U.S. patent application number 12/519077 was filed with the patent office on 2010-06-10 for radiation system and lithographic apparatus.
This patent application is currently assigned to ASML NETHERLANDS B.V.. Invention is credited to Vadim Yevgenyevich Banine, Paul Peter Anna Antonius Brom, Theodorus Petrus Maria Cadee, Denis Alexandrovich Glushkov, Vladimir Vitalevitch Ivanov, Derk Jan Wilfred Klunder, Konstantin Nikolaevitch Koshelev, Vladimir Mihail vitch Krivtsun, Wouter Anthon Soer, Maarten Marinus Johannes Wilhelmus Van Herpen, Arnoud Cornelis Wassink.
Application Number | 20100141909 12/519077 |
Document ID | / |
Family ID | 39052602 |
Filed Date | 2010-06-10 |
United States Patent
Application |
20100141909 |
Kind Code |
A1 |
Wassink; Arnoud Cornelis ;
et al. |
June 10, 2010 |
RADIATION SYSTEM AND LITHOGRAPHIC APPARATUS
Abstract
A radiation system for generating a beam of radiation that
defines an optical axis is provided. The radiation system includes
a plasma produced discharge source for generating EUV radiation.
The discharge source includes a pair of electrodes constructed and
arranged to be provided with a voltage difference, and a system for
producing a plasma between the pair of electrodes so as to provide
a discharge in the plasma between the electrodes. The radiation
system also includes a debris catching shield for catching debris
from the electrodes. The debris catching shield is constructed and
arranged to shield the electrodes from a line of sight provided in
a predetermined spherical angle relative the optical axis, and to
provide an aperture to a central area between the electrodes in the
line of sight.
Inventors: |
Wassink; Arnoud Cornelis;
(Veldhoven, NL) ; Banine; Vadim Yevgenyevich;
(Helmond, NL) ; Ivanov; Vladimir Vitalevitch;
(Moscow, RU) ; Koshelev; Konstantin Nikolaevitch;
(Troitsk, RU) ; Cadee; Theodorus Petrus Maria;
(Vlierden, NL) ; Krivtsun; Vladimir Mihail vitch;
(Troitsk, RU) ; Klunder; Derk Jan Wilfred;
(Geldrop, NL) ; Van Herpen; Maarten Marinus Johannes
Wilhelmus; (Heesch, NL) ; Brom; Paul Peter Anna
Antonius; (Eindhoven, NL) ; Soer; Wouter Anthon;
(Nijmegen, NL) ; Glushkov; Denis Alexandrovich;
(Witten, DE) |
Correspondence
Address: |
PILLSBURY WINTHROP SHAW PITTMAN, LLP
P.O. BOX 10500
MCLEAN
VA
22102
US
|
Assignee: |
ASML NETHERLANDS B.V.
Veldhoven
NL
|
Family ID: |
39052602 |
Appl. No.: |
12/519077 |
Filed: |
November 27, 2007 |
PCT Filed: |
November 27, 2007 |
PCT NO: |
PCT/NL2007/050598 |
371 Date: |
January 25, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11637936 |
Dec 13, 2006 |
7696492 |
|
|
12519077 |
|
|
|
|
Current U.S.
Class: |
355/30 ;
250/504R |
Current CPC
Class: |
G03F 7/70858 20130101;
G03F 7/70983 20130101; G03F 7/70883 20130101; G03F 7/70925
20130101; H05G 2/003 20130101; G03F 7/70933 20130101; G03F 7/70916
20130101; H05G 2/005 20130101 |
Class at
Publication: |
355/30 ;
250/504.R |
International
Class: |
G03B 27/54 20060101
G03B027/54; G21K 5/04 20060101 G21K005/04 |
Claims
1. A radiation system for generating a beam of radiation in a
radiation space, the radiation system comprising: a plasma produced
discharge source constructed and arranged to generate extreme
ultraviolet radiation, the discharge source comprising a pair of
electrodes constructed and arranged to be provided with a voltage
difference, and a system constructed and arranged to produce a
discharge between said pair of electrodes so as to provide a pinch
plasma between said electrodes; and a debris catching shield
constructed and arranged to catch debris from said electrodes, to
shield said electrodes from a line of sight provided in the
radiation space, and to provide an aperture to a central area
between said electrodes in said line of sight.
2. (canceled)
3. A radiation system according to claim 1, wherein the debris
catching shield comprises at least one fluid jet.
4. A radiation system according to claim 3, wherein the fluid jet
comprises molten tin or a tin compound.
5. (canceled)
6. A radiation system according to claim 1, wherein the debris
catching shield is provided by a pair of fluid jets, arranged
oppositely and generally parallel to a longitudinal axis of the
electrodes.
7. A radiation system according to claim 3, wherein the debris
catching shield comprises a plurality of fluid jets, arranged in
radial direction relative from the central area.
8. (canceled)
9. (canceled)
10. (canceled)
11. (canceled)
12. A radiation system according to claim 1, wherein the debris
catching shield comprises a static configuration of generally
radially oriented platelets, relative to said central area, wherein
the platelets are oriented to shield the electrodes from a line of
sight provided between said platelets.
13. A radiation system according to claim 12, wherein a distance
between the platelets is increased relative to distances away from
the optical axis.
14. (canceled)
15. A radiation system according to claim 12, further comprising an
electromagnetic deflecting field unit disposed for applying an
electromagnetic deflecting field between the electrodes and the
shield.
16. A radiation system according to claim 15, wherein said
electromagnetic deflecting field unit provides a static magnetic
field.
17. (canceled)
18. (canceled)
19. A radiation system according to claim 12, further comprising a
hydrogen radical supply system for guiding hydrogen radicals
through said platelets.
20. (canceled)
21. (canceled)
22. (canceled)
23. A radiation system according to claim 12 wherein at least some
of the platelets are provided by a fluid jet.
24. A radiation system according to claim 23, wherein the fluid jet
comprises molten tin or a tin compound.
25. (canceled)
26. A radiation system according to claim 1, further comprising a
heating system that can be selectively heated for elevating a
temperature of said debris catching shield to a temperature for
evaporating said debris from said debris catching shield; and a gas
supply system for providing a gas flow to evacuate said evaporated
debris from said debris catching shield.
27. (canceled)
28. (canceled)
29. (canceled)
30. A radiation system according to claim 12, wherein the platelets
are provided as a material of porous characteristics for removing
said debris from said platelets through capillary action.
31. A radiation system according to claim 12, further comprising an
excitator for removing said debris from said platelets through
mechanical excitation of said platelets.
32. (canceled)
33. (canceled)
34. (canceled)
35. A radiation system according to claim 1, wherein the system
that is constructed and arranged to produce a discharge between
said pair of electrodes comprises a laser.
36. A radiation system according to claim 1, wherein the electrodes
define a discharge axis interconnecting said electrodes and wherein
the radiation space is substantially bounded between two mutually
reversely oriented cones relative to the discharge axis, the cones
having their apex substantially in the central area between the
electrodes.
37. (canceled)
38. A radiation system according to claim 12, wherein platelets
have concentric conical surfaces and/or comprise at least one
planar section.
39. (canceled)
40. (canceled)
41. A radiation system according to claim 12, comprising a wiping
module provided with a multiple number of wiping elements movable
along respective platelet surfaces.
