Thin Solid Films 444(2003) 153–157

0040-6090/03/$ - see front matter� 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0040-6090(03)01094-0

Surface OH groups governing surface chemical properties of SiO thin2

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(SiO ) thin films. The SiO2 2

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 fromthe adsorption of organic substances in the atmosphere was significantly altered by metal doping. The surface reactivity of themetal doped films with a polyfluoroalkyl isocyanate silane was also changed. It was found that these alterations were due to thedifference in surface OH group density of the film. Furthermore, there was a tendency that the surface OH group density increasedalong with the amount of non-bridging oxygens in the films. The formation ability of non-bridging oxygens may be closelyrelated with the bonding nature and co-ordination number of the doped element with oxygens in the SiO structure.2

� 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 (SiO ) thin films are widely used in2

various fields such as passivation layers of electronicdevices, protection layers of magnetic or optical disksand anti-reflective(AR) coatings of displays, becauseof their excellent chemical stability and optical trans-mittance with low refractive indexw1–4x. The surfaceproperties of the film may be changed when organicsubstances are adsorbed on the film from the atmos-phere, causing serious problems for product quality.When other materials are deposited onto the films whereorganic substances have been adsorbed, the adhesion isweakened at the interface between layered films. There-fore, it is very important to control the surface propertiesof the film to obtain high quality products.In previous papersw5,6x, we have investigated the

relationship between the wettability and the surface OHgroup density of metal oxide films or commercial glassesand reported that the surface OH groups play an impor-tant role on the surface chemical properties since they

*Corresponding author. Tel.:q81-45-374-8755; fax:q81-45-374-8863.

E-mail address: takeda@agc.co.jp(S. Takeda).

can work as effective adsorptive or reactive sites. Fur-thermore, it has been found that the hydrophobicity,resulting from the adsorption of organic substances inthe atmosphere was different among the films andglasses and that the origin of the variation was attributedto the difference in the amount of adsorbed carbonsubstances on the surfaces. The amount of the carbonsubstances adsorbed from the atmosphere has beendependent on the surface OH group density of the filmsor glasses.In the present study, we applied these findings to

modify the surface properties of sputtered SiO thin2

films. A part of this work was reported in Ref.w7x. Thepurpose of this study is to modify the surface chemicalproperties of the films by controlling the surface OHgroup density of the films without significant change inoptical properties from the visible to near-IR region.Therefore, the effects of metal doping in the films onthe surface OH group density were investigated. Insputtering technology, the doping can be easily per-formed using metal-doped targets. The aluminium(Al),titanium (Ti) or zirconium (Zr)-doped SiO thin film2

was prepared from a metal-doped silicon target. Therelationship between the surface OH group density and

154 S. Takeda, M. Fukawa / Thin Solid Films 444 (2003) 153–157

Table 1The secondary ion intensity ratios of(OH )y O which represent17 y 16 y

surface OH group density obtained from TOF-SIMS measurementsand the surface roughness(rms) obtained by AFM measurements forthe films

Sample (OH )y O17 y 16 y rmsynm

SiO2 0.409"0.003 1.1"0.2Al-doped SiO2 0.420"0.003 1.0"0.2Ti-doped SiO2 0.484"0.003 1.1"0.2Zr-doped SiO2 0.491"0.003 1.0"0.2

the surface chemical properties such as the wettabilityand the surface reactivity with a polyfluoroalkyl iso-cyanate silane were investigated by contact angle meas-urements, X-ray photoelectron spectroscopy(XPS),time-of-flight secondary ion mass spectrometry(TOF-SIMS), Fourier transform infrared reflectance spectro-scopy(FTIRRS) and atomic force microscopy(AFM).From the results obtained, the effectiveness of metaldoping for the surface modification of sputtered SiO2

films was discussed. Furthermore, we explored the effectof metal doping on the formation of surface OH groupfrom XPS O1s and FTIRRS spectra analyses.

