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Evolution of pores in mesoporous silica films: Porogen loading effect
Applied Surface Science 255 (2008) 183–186
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Evolution of pores in mesoporous silica films: Porogen loading effect Chunqing He a,*, Toshitaka Oka a, Yoshinori Kobayashi a,*, Nagayasu Oshima b, Toshiyuki Ohdaira b, Atsushi Kinomura b, Ryoichi Suzuki b a b
National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan
A R T I C L E I N F O
A B S T R A C T
Article history:
Mesoporous silica films were synthesized via a sol–gel process under an acidic condition. Various amounts of triblock copolymer F38 were loaded to precursor sols as the pore generator. The evolution of the pores generated by porogen decomposition was investigated as a function of F38 loading by positron annihilation gamma-ray energy spectroscopy and positron annihilation lifetime spectroscopy based on slow positron beams. The threshold of pore percolation is found to be around 10 wt% of F38 loading by positron annihilation gamma-ray energy spectroscopy. Positron annihilation lifetime spectroscopy in the films show that the pore size increases from 1 nm to 3 nm with increasing F38 loading from 5 wt% to 30 wt%. ß 2008 Elsevier B.V. All rights reserved.
Available online 24 May 2008 PACS: 78.70Bj Keywords: Positronium Lifetime Gamma-ray energy spectroscopy Thin film Porogen
1. Introduction
2. Experimental
Recently, pore introduction into thin films has become an important issue not only in low dielectric constant (k) interlayer insulators for ultra large-scale integrated (ULSI) circuits, but also in functional porous films for gas sensors and bio-encapsulators. Because of poor sensitivity of most conventional techniques, positron annihilation gamma-ray energy spectroscopy (PAGES) and positron annihilation lifetime spectroscopy (PALS) based on variable-energy slow positron beams emerged as very useful techniques for thin films. These techniques have been successfully applied to the studies of various polymeric and porous silica films [1,2]. In this work, mesoporous silica films were prepared via a sol– gel process using various amounts of a triblock copolymer as the pore generator. The evolution of pores in these films as a function of the porogen loading was investigated by PAGES and PALS combined with slow positron beams.
2.1. Sample preparation Mesostructural silica films were synthesized via a sol–gel process [3], using tetraethoxysilane [Si(OC2H5)4, TEOS] as the network skeleton precursor and various amounts of triblock copolymer F38 (BASF surfactant) as the structure-directing agent, respectively. The coating solutions were prepared by the addition of an ethanol solution of the triblock copolymer to silica sols made by an acid-catalyzed process. The precursor sols were stirred at around 90 8C for 1 h, followed by aging for 30 min, then spincoated on polished Si (1 0 0) wafers. The deposited films were cured in an oven at 100 8C for 3 h and subsequently calcined at 450 8C for 3 h in order to introduce pores by polymer porogen decomposition. Hereafter, a silica film prepared with a particular amount of F38 is denoted with its loading percentage after ‘‘F38‘‘. For example, the film prepared with 5 wt% F38 is referred to F38-5. 2.2. PAGES and PALS combined with positron beam
* Corresponding author. E-mail addresses: [email protected] (C. He), [email protected] (Y. Kobayashi). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.05.302
Positron annihilation gamma-ray energy spectra for 200–300nm thick uncapped films were recorded using a magnetically guided positron beam with a Ge detector over positron incident energies from 80 eV to 30 keV. Positron annihilation lifetime
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C. He et al. / Applied Surface Science 255 (2008) 183–186
spectra were measured at a fixed incident positron energy of 2 keV for the films capped with 20-nm thick nonporous SiO2, using PALS based on the intense pulsed positron beam at the National Institute of Advanced Industrial Science and Technology (AIST). Prior to the PAGES and PALS experiments, each sample was baked for about 30 min at 250 8C under N2 gas flow to remove possible adsorbates, for instance, water molecules. The 3g-annihilation fractions (I3g) were calculated from the obtained gamma-ray energy spectra as previously reported [4]. The lifetime spectra were analyzed into a continuous lifetime distribution using the CONTIN program [5]. The long-lived lifetimes (>1 ns) were attributed to the annihilation of o-Ps in the micro-/meso-pores. 3. Results and discussion
increasing Ein up to 1 keV, beyond which it remains unchanged. In other films, I3g initially increases and then decreases with increasing Ein. I3g for F38-15 increases slightly with Ein up to 0.4 keV and then decreases. The initial increase of I3g becomes more significant for the films prepared with higher loadings of F38, especially, F38-25 and F38-30. The increase and decrease of I3g are due to Ps formation and Ps escaping from the near surface regions, as interpreted in previous reports [6]. As far as the majority of positrons are implanted in the films, not near the film–substrate interface, the slower decrease of I3g suggests a high Ps diffusion coefficient through interconnected pores. Hence, the I3g profiles indicate that the pores in F38-5 and F38-10 are isolated, but those in F38-25 and F38-30 are highly connected. Isolated and connected pores may coexist in F38-15 and F38-20.
