CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to U.S. Application No. 62/845,007, filed May 8, 2019 and titled PROTECTION SYSTEM FOR AN EXTREME ULTRAVIOLET LIGHT SOURCE, and which is incorporated herein in its entirety by reference.

TECHNICAL FIELD

[0002] This disclosure relates to a protection system for an extreme ultraviolet (EUV) light source.

BACKGROUND

[0003] Extreme ultraviolet (“EUV”) light, 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, 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, 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, a target delivery system for an extreme ultraviolet light (EUV) source includes: a conduit including an exterior, an interior conduit region, and an end that defines an orifice. The interior conduit region is configured to receive a target material that emits EUV light when in a plasma state, and the orifice is configured to provide the target material to an interior of a vacuum chamber. The target delivery system also includes a protection system configured to flow a protection gas away from the end that defines an office and toward the interior of the vacuum chamber. The flowing protection gas is configured to direct one or more contaminant substances away from the end that defines the orifice.

[0006] Implementations may include one or more of the following features. The protection gas may include an inert gas or a reacting gas.

[0007] The protection gas may include molecular hydrogen (¾).

[0008] The protection system may be configured to flow the protection gas along the exterior of the conduit. The protection system may include a body including a sidewall that surrounds at least a portion of the exterior of the conduit, the body defining an open end region aligned with the orifice of the conduit. The protection gas may flow in an open space between the exterior of the conduit and an inner wall of the sidewall, and the protection gas may flow through the open end region to exit the body. The sidewall may include at least one port in fluid communication with the open space, the port being configured to fluidly couple to a gas supply that holds the protection gas.

[0009] The protection system includes at least one gas source.

[0010] Target delivery system also may include a temperature control block that at least partially surrounds the exterior of the conduit, and the protection system may include a body that surrounds at least a portion of the temperature control block, the body defining an open end region aligned with the orifice of the conduit. The protection gas may flow in an open space between the temperature control block and an inner wall of the body, and the protection gas may exit the body through the open end region.

[0011] The one or more contaminant substances may include moving substances, and the flowing fluid may be configured to reduce interactions between the one or more moving contaminant substances and the end of the conduit by changing a direction of the motion of the one or more moving contaminant substances away from the end of the conduit.

[0012] The one or more contaminant substances may include moving substances, and the flowing fluid may be configured to prevent interactions between the one or more moving contaminant substances and the end of the conduit by changing a direction of the motion of the one or more moving contaminant substances away from the end of the conduit. [0013] The one or more contaminant substances may include one or more of a gas, liquid, vapor, and particles.

[0014] The one or more contaminant substances may include silicon (Si) or silicon dioxide (SiOi).

[0015] The one or more contaminant substances may include oxygen, water, or carbon dioxide

(C0 ₂ ).

[0016] The conduit may include a capillary tube.

[0017] The protection system may include a diffuser device that includes a plurality of openings, each opening may be configured to direct the protection gas away from the end that defines the orifice. The plurality of openings may surround the exterior of the conduit and may be uniformly distributed relative to the exterior of the conduit.

[0018] In another aspect, a method of protecting an orifice of a target material delivery system includes: passing target material through an orifice to provide a stream of targets to an interior of a vacuum chamber, each target in the stream including a target material that emits EUV light when in a plasma state; and flowing a protection gas in the target material delivery system and into the interior of the vacuum chamber away from the orifice, the protection gas directing one or more contaminant substances away from the orifice.

[0019] Implementations may include one or more of the following features. In some

implementations, the flowing protection gas does not change a trajectory of the stream of targets. The flowing protection gas may have a motion component along a direction of travel of the stream of targets.

[0020] Flowing the protection gas may include flowing the protection gas in an open space between a conduit that defines the orifice and a body that surrounds the conduit. The protection gas may flow into the open space at a port in the body and may flow out of the space through an open end region that is defined by the body and is aligned with the orifice.

[0021] The protection gas may have a uniform volumetric flow rate at an aperture between the orifice and the interior of the vacuum chamber.

[0022] The method also may include determining a state of an extreme ultraviolet light source that includes the target material delivery system; and determining which of a plurality of protection gasses to use as the protection gas based on the determined state. [0023] Implementations of any of the techniques described above may include an EUV light source, a target supply 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

[0024] FIG. 1 is a block diagram of an example of an EUV light source.

[0025] FIG. 2A is a side cross-sectional view of an example of a supply system.

[0026] FIG. 2B is a bottom view of the supply system of FIG. 2A along a line 2A— 2 A’ of FIG. 2A.

[0027] FIG. 2C is a side cross-sectional view of another example of a supply system.

[0028] FIG. 2D is a bottom view of the supply system of FIG. 2C along a line 2C— 2C’ of FIG. 2C.

[0029] FIG. 3A is a side cross-sectional view of another example of a supply system.

[0030] FIG. 3B is a bottom view of the supply system of FIG. 3A.

[0031] FIG. 4A is a side cross-sectional view of another example of a supply system.

[0032] FIG. 4B is a bottom view of the supply system of FIG. 5A.

[0033] FIGS. 5 A and 5B are flow charts of example processes related to flowing a protection gas.

[0034] FIGS. 6 and 7 are block diagrams of an example of a lithographic apparatus.

[0035] FIG. 8 is a block diagram of an example of an EUV light source.

DETAIFED DESCRIPTION

[0036] Referring to FIG. 1, a block diagram of an EUV light source 100 that includes a supply system 110 is shown. The supply system 110 includes a protection system 130, which protects the supply system 110 by directing a protection gas 131 (shown with a dash-dot line style in FIGS. 1, 3A, and 4A) away from the supply system 110. The protection gas 131 carries a contaminant substance 150 (shown as shaded circles in FIG. 1) away from the supply system 110 and/or prevents the contaminant substance 150 from reaching the supply system 110.

[0037] The supply system 110 emits a stream 121 of targets such that a target 121p is delivered to a plasma formation location 123 in a vacuum chamber 109. The target 121p includes target material, which is any material that has an emission line in the extreme ultraviolet (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, SnBn, SntE; as a tin alloy, for example, tin- gallium alloys, tin-indium alloys, tin-indium-gallium alloys, or any combination of these alloys.

[0038] The plasma formation location 123 receives a light beam 106. The light beam 106 is generated by an optical source 105 and delivered to the vacuum chamber 109 via an optical path 107. An interaction between the light beam 106 and the target material in the target 121p produces a plasma 196 that emits EUV light 197. The EUV light 197 is directed toward a lithography tool 199 by an optical element 198.

[0039] The supply system 110 includes a conduit 112. The conduit 112 is a three-dimensional object, such as a tube or cylinder, defined by a sidewall 114. The sidewall 114 extends from a first end 115 to a second end 116. The second end 116 includes an orifice 117 that passes through the sidewall 114 into an interior of the conduit 112 and fluidly couples to a reservoir 140. The reservoir 140 holds a target mixture 141, which includes the target material and also may include impurities. In operational use, the target mixture 141 flows in the conduit 112 and is emitted from the orifice 117 as the stream 121.

