U.S. patent application number 16/626125 was filed with the patent office on 2020-04-23 for radiation source module and lithographic apparatus.
This patent application is currently assigned to ASML Netherlands B.V.. The applicant listed for this patent is ASML Netherlands B.V.. Invention is credited to Yue MA, Gunes NAKIBOGLU, Hrishikesh PATEL, Albert Pieter RIJPMA, Antonius Johannus VAN DER NET, Rens Henricus VERHEES, Zongquan YANG.
Application Number | 20200124976 16/626125 |
Document ID | / |
Family ID | 62597491 |
Filed Date | 2020-04-23 |
United States Patent
Application |
20200124976 |
Kind Code |
A1 |
PATEL; Hrishikesh ; et
al. |
April 23, 2020 |
Radiation Source Module and Lithographic Apparatus
Abstract
A radiation source comprising: a fuel supply device configured
to supply fuel; an excitation device configured to excite the fuel
into a plasma; a collector configured to collect radiation emitted
by the plasma and to direct the radiation to a beam exit; a debris
mitigation system configured to collect debris generated by the
plasma, the debris mitigation system having a component having a
conduit passing therethrough; and a temperature control system
configured to selectively increase or decrease the temperature of
the component by selectively heating or cooling a thermal transfer
fluid circulating through the conduit.
Inventors: |
PATEL; Hrishikesh;
(Eindhoven, NL) ; MA; Yue; (Escondido, CA)
; NAKIBOGLU; Gunes; (Eindhoven, NL) ; RIJPMA;
Albert Pieter; (Veldhoven, NL) ; VAN DER NET;
Antonius Johannus; (Tilburg, NL) ; VERHEES; Rens
Henricus; (Eindhoven, NL) ; YANG; Zongquan;
(Eindhoven, NL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ASML Netherlands B.V. |
Veldhoven |
|
NL |
|
|
Assignee: |
ASML Netherlands B.V.
Veldhoven
NL
|
Family ID: |
62597491 |
Appl. No.: |
16/626125 |
Filed: |
June 8, 2018 |
PCT Filed: |
June 8, 2018 |
PCT NO: |
PCT/EP2018/065113 |
371 Date: |
December 23, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62523911 |
Jun 23, 2017 |
|
|
|
62569105 |
Oct 6, 2017 |
|
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|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G03F 7/70916 20130101;
H05G 2/008 20130101; G03F 7/70033 20130101; H05G 2/005
20130101 |
International
Class: |
G03F 7/20 20060101
G03F007/20; H05G 2/00 20060101 H05G002/00 |
Claims
1-27. (canceled)
28. A radiation source comprising: a fuel supply device configured
to supply fuel; an excitation device configured to excite the fuel
into a plasma; a collector configured to collect radiation emitted
by the plasma and to direct the radiation to a beam exit; a debris
mitigation system configured to collect debris generated by the
plasma, the debris mitigation system having a component having a
conduit passing therethrough; and a temperature control system
configured to selectively increase or decrease the temperature of
the component by selectively heating or cooling a thermal transfer
fluid circulating through the conduit.
29. The radiation source of claim 28, wherein the temperature
control system is operative in a first mode to cool the component
to a first temperature that is below the melting point of the fuel
and is operative in a second mode to heat the component to a second
temperature that is above the melting point of the fuel.
30. The radiation source of claim 28, wherein the thermal transfer
fluid comprises water.
31. The radiation source of claim 30, wherein the thermal transfer
fluid is at a pressure greater than atmospheric or greater than
about 20 bar.
32. The radiation source of claim 28, wherein the thermal transfer
fluid comprises air, artificial air, or nitrogen.
33. The radiation source of claim 32, wherein the temperature
control system comprises a heat exchanger configured to transfer
heat from thermal transfer fluid coming from the component to heat
transfer fluid heading towards the component.
34. The radiation source of claim 33, wherein the temperature
control system comprises an active flow control device on the cool
side of the heat exchanger.
35. The radiation source of claim 28, further comprising: a vacuum
chamber enclosing the collector and debris mitigation system; and
wherein all active flow control devices of the temperature control
system are outside the vacuum chamber.
36. The radiation source of claim 28, wherein the component
comprises a source chamber wall, an obscuration member, a scrubber,
a vane, a heat shield, a shroud for a fuel supply trajectory, a
droplet catcher or a debris bucket.
37. The radiation source of claim 28, wherein: the debris
mitigation system comprises a plurality of components each having a
conduit passing therethrough, and the temperature control system
comprises a plurality of independently controllable circuits
configured to circulate the thermal transfer fluid through
respective components.
38. The radiation source of claim 37, wherein the plurality of
independently controllable circuits comprises a first circuit
configured to maintain a first set of components at a temperature
lower than the melting point of the fuel and a second set of
components at a temperature higher than the melting point of the
fuel.
39. The radiation source of claim 28, wherein the debris mitigation
system comprises a plurality of components each having a conduit
passing therethrough, and the temperature control system comprises
a circuit configured to circulate the thermal transfer fluid
through a plurality of the components in series.
40. The radiation source of claim 28, wherein the fuel supply
device comprises a droplet generator configured to supply droplets
of tin as the fuel.
