Methods and apparatuses for receiving target material in a radiation source

Abstract

A radiation source includes a vessel coupled to an exhaust; a droplet generator aligned with a droplet catch; a reservoir coupled to the droplet catch; and a conduit providing fluid communication between the exhaust and the reservoir. A method of operating a radiation source includes flowing under gravity droplet material from a droplet catch in a radiation source to a reservoir and flowing under gravity droplet material from an exhaust in the radiation source to the reservoir.

Description

METHODS AND APPARATUSES FOR RECEIVING TARGET MATERIAL IN A RADIATION SOURCECROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US Application No. 63 / 712,751, filed on October 28, 2024, titled METHODS AND APPARATUSES FOR RECEIVING TARGET MATERIAL IN A RADIATION SOURCE, and US Application No. 63 / 900,264, filed on October 16, 2025, titled METHODS AND APPARATUSES FOR RECEIVING TARGET MATERIAL IN A RADIATION SOURCE, which are incorporated herein by reference in their entireties.FIELD

[0002] The disclosed subject matter relates to methods and apparatuses for receiving target material used during the operation of a radiation source.BACKGROUND

[0003] Extreme ultraviolet (EUV) radiation generated by a radiation source can be used by exposure tools for semiconductor manufacturing processes. Examples of such exposure tools can include a lithographic apparatus and a metrology apparatus, more specifically a wafer inspection apparatus and mask inspection apparatus, and even more specifically an actinic mask inspection apparatus.

[0004] The lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. The lithographic apparatus can be used, for example, in the manufacture of logic devices and memory devices. The lithographic apparatus projects a pattern at a patterning device (e.g., a mask / reticle) onto a layer of radiation-sensitive material (e.g., a photoresist or resist) provided on the substrate. To project the pattern on the substrate the lithographic apparatus may use electromagnetic radiation.

[0005] The metrology apparatus provides inspection and measurement functions on the wafer and / or the reticle. The metrology apparatus is designed to detect and quantify defects, as well as to measure critical dimensions and analyze chemical compositions, ensuring that the manufacturing process or a quality of the reticle meets a rigorous quality and accuracy standards required at advanced technology nodes.

[0006] The wavelength of the EUV radiation determines the minimum size achievable on the substrate or the resolution of target detection on the reticle. Methods to produce the EUV radiation include, but are not necessarily limited to, converting a source material that has a chemical element with an emission line in the EUV range into a plasma state. Elements can include, but are not necessarily limited to, xenon, lithium, and tin. The EUV range includes wavelengths from about 120 nm down to about 10 nm, including the 13.5 nm emission from tin plasmas.

[0007] In one method, often termed laser-produced plasma ("LPP"), the desired plasma can be produced by irradiating a source material, for example, in the form of a droplet, stream, or wire, with pulses of light produced using a laser. In another method, often termed discharge produced plasma ("DPP"), the plasma can be generated by positioning source material having an appropriate emission line between a pair of electrodes and causing an electrical discharge to occur between the electrodes. EUV imaging and inspection processes are relatively high value processes and downtime is accordingly expensive and thus to be minimized where possible.SUMMARY

[0008] In some general aspects, a radiation source includes a vessel coupled to an exhaust; a droplet generator aligned with a droplet catch; a reservoir coupled to the droplet catch; and a conduit providing fluid communication between the exhaust and the reservoir.

[0009] Implementations can include one or more of the following.

[0010] The radiation source can further include a vacuum chamber enveloping the vessel, and the reservoir can be arranged outside the vacuum chamber. The conduit can be disposed between the vessel and the vacuum chamber. The conduit can provide fluid communication between the exhaust and the reservoir through the droplet catch.

[0011] The conduit can provide fluid communication between the exhaust and the droplet catch through a passage within the body of an entrance aperture structure of the droplet catch. An end of the conduit proximal to the droplet catch can include a closed end wall and a side wall with an opening therethrough. The conduit proximal to the droplet catch can extend through an opening into a chamber within the body of the entrance aperture structure of the droplet catch and the passage within the body of the aperture extends from the chamber to an outlet positioned on an inside surface of the aperture outside of a path of travel of droplets through the aperture. The opening can larger than an outer cross section of the conduit at the opening.

[0012] The chamber within the body of the entrance aperture structure of the droplet catch can be positioned radially outward from an attachment flange of the entrance aperture structure. The conduit can include conduit segments.

[0013] The radiation source can further include a passage coupling the reservoir to the droplet catch and the conduit can provide fluid communication between the exhaust and the passage. The conduit can extend from the exhaust to the reservoir. The conduit can partially encircle the vessel. The conduit can include multiple inlets. The radiation source can further include one or more additional conduits configured to provide fluid communication between the reservoir and additional locations within the radiation source. The conduit can be configured to reduce a velocity of fluid flowing through the conduit. Along a gravitational field, an outlet of the conduit can be positioned between an upper portion of the conduit and a lower portion of the conduit. The radiation source can further include a heater thermally connected to at least a part of the lower portion of the conduit. The radiation sourcecan further include a heater or heaters configured to heat, directly or indirectly, the whole of the conduit. The radiation source can further include a heater or heaters configured to heat the conduit first, or at a greater rate, at end portions thereof and last, or at a lesser rate, at a middle portion thereof.

[0014] The reservoir can be configured to collect both target material that is hit by a main pulse and target material that is missed by a main pulse. The exhaust can include an exhaust receiving chamber coupled to the vessel through an exhaust port. The exhaust receiving chamber can surround the vessel circumferentially. The conduit can provide fluid communication between the exhaust receiving chamber and the reservoir.

[0015] In additional general aspects, a radiation source includes a vessel coupled to an exhaust receiving chamber through an exhaust port extending through the vessel; a droplet generator; a droplet catch aligned with the droplet generator across the vessel; a reservoir connected to the droplet catch; and a conduit connecting from the exhaust receiving chamber to the droplet catch.

[0016] Implementations can include one or more of the following.

[0017] The conduit can extend in a J-shape configuration along a gravitational field. The radiation source can further include a heater thermally connected to the conduit. The exhaust receiving chamber can circumferentially surround the vessel. An inlet of the conduit and an outlet of the conduit can be circumferentially offset from each other as viewed along an optical axis of the radiation source. An inner diameter of the conduit can be greater than ten millimeters (mm).

[0018] In additional general aspects, a method of operating a radiation source includes receiving droplets of a target material from a droplet generator in a radiation source; holding the target material from the droplets in a reservoir; and directing additional target material from an exhaust in the radiation source to the reservoir through a conduit.

