U.S. patent application number 11/682573 was filed with the patent office on 2007-06-28 for advanced illumination system for use in microlithography.
This patent application is currently assigned to ASML Holding N.V.. Invention is credited to Walter Augustyn, Scott Coston, Mark Oskotsky, Lev Ryzhikov, James Tsacoyeanes.
Application Number | 20070146674 11/682573 |
Document ID | / |
Family ID | 29583729 |
Filed Date | 2007-06-28 |
United States Patent
Application |
20070146674 |
Kind Code |
A1 |
Oskotsky; Mark ; et
al. |
June 28, 2007 |
Advanced Illumination System for Use in Microlithography
Abstract
A system for microlithography comprises an illumination source;
an illumination optical system including, in order from an
objective side, (a) a first diffractive optical element that
receives illumination from the illumination source, (b) a zoom
lens, (c) a second diffractive optical element, (d) a condenser
lens, (e) a relay lens, and (f) a reticle, and a projection optical
system for imaging the reticle onto a substrate, wherein the system
for microlithography provides a zoomable numerical aperture.
Inventors: |
Oskotsky; Mark; (Mamaroneck,
NY) ; Ryzhikov; Lev; (Norwalk, CT) ; Coston;
Scott; (New Milford, CT) ; Tsacoyeanes; James;
(Southbury, CT) ; Augustyn; Walter; (Monroe,
CT) |
Correspondence
Address: |
STERNE, KESSLER, GOLDSTEIN & FOX P.L.L.C.
1100 NEW YORK AVENUE, N.W.
WASHINGTON
DC
20005
US
|
Assignee: |
ASML Holding N.V.
Veldhoven
NL
|
Family ID: |
29583729 |
Appl. No.: |
11/682573 |
Filed: |
March 6, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10896022 |
Jul 22, 2004 |
7187430 |
|
|
11682573 |
Mar 6, 2007 |
|
|
|
10166062 |
Jun 11, 2002 |
6813003 |
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|
10896022 |
Jul 22, 2004 |
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Current U.S.
Class: |
355/67 ; 355/53;
355/68 |
Current CPC
Class: |
G03F 7/70158 20130101;
G03F 7/70091 20130101; G03F 7/70183 20130101 |
Class at
Publication: |
355/067 ;
355/068; 355/053 |
International
Class: |
G03B 27/54 20060101
G03B027/54 |
Claims
1-32. (canceled)
33. A system for microlithography comprising: an illumination
source; an illumination optical system including, in order from an
objective side: (a) a first diffractive optical element that
receives illumination from said illumination source; (b) a second
diffractive optical element; (c) a condenser lens; and (d) a relay
lens, wherein an illumination field between the condenser and the
relay lens are continuously adjustable.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of commonly assigned U.S.
patent application Ser. No. 10/166,062, filed Jun. 11, 2002
entitled ADVANCED ILLUMINATION SYSTEM FOR USE IN MICROLITHOGRAPHY,
which is incorporated herein by reference in its entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to microlithography, and more
particularly, to illumination systems for microlithographic
equipment that have high numerical apertures.
[0004] 2. Related Art
[0005] Photolithography (also called microlithography) is used for
manufacturing of semiconductor devices. Photolithography uses
electromagnetic radiation, such as ultraviolet (UV), deep UV or
visible light to generate fine patterns in a semiconductor device
design. Many types of semiconductor devices, such as diodes,
transistors, and integrated circuits, can be fabricated using
photolithographic techniques. Exposure systems or tools are used to
implement photolithographic techniques, such as etching, in
semiconductor fabrication. An exposure system typically includes an
illumination system, a reticle (also called a mask) containing a
circuit pattern, a projection system, and a wafer alignment stage
for aligning a photosensitive resist-covered semiconductor wafer.
The illumination system illuminates a region of the reticle with a
preferably rectangular slot illumination field. The projection
system projects an image of the illuminated region of the reticle
circuit pattern onto the wafer.
[0006] As semiconductor device manufacturing technology advances,
there are ever increasing demands on each component of the
photolithography system used to manufacture the semiconductor
device. This includes the illumination system used to illuminate
the reticle. For example, there is a need to illuminate the reticle
with an illumination field having uniform irradiance. In
step-and-scan photolithography, there is also a need to vary a size
of the illumination field so that the size of the illumination
field can be tailored to different applications and semiconductor
die dimensions.