41. The radiation source of claim 28, wherein the excitation source
comprises a laser.
42. A lithographic apparatus comprising: a radiation source
comprising: a fuel supply device configured to supply fuel; an
excitation device configured to excite the fuel into a plasma; a
collector configured to collect radiation emitted by the plasma and
to direct the radiation to a beam exit; a debris mitigation system
configured to collect debris generated by the plasma, the debris
mitigation system having a component having a conduit passing
therethrough; and a temperature control system configured to
selectively increase or decrease the temperature of the component
by selectively heating or cooling a thermal transfer fluid
circulating through the conduit; an illumination system configured
to illuminate a patterning device with radiation from the radiation
source; and a projection system arranged to project a pattern from
the patterning device onto a substrate.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority of U.S. application
62/523,911 which was filed on Jun. 23, 2017 and U.S. application
62/569,105 which was filed on Oct. 6, 2017 which are incorporated
herein in its entirety by reference.
FIELD
[0002] The present invention relates to radiation sources for EUV
radiation, and to lithographic apparatus using such radiation
sources.
BACKGROUND
[0003] A lithographic apparatus is a machine constructed to apply a
desired pattern onto a substrate. A lithographic apparatus can be
used, for example, in the manufacture of integrated circuits (ICs).
A lithographic apparatus may for example project a pattern from a
patterning device (e.g. a mask) onto a layer of radiation-sensitive
material (resist) provided on a substrate.
[0004] The wavelength of radiation used by a lithographic apparatus
to project a pattern onto a substrate determines the minimum size
of features which can be formed on that substrate. A lithographic
apparatus which uses EUV radiation, being electromagnetic radiation
having a wavelength within the range 5-20 nm, may be used to form
smaller features on a substrate than a conventional lithographic
apparatus (which may for example use electromagnetic radiation with
a wavelength of 193 nm).
[0005] EUV radiation can be generated by a plasma source in which a
fuel, e.g. tin, is excited to form a plasma which then emits
radiation. The fuel may be excited by a laser, in which case the
source is referred to as a laser-produced plasma source (LPP
source), or an electric discharge, in which case the source is
referred to as a discharge-produced plasma source (DPP source). As
well as the useful EUV radiation, a plasma source emits a lot of
other radiation and a lot of particulate debris: ranging in size
from electrons to small particles of the fuel. It is important to
prevent the debris entering the main part of the lithographic
apparatus as contamination of any of the optical elements in the
lithographic apparatus would severely impact its performance.
[0006] Thus the radiation source includes a debris mitigation
system having various elements to capture and otherwise prevent
debris exiting the source module into the rest of the lithographic
apparatus. Some elements, e.g. vanes, of the debris mitigation
system are intended to capture fuel debris and so are maintained at
a temperature below the melting point of the fuel so that when fuel
debris comes into contact with such an element it solidifies.
However, the temperature of such elements must not be too low as
this encourages growth of fuel deposits in undesirable forms, e.g.
tin wool. Other elements of the debris mitigation system are
maintained at a temperature higher than the fuel melting point to
allow accumulated fuel to flow away. In addition, those elements of
the debris mitigation system that are normally maintained below the
fuel melting point must periodically be heated to a temperature
above the fuel melting point to enable accumulated fuel to be
removed.
[0007] Therefore, a known debris mitigation system of a plasma
radiation source has a variety of heating systems and cooling
systems to heat or cool respective elements of the debris
mitigation system to their respective target temperatures. Known
heating systems use electric heaters to supplement the energy
absorbed from the plasma. The electric heaters may be directly
attached to the elements being heated or heat air which is then
used to heat the respective element. Known cooling systems use
water to cool, in some cases with a gas-filled gap between the
element being cooled and the water-filled conduits. The known
heating and cooling systems are complex, inefficient and can result
in undesirably large temperature gradients on elements of the
debris mitigation systems.
SUMMARY
[0008] It is an aim of the invention to provide an improved
radiation source for EUV radiation.
[0009] According to the present invention, there is provided a
radiation source comprising: [0010] a fuel supply device configured
to supply fuel; [0011] an excitation device configured to excite
the fuel into a plasma; [0012] a collector configured to collect
radiation emitted by the plasma and to direct the radiation to a
beam exit; [0013] a debris mitigation system configured to collect
debris generated by the plasma, the debris mitigation system having
a component having a conduit passing therethrough; and [0014] a
temperature control system configured to selectively increase or
decrease the temperature of the component by selectively heating or
cooling a thermal transfer fluid circulating through the
conduit.
