U.S. patent application number 10/236318 was filed with the patent office on 2003-04-24 for methods and devices for evaluating beam blur in subfields projection-exposed by a charged-particle-beam microlithography apparatus.
This patent application is currently assigned to Nikon Corporation. Invention is credited to Yahiro, Takehisa.
Application Number | 20030075690 10/236318 |
Document ID | / |
Family ID | 19094320 |
Filed Date | 2003-04-24 |
United States Patent
Application |
20030075690 |
Kind Code |
A1 |
Yahiro, Takehisa |
April 24, 2003 |
Methods and devices for evaluating beam blur in subfields
projection-exposed by a charged-particle-beam microlithography
apparatus
Abstract
Methods and devices are disclosed for evaluating image
performance in a charged-particle-beam (CPB) microlithography
apparatus. In the disclosed method, multiple knife-edge reference
marks are defined by a plate positioned at an image plane. The
reference marks are illuminated by measurement beamlets formed by
an upstream subfield. The beamlets are scanned over the reference
marks to produce a series of beam-current transmissions. Each
beam-current transmission corresponds to a single beamlet being
scanned over a corresponding reference mark. The beam-current
transmissions are exclusive of one another. The series of
transmissions is then detected and analyzed. The knife-edge
reference marks may be positioned on the plate so that the relative
distance between the reference marks and the corresponding beamlets
progressively increases for each successively scanned mark. A dummy
beam also may be defined on the upstream subfield. The reference
marks of the plate may be apertures or constructed of a
charged-particle-reflecting material.
Inventors: |
Yahiro, Takehisa; (Ageo-shi,
JP) |
Correspondence
Address: |
KLARQUIST SPARKMAN, LLP
One World Trade Center, Suite 1600
121 S.W. Salmon Street
Portland
OR
97204
US
|
Assignee: |
Nikon Corporation
|
Family ID: |
19094320 |
Appl. No.: |
10/236318 |
Filed: |
September 5, 2002 |
Current U.S.
Class: |
250/491.1 ;
250/492.22 |
Current CPC
Class: |
H01J 37/3174 20130101;
H01J 2237/30433 20130101; B82Y 10/00 20130101; B82Y 40/00
20130101 |
Class at
Publication: |
250/491.1 ;
250/492.22 |
International
Class: |
H01J 037/304 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 5, 2001 |
JP |
2001-268320 |
Claims
What is claimed is:
1. A method for evaluating imaging performance in a
charged-particle-beam (CPB) microlithography apparatus, comprising:
defining multiple beam-transmitting measurement marks on a subfield
positioned at an object plane; defining multiple corresponding
reference marks by a plate positioned at an image plane;
illuminating the measurement marks with a charged particle beam to
form multiple measurement beamlets propagating downstream toward
the respective reference marks; projecting the beamlets onto the
plate; scanning the beamlets over the reference marks to produce a
series of beam-current transmissions, each beam-current
transmission corresponding to a respective one of the beamlets
being scanned through a corresponding one of the reference marks,
and each beam-current transmission being exclusive of any other
beam-current transmission; and detecting the series of beam-current
transmissions.
2. The method of claim 1, wherein each of the reference marks
comprises at least one respective knife-edge across which the
respective beamlet is scanned.
3. The method of claim 1, wherein the step of defining multiple
corresponding reference marks comprises defining the reference
marks so that a relative distance between each reference mark and
its corresponding beamlet progressively increases for each
successive mark.
4. The method of claim 3, wherein each reference mark has a
rectangular profile having a major dimension and a minor
dimension.
5. The method of claim 4, wherein the increase of the relative
distance is greater than or equal to the minor dimension of a
previously scanned reference mark.
6. The method of claim 1, further comprising blocking charged
particles that are forward-scattered through the plate while not
blocking charged particles that are transmitted through the
respective reference marks.
7. The method of claim 6, wherein the blocking step is performed by
disposing a beam-limiting diaphragm downstream of the reference
marks, the beam-limiting diaphragm comprising multiple
beam-limiting apertures corresponding to respective reference marks
and having respective opening dimensions sufficient to block the
forward-scattered charged particles.
8. The method of claim 7, wherein the beam-limiting diaphragm is a
first beam-limiting diaphragm, the method further comprising
disposing a second beam-limiting diaphragm downstream of the
reference marks and the first beam-limiting diaphragm, the second
beam-limiting diaphragm comprising a beam-limiting aperture having
an opening dimension sufficient to block charged particles that are
forward-scattered through the plate while not blocking charged
particles that are transmitted through the reference marks.
9. The method of claim 1, further comprising adjusting a
projection-optical system of the CPB-microlithography apparatus in
response to the detected series of beam-current transmissions.
10. The method of claim 9, wherein the adjusting step is performed
so as to minimize blur of an image transferred to the image
plane.
11. The method of claim 10, wherein the adjusting step is performed
so as to minimize a variation in blur within an image of the
subfield transferred to the image plane.
12. The method of claim 1, further comprising defining at least one
dummy pattern on the subfield.
