Measurement system for optical amplification cavity

Abstract

A measurement system for an optical amplification cavity comprises: an input optical element; an imaging system; and a detector. The input optical element is for receiving an input radiation beam and the input optical element: directs a first portion of the input radiation beam along a first optical path; and directs a second portion of the input radiation beam along a second optical path (e.g. into an optical amplification cavity). The imaging system is arranged on the first optical path and is configured to form, in an image plane: a first image and a second image of the first portion of the input radiation beam, the first and second images belonging to 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

[0001] Cross-references to related applications

[0002] This application claims priority to European application 19185779.6, filed on July 11, 2019, the entire contents of which are incorporated herein by reference. Technical Field

[0003] This invention relates to a measurement system for an optical amplification cavity. The optical amplification cavity can be part of a laser system, which in turn can be part of a laser-generated plasma (LPP) radiation source. The LPP radiation source can generate extreme ultraviolet (EUV) radiation and can be part of a photolithography system. Background Technology

[0004] A lithography apparatus is a machine configured to apply a desired pattern onto a substrate. For example, a lithography apparatus can be used in the manufacture of, for example, integrated circuits (ICs). A lithography apparatus can project a pattern at a patterning apparatus (e.g., a mask) onto a radiation-sensitive material (resist) layer disposed on a substrate (e.g., a wafer).

[0005] To project a pattern onto a substrate, a photolithography apparatus can use electromagnetic radiation. The wavelength of this radiation determines the minimum size of the feature that can be formed on the substrate. A photolithography apparatus using extreme ultraviolet (EUV) radiation with wavelengths in the range of 4 to 20 nm (e.g., 6.7 nm or 13.5 nm) can be used to form smaller features on the substrate than a photolithography apparatus using radiation with, for example, a wavelength of about 193 nm.

[0006] EUV radiation used in lithography equipment can be generated by a laser-generated plasma (LPP) radiation source. Within the LPP radiation source, a laser beam can be used to irradiate fuel droplets to generate plasma that will emit EUV radiation. Summary of the Invention

[0007] According to a first aspect of the present invention, a measurement system for an optical amplification cavity is provided, the measurement system comprising: an input optical element for receiving an input radiation beam and the input optical element: 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; an imaging system disposed on the first optical path and configured to form in an image plane: a first image and a second image of the first portion of the input radiation beam, the first image and the second image belonging to two different planes along the first optical path; and a detector disposed in the image plane and operable to detect the first image and the second image.

[0008] The second optical path leads to the optical amplification cavity (and can form the input optical path for the optical amplification cavity). Therefore, in use, the input optics can receive the input radiation beam and guide a second portion of the radiation beam to such an optical amplification cavity, while simultaneously guiding the first portion of the input radiation beam along the first optical path. The measurement system according to the first aspect of the invention is advantageous because it allows the determination of the position and orientation of the input radiation beam (which is to be input to the optical amplification cavity). In particular, as further discussed below, it allows such measurements to be used as part of a feedback alignment process for aligning the input radiation beam with the optical amplification cavity.

[0009] It will be understood that the position and / or orientation of the first portion of the input radiation beam indicates, or relates to, the position and / or orientation of, the second portion of the input radiation beam (which may be the input of the optical amplification cavity). For example, the input optics may be a beam splitter.

[0010] Each of the first and second images can provide information related to the position of the input radiation beam.

[0011] It will be understood that the two different planes imaged onto the detector along the first optical path can be axially spaced along the first optical path. Here, the axial direction should be understood as referring to the direction of the optical path along which radiation or light propagation is followed. Since the first image and the second image belong to two different planes along the first optical path, the combination of the first image and the second image can provide information related to the direction of the input radiation beam.

[0012] The measurement system may further include manipulating optics. The manipulating optics may be arranged to receive the input radiation beam and guide it to the input optical element. The manipulating optics may include an adjustment mechanism operable to control the direction and / or position of the input radiation beam at the input optical element.