[0040] In the example of FIG. 1, the supply system 110 also includes an actuator 135 that is mechanically coupled to the sidewall 114. The actuator 135 may be, for example, a piezoelectric ceramic material such as lead zirconate titanate (PZT) that changes shape in response to the application of voltage. The sidewall 114 is deformed by the actuator 135. Deforming the sidewall 114 modulates a pressure of the target mixture in the conduit 112 and breaks up the target material that flows through the orifice into the stream 121 of targets. The size and spacing of the targets in the stream 121 may be controlled by controlling the frequency and/or amplitude of the deformation applied by the actuator 135. The stream 121 includes a plurality of distinct spherical targets that have a diameter of, for example, 30 micrometers (pm). The supply system 110 may deliver target material to the vacuum chamber 109 in another manner. For example, the supply system 110 may produce a jet of target material that is not broken into individual targets.

[0041] The supply system 110 also includes the protection system 130. The protection system 130 includes a gas direction system 132, which directs the protection gas 131 away from the second end 116. The gas direction system 132 includes a gas management system 167 that includes devices, components, and/or systems that are configured to direct the protection gas 131. For example, the gas direction system 132 may include pumps, flow control devices (such as valves and/or fluid switches), openings through which the protection gas 131 flows, and/or nozzles.

[0042] The gas direction system 132 is fluidly coupled to a gas supply 133 via a fluid connection 134. The gas supply 133 includes a chamber 137 that contains a gas that is used as the protection gas 131. For example, the chamber 137 of the gas supply 133 may contain an inert gas or a reacting gas. An inert gas is a gas that does not react with anything in the vacuum chamber 109. A reacting gas is a gas that is capable of reacting with one or more items that are within the vacuum chamber 109. The protection gas 131 may be, for example, molecular hydrogen (Fh) or argon (Ar). The protection gas 131 may include substances that are not in the gas phase. For example, the protection gas 131 may include solid nanoclusters that are carried with the protection gas 131.

[0043] In some implementations, the chamber 137 contains more than one different gas. For example, the chamber 137 may include a plurality chambers that are not fluidly coupled to each other but are each configured to be fluidly coupled to the fluid connection 134. In these implementations, one chamber may include, for example molecular hydrogen gas (H2) and another chamber may include argon (Ar) gas. In implementations that include more than one chamber 137, the gas management system 167 includes a fluid switch mechanism that allows selection of one of the plurality of chambers 137.

[0044] In the example of FIG. 1, the stream 121 travels generally along the -X direction and the protection gas 131 also flows along the sidewall 114 and into the vacuum chamber 109 in the -X direction. The second end 116 extends generally in the Y-Z plane. Thus, the protection gas 131 flows away from the second end 116 and the orifice 117. By directing the protection gas 131 away from the second end 116, the protection system 130 protects the supply system 110 from the contaminant substance 150. The flow of the protection gas 131 is, in various

implementations, parallel to, or substantially parallel to, the direction of motion of the targets in the stream 121.

[0045] The contaminant substance 150 is any substance that is capable of blocking the orifice 117 and/or adhering to the second end 116 in a manner that blocks the orifice 117. For example, the contaminant substance 150 may be capable of forming a layer on the second end 116 that completely or partially blocks the orifice 117. The contaminant substance 150 moves within the vacuum chamber 109. For example, in the absence of the protection gas 131, the contaminant substance 150 may move toward the second end 116 and/or the orifice 117. In response to an interaction with the protection gas 131, the contaminant substance 150 moves away from the second end 116 and the orifice 117.

[0046] The contaminant substance 150 may include solid, liquid, and/or gaseous matter. The contaminant substance 150 may include more than one type of contaminant substance and/ or more than one substance. For example the contaminant substance 150 may include particles of silica, siloxanes, silicon dioxide (S1O2), tin oxide (SnOi). a gas (such as oxygen), and/or vapor (such as tin vapor). The contaminant substance 150 may include, for example, vapor of silicon (Si), silicon dioxide (S1O2), and/or tin oxide (SnCE) that is capable of being deposited on the second end 216 from the vapor phase. The contaminant substance 150 may arise from components within the vacuum chamber 109 and/or from interaction with items in the vacuum chamber 109 with oxygen, water, and or carbon dioxide (CO2) when the vacuum chamber 109 is depressurized.

[0047] By directing the contaminant substance 150 away from the second end 116, the protection gas 131 prevents or reduces the likelihood of formation of a layer of the contaminant substance 150 on the second end 116. A layer of the contaminant substance 150 on the second end 116 may interfere with the formation of the stream 121. For example, such a layer may completely or partially block the orifice 117, thereby blocking the stream 121 and/or altering the properties of the stream 121. Because the stream 121 includes the targets that are used to produce the EUV light 197, unexpected and/or undesired alterations in the stream 121 may lead to decreased production of the EUV light 197. Thus, by directing the contaminant substance 150 away from the end 116 and the orifice 117, the protection system 130 improves the overall performance of the supply system 110 and the EUV light source 100. In addition to improving the overall performance of the EUV light source 100 during operation, the protection system 130 decreases the downtime of the EUV light source 100. For example, to remove a layer of the contaminant substance 150 from the second end 116, the supply system 110 is removed from the EUV light source 100. Thus, by preventing or reducing the build-up of the contaminant substance 150 on the second end 116, the protection system 130 also reduces the amount of maintenance performed on the supply system 110 and reduces the downtime of the EUV light source 100.

[0048] The EUV light source 100 also includes a control system 160 that governs the operation of the protection system 130. The control system 160 may be coupled to the gas supply 133, the gas management system 167, and/or the gas direction system 132. For example, the control system 160 may control the flow rate of the protection gas 131 by controlling valves and/or pumps within the gas direction system 132 or the gas supply 133. In another example, in implementations in which the gas supply 133 includes a plurality of chambers 137, the control system 160 may be used to control a switch in the gas management system 167. This allows the control system 160 to switch between the chambers such that the protection gas 131 is formed from a gas in one of the chambers at a particular time. The control system 160 also may be coupled to other systems and components of the EUV light source 100, such as the actuator 135, and/or the optical source 105.

[0049] The control system 160 includes an electronic processing module 161, an electronic storage 162, and an I/O interface 163. The electronic processing module 161 includes one or more processors suitable for the execution of a computer program such as a general or special purpose microprocessor, and any one or more processors of any kind of digital computer.

Generally, an electronic processor receives instructions and data from a read-only memory, a random access memory (RAM), or both. The electronic processing module 161 may be any type of suitable electronic processor.

[0050] The electronic storage 162 may be volatile memory, such as RAM, or non-volatile memory. In some implementations, and the electronic storage 162 includes non-volatile and volatile portions or components. The electronic storage 162 may store data and information that is used in the operation of the control system 160. For example, the electronic storage 162 may store information about the operation of the supply system 110 and/or the protection system 130. For example, in some implementations, the electronic storage 162 stores the flow rate at which the protection gas 131 should flow from the gas direction system 132 during typical operation of the EUV light source 100.

