CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Application No. 62/949,144, filed December 17, 2019 and titled TARGET MATERIAL TANK FOR EXTREME ULTRAVIOLET LIGHT SOURCE, and U.S. Application No. 62/986,266, filed March 6, 2020 and titled TARGET MATERIAL TANK FOR EXTREME ULTRAVIOLET LIGHT SOURCE, both of which are incorporated herein in their entireties by reference.

TECHNICAL FIELD

[0002] This disclosure relates to a target material tank for an extreme ultraviolet (EUV) light source.

BACKGROUND

[0003] EUV light may be, for example, electromagnetic radiation having wavelengths of 100 nanometers (nm) or less (also sometimes referred to as soft x-rays), and including light at a wavelength of, for example, 20 nm or less, between 5 and 20 nm, or between 13 and 14 nm, and may be used in photolithography processes to produce extremely small features in substrates, for example, silicon wafers, by initiating polymerization in a resist layer.

[0004] Methods to produce EUV light include, but are not necessarily limited to, converting a material that includes an element, for example, xenon, lithium, or tin, with an emission line in the EUV range in a plasma state. In one such method, often termed laser produced plasma (LPP), the required plasma may be produced by irradiating a target material that is an element, with an emission line in the EUV range in a plasma state, for example, in the form of a droplet, plate, tape, stream, or cluster of material, with an amplified light beam that may be referred to as a drive laser. For this process, the plasma is typically produced in a sealed vessel, for example, a vacuum chamber, and monitored using various types of metrology equipment.

SUMMARY

[0005] In one aspect, an apparatus for an extreme ultraviolet (EUV) light source includes a body. The body includes: a first structure including a first wall; and a second structure including a second wall permanently joined to the first wall. An interior of the body is at least partially defined by the first wall and the second wall. The first wall includes a first metallic material, and the second wall includes a second metallic material that has a different thermal conductivity than the first metallic material. The interior of the body is configured to be fluidly connected to a target supply system of the EUV light source.

[0006] Implementations may include one or more of the following features. [0007] A first end of the first wall and a second end of the second wall may be permanently joined at a brazed interface.

[0008] The first metallic material may include molybdenum (Mo), and the second metallic material may include stainless steel.

[0009] The apparatus may further include a temperature control system configured to control a temperature of at least one of the first wall and the second wall. The temperature control system may include: a heating system configured to be thermally coupled to the first wall; and a cooling system configured to be thermally coupled to the second wall. The thermal conductivity of the second metallic material may be lower than the thermal conductivity of the first metallic material.

[0010] The thermal conductivity of the second metallic material may be lower than the thermal conductivity of the first metallic material. The first wall may extend from a first end to a second end, and the second wall may extend from a first end to a second end. The first end of the first wall may be brazed to the second end of the second wall. The apparatus may further include: an O-ring at the first end of the second wall; and a removable lid configured to be held at the O-ring.

[0011] The thermal conductivity of the second metallic material may be lower than the thermal conductivity of the first metallic material. The first wall may extend from a first end to a second end, and the second wall may extend from a first end to a second end. The first end of the first wall may be brazed to the second end of the second wall. The apparatus may further include at least one port extending from the second wall. The at least one port may include the second metallic material. The second metallic material may include stainless steel.

[0012] The first metallic material may include a first coefficient of thermal expansion, and the second metallic material may include a second coefficient of thermal expansion.

[0013] An outer side of the first wall may be permanently joined to an inner side of the second wall. [0014] The body may further include a third structure including a third wall. The third wall may include an inner surface and an outer surface. The inner surface of the third wall may be permanently joined to an outer surface of the second wall. The third wall may include the second metallic material. The apparatus may further include a temperature control system configured to control a temperature of at least one of the first wall, the second wall, and the third wall. The thermal conductivity of the second metallic material may be lower than the thermal conductivity of the first metallic material. The temperature control system may include: a heating system configured to be thermally coupled to the first wall; and a cooling system configured to be thermally coupled to the second wall and the third wall. The third wall may be between the second wall and the cooling system.

[0015] The apparatus may be a target material tank configured to hold a target material in the interior of the body. The target material may emit EUV light when in a plasma state.

[0016] The apparatus may be a connection assembly. The first structure may include at least a first port, and the second structure may include at least a second port. The first port and the second port may be in fluid communication with each other. The apparatus may be configured to provide a fluid path between an external device coupled to the second port and a reservoir coupled to the first port.

[0017] In another aspect, an EUV light source includes a target supply system. The target supply system includes: a droplet generator configured to produce a stream of targets; at least one apparatus including an interior region configured to be fluidly coupled to the droplet generator; and a vessel configured to receive the targets from the droplet generator. The targets include a target material that emits EUV light when in a plasma state. The apparatus includes: a first structure including a first wall; and a second structure including a second wall permanently joined to the first wall. The interior region is at least partially defined by the first wall and the second wall. The first wall includes a first metallic material, and the second wall includes a second metallic material that has a different thermal conductivity than the first metallic material.

[0018] Implementations may include one or more of the following features.

[0019] The EUV light source may further include a light source configured to produce a pulse of light having an energy sufficient to convert at least some of the target material in a target into the plasma state in which the target material emits EUV light.

[0020] The at least one apparatus may be a target material tank configured to hold the target material in the interior region. The target supply system may further include at least one valve. The at least one valve may be configured to fluidly connect or fluidly disconnect the interior region of the target material tank with the droplet generator.

[0021] The at least one apparatus may be a connection assembly. The first structure may include at least a first port, and the second structure may include at least a second port. The first port and the second port may be in fluid communication with each other. The target supply system may further include: an external device coupled to the second port; and a reservoir coupled to the first port. The reservoir may be configured to hold the target material in an interior cavity and may be fluidly coupled to the droplet generator. The connection assembly may be configured to provide a fluid path between the external device and the reservoir. The external device may be a vacuum system or a gas supply system.

