U.S. patent application number 11/268697 was filed with the patent office on 2006-03-16 for aberration correcting optical system.
This patent application is currently assigned to CANNON KABUSHIKI KAISHA. Invention is credited to Toshiyuki Yoshihara.
Application Number | 20060056038 11/268697 |
Document ID | / |
Family ID | 18353326 |
Filed Date | 2006-03-16 |
United States Patent
Application |
20060056038 |
Kind Code |
A1 |
Yoshihara; Toshiyuki |
March 16, 2006 |
Aberration correcting optical system
Abstract
A projection system for projecting a pattern of a mask onto a
substrate. The projection system includes a projection optical
system disposed between the mask and the substrate, and an optical
element for correcting aberration produced in the projection
optical system. The optical element has different refracting powers
in two orthogonal directions or has a refracting power in one
direction of two orthogonal directions and no refracting power in
the other of the two orthogonal directions. The optical element is
disposed between the mask and the substrate, in which an optical
axis of the optical element is inclined with respect to an optical
axis of the projection optical system.
Inventors: |
Yoshihara; Toshiyuki;
(Utsunomiya-shi, JP) |
Correspondence
Address: |
FITZPATRICK CELLA HARPER & SCINTO
30 ROCKEFELLER PLAZA
NEW YORK
NY
10112
US
|
Assignee: |
CANNON KABUSHIKI KAISHA
Tokyo
JP
|
Family ID: |
18353326 |
Appl. No.: |
11/268697 |
Filed: |
November 8, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10936797 |
Sep 9, 2004 |
6987621 |
|
|
11268697 |
Nov 8, 2005 |
|
|
|
09438491 |
Nov 12, 1999 |
6924937 |
|
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10936797 |
Sep 9, 2004 |
|
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Current U.S.
Class: |
359/649 |
Current CPC
Class: |
G03F 7/70308 20130101;
G03F 7/706 20130101; G02B 5/3083 20130101; G02B 27/0025 20130101;
G03F 7/70483 20130101 |
Class at
Publication: |
359/649 |
International
Class: |
G02B 3/00 20060101
G02B003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 16, 1998 |
JP |
1998-342385 |
Claims
1-20. (canceled)
21. A projection system for projecting a pattern of a mask onto a
substrate, said projection system comprising: a projection optical
system disposed between the mask and the substrate; and an optical
element for correcting aberration produced in said projection
optical system, said optical element having different refracting
powers in two orthogonal directions or having a refracting power in
one direction of two orthogonal directions and no refracting power
in the other of the two orthogonal directions, and said optical
element being disposed between the mask and the substrate, wherein
an optical axis of said optical element is inclined with respect to
an optical axis of said projection optical system.
22. A projection system according to claim 21, wherein said
projection system comprises a plurality of said optical elements,
and wherein one of said plurality of optical elements is used
selectively to change the aberration.
23. A projection system according to claim 21, wherein said optical
element is mainly composed of a transparent material of one of
quartz and fluorite.
24. A projection system according to claim 21, wherein the or each
surface of said optical element, having a refracting power, has a
refractive power not greater than 3.times.10.sup.-7 mm.sup.-1.
25. A projection exposure apparatus, comprising: an illumination
system for illuminating a mask; and a projection system as recited
in claim 21 for projecting a pattern of the mask onto a
substrate.
26. A device manufacturing method, comprising: a process for
transferring a device pattern onto a substrate by use of a
projection exposure apparatus as recited in claim 25.
Description
[0001] This application is a divisional application of copending
U.S. patent application Ser. No. 10/936,797, filed Sep. 9, 2004,
which is a divisional of U.S. patent application Ser. No.
09/438,491, filed Nov. 12, 1999, which issued as U.S. Pat. No.
6,924,937 B2 on Aug. 2, 2005.
FIELD OF THE INVENTION
[0002] This invention relates to an optical system for correcting
aberration of a projection optical system, for use in a projection
exposure apparatus (mask aligner) for the manufacture of
semiconductor devices, for example, to thereby assure better
optical characteristics.
