Contamination control in an EUV light source

Abstract

A method is performed for controlling contamination in a light source. The method includes: obtaining a reference image of an optical element in the light source; obtaining a real-time image of the optical element; comparing the real-time image with the reference image to determine a contamination metric associated with the optical element; and triggering an operation in the light source based on the determined contamination metric to thereby mitigate contamination at the optical element.

Description

CONTAMINATION CONTROL IN AN EUV LIGHT SOURCECROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to US Application No. 63 / 734,406, filed December 16, 2024, titled CONTAMINATION CONTROL IN AN EUV LIGHT SOURCE, which is incorporated herein by reference in its entirety.TECHNICAL FIELD

[0002] The disclosed subject matter relates to method and apparatus for controlling contamination in an extreme ultraviolet (EUV) light source.BACKGROUND

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

[0004] In some general aspects, a method is performed for controlling contamination in a light source. The method includes: obtaining a reference image of an optical element in the light source; obtaining a real-time image of the optical element; comparing the real-time image with the reference image to determine a contamination metric associated with the optical element; and automatically triggering an operation in the light source based on the determined contamination metric to thereby mitigate contamination at the optical element.

[0005] Implementations can include one or more of the following features. For example, the method can include, after automatically triggering the operation in the light source, obtaining an updated real-time image of the optical element and comparing the updated real-time image with the reference image to determine the updated contamination metric associated with the optical elementand automatically triggering an operation in the light source based on the determined updated contamination metric. The method can include, in response to automatically triggering the operation in the light source, increasing a frequency at which a real-time image of the optical element is obtained. The real-time image of the optical element can be obtained by obtaining a far-field intensity distribution of an image of the optical element. The method can also include: calculating a reference image metric indicating an intensity of the reference image; calculating a real-time image metric indicating an intensity of the real-time image; and comparing the real-time image metric with the reference image metric to construct the contamination metric. The real-time image metric can be compared with the reference image metric by determining a ratio of the real-time image metric to the reference image metric or determining a difference between the real-time image metric and the reference image metric. The contamination metric can be determined by isolating a sub-region of the optical element and determining the contamination metric of that sub-region. The sub-region of the optical element can be isolated by isolating a central region of the optical element. The operation in the light source can be automatically triggered by adjusting a pulse shape of a trigger light pulse directed to a target material, the target material producing extreme ultraviolet (EUV) radiation when interacting with the trigger light pulse, the EUV radiation directed to the optical element. The pulse shape of the trigger light pulse directed to the target material can be adjusted by adjusting an energy ahead of a primary pulse shape. The operation in the light source can be automatically triggered by adjusting a pressure of an interior of a vessel in which the optical element is fixed. The method can further include storing one or more of the reference image, the real-time image, and the contamination metric at a location local to the light source. The reference image of the optical element in the light source can be obtained by obtaining an updated reference image of the optical element after the optical element has been used in the light source in operating mode. The real-time image of the optical element can be obtained by obtaining the real-time image of the optical element while the light source is in downtime mode. The operation in the light source can be automatically triggered based on the determined contamination metric by automatically triggering the operation in the light source if the determined contamination metric exceeds a predetermined threshold. The real-time image of the optical element can be obtained by constructing of an image from measurements at two or more portions of the optical element, the measurements taken at subsequent, consecutive moments in time.

[0006] In other general aspects, a method is performed for controlling contamination in a light source. The method includes: comparing a real-time image of an optical element in the light source with a reference image of the optical element in the light source to determine a contamination metric associated with the optical element; automatically triggering an operation in the light source based on the determined contamination metric; and, after automatically triggering the operation in the light source, comparing a next real-time image of the optical element with the reference image to determine an updated contamination metric.

[0007] Implementations can include one or more of the following features. For example, the method can further include, in response to automatically triggering the operation in the light source, increasing a frequency at which a real-time image of the optical element is obtained. The method can further include obtaining the real-time image of the optical element and obtaining the reference image of the optical element. The reference image of the optical element in the light source can be obtained by obtaining the reference image of the optical element while the light source is in downtime mode. The real-time image of the optical element can be obtained by constructing of an image from measurements at two or more portions of the optical element, the measurements taken at subsequent, consecutive moments in time. The method can also include: calculating a reference image metric indicating an intensity of the reference image; calculating a real-time image metric indicating an intensity of the real-time image; and comparing the real-time image metric with the reference image metric to construct the contamination metric. The real-time image metric can be compared with the reference image metric by determining a ratio of the real-time image metric to the reference image metric or determining a difference between the real-time image metric and the reference image metric. The contamination metric can be determined by isolating a sub-region of the optical element and determining the contamination metric of that sub-region. The operation in the light source can be automatically triggered by adjusting a pulse shape of a trigger light pulse directed to a target material, the target material producing extreme ultraviolet (EUV) radiation when interacting with the trigger light pulse, the EUV radiation directed to the optical element. The pulse shape of the trigger light pulse directed to the target material can be adjusted by adjusting an energy ahead of a primary pulse shape. The operation in the light source can be automatically triggered by adjusting a pressure of an interior of a vessel in which the optical element is fixed. The operation in the light source can be automatically triggered based on the determined contamination metric by automatically triggering the operation in the light source if the determined contamination metric exceeds a predetermined threshold.

[0008] In other general aspects, a method is performed for controlling contamination in a light source. The method includes: during a first downtime mode of the light source, obtaining a reference image of an optical element in the light source; during a second downtime mode of the light source that follows the first downtime mode, determining a contamination metric associated with the optical element based on a real-time image and the reference image; during a third downtime mode of the light source that follows the first downtime mode, obtaining an updated reference image of the optical element; and during a fourth downtime mode of the light source that follows the second downtime mode, determining an updated contamination metric associated with the optical element based on an updated real-time image and the updated reference image.

[0009] In other general aspects, an apparatus controls contamination in a light source. The apparatus includes: an image detector configured to obtain images of an optical element in the light source; an image processing system; and an actuation system. The image processing system isconfigured to: receive, from the image detector, a reference image of the optical element and real-time images of the optical element, and compare a current real-time image with the reference image to determine a contamination metric associated with the optical element. The actuation system is configured to: receive the contamination metric from the image processing system, and automatically trigger an operation in the light source to mitigate contamination of the optical element based on the received contamination metric.

