U.S. patent application number 10/269536 was filed with the patent office on 2003-06-26 for exposure apparatus including silica glass for photolithography.
This patent application is currently assigned to Nikon Corporation. Invention is credited to Hiraiwa, Hiroyuki, Jinbo, Hiroki, Komine, Norio.
Application Number | 20030119649 10/269536 |
Document ID | / |
Family ID | 26333474 |
Filed Date | 2003-06-26 |
United States Patent
Application |
20030119649 |
Kind Code |
A1 |
Jinbo, Hiroki ; et
al. |
June 26, 2003 |
Exposure apparatus including silica glass for photolithography
Abstract
A silica glass has a structure determination temperature of 1200
K or lower and an OH group concentration of at least 1,000 ppm. The
silica glass is used for photolithography together with light in a
wavelength region of 400 nm or shorter.
Inventors: |
Jinbo, Hiroki;
(Kawasaki-shi, JP) ; Komine, Norio;
(Sagamihara-shi, JP) ; Hiraiwa, Hiroyuki;
(Yokohama-shi, JP) |
Correspondence
Address: |
PENNIE AND EDMONDS
1155 AVENUE OF THE AMERICAS
NEW YORK
NY
100362711
|
Assignee: |
Nikon Corporation
|
Family ID: |
26333474 |
Appl. No.: |
10/269536 |
Filed: |
October 10, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10269536 |
Oct 10, 2002 |
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09593441 |
Jun 14, 2000 |
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09593441 |
Jun 14, 2000 |
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08581017 |
Jan 3, 1996 |
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6087283 |
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Current U.S.
Class: |
501/53 ;
355/53 |
Current CPC
Class: |
C03C 2203/44 20130101;
G03F 7/70958 20130101; C03B 19/1423 20130101; C03C 2201/23
20130101; C03C 2201/11 20130101; Y02P 40/57 20151101; C03C 2203/52
20130101; C03B 2207/36 20130101; C03B 19/1453 20130101; C03C 4/0085
20130101; C03B 2201/21 20130101; C03B 2201/23 20130101; C03C
2203/42 20130101; C03C 2201/21 20130101; C03C 3/06 20130101 |
Class at
Publication: |
501/53 ;
355/53 |
International
Class: |
C03C 003/04; G03B
027/42 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 6, 1995 |
JP |
000479/1995 |
Jan 13, 1995 |
JP |
004077/1995 |
Claims
What is claimed is:
1. A silica glass for photolithography used together with light in
a wavelength region of 400 nm or shorter, said silica glass having
a structure determination temperature of 1,200 K or lower and an OH
group concentration of at least 1,000 ppm.
2. A silica glass according to claim 1, wherein said silica glass
has a fluorine concentration of at least 300 ppm.
3. A silica glass according to claim 1, wherein said silica glass
has a scattering loss amount of 0.2%/cm or less with respect to ArF
excimer laser.
4. A silica glass according to claim 1, wherein said silica glass
has a scattering loss characteristic which is of center
symmetry.
5. A silica glass according to claim 1, wherein said silica glass
has an internal absorptivity of 0.2%/cm or less at a thickness of
10 mm with respect to ArF excimer laser.
6. A silica glass according to claim 1, wherein said silica glass
has an internal transmittance of 99.6% or more at a thickness of 10
mm with respect to ArF excimer laser.
7. A silica glass according to claim 1, wherein, after being
irradiated with 1.times.10.sup.6 pulses of KrF excimer laser at an
average one-pulse energy density of 400 mJ/cm.sup.2, said silica
glass exhibits an internal transmittance exceeding 99.5% at a
thickness of 10 mm with respect to light having a wavelength of 248
nm.
8. A silica glass according to claim 1, wherein, after being
irradiated with 1.times.10.sup.6 pulses of ArF excimer laser at an
average one-pulse energy density of 100 mJ/cm.sup.2, said silica
glass exhibits an internal transmittance exceeding 99.5% at a
thickness of 10 mm with respect to light having a wavelength of 193
nm.
9. A silica glass according to claim 1, wherein said silica glass
has a birefringence amount of 2 nm/cm or less.
10. A silica glass according to claim 1, wherein said silica glass
has a polarization characteristic and a birefringence
characteristic which are of center symmetry.
11. An optical member used together with light in a wavelength
region of 400 nm or shorter, said optical member comprising a
silica glass according to claim 1.
12. An optical member according to claim 11, wherein said silica
glass has a fluorine concentration of at least 300 ppm.
13. An exposure apparatus using light in a wavelength region of 400
nm or shorter as exposure light, which comprises: a stage allowing
a photosensitive substrate to be held on a main surface thereof; an
illumination optical system for emitting the exposure light of a
predetermined wavelength and transferring a predetermined pattern
of a mask onto said substrate; a projection optical system provided
between a surface on which the mask is disposed and said substrate,
for projecting an image of the pattern of said mask onto said
substrate; and an optical member comprising the silica glass
according to claim 1.
14. An exposure apparatus according to claim 13, wherein said
silica glass has a fluorine concentration of at least 300 ppm.
15. An exposure apparatus according to claim 13, wherein said
illumination optical system comprises said optical member.
16. An exposure apparatus according to claim 13, wherein said
projection optical system comprises said optical member.
17. A method for producing a silica glass having a structure
determination temperature of 1,200 K or lower and an OH group
concentration of at least 1,000 ppm, said method comprising the
steps of: heating a silica glass ingot having an OH group
concentration of 1,000 ppm or more to a temperature of 1,200 to
1,350 K; maintaining said ingot at said temperature for a
predetermined period of time; and then cooling said ingot to a
temperature of 1,000 K or lower at a temperature-lowering rate of
50 K/hr or less to anneal said ingot.
18. A method according to claim 17, further comprising a step of
hydrolyzing a silicon compound in a flame to obtain fine glass
particles, and depositing and melting said fine glass particles to
obtain the silica glass ingot having an OH group concentration of
1,000 ppm or more.
19. A method according to claim 18, wherein a volume ratio of
oxygen gas to hydrogen gas in said flare is 0.4 or more.
20. A method according to claim 17, further comprising the steps
of: hydrolyzing a silicon compound in a flame to obtain fine glass
particles, and depositing and melting said fine glass particles to
obtain a silica glass ingot having an OH group concentration of
1,000 ppm or more; and then cooling said silica glass ingot from a
temperature of at least 1,373 K to a temperature not higher than
1,073 K at a temperature-lowering rate of 50 K/hr or less to
pre-anneal said ingot.
21. A method according to claim 20, wherein a volume ratio of
oxygen gas to hydrogen gas in said flare is 0.4 or more.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a silica glass for
photolithography, optical members including the glass, an exposure
apparatus including the same, and a method for producing the glass.
More particularly, it relates to a silica glass used in
photolithography techniques together with light in a wavelength
region of 400 nm or shorter or, more preferably, 300 nm or shorter,
optical members such as lens and mirror including the glass, an
exposure apparatus including the glass, and a method for producing
the glass.
[0003] 2. Related Background Art
[0004] In recent years, VLSI has been produced with a higher
integration and a higher functionality. Particularly, in the field
of logical VLSI, a larger system has been mounted on a chip,
namely, system-on-chip technique has been in progress. In
conjunction with such a trend, there is a demand for finer
processability and higher integration on a wafer, such as that made
of silicon, which constitutes a substrate for VLSI. In
photolithography techniques according to which fine patterns of
integrated circuits are exposed to light and transferred onto
wafers such as of silicon, exposure apparatuses called stepper are
used.
[0005] In the case of DRAM, as an example of VLSI, with the advance
from LSI to VLSI, as its capacity gradually increases from 1 KB
through 256 KB, 1 MB, 4 MB, and 16 MB to 64 MB, the processing line
width required for the stepper correspondingly becomes finer from
10 .mu.m through 2 .mu.m, 1 .mu.m, 0.8 .mu.m, and 0.5 .mu.m to 0.3
.mu.m.
[0006] Accordingly, it is necessary for a projection lens of the
stepper to have a high resolution and a great depth of focus. The
resolution and the depth of focus are determined by the wavelength
of the light used for exposure and the N.A. (numerical aperture) of
the lens.
[0007] The angle of the diffracted light becomes greater as the
pattern is finer, whereas the diffracted light cannot be captured
when the N.A. of the lens becomes greater. Also, the angle of the
diffracted light becomes smaller in the same pattern as its
exposure wavelength X is shorter, thereby allowing the N.A. to
remain small.
