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`26
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`FIG. 7
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`21
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`FIG. 2
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`2020P00230EP
`
`14
`
`ABSTRACT
`
`There is provided a conditioning system for a lithographic apparatus, said conditioning system being
`
`configured to condition one or more optical elements of the lithographic apparatus, wherein the
`
`conditioning system is configured to have a sub-atmosphcric pressure at the one or morc optical
`
`elements. Also provided are a lithographic apparatus comprising such a conditioning system, the use
`
`of such a conditioning system, a method of conditioning a system, as well as a lithographic method
`
`comprising a sub-atmospheric pressure cooling system
`
`

`

`2020P00230EP
`
`13
`
`11.
`
`12.
`
`The conditioning system according to any preceding claim, wherein said conditioning system
`
`further includes one or more vibration dampers, optionally wherein the one or more vibration
`
`dampersare in the form of one or more hydraulic accumulators.
`
`The conditioning system according to any of claims 9 to 11 when not dependent on claim6,
`
`wherein a or the conditioning fluid reservoir is disposed below the optical element such that the
`
`hydrostatic pressure difference between the optical element and the conditioning fluid reservoir
`
`reduces the pressure at the optical element to below atmospheric pressure.
`
`10
`
`13.
`
`The conditioning system according to any preceding claim, wherein the system comprisesfirst
`
`and second conditioning fluid reservoirs, whercin the first and second conditioning fluid
`
`reservoirs are in fluid connection with one another via a valve which is operable to control the
`
`level of conditioning fluid in the conditioning fluid reservoir whichis in fluid communication
`
`with the optical element.
`
`15
`
`20
`
`14.
`
`15.
`
`16.
`
`A lithographic apparatus comprising the conditioning system according to any of claims 1 to 13.
`
`The use of a sub-atmosphcric pressure conditioning system in a lithographic apparatus, prefcrably
`
`wherein the conditioning systemis a conditioning system according to any of claims 1 to 13.
`
`A method of conditioning a system or sub-system of a lithographic apparatus, said method
`
`including providing a liquid conditioning fluid at sub-atmospheric pressure to the system or sub-
`
`system to be conditioned.
`
`25
`
`17.
`
`The method according to claim 16, wherein the system or sub-system comprises an optical
`
`element, optionally wherein the optical element is a reflector or mirror.
`
`18. A lithographic method comprising projecting a patterned beam of radiation onto a substrate,
`
`wherein the patterned beam is directed or patterned using at least one optical element comprising a
`
`30
`
`conditioning system according to any of claims 1 to 13 or conditioned according to a method of
`claims 16 or 17.
`
`

`

`2020P00230EP
`
`12
`
`CLAIMS
`
`A conditioning system for a lithographic apparatus, said conditioning system being configured to
`
`condition one or more optical clements ofthe lithographic apparatus, wherein the conditioning
`
`systemis configured to have a sub-atmospheric pressure at the one or more optical elements.
`
`The conditioning system according to claim 1, wherein at least one of the optical elements is a
`reflector or a mirror.
`
`The conditioning system according to claim 1 or claim 2, wherein the conditioning system is
`
`configured to operate at between around 0.02 and 0.9 bara, preferably around 0.3 bara.
`
`The conditioning system according to any of claims | to 3, wherein the conditioning system
`
`includes a liquid conditioning fluid, preferably wherein the liquid conditioning fluid is water.
`
`The conditioning system according to any of claims 1 to 4, wherein the system includes a
`
`conditioning fluid reservoir.
`
`The conditioning system according to any of claims 1 to 5, wherein the system includes a pump
`
`downstream of the optical element and a flowrestrictor upstream of the optical element.
`
`The conditioning system according to any of claims | to 6, wherein the system includes a sub-
`
`atmospheric pressure conditioning fluid reservoir.
`
`The conditioning system according to claim 7, whercin the sub-atmospheric pressure conditioning
`
`fluid reservoir is connected to a vacuum pumpand/ora gasinlet.
`
`The conditioning system according to any of claims 1 to 5 and 7 to 8 when not dependent on
`
`claim 6, wherein the system includes a pump upstream of the optical element, and optionally a
`
`flow restrictor between the pump and the optical element.
`
`10.
`
`The conditioning system according to any of claims 7 to 9, wherein the sub-atmospheric pressure
`
`conditioning fluid reservoir includes a deformable separator, preferably wherein the deformable
`
`separator is disposed betweenthe liquid conditioning fluid and a gas.
