U.S. patent number RE33,836 [Application Number 07/573,400] was granted by the patent office on 1992-03-03 for apparatus and method for making large area electronic devices, such as flat panel displays and the like, using correlated, aligned dual optical systems.
This patent grant is currently assigned to MRS Technology, Inc.. Invention is credited to Robert A. McEachern, Griffith L. Resor, III, William C. Schneider, Walter H. Worth.
United States Patent |
RE33,836 |
Resor, III , et al. |
March 3, 1992 |
Apparatus and method for making large area electronic devices, such
as flat panel displays and the like, using correlated, aligned dual
optical systems
Abstract
Apparatus for projecting multiple images from a pair of reticles
(117) to a photo-sensitive coated substrate (1) to produce large
scale electronic devices (2). A pair of parallel and proximate
optical systems (29) are used, the optical systems being positioned
to project in the z-direction upon a movable stage (11) subject to
controlled motion (159, 169) in the x- and y-directions. The
apparatus includes means (225) for determining the coordinates of
motion of the stage relative to images projected from the reticles,
means for using the determined positions to establish a stage
transfer function for the apparatus relative to various positions
of stage, and means (130) for applying the transfer function to
adjust the relative positions of the reticles (117) and substrate
(1) for accurate image projection, and for thereafter projecting an
image upon the substrate. The stage is then stepped to a position
to permit projection of an abutting image, and the transfer
function is used to adjust the relative positions of the reticles
(117) and substrate (1) for accurate image projection for
projecting the abutting image upon the substrate (1). The steps are
repeated until sufficient images have been projected upon the
substrate to make up one integrated layer of a flat panel display
or other large scale electronic device. After treating the
photosensitive layer, subsequent layers are produced in a similar
manner.
Inventors: |
Resor, III; Griffith L. (Acton,
MA), McEachern; Robert A. (Wellesley, MA), Schneider;
William C. (Littleton, MA), Worth; Walter H. (Carlisle,
MA) |
Assignee: |
MRS Technology, Inc.
(Chelmsford, MA)
|
Family
ID: |
26808891 |
Appl.
No.: |
07/573,400 |
Filed: |
August 24, 1990 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
111427 |
Oct 22, 1987 |
04769680 |
Sep 6, 1988 |
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Current U.S.
Class: |
355/43; 355/46;
355/53 |
Current CPC
Class: |
G03F
7/70475 (20130101); G03F 7/70891 (20130101); G03F
7/70791 (20130101); G03F 7/70875 (20130101); G03F
7/70691 (20130101); G03F 7/70275 (20130101); G02F
1/13625 (20210101) |
Current International
Class: |
G03F
7/20 (20060101); G02F 1/1362 (20060101); G02F
1/13 (20060101); G03B 027/52 (); G03B 027/70 () |
Field of
Search: |
;355/43,46,45,53,54
;356/401 ;364/490,559 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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A59-923 |
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Jan 1984 |
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JP |
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60-109228A |
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Jun 1985 |
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JP |
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Other References
M C. King "SPIE Symposium on Microlithography Short Course Notes:
Optical Lithography" Mar. 6, 1990, International Society for
Optical Engineering, pp. 192-193..
|
Primary Examiner: Wintercorn; Richard A.
Attorney, Agent or Firm: Johnson; Haynes N.
Claims
We claim:
1. Apparatus to project images from reticles onto the
photosensitive surface of a single common substrate to produce a
large scale integrated image upon said substrate, said apparatus
including
a movable stage for holding said substrate, means for stepping said
stage in the x- and y-directions, stage calibration means to
calibrate the position of said stage in different stepped positions
and to determine a stage transfer function incorporating said
calibration data,
a pair of parallel optical systems for concurrently projecting dual
images upon said substrate, said systems having optical axes in the
z-direction, each said optical system including a projection
imaging system, an illumination system, a reticle carrier to carry
said reticles, and a reticle chuck positioned to receive said
reticles one at a time from said reticle carrier and to hold each
said reticle within said illumination system during projection of
an image carried by said reticle, each said reticle chuck being
capable of individual adjustment movement in at least the x-, y-,
z-, and .phi.-directions,
reticle calibration means associated with each said reticle chuck
to calibrate the position of a said reticle in said chuck relative
to said stage and to determine a reticle transfer function
incorporating said calibration data,
a computer associated with said stepping means and said optical
systems for controlling same, said computer storing said stage
transfer function and said reticle transfer functions and utilizing
same to adjust said stepping means and said reticle chuck prior to
each said image projection,
whereby said projected images will be properly aligned relative to
one another to produce a unitary, integrated image on said
photosensitive surface.
2. Apparatus to project images from reticles as set forth in claim
1 in which said stage transfer function is an algorithm
incorporating the variations of motion of said stage from
theoretically true positions.
3. Apparatus to project images from reticles as set forth in claim
1 in which said reticle transfer functions are algorithms
incorporating the adjustments required to be made in the positions
of said reticle chucks so that the projected image is correct in
magnification, rotation, size and position.
4. Apparatus to project images from reticles as set forth in claim
1 in which said reticle chucks are capable of adjustment over six
degrees of freedom.
5. Apparatus to project images from reticles as set forth in claim
4, said apparatus including an asymmetric lens in each said optical
system, each said projection imaging system having magnification
adjustment capability on its reticle side and telecentric focus
adjustment capability on its substrate side, and
each said optical system including means for adjusting the spacing
between said lens and said substrate,
whereby adjustment of said spacing focusses said lens and
z-adjustment of said reticle chuck relative to said lens adjusts
said magnification and each projected image can be adjusted for
size, shape, angular orientation, and position.
6. Apparatus to project images from reticles as set forth in claim
1, each said illumination system including a folding mirror to
alter the direction of light before it enters each said lens,
whereby said projection imaging systems may be positioned proximate
to one another.
7. Apparatus to project images from reticles as set forth in claim
1 including means to measure the distance between each said
projection imaging systems and the surface of said substrate and to
vary said distance prior to the projection of each image, whereby
said spacing can be adjusted to maintain said image in focus
regardless of unevenness of said surface.
8. Apparatus to project images from reticles as set forth in claim
1 in which said stage stepping means includes a pair of linear
motors, one of which is positioned to move the stage in one
direction and the other of which is positioned to move the stage in
an orthogonal direction.
9. Apparatus to project images from reticles as set forth in claim
1 in which said stage includes banking pins and a vacuum chuck to
receive and position said substrate, said vacuum chuck being biased
to press said substrate towards said banking pins,
whereby said substrate can be initially positioned in said
stage.
10. Apparatus to project images from reticles as set forth in claim
1 including means to vary the spacing between said two optical
axes.
11. Apparatus to project images from reticles as set forth in claim
1 including means for measuring the intensity of the said image
projected from each said reticle and for varying the exposure to
provide equal exposure dosages to said photosensitive surface from
each said reticle.
12. Apparatus to project images from reticles onto the
photosensitive surface of a single common substrate to produce a
large scale integrated image upon said substrate, said apparatus
including
a movable stage for holding said substrate, means for stepping said
stage in orthogonal directions in the plane of said substrate,
a pair of parallel optical systems mounted proximate to said
substrate for concurrently projecting dual images upon said glass
substrate, said systems having optical axes perpendicular to said
substrate, each said optical system including a reticle carrier to
carry said reticles and a reticle chuck positioned to receive said
reticles one at a time from said reticle carrier and to hold each
said reticle during projection of an image carried by said reticle,
each said reticle chuck being capable of individual adjustment
movement in six degrees of freedom,
a computer associated with said stepping means and said optical
systems for controlling same, said computer storing data pertaining
to the relative positions of said stage and each said reticle and
utilizing same to adjust said stepping means and said reticle chuck
prior to each said image projection,
whereby said projected images will be properly aligned relative to
one another to produce a unitary, integrated image on said
photosensitive surface.
13. Apparatus to project images from reticles as set forth in claim
12 in which each said optical system includes mirror means for
altering the light path, whereby projection lenses may be
positioned more proximate to one another.
14. Apparatus to project images from reticles as set forth in claim
12 including alignment marks on said reticles and sensing means in
said stage for sensing said alignment marks as they are projected
from said reticles.
15. Apparatus to project images from reticles as set forth in claim
14 in which said sensing means is a pop-up sensor adapted to move
between a position corresponding to the level of the upper surface
of substrate and a level below the lower surface of said
substrate.
16. Apparatus to project images from reticles as set forth in claim
12 including a reflective alignment microscope associated with at
least one of said optical columns, whereby said substrate can be
aligned prior to projecting a second layer of images thereupon.
17. Apparatus to project images from reticles as set forth in claim
12 including means for adjusting the positions of said reticles to
compensate for variations in magnification and/or focus caused by
variations in the index of refraction of the surrounding air
affecting the velocity of light.
18. Apparatus to produce large scale integrated images on the
photosensitive surface of a substrate, said apparatus including
a movable stage, means for stepping said stage in the x-and
y-directions,
a pair of parallel and proximate optical systems having axes in the
z-direction and positioned above said stage for projection of
images thereupon, said optical systems including a reticle chuck
operatively associated with each said optical system, for carrying
reticles bearing alignment marks to be projected upon said
stage,
sensing means for comparing the position of said projected
alignment marks on said stage as said stage is stepped throughout
its movement range beneath said optical systems and for determining
and recording the extent of variation of said stage from its
theoretically true x-, y-, and .phi.-positions throughout said
range,
and means for varying the stepping of said stage from theoretical
stepping distances to compensate for said variation in each
position of said stage within said range,
whereby images formed upon said photosensitive surface will
accurately abut with one another.
19. Apparatus to produce large scale integrated images as set forth
in claim 18 in which said sensing means includes a plurality of
sensors mounted in said stage.
20. Apparatus to produce large scale integrated images as set forth
in claim 19 in which at least one of said sensing means is adapted
to move between a position corresponding to the upper surface of
said substrate and a retracted position.
21. Apparatus to produce large scale integrated images as set forth
in claim 18 including
control means for each of said reticle chucks, said control means
being adapted to vary the position of said chuck to adjust the
position of said image to compensate for said variation in the said
positions of said stage.
22. Apparatus to produce large scale integrated images as set forth
in claim 18 in which said reticle chucks have six degrees of
freedom of movement.
23. Apparatus to produce large scale integrated images as set forth
in claim 18 including
a sensor for each said optical system to detect the distance of the
lens in said system from said photosensitive surface and means to
maintain said lens at a constant distance from said surface,
whereby said lenses in said optical system will be maintained in
focus as said stage is moved regardless of variations in height of
said surfaces.
24. Apparatus for making large area electronic devices utilizing
horizontal alignment of images from two optical systems, said
apparatus including
a movable stage adapted for motion in x- and y-directions,
a pair of parallel and proximate optical systems having axes in the
z-direction and positioned to receive reticles in their object
planes and to project images from said reticles upon said
stage,
a reticle carrier associated with each said optical system and a
reticle alignment chuck associated with each said reticle carrier
and adapted to receive a reticle from its respective said carrier
and to hold said reticle in the object plane of its respective said
optical system, and
alignment means associated with said optical systems including
(i) control means associated with each said reticle chuck for
adjusting same in any of six degrees of freedom,
(ii) means for varying the distance between said optical axes,
and
(iii) means for varying the magnification of said optical
systems,
whereby images projected by each said optical systems may be butted
with one another and with images from the other of said system to
produce large scale images suitable for said displays.
25. Apparatus for making large area electronic displays as set
forth in claim 24 in which each said optical system includes an
asymmetric telecentric lens.
26. Apparatus for making large area electronic displays as set
forth in claim 24 in which said reticle carriers are adapted to
carry a plurality of said reticles, and means for individually
interchanging said reticles in said chuck.
27. Apparatus for making large area electronic displays as set
forth in claim 24 including step and repeat means to move said
stage distances equal to one dimension of said projected images,
and means for so aligning said reticle alignment chucks during each
said step that immediately preceding projected said images will
abut with images projected in each said new position.
28. Apparatus for forming abutting images upon the photosensitive
surface of a substrate, said images serving to form a display, said
apparatus including
a movable stage for holding said substrate, said stage including
preliminary alignment means for said substrate,
a pair of reticle chucks adapted to receive and hold reticles for
projection upon said substrate, a light source and an optical
system associated with each said chuck positioned to simultaneously
project an image from each of said reticles held by said chucks
upon different portions of said substrate, said optical systems
having parallel optical axes, and
means for adjusting the relative positions of said chucks and said
optical systems so that multiples of said images are abutting and
in alignment upon said substrate,
whereby said images form a continuum with one another upon said
substrate.
29. Apparatus for forming abutting images as set forth in claim 28
in which said continuum is a single integrated image on said
substrate.
30. Apparatus for forming abutting images as set forth in claim 28
including a stepper associated with said stage adapted to move said
stage in a direction paralleling a line connecting said optical
axes for a distance commensurate with said images sizes, whereby
said images may be repeated upon portions of said photosensitive
surface contiguous with said first images.
31. Apparatus for forming abutting images as set forth in claim 30
in which the line between said optical axes is in the x-direction
and said stepper operates in both the x- and y-directions.
