U.S. patent number RE36,730 [Application Number 09/002,884] was granted by the patent office on 2000-06-13 for projection exposure apparatus having an off-axis alignment system and method of alignment therefor.
This patent grant is currently assigned to Nikon Corporation. Invention is credited to Kenji Nishi.
United States Patent |
RE36,730 |
Nishi |
June 13, 2000 |
**Please see images for:
( Certificate of Correction ) ** |
Projection exposure apparatus having an off-axis alignment system
and method of alignment therefor
Abstract
An exposure apparatus for exposing mask patterns on a sensitive
plate comprises a set (for X and Y direction) of a laser
interferometer for measuring a position of a wafer stage and
satisfying Abbe's condition with respect to a projection lens and a
set (for X and Y direction) of the laser interferometer and
satisfying Abbe's condition with respect to off-axis alignment
system. When a fiducial mark on the wafer stage is positioned
directly under the projection lens, a presetting is performed so
that measuring values by the two sets of laser interferometers are
equal to each other.
Inventors: |
Nishi; Kenji (Kawasaki,
JP) |
Assignee: |
Nikon Corporation (Tokyo,
JP)
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Family
ID: |
27525754 |
Appl.
No.: |
09/002,884 |
Filed: |
January 5, 1998 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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524465 |
Sep 7, 1995 |
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872750 |
Apr 21, 1992 |
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Reissue of: |
998642 |
Dec 29, 1992 |
05243195 |
Sep 7, 1993 |
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Foreign Application Priority Data
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Apr 25, 1991 [JP] |
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3-095599 |
Jul 10, 1991 [JP] |
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3-169781 |
Jul 23, 1991 [JP] |
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3-182557 |
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Current U.S.
Class: |
250/548;
356/401 |
Current CPC
Class: |
G03F
7/70691 (20130101); G03F 9/7088 (20130101) |
Current International
Class: |
G01N
21/86 (20060101); G01N 021/86 () |
Field of
Search: |
;250/548,557,237G,237R
;356/399-401 ;355/53,55 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
IBM Technical Disclosure Bulletin, "Aligning Semiconductor Mask,"
vol. 13, No. 7, Dec. 1970, p. 1816..
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Primary Examiner: Allen; Stephone
Attorney, Agent or Firm: Vorys, Sater, Seymour and Pease
LLP
Parent Case Text
.[.This.]. .Iadd.This reissue application is a continuation of
application Ser. No. 08/524,465 filed Sep. 7, 1995, now abandoned,
which is a reissue application of U.S. Pat. No. 5,243,195 granted
Sep. 7, 1993, which matured from application Ser. No. 07/998,642
filed Dec. 29, 1992, which .Iaddend.is a continuation of
application Ser. No. .Iadd.07/.Iaddend.872,750 filed Apr. 21, 1992,
now abandoned.
Claims
What is claimed is:
1. An exposure apparatus for exposing mask patterns on a sensitive
plate having,
a mask stage holding said mask,
a plate stage movable two dimensionally in accordance with a
rectangular coordinate system while holding said sensitive
plate,
a projection optical system imaging said mask patterns at a
predetermined position on said sensitive plate,
a first mark detecting system for detecting a mask mark formed at a
predetermined position on said mask in the image field of said
projection optical system, and
a second mark detecting system for detecting a plate mark formed on
said sensitive plate in a detection area positioned at
predetermined intervals from the optical axis of said projection
optical system,
said exposure apparatus comprising:
(a) a first fiducial mark member arranged on a part of said plate
stage, having a shape capable of being detected by said first mark
detecting system through said projection optical system;
(b) a second fiducial mark member arranged on a part of said plate
stage, having a shape capable of being detected by said second mark
detecting system; and
(c) a mounting member for mounting said first fiducial mark member
and said second fiducial mark member at constant intervals
recognized in advance so as to position said second fiducial mark
member in the detection area of said second mark detecting system
when said first fiducial mark member is positioned in a
predetermined relationship with respect to the position where said
mask mark is to be present in the image field of said projection
optical system.
2. An exposure apparatus for exposing mask patterns on a sensitive
plate having,
a mask stage holding said mask,
a plate stage movable two dimensionally in accordance with a
rectangular coordinate system while holding said sensitive
plate,
a projection optical system imaging said mask patterns at a
predetermined position on said sensitive plate,
a first mark detecting system for detecting a mask mark formed at a
predetermined position on said mask in the image field of said
projection optical system, and
a second mark detecting system having a detection center positioned
at predetermined intervals apart from the optical axis of said
projection optical system, for detecting a plate mark formed on
said sensitive plate,
said exposure apparatus comprising:
(a) a first fiducial mark member arranged on a part of said plate
stage and capable of being detected by said first mark detecting
system through said projection optical system;
(b) a second fiducial mark member arranged on a part of said plate
stage, and capable or being detected by said second mark detecting
system;
(c) a mounting member for mounting said first fiducial mark member
and said second fiducial mark member at constant intervals
recognized in advance so as to position said second fiducial mark
member in the detection area of said second mark detecting system
when said first fiducial mark member is positioned in a
predetermined relationship with respect to the position where said
mask mark is to be present in the image field of said projection
optical system; and
(d) base line determining means for calculating the base line
distance between the projecting position of said mask patterns by
said projection optical system and the position of the detection
center of said second mark detecting system based on the positional
displacement value between said first fiducial mark member and said
mask mark detected by said first mark detecting system, the
positional displacement value of said second fiducial mark member
against said detection center detected by said second mark
detecting system, and the value of said constant intervals
recognized in advance.
3. An exposure apparatus for exposing mask patterns on a sensitive
plate having a plate stage movable two dimensionally in accordance
with a rectangular coordinate system while holding said sensitive
plate, a projection optical system imaging said mask patterns at a
predetermined position on said sensitive plate, a first mark
detecting system for detecting a mask mark formed at a
predetermined position on said mask in the image field of said
projection optical system, and a second mark detecting system,
having a detection area positioned at predetermined intervals apart
from the optical axis of said projection optical system, for
detecting a plate mark formed on said sensitive plate, said
exposure apparatus comprising:
(a) a first fiducial mark member arranged on a part of said plate
stage and capable of being detected by said first mark detecting
system through said projection optical system;
(b) a second fiducial mark member arranged on a part of said plate
stage, and capable of being detected by said second mark detecting
system;
(c) a mounting member for mounting said first fiducial mark member
and said second fiducial mark member at constant intervals
recognized in advance so as to position said second fiducial mark
member in the detection area of said second mark detecting system
when said first fiducial mark member is positioned in a
predetermined relationship with respect to the position where said
mask mark is to be present in the image field of said projection
optical system;
(d) positioning control means for causing said plate stage to be
stationary on said rectangular coordinate system so as to enable
said mask mark and said first fiducial mark member to be detected
in an aligned state by said first mark detecting system;
(e) means for starting the detecting operation of said second mark
detecting system for said second fiducial mark member while said
positioning control means is activating; and
(f) base line determining means for calculating the base line
distance between the projecting position of said mask patterns by
said projection optical system and the position of the detection
area of said second mark detecting system based on the positional
displacement value of said second fiducial mark member detected by
said second mark detecting system, and the value of said constant
intervals recognized in advance.
4. An exposure apparatus for exposing mask patterns on a sensitive
plate having a plate stage movable two dimensionally in accordance
with a rectangular coordinate system while holding said sensitive
plate, a projection optical system imaging said mask patterns at a
predetermined position on said sensitive plate, a first mark
detecting system for detecting a mask mark formed at a
predetermined position on said mask in the image field of said
projection optical system, and a second mark detecting system,
having a detection area positioned at predetermined intervals apart
from the optical axis of said projection optical system, for
detecting a plate mark formed on said sensitive plate, said
exposure apparatus comprising:
(a) a first fiducial mark member arranged on a part of said plate
stage and capable of being detected by said first mark detecting
system through said projection optical system;
(b) a second fiducial mark member arranged on a part of said plate
stage, and capable of being detected by said second mark detecting
system;
(c) a mounting member for mounting said first fiducial mark member
and said second fiducial mark member at constant intervals
recognized in advance so as to position said second fiducial mark
member in the detection area of said second mark detecting system
when said first fiducial mark member is positioned in a
predetermined relationship with respect to the position where said
mask mark is to be present in the image field of said projection
optical system;
(d) positioning control means for causing said plate stage to be
stationary on said rectangular coordinate system so as to enable
said detection area and said first fiducial mark member to be
detected in an aligned state by said second mark detecting
system;
(e) means for starting the detecting operation of said mask mark
and said second fiducial mark member by said first mark detecting
system while said positioning control means is activating; and
(f) base line determining means for calculating the base line
distance between the projecting position of said mask patterns by
said projection optical system and the position of the detection
area of said second mark detecting system based on the positional
displacement value between said mask mark and said second fiducial
mark member detected by said first mark detecting system, and the
value of said constant intervals recognized in advance.
5. An exposure apparatus for exposing mask patterns on a sensitive
plate including a projection optical system imaging said mask
patterns at a predetermined image plane position on said sensitive
plate, a plate stage movable two dimensionally in accordance with a
rectangular coordinate system parallel to said image plane while
holding said sensitive plate, a first mark detecting system for
detecting a mark on said sensitive plate through a first detecting
area provided in the projection image field of said projection
optical system, a second mark detecting system for detecting a mark
on said sensitive plate through a second detecting area provided
outside the projection image field of said projection optical
system, said exposure apparatus comprising:
(a) a fiducial member mounted on a part of said plate stage with a
first fiducial mark capable of being detected by said first mark
detecting system and a second fiducial mark capable of being
detected by said second mark detecting system, said first and
second fiducial marks being formed at predetermined constant
intervals, wherein said first and second fiducial marks are
provided in a positional relationship substantially the same as the
arrangement of said first and second detecting areas in said
rectangular coordinate system;
(b) a first measuring means of an incremental type having measuring
axes arranged in parallel with each of the two coordinate axes of
said rectangular coordinate system to satisfy the Abbe's condition
with respect to the optical axis of said projection optical system
for measuring the coordinate positions on said rectangular
coordinate system of said plate stage;
(c) a second measuring means of an incremental type having
measuring axes arranged in parallel with each of the two coordinate
axes of said rectangular coordinate system to satisfy the Abbe's
condition with respect to the second detecting area of said second
mark detecting system for measuring the coordinate positions on
said rectangular coordinate system of said plate stage; and
(d) resetting means for resetting each of the measured values of
said first and second measuring means to a same value when said
plate stage is positioned so that said first fiducial mark is
detected by said first mark detecting system and at the same time,
said second fiducial mark is detected by said second mark detecting
system.
6. An exposure apparatus for exposing mask patterns on a sensitive
plate including a projection optical system imaging said mask
patterns at a predetermined image plane position of said sensitive
plate, a plate stage movable two dimensionally in accordance with a
rectangular coordinate system parallel to said image plane while
holding said sensitive plate, a first mark detecting system for
detecting a mask mark on said mask through a detecting area
provided in the projection image field of said projection optical
system, a second mark detecting system for detecting a mark on said
sensitive plate through a detecting area provided outside the
projection image field of said projection optical system, said
exposure apparatus comprising:
(a) a fiducial member provided on a part of said plate stage and
having a fiducial mark detecting by said first mark detecting
system;
(b) positioning control means for driving said plate state and for
causing said plate stage to be stationary in order to arrange said
fiducial mark at a predetermined position in the image field of
said projection optical system for enabling said first mark
detecting system to detect both said mask mark and said fiducial
mark;
(c) a first measuring means of an incremental type having measuring
axes arranged in parallel with each of the two coordinate axes of
said rectangular coordinate system to satisfy the Abbe's condition
with respect to the optical axis of said projection optical system
for measuring the coordinate positions on said rectangular
coordinate system of said plate stage;
(d) a second measuring means of an incremental type having
measuring axes arranged in parallel with each of the two coordinate
axes of said rectangular coordinate system to satisfy the Abbe's
condition with respect to the detection area of said second mark
detecting system for measuring the coordinate positions on said
rectangular coordinate system of said plate stage; and
(e) resetting means for resetting each of the measured values of
said first and second measuring means to a same value when said
plate stage is positioned by said positioning control means.
7. An exposure apparatus for exposing mask patterns on a sensitive
plate including a mask stage movable two dimensionally while
holding said mask, a projection optical system for imaging said
mask patterns on said sensitive plate, a plate stage movable two
dimensionally in a rectangular coordinate system while holding said
sensitive plate, and a first mark detecting system for detecting
mask marks formed at a plurality of positions on said mask in the
projection image field of said projection optical system, said
exposure apparatus comprising:
(a) a fiducial member provided on a part of said plate stage with a
first fiducial marks being formed to be aligned with each of said
mask marks through the projection image field of said projection
optical system;
(b) means for storing values of mounting errors of said fiducial
member on said plate stage in advance with said rectangular
coordinate system as reference;
(c) a first control means for driving said plate stage and for
causing said plate stage to be stationary so as to arrange said
plurality of fiducial marks at predetermined positions in the
projection image field of said projection optical system; and
(d) a second control means for positioning said mask stage on the
basis of the detection of said first mark detecting system so that
said mask marks are displaced by said mounting errors to be aligned
with respect to said plurality of fiducial marks at said
predetermined positions.
8. An exposure apparatus for exposing mask patterns on a sensitive
plate having a mark stage holding said mask, a plate stage movable
two dimensionally in accordance with a rectangular coordinate
system while holding said sensitive plate, a projection optical
system for imaging said mask patterns on said sensitive plate, an
inner-field alignment system for detecting the patterns formed on
an object on said plate stage through the image field of said
projection optical system, and an outer-field alignment system for
detecting the patterns of said object, said exposure apparatus
comprising:
(a) a fiducial member of a low expansion material mounted on a part
of said plate stage with a first fiducial mark capable of being
detected by said inner-field alignment system and a second fiducial
mark capable of being detected by said outer-field alignment system
being formed on the surface thereof, wherein said first and second
fiducial marks are provided in a positional relationship
substantially the same as the arrangement the mark detecting areas
of said inner-field alignment system and said outer-field alignment
system in said rectangular coordinate system;
(b) positioning control means of positioning said plate stage to
cause said first fiducial mark to be stationary at a predetermined
position in the image field of said projection optical system;
and
(c) means for instructing said inner-field alignment system and
said outer-field alignment system to detect respectively said first
fiducial mark and said second fiducial mark almost simultaneously
with said plate stage being at rest.
9. A method for controlling the precision of an exposure apparatus
for exposing mask patterns on a sensitive plate provided with a
mask stage holding said mask, a plate stage movable two
dimensionally in accordance with a rectangular coordinate system
while holding said sensitive plate, an alignment system for
detecting at least one of a mask mark formed on said mask and a
plate mark formed on said sensitive plate for the alignment of said
mask and said sensitive plate, and a fiducial member arranged on a
part of said plate stage with a fiducial mark formed thereon to be
detected by said alignment system, including the following steps
of:
(a) aligning said mask with respect to said fiducial member by
arranging the fiducial mark of said fiducial member at an exposing
position in said rectangular coordinate system and detecting said
mask mark and fiducial mark using said alignment system;
(b) sequentially exposing said mask patterns on said sensitive
plate for each of step positions by moving said plate stage in the
direction of one of the coordinate axes of said rectangular
coordinate system step by step for a predetermined amount, wherein
said mask patterns are partially overlapped in the direction of the
step movement for exposure;
(c) measuring a positional deviating amount of said overlapped
portions of the patterns exposed on said sensitive plate in a
direction intersecting at right angles with said direction of the
step movement; and
(d) calculating the mounting error of said fiducial member on said
plate stage with said rectangular coordinate system as reference on
the basis of said measured positional deviating amount.
10. An exposure apparatus for exposing mask patterns on a sensitive
plate including:
a plate stage movable two dimensionally in accordance with a
rectangular coordinate system while holding said sensitive
plate,
a projection optical system for imaging said mask patterns on said
sensitive plate,
a light emission mark member mounted on a part of said plate stage
for forming a fine light emission mark image on said mask through
said projection optical system,
a first detecting system for detecting variation of the light being
generated when said light emission mark image scans a mark formed
on said mask, and
a second detecting system for detecting a mark on said sensitive
plate by scanning said plate stage through a detecting area
arranged outside the image field of said projection optical system,
comprising the following:
(a) a fiducial member mounted on a part of said plate stage at a
predetermined interval with respect to said light emission mark
member with a fiducial mark formed thereon to be detected by said
second detecting system, where the light emission mark of said
light emission mark member and the fiducial mark of said fiducial
member are provided in a positional relationship substantially the
same as the arrangement of the expected position of the projection
of the mark of said mask by said projection of the mark of said
mask by said projection optical system and the position of the
detecting area of said second detecting system in said rectangular
coordinate system;
(b) driving control means for scanning said plate stage in a
predetermined direction so as to cause said fiducial mark to pass
through the detecting area of said second detecting system while
said light emission image by said projection optical system scans
only a predetermined area including said mask mark; and
(c) means for storing signals output from each of the first
detecting system and said second detecting system while said plate
stage is scanned.
11. An exposure apparatus for exposing mask patterns on a sensitive
plate including:
a plate stage movable two dimensionally in accordance with a
rectangular coordinate system while holding said sensitive plate,
p1 a projection optical system for imaging said mask patterns on
said sensitive plate,
a first detecting system mounted on a part of said plate stage for
detecting photoelectrically mark imaging light from said mask
projected by said projection optical system through a light
receiving area, and
a second detecting system for detecting a mark on said sensitive
plate photoelectrically through a detecting area arranged outside
the image field of said projection optical system, comprising the
following:
(a) a fiducial member mounted on a part of said plate stage at
predetermined interval with respect to said first detecting system
with a fiducial mask formed thereon to be detected by said second
detecting system, wherein the light receiving area of said first
detecting system and the fiducial mark of said fiducial member are
provided in a positional relationship substantially the same as the
arrangement of the expected position of the image mark imaging
light from said mask by said projection optical system and the
position of the detection area of said second detecting system in
said rectangular coordinate system; and
(b) driving control means for driving said plate stage to cause
said fiducial mark to be detected by said second detecting system
substantially at the same time that the mark imaging light from
said mask by said projection optical system is detected by said
first detecting system. .Iadd.
12. An alignment apparatus, comprising:
a movable stage for supporting an object having at least one mark
and for moving in a predetermined coordinate system;
a mark detecting system having a detecting region located at a
predetermined position in said coordinate system, for detecting the
mark positioned into said detecting region and for producing a
deviation value of said mark and a predetermined target point
designated in said detecting region;
an interferometer system including a counter for producing a
measuring value that is variable in response to the movement of
said movable stage; and
a control system for determining control information so as to bring
said movable stage into a predetermined position in said coordinate
system, on the basis of the deviation value from said mark
detecting system and a plurality of the measuring values read out
from said counter at the same time as the detecting by said mark
detecting system, and for controlling said movable stage on the
basis of the determined control information..Iaddend..Iadd.
13. Apparatus according to claim 12, wherein said mark detecting
system includes an objective lens for forming an image of the mark
of the object on a predetermined imaging plane, and an image
pick-up device located at said imaging plane for generating a video
signal, said detecting region being defined by an image receiving
area of said image pick-up device..Iaddend..Iadd.14. Apparatus
according to claim 13, wherein said predetermined target point is
defined by a target mark of a target plate disposed in an optical
path from said objective lens to said image pick-up
device..Iaddend..Iadd.15. Apparatus according to claim 14, wherein
said mark detecting system includes an image processing circuit for
processing said video signal and for producing a deviation amount
of said target mark and the mark of said object..Iaddend..Iadd.16.
Apparatus according to claim 15, wherein said image processing
circuit calculates a mean value of a plurality of the deviation
amounts and generates said mean value as said deviation
value..Iaddend..Iadd.17. A device manufacturing method to transfer
a pattern image of a mask onto a predetermined region of a
substrate in a predetermined aligning state, comprising the steps
of:
(a) detecting a first deviation value of the mask and a fiducial
mark plate, said fiducial mark plate being mounted on a movable
XY-stage supporting the substrate;
(b) detecting a second deviation value of said fiducial mark plate
and a detection center of an alignment system provided for
detecting alignment marks formed on the substrate;
(c) measuring positional information of the XY-stage by reading out
a plurality of measured values produced from an interferometer
system at the same time as the detecting of said second deviation
value;
(d) determining a base line value having relation to a distance
between a center of the mask and the detection center of said
alignment system, on the basis of said first deviation value, said
second deviation value and said positional information; and
(e) controlling the movement of said XY-stage so as to align the
mask and the predetermined region of the substrate relatively, on
the basis of the determined base line value..Iaddend..Iadd.18. A
device manufacturing method to transfer a pattern image of a mask
onto a predetermined region of a substrate in a predetermined
aligning state, comprising the steps of:
(a) aligning a fiducial mark plate and the mask, said fiducial mark
plate being mounted on a movable XY-stage on which the substrate is
supported;
(b) detecting a deviation value of said fiducial mark plate and a
detection center of an alignment system provided for detecting
alignment marks formed on the substrate;
(c) measuring positional information of the XY-stage by reading out
a plurality of measured values produced from an interferometer
system at the same time as the detecting of said deviation
value;
(d) determining a base line value having relation to a distance
between a center of the mask and the detection center of said
alignment system, on the basis of said deviation value and said
positional information; and
(e) controlling the movement of said XY-stage so as to align the
mask and the predetermined region of the substrate relatively, on
the basis of the
determined base line value..Iaddend..Iadd.19. An exposure apparatus
which exposes a photosensitive substrate with an illumination beam
irradiated on a mask through a projection optical system,
comprising:
a movable member disposed at an image plane side of said projection
optical system for supporting said photosensitive substrate;
a mark detection system for detecting a first mark disposed on said
movable member by projecting a light beam having substantially the
same wavelength as said illumination beam and receiving a light
beam reflected by said first mark through said projection optical
system;
an alignment optical system for detecting an alignment mark on said
photosensitive substrate and for detecting a second mark disposed
on said moveable member, said alignment optical system having an
optical axis which is different from an optical axis of said mark
detection system; and
a fiducial plate disposed on said movable member for determining a
base line of said alignment optical system, said fiducial plate
including said first mark and said second mark, both of which are
detected substantially simultaneously..Iaddend..Iadd.20. An
apparatus according to claim 19, wherein said alignment optical
system includes an objective optical system which is independent
from said projection optical system and an image pick-up element
for picking-up an image of said second mark through said objective
optical system..Iaddend..Iadd.21. An apparatus according to claim
20, wherein said alignment optical system projects a wide-bandwidth
light beam, of which a wavelength range is different from that of
said illumination beam, onto said fiducial plate through said
objective optical system..Iaddend..Iadd.22. A method of exposing a
photosensitive substrate with an illumination beam irradiated on a
mask through a projection optical system, comprising the steps
of:
disposing a fiducial plate on which first and second marks are
formed at an image plane side of said projection optical
system;
projecting a light beam having substantially the same wavelength as
said illumination beam toward said first mark to detect a light
beam reflected by said first mark through said projection optical
system; and
detecting said second mark by an off-axis alignment optical system
substantially simultaneously to the detection of said first mark to
determine a base line of said off-axis alignment optical system,
which is disposed independently from said projection optical
system..Iaddend..Iadd.23. A method according to claim 22, wherein
said first mark provides a positional relationship with respect to
a mark on said mask and said second mark provides a positional
relationship with respect to an index mark in said alignment
optical system..Iaddend..Iadd.24. A method according to claim 23,
wherein the base line of said alignment optical system is
determined based on the two positional relationships and positional
information of a stage on which said fiducial plate is
disposed..Iaddend..Iadd.25. An alignment apparatus, comprising:
an interferometer to produce information with respect to relative
movement of a mask and a substrate;
an alignment optical system that outputs a signal corresponding to
a relative deviation between a first mark on the mask and a second
mark on the substrate; and
an alignment controller that determines a positional relationship
between the mask and the substrate based on the signal from said
alignment optical system and a plurality of values of information
produced by said interferometer in response to a detection of the
first and second marks by
said alignment optical system..Iaddend..Iadd.26. An apparatus
according to claim 25, wherein said alignment controller calculates
an equalized value of said plurality of values of information to
determine said positional relationship based on said signal and the
equalized value..Iaddend..Iadd.27. An apparatus according to claim
25, further comprising:
a projection optical system disposed between said mask and said
substrate; and
an off-axis alignment optical system having an optical axis
different from that of the projection optical system to detect a
third mark formed on said substrate..Iaddend..Iadd.28. An alignment
method comprising the steps of:
detecting a relative deviation between a first mark on a mask and a
second mark on a substrate;
measuring a relative position between the mask and the substrate;
and
determining a positional relationship between the mask and the
substrate based on the detected relative deviation and a plurality
of relative positions measured in response to a detection of the
first and second
marks..Iaddend..Iadd.29. A method according to claim 28, wherein
said positional relationship is determined based on said detected
relative deviation and an equalized value of said plurality of
relative positions..Iaddend..Iadd.30. A method according to claim
28, wherein said relative position is measured by an interferometer
which detects a position of said mask in an alignment
direction..Iaddend..Iadd.31. A method according to claim 29,
further comprising:
obtaining positional information of a third mark on said substrate
by an off-axis alignment optical system; and
determining a baseline of the off-axis alignment optical system
based on the determined positional relationship and the obtained
positional information..Iaddend..Iadd.32. A method according to
claim 31, wherein the detection of the relative deviation between
said first mark and said second mark and a detection of said third
mark are performed substantially simultaneously..Iaddend..Iadd.33.
