U.S. patent application number 15/797061 was filed with the patent office on 2018-03-01 for optical component made of quartz glass for use in arf excimer laser lithography and method for producing the component.
The applicant listed for this patent is Heraeus Quarzglas GmbH & Co. KG. Invention is credited to Bodo KUHN.
Application Number | 20180057391 15/797061 |
Document ID | / |
Family ID | 50115904 |
Filed Date | 2018-03-01 |
United States Patent
Application |
20180057391 |
Kind Code |
A1 |
KUHN; Bodo |
March 1, 2018 |
OPTICAL COMPONENT MADE OF QUARTZ GLASS FOR USE IN ArF EXCIMER LASER
LITHOGRAPHY AND METHOD FOR PRODUCING THE COMPONENT
Abstract
An optical component made of synthetic quartz glass includes a
glass structure substantially free of oxygen defect sites and
having a hydrogen content of 0.1.times.10.sup.16 to
1.0.times.10.sup.18 molecules/cm.sup.3, an SiH group content of
less than 2.times.10.sup.17 molecules/cm.sup.3, a hydroxyl group
content of 0.1 to 100 wt. ppm, and an Active temperature of less
than 1070.degree. C. The optical component undergoes a
laser-induced change in the refractive index in response to
irradiation by a radiation with a wavelength of 193 nm using
5.times.10.sup.9 pulses with a pulse width of 125 ns and a
respective energy density of 500 .mu.J/cm.sup.2 at a pulse
repetition frequency of 2000 Hz. The change totals a first measured
value M.sub.193 nm when measured using the applied wavelength of
193 nm and a second measured value M.sub.633 nm when measured using
a measured wavelength of 633 nm. The ratio M.sub.193 nm/M.sub.633
nm is less than 1.7.
Inventors: |
KUHN; Bodo; (Gelnhausen,
DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Heraeus Quarzglas GmbH & Co. KG |
Hanau |
|
DE |
|
|
Family ID: |
50115904 |
Appl. No.: |
15/797061 |
Filed: |
October 30, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14769382 |
Aug 20, 2015 |
9834468 |
|
|
PCT/EP2014/053199 |
Feb 19, 2014 |
|
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15797061 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C03C 2203/54 20130101;
C03C 2201/24 20130101; C03C 2201/11 20130101; G02B 1/00 20130101;
G02B 1/02 20130101; C03B 19/1453 20130101; C03B 32/00 20130101;
C03C 3/06 20130101; Y02P 40/57 20151101; C03C 2201/23 20130101;
C03B 2201/21 20130101; C03B 32/02 20130101; C03C 2201/12 20130101;
C03C 4/04 20130101; C03B 2201/07 20130101; C03B 2201/24 20130101;
C03C 2201/21 20130101; C03C 2204/00 20130101; C03B 2201/075
20130101; C03B 2201/23 20130101 |
International
Class: |
C03C 3/06 20060101
C03C003/06; G02B 1/00 20060101 G02B001/00; C03B 32/02 20060101
C03B032/02; G02B 1/02 20060101 G02B001/02; C03B 32/00 20060101
C03B032/00; C03B 19/14 20060101 C03B019/14; C03C 4/04 20060101
C03C004/04 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 20, 2013 |
DE |
10 2013 101 687.1 |
Claims
1. An optical component made of synthetic quartz glass for use in
ArF excimer laser lithography with an applied wavelength of 193 nm,
the optical component comprising: a glass structure which is
substantially free of oxygen defect sites, the glass structure
having a hydrogen content in the range of 0.1.times.10.sup.16
molecules/cm.sup.3 to 1.0.times.10.sup.18 molecules/cm.sup.3, a
content of SiH groups of less than 2.times.10.sup.17
molecules/cm.sup.3, a content of hydroxyl groups in the range
between 0.1 and 100 wt. ppm, and a fictive temperature of less than
1070.degree. C., wherein the glass structure reacts to irradiation
with radiation of an applied wavelength of 193 nm with 5.times.109
pulses with a pulse width of 125 ns and an energy density of 500
.mu.J/cm2 each time and a pulse repetition frequency of 2000 Hz
with a laser-induced refractive-index change, the amount of which
upon measurement with the applied wavelength of 193 nm yields a
first measured value M.sub.193 nm and upon measurement with a
measurement wavelength of 633 nm yields a second measured value
M.sup.633 nm, and wherein M.sub.193 nm/M.sub.633 nm<1.7.
2. The optical component according to claim 1, wherein M.sub.193
nm/M.sub.633 nm<1.6.
