U.S. patent application number 12/455164 was filed with the patent office on 2009-09-24 for optical component quartz glass.
This patent application is currently assigned to Heraeus Quarzglas GmbH & Co. KG. Invention is credited to Steffen Kaiser, Bodo Kuhn, Stephan Thomas.
Application Number | 20090239732 12/455164 |
Document ID | / |
Family ID | 34934499 |
Filed Date | 2009-09-24 |
United States Patent
Application |
20090239732 |
Kind Code |
A1 |
Kuhn; Bodo ; et al. |
September 24, 2009 |
Optical component quartz glass
Abstract
Starting from an optical component of quartz glass for
transmitting ultraviolet radiation of a wavelength between 190 nm
and 250 nm, with a glass structure essentially without oxygen
defects, a hydrogen content ranging from 0.1.times.10.sup.16
molecules/cm.sup.3 to 5.0.times.10.sup.16 molecules/cm.sup.3, and
with a content of SiH groups of less than 5.times.10.sup.16
molecules/cm.sup.3, to provide such a component which is
particularly well suited for use with linearly polarized UV laser
radiation, the present invention suggests that the component should
have a content of hydroxyl groups ranging from 10 to 250 wt ppm and
a fictive temperature above 1000.degree. C.
Inventors: |
Kuhn; Bodo; (Hanau, DE)
; Thomas; Stephan; (GrossKrotzenburg, DE) ;
Kaiser; Steffen; (Hanau, DE) |
Correspondence
Address: |
BRIAN J. EASTLEY
24 BIRD ROAD
MANSFIELD
MA
02048
US
|
Assignee: |
Heraeus Quarzglas GmbH & Co.
KG
Hanau
DE
|
Family ID: |
34934499 |
Appl. No.: |
12/455164 |
Filed: |
May 29, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11093318 |
Mar 30, 2005 |
7552601 |
|
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12455164 |
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Current U.S.
Class: |
501/43 ;
501/41 |
Current CPC
Class: |
G03F 7/70341 20130101;
G03F 7/70958 20130101; C03C 2203/54 20130101; C03B 2201/23
20130101; C03C 4/0085 20130101; C03C 4/0071 20130101; G03F 7/70966
20130101; G02B 1/00 20130101; C03C 3/06 20130101; C03C 2203/52
20130101; C03B 19/1453 20130101; C03C 2201/23 20130101; C03B
2201/21 20130101; C03C 2203/44 20130101; C03B 19/1469 20130101;
C03C 2201/21 20130101 |
Class at
Publication: |
501/43 ;
501/41 |
International
Class: |
C03C 3/23 20060101
C03C003/23; C03C 3/12 20060101 C03C003/12 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 2, 2004 |
DE |
10 2004 017 031.2 |
Claims
1. An optical component of quartz glass for transmitting
ultraviolet radiation of a wavelength in a range of 190 nm to 250
nm, said optical component comprising quartz glass having a glass
structure substantially without oxygen defects, having a hydrogen
content ranging from 0.1.times.10.sup.16 molecules/cm.sup.3 to
5.0.times.10.sup.16 molecules/cm.sup.3, and having a content of SiH
groups of <less than 5.times.10.sup.16 molecules/cm.sup.3,
wherein the quartz glass has a content of hydroxyl groups ranging
from 10 to 250 wt ppm and a fictive temperature above 1000.degree.
C.
2. The optical component according to claim 1, wherein the fictive
temperature is above 1050.degree. C.
3. The optical component according to claim 1, wherein the content
of hydroxyl groups is in a range of 30 to 200 wt ppm.
4. The optical component according to claim 1, wherein the quartz
glass has a content of fluorine below 100 wt ppm.
5. (canceled)
6. (canceled)
7. (canceled)
8. (canceled)
9. (canceled)
10. (canceled)
11. (canceled)
12. (canceled)
13. (canceled)
14. (canceled)
15. (canceled)
16. The optical component according to claim 1, wherein the fictive
temperature is above 1100.degree. C.
17. The optical component according to claim 3, wherein the content
of hydroxyl groups is below 125 wt ppm.
Description
[0001] The present invention relates to an optical component of
quartz glass for transmitting ultraviolet radiation of a wavelength
between 190 nm and 250 nm with a glass structure essentially
without oxygen defects, a hydrogen content ranging from
0.1.times.10.sup.16 molecules/cm.sup.3 to 5.0.times.10.sup.16
molecules/cm.sup.3 and a content of SiH groups of
<5.times.10.sup.16 molecules/cm.sup.3.
