Radiation source with heat transfer

Abstract

A radiation source includes a vessel, a frame configured at an exterior of the vessel, a plenum structure inside the frame, and a thermally-conductive element between the frame and the plenum structure. The thermally-conductive element defines a first gap maintained between the plenum structure and the thermally-conductive element and a second gap maintained between the thermally-conductive element and the frame.

Description

RADIATION SOURCE WITH HEAT TRANSFERCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US Application No. 63 / 672,225, filed on July 16, 2024, titled RADIATION SOURCE WITH HEAT TRANSFER, and US Application No. 63 / 830,656, filed on June 26, 2025, titled RADIATION SOURCE WITH HEAT TRANSFER, which are incorporated herein by reference in their entirety.TECHNICAL FIELD

[0002] The present disclosure relates to apparatuses and methods for cooling an assembly exposed to heated particles in an extreme ultraviolet (“EUV”) light source during the generation of EUV radiation from plasma.BACKGROUND

[0003] Extreme ultraviolet radiation, for example, electromagnetic radiation having wavelengths of around 50 nm or less (also sometimes referred to as soft x-rays), including radiation at a wavelength of about 13.5 nm, can be used in photolithography processes to produce extremely small features in or on substrates such as silicon wafers. Methods for generating EUV radiation include converting a target material to a plasma state. The target material includes at least one element, for example, xenon, lithium or tin, with one or more emission lines in the EUV portion of the electromagnetic spectrum. The target material can be solid, liquid, or gas. In one such method, often termed laser produced plasma (“LPP”), the required plasma can be produced by using a “source” laser, typically a CO2 laser emitting infrared light at a wavelength at or about 10,600 nanometers (nm), to irradiate with one or more light pulses a target containing one or more EUV line-emitting elements. The plasma is typically produced in a sealed “source vessel” which is typically within a vacuum chamber.SUMMARY

[0004] In some general aspects, a radiation source includes: a vessel; a frame configured at an exterior of the vessel; a plenum structure inside the frame; and a thermally-conductive element between the frame and the plenum structure. The thermally-conductive element defines a first gap maintained between the plenum structure and the thermally-conductive element, and a second gap maintained between the thermally-conductive element and the frame.

[0005] Implementations can include one or more of the following features. For example, The plenum structure can be a part of a debris handling apparatus exterior to the vessel. The plenum structure can define a plenum that is coupled to an exhaust opening defined in a wall of the vessel, the exhaust opening enabling fluid communication between an interior of the vessel and a debris capture assemblyof the debris handling apparatus. The plenum structure can include a hollow portion that has a substantially annular shape and extends around an outer circumference of the vessel.

[0006] A total gap size taken along a direction between the frame and the plenum structure can be the sum of the size of the first gap and the size of the second gap, the total gap size remaining greater than a non-zero threshold size. A thermally-conductive gas can be within the first gap and the second gap. The thermally-conductive gas can include hydrogen molecules. A first distance of the first gap taken along a direction between the plenum structure and thermally-conductive element can be the same as a second distance of the second gap taken along a direction between the thermally-conductive element and the frame. A first distance of the first gap taken along a direction between the plenum structure and thermally-conductive element can be different from a second distance of the second gap taken along a direction between the thermally-conductive element and the frame.

[0007] The thermally-conductive element can include a thermally-conductive metal or metal alloy. The thermally-conductive element can include aluminum, copper, an aluminum alloy, or a copper alloy.

[0008] The radiation source can include an active heating system associated with the plenum structure. The radiation source can further include an active cooling system associated with the frame. The thermally-conductive element between the frame and the plenum structure can be configured to passively reduce heat transfer resistance between the plenum structure and the frame without reducing a distance between the plenum structure and the frame. The thermally-conductive element can include a plurality of sub-elements. The thermally-conductive element can include a roughened surface. The thermally-conductive element can include a thermally -emissive coating on its outer surface.

[0009] The radiation source can include an extreme ultraviolet (EUV) collector mirror in the interior of the vessel, the EUV collector mirror defining a primary focus at which target material interacts with pulses of a light beam to thereby emit EUV radiation and an intermediate focus at which the EUV radiation is directed out of the vessel. The plenum structure can be arranged between the intermediate focus and the primary focus.

[0010] In other general aspects, an apparatus includes: an exhaust structure mounted adjacent a vessel and exposed to debris from an interior of the vessel, an exhaust opening is defined in a wall of a vessel and the exhaust structure, the exhaust opening enabling fluid communication between the interior of the vessel and a debris capture assembly; and a thermally-conductive element between an outer wall of the exhaust structure and a frame to which the vessel is mounted. The thermally- conductive element defines a first gap maintained between the exhaust structure outer wall and the thermally-conductive element, and a second gap maintained between the thermally-conductive element and the frame.

[0011] Implementations can include one or more of the following features. For example, the apparatus can further include a cooling system associated with the frame. The exhaust structure caninclude a hollow portion that has a substantially annular shape and extends around an outer circumference of an exterior of the vessel.

[0012] A total gap size taken along a direction between the frame and the exhaust structure outer wall can be the sum of the size of the first gap and the size of the second gap, the total gap size remaining greater than a non-zero threshold size. A first distance of a first gap taken along a direction between the exhaust structure outer wall and thermally -conductive element can be the same as a second distance of a second gap taken along a direction between the thermally -conductive element and the frame. A first distance of a first gap taken along a direction between the exhaust structure outer wall and thermally -conductive element can be different from a second distance of a second gap taken along a direction between the thermally -conductive element and the frame. The thermally- conductive element can include a thermally-conductive metal or metal alloy. The thermally- conductive element can include magnesium and silicon. The thermally-conductive element can include a first sub-element and a second sub-element. The first sub-element can be arranged along a vessel orientation and the second sub-element can be arranged orthogonal to the vessel orientation. The second sub-element can be positioned between the exhaust structure and a mount associated with a wall of the vessel. The mount can be positioned between the exhaust structure and an output aperture of the vessel. The first sub-element can be physically separate from the second sub-element. The first sub-element can be physically connected to the second sub-element.

[0013] The exhaust structure can include a port defining a first end fluidly coupled to the interior of the vessel, the first end of the port having a substantially circular or oval cross section; and a second end fluidly coupled to the debris capture assembly, the second end of the port having a substantially rectangular or square cross section. The exhaust structure can include a port defining a first end fluidly coupled to the interior of the vessel, the first end of the port having a funnel shape.

[0014] In other general aspects, a method includes: mounting a thermally-conductive element to at least one of a frame and a plenum structure; coupling the frame and the plenum structure such that the thermally-conductive element is fixed between the frame and the plenum structure, and a first gap is maintained between plenum structure and the thermally-conductive element and a second gap is maintained between the thermally-conductive element and the frame; and enabling a thermally- conductive gas to access the first gap and the second gap. A total gap size taken along a direction between the frame and the plenum structure is the sum of the size of the first gap and the size of the second gap, the total gap size remaining greater than a non-zero threshold size.

[0015] Implementations can include one or more of the following features. For example, the method can further include associating an active cooling system with the frame and an active heating system with the plenum structure. The thermally-conductive element can be mounted to at least one of the frame and the plenum structure by mounting a plurality of thermally-conductive sub-elements to at least one of the frame and the plenum structure. The thermally-conductive element between the frame and the plenum structure can be configured to passively reduce heat transfer resistance between theplenum structure and the frame without reducing a distance between the plenum structure and the frame. The method can further include, prior to mounting, sand blasting an outer surface of the thermally-conductive element. The method can also include, prior to mounting, applying a thermallyemissive coating to an outer surface of the thermally-conductive element.

[0016] In other general aspects, a method includes: during a first operation mode, actively heating an exhaust structure, the exhaust structure mounted adjacent a vessel and exposed to debris from an interior of an extreme ultraviolet (EUV) vessel at least during a second operation mode, wherein an exhaust opening is defined in a wall of the vessel and the exhaust structure, the exhaust opening enabling fluid communication between the interior of the vessel and a debris capture assembly; and during the second operation mode, enabling the exhaust structure to passively cool from heat transfer through a thermally-conducting gas between the exhaust structure and a frame. Enabling includes: enabling heat transfer through a first non -zero gap between the exhaust structure and a thermally- conductive element; enabling heat transfer through the thermally-conductive element; and enabling heat transfer through a second non -zero gap between the thermally-conductive element and the frame.