2. Experimental details

SiO thin film doped with Al, Ti or Zr was deposited2

onto soda-lime–silica glass by reactive RF magnetronsputtering with a thickness of;40 nm. The siliconmetal target doped with Al, Ti or Zr was used. Theconcentration of doped element in the film was adjustedat ;3 at.%, because increases in the doped concentra-tion cause significant change in optical properties. Thedopant concentration was determined by XPS. Undoped-SiO thin film was also prepared from a pure silicon2

metal target.The wettability of the films was evaluated through

contact angle measurements using a goniometer as afunction of elapsed time. The measurements were carriedout five times for each sample under the same condi-tions. The accuracy was within"28. Prior to themeasurements, the films were stored in a desiccatorwhose atmosphere was controlled at 508C, 95%RH. Inthe evaluation of reactivity of the film surface, a poly-fluoroalkyl isocyanate silanewC F C H Si(NCO) x was8 17 2 4 3

used as a reactive agentw8,9x. This material reacts withsurface OH groups at room temperature. Prior to thetreatment, the sample surfaces were cleaned using UVyO for 10 min to remove contaminants. Thereafter, the3

sample was immersed into hydrochlorofluorocarbon(HCFC; Asahi Glass AK-225) solution containing 1wt.% wC F C H Si(NCO) x for 1 min and then rinsed8 17 2 4 3

with the HCFC for 1 min. After this treatment, thesurface fluorine concentration was subsequently quanti-fied by XPS. Observation of surface morphology andthe quantitative analysis of the surface roughness of thefilms were performed using AFM(Seiko SPI 3700).The optical transmission spectra of the films weremeasured at room temperature in air using a dual beamspectrometer(Shimazu UV3100).The surface OH group density was evaluated by TOF-

SIMS (EVANSE-PHI TSF2000). The pulsed Ga pri-q

mary ion beam was operated at 15 keV, 1.2 nA andrastered over the area 80=80 mm . In general, FT-IR2

spectroscopy is well known as a useful analytical toolfor characterizing OH groupsw10x, although, it is almostimpossible to detect information selectively from the

surface of the films. On the other hand, chemical shiftanalysis in XPS has been successfully used to elucidatethe oxidation state near the surface region. However, incase of SiO , a curve-fitting technique should be applied,2

since no significant shift in binding energy is observedfor the oxide and hydroxide speciesw11x. Unfortunately,this procedure can sometimes lead to ambiguous results.In the present study, we applied a TOF-SIMS techniqueto evaluate the surface OH groups on the SiO film,2

because TOF-SIMS has excellent sensitivity and highmass resolutionw12x, so that the detailed chemicalinformation from the surface could be obtained. Namely,the surface OH group could be precisely monitored.However, a standard sample should be necessary inorder to obtain the absolute concentration of the surfaceOH groups in the SIMS analysisw13x. According to theresearch in Ref.w8x, the absolute concentration of thesilanol group(SiOH) was determined by using Static-SIMS and XPS with a curve-fitting technique. Therefore,in this study, we used (OH )y O ratio as an17 y 16 y

indication of the surface OH group density, because therelative surface OH group density could be obtainedeasily and quickly. The accuracy of(OH )y O ratio17 y 16 y

was within"0.003.The formation ability of the surface OH group on the

films was evaluated by XPS(PHI 5500) and infraredreflection spectroscopy(Nicolet 740 spectrophotome-ter). XPS measurements were carried out with a mon-ochromatized AlKa source. The energy axis of thespectrometer was calibrated by the position of the Ag3d peak at 367.9 eVwfull-width at half maximum5y2

(FWHM); 0.429 eVx. The detection angle of the X-rayphotoelectrons was 758 to the normal of the samplesurface. The binding energy was referenced to the C 1speak at 284.6 eV. In the IR reflection measurements, theangle of incidence was fixed at 118 to the normal of thesurface and the spectral resolution was 2 cm .y1

3. Results and discussion

3.1. Surface OH group density and optical properties

The surface OH group density of the films obtainedby TOF-SIMS measurements are shown in Table 1. Itis found that the secondary ion intensity ratio of

155S. Takeda, M. Fukawa / Thin Solid Films 444 (2003) 153–157

Fig. 1. Transmission spectra for undoped and metal-doped films.Fig. 2. The contact angle of water droplets for undoped and metal-doped films as a function of elapsed time.