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`2020P00230EP
`
`11
`
`optical element 19. The hydrostatic pressure difference 25 causes a reduced pressure on the return side
`
`of the optical element 19. The pump 21 compensates for this and is able to provide conditioning fluid
`
`to the supply side of the optical element 19. The dotted box surrounding the conditioning fluid reservoir
`
`16, the pump 21, and the flow restrictor 18 indicates that these clements are located below the optical
`
`elementand that the exact positioning of such elements relative to the optical element 19 is schematic
`
`only.
`
`It will be appreciated that the flowrestrictor 18 may be provided adjacent the return side of the
`
`optical element 19.
`
`[00055]
`
`Figure 8 depicts yet another embodimentof a conditioning system according to the present
`
`invention.
`
`In this embodiment,
`
`there are two conditioning fluid reservoirs l6a, 16b. The two
`
`conditioning fluid reservoirs 16a, 16b are connected via a valve 26 which is operable to control the
`
`water Icvel within conditioning fluid reservoir 16a. This in turn controls the hydrostatic pressure of the
`
`conditioning fluid which is provided to the optical element 19 in combination with the water pump 21
`
`which is connected to the return side of the optical element.
`
`[00056]
`
`In summary, the present invention provides for a sub-atmospheric conditioning system
`
`which enables the conditioning of critical components of a lithographic apparatus and enables such
`
`components directly and therefore increases thermal efficiency and enables improved performance at
`
`higher thermal loads without causing unwanted deformation of the optical elements. The present
`
`invention has particular, but not exclusive application, to the cooling of optical clement of lithographic
`
`apparatuses.
`
`[00057] While specific embodiments of the invention have been described above,
`
`it will be
`
`appreciated that the invention may bepracticed otherwise than as described.
`
`[00058]
`
`The descriptions above are intended to beillustrative, not limiting. Thusit 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.
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`2020P00230EP
`
`10
`
`system 15. The flowrestrictor 18 can be of any suitable design and the present invention is not
`
`particularly limited by the exact nature of the flow restrictor. The flow restrictor 18 functions to provide
`
`the required pressure drop at the optical element 19 such that the conditioning fluid at or within the
`
`optical clement 19 is below atmospheric pressure.
`
`In any of the depicted cmbodiments, the optical
`
`element 19 which is being conditioned contains at least one channel through which conditioning fluid
`
`may pass in order to absorb heat from or provide heat to the optical element 19 and transport it away
`
`thereby conditioning the optical element 19. Thelevel of the conditioning fluid 17 in the conditioning
`
`fluid reservoir 16 is schematic only and may be above or below the point at which conditioning fluid
`
`17 is returned via pump 21.
`
`[00050]
`
`Figure 3 depicts a similar conditioning system as Figure 2 albeit with the difference that
`
`the conditioning fluid reservoir 16 is not at atmospheric pressure.
`
`Instead, the system is scaled such
`
`that ii is taken downto the desired sub-atmospheric pressure and then closed off so that it remains al
`
`sub-atmospheric pressure. A gas connection 22 may be provided which is able to reduce the pressure
`
`within the system should it increase above desired levels. Similarly, additional gas or conditioning fluid
`
`may be addedto the system if the pressure falls too much or more conditioning fluid is required.
`
`[00051]
`
`Figure 4 depicts a similar conditioning system to Figure 3 albeit with a deformable
`
`separator 23, which may be in the form of a membrane, which separates the conditioning fluid 17 from
`
`the gas aboveit within the conditioning fluid reservoir 16. The scparator 23 prevents cvaporation of
`
`any of the conditioning fluid and therefore avoids the loss of conditioning fluid and also avoids the need
`
`to have a vacuum pumpor controlled gas inlet. The separator 23 also dampens flow induced vibration
`
`within the system.
`
`[00052]
`
`Figure 5 depicts a further embodimentof a conditioning system 15 in accordance with the
`
`present invention. This embodiment is similar to that of Figure 2 albeit having a closed system and
`
`having a sub-atmospheric pressure conditioning fluid reservoir 16. The conditioning fluid reservoir 16
`
`is maintained at below atmosphcric pressure by vacuum pump 24. Vacuum pump 24is configured to
`
`reduce the pressure within the system by removing gas present therein. A controlled gas inlet may be
`
`provided which can add or remove gas from the system as required.
`
`[00053]
`
`Figure 6 depicts an embodiment which is similar to that of Figure 5 albeit the pump 21
`
`which is used to move the conditioning fluid through the system is disposed between the conditioning
`
`fluid reservoir 16 and the inlet of the optical element 19.
