NL2012129A - Radiation source that jets up liquid fuel to form plasma for generating radiation and recycle liquid fuel. - Google Patents

Description

RADIATION SOURCE THAT JETS UP LIQUID FUEL TO FORM PLASMA FOR GENERATING RADIATION AND RECYCLE LIQUID FUEL

Field

[0001] The present invention relates to a high-energy radiation source that jets up a liquid fuel to form plasma for generating radiation and recycle the unused liquid fuel. Such a radiation source may be used with an illumination system of, for example, a lithographic apparatus such as a reflective photolithographic exposure system.

Related Art

[0002] 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. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction), while synchronously scanning the substrate parallel or anti-parallel to this direction.

[0003] An optical source in the lithographic apparatus generates high energy radiation for patterning the radiation-sensitive material (resist) provided on the substrate. The high energy radiation is typically emitted from a molten metal material that has been struck with an incident beam of radiation. The molten material absorbs the energy from the incident radiation beam and the energy is dissipated as a high energy radiation beam. One technique launches droplets of the molten material, tracks the trajectory of the droplets, and fires a laser to hit a droplet as it flies through a vacuum chamber. Another technique generates a mist of molten material and fires a laser through the mist to generate the high energy radiation. These techniques are inefficient as much of the molten metal is wasted and requires manual cleaning of unused molten metal and any by-products from the source chamber. Additionally, not the entire incident radiation beam is absorbed contributing to further energy waste.

SUMMARY

[0004] Accordingly, there is a need for improved systems and methods for addressing the wastefulness and inefficiency of generating high energy radiation with molten metal.

[0005] In one embodiment, a radiation source that jets up a liquid fuel for plasma generation is provided. The radiation source includes a first reservoir, a housing coupled to the first reservoir, a second reservoir, a laser, and a collector. The first reservoir holds a liquid fuel and the liquid fuel moves through a channel towards a distal end of the channel and forms a bump of liquid fuel at the distal end of the channel. The channel passes through a housing and the distal end of the channel is at an outer surface of the housing. The second reservoir collects at least a portion of the liquid fuel that has moved out of the distal end of the channel. The housing is designed such that the outer surface enables an unused portion of the liquid fuel to flow down over the outer surface. The laser is constructed and arranged to generate a laser beam that is directed at the bump of the liquid fuel for forming plasma to generate radiation. The collector is constructed and arranged to collect the radiation generated by the plasma formed at or near the bump of the liquid fuel when the laser beam collides with the bump of the liquid fuel. The collector also reflects the radiation substantially along the optical axis of the radiation source.

[0006] In another embodiment, a lithographic apparatus includes an illumination system configured to generate a radiation beam. The lithographic apparatus further includes a support configured to hold a patterning device. The patterning device is configured to impart the radiation beam with a pattern in its cross-section to form a patterned radiation beam. A substrate table holds a substrate, and a projection system is configured to project the patterned radiation beam onto a target portion of the substrate. The illumination system includes a radiation source that jets up a liquid fuel for plasma generation for generating the radiation. The radiation source includes a first reservoir, a housing coupled to the first reservoir, a second reservoir, a laser, and a collector. The first reservoir holds a liquid fuel and the liquid fuel moves through a channel towards a distal end of the channel and forms a bump of liquid fuel at the distal end of the channel. The channel passes through a housing and the distal end of the channel is at an outer surface of the housing. The second reservoir collects at least a portion of the liquid fuel that has moved out of the distal end of the channel. The housing is designed such that the outer surface enables an unused portion of the liquid fuel to flow down over the outer surface. The laser is constructed and arranged to generate a laser beam that is directed at the bump of the liquid fuel for forming plasma to generate radiation. The collector is constructed and arranged to collect the radiation generated by the plasma formed at or near the bump of the liquid fuel when the laser beam collides with the bump of the liquid fuel. The collector also reflects the radiation substantially along the optical axis of the radiation source.

[0007] In another embodiment, a method of operating a radiation source within an illumination system of a lithographic apparatus is presented. The method includes providing a liquid fuel in a first reservoir, wherein the first reservoir is coupled to a housing having a channel with a distal end, and wherein the housing has an outer surface that enables an unused portion of the liquid fuel to flow down over the outer surface. The method further includes delivering the liquid to the channel and ejecting the liquid fuel from a distal end of the channel. Next, at least a portion of the ejected liquid fuel is excited with a radiation beam for forming plasma to generate radiation. The method further includes collecting the radiation generated by the plasma using at least a portion of the ejected liquid fuel and collecting at least a portion of the unused portion of the ejected liquid fuel in a second reservoir.

[0008] According to one embodiment, a radiation source is provided to generate radiation. The radiation source comprises a fuel generator. The fuel generator includes a first reservoir to hold a fuel, a channel having a tip such that the channel is coupled to the first reservoir to direct the fuel to a plasma generation site and a pump coupled to the channel to generate a continuous flow of the fuel from the channel such that the fuel bubbles out of the channel and forms a target over the tip of the channel. The radiation source further includes a laser constructed and arranged to generate a single laser beam that is directed to the plasma generation site. The radiation source further includes a controller configured to control the fuel generator to control the flow of the fuel from the channel and/or to control the laser to control the direction of the laser beam. And the radiation source further includes a collector constructed and arranged to collect radiation generated by a plasma formed at the plasma formation site when the laser beam and a portion of the fuel collide. The collector is being configured to reflect the radiation substantially along an optical axis of the radiation source and the laser beam is directed to the plasma generation site through an aperture provided in the collector.

[0009] In one embodiment, the radiation source is configured to generate extreme ultra-violet (EUV) radiation and the channel is passing through a housing. The radiation source may further comprise a valve coupled to the channel and configured to control a rate of flow of the fuel in the channel. For example, the fuel may be molten tin. The fuel may form a dome at the tip of the channel. The pump may be further configured to apply a force to the fuel to substantially stabilize the formation of the dome at the tip of the channel.