An alignment method comprising the steps of:
detecting a relative deviation between a first mark on a mask and a
second mark on a substrate;
measuring a relative position between the mask and the substrate;
and
adjusting a positional relationship of the mask and the substrate
based on the detected relative deviation and a plurality of
relative positions measured in response to a detection of the first
and second
marks..Iaddend..Iadd.34. A projection exposure apparatus which
exposes a substrate with a projected image of a mask through a
projection optical system, comprising:
a first mark detection system which has a detection area at the
image side of said projection optical system to detect a mark on
said substrate through an optical element different from the
projection optical system;
a second mark detection system which detects a mark disposed at the
image side of said projection optical system through the projection
optical system; and
a movable member disposed at the image side of said projection
optical system, on which a first mark and a second mark are
arranged in such a positional relationship that allows the
detection of said first mark by said first mark detection system
and the detection of said second mark by said second mark detection
system substantially simultaneously..Iaddend..Iadd.35. An apparatus
according to claim 34, wherein a distance between said first and
second marks is set such that said second mark falls within a
detection area of said second mark detection system when the
detection of said first mark by said first mark detection system is
effected..Iaddend..Iadd.36. An apparatus according to claim 34,
wherein a distance between said first and second marks is set such
that the detection of the first mark by said first mark detection
system and the detection of the second mark by said second mark
detection system can be effected without movement of said movable
member..Iaddend..Iadd.37. An apparatus according to claim 34,
wherein a distance between said first and second marks is set
substantially corresponding to a distance between a detection area
of said first mark detection system and a detection area of said
second mark detection system..Iaddend..Iadd.38. An apparatus
according to claim 34, wherein said second mark detection system
detects a mark on said mask as well as said second mark and a
distance between said first and second marks is set substantially
corresponding to a distance between a position of said mark on the
mask detected by said second mark detection system within a
projection field of said projection optical system and a center of
detection of said first mark detection system..Iaddend..Iadd.39. An
apparatus according to claim 34, further comprising:
an interferometer having at least one measurement axis which passes
through a center of detection of said first mark detection system,
wherein an output of said interferometer is used upon detection of
said first mark by said first mark detection
system..Iaddend..Iadd.40. An apparatus according to claim 34,
wherein said movable member is a substrate stage which holds said
substrate..Iaddend..Iadd.41. An apparatus according to claim 34,
further comprising a mask stage which holds said mask, wherein said
apparatus scanningly exposes said substrate with said projected
image by moving said mask and said substrate in a direction
perpendicular to an optical axis of said projection optical
system..Iaddend..Iadd.42. An apparatus according to claim 34,
further comprising a mask stage which holds said mask, wherein said
apparatus transfers a pattern on said mask onto said substrate in
accordance with a step and scan scheme..Iaddend..Iadd.43. An
apparatus according to claim 34, further comprising:
a drive system which moves said movable member; and
a controller connected to said drive system, which servo-locks,
upon detection of said second mark by said second mark detection
system, said movable member based on an output of said first mark
detection system..Iaddend..Iadd.44. An apparatus according to claim
43, wherein said controller servo-locks said movable member such
that a center of detection of said first mark detection system and
said first mark substantially coincide..Iaddend..Iadd.45. An
apparatus according to claim 34, wherein said first and second
marks are disposed on said movable member separately
at positions different from each other..Iaddend..Iadd.46. An
apparatus according to claim 45, wherein said first and second
marks are formed on a single fiducial plate disposed on said
movable member..Iaddend..Iadd.47. An apparatus according to claim
46, wherein said measurement system uses a
mounting error of said fiducial plate on said movable member and
positioning errors of said first and second marks on said
fiducial
plate..Iaddend..Iadd.48. An apparatus according to claim 34,
further comprising:
a measurement system connected to said first and second detection
systems to determine a baseline amount of said first detection
system by detecting said first and second marks..Iaddend..Iadd.49.
An apparatus according to claim 48, wherein said first mark
detection system detects said first mark plural times and said
measurement system uses, upon determination of said baseline
amount, a plurality of detection values of said first detection
system..Iaddend..Iadd.50. An apparatus according to claim 48,
wherein said second mark detection system detects said second mark
plural times and said measurement system uses, upon determination
of said baseline amount, a plurality of detection values of said
second detection system..Iaddend..Iadd.51. An apparatus according
to claim 48, wherein said first and second detection systems detect
said first and second marks simultaneously, and said measurement
system has an adjusting device which adjusts a positional
relationship between said first and second detection systems and
said movable member prior to said simultaneous
detection..Iaddend..Iadd.52. An apparatus according to claim 51,
wherein said adjusting device comprises a drive system which moves
said movable member and a position detection system which detects
position information of said movable member to control the movement
of said movable member based on the dispositions of said first and
second mark detection
systems..Iaddend..Iadd.53. An apparatus according to claim 48,
further comprising:
a drive system which moves said movable member;
a detection device which detects position information of said
movable member; and
a controller, connected to said drive system and said detection
device, which controls said drive system based on the position
information detected by said detection device to servo-lock said
movable member at the time of detection of said first mark or said
second mark by said first mark detection system or said second mark
detection system..Iaddend..Iadd.54. An apparatus according to claim
53, wherein said detection device includes an interferometer which
radiates a laser beam onto a reflection surface arranged on said
movable member..Iaddend..Iadd.55. An apparatus according to claim
54, wherein said detection device includes first and second
interferometers, having measurement axes which intersect each other
on an optical axis of said projection optical system, and a third
interferometer having a measurement axis that passes through a
center of detection of said first mark detection system and is
parallel to one of the measurement axes of said first and second
interferometers..Iaddend..Iadd.56. An apparatus according to claim
53, wherein said detection device includes a third mark detection
system which detects a third mark arranged on said movable
member..Iaddend..Iadd.57. An apparatus according to claim 56,
wherein said third mark detection system detects said third mark
through said projection optical system..Iaddend..Iadd.58. An
apparatus according to claim 56, wherein said third mark detection
system detects said third mark and a mark on said
mask..Iaddend..Iadd.59. An apparatus according to claim 56, wherein
said third mark is arranged on a fiducial plate on which said first
and second marks are also formed..Iaddend..Iadd.60. An apparatus
according to claim 56, wherein said third mark detection system
detects said third mark through an objective optical system which
is provided independently of said projection optical
system..Iaddend..Iadd.61. An apparatus according to claim 60,
wherein said third detection system has a center of detection
outside of a projection field of said projection
optical system..Iaddend..Iadd.62. An exposure apparatus which
exposes a substrate with an energy beam through a mask,
comprising:
a first mark detection system which detects a mark on said
substrate through a first optical system different from a second
optical system through which the energy beam passes;
a second mark detection system which detects a mark on said mask;
and
a mark plate which can be detected by said first and second
detection
systems simultaneously..Iaddend..Iadd.63. An apparatus according to
claim 62, wherein said mark plate is formed with a first mark to be
detected by said first mark detection system and a second mark to
be detected by said second mark detection system, the first and
second marks being formed separately from each
other..Iaddend..Iadd.64. An apparatus according to claim 62,
further comprising a movable member on which said mark plate is
arranged and which is arranged on a coordinate system which defines
a movement of said substrate..Iaddend..Iadd.65. An apparatus
according to claim 64, wherein said movable member is a substrate
stage which holds
said substrate..Iaddend..Iadd.66. An exposure apparatus which
exposes a substrate with an energy beam through a mask,
comprising:
a first mark detection system which detects a mark on said
substrate through a first optical system different from a second
optical system through which the energy beam passes;
a second mark detection system which detects a mark on said mask;
and
a movable member formed with a fiducial mark in such a way that
detections of said fiducial mark by said first and second mark
detection systems are effected at substantially the same
position..Iaddend..Iadd.67. An apparatus according to claim 66,
wherein said fiducial mark is detected by said first and second
mark detection systems substantially
simultaneously..Iaddend..Iadd.68. An apparatus according to claim
66, further comprising an interferometer which detects position
information of said movable member, wherein, upon measurement of a
baseline of said first mark detection system, results of detection
of said fiducial mark by said first and second mark detection
system, said position information of said movable member and
information concerning a rotation amount of said movable member
obtained from said interferometer are used..Iaddend..Iadd.69. An
apparatus according to claim 66, wherein said movable member is a
substrate stage which holds said substrate..Iaddend..Iadd.70. An
apparatus according to claim 66, further comprising:
a mask stage which holds said mask; and
an interferometer which detects position information of said mask
stage,
wherein an output of said interferometer is used upon baseline
measurement of said first mark detection system..Iaddend..Iadd.71.
An apparatus according to claim 70, wherein position information of
said mask stage is detected plural times by said interferometer and
a plurality of detection results of position information are used
upon said baseline measurement..Iaddend..Iadd.72. An apparatus
according to claim 66, wherein said second optical system includes
a projection optical system which projects an image of a pattern on
said mask onto said substrate, and said first mark detection system
has a detection area outside of a field of said projection optical
system..Iaddend..Iadd.73. An apparatus according to claim 72,
wherein said first mark detection system is an off-axis system
which has an objective optical system which is provided
independently of said projection optical system and which is
provided in said first optical system..Iaddend..Iadd.74. An
apparatus according to claim 66, wherein said fiducial mark
comprises at least a first mark to be detected by said first mark
detection system and a second mark to be detected by said second
mark detection system, said first mark and said second mark being
formed on different positions on said movable
member..Iaddend..Iadd.75. An apparatus according to claim 74,
further comprising a plate arranged on said movable member, wherein
said first and second marks are formed on said
plate..Iaddend..Iadd.76. An apparatus according to claim 66,
further comprising an interferometer which detects position
information of said movable member, wherein, upon measurement of a
baseline of said first mark detection system, results of detection
of said fiducial mark by said first and second mark detection
systems and information concerning a rotation amount of said
movable member obtained from said interferometer are
used..Iaddend..Iadd.77. An apparatus according to claim 76, further
comprising a drive system which servo-locks, upon detection of said
fiducial mark by said first mark detection system or said second
mark detection system, said movable member using an output of said
interferometer..Iaddend..Iadd.78. An apparatus according to claim
76, wherein said first mark detection system detects said fiducial
mark plural times and a plurality of results of detection of said
fiducial mark by said first mark detection system are used upon
said baseline measurement..Iaddend..Iadd.79. An apparatus according
to claim 78, wherein said second mark detection system detects said
fiducial mark plural times and a plurality of results of detection
of said fiducial mark by said second mark detection system are used
upon said baseline measurement..Iaddend..Iadd.80. An apparatus
according to claim 76, further comprising:
a mark detection system which detects a specific mark arranged on
said movable member; and
a drive system which servo-locks, upon detection of said fiducial
mark by said first and second mark detection system, said movable
member using an output of said mark detection system which detects
the specific
mark..Iaddend..Iadd.81. An apparatus according to claim 80, wherein
said mark detection system which detects the specific mark is one
of said first
and second mark detection systems..Iaddend..Iadd.82. An exposure
method of exposing a substrate with an energy beam through a mask,
comprising:
controlling movement of a mark plate formed with first and second
marks such that detection of said first mark by a first mark
detection system and detection of said second mark by a second mark
detection system can be substantially simultaneously effected, in
order to measure a baseline of said first mark detection system,
which detects a mark on said substrate through a first optical
system different from a second optical system through which the
energy beam passes; and
effecting exposure of said substrate after said baseline
measurement..Iaddend..Iadd.83. An exposure method of exposing a
substrate with an energy beam through a mask, comprising:
positioning a mark plate formed with first and second marks such
that detection of said first mark by a first mark detection system
and detection of said second mark by a second mark detection system
can be effected at substantially the same position, in order to
measure a baseline of said first mark detection system, which
detects a mark on said substrate through a first optical system
different from a second optical system through which the energy
beam passes; and
effecting exposure of said substrate after said baseline
measurement..Iaddend..Iadd.84. A method according to claim 83,
wherein information concerning a rotation amount of said mark plate
is used upon the baseline measurement..Iaddend..Iadd.85. A method
according to claim 83, wherein upon the baseline measurement, said
mark plate is servo-locked in accordance with information regarding
its position..Iaddend..Iadd.86. A method according to claim 83,
wherein upon exposure of said substrate, each of said mask and said
substrate is moved with respect to said energy
beam..Iaddend..Iadd.87. A method according to claim 83, wherein the
detection of said first mark by said first mark detection system is
effected plural times and a plurality of detection results by said
first mark detection system are used upon the baseline
measurement..Iaddend..Iadd.88. A method according to claim 87,
wherein the detection of said second mark by said second mark
detection system is effected plural times and a plurality of
detection results by said second mark detection system are used
upon said baseline measurement..Iaddend..Iadd.89. A method
according to claim 83, wherein position information of said mask is
used upon the baseline measurement..Iaddend..Iadd.90. A method
according to claim 89, wherein position information of said mask in
detected plural times and a plurality of detection results of said
position information are used upon said baseline
measurement..Iaddend..Iadd.91. A projection exposure apparatus
which exposes a substrate with a projected image of a mask through
a projection optical system, comprising:
a substrate stage disposed at the image side of said projection
optical system, which holds said substrate;
first and second interferometers having measurement axes parallel
to each other and perpendicular to a reflection surface provided on
said substrate stage;
a mark detection system which detects a mark disposed on said
substrate stage through said projection optical system; and
a controller connected to said first and second interferometers,
said controller determining, upon detection of said mark by said
mark detection system, the relationship of respective measurement
values of said first
and second interferometers..Iaddend..Iadd.92. An apparatus
according to claim 91, wherein said mark detection system detects
said mark on said substrate stage and a mark on said
mask..Iaddend..Iadd.93. An apparatus according to claim 91, wherein
said mark is formed on a fiducial plate arranged on said substrate
stage..Iaddend..Iadd.94. An apparatus according to claim 91,
further comprising an alignment system which has a center of
detection, on the image side of said projection optical system, at
a
position different from said mark detection
system..Iaddend..Iadd.95. An apparatus according to claim 94,
wherein said mark comprises a first mark portion to be detected by
said alignment system and a second mark portion to be detected by
said mark detection system, said first and second mark portions
being formed on a same plate in a positional relationship
corresponding to an arrangement of said alignment system and said
mark detection system..Iaddend..Iadd.96. An apparatus according to
claim 94, wherein a center of detection of said alignment system is
located on a measurement axis of said first interferometer and a
measurement axis of said second interferometer intersects an
optical axis of said projection optical system..Iaddend..Iadd.97.
An apparatus according to claim 94, wherein a center of detection
of said alignment system is disposed within a field of said
projection optical system..Iaddend..Iadd.98. An apparatus according
to claim 94, wherein a center of detection of said alignment system
is disposed outside of a field of said projection optical
system..Iaddend..Iadd.99. An apparatus according to claim 94,
wherein said alignment system has an objective optical system
provided independently of said projection optical system, wherein
said alignment system detects a mark on said substrate through said
objective optical system..Iaddend..Iadd.100. An exposure apparatus
which exposes a substrate with an energy beam through a mask,
comprising:
a first mark detection system to detect a mark on the
substrate;
a second mark detection system to detect a mark on the mask;
a movable member formed with a fiducial mark to be detected by said
first and second detection systems;
an interferometer to detect position information of said movable
member; and
a measurement device connected to said first and second mark
detection systems, said measurement device determining a baseline
amount of said first mark detection system based on a plurality of
detection results of the position information obtained from said
interferometer upon detections of said fiducial mark by said first
and second mark detection systems and detection results of said
first and second mark detection systems..Iaddend..Iadd.101. An
apparatus according to claim 100, wherein said fiducial mark
comprises a first mark to be detected by said first mark detection
system and a second mark to be detected by said second mark
detection system, said first and second marks being formed on the
same plate with a predetermined positional
relationship..Iaddend..Iadd.102. An apparatus according to claim
100, further comprising a projection optical system disposed
between said mask and said substrate, wherein said second mark
detection system detects said fiducial mark and a mark on said mask
through said projection optical system, and said first mark
detection system has a center of detection at a position different
from a projection point of said mark on said mask by said
projection optical system..Iaddend..Iadd.103. An apparatus
according to claim 102, wherein a center of detection of said first
mark detection system is disposed within
a field of said projection optical system..Iaddend..Iadd.104. An
apparatus according to claim 102, wherein said first mark detection
system has an objective optical system provided independently of
said projection optical system, and said first mark detection
system detects a mark on said substrate through said objective
optical system..Iaddend..Iadd.105. An exposure method of exposing a
substrate with an energy beam through a mask, comprising:
detecting a mark plate respectively by a first mark detection
system for detecting a mark on said substrate and a second mark
detection system for detecting a mark on said mask;
detecting, upon detection of said mark plate, position information
of said mark plate plural times; and
determining a baseline amount of said first mark detection system
based on detection results of said first and second mark detection
systems and a plurality of detection results of the position
information..Iaddend..Iadd.106. A method according to claim 105,
wherein said mark plate is detected by said first and second mark
detection systems substantially simultaneously..Iaddend..Iadd.107.
A method according to claim 105, wherein the detections of said
mark plate by said first and second detection systems are effected
without movement of said mark plate..Iaddend..Iadd.108. A method
according to claim 105, further comprising:
detecting position information of said mask upon detection of said
mark plate, wherein said position information of the mask is used
upon
determining said baseline amount..Iaddend..Iadd.109. An exposure
method of exposing a substrate with a projected image of a mask
through a projection optical system, comprising:
determining, while detecting a mark on a movable member through
said projection optical system, a relationship between measurement
values of first and second interferometers which have measurement
axes parallel to each other and perpendicular to a reflection
surface of said movable member; and
controlling movement of said substrate based on at least one of the
measurement values of said first and second
interferometers..Iaddend..Iadd.110. A method according to claim
109, wherein said movable member is positioned such that said mark
is detected together with at least one specific mark formed on said
mask..Iaddend..Iadd.111. A method according to claim 110, further
comprising:
detecting said mark on said movable member with an alignment system
for detecting a mark on said substrate; and
determining a baseline amount of said alignment system based on a
detection result by said alignment system and detection results of
said mark on said
movable member and said specific mark..Iaddend..Iadd.112. A method
according to claim 111, wherein a relationship between the
measurement axis of said first interferometer and a center of
detection of said alignment system is determined, and an output of
said first interferometer is used upon detection of said mark on
said substrate by said alignment system..Iaddend..Iadd.113. A
method according to claim 112, wherein a relationship between the
measurement axis of said second interferometer and an optical axis
of said projection optical system is determined, and an output of
said second interferometer is used at the time of exposure of said
substrate..Iaddend.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a projection exposure apparatus
for causing the sensitive layer coated on a substrate, such as a
water for fabricating semi-conductor device and a glass plate for
fabricating liquid crystal display device, to be exposed with the
patterned image on a mask or reticle. More particularly, the
invention relates to a projection exposure apparatus provided with
an off-axis type alignment system whereby to observe mark patterns
and others on a substrate through an objective optical system
dedicatedly fixed outside a projection optical system or a
projection optical system only.
2. Related Background Art
An example of the conventional projection exposure apparatus
provided with an off-axis type alignment system (hereinafter
expediently referred to as stepper) is disclosed in U.S. Pat. No.
4,452,526 or in Patent Abstracts of Japan, Vol. 2, No. 92, Jul. 28,
1978, p. 436 E78, JP-A-53-56975.
Also, the fundamental concept of the off-axis system is disclosed
in a paper entitled "ALIGNING SEMI-CONDUCTOR MASKS" in IBM
Technical Disclosure Bulletin Vol. 13, No. 7, December 1970, p.
1816. Further, in U.S. Pat. No. 4,677,301 filed by the same
assignee hereof for the present invention, there is proposed an
alignment method for a sensitive substrate using both of two
alignment systems of the off-axis type. One of the alignment
systems is a system for detecting the mark on the sensitive plate
appearing at a predetermined position in the image field of the
projection optical system only through the projection optical
system which performs a focusing of the original pattern on the
mask or (reticle) onto the sensitive plate (hereinafter referred to
as inner-field off-axis alignment system). The other is a system
for detecting the mark on the sensitive plate through the dedicated
objective optical system fixed to the outside of the projection
optical system (hereinafter referred to as outer-field off-axis
alignment system). The provision of such two kinds of off-axis
alignment systems is also disclosed in the aforesaid U.S. Pat. No.
4,452,526.
The above-mentioned conventional apparatus has a fiducial plate
with a fiducial mark as a reference fixedly on the wafer stage
which is secondarily movably by a step and repeat method. This
fiducial plate is used for measuring the distance between the
off-axis alignment system and projection optical system, that is,
the base line value between them. Now, in conjunction with FIG. 1,
the principle of the conventional base line measurement will be
described. In this description, the off-axis alignment system is
assumed to be of the outer-field type.
In FIG. 1, a main condenser lens ICL illuminates the reticle (mask)
R evenly at the time of exposure. The reticle R is supported by a
reticle stage RST. This reticle stage RST is driven to enable the
center CC of the reticle R to be aligned with the optical axis AX
of the projection optical system (hereinafter referred to as
projection lens) PL. Meanwhile, on a wafer stage WST, the fiducial
mark FM, which is equivalent to the alignment mark formed on the
wafer surface, is provided, and when the stage WST is positioned to
allow this fiducial mark FM to arrive at a predetermined position
in the projection field of the projection lens PL, the mark R on
the reticle R and fiducial mark FM are detected at the same
time by the alignment system DDA of a TTL (through the lens) type
provided above the reticle R. The distance La between the mark RM
and the center CC of the reticle R is a predetermined value defined
at the time of designing. Therefore, the distance between the
projection point of the mark RM on the image formation side (wafer
side) of the projection lens PL and the projection point of the
center CC becomes La/M, where M is magnification of the projection
lens PL observed from the wafer side to the reticle side, and in
the case of a 1/5 reduction projecting lens, M=5.
