Method for producing silica glass for use with light in a vacuum ultraviolet wavelength range

A method for producing a synthetic silica glass for use with vacuum ultraviolet light comprises the steps of: (a) producing a soot preform; (b) heating the soot preform in an atmosphere containing fluorine to obtain a fluorine-doped soot preform; (c) consolidating the fluorine-doped soot preform to obtain a fluorine-doped synthetic silica glass; and (d) heating the fluorine-doped synthetic silica glass in an atmosphere containing hydrogen gas to obtain a synthetic silica glass doped with fluorine and hydrogen molecules. A synthetic silica glass having both a high transmittance and high ultraviolet light resistance with respect to light in the vacuum ultraviolet wavelength range can be produced.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method for producing a silica glass used 
for an optical system using light in an ultraviolet wavelength range, 
typically photolithography. More specifically, the present invention 
relates to a method for producing a silica glass used for an optical 
system using light in a vacuum ultraviolet wavelength range of 200 nm or 
less, especially for ArF excimer laser (193 nm) lithography, F.sub.2 laser 
(157 nm) lithography, and the like. Further, the present invention relates 
to a synthetic silica glass produced by the method and an optical member 
comprising the synthetic silica glass. 
2. Related Background Art 
Conventionally, an exposure apparatus called a stepper has been used in a 
photolithographic technique of exposing/transferring a fine pattern of an 
integrated circuit onto a wafer such as a silicon wafer. With the recent 
increase in the integration degree of LSIs, the wavelength of a light 
source for this stepper has been shortened from a g-line (436 nm) to an 
i-line (365 nm), a KrF excimer laser beam (248 nm), and an ArF excimer 
laser beam (193 nm). In general, as an optical material used for a lens of 
an illumination optical system or a projection optical system of a 
stepper, a material having ultraviolet light resistance and a high 
transmittance with respect to light in a wavelength range shorter than 
that of an i-line is required. For this reason, synthetic silica glass is 
used. 
However, in a case where light in a vacuum ultraviolet wavelength range is 
used, there is an absorption due to various factors in even silica glass. 
In a high-precision optical system such as a stepper or an optical system 
using an F.sub.2 laser with a shorter wavelength (157 nm), even slight 
absorption causes generation of heat or fluorescence, resulting in a 
deterioration in optical performance. 
Silica glass for use with light in such an ultraviolet wavelength range, 
and a method for producing the silica glass have been disclosed in, e.g., 
U.S. Pat. No. 5,086,352 and European Patent Application Publication No. 
(EP-A1) 0 488 320. 
SUMMARY OF THE INVENTION 
The present inventors have found that the following problems have been 
posed in such a conventional silica glass, and satisfactory 
characteristics have not been attained with regard to transmittance and 
ultraviolet light resistance with respect to vacuum ultraviolet light. 
In a photolithographic technique using light in the vacuum ultraviolet 
wavelength range, since light in the vacuum ultraviolet wavelength range 
is higher in energy than an i-line or a KrF excimer laser beam, a very 
large load is imposed on a lens material and the like. For this reason, a 
lens, an optical system and the like which use the conventional synthetic 
silica glass have short service lives, and such an optical system 
undergoes a considerable deterioration in performance. 
Further, a conventional synthetic silica glass obtained by a so-called VAD 
method often has structural defects. For example, an oxygen deficient-type 
defect (Si--Si bond) itself has an absorption band of 163 nm. In addition, 
upon irradiation of an ultraviolet light having high energy such as an 
excimer laser beam, an absorption band based on a structural defect called 
an E' center (Si) is formed at a wavelength of 215 nm. 
It is an object of the present invention to provide a synthetic silica 
glass having both a high transmittance and high ultraviolet light 
resistance with respect to light in the vacuum ultraviolet wavelength 
range, which glass can be suitably used for a high-precision optical 
system for photolithography using light in the vacuum ultraviolet 
wavelength range. 
The present inventors made attempts to dope hydrogen molecules in glass to 
improve the ultraviolet light resistance and also dope fluorine in the 
glass to eliminate absorption caused by a structural defect (to ensure a 
high initial transmittance) and improve the ultraviolet light resistance. 
Upon repeating experiments on hydrogen- and fluorine-doping, the inventors 
have found that a synthetic silica glass is allowed to contain hydrogen in 
the form of molecules together with fluorine by means of doping fluorine 
in a so-called soot preform, consolidating the preform, and then doping 
hydrogen molecules therein, and that a silica glass superior in 
ultraviolet light resistance to a silica glass doped with hydrogen 
molecules in the absence of fluorine can be obtained. 
The present invention provides a method for producing a synthetic silica 
glass for use with light in a vacuum ultraviolet wavelength range, 
comprising the steps of: 
(a) hydrolyzing a silicon compound in a flame to obtain fine glass 
particles, and depositing the fine glass particles to form a porous glass 
mass; 
(b) heating the porous glass mass in an atmosphere containing fluorine to 
obtain a fluorine-doped porous glass mass; 
(c) consolidating the fluorine-doped porous glass mass to obtain a 
fluorine-doped synthetic silica glass; and 
(d) heating the fluorine-doped synthetic silica glass in an atmosphere 
containing hydrogen gas to obtain a synthetic silica glass doped with 
fluorine and hydrogen (hydrogen molecules). 