[0051] The electronic storage 162 also stores instructions, such as one or more computer programs, that, when executed, cause the electronic processing module 161 to communicate with components in the supply system 110 and/or the protection system 130. For example, the electronic storage 162 may store instructions that cause the electronic processing module 161 to provide modulation signals that are sufficient to cause the actuator 135 to vibrate the conduit.

[0052] The I/O interface 163 is any type of interface that allows the control system 160 to receive or send information or data. For example, the I/O interface 163 may be a keyboard, mouse, or other computer peripheral device that enables an operator to operate and/or program the control system 160. The I/O interface 163 may include devices that produce a perceivable alert such as a light or a speaker. Furthermore, the I/O interface 163 may include a

communications interface such as a universal serial port (USB), a network connection, or any other interface that allows communication with the control system 160.

[0053] FIG. 2A is a side cross-sectional view of a supply system 210, which includes a conduit 212 and a gas direction system 232, in an X-Z plane. FIG. 2B is a bottom view of the conduit 212 and the gas direction system 232 in a Y-Z plane as seen from the line 2A— 2 A’ of FIG. 2A. In FIG. 2B, the X direction is into the page. The conduit 212 may be used in the EUV light source 100 (FIG. 1). The conduit includes a sidewall 214 that extends along the X direction from a first end 215 to a second end 216. The sidewall 214 forms the conduit 212, which is a three-dimensional object that is generally cylindrical and has a generally cone-shaped nozzle 250 at the end 216. The conduit 212 may be, for example, a capillary tube.

[0054] The sidewall 214 includes an inner surface 253 and an exterior wall 254. The inner surface 253 defines an interior region 258 (FIGS. 2A and 2B) that is in fluid communication with the nozzle 250. The nozzle 250 narrows along the -X direction to define an orifice 217. In the example of FIGS. 2A and 2B, the nozzle 250 is generally cone shaped and the orifice 217 is at the apex of the cone. The interior region 258 is fluidly coupled to a reservoir (such as the reservoir 140 of FIG. 1) that holds a target mixture (such as the target mixture 141 of FIG. 1), and the target mixture flows in the interior region 258 of the conduit 212 and through the orifice 217 in the -X direction.

[0055] The gas direction system 232 is a three-dimensional body that encloses a space 238. The space is fluidly coupled to the gas supply 133. The gas direction system 232 includes a plurality of openings 236 that pass through a bottom portion 239 of the gas direction system 232. For simplicity, only one of the openings 236 is labeled in FIGS. 2 A and 2B. The protection gas 131 flows from the gas supply 133 into the space 238 and flows out of the openings 236 in the -X direction. The flow rate and direction of the protection gas 131 that leaves each of the openings 236 is substantially the same.

[0056] In the example of FIG. 2B, the openings 236 are arranged in a rectilinear grid in the bottom portion 239. However, other implementations are possible. For example, the openings 236 may be arranged in a random pattern. Moreover, the openings 236 may have any shape. In the example of FIG. 2B, the each opening 236 is a circle in the Y-Z plane. In other

implementations, the openings 236 may be elliptical or the openings may form concentric circles that are centered on the orifice 217.

[0057] Furthermore, the gas direction system 232 may have any shape. In the example of FIG. 2B, the gas direction system 232 is a cylindrical body with a circular cross-section in the Y-Z plane. In other implementations, the gas direction system 232 may have, for example, a square or rectangular cross-section in the Y-Z plane.

[0058] In the example of FIGS. 2A and 2B, the gas direction system 232 is a single element that includes a plurality of openings 236, each of which directs the protection gas 131 in a direction that is away from the nozzle 250. However, other implementations are possible. For example, the gas direction system 232 may be a collection of discrete gas collection systems, each of which are individually fluidly coupled to the gas supply 133 or to separate gas supplies. In these implementations, each of the gas direction systems is individually controllable (for example, with the control system 160 of FIG. 1) to provide a stream of the protection gas 131.

[0059] Moreover, the supply system 210 may include additional components that are not shown in FIGS. 2A and 2B. For example, the supply system 210 may include an actuator (such as the actuator 135 of FIG. 1) that is external to the conduit 212.

[0060] FIGS. 2C and 2D show a supply system 2 IOC that is the same as the actuator 210 except the supply system 2 IOC includes an actuator 235 that is mounted to and surrounds a portion of the exterior wall 254. The actuator 235 is shown with cross-hatch shading in FIGS. 2C and 2D. FIG. 2C is a cross-sectional view of the supply system 210C in the X-Z plane. FIG. 2D is a bottom view of the supply system 2 IOC from the perspective of the line 2C— 2C’ of FIG. 2C. The supply system 2 IOC is another example of an implementation of the supply system 110 of FIG. 1. The actuator 235 is an example of an implementation of the actuator 135 (FIG. 1).

[0061] The actuator 235 is a three-dimensional cylindrical body that includes an inner surface 259 and an outer surface 257. The inner surface 257 of the actuator 235 is mechanically coupled to a portion of the exterior wall 254 of the conduit 212 with, for example, an adhesive material. The actuator 235 surrounds a portion the exterior wall 254. As shown in FIG. 2D, the actuator 235 has a circular cross section in the Y-Z plane.

[0062] The actuator 235 may be made of a solid material, such as PZT, that the protection gas 131 does not penetrate. The adhesive that couples the actuator 235 may be, for example, an epoxy resin that is also generally not penetrable by the protection gas 131. Thus, under ordinary operation while the actuator 235 is properly coupled to the exterior wall 254, the protection gas 131 flows around the actuator 235. In implementations such as shown in FIGS. 2C and 2D) in which the actuator 235 is between the gas direction system 232 and the orifice 217, the protection gas 131 flows along the exterior wall 254 except at the portions where the actuator 235 is attached to the conduit 212. At the portions where the actuator 235 is attached to the conduit 212, the protection gas 131 still flows along the conduit 212 but the protection gas 131 flows around the exterior surface 357 of the of the actuator 235. In other words, the protection gas 131 flowing along the exterior of the conduit 212 includes the scenario in with the protection gas 131 flows around an item (such as the actuator 235) attached to the exterior wall 245 of the conduit 212 that is not penetrable by the protection gas 131.

[0063] Referring to FIGS. 3 A and 3B, a supply system 310 is shown. The supply system 310 is another example of the supply system 110. The supply system 310 may be used in the EUV light source 100 of FIG. 1. For example, the supply system 310 may be mounted to the vacuum chamber 109 and used to produce the stream 121 of targets. FIG. 3A is a cross-sectional view of the supply system 310 in the X-Z plane. FIG. 3B is a view of an end 379 of the supply system 310 in the Y-Z plane. In FIG. 3B, the X direction is into the page.