32. Apparatus for forming abutting images as set forth in claim 28
in which said means for adjusting the relative positions of said
carriers and said stage include step and repeat means to move said
stage a distance substantially equal to one dimension of said
projected images.
33. Apparatus for forming abutting images as set forth in claim 28
in which the spacing between the images of said optical systems
leaves a remainder space and including a reticle image to fill and
abut said space
34. Apparatus for forming abutting images as set forth in claim 28
including reticle carriers to carry a plurality of reticles and
means for transferring said reticles between said carriers and said
chucks.
35. Apparatus for forming abutting images as set forth in claim 28
including means associated with said optical systems for adjusting
the distances of the respective said optical systems from said
substrate to maintain constant distances from the surfaces of said
substrate.
36. Apparatus for forming abutting images as set forth in claim 28
including laser interferometer means for determining the axial
spacing between said optical systems.
37. Apparatus for producing large area electronic devices and
having rapid throughput, said apparatus including
a frame,
a stage carried by said frame for motion in x- and y-directions,
said stage being adapted to hold a substrate,
a pair of parallel and proximate optical systems having lenses with
their axe in the z-direction and positioned to project images upon
said substrate held by said stage,
sensors associated with said stage for determining the x-, y-, and
.phi.-positions of said stage relative to said optical systems,
a pair of reticle alignment chucks mounted on said frame, each said
chuck being operatively associated with one of said optical
systems, said reticle chucks being capable of adjustment in six
degrees of freedom,
a stage alignment function providing coordinates of motion of said
stage relative to each said reticle chuck,
stepping means for said stage, and
control means to adjust the position of said reticle chucks in
accordance with said stage alignment function,
whereby multiple abutting images can be projected through said
optical systems to create a uniform, integral image upon said
substrate.
38. Apparatus for producing large area electronic devices as set
forth in claim 37 including sensors for determining the relative
axial positions of said lenses whereby said control means can
adjust the position of said reticle chucks in accordance with said
relative positions.
39. Apparatus for producing large area devices as set forth in
claim 38 in which said lens position detectors are laser
interferometers.
40. In an apparatus adapted to project simultaneous images, from
reticles carried by a pair of parallel optical columns, upon a
common substrate, to produce a single unified image, said columns
being mounted with axes perpendicular to a stage carrying a
substrate, said stage being adapted for stepping motion in x- and
y- directions orthogonal to the axes of said columns, that
improvement including
an asymmetric projecting lens in each said optical column for
projecting said images, each said lens having magnification
adjustment capability on its reticle side and focus adjustment
capability on its substrate side, and separate means for each said
lens for adjusting the axial spacing between said lenses and said
substrate and separate means for each said lens for adjusting the
spacing between said lenses and said reticles,
whereby the focus and magnification of each said lens can be
independently adjusted for better alignment of said projected
images.
41. In an apparatus adapted to project simultaneous images as set
forth in claim 40, that further improvement including means for
measuring the velocity of light and further means for adjusting the
spacing of the lens from the substrate to adjust for focus changes
based upon changes in said velocity of light.
42. In an apparatus adapted to project simultaneous images as set
forth in claim 41 in which said means for adjusting for focus
changes is an alignment chuck adapted to carry said reticle and
having six degree of freedom of movement.
43. In an apparatus as set forth in claim 40, a folding mirror
within said optical columns whereby light therein will be
redirected permitting said axes to be positioned adjacent to one
another.
44. In an apparatus adapted to project simultaneous images upon a
common substrate, from reticles carried by a pair of optical
columns having illuminators and lenses therein, to produce a
unified image, said lenses being mounted with their axes parallel
and perpendicular to a stage carrying said substrate, said stage
being orthogonal to the axes of said lenses, that improvement
including a folding mirror within said optical columns between said
lenses and said condensers to redirect the light from said
illuminator to said lenses, whereby said lenses may be positioned
proximate to one another.
45. In an apparatus adapted to project simultaneous images, from
reticles carried by a pair of parallel optical columns, upon a
common substrate, to produce a single unified image, said columns
being mounted with axes perpendicular to a stage carrying a
substrate, said stage being adapted for stepping motion in
orthogonal directions perpendicular to the axes of said columns,
that improvement including
an asymmetric projecting lens in each said optical column for
projecting said images, each said lens having magnification
adjustment capability on its reticle side and focus adjustment
capability on its substrate side,
a reticle chuck in each said optical column positioned to hold a
reticle during projection of a said image, each said reticle chuck
being independently adjustable in six degrees of freedom to permit
positioning of said reticle for control of magnification, rotation,
size and shape, and
control means to control the adjustment of each said reticle,
whereby images from each said optical column can be adjusted
independently to permit proper alignment for the creation of a
single integral image on said substrate formed from multiple
simultaneous images projected from said two columns.
46. In an apparatus as set forth in claim 45 in which said reticles
chucks are adjusted prior to each exposure.
47. In an apparatus as set forth in claim 45 in which said reticles
chucks are adjusted upon power-up.
48. In an apparatus adapted to project simultaneous images as set
forth in claim 45, that improvement in which said control means
adjusts the said reticles during the period between each said
simultaneous projection and each stepping of said stage.
49. The method of making displays on a photo-sensitive coated
substrate carried by a movable stage, and using a pair of parallel
optical columns with axes perpendicular to said stage, including
the steps of
positioning said substrate on said stage,
simultaneously projecting a pair of aligned images, one from each
of said optical systems upon said stage,
stepping said stage a predetermined distance in a given direction
and thereafter again projecting said images upon said substrate,
each of said new images being in abutting and aligned relationship
with one of said previously projected images, and
repeating said stepping and projecting steps until said substrate
carries an integral image layer formed of a plurality of said
abutting images,
whereby one layer of an integral display has been projected upon
said photosensitive substrate.
50. The method of making displays as set forth in claim 49
including the step of moving said stage in a direction orthogonal
to said given direction.
51. The method of making displays as set forth in claim 49
including the step of adjusting the size of said aligned images
such that no remainder space remains between the totalities of
images projected by the two said optical systems.
52. The method of making displays as set forth in claim 49
including the step of projecting a remainder image.
53. The method of making displays a set forth in claim 49 including
the step of adjusting the exposures of the two said images to make
them equal.
54. The method of making displays as set forth in claim 49
including the steps of etching said photoresist and recoating it,
positioning said substrate upon said stage, aligning it to receive
a second image, and
thereafter simultaneously projecting said second image on said
substrate, stepping and repeating it as before, to create a second
integral image layer on said substrate.
55. The method of making displays a set forth in claim 49 including
the step of panel scaling, aligning it to receive a second image,
and
thereafter simultaneously projecting said second image on said
substrate, stepping and repeating it as before, to create a second
integral image layer on said substrate.
56. The method of correcting for variations in translatory movement
of a movable stage used in apparatus for making displays, said
apparatus including a pair of parallel and proximate optical
columns to project images upon a substrate carried by said stage,
said method including
determining the actual positions of said stage as it is moved
throughout its range of movement,
determining the apparent position of said stage as determined by
projecting images from said optical columns to sensors on said
stage,
comparing the differences between said actual and apparent
positions and recording them as a transfer function, and
varying the position of said stage relative to said optical columns
in accordance with said transfer function,
whereby images projected by said optical columns upon said
substrate will accurately abut.
57. The method of aligning multiple images being projected from a
reticle to a photo-sensitive coated substrate using a pair of
parallel and proximate optical systems, said systems being
positioned to project in the z-direction upon a movable stage
subject to controlled motion in the x-, y-, and .phi. directions,
said method including the steps of
determining the coordinates of motion of said stage relative to
images projected from said reticles,
determining the reticle coordinates of each said reticle by
projection of images upon sensors positioned in known locations in
said stage,
using said determined coordinates of said stage and said determined
reticle coordinates to determine a first transfer function for said
system relative to positions of said stage,
positioning a said substrate in said stage and applying said first
transfer function to adjust the relative positions of said reticles
and said substrate for accurate image projection, and thereafter
projecting an image upon said substrate,
stepping said stage to a position to permit projection of an
abutting image, and again applying said first transfer function to
adjust the relative positions of said reticles and said substrate
for accurate image projection, and thereafter projecting an image
upon said substrate, and
repeating the steps of stepping said stage, applying said first
transfer function, and projecting said image until sufficient
images have been projected upon said substrate to make up an
integrated level of a display.
58. The method of aligning multiple images, as set forth in claim
57, including the steps of
determining second reticle coordinates for a second reticle for
each said optical system,
developing said first layer of photo-resist and recoating said
substrate,
positioning said recoated substrate in said stage and determining
the difference in alignment of said substrate on said stage
relative to the initial alignment of said substrate to said stage
to provide an alignment transfer function,
using said determined stage coordinates, said determined second
reticle coordinates, and said alignment transfer function to create
a second transfer function for said system relative to positions of
said stage,
applying said second transfer function to adjust the relative
positions of said second reticle and said substrate for accurate
image projection, and thereafter projecting an image upon said
substrate,
stepping said stage to a position to permit projection of an
abutting image on said substrate, and again applying said second
transfer function to adjust the relative positions of said reticles
and said substrate for accurate image projection, and thereafter
projecting an image upon said substrate, and
repeating the steps of stepping said stage, applying said second
transfer function, and projecting said image until sufficient
images have been projected upon said substrate to make up a second
level of a display.
59. A flat panel display containing a multiplicity of repetitive
stitched images and a plurality of layers, said display
including
a substrate,
a plurality of aligned, etched layers formed using photoresist
material upon said substrate, each of said layers having been
formed by simultaneous projection of images upon said photoresist
material from a reticle carried in each of at least a pair of
optical columns, said images each covering a small portion of said
substrate but having been repeatedly stepped and reprojected from
each said optical column over said substrate so as to form a
continuum of interfitting images to form a total, unitary image on
said photoresist of said layer,
said images from each of said optical columns having been
separately adjusted before each stepping and projection to match
and abut one another in respect to size, shape, rotation, and
magnification,
whereby said flat panel display is formed of a plurality of
unitary, integral layer of circuitry.
60. A flat panel display as set forth in claim 59 in which the
dimensions of the images projected on layers of said photoresist
after said first layer have been scale adjusted to compensate for
dimensional changes in said preceding layer. .Iadd.
61. Apparatus to project images from reticles onto the
photosensitive surface of a substrate to produce a large scale
integrated image upon said substrate, said apparatus including
a movable stage for holding said substrate, means for stepping said
stage in the x- and y-directions, stage calibration means to
calibrate the position of said stage in different stepped positions
and to determine a stage transfer function incorporating said
calibration data,
an optical system for projecting an image upon said substrate, said
system having its optical axis in the z-direction, said optical
system including a projection imaging system, an illumination
system, a reticle carrier to carry said reticles, and a reticle
chuck positioned to receive said reticles one at a time from said
reticle carrier and to hold each said reticle within said
illumination system during projection of an image carried by said
reticle, said reticle chuck being capable of individual adjustment
movement in at least the x-, y-, z-, and .PHI.-directions,
reticle calibration means associated with said reticle chuck to
calibrate the position of a said reticle in said chuck relative to
said stage and to determine a reticle transfer function
incorporating said calibration data,
a computer associated with said stepping means and said optical
system for controlling same, said computer storing said stage
transfer function and said reticle transfer function and utilizing
same to adjust said stepping means and said reticle chuck prior to
each said image projection,
whereby said projected images will be properly aligned to produce a
unitary, integrated image on said photosensitive surface. .Iaddend.
.Iadd.62. Apparatus to project images from reticles as set forth in
claim 61 in which said stage transfer function is an algorithm
incorporating the variations of motion of said stage from
theoretically true positions.
.Iaddend. .Iadd.63. Apparatus to project images from reticles as
set forth in claim 61 in which said reticle transfer function is an
algorithm incorporating the adjustments required to be made in the
position of said reticle chuck so that the projected image is
correct in magnification, rotation, size and position. .Iaddend.
.Iadd.64. Apparatus to project images from reticles as set forth in
claim 61, said apparatus including an asymmetric lens in said
optical system, said projection imaging system having magnification
adjustment capability on its reticle side and telecentric focus
adjustment capability on its substrate side, and
said optical system including means for adjusting the spacing
between said lens and said substrate,
whereby adjustment of said spacing focusses said lens and
z-adjustment of said reticle chuck relative to said lens adjusts
said magnification and each projected image can be adjusted for
size, shape, angular orientation,
and position. .Iaddend. .Iadd.65. Apparatus to project images from
reticles onto the photosensitive surface of a substrate to produce
a large scale integrated image upon said substrate, said apparatus
including
a movable stage for holding said substrate, means for stepping said
stage in orthogonal directions in the plane of said substrate,
an optical system mounted proximate to said substrate for
projecting images upon said substrate, said system having an
optical axis perpendicular to said substrate, said optical system
including a reticle carrier to carry said reticles and a reticle
chuck positioned to receive said reticles one at a time from said
reticle carrier and to hold each said reticle during projection of
an image carried by said reticle, said reticle chuck being capable
of individual adjustment movement in at least four degrees of
freedom,
a computer associated with said stepping means and said optical
system for controlling same, said computer storing data pertaining
to the relative positions of said stage and said reticle and
utilizing same to adjust said stepping means and said reticle chuck
prior to each said image projection,
whereby said projected image will be properly aligned to produce a
unitary, integrated image on said photosensitive surface. .Iaddend.
