WO2021239357A1

WO2021239357A1 - Alignment of extreme ultraviolet light source

Info

Publication number: WO2021239357A1
Application number: PCT/EP2021/060794
Authority: WO
Inventors: Alexander Anthony SCHAFGANS; John Tom STEWART IV
Original assignee: Asml Netherlands B.V.
Priority date: 2020-05-28
Filing date: 2021-04-26
Publication date: 2021-12-02
Also published as: TW202202945A; CN115669232A; KR20230016620A

Abstract

An apparatus includes: a vessel comprising an interior space; a material supply system configured to provide targets comprising target material along a path in the interior space; a detection system configured to detect scattered light produced by an interaction between a metrology light beam and one of the targets and to produce information based on the interaction; and a control system configured to adjust the path based on the information produced by the detection system. The target material produces extreme ultraviolet (EUV) light in a plasma state. The metrology light beam does not convert the target material to the plasma state.

Description

ALIGNMENT OF EXTREME ULTRAVIOLET LIGHT SOURCE

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Application No. 63/031,265 filed May 28, 2020 and titled ALIGNMENT OF EXTREME ULTRAVIOLET LIGHT SOURCE, which is incorporated herein in its entirety by reference.

TECHNICAL FIELD

[0002] This disclosure relates to techniques for aligning 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, may be used in photolithography processes to produce extremely small features in substrates, for example, silicon wafers, by initiating polymerization in a resist layer. 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

[0004] In one aspect, an apparatus includes: a vessel including an interior space; a material supply system configured to provide targets including target material along a path in the interior space; a detection system configured to detect scattered light produced by an interaction between a metrology light beam and one of the targets and to produce information based on the interaction; and a control system configured to adjust the path based on the information produced by the detection system. The target material produces extreme ultraviolet (EUV) light in a plasma state. The metrology light beam does not convert the target material to the plasma state.

[0005] Implementations may include one or more of the following features.

[0006] The apparatus may further include a reflective optical element in the interior space. The reflective optical element may include a primary focus and an intermediate focus. The control system may be configured to adjust the path such that the path intersects the primary focus of the reflective optical element. The control system may further be configured to adjust a turning element that directs light along a direction of propagation in the vessel such that the direction of propagation intersects the primary focus. The detection system may be configured to detect the scattered light after the scattered light has been reflected from the reflective optical element. The detection system may include an imaging sensor, and the detection system may be configured to image the scattered light after the scattered light has been reflected from the reflective optical element. The detection system may include a component at the intermediate focus or at a location between the intermediate focus and a scanner apparatus that receives light from the apparatus. The component may include a mask, and the detection system may further include a detector configured to detect light transmitted by the mask.

The component may include a detector configured to be placed at the intermediate focus or removed from the intermediate focus. The component may include an imaging plane. The imaging plane may include a material that causes the scattered light to be sensed by an imaging system. The imaging plane may include ground glass. The imaging plane may include a membrane or a Pellicle. The imaging plane may substantially block target material debris.

[0007] Each point along the path may be a location in a three-dimensional coordinate system, and the control system may be configured to adjust the location of at least one of the points of the path in at least two of the three dimensions. The control system may be configured to adjust the location of the path by controlling the material supply system.

[0008] The apparatus may further include a metrology light source. The metrology light source may be a laser configured to produce visible light or a laser configured to produce infrared light. The metrology light source may be a pulsed light source, and the metrology light beam may include a plurality of pulses of light. The metrology light source may be a continuous-wave light source, and the metrology light beam may include a continuous-wave beam of light.

[0009] In another aspect, an extreme ultraviolet (EUV) light source includes: a vessel; a target material supply system configured to provide targets to an interior of the vessel; and an apparatus.

The targets include a target material that emits EUV light when in a plasma state and the targets travel in the interior of the vessel along a path. The apparatus is configured to align the path of the targets with a focus of an optical element in the interior of the vessel. The apparatus includes: a detection system configured to detect scattered light produced by an interaction between a metrology light beam and one of the targets and to produce information based on the interaction; and a control system configured to adjust the path of the targets based on the information produced by the detection system. The metrology light beam does not convert the target material to the plasma state, and the one of the targets is configured to act as a point source of light when illuminated by the metrology light beam. [0010] Implementations may include one or more of the following features.

[0011] The EUV light source may further include a turning element configured to receive an amplified light beam that has an energy sufficient to convert at least some of the target material to the plasma state. [0012] The EUV light source may further include a final turning element that may include a turning mirror configured to adjust a direction of an incident light beam. The final turning mirror may be outside of the vessel.

[0013] The optical element may be a reflective optical element that may include a primary focus and an intermediate focus, and the apparatus may be configured to align the path of the targets with the primary focus of the reflective optical element. The detection system may include: a first sensor configured to detect an image of the one of the targets at the intermediate focus; and a second sensor. The reflective optical element may be between the second sensor and the first sensor.

[0014] In another aspect, a method includes: directing targets along a path in an interior of a vessel; providing a metrology light beam to the interior of the vessel to produce scattered light; detecting the scattered light at a detection system; and adjusting the path based on information about the scattered light from the detection system. Each target includes target material that emits EUV light in a plasma state. The scattered light arises from an interaction between the metrology light beam and one of the targets.

[0015] In another aspect, an apparatus for an optical system includes: a detection system including a component configured to receive scattered metrology light; and a control system. The scattered metrology light includes light scattered from a target. Each target travels on a path, each target includes target material that reflects the metrology light, and each target is configured to act as a point source of light when illuminated by the metrology light. The control system is configured to adjust the path of each target to intersect with a primary focus of the optical system.

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

[0017] The target may include target material that emits EUV light when in a plasma state, and the target may be one target in a stream of moving targets. The target may be substantially spherical. [0018] In another aspect, a method for inspecting an optical element in a vessel of an extreme ultraviolet (EUV) light source includes: illuminating a target with a metrology light beam to scatter the metrology beam from the target; imaging an optical element in the vessel that receives the scattered metrology beam; and inspecting the optical element based on the imaged optical element. The target includes target material that emits EUV light when in a plasma state, and the metrology light beam does not convert the target material to the plasma state.

[0019] In another aspect, a method includes: providing a metrology light beam to an interior of a vessel; imaging scattered light generated by an interaction between the metrology light beam and a target material droplet to determine information about the path, the scattered light reflected from an optical element that is associated with a primary focus and a secondary focus; adjusting the path based on the information such that the path intersects the primary focus of the optical element; and adjusting a turning element to direct light along a direction of propagation that intersects the primary focus.