[0064] The supply system 310 includes the conduit 212 (discussed above with respect to FIGS. 2A and 2B) and a housing 370. The housing 370 is a three-dimensional body that surrounds the conduit 212. The housing 370 includes a sidewall 371. The sidewall 371 defines a fluid port 372 that is coupled to the gas supply 133. The fluid port 372 may connect to a fluid connection (such as the fluid connection 134 of FIG. 1), or the fluid port 372 may be directly coupled to the gas supply 133. The sidewall 371 also defines an open region 378 at the end 379.

[0065] The fluid port 372 is open to an interior 373 of the housing 370. The conduit 212 is within the interior 373. The conduit 212 and the housing 370 are positioned relative to each other such that the orifice 217 of the conduit 212 is aligned with the open region 378 along the X direction. The alignment of the orifice 217 and the open region 378 allows the target material that is emitted from the orifice 217 to exit the housing 370 along the -X direction.

[0066] The sidewall 371 includes an inner wall 375 that is separated from the exterior wall 254 of the conduit 212 to form an open space 376. The open space 376 is the portion of the interior 373 (within the housing 370) that is between the exterior wall 254 of the conduit 212 and the inner wall 375. The open space 376 is fluidly coupled to the fluid port 372. The protection gas 131 in the supply 133 is held at a higher pressure than the pressure in the vacuum chamber 109, thus the protection gas 131 flows from the gas supply 133 into the fluid port 372 and into the open space 376. The pressure in the vacuum chamber 109 is lower than the pressure in the interior 373. The protection gas 131 flows along the exterior wall 254 of the conduit 212 generally in the -X direction and exits the housing 370 through the open region 378. The protection gas 131 and the target material both flow through the open region 378 in the same general direction. In the example of FIGS. 3 A and 3B, the target material and the protection gas 131 move through the open region 378 generally in the -X direction.

[0067] The protection gas 131 may have a uniform flow rate at all points in the Y-Z plane at the open region 378 and does not substantially disturb the trajectory of the target material that passes through the open region 378. The flow of the protection gas 131 in the X-Y plane at the open region 378 is referred to as the gas flow field. Although the gas flow field may influence the trajectories of the individual targets that are emitted from the orifice 317, the properties (for example, flow rate and direction) of the gas flow field are such that the trajectories of the individual targets are not changed substantially. For example, the gas flow field does not cause the target trajectories to deviate so much that the targets in the stream 121 are not delivered to the plasma formation site 123 (FIG. 1). Additionally, the mass and density of the targets emitted from the orifice 317 is much higher than the mass and density of the protection gas 131. The ratio of the mass and/or density of the targets as compared to the protection gas 131 also minimizes the impact of the protection gas 131 on the trajectories of the targets.

[0068] The flow rate of the protection gas 131 is sufficient to move the contaminant substance 150 (FIG. 1) away from the open region 378 and/or sufficient to prevent the contaminant substance 150 from moving through the open region 378 and entering the open space 376. The flow rate of the protection gas 131 is higher than the diffusion speed of the contaminant substance 150. Diffusion is the net movement of material (for example, molecules or atoms that are in the contaminant substance 150) from a region of higher concentration (or high chemical potential) to a region of lower concentration (or low chemical potential). Diffusion is driven by a gradient in chemical potential of the diffusing species. The protection gas 131 may be considered to mitigate the movement of the contaminant substance 150 into the orifice 317 by the Peclet effect, which is quantified by the Peclet number (Pe). The Peclet number is the ratio of the advective transport rate to the diffusive transport rate. The advective transport rate is the transport rate of the protection gas 131. The diffusive transport rate is the rate of diffusion of the contaminant substance 150. As the Peclet number increases, the protection gas 131 is more likely to dominate the interaction between the gas 131 and the contaminant substance 150, and the protection gas 131 is more likely to push the contaminant substance 150 away from the open region 378. The Peclet number may be increased by increasing the flow velocity of the protection gas 131 and/or increasing the characteristic length (for example, the length along the X direction that the protection gas 131 flows in the supply system 310). Thus, the Perclet number and mitigation of the contaminant substance 150 is controllable through the design of the housing 371 and/or the control of the flow rate of the protection gas 131.

[0069] In some implementations, the flow rate of the protection gas 131 may be between 1 and 50 standard liter per minute (slm). The flow rate of the protection gas 131 at the open region 378 depends on the extent of the open space 376 in the X direction, the flow rate of the protection gas 131 that enters the fluid port 372 from the gas supply 133, the physical properties of the inner wall 375, and the size of the open region 378 in the Y-Z plane.

[0070] The example of FIGS. 3 A and 3B includes one gas port 372. However, in other implementations, more gas ports may be used. For example, a plurality of gas ports 372 may be included. The plurality of gas ports 372 may be circumferentially spaced and equidistant from each other in the Y-Z plane. Moreover, in some implementations, the supply system 310 includes an actuator that is attached to and surrounds a portion of the exterior wall 254, such as the actuator 237 (FIGS. 2C and 2D). When the actuator is between the gas port 372 and the orifice 217, the protection gas 131 flows along the exterior wall 254 and/or the exterior surface of the actuator. The protection gas 131 does not flow between the actuator and the exterior wall 254 when the actuator is attached to the exterior wall 254.

[0071] FIGS. 4A and 4B show a supply system 410. The supply system 410 is another example of an implementation of the supply system 110. The supply system 410 may be used in the EUV light source 100 of FIG. 1. For example, the supply system 410 may be mounted to the vacuum chamber 109 and used to produce the stream 121 of targets. FIG. 4A is a cross-sectional view of the supply system 410 in the X-Z plane. FIG. 4B is a view of an end 479 of the supply system 410 in the Y-Z plane. In FIG. 4B, the X direction is into the page. The supply system 410 is similar to the supply system 310 (FIGS. 3A and 3B), except the supply system 410 includes a temperature control block 480 that controls a temperature of the conduit 212 and/or the nozzle 250.

[0072] The supply system 410 includes a housing 470. The housing 470 includes a sidewall 471 that defines an interior 473. The conduit 212 (FIGS. 2 A and 2B) is in the interior 473. The conduit 212 and the housing 470 are positioned relative to each other such that the orifice 217 is aligned with an open region 478 at an end 479 of the housing. The stream 121 that is emitted from the orifice 217 exits the housing 470 along the -X direction.

[0073] The supply system 410 includes the temperature control block 480. The temperature control block 480 is in the interior 473. The temperature control block 480 surrounds at least a portion of the exterior wall 254 of the conduit 212. In the example of FIGS. 4A and 4B, the temperature control block 480 is a cylindrical body has a longitudinal axis along the X direction that is concentric with a longitudinal axis of the conduit 212 along the X direction.