[0022] In another aspect, a target supply system for an EUV light source includes: a droplet generator configured to produce a stream of targets; and at least one apparatus comprising an interior region configured to be fluidly coupled to the droplet generator. The targets include a target material that emits EUV light when in a plasma state. The apparatus includes: a first structure including a first wall; and a second structure including a second wall permanently joined to the first wall. The interior region is at least partially defined by the first wall and the second wall. The first wall includes a first metallic material, and the second wall includes a second metallic material that has a different thermal conductivity than the first metallic material.

[0023] Implementations may include one or more of the following features. [0024] The at least one apparatus may be a target material tank configured to hold the target material in the interior region.

[0025] The target supply system may further include at least one valve. The at least one valve may be configured to fluidly connect or fluidly disconnect the interior region of the at least one apparatus with the droplet generator.

[0026] The at least one apparatus may be a connection assembly. The first structure may include at least a first port, and the second structure may include at least a second port. The first port and the second port may be in fluid communication with each other.

[0027] Implementations of any of the techniques described above may include an EUV light source, a system, a method, a process, a device, or an apparatus. 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

[0028] FIG. 1 is a block diagram of an extreme ultraviolet (EUV) light source.

[0029] FIG. 2 is a block diagram of a target material tank.

[0030] FIG. 3 is a block diagram of a metallic connection assembly.

[0031] FIG. 4 is a block diagram of a target supply system.

[0032] FIG. 5 is a block diagram of another target supply system.

[0033] FIG. 6A is a cross-sectional view of another target material tank in the X-Y plane.

[0034] FIG. 6B is a cross-section view of the target material tank of FIG. 6A in the X-Z plane.

[0035] FIG. 6C is a perspective view of the target material tank of FIG. 6A.

[0036] FIG. 7 is a block diagram of another EUV light source.

DETAIFED DESCRIPTION

[0037] Referring to FIG. 1, a block diagram of an extreme ultraviolet (EUV) light source 100 is shown. The light source 100 includes a vessel 109 (for example, a vacuum chamber or evacuated vessel), a light source 105 that produces a light beam 106, and a target supply system 140. The target supply system 140 includes a target material tank 144 that has an interior region 103 configured to hold target material. The target supply system 140 also includes a droplet generator 142 that receives target material from the interior 103 of the target material tank 144. The target supply system 140 also includes a pressure management system 130.

[0038] The target material tank 144 is made from more than one metallic material. In the example shown in FIG. 1, the target material tank 144 includes a first structure 146 and a second structure 148 that are permanently joined at an interface 150. The interface 150 may be, for example, a brazed interface. The first structure 146 and the second structure 148 are three-dimensional solid structures, each of which is made of a different metallic material. As discussed below, using more than one metallic material to form the target material tank 144 enhances the safety and usability of the target material tank 144, and allows a greater number of options for components.

[0039] In operational use, the droplet generator 142 delivers a stream 121 of targets to an interior 101 of the vessel 109. The droplet generator 142 includes a droplet delivery system such as a nozzle and may include one or more pressurized vessels that include the liquid target material delivered from that target material tank 144. An interaction between a light beam 106 and target material in a target 121p (which is one of the targets in the stream 121) at a plasma generation site 123 produces a plasma 196 that emits EUV light 197. The light beam 106 is generated by an optical source 105 and delivered to the interior 101 via an optical path 107. The plasma 196 that is generated by the interaction between the light beam 106 and the target material in the target 121p is supplied to a lithography tool 199. The target 121p includes target material, which is any material that has an emission line in the EUV range when in a plasma state. The target material may be, for example, tin, lithium, or xenon. Other materials may be used as the target material. For example, the element tin may be used as pure tin (Sn); as a tin compound, for example, SnBr4, SnBr2, SntF; as a tin alloy, for example, tin-gallium alloys, tin-indium alloys, tin-indium-gallium alloys, or any combination of these alloys.

[0040] The interior 103 of the tank 144 is fluidly coupled to the droplet generator 142 by a fluid communication connection 155. The fluid communication connection 155 is any type of device, structure, or system that allows target material to flow from the interior 103 of the tank 144 to the droplet generator 142. For example, the fluid communication connection 155 may be a tube or a pipe, or a combination of such elements. The fluid communication connection 155 may be made of any material that is capable of transporting the target material. The fluid communication connection 155 may include a regulation device, such as the regulation device 452a as shown in FIG. 4. The regulation device may be configured to control the flow of the target material through the fluid communication connection 155 by, for example, opening, closing, or partially obstructing the fluid communication connection 155. In some embodiments, the regulation device may include a freeze valve. In other implementations, the target material tank 144 is directly adjacent to the droplet generator 142 such that the target material tank 144 supplies the target material to the droplet generator 142 without the use of the fluid communication connection 155.

[0041] Referring to FIG. 2, a side cross-sectional view of a target material tank 244 is shown. The target material tank 244 is an implementation of the target material tank 144 (FIG. 1).