3. The optical component according to claim 1, wherein the fictive
temperature is less than 1055.degree. C.
4. The optical component according to claim 1, wherein the content
of hydroxyl groups is between 10 and 60 wt. ppm.
5. The optical component according to claim 1, wherein the glass
structure has a content of fluorine of less than 10 wt. ppm.
6. The optical component according to claim 1, wherein the glass
structure has a content of chlorine of less than 0.1 wt. ppm.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Division of co-pending U.S. patent
application Ser. No. 14/769,382 filed Aug. 20, 2015, which was a
Section 371 of International Application No. PCT/EP2014/053199,
filed Feb. 19, 2014, which was published in the German language on
Aug. 28, 2014, under International Publication No. WO 2014/128148
A3 and the disclosure of which is incorporated herein by
reference.
BACKGROUND OF THE INVENTION
[0002] Optical components made of synthetic quartz glass and a
method for producing the same are known from WO 2009/106134 A1. The
optical component has a glass structure which is substantially free
from chlorine, oxygen defect sites and SiH groups (below the
detection limit of 5.times.10.sup.16 molecules/cm.sup.3). Within a
diameter of 280 mm (CA area), the component exhibits a mean
hydrogen content of about 3.times.10.sup.16 molecules/cm.sup.3 and
a hydroxyl group content of 25 wt. ppm.
[0003] For the production of the component, a SiO.sub.2 soot body
is dried such that a mean hydroxyl group content of less than 60
wt. ppm is obtained in the quartz glass produced therefrom. Prior
to vitrification, the soot body is subjected to a conditioning
treatment including a treatment with nitrogen oxide. For reducing
mechanical stresses, the quartz glass blank is subjected to an
annealing temperature and is finally loaded with hydrogen in an
atmosphere of 80 vol.-% nitrogen and 20 vol.-% hydrogen at
400.degree. C. at an absolute pressure of 1 bar for a duration of
80 hours.
[0004] Due to the manufacturing process, the synthetic quartz glass
produced in this way contains nitrogen, which is chemically bound
in the glass network. It shows an advantageous damage behavior
vis-a-vis shortwave UV laser radiation especially with respect to
the so-called "compaction".
[0005] With the damage behavior of the "compaction", a local
increase in density is observed in the volume penetrated by
radiation during or after high-energy UV laser irradiation of the
glass. This causes a local increase in the refractive index which
is progressing during continuous irradiation and thereby leads to
an increasing deterioration of the imaging properties of the
optical component and, in the end, to a premature failure of the
component.
[0006] For the sake of simplicity, the changes in the refractive
index distribution due to compaction are often determined not at
the applied wavelength, e.g. at 193 nm, but by using a Fizeau
interferometer equipped with a helium-neon laser with a measurement
wavelength of 633 nm (more exactly: at a wavelength of 632.8
nm).
[0007] It has now been found that, despite identical or similar
measured values of their compaction at a measurement wavelength of
633 nm, quartz glasses can surprisingly show different damage
behaviors at a measurement wavelength at 193 nm. This particularly
poses problems whenever the quartz glass to be measured hints at a
quite acceptable compaction behavior at a measurement wavelength of
633 nm, but, upon use with the applied wavelength, unexpectedly
shows much poorer values or is even unusable.
BRIEF SUMMARY OF THE INVENTION
[0008] It is an objective of the present invention to provide an
optical component for use in ArF excimer laser lithography with an
applied wavelength of 193 nm, wherein the component, starting from
a measurement of the compaction behavior at a measurement
wavelength of 633 nm, permits a reliable prediction of the
compaction behavior during use with UV laser radiation of the
applied wavelength.
[0009] Furthermore, it is an objective of the present invention to
provide a method which is suited for producing such an optical
component.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0010] The foregoing summary, as well as the following detailed
description of the invention, will be better understood when read
in conjunction with the appended drawings. For the purpose of
illustrating the invention, there are shown in the drawings
embodiments which are presently preferred. It should be understood,
however, that the invention is not limited to the precise
arrangements and instrumentalities shown.
[0011] FIG. 1 shows a diagram in which the ratio of measured values
(namely the maximum refractive-index increase) of a damage by
compaction is plotted against the irradiation dose upon measurement
with a measurement wavelength of 193 nm and with a measurement
wavelength of 633 nm;
[0012] FIG. 2 shows a diagram in which the ratio M.sub.193
nm/M.sub.633 nm determined on the basis of a model calculation is
plotted against measured values of the ratio; and
[0013] FIG. 3 shows a result of the model calculation which
illustrates the dependence of the ratio M.sub.193 nm/M.sub.633 nm
on the hydrogen content of the quartz glass and the hydrogen
loading temperature in a three-dimensional representation.
