correlation of silica glass properties with the infrared spectra

Ž .Journal of Non-Crystalline Solids 209 1997 166–174

Correlation of silica glass properties with the infrared spectra

Anand Agarwal 1, Minoru Tomozawa )

Materials Science and Engineering Department, Rensselaer Polytechnic Institute, Troy, NY 12181, USA

Received 5 February 1996; revised 29 May 1996

Abstract

Ž .Infrared IR spectroscopy has been used to interpret structural changes in silica glasses. Specifically, a shift of the Si–Ostretching band in IR spectra is used to monitor changes in average Si–O–Si bond angle in the glass structure. A similarstructural change is induced by the change of fictive temperature, hydrostatic pressure or compressive stress, with theaverage Si–O–Si bond angle decreasing with the increase of these parameters. It is anticipated that these similar structuralchanges would produce a similar change in glass properties. In order to confirm this expectation, HF etch rates of silicaglasses were measured as a function of fictive temperature and stress. The experimental results on HF etch rates, togetherwith changes in other glass properties in the literature, were compared with the change in glass structure revealed by IRspectroscopy. It was found that the similar structural change is accompanied by the consistent changes in a variety of glassproperties. Monitoring the IR spectra of a silica glass sample, therefore, can be used to deduce changes in glass properties.

1. Introduction

Infrared spectroscopy has been used to interpretw xstructural modifications in silica glasses 1–6 . Many

properties of silica glasses are expected to vary withchanges in glass structure. In this study, the univer-sality of structure–property relationships is tested forsilica glasses by comparing their structure modifiedby thermal or mechanical treatment with the corre-sponding property changes. We show that the changein infrared spectroscopy can be used to predictchanges in glass properties.

) Corresponding author. Tel.: q1-518 276 6451; fax: q1-518276 8554; e-mail: [email protected].

1 Currently with Sterlite Communications Ltd., Waluj, Au-rangabad-431136, India.

2. IR spectroscopy and silica glass structure

The fundamental Si–O stretching band at ;1100cmy1 is the predominant structural band in the IRabsorption spectra of silica glass. After the pioneer-

w xing computational work of Bell et al. 1 , structuralw xmodels developed by various researchers 2–4

showed that the position of the Si–O stretching banddepends primarily on the average Si–O–Si bondangle, u , in the glass structure. Using experi-Si – O – Si

w xmental data from several sources, Devine 5 showedthat the position of the Si–O stretching band isdirectly correlated with the average Si–O–Si bondangle. The band was found to shift to higher frequen-cies with increase in average Si–O–Si bond angle inthe glass structure. Due to its high absorption coeffi-cient, the Si–O stretching absorption IR band obser-vation requires extremely thin specimens while the

0022-3093r97r$17.00 Copyright q 1997 Elsevier Science B.V. All rights reserved.Ž .PII S0022-3093 96 00542-X

( )A. Agarwal, M. TomozawarJournal of Non-Crystalline Solids 209 1997 166–174 167

observation of the corresponding IR reflection modeat ;1122 cmy1 can be made conveniently even forbulk silica glass samples since the latter probes only

w xa thin surface layer of the specimen 6 . Both the IRabsorption band at ;1100 cmy1 and the IR reflec-tion band at ;1122 cmy1 are due to the asymmetricstretching vibration of the Si–O bond and can beconverted from one to the other using Kramers–

Ž w x.Kroning transformation for example, Ref. 7 .¨Glass structures can be modified by various ther-

mal and mechanical processes. Here, the structuralŽ .changes induced by a modification of fictive tem-

Ž .perature for bulk silica glass samples, b applicationŽ .of hydrostatic pressure in silica powders, and c

compressive stresses in silica films thermally grownon silicon substrates in the ‘anomalous regions’ ofsilica glass are considered. These ‘anomalous re-gions’ are the range of fictive temperatures from1000 to 15008C, where the density of silica glass

w xincreases with increasing fictive temperature 8–11and a pressure range from 0 to 2 MPa, where thecompressibility of silica glass increases with increas-

w xing pressure 12 . Structure and properties of ther-mally treated glasses were mainly studied at lowertemperature where further structural change wouldnot take place during the measurements while thoseof mechanically treated glasses were studied underapplied pressure or stress as well as under atmo-spheric pressure as a densified glass.