Further, the present invention provides a fluorine- and hydrogen 
molecules-doped synthetic silica glass for use with light in a vacuum 
ultraviolet wavelength range, which is produced by the above method of the 
present invention. 
Furthermore, the present invention provides an optical member for use with 
light in a vacuum ultraviolet wavelength range, which comprises a 
fluorine- and hydrogen molecules-doped synthetic silica glass produced by 
the above method of the present invention. 
Moreover, the present invention provides an exposure apparatus for use with 
light in a vacuum ultraviolet wavelength range as exposure light, which 
comprises: 
a stage allowing a photosensitive substrate to be held on a main surface 
thereof; 
an illumination optical system for emitting the exposure light of a 
predetermined wavelength and transferring a predetermined pattern of a 
mask onto said substrate; 
a projection optical system provided between a surface on which the mask is 
disposed and said substrate, for projecting an image of the pattern of 
said mask onto said substrate; and 
an optical member comprising a fluorine- and hydrogen molecules-doped 
synthetic silica glass which is produced by the above-mentioned method of 
the present invention. 
The present invention will be more fully understood from the detailed 
description given hereinbelow and the accompanying drawings, which are 
given by way of illustration only and are not to be considered as limiting 
the present invention. 
Further scope of applicability of the present invention will become 
apparent from the detailed description given hereinafter. However, it 
should be understood that the detailed description and specific examples, 
while indicating preferred embodiments of the invention, are given by way 
of illustration only, since various changes and modifications within the 
spirit and scope of the invention will be apparent to those skilled in the 
art from this detailed description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The method for producing a synthetic silica glass for use with light in a 
vacuum ultraviolet wavelength range according to the present invention 
will be described first. 
In the method of the present invention, (a) fine glass particles (so-called 
soot) are obtained by causing hydrolytic reaction of a silicon compound 
such as SiCl.sub.4 in an oxygen-hydrogen flame, and the fine glass 
particles are deposited to form a porous glass mass (a so-called soot 
preform). The method for forming such a porous glass mass is not 
specifically defined, and so are the conditions for the formation. A 
so-called VAD (Vapor phase Axial Deposition) method, a so-called OVD 
(Outside Vapor Deposition) method, or the like is properly used. 
In the present invention, (b) the porous glass mass is heated in an 
atmosphere containing fluorine to obtain a fluorine-doped porous glass 
mass. As this atmosphere containing fluorine, an inert gas atmosphere 
containing 0.1 to 100 vol % of a fluorine compound gas such as SiF.sub.4 
is preferable. In addition, the pressure set in this fluorine-doping 
treatment is preferably 0.1 to 10 atm; and the temperature set in this 
fluorine-doping treatment is preferably 1,000.degree. to 1,700.degree. C. 
It tends to be difficult to dope a sufficient amount of fluorine outside 
the above range of conditions. 
In the present invention, (c) the fluorine-doped porous glass mass is 
consolidated to obtain a fluorine-doped synthetic silica glass. In 
general, a porous glass mass is consolidated in an inert gas atmosphere 
such as an He atmosphere at a temperature near the softening point 
(preferably the melting point) of the glass or higher. In the method of 
the present invention, the porous glass mass is preferably consolidated in 
an atmosphere containing fluorine. This is because consolidation in an 
atmosphere containing fluorine tends to increase and maintain the amount 
of fluorine doped in the glass. As this atmosphere containing fluorine, an 
inert gas atmosphere containing 0.1 to 100 vol % of a fluorine compound 
gas such as SiF.sub.4 is preferable. In addition, the pressure set in this 
consolidation process is preferably 0.1 to 10 atm. Note that, in a case 
where the porous glass mass is consolidated in an atmosphere containing 
fluorine, the fluorine-doping step {step (b)} and the consolidation step 
{step (c)} can be performed as a single step. 
In the method of the present invention, (d) the fluorine-doped synthetic 
silica glass is heated in an atmosphere containing hydrogen gas to obtain 
a synthetic silica glass doped with fluorine and hydrogen molecules. As 
this atmosphere containing hydrogen gas, an inert gas atmosphere 
containing 0.1 to 100 vol % of hydrogen gas is preferable. In addition, 
the pressure set in this hydrogen molecules-doping treatment is preferably 
0.1 to 10 atm. It tends to be difficult to dope a sufficient amount of 
hydrogen molecules outside the above range of conditions. 
The temperature set during the above hydrogen molecules-doping treatment 
{step (d)} is preferably not more than 500.degree. C., more preferably 
0.degree. to 500.degree. C., and most preferably 300.degree. to 
500.degree. C. The reason why this temperature range is preferable will be 
described later. 
