Measurement system for use with an optical amplification cavity

Abstract

A measurement system for use with an optical amplifier cavity includes an input optics, an imaging system, and a detector. The input optics is for receiving an input radiation beam and for directing a first portion of the input radiation beam along a first optical path and a second portion of the input radiation beam along a second optical path (e.g., into the optical amplifier cavity). The imaging system is disposed on the first optical path and configured to form first and second images of the first portion of the input radiation beam in an image plane, the first and second images being at two different planes along the first optical path. The detector is disposed in the image plane and is operable to detect the first and second images.

Description

Technical Field

[0001] Cross - reference to related applications

[0001] This application claims the priority of European Application No. 19185779.6 filed on July 11, 2019, the entire content of which is incorporated herein by reference.

[0002]

[0002] The present invention relates to a measurement system for use with an optical amplification cavity. The optical amplification cavity may form part of a laser system, and the laser system may form part of a laser - produced plasma (LPP) radiation source. The LPP radiation source may generate extreme ultraviolet (EUV) light and may form part of a lithography system.

Background Art

[0003]

[0003] A lithography apparatus is a machine constructed to apply a desired pattern onto a substrate. The lithography apparatus can be used, for example, in the manufacture of integrated circuits (ICs). The lithography apparatus can project a pattern in a patterning device (e.g., a mask) onto a layer of radiation - sensitive material (resist) provided on a substrate.

[0004]

[0004] To project a pattern onto a substrate, a lithography apparatus can use electromagnetic radiation. The wavelength of this radiation determines the minimum size of the features that can be formed on the substrate. A lithography apparatus using extreme ultraviolet (EUV) radiation having a wavelength in the range of 4 - 20 nm, for example 6.7 nm or 13.5 nm, can be used to form smaller features on a substrate than a lithography apparatus using radiation having a wavelength of, for example, 193 nm.

[0005]

[0005] EUV light for a lithography apparatus can be generated by a laser - produced plasma (LPP) radiation source. Inside the LPP radiation source, fuel droplets can be irradiated with a laser beam to generate a plasma that emits EUV light.

Summary of the Invention

[0006]

[0006] According to a first aspect of the present invention, there is provided a measurement system for use with an optical amplification cavity, the measurement system comprising an input optical element for receiving an input radiation beam and for guiding a first portion of the input radiation beam along a first optical path and a second portion of the input radiation beam along a second optical path, and an imaging system disposed on the first optical path and configured to form first and second images of the first portion of the input radiation beam within an image plane, the first and second images being of two different surfaces along the first optical path, the imaging system, and a detector disposed within the image plane and operable to detect the first and second images.

[0007]

[0007] The second optical path leads to (and may form an input optical path to) the optical amplification cavity. Thus, in use, while the input optical element receives the input radiation beam and guides the first portion of the input radiation beam along the first optical path, the second portion of the radiation beam is also guided to such an optical amplification cavity. The measurement system according to the first aspect of the present invention can determine the position and direction of the input radiation beam (to be input into the optical amplification cavity). In particular, as will be further considered below, make such measurements available for use as part of a feedback alignment process for aligning an input radiation beam comprising an optical amplification cavity.

[0008]

[0008] It will be appreciated that the position and / or direction of the first portion of the input radiation beam indicates or is related to the position and / or direction of the second portion of the input radiation beam (which may be an input to the optical amplification cavity). For example, the input optical element may be a beam splitter.

[0009]

[0009] Each of the first and second images can provide information regarding the position of the input radiation beam.

[0010] It will be appreciated that two different surfaces along a first optical path imaged on a detector may be arranged axially spaced along the first optical path. Here, the axial direction is to be understood as the direction along the optical path along which radiation or light propagates. Since the first and second images are of two different surfaces along the first optical path, information regarding the direction of the input radiation beam can be provided by combining the first and second images.

[0011]

[0011] The measurement system may further comprise a steering optical system. The steering optical system may be arranged to receive the input radiation beam and to direct the input radiation beam to the input optical element. The steering optical system may comprise an adjustment mechanism operable to control the direction and / or position of the input radiation beam at the input optical element.

[0012]

[0012] Advantageously, such an arrangement enables the position and / or direction of the radiation beam input into the optical amplification cavity to be optimized. The adjustment mechanism of the steering optical system can be used to control the direction and / or position of the input radiation beam at the input optical element. At the same time, the position and / or direction of the radiation beam can be monitored using the first and second images formed on the detector. Thus, the user can align the input radiation beam with the optical amplification cavity using the adjustment mechanism of the steering optical system. For example, the user can control the position and / or direction of the input radiation beam using the adjustment mechanism of the steering optical system until the first and second images have a desired position and / or shape.

[0013]

[0013] It will be appreciated that the steering optical system may comprise any system of optics capable of controlling the direction and / or position of the input radiation beam. In one embodiment, the steering optical system may comprise two movable mirrors arranged to sequentially receive the input radiation beam. For example, each of the movable mirrors may be rotatable and / or translatable. In one embodiment, each of the movable mirrors may be rotatable about two mutually perpendicular axes.

[0014]

[0014] The measurement system may further comprise a display for displaying an image of the input radiation beam in two different (axially spaced) planes along the first optical path. This can provide a visual guide to the user performing the alignment process.

[0015]

[0015] The measurement system may further comprise a memory operable to store information regarding the nominal direction and / or nominal position of the input radiation beam.

[0016]

[0016] For example, the memory may store information regarding the first and second images that would be formed when the input radiation beam indicates the nominal direction and / or is in the nominal position. The memory may store the first and second images that would be formed when the input radiation beam indicates the nominal direction and / or is in the nominal position. Additionally or alternatively, the memory may store information regarding the first and second images, such as their central positions and the like.

[0017]

[0017] The measurement system may further comprise a feedback loop operable to use an adjustment mechanism of the steering optics to control the position and / or direction of the input radiation beam until the position and / or direction of the input radiation beam substantially coincides with the nominal direction and / or nominal position of the input radiation beam.

[0018]

[0018] Such an arrangement can advantageously substantially automate the alignment process for the optical amplification cavity.

[0019]

[0019] The display is further operable to display at least one visual marker for each of the first and second images, and the at least one visual marker indicates the position and / or shape of one of the first and second images when the position and / or direction of the input radiation beam substantially coincides with the nominal direction and / or nominal position of the input radiation beam.

[0020]

[0020] For example, in order to indicate the desired or nominal position of the center of each of the first and second images, a marker (e.g., a cross) may be displayed on the display. The user can control the position and / or direction of the input radiation beam using the adjustment mechanism of the steering optical system until the center of each of the first and second images coincides with one of the markers.

