- Email: [email protected]
Structural role of fluorine in amorphous silica
Journal of Non-Crystalline Solids 349 (2004) 10–15 www.elsevier.com/locate/jnoncrysol
Structural role of fluorine in amorphous silica Randall E. Youngman *, Sabyasachi Sen SP-AR-02-4, Corning Incorporated, Science & Technology, Corning, NY 14831, USA Available online 2 November 2004
Abstract The composition and temperature dependence of the coordination environments of F atoms in fluorinated silica glasses have been studied using high-resolution 19F and 29Si NMR spectroscopies. 19F MAS and wideline NMR spectra have revealed the presence of two distinct types of fluorine environments in glasses containing 1–3.3 wt% fluorine. The majority of the fluorine environments are formed by replacing one of the bridging oxygens around a silicon atom with a non-bridging fluorine atom, forming SiO3/2F polyhedra. The less abundant species is found to be highly unusual in that it involves bonding of a non-bridging fluorine to a silicon atom that is already coordinated to four bridging oxygens, yielding a fivefold coordinated silicon of the type SiO4/2F. The relative concentration of the SiO4/2F species in these glasses is found to increase with both increasing fluorine content and fictive temperature. The possible role of these structural elements in controlling diverse physical properties of these glasses such as the optical absorption edge and viscosity is discussed. Ó 2004 Elsevier B.V. All rights reserved. PACS: 61.43.Fs; 82.56.Hg; 82.56.Ub
1. Introduction Fluorine-doped silica glasses are utilized in a variety of technological applications, due mainly to beneficial changes in optical and physical properties resulting from addition of small quantities of fluorine to pure amorphous silica. Fluorine is one of only two dopants that decreases the refractive index of silica, the other being boron [1]. This has resulted in widespread application of fluorine doping of silica to control the refractive index profile of optical fibers [2–5]. Fluorine also lowers the static dielectric constant of silica, from es = 4.2 to values as low as 3.2 [6–8]. Therefore, F-doped silica is an extremely important material with low dielectric constant, used in microprocessors and logic devices [9]. Another potential technological use of fluorine-doped silica is in the field of photolithography, especially as a photomask *
Corresponding author. Tel.: +1 607 974 2970; fax: +1 607 974 9474. E-mail address: [email protected] (R.E. Youngman). 0022-3093/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.08.255
material for next-generation systems based on 157 nm technology [10,11]. Fluorine blueshifts the absorption edge of pure silica and appears to reduce color-center formation in glasses exposed to deep UV radiation [12,13]. Finally, these materials have been studied for their role in catalysis, as fluorination can often increase the surface area of silicas [14,15]. Even though fluorinated silica is of great importance in the above technologies, the structure of this material has not been well characterized. There are a number of studies based on Raman and Infrared (IR) spectroscopies, which all show the formation of Si–F bonds in the network. Raman spectra of F-doped silica contain a band around 945 cm1 which increases in intensity with increasing fluorine content [16,17]. This band has been assigned to the stretching mode of Si–F, where the fluorine has replaced a bridging oxygen on the silicon tetrahedron. In addition to this new spectral feature, these Raman studies also demonstrate the effect of fluorine on lowering the fictive temperature of the silica glasses. The D1 and D2 defect bands in pure silica, commonly
R.E. Youngman, S. Sen / Journal of Non-Crystalline Solids 349 (2004) 10–15
used to quantify the fictive temperature of the glass, are reduced and eventually eliminated as a function of increasing fluorine doping in amorphous silica [16,18]. IR data support the formation of Si–F groups in Fdoped silica, but the absence of bending and scissor modes precludes the formation of silicon tetrahedra with multiple fluorine atoms (i.e. SiO2/2F2 or SiO1/2F3) [7]. There is also one study of fluorinated silica based on 19 F and 29Si Nuclear Magnetic Resonance (NMR) spectroscopies, in which the authors utilized wideline 19F NMR to examine the local symmetry of the fluorine environments in a glass containing approximately 1 wt% fluorine [19]. The results of this early work are consistent with formation of Si–F bonding and silicon tetrahedra with a single fluorine atom. In addition, some of their findings are consistent with multiple fluorine atoms in close spatial proximity, which was interpreted as evidence for a small fraction of SiO2/2F2 tetrahedra. We have recently studied the short-range atomic environment around F atoms in a silica glass containing 3 wt% fluorine [20]. The combination of high-resolution multi-nuclear NMR and Molecular Dynamics (MD) simulations was instrumental in uncovering important new details of the structure of these glasses. Our 19F NMR results revealed the presence of two fluorine environments. Additional evidence from 19F ! 29Si CrossPolarization Magic-Angle Spinning (CPMAS) NMR showed two silicon resonances related to these fluorine sites, assigned to SiO3/2F and SiO4/2F polyhedra. The SiO3/2F unit is a four-coordinated silicon atom bonded to three bridging oxygens and one non-bridging fluorine whereas the SiO4/2F unit is a penta-coordinated silicon atom bonded to four bridging oxygens and one nonbridging fluorine. In this report, we expand upon this study of fluorinated silica glasses using mainly high-resolution 19F NMR spectroscopy. We present results for glasses with varying fluorine contents (0.5–3.3 wt% F) and examine the effect of fictive temperature on the fluorine speciation.
2. Experimental procedures The fluorine-doped silica glasses were made using typical Chemical Vapor Deposition process with a methane/oxygen burner and SiCl4 as the silica precursors. SiF4 gas was used for fluorine doping. Fluorine contents in all glasses were determined using Electron Probe Microanalysis (EPMA). A bulk piece of the glass sample with the highest fluorine content of 3.3 wt% F was fiberized at 2000 °C in a draw-tower to study the effect of increasing fictive temperature on fluorine speciation. 19 F MAS NMR spectra of the crushed glass samples were acquired at a magnetic field strength of 4.7 T (188 MHz resonance frequency) using a 4 mm MAS
11
NMR probe with sample spinning rates of 15–18 kHz. Increasing the spinning speed to up to 30 kHz with a 2.5 mm MAS NMR probe did not significantly improve the spectral resolution, so data were collected using the larger volume rotors to capitalize on better signal to noise levels. Approximately 160–480 scans were signal averaged using a p/4 pulse (3.1 ls) and recycle delays of 1800–3000 s along with a background suppression pulse sequence. These parameters were found to be sufficient for obtaining quantitative 19F MAS NMR spectra, as the longer recycle delays did not change the appearance of the data. 19F MAS NMR data were processed with minimal line broadening (100 Hz) and the chemical shifts were referenced to that of CFCl3. Wideline 19F NMR spectra were collected with a 4 mm MAS NMR probe without sample spinning using similar pulse widths and recycle delays as described above. 29 Si NMR spectra were collected at 4.7 T (39.7 MHz resonance frequency) using a 9.5 mm MAS NMR probe and sample spinning rates of 3–4 kHz. The 29Si MAS NMR data were acquired with p/4 pulse widths of 3 ls and recycle delays of 900 s, all in conjunction with 19F decoupling during data acquisition. Signal averaging of nominally 320 scans resulted in sufficient signal levels. 19 F ! 29Si CPMAS NMR data were acquired with traditional Hartmann–Hahn matching between the two nuclides. The 19F excitation pulse width was 3 ls (p/4) and the recycle delay was 180 s. 29Si NMR data were processed with 50 Hz line broadening and were referenced to the chemical shift of tetramethyl silane (TMS).