[0003] The need for higher density of a semiconductor device such
as an IC or LSI has been increased more and more. In reduction type
projection exposure apparatuses (steppers) wherein an image of a
circuit pattern of a mask (reticle) is formed on a photosensitive
substrate (wafer) through a projection optical system and the
photosensitive substrate is exposed in accordance with a
step-and-repeat method, or in reduction type projection exposure
apparatuses (scanners) wherein a photosensitive substrate is
exposed with a circuit pattern of a mask (reticle) in accordance
with a step-and-scan method, many improvements have been attempted
with respect to the resolution of a projection optical system and
the precision of pattern registration for repeated projection
exposures, printing different patterns upon the same region, to
thereby meet the requirements of higher density integration.
[0004] In projection exposure apparatuses, the resolving
performance of a projection optical system in relation to a pattern
image, as well as the pattern registration precision, rely upon
many factors such as optical performance of the projection optical
system, stage precision, alignment precision, and so on. The
optical performance of a projection optical system, which directly
concerns the registration precision, involves a projection
magnification error and distortion.
[0005] On the other hand, because of recent advancement in the
optical design technique, it becomes possible to design a
projection optical system having a large numerical aperture and a
wide exposure region, yet with very small residual aberration.
[0006] Practically, however, a projection optical system for an
exposure apparatus, as a product, will have aberrations
attributable to errors in relation to an optical material or
materials used, or to errors involved in the machining or
assembling of optical systems, in addition to the residual
aberration remaining in the design. In consideration of this, in
the manufacture of a projection optical system for an exposure
apparatus, fine adjustments for various components are made after
the assemblage is once completed, to minimize the aberration of the
projection optical system due to any errors in manufacture to a
predetermined level.
[0007] Japanese Laid-Open Patent Application, Laid-Open No.
121816/1989, shows an example of fine adjustment of aberration of a
projection system, wherein an aberration adjusting optical system
comprising a light transmissive parallel flat plate is inserted
onto an optical path between an image side of a projection optical
system, having telecentricity, and an imaging plane thereof, to
thereby adjust spherical aberration and on-axis coma aberration of
the projection optical system. In this structure, the spherical
aberration of the projection optical system can be adjusted by
changing the thickness of the parallel flat plate, while the
on-axis coma aberration can be adjusted by tilting the parallel
flat plate.
[0008] Japanese Laid-Open Patent Application, Laid-Open No.
27743/1998, shows another example of fine adjustment of aberration
of a projection system, wherein an aberration adjusting optical
system is provided on an optical path between an image side of a
projection optical system, having telecentricity, and an imaging
plane thereof. The aberration adjusting optical system comprises
two parallel flat plates having the same refractive index and the
same thickness and being inclined with respect to an optical axis
in opposite directions and by the same angle, means for changing
the tilt angles of these two transparent flat plates in opposite
directions and by the same amount, means for rotating the whole
adjusting optical system integrally about the optical axis of the
projection optical system, and means for tilting the whole
adjusting optical system integrally in a desired direction. This
aberration adjusting optical system adjusts spherical aberration,
on-axis astigmatism, and on-axis coma aberration, individually.
[0009] On the other hand, a registration error in an exposure
apparatus is produced as a consequence (D) of all of or a part of
an alignment error component (A), an image distortion (distortion)
error component (B), and a magnification error component (C). Among
them, the alignment error component can be removed by relative
positional adjustment (alignment) of a reticle and a wafer.
However, the image distortion error component (B) and the
magnification error component (C) cannot be removed by the
alignment adjustment. Thus, reduction of these imaging errors is
strongly required for exposure apparatuses.
[0010] In regard to evaluation of an image distortion error
component and a magnification error component such as described
above, a component being isotropic around an optical axis of a
projection optical system and being in first-order proportion to
the distance in a radial direction from the optical axis, is
defined as a magnification error component. On the other hand, the
image distortion component (B) can be classified into some
components such as a third-order component (B1) proportional to a
cube of "h", a fifth-order component (B2) being proportional to a
fifth power of "h", and a component (aspect magnification error)
(B3) concerning a difference in magnification between two
orthogonal directions upon an image plane, for example.
[0011] Among these image distortion components, no aspect
magnification error (B3) is produced, in design, in a projection
optical system, which is revolutionally symmetrical with respect to
an optical axis. If, however, the surface of the product is not
completely revolutionally symmetrical due to any errors in lens
production, particularly in a case in which the curvature radius of
a lens changes along its circumferential direction, there occurs an
aspect magnification different (B3) as an ordinary large distortion
component.
