41

OH and Interstitial−H2 Concentration Dependenceof Intensities of X−ray−Induced Absorptionof Type−III Fused Silica
Hideharu HORIKOSHI
Nobu KUZUUHydroxyl and interstitial−H2 concentration dependence of X−ray−induced absorption of type−III fused silica up
to an irradiation time of 6 h was studied. Induced absorption spectra in the fused silica irradiated with X−raysfrom a Rh target with a dosage of 2×104 C kg−1 h−1 were reproduced by six Gaussian absorption bands at 3.8, 4.8,
5.0, 5.4, 5.8 and 6.5 eV. Intensities of these absorption bands increased with the power of the irradiation time. OH
and interstitial−H2 concentration dependence at 1 h and 6 h are similar in the doubly logarithmic plot. A linear
relationship between the intensities of each absorption component and the H2 concentration was observed, while
no clear dependence of these intensities on the OH concentration was observed. Intensities of absorption bands
decreased linearly as the H2 concentration increased except for 5.0−eV band in which the intensity increased as
the H2 concentration increased.

4]. Point defects are unfavorable in practical

1．Introduction

applications in UV optical materials because they cause

Vitreous silica (v− SiO 2 ) has a high optical

optical absorption in the UV region. Two effects on thetransmission from the vacuum ultraviolet to the near

suppression of the ArF− and KrF−excimer−laser−

infrared [1]. Utilization of this optical property allows

induced absorption have been proposed: one is thethis material to be used in optical materials such as

effect of ≡Si−OH structure [5−7] and another is

optical waveguides, lenses and prisms for ultraviolet

interstitial−hydrogen molecules [8, 9].

(UV) light. v−SiO2 can be divided into synthetic fused

Point defects are also induced by irradiating with γ−

silica (SFS) synthesized from liquid silicon compounds

rays and X−rays [3, 4]. A study of the irradiation timeand fused quartz (FQ) produced by melting naturaldependence of the absorption intensity of X−ray−quartz powder [1]. SFS can be further divided into

induced absorption provides useful information toseveral types [2]: type−III SFSs produced directly by

understand the characteristics of the radiation−induced

flame hydrolysis of silicon compounds such as SiCl 4,

point defects in SFSs. Nakamura et al. studied the effectthe soot− remelting method, the plasma chemical vaporof heat treatment on the X−ray induced absorption ofdeposition (CVD) method and the sol−gel method.

type−III SFS [10, 11]. Kuzuu and Murahara studied theMost optical SFSs are type−III SFS or soot−remelted

X−ray induced absorption of type−III SFS synthesized

silica produced by the vapor axial−phase deposition

under reducing and oxidizing conditions [12]. They

(VAD) method.

showed that the absorption spectra of SFS synthesized
19

20

−3

(400−

under reducing condition has a peak at 5.7 eV, and an

1500 mass ppm) of OH. Most optical materials for KrF−

SFS synthesized under an oxidizing condition has aand ArF−excimer lasers are type−III SFS because thispeak at 5.7 eV, a shoulder at 4.8 eV and a slight peakmaterial is resistant to UV irradiation [2]. Point defects

at 2.0 eV. The characteristics of these absorption bands

might be induced by irradiating with excimer lasers [3,were discussed based on a model proposed forType−III SFS contains 3×10 −1×10 cm（ 41 ）42

TOSOH Research ＆ Technology Review Vol.52（2008）

describing the ArF−excimer−laser−induced absorption

This OH concentration dependence of the power index

of the same kinds of SFSs [13, 14]. Kuzuu studied theis the origin of the difference in the OH concentration

OH concentration dependence of the X−ray−induced

dependence of the absorption component depending on

absorption of type−III SFSs synthesized under reducing

the irradiation times.
As mentioned before, the characteristics of theconditions up to the irradiation time of 3 h [15]. The
induced absorption spectra were reproduced by fitting

excimer−laser−induced absorption in type−III SFSs

five Gaussian absorption bands at 6.5, 5.8, 5.4, 5.0, 4.8

depend both on OH and interstitial−H2 concentrations.

eV, respectively, and the origin of the OH concentration

In a previous paper [15], however, we focused only on

dependence was discussed. At that time, the author

the OH concentration dependence in excimer−laser−considered only the OH concentration dependence [15]induced absorption in type−III SFSs. The nature of thebut did not consider the effect of the interstitial−H2interstitial−H2 concentration dependence in Ref. [15] ismolecules. Based on the absorption bands proposed in

unknown because we did not measure the interstitial−

Ref. [15], Kuzuu et al. analyzed X−ray− and ArF−laser−

H 2 concentration. Therefore, the change in OH

induced absorptions in type−III SFSs synthesized underconcentration dependence might possibly be due to the

reducing and oxidizing conditions [17]. The present

effect of interstitial−H2 molecules. We then studied OH

authors studied the OH concentration dependence ofand interstitial−H2 concentration dependence of the X−

X−ray−induced absorption of type−III and VAD SFS thatray−induced absorption using type−III SFSs containing

contain various amounts of OH up to the irradiation

various amounts of OH and interstitial−H2.

time of 6 h [16]. In addition to the absorption bands
used in our foregoing studies [15, 17], an absorption
band at 3.8−eV, which might be due to interstitial−Cl2

