U.S. patent application number 10/139362 was filed with the patent office on 2002-10-31 for illuminating apparatus.
This patent application is currently assigned to Nikon Corporation. Invention is credited to Hagiwara, Shigeru, Hiraiwa, Hiroyuki, Mori, Takashi.
Application Number | 20020159142 10/139362 |
Document ID | / |
Family ID | 17441638 |
Filed Date | 2002-10-31 |
United States Patent
Application |
20020159142 |
Kind Code |
A1 |
Hagiwara, Shigeru ; et
al. |
October 31, 2002 |
Illuminating apparatus
Abstract
An illuminating apparatus is disclosed which comprises a light
source, an optical system for condensing light emitted from the
light source and illuminating an object with the condensed light,
and an optical member which absorbs light having wavelengths from
260 to 340 nm among the light emitted from the light source,
wherein the optical member is made of glass or crystalline material
to which metal is doped.
Inventors: |
Hagiwara, Shigeru;
(Kawasaki-shi, JP) ; Hiraiwa, Hiroyuki;
(Yokohama-shi, JP) ; Mori, Takashi; (Fujisawa-shi,
JP) |
Correspondence
Address: |
Mitchell W. Shapiro
Miles & Stockbridge P.C.
Suite 500
1751 Pinnacle Drive
McLean
VA
22102-3833
US
|
Assignee: |
Nikon Corporation
|
Family ID: |
17441638 |
Appl. No.: |
10/139362 |
Filed: |
May 7, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10139362 |
May 7, 2002 |
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09584266 |
Jun 1, 2000 |
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09584266 |
Jun 1, 2000 |
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09131320 |
Aug 7, 1998 |
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6108126 |
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09131320 |
Aug 7, 1998 |
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08842514 |
Apr 24, 1997 |
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08842514 |
Apr 24, 1997 |
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08328816 |
Oct 25, 1994 |
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Current U.S.
Class: |
359/350 ;
359/361; 359/614 |
Current CPC
Class: |
G03F 7/70916 20130101;
H01J 61/40 20130101; H01J 61/30 20130101; H01J 61/302 20130101;
H01J 61/86 20130101; G03F 7/70016 20130101 |
Class at
Publication: |
359/350 ;
359/361; 359/614 |
International
Class: |
G02B 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 26, 1993 |
JP |
5-267208 |
Claims
What is claimed is:
1. An illuminating apparatus comprising: a light source; an optical
system disposed in an optical path of a light emitted from said
light source to condense the light emitted from the light source
and illuminate an object with the condensed light; and an optical
member disposed in said optical path, and exhibiting an absorbing
property for light having wavelength from 260 to 340 nm among the
light emitted from the light source, to suppress formation of
ammonium sulfate that would otherwise form in the absence of said
optical member, wherein said optical member is made of glass or
crystalline material doped with an impurity that has said absorbing
property.
2. An apparatus according to claim 1, wherein said impurity
comprises at least one of Na, Fe, Pb, Al, Rb and Cs.
3. An illuminating apparatus according to claim 1, wherein said
object is a mask on which a pattern is formed, and the illuminating
apparatus is provided in an apparatus for transferring the pattern
on the mask onto a photosensitive substrate.
4. An illuminating apparatus comprising: a light source; an optical
system for condensing light emitted from the light source and
illuminating an object with condensed light; and an optical member
in which fluid absorbing light having wavelength from 260 to 340 nm
among the light emitted from the light source is filled.
5. An illuminating apparatus according to claim 4, wherein said
fluid is one of gaseous rubidium, gaseous caesium and ozone
gas.
6. An illuminating apparatus according to claim 4, wherein said
object is a mask on which a pattern is formed, and the illuminating
apparatus is provided in an apparatus for transferring the pattern
on the mask onto a photosensitive substrate.
7. An apparatus according to claim 1, wherein said optical member
is made of crystalline material doped as aforesaid, and said
crystalline material is one of fluorite (CaF.sub.2) and magnesium
fluoride (MgF.sub.2).
8. An apparatus according to claim 1, wherein said light source
comprises a discharge lamp and a light reflecting-condensing member
to reflect and condense the light from the discharge lamp so as to
direct the light to said object.
