U.S. patent application number 12/967175 was filed with the patent office on 2011-06-30 for titania-doped quartz glass and making method.
This patent application is currently assigned to SHIN-ETSU CHEMICAL CO., LTD.. Invention is credited to Masanobu Ezaki, Shigeru Maida, Hisatoshi Otsuka, Tetsuji Ueda.
Application Number | 20110159413 12/967175 |
Document ID | / |
Family ID | 44187973 |
Filed Date | 2011-06-30 |
United States Patent
Application |
20110159413 |
Kind Code |
A1 |
Maida; Shigeru ; et
al. |
June 30, 2011 |
TITANIA-DOPED QUARTZ GLASS AND MAKING METHOD
Abstract
A titania-doped quartz glass which experiences a reduction in OH
group concentration of less than or equal to 100 ppm upon heat
treatment at 900.degree. C. for 100 hours is suitable as the EUV
lithography member.
Inventors: |
Maida; Shigeru; (Joetsu-shi,
JP) ; Otsuka; Hisatoshi; (Joetsu-shi, JP) ;
Ueda; Tetsuji; (Koriyama-shi, JP) ; Ezaki;
Masanobu; (Koriyama-shi, JP) |
Assignee: |
SHIN-ETSU CHEMICAL CO.,
LTD.
Tokyo
JP
|
Family ID: |
44187973 |
Appl. No.: |
12/967175 |
Filed: |
December 14, 2010 |
Current U.S.
Class: |
430/5 ; 501/53;
65/17.4 |
Current CPC
Class: |
G03F 7/2008 20130101;
C03C 3/06 20130101; B82Y 40/00 20130101; G03F 1/24 20130101; C03B
2207/12 20130101; C03B 2207/06 20130101; B82Y 10/00 20130101; C03B
2201/42 20130101; C03B 19/1453 20130101; C03B 19/1423 20130101;
C03C 2201/06 20130101; C03B 2201/21 20130101; C03B 2207/36
20130101; C03B 2201/23 20130101 |
Class at
Publication: |
430/5 ; 65/17.4;
501/53 |
International
Class: |
G03F 1/00 20060101
G03F001/00; C03B 19/00 20060101 C03B019/00; C03C 3/04 20060101
C03C003/04 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 25, 2009 |
JP |
2009-294557 |
Claims
1. A titania-doped quartz glass which experiences a reduction in OH
group concentration of less than or equal to 100 ppm upon heat
treatment at 900.degree. C. for 100 hours.
2. The titania-doped quartz glass of claim 1 wherein the difference
between maximum and minimum reductions of OH group concentration
upon the 900.degree. C./100-hr heat treatment is less than or equal
to 50 ppm.
3. The titania-doped quartz glass of claim 1, having an OH group
concentration of 300 ppm to 950 ppm after the 900.degree. C./100-hr
heat treatment.
4. The titania-doped quartz glass of claim 1, having an OH group
concentration gradient of less than or equal to 100 ppm/cm after
the 900.degree. C./100-hr heat treatment.
5. The titania-doped quartz glass of claim 1, having a hydrogen
molecule concentration of less than or equal to 5.times.10.sup.17
molecules/cm.sup.3.
6. The titania-doped quartz glass of claim 1, containing 3 to 10%
by weight of titania.
7. An EUV lithographic member comprising the titania-doped quartz
glass of claim 1.
8. The member of claim 7, which is a EUV lithographic photomask
substrate.
9. The member of claim 7, which is a mirror in a reflecting optical
system of a EUV lithography apparatus.
10. A method for preparing a titania-doped quartz glass, comprising
the steps of subjecting a silicon-providing reactant gas and a
titanium-providing reactant gas to oxidation or flame hydrolysis
with the aid of a combustible gas and a combustion-supporting gas,
to thereby form synthetic silica-titania fine particles, depositing
the silica-titania fine particles on a rotating target, and
concurrently melting and vitrifying the deposited particles into
titania-doped quartz glass, the method further comprising the step
of feeding oxygen gas as the combustion-supporting gas through a
central tube of a burner in admixture with the silicon-providing
reactant gas and the titanium-providing reactant gas in a molar
ratio of oxygen gas to the sum of the silicon-providing reactant
gas and the titanium-providing reactant gas of at least 5.
11. A method for preparing a titania-doped quartz glass, comprising
the steps of subjecting a silicon-providing reactant gas and a
titanium-providing reactant gas to oxidation or flame hydrolysis
with the aid of a combustible gas and a combustion-supporting gas,
to thereby form synthetic silica-titania fine particles, depositing
the silica-titania fine particles on a rotating target, and
concurrently melting and vitrifying the deposited particles into
titania-doped quartz glass, the method further comprising the step
of injecting hydrogen gas as the combustible gas through one or
more hydrogen gas feed tubes of a burner at a linear velocity of
less than or equal to 100 m/sec.
12. The method of claim 10 wherein the flow rates of the
combustible gas, the combustion-supporting gas, the
silicon-providing reactant gas and the titanium-providing reactant
gas are controlled so that respective variations of the flow rates
may fall within .+-.1%, the temperatures of cooling air introducing
from the outside of a quartz glass manufacturing furnace thereinto,
exhaust gas from the furnace, and ambient air surrounding the
furnace are controlled so that respective variations of the
temperatures may fall within .+-.2.5.degree. C., and the target is
rotated at a rotational speed of at least 5 rpm when the
silica-titania fine particles are deposited on the rotating target.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This non-provisional application claims priority under 35
U.S.C. .sctn.119(a) on Patent Application No. 2009-294557 filed in
Japan on Dec. 25, 2009, the entire contents of which are hereby
incorporated by reference.
TECHNICAL FIELD
[0002] This invention relates to titania-doped quartz glass with
low thermal expansion which is suited for use in the EUV
lithography, and a method for manufacturing the same.
