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Surface OH groups governing surface chemical properties of SiO2 thin films deposited by RF magnetron sputtering
Thin Solid Films 444 (2003) 153–157
Surface OH groups governing surface chemical properties of SiO2 thin films deposited by RF magnetron sputtering Satoshi Takeda*, Makoto Fukawa Research Center, Asahi Glass Company Limited, 1150 Hazawa-cho, Kanagawa-ku, Yokohama 221-8755, Japan Received 12 May 2002; received in revised form 11 June 2003; accepted 27 June 2003
Abstract We investigated the effects of metal doping on the surface chemical properties of silicon oxide (SiO2 ) thin films. The SiO2 thin films, doped with aluminium (Al), titanium (Ti) or zirconium (Zr), were deposited onto glass by RF magnetron sputtering. The contact angle of water droplets was measured for the films as a function of elapsed time. The hydrophobicity, resulting from the adsorption of organic substances in the atmosphere was significantly altered by metal doping. The surface reactivity of the metal doped films with a polyfluoroalkyl isocyanate silane was also changed. It was found that these alterations were due to the difference in surface OH group density of the film. Furthermore, there was a tendency that the surface OH group density increased along with the amount of non-bridging oxygens in the films. The formation ability of non-bridging oxygens may be closely related with the bonding nature and co-ordination number of the doped element with oxygens in the SiO2 structure. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Secondary ion mass spectrometry; Silicon oxide; Sputtering; Wetting; X-ray photoelectron spectroscopy (XPS)
1. Introduction Silicon oxide (SiO2) thin films are widely used in various fields such as passivation layers of electronic devices, protection layers of magnetic or optical disks and anti-reflective (AR) coatings of displays, because of their excellent chemical stability and optical transmittance with low refractive index w1–4x. The surface properties of the film may be changed when organic substances are adsorbed on the film from the atmosphere, causing serious problems for product quality. When other materials are deposited onto the films where organic substances have been adsorbed, the adhesion is weakened at the interface between layered films. Therefore, it is very important to control the surface properties of the film to obtain high quality products. In previous papers w5,6x, we have investigated the relationship between the wettability and the surface OH group density of metal oxide films or commercial glasses and reported that the surface OH groups play an important role on the surface chemical properties since they *Corresponding author. Tel.: q81-45-374-8755; fax: q81-45-3748863. E-mail address: [email protected] (S. Takeda).
can work as effective adsorptive or reactive sites. Furthermore, it has been found that the hydrophobicity, resulting from the adsorption of organic substances in the atmosphere was different among the films and glasses and that the origin of the variation was attributed to the difference in the amount of adsorbed carbon substances on the surfaces. The amount of the carbon substances adsorbed from the atmosphere has been dependent on the surface OH group density of the films or glasses. In the present study, we applied these findings to modify the surface properties of sputtered SiO2 thin films. A part of this work was reported in Ref. w7x. The purpose of this study is to modify the surface chemical properties of the films by controlling the surface OH group density of the films without significant change in optical properties from the visible to near-IR region. Therefore, the effects of metal doping in the films on the surface OH group density were investigated. In sputtering technology, the doping can be easily performed using metal-doped targets. The aluminium (Al), titanium (Ti) or zirconium (Zr)-doped SiO2 thin film was prepared from a metal-doped silicon target. The relationship between the surface OH group density and
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the surface chemical properties such as the wettability and the surface reactivity with a polyfluoroalkyl isocyanate silane were investigated by contact angle measurements, X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (TOFSIMS), Fourier transform infrared reflectance spectroscopy (FTIRRS) and atomic force microscopy (AFM). From the results obtained, the effectiveness of metal doping for the surface modification of sputtered SiO2 films was discussed. Furthermore, we explored the effect of metal doping on the formation of surface OH group from XPS O1s and FTIRRS spectra analyses. 2. Experimental details SiO2 thin film doped with Al, Ti or Zr was deposited onto soda-lime–silica glass by reactive RF magnetron sputtering with a thickness of ;40 nm. The silicon metal target doped with Al, Ti or Zr was used. The concentration of doped element in the film was adjusted at ;3 at.%, because increases in the doped concentration cause significant change in optical properties. The dopant concentration was determined by XPS. UndopedSiO2 thin film was also prepared from a pure silicon metal target. The wettability of the films was evaluated through contact angle measurements using a goniometer as a function of elapsed time. The measurements were carried out five times for each sample under the same conditions. The accuracy was within "28. Prior to the measurements, the films were stored in a desiccator whose atmosphere was controlled at 50 8C, 95%RH. In the evaluation of reactivity of the film surface, a polyfluoroalkyl isocyanate silane wC8F17C2H4Si(NCO)3 x was used as a reactive agent w8,9x. This material reacts with surface OH groups at room temperature. Prior to the treatment, the sample surfaces were cleaned using UVy O3 for 10 min to remove contaminants. Thereafter, the sample was immersed into hydrochlorofluorocarbon (HCFC; Asahi Glass AK-225) solution containing 1 wt.