Title: Projection optical system
Abstract: Radiation-induced damage to a lens material in a projection exposure system is reduced by selection of maximum design fluence values HD for lenses and at least one lens made of a material having a characteristic transition point TRC after exposure to a given amount of radiation, wherein, for instance, TRC<0.8. HD among other relationships and/or characteristics of the lenses.
Patent Number: 6,992,753 Issued on 01/31/2006 to Krähmer, et al.
| Inventors:
|
Krähmer; Daniel (Aalen, DE);
Eva; Eric (Aalen, DE)
|
| Assignee:
|
Carl Zeiss SMT AG (Oberkochen, DE)
|
| Appl. No.:
|
743815 |
| Filed:
|
December 24, 2003 |
| Current U.S. Class: |
355/53; 355/67; 501/54; 359/350; 359/833; 65/17.3; 65/17.4; 65/30.1; 430/311 |
| Current Intern'l Class: |
G03B 27/42 (20060101) |
References Cited [Referenced By]
U.S. Patent Documents
| 5229872 | Jul., 1993 | Mumola.
| |
| 5296891 | Mar., 1994 | Vogt et al.
| |
| 5523193 | Jun., 1996 | Nelson.
| |
| 6295841 | Oct., 2001 | Allan et al.
| |
| 6339505 | Jan., 2002 | Bates.
| |
| 6376401 | Apr., 2002 | Kondo et al.
| |
| 6451719 | Sep., 2002 | Yamagata.
| |
| 6543254 | Apr., 2003 | Allan et al.
| |
| 6782716 | Aug., 2004 | Moore et al.
| |
| 2001/0030798 | Oct., 2001 | Fujinoki et al.
| |
| 2003/0037568 | Feb., 2003 | Fujiwara et al.
| |
| 2003/0051507 | Mar., 2003 | Ikuta et al.
| |
| 2003/0115904 | Jun., 2003 | Kuhn et al.
| |
| 2003/0115905 | Jun., 2003 | Kuhn et al.
| |
| 2003/0119652 | Jun., 2003 | Kuhn et al.
| |
| 2004/0235635 | Nov., 2004 | Borrelli et al.
| |
| 2005/0068644 | Mar., 2005 | Ikuta et al.
| |
| Foreign Patent Documents |
| 10159959 | Jun., 2003 | DE.
| |
| 10159961 | Jun., 2003 | DE.
| |
| 10159962 | Jul., 2003 | DE.
| |
| 0483752 | May., 1992 | EP.
| |
| 123091 | Aug., 2002 | EP.
| |
Other References
C. Van Peski, "Behavior of Fused Silica Under 193 nm Irradiation", International
Sematech publication, Technology Transfer #00073974A-TR, Jul. 25, 2000,International Sematech.
J. M. Algots et al., "Compaction and Rarefaction of Fused Silica with 193-nm
Excimer Laser Exposure", Optical Microlithography XVI, Feb. 25-28, 2003, final
draft submitted for Proceedings of SPIE vol. #5040, corresponding pp. 1639-1650,
SPIE—The International Society for Optical Engineering.
V.S. Khotimchenko, et al., "Determining the Content of Hydrogen Dissolved in
Quartz Glass Using the Methods of Raman Scattering and Mass Spectrometry", Zhumal
Prikladnoi Spektroskopii, 46(6), pp. 987-991, 1986 (with English-language summary
at end).
D.M. Dodd, et al., "Optical Determinations of OH in Fused Silica", J. Appl. Physics,
p. 3911, 1966.
|
Primary Examiner: Rutledge; D.
Attorney, Agent or Firm: Jones Day
Claims
We claim:
1. A projection exposure apparatus comprising:
an illumination optical system including a light source for illuminating a patterning structure;
a projection optical system for projecting an image of the illuminated patterning
structure onto a substrate;
the illumination optical system and the projection optical system each comprising
a plurality of lenses;
wherein each of the lenses has a maximum design fluence value associated therewith,
the maximum design fluence value representing a predetermined expected maximum
fluence that the respective lens will be exposed to during a standard mode of operation
of the projection exposure apparatus;
at least one lens being made of a lens material selected from a first group of
lens materials wherein, after exposure to a given number of pulses of radiation
of a given pulse length, each lens material of the first group has a characteristic
transition point representative of a fluence value,
wherein each lens material of the first group will have rarefied after exposure
to the given number of pulses of radiation of the given pulse length having any
fluence value below its transition point, and wherein each lens material of the
first group will have densified after exposure to the given number of pulses of
radiation of the given pulse length having any fluence value above its transition
point; and
wherein the transition point of each lens material of the first group satisfies
the following condition:
TRC<0.8
·HD
wherein T
RC represents the transition point of the respective lens
material, H
D represents the design fluence value of the respective lens, and
wherein at least one lens is made of a first fused silica material comprised
in the first group of lens materials, wherein a transmittance of the first fused
silica material obeys the relationship
T=10
-(ko+kind)·l
with
T denoting the transmittance,
k
0 being an absorption coefficient of the first fused silica material
before exposure to light of a wavelength of 193.4 nm,
k
0+k
ind being a total absorption coefficient after the
first fused silica material has been exposed to 160 million pulses of radiation
having a fluence of 5 mJ/cm
2 and a wavelength of 193.4 nm,
and l being a length of a path of light through the first fused silica material,
and wherein the following condition is fulfilled:
kind<10
-3 cm
-1.
