U.S. patent application number 09/870734 was filed with the patent office on 2001-10-11 for interferometer system and method of manufacturing projection optical system using same.
This patent application is currently assigned to NIKON CORPORATION. Invention is credited to Gemma, Takashi, Ichihara, Hiroshi, Ichikawa, Hajime, Nakayama, Shigerur.
Application Number | 20010028462 09/870734 |
Document ID | / |
Family ID | 26548380 |
Filed Date | 2001-10-11 |
United States Patent
Application |
20010028462 |
Kind Code |
A1 |
Ichihara, Hiroshi ; et
al. |
October 11, 2001 |
Interferometer system and method of manufacturing projection
optical system using same
Abstract
A method of manufacturing a projection optical system (37) for
projecting a pattern from a reticle to a photosensitive substrate,
comprising a surface-shape-measuring step wherein the shape of an
optical test surface (38) of an optical element (36) which is a
component in the projection optical system is measured by causing
interference between light from the optical surface (38) and light
from an aspheric reference surface (70) while the optical test
surface (38) and said reference surface (70) are held in integral
fashion in close mutual proximity. A wavefront-aberration-measuring
step is included, wherein the optical element is assembled in the
projection optical system and the wavefront aberration of the
projection optical system is measured. A surface correction
calculation step is also included wherein the amount by which the
shape of the optical test surface should be corrected is calculated
based on wavefront aberration data obtained at the
wavefront-aberration-measuring step and surface shape data obtained
from the surface-shape-measuring step. The method also includes a
surface shape correction step wherein the shape of the optical test
surface is corrected based on calculation performed at the surface
correction calculation step. Surface shape measuring interferometer
systems and wavefront-aberration-measuring interferometer systems
(22J-22Q) used in performing the manufacturing method are also
disclosed.
Inventors: |
Ichihara, Hiroshi;
(Yokohama-shi, JP) ; Gemma, Takashi; (Shibuya-ku,
JP) ; Nakayama, Shigerur; (Kawasaki-shi, JP) ;
Ichikawa, Hajime; (Yokohama-shi, JP) |
Correspondence
Address: |
OLIFF & BERRIDGE, PLC
P.O. Box 19928
Alexandria
VA
22320
US
|
Assignee: |
NIKON CORPORATION
|
Family ID: |
26548380 |
Appl. No.: |
09/870734 |
Filed: |
June 1, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09870734 |
Jun 1, 2001 |
|
|
|
09401552 |
Sep 22, 1999 |
|
|
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Current U.S.
Class: |
356/512 ;
356/513; 356/521; 378/36 |
Current CPC
Class: |
G01B 9/02039 20130101;
G01M 11/0214 20130101; G01B 9/02057 20130101; G01M 11/0264
20130101; G01B 11/2441 20130101; G01M 11/0271 20130101; G03F 7/706
20130101; G01M 11/025 20130101; G01B 9/02072 20130401 |
Class at
Publication: |
356/512 ;
356/521; 378/36; 356/513 |
International
Class: |
G01B 009/02 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 22, 1998 |
JP |
10-268582 |
Sep 22, 1998 |
JP |
10-268793 |
Claims
What is claimed is:
1. An interferometer capable of measuring a surface shape of a
target surface as compared to a reflector standard, comprising: a.
a light source capable of generating a light beam; b. a reference
surface arranged downstream of said light source for reflecting
said light beam so as to form a reference wavefront; c. a null
element arranged downstream of said reference surface for forming a
desired null wavefront from said light beam and arranged such that
said null wavefront is incident the target surface so as to form a
measurement wavefront and is incident the reflector standard when
alternately arranged in place of the target surface so as to form a
reflector standard wavefront; and d. a detector arranged so as to
detect interference fringes caused by interference between said
measurement wavefront and said reference wavefront taking into
account said reflector standard wavefront.
2. An interferometer according to claim 1, wherein: a. the target
surface is an aspheric surface; and b. said detector measures the
shape of said measurement wavefront over a desired range by
analyzing a plurality of interference fringe patterns representing
different subregions of said measurement wavefront which are
obtained by changing the positional relationship between said
measurement wavefront and said reflector standard.
3. An interferometer capable of measuring a surface shape of a
target surface as compared to a reflector standard, comprising: a.
a point light source formed by irradiating a reflective surface so
as to form an outgoing spherical wave; b. a reference surface
arranged downstream of said point light source for reflecting light
from said light source so as to form a reference wavefront; c. a
null element arranged downstream of said reference surface for
forming a desired null wavefront and arranged such that said null
wavefront is incident the target surface so as to form a
measurement wavefront, and the reflector standard alternately
arranged in place of the target surface so as to form a reflector
standard wavefront; d. wherein said null wavefront is determined by
reflecting and folding said measurement wavefront from said
reflector standard by said reflecting surface, causing said
measurement wavefront to interfere with said reference wavefront
thereby causing interference; and e. a detector arranged so as to
detect interference fringes caused by said interference.
4. An interferometer according to claim 3, wherein: a. the target
surface is an aspheric surface; and b. said detector measures the
shape of said measurement wavefront over a desired range by
analyzing a plurality of interference fringe patterns representing
different subregions of said measurement wavefront which are
obtained by changing the positional relationship between said
measurement wavefront and said reflector standard.
5. An interferometer capable of measuring a surface shape of a
target surface as compared to a reflector standard, comprising: a.
a point light source having a reflective surface, for forming an
outgoing spherical wavefront, and that is alternately arrangeable
in placed of the target surface; b. a reference surface arranged
downstream of said point light source for reflecting light from
said light source so as to form a reference wavefront; c. a null
element arranged downstream of said reference surface for forming a
desired null wavefront and arranged such that said null wavefront
is incident the target surface so as to form a measurement
wavefront, and the reflector standard alternately arranged in place
of the target surface so as to form a reflector standard wavefront;
d. wherein said null wavefront is determined by passing said
spherical wavefront from said point light source through said null
element and performing an interferometric measurement; and e. a
detector arranged so as to detect interference fringes created by
the interference between said measured wavefront and said reference
wavefront, taking into account said null wavefront.
6. An interferometer capable of measuring a surface shape of a
target surface as compared to a reflector standard, comprising: a.
a point light source formed by irradiating a reflective surface so
as to form an outgoing spherical wave; b. a reference surface
arranged downstream of said point light source for reflecting light
from said light source so as to form a reference wavefront; c. a
null element arranged downstream of said reference surface for
forming a desired null wavefront and arranged such that said null
wavefront is incident the target surface so as to form a
measurement wavefront, and the reflector standard alternately
arranged in place of the target surface so as to form a reflector
standard wavefront; d. wherein said null wavefront is determined by
passing said spherical wavefront from said point light source
arranged in place of the target surface through said null element
and performing an interferometric measurement; and e. a detector
arranged so as to detect interference fringes created by the
interference between said measured wavefront and said reference
wavefront, taking into account said null wavefront.
7. A method of manufacturing a projection optical system capable of
projecting a pattern from a reticle onto a photosensitive
substrate, comprising the steps of: a. measuring a shape of a test
surface of an optical element that is a component of the projection
optical system by causing interference between light from said test
surface and light from an aspheric reference surface while said
test surface and said aspheric reference surface are held
integrally and in close proximity to one another; b. assembling
said optical element in the projection optical system and measuring
the wavefront aberration of the projection optical system; c.
determining an amount by which said shape of said test surface
should be corrected based on said measured wavefront aberration
obtained in said step b; and d. correcting said shape of said test
surface based on said amount by which said shape of said test
surface should be corrected as determined in said step c.
8. A method according to claim 7, wherein: a. said step c further
includes the step of calculating an error in said shape of said
test surface as measured in said step a, based on said measured
wavefront aberration obtained in said step b; and b. said amount by
which said shape of said test surface should be corrected is
determined from said calculated error, said measured wavefront
aberration and said shape of said test surface.
9. A method according to claim 7, wherein: a. said step of
calculating an error in said shape of said test surface includes
separating said measured wavefront aberration into an test surface
positional error component, a test surface shape error component,
and a residual component when said positional error component is
substantially corrected; and b. wherein error in said surface shape
is calculated based on a component in said residual component
attributable to said shape error component and said shape of said
test surface.
10. A Fizeau interferometer for measuring the shape of an optical
surface of an optical element, comprising: a. a light source for
forming a light beam along an optical path; b. an aspherical
reference surface arranged in said optical path downstream from
said light source; c. a null element arranged in an said optical
path; and d. a holding member that holds said reference surface and
the optical surface as a single unit such that said reference
surface and the optical surface are brought close together so that
light from said reference surface and the optical surface
interferes.
11. An interferometer according to claim 10, further including a
main body unit that supplies light to said reference surface and
said optical surface, and that causes the interference of light
that travels via said reference surface and the light that travels
via said optical surface, and wherein said holding member and said
main body unit are spatially separated.
12. An interferometer according to claim 10, wherein said reference
surface and the optical surface are separated by a spacing that is
less than 1 mm.
13. An interferometer according to claim 10, wherein said reference
surface and the optical surface are separated by a spacing that is
variable.
14. An interferometer according to claim 10, further including a
position detection system that detects the positional relationship
between said reference surface and the optical surface.
15. An interferometer according to claim 10, wherein said reference
surface and the optical surface are separated by a fixed spacing
that is less than 10 .mu.m.
16. An apparatus for measuring wavefront aberration of an optical
system having an object plane and an image plane, comprising: a. a
light source for supplying light of a predetermined wavelength. b.
a first pinhole member capable of forming a first spherical
wavefront from said light arranged at one of said object plane and
said image plane, said first pinhole member having a plurality of
first pinholes arrayed in two dimensions along a surface
perpendicular to an optical axis of the optical system; c. a second
pinhole member arranged at the opposite one of said object plane
and said image plane of said first pinhole member, said second
pinhole member having a plurality of second pinholes arrayed at a
position corresponding to the imaging position where said plurality
of first pinholes is imaged by said optical system; d. a
diffraction grating arranged in the optical path between said first
and second pinhole members; e. a diffracted light plate member that
selectively transmits diffracted light of one or more higher
predetermined diffraction orders associated with said diffraction
grating; and f. a detector arranged to detect interference fringes
arising from the interference between a second spherical wavefront
generated by a zeroeth diffraction order passing through said
second pinhole member and said one or more higher predetermined
diffraction orders passing through said diffracted light plate
member.
17. An apparatus according to claim 16, wherein said light source
is one among the group of light sources consisting of: synchrotron,
laser and laser plasma X-ray.
18. An apparatus according to claim 17, wherein: a. said light
source is a laser plasma X-ray source; and b. said first plurality
of pinholes comprises pinhole groups comprising a plurality of
pinholes.
19. An apparatus according to claim 18, wherein said plurality of
pinholes in said pinhole groups is arrayed along a predetermined
one-dimensional direction.
20. An apparatus according to claim 19, wherein said plurality of
pinholes has an array pitch that is 10 to 25 times the Airy disk
radius determined by the numerical aperture on said first pinhole
side of the optical system and said predetermined wavelength.
21. An apparatus according to claim 18, wherein said plurality of
pinholes constituting said pinhole group each has a slit-shaped
aperture extending in a predetermined one-dimensional
direction.
22. An apparatus according to claim 16, further including: a. first
selective illumination means for selectively illuminating a first
portion of said plurality of first pinholes of said first pinhole
member; and b. selective light receiving means for selectively
receiving said second spherical wavefront from a second portion of
said plurality of second pinholes corresponding to said first
portion, and diffracted light of said predetermined order that
passes through said second portion.
23. An apparatus according to claim 16, further including a fringe
scanning means for moving said diffraction grating so as to perform
fringe scanning.
24. An apparatus for measuring wavefront aberration of an optical
system having an object plane and an image plane, comprising: a. a
light source for supplying coherent light; b. a beam splitter that
splits said coherent light into first and second light beams each
having an associated optical path length; c. a first pinhole
member, arranged in one of the object plane and image plane, that
uses said first light beam to generate a first spherical wave; d. a
pinhole mirror having a plurality of apertures arrayed
two-dimensionally in the opposite one of the object plane and the
image plane where said first pinhole member is arranged, that
transmits said second beam, and a reflective portion that reflects
said first light beam from said optical system; and e. wherein the
wavefront aberration of the optical system is calculated based on
interference fringes generated by interference arising from a
second spherical wave generated by said plurality of apertures of
said pinhole mirror based on said second light beam, and said first
light beam reflected by said reflective portion of said pinhole
mirror.
25. An apparatus according to claim 24, further including a fringe
scanning means that changes at least one of said optical path
lengths of said first and second light beams.
26. An apparatus for measuring wavefront aberration of an optical
system having an image plane and an object plane, comprising: a. a
laser plasma X-ray light source; b. a first pinhole member provided
with a first pinhole group comprising a plurality of first pinholes
that generates a plurality of first spherical waves based on said
light; c. a second pinhole member provided with a second pinhole
group comprising a plurality of second pinholes arranged at the
imaging position where said first pinhole member is imaged by the
optical system; d. a diffraction grating arranged in the optical
path between said first and second pinhole members, and arranged so
that zeroeth order diffracted light passing through said first
pinhole group reaches said second pinhole group. e. diffracted
light selection means for selectively transmitting diffracted light
of a predetermined order from among diffracted light of first order
and higher order from said diffraction grating; and f. a detector
for detecting interference fringes obtained by interference of a
second spherical wave generated by said zeroeth order diffracted
light passing through said second pinhole group, and said
diffracted light of said predetermined order passing through said
diffracted light selection means.
27. An apparatus according to claim 26, wherein: a. said first
pinhole member includes a plurality of first slit groups; b. said
second pinhole member includes a plurality of said second pinhole
groups arranged corresponding to the imaging position where said
plurality of first pinhole groups is imaged by the optical system;
and c. said diffracted light selection means is a plate having a
plurality of apertures for selectively transmitting said plurality
of diffracted light of a predetermined order generated by passing
through said diffraction grating a plurality of light beams
proceeding to a plurality of imaging positions.