[0015] According to another aspect of the present invention, there
is provided a vacuum chamber comprising: [0016] a vacuum chamber
wall comprising a first wall layer and a second wall layer defining
a gap therebetween, wherein the first wall layer is subjected to a
heat source and the second wall layer is subjected to a cooling
source; [0017] a gas supply device connected to the gap and
configured to supply a gas thereto; and [0018] a controller
connected to the gas supply device and configured to control the
pressure of the gas in the gap so as to control heat flow across
the vacuum chamber wall.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] Embodiments of the invention will now be described, by way
of example only, with reference to the accompanying schematic
drawings, in which:
[0020] FIG. 1 depicts a lithographic system comprising a
lithographic apparatus and a radiation source according to an
embodiment of the invention;
[0021] FIG. 2 depicts a radiation source according to an embodiment
of the invention;
[0022] FIG. 3 is a graph of the boiling point of water as a
function of pressure;
[0023] FIG. 4 depicts a radiation source according to another
embodiment of the invention;
[0024] FIG. 5 depicts a temperature control arrangement for the cap
of the radiation source;
[0025] FIG. 6 depicts an embodiment of the cap of the radiation
source;
[0026] FIG. 7 depicts the cap of FIG. 6 with end plates
removed;
[0027] FIG. 8 depicts the cap of FIG. 6 in cross-section;
[0028] FIG. 9 depicts another embodiment of the cap of the
radiation source; and
[0029] FIG. 10 depicts the inner layer of the cap of FIG. 9.
DETAILED DESCRIPTION
[0030] FIG. 1 shows a lithographic system including a multilayer
reflector according to one embodiment of the invention. The
lithographic system comprises a radiation source SO and a
lithographic apparatus LA. The radiation source SO is configured to
generate an extreme ultraviolet (EUV) radiation beam B. The
lithographic apparatus LA comprises an illumination system IL, a
support structure MT configured to support a patterning device MA
(e.g. a mask), a projection system PS and a substrate table WT
configured to support a substrate W. The illumination system IL is
configured to condition the radiation beam B before it is incident
upon the patterning device MA. The projection system is configured
to project the radiation beam B (now patterned by the mask MA) onto
the substrate W. The substrate W may include previously formed
patterns. Where this is the case, the lithographic apparatus aligns
the patterned radiation beam B with a pattern previously formed on
the substrate W.
[0031] The radiation source SO, illumination system IL, and
projection system PS may all be constructed and arranged such that
they can be isolated from the external environment. A gas at a
pressure below atmospheric pressure (e.g. hydrogen) may be provided
in the radiation source SO. A vacuum may be provided in
illumination system IL and/or the projection system PS. A small
amount of gas (e.g. hydrogen) at a pressure well below atmospheric
pressure may be provided in the illumination system IL and/or the
projection system PS.
[0032] The radiation source SO shown in FIG. 1 is of a type which
may be referred to as a laser produced plasma (LPP) source). A
laser 1, which may for example be a CO.sub.2 laser, is arranged to
deposit energy via a laser beam 2 into a fuel, such as tin (Sn)
which is provided from a fuel emitter 3. Although tin is referred
to in the following description, any suitable fuel may be used. The
fuel may for example be in liquid form, and may for example be a
metal or alloy. The fuel emitter 3 may comprise a nozzle configured
to direct tin, e.g. in the form of droplets, along a trajectory
towards a plasma formation region 4. The laser beam 2 is incident
upon the tin at the plasma formation region 4. The deposition of
laser energy into the tin creates a plasma 7 at the plasma
formation region 4. Radiation, including EUV radiation, is emitted
from the plasma 7 during de-excitation and recombination of ions of
the plasma.
[0033] The EUV radiation is collected and focused by a near normal
incidence radiation collector 5 (sometimes referred to more
generally as a normal incidence radiation collector). The collector
5 may have a multilayer structure (described further below) which
is arranged to reflect EUV radiation (e.g. EUV radiation having a
desired wavelength such as 13.5 nm). The collector 5 may have an
elliptical configuration, having two ellipse focal points. A first
focal point may be at the plasma formation region 4, and a second
focal point may be at an intermediate focus 6, as discussed
below.
[0034] The laser 1 may be separate from the radiation source SO.
Where this is the case, the laser beam 2 may be passed from the
laser 1 to the radiation source SO with the aid of a beam delivery
system (not shown) comprising, for example, suitable directing
mirrors and/or a beam expander, and/or other optics. The laser 1
and the radiation source SO may together be considered to be a
radiation system.
[0035] Radiation that is reflected by the collector 5 forms a
radiation beam B. The radiation beam B is focused at point 6 to
form an image of the plasma formation region 4, which acts as a
virtual radiation source for the illumination system IL. The point
6 at which the radiation beam B is focused may be referred to as
the intermediate focus. The radiation source SO is arranged such
that the intermediate focus 6 is located at or near to an opening
(beam exit) 8 in an enclosing structure 9 of the radiation
source.
[0036] The radiation beam B passes from the radiation source SO
into the illumination system IL, which is configured to condition
the radiation beam. The illumination system IL may include a
facetted field mirror device 10 and a facetted pupil mirror device
11. The faceted field mirror device 10 and faceted pupil mirror
device 11 together provide the radiation beam B with a desired
cross-sectional shape and a desired angular distribution. The
radiation beam B passes from the illumination system IL and is
incident upon the patterning device MA held by the support
structure MT. The patterning device MA reflects and patterns the
radiation beam B. The illumination system IL may include other
mirrors or devices in addition to or instead of the faceted field
mirror device 10 and faceted pupil mirror device 11. The faceted
field mirror device 10, faceted pupil mirror device 11 and other
reflectors of the illumination system may have a multilayer
structure as described further below.