[0042] The target material tank 244 includes a first structure 246 and a second structure 248. The first structure 246 and the second structure 248 are solid, three-dimensional bodies that partially define an interior region 203 within the target material tank 244. The interior region 203 holds a target material mixture 220. The target material mixture 220 includes the target material that emits EUV light when in a plasma state and also may include various impurities. [0043] The first structure 246 is made of a first metallic material. The second structure 248 is made of a second metallic material. The first and second metallic materials are different metallic materials. The first metallic material and the second metallic material have different thermal conductivities. The thermal conductivity of the second metallic material may be lower than the thermal conductivity of the first metallic material. For example, the first metallic material may be molybdenum (Mo), which has a thermal conductivity of 138 Watts per meter-Kelvin at 20° Celsius, and the second metallic material may be stainless steel, which has a thermal conductivity of 14.4 Watts per meter-Kelvin at 20° Celsius. As discussed below, this configuration allows the tank 244 to be operated in a safe and efficient manner, and also provides for greater flexibility in the configuration of various components of the tank 244. [0044] Moreover, the first metallic material and the second metallic material may have different coefficients of thermal expansion. The coefficient of thermal expansion is a material property that defines the extent to which a material expands when heated or contracts when cooled. For example, the first metallic material may be molybdenum, which has a coefficient of thermal expansion of 4.8 x 10 ⁶ per Kelvin at 25° Celsius, and the second metallic material may be stainless steel, which has a the second coefficient of thermal expansion being approximately 14.4 x 10 ⁶ per Kelvin. Some sealing techniques are not robust enough to prevent or minimize relative movement between two joined metals that each have a different coefficient of thermal expansion in the presence of a temperature change. On the other hand, the first wall 216 and the second wall 218 may be joined at an interface 250 by brazing. Brazing forms a seal that is robust enough to minimize or prevent relative movement between two joined metals that have different coefficients of thermal expansion in the presence of a temperature change. In particular, brazing forms a seal that minimizes or prevents relative movement between the first wall 216 and the second wall 218 during temperature changes. This reduces wear at the interface 250 and improves the sealing properties of the interface 250.

[0045] The first structure 246 includes a first wall 216 and a base portion 217. The first wall 216 extends in the Y direction from a second end 216b, which is joined to the base portion 217, to a first end 216a. The base portion 217 defines a port 233 that is made of the first metallic material. The port 233 fluidly couples the interior region 203 to a fluid communication connection, such as, for example, a target material tank connection 455 (FIG. 4) or a fluid communication connection 555 (FIG. 5). The port 233 allows fluid (for example, target material) in the interior region 203 to flow from the target material tank 244 into the fluid communication connection.

[0046] The first wall 216 and the base portion 217 are made of the first metallic material. The second structure 248 includes a second wall 218. The second wall 218 is made of the second metallic material. The second wall 218 extends in the Y direction from a second end 218b to a first end 218a. Each of the first wall 216 and the second wall 218 is a three-dimensional, solid body. The first wall 216 and the second wall 218 have approximately the same cross-sectional size shape in the X-Z (the plane that is in and out of the page in the example of FIG. 2). For example, the first wall 216 and the second wall 218 may have a circular, square, or rectangular cross-section in the X-Z plane. The tank 244 may be, for example, a cuboid or a cylinder. In the example of FIG. 2, the first wall 216 and the second wall 218 have approximately the same diameter in the X-Z plane, and the diameter of the walls 216 and 218 is substantially constant in the Y directions. Other implementations are possible. For example, the diameter of the wall 216 and/or the wall 218 in the X-Y plane may vary along the Y direction. Moreover, in some implementations, the base portion 217 is not part of the first structure 246. In these implementations, the first structure 246 is open at the second end 216b. For example, the first structure 246 may be shaped approximately as a half-sphere.

[0047] The second wall 218 is permanently joined to the first wall 216 at the interface 250. For example, the first wall 216 and the second wall 218 may be permanently joined by melting a filler metallic material (the filler metallic material having a lower melting point than the first metallic material and the second metallic material) between the first wall 216 and the second wall 218 such that when the filler metallic material cools to a solid form, the first wall 216 and the second wall 218 are permanently joined. This method of permanently joining the first metallic material to the second metallic material is referred to as brazing. Brazing forms a robust seal that prevents moisture and oxygen from entering the interior region 203. In the example shown in FIG. 2, the first end 216a of the first wall 216 is brazed to the second end 218b of the second wall 218 at the interface 250. The interface 250 is a continuous joint such that the tank 244 is a single, sealed body.

[0048] In the example shown in FIG. 2, an O-ring 212 is positioned at the first end 218a of the second wall 218. The O-ring 212 is a continuous piece or loop of rubber or elastomer that surrounds the circumference of the inner side of the first end 218a of the second wall 218. The O-ring 212 is used to hermetically seal a top portion 215 to the second wall 218. The top portion 215 may be a removable lid (referred to as the removable lid 215) that is configured to be held at the O-ring 212. The O-ring is thermally compatible with the second metallic material which may not be elevated to the same high temperature as the first metallic material when target material in a molten state is within interior 203 of target material tank 244. The O-ring reduces leaking of the target material 220 when the removable lid is closed on the target material tank 244. The removable lid 215 allows target material to be placed in the interior region 203 of the target material tank 244. For example, target material may be placed in the interior region 203 by opening the removable lid 215, placing target material into the target material tank 244, and then resealing the removable lid 215 to the O-ring 212. The removable lid 215 is made of the second metallic material.

[0049] The target material tank 244 includes a temperature control system 231. The temperature control system 231 controls a temperature of the first wall 216 and/or the second wall 218. By controlling the temperature of the first wall 216 and or the second wall 218, the temperature control system 231 also controls the temperature in the interior region 203. [0050] The temperature control system 231 includes a heating system 232 that is configured to be thermally coupled to the first wall 216. The heating system 232 may be a plurality of discrete heating elements positioned at various positions relative to the first wall 216 or may be a single heating element. The heating system 232 may be thermally coupled to the first wall 216 by direct physical contact, but this is not necessarily the case. By heating the first wall 216, the heating system 232 also heats the interior region 203, which includes the target material 220. This allows the target material 220 to be transformed into or maintained in a melted, fluid, or molten state in which the target material 220 is able to flow.