[0021]

[0021] It will be appreciated that the imaging system may comprise any system of optics operable to split the radiation beam into two parts and to form the first and second images from the two parts.

[0022]

[0022] The imaging system may comprise a lens having two surfaces each provided with a reflective coating, at least one of the two surfaces being curved, the second part of the input radiation beam being incident off-axis on the second lens, and the first and second images being formed from separate parts of the second part of the input radiation beam, the separate parts experiencing different numbers of reflections from the two surfaces.

[0023]

[0023] It will be appreciated that the axis of the lens may be the axis of rotation of the lens. The fact that the radiation beam is incident off-axis on the second lens is intended to mean that the radiation beam is incident at a position at a non-zero distance from the axis of the second lens.

[0024]

[0024] In such an arrangement, the first transmitted portion of the radiation incident on the lens is transmitted through both the first and second surfaces of the lens. If the reflectivity of the coatings on the two surfaces is R, the portion of the incident radiation forming this first transmitted portion is given by (1 - R) 2 and may be called the zero-order beam.

[0025]

[0025] Further, a second transmitted portion of the radiation incident on the lens is transmitted at the first surface, reflected internally at the second surface, reflected internally at the first surface, and then transmitted at the second surface. The portion of the incident radiation that forms this second transmitted portion is R 2 (1-R) 2 as given by. The second transmitted portion may be referred to as the primary beam.

[0026]

[0026] Since a second portion of the input radiation beam is incident on the lens off-axis (i.e., at a position at some non-zero distance from the axis of the lens), the first and second transmitted portions are spatially separated and will thus be incident on different portions of the detector.

[0027]

[0027] Further, the first and second transmitted portions pass through different paths within the lens. In particular, the second transmitted portion experiences two additional reflections from the two surfaces of the lens, at least one of which is curved. Thus, in effect, the first and second transmitted portions experience different amounts of optical power. Equally, the focal lengths of the lens for the first and second transmitted portions are different. As a result, in the plane of the detector, the first and second transmitted portions are images of the first portion of the input radiation beam from two different planes along the first optical path.

[0028]

[0028] The first image can be formed from the first transmitted portion of the radiation incident on the lens that is transmitted at both the first and second surfaces of the lens. The second image can be formed from the second transmitted portion of the radiation incident on the lens that is transmitted at the first surface, reflected internally at the second surface, reflected internally at the first surface, and then transmitted at the second surface.

[0029]

[0029] The radii of curvature of the first and second surfaces may be such that the diameter of the first image in the image plane is larger than the diameter of the second image in the image plane.

[0030]

[0030] Since the first transmissive portion (where the first image is formed) receives less reflection than the second transmissive portion (where the second image is formed), the intensity of the first image is greater than the intensity of the second image. However, if the diameter of the first image in the image plane is larger than the diameter of the second image in the image plane, the intensity densities of the two images can remain the same (and can also match the dynamic range of the detector).

[0031]

[0031] The first and second surfaces can have a reflectance of R, and the radii of curvature of the first and second surfaces can be such that the diameter of the first image in the image plane is R times larger than the diameter of the second image in the image plane.

[0032]

[0032] The reflectivities of the reflective coatings on the first and second surfaces of the lens, and the radii of curvature of the first and second surfaces of the lens, can be such that the intensity densities of the two images are substantially the same.

[0033]

[0033] The measurement system can further include a color optical system arranged on the first optical path and configured to split the input radiation beam into at least two wavelength components and direct at least two components to different parts of the detector.

[0034]

[0034] For example, the input radiation beam can comprise two different wavelengths of radiation. Both of these two different wavelengths of radiation can be directed into the optical amplification cavity.

[0035]

[0035] It will be understood that for each of the at least two wavelength components, a separate steering optical system of the type described above (and comprising a separate adjustment mechanism operable to control the direction and / or position of each of the at least two wavelength components of the input radiation beam) can be provided.

[0036]

[0036] For each of at least two wavelength components, a separate detector and / or display may be provided. Alternatively, each of at least two wavelength components may be detected by different parts of a single detector and / or may be displayed on a single display.

[0037]

[0037] The first and second spatially separated images are formed by an imaging system. Additionally, the color optical system is arranged to further divide each of these into two parts (having different wavelengths). It will be appreciated that the color optical system may divide the radiation in a different (e.g., orthogonal) direction to the division performed by the imaging system. As a result, four images are formed on the detector, namely, the first and second images for the first wavelength and the first and second images for the second wavelength.

[0038]

[0038] The color optical system may comprise an optical element having first and second opposing surfaces, and the first and second opposing surfaces may be arranged at a non-zero angle to each other. A coating may be provided on the first surface that is reflective for the first wavelength of the radiation and transmissive for the second wavelength of the radiation. The second surface may be reflective for the second wavelength of the radiation.

[0039]

[0039] Such an optical element is capable of receiving an input radiation beam comprising a mixture of the first and second wavelengths (e.g., incident on the first surface at a non-zero angle of incidence), and of directing the first and second wavelength components to separate locations.

[0040]

[0040] Such an optical element is particularly advantageous because it performs multiple functions and thus saves space. An alternative color optical system may comprise, for example, two dichroic mirrors, beam splitters, and optical components to ensure that the two wavelength components travel equal optical path lengths.

[0041]

[0041] A coating that is reflective for a second wavelength of radiation may be provided on the second surface of the optical element. The coating on the second surface of the optical element may also be transmissive for a first wavelength of radiation.

[0042]

[0042] According to a second aspect of the invention, a system is provided that comprises an amplification cavity and a measurement system of any of the claims already described, the amplification cavity being arranged along a second optical path.

[0043]

[0043] In such an arrangement, a second portion of the input radiation beam (guided along the second optical path by the input optical element) is received by the amplification cavity. The amplification cavity is arranged to amplify the second portion of the input radiation beam.

[0044]

[0044] In some embodiments, the system may comprise two measurement systems, for example one at each end of the amplification cavity, according to the first aspect of the invention. The amplification cavity may be arranged along the second optical path of each of the two measurement systems according to the first aspect of the invention such that radiation can be received from each of the two input optical elements.

[0045]

[0045] The amplification cavity may comprise two coaxial generally cylindrical electrodes, a gain medium provided in a generally tubular cavity defined between the two coaxial generally cylindrical electrodes, and generally annular mirrors disposed at each end of the cavity.

[0046]

[0046] Each mirror may be provided with an aperture to allow the laser beam to enter and exit the cavity. Generally annular (and thus generally closing a generally tubular cavity defined between two generally coaxial and generally cylindrical electrodes), the shape of the two mirrors (and the initial position and direction of the input laser beam propagating along the second optical path) are arranged such that when the laser beam travels between the two mirrors, its trajectory is also azimuthally processed about the axis of the amplification cavity. For example, one of the mirrors may generally be conical in shape and the other mirror may generally be helical in shape.

