U.S. patent number RE36,799 [Application Number 08/967,345] was granted by the patent office on 2000-08-01 for projection optical apparatus using plural wavelengths of light.
This patent grant is currently assigned to Nikon Corporation. Invention is credited to Kenji Nishi.
United States Patent |
RE36,799 |
Nishi |
August 1, 2000 |
Projection optical apparatus using plural wavelengths of light
Abstract
A projection optical apparatus comprising a projection optical
system for projectively focusing a pattern image of a mask under
illumination by light of first wavelength onto a sensitive
substrate, a stage holding the sensitive substrate, a fiducial
plate disposed on the stage, a first mark detector for illuminating
light of second wavelength different from the first wavelength,
through a first mark area formed on the mask and the projection
optical system, onto .[.the sensitive substrate or.]. a second mark
area formed on the fiducial plate, then detecting optical
information produced from the second mark area, a fourth mark area
formed on .[.the sensitive substrate or.]. the fiducial plate and
arranged in a predetermined positional relationship relative to the
second mark area, a third mark area formed on the mask and arranged
in a predetermined positional relationship relative to the first
mark area, a second mark detector for illuminating the light of
first wavelength onto the fourth mark area through the third mark
area and the projection optical system, and then detecting optical
information produced from the fourth mark area, under a condition
that the first mark detector is detecting the optical information
produced from the second mark area, and an error detector for
detecting detection errors due to a distortion at respective
positions in the view field of the projection optical system where
the first mark area and the second mark area are present, based on
the detected results by the first mark detector and the second mark
detector.
Inventors: |
Nishi; Kenji (Kawasaki,
JP) |
Assignee: |
Nikon Corporation (Tokyo,
JP)
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Family
ID: |
27473281 |
Appl.
No.: |
08/967,345 |
Filed: |
October 27, 1997 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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571631 |
Dec 13, 1995 |
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288767 |
Aug 11, 1994 |
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Reissue of: |
711625 |
Jun 6, 1991 |
05138176 |
Aug 11, 1992 |
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Foreign Application Priority Data
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Jun 13, 1990 [JP] |
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2-154287 |
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Current U.S.
Class: |
250/548;
356/401 |
Current CPC
Class: |
G03F
7/706 (20130101); G03F 9/7049 (20130101); G03F
7/70633 (20130101) |
Current International
Class: |
G03F
7/20 (20060101); G03F 9/00 (20060101); G01B
011/00 () |
Field of
Search: |
;250/548,557,237R
;356/399,400,401 ;355/53 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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63-283129 |
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Nov 1988 |
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JP |
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4-321214 |
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Nov 1992 |
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JP |
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Primary Examiner: Allen; Stephone
Attorney, Agent or Firm: Vorys, Sater, Semyour and Pease
LLP
Parent Case Text
.Iadd.This is a continuation of reissue application Ser. No.
08/571,631 filed Dec. 13, 1995, which is a continuation of reissue
application Ser. No. 08/288,767 filed Aug. 11, 1994, both now
abandoned..Iaddend.
Claims
I claim:
1. A projection optical apparatus comprising:
a projection optical system for projectively focusing a pattern
image of a mask under illumination by a light of first wavelength
characteristic onto a sensitive substrate,
a stage for holding said sensitive substrate,
a fiducial plate disposed on said stage,
first mark detection means for illuminating a light of second
wavelength characteristic different from said first wavelength
characteristic, through a first mark area formed on said mask and
said projection optical system, onto .[.said sensitive substrate
or.]. a second mark area formed on said fiducial plate, and then
detecting optical information produced from said second mark
area,
a fourth mark area formed on .[.said sensitive substrate or.]. said
fiducial plate and arranged in a predetermined positional
relationship relative to said second mark area,
a third mark area formed on said mask and arranged in a
predetermined positional relationship relative to said first mark
area,
second mark detection means for illuminating said light of first
wavelength characteristic onto said fourth mark area through said
third mark area and said projection optical system, and then
detecting optical information produced from said fourth mark area,
under a condition that said first mark detection means is detecting
the optical information produced from said second mark area,
and
error detection means for detecting detection errors due to a
distortion at respective positions in the view field of said
projection optical system where said first mark area and said
second mark area are present, based on the detected results by said
first mark detection means and said second mark detection
means.
2. A projection optical apparatus according to claim 1,
wherein:
said first mark detection means has a first object lens system
movable depending on position change of said first mark area on
said mask, and
.[.the.]. .Iadd.a .Iaddend.uniform mark pattern is formed in said
second mark area over a movable range of said first object lens
system.
3. A projection optical apparatus comprising:
a projection optical system disposed between first and second
planes such that said first and second planes are optically
conjugate to each other under a first wavelength
characteristic,
alignment mark means including first and third mark means disposed
on said first plane with a predetermined positional relationship
therebetween, and second and fourth mark means disposed on said
second plane with a predetermined positional relationship
therebetween;
first mark detection means for illuminating a light of second
wavelength characteristic different from said first wavelength
characteristic onto said second mark means through said first mark
means and said projection optical system, and then detecting first
optical information produced from said second mark means,
second mark detection means for illuminating said light of first
wavelength characteristic onto said fourth mark means through said
third mark means and said projection optical system, and then
detecting second optical information produced from said fourth mark
means, and
means for creating information of alignment errors developed
between said first and second planes in the directions enough to
define a plane, based on said first and second optical information.
.Iadd.
4. A projection exposure apparatus for exposing a pattern area of a
mask irradiated with first illumination light onto a substrate
through a projection optical system, the apparatus comprising:
(a) a movable stare supporting the substrate and moving in a
predetermined plane perpendicular to an optical axis of said
projection optical system;
(b) a mask holder supporting the mask which has a mask mark formed
at a peripheral position of the pattern area;
(c) a fiducial plate mounted on said movable stage and having first
and second fiducial marks on its top face, the first fiducial mark
being aligned through said projection optical system with the mask
mark when said movable stage is located at a predetermined
reference position with respect to the image field of said
projection optical system;
(d) a first alignment system irradiating the mask mark and the
first fiducial mark with the first illumination light and receiving
a light from the mask mark and a light from the first fiducial mark
through said projection optical system, to detect a positional
relationship between the mask mark and the first fiducial mark,
when said movable stage is; located near the reference position;
and
(e) a second alignment system including a predetermined detecting
reference, irradiating the second fiducial mark with second
illumination light which has different wavelength from the first
illumination light and receiving a light from the second fiducial
mark through said projection optical system, to detect a positional
relationship between the detecting reference and the second
fiducial mark, when said movable stage is located near the
reference position..Iaddend..Iadd.5. A projection exposure
apparatus for exposing a pattern area of a mask irradiated with
first illumination light onto a substrate through a projection
optical system, the apparatus comprising:
(a) a movable stage supporting the substrate and moving in a plane
perpendicular to an optical axis of said projection optical
system;
(b) a fiducial plate fixedly mounted on a portion of said movable
stage and having first and second fiducial patterns on its surface,
said first and second fiducial patterns being arranged with
predetermined positional relationship;
(c) a first alignment optical system detecting a light from said
first fiducial pattern through said projection optical system and a
peripheral portion of the pattern area of said mask and a light
from a mask mark formed within the peripheral portion of said mask,
when said mask mark and the first fiducial pattern are irradiated
with a light of a same wavelength as the first illumination
light;
(d) a controller driving and positioning said movable stage so that
the first fiducial pattern is substantially aligned with the mask
mark and simultaneously the second fiducial pattern is located at a
predetermined position in the image field of said projection
optical system; and
(e) a second alignment optical system detecting a light from the
second fiducial pattern through said projection optical system,
when the second fiducial pattern is irradiated with a light having
different wavelength than the first illumination light;
wherein said movable stage is stationary until said first and
second alignment optical systems complete the light detecting
operations..Iaddend..Iadd.6. A method for examining a variation of
the base line distance between a center point of a reticle and a
detecting center of an alignment system which detects positional
deviation of a mark formed on a substrate for aligning the
substrate through a projection optical system, the method
comprising the steps of:
(a) providing a fiducial plate within the image field of said
projection optical system instead of the substrate, said fiducial
plate having a first fiducial mark to be aligned with a reticle
mark through said projection optical system and a second fiducial
mark disposed at a predetermined positional relationship with
respect to the first fiducial mark;
(b) positioning said fiducial plate at a reference position so that
the first fiducial mark and the reticle mark are substantially
aligned with each other through said projection optical system;
and
(c) detecting the positional deviation between the detecting center
of said alignment system and the second fiducial mark to determine
the variation
of said base line distance while said fiducial plate is stationary
at the