[0017] Implementations can include one or more of the following features. For example, during the second operation mode, enabling the exhaust structure to passively cool can include enabling at least a portion of the exhaust structure to cool to a temperature that is less than a melting temperature of a target material in the interior of the EUV vessel. During the first operation mode, an EUV light beam can be generated, and, during the second operation mode, the EUV light beam can be generated.DRAWING DESCRIPTION

[0018] The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the present disclosure and, together with the description, further serve to explain the principles of the present disclosure and to enable a person skilled in the relevant art(s) to make and use implementations described herein.

[0019] Fig. 1 is a schematic block diagram of an implementation of a radiation source including a thermally-conductive element between a frame of a vessel and an assembly inside the frame;

[0020] Fig. 2 is a schematic close-up cross-sectional view of an implementation of a thermally- conductive element that includes a roughened surface that can be on one or both sides of the element;

[0021] Fig. 3 is a schematic close-up cross-sectional view of an implementation of a thermally- conductive element designed with a thermally-emissive coating applied to one or more outer sides of the element;

[0022] Fig. 4 is a schematic close-up view of an implementation of a radiation source including a thermally-conductive element between a frame of a vessel and an assembly inside the frame, the thermally-conductive element including a plurality of sub-elements;

[0023] Fig. 5A is a schematic close-up cross-sectional view of an implementation of a thermally- conductive element between a frame of a vessel and an assembly inside the frame, in which a distanceof a first gap between the assembly and the thermally-conductive element taken along a heat flow axis is less than a distance of a second gap between the thermally-conductive element and the frame taken along the heat flow axis;

[0024] Fig. 5B is a schematic close-up cross-sectional view of an implementation of a thermally- conductive element between a frame of a vessel and an assembly inside the frame, in which a distance of a first gap between the assembly and the thermally-conductive element taken along a heat flow axis is greater than a distance of a second gap between the thermally-conductive element and the frame taken along the heat flow axis;

[0025] Fig. 6A is a schematic illustration of an implementation of a radiation source in which a vessel is conically-shaped or frustum-shaped, and a thermally-conductive element is between a frame of the vessel and an exhaust structure having an annular shape;

[0026] Fig. 6B is a schematic illustration of the radiation source of Fig. 6A showing a close-up view of the thermally-conductive element, the frame, and the exhaust structure;

[0027] Fig. 7A is a first perspective view of an implementation of an exhaust structure having an annular shape;

[0028] Fig. 7B is a second perspective view of the exhaust structure of Fig. 7A;

[0029] Fig. 7C is a perspective view of the exhaust structure of Fig. 7B in which a thermally- conductive element is overlaid onto the exhaust structure;

[0030] Fig. 8 A is a first perspective view of an implementation of an exhaust structure in which an active heating system is configured relative to a circumferential wall of the exhaust structure;

[0031] Fig. 8B is a second perspective view of the exhaust structure of Fig. 8A;

[0032] Fig. 9 is a flow chart of a procedure for assembling or building a heat transfer system for an assembly (for example, a plenum structure) of a radiation source;

[0033] Fig. 10 is a flow chart of a procedure for operating a radiation source in standard operation mode, in which an assembly is passively cooled from a passive heat transfer through a thermally- conducting gas between an exhaust structure and a frame, and in clean operation mode, in which the exhaust structure is actively heated;

[0034] Fig. 11A is a schematic illustration of a close-up view of an assembly and a frame, in which passive cooling during standard operation mode of Fig. 10 occurs due to heat transfer through a thermally-conductive gas between the assembly and the frame;

[0035] Fig. 1 IB is a schematic illustration of the close-up view of an assembly and a frame, in which passive cooling during standard operation mode of Fig. 10 occurs due to heat transfer through a thermally-conductive gas and through a thermally-conductive element between the assembly and the frame;

[0036] Fig. 12 is a schematic illustration of an implementation of a radiation source in which an exhaust structure is not a part of an annular shape but is a port that extends generally along a direction between an interior of a vessel and a debris capture assembly of a debris handling apparatus;

[0037] Fig. 13 is a schematic illustration of the radiation source of Fig. 6A shown with more detail related to operation during the standard operation mode in which the vessel operates to produce a light beam and debris can be formed during the production of the EUV radiation within the interior of the vessel;

[0038] Fig. 14 is a schematic illustration of a close-up view of a radiation source in which there are two or more frames adjacent a vessel and an assembly, with thermally -conductive elements between each frame and the assembly;

[0039] Fig. 15A is a schematic illustration of a radiation source including a thermally-conductive element in two pieces, a first piece arranged along a gap between an assembly and a frame and a second piece arranged between the assembly and a mount of wall of a vessel;

[0040] Fig. 15B is a perspective view of the assembly of Fig. 15A and the thermally-conductive element overlaid on the assembly;

[0041] Fig. 16 is a schematic illustration of an implementation of a radiation source in which an exhaust structure is not a part of an annular shape but is a port that extends generally along a direction between an interior of a vessel and a debris capture assembly of a debris handling apparatus;

[0042] Fig. 17A is a close-up view of a thermally-conductive element between a frame and a transition wall of the radiation source;

[0043] Fig. 17B is a cross-sectional view of a first end of the transition wall of Fig. 17A; and

[0044] Fig. 17C is a cross-sectional view of a second end of the transition wall of Fig. 17A.

[0045] The features of the present disclosure will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and / or structurally similar elements. Additionally, generally, the left-most digit(s) of a reference number identifies the drawing in which the reference number first appears. Unless otherwise indicated, the drawings provided throughout the disclosure should not be interpreted as to-scale drawings.DETAILED DESCRIPTION

[0046] Referring to Fig. 1, a radiation source 100 is configured to produce an ultraviolet (UV) light beam 101 configured for use by an output device 103. The radiation source 100 includes a vessel 105 including one or more walls extending along three dimensions, at least one of the walls defining an opening 102 through which the light beam 101 exits an interior 106 of the vessel 105. While the radiation source 100 is depicted schematically in two dimensions in Fig. 1, its components extend in three dimensions. In some implementations, the vessel 105 has a frustum conical or a tapered cylinder shape from a lateral view. The radiation source 100 includes a frame 120 configured at an exterior of the vessel 105 and an assembly 140 inside the frame 120. The interior 106 of the vessel 105 (and the assembly 140) are held under low pressure vacuum conditions.

[0047] The radiation source 100 includes a thermally-conductive element 160 between the frame 120 and the assembly 140. In some implementations, a heatable passthrough is arranged to the vessel 105 and coupled to the assembly 140. The thermally-conductive element 160 defines a first gap 161a maintained between the assembly 140 and the thermally-conductive element 160 and a second gap 161b maintained between the thermally-conductive element 160 and the frame 120. In various implementations, a dimension of the first gap 161a (taken along a heat flow axis Z_140) ranges from about 4 millimeter (mm) to about 8 mm and a dimension of the second gap 161b (taken along the heat flow axis Z 140) ranges from about 0.5 mm to about 2 mm. The dimensions of the first and second gaps are selected to prevent the thermally-conductive element 160 from contacting the assembly 140 or the frame 120 during operation.

[0048] In some implementations, the assembly 140 is part of a larger apparatus that is configured to be exposed to a hot material (such as a vapor or a gas) 107 and one or more additional gas flows that exit an interior 106 of the vessel 105. In some implementations, the assembly 140 functions to provide a flow plenum for facilitating the flow of the hot material 107 and / or a liquidized material. In some implementations, the assembly 140 functions as a plenum structure or an exhaust structure of an exhaust module for removing gaseous and particulate contaminants from the vessel 105.

[0049] The assembly 140 and the vessel 105 operate in two modes. The first mode is a clean / hot operation mode in which the hot material 107 is removed from the assembly 140. The vessel 105 can be active (that is, producing the light beam 101) during the clean operation mode. In the clean operation mode, a heater coupled to the assembly 140 is turned on and the hot material 107 can be removed from the assembly 140 by actively heating the assembly 140 in order to drip off the material 107 that has accumulated in the assembly 140 or in other components downstream of the assembly 140 (but not shown in Fig. 1).