.Iadd.66. Apparatus to project images from reticles as set forth in
claim 65 in which said sensing means is a pop-up sensor adapted to
move between a position corresponding to the level of the upper
surface of substrate and a level below the lower surface of said
substrate. .Iaddend. .Iadd.67. Apparatus to project images from
reticles as set forth in claim 65 including means for adjusting the
positions of said reticles to compensate for variations in
magnification and/or focus caused by variations in the index of
refraction of the surrounding air affecting the velocity of
light. .Iaddend. .Iadd.68. Apparatus to produce a large scale
integrated image on the photosensitive surface of a substrate, said
apparatus including
a movable stage, means for stepping said stage in the x- and
y-directions,
an optical system having its axis in the z-direction and positioned
above said stage for projection of images thereupon, said optical
system including a reticle chuck operatively associated with said
optical system for carrying reticles bearing alignment marks to be
projected upon said stage,
sensing means for comparing the position of said projected
alignment marks on said stage as said stage is stepped throughout
its movement range beneath said optical system and for determining
and recording the extent of variation of said stage from its
theoretically true x-, y-, and .PHI.-positions throughout said
range,
and means for varying the stepping of said stage from theoretical
stepping distances to compensate for said variation in each
position of said stage within said range,
whereby images formed upon said photosensitive surface will
accurately abut with one another. .Iaddend. .Iadd.69. Apparatus to
produce a large scale integrated image as set forth in claim 68 in
which said sensing means includes a plurality of sensors mounted in
said stage. .Iaddend. .Iadd.70. Apparatus to produce large scale
integrated images as set forth in claim 68 in which at least one of
said sensing means is adapted to move between a position
corresponding to the upper surface of said substrate and a
retracted position. .Iaddend. .Iadd.71. Apparatus to produce a
large scale integrated image as set forth in claim 68 including
control means for said reticle chuck, said control means being
adapted to vary the position of said chuck to adjust the position
of said image to compensate for said variation in the said
positions of said stage. .Iaddend. .Iadd.72. Apparatus for
producing large area electronic devices and having rapid
throughput, said apparatus including
a frame,
a stage carried by said frame for motion in x- and y-directions,
said stage being adapted to hold a substrate,
an optical system having a lens with its axis in the z-direction
and positioned to project images upon said substrate held by said
stage,
sensors associated with said stage for determining the x-, y-, and
.PHI.-positions of said stage relative to said optical systems,
a reticle alignment chuck mounted on said frame, said chuck being
operatively associated with said optical system, said reticle chuck
being capable of adjustment in at least four degrees of
freedom,
a stage alignment function providing coordinates of motion of said
stage relative to said reticle chuck,
stepping means for said stage, and
control means to adjust the position of said reticle chuck in
accordanace with said stage alignment function,
whereby abutting images can be projected through said optical
system to
create a uniform, integral image upon said substrate. .Iaddend.
.Iadd.73. The method of correcting for variations in translatory
movement of a movable stage used in apparatus for making displays,
said apparatus including an optical column to project images upon a
substrate carried by said stage, said method including
determining the actual positions of said stage as it is moved
throughout its range of movement,
determining the apparent position of said stage as determined by
projecting an image from said optical column to sensors on said
stage,
comparing the differences between said actual and apparent
positions and recording them as a transfer function, and
varying the position of said stage relative to said optical column
in accordance with said transfer function,
whereby images projected by said optical column upon said substrate
will accurately abut. .Iaddend. .Iadd.74. The method of making
displays as set forth in claim 73 including the step of panel
scaling, aligning said stage to receive a second image, and
thereafter simultaneously projecting said second image on said
substrate, stepping and repeating it as before, to create a second
integral image
layer on said substrate. .Iaddend. .Iadd.75. The method of aligning
multiple images being projected from reticles to a photo-sensitive
coated substrate using an optical system, said system being
positioned to project in the z-direction upon a movable stage
subject to controlled motion in the x-, y-, and .PHI. directions,
said method including the steps of
determining the coordinates of motion of said stage relative to
images projected from said reticles,
positioning a said substrate in said stage and thereafter
projecting multiple abutting images upon said substrate,
determining reticle coordinates for a second said reticle for said
optical system,
developing said photo-sensitive coated substrate and recoating said
substrate,
positioning said recoated substrate in said stage and determining
the difference in alignment of said substrate on said stage
relative to the initial alignment of said substrate to said stage
to provide an alignment transfer function,
using said determined stage coordinates, said determined second
reticle coordinates, and said alignment transfer function to create
a transfer function for said system relative to positions of said
stage,
applying said last-named transfer function to adjust the relative
positions of said second reticle and said substrate for accurate
image projection, and thereafter projecting an image upon said
substrate,
stepping said stage to a position to permit projection of an
abutting image on said substrate, and again applying said transfer
function to adjust the relative positions of said reticles and said
substrate for accurate image projection, and thereafter projecting
an image upon said substrate, and
repeating the steps of stepping said stage, applying said second
transfer function, and projecting said image until sufficient
images have been projected upon said substrate to make up a second
level of a display. .Iaddend.
Description
This is a reissue of U.S. Pat. No. 4,769,680. .Iaddend.
FIELD OF THE INVENTION
This application describes apparatus and methods for the production
of large area electronic devices (LAED's), such as those required
for flat panel displays (FPD's), image scanning arrays for
facsimile machines or copiers, and print head arrays.
FPD's, the most commercially advanced of these applications, now
measure 4.5 cm.times.6.0 cm, but will soon be as large as 30
cm.times.40 cm. They are typically 2.5 mm thick. Generally these
devices are formed on a glass substrate, not on single crystal
silicon. Each device contains many picture elements (pixels), often
made using liquid crystal display (LCD) materials.
Each pixel is controlled by digital matrix circuitry at the edge of
the device, and a thin film transistor (TFT) at each pixel. The
TFT's measure about 5 micrometers (ums) in their smallest
horizontal dimension. TFT's are multi-layered structures, much like
IC's; and the lateral tolerance between vertically-spaced layers
must be one micrometer or better. Small displays contain typically
100,000 pixels (and TFT's) in one integrated circuit; the largest
devices will contain 4,000,000 TFT's.
The large area of such devices, combined with the small imaging
requirements (5 um), and tight tolerance between layers create
difficult lithography problems that can best be solved with
"stepping-aligner" technology.
This invention is related to wafer stepping, a technique used to
make IC's. However, a scaled up wafer stepper will not provide the
speed, tolerance, or low production cost required for these large
area devices. Performance gains of 5 to 10.times. over current
practice are required.
The intended applications differ from wafer stepping applications
in that (a) images must be printed in a controlled manner on
unstable, amorphous substrate materials, often transparent glass,
(b) there are no "scribe lines" between images; the entire array of
images must be oriented and dimensioned to abut, so that the
composed array of images functions as one single large area
electronic device, (c) the array must be composed of multiple image
patterns per layer; up to 9 separate images may be required on one
layer (IC's typically use only one pattern per layer), (d) the
resulting product is viewed directly by people; therefore, image
quality and abutting must meet difficult perception tolerances not
relevant in IC manufacture.
The above issues require new approaches in machine design, machine
control, set up methods, and operating methods, all as will be
disclosed in this document.
BACKGROUND OF THE INVENTION
The apparatus and methods of this invention are directed to the
rapid creation of precisely aligned layers over large areas of high
resolution photoresist images on amorphous substrates, often
transparent glass substrates. Significant speed, cost, and
alignment tolerance improvements are provided for fabrication of
LAED's.
The large size of substrates used to make LAED's (compared to wafer
substrates used in IC manufacture) allows one to use two
coordinated optical columns, i.e., camera and lens systems, to
print the pattern. These two columns, when properly aligned, each
print roughly half of an LAED, substantially doubling printing
speed over conventional single column steppers since two lenses
working simultaneously print twice the area at a time.
When stepping aligners are used to generate arrays of IC patterns,
the patterns are later cut into individual chips. The space left
during patterning for later cutting, the "scribe line," is often
used for local alignment. The stepper disclosed in U.S. Pat. No.
4,040,736, Johannsmier et al., describes such an alignment system.
This approach relieves the performance burden placed on stage
stepping accuracy, by allowing continuous realignment of the
stepping pattern within the array.
The manufacture of LAED's, such as flat panel displays, is
substantially different. All of the individual images must abut to
tight horizontal tolerances to form an overall integrated, uniform,
and precisely interconnected, correlated circuit pattern, with no
perceivable joints. Due to the absence of spacing between images,
one normally cannot use an alignment mark between images each time
the stage is stepped. Rather, one set of alignment marks for the
entire array, placed around the outside edge, is used. As a result,
an order of magnitude improvement in stage metrology is needed to
maintain vertical alignment tolerances. The apparatus of this
invention includes special sensor subsystems, appropriate system
control software, and machine setup methods to accomplish this
improvement.
The behavior of potentially unstable amorphous substrates must also
be corrected for in the apparatus, to achieve proper coordination
of the optical columns (cameras) when printing later levels of the
LAED pattern. After one level of patterning, the partially
completed circuit (on the substrate) will be cycled through a thin
film process, usually involving a significant temperature cycle.
After such a process, the substrate and circuit pattern will likely
change in overall size. This "scale" change is measured and
compensated for in the system control software, using correcting
mechanisms provided in the apparatus.
Historically, the integrated circuit mask-making industry did at
one time use multi-barreled repeaters, that is, units with banks of
six or nine lenses. For example, Baggaley U.S. Pat. No. 3,563,648
discloses the use of nine parallel optical columns directing images
to nine separate masks. The use of multi-barreled repeaters was
unsuccessful, however, and the practice ceased about 1974. The
problem had been that stage travel was measured in one place and
the lens barrels were in another, so inaccuracy resulted. This is
because there is a radial component of stage motion (yaw) which
causes misalignment. This arises because motion of a stage is not
in an exact straight line, but can be off over a short distance by
as much as two arc seconds. This could create an error in the
projected image of as much as 1.6 um. Since, in making flat panel
displays, we are dealing in error factors as low as 0.2 um, an
error factor of as much as 1.6 um is unacceptable.
Early steppers such as that made by Baggaley et al. were also fixed
focus cameras. The lenses used in these tools were non-telecentric.
The magnification of the projected images varied by as much as 3.0
um, due to stage and plate motion up and down under the fixed focus
columns. In the apparatus of our invention, asymmetrical
telecentric lenses are used, and individual focus control and
motion is provided for each optical column to overcome these
problems.
Finally, the stepper built by Baggaley, et al., imaged a separate
plate for each column. The relationship among the columns (cameras)
was therefore not important; the images from the several columns
were never integrated into one contiguous image on one plate. The
apparatus of our invention must successfully project images in
exact spacing, shape, size, and orientation onto one plate, so that
an integrated large area electronic device is created from
precisely joined images. A method for precisely setting and
maintaining the absolute column magnification and spacing must be
provided.
It should be noted, also, that Baggaley had no way of repositioning
a substrate and no method of adjusting optical columns relative to
one another.
Another example of multiple optical columns will be found in Fox
U.S. Pat. No. 3,722,996, which discloses the use of dual optical
columns. These, however, were not used together; rather, one was
for a pattern generating mode and the other for a photorepeater
mode. In reality, this was simply two separate machines combined
together for economy; and the two units were never used in
parallel.
Interferometer systems for controlling stage positioning are
described in Baggaley U.S. Pat. No. 3,563,648 and Fox U.S. Pat. No.
3,772,996. In these patents the use of two interferometers on one
machine is described, one for each axis.
The use of multiple alignment or reference marks on a wafer, at
least one for each chip being etched, will be found in Van Peski et
al. U.S. Pat. No. 4,521,114; Meshman et al. U.S. Pat. No.
4,550,374; Suzuki et al. U.S. Pat. No. 4,620,785; Phillips U.S.
Pat. No. 4,585,337; and Tanimoto U.S. Pat. No. 4,629,313.
BRIEF SUMMARY OF THE INVENTION
We utilize a pair of parallel and proximate optical columns (camera
systems) to simultaneously project two images upon a single glass
substrate. Due to physical limitations, the images from the two
columns do not abut in a given projection; but images from a given
optical column abut when the stage is stepped, and, after several
steppings, the sets of images from the two columns abut to form a
continuum of images in both x- and y-directions. This precise
butting of images requires a type of alignment control not found in
the usual wafer stepper. The images from the two cameras must not
only be properly positioned, but must also be of precisely the same
size, shape, and orientation, so that they can properly abut to
form a large, uniform, integrated image.
While simple in concept, the use of multiple optical columns is in
fact complex. Many functions normally assigned to stage mechanisms,
such as focus motion, must be assigned to the individual columns.