`
`In this embodiment, conditioning fluid 17 is
`
`pumped by pump 21 via restrictor 18 into the optical element 19. The outlet of the optical element 19
`
`is fluidly connected to the sub-atmospheric pressure conditioning fluid reservoir 16. Vacuum pump 24
`
`and gas connector 22 are operable to remove and introduce gas as appropriate in order to control the
`
`pressure within the system.
`
`[00054]
`
`Figure 7 depicts yet another embodimentof a conditioning system according to the present
`
`invention.
`
`In this embodiment, the low pressure is generated by utilizing the hydrostatic pressure
`
`difference 25 caused by the conditioning fluid reservoir 16 being located at a lower height than the
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`2020P00230EP
`
`9
`
`and is incident upon the patterning device MA held bythe support structure MT. Thepatterning device
`
`MAreflects 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.
`
`[00045]
`
`Following reflection from the patterning device MAthe patterned radiation beam B enters
`
`the projection system PS. The projection system comprises a plurality of mirrors 13, 14 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 13, 14 in Figure 1, the
`
`projection system mayinclude any numberof mirrors (c.g. six mirrors).
`
`[00046]
`
`In use the optical elements of the lithographic apparatus, such as mirrors or reflectors, are
`
`heated by the radiation and it is therefore necessary to condition such optical elements. As such, a
`
`conditioning system according to the present invention is integrated into the lithographic apparatus to
`
`provide the required conditioning. Conditioning usually requires the removal of thermal energy from
`
`10
`
`15
`
`the optical elements as they heat up during use.
`
`[00047]
`
`The radiation sources SO shown in Figure 1 may include components which are not
`
`illustrated. For cxample, 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.
`
`20
`
`[00048]
`invention.
`
`Figures 2 to 8 are schematic depictions of conditioning systems according to the present
`
`25
`
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`
`35
`
`[00049]
`
`Figure 2 depicts an embodiment of a condilioning system 15 in accordance with a [first
`
`embodiment of the present invention. The conditioning system 15 includes a conditioning fluid
`
`reservoir 16. In the depicted example, the top of the conditioning fluid reservoir 16 is shown as being
`
`open. Whilst in practice, the conditioning fluid reservoir may or may not be open,it is depicted as being
`
`open to demonstrate that the conditioning fluid 17 therein, which may be water, is not pressurized above
`
`atmospheric pressure and may be at ambient pressure. The conditioning fluid reservoir 17 is connected
`
`to an optical element 19 via optional flow restrictor 18. An optional pressure sensor 20 may be provided
`
`to measure the pressure of the conditioning fluid as it enters the optical element 19 or inside the optical
`
`element 19. A pump 21 is connected between the optical element 19 and the conditioning fluid reservoir
`
`16. In use, conditioning fluid 17 from the conditioning fluid reservoir 16 (which mayalso be reterred
`
`to as a conditioning fluid vessel or container) passes into an optical element 19 via the pressure
`
`difference between reservoir outlet and the pumpinlet, created by the pump.
`
`In other embodiments,
`
`the flow relies on gravity based on the relative position of the tank and optical element. A pump 21
`
`connected to an output side of the optical element 19 pumps the conditioning fluid 17 back to the
`
`conditioning fluid reservoir 16 whereit is able to subsequently be recirculated through the conditioning
`
`

`

`2020P00230EP
`
`8
`
`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.
`
`[00039]
`
`The radiation source SO, illumination system IL, and projection system PS mayall 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.
`
`[00040]
`
`The radiation source SO shownin Figure | is of a type which maybe referred to as a laser
`
`produced plasma (LPP) source. A laser, which may for example be a COzlaser, is arranged to deposit
`
`encrgy via a lascr beaminto a fucl, such as tin (Sn) which is provided from a fucl emitter. Althoughtin
`
`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 may comprise a nozzle
`
`configured to direct tin, e.g. in the form of droplets, along a trajectory towards a plasma formation
`
`region. The laser beam is incident upon the tin at the plasma formation region. The deposition of laser
`
`energy into the tin creates a plasmaat the plasma formation region. Radiation, including EUV radiation,
`
`is emitted from the plasma during de-excitation and recombination of ions of the plasma.
