WO2024235539A1 - Magnetic levitation positioner assembly for use in a lithographic apparatus, lithographic apparatus with the magnetic levitation positioner assembly and method of manufacturing devices using the lithographic apparatus - Google Patents

Abstract

An assembly for use in a lithographic apparatus, the assembly comprising: a thermal conditioning plate comprising an internal passage network for flowing a cryogen therein; and a plurality of superconducting coils for magnetically levitating and linearly displacing a stage thereon, wherein the plurality of superconducting coils is in thermal contact with an external surface of the thermal conditioning plate.

Description

MAGNETIC LEVITATION POSITIONER ASSEMBLY, LITHOGRAPHIC APPARATUS, AND METHOD OF MANUFACTURING DEVICES CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application claims priority of EP application 23173737.0 which was filed on 16 May 2023 and which is incorporated herein in its entirety by reference. FIELD [0002] The present invention relates to a magnetic levitation positioner assembly for use in a lithographic apparatus. 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 (also often referred to as “design layout” or “design”) of a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate (e.g., a wafer). [0004] As semiconductor manufacturing processes continue to advance, the dimensions of circuit elements have continually been reduced while the amount of functional elements, such as transistors, per device has been steadily increasing over decades, following a trend commonly referred to as “Moore’s law”. To keep up with Moore’s law the semiconductor industry is chasing technologies that enable to create increasingly smaller features. To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which are patterned on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm and 13.5 nm. [0005] A lithographic apparatus may include an illumination system for providing a projection beam of radiation, and a support structure for supporting a patterning device. The patterning device may serve to impart the projection beam with a pattern in its cross-section. The apparatus may also include a projection system for projecting the patterned beam onto a target portion of a substrate. [0006] In order to provide the substrate with a desired pattern, both the patterning device and the substrate are moved with respect to the projection system as well as with respect to each other. For this purpose, linear and/or planar motor systems are used based on current conducting coils and permanent magnets at room temperature or higher due to heat developed by the coils. Linear and planar motor systems typically have a stator part and a mover part which is movable in controlled manner with respect to the stator part by manipulating the current through the current conducting coils. Company Secret [0007] For example, in some lithographic apparatuses, the surface of a long-stroke substrate positioner assembly is equipped with strong permanent magnets in a checkerboard pattern on the stator side. The substrate holder contains electromagnets (e.g. in a Lorentz actuator) and levitates over the stator. Replacing the permanent magnets on the stator with superconducting electromagnets can increase the magnetic field density, and therefore provide significant throughput improvement. However, realising high field density superconductive coils requires cryogenic cooling. [0008] Attempts have been made to realise a positioner with superconducting electromagnets stage. For example, US 2021/0223706 A1 discloses a cryostat design with superconducting coils that is applicable for lithography. Here, a first stage of a cryocooler is attached directly to a spacer plate at 80K, and a second stage of the cryocooler is attached to an inner plate at <30K on which the superconducting coils are mounted. In this design, the cryocooler acts a heat sink for the components at these two temperature levels. The cryocooler cold end has spot connections with the cold surfaces, i.e. the spacer plate and the inner plate. [0009] However, as found by the present inventors, in US 2021/0223706 A1, the cooling capability of the cryocooler is limited by the number of spot connections that the cryocooler makes on the cold surfaces, as well as the thermal conductivity between these connections and the generated/absorbed heat. Due to the generated heat near the superconducting coils on top of the inner plate, given that the average dimensions of the inner plate can be larger than a metre, even if the cryocooler is centrally positioned, the temperature distribution of the superconducting coils can have large variations. As found by the present inventors, if the temperature variations are sufficiently large, they can potentially harm the performance of the positioner. In particular, the spatial temperature variations could generate unwanted variations in electromagnetic performance over the superconducting coils. SUMMARY OF THE INVENTION [0010] Therefore, an object of the present invention is to provide a cryogenic positioner with improved temperature distribution. [0011] Another object of the present invention is to provide a cryogenic positioner with efficient use of cooling power. [0012] According to an aspect of the present invention, there is provided an assembly for use in a lithographic apparatus, the assembly comprising: a thermal conditioning plate comprising an internal passage network for flowing a cryogen therein; and a plurality of superconducting coils for magnetically levitating and linearly displacing a stage thereon, wherein the plurality of superconducting coils is in thermal contact with an external surface of the thermal conditioning plate. Company Secret BRIEF DESCRIPTION OF THE DRAWINGS [0013] 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. [0014] Figure 1 depicts a lithographic apparatus in accordance with an embodiment of the present invention. [0015] Figure 2 depicts a positioner assembly in accordance with an embodiment of the present invention. [0016] Figure 3 depicts a thermal conditioning plate in accordance with an embodiment of the present invention. [0017] Figure 4 depicts a cross-section of a flexible conduit in accordance with an embodiment of the present invention. [0018] Figures 5A and 5B depict arrangements of an internal passage network in accordance with embodiments of the present invention. [0019] The features shown in the Figures are not necessarily to scale, and the size and/or arrangement depicted is not limiting. It will be understood that the Figures include optional features which may not be essential to the invention. Furthermore, not all of the features of the apparatus are depicted in each of the figures, and the Figures may only show some of the components relevant for describing a particular feature. DETAILED DESCRIPTION [0020] In the present document, the terms “radiation” and “beam” are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g., with a wavelength of 436, 405, 365, 248, 193, 157, 126 or 13.5 nm). [0021] The term “reticle”, “mask” or “patterning device” as employed in this text may be broadly interpreted as referring to a generic patterning device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term “light valve” can also be used in this context. Besides the classic mask (transmissive or reflective, binary, phase-shifting, hybrid, etc.), examples of other such patterning devices include a programmable mirror array and a programmable LCD array. [0022] Figure 1 schematically depicts a lithographic apparatus LA. The lithographic apparatus includes an illumination system (also referred to as illuminator) IL configured to condition a radiation beam B (e.g., EUV radiation or DUV radiation), a mask support (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA in accordance with certain parameters, a substrate support (e.g., a substrate table or a substrate holder) WT constructed to hold a substrate (e.g., a resist coated wafer) W and connected to a second positioner PW configured to accurately position the Company Secret substrate support WT in accordance with certain parameters, and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W. [0023] In operation, the illumination system IL receives the radiation beam B from a radiation source SO, e.g., via a beam delivery system BD. The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and/or other types of optical components, or any combination thereof, for directing, shaping, and/or controlling radiation. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in its cross section at a plane of the patterning device MA. [0024] The term “projection system” PS used herein should be broadly interpreted as encompassing various types of projection system, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and/or electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, and/or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system” PS. [0025] The lithographic apparatus may be of a type wherein at least a portion of the substrate W may be covered by an immersion liquid having a relatively high refractive index, e.g., water, so as to fill an immersion space between the projection system PS and the substrate W – which is also referred to as immersion lithography. More information on immersion techniques is given in US 6,952,253, which is incorporated herein by reference. [0026] The lithographic apparatus may be of a type having two or more substrate supports WT (also named “dual stage”). In such “multiple stage” machine, the substrate supports WT may be used in parallel, and/or steps in preparation of a subsequent exposure of the substrate W may be carried out on the substrate W located on one of the substrate support WT while another substrate W on the other substrate support WT is being used for exposing a pattern on the other substrate W. [0027] In addition to the substrate support WT, the lithographic apparatus may comprise a measurement stage (not depicted in Figure 1). The measurement stage is arranged to hold a sensor and/or a cleaning device. The sensor may be arranged to measure a property of the projection system PS or a property of the radiation beam B. The measurement stage may hold multiple sensors. The cleaning device may be arranged to clean part of the lithographic apparatus, for example a part of the projection system PS or a part of a system that provides the immersion liquid. The measurement stage may move beneath the projection system PS when the substrate support WT is away from the projection system PS. [0028] In operation, the radiation beam B is incident on the patterning device, e.g., mask, MA which is held on the mask support MT, and is patterned by the pattern (design layout) present on Company Secret patterning device MA. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and a position measurement system PMS, the substrate support WT can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B at a focused and aligned position. Similarly, the first positioner PM and possibly another position sensor (which is not explicitly depicted in Figure 1) may be used to accurately position the patterning device MA with respect to the path of the radiation beam B. Patterning device MA and substrate W may be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. Although the substrate alignment marks P1, P2 as illustrated occupy dedicated target portions C, they may be located in spaces between target portions C. Substrate alignment marks P1, P2 are known as scribe-lane alignment marks when these are located between the target portions C. [0029] In this specification, a Cartesian coordinate system is used. The Cartesian coordinate system has three axis, i.e., an x-axis, a y-axis and a z-axis. Each of the three axes is orthogonal to the other two axes. A rotation around the x-axis is referred to as an Rx-rotation. A rotation around the y- axis is referred to as an Ry-rotation. A rotation around about the z-axis is referred to as an Rz- rotation. The x-axis and the y-axis define a horizontal plane, whereas the z-axis is in a vertical direction. The Cartesian coordinate system is not limiting the invention and is used for clarification only. Instead, another coordinate system, such as a cylindrical coordinate system, may be used to clarify the invention. The orientation of the Cartesian coordinate system may be different, for example, such that the z-axis has a component along the horizontal plane. [0030] As noted above, in order to provide the substrate with a desired pattern, both the patterning device MA and the substrate W are moved with respect to the projection system PS as well as with respect to each other. For this purpose, attempts have been made to realise a positioner with superconducting electromagnets stage. For example, US 2021/0223706 A1, which is herein incorporated by reference in its entirety, discloses a cryostat design with superconducting coils that is applicable for lithography. Here, a first stage of a cryocooler is attached directly to a spacer plate at 80K, and a second stage of the cryocooler is attached to an inner plate at <30K on which the superconducting coils are mounted. In this design, the cryocooler acts a heat sink for the components at these two temperature levels. The cryocooler cold end has spot connections with the cold surfaces, i.e. the spacer plate and the inner plate. [0031] However, as found by the present inventors, in US 2021/0223706 A1, the cooling capability of the cryocooler is limited by the number of spot connections that the cryocooler makes on the cold surfaces, as well as the thermal conductivity between these connections and the generated/absorbed heat. Due to the generated heat near the superconducting coils on top of the inner plate, given that the average dimensions of the inner plate are larger than a metre, even if the cryocooler is centrally positioned, the temperature distribution of the superconducting coils can have large variations. As found by the present inventors, if the temperature variations are sufficiently large, they can potentially Company Secret harm the performance of the positioner. In particular, the spatial temperature variations could generate unwanted variations in electromagnetic performance over the superconducting coils. [0032] To compensate for the temperature variations, the a more powerful cryocooler may be employed so as to keep the warmest coil at the target temperature, but this leads to a significant increase in required cooling power. Alternatively, the number of connections between the cryocooler and the cold surface may be increased, such as by providing multiple cryocoolers. However, as the positioner is subject to sharp accelerations (e.g., greater than 10 ms^-2 on the stator side) during operation, and the cryocooler has to move with the positioner, increasing the number of connections would increase the overall mass of the positioner, which would result in mechanical difficulties. Specifically, such acceleration forces may be harmful for the mechanical connection between the long and slender cryocooler and the relatively rigid positioner. Another difficulty associated with increasing the number of connections is a lack of space. As a result, these design modifications have their drawbacks. [0033] Figure 2 depicts a positioner assembly 1 in accordance with the present invention. The positioner assembly 1 comprises a thermal conditioning plate 2 and a plurality of superconducting coils 5. The plurality of superconducting coils 5 are for magnetically levitating and linearly displacing stage 9 thereon. The positioner assembly 1 and the stage 9 may be any positioner and stage within a lithographic apparatus. For example, the stage 9 may be a mask support MT for the patterning device MA and the positioner assembly 1 may be part of the first positioner PM. Alternatively or additionally, the stage 9 may be the substrate support WT, and the positioner assembly 1 may be the second positioner PW. [0034] As shown in Figure 3, the thermal conditioning plate 2 may comprise an internal passage network 22 for flowing a cryogen therein. As a result, the thermal conditioning plate 2 may be thermally conditioned, e.g., cooled, by the cryogen flowing within the internal passage network 22. As shown, the plurality of conducting coils 5 is in thermal contact with an external surface 21 of the thermal conditioning plate 2. More specifically, the plurality of superconducting coils 5 may be disposed on the external surface 21 of the thermal conditioning plate 2. The plurality of superconducting coils 5 may be directly disposed on the external surface 21. The superconducting coils 5 may be in thermal contact with the external surface 21 of the thermal conditioning plate 2 without coming into direct contact with (e.g., being submerged in) the cryogen. Alternatively, the superconducting coils 5 may be embedded in the thermal conditioning plate 2, and may be not in direct contact with (e.g., being submerged in) the cryogen [0035] Furthermore, as shown in Figure 3, the internal passage network 22 may be substantially co- extensive with the plurality of superconducting coils 5. That is, the internal passage network 22 may have a lateral extent which is similar to or substantially the same as the lateral extent of the plurality superconducting coils 5. This may help improve the uniformity of the thermally conditioning, e.g., cooling provided to the superconducting coils 5. Company Secret [0036] Because the cryogen is flowed in the internal passage network 22 within the thermal conditioning plate 2, compared with