U.S. patent application number 09/769558 was filed with the patent office on 2001-12-20 for charged-particle-beam microlithography methods exhibiting reduced thermal deformation of mark-defining member.
Invention is credited to Okino, Teruaki.
Application Number | 20010052578 09/769558 |
Document ID | / |
Family ID | 15935805 |
Filed Date | 2001-12-20 |
United States Patent
Application |
20010052578 |
Kind Code |
A1 |
Okino, Teruaki |
December 20, 2001 |
Charged-particle-beam microlithography methods exhibiting reduced
thermal deformation of mark-defining member
Abstract
Methods and apparatus are disclosed for reducing thermal
deformation of "upstream" marks (as used for alignment and/or
calibration) situated on a reticle or on a reticle plane (e.g., on
the reticle stage), thereby facilitating more accurate transfer of
the reticle pattern to a sensitized substrate (e.g., semiconductor
wafer) using a charged particle beam (e.g., electron beam). The
charged particle beam illuminates an upstream mark situated on the
reticle or on a reticle plane and projects an image of the
illuminated upstream mark onto a corresponding "downstream" mark
situated on a substrate plane. A shield is situated upstream of the
upstream mark and serves to block downstream passage of the charged
particle beam except to illuminate the upstream mark or a portion
of the upstream mark. The upstream mark can be situated on the
reticle or on a mark member situated in the reticle plane.
Inventors: |
Okino, Teruaki;
(Kamakura-shi, JP) |
Correspondence
Address: |
KLARQUIST SPARKMAN CAMPBELL
LEIGH & WHINSTON, LLP
One World Trade Center, Suite 1600
121 S.W. Salmon Street
Portland
OR
97204
US
|
Family ID: |
15935805 |
Appl. No.: |
09/769558 |
Filed: |
January 24, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09769558 |
Jan 24, 2001 |
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09326484 |
Jun 4, 1999 |
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6207962 |
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Current U.S.
Class: |
250/492.3 |
Current CPC
Class: |
H01J 2237/31793
20130101; B82Y 10/00 20130101; B82Y 40/00 20130101; H01J 37/3174
20130101; H01J 37/3045 20130101 |
Class at
Publication: |
250/492.3 |
International
Class: |
G21G 005/00; A61N
005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 5, 1998 |
JP |
10-172113 |
Claims
What is claimed is:
1. A charged-particle-beam projection-exposure apparatus,
comprising: (a) an illumination optical system situated and
configured to direct a charged-particle illumination beam along an
optical axis from a source to a selected region on a reticle, the
reticle being situated at a reticle plane orthogonal to the optical
axis; (b) a projection-optical system situated and configured to
direct a charged-particle imaging beam from the reticle to a
sensitized substrate so as to transfer the pattern portion defined
by the selected exposure unit to the substrate; (c) at least one
upstream mark situated on the reticle plane so as to be selectively
irradiated by the illumination beam; and (d) a shield situated
between the source and the upstream mark, the shield defining an
aperture that transmits a portion of the illumination beam to the
upstream mark while blocking other portions of the illumination
beam.
2. The apparatus of claim 1, wherein the upstream mark is situated
on the reticle.
3. The apparatus of claim 2, wherein: the reticle comprises
multiple upstream marks distributed over the reticle; and the
shield defines multiple apertures each corresponding to a
respective individual upstream mark on the reticle.
4. The apparatus of claim 1, further comprising a mark member
separate from the reticle, wherein at least one upstream mark is
situated on the mark member.
5. The apparatus of claim 4, wherein the shield extends over the
mark member.
6. The apparatus of claim 1, wherein the upstream mark comprises
multiple mark portions.
7. The apparatus of claim 6, wherein the aperture defined by the
shield is sized, whenever the aperture is axially registered with
the upstream mark, to circumscribe all the mark portions
collectively.
8. The apparatus of claim 6, wherein the shield defines multiple
apertures each corresponding to a respective individual mark
portion.