[0010] Implementations can include one or more of the following features. For example, the apparatus can further include an image collection apparatus configured to switch between two or more imaging states, each imaging state directing light from a portion of the optical element to the image detector. The image detector can be configured to obtain an image of the optical element based on the two or more imaging states. In a first imaging state, light from a central region of the optical element can be directed to the image detector and, in a second imaging state, light from an outer region of the optical element can be directed to the image detector. The image detector can be configured obtain the image of the optical element by calculating a ratio between a first measurement while the image collection apparatus is in the first imaging state and a second measurement while the image collection apparatus is in the second imaging state. The image detector can be configured to generate a far-field intensity distribution of an image of the optical element. The image processing system can be configured to: calculate a reference image metric indicating an intensity of the reference image; calculate a real-time image metric indicating an intensity of the real-time image; and compare the real-time image metric with the reference image metric to construct the contamination metric. The real-time image metric can be compared with the reference image metric by determining a ratio of the real-time image metric to the reference image metric or determining a difference between the realtime image metric and the reference image metric. The contamination metric can be determined by isolating a sub-region of the optical element and determining the contamination metric of that subregion. The actuation system can be configured to automatically trigger the operation in the light source by adjusting a pulse shape of a trigger light pulse directed to a target material, the target material producing extreme ultraviolet (EUV) radiation when interacting with the trigger light pulse, the EUV radiation directed to the optical element. The pulse shape of the trigger light pulse can be adjusted by adjusting an amount of energy arriving ahead of a main pulse. The actuation system can be configured to automatically trigger the operation in the light source by adjusting a pressure of an interior of a vessel in which the optical element is fixed.

[0011] 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

[0012] The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the present disclosure and, together with the description, further serve to explain the principles of the present disclosure and to enable a person skilled in the relevant art(s) to make and use implementations described herein.

[0013] Fig. 1A is a block diagram of an apparatus configured for quantifying and controlling contamination in a light source, in which the apparatus is active during a downtime mode of the light source;

[0014] Fig. IB is a block diagram of the apparatus of Fig. 1A, in which the apparatus is in standby during a production mode of the light source;

[0015] Fig. 2A is a schematic block diagram of an implementation of an apparatus configured for quantifying and controlling contamination in a light source that is an extreme ultraviolet (EUV) light source, the apparatus being in standby mode during a production mode of the EUV light source;

[0016] Fig. 2B is a schematic block diagram of the apparatus of Fig. 2A, the apparatus being active during a downtime mode of the EUV light source;

[0017] Fig. 3 A is a rear perspective view of an implementation of a collector mirror that can be used in the EUV light source of Figs. 2A and 2B;

[0018] Fig. 3B is a front perspective view of the collector mirror of Fig. 3A;

[0019] Fig. 3C is a side cross-sectional view of the collector mirror of Figs. 3A and 3B taken along plane 3C-3C;

[0020] Fig. 3D is a front plan view of the collector mirror of Figs. 3A-3C;

[0021] Fig. 4 is a block diagram of an implementation of an image processing system that can be a part of the apparatus of Figs. 1 A-2B;

[0022] Fig. 5 A is a perspective view of an implementation of an image detector that can be a part of the apparatus of Figs. 1A-2B;

[0023] Fig. 5B is a side cross-sectional view of the image detector of Fig. 5A taken along plane 5B-5B;

[0024] Fig. 6 is a schematic block diagram of an implementation of an apparatus configured for quantifying and controlling contamination in an EUV light source, the apparatus including an image detector, an actuation system, and an image processing system;

[0025] Fig. 7 is a perspective view of an implementation of mirror that can be a part of a photolithography exposure apparatus of the EUV light source of Fig. 6;

[0026] Fig. 8 is a flow chart of a procedure performed by an apparatus for quantifying and controlling contamination in an EUV light source;

[0027] Fig. 9 is a flow chart of an implementation of a procedure performed by an apparatus for quantifying and controlling contamination in an EUV light source;

[0028] Fig. 10A is a rear perspective view of an implementation of a collector mirror that can be used in the EUV light source of Figs. 2A, 2B, and 6;

[0029] Fig. 10B is a front perspective view of the collector mirror of Fig. 10A;

[0030] Fig. 10C is a side cross-sectional view of the collector mirror of Figs. 10A and 10B taken along plane 10C-10C;

[0031] Fig. 10D is a front plan view of the collector mirror of Figs. 10A-10C; and

[0032] Fig. 11 is a flow chart of an implementation of a procedure performed by an apparatus for quantifying and controlling contamination in an EUV light source.

[0033] The features of the present disclosure will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and / or structurally similar elements. Additionally, generally, the left-most digit(s) of a reference number identifies the drawing in which the reference number first appears. Unless otherwise indicated, the drawings provided throughout the disclosure should not be interpreted as to-scale drawings.DESCRIPTION

[0034] Referring to Figs. 1A and IB, an apparatus 100 is configured for quantifying and controlling contamination in a light source 150. The apparatus 100 includes an image detector 105, an actuation system 115, and an image processing system 125 in communication with the image detector105 and the actuation system 115. The image detector 105 is configured to obtain one or more images106 of an optical element 155 in the light source 150. The image processing system 125 is configured to receive, from the image detector 105, a reference image of the optical element 155 and one or more real-time images of the optical element 155. The image processing system 125 is configured to compare a current real-time image with the reference image to determine a contamination metric associated with the optical element 155. The actuation system 115 is configured to receive the determined contamination metric from the image processing system 125 and automatically trigger an operation in the light source 150. The operation that is triggered is based on the received contamination metric and also mitigates contamination of the optical element 155. Specifically, the apparatus 100 can be configured for quantifying and controlling contamination of a surface 155s of the optical element 155. Thus, the images 106 can be taken of the surface 155s of the optical element 155. Contamination of the surface 155s can be caused by debris within the light source 150, and such debris can reduce the performance of the surface 155s and can therefore reduce the overall efficiency of the light source 150.

[0035] The apparatus 100 is active during a downtime mode of the light source 150. The downtime mode is depicted in Fig. 1A. While active, the image detector 105 is obtaining one or more real-time images of the optical element 155, the image processing system 125 is analyzing the real-time images, and the actuation system 115 is triggering the operation in the light source 150. In the downtime mode, the light source 150 is not producing a light beam for standard use by an output apparatus 151. For example, the light source 150 can produce a stable light beam (such as a stable extreme ultraviolet EUV light beam) at a very low brightness or power that would not be suitable for use by the output apparatus 151 but is bright enough to illuminate the surface 155s of the optical element 155. As another example, a secondary light source can be configured to illuminate the surface 155s of the optical element 155.

[0036] Referring to Fig. IB, the apparatus 100 is in standby during a production mode of the light source 150. While in standby, the image detector 105 is not capturing images of the optical element 155, the image processing system 125 is not analyzing the images, and the actuation system 115 is not triggering an operation in the light source 150. In the production mode, the light source 150 produces a light beam 152 for standard use by the output apparatus 151.