[0051] The temperature control system 231 also includes a cooling system 234 that is configured to be thermally coupled to the second wall 218. The cooling system 234 may be a plurality of discrete cooling elements positioned at various positions relative to the second wall 218 or may be a single cooling element. For example, the cooling system 234 may include a channel of fluid, such as water or air, inside a tube. The cooling system 234 cools the second wall 218 and the removable lid 215. For example, the cooling system 234 may be cooled to a touch-safe temperature of, for example, about 40° to 50° C.

[0052] The relatively low thermal conductivity of the second metallic material allows the wall 218 to be cooled to a touch-safe temperature. This allows an operator to manipulate the removable lid 215 and interact with the tank 214 in a safe manner. For example, the target material 220 may be replenished or installed by opening the lid 215 and placing a solid block of the target material in the interior 203. By cooling the removable lid 215 to a touch-safe temperature, the replacement procedure may be performed safely and efficiently without reducing the temperature of the first wall 216. Moreover, the ability to reduce the temperature of the second wall 218 (because of its relatively low thermal conductivity) allows a greater number of materials to be used as the O-ring 212. In particular, because the second wall 218 and the removable lid 215 are able to be cooled to a touch-safe temperature, the O- ring 212 does not necessarily have to withstand high temperatures and may be made from a relatively less thermally robust material. Moreover, because the first structure 246 and the port 233 are made of the first metallic material, connectors, fasteners, or other joining devices that couple to the port 233 also may be made of the first metallic material.

[0053] The target material tank 244 also includes a vacuum port 236. The vacuum port 236 is configured to connect to a vacuum system (not shown). This allows the interior region 203 to be maintained at sub-atmospheric pressures or a particular pressure desired by the user.

[0054] Referring to FIG. 3, a side cross-sectional view of a metallic connection assembly 344 is shown. The metallic connection assembly 344 includes a first structure 346 that is permanently attached to a second structure 348 at an interface 350. The first structure 346 and the second structure 348 are solid, three-dimensional bodies that at least partially define an interior region 303. [0055] The first structure 346 is made of a first metallic material. The second structure 348 is made of a second metallic material. The first metallic material and the second metallic material are different metallic materials. The first metallic material and the second metallic material have different thermal conductivities. The thermal conductivity of the second metallic material may be lower than the thermal conductivity of the first metallic material. Moreover, the first metallic material and the second metallic material may have different coefficients of thermal expansion. The first metallic material may be, for example, molybdenum, and the second metallic material may be, for example, stainless steel. As discussed below, this provides for greater flexibility in the configuration of the various components of the metallic connection assembly 344.

[0056] The first structure 346 includes a first wall 316. The first wall 316 extends in the Y direction from a second end 316b to a first end 316a. The first structure 346 is open at the second end 316b. The open second end 316b is labeled 335 in FIG. 3 and is referred to as the opening 335. The opening 335 provides a fluid connection point for the metallic connection assembly 344. Although in the example shown in FIG. 3 the first structure 346 is open at the second end 316b, other implementations are possible. For example, the first structure 346 may include a base portion that defines a port, similar to the base portion 217 and the port 233 of FIG. 2.

[0057] The first wall 316 includes an outer side 316c that faces away from the interior region 303. The second structure 348 includes a second wall 318. The second wall 318 extends from a second end 318b to a first end 318a. The second wall 318 includes an inner side 318c that faces the interior region 303. The first wall 316 and the second wall 318 are three-dimensional, solid bodies that have approximately equal cross-sectional sizes and shapes in the X-Z plane. For example, the first wall 316 and the second wall 318 may have circular, square, or rectangular cross-sections in a plane that is in and out of the page. The first end 316a of the first wall 316 and the second end 318b of the second wall 318 are permanently joined at the interface 350. The interface 350 may be formed by brazing. In the example of FIG. 3, the outer side 316c of the first wall 316 at the first end 316a is brazed directly to the inner side 318c of the second wall 318 at the second end 318b to form the interface 350.

[0058] The second structure 348 also includes a top portion 315. The top portion 315 and the second wall 318 define one or more ports 338. The top portion 315 also defines a gas or vacuum port 336. Fluid (such as a gas, liquid, or a flowable substance that includes gas and/or liquid) is able to flow through the interior region 303 of the metallic connection assembly 344. For example, fluid is able to flow from the port 336 to the opening 335 and or from one or more of the ports 338, into the interior region 303, and through the opening 335. The port 336 may connect to a vacuum system (not shown) configured to control the pressure in the interior region. The port 336 also may connect to a gas supply system (not shown) that supplies gas to the interior region 303. In the example shown, the port 336 includes a gasket 339 that aids in connecting the portion 336 to an external device. The port 338 may be connected to an external device, such as, for example, a gas supply system or a vacuum system. The second end 316b of the first wall 316 may be connected to a tank or a reservoir, such as the reservoir 547 (FIG. 5).

[0059] The configuration of the metallic connection assembly 344 allows the port 388 and the port 336 to be made of a material that is the same as or is thermally similar to the material used on many external devices. Because the second structure 348, the port 336, and the port 338 are made of the second metallic material, the connectors used to connect to the port 338 and/or the port 336 also may be made of the second metallic material. For example, the second metallic material may be stainless steel and the first metallic material may be molybdenum (Mo). Many external devices use stainless steel connectors. In this example, the port 338 and the port 336 is connected to external devices that have stainless steel connectors at a stainless steel-stainless steel interface. In this example, the connection interface is made between two identical metals. This allows the entire assembly to be heated to relatively high temperatures (for example, 300° C or higher) without degrading the connectors. Moreover, this is in contrast to some prior systems in which the entire structure (including ports for connecting to external devices) is made of a metallic material with a relatively high thermal conductivity (such as Mo) that is greater than the thermal conductivity of the material (for example, stainless steel) of the connectors generally used to connect external devices to the ports. In these prior systems, the connectors of the external devices and the ports on the structure are made of different metals, which may lead to degradation of the connection.