[0008] The resolution and the depth of focus are expressed as
indicated by the following equations:
resolution=k1.multidot..lambda./N.A.
depth of focus=k2.multidot..lambda./N.A.
[0009] wherein k1 and k2 are constants of proportionality.
[0010] In order to improve the resolution, either the N.A. is
increased or .lambda. is shortened. However, as can be seen from
the above equations, it is advantageous, in terms of the depth of
focus, to shorten .lambda.. In view of these points of view,
wavelength of light sources becomes shorter from g-line (436 nm) to
i-line (365 nm) and further to KrF excimer laser beam (248 nm) and
ArF excimer laser beam (193 nm).
[0011] Also, since the optical system loaded in the stepper is
constituted by a combination of numerous optical members such as
lenses, even when each lens sheet has a small transmission loss,
such a loss is multiplied by the number of the lens sheets used,
thereby decreasing the amount of light at the irradiated surface.
Accordingly, it is necessary for the optical member to have a high
degree of transmittance.
[0012] Therefore, in the steppers using light in a wavelength
region of 400 nm or shorter, optical glass made by a specific
method in view of the shortening of wavelength as well as the
transmission loss due to the combination of the optical members is
used. Also, in the steppers using light in a wavelength region of
300 nm or shorter, it has been proposed to use synthetic silica
glass and a fluoride single crystal such as CaF.sub.2
(fluorite).
[0013] As a specific method for measuring internal transmittance,
for example, a method of measuring transmittance of optical glass
is known from JOGIS 17-1982. Here, the internal transmittance is
calculated by the following equation: 1 log = - log T1 - log T2 d
.times. 10 ( 1 )
[0014] wherein .tau. is internal transmittance of the glass when
its thickness is 10 mm; d is difference in thickness of a sample;
and T1 and T2 are spectral transmission factors of the glass having
sample thickness values of 3 mm and 10 mm, respectively, including
their reflection loss.
SUMMARY OF THE INVENTION
[0015] However, the inventors have found that, in the optical
members composed of the conventional silica glass whose internal
transmittance is defined in this manner, although a certain
magnitude of the resolution is secured in terms of their
specification, contrast of an image resulting therefrom may be so
unfavorable that a sufficiently vivid image cannot be obtained.
[0016] Here, the contrast is defined by the following equation: 2
contrast = I max - I min I max + I min ( 2 )
[0017] wherein Imax is maximum value of optical intensity on a
wafer surface and Imin is minimum value of the optical intensity on
the wafer surface.
[0018] The object of the invention is to provide a silica glass for
photolithography which can overcome the foregoing shortcomings of
the prior art and can realize a sufficiently fine and vivid
exposure and transfer pattern with a favorable contrast.
[0019] Accordingly, the inventors have studied, among the
transmission loss factors in the silica glass (optical member) used
for photolithography techniques and the like, factors for
decreasing the contrast of image. As a result, it has been found
that not only the optical absorption at the silica glass but also
its optical scattering causes the transmission loss and that the
amount of loss in light based on such optical scattering
(scattering loss amount) can be sufficiently suppressed when the
structure determination temperature in the silica glass containing
at least a predetermined amount of OH group is reduced at least to
a predetermined level. Thus, the present invention has been
accomplished.
[0020] The silica glass (fused silica, quartz glass) of the present
invention is used for photolithography together with light in a
wavelength region of 400 nm or shorter and is characterized in that
it has a structure determination temperature of 1,200 K or lower
and an OH group concentration of at least 1,000 ppm.
[0021] Further, the optical member (optical component) of the
present invention is an optical member used for photolithography
together with light in a wavelength region of 400 nm or shorter and
is characterized in that it includes the above-mentioned silica
glass of the present invention.
[0022] Furthermore, the exposure apparatus (exposing device) of the
present invention is an exposure apparatus which uses light in a
wavelength region of 400 nm or shorter as exposure light and is
characterized in that it is provided with the optical member
including the above-mentioned silica glass of the present
invention.
[0023] Moreover, the method for producing the silica glass in
accordance with the present invention is characterized in that it
comprises the steps of heating a silica glass ingot having an OH
group concentration of at least 1,000 ppm to a temperature of 1,200
to 1,350 K, maintaining the ingot at that temperature for a
predetermined period of time, and then cooling the ingot to a
temperature of 1,000 K or lower at a temperature-lowering rate
(cooling rate) of 50 K/hr or less to anneal the ingot, whereby
making it possible to produce a silica glass having a structure
determination temperature of 1,200 K or lower and an OH group
concentration of at least 1,000 ppm.
[0024] The "structure determination temperature" herein used is a
factor introduced as a parameter which expresses structural
stability of silica glass and will be explained in detail below.
The fluctuation in density of silica glass at room temperature,
namely, structural stability is determined by density of the silica
glass in the state of melt at high temperatures and density and
structure of the silica glass when the density and the structure
are frozen at around the glass transition point in the process of
cooling. That is, thermodynamic density and structure corresponding
to the temperature at which the density and structure are frozen
are also retained at room temperature. The temperature when the
density and structure are frozen is defined to be "structure
determination temperature" in the present invention.
[0025] The structure determination temperature can be obtained in
the following manner. First, a plurality of silica glass test
pieces are retained at a plurality of temperatures within the range
of 1073-1700 K for a period longer than the structure relaxation
time (a time required for the structure of the silica glass being
relaxed at that temperature) in the air in a tubular oven as shown
in the accompanying FIG. 1, thereby to allow the structure of the
respective test pieces to reach the structure at the retention
temperature. As a result, each of the test pieces has a structure
which is in the thermal equilibrium state at the retention
temperature. In FIG. 1, 101 indicates a test piece, 102 indicates a
silica glass tube, 103 indicates a heater, 104 indicates a
thermocouple, 105 indicates a beaker, and 106 indicates liquid
nitrogen.
[0026] Then, the test pieces are introduced not into water, but
into liquid nitrogen in 0.2 second to quench them. If they are
introduced into water, quenching is insufficient and structural
relaxation occurs in the process of cooling, and the structure at
the retention temperature cannot be fixed. Moreover, it can be
considered that adverse effect may occur due to the reaction
between water and the silica glass. In the present invention,
super-quenching can be attained by introducing the test pieces into
liquid nitrogen as compared with introduction into water, and by
this operation, it becomes possible to fix the structure of the
test pieces to the structure at the retention time. In this way,
for the first time, the structure determination temperature can be
allowed to coincide with the retention temperature.
[0027] The thus obtained test pieces having various structure
determination temperatures (equal to the retention temperatures
here) are subjected to measurement of Raman scattering, and 606
cm.sup.-1 line intensity is obtained as a ratio to 800 cm.sup.-1
line intensity. A graph is prepared with employing as a variable
the structure determination temperature for 606 cm.sup.-1 line
intensity and this is used as a calibration curve. A structure
determination temperature of a test piece of which the structure
determination temperature is unknown can be inversely calculated
from the measured 606 cm.sup.-1 line intensity using the
calibration curve. In the present invention, a temperature obtained
in the above manner on a silica glass the structure determination
temperature of which is unknown is employed as the structure
determination temperature of the silica glass.
[0028] The present invention will be more fully understood from the
detailed description given hereinbelow and the accompanying
drawings, which are given by way of illustration only and are not
to be considered as limiting the present invention.
[0029] Further scope of applicability of the present invention will
become apparent from the detailed description given hereinafter.
However, it should be understood that the detailed description and
specific examples, while indicating preferred embodiments of the
invention, are given by way of illustration only, since various
changes and modifications within the spirit and scope of the
invention will be apparent to those skilled in the art from this
detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1 is a schematic view showing an example of an
apparatus used for measuring the structure determination
temperature in accordance with the present invention;
[0031] FIG. 2 is a block diagram showing the basic structure of an
example of the exposure apparatus of the present invention.
[0032] FIG. 3 is a conceptual view showing an integrating sphere
type measurement apparatus for scattering light;
[0033] FIG. 4 is a conceptual view showing a goniophotometry type
measurement apparatus for scattering light;
[0034] FIG. 5 is a conceptual view showing an ellipsoidal mirror
type measurement apparatus for scattering light;
[0035] FIG. 6 is a graph showing the relationship between
scattering loss amount and contrast;
[0036] FIG. 7 is a graph showing the relationship between
wavelength and scattering loss;
[0037] FIG. 8 is a graph showing the relationship between structure
determination temperature and scattering loss;
[0038] FIG. 9 is a graph showing the relationship between
refractive index and scattering loss;
[0039] FIG. 10 is a schematic view showing an example of an
apparatus for producing a silica glass ingot in accordance with the
present invention;
[0040] FIG. 11 is a bottom view showing an example of a burner for
producing a silica glass ingot in accordance with the present
invention;
[0041] FIG. 12 is a perspective view showing an example of an
annealing furnace in accordance with the present invention; and
[0042] FIG. 13 is a graph showing the relationship between
structure determination temperature and scattering loss.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0043] First, the silica glass in accordance with the present
invention will be explained.