9. An apparatus according to claim 8, wherein said optical system
includes a wavelength selecting element which selects a light of a
predetermined wavelength band among the light emitted from said
light source, and wherein said optical member is disposed between
said reflecting-condensing member and said wavelength selecting
element.
10. An exposure apparatus for transferring a pattern formed on a
mask onto a substrate with a light emitted from a light source,
comprising: an illumination optical system disposed between said
light source and said mask, and which illuminates said mask with
the light emitted from the light source; and an optical member
disposed in said optical path, and exhibiting an absorbing property
for light having wavelength from 260 to 340 nm among the light
emitted from the light source, to suppress formation of ammonium
sulfate that would otherwise form in the absence of said optical
member, wherein said optical member is made of glass or crystalline
material doped with an impurity that has said absorbing
property.
11. An apparatus according to claim 10, wherein said light source
comprises a discharge lamp and a light reflecting-condensing member
to reflect and condense the light from the discharge lamp.
12. An apparatus according to claim 11, wherein said illumination
optical system includes a wavelength selecting element which
selects a light of predetermined wavelength band among the light
emitted from said light source, and wherein said optical member is
disposed between said reflecting-condensing member and said
wavelength selecting element.
13. An apparatus according to claim 12, wherein said impurity is at
least one of Na, Fe, Pb, Al, Rb and Cs.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an illuminating apparatus
for illuminating an object with light emitted from a discharge lamp
such as a mercury lamp, and so on. The illuminating apparatus
according to the present invention is preferably applied especially
to an illuminating optical system in an exposure apparatus for
manufacturing semiconductors.
[0003] 2. Related Background Art
[0004] Illuminating apparatus for illuminating objects with light
emitted from discharge lamps have been used for various purposes in
various fields. Among them, reduce-projection-type exposure
apparatus (such as steppers, aligners, and so on), in order to
manufacture semiconductor elements such as LSIs and liquid crystal
display elements according to the photo-lithography technique,
illuminating apparatus which illuminates reticles on which
transferring pattern is formed with light of a certain wavelength
(i line having a wavelength of 365 nm, g line having a wavelength
of 436 nm, and so on) emitted from extra-high pressure mercury
lamps.
[0005] Much effort is being made in order to transfer much finer
pattern on a photosensitive substrate with higher resolution with
such a reduce-projection-type exposure apparatus. In general,
resolution R and depth of focus DOF of a projection-type exposure
apparatus can be expressed as follows:
R=k.sub.1.multidot..lambda./NA (1)
DOF=k.sub.2.multidot..lambda./NA.sup.2 (2)
[0006] wherein NA is the numerical aperture of the projection
optical system, .lambda. is the wavelength of the exposure light,
k.sub.1 and k.sub.2 are coefficients determined by processes
employed. As is understood from these two formulas, finer pattern
can be realized either
[0007] (1) by increasing the numeral aperture NA of the projection
optical system, or
[0008] (2) by shortening the wavelength .lambda. (exposure
wavelength) of exposure light.
[0009] With respect to the former technique (1), projection optical
systems with a large numerical aperture from 0.5 to 0.6 have been
already realized, which improves resolution. By only increasing the
numerical aperture NA of the projection optical system, however,
the depth of focus DOF must be reduced in inverse proportion to the
square of the numerical aperture NA, as is understood from the
formula (2). In typical semiconductor processes in practical use, a
wafer which is to be subjected to exposure of a circuit pattern has
irreguralities on its surface formed in the previous process. And
flatness of the wafer itself inevitably has errors. Accordingly,
sufficient depth of focus DOF have to be obtained.
[0010] On the other hand, with respect to the technique (2), the
depth of focus DOF varies in proportion to the wavelength .lambda.
of exposure light, as is clearly understood from the formula (2).
Accordingly, it is more preferable to shorten the exposure
wavelength .lambda. in order to improve resolution because
sufficiently large depth of focus can be obtained. As a result, the
emission line of a mercury lamp called i-line (having a wavelength
of 365 nm) has almost replaced, as the exposure light used in the
projection exposure apparatus, the emission line of the mercury
lamp called g-line (having a wavelength of 436 nm).