[0013] Advantageously, such an arrangement allows for optimization of the position and / or orientation of the radiation beam input into the optical amplification cavity. The adjustment mechanism of the manipulating optics can be used to control the orientation and / or position of the input radiation beam at the input optics. Simultaneously, the position and / or orientation of the radiation beam can be monitored using the first and second images formed on the detector. Therefore, the user can use the adjustment mechanism of the manipulating optics to align the input radiation beam with the optical amplification cavity. For example, the user can use the adjustment mechanism of the manipulating optics to control the position and / or orientation of the input radiation beam until the first and second images have the desired position and / or shape.

[0014] It will be understood that the manipulation optics may include any optical system that allows control of the direction and / or position of the input radiation beam. In one embodiment, the manipulation optics may include two movable mirrors arranged to receive the input radiation beam in sequence. For example, each of the movable mirrors may be rotatable and / or translational. In one embodiment, each of the movable mirrors is capable of rotating about two mutually orthogonal axes.

[0015] The measurement system may also include 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 visual guidance or direction to a user performing the alignment process.

[0016] The measurement system may also include a memory operable to store information relating to the nominal direction and / or nominal position of the input radiation beam.

[0017] For example, the memory may store information relating to the first and second images that will be formed when the input radiation beam is pointed in the nominal direction and / or at the nominal position. The memory may store the first and second images that will be formed when the input radiation beam is pointed in the nominal direction and / or at the nominal position. Additionally or alternatively, the memory may store information relating to these first and second images (such as, for example, their center positions).

[0018] The measurement system may further include a feedback loop operable to control the position and / or orientation of the input radiation beam using the adjustment mechanism of the manipulating optics until the position and / or orientation of the input radiation beam is substantially aligned with the nominal orientation and / or nominal position of the input radiation beam.

[0019] This arrangement allows for a largely automated alignment process for the optical amplification cavity.

[0020] The display may also be operable to display at least one visual identifier for each of the first and second images, the at least one visual identifier indicating the position and / or shape of each of the first and second images when the position and / or direction of the input radiation beam is substantially aligned with the nominal direction and / or nominal position of the input radiation beam.

[0021] For example, markings (e.g., a cross) can be displayed on the display to indicate the desired or nominal position of the center of each of the first and second images. The user can use the adjustment mechanism of the manipulating optics to control the position and / or direction of the input radiation beam until the center of each of the first and second images coincides with one of the markings.

[0022] It will be understood that the imaging system may include any optical device system operable to split the radiation beam into two parts and form a first image and a second image from the two parts.

[0023] The imaging system may include a lens having two surfaces, each having a reflective coating, at least one of the two surfaces being curved, and wherein a second portion of the input radiation beam is incident off-axis onto the second lens, and wherein a first image and a second image are formed by discrete portions of the second portion of the input radiation beam, the discrete portions undergoing different numbers of reflections from the two surfaces.

[0024] It will be understood that the axis of the lens can be the rotation axis of the lens. It will also be understood that off-axis radiation incident on the second lens means that the radiation beam is incident at a position that is a non-zero distance from the axis of the second lens.

[0025] With this arrangement, a first transmissive portion of the radiation incident on the lens is transmitted at both the first and second surfaces of the lens. If the reflectivity of the coating on these two surfaces is R, then the fraction of the incident radiation forming this first transmissive portion, i.e., the fraction, is determined by (1-R). 2 The first transmission portion can be referred to as the zero-order beam.

[0026] Furthermore, the second transmissive portion of the radiation incident on the lens is transmitted at the first surface, internally reflected at the second surface, internally reflected again at the first surface, and then transmitted again at the second surface. The fraction of the incident radiation forming this second transmissive portion, i.e., the proportion, is determined by R. 2 (1-R) 2 The second transmission portion can be referred to as a first-order beam.

[0027] Since the second portion of the input radiation beam is incident off-axis onto the lens (i.e., at a non-zero distance from the axis of the lens), the first and second transmission portions are spatially separated and thus will be incident on different portions of the detector.

[0028] Furthermore, the first and second transmissive portions travel through different paths within the lens. Specifically, the second transmissive portion experiences two additional reflections from the two surfaces of the lens (at least one of which is curved). Therefore, in practice, the first and second transmissive portions experience different amounts of optical power. Equivalently, the focal length of the lens is different for the first and second transmissive portions. Consequently, in the plane of the detector, the first and second transmissive portions are images of the first portion of the input radiation beam from two different planes along the first optical path.