[0037] The apparatus 100 relies on a feedback apparatus to provide active adjustment of the light source 150 and provide real-time contamination control of the optical element 155. In particular, the image processing system 125 compares the current real-time image (the feedback) with the reference image, and further determines the contamination metric based on this comparison. Moreover, the actuation system 115 automatically triggers the operation in the light source 150 based on this determined contamination metric. And, there is no need for approval, which takes significant time, for example, from a human operator, to trigger the operation in the light source 150. Additionally, the apparatus 100 can be configured to store and process the images (both the real-time images and the reference image) at a location within or local to the light source 150 or within or local to the output apparatus 151. This local data, which is not transferred remotely from the light source 150 or the output apparatus 151, is available when needed. Because of these features, the time during which the light source 150 is in downtime mode (Fig. 1A) in order to perform the analysis by the apparatus 100 is reduced and there is more time available for the light source 150 to operate in production mode (Fig. IB). The apparatus 100 therefore provides a rapid response to changes in performance of the optical element 155.

[0038] Referring to Fig. 2A, in some implementations, the light source 150 is an extreme ultraviolet (EUV) light source 250 configured to produce an EUV light beam 252 as the light beam 152. The EUV light source 250 is operating in production mode in Fig. 2A. In production mode, the light beam 252 is supplied to an output apparatus 251 as the output apparatus 151. In some implementations, the output apparatus 251 is a photolithography exposure apparatus, an inspection apparatus, or a metrology apparatus. The output apparatus 251 includes a set of optical elements 269, a reticle or mask 265, and a support structure 266 configured to hold a substrate 264 at an imaging plane Pi when in operation. The set of optical elements 269 includes one or more reflective optics configured to direct the light beam 252 along a path toward the reticle / mask 265 and to direct the lightbeam 252 from the reticle / mask 265 to the substrate 264 so that the light is properly shaped at the imaging plane Pi.

[0039] The EUV light source 250 is an implementation of a laser produced plasma (“LPP”) EUV radiation source 150. As shown, the EUV light source 250 includes a pulsed or continuous laser source 253, which can, for example, include one or more pulsed gas discharge CO2 lasers that produce one or more beams 254 of radiation at a wavelength generally below 20 pm, for example, in the range of from about 10.6 pm to about 0.5 pm or less.

[0040] The EUV light source 250 also includes a target delivery system 256 for delivering target material in the form of liquid droplets or a continuous liquid stream. In this example, the target material is a liquid. The target material can be made up of tin or a tin compound, although other materials can be used. The target delivery system 256 introduces fuel targets (such as droplets) 257 of the target material into an interior 258 of a source vessel 259 to an irradiation region 260 where the target material can be irradiated by the one or more beams 254 to produce plasma 268. A beam 254 of radiation includes a train of trigger light pulses, and each trigger light pulse interacts with the target material in the droplet 257 (or a modified form of the droplet 257) to produce the plasma 268. In some implementations, the source vessel 259 has a substantially conical configuration.

[0041] The plasma 268 emits EUV light 261 that is collected and directed out of the source vessel 259 as the EUV light beam 252 to the output apparatus 251. In general, the irradiation region 260 coincides with the primary focus of a collector mirror 255. The collector mirror 255 is an optical element that collects and directs the EUV light 261 out of the source vessel 259 through an intermediate focus 263 of the collector mirror 255.

[0042] In some implementations, an apparatus 200 can be configured to control contamination of the collector mirror 255. Like the apparatus 100, the apparatus 200 operates while the light source 250 is in downtime mode, which is shown in Fig. 2B. The apparatus 200 is shown in dashed lines in Fig. 2A to depict that the apparatus 200 is in standby (while the light source 250 is in downtime mode). The apparatus 200 includes an image detector 205, an actuation system 215, and an image processing system 225 in communication with the image detector 205 and the actuation system 215. While in standby, the image detector 205 can be moved out of an imaging position to enable the EUV light beam 252 to reach the substrate 264. Operation of the apparatus 200 is discussed in more detail with reference to Fig. 2B.

[0043] The collector mirror 255 can be a normal incidence reflector. The collector mirror 255 can be implemented as a multilayer mirror (MLM). The MLM can be fabricated by depositing many pairs of Mo and Si layers on a substrate with additional thin barrier layers, for example B4C, ZrC, SisN4 or C, deposited at each interface between layer pairs to effectively block thermally induced interlayer diffusion. The collector mirror 255 can be formed of other layers of material in other implementations. The collector mirror 255 can be in the form of a prolate ellipsoid, with a central aperture 262 to allow the one or more beams 254 to pass through and reach the irradiation region 260.The collector mirror 255 can be in the shape of an ellipsoid that has a first focus (for example, the primary focus) at the irradiation region 260 and a second focus (for example, the intermediate focus 263) where the EUV light beam 252 can be output from the source vessel 259. The output apparatus 251 uses the radiation from the EUV light beam 252, for example, to process the substrate (such as a silicon wafer) 264 in a known manner using the reticle or mask 265. The substrate 264 is then additionally processed in a known manner to obtain integrated circuit devices. The substrate 264 is held on the support structure 266. In some implementations, the radiation of the EUV light beam 252 is used to inspect a surface of the reticle / mask 265 or the substrate 264.

[0044] The light source 250 can include one or more optical elements within the interior 258 of the source vessel 259 in addition to the collector mirror 255.

[0045] During operation of the light source 250, debris can be partially produced from leftover or remaining target matter in the source vessel 259. In particular, the leftover or remaining target matter can be target matter that is not converted into the plasma 268 in the irradiation region 260 and / or the leftover or remaining target matter can be produced from plasma 268 that reverts back into target matter. The process of generating the EUV light 261 relies on converting the matter in the droplets 257 (or modified forms of the droplets 257) into plasma 268. A large amount of remaining or leftover target matter can be produced in this process. Different phases of the target matter tend to deposit on surfaces of various objects inside the source vessel 259. The target matter that remains or is leftover can travel through the source vessel 259 and coat various objects such as walls, optical elements, and components within the source vessel 259. The debris that forms on the surfaces of these objects can include vapor residue, ions, particles, and / or clusters of matter formed from the target matter. This debris can severely impair the performance of the EUV light source 250 by blocking the EUV light 261 or the EUV light beam 252, or by contaminating the objects within the source vessel 259.

[0046] For example, the debris can form a coating on a reflective surface 255s of the collector mirror 255, such coating acting to effectively block the surface 255s. The reflective surface 255s is an optical surface that is meant to interact with the EUV light 261 in the source vessel 259; accordingly, the efficiency of the surface 255 s will drop as it becomes coated with debris. Moreover, the debris can cause the surface 255s and the collector mirror 255 to heat up, which can lead to the debris being ejected from the surface 255s and onto other elements within the source vessel 259. The debris can cause other problems that lead to a reduction in the production of EUV light 261. For example, the debris can be flaked off, dropped off, spit off, or dripped off the surface 255 s. In summary, the presence of such debris can reduce the performance of the surfaces within the source vessel 259 (such as the surface 255 s) and can therefore reduce the overall efficiency of the EUV light source 250 and production of the EUV light beam 252.