[0012] An aspect magnification error (B3) of a product projection
optical system may be reduced by adjusting some lenses on the basis
of measuring the shapes of the surfaces of the lenses of the
projection optical system by use of an interferometer, for example,
and performing simulations for rotating the lenses about an optical
axis so that the aspect magnification error and the on-axis
astigmatism of the projection optical system can be minimized, in
total, due to cancellation by the elements of the projection
optical system. However, practically, it is still difficult to
suppress both the aspect magnification error and the on-axis
astigmatism, to a very low level at the same time.
[0013] Japanese Laid-Open Patent Application, Laid-Open No.
183190/1995, shows a projection exposure apparatus having an
illumination optical system for illuminating a reticle and a
projection optical system for projecting a pattern of the reticle,
illuminated with the illumination optical system, onto a wafer at a
predetermined reduction magnification. Optical means, having a
refracting power which is revolutionally asymmetrical with respect
to an optical axis of the projection optical system, is disposed
between the reticle and the wafer. This optical means is made
rotatable about the optical axis of the projection optical system
or it is made movable along the optical axis of the projection
optical system, so as to correct any optical characteristic
remaining in the projection optical system and being revolutionally
asymmetrical with respect to the optical axis. However, on-axis
coma aberration such as described above cannot be corrected with
this optical means.
SUMMARY OF THE INVENTION
[0014] It is an object of the present invention to provide an
optical system by which on-axis coma aberration or an aspect
magnification error can be corrected.
[0015] It is another object of the present invention to provide a
projection exposure apparatus having such an optical system.
[0016] In accordance with a first aspect of the present invention,
there is provided an aberration changing optical system for
changing an aberration, characterized by an optical element having
at least one of a cylindrical surface and a toric surface and being
rotatable about and tiltable to an optical axis of said optical
system.
[0017] In accordance with a second aspect of the present invention,
there is provided an aberration changing optical system for
changing an aberration, characterized by an optical element having
different refracting powers in two orthogonal directions (or
sectional planes) or having a refracting power only in one
direction, said optical element being rotatable about and tiltable
to an optical axis of said optical system.
[0018] In these aspects of the present invention, there may be a
plurality of optical elements each being as aforesaid, and wherein
said optical elements are selectively used.
[0019] There may be a second optical element having at least one of
a cylindrical surface and a toric surface and being rotatable about
and tiltable to the optical axis of said optical system integrally
with the first-mentioned optical element, said second optical
element further being tiltable in an opposite direction to the
first-mentioned optical element.
[0020] There may be a parallel flat plate being rotatable about and
tiltable to said optical axis of said optical system integrally
with the optical element, said parallel flat plate further being
tiltable in an opposite direction to said optical element.
[0021] The optical element may be mainly composed of a transparent
material such as quartz or fluorite, for example.
[0022] The or each surface of said optical element, having a
refracting power, may have a refractive power not greater than
3.times.10.sup.-7 mm.sup.-1.
[0023] In accordance with a third aspect of the present invention,
there is provided a projection system comprising a projection
optical system, and an aberration changing optical system, as
described above, for correcting an aberration to be produced in
said projection optical system.
[0024] In accordance with a fourth aspect of the present invention,
there is provided a projection exposure apparatus comprising an
illumination system, and a projection system for projecting a
pattern of a mask onto a substrate in cooperation with said
illumination system, said projection system including a projection
optical system and an aberration changing optical system, as
described above, for correcting an aberration to be produced in
said projection optical system.
[0025] In accordance with a fifth aspect of the present invention,
there is provided a device manufacturing method, including a
process for transferring a device pattern onto a wafer by use of a
projection exposure apparatus as described above.
[0026] In accordance with a sixth aspect of the present invention,
there is provided an optical system characterized by an optical
element having at least one of a cylindrical surface and a toric
surface and being inclined with respect to an optical axis.
[0027] In accordance with a seventh aspect of the present
invention, there is provided an optical system characterized by an
optical element having different refracting powers in two
orthogonal directions (or sectional planes) or having a refracting
power only in one direction, said optical element being inclined
with respect to an optical axis.
[0028] In the sixth and seventh aspects of the present invention,
described above, there may be a plurality of optical elements each
being as aforesaid, and wherein said optical elements are
selectively used.
