WO2023016734A1

WO2023016734A1 - Thermal conditioning apparatus and method

Info

Publication number: WO2023016734A1
Application number: PCT/EP2022/069419
Authority: WO
Inventors: Wenjie JIN; Joost André KLUGKIST; Koos VAN BERKEL; Joris Dominicus Bastiaan Johannes VAN DEN BOOM; Mauritius Gerardus Elisabeth SCHNEIDERS; Victor Sebastiaan DOLK
Original assignee: Asml Netherlands B.V.
Priority date: 2021-08-12
Filing date: 2022-07-12
Publication date: 2023-02-16
Also published as: TW202314999A

Abstract

A thermal conditioning apparatus, said thermal conditioning apparatus including at least one channel, wherein a thermal conditioning fluid and a solid thermal conductor are disposed within the at least one channel. Also described is a system or sub-system including such a thermal conditioning apparatus, and a lithographic apparatus including such a thermal conditioning apparatus, system or sub-system. A method of conditioning a system or sub-system, said method including providing a thermal conditioning fluid and a solid thermal conductor within a channel that is in thermal communication with the system or sub-system, and transferring thermal energy between the system or sub-system and the solid thermal conductor via the thermal conditioning fluid is also provided. The use of such apparatuses or methods is also provided.

Description

THERMAL CONDITIONING APPARATUS AND METHOD

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 21191043.5 which was filed on August 12, 2021 and EP application 22152539.7 which was filed on January 20, 2022 and EP application 22165488.2 which was filed on March 30, 2022 which were incorporated herein in its entirety by reference.

FIELD

[0002] The present invention relates to a thermal conditioning apparatus, a system or sub-system comprising such thermal conditioning apparatus, a method of conditioning a system or sub-system, the use of such a thermal conditioning apparatus, system, sub-system, or method in a lithographic apparatus or process, and a lithographic method utilising any of the aforesaid. The present invention has particular, but not exclusive, application in lithographic apparatuses and methods.

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 4-20 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] Collecting EUV radiation into a beam, directing it onto a patterning device (e.g. a mask) and projecting the patterned beam onto a substrate is difficult because it is not possible to make a refractive optical element for EUV radiation. Therefore, these functions have to be performed using reflectors (i.e. mirrors). Even constructing a reflector for EUV radiation is difficult. The best available normal incidence reflector for EUV radiation is a multi-layer reflector (also known as a distributed Bragg reflector) which comprises a large number of layers which alternate between a relatively high refractive index layer and a relatively low refractive index layer. Each period, consisting of a high refractive index layer and a low refractive index layer, has a thickness equal to half the wavelength ( /2) of the radiation to be reflected so that there is constructive interference between the radiation reflected at the high to low refractive index boundaries. Such a multilayer reflector still does not achieve a particularly high reflectivity and a substantial proportion of the incident radiation is absorbed by the multilayer reflector.

[0006] The absorbed radiation, including infra-red radiation also emitted by the radiation source, can cause the temperature of the multilayer reflector to rise. Known multilayer reflectors are formed on substrates made of materials having a very low coefficient of thermal expansivity, for example ULE™ or cordierite. In some cases, the cross-section of the beam when incident on a reflector may be small enough that localized heating of the reflector causes undesirable deformation of the surface figure of the reflector. Such deformation can cause imaging errors and the constant desire to image ever smaller features means that the amount of deformation that can be tolerated will only reduce. Unwanted heating of components is not limited to reflectors or mirrors, and there may be further elements, systems, or sub-systems within a lithographic apparatus which require cooling.

[0007] Existing reflectors in the projection systems of lithographic apparatuses, particularly EUV apparatuses, are cooled passively, i.e. by radiation, conduction, and convection. However, none of these modes of cooling allows a high rate of heat transfer. In particular, the reflectors are generally in a high vacuum or a low pressure of hydrogen so that heat transfer by convection is minimal. Active cooling of reflectors has been avoided because of the risk of introducing vibrations, which could easily be more problematic than the distortion caused by the localized heat rise.

[0008] The present invention has been devised to provide an improved or alternative conditioning system. The present invention has particular, but not exclusive, application as a conditioning system for a component of a lithographic apparatus. The following description makes reference to lithographic apparatuses, but it will be appreciated that the present invention may be used to condition other apparatuses.

SUMMARY OF THE INVENTION

[0009] According to a first aspect of the present invention, there is provided a thermal conditioning apparatus, said thermal conditioning apparatus including at least one channel, wherein a thermal conditioning fluid - or in other words a heat transfer fluid - and a solid thermal conductor are disposed within the at least one channel.

[00010] As is generally known, a thermal conditioning fluid can be passed through channels within a component in order to actively condition the component in need of thermal conditioning. The thermal conditioning fluid is able to absorb thermal energy from the component to result in a heated thermal conditioning fluid. The heated thermal conditioning fluid is then passed away from the component, thereby removing thermal energy from the component and controlling the temperature of the component. The thermal energy is removed from the thermal conditioning fluid at a location separate from the component being conditioned, and the cooled thermal conditioning fluid is then able to be passed through the component being conditioned again. Whilst this is an efficient way to condition a component, there are some drawbacks. Perhaps the main issue with existing systems is that the flow of thermal conditioning fluid can result in turbulence and vibrations within the component being conditioned. The turbulence and vibrations can be detrimental to the operation of certain apparatuses, such as lithographic apparatuses, particularly with regards to the operation of the elements which interact with the radiation beam used in lithographic apparatuses. Additionally, a thermal conditioning fluid necessarily applies pressure to the channels of the component and this can result in deformation of optical surfaces, which may be located in a vacuum. It is necessary for the thermal conditioning fluid to be pressurized in order to prevent cavitation of the thermal conditioning fluid at the maximum temperature to which the thermal conditioning fluid is exposed. This is known as print-through. Due to the extremely high precision of optical elements required by lithographic apparatuses, it is necessary for the thermal conditioning fluid to be pressurized at a substantially stable pressure as even minor print- through variations caused by variation of the pressure in the conditioning channels of the said elements can be detrimental to the operation of the lithographic apparatus. The present invention may reduce print-through of the mentioned optical elements by reducing the maximum temperature of any local hot spots, thereby allowing a lowering of the pressure of the thermal conditioning fluid.