[0011] FIG. 12 shows an example of the conventional illuminating
apparatus used in a projection exposure apparatus, in which a
mercury lamp is used as the light source, the emission point of the
mercury lamp 1 is arranged at a first focal point F1 inside an
ellipsoidal mirror 2. The inner surface of the ellipsoidal mirror 2
on which aluminum or plurality of layers of various dielectric
materials are deposited serves as a reflecting surface. The light L
emitted from the mercury lamp 1 is reflected by the inner surface
of the ellipsoidal mirror 2 toward a mirror 3. On the reflecting
surface of the mirror 3, also aluminum or plurality of layers of
various dielectric materials are deposited. The light reflected by
the mirror 3 is condensed at a second focal point F2 of the
ellipsoidal mirror 2. Thus, a light source image is formed at the
second focal point F2.
[0012] Light diverging from the second focal point F2 is
substantially collimated by a collimator lens 4, and then is
incident on a band-pass filter 5 of narrow-band type, which selects
light having wavelength in a certain range as illuminating light.
The illuminating light is incident on a fly's-eye lens 6, which
forms a number of secondary light sources in its rear (reticle
side) focal plane. Light beams diverging from these secondary light
sources are reflected by a mirror 7, condensed by a condenser lens
8. The pattern forming surface of a reticle 9 is illuminated
superimposedly with a number of light beams condensed by the
condenser lens 8. Note that aluminum or plurality of layers of
various dielectric materials are deposited also on the reflecting
surface of the mirror 7.
[0013] As the optical path is bent by the mirrors 3 and 7, the size
of the optical system is small. The inner surface of the
ellipsoidal mirror 2 serving as a converging mirror and the
reflecting surfaces of the mirrors 3 and 7 are designed to have
maximum reflectance values with respect to the wavelength of the
exposure light.
[0014] As the mercury lamp, an extra-high pressure mercury lamp is
used. FIG. 13 shows the distribution of the emission spectrum of
this extra-high pressure mercury lamp. FIG. 14A shows the relation
between wavelengths and the reflectance of an aluminum reflecting
mirror on which aluminum is deposited to form a reflecting surface.
FIG. 14B shows the relation between wavelengths and the reflectance
of a typical reflecting mirror according to the prior art on which
plurality of layers of dielectric materials are deposited to form a
reflecting surface. Further, FIG. 15 shows the relation between
wavelengths and the transmittance of the band-pass filter 5 when i
line is the exposure light. In the above-mentioned construction,
the pattern of the reticle 9 is illuminated with illuminating light
(i line) with a uniform distribution of illuminance. And the image
of the pattern is formed on the photosensitive substrate via the
projection optical system (not shown in the drawing).
[0015] When the illuminating apparatus with the above-mentioned
construction is used in the ambient atmosphere, white powder
adheres to the surfaces of the optical members arranged between the
mercury lamp 1 and the band-pass filter 5, that is, the surfaces of
the ellipsoidal mirror 2, the mirror 3 and the collimator lens 4,
including the entrance plane of the band-pass filter 5. Because of
this white powder, the reflectance values and the transmittance of
light L of these optical members decrease to reduce the
illumination efficiency. Analysis shows that the white powder is
ammonium sulfate, (NH.sub.4).sub.2SO.sub.4 and that materials
concerning the formation of ammonium sulfate do not originally
exist in the illuminating apparatus but come from the ambient
atmosphere.
[0016] A method to solve the above problem is disclosed in U.S.
Pat. No. 5,207,505. According to this method, said optical members
are heated and maintained beyond 120.degree. C. because ammonium
sulfate decomposes beyond this temperature. ("Encyclopedia of
Chemistry", Vol. 9, P690, Kyoritsu Pub., 1964) It is rather easy to
heat up and maintain the ellipsoidal mirror 2 at such a high
temperature because the mercury lamp 1 arranged near the
ellipsoidal mirror 2 serves as an effective heat source. The other
optical members, however, have to be heated by an additional, very
effective heat source. As a semiconductor exposure apparatus
requires especially strict temperature control, exhaust of heat is
very difficult in practical use.