28. An apparatus according to claim 27, further including: a. first
selective illumination means for selectively illuminating a first
portion of said plurality of first pinholes of said first pinhole
member; and b. selective light receiving means for selectively
receiving said second ideal spherical wavefront from a second
portion of said plurality of second pinholes corresponding to said
first portion, and diffracted light of said predetermined order
that passes through said second portion.
29. An apparatus according to claim 27, further including a second
selective illumination means for selectively illuminating said
portion of said second pinhole groups from among said plurality of
second pinhole groups.
30. An apparatus for measuring wavefront aberration in an optical
system having an image plane and an object plane, comprising: a. a
laser plasma X-ray light source capable of generating X-ray light;
b. a first slit member provided with a first slit group comprising
a plurality of first slits that generates a plurality of first
one-dimensional spherical waves based on said X-ray light from said
light source; c. a second slit member provided with a second slit
group comprising a plurality of second slits arranged at an imaging
position where said first slit member is imaged by the optical
system; d. a diffraction grating arranged in the optical path
between said first and second slit members, and arranged so that a
zeroeth order diffracted light of said X-ray light passing through
said first slit group reaches said second slit group; e. diffracted
light selection means for selectively transmitting diffracted X-ray
light of non-zero order; and f. a detector that detects
interference fringes arising from interference between a second
one-dimensional spherical wave generated when said zeroeth order
X-ray light passes through said second slit group, and of said
non-zeroeth order diffracted X-ray light passing through said
diffracted light selection means.
31. An apparatus according to claim 30, wherein: a. said first slit
member includes a plurality of said first slit groups; b. said
second slit member includes a plurality of second slit groups
arranged corresponding to a plurality of imaging positions where
said plurality of first slit groups is imaged by the optical
system; and c. said diffracted light selection means has a
plurality of apertures for selectively transmitting a plurality of
diffracted X-ray light generated by passing through said
diffraction grating a plurality of light beams proceeding to said
plurality of imaging positions.
32. An apparatus according to claim 31, further including: a. first
selective illumination means for selectively illuminating a first
portion of said plurality of first slit groups; and b. selective
light receiving means for selectively receiving said second
one-dimensional spherical wavefront from a second portion of said
plurality of second slit groups corresponding to said first
portion, and diffracted light of said predetermined order that
passes through said second portion.
33. An apparatus according to claim 32, further including a second
selective illumination means for selectively illuminating said
portion of said second slit from among said plurality of second
slit groups.
34. An apparatus for measuring wavefront aberration in an optical
system, comprising: a. a laser plasma X-ray light source for
generating X-ray light; b. a slit member provided with a first slit
group comprising a plurality of slits that generates a plurality of
one-dimensional spherical waves based on said light from said light
source; c. a diffraction grating arranged in the optical path
between said slit member and an imaging position where said slit
member is imaged by the optical system, and that generated
diffracted light from light passing therethrough; d. diffracted
light selection means for selectively transmitting diffracted light
of a predetermined order and diffracted light of an order different
than said predetermined order; and e. a detector arranged so as to
detect interference fringes from interference between said
diffracted light of a predetermined order and said diffracted light
of an order different than said predetermined order to allow the
calculation of the wavefront aberration from said interference
fringes.
35. An apparatus for measuring wavefront aberration in an optical
system having an incident-side numerical aperture NA, an object
plane and an image plane, the apparatus comprising: a. an X-ray
light source for generating X-ray light having a wavelength
.lambda.; b. a first pinhole plate having an aperture smaller than
0.6 .lambda./NA arranged at the object plane; c. a Hartmann plate
arranged between said first pinhole plate and the image plane, said
Hartmann plate having a plurality of apertures; d. a detector
arranged adjacent said image plane so as to detect a position of a
plurality of ray groups passing through said plurality of apertures
of said Hartmann plate; and e. wherein the wavefront aberration is
calculated based on said position of said plurality of ray groups
that drive on said detector.
36. An apparatus according to claim 35, wherein said Hartmann plate
is arranged between the optical system and the image plane.
37. An apparatus according to claim 36, further including: a. first
selective illumination means for selectively illuminating a portion
of slit groups from among said plurality of slit groups; b.
detector position changing means that changes the detection
position of said detector so as to detect said ray group based on
light passing through said portion of slit groups, wherein said
slit member is provided with a plurality of said slit groups.
38. An interferometer calibration method for measuring a surface
shape of an optical element of an optical system, the method
comprising the steps of: a. interferometrically measuring the
surface shape of the optical element to obtain a surface shape
measurement value; b. assembling the optical system by including
the optical element in the optical system; c. measuring a wavefront
aberration of the optical system; d. separating said wavefront
aberration into a component corresponding to positional error of
the surface shape and a component corresponding to surface shape
error; e. correcting said positional error component and
calculating said surface shape error component; and f. correcting
said surface shape measurement value using said surface shape error
component as calculated in said step e.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to an interferometer system
for measuring the shape of an aspheric surface of an optical
element in an optical system and for measuring the wavefront
aberration of such an optical system, particularly in connection
with manufacture of a projection optical system suited to for use
in an exposure apparatus employing soft-X-ray (EUV) exposure
light.
BACKGROUND OF THE INVENTION
[0002] Light of wavelength 193 nm or longer has hitherto been used
as the exposure light in lithographic equipment used when
manufacturing semiconductor devices such as integrated circuits,
liquid crystal displays, and thin film magnetic heads. The surfaces
of lenses used in projection optical systems of such lithographic
equipment are normally spherical, and the accuracy in the lens
shape is 1 to 2 nm RMS (root mean square).
[0003] With the advance in microminiaturization of the patterns on
semiconductor devices in recent years, there has been a demand for
exposure apparatus that use wavelengths shorter than those used
heretofore to achieve even greater microminiaturization. In
particular, there has been a demand for the development and
manufacture of projection exposure apparatus that use soft X-rays
of wavelength of 11 to 13 nm.
[0004] Lenses (i.e., dioptric optical elements) cannot be used in
the EUV wavelength region due to absorption, so catoptric
projection optical systems (i.e., systems comprising only
reflective surfaces) are employed. In addition, since a reflectance
of only about 70% can be expected from reflective surfaces in the
soft X-ray wavelength region, only three to six reflective surfaces
can be used in a practical projection optical system.
[0005] Accordingly, to make an EUV projection optical system
aberration-free with just a few reflective surfaces, all reflective
surfaces are made aspheric. Furthermore, in the case of a
projection optical system having four reflective surfaces, a
reflective surface shape accuracy of 0.23 nm RMS is required. One
method of forming an aspheric surface shape with this accuracy is
to measure the actual surface shape using an interferometer and to
use a corrective grinding machine to grind the surface to the
desired shape.
[0006] In a conventional surface-shape-measuring interferometer,
measurement repeatability is accurate to 0.3 nm RMS, the absolute
accuracy for a spherical surface is 1 nm RMS, and the absolute
accuracy of an aspheric surface is approximately 10 nm RMS.
Therefore, the required accuracy cannot possibly be satisfied. As a
result, a projection optical system designed to have a desired
performance cannot be manufactured.
[0007] So-called null interferometric measurement using a null
(compensating) element has hitherto been conducted for the
measurement of aspheric surface shapes. Null lenses that use
spherical lenses comprising spherical surfaces, and zone plates
wherein annular diffraction gratings are formed on plane plates
have principally been used as null elements.
[0008] FIG. 1 shows a conventional interferometer system 122
arrangement for null measurement using a null (compensating)
element 132. The interferometric measurement described herein is a
slightly modified version of a Fizeau interferometric measurement.
Namely, a plane wave 126 emitted from an interferometric light
source 124 is partially reflected by a high-precision Fizeau
surface 130 formed on a Fizeau plane plate 128. The component of
plane wave 126 transmitted through Fizeau surface 130 is converted
into measurement wavefront (null wavefront) 134 by null element 132
and assumes a desired aspheric design shape at a measurement
reference position RP, following which it arrives at a test surface
138 of a test object 136 previously set at the reference position.
The light arriving at test surface 138 is reflected therefrom and
interferes with the light component reflected from Fizeau surface
130, and forms monochromatic interference fringes inside
interferometer system 122. These interference fringes are detected
by a detector such as a CCD (not shown). A signal outputted by the
detector is analyzed by an information processing system (not
shown) that processes the interferometer information contained in
the output signal. Similar measurements can be performed using a
Twyman-Green interferometer.
[0009] To accurately ascertain the shape of test surface 138, the
null element 132 must be manufactured with advanced technology,
since there must be no error in the null wavefront. Specifically,
this means that the optical characteristics of the null element 132
must be measured beforehand with high precision. Based on these
measurements, the shape of null wavefront 134 is then determined by
ray tracing. This results in the manufacture of null element 132
taking a long time. Consequently, the measurement of the desired
aspheric surface takes a long time.
[0010] FIG. 2 shows another example of a conventional Fizeau
interferometer 222. Referring to FIG. 2, laser light from laser 224
passes through a lens system 226 to become a collimated light beam
of a prescribed diameter and is incident Fizeau plate 228. Rear
side 230 of Fizeau plate 228 is accurately ground to a highly flat
surface, and the component of the incident light reflected by rear
side 230 of Fizeau plate 228 becomes a reference beam having a
plane wavefront. The component of incident light transmitted
through a Fizeau plate 228 passes through null element 232, where
the plane wavefront where the plane wavefront is converted to a
desired aspheric wavefront. The aspheric wavefront is then incident
in perpendicular fashion an aspheric test surface 238. The light
reflected by test surface 238 returns along the original optical
path, is superimposed on the reference light beam, reflects off a
beam splitting element 256 in lens system 226, and forms
interference fringes on a CCD detector 260. By processing these
interference fringes by a computer (not shown), the shape error can
be measured.
[0011] A problem with interferometer 222 is deterioration, in
absolute accuracy, due to null element 232. A null element
comprising a number of high-precision lenses (e.g., lenses 234 and
236) a CGH (computer-generated hologram), or the like is ordinarily
used as null element 232, and manufacturing errors on the order of
10 nm RMS typically result.
[0012] Since interferometer 222 tends to be affected by vibration
and air fluctuations due to the separation of reference surface 230
(i.e., rear side of Fizeau plate 228) and test surface 238.
Repeatability is also poor, at 0.3 nm RMS. Furthermore, in
measuring an aspheric surface, alignment of null element 232 and
test surface 238 is critical. Measurement repeatability
deteriorates by several nanometers if alignment accuracy is
poor.
SUMMARY OF THE INVENTION
[0013] The present invention relates to an interferometer system
for measuring the shape of an aspheric surface of an optical
element in an optical system and for measuring the wavefront
aberration of such an optical system, particularly in connection
with manufacture of a projection optical system suited to for use
in an exposure apparatus employing soft-X-ray (EUV) exposure
light.
[0014] The goal of the present invention is to overcome the
above-described deficiencies in the prior art so as to permit fast
and accurate calibration of a null wavefront corresponding to an
aspheric surface accurate to very high dimensional tolerances.
[0015] Another goal of the present invention is to manufacture a
projection optical system having excellent performance.
[0016] Additional goals of the present invention are to provide an
aspheric-surface-shape measuring interferometer having good
reproducibility, to measure wavefront aberration with high
precision and to permit calibration of an aspheric-surface-shape
measuring interferometer so as to improve absolute accuracy in
precision surface measurements.
[0017] Accordingly, a first aspect of the invention is an
interferometer capable of measuring a surface shape of a target
surface as compared to a reflector standard. The interferometer
comprises a light source capable of generating a light beam, and a
reference surface arranged downstream of the light source for
reflecting the light beam so as to form a reference wavefront. The
interferometer further includes a null element arranged downstream
of the reference surface for forming a desired null wavefront from
the light beam. The null element is arranged such that the null
wavefront is incident the target surface so as to form a
measurement wavefront and is also incident the reflector standard
when the latter is alternately arranged in place of the target
surface so as to form a reflector standard wavefront. The
interferometer further includes a detector arranged so as to detect
interference fringes caused by interference between the measurement
wavefront and the reference wavefront. The detection of the
interference fringes takes into account the reflector standard
wavefront.
[0018] A second aspect of the invention is a method of
manufacturing a projection optical system capable of projecting a
pattern from a reticle onto a photosensitive substrate. The method
comprises the steps of first measuring a shape of a test surface of
an optical element that is a component of the projection optical
system by causing interference between light from the test surface
and light from an aspheric reference surface while the test surface
and the aspheric reference surface are held integrally and in close
proximity to one another. The next step is assembling the optical
element in the projection optical system and measuring the
wavefront aberration of the projection optical system. The next
step is then determining an amount by which the shape of the test
surface should be corrected based on the measured wavefront
aberration obtained in the step b. Then, the final step is
correcting the shape of the test surface based on the amount by
which the shape of the test surface should be corrected as
determined above.
[0019] A third aspect of the invention is an interferometer for
measuring wavefront aberration of an optical system having an
object plane and an image plane. The interferometer comprises a
light source for supplying light of a predetermined wavelength, a
first pinhole member capable of forming a first spherical wavefront
from the light arranged at one of the object plane and the image
plane. The first pinhole member has a plurality of first pinholes
arrayed in two dimensions along a surface perpendicular to an
optical axis of the optical system. The interferometer further
includes a second pinhole member arranged at the opposite one of
the object plane and the image plane of the first pinhole member.
The second pinhole member has a plurality of second pinholes
arrayed at a position corresponding to the imaging position where
the plurality of first pinholes is imaged by the optical system.
The interferometer also includes a diffraction grating arranged in
the optical path between the first and second pinhole members, and
a diffracted light plate member that selectively transmits
diffracted light of one or more higher predetermined diffraction
orders associated with the diffraction grating. The interferometer
also includes a detector arranged to detect interference fringes
arising from the interference between a second spherical wavefront
generated by a zeroeth diffraction order passing through the second
pinhole member and the one or more higher predetermined diffraction
orders passing through the diffracted light plate member.