2． Experimental procedure

molecules, appeared at long irradiation times. The

Twelve samples (A−L) containing various amounts

intensity of each absorption component increases withof OH and H2 contents shown in Table 1 were used. All

the power of the irradiation time. At irradiation times

samples used are type−III SFSs. Samples were cut to ashorter than 2 h, the intensities of the 5.8−, 5.4− andsize of 1×3×1 cm 3, and two facing surfaces with a5.0− eV bands decrease with increasing OH

thickness of 1 cm were optically polished.

concentration. At irradiation times longer than 2 h, on

X−rays from a Rh target operated at 50 kV and 50 mA

the other hand, each of these intensities shows awere utilized with a Rigaku System 3270 fluorescent X−−3minimum at an OH concentration of 〜
〜5×10 cm . The

ray analyzer. The dosage was 2×104 C kg−1 h−1. X−ray

power index of the time dependence also shows a

exposure was up to 6 h (1.2×10 5 C kg −1). Induced

minimum at an OH concentration of 〜
〜 5×10 17cm −3.

absorption spectra were measured with a Shimadzu

18

Table１

Sample
A
B
C
D
E
F
G
H
I
J
K
L

Samples used in the present study

OH conc.
（1019cm−3）
After
Symbola）
Before
11
11
□
11
11
□
9.4
9.4
▽
9.0
9.0
▽
9.0
9.1
▽
7.8
7.8
△
7.7
7.7
△
7.5
7.6
△
5.7
5.7
○
5.3
5.4
○
5.3
5.3
○
4.0
4.0
×

H2 conc.
（1018cm−3）
After
Symbolb）
Before
＜0.05
0.09
○
＜0.05
0.71
△
＜0.05
0.18
○
＜0.05
1.2
△
0.93
2.5
▽
＜0.05
0.16
○
0.06
1.3
△
1.4
3.1
▽
＜0.05
0.41
○
＜0.05
1.2
△
1.8
3.4
▽
8.0
10
□

a）Symbols are the same as used in Figs.３ and ５.
b）Symbols are the same as used in Figs.４ and ６.

（ 42 ）≡Si−H conc.
（1018cm−3）
Before
After
＜0.05
＜0.05
＜0.05
0.05
＜0.05
0.05
＜0.05
0.2
＜0.05
0.8
＜0.05
＜0.05
＜0.05
0.3
＜0.05
0.6
＜0.05
＜0.05
＜0.05
＜0.05
＜0.05
0.6
0.3
2

東ソー研究・技術報告 第52巻（2008）I ∝ tα, (1)UV3100−type spectrophotometer within 5 min after

where I is the absorption intensity, t is the irradiationeach irradiation. The OH contents were determined by
the peak intensity at 3650 cm
3coefficient of 77.5 dm mol

−1

cm

43

−1

with an extinction

time, and α is a constant. Hereafter, we will refer to α−1[18] using a Perkinthepower index. In a previous paper [16],however, the time dependence curve was convex inInterstitial−H2 and ≡Si−H contents were measured

low−OH−containing VAD silicas. This could be due towith a Jasco NRS−2000 Raman spectrophotometer [19].

the error in the measurement in large absorption at
Absorption coefficient［cm−1］

Elmer System 2000 FT− IR spectrophotometer.

3．Results
An example of the X−ray−induced absorption isshown in Fig.1. All absorption spectra were reproducedby six Gaussian bands at 3.8, 4.8, 5.0, 5.4, 5.8, and 6.5
eV, whose peaks and full widths at half maxima(FWHMs) are shown in Table 2. The irradiation time

0.8
SampleⅠ，6 h
0.6
0.4
0.2
0
3.0

4.0
5.0
Photon energy［eV］

6.0

dependence of each absorption component is shown in
Fig.１

Fig. 2. The curve of each component is linear in doubly
logarithmic plot,

An example of Gaussian peak decompositionof X−ray−induced absorption;：observation,：calculation

100

100

Absorption coefficient［cm−1］

A

B

C

D

10−1

10−1

10−2

10−2

10−3

10−3

100

100
E

F

G

H

10−1

10−1

10−2

10−2

10−3

10−3

100

100
I

J

K

L

10−1

10−1

10−2

10−2

10−3

10−3

10−4

103

104

103

104

103

104

103

104

Irradiation time［s］

Fig.２

Irradiation time dependence of each absorption components;
×：6.5 eV, ○：5.8 eV, △：5.4 eV, ▽：5.0 eV, □：4.8 eV, ◇：3.8 eV（ 43 ）10−4

44

TOSOH Research ＆ Technology Review Vol.52（2008）

Table２
Peak
（eV）

FWHM
（eV）

3.77

0.69

3.77
4.75
5.01
5.41
5.81
6.50

0.69
1.06
0.32
0.62
0.62
0.74

Defects
Interstitial
Cl2
POL
NBOHC
ODC
（Ⅱ）
E' β
E' γ
Unknown

Absorption components and their origins
Structure

Absorption cross sectiona）
（cm2）

Cl2

1×10−19≡Si−O−O−Si≡≡Si−O・≡Si・・・Si≡≡Si−H ≡Si・
≡Si・

2×10−17 b）
2×10−17
6×10−17 b）
6×10−17 b）

Oscillator
Strength

0.2
0.4
0.4

a）The absorption cross sections are after those in Ref.4.
b）Calculated from the oscillator strength shown in this table.

