U.S. patent application number 11/591673 was filed with the patent office on 2007-07-26 for lithographic apparatus and device manufacturing method.
This patent application is currently assigned to ASML NETHERLANDS B.V.. Invention is credited to Gerardus Wilhelmus Petrus Baas, Hako Botma, Hendrik Antony Johannes Neerhof, Marius Ravensbergen.
Application Number | 20070170376 11/591673 |
Document ID | / |
Family ID | 37854154 |
Filed Date | 2007-07-26 |
United States Patent
Application |
20070170376 |
Kind Code |
A1 |
Neerhof; Hendrik Antony Johannes ;
et al. |
July 26, 2007 |
Lithographic apparatus and device manufacturing method
Abstract
An attenuation adjustment device is disclosed that includes a
plurality of members configured to cast penumbras in a radiation
beam illuminating a patterning device in a lithography apparatus.
Furthermore, an attenuation control device may be provided to
adjust the members in such a manner as to control attenuation of a
radiation beam projected onto a target portion of a substrate
across the cross-section of the radiation beam.
Inventors: |
Neerhof; Hendrik Antony
Johannes; (Eindhoven, NL) ; Botma; Hako;
(Eindhoven, NL) ; Ravensbergen; Marius; (Bergeijk,
NL) ; Baas; Gerardus Wilhelmus Petrus; (Weert,
NL) |
Correspondence
Address: |
PILLSBURY WINTHROP SHAW PITTMAN, LLP
P.O. BOX 10500
MCLEAN
VA
22102
US
|
Assignee: |
ASML NETHERLANDS B.V.
Veldhoven
NL
|
Family ID: |
37854154 |
Appl. No.: |
11/591673 |
Filed: |
November 2, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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11224303 |
Sep 13, 2005 |
|
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|
11591673 |
Nov 2, 2006 |
|
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Current U.S.
Class: |
250/504R |
Current CPC
Class: |
G03F 7/70083 20130101;
G03F 7/70191 20130101 |
Class at
Publication: |
250/504.00R |
International
Class: |
G01J 3/10 20060101
G01J003/10 |
Claims
1. A lithographic apparatus, comprising: an illumination system
configured to condition a radiation beam; a support constructed to
hold a patterning device, the patterning device being constructed
to impart a cross-sectional pattern to the radiation beam to form a
patterned radiation beam; a substrate table constructed to hold a
substrate; a projection system configured to project the patterned
radiation beam onto a target portion of the substrate; and an
attenuation adjustment device comprising a plurality of members
configured to cast penumbras in the radiation beam illuminating the
patterning device, each member configured to increase or decrease
its penumbra by displacement of a portion of the member in a
direction substantially perpendicular to an axis of the member.
2. The apparatus of claim 1, wherein each member comprises a
central structure along the axis, a first structure movable along
the central structure, and a flexible material attached to the
first structure, wherein the portion of the member comprises the
flexible material and movement of the first structure causes
displacement of the flexible material in the direction.
3. The apparatus of claim 2, wherein the first structure is
attached to one end of the flexible material and another end of the
flexible material is fixed relative the central structure at a
position displaced from the first structure, the flexible material
extending between the first structure and the position along at
least a portion of the central structure.
4. The apparatus of claim 2, wherein the first structure is
attached to one end of the flexible material and further comprising
a second structure movable along the central member to which
another end of the flexible material is attached.
5. The apparatus of claim 4, wherein both the first structure and
the second structure are movable in a same direction to displace
the portion of the member in a direction substantially parallel to
the axis.
6. The apparatus of claim 4, wherein the first structure and the
second structure are movable towards each other to cause
displacement of the flexible material in the direction.
7. The apparatus of claim 1, wherein at least one of the plurality
of members is further configured displace the portion of the member
in a direction substantially parallel to the axis of the
member.
8. The apparatus of claim 1, further comprising a scanning system
configured to provide relative movement between the radiation beam
and the target portion of the substrate in a scanning direction,
the members being distributed along a path transverse to the
scanning direction.
9. The apparatus of claim 8, wherein the scanning system comprises
a slit extending across the path through which the radiation beam
is to be projected onto the target portion of the substrate, and
further comprising an attenuation control device arranged to adjust
the members by different amounts in such a manner that an intensity
of the radiation beam is substantially constant over a length of
the slit.
10. The apparatus of claim 8, further comprising an attenuation
control device arranged to adjust the members in such a manner as
to permit an intensity of the radiation beam projected onto the
target portion of the substrate to be varied in a direction
transverse to the scanning direction during the scanning.
11. The apparatus of claim 8, further comprising an attenuation
control device arranged to adjust the members in such a manner as
to permit an intensity of the radiation beam projected onto the
target portion of the substrate to be varied both in the scanning
direction and in a direction transverse to the scanning
direction.
12. The apparatus of claim 1, further comprising a position
detector configured to provide an output indicative of a position
of each member in dependence on detection of a beam of detecting
radiation reaching the position detector after attenuation or
redirection by the member.
13. The apparatus of claim 12, further comprising an attenuation
control device configured to use feedback control to supply a
control signal to at least one of the members to drive that member
to an adjustment position according to the output indicative of the
position of that member received from the position detector.
14. The apparatus of claim 12, further comprising a detection vane
portion of each member spaced from the portion of the member, the
detection vane portion of each member configured to attenuate the
beam of detecting radiation detected by the position detector.
15. A lithographic apparatus, comprising: an illumination system
configured to condition a radiation beam; a support constructed to
hold a patterning device, the patterning device being constructed
to impart a cross-sectional pattern to the radiation beam to form a
patterned radiation beam; a substrate table constructed to hold a
substrate and move the substrate in a scanning direction; a
projection system configured to project the patterned radiation
beam onto a target portion of the substrate; an attenuation
adjustment device comprising a plurality of members configured to
cast penumbras in the radiation beam illuminating the patterning
device; and an attenuation control device configured to adjust the
members so as to control attenuation of the radiation beam, during
scanning projection of the patterned radiation beam, in the
scanning direction across the radiation beam and in a second
direction across the radiation beam substantially perpendicular to
the scanning direction, the attenuation control device comprising a
respective position detector configured to provide an output
indicative of position of each member in dependence on detection of
a beam of detecting radiation reaching the position detector after
attenuation by the member.
16. The apparatus of claim 15, wherein each member is configured to
increase or decrease its penumbra by displacement of a portion of
the member in a direction substantially perpendicular to an axis of
the member.
17. The apparatus of claim 16, wherein at least one of the
plurality of members is further configured displace the portion of
the member in a direction substantially parallel to the axis of the
member.
18. The apparatus of claim 15, further comprising a slit extending
across the path through which the radiation beam is to be projected
onto the target portion of the substrate, and wherein the
attenuation control device is arranged to adjust the members by
different amounts in such a manner that an intensity of the
radiation beam is substantially constant over a length of the
slit.