Fig. 1(a) shows the 3g-annihilation fractions (I3g) of the uncapped films, determined from positron annihilation energy spectra as functions of positron incident energy and porogen loading. At low positron incident energies, high I3g values are observed, in particular, for films prepared with higher F38 loadings because some Ps formed near the surface can diffuse out through the mesopores, and undergo 3g-annihilation in vacuum. Above Ein 10 keV, no 3g-annihilation is observed for any of the films, because the majority of the positrons annihilate in the Si substrate without Ps formation. Below Ein 3 keV, the variations of I3g as a function of positron incident energy depend on the F38 loading. I3g for F38-5 and F3810 decreases, respectively, from 10% and 12% to 2% and 3%, with
Fig. 1. (a) I3g for uncapped films prepared with various loadings of F38 versus incident positron energy and (b) versus porogen loading at 2 keV.
Fig. 2. (a) Lifetime spectra and (b) Ps lifetime distributions for the silica films prepared by various loadings of F38.
C. He et al. / Applied Surface Science 255 (2008) 183–186
Fig. 1(b) shows I3g at 2 keV as a function of F38 loading. Initially it is only a few percent and at a loading of 10 wt% it starts increasing linearly and reaches 30% at 25 wt% loading. Above 25 wt% F38 loading it remains essentially unchanged. This result suggests that the pore percolation occurs around 10 wt% F38 and the pore connectivity is saturated above 25 wt% loading. Fig. 2 shows the positron lifetime spectra and Ps lifetime distributions for the silica films templated with different amounts of F38. In Fig. 2(a), longer Ps lifetimes are observable for the silica films templated with higher amounts of F38, indicative of larger pores in them. As depicted in Fig. 2(b), the Ps lifetime distribution for F38-5 shows two dispersions around 2–3 ns and 8 ns. For F38-10, the peak of the longer lifetimes is shifted to a larger value. Above 15% F38 loading, one observes three Ps-distributions. The peak positions of the shorter two dispersions are essentially the same for all films. The peak of the longest dispersion moves upward with increasing F38 loading. The peak Ps lifetimes and the full widths at half maximum (FWHM) of the longest Ps lifetimes are displayed as a function of F38 loading in Fig. 3. An abrupt increase in Ps lifetime is seen at a progen loading of 10 wt%, which can be related to the onset of pore percolation. Below the pore percolation threshold, the width of Ps lifetime distribution is about 10 ns. Above the threshold, much wider distributions with FWHMs of about 15 ns are seen. The wider distributions may imply that Ps atoms annihilate in different locations of the well-connected pores with somewhat different lifetimes in these films. The long o-Ps lifetimes can be related to the mesopore size by various versions of the extended Tao-Eldrup model [7–10]. However, the o-Ps lifetime is influenced not only by the pore size but also by the pore geometry/dimensionality [8–10] and Ps– pore surface interaction [11,12]. If the pore dimensionality is taken into account, the pore size of silica films with pores covered by –OH groups as the present porous films can be evaluated with the rectangular Tao-Eldrup model assuming an electron layer thickness of 0.18 nm [9,10]. In view of the Ps emission profiles in Fig. 1, it seems that the pores are ‘‘isolated’’ in F38-5 and F38-10, whereas in other films templated with higher porogen loadings connected pores with long Ps diffusion lengths are present. Hence, we calculated the pore size on the assumptions of ‘‘isolated’’ cubic pores in F38-5, F38-10 and long rectangular pores in other films. It is seen from Fig. 4 that the pore size increases remarkably from
185
Fig. 4. Pore size versus F38 loading.