In the method of the present invention, since the above porous glass mass 
(soot preform) is doped with fluorine, an incomplete structure (bond) in 
the porous glass mass can be terminated by fluorine. In performing a 
dehydration or consolidation process for a porous glass mass synthesized 
by the VAD method, in particular, the atmosphere tends to become an oxygen 
deficient atmosphere, and an Si--Si bond having an absorption band at a 
wavelength of 163 nm tends to be formed. According to the present 
invention, under the presence of F, the Si--Si bond is cut, and the 
structure can be terminated by an Si--F bond, thereby canceling the 
formation of the above absorption band. In addition, an Si--F bond is 
higher in bond energy (binding energy) than an Si--H or Si--Cl bond and 
can stably maintain its structure even if it receives the strong energy of 
ultraviolet light. 
In the method of the present invention, after the porous glass mass is 
consolidated, the glass is heated in a hydrogen gas atmosphere within a 
temperature range of preferably not more than 500.degree. C. Doping of 
hydrogen molecules can be performed within the temperature range from room 
temperature to 2,500 K (2,227.degree. C.) from the thermodynamic 
viewpoint. When hydrogen molecules-doping is performed at a relatively low 
temperature, and preferably 500.degree. C. or lower, hydrogen 
molecules-doping can be performed in a state of hydrogen molecules without 
forming an Si--H bond which tends to be cut and become an E' center upon 
irradiation of light in an ultraviolet wavelength range, and without 
reducing the number of Si--F bonds. Therefore, the E' center formed upon 
irradiation of the ultraviolet light is terminated by doped hydrogen 
molecules to obtain better ultraviolet light resistance, in addition to 
the above strong structure. When such a heat treatment in a hydrogen 
atmosphere is performed at a temperature of more than 500.degree. C., 
Si--H bonds tend to be formed, and thus the ultraviolet light resistance 
tends to deteriorate. For this reason, it is not preferable that the above 
heat treatment temperature be further increased. 
In the method of the present invention, between the steps (a) and (b) 
described above, (f) the porous glass mass is preferably heated in an 
atmosphere containing chlorine, more preferably in an inert gas atmosphere 
containing chlorine gas and/or a chlorine compound. This process is a 
so-called dehydration process of removing moisture from the porous glass 
mass obtained by the hydrolytic method before the consolidation process. 
With this dehydration process, the OH group concentration in the synthetic 
silica glass after the consolidation process can be decreased to 100 ppb 
or less. 
If the porous glass mass is consolidated without sufficiently performing 
such a dehydration process, the resultant synthetic silica glass tends to 
have a weak structure. This may be because that weak bonds and unstable 
structures are present in the glass. If many OH groups are present in the 
porous glass mass, HF tends to be formed in the subsequent fluorine-doping 
treatment. In addition, with the above dehydration process, metal 
impurities can be removed as chlorides from the glass. In the method of 
the present invention, therefore, as described above, the dehydration 
process for a porous glass mass is preferably performed before the 
fluorine-doping treatment. 
Furthermore, in the method of the present invention, between the steps (c) 
and (d) described above, (e) the fluorine-doped synthetic silica glass may 
be heated in an inert gas atmosphere having a partial oxygen pressure of 
10.sup.0 to 10.sup.-10 atm at a temperature of 700.degree. C. or more. 
When the fluorine-doped silica glass is heated under a partial oxygen 
pressure of 10.sup.0 to 10.sup.-10 atm, a synthetic silica glass having an 
OH group concentration of about 200 to 300 ppb or more can be obtained. 
Although the heat treatment under a partial oxygen pressure is performed 
to eliminate the distortion of the glass, the main purpose of this 
treatment is to form a small amount of OH groups in the fluorine-doped 
silica glass. Although the formation process of OH groups in this 
treatment is not clarified well, it is assumed that Si--OH bonds are 
formed upon reaction between a small amount of H.sub.2 O left in the glass 
structure and a small amount of Si--Si bonds which do not appear in 
absorption. 
A synthetic silica glass for use with light in a vacuum ultraviolet 
wavelength range according to the present invention will be described 
next. 
The synthetic silica glass of the present invention is obtained by the 
method of the present invention described above. This synthetic silica 
glass is doped with both fluorine and hydrogen (hydrogen molecules). The 
ultraviolet light resistance of the synthetic silica glass of the present 
invention is greatly improved by the synergistic effect of the ultraviolet 
light resistance characteristics of the fluorine and hydrogen molecules 
doped in the glass. The fluorine concentration in the synthetic silica 
glass of the present invention is preferably 100 ppm or more, more 
preferably 100 to 30,000 ppm, and most preferably 500 to 30,000 ppm. The 
hydrogen molecule concentration in the synthetic silica glass of the 
present invention is preferably 1.times.10.sup.17 molecules/cm.sup.3 or 
more, and more preferably 1.times.10.sup.17 to 1.times.10.sup.19 
molecules/cm.sup.3. Desired characteristics tend to be difficult to obtain 
outside the above ranges of the contents of the respective elements. 
In addition, the OH group concentration in the synthetic silica glass of 
the present invention is preferably 100 ppm or less, and more preferably 
10 ppb to 100 ppm. 