[0047]

[0047] The system may further comprise a seed laser operable to output a seed laser beam, and the input optical element is arranged to receive the seed laser beam as the input radiation beam.

[0048]

[0048] In some embodiments, the system may comprise two seed lasers each operable to output a seed laser beam. According to a first aspect of the invention, the input optical element of at least one measurement system may be arranged to receive the seed laser beam of each of the two seed lasers as the input radiation beam.

[0049]

[0049] According to a third aspect of the invention, a laser system is provided comprising a system according to a second aspect of the invention.

[0050]

[0050] The laser system may further comprise an amplification chain. The amplification chain may comprise a chain of resonators. The laser system may form part of a laser-generated plasma radiation source.

[0051]

[0051] According to a fourth aspect of the invention, a fuel injector operable to generate a fuel target in a plasma formation region and a laser system according to a third aspect of the invention arranged to irradiate the fuel target in the plasma formation region to generate a plasma are provided.

[0052]

[0052] According to a fifth aspect of the present invention, there is provided a lithography system comprising a laser-generated plasma radiation source according to a fourth aspect of the present invention and a lithography apparatus.

[0053]

[0053] According to a sixth aspect of the present invention, there is provided a method for aligning an input radiation beam with an amplification cavity, the method comprising receiving the input radiation beam, guiding a first portion of the input radiation beam along a first optical path and guiding a second portion of the input radiation beam along a second optical path for reception by the amplification cavity, forming first and second images of the first portion of the input radiation beam within an image plane, the first and second images being of two different surfaces along the first optical path, and detecting the first and second images within the image plane.

[0054]

[0054] The method according to the sixth aspect of the present invention can be implemented using the system according to the first aspect of the present invention.

[0055]

[0055] The method further includes controlling the direction and / or position of the input radiation beam.

[0056]

[0056] For example, it is possible to monitor the position and / or direction of the radiation beam using the first and second images, while the direction and / or position of the input radiation beam is controllable.

[0057]

[0057] The method may further include comparing at least one feature of each of the first and second images with a nominal value of the at least one feature, the nominal value indicating a nominal direction and / or nominal position of the input radiation beam.

[0058]

[0058] The method may further include controlling the position and / or direction of the input radiation beam until at least one feature of each of the first and second images substantially matches the nominal value of the at least one feature.

[0059]

[0059] Embodiments of the present invention will be described below by way of example only with reference to the accompanying schematic diagrams.

Brief Description of the Drawings

[0060]

Figure 1

Figure 2

Figure 3

Figure 3A

Figure 3B

Figure 4

Figure 5

Figure 6

Figure 7

Embodiments for Carrying Out the Invention

[0061]

[0060] FIG. 1 shows a lithography system comprising a radiation source SO and a lithography apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithography apparatus LA. The lithography apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS, and a substrate table WT configured to support a substrate W.

[0062]

[0061] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident on the patterning device MA. In addition, the illumination system IL can include a facet field mirror device 10 and a facet pupil mirror device 11. The facet field mirror device 10 and the facet pupil mirror device 11 together provide an EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL can include other mirrors or devices in addition to, or instead of, the facet field mirror device 10 and the facet pupil mirror device 11.

[0063]

[0062] After being thus adjusted, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B' is generated. The projection system PS is configured to project the patterned EUV radiation beam B' onto the substrate W. To that end, the projection system PS can comprise a plurality of mirrors 13, 14 configured to project the patterned EUV radiation beam B' onto the substrate W held by the substrate table WT. The projection system PS is capable of applying a reduction factor to the patterned EUV radiation beam B', and thus forms an image with features smaller than the corresponding features on the patterning device MA. For example, reduction factors of 4 or 8 are applicable. The projection system PS is shown in FIG. 1 as having only two mirrors 13, 14, but the projection system PS can include a different number of mirrors (for example, six or eight mirrors).

[0064]

[0063] The substrate W can include a previously formed pattern. In such a case, the lithographic apparatus LA aligns the image formed by the patterned EUV radiation beam B' with the pattern previously formed on the substrate W.

[0065]

[0064] It is possible to provide a relatively low vacuum, i.e., a small amount of gas (e.g., hydrogen) at a pressure far below atmospheric pressure, within the radiation source SO, within the illumination system IL, and / or within the projection system PS.

[0066]

[0065] The radiation source SO shown in FIG. 1 is, for example, of a type that may be referred to as a laser-produced plasma (LPP) source. A laser system 1 capable of including, for example, a CO2 laser is arranged to deposit energy via a laser beam 2 into a fuel such as tin (Sn) provided from, for example, a fuel injector 3. Although tin is referred to in the following description, any suitable fuel can be used. The fuel may be, for example, in liquid form, but can be, for example, a metal or an alloy. The fuel injector 3 can be provided with a nozzle configured to direct tin, for example, in the form of droplets, along a trajectory towards the plasma formation region 4. The laser beam 2 is incident on the tin in the plasma formation region 4. The accumulation of laser energy in the tin creates a tin plasma 7 in the plasma formation region 4. Radiation including EUV radiation is emitted from the plasma 7 during the de-excitation of electrons and recombination with plasma ions.

[0067]

[0066] The laser beam 2 incident on the tin in the plasma formation region 4 can be a pulsed laser beam. The laser beam 2 incident on the tin in the plasma formation region 4 can be referred to as the main laser beam, and the individual pulses of this laser beam 2 can be referred to as main pulses.

[0068]

[0067] Before the main laser beam 2 is incident on the tin in the plasma formation region 4, another prepulse laser beam can be incident on the tin. The prepulse laser beam can act to change the shape of the tin so as to increase the conversion efficiency when the main pulse is (substantially) incident on the tin.

[0069]

[0068] EUV radiation from the plasma is collected and focused by the collector 5. The collector 5 comprises, for example, a near-normal incidence radiation collector 5 (sometimes more generally referred to as a normal incidence radiation collector). The collector 5 can have a multilayer mirror structure arranged to reflect EUV radiation (e.g., EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 can have an elliptical configuration with two foci. As discussed below, the first of the foci can be in the plasma formation region 4 and the second of the foci can be in the intermediate focus 6.

[0070]

[0069] The laser system 1 can be spatially separated from the radiation source SO. In such a case, the laser beam 2 can be passed from the laser system 1 to the radiation source SO with the aid of a beam delivery system (not shown) comprising, for example, suitable steering mirrors and / or beam expanders, and / or other optical systems. The laser system 1, the radiation source SO, and the beam delivery system can all be considered to be a radiation system.