[0047] The apparatus 200 is configured for quantifying and controlling contamination of the surface 255s in the light source 250 while the light source 250 is in downtime mode (Fig. 2B). The apparatus 200 can quantify the contamination of the surface 255s of the collector mirror 255 and cantrigger the operation in the EUV light source 250 within minutes. The image processing system 225 determines the contamination metric associated with the surface 255s of the collector mirror 255 by quantifying a loss of reflectivity by analyzing the real-time images of the surface 255s.

[0048] An implementation 355 of the collector mirror 255 is shown in Figs. 3A-3D. As shown, the collector mirror 355 can be, for example, an ellipsoidal mirror that has a primary focus at the irradiation region 260 and a secondary or intermediate focus 263 (Fig. 2A). This means that a plane section (such as plane section 3C-3C) is in the shape of an ellipses or a circle. Thus, the plane section 3C-3C cuts through the reflective surface 355s, and it is formed from a portion of an ellipse. A plan view of the optical collector 355 (Fig. 3D) shows that the edge of the reflective surface 355s forms a circular shape. The collector mirror 355 can be in the form of a prolate ellipsoid, with a central aperture 362 to allow the one or more beams 254 to pass through and reach the irradiation region 260 (Figs. 2A and 2B).

[0049] Referring to Fig. 2B, the apparatus 200 is configured for quantifying and controlling contamination of the reflective surface 255 s of the collector mirror 255 in the light source 250. The apparatus 200 is shown in Fig. 2B as being active during a downtime mode of the light source 250. In some implementations in the downtime mode, the light source 250 is not producing any plasma 268 and therefore is not producing the EUV light 261 or the EUV light beam 252 for standard use by the output apparatus 251 to process the substrate 264. In some implementations in the downtime mode, the light source 250 produces a lower-power EUV light beam 252 that is not suitable for standard use by the output apparatus 251 to process the substrate 264. In any case, enough light illuminates the surface 255 s of the collector mirror 255 to enable imaging of the surface 255s.

[0050] The apparatus 200 includes the image detector 205, the actuation system 215, and the image processing system 225 in communication with the image detector 205 and the actuation system 215. The image detector 205 is configured to obtain a current image 206c of the surface 255s of the collector mirror 255 in the light source 250. In some implementations, the image detector 205 is positioned at the support structure 266 so that it aligns with the imaging plane Pi defined by the set of optical elements 269. The substrate 264 is not present because the light source 250 is in downtime mode. In some implementations, the image detector 205 is configured to capture a current image 206c of one or more portions of the surface 255s. In some implementations, the image detector 205 is configured to reflect a spatial profile of light intensity of the current image 206c of the surface 255s. In some embodiments, the spatial profile of light intensity is measured at a distance from the source vessel 259, for example, in the output apparatus 251 or near the reticle / mask 265. In such a way, the spatial profile of light intensity is measured in a different chamber from that where the optical element (e.g., the collector mirror 255) is located. In some embodiments, the spatial profile of light intensity is also called as a far-field intensity distribution of the current image 206c of the surface 255s. The image detector 205 can include one or more two-dimensional recording devices that generate a two- dimensional representation of the surface 255s. Thus, for example, the image detector 205 can includea two-dimensional array of cameras, such array defining an imaging plane. Each camera includes a plurality of photo sites (or pixels). The light is directed onto the photo-sensitive area of each pixel where it is converted into electrons that are collected into a voltage signal and the array of these signals forms the two-dimensional image of the surface 255s (or the portion of the surface 255s).

[0051] In some implementations, each camera is a complementary metal -oxide semiconductor (CMOS) having an array of 512 x 512 rectangular or square pixels, with each pixel having a surface area of 25 micrometers (pm) x 25 pm. In other implementations, the camera is a charged coupled device (CCD) or an infrared camera. Each camera can have an exposure time on the order of nanoseconds (ns), for example, between 20-200 ns, or between 30-50 ns, or even greater and up to about 300 ps. In some implementations, each camera has a pixel size on the order of micrometers (pm), for example, about 5 pm, about 8 pm, in a range of 5 - 30 pm. Each camera can have a gain that is anywhere between 0-12 dB, or about 1.0 dB.

[0052] The image processing system 225 is configured to receive, from the image detector 205 or stored within memory (for example, within the image processing system 225), a reference image 206r of the surface 255 s of the collector mirror 255 and one or more real-time images of the surface 255s of the collector mirror 255. The image processing system 225 is configured to compare a current real-time image 206c with the reference image 206r to determine a contamination metric associated with the collector mirror 255. In some implementations, the image processing system 225 is physically and functionally integrated with the light source 250. In some implementations, the image processing system 225 is physically and functionally integrated with the output apparatus 251.

[0053] Referring to Fig. 4, an implementation 425 of the image processing system 225 is described. The image processing system 425 includes, among other possible features, an image processing module 426, an actuation module 427, memory 428, and one or more processors 429 that can be programmable. The memory 428 can be read-only memory and / or random-access memory. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of non-volatile memory, including, by way of example, semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto -optical disks; and CD-ROM disks. The one or more processors 429 can each execute a program of instructions to perform desired functions by operating on input data and generating appropriate output. Generally, the processor receives instructions and data from modules and / or memory 428. Any of the foregoing may be supplemented by, or incorporated in, specially designed microprocessors or ASICs (application-specific integrated circuits).

[0054] Each module 426 or 427 can include one or more computer program products tangibly embodied in a machine -readable storage device (such as memory 428) for execution by a processor 429. Connections between modules 426, 427, the memory 428, and the one or more processors 429within the system 425 and between the system 425 and other components of the apparatus 200 can be wired or wireless.

[0055] The image processing module 426 includes a comparison sub-module 426a and an image analysis sub-module 426b that receives the output of the comparison sub-module 426a. The comparison sub-module 426a receives the current image 406c from the image detector 205 and the reference image 406r from memory 428 (or from the image detector 205, if it is stored at the image detector 205) and outputs a comparison image 406n based on this comparison. The image analysis sub-module 426b receives the comparison image 406n and performs further analysis on the comparison image 406n to extract a measure of degradation (such as a contamination metric) to the surface 255s. The contamination metric is provided to the actuation module 427, which applies a logic analysis to the contamination metric to determine which action to take and therefore the instruction to provide to the actuation system 215.