[00011] The present invention provides a solid thermal conductor and a thermal conditioning fluid disposed within a channel. The channel may be provided in the component itself or in a separate element in thermal communication with the component being conditioned. The solid thermal conductor is configured to conduct thermal energy away from or to the component being conditioned. The solid thermal conductor is able to transfer heat readily. As such, the solid thermal conductor may reduce any local hot-spots and allow the thermal conditioning fluid to be provided at a lower pressure, thereby reducing the likelihood of print-through. The thermal conductivity of the solid thermal conductor is greater than that of the material comprising the component being conditioned. In the case of a mirror, the mirror comprises ultra-low expansion glass or cordierite, which transfers heat slowly. The thermal conditioning fluid provides a path for the transfer of thermal energy between a wall of the channel and the solid thermal conductor. Since the coefficient of thermal expansion of the component and the solid thermal conductor will be different, the thermal conditioning fluid is able to accommodate any differential expansion of the two and thereby prevent aberrations introduced by the expansion of the solid causing aberrations by print-through, and to also prevent damage to either component whilst still providing a continuous path for thermal energy to pass. The solid thermal conductor is a thermal conductor in solid state, i.e. it is solid in the sense that it is not a fluid, but this does not necessarily require that it is a unitary block with no cavities. The solid thermal conductor may include one or more cavities or openings. The solid thermal conductor may be or may comprise a heat pipe. As a heat pipe, the solid thermal conductor may comprise a tube having a fluid therein which is able to rapidly transfer thermal energy along the heat pipe. The solid thermal conductor may have a thermal conductivity of more than ten times, more than twenty times, more than fifty times, more than a hundred times, more than a thousand times, more than ten thousand times, more than one hundred thousand times, or more than a million times that of the material primarily comprising the component which is being conditioned. Due to the extremely high thermal conductivity of heat pipes, the thermal resistance of heat pipes can effectively be ignored compared to the conduction through the body of the mirror, which usually comprises ultra-low expansion glass, and the conductive fluid, such as water. This being the case, heat pipes are able to replace flowing water in any workable configuration which uses water flow to cool the mirror. The thermal conductivity of the solid thermal conductor may be at least 175 W/m K greater than the thermal conductivity of the component which is being thermally conditioned. In the case of optical elements, these may generally comprise ultra-low expansion glass or cordierite. The solid thermal conductor may only be partially disposed within the channel. In such cases, the solid thermal conductor extends outside of the channel. The solid thermal conductor may be comprised in a component which is being conditioned. For example, where the component being conditioned is a mirror, the solid thermal conductor may be provided within one or more of the conditioning channels that are comprised in the mirror. The thermal conditioning apparatus according to the first aspect of the present invention may be separately formed from the component being conditioned, and may be put into thermal communication with the component being conditioned by any suitable means. The solid thermal conductor may be in the form of a plate. The solid thermal conductor may be in the form of a heat spreader. In some systems, the thermal loading applied to an optical element, such as a mirror, is non- uniform. As such, barring any intervention, there would be differential heating across the surface of the optical element. Such differential heating could lead to heating induced aberrations, which are not desirable. In some systems, additional heaters may be provided to compensate for the differential heating of the surface or actuators may be provided which deform the mirrors to correct for any heating induced aberrations. However, the performance of additional heaters is limited by the number of heaters which can be provided and there is not a great deal of space within a lithographic apparatus to work with. In addition, which deformable mirrors are possible, the actuators used to deform the mirrors are themselves sensitive to temperature variation. Also, if the mirrors are too thick, it is not possible to provide the required resolution via deformation. By providing a thermally conductive layer, it is possible to reduce or eliminate hot spots on a surface of an optical element and thereby reduce or eliminate heating induced aberrations. This can be a passive system as it does not require heaters, sensors, or actuators, or complicated control systems. A heat spreader is configured to conduct thermal energy away from areas of high thermal load and redistribute them to areas of lower thermal load, thereby reducing in-plane temperature variations and thereby reduce heating induced aberrations. The heat spreader is preferably sized to cover the entire optical footprint of the radiation falling upon the optical element. Even so, the material of the heat spreader and the optical element, such as ultra-low expansion glass, have different coefficients of thermal expansion and stiffness. As such, it is not possible to simply affix a heat spreader to the optical element. The present invention provides for a thermal conditioning fluid in the channel containing the solid thermal conductor, which could be a heat spreader. As such, the heat spreader and the body of the optical element being cooled are decoupled. Thus due to decoupling by the thermal conditioning fluid, the heat spreader is able to expand or shrink without causing damage. The differential pressure of the thermal conditioning fluid caused by the expansion may be accommodated by any suitable means described herein, and may, for example, include a gas pocket. Whilst cooling by flowing water is not required, it will be appreciated that the heat spreader could be combined with flowing water cooling, if desired. The heat spreader may be coated with a graphene layer. The graphene layer may be provided on all faces of the heat spreader. A heater and/or cooler could be provided in thermal communication with the heat spreader. A temperature sensor could be provided in thermal communication with the heat spreader. As described, a heat spreader makes the temperature profile more uniform by quickly distributing heat over the plane. The average temperature depends on the input heat load, which varies amongst different use cases. Therefore, by providing active and direct thermal control over the heat spreader, the temperature of the heat spreader can be kept constant and therefore the aberrations can be reduced or eliminated. The heater could be a sub-micron thickness platinum film temperature sensor with an integrated heating circuit. The cooler could be a thermoelectric heater/cooler. According to the Peltier effect, it is possible to either heat or cool the heat spreader depending on the applied electric current. A thermally conductive channel can be provided to bridge the cooler and the outer surface of the mirror to enhance performance.