42. (canceled)
43. (canceled)
44. (canceled)
45. (canceled)
46. (canceled)
47. A lithographic apparatus comprising: a radiation system
constructed and arranged to generate a beam of radiation defining
in a radiation space, the radiation system comprising: a plasma
produced discharge source constructed and arranged to generate
extreme ultraviolet radiation, the discharge source comprising a
pair of electrodes constructed and arranged to be provided with a
voltage difference, and a system constructed and arranged to
produce a discharge between said pair of electrodes so as to
provide a pinch plasma between said electrodes; and a debris
catching shield constructed and arranged to catch debris from said
electrodes, to shield said electrodes from a line of sight provided
in the radiation space, and to provide an aperture to a central
area between said electrodes in said line of sight; a patterning
device constructed and arranged to pattern the beam of radiation;
and a projection system constructed and arranged to project the
patterned beam of radiation onto a substrate.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is the U.S. National Phase of
PCT/NL2007/050598, filed Nov. 27, 2007, which claims benefit and
priority to U.S. application Ser. No. 11/637,936, filed on Dec. 13,
2006. Both priority applications are hereby incorporated in their
entirety by reference.
FIELD
[0002] The present invention relates to a radiation system and a
lithographic apparatus that includes a radiation system.
BACKGROUND
[0003] A lithographic apparatus is a machine that applies a desired
pattern onto a substrate, usually onto a target portion of the
substrate. A 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 to be formed
on an individual layer of the IC. This pattern can be transferred
onto a target portion (e.g. comprising part of, one, or several
dies) on a substrate (e.g. a silicon wafer). Transfer of the
pattern is typically via imaging onto a layer of
radiation-sensitive material (resist) provided on the substrate. In
general, a single substrate will contain a network of adjacent
target portions that are successively patterned. Known lithographic
apparatus include so-called steppers, in which each target portion
is irradiated by exposing an entire pattern onto the target portion
at one time, and so-called scanners, in which each target portion
is irradiated by scanning the pattern through a radiation 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 addition to EUV radiation, radiation sources used in EUV
lithography generate contaminant material that may be harmful for
the optics and the working environment in which the lithographic
process is carried out. Such is especially the case for EUV sources
operating via a plasma produced discharge source, such as a plasma
tin source. Such a source typically comprises a pair of electrodes
to which a voltage difference can be applied. In addition, a vapor
is produced, for example, by a laser beam that is targeted to, for
example, one of the electrodes. Accordingly, a discharge will occur
between the electrodes, generating a plasma, and which causes a
so-called pinch in which EUV radiation is produced. In addition to
this radiation, the discharge source typically produces debris
particles, among which can be all kinds of microparticles varying
in size from atomic to complex particles, which can be both charged
and uncharged. It is desired to limit the contamination of the
optical system that is arranged to condition the beams of radiation
coming from an EUV source from this debris. Conventional shielding
of the optical system primarily includes a system comprising a high
number of closely packet foils aligned parallel to the direction of
the light generated by the EUV source. A so-called foil trap, for
instance, as disclosed in EP1491963, uses a high number of closely
packed foils aligned generally parallel to the direction of the
light generated by the EUV source. Contaminant debris, such as
micro-particles, nano-particles and ions can be trapped in walls
provided by the foil plates. Thus, the foil trap functions as a
contamination barrier trapping contaminant material from the
source. Due to the arrangement of the platelets, the foil trap is
transparent for light, but will capture debris either because it is
not travelling parallel to the platelets, or because of a
randomized motion caused by a buffer gas. It is desirable to
improve the shielding of the radiation system, because some
(directed, ballistic) particles may still transmit through the foil
trap.
SUMMARY
[0005] According to an aspect of the invention there is provided a
radiation system for generating a beam of radiation that defines an
optical axis. The radiation system includes a plasma produced
discharge source constructed and arranged to generate EUV
radiation. The discharge source includes a pair of electrodes
constructed and arranged to be provided with a voltage difference,
and a system constructed and arranged to produce a discharge
between the pair of electrodes so as to provide a pinch plasma
between the electrodes. The radiation system also includes a debris
catching shield constructed and arranged to catch debris from the
electrodes, to shield the electrodes from a line of sight provided
in a predetermined spherical angle relative the optical axis, and
to provide an aperture to a central area between the electrodes in
the line of sight.
[0006] According to an aspect of the invention, there is provided a
lithographic apparatus that includes a radiation system for
generating a beam of radiation that defines an optical axis. The
radiation system includes a plasma produced discharge source
constructed and arranged to generate EUV radiation. The discharge
source includes a pair of electrodes constructed and arranged to be
provided with a voltage difference, and a system constructed and
arranged to produce a discharge between the pair of electrodes so
as to provide a pinch plasma between the electrodes. The radiation
system also includes a debris catching shield constructed and
arranged to catch debris from the electrodes, to shield the
electrodes from a line of sight provided in a predetermined
spherical angle relative the optical axis, and to provide an
aperture to a central area between the electrodes in the line of
sight. The lithographic apparatus also includes a patterning device
constructed and arranged to pattern the beam of radiation, and a
projection system constructed and arranged to project the patterned
beam of radiation onto a substrate.
[0007] Other aspects, features, and advantages of the present
invention will become apparent from the following detailed
description, the accompanying drawings, and the appended
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] 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:
[0009] FIG. 1 depicts a lithographic apparatus according to an
embodiment of the invention;
[0010] FIG. 2 depicts a schematic first embodiment of a radiation
system of the lithographic apparatus of FIG. 1 according to an
aspect of the invention;
[0011] FIG. 3 shows schematically a second embodiment according to
an aspect of the invention;
[0012] FIG. 4 shows a further embodiment according to an aspect of
the invention;
[0013] FIG. 5 shows a modification of the arrangement described
with reference to FIG. 4;
[0014] FIG. 6 shows an alternative modification of the arrangement
described with reference to FIG. 4;
[0015] FIG. 7 schematically shows a deflection principle of debris
from the EUV source;
[0016] FIG. 8 schematically shows a quadrupole magnet arrangement
for providing debris deflection;
[0017] FIGS. 9a-c illustrate a further embodiment of the
arrangement of FIG. 4;
[0018] FIG. 10 shows a graph related to a thermal cleaning of the
radiation system;
[0019] FIG. 11 shows an embodiment of the thermal cleaning
principle referred with respect to FIG. 10;
[0020] FIG. 12 shows another embodiment of the thermal cleaning
principle referred with respect to FIG. 10;
[0021] FIGS. 13a-e show embodiments of continuous and droplet fluid
jets;
[0022] FIG. 14 shows a schematic perspective view of a radiation
system according to an embodiment of the invention;
[0023] FIG. 15 shows a schematic perspective view of a cross
section of the radiation system of FIG. 14;
[0024] FIG. 16 shows a schematic perspective view of a wiping
module of a radiation system according to an aspect of the
invention;
[0025] FIG. 17 shows a schematic top view of the wiping module of
FIG. 16;
[0026] FIG. 18 shows a schematic cross-sectional side view of the
wiping module of FIG. 16;
[0027] FIG. 19 shows a schematic cross-sectional side view of a
wiping module of a radiation system according to another aspect of
the invention;
[0028] FIG. 20 shows a schematic perspective view of a wiping
module of a radiation system according to a further aspect of the
invention;
[0029] FIG. 21 shows a schematic cross-sectional side view of a
radiation system according to an embodiment according to the
invention; and
[0030] FIG. 22 shows a diagram of collectable optical power as a
function of an opening semi-angle of a debris catching shield.
DETAILED DESCRIPTION
[0031] FIG. 1 schematically depicts a lithographic apparatus
according to one embodiment of the invention. The apparatus
comprises: an illumination system (illuminator) IL configured to
condition a radiation beam B (e.g. UV radiation or EUV radiation);
a support structure (e.g. a mask table) MT constructed to support a
patterning device (e.g. a mask) MA and connected to a first
positioner PM configured to accurately position the patterning
device in accordance with certain parameters; a substrate table
(e.g. a wafer table) WT constructed to hold a substrate (e.g. a
resist-coated wafer) W and connected to a second positioner PW
configured to accurately position the substrate in accordance with
certain parameters; and a projection system (e.g. a refractive or
reflective projection lens system) PS configured to project a
pattern imparted to the radiation beam B by patterning device MA
onto a target portion C (e.g. comprising one or more dies) of the
substrate W.