Fig. 3. The relationship between the secondary ion intensity ratio of(OH )y O and(a) the contact angle of water droplets at 14 days17 y 16 y

elapsed or(b) the fluorine concentration determined by XPS for thefilms.

(OH )y O is different among the films, indicating17 y 16 y

that the surface OH group density can be effectivelyvaried by the metal dopant. Fig. 1 shows the opticaltransmission spectra of the films. The transmittance inthe wavelength region of 350–1500 nm for Ti or Zrdoped films is almost same as that of non-doped film.The difference in average transmittance between Aldoped and non-doped film was within 1% in this region.These results suggest that the surface OH group densitycan be modified by the doped metal without significantchange in the average transmittance from the visible tonear-IR region. In the UV region, the difference in thetransmittance between metal doped and non-doped filmsshould increase, because OH has UV absorption due toan electronic transition.

3.2. Relationship between wettability and surface OHgroup density

Fig. 2 shows the contact angle of water droplets forthe films as a function of elapsed time. The contactangles were-58 for all the films immediately afterUVyO cleaning. In general, the contact angle of water3

droplets is close to-58 for a glass surface withoutcontaminants, because the surface energy of the oxideis essentially very large compared with that of waterw14,15x. Therefore, the surfaces are considered to becontaminant free immediately after UVyO cleaning.3

The contact angles of all the films gradually increasedwith time and reached at an asymptotic value(us) in;7 days elapsed. The value remains constant for thefollowing 7 days. The hydrophobicities(us) are differentamong the films. These results indicate that, the wetta-bility is significantly altered by metal doping.It is known that the contact angle is affected by

surface roughness as well as by surface contaminationw16x. However, no significant change in the surfaceroughness(rms) examined by AFM is observed afterthe doping, as shown in Table 1. This result indicatesthat the difference in theus is not due to the surfaceroughness, but due to the cleanliness of the surface.

That is, the increase in the contact angle results fromthe adsorption of organic substances in the atmosphereand the difference in theus is caused by the differencein the amount of these adsorbed organic substances.This suggests that, the surface wettability, one of theimportant surface chemical properties is altered bydoping.In order to clarify the origin of this difference,us is

plotted against the (OH )y O ratio, which repre-17 y 16 y

sents the surface OH group density of the film in Fig.3a. Theus increases steadily with the increase in thesurface OH group density. This fact indicates that thehydrophobicity, resulting from the adsorption of organicsubstances from the atmosphere, depends on the surface

156 S. Takeda, M. Fukawa / Thin Solid Films 444 (2003) 153–157

Fig. 4. High-resolution O1s XPS spectra for undoped and metal-dopedfilms.

Fig. 5. Infrared reflection spectra for undoped and metal-doped filmsin the frequency region of(a) 800–1300 cm and(b) 820–1020y1

cm .y1

OH group density of the film. That is, the surface OHgroup can work as an effective adsorptive site fororganic substances and seems to be one of the importantfactors for modifying the surface adsorption propertiesof film.