% wC8F17C2H4Si(NCO)3 x for 1 min and then rinsed with the HCFC for 1 min. After this treatment, the surface fluorine concentration was subsequently quantified by XPS. Observation of surface morphology and the quantitative analysis of the surface roughness of the films were performed using AFM (Seiko SPI 3700). The optical transmission spectra of the films were measured at room temperature in air using a dual beam spectrometer (Shimazu UV3100). The surface OH group density was evaluated by TOFSIMS (EVANSE-PHI TSF2000). The pulsed Gaq primary ion beam was operated at 15 keV, 1.2 nA and rastered over the area 80=80 mm2. In general, FT-IR spectroscopy is well known as a useful analytical tool for characterizing OH groups w10x, although, it is almost impossible to detect information selectively from the
Table 1 The secondary ion intensity ratios of 17(OHy)y16Oy which represent surface OH group density obtained from TOF-SIMS measurements and the surface roughness (rms) obtained by AFM measurements for the films Sample
17
(OHy)y 16Oy
SiO2 Al-doped SiO2 Ti-doped SiO2 Zr-doped SiO2
0.409"0.003 0.420"0.003 0.484"0.003 0.491"0.003
rmsynm 1.1"0.2 1.0"0.2 1.1"0.2 1.0"0.2
surface of the films. On the other hand, chemical shift analysis in XPS has been successfully used to elucidate the oxidation state near the surface region. However, in case of SiO2, a curve-fitting technique should be applied, since no significant shift in binding energy is observed for the oxide and hydroxide species w11x. Unfortunately, this procedure can sometimes lead to ambiguous results. In the present study, we applied a TOF-SIMS technique to evaluate the surface OH groups on the SiO2 film, because TOF-SIMS has excellent sensitivity and high mass resolution w12x, so that the detailed chemical information from the surface could be obtained. Namely, the surface OH group could be precisely monitored. However, a standard sample should be necessary in order to obtain the absolute concentration of the surface OH groups in the SIMS analysis w13x. According to the research in Ref. w8x, the absolute concentration of the silanol group (SiOH) was determined by using StaticSIMS and XPS with a curve-fitting technique. Therefore, in this study, we used 17(OHy)y 16Oy ratio as an indication of the surface OH group density, because the relative surface OH group density could be obtained easily and quickly. The accuracy of 17(OHy)y16Oy ratio was within "0.003. The formation ability of the surface OH group on the films was evaluated by XPS (PHI 5500) and infrared reflection spectroscopy (Nicolet 740 spectrophotometer). XPS measurements were carried out with a monochromatized AlKa source. The energy axis of the spectrometer was calibrated by the position of the Ag 3d5y2 peak at 367.9 eV wfull-width at half maximum (FWHM); 0.429 eVx. The detection angle of the X-ray photoelectrons was 758 to the normal of the sample surface. The binding energy was referenced to the C 1s peak at 284.6 eV. In the IR reflection measurements, the angle of incidence was fixed at 118 to the normal of the surface and the spectral resolution was 2 cmy1. 3. Results and discussion 3.1. Surface OH group density and optical properties The surface OH group density of the films obtained by TOF-SIMS measurements are shown in Table 1. It is found that the secondary ion intensity ratio of
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Fig. 1. Transmission spectra for undoped and metal-doped films. 17
(OHy)y 16Oy is different among the films, indicating that the surface OH group density can be effectively varied by the metal dopant. Fig. 1 shows the optical transmission spectra of the films. The transmittance in the wavelength region of 350–1500 nm for Ti or Zr doped films is almost same as that of non-doped film. The difference in average transmittance between Al doped and non-doped film was within 1% in this region. These results suggest that the surface OH group density can be modified by the doped metal without significant change in the average transmittance from the visible to near-IR region. In the UV region, the difference in the transmittance between metal doped and non-doped films should increase, because OH has UV absorption due to an electronic transition. 3.2. Relationship between wettability and surface OH group density Fig. 2 shows the contact angle of water droplets for the films as a function of elapsed time. The contact angles were -58 for all the films immediately after UVyO3 cleaning. In general, the contact angle of water droplets is close to -58 for a glass surface without contaminants, because the surface energy of the oxide is essentially very large compared with that of water w14,15x. Therefore, the surfaces are considered to be contaminant free immediately after UVyO3 cleaning. The contact angles of all the films gradually increased with time and reached at an asymptotic value (us) in ;7 days elapsed. The value remains constant for the following 7 days. The hydrophobicities (us) are different among the films. These results indicate that, the wettability is significantly altered by metal doping. It is known that the contact angle is affected by surface roughness as well as by surface contamination w16x. However, no significant change in the surface roughness (rms) examined by AFM is observed after the doping, as shown in Table 1. This result indicates that the difference in the us is not due to the surface roughness, but due to the cleanliness of the surface.