BACKGROUND ART
[0003] In the advanced lithography process for the fabrication of
semiconductor devices, a light source of shorter wavelength is used
for exposure. A subsequent transition to lithography using extreme
ultraviolet (EUV) is regarded promising. Since the EUV lithography
uses a reflecting optical system and a short wavelength light
source, the lithography accuracy can be adversely affected even by
a slight thermal expansion of each member (e.g., substrate) in the
lithographic optical system induced by the heat that has reached
there. Accordingly, members like reflecting mirrors, masks, and
stages must be made of low expansion materials. Titania-doped
quartz glass is known as a typical low expansion material. The
addition of a certain amount of titania makes it possible to
minimize the thermal expansion of quartz glass.
[0004] The EUV lithography members must also have a uniform
distribution of low thermal expansion. To gain a uniform
distribution of low thermal expansion, it is of the first priority
that titania-doped quartz glass have a uniform content of titania.
For example, JP-A 2004-315351 discloses titania-doped quartz glass
in which a difference between maximum and minimum TiO.sub.2
concentrations is less than or equal to 0.06% by weight in a range
of 30 mm.times.30 mm, and a variation (.DELTA.n) of refractive
index which varies with the TiO.sub.2 concentration in quartz glass
is less than or equal to 2.times.10.sup.-4 in a range of 30
mm.times.30 mm.
[0005] Also an OH group concentration in titania-doped quartz glass
is known as one of the physical properties having impact on the low
thermal expansion of titania-doped quartz glass. WO 2005/114328
discloses a quartz glass blank having a mean OH content of 700 to
1,000 wt ppm, wherein the variation of the OH content, averaged
over the thickness of the quartz glass blank, is within .+-.50 ppm
in the area of the main functional direction. Then the optical and
thermal properties of quartz glass are kept as homogeneous as
possible.
[0006] JP-A 2005-022954 describes that the fictive temperature of
glass is correlated to the extent of a zero expansion temperature
range that is a temperature range in which the coefficient of
thermal expansion (CTE) of glass becomes substantially zero (0).
For the purpose of broadening the zero expansion temperature range,
the fictive temperature is preferably up to 950.degree. C., more
preferably up to 900.degree. C., and even more preferably up to
850.degree. C. Since a high OH group concentration in glass
indicates fast structural relaxation, the manufacture of a glass
block having a large diameter enough to have a temperature
distribution tends to entail a fictive temperature distribution.
Thus the OH group concentration is preferably up to 600 ppm, more
preferably up to 400 ppm, and even more preferably up to 200 ppm.
In addition, if the OH group concentration varies over a wide
range, the structural relaxation time may substantially vary at
different positions, inviting a difference in fictive temperature.
Thus the variation of the OH group concentration in titania-doped
quartz glass is preferably within 50 ppm, more preferably within 30
ppm, and even more preferably within 10 ppm.
[0007] As discussed above, the OH group concentration in
titania-doped quartz glass has an outstanding impact on low thermal
expansion. It is thus important to specify the absolute amount and
distribution of the OH group concentration in titania-doped quartz
glass. It would be desirable to minimize the variation of the OH
group concentration.
CITATION LIST
[0008] Patent Document 1: JP-A 2004-315351 [0009] Patent Document
2: WO 2005/114328 (JP-A 2008-505827) [0010] Patent Document 3: JP-A
2005-022954 [0011] Patent Document 4: JP-A H07-267662
DISCLOSURE OF INVENTION
[0012] It would be desirable to have a titania-doped quartz glass
which experiences a little or substantially no change of its OH
group concentration upon heat treatment at 900.degree. C. for 100
hours and is suited as members for use in the EUV lithography and a
method for preparing the same.
[0013] The inventors made a study on the heat treatment of
titania-doped quartz glass. Sometimes glass changed its OH group
concentration upon heat treatment at 900.degree. C. for 100 hours.
Such glass with a noticeable change of OH group concentration is
unsuited as EUV lithography members. Titania-doped quartz glass
which experiences a little or substantially no change of its OH
group concentration upon heat treatment at 900.degree. C. for 100
hours is suited as EUV lithography members.
[0014] It is known in the art that the concentration of OH groups
in quartz glass can be reduced by heat treating a porous silica
matrix, which is prepared by the indirect or soot method, for
example, under high-temperature vacuum conditions or in a
chlorine-containing atmosphere. The OH group concentration may be
changed as long as quartz glass has not been converted into
transparent glass.
[0015] However, it is known that the OH group concentration in
quartz glass which has been converted into transparent glass may
not be substantially changed in absolute amount and distribution by
any simple heat treatment but special heat treatments such as
hydrothermal treatment, homogenization treatment described in JP-A
H07-267662, and heat treatment at high temperature and high
pressure in a hydrogen-containing atmosphere. Specifically, the
absolute amount and distribution of OH group concentration in
quartz glass are decided depending on a particular preparation
method and parameters during preparation including gas feed rates,
temperature distribution of the growth front, and atmosphere.
[0016] The behavior of titania-doped quartz glass is different. The
absolute amount and distribution of OH group concentration in
titania-doped quartz glass depend on a particular preparation
method and parameters during preparation as in the case of quartz
glass. The OH group concentration in titania-doped quartz glass
which has been converted into transparent glass may be changed by
simple heat treatment. Even though the variation of OH group
concentration in titania-doped quartz glass is reduced as described
in the above-referred patent document, this glass will change the
OH group concentration when heated, resulting in a varying OH group
concentration.
[0017] Under the circumstances, the inventors sought for
titania-doped quartz glass which, after vitrification, experiences
a little or substantially no change of OH group concentration upon
simple heat treatment. When titania-doped quartz glass is prepared
by subjecting a silicon-providing reactant gas and a
titanium-providing reactant gas to oxidation or flame hydrolysis
with the aid of a combustible gas and a combustion-supporting gas,
to thereby synthesize synthetic silica-titania fine particles,
depositing the silica-titania fine particles on a rotating target,
and concurrently melting and vitrifying the deposited particles
into titania-doped quartz glass, an improvement is made by feeding
oxygen gas as the combustion-supporting gas through a central tube
of a burner in admixture with the silicon-providing reactant gas
and the titanium-providing reactant gas in a molar ratio of oxygen
gas to the sum of the silicon-providing reactant gas and the
titanium-providing reactant gas of at least 5, and preferably
further injecting hydrogen gas as the combustible gas through one
or more hydrogen gas feed tubes of the burner at a linear velocity
of less than or equal to 100 m/sec. The resulting titania-doped
quartz glass experiences a reduction in OH group concentration of
less than or equal to 100 ppm upon heat treatment at 900.degree. C.
for 100 hours. Preferably the difference between maximum and
minimum reductions of OH group concentration is less than or equal
to 50 ppm. There is obtained a titania-doped quartz glass whose OH
group concentration shows no substantial change upon simple heat
treatment and a minimal variation. The invention is predicated on
these findings.