9. In a microlithography method utilizing a charged-particle
illumination beam to irradiate a portion of a pattern defined by a
reticle situated on a reticle plane and a projection-optical system
to direct a corresponding charged-particle imaging beam from the
irradiated portion to a sensitized substrate situated on a
substrate plane, an improved beam-alignment or calibration method
comprising the steps: (a) defining at least one upstream mark on
the reticle plane and at least one downstream mark on the substrate
plane, each upstream mark being selectively registrable with a
downstream mark; (b) providing a shield upstream of an upstream
mark, the shield (i) serving to block downstream propagation of the
illumination beam, and (ii) defining an aperture having a size and
profile sufficient to pass therethrough only a portion of the
illumination beam sufficient to irradiate the upstream mark; and
(c) when irradiating an upstream mark with the illumination beam,
passing the illumination beam through the aperture of the shield
before the illumination beam reaches the upstream mark.
10. The method of claim 9, wherein: step (a) comprises defining at
least one upstream mark on the reticle; and step (b) comprises
extending the shield over the reticle.
11. The method of claim 10, wherein: step (a) comprises defining at
least one upstream mark on a mark member; and step (b) comprises
extending the shield over the mark member.
Description
FIELD OF THE INVENTION
[0001] The present invention pertains, inter alia, to
charged-particle-beam (CPB) microlithography apparatus and methods
as used for transferring a pattern, defined on a reticle, to a
sensitized substrate. Such apparatus and methods have especial
utility in the manufacture of integrated circuits, displays, and
the like. The invention also pertains to methods and apparatus for
calibrating and adjusting a CPB projection-optical system and for
aligning the substrate and reticle with each other for accurate
pattern transfer. The invention also pertains to methods and
apparatus for reducing thermal deformation of a member, such as the
reticle or a movable stage, that defines an alignment or
calibration mark.
[0002] As used herein, the term "reticle" pertains not only to
reticles and masks that define an actual pattern to be transferred
to a substrate, but also to aperture plates and the like as used
in, for example: variable-shaped-beam projection-exposure systems,
character projection systems, and "divided" projection-exposure
systems. In "divided" projection-exposure systems, the reticle is
divided or segmented into multiple "exposure units" (e.g.,
subfields, stripes, or other subdivisions) that are individually
and sequentially exposed onto the substrate on which the images of
individual exposure units are "stitched" together contiguously to
form the complete pattern on the substrate.
BACKGROUND OF THE INVENTION
[0003] Various methods and apparatus are under current research and
development for transferring, using a charged particle beam, a
pattern defined by a reticle or mask onto a sensitized substrate by
microlithography. Representative charged particle beams used in
such systems include electron beams and ion beams. Electron-beam
systems have been the subject of most such effort; hence, the
following summary is in the context of electron-beam systems.
[0004] Charged-particle-beam (CPB) microlithography systems, such
as electron-beam writing systems, offer tantalizing prospects of
improved accuracy and resolution of pattern transfer, but exhibit
disappointingly low throughput. Consequently, much contemporary
research and development has focused on overcoming this
disadvantage. Examples of various conventional approaches include
"cell-projection," "character projection," and "block projection"
(collectively termed "partial-block" pattern transfer).
[0005] Partial-block pattern transfer is especially used whenever
the pattern to be transferred to the substrate comprises a region
in which a basic pattern unit is repeated many times. For example,
partial-block pattern transfer is generally used for patterns
having large memory circuits, such as DRAMs. In such patterns, the
basic pattern unit is very small, having measurements on the
substrate of, for example, (10 .mu.m).sup.2 (i.e., 10
.mu.m.times.10 .mu.m). The basic pattern unit is defined on one or
several exposure units on the reticle and the exposure units are
repeatedly exposed many times onto the substrate to form the
pattern on the substrate. Unfortunately, partial-block pattern
transfer tends to be employed only for repeated portions of the
pattern. Portions of the pattern that are not repeated are
transferred onto the substrate using a different method, such as
the variable-shaped-beam method. Therefore, partial-block
pattern-transfer has a throughput that is too low, especially for
large-scale production of integrated circuits.
[0006] A conventional approach that has been investigated in an
effort to achieve a higher throughput than partial-block
pattern-transfer methods is a projection microlithography method in
which the entire reticle pattern for a complete die (or even
multiple dies) is projection-exposed onto the substrate in a single
"shot." In such a method, the reticle defines a complete pattern,
such as for a particular layer in an entire integrated circuit. The
image of the reticle pattern as formed on the substrate is
"demagnified" by which is meant that the image is smaller than the
pattern on the reticle by a "demagnification factor" (typically 4:1
or 5:1). To form the image on the substrate, a projection lens is
situated between the reticle and the substrate. Whereas this
approach offers prospects of excellent throughput, it to date has
exhibited excessive aberrations and the like, especially of
peripheral regions of the projected pattern. In addition, it is
extremely difficult to manufacture a reticle defining an entire
pattern that can be exposed in one shot.