[0029] Also, there may be a second optical element having at least
one of a cylindrical surface and a toric surface and being inclined
with respect to the optical axis and in an opposite direction to
the first-mentioned optical element.
[0030] There may be a parallel flat plate being inclined with
respect to the optical axis and in an opposite direction to said
optical element.
[0031] The optical element may be mainly composed of a transparent
material such as quartz or fluorite, for example.
[0032] The or each surface of said optical element, having a
refracting power, may have a refractive power not greater than
3.times.10.sup.-7 mm.sup.-1.
[0033] In accordance with an eighth aspect of the present
invention, there is provided a projection system, comprising: a
projection optical system; and an optical system according to the
sixth or seventh aspect of the present invention, for correcting an
aberration to be produced in said projection optical system.
[0034] In accordance with a ninth aspect of the present invention,
there is provided a projection exposure apparatus comprising an
illumination system, and a projection system for projecting a
pattern of a mask onto a substrate in cooperation with said
illumination system, said projection system including a projection
optical system and an optical system according to the sixth or
seventh aspects of the present invention, for correcting an
aberration to be produced in said projection optical system.
[0035] In accordance with a tenth aspect of the present invention,
there is provided a device manufacturing method, including a
process for transferring a device pattern onto a wafer by use of a
projection exposure apparatus as described just above.
[0036] These and other objects, features and advantages of the
present invention will become more apparent upon a consideration of
the following description of the preferred embodiments of the
present invention taken in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0037] FIG. 1 is a schematic view for explaining an aberration
adjusting optical system according to an embodiment of the present
invention.
[0038] FIG. 2 is a schematic view of a projection exposure
apparatus according to an embodiment of the present invention.
[0039] FIG. 3 is a schematic view of an aberration adjusting
optical system according to another embodiment of the present
invention.
[0040] FIG. 4 is a schematic view of an aberration adjusting
optical system according to a further embodiment of the present
invention.
[0041] FIG. 5 is a flow chart of semiconductor device manufacturing
processes.
[0042] FIG. 6 is a flow chart for explaining details of a wafer
process, included in the procedure of FIG. 5.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0043] FIG. 1 is a schematic view of an aberration adjusting
optical system according to a first embodiment of the present
invention, and it shows optical paths in a portion of a projection
system having such an aberration adjusting optical system.
[0044] As shown in FIG. 1, there is an aberration changing optical
system 13 comprising an anamorphic optical element and being
disposed between an image plane 12 of a projection optical system
17 and a lens 11, of the projection optical system 17, which is
closest to the image plane thereof. The optical element 13 is
rotatable about an optical axis and also is tiltable about an axis
perpendicular to the optical axis. Also, the optical element 13 is
mainly composed of transparent material, such as quartz or
fluorite, for example, having a high transmission factor with
respect to light of a wavelength greater than 250 nm.
[0045] In FIG. 1, the projection optical system 17 is telecentric
on its image plane side, and the orientation of chief rays of
imaging lights is parallel to an optical axis 14. A projection
optical system of an exposure apparatus, for example, for use in
the manufacture of semiconductor devices may preferably comprise
such an optical system being telecentric on its exit side, to
prevent a change in imaging magnification of a device pattern
image, depending on the wafer surface position with respect to the
optical axis direction.
[0046] In this embodiment, the aberration changing optical system
includes an optical element 13, which has different refracting
powers with respect to orthogonal directions. The optical element
13 is made rotatable about the optical axis, and also it is made
tiltable about an axis perpendicular to the optical axis, by which
any aspect magnification error of the projection system can be
adjusted.
[0047] As regards the provision of the optical element with a
refracting power only in one direction, a cylindrical surface or a
diffractive surface, such as a binary optics (BO) element, may be
used. The surface that applies a refracting power may be defined
only on one surface of the optical element at the image side or
object side thereof. Alternatively, both surfaces may be formed to
have a refracting power in the same direction.
[0048] In a case in which the aberration changing optical system
13, which comprises a single optical element (13) has a surface
13a, on the object side, and a surface 13b, on the image side,
which are flat and are disposed in parallel to each other (i.e., a
case of a parallel flat plate), a single chief ray 15 impinges at a
reference position 12a upon the image plane 12, spaced from the
optical axis 14 by a distance d.sub.0.