[0007] Some illumination systems include an array or diffractive
scattering optical element positioned before the reticle. The
scattering optical element produces a desired angular light
distribution that is subsequently imaged or relayed to the
reticle.
[0008] Additionally, commonly-used die dimensions are 26.times.5
mm, 17.times.5 mm, and 11.times.5 mm. Thus, a standard zoom lens
needs to accommodate variation in the size of the illumination
field. However, a particular problem arises in the field of
microlithography, where different features that are required to be
formed on the semiconductor substrate require variable partial
coherence on the part of the exposure optics. Specifically, partial
coherence (.sigma.), which in microlithography is commonly defined
as the ratio of a numerical aperture of the illumination optics and
a numerical aperture of the projection system, needs to vary
depending on the nature of the feature being formed on the
semiconductor substrate, e.g., the .sigma. for trench formation may
be different from the .sigma. for line formation.
[0009] Accordingly, a need exists for a simple microlithographic
system that can vary the partial coherence parameter over a large
range, while simultaneously being able to accommodate different
field sizes.
SUMMARY OF THE INVENTION
[0010] The present invention is directed to a microlithographic
system that has variable partial coherence and field size.
[0011] One advantage of the present invention is being able to
provide a microlithographic system with continuously adjustable
partial coherence and discretely adjustable field size.
[0012] Another advantage of the present invention is being able to
provide a microlithographic system where both partial coherence and
field size can vary continuously.
[0013] Another advantage of the present invention is being able to
provide a microlithographic system that can achieve the above
objectives with the use of simple optics.
[0014] Additional features and advantages of the invention will be
set forth in the description which follows, and in part will be
apparent from the description, or may be learned by practice of the
invention. The objectives and other advantages of the invention
will be realized and attained by the structure particularly pointed
out in the written description and claims hereof as well as the
appended drawings.
[0015] To achieve these and other advantages and in accordance with
the purpose of the present invention, as embodied and broadly
described, there is provided a system for microlithography
comprising an illumination source; an illumination optical system
including, in order from an objective side, (a) a first diffractive
optical element that receives illumination from the illumination
source, (b) a zoom lens, (c) a second diffractive optical element,
(d) a condenser lens, (e) a relay lens, and (f) a reticle, and a
projection optical system for imaging the reticle onto a substrate,
wherein the system for microlithography provides a zoomable
numerical aperture.
[0016] In another aspect of the present invention there is provided
a system for microlithography comprising an illumination source, an
illumination optical system that receives illumination from the
illumination source, and a projection optical system that receives
illumination from the illumination system, wherein a ratio of a
numerical aperture of the illumination system and a numerical
aperture of the projection optical system is continuously variable
while a field size is discretely variable.
[0017] In another aspect of the present invention there is provided
an illumination system for microlithography comprising, in order
from an objective side a first diffractive optical element, a zoom
lens, a second diffractive optical element having a rectangular
numerical aperture, a condenser lens, and a relay lens.
[0018] In another aspect of the present invention there is provided
a system for microlithography comprising an illumination system
including, in order from an objective side, (a) a zoom lens having
a first diffractive optical element on a first side, and a second
diffractive optical element on a second side, (b) a condenser lens,
and (c) a relay lens, and a projection optical system, wherein a
ratio of a numerical aperture of the illumination system and a
numerical aperture of the projection optical system is continuously
variable.
[0019] It is to be understood that both the foregoing general
description and the following detailed description are exemplary
and explanatory and are intended to provide further explanation of
the invention as claimed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] The accompanying drawings, which are included to provide a
further understanding of the invention and are incorporated in and
constitute a part of this specification, illustrate embodiments of
the invention and together with the description serve to explain
the principles of the invention. In the drawings:
[0021] FIG. 1 is a schematic illustration of one embodiment of the
present invention;
[0022] FIG. 2 is another illustration of the embodiment of FIG. 1,
showing the lens arrangement;
[0023] FIG. 3 is a schematic illustration of another embodiment of
the present invention;
[0024] FIGS. 4A-4C are a ray trace diagrams illustrating a
condenser lens used in an embodiment of the present invention;
[0025] FIGS. 5A-5B are a ray trace diagrams illustrating a relay
lens used in an embodiment of the present invention;
[0026] FIGS. 6A-6B are a ray trace diagrams illustrating a zoom
lens used in an embodiment of the present invention;
[0027] FIG. 7 illustrates an overall design of the illumination
system, such as that shown in FIG. 1;
DETAILED DESCRIPTION OF THE INVENTION
[0028] Reference will now be made in detail to the preferred
embodiments of the present invention, examples of which are
illustrated in the accompanying drawings.