[0019] Implementations can include one or more of the following.

[0020] Directing the additional target material can include flowing the additional target material through a first part of the conduit along a first path having one or more components in the direction of gravity, then flowing the additional target material by a gravitationally -produced pressure differential through a second part of the conduit along a second path having one or more components opposite the direction of gravity. The method can further include maintaining a liquid level of the second target material within a lower portion of the conduit. The method can further include heating at least a part of the lower portion of the conduit.

[0021] In additional general aspects, a method of operating a radiation source includes flowing under gravity droplet material from a droplet catch in the radiation source to a reservoir and flowing under gravity droplet material from an exhaust in the radiation source to the reservoir.

[0022] Implementations can include one or more of the following.

[0023] The method can further include, while flowing, maintaining a pressure within the reservoir at or near a pressure within the radiation source. Flowing droplet material from the exhaust can include flowing the droplet material in a conduit having a conduit outlet gravitationally higher than a portionof the conduit.

[0024] In additional aspects, a method of operating a radiation source includes flowing under gravity droplet material from a droplet catch in the radiation source to a pump; flowing under gravity droplet material from an exhaust in the radiation source to the pump; and pumping the droplet material into a reservoir, the reservoir being at higher pressure than a pressure in the radiation source.

[0025] The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims.DRAWING DESCRIPTION

[0026] FIG. 1 is a schematic cross-sectional diagram of an EUV radiation source as part of a processing apparatus that uses EUV light.

[0027] FIGS. 2A and 2B are elevation-view diagrams of an EUV radiation source viewed from two differing perspectives.

[0028] FIG. 3 is a diagrammatic view of elements of another EUV radiation source taken along the optical axis thereof.

[0029] FIG. 4 is a diagrammatic view of some components of another EUV radiation source.

[0030] FIG. 5 is a diagrammatic cutaway view showing another radiation source.

[0031] FIG. 6 is a diagrammatic view of another EUV radiation source.

[0032] FIG. 7 is a diagrammatic of yet another EUV radiation source.

[0033] FIGS. 8 A and 8B are cross-sectional schematic diagrams illustrating an implementation of a process and an apparatus useful in moving liquid target material from a low pressure environment inside a vacuum chamber wall to an atmospheric pressure environment containing a reservoir for the target material.

[0034] FIGS. 9A, 9B, 9C, and 9D are cross-sectional schematic diagrams illustrating an implementation of another process and an apparatus useful in moving liquid target material from a low pressure environment inside a vacuum chamber wall to an atmospheric pressure environment containing a reservoir for the target material.

[0035] FIG. 10 is a diagrammatic view of some components of another EUV radiation source.

[0036] FIG. 11A is a diagrammatic view of a conduit and a cooperating droplet catch.

[0037] FIG. 1 IB is a diagrammatic view of an entrance aperture structure of a droplet catch.

[0038] FIG. 11C is a diagrammatic cross-section and side view of the entrance aperture structure of FIG. 1 IB, taken along the line shown in FIG. 1 IB.

[0039] FIG. 1 ID is a diagrammatic cross-section and side view of the entrance aperture structure of FIG. 1 IB, taken along the line shown in FIG. 1 IB.DETAILED DESCRIPTION

[0040] FIG. 1 schematically depicts, in cross-section, an extreme ultraviolet (EUV) radiation source 100 as part of a processing apparatus 102 that uses EUV light. Reference axes include a z-coordinate axis that extends along the direction of an optical axis A in FIG. 1, and x and y axes as shown. The force G due to gravity is in the direction shown by the arrow, downward in the plane of the page, but other orientations are used in other implementations. The EUV radiation source 100 of FIG. 1 is of a type that may be referred to as a laser produced plasma (LPP) source. The processing apparatus 102 of which the EUV radiation source 100 is a part can be a lithographic exposure apparatus or a metrology apparatus, for example. The EUV radiation source 100 includes a vessel 110 that is at least partially enclosed within a vacuum chamber 180 for establishing and maintaining a low-pressure environment within the vessel 110 during operation of the EUV radiation source 100.

[0041] The vessel 110 receives light pulses from a laser or laser system 112. The laser 112 can be a gas-medium laser such as a CO2 laser, a solid-state -medium laser, or other laser or laser system. The laser 112 is configured to deliver and deposit light energy into target material, such as tin (Sn) or tin- containing droplets (not shown in FIG. 1), at a plasma formation region 126. The laser 112 deposits energy into the target material (not shown) such as via one or more light pulses, for example, one or more pre-pulses 118, a rarefication pulse 119 (sometimes termed a rarefaction pulse), and a main pulse 121, in order to produce a plasma 124 that generates EUV radiation. A target in the form of a droplet that is hit by a main pulse is typically significantly ionized, producing plasma. Note that the particular laser and laser configuration is not intended to be limiting. In other implementations, for example, different numbers of lasers are used, such as three lasers, or two lasers, for example, to provide the various pulses. The light pulses ionize the target material, generating a form of highly energized plasma 124 that emits EUV radiation 129. The plasma formation region 126 coincides or essentially coincides with the primary focus of a collector mirror or collector 128 having an optical axis A. The collector 128 redirects the EUV radiation 129 or a significant part thereof to a secondary focus 132 at or after which the EUV radiation is received by an illumination system 134 of the processing apparatus 102.

[0042] A reservoir 160 positioned outside the vacuum chamber 180 receives used and unused target material from the radiation source 100, including both target material irradiated by the laser 112 and target material not irradiated by the laser 112, as will be explained in more detail below.

[0043] Although tin is referred to herein, any suitable material may be used for the target material 116. The target material may for example be in liquid form, such as in the form of a tin -containing droplet or stream. In general, the target material may be a material that, when in an ionized state, emits EUV radiation.

[0044] FIGS. 2A and 2B are schematic elevation-view diagrams showing aspects of an implementation of an EUV radiation source in the form of radiation source 200. The radiation source 200 can be used within a processing apparatus, such as processing apparatus 102 of FIG. 1 .

[0045] FIG. 2A shows a view of the radiation source 200 similar to the view of EUV radiation source 100 of FIG. 1, namely, a view looking along the x axis, with the y and z axes within or parallel to the plane of FIG. 2A, as indicated. The force G due to gravity is downward along the plane of FIG. 2A, as indicated.