[0071]

[0070] The radiation reflected by the collector 5 forms an EUV radiation beam B. The EUV radiation beam B is focused at the intermediate focus 6 in order to form an image at the intermediate focus 6 of the plasma present in the plasma formation region 4. The image at the intermediate focus 6 acts as a virtual radiation source for the illumination system IL. The radiation source SO is arranged such that the intermediate focus 6 is located at or near the aperture 8 within the enclosure 9 of the radiation source SO.

[0072]

[0071] Figure 1 shows the radiation source SO as a laser-produced plasma (LPP) source, but any suitable radiation source such as a discharge-produced plasma (DPP) source or a free electron laser (FEL) can be used to generate EUV radiation.

[0073]

[0072] The laser system 1 may include a seed module (which may be referred to as a high-power seed module), and an amplification chain. The seed module may be operable to generate a prepulse laser beam and a main pulse laser beam. The amplification chain may be operable to receive the prepulse laser beam and the main pulse laser beam from the seed module, and to increase the power of each of the prepulse laser beam and the main pulse laser beam. The amplification chain may include a chain of resonators.

[0074]

[0073] The seed module may include two seed lasers, a prepulse seed laser and a main pulse seed laser. The seed module may further include an amplification cavity arranged to amplify each of the prepulse laser beam and the main pulse laser beam.

[0075]

[0074] FIG. 2 schematically shows a seed module 100 that may form part of the laser system 1 shown in FIG. 1. The second module includes a main pulse seed laser 102, a prepulse seed laser 104, and an amplification cavity 106.

[0076]

[0075] The amplification cavity 106 is arranged to amplify the output of each of the main pulse seed laser 102 and the prepulse seed laser 104. In the illustrated example, the output of the main pulse seed laser 102 passes through the amplification cavity 106 twice, once in one direction and then, following reflection, in the opposite direction before being output as the main pulse laser beam 110. The output of the prepulse seed laser 104 passes through the amplification cavity 106 once before being output as the prepulse laser beam 112.

[0077]

[0076] The laser beams propagating through the amplification cavity 106 are shown spatially separated, but it should be noted that this is merely for the purpose of clearly distinguishing them. As will be further explained below, each of the laser beams actually follows substantially the same path through the amplification cavity 106.

[0078]

[0077] The seed module 100 further comprises an optical diode system 108 arranged to prevent or at least substantially reduce back-reflection from incident on either the main pulse seed laser 102 or the prepulse seed laser 104. Such back-reflection incident on either the main pulse seed laser 102 or the prepulse seed laser 104 would adversely affect the stability of their outputs.

[0079]

[0078] The amplification cavity 106 is of a type having two coaxial generally cylindrical electrodes. A gain medium (e.g., CO2) is provided generally within a tubular cavity between the two electrodes. At each end of the cavity, two generally annular mirrors are provided. Each of the mirrors is provided with an aperture to allow the laser beam to enter and exit the cavity.

[0080]

[0079] The input laser beam passes through the aperture in the mirror at the first end of the amplification cavity 106. The laser beam then propagates so as to travel to and fro through the generally tubular cavity and is reflected by the two mirrors. The shape of the mirrors and the initial position and direction of the input laser beam are arranged such that when the laser beam travels to and fro between the two mirrors, its path is also azimuthally processed about the axis of the amplification cavity 106. For example, one of the mirrors may generally be conical in shape and the other mirror may generally be helical in shape. Finally, the laser beam is (azimuthally) aligned with the aperture in the mirror at the second end of the amplification cavity 106, and the (amplified) laser beam exits the amplification cavity 106.

[0081]

[0080] Such an amplification cavity 106 is particularly suitable for high-power application examples. However, the quality of the output laser beam from such an amplification cavity 106 strongly depends on the initial input trajectory (both position and direction) of the input laser beam. Therefore, at the time of installation or after maintenance, it is important that the main pulse seed laser 102 and the prepulse seed laser 104 are aligned or realigned with the amplification cavity 106.

[0082]

[0081] Some embodiments of the present invention provide a novel measurement system to achieve accurate alignment between a seed laser (e.g., the main pulse seed laser 102 or the prepulse seed laser 104) and the amplification cavity 106.

[0083]

[0082] Generally, the novel measurement system can be provided at one end of the amplification cavity 106 and can enable accurate alignment between the input seed laser beam and that end of the amplification cavity 106. In the case where the amplification cavity 106 is arranged to receive seed laser beams propagating in opposite directions, two novel measurement systems can be provided, one at each end of the amplification cavity 106.

[0084]

[0083] FIG. 3 shows a system 120 including an amplification cavity 106, a first measurement system 122, and a second measurement system 124.

[0085]

[0084] The first measurement system 122 is provided at one end of the amplification cavity 106 that receives the output of the main pulse seed laser 102 (note that this end is on the right side in FIG. 2 but on the left side in FIG. 3).

[0086]

[0085] The first measurement system 122 includes a beam splitter 126. The beam splitter 126 can be regarded as an input optical element for receiving the input radiation beam 127 and guiding the first part of the input radiation beam along the first optical path and guiding the second part of the input radiation beam along the second optical path. In particular, the beam splitter 126 is arranged to receive the output of the main pulse seed laser 102 and guide the first part 128 of the input radiation beam 127 along the first optical path and guide the second part 130 of the input radiation beam 127 (into the amplification cavity 106) along the second optical path.

[0087]

[0086] The first measurement system 122 further includes a first lens 132, a second lens 134, and a detector 136. The first lens 132 and the second lens 134 form an imaging system. In particular, as will be further described below (with reference to FIG. 4), the first lens 132 and the second lens 134 are configured to form the first and second images of the first part 128 of the input radiation beam 127 in the plane of the detector 136, and the first and second images are from two different planes along the first optical path, forming an imaging system.

[0088]

[0087] Optionally, two mirrors 138, 140 and a transparent window 142 are provided between the second lens 134 and the detector 140.

[0089]

[0088] The detector 136 can be in the form of a camera and can include a two-dimensional array of sensing elements. Each sensing element can be operable to detect different pixels of the image.

[0090] Optionally, the first measurement system 122 further comprises a polarizer 144 (e.g., including one or more thin film polarizers). The polarizer 144 may be arranged to transmit a first portion 128 of the output of the main pulse seed laser 102 and may be arranged to block any retroreflected portions of the main pulse laser beam 110 and the prepulse laser beam 112.