[0032] The illumination and projection systems may include various
types of optical components, such as refractive, reflective,
diffractive or other types of optical components, or any
combination thereof, for directing, shaping, or controlling
radiation.
[0033] The support structure supports, i.e. bears the weight of,
the patterning device. It 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."
[0034] 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.
[0035] 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.
[0036] The term "projection system" used herein should be broadly
interpreted as encompassing any type of projection system,
including refractive, reflective, catadioptric, or any combination
thereof, as appropriate for the exposure radiation being used. Any
use of the term "projection lens" herein may be considered as
synonymous with the more general term "projection system".
[0037] As here depicted, the apparatus is of a reflective type
(e.g. employing a reflective mask). Alternatively, the apparatus
may be of a transmissive type (e.g. employing a transmissive
mask).
[0038] The lithographic apparatus may be of a type having two (dual
stage) or more substrate tables (and/or two or more mask tables).
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.
[0039] 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. 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 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.
[0040] 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 may be used to condition the radiation beam, to have a
desired uniformity and intensity distribution in its
cross-section.
[0041] The radiation beam B 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 mask MA, the radiation beam B 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 IF2 (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 B. Similarly, the first positioner
PM and another position sensor IF1 can be used to accurately
position the mask MA with respect to the path of the radiation beam
B, e.g. after mechanical retrieval from a mask library, or during a
scan. In general, movement of the mask table 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 mask table MT may be
connected to a short-stroke actuator only, or may be fixed. Mask MA
and substrate W may be aligned using mask 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 mask MA, the mask
alignment marks may be located between the dies.
[0042] The depicted apparatus could be used in at least one of the
following modes:
[0043] 1. In step mode, the mask table 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.
[0044] 2. In scan mode, the mask table 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 mask table 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.
[0045] 3. In another mode, the mask table 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.
[0046] Combinations and/or variations on the above described modes
of use or entirely different modes of use may also be employed.
[0047] In FIG. 2 a schematic first embodiment is shown of a
radiation system according to an aspect of the invention. In
particular, there is shown a radiation system 1 for generating a
beam of radiation 2 in a radiation space. The radiation space is
bounded by a predetermined spherical angle relative to an optical
axis 3. The radiation system 1 includes a plasma produced discharge
source 4 for generating EUV radiation. The discharge source
includes a pair of electrodes 5 that are constructed and arranged
to be provided with a voltage difference, and a system that
typically includes a laser 6 for producing a vapor between the pair
of electrodes so as to provide a discharge 7 between the electrodes
5. It has been found that debris 8 coming from the radiation system
1 is primarily produced on or near the electrodes 5. These effects
also cause a generation of a so-called pinch which develops between
the electrodes 5. Typically, the EUV light that is generated is
produced by an electron transition in a Tin atom (or another
suitable material, for example, Lithium or Xenon), which is ionized
multiple times of electrons in the discharge process. It was found
that debris particles 8, in particular, ballistic particles of the
kind that may contaminate the downstream optics, are mainly
produced on or near the electrodes 5 in debris producing zones 9,
where the central EUV source light is mainly produced in the pinch
zone 10 that is distanced from the debris producing zones 9. Thus,
for a plasma produced discharge source 4, the debris producing
zones 9 are typically distanced from the EUV radiation producing
pinch zone 10. This effect can be utilized by the illustrated
embodiment, which according to an aspect of the invention comprises
a shield 11 to shield the electrodes 5 from a line of sight
provided in a predetermined spherical angle relative the optical
axis 3 and to provide an aperture 12 to a central area between the
electrodes in the line of sight. Accordingly, debris 8, which is
generated in the debris producing zone 9 initially (in the absence
of additional electromagnetic fields, however, see the embodiment
illustrated in FIG. 5-FIG. 7) travels substantially in straight
lines from the zone 9. Thus, a shield 11 that shields the
electrodes 5 from a line of sight in a predetermined spherical
angle around the optical axis 3 is able to trap these debris
particles 8, so that in the line of sight a substantial amount of
debris 8 is prevented from entering downstream optics (not shown).
Additionally, the shield 11 substantially does not shield the
radiation coming from the EUV radiation producing pinch zone 10,
since it provides an aperture 12 to a central area (conforming to a
designated pinch zone 10) between the electrodes 5 in the line of
sight, which accordingly can travel into the downstream optics
substantially unhindered by the shield 11. In this way, the debris
(which comes from the electrodes) may be stopped by the shield,
without stopping the EUV radiation. Practically, it is convenient
to shield both electrodes, since it is probable that both
electrodes generate debris-producing zones that can attribute in
debris 8 production.
[0048] The shielding effect can be further optimized by placing the
shields 11 close enough, preferably, a distance ranging between 0.5
and 25 mm to any of the electrodes, to shield a maximum spherical
angle of the debris producing zone 9.
[0049] To minimize a distance with the electrodes, the heat load
will be so high on the shield 11 that it is preferably provided as
a fluid jet 13, for example, of molten Tin. Such a jet could have a
length of about 75 mm and a thickness of several mm, for example
ranging from 0.5 to 3 mm. It is noted that fluid jets are per se
known from US 2006-0011864 which discloses electrodes in a plasma
discharge source in the form of fluid jets, however, there is not
disclosed a shield or at least one fluid jet provided near an
electrode of a pair of electrodes. Accordingly, preferably, the
debris catching shield 11 is provided, as illustrated, by a pair of
fluid jets 13, arranged oppositely and generally parallel to a
longitudinal axis of the electrodes 5. It may however, in certain
embodiments, possible to direct the plasma production substantially
towards one of the electrodes 5, which one electrode will
accordingly be a major contributor in producing debris 8. Such
debris may vary in size and travel speed. For instance, one can
have micro-particles: these are micron-sized particles with
relatively low velocities. In addition, there may be produced
nano-particles, which are nanometer-sized particles with typically
quite high velocities; atomic debris, which are individual atoms
that act as gaseous particles; and ions, which are ionised
high-velocity atoms.
[0050] It is noted that in one embodiment, the fluid jet 13 may be
provided near an electrode of the pair of electrodes without
substantially being configured to shield the electrodes from a line
of sight provided in a predetermined spherical angle relative the
optical axis and to provide an aperture to a central area 10
between the electrodes in the line of sight (unlike the embodiment
shown in FIG. 2). In such an embodiment, according to another
aspect of the invention, the fluid jet 13 may be accelerating the
recombination rate of the plasma, which may increase the frequency
of the EUV source 4 and accordingly may provide a higher power
output of the radiation system. Specifically, the fluid jet 13 may
comprise molten Tin, although other materials may be feasible to
provide the same recombining effect, including, for example water
or a liquid gas, such as liquid nitrogen or liquid argon. An
advantage of the latter is that it may evaporate and thus may leave
no further traces in the system. Additionally, the fluid is
preferably of an electrically conductive material and may be kept
at ground potential, although other materials, such as argon and
nitrogen may also be used.
[0051] The advantage of the fluid jets is that the obstruction is
continuously replaced and can thus withstand very high heat loads.
However, in other embodiments, it may be possible, to provide a
shield 11 that is positioned at generally the same distance nearby
the electrodes 5 as discussed hereabove with reference to FIG. 2,
but that is not formed by a fluid jet, but by a moving element (not
shown), for example, an axially moving metal strip, that moves
generally parallel to an electrode longitudinal axis, and which may
be cooled by providing coolant in a container, for guiding the
moving element there through.