Also, on the outside (outer field of projection) of the projection
lens PL, an off-axis wafer alignment system OWA is fixedly mounted.
The optical axis of the wafer alignment system OWA is in parallel
with the optical axis AX of the projection lens PL on the side of
the projection image plane (on wafer plane). Then, inside the wafer
alignment system OWA, there is provided a target mark TM on the
glass plate, which serves to be the reference when the mark on the
wafer or the fiducial mark FM is aligned. This target mark TM is
arranged substantially in conjugation with the projected image
plane (wafer surface or the surface of the fiducial mark FM).
Now, the base line value BL is obtained as shown in FIG. 1 by
measuring the position X.sub.1 of the stage WST when the reticle
mark RM and fiducial mark FM are aligned as well as the position
X.sub.2 of the stage WST when the target mark TM and fiducial mark
FM are aligned by the use of an interferometer and others and then
calculated the difference (X.sub.1 -X.sub.2) This base line value
BL will be a reference value when the mark on the wafer is
transferred to the position just below the projection lens PL by an
alignment conducted by the wafer alignment system OWA later. In
other words, given the distance between the center of one shot (an
area to be exposed) on the wafer and the mark on the wafer as XP,
and the position of the wafer stage WST when the wafer mark and the
target mark TM are matched as X.sub.3, the wafer stage WST should
be driven to the position obtained by an expression given below in
order to coincide with the center of the shot with the center CC of
the reticle.
In this respect, this expression represents the position in only
one dimensional direction as a principle, and in practice, a
two-dimensional consideration should be given. Moreover, the
calculating method should be different depending on the
arrangements of the TTL alignment system DDA (that is, the
arrangement of the mark RM), wafer alignment system OWA, and
others.
In any case, subsequent to having detected the mark position on the
wafer using the wafer alignment system OWA of the off-axis type,
the wafer stage WST is driven for a predetermined amount. Hence
conducting exposure immediately after the pattern on the reticle R
has been overlapped with the shot area on the wafer accurately.
In a conventional technique such as described above, when the
positional relationship between the detected center point (the
center of the target mark TM) of the off-axis alignment system OWA
and the projection point of the mark RM on the reticle R by the
projection lens PL (base line value BL) is measured, its relative
distance is obtained by a laser interferometer while driving the
wafer stage WST. Consequently, due to the running accuracy of the
wafer stage WST, air fluctuation on the optical path of the laser
beam of the laser interferometer, and other unavoidable causes,
there is a limit to the improvement of the precision with which to
measure the base line value. Also, it is necessary to drive the
wafer stage WST for positioning the fiducial mark FM in the
detection area in the TTL alignment system DDA and also to drive
the wafer stage WST for positioning the fiducial mark FM in the
detection center of the off-axis alignment system OWA. Thus there
is a limit to the increase in the speed with which to execute the
base line measurement process.
Further in the conventional stepper, the extended line of the
measuring axis (beam optical axis) of the laser interferometer for
measuring the position of the wafer stage WST is simply set to be
perpendicular to the optical axis of the projection lens both in
the direction X and direction Y as shown in U.S. Pat. No.
4,677,301. Accordingly, it is considered difficult to implement the
direction of the mark detection so that Abbe's error (sine error)
is always zero when the off-axis alignment system OWA is used for
detecting various marks. Then, it may also be considered to provide
a combination of laser interferometers by arranging a set of a
laser interferometer thereby to make the Abbe's error zero with
respect to the optical axis of the projection lens and a set of a
laser interferometer thereby to make the Abbe's error zero with
respect to the detection center of the off-axis alignment system
OWA.
The application for a patent on an apparatus for which a conception
such as this is materialized has been already filed by the inventor
et al hereof and is issued as U.S. Pat. No. 5,003,342.
In this case, the two sets of the laser interferometers are used by
switching them for the stage position measurement for the wafer
alignment using the off-axis alignment system OWA and for the stage
position measurement at the time of projection exposure. However,
the adjustability (coordination) of the values in measuring both
positions must be taken into account. Otherwise, errors may result
inevitably.
In the above-mentioned U.S. Pat. No. 5,003,342, in order to
effectuate the adjustability required, one set of the
interferometer is reset while the fiducial mark plate is positioned
just below the projection lens and the other set of the
interferometer is reset while the fiducial mark plate is positioned
just below the off-axis alignment system. Nevertheless, during each
operation of the two sets of the interferometers, the wafer state
is caused to be driven for the predetermined amount. Strictly
speaking, there still remain errors in the running accuracy of the
wafer stage, particularly depending on its yawing, and errors due
to the air fluctuation (refraction index disturbance) in the
optical path of the laser interferometers.
SUMMARY OF THE INVENTION
It is an object of the present invention to improve the total
accuracy of a projection exposure apparatus having an off-axis
alignment system.
It is another object of the present invention to provide a
projection exposure apparatus in which the measurement precision is
improved for the base line value of its off-axis alignment system,
and the method of alignment therefor.
It is still another object of the present invention to shorten the
time required for the base line measurement.
It is a further object of the present invention to obtain a
structure which is not affected by the generation of errors due to
the air disturbance in the optical path of the laser interferometer
when the base line measurement is performed.
It is still a further object of the present invention to reduce the
systematic errors for the apparatus provided with a laser
interferometer capable of satisfying the Abbe's condition with
respect to its projection optical system as well as with a laser
interferometer capable of satisfying the Abbe's condition with
respect to its off-axis alignment system.
It is still a further object of the present invention to obtain a
structure capable of performing the alignment of the mask (reticle)
for the main body of the apparatus and the base line measurement
almost simultaneously without driving the stage.
It is still a further object of the present invention to obtain an
alignment sequence thereby to restrain the reduction of throughput
even if the base line measurement is executed at each time of
exchanging sensitive plates.
In order to achieve the above-mentioned objects, a first fiducial
mark (FM.sub.1) which can be detected by an off-axis alignment
system (OWA) and a second fiducial mark (FM.sub.2) which can be
detected by an alignment system than a projection optical system
are provided in parallel with a predetermined position relationship
required for designing according to the present invention. Then, at
the same time that the positional deviation between the marks
(RM.sub.1 and RM.sub.2) on the reticle (mask) and the second
fiducial mark (FM.sub.2) on the stage is detected by the alignment
system through the projection optical system, the positional
deviation of the first fiducial mark (FM.sub.1) from the detection
center is detected by the off-axis alignment system (OWA). Further,
in accordance with the designed interval (including manufacturing
errors) between the first fiducial mark (FM.sub.1) and second
fiducial mark (FM.sub.2) and each of the detected positional
deviations, the relative distance between the projection point of
the mark on the reticle by the projection system and the detected
center point by the off-axis alignment system OWA, that is, a base
line value, is calculated.
In the present invention, the alignment system through the
projection optical system and the off-axis alignment system are
used at the same time, thus making it possible to execute the base
line measurement highly accurately without being affected by the
running accuracy of the stage and the respective positioning
accuracies required. Also, there is an advantage that the
measurement is not dependent on the measuring value of the laser
interferometer for measuring the stage position because the base
line value is not measured on the basis of the driving amount of
the wafer stage which is only obtainable when it is driven as in
the conventional art.
Furthermore, in the present invention, at the same time that the
positional detection of the reticle (mask), detection of the amount
of the positional deviation, or the reticle alignment is performed
in accordance with each of the marks provided on the stage as
references, the base line measurement is executed with the fiducial
plate as its reference. Hence, all the references are coordinated
and at the same time, there is an advantage that a series of
processes is completed in a short period. In other words, it is
possible to obtain both effects of a higher accuracy and higher
throughput.
In another invention of the application hereof, a fiducial plate
FP, with the fiducial mark FM.sub.2 to be aligned with the mark RM
on the reticle R and the fiducial mark FM.sub.1 to be aligned with
the detection center of the off-axis alignment system OWA being
formed together thereon, is provided on the wafer stage WST. Then,
the structure is arranged so that when the base line is measured,
the positional deviation between the reticle R and fiducial plate
FP is obtained while the wafer stage WST is at rest and at the same
time, the positional deviation between the detection center of the
off-axis alignment system OWA and the fiducial plate FP is
obtained. Further, the structure is arranged so that a pair of
interferometers (IFX and IFY.sub.1) which can satisfy the Abbe's
condition with respect to the off-axis alignment system OWA and a
pair of interferometers (IFX and IFY.sub.2) which can satisfy the
Abbe's condition with respect to the projection optical system are
provided, and inner counters are arranged to be presettable with
each other so as to make the measuring values of the
above-mentioned two sets of the interferometers equal at the
position of the wafer stage at the time of the above-mentioned base
line measurement.
When the position of the reticle R is measured using the fiducial
plate FP at the time of the base line measurement, the imaginary
line connecting the reference points of the two interferometers
using for the same direction, that is, Y-direction measurement, for
example, can be in parallel precisely with the reflection plane of
the movable mirror (IMy) in the direction Y on the wafer stage just
by presetting the measuring values of the two sets of the
interferometers to be equal.
Furthermore, in the present invention, the structure is arranged so
that the inner counters of the two sets of the interferometers can
be preset with each other when the reticle R is in a state that it
can be aligned to the fiducial plate FP. Hence, there is an
advantage that it is unnecessary to provide any corrective
calculation in consideration of the yawing of the stage and the
like once the presetting has been performed.
Moreover, according to the present invention, the reticle alignment
and base line measurement can be performed almost simultaneously.
Therefore, even if a sequence is arranged so as to measure the base
line value at each time of exchanging wafers, there is no
possibility that the throughput is reduced. As a result, it becomes
possible to confirm at a high speed for correction any long-term
drift of the base line and the positional drift of the reticle
holder resulting from the illumination of the exposure light to the
reticle.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a view illustrating the state of a base line measurement
in a conventional projection exposure apparatus.
FIG. 2 is a perspective view illustrating the structure of a
projection exposure apparatus according to an embodiment of the
present invention.
FIG. 3 is a plan view showing the arrangement of a fiducial mark
plate on a wafer stage.
FIG. 4 is a plan view showing the arrangement of various marks on
the fiducial mark plate.
FIG. 5 is a plan view showing the arrangement relationship between
the image field of a projection lens, reticle pattern, and fiducial
mark.
FIG. 6 is a view showing an example of the shape of a reticle
alignment mark.
FIG. 7 is a view showing the structure of a TTR alignment
system.
FIG. 8 is a view showing the structure of a TTL alignment
system.
FIG. 9 is a view showing the pattern arrangement of the indication
plane of an off-axis alignment system.
FIG. 10 is a view showing the structure of the off-axis alignment
system.
FIG. 11 is an enlargement of the fiducial mark FM.sub.1 on the
fiducial mark plate.
FIG. 12 is an enlargement of the fiducial marks FM.sub.2, LIM, and
LSM on the fiducial mark plate.
FIGS. 13A and 13B are views illustrating the error in taking the
fiducial mark plate to the wafer stage and the measurement method
therefor
FIG. 14 is a view illustrating the typical sequence for the
apparatus.
FIG. 15 is a view illustrating the typical sequence for the
apparatus.
FIGS. 16A and 16B are views showing examples of the waveforms
detected by the LSA system and ISS system.
FIG. 17 is a table showing the constant values and measured values
required for the base line control.
FIG. 18 is a plan view showing the shot arrangement and the wafer
mark arrangement on a
FIG. 19 is a view illustrating the principle of the mutual
presetting of the two interferometers in the direction Y.
FIG. 20 is a circuit block diagram showing an example of performing
the preset of the interferometers.
FIG. 21 is a view illustrating another pattern of the emission mark
on the fiducial mark plate.
FIG. 22 is a plane view showing another arrangement of the off-axis
alignment system.
FIG. 23 is a perspective view illustrating the structure of a
projection exposure apparatus according to a second embodiment of
the present invention.
FIG. 24 is a view illustrating the structure of the apparatus shown
in FIG. 23 on the wafer stage and the structure of its control
system.
FIG. 25 is a view illustrating the operation of the second
embodiment according to the present invention.
FIG. 26 is a view illustrating the shot arrangement on the wafer
when the mounting error of the fiducial plate is obtained by a
pilot exposure.
FIGS. 27A, 27B and 27C are views illustrating the pattern
arrangement of the test reticle for the chip rotation measurement
and the method of stepping therefor.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Hereinafter, in conjunction with the accompanying drawings, the
present invention will be described in detail.
FIG. 2 is a perspective view illustrating the structure of a
projection exposure apparatus according to a first embodiment of
the present invention, in which the same reference marks are given
to the members which are the same as those appearing in FIG. 1. In
FIG. 2, there are provided on the reticle R, a pattern area PA with
the circuit patterns to be exposed on the wafer, and the reticle
marks RM.sub.1 and RM.sub.2 for alignment. These reticle marks
RM.sub.1 and RM.sub.2 are photoelectrically detected through the
object lenses 1A and 1B of a first TTL alignment system
respectively. Also, a reticle stage RST is movably driven by a
motor and others of the driving system, which is not shown in FIG.
2, in two dimensional (X, Y, and .theta.) directions and its
driving amounts or driving positions are sequentially detected by
three laser interferometers IRX, IRY, and IR.theta.. The rotational
driving amount of the reticle stage RST around the coordinate axis
Z (which is parallel with the optical axis AX) is obtained by the
difference between the measures values of the interferometers IRY
and IR.theta., and the translational driving amount in the Y axis
direction is obtained by the arithmetic mean of the measured values
of the interferometers IRY ad IR.theta. while the translational
driving amount in the X axis direction is obtained by the
interferometer IRX.
In the present embodiment, a second TTL alignment system which
detects marks on the wafer W only through the projection lens PL is
provided separately for the use in the direction X and direction Y.
The second TTL alignment system for the X direction use includes a
mirror 2X and object lens 3X and others fixed between the reticle
stage RST and projection lens PL. The second TTL alignment system
for the Y direction use includes likewise a mirror 2Y and object
lens 3Y and others.
In the present embodiment, the first TTL alignment system which
includes the object lenses 1A and 1B is hereinafter referred to as
a TTR (through the reticle) alignment system while the second TTL
alignment system which includes the object lenses 3X and 3Y are
simply referred to as a TTL alignment system.
Now, on the two sides of the wafer stage WST where the wafer W is
mounted, a movable mirror IMx which reflects the beam from the
laser interferometer IFX and a movable mirror IMy which reflects
the beam respectively from the laser interferometers IFY.sub.1 and
IFY.sub.2 are fixed. The beam from the interferometer IFX is
perpendicular to the reflecting plane of the movable mirror IMx
which is extended in the direction Y, and the extended line of the
beam is rectangular to the extended line of the optical axis AX of
the projection lens PL. The beam from the interferometer IFY.sub.2
is perpendicular to the reflecting plane of the movable mirror IMy
which is extended in the direction X, and the extended line of the
beam is also rectangular to the extended line of the optical axis
AX. The beam from another interferometer IFY.sub.1 is perpendicular
to the reflecting plane of the movable mirror IMy, and is in
parallel with the beam from the interferometer IFY.sub.2.
Also, the off-axis wafer alignment system includes a reflective
prism (or mirror) 4A fixed in the closest vicinity of the lower end
portion of the projection lens PL, an object lens 4B, and others.
The light receiving system 4C of the wafer alignment system
includes a target plate formed target mark TM in its inside and CCD
camera for taking the marks of wafer focused on the target mark
plate through the prism 4A and object lens 4B. In the present
embodiment, the optical axis of the object lens 4B falling on the
wafer stage WST through the prism 4A, and the optical axis AX of
the projection lens PL are set apart for a predetermined distance
only in the direction X while these optical axes are set with
almost no positional difference in the direction Y.
The extended line of the optical axis of the object lens 4B which
falls on the wafer stage WST is rectangular to the extended line of
the beam from the interferometer IFX and the extended line of the
beam from the interferometer IFY.sub.1, respectively. An
arrangement of the interferometers such as this is disclosed in
detail in U.S. Pat. No. 5,003,342.
On the wafer stage WST, there is fixedly provided a fiducial plate
FP with two fiducial marks FM1 and FM2 being arranged for measuring
the base line. The fiducial plate FP is arranged at the corner
surrounded by the two movable mirrors IMx and IMy on the wafer
stage WST. This plate is formed with a transparent material of a
low expansion coefficient such as a fused quartz plate on which a
light shielding layer is formed, and the part thereof is etched to
provide the fiducial marks FM1 and FM2. The fiducial mark FM.sub.1
can be detected by the off-axis wafer alignment system (4A, 4B, and
4C) while the fiducial mark FM2 can be detected by the TTR
alignment system (1A and 1B) or TTL alignment system (2X and 3X; 2Y
and 3Y).
The distance between these fiducial marks FM1 and FM2 in the
direction X is defined with precision of sub-micron order but if
there is any residual amount of arrangement error, it is assumed
that such a value has been precisely measured beforehand and that
has been obtained as an apparatus constant value.
FIG. 3 is a plan view showing the arrangement of each member on the
wafer stage SWT. The wafer W is mounted on a finely rotational
wafer holder WH on the wafer stage WST, and is absorbed by vacuum.
In the present embodiment the linear knotch (orientation flat) OF
of the wafer W is prealigned mechanically to be in parallel with
the axis X. Then, the wafer is mounted on the wafer holder WH.
As shown in FIG. 3, the center ponit of the diameter of the lowest
end of the lens barrel of the projection lens PL (that is optical
axis AX) and the field of the objective lens 4B are arranged as
close as possible. When the projection lens PL and the fiducial
plate FP are thus arranged, the wafer W is driven diagonally to the
lowest right-hand side in FIG. 3 from just below the projection
lens PL. Therefore, in this state, it is possible to load or unload
the wafer W. An apparatus with an arrangement of the kind is
disclosed in U.S. Pat. No. 4,897,553.
FIG. 4 is a plan view showing the detailed mark arrangement of the
fiducial marks FM1 and FM2 on the fiducial plate FP.
In FIG. 4, the intersecting point of the straight line LX which is
in parallel with the X axis and a straight line LY.sub.2 is the
center of the fiducial mark FM.sub.2, and when the base line is
measured, this intersecting point is substantially coincided with
the optical axis AX of the projection lens PL. In the present
embodiment, an light emission type cross-shaped slit mark IFS is
arranged on the intersecting point. Then, an illuminating light
having the same wavelength as the exposure light illuminates only a
localized area ISa including the light emission slit mark IFS from
the reverse side of the fiducial plate FP. Also, on the symmetrical
two locations on the straight line LX with respect to the light
emission slit mark IFS, the fiducial marks FM2A and FM2B
corresponding to the respective arrangements of the reticle marks
RM.sub.1 and RM.sub.2 are provided. These marks FM2A and FM2B are
formed by etching the chrome layer on the fiducial plate FP with
the cross type slit respectively. The mark FM2A is aligned with the
reticle mark RM.sub.1 while the mark FM2B is aligned with the
reticle mark RM.sub.2. In this respect, the light emission slit
mark is the same as disclosed in the above-mentioned U.S. Pat No.
4,897,553.
The circular area PIF with the center (intersecting point) of the
light emission slit mark IFS as its origin is a projecting field
area of the projection lens PL, and in the present embodiment, the
mark LIMx which is detectable by the TTL alignment system (2X and
3X) in the direction X as shown in FIG. 2 is arranged on the
straight line LY.sub.2 in the field area PIF while the two marks
LIMy and LSMy which are detectable by the TTL alignment system (2Y
and 3Y) in the direction Y are arranged on the straight line LX in
the field area PIF. As regards the detailed relations of the
respective marks will be described later. In the present
embodiment, however, each of the marks FM2A, FM2B, LIMx, and LIMy
is arranged so that the TTL alignment system (2X and 3X) in the
direction X detects the mark LIMx and the TTL alignment system (2Y
and 3Y) in the direction Y detects the mark LIMy in the state that
the two TTL alignment systems 1A and 1B detect the reticle marks
RM.sub.1 and RM.sub.2 and the fiducial marks FM2A and FM2B at the
same time, respectively.
On the other hand, the straight line LY.sub.1 which is provided
away in the direction X from the straight line LY.sub.2 by a
predetermined distance is in parallel with the Y axis, and on the
intersecting point of the straight lines LY.sub.1 and straight line
Lx, there is formed a large fiducial mark FM1 which can be included
in the field MIF of the objective lens 4B of the off-axis alignment
system. The mark FM1 is an integrated member of a plurality of line
patterns provided in parallel in the direction X and direction Y,
respectively, in order to make a two dimensional alignment
possible. In this respect, as clear from the description set forth
above, the fiducial plate FP is fixed on the wafer stage WST to
enable the straight line LY.sub.1 to be coincided with the center
line (measuring axis) of the beam of the interferometer IFY.sub.1
as much as possible in the X - Y plane as well as the straight line
LT.sub.2 to be matched with the center line (measuring axis) of the
beam of the interferometer IFY.sub.2 as much as possible (that is,
to avoid the occurrence of any rotational deviation as much as
possible).
Further, at the positions symmetrical to the intersecting point of
the straight lines LX and LY.sub.1 on the straight line LX, the two
fiducial marks FM2C and FM2D are provided. The fiducial marks FM2C
and FM2D are of the cross type slit patterns which are exactly the
same shape and size of the fiducial marks FM2A and FM2B. The
distance between them in the direction X are also exactly the same
distance between the marks FM2A and FM2B. In this respect, the mark
LSMx in FIG. 4 is detected by the TTL alignment system (2X and 3X)
in the direction X. Hence, this is provided at the same position as
the X coordinate value of the fiducial mark FM2B.
FIG. 5 is an enlargement of each mark arrangement on the fiducial
mark FM2 side only on the fiducial plate FP and shows the state
that the center of the projecting field area PIF of the projection
lens PL is matched with the intersecting point of the light
emission slit mark IFS.
In FIG. 5, the positional relationship between the peripheral shape
of reticle R which is ideally position in such state and the
peripheral shape of the pattern area PA is also indicated by
two-dot chain lines. The marks LIMx and LIMy for the TTL alignment
system are positioned at the outermost periphery of the projecting
field PIF, but this because of the fact that the mirrors 2X and 2Y
at the leading end of the TTL alignment system are arranged so as
not to shield the projection area of the pattern area PA. In this
state, the fiducial mark FM2A can be aligned with the reticle mark
RM.sub.1, but the reticle mark RM.sub.1 (RM.sub.2 also the same) is
structured with the double slit mark RM.sub.1 y which is extended
in the direction X and the double slit mark RM.sub.1 x which is
extended in the direction Y, and these marks RM.sub.1 y and
RM.sub.1 x are formed as a dark portion in the transparent portion
surrounded by the oblong shielding member SB as shown in FIG. 6.
Among the cross-shaped slits of the fiducial mark FM2A, the slit
which is extended in the direction X is sandwiched by the double
slit marks RM.sub.1 y while the slit which is extended in the
direction Y is sandwiched by the double slit marks RM.sub.1 x,
hence making it possible to achieve the ideal alignment.
Here, the interval k.sub.1 between the center of the fiducial mark
FM2A and the center of the mark LIMy in the direction X and the
interval K.sub.2 the center of the light emission slit mark IFS and
the center of the mark LSMy in the direction X shown in FIG. 5 are
defined to provide a difference just for an offset amount .DELTA.Xk
in the direction X (which value is on the wafer side) when the
light emission slit mark IFS shown in FIG. 6 scans the reticle mark
RM.sub.1 in the direction Y. In other words, it is defined to be
K.sub.1 =K.sub.2 +.DELTA.Xk or K.sub.1 =K.sub.2 -.DELTA.Xk.