[0091] As shown in FIG. 4, the second lens 134 comprises two curved surfaces, a first concave surface 146 and a second convex surface 148, both of which are provided with a reflective coating. The reflective coating has a reflectivity of R for the emission of the prepulse and main pulse laser beams. Further, a second portion 128 of the input radiation beam 127 is incident on the second lens 134 off-axis (i.e., at a position at some non-zero distance from the axis of the second lens 134).

[0092] A first transmitted portion 150 of the incident radiation 128 is transmitted through both the first concave surface 146 and the second convex surface 148. The portion of the incident radiation 128 that forms this first transmitted portion 150 is given by (1 - R). The first transmitted portion 150 may be referred to as the zero-order beam. 2 and is given by. The first transmitted portion 150 may be referred to as the zero-order beam.

[0093] A second transmitted portion 152 of the incident radiation 128 is transmitted through the first concave surface 146, reflected by the second convex surface 148, reflected by the first concave surface 146, and then transmitted through the second convex surface 148. The portion of the incident radiation 128 that forms this second transmitted portion 152 is given by R 2 (1 - R) 2 and is given by. The second transmitted portion 152 may be referred to as the first-order beam.

[0094]

[0093] Since the second portion 128 of the input radiation beam 127 is incident on the second lens 134 off-axis (i.e., at a position at some non-zero distance from the axis of the second lens 134), the first and second transmitted portions 150, 152 are spatially separated and thus will be incident on different portions of the detector 136. The first and second transmitted portions 150, 152 are spatially separated in a direction shown as the y-direction in FIG. 4 (and which lies in the plane of FIG. 3).

[0095]

[0094] Further, the first and second transmitted portions 150, 152 pass through different paths within the second lens 134. In particular, the second transmitted portion 152 experiences two additional reflections from the two curved surfaces 146, 148 of the second lens 134. In effect, the first and second transmitted portions 150, 152 experience different amounts of optical power. Equally, the focal length of the second lens 134 is different for the first and second transmitted portions 150, 152. As a result, within the plane of the detector 136, the first and second transmitted portions 150, 152 are images of the first portion 128 of the input radiation beam 127 from two different surfaces along the first optical path.

[0096]

[0095] FIG. 3A shows an example of the first and second images 180, 182 formed within the image plane 184 where the detector 136 of the first measurement system 122 is disposed. The first image 180 is formed from the first transmitted portion 150 of the second lens 134 of the first measurement system 122. The second image 182 is formed from the second transmitted portion 152 of the second lens 134 of the first measurement system 122. The first and second images 180, 182 are spatially separated in the y-direction.

[0097] It will be appreciated that two different surfaces along the first optical path imaged on the detector 136 are axially spaced apart on the first optical path. Here, the axial direction is to be understood as the direction along the optical path along which the radiation or light propagates. The first and second images 180, 182 are of two different surfaces along the first optical path, and the combination of the first and second images 180, 182 can provide information regarding the direction of the input radiation beam 127.

[0098]

[0097] The first measurement system 122 is advantageous because it can determine the position and direction of the input radiation beam 127, a portion of which is supposed to be the input to the amplification cavity 106. It will be appreciated that the position and / or direction of the first portion 128 of the input radiation beam 127 indicates or is related to the position and / or direction of the second portion 130 of the input radiation beam 127 (which is the input to the amplification cavity 106).

[0099]

[0098] As will be discussed with reference to FIG. 5 below, the first measurement system 122 allows the measured values of the position and direction of the input radiation beam 127 to be used as part of a feedback alignment process to align the input radiation beam 127 with the amplification cavity 106. FIG. 5 shows the first measurement system 122 and the amplification cavity 106 (some parts of the first measurement system 122 are not shown for clarity).

[0100]

[0099] The first measurement system 122 also includes two movable mirrors 154, 156 arranged to sequentially receive the input radiation beam 127. In the present embodiment, each of the movable mirrors is rotatable about two mutually perpendicular axes. The two movable mirrors 154, 156 are regarded as providing a steering optical system, and the steering optical system is arranged to receive the input radiation beam 127 and to direct the input radiation beam 127 to the beam splitter 126. Since each of the two movable mirrors 154, 156 is rotatable about two mutually perpendicular axes, the steering optical system can be regarded as having an adjustment mechanism operable to control the direction and / or position of the input radiation beam 127 at the beam splitter 126. It will be understood that the steering optical system can comprise any optical system capable of controlling the direction and / or position of the input radiation beam.

[0101] [000100] Advantageously, such an arrangement can optimize the position and / or direction of the input radiation beam 127 input into the optical amplification cavity. Using the adjustment mechanism of the steering optical system (e.g., by rotating one or both of the two movable mirrors 154, 156), the direction and / or position of the input radiation beam 127 at the beam splitter 126 can be controlled. At the same time, the first and second images 180, 182 formed on the detector 136 can be used to monitor the position and / or direction of the radiation beam 127. Thus, the user can use the adjustment mechanism of the steering optical system (i.e., rotate one or both of the two movable mirrors 154, 156) to align the input radiation beam 127 with the amplification cavity 106. For example, the user can rotate one or both of the two movable mirrors 154, 156 to control the position and / or direction of the input radiation beam until the first and second images 180, 182 have the desired position and / or shape.

[0102] [000101] In some embodiments, the first measurement system 122 may further comprise a display (e.g., a screen or monitor) for displaying an image formed on the detector 136 of the input radiation beam 127 in the plane spaced apart in two different axial directions along the first optical path. This can provide a useful visual guide to the user performing the alignment process.

[0103] [000102] In some embodiments, the first measurement system 122 may further comprise a memory operable to store information regarding the nominal direction and / or nominal position of the input radiation beam 127. For example, the memory may store information regarding the first and second images that would be formed when the input radiation beam 127 indicates the nominal direction and / or is in the nominal position. The memory may store the first and second images that would be formed when the input radiation beam 127 indicates the nominal direction and / or is in the nominal position. Additionally or alternatively, the memory may store information regarding these first and second images, such as their central positions.

[0104] [000103] In some embodiments, the first measurement system 122 may further comprise a feedback loop operable to use an adjustment mechanism of the steering optics (i.e., the positions of the two movable mirrors 154, 156) to control the position and / or direction of the input radiation beam 127 until the position and / or direction of the input radiation beam 127 substantially coincides with the nominal direction and / or nominal position of the input radiation beam. Such an arrangement can advantageously substantially automate the alignment process for the optical amplification cavity.