[0044] The silica glass in accordance with the present invention is
used for photolithography together with light in a wavelength
region of 400 nm or shorter and is characterized in that it has a
structure determination temperature of 1,200 K or lower and an OH
group concentration of at least 1,000 ppm and preferably within the
range of 1,000 to 1300 ppm.
[0045] In this manner, when the structure determination temperature
of 1,200 K or lower and the OH group concentration of at least
1,000 ppm are specified, a silica glass having a low scattering
loss amount, such as a scattering loss amount of 0.2%/cm or less
with respect to ArF excimer laser, which cannot have been achieved
conventionally, can be obtained, thereby sufficiently preventing
the contrast from decreasing due to flare and ghost resulting from
scattering light.
[0046] In general, optical energy impinging on an object generates
a scattering phenomenon. The scattering phenomenon can be roughly
divided into elastic scattering such as Rayleigh scattering and
Brillouin scattering and inelastic scattering such as Raman
scattering. In particular, when the scattering intensity in an
optical member is high, the scattering light therefrom becomes
flare or ghost so as to decrease the contrast of the image, thereby
causing the optical characteristic to deteriorate.
[0047] The optical scattering, however, has been considered to be
sufficiently less influential than the lowering of the resolution
caused by change in the form or refractive index of the optical
member upon optical absorption and at a level which can be
practically neglected. Actually, in optical instruments using light
in the visible region, the main cause for transmission loss is
optical absorption and, accordingly, when its optical absorption is
set to a level not higher than a certain level, a desirable
resolution is satisfied together with a favorable contrast in the
image.
[0048] The inventors, however, have found that, the optical
scattering can become less negligible as the wavelength of the
incident light has a shorter wavelength, so that, in particular, in
the conventional optical members such as projection lens used for
photolithography, a vivid image cannot be obtained due to flare and
ghost resulting from the scattering light.
[0049] Though there has not been definitely elucidated mechanism by
which the scattering loss with respect to ArF excimer laser is
remarkably suppressed when at least 1,000 ppm of OH group are
introduced into a silica glass having a structure determination
temperature of 1,200 K or less, i.e., a silica glass having nearly
an ideal structure, the inventors consider as explained in the
following. Meanwhile, the structure determination temperature of
the silica glass in accordance with the present invention is quite
lower than that of optical fibers, for example, which is typically
about 1,450 K.
[0050] The silica glass having a high structure determination
temperature is considered to be structurally unstable. Namely, the
bond angle of .ident.Si--O--Si.ident. in the network of silica
glass has a certain distribution because of it being a glass and
this distribution of the bond angle includes structurally unstable
bond angles. This distribution of the bond angle comprises bridged
tetrahedrons made of oxygen atoms and silicon atoms in the silicon
glass and the presence of the unstable bond angles is considered to
be caused by the presence of the tetrahedrons in a distorted state.
Such a distorted bond portion is considered to be readily cut by
irradiation with ultraviolet light to produce the defects such as
detrimental E' center and NBOHC. On the other hand, the silica
glass having a low structure determination temperature is
considered to have few such distorted bond portions.
[0051] Also, the silica glass having an OH group concentration
within the above-mentioned range is structurally more stable than
other kinds of silica glass and its structure determination
temperature tends to become lower.
[0052] The detailed reasons therefor are as follows. Namely, as
mentioned above, the .ident.Si--O--Si.ident. bond angle in the
network of silica glass has a certain distribution because of it
being a glass and it contains structurally unstable distorted bond
portions. However, when OH group within the above-mentioned range
is contained therein, there is no need to make bridges using
unstable bond angle and, accordingly, the tetrahedron can
approximate its most stable structure. Therefore, the silica glass
containing OH group within the above-mentioned range is
structurally more stable than other kinds of silica glass and its
structure determination temperature tends to become lower.
[0053] Accordingly, in the silica glass in accordance with the
present invention in which the OH group concentration is at least
1,000 ppm and the structure determination temperature is 1,200 K or
lower, due to their synergistic effect, the scattering loss amount
of 0.2%/cm or less is attained with respect to ArF excimer
laser.
[0054] Preferably, the silica glass in accordance with the present
invention has a fluorine content of at least 300 ppm. This is
because, under the same annealing condition, the structure
determination temperature tends to become lower when the fluorine
content is at least 300 ppm.
[0055] Further, the total amount of optical scattering and optical
absorption, namely, the transmission loss amount, influences the
light amount on the reticle and wafer, thereby influencing the
decrease in throughput due to the decrease in illumination
intensity or the like. In particular, in the photolithography
optical system, since its resolution is maximized to the limit, the
number of lens sheets for correcting various kinds of wave front
aberrations is large and its optical length is long. Accordingly,
even a minute transmission loss amount (scattering loss amount plus
absorption loss amount) may become influential. For example, in the
optical path length of 1 m, even when the transmission loss amount
is only 0.2%/cm, the total transmission loss amount becomes about
18%.
[0056] Therefore, in the silica glass in accordance with the
present invention, the internal absorptivity in the silica glass
having a thickness of 10 mm with respect to ArF excimer laser is
preferably 0.2%/cm or less. This kind of optical absorption is a
cause for decreasing the resolution as will be explained in the
following. Namely, optical absorption is a phenomenon resulting
from electronic transition caused by photon energy impinging on an
optical member. When optical absorption occurs in the optical
member, its energy is mainly converted into thermal energy, thereby
inflating the optical member or changing its refractive index or
surface condition. As a result, high resolution cannot be obtained.
Further, the optical absorption is accompanied by change in
electronic condition and, during the period by which it is relaxed,
light having a longer wavelength than that of the incident light is
released as fluorescence. When the fluorescence has a wavelength
similar to that of exposure light and its intensity is high, the
contrast of the image is remarkably decreased. Accordingly, in
order to obtain a fine and vivid image with a favorable contrast,
it is preferable to specify the absorption loss amount together
with the scattering loss amount.
[0057] Also, as factors for deteriorating the ultraviolet light
resistance of silica glass, there have been known, for example,
.ident.Si--Si.ident., .ident.Si--O--O--Si.ident., and dissolved
oxygen molecules. These precursors are readily converted into
structural defects such as E' center and NBOHC upon exposure to
ultraviolet light such as excimer laser, thereby causing the
transmittance to decrease. In the silica glass in accordance with
the present invention, it is preferable that there are no
incomplete structures caused by such a deviation from the
stoichiometric ratio. For example, when OH group within the
above-mentioned range is contained therein, the silica glass tends
to contain substantially no oxygen-shortage type defect absorption
bands (7.6- and 5.0-eV absorption bands). Also, when the silica
glass in accordance with the present invention containing at least
5.times.10.sup.16 molecules/cm.sup.3 of hydrogen molecules is
irradiated with 1.times.10.sup.6 pulses of ArF excimer laser at a
one-pulse energy density of 100 mJ/cm.sup.2, substantially no
oxygen-excess type defect absorption band (4.8-eV absorption band)
is generated. Due to the absence of these defects, according to
measurement of transmittance effected by vacuum ultraviolet,
ultraviolet, visible, and infrared spectrophotometers, high
transmittance ratios of at least 99.9% in terms of internal
transmittance (for silica glass having a thickness of 10 mm) for
the light of the wavelength of g-line (436 nm) to i-line (365 nm)
and KrF excimer laser beam (248 nm) and at least 99.6% in terms of
internal transmittance (for silica glass having a thickness of 10
mm) for the light of the wavelength of ArF excimer laser beam (193
nm). Also, after being irradiated with 1.times.10.sup.6 pulses of
KrF excimer laser at an average one-pulse energy density of 400
mJ/cm.sup.2, the internal transmittance of the above-mentioned
silica glass having a thickness of 10 mm exceeds 99.5% with respect
to the light having a wavelength of 248 nm. On the other hand,
after being irradiated with 1.times.10.sup.6 pulses of ArF excimer
laser at an average one-pulse energy density of 100 mJ/cm.sup.2,
the internal transmittance of the above-mentioned silica glass
having a thickness of 10 mm exceeds 99.59 with respect to the light
having a wavelength of 193 nm.