2. The projection exposure apparatus according to claim 1, wherein
0.05
·HD<TRC<0.8
·HD.
3. The projection exposure apparatus according to claim 1, wherein
0.1
·HD<TRC<0.7
·HD.
4. The projection exposure apparatus according to claim 1, wherein
0.001 mJ/cm
2<HD<0.05 mJ/cm
2.
5. The projection exposure apparatus according to claim 1, wherein at least those
lenses are made of the first fused silica material comprised in the first group
of lens materials that fulfil the following condition:
Gi<0.8
·GD
wherein
G
i is an axial thickness of the i
th lens,
and G
D is an average of all axial thickness of the lenses, wherein
the axial thickness of each lens represents a thickness of the lens at a location
on the optical axis.
6. The projection exposure apparatus according to claim 1, wherein at least those
lenses are made of the first fused silica material comprised in the first group
of lens materials that fulfil the following condition:
Gi>1.2
·GD
wherein
G
i is an axial thickness of the i
th lens,
and G
D is an average of all axial thickness of the lenses, wherein
the axial thickness of each lens represents a thickness of the lens at a location
on the optical axis.
7. The projection exposure apparatus according to claim 1, wherein at least those
lenses are made of the first fused silica material comprised in the first group
of lens materials that fulfil the following condition:
Di<0.7
·Dmax
wherein
D
i is an effective diameter of the i
th lens, and
D
max is a maximum effective diameter of a lens in the projection exposure system.
8. The projection exposure apparatus according to claim 1, wherein more than
50% of the total number of lenses in at least one of the illumination optical system
and the projection optical system are made of a lens material selected from the
first group of lens materials.
9. The projection exposure apparatus according to claim 1, wherein more than
75% of the total number of lenses in at least one of the illumination optical system
and the projection optical system are made of a lens material selected from the
first group of lens materials.
10. The projection exposure apparatus according to claim 1, wherein more than
70% of the lenses of the projection optical system are made of the first fused
silica material comprised in the first group of lens materials.
11. The projection exposure apparatus according to claim 1, wherein at least
two lens materials selected from the first group of lens materials are fused silica materials.
12. The projection exposure apparatus according to claim 1, wherein at least
one lens of the plurality of lenses is made of calcium fluoride.
13. The projection exposure apparatus according to claim 1, wherein the light
source is adapted to emit light a substantial amount of which has a wavelength
shorter than 200 nm.
14. The projection exposure apparatus according to claim 13, wherein the light
source is an excimer laser.
15. The projection exposure apparatus according to claim 13, wherein the light
source is an ArF laser.
16. The projection exposure apparatus according to claim 1, wherein the given
number of pulses of radiation is 10 billion and the given pulse length is 25 ns.
17. Projection exposure apparatus comprising:
an illumination optical system including a light source for illuminating a patterning structure;
a projection optical system for projecting an image of the illuminated patterning
structure onto a substrate;
the illumination optical system and the projection optical system each comprising
a plurality of lenses;
wherein each location on a surface of each lens has a design fluence value associated
therewith, the design fluence value representing a predetermined expected fluence
that the respective location on the surface of the lens will be exposed to during
a standard mode of operation of the projection exposure apparatus;
wherein each location on the surface of each lens has a design fluence gradient
value associated therewith, the design fluence gradient value representing a predetermined
expected change of the design fluence value per unit length;
and wherein each lens has a first location with a maximum design gradient product
associated therewith, the maximum design gradient product representing a maximum
product of the design fluence gradient value and the design fluence value at the
location of the design fluence gradient value for the respective lens;
at least one lens being made of a lens material selected from a second group
of lens materials wherein each lens of the second group has a characteristic transition
point after exposure to a given number of pulses of radiation of a given pulse length,
wherein each lens material will have rarefied after exposure to the given number
of pulses of radiation of the given pulse length having any fluence value below
the transition point, and wherein each lens material will densified after exposure
to the given number of pulses of radiation of the given pulse length having any
fluence value above the transition point;
wherein each lens material further has a characteristic minimum value associated
therewith, the minimum value representing a fluence value causing the greatest
extent of rarefaction in the lens material after exposure to the given number of
pulses of radiation of the given pulse length,
and wherein the minimum value H
min of each lens material of the second
group satisfies the following condition:
Hmin<HGHmax
wherein H
GHmax represents the design fluence value at the location
with the maximum design gradient product, and H
min represents the minimum
value of the respective lens material, and
wherein at least one lens is made of a first fused silica material comprised
in the second group of lens materials, wherein a transmittance of the first fused
silica material obeys the relationship
T=10
-(ko+kind)·l
with
T denoting the transmittance,
k
0 being an absorption coefficient of the first fused silica material
before exposure to light of a wavelength of 193.4 nm,
k
0+k
ind being a total absorption coefficient after the
first fused silica material has been exposed to 160 million pulses of radiation
having a fluence of 5 mJ/cm
2 and a wavelength of 193.4 nm,
and l being a length of a path of light through the first fused silica material,
and wherein the following condition is fulfilled:
kind<10
-3 cm
-1.