Fig. 2. The contact angle of water droplets for undoped and metaldoped films as a function of elapsed time.
That is, the increase in the contact angle results from the adsorption of organic substances in the atmosphere and the difference in the us is caused by the difference in the amount of these adsorbed organic substances. This suggests that, the surface wettability, one of the important surface chemical properties is altered by doping. In order to clarify the origin of this difference, us is plotted against the 17(OHy)y 16Oy ratio, which represents the surface OH group density of the film in Fig. 3a. The us increases steadily with the increase in the surface OH group density. This fact indicates that the hydrophobicity, resulting from the adsorption of organic substances from the atmosphere, depends on the surface
Fig. 3. The relationship between the secondary ion intensity ratio of 17 (OHy)y16Oy and (a) the contact angle of water droplets at 14 days elapsed or (b) the fluorine concentration determined by XPS for the films.
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Fig. 4. High-resolution O1s XPS spectra for undoped and metal-doped films.
OH group density of the film. That is, the surface OH group can work as an effective adsorptive site for organic substances and seems to be one of the important factors for modifying the surface adsorption properties of film. 3.3. Relationship between surface reactivity and surface OH group density
that the doped element strongly affects the negative charge density on oxygen atoms at the surface. Accord¨ ing to the research of Bruckner et al. w17,18x, the O1s peak could be separated into two components attributed to bridging oxygen atoms (Si–O–Si; BOs) and nonbridging oxygen atoms (Si–Oy; NBOs) and NBO component was observed at the lower binding energy side of BO component in O1s peak. Namely, the shoulder peak observed in Fig. 4 is considered to result from the formation of NBOs by metal doping. The peak intensity of the NBO component increases for the films having a higher surface OH group density. This result suggests that the formation of surface OH groups is closely related with the number of NBOs. Similar tendencies are observed by IR reflection measurements as shown in Fig. 5. The peak at ;1060 cmy1 is attributed to the stretching vibration of the Si–O–Si bonds (BOs). A shoulder peak at ;930 cmy1 is associated with the stretching vibration of Si–Oy bonds (NBOs). The IR band at ;1060 cmy1 is shifted to lower wavenumbers and the shoulder peak intensity increases for the film having a higher surface OH group density. This indicates that the surface OH group density of the film increases with increasing the number of NBOs w19x. These observations suggest that the metal
The surface fluorine concentration as a result of reaction with a polyfluoroalkyl isocyanate silane wC8F17C2H4Si(NCO)3 x determined by XPS is shown in Fig. 3b. It is found that the surface fluorine concentration is different among the films, indicating that the surface reactivity with the agent is successfully altered by metaldoping. To clarify the origin of this difference, fluorine concentration is plotted against the 17(OHy)y16Oy ratio, which represents the surface OH group density of the films as shown in Fig. 3b. There is a good correlation between them, indicating that the reactivity depends on the surface OH group density of the film. Specifically, the surface OH group can work as an effective reactive site. Here, the fluorine concentration increases with increasing surface OH group density. This indicates that the surface reactivity on the films depends on the number of surface OH groups and not the chemical states. 3.4. Effect of metal doping on surface OH group formation As mentioned previously, the change in the surface OH group density can be induced by metal doping. In order to clarify the origin of this change, XPS O1s analyses were carried out as shown in Fig. 4. The shoulder peak is clearly observed for the metal doped films at lower binding energy side of main peak at ;531.5 eV in the XPS O1s peak, which is not recognized for non-doped film. These observations indicate
Fig. 5. Infrared reflection spectra for undoped and metal-doped films in the frequency region of (a) 800–1300 cmy1 and (b) 820–1020 cmy1.