[0058] Also, it is desirable for the structure determination
temperature distribution in the silica glass of the present
invention to have a center symmetry within the member since this
will render a center symmetry to its scattering loss characteristic
(scattering intensity). In this case, it becomes easy to specify,
at the time of adjusting a lens, lens parts which may cause flare
or ghost, thereby facilitating the optical adjustment. Further, the
contrast can be prevented from fluctuating on the image-forming
surface. Moreover, the silica glass of the present invention
preferably has a birefringence amount of 2 nm/cm or less and
centrally symmetrical polarization and birefringence
characteristics.
[0059] In the silica glass of the present invention, the chlorine
concentration is preferably 50 ppm or less and, in particular, 10
ppm or less. This is because, when the chlorine concentration
exceeds 50 ppm, it tends to become difficult for the OH group
concentration in the silica glass to be maintained at 1,000 ppm or
higher.
[0060] Also, the silica glass preferably has a high quality such
that its concentration of each of metallic impurities (Mg, Ca, Ti,
Cr, Fe, Ni, Cu, Zn, Co, Mn, Na and K) is 50 ppb or less and more
preferably 20 ppb or less. In this case, the above-mentioned
structural defects decrease to form a structure approximating the
ideal structure and also the change in refractive index, change in
surface, and deterioration of transmittance caused by the metallic
impurities become lower such that the ultraviolet light resistance
tends to improve.
[0061] In the following, the optical member and exposure apparatus
of the present invention will be explained. The optical member of
the present invention includes the above-mentioned silica glass of
the present invention in which the structure determination
temperature is 1,200 K or less and the OH group concentration is at
least 1,000 ppm. Such an optical member has no particular
limitation, as long as it includes the above-mentioned silica
glass, and may be such an optical member as lens or prism which is
used together with light in a wavelength region of 400 nm or
shorter. The optical member of the present invention includes
blank. Further, the method for processing the above-mentioned
silica glass of the present invention into the optical member of
the present invention is not restricted in particular, while normal
cutting method or abrasion method, for example, may be
appropriately used.
[0062] Since the optical member of the present invention includes a
silica glass which, as mentioned above, exhibits a very small
scattering loss amount with respect to light of a short wavelength
such as ArF excimer laser beam, as compared with the conventional
optical members, the scattering light is more effectively prevented
from generating and it exhibits a higher resolution. Accordingly,
the silica glass of the present invention is suitably applied to an
optical member such as a lens in a projection optical system of
steppers which requires such a high resolution as 0.25 .mu.m or
less. The silica glass of the present invention is useful not only
for lenses in the projection system of the stepper but also for
lenses in illumination optical systems, for example.
[0063] An exposure apparatus of the present invention will be
described next. The exposure apparatus of the present invention is
provided with the optical member comprising the silica glass of the
present invention and uses light in the wavelength region of 400 nm
or shorter as exposure light, and has no limitation except that it
contains the silica glass as a lens of illumination optical system,
projection optical system, or the like, and is provided with a
light source for emitting light in the wavelength region of 400 nm
or shorter.
[0064] The present invention is preferably applied to the
projection exposure apparatus, such as a so-called stepper, for
projecting an image of patterns of reticle onto a wafer coated with
a photoresist.
[0065] FIG. 2 shows a basic structure of the exposure apparatus
according to the present invention. As shown in FIG. 2, an exposure
apparatus of the present invention comprises at least a wafer stage
301 allowing a photosensitive substrate W to be held on a main
surface 301a thereof, an illumination optical system 302 for
emitting vacuum ultraviolet light of a predetermined wavelength as
exposure light and transferring a predetermined pattern of a mask
(reticle R) onto the substrate W, a light source 303 for supplying
the exposure light to the illumination optical system 302, a
projection optical system (preferably a catadioptric one) 304
provided between a first surface P1 (object plane) on which the
mask R is disposed and a second surface P2 (image plane) to which a
surface of the substrate W is corresponded, for projecting an image
of the pattern of the mask R onto the substrate W. The illumination
optical system 302 includes an alignment optical system 305 for
adjusting a relative positions between the mask R and the wafer W,
and the mask R is disposed on a reticle stage 306 which is movable
in parallel with respect to the main surface of the wafer stage
301. A reticle exchange system 307 conveys and changes a reticle
(mask R) to be set on the reticle stage 306. The reticle exchange
system 307 includes a stage driver for moving the reticle stage 306
in parallel with respect to the main surface 301a of the wafer
stage 301. The projection optical system 304 has a space permitting
an aperture stop 308 to be set therein. The sensitive substrate W
comprises a wafer 309 such as a silicon wafer or a glass plate,
etc., and a photosensitive material 310 such as a photoresist or
the like coating a surface of the wafer 309. The wafer stage 301 is
moved in parallel with respect to a object plane P1 by a stage
control system 311. Further, since a main control section 312 such
as a computer system controls the light source 303, the reticle
exchange system 307, the stage control system 311 or the like, the
exposure apparatus can perform a harmonious action as a whole.
[0066] The exposure apparatus of the present invention comprises an
optical member which comprises the silica glass of the present
invention, for example an optical lens consisting of the
above-mentioned silica glass. More specifically, the exposure
apparatus of the present invention shown in FIG. 2 can include the
optical lens of the present invention as an optical lens 313 in the
illumination optical system 302 and/or an optical lens 314 in the
projection optical system 304.
[0067] Since the exposure apparatus of the present invention is
provided with the optical member made of a silica glass which, as
mentioned above, exhibits a very small scattering loss amount with
respect to light of a short wavelength such as ArF excimer laser
beam, as compared with the conventional optical members, the
contrast of the image is more sufficiently prevented from lowering
due to flare or ghost and it attains a higher resolution.
[0068] The techniques relating to an exposure apparatus of the
present invention are described, for example, in U.S. patent
application Ser. No. 255,927, No. 260,398, No. 299,305, U.S. Pat.
No. 4,497,015, No. 4,666,273, No. 5,194,893, No. 5,253,110, No.
5,333,035, No. 5,365,051, No. 5,379,091, or the like. The reference
of U.S. patent application Ser. No. 255,927 teaches an illumination
optical system (using a laser source) applied to a scan type
exposure apparatus. The reference of U.S. patent application Ser.
No. 260,398 teaches an illumination optical system (using a lamp
source) applied to a scan type exposure apparatus. The reference of
U.S. patent application Ser. No. 299,305 teaches an alignment
optical system applied to a scan type exposure apparatus. The
reference of U.S. Pat. No. 4,497,015 teaches an illumination
optical system (using a lamp source) applied to a scan type
exposure apparatus. The reference of U.S. Pat. No. 4,666,273
teaches a step-and repeat type exposure apparatus capable of using
the catadioptric projection optical system of the present
invention. The reference of U.S. Pat. No. 5,194,893 teaches an
illumination optical system, an illumination region, mask-side and
reticle-side interferometers, a focusing optical system, alignment
optical system, or the like. The reference of U.S. Pat. No.
5,253,110 teaches an illumination optical system (using a laser
source) applied to a step-and-repeat type exposure apparatus. The
'110 reference can be applied to a scan type exposure apparatus.
The reference of U.S. Pat. No. 5,333,035 teaches an application of
an illumination optical system applied to an exposure apparatus.
The reference of U.S. Pat. No. 5,365,051 teaches a auto-focusing
system applied to an exposure apparatus. The reference of U.S. Pat.
No. 5,379,091 teaches an illumination optical system (using a laser
source) applied to a scan type exposure apparatus. These documents
are hereby incorporated by reference.
[0069] Having conducted optical simulation and experiments for
evaluating image-forming properties in cases where the
above-mentioned optical member is used, the inventors have found it
possible to provide an exposure apparatus (photolithography
apparatus) which is substantially prevented from being influenced
by flare or ghost and there is no problem concerning the decrease
of light amount in terms of its property. Based on this finding, in
an optical system constructed by using the optical member of the
present invention, a fine and vivid exposure and transfer pattern
with a line width of 0.25 .mu.m or less has been obtained.
[0070] Thus, the inventors have diligently studied characteristics
of the optical member by which a fine and vivid exposure and
transfer pattern can be obtained in photolithography techniques. As
a result, the inventors have found that, among optical properties
of a projection lens, the transmission loss amount is quite
influential to the image-forming property in cases where the
uniformity in refractive index (.DELTA.n), lens surface accuracy,
and optical thin-film characteristic are at substantially the level
and, more importantly, that the optical properties of the
projection lens cannot be correctly expected unless its
transmission loss is separated into optical absorption and optical
scattering so as to be precisely evaluated. This is because the
optical absorption and the optical scattering generate phenomenons
different from each other, namely, the former contributes to the
deterioration of the image-forming property resulting from the
heating within the lens, whereas the latter contributes to the
deterioration of the contrast resulting from flare or ghost
[0071] Here, explanation will be provided in detail for the optical
scattering in the optical member.