providing cooling at a limited number of connection spots of a cryocooler, the plurality of superconducting coils 5 can be thermally conditioned, e.g., cooled, evenly. In other words, the temperature distribution of the superconducting coils 5 can become more uniform. This, in turn, may result in a more uniform performance of the superconducting coils 5. [0037] Furthermore, because cooling is provided via the internal passage network 22 instead of multiple connection points of multiple cryocoolers, the positioner assembly 1 may be more compact, which may be desirable as space around the positioner assembly 1 may be limited. Specifically, the overall mass of the moving parts of the positioner assembly 1 may be reduced. This may reduce the amount of vibrations generated during operation of the lithographic apparatus. [0038] Furthermore, because the cryogen flows within the thermal conditioning plate 2, the cryogen that is within the thermal conditioning plate 2 at a given moment may provide a buffer capacity against quench events. [0039] A quench event refers to a change of state of an electromagnet from superconducting to non- superconducting, resulting in an increase in electrical resistance. The increase in electrical resistance may initially be small as the electromagnet may only be slightly non-superconducting. However, the increase in electrical resistance may, in turn, causes a certain amount of heat to be generated (by Joule heating) as electricity continues to flow through the coils. In the absence of an adequate heat sink or heat transfer path, this results in a further temperature rise in the electromagnet coils, and a corresponding rise in electrical resistance in a self-reinforced, run-away fashion. In particular, quench may typically initiate at a localised point in an otherwise superconducting electromagnet, and may propagate as a thermal wave to adjacent portions of the electromagnet, thereby causing further portions of the electromagnet to become non-superconducting. A chain reaction may thus ensue, and may cause a total loss of superconductivity in the entire electromagnet. Quench events are therefore undesirable, and may also cause irreversible weakening of the material of the thermal conditioning plate 2. [0040] The likelihood of quench events can be reduced by increasing the heat sink. For example, a passive heat sink may be provided. For example, passive heat sink may be provided by thermally contacting the superconducting coils 5 with a thermal mass, such as a thermally conductive (e.g. copper or aluminium) plate. For example, the material of the thermal conditioning plate 2 may provide some passive heat sink capacity. [0041] However, active heat absorption capacity may be desirable. As noted above, the cryogen that is within the thermal conditioning plate 2 at a given moment may provide additional heat absorption capacity. Specifically, because the material of the thermal conditioning plate 2 may have a reduced heat capacity at cryogenic temperatures, the presence of the cryogen within the thermal conditioning plate 2 may provide additional heat absorbing capacity to the thermal conditioning plate 2 in case of unplanned shutdown, for example. More specifically, the cryogen may be able to Company Secret maintain the thermal conditioning plate 2 at a cryogenic temperature whilst absorbing a large amount of heat from the superconducting coils 5, notably as latent heat of fusion and/or latent heat of vaporisation. Other active techniques such as dump resistors and quench back heaters may be additionally employed. [0042] Another advantage associated with flowing the cryogen in an internal passage network 22 is that sloshing may be reduced. That is, if the internal passage network 22 shown in Figure 3 were replaced by a single plenum in which the cryogen is provided, during operation of the lithographic apparatus, the cryogen may slosh around in the plenum, which may create greater mechanical vibrations. More specifically, when an enclosed volume (e.g. a single plenum) containing a fluid is subjected to acceleration, the fluid itself is also caused to accelerate. This non-flow-induced acceleration (i.e. sloshing) acts as an acoustic source and generates pressure waves within the enclosed volume. These pressure waves may then travel through the fluid line (at the speed of sound) and may introduce dynamic pressures which lead to dynamic deformations (i.e. vibrations). By contrast, by employing an internal passage network 22, any movement of the cryogen is restricted within the passages of the internal passage network 22, which in turn limits mechanical vibrations. [0043] The thermal conditioning plate 2 may be variously constructed. For example, the thermal conditioning plate 2 may comprise a thermally conductive material. Examples of thermally conductive materials include aluminium, copper, aluminium alloys, and copper alloys. The thermal conditioning plate 2 may comprise a material of high stiffness, such as stainless steel. High stiffness may be desirable as it may allow the thermal conditioning plate 2 to withstand sharp acceleration forces during operation of the lithographic apparatus. High stiffness may be achieved through a choice of material and/or constructional design. Depending on the manufacturing method, the thermal conditioning plate 2 may be constructed of a single material, or may be constructed of several materials. In particular, the thermal conditioning plate 2 may be constructed of several materials of different thermal conductivities. [0044] For example, the thermal conditioning plate 2 may be formed by casting, in which case the internal passage network 22 may be formed directly during the casting process. Alternatively, the thermal conditioning plate 2 may be manufactured initially as two plates, with open channels machined into one or both plates, which plates may then be bonded together using e.g., vacuum brazing or other suitable means to close the channels that form the internal passage network 22. Alternatively, the thermal conditioning plate 2 may be formed by fitting pipes that are forged into the required shape into corresponding machined channels on a surface of the thermal conditioning plate 2. The forged pipes may be tightly fitted in the thermal conditioning plate 2 so that the forged pipes and the material of the thermal conditioning plate 2 make a good thermal contact. As another alternative, the thermal conditioning plate 2 may be formed by drilling parallel bores through a solid plate, and closing the surface holes using plugs. As yet another alternative, the thermal conditioning plate 2 may be formed by forging pipes to the required shape of the internal passage network 22, and by casting Company Secret the thermal conditioning plate 2 around the forged pipes. In particular, in this manufacturing method, the pipes may be made of stainless steel, which pipes may be cast in aluminium (or an aluminium alloy), which has a lower melting point, to form the thermal conditioning plate 2. It should be understood that these manufacturing methods are only non-limiting examples; other suitable manufacturing methods may be used to form the thermal conditioning plate 2. [0045] The internal passage network 22 may have different geometries or topologies. Generally, the internal passage network 22 may comprise one or more elongate passages which cover an extent within the thermal conditioning plate 2. Figures 5A and 5B show two non-limiting examples of the internal passage network 22. As shown in Figure 5A, the internal passage network 22 may have a serpentine shape. In one example, the internal passage network 22 may be a single continuous passage with no branches. Alternatively, the internal passage network 22 may have branches. For example, Figure 5B shows an internal passage network 22 which comprises a series of passages which are connected in parallel. In this arrangement, a restrictor 223 may be provided at or near the entrance to each individual parallel passage, so that the internal passage network 22 may be calibrated to provide even cryogen flow through the parallel passages. Furthermore, in the arrangement of Figure 5B, the cryogen supply line 61 and the cryogen return line 62 may be provided at opposite ends of the series of parallel passages, which may help equalise the flow amongst the parallel passages. [0046] The internal passage network 22 may be formed into any other suitable geometries. For example, the internal passage network 22 may have a labyrinthine geometry. The elongate shape of the internal passage network 22 may help reduce sloshing. Furthermore, the presence of bends in the internal passage network 22 may also help reduce sloshing. Specifically, sloshing may be reduced by reducing the number and/or length of straight passages. [0047] It