reference position..Iaddend..Iadd.7. A method for aligning a
substrate and a mask having a circuit pattern, and for exposing a
an image of the circuit pattern onto the substrate through a
projection optical system, the method comprising the steps of:
(a) detecting a mask mark formed on the mask and a first fiducial
mark disposed on a movable stage through said projection optical
system with a first alignment system using a first wavelength of
light and, substantially at the same time, detecting a second
fiducial mark disposed on said movable stage through said
projection optical system with a second alignment system using a
different wavelength of light, when said movable stage is
stationary at a baseline measurement position;
(b) determining a positional relationship between a detecting
reference of said second alignment system and the circuit pattern
of the mask in an image field of said projection optical system
based on detection results obtained in step (a);
(c) detecting an alignment mark formed on the substrate, which is
mounted on said movable stage, with said second alignment
system;
(d) determining a positional relationship between the detecting
reference of paid second alignment system and the alignment mark of
the substrate in the image field of said projection optical system
based on a detection result obtained in step (c); and
(e) positioning said movable stage to alit a shot area of the
substrate and the image of the circuit pattern based on the
positional relationships determined in steps (b) and (d), and
exposing the shot area of the
substrate..Iaddend..Iadd.8. A method according to claim 7, wherein,
in the step (a), said first alignment system generates a first
deviation signal representing a positional relationship of the mask
mark and the first fiducial mark and, substantially at the game
time, said second alignment system generates a second deviation
signal representing a positional relationship of the detecting
reference of said second
alignment system and the second fiducial mark..Iaddend..Iadd.9. A
method according to claim 7, wherein the substrate has first and
second X-direction alignment marks and first and second Y-direction
alignment marks, and said second alignment system includes a first
objective lens for detecting the first X-direction aliment mark, a
second objective lens for detecting the first Y-direction alignment
mark, a third objective lens for detecting the second X-direction
alignment mark, and a fourth objective lens for detecting the
second Y-direction alignment mark..Iaddend..Iadd.10. A method
according to claim 9, wherein the step (c) includes detecting at
least one of said first and second X-direction alignment marks and
at least one of said first and second Y-direction alignment marks,
and the step (d) includes determining positional relationships
between the detecting reference of said alignment system and each
of said one X-direction alignment mark and said one Y-direction
alignment mark..Iaddend..Iadd.11. A method according to claim 7,
wherein the steps (a) and (b) are performed after operation of an
adjustment unit which adjusts a magnification or a distortion
characteristic of said
projection optical system..Iaddend..Iadd.12. A projection exposure
apparatus comprising:
a projection optical system disposed between a mask and a substrate
to project on the substrate an illumination light irradiating the
mask;
a plate having fiducial marking formed thereon and disposed in an
image field of said projection optical system;
a first alignment system irradiating a portion of said fiducial
marking with a first alignment light different from said
illumination light in wavelength to detect optical information
produced from said fiducial marking through said projection optical
system;
a second alignment system irradiating a portion of said fiducial
marking with a second alignment light having substantially the same
wavelength as said illumination light through said projection
optical system to detect a positional relationship between said
mask and said plate; and
an alignment controller connected to said first and second
alignment systems to detect an offset amount of said first
alignment system caused
by said projection optical system..Iaddend..Iadd.13. An apparatus
according to claim 12, wherein
said fiducial marking includes two mark patterns disposed at a
predetermined positional relationship so that said two mark
patterns are detected substantially at the same time by said first
and second alignment systems, respectively..Iaddend..Iadd.14. An
apparatus according to claim 12, wherein
said first alignment system includes a photodetector which
receives, through a mark area on said mask, diffraction light
produced from said fiducial marking and passing through said
projection optical system..Iaddend..Iadd.15. An apparatus according
to claim 12, further comprising:
an adjustment unit connected to said projection optical system to
adjust an optical characteristic of said projection optical
system,
wherein said alignment controller determines the offset amount of
said first alignment system after an adjustment of said optical
characteristic..Iaddend..Iadd.16. A projection exposure apparatus
comprising:
a projection optical system disposed between a mask and a substrate
to project on the substrate an illumination light irradiating the
mask;
a first alignment system irradiating a first fiducial mark with a
first alignment light different from said illumination light in
wavelength to detect optical information produced from said first
fiducial mark through said projection optical system;
a second alignment system irradiating a second fiducial mark with a
second alignment light having substantially the same wavelength as
said illumination light through said projection optical system to
detect a positional relationship between said second fiducial mark
and a mark on said mask; and
a plate disposed on a substrate side with respect to said
projection optical system, said first and second fiducial marks
being formed on said plate at a predetermined positional
relationship so that said first and second alignment systems
respectively detect said first and second
fiducial marks substantially at the same time..Iaddend..Iadd.17. An
apparatus according to claim 16, further comprising:
an alignment controller connected to said first and second
alignment systems to determine a base line amount of said first
alignment system..Iaddend..Iadd.18. A projection exposure method
for protecting an illumination light irradiating a mask through a
projection optical system on a substrate to transfer a pattern of
said mask onto said substrate, comprising the steps of:
disposing a plate formed with a first fiducial mark and a second
fiducial mark in an image field of said projection optical
system;
detecting said first fiducial mark through said projection optical
system by a first alignment system which uses a first beam
different from said illumination light in wavelength;
detecting said second fiducial mark through said projection optical
system by a second alignment system which uses a second beam having
substantially the same wavelength as said illumination light;
and
determining a base line amount of said first alignment system based
on outputs of said first and second alignment
systems..Iaddend..Iadd.19. A method according to claim 18,
wherein
said step of detecting said first fiducial mark by said first
alignment system and said step of detecting said second fiducial
mark by said second alignment system are performed substantially at
the same time..Iaddend..Iadd.20. A method according to claim 19,
wherein
said plate is substantially at rest during said steps of detecting
said first and second fiducial marks..Iaddend..Iadd.21. A method
according to claim 18, wherein
said base line amount of said first alignment system is determined
after an adjustment of an optical characteristic of said projection
optical system or a position of said first alignment
system..Iaddend..Iadd.22. A projection exposure method for
protecting an illumination light irradiating a mask through a
projection optical system on a substrate to transfer a pattern of
said mask on said substrate, comprising the steps of:
disposing a plate formed with fiducial marking in an image field of
said projection optical system;
detecting said fiducial marking through said projection optical
system by a first alignment system which uses a first beam
different from said illumination light in wavelength and by a
second alignment system which uses a second beam having
substantially the same wavelength as said illumination light,
respectively; and
determining a base line amount of said first alignment system based
on outputs of said first and second alignment systems..Iaddend.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a projection exposure apparatus
used in the lithography process for semiconductor devices, liquid
display devices or the like, and more particularly to an alignment
apparatus which can reduce an extent of alignment errors
attributable to distortion caused by a projection system of the
projection exposure apparatus.
2. Related Background Art
Recently, steppers mounting projection lenses of large numerical
aperture thereon have been widely used as apparatus for printing a
pattern of masks (reticles) on semiconductor wafers with resolution
on the order of submicrons.
In such a stepper, a chip pattern (shot area) already formed on the
wafer and a reticle pattern newly exposed in superposed relation
must be aligned with each other at overall precision less than a
fraction of the minimum line width. Of late, therefore, steppers
mounting alignment apparatus (sensors) with an ability of higher
precision thereon have been researched and developed for practical
use.
In the future, it is believed that those steppers which will be
primarily used in producing 4 Mbit D-RAMs and 16 Mbit D-RAMs will
of necessity have a die-by-die exposure mode using the TTR
(Through-The-Reticle) method in which a mark on the reticle and a
mark in each shot area on the wafer are successively detected and
aligned with each other, followed by printing.
While various techniques have so far been proposed to implement the
TTR alignment method, most promising one is a different wavelength
TTR alignment method in which the reticle mark and the wafer mark
are simultaneously detected by using an illumination light
different in wavelength from an exposure light. This type alignment
method is advantageous in that because there will not occur a
phenomenon for a resist layer on the wafer to strongly absorb the
exposure light, the mark
can be stably detected even for such wafers as having a resist
impregnated with dyes or a multi-layered resist, and the resist in
a mark area can be prevented from being exposed or sensitized upon
illumination for alignment. Typical techniques (projection exposure
apparatus) well known as to implement the different wavelength TTR
alignment method are disclosed in U.S. Pat. Nos. 4,251,160,
4,269,505, 4,492,459 or 4,473,293.
In any of those typical projection exposure apparatus, however, an
optical system for correcting chromatic aberration of the
illumination light having the different wavelength for alignment is
disposed between a reticle and a projection lens. Such a correction
optical system serves to maintain the reticle mark and the wafer
mark in focused relation to each other under the illumination light
having the different wavelength, but has suffered from an intrinsic
problem that stability is insufficient and precise reproducibility
cannot be obtained in the alignment.