[0018] An object of the present invention is to provide a
titania-doped quartz glass which experiences a little or
substantially no change of OH group concentration upon heat
treatment at 900.degree. C. for 100 hours, and a method for
preparing the same.
[0019] In one aspect, the invention provides a titania-doped quartz
glass which experiences a reduction in OH group concentration of
less than or equal to 100 ppm upon heat treatment at 900.degree. C.
for 100 hours.
[0020] In a preferred embodiment, the difference between maximum
and minimum reductions of OH group concentration upon the
900.degree. C./100-hr heat treatment is less than or equal to 50
ppm. In a preferred embodiment, the titania-doped quartz glass has
an OH group concentration of 300 ppm to 950 ppm after the
900.degree. C./100-hr heat treatment, an OH group concentration
gradient of less than or equal to 100 ppm/cm after the 900.degree.
C./100-hr heat treatment, or a hydrogen molecule concentration of
less than or equal to 5.times.10.sup.17 molecules/cm.sup.3, or a
combination of any of the foregoing. Typically the titania-doped
quartz glass contains 3 to 10% by weight of titania.
[0021] In another aspect, the invention provides an EUV
lithographic member comprising the titania-doped quartz glass
defined above.
[0022] The member is typically a EUV lithographic photomask
substrate or a mirror in a reflecting optical system of a EUV
lithography apparatus.
[0023] In a further aspect, the invention provides a method for
preparing a titania-doped quartz glass, comprising the steps of
subjecting a silicon-providing reactant gas and a
titanium-providing reactant gas to oxidation or flame hydrolysis
with the aid of a combustible gas and a combustion-supporting gas,
to thereby form synthetic silica-titania fine particles, depositing
the silica-titania fine particles on a rotating target, and
concurrently melting and vitrifying the deposited particles into
titania-doped quartz glass. In one embodiment, the method further
comprises the step of feeding oxygen gas as the
combustion-supporting gas through a central tube of a burner in
admixture with the silicon-providing reactant gas and the
titanium-providing reactant gas in a molar ratio of oxygen gas to
the sum of the silicon-providing reactant gas and the
titanium-providing reactant gas of at least 5. In another
embodiment, the method further comprises the step of injecting
hydrogen gas as the combustible gas through one or more hydrogen
gas feed tubes of a burner at a linear velocity of less than or
equal to 100 m/sec.
[0024] In a preferred embodiment, the flow rates of the combustible
gas, the combustion-supporting gas, the silicon-providing reactant
gas and the titanium-providing reactant gas are controlled so that
respective variations of the flow rates may fall within .+-.1%, the
temperatures of cooling air introducing from the outside of a
quartz glass manufacturing furnace thereinto, exhaust gas from the
furnace, and ambient air surrounding the furnace are controlled so
that respective variations of the temperatures may fall within
.+-.2.5.degree. C., and the target is rotated at a rotational speed
of at least 5 rpm when the silica-titania fine particles are
deposited on the rotating target.
ADVANTAGEOUS EFFECTS OF INVENTION
[0025] Since titania-doped quartz glass experiences only a minimal
change of OH group concentration upon heat treatment at 900.degree.
C. for 100 hours, it is suited to construct a member for use in the
EUV lithography.
BRIEF DESCRIPTION OF DRAWINGS
[0026] FIG. 1 is a diagram showing an OH group concentration
distribution in titania-doped quartz glass in Example 1 before and
after 900.degree. C./100-hr heat treatment.
[0027] FIG. 2 is a diagram showing an OH group concentration
distribution in titania-doped quartz glass in Example 2 before and
after 900.degree. C./100-hr heat treatment.
[0028] FIG. 3 is a diagram showing an OH group concentration
distribution in titania-doped quartz glass in Example 3 before and
after 900.degree. C./100-hr heat treatment.
[0029] FIG. 4 is a diagram showing an OH group concentration
distribution in titania-doped quartz glass in Example 4 before and
after 900.degree. C./100-hr heat treatment.
[0030] FIG. 5 is a diagram showing an OH group concentration
distribution in titania-doped quartz glass in Comparative Example 1
before and after 900.degree. C./100-hr heat treatment.
[0031] FIG. 6 is a diagram showing an OH group concentration
distribution in titania-doped quartz glass in Comparative Example 2
before and after 900.degree. C./100-hr heat treatment.
[0032] FIG. 7 is a radial cross-sectional view of a gas injection
outlet of a burner for the manufacture of synthetic quartz glass
used in Examples.
[0033] FIG. 8 is a side view, partially in axial cross section, of
the burner of FIG. 7.
DESCRIPTION OF EMBODIMENTS
[0034] Unlike undoped quartz glass, titania-doped quartz glass may
change its OH group concentration upon heat treatment at
900.degree. C. for 100 hours (simply referred to as "900.degree.
C./100-hr heat treatment," hereinafter). A substantial change of
the OH group concentration upon 900.degree. C./100-hr heat
treatment broadens the OH group concentration distribution in
titania-doped quartz glass and at the same time, has impact on both
the fictive temperature and birefringence. As a result, thermal
properties of titania-doped quartz glass are altered. Also, when
titania-doped quartz glass experiencing a substantial change of the
OH group concentration upon 900.degree. C./100-hr heat treatment is
used as a member in the EUV lithography, it is likely to display
thermal hysteresis due to thermal cycling, that is, repeated rise
and fall of the member temperature during EUV light exposure and
interruption of exposure. Therefore, titania-doped quartz glass
experiencing a OH group concentration change in excess of 100 ppm
is inadequate as the EUV lithography member.