[0050] The second mode is a standard or cold operation mode. The vessel 105 operates to produce the light beam 101, and, as a byproduct of operation, hot material 107 is produced that is removed from the interior 106 by way of the assembly 140. In the standard operation mode, the heater coupled to the assembly 140 is not turned on (that is, it is turned off) because it is desired to maintain the assembly 140 at a temperature below a melting point of the hot material 107. By maintaining the assembly 140 at this lower temperature, there is a reduced chance that hot material 107 escapes from an interior of the assembly 140 or escapes through the opening 102 and potentially enters the output device 103 (which can be, for example, an exposure tool for lithography or metrology operations in a semiconductor manufacturing process). The material 107 that enters the output device 103 can contaminate components, such as optics or masks, within the output device 103 and impede the operation of the output device 103 and thus there is a desire to prevent material 107 from escaping through the opening 102.

[0051] In this way, the assembly 140 is cycled between two temperature ranges from the clean operation mode (in which it is actively maintained at the temperature above the melting point of thematerial 107) and the standard operation mode (in which it is maintained at the temperature below the melting point of the material 107). In the standard operation mode, the assembly 140 is maintained at the lower temperature using a passive cooling method. Specifically, the assembly 140 is passively cooled due to passive heat transfer through a thermally-conducting gas 162 that exists between the assembly 140 and the frame 120. In order to improve the cooling rate or heat transfer from the assembly 140 to the frame 120 along a heat flow axis Z_140, without having to modify an actual distance (the gap) Da between the frame 120 and the assembly 140 taken along the direction of the axis Z_140, the thermally-conductive element 160 is held between the frame 120 and the assembly 140. By adding the thermally-conductive element 160, an effective distance (or gap) along the axis Z_140 between the frame 120 and the assembly 140 that is smaller than the actual distance Da is realized without needing to reduce the actual distance Da between the frame 120 and the assembly 140. The effective distance is the actual distance Da minus a thickness T 160 of the thermally- conductive element 160.

[0052] Considering a distance between the assembly 140 and the frame 120, the thickness T160 ranges from about 2 mm to about 7 mm. If the thickness T 160 is greater than 7 mm, a likelihood of physical contact between the thermally-conductive element 160 and either the assembly 140 or the frame 120 increases, in some instances. If the thickness T160 is less than 2 mm, a heat transfer rate decreases, in some instances. To put it another way, the effective distance is the sum of the distance of the first gap 161a and the distance of the second gap 161b taken along the heat flow axis Z_140. And, by reducing this effective distance, the cooling rate from the assembly 140 to the frame 120 is improved enough to enable at least portions of the assembly 140 to be maintained at the temperature below the melting point of the material 107 during the standard operation mode. The thermally- conductive element 160 does so by increasing the effectiveness of gas molecular conduction under vacuum conditions by reducing a larger gap to two smaller gaps and reducing the effective thermal resistance of molecular heat transfer from the plenum structure (the assembly 140) to the frame 120. A heat transfer resistance under vacuum conditions decreases non-linearly with decreasing gap sizes for gaps smaller than 15 mm. As a result, two smaller gaps perform significantly better than a single larger gap. The introduced metal body of the thermally-conductive element 160 is highly conductive and reduces the thermal resistance of the gas volume it occupies within the gap. By improving this cooling rate during standard operation mode (heaters off) and reducing the temperature of the portion of the hot material 107 on the assembly 140 that remains in liquid phase, spitting of the hot material 107 from the assembly 140 (and potentially through the opening 102) is reduced or avoided. In this way, the thermally-conductive element 160 between the frame 120 and the assembly 140 is configured to passively reduce heat transfer resistance between the assembly 140 and the frame 120 without reducing the actual physical distance Da between the assembly 140 and the frame 120. The benefits provided by the use of the element 160 arise in part because the interior 106 of the vessel 105 (and the assembly 140) are held under low pressure vacuum conditions.

[0053] It should be noted that the size of Da can change during operation because the assembly 140 expands in size when it is hot (such as during the clean operation mode). The total gap Da can range from about 9 mm to 15 mm due to temperature and assembly tolerances of the assembly 140. Moreover, it may be beneficial for the second gap 161b to be smaller than the first gap 161a so that the thermally-conductive element 160 is closer to the frame 120 (which can be actively cooled) so that the thermally-conductive element 160 remains cooler.

[0054] There can be a cooling system 130 associated with the frame 120 so that heat that is passively transferred from the assembly 140 to the frame 120 can be removed from the frame 120 by way of the cooling system 130. The cooling system 130 can be an active cooling system. For example, the cooling system 130 can be defined within the body of the frame 120 as a set of flow channels through which cooling fluid is passed.

[0055] In some implementations, the thermally-conductive gas 162 exists within the region between the frame 120 and the assembly 140. For example, the gas 162 can occur naturally in this region. That is, it is not necessary to actively introduce additional gas into this region to provide the gas 162. Accordingly, the thermally-conductive gas 162 exists within the first gap 161a and the second gap 161b. The thermally-conductive gas 162 can be any gaseous material that is thermally conductive. In some implementations, the thermally-conductive gas 162 includes at least one of hydrogen molecules H2, helium He, and other suitable gases.

[0056] The thermally-conductive element 160 is made of a thermally-conductive solid material. For example, the thermally-conductive element 160 can be made of a metal or a metal alloy such as aluminum, copper, an aluminum alloy, or a copper alloy. In some implementations, the thermally- conductive element 160 includes at least one of aluminum, magnesium, silicon, iron, copper, chrome, zinc, titanium, and manganese.

[0057] The thermally-conductive element 160 can be fixed or attached to the frame 120 at one or more locations. For example, as shown in Fig. 1, the thermally-conductive element 160 is fixed to the frame 120 at two distinct locations with respective connection mechanisms 121 1, 121 2. In other implementations, there can be only a single connection mechanism positioned at a single location of the frame 120 or there can be more than two connections mechanisms positioned at respective locations of the frame 120.

[0058] Referring to Fig. 2, in some implementations, a thermally-conductive element 260 can correspond to the thermally-conductive element 160 but can be designed with a roughened surface 263. Moreover, the roughened surface 263 can be on both sides of the element 260; thus, it can be on the side facing the assembly 140 and the first gap 161a and it can be on the side facing the frame 120 and the second gap 161b. The roughened surface 263 can further improve the radiative heat transfer by increasing the surface emissivity and absorptivity.

[0059] Referring to Fig. 3, in some implementations, a thermally-conductive element 360 can correspond to the thermally-conductive element 160 but can be designed with a high emissive coating364 applied to an outer surface. In some implementations, the high emissive coating 364 includes using a plasma electrolytic oxidation or a micro arc oxidation to form a ceramic oxide layer from the element 360. In some implementations, the high emissive coating 364 includes at least one of molybdenum and tungsten. In some implementations, a thickness of the high emissive coating 364 ranges from about 30 micrometers (pm) to about 50 pm. The coating 364 can be applied on both sides of the element 360; thus, it can be on the side facing the assembly 140 and the first gap 161a and it can be on the side facing the frame 120 and the second gap 161b. The coating 364 can further improve radiative heat transfer by increasing surface emissivity and absorptivity.

[0060] Referring to Fig. 4, in some implementations, a thermally-conductive element 460 is designed with a plurality of sub-elements 460i, 460ii. While only two sub-elements 460i, 460ii are shown in Fig. 4, the element 460 can be designed with more than two sub-elements. Each sub-element 460i, 460ii can be fixed or attached to a respective portion of the frame 120. For example, the sub-element 460i is fixed with a connection mechanism 42 li 1 to a first portion of the frame 120 and with a connection mechanism 42 li > 2 to a second portion of the frame 120. And, the sub-element 460ii is fixed with a connection mechanism 42 lii 3 to a third portion of the frame 120. In some implementations, the sub-element 460i is physically connected to the sub-element 460ii. In some implementations, the sub-elements 460i and 460ii are separate.

[0061] Referring again to Fig. 1, in some implementations, the distance of the first gap 161a taken along the heat flow axis Z_140 is the same as the distance of the second gap 161b taken along the heat flow axis Z_140. In other implementations, as shown in Figs. 5A and 5B, the distance of the first gap 161a taken along the heat flow axis Z_140 is different from the distance of the second gap 161b taken along the heat flow axis Z_140. For example, in Fig. 5A, a distance D161bA is greater than a distance D161aA. And, in Fig. 5B, a distance D161bB is less than a distance D161aB. When the frame 120 is coupled to a cooling system, the shorter distance D161bB leads to a greater heat transfer rate as the thermally-conductive element 160 is closer to the frame 120.