[0046] FIG. 2B shows another view of the radiation source 200 of FIG. 2A, looking along the y axis, with the x and z axes lying in or parallel to the plane of the figure, as indicated. The force G due to gravity has a component (of approximate magnitude G cos

[0030] ) within the plane of FIG. 2B, and a component (of approximate magnitude G sin

[0030] ) outward from the plane of FIG. 2B toward the viewer, as indicated. This specific orientation is not intended to be limiting; other orientations are used in other implementations.

[0047] With reference to FIGS. 2A and 2B, the radiation source 200 includes a vessel 210 that is at least partially enclosed within a vacuum chamber 280 for establishing and maintaining a low-pressure environment within the vessel 210 during operation of the EUV radiation source 100. A droplet generator 240 (FIG. 2B) includes a nozzle 242 configured to direct target material droplets 216 along a trajectory within the vessel 210 toward a plasma formation region 226. Light pulses from a laser or laser system (not shown in FIGS. 2A and 2B, but see laser or laser system 112 of FIG. 1, for example) irradiate at least some of the droplets 116 at or near the plasma formation region 226, producing a plasma 224 that generates EUV radiation.

[0048] A collector 228 reflects and directs EUV light from the plasma 226, such as explained with respect to collector 128 ofFIG. 1. The vessel 210 at least partially surrounds the plasma formation region 126 and serves various functions, including but not limited to assisting in containing plasma and target material and any other byproducts of plasma production. In some implementations, the vessel 210 includes a plurality of sub -components with a frustum shape, collectively forming a substantially conical vessel 210. In some implementations, these components include, from the collector 228 upward in the direction of the z axis, a vessel wall 210A, one or more vessel walls 210B, 210C, and a cap 210D. In some implementations, each of vessel walls 210A-210C is a modular device or a liner.

[0049] During plasma production during operation of the radiation source 200, some target material can remain in the form of vapor, particles, and / or small liquid droplets, and ions in the plasma can recombine to form additional vapor or particles. Also during operation of the radiation source 200, some droplets are intentionally not used to produce plasma (intentionally missed by one or light pulses), and the light pulses aimed at droplets intended for use in making plasma may occasionally miss.

[0050] Accordingly, to mitigate contamination within the vessel 210, it is desirable or even necessary to collect vapor and droplets and any other particles that remain after plasma formation (after ionization of a droplet), as well as unused droplets 217, or droplets missed by one or more pulses, particularly droplet missed by a main pulse.

[0051] Unused droplets 217 and droplets missed by one or more pulses, particularly missed by a main pulse, are received by a droplet catch 250. The droplet catch 250 is connected to a reservoir 260 positioned outside the vacuum chamber 280. In the implementation shown, droplet catch 250 is connected to the reservoir 260 by a passage 252. Liquid target material from unused droplets in the droplet catch 250 flows to the reservoir 260 through the passage 252.

[0052] The gas, vapor, plasma remnants, and other byproducts of plasma production can be received (see arrows Al) by an exhaust receiving chamber 220 (shaded region) through an exhaust port 230 extending through the vessel 210 (or through a wall of the vessel 210). In some implementations, the exhaust receiving chamber 220 is coupled to the vessel 210. In the implementation shown, the exhaust receiving chamber 220 takes the form of a toroidal or circumferential structure circumferentially surrounding the vessel 210. An evacuation passage 220ev (visible in the view of FIG. 2A) is positioned opposite the exhaust port 230 and is connected to one or more vacuum pumps (not shown) to transport exhaust away from the exhaust receiving chamber 220 (see arrows A2). In some implementations, the exhaust receiving chamber 220, the exhaust port 230, and the evacuation passage 220ev, together with their associated exhaust components, are collectively referred to as an exhaust of the radiation source 200.

[0053] A conduit 270 provides a fluid connection from the exhaust receiving chamber 220 to the reservoir 260, allowing liquid target material (and any other remnants of plasma production collected in the exhaust receiving chamber 220) to be received in the same reservoir as the liquid target material from unused droplets 217. This allows removal of spent target material (such as tin) from the radiation source (both in the form of unused droplets or remains from plasma generation) from a single location or reservoir. As depicted in FIGS. 2A and 2B, the conduit 270 is arranged between the exhaust port 230 and the collector 228. In some implementations, the conduit 270 extends from an inlet 272 at the exhaust receiving chamber 220 to an outlet 274. In some implementations, when the radiation source 200 excludes the exhaust receiving chamber 220, the conduit 270 extends from the inlet at the exhaust port 230 to an outlet 274. In the implementation shown, the outlet 274 opens into the droplet catch 250. In other implementations, the outlet of the conduit 270 is in other locations in the droplet catch 250, such as locations 274a, an inner rim of an opening of the droplet catch 250 and 274b, a deeper position extending to a volume of the droplet catch 250. In yet other implementations, the outlet of the conduit opens into the passage 252, such as at locations 274c and 274d. In still other implementations, the outlet of the conduit 270 opens directly into the reservoir 260, such as at location 274e.

[0054] Performance of the plasma production process and / or other processes can be assessed and / or monitored during and / or before and after operation of the EUV radiation source 100 by various sensor and / or detector modules, such as sensor-detector modules S-D shown in FIG. 2A. The sensor / detector modules are used to assist in monitoring and controlling the production of EUV light, and can include light detection and / or imaging devices, sources of radiation useful for detection and measurement, droplet detection and localization modules, and the like.

[0055] FIG. 3 is a diagrammatic view showing some elements of an implementation of an EUV radiation source 300. The view of FIG. 3 is taken along the z axis (that is, in the direction of the optical axis of a collector 328), with positive y leftward and positive x downward in the plane of the figure, as indicated. In this implementation and view, the force G due to gravity has a component of magnitude G sin(30) to the right in the plane of the figure, and a larger component G cos(30) out of the page toward the viewer. The back side of the collector 328 is visible in the center of the figure. An opening 328h in the collector allows light pulses to pass through to a plasma formation region (not depicted, but see, for example, FIGS. 1 and 2B). A vessel 310 (or the inner portions thereof that are depicted) is mostly obscured by the collector 328. A droplet generator 340 is aligned with a droplet catch 350 across the vessel 310. The droplet catch 350 is partially obscured by a reservoir 360, with the obscured portion shown in dashed lines. The droplet catch 350 is connected to the reservoir 360 by a passage 352 (obscured by the reservoir 360 but shown in dashed lines). An exhaust receiving chamber similar to the exhaust receiving chamber 220 of FIGS. 2A-2B is also present in this implementation but obscured by the collector 328 and the vessel 310.