[00012] The apparatus may be configured such that, in normal use, the thermal conditioning fluid is substantially static within the at least one channel. Whilst the thermal conditioning fluid may move within the at least one channel during, for example, installation, cleaning, or maintenance, when the lithographic apparatus is operating, the thermal conditioning fluid may be effectively static. In other words, there is no particular flow in a specific direction. Whilst there may be minor movement of the thermal conditioning fluid due to the change in relative volume of the channel and the solid thermal conductor, when the apparatus has achieved a steady state, the thermal conditioning fluid is preferably substantially or entirely stagnant in embodiments. One skilled in the art will appreciate that a variety of embodiments may be used with respect to the flow in the channels. For example, in some embodiments there may be some flow of the thermal conditioning fluid, the contribution of the flow to the removal of thermal energy is low or negligible. For example, the contribution of the flow to the removal of thermal energy may be less than 25%, less than 20%, less than 15%, less than 10%, less than 5%, less than 3%, or less than 1% of the entire amount of thermal energy being removed. This is possible due to the incorporation of the solid thermal conductor, which itself is able to transfer the majority of the thermal energy. In other embodiments, the contribution of flow to the removal of thermal energy may be high. For example, the contribution of the flow to the removal of thermal energy may be more than 50%. In these embodiments, flow induced vibrations will be suppressed to a lesser extent; the main purpose of the solid conductor is to enhance thermal diffusion to reduce the maximum temperature of local hot spots, thereby allowing lowering of the operating pressure of the thermal conditioning fluid, reducing print-through effects. As such, the present invention can be used in different modes, with no flow, with a low degree of flow, or with a high degree of flow. The apparatus of the present invention provides advantages for each mode and it allows the operator to select such different modes. Without the apparatus of the present invention, the operator is restricted to a mode which necessarily relies on a flow of thermal conditioning fluid.

[00013] The thermal conditioning fluid is in other words a heat transfer fluid. The thermal conditioning fluid may be a liquid. The thermal conditioning fluid may be a gas. The thermal conditioning fluid may be water. The thermal conditioning fluid may be a gel. The thermal conditioning fluid may include solid particles. The function of the thermal conditioning fluid is to provide a path for the transfer of thermal energy between the component and the solid thermal conductor and to accommodate any difference in expansion between the two. The fluid may not necessarily be compressible. Where the fluid is incompressible or only slightly compressible, the apparatus may be configured to accommodate expansion of the thermal conditioning fluid. As such, there may be provided a system for accommodating the thermal expansion such as an expansion chamber, a hydraulic accumulator, an expansion vessel, expansion joint, or expansion bellow, which avoids the pressure increasing within the channel and potentially resulting in print-through.

[00014] The solid thermal conductor may be formed from a ceramic. The solid thermal conductor may comprise silicon carbide, silicon nitride, silicon infiltrated silicon carbide, silicon infiltrated silicon nitride, diamond-infiltrated silicon carbide or diamond infiltrated silicon nitride. The solid thermal conductor may be formed from a metal. The metal may comprise aluminium. The solid thermal conductor may be formed from a metal-ceramic. The metal ceramic may be aluminium silicon carbide (AlSiC) or aluminium nitride (AIN). The solid thermal conductor may be selected from diamond or polycrystalline diamond. The solid thermal conductor may be a flexible material, optionally a flexible composite material. The flexible composite material may be polyethylene infiltrated with carbon nano tubes. The solid thermal conductor may be formed of combinations of any of the aforementioned materials. The solid thermal conductor may comprise graphene. This graphene may for example be provided as a layer, such as along the entire solid thermal conductor. The outside of the solid thermal conductor may for example be coated or covered with a graphene layer. Each of these have high coefficients of thermal conductivity. As such, they are able to rapidly transfer thermal energy. They are each able to be formed into different shapes and so can be fitted to channels of various dimensions and shapes. The solid thermal conductor may be a heat pipe. The heat pipe may according to an embodiment be arranged in a said at least one channel to extend through the said at least one channel. According to a further embodiment the internal surface and/or external surface of the heat pipe may comprise a graphene layer. A heat pipe is a tube including a fluid that is able to rapidly transfer thermal energy along the tube. The fluid may transition between a liquid and a gas to efficiently transfer thermal energy. The heat pipe may be a capillary heat pipe, a loop heat pipe, or an oscillating heat pipe. Loop heat pipes are able to operate against gravity and are also able to operate over longer distances than conventional heat pipes. A loop heat pipe may include an evaporator and a condenser connected by a vapour line from the evaporator to the condenser, and a liquid line from the condenser to the evaporator. The loop heat pipe may include a reservoir configured to hold liquid thermal conditioning fluid. The reservoir may be connected to a porous wick within the evaporator that is configured to draw liquid thermal conditioning fluid into the evaporator where it can be vaporized by the thermal load applied to the evaporator. The phase of the thermal conditioning fluid may change from a liquid to a gas after which is can be passed via the vapour line to the condenser where thermal energy is removed and the gaseous thermal conditioning fluid is condensed back into a liquid.

[00015] The solid thermal conductor may be in thermal communication with a heat sink. The thermal communication may be direct or indirect. The indirect thermal communication may include a thermally conductive connection, which is optionally flexible. The heat sink may be passively or actively conditioned, whether cooled or heated. The function of the solid thermal conductor is to conduct thermal energy away from (or to) the component being conditioned, such as a component of a lithographic apparatus. In order for there to be a transfer of thermal energy, a temperature gradient is needed. In order to maintain such a temperature gradient, a heat sink may be provided in thermal communication with the solid thermal conductor.

[00016] The thermal conditioning apparatus may be provided in thermal communication with an optical element of a lithographic apparatus. Whilst the apparatus could be used to condition other elements of a lithographic apparatus, the present invention has particular application to the conditioning of optical elements as distortions or vibrations in optical elements can negatively affect the performance of such optical elements.

[00017] The solid thermal conductor may be continuous. The solid thermal conductor may be discontinuous. Having a continuous thermal conductor will provide the most efficient path for thermal energy, it may not be possible for the thermal conductor to be provided as a unitary piece. Even so, since there is a thermal conditioning fluid in thermal communication with the component being conditioned and the solid thermal conductor is in thermal communication with the thermal conditioning fluid, it is still possible to transfer thermal energy between the component and the solid thermal conductor via the thermal conditioning fluid.

[00018] A wall of the solid thermal conductor may be separated from at least one wall of the at least one channel by from around 0.05 mm to around 6 mm. The separation may be 5 mm or less, 4 mm or less, 3 mm or less, 2 mm or less, or 1 mm or less. The thickness of the layer of thermal conditioning fluid between the wall of the channel and the solid thermal conductor need only be thick enough to accommodate differential expansion of the channel and the solid thermal conductor.

[00019] The solid thermal conductor may be in thermal communication with at least one other channel configured to receive a flow of thermal conditioning fluid. Such a channel may serve as a heat sink. The flow of thermal conditioning fluid may be configured to remove thermal energy from the solid thermal conductor to thereby allow the solid thermal conductor to continue to remove thermal energy from the component being conditioned. The flow of thermal conditioning fluid may therefore be provided away from or remote from the component being conditioned and therefore vibrations caused by the flow of thermal conditioning fluid are less likely to adversely affect operation of the lithographic apparatus.