low enough to measure the absorption.

long irradiation times, because part of the absorption

dependence of the absorption components at irradiation

the present samples, the intensities of absorption aretimes of 1 h and 6 h. Interstitial−H 2 concentration

6.5 eV
6 h

10−1

1 h

100

6 h

1015
1 h

1015
1 h

100
4.8 eV（NBOHC）

6 h

1016

10−1

1015
10−2

1 h

Absorption coefficient［cm−1］

1017
1018
1019
H2 concentration［cm−3］

10−1
5.0 eV（ODC
（Ⅱ））

1015
10−2
1 h
1014
10−3

1017
1018
1019
H2 concentration［cm−3］

3.8 eV（Cl2）
10−2

6 h

10−3

1017
1018
1019
H2 concentration［cm−3］

Fig.３

6 h

1017

1016
1017
1018
1019
H2 concentration［cm−3］

Interstitial-H2 concentration dependence of the intensities and corresponding defectconcentrations at the irradiation times of 1 h and 6 h
The symbols indicate the range of the OH concentrations;
×：4.0×1019cm−3, ○：5.3−5.7×1019cm−3, △：7.6−7.8×1019cm−3,▽：9.0−9.4×1019cm−3, □：11×1019cm−3（ 44 ）

Concentration［cm−3］

10−1

Absorption coefficient［cm−1］

6 hConcentration［cm−3］

1016

Concentration［cm−3］

Absorption coefficient［cm−1］
Absorption coefficient［cm−1］

1017
1018
1019
H2 concentration［cm−3］

100

10−2

1016

10−1

1017
1018
1019
H2 concentration［cm−3］

5.4 eV（ E' β）

5.8 eV（E' γ）

Concentration［cm−3］

100

Concentration［cm−3］of absorption being beyond the measurement range. In

Absorption coefficient［cm−1］

Figure 3 shows the interstitial−H 2 concentration

Absorption coefficient［cm−1］

spectra of these samples is truncated due to the value東ソー研究・技術報告 第52巻（2008）45

dependences at 1 h and 6 h are similar in the doubly

dependence was observed as in the case of interstitial−

logarithmic plot. Therefore, the interstitial− H 2

H2 concentration dependence.

concentration dependence at any irradiation time must

Figure 5 shows the interstitial−H 2 concentration

be similar at least between 1 h and 6 h. The intensity of

dependence of the power indices. They decreaseeach component falls on a curve irrespective of the OH

weakly with increasing H2 concentration except for theconcentration. The intensities of the 5.8−, 5.4−, 4.8− and3.8 eV−band. The power indices for 3.8−eV components

3.8− eV bands decrease with increasing H 2

disperse over large amplitude compared to the otherconcentration. The intensity of the 5.0−eV band, on thecomponents. This could be due to the power index

other hand, increases with increasing H2 content.

having greater errors because the intensity of the 3.8−

Figure 4 shows the OH concentration dependence ofeV components is very weak and exists in the tail of theabsorption components at irradiation times of 1 h and 6

absorption spectra.
index is shown in Fig. 6. The OH concentrationthese data suggest that the OH concentration

dependences are similar among all absorption

dependences at any irradiation time are similar at least

components except for the 3.8−eV band; the values are

between 1 h and 6 h. No apparent OH concentration

19
approximately constant (〜〜1) except at [OH] = 4×10

6 h

10−1
1 h

100

10−1
1015
1 h

10−1
15

10
1 h
5
10
OH content［1019cm−3］100
4.8 eV（NBOHC）

6 h
1016

−1

10

1015
1 h
10−2

Fig.４

Absorption coefficient［cm−1］

10−2

Absorption coefficient［cm−1］

10

Concentration［cm−3］

16

Concentration［cm−3］

Absorption coefficient［cm−1］
Absorption coefficient［cm−1］

5
10
OH content［1019cm−3］

100
6 h

6 h
1016

5
10
OH content［1019cm−3］

5.4 eV（E' β）

5.8 eV（E'γ）

10−1
（Ⅱ））
5.0 eV（ODC

6 h
1015

10−2

1 h
10−3

1014

5
10
OH content［1019cm−3］

3.8 eV（Cl2）
10−2

6 h

10−3

5
10
OH content［1019cm−3］

Concentration［cm−3］

6.5 eV

1017

1016

Concentration［cm−3］

100

Concentration［cm−3］

are similar in the doubly logarithmic plot. Therefore,

Absorption coefficient［cm−1］

The OH concentration dependence of the power

Absorption coefficient［cm−1］

h. The OH concentration dependences at 1 h and 6 h

510OH content［1019cm−3］

OH concentration dependence of the intensities and corresponding defect concentrations ofeach absorption components at the irradiation times of 1 h and 6 h
The symbols indicate the range of the interstitial−H2 concentrations;○：0.09−0.41×1018cm−3, △：0.7−1.3×1018cm−3, ▽：2.5−3.4×1018cm−3,□：10×1018cm−3（ 45 ）46