[0056] The actuation system 215 is configured to receive the determined contamination metric from the actuation module 427 of the image processing system 425 and then to automatically trigger an operation in the light source 250. The operation that is triggered is based on the received contamination metric and is configured to mitigate contamination of the surface 255 s of the collector mirror 255. In some implementations, the actuation system 215 communicates with the laser source 253 to control aspects of the one or more beams 254. In other implementations, the actuation system 215 communicates with an environment control system 249 associated with the interior 258 of the source vessel 259. For example, the environment control system 249 can control a pressure within the interior 258. The actuation system 215 can cause a change to the pressure by sending a suitable signal to the environment control system.

[0057] Referring to Figs. 5A and 5B, in some implementations, an image detector 505 is configured for collecting or obtaining images of an optical element (such as the surface 255s of the collector mirror 255 of Figs. 2A and 2B). The image detector 505 includes an array 507 of cameras 508i under a light membrane 509. The light membrane 509 is configured to control an amount of light in the image 106 that reaches the array 507 in order to reduce the change of signal saturation in the cameras 508i. The image detector 505 also includes processing electronics 510 in communication with the cameras 508i . The array 507 and the electronics 510 are positioned on a substrate 511. In some implementations, the processing electronics 510 is positioned in the substrate 511. The substrate in Fig. 5A is illustrated as a rectangular, but it is to be understood that other suitable shapes, such as circular, can be utilized.

[0058] Referring to Fig. 6, an implementation 669 of the optical elements that can be used in the output apparatus 251 is described. The set of optical elements 669 includes, for example, a first facet mirror 670 and a second facet mirror 671 arranged to collect the EUV light redirected by the collector mirror 255. In this implementation, the EUV light from the intermediate focus 263 is split into channels by the first facet mirror 670, and the light in these channels propagates through the set ofoptical elements 669 toward the substrate 264 (when the apparatus 200 is in standby mode) or toward the image detector 205 (when the apparatus 200 is active, as shown in Fig. 6). At the imaging plane Pi, light from all of the channels is superimposed. Other optical elements 673 not shown can be positioned within the set of optical elements 669.

[0059] One or more of the first facet mirror 670 and the second facet mirror 671 can constitute an image collection apparatus that is configured to switch between a plurality of imaging states. Each imaging state is configured to direct light from a portion of the optical element (the surface 255 s of the collector mirror 255) to the image detector 205 or the substrate 264. For example, as shown in Fig. 7, the first facet mirror 670 or second facet mirror 671 includes a plurality of reflective optical elements 772-i (such as mirrors), where i is an integer number ranging from 200 to 900. If smaller than 200 units, in some instances, a resolution is insufficient to accurately pinpoint a location and source of contamination. If greater than 900 units, in some instances, a manufacturing cost increases without significant benefits. In some implementations, a number of reflective optical elements of the first facet mirror 670 is the same as a number of reflective optical elements of the second facet mirror 671. In some implementations, a number of reflective optical elements of the first facet mirror 670 is less than that of reflective optical elements of the second facet mirror 671. In some implementations, a number of reflective optical elements of the first facet mirror 670 is more than that of reflective optical elements of the second facet mirror 671. The reflective optical elements 772-i are arranged across a surface that is transverse to a direction of light beam forming the current image 206c. In order to select a portion of the surface 255s to image, one or more of the reflective optical elements 772-i is angled to direct the portion of the image 206c toward the image detector 205. For example, in Fig. 7, element 772-32 is angled to direct one portion of the image 206c toward the image detector 205 while all of the other elements are angled to deflect the other portions of the image 206c away from the image detector 205. In some implementations, a shape of the reflective optical elements 772-i of the first facet mirror 670 and second facet mirror 671 includes circular, oval, square, hexagonal, rectangular, arcuate, or any other shape suitable for reflecting each channel of the EUV light. For example, each of the reflective optical element 772-i of the first facet mirror 670 is arcuate, and each of the reflective optical element 772-i of the second facet mirror 671 is square. In some implementations, an angle of at least one of the reflective optical element 772-i is adjustable. For example, each of the reflective optical element 772-i of the first facet mirror 670 and second facet mirror 671 is rotatable to generate various patterns of EUV light on the image detector 205 or the substrate 264.

[0060] Referring to Fig. 8, a procedure 880 is performed for controlling contamination in a light source, such as the light source 150 of Figs. 1A and IB or the light source 250 of Figs. 2A and 2B. The procedure 880 includes obtaining a reference image of an optical element in the light source (881) and obtaining a real-time image of the optical element (882). The procedure 880 includes comparing the real-time image with the reference image to determine a contamination metricassociated with the optical element (883). The procedure 880 includes automatically triggering an operation in the light source based on the determined contamination metric to thereby mitigate contamination at the optical element within the light source (884). Each of these steps in the procedure 880 will be discussed next with reference to Figs. 2A, 2B, and 4.

[0061] In some implementations, the reference image 206r, 406r of the optical element 155, 255 is provided to the image processing system 425 of the apparatus 100 or 200. The reference image 206r, 406r can be obtained by the image detector 105, 205 by imaging the optical element 155, 255 at a time when the optical element 155, 255 is first installed in the light source 150, 250. Thus, the reference image 206r, 406r can be captured by the image detector 105, 205 before the optical element 155, 255 is used in the light source 150, 250. As discussed above, the image detector 205 can capture the reference image 206r, 406r by imaging a spatial profile of light intensity of an image of optical element 155, 255 measured outside the source vessel 259 (e.g., a far-field intensity distribution of an image of the optical element 155, 255). In this way, the image processing system 425 of the apparatus 100, 200 obtains the reference image 206r, 406r from the image detector 105, 205 (881).

[0062] In some implementations, the reference image 206r, 406r can be updated periodically during operation of the light source 150, 250. For example, the image processing system 425 can store the last real-time image 206c, 406c as a reference image 206r, 406r for use in future analysis.

[0063] In some implementations, the real-time image 206c, 406c of the optical element 155, 255 is provided to the image processing system 425 of the apparatus 100 or 200. The real-time image 206c, 406c can be obtained by the image detector 105, 205 by imaging the optical element 155, 255 in real time after the optical element 155, 255 has been installed in the light source 150, 250. This imaging occurs while the apparatus 100, 200 is active and the light source 150, 250 is in downtime or standby mode. As discussed above, the image detector 205 can capture the real-time image 206c, 406c by imaging a far-field intensity distribution of an image of the optical element 155, 255. In this way, the image processing system 425 of the apparatus 100, 200 obtains the real-time image 206c, 406c from the image detector 105, 205 (882).

[0064] In some implementations, the real-time image 206c, 406c is compared with the reference image 206r, 406r to determine the contamination metric in accordance with the following steps. The comparison sub-module 426a receives the current image 406c from the image detector 205 and the reference image 406r from memory 428 (or from the image detector 205, if it is stored at the image detector 205) and outputs a comparison image 406n based on this comparison. And, the image analysis sub-module 426b receives the comparison image 406n and performs further analysis on the comparison image 406n to extract a measure of degradation (such as a contamination metric) to the surface 255s of the optical element 255 (883).