[0007] Yet another approach that is receiving much current
attention is the "divided" or "partitioned" projection-exposure
approach that utilizes a "divided," "partitioned," or "segmented"
reticle. On such a reticle, the overall reticle pattern is
subdivided into portions termed herein "exposure units." The
exposure units can be of any of various types termed "subfields,"
"stripes," etc., as known in the art. Each exposure unit is
individually and sequentially exposed in a separate "shot" or scan.
The image of each exposure unit is projection-exposed (typically at
a demagnification ratio such as 4:1 or 5:1) using a
projection-optical system situated between the reticle and the
substrate. Even though the projection-optical system typically has
a large optical field, distortions, focal-point errors and other
aberrations, and other faults in projected images of the exposure
units are generally well controlled. Although divided
projection-exposure systems provide lower throughput than systems
that expose the entire reticle in one shot, divided
projection-exposure systems exhibit better exposure accuracy and
image resolution.
[0008] In divided projection exposure, it is necessary to achieve
very accurate alignment of the reticle with the substrate to ensure
that the images of the exposure units are positioned at the
respective locations on the reticle with extremely high accuracy.
To such end, an operation termed "mark detection" is performed such
as during calibration of the optical system and when aligning the
substrate with the reticle before exposing an exposure unit onto
the substrate. During mark detection, an image of one or more
"upstream" marks provided on the reticle or other location on the
reticle stage is projected onto a corresponding "downstream" mark
provided on the substrate or other location on the substrate stage.
The marks are scanned relative to each other to determine the
relative positions of the marks.
[0009] Systems designed for high-resolution pattern transfer, such
as the divided projection-exposure system summarized above, employ
very large acceleration voltages such as between the CPB source and
the reticle. To achieve the requisite high accuracy of mark
detection, either mark scanning must be performed relatively slowly
or a large number of scans must be performed. Consequently, the
cumulative beam energy that strikes the marks and their immediate
surrounding area is very high. This energy is usually dissipated as
localized heating which elevates the temperature and causes thermal
deformation of the vicinity of the marks. Such deformation degrades
the accuracy with which mark positions can be determined, reduces
calibration and alignment accuracy, and reduces the accuracy with
which images of exposure units on the substrate can be stitched
together. The resulting devices manufactured under such conditions
exhibit a higher incidence of defects such as shorts, opens, and
non-uniform resistance values.
SUMMARY OF THE INVENTION
[0010] The present invention solves certain of the problems of
conventional apparatus and methods summarized above and thereby
provide more accurate transfer of a reticle pattern to a
substrate.
[0011] According to a first aspect of the invention,
charged-particle-beam (CPB) microlithography (projection-exposure
or projection-transfer) apparatus are provided. According to a
representative embodiment, such an apparatus comprises an
illumination optical system situated and configured to direct a
charged-particle illumination beam along an optical axis from a
source to a selected region on a reticle. The reticle is situated
at a reticle plane orthogonal to the optical axis. The apparatus
also comprises a projection-optical system situated and configured
to direct a charged-particle imaging beam from the reticle to a
sensitized substrate so as to transfer the pattern portion defined
by the selected exposure unit to the substrate. An "upstream" mark
is situated on the reticle plane so as to be selectively irradiated
by the illumination beam. A shield is situated between the source
and the upstream mark. The shield defines an aperture that
transmits a portion of the illumination beam to the upstream mark
while blocking other portions of the illumination beam from
reaching the reticle plane.
[0012] In the embodiment summarized above, the upstream mark can be
situated on the reticle. In such an instance, the reticle can
comprise multiple upstream marks distributed over the reticle. In
such a configuration, the shield desirably defines multiple
apertures each corresponding to a respective individual upstream
mark on the reticle.
[0013] Alternatively, the upstream mark can be situated on a mark
member separate from the reticle, wherein the upstream mark is
situated on the mark member. In such a configuration, the shield
desirably extends over the mark member. This configuration is
usually used for calibration of the optics of the CPB
projection-exposure apparatus.
[0014] The upstream mark can comprise multiple mark portions. In
such an instance, the aperture defined by the shield can be sized,
whenever the aperture is axially registered with the upstream mark,
to circumscribe all the mark portions collectively. Alternatively,
the shield can define multiple apertures each corresponding to a
respective individual mark portion.