[0010] Consistent with one embodiment, the radiation source may further comprise one or more collection channels. The one or more collection channels may be configured to direct the fuel that has moved out of the tip of the channel into a second reservoir. In the radiation source, an outer surface of the housing may have a curved geometry such that fuel present on the outer surface of the housing is directed towards the one or more collection channels. The second reservoir may be configured to recirculate the fuel within the second reservoir back to the first reservoir. The radiation source may further comprise a plurality of cooling elements that are coupled to the housing.

[0011] Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. It is noted that the invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

[0012] The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the relevant art(s) to make and use the invention.

[0013] FIG. 1A is a schematic illustration of a reflective lithographic apparatus according to an embodiment of the invention.

[0014] FTG. IB is a schematic illustration of a transmissive lithographic apparatus, according to an embodiment of the invention.

[0015] FIG. 1C is a schematic illustration of an extreme ultra-violet (EUV) radiation lithographic apparatus according to an embodiment of the invention.

[0016] FIG. 2A is a schematic illustration of a high-energy radiation source that jets up a liquid fuel for plasma generation, according to an embodiment of the invention.

[0017] FIG. 2B is a schematic illustration of a close-up view of a high-energy radiation source that jets up a liquid fuel for plasma generation, according to an embodiment of the invention.

[0018] FIG. 3 is a schematic illustration of a liquid source bubbler in an illumination system of a lithographic apparatus, according to an embodiment of the invention.

[0019] FIG. 4 is an example method, according to an embodiment of the invention.

[0020] 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. Generally, the drawing in which an element first appears is typically indicated by the leftmost digit(s) in the corresponding reference number.

DETAILED DESCRIPTION

[0021] 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 clauses appended hereto.

[0022] 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 effect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

[0023] Before describing such embodiments in more detail, however, it is instructive to present an example environment in which embodiments of the present invention may be implemented.

Example Reflective and Transmissive Lithographic Systems

[0024] FIGs. 1A and IB are schematic illustrations of a lithographic apparatus 100 and lithographic apparatus 100', respectively, in which embodiments of the present invention may be implemented. Lithographic apparatus 100 and lithographic apparatus 100' each include the following: an illumination system (illuminator) IL configured to condition a radiation beam B (for example, DUV or EUV radiation); a support structure (for example, a mask table) MT configured to support a patterning device (for example, a mask, a reticle, or a dynamic patterning device) MA and connected to a first positioner PM configured to accurately position the patterning device MA; and, a substrate table (for example, a wafer table) WT configured to hold a substrate (for example, a resist coated wafer) W and connected to a second positioner PW configured to accurately position the substrate W. Lithographic apparatuses 100 and 100' also have a projection system PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion (for example, comprising one or more dies) C of the substrate W. In lithographic apparatus 100, the patterning device MA and the projection system PS are reflective. In lithographic apparatus 100', the patterning device MA and the projection system PS are transmissive.

[0025] The illumination system IL 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 the radiation B.

[0026] The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device MA, the design of the lithographic apparatuses 100 and 100', and other conditions, such as whether or not the patterning device MA is held in a vacuum environment. The support structure MT may use mechanical, vacuum, electrostatic, or other clamping techniques to hold the patterning device MA. The support structure MT can be a frame or a table, for example, which can be fixed or movable, as required. The support structure MT can ensure that the patterning device is at a desired position, for example, with respect to the projection system PS.

[0027] The term “patterning device” MA should be broadly interpreted as referring to any device that can be used to impart a radiation beam B with a pattern in its cross-section, such as to create a pattern in the target portion C of the substrate W. The pattern imparted to the radiation beam B can correspond to a particular functional layer in a device being created in the target portion C, such as an integrated circuit.

[0028] The patterning device MA may be transmissive (as in lithographic apparatus 100' of FIG. IB) or reflective (as in lithographic apparatus 100 of FIG. 1A). Examples of patterning devices MA include reticles, 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 the radiation beam B which is reflected by the mirror matrix.

[0029] The term “projection system” PS can encompass any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors, such as the use of an immersion liquid or the use of a vacuum. A vacuum environment can be used for EUV or electron beam radiation since other gases can absorb too much radiation or electrons. A vacuum environment can therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps.

[0030] Lithographic apparatus 100 and/or lithographic apparatus 100' can be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables) WT. In such “multiple stage” machines, the additional substrate tables WT can be used in parallel, or preparatory steps can be carried out on one or more tables while one or more other substrate tables WT are being used for exposure.

[0031] Referring to FIGs. 1A and IB, the illuminator IL receives a radiation beam from a radiation source SO. The source SO and the lithographic apparatuses 100, 100' can be separate entities, for example, when the source SO is an excimer laser. In such cases, the source SO is not considered to form part of the lithographic apparatuses 100 or 100', and the radiation beam B passes from the source SO to the illuminator IL with the aid of a beam delivery system BD (in FIG. IB) including, for example, suitable directing mirrors and/or a beam expander. In other cases, the source SO can be an integral part of the lithographic apparatuses 100, 100'—for example when the source SO is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD, if required, can be referred to as a radiation system.

[0032] The illuminator IL can include an adjuster AD (in FIG. IB) 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 can comprise various other components (in FIG. IB), such as an integrator IN and a condenser CO. The illuminator IL can be used to condition the radiation beam B to have a desired uniformity and intensity distribution in its cross section.