[0065] The comparison sub-module 426a can perform any suitable analysis to determine the comparison image 406n. Examples of the analysis are discussed next.

[0066] In one particular example, with reference to Fig. 9, the comparison sub-module 426a can perform a procedure 983 to determine the comparison image 406n (883). In particular, the comparison sub-module 426a calculates a reference image metric that indicates an intensity of the reference image 206r, 406r (Iref) (987). The comparison sub-module 426a calculates a real-time image metric that indicates an intensity of the real-time image 206c, 406c (Ireal) (988). The comparison sub-module 426a compares the real-time image metric (Ireal) with the reference image metric (Iref) to determine a comparison image 406n (Icomp) from which the contamination metric can be constructed (989).

[0067] In some implementations, the value of Icomp is a ratio of the intensities Ireal and Iref as follows: Icomp=Ireal / Iref. This unitless signal ranges from 0 to about 1.2, where 0 indicates a complete loss of transmission (Ireal is 0), 1 indicates no change in transmission (Ireal = Iref), and a value greater than 1 indicates an improvement in transmission (Ireal is higher than Iref).

[0068] In other implementations, the value of Icomp is a difference between the intensities Ireal and Iref as follows: Icomp=Iref — Ireal. The signal retains the units of the intensity. The value 0 indicates there is no change in transmission.

[0069] In still other implementations, the value of Iref is updated with an ongoing collector state estimate lest.

[0070] In some implementations, the image analysis sub-module 426b can isolate a sub-region of the optical element 155, 255 and thus the analysis focuses only on the sub-region. For example, it is possible that contamination to the optical element 155, 255 occurs mostly or only in a sub-region of the optical element 155, 255. In such a case, there is a benefit to performing the analysis based only on this sub-region. In particular, by performing the analysis based only on this sub-region, the image processing system 425 can perform the analysis more quickly because less data is required to determine the contamination metric. In this case, the contamination metric can be determined at 883 relative to the sub-region.

[0071] For example, as shown in Figs. 10A-10D, the optical element 155 is a collector mirror 1055 similar to the collector mirror 355. The collector mirror 1055 can become contaminated in a subregion 1055sr of its surface 1055s, the sub-region 1055sr being closest to the central aperture 1062, as shown. This sub-region 1055sr is in the central part of the collector mirror 1055. In this particular example, the sub-region 1055srtends to become contaminated more than the remaining part of the surface 1055s because of its position relative to the irradiation region 260 (Fig. 2A).

[0072] Referring again to Fig, 8, as discussed above, the contamination metric determined at 883 is provided to the actuation module 427, which applies a logic analysis to the contamination metric to determine which action to take and therefore the instruction to provide to the actuation system 215 (884). The actuation system 215 is configured to receive the contamination metric and automatically trigger the operation in the light source 150, 250 to mitigate contamination of the optical element 155, 255. The triggering operation is based on the received contamination metric.

[0073] Referring to Fig. 11, for example, in some implementations, the logic analysis corresponds to a procedure 1184. During the procedure 1184, the actuation system 215 determines whether the determined contamination metric (at 883) exceeds a predetermined threshold (1185). For example, if the contamination metric is greater than an upper bound of acceptable contamination, then the actuation system 215 can determine that the metric exceeds the predetermined threshold at 1 185. If the actuation system 215 determines that the contamination metric exceeds the predetermined threshold at 1185, then the actuation system 215 can then automatically trigger the operation in the light source 150, 250 (1186).

[0074] For example, with reference again to Figs. 2A and 2B, in some implementations, the actuation module 427 communicates with the laser source 253. As discussed above, the laser source 253 can include one or more pulsed gas discharge CO2 lasers that produce one or more beams 254 of radiation at a wavelength generally below 20 pm, for example, in the range of about 10.6 pm to about 0.5 pm or less. A brief description of the laser source 253 is provided next. The laser source 253 can be arranged to deliver and deposit light energy in the radiation beam 254 into the fuel targets 257 in the irradiation region 260. The fuel targets 257 can include, for example, tin (Sn) or Xenon (Xe) droplets. The radiation beam 254 can deposit energy into the fuel targets 257 via a one or more light pulses that can be referred to as one or more pre-pulses. The radiation beam 254 can deposit energy into the fuel targets 257 via second and third light pulses that can be referred to as main pulses. In addition, a small amount of energy can arrive just ahead of the main pulse, and this small amount of energy is referred to as the main pulse pedestal. Adjustment to the size of the main pulse pedestal can impact the amount of contamination at the surface 255 s of the collector mirror 255.

[0075] For example, in some implementations, increasing an amplitude (or power) within the main pulse pedestal can lead to a reduction in contamination at the surface 255 s of the collector mirror 255. Conversely, decreasing the amplitude (or power) within the main pulse pedestal can lead to an increase in contamination at the surface 255s of the collector mirror 255. In this way, the actuation module 427 can automatically send a signal to the laser source 253 to make an adjustment to the amplitude or power supplied to the main pulse pedestal to mitigate contamination of the surface 255 s based on the received contamination metric (884). In more general implementations, the actuation module 427 can automatically send a signal to the laser source 253 to adjust a shape of a pre-pulse, the main pulse, and / or the main pulse pedestal to make an adjustment to the amplitude or power supplied to the main pulse pedestal to mitigate contamination of the surface 255s based on the received contamination metric (884).

[0076] As another example, the actuation module 427 can automatically send a signal to the environment control system 249 to adjust a pressure of the interior 258 of the source vessel 259. For example, in some implementations, increasing the pressure of the interior 258 can lead to a reduction or decrease in contamination at the surface 255 s of the collector mirror 255. Conversely, decreasing the pressure of the interior 258 can lead to an increase in contamination at the surface 255s of thecollector mirror 255. In this way, the actuation module 427 can automatically send a signal to the environment control system 249 to make an adjustment to the pressure of the interior 258 to mitigate contamination of the surface 255 s based on the received contamination metric (884).

[0077] Referring again to Fig. 8, the procedure 880 can include other steps not shown. For example, the obtained reference image 206r, 406r (881) and the obtained real-time image 206c, 406c (882) can also be stored at a location local to the light source 150, 250. For example, the obtained reference image 206r, 406r (881) and the obtained real-time image 206c, 406c (882) can be stored in memory 428 within the image processing system 425, which can be local to the light source 150, 250.

[0078] The procedure 880 can be performed periodically during the life of the light source 150, 250. Thus, after the operation in the light source 150, 250 is automatically triggered at 884, there can be a gap in time and then the procedure 880 can start over again at 881. Each time the procedure 880 is performed, an updated (newly generated) contamination metric is determined at 883 and an updated operation can be automatically triggered at 884. The rate at which the procedure 880 is performed (the frequency) can be adjusted. For example, after the operation in the light source 150, 250 is automatically triggered at 884, the frequency at which each refreshed real-time image is obtained at 882 can be increased.