Each camera must utilize a special, asymmetrical lens. This lens
prevents magnification changes on the substrate (image) side of the
lens when slight defocus occurs (due to substrate unflatness), but
enables magnification adjustment on the opposite (object) side of
the lens.
A special six degree of freedom chuck holds the reticle (master
object). Motion of this chuck provides for adjustment of
magnification, trapezoid error, X, Y position, and rotation of the
projected image. The X adjustment also provides for the precise
setting of the distance between the two cameras' images.
Laser interferometer metering of the X and Y stages is used and is
referenced to the optical axis of both columns. This enables proper
placement of the stage under each optical axis. Stage yaw errors
are measured with an extra laser interferometer, and stage yaw is
corrected using a special yaw motion built into the Y stage and
appropriate control software. In this manner, the substrate is
simultaneously positioned in the proper position under the second
camera. By correcting for stage yaw in the stage mechanism, the
possibility of adding additional columns in the future is
simplified.
An in-stage calibration subsystem is used to establish and maintain
the proper correlation between each camera's projected images and
the X, Y motion. System control software and set-up methods provide
proper control of system operations.
As the stage is stepped, array of images from each of the two lens
systems are precisely butted with one another. After a
predetermined number of steps, the images from the two lens systems
also abut, or almost abut, forming the first layer of a full-size,
precise flat panel display. If the display array size is not an
exact multiple of the largest image size that fits through a lens,
then a separate, smaller circuit pattern, equal to the remainder
distance, can be used to fill the intermediate space. In normal
practice it is often easier to make the image size slightly
smaller, equal to the next larger integer division of the array
size, and step an extra row of images of the same size. Using this
method, the column spacing can be fixed, precisely adjusted, yet be
unequal to an exact division of the panel size, and still provide
significant throughput improvement.
Each camera also includes a fast, wheel-like reticle changing
capability, so that multiple patterns can be printed on one
substrate with minimum loss of throughput. For the smaller patterns
printed at the edge of an LAED pattern, a variable, rectangular
field stop assembly (masking system) is provided. This system
allows users to place multiple patterns on one reticle, but print
only one pattern at a time. The offset of each such pattern from
the optical axis is supplied by the user and compensated for in the
control software. The round wheel-like changer mechanism carries
four reticles, and automatically changes reticles. An in-column
alignment system quickly positions each reticle after it is
interchanged. A high-powered mercury arc lamp illuminator provides
exposure energy. The exposure on each illuminator is controlled by
feedback from an intensity sensor on each lamp. In this manner the
exposure energy of each camera can be matched and still provide the
proper exposure dose, even though the optical efficiency of each
camera may differ. Finally, each camera can be moved up and down on
a rigid Z axis, nominally orthogonal to the X, Y plane. This motion
provides precise adjustment of focus for each camera, even though
the substrate may be at slightly different heights under each
camera, due to plate unflatness or stage top runout.
X and Y laser interferometers reference the right-hand lens column
(the lower part of the right-hand camera) to the stage. These two
interferometers provide positioning data which is used by the
system control computer and software to precisely locate the stage
under the right-hand camera. A second Y-axis interferometer is
provided to measure the horizontal translation error caused by
stage yaw. A third Y-axis interferometer is provided to maintain
yaw control but allows the use of a much shorter stage mirror
during some modes of operation.
Two types of alignment systems are provided on the system. To take
advantage of the transparency of many proposed LAED substrates, a
transmission alignment system is included in the X, Y stage, below
the substrate. For opaque substrates or thin films, a reflective
alignment system is mounted on the right-hand lens column above the
substrate.
Calibration data gathered defines a first transfer function for the
first level exposure. When the substrate is reloaded on the system,
an alignment system, either transmission alignment system or
reflective alignment system, determines the X, Y location of the
substrate, as loaded onto the X, Y stage. The rotation of the
substrate, and its X, Y scale, are also determined; and the
orthogonality between the X and Y axes is measured. This new data
is used to modify the calibration database of the machine, creating
a second transfer function for the second layer, and so on. In this
way the apparatus is realigned with the substrate pattern, even
when it is placed incorrectly onto the stage, or its amorphous
nature has allowed a scale change in either X or Y direction, or
both.
Similarly, if two machines are involved, one patterning the first
layer, and the second being used to pattern the second layer, the
small differences between machines is automatically corrected for
by using this six degree of freedom alignment method. (The six
degrees of freedom for alignment are X-, Y-, plate rotation, X
scale, Y scale, and the orthogonality of the X- and Y-axes).
Though this application discloses the use of two optical cameras,
our apparatus may use additional cameras to provide higher
throughput on larger substrates. Our system can also be used to
create multiple separate circuits upon a single substrate as long
as the alignment is set.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is an exploded view of a portion of a flat panel
display.
FIG. 2A is an enlarged plan view of a portion of the display of
FIG. 1. FIG. 2B is a further enlargement of a portion of FIG.
2A.
FIG. 3 is a section of the thin film transistor, taken on line 3--3
of FIG. 2B.
FIG. 4 is a plan view of the image layout (viewed from the back of
the apparatus as in FIG. 6) required to project abutting images
upon a substrate using dual optical columns.
FIG. 5 is a flow chart explaining the methods for exposing
substrates and for handling error calibration.
FIG. 6 is a rear perspective schematic view showing the optical
controls used for aligning the two optical columns with a substrate
for producing image patterns such as are shown in FIG. 4.
FIG. 7 is a front elevation of our imaging system showing the stage
and the dual optical columns.
FIG. 8 is top plan view of the imaging system.
FIG. 9 is a right side elevation of the imaging system.
FIG. 10 is a rear elevation showing the optical columns and the
laser interferometer systems for measurement and calibration.
FIG. 11 is a plan view of the stage for carrying the substrate and
showing the means for moving the stage in x and y directions.
FIG. 12 is a vertical section taken on line 12--12 of FIG. 11
providing further details of the stage-moving means.
FIG. 13 is a partial top plan view of the in-stage calibration
unit.
FIG. 14 is a plan view showing the alignment markings and detector
geometries of the in-stage calibration unit.
FIG. 15 is a vertical section taken on line 15--15 of FIG. 13
showing a portion of the in-stage calibration system.
FIG. 16 is a vertical section taken on line 16--16 of FIG. 14
showing two of the sensors in the in-stage calibration system.
FIG. 17 is a vertical section taken on line 17--17 of FIG. 8
showing placement of the dual lenses relative to the substrate.
FIG. 18 is a plan view of the stage showing the transmission
alignment system and the .phi.-drive used for aligning the
substrate with the reticles.
FIG. 19A is a plan view of a portion of a reticle, showing
alignment slits. FIG. 19B is an illustrative vertical section
showing the relationship of the slits of FIG. 19A to the lens and
the transmission alignment sensor in the stage. FIG. 19C is a plan
view of the four sensors in a quadcell detector in the stage and
showing the relationship of the four sensors to the slits in the
in-stage calibration unit of FIG. 14.
FIG. 20 is a vertical section, taken on line 20--20 of FIG. 8,
showing the optical column and reticle mount for the left-hand
camera. It shows the unit as if split in the middle and shown in
vertical elevation.
FIG. 21 is a vertical section (partially broken away) taken on
lines 21--21 of FIGS. 8 and 22 of the right-hand reticle changer
and field stop assembly.
FIG. 22 is a side elevation of the reticle carrier and field stop
assembly.
FIG. 23 is a partial top plan view of the camera.
FIG. 24 is a rear elevation of the left-hand camera.
FIG. 25 is a side elevation of the camera showing the reticle
alignment chuck.
FIG. 26 is a section taken on line 26--26 of FIG. 25 showing the
reticle on the chuck.
FIG. 27 is a vertical section, partially broken away, taken on line
27--27 of FIG. 7, showing the support system for the right optical
column.
DETAILED DESCRIPTION OF THE INVENTION
1. The Flat Panel Display
The preparation of a flat panel display 2 ("FPD") involves a new
dimension in microlithography. The finished product is not a mask,
made by repeated printing of the image on a reticle, for a mask is
not multi-layered. It is not simply a multi-layered chip, one of a
series made on a single wafer, for it is many times the size of a
chip, for example, 25 cm on a side. It may be thought of as one
large integrated circuit that must be built in parts, in which the
parts are created in at least two places on the same substrate
simultaneously and must be properly oriented and sized to
accurately abut in the completed image. Simultaneous imaging is
necessary to provide enhanced throughput time.
FIG. 1 is an exploded view of a portion of a flat panel display
("FPD"), in this instance a liquid crystal display ("LCD"). It
includes five functional layers. The first is a white light source
22. Next is a first polarizing filter 23A, which is usually mounted
as a thin film directly on the next component, the circuit plate
19. The circuit plate provides electronic control over many small
areas, called pixels 10 (see FIGS. 2A and 2B), across the area of
the display. The next element is the color filter plate 20 which
carries the three primary colors, also organized into pixels 10C.
The back surface of plate 20 is coated with indium tin oxide
("ITO") to act as a ground plate. For each pixel 10C on the color
filter plate there is a corresponding pixel 10 on the circuit plate
19. This arrangement allows each pixel on the circuit plate to
control one pixel on the color filter plate. The last component is
another polarizing light filter 23B, rotated 90.degree. with
respect to the first polarizing filter 23A.
The sandwich of components described above controls the
transmission of light through the display as follows: The volume
between plates 19 and 20 is filled with liquid crystal material 21.
This material has the unusual property that it will rotate the
polarization of light when an electric field is applied across it
between pixel 10 and the ground plate. When a pixel 10 is not
electrically energized, the two polarizing filters cause light
coming through that part of the display to be blocked. When a pixel
is energized, the liquid crystal material 21 at that pixel site
rotates the polarized light so that it can pass through the second
polarizing filter. In this manner individual pixels of the display
are turned on.
The color of the pixel depends upon the color of its companion
pixel on the color filter plate. Colors other than the three
primary colors are achieved by "blending" portions of the primary
colors. This blending of colors requires that the control of each
pixel be proportional, not just on-off, so that various percentages
of each primary color can be selected. This property is called
"grey scale".
In order to achieve a high resolution color picture each pixel 10
must be small. Many pixels are then used to provide a large area
display. For example, one existing LCD display measures about 5.4
cm by 4.6 cm, for a total area of about 24.8 cm. This display uses
90,000 pixels, or about 3630 pixels per square centimeter.
The best LCD displays have a control element at each pixel 10 on
the circuit plate. A transistor is placed in the corner of each
pixel. FIGS. 2A and 2B show enlarged views of a pixel on the
circuit plate 19. When the horizontal gate line 8 is energized
(typically, to 20 volts), the gates of all transistors 6 in that
row are ready to conduct electricity. Some of the vertical data
lines 9 are energized (typically, to 10 volts), and some data lines
are left unenergized. The energized data lines cause the
transistors 6 at the data and gate line intersections to conduct
electrons onto that pixel, turning that pixel on. In normal
practice, another row of the display is written periodically.
During the time that the other rows of the display are being
written the transistors 6 on the inactive rows block the flow of
electrons to or from the pixel. This holds the information pattern
on each row until it is rewritten on the next scan. The nearly
constant presence of charge at the "on" pixels gives excellent
control of the liquid crystals at those points, resulting in good
viewing angle and contrast. The transistor at each pixel serves to
enhance the quality of the displays.
Since the pixels 10 must be invisible to the eye, the transistors 6
in their corners must be even smaller. Electrical properties also
mandate small dimensions for the transistors.
The transistors are typically made from thin films of amorphous
silicon materials, on glass, as shown in FIG. 3. To get acceptable
switching speeds from such "low grade" semiconductor materials,
small gate lengths must be used, typically they are about 5.0 ums.
The lateral relationships between vertically spaced image layers is
important to circuit tolerances; and usually at least 1.0 um
lateral alignment must be achieved. In short, the fabrication of
transistors for active matrix displays requires manufacturers to
achieve integrated circuit tolerances over the area of a full layer
and also between layers.
Older techniques such as contact printing cannot achieve the
desired tolerances over these large areas. Projection techniques
are required. Projection aligners can provide the yield and
patterning required, but no existing projection system can cover
the area of a large display; at best, a 10 to 15 cm area can be
printed in one exposure.
As can be seen, the production of an FPD requires a higher degree
of error control than has been previously obtained. The error
control must reduce the errors to far below those tolerated on a
lesser-sized product, because repeating the pattern adds an error
source and because of the need for butting adjacent image
fields.
2. Composing Image Arrays Using Two Cameras
FIG. 6 shows schematically the use of two lens systems 13 and 15 to
generate one integrated image on substrate 1, as used in the
circuit plate 19 of an active matrix liquid crystal display
(AM/LCD). Note that the limits of projection lens design prevent
spacing the two cameras close enough to be able to produce
immediately butting images. Instead, the images from each camera
abut first to other images produced by that same camera. (For
example, R1 abuts R2, until after several steps of the stage, the
array R1, R2, R3, R4 produced by the first lens system 13 will abut
the array of images L1, L2, L3, and L4 produced by the second
camera, where "R" and "L" represent images from the right and left
cameras, respectively). Roughly speaking, each camera then produces
one-half of a circuit plate's pattern, with the inter-camera joint
occurring between R.sub.4 and L.sub.1, as shown in FIG. 4.