[0062] Referring to Fig. 6A, an implementation of a radiation source 600 includes a conically-shaped or a frustum-shaped vessel 605 including one or more side walls 608 (which can be conically-shaped) and a base 609 inside which an interior 606 is defined. The radiation source 600 includes a frame 620 configured at an exterior of the vessel 605 and surrounding partially or entirely the vessel 605. The radiation source 600 includes an apparatus that includes an exhaust structure 640 (which is an implementation of the assembly 140). The exhaust structure 640 is adjacent the wall 608 of the vessel 605. There can be other components between the wall 608 and the exhaust structure 640 not shown in the drawings. In some implementations, the exhaust structure 640 is machined in two metal pieces, such as stainless steel or other suitable metals, that are bolted together. In some implementations, the exhaust structure 640 can be formed from sheet metal that is bent and then welded together, brazed, and optionally coated with titanium nitride (for example, by way of chemical vapor deposition). The exhaust structure 640 and the wall 608 define an exhaust opening 610. The exhaust opening 610enables fluid communication between the interior 606 and a debris capture assembly 671 of a debris handling apparatus 670. A passthrough is optionally arranged and fits within the exhaust opening 601 defined in the wall 608. In addition, the passthrough is coupled with the interior 606 of the vessel 605 and the exhaust structure 640. The exhaust structure 640 is mounted to be adjacent the wall 608. The exhaust structure 640 is exposed to debris or at least one hot material 607 produced within the interior 606 of the vessel 605.

[0063] Referring also to Figs. 7A and 7B, the exhaust structure 640 has a substantially annular shape that extends around an outer circumference of the exterior of the vessel 605. The exhaust opening 610 is defined in an inner circumferential wall 611 of the exhaust structure 640, as shown in Fig. 7A. In some implementations, the inner circumferential wall 611 of the exhaust structure 640 can correspond to the portion of the wall 608 of the vessel 605 or it can be a separate element. An output port 612 is defined in an outer circumferential wall 613 of the exhaust structure 640, as shown in Fig. 7A. The output port 612 provides a fluid flow path to the debris capture assembly 671. The fluid connection between the output port 612 and the exhaust opening 610 is by way of a hollow circumferential passage 614 that is at the exterior to the vessel 605 (Fig. 6A).

[0064] Referring again to Figs. 6A and 6B, the radiation source 600 includes an optical collector 673, which is a surface reflective to radiation in the extreme ultraviolet (EUV) wavelength range. The collector 673 is in the interior 606 of the vessel 605. The collector 673 defines a primary focus 674 at which target material interacts with pulses of a light (for example, laser) beam to thereby emit EUV radiation. This interaction produces the hot material 607. The collector 673 also defines an intermediate focus 675 that is located at the opening 602. The exhaust structure 640 (which corresponds to an assembly 140) is arranged between the intermediate focus 675 and the primary focus 674.

[0065] The exhaust structure 640 includes a wall 641 that is adjacent or closest to the exhaust opening 610. The hot material 607 mostly impinges upon the wall 641 of the exhaust structure 640 due to the geometric access of the wall 641 to the interior 606. The wall 641 heats up more than the rest of the exhaust structure 640. Thus, when operating in the standard operation mode, the wall 641 heats up to a temperature above the melting point of the hot material 607. And, in order to maintain the wall 641 at a temperature below the melting point of the hot material 607, the wall 641 of the exhaust structure 640 is cooled due to passive heat transfer through a thermally -conducting gas 662 that exists between the exhaust structure 640 and the frame 620. Even if the wall 641 is not totally or partially below the melting point of the hot material 607, the thermally -conductive element 660 helps to drop the temperature close to the melting point to reduce the number of spitted particles by reducing the surface area that the hot material 607 in liquid form occupies on the wall 641 and any spitted particles have reduced energy and momentum when ejected and thus there is a reduced risk of reaching the opening 602.

[0066] The interface between the exhaust structure 640 and the frame 620 is shown more clearly in Fig. 6B. The apparatus also includes a thermally-conductive element 660 between the wall 641 and the frame 620. The thermally-conductive element 660 includes two sub-elements 660i, 660ii. A first gap 661a is maintained between the wall 641 and the sub-elements 660i, 660ii and a second gap 661b is maintained between the sub-elements 660i, 660ii and the frame 620. A thermally-conducting gas 662 exists in the space between the exhaust structure 640 and the frame 620. Referring to Fig. 7C, the thermally-conductive element 660 is shown overlaid on the exterior of the exhaust structure 640.

[0067] As shown in Fig. 6B, a cooling system 630 is associated with the frame 620 so that the heat that is passively transferred from the wall 641 to the frame 620 can be removed from the frame 620 by way of the cooling system 630. The cooling system 630 can be an active cooling system. As shown, the cooling system 630 can be defined within the body of the frame 620 as a set of flow channels through which cooling fluid is passed. In this way, the temperature differential between the wall 641 and the frame 620 driving heat transfer from the wall 641 remains high, since the cooling system 630 helps to remove incoming heat on the frame 620.

[0068] As discussed above, in the clean operation mode, the hot material 107 is removed from the assembly 140 while the vessel 105 is active (that is, producing the light beam). In the clean operation mode, the hot material 107 can be removed from the assembly 140 by actively heating the assembly 140 in order to drip off the material 107 that has accumulated in the assembly 140 or in other components downstream of the assembly 140 (but not shown in Fig. 1). As shown in Figs. 8 A and 8B, in some implementations, an active heating system 845 is configured relative to the inner circumferential wall 611 (not shown) and the outer circumferential wall 613 of the exhaust structure 640. The heating system 845 is in thermal contact surfaces of the exhaust structure 640 such as the inner circumferential wall 611 and / or the outer circumferential wall 613. The heating system 845 can be formed of any suitable heating element such as a resistive wire that is shaped to fit into grooves at an exterior surface of the wall 611 and / or 613.

[0069] Referring to Fig. 9, a procedure 980 is performed for assembling or building a heat transfer system for an assembly (for example, a plenum structure) of a radiation source. The procedure 980 includes mounting a thermally-conductive element to at least one of a frame and an assembly (operation 981). For example, the thermally-conductive element 160 can be mounted to the frame 120 (Fig. 1). The frame and the assembly can be coupled (operation 982). For example, with reference to Fig. 1, the vessel 105 and the assembly 140 can be mounted within the frame 120. In this way, the thermally-conductive element 160 is fixed between the frame 120 and the assembly 140. And, a first gap is maintained between the assembly and the thermally-conductive element and a second gap is maintained between the thermally-conductive element and the frame. The procedure 980 includes enabling a thermally-conductive gas to access the first gap and the second gap (operation 983). For example, the thermally-conductive gas 162 can exist in the first gap 161a and the second gap 161b (Fig. 1).

[0070] Additionally, a total gap size taken along a direction orthogonal to both the frame and the assembly is the sum of the size of the first gap and the size of the second gap. The total gap size remains greater than a non-zero threshold size because of thermal expansion of the assembly 640 and the assembly tolerances to prevent any contact between the assembly 640 and the thermally - conductive element 660 or frame 620.

[0071] The procedure 980 can also include associating an active cooling system with frame and an active heating system with the assembly. For example, the cooling system 130 can be configured relative to the frame 120 (as shown in Fig. 1) and the heating system 845 (as shown in Figs. 8A and 8B) can be configured relative to the exhaust structure 640.

[0072] Once the heat transfer system is assembled, the radiation source can be operated according to a procedure 1085, as discussed with reference to Fig. 10. During standard operation mode 1086, the assembly (such as assembly in Fig. 1 or the exhaust structure 640 of Figs. 6A and 6B) is passively cooled from a passive heat transfer through a thermally-conducting gas between the exhaust structure and a frame. For example, with reference to Figs. 6A and 6B, the exhaust structure 640 is passively cooled due to passive heat transfer through the thermally-conducting gas 662. In particular, the exhaust structure is passively cooled by enabling at least a portion of the exhaust structure 640 to cool to a temperature that is less than a melting temperature of the target material (which contributes to the hot material 607) in the interior of the vessel 605.

[0073] During standard operation mode (operation 1086), heat transfer is through a first non-zero gap between the exhaust structure and a thermally -conductive element (operation 1087), having heat transfer through the thermally-conductive element (operation 1088), and having heat transfer through a second non-zero gap between the thermally-conductive element and the frame (operation 1089). With reference to Figs. 6A and 6B, heat transfer is enabled through the first gap 661a, through the thermally-conductive element 660, and through the second gap 661b.