3. Results The 19F MAS NMR spectra of fluorinated silica glasses typically consist of an asymmetric lineshape centered around 146 ppm, with spinning sidebands at ±85 ppm from this region [20]. A typical 19F MAS NMR spectrum of a fluorinated silica glass containing 3 wt% fluorine is plotted in Fig. 1. The dashed lines correspond to a simulation of the spectrum with two Gaussian components with chemical shifts of 137 and 146 ppm (Fig. 1). The corresponding wideline 19F NMR is shown in Fig. 2. Fig. 3(a) contains the 29Si MAS NMR spectrum of the 3 wt% fluorine-doped silica glass obtained with 19F decoupling. The experimental spectrum has been simulated with a sum of three Gaussian resonances with 29Si chemical shift values of 103, 112 and 125 ppm. Very weak spinning sidebands are located outside of this plotted spectral region. The 19F ! 29Si CPMAS spectrum of this glass obtained with a short contact time of 0.5 ms is shown in Fig. 3(b). This spectrum clearly shows that resonances at 103 and 125 ppm are strongly enhanced in the CPMAS spectrum with respect to that at 112 ppm, indicating that the former two resonances correspond to silicon atoms
12
R.E. Youngman, S. Sen / Journal of Non-Crystalline Solids 349 (2004) 10–15
(a)
--120
--130 19 F
--140 --150 --160 Chemical Shift ( ppm)
--170
(b)
Fig. 1. 19F MAS NMR spectrum of 3 wt% F-doped silica. Dashed lines represent spectral simulation using two Gaussian peaks. --70
--80
--90
--100 29 Si
--110
--120
--130
--140
--150
Chemical Shift (ppm)
Fig. 3. 29Si NMR spectra of 3 wt% F-doped silica. (a) 29Si MAS NMR spectrum with 19F decoupling and (b) 19F ! 29Si CPMAS NMR spectrum using a short contact time of 0.5 ms. Dashed curves correspond to simulation of the spectra with three Gaussian peaks.
0
--50
--100 19 F
Fig. 2. Wideline
--150
--200
--250
--300
Chemical Shift ( ppm)
19
F NMR spectrum of 3 wt% F-doped silica.
bonded to fluorine. An overlay of 19F MAS NMR spectra for silica glasses containing 1 and 3 wt% fluorine is given in Fig. 4. These data are qualitatively similar, although there are subtle intensity differences on the downfield (less negative) side of the 19F lineshapes indicating concentration dependent fluorine speciation in these glasses. Simulations of the 19F MAS NMR spectra of glasses with varying fluorine contents with two Gaussian components at 137 and 146 ppm indicate a systematic variation in the relative fraction of the peak at 137 ppm corresponding to the SiO4/2F species (Fig. 5) [20]. Fig. 6 contains the 19F MAS NMR spectra of bulk and fiber forms of a 3.3 wt% fluorine-doped silica glass that show significant differences in both widths and intensities between these spectra and implies a fictive temperature dependence of fluorine speciation.
--110
--120
--130 19 F
--140
--150
--160
--170
Chemical Shift ( ppm)
Fig. 4. 19F MAS NMR spectra of F-doped silica glasses. The solid and dashed curves correspond to the spectra of silica containing 3 and 1 wt% fluorine, respectively.
4. Discussion Direct examination of the fluorine environments in fluorine-doped silica glasses can be made using 19F NMR spectroscopy [19,20]. Simulation of the high-resolution 19F MAS NMR spectrum of a silica glass doped with 3 wt% fluorine results in partial resolution of two fluorine resonances at 137 and 146 ppm, with relative intensities of 16% and 84%, respectively (Fig. 1). The
R.E. Youngman, S. Sen / Journal of Non-Crystalline Solids 349 (2004) 10–15
30
F in SiO4/2F Groups, %
25 20 15 10 5 0 -5 0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Fluorine Content (wt%) Fig. 5. Plot of the relative fraction of SiO4/2F groups as a function of fluorine content in doped silica glasses. The open symbol corresponds to the 3.3 wt% F-doped silica drawn into a fiber. Uncertainties in the fluorine contents are on the order of the symbol sizes. The dashed line is drawn as a guide to the eye.
--110
--120
--130 19 F
--140
--150
--160
--170
Chemical Shift ( ppm)
Fig. 6. 19F MAS NMR spectra of 3.3 wt% F-doped silica. The solid and dashed curves correspond to bulk glass and fiber, respectively.