[0029] In recent years, photolithography used in semiconductor
device fabrication has been moving to gradually shorter
wavelengths, as device features shrink in size. With feature sizes
shrinking to sub- micron- and sub-0.1 .mu. range, semiconductor
manufacturers have had to shift to the use of ultraviolet light,
and in some cases to soft X-ray lithography (or deep UV). For
example, excimer lasers, which emit light in the 248, 193 and 157
nm range, are increasingly used in semiconductor device
fabrication. The illumination source in modern microlithographic
equipment, as noted above, is typically a visible light laser, an
excimer laser, or possibly a soft X-ray source. (The terms "light"
and "illumination" will be used interchangeably hereafter to refer
to any electromagnetic radiation used for photoresist exposure.)
The use of these wavelengths presents a particular challenge to the
designer of semiconductor manufacturing equipment, and especially
the optics used to focus and shape the beams from the excimer
lasers. In the present invention, fused silica glass is preferred
for 248 and 193 nm sources, while 157 nm sources typically require
optical elements made of calcium fluoride or barium fluoride to
effectively focus and shape the beam.
[0030] The embodiments described utilize both refractive and
reflective optical elements. It will be understood by one of
ordinary skill in the art, however, that the use of reflective
surfaces is frequently dictated by engineering and design concerns,
rather than fundamental principles of the invention. It is
therefore understood that in the description that follows, the use
of reflective (folding) optical elements is needed strictly due to
engineering design choices, and their use is not required in order
to practice the invention.
[0031] FIG. 1 illustrates a basic configuration of one preferred
embodiment of the present invention. It will be appreciated that in
the figures that follow, where appropriate, the dimensions are in
millimeters.
[0032] As may be seen in FIG. 1, this embodiment of the present
invention includes a diffractive optical element 101 (DOE1), which
is illuminated by an illumination source (not shown).
[0033] The first diffractive optical element 101 may be any element
commonly used to produce diffraction, such as 2-D array of
spherical microlenses, a Fresnel lens, a diffraction grating,
etc.
[0034] From a system perspective, as illustrated in FIG. 1, the
numerical aperture of the beam after the first diffractive optical
element 101 is approximately 0.065.
[0035] As may be further seen from FIG. 1, after passing through
the first diffractive optical element 101, the beam then
illuminates a zoom lens 102. In the this embodiment, the zoom lens
102 is a 5.times. zoom spherical lens, with a focal length of
221.5-1107.7 mm. The diameter of the beam at this point is 180 mm.
The zoom lens 102 is further illustrated in FIG. 6. It will be
appreciated by one of ordinary skill in the art that the zoom lens
102 can use more or fewer elements, as required. One (six element
design) is illustrated by the following prescription (a CODE V
output): TABLE-US-00001 RDY THI GLA >OBJ: INFINITY INFINITY STO:
INFINITY 8.000000 2: -25.24705 5.000000 `CaF2` 3: 55.68759
16.548834 4: -48.92714 25.342815 `CaF2` ASP: K: 1.779039 KC: 0 IC:
YES CUF: 0.000000 CCF: 100 A: 0.146865E-05 B: 0.705843E-08 C:
-.823569E-11 D: 0.127469E-13 AC: 0 BC: 0 CC: 0 DC: 0 5: -36.47260
194.914260 6: 170.18706 28.207990 `CaF2` 7: 510.72551 17.527333 8:
141.82233 51.966932 `CaF2` 9: -277.74471 12.376464 ASP: K:
-3.017335 KC: 0 IC: YES CUF: 0.000000 CCF: 100 A: 0.913504E-07 B:
-.173047E-11 C :-.291669E-15 D :0.148478E-19 AC: 0 BC: 0 CC: 0 DC:
0 10: -297.59579 10.000000 `CaF2` 11: 143.26243 1101.010134 12:
-352.19780 11.373314 `CaF2` 13: -154.19122 187.731924 ASP: K:
-500.000000 KC: 0 IC: YES CUF: 0.000000 CCF: 100 A: -.125463E-05 B:
0.451681E-09 C: -.724157E-13 D: 0.418162E-17 AC: 0 BC: 0 CC: 0 DC:
0 IMG: INFINITY 0.000000 SPECIFICATION DATA EPD 27.66000 DIM MM WL
157.63 XAN 0.00000 0.00000 0.00000 YAN 0.00000 1.85600 3.71900 WTF
3.00000 2.00000 2.00000 VUY 0.00000 0.00000 0.00000 VLY 0.00000
0.00000 0.00000 REFRACTIVE INDICES GLASS CODE 157.63 `CaF2`
1.558739 ZOOM DATA POS 1 POS 2 POS 3 VUY F1 0.00000 0.00000 0.00000
VLY F1 0.00000 0.00000 0.00000 VUY F2 0.00000 0.00000 0.00000 VLY
F2 0.00000 0.00000 0.00000 VUX F1 0.00000 0.00000 0.00000 VLX F1
0.00000 0.00000 0.00000 VUX F2 0.00000 0.00000 0.00000 VLX F2
0.00000 0.00000 0.00000 THI S5 194.91426 1.00000 1.00000 THC S5 0 0
0 THI S7 17.52733 86.68062 1.45028 THC S7 0 0 0 THI S9 12.37646
137.13744 222.36778 THC S9 0 0 0 INFINITE CONJUGATES EFL 221.5400
664.6200 1107.7000 BFL 164.6663 35.0875 11.1078 FFL 115.3771
610.2350 1583.8486 FNO 8.0094 24.0282 40.0470 IMG DIS 187.7319
187.7519 187.7319 OAL 1482.2681 1482.2681 1482.2681 PARAXIAL IMAGE
HT 14.4001 43.2004 72.0006 ANG 3.7190 3.7190 3.7190 ENTRANCE PUPIL
DIA 27.6600 27.6600 27.6600 THI 0.0000 0.0000 0.0000 EXIT PUPIL DIA
353.1110 30.1251 19.3446 THI 590.0538 758.9393 785.8026 STO DIA
27.6600 27.6600 27.6600
[0036] As further illustrated in FIG. 1, a fold (mirror) 103 may be
used in this embodiment to manage and reduce overall tool size by
folding the optical path. As noted above, the use of a mirror 103
is optional, and is generally dictated by engineering/design
choices.
[0037] After reflecting off the fold mirror 103, the beam then
illuminates an axicon 104 (working diameter of 170 mm). After
passing through the axicon 104, the beam has a rectangular
numerical aperture of 0.046-0.009 in the Y dimension, and
0.053-0.011 in the X dimension.
[0038] After passing through the axicon 104, the beam then passes
through the second diffractive element (DOE2) 105. The second
diffractive element 105 is preferably a binary diffractive array.
One example is a array of cylindrical micro-lenses. The
specification for the second diffractive optical element 105 may be
as follows: [0039] Coherence length in mm, X&Y: [0040] 248 nm
temporal--no specs. spatial 0.35.times.0.15 [0041] 193 nm
temporal--3, spatial 0.6.times.0.085 [0042] X & Y beam
divergence, mrad [0043] 248 nm.+-.3.5.times..+-.3.5 [0044] 193
nm.+-.1.times..+-.1.75 [0045] Beam size (nm), X & Y;
6.times.16; 20.times.20; 12.times.32
[0046] After passing through the second diffractive array 105, the
numerical aperture of the beam is approximately
0.165.times.0.04.
[0047] The beam then passes through a spherical condenser lens 106.
A condenser Lens 106 usable in this embodiment can have the
following characteristics: TABLE-US-00002 RDY THI GLA >OBJ:
INFINITY INFINITY STO: INFINITY 75.000000 2: 323.84000 5.000000
`CaF2` 3: INFINITY 491.500000 4: -145.94000 5.000000 `CaF2` 5:
106.10000 278.500000 6: -2090.20000 15.000000 `CaF2` 7: -196.34000
50.000000 IMG: INFINITY 0.000000
[0048] In this embodiment, the condenser lens 106 has a focal
length of 340 mm (generally, it is expected that the condenser lens
106 will have a focal length of 300-400 mm), and the illuminated
diameter is 150-30 mm.
[0049] After passing through the spherical condenser lens, the beam
has a zoomable circular numerical aperture of 0.2125-0.043. The
beam then encounters a delimiter 107 (i.e., a stop), such that the
illuminated field of 112.times.24 mm becomes 108.times.22 mm. The
delimiter 107 is optically conjugate with a reticle 109, through
the use of a relay lens 108 (for example, a 1.times. relay, or a
3.times.-4.times. relay). For design purposes, a fold 110 may be
placed within the relay 108. A stop 111 is placed in the center of
the relay lens 108, for a telecentric illumination system.