19. A device manufacturing method, comprising: casting penumbras on
a patterning device using a plurality of members in the path of the
radiation beam, each member configured to increase or decrease its
penumbra by displacement of a portion of the member in a direction
substantially perpendicular to an axis of the member; imparting a
cross-sectional pattern to the radiation beam using the patterning
device to form a patterned radiation beam; and projecting the
patterned radiation beam onto a target portion of a substrate.
20. The method of claim 19, wherein at least one of the plurality
of members is further configured displace the portion of the member
in a direction substantially parallel to the axis of the member.
Description
[0001] This application is a continuation-in-part application of
co-pending U.S. patent application Ser. No. 11/224,303, filed Sep.
13, 2005, which is incorporated herein in its entirety by
reference.
FIELD
[0002] The present invention relates to a lithographic apparatus
and a device manufacturing method.
BACKGROUND
[0003] A lithographic apparatus is a machine that applies a desired
pattern onto a target portion of a substrate. Lithographic
apparatus can be used, for example, in the manufacture of
integrated circuits (ICs). In that instance, a patterning device,
which is alternatively referred to as a mask or a reticle, may be
used to generate a circuit pattern corresponding to an individual
layer of the IC, and this pattern can be imaged onto a target
portion (e.g. comprising part of, one or several dies) of a
substrate (e.g. a silicon wafer) that has a layer of
radiation-sensitive material (resist). In general, a single
substrate will contain a network of adjacent target portions that
are successively exposed. Lithographic apparatus may be of the
transmissive type, where radiation is passed through a patterning
device to generate the pattern, or of the reflective type, where
radiation is reflected from the patterning device to generate the
pattern. Known lithographic apparatus include so-called steppers,
in which each target portion is irradiated by exposing an entire
pattern onto the target portion at once, and so-called scanners, in
which each target portion is irradiated by scanning the pattern
through the beam in a given direction (the scanning-direction)
while synchronously scanning the substrate parallel or
anti-parallel to this direction. It is also possible to transfer
the pattern from the patterning device to the substrate by
imprinting the pattern onto the substrate.
[0004] In general, there is a non-uniformity in the intensity of
radiation which is imaged onto the substrate in such apparatus.
This is typically caused by, for example, the mirrors or lenses of
the projection system having differing reflectivity or transmission
over their surfaces. In the case of conventional lithography,
so-called deep-UV (DUV), a transmissive filter is included which
corrects for this non-uniformity. In the past the properties of the
filter were fixed and could not be changed over time. In newer
systems the filter is adjustable, and can be adjusted to take
account of slow variations in beam uniformity, for example caused
by gradual degradation of lens surfaces.
[0005] A known adjustable uniformity correction unit for DUV
comprises two transmissive plates which are considerably bigger
than the beam. Different transmission profiles are provided on the
plates, so that, when the transmission of the plate is to be
adjusted, the point at which the beam intercepts the plate is
changed by moving the plate. The plates are made from glass and are
heavy, consequently their movement is slow. In any event, they are
designed and intended to be used to correct for very slow
variations.
[0006] In extreme ultraviolet (EUV) lithography, there are no
materials available which can be used in a transmissive way.
Accordingly an arrangement is disclosed in U.S. Pat. No. 6,741,329
in which non-transmissive blades, commonly called venetian blinds
(`blades`), are used to adjust the beam to correct for
non-uniformity in the intensity of radiation imaged onto the
substrate. In the simplest case the blades are in the form of a
series of rectangles that are rotatably mounted and are spread
across the beam. In more complicated cases the blades can have a
more complicated (`asymmetric blades`) shape. In order to reduce
the beam intensity in a given location, the blade at that location
is rotated so that it partially blocks the beam. The blades are
typically located a distance D.gtoreq.B/tan(a sin(NA)) mm below the
reticle where B is the distance between the blades and NA is the
numerical aperture at reticle level. If the blades were to be
located closer to the reticle, then sharp images of the blade edges
would appear on the substrate. Conversely, if the blades were to be
moved further away from the reticle, then the spatial frequency of
the intensity correction provided by the blades would be
reduced.
[0007] The blade arrangement of U.S. Pat. No. 6,741,329 may not
allow the uniformity or the intensity of the radiation incident on
the substrate to be varied in the direction in which the substrate
is scanned by the beam during a scan. Instead the energy per laser
pulse is varied during the scan to generate a varying intensity
profile in the scanning direction. However, unlike DUV lithography
sources, EUV lithography sources are not configured to change their
output power, and there is therefore no simple way in which the
overall intensity of the beam incident on the substrate can be
varied.
SUMMARY
[0008] An aspect of one or more embodiments of the present
invention is to provide a novel lithographic apparatus enabling
control of the intensity of the beam incident on the substrate.
[0009] According to an aspect of the invention, there is provided a
lithographic apparatus comprising: [0010] an illumination system
configured to condition a radiation beam; [0011] a support
constructed to hold a patterning device, the patterning device
being constructed to impart a cross-sectional pattern to the
radiation beam to form a patterned radiation beam; [0012] a
substrate table constructed to hold a substrate; [0013] a
projection system configured to project the patterned radiation
beam onto a target portion of the substrate; and [0014] an
attenuation adjustment device comprising a plurality of members
configured to cast penumbras in the radiation beam illuminating the
patterning device, each member configured to increase or decrease
its penumbra by displacement of a portion of the member in a
direction substantially perpendicular to an axis of the member.
[0015] According to an aspect of the invention, there is provided a
lithographic apparatus, comprising: [0016] an illumination system
configured to condition a radiation beam; [0017] a support
constructed to hold a patterning device, the patterning device
being constructed to impart a cross-sectional pattern to the
radiation beam to form a patterned radiation beam; [0018] a
substrate table constructed to hold a substrate and move the
substrate in a scanning direction; [0019] a projection system
configured to project the patterned radiation beam onto a target
portion of the substrate; [0020] an attenuation adjustment device
comprising a plurality of members configured to cast penumbras in
the radiation beam illuminating the patterning device; and [0021]
an attenuation control device configured to adjust the members so
as to control attenuation of the radiation beam, during scanning
projection of the patterned radiation beam, in the scanning
direction across the radiation beam and in a second direction
across the radiation beam substantially perpendicular to the
scanning direction, the attenuation control device comprising a
respective position detector configured to provide an output
indicative of position of each member in dependence on detection of
a beam of detecting radiation reaching the position detector after
attenuation by the member.
[0022] According to an aspect of the invention, there is provided a
device manufacturing method, comprising: [0023] casting penumbras
on a patterning device using a plurality of members in the path of
the radiation beam, each member configured to increase or decrease
its penumbra by displacement of a portion of the member in a
direction substantially perpendicular to an axis of the member;
[0024] imparting a cross-sectional pattern to the radiation beam
using the patterning device to form a patterned radiation beam; and
[0025] projecting the patterned radiation beam onto a target
portion of a substrate.
