WO2013160021A1

WO2013160021A1 - Flexible conduit for fluid, lithographic apparatus, and method for manufacturing a device

Info

Publication number: WO2013160021A1
Application number: PCT/EP2013/055646
Authority: WO
Inventors: Gerard Frans Jozef Schasfoort
Original assignee: Asml Netherlands B.V.
Priority date: 2012-04-23
Filing date: 2013-03-19
Publication date: 2013-10-31
Also published as: TW201348891A

Abstract

A lithographic apparatus operating at EUV wavelengths comprises various components and subsystems for transferring a pattern from a patterning device (MA) to a substrate (W) by EUV lithography. Such components are housed within vacuum vessels (220, 222, 224 etc) so that EUV radiation is not impeded by the atmosphere. Conduits (264, 266, 268 etc) within the vacuum vessels circulate water or other fluid for cooling purposes. Some of said conduits are flexible conduits. A novel form of flexible conduit (328) comprises an inner layer (330) of polymer, a barrier layer (332) and an outer layer (334) with optional adhesive layers (338, 340) between. The inner and outer layers comprise polymer with low outgassing performance (e.g., PTFE), while the barrier layer prevents permeation of water into the vacuum environment. The barrier layer may be metal, for example Aluminum, and may be less than 100 µm or less than 10µm in thickness.

Description

FLEXIBLE CONDUIT FOR FLUID, LITHOGRAPHIC APPARATUS, AND METHOD

FOR MANUFACTURING A DEVICE

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of US provisional application 61/636,995 which was filed on April 23, 2012, and which is incorporated herein in its entirety by reference.

FIELD

[0002] The present invention relates to a radiation source apparatus, a method of controlling a radiation source, and to lithographic apparatus and a method for manufacturing a device. The invention is particularly applicable to the control of radiation source apparatus for extreme ultraviolet (EUV) radiation.

BACKGROUND

[0003] A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation- sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.

[0004] Lithography is widely recognized as one of the key steps in the manufacture of

ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.

[0005] In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Possible sources include laser-produced plasma (LPP) sources, although other types of source are possible.

[0006] The components and subsystems of a lithographic apparatus may require water or other fluids to be conveyed to and from them, in particular for cooling purposes. When selecting conduits, their length and configuration must be suitable for routing from place to place without obstructing other components. They may need to be flexible for routing, and/or to absorb vibrations or other motions of their endpoints. In a conventional lithographic apparatus, refractive optical components operate within a very clean environment, but essentially at normal air temperatures and pressures. However, within a lithographic apparatus operating at EUV wavelengths, reflective optical elements operating in a vacuum or near-vacuum environment are generally required, because most materials, even a gaseous atmosphere, are highly absorptive of the wanted radiation. Water must be contained against the vacuum pressure. Even a small amount of water vapor escaping leaking from the cooling system within a vacuum vessel will contaminate the atmosphere unacceptably. Components that are suitable for use at normal temperatures and pressures are frequently unsuitable for use in vacuum, because their materials are prone to outgassing. For example, commonly used polymer materials may release organic molecules into the atmosphere, which are highly absorptive of EUV radiation. In other types of vacuum system, contamination by water and/or organic molecules can be undesirable for different reasons.

[0007] In known vacuum apparatuses such as EUV lithographic apparatuses, rigid metal piping is used to transport cooling water. Flexible conduits made of corrugated metal ('flexhoses') are used at places where rigid ducting cannot be used due to flexibility requirements. Flexibility may be required for routing, or because two parts must move for operation or servicing, or to accommodate vibrations in the apparatus or in the conveyed fluid. Metal is used due to strict outgassing requirements. Wall thickness is typically less than 200 μιη to provide the desired flexibility.

[0008] In a lithographic production environment where high throughput and availability of the apparatus are critical to economic operation, leakage that requires shutdown and repair of the apparatus is particularly undesirable. Unfortunately, corrugated metal conduits have proven to be vulnerable to perforation, with water leakage inside vacuum as a result. More rugged, polymer-based hoses known for use in other environments cannot be applied because of their permeability and/or outgassing problems.