1.5 nm to 2.4 nm with increasing the porogen loading up to 15 wt% and then increases more gradually, indicating that the pores develop dominantly by one-dimensional elongation rather than three-dimensional growth with elongation and enlargement. 4. Conclusion Positron annihilation lifetime spectroscopy and gamma-ray energy spectroscopy combined with slow positron beams have been applied to the investigation of pore evolution in mesoporous silica films prepared with various loadings of F38 porogen. 3g-annihilation fractions due to Ps emitted from the films as a function of incident energy indicated the pore percolation occurs around a loading of 10 wt%. Long o-Ps lifetime showed that the pore size increases from 1.5 nm to about 3 nm with increasing F38 loading up to 30 wt%. Below the percolation threshold, the pore size seems to increase considerably with the loading of F38. On the other hand, above the percolation threshold the pores tend to develop one-dimensionally. In addition, the pore size distributions are narrower below the pore percolation threshold. Acknowledgements Financial support to our porous film studies from the New Energy and Industrial Technology Development Organization (NEDO) is formally appreciated. Drs. K. Ito, K. Hirata, H. Togashi and F.H.M. Mohamed are appreciated for their helpful discussions and assistance in experiments. References
Fig. 3. Peak Ps lifetimes in mesopores and the full widths of half maximum (FWHM) of their distributions.
[1] Y. Kobayashi, K. Ito, T. Oka, C. He, H.F.M. Mohamed, R. Suzuki, T. Ohdaira, Appl. Surf. Sci. 255 (2008) 191–193. [2] D.W. Gidley, H.G. Peng, R.S. Vallery, Annu. Rev. Mater. Res. 36 (2006) 49. [3] C. He, M. Muramatsu, T. Ohdaira, N. Oshima, A. Kinomura, R. Suzuki, Y. Kobayashi, Radiat. Phys. Chem. 76 (2007) 204. [4] Y. Kobayashi, W. Zheng, T.B. Chang, K. Hirata, R. Suzuki, T. Ohdaira, K. Ito, J. Appl. Phys. 91 (2002) 1704. [5] S.W. Provencher, Comput. Phys. Commun. 27 (1982) 213.
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C. He et al. / Applied Surface Science 255 (2008) 183–186
[6] M.P. Petkov, M.H. Weber, K.G. Lynn, K.P. Rodbell, Appl. Phys. Lett. 79 (2001) 3884. [7] K. Ito, H. Nakanishi, Y. Ujihira, J. Phys. Chem. B 103 (1999) 4555. [8] T. Goworek, K. Ciesielski, B. Jasinska, J. Wawryszczuk, Chem. Phys. 230 (1998) 305. [9] D.W. Gidley, W.E. Frieze, T.L. Dull, A.F. Yee, E.T. Ryan, H.M. Ho, Phys. Rev. B 60 (8) (1999) R5157.