[0026] According to an aspect of the present invention, there is
provided a lithographic apparatus comprising: [0027] an
illumination system configured to condition a radiation beam;
[0028] a support constructed to hold a patterning device, the
patterning device being constructed to impart a cross-sectional
pattern to the radiation beam to form a patterned radiation beam;
[0029] a substrate table constructed to hold a substrate; a
projection system configured to project the patterned radiation
beam onto a target portion of the substrate; [0030] an attenuation
adjustment device comprising a plurality of members configured to
cast penumbras in the radiation beam illuminating the patterning
device; and [0031] an attenuation control device configured to
adjust the members so as to control attenuation of the radiation
beam across the cross-section of the radiation beam, the
attenuation control device comprising a respective position
detector configured to provide an output indicative of position of
each member in dependence on detection of a beam of detecting
radiation reaching the position detector after attenuation by the
member, [0032] wherein the attenuation control device comprises a
common radiation source configured to generate beams of detecting
radiation to detect positions of the members.
[0033] Thus the attenuation control device, which is typically in
the form of a series of venetian blinds (the `blades`), can be used
to correct for non-uniformity to a high level of accuracy, and to
decrease the overall intensity of the beam. This is useful because,
as previously indicated, EUV lithography sources are not configured
to change their output power. Because a single source is used to
supply detecting radiation to all of the position detectors, the
number of components is decreased, resulting in lower cost, more
space, greater reliability and less heat generation coupled with
better possibilities for cooling. Also inaccuracies due to
fluctuations or temperature variations are largely avoided, and the
mechanical adjustment of the arrangement becomes more
straightforward.
[0034] According to a further aspect of the present invention,
there is provided a lithographic apparatus comprising: [0035] an
illumination system configured to condition a radiation beam;
[0036] a support constructed to hold a patterning device, the
patterning device being constructed to impart a cross-sectional
pattern to the radiation beam to form a patterned radiation beam;
[0037] a substrate table constructed to hold a substrate; [0038] a
projection system configured to project the patterned radiation
beam onto a target portion of the substrate; [0039] an intensity
adjustment device comprising a plurality of members configured to
cast penumbras in the radiation beam illuminating the patterning
device, at least one of the members having a non-rectangular shape;
and [0040] an attenuation control device configured to adjust the
members so as to control attenuation of the radiation beam across
the cross-section of the radiation beam.
[0041] The attenuation control device may comprise a reference
detector configured to provide a reference output in dependence on
detection of a beam of detecting radiation reaching the reference
detector directly from the common radiation source. The reference
detector directly detects the radiation from the source and
provides a reference output signal so that fluctuations of the
radiation source, due to thermal drift, for example, can be
compensated in an electronic control circuit.
[0042] In some embodiments the attenuation control device comprises
a mixing unit configured to receive detecting radiation from the
common radiation source and to emit a respective beam of detecting
radiation through a respective aperture in the unit towards each of
the members. In an embodiment, the mixing unit has reflective walls
configured to multiply reflect detecting radiation from the common
radiation source towards the aperture. In this manner the
characteristics of the radiation from the radiation source are
scrambled due to the multiple internal reflections within the
mixing unit and the amount of radiation passing through each
aperture is dependent on the geometry of the mixing unit and is
substantially unaffected by the source strength or other
characteristics of the source.
[0043] The attenuation control device conveniently comprises a
detection vane portion of each member spaced from a blade part of
the member configured to cast a penumbra in the radiation beam
illuminating the patterning device, the detection vane portion of
each member configured to attenuate the beam of detecting radiation
detected by the associated position detector. Although it is
desired that the detection vane portion is a separate portion of
the member to the blade part configured to cast a penumbra in the
radiation beam, preferably being disposed on a common shaft to the
blade part, it would also be possible for the detection vane
portion to be constituted by the same part of the member as the
blade part.
[0044] In an embodiment, the apparatus includes a scanning system
configured to provide relative movement between the radiation beam
and the target portion of the substrate in a scanning direction,
the members being distributed along a path transverse to the
scanning direction. In this case the scanning system may comprise a
curved slit extending along the path through which the radiation
beam is projected onto the target portion of the substrate, and the
attenuation control device may be adapted to adjust the members by
different amounts in such a manner that the intensity of the
radiation beam is substantially constant over the length of the
slit.
[0045] The attenuation control device may be arranged to adjust the
members in such a manner as to permit the intensity of the
radiation beam projected onto the target portion of the substrate
to be varied both in the scanning direction and in a direction
transverse to the scanning direction.
[0046] Furthermore the attenuation control device may be configured
to use feedback control to supply a control signals to at least one
of the members to drive that member to the an adjustment position
according to the output indicative of the position of that member
received from the position detector.
[0047] The members of the attenuation control device are typically
a series of blades that are tiltable about tilt axes so as to
adjust the widths of the penumbras that they cast and are disposed
with their tilt axes substantially parallel to one another. In the
case in which the members of the attenuation control device are in
the form of venetian blinds, since the blades are very small and
light, they may be rotated quickly and therefore may be used to
provide real time uniformity correction. In general, during
exposure of a target (die), radiation is scanned across the target
in the Y direction. In a prior art arrangement, the non-uniformity
previously measured in the Y direction could be corrected for by
adjusting the intensity of the illumination provided by the DUV
laser (laser pulse energy control). However, in an embodiment of
the present invention, the venetian blinds blades are used in real
time to adjust the uniformity. For example, during exposure of a
target, the blades may be progressively rotated to compensate for a
previously measured ramp in the exposure intensity (as previously
mentioned, the intensity of the radiation generated by the EUV
source cannot be adjusted). In addition to this, the blades may be
adjusted in advance to take account of variation across the X
direction, and are fixed during the scan. In an alternative or
additional embodiment the positions of the blades may be varied
during the scan to take account of variation in the X direction
during the scan.
[0048] In general it is desired to make the intensity of the
radiation incident on the substrate as uniform as possible, and to
keep the same uniform intensity across the entire substrate.
However, other processes that are outside the user's control, such
as chemical processing of the substrate, may have an effect which
varies for different locations on the substrate. Typically, there
may be a difference between the center of the substrate and the
edge of the substrate. An embodiment of the present invention
allows for effect of these processes to be measured and then for
the intensity of the beam to be adjusted to correct for this. For
example, the intensity of illumination at the edge of the substrate
may be controlled to be greater than the intensity of the
illumination at the center of the substrate, to take account of
differences in processing that occur at the edge of the substrate
compared to the center of the substrate.
[0049] However, what would be even more useful would be to be able
to adjust in the Y direction and also in the X direction. An
embodiment of the present invention allows this to be done. Using
an embodiment of the invention, specific areas of the die may be
given, for example, a lower dose than other areas of the die.
