 6 and 7 show the influence of temperature on refractive index and dielectric constant of film. As the temperature increases, the refractive index gradually decreases. It was expected that lower refractive index of the film would lead to lower dielectric constant, but that was not the case as shown in Fig. 7. The mark represents the mean value of six regions where dielectric constants are measured. All values lie in the range of error bar. The dielectric
Fig. 8. Illustration of the formation of –CH 3 species and their transition to –OH.
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H.-S. Yang et al.r Journal of Non-Crystalline Solids 221 (1997) 151–156
constant of 3008C heated film exhibits the minimum value of 2.2.
P is the porosity of the film for silica aerogel w11x. The densities and porosities evaluated indirectly were in the range from 0.7 to 1.1 grcm3 and from 50 to 67%, respectively.
4. Discussion For the films heated above 4008C the appearance of peak at 3736 cmy1 , which presents isolated, single Si–OH species w8x, throws the meaningful information on the properties of film. Even though a considerable amount of –CH 3 species remain still in the case of 3008C heated film, the transition Žoxidation. of pyrolyzed –CH 3 to –OH species occurs in the films heated above 4008C. Fig. 8 illustrates the changes from the formation of substituted –CH 3 species during the surface modification to the transition of –CH 3 to –OH during the heat treatment above 4008C. Organic groups such as –CH 3 contribute hydrophobicity while –OH contribute hydrophilicity w9,10x. The presence of –CH 3 and –OH allows 608C dried and 5008C heated films to be hydrophobic and hydrophilic, respectively. Although films heated more than 4008C have lower refractive index, the hydrophilicity of gel surface with –OH species facilitates the adsorption of atmospheric water molecules into film, which play a significant role in increasing the dielectric constant due to high polarization of water molecule. Other heat treatment atmospheres such as N2 gas could be considered to block oxidation of –CH 3 to –OH species. Whereas the gel, in which additional condensation aggressively occurs, during drying, exhibits irreversible shrinkage, the gel with substituted –CH 3 species leads to spring back during heat treatment resulting in reversible shrinkage. It is difficult for the gases inside gel to escape due to its entangled structure; therefore it is thought that the gel network is compressed by the gases which are expanded during the heat treatment. Consequently the surface modified gel, creating the spring back phenomenon, shows higher pore volume and accordingly lower refractive index. Film densities and porosities were indirectly determined from refractive index, using the relations, r s Ž n y 1.r0.209 and P s wŽ1.458 y n.r0.458x, where n is the refractive index, r is the density and
5. Conclusions Ž1. A new process for preparing low dielectric silica thin film was established. Ž2. Surface modification was successfully performed to retard additional condensations. Ž3. A film, with the dielectric constant of 2.2, was prepared when heated at 3008C. Ž4. Films heated above 3008C had lower refractive index, and their dielectric constant became higher due to hydrophilicity promoting adsorption of water.
Acknowledgements The authors wish to appreciate ETRI ŽElectronic Telecommunicative Research Institute., for financial support of this work.
References w1x S.P. Murarka, Metallization — The Theory and Practice for VLSI and ULSI, Butterworth-Heinemann, 1993. w2x D.M. Smith, R. Deshpande, C.J. Brinker, Mater. Res. Soc. Symp. Proc. 271 Ž1992. 567. w3x S. Wolf, Silicon Processing for the VLSI ERA, vol. 2: Process Integration, Lattice Press, California, 1990. w4x D.E. Bornside, C.W. Macosko, L.E. Scriven, J. Appl. Phys. 66 Ž1989. 5185. w5x S.S. Prakash, C.J. Brinker, A.J. Hurd, J. Non-Cryst. Solids 190 Ž1995. 264. w6x C.J. Pouchert ŽEd.., The Aldrich Library of FT-IR Spectra, vol. 2, Aldrich Chemical, Wisconsin, 1985. w7x C.J. Brinker, G.W. Scherer, Sol–gel Science, Academic Press, New York, 1990. w8x R.K. Iler, The Chemistry of Silica, Wiley, New York, 1979. w9x H. Yokogawa, M. Yokoyama, J. Non-Cryst. Solids 186 Ž1995. 23. w10x K.-H. Lee, S.-Y. Kim, K.-P. Yoo, J. Non-Cryst. Solids 186 Ž1995. 18. w11x L.W. Hrubesh, J.F. Pico, J. Non-Cryst. Solids 188 Ž1995. 46.