13. The method of claim 12, wherein: the charged particle beam
illuminates the dummy pattern to produce a dummy beam propagating
downstream of the measurement marks; and the detecting step further
comprises detecting beam blur of the beamlets attributable to a
space-charge effect caused by the dummy beam.
14. The method of claim 1, wherein the reference marks are defined
as respective apertures in the plate.
15. The method of claim 1, wherein: the reference marks are defined
as respective electron-reflective reference marks on the plate; and
the detecting step is performed using a detector positioned
opposite the reference marks.
16. The method of claim 15, wherein the electron-reflective
reference marks are defined as respective units of a film of heavy
metal formed on the plate.
17. The method of claim 1, wherein the scanning step is performed
in a single scan.
18. A device for evaluating imaging performance in a
charged-particle-beam (CPB) microlithography apparatus, comprising:
multiple beam-transmitting measurement marks disposed on a subfield
positioned at an object plane of the CPB-microlithography
apparatus; multiple reference marks defined by a plate located at
an image plane of the CPB-microlithography apparatus; an
illumination-optical system that directs a charged particle beam
onto the measurement marks so as to form multiple measurement
beamlets propagating downstream of the respective measurement marks
toward the respective reference marks; a projection-lens assembly
that projects the beamlets onto the plate and scans the beamlets
over the respective reference marks to produce a series of
beam-current transmissions, each beam-current transmission
corresponding to one of the beamlets being scanned through a
corresponding one of the reference marks, and each beam-current
transmission being exclusive of any other beam-current
transmission; and a detector that detects the series of
beam-current transmissions.
19. The device of claim 18, wherein each reference mark comprises
at least one knife-edge across which the respective beamlet is
scanned.
20. The device of claim 18, wherein the reference marks are defined
by the plate so that the relative distance between the reference
marks and the corresponding beamlets progressively increases for
each successive reference mark.
21. The device of claim 20, wherein each reference mark has a
rectangular profile including a major dimension and a minor
dimension.
22. The device of claim 21, wherein the increase of the relative
distance is greater than or equal to the minor dimension of a
previously scanned reference mark.
23. The device of claim 18, further comprising a beam-limiting
diaphragm situated downstream of the reference marks, the
beam-limiting diaphragm defining multiple beam-limiting apertures
corresponding to respective reference marks and having respective
opening dimensions sufficient for blocking charged particles that
are forward-scattered through the plate while not blocking charged
particles that are transmitted through the respective reference
marks.
24. The device of claim 23, wherein the beam-limiting diaphragm is
a first beam-limiting diaphragm, the device further comprising a
second beam-limiting diaphragm situated downstream of the first
beam-limiting diaphragm, the second beam-limiting diaphragm
defining a beam-limiting aperture having an opening dimension
sufficient to block charged particles that are forward-scattered
through the plate while not blocking charged particles that are
transmitted through the reference marks.
25. The device of claim 18, wherein the subfield further defines a
dummy pattern configured to form a dummy beam relative to the
beamlets.
26. The device of claim 18, wherein the reference marks are
respective apertures formed in the plate.
27. The device of claim 18, wherein: the reference marks are
respective reflective reference marks; and the detector is
positioned opposite the reflective marks.
28. The device of claim 27, wherein each reflective reference mark
is defined by a film of heavy metal.
29. The device of claim 18, wherein the projection-lens assembly is
configured to scan the beamlets collectively in a single scan.
30. The device of claim 18, wherein the plate is attached to a
wafer stage.
31. A charged-particle-beam microlithography apparatus, comprising
a device as recited in claim 18.
32. In an imaging-performance measurement system of a
charged-particle-beam (CPB) microlithography apparatus, a device
for determining beam blur in a subfield of a pattern
transfer-exposed by the CPB microlithography apparatus, the device
comprising: a plate disposed on a wafer stage of the
CPB-microlithography apparatus; and multiple knife-edge reference
marks defined by the plate and separated from corresponding
beamlets by a relative distance, the relative distance
progressively increasing for each successive reference mark defined
by the plate.
33. The device of claim 32, further comprising means for scanning
the beamlets across the plate, wherein the increase in the relative
distance is such that, when one of the reference marks is
illuminated by a corresponding beamlet, no other reference mark is
illuminated simultaneously.
34. The device of claim 32, wherein each reference mark has a
rectangular profile, including a major dimension and a minor
dimension.
35. The device of claim 34, further comprising means for scanning
the beamlets across the reference marks, wherein the increase of
the relative distance is greater than or equal to the minor
dimension of a previously scanned reference mark.
36. The device of claim 32, wherein the reference marks are
respective apertures defined in the plate.
37. The device of claim 32, wherein: the reference marks are
configured to backscatter charged particles of a respective
incident beamlet; and the detector is situated opposite the
reflective marks.
38. The device of claim 37, wherein the reflective reference marks
are made of a film of heavy metal.
39. In an imaging-performance measurement system of a
charged-particle-beam (CPB) microlithography apparatus, a device
for determining beam blur in a subfield of a pattern
transfer-exposed by the CPB microlithography apparatus, the device
comprising: a reticle stage for holding a reticle comprising
multiple subfields, the reticle stage defining an object plane in
which multiple measurement marks are defined, each measurement mark
being configured to form a respective beamlet from a charged
particle beam incident on the object plane; and a plate disposed on
a wafer stage of the CPB-microlithography apparatus, the plate
defining multiple knife-edge reference marks separated from the
object plane by a relative distance, the relative distance
progressively increasing for each successive reference mark defined
by the plate.