[0015] According to another aspect of the invention, CPB
microlithography methods are provided in which a charged-particle
illumination beam is used to irradiate a portion of a pattern
defined by a reticle situated on a reticle plane. A
projection-optical system is used to direct a corresponding
charged-particle imaging beam from the irradiated portion to a
sensitized substrate situated on a substrate plane. An upstream
mark is defined on the reticle plane and a "downstream" mark is
defined on the substrate plane. The upstream mark is selectively
registrable with the downstream mark to perform beam alignment. A
shield is provided upstream of the upstream mark. The shield (a)
serves to block downstream passage of the illumination beam, and
(b) defines an aperture having a size and profile sufficient to
pass therethrough only a portion of the illumination beam
sufficient to irradiate the upstream mark. When irradiating the
upstream mark with the illumination beam, the illumination beam is
passed through the aperture of the shield before the illumination
beam reaches the upstream mark. The upstream mark can be defined on
the reticle, in which instance the shield desirably extends over
the reticle. Alternatively, the upstream mark can be defined on a
mark member (which can be separate from the reticle), in which
instance the shield desirably extends over the mark member.
[0016] In conventional CPB projection-exposure systems having
utility for, e.g., performing "divided" projection exposure, the
illumination beam as incident on the reticle can have a transverse
profile that is relatively large (e.g., (100 .mu.m).sup.2)-(1000
.mu.m).sup.2). A typical upstream mark is much smaller, on the
order of a few .mu.m square to about a hundred .mu.m square.
Whenever such upstream marks are illuminated by the charged
particle beam during calibration or alignment, the beam that
strikes the upstream mark is much larger in transverse area than
required for illuminating the upstream mark. As summarized above,
the resulting large amount of energy being dissipated in an area
surrounding the upstream mark can cause thermal deformation of the
upstream marks. Whereas it might be possible to reduce the
transverse area of the beam, such a method is impractical because
it requires a very complex irradiation optical system. Apparatus
and methods according to the invention, as summarized above, reduce
the transverse area of the illumination beam actually irradiating
an upstream mark, thereby largely eliminating thermal deformation
of the mark(s).
[0017] The foregoing and additional features and advantages of the
invention will be more readily apparent from the following detailed
description, which proceeds with reference to the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is an elevational section showing certain details of
a reticle and reticle stage of a charged-particle-beam (CPB)
projection-exposure system according to a first representative
embodiment of the present invention.
[0019] FIGS. 2(A)-2(D) are respective plan views depicting certain
relationships between an upstream mark and an illumination beam,
according to the first representative embodiment.
[0020] FIGS. 3(A)-(B) are plan views of details of two respective
example embodiments of a shield of a CPB projection-exposure system
according to the invention.
[0021] FIG. 4 is an elevational section showing certain features of
another representative embodiment in which the illumination beam
passes through a lens from the shield to a mark member.
[0022] FIG. 5 is an elevational schematic drawing showing certain
imaging relationships in an embodiment of an electron-beam
projection-exposure system according to the invention.
DETAILED DESCRIPTION
[0023] Reference is first made to FIG. 5 which depicts a
representative embodiment of a charged-particle-beam (CPB)
projection-exposure apparatus that can include the instant
invention. The FIG. 5 embodiment is discussed below in the context
of an electron-beam system, but it will be understood that any of
various other charged particle beams can be used with such an
apparatus, such as an ion beam.
[0024] In FIG. 5, an electron gun 101 produces an electron beam EB
that propagates in a downstream direction along an optical axis A.
The electron beam EB propagates from the electron gun 101 through
various components (discussed below) to a reticle 110 and then
through other components (discussed below) to a substrate 114.
[0025] Downstream of the electron gun 101 are situated a first
condenser lens 103 and a second condenser lens 105. The electron
beam EB passes through the condenser lenses 103, 105 and is
converged at a crossover image CO1. Downstream of the second
condenser lens 105 is a beam-shaping aperture 106. The beam-shaping
aperture 106 trims the electron beam EB to have a transverse
profile suitable for illuminating an individual exposure unit on
the downstream reticle 110.