[0072] An optical single crystal such as single crystal fluorite
(CaF.sub.2) is regarded as a perfect crystal. Namely, it is assumed
that the whole atoms and ions in the crystal are regularly arranged
with a distance of about 5 A therebetween and that the crystal has
a uniform density. Also, in view of Huygens-Fresnel's principle
concerning the propagation of light, even when the wave front of
light collides with a molecule (i.e., scattering factor) to
generate numerous secondary spherical waves, except for the
scattering light in the direction where light travels straight
ahead, these waves interfere with each other so as to cancel each
other. Accordingly, the scattering loss of the optical single
crystal becomes quite smaller than that of liquid as well as that
of glass or plastic which is in a non-equilibrium state and, when
structural defects, fine particles, and the like do not practically
exist therewithin, its scattering loss amount is considered to be
negligible.
[0073] However, since a melt material is rapidly cooled when
manufacturing a glass, the arrangement of atoms in the melt glass
may be maintained to a certain degree in the cooled glass.
Accordingly, while the glass is a solid in terms of a macroscopic
property, it has a structure of liquid microscopically. Therefore,
like liquid, it is considered that the molecular distribution of
the glass does not have a regularity such as that of a crystal and
has a statistical thermodynamic fluctuation due to its thermal
motion, thereby generating optical scattering. Such optical
scattering is known as Rayleigh scattering.
[0074] In Rayleigh scattering, the scattering intensity is
inversely proportional to the fourth power of wavelength .lambda..
Accordingly, in optical instruments used in a short wavelength
region, Rayleigh scattering of its optical member is influential to
the optical properties. In particular, optical instruments such as
projection lenses used for photolithography where a superfine
resolution is required, flare and ghost caused by transmission loss
and scattering light become problematic.
[0075] Scattering loss amount of a glass can be calculated by the
following equation: 3 s = 8 2 3 4 n 6 p 2 kTs T ( 3 )
[0076] wherein
[0077] s: scattering loss coefficient (/cm)
[0078] p: Pockels coefficient 0.27
[0079] Ts: structure determination temperature (K)
[0080] .beta.T: isothermal compressibility 7E-12 (cm/dyn)
[0081] .rho.: density 2.201 (g/cm.sup.3)
[0082] .lambda.: wavelength (cm)
[0083] k: Boltzmann constant 1.38E-16 (erg/K)
[0084] n: refractive index
[0085] For example, when calculation is made with respect to a
silica glass, for given physical property values of wavelength
.lambda.=193.4 nm, refractive index n=1.5603, and structure
determination temperature Ts=1,273 K; the scattering loss
coefficient is calculated as s=0.001861/cm, namely, the scattering
loss amount is calculated as 0.1861%/cm. In this manner, the
inventors have found that a larger amount is expected than the
actually measured transmission loss amount and that the main cause
for the transmission loss at 193.4 nm is more attributable to
scattering loss than optical absorption.
[0086] Here, in order to correct the Brillouin scattering portion
and to calculate the Rayleigh scattering coefficient, the term of
.beta.T is corrected as follows:
.beta.T-[.beta.T-(.rho.v.infin..sup.2).sup.-1]=5.7E-12 (4)
[0087] wherein
[0088] v.infin.: high-frequency sound velocity 5.92 (cm/s)
[0089] As a result of this calculation, the scattering loss amount
becomes 0.1516%/cm.
[0090] In view of the foregoing, the theoretically calculated value
of the scattering loss is defined as the Rayleigh scattering loss
plus the Brillouin scattering loss. Here, the Brillouin scattering
loss can be calculated when equation (3) is used while using
(v.infin..sup.2).sup.-1 shown in equation (4) in place of .beta.T
and setting Ts at room temperature (298 K). Brillouin scattering is
theoretically estimated as about {fraction (1/20)} with respect to
Rayleigh scattering.
[0091] However, thus obtained scattering loss amount may have been
estimated lower than its actual value since other scattering
factors and inelastic scattering, for example, are not considered.
Also, since the values indicated here are calculated from
theoretical equations and there may be a problem concerning
reliability of physical property values, they should be regarded as
nothing other than estimated values.
[0092] Therefore, in practice, it is necessary for the scattering
loss amount to be measured.
[0093] Here, an apparatus for measuring the scattering loss amount
will be explained in detail.
[0094] Examples of the measurement apparatus include i) integrating
sphere type (FIG. 3) which uses an integrating sphere to measure
the total scattering amount; ii) goniophotometry type (FIG. 4) used
for measuring angular distribution; and iii) ellipsoidal mirror
type (FIG. 5) which uses an ellipsoidal mirror
[0095] Among the above-mentioned types, substantially the common
light source and optical system are used. With respect to the
visible light region, there is used an actual measurement technique
in which He--Ne laser (632.8 nm), Ar.sup.+ ion laser (e.g., 488 nm)
and the like are used as the light source. With respect to the
actual wavelength of ArF excimer laser (193.4 nm), there is used an
actual measurement technique in which D2 lamp, ArF excimer laser
and the like are used as the light source; or another technique in
which Hg lamp emission line is used so as to interpolate the
scattering loss amount at 193.4 nm according to a calculation
equation.
[0096] Preferably, a sample has a cylindrical or prismatic form in
which light-input and light-output surfaces are parallel planes,
while the other surfaces preferably have a surface roughness of 5
.ANG. or less in terms of RMS and a high surface cleanliness. These
characteristics are used in order to eliminate the influence of the
surface scattering and surface absorption.
[0097] The optical scattering and optical absorption in the present
invention refer to internal scattering and internal absorption of
an optical member, respectively.
[0098] In the following, the detection means in each type of the
measurement apparatus will be explained.
[0099] In the type shown in FIG. 3 in which an integrating sphere
is used, a sample (object to be tested) is held at an optical path
portion within the integrating sphere. In this case, it is
preferable that the length of the sample is slightly longer than
that of the optical path length within the integrating sphere. This
feature is used in order to prevent surface-scattered light from
entering the integrating sphere.
[0100] Also, in order to block a measurement system from
surface-reflected and surface-scattered light components, the
parallel plane portion is provided with a wedge of a few minutes or
the system is tilted by a few degrees with respect to the optical
axis. Further, a signal intensity obtained without the sample is
used for zero-point calibration while an ND filter or the like
whose transmittance is accurately secured is used for determining a
calibration curve. As an optical detection device, a photodiode or
photomultiplier, for example, which is highly sensitive and stable
at each measurement wavelength is used.
[0101] FIG. 4 shows an apparatus in which goniophotometry technique
is used for measuring, in principle, angular dependency of
scattering light. In order to use thus configured apparatus to
measure the absolute value of the scattering light, such a value in
the visible light region is calculated on the basis of its relative
value with respect to a material such as benzene whose scattering
loss coefficient is known. In the ultraviolet region, rare gas or
the like which is hard to be influenced by optical absorption is
preferably used.
[0102] For example, based on a relative scattering intensity
comparison of .theta.90 degrees with respect to the optical axis
(R90 ratio: intensity of 90-degree direction with respect to
optical axis), the whole scattering amount can-be estimated by:
16.pi./3.times.R90
[0103] In this case, it is assumed that the angular dependency of
the scattering is of complete Rayleigh scattering.
[0104] A light-input portion of an optical fiber is set in a
.theta.90-degree direction with respect to the optical axis in
order to transfer the scattering light to the detection means,
while a spectrometer using a photodiode array is employed as the
detection means, thereby enabling easy measurement of the R90
relative value. Also, the spectrum of the scattering light can be
confirmed.
[0105] The ellipsoidal mirror type apparatus shown in FIG. 5 is
mainly used for measuring the surface scattering. While this
apparatus is excellent in measuring relative intensity in the
measurement of scattering as well, it is disadvantageous, for
example, in that a complicated correction equation is required for
calculating the absolute value.
[0106] In view of the foregoing, using the actually measured
scattering values attained by the integrating sphere type and
goniophotometry type apparatuses, the inventors have studied the
influences of the scattering loss upon optical properties of a
photolithography apparatus such as its resolution and contrast.