The interaction between the metrology light beam and the target material droplet does not produce a plasma that emits EUV light. [0020] Implementations may include one or more of the following features.

[0021] The method may further include: after adjusting the path and after adjusting the turning element, providing an amplified light beam to the adjusted turning element such that the amplified light beam is provided to the primary focus. The amplified light beam may have an energy sufficient to convert at least some of the target material in the target material droplet into plasma that emits EUV light. The turning element may include a reflective optical element that receives some of the scattered light, and adjusting the turning element may include moving the turning element and imaging the scattered light to align the reflective optical element such that the direction of propagation intersects the primary focus of the optical element. The scattered light may be imaged at the secondary focus of the optical element and at a location that is between a source of the amplified light beam and the optical element.

[0022] 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

[0023] FIG. 1 is a block diagram of an apparatus.

[0024] FIGS. 2A and 2B are block diagrams of another apparatus.

[0025] FIG. 2C is a perspective view of an optical element of the apparatus of FIGS. 2A and 2B. [0026] FIGS. 3A-3E are block diagrams of various example implementations of a first sensor of the apparatus of FIGS. 2 A and 2B.

[0027] FIG. 4 is a flow chart of an example of a process for aligning a light source.

[0028] FIG. 5 is a flow chart of an example of a process for inspecting an optical element.

[0029] FIG. 6A is an image of a reflective surface.

[0030] FIG. 6B is another image of a reflective surface.

[0031] FIG. 7 is a block diagram of an EUV light source.

DETAIFED DESCRIPTION

[0032] FIG. 1 is a block diagram of an apparatus 100. The apparatus 100 is an extreme ultraviolet (EUV) light source that emits EUV light 197. The EUV light 197 is generated by producing a plasma 196 that emits the EUV light 197. The plasma 196 is formed by irradiating a target 122 with an amplified light beam 106. As discussed below, the apparatus 100 includes a detection system 130.

The detection system 130 allows the apparatus 100 to be aligned using reflected or scattered light 198 and without producing the plasma 196. The reflected or scattered light 198 arises from an interaction between a metrology light beam 108 and one of the targets 122. The interaction between the target 122 and the metrology light beam 108 does not disturb the target 122 and does not produce the plasma 196. Aligning the apparatus 100 without producing the plasma 196 allows the apparatus 100 to be aligned more easily and efficiently. For example, the apparatus 100 may be aligned by the manufacturer prior to being installed and used by an end user. This results in reduced time for installation and a more productive experience for the end user. Moreover, because the plasma 196 also emits debris that may damage components, systems, and devices in the apparatus 100, by aligning the apparatus 100 with the light 198 and without producing the plasma 196, the lifetime and overall performance of the apparatus 100 is increased.

[0033] The apparatus 100 includes a vessel 110 that has an interior space 112. The interior space 112 is an evacuated space in which a vacuum is maintained. The apparatus 100 also includes a material supply system 120 that emits targets 122 through a nozzle 113. For simplicity, only one target 122 is labeled in FIG. 1. However, the material supply system 120 may emit more than one target 122. The targets 122 travel in the interior 112 along a path 121. In the example of FIG. 1, the path 121 is generally along the X direction.

[0034] The material supply system 120 delivers, controls, and directs targets 122 along the path 121. The targets 122 are made of a target mixture in the form of, for example, liquid droplets, droplets that include solid particles or clusters, solid particles contained within liquid droplets or solid particles contained within a liquid stream. The targets 122 may be substantially spherical in shape and may be, for example, about 15 pm to 40 pm in diameter. The target mixture includes target material. The target material is any material that has an emission line in the EUV range when converted to a plasma state. The target material may be, for example, water, tin, lithium, and/or xenon, or a substance that includes such a material. For example, the element tin may be used as pure tin (Sn); as a tin compound, for example, SnBr4, SnBr2, SnH4; as a tin alloy, for example, tin-gallium alloys, tin- indium alloys, tin-indium-gallium alloys, or any combination of these alloys. The target mixture also may include impurities such as non-target particles. Thus, in the situation in which there are no impurities, the target mixture is made up of only the target material.

[0035] An interaction between an amplified light beam 106 and one of the targets 122 produces the plasma 196. The amplified light beam 106 is any type of light beam that has an energy sufficient to convert at least some of the target material in the target 122 to the plasma 196. The amplified light beam 106 may be a pulsed laser beam produced by a light source such as the light source 205 of FIGS. 2A and 2B. The metrology light beam 108 also propagates in the interior 112. The metrology light beam 108 interacts with the target 122 and produces the light 198, which is a reflection and/or scattered light, but the interaction does not change a property (such as, for example, size, shape, and or density) of the target 122 or convert the target material to the plasma 196. In other words, the metrology light beam 108 is an optical probe that generates the light 198 by interacting with the target 122 but does not disturb the target 122.

[0036] The detection system 130 detects the light 198 and provides information related to the light 198 to a control system 150. The control system 150 acts on one or more components of the apparatus 100 to align the apparatus 100 using the information from the detection system 130. For example, the control system 150 may control the material supply system 120 to adjust the path 121 such that the path 121 (and the targets 122) intersects the primary focal point of an optical element in the interior 112 (such as the reflective optical element 240 of FIGS. 2A-2C). In another example, the control system 150 adjusts an optical element (such as the optical element 263) that steers the amplified light beam 106 based on the information about the light 198.

[0037] Referring to FIGS. 2A and 2B, a block diagram of an apparatus 200 is shown. The apparatus 200 is an implementation of the apparatus 100 (FIG. 1). FIG. 2A shows the apparatus 200 when the metrology light beam 108 interacts with one of the targets 122 and produces the light 198. FIG. 2B shows the apparatus 200 at a later time when the amplified light beam 106 interacts with another one of the targets 122 and produces the plasma 196.

[0038] The apparatus 200 includes a vessel 210 and the supply system 120, which provides the targets 122 to an interior 212 of the vessel 210. The targets 122 travel along a path 221. The apparatus 200 also includes a metrology light source 209 and a main light source 205. The metrology light source 209 produces the metrology light beam 108. The main light source 205 produces the amplified light beam 106.