chemical shifts of these lines are consistent with direct Si–F bonding in the glass, as formation of oxyfluoride groups (i.e. Si–O–F) would give rise to fluorine resonances near +150 ppm [21,22]. Our earlier study of this particular composition attributed these two peaks at 146 and 137 ppm to SiO3/2F and SiO4/2F structural units, respectively [20]. The most intense resonance, accounting for 84% of the total fluorine in this glass, was formed by replacing a bridging oxygen atom on a silicon tetrahedron with a terminal fluorine atom, thus forming an SiO3/2F structural unit. This assignment is consistent with all prior structural studies of fluorinated silica glass, including the only 19F NMR study prior to our work [19]. The second fluorine resonance at 137 ppm in Fig. 1, accounting for 16% of all F atoms, corresponds to fluo-
13
rine bonded to a silicon atom that is already bonded to four bridging oxygens, thus forming a SiO4/2F structural entity where the silicon atom is five-coordinated. This assignment, made in a previous study by us, is based on 19F ! 29Si CPMAS NMR spectroscopy of the 3 wt% fluorine-doped silica glass (vide infra) and MD simulation [20]. The wideline 19F NMR spectrum of this glass reflects the local symmetry of the Si–F bonding configuration (Fig. 2). The 19F lineshape is typical of a uniaxial powder pattern and is consistent with fluorine atoms in axially symmetric environments in the SiO3/2F and SiO4/2F units. This type of analysis provided some of the strongest evidence for SiO3/2F groups in the earlier work of Duncan et al. [19]. In addition to supporting the presence of SiO3/2F tetrahedra in these glasses, the lineshape in Fig. 2 precludes the presence of certain fluorine environments. For example, if multiple fluorine atoms were attached to the same silicon atom, such as in SiO2/2F2 groups, then the local symmetry of the fluorine atoms would no longer be axially symmetric, leading to a significant distortion of the wideline 19F NMR lineshape. Further support for fluorine bonded to fivefold coordinated silicon atoms is found in the 29Si NMR spectra of these glasses. A typical 29Si MAS NMR spectrum of a fluorinated silica glass is plotted in Fig. 3(a), and consists of three distinct resonances at 103, 112 and 125 ppm [20]. The strongest resonance at 112 ppm is from the usual Q4 tetrahedra in silica-rich glasses [23]. The two weak resonances at 103 and 125 ppm result from Si–F bonding, as these peaks are strongly enhanced under cross-polarization between 29Si and 19F, especially when measured with short contact times (Fig. 3(b)). The relative fractions of these two peaks, as well as their chemical shift values, are consistent with their assignments to SiO3/2F and SiO4/2F groups, respectively [20]. It is somewhat surprising that the fivefold coordinated silicon site (i.e. SiO4/2F) has a chemical shift that differs only slightly from the the 112 ppm value for Q4 sites although studies of fluorine-containing zeolites demonstrated that the chemical shift of fivefold coordinated SiO4/2F sites can vary between 115 and 150 ppm [24]. One reason for the relatively small change in chemical shielding in these fluorinated silica glasses, on the order of 13 ppm, is the relatively long Si–F bond length in the SiO4/2F groups. Previous MD simulations of F-doped silica by us showed this distance ˚ , which is sufficiently long to be approximately 1.9–2.0 A to lead to only a small change in chemical shielding for the SiO4/2F groups [20]. When taken together, the 29Si MAS and the 19 F ! 29Si CPMAS NMR data indicate that the structure of fluorine-doped silica glasses consists mainly of Q4 silica tetrahedra, with small amounts of SiO3/2F tetrahedra and trace levels of SiO4/2F groups. It should be noted that the peak position of the SiO4/2F group at
14
R.E. Youngman, S. Sen / Journal of Non-Crystalline Solids 349 (2004) 10–15
125 ppm in Fig. 3(a) appears to change slightly (122 ppm) when the 29Si CPMAS NMR spectra are simulated (cf. Fig. 3(b)). This apparent discrepancy is due to the substantial overlap of the broad 29Si NMR resonances and the inherent inaccuracy in fitting such data, but does not affect our assignment of the three distinct peaks. However, as discussed in our previous work on these types of glasses, somewhat self-consistent and partially constrained fitting of these 29Si NMR spectra can be achieved by making use of the known chemical shift values and line width of the Q4 resonance of fluorine-free silica [20]. Our earlier study of fluorinated silica glass focused entirely on the structure of a glass containing 3 wt% fluorine. One of the obvious questions about the structure of fluorinated silica, especially in light of the strong dependence of physical and optical properties on fluorine content, involves changes in network structure with fluorine content. The application of high-resolution 19F MAS NMR spectroscopy allows for a closer examination of the fluorine environments in glasses with different fluorine contents. Simulation of the 19F MAS NMR spectra of 1 and 3 wt% fluorine-doped silica glasses, as shown in Fig. 4, based on two Gaussian peaks at 137 and 146 ppm with similar peak widths, results in relative peak intensities of 8% and 14% for the peak at 137 ppm for the 1% and 3% fluorine-doped glasses, respectively. Thus, based on the peak assignments discussed above, it appears that a decrease in fluorine content from 3 to 1 wt% results in a decrease of the relative fraction of the SiO4/2F structural element in these glasses. The compositional dependence of these two fluorine environments was also studied for other glasses in the range of 0.5–3.3 wt% fluorine. Similar two-peak simulations of the 19F MAS NMR spectra resulted in the quantification of SiO3/2F and SiO4/2F groups as a function of fluorine content. The relative fraction of fluorine incorporated into SiO4/2F groups is plotted in Fig. 5 as a function of the fluorine content in these glasses. This structural element is essentially absent from the structure of glasses containing 0.5 wt% fluorine, and can only be detected in glasses with higher fluorine levels. Above approximately 1.6 wt% fluorine, the fraction of fluorine in SiO4/2F groups appears to reach a constant value with no further change for higher levels of fluorine (Fig. 5). The improved UV transparency of fluorine-doped silica glass is most apparent for glasses with small quantities of fluorine. The addition of 1 mol% (approximately 0.33 wt%) fluorine to pure silica results in a relatively large blueshift of the absorption edge [12]. Further additions of fluorine, ranging from 1.5 to 7.3 mol%, do little to improve the UV transmission of these glasses, especially at 157 nm. Moreover, the resistance of these glasses against formation of color centers on laser exposure also increases on initial addition of fluorine to up to