Ambient-dried low dielectric SiO 2 aerogel thin film Hee-Sun Yang, Se-Young Choi ) , Sang-Hoon Hyun, Hyung-Ho Park, Jung-Kyun Hong Department of Ceramic Engineering, Yonsei UniÕersity, 134, Shinchon-dong, Sudaemoon-ku, Seoul 120-749, South Korea Received 11 March 1997; revised 10 June 1997
Abstract Silica sol synthesized by two steps was deposited on SiŽ100. substrate by spin coating. After aging and washing the wet gel film was modified with TMCS Žtrimethylchlorosilane. to prevent additional condensation during drying. By detecting –CH 3ŽC–H. substituted for –OH species, surface modification was identified by DTA and FTIR. Films dried were heated at different temperatures to investigate heating temperature effects on silica thin film properties. When heated in the range of temperatures Ž200–5008C., crack-free silica thin films with refractive indices of 1.23–1.15 and dielectric constant 2.2–2.8 were synthesized. q 1997 Elsevier Science B.V.
1. Introduction Silica aerogels have unusual properties such as high porosity, large surface area, low density, low thermal conductivity and low dielectric constant. Therefore a number of applications have been proposed. Among applications proposed, silica aerogel as a dielectrics has been investigated. In multilevel metallization, dielectrics play an important role in providing the isolation between metal lines and layers. The capacitance of dielectrics, which is a very significant contributor to interconnection delays, is related to the cross-sectional area and length of the capacitor and to the dielectric constant of the dielectric layer. Lowering of capacitance can be achieved by using materials of low dielectric constants w1x. Commonly used dielectrics have a dielectric constant in the range of 3.7–4.1 )
Corresponding author. Tel.: q82-2 361 2851; fax: q82-2 365 0821; e-mail: [email protected].
while silica aerogel has the ultralow dielectric constant of less than 1.7. Supercritical drying, a conventional method to synthesize aerogel, costs much energy and does not permit continuous process; hence a new method of aerogel synthesis has been attempted w2x. Since the thickness uniformity and planarization of the film across the wafer is required in the microelectric manufacturing processes, spin coating, which is used to deposit photoresist in lithographic stages of microelectric manufacturing, was adopted as a deposition technique w3,4x. During silica sol–gel transition, condensation occurs continuously. The capillary force, resulting from pore fluid evaporation during drying, promotes shrinkage and additional condensation between –OH species. Si–O–Si bonds formed make shrinkage irreversible. Additional condensations in the wet gel can be prevented by developing non-reactive –CH 3 species by means of surface modification with silane compound such as TMCS Žtrimethylchlorosilane..
0022-3093r97r$17.00 q 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 0 9 3 Ž 9 7 . 0 0 3 3 5 - 9
152
H.-S. Yang et al.r Journal of Non-Crystalline Solids 221 (1997) 151–156
Substitution of –CH 3 species for –OH also contributes to gel porosity. Brinker et al. prepared porous silica thin film by redispersing modified wet gel and dipping the substrate into redispersed sol w5x. In this work we prepared aerogel-like silica thin film with the refractive indices in the range of 1.23–1.15 Žequivalent porosity 50–67%. by spin coating with two-step synthesized silica sol.
2. Experimental procedure Silica sols were prepared by two-step acidrbase catalysis. In the first step, TEOS Žtetraethoxysilane, Aldrich, USA., ethanol Žethyl alcohol, Duksan, Korea., H 2 O and HCl were combined in the molar ratio 1:3:1:1.4 = 10y3 and mixed at room temperature for 90 min. In the second step, a mixture of 14 ml of ethanol and 2 ml of 0.05 M NH 4 OH was added into 10 ml of stock solution and mixed for 30 min. Viscosities in the range of 8–14 cP were chosen for spin coating. Spin coating conditions were optimized 2000 rpm and 10 s. Spin coating was conducted within an atmosphere saturated with ethanol vapor to limit the rapid solvent evaporation. Immediately after spinning, the coated substrate was moved into a container saturated with ethanol atmosphere and the network was strengthened through aging. Pore fluids in the wet gel film were alternately exchanged with ethanol and n-hexane ŽDuksan, Korea. to facilitate the following surface modification ŽTMCS reacts with H 2 O and ethanol but does not react with n-hexane although it has good miscibility with n-hexane.. Subsequent modification proceeded by immersing coated substrate in 6 vol.% TMCS Žtrimethylchlorosilane, Lancaster, England.rn-hexane solution at 608C for 12 h. Modified wet gel film was washed in n-hexane for 12 h, dried at 608C and heated to temperatures in the range of 200–5008C Žheating rate; 18Crmin, soaking time; 2 h, cooling rate; 28Crmin.. Fourier-transform infrared spectroscopy ŽFTIR; Jasco, FTIR-300Z, France. was employed to distinguish the unmodified and modified silica thin film and observe the compositional change of films upon heating. Differential thermal analysis ŽDTA; TGrDTA-92, Setaram, France., the result of which presented silica gel monolith prepared under the
same conditions as thin film, was used to supplement FTIR. Ellipsometry ŽL117, Gaertner, USA. was used to determine the refractive index and thickness of the films ŽSEM was used to supplement ellipsometric data for the film thickness.. The morphology of films was characterized by scanning electron microscopy ŽSEM; Hitachi, H600, Japan.. The filmrsubstrate coated with a Au electrode was made to form MIS Žmetal–insulator–semiconductor. structure, and an impedencergain-phase analyzer ŽHP, 4194 A, USA. was used to measure film dielectric constant.