In the last several years, a method permitting the different
wavelength TTR alignment without using such .[.an.]. .Iadd.a
.Iaddend.correction optical system with less stability and
reproducibility has been proposed in U.S. Pat. No. 4,880,310 or
Japanese Patent Laid Open No. 63-283129 (corresponding to U.S.
application Ser. No. 192,784 filed on May 10, 1988). In an
alignment system disclosed in the above reference, beams for
illuminating the reticle mark and the wafer mark are simultaneously
focused by a two-focusing element and an object lens on two planes,
respectively. One plane is coincident with a pattern plane (mark
plane) of the reticle, whereas the other .[.plate.]. .Iadd.plane
.Iaddend.is coincident with a wafer conjugate plane in a space away
from the reticle pattern plane by a distance corresponding to the
amount of axial (on-axis) chromatic aberration of the projection
lens.
Adopting the above disclosed method eliminates the need of
providing an optical element, other than the projection lens, in an
optical path for alignment between the reticle and the wafer, and
permits the TTR alignment as if the exposure light is used.
However, the latest projection lens is corrected in its various
aberrations satisfactorily for only the exposure light, but
exhibits both axial chromatic aberration and magnification
chromatic aberration. Even if use of the two-focusing element
succeeds in correcting the axial chromatic aberration, the
magnification chromatic aberration cannot be always corrected
satisfactorily, thereby requiring it to remove an alignment error
(offset) attributable to the magnification chromatic aberration by
some method. For this reason, a technique of coping with the
magnification chromatic aberration (distortion) has been proposed
in U.S. Pat. Nos. 4,780,913 and 4,856,905, by way of example. Of
them, U.S. Pat. No. 4,780,913 discloses such a technique that a
reference mark capable of exiting light rays at two wavelengths,
i.e., the exposure light at one wavelength and the illumination
light for alignment at a different wavelength, is provided on a
wafer stage and moved on the image plane side of a projection lens
to scan a retroprojected image of the reference mark on the reticle
side for determining a position of the reticle mark, thereby
preparing a distortion map in the view field of the projection lens
beforehand. On the other hand, U.S. Pat. No. 4,856,905 discloses
such a technique that the exposure light is introduced in the form
of a beam to an illumination beam transmitting system (comprising a
scanner mirror, two-focusing element, object lens, etc.) of a
different wavelength TTR alignment system, allowing an illumination
beam at the different wavelength and an illumination beam at the
wavelength of the exposure light to be scanned simultaneously,
whereby data of light information are photoelectrically detected
from respective reference marks on the reticle mark and the wafer
stage to determine a distortion at the alignment point based on
differences in the amount of position deviation detected for each
wavelength of the beam.
The aforementioned prior arts for coping with the magnification
chromatic aberration are arranged so that both the illumination
light for alignment and the illumination light for exposure pass
through the same position in the view field of the projection lens.
Specifically, in U.S. Pat. No. 4,780,913, two types of illumination
light are required to be introduced to the rear side of the same
reference mark through optical fibers or the like. In U.S. Pat. No.
4,856,905, the illumination light for exposure is required to be
introduced to the different wavelength TTR alignment system.
That structure has suffered from problems as follows. An
arrangement of the alignment optical system is complicated, which
leads to a difficulty in manufacture that a severer level is
required in the performance of constituent optical elements
(particularly, achromatism). In addition, it is difficult to make a
match between various conditions of the optical system (such as a
sigma value, number of aperture and tele-centricity) under the
illumination light for exposure and various conditions of the
optical system under the illumination light for alignment, thus
rendering it hard to know a precise distortion error enough for
practical use. Moreover, an actual exposure apparatus (stepper) of
even 1/5 reduction type is designed so as to accommodate an
exposure of a wide field (view field) on the order of 15.times.15
mm to 20.times.20 mm. But, exposure areas of reticle patterns
employed by stepper users are versatile in size, and the position
of the alignment mark in the projection field is naturally changed
variously depending on the size. Because the distortion amount of
the projection lens with respect to an ideal lattice under the
illumination light for exposure is also changed depending on change
in the alignment position, there has been another problem that the
difference between distortion characteristics under the
illumination light for alignment and the illumination light for
exposure, which has been determined at only the alignment position
is not enough for satisfactory correction, taking into account the
fact that the reticle pattern must be superposed with the shot area
over the entire wide field.
Further, the apparatus disclosed in the above cited U.S. Pat. No.
4,856,905 has a specific problem as follows. Where the illumination
light for exposure and the illumination light for alignment are
separated from each other depending on their ranges of wavelength
by a dichroic mirror obliquely disposed above the reticle at an
angle of 45.degree., if transmissivity (or reflectivity) of the
dichroic mirror for the illumination light for alignment is set
very high, the illumination light for exposure to be detected by
the different wavelength TTR alignment system could not pass
through (or be reflected by) the TTR alignment system in its large
part, making is difficult to detect the mark. On the other hand, if
wavelength characteristics of the dichroic mirror are selected to
give some degree of transmissivity (or reflectivity) to the
illumination light for exposure as well, the amount or intensity of
light directing toward the reticle from an exposure light
illuminating system during the exposure would now be reduced
correspondingly.
SUMMARY OF THE INVENTION
The present invention has been achieved in view of the various
problems as stated above, and has for its object to provide a
projection optical apparatus equipped with a different wavelength
TTR alignment system which can detect and correct an alignment
offset attributable to, particularly, a distortion with high
precision.
To achieve the above .[.problem.]. .Iadd.object.Iaddend., with the
present invention, a fiducial mark (fourth mark area) to be
detected under an illumination light for exposure (light of first
wavelength characteristic) and a fiducial mark (second mark area)
to be detected under an illumination light for alignment (a light
of second wavelength characteristic) are arranged on a fiducial
plate to be positionally separate from each other, the fiducial
plate being fixed on a stage on which a sensitive plate such as a
wafer is rested. Furthermore, a reticle mark (third mark area) to
be detected under the illumination light for exposure and a reticle
mark (first mark area) to be detected under the illumination light
for alignment are disposed on a reticle (mask) respectively
corresponding to the separate positions of the associated fiducial
marks on the fiducial plate. A different wavelength TTR alignment
system (first mark detection means) for detecting the reticle mark
(first mark area) and the fiducial mark (second mark area) under
the illumination light for alignment, and an exposure wavelength
TTR alignment system (second mark detection means) for detecting
the reticle mark (third mark area) and the fiducial mark (fourth
mark area) under the illumination light for exposure are arranged
as independent optical systems of each other and to be capable of
detecting the corresponding marks simultaneously by both the TTR
alignment systems.
FIG. 1 shows a schematic configuration of a stepper for explaining
the principles of the present invention. A circuit pattern area PA
on a reticle R is projected to one shot area on a wafer (not shown
in FIG. 1) by a projection lens PL which is tele-centric on both
sides. Around the pattern area PA on the reticle R, there are
provided marks Au, Al to be respectively detected by TTR alignment
systems AO.sub.2x, AO.sub.2y each using an illumination light of
different wavelength .[.(.lambda..sub.2x).]. (.Iadd..lambda..sub.2
.Iaddend.). The mark Au to be detected by the alignment system
AO.sub.2x is used for alignment in the X direction, whereas the
mark Al to be detected by the alignment system AO.sub.2y is used
for alignment in the Y direction. These marks Au and Al are formed
midway one side extending in the X direction of the pattern area PA
and another side extending in the Y direction thereof,
respectively. The different wavelength TTR alignment systems
AO.sub.2x, AO.sub.2y are each able to move an alignment position in
the X and Y directions. Further, at a location outside the pattern
area PA on the reticle R, but within the view field of the
projection lens PL, there is formed a mark RMr to be detected by a
TTR alignment system AO.sub.1 using an illumination light of
exposure wavelength .lambda..sub.1. The position of the mark RMr is
always fixedly located regardless of reticles having different
sizes of the pattern area PA. Then, the exposure light for
uniformly illuminating the pattern area PA during the exposure is
set not to be shielded by a distal end portion of the. TTR
alignment system AO.sub.1. Accordingly, even in the case where a
variety of reticles having different sizes of the pattern area PA
are exchanged from one to another, the TTR alignment system
AO.sub.1 is not required to be moved and can be held fixed on the
apparatus.
Meanwhile, below the projection lens PL, there is located a
fiducial (reference mark) plate FP fixedly disposed on wafer stage.
The surface of the fiducial plate FP is conjugate to the reticle R
with respect to the projection lens PL under the exposure
wavelength. A fiducial mark FMr to be detected at the same time as
the mark RMr on the reticle R and fiducial marks Fu, Fl to be
detected at the same time as the marks Au, Al on the reticle R,
respectively, are formed on the surface of the fiducial plate FP by
etching of a chromium layer, etc. The mark Fu corresponds to the
mark Au and is formed in a wide area enough to cover any positions
to which the mark Au is movable on the reticle R. This also equally
applies to the mark Fl.