18. The projection exposure apparatus according to claim 17, wherein the transition
point of each lens material of the second group satisfies the following condition:
TRC<HGHmax
wherein H
GHmax represents the design fluence value at the location
with the maximum design gradient product, and T
RC represents the transition
point of the respective lens material.
19. The projection exposure apparatus according to claim 17, wherein the at least
one lens is a lens in the projection optical system.
20. The projection exposure apparatus according to claim 17,
wherein the projection optical system has a pupil plane,
wherein the projection optical system has an optical axis at least in a region
of the pupil plane, and
wherein the lens is located at a distance from the pupil plane along the optical
axis and
wherein the distance is shorter than a free diameter of the lens.
21. The projection exposure apparatus according to claim 20, wherein the distance
is shorter than half of the free diameter of the lens.
22. The projection exposure apparatus according to claim 17, wherein more than
50% of a total number of the plurality of lenses in at least one of the illumination
optical system and the projection optical system are made of a lens material selected
from the second group of lens materials.
23. The projection exposure apparatus according to claim 22, wherein more than
75% of the total number of lenses in at least one of the illumination optical system
and the projection optical system are made of a lens material selected from the
second group of lens materials.
24. The projection exposure apparatus according to claim 17, wherein more than
70% of the lenses in at least one of the illumination optical system and the projection
optical system are made of the first fused silica material comprised in the second
group of lens materials.
25. The projection exposure apparatus according to claim 17, wherein at least
two lens materials selected from the second group of lens materials are fused silica materials.
26. The projection exposure apparatus according to claim 17, wherein at least
one lens of the plurality of lenses is made of calcium fluoride.
27. The projection exposure apparatus according to claim 17, wherein the light
source is adapted to emit light a substantial amount of which has a wavelength
shorter than 200 nm.
28. The projection exposure apparatus according to claim 27, wherein the light
source is an excimer laser.
29. The projection exposure apparatus according to claim 27, wherein the light
source is an ArF laser.
30. The projection exposure apparatus according to claim 17, wherein the given
number of pulses of radiation is 10 billion and the given pulse length is 25 ns.
31. A projection exposure apparatus comprising:
an illumination optical system including a light source for illuminating a patterning structure;
a projection optical system for projecting an image of the illuminated patterning
structure onto a substrate;
the illumination optical system and the projection optical system each comprising
a plurality of lenses;
wherein each of the lenses has a maximum design fluence value associated therewith,
the maximum design fluence value representing a predetermined expected maximum
fluence that the respective lens will be exposed to during a standard mode of operation
of the projection exposure apparatus;
at least one lens being made of a lens material selected from a first group of
lens materials wherein, after exposure to a given number of pulses of radiation
of a given pulse length, each lens material of the first group has a characteristic
transition point representative of a fluence value,
wherein each lens material of the first group will have rarefied after exposure
to the given number of pulses of radiation of the given pulse length having any
fluence value below its transition point, and wherein each lens material of the
first group will have densified after exposure to the given number of pulses of
radiation of the given pulse length having any fluence value above its transition
point; and
wherein the transition point of each lens material of the first group satisfies
the following condition:
TRC<0.8
·HD
wherein T
RC represents the transition point of the respective lens
material, H
D represents the design fluence value of the respective lens, and
wherein at least one lens is made of a first fused silica material comprised
in the first group of lens materials, which first fused silica material comprises
a fused silica material manufactured by depositing SiO
2-particles to
form a porous soot body, followed by vitrification, which first fused silica material
has a H
2-content of about 5·10
15 molecules/cm
3
or more.
32. A projection exposure apparatus comprising:
an illumination optical system including a light source for illuminating a patterning structure;
a projection optical system for projecting an image of the illuminated patterning
structure onto a substrate;
the illumination optical system and the projection optical system each comprising
a plurality of lenses;
wherein each of the lenses has a maximum design fluence value associated therewith,
the maximum design fluence value representing a predetermined expected maximum
fluence that the respective lens will be exposed to during a standard mode of operation
of the projection exposure apparatus;
at least one lens being made of a lens material selected from a first group of
lens materials wherein, after exposure to a given number of pulses of radiation
of a given pulse length, each lens material of the first group has a characteristic
transition point representative of a fluence value,
wherein each lens material of the first group will have rarefied after exposure
to the given number of pulses of radiation of the given pulse length having any
fluence value below its transition point, and wherein each lens material of the
first group will have densified after exposure to the given number of pulses of
radiation of the given pulse length having any fluence value above its transition
point; and
wherein the transition point of each lens material of the first group satisfies
the following condition:
TRC<0.8
·HD
wherein T
RC represents the transition point of the respective lens
material, H
D represents the design fluence value of the respective lens, and
wherein at least one lens is made of a first fused silica material comprised
in the first group of lens materials, which first fused silica material comprises
a fused silica material manufactured by depositing SiO
2-particles to
form a porous soot body, followed by vitrification, which first fused silica material
has an OH-content of about 50 ppm by weight or less.