Further, the central position of the mark LSMx in the direction X
which is detectable by the TTL alignment system in the direction X
coincides with the central position of the fiducial mark FM2B in
the direction X. This is a condition that is satisfied when the
intervals K.sub.3 in the direction X between each of the center
points of the fiducial marks FM2A and FM2B in the two locations and
the center of the light emission slit mark IFS are both equal.
Also, the position of the mark LSMx in the direction Y is
substantially equal to the position of the mark LIMx in the
direction Y. Strictly speaking, however, if the interval between
the center of the light emission mark IFS and the center of the
mark LIMx in the direction Y is given as K.sub.4 and the center of
the light emission mark IFS and the center of the mark LSMx in the
direction Y is given as K.sub.5, these are defined to be a
relationship K.sub.4 =K.sub.5 +.DELTA.Yk or K.sub.4 =K.sub.5
-.DELTA.Yk. Here, .DELTA.Yk is an offset amount in the direction Y
when the light emission slit mark IFS scans the double slit mark
RM.sub.1 x in the direction X as shown in FIG. 6.
Subsequently, in reference to FIG. 7, the detailed structure of the
TTR alignment system (1A) will be described. A total reflection
mirror 100 is provided slantly at an angle of 45 degrees above the
reticle mark RM.sub.1 to set the optical axis of the objective lens
101 perpendicular to the reticle R. This TTR alignment system has a
self-illuminating system for a coaxial falling illumination,
comprising a beam splitter 102, a light source 103 for generating
light of exposure wavelength, a shutter 104 for switching the
shield and passage of the illuminating light, an optical fiber 105
for conducting the illuminating light, a condenser lens 106 for
converging the illuminating light from the emission ends of the
optical fiber 105 to illuminate an illumination field aperture 107
evenly, and a lens system 109 for transmitting the illuminating
light from the field aperture 107 to the objective lens 101 on
Koehler illumination condition. In this way, the objective lens 101
illuminates only the inside of the shielding zone SB in which the
mark RM.sub.1 of the reticle R. Thus, the reflecting light from the
mark RM.sub.1 is reflected by the beam splitter 102 through the
mirror 100 and objective lens 101 to entire the image formation
lens 110. The imaging light of the mark RM.sub.1 is divided into
two by a half mirror 111 to enable an enlarged image to be formed
by the image formation lens 110 respectively on the image formation
planes of a CCD camera 112X for X-direction detection and a CCD
camera 112Y for Y-direction detection. The CCD cameras 112X and
112Y are arranged to provide the directions of its horizontal
scanning to be rectangular to each other with respect to the
enlarged image of the mark RM.sub.1.
At this juncture, when the fiducial mark FM2A on the fiducial plate
FP is position directly below the inner area of the shielding zone
SB including the mark RM.sub.1, the CCD 112X and CCD 112Y take the
cross-shaped slit of the fiducial mark FM2A as black line. Then an
image processing circuit 113X executes a digital waveform process
of image signals from the CCD camera 112X to obtain the positional
deviation amount of the X-direction (horizontal scanning direction)
between the slit of the fiducial mark FM2A which is extended in the
direction Y and the double slit mark RM.sub.1 x of the reticle mark
RM.sub.1. An image processing circuit 113Y executes a digital
waveform process of image signals from the CCD camera 112Y to
obtain the positional deviation amount of the Y-direction
(horizontal scanning direction) between the slit of the fiducial
mark FM2A which is extended in the direction X and the double slit
mark RM.sub.1 y of the reticle mark RM.sub.1. A main control system
114 controls the driving system 115 of the reticle stage RST to
correct the position of the reticle R if the positional deviation
amounts of the fiducial mark FM2A and reticle mark RM.sub.1 in the
directions X and Y thus obtained by the processing circuits 113X
and 113Y are beyond the predetermined tolerance range.
The driving system 115 detects the positions (X, Y, and .theta.) of
the reticle stage RST by the three interferometers IRX, IRY, and
IR.theta. (shown in FIG. 2) before correction of the stage RST, and
obtained the measuring values to be detected by the three
interferometers IRX, IRY, and IR.theta. after correction by
computation. Therefore, the driving system 115 serves to position
the reticle stage RST by its servo control so as to make each of
the measuring values of the three interferometers IRX, IRY, and
IR.theta. the measuring value which should be detected after
correction of the stage RST. Also, the main control system 114
controls the driving system 116 which performs the servo control on
the position of the wafer stage WST on the basis of the measuring
values of the interferometers IFX, IFY.sub.1, or IFY.sub.2.
Now, as shown in FIG. 7, there is provided in the TTL alignment
system 1A, a light receiving system for the light emission mark for
detecting the illuminating light from the light emission mark IFS
on the fiducial plate FP through the projection lens PL, the
transparent portion in the inside of the light shield zone SB of
the reticle R, the mirror 100, the objective lens 101, the beam
splitter 102, the lens system 109 and the beam splitter 108. This
light receiving system for the light emission mark comprises a lens
system 120, a photoelectric sensor (photomultiplier) 121 and
others, and the light receiving plane of the photoelectric sensor
121 is conjugately arranged with the pupil plane EP of the
projection lens PL and the pupil plane formed between the objective
lens 101 and the lens system 109. The photoelectric sensor 121
detects photoelectrically the amount of passing light which is
varied when the light emission mark IFS scans the reticle mark
RM.sub.1 (or RM.sub.2) to output photoelectric signals SSD in
accordance with its variations. The process of this photoelectric
signals SSD is executed by the digital sampling of the signal
waveforms in response to the up down pulses (one pulse per driving
amount of 0.02 .mu.m, for example) being output from the
interferometers IFX and IFY.sub.2 accompanying the scanning of the
wafer stage WST for storage in a memory.
Now, in reference to FIG. 8, an example of the structure of the TTL
alignment systems (2Y and 3Y) in FIG. 2 will be described. The TTL
alignment system used for the present embodiment prevent the effect
produced by the wafer resist layer of the wafer W when the mark
reflection light is detected as well as the sensitizing the resist
layer by utilizing the red light from an He-Ne laser source 130 as
the mark illumination light. Further, in this TTL alignment system,
two alignment sensors each having a different mark detection
principle are incorporated so that the two alignment sensors can be
used alternatively by sharing the objective lens 3Y. A structure of
the kind is precisely disclosed in an application Ser. No. 55,504
filed on Apr. 6, 1990 which is currently copending. Hence, the
description thereof will be made briefly here.
The He-Ne laser light from the laser source 130 is divided by the
beam splitter 131 to arrive at shutter 132A and 132B which are
opened and closed complementarily. In FIG. 8, the shutter 132A is
opened while the shutter 132B is closed. The laser light enters the
light transmission system 133A of a double beam interference
alignment system (L1A). This light transmission system 133A divides
the incident beam into two laser beams, and outputs the two laser
beams with a constant frequency differential by the use of an
acousto-optic modulation (AOM) element. In the case represented by
FIG. 8, the two laser beams output from the light transmission
system 133A are paralleled in the direction perpendicular to the
plane of FIG. 8. These two laser beams are reflected by a half
mirror 134 and are further divided into two by a beam splitter 135.
The two laser beams reflected by the beam splitter 135 are caused
to intersect on the diaphragm APA of the conjugation plane of the
wafer by the objective lens 3Y. The two parallel laser beams
passing the diaphragm APA with predetermined intersection angle are
reflected by a mirror 2Y to enter the projection lens PL, and are
again caused to intersect on the wafer W or on the fiducial plate
FP. In the area where these two laser beams are caused to
intersect, one dimensional interference fringes are produced. The
interference fringes thus produced flow in the direction of the
pitches of the interference fringes at a velocity corresponding to
the frequency differential of the two beams.
Here, the marks LIMy and LIMx shown in FIG. 4 and FIG. 5 are
assumed to be diffraction gratings in parallel with the
interference fringes. Then, from the marks LIMx and LIMy of the
diffraction grating type, the interference beat light capable of
varying its intensity in response to the frequency differential are
generated. Now, if the pitches of the diffraction gratings of the
marks LIMx and LIMy and the pitches of the interference fringes are
arranged to be in a certain constant relation, the interference
beat light are generated vertically from the wafer W or fiducial
plate FP and caused to return through the projection lens PL along
the optical path of the two transmission beams sequentially from
the mirror 2Y, diaphragm APA and the objective lens 3Y. Then, the
interference beat light are transmitted partially through the beam
splitter 135 to reach a photoelectric detector 139. The light
receiving plane of the photoelectric detector 139 is arranged
almost conjugately with the pupil plane EP of the projection lens
PL. Also, on the light receiving plane of the photoelectric
detector 139, a plurality of photoelectric elements (photodiode,
phototransistor, and others) are arranged apart from each other.
The interference beat light are received by the photoelectric
element positioned in the center (the center of pupil) of the
photoelectric detector 139. The photoelectric singals becomes
signals of alternating current having the sine waveform which is of
the same frequency as the heating frequency, and are inputted into
a phase difference measuring circuit 140 as the measuring
signal.
Also, the two transmission light beams which have been transmitted
through the beam splitter 135 become parallel beams on the
reference grating plate 137 of the transmission type by an inverted
Fourier transform lens 136 for intersecting. Therefore, on the
reference grating plate 137, one dimensional interference fringes
are formed. The interference fringes are caused to flow in one
direction at a velocity in response to the beating frequency. The
photoelectric element 138 receives the interference light of .+-.
primary diffraction light coaxially generated from the reference
grating plate 137 or either the interference light of zeroth light
or the interference light of secondary diffraction light. The
intensities of these rays of interference light are also varied
into the sine waveform by the frequency which is equal to the
beating frequency, and the photoelectric element 138 outputs
signals of alternating current having the same frequency as the
beating frequency to the phase difference measuring circuit 140 as
the reference signal.
The phase difference measuring circuit 140 obtains the phase
difference .DELTA..phi. (within .+-.180.degree.) between the
measuring signal from the photoelectric detector 139 and the
reference signal from the photoelectric element 138, and outputs
the positional deviation about Y direction of the mark LIMy on the
fiducial plate FP corresponding to the phase difference
.DELTA..phi. (or an equivalent mark on the wafer), that is, outputs
the information SSB of the positional deviation amount in the
direction of the grating pitches to the main control system 114 in
FIG. 7. The resolution of the positional deviation detection is
determined by the relationship between the pitches of the mark LIMy
and the pitches of the interference fringes illuminated on this
mark, and the resolution of the phase difference detection circuit.
However, assuming that the resolution of the phase difference
detection is .+-.1.degree. and given the grating pitch Pg of the
mark LIMy as 8 .mu.m, and the pitch Pf of the interference fringe
as Pg/2, the detection resolution of the positional deviation is
expressed as .+-.(1.degree./180.degree.).times.(Pg/4). Hence the
resolution being approximately .+-.0.01 .mu.m.
The main control system 114 in FIG. 7 performs the servo control of
the driving system 116 of the wafer stage WST on the basis of the
positional deviation information SSB from the TTL alignment system
of a high resolution L1A type such as this in order to servo lock
the wafer stage WST so that the mark LIMy on the fiducial plate FP
is always driven to maintain a predetermined positional
relationship with the reference grating plate 137.
However, when the servo lock is given, it is not necessarily
required to convert the phase difference to the positional
deviation amount if only the phase difference of the signals
respectively from the photoelectric element 138 and photoelectric
detector 139 is stabilized at a predetermined value, and it is
possible to perform the servo lock by detecting only the variation
of the phase difference.
Another detection method for the TTL alignment system is such that
as disclosed in the aforesaid application Ser. No. 505,504
currently copending, the mark is scanned against the slit type
laser spot light which is extended in the direction rectangular to
the direction of the mark detection, and the signal level
obtainable by detecting the diffraction and scattered rays of light
being generated from the mark photoelectrically is digitally
sampled in response to the up down pules from the interferometers
IFX and IFY.sub.2, which are generated accompanying the drawing of
the wafer stage WST for the mark scanning.
In FIG. 8, the laser beam enters the light transmission system 133B
of the laser step alignment (LSA) type when the shutter 132B is
opened while the shutter 132A is closed. The incident beam is
formed to be or a slit type having the beam cross-section at the
converging point being extended in one direction by the functions
of a beam expander and a cylindrical lens. The incident beam thus
formed enters the projection lens PL through the beam splitters 134
and 135, the lens system 3Y, and mirror 2Y. At this juncture, the
diagram APA is conjugated with the surface of wafer or the fiducial
plate FP under the wavelength of the He-Ne laser light, and the
beam is converged thereto in the slit shape. In a case of TTL
alignment system represented in FIG. 8, the beam spot produced by
the LSA method is formed in the slit shape which is extended in the
direction X at a rest position in the projection field PIF. When
the wafer stage WST is scanned in the direction Y and the mark LSMy
on the fiducial plate FP crosses the beam spot, the diffraction
light and rays of the scattered light generated from this mark LSMy
are caused to arrive at the photoelectric detector 139 through the
projection lens PL, mirror 2Y, objective lens 3Y and beam splitter
135 and are received by photoelectric elements on the circumference
other than the central photoelectric element. The photoelectric
signals from these photoelectric elements are inputted into an LSA
processing circuit 142 so as to be sampled digitally in response to
the up down pulse signals UDP from the interferometer IFY.sub.2 (or
IFY.sub.1) of the wafer stage WST. The processing circuit 142
stores the digitally sampled signal waveform in a memory and
calculates the Y coordinate value of the wafer stage WST at the
time when the center point of the slit shaped spot light of the LSA
type in the direction Y and the center point of the mark LSMy in
the direction Y are precisely matched in accordance with the stored
waveforms. This coordinate value thus calculated is output as a
mark positional information SSA. This information SSA is
transmitted to the main control system 114 shown in FIG. 7 for the
use of the driving control of the driving system 116 of the wafer
stage WST.
Also, in the LSA processing circuit 142, there are provided the
memory which performs the digital sampling of the photoelectric
signals SSD from the photoelectric sensor 121 in FIG. 7 in response
to the up down pulse signals UDP, and the circuit whereby to give a
high-speed computation process to the signal waveforms stored in
the memory. With these, the processing circuit outputs to the main
control system 114 a coordinate value of the wafer stage WST as a
projection positional information SSC of the reticle mark RM.sub.1
when the projected image of the reticle mark RM.sub.1 by the
projection lens PL has been precisely matched with the light
emission mark IFS.
Subsequently, in reference to FIG. 9 and FIG. 10, the structure of
the off-axis alignment system OWA will be described in detail. FIG.
10 illustrates the structure of the off-axis alignment system OWA,
and a reference mark IMP designates the wafer surface or the
surface of the fiducial plate FP. An image on the surface area
positioned in the field MIF of the objective lens 4B is formed on
an index plate 4F through a prism mirror 4A, objective lens 4B,
mirror 4C lens system 4D, and half mirror 4E. The light which
illuminates the surface IMP advances through the half mirror 4E and
then progressively through the lens system 4D, mirror 4C, the
objective lens 4B, and the prism mirror 4A to the surface IMP. The
illuminating light has a band width of approximately 300 nm which
is extremely low in sensitivity to the resist layer of the
wafer.
As shown in FIG. 9, the index plate 4F is formed by arranging index
marks TMX.sub.1, TMX.sub.2, TMY.sub.1, and TMY.sub.2 comprising a
plurality of line patterns (four lines, for example) with shielding
portions on a transparent glass. FIG. 9 shows a state that the
intersecting point of the straight lines LX and LY.sub.1 provided
on the fiducial plate FP and the center of the index plate 4F are
matched. The index marks TMX.sub.1 and TMX.sub.2 are arranged to
sandwich the fiducial mark FM.sub.1 on the fiducial plate FP in the
direction X while the index marks TMY.sub.1 and TMY.sub.2 are
arranged to sandwich the fiducial mark FM1 in the direction Y.
Now, the images of the respective index marks on the index plate 4F
and the fiducial mark FM.sub.1 (or mark on the wafer) are formed in
enlargement on the two CCD cameras 4X and 4Y through the image
formation lens 4G for taking image and half mirror 4H.
The image taking area of the CCD camera 4X is provided in the area
40X in FIG. 9 on the index plate 4F while the image taking area of
the CCD camera 4Y is provided in the area 40Y. Then, the horizontal
scanning line of the CCD camera 4X is defined in the direction X
which is rectangular to the line patterns of the index marks
TMX.sub.1 and TMX.sub.2 while the horizontal scanning line of the
CCD camera 4Y is defined in the direction Y which is rectangular to
the line patterns of the index marks TMY.sub.1 and TMY.sub.2. The
image signals (composite video signals) from each of the CCD
cameras 4X and 4Y are processed by a waveform processing circuit
including a circuit for digitally sampling the signal level for
each pixel, a circuit for adding the image signals (digital values)
obtainable from each of the plural horizontal scanning lines to
obtain its average, a circuit for computing each of the positional
deviation amounts of the index marks TM and fiducial mark FM.sub.1
in the directions X and Y at a high speed, and others. Then, the
information of the positional deviation amounts is transmitted to
the main control system 114 in FIG. 7 as an information SSE.
In this respect, the detection center or the off-axis alignment
system OWA in the present embodiment is meant to be the bisector
point of the two index marks TMX.sub.1 and TMX.sub.2 in the
direction X as to the X direction, for example, and in the
direction Y, the bisector point of the two index marks TMY.sub.1
and TMY.sub.2 in the direction Y. In some cases, however, only the
center point of the index mark TMX.sub.2 or TMX.sub.1 in the
direction X may be used instead of the bisector point of the two
index marks TMX.sub.1 and TMX.sub.2, for example
FIG. 11 is an enlargement of the fiducial mark FM.sub.1 formed on
the fiducial plate FP, which is formed as a two dimensional pattern
in which a plurality or line patterns extending in the direction Y
are arranged at predetermined pitches in the direction X while the
line patterns extending in the direction X are arranged at a
predetermined pitches in the direction Y.
For the positional detection of this fiducial mark FM.sub.1 in the
direction X, the image signals from the CCD camera 4X are analyzed
by the waveform processing circuit, and the averaged position of
each of the detected positions (pixel positions) of the plural line
patterns arranged in the direction X is defined as the X
directional position of the fiducial mark FM. Then, the deviation
amount with respect to the central position of the index marks
TMX.sub.1 and TMX.sub.2 are obtained. The detection of the fiducial
mark FM.sub.1 in the direction Y is conducted by the CCD camera 4Y
in the same manner.
Now, as described earlier in conjunction with FIG. 5, the
arrangement of each of the marks on the fiducial plate FP to be
detected by the TTR alignment system and TTL alignment system is
defined to be in a predetermined positional relationship. Here, in
reference to FIG. 12, the description will be made further thereof.
FIG. 12 is an enlargement showing each of the marks positioned on
the straight line LX. The mark LIMy is a diffraction grating in
which grating elements are arranged in the direction Y at a
predetermined pitch (8 .mu.m, for example). The mark
LSMy is a two dimensional grating pattern in which fine square dot
patterns are arranged in the direction X at pitches PSx and in the
direction Y at pitches PSy as shown in a circle in enlargement. The
mark LSMy is the one to be detected by the beam spot of the TTL
alignment system of LSA type in the direction Y, and the beam spot
is of a slit type extending in the direction X and its beam width
in the direction Y is substantially the same as the dimension of
the dot pattern in the direction Y. In this respect, the pitches
PSx in the direction X contribute to the generatin of the
diffraction light at the time of the mark detection. The pitches
PSy the direction Y serve to provide a multimark by arranging a
plurality of diffraction marks in the direction Y. Therefore, if
there is no need for providing any multimark, it sufficies if only
dot pattern group of one kind is arranged on the straight line
LX.
Also, while the pitch PSx in the direction X is uniformly
determined by the diffraction angle of the primary diffraction
light required for the wavelength of the beam spot, the pitch PSy
in the direction Y can be the same as or larger than the pitch
PSx.
Now, as described in conjunction with FIG. 5, the interval K.sub.1
between the center point of the marl LIMy in the direction X and
the center point of the fiducial mark FM2A in the direction X and
the interval K.sub.2 between the center point of the light emission
mark IFS in the direction X and the center point of the mark LSMy
in the direction X are in a relation ship K.sub.1 =K.sub.2
.+-..DELTA.Xk. This condition is only required for the substantial
matching of the center of the mark detection area (the illuminating
area of the interference fringes) or the TTL alignment system of
L1A type according to the present embodiment and the center point
of the mark detection (beam spot) of the TTL alignment system of
LSA type. Therefore, the relationship is not necessarily confined
to the above-mentioned condition.
The alignment system described in conjunction with FIG. 8 is also
structured exactly in the same way in the direction X, and the
positional information of each of the marks in the direction X is
transmitted to the main control system 114.
Now, before the base line measurement and the operation of each
alignment by the apparatus (stepper) according to the present
embodiment will be described, the correction of errors that may
take place when the fiducial plate FP is mounted on the wafer stage
WST is described. Of the possible errors in mounting the fiducial
plate FP, the one which will affect the accuracy ultimately is a
rotational error which is residual in the X-Y coordinate system of
the fiducial plate FP.
Traditionally, for the mounting of a fiducial plate of the kind on
a wafer stage, it has been proposed that the plate is fixed with a
set screw and others to make fine adjustments possible (such as
disclosed in U.S. Pat. No. 4,414,749). However in consideration of
changes in the elapsing time, it is extremely disadvantageous to
adopt a fixing method of fiducial plate with a fine adjustment
mechanism from the viewpoint of stability in accuracy. Accordingly,
it is desirable to fix the fiducial plate FP on the wafer stage
firmly without allowing even a slightest displacement (nm
order).
In any fixing methods to be adopted, it is prepared to obtain the
amount of the residual rotation error in advance for the fiducial
plate FP according to the present embodiment.
The residual rotation error referred to here is meant to be the
degree of the parallelization between the straight line LX provided
on the fiducial plate FP shown in FIG. 4 and the reflective plane
of the movable mirror IMy shown in FIG. 3. The control of the
coordinating position for the wafer stage WST is carried out all by
the operations of the interferometers IFX, IFY (or IFY.sub.2) as
reference, and it is equally mentioned that each of the reflecting
planes of the movable mirrors IMx and IMy serves to be the
reference for measuring the coordinating position. Hence, the
parallelism between the reflection plane of the movable mirror IMy
and the straight line LX on the fiducial plate FP presents a
problem. Also, as a mounting error, there is possible displacement
(translational error) in parallelism in each direction, that is the
Y direction which is perpendicular to the reflecting plane of the
movable mirror IMy and the X direction which is perpendicular to
the reflecting plane of the movable mirror IMx. However, this can
be corrected when positioning the wafer stage WST, and those
translational error presents almost no problem.
Now, the residual rotational error of the fiducial plate FP can be
obtained either by the self check conducted by the stepper in FIG.
2 or by a pilot exposure using a wafer. Here, as an example, the
self check method will be described. Among the alignment sensors
for the stepper shown in FIG. 2, it is only the off-axis type wafer
alignment system OWA that while the mark detection in the direction
Y is possible, the Abbe's condition can be satisfied with respect
to either one of the two interferometers IFY.sub.1 and IFY.sub.2.
Accordingly, in the present embodiment, the mark detection function
of the alignment system OWA in the direction Y will be used with
the interferometer IFY.sub.1 as reference. At first, the coordinate
positions of the fiducial marks FM2A and FM2D on the fiducial plate
FP in the direction Y respectively are measured by the off-axis
alignment system OWA. To this end, as shown in FIG. 13A, a bar mark
which is extended in the direction X of the fiducial mark FM2D is
positioned in the field of the objective lens 4B to obtain the
positional deviation amount in the direction Y between the index
marks TMY.sub.1 and TMY.sub.2 shown in FIG. 9. In this case, it may
be possible to align the bar mark which is extend in the direction
X of the fiducial mark FM2D only with either one of the index marks
TMY.sub.1 and TWY.sub.2. Here, in FIG. 13, it is assumed that the
straight line LX on the fiducial plate FP and movable mirror IMy is
relatively rotated by .theta.f, and its representation is
exaggerated.