[0033] Referring to FIG. 1 A, the radiation beam B is incident on the patterning device (for example, mask) MA, which is held on the support structure (for example, mask table) MT, and is patterned by the patterning device MA. In lithographic apparatus 100, the radiation beam B is reflected from the patterning device (for example, mask) MA. After being reflected from the patterning device (for example, mask) MA, the radiation beam B passes through the projection system PS, which focuses the radiation beam B onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF2 (for example, an interferometric device, linear encoder, or capacitive sensor), the substrate table WT can be moved accurately (for example, 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 IF1 can be used to accurately position the patterning device (for example, mask) MA with respect to the path of the radiation beam B. Patterning device (for example, mask) MA and substrate W can be aligned using mask alignment marks Ml, M2 and substrate alignment marks PI, P2.

[0034] Referring to FIG. IB, the radiation beam B is incident on the patterning device (for example, mask MA), which is held on the support structure (for example, mask table MT), and is patterned by the patterning device. 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. The projection system has a pupil PPU conjugate to an illumination system pupil IPU. Portions of radiation emanate from the intensity distribution at the illumination system pupil IPU and traverse a mask pattern without being affected by diffraction at a mask pattern create an image of the intensity distribution at the illumination system pupil IPU.

[0035] With the aid of the second positioner PW and position sensor IF (for example, an interferometric device, linear encoder, or capacitive sensor), the substrate table WT can be moved accurately (for example, 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 (not shown in FIG. IB) can be used to accurately position the mask MA with respect to the path of the radiation beam B (for example, after mechanical retrieval from a mask library or during a scan).

[0036] In general, movement of the mask table MT can be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the first positioner PM. Similarly, movement of the substrate table WT can be realized using a long-stroke module and a short-stroke module, which form part of the second positioner PW. In the case of a stepper (as opposed to a scanner), the mask table MT can be connected to a short-stroke actuator only or can be fixed. Mask MA and substrate W can be aligned using mask alignment marks Ml, M2, and substrate alignment marks PI, P2. Although the substrate alignment marks (as illustrated) occupy dedicated target portions, they can be located in spaces between target portions (known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the mask MA, the mask alignment marks can be located between the dies.

[0037] Mask table MT and patterning device MA can be in a vacuum chamber, where an in-vacuum robot IVR can be used to move patterning devices such as a mask in and out of vacuum chamber. Alternatively, when mask table MT and patterning device MA are outside of the vacuum chamber, an out-of-vacuum robot can be used for various transportation operations, similar to the in-vacuum robot IVR. Both the invacuum and out-of-vacuum robots need to be calibrated for a smooth transfer of any payload (e.g., mask) to a fixed kinematic mount of a transfer station.

[0038] The lithographic apparatuses 100 and 100' can be used in at least one of the following modes:

[0039] 1. In step mode, the support structure (for example, mask table) MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam B 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.

[0040] 2. In scan mode, the support structure (for example, mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam B 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 (for example, mask table) MT can be determined by the (de-)magnification and image reversal characteristics of the projection system PS.

[0041] 3. In another mode, the support stmcture (for example, mask table) MT is kept substantially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam B is projected onto a target portion C. A pulsed radiation source SO can be 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 a programmable patterning device, such as a programmable mirror array of a type as referred to herein.

[0042] Combinations and/or variations on the described modes of use or entirely different modes of use can also be employed.

[0043] Although specific reference can 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 can 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), and thin-film magnetic heads. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein can be considered as synonymous with the more general terms “substrate” or “target portion,” respectively. The substrate referred to herein can 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 can be applied to such and other substrate processing tools. Further, the substrate can be processed more than once, for example, in order to create a multi-layer IC, so that the term substrate used herein can also refer to a substrate that already contains one or multiple processed layers.

[0044] In a further embodiment, lithographic apparatus 100 includes an extreme ultraviolet (EUV) source, which is configured to generate a beam of EUV radiation for EUV lithography. In general, the EUV source is configured in a radiation system (see below), and a corresponding illumination system is configured to condition the EUV radiation beam of the EUV source.

[0045] In the embodiments described herein, the terms “lens” and “lens element,” where the context allows, can refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic, and electrostatic optical components.

[0046] Further, the terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (for example, having a wavelength λ of 365, 248, 193, 157 or 126 nm), extreme ultraviolet (EUV or soft X-ray) radiation (for example, having a wavelength in the range of 5-20 nm such as, for example, 13.5 nm), or hard X-ray working at less than 5 nm, as well as particle beams, such as ion beams or electron beams. Generally, radiation having wavelengths between about 780-3000 nm (or larger) is considered IR radiation. UV refers to radiation with wavelengths of approximately 100-400 nm. Within lithography, the term “UV” also applies to the wavelengths that can be produced by a mercury discharge lamp: G-line 436 nm; H-line 405 nm; and/or, I-line 365 nm. Vacuum UV, or VUV (i.e., UV absorbed by gas), refers to radiation having a wavelength of approximately 100-200 nm. Deep UV (DUV) generally refers to radiation having wavelengths ranging from 126 nm to 428 nm, and in an embodiment, an excimer laser can generate DUV radiation used within a lithographic apparatus. It should be appreciated that radiation having a wavelength in the range of, for example, 5-20 nm relates to radiation with a certain wavelength band, of which at least part is in the range of 5-20 nm.

[0047] FIG. 1C shows a projection apparatus in more detail, comprising a radiation system 42, an illumination optics unit 44, and the projection system PS. The radiation system 42 comprises the radiation source SO which may be formed by a discharge plasma. EUV radiation may be produced by a gas or vapor, for example Xe gas, Li vapor or Sn vapor in which a very hot plasma is created to emit radiation in the EUV range of the electromagnetic spectrum. The very hot plasma is created by causing an at least partially ionized plasma by, for example, an electrical discharge. Partial pressures of, for example, 10 Pa of Xe, Li, Sn vapor or any other suitable gas or vapor may be required for efficient generation of the radiation. The radiation emitted by radiation source SO is passed from a source chamber 47 into a collector chamber 48 via a gas barrier or contaminant trap 49 which is positioned in or behind an opening in source chamber 47. The gas barrier 49 may comprise a channel stmcture.