w xGerber and Himmel 13 carried out an X-raydiffraction study to determine the effect of fictivetemperature on silica glass structure. They found thatwith decreasing fictive temperature, the radial distri-bution curves became more ‘structured’ and acquiredadded details at large values of distance betweenatoms. On the basis of their results, Gerber and

w xHimmel 13 suggested that an increase in fictivetemperature broadens the distribution of Si–O–Sibond angles, u , and shifts the distributionSi – O – Si

maximum toward the smaller angles. Correspond-ingly, the 1122 cmy1 IR reflection band shifts tolower frequencies with increasing fictive temperaturew x6 .

A similar decrease in the Si–O–Si bond angle,u , has been observed to take place when silicaSi – O – Si

w xglass is subjected to hydrostatic pressure 14–16 andthe corresponding 1100 cmy1 IR absorption band

w xhas been observed 17–19 to shift to lower frequen-

cies for silica powders under increasing hydrostaticpressure. For silica films thermally grown on silicon,

w xthe IR band shifts to lower frequencies 20 withdecreasing growth temperature which corresponds toincreasing biaxial compressive stresses in the film.ŽThe effect of stress on peak-position predominatesover any effect of fictive temperature on peak-posi-tion in thin films. If the effect of stress is absent, theoxide film grown at lower temperature would give ahigher IR peak frequency due to its lower density or

.lower effective fictive temperature. These corre-sponding structural changes revealed by IR spec-troscopy are summarized in Fig. 1.

Thus, increase in fictive temperature, hydrostaticpressure or compressive stress, all lead to decreasing

y1 Ž .Fig. 1. Peak position of the 1100 cm band versus a fictivetemperature of Furukawa silica plates scanned in IR reflection

Ž w x. Ž .mode after Ref. 6 , b hydrostatic pressure of silica powdersŽ w x. Ž .scanned in IR absorption mode after Ref. 17 , and c growth

Ž .temperature compressive stress of silica films scanned in IRŽ w x.absorption mode after Ref. 20 . Lines in the figures are guides

to the eye.

( )A. Agarwal, M. TomozawarJournal of Non-Crystalline Solids 209 1997 166–174168

u , consequently shifting the infrared Si–OSi – O – Si

stretching band to lower frequencies. Since the struc-tural changes in vitreous silica are dominated by

w xchanges in the Si–O–Si bond angle 21 , an increasein fictive temperature, pressure or compressive stressis expected to affect the glass properties in a similarmanner. In order to test this hypothesis HF etchingrates of silica glasses with different fictive tempera-ture and different stress states were determined.

3. Experimental procedure

The chemical durability of silica glass is normallyinferred from the dissolution rate of the glass in anappropriate etchant. To study the effect of fictivetemperature on chemical durability, TO8 silica glass

Ž .samples Heraeus Amersil, Sayerville, NJ with fic-tive temperatures of 1000, 1200 and 14008C wereprepared by heat-treating the glass samples at respec-tive temperatures and quenching as described in an

w xearlier paper 6 . These samples were etched in aŽsolution containing 32.5% HF and 32.5% H SO by2 4

.weight at room temperature. The thickness removedwas calculated from the weight loss, surface area anddensity of the samples. For each fictive temperature,

Žthree identical square silica plate samples approxi-.mate dimension, 15 mm=15 mm=1.4 mm were

tested. The weight loss measurement of each samplewas performed thrice using a Mettler balance with areproducibility within 0.00005 g and averaged andthe thickness removed was thereby calculated to aprecision of "0.1 mm.