[0049] The aberration changing optical system 13 of this embodiment
is formed with an object side surface 13a being flat and an image
side surface 13b of a cylindrical surface having a convex shape
toward the image side and having a very large curvature radius r.
The generating line of the cylindrical surface extends in a Y-axis
direction. Thus, by rotating it around the optical axis and by
tilting it about an axis parallel to the generating line and
perpendicular to the optical axis, the aspect magnification
difference can be changed, such that adjustment of aspect
magnification of the projection system is accomplished.
[0050] When a chief ray 15 is refracted by the cylindrical surface
13b in which the largest image height of the projection optical
system 17 is h.sub.max and the refractive index of the glass
material (material) of the aberration changing optical system 13 is
n, the angle .theta..sub.0 of light emitted therefrom, with respect
to the chief ray 15, can be expressed by the following equation:
.theta..sub.0.apprxeq.(n-1)h.sub.max/r. (1)
[0051] Further, when the distance form the cylindrical surface 13b
to the image plane 12 is S.sub.k, the amount of shift .DELTA.d of
the imaging position, upon the image plane 12 as the chief ray 16
impinges on the image plane 12, from the reference position 12a
thereon can be given by:
.DELTA.d.apprxeq.S.sub.k.theta..sub.0.apprxeq.(n-1)S.sub.kh.sub.max/r.
(2) The shift amount .DELTA.d above corresponds to a change in
imaging magnification.
[0052] On the other hand, as regards the position of incidence,
upon the image plane, of the chief ray with respect to the
generating-line direction (Y direction) perpendicular to the
direction (X direction) of the curvature r of the cylindrical
surface 13b, since the chief ray is not refracted by the
cylindrical surface 13b in the Y direction, it does not shift from
the reference position 12a. Thus, no change occurs in the imaging
magnification. In accordance with the principle described above,
the imaging magnification can be changed only in the direction (X
direction) in which the cylindrical surface 13b has a curvature,
and thus, correction of aspect magnification difference with the
projection optical system 17 is accomplished.
[0053] In this embodiment, when the distance S.sub.k from the
cylindrical surface 13b to the image plane is 36 mm, the largest
image height h.sub.max of the projection optical system 16 is 15.6
mm, the refractive index n of the material of the aberration
changing optical system 13 is 1.5, and the aspect magnification
difference (a difference at the largest image height position),
which can be adjusted through the aberration adjusting optical
system 13 is 0.05 micron, then, from equation (2), the curvature
radius r required for the cylindrical surface 13b is about
5.0.times.10.sup.6 mm.
[0054] Here, the refracting power o to a light ray passing through
the cylindrical surface 13b can be given by the following equation:
o=|(n-r)/r|. (3)
[0055] The refracting power o above takes a positive value only, as
a common definition for the light entrance side surface and the
light exit side surface of the aberration changing optical system
13. The refracting power of the cylindrical surface 13b in the
above-described embodiment is, from equation (3),
o=1.0.times.10.sup.-7 mm.sup.-1. If the refracting power o is
sufficiently small, an additional aberration amount other than the
aspect magnification difference through the aberration changing
optical system 13 is small, such that there occurs no problem in
practice. If, on the other hand, the refracting power o becomes
larger than o=3.0.times.10.sup.-7 mm.sup.-1, the addition of other
aberration through the aberration changing optical system 13
becomes notable and cannot be disregarded. Therefore, preferably, a
condition o.ltoreq.3.0.times.10.sup.-7 mm.sup.-1 should be
satisfied.
[0056] On the other hand, it will be readily understood that, even
if the refracting power is sufficiently weak, not only the aspect
magnification difference but also other aberrations such as a
spherical aberration, which are influential to the imaging
characteristic, are produced due to the aberration changing optical
system 13. Therefore, it is necessary to design the projection
optical system 17 while taking into account the influence of the
aberration changing optical system 13. Further, in consideration of
the property that the spherical aberration of the projection
optical system 17 depends on the thickness of the aberration
changing optical system 13, the spherical aberration of the
projection optical system 17 as a product may be measured in
practice and then a best thickness of the aberration changing
optical system 13 may be determined. Further, the on-axis coma
aberration of the projection optical system 17 may be measured and,
on the basis of the result thereof, a best tilt angle of the
aberration changing optical system 13 may be determined. Then, by
tilting the aberration changing optical system 13 about an axis
orthogonal to the optical axis, the on-axis coma aberration can be
adjusted.