With an OH group concentration of 100 ppm or less, a high transmittance and 
a high ultraviolet light resistance in a vacuum ultraviolet wavelength 
range tend to be ensured. More specifically, as the concentration of OH 
groups contained in the silica glass increases, structural defects unique 
to the silica glass, which are left even after F-doping, are terminated by 
the OH groups, and hence a higher transmittance and better ultraviolet 
light resistance can be obtained with respect to light in the ultraviolet 
wavelength range. If, however, the silica glass contains more than 100 ppm 
of OH groups, the absorption edge in the vacuum ultraviolet wavelength 
range of the resultant silica glass shifts toward the long-wavelength 
side, and thus it becomes difficult to use the resultant silica glass in a 
high-precision optical system. 
In addition, such OH groups serve to stabilize a glass structure. When a 
laser beam is irradiated on the glass, the OH groups suppress formation of 
an absorption band and improve the ultraviolet light resistance (laser 
resistance). Furthermore, under the presence of such OH groups, initial 
absorption in the resultant silica glass is suppressed. For this reason, a 
silica glass for use with light in an ultraviolet wavelength range such as 
a KrF or ArF laser beam may have an OH group concentration of 10 ppb to 
1,000 ppm, and preferably 10 ppb to 100 ppm. 
In contrast to this, since an OH group absorption band is present near a 
wavelength of 150 nm, a silica glass for used with light in a vacuum 
ultraviolet wavelength range such as an F.sub.2 laser beam tends to 
decrease in initial transmittance with an increase in OH group 
concentration. In particular, in this case, therefore, the OH group 
concentration is preferably 10 ppb to 20 ppm. 
In the synthetic silica glass of the present invention, metal impurities 
such as alkali metals, alkali earth metals, and transition metals are the 
cause of a structural defect having an absorption band in the ultraviolet 
wavelength range. More specifically, since bonds of the metal impurities 
are easily cut by irradiation of high-energy light such as ultraviolet 
light or vacuum ultraviolet light, the ultraviolet light resistance is 
greatly degraded. For this reason, such metal impurities contained in the 
glass are preferably minimized (preferably to 50 ppb or less). 
An optical member for use with vacuum ultraviolet light according to the 
present invention will be described next. 
The optical member of the present invention comprises the fluorine- and 
hydrogen molecules-doped synthetic silica glass obtained by the method of 
the present invention. The optical member of the present invention is not 
specifically defined except that the member comprises the above-mentioned 
synthetic silica glass. That is, the optical member includes various 
optical members for use with light in a vacuum ultraviolet wavelength 
range, e.g., a lens, a prism and the like used in an exposure apparatus 
such as a stepper. Further, the optical member of the present invention 
also includes blanks. In addition, a method for processing the synthetic 
silica glass of the present invention into the optical member of the 
present invention is not specifically defined, and a general cutting 
method, a general polishing method, and the like are employed. 
The optical member of the present invention contains the above-mentioned 
synthetic silica glass of the present invention which has both a high 
ultraviolet light resistance and a high transmittance with respect to 
vacuum ultraviolet light owing to the synergistic effect of the fluorine 
and hydrogen (hydrogen molecules) doped in the glass. For this reason, the 
optical member of the present invention achieves a longer service life 
than the conventional optical members. 
An exposure apparatus of the present invention will be described next. 
The present invention is preferably applied to the projection exposure 
apparatus, such as a so-called stepper, for projecting an image of 
patterns of reticle onto a wafer coated with a photoresist. 
FIG. 1 shows a basic structure of the exposure apparatus according to the 
present invention. As shown in FIG. 1, an exposure apparatus of the 
present invention comprises at least a wafer stage 3 allowing a 
photosensitive substrate W to be held on a main surface 3a thereof, an 
illumination optical system 1 for emitting vacuum ultraviolet light of a 
predetermined wavelength as exposure light and transferring a 
predetermined pattern of a mask (reticle R) onto the substrate W, a light 
source 100 for supplying the exposure light to the illumination optical 
system 1, a projection optical system (preferably a catadioptric one) 5 
provided between a first surface P1 (object plane) on which the mask R is 
disposed and a second surface P2 (image plane) to which a surface of the 
substrate W is corresponded, for projecting an image of the pattern of the 
mask R onto the substrate W. The illumination optical system 1 includes an 
alignment optical system 110 for adjusting a relative positions between 
the mask R and the wafer W, and the mask R is disposed on a reticle stage 
2 which is movable in parallel with respect to the main surface of the 
wafer stage 3. A reticle exchange system 200 conveys and changes a reticle 
(mask R) to be set on the reticle stage 2. The reticle exchange system 200 
includes a stage driver for moving the reticle stage 2 in parallel with 
respect to the main surface 3a of the wafer stage 3. The projection 
optical system 5 has a space permitting an aperture stop 6 to be set 
therein. The sensitive substrate W comprises a wafer 8 such as a silicon 
wafer or a glass plate, etc., and a photosensitive material 7 such as a 
photoresist or the like coating a surface of the wafer 8. The wafer stage 
3 is moved in parallel with respect to a object plane P1 by a stage 
control system 300. Further, since a main control section 400 such as a 
computer system controls the light source 100, the reticle exchange system 
200, the stage control system 300 or the like, the exposure apparatus can 
perform a harmonious action as a whole. 