40. The device of claim 39, further comprising means for scanning
the beamlets across the plate, wherein the increase in the relative
distance is such that, when one of the reference marks is
illuminated by a corresponding beamlet, no other reference mark is
illuminated simultaneously.
41. The device of claim 39, wherein: each reference mark has a
rectangular profile, including a major dimension and a minor
dimension; and the device further comprising means for scanning the
beamlets across the reference marks, wherein the increase of the
relative distance is greater than or equal to the minor dimension
of a previously scanned reference mark.
Description
FIELD
[0001] This disclosure pertains to microlithography
(transfer-exposure of a pattern to a sensitive substrate).
Microlithography is a key technology used in the fabrication of
microelectronic devices such as integrated circuits, displays, and
micromachines. More specifically, this disclosure relates to
microlithography performed using a charged particle beam, such as
an electron beam or an ion beam. Even more specifically, this
disclosure pertains to methods and devices for evaluating the
imaging performance of a CPB-microlithography apparatus.
BACKGROUND
[0002] Conventional charged-particle-beam (CPB) microlithography
systems (typically using an electron beam as an exemplary charged
particle beam) suffer from low throughput (i.e., the number of
production units, such as wafers, that can be processed per unit
time). To increase throughput, CPB projection-exposure apparatus
(e.g., electron-beam steppers, etc.) have been developed that are
capable of transferring large portions of the pattern to the
substrate in one exposure "shot."In one such apparatus, termed a
"divided-reticle" CPB-microlithography apparatus, the pattern is
defined on a reticle divided into a large number of "subfields,"
each defining a respective portion of the pattern. Modern
divided-reticle CPB-microlithography apparatus are configured to
expose subfields measuring about 250 .mu.m square (as exposed on
the substrate). Simulation studies have revealed that the
distribution of beam blur over such a large area is uneven.
Simulation studies also have revealed that, whenever the current of
a charged particle beam is increased in order to increase the
throughput of the microlithography apparatus, space-charge effects
also produce an uneven distribution of beam blur across the
subfield. As a result, it is necessary to measure the distribution
of beam blur at various points of the subfield with extremely high
accuracy and precision. Based on these measurements, appropriate
corrective adjustments can be made to the beam (e.g., of focal
point, astigmatism, magnification, rotation, etc.), allowing the
imaging performance of the microlithography apparatus to be
improved.
[0003] A conventional technique for measuring imaging performance
is shown with reference to FIGS. 10-13. Referring first to FIG. 10,
it is understood that an illumination-beam source and a reticle,
although not shown, are located upstream of the components shown in
the figure (i.e., above the plane of the page). The reticle is
positioned in the "object plane". The multiple beamlets EB depicted
in FIG. 10 are small electron beams produced by transmission of the
illumination beam through respective rectangular measurement marks
located in a subfield of the reticle. Hence, the beamlets EB that
have passed through the measurement marks have rectangular
transverse profiles. The beamlets EB are incident on a plate 102
that defines "knife-edge" reference marks 103a, 103b. The plate 102
is disposed on a wafer stage, which is positioned in the plane
where the transferred image is to be formed (i.e., the "image
plane"). The reference marks 103a, 103b are typically rectangular
in profile and are configured as respective through-holes defined
by the plate 102. The reference marks 103a, 103b define respective
"knife-edges" 101a, 101b on which the beamlets EB are incident. The
reference mark 103a is used for measuring beam blur along the
X-direction of the plate 102, and the reference mark 103b is used
for measuring beam blur along the Y-direction. In FIG. 10, nine
X-direction reference marks 103a and nine Y-direction reference
marks 103b are shown.
[0004] The knife-edge 101a is shown in FIG. 11. A beam-limiting
diaphragm 105b is disposed downstream of the knife-edge 101a. The
diaphragm 105b is made of a sufficiently thick, conductive metal
plate that absorbs electrons of an incident beamlet EB. The
beam-limiting diaphragm 105b defines a beam-limiting aperture 105.
An electron detector (sensor) 106 is disposed downstream of the
beam-limiting aperture 105. As shown in FIG. 12, beam currents
detected by the electron detector 106 are amplified by a
pre-amplifier 107, converted to an output waveform by a
differentiation circuit 108, and displayed on an oscilloscope 109
or analogous display.
[0005] As shown in FIG. 12, a beamlet EB is incident in a scanning
manner over the knife-edge 101a and the reference mark 103a. As the
beamlet EB is scanned in a direction indicated by the respective
arrow (labeled "SCAN" and extending to the right in FIG. 12),
electrons e1, which are transmitted through the reference mark
103a, and a portion of electrons e2, which are forward-scattered
through the plate 102, are transmitted through the beam-limiting
aperture 105. Thus, most of the forward-scattered electrons e2 are
blocked by the beam-limiting diaphragm 105b, and most of the
electrons transmitted through to the detector 106 are the
non-scattered electrons el. The distribution of beam blur (i.e.,
blur variation, or ".DELTA.blur") in the subfield can be measured
by sequentially performing this measurement method for each of the
nine knife-edge reference marks shown in FIG. 10.