BRIEF DESCRIPTION OF THE DRAWINGS
[0050] Embodiments of the invention will now be described, by way
of example only, with reference to the accompanying schematic
drawings in which corresponding reference symbols indicate
corresponding parts and in which:
[0051] FIG. 1 diagrammatically shows a lithographic apparatus
having a reflective patterning device;
[0052] FIG. 2 is a perspective view of an attenuation control
device;
[0053] FIG. 3 is a diagram of a control arrangement for controlling
the positions of the blades of an attenuation control device used
in a lithographic apparatus in an embodiment of the invention;
[0054] FIG. 4 shows a detail of the attenuation control device;
[0055] FIG. 5 is an explanatory diagram indicating the difference
in the radiation received by the angle detector depending on the
angle of the blade;
[0056] FIG. 6 is a diagram showing parts of the control arrangement
of the attenuation control device;
[0057] FIG. 7 shows several shapes of attenuation blade: (a)
rectangular (symmetric), (b) asymmetric, (c) partially
transmissive, (d) compound, and (e) 3-dimensional; and
[0058] FIG. 8 illustrates an alternate embodiment of the present
invention;
[0059] FIGS. 9(a) and (c) are top views of an attenuation structure
of an attenuation control device in respectively two different
operational conditions;
[0060] FIGS. 9(b) and (d) are respective side views of the
attenuation structure depicted in FIGS. 9(a) and (c);
[0061] FIG. 10 is a top view of an embodiment of an attenuation
control device, comprising a plurality of attenuation structures,
positioned in relation to an illumination field; and
[0062] FIG. 11 is a top view of another embodiment of an
attenuation control device, comprising a plurality of attenuation
structures, positioned in relation to an illumination field.
[0063] While the invention is susceptible to various modifications
and alternative forms, specific embodiments thereof have been shown
by way of example in the drawings and will herein be described in
detail. It should be understood, however, that this specification
is not intended to limit the invention to the particular forms
disclosed herein, but on the contrary, the intention is to cover
all modifications, equivalents, and alternatives falling within the
scope of the invention, as defined by the appended claims.
DETAILED DESCRIPTION
[0064] FIG. 1 schematically depict an example of a lithographic
apparatus. The apparatus includes:
[0065] an illumination system (illuminator) IL configured to
provide a beam PB of radiation (e.g. UV radiation);
[0066] a support structure (e.g. a mask table) MT configured to
hold a patterning device (e.g. a mask) MA and connected to first
positioner PM configured to accurately position the patterning
device with respect to item PL;
[0067] a substrate table (e.g. a wafer table) WT configured to hold
a substrate (e.g. a resist-coated wafer) W and connected to a
second positioner PW configured to accurately position the
substrate with respect to item PL; and
[0068] a projection system (e.g. a refractive or reflective
projection lens) PL configured to image a pattern imparted to the
beam PB by the patterning device MA onto a target portion C (e.g.
comprising one or more dies) of the substrate W.
[0069] The illumination system may include various types of optical
components, such as refractive, reflective, magnetic,
electromagnetic, electrostatic or other types of optical
components, or any combination thereof, for directing, shaping, or
controlling radiation.
[0070] The support structure holds the patterning device in a
manner that depends on the orientation of the patterning device,
the design of the lithographic apparatus, and other conditions,
such as for example whether or not the patterning device is held in
a vacuum environment. The support structure can use mechanical,
vacuum, electrostatic or other clamping techniques to hold the
patterning device. The support structure may be a frame or a table,
for example, which may be fixed or movable as required. The support
structure may ensure that the patterning device is at a desired
position, for example with respect to the projection system. Any
use of the terms "reticle" or "mask" herein may be considered
synonymous with the more general term "patterning device".
[0071] The term "patterning device" used herein should be broadly
interpreted as referring to any device that can be used to impart a
radiation beam with a pattern in its cross-section such as to
create a pattern in a target portion of the substrate. It should be
noted that the pattern imparted to the radiation beam may not
exactly correspond to the desired pattern in the target portion of
the substrate, for example if the pattern includes phase-shifting
features or so called assist features. Generally, the pattern
imparted to the radiation beam will correspond to a particular
functional layer in a device being created in the target portion,
such as an integrated circuit.
[0072] The patterning device may be transmissive or reflective.
Examples of patterning devices include masks, programmable mirror
arrays, and programmable LCD panels. Masks are well known in
lithography, and include mask types such as binary, alternating
phase-shift, and attenuated phase-shift, as well as various hybrid
mask types. An example of a programmable mirror array employs a
matrix arrangement of small mirrors, each of which can be
individually tilted so as to reflect an incoming radiation beam in
different directions. The tilted mirrors impart a pattern in a
radiation beam which is reflected by the mirror matrix.
[0073] The term "projection system" used herein should be broadly
interpreted as encompassing any type of projection system,
including refractive, reflective, catadioptric, magnetic,
electromagnetic and electrostatic optical systems, or any
combination thereof, as appropriate for the exposure radiation
being used, or for other factors such as the use of an immersion
liquid or the use of a vacuum. Any use of the term "projection
lens" herein may be considered as synonymous with the more general
term "projection system".
[0074] As here depicted, the apparatus is of a reflective type
(e.g. employing a programmable mirror array of a type as referred
to above, or employing a reflective mask). Alternatively, the
apparatus may be of a transmissive type.
[0075] The lithographic apparatus may be of a type having two (dual
stage) or more substrate tables (and/or two or more support
structures). In such "multiple stage" machines the additional
tables may be used in parallel, or preparatory steps may be carried
out on one or more tables while one or more other tables are being
used for exposure.
[0076] The lithographic apparatus may also be of a type wherein at
least a portion of the substrate may be covered by a liquid having
a relatively high refractive index, e.g. water, so as to fill a
space between the projection system and the substrate. An immersion
liquid may also be applied to other spaces in the lithographic
apparatus, for example, between the mask and the projection system.
Immersion techniques are well known in the art for increasing the
numerical aperture of projection systems. The term "immersion" as
used herein does not mean that a structure, such as a substrate,
must be submerged in liquid, but rather only means that liquid is
located between the projection system and the substrate during
exposure.
[0077] Referring to FIG. 1, the illuminator IL receives a radiation
beam from a radiation source SO. The source and the lithographic
apparatus may be separate entities, for example when the source is
an excimer laser, as may be the case for transmissive apparatus. In
such cases, the source is not considered to form part of the
lithographic apparatus and the radiation beam is passed from the
source SO to the illuminator IL with the aid of a beam delivery
system BD comprising, for example, suitable directing mirrors
and/or a beam expander. In other cases the source may be an
integral part of the lithographic apparatus, for example when the
source is a mercury lamp. The source SO and the illuminator IL,
together with the beam delivery system BD if required, may be
referred to as a radiation system.
[0078] The illuminator IL may comprise an adjuster for adjusting
the angular intensity distribution of the radiation beam.