[0039] The apparatus 200 also includes an optical element 240 in the interior 212. FIGS. 2 A and 2B show a side cross-sectional view of the optical element 240. FIG. 2C shows a perspective view of the optical element 240. The optical element 240 is an elliptical mirror that has a reflective surface 241 and a through hole 245. The reflective surface 241 is curved. The through hole 245 is approximately in the center of the reflective surface 241 and passes all the way through the optical element 240. The through hole 245 provides a passage for the amplified light beam 106. A point source of light at the primary focus 242 is focused by the reflective surface 241 to an intermediate focus 243. The primary focus 242 is between the reflective surface 241 and the intermediate focus 243. The reflective surface 241 is made of a material that reflects the EUV light 197 and the light 198. Thus, the reflective surface 241 reflects incident EUV light 197 and incident light 198 to the intermediate focus 243 and toward a scanner apparatus 299.

[0040] To optimize the amount of EUV light 197 that is collected by the optical element 240 and provided to the scanner apparatus 299, the path 221 and the propagation path of the amplified light beam 106 are aligned with the primary focus 242. In other words, the path 221 and the light beam 106 are directed to pass through the primary focus 242. Traditionally, the path 221 and the amplified light beam 106 were aligned with the primary focus 242 using the EUV light 197. For example, some prior systems repeatedly generate the plasma 196 and measure the amount of the EUV light 197 produced while adjusting the path 221 and/or the propagation direction of the amplified light beam 106. When the greatest amount of EUV light 197 was produced, the amplified light beam 106 and the path 221 were assumed to be aligned or nearly aligned with the primary focus 242. Such an approach requires the generation of the plasma 196, which may damage the surface 241 or other objects in the interior 212. Moreover, because each system has its own misalignment with the primary focus 242, the traditional approach does not locate the primary focus 242 relative to all systems that should be aligned to the primary focus 242. Furthermore, the traditional approach needs to be repeated any time that a component that is aligned with the primary focus 242 is replaced or moved for repair.

[0041] On the other hand, the apparatus 200 uses the light 198, which does not require the production of the plasma 196, to align the path 221 and the amplified light beam 106 with the primary focus 242. By using the light 198 instead of the EUV light 197, the apparatus 200 avoids the contamination and damage that can occur with the traditional approach. Furthermore, because the EUV light 197 is not used for the alignment, the apparatus 200 may be aligned without producing any of the plasma 196. Thus, the apparatus 200 may be aligned by the manufacturer and prior to installation or use by an end- user. Moreover, using the light 198 allows multiple systems (the supply system 120, the main light source 205, and the metrology light source 209) to be aligned to a common point (the primary focus 242). In other words, the light scattered from a target 122 is used to identify the location of the primary focus 242 relative to multiple systems and components, allowing the supply system 120, the metrology light source 209, and the main light source 205 to be aligned with the primary focus 242 without producing the plasma 196.

[0042] The configuration of the apparatus 200 is discussed in greater detail prior to discussing examples of aligning the apparatus 200 using the light 198.

[0043] Referring again to FIG. 2A, the metrology light source 209 is coupled to an exterior region 211 of the vessel 210 at a viewport 207. The viewport 207 is sealed at the exterior region 211 such that a vacuum may be maintained in the interior 212. The viewport 207 includes a window or other element that is transparent to the wavelengths in the metrology light beam 108. The metrology light source 209 generates the metrology light beam 108, and the light beam 108 passes through the transparent portion of the viewport 207 and propagates in the interior 212. The metrology light source 209 may be, for example, a laser. The metrology light beam 108 may be a continuous light beam or may be a pulsed light beam. The metrology light beam 108 may include wavelengths in the visible region (about 380 to 740 nanometers (nm)) and/or the near infrared (NIR) region (about 0.7 pm to 5 pm). For example, the metrology light source 209 may be a laser that produces light having a center wavelength of 523 nm, 808 nm, 820 nm, 908nm, 980 nm, 1064 nm, 1070 nm, or 1550 nm.

[0044] Referring to FIG. 2B, the main light source 205 emits the amplified light beam 106. The main light source 205 may be any type of light source that is capable of emitting the amplified light beam 106. For example, the main light source 205 may be a carbon dioxide (CO2) laser. The center wavelength of the amplified light beam 106 may be, for example, 10.6 pm or another wavelength between 9 pm and 11 pm. The amplified light beam 106 may be a pulsed light beam.

[0045] The light beam 106 travels to the interior space 212 via an optical path 265. The optical path 265 is defined by optical elements 263 and 264. In the example shown, the optical elements 263 and 264 are reflective optical elements such as mirrors. However, any type of optical element that directs light may be used as the elements 263 and 264. In the example of FIGS. 2 A and 2B, two optical elements are shown. However, fewer or more optical elements may define the path 265. In the example of FIGS. 2A and 2B, the optical elements 263 and 264 are outside of the vessel 210. However, in other implementations, the optical element 264 is inside the interior 212. In some implementations, both of the optical elements 264 and 263 are in the interior 212.

[0046] The optical element 263 is coupled to an actuator 262 and the optical element 264 is coupled to an actuator 267. Each actuator 262 and 267 is any type of device that is controllable to move the optical element 263 and 264, respectively. For example, each actuator 262 and 267 may include motors, hinges, translation stages, or a combination of such elements. Each actuator 262 and 267 may be manually adjustable or each actuator 262 and 267 may be coupled to a control system 250 for electronic control. The direction of propagation of the amplified light beam 106 in the interior 212 is adjusted by controlling one or more of the actuators 262 and 267.

[0047] The apparatus 200 also includes a detection system 230, which receives information about the light 198 from one or more sensors. In the example of FIGS. 2A and 2B, the detection system 230 includes a first sensor 231 and a second sensor 238. The first sensor 231 includes an active element 232 that is configured to sense the light 198 at or near the intermediate focus 243. The active element 232 is any type of active element that is sensitive to the light 198. The active element 232 may be a single element that produces an amount of sensed light at a particular location, or the active element 232 may be a two-dimensional array of sensors that produces two-dimensional image data. FIGS. 3A-3E relate to various example implementations of the first sensor 231.