To study the effect of compressive stresses onchemical durability, etching studies were performedon a thermally grown silica film on a silicon sub-strate. The film was wet oxidized at 11008C to athickness of about 1 mm on a 5 inch silicon waferand was found to be under compressive stress atroom temperature. The film was consecutively etched

Ž .in 10:1 BOE Buffered Oxide Etch: 10 vol% HFand the film thickness was measured using an ellip-someter at five different locations on the wafer andaveraged. At the same time, the average compressivestress in the remaining film was also measured as afunction of film thickness, using a thin film stress

Žmeasuring apparatus Flexus Incorporated, Sunny-.dale, CA . This apparatus essentially measures the

changes in the radius of curvature of a substratecreated by a stressed thin film on its surface. Theaverage stress in the thin film is calculated from thesubstrate radius of curvature using the following

w xequation 22,23 :2Eh 1 1

ss y 1Ž .6 1yn t R RŽ . t i

where, E and n are Young’s modulus and Poisson’sratio, respectively, for the silicon substrate. The filmthickness is represented by t, and the substrate thick-ness by h. R is the radius of curvature of substratei

without the silica film, and R is the radius oft

curvature of the substrate with the film.

4. Experimental results

As shown in Fig. 2, the etch rate of silica glasswas found to increase with increasing fictive temper-ature. The data point in this figure represents anaverage of three measurements and the error bars of" one standard deviation are smaller than the sym-bol size. The lines are linear regression fit to the dataand the slope of the line indicates the etch rate of theglass. The results clearly demonstrate that a silicaglass with a higher fictive temperature has a higheretch rate. For example, the glass with fictive temper-ature 14008C has, approximately, a 40% higher etch

Fig. 2. Thickness removed versus etch time for TO8 silica glasseswith different fictive temperatures etched with 32.5%HF–32.5%H SO etchant. Lines in the figure are linear regression fits2 4

to the data.


rate than the glass with fictive temperature 10008C.Since the silica glass density increases with increas-ing fictive temperature, the observed result of thefictive temperature effect should be qualitatively

w xsimilar to the pressure effect. Ito and Tomozawa 24have measured the dissolution rate of silica glass inwater as a function of hydrostatic pressure at 2858C,and found the rate to increase with increasing pres-sure.

Ž . Ž .Fig. 3 a and b , respectively, show the etch rateand average compressive stress as a function of filmthickness. The film thickness was measured at fivedifferent points on the wafer and the etch rate wascalculated from these thickness measurements. The

Ž . Ž .error bars in Fig. 3 a for etch rate and Fig. 3 b forfilm stress correspond to " one standard deviationof these thickness measurement. The glass etch rateis found to increase significantly below a film thick-ness of 50 nm. The average compressive film stressis also found to increase in this region. Since themeasured stress represents the average stress in theremaining film, the stress at particular location is

Ž .related to the derivative of the data shown. Fig. 3 bstill shows a high compressive stress in the silicafilm near the interface with silicon. The presentmeasurement indicates that the etch rate increaseswith increasing compressive stresses in the film.

The results presented here show an increase in

Fig. 3. Etch rate and stress in thermally grown SiO films on Si obtained by successively thinning the film plotted as a function of2Ž . Ž .remaining film thickness. a Etch rate as a function of distance from the interface, b corresponding average stress in the remaining film.


silica glass etch rate with increase in its fictivetemperature, applied hydrostatic pressure, or com-pressive stress. In terms of glass structure, these

Žresults suggest an increasing etch rate or decreasing.chemical durability of the glass with decreasing

Si–O–Si bond angle. This behavior is expected,w xsince it is known 25–27 that silicate structures with

small Si–O–Si bond angles, and consequentlystrained Si–O bonds, are highly reactive comparedto structure with unstrained bonds.

5. Discussion

The chemical etch rate results shown above clearlydemonstrated that the similar structural change in-duced by fictive temperature change, hydrostaticpressure and compressive stress produces a similarchange in a chemical property. In order to check theuniversality of the observed trend, changes in variousother properties of silica glasses induced by thermaland mechanical processes reported in the literaturewere compared.

5.1. Density

In the temperature range of 1000 to 15008C,w xDouglas and Isard 28 found ‘‘the equilibrium den-

sity of fused silica to increase with increasing tem-perature’’. For different types of silica glasses, it was

w xsubsequently shown 9–11 that density increaseswith increasing fictive temperature and that a max-

w xima in density is observed 9,10 in the fictive tem-perature region of 1500–16008C. Fig. 4 shows the

w xdensity values measured 8,9 at room temperature,for Homosil silica glass samples, after quenching thesamples from their heat-treatment temperature, whichis also their fictive temperature, T . Also, included inf

the figure is the calculated density at the fictivetemperature, T , using thermal expansion coefficientf

data. These data show that the density of sampleswith fictive temperature-15008C increases with in-creasing fictive temperature. The density of silica

w xpowders has also been observed 18,19,29 to in-crease with increasing applied hydrostatic pressure.When the hydrostatic pressure exceeded a certainvalue, permanent densification resulted even after therelease of the pressure.