In any case, with the index marks TMY.sub.1 and TMY.sub.2 as
reference the deviation amount .DELTA.YFd in the direction Y of the
fiducial FM2D is detected by on the basis of the image signals from
the CCD camera 4Y shown in FIG. 10. The positional deviation amount
is obtainable as an information SSE inputted in the main control
system 114 shown in FIG. 7.
Simultaneously, the measured values YA1 and YA2 of the
interferometers IFY.sub.1 and IFY.sub.2 when the fiducial mark FM2D
is being detected by the objective lens 4B are stored in the main
control system 114.
Subsequently, the wafer stage WST is driven in the direction X by a
predetermined amount Lfp, and the bar mark which is extended in the
direction X of the fiducial mark FM2A is positioned with respect to
the index marks TMY.sub.1 and TMY.sub.2 of the off-axis alignment
system OWA. This state is illustrated in FIG. 13B.
The predetermined amount Lfp is defined at this juncture to be
equal to the designed interval between the fiducial marks FM2A and
FM2D in the direction X.
Then, likewise, the deviation amount .DELTA.YFa of the fiducial
mark FM2A in the direction Y and each of the measured values
YB.sub.1 and YB.sub.2 of the interferometers IFY.sub.1 and
IFY.sub.2 are obtained.
With the above-mentioned operation, the measuring preparation work
has been completed, and it is now possible to obtain the amount
.theta.f of the residual rotation error by computation.
At first, a rotational error .theta.f' can be obtained by an
equation given below approximately on the assumption that there is
no yawing when the wafer stage WST is driven in the direction X for
a predetermined amount Lfp. ##EQU1## However, if any yawing takes
place, a fine rotational error portion .DELTA..theta.y of the wafer
stage WST due to this yawing should be included in the equation
(1).
Consequently, the real residual rotation error .theta.f can be
expressed by the following equation:
The rotational error .DELTA..theta.y due to the yawing can be
obtained by:
Here, LB is the interval between the respective measuring axes in
the direction X of the two interferometers IFY.sub.1 and
IFY.sub.2.
Now, let it be assumed that the same measuring is performed using
the fiducial mark FM2C instead of the fiducial mark FM2D. Then, the
designed interval between the fiducial marks FM2A and FM2C in the
direction X becomes equal to the interval LB between the
interferometers IFY.sub.1 and IFY.sub.2 in the direction X.
Therefore, the predetermined driving amount Lfp or the wafer stage
WST also becomes Lfp=LB.
Because of this, when the fiducial marks FM2A and FM2C (or FM2B and
FM2D) are used, the equation (3) will be as follows:
Therefore, the residual rotation error .theta.f can be obtained as
follows from the equations (1), (2), and (4):
In other words, when the interval between the two fiducial marks in
the direction X used for measuring is equal to the interval LB
between the two interferometers IFY.sub.1 and IFY.sub.2 in the
direction X, it becomes unnecessary to monitor the measured values
(YA.sub.1 and YB.sub.1) of the interferometer IFY.sub.1 which have
been regarded as reference.
Now, the residual rotational error .theta.f of the fiducial plate
FP for the movable mirror IMy is obtainable as set forth above.
This value is stored in the main control system 114 In this
respect, four fiducial marks are provided along the straight line
LX on the fiducial plate FP Thus, the residual rotational error is
obtained using two arbitrary fiducial marks out of the four, and it
may be possible to adopt its mean value For example, an arithmetic
mean value (.theta.f.sub.1 +.theta.f.sub.2)/2 can be a residual
rotational error of the fiducial plate FP where .theta.f.sub.1 is a
rotational error obtained by detection of the fiducial marks FM2A
and FM2C and .theta.f.sub.2 is a rotational error obtained by
detection of the fiducial marks FM2B and FM2D. Further, on the
straight line LX, the marks LIMy, LSMy, IFS, FM.sub.1 are arranged.
Therefore, it may be possible to detect any two of them by the
off-axis alignment system OWA for the measurement of the mark
position in the direction Y. In any events, the distance between
the marks in two locations to be measured in the direction X should
be desirably be as long as possible for securing its accuracy
Also, the measuring method for the residual rotation error by the
self check set forth above is an example. Some other method by the
self check is conceivable, which will be referred to when the
operational sequence is described later.
Moreover, the above-mentioned measuring method is such as obtaining
the residual rotational error .theta.f. However, as the .theta.f is
detected as an offset value when the wafer alignment is conducted
by the off-axis alignment system OWA, a method for obtaining the
.theta.f by examining the vernier after exposure is conceivable In
other words, using the off-axis alignment system OWA an overlapped
exposure is performed on the test wafer, it is possible to obtain
the residual mounting error .theta.f by reading the vernier for
checking the overlapping accuracy in the directions X and Y
subsequent to the resist pattern having been developed. Of this
method, the detailed description will be made later.
Now, the operation of the base line measurement by the apparatus
according to the present embodiment will be described. The
operation described herein is a typical one and some of its
variations will be described later colletively.
FIG. 14 and FIG. 15 are flowcharts showing the typical sequence
This typical sequence is mainly controlled by the main control
system 114 integrally.
At first, the reticle R which is stored in a predetermined storage
position is transferred automatically or manually for its loading
to the reticle stage RST only with the precisions mechanically
available for delivering and positioning (step 500). In this case,
the loading precision of the reticle R should desirably be less
than .+-.2 mm provided that the window area (inner side of the
shielding zone SB) shown in FIG. 6 is approximately 5 mm square in
its size and the length of the double slit marks RM.sub.1 x and
RM.sub.2 y is approximately 4 mm.
Then, the main control system 114 executes a reticle search for the
preliminarily rough alignment (prealignment) of the position of
reticle R so that the marks RM.sub.1 and RM.sub.2 of the reticle R
can be normally detected by the TTL alignment systems 1A and 1B.
For this reticle search, there are two kinds, SRA method and IFS
method, as shown in steps 504 and 506 in FIG. 14. In step 502, the
mode selection is executed. The prealignment by the IFS method to
be selected in the step 504 is such that as shown in FIG. 6, the
wafer stage WST is driven by a large stroke (several mm, for
example) for searching in the directions X and Y so that the
possible position of the reticle mark RM.sub.1 or RM.sub.2 is
searched by the light emission mark IFS while the position of the
reticle stage RST is kept stationary, and then the positions of the
reticle marks RM.sub.1 and RM.sub.2 are roughly detected by the
interferometers IFX, and IFY.sub.2 thereby to obtain the deviation
amount of the detected positions from the designed positions (where
they are supposed to be located), hence making it possible to drive
finely the reticle stage RST depending on the interferometers IRX,
IRY, and IR.theta. for the reticle stage RST.
On the other hand, the prealignment by the SRA method to be
selected in the step 506 is executed as given below The blank
surface of the fiducial plate FP is arranged directly below the
possible positions of the reticle marks RM.sub.1 and RM.sub.2, and
in such a state, the TTR alignment systems 1A and 1B are used to
allow the CCD cameras 112X and 112Y (FIG. 7) to take the image of
patterns on the reticle R. Thus, the image signal waveforms
corresponding to the horizontal scanning line in the first flame
are stored in a memory. Then, the reticle stage RST is driven for a
predetermined amount in the direction X or direction Y by the
driving system 115 in accordance with the measured values of the
interferometers IRX, IRY, and IR.theta.. Subsequently, the image
signal waveforms of the second flame are received from the CCD
cameras to connect them to the signal waveforms of the first flame.
After that, the joined image signal waveforms are analyzed to
obtain each position of the reticle mars RM.sub.1 and RM.sub.2.
Hence, with this method, the deviation amounts from the designed
positions are obtained for the required driving of the reticle
stage RST.
In any one of the search modes, the center of each of the reticle
marks RM.sub.1 and RM.sub.2 of the reticle R can be prealigned with
the center of the photographing area for each of the CCD cameras
112X and 112Y provided respectively for the two TTR alignment
systems 1A and 1B at a high speed with an accuracy of approximately
several .mu.m.
Now, the main control system 114 enters the reticle alignment
operation beginning at the step 508. Before that, however, the
driving system 116 is controlled in accordance with the measured
values of the interferometers IFX, IFY.sub.2 (or IFY.sub.1) to
position the wafer stage WST so as to set each of the two fiducial
marks FM2A and FM2B at the designed positions in the field PIM of
the projection lens PL. When the position of the wafer stage WST is
set, the fiducial mark FM2A (FM2B) is taken by the CCD cameras 112X
and 112Y in a state that it is substantially aligned with the
reticle mark RM.sub.1 (RM.sub.2). At this stage, the processing
circuits 113X and 113Y in FIG. 7 are actuated to calculate the
positional deviation amounts (.DELTA.XR.sub.1 and .DELTA.YR.sub.1)
in the directions X and Y of the reticle mark RM.sub.1 with respect
to the fiducial mark FM2A as well as the positional deviation
amounts (.DELTA.XR.sub.2 and .DELTA.YR.sub.2) in the directions X
and Y of the reticle mark RM.sub.2 with respect to the fiducial
mark FM2B.
Now, in step 510, whether each of the positional deviation amounts
is within the tolerance value or not is determined. If it is beyond
the tolerance value, the process proceeds to step 512. At this
juncture, as clear from the shapes and arrangements of the two
reticle marks RM.sub.1
and RM.sub.2, it is possible to achieve the alignment of the
reticle R in the direction X by making the polarities and absolute
values of the deviation amounts .DELTA.XR.sub.1 and .DELTA.XR.sub.2
equal provided that it is given as positive if each of the center
points of the reticle marks RM.sub.1 and RM.sub.2 is respectively
deviated toward the reticle center CC with respect to each of the
center points of the fiducial marks FM2A and FM2B, and given as
negative if deviated in the opposite direction.
Likewise, it is possible to achieve the alignment of the reticle R
in the direction Y and direction .theta. by making the polarities
and absolute values of the deviation amounts .DELTA.YR.sub.1 and
.DELTA.YR.sub.2 in the direction Y equal provided that it is given
as positive if the center point of each of the reticle marks
RM.sub.1 and RM.sub.2 is deviated toward the positive direction of
the Y axis of the static coordinate system. The deviation amount
.DELTA..theta.R of the reticle R in the direction .theta.
(rotational direction) can be obtained by the following equation
from the deviating amounts .DELTA.YR.sub.1 and .DELTA.YR2 (real
dimensions on the reticle) in the direction Y where the interval
between the reticle marks RM.sub.1 and RM.sub.2 in the direction X
is Lrm:
However, there should be no problem even if the evaluation of the
deviating amount of the reticle R in the direction .theta. is
simply made by obtaining the absolute value .DELTA.YR.sub.1
-.DELTA.YR.sub.2 because the interval Lrm is accurately defined in
advance for any one of the reticles. From the above, if the
deviating amounts of the reticle R in the directions X, Y, and
.theta. are greater than the allowable amounts, the reticle stage
RST is driven finely in step 512. At this juncture, the required
amounts for such fine drivings in the direction X, direction Y, and
direction .theta. are calculated on the basis of the each of the
deviating amounts (.DELTA.XR.sub.1, .DELTA.YR.sub.1) and
(.DELTA.XR.sub.2, .DELTA.YR.sub.2). Therefore, while monitoring the
positions of the reticle stage RST by the three interferometers
IRX, IRY, and IR.theta., the stage is finely driven to the
correcting postions. This method is the so-called open control
system, and if the control accuracy of the driving system 115 and
the positioning accuracy of the reticle stage RST are sufficiently
high and stable, it is possible to align the reticle R at the
target position precisely with only one-time positional deviation
measurement (step 508) and one-time positional correction (step
512). However, whether or not the alignment at the target position
has been executed by the positional correction must be verified, it
is necessary for the main control system 114 to repeat the
operations beginning at the step 508.
With the above-mentioned steps 508 to 510, the reticle R is aligned
with respect to the designed coordinating positions of the fiducial
marks FM2A and FM2B on the fiducial plate FP.
Now, the reticle R is thus aligned with respect to the fiducial
marks FM2A and FM2B, but due to the constant residual rotation
error .theta.f that the fiducial plate FP has against the
reflecting plane of the movable mirror as described earlier in
conjunction with FIG. 13, the reticle R is, strictly speaking,
rotated by .theta.f after the alignment with respect to the
reflecting plane of the movable mirror.
Therefore, in the step 512 where the reticle stage RST is finely
driven, it is arranged that the alignment position with to the
fiducial mark FM2A of the reticle mark RM.sub.1 is further allowed
to have offsets of (.DELTA.Ox.sub.1, .DELTA.Oy.sub.1) and the
alignment position with respect to the fiducial mark FM2B of the
reticle mark RM.sub.2 is further allowed to have offsets of
(.DELTA.Ox.sub.2, .DELTA.Oy.sub.2) Here, the offsets Ox.sub.1 and
Ox.sub.2 in the direction X can be zero for both of them while the
offsets .DELTA.Oy.sub.1 and .DELTA.Oy.sub.2 in the direction Y can
be defined as follows:
Consequently, when the reticle R is aligned with the fiducial plate
FP as reference, the final condition with the mounting error
(.theta.f) of the fiducial plate FP having been taken into account
is provided with the positional deviating amounts defined as
follows when each of the marks are detected in the TTR alignment
system:
For the establishment of the final alignment positions with these
offsets having been executed, it may be possible to adopt the open
control system using the interferometers IRX, IRY, and IR.theta.
for the reticle or to drive the reticle stage RST with the closed
loop control using the positional deviating values
(.DELTA.YR.sub.1, .DELTA.YR.sub.2) obtainable from the each of the
processing circuits 113X and 113Y of the TTR alignment system as
deflecting signals and the above-mentioned final positional
deviating amounts (.DELTA.ORy.sub.1, .DELTA.ORy.sub.2) as target
values.
Now, there is a method in which the reticle marks RM.sub.1,
RM.sub.2 and the TTR alignment system are used besides the methods
described in the case of obtaining the residual rotational error
.theta.f of the fiducial plate FP and in conjunction with FIG. 13.
Such method can be implemented by providing additionally a step
before the step 508 in the flowchart shown in FIG. 14, in which the
reticle marks RM.sub.1, RM.sub.2 and the fiducial marks FM2C, FM2D
are aligned by the TTR method.
In other words, when the prealignment of the reticle R is completed
in the step 504 or step 506, the coordinate positions of the
fiducial marks FM2C and FM2D are measured with the reticle marks
RM.sub.1 and RM.sub.2 as tentative reference points because the
reticle R has been set with a precision of .+-.several .mu.m. At
this juncture, the reticle marks RM.sub.1 and RM.sub.2 are
positioned almost symmetrically to the optical axis AX of the
projection lens PL in the direction X. Strict speaking, therefore,
an Abbe's error is included in each of the positional deviating
amount .DELTA.Y2C of the reticle mark RM.sub.1 and the fiducial
mark FM2C in the direction Y detected by the TTR alignment system
1A and the positional deviating amount .DELTA.Y2D of the reticle
mark RM.sub.2 and the fiducial mark FM2D in the direction Y by the
TTL alignment system 1B. However, if the arithmetic mean value
.DELTA.YRC [(.DELTA.Y2C+.DELTA.Y2D)/2], which expresses the
deviating amount between the center point of the reticle R and the
center point of the fiducial mark FM1 in the direction Y, is
obtained, the Abbe's error portion should be canceled by the
arithmetic mean.
Therefore, the measured value Yfm.sub.1 of the interferometer
IFY.sub.2 is stored when the deviating values .DELTA.Y2C and
.DELTA.Y2D are being detected by the TTR alignment systems 1A and
1B to obtain the value YF.sub.1 =Yfm.sub.1 -.DELTA.YRC. Then, it
becomes possible to obtain the Y coordinate value YF.sub.1 of the
center point of the fiducial mark FM1 (the center point of the
fiducial marks FM2C and FM2D in the direction X) with the center
point of the reticle R as reference.
As regards the direction X, it sufficies if only the deviating
amount .DELTA.XRC[(.DELTA.X2C-.DELTA.X2D)/2] between the center
point of the reticle R and the center point of the fiducial mark
FM1 in the direction X is obtained in consideration of the
orientation (positive or negative) thereof on the basis of the
deviating amount .DELTA.X2C detected by the TTR alignment system 1A
and the deviating amount .DELTA.X2D detected by the TTR alignment
system 1B. At this juncture, the X coordinate position of the wafer
stage WST is detected as Yfm.sub.1 by the interferometer IFX. Then,
by working out XF.sub.1 =Yfm.sub.1 -.DELTA.XRC, it is possible to
obtain the X coordinate value XF.sub.1 of the center point of the
fiducial mark FM1 with the center point of the reticle R as
reference.
The coordinate values (XF.sub.1, YF.sub.1) obtained as set forth
above are the values including the distance from each of the
reflecting planes of the movable mirrors IMy and IMx to the center
point of the fiducial mark FM1 with the interferometers IFX and
IFY.sub.2 as references.
Now, the wafer stage WST is driven to execute step 508 in FIG. 14.
As described earlier, in the step 508, each of the positional
deviating amounts between the reticle marks RM.sub.1, RM.sub.2 and
the fiducial my FM2A, FM2B are obtained by the TTR alignment
systems 1A and 1B at first. The positional deviating amounts of the
fiducial mark FM2B with respect to the reticle mark RM.sub.1 are
(.DELTA.XR.sub.1, .DELTA.YR.sub.1) while the positional deviating
amounts of the fiducial mark FM2B with respect to the reticle mark
RM.sub.2 are (.DELTA.XR.sub.2, .DELTA.YR.sub.2). At this juncture,
the coordinate values (Xfm.sub.2, Yfm.sub.2) of the wafer stage WST
obtained at the time of detection of the fiducial marks FM2A, FM2B
by the TTR alignment system are stored from the interferometers IFX
and IFY.sub.2 although this has not been required in the step 508
in FIG. 14.
From the above-mentioned measurement results, the main control
system 114 obtains the coordinate values (XF.sub.2, YF.sub.2) of
the center point of the fiducial mark FM.sub.2 (the center points
of the marks FM2A and FM2B in the direction X) with the center
point of the reticle R as reference as follows:
The coordinate values (XF.sub.2, YF.sub.2) are the values including
the distance from each of the reflecting planes of the movable
mirrors IMy and IMx to the center point of the fiducial mark FM2
with the interferometers IFY.sub.2 and IFX as references.
Therefore, the mounting error (rotational error) .theta.f' of the
fiducial plate FP including the yawing amount .DELTA..theta.y, when
the wafer stage WST is driven from the detecting position of the
fiducial marks FM2C and FM2D to the detecting position of the
fiducial marks FM2A and FM2B, will be calculated by the following
equation:
In this case, the variation of the difference between the measured
value of the interferometer IFY.sub.1 and the measuring value of
the interferometer IFY.sub.2 corresponds to the yawing amount
.DELTA..theta.y. Consequently, the real mounting error .theta.f can
be obtained with such a correction as provided by the aforesaid
equation (2).
During the period the above computation is being executed, the main
control system 114 will execute the next steps 510 and 512. In
other words, when the mounting error .theta.f of the fiducial plate
FP is obtained in the sequence in FIG. 14 as describe above, only
the positional deviating amounts (.DELTA.XR.sub.1, .DELTA.RY.sub.1)
and (.DELTA.XR.sub.2, .DELTA.RY.sub.2) which are initially measured
in the step 508 are needed.
Subsequently, the main control system 114 will execute the
operations beginning at step 516 as shown in FIG. 15. In step 516,
whether the position of the fiducial plate FP is servo locked in
accordance with the measured values obtained by the interferometers
IFX, IFY.sub.2 (or IFY.sub.1) for the wafer stage WST or servo
locked by the LIA method of the TTL alignment system is selected.
If the servo lock using the interferometers is selected, the
process will proceed to step 518 to store the coordinate values of
the wafer stage WST at the time or the completion of the reticle
alignment and perform the required servo control of the driving
system 116 of the wafer stage WST so that the measured values of
the interferometers IFX, IFY.sub.2 (or IFY.sub.1) are always
matched with the stored values. If the servo lock by the LIA method
is selected, the process will proceed to step 520 to set the
shutters 132A and 132B shown in FIG. 8 as in the state shown in
FIG. 8 and allow the respective marks LIMx and LIMy on the fiducial
plate FP to be illuminated with interference fringes. Then, the
wafer stage WST is servo controlled by the phase difference
measuring circuit 140 so that the phase difference with the
fiducial signals is always kept to be in a predetermined value
respectively in the direction X and direction Y. In the case of the
LIA method, the two marks LIMx and LIMy on the fiducial plate FP
are aligned to the reference grating plate 138 fixed inside the TTL
alignment systems
While it is possible to servo lock the wafer stage WST with
substantially the same precision irrespective of the interferometer
mode on the basis of the measured values of the interferometers
IFX, IFY.sub.2 (or IFY.sub.1) or the LIA mode in accordance with
the TTL alignment, it has been ascertained according to experiments
or simulations that the LIA mode is more stable than the
interferometer mode. In general, the moving stroke of the wafer
stage WST in the directions X and Y is greater than the diameter of
the wafer. The stroke is required to be 30 cm or more, for example.
Consequently, the length of the light path of laser beam from the
interferometers IFX and IFY.sub.2, which is exposed to the
atmosphere is as much as several tens cm or more. Because of this,
in spite of the wafer stage WST being strictly stationary, the
values of the inner counters of the interferometers are varied in
the order of 1/100 .mu.m to 1/10 .mu.m if a diffraction index
locally fluctuates in the air on the optical path. Therefore, when
the servo lock is performed in order to make the counter values of
the interferometers constant, the position of the wafer stage WST
may be finely moved due to the fluctuation of the diffraction index
within a range of approximately .+-.0.08 .mu.m in some cases. The
fluctuation of the diffraction index takes place when lumps of air
having temperature differential are caused to pass slowly through
the optical path of the laser beam from the interferometer. The
interferometer for the wafer stage has an ambient disadvantage such
as this and in some cases, it lacks stability as compared with the
LIA method. It is possible to provide a cover for the beam used for
the LIA method so that it is not exposed to the atmosphere
eventually. Further, if the beam is not exposed, there hardly
occurs any fluctuation of diffraction index because the space
between the reticle and projection lens and the space between the
projection lens and wafer are approximately several cm only.
In consideration of the above, it is preferable to use the TTL
alignment system as much as possible if the position of the
fiducial plate FP (wafer stage WST) can be controlled by servo
using the this alignment system in a state where the fiducial marks
FM2A and FM2B are being detected by the TTR alignment systems.
Now, the main control system 114 performs the fiducial mark
detections in step 522 using the TTR alignment system and off-axis
alignment system simultaneously. In general, the reticle stage RST
is finely driven to the target position in the earlier step 510,
and when the alignment is completed, the reticle stage RST is fixed
by a vacuum absorption and others to the column side which becomes
its base. When this absorption is conducted, the reticle stage RST
may be deviated laterally for a slight amount in some cases. The
lateral deviation is extremely small, but it can be one of errors
in view of the base line control and careful attention should be
given. Such a care may be taken by repeating the measuring
operation in the step 508 using the CCD cameras 112X and 112Y for
the TTR alignment systems or by monitoring the variations of the
measured values of the interferometers IRX, IRY, and IR.theta. from
the completion of the reticle alignment. In the present embodiment,
however, the arrangement has been made so that the base line value
is controlled including such a lateral deviation. As a result, it
is unnecessary to obtain the lateral deviation amount individually
for its particular purpose.