[0079] Referring again to Figs. 10A and 10B, in some implementations, the image detector 250 is configured to obtain an image (such as the reference image 206r, 406r or the real-time image 206c, 406c) based on two or more imaging states. For example, in a first imaging state, light from the central region (the sub-region 1055sr) can be directed to the image detector 205. In a second imaging state, light from the outer region 1055 or can be directed to the image detector 205. The image detector 205 can calculate a ratio between a first measurement taken while in the first imaging state and a second measurement taken while in the second imaging state.

[0080] Referring again to Fig. 8, in some implementations, the actuation system 115, 215 can automatically trigger the operation in the light source 150, 250 at 884 only to stop further degradation due to contamination of the optical element 155, 255. Thus, if there is little to no contamination of the optical element 155, 255, then no triggered operation is needed at 884. The actuation system 115, 215 can trigger operations at 884 in discrete steps rather than according to a proportional response to the amount of contamination (as determined by the contamination metric) at 883). While such a control can mean that it takes longer to reduce the contamination, it also reduces overcompensation.

[0081] The implementations can be further described using the following clauses:1. A method for controlling contamination in a light source, the method comprising: obtaining a reference image of an optical element in the light source; obtaining a real-time image of the optical element; comparing the real-time image with the reference image to determine a contamination metric associated with the optical element; andtriggering an operation in the light source based on the determined contamination metric to thereby mitigate contamination at the optical element.2. The method of clause 1, further comprising, after triggering the operation in the light source, obtaining an updated real-time image of the optical element and comparing the updated real-time image with the reference image to determine the updated contamination metric associated with the optical element and triggering an operation in the light source based on the determined updated contamination metric.3. The method of clause 1, further comprising, in response to triggering the operation in the light source, increasing a frequency at which an updated real-time image of the optical element is obtained.4. The method of clause 1, wherein obtaining the real-time image of the optical element comprises obtaining a spatial profile of light intensity of an image of the optical element.5. The method of clause 1, further comprising: calculating a reference image metric indicating an intensity of the reference image; calculating a real-time image metric indicating an intensity of the real-time image; and comparing the real-time image metric with the reference image metric to construct the contamination metric.6. The method of clause 5, wherein comparing the real-time image metric with the reference image metric comprises determining a ratio of the real-time image metric to the reference image metric or determining a difference between the real-time image metric and the reference image metric.7. The method of clause 1, wherein determining the contamination metric comprises isolating a subregion of the optical element and determining the contamination metric of that sub-region.8. The method of clause 7, wherein isolating the sub-region of the optical element comprises isolating a central region of the optical element.9. The method of clause 1, wherein triggering the operation in the light source comprises adjusting a pulse shape of a trigger light pulse directed to a target material, the target material producing extreme ultraviolet (EUV) radiation when interacting with the trigger light pulse, the EUV radiation directed to the optical element.10. The method of clause 9, wherein adjusting the pulse shape of the trigger light pulse directed to the target material comprises adjusting an energy ahead of a primary pulse shape.11. The method of clause 1, wherein triggering the operation in the light source comprises adjusting a pressure of an interior of a vessel in which the optical element is fixed.12. The method of clause 1, further comprising storing one or more of the reference image, the realtime image, and the contamination metric at a location local to the light source.13. The method of clause 1, wherein obtaining the reference image of the optical element in the light source comprises obtaining an updated reference image of the optical element after the optical element has been used in the light source in operating mode.14. The method of clause 1, wherein obtaining the real-time image of the optical element comprises obtaining the real-time image of the optical element while the light source is in downtime mode.15. The method of clause 1, wherein triggering the operation in the light source based on the determined contamination metric comprises triggering the operation in the light source if the determined contamination metric exceeds a predetermined threshold.16. The method of clause 1, wherein obtaining the real-time image of the optical element comprises a construction of an image from measurements at two or more portions of the optical element, the measurements taken at subsequent, consecutive moments in time.17. A method for controlling contamination in a light source, the method comprising: comparing a real-time image of an optical element in the light source with a reference image of the optical element in the light source to determine a contamination metric associated with the optical element; triggering an operation in the light source based on the determined contamination metric; and after triggering the operation in the light source, comparing a next real-time image of the optical element with the reference image to determine an updated contamination metric.18. The method of clause 17, further comprising, in response to triggering the operation in the light source, increasing a frequency at which a refreshed real-time image of the optical element is obtained.19. The method of clause 17, further comprising obtaining the real-time image of the optical element and obtaining the reference image of the optical element.20. The method of clause 19, wherein obtaining the reference image of the optical element in the light source comprises obtaining the reference image of the optical element while the light source is in downtime mode.21. The method of clause 19, wherein obtaining the real-time image of the optical element comprises constructing of an image from measurements at two or more portions of the optical element, the measurements taken at subsequent, consecutive moments in time.22. The method of clause 17, further comprising: calculating a reference image metric indicating an intensity of the reference image; calculating a real-time image metric indicating an intensity of the real-time image; and comparing the real-time image metric with the reference image metric to construct the contamination metric.23. The method of clause 22, wherein comparing the real-time image metric with the reference image metric comprises determining a ratio of the real-time image metric to the reference image metric or determining a difference between the real-time image metric and the reference image metric.24. The method of clause 17, wherein determining the contamination metric comprises isolating a sub-region of the optical element and determining the contamination metric of that sub-region.25. The method of clause 17, wherein triggering the operation in the light source comprises adjusting a pulse shape of a trigger light pulse directed to a target material, the target material producingextreme ultraviolet (EUV) radiation when interacting with the trigger light pulse, the EUV radiation directed to the optical element.26. The method of clause 25, wherein adjusting the pulse shape of the trigger light pulse directed to the target material comprises adjusting an energy ahead of a primary pulse shape.27. The method of clause 17, wherein triggering the operation in the light source comprises adjusting a pressure of an interior of a vessel in which the optical element is fixed.28. The method of clause 17, wherein triggering the operation in the light source based on the determined contamination metric comprises triggering the operation in the light source if the determined contamination metric exceeds a predetermined threshold.29. A method for controlling contamination in a light source, the method comprising: during a first downtime mode of the light source, obtaining a reference image of an optical element in the light source; during a second downtime mode of the light source that follows the first downtime mode, determining a contamination metric associated with the optical element based on a real-time image and the reference image; during a third downtime mode of the light source that follows the first downtime mode, obtaining an updated reference image of the optical element; and during a fourth downtime mode of the light source that follows the second downtime mode, determining an updated contamination metric associated with the optical element based on an updated real-time image and the updated reference image.30. An apparatus for controlling contamination in a light source, the apparatus comprising: an image detector configured to obtain images of an optical element in the light source; an image processing system configured to: receive, from the image detector, a reference image of the optical element and real-time images of the optical element, and compare a current real-time image with the reference image to determine a contamination metric associated with the optical element; and an actuation system configured to: receive the contamination metric from the image processing system, and automatically trigger an operation in the light source to mitigate contamination of the optical element based on the received contamination metric.31. The apparatus of clause 30, further comprising an image collection apparatus configured to switch between two or more imaging states, each imaging state directing light from a portion of the optical element to the image detector.32. The apparatus of clause 31, wherein the image detector is configured to obtain an image of the optical element based on the two or more imaging states.33. The apparatus of clause 32, wherein, in a first imaging state, light from a central region of the optical element is directed to the image detector and, in a second imaging state, light from an outer region of the optical element is directed to the image detector.34. The apparatus of clause 33, wherein the image detector is configured obtain the image of the optical element by calculating a ratio between a first measurement while the image collection apparatus is in the first imaging state and a second measurement while the image collection apparatus is in the second imaging state.35. The apparatus of clause 30, wherein the image detector is configured to generate a far-field intensity distribution of an image of the optical element.36. The apparatus of clause 30, wherein the image processing system is configured to: calculate a reference image metric indicating an intensity of the reference image; calculate a real-time image metric indicating an intensity of the real-time image; and compare the real-time image metric with the reference image metric to construct the contamination metric.37. The apparatus of clause 36, wherein comparing the real-time image metric with the reference image metric comprises determining a ratio of the real-time image metric to the reference image metric or determining a difference between the real-time image metric and the reference image metric.38. The apparatus of clause 30, wherein determining the contamination metric comprises isolating a sub-region of the optical element and determining the contamination metric of that sub-region.39. The apparatus of clause 30, wherein the actuation system being configured to automatically trigger the operation in the light source comprises adjusting a pulse shape of a trigger light pulse directed to a target material, the target material producing extreme ultraviolet (EUV) radiation when interacting with the trigger light pulse, the EUV radiation directed to the optical element.40. The apparatus of clause 39, wherein adjusting the pulse shape of the trigger light pulse comprises adjusting an amount of energy arriving ahead of a main pulse.41. The apparatus of clause 30, wherein the actuation system being configured to automatically trigger the operation in the light source comprises adjusting a pressure of an interior of a vessel in which the optical element is fixed.