[0002] Furthermore, the present invention relates to a method for
producing such an optical component of quartz glass, and to the use
thereof.
[0003] Methods for producing synthetic quartz glass by oxidation or
by flame hydrolysis of silicon-containing start substances are
generally known under the names VAD (vapor-phase axial deposition),
OVD (outside vapor phase deposition), MCVD (modified chemical vapor
deposition) and PCVD (or also PECVD; plasma enhanced chemical vapor
deposition) methods. In all of these procedures SiO.sub.2 particles
are normally produced by means of a burner and deposited in layers
on a carrier which is moved relative to a reaction zone. At a
sufficiently high temperature in the area of the carrier surface,
"direct vitrification" of the SiO.sub.2 particles takes place. By
contrast, in the so-called "soot method" the temperature is so low
during deposition of the SiO.sub.2 particles that a porous soot
layer is obtained which is sintered in a separate step of the
method to obtain transparent quartz glass. Both direct
vitrification and soot method produce a dense transparent synthetic
quartz glass of high purity in the form of rods, blocks, tubes or
plates, which are further processed into optical components such as
lenses, windows, filters, mask plates for use, for instance, in
microlithography.
[0004] EP-A 401 845 describes methods for producing plate-shaped
quartz glass blanks by direct vitrification and according to the
soot method. To reduce mechanical stresses inside the blanks and to
achieve a homogeneous distribution of the fictive temperature, the
blanks are normally annealed with great care. An annealing program
is suggested in which the blank is subjected to a holding time of
50 hours at a temperature of about 1100.degree. C. and is
subsequently cooled in a slow cooling step at a cooling rate of
2.degree./h to 900.degree. and then cooled in the closed furnace to
room temperature.
[0005] A similar method for producing a component of synthetic
quartz glass for use in microlithography according to the soot
method is also known form EP 1 125 897 A1.
[0006] A quartz glass blank for an optical component of the
above-mentioned type is described in DE 101 59 961 C2. Such optical
components of quartz glass are used for transmitting high-energy
ultraviolet laser radiation, for instance in the form of optical
exposure systems in microlithography devices for producing
large-scale integrated circuits in semiconductor chips. The
exposure systems of modern microlithography devices are equipped
with excimer lasers emitting high-energy pulsed UV radiation of a
wavelength of 248 nm (KrF laser) or 193 nm (ArF laser).
[0007] In microlithographic projection exposure systems the demand
is in general made that a light distribution provided in the area
of a pupil plane of the exposure system should be transmitted as
homogeneously as possible and in an angle-maintaining manner into a
pupil plane of the projection lens conjugated relative to the pupil
plane of the exposure system. Each change in the angular spectrum
that is created in the optical path leads to a distortion of the
intensity distribution in the lens pupil, which leads to an
asymmetrical irradiation and thus to a deterioration of the imaging
performance. Linearly polarizing light sources, such as excimer
lasers, normally have a degree of polarization of about 90% to 95%.
With the help of a .lamda./4 plate the light is circularly
polarized and should ideally be maintained in this circular state
up to the wafer to be exposed.
[0008] In this context birefringence plays an important role
because it impairs the imaging fidelity of optical components of
quartz glass. Stress birefringence in the quartz glass is, for
instance, created during inhomogeneous cooling of the blank used
for the optical component to be produced, or by the UV irradiation
itself.
[0009] Recently, experiments were carried out with projection
systems which operate with a technique called "immersion
lithography". The gap between the image plane and the last optical
component of the lens system is here filled with a liquid (usually
deionized water) with a higher refractive index than air, ideally
with the refractive index of the quartz glass at the wavelength
used. The higher refractive index of the liquid in comparison with
air entails a greater numerical aperture of the optical component,
thereby improving the imaging characteristics. "Immersion
lithography" is however polarization sensitive; the best results
will be obtained when linearly polarized laser radiation is used
and not, as is otherwise standard practice, completely or partly
circularly polarized laser radiation. It is described in "N. F.
Borelli, C. M. Smith, J. J. Price, D. C. Allan "Polarized excimer
laser-induced birefringence in silica", Applied Physics Letters,
Vol. 80, No. 2, (2002), p. 219-221" that the use of linearly
polarized UV laser radiation seriously damages the glass structure
of the optical quartz glass component, which will be explained in
more detail in the following.
[0010] The so-called "compaction" of the quartz glass after
irradiation with short-wave UV radiation is expressed in a local
density increase of the glass in the irradiated volume. This leads
to a locally inhomogeneous rise of the refractive index and thus to
a deterioration of the imaging characteristics of the optical
component. It has now been found that circularly polarized UV
radiation effects a rather isotropic density change, and linearly
polarized UV radiation a rather anisotropic density change. The
difference will be explained with reference to FIG. 4.