Then, in the configuration of FIG. 1, when the fiducial plate FP is
positioned such that the mark RMr (the third mark area) and the
fiducial mark FMr (the fourth mark area) are simultaneously
detected by the TTR alignment system AO.sub.1, the mark Au (the
first mark area) and the fiducial mark Fu (the second mark area)
are simultaneously detected by the TTR alignment system AO.sub.1,
for example. Since the positional relationship between the fiducial
marks FMr and Fu on the fiducial plate FP, as well as the
positional relationship between the marks RMr and Au on the reticle
R are precisely known in advance, a difference between the amount
or alignment error detected by the different wavelength TTR
alignment system AO.sub.2X and the amount of alignment error
detected by the exposure wavelength TTR alignment system AO.sub.1
represents an offset amount in the X direction attributable to
chromatic aberration at the position of the mark Au.
By storing this offset amount, therefore, the error can be easily
corrected when the reticle R and the wafer are later aligned with
each other by using the TTR alignment system AO.sub.2X.
In this way, with the different wavelength TTR alignment system and
the exposure wavelength TTR alignment system being arranged
independently of each other, but to carry out the simultaneous mark
detecting operation, an influence of distortion can be corrected
more strictly and reliably.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a configuration for explaining the
principles of a stepper according to an embodiment of the present
invention;
FIG. 2 is a view showing a detailed configuration of the stepper
according to the embodiment of the present invention:
FIGS. 3 and 4 are views for explaining an optical system of a
different wavelength TTR alignment system;
FIG. 5 is a plan view showing an arrangement of reticle grating
marks;
FIG. 6 is a plan view showing an arrangement of a wafer grating
marks;
FIG. 7A is a plan view showing a fiducial grating for a
reticle;
FIG. 7B is a plan view showing a field iris for a wafer;
FIG. 8 is a view for explaining an optical system of the different
wavelength TTR alignment system;
FIG. 9 is a perspective view showing a practical arrangement of the
different wavelength TTR alignment system and an exposure
wavelength TTR alignment system;
FIG. 10 is a plan view showing a practical pattern arrangement of
the reticle;
FIG. 11 is a plan view showing a pattern arrangement on a fiducial
mark plate; and
FIGS. 12A to 12C are views showing distortion characteristics at
the wavelength of an exposure light and a different wavelength.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Hereinafter, a configuration of a stepper according to one
preferred embodiment of the present invention will be described
with reference to FIGS. 2 to 11.
While this embodiment employs a different wavelength TTR alignment
system of two-beam interference type using gratings, as disclosed
in the above cited U.S. application Ser. No. 192,784, the present
invention is also applicable to other types of alignment system
(including an image detecting method or spot scanning method)
exactly in the same manner.
In FIG. 2, a dichroic mirror DM is obliquely disposed above a
reticle R at an angle of 45.degree. to bend an optical axis AX of a
projection lens PL at a right angle or horizontally on the drawing.
The dichroic mirror DM reflects an exposure light propagating along
the optical axis AX from an illumination system for exposure (not
shown) at a percentage greater than about 90% so that the reflected
light is directed toward a pattern area PA on a reticle R. The
reticle R is held on a reticle stage RS which is finely movable in
the two dimensional directions (X, Y, .theta. directions), the
reticle stage RS being positioned by a driver 1. On the other hand,
an XY stage ST for holding a wafer W and a fiducial plate FP
thereon is disposed below the projection lens PL and movable in the
X and Y directions by a motor 2, the coordinate position of the
stage ST being successively measured by a laser interferometer 3.
Positioning of the stage ST is performed by a stage driver circuit
4 which monitors the measured value of the interferometer 3 and
drives the motor 2. The stage driver circuit 4 controls the
movement and positioning of the stage ST based, on a command from a
main controller 5. The main controller 5 also supervises control of
the driver 1.
While a different wavelength TTR alignment system of the this
embodiment is constructed above the dichroic mirror DM in two to
four eyes (representing the number of object lenses), FIG. 2 shows
only one eye, i.e., the system corresponding to AO.sub.2X in FIG.
1.
A tele-centric object lens OBJu and a mirror M.sub.1 are held by a
holder 11 which is movable in the X and Y directions by the driver
10 together with the object lens and the mirror.
The object lens OBJu is arranged to be out of interference with the
dichroic mirror DM and to have its optical axis vertical to the
reticle R.
An illumination light for the different wavelength alignment is
emitted in the form of a linearly polarized beam LB from a laser
light source 12 such as a helium-neon or argon ion laser, and
enters a two-beam forming frequency shifter unit 13. In the unit
13, there are disposed means for dividing the beam LB into two
beams, two AOMs (acousto-optic modulators) 130A, 130B for
respectively applying high-frequency modulations (frequency shifts)
to the divided two beams, small lenses 131A, 131B for focusing the
respective frequency shifted beams, and so on. The AOM 130A is
driven at frequency f.sub.1 (for example, 80 MHz) and a 1st order
diffracted beam LB.sub.1 therefrom is taken out through the small
lens 131A, whereas the AOM 130B is driven at frequency f.sub.2 (for
example, 80.03 MHz) and a 1st order diffracted beam LB.sub.2
therefrom is taken out through the small lens 131B. The two beams
LB.sub.1, LB.sub.2 have their principal rays set parallel to the
optical axis of the alignment system and symmetrical about the
optical axis, and are each divided by a beam splitter 14 into two
beams. A pair of divided beams L.sub.r1, L.sub.r2 having passed
through the beam splitter 14 are converted by condenser lens
(inverse Fourier transform lens) 15 into fluxes of parallel light
which cross each other in its focus plane on the rear or downstream
side. A reference grating 16 is disposed in the rear focus plane so
as to produce an interference fringe with the pitch depending on
the crossing angle and wavelength of the two beams L.sub.r1,
L.sub.r2. The interference fringe moves over the reference grating
16 in one direction at a speed depending on a frequency difference
.DELTA.f (30 KHz) between the two beams L.sub.r1 and L.sub.r2. On
the reference grating 16, there is provided a transmission type
diffraction grating with the pitch being equal to the pitch of the
interference fringe. Accordingly, the reference grating 16 produces
an interference light DR.sub.1 between a 0th order light of the
beam L.sub.r1 and a 1st order light created from the beam L.sub.r2,
as well as an interference light DR.sub.2 between a 0th order light
of the beam L.sub.r2 and a 1st order light created from the beam
L.sub.r1, the amounts of the interference lights DR.sub.1, DR.sub.2
being detected by a photoelectric element 17. Here, the intensity
of the interference lights DR.sub.1, DR.sub.2 is changed
sinusoidally at the frequency difference .DELTA.f (30 KHz), i.e.,
beat frequency, so that an output signal SR from the photoelectric
element 17 becomes an AC signal of 30 KHz. This signal SR is used
as a reference signal for phase comparison in the alignment
process.
FIG. 3 is a view showing optical paths of the beams LB.sub.1,
LB.sub.2 and behaviors thereof in detail from the lens 15 to the
photoelectric element 17. First, the two beams LB.sub.1, LB.sub.2
are respectively condensed by the action of the small lenses 131A,
131B in a plane EPa to become beam waists. The plane EPa is a
Fourier plane in coincidence with a focus plane of the lens 15 on
the front or upstream side, so that the beams L.sub.r1, L.sub.r2
propagate from the lens 15 in the form of parallel light fluxes to
cross each other on the reference grating 16. Respective positions
of the beams L.sub.r1, L.sub.r2 in the plane EPa are set to be
symmetrical about the optical axis AXa of the alignment system, and
the distance of each beam position from the optical axis AXa
determines an incident angle .theta. of the beams L.sub.r1,
L.sub.r2 with respect to the reference grating 16.
Following the general analytical theory, a diffraction angle
.alpha. of .+-.1st order diffracted lights with respect to a 0th
order light, produced by a coherent beam of wavelength .lambda.
impinging upon a one-dimensional grating with a pitch Pg, is
uniquely determined by sin .alpha.=.lambda./Pg. Therefore, by
setting the pitch Pg of the reference grating 16 such that the
diffraction angle .alpha. of the 1st order diffracted light becomes
just 2.theta. with respect to the 0th order light of the beam
L.sub.r1 (i.e., the interference light DR.sub.1), the 1st order
diffracted light of the beam L.sub.r1 produces coaxially with the
0th order light of the beam L.sub.r2 (i.e., the interference light
DR.sub.2).
At this time, the pitch of the interference fringe produced on the
reference grating 16 is equal to the grating pitch Pg.