The exposure apparatus of the present invention comprises an optical member 
which comprises a fluorine- and hydrogen molecules-doped synthetic silica 
glass produced by the method of the present invention, for example an 
optical lens consisting of the above-mentioned synthetic silica glass. 
More specifically, the exposure apparatus of the present invention shown 
in FIG. 1 can include the optical lens of the present invention as an 
optical lens 9 in the illumination optical system 1 and/or an optical lens 
10 in the projection optical system 5. 
The exposure apparatus of the present invention includes the 
above-mentioned optical member comprising the synthetic silica glass of 
the present invention which has both a high ultraviolet light resistance 
and a high transmittance with respect to light in a vacuum ultraviolet 
wavelength range owing to the synergistic effect of the fluorine and 
hydrogen (hydrogen molecules) doped in the glass. For this reason, the 
exposure apparatus of the present invention achieves a longer service life 
than the conventional exposure apparatuses. 
The techniques relating to an exposure apparatus of the present invention 
are described, for example, in U.S. patent application Ser. No. 255,927, 
No. 260,398, No. 299,305, U.S. Pat. No. 4,497,015, No. 4,666,273, No. 
5,194,893, No. 5,253,110, No. 5,333,035, No. 5,365,051, No. 5,379,091, or 
the like. The reference of U.S. patent application Ser. No. 255,927 
teaches an illumination optical system (using a laser source) applied to a 
scan type exposure apparatus. The reference of U.S. patent application 
Ser. No. 260,398 teaches an illumination optical system (using a lamp 
source) applied to a scan type exposure apparatus. The reference of U.S. 
patent application Ser. No. 299,305 teaches an alignment optical system 
applied to a scan type exposure apparatus. The reference of U.S. Pat. No. 
4,497,015 teaches an illumination optical system (using a lamp source) 
applied to a scan type exposure apparatus. The reference of U.S. Pat. No. 
4,666,273 teaches a step-and repeat type exposure apparatus capable of 
using the catadioptric projection optical system of the present invention. 
The reference of U.S. Pat. No. 5,194,893 teaches an illumination optical 
system, an illumination region, mask-side and reticle-side 
interferometers, a focusing optical system, alignment optical system, or 
the like. The reference of U.S. Pat. No. 5,253,110 teaches an illumination 
optical system (using a laser source) applied to a step-and-repeat type 
exposure apparatus. The '110 reference can be applied to a scan type 
exposure apparatus. The reference of U.S. Pat. No. 5,333,035 teaches an 
application of an illumination optical system applied to an exposure 
apparatus. The reference of U.S. Pat. No. 5,365,051 teaches a 
auto-focusing system applied to an exposure apparatus. The reference of 
U.S. Pat. No. 5,379,091 teaches an illumination optical system (using a 
laser source) applied to a scan type exposure apparatus. These documents 
are hereby incorporated by reference. 
As explained above, according to the method of the present invention, it 
becomes possible to produce a synthetic silica glass having both a high 
transmittance and high ultraviolet light resistance with respect to light 
in the vacuum ultraviolet wavelength range. Therefore, according to the 
present invention, it becomes possible to obtain an optical member which 
has both a high transmittance and high ultraviolet light resistance with 
respect to light in the vacuum ultraviolet wavelength range, and thus 
which has a long service life. 
Further, according to the present invention, a silica glass which had a 
high ultraviolet light resistance and no fluorescence band of 650 nm near 
an He--Ne laser wavelength (633 nm) used for wafer alignment can be 
obtained, with the good vacuum ultraviolet transmittance characteristics 
of a fluorine-doped silica glass being maintained. 
By use of the above-mentioned silica glass or optical member of the present 
invention, an improvement in performance and an increase in service life 
of a photolithographic apparatus, such as an exposure apparatus, which 
uses light in a vacuum ultraviolet wavelength range such as an F.sub.2 
laser beam can be achieved. 
Further, in comparison with the conventional synthetic silica glass 
containing hydrogen molecules which contains a large amount of OH groups 
(at least 100 ppm) as the essential condition, the silica glass of the 
present invention is superior in ultraviolet light resistance. 
In addition, although such a large amount of OH groups decrease the initial 
transmittance in the vacuum ultraviolet wavelength range (near the 
absorption end), according to the present invention, a silica glass having 
a high transmittance in the vacuum ultraviolet wavelength range can be 
obtained, since such OH groups are not indispensable and fluorine is doped 
in the silica glass of the present invention. 
EXAMPLES 1 AND 2 AND COMATIVE EXAMPLES 1-4 
(Example 1) 
A synthetic silica glass sample of the present invention was produced in 
accordance with a series of steps (sequence) shown in FIG. 2. In this 
example, heat treatment (the fifth step) under the presence of oxygen was 
not performed. 