[0026] Desirably, the beam-shaping aperture 106 trims the electron
beam EB to have a transverse profile slightly larger than the area
and profile of the exposure unit. For example, the beam-shaping
aperture 106 can shape the electron beam to have a square profile
measuring slightly more than one millimeter on a side as projected
onto the reticle 110, for illuminating an exposure unit measuring
exactly 1 mm square.
[0027] A blanking aperture 107 is situated at the same axial
position, downstream of the beam-shaping aperture 106, as the
crossover image CO1. Immediately downstream of the blanking
aperture 107 is a deflector 108. A collimating lens 109 forms an
image of the beam-shaping aperture 106 on the illuminated exposure
unit on the reticle 110.
[0028] As used herein, an "illumination beam" denotes the charged
particle beam EB between the electron gun 101 and the reticle 110,
and an "imaging beam" denotes the charged particle beam between the
reticle 110 and the substrate 114. Similarly, the
"illumination-optical system" denotes the optical system located
between the source 101 and the reticle 110, and the
"projection-optical system" denotes the optical system located
between the reticle 110 and the substrate 114.
[0029] The deflector 108 sequentially scans the electron beam EB
primarily in the X direction of FIG. 5 so as to illuminate, within
the optical field of the illumination-optical system, a desired
exposure unit on the reticle 110.
[0030] With respect to the reticle 110, although only one exposure
unit (through which the optical axis A passes) is shown in FIG. 5,
the reticle 110 actually extends outward in the X-Y plane
(perpendicular to the optical axis) and typically comprises a large
number of exposure units. As the exposure units are sequentially
illuminated by the electron beam, the deflector 108 scans the
electron beam, as discussed above, across the optical field of the
illumination-optical system.
[0031] Provided downstream of the reticle 110 are first and second
projection lenses 112 and 113 and a deflector 131. The projection
lenses are preferably configured as a "Symmetric Magnetic Doublet"
or "SMD." As each exposure unit on the reticle 110 is illuminated
by the illumination beam, the beam passes through the illuminated
exposure unit and thus acquires an ability to form an image of the
illuminated exposure unit. The resulting imaging beam is
demagnified by passage through the projection lenses 112, 113 and
deflected as required by the deflectors 131 to form an image of the
illuminated exposure unit at the desired location on the substrate
114.
[0032] The reticle 110 is mounted on a reticle stage 111 that is
movable within an X-Y plane. In a similar manner, the substrate
(e.g., a semiconductor wafer) 114 is mounted on a wafer stage 115
that is also movable within a respective X-Y plane. Hence,
continuous scanning of the exposure units of the reticle pattern
can be performed (assuming the projection lenses 112, 113 are
configured as an SMD) by scanning the reticle stage 111 and the
wafer stage 115 in opposite directions along the Y axis. Both the
reticle stage 111 and wafer stage 115 include highly accurate
position-measurement systems employing laser interferometers as
known in the art. The position-measurement systems, in concert with
beam alignments and adjustments performed by the various deflectors
of the illumination and projection optical systems, enable the
images of the exposure units as formed on the substrate 114 to be
accurately stitched together.
[0033] The upstream-facing surface of the substrate 114 is coated
with a suitable resist so as to be imprintable with the projected
image of the substrate pattern. To effect such imprinting, the
substrate 114 must be exposed with a proper dosage of the imaging
beam.
[0034] Situated upstream of the substrate 114 is a
backscattered-electron detector 133 used for mark detection, as
discussed below.
[0035] FIG. 1 shows the vicinity of a reticle stage according to a
first representative embodiment of the invention. As shown in FIG.
1, a reticle 1 is mounted on a reticle stage 3. A mark member 5 is
situated adjacent the reticle on the reticle stage 3. The
upstream-facing surfaces of the mark member 5 and the reticle 1 are
desirably co-planar in a "reticle plane" that is orthogonal to the
optical axis. The mark member 5 desirably is made of silicon about
800 .mu.m in thickness and defines one or more "upstream" marks,
such as shown in FIGS. 2(A)-2(D), useful for alignment and
calibration purposes, for example. Whenever the charged particle
beam 8 impinges on an upstream mark, some of the particles in the
beam pass through the upstream mark and are projected onto a
respective region on the substrate or wafer stage. The
upstream-facing surface on the substrate or on the wafer stage
where the upstream mark is projected desirably is situated in a
"substrate plane" orthogonal to the optical axis.