[10] T.L. Dull, W.E. Frieze, D.W. Gidley, J. Sun, A.F. Yee, J. Phys. Chem. B 105 (2001) 4657. [11] C. He, T. Ohdaira, N. Oshima, M. Muramatsu, A. Kinomura, R. Suzuki, T. Oka, Y. Kobayashi, Phys. Rev. B 75 (2007) 195404. [12] C. He, T. Oka, Y. Kobayashi, T. Ohdaira, N. Oshima, A. Kinomura, R. Suzuki, Appl. Phys. Lett. 91 (2007) 024102.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Evolution of pores in mesoporous silica films: Porogen loading effect Chunqing He a,*, Toshitaka Oka a, Yoshinori Kobayashi a,*, Nagayasu Oshima b, Toshiyuki Ohdaira b, Atsushi Kinomura b, Ryoichi Suzuki b a b
National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan
A R T I C L E I N F O
A B S T R A C T
Article history:
Mesoporous silica films were synthesized via a sol–gel process under an acidic condition. Various amounts of triblock copolymer F38 were loaded to precursor sols as the pore generator. The evolution of the pores generated by porogen decomposition was investigated as a function of F38 loading by positron annihilation gamma-ray energy spectroscopy and positron annihilation lifetime spectroscopy based on slow positron beams. The threshold of pore percolation is found to be around 10 wt% of F38 loading by positron annihilation gamma-ray energy spectroscopy. Positron annihilation lifetime spectroscopy in the films show that the pore size increases from 1 nm to 3 nm with increasing F38 loading from 5 wt% to 30 wt%. ß 2008 Elsevier B.V. All rights reserved.
Available online 24 May 2008 PACS: 78.70Bj Keywords: Positronium Lifetime Gamma-ray energy spectroscopy Thin film Porogen
1. Introduction
2. Experimental
Recently, pore introduction into thin films has become an important issue not only in low dielectric constant (k) interlayer insulators for ultra large-scale integrated (ULSI) circuits, but also in functional porous films for gas sensors and bio-encapsulators. Because of poor sensitivity of most conventional techniques, positron annihilation gamma-ray energy spectroscopy (PAGES) and positron annihilation lifetime spectroscopy (PALS) based on variable-energy slow positron beams emerged as very useful techniques for thin films. These techniques have been successfully applied to the studies of various polymeric and porous silica films [1,2]. In this work, mesoporous silica films were prepared via a sol– gel process using various amounts of a triblock copolymer as the pore generator. The evolution of pores in these films as a function of the porogen loading was investigated by PAGES and PALS combined with slow positron beams.
2.1. Sample preparation Mesostructural silica films were synthesized via a sol–gel process [3], using tetraethoxysilane [Si(OC2H5)4, TEOS] as the network skeleton precursor and various amounts of triblock copolymer F38 (BASF surfactant) as the structure-directing agent, respectively. The coating solutions were prepared by the addition of an ethanol solution of the triblock copolymer to silica sols made by an acid-catalyzed process. The precursor sols were stirred at around 90 8C for 1 h, followed by aging for 30 min, then spincoated on polished Si (1 0 0) wafers. The deposited films were cured in an oven at 100 8C for 3 h and subsequently calcined at 450 8C for 3 h in order to introduce pores by polymer porogen decomposition. Hereafter, a silica film prepared with a particular amount of F38 is denoted with its loading percentage after ‘‘F38‘‘. For example, the film prepared with 5 wt% F38 is referred to F38-5. 2.2. PAGES and PALS combined with positron beam
* Corresponding author. E-mail addresses: [email protected] (C. He), [email protected] (Y. Kobayashi). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.05.302
Positron annihilation gamma-ray energy spectra for 200–300nm thick uncapped films were recorded using a magnetically guided positron beam with a Ge detector over positron incident energies from 80 eV to 30 keV. Positron annihilation lifetime
184
C. He et al. / Applied Surface Science 255 (2008) 183–186
spectra were measured at a fixed incident positron energy of 2 keV for the films capped with 20-nm thick nonporous SiO2, using PALS based on the intense pulsed positron beam at the National Institute of Advanced Industrial Science and Technology (AIST). Prior to the PAGES and PALS experiments, each sample was baked for about 30 min at 250 8C under N2 gas flow to remove possible adsorbates, for instance, water molecules. The 3g-annihilation fractions (I3g) were calculated from the obtained gamma-ray energy spectra as previously reported [4]. The lifetime spectra were analyzed into a continuous lifetime distribution using the CONTIN program [5]. The long-lived lifetimes (>1 ns) were attributed to the annihilation of o-Ps in the micro-/meso-pores. 3. Results and discussion
increasing Ein up to 1 keV, beyond which it remains unchanged. In other films, I3g initially increases and then decreases with increasing Ein. I3g for F38-15 increases slightly with Ein up to 0.4 keV and then decreases. The initial increase of I3g becomes more significant for the films prepared with higher loadings of F38, especially, F38-25 and F38-30. The increase and decrease of I3g are due to Ps formation and Ps escaping from the near surface regions, as interpreted in previous reports [6]. As far as the majority of positrons are implanted in the films, not near the film–substrate interface, the slower decrease of I3g suggests a high Ps diffusion coefficient through interconnected pores. Hence, the I3g profiles indicate that the pores in F38-5 and F38-10 are isolated, but those in F38-25 and F38-30 are highly connected. Isolated and connected pores may coexist in F38-15 and F38-20.