[0048] The collector chamber 48 comprises a radiation collector 50 (also called collector mirror or collector) which may be formed by a grazing incidence collector. Radiation collector 50 has an upstream radiation collector side 50a and a downstream radiation collector side 50b. Radiation passed by collector 50 can be reflected off a grating spectral filter 51 to be focused in a virtual source point 52 at an aperture in the collector chamber 48. From collector chamber 48, a beam of radiation 56 is reflected in illumination optics unit 44 via normal incidence reflectors 53, 54 onto a reticle or mask positioned on reticle or mask table MT. A patterned beam 57 is formed which is imaged in projection system PS via reflective elements 58, 59 onto wafer stage or substrate table WT. More elements than shown may generally be present in illumination optics unit 44 and projection system PS. Grating spectral filter 51 may optionally be present, depending upon the type of lithographic apparatus. Further, there may be more mirrors present than those shown in the figures, for example there may be 1-4 more reflective elements present than 58, 59. Radiation collectors 50 are known from the prior art. Reference number 180 indicates a space between two reflectors, e.g. between reflectors 142 and 143.

[0049] Instead of a grazing incidence mirror as collector mirror 50, also a normal incidence collector may be applied. Collector mirror 50, as described herein in an embodiment in more detail as nested collector with reflectors 142, 143, and 146, is herein further used as example of a collector. Hence, where applicable, collector mirror 50 as grazing incidence collector may also be interpreted as collector in general and in an embodiment also as normal incidence collector.

[0050] Further, instead of a grating 51, as schematically depicted in FIG. 1C, also a transmissive optical filter may be applied. Optical filters transmissive for EUV and less transmissive for or even substantially absorbing UV radiation are known in the art. Hence, "grating spectral purity filter" is herein further indicated as "spectral purity filter" which includes gratings or transmissive filters. Not depicted in schematic drawings 3a and 3b, but also included as optional optical element may be EUV transmissive optical filters, for instance configured upstream of collector mirror 50, or optical EUV transmissive filters in illumination unit 44 and/or projection system PS.

[0051] Herein the terms "upstream" and "downstream" with respect to optical elements indicate positions "optically upstream" and "optically downstream" respectively. The beam of radiation B passes through lithographic apparatus 1. Following the light path that beam of radiation B traverses through lithographic apparatus 1, a first optical elements closer to source SO than a second optical element is configured upstream of the second optical element; the second optical element is configured downstream of the first optical element. For instance, collector mirror 50 is configured upstream of spectral filter 51, whereas optical element 53 is configured downstream of spectral filter 51.

[0052] All optical elements shown in FIG. 1C (and optical elements not shown in the schematic drawing of this embodiment) are vulnerable to deposition of contaminants produced by source SO, for example, Sn. This is the case for the radiation collector 50 and, if present, the spectral purity filter 51. Hence, a cleaning device may be used to clean one or more of these optical elements as well as a cleaning method may be applied to those optical elements, but also to normal incidence reflectors 53, 54 and reflective elements 58, 59 or other optical elements, for example additional mirrors, gratings, etc.

[0053] Radiation collector 50 may be a grazing incidence collector. The collector 50 is aligned along an optical axis O. The source SO or an image thereof is located on optical axis O. The radiation collector 50 may comprise reflectors 142, 143, 146 (also known as a Wolter-type reflector comprising several Wolter-type reflectors). Sometimes they are also called a shell. These reflectors 142, 143, 146 may be nested and rotationally symmetric about optical axis 0. In FIG. 1C, an inner reflector is indicated by reference number 142, an intermediate reflector is indicated by reference number 143, and an outer reflector is indicated by reference number 146. The radiation collector 50 encloses a certain volume, i.e. the volume within the outer reflector(s) 146. Usually, this volume within outer retlector(s) 146 is circumferentially closed, although small openings may be present. All the reflectors 142, 143 and 146 comprise surfaces of which at least part comprises a reflective layer or a number of reflective layers. Hence, reflectors 142, 143 and 146 (more reflectors may be present and embodiments of radiation collectors (also called collector mirrors) 50 having more than 3 reflectors or shells are comprised herein), are at least partly designed for reflecting and collecting EUV radiation from source SO, and at least part of the reflector may not be designed to reflect and collect EUV radiation. For example, at least part of the back side of the reflectors may not be designed to reflect and collect EUV radiation. The latter part may also be called back side. On the surface of these reflective layers, there may in addition be a cap layer for protection or as optical filter provided on at least part of the surface of the reflective layers.

[0054] The radiation collector 50 may be placed in the vicinity of the source SO or an image of the source SO. Each reflector 142, 143, 146 may comprise at least two adjacent reflecting surfaces, the reflecting surfaces further from the source SO being placed at smaller angles to the optical axis O than the reflecting surface that is closer to the source SO. In this way, a grazing incidence collector 50 is configured to generate a beam of (E)UV radiation propagating along the optical axis O. At least two reflectors may be placed substantially coaxially and extend substantially rotationally symmetric about the optical axis O. It should be appreciated that radiation collector 50 may have further features on the external surface of outer reflector 146 or further features around outer reflector 146, for example a protective holder, a heater, etc.

