Optical spectroscopic analyses of OH incorporation into SiO 2 filmsdeposited from O 2Õtetraethoxysilane plasmas

A. Goullet,a) C. Vallee,b) A. Granier, and G. TurbanLaboratoire des Plasmas et des Couches Minces, IMN, UMR 6502, CNRS-Universite´ de Nantes,2 rue de la Houssinie`re, BP 32229, 44322 Nantes Cedex 3, France

~Received 4 May 1999; accepted 19 May 2000!

Silicon dioxide thin films are deposited on~100! silicon substrates at low pressure~5 mTorr!, fromO2/tetraethoxysilane~TEOS! helicon plasmas. The reactor is operated at 300 W radio frequencypower without any intentional heating or biasing of the substrate. The samples are characterizedusing infrared spectroscopy, ultraviolet-visible ellipsometry, and complementary densitymeasurements. Changes in film properties are observed varying the TEOS fraction in the gasmixture. Good quality SiO2 films, insensitive to postdeposition exposure to atmospheric water, aredeposited for low TEOS fractions~,5%! in the mixture. As the O2 flow rate decreases, porous SiO2

and polymeric SiOxCyHz samples are successively obtained. Aging over 5 months of intentionallyproduced porous films has been investigated using Fourier-transform infrared spectroscopy. The2900–3800 cm21 OH absorption stretching band is quantitatively analyzed with threedeconvolution bands. These films are hygroscopic and they show changes in the infrared spectraindicating an incorporation of additional highly associated hydroxyl groups. In addition, thedevelopment of the 935 cm21 Si–OH stretching band and the evolution of the Si–O–Sistretchingpeak are due to interactions between the airborne absorbed water and silica network. On the otherhand, isolated silanol species are rather insensitive to the postdeposition exposure to the atmosphericwater. The respective contribution of growth induced and after growthex situincorporated Si–OHgroups is established. Using the Bruggeman effective medium approximation, we found that watermolecules account well for the dielectric properties of these highly associated SiOH groups mainlyoriginating from postdeposition silica hydrolysis. ©2000 American Vacuum Society.@S0734-2101~00!05305-7#

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I. INTRODUCTION

Plasma enhanced chemical vapor deposition~PECVD! isan attractive low temperature growth technique and silicdioxide (SiO2) thin films deposited from tetraethoxysilan~TEOS! are widely used in microelectronics as intermedielectrics because of their high step coverage capabilitywell as their good electrical and mechanical properties. Hoever, low temperature PECVD processes can lead to pofilms which react with atmospheric water when exposedthe ambient atmosphere. This results in the presence oditional Si–OH groups that are generated outside ofdeposition chamber. They play an important role in the dradation of metal–oxide–semiconductor devices and thea particular interest to correlate the shape of these infraabsorption bands to their concentrations.1,2

When SiO2 film stability is affected over time, it is welrevealed in the OH stretching absorption region~2800–3900cm21!. However, in most cases,in situ Fourier-transforminfrared ~FTIR! investigation of film properties cannot beasily performed. So there is some difficulty in separatthe contribution of OH bonds incorporated during depositand additional OH groups which stem fromex situexposure

a!Author to whom correspondence should be addressed; electronicAntoine.Goullet@cnrs-imn.fr

b!Present address: Laboratoire d’Electrostatique et de Mate´riaux Dielec-triques, CNRS-Universite´ Joseph Fourier, 25 rue des Martyrs, BP 16F-38042 Grenoble Cedex 9, France.

2452 J. Vac. Sci. Technol. A 18 „5…, SepÕOct 2000 0734-2101 Õ200

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to atmospheric water. On the other hand, ultraviolet~UV!-visible spectroscopic ellipsometry is rather easy to hanand well suited forin situ measurements. It is a usefucomplementary characterization tool to investigate the incporation of water in porous silicon dioxide films.3,4

In this study, films with various quality have been depoited in a helicon reactor from O2/TEOS plasmas. As theTEOS fraction (XTEOS) increases in the mixture, films successively evolve from good to porous SiO2 and finally toorganic SiOxCyHz films. This article is focused on mediumterm ~over 5 months! aging of porous SiO2 films and iscomplementary to our previous one5 concerned with short-term changes induced by exposure to atmospheric water.experimental aspects of this study are presented in Secwhereas the experimental results are presented and anain Sec. III. Structural properties of deposited films are dscribed in Sec. III A and aging of porous films is studiedSec. III B.