[0082] Other implementations are within the scope of the following claims.

Claims

CLAIMS1. A method for controlling contamination in a light source, the method comprising: obtaining a reference image of an optical element in the light source; obtaining a real-time image of the optical element; comparing the real-time image with the reference image to determine a contamination metric associated with the optical element; and triggering an operation in the light source based on the determined contamination metric to thereby mitigate contamination at the optical element.

2. The method of claim 1, further comprising, after triggering the operation in the light source, obtaining an updated real-time image of the optical element and comparing the updated realtime image with the reference image to determine the updated contamination metric associated with the optical element and triggering an operation in the light source based on the determined updated contamination metric.

3. The method of claim 1, further comprising, in response to triggering the operation in the light source, increasing a frequency at which an updated real-time image of the optical element is obtained.

4. The method of claim 1, wherein obtaining the real-time image of the optical element comprises obtaining a spatial profile of light intensity of the optical element.

5. The method of claim 1, further comprising: calculating a reference image metric indicating an intensity of the reference image; calculating a real-time image metric indicating an intensity of the real-time image; and comparing the real-time image metric with the reference image metric to construct the contamination metric.

6. The method of claim 5, wherein comparing the real-time image metric with the reference image metric comprises determining a ratio of the real-time image metric to the reference image metric or determining a difference between the real-time image metric and the reference image metric.

7. The method of claim 1, wherein determining the contamination metric comprises isolating a sub-region of the optical element and determining the contamination metric of that sub-region.

8. The method of claim 7, wherein isolating the sub-region of the optical element comprises isolating a central region of the optical element.

9. The method of claim 1, wherein triggering the operation in the light source comprises adjusting a pulse shape of a trigger light pulse directed to a target material, the target material producing extreme ultraviolet (EUV) radiation when interacting with the trigger light pulse, the EUV radiation directed to the optical element.

10. The method of claim 1, wherein triggering the operation in the light source comprises adjusting a pressure of an interior of a vessel in which the optical element is fixed.

11. A method for controlling contamination in a light source, the method comprising: comparing a real-time image of an optical element in the light source with a reference image of the optical element in the light source to determine a contamination metric associated with the optical element; triggering an operation in the light source based on the determined contamination metric; and after triggering the operation in the light source, comparing a next real-time image of the optical element with the reference image to determine an updated contamination metric.

12. The method of claim 11, further comprising, in response to triggering the operation in the light source, increasing a frequency at which a refreshed real-time image of the optical element is obtained.

13. The method of claim 11, further comprising obtaining the real-time image of the optical element and obtaining the reference image of the optical element.

14. The method of claim 13, wherein obtaining the reference image of the optical element in the light source comprises obtaining the reference image of the optical element while the light source is in downtime mode.

15. The method of claim 13, wherein obtaining the real-time image of the optical element comprises constructing of an image from measurements at two or more portions of the optical element, the measurements taken at subsequent, consecutive moments in time.

16. The method of claim 11, further comprising: calculating a reference image metric indicating an intensity of the reference image; calculating a real-time image metric indicating an intensity of the real-time image; andcomparing the real-time image metric with the reference image metric to construct the contamination metric, wherein comparing the real-time image metric with the reference image metric comprises determining a ratio of the real-time image metric to the reference image metric or determining a difference between the real-time image metric and the reference image metric.

17. The method of claim 11, wherein determining the contamination metric comprises isolating a sub-region of the optical element and determining the contamination metric of that subregion.

18. An apparatus for controlling contamination in a light source, the apparatus comprising: an image detector configured to obtain images of an optical element in the light source; an image processing system configured to: receive, from the image detector, a reference image of the optical element and realtime images of the optical element, and compare a current real-time image with the reference image to determine a contamination metric associated with the optical element; and an actuation system configured to: receive the contamination metric from the image processing system, and automatically trigger an operation in the light source to mitigate contamination of the optical element based on the received contamination metric.

19. The apparatus of claim 18, further comprising an image collection apparatus configured to switch between two or more imaging states, each imaging state directing light from a portion of the optical element to the image detector, wherein the image detector is configured to obtain an image of the optical element based on the two or more imaging states.

20. The apparatus of claim 18, wherein, in a first imaging state, light from a central region of the optical element is directed to the image detector and, in a second imaging state, light from an outer region of the optical element is directed to the image detector.

Images