[0050] The relay lens 108 is used to conjugate a plane of a
delimiter 107 with a plane of a reticle 109. An example of a
1.times. relay lens 108 prescription is shown below (here, a
10-element design): TABLE-US-00003 RDY THI GLA >OBJ: INFINITY
73.362171 AIR 1: 169.24669 15.000000 `NCaF2` ASP K: -0.916442 IC:
YES CUF: 0.000000 A: 0.000000E+00 B: 0.000030E+00 C: 0.000000E+00
D: 0.000000E+00 2: 297.03762 280.000000 3: 607.71047 32.530979
`NCaF2` 4: -296.65731 1.000000 CON: K: -2.313366 5: 172.28333
33.841572 `NCaF2` 6: 4765.41367 1.000000 AIR 7: 129.90270 40.919042
`NCaF2` 8: 103.26821 29.576441 9: -306.34576 8.000000 `NCaF2` 10:
162.90100 15.103930 STO: INFINITY 15.104002 12: -162.90100 8.000000
`NCaF2` 13: 306.34576 29.576441 14: -103.26821 40.919042 `NCaF2`
15: -129.90270 1.000000 16: -4765.41367 33.841572 `NCaF2` 17:
-172.28333 1.000000 18: 296.65731 32.530979 `NCaF2` CON: K:
-2.313366 19: -607.71047 280.000000 20: -297.03762 15.000000
`NCaF2` 21: -169.24669 73.362171 ASP K: -0.916442 IC: YES CUF:
0.000000 A: 0.000000E+00 B:0.000000E+00 C:0.000000E+00
D:0.000000E+00 IMG: INFINITY 0.000000 AIR XDE: 0.000000 YDE:
0.000000 ZDE: 0.000000 DAR ADE: 0.000000 BDE: 0.000000 CDE:
0.000000
[0051] A projection optical system (not shown) images the reticle
down onto the semiconductor wafer (typically reducing image size by
4.times., to 26.times.5 mm, 17.times.5 mm, or 11.times.5 mm).
[0052] It will be appreciated by one of ordinary skill in the art
that the use of the axicon 104 in such a system improves the
system's optical properties, but the invention may work without it.
It will also be appreciated by one of ordinary skill in the art
that the positions of the axicon 104 and the second diffractive
element 105 can be reversed (i.e., the axicon 104 may be downstream
from the second diffractive element 104), although it is believed
at the present time that the arrangement shown in FIG. 1 is
preferred.
[0053] FIG. 2 illustrates in greater detail the arrangement of the
optical elements of the illumination system. In particular, FIG. 2
shows the zoom lens 102 (shown as a 5-element design) and its
constituent elements 102a, 102b, 102c, 102d and 102e. FIG. 2
further shows the constituent elements of the condenser lens 106
(shown here as a four-element lens), and the 1.times. relay 108
(shown here as an 8-element design). It further illustrates the
position of the .lamda./4 plate, and the reticle (mask) 109, which
is optically conjugate with the plane of the delimiter 107 through
the relay lens 108.
[0054] FIG. 7 is another illustration of the embodiment of FIG. 1,
showing additional elements commonly found in a real-life
microlithography system. All the optical elements illustrated in
FIG. 1 are shown in FIG. 7, using the same reference numerals. In
addition, FIG. 7 also shows a changer unit 701 for the second
diffractive optical element 105. It is anticipated that in order to
achieve different field sizes, different diffractive optical
elements, having different numerical apertures, may need to be
used. Accordingly, the changer unit 701 illustrated in FIG. 7 can
be used for that purpose. It will also be appreciated that a
similar changer unit may be used for the first diffractive optical
element 101, if necessary.
[0055] FIG. 7 also illustrates the dynamic adjustable slit 702,
which is part of the delimiter 107 assembly. The adjustable slit
702 is further described in U.S. Pat. No. 5,966,202, which is
incorporated by reference herein. Together with the field framing
assembly 704, they are used to ensure that the proper beam size
exists at the delimeter plane, which is optically conjugate with
the reticle plane.
[0056] FIG. 7 also illustrates the cleanup aperture assembly 703,
which is used as a telecentric stop at the center of the relay
lens. (See also U.S. Pat. No. 6,307,619, which is incorporated by
reference herein).
[0057] FIG. 7 also illustrates the position of the .lamda./4 plate
112, above plane of the reticle 108 and below the last optical
element (lens) of the relay lens 108.