The photoelectric element 17 has light receiving surfaces 170A,
170B for separately receiving the two interference beams DR.sub.1,
DR.sub.2. Respective photoelectric signals from the light receiving
surfaces 170A, 170B are added to each other by an adder 171 in an
analog manner and the resulting sum is outputted as the reference
signal SR.
Returning again to the explanation of FIG. 2, two beams L.sub.m1,
L.sub.m2 reflected by the beam splitter 14 are further reflected by
another beam splitter 18 to enter the object lens OBJu through the
mirror M.sub.1. The object lens OBJu converts the two beams
L.sub.m1, L.sub.m2 into respective fluxes of parallel light which
cross each other in a spatial plane IP. The plane IP is spaced from
the reticle R by a distance corresponding to the amount .DELTA.L of
axial chromatic aberration in the direction of the optical axis AX,
and is conjugate to the wafer W or the fiducial plate FP under the
wavelength (.lambda..sub.2) of the illumination beam LB for
alignment. The two beams L.sub.m1, L.sub.m2 having crossed each
other in the plane IP are spaced from each other in a mark area Au
on the reticle R and, after becoming beam waists in a pupil EP of
the projection lens PL, they cross each other again on the fiducial
plate FP (or the wafer W) in the form of parallel light fluxes.
FIG. 4 is a view of optical paths showing behavior of the beams
L.sub.m1, L.sub.m2. The focus plane of the object lens OBJu on the
front or upstream side is coincident with the plane EPa shown in
FIG. 3, so that the two beams L.sub.m1, L.sub.m2 turn to beam
waists in the plane EPa. The two beams L.sub.m1, L.sub.m2 exiting
from the object lens OBJu become fluxes of parallel light and cross
each other in the plane IP, i.e., in the rear focal plane of the
object lens OBJu. Thereafter, the beam L.sub.m1 illuminates a
reticle grating mark Aua in the mark area Au on the reticle R and
also passes through a transparent portion of the mark area Au,
followed by entering the projection lens PL. Then, the beam
L.sub.m1 becomes a flux of parallel light again which obliquely
illuminates a grating in a mark area Fu on the fiducial plate FP
(or the wafer W).
Likewise, the beam L.sub.m2 illuminates a reticle grating mark Aub
in the mark area Au on the reticle R and also passes through a
transparent portion of the mark area Au, following which the beam
L.sub.m2 obliquely illuminates the grating in the mark area Fu on
the fiducial plate FP (or the wafer W) in a direction symmetrical
to the beam L.sub.m1.
As shown in FIG. 4, the two beams L.sub.m1, L.sub.m2 become fluxes
of converging light in a pupil space (comprising the optical path
between the object lens OBJu and the small lenses 131A, 131B and
the interior of the projection lens PL) with their principal rays
being parallel to the optical axis AXa, and then become fluxes of
parallel light in an image space (comprising the optical path
between the object lens OBJu and the projection lens PL and the
optical path between the projection lens PL and the fiducial plate
FP or the wafer W) with their principal rays crossing each other in
a focal plane (image plane) of the projection lens PL.
From the mark area Fu on the fiducial plate FP, an interference
light BT is vertically produced in the upward direction. The
interference light BT is resulted from interference between a 1st
order diffracted light vertically produced from the mark Fu upon
irradiation of the beam L.sub.m1 and a 1st order diffracted light
vertically produced from the mark Fu upon irradiation of the beam
L.sub.m2, the interference light BT being a flux of parallel light
in the image space. After converging into, a beam waist at the
center of the pupil EP of the projection lens PL, the interference
light BT reversely propagates along the optical axis Axa of the
object lens OBJu while passing through a transparent portion
(between the marks Aua and Aub) at the center of the mark area on
the reticle R in the form of a parallel light flux. The
interference light BT converges again into a beam waist at the
center of the front focal plane of the object lens PL, that is, the
plane EPa conjugate to the pupil EP of the projection lens PL,
followed by returning toward a light receiving system while passing
through the mirror M.sub.1 and the beam splitter 18 in this order
as shown in FIG. 2. Structure of the mark area Au on the reticle R
will now be explained with reference to FIGS. 5 and 6. As shown in
FIG. 5, a light shielding band ESB of constant width is formed all
around the circuit pattern area PA on the reticle R. At a part of
the light shielding band ESB, there are arranged two grating marks
Aua, Aub with a light shielding portion ESB' therebetween. The
marks Aua and Aub have the same pitch, and their width in a
direction perpendicular to the pitch direction is set about a half
the total width of the transparent window. The width of the light
shielding portion ESB' is also set about a half the total width of
the transparent window. A grating mark WMu on the wafer W is
located in a portion WMu' in the mark area under the wavelength for
alignment. The two beams L.sub.m1, L.sub.m2 respectively illuminate
the grating marks Aua and Aub in a rectangular shape. Meanwhile, as
shown in FIG. 6, the beam L.sub.m1 having passed through the
transparent portion adjacent to the mark Aua on the same side as
the pattern area PA illuminates the wafer mark WMu, and the beam
L.sub.m2 having passed through the transparent portion adjacent to
the mark Aub on the same side as the pattern area PA also
illuminates the wafer mark WMu. The interference light BT
vertically produced from the mark WMu returns to the object lens
OBJu after passing through the portion WMu' in the plane of the
reticle R as shown in FIG. 6.
When the beams L.sub.m1, L.sub.m2 respectively illuminate the
reticle grating marks Aua, Aub in FIG. 4, higher order diffracted
lights are also reflected along with 0th order lights
D.sub.01,D.sub.02. In this embodiment, the pitch of the mark Aua
(Aub) is set so that assuming an incident angle of the beam
L.sub.m1 (L.sub.m2) upon the grating mark Aua (Aub) be .theta.',
the diffraction angle of the 1st order diffracted light with
respect to the 0th order light D.sub.01 (D.sub.02) becomes exactly
2.theta.'. With such setting, the grating mark Aua produces a 1st
order diffracted light D.sub.11 (with the frequency shift f.sub.1)
propagating along the optical path of the beam L.sub.m1 exactly in
a reversed direction, whereas the grating mark Aub produces a 1st
order diffracted light D.sub.12 (with the frequency shift f.sub.2)
propagating along the optical path of the beam L.sub.m2 exactly in
a reversed direction. Accordingly, along with the interference
light BT, the 1st order diffracted beams D.sub.11, D.sub.12 are
also returned toward the light receiving system through the object
lens OBJu. Referring to FIG. 2 again, the interference light BT and
the 1st order diffracted lights D.sub.11, D.sub.12 pass through the
beam splitter 18 and enter a condensing lens system 19. The lens
system 19 is an inverse Fourier transform lens which converts all
of the interference light BT and the 1st order diffracted lights
D.sub.11, D.sub.12 into fluxes of parallel light and causes them to
cross each other in a focal plane (image conjugate plane) of the
lens system 19. The three beams having passed through the lens
system 19 are each divided into two parts by a beam splitter 20.
Respective parts having passed through the beam splitter 20 reach a
fiducial grating plate 21 for the reticle, whereas the reflected
parts reach a field iris 22. The fiducial grating plate 21 and the
field iris 22 are both conjugate to the .[.plate.]. .Iadd.plane
.Iaddend.IP (i.e., the plane conjugate to the fiducial plate FP or
the wafer W). Accordingly, at the fiducial grating plate 21, the
1st order diffracted lights D.sub.11, D.sub.12 cross each other to
produce an interference fringe in the crossed area. The resulting
interference fringe of course flows or drifts one-dimensionally at
the beat frequency .DELTA.f (30 KHz). As shown in FIG. 7A,
therefore, a transmission type diffraction grating 21A is provided
on the fiducial grating plate 21 covered with a chromium layer, and
an interference light BTr between two diffracted lights .Iadd.is
.Iaddend.coaxially produced from the grating 21A. This behavior
will now be described by referring to FIG. 8. Because of meeting an
image conjugate relation, the fiducial grating plate 21 is
conjugate to the plane IP and the wafer mark WMu (or the surface of
the fiducial plate FP) so that the interference light BT also
impinges upon the fiducial grating plate 21 in addition to the 1st
order diffracted lights D.sub.11, D.sub.12. However, since the
wafer mark WMu and the reticle marks Aua, Aub are arranged in the
X-Y plane to be shifted laterally, the interference light BT
returns to a portion (light shielding portion) 21B on the fiducial
grating plate 21, which portion is adjacent to the grating 21A
where the two 1st order diffracted lights D.sub.11, D.sub.12 cross
each other, as shown in FIG. 7A.
Accordingly, by just setting the position and size of the grating
21A on the fiducial grating plate 21 in match with the size of the
mark Aua or Aub, the interference light BT from the wafer mark WMu
can be shielded.