DETAILED DESCRIPTION OF THE INVENTION
[0014] An embodiment of the present invention relates to an optical
component made of synthetic quartz glass for use in ArF excimer
laser lithography with an applied wavelength of 193 nm, with a
glass structure substantially without oxygen defect sites, a
hydrogen content in the range of 0.1.times.10.sup.16
molecules/cm.sup.3 to 1.0.times.10.sup.18 molecules/cm.sup.3 and a
content of SiH groups of less than 2.times.10.sup.17
molecules/cm.sup.3 and with a content of hydroxyl groups in the
range between 0.1 and 100 wt. ppm, wherein the glass structure has
a fictive temperature of less than 1070.degree. C. Another
embodiment of the present invention relates to a method for
producing such an optical component.
[0015] The glass structure of the optical component according to
the present invention preferably reacts to irradiation with
radiation of a wavelength of 193 nm with 5.times.10.sup.9 pulses
with a pulse width of 125 ns and an energy density of 500
.mu.J/cm.sup.2 each time and a pulse repetition frequency of 2000
Hz with a laser-induced refractive-index change, the amount of
which upon measurement with the applied wavelength of 193 nm yields
a first measured value M.sub.163nm and upon measurement with a
measurement wavelength of 633 nm a second measured value M.sub.633
nm, where: M.sub.193 nm/M.sub.633 nm<1.7.
[0016] The dose of irradiation with radiation of the applied
wavelength is defined by the pulse number of the laser pulses,
their pulse width and energy density and the pulse repetition
frequency. With this irradiation dose, the component according to
the invention exhibits a compaction behavior with the following
typical features: [0017] (a) After the above-specified irradiation
dose, the respective measured values M.sub.193 nm and M.sub.633 nm
of the measurements of the compaction behavior at 193 nm and at 633
nm show a ratio M.sub.193 nm/M.sub.633 nm that is smaller than 1.7.
This is a comparatively small ratio M.sub.193 nm/M.sub.633 nm. It
has been found that the small ratio represents not only a high
compaction resistance of the glass, but is also an indispensable
condition for the predictability of the compaction itself [0018]
(b) It has been found that in the quartz glass according to the
invention, this small ratio of M.sub.193 nm/M.sub.633 nm which
satisfies condition (a) remains almost constant, even if the
irradiation dose exceeds the above-specified dose. In view of the
constant ratio, it is possible to calculate the compaction due to
irradiation with the applied wavelength also for every higher
radiation dose than the above-specified one in an exact manner or
at least with sufficient accuracy, namely on the basis of a
measurement at 633 nm.
[0019] If the fulfillment of condition (a) is known for sure, it is
thus possible due to condition (b) to reliably predict, by
measurements at a wavelength of 633 nm, the compaction behavior
upon use of the quartz glass with UV laser radiation of 193 nm. The
glass structure is substantially free of oxygen defect sites, such
that the concentrations of oxygen deficient defects and oxygen
excess defects in the glass structure are below the detection limit
of the Shelby method.
[0020] The Shelby detection method is published in "Reaction of
hydrogen with hydroxyl-free vitreous silica" (J. Appl. Phys., Vol.
51, No. 5 (May 1980), pp. 2589-2593). Quantitatively, this results
in a number of oxygen deficient defects or oxygen excess defects in
the glass structure of not more than about 10.sup.17 per gram
quartz glass.
[0021] On the basis of this method, the content of SiH groups is
also determined, with a calibration being carried out on the basis
of the chemical reaction: Si--O-Si+H.sub.2.fwdarw.Si-H+Si-OH. SiH
groups and hydrogen are in a mutual thermodynamic equilibrium. The
content of SiH groups is less than 2.times.10.sup.17
molecules/cm.sup.3 in the quartz glass of the component according
to the invention, and the hydrogen content is in the range of
0.1.times.10.sup.16 molecules/cm.sup.3 to 1.0.times.10.sup.18
molecules/cm.sup.3.
[0022] Hence, SiH-- groups are formed by reaction with molecular
hydrogen with breakdown of the SiO.sub.2 network. They are not
desired because a so-called E' center and atomic hydrogen may
evolve from them upon irradiation with energy-rich UV light. The E'
center causes an increased absorption at a wavelength of 210 nm and
is also disadvantageously noticed in the neighboring UV wavelength
range.