SUMMARY

[0009] The invention aims to provide novel flexible conduits for use conveying water or other fluids in specialized vacuum or near- vacuum environments.

[0010] The invention in a first aspect provides a flexible conduit for conveying fluids in a vacuum environment, the conduit including:

an inner polymer layer;

an outer polymer layer; and

- a barrier layer enclosed between the inner and outer polymer layers, wherein the barrier layer is resistant to permeation by water from within the conduit to said vacuum environment, and wherein at least the outer polymer layer is made of vacuum-compatible material resistant to outgassing within a vacuum environment.

[0011] The barrier layer may for example comprise one or more metal layers. A thickness of the or each metal layer may be less than 200 μιη, optionally less than 100 μιη, less than 50 μιη or less than 10 μιη.

[0012] The inner and/or outer polymer layer may comprise PTFE. Outgassing performance requirements can be specified using known test methods. The novel conduit can be used in a wide range of applications, but is particularly suited for use at various places within an EUV optical apparatus such as a lithographic apparatus, where high standards of performance and reliability are especially important.

[0013] The invention accordingly provides a lithographic apparatus for use in applying a pattern to a substrate, the lithographic apparatus comprising in one or more vacuum vessels:

a radiation source apparatus for generating a beam of EUV radiation an illumination system for receiving a beam of EUV radiation from the radiation source apparatus and for conditioning the beam to illuminate a target area of a patterning device;

- a support constructed to support a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross- section to form a patterned radiation beam;

a substrate table constructed to hold a substrate; and a projection system for producing an image of the illuminated patterning device on a substrate, in order to transfer a pattern from said patterning device to said substrate by EUV lithography,

wherein the apparatus includes a fluid supply system for providing fluid to at least one component operating in one or more of said vacuum vessels, said fluid supply system comprising a conduit according to the invention, as set forth above.

[0014] The fluid may be cooling fluid, for example water.

[0015] The invention further provides a method of manufacturing a device using such a lithographic apparatus.

[0016] These aspects of the invention and various optional features and implementations thereof will be understood by the skilled reader from the description of examples that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] 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:

[0018] Figure 1 depicts schematically a lithographic apparatus including an illumination system according to an embodiment of the invention;

[0019] Figure 2 is a more detailed view of the apparatus 100, showing schematically components of water distribution systems within different vacuum enclosures;

[0020] Figure 3 shows at (a) and (b) two known types of conduit for water in vacuum enclosures; and

[0021] Figure 4 shows a novel flexible conduit for use in vacuum enclosures, including a cut away detail.

[0022] 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 OF EXEMPLARY EMBODIMENTS

[0023] This specification discloses one or more embodiments that incorporate the features of this invention. The disclosed embodiment(s) merely exemplify the invention. The scope of the invention is not limited to the disclosed embodiment(s). The invention is defined by the claims appended hereto. [0024] The embodiment(s) described, and references in the specification to "one embodiment," "an embodiment," "an example embodiment," etc., indicate that the embodiment(s) described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is understood that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

[0025] Figure 1 schematically depicts a lithographic apparatus 100 including a source collector module SO, which forms a radiation source apparatus according to one embodiment of the invention. The apparatus comprises:

an illumination system (illuminator) IL configured to condition a radiation beam B (e.g., EUV radiation).

- a support structure (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask or a reticle) MA and connected to a first positioner PM configured to accurately position the patterning device;

a substrate table (e.g., a wafer table) WT constructed to hold a substrate (e.g., a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate; and

a projection system (e.g., a reflective projection 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.

[0026] The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.

[0027] The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure may be a frame or a table, for example, which may be fixed or movable as required. The support structure may ensure that the patterning device is at a desired position, for example with respect to the projection system.

[0028] The term "patterning device" should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. The pattern imparted to the radiation beam may correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

[0029] The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam that is reflected by the mirror matrix.

[0030] The projection system, like the illumination system, may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of a vacuum. It may be desired to use a vacuum for EUV radiation since other gases may absorb too much radiation. A vacuum environment may therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps.