[0011] The diagram of FIG. 4a) schematically shows a volume element
40 (symbolized by its position and extension along the x-axis)
which is irradiated with UV radiation of an energy density of 0.08
(in relative units) (circular irradiation spot).
[0012] FIG. 4b) shows the result of irradiation upon use of
circularly polarized UV radiation. After irradiation the density of
the irradiated volume element is, on the whole, higher than the
density of the surrounding quartz glass (isotropic density change).
In the area of the transition between compacted and non-compacted
material, stresses are therefore created that are optically
expressed as stress birefringence. In the two-dimensional
illustration of FIG. 4b, these stresses are illustrated around the
edge of the circular irradiation spot as maxima 41, 42 of the
stress birefringence. In a top view on the volume element 40, the
maxima 41, 42 belong to a ring extending around the volume 40. Once
produced, this isotropic density and refractive index change
(stress birefringence) effects a change in the imaging
characteristics of the lens. Due to its circular symmetry said
change, however, has substantially the same effect during later use
of the component; it can thus be calculated.
[0013] By contrast, an irradiation of the volume element 40 with a
linearly polarized UV laser radiation effects an anisotropic
density change, as outlined in FIG. 4c). A maximum of the density
change, and thus also a maximum 43 of the birefringence generated
thereby, is here produced that shows a preferred direction in the
direction of the polarization vector of the incident UV radiation.
The anisotropic density and refractive index change produced
thereby is not substantially radially symmetrical and also effects
a change in the imaging characteristics of the component. This
change is disadvantageous--especially upon a change in the
polarization direction of the transmitted UV radiation, which must
be expected in the course of the lifetime of the component, because
its influence on imaging can hardly be calculated. Therefore, such
a pre-damaged quartz glass component is hardly suited for other
applications, which limits the service life of the optical
component.
[0014] It is the object of the present invention to provide an
optical component which is particularly suited for use with
linearly polarized UV laser radiation, and which even after use
with linearly polarized radiation can still be used in a variable
manner. Moreover, it is the object of the present invention to
indicate a method for producing such an optical component, and a
special use therefor.
[0015] As for the optical component, this object is achieved
according to the invention by an embodiment of the component which
combines the following properties: [0016] a glass structure
essentially without oxygen defects, [0017] an H.sub.2 content
ranging from 0.1.times.10.sup.16 molecules/cm.sup.3 to
5.0.times.10.sup.16 molecules/cm.sup.3, [0018] a content of SiH
groups of less than 5.times.10.sup.16 molecules/cm.sup.3, [0019] a
content of hydroxyl groups ranging from 10 to 250 wt ppm, and
[0020] a fictive temperature above 1000.degree. C.
[0021] Ideally, the properties (fictive temperature) are constant
over the used volume of the optical component and the indicated
components are evenly distributed. The concentration and
temperatures indicated above are mean values within the optically
used range of the component (also designated as "CA (clear
aperture) area" or "optically used volume").
[0022] A glass structure that is substantially free from oxygen
defects is here understood to mean a glass structure in which the
concentrations of oxygen deficiency defects and excess oxygen
defects are below the detection limit of the method of Shelby. Said
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 yields a number of
oxygen deficiency defects or excess oxygen defects in the glass
structure of not more than 10.sup.17 per gram quartz glass.
[0023] The hydrogen content (H.sub.2 content) is determined by a
Raman measurement, which was first suggested by 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 SiH groups is determined by means of Raman
spectroscopy, a calibration being carried out on the basis of a
chemical reaction: Si--O--Si+H.sub.2.fwdarw.Si--H+Si--OH, as
described in Shelby "Reaction of hydrogen with hydroxyl-free
vitreous silica" (J. Appl. Phys., Vol. 51, No. 5 (May 1980), pp.
2589-2593).
[0025] The hydroxyl group content (OH content) follows from a
measurement of the IR absorption according to the method of D. M.
Dodd et al. ("Optical Determinations of OH in Fused Silica", p.
3911).
[0026] The fictive temperature is a parameter which characterizes
the specific network structure of the quartz glass. A standard
measuring method for determining the fictive temperature by way of
a measurement of the Raman scattering intensity at a wavelength of
about 606 cm.sup.-1 is described in "Ch. Pfleiderer et al.: "The
UV-induced 210 nm absorption band in fused silica with different
thermal history and stoichiometry"; J. Non-Cryst. Solids 159 (1993)
145-143".