The temperature control block 480 is made of a material that may be heated or cooled. The temperature control block 480 is made from any thermally conductive material. The temperature control block 480 is thermally coupled to a controllable heater and/or cooler such that the temperature of the control block 480 is controllable. The temperature control block 480 may be, for example, a solid block of a material that is resistant to corrosion from the target material. In implementations in which the target material includes tin the temperature control block 480 may be, for example, molybdenum (Mo).

[0074] The temperature control block 480 is close enough to the exterior wall 254 to influence the temperature of the exterior wall 254 (and thus the conduit 212) but the temperature control block 480 does not touch the exterior wall 254. When the temperature control block 480 is warmer than the conduit 212, the temperature control block 480 heats the conduit 212. Heating the conduit 212 may, for example, encourage the target material in the conduit 212 to flow more efficiently. When the temperature control block 480 is cooler than the conduit 212, the temperature control block 480 reduces the temperature of the conduit 212. [0075] The temperature control block 480 and the exterior wall 254 are not in direct physical contact, and there is an open space 481 between the temperature control block 480 and the exterior wall 254. Fluid (such as the protection gas 131) is able to flow in the open space 481 between the temperature control block 480 and the wall 254. The temperature control block 480 may be mounted to an inner wall 475 of the housing 470 or to the reservoir 140. Thus, the temperature control block 480 does not necessarily touch the inner wall 475, and a fluid (such as the protection gas 131) may flow between the temperature control block 480 and the inner wall 475. In the implementation of FIGS. 4A and 4B, an open space 482 is between the temperature control block 480 and the inner wall 475 and the open space 481 is between the temperature control block 480 and the exterior wall 254 of the conduit 212. The protection fluid 131 flows in the open space 481 and the open space 482.

[0076] The sidewall 471 defines a fluid port 472, which is fluidly coupled to the gas supply 133 and the interior 473. The protection gas 131 flows from the gas supply 133 into the interior 473. The pressure in the vacuum chamber 109 is lower than the pressure in the interior 473, and the protection gas 131 is drawn through the open space 481 and the open space 482.

[0077] The protection gas 131 flows through the open region 478 generally in the -X direction and into the vacuum chamber 109. The protection gas 131 and the stream 121 flow through the open region 478 in generally the same direction (the -X direction). Thus, the protection gas 131 flows through the open region 478 and into the vacuum chamber 109 in a direction that is away from the orifice 217 and is away from the end 479. The direction of flow of the protection gas 131 discourages or prevents the contaminant substance 150 from entering the housing 470 through the open region 478 and reduces the likelihood of the orifice 217 becoming blocked by the contaminant substance 150.

[0078] The protection gas 131 has a uniform flow rate at all points in the Y-Z plane at the open region 478 and thus does not disturb the trajectory of the target material that passes through the open region 478. The flow rate of the protection gas 131 is sufficient to move the contaminant substance 150 away from the open region 478 and/or sufficient to prevent the contaminant substance 150 from moving through the open region 478 into the interior 473. For example, the flow rate of the protection gas 131 may be between 1 and 50 standard liter per minute (slm). The flow rate of the protection gas 131 at the open region 478 depends on the extent of the open spaces 481 and 482 in the X direction, the dimensions and placement of the temperature control block 480, the flow rate of the protection gas 131 that enters the fluid port 472 from the gas supply 133, the pressure difference between the interior 473 and the vacuum chamber 109, the physical properties of the inner wall 475, and the size of the open region 478 in the Y-Z plane.

[0079] Other implementations are possible. For example, the temperature control block 480 may be mounted to the inner wall 475 such that fluid does not flow between the inner wall 475 and the temperature control block 480. In these implementations, the protection fluid 131 flows only in the open space 481.

[0080] Moreover, the supply system 410 may include a gas direction system such as the gas direction system 232 of FIGS. 2 A and 2B. The gas direction system 232 may be mounted on the temperature control block 480 and mounted in the x direction relative to the open region 378.

For example, the gas direction system 232 may be mounted at the end of the temperature control block 480 that is closest to the open region 378. In implementations that include the gas direction system 232, the gas direction system 232 has a diameter in the Y-Z plane that is slightly larger than the diameter of the open region 378 in the Y-Z plane.

[0081] Furthermore, in some implementations, the supply system 410 includes a three- dimensional actuator that is attached to and surrounds a portion of the exterior wall 254, such as the actuator 237 (FIGS. 2C and 2D). When the actuator is between the gas port 472 and the orifice 217, the protection gas 131 flows along the exterior wall 254 and/or the exterior surface of the actuator. The temperature control block 480 is not connected to the actuator. Thus, in these implementations, all or part of the space 481 may be between the temperature control block 480 and the actuator. The protection gas 131 does not flow between the actuator and the exterior wall 254 when the actuator is attached to the exterior wall 254.

[0082] Referring to FIG. 5, a flow chart of a process 500 is shown. The process 500 is an example of a process for protecting an orifice of a supply system, such as the orifice 217 of the conduit 212 (FIG. 2). FIG. 5 is discussed with respect to the orifice 217. However, the process 500 may be used to protect other orifices, such as the orifice 117 of FIG. 1.

[0083] Target material is passed through the orifice 217 to form the stream 121 (510). The stream 121 moves away from the orifice 217 along a trajectory. The trajectory may be, for example, in the -X direction as shown in FIGS. 1, 3 A, and 4A. The protection gas 131 flows in a direction that is away from the orifice 217 (520). By flowing away from the orifice 217, the protection gas 131 discourages or prevents the contaminant substance 150 from reaching the orifice 217 and/or from forming a layer of the contaminant substance 150 on an exterior of the nozzle 250 (FIG. 2A). In this way, the protection gas 131 protects the orifice 217 and ensures that the stream 121 is produced in the manner expected. Furthermore, the flowing protection gas 131 protects the orifice 217 when the vacuum chamber 109 is vented and oxygen enters the chamber 109. When oxygen enters the chamber 109, metallic material (such as tin) oxidizes and may clog or block the orifice 217. The protection gas 131 acts to keep oxygen away from the orifice 217.

[0084] The protection gas 131 may flow along the exterior wall 254. For example, and referring also to FIG. 2A, the protection gas 131 may flow from the gas direction system 232 and along the exterior wall 254 in the -X direction. The protection gas 131 continues to flow along the nozzle 250 and into the vacuum chamber 109 in the -X direction. Thus, the protection gas 131 flows along the exterior wall 254 and away from the orifice 217.

[0085] The protection gas 131 may flow in an open space between the conduit 212 and an interior wall of a housing that surrounds the conduit. For example, as shown in FIG. 3A, the protection gas 131 may flow in the open space 376, which is between the exterior wall 254 of the conduit 212 and the inner wall 375 of the housing 370.

[0086] As discussed with respect to FIG. 1, in some implementations, the gas supply 133 includes more than one chamber 137, and each chamber may include a different gas for use as the protection gas 131. In these implementations, the control system 160 may select a particular one of the chambers 137 to supply the protection gas 131. FIG. 5B shows a process 515 that may be performed with the process 500 or independently of the process 500. For example, the process 515 may be performed after performing (510) and before performing (520). In other examples, the process 515 is performed independently of the process 500. The process 500 may be performed by one or more electronic processors in the electronic processing module 161.