[0031] As here depicted, the apparatus is of a reflective type (e.g., employing a reflective mask).

[0032] The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such "multiple stage" machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.

[0033] Referring to Figure 1, the illuminator IL receives an extreme ultra violet radiation beam from the source collector module SO. Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has at least one element, e.g., xenon, lithium or tin, with one or more emission lines in the EUV range. In one such method, often termed laser produced plasma ("LPP") the required plasma can be produced by irradiating a fuel, such as a droplet, stream or cluster of material having the required line-emitting element, with a laser beam. The source collector module SO may be part of an EUV radiation system including a laser, not shown in Figure 1, for providing the laser beam exciting the fuel. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector, disposed in the source collector module. The laser and the source collector module may be separate entities, for example when a C0₂ laser is used to provide the laser beam for fuel excitation.

[0034] In such cases, the laser is not considered to form part of the lithographic apparatus and the radiation beam is passed from the laser to the source collector module with the aid of a beam delivery system comprising, for example, suitable directing mirrors and/or a beam expander. In other cases the source may be an integral part of the source collector module, for example when the source is a discharge produced plasma EUV generator, often termed as a DPP source.

[0035] The illuminator IL may comprise an adjuster for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL may comprise various other components, such as facetted field and pupil mirror devices. The illuminator may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross-section.

[0036] The radiation beam B is incident on the patterning device (e.g., mask) MA, which is held on the support structure (e.g., mask table) MT, and is patterned by the patterning device. After being reflected from the patterning device (e.g., mask) MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor PS2 (e.g., an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor PS1 can be used to accurately position the patterning device (e.g., mask) MA with respect to the path of the radiation beam B. Patterning device (e.g., mask) MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks PI, P2.

[0037] The depicted apparatus could be used in at least one of the following modes: [0038] 1. In step mode, the support structure (e.g., mask table) MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one time (i.e., a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed.

[0039] 2. In scan mode, the support structure (e.g., mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e., a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure (e.g., mask table) MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS.

[0040] 3. In another mode, the support structure (e.g., mask table) MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.

[0041] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed. The embodiments to be illustrated involve scanning, as in the modes 2 and 3 just mentioned.

[0042] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms "wafer" or "die" herein may be considered as synonymous with the more general terms "substrate" or "target portion," respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

[0043] Figure 2 shows an example of the apparatus 100 in more detail, including the source collector module SO, the illumination system IL, and the projection system PS. The source collector module SO is constructed and arranged such that a vacuum environment can be maintained in an enclosing structure 220 of the source collector module SO. The systems IL and PS are likewise contained within vacuum environments 222, 224 of their own. An EUV radiation emitting plasma 226 may be formed by a laser produced LPP plasma source, comprising a fuel supply 228 and a laser energy source 230The function of source collector module SO is to deliver EUV radiation beam 20 from the plasma 226 such that it is focused in a virtual source point. The virtual source point is commonly referred to as the intermediate focus (IF), and the source collector module is arranged such that the intermediate focus IF is located at or near an aperture 232 that connects the enclosing structures 220 and 222. The virtual source point IF is an image of the radiation emitting plasma 226.

[0044] From the aperture 232 at the intermediate focus IF, the radiation traverses the illumination system IL, which in this example includes a facetted field mirror device 22 and a facetted pupil mirror device 24. These devices form a so-called "fly's eye" illuminator, which is arranged to provide a desired angular distribution of the radiation beam 21, at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA. Upon reflection of the beam 21 at the patterning device MA, held by the support structure (mask table) MT, a patterned beam 26 is formed and the patterned beam 26 is imaged by the projection system PS via reflective elements 28, 30 onto a substrate W held by the wafer stage or substrate table WT. More elements than shown may generally be present in illumination system IL and projection system PS. Further, there may be more mirrors present than those shown in the Figures. For example there may be one to six additional reflective elements present in the illumination system IL and/or the projection system PS, besides those shown in Figure 2.