Now, in the step 522, the fiducial mark FM1 on the fiducial plate
FP has already been positioned in the detection area of the
off-axis alignment system OWA.
Therefore, the main control system 114 obtains the positional
deviating amounts (.DELTA.XF, .DELTA.YF) of the index mark TM in
the index plate 4F and the fiducial mark FM1 in the directions X
and Y as real dimensions on the wafer using the CCD cameras 4X and
4Y on the off-axis alignment system as shown in FIG. 10. At the
same time, using the CCD cameras 112X and 112Y on the TTR alignment
system, the positional deviating amounts (.DELTA.XR.sub.1,
.DELTA.YR.sub.1) of the reticle mark RM.sub.1 and fiducial mark
FM2A and the positional deviating amounts (.DELTA.XR.sub.2,
.DELTA.YR2) of the reticle mark RM.sub.2 and fiducial mark FM2B are
measured as real dimensions on the wafer side. At this juncture,
the
processing circuits 113X, 113Y, and others are controlled in order
to match the timing as much as possible for the storage of the
image signal waveforms corresponding to the photographed images of
the marks because the CCD cameras are of the photoelectric type for
the TTR system and off-axis system as well. In general, however,
the CCD camera outputs the image singals for one frame portion per
1/30 seconds. It is therefore unnecessary to synchronize the
reception of the image signals strictly by the frame unit for the
TTR system and the off-axis system. In other words, the required
receptions of the signals are carry out almost simultaneously. It
will be good enough if the signals are received within several
seconds (preferably within a second), for example. In this respect,
when the position of the fiducial plate FP is servo locked, it is
necessary to set the reception of the image signal waveforms by the
TTR system and that of the image signal waveforms by the off-axis
system to be in an interval sufficinetly shorter than the time for
the positional variation of the wafer stage due to the fluctuation
of the air diffraction index.
Subsequently in step 524, the main control system 114 releases the
servo lock of the wafer stage WST and proceeds to the operation in
step 526 in which the driving (scanning) of the wafer stage WST is
started for detecting each of the marks on the fiducial plate FP by
the use of the LSA method and IFS method simultaneously.
In this step 526, the wafer stage WST is driven so that as
described in conjunction with FIG. 6 and FIG. 5 earlier, the light
emission slit mark IFS scans the reticle mark RM.sub.1 two
dimensionally. The wafer stage WST is at first positioned so as to
set the light emission slit mark IFS to be in the positional
relationship as shown in FIG. 6. Then, the beam spot of the slit
type extended in the direction X by the TTL alignment system of the
LSA method is at a deviated position in the direction Y with
respect to the mark LSMy on the fiducial plate FP. Now, from such
state, if the wafer stage WST is scanned in the direction Y, the
waveforms of the photoelectric signal from the photoelectric
detector 139 of the LSA method as well as of the photoelectric
signal SSD from the photoelectric element 121 of the IFS method
become as shown in FIG. 16. FIG. 16A shows the detected waveform of
the mark LSMy received in the memory by the LSA method. Here, as
the mark LSMy is formed with five pieces of diffraction grating
patterns, five peaks are produced on the signal waveform. The
processing circuit 142 shown in FIG. 8 obtains each of the
gravitational positions for the five peak waveforms and works out
its mean value as the Y coordinate position for the mark LSMy.
On the other hand, the signals SSD obtainable by the IFS method
include the two-bottom waveform portions as shown in FIG. 16B with
respect to the double slit mark RM.sub.1 y of the reticle mark
RM.sub.1. The processing circuit 142 obtains the respective center
points of the two bottom waveforms in the signal waveforms in FIG.
16B in order to work out its center point as the central coordinate
position YIf of the projected image of the double slit mark
RM.sub.1 y in the direction Y.
Likewise, as indicated in FIG. 6 by an arrow in the direction X,
the light emission slight mark IFS is driven to scan the double
slit mark RM.sub.1 x of the reticle mark RM.sub.1. At this
juncture, the slit type spot by the LSA method of the TTL alignment
system in the direction X is being scanned by the mark LSMx on the
fiducial plate FP simultaneously, so that the same waveforms as
shown in FIG. 16 are obtained. In this case, the X coordinate value
of the mark LSMx detected by the LSA method in the direction X is
XLs while the X coordinate value of the double slit mark RM.sub.1 x
detected by the IFS method is XIf.
As shown in FIG. 16, the difference between the coordinate
positions YLs and YIf is the base line value between the center
point detected by the LSA method of the TTL alignment system in the
direction Y and the projected point of the center CC of the reticle
R in the direction Y.
Subsequently, the main control system 114 operates the computation
to obtain the base line value in step 528. The parameters required
for this computation are separated into the measured real values
obtained by the measurements as shown in the table in FIG. 17 and
the constant values predetermined for the system design. For the
measured real values shown in the table in FIG. 17, [TTR-A] means
the TTR alignment system 1A in FIG. 2 while [TTR-B] means the TTR
alignment system 1B. Also, the measured real values for each of the
alignment systems are classified by the positional deviating
amounts or mark coordinate positions in the direction X and
direction Y. On the other hand, for the designed constant values,
each of the distances (.DELTA.Xfa, .DELTA.Yfa) in the directions X
and Y between the center point of the fiducial mark FM1 and the
fiducial mark FM2A and each of the distances (.DELTA.Xfb,
.DELTA.Yfb) in the directions X and Y between the center point of
the fiducial mark FM1 and the fiducial mark FM2B are precisely
measured in advance with the straight line LX as reference and are
stored.
The main control stem 114 works out the distance from the bisector
point of the line portion connecting each of the center points of
the fiducial marks FM2A and FM2B to the center point of the
fiducial mark FM1 in the direction X on the basis of the constant
values .DELTA.Xfa and .DELTA.Xfb as follows:
Then, the main control system 114 obtains 1/2 of the difference
between the deviating amount .DELTA.XR.sub.1 in the direction X
obtainable from TTR-A and the deviating amount .DELTA.XR.sub.2
obtainable from TTR-B as the deviating amount .DELTA.Xcc on the
dimension for the wafer side as follows:
Here, the values .DELTA.XR.sub.1 and .DELTA.XR.sub.2 are assumed to
be positive when the reticle marks RM.sub.1 and RM.sub.2 are
deviated toward the center of the reticle with respective to the
fiducial marks FM2A and FM2B respectively, and negative when
deviated in the opposite direction.
If the value .DELTA.Xcc thus obtained by the equation (9) is zero,
it is considered that the projection point of the center CC of the
reticle R is precisely matched with the bisector point in the
direction X of each of the center points of the two fiducial marks
FM2A and FM2B.
Then, on the basis of the measured real value .DELTA.XF and the
calculated values LF and .DELTA.Xcc, the main control system 114
works out the distance BLOx from the projection point on the XY
coordinate plane of the center CC of the reticle R to the center
point (bisector point between the index marks TMX.sub.1 and
TMX.sub.2) on the XY coordinate plane of the index plate 4F of the
off-axis alignment system OWA in the direction X as the base line
value in the direction X with respect to the off-axis alignment
system OWA as follows:
Here, the .DELTA.XF is assumed to take the positive value when the
fiducial mark FM1 is detected and found to be deviated toward the
direction of the projection lens PL (fiducial marks FM2A and FM2B)
with respect to the bisector point of the index marks TMX.sub.1 and
TMX.sub.2 in the direction X, and the negative value when detected
and found to be deviated in the opposite direction.
Subsequently, on the basis of the measured real values
.DELTA.YR.sub.1 and .DELTA.YR.sub.2, the main control system 114
obtains the deviating amount .DELTA.Ycc in the direction Y between
the projection point of the center point CC of the reticle R and
the bisector point (provided substantially on the straight line
LY.sub.2) of the line portion connecting the center point of the
fiducial mark FM2A and the center point of the fiducial mark FM2B
as follows:
Here, the .DELTA.YR.sub.1 and .DELTA.YR.sub.2 are assumed to take
the positive value when the reticle marks RM.sub.1 and RM.sub.2 are
deviated in the positive direction Y in FIG. 4 (in the above on the
plane of FIG. 4) with respect to the respective fiducial marks
FM2A, FM2B and the negative value when deviated in the opposite
direction. This deviating amount .DELTA.Ycc becomes zero when the
projection point of the center CC of the reticle R and the bisector
point of the line portion connecting the fiducial marks FM2A and
FM2B are matched precisely.
Further, on the basis of the constant values .DELTA.Yfa and
.DELTA.Yfb, the main control system 114 obtains the deviating value
.DELTA.Yf.sub.2 in the direction Y between the bisector point of
the line portion connecting each of the center points of the
fiducial marks FM2A, FM2B and the center point of the fiducial mark
FM1 as follows:
On the basis of the above-mentioned calculated values .DELTA.Ycc
and .DELTA.Yf.sub.2 and measured real value .DELTA.YF. The main
control system 114 works out the distance BLOy in the direction Y
between the projection point of the center CC of the reticle R and
the center point (bisector point between the index marks TMY.sub.1
and TMY.sub.2) of the index plate 4F of the off-axis alignment
system OWA in the direction Y as the base line value in the
direction Y with respect to the off-axis alignment system OWA as
follows:
From the above-mentioned calculation, the base line values (BLOx
and BLOy) of the off-axis alignment system OWA is obtained Then,
the main control system 114 calculates the base line values (BLTx
and BLTy) or the TTL alignment system of the LSA method. The base
line value BLTy for the TTL alignment system of the LSA method in
the direction Y is a deviating amount between the center point of
the slit type beam spot in the direction Y and the projection point
of the center CC of the reticle R in the direction Y and is
obtainable by the following equation:
Likewise, the base line value BLTx for the TTL alignment system of
the LSA method in the direction X is a deviating amount between the
center point of the slit type beam spot in the direction X and the
projection point of the center CC of the reticle R in the direction
X and is obtainable by the following equation:
However, the values thus obtained by the equations (14) and (15)
include the arrangement error .DELTA.Ysm in the direction Y between
the center of the light emission mark IFS and the mark LSMy on the
fiducial plate FP and the arrangement error .DELTA.Xsm in the
direction X between the center of the light emission mark IFS and
the mark LSMx on the fiducial plate FP. Therefore, if these errors
cannot be neglected, these should be stored as constant values in
advance, and the equations (14) and (15) are modified respectively
as the equations (14') and (15') given below.
With the sequence set forth above, the base line measurement is
completed, and the prealigned wafer W is mounted on the wafer stage
WST.
On the wafer W, a plurality of exposure areas, that is, the shot
area in which the pattern area PA of the reticle R is projected,
are arranged two dimensionally. Then, in each of the shot areas the
alignment marks to be detected by the off-axis alignment system OWA
or TTL alignment systems (2X and 3X; 2Y and 3Y) are formed in a
predetermined relationship with respect to the center point of the
shot area. In many cases, these alignment marks on the wafer are
provided within a street line. For the practical wafer alignment
methods, there have been known traditionally several method or
sequence. Here, therefore, the description of those known methods
and sequences is omitted. Only the fundamental wafer alignment will
be described.
FIG. 18 shows the arrangement of the marks and shot areas on the
wafer W, and the interval in the direction X between the center SCn
of the sot area SAn and the mark WMx to be detected in the
direction X is established as .DELTA.Xwm and the interval in the
direction Y between the center SCn and the mark WMy to be detected
in the direction Y are established as .DELTA.Ywm in designing. At
first, when the off-axis alignment system OWA is used, the wafer
stage WST is positioned so as to allow the mark WMx in an arbitrary
shot area SAn to be sandwiched between the index marks TMX.sub.1
and TMX.sub.2 in the detecting area of the off-axis alignment
system OWA. Here, the marks WMx and WMy are assumed to be of the
same maltiline patterns as the fiducial mark FM1.
Then, the main control system 114 reads the coordinate position Xm
of the wafer stage WST positioned in the direction X from the
interferometer IFX.
Further, the image signals from the CCD camera 4X in the off-axis
alignment system OWA are processed to detect the deviating amount
.DELTA.Xp between the center point of the index plate 4F and the
center point of the mark WMx in the direction X. Subsequently, the
wafer stage WST is driven to position the wafer stage WST so that
the wafer mark WMy is allowed to be sandwiched between the index
marks TMY.sub.1 and TMY.sub.2 of the off-axis alignment system. At
this juncture, the coordinate position Ym in the direction Y is
read from the interferometer IFY.sub.1. Then, the deviating amount
.DELTA.Yp in the direction Y between the center point of the index
plate 4F and the center point of the mark WMy is obtained by the
photographing of the CCD camera 4Y.
When the above-mentioned mark position detection is completed, the
coordinate positions (Xe and Ye) of the wafer stage WST are
obtained by the computation according to the equations given below
to enable the center SCn of the shot area SAn to be matched with
the projection point of the center CC of the reticle R when
exposed.
In this respect, if the marks WMx and WMy are detected by the TTL
system of the LSA method, the stage coordinate positions of
exposure can be obtained by the following equations provided that
each of the detecting positions of the marks WMx and WMy by the LSA
method is given as Xm and Ym:
According to the description set forth above, there are no Abbe's
errors included in the coordinate positions Xm and Ym of the wafer
stage WST and the deviating amounts .DELTA.Xp and .DELTA.Yp of the
mark positions at the time of the mark detection if the measured
values of the two interferometers IFX and IFY.sub.1 are used for
the two dimensional mark positioning detection using the off-axis
alignment system OWA because both of the measured values of the
interferometers IFX and IFY.sub.1 are defined to be rectangular
even at the detection center point in the static coordinate system
of the off-axis alignment system OWA.
Therefore, when the wafer marks and fiducial marks are detected
using the off-axis alignment system OWA, it is important to use the
interferometer IFY.sub.1 which can satisfy the Abbe's condition
with respect to the alignment system OWA rather than the
interferometer IFY.sub.2 whcih can satisfy the Abbe's condition
with respect to the projection lens PL.
However, in order to use the interferometers IFX and IFY.sub.2
which can satisfy the Abbe's condition with respect to the
projection lens PL and the interferometers IFX and IFY.sub.1 which
can satisfy the Abbe's condition with respect to the off-axis
alignment system OWA by switching them without any modifications,
it is necessary to perform the resetting (or presetting) of each of
the inner counters of the two interferometers IFY.sub.1 and
IFY.sub.2 in a specific state. To state the conclusion first, the
values of the respective inner counter of the two interferometers
IFY.sub.1 and IFY.sub.2 should be preset so that they are equal to
the value of either one of them at the position where the wafer
stage WST is stationary so as to enable the detection of the
fiducial mark FM2 through the projection lens PL to be conducted at
the same time of the detection of the fiducial mark FM1 being
performed through the off-axis alignment system OWA. Therefore,
according to the sequences in FIG. 14 and FIG. 15, it is necessary
to add the counter presetting operation (or counter data latching
operation) of the two interferometers IFY.sub.1 and IFY.sub.2 and
at the same time, and to operate the computation to correct the
base line value due to the mounting error .theta.f of the fiducial
plate FP as described earlier. Subsequently, therefore, the
specific example will be described hereunder.
At first, the reticle alignment is executed in the steps 508, 510,
and 512 in FIG. 14. At this junction, in consideration of the
mounting error .theta.f of the fiducial plate FP, the alignment
positions of the reticle marks RM.sub.1 and RM.sub.2 in the
direction X are set to be .DELTA.XR.sub.1 =.DELTA.XR.sub.2 and to
take the zero value while the alignment positions in the direction
Y are set to be .DELTA.YR.sub.1 .fwdarw..DELTA.Oy.sub.1 and
.DELTA.YR.sub.2 .fwdarw..DELTA.Oy.sub.2 respectively. In other
words, the reticle R is aligned so that the line connecting the two
reticle marks RM.sub.1 and RM.sub.2 is paralleled to the reflecting
plane of the movable mirror IMy.
After that, the base line error is measured, and once the reticle
alignment has been accomplished, the servo lock is actuated so that
the wafer stage WST is not moved. Now, with such servo lock state
in view, an error Ly (.DELTA..theta.a+.DELTA..theta.r) is present
between the measured value Le of the interferometer IFY.sub.2 which
can satisfy the Abbe's condition with respect to the projection
lens PL and the measured value Lf of the interferometer IFY.sub.1
which can satisfy the Abbe's condition with respect to the off-axis
alignment system OWA. Here, the Ly is the interval in the direction
X (same as LB in FIG. 13) between each of the measuring axes of the
two interferometers IFY.sub.1 and IFY.sub.2, and the rotational
error .DELTA..theta.a is a fine rotational error from the ideal
position (ideal X axis) of the reflecting plane of the movable
mirror IMy resulting from the position of the wafer stage WST at
the time of the base line measurement. Also, the rotational error
.DELTA..theta.r is a fine rotational error from the ideal position
(X axis) of the reflecting plane of the movable mirror IMy when the
wafer stage WST is driven to its predetermined home position. These
errors .DELTA..theta.a, and .DELTA..theta.r cannot be measured
directly and independently. Usually, however, it is possible to
measure them as the varied amount from the reset position of the
synthesized value of the .DELTA..theta.a and .DELTA..theta.r by
resetting (or presetting) the inner counters of the interferometers
IFY.sub.1 and IFY.sub.2 simultaneously when the wafer stage WST has
reached its home position. In other words, the varied amount of
.DELTA..theta.a+.DELTA..theta.r can be measured as a yawing value
with the reset position as reference.
Therefore, in a state that the position of the wafer stage WST is
being monitored or controlled by the interferometer IFY.sub.2
capable of satisfying the Abbe's condition with respect to the
projection lens, the error Lt-Le=Ly
(.DELTA..theta.a+.DELTA..theta.r) is included in he measured value
Lf of the other interferometer IFY.sub.1 naturally, and it is
neither possible to incorporate the measured value Lf of the
interferometer IFY.sub.1 with the measurement of the base line
value as a real value as it is, nor is it possible to transfer the
control of the wafer stage WST to the control by the interferometer
IFY.sub.1 as it is.
As a result, after storing the difference between the measured
value Lf or the interferometer IFY.sub.1 and the measured value Le
of the interferometer IFY.sub.2 with the wafer stage WST being
servo locked subsequent to the positioning of the fiducial plate FP
as .DELTA.Lyw [Ly(.DELTA..theta.a+.DELTA..theta.r)] when the base
line is measured, the measured value of inner counter of the
interferometer IFY.sub.1 is replaced (preset) from the value Lf to
the value Le. Thus, in performing control thereafter, there will be
no problem at all even if the control based on the interferometer
IFY.sub.2 used for positioning the wafer stage WST for exposure is
switched over to the control based on the interferometer IFY.sub.1
used for the off-axis alignment.
FIG. 19 illustrates this state exaggeratedly. In FIG. 19, the line
LX connecting the two fiducial marks FM2A and FM2B is rotated by an
error .theta.f with respect to the line Lrc which is in parallel
with the reflecting plane of the movable mirror IMy. When the
reticle R is aligned, the reticle mark RM.sub.1 is positioned by
offsetting itself just by .DELTA.Oy.sub.1 with respect to the
fiducial mark FM2A while reticle mark RM.sub.2 is positioned by
offsetting itself just by .DELTA.Oy.sub.2 with respect to the
fiducial mark FM2B. Thus, the line portion connecting the reticle
marks RM.sub.1 and RM.sub.2 is in parallel with the line Lrc
eventually. In FIG. 19, the line Lrc is defined to run through the
reticle center CC, the reticle marks RM.sub.1, RM.sub.2, and the
center CC are positioned on the line Lrc.
Now, in this state, the two interferometers IFY.sub.1 and IFY.sub.2
should be preset to the same counting value Le but as shown in FIG.
19, references for the two interferometers IFY.sub.1 and IFY.sub.2
after the presetting are modified to the reference line Lir'. In
FIG. 19, the line Lir represents the reference in a state of the
interferometers IFY.sub.1 and IFY.sub.2 being preset to a same
value when the wafer stage WST comes to its home position, for
example. In other words, it can be considered that the distance
from this imaginary reference line Lir or Lir' to the movable
mirror IMy is being measured by the interferometers IFY.sub.1 and
IFY.sub.2. Accordingly, immediately after the presetting is
completed, the reference line Lir', the reflecting plane of the
movable mirror IMy, and the line Lrc are in parallel to one
another. In this connection, if an attempt is made to find the
yawing of the wafer stage WST from the difference in the measured
values of the two interferometers IFY.sub.1 and IFY.sub.2
subsequent to the presetting, the reference required for the yawing
value should be modified to the line Lir' in FIG. 19. In other
words, the line which is in parallel with the reflecting plane of
the movable mirror IMy when fiducial plate FP is positioned
directly below the projection lens PL and the off-axis alignment
system at the time of the base line measurement becomes the
reference for the measurement of the yawing value thereafter.
Further, in the base line measurement, the positional deviating
amounts (.DELTA.XF and .DELTA.YF) between the fiducial mark FM1 and
index mark TM are obtained by the off-axis alignment system as
shown in FIG. 17. In FIG. 19, a point Of designates the detection
center point defined by the index mark TM of the off-axis alignment
system.
Here, the real base line value is determined by the positional
relationship between the center point CC of the reticle R and the
detection center point Of, but if the mounting error .theta.f of
the fiducial plate FP should be extremely small, the base line
value in the direction X is determined by the constant value
.DELTA.Xfa (distance between FM1 and FM2A), constant value
.DELTA.Xfb (distance between FM1 and FM2B), deviating amount of the
center point CC at the time of the reticle alignment in the
direction X, and deviating amount .DELTA.XF detected by the
off-axis alignment system as shown earlier in FIG. 17.
In other words, given the distance from the bisection point of the
two fiducial marks FM2A and FM2B in the direction X to the center
point of the fiducial mark FM1 on the line LX as LF, this LF can be
obtained as followed in the same manner of the equation (8).
Also, the residual deviating amount .DELTA.Xcc of the center point
CC in the direction X with respect to the intermediate point of the
fiducial mark FM2 at the time of the reticle alignment can be
obtained from the measured real values .DELTA.XR.sub.1 and A
XR.sub.2 in FIG. 17 in the same manner of the equation (9).
Consequently, the real base line value BLOx in the direction can be
obtained as followed in the same manner of equation (10).
On the other hand, as to the base line value BLOy in the direction
Y, it is impossible to guarantee the accuracy when applying the
equation (13) described earlier as it is because there occurs a
sine error (deviating amount in the direction Y) due to the
mounting error .theta.f.
Here, again, FIG. 19 is referred to for consideration. At first,
there is no problem for the positional control of the wafer stage
using either one of the two interferometers IFY.sub.1 and IFY.sub.2
if only they have already been preset. For example, let it be
assumed that the wafer stage WST is driven in the direction X by
the distance LF (strictly speaking, by the distance LF-.DELTA.Xcc)
from the state where the a specific point on the wafer is
positioned directly below the center point CC of the reticle R
without changing the measured value of the interferometer
IFY.sub.1. Then, the specific point on the wafer is positioned the
point Pc in FIG. 19. Consequently, the real base line value BLOy to
be controlled in the direction Y is an interval between the
detection center point Of of the off-axis alignment system and the
point Pc in the direction Y.
Now, since the mounting error .theta.f of the fiducial plate FP has
already been obtained, the deviating amount .DELTA.Yfc between the
point Pc and the fiducial mark FM1 in the direction Y can be
obtained by the following equation provided that the .theta.f is
sufficiently small:
Therefore, the base line value BLOy in the direction Y with the
mounting error .theta.f being taken into account can be expressed
in the following equation with a modification to the equation
(13):
In this respect, the values .DELTA.Ycc and .DELTA.Yf.sub.2 are
obtained from the equations (11) and (12).