[0020] A fourth aspect of the invention is an interferometer
calibration method for measuring a surface shape of an optical
element of an optical system. The method comprises the steps of
first, interferometrically measuring the surface shape of the
optical element to obtain a surface shape measurement value, then
assembling the optical system by including the optical element in
the optical system, then measuring a wavefront aberration of the
optical system, then separating the wavefront aberration into a
component corresponding to positional error of the surface shape
and a component corresponding to surface shape error, then
correcting the positional error component and calculating the
surface shape error component, and then finally correcting the
surface shape measurement value using the surface shape error
component as previously calculated
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a schematic optical diagram of a first
conventional surface-shape-measuring interferometer according to
the prior art;
[0022] FIG. 2 is a schematic optical diagram of a second
conventional surface-shape-measuring interferometer according to
the prior art;
[0023] FIGS. 3a and 3b are schematic optical diagrams of first and
second surface-shape-measuring interferometers of a first
embodiment according to a first aspect of the present
invention;
[0024] FIGS. 4a and 4b are schematic optical diagrams of third and
fourth surface-shape-measuring interferometers of a first
embodiment according to a first aspect of the present
invention;
[0025] FIGS. 5a and 5b are schematic optical diagrams of fifth and
sixth surface-shape-measuring interferometers of a second
embodiment according to a first aspect of the present
invention;
[0026] FIG. 6 is a schematic optical diagram of a seventh
surface-shape-measuring interferometer of a third embodiment
according to a first aspect of the present invention;
[0027] FIG. 7 is a schematic optical diagram of an eighth
surface-shape-measuring interferometer of a fourth embodiment
according to a second aspect of the present invention;
[0028] FIGS. 8a and 8b are cross-sectional diagrams of the main
components of the holder assembly of the surface-shape-measuring
interferometer of FIG. 7;
[0029] FIG. 9 is a schematic optical diagram of a ninth
surface-shape-measuring interferometer that is a variation of the
surface-shape-measuring interferometer of FIG. 7;
[0030] FIG. 10a is a schematic optical diagram of a first
wavefront-aberration-measuring interferometer for explaining the
principle of a fifth embodiment according to a third aspect of the
present invention;
[0031] FIG. 10b is a cross-sectional diagram of a second
semitransparent film with a pinhole plate in the interferometer of
FIG. 10a;
[0032] FIG. 11a is a schematic optical diagram of a second
wavefront-aberration-measuring interferometer that is a variation
of the wavefront-aberration-measuring interferometer of FIG.
10a;
[0033] FIG. 11b is a plan view of the second dual hole plate in the
interferometer of FIG. 11a;
[0034] FIG. 11c is a cross-sectional diagram explaining the
operation of the second dual hole plate in the interferometer of
FIGS. 11a and 11b;
[0035] FIG. 12 is a schematic optical diagram of a third
wavefront-aberration-measuring interferometer of a fifth embodiment
according to a third aspect of the present invention;
[0036] FIG. 13a is a plan view of a first embodiment of the first
pinhole array plate of the interferometer of FIG. 12;
[0037] FIG. 13b is a plan view of a first embodiment of the second
dual hole array plate of the interferometer of FIG. 12;
[0038] FIG. 14a is a plan view of a second embodiment of the first
pinhole array plate, being a variation on the first embodiment of
the first pinhole array plate of FIG. 13a;
[0039] FIG. 14b is a plan view of a second embodiment of the second
dual hole array plate, being a variation on the first embodiment of
the second dual hole array plate of FIG. 13b;
[0040] FIG. 15a is a schematic optical diagram of fourth
wavefront-aberration-measuring apparatus of a sixth embodiment
according to the present invention;
[0041] FIG. 15b is a plan view of second Hartmann plate of the
apparatus shown in FIG. 15a;
[0042] FIG. 16a is a schematic optical diagram of a fifth
wavefront-aberration-measuring interferometer of a seventh
embodiment according to a third aspect of the present
invention;
[0043] FIG. 16b is a plan view of the first pinhole cluster plate
of the in interferometer of FIG. 16a;
[0044] FIG. 16c is a plan view of the second dual hole cluster
plate of the in interferometer of FIG. 16a;
[0045] FIG. 17a is a plan view of the first pinhole row plate of an
eighth embodiment according to a third aspect of the present
invention;
[0046] FIG. 17b is a plan view of the second dual hole row plate in
an eighth embodiment according to a third aspect of the present
invention;
[0047] FIG. 18a is a plan view of the first slit plate of a ninth
embodiment according to a third aspect of the present
invention;
[0048] FIG. 18b is a plan view of the second dual slit plate of a
ninth embodiment according to a third aspect of the present
invention;
[0049] FIG. 19 is a schematic optical diagram of a sixth
wavefront-aberration-measuring interferometer of a tenth embodiment
according to a third aspect of the present invention;
[0050] FIG. 20a is a schematic optical diagram of seventh
wavefront-aberration-measuring interferometer of an eleventh
embodiment according to a third aspect of the present
invention;
[0051] FIG. 20b is a cross-sectional diagram of the second pinhole
mirror plate in the interferometer of FIG. 20a;
[0052] FIG. 21a is a plan view of the first pinhole array plate
used in a variation of the interferometer of FIG. 20a;
[0053] FIG. 21b is a plan view of second pinhole mirror array plate
63 in a variation on interferometer 22Q shown in FIG. 20a;
[0054] FIG. 22 is a schematic optical diagram of an eighth
wavefront-aberration-measuring interferometer that is a variation
of the interferometer of FIG. 20a;
[0055] FIG. 23 is a schematic optical diagram of a
wavefront-aberration-me- asuring apparatus serving as a comparative
example for illustrating the advantage of interferometers of FIGS.
20a and 22;
[0056] FIG. 24 is a flowchart indicating an exemplary method for
calibrating the aspheric-surface-shape measuring interferometer of
FIG. 7 using the wavefront-aberration-measuring interferometer FIG.
10a; and
[0057] FIG. 25 is a cross-sectional showing a small tool grinding
apparatus used in the interferometer calibration method indicated
in the flowchart of FIG. 24.
DETAILED DESCRIPTION OF THE INVENTION
[0058] The present invention relates to an interferometer system
for measuring the shape of an aspheric surface of an optical
element in an optical system and for measuring the wavefront
aberration of such an optical system, particularly in connection
with manufacture of a projection optical system suited to for use
in an exposure apparatus employing soft-X-ray (EUV) exposure
light.
[0059] Referring to FIGS. 3a and 3b, the principle of operation of
an interferometer system according to a first aspect of the present
invention is now discussed.
[0060] Compared with prior art interferometer 122 shown in FIG. 1,
first and second interferometer systems 22A and 22B, shown in FIGS.
3a and 3b, respectively, according to the first aspect of the
present invention have a reflective standard 40 with a separately
and accurately calibrated spherical reflective surface 42 arranged
in place of test surface (aspheric surface) 138 of test object 136
(see FIG. 1).
[0061] Interferometer 22A shown in FIG. 3a further differs from
prior art interferometer 122 of FIG. 1 in that a wavefront 45
incident null element 32 is a spherical wavefront from a Fizeau
lens 44, and in that a Fizeau surface 46 is used as the reference
surface. Fizeau lens 42 need not be limited to a convergent system
as shown, but may also be a divergent system. Interferometer 22B
shown in FIG. 3b is an example wherein a wavefront incident null
element 32 is a plane wave 26, as in the case of prior art
interferometer 122 shown in FIG. 1. A flat Fizeau surface 30 of a
Fizeau lens 28 is used as the reference surface. Interferometer 22B
differs from prior art interferometer 122 of FIG. 1 in that the
light beam converted by null element 32 is a convergent light beam,
and in that it permits measurement of concave surfaces as well as
convex surfaces. A method of calibrating null wavefront in this
case is to use a concave reflective surface to calibrate the
wavefront as it diverges after having first converged, and then to
reverse calculate the shape of the null wavefront 34 at the
position where it is actually used (heavy line in drawing) based on
the calibrated wavefront shape. High-precision calibration is
possible if a pinhole interferometer (i.e., a point diffraction
interferometer, hereinafter referred to as a "PDI," discussed
further below) is used to calibrate the concave reflective
surface.
[0062] If the amount of asphericity of surface 42 is small, then
the entire surface can be measured all at once. However, in the
case of an aspheric surface that unfortunately generates
interference fringes exceeding the resolution of the interferometer
CCD, data for the entire surface can be obtained in the same manner
by applying the so-called wavefront synthesis technique. This
technique involves axially displacing reflective standard 40
relative to null wavefront 34, conducting interferometric
measurements on a plurality of annular wavefront data, and joining
the redundant regions of each of the data so they overlap without
excess.
[0063] First Embodiment
[0064] Referring now to FIGS. 4a and 4b, third and fourth
surface-shape-measuring interferometers 22C and 22D of a first
embodiment according to a first aspect of the present invention are
now described, wherein a PDI 52 employing an ideal spherical
wavefront from a point light source 54 is used to measure null
element 32 in Fizeau (aspheric-surface-measuring) interferometer
(i.e., first interferometer system) 22A shown in FIG. 3a.
[0065] Interferometer 22C shown in FIG. 4a employs a divergent null
element 32, and interferometer 22D shown in FIG. 4b employs a
convergent null element 32. The latter is adopted when calibrating
the wavefront 34 for measurement of a convex surface.
[0066] Since spherical wavefront 45 incident null element 32 in
interferometers 22C and 22D of FIGS. 4a and 4b is an ideal
spherical wavefront from a point light source 54, it is possible to
simultaneously ascertain the shape of null wavefront 34 as well as
the transmission characteristics of null element 32.
[0067] Second Embodiment
[0068] Referring now to FIGS. 5a and 5b, fifth and sixth
surface-shape-measuring interferometers 22E and 22F of a second
embodiment according to a first aspect of the present invention are
used to measure null element 32 generating a convergent null
wavefront 34, as the case at interferometer 22B shown in FIG. 3b.
Interferometer 22E of FIG. 5a uses a spherical wavefront 45 as the
wavefront from Fizeau surface 46 incident null element 32.
Interferometer 22F in FIG. 5b uses a plane wave 26 therefor. It
does not matter whether spherical wavefront 45 is a convergent
light beam or a divergent light beam. Furthermore, use of PDI 52
replaces calibration using a reflective surface. PDI 52 corresponds
to a point light source of the present invention.
[0069] To perform measurements with PDI 52, taking the case in
which null wavefront 34 is convergent, pinhole 54 of PDI 52 is
positioned so as to approximately coincide with the point of
convergence of null wavefront 34. As a result, null wavefront 34,
which is reflected from a reflective surface 56 surrounding pinhole
54, and the ideal spherical wavefront produced by the light leaving
pinhole 54 will form interference fringes.
[0070] Third Embodiment
[0071] Referring now to FIG. 6 a seventh surface-shape-measuring
interferometer 22G is a third embodiment according to a first
aspect of the present invention and is similar to interferometer
22E of FIG. 5a, except that a PDIs 52A is used in place of a Fizeau
lens 44 that there had generated a spherical wavefront. A second
PDI 52B is also used for measurement light. In interferometer 22E
and 22F shown in FIGS. 5a and 5b, respectively, there is a
possibility that during operation of Fizeau interferometer 22E or
22F, the measurement light signal from PDI 52 will be lost in
noise. In this case, it is preferable to in addition employ a
polarizing element to reduce noise and improve the usable
signal.
[0072] The measurement arrangement in interferometer 22G shown in
FIG. 6 has the advantage that pinhole 54B that forms the point
light source of second PDI 52B acts to reduce noise and improve the
usable signal. This permits not only the shape of null wavefront 34
and the transmission characteristics of null element 32 to be
accurately calibrated, but also permits the transmission
characteristics of two PDIs 52A and 52B to be calibrated in both
the forward and backward directions. Accordingly, accuracy can be
further improved.
[0073] To actually use one of the aforementioned interferometers
22C-22G to measure a test surface 38 after calibration has thus
been performed, reflective standard 40, point light source forming
means, PDIs 52 or the like are removed and these are replaced with
the original test surface 38 and a light source 48, following which
measurements may be performed.
[0074] As described above, interferometers 22C-22G of the first
through third embodiments according to a first aspect of the
present invention make it possible to calibrate an aspheric null
element 32 with high precision and in a short period of time.
[0075] Fourth Embodiment
[0076] FIG. 7 shows an eighth surface-shape-measuring
interferometer 22H of a fourth embodiment according to a second
aspect of the present invention. FIGS. 8a and 8b show the principal
parts of interferometer 22H of FIG. 7. Interferometer 22H shown in
FIG. 7 is capable of measuring the shape of an aspheric
surface.
[0077] Referring to FIG. 7, laser light from a laser 24 is changed
into a collimated beam of a prescribed diameter by way of a lens
system 58, and is then incident null element 32. Null element 32
emits a wavefront having a shape substantially identical to that of
test surface 38, and the wavefront, having been converted to a
prescribed aspheric surface shape, is incident in perpendicular
fashion, an aspheric reference surface 70 and aspheric test surface
38. Furthermore, aspheric reference surface 70 has substantially
the same shape as aspheric test surface 38 (with, however,
concavity and convexity reversed). The light incident aspheric
reference surface 70 is amplitude-divided, with one wavefront
proceeding to test surface 38 and the other wavefront returning
along the original optical path to serve as reference
wavefront.
[0078] Aspheric reference surface 70 is arranged proximate test
surface 38, and aspheric reference surface 70 and test surface 38
have mutually complementary shapes. Aspheric reference surface 70
and test surface 38 are supported in integral fashion by a holder
72.
[0079] Furthermore, light from aspheric reference surface 70 is
reflected by test surface 38, and is again incident aspheric
reference surface 70 as the measurement wavefront.
[0080] After the abovementioned reference wavefront and measurement
wavefront exit from the reference optical element 76 upon which
aspheric reference surface 70 is formed, they are incident null
element 32, are reflected by a beam splitter 74 within lens system
58, and then form interference fringes on detector 60 comprising a
CCD or other such image pickup element. By processing these
interference fringes with a computer CU electronically connected to
detector 60, the shape error of test surface 38 can be
measured.
[0081] In interferometer 22H shown in FIG. 7, a main body, which
includes the elements from laser 24 to null element 32, and holder
72, are supported by separate members so as to be spatially
separated.