[0052] FIG. 3 shows schematically an embodiment of the invention,
showing a shield in the form of a plurality of fluid jets 13,
arranged in radial direction relative from the central area 10
between electrodes 5 in the line of sight. In such an embodiment,
the fluid jets 13 are provided adjacent to each other, and may be
generally aligned to form a static configuration of generally
radially oriented platelets 14, relative to the central area 10.
Although within the general context of the invention, preferably,
these platelets are oriented to shield the electrodes 5 from a line
of sight provided between the platelets 14, this embodiment may
also have practical applications with the platelets oriented to
include the electrodes 5 in a line of sight provided between the
platelets 14. These applications may benefit from the heat load
capacity of the shield 11 that is provided by the fluid jets 13. A
further advantage is that the jets 13 by nature are not
contaminated by debris depositions since they are continuously
renewed. This is in contrast with a conventional foil trap solution
where solid platelets 14 (foils) are used to provide shielding from
debris 8. These conventional platelets therefore may suffer from
contamination which may hinder a proper transmission of the EUV
radiation.
[0053] In particular, especially for plasma produced discharge
sources 4 operated with Tin plasma, a suitable material for the
fluid jets may also be Tin or a compound comprising Tin, such as
for example Ga--In--Sn, which may be suitable to have a lower
melting point and easier handling properties. Furthermore, although
FIG. 3 shows an embodiment wherein the jets 13 are dimensioned with
a general circular form, other form, including strip forms may be
feasible, thus providing a shield 11 comprising platelets 14 in the
form of single jets, generally of the form as depicted in FIG. 4. A
thickness of such liquid foil may be typically 0.5-1 mm, which is
slightly thicker than conventional foil thicknesses that are about
0.1 mm thick. It is noted that thin liquid foils are discussed in
T. Inamura, H. Tamura, H. Sakamoto, "Characteristics of Liquid Film
and Spray Injected from swirl Coaxial Injector"; Journal of
Propulsion and Power 19 (4), 623-639 (2003). In this publication,
cone-shaped foils are produced. However, preferably, according to
an aspect of the invention, a slit-shaped nozzle is used, in
particular, for providing straight-formed jets that are radially
oriented relative to a centre zone 10 wherein a pinch can develop.
In addition, this static embodiment may be combined with a rotating
foil trap, known per se from EP1491963 and, of course, with other
embodiments described in the current document.
[0054] Under certain circumstances, fluid jets may not be
stable--i.e. they may spontaneously divide into droplets with a
diameter approximately equal to the jet diameter. This means that
it may only be possible to create continuous jets if the diameter
is relatively large (>.about.0.5 mm). Therefore, it may be
advantageous to use jets that intentionally consist of closely
spaced droplets that can have a very small and controllable size,
with a controllable distance between droplets. The ability to
create such stable droplet chains (40 .mu.m diameter with about 40
.mu.m distance) was presented in the EUVL Sematech conference in
Barcelona (Conference 7870, 17 Oct. 2006) by David Brandt (session
3-SO-04) for use as a laser target in a LPP EUV source.
[0055] The stability of the droplet chains means that different
configurations may be employed, depending upon which functional
aspects (recombination and/or debris catching) need to be
optimized. FIGS. 13a-e show examples of such configurations. FIG.
13a depicts a continuous jet 13 in which the recombination surface
is moving in the direction T. FIG. 13b depicts a stable train of
droplets 113, moving in direction T, which for the purposes of this
invention may be considered to be a jet 13. The stability of the
droplet chains means that these chains may be positioned adjacent
to each other to add an extra degree of flexibility when
implementing the invention. FIG. 13c shows two adjacent chains of
droplets 113, effectively creating a jet 13, extended in one
direction compared to the jet 13 of FIG. 13b. A disadvantage of a
droplet chain is that debris has a possible path to pass through
the fluid jet. FIG. 13d and FIG. 13e show how the droplet chains
can be shifted in the direction of movement T with respect to each
other to effectively create a virtual continuous jet 13 for debris
having a trajectory in the plane of the figure and perpendicular to
the direction of movement T of the jet.
[0056] FIG. 4 in addition shows a further embodiment according to
an aspect of the invention, wherein the debris catching shield,
herebelow also indicated as a foil trap 15 comprises a static
configuration of generally radially oriented platelets 14, relative
to the central area 10, wherein the platelets 14 are oriented to
shield the electrodes 5 from a line of sight provided between the
platelets 14. In this embodiment, at least some of the platelets
are of a solid nature, in particular, of foils used in a so called
conventional foil trap. It is noted that WO 99/42904 A1 discloses a
foil trap of generally the same configuration; however, the
publication does not discuss that the platelets 14 are configured
to shield the electrodes 5 from a line of sight provided in a
predetermined spherical angle relative the optical axis and to
provide an aperture to a central area 10 between the electrodes in
the line of sight. In comparison with conventional rotating foil
traps of the type as disclosed in EP1491963, this static foil trap
configuration may have an advantage in easier cooling properties,
since, in an embodiment, this static foil trap configuration can be
cooled using static coolant circuits devised on or in proximity of
the platelets 14. Since the configuration is static, accordingly,
cooling may be much simpler and therefore, the configuration can be
easily scaled to higher power levels of the source. In addition,
this configuration has as a benefit that it does not require moving
parts, which may provide constructional advantages since the
required strength and dimensions of the platelets 14 may be of a
different order than the rotating conventional construction, which
requires complex parts such as air bearings and high tension
materials that can withstand centrifugal tension forces applied to
the platelets. Thus, according to the proposed embodiment, the
radially oriented platelets 14 are aiming at the pinch zone 10 thus
substantially unhindering transmittance of EUV-radiation 16. This
foil trap 15 will fill up with debris at certain locations so a
slow rotation around the optical axis (e.g. once a day) could be
useful to make sure no debris will contaminate the next foil trap
15 or other optics. This may be useful, since in a preferred
embodiment, the optical axis may be 45 degrees with respect to a
level plane. This principle could also be designed in combinations
of concentric circles and plates. In addition, the geometry of the
depicted embodiment, including static radially oriented platelets
14, may have stacking dimensions that have high gas resistance
wherein a distance between the platelets may be in an order of
0.5-2 mm, preferably about 1 mm. Accordingly, atomic debris may be
trapped easier. Also, a high gas resistance may help to allow a
lower buffer gas pressure near the pinch zone 10, which may
resulting in a higher efficiency EUV power. Typically, such a
buffer gas may be Argon gas.
[0057] In addition to the thermal cleaning techniques illustrated
with reference to the FIG. 10-FIG. 12 presented herebelow, the
platelets 14 may provided as a material of porous characteristics
for removing the debris from the platelets through capillary
action. For instance, by using foil material with porous
characteristics (e.g. sintered materials) Tin can be taken out of
the optical path and drained (or buffered in an exchangeable
element). Accordingly, lifetime of the debris suppression system
may be increased and downtime due to foil trap cleaning may be
minimized.
[0058] In addition to the above-discussed cleaning technique, the
radiation system may comprise an excitator 17 (see FIG. 4) for
removing the debris from the platelets 14 through mechanical
excitation of the platelets 14. For example, by rotating the module
fast enough (.about.2000-3000 RPM as an indication) on a
temporarily basis, the tin may be spun of the relevant foils, and
may be caught by a getter 18. In an embodiment, the revolution axis
is the optical axis, but other axes of revolution may also be
possible. A combination of rotation and vibration is also an
option. Accordingly, the excitator may comprise a centrifuge for
removing the debris from the platelets through centrifugal action
and advantageously a getter 18 for catching debris 8 removed from
the platelets.