[0105] [000104] The display may be operable to display at least one visual marker for each of the first and second images 180, 182. The at least one visual marker may indicate the position and / or shape of one of the first and second images 180, 182 when the position and / or direction of the input radiation beam 127 substantially coincides with the nominal direction and / or nominal position of the input radiation beam 127. For example, a marker (e.g., a cross) may be displayed on the display to indicate the desired or nominal position of the center of each of the first and second images 180, 182. The user can use the adjustment mechanism of the steering optical system to control the position and / or direction of the input radiation beam 127 until the center of each of the first and second images 180, 182 coincides with one of the markers (i.e., it is possible to control the orientation of the two movable mirrors 154, 156). Alternatively, a marker (e.g., a circle) may be displayed on the display to indicate the desired or nominal position of the edge of each of the first and second images 180, 182. The user can use the adjustment mechanism of the steering optical system to control the position and / or direction of the input radiation beam 127 until the edge of each of the first and second images 180, 182 coincides with one of the markers (i.e., it is possible to control the orientation of the two movable mirrors 154, 156).

[0106] [000105] However, the optical system that forms the two images 180, 182 of the input radiation beam 127 in two different planes includes the aforementioned first and second lenses 132, 134. It will be appreciated that in an alternative embodiment, this imaging system may comprise any optical system operable to split the radiation beam into two parts and form the first and second images 180, 182 from these two parts. However, the use of the second lens 134 in particular provides an advantageous arrangement for several reasons, as will be considered next.

[0107] [000106] First, the second lens 134 is a single component operable to perform both (a) splitting the incident radiation into two portions (first and second transmitted portions 150, 152), and (b) focusing these portions at different focal lengths (so as to form two different images 180, 182).

[0108] [000107] As described above, the reflectivity R of the reflective coatings on the first and second surfaces 146, 148 determines the relative intensities of the first and second transmitted portions 150, 152. Further, the radii of curvature of the first and second surfaces 146, 148, together with the focal length of the first lens 132, determine the focal length of the imaging system experienced by the first and second transmitted portions 150, 152. Equally, the radii of curvature of the first and second surfaces 146, 148, together with the focal length of the first lens 132, determine the sizes of the two images 180, 182 formed on the detector 136. Note that the focal length of the first lens 132 is the same for both the first and second transmitted portions 150, 152. In contrast, the focal length of the second lens 134 is different for the first and second transmitted portions 150, 152.

[0109] [000108] In some embodiments, the reflectivity R of the reflective coatings on the first and second surfaces 146, 148 and the radii of curvature of the first and second surfaces 146, 148 are selected to ensure that the intensity densities of the two images 180, 182 are substantially the same. For example, the intensity of the first transmitted portion 150 is proportional to (1 - R) 2 and the intensity of the second transmitted portion 152 is proportional to R 2 (1 - R) 2 such that the ratio of the intensity of the second transmitted portion 152 to the intensity of the first transmitted portion 150 is R 2It becomes. When R = 1 / 3, the intensity of the second transmission portion 152 is 1 / 9 of the intensity of the first transmission portion 150. Therefore, the radii of curvature of the first and second surfaces 146, 148 can be selected such that the area of the first image 180 is 9 times that of the second image 182. Equally, the radii of curvature of the first and second surfaces 146, 148 can be selected such that the diameter of the first image 180 is 3 times the diameter of the second image 182. Generally, it may be desirable to ensure that the ratio of the diameter of the second image 182 to the diameter of the first image 180 is R. By doing so, it can be ensured that both the first and second images 180, 182 match the dynamic range of the detector 136.

[0110] [000109] Next, the second measurement system 124 of FIG. 3 will be described. The second measurement system 124 shares some features common to the aforementioned first measurement system 122. When the features are generally equivalent, they share common reference numerals. Only the differences between the second measurement system 124 and the first measurement system 122 will be described in detail below.

[0111] [000110] The second measurement system 124 is provided at an end of the amplification cavity 106 that receives a second path of the output of the prepulse seed laser 104 and the output of the main pulse seed laser 102.

[0112] [000111] The second measurement system 124 does not have the optional polarizer 144 of the first measurement system 122. Instead, the second measurement system 124 includes a dichroic mirror 158. The dichroic mirror is arranged to have a lower transmittance for the output of the main pulse seed laser 102 and the output of the prepulse seed laser 104. In particular, the dichroic mirror 158 can be arranged to at least partially attenuate the main pulse beam such that the intensity of the main pulse beam is reduced to a level similar to the intensity of the prepulse beam. It should be noted that while the dichroic mirror 158 can partially attenuate the main pulse beam, there may be no significant attenuation of the prepulse beam by the dichroic mirror 158. In this way, it can be ensured that the images of both the prepulse beam and the main pulse beam can match the dynamic range of the detector 136.

[0113] [000112] In addition, the first mirror 138, which is downstream of the second lens 134 in the first measurement system 122, has been replaced with a novel dichroic wedge 160. Next, the dichroic wedge 160 will be described with reference to FIG. 6.

[0114] [000113] The dichroic wedge 160 is an optical element having first and second opposing surfaces 162, 164. The first and second opposing surfaces 162, 164 are arranged at a non-zero angle to each other. The first surface 162 is provided with a coating that is reflective with respect to a first wavelength of radiation (e.g., the main pulse laser) and transmissive with respect to a second wavelength of radiation (e.g., the prepulse laser). The second surface 164 is reflective with respect to the second wavelength of radiation (e.g., the prepulse laser). For example, this reflection can be the result of reflection at the interface between the second surface 164 of the dichroic wedge 160 and the surrounding medium. Optionally, the second surface 164 can be provided with a coating that is reflective with respect to the second wavelength of radiation (e.g., the prepulse laser). Optionally, the coating provided on the second surface 164 can also be transmissive with respect to the first wavelength of radiation (e.g., the main pulse laser).

[0115] [000114] In use, the dichroic wedge 160 is arranged to receive (from the second lens 134) an input radiation beam that includes a mixture of the first and second wavelengths (e.g., a mixture of prepulse and main pulse radiation). This mixture generally impinges on the first surface 162 at a non-zero angle of incidence. Since the first and second opposing surfaces 162, 164 are arranged at a non-zero angle to each other, and since the first and second opposing surfaces 162, 164 reflect and transmit the two wavelength components differently, the dichroic wedge 160 is operable to direct the first and second wavelength components to separate locations.