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doping causes the increase in the number of NBOs in the film, resulting in increasing the formation of the surface OH groups. The difference in the formation ability of NBOs among the doped elements may be due to the bonding nature of the doped element with oxygens in the SiO2 structure. According to the Pauling’s rules, if the bond is ionic, the formation of NBOs is required so as to meet the local electroneutrality around the metal ions w20x. On the contrary, if the bond is covalent, NBOs are not formed. In this study, since the bond of Ti or Zr is considered to be rather ionic than that of Al, the number of NBOs for Ti or Zr doped film become larger than that of Al doped film. Consequently, the surface OH group density of Ti or Zr doped film becomes higher than that of Al doped film. Similarly, since the bond of Al is considered to be rather ionic than that of Si, the Al doped film have larger NBOs than that of non-doped film, resulting in the difference in the surface OH group density between them. However, it is known that the bond of Al could be both ionic and covalent in silicate glass, depending on the co-ordination number of oxygens around Al ions w21x. Therefore, further investigation about the co-ordination structure of doped metal element in SiO2 structure should be necessary for the conclusive discussion about this subject. 4. Conclusions In this paper, we have investigated the effects of metal doping in sputtered SiO2 thin films on surface properties such as wettability and reactivity with a polyfluoroalkyl isocyanate silane. The wettability and the surface reactivity were significantly altered by metal doping. It is found that these alterations are due to the change in the surface OH group density of the films and that the surface OH group density increases with increasing the number of non-bridging oxygens in the SiO2 film. In addition, no significant transmittance change was observed in the wavelength region of 350– 1500 nm when the dopant concentration is lower than ;3 at.% and the film thickness is ;40 nm. This suggests that the metal doping is the effective way to modify the surface properties without significant change
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in optical transmittance from the visible to near-IR region. We expect that these findings offer a novel method to control and design the surface properties of SiO2 thin films. Acknowledgments The authors are grateful to Prof. Hideo Hosono of Tokyo Institute of Technology for his valuable comments about the formation of NBOs in SiO2 structure. References w1x H.A. Macleod, Thin-Film Optical Filters, McGraw-Hill, New York, 1989, p. 71, Chapter 3. w2x J.D. Rancourt, Optical Thin Films User’s Handbook, McGrawHill, New York, 1989, p. 78, Chapter 4. w3x T. Oyama, H. Ohsaki, Y. Tachibana, Y. Hayashi, Y. Ono, N. Horie, Thin Solid Films 351 (1999) 235. w4x K. Toki, K. Kusakabe, T. Odani, S. Kobuna, Y. Shimizu, Thin Solid Films 281–282 (1996) 401. w5x S. Takeda, M. Fukawa, Y. Hayashi, K. Matsumoto, Thin Solid Films 339 (1999) 220. w6x S. Takeda, K. Yamamoto, Y. Hayasaka, K. Matsumoto, J. NonCryst. Solids 249 (1999) 41, inbid, 258 (1999) 244. w7x S. Takeda, M. Fukawa, Mater. Res. Soc. Symp. Proc. 740 (2003) 443. w8x Y. Hayashi, T. Yoneda, K. Matsumoto, J. Ceram. Soc. Jpn. 102 (1994) 206. w9x T. Yoneda, T. Morimoto, Thin Solid Films 351 (1999) 279. w10x K. Kiier, J.H. Shen, A.C. Zettlemoyer, J. Phys. Chem. 77 (1973) 1458. w11x D. Sprenger, H. Bach, W. Meisel, P. Gutlich, J. Non-Cryst. Solids 126 (1990) 111. w12x A. Benninghoven, Angew. Chem. Int. Ed. Engl. 33 (1994) 1023. w13x R.G. Wioson, F.A. Stevie, C.W. Magee, Secondary Ion Mass Spectrometry, John Wiley and Sons, New York, 1989. w14x D.A. Olsen, A.J. Osterass, J. Phys. Chem. 68 (1964) 2730. w15x S.J. Glegg, The Surface Chemistry of Solids, Rheinhold, New York, 1961. w16x R.W. Wenzel, Ind. Eng. Chem. 55 (1963) 18. w17x R. Bruckner, ¨ H.-U. Chun, H. Goretzki, Glastech. Ber. 49 (1976) 211. w18x R. Bruckner, ¨ H.-U. Chun, H. Goretzki, Glastech. Ber. 51 (1978) 1. w19x D. Crozier, R.W. Douglas, Phys. Chem. Glasses 6 (1965) 240. w20x H. Rawson, Inorganic Glass-Forming System, Academic Press, London and New York, 1967. w21x H. Moore, P.W. McMillan, J. Soc. Glass Tech. 40 (1956) 66T.