Fig. 4. Density of Homosil silica glass as a function of the glassfictive temperature. The figure includes both the actual densitymeasurements made at room temperature as well as the calculateddensity at each corresponding fictive temperature. For the calcula-tions, thermal expansion data for the temperature interval 0–

Ž10008C were used. Lines in the figure are guides to the eye afterw x.Ref. 9 .

5.2. RefractiÕe index

Since density and refractive index are linearlyw xcorrelated 29 , a similar change of refractive index

with fictive temperature and pressure would be ex-w xpected. Bruckner 8 showed that the room-tempera-¨

ture refractive index of silica glass increases withincrease in fictive temperature in the 1000–15008Cfictive temperature range. The refractive index in-creases also for silica powders with increasing ap-

w xplied pressure 19,29 and for silica films with de-Žcreasing growth temperature or increasing biaxial

. w xcompressive stresses 20,30 .

5.3. Coefficient of thermal expansion

Another related property which shows a fictivetemperature dependence analogous to density andrefractive index is the thermal expansion coefficientof silica glass. The average thermal expansion coeffi-

w xcient was found 8 to increase with increasing fic-tive temperature of the glass with the maxima ataround 15008C. Also, the thermal expansion coeffi-cient of silica glass between 11 and 3908C was found


Ž w x w x.Ref. 90 of 8 to increase as the pressure wasincreased from 0 to 10 kbar.

5.4. Viscosity

w xHetherington et al. 31 prepared silica glasseswith different fictive temperatures and measured theirviscosity over a temperature range of 1000 to 14008C.They found that at any given temperature, the viscos-ity is less for glasses with higher fictive temperaturein the fictive temperature range of 1000 to 14008C.

w xAlso, Scarfe et al. 32 showed that for many silicatemelts, the viscosity decreases with increasing pres-sure. No direct evidence of viscosity–pressure rela-tionship is available for vitreous silica. However, for

w xNa O–SiO glasses, it was found 32 that change in2 2

SiO content changed the trends in viscosity–pres-2

sure relationship. While a low silica-content melt,e.g. Na OPSiO , shows essentially an increasing2 2

viscosity with increasing applied pressure, a highersilica-content melt, e.g. Na OP3SiO , exhibited a2 2

decreasing viscosity with increasing pressure. Fromthe trend, it is expected that the viscosity of a silicamelt will decrease with increasing pressure. Also,

w xGupta 33 suggested, using the Adam–Gibbs theoryof viscosity, that silica glass shows a decrease inviscosity with increasing pressure.

5.5. Water diffusion

w xDoremus 34 proposed that structural informationfor silicate glasses can be obtained by transportproperty measurements, such as the diffusion ofgases, in the glass. Since the diffusion behavior ofany species is affected by the structure of the glass, itwould be expected that any change in structure willaffect the diffusion coefficient.

w xRoberts and Roberts 35 studied the effect offictive temperature on the diffusion behavior of wa-ter in silica glass. They hydrothermally treated glasssamples, with fictive temperatures of 1100 and13008C, at various temperatures. They found that, inthe temperature range of ;700 to 9008C, a glasswith a higher fictive temperature has a lower diffu-sion coefficient of water but a higher hydroxyl sur-face concentration than a glass with lower fictive

w xtemperature. Also, Nogami and Tomozawa 36 con-ducted water diffusion studies as a function of ap-plied hydrostatic pressure at low temperatures and

found that the water diffusion coefficient decreasedand surface hydroxyl concentration increased withincreasing applied hydrostatic pressure to the glass.

w xDevine et al. 37,38 investigated the kinetics ofannihilation of various point defects such as oxygenvacancies created by radiation in silica glasses, andfound that densified glasses require a higher temper-ature for annihilation than undensified glasses. Thisdifference was attributed to the slower diffusion rateof oxidants such as oxygen or water into the densi-fied glasses.