[0006] A second conventional technique for measuring imaging
performance is shown with reference to FIGS. 13-14. In FIG. 13,
beamlets EB, as described above, are incident on a plate 102',
which defines multiple reflective reference marks 103a', 103b'.
Each reference mark 103a', 103b' is made of a thin film of heavy
metal (e.g., Ta, W, etc.). The reference mark 103a' is used to
measure beam blur along the X-direction of the plate 102', and the
reference mark 103b' is used to measure beam blur along the
Y-direction. In FIG. 13, nine X-direction reference marks 103a' and
nine Y-direction reference marks 103b' are shown. FIG. 14 shows an
electron detector 116 disposed above the reference marks 103a',
103b'. The detector 116 detects electrons that are incident on and
reflected by the reference marks 103a', 103b' as the beamlet EB is
scanned across the reference marks. The distribution of blur in the
subfield is measured by sequentially performing the measurement
method described above for each of the nine reference marks shown
in FIG. 13.
[0007] In both of the conventional imaging-performance measurement
techniques described above, beam blur resulting at each of the
reference marks is measured individually (i.e., one at a time).
Therefore, in order to measure the distribution of beam blur across
an entire subfield, a large amount of time is needed.
SUMMARY
[0008] In view of the shortcomings of the prior art as summarized
above, the present disclosure provides, inter alia, methods and
devices for evaluating imaging performance in a
charged-particle-beam (CPB) microlithography apparatus. In general,
the disclosed methods enable the imaging performance of a
CPB-microlithography apparatus to be evaluated quickly and with a
high degree of accuracy. Specifically, the disclosed methods enable
the beam blur at multiple locations of a projected subfield to be
measured nearly simultaneously. Thus, the projection-optical system
of the CPB exposure apparatus can be adjusted to correct for the
beam blur quickly and easily.
[0009] A first aspect of the invention is set forth in the context
of methods for measuring imaging performance. In an embodiment of
the method, multiple beam-transmitting measurement marks are
defined on a subfield positioned at an object plane, and multiple
corresponding knife-edge reference marks are defined on a plate
positioned at an image plane. The multiple measurement marks are
illuminated simultaneously with a charged particle beam to form
multiple measurement beamlets that propagate downstream toward the
reference marks. The beamlets are projected onto the plate and
scanned over the reference marks to produce a series of
beam-current transmissions. The scanning of the reference marks can
be performed in a single scan. Each beam-current transmission
corresponds to one of the beamlets being scanned through a
corresponding one of the reference marks, and is exclusive of any
other beam-current transmission. The beam-current transmissions are
detected by a detector. The signal input to the detector is
time-divided for each of the reference marks and its corresponding
measurement beamlet. The projection-optical system of the
CPB-microlithography apparatus may be adjusted in accordance with
the detected beam-current transmissions. The projection-optical
system may be adjusted so that the maximum beam blur of a
transferred image is minimized or so that the variation in blur is
minimized.
[0010] In one embodiment, the reference marks have at least one
knife-edge across which the corresponding beamlet is scanned. In
another embodiment, the reference marks on the plate are positioned
so that the relative distance between the reference marks and the
corresponding beamlets progressively increases for each successive
reference mark. For instance, the reference marks have a
rectangular profile having a major and a minor dimension, wherein
the increase of the relative distance is greater than or equal to
the minor dimension of the reference mark that was scanned
previously.
[0011] In another embodiment of the method, a beam-limiting
diaphragm is positioned downstream of the reference marks. The
beam-limiting diaphragm includes multiple beam-limiting apertures
that correspond to respective reference marks. The beam-limiting
apertures have respective "opening dimensions" (diameters, if the
apertures are round) sufficient to block charged particles that are
forward-scattered through the plate while not blocking charged
particles that are transmitted through the reference marks. A
second beam-limiting diaphragm also may be included, positioned
downstream of the first beam-limiting diaphragm. The second
beam-limiting diaphragm can include an aperture having an opening
dimension sufficient to block charged particles that are
forward-scattered through the plate while not blocking charged
particles that are transmitted through the reference marks. By
using the beam-limiting diaphragm(s) to block out unwanted charged
particles, the beam-blur measurements made by the detector are made
with high precision and nearly ideal contrast.
[0012] For performing these measurements while taking into account
space-charge effects, a dummy pattern can be defined in a subfield
of a reticle or analogous structure positioned at an object plane.
Whenever a charged particle beam illuminates the subfield, the
dummy pattern produces a dummy beam that propagates downstream,
parallel to the beamlets, from the measurement marks. The dummy
beam exerts a Coulomb force on the measurement beamlets and allows
beam blur to be evaluated while taking into account space-charge
effects.
[0013] The reference marks can be made of a material that
backscatters charged particles of the incident beam. For instance,
the reference marks can be made of a thin film of heavy metal
(e.g., Ta or W). In an embodiment of a method performed using such
reference marks, the detectors measuring the beam current are
positioned opposite the reference marks in order to detect the
reflected electrons.