TOSOH Research ＆ Technology Review Vol.52（2008）

1.5

1.5
5.8 eV（E' γ）

1.0

Power index

Power index

6.5 eV

0.5

0

1017

1018

1.0

0.5

0

1019

1017

−3

H2 concentration［cm ］
1.5

1.5

5.0 eV（ODC
（Ⅱ））

1.0

Power index

Power index

5.4 eV（E' β）

0.5

1017

1018

1.0

0.5

0

1019

1017

H2 concentration［cm−3］

1019

3.0
4.8 eV（NBOHC）

3.8 eV（Cl2）

1.0Power index

Power index

1018
H2 concentration［cm−3］

1.5

0.5

0

1019
−3

H2 concentration［cm ］

0

1018

1017

1018

1.0

0

1019
−3

1017

1018

1019
−3

H2 concentration［cm ］

Fig.５

2.0

H2 concentration［cm ］

Interstitial−H2 concentration dependence of the power index of the irradiation time
The symbols indicate the range of the OH concentrations;
×：4.0×1019cm−3, ○：5.3−5.7×1019cm−3, △：7.6−7.8×1019cm−3,▽：9.0−9.4×1019cm−3, □：11×1019cm−3

cm − 3 which was less than those at higher OH

by five Gaussian absorption bands at 6.5, 5.8, 5.4, 5.0

concentration. The OH concentration dependence ofand 4.8 eV, respectively. The absorption bands used in

the power index for the 3.8−eV band is complicated as

this paper include the absorption bands used in thesein the case of the interstitial−H 2 concentration

papers. In addition to these absorption bands, andependence.

absorption band at 3.8 eV is assumed in the previous[16] and the present papers. The peaks and full width at
half maxima (FWHMs) shown in Table 2 are the same

4．Discussion

as in Ref. [16].

［1］Origin of the absorption components

The absorption bands near 5 eV are called the B2

One of the present authors and colleagues showedbands. At least two kinds of B2 bands, a B2α band withthat X−ray− [15−17], γ−ray− [20] and excimer− laser−a peak at 5.02 eV and an FWHM of 0.32 eV and a B2β[17, 21] induced absorption spectra can be reproduced

band with a peak at 5.15 eV and an FWHM of 0.42 eV,（ 46 ）

東ソー研究・技術報告 第52巻（2008）

1.5

1.5
5.8 eV（E' γ）

1.0

Power index

Power index

6.5 eV

0.5

0

5

1.0

0.5

0

10

OH content［1019cm−3］

5.0 eV（ODC
（Ⅱ））

1.0

Power index

Power index

10

1.5
5.4 eV（E' β）

0.5

0

5

1.0

0.5

0

10

OH content［1019cm−3］

5

10

OH content［1019cm−3］

1.5

3.0
4.8 eV（NBOHC）

3.8 eV（Cl2）

1.0Power index

Power index

5

OH content［1019cm−3］

1.5

0.5

0

5

2.0

1.0

0

10

OH content［1019cm−3］

Fig.６

47

5

10

OH content［1019cm−3］

OH concentration dependence of the power index to time dependence of each absorption component
The symbols indicate the range of the interstitial−H2 concentrations;○：0.09−0.41×1018cm−3, △：0.7−1.3×1018cm−3, ▽：2.5−3.4×1018cm−3,□：10×1018cm−3

exist [4, 22]. The origin of the B 2α band is anwhich is related to hydrogen. A possible structure is ≡unrelaxed oxygen vacancy ≡Si･･･Si≡ called ODC(II)Si−H ≡Si･. The absorption bands due to two kinds of[4]. Here,E′centers, at 5.4 and 5.8 eV, respectively, have the≡" represents covalent bonds to threeoxygen atoms, andrepresents a vacancy atsame value of FWHM, 0.62 eV. Some reports assume

which an oxygen atom is removed from an ≡Si−O−Si

that the FWHM of the E′ centers is 0.8 eV [4]. In≡ structure. ODC(II) was named by Imai et al. [23] asprevious papers, we also used 0.8 eV as the FWHM of

distinct from the ≡Si−Si≡ structure called ODC(I), in

the 5.8−eV bands induced by irradiating with X−raysboth of which ODC stands foroxygen deficient[15, 17], an ArF−excimer−laser [17, 21], and γ−rays. ODC(I) causes an absorption band at 7.6 eV.

[20]. However, KrF−excimer−laser−induced absorption

The origin of the 5.8−eV band is the E′
γ center (≡cannot be reproduced if we assume the FWHM of thecenter･･･

represents an unpaired electron. The

5.8−eV band to be 0.8 eV [7]. If we assume the FWHM

5.4−eV band may be caused by the E′
β center [4],of the 5.8− and 5.4−eV bands to be 0.62 eV, theSi･), where･（ 47 ）48

TOSOH Research ＆ Technology Review Vol.52（2008）

absorption spectra for various types of SFSs irradiatednot assume the 3.8−eV band [15, 17]. Because thewith X−rays, γ−rays and excimer−lasers can be

maximum irradiation time in these papers was 3 h [12,reproduced [19]. The origin of the 6.5−eV band is15, 17], the intensity of the 3.8−eV band was still weakunknown. This absorption component is used tocompared to the other absorption components. In fact,

reproduce the effects of absorption at the energy

the 3.8−eV band intensity in the present study at an

higher than 6.2 eV.