Surface OH groups governing surface chemical properties of SiO2 thin films deposited by RF magnetron sputtering Satoshi Takeda*, Makoto Fukawa Research Center, Asahi Glass Company Limited, 1150 Hazawa-cho, Kanagawa-ku, Yokohama 221-8755, Japan Received 12 May 2002; received in revised form 11 June 2003; accepted 27 June 2003
Abstract We investigated the effects of metal doping on the surface chemical properties of silicon oxide (SiO2 ) thin films. The SiO2 thin films, doped with aluminium (Al), titanium (Ti) or zirconium (Zr), were deposited onto glass by RF magnetron sputtering. The contact angle of water droplets was measured for the films as a function of elapsed time. The hydrophobicity, resulting from the adsorption of organic substances in the atmosphere was significantly altered by metal doping. The surface reactivity of the metal doped films with a polyfluoroalkyl isocyanate silane was also changed. It was found that these alterations were due to the difference in surface OH group density of the film. Furthermore, there was a tendency that the surface OH group density increased along with the amount of non-bridging oxygens in the films. The formation ability of non-bridging oxygens may be closely related with the bonding nature and co-ordination number of the doped element with oxygens in the SiO2 structure. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Secondary ion mass spectrometry; Silicon oxide; Sputtering; Wetting; X-ray photoelectron spectroscopy (XPS)
1. Introduction Silicon oxide (SiO2) thin films are widely used in various fields such as passivation layers of electronic devices, protection layers of magnetic or optical disks and anti-reflective (AR) coatings of displays, because of their excellent chemical stability and optical transmittance with low refractive index w1–4x. The surface properties of the film may be changed when organic substances are adsorbed on the film from the atmosphere, causing serious problems for product quality. When other materials are deposited onto the films where organic substances have been adsorbed, the adhesion is weakened at the interface between layered films. Therefore, it is very important to control the surface properties of the film to obtain high quality products. In previous papers w5,6x, we have investigated the relationship between the wettability and the surface OH group density of metal oxide films or commercial glasses and reported that the surface OH groups play an important role on the surface chemical properties since they *Corresponding author. Tel.: q81-45-374-8755; fax: q81-45-3748863. E-mail address: [email protected] (S. Takeda).
can work as effective adsorptive or reactive sites. Furthermore, it has been found that the hydrophobicity, resulting from the adsorption of organic substances in the atmosphere was different among the films and glasses and that the origin of the variation was attributed to the difference in the amount of adsorbed carbon substances on the surfaces. The amount of the carbon substances adsorbed from the atmosphere has been dependent on the surface OH group density of the films or glasses. In the present study, we applied these findings to modify the surface properties of sputtered SiO2 thin films. A part of this work was reported in Ref. w7x. The purpose of this study is to modify the surface chemical properties of the films by controlling the surface OH group density of the films without significant change in optical properties from the visible to near-IR region. Therefore, the effects of metal doping in the films on the surface OH group density were investigated. In sputtering technology, the doping can be easily performed using metal-doped targets. The aluminium (Al), titanium (Ti) or zirconium (Zr)-doped SiO2 thin film was prepared from a metal-doped silicon target. The relationship between the surface OH group density and
0040-6090/03/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0040-6090(03)01094-0
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the surface chemical properties such as the wettability and the surface reactivity with a polyfluoroalkyl isocyanate silane were investigated by contact angle measurements, X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (TOFSIMS), Fourier transform infrared reflectance spectroscopy (FTIRRS) and atomic force microscopy (AFM). From the results obtained, the effectiveness of metal doping for the surface modification of sputtered SiO2 films was discussed. Furthermore, we explored the effect of metal doping on the formation of surface OH group from XPS O1s and FTIRRS spectra analyses. 2. Experimental details SiO2 thin film doped with Al, Ti or Zr was deposited onto soda-lime–silica glass by reactive RF magnetron sputtering with a thickness of ;40 nm. The silicon metal target doped with Al, Ti or Zr was used. The concentration of doped element in the film was adjusted at ;3 at.%, because increases in the doped concentration cause significant change in optical properties. The dopant concentration was determined by XPS. UndopedSiO2 thin film was also prepared from a pure silicon metal target. The wettability of the films was evaluated through contact angle measurements using a goniometer as a function of elapsed time. The measurements were carried out five times for each sample under the same conditions. The accuracy was within "28. Prior to the measurements, the films were stored in a desiccator whose atmosphere was controlled at 50 8C, 95%RH. In the evaluation of reactivity of the film surface, a polyfluoroalkyl isocyanate silane wC8F17C2H4Si(NCO)3 x was used as a reactive agent w8,9x. This material reacts with surface OH groups at room temperature. Prior to the treatment, the sample surfaces were cleaned using UVy O3 for 10 min to remove contaminants. Thereafter, the sample was immersed into hydrochlorofluorocarbon (HCFC; Asahi Glass AK-225) solution containing 1 wt.