[0014] According to another aspect of the invention, devices are
provided for evaluating image performance of a CPB-microlithography
device utilizing a method for evaluating imaging performance, as
summarized above. An embodiment of the device includes multiple
beam-transmitting measurement marks disposed on a subfield
positioned at an object plane of the CPB-microlithography
apparatus. The device further includes multiple reference marks
defined on a plate located at an image plane of the
CPB-microlithography apparatus. An illumination-optical assembly is
situated and configured to direct a charged particle beam onto the
measurement marks so as to form multiple measurement beamlets
propagating downstream of the measurement marks toward respective
reference marks. A projection-lens assembly is situated and
configured to project the beamlets onto the plate and to scan the
beamlets over the respective reference marks to produce a series of
beam-current transmissions. Each beam-current transmission
corresponds to a respective one of the beamlets being scanned
through a corresponding one of the reference marks. Additionally,
each beam-current transmission is exclusive of any other
beam-current transmission. The device also includes a detector
situated and configured to detect the beam-current transmissions.
The device further may incorporate any of the various features
summarized above.
[0015] According to yet another aspect of the invention, devices
are provided for use in an imaging-performance measurement system,
such as the system summarized above. An embodiment of such a device
includes a plate disposed on a wafer stage of the
CPB-microlithography apparatus. Multiple knife-edge reference marks
are defined by the plate. Each reference mark is separated from the
incidence locus of the corresponding beamlets by a respective
relative distance, wherein the relative distance progressively
increases for each successive mark. Each of the reference marks can
have a rectangular profile including a major dimension and a minor
dimension. The increase in relative distance may be greater than or
equal to the minor dimension of a previously scanned reference
mark. In another embodiment, the reference marks are positioned so
that, whenever one of the reference marks is illuminated by the
reference mark's corresponding beamlet, no other reference mark is
illuminated simultaneously.
[0016] 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
[0017] FIG. 1 is a schematic plan view of a plate illustrating the
size and shape relationships of incident electron beamlets EBa-EBi
and corresponding knife-edge reference marks in accordance with the
first representative embodiment.
[0018] FIG. 2(A) is a schematic elevational view, with an
accompanying block diagram, illustrating the manner in which
imaging performance is measured according to the first
representative embodiment.
[0019] FIG. 2(B) is a schematic elevational diagram showing the
angles .alpha. and .theta. with respect to a beam incident on edges
of respective apertures in the plate 2 and diaphragm 4.
[0020] FIG. 3(A) is a schematic plan view of a beamlet EB being
scanned over a knife-edge of the corresponding reference mark, as
discussed with respect to the first representative embodiment.
[0021] FIG. 3(B) depicts an exemplary plot of a single beam-current
waveform detected by an electron detector situated downstream of
the reference marks in FIG. 3(A).
[0022] FIG. 3(C) depicts an exemplary plot of a single differential
waveform derived from the plot in FIG. 3(B), showing both the ideal
waveform and the actual waveform.
[0023] FIG. 3(D) is an enlargement of the rising portion of the
actual differential waveform in FIG. 3(C).
[0024] FIG. 4(A) depicts an exemplary plot of a series of
beam-current waveforms measured in accordance with the first
representative embodiment.
[0025] FIG. 4(B) depicts an exemplary plot of the differential
waveform derived from the plot of FIG. 4(A).
[0026] FIG. 5 is an oblique elevational view schematically
illustrating the method of measuring beam blur according to the
first representative embodiment.
[0027] FIG. 6 is a schematic plan view of a subfield of a reticle
defining multiple measurement marks that form respective measuring
beamlets.
[0028] FIG. 7 is a schematic plan view of a subfield of a reticle
defining multiple measurement marks that form respective measuring
beamlets and a dummy beam in accordance with the second
representative embodiment.
[0029] FIG. 8 is a schematic plan view of a plate illustrating
relative size and shape relationships of electron beamlets,
knife-edge reference marks, and the dummy beam in accordance with
the second representative embodiment.
[0030] FIG. 9 is a schematic plan view of a plate illustrating
relative size and shape relationships of electron beamlets and
respective knife-edge reflective reference marks in accordance with
the third representative embodiment.
[0031] FIG. 10 is a schematic plan view of a plate showing relative
size and shape relationships of two electron beamlets with
respective knife-edge reference marks in a first conventional
method for measuring imaging performance.
[0032] FIG. 11 is a schematic oblique view illustrating the first
conventional method for measuring imaging performance.
[0033] FIG. 12 is a schematic elevational view, with an
accompanying block diagram, of the first conventional method for
measuring imaging performance (corresponding to the method shown in
FIG. 11).
[0034] FIG. 13 is a schematic plan view of a plate showing relative
size and shape relationships of two electron beamlets with the
multiple reflective reference marks defined on the plate, in a
second conventional method for measuring imaging performance.
[0035] FIG. 14 is a schematic oblique view illustrating the second
conventional method for measuring imaging performance, using the
plate shown in FIG. 13.