Fig. 1(a) shows the 3g-annihilation fractions (I3g) of the uncapped films, determined from positron annihilation energy spectra as functions of positron incident energy and porogen loading. At low positron incident energies, high I3g values are observed, in particular, for films prepared with higher F38 loadings because some Ps formed near the surface can diffuse out through the mesopores, and undergo 3g-annihilation in vacuum. Above Ein 10 keV, no 3g-annihilation is observed for any of the films, because the majority of the positrons annihilate in the Si substrate without Ps formation. Below Ein 3 keV, the variations of I3g as a function of positron incident energy depend on the F38 loading. I3g for F38-5 and F3810 decreases, respectively, from 10% and 12% to 2% and 3%, with
Fig. 1. (a) I3g for uncapped films prepared with various loadings of F38 versus incident positron energy and (b) versus porogen loading at 2 keV.
Fig. 2. (a) Lifetime spectra and (b) Ps lifetime distributions for the silica films prepared by various loadings of F38.
C. He et al. / Applied Surface Science 255 (2008) 183–186
Fig. 1(b) shows I3g at 2 keV as a function of F38 loading. Initially it is only a few percent and at a loading of 10 wt% it starts increasing linearly and reaches 30% at 25 wt% loading. Above 25 wt% F38 loading it remains essentially unchanged. This result suggests that the pore percolation occurs around 10 wt% F38 and the pore connectivity is saturated above 25 wt% loading. Fig. 2 shows the positron lifetime spectra and Ps lifetime distributions for the silica films templated with different amounts of F38. In Fig. 2(a), longer Ps lifetimes are observable for the silica films templated with higher amounts of F38, indicative of larger pores in them. As depicted in Fig. 2(b), the Ps lifetime distribution for F38-5 shows two dispersions around 2–3 ns and 8 ns. For F38-10, the peak of the longer lifetimes is shifted to a larger value. Above 15% F38 loading, one observes three Ps-distributions. The peak positions of the shorter two dispersions are essentially the same for all films. The peak of the longest dispersion moves upward with increasing F38 loading. The peak Ps lifetimes and the full widths at half maximum (FWHM) of the longest Ps lifetimes are displayed as a function of F38 loading in Fig. 3. An abrupt increase in Ps lifetime is seen at a progen loading of 10 wt%, which can be related to the onset of pore percolation. Below the pore percolation threshold, the width of Ps lifetime distribution is about 10 ns. Above the threshold, much wider distributions with FWHMs of about 15 ns are seen. The wider distributions may imply that Ps atoms annihilate in different locations of the well-connected pores with somewhat different lifetimes in these films. The long o-Ps lifetimes can be related to the mesopore size by various versions of the extended Tao-Eldrup model [7–10]. However, the o-Ps lifetime is influenced not only by the pore size but also by the pore geometry/dimensionality [8–10] and Ps– pore surface interaction [11,12]. If the pore dimensionality is taken into account, the pore size of silica films with pores covered by –OH groups as the present porous films can be evaluated with the rectangular Tao-Eldrup model assuming an electron layer thickness of 0.18 nm [9,10]. In view of the Ps emission profiles in Fig. 1, it seems that the pores are ‘‘isolated’’ in F38-5 and F38-10, whereas in other films templated with higher porogen loadings connected pores with long Ps diffusion lengths are present. Hence, we calculated the pore size on the assumptions of ‘‘isolated’’ cubic pores in F38-5, F38-10 and long rectangular pores in other films. It is seen from Fig. 4 that the pore size increases remarkably from
185
Fig. 4. Pore size versus F38 loading.