[0023] The hydrogen content (H.sub.2) content is determined with
the help of a Raman measurement, as suggested in "Khotimchenko et
al.; Determining the Content of Hydrogen Dissolved in Quartz Glass
Using the Methods of Raman Scattering and Mass Spectrometry,"
Zhurnal Prikladnoi Spektroskopii, Vol. 46, No. 6 (June 1987), pp.
987-991.
[0024] The content of hydroxyl groups is in the range between 0.1
and 100 wt. ppm, preferably between 10 and 60 wt. ppm. The hydroxyl
group content is obtained from the measurement of the IR absorption
according to the method of D. M. Dodd et al. (see, e.g., "Optical
Determinations of OH in Fused Silica", (1966), pp. 3911).
[0025] With a decreasing hydroxyl group content, the viscosity of
quartz glass is increasing. The low hydroxyl group content of less
than 100 wt. ppm leads to a more rigid glass structure and improves
the behavior toward a local anisotropic density change,
particularly in the case of linearly polarized UV radiation. It has
also been assumed that the density change upon compaction is
accompanied by a rearrangement of hydroxyl groups, this
rearrangement mechanism being the more likely and easier, the more
hydroxyl groups are available.
[0026] The fictive temperature of the glass structure is less than
1070.degree. C., preferably less than 1055.degree. C. Its
measurement method, based on a measurement of the Raman scattering
intensity at a wave number of about 606 cm.sup.-1, is described
"The UV-induced 210 nm absorption band in fused Silica with
different thermal history and stoichiometry," Ch. Pfleiderer et.
al., J. Non-Cryst. Solids 159 (1993) 145-143.
[0027] The content of fluorine is preferably less than 10 wt. ppm
and the content of chlorine is preferably less than 0.1 wt. ppm.
Halogens may react with the SiO.sub.2 glass network with breakdown
and can thereby weaken the network.
[0028] It has been found that the predictability of the compaction
behavior upon use of the quartz glass with UV laser radiation of
193 nm is more reliable by measurement at a wavelength of 633 nm,
as the ratio of first and second measured value is relatively
smaller.
[0029] Therefore, in a preferred embodiment of the component, upon
measurement of the laser-induced refractive index change, the
following is applicable to the ratio of the first measured value
M.sub.193 nm and the second measured value M.sub.633 nm:M.sub.193
nm/M.sub.633 nm<1.6, and more preferably M.sub.193 nm/M.sub.633
nm<1.55.
[0030] In one embodiment according to the invention, a method for
producing the optical component comprises: [0031] a) producing a
porous soot body of SiO.sub.2 by flame hydrolysis of a
silicon-containing start sub stance, [0032] b) drying the soot
body, [0033] c) treating the soot body in an oxidizing atmosphere
containing N.sub.2O, with the proviso that a hydroxyl group content
is set in the soot body by drying according to method step (b) and
by treatment according to method step (c) in such a manner that due
to [0034] d) subsequent sintering of the soot body, a semifinished
product of quartz glass is obtained that has a mean hydroxyl group
content in the range between 0.1 and 100 wt. ppm, [0035] e) shaping
the semifinished product into a blank of quartz glass and annealing
the blank, such that the blank has a mean fictive temperature of
less than 1070.degree. C., and [0036] f) loading the blank with
hydrogen by heating in a hydrogen-containing atmosphere at a
temperature below 400.degree. C. while producing a mean hydrogen
content in the range of 0.1.times.10.sup.16 molecules/cm.sup.3 to
1.0.times.10.sup.18 molecules/cm.sup.3.
[0037] The quartz glass for the optical component according to the
invention is produced according to the so-called "soot method". A
porous body of SiO.sub.2 soot (here called "soot body") is obtained
as an intermediate product. The porosity of the soot body makes it
possible to change the chemical composition, and thus also directly
the SiO.sub.2 network structure, and to adapt it to special
demands. Specifically, the concentrations of hydroxyl groups and
halogens can be reduced and set to predetermined values, or
components such as oxygen or nitrogen may be added.
[0038] Drying of the soot body is carried out by heating below the
vitrification temperature either in a halogen-containing atmosphere
or, preferably, under vacuum. Drying leads to a reduction of the
hydroxyl groups, which are contained in the soot body due to the
manufacturing process, to the predetermined value. The reduction is
preferably as uniform as possible. Ideally, the subsequent
treatment steps no longer have any significant influence on the
hydroxyl group content.