3. Results Silica sol, which has a water:TEOS molar ratio of 1 and final pH range of 8.5–8.7, exhibits about 2 h gelation time right after two steps preparation. The prepared silica sol was allowed to stand at room temperature until it had the appropriate viscosity for spin coating. Fig. 1 shows the change of viscosity versus time measured with two-step synthesized sol. As shown in Fig. 1, the viscosity range for the coating lies in between near 6 and 17.5 cP. Therefore
Fig. 1. Viscosity changes of SiO 2 sol according to time Žat room temperature.. Each point represents the mean value from three times measurement.
H.-S. Yang et al.r Journal of Non-Crystalline Solids 221 (1997) 151–156
the viscosities in the range of 8–14 cP were established for the spin coating. Films coated below near 6 cP exhibited non-homogeneous quality and those above 17.5 cP exhibited a cracked surface. Immediately after spin coating under the saturated ethanol atmosphere aging process for the condensation reaction under ethanol atmosphere is needed to enhance films adhesion and strengthen the network. Films without this aging show poor adhesion and disappear during ethanol washing. Fig. 2 shows FTIR spectra of Ža. unmodified and Žb. modified films. The two insets indicate magnified regions of C–H. While the 2976, 2933 and 796 cmy1 peaks indicate C–H bonding due to ethoxy groups Ž –OC 2 H 5 ., the 2968 and 852 cmy1 peaks C–H bonding due to the presence of Si–CH 3 w6x.
153
Fig. 3 shows DTA curve of 608C dried silica gel monolith which experienced surface modification. It is believed that the exothermic peak in the vicinity of 3108C is a consequence of oxidation of –CH 3 which resulted from modification. In addition SEM photographs shown in Fig. 4 exhibit the significant morphological distinction between unmodified and modified films. The substrate coated with silica film was divided into two pieces for comparison. One does not undergo modification and the other undergoes it. This presents the effect of modification that prevents additional condensation between –OH species resulting from the capillary force when drying w7x. Whereas the unmodified film has densesurface morphology and thickness of 180 nm approximately. The modified film has porous-surface mor-
Fig. 2. Comparison of FTIR spectra of Ža. unmodified and Žb. modified 608C dried films.
154
H.-S. Yang et al.r Journal of Non-Crystalline Solids 221 (1997) 151–156
Fig. 3. DTA curve for modified SiO 2 gel Žmonolith type..
Fig. 5. Changes of FTIR spectra of modified films according to temperature.
phology and thickness over twice that of the unmodified film. In a multilevel metallization scheme when alu-
minum is present as a metal layer on the wafer surface, the maximum temperature of the intermetal dielectric layer is limited to ; 4508C w3x. Therefore
Fig. 4. SEM photographs of planar and cross-sectional views of Ža. unmodified and Žb. modified 608C dried films, respectively.
H.-S. Yang et al.r Journal of Non-Crystalline Solids 221 (1997) 151–156
155
Fig. 6. Changes of refractive index of SiO 2 thin film according to temperature.
Fig. 7. Changes of dielectric constant of SiO 2 thin film according to temperature.
the influence of temperature was considered to ; 5008C. The FTIR spectra of films heated in the different range of temperatures are shown in Fig. 5. The remarkable change of peak lies in 2968 and 3736 cmy1 which correspond to C–H and Si–OH bonding, respectively w6,8x. It is observed that –CH 3 species has the decreasing tendency with temperature but do not disappear completely with heat treatment to 5008C. As shown in the DTA curve, the continuous exothermic reaction proceeds to ; 5008C. It is
clear from this result that oxidation of organic groups Ž –CH 3 , –OC 2 H 5 . does not cease to ; 5008C. Figs. 6 and 7 show the influence of temperature on refractive index and dielectric constant of film. As the temperature increases, the refractive index gradually decreases. It was expected that lower refractive index of the film would lead to lower dielectric constant, but that was not the case as shown in Fig. 7. The mark represents the mean value of six regions where dielectric constants are measured. All values lie in the range of error bar. The dielectric
Fig. 8. Illustration of the formation of –CH 3 species and their transition to –OH.