[0037] Following reflection from the patterning device MA the
patterned radiation beam B enters the projection system PS. The
patterning device may include a reflector having a multilayer
structure as described further below. The projection system
comprises a plurality of mirrors which are configured to project
the radiation beam B onto a substrate W held by the substrate table
WT. The projection system PS may apply a reduction factor to the
radiation beam, forming an image with features that are smaller
than corresponding features on the patterning device MA. A
reduction factor of 4 may for example be applied. Although the
projection system PS has two mirrors in FIG. 1, the projection
system may include any number of mirrors (e.g. six mirrors). The
mirrors, and any other reflectors of the projection system PS, may
have a multilayer structure as described further below.
[0038] The radiation source SO shown in FIG. 1 may include
components which are not illustrated. For example, a spectral
filter may be provided in the radiation source. The spectral filter
may be substantially transmissive for EUV radiation but
substantially blocking for other wavelengths of radiation such as
infrared radiation.
[0039] FIG. 2 shows in more detail the radiation source SO. In
addition to the components identified above, the radiation source
SO comprises a debris mitigation system 20 and a temperature
control system 300. The debris mitigation system 20 comprises
various components located within the enclosing structure 9 (also
referred to as a vacuum chamber) of the radiation source SO. The
relevant components of the debris mitigation system 20 are provided
with conduits 301 for the circulation of a thermal transfer fluid
therethrough. The conduits 301 are connected to the temperature
control system 300 which is mostly located outside the vacuum
chamber 9. In particular, all active control devices of the
temperature control system, such as valves and pumps are located
outside the vacuum chamber 9.
[0040] The temperature control system 300 is configured to
selectively heat or cool the thermal transfer fluid in order to
heat or cool respective components of the debris mitigation system
20. When the radiation source is operating, some components of the
debris mitigation system experience a high heat load since the
efficiency of conversion of the energy used to excite plasma 7 into
useful EUV radiation exiting through beam exit 8 is quite low.
Therefore, in periods of operation of the radiation source, various
components of the debris mitigation system require continuous
cooling. As will be discussed further below, some components of the
debris mitigation system 20 are desirably cooled to a temperature
below the melting point of the fuel (e.g. tin) whilst others are
desirably maintained at a higher temperature, above the melting
point of the fuel.
[0041] Since debris accumulates on various components of the debris
mitigation system 20, periodically such components of the radiation
system 20 are heated to a temperature above the melting point of
the fuel in order that the accumulated fuel can be melted and drain
away. Since during this period the radiation source is not
operating, a substantial heat input may be required to raise the
temperature of the components of the debris mitigation system 20
sufficiently.
[0042] Generally, the largest component of the debris mitigation
system 20 is a set of vanes 21 set in a conical arrangement around
the path of radiation from the collector 5 to the beam exit 8.
Vanes 21 collect the majority of debris ejected from the plasma 7
and are configured to minimise splashing of debris therefrom.
[0043] The debris mitigation system 20 also includes a debris
bucket 22 located to collect liquid fuel running off the vanes 21
and a droplet catcher 23 located on the trajectory of droplets
emitted by the droplet generator 3 after the plasma formation
location 4. There is also a shroud 24 to protect the trajectory of
droplets between the droplet generator 3 and plasma forming
location 4. An obscuration member 26 (sometimes referred to a
horizontal obscuration bar) is located directly on the optical axis
of the radiation source between the plasma formation location 4 and
beam exit 8 to block particulate debris traveling directly from the
plasma 7 to the beam exit 8. The obscuration member 26 may comprise
a disk supported by a strut projecting from one of the vanes 21. An
exhaust and scrubber 27 is provided to extract gas from the vacuum
chamber 9 and remove debris from the extracted gas. The upper part
of the vacuum chamber includes a conical part 9A and a cap 9B
closely surrounding the intermediate focus and defining the beam
exit 8.
[0044] As mentioned, the vanes 21 receive the largest portion of
the fuel debris in use and are cooled so that the fuel debris
solidifies thereon and is thus collected rather than allowed to
circulate within the lithographic apparatus. Solid fuel can
therefore accumulate on the vanes quite rapidly and it is desirable
to control the temperature of the vanes 21 in order to control the
form of growth of the solidified fuel thereon. If the fuel is tin,
then if the vanes 21 are too cool, e.g. about 150.degree. C., the
accumulating tin forms tin wool. Tin wool is a relatively low
density form of tin comprising thin tin fibres. An accumulation of
tin in the form of tin wool therefore projects further from the
vanes than if the tin were to accumulate in a more compact form.
Therefore, there is a risk that the tin wool will intercept the
beam of useful radiation from the collector 5 to beam exit 8.
[0045] The temperature of the components of the debris mitigation
system, especially the vanes, also influences other undesirable
phenomena, for example tin spitting and tin dripping. Tin spitting
occurs when molten tin on a surface reacts with hydrogen present in
the vacuum chamber and results in tin particles being ejected into
the vacuum chamber where they may absorb energy from the beam or
travel into other parts of the lithographic apparatus. Tin dripping
occurs when large amounts of molten tin accumulate and detach from
a surface. This can result in large drops of tin falling onto the
collector 5, reducing its reflectivity. Known cooling systems for
components of the debris mitigation system can still result in
large temperature gradients across certain components so that it
becomes difficult to control the temperature of the components
sufficiently to reduce the undesirable phenomena.