−3−3
irradiation time of 3 h, 〜
〜10 cm , is two orders less

The 4.8−eV band is an oxygen−related center.

than those of the 5.8− and 5.4−eV bands. Nishikawa et

Several origins have been proposed [3, 4]; non−al. proposed that the 3.8−eV band is due to a peroxy

bridging oxygen hole center (NBOHC [≡Si−O･]),linkage (POL [≡Si−O−O−Si≡]) [24]. Awazu et al.

peroxy radical (POR [≡Si−O−O･]) and interstitial−[25] showed that the 3.8−eV band was induced in VAD

O3 molecule. The FWHM of the 4.8−eV band due tosilica vitrified in an ambient mixture of Cl2 and He gas

POR is 0.8 eV, while that of the others is 1.05 eV [4].

and that the intensity of this band increased withNBOHC can be produced by bond breakage, as

increasing Cl2 concentration in the ambient atmospherefollows :during the vitrification process. Based on this≡Si−O−Si≡→≡Si･･O−Si≡.

(2)

experimental evidence, they proposed that the 3.8−eVThe same amount of E′center as NBOHC is also

band is due to the interstitial−Cl2. In a previous paper

produced in this process. In a previous paper [16] on[16], we discussed whether the origin of the 3.8−eV

X−ray−induced absorption of various kinds of SFSs, weband is POL or the interstitial− Cl 2. POL could beassumed that the main process of the 4.8−eV band isformed by the reaction of ODC(II) and an interstitial−

NBOHC, because the estimated values of E′centers

O2 molecule. Because the POL will be changed into a

and NBOHCs in samples containing OH at greater thanpair of NBOHC or a peroxy radical (POR) and an E′

4×10

18

are of the same order. In the present

center by the irradiation, the intensity of the 3.8−eVpaper, these amounts are also of the same order in theband must be saturated or decreased due to thesample with the interstitial−H2 contents of 0.09−0.41×

destruction of the POL. Therefore, the interstitial−Cl2 is

18

cm

−3

−3

(Symbol ○ in Fig. 4). The NBOHCs in thea more probable candidate for the origin of the 3.8 eV−other samples are less than that of the E′ centersband. In Figs. 3 and 4, the concentration of Cl 2 iswhich may be due to the effect of interstitial H 2 −tentatively indicated.

10cm

molecules to suppress the creation of the NBOHC. In
our previous papers on X−ray− and ArF−laser−induced［ 2］ Effects of ≡ Si− OH and interstitial− H 2absorption of type−III silicas, we assumed that the 4.8−

molecules on the defects creationeV band was due to interstitial−ozone [12−14, 17]. In

Hydroxyl groups and interstitial−H2 molecules arethese papers, the 4.8−eV band is apparent only inconsidered to suppress the increment in the excimer−

samples synthesized under oxidizing conditions [12, 14,laser−induced absorption of v−SiO2. One of the present

17] in which oxygen molecules could dissolve into theauthors and coworkers studied the hydroxyl−content−

glass network during the glass formation process,dependence of ArF− and KrF−excimer−laser−induced

because an excess amount of oxygen molecules exists

absorption and showed that the absorption intensities

in the flame used for the hydrolysis. The intensities ofdecreased with increasing hydroxyl content [5−7].

the 4.8−eV band in fused silicas produced under

The authors considered that the OH group makes the

reducing conditions are not so strong [12, 13, 20]. The

glass network softer and prevents bond breakage uponpresent samples are also type−III silicas produced

irradiation. Yamagata showed that the interstitial−H2

under a reducing condition. In these samples, the 4.8−

molecules make the SFS resistant to the excimer−lasereV band might be due to NBOHC because an excess

irradiation [8]. The interstitial−H2 molecules react withamount of hydrogen molecules exists in the flame andthe dangling bonds formed by the irradiation throughan oxygen molecule can hardly be dissolved during thethe process of Eq. (2), and the defect structure

glass formation process.

becomes optically inactive. The E′ centers andIn our previous papers except for Ref. [16], we did

NBOHCs on the right side in Eq. (2) react with the H2（ 48 ）

東ソー研究・技術報告 第52巻（2008）49

molecule as≡Si･ ･O−Si≡ + H2 → ≡Si−H H−O−Si≡.

(3)
Concentration［cm−3］

Through this reaction, creation of optical absorption
due to E′centers and NBOHCs is suppressed.
X −r a y −i n d u c e d a b s o r p t i o n i n S F S s w a s
characteristics similar to the case of excimer−laser−induced absorption. The absorption intensity of X−ray−induced absorption and the probability of bond

1016
NBOHC
6 h
1015
1 h
ODC
（Ⅱ）

breakage are greater than those in excimer−laser−1014

induced irradiation. The feature of the absorption peakintensity can be described by that of the 5.8−eV band,

1017

because this band is dominant among all absorption

1018

1019
18

−3

H2 concentration［10 cm ］

components. In a previous paper [16], we showed thatFig.７

the induced absorption intensity decreases withincreasing OH content at irradiation times shorter than
2 h among SFSs containing more than 5×1017 cm−3 ofInterstitial−H2 concentration dependence of theconcentration of NBOHC and ODC(II) at theirradiation times of 1 h and 6 h;□：NBOHC 1 h, ■：NBOHC 6 h,○：ODC(II) 1 h, ●：ODC(II) 6 h

OH. At irradiation times longer than 2 h, on the other
hand, the induced absorption intensity increased with

The slopes of the ODC(II) and NBOHC concentrations

increasing OH concentration. The change in the sign ofversus interstitial−H2 concentration, respectively, arethe slope of the OH concentration dependence of thethe same in absolute value and opposite in sign. Thisabsorption intensity is due to the increment in thefact indicates that the product of the ODC(II) andpower index with the increment in OH concentration.