[0027] In comparison with the quartz glass described in DE 101 59
961 C2, the quartz glass of the optical component of the invention
is characterized by a comparatively low OH content noz exceeding
250 wt ppm, and particularly by a high fictive temperature.
[0028] Surprisingly, it has been found that an optical component
made from a quartz glass having the above-indicated properties will
only experience a small anisotropic density change upon use with
linearly polarized UV laser radiation.
[0029] This is said to be due to the comparatively low hydroxyl
group content of the quartz glass and its relatively high fictive
temperature. With a decreasing hydroxyl group content of a quartz
glass the viscosity thereof is increasing. On the other hand, it is
known that quartz glass (with a high fictive temperature) which is
rapidly cooled from the temperature range between 1000.degree. C.
and 1500.degree. C. has a lower specific volume and thus a higher
specific density than quartz glass (with a low fictive temperature)
which is cooled at a slow rate. According to "R. Bruckner, Silicon
Dioxide; Encyclopedia of Applied Physics, Vol. 18 (1997), pp.
101-131", this effect is due to an anomaly of synthetic quartz
glass in the case of which the evolution of the specific volume in
the range between 1000.degree. and 1500.degree. C. has a negative
temperature coefficient, i.e., the specific volume of quartz glass
increases in this temperature range with a decreasing temperature,
or in other words, the quartz glass rapidly cooled from the said
temperature range and having a high fictive temperature has a
higher density than quartz glass which is cooled at a slow rate and
has a low fictive temperature. The density of the quartz glass
which is also higher due to the higher fictive temperature acts
like an "anticipated" compaction of the glass structure on the
whole. In this respect the compact network structure counteracts
the effect of a local compaction upon UV radiation. It has been
found that the portion of a compaction that is due to an isotropic
density change can thus be reduced, and it must be expected that
this will also reduce the risk of an anisotropic density change
with respect to linearly polarized UV radiation.
[0030] Apart from the enhanced viscosity, the low OH content may
also show another important aspect with respect to the prevention
of an anisotropic density change. It is assumed that the change in
density is accompanied by a rearrangement of hydroxyl groups, this
rearrangement mechanism being all the more likely and easier the
more hydroxyl groups are available. The low hydroxyl group content
and the increased density (high fictive temperature) of the quartz
glass therefore reduce the sensitivity of the glass structure over
a local anisotropic density change. The quartz glass component of
the invention thus withstands the compaction effect of UV radiation
in a better way than the known quartz glass qualities, so that it
is particularly well suited for use in the transmission of linearly
polarized UV radiation having a wavelength of between 190 nm and
250 nm.
[0031] It has turned out to be particularly advantageous when the
quartz glass has a fictive temperature above 1050.degree. C.,
preferably above 1100.degree. C.
[0032] The higher the fictive temperature of the quartz glass, the
higher is its density and the more pronounced the above-described
effect of the "anticipated" compaction of the quartz glass on the
whole, and thus the resistance to a local anisotropic density
increase by linearly polarized UV radiation. At very high fictive
temperatures (>1200.degree. C.) this positive effect, however,
may be impaired by excessively high and thermally created stress
birefringence.
[0033] As for a high viscosity of the quartz glass, preference is
given to an embodiment of the optical component in which the quartz
glass has a content of hydroxyl groups between 30 and 200 wt ppm,
preferably below 125 wt ppm.
[0034] The low hydroxyl group content effects an increase in
viscosity. The accompanying improvement of the behavior over a
local anisotropic density change is surprising insofar as it is
assumed in the above-mentioned DE 101 59 961 C2 that a quartz glass
having a hydroxyl group content of less than 125 wt ppm, as is
typical of the quartz glass produced according to the soot method,
tends to compaction.
[0035] The viscosity increasing effect of the comparatively low
hydroxyl group content can be compensated by a high fluorine
content completely or in part. Therefore, the quartz glass for the
optical component of the invention has preferably a content of
fluorine of less than 100 wt ppm. Moreover, fluorine reduces the
refractive index of quartz glass so that the variability during use
is reduced in the case of a quartz glass doped with fluorine
(.gtoreq.100 wt ppm).
[0036] As for the method, the above-indicated object is achieved
according to the invention by a method comprising the following
steps: [0037] producing an SiO.sub.2 soot body, [0038] vitrifying
the soot body under vacuum with formation of a cylindrical quartz
glass blank with a hydroxyl group content ranging between 10 and
250 wt ppm, preferably between 30 and 200 wt ppm, and particularly
preferably below 125 wt ppm, [0039] annealing the quartz glass
blank with formation of a quartz glass cylinder, with a fictive
temperature above 1000.degree. C., preferably above 1050.degree.