[0048] FIGS. 3 A and 3B show an implementation in which the first sensor 231 is a movable sensor 331 A that includes the active sensing element 232. The movable sensor 331 A is mechanically mounted to a moving system 334 that enables the movable sensor 331 A to move. The mounting system 334 may be an arm or a track, for example. The mounting system 334 may be manually controllable or may be coupled to the control system 250. When activated, the mounting system 334 moves the first sensor 331A from a first location (FIG. 3A) to a second location (FIG. 3B). When the sensor 331A is in the first location, the sensor 331A is displaced in the X direction relative to the intermediate focus 243. When the sensor 331 A is in the second location, the active sensing element 232 is at the intermediate focus 243. Thus, when the sensor 331A is in the second location, the active element 232 is positioned to capture light reflected from the reflective surface 241. When the sensor 331 A is in the first location, the active element 232 is not positioned to capture light reflected from the reflective surface 241 and the sensor 331 A does not block light reflected from the surface 241. As discussed above, during operational use of the apparatus 200, the EUV light 197 passes through the intermediate focus 243 and into the scanner apparatus 299. Thus, when the sensor 331 A is in the first location, it does not prevent the EUV light 197 from reaching the scanner apparatus 299. In other words, the sensor 331 A may be positioned at the first location (FIG. 3A) when the apparatus 200 is being used to produce the EUV light 197 and at the second location (FIG. 3B) when the apparatus 200 is being aligned.

[0049] FIG. 3C is a block diagram of a sensor 331C and a component 337. The sensor 331C is another implementation of the sensor 231 (FIGS. 2A and 2B), and the sensor 331C includes the active sensing element 232. The sensor 331C is positioned away from the intermediate focus 243 and the component 337 is used to scatter the light 198 to the active sensing element 232. The component 337 is any object that scatters the light 198. For example, the component 337 may be ground glass, an imaging screen, a Pellicle, or a membrane. The component 337 may be positioned at the intermediate focus 243 or a location that is displaced in the Z or -Z direction relative to the intermediate focus 243. In the example shown in FIG. 3C, the component is displaced in the Z direction relative to the intermediate focus 243.

[0050] The component 337 may be a moveable component that is positioned at or near the intermediate focus 243 only when the light 198 is being measured and is removed when the EUV light 197 is produced. In some implementations, the component 337 scatters wavelengths in the light 198 but transmits the EUV light 197. In these implementations, the component 337 does not interfere with the delivery of the EUV light 197 to the scanner apparatus 299 and remains in place while the apparatus 200 is being used to produce the EUV light 197. Moreover, this type of component 337 also may prevent or inhibit debris from the plasma 196 from reaching the scanner 299.

[0051] FIG. 3D is a perspective view of the reflective surface 241, a moveable sensor 331D, and a component 337D. The component 337D is a mask. FIG. 3E is a block diagram of the mask 337D in the X-Y plane. The mask 337D includes a transparent portion 346 and an opaque portion 348. The transparent portion 346 is made of a material that is transparent or transmissive at the wavelengths in the light 198. For example, the transparent portion 346 may be made of quartz or plastic. The opaque portion 348 is made of a material that blocks or does not transmit the wavelengths in the light 198. [0052] A center 347 of the transparent portion 346 of the mask 337D is placed at the intermediate focus 243. The center 347 is aligned with a center of the optical element 240 in the X and Y directions. The transparent portion 346 has the same shape as a focused image of the reflective surface 241. The focused image of the reflective surface 241 is formed at the intermediate focus 243 when a point source at the primary focus 242 illuminates the reflective surface 241. The target 122 acts as a point source when illuminated by the metrology light beam 109. Thus, when the target 122 is at the primary focus 242, the image of the reflective surface 241 is in focus at the intermediate focus 243. The amount of light that is transmitted by the transparent portion 346 is greatest when the target 122 is at the primary focus 242.

[0053] The moveable sensor 331D is moveable along the X and -X directions and/or the Y and -Y directions. When positioned to measure light that passes through the transparent portion 346, the moveable sensor 331D is aligned with the center 347 of the transparent portion 346 in the X-Y plane and displaced in the Z direction relative to the center of the transparent portion 346. When the apparatus 200 is not being aligned, the mask 337D and the sensor 331D may be moved along with X or -X and/or the Y and -Y directions such that they no longer interact with light that is reflected from the reflective surface 241.

[0054] Returning to FIGS. 2A and 2B, the detection system 230 also includes a second sensor 238. The second sensor 238 includes an active element 239, which is sensitive to the wavelengths in the light 198. The active element 239 may be, for example, an imaging sensor that captures two- dimensional data. The second sensor 238 is positioned along the optical path 265. A portion of the light 198 propagates through the through hole 245 in the -Z direction and along the optical path 265. This portion of the light 198 is referred to as the return light 198 or the reverse light 198. The second sensor 238 is used to detect the return light 198. For example, the active element 239 may be positioned to receive reflections of the return light 198 from the reflective optical element 263.

[0055] The apparatus 200 is coupled to a control system 250. The control system 250 includes an electronic processing module 251, an electronic storage 252, and an I/O interface 253. The electronic processing module 251 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 251 may include any type of electronic processor. The electronic processor or processors of the electronic processing module 251 execute instructions and access data stored on the electronic storage 252. The electronic processor or processors are also capable of writing data to the electronic storage 252.

[0056] The electronic storage 252 is any type of computer-readable or machine -readable medium. For example, the electronic storage 252 may be volatile memory, such as RAM, or non-volatile memory. In some implementations, and the electronic storage 252 includes non-volatile and volatile portions or components. The electronic storage 252 may store data and information that is used in the operation of the control system 250. The electronic storage 252 also may store instructions (for example, in the form of a computer program) that cause the control system 250 to interact with components and subsystems in the apparatus 200 and or the scanner apparatus 299. For example, the instructions may be instructions that cause the electronic processing module 251 to provide a command signal to the material supply 120 to change the direction of the path 221 and/or to the actuators 262 and 267 to change the direction of propagation of the amplified light beam 106. The electronic storage 252 also may store information received from the apparatus 200 and/or the scanner apparatus 299. The electronic storage 252 also stores instructions that implement the processes discussed with respect to FIGS. 4 and 5.

[0057] The I/O interface 253 is any kind of interface that allows the control system 250 to exchange data and signals with an operator, other devices, and/or an automated process running on another electronic device. For example, in implementations in which rules or instructions stored on the electronic storage 252 may be edited, the edits may be made through the I/O interface 253. The I/O interface 253 may include one or more of a visual display, a keyboard, and a communications interface, such as a parallel port, a Universal Serial Bus (USB) connection, and/or any type of network interface, such as, for example, Ethernet. The I/O interface 253 also may allow communication without physical contact through, for example, an IEEE 802.11, Bluetooth, or a near-field communication (NFC) connection.