[0059] Also, the foil could be externally excitated (longitudinal
waves) so a flow of tin in a predefined direction may be present.
Also (directional) accelerations/vibrations can be used to give
excitation profile(s) (pending between stick/slip effect of the
droplets) to the entire module instead of each separate foil.
[0060] FIG. 5 discloses a further embodiment of the arrangement
described with reference to FIG. 4. In this embodiment, a
deflecting electromagnetic field unit 19 is disposed between the
electrodes 5 and a shield, in this embodiment illustrated as foil
trap 15. By applying an electromagnetic field, charged debris
particles 8 traveling from the debris producing zones 9 can be
deflected, which accordingly can be used to virtually expand the
distance between the EUV radiation producing pinch zone 10 and the
debris producing zones 9 as will be made even more clear with
reference to FIG. 7. In FIG. 5, the deflecting field is produced by
a pair of electrodes 20 arranged oppositely to the optical axis.
Accordingly, a static electric field is generated according to
which the electrically charged particles can be deflected.
[0061] In FIG. 6, in contrast to the embodiment depicted in FIG. 5,
or in addition to it, the electromagnetic deflecting field is
provided as a static magnetic field 21, due to magnet elements 26
(see FIG. 8) arranged around the optic axis 3. For a front view of
this configuration, see FIG. 8. Although various static field
configurations are feasible, an optimally defined field is provided
as a quadrupole field, arranged for deflecting substantially all
electrically charged particles 8 traveling generally in a direction
towards the optical system (not shown), towards a plane 22 oriented
along the radially oriented platelets 14 and generally parallel to
a length axis of the electrodes 5. Preferably, as is also shown in
the Figure, this plane 22 is provided along the optical axis 3.
However, it may be possible to select another region that is more
off axis to deflect the particles thereto. Accordingly, charged
debris particles can be deflected more easily towards the platelets
14 of the foil trap 15, which virtually increases the distance
between the electrodes 5. Consequently, fewer platelets 14 may be
needed to achieve a given extent of debris suppression.
Accordingly, a typical distance may range between 0.5 and 3 mm,
preferably about 2 mm. This significantly increases the optical
transmission of the foil trap.
[0062] The principle of operation in FIG. 6 is as follows. The
rectangle 10 indicates an acceptance width of the foil trap in the
absence of a magnetic field and is accordingly generally
corresponding to a zone 10 from where EUV radiation is produced.
However, particles 8 generated near the edges of the zone 10
(accordingly, produced from a debris producing zone 9) may travel
unhindered through the shield, in this embodiment illustrated as
foil trap 15, without being intercepted, as illustrated by the
trajectory 23.
[0063] By applying a magnetic field of the type as indicated (with
a conventional arrow indication), such debris particles 8 are
deflected towards the optical axis 3. For example, the particle
with trajectory 23 may be deflected to follow the solid line 24 and
no longer be transmitted through the foil trap 15. This is because
on entrance of the foil trap, the particle appears to originate
from a point outside the acceptance width 10 as indicated by the
other dashed line 25. In other words, the application of the
magnetic field effectively narrows down an effective acceptance
width of the shield, which width defines a zone from where debris
particles could enter the system unhindered. Accordingly, for a
given dimensioning of the acceptance width, the optical
transmission may be improved by reducing the number of platelets 11
and applying a magnetic field.
[0064] A typical distance for the acceptance width of the foil trap
in the absence of a magnetic field may be ranging from about 0.5 to
about 2 mm, preferably about 1 mm. For typical foil trap dimensions
(inner radius 30 mm, relative to a central zone 10, outer radius
139 mm), this leads to a foil trap with 137 foils having an optical
transmission of approximately 63%. As the Figure shows, in a
preferred embodiment, the distance d, d' between the platelets 14
may vary, wherein typically a distance d towards the optical axis 3
may increase relative to distances d' away from the optical axis
3.
[0065] FIG. 7 shows how the source of the particles, that is, the
debris producing zone 9 can be virtually shifted over a distance d
to a virtual debris producing zone 9' by applying the magnetic
field. Accordingly, an effective acceptance width may be
reduced.
[0066] In the presence of a magnetic field B, a particle with
charge q and velocity v experiences a Lorentz force given by
F=qv.times.B (1)
[0067] Consequently, if the direction of the magnetic field is
perpendicular to the velocity, the particle follows a circular
trajectory with radius R equal to
R = mv qB ( 2 ) ##EQU00001##
[0068] In the present embodiment, the angular deflection .alpha.
due to the magnetic field depends on the distance over which the
field is applied, which is approximately equal to the inner radius
of the foil trap r.sub.0. The deflection angle is given by sin
.alpha.=r.sub.0/R as shown in FIG. 3. The apparent point of
departure of the particle is accordingly displaced over a distance
d given by
d=r.sub.0 sin .alpha.-R(1-cos .alpha.) (3)
which for small values of .alpha. reduces to
d = r 0 2 2 R ( 4 ) ##EQU00002##
[0069] By substituting Eq. (2), the following expression relating
the displacement d to the characteristic parameters q, m and v of
the debris particles is obtained:
d = qBr 0 2 2 mv ( 5 ) ##EQU00003##
[0070] Using permanent magnets or electromagnets, a magnetic field
of the order of 1 T can fairly easily be achieved. When a magnetic
field is applied so that the displacement d is equal to 0.5 mm for
a certain type of debris, the acceptance width for that debris
accordingly effectively decreases by a factor of 2 compared to the
earlier mentioned value of 1 mm acceptance width. One can therefore
construct a foil trap that has an acceptance width of 2 mm and
still obtain the same extent of debris mitigation. Such a foil trap
may have only 69 foils and an optical transmission of 70%. Thus,
the optical transmission is significantly improved by applying a
magnetic field.
[0071] FIG. 8 shows a front view, seen along the optic axis, of the
electrodes 5 and a quadrupole magnet configuration of magnets 26.
In this configuration, the North-South lines of opposing magnets 26
are oriented alternating and generally parallel to the longitudinal
axis of the electrodes 5. Accordingly, a magnetic field may be
produced that follows the orientation depicted in FIG. 6, that is,
with a general direction of the magnetic field on either sides of
the optic axis 3 in a plane generally parallel to the length axis
of the electrodes, to deflect the particles inwards towards a plane
22 coaxial with the optic axis 3. Accordingly, for typical
configurations, positively charged particles are focused to a
vertical plane (by focusing in the horizontal direction and
spreading in the vertical direction). Alternatively, a similar (but
less well-defined) deflecting field may be obtained by placing two
identical magnetic poles on opposite sides of the optical axis.
[0072] FIG. 14 shows a schematic perspective view of a further
embodiment of a radiation system 1 according to an aspect of the
invention. The radiation system 1 is arranged for generating a beam
of radiation in a radiation space. FIG. 15 shows a schematic
perspective view of a cross section of the radiation system 1 of
FIG. 14. Similar to the radiation system shown in FIG. 2, the
radiation system 1 shown in FIGS. 14 and 15 comprises a plasma
produced discharge source for generating EUV radiation. The
discharge source includes a pair of electrodes 5 that are
constructed and arranged to be provided with a voltage difference,
and a system that typically includes a laser for producing a vapor
between the pair of electrodes 5 so as to provide a discharge
between the electrodes. Further, the electrodes 5 define a
discharge axis 40 interconnecting said electrodes 5. The discharge
axis 40 traverses the central area between the electrodes. The
radiation space is substantially bounded between two mutually
reversely oriented cones 41, 42 relative to the discharge axis 40,
the cones 41, 42 having their apex 43 substantially in the central
area between the electrodes 5. The two cones 41, 42 have a diabolo
type appearance. The radiation system 1 further comprises a debris
catching shield constructed and arranged to catch debris from said
electrodes 5 from a line of sight provided in the radiation space
44 bounded between the two cones 41, 42, and to provide an aperture
to the central area between the electrodes in said line of sight.