More specifically, first of all, SiCl.sub.4 was hydrolyzed in an 
oxygen-hydrogen flame by using a ring burner having a quintuple-tube 
structure under the following conditions to obtain fine glass particles 
(SiO.sub.2 soot), and the fine glass particles were deposited to 
synthesize a porous glass mass (soot preform) having a diameter of 180 mm 
and a length of 500 mm in 20 hours (the first step). 
______________________________________ 
Gas composition! (1: inner ring to 5: outer ring) 
______________________________________ 
1 silicon tetrachloride 
10 g/min + He carrier 
1 slm 
2 oxygen 5 slm 
3 hydrogen 10 slm 
4 oxygen 15 slm 
5 hydrogen 40 slm 
______________________________________ 
The above porous glass mass was dehydrated (the second step) under the 
conditions shown in Table 1. The porous glass mass was then doped with 
fluorine in an SiF.sub.4 atmosphere (the third step), and consolidated 
(the fourth step), under the conditions shown in Table 1. The resultant 
silica glass member was cut and polished to produce a sample having a 
diameter of 60 mm and a thickness of 10 mm. When the fluorine 
concentration in the sample was measured by colorimetry, the concentration 
was 1 wt %. This sample was then treated by heat in a hydrogen gas 
atmosphere at 400.degree. C. for 60 hours under the conditions shown in 
Table 1 (the sixth step). As a result, the fluorine concentration in the 
sample underwent no change, and the hydrogen molecule concentration in the 
sample was 5.times.10.sup.17 molecules/cm.sup.3. The OH group 
concentration was about 75 ppb. 
The hydrogen molecule concentration was calculated as a contained-hydrogen 
concentration C (H.sub.2 molecules/cm.sup.3 glass) according to the 
following equation by obtaining the ratio of scattering intensities I of 
4,135 cm.sup.-1 and 800 cm.sup.-1 using the argon ion laser Raman 
scattering measuring device disclosed in Zhurnal Prikladoni Spektroskopii, 
Vol. 46, No. 6, pp. 987-991, June, 1987: 
EQU C=I(4,135 cm.sup.-1)!/I(800 cm.sup.-1)!.times.k 
where k is a constant, and k=1.22.times.10.sup.21. 
The OH group concentration was obtained by measuring absorption of 2.73 
.mu.m with an infrared spectrophotometer. According to this measuring 
method in which the sample has a thickness of 10 mm, it is possible to 
make sure of the presence of OH group of 10 ppb or more, and it is 
possible to determine accurately the OH group concentration of 100 ppb or 
more. 
(Comparative Example 1) 
Following the same procedures as in Example 1, SiCl.sub.4 was hydrolyzed in 
an oxygen-hydrogen flame, and SiO.sub.2 soots were deposited to obtain a 
porous glass mass (the first step). The porous glass mass was then treated 
by heat in a hydrogen gas atmosphere under the conditions shown in Table 1 
(the second step). Thereafter, the porous glass mass was doped with 
fluorine in an SiF.sub.4 atmosphere (the third step), and then 
consolidated (the fourth step), under the conditions shown Table 1. The 
resultant silica glass member was cut and polished to produce a sample 
having a diameter of 60 mm and thickness of 10 mm. The fluorine 
concentration and hydrogen molecule concentration in the sample were 
measured. The fluorine concentration was 500 ppm, and the hydrogen 
molecule concentration was below the detection limit (1.times.10.sup.16 
molecules/cm.sup.3). The OH group concentration was about 75 ppb. 
(Example 2) 
Following the same procedures as in Example 1, SiCl.sub.4 was hydrolyzed in 
an oxygen-hydrogen flame, and SiO.sub.2 soots were deposited to obtain a 
porous glass mass (the first step). The porous glass mass was then 
dehydrated (the second step), doped with fluorine in an SiF.sub.4 
atmosphere (the third step), and then consolidated (the fourth step), 
under the conditions shown in Table 1. The resultant silica glass member 
was cut and polished to produce a sample having a diameter of 60 mm and a 
thickness of 10 mm. The measured fluorine concentration in the sample was 
1 wt %. This sample was treated by heat in an Ar gas atmosphere having a 
partial oxygen pressure of 10.sup.-4 atm at 1,050.degree. C. for 60 hours 
(the fifth step), and, thereafter, the sample was further treated by heat 
in a hydrogen gas atmosphere at 400.degree. C. for 60 hours (the sixth 
step), under the conditions shown in Table 1. As a result, the fluorine 
concentration in the sample underwent no change before and after the 
treatment, and the hydrogen molecule concentration in the sample was 
7.times.10.sup.17 molecules/cm.sup.3. The OH group concentration, which 
was about 75 ppb before the treatment of the fifth step, increased to 1 
ppm after the treatment of the fifth and sixth steps. 
(Comparative Example 2) 
A synthetic silica glass sample was obtained following the same procedures 
as in Example 1 except that an atmosphere containing no SiF.sub.4 was used 
in the third and fourth steps, and the sixth step was not performed. 
The fluorine concentration in the resultant sample was below the detection 
limit (1 ppm), and the hydrogen molecule concentration was also below the 
detection limit (1.times.10.sup.16 molecules/cm.sup.3). The OH group 
concentration was about 75 ppb. 