Example Embodiments of a High-Energy Radiation Source that Jets Up a Liquid Fuel for Plasma Generation

[0055] Example embodiments of a high-energy radiation source that jets up a fuel for plasma generation are described herein. A high-energy radiation source that jets up a liquid fuel for plasma generation is used within radiation source SO as illustrated in FIGs. 1A and IB to produce EUV radiation, according to one embodiment. For example, according to one embodiment, in a radiation source, a liquid tin may be pumped upwards through a narrow tube with its outlet at the tip of a dome. As the tin exits the tube a bump or drop is created on the top of the dome where it is hit by a laser beam and emits EUV radiation. Excess tin may roll down the side of the dome and can be captured and reused. This solution may improve the control of the tin supply and permit constant EUV emission. In other words, a fuel supply for generation of EUV radiation is provided with an integrated tin recovery structure such that a fuel is constantly supplied to an excitation point that is located at the exit of a dome-shaped supply unit. In one embodiment, such a radiation source may result in reduced contamination and improved control of the fuel supply.

[0056] FIG. 2 A illustrates a high-energy radiation source 200 that jets up a fuel for plasma generation to generate radiation such as extreme ultra-violet (EUV) radiation, according to an embodiment of the invention. High-energy radiation source that jets up a fuel for plasma generation 200 includes a housing 202, a channel 204, a first reservoir 206, a pump 210, collection channels 212, and a second reservoir 214. It should be understood that the relative placement, shapes, and sizes of each component illustrated in FIG. 2 is not intended to be limiting and could be altered without deviating from the spirit or scope of the invention as would be apparent to one having skill in the relevant art(s).

[0057] A liquid fuel 218 is stored in first reservoir 206, according to an embodiment. Liquid fuel 218 is molten tin, in one example. The phase of the molten tin is such that it exhibits an ability to flow given an applied force. Pump 210 may represent any type or number of different liquid pumps, such as, for example, a liquid-ring pump. In an embodiment, pump 210 is arranged to force liquid fuel 218 through channel 204 towards a distal end 205. Pump 210 may be operated to move liquid fuel 218 at any speed continuously, or may operate at pulsed speeds. In one example, a sensor (not shown) measures the speed of liquid fuel 218 in channel 204 and feeds the data back to a controller 219. Controller 219 may use the sensor data to control the actuation of pump 210.

[0058] Liquid fuel 218 is pumped through channel 204 and exits at distal end 205, according to an embodiment. The force applied to liquid fuel 218 may cause a substantially spherical dome of liquid to form at distal end 205. In one example, the applied force from pump 210 balances surface tension forces from liquid 218 and the dome would be substantially stabilized at distal end 205. In another example, liquid fuel 218 is continuously being ejected from distal end 205 and running down the sides of housing 202. Alternatively, liquid 218 may be ejected forcefully from distal end 205 and fall back down around housing 202 (similar to a geyser). In another example, liquid fuel 218 flows out from distal end 205 and continues to flow down the outer surface of housing 202 without forming any dome-like shape. The outer surface of housing 202 may be curved as illustrated in FIG. 2A to facilitate liquid 218 mnning down the sides of housing 202 after being ejected from channel 204. Other geometries for the outer surface of housing 202 may be contemplated as well, including etched grooves and more angular walls.

[0059] FIG. 2B provides a magnified view of housing 202 with liquid fuel 218 filling channel 204 and forming bump 220, according to an embodiment. Also illustrated are a plurality of liquid droplets 222 that have detached from bump 220. Liquid droplets 222 mn down the curved outer surface of housing 202 and towards collection channels 212, according to an embodiment. It should be understood that the liquid mnning down the outer surface of housing 202 may also still be attached in a continuous stream from bump 220.

[0060] Bump 220 may be a portion of any elliptical or spherical shape. For example, a radius of bump 220 does not need to be the same as a radius of distal end 205. Bump 220 may have an oblong shape. Furthermore, the surface of bump 220 does not need to be smooth and may exhibit signs of turbulence, according to some embodiments.

[0061] Returning to FIG. 2A, collection channels 212 are disposed around housing 202 in such a manner so as to capture liquid fuel 218 that has moved out of channel 204, according to an embodiment. In one example, collection channels 212 direct the captured liquid to a second reservoir 214. Second reservoir 214 may provide, or be coupled to, filtering components to remove any by-products or other contaminants from the captured liquid. In one example, captured liquid in second reservoir 214 may be re-circulated to first reservoir 206 in an effort to re-use the captured liquid again in the system.

[0062] High-energy radiation source 200 that jets up the liquid fuel 218 for plasma generation may also include optional components such as a valve 208 and side structures 216, according to embodiments. Valve 208 may be coupled to channel 204 and be used to control a flow rate of liquid fuel 218 within channel 204. In one example, a size of bump 220 is controlled by adjusting valve 208. In another embodiment, the environment outside of housing 202 is at vacuum pressure and valve 208 controls how much of liquid fuel 218 is sucked up channel 204 due to the pressure differential. In an example, the amount that valve 208 opens is proportional to the flow rate of liquid fuel 218 up channel 204. In a situation where a vacuum environment is used to suck liquid fuel 218 up channel 204, there may be no need for pump 210.

[0063] Side structures 216 may be disposed around housing 202 in order to help guide any liquid that has exited from channel 204 into collection channels 212. For example, if some of the liquid sloshes off of the outer surface of housing 202, it may hit the surface of side structure 216 and fall towards one of collection channels 212. Side structure 216 may be designed as a ring that substantially surrounds housing 202, or as discrete structures on various sides of housing 202. In an embodiment, the surfaces of side structures 216 have a curved geometry, such as the dome-shaped geometry illustrated in FIG. 2A. Other geometries for the outer surface of side structures 216 may be contemplated as well, including etched grooves and more angular walls.