[0039] A treatment step of the porous body in an oxidizing
atmosphere containing N.sub.2O is essential for the compaction
behavior of the quartz glass. Dinitrogen monoxide (N.sub.2O)
decomposes at a high temperature into oxygen and reactive nitrogen
atoms and compounds that are able to react with and saturate the
defect sites of the quartz-glass network structure, thereby
eliminating the defect sites. The glass network is thereby
strengthened.
[0040] Apart from the oxidative treatment with N.sub.2O,
after-treatments of the quartz glass blank obtained from the soot
body after vitrification make a further substantial contribution to
this effect. On the one hand, a mean fictive temperature of less
than 1070.degree. C. is set by annealing the blank; on the other
hand, the blank is loaded with hydrogen by heating in a
hydrogen-containing atmosphere.
[0041] Subsequently, the soot body is vitrified under vacuum into a
cylindrical quartz-glass blank. Molecular hydrogen which is
introduced into the quartz glass in the flame hydrolysis method and
which otherwise would further react forming undesired SiH groups in
the subsequent hot treatment steps is removed by the vacuum.
[0042] After vitrification, a quartz glass blank with a hydroxyl
group content ranging between 0.1 and 100 wt. ppm is obtained. The
quartz glass blank is substantially free of SiH groups and of
hydrogen (i.e., the content of the two components is below the
detection limit).
To reduce mechanical stresses as well as birefringence and to
produce a compaction-resistant glass structure, the quartz glass
blank is subjected to an annealing treatment which is carried out
such that, measured over the volume, a mean fictive temperature of
less than 1070.degree. C., preferably less than 1055.degree. C., is
obtained. It has been found that a comparatively dense network
structure is thereby produced, which counteracts further (local)
compaction by UV radiation.
[0043] The defect-healing action of hydrogen is known. Therefore,
depending on the application and on the projected service life of
an optical component, a certain hydrogen content is often
predetermined, even if other disadvantages have to be accepted in
return. After annealing, the hydrogen content of the quartz glass
is, however, below the resolution limit of the measurement method.
The quartz glass is subsequently loaded with hydrogen. Preferably,
the loading with hydrogen is carried out at a temperature below
400.degree. C., preferably below 350.degree. C., because in a
thermodynamic equilibrium, SiH groups are formed at elevated
temperatures in the quartz glass at the presence of hydrogen at
elevated temperatures. The particularly low loading temperature
prevents or avoids this. Upon irradiation with energy-rich UV
light, SiH groups may form so-called E' centers which, in turn,
cause an enhanced absorption at a wavelength of 210 nm, which can
be disadvantageously noticed also in the neighboring wavelength
range of the applied radiation.
[0044] A mean hydrogen content in the range of 0.1.times.10.sup.16
molecules/cm.sup.3 to 1.0.times.10.sup.18 molecules/cm.sup.3 is
set. Due to the low loading temperature, a high hydrogen partial
pressure is instrumental in achieving an adequate hydrogen loading
within economically reasonable treatment periods. The hydrogen
partial pressure is therefore preferably between 1 and 150 bar.
[0045] An increased pressure accelerates not only hydrogen loading,
but may also contribute to a somewhat compacter glass structure of
increased density that is resistant to local anisotropic density
change.
[0046] The outcome of the manufacturing method is a cylinder of
quartz glass with a specific compaction behavior. After irradiation
with the specific radiation dose of the wavelength of 193 nm (with
5.times.10.sup.9 pulses with a pulse width of 125 ns and an energy
density of 500 .mu.J/cm.sup.2 each time and a pulse width
repetition frequency of 2000 Hz), the quartz glass reacts with a
maximum value of the laser-induced refractive index change, the
amount of which upon measurement with the applied wavelength of 193
nm yields a first measured value M.sub.193 nm and upon measurement
with a measurement wavelength of 633 nm yields a second measured
value M.sub.633 nm, where M.sub.193 nm/M.sub.633 nm<1.7; and
preferably, M.sub.193 nm/M.sub.633 nm is less than 1.6, and more
preferably M.sub.193 nm/M.sub.633 nm is less than 1.55.
[0047] It is important, particularly at a sufficiently small ratio
value, that the ratio remains constant upon further irradiation
under the same irradiation conditions, which permits a reliable
prediction of the damage upon further irradiation with the applied
radiation, based on a measurement at 633 nm.
[0048] The level of the constant value of the ratio M.sub.193
nm/M.sub.633 nm is particularly strongly influenced by the maximum
loading temperature in the hydrogen treatment and by the mean
hydrogen concentration produced thereby in the quartz glass. The
hydrogen loading temperature can be regarded as a measure of the
number of defect sites and SiH groups.