[0116] [000115] In particular, upon incidence on the first surface 162, the first wavelength of the radiation (e.g., the main pulse laser) is reflected so as to form the first output beam 166. The second wavelength of the radiation (e.g., the prepulse laser) is transmitted and propagates through the dichroic wedge 160. The second wavelength of the radiation then impinges on the second surface 164 and is reflected so as to form a second output beam 168 that is transmitted by the first surface 162. It should be noted that any portion of the first wavelength of the radiation transmitted by the first surface 162 is transmitted by the second surface 164 and thus tends not to form part of the second output beam 168. Furthermore, any portion of the first wavelength of the radiation transmitted by the first surface 162 and then reflected by the second surface 164 is reflected by the first surface 162 (back into the dichroic wedge 160) and also tends not to form part of the second output beam 168. Since the first and second opposing surfaces 162, 164 are arranged at a non-zero angle θ with respect to each other, the first and second output beams 166, 168 diverge and will be spatially separated from each other in the far field.

[0117] [000116] The dichroic wedge is configured to split an input radiation beam into at least two wavelength components and direct the at least two components to different portions of the detector 136. For example, the input radiation beam may include two different wavelengths of radiation. The two different wavelengths of radiation may both be directed into the amplification cavity 106.

[0118] [000117] It will be appreciated that a separate steering optical system (of the type described above and operable to control the direction and / or position of each of the at least two wavelength components of the input radiation beam) may be provided for each of the at least two wavelength components.

[0119] [000118] FIG. 3B shows an example of four images 186, 188, 190, 192 formed in the image plane 194 in which the detector 136 of the second measurement system 124 is disposed.

[0120] [000119] As described above, the second lens 134 serves to split the incoming radiation beam into two spatially separated portions (the first transmission portion 150 and the second transmission portion 152) of the detector 136. The dichroic wedge 160 further splits each of these into two portions (the prepulse portion and the main pulse portion). It will be appreciated that the dichroic wedge 160 splits the radiation in a direction different (e.g., orthogonal) to the split performed by the second lens 134. As described above, the second lens 134 splits the incoming radiation beam into two portions (the first transmission portion 150 and the second transmission portion 152) spatially separated in the y direction. The dichroic wedge 160 further splits each of these into two portions (the prepulse portion and the main pulse portion) spatially separated in the x direction.

[0121] [000120] As a result, four images 186, 188, 190, 192 of the first and second images 186, 188 for the prepulse radiation and the first and second images 190, 192 for the main pulse radiation are formed within the image plane 194 of the detector 136.

[0122] [000121] The dichroic wedge 160 is particularly advantageous because it can perform multiple functions and thus save space. As shown in FIG. 7, an alternative color optical system 170 may include, for example, two dichroic mirrors 172, 174, a beam splitter 176, and an optical component 178 to ensure that two wavelength components travel equal optical path lengths (such that the same two surfaces are imaged for the main pulse radiation and the prepulse radiation). The dichroic wedge 160 can perform these multiple functions and thus save space.

[0123] [000122] Although this document particularly refers to the use of a lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described in this document has other applications. Other possible applications are in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc.

[0124] [000123] Although specific reference has been made in this document to embodiments of the invention in relation to a lithographic apparatus, embodiments of the invention can also be used in other apparatuses. Embodiments of the invention may form part of any apparatus for measuring or processing an object such as a mask inspection apparatus, a metrology apparatus, or a wafer (or other substrate) or a mask (or other patterning device). These apparatuses may generally be referred to as lithographic tools. Such lithographic tools can use vacuum conditions or ambient (non-vacuum) conditions.

[0125] [000124] Where context permits, embodiments of the present invention can be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the present invention can also be implemented as instructions stored on a machine-readable medium that can be read and executed by one or more processors. A machine-readable medium can include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium can include read-only memory (ROM), random access memory (RAM), magnetic storage media, optical storage media, flash memory devices, electrical, optical, acoustic, or other forms of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, such descriptions are for convenience only, and such actions actually result from computing devices, processors, controllers, or other devices that execute firmware, software, routines, instructions, etc., and it should be understood that when executed, actuators or other devices can interact with the physical world.

[0126] [000125] Having described specific embodiments of the present invention above, it will be understood that the present invention can be practiced in ways different from those described. The above description is illustrative and not restrictive. Therefore, The following clauses it will be apparent to those skilled in the art that the present invention as described can be modified without departing from it. 1. A measurement system for use with an optical amplification cavity, comprising: an input optical element for receiving an input radiation beam and for guiding a first portion of the input radiation beam along a first optical path and a second portion of the input radiation beam along a second optical path; an imaging system disposed on the first optical path and configured to form first and second images of the first portion of the input radiation beam in an image plane, the first and second images being from two different surfaces along the first optical path; a detector disposed in the image plane and operable to detect the first and second images; The measurement system comprising the above elements. 2. Further comprising a steering optical system, The steering optical system is arranged to receive the input radiation beam and to direct the input radiation beam to the input optical element, The measurement system according to clause 1, wherein the steering optical system comprises an adjustment mechanism operable to control the direction and / or position of the input radiation beam at the input optical element. 3. The measurement system according to clause 1 or 2, further comprising a display for displaying an image of the input radiation beam in two different planes along the first optical path. 4. The measurement system according to any one of clauses 1 to 3, further comprising a memory operable to store information regarding the nominal direction and / or nominal position of the input radiation beam. 5. When dependent on clause 2, the measurement system according to clause 4, comprising a feedback loop operable to use the adjustment mechanism of the steering optical system to control the position and / or direction of the input radiation beam until the position and / or direction of the input radiation beam substantially coincides with the nominal direction and / or nominal position of the input radiation beam. 6. The display is further operable to display at least one visual marker for each of the first and second images, When dependent on clause 3, the measurement system according to clause 4 or 5, wherein at least one visual marker indicates the position and / or shape of one of the first and second images when the position and / or direction of the input radiation beam substantially coincides with the nominal direction and / or nominal position of the input radiation beam. 7. The imaging system comprises a lens having two surfaces each provided with a reflective coating, At least one of the two surfaces is curved. The second part of the input radiation beam is incident off-axis on the second lens and the first and second images are formed from separate portions of the second part of the input radiation beam, the separate portions experiencing different numbers of reflections from the two surfaces, a measurement system according to any one of clauses 1 to 6. 8. The first image is formed from a first transmitted portion of the radiation incident on the lens that is transmitted through both the first and second surfaces of the lens. The second image is formed from a second transmitted portion of the radiation incident on the lens that is transmitted through the first surface, reflected internally from the second surface, reflected internally from the first surface, and then transmitted through the second surface, a measurement system according to clause 7. 9. The radii of curvature of the first and second surfaces are such that the diameter of the first image in the image plane is greater than the diameter of the second image in the image plane, a measurement system according to clause 8. 10. The first and second surfaces have a reflectivity of R, the radii of curvature of the first and second surfaces are such that the diameter of the first image in the image plane is R times greater than the diameter of the second image in the image plane, a measurement system according to clause 9. 11. The reflectivity of the reflective coatings on the first and second surfaces of the lens and the radii of curvature of the first and second surfaces of the lens are such that the intensity densities of the two images are substantially the same, a measurement system according to any one of clauses 7 to 9. 12. Further comprising a color optical system arranged on the first optical path and configured to split the input radiation beam into at least two wavelength components and to direct the at least two components to different portions of the detector, a measurement system according to any one of clauses 1 to 11. 13. The color optical system comprises an optical element having first and second opposing surfaces, the first and second opposing surfaces being arranged at a non-zero angle to each other, the first surface being provided with a coating that is reflective for a first wavelength of the radiation and transmissive for a second wavelength of the radiation, the second surface being reflective for the second wavelength of the radiation, a measurement system according to clause 12. 14. The second surface of the optical element is provided with a coating that is reflective for the second wavelength of the radiation, a measurement system according to clause 13. 15. An amplification cavity and a measurement system according to any one of clauses 1 to 14, wherein the amplification cavity is arranged along the second optical path, comprising a system. 16. The amplification cavity is two coaxial generally cylindrical electrodes and A gain medium provided within a generally tubular cavity defined between two generally coaxial and generally cylindrical electrodes, Generally annular mirrors disposed at each end of the cavity, The system according to claim 15, comprising. 17. The system further comprises a seed laser operable to output a seed laser beam, The system according to claim 15 or 16, wherein the input optical element is arranged to receive the seed laser beam as an input radiation beam. 18. A laser system comprising the system according to any one of claims 15 to 17. 19. A fuel ejector operable to generate a fuel target in a plasma formation region, The laser system according to claim 18, arranged to irradiate the fuel target in the plasma formation region so as to generate a plasma, A laser-generated plasma radiation source comprising. 20. The laser-generated plasma radiation source according to claim 19, A lithography apparatus, A lithography system comprising. 21. A method for aligning an input radiation beam with an amplification cavity, comprising: Receiving an input radiation beam, Directing a first portion of the input radiation beam along a first optical path and directing a second portion of the input radiation beam along a second optical path for reception by the amplification cavity, Forming first and second images of the first portion of the input radiation beam in an image plane, the first and second images being from two different planes along the first optical path, Detecting the first and second images in the image plane, A method comprising. 22. The method according to claim 21, further comprising controlling the direction and / or position of the input radiation beam. 23. Further comprising comparing at least one feature of each of the first and second images with a nominal value of the at least one feature, the nominal value indicating a nominal direction and / or nominal position of the input radiation beam, the method according to claim 21 or 22. 24. The method according to claim 23 when dependent on claim 22, further comprising controlling the position and / or direction of the input radiation beam until at least one feature of each of the first and second images substantially coincides with the nominal value of the at least one feature.