% wC8F17C2H4Si(NCO)3 x for 1 min and then rinsed with the HCFC for 1 min. After this treatment, the surface fluorine concentration was subsequently quantified by XPS. Observation of surface morphology and the quantitative analysis of the surface roughness of the films were performed using AFM (Seiko SPI 3700). The optical transmission spectra of the films were measured at room temperature in air using a dual beam spectrometer (Shimazu UV3100). The surface OH group density was evaluated by TOFSIMS (EVANSE-PHI TSF2000). The pulsed Gaq primary ion beam was operated at 15 keV, 1.2 nA and rastered over the area 80=80 mm2. In general, FT-IR spectroscopy is well known as a useful analytical tool for characterizing OH groups w10x, although, it is almost impossible to detect information selectively from the
Table 1 The secondary ion intensity ratios of 17(OHy)y16Oy which represent surface OH group density obtained from TOF-SIMS measurements and the surface roughness (rms) obtained by AFM measurements for the films Sample
17
(OHy)y 16Oy
SiO2 Al-doped SiO2 Ti-doped SiO2 Zr-doped SiO2
0.409"0.003 0.420"0.003 0.484"0.003 0.491"0.003
rmsynm 1.1"0.2 1.0"0.2 1.1"0.2 1.0"0.2
surface of the films. On the other hand, chemical shift analysis in XPS has been successfully used to elucidate the oxidation state near the surface region. However, in case of SiO2, a curve-fitting technique should be applied, since no significant shift in binding energy is observed for the oxide and hydroxide species w11x. Unfortunately, this procedure can sometimes lead to ambiguous results. In the present study, we applied a TOF-SIMS technique to evaluate the surface OH groups on the SiO2 film, because TOF-SIMS has excellent sensitivity and high mass resolution w12x, so that the detailed chemical information from the surface could be obtained. Namely, the surface OH group could be precisely monitored. However, a standard sample should be necessary in order to obtain the absolute concentration of the surface OH groups in the SIMS analysis w13x. According to the research in Ref. w8x, the absolute concentration of the silanol group (SiOH) was determined by using StaticSIMS and XPS with a curve-fitting technique. Therefore, in this study, we used 17(OHy)y 16Oy ratio as an indication of the surface OH group density, because the relative surface OH group density could be obtained easily and quickly. The accuracy of 17(OHy)y16Oy ratio was within "0.003. The formation ability of the surface OH group on the films was evaluated by XPS (PHI 5500) and infrared reflection spectroscopy (Nicolet 740 spectrophotometer). XPS measurements were carried out with a monochromatized AlKa source. The energy axis of the spectrometer was calibrated by the position of the Ag 3d5y2 peak at 367.9 eV wfull-width at half maximum (FWHM); 0.429 eVx. The detection angle of the X-ray photoelectrons was 758 to the normal of the sample surface. The binding energy was referenced to the C 1s peak at 284.6 eV. In the IR reflection measurements, the angle of incidence was fixed at 118 to the normal of the surface and the spectral resolution was 2 cmy1. 3. Results and discussion 3.1. Surface OH group density and optical properties The surface OH group density of the films obtained by TOF-SIMS measurements are shown in Table 1. It is found that the secondary ion intensity ratio of
S. Takeda, M. Fukawa / Thin Solid Films 444 (2003) 153–157
155
Fig. 1. Transmission spectra for undoped and metal-doped films. 17
(OHy)y 16Oy is different among the films, indicating that the surface OH group density can be effectively varied by the metal dopant. Fig. 1 shows the optical transmission spectra of the films. The transmittance in the wavelength region of 350–1500 nm for Ti or Zr doped films is almost same as that of non-doped film. The difference in average transmittance between Al doped and non-doped film was within 1% in this region. These results suggest that the surface OH group density can be modified by the doped metal without significant change in the average transmittance from the visible to near-IR region. In the UV region, the difference in the transmittance between metal doped and non-doped films should increase, because OH has UV absorption due to an electronic transition. 3.2. Relationship between wettability and surface OH group density Fig. 2 shows the contact angle of water droplets for the films as a function of elapsed time. The contact angles were -58 for all the films immediately after UVyO3 cleaning. In general, the contact angle of water droplets is close to -58 for a glass surface without contaminants, because the surface energy of the oxide is essentially very large compared with that of water w14,15x. Therefore, the surfaces are considered to be contaminant free immediately after UVyO3 cleaning. The contact angles of all the films gradually increased with time and reached at an asymptotic value (us) in ;7 days elapsed. The value remains constant for the following 7 days. The hydrophobicities (us) are different among the films. These results indicate that, the wettability is significantly altered by metal doping. It is known that the contact angle is affected by surface roughness as well as by surface contamination w16x. However, no significant change in the surface roughness (rms) examined by AFM is observed after the doping, as shown in Table 1. This result indicates that the difference in the us is not due to the surface roughness, but due to the cleanliness of the surface.