Respective 0th order lights BTo of the 1st order diffracted lights
D.sub.11, D.sub.12 illuminating the grating 21A on the fiducial
grating plate 21 propagate so as to deviate from a photoelectric
element 23, and only the interference light BTr between two 1st
order diffracted lights vertically produced from the grating 21A is
received by the photoelectric element 23. This technique is the
same as the case of taking out the interference light BT from the
wafer mark WMu as shown in FIG. 4. In either case, the grating
pitch of the wafer mark WMu or the grating 21A is set exactly two
times the pitch of the interference fringe produced thereon.
The intensity of the interference light BTr thus received by the
photoelectric element 23 is changed sinusoidally at the beat
frequency .DELTA.f (30 KHz). Therefore, an output signal Sm of the
photoelectric element 23 becomes an AC signal which is linearly
changed in a phase difference relative to the reference signal SR
depending on the amount of displacement of the reticle marks Aua,
Aub in the pitch direction on the basis of the reference grating
16.
On the other hand, the interference light BT and the 1st order
diffracted lights D.sub.11, D.sub.12, all reflected by the beam
splitter 20 shown in FIG. 2, reaches the field iris 22. The iris 22
is formed with an opening 22A which allows only the interference
light BT to pass therethrough, as shown in FIG. 7B, while the two
1st order diffracted lights D.sub.11, D.sub.12 are shielded by a
light shielding portion 22B.
The interference light BT having passed through the iris 22 reaches
a photoelectric element 26 through a mirror 24 and a condenser lens
25. The photoelectric element 26 produces an output signal Sw
depending on change in the intensity of the interference light BT.
This output signal Sw also becomes an AC signal of which level is
changed sinusoidally at the beat frequency .DELTA.f, the phase of
the AC signal relative to the reference signal SR being changed in
proportion to the amount of deviation of the mark WMu on the wafer
W or the fiducial mark Fu on the fiducial plate FP from the
reference grating 16.
The reference signal SR and the output signals Sm, Sw are inputted
to a phase difference measurement unit 27 which determines a phase
difference .phi.m of the output signal Sm relative to the reference
signal SR, determines a phase difference .phi.w of the output
signal Sw relative to the reference signal SR, and further a
difference therebetween .DELTA..phi.=.phi.m-.phi.w. In the case of
this embodiment, since the diffraction grating pitch on the
fiducial plate FP (or the wafer W) is two times the pitch of the
interference fringe produced thereon, one period (.+-.180.degree.)
of the phase difference .DELTA..phi. corresponds to 1/2 (.+-.1/4
pitch) of the diffraction grating pitch. Based on the phase
difference .DELTA..phi., the measurement unit 27 calculates
position correction amounts (or position shift amounts) .DELTA.X,
.DELTA.Y of the water stage ST or the reticle stage RS, values of
those amounts being delivered to the main controller 5.
In the foregoing, the system from the driver 10 to the measurement
unit 27, including the object lens OBJu, corresponds to first mark
detecting means in the present invention.
Meanwhile, in FIG. 2, the mark RMr on the reticle R is detected by
a TTR alignment system for exposure (corresponding to AO.sub.1 in
FIG. 1) which comprises a mirror M.sub.2, an object lens OBr, a
beam splitter 30, a lens system 31, an illumination field iris 32,
a condenser lens 33, a fiber 34, a focusing lens 35, a beam
splitter 36, and CCD image sensors 37A, 37B. The fiber 34 emits an
illumination light at the exposure wavelength to
uniformly irradiate the iris 32 through the condenser lens 33. The
illumination light having passed through an opening of the iris 32
enters the object lens OBr through the lens system 31 and the beam
splitter 30, following which it is bent by the mirror M.sub.2 at a
right angle to vertically illuminate a local area of the reticle R
including the mark RMr downwards. The iris 32 is conjugate to the
reticle R so that an opening image or the iris 32 is focused on the
reticle R. The beam splitter 30 is located near a front focus plane
or the tele-centric object lens OBr, i.e., a plane conjugate to the
pupil EP of the projection lens FL, thereby reflecting a part of
the light returned from the object lens OBr toward the focusing
lens 35. The CCD image sensors 37A, 37B have their light receiving
surfaces which are conjugate to the reticle R through the object
lens OBr and the focusing lens 35, and also conjugate to the wafer
W or the fiducial plate FP through the projection lens PL.
Incidentally, an exit end of the fiber 34 is located to be
conjugate to the pupil EP of the projection lens PL for achievement
of the Keller's illumination. In the case of this embodiment, the
optical path between the object lens OBr and the beam splitter 30
exhibits an afocal system and, therefore, a deviation in the
position of the mark RMr can be compensated for by arranging the
object lens OBr and the mirror M.sub.2 integrally with each other
in such a manner as able to move together horizontally in FIG. 2.
But, in consideration of stability of the system, the mirror
M.sub.2 is here fixedly located at a position outside the pattern
area PA having the maximum dimension possibly expected on the
reticle R.
CCD image sensors 37A, 37B are arranged to have an angular spacing
of 90.degree. therebetween so that their horizontal scan lines
coincide with the X and Y directions, respectively, for making
position measurements of the crossshaped mark RMr in the X and Y
directions separately. The reason is to avoid a difference in
resolution as developed when using a single CCD image sensor to
detect shifts of the mark image in both the horizontal and vertical
directions, because a usual CCD image sensor .[.have.]. .Iadd.has
.Iaddend.different degrees of pixel resolution in the horizontal
and vertical directions. An image processing unit 38 receives
respective image signals (video signals) from the CCD image sensors
37A, 37B, detects a position shift amount of the mark RMr on the
reticle R from the fiducial mark FMr on the fiducial plate FP, and
then .[.deliver.]. .Iadd.delivers .Iaddend.information of the
position shift amount to the main controller 5.
In addition to the above configuration, there are also provided a
global mark detection system 40 of off-axis type for detecting a
global aligment mark on the wafer W, a latent image in a resist
layer or the respective fiducial marks on the fiducial plate FP,
and a processing unit 42 for processing a signal from the system
40. Furthermore, adjustment units 50A, 50B for adjusting or
correcting various focusing characteristics of the projection lens
PL are provided and supervised under control of the main controller
5. The adjustment unit 50A has a function of changing
magnification, focus position, distortion, etc. of the projection
lens PL by, for example, controlling pressure of a predetermined
air chamber in the projection lens PL. The adjustment unit 50B has
a function of finely moving in the axial direction or tilting a
lens element as one component of the projection lens PL (for
example, a field lens on the reticle side).
The different wavelength TTR alignment system and the exposure
light TTR alignment system are preferably arranged as schematically
shown in FIG. 9. As will be seen from FIG. 9, reticle grating mark
areas Au, Al, Ar, Ad, each being arranged as shown in FIG. 5, are
formed in the respective sides of the light shielding band ESB
around the pattern area PA on the reticle R. The mark areas Au, Ad
are used for alignment in the X direction plotted in FIG. 9,
whereas the mark areas Ar, Al are used for alignment in the Y
direction.
Outside the light shielding band ESB, there is also provided a
reticle mark RMl similar to the reticle mark RMr at a symmetrical
position.
Therefore, object lenses OBr, OBl, of two exposure light TTR
alignment systems are arranged to respectively detect the marks
RMr, RMl below the dichroic mirror DM, and object lenses OBJu,
OBJd, OBJr, OBJl of four different wavelength TTR alignment systems
are arranged to respectively detect the mark areas Au, Ad, Ar, Al
through the dichroic mirror DM.
When the reticle R and the wafer W are actually aligned with each
other on a die-by-die basis by using the different wavelength TTR
alignment systems, the four eyes are not always required to be used
simultaneously, but three or two eyes may be used instead. This is
relied on the fact that even in the case where any of corresponding
marks (WMu, WMd, WMr, WMl) in one shot area on the wafer is
defective, if one eye in the X direction and one eye in the Y
direction at minimum properly output photoelectric signals, an
alignment error (interruption of the sequence) can be avoided to
the utmost by executing the alignment for the shot area of
interest.
The mark arrangement on the reticle R as shown in FIG. 9 and the
mark arrangement on the fiducial plate FP suitable for this
embodiment will be next described with reference to FIGS. 10 and
11.
FIG. 10 shows one preferred example of a pattern layout on the
reticle R, in which the pattern area PA having the maximum
dimension projectable by the projection lens PL is supposed. The
cross-shaped reticle marks RMr, .[.RM.]. .Iadd.RMl .Iaddend.are
provided on a line passing the center Rcc of the pattern area PA on
the reticle R and extending parallel to the X axis. These marks
RMr, RMl are set substantially at the respective centers of the
view fields of the object lenses OBr, OBl. In the reticle R shown
in FIG. 10, the four mark areas Au, Ad, Ar, Al are each located at
a position farthest from the reticle center Rcc. Stated otherwise,
rectangular areas Su, Sd, Sr, Sl indicated by broken lines in FIG.