NBOHC concentration, [ODC(II)][NBOHC], isIn a previous study [16], we used VAD silicas and aconstant at each irradiation time. In fact, the product is

type−III silica. In the present study, on the other hand,

3.6±0.8×1029 cm−6 at irradiation time of 1 h and 0.92±

only type−III silicas are used. In type−III silicas, no0.20×1031 cm−6 at that of 6 h.

strong dependency on OH concentration is observed,

In a previous paper [15], the intensities of the 5.4−

as shown in Fig. 4. The absorption intensity decreased

and 5.0−eV bands in type−III SFSs decreased almost

with increasing interstitial−H2 concentration except for

linearly with increasing OH content. The intensity of

the 5.0−eV band as shown in Fig. 3. Only the intensity

the 5.8−eV band decreased with increasing OH content

of 5.0− eV band increases with increasing H 2

at an irradiation time shorter than 2 h. At an irradiation

concentration. However, almost no contribution of thetime longer than 3 h, on the other hand, the 5.8−eVincrement in the 5.0−eV band intensity to the peakband intensity increased with increasing OH

intensities of the induced absorption is seen, because

concentration up to 6×1019 cm−3 of OH and saturated at

the intensity of this band is quite small compared to thehigher OH concentration. The intensity of the 4.8−eVother components. The increment in the 5.0−eV band,

band increased almost linearly with increasing OH

however, must be an important phenomenon withconcentration at all irradiation times measured. We

respect to understanding the defect creation

assumed that the FWHMs of 6.5−, 5.8−, 5.4−, 5.0− andmechanism. The intensity of the 4.8−eV band decreases

4.8−eV bands are 0.74, 0.80, 0.61, 0.31 and 1.12 eV,with increasing interstitial−H 2 concentration. The

respectively. The FWHM of 5.8−eV band (0.80 eV) was

origins of the 5.0− and 4.8−eV bands must be due tolarger than that used in the present study (0.62 eV).

ODC(II) and NBOHC, respectively. Interstitial−H 2

The reason for the different value of the FWHM used isconcentration dependence on the population of thesedescribed in Sec. 4. [1]. Although the values ofdefects at irradiation times of 1 h and 6 h is shown in

FWHMs between the previous and the present studies

Fig. 7. At each irradiation time, the H2 concentration

are different, the feature of the OH concentration

dependence of every defect concentration is similar,

dependence of each absorption component must not bei.e., the slope is the same in the doubly logarithmic plot.

so different. In a previous study [15], we did not focus（ 49 ）50

TOSOH Research ＆ Technology Review Vol.52（2008）

on the effects of interstitial−H2 molecules. Therefore,

between the ≡Si−H content after irradiation with X−

the intensity of the absorption components could berays for 6 h and the H2 content before irradiation asaffected by interstitial hydrogen; a possibility is that theshown in Fig. 8. The line drawn in Fig. 8 representsinterstitial−H2 concentration increased with increasing
OH content. In that paper [15], we used samples
containing 3.8, 5.4, 6.0 and 8.7×10

19−3[Si−H]6h ＝ 0.20[H2]0 , (5)where [Si−H]6h represents the ≡Si−H concentrationof OH. A

after irradiation for 6 h and [H 2 ] 0 represents theslight inversion of the order of the intensity wasinterstitial hydrogen concentration before irradiation.

observed between the samples with 5.4 and 6.0×1019

This relation shows that the ≡Si−H content aftercm−3cm OH. This inversion could be due to the effect of H2irradiation is equal to the 20％ of the initial H2 content.

molecules. However, we cannot confirm that now,

Therefore 10％ of the H atoms, i.e. 10％ of the H 2

because we cannot obtain the same samples.

molecules, before irradiation becomes ≡ Si− H
structure after irradiation of 6 h.

［ 3］ Changes in interstitial− H 2 and ≡ Si− H
concentrations upon irradiation

Figure 9 shows the increment in ≡ Si− H
concentration versus the decrement in H2 content upon

Interstitial−H2 molecules may form ≡Si−OH and ≡irradiation for 6 h. Although the maximum value of

Si−H by reaction with defect structures. In addition to[H2]0 before irradiation is 1×1019 cm−3, the maximum

the reaction expressed in Eq. (3), the following process

value of the change in [H2], −Δ[H2], remains 2×1018

must be considered.

cm−3, which corresponds to the relation of Eq. (5); the≡Si･･･Si≡ + H2 → ≡Si−H H−Si≡.

change is about 20％ of the amount of initial H 2(4)Imai et al. [23] showed that the ≡S−H structures areconcentration. When the increment in H2, −Δ[H2], iscreated by heating a ODC(II)−containing SFS withless than 1×1018 cm−3, almost no ≡Si−H is induced.