SUMMARY OF THE INVENTION
[0017] In consideration of the above-mentioned problems, the
present invention was made. And the object of the present invention
is to provide an illuminating apparatus which condenses light
emitted from a discharge lamp with a converging mirror and
illuminates an object with the light led through optical members,
wherein white powder of ammonium sulfate which adheres to the
optical members can be reduced without newly adding an effective
heat source nor a mechanism for exhausting gaseous impurities.
[0018] With reference to FIG. 10, an illuminating apparatus
according to the present invention comprises;
[0019] (a) a light source 1;
[0020] (b) an optical system consisting of optical members 2 to 8
for condensing light emitted from the light source 1 and
illuminating an object 9 with said condensed light; and
[0021] (c) an optical member 20 for absorbing light having
wavelengths in a range from 260 to 340 nm among light emitted from
the light source 1,
[0022] wherein the optical member 20 is made of glass or
crystalline material to which metal is doped.
[0023] Another illuminating apparatus, also with reference to FIG.
10, according to the present invention comprises:
[0024] (a) a light source 1;
[0025] (b) an optical system consisting of optical members 2 to 8
for condensing light emitted from the light source 1 and
illuminating an object 9 with said condensed light; and
[0026] (c) an optical member 20 in which a fluid absorbing light
having wavelengths in a range from 260 to 340 nm among light
emitted from the light source 1 is filled.
[0027] Still another illuminating apparatus according to the
present invention, with reference to FIGS. 7 and 12, comprises;
[0028] (a) a lamp having a pair of electrodes 13A and 13B the bulb
(10, 22) of which shields light having wavelengths in a range from
260 to 340 nm among light emitted from said pair of electrodes 13A
and 13B; and
[0029] (b) an optical system consisting of optical members 2 to 8
for condensing light emitted from the lamp and illuminating an
object 9 with said condensed light.
[0030] Now basic principles of the present invention will be
described. The inventors of the present invention carried out a
further examination on the formation processes of white powder of
ammonium sulfate from trace substances in the atmosphere.
[0031] Trace substances such as sulfur dioxide SO.sub.2 (sulfurous
acid) and ammonia NH.sub.3 together with oxygen O.sub.2 and water
vapor H.sub.2O are common in the clean room in which the
semiconductor exposure apparatus is used as well as in the air. It
is probable that these substances react with one another with the
help of ultraviolet rays having energy h.nu. (h is Planck's
constant, and .nu. is frequency) as follows.
[0032] (1) sulfer dioxide SO.sub.2 is activated by energy of
ultraviolet rays to be activated sulfur dioxide SO.sub.2*; 1
[0033] (2) The resultant activated sulfur dioxide SO.sub.2* is
oxidized to be sulfur trioxide SO.sub.3;
2SO.sub.2*+O.sub.2.fwdarw.2SO.sub.3.
[0034] (3) The resultant sulfur trioxide SO.sub.3 reacts with water
H.sub.2O to be sulfuric acid;
SO.sub.3+H.sub.2O.fwdarw.H.sub.2SO.sub.4.
[0035] (4) On the other hand, ammonia NH.sub.3 reacts with water
H.sub.2O to be ammonium hydroxide;
NH.sub.3+H.sub.2O.fwdarw.NH.sub.4OH.
[0036] (5) The sulfuric acid from the process (3) is neutralized
with the ammonium hydroxide from the process (4) to form ammonium
sulfate;
H.sub.2SO.sub.4+2NH.sub.4OH.fwdarw.(NH.sub.4).sub.2SO.sub.4+2H.sub.2O.
[0037] The above examination was carried out on the basis of a
literature, "Chiba Univ. Environmental Sci. Res. Rep." Vol. 1, No.
1, pp 165-177.
[0038] The inventors of the present invention took notice of the
reaction (1) among the above reactions in order to find a way to
inhibit the formation of ammonium sulfate. According to another
literature (H. Okabe: "Photochemistry of Small Molecules" P248,
Wiley-Inter Science, 1978), sulfur dioxide has the following four
absorption bands:
[0039] (1) 105-180 nm
[0040] (2) 180-240 nm
[0041] (3) 260-340 nm
[0042] (4) 340-390 nm
[0043] Since the ultra-high pressure mercury lamp emits little
amount of light having a wavelength of 240 nm or shorter
wavelengths, and at the same time since the white powder is found
only in the optical path down to the entrance plane of the
band-pass filter and not from the band-pass filter downward in the
optical path, ultra-violet rays having wavelength in a range from
260 nm to 340 nm is thought to be the main factor of the reaction.