[0074] If, at any time, there is a request to clean the exhaust structure 640 or any part of the debris handling apparatus 670 (operation 1090), then the radiation source is operated in clean operation mode 1091. In clean operation mode (operation 1091), the exhaust structure is actively heated (operation 1092) in order to remove the hot material 607 from the exhaust structure (or from any part of the debris handling apparatus) (operation 1093). For example, the exhaust structure 640 can be actively heated (operation 1092) using the heating system 845 (Figs. 8 A and 8B).

[0075] In general, with reference again to Fig. 6A, the debris handling apparatus 670 is configured for controlling debris within a controlled-environment of the interior 606. The debris handling apparatus 670 enables the removal of debris from the interior 606 of the vessel 605 and thus acts to clean the interior 606. The debris handling apparatus 670 controls debris within the interior 606 by removing debris from the interior 606 and transporting the debris to one or more locations outside the interior 606. The radiation source 600 is shown in schematic cross section in the X-Z plane, with the Z axis in the plane of the page and aligned with an optical axis that extends between the primary focus674 and the intermediate focus 675 and the X axis perpendicular to the Z axis in the plane of the page. The vessel 605 can be generally symmetrical about the Z axis such that it includes a conical shape that tapers from the primary focus 674 to the intermediate focus 675.

[0076] For example, as shown in Fig. 11A, the exhaust assembly 640 and the frame 620 of Figs. 6A and 6B are shown without thermally -conductive element 660 (during standard operation mode). The hot material 607 impinging upon the assembly 640 causes the assembly 640 to heat up. While there is some passive cooling from the assembly 640 to the frame 620 through the thermally -conducting gas 662 (as depicted by the arrows), there is not enough to prevent spitting of particles 649 of the deposited debris from the assembly 640 because the assembly 640 remains too hot. As shown in Fig.1 IB, in order to improve the cooling rate or heat transfer from the assembly 640 to the frame 620 along a heat flow axis Z_1140, without having to modify an actual physical distance (the gap) Da between the frame 620 and the assembly 640 taken along the direction of the axis Z_640, the thermally-conductive element 660 is held between the frame 120 and the assembly 640. By adding the thermally-conductive element 660, an effective distance (or gap) along the axis Z_1140 between the frame 620 and the assembly 640 that is smaller than the actual distance Da is realized without needing to reduce the actual distance Da between the frame 620 and the assembly 640. By reducing this effective distance, the cooling rate from the assembly 640 to the frame 620 is improved enough to enable at least portions if not the entirety of the assembly 640 to be maintained at the temperature below the melting point of the material 607 during the standard operation mode. By improving this cooling rate during standard operation mode, spitting particles 649 of the debris deposited on the assembly 640 (from the hot material 607) is reduced or avoided. The spitting propensity of a surface is strongly corelated with the temperature of the surface (in particular, the surface of the wall 641 of the assembly 640). With the incorporation of the element 660 in the gap, the reduction in temperature of the assembly 640 during standard operation mode can result in a reduction of spitting rates by reducing the energy and momentum of the spitted particles. And, reducing the momentum of the spitted particles reduces the probability of a spitted particle making it through the opening 602 in Fig. 6A. That is an added benefit to the direct reduction of the total surface area of the assembly 640 that remains at a temperature higher than the melting point of the hot material 607.

[0077] It should be noted that the benefits provided by the use of the element 660 (in Fig. 1 IB) arise because the interior 606 of the vessel 605 (and the assembly 640) are held under low pressure vacuum conditions.

[0078] Depending on the design of the radiation source 100, the optical axis OA (and the Z axis) can be positioned in different directions relative to the gravitational direction G. Referring to Fig. 12, in a radiation source 1200, an exhaust structure 1240 is not a part of an annular shape but is a port that extends generally along the direction between an interior 1206 of a vessel 1205 and a debris capture assembly 1271 of a debris handling apparatus 1270. The exhaust structure 1240 is mounted to be adjacent a wall 1208 of the vessel 1205. The exhaust structure 1240 and the wall 1208 define anexhaust opening 1210. There may be other components between the wall 1208 and the exhaust structure 1240 not shown in the drawings. The exhaust opening 1210 enables fluid communication between the interior 1206 and the debris capture assembly 1271 of the debris handling apparatus 1270. The exhaust structure 1240 is mounted to be adjacent the wall 1208. The exhaust structure 1240 is exposed to debris or hot material 1207 produced within the interior 1206 of the vessel 1205.

[0079] The radiation source 1200 includes a frame 1220 configured at an exterior of the vessel 1205. The vessel 1205 includes one or more walls 1208 extending along three dimensions, at least one of the walls defining an opening 1202 through which the light beam (not shown in Fig. 12) exits the interior 1206 of the vessel 1205. While the radiation source 1200 is depicted schematically in two dimensions in Fig. 12, its components extend in three dimensions. In some implementations, the vessel 1205 has a frustum conical or a tapered cylinder shape from a lateral view. The radiation source 1200 includes an optical collector 1273, which is designed similarly to the collector 672. Thus, the collector 1273 defines a primary focus 1274 at which target material interacts with pulses of a light (for example, laser) beam to thereby emit EUV radiation. This interaction produces the hot material 1207. The collector 1273 also defines an intermediate focus 1275 that is located at the opening 1202. The exhaust structure 1240 (which corresponds to an assembly 140) is arranged between the intermediate focus 1275 and the primary focus 1274.

[0080] The exhaust structure 1240 can be formed from stainless steel and by a process of three- dimensional printing in some implementations. The interface between the exhaust structure 1240 and the frame 1220 includes a thermally -conductive element 1260. The thermally -conductive element 1260 includes two sub-elements. A first gap is maintained between the wall 1241 of the exhaust structure 1240 and the sub-elements of the thermally -conductive element 1260 and a second gap is maintained between the sub-elements of the thermally-conductive element 1260 and the frame 1220. A thermally-conducting gas 1262 exists in the space between the wall 1241 and the frame 1220.

[0081] Additionally, a cooling system 1230 is associated with the frame 1220 so that the heat is passively transferred from the wall 1241 to the frame 1220 can be removed from the frame 1220 by way of the cooling system 1230. The cooling system 1230 can be an active cooling system. As shown, the cooling system 1230 can be defined within the body of the frame 1220 as a set of flow channels through which cooling fluid is passed. In this way, heat is transferred with reduced resistance from the wall 1241 to the frame 1220 along the paths.

[0082] With reference to Fig. 13, the radiation source 600 is shown with more detail related to operation during the standard operation mode in which the vessel 605 operates to produce a light beam 601. Debris can be formed during the production of the EUV radiation within the interior 606 of the vessel 605, as follows. Targets 615 that are delivered to an irradiation site (at the primary focus 674) individually interact with one or more pulses of light in a light beam 616. Plasma 617 is produced from this interaction, and the plasma 617 produces EUV radiation 618. The EUV radiation 618 is collected by the collector 673, which can be a near-normal incidence collector mirror definedby the optical axis OA and having a reflective surface 673r that can be in the form of a prolate spheroid (that is, an ellipse rotated about its major axis). In this way, the collector 673 defines the primary focus 674 at the irradiation site and the intermediate focus 675 at the exit or opening 602 of the vessel 605. The collector 673 outputs the EUV radiation 618 as the light beam 601.

[0083] The targets 615 include an EUV emitting target material such as, but not limited to, tin, lithium, xenon, or combinations thereof. The targets 615 at the irradiation site can be in the form of liquid droplets, or alternatively can be solid particles or solid particles contained within liquid droplets. For example, the element tin can be present as a target in the form of pure tin; a tin compound such as Sn Br4. SnBr2, SnH : a tin alloy such as tin -gallium alloys, tin-indium alloys, or tin- indium-gallium alloys; or a combination thereof. High energy ions and / or particles and vapor of or containing target material can be formed during the production of the EUV radiation 618. For example, particles of target material and high energy ions and vapor are byproducts from light-based vaporization or ablation processes that can occur when the targets 615 interact with the pulses of the light beam 616, and these byproducts are debris within the interior 606 of the vessel 605. Debris can contaminate the reflective surface 673r of the collector 673. For example, particles of target material and energetic ions and vapor (all debris) can cause physical damage and localized heating of the reflective surface 673r of the collector 673. A debris handling apparatus (not shown) is designed, in combination with other components that direct the debris within the interior 606 of the vessel 605 toward the debris handling apparatus, to remove the debris from the interior 606 of the vessel 605 and to prevent the debris from re-entering the interior 606 after it has been removed.