[0056] A conduit 370 extends from the (obscured) exhaust receiving chamber to the droplet catch 350, beginning at an inlet 372 (obscured, but shown in dashes lines) terminating in an outlet 374. In some implementations such as the one shown here, the conduit 370 is coupled to the reservoir 360 through the droplet catch 350. In some other implementations, the conduit 370 is directly connected to the reservoir 360, or to the passage 352. In some implementations such as the one shown, the inlet 372 and the outlet 374 are arranged circumferentially offset as viewed along the optical axis A (the positive z axis of the figure) of the EUV radiation source 300 (or of the collector 328 thereof). The conduit 370 can be considered to include a descending portion 370de running principally in the direction of gravity, and a circumferential portion 370re running principally in a circumferential direction around the axis of the vessel 310 (or of the collector 328) and principally parallel to the plane of the figure. In some implementations, the circumferential portion 370re can be spaced apart from the vessel 310, such as by a radial distance DA 1. Regions at or near the exterior of the vessel 310 tend to be highly valued for positioning of sensing and metrology equipment, much of which can require a line of sight to the plasma formation region or to regions nearby. Spacing of the circumferential portion 370re of the conduit 370 from the vessel 410 helps provide room for sensor / detector modules such as sensor / detector modules S-D shown in FIG. 2A and / or other such equipment.

[0057] FIG. 4 shows a vessel 410 and some associated components of an EUV radiation source 400. The collector is omitted in FIG. 4 for clarity. In the implementation of FIG. 4, an exhaust receiving chamber 420 is connected to the interior of the vessel through an exhaust port 430 to receive exhaust from the vessel 410 (arrows A3). The exhaust receiving chamber 420 is relatively smaller and does not surround the vessel 410 circumferentially. This illustrates that the present disclosure can be applied with non-circumferential exhaust receiving chamber if desired. An evacuation passage 420evreceives (or “pulls”) exhaust from the receiving chamber 420 (arrows A4). In the implementation of FIG. 4 and similar implementations, a conduit 470 extends from the exhaust receiving chamber 420 generally (but not necessarily everywhere) in a gravitationally downward direction. Specifically, the outlet 474 of the conduit 470 is positioned along the gravitational field between an upper portion UP of the conduit 470 (above the dashed line U of FIG. 4 at an upper edge of the outlet 474) and a lower portion LP of the conduit 470 (below the dashed line L at a lower edge of the outlet 474 of FIG. 4). In addition or alternatively, the conduit 470 is configured such that: (1) target material from the exhaust or exhaust receiving chamber 420 flows through a first part of the conduit 470 (prior to a dividing line DL at the lowest point of the conduit 470) along a first path having one or more components in the direction of gravity, then (2) the target material flows by a gravitationally-produced pressure differential through a second part of the conduit 470 (after the dividing line DL) along a second path having one or more components opposite the direction of gravity. In some implementations, the conduit 470 extends in a J-shaped (or equivalently, in a left-right reversed J-shaped) configuration along the gravitational direction. In comparison with other approaches, the J-shaped configuration helps to reduce a velocity of fluid flowing through the conduit 470, reducing molten target material ejecting and contaminating an interior of the vessel 410. This can include maintaining a liquid level of the target material within a lower portion of the conduit (the bottom of the curve of the “J”-shape or the lower portion LP of the conduit 470) to reduce a velocity of fluid flowing through the conduit.

[0058] In the implementation of FIG. 4 and similar implementations, at least a portion of the conduit 470 is spaced apart from the vessel 410, such as by a distance DA2. Regions at or near the exterior of the vessel 410 tend to be highly valued for positioning of sensing and metrology, especially for sensing and metrology requiring a line of sight to the plasma formation region. Spacing of the conduit 470 from the vessel 410 helps provide room for sensor / detector modules such as sensor / detector modules S-D shown in FIG. 2A and / or other such equipment. In some implementations, an inner diameter of the conduit 470 is equal to or greater than ten millimeters (mm). If the inner diameter is less than 10 mm, in some instances, capillary forces increase, resulting in clogs within the conduit.

[0059] Optionally, an EUV radiation source such as radiation source 400 of FIG. 4 includes a heater H configured to be thermally connected with the conduit 470, or at least with a part of the lower portion LP of the conduit 40. A resistance heater is represented in FIG. 4, but any suitable type of heater may be used. The heater H can be operated to ensure that target material such as liquid tin collected within the conduit 470 remains in a liquid state, allowing the material in the conduit to flow. In some implementations, the second part of the conduit 470 is straight or curved. In some implementations, the second part of the conduit 470 is angled, with an end proximal to the droplet catch 450 positioned upward or angled upward.

[0060] FIG. 5 is a schematically illustrated cutaway showing some elements of a radiation source 500. A vessel 510 is shown in diagrammatic perspective with its interior surface 510s exposed. A circumferential exhaust port 530c extends through the vessel 510 (or through the wall thereof) andconnects to a circumferential exhaust receiving chamber 520, the inside 520i of which can be seen in the cutaway view of the figure. The circumferential exhaust port 530c is screened on the interior surface 510s of the vessel 510, as indicated by the cross-hatching in FIG. 5. This illustrates that the present disclosure can be applied with a circumferential exhaust port 530c, in which the exhaust port 530c extends around more than 1 / 4 of a circumference of the vessel such as vessel 510, or even around the whole circumference of the vessel, if desired. Alternatively, the exhaust port, as in aspects described previously, may extend circumferentially 1 / 4 of the circumference of the vessel or less.

[0061] A conduit 570 extends from the exhaust receiving chamber 520 (beginning at an inlet 572) to a reservoir 560. In some implementations, the conduit 570 is coupled to the reservoir 560 through a droplet catch 550. In some implementations, as in the implementation of FIG. 5, the conduit 570 is directly connected to the reservoir 560.