[0060] Referring to FIG. 4, a target supply system 440 is shown. The target supply system 440 is an example of a system in which a multi-metal or bi-metal tank or apparatus may be used. The target supply system 440 includes a priming tank 416, a target material tank 444, a reservoir 447, and the droplet generator 142 (FIG. 1). In some embodiments, there may be multiple reservoirs 447 and the one or more reservoirs may be pressurized. The target material tank 444 is made of two or more different metallic materials and may be, for example, the tank 144 or 244. The fluid communication connection 455 fluidly connects the priming tank 416, the target material tank 444, the reservoir 447 , and the droplet generator 142. The fluid communication connection 455 is an implementation of the fluid communication connection 155 (FIG. 1). In the example of FIG. 4, the tank 444 is a refill tank.

[0061] The fluid communication connection 455 includes a regulation device 452a between the tank 444 and the reservoir 447. The regulation device 452a is configured to regulate, direct, or control the flow of the target material from the target material tank 444 to the reservoir 447. In some implementations, the fluid communication connection 455 also includes a regulation device 452b between the priming tank 416 and the target material tank 444. In implementations that include the regulation device 452b, the regulation device 452b is configured to regulate, direct, or control the flow of the target material from the priming tank 416 to the target material tank 444. Each regulation device 452a, 452b may be, for example, a valve. If each regulation device 452a, 452b is a valve, then the valve may be a fluid valve which may be, for example, hydraulic, pneumatic, manually operated, solenoid- driven, or motor-driven. In some implementations, one or both of the regulation devices 452a, 452b is or includes a freeze valve.

[0062] The priming tank 416 supplies the target material tank 444 with the target material, and the tank 444 supplies the reservoir 447. The reservoir 447 supplies the droplet generator 142 with the target material. The target supply system 440 is configured to allow the droplet generator 142 to operate while the tank 444 is replenished. For example, the tank 444 may be replenished when the regulation device 452 is in a state that prevents the target material from flowing between the target material tank 444 and the reservoir 447. During this time, the priming tank 416 either replenishes the target material tank 444 with the target material or produces the fluid target material from the solid target material. However, the reservoir 447 continues to supply target material to the droplet generator 142. When the supply of the target material in the reservoir 447 is low, the regulation device 452 changes state to allow the target material to flow from the target material tank 444 to the reservoir 447. Target material flows into the reservoir 447 and the reservoir 447 is thereby replenished while the droplet generator 142 continues to produce the stream 121 (FIG. 1).

[0063] The target supply system 440 is one example of a system in which a tank that is made of more than one metallic material (such as the tank 144 or 244) may be used. Other implementations are possible. For example, the target supply system 440 may include two or more separate reservoirs between the target material tank 444 and the droplet generator 142. The two or more reservoirs are fluidly connected to each other, the droplet generator 142, and the target material tank 444 via the fluid communication connection 455, and may include regulation devices between each of these elements. Moreover, a tank made of more than one metal may be used in ways in an EUV light source other than the configuration shown in FIG. 4. For example, the tank may be directly connected to the droplet generator 142 such that the bi-metal or multi-metal tank acts as a reservoir.

[0064] Referring to FIG. 5, a target supply system 540 is shown. The target supply system 540 is another example of a system in which a multi-metal or bi-metal tank or apparatus may be used. The target supply system 540 includes the target material tank 244, the metallic connection assembly 344, the reservoir 547, the fluid communication connection 555, and the droplet generator 142. The fluid communication connection 555 fluidly connects the target material tank 244, the reservoir 547, and the droplet generator 142. For example, the fluid communication connection 555 may be a tube or a pipe, or a combination of such elements. In the example of FIG. 5, the target material tank 244 acts as a refill tank. Specifically, the target material tank 244 is configured to refill the reservoir 547 with target material through the fluid communication connection 555 when the supply of target material is low in the reservoir 547. The metallic connection assembly 344 provides a gas connection to the reservoir 547.

[0065] A regulation device 552a is coupled to the fluid communication connection 555 between the tank 244 and the reservoir 547. The regulation device 552b is coupled to the fluid communication connection 555 between the reservoir 547 and the droplet generator 142. The regulation devices 552a and 552b are configured to regulate, direct, or control the flow of the target material from the target material tank 244 to the reservoir 547 and from the reservoir 547 to the droplet generator 142, respectively.

[0066] Each of the regulation devices 552a, 552b may be, for example, a valve. If each of the regulation devices 552a, 552b is a valve, then the valve may be a fluid valve which may be, for example, hydraulic, pneumatic, manually operated, solenoid-driven, or motor-driven. In some implementations, each of the regulation devices 552a, 552b is or includes a freeze valve.

[0067] The fluid communication connection 555 and each of the regulation devices 552a and 552b may be made of the first metallic material such that the first structure 246 of the tank 244, the fluid communication connection 555, and each of the regulation devices 552a and 552b are made of a material that does not adversely affect the target material.

[0068] The target material tank 244 holds target material. The target material flows out of the target material tank 244 through the port 233, into the fluid communication connection 555, and into the reservoir 547. The reservoir 547 supplies the droplet generator 142 with the target material. The target supply system 540 allows the droplet generator 142 to operate while the tank 244 is being replenished. For example, the tank 244 may be replenished when the regulation device 552a is in a state that prevents the target material from flowing between the target material tank 244 and the reservoir 547. During this time, the target material tank 244 is replenished with the solid target material and produces the fluid target material from the solid target material. For example, as discussed above, the top portion 215 of the tank 244 may be a removable lid 215. In these implementations, the tank 244 is replenished with solid target material by opening the top portion 215, placing the solid target material in the tank 244, and resealing the lid 215 to the tank 244. However, the reservoir 547 may still contain an adequate supply of the target material and continue to supply target material to the droplet generator 142. When the supply of the target material in the reservoir 547 is low, the regulation device 552a changes state to allow the target material to flow from the target material tank 244 to the reservoir 547. Target material flows into the reservoir 547 thereby replenishing the reservoir 547 while the droplet generator 142 continues to produce the stream 121 (FIG. 1).