[00020] According to a second aspect of the present invention, there is provided a system or subsystem comprising the thermal conditioning apparatus according to the first aspect of the present invention. The system or sub-system may be a system or sub-system of a lithographic apparatus. The system or sub-system may further comprise a component to be thermally conditioned, which component is provided with the thermal conditioning apparatus.

[00021] The system or sub-system may be an optical element. The optical element may be a mirror, a reticle, a sensor, or a fiducial. The system or sub-system may be a reticle stage clamp, a wafer stage, a reticle stage, a wafer stage clamp, or a frame to mount a component of a lithographic apparatus.

[00022] By providing a system or sub-system with the thermal conditioning apparatus according to the first aspect of the present invention, the performance of such system or sub-system may be improved as print-through and vibrations are reduced.

[00023] According to a third aspect of the present invention, there is provided a lithographic apparatus comprising the thermal conditioning apparatus according to the first aspect of the present invention or a system or sub-system according to the second aspect of the present invention.

[00024] Again, such a lithographic apparatus is less susceptible to print-through and vibrations induced by the flow of fluid therein.

[00025] According to a fourth aspect of the present invention, there is provided a method of thermally conditioning a system or sub-system of a lithographic apparatus, said method including providing a thermal conditioning fluid and a solid thermal conductor within a channel that is in thermal communication with the system or sub-system, and transferring thermal energy between the system or sub-system and the solid thermal conductor via the thermal conditioning fluid.

[00026] The method provides for the thermal conditioning of a system or sub-system of a lithographic apparatus that effects conditioning by transferring thermal energy to a solid thermal conductor via a thermal conditioning fluid. This method avoids the need to provide a flow of thermal conditioning fluid, which can result in unwanted vibrations. The high thermal conductivity of the solid thermal conductor allows thermal energy to be removed efficiently and so localized hot spots are avoided and the maximum temperature to which the thermal conditioning fluid is exposed is reduced, thereby allowing a lower pressure of conditioning fluid to be used, which in turn reduces print-through. Such a method provides for a wider range of operating modes. Due to the provision of the solid thermal conductor, it is possible to provide a method in which there is effectively no flow of thermal conditioning fluid. This reduces or eliminates flow induced vibrations. A small flow of thermal conditioning fluid may be used, with the flow being such that it is not the primary contributor to the removal of thermal energy. A high flow mode may be used, with the flow of thermal conditioning fluid being the primary contributor (i.e. 50% of greater) to the removal of thermal energy. The high flow mode may not reduce flow induced vibrations as much as other modes, but allows the thermal conditioning fluid to be provided at a lower pressure than would be the case where no solid thermal conductor is provided.

[00027] The method may include providing a heat sink in thermal communication with the solid thermal conductor. The heat sink may be in the form of a flow of thermal conditioning fluid. The thermal communication may be provided by a flow of thermal conditioning fluid. The thermal communication may be provided by another flexible solid thermal conductor. The thermal communication may be provided by another rigid solid thermal conductor.

[00028] According to a fifth aspect of the present invention, there is provided the use of a thermal conditioning apparatus according to the first aspect of the present invention, a system or sub-system according to the second aspect of the present invention, a lithographic apparatus in accordance with the third aspect of the present invention, or a method according to the fourth aspect of the present invention in a lithographic apparatus or process.

[00029] According to a sixth aspect of the present invention, there is provided 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 thermal conditioning apparatus according to the first aspect of the present invention, a system or sub-system according to the second aspect of the present invention, or conditioned according to a method of the fourth aspect of the present invention.

[00030] According to a seventh aspect of the present invention, there is provided an optical element for a lithographic apparatus, said optical element including a heat spreader.

[00031] As described in relation to the other aspects of the present invention, a heat spreader is able to isothermalise the optical element to reduce the presence of hot spots and the associated heating induced aberrations.

[00032] The heat spreader may be disposed beneath the surface of the optical element. This protects the heat spreader from the environment to which the optical element is exposed. In other embodiments, the heat spreader may be provided around the outer surface of the optical element to provide uniform heating of the optical element.

[00033] The heat spreader may comprise graphene. Graphene is highly thermally conductive and can therefore act as an efficient heat spreader.

[00034] The heat spreader may be disposed within a channel comprising a thermal conditioning fluid. As described above in respect of the other aspects of the present invention, the thermal conditioning fluid is able to accommodate relative contraction and expansion between the body of the optical element and the heat spreader.

[00035] The heat spreader may be in thermal communication with a heater and/or cooler. Whilst the heat spreader serves to reduce in-plane temperature variation, it does not change the amount of thermal energy being handled. By providing a cooler, it is possible to remove thermal energy from the system. Similarly, by providing a heater, it is possible to add thermal energy in order to reduce temperature differential across the surface of the optical element.

[00036] The heating and/or cooling device may include a Peltier device. This allows cooling or heating depending on the electrical current provided.

[00037] The heating and/or cooling device may be controlled via a temperature sensor. As such, when the temperature reaches a predetermined value, the heater or cooler can be activated to provide the required heating or cooling effect.

[00038] 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

[00039] 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:

[00040] Figure 1 depicts a lithographic apparatus according to an embodiment of the invention;

[00041] Figures 2a to 2c are schematic representations of different cooling modes, with Figure 2c depicting a cooling mode according to the present invention;

[00042] Figure 3 depicts a cross-section of a thermal conditioning apparatus according to the present invention;

[00043] Figure 4a and 4b depict a side view and a top view of thermal conditioning apparatuses according to the present invention;

[00044] Figure 5 depicts an embodiment of a thermal conditioning apparatus according to the present invention; and

[00045] Figures 6 to 9 depict various embodiments of thermal conditioning apparatuses according to the present invention;

[00046] Figure 10 depicts an embodiment of a thermal conditioning apparatus according to the present invention including means for accommodating expansion of the thermal conditioning fluid;

[00047] Figures I la and 1 lb depict an embodiment of a thermal conditioning apparatus according to the present invention including an oscillating heat pipe;

[00048] Figure 12 depicts an embodiment of a thermal conditioning apparatus according to the present invention similar to that depicted in Figure 4;

[00049] Figure 13 depict an embodiment of a thermal conditioning apparatus according to the present invention including a heat spreader;

[00050] Figure 14 depicts a simulated temperature distribution of a nominal optical surface, an optical surface including a graphene heat spreader and an optical surface including water cooling; [00051] Figure 15 is a bar chart showing a comparison of normalised imaging key performance indicators comparing the nominal case, the case including a graphene heat spreader, and the case including two different depths of water cooling; and

[00052] Figure 16 is a bar chart showing a comparison of the effect of having no heat spreader, a heat spreader, and a heat spreader with thermal control.