[0029] The first image can be formed by a first transmissive portion of the radiation incident on the lens, the first transmissive portion being transmitted over both the first and second surfaces of the lens. The second image can be formed by a second transmissive portion of the radiation incident on the lens, the second transmissive portion being transmitted over the first surface, internally reflected over the second surface, internally reflected over the first surface, and then transmitted over the second surface.

[0030] The radii of curvature of the first surface and the second surface 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.

[0031] Because the first transmissive portion (from which the first image is formed) undergoes fewer reflections than the second transmissive portion (from which the second image is formed), the intensity of the first image will be 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 still be matched (and both can match the dynamic range of the detector).

[0032] The first surface and the second surface may have a reflectivity of R, and the radii of curvature of the first surface and the second surface may 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.

[0033] The reflectivity of the reflective coatings on the first and second surfaces of the lens, as well as the radii of curvature of the first and second surfaces of the lens, can make the intensity densities of the two images substantially matched.

[0034] The measurement system may further include a dichroic optics arranged on the first optical path, and the dichroic optics are configured to split the input radiation beam into at least two wavelength components and guide the at least two components to different parts of the detector.

[0035] For example, the input radiation beam may include two different wavelengths of radiation. Both different wavelengths of radiation can be guided into the optical amplification cavity.

[0036] It will be understood that each of the at least two wavelength components may be provided with discrete manipulating optics of the type described above (and include discrete adjustment mechanisms operable to control the direction and / or position of each of the at least two wavelength components of the input radiation beam).

[0037] Each of the at least two wavelength components may be provided with a separate detector and / or display. Alternatively, each of the at least two wavelength components may be detected by a single detector (different parts) and / or may be displayed on a single display.

[0038] Spatially separated first and second images are formed by the imaging system. Furthermore, the dichroic optics are arranged to further split each of these images into two parts (with different wavelengths). It should be understood that the dichroic optics can split the radiation in a direction different from (e.g., orthogonal) to the split performed by the imaging system. As a result, four images are formed on the detector: a first and second image for the first wavelength, and a first and second image for the second wavelength.

[0039] The dichroic optics device may include optical elements comprising opposing first and second surfaces, which may be arranged at a non-zero angle relative to each other. The first surface may have a coating that is reflective to radiation of a first wavelength and transmissive to radiation of a second wavelength. The second surface may be reflective to radiation of the second wavelength.

[0040] Such an optical element can receive an input radiation beam comprising a mixture of a first wavelength and a second wavelength (e.g., incident on the first surface at a non-zero angle of incidence) and direct the first wavelength component and the second wavelength component to discrete locations.

[0041] This type of optical element is particularly advantageous because it can perform multiple functions and thus saves space. For example, an alternative dichroic optics device may include two dichroic mirrors, a beam splitter, and optical components to ensure that the two wavelength components travel equal optical path lengths or optical distances.

[0042] The second surface of the optical element may have a coating that is reflective to radiation of the second wavelength. The coating on the second surface of the optical element may also be transmissive to radiation of the first wavelength.

[0043] According to a second aspect of the present invention, a system is provided, comprising: an amplifying cavity; and a measurement system according to any of the preceding claims, wherein the amplifying cavity is arranged along the second optical path.

[0044] With this arrangement, the second portion of the input radiation beam (guided by the input optical element along the second optical path) is received by the amplifying cavity. The amplifying cavity is arranged to amplify the second portion of the input radiation beam.

[0045] In some embodiments, the system may include two measurement systems according to the first aspect of the invention, for example, one at each end of the amplifying cavity. The amplifying 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 the radiation can be received from each of the two input optical elements.

[0046] The amplification cavity may include: two coaxial generally cylindrical electrodes; a gain medium disposed in a generally tubular cavity defined between the two coaxial generally cylindrical electrodes; and a generally annular mirror disposed at each end of the cavity.