[0082] Interferometer 22H shown in FIG. 7 is basically a Fizeau
interferometer, but it has several significant advantages over
prior art Fizeau interferometer 222 of FIG. 2. The causes of the
degradation in the measurement reproducibility in a conventional
interferometer such as interferometer 122 of FIG. 1 or
interferometer 222 of FIG. 2 include air fluctuations, vibration,
sound, air pressure fluctuations, temperature fluctuations,
detector noise, nonlinear errors and amplitude errors in the fringe
scan, reproducibility of positioning the specimen, reproducibility
of strain in the specimen due to the specimen holder, and
aberrations in the optical system. Among these, air fluctuations,
vibration, sound, air pressure fluctuations, temperature
fluctuations, and optical system aberrations can be significantly
reduced by bringing test surface 38 and reference surface 70 close
together and physically joining them in integral fashion, as in
interferometer 22H of the fourth embodiment of the present
invention shown in FIG. 7.
[0083] Particularly with respect to interferometer 22H of in FIG.
7, while null element 32 is used therein, measurement accuracy is
not affected by either the accuracy of null element 32 or the
accuracy of alignment between null element 32 and test surface 38.
This is because null element 32 functions to deliver a wavefront
having an aspheric shape more or less identical to aspheric
reference surface 70 to that aspheric reference surface 70, but
does not directly function to deliver an aspherically shaped
wavefront to test surface 38. Accordingly, although null element 32
is not an essential component in interferometer 22H, it is
preferable to use null element 32 so as to improve measurement
accuracy.
[0084] The positional reproducibility of test object 36 in
interferometer 22H is ensured through use of a position sensor PS
(electronic micrometer or the like), not shown, arranged around
test object 36, and the reproducibility of strain in the test
specimen 36 from the specimen holder 72 is improved by constructing
the specimen holder 72 such that support is effected in three-point
or multi-point fashion.
[0085] In addition, the close proximity of test surface 38 and
reference surface 70 makes detection of alignment error easier and
enables high-precision alignment. Detector noise can be
sufficiently reduced by cooling detector 60 and by integrating the
data. Nonlinear errors and amplitude errors during fringe scans can
be eliminated by using a digital-readout piezoelectric element, and
by processing the signal such that there are an increased number of
packets during fringe scans. Adoption of the above-described
constitution in interferometer 22H permits attainment of
repeatabilities of 0.05 nm RMS or better, and permits attainment of
measurement reproducibilities, including alignment error, changes
occurring over time, and so forth, of 0.1 nm RMS or better.
[0086] A remaining problem with interferometer 22H is absolute
accuracy, which is dependant on the surface accuracy of reference
aspheric surface 70. This error is a systematic error associated
with the interferometer 22H. Below are described ways to correct
this error (i.e., how calibration to offset this error.
[0087] Interferometer 22H, while based on conventional Fizeau
interferometer 222 shown in FIG. 2, is different from the
conventional Fizeau interferometer in the following respects.
Fizeau (reference) surface 70 of interferometer 22H is an aspheric
surface, its shape being such that convexity and concavity are
reversed with respect to test surface 38 arranged in close
proximity to Fizeau surface 70. The constitution is such that
reference element 76 is separated from the optical system, and such
that the (Fizeau) reference optical element 76 is physically
connected in integral fashion to test object 36. This constitution
significantly improves repeatability and measurement
reproducibility as compared with that of above-described
conventional interferometer 222 shown in FIG. 2.
[0088] FIGS. 8a and 8b show two exemplary configurations for holder
assembly 72 of interferometer 22H of FIG. 7. FIG. 8a shows an
exemplary configuration wherein the spacing between test surface 38
and aspheric reference surface 70 is variable, and FIG. 8b shows an
exemplary configuration wherein the spacing is fixed.
[0089] Referring to FIG. 8a, reference element 76 with aspheric
reference surface 70 is held by reference element holder 72H, which
is disposed separately from the interferometer 22H main body. A
piezoelectric element 72P is provided on reference element holder
72H. A test object holder 72T, which holds test object 36, is
mounted to reference element holder 72H by way of piezoelectric
element 72P. By driving piezoelectric element 72P, the spacing
between aspheric reference surface 70 and test surface 38 can be
adjusted. Furthermore, this variable spacing can also be exploited
to perform a fringe scan, which is a conventional method of
analyzing interference fringes.
[0090] The exemplary configuration of holder assembly 72 shown in
FIG. 8b is similar to the exemplary configuration shown in FIG. 8a
in that reference element 76 with aspheric reference surface 70 is
held by reference element holder 72H. However, holder assembly 72
of FIG. 8b has spacers 72S directly vacuum-deposited at three
locations on aspheric reference surface 70. Spacers 72S are 1 to 3
.mu.m in thickness, this thickness being identical at all three
locations. Furthermore, spacers 72S are provided so that they
trisect the circumference about an axis Ax in the vertical
direction of the paper surface in FIG. 8b. Test surface 38 is
mounted on (three) spacers 72S. The spacing between aspheric
reference surface 70 and test surface 38 can thereby be kept
constant and the strain in test surface 38 due to gravity can also
be kept constant. If the exemplary configuration shown in FIG. 8b
is employed, it is possible to perform a fringe scan for analyzing
interference fringes by varying laser wavelength, which has the
additional benefit of eliminating the likelihood that the
interferometer will be affected by mechanical vibration or the
like.
[0091] It is preferable that test object 36 be held in holder
assembly 72 in the same manner as it is held in the optical system
of which it is an optical component. It is also preferable that
test object 36 be held in holder assembly 72 in the same
orientation with respect to gravity as it is held in the optical
system of which it is an optical component. This will make it
possible to carry out meaningful measurements despite changes in
surface shape which may occur due to the action of strain on test
surface 38 when test object 36 is actually incorporated into an
optical system.
[0092] It is also preferable to make the spacing between aspheric
reference surface 70 and test surface 38 less than 1 mm. If this
spacing exceeds 1 mm, the impact of air fluctuations, vibration,
sound, air pressure fluctuations, temperature fluctuations and
optical system aberrations increases, leading to a deterioration in
measurement accuracy. To further improve measurement accuracy, it
is preferable to set the spacing between aspheric reference surface
70 and test surface 38 to be less than 100 .mu.m.
[0093] In addition, if the spacing between aspheric reference
surface 70 and test surface 38 is fixed as in FIG. 8b, it is
preferable to set this spacing to be less than 10 .mu.m.
[0094] Variation on Fourth Embodiment
[0095] In the exemplary configuration shown in FIG. 8a and
discussed above, the spacing between test surface 38 and aspheric
reference surface 70 may be detected by the following
techniques.
[0096] Referring now also to FIG. 9, a ninth
surface-shape-measuring interferometer 221 is a variation on the
above-described interferometer 22H of the fourth embodiment shown
in FIG. 7. In interferometer 221, elements similar in function to
elements as those in interferometer 22H have been given the same
reference numerals and so a description thereof is omitted.
[0097] Interferometer 221 shown in FIG. 9 differs from
interferometer 22H shown in FIG. 7 in that a shearing
interferometer 80 is provided behind test surface 38 (i.e., at the
side opposite from aspheric reference surface 70). Shearing
interferometer 80 guides light from a white light source 80S to
test surface 38 and aspheric reference surface 70 by way of a beam
splitter 80BS. Light reflected by test surface 38 and light
reflected by aspheric reference surface 70 passes through beam
splitter 80BS, and is horizontally displaced by a birefringent
member 80BR. The latter may be, for example, a Wollaston prism. The
light then passes through an analyzer 80A and forms an interference
pattern on detector 60, such as a CCD. The spacing between test
surface 38 and aspheric reference surface 70 can be detected by
monitoring the change in the interference pattern on detector 60.
In addition, in interferometer 22I shown in FIG. 9, optical element
36 having test surface 38 is preferably made of an optically
transmissive material such as, for example, quartz or Zerodur.
[0098] Fifth Embodiment
[0099] Referring now to FIGS. 10a-14, we describe a fifth
embodiment according to a third aspect of the present invention.
FIGS. 10a and 11a show first and second
wavefront-aberration-measuring interferometers 22J and 22K. FIGS.
12-14b show exemplary configurations of a third
wavefront-aberration-measuring interferometer 22L according to the
fifth embodiment.
[0100] Interferometers 22J, 22K, and 22L, respectively shown in
FIGS. 10a, 11a, and 12, are not "Fizeau"
aspheric-surface-shape-measuring interferometers for measuring the
surface shape of a test surface 38 of a test object 36 previously
removed from an optical system of which it is an optical component,
as were interferometers 122, 222, and 22A-22I shown in FIGS. 1-9.
Rather, they are wavefront-aberration-measuring interferometers for
measuring the wavefront aberration produced by an optical system.
Note that for the sake of convenience, the term "interferometer" is
used to refer to either an aspheric-surface-shape-mea- suring
interferometer, a wavefront-aberration-measuring interferometer, or
to both, when the meaning is clear from context.
[0101] The wavefront-aberration-measuring interferometers 22J-22L
according to the fifth embodiment of the present invention use
light corresponding to an exposure wavelength in the soft X-ray
region to measure wavefront aberration of a projection optical
system.
[0102] Referring to FIGS. 10a-11c, the principle of the
wavefront-aberration-measuring interferometer of the fifth
embodiment according to a second aspect of the present invention is
now described.
[0103] With reference to FIG. 10a, light from a synchrotron orbital
radiation (hereinafter "SOR") undulator (not shown) passes through
a spectroscope (not shown) to form quasimonochromatic light 84
having a wavelength around 13 nm. Light 84 is condensed by a
condenser mirror 64 and is incident a first pinhole plate 86. First
pinhole plate 86 has an aperture (pinhole) 86o of a size smaller
than the size of the Airy disk as determined from the numerical
aperture on the incident side (first pinhole plate 86 side) of an
optical system 37 under test. The size of the Airy disk is given by
0.6 .lambda./NA, where NA is the incident-side numerical aperture
of optical system 37, and .lambda. is the wavelength of
quasimonochromatic light 84.
[0104] Light having a wavefront which can be regarded as that of an
ideal spherical wavefront will exit first pinhole plate 86. Light
from first pinhole plate 86 is then incident optical system 37, and
then arrives at a pinhole plate 88 having an aperture 88o arranged
at an image plane IP of optical system 37. First pinhole plate 86
and second pinhole plate 88 are arranged at locations made mutually
conjugate by optical system 37, i.e., at locations corresponding to
what would be an object point and an image point if optical system
37 were actually used to image an object.
[0105] Referring to FIG. 10b, pinhole plate 88 comprises a
semitransparent film 88F provided on a substrate 88S which is
optically transmissive at the wavelength of emitted
quasimonochromatic light 84, and aperture 88o wherein
semitransparent film 88F is not provided. Accordingly, a portion of
the wavefront incident pinhole plate 88 is transmitted without
alteration of the wavefront, and another portion undergoes
diffraction at aperture 88o. Accordingly, if the size of aperture
88o is sufficiently small, the light diffracted at aperture 88o can
be regarded as an ideal spherical wavefront.
[0106] Referring again to FIG. 10a, detector 60 is arranged on the
exit side of pinhole plate 88 (i.e., at the side thereof opposite
from optical system 37). Interference fringes are formed on
detector 60 due to interference between the ideal spherical
wavefront from aperture 88o and the transmitted wavefront from
semitransparent film 88F. The transmitted wavefront from
semitransparent film 88F corresponds in shape to the wavefront
aberration of optical system 37. The interference fringes on
detector 60 assume a shape corresponding to the deviation of this
transmitted wavefront from an ideal spherical wavefront (ie., the
wavefront from aperture 88o). Accordingly, the wavefront aberration
of optical system 37 can be determined by analyzing, in a computer
CU electrically connected to detector 60, the interference fringes
detected by detector 60.
[0107] FIGS. 11a is a fourth wavefront-aberration-measuring
interferometer 22K employing an SOR undulator light source and
which is a variation of wavefront-aberration-measuring
interferometer 22J of FIG. 10a. Note that in FIGS. 11a-11c,
elements similar in function to elements appearing in FIGS. 10a and
10b are given the same reference numerals as in FIGS. 10a and 10b.
Interferometer 22K makes use of a measurement technique of higher
precision than that of interferometer 22J. Interferometer 22K in
FIG. 11a differs from interferometer 22J in FIG. 10a in that a
second dual hole plate 90 is arranged in place of second pinhole
plate 88, and a diffraction grating 62 is inserted between first
pinhole plate 86 and second dual hole plate 90.
[0108] FIG. 11b shows the constitution of second dual hole plate
90, and FIG. 11c is a diagram for explaining the functions of
diffraction grating 62 and second dual hole plate 90. Referring to
FIG. 11b, second dual hole plate 90 has microscopic aperture 90o
that functions as a pinhole, and an aperture 92 that is larger than
pinhole 90o. Pinhole 90o and aperture 92 are formed such that if
second dual hole plate 90 is at the location of image plane IP of
optical system 37, pinhole 90o is positioned in the optical path of
the zeroeth-order peak P0 of the diffraction pattern produced by
diffraction grating 62. In addition, aperture 92 is positioned in
the optical path of a first-order peak P1 of the diffraction
pattern produced by diffraction grating 62, as shown in FIG.
11c.
[0109] Accordingly, zeroeth-order peak P0 is diffracted by pinhole
90o, forming an ideal spherical wavefront 45 which then proceeds to
detector 60. In addition, a wavefront 45' associated with
first-order peak P1, which contains information about the wavefront
aberration of optical system 37, passes through aperture 92 without
alteration, and proceeds to detector 60. At this time,
zeroeth-order peak P0 and first-order peak P1 have wavefronts 45
and 45', respectively, corresponding to the wavefront aberration of
optical system 37. Wavefront 45 of the light that passes through
pinhole 90o, is converted to an ideal spherical wavefront. However,
wavefront 45' passing through aperture 92 does not undergo any
significant amount of diffraction, and so has a wavefront shape
corresponding to the wavefront aberration of optical system 37.
Accordingly, interference fringes due to interference between ideal
spherical wavefront 45 from pinhole 90o and measurement wavefront
'45 from aperture 92 are formed on detector 60. The profile of the
interference fringes formed on detector 60 will correspond to the
deviation of the measurement wave from an ideal spherical wavefront
45, and wavefront 45' containing aberration information of optical
system 37 can be determined by analyzing these interference
fringes, as in the case for interferometer 22J of FIG. 10a.