Accordingly, if ultraviolet rays having said wavelength from 260 nm
to 340 nm can be shielded in the vicinity of the mercury lamp,
adhesion of ammonium sulfate which hinders illumination efficiency
can be reduced.
BRIEF DESCRIPTION OF THE DRAWINGS
[0044] FIG. 1A is a cross-sectional view showing the structure of
the mercury lamp used in the first embodiment of the illuminating
apparatus according to the present invention.
[0045] FIG. 1B is a cross-sectional view showing a modification of
the mercury lamp shown in FIG. 1A.
[0046] FIG. 2 is a chart showing the absorption cross section of
vaporous rubidium (Rb).
[0047] FIG. 3 is a chart showing the absorption cross section of
vaporous caesium (Cs).
[0048] FIG. 4 is a chart showing the absorption cross section of
ozone gas (O.sub.3) and that of gaseous oxygen (O.sub.2).
[0049] FIG. 5 is a cross-sectional view showing the structure of
the mercury lamp used in the second embodiment of the illuminating
apparatus according to the present invention.
[0050] FIG. 6 is a chart showing transmittance characteristics of
glass material LF5W.
[0051] FIG. 7 is a cross-sectional view showing the structure of
the mercury lamp used in the third embodiment of the illuminating
apparatus according to the present invention.
[0052] FIG. 8 is a cross-sectional view of the multilayered film
with which the surface of the substrate is coated.
[0053] FIG. 9 is a chart showing an example of reflectance
characteristics of the multilayered film used in the third
embodiment.
[0054] FIG. 10 is a schematic view showing the construction of the
fourth embodiment of the illuminating apparatus according to the
present invention.
[0055] FIG. 11 is a perspective view showing the broken-out section
of a box member 20 used in the fourth embodiment.
[0056] FIG. 12 is a schematic view showing the construction of a
conventional illuminating apparatus.
[0057] FIG. 13 is a chart showing the emission spectrum
distribution of a ultra-high pressure mercury lamp.
[0058] FIG. 14A is a chart showing the reflectance characteristics
of a conventional aluminum reflecting mirror.
[0059] FIG. 14B is a chart showing the reflectance characteristics
of a typical reflecting mirror coated with a multilayered film of
dielectric substances.
[0060] FIG. 15 is a chart showing the transmittance characteristics
of a conventional band-pass filter.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0061] Now, the first embodiment according to the present invention
will be described. In this embodiment of the illuminating apparatus
differs from the conventional illuminating apparatus in that the
mercury lamp 1 is replaced by a new one having double-bulb
structure. First, this double-bulb mercury lamp used in this
embodiment will be described.
[0062] FIG. 1A shows the mercury lamp used in this embodiment. A
tubular inner bulb 11 has a spherical portion in the middle and the
open ends one sealed by bases 12A and 12B, respectively. Electrodes
13A and 13B are inserted through the bases 12A and 12B,
respectively, into the hollow inside the inner bulb 11. Also
substances necessary for emission of the mercury lamp are filled in
the hollow inside the inner bulb 11. Thus, the inner bulb 11 with
other necessary components functions as an ordinary ultra-high
pressure mercury lamp. Further, a tubular outer bulb 19 also having
a spherical portion in the middle surrounds the inner bulb 11. The
doughnut-shaped openings at both ends of the outer bulb 14 are
sealed by bases 15A and 15B, respectively. And a gas which absorbs
light having wavelengths in a range from 260 to 340 nm is filled in
a space S between the inner bulb 11 and the outer bulb 14.