1 mol% and the effect saturates at higher fluorine concentrations [12]. It was hypothesized in these studies that the role of initial addition of fluorine is to widen the optical band-gap by removal of the strained bonds in the structure of amorphous silica that may have high-energy electronic states associated with them. These strained bonds are also likely to be prone to scission by laser irradiation to form color centers. However, once these strained bonds are removed on initial addition of fluorine, further increases in fluorine would not have a significant beneficial effect. It is interesting to note in this regard that there is also a change in the concentration dependence of the fluorine speciation behavior in this range as observed in this study, that may lend support to this strained-bond hypothesis. For example, at low levels of fluorine (60.5 wt%) in silica, there is only the formation of SiO3/2F tetrahedra in response to the doping (see Fig. 5). The formation of SiO3/2F units will be energetically preferred as it requires strong structural modification of the glass network and breaking of the Si–O–Si linkages, thereby removing strained bonds from the structure. On the other hand, further addition of fluorine results in the onset of formation of the SiO4/2F groups that does not require extensive structural modification of the network. This result is possibly indicative of the fact that removal of significantly strained Si–O– Si linkages is essentially complete at this level of fluorine and the energetic difference between the two types of fluorine environments is not significant enough to warrant the formation of solely SiO3/2F units. The response of glass structure to changes in fictive temperature has also been investigated with high-resolution 19F MAS NMR spectroscopy. The 19F MAS NMR spectra in Fig. 6 represent data from the same glass, but in bulk and fiber forms, thus providing two different fluorine-doped silica samples with identical composition and different fictive temperatures. The higher fictive temperature induced in the fiberized glass appears to change the 19F MAS NMR spectrum in a manner similar to changes in fluorine content (cf. Fig. 4). Deconvolution of the two 19F resonances corresponding to SiO3/2F and SiO4/2F groups shows that the fiber version of the 3.3 wt% fluorine-doped silica glass has a higher fraction of the fivefold coordinated silicon environments than in the bulk form (Fig. 5). This change in relative fraction of fluorine in SiO4/2F groups corresponds to an increase of approximately 9%. A similar fictive temperature dependence of the concentration of penta-coordinated silicon atoms was also observed at ambient pressure in K2Si4O9 glass in a previous study, although the concentration of the penta-coordinated species in this glass was only 60.1% of all silicon atoms [25]. In contrast, the concentration of penta-coordinated silicon species in fluorinated silica glasses can be up to 2% of all silicon atoms (Fig. 3(a)). The strong increase in SiO4/2F concentration with higher fictive temperature indicates that
R.E. Youngman, S. Sen / Journal of Non-Crystalline Solids 349 (2004) 10–15
such structural elements play an important role in weakening the network at high temperatures and thereby lowering the viscosity of fluorinated silica liquids at high temperatures. Previous MD simulation studies have emphasized the role of fivefold coordinated silicon as transition states in the silicate liquids which are temporarily formed during bond breaking and re-forming associated with Q-species exchange to allow for cooperative motion and viscous flow of the silicate network [26]. It is tempting to speculate that SiO4/2F species in fluorinated silica plays a similar role. It may be noted here that the two factors that control the concentration of the SiO4/2F groups in fluorinated silica, i.e. the fluorine concentration and the fictive temperature, are also dependent on each other. An increase in fluorine content leads to a decrease in the fictive temperature of fluorinated silica glasses made under similar conditions. Therefore, while increasing fluorine concentration apparently contributes to a rise in the SiO4/2F population, the concomitant decrease in fictive temperature is likely to reduce the magnitude of such an effect. Therefore, our findings on the compositional dependence of the fluorine speciation may be rather conservative and in actuality, for glasses having different fluorine contents but the same fictive temperatures, the difference in fluorine speciation, particularly with regard to SiO4/2F species, may be much greater than that measured in this work. Additional studies are currently underway to separate the effect of fluorine concentration from that of fictive temperature on fluorine speciation.
5. Conclusions The structural role of fluorine in amorphous silica has been studied as a function of fluorine content and fictive temperature, using 19F and 29Si NMR spectroscopies. For all glasses containing P1 wt% fluorine, the fluorine atoms are incorporated into the glass structure through the formation of Si–F terminal bonds on tetrahedral and penta-coordinated silicon atoms. The replacement of a single bridging oxygen atom with a terminal fluorine, forming SiO3/2F tetrahedra, results in depolymerization of the silicate network and can explain the effect of addition of fluorine in lowering the viscosity of silica. The minor fluorine type, present as SiO4/2F groups, increases in relative concentration with increasing fluorine content until approximately 1.5 wt% fluorine, at which point it remains unchanged with increasing fluorine. These fluorine environments also increase in population with increasing fictive temperature. The composition and fictive temperature dependence of the fluorine speciation may play an
15
important role in explaining a wide range of physical properties of these glasses including optical absorption edge and temperature dependence of viscosity.
Acknowledgments The authors would like to acknowledge Drs Charlene Smith, Lisa Moore, Steven Dawes and Marc Whalen for useful conversation and some of the glass samples.
References [1] Y. Haung, A. Sarker, P. Schultz, J. Non-Cryst. Solids 27 (1978) 29. [2] S. Shiraishi, K. Fujiwara, S. Kurosaki, US Patent 4,082,420. [3] J.W. Fleming, D.L. Wood, Appl. Opt. 22 (1983) 3102. [4] M. Ogai, A. Iino, K. Matsubara, J. Tamura, M. Koguchi, S. Nakamura, E. Kinoshita, J. Lightwave Technol. 6 (1988) 1455. [5] M. Kyoto, Y. Ohoga, S. Ishikawa, Y. Ishiguro, J. Mater. Sci. 28 (1993) 2738. [6] M.J. Shapiro, S.V. Nguyen, T. Matsuda, D. Dobuzinsky, Thin Solid Films 270 (1995) 503. [7] G. Lucovsky, H. Yang, J. Vac. Sci. Technol. A 15 (1997) 836. [8] S.M. Han, E.S. Aydil, J. Appl. Phys. 83 (1998) 2172. [9] J.H. Kim, S.H. Seo, S.M. Yun, H.Y. Chang, K.M. Lee, C.K. Choi, Appl. Phys. Lett. 68 (1996) 1507. [10] J.A. McClay, A.S.L. McIntyre, Solid State Technol. 42 (1999) 57. [11] C.M. Smith, L.A. Moore, in: Emerging Lithographic Technologies III, in: Y. Vladimirsky (Ed.), Proceedings of SPIE, vol. 3676, 1999, p. 834. [12] H. Hosono, M. Mizuguchi, L. Skuja, T. Ogawa, Opt. Lett. 24 (1999) 1549. [13] M. Mizuguchi, L. Skuja, H. Hosono, T. Ogawa, J. Vac. Sci. Technol. B 17 (1999) 3280. [14] T. Katsuo, Y. Satohito, T. Kimio, Bull. Jpn. Pet. Inst. 12 (1970) 136. [15] C.V.A. Duke, J.M. Miller, J.H. Clark, A.P. Kybett, Spectrochim. Acta A 46 (1990) 1381. [16] P. Dumas, J. Corset, W. Carvalho, Y. Levy, Y. Neuman, J. NonCryst. Solids 47 (1982) 239. [17] C.A.M. Mulder, R.K. Janssen, P. Bachmann, D. Leers, J. NonCryst. Solids 72 (1985) 243. [18] M. Yoshikawa, K. Iwagami, N. Morita, T. Matsunobe, H. Ishida, Thin Solid Films 310 (1997) 167. [19] T.M. Duncan, D.C. Douglass, R. Csencsits, K.L. Walker, J. Appl. Phys. 60 (1986) 130. [20] R.E. Youngman, S. Sen, J. Non-Cryst. Solids 337 (2004) 182. [21] J.W. Emsley, L. Phillips, Prog. Nucl. Magn. Reson. Spectrosc. 7 (1971) 1. [22] V. Wray, Annu. Rep. NMR Spectrosc. 10A (1980) 1. [23] L.F. Gladden, T.A. Carpenter, S.R. Elliott, Philos. Mag. B 53 (1986) L81. [24] H. Koller, A. Wo¨lker, L.A. Villaescusa, M.J. Dı´az-Caban˜as, S. Valencia, M.A. Camblor, J. Am. Chem. Soc. 121 (1999) 3368. [25] J.F. Stebbins, Nature 351 (1991) 638. [26] S. Brawer, Relaxation in Viscous Liquids and Glasses, American Ceramic Society, Columbus, OH, 1985.