[0035] When titania-doped quartz glass experiences a substantial
change of the OH group concentration upon 900.degree. C./100-hr
heat treatment, crystalline silica often forms in the titania-doped
quartz glass as a result of the heat treatment. It is believed that
once fine titania grains form within the ingot during preparation
of titania-doped quartz glass, crystalline silica grows on the
titania grains serving as nuclei during the heat treatment.
Formation of titania grains as inclusions in the titania-doped
quartz glass and crystalline silica is inadequate as the EUV
lithography member which is required to have a high accuracy,
cleanness, and stable thermal properties at its surface.
[0036] In contrast, titania-doped quartz glass of the invention
experiences a minimal change of the OH group concentration upon
900.degree. C./100-hr heat treatment and is adequate as the EUV
lithography member.
[0037] According to the invention, such titania-doped quartz glass
may be prepared by feeding a combustible gas containing hydrogen
and a combustion-supporting gas containing oxygen to a burner built
in a quartz glass-manufacturing furnace, burning the gases to form
an oxyhydrogen flame at the burner tip, feeding a silicon-providing
reactant gas and a titanium-providing reactant gas into the flame
for subjecting the gases to oxidation or flame hydrolysis to
thereby form silica, titania and composite fine particles,
depositing the fine particles on a rotating target disposed forward
of the burner, and concurrently melting and vitrifying the
deposited particles into titania-doped quartz glass to form an
ingot, shaping the ingot into a predetermined shape, and annealing
the shaped ingot. In a preferred embodiment, the flow rates of the
combustible gas, the combustion-supporting gas, the
silicon-providing reactant gas and the titanium-providing reactant
gas are controlled so that respective variations of the flow rates
may fall within .+-.1%, the temperatures of cooling air introducing
from the outside of the furnace thereinto, exhaust gas from the
furnace, and ambient air surrounding the furnace are controlled so
that respective variations of the temperatures may fall within
.+-.2.5.degree. C., and the target is rotated at a rotational speed
of at least 5 rpm when the silica-titania fine particles are
deposited on the rotating target.
[0038] The quartz glass-manufacturing furnace may be of vertical or
horizontal type. The target of a seed or similar material is
rotated at a rotational speed of at least 5 rpm, preferably at
least 15 rpm, and more preferably at least 30 rpm. This is because
striae, strains or structurally or compositionally non-uniform
zones generate, depending largely on the unevenness of temperature
in a portion where titania-doped quartz glass grows on the rotating
target. Then the generation of structurally or compositionally
non-uniform zones in titania-doped quartz glass can be inhibited by
increasing the rotational speed of the target so that an even
temperature may be available in a portion where titania-doped
quartz glass grows. The upper limit of rotational speed of the
target may be selected as appropriate although it is usually up to
300 rpm, specifically up to 200 rpm. Control of the rotational
speed of the target is important in reducing the gradient of the OH
group concentration of titania-doped quartz glass.
[0039] The generation of structurally or compositionally
non-uniform zones in titania-doped quartz glass can be inhibited by
supplying the silicon-providing reactant gas, titanium-providing
reactant gas, combustible gas, and combustion-supporting gas at
steady rates. To this end, in one preferred embodiment, the flow
rates of the silicon-providing reactant gas, titanium-providing
reactant gas, combustible gas, and combustion-supporting gas are
each controlled within a variation of .+-.1%, more preferably
.+-.0.5%, and even more preferably .+-.0.25%.
[0040] The generation of structurally or compositionally
non-uniform zones in titania-doped quartz glass can also be
inhibited by supplying the silicon-providing reactant gas and the
titanium-providing reactant gas into a common nozzle of the burner
along with the combustion-supporting gas during the preparation of
titania-doped quartz glass. The silicon-providing reactant gas, the
titanium-providing reactant gas and the combustion-supporting gas
are previously mixed to form a premix, which is preferably made
uniform in composition by a line mixer or the like.
[0041] Specifically, the silicon-providing reactant gas and the
titanium-providing reactant gas are fed through a central tube of a
burner, and oxygen gas as the combustion-supporting gas is fed
through the central tube in admixture with the silicon-providing
reactant gas and the titanium-providing reactant gas. In the
mixture, oxygen gas is present in a molar ratio of oxygen gas to
the sum of the silicon-providing reactant gas and the
titanium-providing reactant gas of at least 5, more preferably at
least 7.5, and even more preferably at least 10. If a molar ratio
of oxygen gas as the combustion-supporting gas to the sum of the
silicon-providing reactant gas and the titanium-providing reactant
gas is less than 5, the titania-doped quartz glass prepared under
such conditions tends to experience a substantial reduction of the
OH group concentration upon 900.degree. C./100-hr heat treatment.
The upper limit of the molar ratio is usually up to 30, and
preferably up to 20.
[0042] In another preferred embodiment, hydrogen gas as the
combustible gas is injected through one or more hydrogen gas feed
tubes of the burner at a linear velocity of less than or equal to
100 m/sec, more preferably less than or equal to 90 m/sec, and even
more preferably less than or equal to 80 m/sec for each tube. If
the linear velocity of hydrogen gas injected as the combustible gas
through the burner is higher than 100 m/sec, the titania-doped
quartz glass prepared under such conditions tends to experience a
substantial reduction of the OH group concentration upon
900.degree. C./100-hr heat treatment. The lower limit of the linear
velocity is usually at least 10 m/sec, and preferably at least 20
m/sec. Control of the molar ratio of oxygen gas to the sum of the
silicon-providing reactant gas and the titanium-providing reactant
gas and the linear velocity of hydrogen gas is important in
reducing the reduction-on-heating of the OH group concentration of
titania-doped quartz glass.
[0043] The burner used herein may be the one illustrated in FIGS. 1
and 2 of JP-A 2005-187319, but is not limited thereto. The burner
comprises a main burner including a multi-fold tube of at least
triple-tube structure, a shell tube surrounding the multi-fold
tube, and a plurality of nozzles inside the shell tube. The burner
further comprises a dual tube disposed outside the main burner.