[0064] FIG. 3 illustrates an example of high-energy radiation source 200 that jets up the liquid fuel 218 for plasma generation within an optical source of a lithographic apparatus, according to an embodiment. In other examples, high-energy radiation source 200 may be used within any device that generates high energy radiation. In one example, bump 220 is formed at the end of housing 202 in order to be struck with a first beam of radiation 302. Also illustrated is captured liquid fuel 312 within second reservoir 214, according to an embodiment. Captured liquid 312 may include liquid that has run out over the outer surface of housing 202 without contact with first beam of radiation 302, and also any by-products generated as a result of first beam of radiation 302 interacting with the liquid within bump 220. A laser source such as a xenon or argon based excimer laser (not shown) may be used to generate first beam of radiation 302. In one example, first beam of radiation 302 is generated from a laser having a power around 20 kilowatts. Since the placement of bump 220 is substantially static, the trajectory of first beam of radiation 302 may also be fixed. In another embodiment, the trajectory of first beam of radiation 302 may be controlled via a controller, such as, for example, controller 219 from FIG. 2A. In this way, the trajectory of beam of radiation 302 may be altered if the flow rate of liquid fuel 218 is changed, since changing the flow rate of liquid fuel 218 may change the size of bump 220.

[0065] The energy from first beam of radiation 302 is partially absorbed at bump 220, and a second beam of radiation 304 is emitted, according to an embodiment. In one example, second beam of radiation 304 represents extreme ultraviolet (EUV) radiation emitted from a bump 220 of molten tin. Second beam of radiation 304 may emit from substantially all angles around bump 220. In another example, the angle of incidence that first beam of radiation 302 has on bump 220 may control which angles have the highest intensity of emitted radiation from bump 220. The trajectory of second beam of radiation 304 may be redirected using an optical element 306. Optical element 306 may be a mirror designed to reflect second beam of radiation 304, or optical element 306 may be a prism or lens designed to refract the trajectory angle of second beam of radiation 304.

[0066] In another example, optical element 306 includes index modulating elements for altering the trajectory of second beam of radiation 304. The new trajectory of second beam of radiation 304, after interaction with optical element 306, may be towards other optical elements in an illumination system within a lithographic apparatus, according to some embodiments. Optical element 306 may be positioned to collect a large portion of the emitted radiation and re-direct the radiation in the same direction. The angle of incidence of first beam of radiation 302 on bump 220 may be set based on, for example, the placement and geometry of optical element 306, in order to optimize the collection of emitted radiation by optical element 306.

[0067] In one embodiment, an environment 301 is at vacuum pressure around bump 220. The vacuum pressure environment 301 is more conducive to the generation of second beam of radiation 304 with minimal disruptions, according to an embodiment. Energy absorbed at bump 220 is also dissipated as heat. Heat may dissipate fuel through the vacuum environment 301 as radiation. As such, optical element 306 may absorb the radiated heat. In one embodiment, a plurality of cooling elements 308 is coupled to optical element 306. Cooling elements 308 may represent cooling tubes, thermoelectric cooling devices, or forced air for modulating the temperature of optical element 306. In another example, cooling elements 308 are heat dissipation fins or include any other geometry to maximize exposed surface area.

[0068] A more efficient form of heat dissipation is conduction. In an embodiment, the design of high-energy radiation source 200 allows for bump 220 to still be substantially in contact with housing 202. Heat generated at bump 220 may be dissipated via conduction through housing 202. Housing 202 may comprise a suitable material with a high thermal conductivity such as copper, aluminum, or iron. In another embodiment, housing 202 may include a material with a high thermal conductivity as a thin layer over the outer surface of housing 202. Housing 202 may include cooling elements 310 coupled to a portion of housing 202. In an embodiment, cooling elements 310 are coupled to a lower portion of housing 202 as illustrated in HG. 3. Heat that has been conducted through housing 202 may be more efficiently dissipated by cooling elements 310. Cooling elements 310 may represent cooling tubes, thermoelectric cooling devices, or forced air for modulating the temperature of housing 202. In another example, cooling elements 310 are heat dissipation fins or include any other geometry to maximize exposed surface area.

Exemplary Method of Operation

[0069] FIG. 4 is a flow diagram of a method 400 for operating a high-energy radiation source that jets up a liquid fuel for plasma generation, according to embodiments of the present invention. It is to be appreciated that operations in method 400 may be performed in another order, and that not all operations shown may be required. In one example, various components of high-energy radiation source 200 may be involved in the execution of the steps of method 400.

[0070] Method 400 begins at step 402 where a liquid fuel is provided in a first reservoir, according to an embodiment. The first reservoir is coupled to a housing having a channel with a distal end passing through the housing. In an embodiment, an outer surface of the housing is shaped such that an unused portion of the liquid fuel flows down over the outer surface. The term “unused” in this context may refer to any portion of the liquid fuel that has not been consumed during the formation of plasma for generating radiation.

[0071] At step 404, a liquid fuel is delivered to the channel, according to an embodiment. The liquid fuel may be delivered to the channel by a pump coupled to the first reservoir, or the liquid fuel may be drawn through the channel via a pressure differential. The pressure differential may be caused by a vacuum environment being created at one end of the channel. In another embodiment, a valve coupled to the channel may control a rate of flow of the liquid fuel as it moves through the channel.

[0072] At step 406, the liquid fuel is ejected from the distal end of the channel, according to an embodiment. In one example, the liquid fuel forms a bump shape at the distal end of the channel. A relative size of the bump may be controlled via a pressure applied to the liquid. The pressure may be controlled by one or more pumps and/or valves coupled to the channel. In one example, the liquid fuel is continuously moving out from the distal end of the channel, while in another example the liquid fuel remains substantially stable in a dome shape at the distal end of the channel.

[0073] At step 408, at least a portion of the ejected liquid fuel is excited by a first beam of radiation, according to an embodiment. The ejected liquid fuel may be excited when the first beam of radiation is incident upon a portion of the ejected liquid fuel. The first beam of radiation may be generated by an excimer laser. The excitation of, for example, molten tin, may produce heat as well as form a plasma that emits radiation that is used to form a second beam of radiation. The heat may be dissipated via thermal conduction through a housing in contact with at least a portion of the ejected liquid fuel.