Also, based on the results thereof, optical simulation and
experiments for evaluating image-forming property have been
effected.
[0107] FIG. 6 shows the relationship between the total scattering
loss amount and contrast in an optical system in the
photolithography apparatus obtained by the experiments for
evaluating image-forming property. As shown in this chart, a very
good correlation has been obtained between them.
[0108] Here, the standard value of the scattering loss amount,
0.2%/cm, is a value calculated from the following equation: 4 S0 L
0.2 ( % / cm ) ( 5 )
[0109] wherein
[0110] S0: maximum value (%) of total scattering loss allowed for
obtaining a required contrast
[0111] L: total optical path length of optical system (cm)
total scattering loss=(total scattering loss intensity)/(incident
light intensity).times.100 (%)
[0112] Namely, the image-forming evaluation experiments have
confirmed that the image-forming property of a stepper is
remarkably influenced by not only the absorption loss but
scattering loss and that, when the scattering loss amount is
0.2%/cm or less, flare and ghost have substantially no influence
and the, decrease in light amount is at a level which is not
problematic in terms of the property without influencing the
image-forming property.
[0113] Further, when the scattering loss amount is 0.2%/cm or less
with respect to light of 193.4 nm, since the scattering loss amount
is inversely proportional to wavelength .lambda..sup.4 while being
proportional to refractive index n.sup.6, the scattering loss
amount becomes smaller as the wavelength is longer for light in a
wavelength region longer than 193.4 nm, thereby satisfying the
standard required for the present invention. This fact has been
confirmed by equation (3) as well as by the results of the
experiments.
[0114] By contrast, from the scattering loss amount in a visible
region, for example, at wavelengths of He--Ne laser (632.8 nm) and
Ar.sup.+ ion laser (e g., 488 nm), the scattering loss amount at
193.4 nm can be calculated by using the inversely proportional rule
with respect to wavelength .lambda..sup.4 and the proportional rule
with respect to refractive index n.sup.6, thereby judging whether
the standard in accordance with the present invention, i.e., the
scattering loss amount of 0.2%/cm or less, is satisfied or not.
[0115] FIG. 7 shows the results obtained when the actually measured
values of various kinds of silica glass used for photolithography
are compared with theoretically calculated values of scattering
loss amount calculated by using equation (3) in which structure
determination temperature Ts is set at 1,273 K. As shown in this
chart, the actually measured values are higher than the
theoretically calculated values and exhibit a large fluctuation. It
has been found that, at 193.4 nm, due to such a fluctuation, the
conventionally used silica glasses for photolithography exceed the
standard for the scattering loss amount in the present invention
which is 0.2%/cm or less. By contrast, as can be seen from Examples
which will be described later, the silica glass of the present
invention can attain a scattering loss amount of 0.2%/cm or less
even with respect to light of 193.4 nm.
[0116] Also, the inventors have confirmed the relationship between
structure determination temperature Ts and the scattering loss.
FIG. 8 shows the results thereof. Here, the actually measured
values are also slightly higher than the theoretical values. This
phenomenon is assumed to be the results of optical scattering
(e.g., influence of particulate or colloidal scattering factors
such as optical glass and influence of inelastic scattering) and
lack of reliability in physical properties used for theoretical
calculation.
[0117] Further, the inventors have confirmed the relationship
between the scattering loss and the change in refractive index
caused by change in OH group and F concentration in the silica
glass and HIP processing. FIG. 9 shows the results thereof. As
shown in this chart, the actually measured values are higher than
the theoretically calculated values. Also, the scattering loss
amount has been found to be dependent on refractive index. Further,
it has been discovered that, in order to satisfy the standard for
the scattering loss in accordance with the present invention which
is 0.2%/cm or less, the refractive index is preferably less than
1.56 with respect to light of 193.4 nm.
[0118] Next, the method for producing the silica glass of the
present invention will be explained.
[0119] In the method for producing the silica glass of the present
invention, a silica glass ingot having an OH group concentration of
1,000 ppm or more is heated to a temperature of 1200-1350 K and the
ingot is retained at this temperature for a given period of time.
When the retention temperature exceeds 1350 K, the surface of the
silica glass is degraded, and it would become necessary to spend
very long period of time for lowering the structure determination
temperature of the silica glass to 1200 K or lower. When the
retention temperature is lower than 1200 K, the structure
determination temperature cannot be lowered to 1200 K or lower in a
given period, and, furthermore, annealing is insufficient and
strain cannot be removed. The retention time is preferably a period
of longer than the structure relaxation time at the retention
temperature, especially preferably 1-24 hours. For example, in the
case of the silica glass having a structure determination
temperature of 1300 K or higher and containing OH group in an
amount of about 1000 ppm, the structure relaxation time at 1273 K
is 280 seconds. The heating rate (temperature-rising rate) does not
affect the properties of the resulting silica glass, but is
preferably less than about 150 K/hr.
[0120] Then, the above silica glass ingot is cooled to a
temperature (annealing completion temperature (a.c.t.) ) of 1000 K
or lower, preferably 873 K or lower, more preferably 473 K or
lower, at a cooling rate (annealing rate or temperature-lowering
rate for annealing) of 50 k/hr or less, preferably 20 K/hr or less,
thereby to anneal the ingot. When the annealing completion
temperature is higher than 1000 K or the annealing rate
(temperature-lowering rate) is higher than 50 k/hr, the structure
determination temperature cannot be lowered to 1200 K or lower and,
besides, strain cannot be sufficiently removed.
[0121] After the ingot reaches the above annealing completion
temperature, usually it is air-cooled or spontaneously cooled to
room temperature, though this is not essential. The atmosphere of
the above annealing step is unlimited and may be air. The pressure
is also unlimited and may be atmospheric pressure.
[0122] Further, the method of the present invention preferably
additionally includes, prior to the annealing step, a step of
hydrolyzing a silicon compound such as SiCl.sub.1, SiHCl.sub.3,
SiF.sub.4, or the like in a flame (preferably oxy-hydrogen flame)
to obtain fine glass particles (glass soot) and depositing and
fusing the fine glass particles to obtain the silica glass ingot
having an OH group concentration of 1,000 ppm or more.
[0123] Moreover, preferably, the method of the present invention
further comprises a step of descending the temperature of the
above-mentioned silica glass ingot from a temperature of at least
1,373 K to a temperature of 1073 K or lower, preferably to a
temperature of 773 K or lower, more preferably to room temperature,
at a rate of 50 K/hr or less, preferably 20 K/hr or less, and more
preferably 10 K/hr, thereby to pre-anneal the silica glass ingot.
When the silica glass ingot is pre-annealed in this manner, the
structure determination temperature of the silica glass tends to
become lower.
[0124] As mentioned above, the silica glass ingot of the present
invention is preferably prepared by the above-mentioned direct
method, namely, oxy-hydrogen flame hydrolysis. That is,
.ident.Si--Si.ident. bond, .ident.Si--O--O--Si.ident. bond and the
like are known as the precursors which cause formation of
structural defects when synthetic silica glass is irradiated with
ultraviolet light, and synthetic glasses obtained by so-called soot
methods (VAD method, OVD method) or plasma method have these
precursors. On the other hand, synthetic silica glasses produced by
the direct method have no incomplete structures of oxygen-shortage
or excessive type formed by deviation from the stoichiometric
ratio. Furthermore, high purity of low metallic impurities can
generally be attained in the synthetic silica glasses produced by
the direct method. Moreover, since the silica glasses synthesized
by the direct method generally contain more than several hundred
ppm of OH group, they are structurally more stable as compared with
silica glasses containing no OH group.
[0125] The silica glass synthesized by the so-called direct method
which comprises hydrolyzing silicon chloride with oxy-hydrogen
flame and depositing the resulting silica fine glass particles on a
target and melting it to form a silica glass ingot has a structure
determination temperature of 1300 K or higher at the state just
after synthesis.
[0126] In order to obtain a silica glass ingot having an OH group
concentration of 1,000 ppm or more by the direct method, it is
preferred that the volume ratio of oxygen gas to hydrogen gas
(O.sub.2/H.sub.2) in the flame is at least 0.4, more preferably
0.42-0.5. When this ratio (oxygen hydrogen gas ratio) is less than
0.4, it tends to occur that the resulting silica glass ingot does
not contain 1,000 ppm or more of OH group.
[0127] Furthermore, in the method of the present invention, the
effect of the above-mentioned annealing is attained more
effectively and uniformly by cutting the silica glass ingot to make
blanks having a given size, preferably 200-400 mm in diameter and
40-150 mm in thickness, and then, annealing them.
