U.S. patent application number 11/285393 was filed with the patent office on 2007-05-24 for radiation system and lithographic apparatus.
This patent application is currently assigned to ASML NETHERLANDS B.V.. Invention is credited to Wilhelmus Josephus Box, Niels Machiel Driessen, Derk Jan Wilfred Klunder, Maarten Marinus Johannes Wilhelmus Van Herpen.
Application Number | 20070115443 11/285393 |
Document ID | / |
Family ID | 38053116 |
Filed Date | 2007-05-24 |
United States Patent
Application |
20070115443 |
Kind Code |
A1 |
Box; Wilhelmus Josephus ; et
al. |
May 24, 2007 |
Radiation system and lithographic apparatus
Abstract
The invention relates to a radiation system for generating a
beam of radiation. The radiation system includes an extreme
ultraviolet source for generating extreme ultraviolet radiation, a
contamination barrier for trapping contamination from the radiation
source, and a temperature sensor for sensing a temperature of the
contamination barrier.
Inventors: |
Box; Wilhelmus Josephus;
(Belgium, NL) ; Klunder; Derk Jan Wilfred;
(Geldrop, NL) ; Wilhelmus Van Herpen; Maarten Marinus
Johannes; (Heesch, NL) ; Driessen; Niels Machiel;
(Valkenswaard, 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: |
38053116 |
Appl. No.: |
11/285393 |
Filed: |
November 23, 2005 |
Current U.S.
Class: |
355/30 ;
356/237.2 |
Current CPC
Class: |
G03F 7/70858 20130101;
G03F 7/70916 20130101 |
Class at
Publication: |
355/030 ;
356/237.2 |
International
Class: |
G03B 27/52 20060101
G03B027/52 |
Claims
1. A radiation system for generating a beam of radiation
comprising: an extreme ultraviolet radiation source for generating
extreme ultraviolet radiation; a contamination barrier for trapping
contamination from said radiation source; and a temperature sensor
for measuring a temperature of said contamination barrier.
2. A radiation system according to claim 1, wherein the temperature
sensor comprises a probe facing the contamination barrier and is
arranged to be heated contactlessly by said contamination
barrier.
3. A radiation system according to claim 2, wherein said probe is
formed to substantially conform to a geometry of the contamination
barrier.
4. A radiation system according to claim 2, wherein said probe is
formed to substantially conform to a blackbody geometry.
5. A radiation system according to claim 4, wherein said blackbody
geometry is provided by a concave shape for providing a high
apparent emissivity.
6. A radiation system according to claim 2, wherein said probe is
thermally isolated on sides not facing the contamination
barrier.
7. A radiation system according to claim 2, wherein said probe
comprises a metal plate.
8. A radiation system according to claim 7, wherein said metal
plate is tin covered.
9. A radiation system according to claim 2, wherein a thermocouple
sensor is coupled to said probe and is arranged to be heated only
by direct thermal contact with the probe.
10. A radiation system according to claim 1, further comprising a
heater at least partly enclosing said contamination barrier.
11. A radiation system according to claim 1, wherein said
temperature sensor is coupled directly to the contamination
barrier.
12. A radiation system according to claim 11, wherein said
contamination barrier is of a rotating type, and wherein said
temperature sensor is coupled to a rotation axis of the
contamination barrier.
13. A radiation system according to claim 1, wherein said
contamination barrier is of a rotating foil trap type.
14. A radiation system according to claim 1, wherein said extreme
ultraviolet radiation source is a laser induced plasma source.
15. A radiation system according to claim 14, wherein said plasma
source comprises Tin, Lithium, or Xenon.
16. A radiation system for generating a beam of radiation
comprising: an extreme ultraviolet radiation source for generating
extreme ultraviolet radiation; a contamination barrier for trapping
contamination from said radiation source; and a temperature control
system for controlling the temperature of said contamination
barrier.
17. A radiation system according to claim 16, wherein said
temperature control system comprises a temperature sensor for
measuring the temperature of the contamination barrier.