10 respectively .[.stand for one.]. .Iadd.are .Iaddend.examples of
ranges where the optical axes (detection centers) of the object
lenses OBJu, OBJd, OBJr, OBJl of the four different wavelength TTR
alignment systems are movable. Then, in the case of FIG. 10, the
mark areas Au, Ad, Ar, Al are each provided at the outermost
position of the movable range of the corresponding object lens.
FIG. 10 is illustrated by way of example and, depending on cases,
the movable ranges Sr, Sl of the object lenses OBJr, OBJl may
protrude into a region between the movable ranges Su, Sd of the
object lenses OBJu, OBJd from the right and left.
FIG. 11 shows the mark arrangement on the fiducial plate FP with
double cross-shaped fiducial marks FMr, FMl provided on the left
and right sides of the center in the X direction. The
center-to-center distance between the two fiducial marks FMr and
FMl is set equal to a value resulted from multiplying the
center-to-center distance between the two marks RMr and RMl on the
reticle by the projection magnification (l/M). Accordingly, when
the center Fcc of the fiducial plate FP is made coincident with the
reticle center Rcc, the fiducial mark FMr and the reticle mark RMr
are simultaneously observed by the object lens OBr of one exposure
light TTR alignment system under a condition that the reticle mark
RMr is positioned between the double lines of the fiducial mark
FMr, and the fiducial mark .[.FM.]. .Iadd.FMl .Iaddend.and the
reticle mark RMl are simultaneously observed by the object lens OBl
of the other exposure light TTR alignment system under a condition
that the reticle mark RMl is positioned between the double lines of
the fiducial mark FMl.
On the fiducial plate FP, there are also provided fiducial mark
areas Fu, Fd, Fr, Fl in which diffraction gratings are engraved in
positions and sizes respectively corresponding to the movable
ranges Su, Sd, Sr, Sl on the reticle R when the center Fcc of the
fiducial plate FP is made coincident with the reticle center Rcc.
In the fiducial mark area Fu, for example, a group of grating lines
engraved in the X direction with the constant pitch are formed
similarly to the mark WMu on the wafer W and used for detecting a
relative position shift in the X direction from the mark area Au on
the reticle R. This equally applies to the other fiducial mark
areas Fd, Fr, Fl. Accordingly, even if the size of the pattern area
PA (or the light shielding band ESB) is changed upon replacement of
the reticle R with another one, it is possible to simultaneously
detect an X-directional shift of the mark area Au from the fiducial
mark Fu, an X-directional shift of the mark area Ad from the
fiducial mark Fd, a Y-directional shift of the mark area Ar from
the fiducial mark Fr, and a Y-directional shift of the mark area Al
from the fiducial mark Fl by the four eyes (OBJu, OBJd, OBJr, OBJl)
so long as the mark areas Au. Ad, Ar, Al on the reticle are present
within the movable ranges Su, Sd, Sr, Sl, respectively.
In FIG. 11, the two groups of plural grating lines making up the
fiducial marks Fr and Fl on the left and right sides correspond to
each other in one-to-one relation with respect to the Y direction,
whereas the two groups of plural grating lines making up the
fiducial marks Fu and Fd on the upper and lower sides correspond to
each other in one-to-one relation with respect to the X direction.
Further, the center-to-center spacing in the X direction between
the mark area Au and Ad shown in FIG. 10 is accurately equal to
integer times as much as the grating pitch of the fiducial marks
Fu, Fd, whereas the center-to-center spacing in the Y direction
between the mark area Ar and Al is accurately equal to integer
times as much as the grating pitch of the fiducial marks Fr,
Fl.
Operation of this embodiment, that is, base line measurement of the
exposure light TTR alignment system and the different wavelength
TTR alignment system in consideration of a distortion, will be
described below. At first, an arbitrary reticle (such as shown in
FIG. 10, for example) is set on the reticle stage RS and the
reticle alignment is performed by picking up images of the reticle
marks RMr. RMl and the marks FMr, FMl on the fiducial plate FP by
the CCD sensor elements 37A, 37B of the exposure light TTR
alignment systems.
Then, the object lenses OBJu, OBJd, OBJr, OBJl (i.e., the holders
11) of the four different wavelength alignment systems are set at
positions respectively corresponding to the mark areas Au, Ad, Ar,
Al, followed by checking shifts in illumination position of the two
beams L.sub.m1, L.sub.m2 in the X and Y directions and a
tele-centric error of the two beams L.sub.m1, L.sub.m2 by the use
of the fiducial marks Fu, Fd, Fr, Fl on the fiducial plate FP.
After completion of the checking, the four different wavelength
alignment systems each determines amounts of relative position
shifts between the reticle R and the fiducial plate FP at that
position. In other words, the shift amounts in the X direction
between the fiducial mark Fu (and Fd) and the reticle mark Au (and
Ad), as well as the shift amounts in the Y direction between the
fiducial mark Fr (and Fl) and the reticle mark Ar (and Al) are
detected through the respective measurement units 27.
Based on the detected shift amounts, the main controller 5 controls
the driver 1 and the stage driver 4 for servo-driving the reticle
stage RS or the wafer stage ST. Since the different wavelength
alignment systems of this embodiment are each of the heterodyne
type allowing successive measurement of the relative position shift
amount even in a condition that the reticle mark and the fiducial
mark remain at rest.[.. The.]..Iadd., the .Iaddend.measurement
units 27 continue successively outputting the information on the
relative position shifts (such as in the X, Y and rotating
directions). Therefore, the alignment between the reticle R and the
fiducial plate FP is continued so that all the phase differences
.DELTA..phi. measured by the measurement units 27 of the four
different wavelength alignment systems become zero (or a fixed
value). During the above step of the different wavelength
alignment, the image processing units 38 of the exposure light TTR
alignment systems continue successively (at certain intervals of
time) outputting the position shift amounts (.DELTA.Xr, .DELTA.Yr)
in the X, Y directions between the reticle mark RMr and the
fiducial mark FMr, as well as the position shift amounts
(.DELTA.Xl, .DELTA.Yl) in the X, Y directions between the reticle
mark RMl and the fiducial mark FMl. It is to be noted that in the
case of a large distortion, the corresponding shift amount is added
as an offset amount to a design value beforehand. Thus, the shift
amounts (.DELTA.Xr, .DELTA.Yr), (.DELTA.Xl, .DELTA.Yl) determined
by the exposure light TTR alignment systems correspond to averages
of distortion differences at the alignment positions (i.e., the
marks Au, Ad, Ar, Al) between the wavelength of the exposure light
and the different wavelength therefrom. To put it in more detail,
when the results of detecting two pairs of the reticle mark Au (the
fiducial mark Fu) and the reticle mark Ad (the fiducial mark Fd)
show that the phase differences .DELTA..phi. therebetween are both
zero, the shift amounts (.DELTA.Xr, .DELTA.Yr), (.DELTA.Xl,
.DELTA.Yl) detected by the image processing units 38 are stored
plural times, following which the stored results area averaged to
determine overall alignment errors (in X, Y and .theta. directions)
caused by a distortion difference between the reticle R and the
fiducial plate FP. In the case of this embodiment, the reticle R
and the fiducial plate FP are simultaneously aligned with each
other by using four eyes of the different wavelength TTR alignment
systems.[., the.]..Iadd.. The .Iaddend.reticle R and the fiducial
plate FP exhibit, as a consequence of the alignment, slight errors
in the X, Y and .theta. directions depending on distortion
characteristics of the projection lens PL at the different
wavelength. These errors can be assumed as fixed offsets so long as
the reticle marks Au, Ad, Ar, Al are not changed in their
positions. Therefore, by detecting the shift amounts between the
marks RMr, RMl and the fiducial marks FMr, FMl using the exposure
light TTR alignment systems, the resulting shift amounts give
offset amounts in the X, Y and .theta. directions which include the
distortion amounts of the projection lens at the positions of the
marks RMr, RMl under the exposure light.
Accordingly, when actually carrying out alignment of the shot area
on the wafer W by the different wavelength TTR alignment systems
thereafter, it is only required to control the reticle stage RS or
the wafer stage ST so that the aligned position is shifted by the
above offset amounts to reach the true alignment position. The
offset amounts caused by the distortion difference in the X, Y and
.theta. (rotating) directions are calculated in the main controller
5 based on information of the shift amounts from the image
processing unit 38, and stored until the reticle R will be
realigned or replaced with another one.