[0063] As described before, the ultra-high pressure mercury lamp
used in the projection exposure apparatus has the emission spectrum
distribution shown in FIG. 13. As is clearly shown in FIG. 13, the
ultra-high pressure mercury lamp has distributions in a wavelength
range from 260 to 340 nm, that is, the wavelength range causing
adhesion of the white powder (blurring phenomenon). In order to
prevent emission of light in said wavelength range, the mercury
lamp has the double-bulb structure and the gas which absorbs light
having wavelength in the range from 260 to 340 nm is filled in the
space S between the inner bulb 11 and the outer bulb 14, as
described above. Gases having such proper absorption
characteristics include metallic vapour of rubidium, caesium, and
so on.
[0064] According to a literature (R. D. Hudson and L. J. Kieffer,
"Compilation of Atomic Ultraviolet Photoabsorption Cross Sections
for Wavelengths Between 3000 and 10 .ANG.", Atomic Data 2, pp
205-262 (1971) especially, see p. 235 and p. 253), FIG. 2 shows the
absorption cross section spectrum of vaporous rubidium. According
to the same literature, FIG. 3 shows the absorption cross section
spectrum of vaporous caesium. As is shown in FIGS. 2 and 3, both
vaporous rubidium and vaporous caesium have large absorption cross
sections for wavelength of 340 nm and shorter wavelengths.
Accordingly, if such metallic vapor is sealed in the space S of the
double-bulb structure shown in FIGS. 1A and 1B, the light in said
wavelength range causing the blurring phenomenon can be selectively
removed from the light emitted from the inner bulb serving as an
ultra-high pressure mercury lamp.
[0065] Gaseous ozone has absorption characteristics similar to
those of the above metallic vapor. The absorption cross section
spectrum of gaseous oxygen (O.sub.2) and ozone gas (O.sub.3) are
shown in FIG. 4, in which reference numeral 17 indicates the
absorption cross section spectrum of gaseous oxygen and reference
numeral 18 indicates that of ozone gas. As is clearly shown is FIG.
4, the absorption spectrum of ozone gas (O.sub.3) has ideal
absorption characteristics for wavelength of or shorter than 340
nm. The ozone gas, however, unlike metallic vapor, dissociates to
be O and O.sub.2 in photochemical reactions. Photochemical
reactions of ozone and oxygen occurs as shown in the following (in
the following reaction formulas M, which is called a third body any
atom, molecule or ion except an oxygen atom, for example, a
molecule of oxygen (O.sub.2) or nitrogen (N.sub.2)). 2
[0066] Ozone O.sub.3 and/or oxygen O.sub.2 filled in the space S
shown in FIG. 1A react as described above until the mixture of
gases reaches a chemical equilibrium. The final density of ozone
should be controlled in consideration of all the reaction and the
final chemical equilibrium. In short, the final density of ozone
after these photochemical reactions settles in a certain range
regardless of any initial densities of ozone. The absorption
efficiency for wavelength of 340 nm and shorter wavelengths in the
state of chemical equilibrium is obtained from the chart of FIG. 4
by calculating molecular densities of O.sub.3 and O.sub.2. A full
detail of the calculation is not given here, but an outline thereof
is as follows. For example, negligible reactions with respect to
reaction energy and so on are put out of account. And molecular
densities are approximately calculated from, for example, the
following formula expressing the condition of chemical equilibrium:
1 n t = 0 ( 3 )
[0067] wherein n is concentration of each substance.
[0068] The dissociation rate J of O.sub.3 or O.sub.2 can be
calculated as follows: 2 J = 0 max N ( ) ( ) ( 4 )
[0069] wherein J's dimension is [1/sec], 3 N ( ) [ cm - 1 , sec - 1
, cm - 1 ]
[0070] is the number of photons passing per second per unit
wavelength per unit area, .sigma.(.lambda.)[cm.sup.-2] is the
photoelectric absorption cross section of a molecule, and
.lambda.max is the maximum wavelength of .lambda. in the above
reactions.
[0071] The reaction rate of each reaction can be obtained from
well-known literature. Light absorption efficiency can be promoted
by increasing pressure of the gas filled in the double-bulb
structure shown in FIG. 1A. But temperature rising caused by light
absorption must be taken into account. That is, both the inner bulb
11 and the outer bulb 14 have to be made of glass material having a
small coefficient of thermal expansion as well as enough
strength.