1.5 nm to 2.4 nm with increasing the porogen loading up to 15 wt% and then increases more gradually, indicating that the pores develop dominantly by one-dimensional elongation rather than three-dimensional growth with elongation and enlargement. 4. Conclusion Positron annihilation lifetime spectroscopy and gamma-ray energy spectroscopy combined with slow positron beams have been applied to the investigation of pore evolution in mesoporous silica films prepared with various loadings of F38 porogen. 3g-annihilation fractions due to Ps emitted from the films as a function of incident energy indicated the pore percolation occurs around a loading of 10 wt%. Long o-Ps lifetime showed that the pore size increases from 1.5 nm to about 3 nm with increasing F38 loading up to 30 wt%. Below the percolation threshold, the pore size seems to increase considerably with the loading of F38. On the other hand, above the percolation threshold the pores tend to develop one-dimensionally. In addition, the pore size distributions are narrower below the pore percolation threshold. Acknowledgements Financial support to our porous film studies from the New Energy and Industrial Technology Development Organization (NEDO) is formally appreciated. Drs. K. Ito, K. Hirata, H. Togashi and F.H.M. Mohamed are appreciated for their helpful discussions and assistance in experiments. References
Fig. 3. Peak Ps lifetimes in mesopores and the full widths of half maximum (FWHM) of their distributions.
[1] Y. Kobayashi, K. Ito, T. Oka, C. He, H.F.M. Mohamed, R. Suzuki, T. Ohdaira, Appl. Surf. Sci. 255 (2008) 191–193. [2] D.W. Gidley, H.G. Peng, R.S. Vallery, Annu. Rev. Mater. Res. 36 (2006) 49. [3] C. He, M. Muramatsu, T. Ohdaira, N. Oshima, A. Kinomura, R. Suzuki, Y. Kobayashi, Radiat. Phys. Chem. 76 (2007) 204. [4] Y. Kobayashi, W. Zheng, T.B. Chang, K. Hirata, R. Suzuki, T. Ohdaira, K. Ito, J. Appl. Phys. 91 (2002) 1704. [5] S.W. Provencher, Comput. Phys. Commun. 27 (1982) 213.
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C. He et al. / Applied Surface Science 255 (2008) 183–186
[6] M.P. Petkov, M.H. Weber, K.G. Lynn, K.P. Rodbell, Appl. Phys. Lett. 79 (2001) 3884. [7] K. Ito, H. Nakanishi, Y. Ujihira, J. Phys. Chem. B 103 (1999) 4555. [8] T. Goworek, K. Ciesielski, B. Jasinska, J. Wawryszczuk, Chem. Phys. 230 (1998) 305. [9] D.W. Gidley, W.E. Frieze, T.L. Dull, A.F. Yee, E.T. Ryan, H.M. Ho, Phys. Rev. B 60 (8) (1999) R5157.
[10] T.L. Dull, W.E. Frieze, D.W. Gidley, J. Sun, A.F. Yee, J. Phys. Chem. B 105 (2001) 4657. [11] C. He, T. Ohdaira, N. Oshima, M. Muramatsu, A. Kinomura, R. Suzuki, T. Oka, Y. Kobayashi, Phys. Rev. B 75 (2007) 195404. [12] C. He, T. Oka, Y. Kobayashi, T. Ohdaira, N. Oshima, A. Kinomura, R. Suzuki, Appl. Phys. Lett. 91 (2007) 024102.