II. EXPERIMENT

The radio frequency~13.56 MHz! helicon reactor, whichis described in detail in Ref. 6, is made of a source attacto a diffusion chamber where a~100! one side polished thick~;500 mm! p-type silicon substrate with a 10–30V cm re-sistivity is positioned. It is operated at 5 mTorr, 300 W wia total gas flow rate of 37 sccm. The oxygen gas is intduced at the top of the source while the TEOS vapor

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injected through a dispersal ring, a few centimeters abovesubstrate. During the deposition process, the substratgrounded and is not heated.

The film growth rate is monitored using a phase modlated ellipsometer UVISEL~ISA Jobin Yvon!. The ellipsom-eter is operated either in spectroscopic mode~1.5–5 eV! or akinetic mode~at fixed wavelengths!. The real time evolutionof the ellipsometric parameters~C, D! is recorded in thekinetic mode at a fixed wavelength whereas spectroscellipsometry~SE! measurements are performedin situ andexsitu ~after deposition!. FTIR spectra~400–4000 cm21! areobtained with a spectrometer~Nicolet 20 SXC! equippedwith a HgCdTe detector. The thickness of the samplesfixed ~;500 nm! to avoid thickness effects which appearthe FTIR spectra. The samples are weighed before andthe deposition using a Mettler AT20 microbalance~610 mg!and the precision on the density of the films is estimated60.1 g cm23.

III. RESULTS AND DISCUSSION

A. Properties of films deposited as a function of theTEOS fraction

1. Ellipsometry

Films deposited from TEOS/O2 mixtures have been analyzed by performing room temperature measurements adepositionin situ ~under vacuum! andex situ2 months later.The refractive indices measuredin situ (nin) andex situ(nex)are plotted in Fig. 1 as a function ofXTEOS. The evolution ofellipsometric data and deposition rate is very similar toone obtained when TEOS was injected close to the walthe reactor diffusion chamber and has already bdiscussed.4

WhenXTEOS is less than 22%,C andD in situ values arewell fitted assuming that the film is a homogeneous mixtof voids and amorphous SiO2. UVISEL reference data forUV-visible ~1.5–5 eV! dielectric functions~Aspnes! of SiO2

and c-Si were used for modeling the spectra. Good quavoid free SiO2 films can be obtained forXTEOS,5%. Thedecrease in the refractive indexnin , from 1.46 to 1.40, is

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well accounted for by the increase in the fraction of incoporated voids, from 0% to 13%. For higherXTEOSvalues, therefractive index increases due to the incorporation of carspecies. The differences observed betweennin andnex valueswere found to be due to postdeposition incorporationwater4 which results innex.nin .

2. Infrared absorption spectroscopy

All spectra exhibited a slopping base line due to optiinterference fringes generated by the optical system coning of a homogeneous isotropic film upon a ‘‘thick’’ substrate. The corresponding transmission coefficientT is afunction of the wavelength,l, the real refractive index of thesubstrate (nSi), the complex index (n2 ik) and thickness~d!of the film. The spectra are recorded in normal incidencethat in the case of transverse optical~TO! modes7 T can beexpressed for the@air~1!/film~2!/substrate~3!# configurationas

FIG. 1. Variation of the in situ~s!, ex situ ~d! 2 months later, refractiveindices at 1.96 eV and the fraction of voids~n! as a function of the TEOSflow rate.

T~l!5ut12u2ut23u2nSi

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~n2 ik !1nSi.

In this expression, the imaginary part of the silicon substrrefractive index is not considered. The transmission baseresulting from the interference effect is fitted in the transpent spectral range by settingk50 and using a constant valuof the refractive indexn.

Figure 2 shows typical infrared transmission spectra

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2454 Goullet et al. : Optical spectroscopic analyses 2454

samples prepared at high dilution~3%! or low dilution ~50%!of TEOS in oxygen. The bands at 2340 and 2360 cm21 cor-respond to asymmetric stretching vibrations of gaseous2molecule8 present in the apparatus.