`
`[00041]
`
`The EUV radiation is collected and focused by a near normal incidence radiation collector
`
`(sometimes referred to more generally as a normal incidence radiation collector). The collector may
`
`have a multilayer structure which is arranged to reflect EUV radiation (e.g. EUV radiation having a
`
`desired wavelength such as 13.5 nm). The collector may have anelliptical configuration, having two
`
`ellipse focal points. A first focal point may be at the plasma formation region, and a secondfocal point
`
`may be ai an intermediate focus, as discussed below.
`
`[00042]
`
`The laser may be separated from the radiation source SO. Where this is the case, the laser
`
`beam may be passed from the laser 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 and the radiation source SO maytogether be considered to be a radiation system.
`
`[00043]
`
`Radiation that is reflected by the collector forms a radiation beam B. The radiation beam
`
`B is focused at a point to form an imageof the plasma formation region, which acts as a virtual radiation
`
`source for the illumination system IL. The point 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
`
`tocusis located at or near to an opening in an enclosing structure ofthe radiation source.
`
`[00044]
`
`The radiation beam B passes fromthe radiation source SO into the illumination systemIL,
`
`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
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`2020P00230EP
`
`7
`
`such as pipes and manifolds may be provided, to allow the conditioning fluid to be provided to the
`
`optical elements.
`
`[00026]
`
`According to a fifth aspect of the present invention, there is provided a lithographic method
`
`comprising projecting a patterned beam ofradiation onto a substrate, wherein the patterned beam is
`
`directed or patterned using at least one optical element comprising a conditioning system according to
`
`any aspect of the present invention.
`
`[00027]
`
`It will be appreciated that features described in respect of one aspect or embodiment may
`
`be combined with any features described in respect of another aspect or embodiment and all such
`
`combinations are expressly considered and disclosed herein.
`
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`[00028]
`
`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:
`
`[00029]
`
`Figure | depicts a lithographic apparatus according to an embodimentof the invention;
`
`[00030]
`
`Figure 2 depicts a first embodiment of a system according to the present invention;
`
`[00031]
`
`Figure 3 depicts a second embodimentof a system according to the present invention;
`
`[00032]
`
`Figure 4 depicts a third cmbodiment of a system according to the present invention;
`
`[00033]
`
`Figure 5 depicts a fourth embodiment of a system according to the present invention;
`
`[00034]
`
`Figure 6 depicts a fifth embodimentof a system according to the present invention;
`
`[00035]
`
`Figure 7 depicts a sixth embodiment of a system according to the present invention; and
`
`[00036]
`
`Figure 8 depicts a seventh embodiment of a system according to the present invention.
`
`[00037]
`
`The features and advantages of the present invention will become more apparent fromthe
`
`detailed description set forth below when taken in conjunction with the drawings, in which like
`
`reference characters identify corresponding clements throughout.
`
`In the drawings, like reference
`
`numbers generally indicate identical, functionally similar, and/or structurally similar elements.
`
`DETAILED DESCRIPTION
`
`[00038]
`
`Figure 1 showsa lithographic according to an embodimentof the present 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 mayinclude
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`2020P00230EP
`
`6
`
`[00020]
`
`The conditioning system may include one or more vibration dampers, which mayserve to
`
`dampen pressure control error or flow induced vibrations. The one or more vibration dampers may be
`
`in the form of one or more hydraulic accumulators. As the optical elements are also very sensitive to
`
`vibrations, it is desirable to reduce or climinate vibrations where possible. By providing one or more
`
`vibration dampers, which maybe in the form of one or more hydraulic accumulators, any vibrations
`can be reduced.
`
`[00021]
`
`In embodiments, a or the conditioning fluid reservoir may be disposed below the optical
`
`element such that the hydrostatic pressure difference between the optical element and the conditioning
`
`fluid reservoir reduces the pressure present at the optical element to below atmospheric pressure. As
`
`such, if the conditioning fluid reservoir is provided below the height of the return side of the optical
`
`clement, the hydrostatic pressure difference is sufficient to reduce the pressure of the conditioning fluid
`
`al the optical element to below atmospheric pressure. A pump may be provided to supply conditioning
`
`fluid to the optical element and the pressure being over atmospheric pressure is avoided by the
`
`hydrostatic pressure difference. There may be a pressure control device between the optical element
`
`and the conditioning fluid reservoir. The pressure control device may be configured to control the flow
`
`of conditioning fluid therethrough in order to control the pressure at the optical element. Optionally,
`
`there may be provided a pressure sensor configured to control the pressure control device. A pressure
`
`controller may be provided which receives an input from a pressure sensor and is operable to control
`
`the pump and/or a gasinlet flow controller.