[0110] With continuing reference to FIG. 11a, a fringe scan for
high-precision measurement can be performed by moving diffraction
grating 62 by operatively connecting the latter to a diffraction
grating driving unit DU. In interferometer 22K, diffraction grating
62 is shown arranged in the optical path between optical system 37
and second dual hole plate 90. However, diffraction grating 62 may
be arranged in the optical path anywhere between first pinhole
plate 86 and second dual hole plate 90. For example, it is possible
to arrange diffraction grating 62 in the optical path between first
pinhole plate 86 and optical system 37. In addition, while the
above-described embodiment of interferometer 22K employed two
diffraction orders P0 and P1 of the diffraction pattern produced by
diffraction grating 62, the present invention is not limited to two
such orders or of combinations of the zeroeth-order and
first-order.
[0111] Referring now to FIG. 12, a fifth
wavefront-aberration-measuring interferometer 22L, which represents
a fifth embodiment according to the second aspect of the present
invention for measuring the wavefront aberration of an optical
system 37 based on the principle explained above with reference to
FIGS. 10a-11c, is now described. In FIG. 12, elements similar in
function to elements appearing in FIGS. 10a-11c are given the same
reference numerals as in FIGS. 10a and 10b.
[0112] In interferometers 22J and 22K shown in FIGS. 10a and 11a,
the aberration of optical system 37 can only be measured at one
point in image plane IP. To accurately ascertain the aberration of
an optical system, it is necessary to measure a plurality of image
points. To measure a plurality of image points in interferometers
22J and 22K, one conceivable method of performing measurements
would involve moving first pinhole plate 86 and second pinhole
plate 88. or second dual hole plate 90, to a number of prescribed
positions. In this case, since the pinholes are extremely small,
there is a risk that the pinholes will be affected by the vibration
of the movement mechanism that moves the pinholes, and that
particularly for pinholes on the image side, it will not be
possible to make light pass through these pinholes stably. This
makes good measurements extremely problematic. In addition, if
pinholes are moved, it becomes difficult to measure the pinhole
positions with good accuracy. Further, there is a risk that the
accuracy with which aberration (particularly distortion), is
measured will no longer be sufficient, particularly for image
points.
[0113] In interferometer 22L, a first pinhole array plate 93,
wherein pinholes are arrayed in two dimensions, is used in place of
first pinhole plate 86 of interferometer 22K shown in FIG. 11a.
[0114] Referring to FIG. 12, light from an SOR undulator (not
shown) passes through an analyzer (not shown) to form
quasimonochromatic light 84 having of wavelength around 13 nm. This
light is condensed by condenser mirror 64 and is incident first
pinhole array plate 93. Unlike wavefront-aberration-measuring
interferometers 22J and 22K shown in FIGS. 10a and 11a,
interferometer 22L shown in FIG. 12 is constituted such that light
is incident the image plane IP side, not the object plane OP side,
of optical system 37, the reason for which is discussed below.
[0115] Turning briefly to FIG. 13a, first pinhole array plate 93
comprises an array or matrix of pinhole apertures (pinholes) 93o of
a size well smaller than the size of the Airy disk 0.6 .lambda./NA,
as determined from the numerical aperture (imagewise numerical
aperture) NA at the incident side of optical system 37. The
positions of pinholes 93o correspond to the locations of image
points of optical system 37 for which measurement of wavefront
aberration is desired.
[0116] Returning now to FIG. 12, condenser mirror 64 is provided on
a condenser mirror stage 66, which is capable of movement parallel
to image plane IP of optical system 37. By moving condenser mirror
stage 66, any desired pinhole 93o on first pinhole array plate 93
can be selectively illuminated. An illuminated pinhole 93o
corresponds to a measurement point. Furthermore, the position at
which quasimonochromatic light 84 is incident first pinhole array
plate 93 will change with the movement of condenser mirror stage
66. In addition, it is also possible to collectively illuminate a
plurality of pinholes 93o on first pinhole array plate 93 instead
of, or in addition to, illuminating just one of the pinholes.
Nonetheless, in the description below, it is assumed for the sake
of convenience, that only one pinhole 93o is illuminated.
[0117] Referring now also to FIG. 13b, second dual hole array plate
94 is located in object plane OP, ie., arranged at the position at
which optical system 37 images first pinhole array plate 93. Second
dual hole array plate 94 has a plurality of pinhole apertures
(pinholes) 94o provided in a matrix at positions at which the
plurality of pinholes 93o of first pinhole array plate 93 are
imaged, and a plurality of apertures 95 provided in a matrix such
that each is separated by a prescribed distance from each of the
plurality of pinholes 94o. Furthermore, each of the plurality of
pinholes 94o has the same function as pinholes 90o in FIG. 11b, and
each of the plurality of apertures 95 has the same function as
aperture 92 in FIG. 11b.
[0118] Referring again to FIG. 12, light having a wavefront 45,
which can be regarded as that of an ideal spherical wavefront,
exits an illuminated pinhole 93o, and is incident optical system
37. This light passes through optical system 37 and is diffracted
by diffraction grating 62 arranged between optical system 37 and
object plane IP. Zeroeth-order peak P0 (not shown in FIG. 12) of
the diffraction pattern arrives at pinhole 94o on second dual hole
array plate 94 corresponding to the illuminated pinhole 93o on
first pinhole array plate 93. First-order peak P1 (not shown on
FIG. 12) of the diffraction pattern arrives at aperture 94o on
second dual hole array plate 94 corresponding to the illuminated
pinhole 93o on first pinhole array plate 93. Light that passes
through pinhole 94o and the light that passes through aperture 95
mutually interfere.
[0119] With continuing reference to FIG. 12, detector 60, is
attached to a detector stage 68 which is capable of movement
parallel to object plane OP, is arranged at the exit side of second
dual hole array plate 94. Detector stage 68 is constituted so that
it is linked with and moves with condenser mirror stage 66, and
such that only pinhole 94o and apertures 95, corresponding to
illuminated pinhole 93o, can be seen from detector 60. Accordingly,
the interference fringes due to the light only from pinhole 94o and
aperture 95, corresponding to the illuminated pinhole 93o, are
formed on detector 60. By analyzing these interference fringes, the
wavefront aberration at image plane IP location corresponding to
illuminated pinhole 93o can be determined.
[0120] In interferometer 22L of FIG. 12, first pinhole array plate
93 and second dual hole array plate 94 are physically grounded
(i.e., secured so as to be stationary) with respect to optical
system 37. Thus, stable measurements can be performed without being
affected by vibrations caused by the movement of stages 66, 68
during actual measurements.
[0121] First pinhole array plate 93 is mounted on a vertical stage
67, which is capable of causing first pinhole array plate 93 to
move in jogged (i.e., incremental) fashion in a direction parallel
to the optical axis of optical system 37. Vertical stage 67 is
preferably secured to the same frame (not shown) that supports
optical system 37. In addition, second dual hole array plate 94 is
mounted on an XY stage 69, which is capable of causing second dual
hole array plate 94 to move in jogged fashion within object plane
OP of optical system 37. XY stage 69 is attached to the
abovementioned frame by way of a piezoelectric element. Adjustment
of focus can be performed by using vertical stage 67 to move first
pinhole array plate 93. If there is distortion in optical system
37, XY stage 69 can be used to align the position of pinhole 94o.
Furthermore, a length measuring interferometer or other such
microdisplacement sensor (not shown) may be provided on XY stage
69, permitting distortion in optical system 37 to be measured based
on the output from the microdisplacement sensor. Furthermore, in
the present embodiment, the positions of the plurality of pinholes
93o of first pinhole array plate 93 and the plurality of pinholes
94o of second dual hole array plate 94 are accurately measured
beforehand using a coordinate measuring interferometer.
[0122] Although the position of pinhole 94o is moved in
interferometer 22L, this pinhole can be positioned with good
accuracy since the stroke of this movement is small. Furthermore,
interferometer 22L is constituted such that pinhole 94o, on the
object plane OP side of optical system 37 is moved when optical
system 37 has a reduction magnification of -1/.beta.. Thus, the
positioning accuracy of pinhole 94o can be relaxed by the factor
.vertline.-1/.beta..vertline. as compared with the case in which
pinhole 93o, on the image plane IP side of optical system 37, is
moved. Interferometer 22L is not constituted so that pinhole 93o is
moved and the amount of movement of pinhole 94o is in a range
wherein positioning accuracy can be maintained. Thus, stable
measurement can be achieved, and the measurement accuracy of
aberration, particularly distortion, at the imaged location can be
made sufficient.
[0123] In interferometer 22L shown in FIG. 12, the plurality of
pinholes 93o corresponding to positions for measurement of the
wavefront aberration of optical system 37 are shown arranged in a
matrix. However, the arrangement of pinholes 93o is not limited to
a typical square or rectangular matrix. For example, referring to
FIG. 14a, if the field (exposure field) EF of optical system 37 is
arcuate, as shown in FIGS. 14a and 14b, then a pinhole plate 93'
having pinholes 93o may be arranged with a prescribed spacing at an
object height (image height) of the same height as that of optical
system 37. Also, as shown in FIG. 14b, the arrangement of the
pinholes 94o and apertures 95 in second dual hole array plate 94'
will have to be prealigned with pinholes 93o of the first pinhole
array plate 93.
[0124] While diffraction grating 62 in interferometer 22L of FIG.
12 is arranged in the optical path between optical system 37 and
second dual hole array plate 94, diffraction grating 62 may also be
arranged in the optical path between first pinhole array plate 93
and second dual hole array plate 94. For example, it is possible
for diffraction grating 62 to be arranged between first pinhole
array plate 93 and optical system 37. In addition, while
interferometer 22L shown in FIG. 12 employs two peaks of the
diffraction pattern produced by diffraction grating 62, i.e.,
zeroeth-order peak P0 of the diffraction pattern and first-order
peak P1 of the diffraction pattern, the present invention is not
limited to employment of two such peaks, or of employment of
combinations of zeroeth-order and first-order peaks.
[0125] Sixth Embodiment
[0126] Referring now to FIGS. 15a and 15b, a fourth
wavefront-aberration-measuring apparatus 22M of a sixth embodiment
according to the present invention is now described. Apparatus 22M
uses a soft X-ray exposure wavelength to measure the wavefront
aberration of an optical system 37. Note that in FIGS. 15a and 15b,
elements similar in function to elements appearing in FIGS. 10a-14b
are given the same reference numerals as in FIGS. 10a-14b.
[0127] Referring to FIG. 15a, light from an SOR undulator (not
shown) passes through an analyzer (not shown) to form
quasimonochromatic light 84 having a wavelength around 13 nm, which
is condensed by a condenser mirror 64 and is incident first pinhole
plate 86. First pinhole plate 86 has an aperture of a size well
smaller than the size of the Airy disk, 0.6 .lambda./NA, where
.lambda. is the wavelength of quasimonochromatic light 84 and NA is
the numerical aperture on the incident side (first pinhole plate 86
side) of optical system 37. Accordingly, the light that exits first
pinhole plate 86 can be regarded as having the wavefront of an
ideal spherical wavefront.
[0128] In apparatus 22M, a second Hartmann plate 96 having a
plurality of apertures 96o, as shown in FIG. 15b, is arranged
between the location of image plane IP of optical system 37 (a
location made conjugate to first pinhole plate 86 by optical system
37) and optical system 37.
[0129] Returning to FIG. 15a, the light beam from first pinhole
plate 86, upon exiting optical system 37, forms, due to the action
of the plurality of apertures 96o of second Hartmann plate 96, a
plurality of ray groups RG that are the same in number as the
number of apertures 96o. Ray groups RG then proceed to image plane
IP of optical system 37. Ray groups RG converge at image plane IP
of optical system under test 37 and arrive at detector 60 in a
divergent state. If the plane of the pupil (not shown) of optical
system under test 37 is subdivided into a plurality of sections,
ray groups RG that pass through the plurality of apertures 96o on
second Hartmann plate 96 respectively correspond to rays passing
through each such pupil section. As a result, the lateral
aberration of optical system 37 can be determined if the position
at which each of ray groups RG arrives at detector 60 is detected.
The wavefront aberration of optical system 37 can be determined
from this lateral aberration.
[0130] In apparatus 22M, the plurality of apertures 96o provided on
second Hartmann plate 96 are arranged in a matrix as shown in FIG.
I5b. However, the present invention is not limited to this
arrangement. In addition, while in apparatus 22M second Hartmann
plate 96 is arranged between optical system 37 and image plane IP
second Hartmann plate 96 may also be located between first pinhole
plate 86 and image plane IP, it being possible, for example, for
second Hartmann plate 96 to be arranged in the optical path between
first pinhole plate 86 and optical system 37.
[0131] Seventh Embodiment
[0132] Referring now to FIGS. 16a-16c, a fifth
wavefront-aberration-measur- ing interferometer 22N of a seventh
embodiment according to a third aspect of the present invention is
described. Interferometers 22J, 22K, 22L, and 22M of the fifth and
sixth embodiments discussed above are
wavefront-aberration-measuring interferometers which employ an SOR
undulator (not shown) as a light source. Although accuracy can be
made extremely high if an SOR undulator is used as a light source,
the apparatus itself becomes excessively large, and it is generally
extremely difficult to use in a factory. Thus, referring to FIG.
16a, in interferometer 22N discussed in further detail below, a
laser plasma X-ray (hereinafter "LPX") source 21 is used in place
of an SOR undulator as light source. LPX source 21 generates
high-temperature plasma from a target 25 when high-intensity pulsed
laser light is focused on target 25. X-rays present within this
plasma are then used. In interferometer 22N, light emitted from LPX
source 21 is divided into spectral components by a spectroscope
(not shown), and light 27 of only a prescribed wavelength (e.g., 13
nm) is extracted. Light 27 is used as the light for
wavefront-aberration-measuring interferometer 22N.