EXAMPLES 1-14 and COMPARATIVE EXAMPLES 1-10
[0128] A silica glass ingot was produced using the apparatus for
producing silica glass as shown in FIG. 10. That is, high purity
silicon tetrachloride A (starting material) (Examples 1-11 and
Comparative Examples 1-8), or silicon tetrachloride A and silicon
tetrafluoride B (starting material) (Examples 12-14 and Comparative
Examples 9-10), which are fed from silicon compound bomb 401, was
mixed with a carrier gas fed from oxygen gas bomb 403 in baking
system 402, and the mixture was fed to silica glass burner 406
together with hydrogen gas fed from hydrogen bomb 404 and oxygen
gas fed from oxygen gas bomb 405. Oxygen gas and hydrogen gas in
flow rates shown in Table 1 were mixed and burnt in the burner 406
and the starting material gas in the flow rate shown in Table 1 was
diluted with a carrier gas (oxygen gas) and ejected from the
central part to obtain silica fine glass particles (SiO.sub.2 fine
particles). The silica fine glass particles were deposited and
molten on target 408 surrounded by refractory 407 and then cooled
to room temperature at a temperature-lowering rate (cooling rate at
pre-annealing) shown in Table 2 thereby to obtain silica glass
ingot 409 (500 mm long) having the composition shown in Table 1. In
this case, the upper face (synthesis face) was covered with flame
and the target 408 was lowered at a constant speed with rotating
and rocking at a constant period. The structure determination
temperature of the silica glass at this stage was 1400 K. The
reference numeral 410 in FIG. 10 indicates a mass flow controller
and R in-Table 1 indicates an oxygen hydrogen ratio
(O.sub.2/H.sub.2).
[0129] The burner 406 has quintuple tube structure as shown in FIG.
11, and 501 indicates an ejection port for starting material and
carrier gas, 502 indicates an ejection port for inner side oxygen
gas (OI), 503 indicates an ejection port for inner side hydrogen
gas (HI), 504 indicates an ejection port for outer side oxygen gas
(OO), and 505 indicates an ejection port for outside hydrogen gas
(HO). The size (mm) of the ejection port is as follows.
1 Burner A Inner diameter Outer diameter 501 6.0 9.0 502 12.0 15.0
503 17.0 20.0 504 3.5 6.0 505 59.0 63.0
[0130]
2 Burner B Inner diameter Outer diameter 501 3.5 6.5 502 95 12.5
503 14.5 17.5 504 3.5 6.0 505 59.0 63.0
[0131]
3 Burner C Inner diameter Outer diameter 501 2.0 5.0 502 8.5 11.5
503 14.5 17.5 504 3.5 6.0 505 59.0 63.0
[0132] Then, a test piece to be irradiated with ArF excimer laser
beam (60 mm in diameter and 10 mm in thickness, the opposite two
sides being subjected to optical abrasion) was prepared from each
of the resulting ingots. The test piece was placed in an annealing
furnace made of an insulating firebrick as shown in FIG. 12 and
heated to retention temperature from room temperature at a heating
rate shown in Table 2. After lapse of the retention time, it was
cooled to the annealing completion temperature from the retention
temperature at an annealing rate (temperature-lowering rate) shown
in Table 2 and, thereafter, spontaneously cooled to room
temperature. The cooling rate after a.c.t. shown in Table 2 is a
cooling rate one hour after starting of the spontaneous cooling.
Moreover, in FIG. 12, 601 indicates a test piece, 602 indicates an
annealing furnace, 603 indicates a stand comprising a silica glass
and legs made of a firebrick, and 604 indicates a rod-like SiC
heating element.
4 TABLE 1 Starting Material Grow- Inner Gas Inner Side Outer Side
ing Diam- Diameter F Flow Oxygen .multidot. Oxygen .multidot. Rate
eter of Tube for Example/ H.sub.2 Concen- Flow Velocity Hydrogen
Gas Hydrogen Gas of of Starting Comparative Concentration tration
Rate [g/min/ HI OI HO OO Ingot Ingot Material Example
[mol./cm.sup.3] [ppm] Kind [g/min] cm.sup.2] [slm] [slm] R [slm]
[slm] R [mm/hr] [mm] [mm.phi.] Ex. 1-6 About 1.1 .times. 10.sup.17
0 A 30 330 70 30.8 0.44 200 78 0.44 0.6 200 6.0 Comp. Ex. 1-3 Ex. 7
<1 .times. 10.sup.16 0 A 30 330 70 35 0.5 200 100 0.5 0.6 200
6.0 Ex. 8 About 1.3 .times. 10.sup.17 0 A 30 330 70 28 0.40 200 78
0.44 0.6 200 6.0 Ex. 9-11 About 1.5 .times. 10.sup.17 0 A 30 330 70
29.4 0.42 200 78 0.44 0.6 200 6.0 Comp. Ex. 4 Comp. Ex. 5-6 About
2.1 .times. 10.sup.18 0 A 30 330 150 45 0.3 360 158 0.44 4.0 250
3.5 Comp. Ex. 7-8 About 2.1 .times. 10.sup.18 0 A 30 330 150 45 0.3
360 158 0.44 8.0 250 2.0 Ex. 12-13 <1 .times. 10.sup.10 350 A 20
330 70 35 0.5 200 100 0.5 0.6 200 6.0 Comp. Ex. 9 B 6.1 Comp. Ex.
10 About 2.1 .times. 10.sup.18 350 A* 20 330 150 45 0.3 360 158
0.44 4.0 250 3.5 B 6.1 Ex. 14 About 2.1 .times. 10.sup.18 800 A 10
330 150 45 0.3 200 78 0.44 0.6 200 6.0 B 12.2
[0133]
5 TABLE 2 Annealing Pre- Annealing Cooling Annealing Heat- Reten-
Reten- Anneal- Completion Rate Example/ Cooling ing tion tion ing
Temperature After Comparative Rate Rate Temp. Time Rate (a.c.t)
a.c.t. Example [K/hr] [K/hr] [K] [hr] [K/hr] [K] [K/hr] Ex. 1 125
100 1223 10 1.0 773 80 Ex. 2 125 100 1273 10 1.0 773 80 Ex. 3 125
100 1273 5 1.0 773 80 Ex. 4 125 100 1273 10 5 773 80 Ex. 5 125 100
1273 10 10 773 80 Ex. 6 125 100 1273 10 10 993 80 Comp. Ex. 1 125
100 1273 10 10 1173 80 Comp. Ex. 2 125 100 1273 10 100 773 80 Comp.
Ex. 3 125 100 1373 10 10 773 80 Ex. 7 50 100 1273 10 7 773 80 Ex. 8
50 100 1273 10 10 773 80 Ex. 9 50 100 1273 10 20 773 80 Comp. Ex. 4
50 100 1273 10 100 773 80 Ex. 10 125 100 1223 10 1.0 773 80 Comp.
Ex. 5 125 100 1223 10 1.0 *773 80 Ex. 11 125 100 1273 10 10 773 80
Comp. Ex. 6 125 100 1273 10 10 773 80 Comp. Ex. 7 125 100 1273 10
100 773 80 Comp. Ex. 8 50 100 1373 10 10 773 80 Ex. 12 50 100 1223
10 1.0 773 80 Ex. 13 125 100 1223 10 10 773 80 Comp. Ex. 9 125 100
1373 10 100 773 80 Comp. Ex. 10 125 100 1273 10 10 773 80 Ex. 14
125 100 1223 10 10 773 80
[0134]
6TABLE 3 Example/ OH Group F H.sub.2 Scattering Comparative Ts
Concentration Concentration Concentration Loss Example [K] [ppm]
[ppm] [mol./cm.sup.3] [%/cm] Ex. 1 1023 1200 0 1 .times. 10.sup.17
0.10 Ex. 2 1073 1200 0 1 .times. 10.sup.17 0.12 Ex. 3 1123 1200 0 1
.times. 10.sup.17 0.15 Ex. 4 1150 1200 0 1 .times. 10.sup.17 0.17
Ex. 5 1173 1200 0 1 .times. 10.sup.17 0.18 Ex. 6 1198 1200 0 1
.times. 10.sup.17 0.19 Comp. Ex. 1 1223 1200 0 1 .times. 10.sup.17
0.23 Comp. Ex. 2 1273 1200 0 1 .times. 10.sup.17 0.23 Comp. Ex. 3
1223 1200 0 <1 .times. 10.sup.16 0.24 Ex. 7 1223 1250 0 <1
.times. 10.sup.16 0.1 Ex. 8 1150 1010 0 1.2 .times. 10.sup.17 0.15
Ex. 9 1193 1050 0 1.4 .times. 10.sup.17 0.18 Comp. Ex. 4 1223 1050
0 1.4 .times. 10.sup.17 0.23 Ex. 10 1023 1050 0 1.4 .times.