As set forth above, the two interferometers IFY.sub.1 and IFY.sub.2
are preset to a same value at the time of the base line measurement
by the fiducial plate FP and at the same time, a correction is
additionally given to the calculation of the base line value in
accordance with the mounting error .theta.f as well as the
alignment of the reticle R is implemented with respect to the
fiducial marks on the fiducial plate FP in the state of the base
line measurement. In this way, all the erroneous elements are
canceled, hence enabling the provision of a significantly higher
accuracy than the conventional one for the base line
measurement.
In this respect, when the base line measurement is operated, the
measured values of the inner counter are sampled approximately
several tens times per approximately second by the computor even in
the case where the halting position of the wafer stage WST is being
read by the interferometer IFY.sub.1, and by equalizing them, the
error due to the fluctuation is experimentally reduced to
approximately 0.03 .mu.m to 0.012 .mu.m.
Also, when the alignment marks WMx, WMy, and others of the wafer W
are detected by the off-axis alignment system OWA as in FIG. 18,
the positioning of the wafer stage WST is controlled by the
interferometers IFY.sub.1 and IFX. At this juncture, the yawing of
the wafer stage WST may take place in some cases. The occurrence of
the yawing in this case, however, will not affect the ultimate
alignment accuracy (the registration accuracy between the reticle
and shot on the wafer) after the two interferometers IFY.sub.1 and
IFY.sub.2 are preset.
FIG. 20 shows an example of the implementation of the mutual
presetting hard ware of the circuit of the two interferometers
IFY.sub.1 and IFY.sub.2 described in conjunction with FIG. 19. The
same function can be implemented by the same conception in
computation on a soft ware.
In FIG. 20, the up down counter (UDC) 200 is the inner counter of
the interferometer IFY.sub.1 to perform the reversible counting of
the up pulse UP1 and down pulse DP1 generated accompanying the Y
direction driving of the wafer stage WST. The up and down counter
(UDC) 202 is the inner counter of the interferometer IFY.sub.2 to
perform likewise the reversible counting of the up pulse UP2 and
down pulse DP2. Each of the counted values of the UDC 200 and UDC
202 is output to the main control system 114 as parallel 24-bit Y
coordinate values DY.sub.1 and DY.sub.2, for example. The latch
circuits (LT) 204 and 206 keep on holding the counting values
DY.sub.1 and DY.sub.2 when each of the counting values DY.sub.1 and
DY.sub.2 is inputted and at the same time, the latch pulses S.sub.1
a and S.sub.2 a are received from the main control system 114.
Here, the output value of the LT 204 is applied as a preset value
to the UDC 202 while the output value of the LT 206 is applied as a
preset value to the UDC 200. The UDCs 200 and 202 are preset to the
presetting values in response to the load pulses S.sub.1 b and
S.sub.2 b from the main control system 114, respectively.
As described earlier, when the measured value Le (DY.sub.2) of the
interferometer IFY.sub.2 is preset to the interferometer IFY.sub.1,
the latch pulse S.sub.2 a is output to the LT 206 and the load
pulse S.sub.1 b is output to the UDC 200 after a predetermined
delay (.mu.Sec order). In the case of the circuit in FIG. 20, an
inverted presetting is possible as a matter of course. It is also
possible to preset the measured value Lf (DY.sub.1) to the UDC 202
of the interferometer IFY.sub.2. In this respect, the coordinate
measurement using interferometers is relative because of the
interferometer is a type of incremental measuring. So, it may
therefore be possible to reset the UDCs 200 and 202 to zero
simultaneously instead of the presetting of the UDC 200 of the
interferometer IFY.sub.1 and UDC 202 of the interferometer
IFY.sub.2 or to reset them to a predetermined value at the same
time irrespective of the measured values Le and Lf.
Now, while the measurement operation of the base line value shown
in the above-mentioned example is executed after the precise
reticle alignment has been completed a illustrated in FIG. 14 and
FIG. 15, it may be possible to perform the base line measurement in
the stage where the reticle has been aligned roughly.
For example, the reticle is roughly aligned by the SRA method or
IFS method to the location where the reticle marks RM.sub.1 and
RM.sub.2 can be detected by the TTR alignment systems 1A and 1B in
the step 504 or 506 in FIG. 14. Then, the step 508 in FIG. 14 and
the step 522 in FIG. 15 are operated simultaneously to obtain the
positional deviating amounts (.DELTA.XR.sub.1 and .DELTA.YR.sub.1)
between the fiducial mark FM2B and reticle mark RM.sub.1, the
positional deviating amounts (.DELTA.XR.sub.2 and .DELTA.YR.sub.2)
between the fiducial mark FM2A and reticle mark RM.sub.2, and the
positional deviating amounts (.DELTA.XF and .DELTA.YF) between the
fiducial mark FM1 and index mark of the off-axis alignment
system.
At this juncture, the fiducial plate FP is servo locked either in
the interferometer mode or LIA mode. In consideration of the fine
movement of the wafer stage WST, however, the positional deviating
amount detections respectively by the TTR alignment system and
off-axis alignment system should be repeated several times to
obtain its means values. By this equalization, the error values
which are generated at random are reduced. Then, once each of the
positional deviating amounts is thus obtained, it becomes possible
to know thereafter just be computation the relative positional
relationship between the projection point at the center point CC
(or the marks RM.sub.1 and RM.sub.2) of the reticle R and the
detection center point of the off-axis alignment system OWA.
Further, the position rough alignment position of the reticle stage
RST in this state is read from the measured values of the
interferometers IRX, IRY, and IR.theta. for storage. It is also
desirable to execute the equalization of this reading.
Then, on the basis of the positional deviating amounts previously
measured (.DELTA.XR.sub.1 and .DELTA.YR.sub.1), (.DELTA.XR.sub.2
and .DELTA.YR.sub.2), (.DELTA.XF and .DELTA.YF) and the
predetermined constant values, the deviating amounts (in the
directions X, Y, and .theta.) between the projection point at the
center CC of the reticle and the center point of the fiducial mark
FM2 (the bisector point between the mark FM2A and FM2B) which
should take place when the detection center of the off-axis
alignment system OWA is matched with the center of the fiducial
mark FM1 (.DELTA.XF=0 and .DELTA.YF=0) are calculated. Then, the
reticle stage RST is finely driven from the stored rough alignment
position by the deviating amounts thus worked out by the use of the
interferometers IRX IRY, and IR.theta.. In this way, the reticle R
is precisely aligned with respect to the detection center of the
off-axis alignment system OWA. Thereafter, the sequence beginning
at the step 524 shown in FIG. 15 is taken by the main control
system 114.
As described above, if there is provided a sensor (interferometer
or alignment system) capable of measuring the positional deviating
amount of the reticle stage RST (that is, reticle R) over a
comparatively long range
(.+-.several mm, for example), the reticle R can be finely aligned
after the detection of each of the fiducial marks for measuring the
base line is operated, at the same time the rough alignment
position being stored. In this way, the throughput will be improved
more that the sequences shown in FIG. 14 and FIG. 15.
In the embodiment according to the present invention, the TTL
alignment system of the LIA method is employed for the servo lock
of the fiducial plate FP. It is also necessary to operate the base
line control regarding this TTL alignment system of the LIA method
itself with the center CC of the reticle R. Now, assuming that the
TTL, alignment system of the LIA method is used for the detection
of the marks on the wafer W, it should be good enough only to store
the phase errors .DELTA..phi.x and .DELTA..phi.y respectively for
the marks LIMx and LIMy detected by the TTL alignment systems of
the LIA methods 3X and 3Y as the portions corresponding to the base
line error values with respect to the center CC of the reticle R
when the reticle marks RM.sub.1 and RM.sub.2 detected by the TTR
alignment systems 1A and 1B and the fiducial marks FM2A and FM2B
are precisely matched, respectively.
Subsequently, examples of the modifications of the present
embodiment will be described.
In the steps 508 to 512 of the sequences shown in FIG. 14 and FIG.
15, the reticle alignment is completely implemented by the use of
the TTR alignment systems 1A and 1B. However, it is possible to
omit such operation to a certain extent. As shown in FIG. 2, the
positional deviations of the reticle R in the directions X, Y, and
.theta., are being monitored by the iterferometers IRX, IRY, and
IR.theta. successively, and with the searching operation by IFS
method in the step 504, the respective coordinates of the
projection points of the reticle marks RM.sub.1 and RM.sub.2 are
detected by the interferometers on the wafer stage side to work out
the deviating amounts of the reticle R form the designed positions
in the directions X, Y, and .theta.. Then, it may be possible to
driven the reticle stage RST finely depending on the
interferometers on the reticle side to make the correction of such
deviating amounts possible. In this case, assuming that the
measuring resolution of the interferometers IRX, IRY, and IR.theta.
on the reticle side are sufficiently high (0.005 .mu.m, for
example), the positioning of the reticle R should be executed high
accuracy.
Also, while the off-axis alignment system OWA used for the present
embodiment is a static type alignment system where the mark
detection is operated with the wafer state WST being at rest, the
same effect is obtainable for a scanning type alignment system
where the mark detection is operated by moving the wafer stage WST
such as a TTL alignment system of the LSA method or IFS method. For
example, if the off-axis alignment system OWA is modified to be a
system in which the marks on the wafer stage WST are detected by
scanning with the irradiation to the wafer W of the laser beam the
spot of which is formed to be of a slit type, it should be good
enough only to make the arrangement of each of the marks on the
fiducial plate so that the light emission mark IFS can scan the
reticle marks RM.sub.1 or RM.sub.2 at the same time that the wafer
stage WST is moved to cause the fiducial mark FM.sub.1 on the
fiducial plate FP to cross such beam spot.
Moreover, if the LIA method is incorporated in the off-axis
alignment system OWA and the fiducial mark FM.sub.1 on the fiducial
plate FP is arranged to be the same diffraction grating as the
marks LIMx and LIMy, it may be possible to servo lock the wafer
stage WST on the basis of the resultant detection of the phase
difference measurement circuit so as to align the fiducial mark
FM.sub.1, which is detected by the off-axis alignment system OWA,
with the reference grating of the LIA in the off-axis alignment
system at all times. In this case, it is possible to calculate the
base line value just by obtaining each of the positional deviating
amounts between the fiducial marks FM2A and FM2B and reticle marks
RM.sub.1 and RM.sub.2 by the TTR alignment systems IA and 1B in a
state that the detection center of the off-axis alignment system
OWA is precisely matched with the center of the fiducial mark
FM1.
Also, using CCD cameras for a TTL alignment system, the mark images
on the wafer or fiducial plate FP are reversely projected by the
projection lens, and both the images thus projected and the images
of the index marks provided in the optical path of the TTL
alignment system are photographed for the detection of the
positional deviating amounts. Such a method for detecting the mark
positions may be employed. When this method is used, the control of
the base line value should be performed between the projection
point toward the wafer side of the center point (the detection
center point) of the index mark in the optical path of the TTL
alignment system and the center of the reticle marks RM.sub.1 and
RM.sub.2 (or the center CC of the reticle).
Now, the description has been made of the IFS method shown in the
present embodiment solely as a stage scan, that is, a scanning type
alignment system. However, it may be possible to make it a static
type alignment system. To this end, the light emission mark IFS on
the fiducial plate FP should be modified from the slit type to a
rectangular light emission area. Then, the light emission area
which is sufficiently larger than the width of the double slit is
positioned directly below the double slit RM.sub.1 y (or RM.sub.1
x) of the reticle mark shown in FIG. 6, and using TTR alignment
system disposed above the reticle R, the portion of the mark
RM.sub.1 y (or RM.sub.1 x) is observed by the CCD camera and
others. Hence obtaining the image signals having the same waveforms
as those shown in FIG. 16B. At this juncture, if there is no index
mark which can serve as an reference in the TTR alignment system,
it may be possible to obtain the deviating amount of the double
slit mark RM.sub.1 y (or RM.sub.1 x) with the position of a
specific pixel of the CCD camera as reference. Also, with this
method, the projection point of the center of the reticle mark
RM.sub.1 (or RM.sub.2) can be calculated on the basis of its
deviating amount and the coordinate value of the wafer stage WST
obtainable when the rectangular light emission area is positioned.
Further, as shown in FIG. 21, light shielding slit patterns SSP are
provided on a part of the rectangular light emission area PIF for
measuring the amount of its deviation with respect to the double
slit mark RM.sub.1 y (or RM.sub.1 x) Then, the light emission area
PIF is photographed by the CCD camera of the TTR alignment system
so that the positional deviating amounts may be obtained by the
dark lines of the double slit mark RM.sub.1 y and the dark lines of
the slit patterns SSP.
FIG. 22 shows an example of a variation of the arrangement of the
fiducial plate FP on the wafer stage WST and the arrangement of the
off-axis alignment system, in which the objective lens 4B in the
off-axis alignment system is positioned below the projection lens
PL in the plane of FIG. 22. This position is at the front side of
the main body of the apparatus and corresponds to the wafer loading
direction. Of the reference marks of parts in FIG. 22, those other
than IFY, IFX.sub.1, IFX.sub.2 which designate the interferometers
for measuring the position of the wafer stage WST are the same as
the reference marks of parts appearing in FIG. 3. In FIG. 22, the
line portion connecting the position of the optical axis of the
projection lens PL and the detection center (substantially the same
as the optical axis position of the objective lens 4B) of the
off-axis alignment system OWA is in parallel with the Y axis.
Therefore, one interferometer IFY is arranged in the direction Y
while two interferometers IFX.sub.1 and IFX.sub.2 are arranged in
the direction X. To match with them, the arrangement of each of the
marks on the difucial plate FP is modified so that the line portion
connecting each of the center points of the fiducial mark FM1 and
fiducial mark FM2 can be paralleled to the Y axis.
In this case shown in FIG. 22, too, when the marks on the wafer,
fiducial mark FM1, or the like is detected by the off-axis
alignment system OWA, the interferometers IFX.sub.1 and IFY which
can satisfy the Abbe's condition are used, and the interferometers
IFX.sub.2 and IFY are used when the wafer stage is positioned for
exposure. In other words, in detecting the marks by the off-axis
alignment system OWA, the positional coordinate value in the
direction X measured by the interferometer IFX.sub.1 becomes
correspondent to the positional coordinate value measured by the
interferometer IFX.sub.2. This correspondence is performed exactly
in the same manner as the reciprocative mutual presetting of the
interferometers IFY.sub.1 and IFY.sub.2 as described in conjunction
with FIG. 19.
The exposure apparatus described in the embodiment mentioned above
is a stepper which causes the projected image in the pattern area
PA on the reticle R to be exposed on the wafer W by a
step-and-repeat method. The present invention is also equally
applicable to an exposure apparatus of a step-scan method whereby
to scan the reticle and wafer simultaneously in the direction
perpendicular to the optical axis of the projection optical
system.
Also, it is possible to apply the same positioning systems to an
X-ray aligner, X-ray stepper, and the like using an X-ray souce of
SOR and others.
FIG. 23 illustrates the structure of the a projection exposure
apparatus according to a second embodiment of the present
invention. FIG. 24 is a block diagram showing the arrangements of
the wafer stage and laser interferometers and the control system of
the apparatus shown in FIG. 23.
In FIG. 23, a reticle R having a predetermined pattern area PA is
held on a reticle stage which is not shown, and is positioned so
that the optical axis AX of a project lens PL can pass through the
center point of the pattern area PA. This reticle stage is finely
driven by a motor in the direction X, direction Y, and direction
.theta. (rotation around the optical axis AX) to drive the reticle
R for the alignment with the wafer through the projection lens PL
(die by die alignment) or for the alignment of the reticle R itself
with respect to the apparatus (reticle alignment). Also, in four
locations in the circumference of the pattern area PA of the
reticle R, the reticle alignment (or die by die alignment) marks
RMx.sub.1, RMx.sub.2, RMy.sub.1 and RMy.sub.2 are provided. The
marks RMx.sub.1, RMx.sub.2 are used for positioning in the
direction of the X axis of the rectangular coordinate system xy on
the reticle side or the positional deviation detection while the
marks RMy.sub.1 and RMy.sub.2 are used for positioning in the
direction of the y axis of the rectangular coordinate system xy or
the positional deviation detection. These four marks are all
arranged in the field of the projection lens PL.
Now, as shown in FIG. 24, the projection image of the pattern area
PA of the reticle R is focused on the wafer W which serves as an
exposure substrate. The wafer W is held on the wafer stage WST
through a wafer holder WH. The wafer stage is driven in parallel in
the rectangular coordinate system XY by the step-and-repeat method
or step-and-scan method. The holder WH can be finely driven in the
direction .theta. on the stage WST to correct the rotation after
the prealignment of the wafer W. Also, on the two sides surrounding
the stage WST, there are fixedly provided a movable mirror IMy
having a reflecting plane in parallel with the X axis of the
rectangular coordinate system XY and a movable mirror IMx having a
reflecting plane in parallel with the Y axis. To the reflecting
plane of the movable mirror IMy, the laser beam from the
interferometer IFY is irradiated in parallel with the Y axis. The
interferometer IFY causes the reflected beam from the driving
mirror IMY to interfere with the reflected beam from a reference
mirror fixed at a predetermined position to measure the driving
distance from the movable mirror IMy (that is, the reference
position of the wafer stage WST) in the direction Y.
Likewise, two laser interferometers IFX.sub.1 and IFX.sub.2 are
provided oppositely in the direction X with respect to the movable
mirror IMx. Of these the measurement axis which is the center of
the laser beam of the interferometer IFX.sub.2 and each of the
extended lines of the measurement axes (beam centers) of the
interferometers IFY are provided to intersect at right angles at
the position of the optical axis AX of the projection lens PL.
In FIG. 24, a circle If around the optical axis AX is the image
field of the projection lens PL, and the fiducial plate FP which is
large enough to include the image field If is fixed to the stage
WST at a height substantially the same level as the surface on the
wafer W.
On this fiducial plate FP, there are formed four fiducial marks
FMX.sub.1, FMX.sub.2, FMY.sub.1 and FMY.sub.2 as shown in FIG. 23,
which are aligned with each of the projected images of the four
marks RMx.sub.1, RMx.sub.2, RMy.sub.1 and RMy.sub.2 of the reticle
R when its center Fcc is matched with the optical axis AX of the
projection lens PL. These four fiducial marks FM are detected by
each of the TTR alignment systems (mark detection means) DDX.sub.1,
DDX.sub.2, DDY.sub.1, and DDY.sub.2 through the projection lens PL
and reticle R together with the corresponding four reticle marks
RM. For this TTR alignment system, several methods are made
practicable and publicly known. As a typical TTR system thereof,
there are such a method, as shown in FIG. 7, too, in which the
images of the fiducial marks FM reversely projected to the reticle
R side by the projection lens P1 and the images of the reticle
marks RM are photographed for the detection of the positional
deviation on the basis of the image signals thus obtained, and a
method in which the spot light of converged laser beams is
irradiated on the fiducial plate FP through the reticle R and
projection lens PL, and the diffraction rays or scattered rays of
light generated from each of the marks at the time of one
dimensional scanning which crosses the reticle marks RM and
fiducial marks FM are selectively detected by photoelectrically in
the plane conjugated with the pupil EP of the projection lens PL to
detect the positional deviating amounts between the fiducial marks
FM and reticle marks RM and others. In addition, the TTL alignment
system may be used for the detection of the positional deviation
between the alignment marks (which should be wafer marks) attached
to the shot area on the wafer W and the reticle marks RM.
Now, in the present embodiment, the off-axis alignment system OWA
is fixed outside the projection lens PL as shown in FIG. 23
earlier. This off-axis alignment system OWA includes a microscopic
objective lens 4B to detect the alignment marks on the wafer W as
images, and television camera (CCD and others) 4X and 4Y for
photographing the images in enlargement as in the case shown in
conjunction with FIG. 10. Also, since the alignment system OWA is
separately provided from the projection lens PL, there is provided
an optical system (objective lens) for which an achromatic process
has been given independently, and the light evenly illuminating the
observation area on the wafer W through the objective lens 4B is
produced by a halogen lamp having a wavelength width of
approximately 200 to 350 nm. A case is of course taken so that the
light sensitive wavelength band of the resist on the wafer W is not
exposed to such illuminating light.
Inside an off-axis alignment system OWA such as this, index marks
are provided for detecting the positional deviation of the marks on
the wafer W as shown in FIG. 9. The enlarged images of the index
marks and wafer marks are photographed by the television cameras.
Therefore, when the mark positions on the wafer W are detected by
the off-axis alignment system OWA, the coordinate positions (Xw and
Yw) of the wafer stage WST are stored while the index marks TM and
wafer marks WM are being photographed by the television cameras as
in the case of the first embodiment. Then, after obtaining the
positional deviating amounts (.DELTA.X and .DELTA.Y) of the index
marks TM and wafer marks WM in the directions X and Y by the image
signal processing, these amounts must be worked out as the
coordinate values (Xw-.DELTA.X and Yw-.DELTA.Y). Because there is
such a necessity to read the coordinate positions of the wafer
stage WST, the center of the detection point Pd (the same as Of in
FIG. 19) of the off-axis alignment system OWA is set at the point
where the extended line of the measurement axis of the
interferometer IFY in the direction Y and the extended line of the
measurement axis of the interferometer IFX.sub.1 in the direction X
intersect at right angles as shown in FIG. 24, too. Here, the
center point Pd of the detection for the off-axis alignment system
OWA is not necessarily the position of the optical axis of the
objective lens, but it is rather an imagenary point determined by
the inner index marks. In other words, when the index marks are
projected on the wafer W through the objective lens, it is
considered that the center point Pd of the detection is present at
a predetermined position with the projection points of such index
marks as reference, and that the positional deviating amounts
(.DELTA.X and .DELTA.Y) with respect to the center point Pd of the
detection is being obtained by the off-axis alignment system
OWA.
When the interferometers IFY and IFX.sub.1 of the off-axis
alignment system OWA are arranged as described above, the Abbe's
error (sine error) becomes zero at the time of the mark position
detection by the alignment system OWA. Likewise, the arrangements
of the interferometers IFY and IFX.sub.2 can be made to satisfy the
Abbe's condition with respect to the projection lens PL.
Now, in FIG. 24, the up down pulses UDP1 output from the
interferometer IFX.sub.1 are reversibly counted by the up down
counter (UDC) 211 which can be preset, and its counted value
DX.sub.1 is output to the main control system 214 for the stage
control as a coordinate value of the stage WST in the direction X.
Likewise, in the interferometer IFX.sub.2, the up down pulses
UPD.sub.2 are counted by the UDC 212 which can be preset and its
counted value DX.sub.2 is output to the main control system 214 as
a coordinate value of the stage WST in the direction X. Likewise,
in the interferometer IFY.sub.1, the up down pulses are counted by
the UDC 210 and its counted value DY is transmitted to the main
control system 214. The up down pulses from each of the these
interferometers are defined to be one pulse per driving of the
stage WST of 0.01 .mu.m, for example.
The main control system 214 outputs preset (or reset) signals
S.sub.1, S.sub.2, and S.sub.3 to the UDCs 210, 211, and 212,
respectively. To the UDCs 211 and 212, among them, particular
presettings are conducted in response to the preset signals S.sub.1
and S.sub.2. In other words, when the preset signal (pulse) S.sub.1
is output, the counted value DX.sub.2 of the UDC 212 is preset to
the UDC 211 while the preset signal (pulse) S.sub.2 is output, the
counted value DX.sub.1 of the UDC 211 is present to the UDC 212. In
this respect, zero reset signal is transmitted from the main
control system 214 to the UDCs 210, 211, and 212 when the wafer
stage WST is positioned at its home position.