C., and particularly preferably above 1100.degree. C., which
surrounds a contour of the optical component to be produced with an
overdimension, [0040] removing part of the axial overdimension in
the area of the faces of the quartz glass cylinder, [0041] loading
the quartz glass cylinder with hydrogen by heating in a
hydrogen-containing atmosphere at a temperature below 500.degree.
C. with generation of a mean hydrogen content in the range of
0.1.times.10.sup.16 molecules/cm.sup.3 to 5.0.times.10.sup.16
molecules/cm.sup.3.
[0042] "Direct vitrification" normally yields quartz glass having
an OH content of 450 to 1200 wt ppm, whereas rather low OH contents
ranging between a few wt ppm and 300 wt ppm are typical of quartz
glass produced according to the "soot method". The quartz glass for
the optical component according to the invention is therefore
preferably produced by means of the "soot method". In this method
an SiO.sub.2 soot body is produced as an intermediate product
having a hydroxyl group content that can be adjusted in a simple
way to a predetermined value through the duration and intensity of
a dehydration treatment.
[0043] The soot body is vitrified under vacuum with formation of a
cylindrical quartz glass blank. Molecular hydrogen is removed by
way of the vacuum. This hydrogen is introduced into the quartz
glass during the flame hydrolysis method due to the production
process and would otherwise further react in subsequent heat
treatment steps to form undesired SiH groups which in the course of
the further treatment steps would be noticed in a disadvantageous
way and would lead to a deterioration of the damage behavior of the
quartz glass component. The vacuum serves to accelerate the
degasification operation.
[0044] After vitrification a quartz glass blank is present with a
hydroxyl group content in the range between 10 and 250 wt ppm,
preferably between 30 and 200 wt ppm, and particularly preferably
below 125 wt ppm, and is substantially free of SiH groups and
hydrogen (the content of both components is below the detection
limit).
[0045] The quartz glass blank is subsequently annealed, attention
being paid to the adjustment of a fictive temperature above
1000.degree. C., preferably above 1050.degree. C., and particularly
preferably above 1100.degree. C. The predetermined fictive
temperature can be maintained by the measures that the quartz glass
blank is held at a temperature within the range of the desired
fictive temperature until the setting of the structural balance and
is then cooled rapidly, or that the blank is cooled at a
sufficiently fast rate from a temperature above the fictive
temperature to be set. Attention must here be paid on the one hand
that the desired high fictive temperature is maintained and that no
stress birefringence is produced on the other hand. The one
precondition (high fictive temperature) is taken into account
through the lower limit of a cooling rate, and the other
precondition (low stress birefringence) through a corresponding
lower limit which will be explained in more detail further
below.
[0046] Due to the setting of a comparatively high fictive
temperature the quartz glass cylinder obtained exhibits residual
stresses which are above all noticed in the more rapidly cooling
peripheral portion of the component. Therefore, a portion which
pertains to the overdimension surrounding the contour of the
optical component to be produced is removed from both faces of the
cylinder. Due to the previous removal of this overdimension (or a
part thereof), the loading duration during subsequent loading of
the quartz glass cylinder with hydrogen is shortened, the loading
duration being required for setting a mean hydrogen content ranging
from 0.1.times.10.sup.16 molecules/cm.sup.3 to 5.0.times.10.sup.16
molecules/cm.sup.3.
[0047] It is known that hydrogen has a healing effect with respect
to defects created by UV irradiation in the quartz glass. In the
method of the invention, the hydrogen content is however reduced to
a considerable extent, e.g. due to the above-explained vacuum
treatment of the soot body. Therefore, the quartz glass is
subsequently loaded with hydrogen. Hydrogen loading takes place at
a low temperature below 500.degree. C. to reduce the formation of
SiH groups. SiH groups in the quartz glass are undesired because a
so-called E' center and atomic hydrogen are formed therefrom upon
irradiation with high-energy UV light. The E' center effects an
increased absorption at a wavelength of 210 nm and is unfavorably
noticed in the adjoining UV wavelength range as well. Due to
thermodynamic conditions SiH groups are increasingly formed at
elevated temperatures (500.degree. C.-800.degree. C.) in the
presence of hydrogen, and the comparatively low OH content of the
quartz glass also shifts the balance towards SiH formation.