At first, it may seem that for every pattern size the inter-camera
spacing must be adjusted so that the arrays made with each camera
will also join in the middle to make a display of the desired size.
This is impractical, since each camera is large, heavy, and too
many precision adjustments would have to be made every time the
display size changed.
An alternative method is to use one camera to step a special
reticle image into the remainder space 7, a "remainder image." (See
FIG. 4). This remainder image may be designed by the user to
exactly fill space 7 a row at a time or may overlap the already
projected arrays from each camera, taking advantage of the
repetitive nature of the patterns. However, a higher throughput
method is to produce reticle artwork for each camera's array that
is smaller than the maximum allowed by the lens field of each
camera, but is an integer subdivision of the inter-camera spacing.
For example, in FIG. 4, if we assume that the optical columns have
a minimum spacing 3 (FIG. 6) of 165 mm. and that the pixel images 5
(R.sub.1, R.sub.2, R.sub.3, and R.sub.4 for the right-hand column;
and L.sub.1, L.sub.2, L.sub.3, and L.sub.4 for the left-hand
column) are 35 mm. long in the stepping direction, then four images
5 will be required from each column to fill the space as the stage
is stepped in the x-direction, and there will be a remainder space
7 of 25 mm. (165 mm.-[4.times.35 mm.]=25 mm.).
This remainder space 7 could be filled with a separate 25 mm image,
and, here, a fifth stage step for each horizontal row. However,
this would require a change of reticle 117, taking approximately 10
seconds. By making each reticle image 33 mm in the X-direction,
exactly five steps in X will complete both arrays and the whole
pattern, without any added stage steps, and avoiding the reticle
change. By using either method, the apparent need for infinitely
adjustable inter-column spacing can be avoided without decreasing
throughput, but greatly simplifying machine design, setup, and
operation.
After the first horizontal row has been imaged, the stage steps in
Y by an amount that is precisely matched to the Y-dimension of
image 5, and a new row of X images is projected as before, but
stepping in the reverse X direction, to avoid the time needed to
return to the opposite end of travel. As can be seen, it is
critical that the sizes, shapes, position, and rotation of the
projected images 5 be correlated. Not only must the images from the
right-hand lens 13, namely R1, R2, R3, etc., join correctly with
each other (and the same with the left images), but the farthest
right lens image, R4, must properly join with the nearest left lens
image, L1. This requires very accurate setup and maintenance of the
length relationships among the several coordinate systems.
Alternatively, each single X-direction step can be followed by
projection of all images in the Y-direction, followed by the next
X-step.
3. System Overview
FIGS. 7-10 and 17 show the stage and the dual optical systems of
our imaging system. Light for exposure is provided by illuminators
90. This light passes through field stop assembly 121, through the
reticle 117, reflects off folding mirror 99, and passes through the
main projection lens 13 or 15 to the substrate 1. The main
projection lens 13 (or 15) images the pattern on reticle 117 onto
the substrate 1. Use of the folding mirror 99 in the illumination
system permits the two lenses to be placed closer to one another
than would be the case if the mirrors were absent, since the
diameter of the condensing lenses 95 is greater than the diameter
of lenses 13 and 15.
The asymmetric telecentric design of main projection lens 13 (or
15), the autofocus sensor 213 (FIG. 17), and Z axis drive 105
maintain precise control of focus of the projected image, even when
the surface of substrate moves up and down slightly due to material
tolerances or stage motion. The optical systems 29 rest on a large
(1,000 kg) granite bridge structure 75, mounted on legs 77, which
in turn are mounted on the large (3,000 kg) granite base 51. This
structure provides the rigid, stable platform required for such a
large and precise stepping and imaging system. Base 51 rests on
commercial vibration isolation mounts 53.
Reticle 117 is held on a six degree of freedom reticle chuck
alignment chuck 130. (By "six degree of freedom" we mean motion in
x-, y-, and z- directions and rotation about .phi..sub.x,
.phi..sub.y, and .phi..sub.z axes). This chuck's motions permit
controlled, programmable motion of reticles as needed to correct
magnification, trapezoid error, X, Y, rotation, and inter-image
spacing. Since the main projection lenses 13 and 15 are
asymmetrical, adjustment of magnification and trapezoid error can
be made by varying the lens to reticle distance using actuators 84
and, of focus, by varying the lens to substrate distance.
Adjustment of chuck 130 also serves to correlate image spacing and
rotation; for example, rotation can be used to align images to
compensate for rotation of the substrate.
Substrates are placed on stage 11Y by a commercial automatic
substrate handling system (not shown). As shown in FIGS. 7, 9, 11,
12, and 17, stage 11Y is partly mounted upon the X stage 11X, and
partly references the main surface of the base 51. This arrangement
makes for a more compact machine design than the usual design, and
provides for easier maintenance, as the Y stage can be easily
removed to the rear. The X stage provides the Y axis guide 165, and
drives the two stages in X. Both stages use commercial,
frictionless linear motors 159 and 169 to step the stages across
desired distances.
The Y stage, shown in plan view in FIG. 11, contains a banking
chuck 189 and vacuum line 190 for drawing substrate 1 against
banking pins 187. In this manner, substrates are prealigned on the
stages. Provision has been made for substrates as small as 300 mm
and as large as 450 mm, square or rectangular.
The in-stage calibration unit 227 is located under the Y stage, as
shown in FIG. 11. This unit is normally kept just below the
substrate chucking surface of stage 11Y, but is raised to the image
plane 30 when used for calibration of projected images.
The yaw adjustment mechanism 200 is mounted to the stage 11Y as
well. Bearings 57 provide the lift support of stage 11Y; Y guide
bearings 203 and Y guide surface 204 provide guide control for
stage 11Y. Transmission alignment system units 225A, B, C, D, and
E, described below, are placed in the stage, under the edge of
substrate 1, where they can be used for transmission alignment of
transparent substrates, directly referencing substrate 1, when
partly processed, to reticles 117L and 117R.
Stage positioning is controlled by laser interferometers 17A-17D
(FIGS. 6-10). Our system uses four interferometers for stage
control, one, 17D, for X position, and three in Y, which is not
typical. Interferometer 17B controls the position of the stage in
Y, with reference to reflector 14, mounted on the right hand lens
13, and stage mirror 12Y. Interferometer 17C measures the Y
translation error under left-hand lens 15 (with reference to
reflector 16) caused by stage yaw and, together with the yaw
correction mechanism 200, controls stage yaw errors. Interferometer
17A is used when the stage travels to its full right position,
instead of interferometer 17A, so that stage mirror 12Y can be made
shorter and yet provide the full travel in X with active yaw
control. Stage mirror 12X provides for X position referencing and
works in conjunction with interferometer 17D and reflector 18 on
lens 13. Both stage mirrors are mounted on stage 11Y and are
rigidly connected to substrate 1 during stepping to provide the
best possible measurement of the position of substrate 1. (Normally
a rotation mechanism for aligning the substrate to the mirrors
would be included on top of stage 11Y. However, such rotation
mechanisms contribute significantly to measurement errors and
decrease stepping speed, so are to be avoided. The novel yaw
control 200 of this system can be used, as well as chucks 130, to
circumvent this problem).
To provide the needed long-term stability, the entire system is
placed in a temperature and particle controlled environmental
enclosure (not shown). A computer and associated electronics
control the system and are housed in separate electronic racks (not
shown) outside the environmental enclosure.
4. The Dual Optical Systems
As shown in FIG. 7, the dual optical systems 29 are mounted on
granite bridge 75, above the motions of the machine. One optical
system is the approximate mirror image of the other, with the
exception of mirror 18 and the reflective alignment system 241
which are mounted only on the right-handed camera, on lens 13. The
detailed description will therefore only be given for one camera
but should be understood to apply to both. Subassemblies will be
taken in their order of appearance, proceeding up from substrate
1.
Each camera contains a main projection lens 13 or 15 (FIGS. 6, 7,
10, 17, and 20). This lens is asymmetrical, being telecentric to
within 1.degree. on the image (substrate) side and non-telecentric
by 10.degree. on the object (reticle) side. These values describe
the approximate angle of a ray at the edge of the image and object,
respectively, where 0.degree. represents a ray that is exactly
perpendicular to the image or object planes. The tangent of these
angles properly predicts image size change with motion along the
optical axis. (The tangent of 1.degree. is 0.017). When slight
defocussing occurs (10 ums typically, due to adjustment errors,
plate unflatness, or stage top runout) images retain their desired
size to within acceptable limits. In this manner magnification
stability is achieved, even on inexpensive production substrates.
On the object side, the 10.degree. of non-telecentricity equates to
1 um of magnification change across the image field diameter for
each 5.7 ums of motion of the reticle 117 in the z-direction with
respect to lens 13. Non-telecentricity on the object side permits
control of magnification.
This design allows use of a six degree of freedom chuck 130 to
adjust magnification of each camera independently and precisely,
rather than rely upon having lenses of identical focal length
(difficult to achieve within the necessary tolerances). Fold mirror
99 folds the optical path so that lenses 13 and 15 can be
positioned more closely to one another without the reticle carriers
115, chucks 130, and condensing lenses 95 interfering with one
another; and closer lens positioning enhances image accuracy.
FIG. 23 shows the details of the six degree of freedom reticle
chuck 130. The entire assembly mounts on support flexures 132 above
the main projection lens 13, directly on the lens support 26. Three
piezoelectric drivers 84 control the distance between the lens and
the reticle. These drivers move small distances parallel to the
optical axis to correct magnification errors. If any two drivers 84
are driven relative to the third, the tip and tilt (.phi..sub.x and
.phi..sub.y) of the reticle chuck 125 is adjusted. This eliminates
trapezoidal error in the imaging system. Piezo drives 84 position
intermediate frame 140, which supports voice coil drivers 129, 131,
and 133, flexure assembly 120, and reticle chuck 125. (The coil
drivers may be of the type disclosed in Borner U.S. Pat. No.
3,569,718). Voice coils 129 and 131 move together to adjust the
alignment of reticle 117 in the X direction. If they move different
amounts, they also serve to adjust alignment in the .phi.-direction
(rotation about the optical axis). Coil 133 is used for Y-direction
adjustment. Each piezoactuator and voice coil contains a local
position transducer which enables the control computer to
reposition the drive at the correct alignment position (stored in
the calibration data base 33DB) after power up or
recalibration.
Reticle carrier 115, shown in FIGS. 7-10, 20-22, and 27, holds four
reticles 117 in openings 116. When a new reticle is needed, carrier
115 is driven by air cylinder 128 toward the reticle (to the left
in FIG. 21), moving on slide 122. Pneumatic controls turn on vacuum
on reticle carrier 115 and release the vacuum in reticle chuck 125.
In this manner reticle 117 is passed to carrier 115. Air cylinder
128 then returns slide 122 and carrier 115 to the reticle changing
position (to the right).
Reticle carrier drive assembly 124 includes reticle carrier 115
which rotates, driven by motor 123, to place the new reticle in
position opposite chuck 125. Carrier 115 is again moved to the left
onto locating pin 126. The reticle is then handed off to reticle
chuck 125 by reversing the above sequence. In-column alignment
reference marks 134, seen in FIG. 27, are then used to precisely
align the reticle to the top of the optical column 26. If
subsequent checks of reticle alignment, using the in-stage
calibration unit 227, as described below, show that reticle
alignment to the in-column marks 134 is incorrect, a programmable
offset is entered into the system calibration database 33DB and is
used to offset the alignment of the in-column system the correct
amount, re-establishing its proper calibration and alignment.
The above subsystems, lens 13, mirror 99, six degree of freedom
stage 130, and reticle carrier 115 are all mounted on lens support
26, which is connected to camera support 83 by the Z-axis air
bearings 106 (FIGS. 23 and 24). The Z-axis drive 105 (FIG. 27)
supports the whole assembly, and provides Z-axis motions. Air
cylinder 109 can be used to raise the whole assembly nearly 50 mm,
allowing easy exchange of substrates under the lens. Fine drive of
Z, for automatic correction of focus, is provided by voice coil
112, which drives pivoted support arm 110, mounted on pivot
flexures 111. Movement of arm 110 drives lens support 26 through
flexure linkages 107 and 108.
Autofocus is provided by autofocus sensor 213 mounted in close
proximity to the bottom of lenses 13 and 15 (FIG. 17). The
autofocus sensor 213 monitors the distance between the bottom of
the lens and the top of the substrate. The error signal developed
by the sensor is used to drive the Z-axis drive 105. The autofocus
sensor projects a beam of visible or invisible light (of a
frequency that will not affect the photosensitive coating on the
substrate) from a light source 215 onto substrate 1 to a point
directly under the lens 13 on its optical axis. It is received and
reflected back by mirror 218 to the substrate and then to a
collector. The beam is then collected by the sensor in a manner
similar to that in common use on wafer steppers (such as disclosed
in Tigreat U.S. Pat. No. 4,447,185). By proper arrangement of
slits, lenses and detectors, small changes in lens to substrate
spacing are monitored and corrected.