Referring to FIGS. 7 and 8, a main burner 7 includes a multi-tube
assembly 1 of triple-tube structure consisting of a central tube 2,
a first enclosure tube 3 enclosing the central tube 2, and a second
enclosure tube 4 enclosing the first enclosure tube 3. The main
burner 7 also includes a shell tube 5 surrounding the triple-tube
assembly 1, and a plurality of nozzles 6 which are disposed inside
the shell tube 5 and distributed between the shell tube 5 and the
triple-tube assembly 1. A dual tube 8 consisting of an outer tube 9
and an inner tube 10 disposed inside the outer tube 9 is disposed
outside the main burner 7 and surrounds at least the front opening
of the main burner 7. The dual tube 8 is configured such that a tip
portion of the outer tube 9 surrounds the front opening of the main
burner 7 and axially extends forward thereof to provide a guard for
preventing the gas stream from the main burner 7 from spreading
laterally. A tip portion of the inner tube 10 is radially
coextensive with the front opening of the main burner 7. It is
noted that the tip portion of the inner tube 10 may be disposed
backward of the front opening of the main burner 7
[0044] With the tube arrangement illustrated above, the
silicon-providing reactant gas and the titanium-providing reactant
gas are fed and flowed through the central tube 2, and oxygen gas
is also fed and flowed through the central tube 2. The
combustion-supporting gas such as oxygen gas is fed and flowed
through the first enclosure tube 3. The combustible gas such as
hydrogen gas is fed and flowed through the second enclosure tube 4.
Further, the combustion-supporting gas such as oxygen gas is fed
and flowed through the nozzles 6 and the dual tube 8 (between outer
and inner tubes 9 and 10). The combustible gas such as hydrogen gas
is fed and flowed through the shell tube 5 so that the gas flows
around the nozzles 6.
[0045] The oxygen gas prescribed as being fed in a molar ratio of
at least 5 relative to the sum of silicon-providing reactant gas
and titanium-providing reactant gas is an oxygen gas fraction
through the central tube 2. The silicon-providing reactant gas is
preferably fed at a flow rate of 500 to 3,000 g/hr, more preferably
1,000 to 2,000 g/hr, and the titanium-providing reactant gas is
preferably fed at a flow rate of 85 to 500 g/hr, more preferably
150 to 350 g/hr. A weight ratio of the titanium-providing reactant
gas to the silicon-providing reactant gas is preferably between 0.8
and 0.15, more preferably between 0.11 and 0.13 so that
titania-doped quartz glass may contain 3 to 10% by weight,
specifically 5 to 8% by weight of titania.
[0046] While the proportion of oxygen gas in the gas flow through
the central tube 2 is as defined above, the flow rates of oxygen
gas through tubes other than the central tube 2 may be determined
so as to total 0.8 to 1.1 times the stoichiometry for reaction with
hydrogen gas.
[0047] Control of gas flow rates is important in minimizing the
difference between maximum and minimum reductions of OH group
concentration in titania-doped quartz glass and reducing the
hydrogen molecule concentration in titania-doped quartz glass.
Control of titania content is important in enabling preparation at
the temperature at which the CTE of titania-doped quartz glass
becomes zero.
[0048] While the second enclosure tube 4 and the shell tube 5 serve
as hydrogen gas feed tubes, hydrogen gas flows through each of the
enclosure tube and shell tube at a linear velocity of up to 100
m/sec. When hydrogen gas is fed through more than two tubes, a
linear velocity of hydrogen gas through a respective one of the
tubes is up to 100 m/sec.
[0049] The silicon-providing reactant gas used herein may be
selected from well-known organosilicon compounds, for example,
silicon tetrachloride, chlorosilanes such as dimethyldichlorosilane
and methyltrichlorosilane, and alkoxysilanes such as
tetramethoxysilane, tetraethoxysilane, and
methyltrimethoxysilane.
[0050] The titanium-providing reactant gas used herein may also be
selected from well-known compounds, for example, titanium halides
such as titanium tetrachloride and titanium tetrabromide, and
titanium alkoxides such as tetraethoxytitanium,
tetraisopropoxytitanium, tetra-n-propoxytitanium,
tetra-n-butoxytitanium, tetra-sec-butoxytitanium, and
tetra-t-butoxytitanium.
[0051] On the other hand, the combustible gas used herein may be
one containing hydrogen, optionally in combination with another gas
such as carbon monoxide, methane or propane. The
combustion-supporting gas used herein may be one containing
oxygen.
[0052] As described above, titania-doped quartz glass is prepared
by feeding a combustible gas containing hydrogen and a
combustion-supporting gas containing oxygen to a burner built in a
quartz glass-manufacturing furnace, burning the gases to form an
oxyhydrogen flame at the burner tip, feeding a silicon-providing
reactant gas and a titanium-providing reactant gas into the flame
to subject the gases to oxidation or flame hydrolysis to thereby
form silica, titania and composite fine particles, depositing the
fine particles on a rotating target disposed forward of the burner,
and concurrently melting and vitrifying the deposited particles
into titania-doped quartz glass to form an ingot. A sample is cut
out of the titania-doped quartz glass ingot and measured for OH
group concentration. The sample is then heat treated at 900.degree.
C. for 100 hours, after which it is measured again for OH group
concentration. A reduction of OH group concentration is
computed.
[0053] In order that the titania-doped quartz glass ingot be shaped
into a desired shape suited for a particular EUV lithography member
such as a mirror, stage or photomask substrate, it is first shaped
at a temperature of 1,500 to 1,800.degree. C. for 1 to 10 hours.
Preferably shaping is conducted such that the axis of the shape is
parallel to the growth axis of the titania-doped quartz glass grown
in the manufacture furnace. After the shaping, the titania-doped
quartz glass is annealed. The annealing step is effective for
mitigating any thermal strain induced in the titania-doped quartz
glass by shaping. Annealing may be conducted under well-known
conditions, for example, by holding at a temperature of 700 to
1,300.degree. C. in air for 1 to 200 hours. This may be followed by
slow cooling, for example, at a rate of 1 to 20.degree. C./hr from
the annealing temperature to 500.degree. C. The annealing treatment
may reduce the fictive temperature of titania-doped quartz glass.