33. The projection exposure apparatus according to claim 32, wherein the first
fused silica material has a H
2-content of about 10
15 molecules/cm
3
or more.
34. Projection exposure apparatus comprising:
an illumination optical system including a light source for illuminating a patterning structure;
a projection optical system for projecting an image of the illuminated patterning
structure onto a substrate;
the illumination optical system and the projection optical system each comprising
a plurality of lenses;
wherein each location on a surface of each lens has a design fluence value associated
therewith, the design fluence value representing a predetermined expected fluence
that the respective location on the surface of the lens will be exposed to during
a standard mode of operation of the projection exposure apparatus;
wherein each location on the surface of each lens has a design, fluence gradient
value associated therewith, the design fluence gradient value representing a predetermined
expected change of the design fluence value per unit length;
and wherein each lens has a first location with a maximum design gradient product
associated therewith, the maximum design gradient product representing a maximum
product of the design fluence gradient value and the design fluence value at the
location of the design fluence gradient value for the respective lens;
at least one lens being made of a lens material selected from a second group
of lens materials wherein each lens of the second group has a characteristic transition
point after exposure to a given number of pulses of radiation of a given pulse length,
wherein each lens material will have rarefied after exposure to the given number
of pulses of radiation of the given pulse length having any fluence value below
the transition point, and wherein each lens material will densified after exposure
to the given number of pulses of radiation of the given pulse length having any
fluence value above the transition point;
wherein each lens material further has a characteristic minimum value associated
therewith, the minimum value representing a fluence value causing the greatest
extent of rarefaction in the lens material after exposure to the given number of
pulses of radiation of the given pulse length,
and wherein the minimum value H
min of each lens material of the second
group satisfies the following condition:
Hmin<HGHmax
wherein H
GHmax represents the design fluence value at the location
with the maximum design gradient product, and H
min represents the minimum
value of the respective lens material, and
wherein at least one lens is made of a first fused silica material comprised
in the second group of lens materials, which first fused silica material comprises
a fused silica material manufactured by depositing SiO
2-particles to
form a porous soot body, followed by vitrification, which first fused silica material
has a H
2-content of about 5·10
15 molecules/cm
3
or more.
35. Projection exposure apparatus comprising:
an illumination optical system including a light source for illuminating a patterning structure;
a projection optical system for projecting an image of the illuminated patterning
structure onto a substrate;
the illumination optical system and the projection optical system each comprising
a plurality of lenses;
wherein each location on a surface of each lens has a design fluence value associated
therewith, the design fluence value representing a predetermined expected fluence
that the respective location on the surface of the lens will be exposed to during
a standard mode of operation of the projection exposure apparatus;
wherein each location on the surface of each lens has a design fluence gradient
value associated therewith, the design fluence gradient value representing a predetermined
expected change of the design fluence value per unit length;
and wherein each lens has a second location with a maximum design gradient product
associated therewith, the maximum design gradient product representing a maximum
product of the design fluence gradient value and the design fluence value at the
location of the design fluence gradient value for the respective lens;
at least one lens being made of a lens material selected from a second group
of lens materials wherein each lens of the second group has a characteristic transition
point after exposure to a given number of pulses of radiation of a given pulse length,
wherein each lens material will have rarefied after exposure to the given number
of pulses of radiation of the given pulse length having any fluence value below
the transition point, and wherein each lens material will densified after exposure
to the given number of pulses of radiation of the given pulse length having any
fluence value above the transition point;
wherein each lens material further has a characteristic minimum value associated
therewith, the minimum value representing a fluence value causing the greatest
extent of rarefaction in the lens material after exposure to the given number of
pulses of radiation of the given pulse length,
and wherein the minimum value H
min of each lens material of the second
group satisfies the following condition:
Hmin<HGHmax
wherein H
GHmax represents the design fluence value at the location
with the maximum design gradient product, and H
min represents the minimum
value of the respective lens material, and
wherein at least one lens is made, of a first fused silica material comprised
in the second group of lens materials, which first fused silica material comprises
a fused silica material manufactured by depositing SiO
2-particles to
form a porous soot body, followed by vitrification, which first fused silica material
has an OH-content of about 50 ppm by weight or less.
36. The projection exposure apparatus according to claim 35, wherein the first
fused silica material has a H
2-content of about 10
15 molecules/cm
3
or more.
37. A projection exposure apparatus comprising:
an illumination optical system including a light source for illuminating a patterning structure;
a projection optical system for projecting an image of the illuminated patterning
structure onto a substrate;
the illumination optical system and the projection optical system each comprising
a plurality of lenses;
wherein each of the lenses has a maximum design fluence value associated therewith,
the maximum design fluence value representing a predetermined expected maximum
fluence that the respective lens will be exposed to during a standard mode of operation
of the projection exposure apparatus;
wherein more than 50% of a total number of the plurality of lenses in at least
one of the illumination optical system and the projection optical system are made
of a lens material selected from a first group of lens materials wherein, after
exposure to a given number of pulses of radiation of a given pulse length, each
lens material of the first group has a characteristic transition point representative
of a fluence value,
wherein each lens material of the first group will have rarefied after exposure
to the given number of pulses of radiation of the given pulse length having any
fluence value below its transition point, and wherein each lens material of the
first group will have densified after exposure to the given number of pulses of
radiation of the given pulse length having any fluence value above its transition
point; and
wherein the transition point of each lens material of the first group satisfies
the following condition:
TRC<0.8
·HD
wherein T
RC represents the transition point of the respective lens
material, H
D represents the design fluence value of the respective lens.