Now, the TTR alignment processing system 216 works out various
alignment errors and positional correction amounts of the reticle
stage or wafer stage WST on the basis of the deviation information
SX.sub.1, SX.sub.2, SY.sub.1, and SY.sub.2 for the mark positions
detected by each of the TTR alignment systems DDX.sub.1, DDX.sub.2,
DDY.sub.1, and DDY.sub.2 to output the results to the main control
system 214. The off-axis alignment processing system 218 works out
the center point of the coordinates of the shot area on the wafer
W, the regularity of the shot area arrangements, the expansion,
contraction, distortion of the wafer W, and others on the basis or
each information of the positional deviations SGx and SGy of the
plural wafer marks in the directions X and Y detected by the
alignment system OWA and outputs such results to the principal
control system 214. The stage driver 220 controls motors 221 and
222 for driving the wafer stage WST in the direction X and
direction Y, respectively, to position the wafer stage WST
precisely in accordance with the results output from the aforesaid
processing systems 216 and 218 as well as with the coordinate
values (DX.sub.1, DX.sub.2, and DY) from the UDCs 210, 211 and
212.
Subsequently, the operation of the present embodiment will be
described. However, as the entire sequence thereof is mostly known,
its details will be omitted. Only its characteristic sequence will
be described below.
At first, the reticle R is mounted on the reticle stage before
mounting the prealigned wafer W on the holder WH. The reticle R is
also held on the reticle stage by the mechanical prealignment. At
this juncture, it is assumed that the UDCs 210, 211, 212 of the
interferometers IFY, IFX.sub.1 and IFX.sub.2 are zero reset because
the wafer stage WST is at its home position.
Then, the main control system 214 controls the motor 221 and 222 to
position the wafer stage WST through the stage driver 220 as shown
in FIG. 23 or in FIG. 24. The position at which the wafer stage WST
is set as shown in FIG. 23 or in FIG. 24 is predetermined with
respect to the home position. Accordingly, the center point Fcc of
the fiducial plate FP is positioned with an accuracy of
approximately several .mu.m with respect to the optical axis AX of
the projection lens PL. When this positioning is accomplished, the
wafer stage WST is servo locked under control of the
interferometers IFY and IFX.sub.2.
After that, using the TTR alignment system DDX.sub.1, DDX.sub.2,
DDY.sub.1, and DDY.sub.2, the positional deviating amounts
SX.sub.1, SX.sub.2, SY.sub.1, and SY.sub.2 between the reticle
marks RM and fiducial marks FM are obtained. Based on these
positional deviating amounts, the processing system 216 works out
the positional deviating amount between the center point of the
reticle R and the center point Fcc of the fiducial plate FP in the
directions X and Y and the relative rotational error amount in the
direction .theta.. The main control system 214 drives motors (not
shown) and others to drive the reticle stage finely so that the
positional deviating amount and rotational error amount are both
driven into zero values. The driving of the reticle stage can be
controlled either with a closed loop control based on the
information of the positional deviating amounts and rotational
error amounts obtainable from the processing system 216 as the
position deviation of the marks are sequentially detected by the
TTR alignment systems, or with an open control if there are
provided sensors capable of measuring the position of the reticle
stage highly precisely.
With the operation set forth above, the reticle R is aligned with
the fiducial mark FM on the fiducial plate FP as reference.
At the position of the wafer stage WST where the reticle R is
precisely aligned with respect to the fiducial plate FP, each of
the UDCs 211 and 212 for the two interferometers IFX.sub.1 and
IFX.sub.2 are preset with each other. More specifically, the main
control system 214 outputs the present signal S.sub.1 so that the
measured value DX.sub.2 of the UDC 212 of the interferometer
IFX.sub.2 being used for servo locking the wafer stage WST can be
preset to the counter UDC 211 of the other interferometer IFX.sub.1
at this juncture.
Immediately before this operation, however, the measured value
DX.sub.1 of the counter UDC 211 is read by the main control system
214 to work out the difference with the measured value DX.sub.2 for
the storage of such difference or the measured value DX.sub.1 thus
read. These stored values are needed when the reciprocative
relationship of the measured values between the interferometers
IFX.sub.1 and IFX.sub.2 is restored to the original stage.
The operation which has been described follows the sequence
characteristic of the present embodiment, and after this, the wafer
stage WST is driven to allow the off-axis system OWA to detect the
fiducial marks FM on the fiducial plate FP sequentially for
positioning. Each position of the wafer stage WST then is measured
by the interferometers IFX.sub.1 and IFY for storage. Further, the
positional deviating amount for each of the fiducial marks FM is
obtained by the off-axis alignment processing system 218 with
respect to the detection center Pd. Then, on the basis of the
information of the positional deviating amounts and stored
respective positions of the stage, the relative positional
relationship (base line) between the projection point of the center
point of the reticle R and the center point Pd of the detection on
the rectangular coordinate system XY is computed and stored.
After that, the wafer W is mounted on the holder WH Then, the
interferometers IFX.sub.1 and IFY for measuring the coordinate
position of the wafer stage WST are used when the wafer marks
provided with several shot areas on the wafer W are detected by the
off-axis alignment system, and the positional control of the wafer
stage WST is performed in accordance with the measured values of
the interferometers IFX.sub.2 and IFY at the time of the
exposure.
In other words, at the position of the wafer stage WST where the
reticle R can be aligned with the fiducial plate FP as reference,
the respective inner counters (UDC 211 and UDC 212) of the two
interferometers IFX.sub.1 and IFX.sub.2 are preset to a same value.
Then, after that, it is possible to introduce the value measured by
the interferometer IFX.sub.1 at the time of the mark detection by
the off-axis alignment system OWA as it is as the required value
for positioning the wafer stage WST on the basis of the measured
value of the interferometer IFX.sub.2 at the time of exposure, that
is, no positional error results from the yawing of the wafer stage
WST during the period from the off-axis alignment to the
exposure.
Now, in reference to FIG. 25, the sequence set forth above will be
described further in detail. FIG. 25 is an exaggerated view showing
the arrangement relationship between the fiducial marks FM and
movable mirror IMx and interferometers IFY, IFX.sub.1, and
IFX.sub.2.
Firstly, the measurement axis of the interferometer IFY and each of
the measurement axes of the interferometers IFX.sub.1 and IFX.sub.2
are assumed to be precisely rectangular while the intersecting
point (Pd) of the detection center PDx of the off-axis alignment
system OWA in the direction X and the detection center PDy in the
direction Y and the center point Fcc of the fiducial plate FP are
assumed to be positioned on the measurement axis of the
interferometer IFY. Now, when the fiducial plate FP is mounted on
the wafer stage WST, it has a rotational error .theta.f with
respect to the reflecting plane of the movable mirror IMx.
Accordingly, this rotational error .theta.f is precisely measured
in advance as in the case of the first embodiment. Then, when the
reticle stage is positioned to correct this rotational error
.theta.f at the time of the reticle alignment, the reticle R is
aligned to the reflecting plane of the movable mirror IMx without
any rotation as shown at r.sub.2 in FIG. 25. In this respect, a
indicium r.sub.1 in FIG. 25 designates the position of the reticle
when the reticle marks RM are aligned with respect to the fiducial
marks FM with zero errors. The straight line K.sub.1 which runs
through the center point Fcc represents a line extending in the
direction Y through the center of the reticle R, and when the
reticle R is aligned to have corrected the error .theta.f, the
straight line K.sub.1 is in parallel with the reflecting plane of
the movable mirror IMx.
On the other hand, the servo control for positioning the wafer
stage WST in this state is being executed in accordance with the
interferometers IFX.sub.2 and IFY. However, given the measured
value of the interferometer IFX (DX.sub.2) as Le, and the measured
value of the interferometer IFX.sub.1 (DX.sub.1) as Lf, the main
control system 214 calculates its difference Ly using the value
(Lf-.DELTA.Le) and stores .DELTA.Ly. The measured values Le and Lf
are based on the position of the reflecting plane of the movable
mirror IMx when the wafer stage WST is at its home position, and
its reference is indicated by a straight line Lir in FIG. 25. Now,
after the difference .DELTA.Ly is stored, the UDC 211 is preset to
make the measured value Lf of the interferometer IFX.sub.1 equal to
the measured value Le of the interferometer IFX.sub.2. Then, the
reference of the two interferometers IFX.sub.1 and IFX.sub.2 are
modified to be as indicated by a straight line Lir'. This straight
line Lir' is in parallel with the reflecting plane of the movable
mirror IMx and the straight line K.sub.1 as well. Thus, thereafter,
the marks on the wafer W or the fiducial marks FM on the fiducial
plate FP are detected by the off-axis alignment system OWA, and the
coordinate value obtainable by detecting the positions thereof in
the direction X using the interferometer IFX.sub.1 corresponds one
to one as it is to be value to be measured by the interferometer
IFX.sub.2. Hence making it possible to switch over the positioning
control of the wafer stage WST at the time of exposure to the
control by the interferometers IFX.sub.2 and IFY without operating
any corrective calculation in consideration of the yawing and
others of the stage.
In this respect, when the wafer stage WST is stepped in the
direction Y, for example, by the step-and-repeat method, the stage
WST is driven along the direction in which the reflecting plane of
the movable mirror IMx is set. Because of this, if the reticle R is
aligned with the correction of the error .theta.f at r.sub.2, the
rotation per shot area, that is, the so-called chip rotation, is
reduced to an extent which is negligible with respect to the
arrangement coordinate system determined by the arrangement of a
plurality of shot areas exposed on the wafer W.
Also, in the present embodiment, while only one off-axis alignment
system OWA is arranged, if the arrangement of two or more systems
is desired, the interferometers which can satisfy the Abbe's
condition likewise should only be arranged more. Further, it may be
possible to arrange the interferometer to satisfy the Abbe's
condition for the TTL inner-field off-axis alignment system having
the center point of detection in the circumference inside the
larger image field of the projection lens. Also, if there are
arranged two interferometers in the direction X and two
interferometers in the direction Y as well, the reciprocative
presets are conducted respectively in the direction X and direction
Y. Also, in FIG. 25, while the description is made of the case
where the measured value Le (DX.sub.2) of the UDC 212 of the
interferometer IFX.sub.2 is preset to the UDC 211 of the
interferometer IFX.sub.1, it may be possible to conduct the
presetting in the reverse direction. In other words, the measured
value Lf of the UDC 211 can be preset to the UDC 212. In this case,
however, it is necessary to switch over the positioning control of
the wafer stage WST to before the resetting.
Now, as the TTR alignment systems DDX.sub.1, DDX.sub.2, DDY.sub.1,
and DDY.sub.2 shown in FIG. 23 or off-axis alignment system OWA, it
may be possible to employ a twin-beam interference alignment system
(same as LIA type) in which, as disclosed in the application Ser.
No. 483,820 filed on Feb. 23, 1990, for example, two coherent beams
are irradiated so that they are intersected at a predetermined
angle on the reticle or wafer to produce interference fringes on
the grating patterns on the reticle or the wafer, and then the
coordinate position of the grating patterns on the wafer or the
relative positional deviating amounts between the grating patterns
of the reticle and the grating patterns on the wafer are obtained
by photoelectrically detecting the diffracting light (.+-.first
order diffraction light, or zeroth light, .+-.second order
diffraction light) generated by the grating patterns. Particularly,
this system is extremely effective when the reticle R is aligned
using the fiducial plate FP. In other words, each of the fiducial
marks FM on the fiducial plate FP is formed into the grating
pattern and each of the reticle marks RM on the reticle R is also
formed into the grating pattern as well. Then, the positional
deviating amount between each of the reticle marks RM and each of
the fiducial marks FM is obtained by the TTR alignment system of
the twin-beam interference alignment method. The measuring
resolution of this positional deviating amount can be higher than
the resolution (.+-.0.01 .mu.m) of the interferometers IFX.sub.1,
IFX.sub.2, and IFY.
Then, it should be good enough if the offset amount of each of the
marks necessitated by the mounting error .theta.f of the fiducial
plate FP is added to the positional deviating amount thus obtained
for the execution of the servo control of the reticle stage in the
directins X, Y, and .theta..
In this respect, it may be possible to make an arrangement so that
subsequent to the alignment of the reticle R, the control of the
stage driver 220 is started only on the basis of the positional
deviating amount detected by the TTR alignment system in order to
servo control the position of the fiducial plate FP with respect to
the reticle R by the TTR alignment system of the twin-beam
interference alignment method thereby to preset the 20 measured
values of the two interferometers IFX.sub.1 and IFX.sub.2 equally
while the position of the wafer stage WST is being servo locked
with the aligned reticle R as reference. In this case, the two
interferometers IFX.sub.1 and IFX.sub.2 do not contribute to the
positioning control of the wafer stage WST. Therefore, both of them
can be preset.
Also, regarding the reticle alignment or the checking of the
residual errors subsequent to the reticle alignment, it may be
possible to employ besides the TTR alignment system a method in
which he relative positions of the images of the light emission
marks and the reticle marks RM are detected by projecting the light
emission type marks on the fiducial plate FP of the wafer stage WST
to the reticle R inversely, or a method in which photoelectric
elements are provided below the slit type marks of the fiducial
plate FP and the projected images of the reticle marks RM focused
on the fiducial plate are received by such photoelectric elements
for the positional detection of the projected images.
Now, as a method to measure precisely the mounting error .theta.f
of the fiducial plate FP in advance, there is a method using a
pilot exposure besides the method described in the first embodiment
in this case, a test reticle or the like for the evaluation of the
stepper is aligned precisely with respect to the fiducial plate FP
using the TTR alignment systems DDX.sub.1, DDX.sub.2, DDY.sub.1,
and DDY.sub.2 shown in FIG. 23. In other words, in this reticle
alignment, the test reticle is positioned with respect to the
fiducial plate FP with the accompanying mounting error .theta.f. As
a result, the test reticle is set at r.sub.1 in FIG. 25.
Then, in such a state, the test wafer (bare silicon or the like)
coated with resist is prealigned on the wafer holder WH with flat
OF as reference for mounting. Then, at the pitches corresponding to
the size of the projected images on the pattern area PA of the test
reticle, the wafer stage WST is stepped in the direction X and
direction Y to repeat the exposure in this case, the driving of the
wafer stage WST in the direction X and direction Y must follow the
reflecting planes of the movable mirrors IMx and IMy.
FIG. 26 is an exaggerated view showing the arrangement of the shot
area TS of the test reticle to be formed on the test wafer W at the
time of exposure. In FIG. 26, the rectangular coordinate systems Xr
and Yr are the internal coordinate systems of the test reticle, and
there is no rotational deviation for each of the plural shot areas
TS on the wafer W with respect to the coordinate systems Xr and Yr.
However, the stepping of the wafer stage WST by the step-and-repeat
method is executed following the movable mirrors IMx and IMy.
Accordingly, the arrangement coordinate systems (rectangular)
.alpha..beta. regulated by the center point of each of the shot
areas are in parallel with each of the reflecting planes of the
moving mirrors IMx and IMy. Consequently, as clear from FIG. 26,
each of the shot areas TS on the wafer looks as if a chip rotation
has taken place when observed with the arrangement coordinate
systems .alpha..beta. as reference.
Therefore, subsequent to the development of the wafer to which the
pilot exposure has been given, the averaged rotational error amount
.DELTA..theta.c of each of the shot areas TS with respect to the
arrangement coordinate systems .alpha..beta. is obtained by the use
of the measuring function of the stepper or some other measuring
instrument using the test mark (vernier pattern or the like) in
each of the shot areas TS. Then, the error amount .DELTA..theta.c
must essentially be the same as the mounting error .theta.f.
There is in this respect a technique for performing a special
stepping when the pilot exposure is conducted for measuring the
chip rotation. In this method, the stepping is conducted in such a
manner that the vernier patterns provided at plural locations on
the outermost circumference of the pattern areas PA of the test
reticle are caused to overlap with each other when adjacent shot
area is exposed. The specific example thereof is shown in FIGS.
27A, 27B and 27C.
FIG. 27A illustrates an example of the vernier pattern arrangement
in the pattern area PA of a test reticle TR. On each of the four
sides of the circumference of the area PA, a plurality of vernier
patterns GP of a diffraction grating type shown in FIG. 27B are
formed on the predetermined positions. Among these vernier patterns
GP, those formed along the side of the pattern area PA in parallel
with the axis Yr of the coordinate system Xr and Yr are set so as
to allow its pitch direction of the diffraction grating to be
matched with the direction Yr while those fomred along the side of
the pattern area PA in parallel with the axis Xr are set so as to
allow its pitch direction of the diffraction grating to be matched
with the direction Xr. Also, each of the plural vernier patterns GP
is formed at a predetermined position with respect to the center
point CR of the test reticle TR. Then, it is assumed that this
positional relationship has been precisely measured by some other
measuring instrument and obtained as an arrangement precision
(error) of the verniear of the test reticle.
Now, the vernier pattern GP of a diffraction grating type shown in
FIG. 27B is formed with a constant pitch (4 .mu.m on the wafer, for
example) to present lines and spaces, and the width of each of the
lines and spaces is 2 .mu.m on the wafer, for example.
If the pattern areas PA having vernier patterns GP such as this are
exposed to the wafer by stepping, the adjacent shot areas
themselves are driven in step with parts thereof being overlapped
in the direction X and direction Y as shown in FIG. 27C. FIG. 27C
is an exaggerated view showing the arrangement of one specific shot
area TSa of the plural shot areas shown in FIG. 26, in which
adjacent four shot areas TS.sub.1, TS.sub.2, TS.sub.3, and TS.sub.4
are arranged. As clear from FIG. 26, too, if any chip rotation is
present, the adjacent shot areas themselves are observed as if they
have created relative positional deviations in the direction
substantially perpendicular to the stepping direction. For example,
in FIG. 27C the shot areas TS.sub.1 and TS.sub.3 formed by stepping
in the direction .alpha. (X) are deviated in the direction .beta.
(Y) with respect to the specific shot area TSa while the shot areas
TS.sub.2 and TS.sub.4 formed by stepping in the direction .beta.
(Y) are deviated in the direction .alpha. (X) with respect to the
specific shot area TSa. Then, the stepping is performed so that the
deviating direction due to the chip rotation and the pitch
direction of the grating vernier pattern GP are matched to the
overlapping portion (slanted portion) each of the shot areas and
each of the vernier patterns of the adjacent shot areas is arranged
in parallel with a slight deviation.
When the wafer thus exposed is developed, the resist images of a
pair of grating type vernier patterns GP are formed on each of the
four sides on the circumference of the shot area TSa. The pair of
the vernier resist patterns have slight positional deviations in
the pitch direction. And measuring this positional deviating amount
with a high accuracy, the chip rotation amount .DELTA..theta.c,
that is, the mounting error .theta.f is obtained.
Now, a specific example is provided. Among the vernier patterns GP
of the test reticle TR in FIG. 27A, an attention should be given to
the two vernier patterns GPa and GPb which are positioned
symmetrically with respect to the center point CR on the line
running through the reticle center CR in parallel with the axis Xr.
At this juncture, in the overlapped portion (assumed to be the
overlapped portion on the left-hand side) between the shot area TSa
and shot area TS.sub.1, the vernier resist pattern GPb of the area
TS.sub.1 is deviated to the positive position in the direction Y
with respect to the vernier resist pattern GPa provided to the area
TSa while in the overlapped portion (assumed to be the overlapped
portion on the right-hand side) between the shot area TS.sub.a and
shot area TS.sub.3, the vernier resist pattern GPa of the area
T.sub.3 is deviated to the negative position in the direction Y
with respect to the vernier resist pattern GPb provided to the area
TSa.
Now, given the value obtainable by converting the interval between
the vernier patterns GPa and GPb in the direction Xr on the test
reticle into the wafer side as LD, the positional deviating amount
in the direction Y obtainable in the overlapped portion on the
left-hand side as .DELTA.Yg.sub.1, and the positional deviating
amount in the direction Y obtainable in the overlapped portion on
the right-hand side as Yg.sub.2, the chip rotation .DELTA..theta.c
(=.theta.f) can be worked out by the following equation:
With a technique such as this, the relative positional deviating
amounts of other plural vernier resist patterns GP are obtained in
the specific shot area TSa in order to calculate the plural chip
rotation amounts .DELTA..dwnarw.c. Then, by averaging them, it is
possible to obtain the mounting error .theta.f with a higher
precision. Also, the specific shot area TSa is arranged in the
plural different positions on the wafer so as to obtain the
averaged chip rotation amount .DELTA..theta.c in each of such
plural positions. Then, by averaging them further, it is possible
to reduce the effect due to the random stepping errors of the wafer
stage WST.
In this respect, if the two of the resist images of the vernier
patterns GP in FIG. 27B are arranged in parallel in the direction
rectangular to the pitch direction, the positional deviating
amounts of the two vernier patterns in the pitch direction can be
measured with an extremely high precision by the use of a
measurement instrument for the positional deviation using the
interference fringes as disclosed in U.S. Pat. No. 4,710,026, for
example.
According to the method disclosed in this publication, two laser
beams symmetrically slanted in the pitch direction are irradiated
to each of the two vernier resist patterns simultaneously to
produce the interference fringes having pitches 1/2 of the grating
pitches Pg of the vernier resist patterns. Then, a pair of
frequency differentials .DELTA.f are given between the two laser
beams simultaneously. At this juncture, the interference light of
.+-.first order diffraction light generated vertically from each of
the vernier resist patterns becomes bearing light having a beat
frequency .DELTA.f. Then, two alternating current signals (sine
waveform) having the frequency .DELTA.f can be obtained by
photoelectrically detecting the interference light individually.
However, if the two vernier resist patterns have positional
deviations in the pitch direction (within 1/2 of the grating pitch
Pg), a phase difference .phi. is generated between the two
alternating current signals in proportion to the positional
deviating amounts. Therefore, using a phase difference meter or the
like, the phase difference .phi. between the two alternating
current signals (within .+-.180.degree.) is measured, and the
result can be converted into the positional deviating amount. In
this connection, assuming that Fourier integration is used for the
phase difference measurement, it is possible to obtain
approximately .+-.0.5 stably as the resolution of the phase
difference measurement. Now, if the grating pitch Pg is 4 .mu.m on
the wafer, the positional deviaiton of Pg/2 corresponds to
360.degree. of the phase difference (.+-.Pg/4 corresponds to
.+-.180.degree.). Accordingly, when the phase measurement
resolution is .+-.0.5.degree., the positional deviation detecting
resolution is .+-.Pg/4 (4.multidot.180/0.5), which is approximately
.+-.2.8 nm. This value is higher by one digit than the usual
resolution of the laser interferometer for measurement use.
Therefore, if the positional deviation measuring function using
such interference fringes, that is, the same function of the LIA
system described in the aforesaid first embodiment, is incorporated
as an alignment system of a stepper, it is possible to obtain the
chip rotation .DELTA..theta.c and the mounting error .theta.f of
the fiducial plate FP immediately by mounting the developed wafer
on the stepper for an automatic measurement. As a result, there is
an advantage that the operations required to store such values in
the stepper as apparatus constants can be automated. Also, with a
stepper provided with such a high resolution and a highly precise
self measuring system, it is possible to obtain the mounting error
of the fiducial plate FP by measuring the positional relations of
the fiducial marks directly. In this case, it is necessary to
provide the marks of the same diffraction grating type as shown in
FIG. 27B partially with the fiducial marks.
* * * * *