Fig. 2. The contact angle of water droplets for undoped and metaldoped films as a function of elapsed time.
That is, the increase in the contact angle results from the adsorption of organic substances in the atmosphere and the difference in the us is caused by the difference in the amount of these adsorbed organic substances. This suggests that, the surface wettability, one of the important surface chemical properties is altered by doping. In order to clarify the origin of this difference, us is plotted against the 17(OHy)y 16Oy ratio, which represents the surface OH group density of the film in Fig. 3a. The us increases steadily with the increase in the surface OH group density. This fact indicates that the hydrophobicity, resulting from the adsorption of organic substances from the atmosphere, depends on the surface
Fig. 3. The relationship between the secondary ion intensity ratio of 17 (OHy)y16Oy and (a) the contact angle of water droplets at 14 days elapsed or (b) the fluorine concentration determined by XPS for the films.
156
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Fig. 4. High-resolution O1s XPS spectra for undoped and metal-doped films.
OH group density of the film. That is, the surface OH group can work as an effective adsorptive site for organic substances and seems to be one of the important factors for modifying the surface adsorption properties of film. 3.3. Relationship between surface reactivity and surface OH group density
that the doped element strongly affects the negative charge density on oxygen atoms at the surface. Accord¨ ing to the research of Bruckner et al. w17,18x, the O1s peak could be separated into two components attributed to bridging oxygen atoms (Si–O–Si; BOs) and nonbridging oxygen atoms (Si–Oy; NBOs) and NBO component was observed at the lower binding energy side of BO component in O1s peak. Namely, the shoulder peak observed in Fig. 4 is considered to result from the formation of NBOs by metal doping. The peak intensity of the NBO component increases for the films having a higher surface OH group density. This result suggests that the formation of surface OH groups is closely related with the number of NBOs. Similar tendencies are observed by IR reflection measurements as shown in Fig. 5. The peak at ;1060 cmy1 is attributed to the stretching vibration of the Si–O–Si bonds (BOs). A shoulder peak at ;930 cmy1 is associated with the stretching vibration of Si–Oy bonds (NBOs). The IR band at ;1060 cmy1 is shifted to lower wavenumbers and the shoulder peak intensity increases for the film having a higher surface OH group density. This indicates that the surface OH group density of the film increases with increasing the number of NBOs w19x. These observations suggest that the metal
The surface fluorine concentration as a result of reaction with a polyfluoroalkyl isocyanate silane wC8F17C2H4Si(NCO)3 x determined by XPS is shown in Fig. 3b. It is found that the surface fluorine concentration is different among the films, indicating that the surface reactivity with the agent is successfully altered by metaldoping. To clarify the origin of this difference, fluorine concentration is plotted against the 17(OHy)y16Oy ratio, which represents the surface OH group density of the films as shown in Fig. 3b. There is a good correlation between them, indicating that the reactivity depends on the surface OH group density of the film. Specifically, the surface OH group can work as an effective reactive site. Here, the fluorine concentration increases with increasing surface OH group density. This indicates that the surface reactivity on the films depends on the number of surface OH groups and not the chemical states. 3.4. Effect of metal doping on surface OH group formation As mentioned previously, the change in the surface OH group density can be induced by metal doping. In order to clarify the origin of this change, XPS O1s analyses were carried out as shown in Fig. 4. The shoulder peak is clearly observed for the metal doped films at lower binding energy side of main peak at ;531.5 eV in the XPS O1s peak, which is not recognized for non-doped film. These observations indicate
Fig. 5. Infrared reflection spectra for undoped and metal-doped films in the frequency region of (a) 800–1300 cmy1 and (b) 820–1020 cmy1.