As the distortion difference in the projection lens PL between the
wavelength of the exposure light and the different wavelength
therefrom may be changed upon the adjustment unit 50B being
actuated, it is desirable that immediately after actuating the
adjustment unit 50B to a large extent, the offset amounts are
measured again by using the fiducial plate FP.
With this embodiment as stated above, since the mark areas Au, Ad,
Ar, Al for the different wavelength TTR alignment are respectively
provided on the four sides of the reticle R and the corresponding
fiducial marks Fu, Fd, Fr, Fl on the fiducial plate FP are
simultaneously detected, it is possible to precisely determine the
overall alignment errors through TTR which are caused by an
influence of distortions at the different wavelength. Further, with
this embodiment, since the different wavelength TTR alignment
systems and the exposure light TTR alignment systems are
simultaneously operated through the projection lens PL and the
values measured by the interferometer 3 on the wafer stage ST are
not used at all, errors due to fluctuating air in the atmosphere,
which would raise a problem associated with the interferometer 3,
will not be involved so that the offset amounts can be measured
with very high precision.
In addition, since this embodiment employs the different wavelength
TTR alignment systems of heterodyne type having extremely high
resolution, the highly accurate measurement is achieved. For
example, assuming that the pitch of the diffraction gratings on the
fiducial plate FP is on the order of 4 .mu.m, the phase difference
detectable range (.+-.180.degree.) is given .+-.1 .mu.m. Also,
assuming that the practical phase measurement resolution is
.+-.2.degree. in consideration of noises and so on, the position
shift detecting resolution becomes as high as about .+-.0.01
.mu.m.
Consequently, with such an arrangement that the reticle R and the
fiducial plate FP are subjected to alignment servo control by using
the different wavelength TTR alignment systems of heterodyne type,
the highly stable
positioning can be achieved.
While the projection lens PL of this embodiment has been explained
as being tele-centric on both sides thereof, it may of course be
tele-centric on either side only. In the case of a projection lens
being tele-centric on both sides, the optical axis of the object
lens of the exposure light TTR alignment system is vertical to the
reticle surface and also coincident with the principal ray passing
the pupil center of the projection lens PL. Therefore, if the mark
patterns on the reticle are formed of reflective chromium layers,
the light regularly reflected by the patterns are so strongly
detected by the CCD image sensors that both the reticle mark RMr,
RMl and the fiducial mark FMr, FMl may appear bright. In the case
where the fiducial plate FP made of quartz glass or the like is
entirely covered with a chromium layer and the fiducial marks FMr,
FMl are formed by removing the chromium layer by etching or the
like into desired patterns, it may happen that the fiducial marks
FMr, FMl look black, but the whole of the surroundings become
bright, thereby greatly lowering the contrast of the reticle marks
RMr, RMl. In this case, an aperture iris (spatial filter) having a
ring-shaped opening may be disposed in the illumination optical
path of each exposure light TTR alignment system, e.g., in the
pupil conjugate plane between the beam splitter 30 and the lens
system 31 in FIG. 2, for illuminating the reticle R in the dark
field. With this arrangement, dark field images are focused on the
CCD image sensors such that only respective edges of the reticle
marks RMr, RMl and the fiducial marks FMr, FMl glint brightly.
Moreover, the spatial filter disposed in the pupil conjugate plane
may be formed of a liquid crystal, electrochromic (EC) or the like
in which multiple ring-like openings arc patterned in concentric
relation, allowing the illumination to be switched between the dark
field and the light field. It is also possible to change the number
of aperture for the illumination light.
A manner of improving superposition accuracy when using the
apparatus as shown in FIG. 2, in particular, a manner of improving
the matching between different units of the apparatus, will be
described below.
FIG. 12A exaggeratedly represents distortion characteristics
(broken lines) of the projection lens PL under the exposure
wavelength and distortion characteristics (one-dot-chain lines)
thereof under the different wavelength (i.e., the wavelength of the
alignment light) on the basis of an ideal lattice (solid
lines).
A distortion map like that can be drawn by, for example, making a
trial print using a test reticle. Note that the distortion map
under the different wavelength cannot be obtained by the trial
printing, but can be obtained by using the method similar to the
above stated embodiment in a combined manner.
At first, a test reticle having vernier marks for superposition
measurement at respective ideal lattice points in the pattern area
is prepared and aligned by the exposure light TTR alignment system.
Then, a reticle blind in the exposure illumination system is fully
opened and the test reticle is exposed onto a dummy wafer (such as
a photosensitive resist layer, photo-chromic layer or bare silicon
wafer coated with an opto-magnetic medium).
Next, the reticle blind is narrowed so as to illuminate only the
vernier mark provided at the center of the test reticle. Following
that, while stepping the wafer stage ST at the pitch of the ideal
lattice points, an exposure is made in superposed relation to each
latent image of the vernier mark on the test reticle having been
exposed in advance on a step-by-step basis.
In this case, on assumptions that the stepping of the wafer stage
ST is coincident with the division pitch of the ideal lattice, the
distortion characteristics under the exposure wavelength on the
basis of the ideal lattice can be determined by measuring the
superposition accuracy between the latent image of each vernier
mark exposed at the first time and the latent image of the same
vernier mark printed at the second time in superposing relation. In
the above measurement, the latent images of the vernier marks on
the dummy wafer may be detected by the global mark detection system
40 of off-axis type shown in FIG. 2, or by the exposure light TTR
alignment system after properly modifying a shape of the vernier
mark. Where the dummy wafer is formed of a usual photoresist layer,
the measurement may be performed by the different wavelength TTR
alignment system, etc. after once developing the dummy wafer to
build resist images of the vernier marks.
Then, amounts of superposition errors are determined at each of the
ideal lattice points in that way, and the resulting error amounts
are statistically processed by the method of least squares for
determining the respective offsets in the X, Y and .theta.
directions. Based on those offsets, the shift amounts
(.DELTA.OF.sub.x1, .DELTA.OF.sub.y1), (.DELTA.OF.sub.x2,
.DELTA.OF.sub.y2) of the images RMr', RMl' of the reticle marks
RMr, RMl under the exposure wavelength, indicated by broken lines
in FIGS. 12B and 12C, from the ideal positions (solid lines) can be
presumed as system offsets.
Consequently, when aligning the reticle marks RMr, RMl by the
exposure light TTR alignment systems, it is possible to always make
an exposure on the distortion map with the least errors from the
ideal lattice, by taking into account the system offsets.
Further, since the difference in distortion between the exposure
wavelength and the different wavelength can be determined following
the above first embodiment, the reticle pattern can be exposed in
superposed relation to each shot area on the wafer under a
condition closest to the ideal lattice, by further taking into
account (or compensating for) the distortion difference, even in
the case of die-by-die alignment using the different wavelength TTR
alignment systems. This implies that the matching between different
units of plural steppers jointly constituting a single
semiconductor manufacture line can be achieved on the order of the
ideal lattice, and hence that the matching accuracy in the
manufacture line can be improved.
Since the above embodiment is premised on using the stepper shown
in FIG. 2, the detection center of the exposure light TTR alignment
system is fixedly positioned in the view field of the projection
lens. Therefore, after determining and compensating for the system
offsets (.DELTA.OF.sub.x1, .DELTA.OF.sub.y1), (.DELTA.OF.sub.x2,
.DELTA.OF.sub.y2) during the reticle alignment beforehand so that
the distortion map under the exposure wavelength becomes closest to
the ideal lattice, as explained in connection with FIGS. 12B and
12C, distortion errors under the different wavelength may be
determined at each alignment position using the different
wavelength TTR alignment system.
According to the present invention, as described above, since a
different wavelength TTR alignment system and an exposure light TTR
alignment system are arranged to be separated from each other and
to simultaneously detect a group of fiducial marks located on the
image plane of a projection optical system, the distortion
difference as developed in when using the different wavelength can
be precisely measured. In other words, since the alignment light at
the exposure wavelength and the alignment light at the different
wavelength are detected by the respective TTR alignment systems
exactly at the same timing through the projection optical system,
the detected results commonly include measurement errors due to
minute fluctuations in air flows, temperature distribution, etc.
within the optical paths, thereby allowing those measurement errors
to be canceled out. Another advantage is in that since the present
invention does not rely on the method using a laser interferometer
while running a wafer stage or the like, the results will not be
affected by measurement accuracy (reproducibility) of the laser
interferometer itself.
Further, according to the present invention, since a plurality of
TTR alignment systems can be simultaneously operated using the
group of fiducial marks, it is also possible to implement beam
positioning, tele-centric checking and focus checking at the same
time, which are necessary upon the an object lens of the TTR
alignment system being moved, resulting in an advantage of
increasing a throughput in setting of the TTR alignment system.
* * * * *