[00053] The features and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements.

DETAILED DESCRIPTION

[00054] Figure 1 shows a lithographic system according to an embodiment of 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 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.

[00055] 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.

[00056] The radiation source SO shown in Figure 1 is of a type which may be referred to as a laser produced plasma (LPP) source. A laser, which may for example be a CO2 laser, is arranged to deposit energy via a laser beam into a fuel, such as tin (Sn) which is provided from a fuel emitter. 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 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 plasma at the plasma formation region. Radiation, including EUV radiation, is emitted from the plasma during de-excitation and recombination of ions of the plasma. [00057] 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 an elliptical configuration, having two ellipse focal points. A first focal point may be at the plasma formation region, and a second focal point may be at an intermediate focus, as discussed below.

[00058] 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 may together be considered to be a radiation system.

[00059] Radiation that is reflected by the collector forms a radiation beam B. The radiation beam B is focused at a point to form an image of 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 focus is located at or near to an opening in an enclosing structure of the radiation source.

[00060] 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.

[00061] Following reflection from the patterning device MA the 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 may include any number of mirrors (e.g. six mirrors).

[00062] 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 thermal conditioning apparatus according to the present invention is integrated into the lithographic apparatus to provide the required thermal conditioning. Thermal conditioning usually requires the removal of thermal energy from the optical elements as they heat up in use and/or are subject to temperature variations. [00063] The radiation sources SO shown in Figure 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.

[00064] Figures 2a to 2c depict various cooling modes. Figure 2a depicts a cooling mode in which thermal energy Q passes through a mirror 15. The mirror 15 is generally formed from ultra-low expansion glass or cordierite which has a high thermal resistance. Due to the high thermal resistance, heat is transmitted very slowly and so the material heats up. The mirror 15 cools by several heat flows: radiative heat transfer, by convective heat transfer by the Hz gas surrounding the mirror, and by conductive heat towards the cooler mirror side surfaces and the cooler bottom surface. The distance L between the front of the mirror and the bottom surface through which the heat has to be transferred is the full depth of the mirror 15. These heat flows are inefficient ways of cooling and so the temperature of the mirror 15 in use can be higher than desired. In this case, when switching illumination modes, the changed heat load distribution causes a greater than desired changed spatial thermal gradient between the optical surface and the outer surfaces of the mirror 15, causing aberrations introduced by the changed thermal expansion of the solid. Figure 2b depicts a cooling mode in which the mirror 15 is provided with a channel 16 through which a thermal conditioning fluid 17, for example water, is flowed. The direction of flow is depicted by the arrows labelled F. Due to the presence of the channel 16, the distance L through which the heat has to be transferred is reduced as compared to Figure 2a. The thermal conditioning fluid 17 is able to take the heat away from the mirror 15, thereby cooling the mirror 15. However, a flowing thermal conditioning fluid 17 can cause flow induced vibrations. Figure 2c depicts an embodiment of a thermal conditioning apparatus in accordance with the present invention in which a solid thermal conductor 18 is provided within the channel 16. The thermal conditioning fluid 17 is provided between the wall of the channel 16 and the solid thermal conductor 18. The thermal conditioning fluid 17 is substantially static, although is able to move to accommodate differences in thermal expansion of the walls of the channel 16 or mirror 15 and the solid thermal conductor 18. Since the solid thermal conductor 18 is configured to conduct thermal energy much more quickly than the material of the mirror 15, it is possible to remove thermal energy from the mirror 15 without the need for a flow of thermal conditioning fluid 17. Since the thermal conditioning fluid 17 is substantially static, it is possible to avoid flow induced vibrations. An expansion chamber, a hydraulic accumulator, an expansion vessel, an expansion joint, or an expansion bellow (not shown) may be provided at any suitable location to accommodate changes in volume of the thermal conditioning fluid. Whilst the detailed description refers to a mirror, it will be appreciated that the present invention may equally be applied to a reticle stage clamp, a wafer stage, a reticle stage, a wafer stage clamp, or a frame to mount a component of a lithographic apparatus. In an embodiment, the solid thermal conductor 18 can include a heat pipe, e.g. a heat pipe immersed in the thermal conditioning fluid 17. In an embodiment, the heat pipe can comprise a thermally conductive layer, e.g. at an outer surface or at an inner surface of the heat pipe. The thermally conductive layer can e.g. comprise graphene.

[00065] Figure 3 is a cross-sectional view of a thermal conditioning apparatus according to an embodiment of the present invention. As shown, the solid thermal conductor 18 is disposed within a channel 16 within the mirror 15 and is surrounded by a layer of thermal conditioning fluid 17. Although the cross-sectional shape of the channel 16 and the solid thermal conductor 18 is shown as square, the invention is not particularly limited to the cross-sectional shape of either of these features. Indeed the cross-sectional shape may be any shape. The solid thermal conductor 18 is shown as being a unitary piece, but it may include one or more cavities or openings. In addition, although the cross-sectional view shows the solid conductor 18 centered in the channel 16, the invention is not particularly limited to this positioning. It will be appreciated that the solid conductor 18 may have any positioning within the channel 16. The solid thermal conductor 18 does not necessarily need to be entirely surrounded by the thermal conditioning fluid 17.

[00066] Figure 4a depicts an embodiment of a thermal conditioning apparatus according to the present invention in which the solid thermal conductor 18 is not continuous. Even though the solid thermal conductor 18 may not be continuous, the thermal conditioning fluid 17 is able to transfer thermal energy to or from the solid thermal conductor 18 to effect conditioning. Figure 4b depicts a plan view of an embodiment of a thermal conditioning apparatus according to the present invention showing multiple channels 16 (not all labelled) including solid thermal conductors 18 and thermal conditioning fluid 17.