(Comparative Example 3) 
A synthetic silica glass sample was obtained following the same procedures 
as in Example 1 except that the sixth step was not performed. 
The fluorine concentration in the resultant sample was 1 wt %, and the 
hydrogen molecule concentration was below the detection limit 
(1.times.10.sup.16 molecules/cm.sup.3). The OH group concentration was 
about 75 ppb. 
(Comparative Example 4) 
A synthetic silica glass sample was obtained following the same procedures 
as in Example 1 except that an atmosphere containing no SiF.sub.4 was used 
in the third and fourth steps. 
The fluorine concentration in the resultant sample was below the detection 
limit (1 ppm), and the hydrogen molecule concentration was 
5.times.10.sup.17 molecules/cm.sup.3. The OH group concentration was about 
75 ppb. 
TABLE 1 
__________________________________________________________________________ 
Composition of Temper- 
Pres- 
Period 
Example 
surrounding gas ature 
sure 
of time 
STEP 
Comp. Ex. 
(flow rate slm!) 
.degree.C.! 
atm! 
hours! 
__________________________________________________________________________ 
The Example. 1 
Cl.sub.2 
(1.0) 
+ He 
(20.0) 
1100 1.0 
20 
second 
Example. 2 
Cl.sub.2 
(1.0) 
+ He 
(20.0) 
1100 1.0 
20 
step 
Comp. Ex. 1 
H.sub.2 
(5.0) -- 
-- 1100 1.0 
20 
Comp. Ex. 2 
Cl.sub.2 
(1.0) 
+ He 
(20.0) 
1100 1.0 
20 
Comp. Ex. 3 
Cl.sub.2 
(1.0) 
+ He 
(20.0) 
1100 1.0 
20 
Comp. Ex. 4 
Cl.sub.2 
(1.0) 
+ He 
(20.0) 
1100 1.0 
20 
The Example. 1 
SiF.sub.4 
(0.6) 
+ He 
(20.0) 
1300 1.0 
20 
third 
Example. 2 
SiF.sub.4 
(0.6) 
+ He 
(20.0) 
1300 1.0 
20 
step 
Comp. Ex. 1 
SiF.sub.4 
(0.6) 
+ He 
(20.0) 
1300 1.0 
20 
Comp. Ex. 2 
-- -- He 
(20.0) 
1300 1.0 
20 
Comp. Ex. 3 
SiF.sub.4 
(0.6) 
+ He 
(20.0) 
1300 1.0 
20 
Comp. Ex. 4 
-- -- He 
(20.0) 
1300 1.0 
20 
The Example. 1 
SiF.sub.4 
(0.6) 
+ He 
(20.0) 
1650 1.0 
15 
fourth 
Example. 2 
SiF.sub.4 
(0.6) 
+ He 
(20.0) 
1650 1.0 
15 
step 
Comp. Ex. 1 
SiF.sub.4 
(0.6) 
+ He 
(20.0) 
1650 1.0 
15 
Cbmp. Ex. 2 
-- -- He 
(20.0) 
1650 1.0 
15 
Comp. Ex. 3 
SiF.sub.4 
(0.6) 
+ He 
(20.0) 
1650 1.0 
15 
Comp. Ex. 4 
-- -- He 
(20.0) 
1650 1.0 
15 
The Example. 1 
-- -- -- 
-- -- -- -- 
fifth 
Example. 2 
O.sub.2 
(10.sup.-4 atm) 
+ Ar 
(200.0) 
1050 1.0 
60 
step 
Comp. Ex. 1 
-- -- -- 
-- -- -- -- 
Comp. Ex. 2 
-- -- -- 
-- -- -- -- 
Comp. Ex. 3 
-- -- -- 
-- -- -- -- 
Comp. Ex. 4 
-- -- -- 
-- -- -- -- 
The Example. 1 
H.sub.2 
(2.0) -- 
-- 400 6.0 
60 
sixth 
Example. 2 
H.sub.2 
(2.0) -- 
-- 400 6.0 
60 
step 
Comp. Ex. 1 
-- -- -- 
-- -- -- -- 
Comp. Ex. 2 
-- -- -- 
-- -- -- -- 
Comp. Ex. 3 
-- -- -- 
-- -- -- -- 
Comp. Ex. 4 
H.sub.2 
(2.0) -- 
-- 400 6.0 
60 
__________________________________________________________________________ 
Each of the synthetic silica glass samples obtained by Examples 1 and 2 and 
Comparative Examples 1 to 4 was irradiated with an F.sub.2 laser beam 
under the following conditions, and a change in absorption coefficient 
thereof was measured. Table 2 and FIG. 3 show the measurement result. 
Irradiation Conditions! 
F.sub.2 laser beam: wavelength=157 nm 
energy density: 25 mJ/cm.sup.2 
repetition frequency: 50 Hz 
In addition, each of the synthetic silica glass samples obtained by 
Examples 1 and 2 and Comparative Examples 1 to 4 was irradiated with an 
ArF laser beam under the following conditions, and a change in absorption 
coefficient thereof was measured. Table 3 and FIG. 4 show the measurement 
result. 