[0072] The gas which absorbs light having wavelength from 260 to
340 nm may be circulated through the space S between the inner bulb
11 and the outer bulb 14, as shown in FIG. 1B. In this case, the
gas is supplied through a pipe 16A into the space S by a gas
supplier (not shown), wherein conditions of the gas (density,
pressure, flow velocity, temperature, and so on) must be well
controlled. The gas is exhausted through another pipe 16B to an
exhaust system (not shown). By circulating the gas through the
double-bulb structure, high light absorption efficiency can be
maintained.
[0073] When the structure shown in FIG. 1B is adapted, additional
systems are required to monitor and control the pressure and the
temperature of the gas circulated through the double-bulb
structure. The systems for monitoring and controlling the pressure
and the temperature of metallic vapor are very large. So metallic
vapor is preferably filled in the double-bulb structure, as shown
in FIG. 1A, when it is desirable to simplify the construction of
the whole apparatus. Accordingly, in practice, ozone gas is usually
circulated through the double-bulb structure shown in FIG. 1B. In
this case, however, the density of ozone circulated through the
double-bulb structure has to be newly calculated. If the time
required to reach the equilibrium is much longer than the time
during which the gas remains inside the double-bulb structure, the
initial density of ozone has to be high. Otherwise, the flow
velocity is changed to obtain desirable densities of ozone.
[0074] Next, the second embodiment according to the present
invention will be described with reference to FIGS. 5 and 6. This
embodiment has construction similar to that shown in FIG. 12,
wherein an impurity having certain absorption characteristics is
doped in the bulb of the mercury lamp 1. First, the structure of
the mercury lamp used in this embodiment will be described.
[0075] FIG. 5 shows the mercury lamp of this embodiment. A tubular
bulb 19 has a spherical portion in the middle. The openings of the
bulb 19 are sealed by bases 12A and 12B. Electrodes 13A and 13B are
inserted in the hollow inside the bulb 19 through the bases 12A and
12B, respectively. Thus the bulb 19 with other necessary components
functions as an ordinary ultra-high pressure mercury lamp. An
impurity which absorbs light having wavelength of 340 nm and
shorter wavelengths is doped in quartz glass, of which the bulb 19
of the lamp 1 is made.
[0076] One of materials which are preferably doped in quartz glass
is sodium Na. Sodium Na, however crystallize SiO.sub.2 at high
temperatures, which blurs the bulb 19. Accordingly, the bulb 19 has
to be kept at a temperature of 1000.degree. C. or lower. Other
preferable materials to be doped in quartz glass includes iron Fe,
lead Pb, aluminum Al, rubidium Rb, caesium Cs, and so on.
[0077] The bulb 19 can be made of materials on the market. For
example, ULETM titanium silicate glass (manufactured by Corning
Co., Commodity No. 7971) can be used without doping an impurity.
This ULETM titanium silicate glass absorbs light having a
wavelength of 300 nm and shorter wavelength, so the lamp can be
effectively prevented from being blurred.
[0078] Also glass material LF5W manufactured by Ohara Co. is
useful. This glass material LF5W exhibits light transmittance
characteristics shown in FIG. 6. The transmittance of this material
having a thickness of 10 mm for the light having a wavelength 365
nm (i line) is 0.994, from which reflection loss has already
subtracted. This glass material having said characteristics can
satisfy conditions required according to this embodiment. This
glass material, however, causes solarization when used at low
temperatures. In addition, it can not be used at 400.degree. C. or
higher temperatures. Accordingly, the bulb 19 has to be controlled
in the temperature range from 100.degree. C. to 400.degree. C.
[0079] Now, the third embodiment of the present invention will be
described with reference to FIGS. 7, 8 and 9. This embodiment also
has construction similar to that shown in FIG. 12, wherein the
glass of the mercury lamp 1 is coated with a multilayered film. The
same members as those of the previous second embodiment are
indicated by the same reference numerals and detailed description
thereof is omitted. First, the structure of the mercury lamp used
in this embodiment will be described.