At low TEOS fraction (XTEOS,5%), theFTIR spectraare very similar to those of thermal oxide. The only weadditional component is assigned to OH stretching of ilated SiOH groups. AsXTEOS increases between 5% an22%, SiOH vibration appears at about 935 cm21 while thehigh energy OH stretching band broadens and is shidown to about 3400 cm21 due to associated SiOH groupThe presence of ‘‘free water’’ could not be confirmed by tweak bending vibration near 1650 cm21. When XTEOS

.22%, carbon species are first detected with a C5O absorp-tion peak at;1700 cm21. Finally, whenXTEOS550%, nu-merous CH stretching bands appear at 2875, 2940, andcm21 along with very weak peaks at 1385, 1410, and 14assigned to CH3 and CH2 deformation modes.9 As alreadyobserved in our previous study,4 there is no evidence oO3SiH absorption at 2260 cm21. We observed an increasethe intensity of the bands related to CH species which cresponds to a decrease in the intensity of the Si–O–Sistretching peak.

3. Film density

For a given material, the evolution of the refractive ind~n! and density~r! can generally be correlated provided tcomposition of the sample under study is basically constThe variation ofr as a function ofn is often described by theLorentz–Lorenz formula

r5K~n221!

~n212!, ~2!

where K is a constant,K58.1145 for a thermal oxide10

grown at 920 °C~n51.465,r52.25!. The values ofr are

FIG. 2. Interference effect corresponding to a 500 nm SiO2 film deposited inan oxygen rich plasma~a! (XTEOS53%). FTIR transmission spectra withouinterference effect for films deposited in oxygen rich~b!, (XTEOS53%) andTEOS rich~c! (XTEOS550%) plasmas.

J. Vac. Sci. Technol. A, Vol. 18, No. 5, Sep ÕOct 2000

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displayed in Fig. 3 versus (n221)/(n212), wheren is therefractive index measured at 1.96 eV. For oxygen rich mtures, the values ofr are close to thermal oxide whereasrdecreases asXTEOS increases. A linear relationship is observed as long asXTEOS is less than 33%. The correspondinfilms are initially described by a homogeneous mixturea-SiO21voids. So, in spite of slight changes in film compsition which result from postdeposition incorporation of wter, the law is verified. However, the valueK'9 indicatesthat the structure of our plasma deposited SiO2 films isslightly different from the thermal oxide one. As expectethe films deposited atXTEOS533% and 50% do not followthe same law. In this case, the films are organic wSiOxCyHz composition and their characteristics are quite dferent from the inorganic ones.

B. Incorporation of water in porous films

1. Interaction of absorbed atmospheric water withporous films

We have investigated the stability of PECVD SiO2 thinfilms upon exposure to atmospheric water by phase molated ellipsometry, following the approach reported in oprevious study.5 Real time evolution of kineticD5 f (C)curves exhibits two typical behaviors for as deposited carfree films exposed to the ambient. Significant changesobserved for porous films (XTEOS510%) whereas no shift isobserved for good quality films (XTEOS,5%). Fast changesin C andD occur in the first seconds which are followed ba quite slower evolution. Simulation results clearly indicathat the evolution ofC andD results from incorporation ofairborne water which occurs in porous films exposed toambient. We found that the initial homogeneous mixturea-SiO21voids can be described bya-SiO21H2O after 20min exposure to the atmospheric water. Then, slight vations are detected a few hours later corresponding to

FIG. 3. Lorentz–Lorenz plot of film density for the various TEOS fractionThe data was taken 2 months after deposition. The straight line correspto a linear fit betweenXTEOS53 andXTEOS522%.

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2455 Goullet et al. : Optical spectroscopic analyses 2455

amount of water greater than the initial fraction of voidThis suggests that silanol species may be created by chcal reaction of SiO2 with H2O.5 Unfortunately, real time el-lipsometric measurements cannot be easily performed insame conditions during days or weeks. Furthermore, vations of the ellipsometric parameters after days or weeksposure to the ambient is not reasonably detectable. Coquently, the quantification of water from kinetic ospectroscopic measurements is critical. Indeed the imagipart of water refractive index is negligible in the 1.5–5 erange so that it is not well suited to evidence aging. Oncontrary, the signature of water is well identified in thmid-IR range11 with a strong feature at 3400 cm21.

To investigate the presence ofin situ incorporated wateror SiOH species during the deposition, three different fildeposited on silicon have been analyzed.5 Sample 1 (S1) ismade of a 500 nm porous SiO2 film ~F1 , XTEOS510%!. Theevolution of its OH stretching band recorded over 5 monfor different times of exposure to atmospheric watershown in Fig. 4. The band intensity clearly increases be3650 cm21 while SiOH species at 3650 cm21 appear to beinsensitive to aging.