[0057] FIG. 2 illustrates an optical arrangement having an
aberration adjusting optical system according to an embodiment of
the present invention, wherein the optical system of the present
invention is incorporated into a projection exposure apparatus for
semiconductor device manufacture. This embodiment can be applied to
a projection exposure apparatus of either a step-and-repeat type or
a step-and-scan type.
[0058] Denoted in FIG. 2 at 21 is a light source, which comprises a
laser, such as an excimer laser. Denoted at 22 is an illumination
optical system for uniformly illuminating a reticle 23 (surface to
be illuminated) with light from the light source 21. Denoted at 23
is a reticle, and denoted at 24 is a projection lens for projecting
a pattern of the reticle 23 onto a wafer 24 in a reduced scale.
Denoted at 25 is the surface of the wafer, which is placed on the
image plane.
[0059] There is an aberration changing optical system 13, such as
described with reference to FIG. 1, which is disposed between the
projection lens 24 and the wafer 25. The aberration changing
optical system 13 can be rotated around the optical axis and also
tilted about any axis orthogonal to the optical axis, by means of a
driving system 130 or manually. With this structure, aberrations of
the mask projecting system, including the projection optical system
24 and the optical system 13, can be adjusted.
[0060] FIG. 3 is a schematic view of a main portion of an
aberration changing optical system according to a second embodiment
of the present invention. In this embodiment, the aberration
changing optical system 13 comprises two optical elements 311 and
312 having the same refractive index and the same thickness and
being inclined with respect to the optical axis by the same angle,
but in opposite directions.
[0061] The first optical element 311 placed at the object side has
a surface 311a on the object side and a surface 311b on the image
side, which are flat and are disposed parallel to each other. The
second optical element 312 placed at the image side has flat
surface 312a on the image side and a cylindrical surface 312b, on
the image side, of a convex shape facing toward the image side.
[0062] Further, there are first rotating means for rotationally
moving the second optical element 312, having a protruded
cylindrical shape on one surface thereof, about an axis
perpendicular to its flat surface 312a, and first tilting means for
changing the tilt angles of the two optical elements 311 and 312 in
opposite directions, but by the same amount. Additionally, there
are second rotating means for rotationally moving the whole
aberration changing optical system 13 integrally about the optical
axis of the projection optical system, and second tilting means for
tilting the whole aberration changing optical system integrally in
a desired direction.
[0063] With the use of the aberration changing optical system of
this embodiment, in addition to the aspect magnification error,
other aberrations such as a spherical aberration, an on-axis coma
aberration and an on-axis astigmatism, for example, can be
adjusted.
[0064] More specifically, the aspect magnification error can be
adjusted in accordance with the same principle as in the first
embodiment. The spherical aberration can be adjusted by changing
the central thickness of the two optical elements 311 and 312, by
changing one by another. The on-axis astigmatism can be adjusted by
rotating the aberration changing optical system 13 as a whole
integrally, about the optical axis of the projection optical
system, and by changing the tilt angles of the two optical elements
311 and 312 in opposite directions and by the same amount. Further,
the on-axis coma aberration can be adjusted by tilting the
aberration changing optical system 13 integrally.
[0065] An example of aspect magnification error adjustment, through
the optical system 13 of this embodiment, will be explained with
reference to FIG. 1 and with respect to specific numerical
data.
[0066] In this example, the distance S.sub.k from the cylindrical
surface 312b to the image plane is 36 mm, the largest image height
h.sub.max of the projection optical system is 15.6 mm, the
refractive index n of the materials of the optical elements 311 and
312 is 1.5, and the aspect magnification difference at the largest
image height position, which can be adjusted by the aberration
changing optical system 13 of this embodiment, is 0.05 micron. The
required curvature radius r of the cylindrical surface 312b can be
calculated in accordance with equation (2), as in the first
embodiment, and it is about 5.0.times.10.sup.6 mm.
[0067] In a case in which the central thickness of the two optical
elements 311 and 312 of the aberration changing optical system 13,
in the optical axis direction, is 8 mm, the air spacing between the
two optical elements 311 and 312 is 2 mm, the exposure wavelength
is 248 nm, and the numeral aperture of the projection optical
system 17 is 0.6, and when the tilt angles of the two optical
elements 311 and 312 are changed in opposite directions by 10',
respectively, the on-axis astigmatism of the projection optical
system changes by about 0.06 micron.