38. Projection exposure apparatus comprising:
an illumination optical system including a light source for illuminating a patterning structure;
a projection optical system for projecting an image of the illuminated patterning
structure onto a substrate;
the illumination optical system and the projection optical system each comprising
a plurality of lenses;
wherein each location on a surface of each lens has a design fluence value associated
therewith, the design fluence value representing a predetermined expected fluence
that the respective location on the surface of the lens will be exposed to during
a standard mode of operation of the projection exposure apparatus;
wherein each location on the surface of each lens has a design fluence gradient
value associated therewith, the design fluence gradient value representing a predetermined
expected change of the design fluence value per unit length;
and wherein each lens has a first location with a maximum design gradient product
associated therewith, the maximum design gradient product representing a maximum
product of the design fluence gradient value and the design fluence value at the
location of the design fluence gradient value for the respective lens;
wherein more than 50% of a total number of the plurality of lenses in at least
one of the illumination optical system and the projection optical system are made
of a lens material selected from a second group of lens materials wherein each
lens of the second group has a characteristic transition point after exposure to
a given number of pulses of radiation of a given pulse length,
wherein each lens material will have rarefied after exposure to the given number
of pulses of radiation of the given pulse length having any fluence value below
the transition point, and wherein each lens material will densified after exposure
to the given number of pulses of radiation of the given pulse length having any
fluence value above the transition point;
wherein each lens material further has a characteristic minimum value associated
therewith, the minimum value representing a fluence value causing the greatest
extent of rarefaction in the lens material after exposure to the given number of
pulses of radiation of the given pulse length,
and wherein the minimum value H
min of each lens material of the second
group satisfies the following condition:
Hmin<HGHmax
wherein H
GHmax represents the design fluence value at the location
with the maximum design gradient product, and H
min represents the minimum
value of the respective lens material.
Description
BACKGROUND OF THE INVENTION
1. Field of Industrial Applicability
The present invention generally relates to a projection exposure apparatus for
photolithography, and, more particularly, to a projection exposure apparatus adapted
to reduce a decrease in performance due to radiation-induced damage of lens material(s)
comprised therein.
2. Prior Art
Lithographic processes are commonly used in the manufacture of semiconductor
elements, such as integrated circuits (ICs), LSIs, liquid crystal elements, micropatterened
members such as thin-film magnetic heads and micromechanical components. A projection
exposure apparatus used for lithography generally comprises an illumination optical
system with a light source and a projection optical system. Light from the illumination
optical system illuminates a patterning structure (mask) with a given pattern and
the projection optical system transfers an image of the patterning structure pattern
onto a photo-sensitive substrate. The image of the mask may be reduced in size
by the projection optical system so as to project a smaller image of the patterning
structure onto the substrate.
The trend towards ever more sophisticated semiconductor devices requires semiconductor
elements of smaller size and higher complexity which, in turn, makes higher demands
on the resolution achievable in projection optical systems. Improved resolution
is generally achieved by increasing the numerical aperture of the projection optical
system as well as decreasing the wavelength of the exposure radiation (illumination
light), with recently used illumination light having wavelengths of 248 nm and below.
However, the transition to ever shorter wavelengths is associated with a
number of problems. In particular, the number of suitable materials for lenses
in the projection optical system as well as in the illumination optical system
having a large enough transmittance at short wavelengths is very limited. Furthermore,
the materials that are satisfactory in terms of transmittance often suffer damage
upon exposure to radiation. Whilst it was, at first, observed that certain lens
materials will undergo compaction, i.e. densification, it was later noticed that
some materials show the opposite effect, i.e. rarefaction, which involves an expansion
of the lens material.
Fused silica (sometimes referred to as "quartz" in a more general manner) is
the most common material used in recently introduced projection exposure apparatus
employing short wavelengths for exposure (193 nm and 248 nm being the most common).
Upon exposure to high intensity radiation, exposed areas of a lens of a given material
have been found to undergo a change in density, in particular densification or
rarefaction. The change in density of an exposed area in a lens in a projection
exposure apparatus can generally be assumed to have a detrimental effect on the
optical properties of the lens. In particular, wavefront distortion is indicative
of densification or rarefaction and can be measured and determined by suitable
interferometric methods, for example. An increase in density, for instance, of
the lens material shortens the physical path through the material, but also alters
the refractive index, which is generally increased to a greater extent, so that
the net effect is an increase in the optical path. For rarefaction phenomena, the
opposite applies. Usually, densification and rarefaction are quantified in terms
of the change of the product of refractive index and path length, commonly referred
to as optical path length difference OPD.