[0087] A state of the EUV light source 100 is determined (516). The state of the EUV light source 100 may be, for example, an operating mode. The state may be, for example, typical operation or a vented state. In typical operation, the stream 121 is produced as expected and the vacuum chamber 109 is sealed. In the vented state, the vacuum chamber 109 is open and oxygen is in the chamber. The state of the EUV light source 100 may be determined from, for example, an oxygen sensor in the vacuum chamber 109 or from an input made at the I/O interface 163 by the operator. [0088] The protection gas 131 to use is determined based on the determined state (518). For example, the instructions 162 may store a database or lookup table that stores a relationship between particular chambers 137 and the possible states of the EUV light source 100. For example, the database may define a relationship between a first chamber 137 and the typical operating state, and a second, different chamber 137 and the vented state. The database maybe be created and stored when the EUV light source 100 is manufactured or programmed by the operator.

[0089] The control system 160 determines which chamber to couple to the fluid connection 134. For example, the control system 160 may operate a fluid switch that connects the fluid connection 134 to the first chamber 137 during typical operation and to the second chamber 137 during the vented state. In this example, the first chamber 137 may contain molecular hydrogen (Fb) gas and the second chamber 137 may contain argon (Ar). The Fb gas has a lower mass than the Ar gas. Thus, in this example, the protection gas 131 to have a relatively low mass during typical operation, such that the stream 121 is not substantially disturbed, and a relatively high mass during the vented state, such that oxygen is pushed away from the supply system, thereby preventing or reducing oxidation of the components of the supply system.

[0090] FIGS. 6 and 7 are an example of an EUV lithographic apparatus that may use the control systems and/or supply systems discussed above. FIG. 8 is an example of an EUV light source that may use the control systems and/or supply systems discussed above.

[0091] FIG. 6 is a block diagram of a lithographic apparatus 700 that includes a source collector module SO. The lithographic apparatus 700 includes:

• an illumination system (illuminator) IL configured to condition a radiation beam B (for example, EUV radiation).

• a support structure (for example, a mask table) MT constructed to support a patterning device (for example, a mask or a reticle) MA and connected to a first positioner PM configured to accurately position the patterning device;

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

• a projection system (for example, a reflective projection system) PS

configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (for example, including one or more dies) of the substrate W.

[0092] The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.

[0093] The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure may use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure may be a frame or a table, for example, which may be fixed or movable as required. The support structure may ensure that the patterning device is at a desired position, for example with respect to the projection system.

[0094] The term“patterning device” should be broadly interpreted as referring to any device that may be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. The pattern imparted to the radiation beam may correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

[0095] The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam which is reflected by the mirror matrix.

[0096] The projection system PS, like the illumination system IL, may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of a vacuum. It may be desired to use a vacuum for EUV radiation since other gases may absorb too much radiation. A vacuum environment may therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps.

[0097] In the example of FIGS. 6 and 7, the apparatus is of a reflective type (for example, employing a reflective mask). The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more patterning device tables). In such“multiple stage” machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.

[0098] Referring to FIG. 6, the illuminator IL receives an extreme ultraviolet radiation beam from the source collector module SO. Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has at least one element, for example, xenon, lithium or tin, with one or more emission lines in the EUV range. In one such method, often termed laser produced plasma (“LPP”) the required plasma is produced by irradiating a fuel, such as a droplet, stream or cluster of material having the required line- emitting element, with a laser beam. The source collector module SO may be part of an EUV radiation system including a laser, not shown in FIG. 6, for providing the laser beam exciting the fuel. The resulting plasma emits output radiation, for example, EUV radiation, which is collected using a radiation collector, disposed in the source collector module. The laser and the source collector module may be separate entities, for example when a carbon dioxide (CO2) laser is used to provide the laser beam for fuel excitation.

[0099] In such cases, the laser is not considered to form part of the lithographic apparatus and the radiation beam is passed from the laser to the source collector module with the aid of a beam delivery system including, for example, suitable directing mirrors and/or a beam expander. In other cases the source may be an integral part of the source collector module, for example when the source is a discharge produced plasma EUV generator, often termed as a DPP source.

[0100] The illuminator IL may comprise an adjuster for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as s-outer and s-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL may comprise various other components, such as facetted field and pupil mirror devices. The illuminator IL may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross-section. [0101] The radiation beam B is incident on the patterning device (for example, mask) MA, which is held on the support structure (for example, mask table) MT, and is patterned by the patterning device. After being reflected from the patterning device (for example, mask) MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor PS2 (for example, an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, for example, so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor PS1 can be used to accurately position the patterning device (for example mask) MA with respect to the path of the radiation beam B. Patterning device (for example mask) MA and substrate W may be aligned using patterning device alignment marks Ml, M2 and substrate alignment marks PI, P2.

[0102] The depicted apparatus may be used in at least one of the following modes:

1. In step mode, the support structure (for example, mask table) MT and the

substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one time (that is, a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed.

2. In scan mode, the support structure (for example, mask table) MT and the

substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (that is, a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure (for example, mask table) MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS.

3. In another mode, the support structure (for example, mask table) MT is kept

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

[0103] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

[0104] FIG. 7 shows an implementation of the lithographic apparatus 700 in more detail, including the source collector module SO, the illumination system IL, and the projection system PS. The source collector module SO is constructed and arranged such that a vacuum

environment can be maintained in an enclosing structure 720 of the source collector module SO. The systems IL and PS are likewise contained within vacuum environments of their own. An EUV radiation emitting plasma 2 may be formed by a laser produced LPP plasma source. The function of source collector module SO is to deliver EUV radiation beam 20 from the plasma 2 such that it is focused in a virtual source point. The virtual source point is commonly referred to as the intermediate focus (IF), and the source collector module is arranged such that the intermediate focus IF is located at or near an aperture 721 in the enclosing structure 720. The virtual source point IF is an image of the radiation emitting plasma 2.

[0105] From the aperture 721 at the intermediate focus IF, the radiation traverses the

illumination system IF, which in this example includes a facetted field mirror device 22 and a facetted pupil mirror device 24. These devices form a so-called“fly’s eye” illuminator, which is arranged to provide a desired angular distribution of the radiation beam 21, at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA (as shown by reference 760). Upon reflection of the beam 21 at the patterning device MA, held by the support structure (mask table) MT, a patterned beam 26 is formed and the patterned beam 26 is imaged by the projection system PS via reflective elements 28, 30 onto a substrate W held by the substrate table WT. To expose a target portion C on substrate W, pulses of radiation are generated while substrate table WT and patterning device table MT perform synchronized movements to scan the pattern on patterning device MA through the slit of illumination.