[0058] The control system 250 is coupled to various components of the apparatus 200 through a data connection 254. The data connection 254 is shown with a dashed line in FIGS. 2A and 2B. In FIGS. 2A and 2B, the material supply system 120, the detection system 230, and the actuator 262 are shown as being coupled to the control system 250. However, other components of the apparatus 200 may be coupled to the control system 250. For example, the actuator 267 may be coupled to the control system 250. Moreover, the scanner apparatus 299 and or the main light source 205 may be coupled to the control system 250.

[0059] The data connection 254 is any type of connection that allows transmission of data, signals, and or information. For example, the data connection 254 may be a physical cable or other physical data conduit (such as a cable that supports transmission of data based IEEE 802.3), a wireless data connection (such as a data connection that provides data via IEEE 802.11 or Bluetooth), or a combination of wired and wireless data connections.

[0060] The apparatus 200 is provided as an example, and other implementations are possible. For example, the apparatus 200 may be packaged separately from the control system 250, and the apparatus 200 does not necessarily include the control system 250. The control system 250 is shown as a single control system. However, the control system 250 may be implemented as a plurality of control systems. For example, the control system 250 may be implemented as a plurality of local control systems that are each associated with a particular component or sub-system of the apparatus 200, where each of the local control systems communicate with a host controller.

[0061] Similarly, the apparatus 200 may be packaged separately from the scanner apparatus 299 and the main light source 205, and the apparatus 200 does not necessarily include the scanner apparatus 299 or the main light source 205. In some implementations, the apparatus 200 includes an additional light source that provides a pre-pulse light beam that interacts with the target 122 prior to the amplified light beam 106 interacting with that target 122. The pre-pulse light beam is used to change one or more properties (such as size, shape, and or density) of the target 122. Moreover, the apparatus 200 may include additional metrology light sources other than the metrology light source 209. The other metrology light sources may be mounted to the vessel 210.

[0062] FIG. 4 is a flow chart of a process 400. The process 400 is an example of a process for aligning a light source. The process 400 is discussed with respect to the apparatus 200 (FIGS. 2A- 2C). The process 400 may be performed by the control system 250. For example, the process 400 may be performed by one or more electronic processors in the processing module 251. The process 400 may be performed before the apparatus 200 is shipped to an end user or before the apparatus 200 is installed at the end user’ s site. However, the process 400 may be performed during the lifetime of the apparatus 200. For example, the process 400 may be performed by the end user after the apparatus 200 has been installed at the end user’s site.

[0063] The process 400 uses the light 198 to align the apparatus 200 and does not rely on production of the plasma 196. The target 122 acts as a point source of light when illuminated by the metrology beam 209, and the light 198 that arises from the interaction is used to align the apparatus 200.

[0064] A first sensor is provided (410). The first sensor measures the light 198 at or near the intermediate focus 243. For example, the first sensor 331 A may be moved such that the active element 232 is at the intermediate focus 243, as shown in FIG. 3B. In another example, the component 337 is installed at or near the intermediate focus 243 such that the light 198 that is received at the intermediate focus 243 is directed to the active element 232. In yet another example, the first sensor 33 ID is positioned in the Z direction relative to the mask 337D, as shown in FIG. 3D. [0065] A metrology light beam is provided to the interior 212 (420). The metrology light beam is any light beam that acts as an optical probe and generates the light 198 without disturbing the target 122. The metrology light beam may be, for example, the light beam 108. The metrology light beam 108 interacts with the target 122 to produce the light 198. The metrology light 108 is provided to the interior 212 at a time that is synchronized with the target 122 when the target 122 is at the primary focus 242.

[0066] As discussed above, the light 198 is reflected and/or scattered light that is generated when the light beam 108 is incident on the target 122. The target 122 is substantially spherical and acts as a point source of light by reflecting the metrology light beam 108. At least some of the light 198 is incident on the reflective surface 241 and is reflected toward the intermediate focus 243. The light 198 that arrives at the intermediate focus 243 is detected by the active sensing element 232 of the first sensor 231. The sensor 231 generates information or data based on the detected light 198 and provides the information to the control system 250. The information may be, for example, two- dimensional image data that represents an image of the reflective optical surface 241 at the intermediate focus 243. In another example, the information about the light 198 is the intensity of the light 198 at or near the intermediate focus 243.

[0067] The information from the first sensor 231 is analyzed (430). As discussed above, when the target 122 is at the primary focus 242, the image of the reflective surface 241 is in focus at the intermediate focus 243. The target 122 travels along the path 221. Thus, when the target 122 is at the primary focus 242, the path 221 also intersects the primary focus 242. By analyzing the information from the first sensor 231, the process 400 determines whether the path 221 intersects the primary focus 242. In implementations in which the information from the first sensor 231 is two-dimensional image data, the images collected by the sensor 231 are analyzed to determine when the reflective surface 241 is in focus. For example, the reflective surface 241 may be deemed to be in focus when the imaged edge of the reflective surface 241 has a shape that is closest to the actual shape of the edge of the reflective surface 241. In another example, when the mask 337D is used, the reflective surface 241 is deemed to be in focus when the amount of light detected by the active element 232 is greatest. In implementations in which the mask 337D is used, the target 122 is at the primary focus 242 when the greatest amount of light is transmitted by the transparent portion 346.

[0068] Based on the analysis, it is determined whether or not the path 221 intersects the primary focus 242 (440). If the path 221 does not intersect the primary focus 242, then the path 221 is adjusted (450). For example, if the information from the first sensor 231 indicates that light is being reflected only from a small center portion of the reflective surface 241, or if the light is over-filling the reflective surface 241, or if the distribution of reflected light is not centered on the center of reflective surface 241, then path 221 can be adjusted in a direction and degree that is expected to better align target material droplet 122 onto the primary focus 242. The path 221 is adjusted in the Y and Z directions by adjusting the target supply system 120. For example, the control system 150 may issue a command to move the nozzle 113 to adjust the path 221 in the Y and/or Z directions. The nozzle 113 may be moved by, for example, driving an actuator (not shown) that is mechanically mounted to the nozzle 113. The process 400 performs (420)-(450) until the path 221 is aligned with the primary focus 242 in the Y and Z directions.