[0036] Situated upstream of the mark member 5 is a shield 7. The
shield 7 desirably is made of an electrically conductive material
such as tantalum or molybdenum having a thickness of approximately
0.1 to 1 mm in this embodiment. The shield 7 is supported relative
to the reticle stage 3 by a leg portion 7b from which a shield
plate 7c extends in a cantilever manner so as to cover the mark
member 5. The gap between the mark member 5 and the shield 7 is
desirably within the range of approximately 0.1 mm to several mm.
Alternatively, a separate leg portion 7b can be placed along each
of at least two edges of the shield plate 7c, or the shield plate
can be supported relative to the reticle stage 3 in any of various
other suitable ways. Flanking the shield 7b is a laser mirror 9
used by the position-measurement system of the reticle stage
discussed above.
[0037] The shield plate 7c defines an aperture 7a that is desirably
slightly larger than the upstream mark on the mark member 5. The
aperture 7a desirably is located in the center of the shield plate
7c and axially registered with the upstream mark on the mark member
5. The aperture 7a is discussed further below, with reference to
FIGS. 3(A) and 3(B).
[0038] The reticle 1 also can be covered with a shield 6 that
defines apertures 6a in locations on the shield 6 that correspond
to the locations of corresponding upstream marks on the reticle
1.
[0039] Representative relationships between an upstream mark and
the illumination beam are depicted in FIGS. 2(A)-2(D). FIG. 2(A)
shows the area encompassed by a single exposure unit 11, with the
superposed transverse profile of the illumination beam 13. (The
exposure-unit area 11 encompasses that portion of the overall
reticle pattern transferred from the reticle 1 to the substrate in
a given instant of time.) For divided projection exposure, a
typical exposure-unit area 11 would be square or rectangular in
profile and have an area (on the reticle) of approximately (100
.mu.m).sup.2 to (1000 .mu.m).sup.2. With a demagnification ratio of
4:1, for example, such an exposure unit would illuminate an area of
approximately (25 .mu.m).sup.2 to (250 .mu.m).sup.2, respectively,
on the substrate. For a shaped-beam single-shot transfer technique
such as cell projection, the typical exposure-unit area 11 would
measure (100 .mu.m).sup.2 to (200 .mu.m).sup.2 on the reticle. With
a demagnification ratio of 25:1, for example, such an exposure unit
would illuminate an area of about (5 .mu.m).sup.2 on the substrate.
In FIGS. 2(A)-2(D), the upstream marks are formed on the same
membrane region of the reticle as the pattern to be
projection-transferred to the substrate.
[0040] The transverse area of the illumination beam 13 is slightly
larger than the exposure unit 11. For example, if the exposure unit
11 were a square measuring 1000 .mu.m.times.1000 .mu.m, then the
transverse area of the illumination beam 13 would be a square
measuring about 1100 .mu.m.times.1100 .mu.m.
[0041] FIG. 2(B) shows a relatively large (relative to the aperture
21) upstream mark 23 that has especial utility for aligning and
calibrating the main field of the illumination and imaging optical
systems. The mark 23 is configured as a line-and-space pattern in
which each line has a width of, by way of example, 1.6 .mu.m, a
length of 50 .mu.m and spacing therebetween of 3.2 .mu.m. The
illumination beam illuminates the upstream mark 23. As the
illumination beam illuminates the mark 23, the portion of the beam
passing through the mark is projected onto the substrate (or other
suitable location on the substrate plane). The projection is
performed such that the projected image of the upstream mark 23
overlays a corresponding "downstream" mark on the substrate (or
substrate plane). The image of the upstream mark 23 is scanned onto
the downstream mark by the deflector 131 (FIG. 5). The
backscattered-electron detector 133 (FIG. 5) detects backscattered
electrons propagating from the overlaying marks. Based on the
resulting detection signal relative to the scan signal, a
measurement is performed in which a mark pattern previously
imprinted on the substrate or substrate plane is aligned so as to
be in registration with the newly projected mark pattern.
Alternatively, a calibration can be performed in which one or more
of demagnification ratio, rotation, distortion, lateral position,
and focus position, for example, is adjusted as required.
[0042] FIG. 2(C) shows a relatively small (relative to the aperture
31) upstream mark 33 that has especial utility for calibrations and
corrections of distortion of exposure units as projected onto the
substrate. The upstream mark 33 is further detailed in the
enlargement shown in FIG. 2(D), in which the mark comprises
multiple lines 35 each having, by way of example, a width of
several .mu.m, a length of about 10 .mu.m, and spaces therebetween
each having a width of 2 .mu.m.