Generally, at least the outer and/or inner radial extent (commonly
referred to as .sigma.-outer and .sigma.-inner, respectively) of
the intensity distribution in a pupil plane of the illuminator can
be adjusted. In addition, the illuminator IL may comprise various
other components, such as an integrator and a condenser. The
illuminator provides a conditioned beam of radiation, referred to
as the beam PB, having a desired uniformity and intensity
distribution in its cross-section.
[0079] The radiation beam PB is incident on the patterning device
(e.g., mask) MA, which is held on the support structure (e.g., mask
table) MT, and is patterned by the patterning device. Having
traversed the patterning device MA, the radiation beam PB passes
through the projection system PS, which focuses the beam onto a
target portion C of the substrate W. With the aid of the second
positioner PW and position sensor IF (e.g. an interferometric
device, linear encoder or capacitive sensor), the substrate table
WT can be moved accurately, e.g. so as to position different target
portions C in the path of the radiation beam PB. Similarly, the
first positioner PM and another position sensor (which is not
explicitly depicted in FIG. 1) can be used to accurately position
the patterning device MA with respect to the path of the radiation
beam PB, e.g. after mechanical retrieval from a mask library, or
during a scan. In general, movement of the support structure MT may
be realized with the aid of a long-stroke module (coarse
positioning) and a short-stroke module (fine positioning), which
form part of the first positioner PM. Similarly, movement of the
substrate table WT may be realized using a long-stroke module and a
short-stroke module, which form part of the second positioner PW.
In the case of a stepper (as opposed to a scanner) the support
structure MT may be connected to a short-stroke actuator only, or
may be fixed. Patterning device MA and substrate W may be aligned
using patterning device alignment marks M1, M2 and substrate
alignment marks P1, P2. Although the substrate alignment marks as
illustrated occupy dedicated target portions, they may be located
in spaces between target portions (these are known as scribe-lane
alignment marks). Similarly, in situations in which more than one
die is provided on the patterning device MA, the patterning device
alignment marks may be located between the dies.
[0080] The depicted apparatus could be used in at least one of the
following modes:
[0081] 1. In step mode, the support structure MT and the substrate
table WT are kept essentially stationary, while an entire pattern
imparted to the radiation beam is projected onto a target portion C
at one time (i.e. a single static exposure). The substrate table WT
is then shifted in the X and/or Y direction so that a different
target portion C can be exposed. In step mode, the maximum size of
the exposure field limits the size of the target portion C imaged
in a single static exposure.
[0082] 2. In scan mode, the support structure MT and the substrate
table WT are scanned synchronously while a pattern imparted to the
radiation beam is projected onto a target portion C (i.e. a single
dynamic exposure). The velocity and direction of the substrate
table WT relative to the support structure MT may be determined by
the (de-)magnification and image reversal characteristics of the
projection system PS. In scan mode, the maximum size of the
exposure field limits the width (in the non-scanning direction) of
the target portion in a single dynamic exposure, whereas the length
of the scanning motion determines the height (in the scanning
direction) of the target portion.
[0083] 3. In another mode, the support structure MT is kept
essentially stationary holding a programmable patterning device,
and the substrate table WT is moved or scanned while a pattern
imparted to the radiation beam is projected onto a target portion
C. In this mode, generally a pulsed radiation source is employed
and the programmable patterning device is updated as required after
each movement of the substrate table WT or in between successive
radiation pulses during a scan. This mode of operation can be
readily applied to maskless lithography that utilizes programmable
patterning device, such as a programmable mirror array of a type as
referred to above.
[0084] In scan mode, the support structure MT is movable in a given
direction (the so-called "scan direction", e.g., the Y direction)
with a speed v, so that the beam PB is caused to scan over a
patterning device image. Concurrently, the substrate table WT is
simultaneously moved in the same or opposite direction at a speed
V=Mv, in which M is the magnification of the projection system PL
(typically, M=1/4 or 1/5). In this manner, a relatively large
target portion C can be exposed, without having to compromise on
resolution.
[0085] As illustrated in FIGS. 2 and 4, an attenuation control
device 10 may comprise a plurality of blades 11 disposed in the
illumination system IL in the path of the beam PB. The attenuation
control device 10 is situated at an optical distance d from the
patterning device MA, or a plane conjugate with the patterning
device MA, such that the blades would be out of focus at patterning
device level and also not in a pupil plane of the illumination
system. In general, the attenuation control device should be closer
to the patterning device, or a conjugate plane thereof, than to a
pupil plane. If the illumination system contains an intermediate
image plane, the blades may be positioned closer to that than to a
pupil plane. In an illumination system utilizing field and pupil
facet mirrors to provide uniformity, the attenuation control device
may be positioned after the field facet mirrors.
[0086] The blades 11 extend partially or wholly across the beam so
that their half shadows extend partially or wholly across the width
of the illumination field IFL (along the scanning direction of the
apparatus), substantially perpendicular to its longitudinal axis.
Usually the blades extend over the whole slit but, when there is a
strong telecentricity gradient near the edges of the slit, the edge
area is desirably not blocked. The blades are spaced apart a
distance such that their half shadows at patterning device level
are overlapping (though it may be sufficient that they are
adjacent) and must be sufficient in number so that their half
shadows cover the entire illumination field. The shadow profiles of
the blades tail-off and the tail portions overlap. Rotating the
blades, to increase their effective widths, darkens their shadow
profiles. Actuators 12 are positioned to selectively rotate the
corresponding blades 11.
[0087] The illumination slit that is used to expose the substrate
during scanning is usually curved, as shown in FIG. 2. The blades
11 are oriented with a fixed angle of typically around 60 degrees
with respect to the non-scanning direction or x-axis. Due to this
angle the shadow of each blade is beneficially spread out in the
non-scanning x-direction. Since the overlap of the blades with the
slit is different on the left hand side to the right hand side of
the slit, the transmittance of the blades is not the same. The
blade on the left hand side of FIG. 2 is more nearly perpendicular
to the slit, leading to a narrower spatial profile with a
relatively low transmission, whereas the blade on the right hand
side of FIG. 2 is more nearly parallel to the slit, leading to a
broader spatial profile with a relatively high transmission. By
rotating the blades progressively less from left to right with
respect to the x-axis, their peak transmission can be made the
same. For example, at the left hand end of the slit the blades may
be mounted at 60.degree. relative to the x-axis, whereas at the
right hand side of the slit the blades may be mounted at 45.degree.
relative to the x-axis.
[0088] Rotation of one of the blades 11 from the maximally open
position shown in FIG. 5(c) causes its effective width in the beam
to increase, thereby blocking a greater portion of the incident
radiation. In an embodiment, the blades are made of a material
absorbent of the radiation of the beam so as to minimize scattered
stray radiation (or have an anti-reflection coating). Accordingly,
the angle of inclination of individual ones of the blades 11 can be
adjusted to absorb a greater portion of incident radiation in
regions of the beam where the incident intensity is higher so as to
increase uniformity of illumination. The angle of the blades can be
varied to reduce the intensity in the half shadow by up to about
10% without unduly affecting telecentricity. For a blade disposed
at 90 mm from the patterning device in an apparatus with NA=0.25
and .sigma.=0.5 using EUV radiation, the radius of the half shadow
at patterning device level is 3 mm so that about 35 blades would be
used to cover an illumination field of length 104 mm, for example.