should be understood that the serpentine shape of Figure 5A and the parallel-passage arrangement of Figure 5B may be combined in a single internal passage network 22. It should also be understood that the internal passage network 22 may comprise several subsections, each of which may have the same or different geometry, and the subsections may be interconnected or isolated from one another. Furthermore, different subsections may be arranged laterally within the thermal conditioning plate 2, and/or may be stacked in the thickness direction of the thermal conditioning plate 2. [0048] Different cryogens can be used to thermally condition, e.g., cool the thermal conditioning plate 2. Table 1 below lists examples of suitable cryogens together with some of their relevant thermodynamic properties. The ranges between their triple point and critical point identify their potential use for a flow boiling application. “Flow boiling” refers to evaporative cooling whereby a continuous flow of coolant evaporates as it flows through/past a heat source. By contrast, “pool boiling” refers to evaporative cooling whereby a non-flowing pool of coolant evaporates as it absorbs heat. Flow boiling is generally a more efficient cooling mechanism (in terms of Watts per unit amount of coolant). In particular, helium is suitable for a multiphase flow heat transfer application Company Secret (albeit at superfluid conditions, lower than the λ point) down to 1.9 K. Alternatively, a multiphase flow cooling application with a target temperature of 20 K can be achieved using para-hydrogen. For another example, for an operation temperature of 25 K, liquid neon or hydrogen may be used. For 60 K, coolants such as fluorine or oxygen may be used. Generally, the coolant may be chosen so that the operation temperature falls above the triple point (so that the coolant does not solidify) and below the critical temperature (so that the coolant does not evaporate) of the coolant. Para- Property Helium _Hydrogen Neon Nitrogen Air Fluorine Argon Oxygen Triple Point Temperature - 2.17 13.8 24.56 63.2 60.6 53.5 83.8 54.4 [K] 7 (λ) Triple Point Pressure - [kPa] 4.73 (λ) 7.00 43.00 12.80 7.04 0.25 68.60 0.15 Critical temperature [K] 5.2 32.9 44.4 126.1 133.3 144.3 150.9 154.6 Critical pressure [_MPa] 0.23 1.28 2.71 3.38 3.9 5.22 4.87 5.06 Normal (1 atm) Boiling Point 4.2 20.3 27.1 77.3 78.9 84.95 87.3 90.2 [K] NBP Heat of vaporization 20.7 445.4 85.7 198.8 205.1 157.4 161.6 213.1 [kJ/kg] Table 1 [0049] By way of a non-limiting example, in a 1.2 m to 2.4 m wide and 40 mm thick thermal conditioning plate 2 constructed out of aluminium, with an internal passage network 22 having passages of 20 mm diameter, assuming a modest heat transfer coefficient (e.g., 20 Wm^-2K^-1 at 20K), the temperature variance across the external surface 21 may be less than 0.02 K with the present invention. At 20 K, liquid hydrogen is at its boiling point at 0.934 bar. At this pressure and temperature, liquid hydrogen has a latent heat of 445 kJ/kg and a density of around 71.1 kg/m³. Assuming a flow velocity of 0.1 m/s and a pipe diameter of 20 mm, the mass flowrate can be calculated as 2.2 g/s. Assuming complete evaporation of the liquid cryogen, the cooling capacity due to phase change is estimated to be 1 kW at 20 K, which is much larger than achievable by known arrangements in which a cryocooler is in thermal contact with the thermal conditioning plate at a spot connection. Furthermore, in this example, the cryogen may provide a surplus cooling capacity that can be used as a margin against quench events or power disruptions. For example, assuming a normal heat load of 100 W at 20K, only about 10% of the cryogenic fluid would evaporate during normal Company Secret operation and remaining 900 W may serve as a safety margin for a potential quench event. It should be understood that different dimensions, choice of cryogen, operating temperature, flow rates and flow velocities may be used depending on the specific implementation of the present invention. [0050] As noted above, the internal passage network 22 may comprise multiple sections. For example, as shown in Fig.3, the internal passage network 22 may comprise a first passage section 221 and a second passage section 222. As shown, the first passage section 221 may be located relatively close to the plurality of superconducting coils 5, and the second passage 222 may be located relatively further away from the superconducting coils 5. Furthermore, the second passage section 222 may be downstream of the first passage section 221. That is, the cryogen may first flow through the first passage section 221 before flowing through the second passage section 222. The temperature of the cryogen may absorb heat from the superconducting coils 5 as it flows through the first passage section 221, and then through the second passage section 222. As the first passage section 221 may be located closer to the plurality of superconducting coils 5, heat from the superconducting coils 5 may be effectively transferred into the cryogen in the first passage section 221, which may be at a lower temperature than the cryogen in the second passage 222. [0051] In order for the thermal conditioning plate 2 to reach and remain at cryogenic temperatures without requiring excessive cooling power, it may be beneficial to thermally insulate the thermal conditioning plate 2 and the plurality of superconducting coils 5 from the outside environment. For example, as shown in Figure 2, the positioner assembly 1 may further comprise a vacuum vessel 4. The vacuum vessel 4 may enclose the thermal conditioning plate 2 and the plurality of superconducting coils 5. [0052] Vacuum may be applied to the space within the vacuum vessel 4. A vacuum pump (not shown) may be connected to the volume of space within the vacuum vessel 4 for this purpose. A vacuum pump may also help prevent a build-up of solids on cold surfaces. Specifically, when a surface reaches cryogenic temperatures (e.g. less than 44K), any molecule (except helium, neon or hydrogen, for example) that comes into contact with the cold surface will solidify and will no longer be present as a free gas. This may in turn reduce the pressure within the vacuum vessel 4 even without the help of a vacuum pump. However, it may be undesirable to allow solidified condensates to build up because this may degrade thermal performance and/or create electrical short circuits (e.g. between the thermal conditioning plate 2 and the vacuum vessel 4). To avoid a build-up of solids, the vacuum pump may be used to evacuate molecules from within the vacuum vessel 4 before cooldown. [0053] Furthermore, as molecules that are present in the vacuum vessel 4 would promote convective heat transfer, the presence of molecules may lengthen the time it takes to cool the thermal conditioning plate 2 down to target temperature. Therefore, by evacuating the vacuum vessel 4 before cooldown, the time and energy it takes to cool the thermal conditioning plate 2 may be reduced. [0054] Although vacuum is an effective means of heat insulation, other means of heat ventilation may be used instead. Company Secret [0055] The positioner assembly 1 may alternatively or additionally comprise a thermal shield 3 to improve thermal insulation. As shown in Figure 2, the thermal shield 3 may enclose the thermal conditioning plate 2 and the plurality of superconducting coils 5. Where a vacuum vessel 4 is present, the thermal shield 3 may also be enclosed within the vacuum vessel 4. [0056] With this arrangement, by way of a non-limiting example, it may be possible to maintain the thermal conditioning plate 2 and the plurality of superconducting coils 5 at around 44 K or less, preferably 10 to 35 K, preferably 20 K or less. The thermal shield 3 may be maintained at around 50 to 90 K. The vacuum vessel 4 may be maintained at room temperature, i.e. around 295 K. It should be understood that the vacuum applied within the vacuum vessel 4 need not be a perfect vacuum. For example, the pressure within the vacuum vessel 4 may be maintained at around 10 Pa or less. [0057] To further improve thermal insulation for the thermal conditioning plate 2, the thermal shield 3 may also comprise an internal passage network 32 for flowing a cryogen therein. This may further reduce the temperature within the thermal shield 3, which may help ensure that the thermal conditioning plate 2 and the superconducting coils 5 can be maintained at the requisite cryogenic temperature. To further improve thermal insulation, additional thermal shields similar to thermal shield 3 may be provided, subject to the availability of space. [0058] As shown in Figure 2, the vacuum vessel 4 may comprise a penetration 43 through which the cryogen is supplied to the internal passage network 22 of the thermal