[0069] Additionally, the path 221 is aligned with the primary focus 242 in the X direction using the information from the first sensor 231. The X coordinate of the primary focus 242 is found by scanning (moving) the metrology light beam 108 along the X direction and/or -X direction until the primary focus 242 is found.

[0070] The light 198 also may be used to align the second sensor 238 with the primary focus 242.

By aligning the second sensor 238 with the primary focus 242, the propagation direction of the amplified light beam 106 is also aligned with the primary focus 242. To align the second sensor 238 with the primary focus 242, the return light 198 (which is the portion of the light 198 that travels through the through hole 245 and onto the path 265) is imaged by the second sensor 238. The second sensor 238 produces information about the detected return light 198. The information from the second sensor 238 is analyzed (460). When the target material droplet 122 is at the primary focus 242, the target 122 should appear as a point in an image of the target 122 produced by the second sensor 238. Because the path 221 was aligned with the primary focus 242 using (420)-(450) as discussed above, if the image of the target 122 produced by the second sensor 238 does not show the target 122 as a point, then the optical element 263 is not aligned with the primary focus 242. When the optical element 263 is not aligned with the primary focus 242, the optical element 263 will not direct a forward-going light beam (such as the amplified light beam 106) to the primary focus 242. [0071] The analysis of the information from the second sensor 238 is used to determine whether or not the target 122 appears as a point in the image and to determine whether the second sensor 238 is aligned with the primary focus 242 (470). Analyzing the information from the second sensor 238 may include applying a shape filter other spatial filter to the image data from the second sensor 238. The shape filter or the mask have the shape of the image of the target 122 when the target 122 appears as a point source in the image data. Alternatively or additionally, the light 198 may be compared to a fiducial marker on the second sensor 238 to determine whether or not the second sensor 238 is aligned with the primary focus 242.

[0072] If the target 122 appears as a point in the image from the second sensor 238, then the optical element 263 is aligned relative to the primary focus 242 and is not adjusted. The process 400 ends. If the target 122 does not appear as a point in the image from the second sensor, then the optical element 263 is adjusted (480). The control system 250 issues a command that causes the actuator 262 to move the optical element 263. Another target 122 is irradiated with the metrology light beam 108, and the information from the second sensor 238 is analyzed again (460). The control system 250 continues to command the actuator 262 to adjust the optical element 263 until the target 122 appears as a point in the image produced by the second sensor 238.

[0073] When the apparatus 200 is used to produce the EUV light 197, the optical element 263 delivers the amplified light beam 106 to the interior 212. By aligning the second sensor 238 to the primary focus 242 as discussed relative to (460)-(480), the optical element 263 also is aligned with the primary focus 242. Therefore, the optical element 263 also delivers a forward-going light beam (such as the amplified light beam 106) to the primary focus 242. In this way, the amplified light beam 106 is aligned with the primary focus 242 without having to actually deliver the amplified light beam 106 to the interior 212 and without producing the plasma 196 that emits the EUV light 197. Furthermore, although the main light source 205 is shown with the apparatus 200 in FIGS. 2A and 2B, the main light source 205 is not needed to align the amplified light beam 106 with the primary focus 242.

[0074] In implementations, in which the light beam 106 (and other light beams that are directed into the interior 212 by the optical element 263 (such as a pre -pulse light beam) are pulsed light beams, the production of the plasma 196 is optimized when the pulses arrive at the primary focus 242 at the same time as one of the targets 122. The metrology light beam 108 also may be used to determine the timing of the light pulses. Specifically, the metrology light beam 108 may be used to determine when the pulses of the light beam 106 (or pulses of a pre -pulse light beam) should arrive at the primary focus 242 such that the light pulses and the target 122 are at the primary focus 242 at the same time. [0075] After the target path 221 and the metrology light beam 108 have been aligned to the primary focus 242 using the process 400, pulses of the metrology light beam 108 are directed toward the primary focus 242. The light 198 (which is the reflection and/or scatter of the pulses of the metrology light beam 108 off of one of the targets 122) is time-resolved by detecting the light 198 at the second sensor 238 (or at another detector, such as a photodiode) that receives light from the path 265. The reflected pulses (the light 198) determine precisely when the irradiated one of the targets 122 passes through the primary focus 242. There is a delay between the time at which a pulse of the light beam 106 (or other forward-going light pulse that propagates on the path 265) is formed and the time at which that forward-going light pulse arrives at the primary focus 242. This delay is referred to as the pulse delay time and is equal to the speed of light in the medium multiplied by the path length between the source of the pulse and the primary focus 242. The time at which the target 122 passes through the primary focus 242 minus the pulse delay time provides the time at which the source of the forward-going pulse (such as the main source 205) should be activated, stimulated, or fired to produce the pulse. The control system 250 may determine the pulse delay time and the time at which the source of the forward-going pulse is activated. Moreover, the targets 122 may be emitted from the material supply system 120 at regular intervals. The light 198 also may be used to confirm or measure the rate of emission of targets, and the knowledge of the rate may be used to set the timing of the production of light pulses.

[0076] FIG. 5 is a flow chart of a process 500. The process 500 is an example of a process for inspecting the optical element 240. The process 500 is discussed with respect to the apparatus 200. The process 500 may be performed by the control system 250. For example, the process 500 may be performed by one or more electronic processors in the processing module 251. The process 500 may be performed after the path 221 has been aligned with the primary focus 242 and after the apparatus 200 has been used to produce the plasma 196.

[0077] One of the targets 122 is illuminated with the metrology light beam 108 (510). The metrology light beam 108 is scattered and/or reflected from the target 122 to produce the light 198. The light 198 reflects off of the reflective surface 241 and is imaged at the first sensor 231 (520). The first sensor 231 produces image data of the reflective surface 241. The reflective surface 241 is inspected based on the produced image data (530).

[0078] As discussed above, the target 122 acts as a point source and, when the target 122 is illuminated while at the primary focus 242, the light 198 that is reflected from the reflective surface 241 produces a focused image of the reflective surface 241 at the intermediate focus 243. This focused image may be used to inspect the reflective surface 241. For example, the focused image may show damage and debris that have formed on the reflective surface 241 from forming the plasma 196.