[0046] As shown in FIG. 2, a temperature control system 300
according to an embodiment of the invention circulates a thermal
transfer fluid, in this case water, through conduits 301 provided
within relevant components of the debris mitigation system 20. In
an embodiment, the temperature conditioning system 300 is
configured to circulate the thermal transfer fluid through some or
all of the following components of the debris mitigation system 20:
vanes 21, shroud 24, scrubber 27, debris bucket 22, droplet catcher
23, intermediate focus cap 9B, lower cone 28, droplet generator 3
and the heat shield 29.
[0047] In the embodiment of FIG. 2, the temperature control system
300 comprises two high pressure water circuits 310a, 310b. In some
cases a single circuit can be used, in other cases more than two
circuits may be desirable.
[0048] High pressure water circuit 310a is configured to control
the temperature of the vanes 21, obscuration 26 and scrubber 27. An
electric heater 302 is provided to heat the water as necessary
whilst heat exchanger 306 is used to cool the circulating water
when necessary. Heat exchanger 306 is connected to the main water
supply 320 that is used for cooling other parts of the lithographic
apparatus. Valve 304 is used to control whether returning water is
sent to heat exchanger 306. A pump 305 is provided in high pressure
water circuit 310a to circulate the water and maintain an
appropriate pressure within the circuit to prevent the water
boiling. It can be seen from FIG. 3, which is a graph of the
boiling temperature of water as a function of pressure, that by
maintaining the pressure within the high pressure water circuit 310
at about 50 bar it can be ensured that the water can be circulated
at a temperature in the range of from 200 to 250.degree. C. without
risk of it boiling.
[0049] In addition, the control valves 303, 332 and 331 are
provided so that cool water, e.g. at about 22.degree. C., can be
provided from main water supply 320 into the high pressure water
circuit 310a if it is desired to rapidly cool down the relevant
components of the debris mitigation system 20.
[0050] The second high pressure water circuit 310b in this
embodiment is configured to control the temperature of the droplet
generator 3, the debris bucket 22 and the droplet catcher 23. These
components are desirably maintained at a higher temperature, above
the melting point of the fuel, than the vanes 21 and obscuration
member 26 which are temperature controlled using the first high
pressure water circuit 310a. The second high pressure water circuit
310b comprises an electric heater 311 to heat the water as
necessary and a heat exchanger 316 to cool the water when necessary
by exchanging heat with water from the main water supply 320. Flow
control valve 313 and pump 314 function to control flow of the
water within the circuit and maintain it at an appropriate
pressure. Similarly with first high pressure water circuit 310a,
valves 312 and 315 are provided to enable cool water from the main
supply 320 to be introduced directly into the circuit when it is
desired to cool it down rapidly.
[0051] In an embodiment of the present invention using tin as the
fuel, first circuit 310a is configured to maintain the relevant
components of the debris mitigation system 20 at about 200.degree.
C. Therefore, water is supplied to the conduits 301 at a
temperature of 200.degree. C. Since the components such as the
vanes 21 and obscuration member 26 experience a high heat load when
the radiation source is operating, the water returning from the
conduits 301 may be at an elevated temperature. By maintaining a
flow rate of about 10 to 50 lpm it can be ensured that the
temperature of the returning water does not rise above about
210.degree. C. Different temperature ranges may apply if an
alternative fuel is used and different flow rates may be
appropriate depending on the power and conversion efficiency of the
source. When it is desired to remove accumulated fuel from the
vanes 21 and horizontal obscuration bar 26, the water entering the
conduits 301 is heated to a higher temperature, e.g. about
250.degree. C., in order to melt the accumulated tin.
[0052] In the second high pressure water circuit 310b it is desired
to maintain the relevant components above the melting point of the
fuel so that water is supplied to the conduits 301 at a temperature
of about 250.degree. C. Since the water aims to heat the components
such as the droplet catcher 3, some cooling of the water occurs as
it flows around the circuit. By maintaining a flow rate in the
range of about 10 to 50 lpm, sufficient heat can be transferred to
the relevant components whilst ensuring that the returning liquid
does not drop in temperature too far, e.g. no lower than about
245.degree. C.
[0053] This embodiment of the invention provides a number of
advantages compared to prior art systems. By the use of high
pressure water circuits to control the temperature, it is possible
to achieve excellent control over temperature gradients and to
readily change the temperature set point. Heating up and cooling
down times can be reduced, improving throughput of the apparatus.
Embodiments of the present invention are readily serviceable since
all relevant controls and active components can be located outside
the vacuum chamber. No parts that are likely to require replacement
or servicing need be located within the vacuum chamber. By using
the same fluid for both heating and cooling the components of the
debris management system, the overall system is considerably
simplified and the number of components required is reduced. The
above described arrangement is also efficient in its use of
electrical power.
[0054] It should be noted that in the above described embodiment of
the present invention another liquid, such as an oil, may be
substituted for the water as the thermal transfer fluid. However,
water is desirable as having a high heat capacity and fewer safety
issues than, for example, oil.