conditioning plate 2. For example, as shown in Figure 2, a cryogen supply line 61 may be provided through the penetration 43, and may be connected to the internal passage network 22 of the thermal conditioning plate 2. Cryogen for the internal passage network 32 of the thermal shield 3 may similarly be supplied through the penetration 43 of the vacuum vessel 4. More generally, any required number of cryogen supplies may be provided through the penetration 43 of the vacuum vessel 4. Compared with contacting multiple cryocoolers with the thermal conditioning plate at multiple spot connections, which requires a corresponding number of penetrations, the present arrangement may reduce the number of penetrations required. [0059] Furthermore, as shown in Figure 2, the positioner assembly 1 may comprise a flexible conduit 6 provided through the penetration 43 of the vacuum vessel 4. Cryogen may be supplied to internal passage network 22 of the thermal conditioning plate 2 and/or to the internal passage network 32 of the thermal shield 3 through this flexible conduit 6. The flexibility of flexible conduit 6 may be beneficial for accommodating movements of the thermal conditioning plate 2, the thermal shield 3, and/or the vacuum vessel 4 during operation of the lithographic apparatus. The thermal conditioning plate 2 may move as much as 10 to 100 mm, e.g.50 mm. This may be caused by movements of the vacuum vessel 4 during scanning operation of the lithographic apparatus. Alternatively or additionally, the thermal conditioning plate 2 may move due to thermal shrinkage. Furthermore, in order to accommodate such movements, one or more of the thermal conditioning plates 2, the thermal shield 3, and the vacuum vessel 4 may be mechanically suspended rather than rigidly fixed. For Company Secret example, flexible mechanical supports (not shown) may mechanically connect the thermal conditioning plate 2 and the vacuum vessel 4. These mechanical supports may be provided through clearance holes in the thermal shield 3. The vacuum vessel 4, in turn, may also be similarly attached to the lithographic apparatus using flexible mechanical supports. Generally, it may be desirable for these mechanical supports to have low thermal conduction so as to reduce the amount of heat from the external environment reaching into the thermal conditioning plate 2. For example, these mechanical supports may comprise materials having low thermal conductivity. The mechanical supports may also have a small cross-sectional area. [0060] In the simplest case, the cryogen supplied to the internal passage network 22 may be vented to the environment after it has passed through the internal passage network 22. However, as this may be wasteful, the cryogen supplied to the internal passage network 22 may be recycled. For example, as shown in Figure 2, in addition to a cryogen supply line 61 through which the cryogen flows towards the internal passage network 22, the positioner assembly 1 may also comprise a cryogen return line 62 through which the cryogen flows away from the internal passage network 22. As shown in Figure 2, both the cryogen supply line 61 and the cryogen return line 62 may be provided within the flexible conduit 6. That is, both the cryogen supply and return lines 61, 62 may be provided through the same penetration 43 of the vacuum vessel 4. Compared with providing the cryogen supply and return lines 61, 62 through separate flexible conduits and separate penetrations, providing the cryogen supply and return lines 61, 62 through the same flexible conduit 6 or the same penetration 43 may reduce the number of flexible conduits and penetrations required. This, in turn, may reduce the amount of heat from the external environment reaching into the thermal conditioning plate 2. [0061] In order to provide electrical power to the plurality of superconducting coils 5, power supply leads 63 may be used. As shown in Figure 2, the power supply leads 63 may also be provided within the flexible conduit 6. Specifically, the power supply leads 63 may be provided in the same flexible conduit 6 as the cryogen supply line 61. The power supply leads 63 may generally be required to carry a large amount of electric current, e.g., 1 kA to 5 kA, or at least 2 kA. In view of the large electric current carried by the power supply leads 63, it may be beneficial to cool the power supply leads, so as to reduce resistive losses. For this purpose, the power supply leads 63 may be surrounded by cryogen. In particular, the power supply leads 63 may be surrounded by the same cryogen as that supplied to the internal passage network 22 of the thermal conditioning plate 2. The power supply leads 63 may be superconducting. A further benefit of using superconducting power supply leads 63 is that the diameter of the power supply leads 63 may be reduced without increasing resistive losses. [0062] More specifically, as shown in Figure 2, the power supply leads 63 may be provided within the cryogen supply line 61 and/or the cryogen return line 62. Advantageously, as shown in Figure 4, each of the cryogen supply lines 61 and the cryogen return line 62 may have one of the power supply leads 63 provided within. More particularly, as shown in Figure 4, one power lead 63 may be Company Secret surrounded by cryogen 610 in the cryogen supply line 61, and another power lead 63 may be surrounded by cryogen 620 in cryogen return line 62. By providing one power supply lead 63 in each of the cryogen supply line 61 and the cryogen return line 62, instead of providing two (or more) power supply leads 63 in a single cryogen supply or return lines 61, 62, the cryogen supply line 61 and/or the cryogen return line 62 may have a relatively smaller diameter, which may render the cryogen supply line 61 and/or the cryogen return line 62 more flexible. A greater flexibility may enable the cryogen supply line 61 and the cryogen return line 62 to better accommodate movements of the thermal conditioning plate 2, and/or the thermal shield 3, and/or the vacuum vessel 4. Furthermore, by combining power supply, cryogen supply and vacuum supply in the same flexible conduit 6, compactness may be improved. [0063] As the flexible conduit 6 may be of a certain length necessary for accommodating the movements of the thermal conditioning plate 2, the thermal shield 3, and/or the vacuum vessel 4, the cryogen may absorb heat from the external environment whilst flowing in the flexible conduit 6. In order to reduce the amount of heat form the external environment reaching into the cryogen within the flexible conduit 6, vacuum may be applied to the space 60 between the cryogen supply and return lines 61, 62 and the flexible conduit 6. In one arrangement, as shown in Figure 2, this interstitial space 60 (i.e. the space between the cryogen supply and return lines 61, 62 and the flexible conduit 6) may be fluidly connected to the internal volume of the vacuum vessel 4. With this arrangement, vacuum may be applied to the internal volume of the vacuum vessel 4 and the interstitial space 60 of the flexible conduit 6 using a common vacuum source, such as a vacuum pump. Alternatively, the interstitial space 60 of the flexible conduit 6 and the internal volume of the vacuum vessel 4 may be fluidly non-connected, and vacuum may be applied separately. [0064] In order to reuse the cryogen, the positioner assembly 1 may comprise a cryogen recycling unit 7, as shown in Figure 2. The cryogen recycling unit 7 may be configured to receive the cryogen from the internal passage network 22, cool the received cryogen, and supply the cooled cryogen to the internal passage network 22. For example, the cryogen received from the internal passage network 22 may be cooled by a cryocooler 73 before being returned to the internal passage network 22. [0065] In addition, the cryogen recycling unit 7 may comprise a control system (not shown) to control the flow rate, pressure, and/or temperature of the cryogen supplied to the internal passage network 22. [0066] For example, it may be desirable to keep the cryogen in the liquid state throughout the internal passage 22 of the thermal conditioning plate 2. Therefore, the control system may be configured to control one or more of flow rate, pressure and temperature of the cryogen supplied to the internal passage network 22 so that the temperature of the cryogen received from the internal passage network 22 is at or below the boiling point of the cryogen. This may be desirable because the cryogen in its liquid state may have a greater volume metric heat capacity than in the gaseous state. Furthermore, by ensuring that the entire internal passage network 22 is filled with the cryogen in its Company Secret liquid state, the cryogen within the internal passage network 22 may have a greater capacity of heat absorption, so