`
`[00022]
`
`In embodiments, the system may comprise first and second conditioning fluid reservoirs.
`
`The first and second conditioning fluid reservoirs may be in fluid communication with one another via
`
`a valve. The valve may be operable to control the level of conditioning fluid in the coolant reservoir
`
`whichis in fluid communication with the optical element to control the pressure.
`
`In an embodiment,
`
`the level of conditioning fluid in the conditioning fluid reservoir, and therefore the static pressure on
`
`the optical clement, may be controlled by the provision of a conditioning fluid overflow in which the
`
`height of the conditioning fluid overflow is related to the height of the optical element.
`
`[00023]
`
`According to a second aspect of the present invention, there is provided a lithographic
`
`apparatus comprising a conditioning system according to the first aspect of the present invention. The
`
`lithographic apparatus may be an EUV lithographic apparatus.
`
`[00024]
`
`According to a third aspect of the present invention, there is provided the use of a sub-
`
`atmospheric pressure conditioning system in a lithographic apparatus. Preferably, the conditioning
`
`system is a conditioning system according to the first aspect of the present invention.
`
`[00025]
`
`According to a fourth aspect of the present invention, there is provided a method of
`
`conditioning a system or sub-system of a lithographic apparatus, wherein said method includes
`
`providing a liquid conditioning fluid at sub-atmospheric pressure to the system or sub-system to be
`
`conditioned. The system or sub-system may comprise an optical element, such as, for example a
`
`reflector or mirror. There may be one or more than one optical elements. Suitable distribution means,
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`2020P00230EP
`
`5
`
`[00016]
`
`The system mayinclude a pump downstream of the optical element and a flow restrictor
`
`upstream of the optical element. As such, the suction side of the pump maybein fluid connection with
`
`a conditioning channelof the optical element such that when the pumpis in operation, a pressure which
`
`is below atmospheric pressure is created at the optical clement. The presence ofthe flow restrictor
`
`upstream of the optical element ensures that the pressure at the optical element is below atmospheric
`
`pressure. The present inventionis not particularly limited by the exact dimensionsor nature of the flow
`
`restrictor since these will depend on the dimensions of the pipes which carry the conditioning fluid, the
`
`required flow rate, and the required pressure of the system, but these can be determined routinely by
`
`the skilled person and the pressure at the optical element being conditioned can be measured routinely
`
`to confirm the pressure therein.
`
`[00017]
`
`The system may include a sub-atmosphceric pressure conditioning fluid rescrvoir. As such,
`
`the system may comprise a conditioning fluid reservoir which is al sub-atmospheric pressure. The sub-
`
`atmospheric conditioning fluid reservoir may be connected to a vacuum pump. The vacuum pump may
`
`be configured to allow control of the pressure within the conditioning fluid reservoir by removing gases
`
`from the reservoir to provide the desired sub-atmospheric pressure. A gas inlet may optionally be
`
`provided which allows a gas, such as air or nitrogen, to be introduced into the conditioning fluid
`
`reservoir to increase the pressure should it become too low. The system may include a controller which
`
`is operable to control the vacuum pumpand/or gas inlct in order to achieve the desired sub-atmospheric
`
`pressure.
`
`[00018]
`
`In some embodiments which include a sub-atmospheric conditioning fluid reservoir, a
`
`pumpis provided downstream of the optical element and a flow restrictor is provided upstream of the
`
`optical element.
`
`In other embodiments including a sub-atmospheric pressure conditioning fluid
`
`reservoir, the pump may be provided upstreamof the optical element since the downstream portion of
`
`the conditioning system is connected to the sub-atmospheric conditioning fluid reservoir and therefore
`
`the pressure at the optical clement is able to remain sub-atmosphcric. Optionally, a flow restrictor is
`
`provided upstream of the pump and downstreamof the optical element, namely between the pump and
`
`the optical element, in order to control the flow of conditioning fluid and avoid the pressure at the
`
`optical element from exceeding atmospheric pressure.
`
`[00019]
`
`The sub-atmospheric pressure conditioning fluid reservoir may include a deformable
`
`separator between the liquid conditioning fluid and a gas. The deformable separator may be in the form
`
`of amembrane. By having a deformable separator between the conditioning fluid and a gas within the
`
`sub-atmospheric pressure conditioning fluid reservoir, the sub-atmospheric pressure can be maintained
`
`without any evaporation of the conditioning fluid. The gas above the deformable separator may have
`
`its pressure adjusted to control the pressure within the system. The composition of the gas is not
`
`particularly limiting on the present invention, and may, for example, be air or nitrogen. The pressure
`
`of the gas can be adjusted to adjust the pressure within the system. In addition, the separator is able to
`
`be deformed to take account of any pressure changes and to also act as a vibration damper.