Structural role of fluorine in amorphous silica Randall E. Youngman *, Sabyasachi Sen SP-AR-02-4, Corning Incorporated, Science & Technology, Corning, NY 14831, USA Available online 2 November 2004
Abstract The composition and temperature dependence of the coordination environments of F atoms in fluorinated silica glasses have been studied using high-resolution 19F and 29Si NMR spectroscopies. 19F MAS and wideline NMR spectra have revealed the presence of two distinct types of fluorine environments in glasses containing 1–3.3 wt% fluorine. The majority of the fluorine environments are formed by replacing one of the bridging oxygens around a silicon atom with a non-bridging fluorine atom, forming SiO3/2F polyhedra. The less abundant species is found to be highly unusual in that it involves bonding of a non-bridging fluorine to a silicon atom that is already coordinated to four bridging oxygens, yielding a fivefold coordinated silicon of the type SiO4/2F. The relative concentration of the SiO4/2F species in these glasses is found to increase with both increasing fluorine content and fictive temperature. The possible role of these structural elements in controlling diverse physical properties of these glasses such as the optical absorption edge and viscosity is discussed. Ó 2004 Elsevier B.V. All rights reserved. PACS: 61.43.Fs; 82.56.Hg; 82.56.Ub
1. Introduction Fluorine-doped silica glasses are utilized in a variety of technological applications, due mainly to beneficial changes in optical and physical properties resulting from addition of small quantities of fluorine to pure amorphous silica. Fluorine is one of only two dopants that decreases the refractive index of silica, the other being boron [1]. This has resulted in widespread application of fluorine doping of silica to control the refractive index profile of optical fibers [2–5]. Fluorine also lowers the static dielectric constant of silica, from es = 4.2 to values as low as 3.2 [6–8]. Therefore, F-doped silica is an extremely important material with low dielectric constant, used in microprocessors and logic devices [9]. Another potential technological use of fluorine-doped silica is in the field of photolithography, especially as a photomask *
Corresponding author. Tel.: +1 607 974 2970; fax: +1 607 974 9474. E-mail address: [email protected] (R.E. Youngman). 0022-3093/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.08.255
material for next-generation systems based on 157 nm technology [10,11]. Fluorine blueshifts the absorption edge of pure silica and appears to reduce color-center formation in glasses exposed to deep UV radiation [12,13]. Finally, these materials have been studied for their role in catalysis, as fluorination can often increase the surface area of silicas [14,15]. Even though fluorinated silica is of great importance in the above technologies, the structure of this material has not been well characterized. There are a number of studies based on Raman and Infrared (IR) spectroscopies, which all show the formation of Si–F bonds in the network. Raman spectra of F-doped silica contain a band around 945 cm1 which increases in intensity with increasing fluorine content [16,17]. This band has been assigned to the stretching mode of Si–F, where the fluorine has replaced a bridging oxygen on the silicon tetrahedron. In addition to this new spectral feature, these Raman studies also demonstrate the effect of fluorine on lowering the fictive temperature of the silica glasses. The D1 and D2 defect bands in pure silica, commonly
R.E. Youngman, S. Sen / Journal of Non-Crystalline Solids 349 (2004) 10–15
used to quantify the fictive temperature of the glass, are reduced and eventually eliminated as a function of increasing fluorine doping in amorphous silica [16,18]. IR data support the formation of Si–F groups in Fdoped silica, but the absence of bending and scissor modes precludes the formation of silicon tetrahedra with multiple fluorine atoms (i.e. SiO2/2F2 or SiO1/2F3) [7]. There is also one study of fluorinated silica based on 19 F and 29Si Nuclear Magnetic Resonance (NMR) spectroscopies, in which the authors utilized wideline 19F NMR to examine the local symmetry of the fluorine environments in a glass containing approximately 1 wt% fluorine [19]. The results of this early work are consistent with formation of Si–F bonding and silicon tetrahedra with a single fluorine atom. In addition, some of their findings are consistent with multiple fluorine atoms in close spatial proximity, which was interpreted as evidence for a small fraction of SiO2/2F2 tetrahedra. We have recently studied the short-range atomic environment around F atoms in a silica glass containing 3 wt% fluorine [20]. The combination of high-resolution multi-nuclear NMR and Molecular Dynamics (MD) simulations was instrumental in uncovering important new details of the structure of these glasses. Our 19F NMR results revealed the presence of two fluorine environments. Additional evidence from 19F ! 29Si CrossPolarization Magic-Angle Spinning (CPMAS) NMR showed two silicon resonances related to these fluorine sites, assigned to SiO3/2F and SiO4/2F polyhedra. The SiO3/2F unit is a four-coordinated silicon atom bonded to three bridging oxygens and one non-bridging fluorine whereas the SiO4/2F unit is a penta-coordinated silicon atom bonded to four bridging oxygens and one nonbridging fluorine. In this report, we expand upon this study of fluorinated silica glasses using mainly high-resolution 19F NMR spectroscopy. We present results for glasses with varying fluorine contents (0.5–3.3 wt% F) and examine the effect of fictive temperature on the fluorine speciation.
2. Experimental procedures The fluorine-doped silica glasses were made using typical Chemical Vapor Deposition process with a methane/oxygen burner and SiCl4 as the silica precursors. SiF4 gas was used for fluorine doping. Fluorine contents in all glasses were determined using Electron Probe Microanalysis (EPMA). A bulk piece of the glass sample with the highest fluorine content of 3.3 wt% F was fiberized at 2000 °C in a draw-tower to study the effect of increasing fictive temperature on fluorine speciation. 19 F MAS NMR spectra of the crushed glass samples were acquired at a magnetic field strength of 4.7 T (188 MHz resonance frequency) using a 4 mm MAS
11
NMR probe with sample spinning rates of 15–18 kHz. Increasing the spinning speed to up to 30 kHz with a 2.5 mm MAS NMR probe did not significantly improve the spectral resolution, so data were collected using the larger volume rotors to capitalize on better signal to noise levels. Approximately 160–480 scans were signal averaged using a p/4 pulse (3.1 ls) and recycle delays of 1800–3000 s along with a background suppression pulse sequence. These parameters were found to be sufficient for obtaining quantitative 19F MAS NMR spectra, as the longer recycle delays did not change the appearance of the data. 19F MAS NMR data were processed with minimal line broadening (100 Hz) and the chemical shifts were referenced to that of CFCl3. Wideline 19F NMR spectra were collected with a 4 mm MAS NMR probe without sample spinning using similar pulse widths and recycle delays as described above. 29 Si NMR spectra were collected at 4.7 T (39.7 MHz resonance frequency) using a 9.5 mm MAS NMR probe and sample spinning rates of 3–4 kHz. The 29Si MAS NMR data were acquired with p/4 pulse widths of 3 ls and recycle delays of 900 s, all in conjunction with 19F decoupling during data acquisition. Signal averaging of nominally 320 scans resulted in sufficient signal levels. 19 F ! 29Si CPMAS NMR data were acquired with traditional Hartmann–Hahn matching between the two nuclides. The 19F excitation pulse width was 3 ls (p/4) and the recycle delay was 180 s. 29Si NMR data were processed with 50 Hz line broadening and were referenced to the chemical shift of tetramethyl silane (TMS).