[0049] The optical component for use in microlithography for the
applied wavelength of 193 nm is obtained from the quartz glass
cylinder by standard after-treatment steps, such as cutting,
grinding, polishing.
[0050] To avoid contact of the SiO.sub.2 network with
halogen-containing drying reagents, the drying of the soot body
according to method step (b) is preferably carried out purely
thermally under vacuum or in inert gas, and comprises a treatment
of the soot body at a drying temperature in the range between
100.degree. C. and 1350.degree. C., preferably at not more than
1300.degree. C.
[0051] An input of halogens into the soot body is avoided by
dispensing with halogen-containing drying reagents, so that these
do not have to be removed again later. On the other hand, oxygen
defects are created due to the long-winded thermal treatment under
reducing conditions, because a suitable substituent is not directly
available for the removed OH groups. The oxygen defects impair the
UV radiation resistance of the quartz glass.
[0052] Accordingly, after completion of the drying treatment, the
soot body is treated in N.sub.2O-containing atmosphere at the same
or a lower temperature. Treatment temperatures of less than
600.degree. C., preferably below 500.degree. C., are particularly
useful. The N.sub.2O content of the atmosphere is between 0.1 and
10 vol.-%, preferably between 0.5 and 5 vol.-%; the treatment
duration is at least 10 h.
[0053] At nitrogen oxide contents below 0.1 vol.-%, a small
oxidative effect is achieved, and at nitrogen oxide contents of
more than 10 vol.-%, the SiO.sub.2 network may be overloaded with
nitrogen and bubbles may form in the subsequent vitrification. The
treatment is carried out at such a low temperature that the
porosity of the soot body is maintained. At treatment temperatures
of less than 200.degree. C., the reactivity of N.sub.2O is however
very low and long treatment periods are needed for achieving a
noticeable effect with respect to the saturation of oxygen
defects.
[0054] Sample Preparation
[0055] A soot body is produced by flame hydrolysis of SiCl.sub.4
and the OVD method. The soot body is dehydrated under vacuum at a
temperature of 1200.degree. C. for 50 hours in a heating furnace
having a heating element of graphite. The graphite in the heating
furnace produces reducing conditions. If the soot body is
immediately vitrified after this treatment stage, quartz glass that
includes oxygen defects in the order of 1.7.times.10.sup.16
cm.sup.-3 are obtained.
[0056] The thermally dried soot body is subsequently heated in an
oxidizing atmosphere. The soot body is continuously heated in a
treatment chamber with a treatment gas of dinitrogen monoxide
(N.sub.2O; 1.5 vol.-%) in a carrier gas stream of nitrogen to a
temperature of 450.degree. C. and kept at this temperature for 20
hours.
[0057] Subsequently, the dried and aftertreated soot body is
vitrified in a sintering furnace at a temperature of about
1400.degree. C. under vacuum (10.sup.-2 mbar) into a transparent
quartz glass blank. The blank is subsequently homogenized by
thermo-mechanical homogenization (twisting) and formation of a
quartz glass cylinder.
[0058] After completion of the homogenization treatment, the
hydroxyl group content of the soot body is about 25 wt. ppm.
[0059] To reduce mechanical stresses as well as birefringence and
to produce a compaction-resistant glass structure, the quartz glass
cylinder is subjected to an annealing treatment in which the quartz
glass cylinder is heated in air and at an atmospheric pressure to
1190.degree. C. for a holding period of 8 hours and is subsequently
cooled at a cooling rate of 4.degree. C./hour to a temperature of
1050.degree. C. and is kept at the lower temperature for 4 hours.
Thereupon, the quartz glass cylinder is cooled at a higher cooling
rate of 50.degree. C./hour to a temperature of 300.degree. C.,
whereupon the furnace is switched off and the quartz glass cylinder
is allowed to cool freely in the furnace.
[0060] The quartz glass cylinder treated in this way has an outer
diameter of 350 mm and a thickness of 60 mm. Measured over the
thickness, a mean fictive temperature of 1065.degree. C. is
obtained.
[0061] The quartz glass cylinder is subsequently loaded with
hydrogen. The two-stage treatment is carried out in an atmosphere
of 100 vol.-% hydrogen by heating at a temperature T.sub.loading of
380.degree. C., first at a pressure p1.sub.loading of 11 bar and
for a holding period t1.sub.loading of 30 hours, and subsequently
at a pressure p2.sub.loading of 1 bar and for a holding period
t2.sub.loading of 80 hours.