[0058] Although the preferred embodiments of the present invention
describe a system used for exposure of discrete field sizes
(26.times.5 mm, 17.times.5 mm, and 11.times.5 mm), it is expected
that the system can be made to have a continuously variable field
size. This could be accomplished by the addition of other
diffractive optical elements in the optical path, similar to the
second diffractive optical element 105. By the addition of one or
two such elements, (e.g., additional binary diffractive arrays, or
cylindrical microlens arrays), which may be placed between the
condenser lens and the second diffractive optical element, and by
adjusting its position along the optical axis, it is possible to
achieve a microlithographic system that has both a continuously
variable partial coherence, and a continuously variable field size
at the wafer.
[0059] The use of a projection optical system (not illustrated in
the figures) is well-known in the art, and is typically a 4.times.
lens that reduces the reticle image down onto the wafer.
[0060] The description of another embodiment below, and the
corresponding figures, use the same reference numerals to designate
the same elements as in the embodiment of FIG. 1. FIG. 3
illustrates the basic configuration of another preferred embodiment
of the present invention. As may be seen in FIG. 3, this embodiment
of the present invention includes a diffractive optical element
101, which is illuminated by an illumination source (not
shown).
[0061] The first diffractive optical element (DOE1) 101 may be any
refractive or reflective element commonly used to produce
diffraction, such as an array of spherical microlenses, a Fresnel
lens, a diffraction grating, etc. The numerical aperture of the
beam after the first diffractive optical element 101 is
approximately 0.065 (circular).
[0062] As may be further seen from 102, after passing through DOE1
101, light then illuminates a zoom lens 102. In this embodiment,
the zoom lens 102 is a 5.times. zoom spherical lens, with a focal
length of 196-982 mm. The diameter of the beam at this point is 135
mm. In this embodiment, the zoom lens 102 is a five-element
lens.
[0063] After passing though the zoom lens 102 and reflecting off a
fold mirror 103, the beam then illuminates an axicon 104. After
passing through the axicon 104, the beam has a rectangular
numerical aperture of 0.46-0.009 in the Y dimension, and
0.053-0.011 in the X dimension.
[0064] After passing through the axicon 104, the beam then passes
through the second diffractive element (DOE2) 105 (beam diameter
135 mm). The second diffractive element 105 is preferably a binary
diffractive array. One example is a array of cylindrical
micro-lenses. After passing through the second diffractive array
105, the numerical aperture of the beam becomes 0.2.times.0.04.
[0065] The beam then passes through a condenser lens 106. In this
embodiment, the condenser lens 106 has a focal length of 300 mm,
and the illuminated diameter is 120-25 mm.
[0066] After passing through the spherical condenser lens, the beam
has a zoomable circular numerical aperture of 0.2125-0.043. The
beam then encounters a delimiter 107 (i.e.; a stop), such that the
illuminated field of 120.times.24 mm becomes 108.times.20 mm, The
delimiter 107 is optically conjugate with a reticle 111, through
the use of a relay lens 108. The relay lens 108 is used to
conjugate the plane of the delimiter 107 with the plane of the
reticle. For design purposes, a fold 110 may be placed within the
relay lens 108. A stop 109 is placed in the center of the relay
lens, for a telecentric illumination system.
[0067] A projection optical system (not shown) images the reticle
111 down onto the semiconductor wafer (typically reducing image
size by 4.times.).
[0068] It will be appreciated by one of ordinary skill in the art
that a relay lens is not always necessary to practice the
invention, since the optical planes of the reticle and the
delimiter are conjugate with each other. However, in most practical
systems, a relay lens is used in order to ensure proper size of the
field at the reticle plane, due to mechanical constraints.
[0069] Additionally, it will be appreciated that the field size may
also be made continuous through the use of additional second
diffractive elements, similar in nature to the second diffractive
element 105 described above. Alternatively, a more complex zoom
lens, or the use of a second zoom lens, may be used to achieve the
same purpose.
[0070] Further, it will be appreciated that the present invention
allows for the use of an even lower partial coherence .sigma.,
e.g., 0.001, if needed. A more complex zoom lens (or multiple zoom
lenses) would be needed to achieve this.
[0071] It will be apparent to those skilled in the art that various
modifications and variations can be made to the present invention
without departing from the spirit or scope of the invention. Thus,
it is intended that the present invention cover the modifications
and variations of this invention provided they come within the
scope of the appended claims and their equivalents.
* * * * *