The effect of uniaxially applied stress on waterw xdiffusion has been found 39 to depend on tempera-

ture. The water diffusion coefficient decreased andthe surface hydroxyl concentration increased underuniaxially compressive stresses at low temperaturesŽ . w x-2508C 36,39 , while the opposite trend wasobserved under uniaxially compressive stresses at

Ž .high temperatures )6508C . The low temperaturebehavior was attributed to the effect of stress on thekinetics of water uptake, and the high temperaturebehavior was attributed to the effect of applied stresson the glass–water reaction equilibrium. Since waterdiffusion into silica glass at high temperature can

w xcause a fast structural relaxation 40 and stressŽ .relaxation especially in a thin surface layer , only

Ž .low temperature or short time diffusion behaviorwould be appropriate for investigation of purelystress effect.

The diffusion of water in silica glass involves themotion of water molecules and their reaction withthe glass network, with the reaction product, hy-droxyl, being immobile. Thus the reaction sites ofthe glass become diffusion traps and the higherconcentration of the reaction sites will produce alower diffusion coefficient and higher surface hy-droxyl concentration. Since an increase in fictive

w x w xtemperature 41 or pressure 42 leads to an increasein defect or defect precursor concentrations in silicaglass, it is plausible that the observed difference indiffusion coefficients under these treatments is dueto a difference in concentrations of the defect ordefect precursor with high reactivity in the glass.

5.6. Mechanical properties

The room-temperature microhardness of silicaglasses was found to increase with increasing fictive


temperature. Similarly, microhardness of densifiedsilica glasses measured at room temperature underatmospheric pressure increases with the glass densityw x19 . Both can be attributed to the higher density of

w xglasses. Fraser 10 conducted detailed experimentsto determine the density, Poisson’s ratio and acousti-cally determined transverse and longitudinal veloci-ties for different types of silica glasses as a functionof fictive temperature. He found that, in the fictivetemperature range of 1000 to 15008C, the Poisson’sratio and longitudinal velocity increase, while thetransverse velocity decreases, with increasing fictivetemperature. From these results, it was calculatedthat the shear modulus decreases while Young’smodulus and the bulk modulus increase with increas-ing fictive temperature.

The elastic moduli were measured in the appliedpressure range of 0 to 2 GPa and all were found to

w xdecrease with increasing pressure 43 . These datacannot be directly compared with those of glasseswith different fictive temperature since mechanicalproperties under pressure, unlike other properties, is

w xa non-linear effect 43 due to added effects ofhydrostatic pressure and stress for mechanical mea-surements. Mechanical stress of densified glass mea-sured under atmospheric pressure should be com-pared with those of glasses with different fictivetemperature. Such data cannot be found in the litera-ture but available data indicate a consistent trendwith the fictive temperature effect. For example,

w xGrimsditch 44 measured the Brillouin frequencyshift of longitudinal waves during pressure variationand found an irreversible frequency shift when asilica glass is subjected to 17 GPa pressure andreleased. When this sample was subjected to pressureagain, the frequency shift traced the shift observedduring pressure release. Correspondingly, the volumechange caused by pressure observed by Meade and

w xJeanloz 45 appears to follow the same trend, withthe percent volume change being much smaller inthe second cycle compared with the first cycle,indicating the compressibility of the densified glassesis smaller compared with the undensified glass. Thisis consistent with the behavior of a glass with higherfictive temperature.

From molecular-dynamics calculations, Graboww xet al. 46 suggested that silica glasses with higher

fictive temperatures have a higher fracture strength.

w xRecent experimental results 47 have also shownthat silica glasses with higher fictive temperatureshave a higher inert strength and a greater dynamicfatigue resistance. Correspondingly, densified glassesprepared by high pressure treatment are expected toexhibit better fatigue resistance than undensifiedglasses.

5.7. Summary of structure–property correlations

All of the above-mentioned effects of fictive tem-perature, pressure and compressive stress on silicaglass structure and properties in the anomalous re-gion, i.e. a fictive temperature range of 1000–15008Care summarized in Table 1.