[0079] Moreover, the image produced by the light 198 is a much clearer image of the reflective surface 241 than an image of the reflective surface 241 made with the EUV light 197. FIGS. 6 A and 6B are examples of respective images 687A and 687B of the reflective surface 241. FIG. 6A is an image 687A of the reflective surface 241 made with the EUV light 197. FIG. 6B is an image 687B of the reflective surface 241 made with the light 198 in an implementation in which the metrology light beam 108 had a wavelength of 1070 nm. The horizontal bar in the middle region of the image 687A (FIG. 6 A) is the EUV detector. The image of the reflective surface 241 made with the light 198 is much clearer and is more suitable for inspecting the reflective surface 241. For example, a defect 689 is present in the images 687A and 687B. However, the defect 689 is much clearer in the image 687B. The image 687B (which is the image of the reflective surface 241 obtained based on the light 198) allows for more effective inspection of the reflective surface 241, and may allow such inspection to occur through automatic analysis (for example, by a computer program implemented on the control system 250) or through manual inspection of the image without removing the optical element 240 from the vessel 210.

[0080] 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 processes 400 and 500 may be performed with the EUV light source 700 by supplying a metrology light beam (such as the metrology light beam 108) into an interior 707 of a vacuum chamber 730 of the light source 700.

[0081] The EUV light source 700 includes a target supply system 727. The EUV light source 700 may include a metrology light source that produces a metrology light beam, such as the metrology light source 209 that produces the metrology light beam 108 (FIG. 2A). The EUV light source 700 may also include a detection system, such as the detection system 130 (FIG. 1) or the detection system 230 (FIGS. 2A and 2B), that detects scattered light produced by an interaction between the metrology light beam and one of the targets provided from the target supply system 727.

[0082] 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 122 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 the 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.

[0083] 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.

[0084] 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.

[0085] 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.

[0086] 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.

[0087] 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.

[0088] 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.

[0089] 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.

[0090] 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.

[0091] 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 system 715 includes suitable optical feedback to form a laser cavity.

[0092] Other aspects of the invention are set out in the following numbered clauses.

1. An apparatus comprising: a vessel comprising an interior space; a material supply system configured to provide targets comprising target material along a path in the interior space, wherein the target material produces extreme ultraviolet (EUV) light when in a plasma state; a detection system configured to detect scattered light produced by an interaction between a metrology light beam and one of the targets and to produce information based on the interaction, wherein the metrology light beam does not convert the target material to the plasma state; and a control system configured to adjust the path based on the information produced by the detection system.

2. The apparatus of clause 1, further comprising: a reflective optical element in the interior space, the reflective optical element comprising a primary focus and an intermediate focus, and wherein the control system is configured to adjust the path such that the path intersects the primary focus of the reflective optical element.

3. The apparatus of clause 2, wherein the control system is further configured to adjust a turning element that directs light along a direction of propagation in the vessel such that the direction of propagation intersects the primary focus.

4. The apparatus of clause 2, wherein the detection system is configured to detect the scattered light after the scattered light has been reflected from the reflective optical element.

5. The apparatus of clause 2, wherein the detection system comprises an imaging sensor, and the detection system is configured to image the scattered light after the scattered light has been reflected from the reflective optical element.

6. The apparatus of clause 2, wherein the detection system comprises a component at the intermediate focus or at a location between the intermediate focus and a scanner apparatus that receives light from the apparatus.

7. The apparatus of clause 6, wherein the component comprises a mask, and the detection system further comprises a detector configured to detect light transmitted by the mask.

8. The apparatus of clause 6, wherein the component comprises a detector configured to be placed at the intermediate focus or removed from the intermediate focus.

9. The apparatus of clause 6, wherein the component comprises an imaging plane, the imaging plane comprising a material that causes the scattered light to be sensed by an imaging system.

10. The apparatus of clause 9, wherein the imaging plane comprises ground glass.

11. The apparatus of clause 9, wherein the imaging plane comprises a membrane or a Pellicle.

12. The apparatus of clause 9, wherein the imaging plane substantially blocks target material debris.

13. The apparatus of clause 1, wherein each point along the path is a location in a three- dimensional coordinate system, and the control system is configured to adjust the location of at least one of the points of the path in at least two of the three dimensions. 14. The apparatus of clause 13, wherein the control system is configured to adjust the location of the path by controlling the material supply system.

15. The apparatus of clause 1, further comprising a metrology light source.

16. The apparatus of clause 15, wherein the metrology light source is a laser configured to produce visible light or a laser configured to produce infrared light.

17. The apparatus of clause 15, wherein the metrology light source is a pulsed light source, and the metrology light beam comprises a plurality of pulses of light.

18. The apparatus of clause 15, wherein the metrology light source is a continuous-wave light source, and the metrology light beam comprises a continuous-wave beam of light.

19. An extreme ultraviolet (EUV) light source comprising: a vessel; a target material supply system configured to provide targets to an interior of the vessel, wherein the targets comprise a target material that emits EUV light when in a plasma state and the targets travel in the interior of the vessel along a path; and an apparatus configured to align the path of the targets with a focus of an optical element in the interior of the vessel, the apparatus comprising: a detection system configured to detect scattered light produced by an interaction between a metrology light beam and one of the targets and to produce information based on the interaction, wherein the metrology light beam does not convert the target material to the plasma state, and the one of the targets is configured to act as a point source of light when illuminated by the metrology light beam; and a control system configured to adjust the path of the targets based on the information produced by the detection system.

20. The EUV light source of clause 19, further comprising a turning element configured to receive an amplified light beam that has an energy sufficient to convert at least some of the target material to the plasma state.

21. The EUV light source of clause 19, further comprising a final turning element that comprises a turning mirror configured to adjust a direction of an incident light beam.

22. The EUV light source of clause 21, wherein the final turning mirror is outside of the vessel.

23. The EUV light source of clause 19, wherein the optical element is a reflective optical element comprising a primary focus and an intermediate focus, and the apparatus is configured to align the path of the targets with the primary focus of the reflective optical element.

24. The EUV light source of clause 23, wherein the detection system comprises a first sensor configured to detect an image of the one of the targets at the intermediate focus, and a second sensor, wherein the reflective optical element is between the second sensor and the first sensor.

25. A method comprising: directing targets along a path in an interior of a vessel, each target comprising target material that emits EUV light when in a plasma state; providing a metrology light beam to the interior of the vessel to produce scattered light, the scattered light arising from an interaction between the metrology light beam and one of the targets; detecting the scattered light at a detection system; and adjusting the path based on information about the scattered light from the detection system.