The individual focus sensors mounted on each lens allow individual
focus of each camera, thereby correcting for substrate and stage
height variations that occur between cameras. By providing
constant, sharp focus, the total panel image can be built up of
precisely controlled image pieces that match each other so closely
that the subtle differences that remain are invisible to the human
eye.
The spacing between the two cameras is adjustable, so that a
standard value of column spacing 3 (FIGS. 4 and 6) (such as our
165.000 mm spacing) can be provided on all machines. A differential
screw drive is fixed at the front of the right-hand camera to move
it in the x-direction; the left-hand camera is considered fixed.
Bridge support 79 acts as a guiding surface for this motion. The
two cameras are placed on the bridge in approximate position, a
test reticle is aligned on each camera, and the instage calibration
unit 227 is used to measure the remaining error in the column
spacing 3. The differential screw drive is then moved the desired
amount, to remove most of the remaining error.
It is assumed in the system design that after the above X axis
adjustment, some small (about 2.0 um) error may remain in both X
and Y axes. The motion of the reticle alignment chuck 125 includes
enough travel in the X and Y directions to accommodate the
remaining adjustment. After the physical adjustment described
above, the pop-up calibration unit 227 and the X, Y laser metered
stages will be used to locate projected reticle images 5 for each
column, as described below. Any error in the exact column spacing
can be calculated from the measurement data, and used to offset the
reticle alignment origin (stored in the calibration database 33DB)
in the direction and amount required to bring the column spacing
into exact adjustment. In this manner, the column spacing is set
and maintained at an exact value.
Each reticle may contain more than one image pattern, even though
only one is to used at a given time. Field stop assembly 121 (FIGS.
7, 8, 21, and 22) is positioned in the optical path between the
reticle carrier 115 and the condenser 95. It serves to delineate
the portion of a particular reticle that is to be used. Assembly
121 includes a pair of horizontal blades 137, driven by motors 149,
guided by ball slide 146, belts 139, and guide slot 148; also, a
pair of vertical blades 141, driven by motors 150 guided by ball
slide 145, belts 143, and vertical guide slots 147. These blades
may form any shape rectangle, providing a field stop for any part
of the reticle. Assembly 121 is not, and need not be, in the plane
of focus of lens 13 or 15 since the patterns on the reticle are
spaced and so need not be exactly delineated by the field stop
assembly. (A form of assembly is disclosed in Hill U.S. Pat. No.
3,980,407, though the Hill structure, in contrast to ours, only
moves symmetrically).
Finally, each optical system 29 includes an illuminator housing 90,
which contains a mercury lamp 91, mirrors 93 to direct the light,
and exposure control shutter 97, and a condensing lens 95 which
directs the light through the field stop assembly 121 to reticle
117 and from there off mirror 99, through lens 13 or 15 onto
substrate 1. A sensor is included in each illuminator to monitor
exposure dose, so that the exposure energy for each system will be
the same even when the lamp outputs and the optical efficiencies of
the two cameras differ.
5. The Stage Motions And Support
Patterning a 450 mm square area requires a large, and, therefore,
heavy stage. Aluminum stages weighing nearly 100 kg are used. While
the use of two cameras simultaneously can reduce the travel
required in one axis by 2X, initial calibration is best
accomplished using 450 mm of travel in both X and Y. A massive,
stiff structure is therefore required to provide rapid stepping and
stable optical systems. To achieve this, a structure weighing
nearly 5,000 kg is used (FIGS. 7-10, 20, and 27). Granite base 51,
weighing 3,000 kg, rests on commercial vibration isolators 53,
which reduces transmission of building vibrations. Nodular legs 77
(Mehanite)support bridge 75. Granite bridge 75 weighs nearly 1,000
kg and supports both optical systems 29. Each optical system,
including light source, weighs about 250 kg. Legs 77 are stiff
enough to maintain high servo-drive bandwidth. The roughly 75 kg of
assemblies mounted to the lens support casting 26 are supported by
Z-axis drives 105. This entire structural assembly provides the
stability and stiffness needed to step large stages rapidly, image
high resolution patterns, and maintain system calibration.
X-motion stage 11X moves along the top of base 51, supported on
frictionless air bearings 55, and guided by air bearings 158 which
move along the X-axis guide keys mounted in a slot in base 51, as
shown in FIGS. 7 and 11. The guide bearings provide torsional
stiffness of the X-axis motion stage 11X. Stiffness of 10,000 kg/mm
are needed in these bearings to provide high gain servo control of
the combined X and Y axes.
X-motion stage 11X is driven by a commercial linear motor 159 (FIG.
9). The stator 160 for motor 159 consists of two rows of permanent
magnets mounted to base 51. The armature 161 consists of a set of
movable copper coils mounted from the stage so that it is centered
between these two rows of magnets. Current in the coils provides
thrust to move the stage. Precise control of the current, provided
by digital servo loops, fine digital-to-analog converters (DAC's),
and linear power amplifiers allows high speed coarse positioning
and lower speed fine positioning to 0.10 um with no moving friction
parts to wear out or inhibit precise stage positioning. Stops limit
the motion of the stage at the ends. This design provides the
capability needed to move the heavy stages precisely, without
particle contamination, for the more than 20,000,000 steps per year
anticipated in high volume production applications.
The Y guide key 165 is an integral part of the X stage 11X, and
provides orthogonal motion guidance for the Y stage 11Y (FIGS. 11
and 12). The Y stage is supported on three lift bearings 59. Two of
these lift bearings 59 ride directly on the base 51, at the rear of
the stage, under mirror 12Y, as can best be seen in FIG. 10. The
third Y lift bearing 59 rides on top of the X stage. This design
allows a 25% reduction of the size and weight of the Y stage, and
permits easy service access from the rear. Linear motor 169,
mounted within the Y guide key 165, provides the drive force to
move stage 11Y in the Y direction; stator 171 is mounted to stage
11X, armature 170 is mounted under stage 11Y. Control of motor 169
is the same as described above for the X-axis linear motor 159.
As can be seen in FIGS. 6-10, laser interferometers 17, using laser
24, reference stage mirrors 12 to determine stage position.
(Interferometers are of the type disclosed in Sommargen U.S. Pat.
Nos. 4,688,940 and 4,693,605). Interferometer 17D works with stage
mirror 12X and mirror 18 mounted on lens 13 to monitor X position.
Interferometer 17B works with stage mirror 12Y and mirror 14
mounted on the rear of lens 13 to monitor the Y position of the
stages relative to the right-hand lens. Interferometer 17C works
with stage mirror 12Y and mirror 16 mounted on the rear of lens 15
to monitor the Y position of the stage under the left-hand
lens.
Due to the nature of X and Y motions, small yaw rotations will
occur in the as-built stages. Thus the Y positions determined by
interferometers 17B and 17C will differ slightly. The system
controls consider the true position to be that measured by
interferometer 17B. The difference in distance measurements between
the two interferometers represents the error in Y under lens 15 due
to stage yaw. If left uncorrected, this error can exceed 1.6 um,
which is too large.
To meet the tolerances required, this yaw error must be not only
measured, but corrected, FIG. 18 provides a cutaway view through
the Y stage and shows the yaw correcting guide mechanism 200. The
front two guide bearings 203 are fixed, while the rear two are
mounted to lever arms 199. These levers are attached to the Y stage
through pivots 201; they are driven by voice coils 197, as shown.
Servo current applied to the coils causes the Y stage to rotate
slightly, until the yaw error at lens 15 is removed.
When stage 11Y is moved in X to the far right, see FIG. 10, the
laser beam from interferometer 17C will fall off the left end of
mirror 12Y. During normal two camera operation, such travel is not
required, and mirror 12Y need not be made longer. However, during
initial system calibration the full travel is used. In this case,
the mirror 12Y is too short, and active yaw correction is lost.
Rather than make mirror 12Y longer, making stage 11Y larger and
slower, and making the whole machine larger too, a third
interferometer 17A is provided to the right of 17B. As can be seen
in FIG. 6, the beam from 17A picks up mirror 12Y just before the
beam 17C passes off the other end of mirror 12Y. While all three
beams are on mirror 12Y, the control of stage yaw position is
passed by the system controls from interferometer 17C to 17A. In
this manner active yaw control is maintained. While interferometer
17A does not reference any optical column, it need not, because in
this range of travel, only lens 13 is in a position to image onto
the substrate 1, lens 15's optical axis having passed off the left
edge of the substrate a approximately the same time as the beam
from interferometer 17C passed off mirror 12Y.
The Y stage carries several sub-assemblies (FIGS. 11 and 18). Lift
pins 188 are pneumatically driven and lift substrate 1 so that it
can be removed from the stage by automatic material handlers. When
a substrate is loaded onto the stage, it is placed on top of lift
pins 188. These pins then lower the substrate to the stage surface.
The vacuum line 190 in banking chuck 189 is turned on; the
mechanism attached to banking chuck 189, which is below the stage,
then pushes substrate 1 gently against banking pins 187 which
reference the front and left edge of the substrate, thereby
locating substrate 1 in approximate prealignment on top of stage
11Y. As shown in FIG. 11, three or five transmission alignment
system units 225A-225E are mounted in the stage under the edge of
the substrate. The location of the in-stage calibration unit 227 is
also shown.
Stages 11 provide no separate mechanism for rotating substrate 1
with respect to the stage mirrors. Such mechanisms are common on
wafer steppers, but detract from stepping and positioning
performance. The large size of LAED substrates, as compared to
silicon wafers, and the fact that their edges are flat and
nominally orthogonal, allows the plate loading and prealignment
just described to position LAED substrates adequately so that only
fine correction is required. In this apparatus, this fine
correction can be made using the X, Y and yaw mechanisms just
described or the six degree of freedom chucks 130.
6. Sensing And Referencing Systems
Systems built into our apparatus provide substrate alignment and
system setup and calibration. The methods which employ these
sensors and referencing systems will be described in the next
section. The construction of each sensor and reference is described
here.
Stage 11Y contains five transmission alignment system sensors, as
shown in FIG. 11. In general, sensor 225A, mounted in the corner,
and sensors 225B and 225C will be used for smaller substrates,
while sensors 225A, 225D, and 225E will be used for larger
substrates, though all five may be used on large substrates. All
five sensors are the same; a sectional view of one is shown in FIG.
19.
Reticle 117 contains transmission alignment system slits 226, as
shown in FIG. 19A. The surrounding area is opaque; slits 226 are
clear, creating small slits of light when shutter 97 is opened.
Field stop assembly 121 is used to mask all but the transmission
alignment system slits of the reticle, so that undesired exposure
of the main pattern doesn't take place during transmission
alignment system use.
The light from slits 226 is imaged by lens 13 or 15 onto substrate
1, where a corresponding set of four slits 222 partly blocks the
light from the reticle, as shown in FIG. 19B. Light passing around
substrate slits 222 is gathered by lenses 223 and focused onto
quadcell detector 224, as shown.
The arrangement of the four detection cells of detector 224 is
shown in FIG. 19C. As can be seen, the energy from each reticle
slit 226 falls separately onto its respective portion of the quad
cell detector. When a substrate is loaded onto the stage, banking
chuck 189 pre-aligns the substrate so that slits 222 fall roughly
over transmission alignment system unit 225 at all locations. Quad
cell detector 224 is large enough so that precise prealignment of
substrate 1 to detector 224 is not needed. The stage is then moved
in X and Y directions until transmission alignment system unit 225
is placed under the image of reticle slit 226 projected by lens 15
from a reticle aligned on the left camera. At this point in the
procedure some misalignment will exist. For example, slit 226Y1
could fall entirely onto quadcell 224Y1, while slit 226Y2 falls
entirely onto the opaque area of substrate 1. By comparing the
signal from 224Y1 and 224Y2, this imbalance can be detected. It can
also be determined which direction of misalignment exists. By
moving stage 11Y to the rear, in this example, slits 222Y1 and
222Y2 can be moved until they are centered directly under slits
226Y1 and 226Y2. The Y position at which this balance is achieved
is recorded in the system data base 32DB, as the desired Y location
for alignment at location 225A.
The same process is repeated for the X direction at location 225A.
This process is then carried out at the other transmission
alignment system locations, using whichever lens (13 or 15) and
reticle that is appropriate. Note, in actual practice, several
iterations of X and Y alignment at any one transmission alignment
system location are required before precise alignment of both axes
is achieved. Also, the area around slits 222 need not be opaque; it
simply must block some of the light energy from slits 226, enough
to create a measurable asymmetry in the misaligned images.
The transmission alignment system directly references projected
images at the exposure wavelength. This is the most direct
alignment method, and, therefore, the preferred method. By making
the transmission alignment system sensors small, multiple sensors
can be easily included around the stage area, thereby avoiding the
need for a large hole and viewing microscope intruding up through
the stage assembly, as is common on most transmission viewing
systems. Thus, the compactness of the transmission alignment system
units is a key to the practical use of transmitted light for
alignment. It should be noted that at the end of the transmission
alignment system alignment process just described, the location of
the substrate in X, Y and rotation is known from the three position
measurements just made. The X length, the Y length, and the angle
between X and Y can also be determined. In this manner, six degrees
of freedom of alignment of the substrate can be achieved. The use
of this data to place the next layer of images correctly over the
existing layer(s) is described later.