[0084] The interior 606 can accommodate gas flows that can be used as a buffer gas for debris and / or vapor emanating from the irradiation site. In some implementations, hydrogen (H2) is used as a hydrogen gas flow within the interior 606 since hydrogen is relatively transparent to EUV radiation having a wavelength of about 13.5 nanometers (nm). Hydrogen gas can be introduced into the interior 606 to slow down and / or guide energetic debris (such as ions, atoms, and clusters) of target material created by irradiation of the targets 615 at the irradiation site and by the resulting plasma 617. The debris (which is highly energetic) is slowed down (or de-energized) by collisions with the gas molecules (the H2 molecules) in the flow. This gas flow can be directed to prevent debris produced at the irradiation site 606 from reaching the reflective surface 673r of the collector 673. Thus, this gas flow can reduce damage to the collector 673 otherwise caused by vapor deposition, implantation, and deposition of sputtered target material. The gas flow can therefore direct the debris (which is a hot material 607) toward the exhaust structure 640. The details of the frame 620 and other aspects of the passive cooling system are not shown in Fig. 13.

[0085] When targets 615 that are tin or tin-containing are used, the use of hydrogen gas for gas flow with tin targets 615 can result in another potential source of contamination in the interior 606 of the vessel 605. In particular, molten tin can be ejected or spit from surfaces in the interior 606 that are coated or subject to coating with the molten tin when hydrogen bubbles form and grow in or under themolten tin and then burst. One way to prevent tin spitting is to prevent molten target material (hot material 607) from accumulating on any surface within the interior 606 of the vessel 605 by keeping the temperature of the surface below or well below the melting point of the target material. The melting point of tin is about 232 °C. For example, some surfaces of objects or portions of the interior 606 can be maintained at a temperature below 232 °C, or in a temperature in the range of about 50 °C to about 110 °C. Any tin that deposits on a surface maintained at that temperature is kept in solid form and prevents or resists spitting. And, because the debris handling apparatus is not within the interior 606, it is possible to maintain surfaces and the interior 606 at such low temperatures.

[0086] The deposition of debris onto cold surfaces within the interior 606 can shorten the length of service intervals of the EUV radiation source 600. Growth of such deposits on cold surfaces and accumulation of liquid tin on surfaces can be reduced by the use of gas flows inside the interior 606. Moreover, different gas flows can be used. For example, one or more gas flows can be directed along the reflective surface 673r of the collector 673. As another example, one or more gas flows can be directed through apertures generally perpendicular to a surface to be protected within the interior 606. Gas flows can be directed through one or more walls 608 of the vessel 605. As a further example, protective gas flows can be parallel to, or have a component of flow directed parallel to, the surface to be protected (such as in regions near the intermediate focus 675). As a further example, a gas flow (referred to as a dynamic gas lock) can be used to prevent material from leaving the vessel 605 in the region of the intermediate focus 675. A dynamic gas lock can produce a gas flow from the region of the intermediate focus 675 toward the irradiation site. In general, a stable guided flow that flows away from the collector 673 can be formed from a combination of one or more of these flows. This guided flow helps contain and carry away from the collector 673 the debris, which includes hot material 607 such as vapor, ions, and micro and nano-particles, and is generated from the targets 615 during production of plasma 617. Other flows can be formed. Moreover, although the pathways of the hot material 607 are depicted with merely a few pathways in Figs. 6A and 13 (and other drawings), it should be understood that the pathways can extend along many and multiple pathways.

[0087] Low pressures are used within the interior 606, and pressure differentials at the exhaust opening 610 are not large. Nevertheless, a small pressure differential at the exhaust opening 610 can be produced by vacuum pumping the exhaust opening 610 (by way of an exhaust pump (not shown, which is in fluid communication with the exhaust opening 610). The stable guided flow of the debris entrained and contained in the flow of the hot material 607 is thereby directed into the exhaust opening 610 and out of the interior 606, thus substantially reducing the amount of debris contacting surfaces within the interior 606. The debris is directed into the exhaust opening 610, and it can be captured downstream at the debris capture assembly 671.

[0088] The debris handling apparatus can also include additional heaters configured to maintain a temperature of the debris capture assembly 671 above a melting point of the material of the debris in order to prevent the debris from sticking within the assembly 671. Additionally, the debris handlingapparatus includes a drainage apparatus configured to drain any debris that is collected in the away from the apparatus 671 . In at least one implementation, the drainage apparatus relies on gravity to remove the debris from the apparatus 671 but it is configured to function independently of the relative gravitational position between the exhaust opening 610 and the assembly 671.

[0089] Referring to Fig. 14, in other implementations, there can be two or more frames in a radiation source 1400. For example, an ancillary frame 1420a can be positioned adjacent to a frame 1420. The ancillary frame 1420a is configured to include an active cooling system, and the frame 1420 is closer to the opening 102 than the ancillary frame 1420a. In these implementations, a thermally-conductive element 1460 can be arranged between both the ancillary frame 1420a and the frame 1420. The thermally-conductive element 1460 can be made up of a plurality of pieces or sub-elements (such as shown in Fig. 4). For example, one sub-element 1460a can be between the ancillary frame 1420a and the assembly 140 while another sub-element 1460 is between the frame 1420 and the assembly 140. A lateral dimension (that is, a circumferential measurement around the vessel 105) of the sub-element 1460a between the ancillary frame 1420a and the assembly 140 can be smaller than or greater than that of the sub-element 1460 between the frame 1420 and the assembly 140. In some implementations, the sub-elements 1460 and 1460a are physically connected. In some implementations, the sub-elements 1460 and 1460a are separate.

[0090] Referring to Fig. 15 A, in some implementations of a radiation source 1500, a thermally- conductive element 1560 includes two pieces. A first piece 1560a of the thermally-conductive element 1560 is arranged along the gap between the assembly 640 and a frame 1520. A second piece 1560b of the thermally-conductive element 1560 is arranged between the assembly 640 and a mount 1528. The mount 1528 is positioned between the assembly 640 and the frame 1520, and is configured to mount or fix a portion 1529 of the wall 608 of the vessel 605 to the lower part of the wall 608 of the vessel 605. The second piece 1560b of the thermally-conductive element 1560 is transversely positioned relative to the first piece 1560a of the thermally-conductive element 1560. As shown in Fig. 15A, the first piece 1560a is a stepped plate. In other implementations, the first piece 1560a is a curved plate. In the implementation of Fig. 15 A, the second piece 1560b is a flat plate. In some implementations, as shown in Fig. 14B, the second piece 1560b has an arc shape or a moon shape with respect to a view from the opening 102 (that is, a view along the Z axis of the vessel 605). In other words, the second piece 1560b is partially positioned between the assembly 640 and the mount 1528. In some implementations, the first piece 1560a and the second piece 1560b are physically connected. In some implementations, the first piece 1560a and the second piece 1560b are separate pieces.

[0091] Referring to Fig. 15B, the thermally-conductive element 1460 is shown overlaid on the assembly 640.

[0092] In various implementations, the thermally-conductive element 1460 can include one or more curved pieces or plates, which can be arc shaped, moon shaped, or wavy shaped. The shape of thethermally-conductive element 1460 can conform to the shape of one or more of the exhaust structure 640 and the frame (not shown).

[0093] Referring to Fig. 16, in some implementations of a radiation source 1600, the optical axis OA (and the Z axis) is positioned in a direction such that the Z axis is closer to 90° relative to the gravitational direction G. In other implementations, the OA is oriented such that the Z axis is not closer to 90° relative to the gravitational direction G. Similar to the exhaust structure 1240, in some implementations, an exhaust structure 1640 is not a part of an annular shape but is a port that extends generally along the direction between an interior 1606 of a vessel 1605 and a debris capture assembly 1671 of a debris handling apparatus 1670. The exhaust structure 1640 is mounted to be adjacent a wall 1608 of the vessel 1605. The exhaust structure 1640 and the wall 1608 define an exhaust opening 1610. There can be other components between the wall 1608 and the exhaust structure 1640 not shown in the drawings. For example, a portion of an annular shape can be between the wall 1608 and the exhaust structure 1640. The exhaust opening 1610 enables fluid communication between the interior 1606 and the debris capture assembly 1671 of the debris handling apparatus 1670. The exhaust structure 1640 is mounted to be adjacent the wall 1608. The exhaust structure 1640 is exposed to debris or hot material 1607 produced within the interior 1606 of the vessel 1605.