[0133] The intensity of LPX source 21 is smaller than that of the
SOR undulator by an order of magnitude. Consequently, in
interferometer 22N, first pinhole plate 86, which had only a single
aperture in interferometers 22J, 22K, 22L, and 22M of the fifth and
sixth embodiments shown in FIGS. 10a-15b and discussed above, is
replaced with a first pinhole cluster plate 87. The latter includes
a plurality of pinhole clusters 87c, each of which contains a
plurality of pinholes 87o clustered together in a microlocation, as
shown in FIG. 16b.
[0134] Referring again to FIG. 16a, in LPX source 21, a laser light
source 23 supplies high-intensity pulsed laser light of a
wavelength in the range from the infrared region to the visible
region. Laser light source 23 may be, for example, a YAG laser
excited by a semiconductor laser, an excimer laser, or the like.
This laser light is condensed by a condenser optical system 29 onto
target 25. Target 25 receives the high-intensity laser light, rises
in temperature and is excited to the plasma state, and emits X-rays
27 during transitions to a lower potential state. By passing X-rays
27 through a spectroscope (not shown), quasimonochromatic light 27
only of wavelength 13 nm is extracted, which is then acted on by
condenser mirror 64 and irradiates a pinhole cluster 87c on first
pinhole cluster plate 87.
[0135] Referring again to FIG. 16b, first pinhole cluster plate 87
has pinhole clusters 87c, each of which comprises a plurality of
pinholes 87o clustered in a microlocation at a position for which
the wavefront aberration of optical system 37 is to be measured.
Note that in FIG. 16b, pinhole cluster 87c is shown as having only
four pinholes 87o. However, pinhole cluster 87c preferably actually
comprises one hundred or more pinholes 87o. Pinholes 87o are of a
size much smaller than the size of the Airy disk 0.6 .lambda./NA,
where .lambda. is the wavelength of quasimonochromatic light 27 and
NA is the numerical aperture on the incident side (first pinhole
cluster plate 87 side) of optical system 37. In addition, FIG. 16b
shows an exemplary schematic arrangement wherein a plurality of
pinhole clusters 87c are formed on first pinhole cluster plate 87.
In practice the positions at which pinhole clusters 87c are formed
to correspond to the positions of object points of optical system
37 for which measurement is desired.
[0136] Returning to FIG. 16a, the entire region of one pinhole
cluster 87c on first pinhole cluster plate 87 is illuminated by
quasimonochromatic light 27. A plurality of ideal spherical
wavefronts are generated from the numerous pinholes 87o of the
illuminated pinhole cluster 87c. The plurality of ideal spherical
wavefronts passes through optical system 37, and then proceeds to
and converges at image plane IP of optical system 37, which
position is made conjugate to first pinhole cluster plate 87 by
optical system 37.
[0137] Although not shown in FIGS. 16a-16c, in interferometer 22N
one of pinhole clusters 87c on first pinhole cluster plate 87 is
selectively illuminated, just as in the case of interferometers
22J, 22K, and 22L of the fifth embodiment, discussed above.
[0138] In interferometer 22N diffraction grating 62 is arranged
between optical system 37 and the location of the image plane IP of
optical system 37. The light that exits optical system 37 and
passes through diffraction grating 62 is diffracted by diffraction
grating 62 and proceeds to a second dual hole cluster plate 89.
[0139] FIG. 16c shows a preferred constitution of second dual hole
cluster plate 89. Second dual hole cluster plate 89 has pinhole
cluster 89c comprising a plurality of pinholes 89o provided in
one-to-one correspondence with the pinholes 87o of which plurality
of pinhole clusters 87c on first pinhole cluster plate 87 are each
comprised, and a plurality of apertures 89a provided in one-to-one
correspondence with the plurality of pinhole clusters 87c. In other
words, one aperture 89a corresponds to one pinhole cluster 87c
comprising a plurality of pinholes 87o.
[0140] At this time, if second dual hole cluster plate 89 is
arranged at image plane IP, then plurality of pinhole clusters 89c
and plurality of apertures 89a will be positionally related so that
pinhole cluster 89c is positioned in the optical path of the
zeroeth-order peak P0 of the diffraction pattern produced by
diffraction grating 62, and so that aperture 89a is positioned in
the optical path of first-order peak P1 of the diffraction pattern
produced by diffraction grating 62.
[0141] Accordingly, the ideal spherical wavefronts from pinhole
cluster 87c on first pinhole cluster plate 87 pass through optical
system 37 and are then diffracted by diffraction grating 62. Of the
light produced by this diffraction, zeroeth-order peak P0 of the
diffraction pattern arrives at the pinhole cluster 89c on second
dual hole cluster plate 89, which corresponds to illuminated
pinhole cluster 87c. In addition, first-order peak P1 of the
diffraction pattern arrives at the aperture 89a on second dual hole
cluster plate 89, which corresponds to illuminated pinhole cluster
87c. Zeroeth-order peak P0 of the diffraction pattern and
first-order peak P1 of the diffraction pattern have wavefronts
corresponding in shape to the wavefront aberration of optical
system 37. Zeroeth-order peak P0 of the diffraction pattern is
diffracted by pinhole cluster 89c as it passes therethrough and is
converted to a second group of ideal spherical wavefronts.
First-order peak P1 of the diffraction pattern passes through
aperture 89a and exits therefrom without being diffracted. The
light from the second ideal spherical wavefront group and the light
from aperture 89a mutually interfere.
[0142] Accordingly, interference fringes due to interference
between the ideal spherical wavefront group from pinhole cluster
89c and the wavefront from aperture 89a are formed on detector 60
arranged on the exit side of second dual hole cluster plate 89
(i.e., on the side of second dual hole cluster plate 89 opposite
from optical system 37). Furthermore, the interference fringes on
detector 60 form a shape corresponding to the deviation from an
ideal spherical wavefront of the wavefront that passes through
optical system 37. The wavefront aberration of optical system 37
can be determined by analyzing these interference fringes via
computer CU electrically connected to detector 60, just as in the
previously mentioned embodiments.
[0143] Furthermore, although not shown in FIG. 16a, detector 60 is
constituted so as to be capable of movement parallel to image plane
IP of optical system 37 so that it can be made to selectively
receive the light from pinhole cluster 89c and aperture 89a
corresponding to illuminated pinhole cluster 87c, just as in
interferometers 22J, 22K, and 22L of the fifth embodiment,
discussed above. As a result, wavefront aberration can be measured
at a plurality of measurement points within object plane OP of
optical system 37.
[0144] The seventh embodiment of the present invention as described
above can provide a wavefront-aberration-measuring interferometer
22N that can be used even in an ordinary factory.
[0145] Furthermore, while diffraction grating 62 in interferometer
22N of the seventh embodiment shown in FIG. 16a is arranged in the
optical path between optical system 37 and second dual hole cluster
plate 89, diffraction grating 62 may also be arranged in the
optical path between first pinhole cluster plate 87 and second dual
hole cluster plate 89. It being possible, for example, to arrange
diffraction grating 62 in the optical path between first pinhole
cluster plate 87 and optical system 37. Also, while interferometer
22N employs two peaks of the diffraction pattern produced by
diffraction grating 62 (zeroeth-order peak P0 and first-order peak
P1) the present invention is not limited to employment of two such
peaks or of employment of combinations of the zeroeth-order and
first-order peaks.
[0146] Eighth Embodiment
[0147] Referring now to FIGS. 17a and 17b, an eighth embodiment
according to a third aspect of the present invention is described.
Interferometer 22N of the seventh embodiment shown in FIG. 16a and
described above employed pinhole clusters 87c, 89c provided with a
plurality of pinholes 87o, 89o in prescribed microlocations.
However, a pinhole row plate 97 may be used, wherein plate 97
includes a plurality of a pinhole rows 97R wherein a plurality of
pinholes 97o are arranged unidimensionally in a prescribed
direction, as shown in FIG. 17a. In this case, first pinhole row
plate 97 is provided with a plurality of rows 97R of pinholes 97o
arrayed in matrix-like fashion so as to correspond to a plurality
of measurement points in object plane OP or image plane IP of
optical system 37. Although FIG. 17a shows a pinhole row 97R having
only four pinholes 97o, an actual pinhole row 97R comprises 100 or
more pinholes 97o. Pinholes 97o are of a size smaller than the Airy
disk 0.6 .lambda./NA, where .lambda. is the wavelength of
quasimonochromatic light 84 and NA is the numerical aperture on the
incident side of optical system 37 (i.e., on the side thereof at
which first pinhole row plate 97, which here takes the place of
first pinhole cluster plate 87 shown in FIG. 16a, is present).
[0148] Referring back and forth between FIGS. 16a-16c and FIGS.
17a-17b, if first pinhole row plate 97 is used in place of first
pinhole cluster plate 87, then a second dual hole row plate 99
should be used in place of second dual hole cluster plate 89.
Second dual hole row plate 99 has a plurality of pinhole rows 99R,
each of which comprises a plurality of pinholes 99o provided in
one-to-one correspondence with pinholes 97o of which pinhole rows
97R on first pinhole row plate 97 are each comprised. In addition,
plate 99 has a plurality of apertures 99a provided in one-to-one
correspondence with plurality of pinhole rows 97o. Furthermore,
each of the plurality of pinhole rows 99R comprises numerous
pinholes 99o arrayed unidimensionally in a prescribed direction. In
addition, one aperture 99a corresponds to one pinhole row 97R
comprising plurality of pinholes 97o.
[0149] Employment of a pinhole row 97R, 99R thus comprising a
plurality of pinholes 97o, 99o arrayed unidimensionally in a
prescribed direction makes it possible to reduce noise caused by
the intermixing of light among the plurality of pinholes 92o, 94o,
93o, 95o, 96o, 87o, 89o, and measurement accuracy can thereby be
further improved.
[0150] It is also preferable to make the pitch of the plurality of
pinholes arrayed unidimensionally in a prescribed direction be 10
to 25 times the radius of the Airy disk 0.6 .lambda./NA as
determined by the numerical aperture on the first pinhole row plate
97 side of optical system 37. It is further preferable to make it
approximately 16 to 20 times this Airy disk radius.
[0151] Ninth Embodiment
[0152] Referring now to FIGS. 18a and 18b, we describe a ninth
embodiment according to a third aspect of the present invention. It
is possible to use slit-shaped apertures 57s, 59s in place of
pinhole clusters 87c, 89c in interferometer 22N shown in FIG. 16a
and described above. FIGS. 18a and 18b show slit plates 57, 59
provided with pluralities of slit-shaped apertures 57s, 59s.
[0153] In describing the use of first slit plate 57 and second dual
slit plate 59 in place of first pinhole cluster plate 87 and second
dual hole cluster plate 89, to reference is made back and forth
between FIGS. 16a-16c and FIGS. 18a-18b.
[0154] In FIG. 18a, first slit plate 57 is provided with a
plurality of slit-shaped apertures 57s arrayed in matrix-like
fashion so as to correspond to a plurality of measurement points in
object plane OP image plane IP of optical system 37. Furthermore,
the slit shape mentioned in the present embodiment refers to a
shape extending unidimensionally in a prescribed direction, the
overall shape hereof not being limited to rectangular. In addition,
the width in the latitudinal direction of slit-shaped aperture 57s
is of a size well smaller than the size of the Airy disk 0.6
.lambda./NA, where .lambda. is the wavelength of quasimonochromatic
light 27 and NA is the by numerical aperture on the incident side
(on the side of first slit plate 57, which here corresponds to
first pinhole cluster plate 87 in FIG. 16a) of optical system 37.
Upon illumination of a slit-shaped aperture 57s, the wavefront
emitted therefrom will be such that its cross section in the short
direction of the slit-shaped aperture 57s is the same as that of an
ideal spherical wavefront (i.e., this then can be said to represent
a one-dimensional ideal spherical wavefront).
[0155] If first slit plate 57 shown in FIG. 18a is used in place of
first pinhole cluster plate 87 shown in FIG. 16b, then second dual
slit plate 59 shown in FIG. 18b should be used in place of second
dual hole cluster plate 89. Second dual slit plate 59 comprises a
plurality of slit-shaped apertures 59s provided in one-to-one
correspondence with the plurality of slit-shaped apertures 57s on
first slit plate 57, and a plurality of apertures 59a provided in
one-to-one correspondence with the plurality of slit-shaped
apertures 57s on first slit plate 57.
[0156] In the ninth embodiment of the invention, slit plates 57, 59
shown in FIGS. 18a and 18b are incorporated in
wavefront-aberration-measuring interferometer 22N of the seventh
embodiment shown in FIG. 16a. Operation in this case is as
follows.
[0157] First, one of the plurality of slit-shaped apertures 57s
first slit plate 57 corresponding to a desired measurement point is
illuminated with light 27 from LPX source 21. The wave emitted from
the illuminated slit-shaped aperture 57s is such that a
one-dimensional ideal spherical wavefront is generated in the short
direction of slit-shaped aperture 57s. This one-dimensional ideal
spherical wavefront passes through optical system 37 and is
diffracted by diffraction grating 62. Zeroeth-order peak P0 of the
diffraction pattern arrives at the corresponding slit-shaped
aperture 59s on second dual slit plate 59, and first-order peak P1
of the diffraction pattern arrives at aperture 59a on second dual
slit plate 59.
[0158] Furthermore, a one-dimensional ideal spherical wavefront is
generated in the short direction of the corresponding slit-shaped
aperture 59s on second dual slit plate 59, and a wavefront
corresponding in shape to the wavefront aberration of optical
system 37 passes through aperture 59a. The wavefront of the
one-dimensional ideal spherical wavefront and the wavefront from
the aperture 59a mutually interfere and form interference fringes
on detector 60. The wavefront aberration of optical system 37 can
be measured by analyzing these interference fringes in computer CU.
Furthermore, it is possible in this ninth embodiment that
measurement accuracy will lower in a direction parallel to the long
direction of slits 57s, 59s. If this should be the case, all that
need be done to rectify this is to arrange slit plates 57, 59 and
optical system 37 such that they are rotatable relative to one
another, or to provide a plurality of slit-shaped apertures 57s,
59s having long directions in mutually different orientations in
place of the slit-shaped apertures 57s, 59s shown in FIGS. 18a and
18b.