Where the blades are mounted at an angle of typically 60 degrees to
the scanning direction, even less blades, i.e. 23 blades, are
needed to cover the slit. In another apparatus, e.g. using DUV
radiation, the stand-off distance may be a factor of 4 or 5
less.
[0089] Referring to FIG. 4, each of the blades 11 is made from
molybdenum, is 10 mm long, 2 mm wide and 0.2 mm thick and is
mounted on a rotatable shaft 13. Connected to the shaft 13 of each
blade 11 is a vane 14 to be illuminated by radiation (e.g., visible
light) from a radiation source and having a radiation detector 16
located adjacent it. This is used to measure the orientation of the
blade 11 by detecting the quantity of radiation reaching the
detector 16 from the radiation source which is dependent on the
orientation of the intervening vane 14. The shaft 13 is connected
to a moving magnet 17 surrounded by a yoke 18 and coil 19 of a
motor that is used to rotate the blade 11. One end of the
collection of elements is fixed to a mounting, thereby acting as a
torsion bar, and the other end is mounted in ruby bearings (not
shown).
[0090] The distribution of the intensity across the slit can
determine the desired shape of the attenuation blades. In FIG. 7, a
variety of blades is shown which results in a good uniformity at
substrate level, depending on the radiation distribution at the
working level of the attenuation control device. In general the
rectangular blades in FIG. 7(a) are desired when the intensity
distribution across the slit has the shape of a top hat or has
linear ramps. When the intensity distribution is more
Gaussian--like, an asymmetric blade, as shown in FIG. 7(b),
provides the flattest intensity profile at substrate level. If the
beam has strongly non-telecentric edges, the blades desirably are
shorter so that that area is not attenuated. If the telecentricity
is worst at the center or if for other reasons the center of the
beam must not be disturbed, a holed blade, as shown in FIG. 7(c),
may be advantageous. By placing multiple blades, e.g. 2 blades, as
shown in FIG. 7(d), on a common rotation axis, widely different
attenuation profiles can be generated as a function of rotation
angle. The same applies to the other 3-dimensional shape, as shown
in FIG. 7(e), in this case a helicoid, in that here also the
position of the center of gravity of the attenuation profile can be
moved by rotating the blade.
[0091] In FIG. 8 the beam PB is perpendicular to the page, its
intensity cross-section is Gaussian shaped. The attenuation blades
11 numbered n and n+1 cast penumbras Pn and Pn+1 at patterning
device level. As shown the rectangular blades n and n+1 can be
rotated so that the integrated (attenuated) intensities at
positions xn and xn+1 are identical. In between the blades however
there is still a residual non-uniformity due to the non-linearity
of the PB cross section profile. By deviating from the rectangular
blade shape, i.e. by locally widening the blade n, n+1 or both (the
dashed lines on the blades) the non-uniformity between the blades
can be minimized.
[0092] It will also be appreciated that the exact shape of the
blades is not crucial to an embodiment of the invention, although
the blades should be made as thin as possible to provide minimum
obscuration at their maximally open position. The width of the
blades should be determined in accordance with the accuracy of the
actuators 12 to provide the desired degree of controllability over
the amount of radiation absorbed.
[0093] The actuators 12 may be, for example, piezoelectric
actuators or any other suitable rotary actuator. A linear actuator
driving the rods via a gear arrangement is also possible.
[0094] In an embodiment, as illustrated in FIGS. 9-11, an
attenuation control device 10 comprises a plurality of attenuation
structures 21 disposed in the illumination system IL in the path of
the beam PB. The attenuation control device 10 is situated at an
optical distance from the patterning device MA, or a plane
conjugate with the patterning device MA, such that the attenuation
structures would be out of focus at patterning device level and
also not in a pupil plane of the illumination system. In general,
the attenuation control device should be closer to the patterning
device, or a conjugate plane thereof, than to a pupil plane. If the
illumination system contains an intermediate image plane, the
attenuation structures may be positioned closer to that than to a
pupil plane. In an illumination system utilizing field and pupil
facet mirrors to provide uniformity, the attenuation control device
may be positioned after the field facet mirrors.
[0095] Referring to FIG. 9(a), an attenuation structure 21
comprises a central wire 20 with at least one tube 22, 24 mounted
thereon. The tube 22, 24 is movable along the wire 20 by means of,
for example, an actuator. The wire 20 may be circular or any other
shape and may be flexible or rigid. In an embodiment, there is only
one tube 22 mounted on the wire 20. In an embodiment, there are a
plurality of tubes 22, 24 mounted on the wire 20, one tube 22 of
which is movable along the wire and another tube 24 of which is
fixed. Connected to the tube 22, 24 is a flexible material 26,
which, in an embodiment, comprises a plurality of small flexible
wires. Where there is only one tube 22, the flexible material 26 is
connected at one end to the tube 22 and fixed at another end to the
wire 20 or other structure. Where a plurality of tubes 22, 24 are
provided, the flexible material 26 is connected at one end to a
tube 22 of the plurality of tubes and at another end to another
tube 24 of the plurality of tubes.
[0096] Referring to FIG. 9(b), in an embodiment, the tube 22, 24
completely surrounds the wire 20 although it need not if otherwise
moves in a direction along the wire 20. Similarly, in an
embodiment, the flexible material 26 completely surrounds the wire
20 although it need not if, as discussed below, the material is
sufficient to be displaced to attenuate radiation.
[0097] Referring to FIG. 9(c), operation of the attenuation
structure 11 to attenuate radiation according to an embodiment
involves displacing by the tube 22, 24 to cause the flexible
material 26 to be displaced in a direction (X-direction)
substantially perpendicular to an axis (Y-axis) of the wire 20. In
an embodiment where only tube 22 is movable, then tube 22 is moved
towards the end of the flexible material that is affixed or
otherwise stationary. The movement of tube 22 causes the flexible
material 26 to compress and move outward from the wire 20 as shown
in FIG. 9(c). In an embodiment where tubes 22, 24 are movable, one
tube 22 or 24 may be moved or both tubes 22 and 24 may be moved.
For example, tube 22 may be moved toward tube 24 or tubes 22 and 24
may be moved toward each other, in each case causing the flexible
material 26 to move outward from the wire 20. FIG. 9(d) shows, for
example, the displacement of the flexible material 26 when tube 22
is moved towards tube 24. As will be apparent, tube 22, 24 may be
moved away along the wire 20 to cause the flexible material 26 to
be displaced towards the wire 20.