The examples presented show that all the physi-cal, chemical and mechanical properties of the glasschange in a consistent manner with change in glassstructure brought about by the changes of fictivetemperature, pressure or compressive stress providedthat the measurements were performed in the samemanner. The elastic properties of glass under high

Table 1Qualitative behavior of the effect of increase in fictive tempera-ture, applied pressure or compressive stress on silica glass struc-ture and properties. ≠ indicates an increase while x indicates adecrease. The numbers refer to the reference concerned

Increase in Increase in Increase infictive applied compressivetemperature pressure stress

Structurew x w xu x 13 x 14–16Si – O – Siw x w x w xn x 6 x 17–19 x 201100 position

Propertiesw x w xDensity ≠ 9–11 ≠ 18,19,29w x w x w xRefractive index ≠ 8 ≠ 19,29 ≠ 20,30w x w xŽ .Viscosity x 31 x 32,33 ?w x w xCoefficient of ≠ 8 ≠ 8

thermal expansiona aw xEtch rate ≠ ≠ 24 ≠

w x w x w xWater diffusion x 35 x 36 x 36,39b w xHardness ≠ ≠ 19

w x w xCompressibility x 10 x 45w xShear modulus x 10w xYoung’s modulus ≠ 10w xInert strength ≠ 47

a This study.b Unpublished results.


hydrostatic pressure appear to deviate from this gen-eral trend. However, this effect is because of thenon-linear nature of elastic properties of glasses andit is not appropriate to compare these with linearelastic behavior determined for glasses with differentfictive temperatures. When linear elastic propertieswere compared, for example, using densified glassesunder atmospheric pressure, a consistent trend isexpected.

Along with the decrease in u , the defect orSi – O – Si

defect precursor concentration in the glass also in-w xcreases with increasing fictive temperature 41 and

w xhydrostatic pressure 42 . While the change in etch-ing rates can be explained in terms of changes inSi–O–Si bond angle, the change in kinetic propertiessuch as viscosity and water diffusion coefficientseem to be related to concentration of defects ordefect precursors.

A change in fictive temperature, hydrostatic pres-sure or compressive stress seems to cause similarstructural changes in silica glass which are detectableby a shift in the Si–O stretching infrared band.Monitoring the IR spectra of a silica glass sampleunder these physico–chemical treatments can giveindications to the change in glass properties. Forexample, we found that the IR reflection band posi-tion at ;1122 cmy1 for as-received silica opticalfibers as well as ion-implanted and irradiated glassesto be at lower frequencies compared to silica sam-ples heat-treated in the temperature range of 950 to14008C. From these observations, changes in glassstructure and properties can be predicted. From inde-pendent experimental results, it is known that ion-implanted and irradiated glass samples tend to have a

w xsmaller bond-angle 48 , higher density and refrac-w xtive-index 49,50 as well as lower chemical durabil-

w xity 51 as compared to annealed samples. The ob-served changes in glass structure and properties areconsistent with the trends predicted based on theshift in IR peak positions. Similarly, it is expectedthat the surface of as-received silica optical fibershas a higher density and lower chemical durability ascompared with annealed silica glass.

6. Conclusions

Infrared spectroscopy has been used to monitorstructural changes in silica glass. The changes in

glass structure, caused by changes in fictive tempera-ture, applied pressure or compressive stress, werewell correlated with changes in glass properties. Itwas shown that an increase in fictive temperature,

Ž .hydrostatic pressure or biaxial or uniaxial compres-sive stress reduced the average Si–O–Si bond anglein the glass structure and changed various glassproperties in a consistent manner as long as theproperties were measured under similar conditions.The observed structure–property correlation may brrevealed for silicate glasses also, in view of our

w xobservation 52 that, similar to silica glass, IR re-flection can be used to monitor the fictive tempera-ture of soda–lime silicate glass.

Acknowledgements

This research was supported by the US Depart-ment of Energy under Grant No. DE-FG02-85ER45217. Careful reading of the manuscript byDr. Steven N. Crichton of Rensselaer PolytechnicInstitute is appreciated.

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correlation of silica glass properties with the infrared spectra

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