[00067] Figure 5 depicts an embodiment of a thermal conditioning apparatus according to the present invention in which a flow F of a thermal conditioning fluid 17’, which may or may not be the same thermal conditioning fluid as used to transfer thermal energy Q between the mirror 15 and the solid thermal conductor 18 and accommodate differential expansion thereof, is used to transfer thermal energy Q from the solid thermal conductor 18 and carried away. As shown, the thermal energy Q is transferred into the solid thermal conductor 18 via thermal conditioning fluid 17. Due to the high thermal conductivity of the solid thermal conductor 18, the thermal energy Q is efficiently transferred along the solid thermal conductor 18. The solid thermal conductor 18 is in thermal communication with a heat sink. In the depicted example, the heat sink comprises a flow F of thermal conditioning fluid 17’ which is able to absorb thermal energy Q from the solid thermal conductor 18 and be carried away. The way in which the solid thermal conductor 18 is cooled is not particularly limiting and any suitable means may be used. The heat sink may also be in the form of an apparatus similar to the present invention comprising a solid thermal conductor, which may also be disposed in a channel housing a thermal conditioning fluid. In the embodiment as shown, the thermal conductor 18 has a first part 18.1 that is arranged in a channel 16 in which the conditioning fluid 17 can flow or reside. The thermal energy Q can be transferred from the mirror 15 to the first part 18.1 of the thermal conductor 18, via the conditioning fluid 17. In the embodiment as shown, the heat Q as absorbed by the first part 18.1 of the thermal conductor 18 is transferred to a second part 18.2 of the thermal conductor 18 due to the high thermal conductivity of the thermal conductor. In the embodiment as shown, the second part 18.2 of the thermal conductor is in thermal communication with the heat sink comprising the flow F of thermal conditioning fluid 17’. In the embodiment as shown, the second part 18.2 of the thermal conductor 18 is arranged outside the channel 16 and outside the mirror 15, facilitating arranging a heat transfer from the second part 18.2 to the heat sink.

Figure 6 depicts an embodiment of a thermal conditioning apparatus according to the present invention similar to that depicted in Figure 5, but further depicting a support frame 19 to which the mirror 15 is connected via connections 20. In the embodiment as shown, the thermal conductor 18 has a first part 18.1 that is arranged in a channel 16 in which the conditioning fluid 17 can flow or reside. The thermal energy Q can be transferred from the mirror 15 to the first part 18.1 of the thermal conductor 18, via the conditioning fluid 17 in the channel. In the embodiment as shown, the heat Q as absorbed by the first part 18.1 of the thermal conductor 18 is transferred to a second part 18.2 of the thermal conductor 18 due to the high thermal conductivity of the thermal conductor. In the embodiment as shown, the second part 18.2 of the thermal conductor is provided with a channel comprising the flow F of thermal conditioning fluid 17’. The second part 18.2 is arranged outside the channel 16 and outside the mirror 15, facilitating a heat transfer from the second part 18.2 to the heat sink.

Figure 7 depicts a further embodiment of a thermal conditioning apparatus according to the present invention similar to that depicted in Figures 5 and 6. In this embodiment, a flow of a secondary thermal conditioning fluid 17’ which is operable to cool the solid thermal conductor 18, is at least partially supported by the frame 19. The tube 22 supplying or receiving the flow may be flexible, have flexible elements, or may be non- flexible. Similar to the arrangement of Figure 6, the second part 18.2 of the thermal conductor is provided with a channel comprising the flow F of thermal conditioning fluid 17’. The second part 18.2 is arranged outside the channel 16 and outside the mirror 15, facilitating a heat transfer from the second part 18.2 to the heat sink.

[00068] Figure 8 depicts a further embodiment of a thermal conditioning apparatus according to the present invention similar to that depicted in Figures 5 to 7. In this embodiment, the secondary thermal conditioning fluid 17’, which is operable to cool the solid thermal conductor 18, in particular the second part 18.2 of the thermal conductor 18, is provided in one or more heat pipes 21, which include an evaporator portion 21a and a condenser portion 21b. The evaporator portion 21a and the condenser portion 21b may be connected by a connection tube 22, which may be flexible or may be non-flexible. The frame 19 may itself be cooled by any suitable means to remove thermal energy from the heat pipe 21. It will be appreciated that this figure is schematic and the relative orientation of portions of the heat pipe are shown for optimal understanding of the concepts behind the present invention. Again, similar to the arrangements in Figures 5 to 7, the thermal conductor 18 has a first part 18.1 that is arranged in a channel 16 in which the conditioning fluid 17 can flow or reside. The thermal energy Q can be transferred from the mirror 15 to the first part 18.1 of the thermal conductor 18, via the conditioning fluid 17 in the channel. In the embodiment as shown, the heat Q as absorbed by the first part 18.1 of the thermal conductor 18 is transferred to a second part 18.2 of the thermal conductor 18 due to the high thermal conductivity of the thermal conductor 18.

[00069] Figure 9 depicts a further embodiment of a thermal conditioning apparatus according to the present invention similar to that depicted in Figures 5 to 8, particularly Figure 8. In this embodiment, the heat pipe 21 is in the form of a loop heat pipe 21 which is in thermal communication with the second part 18.2 of the thermal conductor 18. In the embodiment as show, the loop heat pipe 21 comprises an evaporation portion 21a that is in thermal communication with the second part 18.2 of the thermal conductor 18 and a condenser portion 21b connected or in thermal communication with a frame 19, which can e.g. be cooled by any suitable means. In embodiments, there may be separate heat pipes that are connected via thermal straps rather than via connection tube 22. Thermal straps are thermally conductive and are thereby able to transfer thermal energy between heat pipes. The thermal straps may be flexible or non-flexible. In embodiments, the solid thermal conductor 18 may be thermally connected to a heat sink via one or more thermal straps.