that the thermal conditioning plate 2 may be more able to cope with unplanned quench events. [0067] More generally, however, the control system may be configured to control one or more of flow rate, pressure and temperature of the cryogen supplied to the internal passage network 22 such that the cryogen remains in a single phase throughout the internal passage network 22. The single phase may be one of gaseous, liquid and supercritical phase. [0068] Alternatively, the control system may be configured to control one or more of flow rate, pressure and temperature of the cryogen supplied to the internal passage network 22 such that a portion of the cryogen changes from the liquid state to the gaseous state as it flows through the internal passage network 22. In other words, the cryogen may gradually boil as it absorbs latent heat of vaporisation as it flows through the internal passage network 22 of the thermal conditioning plate 2. This arrangement may be beneficial because boiling occurs at a substantially constant temperature, which may in turn help reduce the temperature variations across the thermal conditioning plate 2. Furthermore, the control system may be configured to control the flow rate, pressure and/or the temperature of the cryogen supplied to the internal passage network 22 such that an adequate reserve of liquid cryogen remains at the exit of the internal passage network 22. This reserve of liquid cryogen may provide additional heat absorption capacity in case of unplanned quench events. [0069] A further advantage of allowing a portion of the cryogen to boil as it flows through the internal passage network 22 is that a two-phase flow may have superior vibration damping properties. For example, two-phase flow may help reduce sloshing. In short, interfacial waves between the two phases may act as an energy dissipating mechanism and hence provide additional acoustic and mechanical damping. This phenomenon is further explained in C. Charreton et al., “Two-phase damping for internal flow: Physical mechanism and effect of excitation parameters”, Journal of Fluids and Structures, Volume 56, 2015, Pages 56-74, ISSN 0889-9746, https://doi.org/10.1016/j.jfluidstructs.2015.03.022, which is herein incorporated by reference in its entirety. A damping ratio of about 2 to 3% is achievable using two-phase flow, whereas the actual structural damping in most single-phase systems is much less than 1%. Compared with single-phase flow, a smaller amount of two-phase fluid may be used while maintaining damping performance. [0070] As yet another alternative, the control system may be configured to control one or more of flow rate, temperature and pressure of the cryogen supplied to the internal passage network 22 such that the cryogen is in slush form. In other words, the cryogen supplied to the internal passage network 22 may comprise solid particles of the cryogen suspended in the cryogen in liquid form. The presence of the solid cryogen may cool the superconducting coils 5 to a temperature lower than achievable with the same cryogen in liquid or gaseous form, and may provide a greater reserve of heat absorption capacity for unplanned quench events. At the same time, the use of the cryogen in slush form, as opposed to purely solid, may enable the cryogen to be transported like a liquid. Company Secret [0071] The temperature of the cryogen supplied to the internal passage network 22 may be variously controlled. For example, the temperature of the cryogen may be controlled by varying its pressure. More specifically, a pressure reduction may induce a drop of the saturation temperature of the cryogen. As shown in Figure 2, cryogen may be supplied from the cryogen recycling units 7 to the internal passage network 22 of the thermal conditioning plate 2 through a cryogen supply line 61. In order to control the temperature of the cryogen supplied to the internal passage network 22, the control system may comprise a cryogen pressure controller 71 installed on the supply line 61. The cryogen pressure controller 71 may cause a pressure drop of the cryogen as it flows from an upstream section 611 of the cryogen supply line 61 to a downstream section 612 of the cryogen supply line 61. The cryogen pressure controller 71 may control the amount of pressure drop of the cryogen. By controlling the amount of pressure drop, the temperature of the cryogen in the downstream section 612 of the supply line 61 may be controlled. [0072] Furthermore, the cryogen pressure controller 71 may be located close to the thermal conditioning plate 2. That is, the downstream section 612 of the cryogen supply line 61 may be shorter than the upstream section 611 of the supply line 61. This may be beneficial because the amount of heat absorbed by the cryogen whilst in the cryogen supply line 61 may be difficult to predict. By placing the cryogen pressure controller 71 relatively close to the point of use (i.e. close to the thermal conditioning plate 2), the conditions of the cryogen actually supplied to the internal passage network 22 may be controlled with greater accuracy. [0073] The positioner assembly 1 may further comprise a buffer tank 72 configured to hold a volume of the cryogen. The cryocooler 73 may cool the cryogen using a cooling circuit 731. As shown in Figure 2, the cooling circuit 731 may comprise a heat exchanger 721 for cooling the cryogen in the buffer tank 72. The working fluid within the cooling circuit 731 may be helium in an embodiment. The buffer tank 72 may provide a thermal buffer as a relatively large amount of cryogen can be maintained at the desired temperature during planned or unplanned shutdowns (e.g., during maintenance or short power disruptions). When the shutdown is over, the cool-down process of the system can be shortened compared to an arrangement without a buffer tank 72. [0074] Although the buffer tank 72 and the cryocooler 73 are shown as separate components in Figure 2, it should be understood that these components may be provided within a single unit. It should further be understood that any cryogenic cooler, including off-the-shelf equipment, that is suitable for supplying cryogen to the thermal conditioning plate 2 may be used. [0075] Although the present invention may be implemented using a single cryogen, several cryogens may be used. For example, the positioner assembly 1 may comprise a supply of a first cryogen and a supply of a second cryogen (not shown). The first and second cryogens may be different. In particular, the second cryogen may have a lower boiling point than the first cryogen. The boiling points of the first and second cryogens may be compared at 1 atm. The positioner Company Secret assembly 1 may switch between the first and second cryogens depending on the temperature of the thermal conditioning plate 2 at a given moment. [0076] In particular, when reducing the temperature of the thermal conditioning plate 2 from a higher temperature towards a target temperature, the positioner assembly 1 may initially supply the first cryogen (higher boiling point) in the internal passage network 22, then subsequently switch to the second cryogen (lower boiling point). In particular, the positioner assembly 1 may be configured to supply the first cryogen to the internal passage network 22 as the thermal conditioning plate 2 cools, and may switch to the second cryogen when the temperature of the thermal conditioning plate 2 has reduced to or below the boiling point of the second cryogen. This may ensure that the cryogen supplied to the internal passage network 22 remains in liquid form for a greater portion of the cooling process. This may be beneficial because cryogens in liquid form may have a greater heat capacity than the same cryogens in gaseous form. More particularly, the second cryogen may be chosen to have a greater specific heat capacity than the first cryogen, so that the flow rate of cryogen may be reduced after switching to the second cryogen. [0077] The first and second cryogens may be any two cryogens, e.g., selected from Table 1 above (Table 1 lists the cryogens in ascending order in terms of boiling points from left to right). For example, the first cryogen may be liquid nitrogen, and the second cryogen may be para-hydrogen. As can be seen in Table 1, it may be beneficial to switch to para-hydrogen as the second cryogen because para-hydrogen has a relatively large heat of vaporisation. [0078] For another example, the first cryogen may be liquid nitrogen, and the second cryogen may be one of liquid neon and liquid helium. In one example, the first cryogen may be used to cool the thermal conditioning plate 2 down to about 77 K (e.g. starting from room temperature) before switching to the second cryogen. The second cryogen may be used to further cool the thermal conditioning plate 2 to