[0068] While this embodiment has been described with reference to
an example in which the first optical element 311 of the aberration
changing optical system comprises a parallel flat plate and the
second optical element 312 thereof has a one-side cylindrical
surface, the invention is not limited to this. The first optical
element may have a cylindrical surface on one side thereof, and the
second optical element may comprise a parallel flat plate.
[0069] FIG. 4 is a schematic view of a main portion of an
aberration changing optical system according to a third embodiment
of the present invention. In this embodiment, the aberration
changing optical system 13 comprises two optical elements 411 and
412 having the same refractive index and the same central thickness
and being tilted with respect to the optical axis, in opposite
directions and by the same angle.
[0070] The first optical element 411 placed on the object side has
a flat surface 411 placed on the object side and has a flat surface
411a on the object side and a cylindrical surface 411b, on the
image side, of a convex shape facing toward the image side.
Similarly, the second optical element 412 placed on the image side
has a flat surface 412a on the object side and a cylindrical
surface 412b, in the image side, of a convex shape facing toward
the image side.
[0071] The aberration changing optical system 13 of this embodiment
is provided with first rotating means for rotationally moving these
two optical elements 411 and 412 about axes perpendicular to their
flat faces 411a and 412a, respectively, and first tilting means for
changing the tilt angles of the two optical elements in opposite
directions but by the same amount. Additionally, there are second
rotating means for rotationally moving the whole aberration
changing optical system 13 integrally, about the optical axis of
the projection optical system, and second tilting means for tilting
the whole aberration changing optical system integrally, in a
desired direction.
[0072] With the aberration changing optical system of this
embodiment, by rotating the first optical element 411 about an axis
perpendicular to its flat surface 411a and by rotating the second
optical element 412 about an axis perpendicular to its flat surface
412a, respectively, a change in magnitude and direction of the
aspect magnification difference can be adjusted.
[0073] More specifically, as the direction in which the cylindrical
surface 411b of the first optical element 411 has a curvature is
brought into registration with the direction in which the
cylindrical surface 412b of the second optical element 412 has a
curvature, the amount of change in aspect magnification difference
becomes maximum. If it is made orthogonal to the direction in which
the cylindrical surface 412b of the second optical element 412 has
a curvature, the amount of change in aspect magnification
difference becomes smallest.
[0074] An example of adjustment of an aspect magnification error in
this embodiment will be described with reference to FIG. 1 and with
respect to specific numerical data.
[0075] In this example, simulations were made with the conditions
that: the exposure wavelength is 248 nm, the numerical aperture of
the projection optical system is 0.6, the central thickness of the
two optical elements 411 and 412 of the aberration changing optical
system in the optical axis direction is 8 mm, the air spacing
between the two optical elements 411 and 412 is 2 mm, the curvature
radius of the cylindrical surface 411b of the first optical element
411 is 6.0.times.10.sup.6 mm, the curvature radius of the
cylindrical surface 412b of the second optical element 412 is
5.0.times.10.sup.6 mm, the distance S.sub.k from the cylindrical
surface 412b of the second optical element 412 to the image plane
is 36 mm, the largest image height h.sub.max of the projection
optical system is 15.6 mm, and the refractive index n of the
materials of the optical elements 411 and 412 is 1.5.
[0076] As a result, it has been confirmed that, when the direction
in which the cylindrical surface 411b of the first optical element
411 has a curvature and the direction in which the cylindrical
surface 412b of the second optical element 412 has a curvature, are
placed orthogonal to each other, the aspect magnification
difference imparted by the aberration changing optical system
becomes equal to zero.
[0077] On the other hand, when the first optical element 411 and/or
the second optical element 412 is rotated about an axis
perpendicular to its flat surface so that the direction in which
the cylindrical surface 411b of the first optical element 411 has a
curvature and the direction in which the cylindrical surface 412b
of the second optical element 412 has a curvature are brought into
registration with each other, the amount of aspect magnification
difference imparted by the aberration changing optical system 13
becomes maximum. The amount of change thereof at the largest image
height position is 0.11 micron.