Claims

1. A measurement system for use with an optical amplification cavity, comprising: an input optical element for receiving an input radiation beam and guiding a first portion of the input radiation beam along a first optical path and a second portion of the input radiation beam into the optical amplification cavity along a second optical path; an imaging system disposed on the first optical path and configured to form first and second images of the first portion of the input radiation beam in an image plane, wherein the first and second images are of two different surfaces along the first optical path; a detector disposed in the image plane and operable to detect the first and second images; wherein the first optical path is different from the second optical path; and wherein the optical amplification cavity is not disposed on the first optical path.

2. Further comprising a steering optical system, wherein the steering optical system is arranged to receive the input radiation beam and guide the input radiation beam to the input optical element, and wherein the steering optical system comprises an adjustment mechanism operable to control the direction and / or position of the input radiation beam at the input optical element. The measurement system according to claim 1.

3. A feedback loop operable to use the adjustment mechanism of the steering optical system to control the position and / or direction of the input radiation beam until the position and / or direction of the input radiation beam substantially coincides with the nominal direction and / or nominal position of the input radiation beam. The measurement system according to claim 2.

4. The imaging system comprises a lens having two surfaces each provided with a reflective coating, at least one of the two surfaces being curved, wherein the second portion of the input radiation beam is incident off-axis on a second lens, and wherein the first and second images are formed from separate portions of the second portion of the input radiation beam, the separate portions experiencing different numbers of reflections from the two surfaces. The measurement system according to any one of claims 1 to 3.

5. The first image is formed from a first transmitted portion of the radiation incident on the lens that is transmitted through both the first and second surfaces of the lens. The second image is formed from a second transmitted portion of the radiation incident on the lens that is transmitted through the first surface, reflected internally at the second surface, reflected internally at the first surface, and then transmitted through the second surface, as claimed in claim 4.

6. The measurement system according to claim 5, wherein the radii of curvature of the first and second surfaces are such that the diameter of the first image in the image plane is larger than the diameter of the second image in the image plane.

7. The first and second surfaces have a reflectivity of R, The measurement system according to claim 6, wherein the radii of curvature of the first and second surfaces are such that the diameter of the first image in the image plane is R times larger than the diameter of the second image in the image plane.

8. The measurement system according to any one of claims 4 to 6, wherein the reflectivity of the reflective coating on the first and second surfaces of the lens and the radii of curvature of the first and second surfaces of the lens are such that the intensity densities of the two images are substantially identical.

9. The measurement system according to any one of claims 1 to 8, further comprising a color optical system arranged on the first optical path and configured to split the input radiation beam into at least two wavelength components and direct the at least two components to different parts of the detector.

10. The color optical system comprises an optical element having first and second opposing surfaces, The first and second opposing surfaces are arranged at a non-zero angle to each other, The first surface is provided with a coating that is reflective for a first wavelength of the radiation and transmissive for a second wavelength of the radiation, The measurement system according to claim 9, wherein the second surface is reflective for the second wavelength of the radiation.

11. The measurement system according to claim 10, wherein the second surface of the optical element is provided with a coating that is reflective for the second wavelength of the radiation.

12. An amplification cavity, A measurement system according to any one of claims 1 to 11, wherein the amplification cavity is arranged along the second optical path, Comprising a system.

13. The amplification cavity is, Two coaxial generally cylindrical electrodes, A gain medium provided in a generally tubular cavity defined between the two coaxial generally cylindrical electrodes, Generally annular mirrors disposed at each end of the cavity, The system according to claim 12, comprising: **Claim 14** The system further comprises a seed laser operable to output a seed laser beam, The system according to claim 12 or 13, wherein the input optical element is arranged to receive the seed laser beam as the input radiation beam. **Claim 15** A fuel injector operable to generate a fuel target in a plasma formation region, A laser system comprising the system according to any one of claims 12 to 14, arranged to irradiate the fuel target in the plasma formation region so as to generate the plasma, A laser-generated plasma radiation source, comprising:

Images