10.sup.17 0.11 Comp. Ex. 5 1023 900 0 2 .times. 10.sup.18 0.23 Ex.
11 1173 1050 0 1.4 .times. 10*.sup.17 0.16 Comp. Ex. 6 1173 900 0 2
.times. 10.sup.18 0.22 Comp. Ex. 7 1273 780 0 2 .times. 10.sup.18
0.35 Comp. Ex. 8 1223 780 0 1 .times. 10.sup.17 0.3 Ex. 12 1023
1200 350 <1 .times. 10.sup.16 0.07 Ex. 13 1173 1200 350 <1
.times. 10.sup.16 0.2 Comp. Ex. 9 1223 1200 350 <1 .times.
10.sup.16 0.4 Comp. Ex. 10 1173 900 350 2 .times. 10.sup.18 0.21
Ex. 14 1123 1200 800 2 .times. 10.sup.18 0.10
[0135] Structure determination temperature (Ts), OH group
concentration, F concentration and hydrogen molecule concentration
of these test pieces were measured. The results are shown in Table
3. The structure determination temperature was obtained by
inversely calculating from measured 606 cm.sup.-1 line intensity
value based on the previously prepared calibration curve. The
hydrogen molecule concentration was measured by a laser Raman
photometer. That is, among the Raman scattered lights perpendicular
to the sample which occurred when the sample was irradiated with
Ar+ laser beam (output 800 mW), intensity of 800 cm.sup.-1 and 4135
cm.sup.-1 was measured and the ratio of the intensities was
determined. The OH group concentration was measured by infrared
absorption spectrometry (measurement of absorption by OH group for
1.38 .mu.m). In addition, quantitative analysis of metallic
impurities (Mg, Ca, Ti, Cr, Fe, Ni, Cu, Zn, Co, Mn, Na and K) in
the test pieces was conducted by inductively coupled plasma
spectrometry to find that the concentrations of them were lower
than 20 ppb, respectively.
[0136] Scattering loss amount of each of the test pieces thus
obtained with respect to ArF excimer laser beam was measured. The
results are shown in Table 3. As is clear from Table 3, the silica
glasses of the present invention (Examples 1-14) satisfied the
desired conditions on the scattering loss amount.
[0137] Furthermore, as is clear from FIG. 13, when the OH group
concentration was 1000 ppm or more, the scattering loss amount
extremely decreased by reducing the structure determination
temperature to 1200 K or lower.
[0138] Also, the scattering loss characteristic, polarization
characteristic, and birefringence characteristic of each silica
glass obtained by the Examples exhibited a center symmetry. Their
birefringence amount was 2 nm/cm or less.
[0139] Further, the results of the measurement of various
characteristics of the silica glass obtained by the Examples are as
indicated in the following. Namely, the internal absorptivity of
the above-mentioned silica glass with a thickness of 10 mm was
0.2%/cm or less with respect to ArF excimer laser. The internal
transmittance of the above-mentioned silica glass with a thickness
of 10 mm was 99.8% or more with respect to ArF excimer laser. Also,
after being irradiated with 1.times.10.sup.6 pulses of KrF excimer
laser at an average one-pulse energy density of 400 mJ/cm.sup.2,
the internal transmittance of the above-mentioned silica glass
having a thickness of 10 mm was 99.5.degree.. or more with respect
to light having a wavelength of 248 nm. Further, after being
irradiated with 1.times.10.sup.6 pulses of ArF excimer laser at an
average one pulse energy density of 100 mJ/cm.sup.2, the internal
transmittance of the above-mentioned silica glass having a
thickness of 10 mm was 99.5% or more with respect to light having a
wavelength of 193 nm.
COMPARATIVE EXAMPLE 11
[0140] A test piece of silica glass was prepared in the same manner
as Example 4 except that the holding temperature was set at 1,123
K. Since the structure was not relaxed during the holding time, the
structure determination temperature did not become 1,200 K or
lower. Also, due to insufficient annealing, strain was not
removed.
COMPARATIVE EXAMPLE 12
[0141] A silica glass simply satisfying a specification of a lens
material characteristic of .DELTA.n.ltoreq.2.times.10.sup.-6, a
birefringence amount .ltoreq.2 nm/cm, and an internal transmittance
of 99.6% or more was used to prepare a projection lens for an ArF
excimer laser stepper. The resolution (L/S) of thus prepared lens
was 0.30 .mu.m with respect to the designed L/S of 0.20 .mu.m.
Also, its contrast was so unfavorable that the designed property
could not be obtained. Thus, it was found that the selection of the
optical member according to such a specification alone was
insufficient. It is assumed that, since the absorption loss amount
or scattering loss amount exceeded 0.2%/cm, the internal heating
within the lens caused by optical absorption and the flare
generated by optical scattering became remarkably influential to
the deterioration of L/S.
[0142] Here, L/S is an abbreviation of "line and space" which is a
value generally used as an index for evaluating properties of
semiconductor manufacturing.
[0143] Homogeneity was measured by oil-on-plate technique in which
a He--Ne laser interferometer was used, whereas birefringence was
measured by rotational analyzer technique. Internal transmittance
was measured by a normal spectrophotometer.
EXAMPLE 15
[0144] A silica glass of the present invention satisfying a
specification of a lens material characteristic of
.DELTA.n.ltoreq.2.times.10.sup.-6 and a birefringence amount
.ltoreq.2 nm/cm as well as both scattering loss amount and
absorption loss amount of 0.2%/cm or less was used to prepare a
projection lens for ArF excimer laser stepper. The resolution (L/S)
of thus prepared lens was 0.20 .mu.m with respect to the designed
L/S of 0.20 .mu.m. Also, its contrast was favorable. Thus, by
selecting the optical member according to this specification,
properties approximating the designed values were obtained.
[0145] Homogeneity was measured by oil-on-plate technique in which
a He--Ne laser interferometer was used, whereas birefringence was
measured by phase modulation technique. The silica glass used here
exhibited a 10-mm internal transmittance exceeding 99.6% at 193
nm.
[0146] Also, after being irradiated with 1.times.10.sup.6 pulses of
ArF excimer laser at an average one-pulse energy density of 100
mJ/cm.sup.2, the internal transmittance of the silica glass having
a thickness of 10 mm exceeded 99.5% with respect to light having a
wavelength of 193 nm.
[0147] Further, it was confirmed that, after being irradiated with
1.times.10.sup.6 pulses of KrF excimer laser at an average
one-pulse energy density of 400 mJ/cm.sup.2, the internal
transmittance of the silica glass having a thickness of 10 mm
exceeded 99.5% with respect to light having a wavelength of 248
nm.
[0148] When the lens designed is made for a KrF excimer laser, this
optical member can be used for KrF excimer laser steppers.
[0149] A projection lens made of this optical member has a hydrogen
concentration of 5.times.10.sup.17 molecules/cm.sup.3 or more with
a higher concentration at the center portion than the
periphery.
[0150] This projection lens can be used in manufacturing lines for
256-MB VLSI.
[0151] As explained in the foregoing, the present invention can
provide a silica glass in which the influence of flare and ghost
caused by optical scattering is reduced so as to yield optical
properties approximating the designed resolution defined at the
time of designing a lens, and thus, which achieves high resolution.
Further, the present invention can provide an optical member which
includes this silica glass of the present invention, and thus,
which achieves a favorable contrast. Also, it would be effective in
improving throughput.
[0152] Therefore, the optical member including the silica glass of
the present invention can be applied to projection lenses used for
any of i-line, ArF, and KrF excimer laser steppers using light in
the wavelength region of 400 nm or shorter. Also, in accordance
with the present invention, the performance including resolution
and stability of the photolithography apparatuses can be
improved.
[0153] From the invention thus described, it will be obvious that
the invention may be varied in many ways. Such variations are not
to be regarded as a departure from the spirit and scope of the
invention, and all such modifications as would be obvious to one
skilled in the art are intended for inclusion within the scope of
the following claims.
[0154] The basic Japanese Application Nos. 000479/1995 (7-479)
filed on Jan. 6, 1995 and 004077/1995 (7-4077) filed on Jan. 13,
1995 are hereby incorporated by reference.
* * * * *