[0159] Thus, by using slit-shaped apertures 57s, 59s, it is
possible to further increase light flux as compared with cases
wherein pinhole plates having a single pinhole, or a pinhole
cluster or a pinhole row comprising a plurality of pinholes, are
used. This constitution corresponds to a shearing
interferometer.
[0160] Also, while second dual slit plate 59 makes use of two peaks
of the diffraction pattern produced by diffraction grating 62
(zeroeth-order peak P0 and first-order peak P1), the present
invention is not limited to employment of two such peaks or of
employment of combinations of the zeroeth-order and first-order
peaks thereof.
[0161] Tenth Embodiment
[0162] Referring to FIG. 19, we describe a sixth
wavefront-aberration-meas- uring interferometer 22P of a tenth
embodiment according to a third aspect of the present
invention.
[0163] Interferometer 22P is a variation on the above-discussed
interferometers 22M, 22N in the sixth embodiment shown in FIGS.
15a-16c. An LPX source 21 is used in interferometer 22P of the
tenth embodiment in place of the SOR undulator light source (not
shown) that was used in interferometers 22M, 22N of the sixth
embodiment.
[0164] Referring to FIG. 19, in LPX source 21, laser light source
23 supplies pulsed laser light of a wavelength in the range from
the infrared region to the visible light region. Laser light source
23 may be, for example, a YAG laser excited by a semiconductor
laser, an excimer laser, or the like. This laser light is condensed
by condenser optical system 29 onto target 25. Target 25 receives
the high-intensity laser light, rises in temperature and is excited
to the plasma state, and emits X-rays 27 during transitions to a
lower potential state. By passing X-rays 27 through a spectroscope
(not shown), quasimonochromatic light 27 only of wavelength 13 nm
is extracted, which is then acted on by condenser mirror 64 and
irradiates a pinhole plate 31.
[0165] Pinhole plate 31 has a single aperture much larger (i.e.,
ten or more times) than the diameter of the Airy disk 0.6
.lambda./NA, where .lambda. is the wavelength of quasimonochromatic
light 27 and NA is the numerical aperture on the incident side
(pinhole plate 31 side) of optical system 37. Here, so long as
aperture 31o of pinhole plate 31 can be illuminated such that there
is uniform illuminance within object plane OP of optical system 37
and such that there is uniform illuminance within the cross section
of the light beam incident pinhole plate 31, there is no need to
make the size of the aperture of pinhole plate 31 smaller than the
Airy disk, as is the case for the above-described embodiments.
[0166] In interferometer 22P, illumination is such that there is
uniform illuminance within object plane OP and within the cross
section of the light beam incident pinhole plate 31. Accordingly,
the pinhole plate 31 which is used can have a large aperture 310
such as has been described.
[0167] As in the case in the above-described embodiments, in
interferometer 22P, light exiting from aperture 31o of pinhole
plate 31 can be regarded as having an ideal spherical
wavefront.
[0168] As in the case in interferometer 22M, in interferometer 22P,
second Hartmann plate 96 (see FIG. 15b) having a plurality of
apertures 96o is arranged between image plane IP of optical system
37 (i.e., a location made conjugate to pinhole plate 31 by optical
system 37) and optical system 37.
[0169] With continuing reference to FIG. 19, the light beam from
aperture 31o of pinhole plate 31, upon exiting from optical system
37, forms, due to the action of the plurality of apertures 96o of
second Hartmann plate 96, a plurality of ray groups RG that are the
same in number as the number of apertures 96o. Ray groups RG then
proceed to image plane IP of optical system 37. Ray groups RG
converge at image plane IP and arrive at detector 60 in a divergent
state. If the plane of the pupil (not shown) of optical system 37
is subdivided into a plurality of sections, ray groups RG that pass
through the plurality of apertures 96o on second Hartmann plate 96
respectively correspond to rays passing through each such section.
As a result, the lateral aberration of optical system 37 can be
determined if the position at which each of the ray groups RG
arrives at detector 60 is detected. The wavefront aberration of
optical system 37 can then be determined from this lateral
aberration using computer CU, as describe above.
[0170] Eleventh Embodiment
[0171] Referring now to FIGS. 20a and 20b, a seventh
wavefront-aberration-measuring interferometer 22Q in an eleventh
embodiment according to a third aspect of the present invention is
described.
[0172] Although a light source 21 supplying light in the soft X-ray
wavelength region was used as light source in the above-described
interferometers 22N-22P in the seventh through tenth embodiments,
it may be convenient to use an ordinary laser light source 41 (see
FIG. 20a), not an X-ray source 21 (see FIGS. 16a and 19), when
assembling and adjusting optical system 37 at an ordinary
factory.
[0173] FIG. 20a shows wavefront-aberration-measuring interferometer
22Q of the tenth embodiment which uses a non-X-ray laser light
source 41. FIGS. 20a-23 are intended to assist in explaining the
principle of the eleventh embodiment.
[0174] Referring to FIG. 20a, in interferometer 22Q, laser light
source 41 supplies laser light of a prescribed wavelength. This
laser light is split by a beam splitter 74 adjacent light source
41. One of the beams b1 so split travels by way of two folding
mirrors 35a and 35b to a condenser lens 39, and is guided to first
pinhole plate 86 having a single pinhole 86o. First pinhole plate
86 is arranged at the location of image plane IP of optical system
37. Pinhole 86o is of a size smaller than the diameter of the Airy
disk 0.6 .lambda./NA, where .lambda. is the wavelength of the laser
light and NA is the numerical aperture NA on the incident side
(first pinhole plate 86 side) of optical system 37. Accordingly, a
first ideal spherical wavefront is generated from pinhole 86o of
first pinhole plate 86.
[0175] The first ideal spherical wavefront from first pinhole plate
86 passes through optical system 37 and is guided to second pinhole
mirror plate 33 arranged at a position conjugate to first pinhole
plate 86 by optical system 37.
[0176] Referring to FIG. 20b, second pinhole mirror plate 33
comprises an optically transparent substrate 33S, reflective
surface 33R provided on substrate 33S, and aperture 33o, which is a
region wherein reflective surface 33R is not provided. Furthermore,
aperture 33o of second pinhole mirror plate 33 is of a size smaller
than the diameter of the Airy disk 0.6 .lambda./NA, where .lambda.
is the wavelength of the laser light and NA is the numerical
aperture on the exit side (second pinhole mirror plate 33 side) of
optical system 37.
[0177] Returning again to FIG. 20a, light beam b2 produced by
splitting at beam splitter 74 travels by way of a folding mirror
35c to pass through a condenser lens 49, and is then guided in a
condensed state to the rear side of second pinhole mirror plate 33R
(i.e., the back thereof, if the side on which reflective surface
33R is applied is taken as the front thereof, which is arranged in
object plane OP of optical system 37.
[0178] Accordingly, a second ideal spherical wavefront will be
generated at second pinhole mirror plate 33 when light beam b2 from
the rear side of second pinhole mirror plate 33 passes through
aperture 33o. In addition, the light beam that passes through
optical system 37 is reflected by reflective surface 33R of second
pinhole mirror plate 33. This reflected light has a wavefront
corresponding in shape to the wavefront aberration of optical
system 37.
[0179] The second ideal spherical wavefront from aperture 33o of
second pinhole mirror plate 33 and the reflected light from
reflective surface 33R of second pinhole mirror plate 33 arrive at
detector 60 by way of lens 47, and form interference fringes on
detector 60.
[0180] The interference fringes on detector 60 form a shape
corresponding to the deviation from an ideal spherical wavefront of
the wavefront that passes through optical system 37. The wavefront
aberration of optical system 37 can be determined by analyzing
these interference fringes using computer CU, as described
above.
[0181] In FIGS. 20a and 20b, which illustrate the principle of the
wavefront-aberration-measuring interferometer 22Q of the eleventh
embodiment, one prescribed point in object plane OP (or image plane
IP) of optical system 37 is used as the measurement point. If a
plurality of measurement points are to be measured, then, referring
briefly to FIG. 21a, first pinhole array plate 61 wherein a
plurality of pinholes 61o are arranged in a prescribed array may be
used in place of first pinhole plate 86 of FIG. 20a. In addition, a
second pinhole mirror array plate 63 having a plurality of pinholes
63o and a reflective interstitial surface 63R may be used in place
of second pinhole mirror plate 33 shown in FIGS. 16a and 16b.
[0182] Referring now to FIG. 22, an eighth
wavefront-aberration-measuring interferometer 22R, which is a
variation on wavefront-aberration-measurin- g interferometer 22Q of
the eleventh embodiment wherein the wavefront aberration of optical
system 37 can be measured at a plurality of measurement points, is
described. In FIG. 22, elements similar in function to elements
appearing in FIG. 20a have been given the same reference numerals
as in FIG. 20a and description thereof will be omitted here for the
sake of convenience.
[0183] Referring to FIG. 22 and interferometer 22R, laser light of
a prescribed wavelength from laser light source 41 is split by beam
splitter 74. One of the light beams b1 so split sequentially
travels by way of folding mirror 35a to condenser lens 39 provided
on condenser lens stage 66 capable of movement parallel to the
image plane of optical system 37, thereafter arriving at first
pinhole array plate 61.
[0184] Referring back to FIG. 21a, first pinhole array plate 61 has
a plurality of pinholes 61o arrayed in a matrix. The positions of
the plurality of pinholes 61o correspond to the positions of
measurement points for optical system 37. Furthermore, each of the
plurality of pinholes 61o is of a size smaller than the diameter of
the Airy disk 0.6 .lambda./NA, where .lambda. is the wavelength of
the laser light and the NA is the numerical aperture on the
incident side (first pinhole array plate 61 side) of optical system
37. Accordingly, upon being illuminated, pinhole 61o on first
pinhole array plate 61 will generate an ideal spherical
wavefront.
[0185] Returning again to FIG. 22, as a result of moving condenser
lens stage 66, a desired pinhole 61o on first pinhole array plate
61 is selectively illuminated. Furthermore, the position at which
the laser light is incident folding mirror 35a mounted on condenser
lens stage 66 changes as condenser lens stage 66 is moved. In
addition, instead of one of pinholes 61o, a plurality of pinholes
61o may also be collectively illuminated.
[0186] With continuing reference to FIG. 22, the ideal spherical
wavefront from first pinhole array plate 61 passes through optical
system 37, and is then guided to second pinhole mirror array plate
63, located at a position conjugate to first pinhole array plate 61
by optical system 37.
[0187] Referring briefly again to FIG. 21b, second pinhole mirror
array plate 63 is provided with reflective interstitial surface 63R
arranged such that plurality of pinholes 630 form a matrix, no such
reflective interstitial surface 63R being provided at the locations
of pinholes 63o. Furthermore, each of the plurality of pinholes 63o
of second pinhole mirror array plate 63 is of a size smaller than
the diameter of the Airy disk 0.6 .lambda./NA, where .lambda. is
the wavelength of the laser light and NA is the numerical aperture
on the exit side (second pinhole mirror array plate 63 side) of
optical system 37.
[0188] Returning now to FIG. 22, light beam b2 produced by
splitting at beam splitter 74 sequentially travels by way of
oscillatory folding mirror 45 electrically connected to mirror
oscillating unit MU, and then by way of folding mirror 35 to a
condenser lens 49, and is then guided in a condensed state to the
rear side of second pinhole mirror array plate 63 (i.e., the side
opposite from the side at which reflective interstitial surface 63R
is present), which is arranged in object plane OP of optical system
37.
[0189] Accordingly, an ideal spherical wavefront is generated at
second pinhole mirror array plate 63 when light beam b2 from the
rear side of second pinhole mirror array plate 63 passes through
pinhole 63o. In addition, when the light beam that passes through
optical system 37 is reflected by reflective interstitial surface
63R of second pinhole mirror array plate 63, the reflected light
will have a wavefront corresponding in shape to the wavefront
aberration of optical system 37.
[0190] The ideal spherical wavefront from pinhole 63o of second
pinhole mirror array plate 63 and the light reflected by reflective
interstitial surface 63R of second pinhole mirror array plate 63
arrive at detector 60 by way of another folding mirror 35d and lens
47, and form interference fringes on detector 60.
[0191] The interference fringes on detector 60 form a shape
corresponding to the deviation from an ideal spherical wavefront of
the wavefront that passes through optical system 37. The wavefront
aberration of optical system 37 can be determined by analyzing
these interference fringes using computer CU, as discussed
above.
[0192] In interferometer 22R as a variation on the eleventh
embodiment shown in FIG. 22, detector 60, along with the optical
system which guides the light from second pinhole mirror array
plate 63 to detector 60, and condenser lens 49 are mounted on
Detector stage 68, which is capable of movement parallel to object
plane OP of optical system 37. Detector stage 68 is constituted so
that it is linked and moves with condenser lens stage 66 discussed
above, and only pinhole 63o, corresponding to the illuminated
pinhole 61o, can be seen from detector 60. Accordingly,
interference fringes are formed on detector 60 due to interference
between the light that passes through optical system 37 from
illuminated pinhole 61o and the diffracted light from pinhole 63o
on second pinhole mirror array plate 63 corresponding to the
illuminated pinhole 61o. Accordingly, the wavefront aberration at
the measurement point where the illuminated pinhole 61o is
positioned can be determined by analyzing these interference
fringes.
[0193] Stable measurement can also be performed with interferometer
22R in this variation on the eleventh embodiment shown in FIG. 22,
without being affected by vibrations caused by the movement of
stages 66, 68 during measurement.
[0194] With continuing reference to FIG. 22, first pinhole array
plate 61 is mounted on a vertical stage 67, which is capable of
causing first pinhole array plate 61 to move in jogged (i.e.,
incremental) fashion in a direction parallel to the optical axis of
optical system 37. Vertical stage 67 is secured to the same frame
that supports optical system 37. In addition, second pinhole mirror
array plate 63 is mounted on an XY stage 69, which is capable of
causing second pinhole mirror array plate 63 to move in jogged
fashion within object plane OP of optical system 37. XY stage 69 is
attached to the abovementioned frame by way of a piezoelectric
element. Furthermore, adjustment of focus can be performed by using
vertical stage 67 to move first pinhole array plate 61. If there is
distortion in optical system 37, XY stage 69 can be used to align
the position of pinhole 63o.