[0043] The mark patterns shown in FIGS. 2(B) and 2(C) are
significantly smaller than the transverse profile of the
illumination beam 13. As a result, many (if not most) of the
charged particles in the illumination beam are not used to
illuminate the marks per se but rather used to illuminate the
vicinity of the marks. I.e., most of the charged particles impinge
on the mark member 5 (or the reticle if the upstream marks are
defined on the reticle) and cause localized heating and consequent
thermal deformation of the mark member (or reticle). Such thermal
deformation causes the shapes and positions of the upstream marks
(and of the lines or elements thereof) to change. Such changes
degrade alignment and calibration accuracy, which degrade the
accuracy with which the reticle pattern can be transferred to the
substrate. The shields 6, 7 shown in FIG. 1 alleviate this
problem.
[0044] Details of a shield 6, 7 according to two example
embodiments are shown in FIGS. 3(A) and 3(B), respectively. Turning
first to FIG. 3(A) the shield 6, 7 is shown in plan view. The
perimeter of the shield 6, 7 encloses an area that is larger than
the transverse area and profile of the illumination beam 13. For
example, if the illumination beam 13 has a 1100 .mu.m.times.1100
.mu.m transverse profile, then the shield 6, 7 has at least a
slightly larger area. The center of the shield 6, 7 defines an
aperture 6a, 7a measuring, by way of example, 55 .mu.m.times.55
.mu.m. The aperture 6a, 7a is situated such that the upstream mark
23 (which, by way of example occupies an area of approximately 50
.mu.m.times.50 .mu.m) when viewed axially is approximately centered
in the aperture 6a, 7a. To illuminate the upstream mark 23, the
illumination beam first passes through the aperture 6a, 7a; the
shield 6, 7 blocks most of the illumination beam from reaching
anything downstream other than the upstream mark 23. As a result,
only that portion of the illumination beam that is actually
required to illuminate the upstream mark 23 strikes the mark member
5. The amount of heating imparted to the mark member 5 is thus much
less than if the shield 6, 7 were absent.
[0045] The example embodiment of the shield shown in FIG. 3(B) is
especially useful whenever the space between the lines of the
upstream mark 23 is relatively wide. Rather than having a single
large aperture 6a, 7a, as used in the FIG. 3(A) embodiment, the
shield 6', 7' in the FIG. 3(B) embodiment defines individual
slit-shaped apertures 6a',7a' for each respective line of the mark
23. By way of example, each slit-shaped aperture 6a',7a' has a
width of 5.5 .mu.m and a length of 51 .mu.m. Thus, each slit-shaped
aperture 6a',7a' is slightly larger than the corresponding line of
the mark 23. The FIG. 3(B) configuration further reduces the
electron dose received by regions of the mark member 5 (or reticle)
outside the upstream mark 23. This, in turn, further reduces
thermal deformation of the mark member (or reticle).
[0046] Turning now to FIG. 4 showing another representative
embodiment, a shield 51 defining an aperture 51a is axially
separated from a mark member 57. I.e., the shield 51 is situated
upstream of the mark member 57, and a lens 53 is situated between
the shield and the mark member. An illumination beam 55, having
passed through the aperture 51a in the shield 51 is projected by
the lens 53 onto (and imaged on) an upstream mark 57a on the mark
member 57. In this configuration, the upstream mark 57a on the mark
member (or reticle) is selectively illuminated by the illumination
beam. This avoids thermal deformation of the mark member (or
reticle) due to excessive localized irradiation by the illumination
beam.
[0047] Therefore, the present invention provides a shield situated
over a location on a reticle plane (e.g., a mark member or reticle)
defining an upstream mark. The shield effects more localized
irradiation of the upstream mark during instances in which the
upstream mark is being irradiated by the illumination beam.
Consequently, excess irradiation of the vicinity of the upstream
mark is prevented, which correspondingly reduces thermal
deformation of the mark and increases the accuracy of mark
detection.
[0048] Whereas the invention has been described in connection with
multiple representative embodiments, it will be apparent that the
invention is not limited to those embodiments. On the contrary, the
invention is intended to encompass all alternatives, modifications,
and equivalents as may be encompassed within the spirit and scope
of the invention as defined by the appended claims.
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