Preferably the titania-doped quartz glass has a fictive temperature
of lower than or equal to 1,200.degree. C., more preferably lower
than or equal to 1,150.degree. C., and even more preferably lower
than or equal to 1,100.degree. C. Since the CTE of titania-doped
quartz glass depends on the distribution of fictive temperature, it
preferably has a fictive temperature distribution (.DELTA.FT) of
lower than or equal to 30.degree. C., more preferably lower than or
equal to 20.degree. C., and even more preferably lower than or
equal to 10.degree. C. It is noted that the fictive temperature of
titania-doped quartz glass may be measured by the method described
in J. Non-Cryst. Solids, 185, 191 (1995).
[0054] After the annealing treatment, the titania-doped quartz
glass is worked into a predetermined size by machining or slicing
and then polished by a double-side lapping machine with an abrasive
such as silicon oxide, aluminum oxide, molybdenum oxide, silicon
carbide, diamond, cerium oxide or colloidal silica, thereby forming
an EUV lithography member.
[0055] From the titania-doped quartz glass, EUV lithography
photomask substrates can be formed in which the difference between
the highest and the lowest positions (also referred to as
peak-to-valley (P-V) flatness) within a central region of 142.4
mm.times.142.4 mm square in the substrate surface as polished is up
to 200 nm, preferably up to 100 nm. It is noted that the P-V
flatness may be determined by a Fizeau interferometer (Zygo Mark
IV).
[0056] By the method described above, titania-doped quartz glass is
obtainable which experiences a little change of OH group
concentration upon heating. Specifically the titania-doped quartz
glass of the invention experiences a reduction in OH group
concentration of less than or equal to 100 ppm upon heat treatment
at 900.degree. C. for 100 hours. The reduction of OH group
concentration is more preferably less than or equal to 50 ppm, and
even more preferably less than or equal to 20 ppm. Most preferably
the reduction of OH group concentration is substantially nil, that
is, within the measurement error range before and after the
900.degree. C./100-hr heat treatment.
[0057] In a preferred embodiment of titania-doped quartz glass, the
difference between maximum and minimum reductions of OH group
concentration upon the 900.degree. C./100-hr heat treatment is less
than or equal to 50 ppm, more preferably less than or equal to 20
ppm, and most preferably substantially nil. If a substantial
difference is found in the reduction of OH group concentration upon
the 900.degree. C./100-hr heat treatment, the glass has a
distribution of thermal properties and is inadequate as the EUV
lithography member.
[0058] The titania-doped quartz glass should preferably have an OH
group concentration of from 300 ppm to 950 ppm, more preferably
from 400 ppm to 850 ppm, even more preferably more than 500 ppm and
less than 750 ppm, and most preferably more than 500 ppm and less
than 700 ppm, after the 900.degree. C./100-hr heat treatment. If
the OH group concentration is less than 300 ppm, titania-doped
quartz glass is often colored. Such colored glass is undesired
because it inhibits transmission of laser light used in the
alignment of a member in the EUV lithography. If the OH group
concentration is more than 950 ppm, titania-doped quartz glass
tends to generate unwanted inclusions such as bubbles when shaped
into the desired shape.
[0059] The titania-doped quartz glass should preferably have an OH
group concentration gradient of less than or equal to 100 ppm/cm,
more preferably less than or equal to 50 ppm/cm, and even more
preferably less than or equal to 20 ppm/cm, after the 900.degree.
C./100-hr heat treatment. If the OH group concentration gradient is
greater than the range after the 900.degree. C./100-hr heat
treatment, titania-doped quartz glass even featuring a little
reduction of OH group concentration upon the 900.degree. C./100-hr
heat treatment has a distribution of thermal properties and is
inadequate as the EUV lithography member.
[0060] An OH group concentration of titania-doped quartz glass is
measured by an infrared spectrophotometer FT/IR-300E by Jasco Corp.
Specifically, a glass sample was scanned over a range of 3,000 to
5,000 cm.sup.-1 at a resolution of 2 cm.sup.-1 and an accumulation
count of 20, obtaining an absorption spectrum. A straight line
connecting peaks at 4,762 cm.sup.-1 and 4,202 cm.sup.-1 is used as
the baseline. An absorption coefficient is given as the peak height
near 4,522 cm.sup.-1. The OH group concentration is computed in
accordance with the equation:
OH group concentration (ppm)=(absorption coefficient at 4522
cm.sup.-1)/T.times.4400
wherein T is a thickness (cm) of a sample.
[0061] In a test, measurement was repeated 5 times at the same
position on the glass sample, with a measurement value being an
average of 5 measured values. A measurement value of OH group
concentration varied in a range of .+-.2 ppm at the same
measurement position. When measurement values at the same position
of the same sample were in a range of .+-.2 ppm before and after
the 900.degree. C./100-hr heat treatment, the difference between
maximum and minimum reductions of OH group concentration was
regarded substantially nil. Measurement of OH group concentration
was made at intervals of 5 mm from the center to the outer
periphery of a sample.
[0062] It is noted that the 900.degree. C./100-hr heat treatment is
preferably carried out in a neutral or oxidizing atmosphere, for
example, in air, oxygen, nitrogen or an inert gas such as argon.
The internal pressure of the heat treatment furnace may be an added
pressure, atmospheric (1 atm) or reduced pressure although
atmospheric or reduced pressure is preferred from the standpoints
of installation and safety.
[0063] The titania-doped quartz glass should preferably have a
hydrogen molecule concentration of less than or equal to
5.times.10.sup.17 molecules/cm.sup.3, more preferably less than or
equal to 1.times.10.sup.17 molecules/cm.sup.3. Even more
preferably, in Raman spectroscopy, the peak near 4,135 cm.sup.-1
assigned to hydrogen molecule is below the detection limit. Since
titania-doped quartz glass containing more hydrogen molecules tends
to generate unwanted inclusions such as bubbles when shaped into
the desired shape, it is preferred that the concentration of
hydrogen molecules be as low as possible.
[0064] It is noted that the hydrogen molecule concentration is
measured by a Raman spectrometer NRS-2100 by Jasco Corp. using a
4-W argon ion laser as the exciting light source and according to
the method described in Zurnal Pril; adnoi Spektroskopii Vol. 46,
No. 6, pp 987-991, June 1987. The detection limit is
7.5.times.10.sup.16 molecules/cm.sup.3.