[0106] Each system IF and PS is arranged within its own vacuum or near-vacuum environment, defined by enclosing structures similar to enclosing structure 720. More elements than shown may generally be present in illumination system IF and projection system PS. Further, there may be more mirrors present than those shown. For example there may be one to six additional reflective elements present in the illumination system IL and/or the projection system PS, besides those shown in FIG. 7.

[0107] Considering source collector module SO in more detail, a laser energy source including a laser 723 is arranged to deposit laser energy 724 into a fuel that includes a target material. The target material may be any material that emits EUV radiation in a plasma state, such as xenon (Xe), tin (Sn), or lithium (Li). The plasma 2 is a highly ionized plasma with electron

temperatures of several 10's of electron volts (eV). Higher energy EUV radiation may be generated with other fuel materials, for example, terbium (Tb) and gadolinium (Gd). The energetic radiation generated during de-excitation and recombination of these ions is emitted from the plasma, collected by a near-normal incidence collector 3 and focused on the aperture 721. The plasma 2 and the aperture 721 are located at first and second focal points of collector CO, respectively.

[0108] Although the collector 3 shown in FIG. 7 is a single curved mirror, the collector may take other forms. For example, the collector may be a Schwarzschild collector having two radiation collecting surfaces. In an embodiment, the collector may be a grazing incidence collector which comprises a plurality of substantially cylindrical reflectors nested within one another.

[0109] To deliver the fuel, which, for example, is liquid tin, a droplet generator 726 is arranged within the structure 720, arranged to fire a high frequency stream 728 of droplets towards the desired location of plasma 2. The droplet generator 726 may be, for example, the supply system 110, 210, 310, or 410. In operation, laser energy 724 is delivered in a synchronism with the operation of droplet generator 726, to deliver impulses of radiation to turn each fuel droplet into a plasma 2. The frequency of delivery of droplets may be several kilohertz, for example 50 kHz. In practice, laser energy 724 is delivered in at least two pulses: a pre pulse with limited energy is delivered to the droplet before it reaches the plasma location, in order to vaporize the fuel material into a small cloud, and then a main pulse of laser energy 724 is delivered to the cloud at the desired location, to generate the plasma 2. A trap 730 is provided on the opposite side of the enclosing structure 720, to capture fuel that is not, for whatever reason, turned into plasma.

[0110] The droplet generator 726 comprises a reservoir 701 which contains the fuel liquid (for example, melted tin) and a filter 769 and a nozzle 702. The nozzle 702 is configured to eject droplets of the fuel liquid towards the plasma 2 formation location. The droplets of fuel liquid may be ejected from the nozzle 702 by a combination of pressure within the reservoir 701 and a vibration applied to the nozzle by a piezoelectric actuator (not shown).

[0111] As the skilled reader will know, reference axes X, Y, and Z may be defined for measuring and describing the geometry and behavior of the apparatus, its various components, and the radiation beams 20, 21, 26. At each part of the apparatus, a local reference frame of X,

Y and Z axes may be defined. In the example of FIG. 7, the Z axis broadly coincides with the direction optical axis O at a given point in the system, and is generally normal to the plane of a patterning device (reticle) MA and normal to the plane of substrate W. In the source collector module, the X axis coincides broadly with the direction of fuel stream 728, while the Y axis is orthogonal to that, pointing out of the page as indicated in FIG. 7. On the other hand, in the vicinity of the support structure MT that holds the reticle MA, the X axis is generally transverse to a scanning direction aligned with the Y axis. For convenience, in this area of the schematic diagram FIG. 7, the X axis points out of the page, again as marked. These designations are conventional in the art and will be adopted herein for convenience. In principle, any reference frame can be chosen to describe the apparatus and its behavior.

[0112] Numerous additional components used in the operation of the source collector module and the lithographic apparatus 700 as a whole are present in a typical apparatus, though not illustrated here. These include arrangements for reducing or mitigating the effects of contamination within the enclosed vacuum, for example to prevent deposits of fuel material damaging or impairing the performance of collector 3 and other optics. Other features present but not described in detail are all the sensors, controllers and actuators involved in controlling of the various components and sub-systems of the lithographic apparatus 700.

[0113] Referring to FIG. 8, an implementation of an LPP EUV light source 800 is shown. The light source 800 may be used as the source collector module SO in the lithographic apparatus 700. Furthermore, the optical source 105 of FIG. 1 may be part of the drive laser 815. The drive laser 815 may be used as the laser 723 (FIG. 7).

[0114] The LPP EUV light source 800 is formed by irradiating a target mixture 814 at a plasma formation location 805 with an amplified light beam 810 that travels along a beam path toward the target mixture 814. The target material discussed with respect to FIG. 1 and the targets in the stream 121 of targets discussed with respect to FIG. 1 may be or include the target mixture 814. The plasma formation location 805 is within an interior 807 of a vacuum chamber 830. When the amplified light beam 810 strikes the target mixture 814, a target material within the target mixture 814 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 814. 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.

[0115] The light source 800 includes a drive laser system 815 that produces the amplified light beam 810 due to a population inversion within the gain medium or mediums of the laser system 815. The light source 800 includes a beam delivery system between the laser system 815 and the plasma formation location 805, the beam delivery system including a beam transport system 820 and a focus assembly 822. The beam transport system 820 receives the amplified light beam 810 from the laser system 815, and steers and modifies the amplified light beam 810 as needed and outputs the amplified light beam 810 to the focus assembly 822. The focus assembly 822 receives the amplified light beam 810 and focuses the beam 810 to the plasma formation location 805.

[0116] In some implementations, the laser system 815 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 815 produces an amplified light beam 810 due to the population inversion in the gain media of the laser amplifiers even if there is no laser cavity. Moreover, the laser system 815 may produce an amplified light beam 810 that is a coherent laser beam if there is a laser cavity to provide enough feedback to the laser system 815. The term“amplified light beam” encompasses one or more of: light from the laser system 815 that is merely amplified but not necessarily a coherent laser oscillation and light from the laser system 815 that is amplified and is also a coherent laser oscillation.

[0117] The optical amplifiers in the laser system 815 may include as a gain medium a filling gas that includes CO2 and may amplify light at a wavelength of between about 9100 and about 11000 nm, and in particular, at about 10600 nm, at a gain greater than or equal to 800 times. Suitable amplifiers and lasers for use in the laser system 815 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 815 may also include a cooling system such as water that may be used when operating the laser system 815 at higher powers.

[0118] The light source 800 includes a collector mirror 835 having an aperture 840 to allow the amplified light beam 810 to pass through and reach the plasma formation location 805. The collector mirror 835 may be, for example, an ellipsoidal mirror that has a primary focus at the plasma formation location 805 and a secondary focus at an intermediate location 845 (also called an intermediate focus) where the EUV light may be output from the light source 800 and may be input to, for example, an integrated circuit lithography tool (not shown). The light source 800 may also include an open-ended, hollow conical shroud 850 (for example, a gas cone) that tapers toward the plasma formation location 805 from the collector mirror 835 to reduce the amount of plasma-generated debris that enters the focus assembly 822 and/or the beam transport system 820 while allowing the amplified light beam 810 to reach the plasma formation location 805.