ambient H2. Each sample used in this study contains

When −Δ[H2] is greater than 1×1018 cm−3, Δ[Si−H]

the ≡Si−OH structure whose concentration is moreincreases steeply with increasing −Δ[H2].

than an order greater than that of interstitial−H2 except
for Sample L. Therefore, the OH concentrations of［4］Interstitial−H 2 concentration dependence of

these samples before and after X−ray irradiation areintensities of absorption components

constant within experimental accuracy as shown in

We studied in a previous paper [16] the irradiation

Table 1. Hydride concentrations, on the other hand,time dependence of X−ray−induced absorption of type−change. The concentration of ≡Si−H induced by

III and VAD SFSs. In that paper, only one kind of type−irradiation must increase with increasing interstitial−H2

III silica, corresponding to Sample G in the present

concentration. In fact, a linear relationship is observed

paper, was used. The amounts of E′centers (5.8− and1018

Δ［Si−H］［1018cm−3］［Si−H］6h［cm−3］

2

1017

1

0
1017

Fig.８

1018
［H2］0［cm−3］

1019

0

1

2

−Δ［H2］［1018cm−3］

Relation between the change of the interstitial−H2
concentration before the irradiation [H2]0 and the≡Si−H concentration after irradiation for 6 h[Si−H]6h

Fig.９（ 50 ）

Relation between the change of the interstitial−H2
concentration and the ≡Si−H concentration
before and after irradiation for 6 h

東ソー研究・技術報告 第52巻（2008）

51

5.4−eV bands) and ODC(II) (5.0−eV band) decrease

into NBOHCs by X−ray irradiation. However, the ≡with increasing OH concentration. As for the 4.8−eV

Si−OH structures are stabler than ≡Si−H under X−

band, on the other hand, the difference among samples

ray irradiation. Therefore, the difference in the stability

is quite small compared to the 5.8−, 5.4− and 5.0−eV

on X−ray irradiation between ≡Si−H and ≡Si−OH

bands. This fact suggests that the NBOHCs arecauses the difference in the H2 content dependence of

produced through the bond breakage expressed in Eq.

the intensities between E′centers and NBOHCs.

(2). The interstitial−H2 concentration of the VAD silicas

The intensity of the 5.0−eV band increased within a previous paper [16] was less than the detection

increasing interstitial−H 2 concentration, which islimit, 5×10

16

−3cm , except for a VAD−silica whichcontained 8× 10

16

cm

−3

of interstitial− H 2 . This

opposite to the other components. From ODC(II), E′
centers must be produced by the process of≡Si･･･Si → ≡Si･ ≡Si＋ ,concentration is slightly greater than the detection limitof interstitial−H 2 , 5× 10

16−3cm . Anyway, the H 2

where ≡Si＋

(7)

represents a planar−three−fold−oxygen

concentrations of VAD silicas are quite small compared

coordinated structure [4]. Because the ODC(II)to that of type−III silicas; the H2 concentration of theconcentration increases with increasing interstitial−H2

type−III silica in that paper [16] was 1.3×10

18

−3

cm .

concentration, the amount of E′centers increases with

Therefore, H 2 will not function in the creation ofincreasing interstitial−H2 content. This process also

NBOHC in VAD silicas. NBOHC in type−III silica must

contributes the difference in H2 content dependence

be created through the process of Eq. (2), and ≡Si−H

between the amounts of E′centers and NBOHCs.

and ≡Si−OH are further produced by reacting with
the interstitial−H2 molecule from the process of Eq. (3).

ODC(II) can be created by the knock−on process ofstrained ≡Si−O−Si≡ bond as≡Si−O−Si≡ → ≡Si･･･Si≡ + O .

In the present samples, the intensities of the 5.8−,(8)

5.4−, 4.8− and 3.8−eV bands decrease with increasing

ODC(II)s can also be created by the reaction of

H 2 concentration (Fig. 3). A probable origin of theinterstitial−H2 molecules with strained ≡Si−O−Si≡

decrement in the amounts of E′centers, 5.8− and 5.4−

bonds as≡Si−O−Si≡ + H2 → ≡Si･･･Si≡ + H2O .

eV bands, is the process of Eq. (3); the probability for(9)

the reaction of Eq. (3) to proceed is increased withThis reaction must be the origin of the H2 concentration

increasing interstitial−H2 concentration. The effect of

dependence of the intensity of the 5.0−eV band. By thethe interstitial−H 2 on the suppression of the 4.8−eVreaction between interstitial−H2 molecules and ≡Si−band creation is greater than that on the other

O−Si≡ bonds, the processcomponents; the absolute value of the slope of the≡Si−O−Si≡ + H2 → ≡Si−H H−O−Si≡(10)intensity versus interstitial−H2 concentration is greater

can also proceed. The criterion regarding whether thethan that of the other absorption components. This factprocess of (9) or (10) occurs must depend both on theseems to contradict the hypothesis that NBOHCs are

magnitude of the stress on the ≡Si−O−Si≡ bond andcreated through the process of Eq. (2), because E′

the probability of an encounter between a strained ≡centers produced from the process of Eq. (2) must

Si−O−Si≡ bond and an H2. Anyway, the process ofcause the ≡Si−H structure through the process of Eq.