[0065] The titania-doped quartz glass should preferably has a
titania content of 3 to 10% by weight, more preferably 5 to 8% by
weight in order that the glass undergo low thermal expansion in the
exposure temperature range of the EUV lithography. The titania
content is measured by electron probe microanalysis (EPMA) using a
probe with a diameter of 10 .mu.m. Computation is made on the
assumption that all titanium detected is present as titania
(TiO.sub.2).
[0066] In a preferred embodiment, the titania-doped quartz glass
has a coefficient of thermal expansion (CTE) which becomes zero (0)
at a temperature within the temperature range of 0.degree. C. to
100.degree. C., more preferably within the temperature range of
10.degree. C. to 90.degree. C., even more preferably within the
temperature range of 20.degree. C. to 80.degree. C., further
preferably within the temperature range of 30.degree. C. to
70.degree. C., even further preferably within the temperature range
of 40.degree. C. to 60.degree. C., and most preferably within the
temperature range of 45.degree. C. to 55.degree. C. It is noted
that the CTE and the thermal expansion curve may be determined on a
sample sized 6 mm diameter and 12 mm length and having cannonball
shaped ends by a thermal dilatometer LIX-2 by Ulvac-Riko, Inc.
[0067] The titania-doped quartz glass is suited as a stock for
forming EUV lithography members such as EUV lithography photomask
substrates and reflecting optical system mirrors in EUV lithography
apparatus. It is best suited as EUV lithography photomask
substrates and reflecting optical system mirrors in EUV lithography
apparatus since it enables transfer of a pattern of high image
quality and fine size onto a wafer.
EXAMPLE
[0068] Examples and Comparative Examples are given below for
illustrating the invention although the invention is not limited
thereto.
Examples 1 to 3
[0069] A titania-doped quartz glass ingot was prepared by using a
furnace including a quartz burner as shown in FIGS. 7 and 8,
feeding gases (SiCl.sub.4, TiCl.sub.4, O.sub.2, H.sub.2) to
respective tubes of the burner as shown in Table 1, forming an
oxyhydrogen flame, effecting oxidation or flame hydrolysis of
silicon tetrachloride and titanium tetrachloride in the oxyhydrogen
flame to produce SiO.sub.2 and TiO.sub.2, depositing silica and
titania fine particles on a target, and concurrently melting and
vitrifying the particles. The target was disposed forward of the
burner, rotated at 50 rpm, and retracted at 10 mm/hr. Oxygen gas
was fed in admixture with the silicon-providing reactant gas and
titanium-providing reactant gas through the central tube, and
hydrogen gas was fed through the second enclosure tube and shell
tube. Table 3 reports a molar ratio of oxygen gas to the sum of
silicon-providing reactant gas and titanium-providing reactant gas
and the linear velocity of hydrogen gas flows. The flow rates of
respective gases were kept at a variation of .+-.0.2%. During
preparation of titania-doped quartz glass in the furnace, the
temperatures of inlet air to the furnace, exhaust gas from the
furnace, and ambient air around the furnace were kept at a
variation of .+-.1.degree. C.
[0070] The resulting ingot had a diameter of 110 mm and a length of
400 mm. A disk sample of 10 mm thick was sliced from the ingot,
ground and polished on both the surfaces, after which a OH group
concentration was measured in a radial direction. The sample was
further heat treated for 100 hours in air at 900.degree. C. and
atmospheric pressure, after which a OH group concentration was
measured again in a radial direction. Also a hydrogen molecule
concentration was measured by Raman spectroscopy and a TiO.sub.2
concentration by EPMA. Table 4 reports maximum and minimum
reductions of OH group concentration by the 900.degree. C./100-hr
heat treatment, the difference between the maximum and minimum
reductions of OH group concentration, maximum gradient of OH group
concentration by the 900.degree. C./100-hr heat treatment, hydrogen
molecule concentration, and maximum and minimum of TiO.sub.2
concentration. FIGS. 1 to 3 show a radial distribution of the OH
group concentration before and after the 900.degree. C./100-hr heat
treatment in Examples 1 to 3, respectively.
[0071] The remaining titania-doped quartz glass ingot was shaped by
heating at 1700.degree. C. for 6 hours. It is noted that only the
ingot of Example 3 was cylindrically ground to a depth of 10 mm
from the periphery prior to shaping. The ingot was annealed by
holding in air at 950.degree. C. for 150 hours and then slowly
cooling at a rate of 5.degree. C./hr to 500.degree. C. The annealed
ingot was machined into a square column of 152.4 mm by 152.4 mm,
designated titania-doped quartz glass ingot I. A substrate was
sliced from ingot I. The substrate was polished for 6 hours by a
double-side lapping machine Model 12B (Fujikoshi Machinery Corp.)
using a suede-type polishing pad and cerium oxide abrasive and then
for 1 hour using colloidal silica abrasive instead. This resulted
in a substrate of 1 mm thick having both surfaces mirror polished.
The polished substrate was measured for fictive temperature along a
diagonal, with maximum and minimum values thereof being reported in
Table 4.
[0072] A sample for a thermal expansion test was cut out of the
remaining ingot I from the center within the 152.4 mm.times.152.4
mm square. A thermal expansion curve was determined in a
temperature range of -50.degree. C. to 150.degree. C. The
temperature at which the CTE becomes zero on the thermal expansion
curve, referred to as "zero expansion temperature", is reported in
Table 4.
[0073] Further, a substrate of 6.7 mm thick was sliced from the
remaining ingot I. The substrate was polished for 6 hours by a
double-side lapping machine Model 12B (Fujikoshi Machinery Corp.)
using a suede-type polishing pad and cerium oxide abrasive and then
for 1 hour using colloidal silica abrasive instead. The polished
substrate had a thickness of 6.35 mm. For the substrate thus
obtained, a difference between the highest and lowest positions in
a central region of 142.4 mm.times.142.4 mm square in the substrate
surface was measured using a laser interferometer. The result is
reported in Table 4 as P-V flatness in exposure-accessible
region.