[0062] FIG. 6 is an elevation view like that of FIG. 2A, of a radiation source 600 in which additional conduits are employed. Specifically, a conduit 670, an additional conduit 670a, and a second additional conduit 670b are used. The conduit 670 receives target material such as liquid tin and other byproducts of EUV production from the exhaust receiving chamber 620 and delivers to the droplet catch 650. In this implementation, the conduit 670 does not include a portion of the conduit gravitationally lower than the outlet 674 of the conduit. A first additional conduit 670a receives target material such as liquid tin and other byproducts, if any, from another part of the exhaust or exhaust system, specifically from an evacuation passage 620ev. The first additional conduit 670a includes a portion of the conduit that is gravitationally lower than an outlet 674a thereof, and the outlet 674a opens into a passage 652 between a droplet catch 650 and a reservoir 660. The second additional conduit 670b is used to transport liquid target material from a selected location in the interior of the vessel 610 at which target material can tend to collect, if any. The second additional conduit 670b delivers any target material received into the droplet catch 650. This illustrates that, in various implementations, multiple conduits can be used in the same radiation source, and that they can receive from different locations and deliver to different locations within the radiation source as needed or desired, though all the deliveries end up in the same single reservoir 660. Also illustrated is that some conduits can have lower parts lower than their outlets, and others can have no such lower parts, and both types of conduits can cooperate or coexist within a radiation source.

[0063] FIG. 7 is an elevation view like that of FIG. 2B, of a radiation source 700, in which additional inlets are connected to a single conduit 770. The conduit 770 includes an inlet 772 similar to inlet 272 of FIG. 2B, but further includes a first additional inlet 772ia and a second additional inlet 772ib. Additional inlets 772ia and 772ib are joined to the conduit 770 via branch conduits in the implementation shown. In other implementations, one or more additional inlets are formed directly through the sides or walls of the main conduit. In the implementation shown, both of the additional inlets, along with the inlet 772, receive target material such as liquid tin and other byproducts from an exhaust receiving chamber 720. In other implementations, the additional inlets receive target materialsuch as liquid tin from locations within the radiation source 700 other than an exhaust receiving chamber. Each of the inlets leads through the conduit 770 to the outlet 774 and eventually to the reservoir 760. This illustrates that, in various implementations, multiple inlets can be used with the same conduit.

[0064] FIGS. 8A and 8B illustrate, in the form cross-sectional schematic diagrams, one implementation of a process and apparatus useful in moving liquid target material such as liquid tin from a low pressure environment LP inside a vacuum chamber wall 880w to an atmospheric pressure environment AP in which a reservoir 860 resides and is periodically emptied. In the implementation shown, liquid target material, such as liquid tin (and any byproducts of EUV production included therewith) is first delivered through an outlet 874 of a conduit 870 to a pump. In the implementation shown, a gravity-primed piston pump 881 is employed, having a vertically actuating piston 882 and a side inlet formed by the outlet 874. A bottom outlet is formed by a one-way valve V1W.

[0065] With the piston 882 in the up position as in FIG. 8 A, a charge of liquid target material such as liquid tin (and byproducts) LTc fills the cylinder of the pump 881 by gravity flow from the conduit 870 and the outlet 874. Once the cylinder of the pump 881 is sufficiently full, the piston 882 is actuated, moving downward within the cylinder of the pump 881 and sealing off the outlet 874, preventing back flow. The charge of liquid target material LTc is then pressurized by the downward force of the piston 882 sufficiently to open the one-way valve V1W against a pressure difference between the low pressure environment LP within the vacuum chamber 880 and atmospheric pressure environment AP outside the vacuum chamber 880, causing the charge of liquid target material LTc to flow into the reservoir 860 and join (and raise the level of) the liquid target material LT in the reservoir 860. In other implementations other types of pumps are used in similar fashion to move gravity-collected liquid target material from the lower pressure environment LP of the vacuum chamber into the atmospheric pressure environment AP of the reservoir 860. This and similar implementations thus provide for the removal from the reservoir of the collected target material without depressurizing the vacuum chamber and / or without halting the generation of radiation and the operation of associated production or inspection processes.

[0066] FIGS. 9A through 9D illustrate, in the form cross-sectional schematic diagrams, one implementation of another process and apparatus useful in moving liquid target material such as liquid tin from a low pressure LP inside a vacuum chamber wall 980w to an atmospheric pressure AP in which a reservoir 960 resides. As will be understood from the following, in the process or method of FIGS. 9A-9D, the reservoir 960 is a pressure vessel capable of cycling between a low pressure LP of an environment inside the vacuum chamber wall 980w and an atmospheric pressure AP of an environment outside the vacuum chamber wall 980w.

[0067] Figure 9A is a schematic cross-sectional diagram illustrating an implementation of the operation of the reservoir 960 as the reservoir 960 is filling. A first valve VI between a passage 952 conveying liquid target material such as liquid tin (and plasma production byproducts) is open,allowing liquid target material (and plasma production byproducts) LT to flow from the passage 952 into the reservoir 960 and begin to fill the reservoir. During the filling of the reservoir 960, the reservoir interior is at low pressure LP, allowing the liquid target material to flow freely from the passage 952.

[0068] FIG. 9B represents the operation of the reservoir when the reservoir is sufficiently full. The first valve VI is closed and a second valve V2 is opened between the remaining gas in the reservoir and the external atmospheric pressure AP to allow atmospheric pressure to bleed into the reservoir 960 and bring the reservoir from low pressure LP to atmospheric pressure AP. Once atmospheric pressure is reached within the reservoir 960, the reservoir is detached at the couplers CPI and CP2 and emptied (and cleaned as needed).

[0069] FIG. 9B represents the operation of the reservoir 960 after emptying (and cleaning as needed). The reservoir 960 is reattached at couplers CPI and CP2, the first valve VI and the second valve V2 are in closed position, and a third valve V3 is opened between the reservoir 960 and a source of vacuum or low pressure LP. This state is maintained as the interior of the reservoir 960 goes from atmospheric pressure AP to low pressure LP.

[0070] Then, as shown in FIG. 9D, the third valve is closed and the first valve is reopened, ready again to receive liquid target material (and byproducts) from the passage 952. In other implementations, other hardware and other steps are used to cycle the interior pressure of the reservoir 960. But in every case, providing low pressure in the reservoir during filling allows the reservoir to fill without significant mechanical or electromechanical operations and essentially only under the influence of gravity, resulting in low impact to the mechanical and electrical environment of the associated radiation source. This implementation and similar implementations also provide for the removal from the reservoir of the collected target material without depressurizing the vacuum chamber and / or without halting the generation of radiation and the operation of associated production or inspection processes.