DETAILED DESCRIPTION
[0036] This invention is described below in connection with
representative embodiments that are not intended to be limiting in
any way. Although the various embodiments are described in the
context of utilizing an electron beam as an exemplary charged
particle beam, the general principles set forth herein are
applicable with equal facility to the use of an alternative charged
particle beam, such as an ion beam.
[0037] First Representative Embodiment
[0038] FIGS. 1-6 illustrate a first representative embodiment of
the disclosed imaging-performance measurement methods and devices.
Turning first to FIG. 5, certain optical-system components of an
electron-beam microlithography apparatus are shown in the vicinity
of a wafer stage 16. At the upstream end of the depicted apparatus,
an illumination beam 12 is shown incident on a subfield 11 of a
divided reticle. The illumination beam 12 is emitted from an
upstream electron gun (not shown) and formed by an
illumination-optical system (not shown, but well understood to be
located between the electron gun and the subfield 11) so as to be
collimated as the beam is incident on the subfield 11. The subfield
11 defines multiple "measurement marks" as used for measuring beam
blur. The subfield 11 also can define a respective portion of an
actual lithographic pattern. The subfield 11 is situated at an
"object plane" of the depicted system. Although a single
measurement mark 13 is shown in FIG. 5, it is understood that
multiple measurement marks may be defined on the subfield 11 (e.g.,
nine). In FIG. 5 the measurement mark 13 is a rectangular aperture
(or through-hole) defined in a stencil-type reticle. It is
understood, however, that the measurement mark in this or any other
embodiment described herein may be defined on another type of
reticle used in CPB microlithography (e.g., a scattering-membrane
reticle). As the illumination beam 12 is incident on the
measurement mark 13, a portion of the beam passes through the mark
without experiencing any absorption or scattering of electrons. The
portion of the beam 12 transmitted through the measurement mark 13
is a collimated beamlet EB having a rectangular transverse
profile.
[0039] First and second projection lenses 14, 15, respectively,
define a two-stage projection-lens system disposed downstream of
the subfield 11. A contrast diaphragm 17 is situated between the
projection lenses 14, 15. The beamlet EB formed by the measurement
mark 13 in the subfield 11 is converged by the first projection
lens 14 to form a crossover CO in the center of an aperture 17a
defined by the contrast diaphragm 17. The contrast diaphragm 17
comprises a plate 17b that blocks electrons of the beamlet EB that
were forward-scattered during passage of the illumination beam
through the subfield 11 (i.e., only non-scattered electrons pass
through the aperture 17a).
[0040] A wafer stage 16 is situated downstream of the second
projection lens 15. The wafer stage 16 is configured to hold a
suitable "sensitive" lithographic substrate, such as a
semiconductor wafer. A plate 2 (not drawn to scale) is disposed on
the wafer stage 16 and is configured as a thin silicon film having
a thickness of about 2 .mu.m. Multiple knife-edge reference marks
are defined in the plate 2. The plate 2 on the wafer stage 16 is
located in a plane representing an "image plane" of the depicted
system. Mounted to the upstream-facing surface of the wafer stage
16 is a wafer chuck (not shown but well understood in the art) on
which a wafer or other suitable substrate (not shown) is mounted
for lithographic exposure.
[0041] Turning to FIG. 6, a subfield 11a used for measuring beam
blur along the X-direction is shown. The subfield 11a has an area
of about 1-mm square, for example. Rectangular apertures (or
through-holes) 13a-13i are formed uniformly across the subfield 11a
in an arrangement consisting of three rows and three columns. Each
of the apertures 13a-13i is oriented so that its major dimension
extends in the Y-direction. An additional pattern of separate
rectangular apertures for measuring beam blur in the Y-direction
(i.e., with major dimensions extending in the X-direction) also may
be defined on the subfield 11a.
[0042] FIG. 1 shows a plate 2 defining multiple knife-edge
reference marks 3a-3i according to this embodiment. The plate 2 is
illuminated by the beamlets EB produced by the subfield of FIG. 6.
The plate 2 is positioned at the image plane of the electron-beam
microlithography apparatus and has a size substantially equal to
the size of an image transferred from an upstream subfield (e.g.,
250-.mu.m square). Nine rectangular beamlets EBa-EBi are shown as
projected onto the plate 2 in three equally spaced columns and
three equally spaced rows. Nine reference marks 3a-3i are disposed
on the plate 2 in three columns and three rows. Each of the marks
3a-3i has a major dimension extending in the Y-direction and that
is slightly larger than the major dimension of the corresponding
beamlet. The position of each of the marks 3a-3i relative to the
respective beamlets EBa-EBi is shifted progressively in the
positive X-direction for each successive mark (e.g., the reference
mark 3b has an X-direction position relative to the beamlet EBb
that is slightly greater than the X-direction position of the
reference mark 3a relative to the beamlet EBa, etc.). For each
successive mark, the increase of the relative distance may be
greater than or equal to the minor dimension of the previous
reference mark. Thus, the respective marks 3a-3i are positioned so
that, whenever any one of the beamlets EBa-EBi overlaps its
respective reference mark, no other beamlet is simultaneously
overlapping its respective reference mark. Similar reference marks
may be positioned on the plate 2 for measuring beam blur in the
Y-direction.