Sample 2 ~S2! is made of a 200 nm good quality SiO2

film ~F2 , XTEOS52%!. Finally, a two layer sample (S3) cor-responds toF2 /F1 /c-Si. When S2 is exposed to the ambienthe ellipsometric parameters are unchanged indicatingthe cap layer (F2) is an efficient barrier to atmospheric watvapor. In addition, theex situFTIR absorption spectra of S2

and S3 recorded the following 24 h of atmospheric exposudid not exhibit any variation. These results are shown in F5, where S2 has very little OH. In this way, the difficulty thain situ FTIR measurements cannot be performed is paovercome: the barrier layer of S3 was immediately deposite~a few seconds after the porous layer! so that the OH stretching absorption of S3 is only due to OH bonds incorporateduring deposition. S1 has more OH than sample S3 because

FIG. 4. Time evolution of the normalized OH stretching absorption bandthe 500 nm porous SiO2 film ~F1 , XTEOS510%!, due to postdepositionexposure to atmospheric water.

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S1 has no barrier and was exposed to the ambient. The exin S1 over that of S3 is due to atmospheric exposure. Thbehavior goes along with an increase in the SiOH band935 cm21 which is only related to OH groups obtained frothe atmospheric exposure. In addition, no 1650 cm21

H–O–H band appears. Though the handbook of opticonstants11 indicates thata(3400 cm21)'6.6a(1650 cm21)for water, the band related to scissors should appear inspectra if water exists primarily asH–O–H and iscontainedin some sort of pockets. Hence, the water absorbed in pofilms reacts with the SiO2 network to form H bonded Si–OHgroups. It is concluded that the narrow 3650 cm21 feature isdue to species incorporating during deposition, whereasbroad main part of the absorption band results from podeposition absorbed atmospheric water.

2. Infrared quantitative analysis of OH species

We have carried out a quantitative analysis of the spedisplayed in Fig. 4 and corresponding to S1. The OH stretch-ing band ~2900–3800 cm21! has been deconvoluted usinthree Gaussian bands. All the spectra are nicely fitted anexample of such deconvolution is shown in Fig. 6 corsponding to 8 days exposure to atmospheric water. By cparison with Rojaset al.,12 the lower energy band (G1) atabout 3300 cm21 is shifted ~;100 cm21! towards the lowenergy side while the one at about 3500 cm21 (G2) is muchmore intense. As expected, the third Gaussian band (G3) athigher energy is positioned at about 3650 cm21 and corre-sponds to isolated silanols. TheG1 and G2 may not neces-sarily correspond to unique OH environments, but are usimply to quantify the asymmetrically broadband. Note thasymmetry is common trait of H bonding.

In order to find out if water diffuses in all the film, werecorded~a few minutes after deposition! the FTIR spectra oftwo porous films deposited in the same conditions as S1 butwith different thicknesses~100 and 500 nm!. Figure 7 shows

fFIG. 5. FTIR spectra recorded 5 h after deposition of a 500 nm porous SiO2

film ~F1 , XTEOS510%!, a good quality 200 nm SiO2 film ~F2 , XTEOS

52%!; and a two-layer deposited sample corresponding toF1 /F2 .

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2456 Goullet et al. : Optical spectroscopic analyses 2456

that the absorption coefficient values of the two samplesclose indicating that OH groups can reasonably be conered to be equally shared through out the layer.

We tried to quantify the dielectric properties of thstrongly H-bonded Si–OH groups. No reference dielecfunction of Si–OH groups was available in the literatuNevertheless, the absorption feature on the low wave numside of the OH stretching band which develops with agingpositioned in the same spectral range as for liquid waHence, we attempted to account for the changes on the esometric data~cf. Sec. III B 1! by considering the poroufilms to be a homogeneous mixture ofa-SiO21H2O. Underthis assumption, the volume fraction of absorbed waterf canbe quantified. For a given value off, the effective complexdielectric function« is calculated in the mid-IR range usinthe Bruggeman effective medium approximation~BEMA!

FIG. 6. Absorption coefficienta ~solid line! in the high wave number rangefor F1 after 8 days exposure to atmospheric water. The three deconvoluGaussian bandsG1 , G2 , G3 ~solid lines! and their sumG11G21G3

~dashed lines! are displayed.

FIG. 7. Absorption coefficienta of OH stretching absorption band of a 10and a 500 nm porous film likeF1 .