[0045] Each system IL and PS is arranged within its own vacuum or near-vacuum environment, defined by enclosing structures 222, 224 similar to enclosing structure 220. The mask table MT and substrate table WT are also housed within near-vacuum environments, with enclosing structures not shown here.

[0046] Also shown schematically in Figure 2 are various components for the distribution of fluids, particularly water as an example, which are used for cooling and possible other purposes within the lithographic apparatus and its subsystems. Accurate control of temperature and uniformity of temperature are very important factors in a precision apparatus such as a lithographic apparatus, to avoid thermal displacement and distortion of between and within the mechanical parts. Cooling may be necessary to remove and/or redistribute heat generated by stray radiation, for example IR radiation from laser 230 as well as EUV and other radiation. Cooling may also be necessary to remove heat dissipated by actuators and other components.

[0047] In Figure 2, two cooling units 260 and 262 are shown, just as examples, within the source collector module CO and the illumination system IL. Unit 260 may be for cooling part of the structure 220, where IR radiation impinges on the walls. It may be for cooling a spectral purity filter (not shown), which is designed to absorb the IR radiation while passing the EUV radiation to the intermediate focus. Within the illumination system, unit 262 is for cooling the pupil mirror device 24. This device may include actuators that generate heat, as well as being subject to heating by a portion of the radiation that is not reflected into beam 21.

[0048] Each subsystem SO, IL, PS, MT, WT may include many cooling units in practice. Conduits including the ones labeled 264, 266, 268 are provided for distributing water within the vacuum vessels defined by enclosing structures 220, 222, 224, respectively. These in turn are connected to distribution units 270, 272, 274, which include manifolds, penetrations of the vacuum vessels, and optionally functions such as pumping, valves, filters and flow metering. Further distribution units 278 and conduits 280 are provided outside the vacuum vessels, connected to a master supply conduit 282. These components are all conventional and do not need to be specified or explained further. In practice, flow and return conduits may be required, where only a single conduit is shown in the drawing, for simplicity.

[0049] Within the vacuum vessels, strict performance requirements are imposed on the conduits. They must be impermeable to the water, so that water liquid or vapor do not permeate the conduit walls and contaminate the vacuum environment unacceptably. The materials of the conduits themselves must be compatible with the vacuum environment and with any restrictions on outgassing that are imposed by the desired performance of the apparatus. Many polymers for example will emit organic molecules into a vacuum environment, which would be strongly absorbent of the EUV radiation used in the lithographic apparatus.

[0050] Figure 3 shows two known types of conduits for transporting water or other fluids in a vacuum environment. Figure 3 (a) shows a rigid type of conduit 300 formed from a simple metal pipe 302 with terminations 304, 306 (for example flanges). The pipe may be of stainless steel, for example. Bends 308 have been imparted to the pipe before fitting, so that it can follow a desired route. Rigid pipework of this type is rugged and reasonable in cost, but not always suitable. Flexible conduits are desired sometimes, whether for reasons of routing, or for isolating vibrations and other movement of the endpoints. Accordingly, it is known to use flexible conduits of the type shown schematically in Figure 3 (b). Here the conduit 310 comprises a corrugated metal tube 312 with terminations 314, 316. Such conduits are often referred to as 'flexhoses'. The corrugations in principle provide elasticity in the wall and allow the conduit to be bent to follow a desired route. They can also flex in use, to allow some movement between the endpoints. The construction of corrugated wall is generally of thin metal, typically stainless steel. Wall thickness is typically less than 200 μιη to keep flexibility. The use of metal meets perfectly the requirement for no outgassing. Unfortunately such conduits have proven to be vulnerable to perforation or tearing, with water leakage inside vacuum as a result. Aside from the strain and fatigue imparted on the conduit by routing and vibration, vibrations can be induced also by flows and pressure changes within the conduit.