[0043] FIG. 2(A) is a schematic elevational view of the
imaging-performance measurement method of the first representative
embodiment. Beamlets EBa-EBc are formed by passage of the electron
beam through respective apertures of the upstream subfield (see
FIG. 6) and are incident on the plate 2. The reference marks 3a-3c
of FIG. 1 are shown defining apertures on the plate 2. Each of the
reference marks 3a-3c defines a respective knife-edge 1a-1c. As the
beamlet EBa is scanned over the knife-edge 1a, the electrons e1 of
the beamlet EBa pass through the reference mark 3a without
scattering and propagate downstream.
[0044] A first beam-limiting diaphragm 4 with beam-limiting
apertures 4a-4c that are aligned with the respective reference
marks 1a-1c may be positioned immediately downstream of the plate
2. Although FIG. 2(A) shows only three beam-limiting apertures
4a-4c, the number of such apertures defined by the diaphragm 4
desirably equals the number of reference marks on the plate 2. In
this example, for instance, the diaphragm 4 has nine apertures
4a-4i corresponding to the respective nine reference marks 3a-3i of
the plate 2. The first beam-limiting diaphragm 4 desirably is made
of a conductive metal and is sufficiently thick (e.g., 1 mm) to
ensure absorption of incident beamlets. Desirably, each of the
apertures 4a-4i has a width of approximately 10 .mu.m. The axial
distance between the plate 2 and the first beam-limiting diaphragm
4 is such that the nominal angle .theta. is slightly greater than
the beam-convergence angle .alpha. of the beamlets EBa-EBi as they
pass through the apertures (see FIG. 2(B)). For instance, if the
beam-convergence angle .alpha. of the beam is 5 mrad, the axial
distance between the plate 2 and the first beam-limiting diaphragm
4 might be 1 mm. Thus, when the beamlets EBa-EBi are scanned over
the respective knife-edge reference marks 3a-3i, the electrons e1
passing through the reference marks also pass through the
respective beam-limiting apertures 4a-4i. On the other hand,
electrons e2 that are forward-scattered by the plate 2 or from a
dummy beam (not shown) used to impart a space-charge effect are
blocked almost completely by the first beam-limiting diaphragm 4.
As a result, mostly non-scattered electrons e1 are transmitted
downstream of the first beam-limiting diaphragm 4, thereby
enhancing the contrast of beam-blur measurements performed using
the device.
[0045] A second beam-limiting diaphragm 5 may be disposed at a
position downstream of the first beam-limiting diaphragm 4. By way
of example, this position may be approximately 10 mm to 20 mm below
the first beam-limiting diaphragm 4. The second beam-limiting
diaphragm 5 desirably is made of a conductive metal plate that is
sufficiently thick (e.g., 1 mm) to ensure absorption of incident
electrons. An aperture 5a is defined centrally in the second
beam-limiting diaphragm 5 and has an opening dimension (e.g.,
diameter) sufficient to block forward-scattered electrons not
blocked by the first beam-limiting diaphragm 4 while not blocking
non-scattered electrons. For instance, the aperture 5a can have
dimensions of approximately 200 .mu.m to 500 .mu.m. As a result,
the contrast of the beam-blur measurements is further
increased.
[0046] An electron detector (sensor) 6 is situated downstream of
the plate 2 and the beam-limiting apertures 4a-4i, 5a. The electron
detector 6 desirably comprises a combination of a photomultiplier
and a scintillator, a Faraday cup, or a semiconductor detector. The
electron detector 6 is connected to a pre-amplifier 7, a
differentiation circuit 8, and an oscilloscope (or analogous
display) 9.
[0047] FIGS. 3(A)-3(D) illustrate the manner in which an exemplary
beamlet EB is measured by the electron detector 6 and displayed on
the oscilloscope 9. As shown in FIG. 3(A), the beamlet EB is
scanned over the knife-edge 1 of the reference mark 3 on the plate
2 in the direction of the "SCAN" arrow (i.e., to the right in the
figure). As the beamlet EB passes over the knife-edge 1, the
proportion of the beamlet EB propagating past (downstream of) the
knife-edge progressively increases, indicated by a corresponding
increase in beam current detected by the detector 6. As the
scanning of the beamlet EB through the reference mark continues,
the proportion of the beamlet EB propagating through the reference
mark 3 peaks and eventually decreases, indicated by a corresponding
decrease in beam current detected by the detector 6. Thus, as is
shown in FIG. 3(B), a plot of the transmitted beam current detected
by the detector 6 has a peaked profile. This beam-current
transmission is amplified by the pre-amplifier 7 and converted by
the differentiation circuit 8 to a plot of percentage change versus
time. An exemplary differential waveform W2 output from the
differentiation circuit 8 is shown in FIG. 3(C). Ideally, the
differential waveform W2 has a rectangular profile W1 assuming the
beamlet EB has no blur. In practice, however, the actual
differential waveform W2 has sloped sides resulting from beam blur.
As can be seen in FIG. 3(D), the distance "t" that is used to
quantify beam blur is measured from where the beam intensity of the
differential waveform is 12% of maximum to where the beam intensity
is 88% of maximum.