The susceptibility of fused silica materials to UV-induced damage is correlated
with the materials' chemical and physical properties, which, in turn, are closely
linked to methods of manufacturing and/or treating the material(s).
A lens material that is transparent to UV-radiation and which has, at least so
far, not been found to be subject to such structural alterations that are associated
with changes in optical properties is calcium fluoride, CaF
2. However,
for calcium fluoride to be suitable for use in optical lenses, it needs to be in
the form of single crystals which are not only costly but also technically difficult
to manufacture so that the resulting limited supply somewhat constrains its practical use.
Accordingly, fused silica material is still the most widely used option
in terms of lens materials suited for photolithography with UV-radiation.
The observation of the above described phenomena has been an incentive for both
materials scientists and projection exposure apparatus designers to find solutions
to the problems created by the structural change of the respective lens material
manifesting itself as a density and refractive index change over the lifespan of
a projection exposure system.
In U.S. Pat. No. 6,295,841 B1 by Allan et al., a method of precompacting fused
silica and a method for making a fused silica stepper lens are disclosed, the entire
content of which is incorporated herein by reference.
In U.S. Pat. No. 6,339,505 B1 by Bates, a method for radiation projection and
a lens assembly for semiconductor exposure tools are disclosed, the entire content
of which is incorporated herein by reference. In particular, a lens assembly comprising
a first lens element of a material that densifies upon exposure to radiation and
a second lens material that rarefies upon exposure to radiation are described.
The lens materials are supposed to be selected such that the change in optical
path length difference of one lens will exactly compensate for that of the other
lens. Exact compensation, however, has proven difficult, if not almost impossible
to achieve in practice.
There remains a need for a projection exposure system which is adapted to reduce
a decrease in performance of the optical system comprised therein due to radiation-induced
damage to the lens material(s), and in particular a radiation-induced change in
density of the lens material(s).
SUMMARY OF THE INVENTION
In view of this and other needs, the present invention aims at reducing the problem
of deterioration of optical performance in a projection exposure system due to
radiation-induced damage to lens material(s).
Under a first aspect, the invention provides a projection exposure system comprising:
an illumination optical system including a light source for illuminating a patterning
structure; a projection optical system for projecting an image of the illuminated
patterning structure onto a substrate; the illumination optical system and the
projection optical system each comprising a plurality of lenses; wherein each of
the lenses has a maximum design fluence value (H
D) associated therewith,
the maximum design fluence value (H
D) representing a predetermined expected
maximum fluence that the respective lens will be exposed to during a standard mode
of operation of the projection exposure apparatus; at least one lens being made
of a lens material selected from a first group of lens materials wherein each lens
material of the first group has a characteristic transition point (T
RC)
after exposure to a given number of pulses of radiation of a given pulse length,
wherein each lens material of the first group will have rarified after exposure
to the given number of pulses of radiation of the given pulse length having any
fluence value below its transition point (T
RC), and wherein each lens
material of the first group will have densified after exposure to the given number
of pulses of radiation of the given pulse length having any fluence value above
its transition point (T
RC); and wherein the transition point (T
RC)
of each lens material of the first group satisfies the following condition:
TRC<0.8
·HD
wherein T
RC represents the transition point of the respective
lens material, wherein H
D represents the maximum design fluence value
of the respective lens, and wherein at least one of the following is fulfilled:
- (i) at least one lens is made of a first fused silica material comprised
in the first group of lens materials, wherein a transmittance of the first fused
silica material obeys the relationship
T=10-(ko+kind)·l
- with
- T denoting the transmittance,
- k0 being an absorption coefficient of the first fused
silica material before exposure to light of a wavelength of 193.4 nm,
- k0+kind being a total absorption coefficient
after the first fused silica material has been exposed to 160 million pulses of
radiation having a fluence of 5 mJ/cm2 and a wavelength of 193.4 nm,
- and l being a length of a path of light through the first fused
silica material,
- and wherein the following condition is fulfilled:
kind<10-3 cm-1;
- (ii) at least one lens is made of a first fused silica material comprised
in the first group of lens materials, wherein a transmittance of the first fused
silica material obeys the relationship
T=10-(ko+k′ind)·l
- with
- T denoting the transmittance,
- k0 being an absorption coefficient of the first fused
silica material before exposure to light of a wavelength of 193.4 nm,
- k0+k′ind being a total absorption coefficient
after the first fused silica material has been exposed to 100 billion pulses of
radiation having a fluence of 200 μJ/cm2 and a wavelength of 193.4 nm,
- and l being a length of a path of light through the first fused
silica material,
- and wherein the following condition is fulfilled:
k′ind<10-3 cm-1;
- (iii) at least one lens is made of a first fused silica material comprised
in the first group of lens materials, which first fused silica material comprises
a fused silica material manufactured by depositing SiO2-particles to
form a porous soot body, followed by vitrification, which first fused silica material
has a H2-content of about 5·1015 molecules/cm3
or more;
- (iv) at least one lens is made of a first fused silica material comprised
in the first group of lens materials, which first fused silica material comprises
a fused silica material manufactured by depositing SiO2-particles to
form a porous soot body, followed by vitrification, which first fused silica material
has an OH-content of about 50 ppm by weight or less, and preferably a H2-content
of about 1015 molecules/cm3 or more;
- (v) more than 50% of a total number of the plurality of lenses in at
least one of the illumination optical system and the projection optical system
are made of a lens material selected from the first group of lens materials.