The debris catching shield extends circumferentially around the
discharge axis 40 over at least 180.degree., preferably over at
least 270.degree.. By arranging the shield such that the shield
surrounds the discharge axis 40 over at least 180.degree. the
effective optical output of the plasma source is relatively high. A
beam of radiation generated by the plasma source and passing the
debris catching shield has a larger spherical extension compared
with the embodiment of the radiation system shown in FIG. 2. As a
consequence, the performance of the plasma source output that can
be collected for further processing, increases with respect to the
radiation system shown in FIG. 2. Further, by extending the debris
catching shield circumferentially around the discharge axis 40 up
to 360.degree. an optimal effective optical output is obtained. In
one embodiment, the shield extends over a circumferential range of
approximately 270.degree. to approximately 360.degree., a space
near the discharge axis is available, e.g. for inspection purposes
and/or for arranging devices, such as a system for producing the
vapor between the pair of electrodes and/or a cooling
structure.
[0073] The debris catching shield of the radiation system 1 in FIG.
14 includes a ring shaped or ring section shaped structure that is
substantially rotationally symmetric with respect to the discharge
axis 40. As a consequence, debris suppression can be obtained along
radial directions in a substantial circumferential range around the
discharge axis 40, viz. in a circumferential range of at least
180.degree. around the discharge axis 40. The debris catching
shield comprises a static configuration of generally radially
oriented platelets, relative to the discharge axis 40, wherein the
platelets are oriented to shield the electrodes from a line of
sight provided between the platelets. It appears that good debris
suppression can be obtained along directions having an angle of at
least 45.degree. with respect to the discharge axis 40. Platelets
of the debris catching shield have concentric conical surfaces
and/or comprise at least one planar section.
[0074] In a preferred embodiment, the platelets, also called foils,
have concentric conical surfaces aligned with respect to the
discharge axis 40, with their apex at the central area along the
discharge axis. In another embodiment, the foils can be composed of
a multiple number of planar sections, the foil being aligned with
respect to the discharge axis. For example, each foil may have, in
cross-sections thereof, a hexagonal or octagonal shape.
[0075] FIG. 9 shows a further embodiment of the static
configuration of generally radially oriented platelets 14 described
with reference to FIG. 4. In this embodiment, instead of solid
monolithic platelets 14, in at least some of the platelets 14,
traverses 27 are provided oriented generally transverse to the
platelets 14. This embodiment may provide thermal isolation to the
further downstream platelets 14, as seen from the EUV source 4. In
addition to it, possibly by applying fluid jets as shown in FIG. 3,
preferably on a proximal side of the platelets 14 relative to the
EUV source 4, the heat load to the platelets 14 can be further
managed. In addition, a gas 28 can be guided through the traverses
27 of the platelets 14, which may be used for cleaning purposes of
the platelets 14, for example, a hydrogen radical gas. Accordingly,
the platelets 14 can be cleaned to prevent debris depositing on the
platelets 14, thereby preventing a situation in which EUV light
will no longer be able to pass through the platelets. Preferably,
the foil trap may be cleaned without having to take the foil trap
out of the system. The principle of additional traverses in the
shown foil trap embodiment could also be used for other types of
foil traps, in particular, in non-static foil traps.
[0076] In addition to, or alternatively, the traverses may be used
as a buffer gas to provide a buffer gas zone within a zone in side
the platelets, in order to be able to further trap, for example,
neutral nanoparticles which may diffuse through the platelets 14
and may cause contamination of the optical system provided
downstream (not shown). FIG. 9A shows a side view of an embodiment
with traverses 27, which may be provided with alternating use of
wires 29 and platelet parts 30.
[0077] FIG. 9B shows an embodiment with only wires 29; to provide a
configuration similar to the fluid jet configuration depicted in
FIG. 3. FIG. 9C in addition shows a top view generally seen along
an axis parallel to the length axis of the electrodes 5, of the
platelet embodiment depicted in FIG. 9A. The more open structure of
FIG. 9B has an advantage when integrating foil trap cleaning based
on hydrogen radicals, because it becomes easier to bring the
reactive H radicals to the surface of the foils, and it becomes
easier to transport the reaction products out of the foil trap 15.
However, the drawback is that the flow resistance of the foil trap
15 becomes lower, which may make it more difficult to achieve a
high buffer gas pressure. Therefore one needs to optimize the
amount of openings in the platelets. The preferred embodiment
therefore is in most cases a partially open foil structure, as
shown in FIG. 9A. Furthermore, in a preferred embodiment H cleaning
is integrated with the wired structures shown in the figures by
providing an electric current supply 31, which is connected to at
least some of the wires 29 of a platelet 14. At least some of the
wires 29 in the platelet are now interconnected in order to allow a
current to run through several wires 29 simultaneously. With a high
enough current (for example, 20 A for a 0.4 mm thick wire), the
wires will form a filament that will reach temperatures of about
2000.degree. C. where typically H2 molecules will dissociate,
generating H radicals. These H radicals can then react with Sn to
form gaseous SnH4, which is pumped out of the system. In order to
add H2 to the system, the embodiment therefore further comprises a
H2 gas inlet 32 and the embodiment comprises a vacuum pump 33 to
remove gas from the system (as shown in FIG. 9C).
[0078] Alternatively, it is possible to remove debris from the
capture shield using evaporation. FIG. 10 schematically indicates a
comparison between removal rates by evaporation for lithium and
tin. Along the horizontal axis is plotted the temperature, in
degrees Celsius. Along the vertical axis is plotted the removal
rate (nm/hour). In particular FIG. 10 shows a graph of a
calculation that was performed to calculate the removal rate of tin
and lithium, for temperatures in a range of 200-800.degree. C. In
addition, for tin a removal rate of about 0.1 nm/hour was
calculated for a temperature of about 900 K, and a rate of about
1E5 nm/hour for a temperature of about 1400 K, with an almost
exponential increase. Thus, in a range between these temperature
values, by providing a heating system (which may be EUV source 4)
the debris catching shield, in particular a foil trap 15 of the
kind as shown in FIG. 4 may be selectively heated to elevate a
temperature of the debris shield to a temperature for evaporating
debris from the debris catching shield. In addition a gas supply
system is provided which may in use serve for providing a buffer
gas flow between the platelets, and which may off line be used for
cleaning purposes, in particular, for providing a gas flow to
evacuate evaporated debris from the debris catching shield. A
particular preferable elevation temperature of the debris catching
shield for a tin plasma source may be at least 900 K for offline
cleaning purposes. Accordingly an alternative may be provided for
chemically reactive cleaning, which may be harmful to the optics
system. For a temperature of the platelets 14 of 940 K (667 C) a
Tin evaporation of 0.4 nm/hour may be achievable.
[0079] Advantageously, a lithium plasma source is used since
lithium has a significantly higher vapor pressure than tin (about 9
orders of magnitude) and as a consequence also a significantly
higher removal rate (removal rate of 0.4 nm/hr requires temperature
of only 550 K (277 C). This allows applying evaporative cleaning of
lithium-contaminated surfaces at significantly lower temperatures
than evaporative cleaning of tin-contaminated surfaces; evaporative
cleaning of collector shells contaminated with lithium is
feasible.
[0080] FIG. 11 shows a general schematic illustration of the
cleaning principle explained hereabove with reference to FIG. 10.