[0078] The angle, defined between the direction in which the
cylindrical surface 411b of the first optical element 411 has a
curvature and the direction in which the cylindrical surface 412b
of the second optical element 412 has a curvature, can be adjusted
as desired. Here, the aspect magnification difference to be
imparted by the aberration changing optical system varies
continuously from zero to a maximum. Thus, within a range from zero
to an adjustable largest value, the aspect magnification difference
in a desired amount and direction to be produced by the projection
optical system can be adjusted through the aberration changing
optical system of this embodiment. Further, like the second
embodiment, the spherical aberration, the on-axis coma aberration
and the on-axis astigmatism of the projection optical system can be
adjusted independently of each other, as desired.
[0079] Although the foregoing embodiments have been described with
reference to a case in which the image side surface of an optical
element, constituting the aberration changing optical element
system, has a refracting power, the invention is not limited to
this form. The object side surface may have a refracting power, or
both of the object side surface and the image side surface may have
a refracting power in the same direction.
[0080] Providing a refracting power in one direction of the optical
element is not limited to the use of a smooth cylindrical surface
shape. A diffractive optical element such as a binary optics (BO)
having a one-dimensional refracting power may be used, for
example.
[0081] Alternatively, an anamorphic optical system having different
refracting powers in the X and Y directions may be used for the
aberration changing optical system. As regards the central
thickness of the optical element used in the aberration changing
optical system, if it is not greater than 5 mm, any additional
aberration will be produced due to the influence of weight
deformation of the optical element. If it is not less than 30 mm,
there will occur absorption of exposure light by the optical
element. Thus, preferably, it may be in a range from 5 mm to 30 mm.
Further, in a case in which the aberration changing optical system
comprises plural optical elements, in order to avoid Newton's rings
resulting from interference of light inside the optical system, an
air spacing of 0.1 mm or more should preferably be maintained
between adjacent optical elements.
[0082] Further, the specifications of a projection optical system
having an aberration changing optical system of the present
invention incorporated therein, such as the exposure wavelength and
the numerical aperture, for example, are not limited to those
described hereinbefore.
[0083] Next, an embodiment of a device manufacturing method, which
uses an exposure apparatus having a projection optical system with
an aberration changing optical system, according to any one of the
preceding embodiments, will be explained.
[0084] FIG. 5 is a flow chart of a procedure for the manufacture of
microdevices such as semiconductor chips (e.g., ICs or LSIs),
liquid crystal panels, or CCDs, for example.
[0085] Step 1 is a design process for designing a circuit of a
semiconductor device. Step 2 is a process for making a mask on the
basis of the circuit pattern design. Step 3 is a process for
preparing a wafer by using a material such as silicon. Step 4 is a
wafer process (called a pre-process) wherein, by using the so
prepared mask and wafer, circuits are practically formed on the
wafer through lithography. Step 5, subsequent to this, is an
assembling step (called a post-process), wherein the wafer having
been processed by step 4 is formed into semiconductor chips. This
step includes an assembling (dicing and bonding) process and a
packaging (chip sealing) process. Step 6 is an inspection step
wherein an operation check, a durability check, and so on, for the
semiconductor devices provided by step 5, are carried out. With
these processes, semiconductor devices are completed, and they are
shipped (step 7).
[0086] FIG. 6 is a flow chart showing details of the wafer
process.
[0087] Step 11 is an oxidation process for oxidizing the surface of
a wafer. Step 12 is a CVD process for forming an insulating film on
the wafer surface. Step 13 is an electrode forming process for
forming electrodes upon the wafer by vapor deposition. Step 14 is
an ion implanting process for implanting ions to the wafer. Step 15
is a resist process for applying a resist (photosensitive material)
to the wafer. Step 16 is an exposure process for printing, by
exposure, the circuit pattern of the mask on the wafer through the
exposure apparatus described above. Step 17 is a developing process
for developing the exposed wafer. Step 18 is an etching process for
removing portions other than the developed resist image. Step 19 is
a resist separation process for separating the resist material
remaining on the wafer after being subjected to the etching
process. By repeating these processes, circuit patterns are
superposedly formed on the wafer.
[0088] With these processes, high density microdevices can be
manufactured.
[0089] While the invention has been described with reference to the
structures disclosed herein, it is not confined to the details set
forth and this application is intended to cover such modifications
or changes as may come within the purposes of the improvements or
the scope of the following claims.
* * * * *