18. A radiation system according to claim 16, further comprising a
heater at least partly enclosing said contamination barrier for
heating the contamination barrier.
19. A lithographic apparatus comprising: a radiation system for
generating a beam of radiation, said radiation system comprising an
extreme ultraviolet radiation source for generating extreme
ultraviolet radiation; a contamination barrier for trapping
contamination from said radiation source; and a temperature sensor
for measuring a temperature of said contamination barrier; a
patterning device for patterning the beam of radiation; and a
projection system for projecting the patterned beam of radiation
onto a substrate.
20. A lithographic apparatus according to claim 19, wherein the
radiation system further comprises a temperature control system for
controlling the temperature of said contamination barrier.
Description
FIELD
[0001] The present invention relates to a radiation system and a
lithographic apparatus that includes the radiation system.
Specifically, the invention relates to a radiation system that
includes an extreme ultraviolet radiation source that generates
extreme ultraviolet radiation, and a contamination barrier for
trapping contamination from the radiation source.
BACKGROUND
[0002] 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.
[0003] In addition to extreme ultraviolet (EUV) radiation,
radiation sources used in EUV lithography generate contaminant
material that is 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 laser induced
plasma or discharge plasma. Hence, in EUV lithography, a desire
exists to limit the contamination of the optical system that is
arranged to condition the beams of radiation coming from an EUV
source. To this end, a contamination barrier can be used that traps
the contamination from an EUV source. One example of a
contamination barrier is for instance disclosed in EP 1491963. This
publication describes a so called foil trap, a device that 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 foil plates. Thus, the foil trap
functions as a contamination barrier trapping contaminant material
from the source. Generally, these foil traps are designed to have a
sufficiently large dimension to trap virtually any contaminant
particle traveling through the trap. However, illumination results
in a relatively high heat load of the contamination barrier. A high
temperature can cause many problems, such as tin evaporation and
consequently reduced debris suppression or undesired mechanical
deformation of the contamination barrier. Therefore, the
temperature of the contamination barrier should be monitored during
EUV operation.
SUMMARY
[0004] It is an aspect of the present invention to provide a
radiation system that includes a contamination barrier that
functions at an optimal working temperature. To this end, a
radiation system for generating a beam of radiation is provided.
The radiation system includes an extreme ultraviolet radiation
source for generating extreme ultraviolet radiation, a
contamination barrier for trapping contamination from the radiation
source, and a temperature sensor for measuring a temperature of the
contamination barrier.
[0005] According to a further embodiment of the invention, a
radiation system may be provided with a temperature control system
for controlling the temperature of the contamination barrier.
[0006] According to an embodiment of the invention, a lithographic
apparatus is provided. The lithographic apparatus includes a
radiation system for generating a beam of radiation. The radiation
system includes an extreme ultraviolet radiation source for
generating extreme ultraviolet radiation, a contamination barrier
for trapping contamination from the radiation source, and a
temperature sensor for measuring a temperature of the contamination
barrier. The apparatus also includes a patterning device for
patterning the beam of radiation, and a projection system for
projecting the patterned beam of radiation onto a substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] 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:
[0008] FIG. 1 depicts a lithographic apparatus according to an
embodiment of the invention;
[0009] FIG. 2 depicts a first embodiment of the invention;
[0010] FIG. 3 depicts a second embodiment of the invention;
[0011] FIG. 4 depicts another embodiment of the invention; and
[0012] FIG. 5 depicts another embodiment according to the
invention.
DETAILED DESCRIPTION
[0013] 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
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.
[0014] 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.
[0015] 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."
[0016] The term "patterning device" as 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.
[0017] 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.
[0018] The term "projection system" as 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".
[0019] 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).
[0020] 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.
[0021] 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.
[0022] 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. The source SO and the illuminator IL,
together with the beam delivery system if required, may be referred
to as a radiation system.
[0023] 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.
[0024] 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.
[0025] The depicted apparatus could be used in at least one of the
following modes:
[0026] 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.
[0027] 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.
[0028] 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.