In particular, a platelet 14 is heated, so that debris 8 deposed
thereon will be evaporated. By providing a gas flow 34 along the
platelet 14, the evaporated debris, for example, tin vapor 35, will
be carried away from the platelet, through which the platelet can
be cleaned. Although FIG. 11 has been explained with reference to a
gas flow along a platelet 14 of a foil trap, the cleaning principle
can be used generally, to clean EUV mirror surfaces in particular,
of downstream optical elements such as a collector element.
[0081] In FIG. 11, the object to be cleaned (a platelet 14 or
mirror optic) is heated while a gas is flowing over the mirror in
order to transport the tin vapor away from the mirror. Heating can
be done with a heating device, but it is also possible to
temporarily reduce active cooling of the object, and use the heat
generated by the EUV source.
[0082] In FIG. 12 this technique is used for the collector 36 of an
EUV lithography setup. In this embodiment the collector shells are
heated one-by-one, in order to evaporate the tin from the
reflective side of the collector shell, and to deposit the tin
vapor on the backside of the collector shell below. When a
collector shell 37 is heated, it will typically evaporate tin on
both sides of the shells. This means that also the backside of the
shell will evaporate tin and deposit this on the reflective surface
of the collector shell above. To prevent this it is preferable to
heat the center shell first, and then continue with the next shell,
etc. Thus by cleaning the collector shells in the right order and
controlling the temperature of the collector shell at the same
time, it is possible to minimize (re)deposition on the reflective
surface.
[0083] FIG. 16 shows a schematic perspective view of a wiping
module 60 of a radiation system according to an aspect of the
invention. The wiping module 60 is provided with a multiple number
of substantially parallel oriented wiping elements 61 that are
movable along respective platelet surfaces 62 of a debris catching
shield. FIGS. 17 and 18 show further schematic views of the wiping
module 60, in a top view and a cross-sectional side view,
respectively. A single frame supports the wiping elements 61. In
particular, the wiping module is implemented as a comb-like
structure wherein the individual wiping elements 61 form the
fingers of the comb. The width of the wiping elements 61 are chosen
such that the elements 61 fill an intermediate space 63 between
adjacent platelet surfaces. As consequence, platelet surfaces
arranged opposite to each other can be wiped simultaneously by
performing one or more movements of the wiping module 60 with
respect to the surfaces to be cleaned. A local wiping element width
W substantially equals the intermediate space 63 distance between
two adjacent platelet surfaces. It is noted that in another
embodiment according to an aspect of the invention, the wiping
elements are not oriented substantially parallel, but otherwise,
e.g. mutually deviating arranged for following a surface shape of a
platelet to be cleaned.
[0084] By moving the wiping module 60 along a moving path of a
wiping element 61 with respect to a platelet surface 62
contamination particles, such as Sn contamination, is swept and/or
pushed away from the platelet surface 62. As the spacings between
platelets of the debris catching shield, also called foil trap, can
be small, contamination particles might quickly fill said
intermediate spacings, thereby strongly reducing a transmittance of
the foil trap. This is especially the case with foil traps that are
directly exposed to micro particle debris emitted by an EUV source,
such as described referring to FIGS. 14 and 15. Thus, by applying
the wiping module 60, contamination particles can be removed from
the platelet surfaces, thereby improving the transmittance of the
foil trap.
[0085] As the wiping elements 61 are substantially parallel
oriented, similarly oriented platelet surfaces can be cleaned.
Further, instead of using a single frame for supporting the wiping
elements, multiple supporting elements can be used for supporting
the wiping elements. It is also possible to mutually interconnect
ends of the finger-like wiping elements, thereby obtaining a
plate-like structure having slots for receiving the platelets.
[0086] It is noted in this context that during use of the wiping
module, the wiping elements move with respect to the platelet
surfaces, meaning that the wiping elements move, or the platelets,
or both such that a net relative movement results. Along a moving
path of the wiping elements with respect to the platelet surfaces,
the intermediate spacing distance between two opposite platelet
surfaces remains substantially constant, thereby maintaining an
efficient wiping operation. In an alternative embodiment, the
distance between the opposite platelet surfaces along said path
varies, e.g. for providing locally a low sweep resistance for the
movement of the wiping elements.
[0087] The wiping elements 61 are arranged for performing a
translation and/or swiveling movement with respect to the
respective platelet surfaces 62. In the embodiment shown in FIGS.
16-18, the wiping elements 61 perform a translation, i.e. the
elements 61 move in a moving direction M, substantially
transversely with respect to the plane wherein the wiping elements
extend. The platelets 62 are substantially planar. Further, the
platelet structure of the debris catching shield, the foil trap, is
substantially invariant in the moving direction M, thereby allowing
an efficient cleaning operation of the wiping module 60. The moving
direction M is substantially transverse with respect to both a
discharge axis and an optical axis of the source.
[0088] Viewed from the discharge 7 between the electrodes of the
source, the platelets 62 extend substantially between a fixed
radial inner distance and a fixed radial outer distance, see e.g.
FIG. 18. As can be deduced from the figures, some space is to be
reserved for accommodating the wiping module 60 between the source
and a collector, especially when the wiping elements 61 are at end
positions of the moving path, in FIG. 18 at an uppermost and
lowermost position.
[0089] Depending on an accumulation rate of contamination
particles, e.g. Sn, the wiping module 60 can be moved along the
platelet surfaces of the foil trap at a specific time interval,
e.g. once every 5 minutes. This can be done online during operation
of the source. However, a significant amount of radiation can be
blocked during a wiping action and it may be necessary to
compensate this loss of illumination with a longer illumination
time, e.g. using a feedback system with a dose sensor. In a
non-operational state of the wiping module 60, in a stationary
position, the wiping module is preferably placed outside a
collection angle of the source, in order to counteract any
radiation blocking. As an example, the wiping module can in the
non-operational state be placed in an uppermost or lower position.
Alternatively, the wiping module may be placed on the optical axis,
the position as shown in FIG. 18, so that it is optimally aligned
with source radiation paths, so that optical losses are relatively
small.
[0090] In an embodiment according to an aspect of the invention,
the wiping module further comprises one or more wipers 64 that are
positioned to clean the wiping elements 61 from contamination
particles that are collected during a wiping movement. Preferably,
the wiping module also comprises a collection base 65 to collect
the contamination particles that are removed from the wiping
elements. As shown in FIG. 18, the wipers 64 can be positioned to
clean the wiping elements when the module is in its uppermost
position or in its lowermost position. Alternatively, the wipers
can also be positioned for cleaning the wiping elements in either
the uppermost position or lowermost position. In the embodiment
shown in FIG. 18, the wipers 64 perform a movement along the
surface of the wiping elements 61. By collecting the contamination
particles in the one or more collection bases 65, the particles
such as Sn, can be removed, e.g. for re-use. In another embodiment
according to the invention, the wiping elements 61 are arranged for
moving along a stationary wiper 64, see e.g. FIG. 19 showing a
schematic cross-section view of a wiping module embodiment.
Specifically, the wiper might comprises two wiper sections placed
opposite with respect to each other and defining a receiving
opening for receiving the wiping elements 61. In further
embodiments according to an aspect of the invention, the wiping
elements are cleaned otherwise, e.g. by using a hydrogen or halogen
cleaning or evaporation process.
[0091] FIG. 20 shows a schematic perspective view of a wiping
module 60 of a radiation system according to a further aspect of
the invention. Here, the platelets 14 of the foil trap are curved,
in particular the platelets have concentric conical surfaces
aligned with respect to a discharge axis of the source as explained
referring to FIG. 14. The apex of the platelets are located
substantially at a central area along the discharge axis. In the
embodiment shown in FIG. 20, the wiping elements 61 of the wiping
module 60 are arranged for performing a swiveling movement with
respect to the respective platelet surfaces. The swiveling axis of
the swiveling movement substantially coincides with the discharge
axis of the EUV source. Since the spacing between the platelets is
substantially invariant under swiveling with respect to the
discharge axis, an effective and efficient wiping operation can be
performed. In radiation system shown in FIG. 20 a more compact
construction is obtained. In particular, no substantial additional
space is required for the wiping module in a non-operational state.