an operating temperature, such as down to about 20 K. The second cryogen may thereafter maintain the temperature of the thermal conditioning plate 2. [0079] Where two cryogens are used, the internal passage network 22 of the thermal conditioning plate 2 may comprise first and second sub-networks (not shown) which are fluidly isolated from each other. The positioner assembly 1 may be configured to supply the first cryogen to the first subnetwork, and to supply the second cryogen to the second subnetwork. The positioner assembly 1 may be configured to supply one, but not both, of the first and second cryogens to their respective sub-networks at any given time. As above, the switch between the first and second cryogens may be dependent on the temperature of the thermal conditioning plate 2. By fluidly isolating the first and second sub-networks, the first and second cryogens may be prevented from mixing. Furthermore, the positioner assembly 1 may switch between the first and second cryogens without first having to flush the cryogen supply line 61 and/or the cryogen return line 62. It should be noted that, where more than one cryogen is used, a separate set of cryogen supply and return lines 61, 62 and/or buffer tank 72 and/or cryocooler 73 may be provided for each cryogen. Company Secret [0080] The positioner assembly 1 as variously disclosed above may be part of a lithographic apparatus. The lithographic apparatus may be used in a method of manufacturing devices. [0081] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion", respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains one or multiple processed layers. [0082] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention may be used in other applications. [0083] Aspects of the invention are described in the following numbered clauses. 1. An assembly (1) for use in a lithographic apparatus, the assembly comprising: a thermal conditioning plate (2) comprising an internal passage network (22) for flowing a cryogen therein; and a plurality of superconducting coils (5) for magnetically levitating and linearly displacing a stage (9) thereon, wherein the plurality of superconducting coils is in thermal contact with an external surface (21) of the thermal conditioning plate. 2. The assembly of clause 1, wherein the plurality of superconducting coils is disposed on the external surface of the thermal conditioning plate. 3. The assembly of clause 1, wherein the internal passage network and the plurality of superconducting coils are substantially co-extensive. 4. The assembly of any one of clauses 1 to 3, wherein the thermal conditioning plate comprises a thermally conductive material, optionally aluminium, copper, an aluminium alloy, or a copper alloy. 5. The assembly of any one of the preceding clauses, wherein the internal passage network comprises first and second passage sections (221, 222), and the first passage section (221) is located closer to the plurality of superconducting coils than the second passage section (222) is. 6. The assembly of clause 5, wherein the second passage section is downstream of the first passage section. Company Secret 7. The assembly of any one of the preceding clauses, further comprising a vacuum vessel (4) enclosing the thermal conditioning plate and the plurality of superconducting coils. 8. The assembly of clause 7, further comprising a thermal shield (3) enclosing the thermal conditioning plate and the plurality of superconducting coils, wherein the thermal shield is enclosed within the vacuum vessel. 9. The assembly of clause 8, wherein the thermal shield comprises a second internal passage network (32) for flowing a cryogen therein. 10. The assembly of any one of clauses 7 to 9, wherein the vacuum vessel comprises a penetration (43) through which the cryogen is supplied to the internal passage network. 11. The assembly of any one of clauses 7 to 10, further comprising a flexible conduit (6) provided through the penetration, and the cryogen is supplied to the internal passage network through the flexible conduit. 12. The assembly of clause 11, further comprising a cryogen supply line (61) through which the cryogen is configured to flow to the internal passage network, and a cryogen return line (62) through which the cryogen is configured to flow away from the internal passage network; wherein both the cryogen supply line and the cryogen return line are provided within the flexible conduit. 13. The assembly of clause 11 or 12, further comprising power supply leads (63) configured to supply electrical power to the superconducting coils, wherein the power supply leads are provided within the flexible conduit. 14. The assembly of clause 13, wherein the power supply leads are configured to be surrounded by the cryogen. 15. The assembly of clause 14, wherein the power supply leads are superconducting. 16. The assembly of clause 12, further configured to apply vacuum to the space (60) between the cryogen supply and return lines and the flexible conduit. 17. The assembly of any one of the preceding clauses, further comprising a cryogen recycling unit (7) configured to receive the cryogen from the internal passage network, cool the received cryogen, and supply the cooled cryogen to the internal passage network. 18. The assembly of clause 17, wherein the cryogen recycling unit comprises a control system configured to control one or more of flow rate, pressure and temperature of the cryogen supplied to the internal passage network. 19. The assembly of clause 18, wherein the control system is configured to control one or more of flow rate, pressure and temperature of the cryogen supplied to the internal passage network such that the temperature of the cryogen received from the internal passage network is at or below the boiling point of the cryogen. 20. The assembly of clause 19, wherein the control system is configured to control one or more of flow rate, pressure and temperature of the cryogen supplied to the internal passage network such that Company Secret the cryogen remains in a single phase throughout the internal passage network, the single phase being one of gaseous, liquid and supercritical phase. 21. The assembly of clause 19, wherein the control system is configured to control one or more of flow rate, pressure and temperature of the cryogen supplied to the internal passage network such that a portion of the cryogen changes from the liquid state to the gaseous state as it flows through the internal passage network. 22. The assembly of clause 19, wherein the control system is configured to control one or more of flow rate, temperature and pressure of the cryogen supplied to the internal passage network such that the cryogen supplied to the internal passage network is in slush form. 23. The assembly of any one of clauses 18 to 22, wherein: the assembly further comprises a supply line (61) through which the cryogen is supplied to the internal passage network; and the control system comprises a cryogen pressure controller (71) on the supply line, wherein the cryogen pressure controller is configured to control a pressure drop of the cryogen as the cryogen flows from an upstream section (611) to a downstream section (612) of the supply line. 24. The assembly of any one of clauses 17 to 23, wherein the cryogen recycling unit further comprises a buffer tank (72) configured to hold a volume of the cryogen. 25. The assembly of any one of the preceding clauses, further comprising a supply of a first cryogen and a supply of a second cryogen, wherein the second cryogen has a boiling point, as measured at 1 atm, which is lower than that of the first cryogen; and wherein, when reducing the temperature of the thermal conditioning plate towards a target temperature, the assembly is configured to initially supply the first cryogen in the internal passage network, then subsequently switch to supply the second cryogen in the internal passage network. 26. The assembly of clause 25, wherein the internal passage network comprises a first subnetwork, and a second subnetwork fluidly isolated from the first subnetwork, wherein the assembly is configured to supply the first cryogen to the first subnetwork and supply the second cryogen to the second subnetwork. 27. The assembly of clause 25 or 26, configured to switch from supplying the first cryogen to supplying the second cryogen in the internal passage network when the temperature of the thermal conditioning plate has reduced to or below the boiling point of the second cryogen. 28. A lithographic apparatus comprising the assembly of any one of the preceding clauses. 29. A method of manufacture devices using the lithographic apparatus of clause 28. [0084] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. [0085] 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. Company Secret