The at least one lens made of the first fused silica material may be the at least
one lens being made of a lens material selected from the first group of lens materials
or may be at least one lens out of the at least one lens being made of a lens material
selected from the first group of lens materials.
Under a second aspect, the invention provides a projection exposure apparatus
comprising: an illumination optical system including a light source for illuminating
a patterning structure; a projection optical system for projecting an image of
the illuminated patterning structure onto a substrate; the illumination optical
system and the projection optical system each comprising a plurality of lenses;
wherein each location on a surface of each lens has a design fluence value associated
therewith, the design fluence value representing a predetermined expected fluence
that the respective location on the surface of the lens will be exposed to during
a standard mode of operation of the projection exposure apparatus; wherein each
location on the surface of each lens has a design fluence gradient value associated
therewith, the design fluence gradient value representing a predetermined expected
change of the design fluence value per unit length; and wherein each lens has a
first location with a maximum design gradient product (P
D) associated
therewith, the maximum design gradient product representing a maximum product of
the design fluence gradient value and the design fluence value at the location
of the design fluence gradient value for the respective lens; at least one lens
being made of a lens material selected from a second group of lens materials wherein
each lens of the second group has a characteristic transition point (T
RC)
after exposure to a given number of pulses of radiation of a given pulse length,
wherein each lens material will have rarefied after exposure to the given number
of pulses of radiation of the given pulse length having any fluence value below
the transition point (T
RC) , and wherein each lens material will have
densified after exposure to the given the given number of pulses of radiation of
the given pulse length having any fluence value above the transition point (T
RC);
wherein each lens material further has a characteristic minimum value (H
min)
associated therewith, the minimum value representing a fluence value causing the
greatest extent of rarefaction in the lens material after exposure to the given
number of pulses of radiation of the given pulse length, and wherein the minimum
value H
min of each lens material satisfies the following condition:
Hmin<HGHmax
wherein H
GHmax represents the design fluence value at the location
with the maximum design gradient product (P
D), H
min represents
the minimum value, and wherein at least one of the following is fulfilled:
- (i) at least one lens is made of a first fused silica material comprised
in the second group of lens materials, wherein a transmittance of the first fused
silica material obeys the relationship
T=10-(ko+kind)·l
- with
- T denoting the transmittance,
- k0 being an absorption coefficient of the first fused
silica material before exposure to light of a wavelength of 193.4 nm,
- k0+kind being a total absorption coefficient
after the first fused silica material has been exposed to 160 million pulses of
radiation having a fluence of 5 mJ/cm2 and a wavelength of 193.4 nm,
- and l being a length of a path of light through the first fused
silica material,
- and wherein the following condition is fulfilled:
kind<10-3 cm-1;
- (ii) at least one lens is made of a first fused silica material comprised
in the second group of lens materials, wherein a transmittance of the first fused
silica material obeys the relationship
T=10-(ko+k′ind)·l
- with
- T denoting the transmittance,
- k0 being an absorption coefficient of the first fused
silica material before exposure to light of a wavelength of 193.4 nm,
- k0+k′ind being a total absorption coefficient
after the first fused silica material has been exposed to 100 billion pulses of
radiation having a fluence of 200 μJ/cm2 and a wavelength of 193.4 nm,
- and l being a length of a path of light through the first fused
silica material,
- and wherein the following condition is fulfilled:
k′ind<10-3 cm-1;
- (iii) at least one lens is made of a first fused silica material comprised
in the second group of lens materials, which first fused silica material comprises
a fused silica material manufactured by depositing SiO2-particles to
form a porous soot body, followed by vitrification, which first fused silica material
has a H2-content of about 5·1015 molecules/cm3
or more;
- (iv) at least one lens is made of a first fused silica material comprised
in the second group of lens materials, which first fused silica material comprises
a fused silica material manufactured by depositing SiO2-particles to
form a porous soot body, followed by vitrification, which first fused silica material
has an OH-content of about 50 ppm by weight or less, and preferably a H2-content
of about 1015 molecules/cm3 or more;
- (v) more than 50% of a total number of the plurality of lenses in at
least one of the illumination optical system and the projection optical system
are made of a lens material selected from the second group of lens materials.
The at least one lens made of the first fused silica material may be the at least
one lens being made of a lens material selected from the second group of lens materials
or may be at least one lens out of the at least one lens being made of a lens material
selected from the second group of lens materials.
The term "fluence" refers to a radiation (beam) intensity per unit area and is
usually measured in units of mJ/cm
2.