[0195] Furthermore, a length measuring interferometer or other such
microdisplacement sensor is preferably provided on XY stage 69,
permitting distortion in optical system 37 to be measured based on
the output from the microdisplacement sensor. Furthermore, in the
present embodiment, the positions of the plurality of pinholes 61o
of first pinhole array plate 61 and the plurality of pinholes 63o
of second pinhole mirror array plate 63 are accurately measured
beforehand using a coordinate measuring interferometer.
[0196] In addition, oscillatory folding mirror 45 in interferometer
22R in this variation on the eleventh embodiment shown in FIG. 22
is constituted so as to permit oscillation via mirror oscillation
unit MU, the difference in lengths of the optical paths of the two
beams produced by beam splitter 74 changing in accordance with this
oscillation. As a result, a fringe scan can be executed for
high-precision measurement.
COMPARATIVE EXAMPLE
[0197] Referring to FIG. 23, wavefront-aberration-measuring
interferometer 22S is a comparative example for illustrating the
advantage of interferometers 22Q and 22R of the eleventh
embodiment. Interferometer 22S of the comparative example shown in
FIG. 23 employs an ultraviolet laser 41 instead of the SOR
undulator light source employed in interferometer 22J shown in FIG.
10a. As previously mentioned, measurement accuracy increases as the
wavelength of the light source of the
wavefront-aberration-measuring interferometer is shortened. Since
the wavelength of an ultraviolet laser 41 is approximately 20 times
longer than the working wavelength of optical system 37, the
accuracy of interferometer 22S of the comparative example can be
expected to be 20 times worse than that of interferometer 22J shown
in FIG. 10a.
[0198] However, in interferometers 22Q and 22R of FIGS. 20a and
A22, the reference wavefront is made to travel along an optical
path separate from the measurement wavefront. Thus, measurement can
be performed with a precision higher than is possible with
interferometer 22S of the comparative example shown in FIG. 23.
Thus, in interferometers 22Q and 22R of the eleventh embodiment,
wavefront aberration can be measured with high precision without
the need to use an X-ray source.
[0199] Method of Calibrating Aspheric-Shape-Measuring
Interferometer
[0200] FIG. 24 is a flowchart for assisting in describing a method
for calibrating an aspheric-surface-shape measuring interferometer
of the type shown in FIGS. 1-7. In the course of this calibration,
a wavefront-aberration-measuring interferometer of the type shown
in FIGS. 10a-22 is used to verify the aspheric shape obtained using
the aspheric-surface-shape measuring interferometer. This method or
variations thereof can be applied to any of these interferometers
for the sake of convenience, however, we take the example of
calibration of aspheric-surface-shape measuring interferometer 22H
of the fourth embodiment shown in FIG. 7 using
wavefront-aberration-measuring interferometer 22J of the fifth
embodiment shown in FIG. 10a.
[0201] Before executing step S1 in FIG. 24, the aspheric surface
under test 38 is first machined to a surface accuracy of
approximately 10 nm RMS using well-known technology.
[0202] At step S1 in FIG. 24, the surface shape of the
abovementioned aspheric test surface 38 is measured using
interferometer 22H of the fourth embodiment of the present
invention shown in FIG. 7. Furthermore, interferometer 22H of the
fourth embodiment may also be used starting from the time when the
aspheric surface is first machined. When performing measurements
using interferometer 22H, it is preferable to minimize asymmetric
systematic errors (errors in reference surface 70) by collecting
data at stepped angular rotations obtained by either rotating test
surface 38 about the optical axis with respect to reference surface
70 in stepwise fashion or rotating reference surface 70 about the
optical axis with respect to test surface 38 in stepwise fashion,
and averaging the data obtained.
[0203] At step S2, using the measurement data from step S1,
corrective grinding is performed on the aspheric surface 38 so as
to make the shape of aspheric test surface 38 conform to the design
data. FIG. 25 shows a small tool grinding apparatus 400 for
performing this corrective grinding. Referring to FIG. 25, small
tool grinding apparatus 400 has grinding head 406 provided with a
polisher 410 that rotates, and coil spring 414 that applies a
prescribed pressure to polisher 414. Aspheric test surface 38 is
ground as a result of application of a constant load in a direction
normal to aspheric test surface 38 as optical test element 36 is
rotated. The amount of grinding is proportional to the dwell time
of polisher 410 (i.e., the time that polisher 410 remains at a
given position and grinds). Furthermore, the shape of test surface
38 is measured using interferometer 22H shown in FIG. 7, just as
was performed at step S1. If the result of measurement is that the
measured aspheric shape differs from the design shape, the shape of
test surface 38 is again corrected using small tool grinding
apparatus 82. By repeating this measurement and correction process,
the measured aspheric shape and the design aspheric shape can be
made to coincide.
[0204] At step S3, optical element 36 having test surface 38 shaped
as a result of the operations at step S2 is assembled in the
optical system 37 of which it is an optical component.
[0205] At step S4, the wavefront aberration of the optical system
37 assembled in step S3 is measured. In connection with the
measurement of this wavefront aberration, a PDI (point diffraction
interferometer) employing an SOR (synchrotron orbital radiation)
undulator light source, such as in interferometer 22J shown in FIG.
10a, is used. Since the measurement wavelength of interferometer
22J is short, at about 13 nm, the wavefront aberration of the
optical system can be measured with high precision, specifically to
0.13 nm RMS or better. The constitutions of exemplary
interferometers which may be applied here are described under the
fifth through eleventh embodiments of the present invention shown
in FIGS. 10a-22.
[0206] At step S5, the causes of error in the wavefront aberration
measured at step S4 is broken down into an alignment error
component (for each aspheric surface) and a shape error component
for each surface.
[0207] Specifically, a computer uses, for example, known optical
system automatic correction software, assigns the position of test
surface 38 (spacing, inclination and shift) and the shape of test
surface 38 as variables, initializes the measurement values of the
wavefront aberration, and performs optimization so that the
wavefront aberration approaches zero. The difference between the
position and shape of test surface 38 when optimized and the
position and shape of test surface 38 prior to optimization
corresponds to the alignment error (positional error) and shape
error, respectively.
[0208] At step S6, the alignment error calculated at step S5 is
evaluated to determine whether it is sufficiently small. If it is
not small enough, the flow operation proceeds to step S7 where the
alignment error is adjusted. If it is small enough, the flow
proceeds to step S8.
[0209] At step S7, alignment of optical element 36 in optical
system 37 is adjusted based on the alignment error calculated at
step S5, following which flow returns to step S4.
[0210] Note that the sequence of operations between steps S4 and S7
are repeated until the alignment error calculated at step S5 is
sufficiently small.
[0211] At step S8, the difference between the shape error (shape
error isolated by the most recent iteration of step S5) in the
final wavefront aberration (wavefront aberration as determined by
the most recent iteration of step S4) and the final measured
aspheric surface shape data calculated in step S2 is calculated.
This difference corresponds to the systematic error of
aspheric-surface-shape-measuring interferometer 22H. This error
corresponds to the shape error of reference surface (Fizeau
surface) 70 in the aspheric-surface-shape-measuring (Fizeau-type)
interferometer 22H.
[0212] At step S9, the final aspheric surface shape data measured
at step S2 is corrected by the amount of the systematic error
calculated at step S8, and test surface 38 is reworked using small
tool grinding apparatus 400 based on this corrected aspheric
surface shape data. At this time, optical element 36 having test
surface 38 is removed from optical system 37 of which it is a part
before corrective grinding operations can be carried out.
[0213] After steps S1 through S9 have been executed, optical system
37 is reassembled and the wavefront aberration is measured using
interferometer 22J shown in FIG. 10a. The measured values are again
separated into an alignment error component and a shape error
component for each surface, and the surface error is verified to
determine whether it is smaller than previously measured.
[0214] By numerous repetitions of the series of procedures
including machining of aspheric test surface 38, assembly in
optical system 37, measuring of wavefront aberration, and
determining the systematic error in
aspheric-surface-shape-measuring interferometer 22H as described
above, systematic errors in aspheric-surface-shape-measuring
interferometer 22H can be identified. Furthermore, if such errors
are large (e.g., 2 nm RMS or greater),
aspheric-surface-shape-measuring interferometer 22H must itself be
corrected (i.e., the surface shape of aspheric reference surface 70
must be corrected).
[0215] If the measurement values during subsequent measurements and
machining are continuously corrected by the amount of the
systematic error in aspheric-surface-shape-measuring interferometer
22H as calculated by this procedure and this then used as data
during operations using the corrective grinding apparatus 400, an
aspheric surface 38 can be machined with good accuracy.
[0216] Since measurement accuracy, and in particular
reproducibility, with aspheric-surface-shape-measuring
interferometer 22H of the fourth embodiment are excellent, the
above-described calibration method is extremely effective.
[0217] Furthermore, should existence of systematic errors be
confirmed thereafter as a result of wavefront aberration
measurement based at the exposure wavelength or other such
measurements performed during a production run, systematic error
may be corrected at each such occasion so as to constantly approach
design values.
[0218] In addition, after machining the aspheric surface 38 using
the machining and measurement procedures based on the present
invention, optical system 37 is assembled and a reflective film
(not shown) must be applied to each surface 38 to be made
reflective prior to measurement of the wavefront aberration. The
shape of surface 38 may change under the influence of stress from
the film when applying and removing (e.g., to perform corrective
grinding) the reflective film. Although the reproducibility of this
change should be less than 0.1 nm RMS, this is not attainable.
Nevertheless, the majority of the surface change is second- and
fourth-order components (power components and third-order spherical
aberration components), and the higher-order components are small.
Second-order and fourth-order surface change components can be
compensated to a certain degree by adjusting the surface spacing.
In other words, it is sufficient to ensure that the reproducibility
of the surface changes associated only with higher-order components
are held to 0.1 nm RMS or smaller. This can be accomplished by
sufficient reduction of the stress from the film.
[0219] As described above, the present invention provides an
aspheric-surface-shape measuring interferometer displaying good
reproducibility, and moreover makes it possible to measure
wavefront aberration with high precision. In addition, the present
invention permits improvement in the absolute accuracy of precision
surface measurements in an aspheric-surface-shape measuring
interferometer. In addition, the present invention permits
manufacture of a projection optical system having excellent
performance.
[0220] Adoption of the present invention also makes it possible to
accurately verify the shape of a null wavefront, as well as the
transmission characteristics of such a null wavefront, without the
need to use a reflective standard. Moreover, adoption of an
interferometer system according to the present invention makes it
possible to calibrate an aspheric null element with high precision
and in a short period of time.
[0221] Furthermore, the wavefront-aberration-measuring
interferometers of the fifth through eleventh embodiments of the
present invention discussed above can be assembled as part of an
exposure apparatus. In particular, when an SOR undulator of a
wavelength which may be used for exposure is used as light source
in a wavefront-aberration-measuring interferometers, as was the
case in the fifth and sixth embodiments, this will be favorable
since the light source unit can also serve as the exposure light
source. When a laser plasma X-ray source of a wavelength which may
be used for exposure in a wavefront-aberration-measuring
interferometers, as was the case in the seventh through tenth
embodiments, this will be favorable since the light source unit can
also serve as the exposure light source. In addition, the
wavefront-aberration-measuring interferometers of the eleventh
embodiment of the present invention requires a laser light source
to be furnished separate from the exposure light source. However,
this laser light source can also serve as light source for an
alignment system or as light source for an autofocus system in the
exposure apparatus. In addition, in the wavefront-aberration-measu-
ring interferometers of the fifth through eleventh embodiments of
the present invention, when this light source is shared by the
exposure apparatus, detector 60 serving as detector may also be
fashioned such that it is removable from the exposure apparatus. In
this case, the wavefront aberration of projection optical system 37
can be measured by attaching such a removable unit to the exposure
apparatus in the event that maintenance or the like is required.
Consequently, there will be no need to provide a dedicated
wavefront-aberration-measuring interferometer for each and every
exposure apparatus, permitting reduction in the cost of the
exposure apparatus.
[0222] In addition, while detector 60 has been adopted as detector
in the fifth through tenth embodiments of the present invention
discussed above, a member having a function that converts emitted
light in the soft X-ray region to visible light (for example, a
fluorescent plate) may be provided at the position of the detector
60 and used in place thereof, and the visible light from this
member may be detected by a detector such as a CCD.
[0223] Furthermore, the embodiments of the present invention
discussed above describe a manufacturing method of a projection
optical system 37 in the context of an exposure apparatus that uses
soft X-rays of wavelength around 10 nm as exposure light,
wavefront-aberration-measuring interferometers ideally suited to
the measurement of the wavefront aberration of this projection
optical system 37, surface-shape-measuring interferometers ideally
suited to measurement of the surface shape of a reflective surface
in this projection optical system 37, and a calibration method for
such an interferometer. However, the present invention is not
limited to this soft X-ray wavelength. For example, the present
invention can be applied to a projection optical system or
wavefront-aberration-measuring interferometer for hard X-rays of
wavelength shorter than soft X-rays, and to a
surface-shape-measuring interferometer that measures the surface
shape of an optical element of a hard X-ray projection optical
system, and can also be applied to the vacuum ultraviolet region
(100 to 200 nm) of wavelength longer than soft X-rays. Furthermore,
measurement and manufacturing of a precision much greater than
hitherto possible becomes possible if the present invention is
applied to a vacuum-ultraviolet projection optical system or
wavefront-aberration-measuring interferometer, or to surface shape
measurement of an optical element in a vacuum-ultraviolet
projection optical system.
[0224] Thus, the present invention is not to be limited by the
specific modes for carrying out the invention described above. In
particular, while the present invention has been described in terms
of several aspects, embodiments, modes, and so forth, the present
invention is not limited thereto. In fact, as will be apparent to
one skilled in the art, the present invention can be applied in any
number of combinations and variations without departing from the
spirit and scope of the invention as set forth in the appended
claims, and it is intended to cover all alternatives, modifications
and equivalents as may be included within the spirit and scope of
the invention as defined in the appended claims.
* * * * *