[0094] The radiation source 1600 includes a frame 1620 configured at an exterior of the vessel 1605. The exhaust structure 1640 can be formed from stainless steel and by a process of three-dimensional printing in some implementations. The interface between a transition wall 1641 of the exhaust structure 1640 and the frame 1620 includes a thermally-conductive element 1660. The transition wall 1641 defines a channel between and coupled to the exhaust opening 1610 and the debris capture assembly 1671. In some implementation, the thermally-conductive element 1660 is an integral component that surrounds or substantially surrounds the transitional wall 1641 of the exhaust structure 1640. In some implementations, the thermally-conductive element 1660 includes two separate components that surrounds or substantially surrounds the transitional wall 1641 of the exhaust structure 1640. One component of the thermally-conductive element 1660 is arranged on the side of an opening 1202 or on the side of an intermediate focus 1275, and the other component is arranged on the side of the collector 1273 or on the side of a primary focus 1274. A first gap is maintained between the transition wall 1641 of the exhaust structure 1640 and the thermally-conductive element 1660. Optionally, a second gap is maintained between the thermally-conductive element 1660 and the frame 1620. In some implementations, a thermally-conducting gas 1662 exists in the space between the transition wall 1641 and the frame 1620. In some implementations, the thermally-conductive element 1660 is in direct contact with the frame 1620. In some implementations, a cooling device (not shown) is embedded in the frame 1620.

[0095] As shown in Fig. 17A, the transition wall 1641 is a port defining a first end 1641 1 fluidly coupled to the interior 1606 of the vessel 1605 and a second end 1641 2 fluidly coupled to the debris capture assembly 1671. As shown in the cross-section of Fig. 17B, the first end 1641 1 of the port hasa substantially circular or oval cross section. This circular or oval cross section is best suited for collecting the debris from the interior 1606 of the vessel 1605. The first end 1641 1 is generally in the shape of a funnel. As shown in the cross section of Fig. 17C, the second end 1641 2 has a quadrilateral cross section, such as trapezoid with rounded comers, a substantially rectangular cross section, or a square cross section. This quadrilateral section at the second end 1641 2 matches up with the quadrilateral section of an input structure 1672 of the debris handling apparatus 1670. The thermally-conductive element 1660 should have a geometry that follows or traces the geometry of the transition wall 1641 at least on the side that faces the transition wall 1641.

[0096] Additionally, a cooling system 1630 is associated with the frame 1620 so that the heat is passively transferred from the wall 1641 to the frame 1620 can be removed from the frame 1620 by way of the cooling system 1630. The cooling system 1630 can be an active cooling system. As shown, the cooling system 1630 can be defined within the body of the frame 1620 as a set of flow channels through which cooling fluid is passed. In this way, heat is transferred with reduced resistance from the transition wall 1641 to the frame 1620 along the paths.

[0097] The implementation can be further described using the following clauses.1. A radiation source comprising: a vessel; a frame configured at an exterior of the vessel; a plenum structure inside the frame; and a thermally-conductive element between the frame and the plenum structure, the thermally-conductive element defining a first gap maintained between the plenum structure and the thermally-conductive element and a second gap maintained between the thermally-conductive element and the frame.2. The radiation source of clause 1, wherein the plenum structure is a part of a debris handling apparatus exterior to the vessel.3. The radiation source of clause 2, wherein the plenum structure defines a plenum that is coupled to an exhaust opening defined in a wall of the vessel, the exhaust opening enabling fluid communication between an interior of the vessel and a debris capture assembly of the debris handling apparatus.4. The radiation source of clause 3, wherein the plenum structure comprises a hollow portion that has a substantially annular shape and extends around an outer circumference of the vessel.5. The radiation source of clause 1, wherein a total gap size taken along a direction between the frame and the plenum structure is the sum of the size of the first gap and the size of the second gap, the total gap size remaining greater than a non-zero threshold size.6. The radiation source of clause 5, wherein a thermally-conductive gas is within the first gap and the second gap.7. The radiation source of clause 6, wherein the thermally-conductive gas comprises hydrogen molecules.8. The radiation source of clause 5, wherein a first distance of the first gap taken along a direction between the plenum structure and thermally-conductive element is the same as a second distance of the second gap taken along a direction between the thermally-conductive element and the frame.9. The radiation source of clause 5, wherein a first distance of the first gap taken along a direction between the plenum structure and thermally-conductive element is different from a second distance of the second gap taken along a direction between the thermally-conductive element and the frame.10. The radiation source of clause 1, wherein the thermally-conductive element comprises a thermally-conductive metal or metal alloy.11. The radiation source of clause 1, wherein the thermally-conductive element comprises aluminum, copper, an aluminum alloy, or a copper alloy.12. The radiation source of clause 1, further comprising an active heating system associated with the plenum structure.13. The radiation source of clause 1, further comprising an active cooling system associated with the frame.14. The radiation source of clause 1, wherein the thermally-conductive element between the frame and the plenum structure is configured to passively reduce heat transfer resistance between the plenum structure and the frame without reducing a distance between the plenum structure and the frame.15. The radiation source of clause 1, wherein the thermally-conductive element comprises a plurality of sub-elements.16. The radiation source of clause 1, wherein the thermally-conductive element comprises a roughened surface.17. The radiation source of clause 1, wherein the thermally-conductive element comprises a thermally-emissive coating on its outer surface.18. The radiation source of clause 1, further comprising an extreme ultraviolet (EUV) collector mirror in the interior of the vessel, the EUV collector mirror defining a primary focus at which target material interacts with pulses of a light beam to thereby emit EUV radiation and an intermediate focus at which the EUV radiation is directed out of the vessel.19. The radiation source of clause 18, wherein the plenum structure is arranged between the intermediate focus and the primary focus.20. An apparatus comprising: an exhaust structure mounted adjacent a vessel and exposed to debris from an interior of the vessel, an exhaust opening is defined in a wall of a vessel and the exhaust structure, the exhaust opening enabling fluid communication between the interior of the vessel and a debris capture assembly; and a thermally-conductive element between an outer wall of the exhaust structure and a frame to which the vessel is mounted, the thermally-conductive element defining a first gap maintained between theexhaust structure outer wall and the thermally -conductive element and a second gap maintained between the thermally -conductive element and the frame.21. The apparatus of clause 20, further comprising a cooling system associated with the frame.22. The apparatus of clause 20, wherein the exhaust structure comprises a hollow portion that has a substantially annular shape and extends around an outer circumference of an exterior of the vessel.23. The apparatus of clause 20, wherein a total gap size taken along a direction between the frame and the exhaust structure outer wall is the sum of the size of the first gap and the size of the second gap, the total gap size remaining greater than a non-zero threshold size.24. The apparatus of clause 20, wherein a first distance of a first gap taken along a direction between the exhaust structure outer wall and thermally-conductive element is the same as a second distance of a second gap taken along a direction between the thermally-conductive element and the frame.25. The apparatus of clause 20, wherein a first distance of a first gap taken along a direction between the exhaust structure outer wall and thermally-conductive element is different from a second distance of a second gap taken along a direction between the thermally-conductive element and the frame.26. The apparatus of clause 20, wherein the thermally-conductive element comprises a thermally- conductive metal or metal alloy.27. The apparatus of clause 20, wherein the thermally-conductive element comprises magnesium and silicon.28. The apparatus of clause 20, wherein the thermally-conductive element comprises a first subelement and a second sub-element.29. The apparatus of clause 28, wherein the first sub-element is arranged along a vessel orientation and the second sub-element is arranged orthogonal to the vessel orientation.30. The apparatus of clause 28, wherein the second sub-element is positioned between the exhaust structure and a mount associated with a wall of the vessel.31. The apparatus of clause 30, wherein the mount is positioned between the exhaust structure and an output aperture of the vessel.32. The apparatus of clause 28, wherein the first sub-element is physically separate from the second sub-element.33. The apparatus of clause 28, wherein the first sub-element is physically connected to the second sub-element.34. The apparatus of clause 20, wherein the exhaust structure comprises a port defining a first end fluidly coupled to the interior of the vessel, the first end of the port having a substantially circular or oval cross section; and a second end fluidly coupled to the debris capture assembly, the second end of the port having a substantially rectangular or square cross section.35. The apparatus of clause 20, wherein the exhaust structure comprises a port defining a first end fluidly coupled to the interior of the vessel, the first end of the port having a funnel shape.36. A method comprising:mounting a thermally -conductive element to at least one of a frame and a plenum structure; coupling the frame and the plenum structure such that the thermally -conductive element is fixed between the frame and the plenum structure, and a first gap is maintained between plenum structure and the thermally-conductive element and a second gap is maintained between the thermally- conductive element and the frame; and enabling a thermally-conductive gas to access the first gap and the second gap; wherein a total gap size taken along a direction between the frame and the plenum structure is the sum of the size of the first gap and the size of the second gap, the total gap size remaining greater than a non-zero threshold size.37. The method of clause 36, further comprising associating an active cooling system with the frame and an active heating system with the plenum structure.38. The method of clause 36, wherein mounting the thermally-conductive element to at least one of the frame and the plenum structure comprises mounting a plurality of thermally-conductive subelements to at least one of the frame and the plenum structure.39. The method of clause 36, wherein the thermally-conductive element between the frame and the plenum structure is configured to passively reduce heat transfer resistance between the plenum structure and the frame without reducing a distance between the plenum structure and the frame.40. The method of clause 36, further comprising, prior to mounting, sand blasting an outer surface of the thermally-conductive element.41. The method of clause 36, further comprising, prior to mounting, applying a thermally -emissive coating to an outer surface of the thermally-conductive element.42. A method comprising: during a first operation mode, actively heating an exhaust structure, the exhaust structure mounted adjacent a vessel and exposed to debris from an interior of an extreme ultraviolet (EUV) vessel at least during a second operation mode, wherein an exhaust opening is defined in a wall of the vessel and the exhaust structure, the exhaust opening enabling fluid communication between the interior of the vessel and a debris capture assembly; and during the second operation mode, enabling the exhaust structure to passively cool from heat transfer through a thermally-conducting gas between the exhaust structure and a frame, enabling comprising: enabling heat transfer through a first non-zero gap between the exhaust structure and a thermally- conductive element; enabling heat transfer through the thermally-conductive element; and enabling heat transfer through a second non-zero gap between the thermally-conductive element and the frame.43. The method of clause 42, wherein, during the second operation mode, enabling the exhaust structure to passively cool comprises enabling at least a portion of the exhaust structure to cool to atemperature that is less than a melting temperature of a target material in the interior of the EUV vessel.44. The method of clause 42, wherein during the first operation mode, an EUV light beam is generated, and, during the second operation mode, the EUV light beam is generated.