[0048] The annealing of the quartz glass blank primarily serves to
reduce stresses, to adjust the desired fictive temperature, and
thus a compaction-resistant glass structure, and it preferably
comprises the following method steps: [0049] holding the quartz
glass blank for a first holding period of at least 4 hours at a
first higher annealing temperature which is at least 50.degree. C.
above the fictive temperature of the quartz glass component to be
set, [0050] cooling at a first lower cooling rate to a second lower
annealing temperature which is in the range between +/-20.degree.
C. around the fictive temperature of the quartz glass component to
be set, [0051] holding at the lower annealing temperature for a
second holding period, and [0052] cooling to a predetermined final
temperature below 800.degree. C., preferably below 400.degree. C.,
at a second higher cooling rate which is at least 25.degree.
C./h.
[0053] It has been found that a high fictive temperature is
accompanied by the generation of a comparatively dense network
structure which counteracts a further local compaction by UV
irradiation and particularly an anisotropic density change by
linearly polarized UV radiation. The above-indicated preferred
annealing program includes heating to a temperature clearly above
the fictive temperature (>50.degree. C.), cooling to a
temperature in the range around the fictive temperature to be set,
and then comparatively rapid cooling of the quartz glass blank to a
low temperature below which no essential changes in the glass
structure are to be expected any more.
[0054] This is a comparatively short annealing method, which
although it might entail drawbacks with respect to stress
birefringence effects an enhanced stability with respect to local
compaction by UV radiation and, apart from saving time, has the
further advantage that due to the comparatively short treatment
duration at a high temperature the formation of inhomogeneities due
to out-diffusion of components and contaminations by diffusing
impurities are avoided.
[0055] A particularly compact network structure is obtained when
the first cooling rate is set in the range between 1.degree. C./h
and 10.degree. C./h, and preferably to a value in the range between
3.degree. and 5.degree. C./h.
[0056] As for a compact glass structure, it has also turned out to
be advantageous when the second cooling rate is set in the range
between 25.degree. and 80.degree. C./h, preferably above 40.degree.
C./h.
[0057] The faster the cooling process, the greater are the
above-mentioned advantages with respect to saving time, reduction
of diffusion effects and action of the "previously compacted" glass
structure.
[0058] In a preferred embodiment of the method of the invention,
the second holding time is between 1 hour and 16 hours.
[0059] The quartz glass is once again given the opportunity to
relax. The temperature distribution inside the quartz glass blank
is homogenized and thermal gradients that lead to stress
birefringence are reduced.
[0060] In this connection and also with respect to an adjustment of
a glass structure that is as fast as possible and near the
predetermined fictive temperature, the first holding time is not
more than 50 hours.
[0061] Advantageously, the quartz glass blank is loaded with
hydrogen at a pressure between 1 and 150 bar.
[0062] An increased pressure accelerates hydrogen loading and may
also have an effect on the density in the sense of a more compact
network structure that is more resistant to a local anisotropic
density change.
[0063] For achieving a small formation of SiH groups a procedure is
preferred in which the quartz glass blank is loaded with hydrogen
at a temperature below 400.degree. C., preferably below 350.degree.
C.
[0064] The optical quartz glass component of the invention or the
optical component produced according to the method of the invention
is characterized by low sensitivity to a local anisotropic density
change upon irradiation with short-wave UV radiation. Therefore, it
is preferably used as an optical component in a projection system
of an automatic exposure machine for immersion lithography for the
purpose of transmitting ultraviolet, pulsed and linearly polarized
UV laser radiation of a wavelength between 190 nm and 250 nm.
[0065] The quartz glass component has turned out to be particularly
stable with respect to UV laser radiation of this wavelength if it
has an energy density of less than 300 .mu.J/cm.sup.2, preferably
less than 100 .mu.J/cm2, and a pulse width in time of 50 ns or
more, preferably 150 ns or more.
[0066] The invention shall now be explained in more detail with
reference to embodiments and a drawing, in which
[0067] FIG. 1 shows a diagram regarding the dependence of the UV
radiation-induced birefringence on the energy dose (energy
density.times.pulse number) of the radiation;
[0068] FIG. 2 a diagram regarding the dependence of the UV
radiation-induced birefringence (slope of the straight line of FIG.
1) on the hydroxyl group content of the quartz glass;
[0069] FIG. 3 a diagram regarding the dependence of the UV
radiation-induced birefringence on the pulse number of the
radiation in two quartz glass qualities that differ in their
fictive temperature;
[0070] FIG. 4 a graph for explaining the isotropic and the
anisotropic density change upon UV radiation.