[00070] Figure 10 depicts an embodiment of the present invention in which an exemplary means for accommodating changes in volume of the thermal conditioning fluid 17 is depicted. The change in volume of the thermal conditioning fluid can be accommodated by any suitable means, such as an expansion chamber, a hydraulic accumulator, an expansion vessel, an expansion joint, or an expansion bellow. In the depicted exemplary embodiment, a number of different means of accommodating changes in volume of the thermal conditioning fluid 17 are depicted. It will be appreciated that these may be provided individually or in any combination. As such, although the figure depicts three possibilities, this does not mean that all three are required. The means for accommodating changes in volume of the thermal conditioning fluid may be any of the means described herein. The means for accommodating a change in volume may comprise one or more of a raised weight hydraulic accumulator 23, a bladder hydraulic accumulator 24, and an actuated piston 25. The raised weight hydraulic accumulator 23 functions by providing a piston within a bore that has a predetermined weight. An increase in pressure in the thermal conditioning fluid 17 will move the piston within the bore until such a time as the pressure exerted by the thermal conditioning fluid 17 will equal the force provided by the weight of the piston. The bladder hydraulic accumulator 24 includes a pressurized bladder which can be deformed by the pressure provided by the thermal conditioning fluid 17. Again, as the pressure of the thermal conditioning fluid 17 varies, the bladder will expand or contract to accommodate the change in pressure of the thermal conditioning fluid. A third option is an actuated piston 25 arrangement, which is provided with a pressure sensor 26. When the thermal conditioning fluid expands, the pressure increases. This increase in pressure is detected by the pressure sensor 26, which then controls an actuator that adjusts the position of the piston in order to accommodate the change in volume of the thermal conditioning fluid and thereby control the pressure of the thermal conditioning fluid 17. In the embodiment shown in Figure 10, the thermal conductor 18 has a first part 18.1 that is arranged in a channel 16 in which the conditioning fluid 17 can flow or reside. The thermal energy Q can be transferred from the mirror 15 to the first part 18.1 of the thermal conductor 18, via the conditioning fluid 17. In the embodiment as shown, the heat Q as absorbed by the first part 18.1 of the thermal conductor 18 is transferred to a second part 18.2 of the thermal conductor 18 due to the high thermal conductivity of the thermal conductor. In an embodiment, the second part 18.2 of the thermal conductor 18 can e.g. be conditioned using any conditioning means, e.g. heat sinks or heat pipes, described in Figures 5 to 9.

[00071] Figure 11 depicts an embodiment in which the solid thermal conductor 18 is a heat pipe 21. In this embodiment, the heat pipe 21 is an oscillating heat pipe - also known as pulsating heat pipe - that is formed in a serpentine shape. In a so-called oscillating heat pipe, the oscillation or pulsation takes place inside the heat pipe, the pipe remains motionless. The heat pipe 21 is configured to isothermalise the optical element by transferring thermal energy through the heat pipe 21 to a heat sink 28. The heat sink 28 may be actively or passively heated or cooled, as required. In most cases, the heat sink 28 is configured to remove thermal energy from the heat pipe 21 to thereby allow the conditioning and isothermalisation of the mirror 15. The oscillating heat pipe 21 contains a fluid which is able to transition between a liquid and a gaseous state, as shown by the light and dark areas within the heat pipe 21. In use, the area of the mirror which is under the optical footprint 27 of the radiation falling upon the mirror 15 causes the mirror to heat up. The thermal energy is transferred to the heat pipe 21, where it is conducted rapidly away to heat sink 28 where excess thermal energy can be removed before the heat pipe 21 passes back into the optical footprint 27. In this way, the mirror 15 can be conditioned and the heat pipe 21 is able to remove hot-spots from the mirror 15 and cause the temperature across the mirror to be more uniform. It will be appreciated that the conditioning system may include more than one heat pipes.

[00072] Figure 1 lb depicts on embodiment in which the heat sink 28 is positioned on the opposite side of the mirror 15 to where the thermal load Q is applied. The channel 16 within the mirror 15 can be sealed with a flange or stopper 29 so as to retain the thermal conditioning fluid 17 therein.

[00073] Figure 12 is similar to Figure 2c and Figure 4 and depicts an embodiment including a plurality of heat pipes 21 disposed in channels 16 containing thermal conditioning fluid 17. The part of the heat pipes in the channels 16 are so to say immersed in the thermal conditioning fluid in the channels. The heat pipes 21 are in thermal communication with heat sinks 28 which are configured to either remove or add thermal energy to the system. In this way, the heat pipes 21 are able to condition the mirror 15 and also isothermalise the mirror to reduce or avoid hot spots.

[00074] Figure 13 depicts an embodiment in which a mirror 15 is provided with a heat spreader 30 within a channel 16 therein. The channel 16 also include a thermal conditioning fluid 17. The mirror 15 may also be provided with a gas pocket 31. The gas pocket 31 is provided to accommodate expansion of the thermal conditioning fluid 17 and of the heat spreader 30. The gas pocket 31 may include any suitable gas, such as air or nitrogen. The heat spreader 30 may be in the form of a plate. The heat spreader may comprise graphene. The channel height may be any suitable height, for example around 100 microns. The thermal conditioning fluid 17 may be under sufficient pressure to avoid boiling during normal use.

[00075] Figure 14 depicts a comparison of the simulated temperature differential across an optical element comprising the nominal case, the case including a graphene heat spreader, and the case including water cooling. It can be seen that in the nominal case, there is a temperature differential across the surface of around 10°C with a clear star shaped hot spot in the middle. In contrast, the temperature differential for the water cooled element is much reduced at around 1 - 2°C For the example including a graphene heat spreader, the temperature differential is somewhere between the nominal example and the water cooled example. The simulation was based on a 600W power source and the graphene example was simulated with a 30 micron thick graphene layer placed 3 mm under the optical surface and only thermally coupled to the mirror body. Whilst the water cooling removes the thermal energy resulting in a lower overall temperature, the graphene heat spreader more evenly distributes the heat over the optical element, thereby reducing maximum temperature and also the temperature differential.

[00076] Figure 15 is a bar chart comparing the nominal case (left hand bar in each set of four), the case including a graphene heat spreader located 3mm below the surface of the optical element (the second from left bar in each set of four), the case including water cooling at 5mm from the surface of the optical element (the second from right bar in each set of four), and the case including water cooling at 3mm from the surface of the optical element (the right hand bar in each set of four). These key performance indicators, namely best focus, overlay, and critical dimension, are normalized to the nominal case. The presence of the graphene heat spreader enhances spread results by more than a factor of 2 and would allow the temperature differential to be more easily corrected with heaters to heat the remaining cooler areas. With water cooling, the distance from the surface is limited by print-through and variations therein due to pressure fluctuations. With a heat spreader, there are no inherent pressure fluctuations.

[00077] Figure 16 shows the difference provided by the provision of a heat spreader as well as the provision of a heat spreader including thermal control. The root mean square (rms) wave front error (WFE) is drastically reduced via the presence of a heat spreader and further by the addition of thermal control, whether that is a heater or cooler.