[0047] Each mirror may be provided with an aperture to allow the laser beam to pass into and out of the cavity. Although generally annular (and therefore generally closed within the generally tubular cavity defined between the two coaxial generally cylindrical electrodes), the shapes of the two mirrors (and the initial position and orientation of the input laser beam propagating along the second optical path) are arranged such that as the laser beam reciprocates between the two mirrors, its trajectory is also oriented about the axis of the amplifying cavity. For example, one of the mirrors may be generally conical in shape, while the other may be generally helical.

[0048] The system may further include: a seed laser operable to output a seed laser beam, and the input optics arranged to receive the seed laser beam as the input radiation beam.

[0049] In some embodiments, the system may include two seed lasers, each operable to output a seed laser beam. The input optics of at least one measurement system according to a first aspect of the invention may be arranged to receive the seed laser beam from each of the two seed lasers as an input radiation beam.

[0050] According to a third aspect of the present invention, a laser system is provided, the laser system comprising the system according to a second aspect of the present invention.

[0051] The laser system may also include an amplification chain. The amplification chain may include a resonator chain. The laser system may form part of a laser-generated plasma radiation source.

[0052] According to a fourth aspect of the invention, a laser-generated plasma radiation source is provided, comprising: a fuel emitter for generating a fuel target at a plasma formation region; and a laser system according to a third aspect of the invention, the laser system being arranged to irradiate the fuel target at the plasma formation region to generate plasma.

[0053] According to a fifth aspect of the present invention, a photolithography system is provided, comprising: a laser-generated plasma radiation source according to a fourth aspect of the present invention; and a photolithography apparatus.

[0054] According to a sixth aspect of the present invention, a method for aligning an input radiation beam with an amplifying cavity is provided, the method comprising: receiving an 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 amplifying cavity; forming in an image plane a first image and a second image of the first portion of the input radiation beam, the first image and the second image belonging to two different planes along the first optical path; and detecting the first image and the second image in the image plane.

[0055] The method according to the sixth aspect of the invention can be implemented using the system described in the first aspect of the invention.

[0056] The method may also include controlling the direction and / or position of the input radiation beam.

[0057] For example, the direction and / or position of the input radiation beam can be controlled, while the position and / or direction of the radiation beam can be monitored using the first image and the second image.

[0058] The method may further include: comparing at least one characteristic of each of the first image and the second image with a nominal value of the at least one characteristic, the nominal value indicating the nominal direction and / or nominal position of the input radiation beam.

[0059] The method may further include controlling the position and / or orientation of the input radiation beam until at least one characteristic of each of the first and second images substantially matches the nominal value of the at least one characteristic. Attached Figure Description

[0060] Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings, in which:

[0061] - Figure 1 A lithography system, including lithography equipment and a radiation source, is described;

[0062] - Figure 2 A seed module is schematically shown, which can form Figure 1 Part of the laser system shown;

[0063] - Figure 3 A system including an amplifying cavity is shown, the amplifying cavity being able to form Figure 2 A portion of the seed module shown; a first measurement system; and a second measurement system;

[0064] - Figure 3A Showing the setting Figure 3 Examples of the first and second images formed in the detector image plane of the first measurement system are shown below.

[0065] - Figure 3B Showing the setting Figure 3 Examples of four images formed in the image plane of the detector of the second measurement system are shown below;

[0066] - Figure 4 Showing the formation Figure 3 The second lens and detector are shown as a part of the first measurement system and the second measurement system;

[0067] - Figure 5 Show Figure 3 The system shown includes an amplification cavity and the first measurement system (some parts of the first measurement system are not shown for ease of understanding);

[0068] - Figure 6 Showing the formation Figure 3 The system shown includes a dichroic wedge or dichroic wedge mirror as part of the second measurement system; and

[0069] - Figure 7 Show Figure 6 The dichroic wedge described above can replace an alternative dichroic optical device, which includes two dichroic mirrors, a beam splitter, and optical components arranged to ensure that the two wavelength elements travel equal optical path lengths. Detailed Implementation

[0070] Figure 1A lithography system including a radiation source SO and a lithography apparatus LA is shown. The radiation source SO is configured to generate an EUV radiation beam B and provide the EUV radiation beam B to the lithography apparatus LA. The lithography apparatus LA includes an irradiation system IL, a support structure MT configured to support a patterning apparatus MA (e.g., a mask), a projection system PS, and a substrate stage WT configured to support a substrate W.