[0071] As mentioned above, the orientation relative to gravity as shown in the figures of the various aspects and implementations described herein is not intended to be limiting. Other orientations are useful in other implementations. For instance, FIG. 10 is a diagrammatic view of some components of an EUV radiation source 1000 in which a droplet catch 1050 is positioned gravitationally above an exhaust (or portion thereof) 1020, and gravitationally above an associated inlet 1072 of a conduit 1070 from the exhaust 1020 to a reservoir 1060. Exhaust gases pass through the exhaust 1020 as indicated by the arrows, left-to-right in FIG. 10. Target material from the exhaust flows downward through the conduit 1070 into a reservoir 1060. A passage 1052 from the droplet catch 1050 leads from the droplet catch to an outlet 1052o in the conduit 1070, alternatively to an outlet 1052oa in the reservoir 1060. An alternative implementation of the passage 1052 includes a portion 1052a that extends gravitationally below the associated outlet 1052oa. A passage 1052 with no portion thatextends below the outlet 1052oa can of course be used with the outlet 1052oa, just as a passage with a portion that extends below outlet 1052o can extend below outlet 1052o, as needed or desired.

[0072] FIG. 11A is a diagrammatic view of another implementation of a conduit and a cooperating droplet catch, with the view taken along the positive z-axis as shown by the coordinates in the figure, as in FIG. 3 discussed above. In some implementations, components of gravity can be toward the viewer of the figure, and slightly toward the right, as indicated.

[0073] With reference to FIG. 11A, a conduit 1170 includes a descending portion 1170de running principally in the direction of gravity (indicated by the arrow G), and a circumferential portion 1170re running principally in a circumferential direction around the axis of an associated vessel (not shown). The conduit 1170 can be one single component or can be comprised of conduit segments 1171. The multiple conduit segments 1171 can facilitate installation of the conduit 1170 and removal for repair or replacement. The segments can be curved as shown in the figure or straight with angled joints, for example.

[0074] The conduit 1170 can include or be in thermal contact with a heater H. The heater H can extend along all or almost all of the conduit 1170, such that all of the conduit 1170 can be heated directly or indirectly (such as through some thermal conduction along the conduit) and kept above a desired temperature by the heater H. A desired temperature can be, for example, a temperature at or above the liquidus temperature of tin, e.g., approximately 232° C. The heater H can be arranged to heat from ends 1173 of the conduit 1170 inward toward a central portion 1175 of the conduit 1170. For example, the heater H may include multiple coil heater units in series, with heater units at or near the ends of the conduit 1170 having a higher density of coils per distance along the conduit 1170 than the heater units at or near the central portion of the conduit 1170. Alternatively, one long heating coil can be used, having higher coil density at the ends of the conduit 1170 and lower in the middle. As a further alternative, multiple heater units can be controlled individually to heat in the same manner, namely, to heat the ends 1173 of the conduit 1170 first, progressing gradually to the middle 1175.

[0075] With further reference to FIG. 11A, a droplet catch 1150 includes a catch chamber 1151 defined by a catch chamber wall having an interior surface indicated by the dashed line 1154. The catch chamber 1151 is connected to and closed off or partially closed off by an entrance aperture structure 1162. The entrance aperture structure 1162 includes a flange 1153 for mounting the droplet catch 1150, such as by the use of flange through -holes 1155 (shown in dashed outline). The entrance aperture structure 1162 also includes a conical entrance aperture 1159 (shown in dashed outline) through which droplets not used for plasma production, arriving at the droplet catch 1150 generally in a direction along the arrow 1156, pass into the catch chamber 1151 of the droplet catch 1150. In some implementations, a passage 1152 (of which only an outlet 1176 is shown in FIG. 11A) drains liquid target material such as liquid tin (and any byproducts) collected in the catch chamber 1151 into a reservoir (not shown). The entrance aperture structure 1162 also includes an extended portion 1163 extending radially beyond the through-holes 1155 of the flange 1153. The extended portion forms orcontains a receiving chamber 1157 configured and positioned to receive target material from the conduit 1170. A receiving chamber drain passage not shown in FIG. 11A delivers target material from the receiving chamber 1157 into a portion of the catch chamber 1151 to be drained together with target material from unused droplets through the passage 1152.

[0076] FIG. 1 IB is a diagrammatic view of an entrance aperture structure 1162 of a droplet catch such as the droplet catch 1150 of FIG. 11A, together with a portion of the conduit 1170, taken in the negative x-axis direction. In some implementations, gravity (G) is downward in the plane of the figure as shown (in the same relation to the x-y-z axis as in FIG. 11A).

[0077] With reference to FIG. 1 IB, the entrance aperture structure 1162 is shown viewed in the direction of travel of droplets arriving at the droplet catch 1150 of FIG. 1 IB. In this view multiple mounting holes of the flange can be seen around the perimeter of the entrance aperture structure 1162. The flange 1153 surrounds a cylindrical rim 1166 in which is positioned a conical entrance aperture structure 1159. The conical entrance aperture structure 1159 has a front surface 1159f that slopes inward toward a central aperture 1168 and in the negative x-axis direction as indicated by arrows 1177. Droplets that miss passing directly through the central aperture 1168, if any, can be redirected therethrough by the surface 1159f of the conical entrance aperture structure 1159. The molten or liquid target material from droplets received into the droplet catch through the central aperture 1168 is drained from the droplet catch through a droplet catch drain passage 1152, shown in dashed outline, in the direction shown by the dashed arrow therein. The passage 1152 extends from an inlet 1169 in the back surface of the conical entrance aperture structure 1159 (not visible in the figure) to an outlet 1176, facing downward in the figure, through the rim 1166.

[0078] The extended portion 1163 of the entrance aperture structure 1162 extends radially beyond the through-holes 1155 of the flange 1153. The extended portion can also extend forward from the flange 1153, in the positive x-axis direction in the figure, and can include access holes 1167 by which the through-holes and bolts or screws therein can be reached. The extended portion includes a receiving chamber 1157 that receives target material from the conduit 1170. The conduit 1170 passes, in part, in a general direction around a circumference of a vessel (not shown) (such as the vessel 110 of FIG. 1, 210 of FIG. 2, or 310 of FIG. 3, for example) as indicated by the arrow 1158a. The conduit 1170 also passes below the central aperture 1168, out of the path of any droplets traveling toward the droplet catch, and then upward in a direction at least partly opposite to gravity, as indicated by the arrow 1158b, to the receiving chamber 1157. The last section of the conduit 1170 as it reaches the entrance aperture structure 1162 is not shown in FIG. 1 IB so that details of the extended portion 1163 and receiving chamber 1157 can be seen.