3.3. Relationship between surface reactivity and surfaceOH group density

The surface fluorine concentration as a result ofreaction with a polyfluoroalkyl isocyanate silanewC F C H Si(NCO) x determined by XPS is shown in8 17 2 4 3

Fig. 3b. It is found that the surface fluorine concentrationis different among the films, indicating that the surfacereactivity with the agent is successfully altered by metal-doping.To clarify the origin of this difference, fluorine con-

centration is plotted against the(OH )y O ratio,17 y 16 y

which represents the surface OH group density of thefilms as shown in Fig. 3b. There is a good correlationbetween them, indicating that the reactivity depends onthe surface OH group density of the film. Specifically,the surface OH group can work as an effective reactivesite. Here, the fluorine concentration increases withincreasing surface OH group density. This indicates thatthe surface reactivity on the films depends on thenumber of surface OH groups and not the chemicalstates.

3.4. Effect of metal doping on surface OH groupformation

As mentioned previously, the change in the surfaceOH group density can be induced by metal doping. Inorder to clarify the origin of this change, XPS O1sanalyses were carried out as shown in Fig. 4. Theshoulder peak is clearly observed for the metal dopedfilms at lower binding energy side of main peak at;531.5 eV in the XPS O1s peak, which is not recog-nized for non-doped film. These observations indicate

that the doped element strongly affects the negativecharge 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 attributedto bridging oxygen atoms(Si–O–Si; BOs) and non-bridging oxygen atoms(Si–O ; NBOs) and NBOy

component was observed at the lower binding energyside of BO component in O1s peak. Namely, theshoulder peak observed in Fig. 4 is considered to resultfrom the formation of NBOs by metal doping. The peakintensity of the NBO component increases for the filmshaving a higher surface OH group density. This resultsuggests that the formation of surface OH groups isclosely related with the number of NBOs.Similar tendencies are observed by IR reflection

measurements as shown in Fig. 5. The peak at;1060cm is attributed to the stretching vibration of they1

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 cm is shiftedy1

to lower wavenumbers and the shoulder peak intensityincreases for the film having a higher surface OH groupdensity. This indicates that the surface OH group densityof the film increases with increasing the number ofNBOs w19x. These observations suggest that the metal

157S. Takeda, M. Fukawa / Thin Solid Films 444 (2003) 153–157

doping causes the increase in the number of NBOs inthe film, resulting in increasing the formation of thesurface OH groups.The difference in the formation ability of NBOs

among the doped elements may be due to the bondingnature of the doped element with oxygens in the SiO2

structure. According to the Pauling’s rules, if the bondis ionic, the formation of NBOs is required so as tomeet the local electroneutrality around the metal ionsw20x. On the contrary, if the bond is covalent, NBOs arenot formed. In this study, since the bond of Ti or Zr isconsidered to be rather ionic than that of Al, the numberof NBOs for Ti or Zr doped film become larger thanthat of Al doped film. Consequently, the surface OHgroup density of Ti or Zr doped film becomes higherthan that of Al doped film. Similarly, since the bond ofAl is considered to be rather ionic than that of Si, theAl doped film have larger NBOs than that of non-dopedfilm, resulting in the difference in the surface OH groupdensity between them. However, it is known that thebond of Al could be both ionic and covalent in silicateglass, depending on the co-ordination number of oxy-gens around Al ionsw21x. Therefore, further investiga-tion about the co-ordination structure of doped metalelement in SiO structure should be necessary for the2

conclusive discussion about this subject.

4. Conclusions

In this paper, we have investigated the effects ofmetal doping in sputtered SiO thin films on surface2

properties such as wettability and reactivity with apolyfluoroalkyl isocyanate silane. The wettability andthe surface reactivity were significantly altered by metaldoping. It is found that these alterations are due to thechange in the surface OH group density of the filmsand that the surface OH group density increases withincreasing the number of non-bridging oxygens in theSiO film. In addition, no significant transmittance2

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. Thissuggests that the metal doping is the effective way tomodify the surface properties without significant change

in optical transmittance from the visible to near-IRregion. We expect that these findings offer a novelmethod to control and design the surface properties ofSiO thin films.2

Acknowledgments

The authors are grateful to Prof. Hideo Hosono ofTokyo Institute of Technology for his valuable commentsabout the formation of NBOs in SiO structure.2

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