[0055] A further embodiment of a radiation source according to the
present invention is depicted in FIG. 4. Parts of the radiation
source of FIG. 4 that are the same as in the previous embodiment,
in particular relating to the debris mitigation system, are
indicated by like references and not described further herein for
the sake of brevity. The radiation source SO of FIG. 4 can be used
with a lithographic apparatus LA as depicted in FIG. 1.
[0056] In the radiation source SO of FIG. 4 heated gas, e.g. air,
is used as the thermal transfer fluid. In embodiments of the
present invention the heated air can be atmospheric air sourced
from the environment around the apparatus or artificial air. As in
the embodiment of FIG. 2, the thermal transfer fluid, in this case
gas, is conducted through conduits 401 provided within components
of the debris mitigation system 20. The same advantages, of
simplification of manufacture and operation, as in the first
embodiment are thereby achieved.
[0057] Temperature conditioning system 400 of the embodiment of
FIG. 4 comprises a fan 407 that supplies pressurised air to a
manifold from which control valves 408a, 408b, 408c control the
flow of gas towards conduits 401 in respective components of the
debris mitigation system 20. Gas flowing towards the conduit 401 is
first heated by heat exchanger 406 to recover heat from gas that is
returning from the conduit 401. It is then passed through an
electric heater 402 which is controlled to heat the gas further if
required, especially during a heat up phase. Gas that has flowed
through the conduits 401 is combined into a single return path. The
return path is configured so that the gas can pass through a cooler
403 if it is desired to remove heat from the system, or pass
through bypass channel 405 to avoid the cooler 403, e.g. during a
heat up phase. Control valve 404 is provided to control whether the
return gas flows through the cooler 403 or the bypass channel 405.
Thereafter, the returning gas passes through the heat exchanger 406
to give up heat towards the gas flowing towards the conduits 401.
Returning gas is then recirculated by pump 407.
[0058] By providing heat exchanger 406 and positioning pump 407 and
control valves 408a to 408c on the cold side of heat exchanger 406
it is not necessary to provide pumps and valves capable of
withstanding elevated temperatures and therefore the serviceability
and cost of the system is reduced.
[0059] Components of the debris mitigation system whose
temperatures can be controlled in this embodiment include: the
lower cone, the obscuration member, the scrubber, vanes 21, the
heat shield, the debris bucket and the droplet catcher. For the
sake of clarity, not all of these components are shown in FIG. 4.
For larger components, such as the vanes 21, multiple conduits 401
may be provided in parallel as necessary to ensure even heat flow.
The number of separate circuits can be varied according to the
required heat flows and desired set points. As well as providing a
single heater 402, separate heaters can be provided on each flow
channel. If desired, the return flow paths from different circuits
can be combined after the heat exchanger 406.
[0060] In view of the relatively low heat capacity of a gas such as
air, compared to the water used in the embodiment of FIG. 2, a
higher flow rate may be required. In an embodiment, the total flow
rate of gas may be of the order of 1000 to 3000 slm. A large pump
or a number of pumps may be required to achieve such a flow rate
and it may be desirable therefore to locate these pumps away from
the lithographic apparatus, e.g. in the fab subfloor. Such a
location is advantageous in reducing transmission of vibrations
from the pumps to the lithographic apparatus.
[0061] FIG. 5 depicts an arrangement for controlling the
temperature of the top cone 9b, which can also be referred to as
the cap. Top cone 9b surrounds the exit aperture of the radiation
source and receives a substantial heat load HL from the plasma and
excitation laser beam. It is therefore necessary to extract heat
from the top cone 9b whilst at the same time controlling the
temperature thereof in order to control the accumulation of tin
thereon. Temperature control can be based on feedback control or
feedforward control or a combination of both. Feedforward control
may be based on the operating status of the lithographic apparatus,
e.g. whether exposures are underway. In an embodiment, it is
desirable to control the temperature of the top cone to a target of
about 200.degree. C. at some times and at other times to a target
of about 600.degree. C.
[0062] In an embodiment of the invention at least a part of the top
cone 9b is constructed with two layers, an inner layer 961 and an
outer 962 which together define a gap 99 therebetween. A
temperature control system 900 is arranged to control heat transfer
across the two-layer structure by controlling the pressure of gas
in the gap. Temperature control system 900 includes a gas supply
901 which is connected to gap 99 via a mass flow controller 902.
Mass flow controller 902 may include a controllable valve and a
mass flow meter together with a controller to control the degree of
opening of the valve in order to meet a target mass flow. A
pressure and temperature sensor 903 measures the pressure and
temperature of the gas in the gap 99 and is connected to a
controller 904, e.g. a PID controller. Controller 904 provides a
set point to mass flow controller 902 so as to achieve the desired
pressure and temperature of the gas in the gap 99.
[0063] A vacuum pump 905 is also connected to the gap 99 via
metering valve 906 in order to remove gas from the gap 99 when
required. Gas removed from the gap 99 is sent to the main exhaust
908 of the apparatus. A bypass valve 907 may be provided for use
during equivalent start up.