[0079] The receiving chamber 1157 has an opening 1161 larger than a circumference or a cross section of the conduit 1170. The opening 1161 can be oblong or slot-shaped. In some implementations, a long dimension of the oblong or slot-shaped opening 1161 is aligned with gravity. A gravitationally downward portion of the receiving chamber 1157 can extend below a bottom of theopening 1161. The opening 1161 being larger than a circumference or cross section of the conduit 1170 provides a low or zero stress fluid coupling between the conduit 1170 and the droplet catch, and, together with a long dimension of the opening 1161, provides room for relative motion between the droplet catch and the conduit that can allow the droplet catch to be removed for maintenance or replacement without disturbing or removing the conduit 1170.

[0080] The target material received in the receiving chamber 1157 is drained through a receiving chamber drain passage 1165, shown in dashed outline, with the molten or liquid target material moving in the general direction of the dashed arrows shown therein. The receiving chamber drain passage 1165 passes from the bottom of the receiving chamber 1157 through a part of the extended portion 1163 of the entrance aperture structure 1162, and into and through a portion of the conical entrance aperture structure 1159, to an outlet 1179 in the back surface (not visible) of the conical entrance aperture 1159. The target material collected from the outlet 1179 (e.g., target material from an exhaust chain) joins with other target material, if any, received from droplets captured in the droplet catch, and drains through the droplet catch drain passage 1152 to a reservoir not shown.

[0081] FIG. 11C is a diagrammatic cross-section and side view of the droplet catch entrance aperture structure 1162 of FIG. 1 IB, taken along the line shown in FIG. 1 IB.

[0082] With reference to FIG. 11C, the entrance aperture structure 1162 is shown with a cross section taken through the receiving chamber 1157, showing an internal surface 1157s of the receiving chamber 1157. The flange 1153 and the rim 1166 are visible behind the plane of the cross section, as well as a portion of an outer surface of the extended portion 1163. In some implementations, the conduit 1170, shown in cross section, extends into the receiving chamber 1157 without contacting any surface of the receiving chamber or of the opening 1161 (FIG. 1 IB) into the receiving chamber. The end 1178 of the conduit nearest the droplet catch can be a closed end 1178. The conduit 1170 can have side wall opening, such as an opening 1174 through a downward portion of a side wall of the conduit 1170, to deliver molten or liquid target material into the receiving chamber 1157. The combination of closed end 1178 and side wall opening 1174 can help reduce lateral momentum of molten or liquid target material entering the receiving chamber 1157, reducing or preventing splashing.

[0083] The molten or liquid target material received in the receiving chamber 1157 is drained through the receiving chamber drain passage 1165, shown in dashed outline. Starting at the bottom of the receiving chamber 1157 in the direction shown by the arrow 1165a, the molten or liquid target material flows downward through the receiving chamber drain passage 1165 and then out into the inside of the droplet catch through the outlet 1179 . The molten or liquid target material from inside the droplet catch drains through the droplet catch drain passage 1152, from an inlet 1169 to an outlet 1176.

[0084] FIG. 1 ID is a diagrammatic cross-section and side view of the droplet catch entrance aperture structure 1162 of FIG. 1 IB, taken along the line shown in FIG. 1 IB. A portion of the catch chamber1151 defined by a catch chamber wall 115 Iw is also shown. The flange 1153 and rim 1166 of the catch entrance aperture structure are visible in cross section, along with the conical entrance aperture 1159. A portion of the front surface 1159f of the conical entrance aperture 1159 is visible. A back surface of the conical entrance aperture is flat or otherwise shaped to as to retain in the catch chamber 1151 tin droplets or portions thereof, if any, that may splash on it. A portion of the inside of the droplet catch drain passage 1152 is also visible, extending from the inlet 1169 in the back surface 1159b of the conical entrance aperture 1159 to an outlet 1176 in the rim 1166. The molten or liquid target material that collects in the catch chamber 1151, whether from droplets or from the receiving chamber 1157 (FIGS. 1 IB, 11C), remains below the opening in the conical entrance aperture 1159 and is drained from the region 1133 by the drain passage 1152.

[0085] In addition to the foregoing, advantages produced by aspects of this disclosure include reduced down time in that using apparatuses and / or methods disclosed herein allows removing and / or cleaning both (1) target material such as tin used to make droplets and then plasma and (2) target material such as tin used to make droplets only, or tin from droplets missed or not hit by a main pulse, by accessing only a single location at or in a radiation source. Further, removing used target material such as tin by apparatuses and / or methods disclosed herein can be performed without depressurizing the vacuum chamber and / or during use of the radiation source, further reducing downtime.