3. Results The 19F MAS NMR spectra of fluorinated silica glasses typically consist of an asymmetric lineshape centered around 146 ppm, with spinning sidebands at ±85 ppm from this region [20]. A typical 19F MAS NMR spectrum of a fluorinated silica glass containing 3 wt% fluorine is plotted in Fig. 1. The dashed lines correspond to a simulation of the spectrum with two Gaussian components with chemical shifts of 137 and 146 ppm (Fig. 1). The corresponding wideline 19F NMR is shown in Fig. 2. Fig. 3(a) contains the 29Si MAS NMR spectrum of the 3 wt% fluorine-doped silica glass obtained with 19F decoupling. The experimental spectrum has been simulated with a sum of three Gaussian resonances with 29Si chemical shift values of 103, 112 and 125 ppm. Very weak spinning sidebands are located outside of this plotted spectral region. The 19F ! 29Si CPMAS spectrum of this glass obtained with a short contact time of 0.5 ms is shown in Fig. 3(b). This spectrum clearly shows that resonances at 103 and 125 ppm are strongly enhanced in the CPMAS spectrum with respect to that at 112 ppm, indicating that the former two resonances correspond to silicon atoms
12
R.E. Youngman, S. Sen / Journal of Non-Crystalline Solids 349 (2004) 10–15
(a)
--120
--130 19 F
--140 --150 --160 Chemical Shift ( ppm)
--170
(b)
Fig. 1. 19F MAS NMR spectrum of 3 wt% F-doped silica. Dashed lines represent spectral simulation using two Gaussian peaks. --70
--80
--90
--100 29 Si
--110
--120
--130
--140
--150
Chemical Shift (ppm)
Fig. 3. 29Si NMR spectra of 3 wt% F-doped silica. (a) 29Si MAS NMR spectrum with 19F decoupling and (b) 19F ! 29Si CPMAS NMR spectrum using a short contact time of 0.5 ms. Dashed curves correspond to simulation of the spectra with three Gaussian peaks.
0
--50
--100 19 F
Fig. 2. Wideline
--150
--200
--250
--300
Chemical Shift ( ppm)
19
F NMR spectrum of 3 wt% F-doped silica.
bonded to fluorine. An overlay of 19F MAS NMR spectra for silica glasses containing 1 and 3 wt% fluorine is given in Fig. 4. These data are qualitatively similar, although there are subtle intensity differences on the downfield (less negative) side of the 19F lineshapes indicating concentration dependent fluorine speciation in these glasses. Simulations of the 19F MAS NMR spectra of glasses with varying fluorine contents with two Gaussian components at 137 and 146 ppm indicate a systematic variation in the relative fraction of the peak at 137 ppm corresponding to the SiO4/2F species (Fig. 5) [20]. Fig. 6 contains the 19F MAS NMR spectra of bulk and fiber forms of a 3.3 wt% fluorine-doped silica glass that show significant differences in both widths and intensities between these spectra and implies a fictive temperature dependence of fluorine speciation.
--110
--120
--130 19 F
--140
--150
--160
--170
Chemical Shift ( ppm)
Fig. 4. 19F MAS NMR spectra of F-doped silica glasses. The solid and dashed curves correspond to the spectra of silica containing 3 and 1 wt% fluorine, respectively.
4. Discussion Direct examination of the fluorine environments in fluorine-doped silica glasses can be made using 19F NMR spectroscopy [19,20]. Simulation of the high-resolution 19F MAS NMR spectrum of a silica glass doped with 3 wt% fluorine results in partial resolution of two fluorine resonances at 137 and 146 ppm, with relative intensities of 16% and 84%, respectively (Fig. 1). The
R.E. Youngman, S. Sen / Journal of Non-Crystalline Solids 349 (2004) 10–15
30
F in SiO4/2F Groups, %
25 20 15 10 5 0 -5 0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Fluorine Content (wt%) Fig. 5. Plot of the relative fraction of SiO4/2F groups as a function of fluorine content in doped silica glasses. The open symbol corresponds to the 3.3 wt% F-doped silica drawn into a fiber. Uncertainties in the fluorine contents are on the order of the symbol sizes. The dashed line is drawn as a guide to the eye.
--110
--120
--130 19 F
--140
--150
--160
--170
Chemical Shift ( ppm)
Fig. 6. 19F MAS NMR spectra of 3.3 wt% F-doped silica. The solid and dashed curves correspond to bulk glass and fiber, respectively.