S. Takeda, M. Fukawa / Thin Solid Films 444 (2003) 153–157
doping causes the increase in the number of NBOs in the film, resulting in increasing the formation of the surface OH groups. The difference in the formation ability of NBOs among the doped elements may be due to the bonding nature of the doped element with oxygens in the SiO2 structure. According to the Pauling’s rules, if the bond is ionic, the formation of NBOs is required so as to meet the local electroneutrality around the metal ions w20x. On the contrary, if the bond is covalent, NBOs are not formed. In this study, since the bond of Ti or Zr is considered to be rather ionic than that of Al, the number of NBOs for Ti or Zr doped film become larger than that of Al doped film. Consequently, the surface OH group density of Ti or Zr doped film becomes higher than that of Al doped film. Similarly, since the bond of Al is considered to be rather ionic than that of Si, the Al doped film have larger NBOs than that of non-doped film, resulting in the difference in the surface OH group density between them. However, it is known that the bond of Al could be both ionic and covalent in silicate glass, depending on the co-ordination number of oxygens around Al ions w21x. Therefore, further investigation about the co-ordination structure of doped metal element in SiO2 structure should be necessary for the conclusive discussion about this subject. 4. Conclusions In this paper, we have investigated the effects of metal doping in sputtered SiO2 thin films on surface properties such as wettability and reactivity with a polyfluoroalkyl isocyanate silane. The wettability and the surface reactivity were significantly altered by metal doping. It is found that these alterations are due to the change in the surface OH group density of the films and that the surface OH group density increases with increasing the number of non-bridging oxygens in the SiO2 film. In addition, no significant transmittance change was observed in the wavelength region of 350– 1500 nm when the dopant concentration is lower than ;3 at.% and the film thickness is ;40 nm. This suggests that the metal doping is the effective way to modify the surface properties without significant change
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in optical transmittance from the visible to near-IR region. We expect that these findings offer a novel method to control and design the surface properties of SiO2 thin films. Acknowledgments The authors are grateful to Prof. Hideo Hosono of Tokyo Institute of Technology for his valuable comments about the formation of NBOs in SiO2 structure. References w1x H.A. Macleod, Thin-Film Optical Filters, McGraw-Hill, New York, 1989, p. 71, Chapter 3. w2x J.D. Rancourt, Optical Thin Films User’s Handbook, McGrawHill, New York, 1989, p. 78, Chapter 4. w3x T. Oyama, H. Ohsaki, Y. Tachibana, Y. Hayashi, Y. Ono, N. Horie, Thin Solid Films 351 (1999) 235. w4x K. Toki, K. Kusakabe, T. Odani, S. Kobuna, Y. Shimizu, Thin Solid Films 281–282 (1996) 401. w5x S. Takeda, M. Fukawa, Y. Hayashi, K. Matsumoto, Thin Solid Films 339 (1999) 220. w6x S. Takeda, K. Yamamoto, Y. Hayasaka, K. Matsumoto, J. NonCryst. Solids 249 (1999) 41, inbid, 258 (1999) 244. w7x S. Takeda, M. Fukawa, Mater. Res. Soc. Symp. Proc. 740 (2003) 443. w8x Y. Hayashi, T. Yoneda, K. Matsumoto, J. Ceram. Soc. Jpn. 102 (1994) 206. w9x T. Yoneda, T. Morimoto, Thin Solid Films 351 (1999) 279. w10x K. Kiier, J.H. Shen, A.C. Zettlemoyer, J. Phys. Chem. 77 (1973) 1458. w11x D. Sprenger, H. Bach, W. Meisel, P. Gutlich, J. Non-Cryst. Solids 126 (1990) 111. w12x A. Benninghoven, Angew. Chem. Int. Ed. Engl. 33 (1994) 1023. w13x R.G. Wioson, F.A. Stevie, C.W. Magee, Secondary Ion Mass Spectrometry, John Wiley and Sons, New York, 1989. w14x D.A. Olsen, A.J. Osterass, J. Phys. Chem. 68 (1964) 2730. w15x S.J. Glegg, The Surface Chemistry of Solids, Rheinhold, New York, 1961. w16x R.W. Wenzel, Ind. Eng. Chem. 55 (1963) 18. w17x R. Bruckner, ¨ H.-U. Chun, H. Goretzki, Glastech. Ber. 49 (1976) 211. w18x R. Bruckner, ¨ H.-U. Chun, H. Goretzki, Glastech. Ber. 51 (1978) 1. w19x D. Crozier, R.W. Douglas, Phys. Chem. Glasses 6 (1965) 240. w20x H. Rawson, Inorganic Glass-Forming System, Academic Press, London and New York, 1967. w21x H. Moore, P.W. McMillan, J. Soc. Glass Tech. 40 (1956) 66T.