[0086] Aspects and implementations of the present disclosure can be further described using the following numbered clauses:1. A radiation source including: a vessel coupled to an exhaust; a droplet generator aligned with a droplet catch; a reservoir coupled to the droplet catch; and a conduit providing fluid communication between the exhaust and the reservoir.2. The radiation source of clause 1, further including a vacuum chamber enveloping the vessel, wherein the reservoir is arranged outside the vacuum chamber.3. The radiation source of clause 2, wherein the conduit is disposed between the vessel and the vacuum chamber.4. The radiation source of clause 1, wherein the conduit provides fluid communication between the exhaust and the reservoir through the droplet catch.5. The radiation source of clause 4, wherein the conduit provides fluid communication between the exhaust and the droplet catch through a passage within the body of an entrance aperture structure of the droplet catch.6. The radiation source of clause 5, wherein an end of the conduit proximal to the droplet catch includes a closed end wall and a side wall with an opening therethrough.7. The radiation source of clause 5, wherein an end of the conduit proximal to the droplet catch extends through an opening into a chamber within the body of the entrance aperture structure of the droplet catch and the passage within the body of the aperture extends from the chamber to an outlet positioned on an inside surface of the aperture outside of a path of travel of droplets through theaperture.8. The radiation source of clause 7, wherein the opening is larger than an outer cross section of the conduit at the opening.9. The radiation source of clause 7, wherein the chamber within the body of the entrance aperture structure of the droplet catch is positioned radially outward from an attachment flange of the entrance aperture structure.10. The radiation source of clause 5, wherein the conduit includes conduit segments.11. The radiation source of clause 1, wherein the conduit includes conduit segments.12. The radiation source of clause 1, further including a passage coupling the reservoir to the droplet catch and wherein the conduit provides fluid communication between the exhaust and the passage.13. The radiation source of clause 1, wherein the conduit extends from the exhaust to the reservoir.14. The radiation source of clause 1, wherein the conduit partially encircles the vessel.15. The radiation source of clause 1, wherein the conduit includes multiple inlets.16. The radiation source of clause 1, further including one or more additional conduits configured to provide fluid communication between the reservoir and additional locations within the radiation source.17. The radiation source of clause 1, wherein the conduit is configured to reduce a velocity of fluid flowing through the conduit.18. The radiation source of clause 1, wherein, along a gravitational field, an outlet of the conduit is positioned between an upper portion of the conduit and a lower portion of the conduit.19. The radiation source of clause 18, further including a heater thermally connected to at least a part of the lower portion of the conduit.20. The radiation source of clause 18, further including a heater or heaters configured to heat, directly or indirectly, the whole of the conduit.21. The radiation source of clause 18, further including a heater or heaters configured to heat the conduit first or at a greater rate at end portions thereof and last or at a lesser rate at a middle portion thereof.22. The radiation source of clause 1, wherein the reservoir is configured to collect both target material that is hit by a main pulse and target material that is missed by a main pulse.23. The radiation source of clause 1, wherein the exhaust includes an exhaust receiving chamber coupled to the vessel through an exhaust port.24. The radiation source of clause 23, wherein the exhaust receiving chamber surrounds the vessel circumferentially.25. The radiation source of clause 24, wherein the conduit provides fluid communication between the exhaust receiving chamber and the reservoir.26. A radiation source including: a vessel coupled to an exhaust receiving chamber through an exhaust port extending through the vessel; a droplet generator; a droplet catch aligned with the droplet generator across the vessel; a reservoir connected to the droplet catch; and a conduit connecting from the exhaust receiving chamber to the droplet catch.27. The radiation source of clause 26, wherein the conduit extends in a J-shape configuration along a gravitational field.28. The radiation source of clause 26, further including a heater thermally connected to the conduit.29. The radiation source of clause 26, wherein the exhaust receiving chamber circumferentially surrounds the vessel.30. The radiation source of clause 26, wherein an inlet of the conduit and an outlet of the conduit are arranged circumferentially offset as viewed along an optical axis of the radiation source.31. The radiation source of clause 26, wherein an inner diameter of the conduit is greater than ten millimeters (mm).32. A method of operating a radiation source, the method including: receiving droplets of a target material from a droplet generator in a radiation source; holding the target material from the droplets in a reservoir; and directing additional target material from an exhaust in the radiation source to the reservoir through a conduit.33. The method of clause 32, wherein directing the additional target material includes flowing the additional target material through a first part of the conduit along a first path having one or more components in the direction of gravity, then flowing the additional target material by a gravitationally- produced pressure differential through a second part of the conduit along a second path having one or more components opposite the direction of gravity.34. The method of clause 32, further including maintaining a liquid level of the second target material within a lower portion of the conduit.35. The method of clause 34, further including heating at least a part of the lower portion of the conduit.36. A method of operating a radiation source, the method including: flowing under gravity droplet material from a droplet catch in a radiation source to a reservoir; and flowing under gravity droplet material from an exhaust in the radiation source to the reservoir.37. The method of clause 36, further including, while flowing, maintaining a pressure within the reservoir at or near a pressure within the radiation source.38. The method of clause 36, wherein flowing droplet material from the exhaust includes flowing the droplet material in a conduit having a conduit outlet gravitationally higher than a portion of the conduit.39. A method of operating a radiation source, the method including: flowing under gravity droplet material from a droplet catch in the radiation source to a pump; flowing under gravity dropletmaterial from an exhaust in the radiation source to the pump; and pumping the droplet material into a reservoir, the reservoir being at higher pressure than a pressure in the radiation source.

[0087] The above-described aspects and implementations and other implementations are within the scope of the following claims.

Claims

CLAIMS1. A radiation source comprising: a vessel coupled to an exhaust; a droplet generator aligned with a droplet catch; a reservoir coupled to the droplet catch; and a conduit providing fluid communication between the exhaust and the reservoir.

2. The radiation source of claim 1, further comprising a vacuum chamber enveloping the vessel, wherein the reservoir is arranged outside the vacuum chamber.

3. The radiation source of claim 2, wherein the conduit is disposed between the vessel and the vacuum chamber.

4. The radiation source of claim 1, wherein the conduit provides fluid communication between the exhaust and the reservoir through the droplet catch.

5. The radiation source of claim 4, wherein the conduit provides fluid communication between the exhaust and the droplet catch through a passage within the body of an entrance aperture structure of the droplet catch.

6. The radiation source of claim 5, wherein the conduit comprises conduit segments.

7. The radiation source of claim 1, further comprising a passage coupling the reservoir to the droplet catch and wherein the conduit provides fluid communication between the exhaust and the passage.

8. The radiation source of claim 1, wherein the conduit extends from the exhaust to the reservoir.

9. The radiation source of claim 1, wherein the conduit partially encircles the vessel.

10. The radiation source of claim 1, further comprising one or more additional conduits configured to provide fluid communication between the reservoir and additional locations within the radiation source.

11. The radiation source of claim 1, wherein, along a gravitational field, an outlet of the conduit ispositioned between an upper portion of the conduit and a lower portion of the conduit.

12. The radiation source of claim 11, further comprising a heater thermally connected to at least a part of the lower portion of the conduit.

13. The radiation source of claim 11, further comprising a heater or heaters configured to heat the conduit first or at a greater rate at end portions thereof and last or at a lesser rate at a middle portion thereof.

14. A method of operating a radiation source, the method comprising: receiving droplets of a target material from a droplet generator in a radiation source; holding the target material from the droplets in a reservoir; and directing additional target material from an exhaust in the radiation source to the reservoir through a conduit.

15. The method of claim 14, wherein directing the additional target material comprises flowing the additional target material through a first part of the conduit along a first path having one or more components in the direction of gravity, then flowing the additional target material by a gravitationally- produced pressure differential through a second part of the conduit along a second path having one or more components opposite the direction of gravity.

16. The method of claim 14, further comprising maintaining a liquid level of the second target material within a lower portion of the conduit.

17. The method of claim 16, further comprising heating at least a part of the lower portion of the conduit.

18. A method of operating a radiation source, the method comprising: flowing under gravity droplet material from a droplet catch in a radiation source to a reservoir; and flowing under gravity droplet material from an exhaust in the radiation source to the reservoir.

19. The method of claim 18, further comprising, while flowing, maintaining a pressure within the reservoir at or near a pressure within the radiation source.

20. The method of claim 18, wherein flowing droplet material from the exhaust comprises flowing the droplet material in a conduit having a conduit outlet gravitationally higher than a portionof the conduit.

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