chemical shifts of these lines are consistent with direct Si–F bonding in the glass, as formation of oxyfluoride groups (i.e. Si–O–F) would give rise to fluorine resonances near +150 ppm [21,22]. Our earlier study of this particular composition attributed these two peaks at 146 and 137 ppm to SiO3/2F and SiO4/2F structural units, respectively [20]. The most intense resonance, accounting for 84% of the total fluorine in this glass, was formed by replacing a bridging oxygen atom on a silicon tetrahedron with a terminal fluorine atom, thus forming an SiO3/2F structural unit. This assignment is consistent with all prior structural studies of fluorinated silica glass, including the only 19F NMR study prior to our work [19]. The second fluorine resonance at 137 ppm in Fig. 1, accounting for 16% of all F atoms, corresponds to fluo-
13
rine bonded to a silicon atom that is already bonded to four bridging oxygens, thus forming a SiO4/2F structural entity where the silicon atom is five-coordinated. This assignment, made in a previous study by us, is based on 19F ! 29Si CPMAS NMR spectroscopy of the 3 wt% fluorine-doped silica glass (vide infra) and MD simulation [20]. The wideline 19F NMR spectrum of this glass reflects the local symmetry of the Si–F bonding configuration (Fig. 2). The 19F lineshape is typical of a uniaxial powder pattern and is consistent with fluorine atoms in axially symmetric environments in the SiO3/2F and SiO4/2F units. This type of analysis provided some of the strongest evidence for SiO3/2F groups in the earlier work of Duncan et al. [19]. In addition to supporting the presence of SiO3/2F tetrahedra in these glasses, the lineshape in Fig. 2 precludes the presence of certain fluorine environments. For example, if multiple fluorine atoms were attached to the same silicon atom, such as in SiO2/2F2 groups, then the local symmetry of the fluorine atoms would no longer be axially symmetric, leading to a significant distortion of the wideline 19F NMR lineshape. Further support for fluorine bonded to fivefold coordinated silicon atoms is found in the 29Si NMR spectra of these glasses. A typical 29Si MAS NMR spectrum of a fluorinated silica glass is plotted in Fig. 3(a), and consists of three distinct resonances at 103, 112 and 125 ppm [20]. The strongest resonance at 112 ppm is from the usual Q4 tetrahedra in silica-rich glasses [23]. The two weak resonances at 103 and 125 ppm result from Si–F bonding, as these peaks are strongly enhanced under cross-polarization between 29Si and 19F, especially when measured with short contact times (Fig. 3(b)). The relative fractions of these two peaks, as well as their chemical shift values, are consistent with their assignments to SiO3/2F and SiO4/2F groups, respectively [20]. It is somewhat surprising that the fivefold coordinated silicon site (i.e. SiO4/2F) has a chemical shift that differs only slightly from the the 112 ppm value for Q4 sites although studies of fluorine-containing zeolites demonstrated that the chemical shift of fivefold coordinated SiO4/2F sites can vary between 115 and 150 ppm [24]. One reason for the relatively small change in chemical shielding in these fluorinated silica glasses, on the order of 13 ppm, is the relatively long Si–F bond length in the SiO4/2F groups. Previous MD simulations of F-doped silica by us showed this distance ˚ , which is sufficiently long to be approximately 1.9–2.0 A to lead to only a small change in chemical shielding for the SiO4/2F groups [20]. When taken together, the 29Si MAS and the 19 F ! 29Si CPMAS NMR data indicate that the structure of fluorine-doped silica glasses consists mainly of Q4 silica tetrahedra, with small amounts of SiO3/2F tetrahedra and trace levels of SiO4/2F groups. It should be noted that the peak position of the SiO4/2F group at
14
R.E. Youngman, S. Sen / Journal of Non-Crystalline Solids 349 (2004) 10–15
125 ppm in Fig. 3(a) appears to change slightly (122 ppm) when the 29Si CPMAS NMR spectra are simulated (cf. Fig. 3(b)). This apparent discrepancy is due to the substantial overlap of the broad 29Si NMR resonances and the inherent inaccuracy in fitting such data, but does not affect our assignment of the three distinct peaks. However, as discussed in our previous work on these types of glasses, somewhat self-consistent and partially constrained fitting of these 29Si NMR spectra can be achieved by making use of the known chemical shift values and line width of the Q4 resonance of fluorine-free silica [20]. Our earlier study of fluorinated silica glass focused entirely on the structure of a glass containing 3 wt% fluorine. One of the obvious questions about the structure of fluorinated silica, especially in light of the strong dependence of physical and optical properties on fluorine content, involves changes in network structure with fluorine content. The application of high-resolution 19F MAS NMR spectroscopy allows for a closer examination of the fluorine environments in glasses with different fluorine contents. Simulation of the 19F MAS NMR spectra of 1 and 3 wt% fluorine-doped silica glasses, as shown in Fig. 4, based on two Gaussian peaks at 137 and 146 ppm with similar peak widths, results in relative peak intensities of 8% and 14% for the peak at 137 ppm for the 1% and 3% fluorine-doped glasses, respectively. Thus, based on the peak assignments discussed above, it appears that a decrease in fluorine content from 3 to 1 wt% results in a decrease of the relative fraction of the SiO4/2F structural element in these glasses. The compositional dependence of these two fluorine environments was also studied for other glasses in the range of 0.5–3.3 wt% fluorine. Similar two-peak simulations of the 19F MAS NMR spectra resulted in the quantification of SiO3/2F and SiO4/2F groups as a function of fluorine content. The relative fraction of fluorine incorporated into SiO4/2F groups is plotted in Fig. 5 as a function of the fluorine content in these glasses. This structural element is essentially absent from the structure of glasses containing 0.5 wt% fluorine, and can only be detected in glasses with higher fluorine levels. Above approximately 1.6 wt% fluorine, the fraction of fluorine in SiO4/2F groups appears to reach a constant value with no further change for higher levels of fluorine (Fig. 5). The improved UV transparency of fluorine-doped silica glass is most apparent for glasses with small quantities of fluorine. The addition of 1 mol% (approximately 0.33 wt%) fluorine to pure silica results in a relatively large blueshift of the absorption edge [12]. Further additions of fluorine, ranging from 1.5 to 7.3 mol%, do little to improve the UV transmission of these glasses, especially at 157 nm. Moreover, the resistance of these glasses against formation of color centers on laser exposure also increases on initial addition of fluorine to up to