The term "fluence gradient value" refers to the amount of a gradient vector of
the fluence values H in dependence on a location on a surface of a respective lens,
as defined by x- and y-coordinates, which is given by
##EQU1##
The term "patterning structure" as used herein refers broadly to any means suited
for endowing an illuminating light beam with a patterned cross-section, an image
of which pattern (of the illuminated patterning structure) is projected onto the
substrate. The patterning structure may be a mask or a reticle, for example. The
term "reticle" is more generally associated with a mask a reduced image of which
is projected onto the substrate, and the term "mask" generally refers to a non-reducing,
i.e. 1:1 projection exposure. Mask or reticle types include binary, attenuating
and alternating phase shift types, and various hybrid types. The mask/reticle may
transmit or reflect the illumination light beam whilst imparting a patterned cross-section
upon it. Programmable mirror arrays are further examples of patterning structures
suitable for use with the present invention. One example of such an array is described,
for instance, in U.S. Pat. No. 5,296,891, the entire content of which is incorporated
by reference herein. An active, matrix-addressable surface light modulator is provided
with a reflective surface whose individually addressed surface areas reflect incident
light as diffracted light and whose non-addressed surface areas reflect light as
undiffracted light. Undiffracted light is then filtered out and only the diffracted
light permitted to pass to a projection lens. Thus, the matrix-addressable surface
matrix is programmed to impart a desired pattern to the illumination light beam.
An additional example of a programmable mirror array is disclosed in U.S. Pat.
No. 5,523,193, the entire content of which is incorporated by reference herein.
Illumination light strikes a spatial light modulator comprising a plurality of
programmable mirror devices or pixels, which are electronically actuated to a defined
position thus creating a pattern from which the illumination light beam is reflected
towards the substrate or projection optical system, respectively or into an off
position. Programmable LCD arrays are further examples of patterning structures
suitable for use with the present invention. Such an array is disclosed in U.S.
Pat. No. 5,229,872, for instance, the entire content of which is incorporated by
reference herein. An illumination light beam is reflected from a face of a liquid
crystal light valve having a reflective pattern of an image (reproduced from a
cathode ray tube screen) and is directed through a projection optical system towards
a substrate. Generally, light valves or illumination templates are additional terms
used in connection with patterning structures.
In the following, a short summary of the new findings relating to a change of
physical properties of a range of lens materials upon exposure to UV-radiation
is given.
A series of experiments involving a variety of lens materials was carried out.
Samples of each material were exposed to radiation pulses having fluence values
in the order of those that a lens would actually be exposed to in a standard operating
mode in a projection exposure apparatus. More precisely, the fluence values were
chosen to be in the range of 0.01 to 0.5 mJ/cm
2. Each sample was exposed
to 10 billion pulses of a 193 nm wavelength excimer laser light of a given fluence.
The pulse length, or more precisely integral square pulse width, was about 25 ns.
A change in physical properties resulting from the exposure of a sample to UV-radiation
was assessed by measuring a change of the relative optical path length difference,
i.e. the product of refractive index n and path length L through the material,
divided by the thickness of the sample, i.e. the path length L through the sample
before radiation exposure: Δ(n·L)/L. These measurements were carried
out using an interferometric method.
It was found that the different lens materials do not show either rarefaction
or compaction, but rather both phenomena. Which of the two phenomena occurs depends
strongly on the fluence the lens material is exposed to and, at least to a certain
extent, also on the total number of pulses the material was exposed to as well
as the pulse length. In particular, it was found that a lens material can be characterised
by its characteristic transition point (T
RC) after exposure to a given
number of pulses of radiation of constant pulse length and constant wavelength.
When the lens material has been exposed to the given number of pulses of radiation
having fluence values smaller than the one indicated by the transition point (T
RC
), the lens material will have rarefied. In contrast, when the lens material has
been exposed to the given number of pulses of radiation having fluence values above
the transition point (T
RC), the lens material will have densified. At
the transition point (T
RC) itself, no structural change relating to
the density of the lens is observed after exposure to the given number of pulses
of radiation having a fluence value as indicated by the transition point value.
Some materials show relatively little or almost no rarefaction. Those materials
are still meant to be encompassed by the present invention. If no transition point
can be determined for these materials, they are attributed a transition point of
zero for purposes of the present invention.
The transition point (T
RC) itself, however, is not necessarily a constant,
but may vary to some degree with the total number of pulses of radiation of a given
fluence that the lens material has been exposed to, as well as the pulse length
(at constant wavelength of radiation). The value of the transition point (T
RC)
may change by about 10% to 15% during an experiment involving 30 billion pulses,
for instance. Therefore, the transition points (T
RC) and other characteristics
of the lens materials are defined herein as characterising the material behaviour
observed after exposure to a given number of pulses of radiation of a given pulse
length and at a given wavelength. Preferably, the transition point is given for
a specified number of pulses of radiation of a given pulse length, and most preferably
10 billion pulses of radiation of 25 ns pulse length, at constant wavelength. 10
billion pulses are typically used in an experiment sufficient to characterise density
changes in a lens material. However, a different reference point, for instance
a different number of pulses or a corresponding duration of radiation from a light
source having continuous emission, may be chosen for the definition of the materials'
characteristics without departing from the scope of the present invention as defined
by the appended claims. Continuous emission may be regarded as an extreme case
of pulsed emission, with the number of pulses being 1 and the pulse length appr