[0069] The metallic connection assembly 344 supplies the reservoir 547 with a fluid (for example, a gas, a liquid, or a liquid that includes gas) or removes fluid from the reservoir 547. The first structure 346 of the metallic connection assembly 344 is attached to the reservoir 547 with the opening 335 in fluid communication with an interior of the reservoir 547. The port 338 is connected to an external device 551 using a connector 553 that is made of the second metallic material. Thus, the port 338 and the connector 553 are made of the same metallic material. The external device may be, for example, a vacuum system or a gas supply. The reservoir 547 may be made of the first metallic material such that the first structure 346 of the metallic connection assembly 344 and the reservoir 547 are made of the same material.

[0070] The target supply system 540 is one example of a system in which a tank that is made of more than one metallic material, including the tank 244 and the metallic connection assembly 344, may be used. Other implementations are possible. For example, the target supply system 540 may include two or more separate reservoirs between the target material tank 244 and the droplet generator 142. The two or more reservoirs are fluidly connected to each other, the droplet generator 142, and the target material tank 244 via the fluid communication connection 555, and may include regulation devices between each of these elements. Each of the two or more reservoirs may be connected to one or more of the metallic connection assembly 344 to provide one or more gas connections or a vacuum to each of the two or more reservoirs. Moreover, a tank made of more than one metal may be used in ways in an EUV light source other than the configuration shown in FIG. 5. For example, the tank may be directly connected to the droplet generator 142 such that the bi-metal or multi-metal tank acts as a reservoir.

[0071] Referring to FIGS. 6A-6C, another target material tank 644 is shown. FIG. 6A is a side cross- sectional view of the target material tank 644 in the X-Y plane. FIG. 6B is a cross-sectional view of the target material tank 644 in the X-Z plane taken along the line B — B’ of FIG. 6A. FIG. 6C is a perspective view of the target material tank 644. The target material tank 644 is the same as the target material tank 244 (FIG. 2), except the target material tank 644 also includes a third wall 619. The third wall 619 provides an additional barrier between the interior region 203 and the cooling system 234, as discussed below. The presence of the third wall 619 reduces the thermal gradient across the second wall 218.

[0072] The third wall 619 is a three-dimensional, solid body. Fike the second wall 218, the third wall 619 is made of the second metallic material. The third wall 619 extends in the Y direction from a second end 219b to a first end 219a. The third wall 619 includes an inner surface 619c and an outer surface 619d. In the example of FIGS. 6A-6C, the third wall 619 and the second wall 218 each have a circular cross-sectional shape in the X-Z plane. Other implementations are possible. For example, the second wall 218 and the third wall 219 may have a square or rectangular cross-section in the X-Z plane. [0073] The inner surface 619c of the third wall 619 may be permanently joined to an outer surface 218c of the second wall 218. For example, the inner surface 619c and the portion of the outer surface 218c may be permanently joined by melting a filler metallic material (the filler metallic material having a lower melting point than the second metallic material) between the third wall 619 and the second wall 218 such that when the filler metallic material cools to a solid form, the third wall 619 and the second wall 218 are permanently joined.

[0074] The cooling system 234 is thermally coupled to the outer surface 619d. Under some conditions, there is a relatively large temperature difference between the interior region 203 and the cooling system 234. This temperature gradient may cause compressive and tensile stresses to be generated on objects (such as the second wall 218) that are between the interior region 203 and the cooling system 234. The compressive and/or tensile stresses may cause the objects to deform. By placing the third wall 619 between the second wall 218 and the cooling system 234, the distance between the interior 203 and the cooling system 234 is increased and the thermal gradient is reduced. Reducing the thermal gradient decreases the compressive and tensile stresses on the second wall 218 and, therefore, decreases the likelihood of material deformation of the second wall 218.

[0075] Referring to FIG. 7, an implementation of an LPP EUV light source 700 is shown. The LPP EUV light source 700 is an implementation of the EUV light source 100 (FIG. 1). The EUV light source 700 includes a target supply system 727. The target supply system 727 may include a two-metal or multi-metal target material tank, such as the target material tank 144 or 244.

[0076] The LPP EUV light source 700 is formed by irradiating a target mixture 714 at a plasma formation region 705 with an amplified light beam 710 that travels along a beam path toward the target mixture 714. The target material in the targets of the stream 121 discussed with respect to FIG. 1 may be or include the target mixture 714. The plasma formation region 705 is within the interior 707 of a vacuum chamber 730. When the amplified light beam 710 strikes the target mixture 714, a target material within the target mixture 714 is converted into a plasma state that has an element with an emission line in the EUV range. The created plasma has certain characteristics that depend on the composition of the target material within the target mixture 714. These characteristics may include the wavelength of the EUV light produced by the plasma and the type and amount of debris released from the plasma.

[0077] The light source 700 includes a drive laser system 715 that produces the amplified light beam 710 due to a population inversion within the gain medium or mediums of the laser system 715. The light source 700 includes a beam delivery system between the laser system 715 and the plasma formation region 705, the beam delivery system including a beam transport system 720 and a focus assembly 722. The beam transport system 720 receives the amplified light beam 710 from the laser system 715, and steers and modifies the amplified light beam 710 as needed and outputs the amplified light beam 710 to the focus assembly 722. The focus assembly 722 receives the amplified light beam 710 and focuses the beam 710 to the plasma formation region 705.