Claims

CLAIMS 1. An assembly (1) for use in a lithographic apparatus, the assembly comprising: a thermal conditioning plate (2) comprising an internal passage network (22) for flowing a cryogen therein; and a plurality of superconducting coils (5) for magnetically levitating and linearly displacing a stage (9) thereon, wherein the plurality of superconducting coils is in thermal contact with an external surface (21) of the thermal conditioning plate.

2. The assembly of claim 1, wherein the plurality of superconducting coils is disposed on the external surface of the thermal conditioning plate, or wherein the internal passage network and the plurality of superconducting coils are substantially co-extensive.

3. The assembly of claim 1 or 2, wherein the thermal conditioning plate comprises a thermally conductive material, optionally aluminium, copper, an aluminium alloy, or a copper alloy, and/or wherein the internal passage network comprises first and second passage sections (221, 222), and the first passage section (221) is located closer to the plurality of superconducting coils than the second passage section (222) is, and/or further comprising a vacuum vessel (4) enclosing the thermal conditioning plate and the plurality of superconducting coils.

4. The assembly of claim 3, wherein the second passage section is downstream of the first passage section, and/or further comprising a thermal shield (3) enclosing the thermal conditioning plate and the plurality of superconducting coils, wherein the thermal shield is enclosed within the vacuum vessel, and/or wherein the vacuum vessel comprises a penetration (43) through which the cryogen is supplied to the internal passage network, and/or further comprising a flexible conduit (6) provided through the penetration, and the cryogen is supplied to the internal passage network through the flexible conduit.

5. The assembly of claim 4, wherein the thermal shield comprises a second internal passage network (32) for flowing a cryogen therein, and/or further comprising a cryogen supply line (61) through which the cryogen is configured to flow to the internal passage network, and a cryogen return line (62) through which the cryogen is configured to flow away from the internal passage network; wherein both the cryogen supply line and the cryogen return line are provided within the flexible conduit, desirably further configured to apply vacuum to the space (60) between the cryogen supply and return lines and the flexible conduit. Company Secret

6. The assembly of claim 4 or 5, further comprising power supply leads (63) configured to supply electrical power to the superconducting coils, wherein the power supply leads are provided within the flexible conduit, desirably wherein the power supply leads are configured to be surrounded by the cryogen, desirably wherein the power supply leads are superconducting.

7. The assembly of any of the preceding claims, further comprising a cryogen recycling unit (7) configured to receive the cryogen from the internal passage network, cool the received cryogen, and supply the cooled cryogen to the internal passage network.

8. The assembly of claim 7, wherein the cryogen recycling unit comprises a control system configured to control one or more of flow rate, pressure and temperature of the cryogen supplied to the internal passage network, desirably wherein the control system is configured to control one or more of flow rate, pressure and temperature of the cryogen supplied to the internal passage network such that the temperature of the cryogen received from the internal passage network is at or below the boiling point of the cryogen.

9. The assembly of claim 8, wherein the control system is configured to control one or more of flow rate, pressure and temperature of the cryogen supplied to the internal passage network such that the cryogen remains in a single phase throughout the internal passage network, the single phase being one of gaseous, liquid and supercritical phase, or wherein the control system is configured to control one or more of flow rate, pressure and temperature of the cryogen supplied to the internal passage network such that a portion of the cryogen changes from the liquid state to the gaseous state as it flows through the internal passage network, or wherein the control system is configured to control one or more of flow rate, temperature and pressure of the cryogen supplied to the internal passage network such that the cryogen supplied to the internal passage network is in slush form.

10. The assembly of claim 7 or 8, wherein: the assembly further comprises a supply line (61) through which the cryogen is supplied to the internal passage network; and the control system comprises a cryogen pressure controller (71) on the supply line, wherein the cryogen pressure controller is configured to control a pressure drop of the cryogen as the cryogen flows from an upstream section (611) to a downstream section (612) of the supply line.

11. The assembly of any of claims 7 to 10, wherein the cryogen recycling unit further comprises a buffer tank (72) configured to hold a volume of the cryogen. Company Secret

12. The assembly of any of the preceding claims, further comprising a supply of a first cryogen and a supply of a second cryogen, wherein the second cryogen has a boiling point, as measured at 1 atm, which is lower than that of the first cryogen; and wherein, when reducing the temperature of the thermal conditioning plate towards a target temperature, the assembly is configured to initially supply the first cryogen in the internal passage network, then subsequently switch to supply the second cryogen in the internal passage network.

13. The assembly of claim 12, wherein the internal passage network comprises a first subnetwork, and a second subnetwork fluidly isolated from the first subnetwork, wherein the assembly is configured to supply the first cryogen to the first subnetwork and supply the second cryogen to the second subnetwork, and/or configured to switch from supplying the first cryogen to supplying the second cryogen in the internal passage network when the temperature of the thermal conditioning plate has reduced to or below the boiling point of the second cryogen.

14. A lithographic apparatus comprising the assembly of any of the preceding claims.

15. A method of manufacture devices using the lithographic apparatus of claim 14. Company Secret