[0051] Polymer (plastic) tubes are of course well known as alternatives to metal, and can be highly flexible and robust, but they have too high water permeability for use in sensitive vacuum environment. Polymer pipes with internal barrier layers are known for general (non- vacuum) use, where the barrier layer is normally intended to prevent oxygen in the environment from permeating into the liquid. For under-floor heating, for example, a known type of pipe has three layers: an inner layer of (crosslinked) polyethylene; a barrier made of aluminum pipe; and an outer layer of (crosslinked) polyethylene). Plastic pipes with a barrier layer of aluminum 'wrapped' onto a plastic core pipe are known for supplying potable water in environments where it is important to prevent contamination permeating into the water. Such barriers of metal or other effective barrier materials may also be considered to prevent liquid escaping into the environment for vacuum applications. However, known polymer hoses are subject to outgassing of polyethylene that makes them unsuitable for use in sensitive vacuum environments.

[0052] Figure 4 shows a novel type of flexible conduit 400 disclosed here for the first time. Terminations 324 and 326 are joined by a novel form of vacuum-compatible flexible conduit. This comprises an inner layer 330 of polymer; a barrier layer 332 and an outer layer of vacuum-compatible polymer 334. Optionally, an adhesive layer 338 and/or 340 can be provided, to bond the inside and/or outside of the barrier layer to the adjacent polymer layer. The barrier layer 331 may be of metal, for example aluminum. The barrier layer made of metal can be as thin as or thinner than that of the known flexible conduit 300, while the surrounding polymer layers provide strength against perforation and tearing. The metal layer may be for example less than 100 μιη in thickness, even less than 20 μιη or less than 10 μιη. That is much thinner than the wall of the known corrugated hose. The vacuum-compatible polymer layer 334 can be selected to be vacuum-compatible, in particular to meet the strict outgassing requirements of lithography tools or such other application as is desired. An example is to use a polymer like PTFE (PolyTetraFluoroEthane, "Teflon" trade mark) or PCTFE (PolyChloroTetraFluoroEthylene), in a suitable grade of purity to meet outgassing requirements.

[0053] Outgassing specifications can be expressed in terms of standard tests recognized for the particular application. For many applications the standard ASTM Test Method E595-93, "Total Mass Loss and Collected Volatile Condensable Materials from Outgassing in a Vacuum Environment". In Test Method E595, the material sample is heated to 125°C for 24 hours while in a vacuum (typically less than

5 x 10 -^"5 torr or 7 x 10 -^"3 Pascal). Specimen mass is measured before and after the test and the difference is expressed as percent total mass loss (TML ). A small cooled plate (at 25°C) is placed in close proximity to the specimen to collect the volatiles by condensation. This plate is used to determine the percent collected volatile condensable materials (CVCM ). An additional parameter, Water Vapor Regained (WVR ) can also be determined after completion of exposures and measurements for TML and CVCM.

[0054] ASTM Test Method E595 data can be used to identify suitable materials and grades of material for the polymer layers 330, 334 in the novel conduit. Actual surface contamination from the outgassing of materials will, of course, vary with environment and quantity of material used. Levels of TML < 1.0% and CVCM < 0.1% can be specified, for example, but semiconductor applications, and EUV optics, may require lower limits of acceptability. In embodiments of the invention suitable for these different applications, one or both of the polymer layers may be of material chosen to have TML less than 1%, 0.5%, 0.2% or 0.1%. Similarly, one or both of the polymer layers may be of material chosen to have CVCM may be less than 0.1%, 0.05% or 0.01%. Similarly, one or both of the polymer layers may be of material chosen to have WVR% may be less than 0.5%, 0.1%, 0.05% or 0.01%.