Some substrates will be opaque, and must therefore be viewed from
above. A reflective alignment system 241 is provided for such
substrates. It is mounted near the bottom of the right lens 13, as
shown in FIG. 17. This unit consists of a combined dark field and
bright field microscope. A built-in focus sensor is provided to
eliminate small, residual non-telecentric effects in the reflective
alignment system. The image of reflection alignment system mark 228
from the substrate is magnified by this microscope onto a charge
coupled device ("CCD") array which is connected to a commercial
image processor in the system controls. The processor analyzes the
magnified image of mark 228 and determines its location in X and Y
directions; from this analysis alignment corrections can be
determined as above. Again, by measuring the X and Y location of
three separate substrate marks, X, Y, .phi., scale X, and scale Y
alignment can be achieved using the reflective alignment system
unit.
Note that the reflective alignment system unit 241 does not
directly reference marks on reticle 117; instead, the CCD array is
used as a TV camera and provides an intermediate position
reference. As a result, the relationship between reflective
alignment system unit 241 and projected reticle images must be
separately established during system calibration, and maintained
thereafter, even during power downs and restarts. An in-stage
calibration unit 227 is provided in stage 11Y for this purpose.
When no substrate is present on stage 11Y, in-stage calibration
unit 227 (the "pop-up" unit) is raised by air cylinder 231 to place
its top surface, the top surface of glass disc 229, at image plane
30. On wafer steppers, simpler units have been fixed at the image
plane, inside the rectangular range of stage travel, but outside
the circular area to be patterned (See Johannsmeier U.S. Pat. No.
4,414,749 and Tanimoto U.S. Pat. No. 4,629,313). For LAED's this
unused area often doesn't exist; the rectangular substrate may
entirely fill the stage area. If such a sensor unit were mounted
below the substrate, undesirable Abbe offsets would occur. It is
desirable, therefore, to lift the sensor package 227 to the image
plane, as shown in FIG. 15.
Pop-up unit 227 contains three detector subsystems. Detector 235, a
small light meter, is used to measure the intensity of exposure
light coming through lens 13 or 15 at a small portion of the image
field. By moving the X and Y stages around the image field of, say,
lens 13, the uniformity of intensity of illumination for lens 13
can be determined. Shutter dynamics and exposure dose control
behavior can also be measured.
Detector 237, also contained in the pop-up unit, has two narrow
slits, one for the Y-axis and one for the X-axis, aligned with the
respective axes. Filtering and detection are provided below the two
slits. A test reticle, which contains an array of similar test
slits, is used in conjunction with detector 237. The image of these
slits is scanned by the X and Y stages, scanning slit 237Y and then
slit 237X across the image of the test slits; during these scans,
intensity vs. position data is collected and then analyzed to
determine the resolving capability of the lens. If the lens
performance is analyzed by stepping the lens along its optical
axis, the position of best focus can be determined.
By placing the array of test slits at several locations on the test
reticle, resolution and focus can be determined across the field of
either lens, and, in this way, best focus for each lens can be
determined. Once determined, the desired lens to substrate height
is stored in the system calibration data base 33DB, and is then
maintained by autofocus systems 213, mounted at the bottom of each
lens.
Pop-up unit 227 also contains a transmission alignment system
sensor 230, similar to 225 described above described above. Only
now, glass disc 229 carries the transmission alignment system slits
222 normally provided by substrate 1. The test reticle contains an
array of many transmission alignment system slits 226 placed around
the object field. By moving the X, Y stages to the nominal
locations of these slits 226 in the projected image, and then
making a transmission alignment system alignment at each site, the
exact location of the projected images can be determined, as was
described above for the transmission alignment system. In this way
a map of the projected image errors can be made. The system
software analyzes this map, using known techniques, to separate X,
Y, .phi., magnification, and trapezoid errors, and to balance
residual distortion errors. See David S. Holbrook, "Projection
Lens/Column Evaluation For Microlithographic Imaging: A
User-Oriented Approach", Kodak Microelectronics Seminar, 1983,
Kodak Publication No. G-151 (1984). Each of these error amounts is
then used by the system software to determine offsets for the six
degree of freedom alignment chuck 130 mounted above lenses 13 and
15. These offsets correct the image's errors. This procedure, using
transmission alignment system-type detector 230 in pop-up unit 227
to measure and correct projected image placement errors is repeated
until no further improvement can be made. In this way, each camera
is set up automatically by the system controls to have minimum
error, without the need for expensive, slow testing using actual
exposures on substrates.
Sufficient X, Y stage travel is provided so that unit 227 can be
scanned under both cameras, across the full image fields of both
lenses 13 and 15. In this way, the location of images from both
cameras is learned during the calibration procedure, and so is the
spacing 3 between cameras. The measured X and Y error in camera
spacing 3 is then stored in the system calibration database 33DB
and is used to offset the right-hand camera's six degrees of
freedom reticle alignment chuck 130 the necessary amount to align
the two cameras precisely in Y and to space them our preferred
165.000 mm in X.
Finally, the glass disc 229 used with the pop-up unit contains
reflective alignment system alignment marks 228 (FIG. 14). When the
X, Y stages place the pop-up unit 227 at the desired location for
the reflective alignment system 241, an alignment measurement is
made with the reflective alignment system unit. Any error in this
alignment is considered an error in the location of the reflective
alignment system unit. Again, an offset is stored in the system
calibration database 32DB and used to correct later alignments.
Note that by using an in-stage calibration unit 227 to locate the
optical axes for lenses 13 and 15, and to locate the reflective
alignment system unit, these three optical axes can be precisely
located relative to each other. Since only small errors of the
order of a few micrometers are expected, all corrections can be
made in the system software, to bring each axis to its exact
desired position. In this way the indirect referencing problem
inherent in the reflective alignment system 241 is corrected,
projected images from each column are brought into calibration, and
camera spacing 3 is measured and corrected.
Scale 205 which is used for velocity of light (VOL) correction can
be seen in FIG. 17, rigidly mounted on stage mirror 12X.
Conventional means for measuring the index of refraction of air
have at least 1.5 ppm residual errors. Across 450 mm of travel, an
error of some 0.68 ums could occur, which is too large. A better
means for measuring VOL is needed. Prior art reference systems
operate only so long as power is not lost; on power up, they
provide no absolute reference. (See, for example, Hewlett Packard
Technical Data Bulletin on HP 10717A Wavelength Tracker).
The apparatus of our invention includes scale 205, made of zero
expansion material (such as Zerodur) to provide better VOL
reference. The reflective alignment system unit 241 is used to
measure the location of reflective alignment system marks 228B and
228F at the back and front of this scale, respectively. The
original length, which is known, is stored in the system
calibration database 32DB. Upon subsequent power up, or as needed,
this scale is remeasured using the X, Y stages 11 and the
reflective alignment system unit 241. Any change in measured length
represents a change in VOL from any cause. This data is used by the
system software to correct the factor used to convert fringe counts
into millimeters of travel, as is normal in laser
interferometry.
Changes in focus or magnification of lenses 13 and 15 caused by a
change in air index can be compensated for by direct measurement
with the in-stage calibration unit 227, or by deduction, in the
software, using models for the lens behavior derived from the
original lens design modelling. Deductive correction will generally
be quicker, therefore preferred. Note that adaptive behavior can be
included in the software system, whereby the anticipated change is
predicted using the deductive models, and is checked using unit
227. The residual errors detected can be used to modify the
deductive models until good agreement between these models and
actual system performance is achieved.
Yaw adjustment mechanism 200 and interferometers 17 provide no
precise origin for X, Y or .phi. upon power up. Scale 205 serves as
the origin for X and Y. The reflective alignment system unit 227 is
used to measure the X and Y locations of mark 228B, on scale 205.
The system software treats this location, then, as the origin of X
and Y travel. Reestablishment of the .phi. origin is covered in the
next section.
7. Machine Calibration And Correction
The apparatus, once assembled out of its component parts, will not,
due to the precision required, operate to the desired tolerances.
Even if the component parts are built to tight tolerances, this
will be true.
Adjustments not found in normal single column designs must be
anticipated and planned for in the system design and in the system
integration plans. Then, successive iterations of calibration and
adjustment must be carried out on the assembled machine until the
desired level of integrated system performance is achieved. This
level of performance must then be automatically maintained over the
productive life of the machine. This is done through use of the
sensors and reference systems just described, system software, and
transfer functions, as will be described below.
To "photocompose" one circuit pattern from subfield images 5, as is
shown in FIG. 4, the images must be formed accurately, and the
distances between image centers must be stepped properly. The many
adjustments described above provide the means for proper
calibration and correction, but not the method. While many methods
will work, the one presented here is believed to be the most
efficient, and hence the preferred method.
The calibration of the apparatus is usefully divided into three
phases: (1) initial system correction, (2) power-up correction, and
(3) routine operational correction. Initial system correction
begins with the X, Y stages 11, since all other measurements are
referenced to these stages.
Once a system is operational, initial systems calibration is
required. A "perfect" grid plate (with an array of reflective
alignment system marks 228 positioned at known distances from one
another in X- and Y-directions) is loaded onto stage 11Y. The
locations of all these marks is measured with the reflective
alignment system 241. Measured differences are assumed to be errors
in the X, Y stepping matrix, or "grid" of the machine under test.
For example, if the angle between the stage mirrors 11Y and 11X is
incorrect by 2.0 arc sec, then 4.5 ums of error will be found
across the 450 mm of travel. The amount of correction needed to
shift the grid until it exactly matches the perfect grid plate is
stored in the system's calibration data base 32DB and used to
correct subsequent stepping patterns until an exact match has been
made to the grid plate. In this manner, precisely matched stepping
grids can be achieved on all tools, permitting, if desired, the
imaging of successive layers on different tools.
The grid calibration procedure for the first machine built is more
complex than that for successor machines. The problem is that no
grid standard exists for the size area imaged by our apparatus (450
mm square). So the grid standard for the first tool must be
constructed using an iterative method.
After rough calibration by known methods, an X, Y array of
calibration marks is printed on a substrate. By successive steps of
measurement (using the X, Y stages 11X and 11Y and reflective
alignment system 241) the major grid errors can be discovered.
(See, for example, M. R. Raugh, "Absolute Two-Dimensional SubMicron
Metrology For Electron Beam Lithography," SPIE Proceedings, Vol.
480, May 3-4, 1984; J. Freyer et al., "Enhanced Pattern Accuracy
With MEBES III," SPIE Vol. 471, 1984). The matrix of errors so
discovered will be entered into the machine's X, Y stage
corrections data base 32DB (in FIG. 5) as the stage transfer
function, and thereafter used to offset commanded X, Y stage
positions, thereby providing corrected stage placement at all X, Y
locations. In this manner an improved tolerance for the X, Y stage
stepping distances can be achieved. The calibrated machine is then
used to step an X, Y grid pattern on a 450 mm square Zerodur
(thermally stable) plate. This grid plate will record the corrected
X and Y grid of the first machine and become the "perfect" grid
plate used above.
Once the stepping grid has been corrected, the optical cameras 29
can each be corrected. In-stage calibration unit 227 (FIGS. 13, 15,
16, and 18) and the now corrected stages 11 provide the measurement
means. Test reticles, patterned on quartz substrates to tolerances
of 0.1 ums (by using commercial IC mask making E-beam tools) are
used as the positioning reference at the reticle plane. These test
reticles contain transmission alignment system slits 226 (FIGS. 14
and 19) on 10.0000 mm centers, distributed around the 160 mm
circular field. Such a test reticle is placed onto each reticle
chuck 125 and aligned to the in-column alignment references 134 at
the top of each camera. Field stop assembly 121 is opened fully;
shutters 97 are opened to illuminate the reticles. The in-stage
calibration unit 227 is then moved to the nominal position of each
projected image of each transmission alignment system slit 226 in
image plane 30. Deviations from the true position are measured and
recorded in the apparatus' calibration data base 33DB. Subsequent
analysis (See Holbrook, above) determines the amount of X, Y, .phi.
magnification and trapezoid errors found in each camera's projected
images.
Using deductive models built into the system software which
describe subsystem behavior and conversion methods, the proper
adjustments for the six degree of freedom alignment chucks 130 are
calculated (see lens discussion above for an example). The
calculated adjustments are fed back to each of the six degree of
freedom adjustment actuators (described above), so that the desired
correction is made. This is again checked by successive
measurements, using stage 11 and the in-stage calibration unit 227,
until the residual noise level of error is achieved.