SAMPLE PREPARATION
[0071] A soot body is produced by flame hydrolysis of SiCl.sub.4
with the help of the known VAD method. The soot body is dehydrated
at a temperature of 1200.degree. C. in a chlorine-containing
atmosphere and then vitrified at a temperature of about
1750.degree. C. in vacuum (10.sup.-2 mbar) to form a transparent
quartz glass blank. This blank is then homogenized by thermally
mechanical homogenization (twisting) and formation of a quartz
glass cylinder. The quartz glass cylinder has then an OH content of
about 250 wt ppm.
Sample 1
[0072] For reducing stresses and birefringence and for producing a
compaction-resistant glass structure, the quartz glass cylinder is
subjected to an annealing treatment which is particularly
characterized by its shortness. The quartz glass cylinder is here
heated to 1130.degree. C. in air and at atmospheric pressure for a
holding time of 8 hours and then cooled at a cooling rate of
4.degree. C./h to a temperature of 1030.degree. C. and held at this
temperature for 4 hours. Thereupon, the quartz glass cylinder is
cooled at a higher cooling rate of 50.degree./h to a temperature of
300.degree. C., whereupon the furnace is switched off and the
quartz glass cylinder is left to the free cooling of the
furnace.
[0073] The quartz glass cylinder treated in this way has an outer
diameter of 350 mm and a thickness of 60 mm. The quartz glass has a
mean fictive temperature of 1035.degree. C. It has been found that
the cylinder exhibits relatively strong stress birefringence
probably due to the rapid cooling from the temperature of
1030.degree. C., particularly in its peripheral portions. Part of
the overdimension with respect to the component contour, namely a
thickness of 3 mm, is removed from the faces of the quartz glass
cylinder before the next treatment step.
[0074] Thereupon, the quartz glass cylinder is held in a pure
hydrogen atmosphere at 380.degree. C. first at a pressure of 10 bar
for a duration of 22 hours and then at a pressure of 0.07 bar for a
duration of 816 hours.
[0075] The quartz glass cylinder obtained thereafter is
substantially free of oxygen defects and SiH groups (below the
detection limit of 5.times.10.sup.16 molecules/cm.sup.3), and it is
characterized within a diameter of 280 mm (CA area) by a mean
hydrogen content of 2.times.10.sup.16 molecules/cm.sup.3 (outside
thereof about 3.6.times.10.sup.15 molecules/cm.sup.3), a hydroxyl
group content of 250 wt ppm and a mean fictive temperature of
1035.degree. C. The quartz glass is not additionally doped with
fluorine; the fluorine content is below 1 wt ppm.
Sample 2
[0076] Another quartz glass cylinder was produced, as described
with reference to sample 1, but hydrogen loading of the quartz
glass cylinder took place in a pure hydrogen atmosphere in a first
process step at 340.degree. C. and at a pressure of 10 bar for a
duration of 8 hours, and in a second process step at 340.degree. C.
at a pressure of 0.007 bar and for a duration of 1570 hours.
[0077] The quartz glass cylinder obtained thereafter is essentially
free from oxygen defects and SiH groups (below the detection limit
of 5.times.10.sup.16 molecules/cm.sup.3), and it is characterized
within a diameter of 280 mm (CA area) by a mean hydrogen content of
about 2.times.10.sup.15 molecules/cm.sup.3 (outside thereof about
3.times.10.sup.15 molecules/cm.sup.3), a hydroxyl group content of
250 wt ppm and a mean fictive temperature of 1035.degree. C. The
quartz glass is not additionally doped with fluorine; the fluorine
content is below 1 wt ppm.
Sample 3
[0078] Another quartz glass cylinder was produced, as described
above with reference to sample 1, including hydrogen loading, but
the annealing treatment took place with the following heating
program: The quartz glass cylinder is heated in air and at
atmospheric pressure to 1250.degree. C. for a holding time of 8
hours and is subsequently cooled at a cooling rate of 4.degree.
C./h to a temperature of 1130.degree. C., and held at this
temperature for 4 hours. Thereupon, the quartz glass cylinder is
cooled at a higher cooling rate of 70.degree. C./h to a temperature
of 300.degree. C., whereupon the furnace is switched off and the
quartz glass cylinder is left to the free cooling of the
furnace.
[0079] Following hydrogen loading of the quartz glass cylinder,
said cylinder is substantially free from oxygen defects and SiH
groups (below the detection limit of 5.times.10.sup.16
molecules/cm.sup.3), and it is characterized by a hydrogen content
of 2.times.10.sup.16 molecules/cm.sup.3) and a hydroxyl group
content of 250 wt ppm and a mean fictive temperature of
1115.degree. C.