156
H.-S. Yang et al.r Journal of Non-Crystalline Solids 221 (1997) 151–156
constant of 3008C heated film exhibits the minimum value of 2.2.
P is the porosity of the film for silica aerogel w11x. The densities and porosities evaluated indirectly were in the range from 0.7 to 1.1 grcm3 and from 50 to 67%, respectively.
4. Discussion For the films heated above 4008C the appearance of peak at 3736 cmy1 , which presents isolated, single Si–OH species w8x, throws the meaningful information on the properties of film. Even though a considerable amount of –CH 3 species remain still in the case of 3008C heated film, the transition Žoxidation. of pyrolyzed –CH 3 to –OH species occurs in the films heated above 4008C. Fig. 8 illustrates the changes from the formation of substituted –CH 3 species during the surface modification to the transition of –CH 3 to –OH during the heat treatment above 4008C. Organic groups such as –CH 3 contribute hydrophobicity while –OH contribute hydrophilicity w9,10x. The presence of –CH 3 and –OH allows 608C dried and 5008C heated films to be hydrophobic and hydrophilic, respectively. Although films heated more than 4008C have lower refractive index, the hydrophilicity of gel surface with –OH species facilitates the adsorption of atmospheric water molecules into film, which play a significant role in increasing the dielectric constant due to high polarization of water molecule. Other heat treatment atmospheres such as N2 gas could be considered to block oxidation of –CH 3 to –OH species. Whereas the gel, in which additional condensation aggressively occurs, during drying, exhibits irreversible shrinkage, the gel with substituted –CH 3 species leads to spring back during heat treatment resulting in reversible shrinkage. It is difficult for the gases inside gel to escape due to its entangled structure; therefore it is thought that the gel network is compressed by the gases which are expanded during the heat treatment. Consequently the surface modified gel, creating the spring back phenomenon, shows higher pore volume and accordingly lower refractive index. Film densities and porosities were indirectly determined from refractive index, using the relations, r s Ž n y 1.r0.209 and P s wŽ1.458 y n.r0.458x, where n is the refractive index, r is the density and
5. Conclusions Ž1. A new process for preparing low dielectric silica thin film was established. Ž2. Surface modification was successfully performed to retard additional condensations. Ž3. A film, with the dielectric constant of 2.2, was prepared when heated at 3008C. Ž4. Films heated above 3008C had lower refractive index, and their dielectric constant became higher due to hydrophilicity promoting adsorption of water.
Acknowledgements The authors wish to appreciate ETRI ŽElectronic Telecommunicative Research Institute., for financial support of this work.
References w1x S.P. Murarka, Metallization — The Theory and Practice for VLSI and ULSI, Butterworth-Heinemann, 1993. w2x D.M. Smith, R. Deshpande, C.J. Brinker, Mater. Res. Soc. Symp. Proc. 271 Ž1992. 567. w3x S. Wolf, Silicon Processing for the VLSI ERA, vol. 2: Process Integration, Lattice Press, California, 1990. w4x D.E. Bornside, C.W. Macosko, L.E. Scriven, J. Appl. Phys. 66 Ž1989. 5185. w5x S.S. Prakash, C.J. Brinker, A.J. Hurd, J. Non-Cryst. Solids 190 Ž1995. 264. w6x C.J. Pouchert ŽEd.., The Aldrich Library of FT-IR Spectra, vol. 2, Aldrich Chemical, Wisconsin, 1985. w7x C.J. Brinker, G.W. Scherer, Sol–gel Science, Academic Press, New York, 1990. w8x R.K. Iler, The Chemistry of Silica, Wiley, New York, 1979. w9x H. Yokogawa, M. Yokoyama, J. Non-Cryst. Solids 186 Ž1995. 23. w10x K.-H. Lee, S.-Y. Kim, K.-P. Yoo, J. Non-Cryst. Solids 186 Ž1995. 18. w11x L.W. Hrubesh, J.F. Pico, J. Non-Cryst. Solids 188 Ž1995. 46.