[0062] The quartz glass cylinder obtained thereafter is
substantially free of chlorine oxygen defect sites and SiH groups
(below the detection limit of 5.times.10.sup.16
molecules/cm.sup.3), and is distinguished within a diameter of 280
(CA area) by a mean hydrogen content of 40.times.10.sup.16
molecules/cm.sup.3 and a hydroxyl group content of 25 wt. ppm.
[0063] Table 1 summarizes the parameters of the individual method
steps and the measurement results for the above-described Sample 1
and for further Samples 2 to 7 which are produced in a similar
way.
TABLE-US-00001 TABLE 1 Sample 1 2 3 4 5 6 7 T.sub.N2O treatment
(.degree. C.)/ 550/20 550/20 550/20 -- 450/20 -- -- t.sub.N2O
treatment (hours) T1.sub.annealing (.degree. C.)/ 1190/8 1190/8
1190/8 1190/8 1190/8 1190/8 1190/8 t1.sub.annealing (hours)
T2.sub.annealing (.degree. C.)/ 1050/4 1100/4 1050/4 1050/4 980/4
1100/4 1070/4 t2.sub.annealing (hours) T.sub.loading (.degree. C.)
380.degree. C. 400.degree. C. 400 425.degree. C. 380.degree. C.
425.degree. C. 450.degree. C. t1.sub.loading (hours)/ 30 h @ 50 h @
30 h @ 30 h @ 30 h @ 15 h @ 6 h @ p1.sub.loading (bar) 11 bar 100
bar 5 bar 5 bar 11 bar 50 bar 11 bar t2.sub.loading (hours)/ 80 h @
70 h @ 70 h @ 70 h @ 80 h @ 70 h @ 70 h @ p2.sub.loading (bar) 1
bar 25 bar 0.9 bar 0.9 bar 1 bar 6 bar 0.6 bar OH (wt. ppm) 25 21
22 37 35 38 40 Mean value of Tf 1065 1102 1067 1060 1054 1105 1075
(.degree. C.) Hydrogen cont. 40 850 30 30 40 200 20
(.times.10.sup.16 molecules/cm.sup.3) SiH content 6 212 15 20 4 100
20 (.times.10.sup.16 molecules/cm.sup.3) M.sub.193 nm/ M.sub.633 nm
1.55 4.70 1.69 1.75 1.53 15.27 1.75
[0064] Compaction Measurement
[0065] All of the Samples 1-7 were irradiated with radiation of a
wavelength of 193 nm, which is characterized by the following
dose:
[0066] Pulse number: 5.times.10.sup.9 pulses;
[0067] Pulse width: 125 ns;
[0068] Energy density: 500 .mu.J/cm.sup.2 each time; and
[0069] Pulse repetition frequency: 2000 Hz.
[0070] In the sample irradiated in this way, the amount of the
local maximum refractive-index change as compared with the
non-irradiated glass is determined, namely both by measurement with
a measurement wavelength of 193 nm (amount of the maximum
refractive-index change: M.sub.193 nm) and by measurement with a
measurement wavelength of 633 nm (amount of the maximum
refractive-index change: M.sub.633 nm). The ratio of the measured
values M.sub.193 nm/M.sub.633 nm is indicated in the last row of
Table 1.
[0071] The measurement results show that the ratio M.sub.193
nm/M.sub.633 nm can be regarded as a quality reference to a small
and predictable compaction, and is obviously considerably
determined by the parameters in the after-treatment of the soot
body in a N.sub.2O-containing atmosphere and after-treatment of the
vitrified quartz glass blank in an H.sub.2-containing atmosphere.
Also, the intensity of the N.sub.2 treatment for the elimination of
oxygen defect sites and the temperature during hydrogen loading
play a decisive role in preventing SiH groups.
[0072] This is also demonstrated by the further measurement results
discussed hereinafter with reference to the diagrams of FIGS. 1 to
3.
[0073] Specifically, for Sample 3, FIG. 1 shows the development of
the ratio V (M.sub.193 nm/M.sub.633 nm) with the irradiation dose
("dose") as a product of the energy density to the square and pulse
number divided by the pulse width in time in ns (in the unit
(J/cm.sup.2).sup.2/ns).
[0074] Thus, the ratio M.sub.193 nm/M.sub.633 nm first rises
steeply at the irradiation beginning from 1.0 to about 1.69, and
remains thereafter (after a pulse number of about 3.times.108
pulses, or after a dose D of about 3 J/cm.sup.2).sup.2/ns)
approximately constant at this value (hereinafter also called
"final value").