1 mol% and the effect saturates at higher fluorine concentrations [12]. It was hypothesized in these studies that the role of initial addition of fluorine is to widen the optical band-gap by removal of the strained bonds in the structure of amorphous silica that may have high-energy electronic states associated with them. These strained bonds are also likely to be prone to scission by laser irradiation to form color centers. However, once these strained bonds are removed on initial addition of fluorine, further increases in fluorine would not have a significant beneficial effect. It is interesting to note in this regard that there is also a change in the concentration dependence of the fluorine speciation behavior in this range as observed in this study, that may lend support to this strained-bond hypothesis. For example, at low levels of fluorine (60.5 wt%) in silica, there is only the formation of SiO3/2F tetrahedra in response to the doping (see Fig. 5). The formation of SiO3/2F units will be energetically preferred as it requires strong structural modification of the glass network and breaking of the Si–O–Si linkages, thereby removing strained bonds from the structure. On the other hand, further addition of fluorine results in the onset of formation of the SiO4/2F groups that does not require extensive structural modification of the network. This result is possibly indicative of the fact that removal of significantly strained Si–O– Si linkages is essentially complete at this level of fluorine and the energetic difference between the two types of fluorine environments is not significant enough to warrant the formation of solely SiO3/2F units. The response of glass structure to changes in fictive temperature has also been investigated with high-resolution 19F MAS NMR spectroscopy. The 19F MAS NMR spectra in Fig. 6 represent data from the same glass, but in bulk and fiber forms, thus providing two different fluorine-doped silica samples with identical composition and different fictive temperatures. The higher fictive temperature induced in the fiberized glass appears to change the 19F MAS NMR spectrum in a manner similar to changes in fluorine content (cf. Fig. 4). Deconvolution of the two 19F resonances corresponding to SiO3/2F and SiO4/2F groups shows that the fiber version of the 3.3 wt% fluorine-doped silica glass has a higher fraction of the fivefold coordinated silicon environments than in the bulk form (Fig. 5). This change in relative fraction of fluorine in SiO4/2F groups corresponds to an increase of approximately 9%. A similar fictive temperature dependence of the concentration of penta-coordinated silicon atoms was also observed at ambient pressure in K2Si4O9 glass in a previous study, although the concentration of the penta-coordinated species in this glass was only 60.1% of all silicon atoms [25]. In contrast, the concentration of penta-coordinated silicon species in fluorinated silica glasses can be up to 2% of all silicon atoms (Fig. 3(a)). The strong increase in SiO4/2F concentration with higher fictive temperature indicates that
R.E. Youngman, S. Sen / Journal of Non-Crystalline Solids 349 (2004) 10–15
such structural elements play an important role in weakening the network at high temperatures and thereby lowering the viscosity of fluorinated silica liquids at high temperatures. Previous MD simulation studies have emphasized the role of fivefold coordinated silicon as transition states in the silicate liquids which are temporarily formed during bond breaking and re-forming associated with Q-species exchange to allow for cooperative motion and viscous flow of the silicate network [26]. It is tempting to speculate that SiO4/2F species in fluorinated silica plays a similar role. It may be noted here that the two factors that control the concentration of the SiO4/2F groups in fluorinated silica, i.e. the fluorine concentration and the fictive temperature, are also dependent on each other. An increase in fluorine content leads to a decrease in the fictive temperature of fluorinated silica glasses made under similar conditions. Therefore, while increasing fluorine concentration apparently contributes to a rise in the SiO4/2F population, the concomitant decrease in fictive temperature is likely to reduce the magnitude of such an effect. Therefore, our findings on the compositional dependence of the fluorine speciation may be rather conservative and in actuality, for glasses having different fluorine contents but the same fictive temperatures, the difference in fluorine speciation, particularly with regard to SiO4/2F species, may be much greater than that measured in this work. Additional studies are currently underway to separate the effect of fluorine concentration from that of fictive temperature on fluorine speciation.
5. Conclusions The structural role of fluorine in amorphous silica has been studied as a function of fluorine content and fictive temperature, using 19F and 29Si NMR spectroscopies. For all glasses containing P1 wt% fluorine, the fluorine atoms are incorporated into the glass structure through the formation of Si–F terminal bonds on tetrahedral and penta-coordinated silicon atoms. The replacement of a single bridging oxygen atom with a terminal fluorine, forming SiO3/2F tetrahedra, results in depolymerization of the silicate network and can explain the effect of addition of fluorine in lowering the viscosity of silica. The minor fluorine type, present as SiO4/2F groups, increases in relative concentration with increasing fluorine content until approximately 1.5 wt% fluorine, at which point it remains unchanged with increasing fluorine. These fluorine environments also increase in population with increasing fictive temperature. The composition and fictive temperature dependence of the fluorine speciation may play an
15
important role in explaining a wide range of physical properties of these glasses including optical absorption edge and temperature dependence of viscosity.
Acknowledgments The authors would like to acknowledge Drs Charlene Smith, Lisa Moore, Steven Dawes and Marc Whalen for useful conversation and some of the glass samples.
References [1] Y. Haung, A. Sarker, P. Schultz, J. Non-Cryst. Solids 27 (1978) 29. [2] S. Shiraishi, K. Fujiwara, S. Kurosaki, US Patent 4,082,420. [3] J.W. Fleming, D.L. Wood, Appl. Opt. 22 (1983) 3102. [4] M. Ogai, A. Iino, K. Matsubara, J. Tamura, M. Koguchi, S. Nakamura, E. Kinoshita, J. Lightwave Technol. 6 (1988) 1455. [5] M. Kyoto, Y. Ohoga, S. Ishikawa, Y. Ishiguro, J. Mater. Sci. 28 (1993) 2738. [6] M.J. Shapiro, S.V. Nguyen, T. Matsuda, D. Dobuzinsky, Thin Solid Films 270 (1995) 503. [7] G. Lucovsky, H. Yang, J. Vac. Sci. Technol. A 15 (1997) 836. [8] S.M. Han, E.S. Aydil, J. Appl. Phys. 83 (1998) 2172. [9] J.H. Kim, S.H. Seo, S.M. Yun, H.Y. Chang, K.M. Lee, C.K. Choi, Appl. Phys. Lett. 68 (1996) 1507. [10] J.A. McClay, A.S.L. McIntyre, Solid State Technol. 42 (1999) 57. [11] C.M. Smith, L.A. Moore, in: Emerging Lithographic Technologies III, in: Y. Vladimirsky (Ed.), Proceedings of SPIE, vol. 3676, 1999, p. 834. [12] H. Hosono, M. Mizuguchi, L. Skuja, T. Ogawa, Opt. Lett. 24 (1999) 1549. [13] M. Mizuguchi, L. Skuja, H. Hosono, T. Ogawa, J. Vac. Sci. Technol. B 17 (1999) 3280. [14] T. Katsuo, Y. Satohito, T. Kimio, Bull. Jpn. Pet. Inst. 12 (1970) 136. [15] C.V.A. Duke, J.M. Miller, J.H. Clark, A.P. Kybett, Spectrochim. Acta A 46 (1990) 1381. [16] P. Dumas, J. Corset, W. Carvalho, Y. Levy, Y. Neuman, J. NonCryst. Solids 47 (1982) 239. [17] C.A.M. Mulder, R.K. Janssen, P. Bachmann, D. Leers, J. NonCryst. Solids 72 (1985) 243. [18] M. Yoshikawa, K. Iwagami, N. Morita, T. Matsunobe, H. Ishida, Thin Solid Films 310 (1997) 167. [19] T.M. Duncan, D.C. Douglass, R. Csencsits, K.L. Walker, J. Appl. Phys. 60 (1986) 130. [20] R.E. Youngman, S. Sen, J. Non-Cryst. Solids 337 (2004) 182. [21] J.W. Emsley, L. Phillips, Prog. Nucl. Magn. Reson. Spectrosc. 7 (1971) 1. [22] V. Wray, Annu. Rep. NMR Spectrosc. 10A (1980) 1. [23] L.F. Gladden, T.A. Carpenter, S.R. Elliott, Philos. Mag. B 53 (1986) L81. [24] H. Koller, A. Wo¨lker, L.A. Villaescusa, M.J. Dı´az-Caban˜as, S. Valencia, M.A. Camblor, J. Am. Chem. Soc. 121 (1999) 3368. [25] J.F. Stebbins, Nature 351 (1991) 638. [26] S. Brawer, Relaxation in Viscous Liquids and Glasses, American Ceramic Society, Columbus, OH, 1985.