[0080] The mercury lamp of this embodiment shown in FIG. 7 has a
bulb 22 made of ordinary glass material. The outer surface 22a of
the bulb 22 is coated with a multilayered film 10, which reflects
light having wavelengths in a range 260 to 340 nm and transmit
light having wavelength of 350 nm or longer wavelength. In other
words, the multilayered film selectively transmits the light used
as exposure light. An example of the multilayered film having
selectivity with respect to wavelengths is designed as: 4 air / ( 8
H : 4 L : 8 H ) n / substrate ( 5 )
[0081] wherein: H is selected from a group including ZrO.sub.2,
Sc.sub.2O.sub.3, HfO.sub.2, Y.sub.2O.sub.3, and so on; L is
selected from a group including SrO.sub.2, MgF.sub.2, and so on;
the wavelength .lambda. is determined to be about 300 nm; and the
number of layers n is generally from 8 to 16.
[0082] FIG. 8 shows a cross section of such a multilayered film,
wherein the film is formed according to the above design (5) and
the number of layers is 10. As the substrate, materials which
transmit light having a wavelength of 350 nm or longer wavelengths
can be used, including optical glass, quartz glass, fluorite, and
so on. When the material employed as the substrate absorbs light
having a wavelength 340 nm or shorter wavelengths, such light can
be prevented from being transmitted more effectively. By coating
the glass of the mercury lamp with the multilayered film, blurring
of the other optical members in the illuminating apparatus can be
reduced.
[0083] Next, the fourth embodiment of the present invention will be
described with reference to FIGS. 10 and 11. The components in FIG.
10 corresponding to those in FIG. 12 are indicated by the same
reference numerals, and detailed description thereof is omitted. In
this embodiment, an optical filter which absorbs light having
wavelengths from 260 to 340 nm is provided in the optical path of
the illuminating optical system.
[0084] FIG. 10 schematically shows the construction of this
embodiment. As shown in FIG. 10, a box member 20 is arranged
between the ellipsoidal mirror 2 and the mirror 3. The box member
has two flat glass surfaces parallel to each other. FIG. 11 shows a
broken-out section of the box member. The box member 20 has a
hollow space 21, which is arranged to coincide with the optical
path. A gaseous substance which absorbs light having wavelengths
from 260 to 340 nm (cf. description of the first embodiment) is
filled in the hollow space 21. The box member is arranged
preferably in the vicinity of the mercury lamp 1, as shown FIG. 10.
The box member 20 reduces adhesion of the white powder on the
optical members arranged downstream in the optical path from the
box member 20.
[0085] The glass material of the box member 20 may be the glass
material used in the second embodiment, that is, the glass material
which absorbs certain undesirable light. Or the box member 20 may
be replaced by a plane parallel glass which has absorption
characteristics similar to those of the glass materials used in the
second embodiment. In addition to the glass materials used in the
second embodiment, the plane parallel glass provided in the
illuminating optical system may be also made of a crystalline
material (for example, fluorite CaF.sub.2, magnesium fluoride, and
so on) to which the above-mentioned metal (such as Na, Fe, and so
on) is doped.
[0086] This fourth embodiment is useful in case, for example, the
double-bulb structure employed in the first embodiment is difficult
to manufacture.
[0087] The illuminating apparatus according to the present
invention can be applied not only to the projection exposure
apparatus as described but also to a proximity-type exposure
apparatus and a contact-type exposure apparatus, and further any
type of optical apparatus using ultraviolet rays.
[0088] As described before, ammonium sulfate is formed from trace
sulfur dioxide (SO.sub.2) and ammonia (NH.sub.3) existing in the
ambient atmosphere in which the illuminating apparatus is used.
Accordingly, if the illuminating apparatus is installed in a clean
room, sulfur dioxide (SO.sub.2) and/or ammonia (NH.sub.3) may be
removed from the air circulated in the clean room by attaching a
filtering system for removing sulfur dioxide (SO.sub.2) and/or
ammonia (NH.sub.3) to the air conditioning system. Thus, formation
of ammonium sulfate can be reduced.
[0089] The devices of the first to fourth embodiment can be used
separately. But if used in combination, these devices can more
effectively prevent adhesion of the white powder. Note that the
present invention is not limited to the above-mentioned embodiment.
The present invention includes any construction which concerns the
fundamental principles of the present invention.
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