[0029] Combinations and/or variations on the above described modes
of use or entirely different modes of use may also be employed.
[0030] In FIG. 2, a first embodiment of a radiation system 1
according to the invention is depicted. Typically, the radiation
system 1 may comprise a plasma source 2 such as a Tin, Lithium or
Xenon source, however, other sources are possible. The source 2
depicted in FIG. 2 is typically operated in a pulsed way so that
periodically EUV radiation 3 is generated in conjunction with
contamination debris (not shown) traveling from the source 2. A
rotating foil trap device 4 is mounted in front of the EUV source
2. Contaminant debris, such as micro-particles, nano-particles and
ions can be trapped in walls provided by foil plates 5. Thus, the
foil trap 4 functions as a contamination barrier trapping
contaminant material from the source 2. However, due to the heat
irradiation of the foil trap 4 by the source 2, the foil trap is
heated to high temperatures, in particular, temperatures well above
the melting point of Tin. This can be undesirable since such high
temperatures may lead to Tin evaporation, which may cause
contamination of the lithographic system, and consequently a
reduced debris suppression of the foil trap 4 and possibly
undesired mechanical deformation of the foils 5 in the foil trap 4.
Accordingly, the invention proposes a temperature sensor arranged
for measuring a temperature of said contamination barrier. Through
monitoring of the temperature of the foil trap 4, the temperature
thereof can be kept within acceptable bounds, and if the foil trap
temperature becomes too high, appropriate measures can be taken in
an automated manner or semi-automatically, such as shutting down
the source, shielding the source, or reducing a pulse rate of the
source, or increasing a cooling action to keep the temperature of
the foil trap 4 within acceptable limits.
[0031] The arrangement of FIG. 2 shows an embodiment that includes
a contactless temperature sensing arrangement 6, which may greatly
reduce measuring difficulties in contrast to direct temperature
sensing of the foil trap 4. Indeed, a direct temperature sensing
arrangement, though not excluded from the claims of the invention,
may be less advantageous since it may introduce wiring problems,
especially for the embodiments as shown in the drawing, namely, of
a rotating foil trap type. In addition, potential shielding of EUV
radiation due to the temperature sensor can occur. Alternative
embodiments may include a contamination barrier of a rotating type
wherein said temperature sensor is coupled to a rotation axis of
the contamination barrier as a temperature indication.
[0032] The temperature sensor 6 shown in the drawing comprises a
metal plate 7 facing the foil trap 4 to be heated by said
contamination barrier through heat radiation 8. The metal plate 7
acts as a heat sink, acquiring radiated heat from the foil trap 4.
Indirectly, through the heat acquired by the metal plate 7, a
thermocouple sensor 9 is heated that is mounted on the backside 10
of the metal plate 7, i.e. the side that is not facing the foil
trap 4. This side 10 is further thermally isolated from the
environment, to prevent heat leaks from the metal plate 7. Thus,
the thermocouple sensor 9 measures accurately the temperature of
the heated metal plate 7. Accordingly, the temperature can be
measured contactlessly. Radiation theory can relate the temperature
of the metal plate 7 with the temperature of the foil trap 4, via a
calibration factor which can be properly established. A more
detailed discussion of this theory will follow referring to FIG. 4
and FIG. 5 in which an apparent emissivity factor will be taken
into account.
[0033] The illustrated contact-less setup of FIG. 2 is generally
robust to EUV radiation and/or contamination debris, in particular,
Tin that is emitted from a Tin source. Advantageously, the metal
plate 7 comprises a Tin layer and/or is formed conform a geometry
of the contamination barrier. For the rotating foil trap 4
illustrated in the figure, this means that the plate has a form
that is partly cylindrical or conical with a curvature which is
concentric with a rotational axis 11 of the foil trap 4 (as
indicated in FIG. 3). The setup can be dimensioned relatively small
and an estimated accuracy can be about 10 K. Another possibility of
sensing the temperature is via a pyrometer (not shown), which may
be used to directly sense incoming heat radiation. Such a pyrometer
tends to be bulky and sensitive to debris, because the measurement
requires that the path between the foil trap and the pyrometer is
reasonably transparent for infrared radiation. However, through
appropriate shielding against debris or through cleaning or
replacing the optical components a contactless temperature
measurement may be possible.