26. An apparatus for an optical system, the apparatus comprising: a detection system comprising a component configured to receive scattered metrology light, the scattered metrology light comprising light scattered from a target, wherein each target travels on a path, each target comprises target material that reflects the metrology light, and each target is configured to act as a point source of light when illuminated by the metrology light; and the apparatus further comprises a control system configured to adjust the path of each target to intersect with a primary focus of the optical system.

27. The apparatus of clause 26, wherein the target comprises target material that emits EUV light when in a plasma state, and the target is one target in a stream of moving targets.

28. The apparatus of clause 27, wherein the target is substantially spherical.

29. A method for inspecting an optical element in a vessel of an extreme ultraviolet (EUV) light source, the method comprising: illuminating a target with a metrology light beam to scatter the metrology beam from the target, wherein the target comprises target material that emits EUV light when in a plasma state, and the metrology light beam does not convert the target material to the plasma state; imaging an optical element in the vessel that receives the scattered metrology beam; and inspecting the optical element based on the imaged optical element.

30. A method comprising: providing a metrology light beam to an interior of a vessel; imaging scattered light generated by an interaction between the metrology light beam and a target material droplet to determine information about the path, wherein the scattered light is reflected from an optical element that is associated with a primary focus and a secondary focus; adjusting the path based on the information such that the path intersects the primary focus of the optical element; and adjusting a turning element to direct light along a direction of propagation that intersects the primary focus, wherein the interaction between the metrology light beam and the target material droplet does not produce a plasma that emits EUV light.

31. The method of clause 30, further comprising: after adjusting the path and after adjusting the turning element, providing an amplified light beam to the adjusted turning element such that the amplified light beam is provided to the primary focus, and wherein the amplified light beam has an energy sufficient to convert at least some of the target material in the target material droplet into plasma that emits EUV light.

32. The method of clause 31, wherein the turning element comprises a reflective optical element that receives some of the scattered light, and adjusting the turning element comprises moving the turning element and imaging the scattered light to align the reflective optical element such that the direction of propagation intersects the primary focus of the optical element. 33. The method of clause 32, wherein the scattered light is imaged at the secondary focus of the optical element and at a location that is between a source of the amplified light beam and the optical element.

[0093] The above described implementations and other implementations are within the scope of the claims.

Claims

CLAIMS:

2. The apparatus of claim 1, further comprising: a reflective optical element in the interior space, the reflective optical element comprising a primary focus and an intermediate focus, and wherein the control system is configured to adjust the path such that the path intersects the primary focus of the reflective optical element.

3. The apparatus of claim 2, wherein the control system is further configured to adjust a turning element that directs light along a direction of propagation in the vessel such that the direction of propagation intersects the primary focus.

4. The apparatus of claim 2, wherein the detection system is configured to detect the scattered light after the scattered light has been reflected from the reflective optical element.

5. The apparatus of claim 2, wherein the detection system comprises an imaging sensor, and the detection system is configured to image the scattered light after the scattered light has been reflected from the reflective optical element.

6. The apparatus of claim 2, wherein the detection system comprises a component at the intermediate focus or at a location between the intermediate focus and a scanner apparatus that receives light from the apparatus.

7. The apparatus of claim 6, wherein the component comprises a mask, and the detection system further comprises a detector configured to detect light transmitted by the mask.

8. The apparatus of claim 6, wherein the component comprises a detector configured to be placed at the intermediate focus or removed from the intermediate focus.

9. The apparatus of claim 6, wherein the component comprises an imaging plane, the imaging plane comprising a material that causes the scattered light to be sensed by an imaging system.

10. The apparatus of claim 9, wherein the imaging plane comprises ground glass.

11. The apparatus of claim 9, wherein the imaging plane comprises a membrane or a Pellicle.

12. The apparatus of claim 9, wherein the imaging plane substantially blocks target material debris.

13. The apparatus of claim 1, wherein each point along the path is a location in a three- dimensional coordinate system, and the control system is configured to adjust the location of at least one of the points of the path in at least two of the three dimensions.

14. The apparatus of claim 13, wherein the control system is configured to adjust the location of the path by controlling the material supply system.

15. The apparatus of claim 1, further comprising a metrology light source.

16. The apparatus of claim 15, wherein the metrology light source is a laser configured to produce visible light or a laser configured to produce infrared light.

17. The apparatus of claim 15, wherein the metrology light source is a pulsed light source, and the metrology light beam comprises a plurality of pulses of light.

18. The apparatus of claim 15, wherein the metrology light source is a continuous-wave light source, and the metrology light beam comprises a continuous-wave beam of light.

20. The EUV light source of claim 19, further comprising a turning element configured to receive an amplified light beam that has an energy sufficient to convert at least some of the target material to the plasma state.

21. The EUV light source of claim 19, further comprising a final turning element that comprises a turning mirror configured to adjust a direction of an incident light beam.

22. The EUV light source of claim 21, wherein the final turning mirror is outside of the vessel.

23. The EUV light source of claim 19, wherein the optical element is a reflective optical element comprising a primary focus and an intermediate focus, and the apparatus is configured to align the path of the targets with the primary focus of the reflective optical element.

24. The EUV light source of claim 23, wherein the detection system comprises a first sensor configured to detect an image of the one of the targets at the intermediate focus, and a second sensor, wherein the reflective optical element is between the second sensor and the first sensor.

27. The apparatus of claim 26, wherein the target comprises target material that emits EUV light when in a plasma state, and the target is one target in a stream of moving targets.

28. The apparatus of claim 27, wherein the target is substantially spherical.

31. The method of claim 30, further comprising: after adjusting the path and after adjusting the turning element, providing an amplified light beam to the adjusted turning element such that the amplified light beam is provided to the primary focus, and wherein the amplified light beam has an energy sufficient to convert at least some of the target material in the target material droplet into plasma that emits EUV light.

32. The method of claim 31, wherein the turning element comprises a reflective optical element that receives some of the scattered light, and adjusting the turning element comprises moving the turning element and imaging the scattered light to align the reflective optical element such that the direction of propagation intersects the primary focus of the optical element.

33. The method of claim 32, wherein the scattered light is imaged at the secondary focus of the optical element and at a location that is between a source of the amplified light beam and the optical element.