J. Vac. Sci. Technol. A, Vol. 18, No. 5, Sep ÕOct 2000

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11

are known in this mid-IR spectral region. Then the effecticomplex refractive indexn of the film corresponding to(12 f )a-SiO21 f H2O is deduced and the transmission coficient, T@l,n( f )#, is obtained using Eq.~1!. Finally, thevalue off which gives the best fit between the simulated aexperimental spectral region is obtained. Sincea-SiO2 doesnot have any OH in this simulation, the OH feature comonly from the water. As an example, the calculated absbance values after base line correction, are presented in8 between 2900 and 3800 cm21, for f 50.12. For compari-son, the experimental spectrum plotted in Fig. 6 is ashown along with the same two deconvolution bandsG1 andG2 of Fig. 6. We found that the calculated water absorptband corresponds well to the sum of the firstG1 and secondG2 Gaussian profiles. Figure 9 shows the volume fractionwater f as a function of the integrated intensityI (G1)1I (G2), whereI (Gi)5*Gi( n)dn ( i 51,2), over 5 monthsaging for sample S1. A linear regression analysis yieldscorrelation coefficient of 0.99. Consequently, the as idefied fraction of water can be calculated from the deconvotion results using

%H2O50.242111.21131025@ I ~G1!1I ~G2!#. ~4!

Within the margin of error this line goes through zero at torigin as expected. Now let us assume that the fractionHOH units present in a film can be written as

x5f NHOH

~12 f !NSIO21 f NHOH

, ~5!

on

FIG. 8. Comparison in the high wave number range between the absorpcoefficienta ~solid line! for F1 , after 8 days exposure to atmospheric watand the absorption coefficient calculated using the BEMA~dotted line! for afilm composed of (88% SiO2112% H2O). This later corresponds toG1

1G2 ~solid line!. TheseG1 andG2 are the same as in Fig. 6.

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2457 Goullet et al. : Optical spectroscopic analyses 2457

whereNSiOSi andNHOH are the densities of SiOSi and HObonds in thermal silica and liquid water equal to 4.431022

and 3.331022cm23, respectively. The density of HOHbonds in a film can be expressed by

xNHOH5KHOHE a~n !dn, ~6!

where a~n! ~cm21! is the absorption coefficient an*a( n)dn is equal to the calculated intensityI (G1)1I (G2). Combining Eqs.~5! and ~6! yields KHOH50.331016cm21 close to the value of 0.431016cm21 given inRef. 14.

These results appear to be satisfying and show that thighly associated Si–OH groups behave, dielectrically, vmuch like H–O–H molecules themselves. Indeed, Thet al.15 estimated that the O...O separation between the~OH!groups in neighboring silanol groups is only;2.8 Å. Thisdistance is comparable to the average separation of H2O mol-ecules in liquid water~;3.1 Å!. On the other hand, our results show that the polarizability of isolated silanol groumay be very different compared to closely associated silagroups. In these conditions, since isolated SiOH groups cnot be accounted for using the BEMA they have been qutified using the well-known procedure corresponding to E~6! with KSiOH51.431016cm21 ~Ref. 14! and with*a( n)dn5I (G3) in this case.

The H2O and isolated SiOH contents obtained for S1 aregiven in Table I. Since the initial values~t50, just afterdeposition! cannot be directly obtained, they have been c

FIG. 9. Plot of the calculated absorbed H2O volume fraction using theBEMA against the integrated intensityI (G1)1I (G2) for F1 .

TABLE I. Evolution with aging time of the calculated absorbed H2O andisolated SiOH contents in the porous filmF1 . The values att50 correspondto the two-layer filmF1 /F2 .

Time ~h! 0 0.08 0.25 0.75 5 192 3600

% H2O 7 9 10 10.5 11.5 12 16% SiOH 3.5 4 4 6 4.5 4 4

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culated from the spectrum of two layer sample S3. The cal-culation results show an increase in the absorbed water ftion with time from 7%(t50) to 16%~t55 months!. As faras the comparison with ellipsometric results is valid, we fisome discrepancies. On the one hand, FTIR analysis shan increase of;3% in the volume fraction of absorbed watafter ;15 min postdeposition exposure to the ambient.the other hand,;8% of absorbed water is necessary to acount for the ellipsometric data variations over appromately the same time. The problem with ellipsometric simlation results, lies in the fact that no OH bond incorporatedinsitu has been revealed. This might be a reason for the difences observed.