For this purpose, a gas flow may be provided in the shroud that is directed toward the plasma formation location 805.

[0119] The light source 800 may also include a master controller 855 that is connected to a droplet position detection feedback system 856, a laser control system 857, and a beam control system 858. The light source 800 may include one or more target or droplet imagers 860 that provide an output indicative of the position of a droplet, for example, relative to the plasma formation location 805 and provide this output to the droplet position detection feedback system 856, 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 856 thus provides the droplet position error as an input to the master controller 855. The master controller 855 may therefore provide a laser position, direction, and timing correction signal, for example, to the laser control system 857 that may be used, for example, to control the laser timing circuit and/or to the beam control system 858 to control an amplified light beam position and shaping of the beam transport system 820 to change the location and/or focal power of the beam focal spot within the chamber 830. [0120] The supply system 825 includes a target material delivery control system 826 that is operable, in response to a signal from the master controller 855, for example, to modify the release point of the droplets as released by a target material supply apparatus 827 to correct for errors in the droplets arriving at the desired plasma formation location 805.

[0121] Additionally, the light source 800 may include light source detectors 865 and 870 that measures 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 865 generates a feedback signal for use by the master controller 855. 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.

[0122] The light source 800 may also include a guide laser 875 that may be used to align various sections of the light source 800 or to assist in steering the amplified light beam 810 to the plasma formation location 705. In connection with the guide laser 875, the light source 800 includes a metrology system 824 that is placed within the focus assembly 822 to sample a portion of light from the guide laser 875 and the amplified light beam 810. In other implementations, the metrology system 824 is placed within the beam transport system 820. The metrology system 824 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 810. A beam analysis system is formed from the metrology system 824 and the master controller 855 since the master controller 855 analyzes the sampled light from the guide laser 875 and uses this information to adjust components within the focus assembly 822 through the beam control system 858.

[0123] Thus, in summary, the light source 800 produces an amplified light beam 810 that is directed along the beam path to irradiate the target mixture 814 at the plasma formation location 805 to convert the target material within the mixture 814 into plasma that emits light in the EUV range. The amplified light beam 810 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 815. Additionally, the amplified light beam 810 may be a laser beam when the target material provides enough feedback back into the laser system 815 to produce coherent laser light or if the drive laser system 815 includes suitable optical feedback to form a laser cavity.

[0124] Other aspects of the invention are set out in the following numbered clauses.

1. A target delivery system for an extreme ultraviolet light (EUV) source, the system comprising:

a conduit comprising an exterior, an interior conduit region, and an end that defines an orifice, wherein the interior conduit region is configured to receive a target material that emits EUV light when in a plasma state, and the orifice is configured to provide the target material to an interior of a vacuum chamber; and

a protection system configured to flow a protection gas away from the end that defines an office and toward the interior of the vacuum chamber, wherein the flowing protection gas is configured to direct one or more contaminant substances away from the end that defines the orifice.

2. The target delivery system of clause 1, wherein the protection gas comprises an inert gas or a reacting gas.

3. The target delivery system of clause 1, wherein the protection gas comprises molecular hydrogen (¾).

4. The target delivery system of clause 1, wherein the protection system is configured to flow the protection gas along the exterior of the conduit.

5. The target delivery system of clause 4, wherein the protection system comprises:

a body comprising a sidewall that surrounds at least a portion of the exterior of the conduit, the body defining an open end region aligned with the orifice of the conduit, and wherein

the protection gas flows in an open space between the exterior of the conduit and an inner wall of the sidewall, and the protection gas flows through the open end region to exit the body.

6. The target delivery system of clause 5, wherein the sidewall comprises at least one port in fluid communication with the open space, the port being configured to fluidly couple to a gas supply that holds the protection gas.

7. The target delivery system of clause 1, wherein the protection system comprises at least one gas source.

8. The target delivery system of clause 1, wherein the target delivery system further comprises a temperature control block that at least partially surrounds the exterior of the conduit, and the protection system comprises a body that surrounds at least a portion of the temperature control block, the body defining an open end region aligned with the orifice of the conduit, and wherein

the protection gas flows in an open space between the temperature control block and an inner wall of the body, and the protection gas exits the body through the open end region.

9. The target delivery system of clause 1, wherein the one or more contaminant substances comprise moving substances, and the flowing fluid is configured to reduce interactions between the one or more moving contaminant substances and the end of the conduit by changing a direction of the motion of the one or more moving contaminant substances away from the end of the conduit.

10. The target delivery system of clause 1, wherein the one or more contaminant substances comprise moving substances, and the flowing fluid is configured to prevent interactions between the one or more moving contaminant substances and the end of the conduit by changing a direction of the motion of the one or more moving contaminant substances away from the end of the conduit.

11. The target delivery system of clause 1, wherein the one or more contaminant substances comprise one or more of a gas, liquid, vapor, and particles.

12. The target delivery system of clause 1, wherein the one or more contaminant substances comprise silicon (Si) or silicon dioxide (SiCh).

13. The target delivery system of clause 1, wherein the one or more contaminant substances comprise oxygen, water, or carbon dioxide (CO2).

14. The target delivery system of clause 1, wherein the conduit comprises a capillary tube.

15. The target delivery system of clause 1, wherein the protection system comprises a diffuser device comprising a plurality of openings, each opening being configured to direct the protection gas away from the end that defines the orifice.

16. The target delivery system of clause 15, wherein the plurality of openings surround the exterior of the conduit and are uniformly distributed relative to the exterior of the conduit.

17. A method of protecting an orifice of a target material delivery system, the method comprising:

passing target material through an orifice to provide a stream of targets to an interior of a vacuum chamber, each target in the stream comprising a target material that emits EUV light when in a plasma state; and flowing a protection gas in the target material delivery system and into the interior of the vacuum chamber away from the orifice, the protection gas directing one or more contaminant substances away from the orifice.

18. The method of clause 17, wherein the flowing protection gas does not change a trajectory of the stream of targets.

19. The method of clause 18, wherein the flowing protection gas has a motion component along a direction of travel of the stream of targets.

20. The method of clause 17, wherein flowing the protection gas comprises flowing the protection gas in an open space between a conduit that defines the orifice and a body that surrounds the conduit.

21. The method of clause 20, wherein the protection gas flows into the open space at a port in the sidewall and flows out of the space through an open end region that is defined by the body and is aligned with the orifice.

22. The method of clause 17, wherein the protection gas has a uniform volumetric flow rate at an aperture between the orifice and the interior of the vacuum chamber.

23. The method of clause 17, further comprising:

determining a state of an extreme ultraviolet light source that comprises the target material delivery system; and

determining which of a plurality of protection gasses to use as the protection gas based on the determined state.

[0125] Other implementations are within the scope of the claims.