[0098] Depending on the amount of movement of tube 22, 24, the
flexible material 26 can be variably displaced inward and outward
from the wire 20 to control the amount of attenuation effected by
the attenuation structure 21 in the X-direction and to some extent
in the Y-direction. A controller as discussed above may be used to
control the amount of attenuation provided by the attenuation
structure 21.
[0099] In an embodiment, where, for example, a plurality of movable
tubes 22, 24 are provided, the plurality of tubes 22, 24 may be
moved, simultaneously or not, in the same direction to shift the
flexible material 26 along the wire 20. Thus, not only may
attenuation be controlled in a direction (X-direction)
substantially perpendicular to the axis of the wire 20, it may also
be effectively controlled in a direction (Y-direction)
substantially parallel to the axis of the wire 20. Thus,
attenuation may be effectively variably controlled in a X-Y
plane.
[0100] Referring to FIG. 10, an embodiment of an attenuation
control device 10 comprising a plurality of attenuation structures
21 is depicted in relation to an illumination field IFL of a
lithographic apparatus. In this embodiment, only tube 22 of each
attenuation structure 21 is movable along the wire 20 of each
attenuation structure 21 so as to cause the flexible material 26 to
be displaced and thus control attenuation effected by that
attenuation structure 21. Each of the attenuation structures 21 can
be individually controlled to vary the amount and spatial position
of the attenuation provided by that attenuation structure 21. In
this embodiment, the attenuation is primarily controlled in the
X-direction through movement of the flexible material 26 inwards or
outwards from the respective wire 20 of each attenuation structure
21.
[0101] Referring to FIG. 11, an additional or alternative
embodiment of an attenuation control device 10 comprising a
plurality of attenuation structures 21 is depicted in relation to
an illumination field IFL of a lithographic apparatus. In this
embodiment, both tubes 22 and 24 of each attenuation structure 21
is movable along the wire 20 of each attenuation structure 21 so as
to cause the flexible material 26 to be displaced and thus control
attenuation effected by that attenuation structure 21. Each of the
attenuation structures 21 can be individually controlled to vary
the amount and spatial position of the attenuation provided by that
attenuation structure 21. In this embodiment, the attenuation can
be primarily controlled in the X and Y-directions through movement
of the flexible material 26 inwards or outwards from the respective
wire 20 of each attenuation structure 21 and through movement of
the flexible material 26 along the respective wire 20 of each
attenuation structure 21.
[0102] As will be apparent, a plurality of attenuation structures
21 may be provided wherein some of the attenuation structures 21
comprise only tube 22 of each such attenuation structure 21 being
movable along the wire 20 of such attenuation structure 21 and some
of the attenuation structures comprise both tubes 22 and 24 of each
such attenuation structure 21 being movable along the wire 20 of
such attenuation structure 21.
[0103] In an embodiment, attenuation may be completely customized
in the X-Y plane of the illumination field IFL to control, for
example, uniformity of the distribution of radiation. For example,
the flexible material 26 may be moved in the X and Y directions and
selective attenuation structures 21 may have the flexible material
variably adjusted in the X and Y directions so that attenuation of
radiation in throughout X-Y plane of illumination field IFL may be
variably controlled.
[0104] The attenuation structures 21 may have some or many of the
features of the blades 11 and/or actuators 12 described above with
reference FIGS. 2-5 and 7-8.
[0105] Further, measurement may be provided to control the amount
of attenuation provided by the blades 11 and/or attenuation
structures 21. While the following will discuss measurement and/or
control in relation to blades 11, the same or similar principles
may be applied to attenuation structures 21.
[0106] To detect the positions of the blades for the purpose of
controlling the amount of attenuation applied by the blades, each
blade may have an associated position detector configured to detect
a quantity of radiation received from a radiation source providing
a radiation beam that is arranged to be interrupted by a portion of
the blade, or an element connected to the blade so as to rotate
with the blade in such a manner that the quantity of radiation
reaching the position detector is indicative of the orientation of
the blade. The outputs of the position detectors can then be
supplied to an electronic controller to control the actuators, used
to tilt the blades, in such a manner as to accurately orient the
blades according to the degree of attenuation desired. Generally
the number of radiation sources used in such a position detection
arrangement will correspond to the number of blades whose positions
are to be detected. Thus, if in a typical arrangement 30 blades are
provided, the position detection arrangement may include, for
example, 30 light-emitting diodes to emit light and 30 photodiodes
to detect the light after attenuation by the blades. All the
components may be disposed in a vacuum so that, because of the lack
of convection, cooling can present a problem.
[0107] When a high measurement accuracy is required, the use of
multiple radiation sources can be disadvantageous in that the
intensity of radiation emitted can vary from source to source and
with time depending on the different thermal behavior of each
source, which may cause the relationship between the proportion of
radiation received by each detector and the precise orientation of
the blade, as well as the angular distribution of the radiation and
the degree of self-heating, to vary from source to source. The
thermal drift of the light emitting diodes can also render these
unsuitable for use in high measurement accuracy system. The use of
multiple radiation sources is also disadvantageous in so far as it
requires use of a high level of components and cabling, as well as
providing high power consumption and cooling requirements.
[0108] As shown in the three explanatory diagrams of FIG. 5, the
attenuation control device comprises a respective radiation
detector 16 configured to provide an electrical output signal
indicative of the orientation of each blade in dependence on
detection of a beam of detecting radiation emitted by a
light-emitting diode, for example, reaching the radiation detector
16 after attenuation by a vane 14 that may be constituted by the
blade 11 itself or may be a separate part mounted on the same shaft
as the blade. The diagrams (a), (b) and (c) show how the quantity
of radiation received by the detector 16 is a function of the
orientation of the vane 14, although it should be appreciated that
these diagrams exaggerate the range of rotation for the purposes of
illustration and that the full range of rotation is more likely to
be in the range of 40.degree., rather than 90.degree. as shown.
[0109] In one operational mode, the attenuation control device 10
is used to correct for undesired non-uniformities in the beam
provided by the illumination system. When used in this way, such
uniformities can be measured by an appropriate sensor or by
calibration runs. The appropriate blade angles to achieve the
desired uniformity correction are then calculated and the actuators
12 controlled to effect this by a controller 30 (see FIG. 3). The
uniformity of the beam is then re-measured at appropriate intervals
to detect any time varying non-uniformities and the blade angles
adjusted as necessary. For this function the speed of response of
the blade actuators is not crucial but the actuators should
desirably be designed so that the blade positions can be maintained
for relatively long periods without the need for constant
energization of the actuators.
[0110] In another operational mode of the attenuation control
device, the blades are positioned both to correct for
non-uniformity and to decrease the intensity of the beam. This is
particularly useful as the EUV source has no capability to vary the
pulse energy over a large range (unlike DUV lasers). The uniformity
of illumination is optimized by finding the point with minimum
intensity and, by suitable adjustment of the blades, then cutting
off all radiation above that minimum for all other positions in the
slit through which the beam passes to the substrate, the excess
radiation being absorbed by the blades. A similar uniformity
profile, but of lower intensity, may be obtained by cutting off all
radiation above a lower intensity by suitable adjustment of the
blades.