[0098] The above-described implementations and other implementations are within the scope of the following claims.

Claims

CLAIMS1. A radiation source comprising: a vessel; a frame configured at an exterior of the vessel; a plenum structure inside the frame; and a thermally-conductive element between the frame and the plenum structure, the thermally- conductive element defining a first gap maintained between the plenum structure and the thermally- conductive element and a second gap maintained between the thermally-conductive element and the frame.

2. The radiation source of claim 1, wherein the plenum structure is a part of a debris handling apparatus exterior to the vessel.

3. The radiation source of claim 2, wherein the plenum structure defines a plenum that is coupled to an exhaust opening defined in a wall of the vessel, the exhaust opening enabling fluid communication between an interior of the vessel and a debris capture assembly of the debris handling apparatus.

4. The radiation source of claim 3, wherein the plenum structure comprises a hollow portion that has a substantially annular shape and extends around an outer circumference of the vessel.

5. The radiation source of claim 1, wherein a total gap size taken along a direction between the frame and the plenum structure is the sum of the size of the first gap and the size of the second gap, the total gap size remaining greater than a non-zero threshold size.

6. The radiation source of claim 5, wherein a thermally-conductive gas is within the first gap and the second gap.

7. The radiation source of claim 6, wherein the thermally-conductive gas comprises hydrogen molecules.

8. The radiation source of claim 5, wherein a first distance of the first gap taken along a direction between the plenum structure and thermally-conductive element is the same as a second distance of the second gap taken along a direction between the thermally-conductive element and the frame.

9. The radiation source of claim 5, wherein a first distance of the first gap taken along a direction between the plenum structure and thermally-conductive element is different from a second distance of the second gap taken along a direction between the thermally-conductive element and the frame.

10. The radiation source of claim 1, wherein the thermally-conductive element comprises a thermally-conductive metal or metal alloy.

11. The radiation source of claim 1, wherein the thermally-conductive element comprises a roughened surface.

12. The radiation source of claim 1, wherein the thermally-conductive element comprises a thermally-emissive coating on its outer surface.

13. The radiation source of claim 1, further comprising an extreme ultraviolet (EUV) collector mirror in the interior of the vessel, the EUV collector mirror defining a primary focus at which target material interacts with pulses of a light beam to thereby emit EUV radiation and an intermediate focus at which the EUV radiation is directed out of the vessel.

14. An apparatus comprising: an exhaust structure mounted adjacent a vessel and exposed to debris from an interior of the vessel, an exhaust opening is defined in a wall of a vessel and the exhaust structure, the exhaust opening enabling fluid communication between the interior of the vessel and a debris capture assembly; and a thermally-conductive element between an outer wall of the exhaust structure and a frame to which the vessel is mounted, the thermally-conductive element defining a first gap maintained between the exhaust structure outer wall and the thermally-conductive element and a second gap maintained between the thermally-conductive element and the frame.

15. The apparatus of claim 14, further comprising a cooling system associated with the frame.

16. The apparatus of claim 14, wherein the exhaust structure comprises a hollow portion that has a substantially annular shape and extends around an outer circumference of an exterior of the vessel.

17. The apparatus of claim 14, wherein the thermally-conductive element comprises a first sub-element and a second sub-element.

18. The apparatus of claim 17, wherein the first sub-element is arranged along a vessel orientation and the second sub-element is arranged orthogonal to the vessel orientation.

19. The apparatus of claim 17, wherein the second sub-element is positioned between the exhaust structure and a mount associated with a wall of the vessel.

20. The apparatus of claim 17, wherein the first sub-element is physically separate from the second sub-element.

21. The apparatus of claim 17, wherein the first sub-element is physically connected to the second sub-element.

22. The apparatus of claim 14, wherein the exhaust structure comprises a port defining a first end fluidly coupled to the interior of the vessel, the first end of the port having a substantially circular or oval cross section; and a second end fluidly coupled to the debris capture assembly, the second end of the port having a substantially rectangular or square cross section.

23. The apparatus of claim 14, wherein the exhaust structure comprises a port defining a first end fluidly coupled to the interior of the vessel, the first end of the port having a funnel shape.

24. A method comprising: mounting a thermally-conductive element to at least one of a frame and a plenum structure; coupling the frame and the plenum structure such that the thermally-conductive element is fixed between the frame and the plenum structure, and a first gap is maintained between plenum structure and the thermally-conductive element and a second gap is maintained between the thermally-conductive element and the frame; and enabling a thermally-conductive gas to access the first gap and the second gap; wherein a total gap size taken along a direction between the frame and the plenum structure is the sum of the size of the first gap and the size of the second gap, the total gap size remaining greater than a non-zero threshold size.

25. The method of claim 24, wherein mounting the thermally-conductive element to at least one of the frame and the plenum structure comprises mounting a plurality of thermally-conductive sub-elements to at least one of the frame and the plenum structure.

26. The method of claim 24, wherein the thermally-conductive element between the frame and the plenum structure is configured to passively reduce heat transfer resistance between the plenum structure and the frame without reducing a distance between the plenum structure and the frame.

27. The method of claim 24, further comprising, prior to mounting, sand blasting an outer surface of the thermally-conductive element.

28. The method of claim 24, further comprising, prior to mounting, applying a thermallyemissive coating to an outer surface of the thermally-conductive element.

29. A method comprising: during a first operation mode, actively heating an exhaust structure, the exhaust structure mounted adjacent a vessel and exposed to debris from an interior of an extreme ultraviolet (EUV) vessel at least during a second operation mode, wherein an exhaust opening is defined in a wall of the vessel and the exhaust structure, the exhaust opening enabling fluid communication between the interior of the vessel and a debris capture assembly; and during the second operation mode, enabling the exhaust structure to passively cool from heat transfer through a thermally-conducting gas between the exhaust structure and a frame, enabling comprising: enabling heat transfer through a first non-zero gap between the exhaust structure and a thermally-conductive element; enabling heat transfer through the thermally-conductive element; and enabling heat transfer through a second non-zero gap between the thermally- conductive element and the frame.

30. The method of claim 29, wherein, during the second operation mode, enabling the exhaust structure to passively cool comprises enabling at least a portion of the exhaust structure to cool to a temperature that is less than a melting temperature of a target material in the interior of the EUV vessel.

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