[0075] Corresponding tests were carried out for other quartz glass
qualities. Samples 1 and 5 showed similar profiles of the ratio
M.sub.193 nm/M.sub.633 nm with the irradiation dose. An initially
stronger rise of the ratio M.sub.193 nm/M.sub.633 nm to more than
1.7 was found in the remaining samples and also a less constant
profile with an increasing irradiation dose. These are comparative
samples (i.e., Samples 2, 4, 6 and 7).
[0076] In Samples 1, 3 and 5, due to the substantially constant
ratio M.sub.193 nm/M.sub.633 nm, the degree of compaction can be
reliably indicated by continuous measurements at a wavelength of
633 nm by using the quartz glass with UV laser radiation of 193
nm.
[0077] Based on the results of numerous measurements of such a
type, it was found that if the quartz glass has been subjected to
an adequately oxidative treatment under N.sub.2O, the parameters
that influence the final value of the ratio can ultimately be
summarized in the loading temperature in the case of hydrogen
loading (T.sub.loading in .degree. C.) and in the mean hydrogen
concentration (C.sub.H2 in 10.sup.17 molecule/cm.sup.3) produced in
the quartz glass.
[0078] Thus, the temperature during loading of the quartz glass
with hydrogen is of relevance to the formation of SiH groups. The
lower the temperature, the lower the SiH group concentration
evolving in the thermal equilibrium. On the other hand, hydrogen
loading is diffusion-controlled, so that low loading temperatures,
depending on the diffusion length and an acceptable concentration
gradient, require long treatment periods.
[0079] The loading process is energy- and time-consuming and,
therefore, as short and "cold" as possible, but must be carried out
for such a long period as is needed for ensuring a given compaction
behavior of the quartz glass. This estimation has so far been an
empirical one. However, it has been found that the following
equation is suited for estimating the final value for the ratio
M.sub.193 nm/M.sub.633 nm:
M.sub.193 nm/M.sub.633
nm=1.47+0.0345.times.2.sup.((Tloading-400)/25).times.C.sub.H2
(1)
[0080] Thus, at the moment, the final value has a limit of 1.47
that must be reached. Additional contributions are due to the
parameters of the hydrogen loading. Based on the calculation model
(1), the final value can thus be estimated for the ratio M.sub.193
nm/M.sub.633 nm and thus the compaction tendency of the quartz
glass toward UV radiation of a wavelength of 193 nm, particularly
in the case of linearly polarized radiation, and hydrogen loading
can thereby be optimized.
[0081] The validity of this model assumption is demonstrated by
FIG. 2. Referring to FIG. 2, the final value of the ratio M.sub.193
nm/M.sub.633 nm, which is determined on the basis of the
above-indicated model calculation (1), is plotted for some of the
samples of Table 1 against the actually-measured final values after
irradiation with the above-specified irradiation dose. The measured
values are located almost exactly along a straight line with the
slope 1, which can be regarded as proof of the correctness of the
model according to equation (1).
[0082] The result becomes more comprehensible with a look at the
three-dimensional modeling of FIG. 3. Referring to FIG. 3, the
final value of the ratio M.sub.193 nm/M.sub.633 (as a measure of
the compaction tendency of the quartz glass) is plotted against the
hydrogen content of the quartz glass C.sub.H2 in 10.sup.17
molecules/cm.sup.3 (x-axis) and against the temperature
T.sub.loading in .degree. C. with the hydrogen loading (z-axis).
The loading temperature T.sub.loading is a measure of the SiH
concentration at the same time.
[0083] Thus, the compaction tendency normally increases strongly
with the loading temperature and slightly with the hydrogen
concentration. A certain hydrogen concentration is often given.
This specification can basically be met at a high loading
temperature within a short period of time and at a low loading
temperature within a longer period of time. For the former, an
increased compaction tendency follows automatically; for the
latter, there is an economically more troublesome production
process.
[0084] If, in addition to the hydrogen concentration, the maximally
admissible compaction tendency is also given, the highest, but
still acceptable loading temperature can be determined with the
help of the model and the loading period can thus be shortened to a
minimum.
[0085] It will be appreciated by those skilled in the art that
changes could be made to the embodiments described above without
departing from the broad inventive concept thereof. It is
understood, therefore, that this invention is not limited to the
particular embodiments disclosed, but it is intended to cover
modifications within the spirit and scope of the present invention
as defined by the appended claims.
* * * * *