[0074] The titania-doped quartz glass obtained in Example 1
experienced a OH group concentration change within the measurement
error range upon 900.degree. C./100-hr heat treatment. That is, a
reduction of the OH group concentration is substantially nil while
the OH group concentration is at an adequate level. Neither
coloration nor inclusion was observed, the OH group concentration
gradient was low, and the fictive temperature distribution was
restricted. The glass is best suited as EUV lithography
members.
[0075] The titania-doped quartz glass obtained in Example 2
included a region experiencing a relatively large reduction of the
OH group concentration. As a result, the OH group concentration
gradient and the fictive temperature distribution were relatively
noticeable.
[0076] The titania-doped quartz glass obtained in Example 3
included an outer peripheral portion having a high OH group
concentration and containing bubbles. However, a reduction of the
OH group concentration was small. Given that the bubbled region is
machined off, the glass is suited as EUV lithography members.
Example 4
[0077] A titania-doped quartz glass ingot was prepared by the same
procedure as in Example 1 except that the target was rotated at 5
rpm. The ingot was measured for properties as in Example 1, with
the results shown in Table 4. FIG. 4 shows a radial distribution of
the OH group concentration before and after the heat treatment
(air/900.degree. C./atmospheric pressure/100 hours).
[0078] The titania-doped quartz glass obtained in Example 4 showed
an increased OH group concentration gradient because the OH group
concentration abruptly increased in an outer peripheral portion.
However, a reduction of the OH group concentration was small. Thus,
the glass was suited as EUV lithography members.
Comparative Examples 1 and 2
[0079] Titania-doped quartz glass ingots were prepared by the same
procedure as in Example 1 except that the gas feed conditions shown
in Table 2 were used. The ingots were measured for properties as in
Example 1, with the results shown in Table 4. FIGS. 5 and 6 show a
radial distribution of the OH group concentration before and after
the heat treatment (air/900.degree. C./atmospheric pressure/100
hours) in Comparative Examples 1 and 2, respectively.
[0080] The titania-doped quartz glass obtained in Comparative
Example 1 had an adequate OH group concentration, but showed a
substantial reduction of OH group concentration in the overall
ingot.
[0081] The titania-doped quartz glass obtained in Comparative
Example 2 showed a substantial reduction of OH group concentration,
with the reduction having a broad distribution. In a region showing
a substantial reduction of OH group concentration, crystalline
silica was observed after shaping.
TABLE-US-00001 TABLE 1 Example 1, 4 Example 2 Example 3
Cross-sectional Gas flow Cross-sectional Gas flow Cross-sectional
Gas flow Gas area (mm.sup.2) rate (Nm.sup.3/hr) area (mm.sup.2)
rate (Nm.sup.3/hr) area (mm.sup.2) rate (Nm.sup.3/hr) Central tube
SiCl.sub.4 15 1,420 g/hr 15 1,420 g/hr 15 1,420 g/hr TiCl.sub.4
.sup. 200 g/hr .sup. 200 g/hr .sup. 200 g/hr O.sub.2 2.46 2.46 2.46
1st enclosure O.sub.2 30 1 30 1 30 1 tube 2nd enclosure H.sub.2 50
14 50 17 50 14 tube Shell tube H.sub.2 1,700 25 1,700 24 1,700 32
Nozzles O.sub.2 150 13 150 13 150 17 Dual tube O.sub.2 1,090 3
1,090 3 1,090 3
TABLE-US-00002 TABLE 2 Comparative Example 1 Comparative Example 2
Cross-sec- Gas Cross-sec- Gas tional area flow rate tional area
flow rate Gas (mm.sup.2) (Nm.sup.3/hr) (mm.sup.2) (Nm.sup.3/hr)
Central tube SiCl.sub.4 15 1,420 g/hr 15 1,420 g/hr TiCl.sub.4
.sup. 200 g/hr .sup. 200 g/hr O.sub.2 0.92 0.92 1st enclo- O.sub.2
30 1 30 1 sure tube 2nd enclo- H.sub.2 50 14 50 24 sure tube Shell
tube H.sub.2 1,700 25 1,700 24 Nozzles O.sub.2 150 11 150 13 Dual
tube O.sub.2 1,090 2 1,090 3
TABLE-US-00003 TABLE 3 Comparative Example Example 1, 4 2 3 1 2
Molar ratio of O.sub.2 gas 11.6 11.6 11.6 4.4 4.4 to reactant gases
Linear velocity of H.sub.2 gas 77.78 94.44 77.78 77.78 133.33 in
2nd enclosure tube (m/sec) Linear velocity of H.sub.2 gas 4.08 3.92
5.23 4.08 3.92 in shell tube (m/sec)
TABLE-US-00004 TABLE 4 Comparative Example Example 1 2 3 4 1 2
Reduction of Maximum .ltoreq.2 59 11 9 144 190 OH group Minimum
.ltoreq.2 3 .ltoreq.2 2 112 111 concentration (ppm) (Max - Min)
.ltoreq.2 56 .ltoreq.11 7 32 79 OH group Maximum 634 648 969 738
515 522 concentration (ppm) Minimum 602 545 875 587 477 410
Gradient of OH group 19 56 49 110 27 68 concentration (ppm/cm)
Hydrogen molecule concentration N.D. N.D. 4 N.D. N.D. N.D.
(molecules .times. 10.sup.17/cm.sup.3) Fictive temperature Maximum
872 888 853 865 902 898 (.degree. C.) Minimum 860 843 825 814 880
861 (Max - Min) 12 45 28 51 22 37 TiO.sub.2 concentration Maximum
7.1 7.1 7.1 7.1 7.1 7.1 (wt %) Minimum 6.9 6.9 6.9 6.9 6.9 6.9 Zero
expansion temperature (.degree. C.) 43 45 41 38 25 32 P-V flatness
(nm) 58 62 84 73 92 81 Note: N.D. stands for not detected.
[0082] Japanese Patent Application No. 2009-294557 is incorporated
herein by reference.
[0083] Although some preferred embodiments have been described,
many modifications and variations may be made thereto in light of
the above teachings. It is therefore to be understood that the
invention may be practiced otherwise than as specifically described
without departing from the scope of the appended claims.
* * * * *