[0064] In an embodiment, the gap 99 is not perfectly sealed from
the interior of the vacuum chamber 9. Therefore, there will be a
gas exchange GE between the gap 99 and the main part of the vacuum
chamber 9. In many cases, the net gas flow will be out of the gap
99 into the interior of the main vacuum chamber 9 so that a
balancing in flow of gas is necessary to maintain the pressure in
the gap 99 at the desired level. However, in other embodiments the
gap 99 may be effectively completely sealed in which case an inflow
of gas is only required when it is desired to increase the pressure
in the gap 99. Maintaining a flow through the gap 99 can enable
faster control of the gas pressure.
[0065] The heat flow across the gap 99 is dependent on the pressure
of the gas in the gap 99. In an embodiment of the present
invention, the pressure of the gas in the gap 99 is maintained in
the range of from 0.01 Pa to 500 Pa. Desirably the gas in the gap
is in the slip flow regime. In an embodiment the Knudsen number of
the gas in the gap is about 1 or greater. When the Knudsen number
is about 1 or greater, the heat transfer through the gap is
strongly dependent on pressure. The outer layer 962 is maintained
at a relatively low temperature, e.g. by a water-based temperature
conditioning system, so that the interior of the vacuum chamber is
maintained at a desired temperature by balancing the heat sources
within the chamber and the heat transfer across the gap 99. In an
embodiment of the invention, the width of the gap 99 is 3 mm so
that the representative physical length scale L used to calculate
the Knudsen number is about 6 mm. The gap can have a width of from
about 1 mm to 10 mm, desirably 2 mm to 5 mm. If the gas is hydrogen
then the Knudsen number in a 3 mm gap will be greater than 1 at a
pressure of about 10 Pa at 200.degree. C. and at a pressure of
about 20 Pa at 650.degree. C. It can be convenient to use hydrogen
as the gas in the gap in an embodiment of the present invention
because hydrogen is already present within the vacuum chamber 9 as
part of the arrangements for mitigation of plasma debris. However,
it is also possible to use other gases, such as inert gases like
nitrogen, helium, argon and neon, or mixtures thereof.
[0066] FIGS. 6 to 8 show an example of an end cap 9b according to
an embodiment of the present invention. The top cone 9b is, for
example, made of aluminium and may be provided with copper or
molybdenum coatings on the interior surface of the inner layer 961
(which faces the plasma) as well as the surfaces which define the
gap 99. The inner layer 961 and outer layer 962 can be constructed
as nested cones with a space therebetween to form the gap 99. End
plates 964 and 965 are provided to seal the gap. End plates 964,
965 can be formed of a nickel-chromium-based alloy such as Inconel.
The copper or molybdenum coatings on the surfaces defining the gap
99 provide two advantages. Firstly, they are resistant to hydrogen
embrittlement and secondly have a low emissivity so that the
thermal transfer across the gap is better controlled by the
pressure of gas in the gap.
[0067] The top cone 9b can also be constructed from metals such as
aluminium, molybdenum, tungsten, steel, copper, nickel and
composites thereof, or from non-metals such as boron nitride,
aluminum nitride, aluminum oxide, boron carbide, graphite, and
quartz.
[0068] FIGS. 9 and 10 depict an alternative structure for the end
cap 9b. In the embodiment of FIGS. 9 and 10 the inner and outer
layers 961' and 962' are provided with complementary sets of vanes
98. The interleaved vanes 98 have the effect of increasing the area
of the gap 99 so as to increase heat transfer there-across whilst
still maintain the gap at a desired thickness of a few mm, e.g. 1
mm to 10 mm. FIG. 9 shows an end view of the end cap 9b with end
seal plates removed whilst FIG. 10 shows the inner layer 961 only.
It can be seen that the vanes 98 extend in planes containing the
axis of the cone formed by the top cone 9b to aid assembly if the
inner and outer parts are formed as single pieces. Alternative
configurations of vanes can be used if the two-parts of the top
cone are themselves assembled from smaller parts.
[0069] The presence of interleaved vanes can hinder gas flow in the
gap. To improve the uniformity of the pressure in the gap it is
possible to provide multiple gas inlets and/or outlets spaced
around the gap. Alternatively or in addition, one or more
circumferential grooves or openings can be provided to assist gas
flow around the gap.
[0070] In an embodiment, the gap 99 is divided into a plurality of
segments so that the pressure of gas in the segments can be
independently controlled. The gap can be segmented axially to form
rings or circumferentially to form sectors or both. A thermal
barrier can be provided to isolate the first wall layer from an
adjacent part of the vacuum chamber wall.
[0071] If desired, a heater can be provided inside the cone to
enable the vacuum chamber to be maintained at an elevated
temperature when the other heat sources therein are
insufficient.
[0072] The present embodiment enables control of the vacuum chamber
at high temperatures not achievable with other systems. The present
embodiment advantageously locates all components likely to require
servicing, e.g. the valves and pumps, outside the vacuum chamber.
This avoids the need to break the vacuum in the event servicing is
required.