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`2020P00230EP
`
`4
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`operating pressure. As mentioned above, by operating at below atmospheric pressure (1 bara), it is
`
`possible to avoid the problems associated with deformation caused by using pressure above 1 bara and
`
`also allows any conditioning channels to be provided closer to the surface which is to be conditioned
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`than would otherwise be the case, thereby improving conditioning efficiency. Of course,if the pressure
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`is too low, then there is a risk of cavitation and there would not be enough pressure difference to cause
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`conditioning fluid to flow. In addition,if the system goes below the vapour pressure of the conditioning
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`fluid, which may be water, the conditioning fluid may boil at an undesirably low temperature, for
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`example 22°C at 0.02 bara for water. It has been foundthat operating pressures of between around 0.02
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`bara and 0.9 bara provide the correct balance between avoiding deformation, avoiding cavitation,
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`allowing conditioning fluid channels to be close enoughto the surface to be conditioned efficiently, and
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`allowing conditioning fluid flow through the system. The conditioning system of the present invention
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`also allows in some cases the avoidance of the need to manufacture optical elements, such as reflectors,
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`when underpressure. It will be appreciated that at certain points in the conditioning system, the pressure
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`may be around atmospheric or even slightly above atmospheric. Even so, the system is configured such
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`that the pressure at the optical element or elements being conditioned is sub-atmospheric when in use.
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`Since the optical elements are in a very low pressure environment within the lithographic apparatus of
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`around a few pascals of hydrogen, the pressure differential between the conditioning fluid channels
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`within the optical clement and the outside of the optical clementis effectively equal to the pressure of
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`the conditioning fluid within the conditioning fluid channels.
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`[00014]
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`The conditioning system may include a liquid conditioning fluid, preferably water. The
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`conditioning fluid may be provided with one or more additives to improve performance, such as
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`corrosion inhibitors to prevent corrosion within the system. Whilst conditioning fluids such as carbon
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`dioxide may be used in other systems, this usually requires much higher pressures in order to obtain the
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`necessary mass flowto provide effective conditioning and so there are additional safety considerations
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`to take into account and the high pressures required, such as from 20 to 100 bar, are not compatible
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`with avoiding deformation. Whilst other water-cooled systems may describe a broad range of pressures,
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`there is no realization of the suitability of the specific range described herein, nor of operating at sub-
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`atmospheric pressures at the units which are being conditioned, namely at the optical elements.
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`Pressures above atmospheric pressure have previously been used since these can be more readily
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`achieved bythe provision of standard pumping apparatus, which pressurizes water to above atmospheric
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`pressure in order to pump the water through the conditioning system, and the problem of deformation
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`has been addressed by moditying the contiguration of the conditioning channels. Water is preterred
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`dueto its safety, high thermal mass, and availability.
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`[00015]
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`The system may include a conditioning fluid reservoir. The conditioning fluid reservoir
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`maybeatleast partially filled with conditioning fluid, preferably water. By having a conditioning fluid
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`reservoir, there is a thermal mass of conditioning fluid which can be circulated through the system to
`removeheat therefrom or add heat thereto.
`
`10
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`15
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`20
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`25
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`30
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`35
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`

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`2020P00230EP
`
`3
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`elements of a lithographic apparatus, particularly an EUV lithographic apparatus, are very sensitive to
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`any deformations, even very small ones.
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`[00011]
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`It has been found that if the relative pressure between the conditioning system and the
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`outside of the optical clement is greater than atmospheric pressure, there will be deformation of the
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`surface of the optical elements and this deformation impacts the performance of the optical elements.
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`Since the conditioning system is for a lithographic apparatus,
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`the main parts of the lithographic
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`apparatus that are to be conditioned, whether by cooling or heating, are under vacuum or only a very
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`lowpressure of gas, usually hydrogen, and so it has been found that sub-atmospheric pressures are
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`desired to avoid deformation of critical components with the associated loss of performance. Whilstit
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`has previously been mitigated by providing the condition channels deeper within the substrate of the
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`system being conditioned, this impacts on thermal performance and the conditioning provided is
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`insufficient in cases where the conditioning channels need to be deep within a substrate in order to
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`reduce or eliminate deformation. By providing a conditioning system which is configured to operate at
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`sub-atmospheric pressure