[0034] FIG. 3 shows a further embodiment which is a variation of
the embodiment depicted in FIG. 2. In this embodiment, a heater 12
is placed around a rotating foil trap 4 of the kind illustrated in
FIG. 2. The heater 12 may typically comprise a metal hull enclosing
the foil trap 4, of a cylindrical or conical form and concentric
with the symmetry axis of the foil trap 4. In addition, although in
FIG. 2 a foil trap of a rotating type is illustrated, other types,
in particular, static types, may be used also. The heater 12 may be
used to heat the foil trap 4 to a certain desired working
temperature prior to turning on the EUV source 2. In this
embodiment the temperature sensor 6 thus may have a multifold
function including detecting a heater temperature in an
on-condition of the heater 12, and detecting a rotating foil trap
temperature in an off-condition of the heater 12. To this end, the
sensor 6 is coupled to processing circuitry 13, which can provide
appropriate control of the heater 12 and/or the rotating foil trap
temperature. Reference numeral 11 indicates a rotational axis of
the foil trap 4
[0035] FIG. 4 shows a variation of the embodiment of the
contact-less temperature sensing arrangement disclosed in FIG. 2.
The calibration factor referred to previously, is related to the
apparent emissivity fraction .epsilon. known from radiation theory.
This fraction is related to the material and geometric properties
of the radiating body relative to a blackbody radiation. With a
changing surface and or shape of the foil trap 4 and/or probe 14,
this factor may vary, thus impeding proper temperature measurement
of the foil trap 4.
[0036] Where for blackbody radiation, the Stefan Boltzmann relation
holds for the radiated thermal energy W.sub.b per unit surface
area: W.sub.b=.sigma.T.sup.4 Eq 1
[0037] for a radiative body having an apparent emissivity of
.epsilon. the radiated thermal energy per unit surface area is
W=.epsilon.W.sub.b Eq 2
[0038] Accordingly, a temperature relation between the foil trap
T.sub.FT and a measuring probe T.sub.probe holds: T FT = ( probe ft
.times. .eta. ) 1 / 4 .times. T probe Eq .times. .times. 3
##EQU1##
[0039] with .epsilon..sub.probe and .epsilon..sub.FT the apparent
emisivities of the probe 14 and the foil trap 4 respectively, and
.eta. being a correction factor correcting for difference in the
areas of the foil trap and the probe and the resulting difference
in the intensity fraction of thermal radiation reaching the probe
14 radiated from the foil trap 4.
[0040] It can be seen that a change in apparent emissivity of the
probe 14 and/or the foil trap 4 may be of considerable influence to
a measured foil trap temperature. This is especially the case for
Tin operated sources. In particular, a metal plate 7 covered with
tin has an emissivity of 0.05. However, after venting, a formed tin
oxide layer has an emissivity of 0.8. For the foil trap, the
apparent emissivity of is close to 1 when covered with tin oxide,
and about 0.45 when covered with Tin. Accordingly, due to a change
in emissivity, in case of a foil trap covered with pure Tin, the
measured foil trap temperature T.sub.FT.sub.--.sub.measured can be
overestimated with 64% to the actual foil trap temperature T.sub.FT
due to a factor: T FT_measured T FT = ( probe_ox ft_ox ) 1 / 4
.times. ( ft_Sn probe_Sn ) 1 / 4 Eq .times. .times. 4 ##EQU2##
[0041] with .epsilon..sub.probe.sub.--.sub.ox;
.epsilon..sub.FT.sub.--.sub.ox being the apparent emissivity of the
Tin oxidized probe 14 and the Tin oxidized foil trap 4 respectively
(about 0.8 and 1 respectively); and
[0042] .epsilon..sub.probe.sub.--.sub.Sn;
.epsilon..sub.FT.sub.--.sub.Sn being the apparent emissivity of the
probe 14 covered with pure Tin and the foil trap 4 covered with
pure Tin respectively (about 0.05 and 0.45 respectively). This
change in apparent emissivity is most relevant just after starting
the EUV source when the source 2 contaminates foil trap 2 and probe
14 with pure Tin. Calibrating the measurement for the case where
both the probe 14 and the foil trap 4 are covered with pure Tin
will result in a correct measurement after some time.