[0078] In some implementations, the laser system 715 may include one or more optical amplifiers, lasers, and/or lamps for providing one or more main pulses and, in some cases, one or more pre -pulses. Each optical amplifier includes a gain medium capable of optically amplifying the desired wavelength at a high gain, an excitation source, and internal optics. The optical amplifier may or may not have laser mirrors or other feedback devices that form a laser cavity. Thus, the laser system 715 produces an amplified light beam 710 due to the population inversion in the gain media of the laser amplifiers even if there is no laser cavity. Moreover, the laser system 715 may produce an amplified light beam 710 that is a coherent laser beam if there is a laser cavity to provide enough feedback to the laser system 715. The term “amplified light beam” encompasses one or more of: light from the laser system 715 that is merely amplified but not necessarily a coherent laser oscillation; and light from the laser system 715 that is amplified and is also a coherent laser oscillation.

[0079] The optical amplifiers in the laser system 715 may include as a gain medium a filling gas that includes CO2 and may amplify light at a wavelength of between about 9100 nm and about 11000 nm, and in particular, at about 10600 nm, at a gain greater than or equal to 900 times. Suitable amplifiers and lasers for use in the laser system 715 may include a pulsed laser device, for example, a pulsed, gas- discharge CO2 laser device producing radiation at about 9300 nm or about 10600 nm, for example, with DC or RF excitation, operating at relatively high power, for example, lOkW or higher and high pulse repetition rate, for example, 40 kHz or more. The pulse repetition rate may be, for example, 50 kHz. The optical amplifiers in the laser system 715 may also include a cooling system such as water that may be used when operating the laser system 715 at higher powers.

[0080] The light source 700 includes a collector mirror 735 having an aperture 740 to allow the amplified light beam 710 to pass through and reach the plasma formation region 705. The collector mirror 735 may be, for example, an ellipsoidal mirror that has a primary focus at the plasma formation region 705 and a secondary focus at an intermediate location 745 (also called an intermediate focus) where the EUV light may be output from the light source 700 and may be input to, for example, an integrated circuit lithography tool (not shown). The light source 700 may also include an open-ended, hollow conical shroud 750 (for example, a gas cone) that tapers toward the plasma formation region 705 from the collector mirror 735 to reduce the amount of plasma-generated debris that enters the focus assembly 722 and/or the beam transport system 720 while allowing the amplified light beam 710 to reach the plasma formation region 705. For this purpose, a gas flow may be provided in the shroud that is directed toward the plasma formation region 705.

[0081] The light source 700 may also include a master controller 755 that is connected to a droplet position detection feedback system 756, a laser control system 757, and a beam control system 758. The light source 700 may include one or more target or droplet imagers 760 that provide an output indicative of the position of a droplet, for example, relative to the plasma formation region 705 and provide this output to the droplet position detection feedback system 756, which may, for example, compute a droplet position and trajectory from which a droplet position error may be computed either on a droplet by droplet basis or on average. The droplet position detection feedback system 756 thus provides the droplet position error as an input to the master controller 755. The master controller 755 may therefore provide a laser position, direction, and timing correction signal, for example, to the laser control system 757 that may be used, for example, to control the laser timing circuit and/or to the beam control system 758 to control an amplified light beam position and shaping of the beam transport system 720 to change the location and/or focal power of the beam focal spot within the chamber 730. [0082] The supply system 725 includes a target material delivery control system 726 that is operable, in response to a signal from the master controller 755, for example, to modify the release point of the droplets as released by the target supply system 727 to correct for errors in the droplets arriving at the desired plasma formation region 705.

[0083] Additionally, the light source 700 may include light source detectors 765 and 770 that measure one or more EUV light parameters, including but not limited to, pulse energy, energy distribution as a function of wavelength, energy within a particular band of wavelengths, energy outside of a particular band of wavelengths, and angular distribution of EUV intensity and/or average power. The light source detector 765 generates a feedback signal for use by the master controller 755. The feedback signal may be, for example, indicative of the errors in parameters such as the timing and focus of the laser pulses to properly intercept the droplets in the right place and time for effective and efficient EUV light production.

[0084] The light source 700 may also include a guide laser 775 that may be used to align various sections of the light source 700 or to assist in steering the amplified light beam 710 to the plasma formation region 705. In connection with the guide laser 775, the light source 700 includes a metrology system 724 that is placed within the focus assembly 722 to sample a portion of light from the guide laser 775 and the amplified light beam 710. In other implementations, the metrology system 724 is placed within the beam transport system 720. The metrology system 724 may include an optical element that samples or re-directs a subset of the light, such optical element being made out of any material that may withstand the powers of the guide laser beam and the amplified light beam 710. A beam analysis system is formed from the metrology system 724 and the master controller 755 since the master controller 755 analyzes the sampled light from the guide laser 775 and uses this information to adjust components within the focus assembly 722 through the beam control system 758.

[0085] Thus, in summary, the light source 700 produces an amplified light beam 710 that is directed along the beam path to irradiate the target mixture 714 at the plasma formation region 705 to convert the target material within the mixture 714 into plasma that emits light in the EUV range. The amplified light beam 710 operates at a particular wavelength (that is also referred to as a drive laser wavelength) that is determined based on the design and properties of the laser system 715. Additionally, the amplified light beam 710 may be a laser beam when the target material provides enough feedback back into the laser system 715 to produce coherent laser light or if the drive laser system715 includes suitable optical feedback to form a laser cavity.

[0086] Implementations of the disclosure may further be described using the following clauses:

1. An apparatus for an extreme ultraviolet (EUV) light source, the apparatus comprising: a body comprising: a first structure comprising a first wall; and a second structure comprising a second wall permanently joined to the first wall, wherein an interior of the body is at least partially defined by the first wall and the second wall, the first wall comprises a first metallic material, and the second wall comprises a second metallic material that has a different thermal conductivity than the first metallic material, and wherein the interior of the body is configured to be fluidly connected to a target supply system of the EUV light source.