[0111] In a further operational mode of the attenuation control
device, the adjustment of the lightweight blades performs
uniformity correction (in direction X) and variable attenuation (in
direction Y) simultaneously during scanning of a die by the beam
through a scanning slit. In the simplest mode of operation the
blades are adjusted in advance to take account of variation along
the length of the slit (the X direction), and then remain fixed in
these positions during scanning across the die.
[0112] In an alternative mode of operation the positions of the
blades are adjusted during the scanning operation to take account
of uniformity variation in the X direction during the scan.
[0113] In general it is useful to make the illumination of the
substrate as uniform as possible and to keep the same uniform
intensity over the entire substrate. However other processes
outside the user's control, such as the chemical processing of the
substrate for example, may have an effect which varies for
different locations on the substrate. Typically there may be a
difference between the center of the substrate and the edge of the
substrate, and in this case it is possible to measure the effect of
such processes and to then adjust the intensity of the beam to
correct for this effect. For example, the intensity of illumination
at the edge of the substrate may be controlled to be greater than
the intensity of the illumination at the center of the substrate,
to take account of differences in processing that occur at the edge
of the substrate as compared to the center.
[0114] Alternatively the attenuation control device may be used as
a two-dimensional attenuation controller. In this case the
requirement is that the blades can be rotated fast, e.g., within
the exposure time of a die so that there can be different
corrections within a die. With the current concept of very
lightweight blades this is possible, the full range of angles (40
degrees: from -5 to plus 35 degrees) can be travelled within 0.2
sec.
[0115] The control of the attenuation control device is effected by
means of a `closed-loop` control (feedback control) arrangement in
which there is two-way communication between a controller 30 and
the attenuation control device 10 as shown in FIG. 3 in that a
respective feedback signal is sent back to the controller 30 from
an angle detector 16 indicative of the orientation of each blade 11
of the attenuation control device . The feedback signal from each
angle detector 16 dependent on the position of the corresponding
blade 11 is transmitted through an amplifier 31 and an input of an
adder 32 to the other input of which a set point signal is applied,
and the controller 30 sends out an adjustment signal to a
corresponding actuator 12 by way of a further amplifier 33 if the
orientation of the blade is incorrect. At each point in time the
actual angles of all the blades are known to the controller 30.
This method is very accurate but places a heavy burden on
data-transport and on the calculation capabilities.
[0116] As shown diagrammatically on the right hand side of FIG. 6,
the radiation supplied to the vanes 14 for the purpose of detecting
the orientation of the vanes 14 is supplied from a common radiation
source 20, such as a LED and optionally by way of an optical fiber
if the LED is positioned remotely, by way of an optical integrator
21 constituting a mixing unit having diffusely reflective internal
surfaces configured to mix the radiation from the common source 20.
The mixed radiation is emitted from the optical integrator 21
through a respective aperture 22 towards each of the vanes 14. The
row of apertures 22 follows the curved shape of the slit, and the
radiation integrator itself can have any outer shape provided that
all apertures 22 receive the same amount of randomized radiation.
As previously described the quantity of radiation reaching the
angle detector 16 is dependent on the orientation of the vane 14 as
shown diagrammatically in FIG. 6. The optical integrator 21 has
reflective walls to multiply reflect radiation from the common
source 20 towards the apertures 22, as shown diagrammatically in
FIG. 6, and provides the required high homogeneity and stability
with respect to drift over time and temperature to assure stable
reproducible angle measurement. The attenuation control device also
comprises a reference detector 23 configured to provide a reference
output signal in dependence on detection of a beam of detecting
radiation reaching the reference detector directly from the common
radiation source. The reference detector directly detects the
radiation from the source and provides a reference output signal so
that fluctuations of the radiation source, due to thermal drift,
for example, can be compensated in an electronic control
circuit.
[0117] As noted above, the above described measurement and control
concepts may be applied to the attenuation structures 21. So, for
example, the blade 11 may correspond to the flexible material 26
and angles of the blade 11 may correspond to displacement of the
flexible material 27.
[0118] As an example, the flexible material 26, the tube 22, 24, or
some other part moveable with the tube 22, 24 or the flexible
material 26 may be equivalent to the vane 14, i.e., the movement
thereof is used to attenuate a detecting beam of radiation and the
position detector is configured to provide an output indicative of
a position of the attenuation structure in dependence on detection
of the beam of detecting radiation reaching the position detector
after attenuation.
[0119] Alternatively or additionally, a beam of detecting radiation
may be redirected by the flexible material 26, the tube 22, 24, or
some other part (e.g., a mirror) moveable with the tube 22, 24 to a
detector, the detector providing an output indicative of a position
of the attenuation structure in dependence on detection of the beam
of detecting radiation reaching the position detector after
redirection. For example, an interferometer or encoder system may
be used for this purpose. Alternatively or additionally, any other
measurement apparatus may be used to determine the displacement of
the flexible material 26 in X and/or Y directions.
[0120] In each case, with the detected position of the flexible
material 26, attenuation of the radiation beam can be controlled by
adjusting the attenuation structures 21. For example, the
appropriate tube 22, 24 displacement to achieve the desired
uniformity correction can be calculated and the actuator for the
tube 22, 24 is then controlled to effect this.
[0121] Although specific reference may be made in this text to the
use of lithographic apparatus in the manufacture of ICs, it should
be understood that the lithographic apparatus described herein may
have other applications, such as the manufacture of integrated
optical systems, guidance and detection patterns for magnetic
domain memories, liquid-crystal displays (LCDs), thin-film magnetic
heads, etc. The skilled person will appreciate that, in the context
of such alternative applications, any use of the terms "wafer" or
"die" herein may be considered as synonymous with the more general
terms "substrate" or "target portion", respectively. The substrate
referred to herein may be processed, before or after exposure, in
for example a track (a tool that typically applies a layer of
resist to a substrate and develops the exposed resist) or a
metrology or inspection tool. Where applicable, the disclosure
herein may be applied to such and other substrate processing tools.
Further, the substrate may be processed more than once, for example
in order to create a multi-layer IC, so that the term substrate
used herein may also refer to a substrate that already contains
multiple processed layers.
[0122] The terms "radiation" and "beam" used herein encompass all
types of electromagnetic radiation, including ultraviolet (UV)
radiation (e.g. having a wavelength of 365, 248, 193, 157 or 126
nm) and extreme ultra-violet (EUV) radiation (e.g. having a
wavelength in the range of 5-20 nm), as well as particle beams,
such as ion beams or electron beams.
[0123] While specific embodiments of the invention have been
described above, it will be appreciated that the invention may be
practiced otherwise than as described. The description is not
intended to limit the invention.
* * * * *