Note that not only is the size, placement and scale error for each
image detected and corrected by this procedure for each camera, but
the optical axis position of each camera can be determined from the
data as well, and from this data the X and Y separation 3 of the
two cameras can be determined. The absolute separation is not
needed, only the relative separation as compared to the X and Y
stage stepping distances. The above method determines this desired
relationship, by using only one in-stage detector 227 and stepping
it around the image field of both lenses with the one stepping grid
of stage 11. The correction needed in camera separation distance 3
is used to offset the X origin of the right-hand camera's six
degree of freedom stage 130. Using this method, both projected
images are precisely matched to the grid of the stages, and their
separation is precisely set to match the desired 165.000 mm
distance of the X-axis of stage 11X travel. This leaves the tool
properly corrected and ready for use in making LAED's.
The second phase of calibration occurs at power up. Each time the
apparatus is powered up corrections must be made. This is because
some knowledge about exact machine status may have changed during
power down. As has already been discussed, velocity of light (VOL)
correction must be checked in such situations. Each of the
actuators in the system contains a small built-in origin sensor,
with repositioning capability of roughly 1.0 micron. These sensors
serve to reestablish the machine relationships near to the desired,
corrected settings. In addition, all offsets stored in the machine
databases 32DB and 33DB are stored on hard disk and, therefore,
available upon power up. These are used to offset all actuators by
the last measured value. Thus, only one iteration of testing using
the grid plate and in-stage calibration unit 227 is required upon
power up to restore all corrections. In this way the machine is
quickly and easily restored to its corrected performance.
The third phase of calibration occurs during routine operation,
when added corrections may be needed. In particular, a panel
scaling capability may be needed. The above calibration assures
that the coordinate systems of the two projected images and the X,
Y stage coordinate system are aligned with sufficient precision to
allow practical integration of their images into panel circuit
patterns, such as FPD's. However, subsequent panel process steps
may cause the X, Y scale of the panel to change. For example, an
added layer of aluminum can stress the substrate, causing it to
shrink or expand; glass is an amorphous material and will change
dimensions significantly as it undergoes temperature cycling in the
normal process of depositing and etching panel layers.
Consequently, for subsequent panel layers, it is desireable to
measure the X, Y scale of each panel substrate and adjust the
system again, this time to the scale of each panel. The
transmission alignment system and reflective alignment system
alignment systems can be used to make the panel scale measurement,
as described above. This data can then be used by the system
computer to add a final correction to the X, Y stage database, and
to make a final trim adjustment of the column magnification and the
column spacing in database 33DB, so that the scale of the new layer
is matched to the X, Y scale of the individual panel substrate. In
this manner, the images for the new layer will be placed most
accurately above previous layers (FIG. 5). The capability for
measuring and making panel scale adjustments for each panel
substrate already exists in the system, because the above
calibration must be provided for successful two column operation.
Here, it is used for an additional purpose, to improve the
performance of the tool in real production, by providing the panel
scale feature as well.
Finally, alignment of upper layers of printed images to earlier
layers is needed. When substrates are loaded onto the X, Y stages,
their orientation will not be exact. Using either the transmission
alignment system or reflective alignment system alignment systems,
this misorientation can be measured. The X and Y position error
will be used to offset the stepping grid by the correct amount, so
that the new layer of images falls on top of the prior layers.
However, usually the panel substrate will also be misaligned in
.phi. (rotation).
In the past wafer aligners have provided a .phi. stage just for
final rotation adjustment of the substrate, after alignment. On
most stepping aligners, this motion is placed between the X, Y
stage metrology and the substrate. This extra mechanical linkage
causes loss of metrology accuracy and precision, and detracts from
throughput, since it is usually a flexible, hence vibrationprone
subsystem. It detracts substantially from the accuracy needed for
LAED's. The apparatus in our invention has eliminated the separate
.phi. stage, and employs instead the .phi. correction built into
the Y axis stage guidance to rotate the entire stage for the .phi.
correction needed. The mechanical banking used to pre-align each
plate as it is loaded will align plates within 5.0 ums. So only
small .phi. corrections are needed. This precision of prealignment
is achievable with panel substrates because they are square or
rectangular, not round like wafers, and, because they are large,
there is over 30 cm between reference points used for
pre-alignment.
Rotating the stage yaw correction deliberately out of yaw alignment
to correct for plate rotation errors, is one method by which the
panel substrate may be brought into .phi. alignment. Resulting X, Y
positioning errors can be calculated from the known yaw rotation
command, and entered into the X, Y stage calibration database, as
added X, Y offsets. In this way a more rigid, higher throughput,
lower cost system design is achieved without decreasing alignment
performance.
An alternate method for correcting substrate 1 misalignment to the
X and Y axes is to rotate each camera's six degree of freedom
alignment chuck 130 by the measured alignment error angle .phi.,
thereby aligning each image to the orientation of the images on the
substrate. The X and Y origin for each camera will also need to be
adjusted in X and Y by moving chucks 130, so that a line connecting
the optical axes of the two cameras is brought into .phi. alignment
as well. Finally, since the plate is misoriented with respect to
the X and Y axes of the machine, a "stair-step" stepping pattern
will need to be executed, so that the actual stepping pattern of
the stages 11 is made parallel to the orientation of the images on
the substrate.
The procedure used to align substrates, once a single patterned
layer has been created on the substrate, has been described above.
The data acquired during the alignment process is used by the
system control computers to modify the stepping pattern so that the
new layer being printed is placed as exactly as possible on top of
the existing layer or layers. Shifting the X, Y stepping array in
X, Y to overlay the prior layer is a well known practice. However,
the methods used here to correct for .phi., panel scale, and
orthagonality error are novel.
In these ways alignment is achieved for both intra-field and
inter-field relationships, even when subsequent layers are
distorted in panel scale and improperly placed on stage 11Y.
8. Automation of Calibration and Corrections
The apparatus and methods described above are controlled by a
system computer. All data collection methods and precision
adjustments are included in the software and, so, are automatic.
This allows the user to repeat the complex calculation procedure
quickly and precisely, without a high level of skill. It also
allows the user to maximize productive uptime, since complex
calibration sequences are handled automatically by the
computer.
The flow chart in FIG. 5 illustrates the use of these procedures
during normal operations, such as power up 43, stage corrections
32, camera corrections 33, first level patterning 41, and upper
level patterning 42.
Correction data is stored in data bases 32DB and 33DB for the X, Y
motions and the cameras, respectively. The data consists of
coordinate offsets and scaling coefficients which are used to
correct each subsystem so that its operation will be within the
desired tolerances. More generally, the data consists of
mathematical arrays (matrices).
Models of subsystem behavior are also kept in these databases. For
example, the function which describes the nontelecentricity of each
lens 13 or 15 is stored in database 33DB and is used to convert a
desired magnification change (expressed in %) into motion increment
commands (expressed in microns) for drives 84. Since drives 84 are
voltage driven devices, the function that accurately converts
motion increment commands (in microns) into volts for drive 84 is
also included in the data of database 33DB. More generally, these
functions are known as transfer functions, because they transfer
one set of commands into another set, with offset, scale, and
unit-of-measure conversions being accomplished in the process.
Since the input data is often a matrix of numbers, these transfer
functions are also generally expressed as mathematical arrays
(matrices).
Database 32DB contains the correction data and the transfer
functions for the X and Y stages. The primary input for this
database comes from the stage correction procedure 32. Information
which modifies this database comes from VOL and stage .phi. origin
measurements 38, panel alignment and scale measurements 48, and
from real time stage .phi. control 34.
Database 33DB contains the correction data and the transfer
functions for each camera. The primary input for this database
comes from the camera correction procedure 33. Updates come from
reticle selection 45, VOL measurements 38, and autofocus 35.
All control of the apparatus is accomplished through user interface
39 in the form of job commands, which are then stored in the job
command file 40. Commands may be typed in via a computer keyboard
or may be entered via a programmable touch panel. Generally,
complex engineering control is via the computer keyboard, while
routine operating commands, such as "Start," are entered by
touching the displayed graphic on the programmable touch panel. An
exception is the powerup routines. Upon powerup, the apparatus
automatically executes a set of startup commands that include
normal computer diagnostic checks, safety and utility checks,
ending with the initialization of each subsystem on the
apparatus.
Next procedure 38, measuring VOL and aligning the stage .phi.
origin, takes place, so that these critical trim adjustments can be
made. To measure the current index of refraction of air (the VOL
correction), mark 228B on scale 205 is brought under reflective
alignment system unit 241 and its Y-axis location is measured. The
stages 11 then move the known distance to mark 228F. This distance
is calculated from data and functions in database 32DB, namely, the
known length of scale 205, and a prior transfer function used to
convert interferometer fringe counts into millimeters of motion.
Using this process, mark 228F is brought under reflective alignment
system unit 241, and the Y-axis location of mark 228F is also
determined. From prior calibrations the desired length between
marks 228B and 228F is known and stored in database 32DB. Since
scale 205 is made of a material such as Zerodur, which does not
change its dimension with time or with air pressure, etc., any
measured length difference is assumed to be due to a change in air
index, due to pressure changes (or any other index changing
variable, such as temperature, humidity, etc.). The measured error
in the length of scale 205 is therefore used to alter the VOL
scaling factor in the stage transfer function, so that the computed
nominal distance and the actual measured distance from mark 228B to
mark 228F become the same.
On power up, process step 38 causes the yaw drive 200 to set the
stages to a nominal origin for stage .phi. (rotation about Z). When
compared to a line connecting the optical axes of lenses 13 and 15,
the stages may be at a slightly different .phi. orientation each
time the apparatus is started up. This residual error, however,
will be removed by step 33, where the cameras are recalibrated to
match the new .phi. orientation of the stages.
At this point stage correction procedure 32 may be run. Generally,
however, the VOL correction is adequate, assuming that the stages
have been corrected once, at some prior date. If, however, the
stage correction procedure is to be rerun, one would load a grid
calibration plate onto stage 11Y and begin procedure 32. Each grid
plate contains an array of reflective alignment system marks 228 on
a stable substrate, such as Zerodur. The corrected location of each
mark on the grid plate is known and is already in database 32DB,
from prior calibrations. The X and Y stages are moved from mark to
mark by stepping the exact distance contained in database 32DB.
Reflective alignment system unit 241 then measures any residual
error in the location of each mark. Measured deviations are assumed
to be errors in the current X, Y stepping distances. These errors
are stored as an array of correction values and become a transfer
function, used to modify later commanded stepping distances so that
an accurate distance is stepped.
Procedure 33, the camera correction procedure, uses the X, Y
motions as a local measuring machine. The best performance is
achieved if the stage correction procedure 32 has been done first,
as just described. Test reticles 117, which contain an array of
transmission alignment system slits 226 on them are loaded onto
chucks 125 on each camera and aligned to the in-column alignment
references 134. The shutter 97 is opened, in-stage calibration unit
227 is raised to image plane 30. Sensor 23 is moved around each
projected image until the location of the image of each slit 226 is
measured. Offset and scale of each projected image can be
determined from this process. The distance between each camera is
also determined. In this way each camera's projected images are
matched to the X, Y stepping distances and angles.
At this point the apparatus is fully adjusted and is ready to
pattern substrates. First level patterns differ from upper level
patterns in that first level patterns do not have any pattern on
the substrate yet. So no alignment to the substrate is possible.
After loading substrate 1 and banking it against pins 187, using
banking chuck 189, the X, Y stages are stepped to each desired
exposure location directly, using (see step 46) the transfer
function from data base 32DB to modify commanded stepping distances
as required by earlier VOL updates (38) and calibration corrections
(32). In this manner each exposure is placed in its desired,
accurate location.
Upper level patterns require that the new pattern be placed
precisely and accurately above the prior layers of patterning. This
means that the earlier layer(s) must be first located. As described
earlier, offset and scale errors are expected and are corrected
for. By using either the transmission alignment system 225 or the
reflective alignment system alignment system 241, prior level
patterns are located in the coordinate system of stages 11, as has
been described. The data gathered during the alignment process is
used (step 49) to modify the transfer functions in both databases.
For example, if a panel scale change of +2.0 ums over 200 mm
occurred, as measured during alignment step 48, the X and Y
stepping distance transfer function is modified to alter its
scaling function accordingly. Assume that the job commands 40 call
for 5 steps across the measured distance of 200 mm. In this case
each step is increased by +0.4 ums, thereby providing a +2.0 ums
correction across the full distance and matching the new stepping
pattern exactly to the underlying, expanded pattern. Note, in the
above example, that the spacing between the optical images must
also be adjusted using chuck 130. Since the optical image spacing
is generally 165 mm, it must be increased by +1.65 um, which is
accomplished by adding an offset to database 33DB, which in turn
offsets the origin of the six degree of freedom reticle alignment
system 130, so that the proper spacing 3 is provided to exactly
match the substrate's new scale. Similarly, the magnification must
be adjusted +0.001% so that the 40 mm image being stepped is
increased to 40.004 mm to match the new stepping distance. Once
these modifications (see step 49) have been made in the databases,
the revised databases are used to modify job commands (in our
example changing the 40.000 mm stepping distance to 40.004 mm), as
the job is executed, again creating a new pattern level which is
precisely and accurately matched to the underlying layer(s). In
this manner, the apparatus described herein is maintained in
calibration and used to step image arrays with the degree of
control required to produce large arrays of electronic devices
(such as thin film transistors) on amorphous substrates.
* * * * *