2. The apparatus of clause 1, wherein a first end of the first wall and a second end of the second wall are permanently joined at a brazed interface.

3. The apparatus of clause 1, wherein the first metallic material comprises molybdenum (Mo), and the second metallic material comprises stainless steel.

4. The apparatus of clause 1, further comprising: a temperature control system configured to control a temperature of at least one of the first wall and the second wall.

5. The apparatus of clause 4, wherein the temperature control system comprises: a heating system configured to be thermally coupled to the first wall; and a cooling system configured to be thermally coupled to the second wall, wherein the thermal conductivity of the second metallic material is lower than the thermal conductivity of the first metallic material.

6. The apparatus of clause 1, wherein the thermal conductivity of the second metallic material is lower than the thermal conductivity of the first metallic material, the first wall extends from a first end to a second end, the second wall extends from a first end to a second end, the first end of the first wall is brazed to the second end of the second wall, and the apparatus further comprises: an O-ring at the first end of the second wall; and a removable lid configured to be held at the O-ring.

7. The apparatus of clause 1, wherein the thermal conductivity of the second metallic material is lower than the thermal conductivity of the first metallic material, the first wall extends from a first end to a second end, the second wall extends from a first end to a second end, the first end of the first wall is brazed to the second end of the second wall, and the apparatus further comprises: at least one port extending from the second wall, the at least one port comprising the second metallic material.

8. The apparatus of clause 7, wherein the second metallic material comprises stainless steel.

9. The apparatus of clause 1, wherein the first metallic material comprises a first coefficient of thermal expansion, and the second metallic material comprises a second coefficient of thermal expansion.

10. The apparatus of clause 1, wherein an outer side of the first wall is permanently joined to an inner side of the second wall.

11. The apparatus of clause 1, wherein the body further comprises: a third structure comprising a third wall, the third wall comprising an inner surface and an outer surface; and wherein the inner surface of the third wall is permanently joined to an outer surface of the second wall, and the third wall comprises the second metallic material.

12. The apparatus of clause 11, further comprising: a temperature control system configured to control a temperature of at least one of the first wall, the second wall, and the third wall, and wherein the thermal conductivity of the second metallic material is lower than the thermal conductivity of the first metallic material.

13. The apparatus of clause 12, wherein the temperature control system comprises: a heating system configured to be thermally coupled to the first wall; and a cooling system configured to be thermally coupled to the second wall and the third wall, wherein the third wall is between the second wall and the cooling system.

14. The apparatus of clause 1, wherein the apparatus is a target material tank configured to hold a target material in the interior of the body, the target material emitting EUV light when in a plasma state.

15. The apparatus of clause 1, wherein the apparatus is a connection assembly, the first structure comprises at least a first port, the second structure comprises at least a second port, and the first port and the second port are in fluid communication with each other.

16. The apparatus of clause 15, wherein the apparatus is configured to provide a fluid path between an external device coupled to the second port and a reservoir coupled to the first port.

17. An extreme ultraviolet (EUV) light source comprising: a target supply system comprising: a droplet generator configured to produce a stream of targets, wherein the targets comprise a target material that emits EUV light when in a plasma state; and at least one apparatus comprising an interior region configured to be fluidly coupled to the droplet generator, the apparatus comprising: a first structure comprising a first wall; and a second structure comprising a second wall permanently joined to the first wall, wherein the interior region is at least partially defined by the first wall and the second wall, the first wall comprises a first metallic material, and the second wall comprises a second metallic material that has a different thermal conductivity than the first metallic material; and a vessel configured to receive the targets from the droplet generator.

18. The EUV light source of clause 17, further comprising a light source configured to produce a pulse of light having an energy sufficient to convert at least some of the target material in a target into the plasma state in which the target material emits EUV light.

19. The EUV light source of clause 17, wherein the at least one apparatus is a target material tank configured to hold the target material in the interior region. 20. The EUV light source of clause 19, wherein the target supply system further comprises at least one valve, the at least one valve being configured to fluidly connect or fluidly disconnect the interior region of the target material tank with the droplet generator.

21. The EUV light source of clause 17, wherein the at least one apparatus is a connection assembly, the first structure comprises at least a first port, the second structure comprises at least a second port, and the first port and the second port are in fluid communication with each other.

22. The EUV light source of clause 21, wherein the target supply system further comprises: an external device coupled to the second port; and a reservoir coupled to the first port, wherein the reservoir is configured to hold the target material in an interior cavity and be fluidly coupled to the droplet generator; and wherein the connection assembly is configured to provide a fluid path between the external device and the reservoir.

23. The EUV light source of clause 22, wherein the external device is a vacuum system or a gas supply system.

24. A target supply system for an EUV light source, the target supply system comprising: a droplet generator configured to produce a stream of targets, wherein the targets comprise a target material that emits EUV light when in a plasma state; and at least one apparatus comprising an interior region configured to be fluidly coupled to the droplet generator, the apparatus comprising: a first structure comprising a first wall; and a second structure comprising a second wall permanently joined to the first wall, wherein the interior region is at least partially defined by the first wall and the second wall, the first wall comprises a first metallic material, and the second wall comprises a second metallic material that has a different thermal conductivity than the first metallic material.

25. The target supply system of clause 24, wherein the at least one apparatus is a target material tank configured to hold the target material in the interior region.

26. The target supply system of clause 24, further comprising at least one valve, the at least one valve being configured to fluidly connect or fluidly disconnect the interior region of the at least one apparatus with the droplet generator.

27. The target supply system of clause 24, wherein the at least one apparatus is a connection assembly, the first structure comprises at least a first port, the second structure comprises at least a second port, and the first port and the second port are in fluid communication with each other.

[0087] Other implementations are within the scope of the claims.