[0043] In embodiment depicted in FIG. 4, changes in emissivity of
the foil trap 4 and/or the probe 14 can be mitigated by forming the
probe 14 to substantially conform to a part of the geometry of the
contamination barrier 4. Preferably, the probe 14 is formed to
exactly conform to a part of the geometry of the contamination
barrier 4. Thus, a probe 14 is provided having foils 15 arranged
according to the foil structure of the foil trap 4, in particular,
in the embodiment depicted in FIG. 4, a segment 16 of radially
oriented foils 15 where a thermocouple sensor 17 is arranged near
the top 18 of the segment 16, preferably, in a central foil 19. In
the vicinity of the central foil 19 additional thermocouple sensors
may be provided (not shown) for spatial averaging and improving the
sensor reliability. This embodiment has as advantage that it is
less sensititve to the apparent emissivity factor .epsilon.
variations of the foil trap and/or the probe 14.
[0044] Indeed, in this way, the .epsilon. factors in Eq 3 and Eq 4
will be cancelled since the apparent emissivities of
.epsilon..sub.plate and .epsilon..sub.FT will be about equal,
irrespective of whether covered by pure Tin or tin oxide due to the
identical geometry. From Eq 4 it can be seen that in that case the
measured foil trap temperature (which derives from the probe
temperature by proper calibration) equals the actual
temperature.
[0045] As an alternative to the FIG. 4 embodiment, FIG. 5 shows an
arrangement wherein the apparent emissivity of the probe is
increased to a value close to 1 through an appropriate geometric
shape, irrespective of whether the walls are covered with Tin or
with Tin oxide. Accordingly, the probe is formed to substantially
conform to a blackbody geometry, which means it has an apparent
emissivity matching a blackbody. This can be provided by a concave
shape of the probe 20 for providing a high apparent emissivity. In
general, a concave form of a radiating body increases it's apparent
emissivity through reflection of heat radiation and thus increasing
a local heat energy field.
[0046] In the arrangement of FIG. 5, for a foil trap and probe
covered with Tin, the measured temperature equals 0.82 times the
actual foil trap temperature: measurement underestimates the foil
trap temperature with 18%. In other words, increasing the apparent
emissivity of the probe results in an improvement of the accuracy
of the temperature measurement. This apparent emissivity can be
provided by forming a labyrinth of surfaces 21 for generating heat
radiation that can be recycled through reflection and emittance of
the surfaces.
[0047] 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.
[0048] Although specific reference may have been made above to the
use of embodiments of the invention in the context of optical
lithography, it will be appreciated that the invention may be used
in other applications, for example imprint lithography, and where
the context allows, is not limited to optical lithography. In
imprint lithography a topography in a patterning device defines the
pattern created on a substrate. The topography of the patterning
device may be pressed into a layer of resist supplied to the
substrate whereupon the resist is cured by applying electromagnetic
radiation, heat, pressure or a combination thereof. The patterning
device is moved out of the resist leaving a pattern in it after the
resist is cured.
[0049] The terms "radiation" and "beam" used herein encompass all
types of electromagnetic radiation, including ultraviolet (UV)
radiation (e.g. having a wavelength of or about 365, 355, 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.
[0050] 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.
[0051] While specific embodiments of the invention have been
described above, it will be appreciated that the invention may be
practiced otherwise than as described. For example, the invention
may take the form of a computer program containing one or more
sequences of machine-readable instructions describing a method as
disclosed above, or a data storage medium (e.g. semiconductor
memory, magnetic or optical disk) having such a computer program
stored therein.
[0052] 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.
* * * * *