[0055] A range of low-outgassing polymer materials are offered for example by the supplier Boedecker Plastics Inc. An appropriate one can be selected for the present application according not only to its low outgassing performance, but also mechanical properties (e.g., elasticity, dimensional stability etc.), resistance to damage (e.g., by chemical attack, radiation and mechanical abrasion), and ease of forming the conduit layers. Known outgassing polymers that may be considered for this application include: filled PTFE e.g., Semitron® ESd 500HR; PCTFE PolyChloroTetra- FluoroEthylene) e.g., Neoflon® PCTFE; Polyimide e.g., DuPont Vespel® SP-lor Duratron® XP; unfilled PAI (PolyAmidelmide) e.g., Torlon® 4203; glass-filled PAI (PolyAmide-Imide) e.g., Torlon 5530 (30% glass-filled); PEEK (PolyEtherEtherKetone); PPS (PolyPhenylene Sulfide) e.g., Techtron® PPS; PEI (PolyEtherlmide) e.g., Ultem®; filled PEI e.g., Semitron® ESd 410C; PET-P (Polyethylene Terephthalate-Polyester) e.g., Ertalyte®; filled acetal e.g., Semitron® ESd 225. TORLON is a registered trademark of Solvay Advanced Polymers. NEOFLON is a registered trademark of Daikin America. DURATRON, ERTALYTE and SEMITRON are registered trademarks of Quadrant DSM Engineering Plastic Products. VESPEL is a registered trademark of DuPont. ULTEM is a registered trademark of SABIC Innovative Polymers (formerly GE Plastics). PEEK is a trademark of Victrex pic.

[0056] The degree of flexibility required of the conduit is a key parameter for material selection, along with the mechanical strength and fatigue resistance. Depending on the thickness of the layers, several of the above materials may be insufficiently flexible, or too difficult to form by a chosen method of making the conduit. Thermoplastic materials may be preferred, for example. The internal and external polymer layers do not need to be the same polymer. The outgassing requirement is naturally most acute for the outer layer 334. [0057] The methods of manufacturing the novel hose can be selected from known methods. The metal and polymer layers can be formed together by co-extrusion, for example. The metal layer can be formed by depositing a metal coating onto an existing polymer layer, or a polymer layer can be formed by coating onto an existing metal substrate. The metal layer can be made by wrapping a metal foil around a polymer pipe substrate, or by depositing from a chemical or physical vapor, by sputtering etc. In the co-extrusion method or other methods, one or more adhesive layers can be included.

[0058] Although reference is made in this text to lithographic process tools, the invention is equally applicable in other vacuum and near-vacuum environments, whether used in the manufacture of devices or materials, or instrumentation for scientific or other purposes.

[0059] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

1. A flexible conduit for conveying fluids in a vacuum environment, the conduit including:

- an inner polymer layer;

an outer polymer layer; and

a barrier layer enclosed between the inner and outer polymer layers, wherein the barrier layer is resistant to permeation by water from within the conduit to said vacuum environment, and wherein at least the outer polymer layer is made of vacuum-compatible material resistant to outgassing within a vacuum environment.

2. A conduit as claimed in claim 1 wherein said barrier layer comprises one or more metal layers.

3. A conduit as claimed in claim 2 wherein a thickness of the or each metal layer is less than 200 μιη, optionally less than 100 μιη, less than 50 μιη or less than 10 μιη.

4. A conduit as claimed in claim 2 or 3 wherein said barrier layer comprises at least one aluminum layer.

5. A conduit as claimed in any preceding claim wherein said outer polymer layer comprises PTFE.

6. A conduit as claimed in claim 5 wherein said inner polymer layer also comprises PTFE.

7. A conduit as claimed in any preceding claim wherein a bonding material is interposed between said barrier layer and at least one of said inner and outer polymer layers.

8. A lithographic apparatus for use in applying a pattern to a substrate, the lithographic apparatus comprising in one or more vacuum vessels:

- a support constructed to support a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam;

wherein the apparatus includes a fluid supply system for providing fluid to at least one component operating in one or more of said vacuum vessels, said fluid supply system comprising a conduit according any of claims 1 to 7.

9. An apparatus as claimed in claim 8 wherein the fluid is a cooling fluid.

10. An apparatus as claimed in claim 8 or 9 wherein the fluid is water.

11. A method of manufacturing a device, wherein as part of said method an image of a patterning device is projected using EUV radiation onto a substrate, in order to transfer a device pattern from said patterning device to said substrate, wherein said image is projected using a lithographic apparatus as claimed in claim 8, 9, 10 or 11.