Eq. (9) must be promoted by the existence of(3). A probable mechanism to explain this contradiction

interstitial−H2 molecules.

is as follows: If the interstitial−H2 molecule reacts with

The intensity of 3.8−eV band at an irradiation time 6

NBOHCs and E′centers produced by the process ofh decreases with increasing interstitial− H 2

Eq. (2), the same amounts of ≡S−OH and ≡Si−H

concentration as shown in Fig. 3. The data fall on aare produced by the process of Eq. (3). Some of the ≡master curve irrespective of OH concentration. As

Si−H bonds can be broken by X−ray irradiation by the

discussed in Sec. 4. [1], we ascribe the Cl2 to be thereactionorigin of the 3.8−eV band. The H 2 concentration≡Si−H → ≡Si･ + H ,(6)dependence of the intensity of the 3.8−eV band can be

and E′centers are produced again. Some of the ≡Si−

explained as follows: The intensity must decrease with

OHs produced by this mechanism might also change

increasing interstitial−H 2 molecules because HCl（ 51 ）

52

TOSOH Research ＆ Technology Review Vol.52（2008）

molecules are formed by the reaction between Cl2 anddosage of 2× 10 4 C kg −1 were utilized. Induced

H2. Therefore, the 3.8−eV band intensity decreases withabsorption spectra were reproduced by six Gaussian

increasing interstitial−H2 molecules.

absorption bands at 3.8, 4.8, 5.0, 5.4, 5.8 and 6.5 eV.
Hydroxyl and interstitial− hydrogen− molecule［5］Power indices of absorption components

concentration dependences at irradiation times of 1 h

In a previous paper [16], the power index shows aand 6 h of each absorption component wereminimum at an OH concentration of 5×1017 cm−3 andproportional to each other in a doubly logarithmic plot.

increased almost linearly with increasing OH

Apparent interstitial−H2 concentration dependence was17

observed at each irradiation time. OH concentration

cm . These samples were both VAD and type−III

dependence, on the other hand, was not as clear as insilicas. The power indices of the present samples are

the case of H2 concentration dependence. Absorption

almost constant except at the lowest OH concentration

intensities decrease with increasing H2 concentration

as shown in Fig. 6 (Symbol □). As for the 3.8−eV band,

except for the 5.0−eV band. The intensity of the 5.0−eV

however, the dispersion of the data is larger than thoseband, on the other hand, increases with increasing H2of the other components because the intensity of theconcentration. The results indicate that interstitial−H2

concentration at concentrations greater than 5×10
−3

3.8−eV band is quite weak and is located at the tail ofmolecules affect the characteristics of X−ray−induced

the absorption spectra. In a previous paper [16], theabsorption. The effect of the hydroxyl structure on the

power index of the 3.8−eV band was almost three times

induced absorption is not apparent as in the case of

those of the other components. Although the powerinterstitial−H 2. This fact suggests a possibility thatindices for the 3.8−eV band in the present study are

excimer−laser−induced absorption is also affected only

also greater than those of the other components, theby the interstitial−H2 in spite of the apparent effect of ≡value of the power indices of the 3.8−eV band is less

Si−OH on the suppression of the excimer−laser−than three times of that of the other components.

induced absorption.

As shown in Fig. 5, the power indices for all
absorption components decrease with increasing
interstitial−H 2 concentration except for the 3.8−eVReferences

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suppression of the X−ray−induced absorption.

［２］H. Kawazoe, K. Awazu, Y. Ohki, N. Kuzuu,
S. Todoroki, A. Hayashi and H. Fukuda(Eds.),

Similar suppression effects of the interstitial−H 2molecules on the intensities of KrF− and ArF− excimer−

Hishoushitsu Shirika Zairyo Ouyou Handobukku

laser−induced absorption have been proposed [8, 9].

(Handbook for Application of Amorphous Silica),

One of the present authors insisted that the ≡Si−OH

Realize, Tokyo, 1999 (in Japanese)

suppresses the excimer−laser−induced absorption [5−［３］D. L. Griscom, J. Ceram. Soc. Japan, 99, 923(1991)

7]. However, in these papers, no interstitial−H2 effectwas considered. Therefore, we cannot deny the［４］R. Skuja, J. Non−Cryst. Solids, 239, 16 (1998)

possibility that only interstitial−H 2 contributes the［５］Reza Kenkyu, (Rev. Laser Eng.), 23, 396 (1995)(in Japanese)

suppression of the induced absorption; the higher OH
containing sample contains a higher content of［６］N. Kuzuu, Proc. SPIE, 2714, 71 (1996)

interstitial−H 2 molecules. Further study must be［７］N. Kamisugi, N. Kuzuu, Y. Ihara and
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required to examine this possibility.

［８］S. Yamagata, Minaral. J., 15, 333 (1991)［９］D. R. Sempolinski, T. P. Seward, C. Smith,

5．Summary and Conclusion

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著者

氏名 堀 越 秀 春Hideharu HORIKOSHI
入社 昭和61年 4 月 1 日
所属 東ソー・エスジーエム株式会社

氏名 葛 生伸Nobu KUZUU
所属 福井大学
教授

技術部
技術チーム

（ 53 ）

53