Sample 4
[0080] A soot body is produced by flame hydrolysis of SiCl.sub.4
with the help of the known VAD method as explained above. The soot
body is dehydrated at a temperature of 1200.degree. C. in a
chlorine-containing atmosphere and then vitrified at a temperature
of about 1750.degree. C. in vacuum (10.sup.-2 mbar) to form a
transparent quartz glass blank. Due to a more excessive dehydration
treatment (compared to the samples 1 to 3), the OH content is about
120 wt ppm. This blank is then homogenized by thermally mechanical
homogenization (twisting) and formation of a quartz glass cylinder
as explained above.
Measurement Results
[0081] Measurement samples are made from the quartz glass cylinders
produced in this way for determining the resistance of the quartz
glass to irradiation with linearly polarized UV excimer laser
radiation of a wavelength of 193 nm.
[0082] A result of this measurement is shown in FIG. 1. For samples
1 and 2 birefringence is here plotted in nm/cm on the Y-axis, and a
parameter characterizing the energy of the transmitted UV
radiation, namely the product following from the energy density of
the UV radiation in .mu.J/cm.sup.2 and the pulse number, is plotted
on the X-axis.
[0083] Hence, both in the sample having a low hydrogen content
(2.times.10.sup.15 molecules/cm.sup.3) and in the sample having a
higher hydrogen content (3.times.10.sup.16 molecules/cm.sup.3),
birefringence increases approximately linearly with an increasing
product .epsilon..times.P. The slope of the straight line is here
about 3.9.times.10.sup.-13, and it is a measure of the sensitivity
of the quartz glass to linearly polarized UV radiation with respect
to anisotropic changes in its density.
[0084] Corresponding tests were carried out for further quartz
glass samples which have a hydroxyl group content of 30 wt ppm and
of about 480 wt ppm, respectively, and otherwise correspond to
samples 1 and 2. The test results are summarized in the diagram of
FIG. 2. The slope of the straight line is respectively plotted on
the X-axis, as shown for samples 1 and 2 with reference to FIG. 1.
The X-axis shows the respective OH content of the samples in wt
ppm.
[0085] As can clearly be seen, the slope is strongly scaled with
the OH content substantially independently of the hydrogen content
of the sample. This means that with an increasing OH content the
sensitivity of the quartz glass samples greatly increases with
respect to an anisotropic density change upon irradiation with
linearly polarized laser light radiation of a wavelength of 193 nm.
The corresponding resistance of the quartz glass samples 1 and 2
(with an OH content of 250 wt ppm) can just be regarded as
acceptable. At increased OH contents the sensitivity of the quartz
glass with respect to an anisotropic density change is however no
longer acceptable. The best resistance was found in the measurement
samples of quartz glass having an OH content of 30 wt ppm.
[0086] The diagram of FIG. 3 shows the wavefront distortion
indicated as a change in the refractive index based on the distance
.DELTA.nL/L in ppb, depending on the pulse number upon irradiation
of two different quartz glass samples (sample 1 and sample 3) which
differ in their fictive temperature. These samples were exposed to
linearly polarized UV radiation of a wavelength of 193 nm, at a
pulse width of 25 ns and an energy density of 35 .mu.J/cm.sup.2 and
the wavefront distortion produced thereby was measured from time to
time.
[0087] As can be seen therefrom, the wavefront distortion passes at
an increasing pulse number after an initially steep rise into a
distinctly flatter rise, the level of the wavefront distortion in
sample 3 with the high fictive temperature being considerably lower
than in sample 1 with the lower fictive temperature. This
demonstrates that the isotropic portion of the density change due
to linearly polarized radiation depends on the fictive temperature
of the respective quartz glass, and that this portion turns out to
be lower in the sample having the high fictive temperature than in
the sample having the low fictive temperature.
[0088] Optical components made from a quartz glass quality in
accordance with samples 1 to 3 (with a hydroxyl group content
around 250 wt ppm) are particularly suited for use in a projection
system of an automatic exposure machine for immersion lithography
for the purpose of transmitting ultraviolet, pulsed and linearly
polarized UV laser radiation of a wavelength between 190 nm and 250
nm. Even better results can however be expected when the hydroxyl
group content is below 200 wt ppm, preferably below 125 wt ppm as
in sample 4.
[0089] First tests for checking the dependence of the anisotropic
radiation damage on the pulse width of the transmitted laser light
suggest that the quartz glass of the component of the invention has
an improved resistance to pulses with a pulse width of 50 ns (in
comparison with a pulse width of 25 ns). A further improvement of
the radiation resistance was observed with respect to irradiation
with pulse widths of 150 ns.
* * * * *