[0073] In an embodiment, the radiation source of the invention may
form part of a mask inspection apparatus. The mask inspection
apparatus may use EUV radiation to illuminate a mask and use an
imaging sensor to monitor radiation reflected from the mask. Images
received by the imaging sensor are used to determine whether or not
defects are present in the mask. The mask inspection apparatus may
include a processor configured to analyse the image of the mask at
the imaging sensor, and to determine from that analysis whether any
defects are present on the mask. The processor may further be
configured to determine whether a detected mask defect will cause
an unacceptable defect in images projected onto a substrate when
the mask is used by a lithographic apparatus.
[0074] In an embodiment, the radiation source of the invention may
form part of a metrology apparatus. A metrology apparatus may be
used to measure alignment of a projected pattern formed in resist
on a substrate relative to a pattern already present on the
substrate. This measurement of relative alignment may be referred
to as overlay. A metrology apparatus may be used to measure
critical dimension (CD) of a target. A metrology apparatus may for
example be located immediately adjacent to a lithographic apparatus
and may be used to measure the overlay and/or CD before the
substrate (and the resist) has been processed. The metrology
apparatus may use EUV radiation for increased resolution.
[0075] Although specific reference may be made in this text to
embodiments of the invention in the context of a lithographic
apparatus, embodiments of the invention may be used in other
apparatus. Embodiments of the invention may form part of a mask
inspection apparatus, a metrology apparatus, or any apparatus that
measures or processes an object such as a wafer (or other
substrate) or mask (or other patterning device). These apparatus
may be generally referred to as lithographic tools. Such a
lithographic tool may use vacuum conditions or ambient (non-vacuum)
conditions.
[0076] The term "EUV radiation" may be considered to encompass
electromagnetic radiation having a wavelength within the range of
5-20 nm, for example within the range of 13-14 nm. EUV radiation
may have a wavelength of less than 10 nm, for example within the
range of 5-10 nm such as 6.7 nm or 6.8 nm.
[0077] Although the described radiation source is a laser produced
plasma LPP source, the invention may be applied to other types of
radiation source. For example, EUV emitting plasma may be produced
by using an electrical discharge to convert fuel (e.g. tin) to a
plasma state. A radiation source of this type may be referred to as
a discharge produced plasma (DPP) source. The electrical discharge
may be generated by a power supply which may form part of the
radiation source or may be a separate entity that is connected via
an electrical connection to the radiation source SO.
[0078] 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. Possible other applications include the
manufacture of integrated optical systems, guidance and detection
patterns for magnetic domain memories, flat-panel displays (such as
LED or OLED displays), liquid-crystal displays (LCDs), thin-film
magnetic heads, etc.
[0079] While specific embodiments of the invention have been
described above, it will be appreciated that the invention may be
practiced otherwise than as described. The 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. Other aspects of the invention are set out as
in the following numbered clauses: [0080] 1. A vacuum chamber
comprising: [0081] a vacuum chamber wall comprising a first wall
layer and a second wall layer defining a gap therebetween, wherein
the first wall layer is subjected to a heat source and the second
wall layer is subjected to a cooling source; [0082] a gas supply
device connected to the gap and configured to supply a gas thereto;
and [0083] a controller connected to the gas supply device and
configured to control the pressure of the gas in the gap so as to
control heat flow across the vacuum chamber wall. [0084] 2. A
vacuum chamber according to clause 1 wherein the controller is
configured to control the pressure of the gas in the gap to be in
the range of from 0.01 Pa to 500 Pa. [0085] 3. A vacuum chamber
according to clause 1 or 2 wherein the gap has a width in the range
of from 1 to 10 mm, desirably 2 to 5 mm. [0086] 4. A vacuum chamber
according to any one of clause 1, 2 or 3 wherein the gas supply
device is configured to supply a gas selected from the group
consisting of: hydrogen, helium, nitrogen, argon, and mixtures
thereof. [0087] 5. A vacuum chamber according to any one of clauses
1 to 4 wherein the first wall layer and/or the second wall layer is
made of a material selected from the group consisting of aluminium,
molybdenum, tungsten, steel, copper, nickel and composites thereof.
[0088] 6. A vacuum chamber according to any one of clauses 1 to 4
wherein the first wall layer and/or the second wall layer is made
of a material selected from the group consisting of boron nitride,
aluminum nitride, aluminum oxide, boron carbide, graphite, and
quartz. [0089] 7. A vacuum chamber according to any one of clauses
1 to 6 wherein the first wall layer and/or the second wall layer
has a plurality of vanes on a surface defining the gap. [0090] 8. A
vacuum chamber according to clause 7 wherein the first wall layer
and/or the second wall layer has a groove crossing at least one
vane. [0091] 9. A vacuum chamber according to any one of clauses 1
to 8 wherein the gap is divided into a plurality of sections and
the controller is configured to control the pressure of the gas in
each section independently. [0092] 10. A vacuum chamber according
to any one of clauses 1 to 9 wherein the gas supply device
comprises a valve to control the flow of gas into the gap and the
controller is configured to control the valve. [0093] 11. A vacuum
chamber according to any one of clauses 1 to 10 further comprising
a vacuum pump connected to the gap and configured to extract gas
from the gap, and wherein the controller is configured to control
the vacuum pump. [0094] 12. A vacuum chamber according to any one
of clauses 1 to 11 further comprising a thermal barrier to isolate
the first wall layer from an adjacent part of the vacuum chamber
wall.
* * * * *