Irradiation Conditions! 
ArF laser beam: wavelength=193 nm 
energy density: 100 mJ/cm.sup.2 
repetition frequency: 100 Hz 
TABLE 2 
______________________________________ 
Absorption coefficient cm.sup.-1 ! 
Total numbers of irradiated 
pulses .times.10.sup.5 ! 
0 1 2 5 10 
______________________________________ 
Example 1 0 0.009 0.012 0.023 
0.031 
Example 2 0 0.008 0.011 0.020 
0.023 
Comp. Ex. 1 0 0.035 0.055 0.085 
0.105 
Comp. Ex. 2 0 0.060 0.080 0.110 
0.125 
Comp. Ex. 3 0 0.010 0.016 0.032 
0.049 
Comp. Ex. 4 0 0.015 0.025 0.050 
0.065 
______________________________________ 
TABLE 3 
______________________________________ 
Absorption coefficient cm.sup.-1 ! 
Total numbers of irradiated 
pulses .times.10.sup.5 ! 
0 1 2 5 10 
______________________________________ 
Example 1 0 0.0045 0.006 0.011 
0.015 
Example 2 0 0.004 0.005 0.01 0.011 
Comp. Ex. 1 0 0.105 0.165 0.255 
0.315 
Comp. EX. 2 0 0.18 0.24 0.33 0.375 
Comp. Ex. 3 0 0.03 0.048 0.096 
0.13 
Comp. Ex. 4 0 0.045 0.075 0.15 0.195 
______________________________________ 
As is apparent from Tables 2 and 3 and FIGS. 3 and 4, in the silica glass 
samples of Comparative Examples 1 to 4 which were outside the range of the 
present invention, structural defects were produced at the same time when 
laser irradiation was started, and thus considerable decreases in 
ultraviolet transmittance were observed. In contrast to this, the silica 
glass samples of Examples 1 and 2 of the present invention, which were 
doped with both fluorine and hydrogen molecules, exhibited a high 
ultraviolet light resistance. 
Further, when fluorescence bands of 650 nm during irradiation of a laser 
beam were measured with an instantaneous multi-channel photodetector, no 
such bands were observed in the samples of Examples 1 and 2, while 
conspicuous fluorescence bands were observed in the samples of Comparative 
Examples 1 to 4. 
Examples 3-4 and Comparative Examples 5-8 
Following the same procedures as in Example 1, synthetic silica glass 
samples were obtained except that the treatment temperature in the 
hydrogen molecules-doping treatment (the sixth step) was set to 
400.degree. C. (Example 3), 500.degree. C. (Example 4), 600.degree. C. 
(Comparative Example 5), 700.degree. C. (Comparative Example 6), 
800.degree. C. (Comparative Example 7), and 900.degree. C. 
(Comparative Example 8). 
Each of the resultant samples was irradiated with an F.sub.2 laser beam 
under the following conditions, and then an absorption coefficient thereof 
was measured. Table 4 and FIG. 5 show the result. 
Irradiation Conditions! 
F.sub.2 laser beam: wavelength=157 nm 
energy density: 25 mJ/cm.sup.2 
repetition frequency: 50 Hz 
total numbers of irradiated pulses: 1.times.10.sup.6 
TABLE 4 
______________________________________ 
Comp. Comp. Comp. 
Ex. 3 Ex.4 Ex. 5 Ex. 6 Ex. 7 Comp. Ex. 8 
______________________________________ 
Temperature 
400 500 600 700 800 900 
for hydrogen 
molecules- 
doping .degree.C.! 
Absorption 
0.023 0.023 0.040 0.085 0.140 0.15 
coefficient 
cm.sup.-1 ! 
______________________________________ 
As is apparent from Table 4 and FIG. 5, a synthetic silica glass having a 
quite high ultraviolet light resistance was obtained especially when the 
treatment temperature in the hydrogen molecules-doping treatment (the 
sixth step) was 500.degree. C. or less. 
Example 5 
Lenses to-be used in the projection optical system and illumination optical 
system of the semiconductor exposure apparatus shown in FIG. 1 were 
produced by using the silica glass samples obtained in Examples 1 and 2. 
It was confirmed that the resultant lenses were sufficiently satisfied in 
desired design performance. Further, the resultant lenses had a resolving 
power corresponding to a line width of 0.19 .mu.m (using an ArF laser 
beam), and an integrated circuit pattern having practically sufficient 
flatness was obtained by use of the semiconductor exposure apparatus 
comprising the resultant lenses. Additionally, the service lives of the 
above lenses of the illumination optical system and the projection optical 
system according to the present invention were about twice as those of the 
conventional lenses. 
From the invention thus described, it will be obvious that the invention 
may be varied in many ways. Such variations are not to be regarded as a 
departure from the spirit and scope of the invention, and all such 
modifications as would be obvious to one skilled in the art are intended 
for inclusion within the scope of the following claims. 
The basic Japanese Application No. 156302/1994 (6-156302) filed on Jul. 7, 
1994 is hereby incorporated by reference.