[0074] At step 410, the radiation, emitted from at least a portion of the ejected liquid fuel, is collected to form the second beam of radiation, according to an embodiment. In one example, the emitted radiation may be extreme ultraviolet (EUV) radiation emitted from molten tin. The emitted radiation may be redirected or focused by an optical element constructed to form the second beam of radiation. Optical parameters of the formed second beam of radiation such as intensity, wavelength, mode shape, angle, etc., may vary depending on similar optical parameters of the first beam of radiation. In another example, the optical parameters of the second beam of radiation may vary depending on material/electrical parameters such as viscosity, density, band gap, conductivity, etc. of the ejected liquid fuel.

[0075] At step 412, at least a portion of the unused portion of the ejected liquid fuel is collected in a second reservoir, according to an embodiment. The ejected liquid fuel may be collected via one or more channels that guide the ejected liquid fuel into the second reservoir. In an embodiment, an applied pressure differential may force the ejected liquid fuel into one or more channels that lead to the second reservoir. The collection of the ejected liquid fuel may help to reduce build-up of by-products and reduce time and effort spent manually cleaning a chamber substantially surrounding the high-energy radiation source.

[0076] At an optional step 414, the collected liquid fuel is recirculated from the second reservoir back into the first reservoir, according to an embodiment. The steps of method 400 may be carried out again with the re-circulated liquid fuel. In one embodiment, the re-circulated liquid fuel is filtered before being disposed back into the first reservoir.

[0077] It is to be appreciated that the Detailed Description section, and not the Summary and Abstract sections, is intended to be used to interpret the clauses. The Summary and Abstract sections may set forth one or more but not all exemplary embodiments of the present invention as contemplated by the inventor(s), and thus, are not intended to limit the present invention and the appended clauses in any way.

[0078] The present invention has been described above with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed.

[0079] The foregoing description of the specific embodiments will so fully reveal the general nature of the invention that others can, by applying knowledge within the skill of the art, readily modify and/or adapt for various applications such specific embodiments, without undue experimentation, without departing from the general concept of the present invention. Therefore, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein. It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present specification is to be interpreted by the skilled artisan in light of the teachings and guidance.

[0080] The breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following clauses and their equivalents. Other aspects of this invention are set out as in the following numbered clauses: 1. A radiation source having an optical axis, the radiation source comprising: a first reservoir configured to hold a liquid fuel; a housing having an outer surface, the housing being coupled to the first reservoir and having a channel passing through the housing, the channel having a distal end at the outer surface of the housing, wherein the channel is configured to jet up the liquid fuel and form a bump of the liquid fuel at the distal end for generating plasma, and wherein the housing is configured such that the outer surface enables an unused portion of the liquid fuel to flow down over the outer surface; a second reservoir configured to collect at least a portion of the unused portion of the liquid fuel that has moved out of the distal end of the channel; a laser constructed and arranged to generate a laser beam that is directed at the bump of the liquid fuel for forming plasma to generate radiation; and a collector constructed and arranged to collect the radiation generated by the plasma formed at or near the bump of the liquid fuel when the laser beam collides with the bump of the liquid fuel, wherein the collector is configured to reflect the radiation substantially along the optical axis of the radiation source.

2. The radiation source of clause 1 further comprising: a valve coupled to the channel and configured to control a rate of flow of the liquid fuel in the channel.

3. The radiation source of clause 1, wherein the liquid fuel is molten tin.

4. The radiation source of clause 3, wherein the radiation is extreme ultra-violet (EUV) radiation.

5. The radiation source of clause 1, further comprising: a pump configured to move the liquid fuel through the channel towards the distal end.

6. The radiation source of clause 5, wherein the pump is further configured to apply a force to the liquid fuel to substantially stabilize the formation of the bump at the distal end of the channel.

7. The radiation source of clause 5, wherein the pump is further configured to continuously move the liquid fuel through the channel and out the distal end of the channel.

8. The radiation source of clause 1, further comprising: one or more collection channels, wherein the one or more collection channels are configured to direct at least a portion of the unused portion of liquid fuel that has moved out of the distal end of the channel into the second reservoir.

9. The radiation source of clause 8, wherein the outer surface of the housing has a curved geometry such that liquid fuel present on the outer surface of the housing is directed towards the one or more collection channels.

10. The radiation source of clause 1, further comprising: a plurality of cooling elements coupled to the housing.

11. The radiation source of clause 10, wherein the plurality of cooling elements includes one or more cooling tubes.

12. The radiation source of clause 10, wherein the plurality of cooling elements includes one or more heat dissipation structures.

13. The radiation source of clause 1, wherein the second reservoir is configured to recirculate the liquid fuel within the second reservoir back to the first reservoir.

14. The radiation source of clause 14, wherein the second reservoir is coupled to one or more filtering components configured to remove by-products or contaminants from the liquid fuel before it is re-circulated.

15. The radiation source of clause 1, further comprising: a controller configured to control at least one of the flow of the liquid fuel from the channel and a direction of the laser beam.

16. A lithographic apparatus comprising: an illumination system configured to generate a radiation beam; a support constructed to hold 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 configured to project the patterned radiation beam onto a target portion of the substrate, wherein the illumination system includes a radiation source having an optical axis, comprising: a first reservoir configured to hold a liquid fuel; a housing having an outer surface, the housing being coupled to the first reservoir and having a channel passing through the housing, the channel having a distal end at the outer surface of the housing, wherein the channel is configured to jet up the liquid fuel and form a bump of the liquid fuel at the distal end for generating plasma, and wherein the housing is configured such that the outer surface enables a portion of the liquid fuel to flow down over the outer surface; a second reservoir configured to collect at least a portion of the unused portion of the liquid fuel that has moved out of the distal end of the channel; a laser constructed and arranged to generate a laser beam that is directed at the bump of the liquid fuel for forming plasma to generate radiation; and a collector constructed and arranged to collect the radiation generated by the plasma formed at or near the bump of the liquid fuel when the laser beam collides with the bump of the liquid fuel, wherein the collector is configured to reflect the radiation substantially along the optical axis of the radiation source.