As generally assumed in the literature,12,15–18the analysisof the 2900–3800 cm21 experimental OH stretching regioyields different possible local environments for the hydroxgroups. We found thatG11G2 corresponds to the wide absorption band. However, it clearly appears that water umately is converted to Si–~OH! groups. In these conditionswe only distinguish isolated silanol groups which canassociated withG3 on the one hand, and near neighbSi–OH groups associated withG11G2 . Finally, FTIR dataare probably more reliable than UV-visible ellipsometrones to study OH species though the dielectric functionsthe IR range could not be directly obtained.

3. Analysis of SiO2 stretching peak evolution ofsample S1

Briefly, the stretching band of SiO2 is composed of twoTO modes: the TO1 mode at;1070 cm21 and its high fre-quency shoulder~TO2 mode! at ;1200 cm21.19,20 We foundthat three Gaussian profilesG1 , G2 , andG3 located respec-tively at ;1070, 1150, and 1200 cm21 were necessary to fithe SiO2 stretching band.G1 corresponds to TO1 absorptionpeak whereasG21G3 account for its high energy shouldeFigure 10 shows the evolution of the position and FWHMTO1 mode as a function of time. The former increases fro1063 to 1076 cm21 and the latter decreases from 78 to

FIG. 10. Time evolution of the frequency and FWHM of TO1 stretchingSiOSi deconvolution Gaussian band forF1 .

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2458 Goullet et al. : Optical spectroscopic analyses 2458

cm21. The upward shift in the Si–O–Sistretching frequen-cylikely corresponds to a disruption~relaxation! of SiO2 net-work. Indeed, a central force model18 suggests that

nTO15n0 sin~ u !, ~7!

wheren0 is a constant and 2u is the average Si–O–Siangle~equal to 144° in fused silica!. However, the increase innTO1

along with the decrease in the FWHW should not be intpreted as an improvement of the oxide quality. As obserin Fig. 11, the high frequency side shoulder integrated intsity ratio (I sh/I TO1

) increases from 0.285 to 0.35. This evlution suggests an increase in the disorder associated withamorphous silicon dioxide thin films.20–22Tsu23 showed thatthere is a strong coupling between the SiOH bending mand the SiOSi stretching mode. This coupling washes odistinct bending band, but causes the shoulder to increThese results can be explained considering that posamples favor incorporation of water, which partly leadsthe generation of extra near-neighbor SiOH bonds. Indeaccording to Theil,15 as the Si–O–Sibond angle decreasethe Si–O–Sigroups become more reactive which facilitattheir conversion to Si–OH groups. The Si–O–Si bondsmainly involved correspond to the ones associated to thewave number side of this stretching band. In this way,water incorporation induces an increase, and a decreasespectively, in the position, and FWHM of the TO1 mode.

IV. SUMMARY

Films deposited from O2 /TEOS plasmas at 5 mTorr, 30W in a helicon plasma reactor have been charactermainly using infrared spectroscopy and UV visible ellipsoetry. Good quality and porous SiO2 films are obtained whilethe fraction of TEOS does not exceed 22%. The formerinsensitive to the ambient atmosphere exposure wherealater incorporate water. Aging of such unpassivated porfilms has been investigated over 5 months using FTIR sp

FIG. 11. Time evolution of (I sh)/I TO1intensity ratio forF1 .

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troscopy. The layer induced changes have been mainlylyzed in the 2900–3800 cm21 OH stretching band with threeGaussian bands. The intensity of the Gaussian band;3650 cm21, which accounts for isolated silanol groups icorporated during the low pressure deposition process issensitive toex situatmospheric water exposure. The devopment of the 935 cm21 SiOH stretching peak along with thevolution of the Si–O–Sistretching peak is entirely consistent with a disruption~relaxation! of the silica network. Inaddition, the 1650 cm21 bending vibration of free watecould not be evidenced, so that absorption of postdeposiairborne water is followed by silica hydrolysis.

Using the Bruggeman effective approximation, we fouthat that highly associated Si–OH groups, dielectrically bhave very much like free water molecules. Under these cditions, it turns out thatH–O–H molecules are in fact moreappropriate to use when trying to quantify the dielectproperties of highly associated Si–OH groups in the 2903800 cm21 band, than the silanol units themselves.

ACKNOWLEDGMENT

The authors would like to thank the referee for very heful comments.

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