Further, the wiping elements block merely a minimum amount of
radiation during operation as the wiping elements are always
aligned with the central area between the electrodes. In addition,
the cleaning process at extreme positions of the wiping elements
becomes easier.
[0092] According to a further aspect of the invention, the surface
of the wiping elements is treated for enhancing its wetting
properties, e.g. by reduction of oxides or by applying a
coating.
[0093] It is noted that the described wiping module variants can
also be applied in combination with other debris catching shield
types. As an example, such a wiping module can be applied in
combination with a debris catching shield that extends
circumferentially around the discharge axis over at least
180.degree., preferably over at least 270.degree., optionally over
360.degree.. In such an embodiment, the debris catching shield can
be rotated with respect to the discharge axis, thereby performing a
cleaning action by means of a stationary wiping module. Therefore,
according to an aspect of the invention, a radiation system is
provided for generating a beam of radiation in a radiation space,
the radiation system comprising a plasma produced discharge source
constructed and arranged to generate extreme ultraviolet radiation,
the discharge source comprising a pair of electrodes constructed
and arranged to be provided with a voltage difference, and a system
constructed and arranged to produce a discharge between said pair
of electrodes so as to provide a pinch plasma between said
electrodes, a debris catching shield comprising platelets
constructed and arranged to catch debris from said electrodes, and
a wiping module provided with a multiple number of substantially
parallel oriented wiping elements movable along respective surfaces
of said platelets. In a preferred embodiment according to an aspect
of the invention, the intermediate distance between platelet
surfaces is substantially invariant along a moving path of a wiping
element with respect to a platelet surface to be cleaned.
[0094] FIG. 21 shows a schematic cross-sectional side view of a
radiation system according to an embodiment according to the
invention. The radiation system 1 comprises a plasma produced
discharge source and a debris catching shield as explained
referring to FIGS. 14 and 15. The source includes a pair of
electrodes 5 between which electrodes a discharge 7 is generated
during operation of the radiation system 1. In a radiation space
that is bounded between two mutually reversely oriented cones, a
beam of radiation generated passing through a debris catching
shield having a static configuration of generally radially oriented
platelets 14. In the shown embodiment, the platelets 14 form a
ring-shaped foil trap. Further, the system 1 comprises a collector
configuration for modifying a generated beam of radiation, wherein
the collector configuration substantially surrounds the plasma
produced discharge source in a circumferential direction around the
discharge axis. The collector configuration comprises a normal
incidence reflector 44 that extends circumferentially substantially
around the plasma source. In FIG. 21, an upper cross section 44a
and a lower cross section 44b of the reflector 44 is shown. The
reflector 44 is arranged for reflecting the beam of radiation
passed through the foil trap. In the shown embodiment, the
reflector 44 is provided with an elliptic reflector surface so that
the beam 46a, 46b incident upon the reflector surface is
transformed into a converging beam 48a, 48b propagating towards an
intermediate focus point 50. It is noted that the collector
configuration can be arranged to extend over a reduced
circumferential range, e.g. over a circumferential range of
approximately 270.degree. with respect to the plasma source, in
particular if the debris catching shield also does not entirely
enclose the discharge axis 40 in the circumferential orientation.
Further, instead of applying a single normal incidence collector, a
grazing incidence collector, or a combination of a normal incidence
collector and a grazing incidence collector might be applied.
[0095] In addition, it is noted that a collector configuration
substantially surrounding a plasma produced discharge source can
not only be applied in combination with a radiation system
according to the invention having a debris catching shield
constructed and arranged to catch debris from electrodes of a
plasma source, to shield said electrodes from a line of sight
provided in the radiation space, and to provide an aperture to a
central area between said electrodes in said line of sight, but
also in combination with other radiation systems, e.g. provided
with a rotating foil trap configuration. Therefore, according to an
aspect of the invention, a radiation system is provided for
generating a beam of radiation in a radiation space, the radiation
system comprising a plasma produced discharge source constructed
and arranged to generate extreme ultraviolet radiation, the
discharge source comprising a pair of electrodes constructed and
arranged to be provided with a voltage difference, and a system
constructed and arranged to produce a discharge between said pair
of electrodes so as to provide a pinch plasma between said
electrodes, and a collector configuration for modifying a generated
beam of radiation, wherein the collector configuration
substantially surrounds the plasma produced discharge source in a
circumferential direction around discharge axis interconnecting
said electrodes. In a preferred embodiment according to the
invention, the collector configuration extends circumferentially
around the discharge axis over at least 180.degree., preferably
over at least 270.degree., optionally over 360.degree.. In a
further preferred embodiment according to the invention, the
collector configuration is substantially rotationally symmetric
with respect to the discharge axis between the electrodes.
[0096] FIG. 22 shows a diagram of collectable optical power as a
function of an opening semi-angle of a debris catching shield. An
amount of effective, collectable optical power transmitted through
the debris catching shield can be calculated by subtracting the
solid angle allocated to the cones 41, 42 in FIG. 14 from a total
of 4.pi.. The solid angle subtended by a single cone of opening
semi-angle .alpha. is given by 2.pi. (1-cos .alpha.). Hence, the
total solid angle that can be collected is given by:
.OMEGA.=4.pi.-4.pi.(1-cos .alpha.)=4.pi. cos .alpha.=4.pi. sin
.theta. (6)
where .theta. is the opening semi-angle of the foil trap. For
example, a foil trap with .theta.=45.degree. covers 71% of the
total solid angle of 4.pi..
[0097] The amount of power that is actually transmitted through the
debris catching shield can be calculated by integrating the
transmittance of the debris catching shield over the covered solid
angle. The transmittance of the debris catching shield increases
with 0 due to the increasingly dense spacing between the foils.
[0098] FIG. 22 shows a diagram of collectable optical power as a
function of an opening semi-angle of a debris catching shield. The
diagram shows a first curve 80 representing the collectable solid
angle as a function of the semi-angle of the shield according to
equation 6, assuming that optical power is emitted in 4.pi. and
that no losses occur in passing the shield. Further, the diagram
shows a second curve 81 wherein optical losses have been
incorporated according to parameters of a typical foil trap shield.
From the diagram, it can be deduced that, as an example, using a
foil trap with .theta.=45.degree., 45% of the radiation emitted in
4.pi. can be collected after the foil trap. The diagram further
shows a third and fourth curve 82, 83 representing a collectable
power without and with losses in the foil trap, respectively, in a
typical radiation system as shown in FIG. 5, assuming a typical
collection with respect to the optical axis of a beam of radiation.
As can be seen from the diagram, the amount of optical power that
can be collected using a typical ring-shaped foil trap with
.theta.=45.degree. is about four times higher than the collectable
power in the typical radiation system collecting a beam of
radiation using a foil trap as e.g. shown in FIG. 5.
[0099] 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, flat-panel displays, liquid-crystal displays
(LCDs), thin-film magnetic heads, etc. The skilled artisan 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), a metrology tool and/or an 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.
[0100] The term "lens", where the context allows, may refer to any
one or combination of various types of optical components,
including refractive, reflective, magnetic, electromagnetic and
electrostatic optical components.
[0101] The descriptions above are intended to be illustrative, not
limiting. Thus, it will be apparent to one skilled in the art that
modifications may be made to the invention as described without
departing from the scope of the claims set out below.
* * * * *