[00078] In summary, the present invention provides for an apparatus which conditions an element of a lithographic apparatus, particularly an optical element such as a mirror, by transferring thermal energy away from such element via a solid thermal conductor provided within a channel that includes a thermal conditioning fluid. The thermal conditioning fluid is able to transfer the thermal energy and accommodate differential expansion of the components. The thermal conditioning fluid is static, save for the minor movement associated with accommodating differential expansion of the solid thermal conductor and the component being conditioned, and so flow induced vibrations are eliminated. In addition, improved heat dissipation reduces the maximum temperature to which the thermal conditioning fluid is exposed, which allows reduction of the operating pressure. This results in reduced print-through. . This improves the performance of the lithographic apparatus.

[00079] The present invention has particular, but not exclusive application, to the cooling of optical elements of lithographic apparatuses. The present invention also has particular, but not exclusive application, to the conditioning of systems or sub-systems of a lithographic apparatus, such as, for example a reticle stage clamp, a wafer stage, a reticle stage, a wafer stage clamp, or a frame to mount a component of a lithographic apparatus.

[00080] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described.

[00081] 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.

Claims

1. A thermal conditioning apparatus, said thermal conditioning apparatus including at least one channel, wherein a thermal conditioning fluid and a solid thermal conductor are disposed within the at least one channel, wherein the solid thermal conductor is in thermal communication with a heat sink and wherein the solid thermal conductor comprises a first part arranged in the at least one channel and a second part arranged outside the at least one channel.

2. The thermal conditioning apparatus according to claim 1, wherein the apparatus is configured such that, in normal use, the thermal conditioning fluid is substantially static within the at least one channel.

3. The thermal conditioning apparatus according to claim 1 or claim 2, wherein the thermal conditioning fluid is selected from a liquid and a gas, optionally where the thermal conditioning fluid is water.

4. The thermal conditioning apparatus according to any preceding claim, wherein the solid thermal conductor is selected from a ceramic, a metal, a ceramic-metal, or a flexible composite material, optionally wherein the material is selected from silicon carbide, silicon nitride, silicon infiltrated silicon carbide, silicon infiltrated silicon nitride, diamond infiltrated silicon carbide, diamond infiltrated silicon nitride, diamond, polycrystalline diamond, graphene, silicon nitride, aluminium, aluminium silicon carbide, aluminium nitride, polyethylene infiltrated with carbon nanotubes, or combinations thereof, and/or wherein the solid thermal conductor comprises a heat pipe.

5. The thermal conditioning apparatus according to any of the preceding claims, wherein the heat sink is arranged in thermal communication with the second part of the solid thermal conductor.

6. The thermal conditioning apparatus according to claim 5, wherein the second part of the solid thermal conductor comprises a channel to receive a conditioning fluid.

7. the thermal conditioning apparatus according to any of the preceding claims, wherein the heat sink comprises a heat pipe or further thermal conductor arranged in thermal communication with the second part of the solid thermal conductor.

8. The thermal conditioning apparatus according to any preceding claim, wherein the thermal conditioning apparatus is provided in thermal communication with an optical element of a lithographic apparatus.

9. The thermal conditioning apparatus according to any preceding claim, wherein the solid thermal conductor is continuous, or wherein the solid thermal conductor is discontinuous.

10. The thermal conditioning apparatus according to any preceding claim, wherein a wall of the solid thermal conductor is separated from a wall of the at least one channel by from around 0.05 mm to around 6 mm.

11. The thermal conditioning apparatus according to any preceding claim, wherein the solid thermal conductor is in thermal communication with i) at least one other channel configured to receive a flow of thermal conditioning fluid, ii) a heat pipe, and/or iii) another solid thermal conductor, optionally wherein the another solid thermal conductor is flexible or rigid.

12. The thermal conditioning apparatus according to any preceding claim, the apparatus being configured to accommodate expansion of the thermal conditioning fluid, optionally wherein the apparatus includes an expansion chamber, a hydraulic accumulator, an expansion vessel, an expansion joint, or an expansion bellow.

13. The thermal conditioning apparatus according to any preceding claim, wherein the solid thermal conductor comprises a heat pipe and/or a heat spreader.

14. The thermal conditioning apparatus according to any preceding claim, wherein the solid thermal conductor comprises a layer of graphene.

15. A system or sub-system for a lithographic apparatus comprising the thermal conditioning apparatus according to any of claims 1 to 14.

16. The system or sub-system according to claim 15, wherein system or the sub-system is an optical element of a lithographic apparatus, optionally selected from the list consisting of: a mirror, a reticle, a sensor, or a fiducial.

17. The system or sub-system according to claim 16, wherein the system or sub-system is a reticle stage clamp, a wafer stage, a reticle stage, a wafer stage clamp, or a frame to mount a component of a lithographic apparatus.

18. The system or sub-system according to any of claims 15-17, further comprising a component to be thermally conditioned, the component being provided with the thermal conditioning apparatus, the thermal conductivity of the solid thermal conductor is at least 175 W/m K greater than the thermal conductivity of the component.

19. A lithographic apparatus including the thermal conditioning apparatus according to any of claim 1 to 14 or a system or sub-system according to any of claims 15 to 17.

20. A method of conditioning a system or sub-system, said method including providing a thermal conditioning fluid and a solid thermal conductor within a channel that is in thermal communication with the system or sub-system, the solid thermal conductor being in thermal communication with a heat sink and the solid thermal conductor comprising a first part arranged in the channel and a second part arranged outside the channel, and transferring thermal energy between the system or sub-system and the solid thermal conductor via the thermal conditioning fluid.

21. The use of a thermal conditioning apparatus according to any of claims 1 to 14, a system or sub-system according to any of claims 15 to 17, a lithographic apparatus in accordance with claim 19, or a method according to claim 20 in a lithographic apparatus or process.

22. 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 thermal conditioning apparatus according to any of claims 1 to 14, a system or sub-system according to claims 15 to 17, or conditioned according to a method of claim 20.

23. An optical element for a lithographic apparatus, said optical element including a heat spreader.

24. The optical element of claim 23, wherein said heat spreader is disposed beneath the surface of the optical element.

25. The optical element of claim 23 or 24, wherein the heat spreader comprises graphene.

26. The optical element of any of claims 23 to 25, wherein the heat spreader is disposed within a channel comprising a thermal conditioning fluid.

27. The optical element of any of claims 23 to 26, wherein the heat spreader is in thermal communication with a heater and/or cooler.

28. The optical element of claim 27, wherein the heater and/or cooler includes a Peltier device.

29. The optical element of any of claims 27 to 28, wherein the heater and/or cooler is controlled via a temperature sensor.