[0048] As discussed above, the respective positions of the nine
reference marks 3a-3i on the knife-edge plate 2 are shifted
progressively along the X-direction (see FIG. 1). Thus, whenever
the measurement beamlets EBa-EBi are scanned over the respective
reference marks, the resulting beamlets propagate through the
respective apertures 1a-1i one after another, thereby forming a
series of nine contiguous, yet separate, beam-current transmissions
that are detected by the detector 6. Differential processing can be
performed on each of the transmissions so as to create nine
respective differentiation waveforms that can be analyzed to
measure the beam blur at different respective regions of the
subfield. FIG. 4(A) shows nine exemplary beam-current waveforms
corresponding to respective beam-current transmissions passing
through each of the nine reference marks 3a-3i. The beam-current
waveforms can be amplified and converted into plots of percentage
change versus time by the differentiation circuit 8. The resulting
differentiation waveforms are shown in FIG. 4(B). These
differentiation waveforms may be displayed on the oscilloscope 9
(see FIG. 2(A)). Beam adjustment (e.g., calibration of focal point,
astigmatism, magnification, rotation, and/or other parameters) and
evaluation of imaging performance may be performed on the basis of
these differentiation waveforms. Beam adjustments may be made so as
to minimize maximum beam blur or to minimize the overall variation
in beam blur (.DELTA.blur) within the subfield.
[0049] Second Representative Embodiment
[0050] FIGS. 7-8 illustrate a second representative embodiment of
the disclosed methods and devices. In general, the second
representative embodiment is characterized by the use of a dummy
beam, which is defined by the upstream subfield and projected
adjacent to the measuring beamlets. The dummy beam exerts a Coulomb
force on the measurement beamlets, thereby allowing beam blur to be
measured under actual exposure conditions.
[0051] FIG. 7 shows a reticle subfield 21 that illustrates the size
and shape relationships of the patterns used to define the
measuring beamlets and the dummy beam. The subfield 21 includes an
outer area 21a in the shape of a square frame, a center area 21b,
and linking members 21c disposed around the center area to link the
center area 21b with the outer area 21a. In the illustrated
subfield 21, three linking members 21c per single side of the
center area 21b are shown, although it is understood that other
arrangements are possible. A large aperture (or through-hole) 21d
is formed between the outer area 21a and the center area 21b, and
defines the dummy beam. Rectangular apertures 23a-23i are formed at
nine respective locations on the subfield 21 (eight in the outer
area 21a and one in the center area 21b). Each of the apertures
23a-23i is oriented so that its major dimension is aligned with the
Y-axis.
[0052] FIG. 8 shows the pattern from the subfield 21 as projected
onto the plate 2. The portions of the illumination beam passing
through the apertures 23a-23i on the subfield 21 form respective
rectangular beamlets EBa-EBi. Similarly, the portions of the
illumination beam passing through the aperture 21d forms a dummy
beam EB2. The dummy beam EB2 alters the beam current of each of the
beamlets EBa-EBi so that the beam blur in the beamlets can be
corrected for space-charge effects. I.e., the dummy beam EB2 exerts
a Coulomb force on each of the measurement beamlets EBa-EBi, which
allows beam blur to be evaluated while taking into account
space-charge effects.
[0053] Third Representative Embodiment
[0054] FIG. 9 illustrates a third representative embodiment of the
disclosed methods and devices. In general, the third representative
embodiment is similar to any of the embodiments described above
except that, in the third representative embodiment, the reference
marks disposed on the plate are made of an electron-reflective
material. Also, in this embodiment, beam blur is measured by
detectors positioned opposite respective reference marks so as to
detect backscattering of the incident beamlets from the reference
marks. As is shown in FIG. 9, nine rectangular beamlets EBa-EBi are
incident on a plate 32 as a result of passage of an illumination
beam through respective apertures 13a-13i in an upstream subfield
11 (see FIG. 6). The rectangular beamlets EBa-EBi are distributed
uniformly across the plate 32. Reference marks 31a-31i are disposed
in three columns and three rows on the plate 32. Each of the marks
31a-31i is defined by a respective unit of thin film of heavy metal
(e.g., Ta or W) and has a major dimension oriented in the
Y-direction. Each major dimension is slightly larger than the major
dimension of the respective beamlet EBa-EBi. Like the reference
marks described above with respect to FIG. 1, the relative
distances between the reference marks 31a-31i and their respective
beamlets EBa-EBi progressively increases for each successive mark.
In addition to the reference marks 31a-31i, which are used for
measuring beam blur along the X-direction, the plate 32 may include
nine reference marks (not shown) having major dimensions oriented
in the X-direction. These additional reference marks are used for
measuring the beam blur in the Y-direction. The X-direction
reference marks and the Y-direction reference marks may be disposed
collectively on a single plate 32.
[0055] Whereas the invention has been described in connection with
multiple representative embodiments, it will be understood that the
invention is not limited to those embodiments. On the contrary, the
invention is intended to encompass all modifications, alternatives,
and equivalents as may be included within the spirit and scope of
the invention, as defined by the appended claims.
* * * * *