17. The lithographic apparatus of clause 16, wherein the collector comprises a mirror configured to re-direct the generated radiation into the radiation beam.

18. The lithographic apparatus of clause 17, further comprising: one or more cooling elements coupled to the mirror.

19. The lithographic apparatus of clause 16, wherein the radiation source further comprises: a valve coupled to the channel and configured to control a rate of flow of the liquid fuel in the channel.

20. The lithographic apparatus of clause 16, wherein the liquid fuel is molten tin.

21. The lithographic apparatus of clause 16, further comprising: a pump configured to move the liquid fuel through the channel towards the distal end.

22. The lithographic apparatus of clause 21, wherein the pump is further configured to apply a force to the liquid fuel to substantially stabilize the formation of the bump at the distal end of the channel.

23. The lithographic apparatus of clause 21, wherein the pump is further configured to continuously move the liquid fuel through the channel and out the distal end of the channel.

24. The lithographic apparatus of clause 16, wherein the radiation source further comprises: one or more collection channels, wherein the one or more collection channels are configured to direct at least a portion of the unused portion of liquid fuel that has moved out of the distal end of the channel into the second reservoir.

25. The lithographic apparatus of clause 24, wherein the outer surface of the housing has a curved geometry such that liquid fuel present on the outer surface of the housing is directed towards the one or more collection channels.

26. The lithographic apparatus of clause 16, wherein the radiation source further comprises: a plurality of cooling elements coupled to the housing.

27. The lithographic apparatus of clause 16, wherein the second reservoir is configured to re-circulate the liquid fuel within the second reservoir back to the first reservoir.

28. The lithographic apparatus of clause 27, wherein the second reservoir is coupled to one or more filtering components configured to remove by-products or contaminants from the liquid fuel before it is re-circulated.

29. The lithographic apparatus of clause 16, wherein the radiation source further comprises: a controller configured to control at least one of the flow of the liquid fuel from the channel and a direction of the laser beam.

30. A method of operating a radiation source in an illumination system of a lithographic apparatus, the method comprising: providing a liquid fuel in a first reservoir, wherein the first reservoir is coupled to a housing having a channel with a distal end, and wherein the housing has an outer surface that enables an unused portion of the liquid fuel to flow down over the outer surface; delivering the liquid fuel to the channel; ejecting the liquid fuel from the distal end of the channel, such that the ejected liquid fuel forms a bump at the distal end of the channel; exciting at least a portion of the ejected liquid fuel with a radiation beam for forming plasma to generate radiation; collecting the radiation generated by the plasma by using at least a portion of the ejected liquid fuel; and collecting at least a portion of the unused portion of the ejected liquid fuel in a second reservoir.

31. The method clause 30, further comprising: recirculating the liquid fuel within the second reservoir back into the first reservoir.

32. The method clause 30, further comprising: filtering the liquid fuel within the second reservoir using one or more filtering components.

33. A radiation source configured to generate radiation and having an optical axis, the radiation source comprising: a fuel generator comprising: a first reservoir configured to hold a fuel, a housing having an outer surface, the housing being configured such that the outer surface enables a portion of the fuel to flow down over the outer surface, a channel passing through the housing and coupled to the first reservoir, the channel having a distal end at the outer surface of the housing, wherein the channel is configured to jet up the fuel and form a bump of the fuel at the distal end for generating plasma; a laser constructed and arranged to generate a laser beam that is directed at the bump of the fuel; and a collector constructed and arranged to collect radiation generated by the plasma formed at or near the bump of the fuel when the laser beam collides with the bump of the fuel, wherein the collector is configured to reflect the radiation substantially along the optical axis of the radiation source, and wherein the laser beam is directed at the plasma generation site through an aperture provided in the collector.

34. The radiation source of clause 33, wherein the radiation source is configured to generate extreme ultra-violet (EUV) radiation.

35. The radiation source of clause 33, further comprising: a valve coupled to the channel and configured to control a rate of flow of the fuel in the channel.

36. The radiation source of clause 33, wherein the fuel is molten tin.

37. The radiation source of clause 33, further comprising: a pump configured to move the fuel through the channel towards the distal end.

38. The radiation source of clause 37, wherein the pump is further configured to apply a force to the fuel to substantially stabilize the formation of the bump at the distal end of the channel.

39. The radiation source of clause 33, further comprising: one or more collection channels, wherein the one or more collection channels are configured to direct the fuel that has moved out of the distal end of the channel into a second reservoir.

40. The radiation source of clause 39, wherein an outer surface of the housing has a curved geometry such that fuel present on the outer surface of the housing is directed towards the one or more collection channels.

41. The radiation source of clause 39, wherein the second reservoir is configured to recirculate the fuel within the second reservoir back to the first reservoir.

42. The radiation source of clause 33, further comprising: a plurality of cooling elements coupled to the housing.

Claims (1)

A lithography device comprising: an illumination device adapted to provide a radiation beam; a carrier constructed to support a patterning device, the patterning device being capable of applying a pattern in a section of the radiation beam to form a patterned radiation beam; a substrate table constructed to support a substrate; and a projection device adapted to project the patterned radiation beam onto a target area of the substrate, characterized in that the substrate table is adapted to position the target area of the substrate in a focal plane of the projection device.