[0071] The irradiation system IL is configured to adjust the EUV radiation beam B before it is incident on the patterning device MA. Additionally, the irradiation system IL may include a faceted field mirror assembly 10 and a faceted pupil mirror assembly 11. The faceted field mirror assembly 10 and the faceted pupil mirror assembly 11 together provide an EUV radiation beam B with the desired cross-sectional shape and intensity distribution. The irradiation system IL may include other mirrors or devices besides, or in place of, the faceted field mirror assembly 10 and the faceted pupil mirror assembly 111.

[0072] After such adjustment, the EUV radiation beam B interacts with the patterning apparatus 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. For this purpose, the projection system PS may include a plurality of mirrors 13, 14, configured to project the patterned EUV radiation beam B' onto the substrate W held by the substrate stage WT. The projection system PS can apply a reduction factor to the patterned EUV radiation beam B', thus forming an image with features smaller than the corresponding features on the patterning apparatus MA. For example, a reduction factor of 4 or 8 can be applied. Although the projection system PS... Figure 1 The image is shown as having only two reflectors 13, 14, but the projection system PS may include a different number of reflectors (e.g., six or eight reflectors).

[0073] The substrate W may include a previously formed pattern. In this case, the photolithography apparatus LA aligns the image formed by the patterned EUV radiation beam B' with the pattern previously formed on the substrate W.

[0074] A relative vacuum, i.e., a small amount of gas (e.g., hydrogen) at a pressure far below atmospheric pressure, can be provided in the radiation source SO, the irradiation system IL, and / or the projection system PS.

[0075] For example, Figure 1The radiation source shown is, for example, of the type that can be referred to as a laser-generated plasma (LPP) source. For example, a laser system 1, which may include a CO2 laser, is arranged to deposit energy into a fuel, such as tin (Sn) supplied, for example, from a fuel emitter 3, via a laser beam 2. Although tin is mentioned in the following description, any suitable fuel can be used. For example, the fuel can be liquid and can be, for example, a metal or alloy. The fuel emitter 3 may include a nozzle configured to guide tin (e.g., in droplet form) along a trajectory toward the plasma forming region 4. The laser beam 2 is incident on the tin at the plasma forming region 4. The laser energy deposited into the tin generates tin plasma 7 at the plasma forming region 4. During the deexcitation and recombination of electrons with ions in the plasma, radiation, including EUV radiation, is emitted from the plasma 7.

[0076] 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 a single pulse of such laser beam 2 can be referred to as the main pulse.

[0077] Before the main laser beam 2 is incident on the tin at the plasma formation region 4, another pre-pulse laser beam can be incident on the tin. The pre-pulse laser beam can be used to change the shape of the tin when the main pulse (subsequently) is incident on the tin in order to improve conversion efficiency.

[0078] EUV radiation from the plasma is collected and focused by collector 5. Collector 5 includes, for example, a near-normal incident radiation collector 5 (sometimes more generally referred to as a normal incident radiation collector). Collector 5 may have a multi-layered mirror structure arranged to reflect EUV radiation (e.g., EUV radiation with a desired wavelength such as 13.5 nm). Collector 5 may have an elliptical configuration with two foci. The first of these foci may be located at the plasma formation region 4, and the second of these foci may be located at the intermediate focal point 6, as discussed below.

[0079] The laser system 1 can be spatially separated from the radiation source SO. In this case, the laser beam 2 can be transmitted from the laser system 1 to the radiation source SO via a beam transmission system (not shown), which includes, for example, suitable directional mirrors and / or beam expanders, and / or other optical devices. The laser system 1, the radiation source SO, and the beam transmission system can be considered together as a radiation system.

[0080] The radiation reflected by the collector 5 forms the EUV radiation beam B. The EUV radiation beam B is focused at an intermediate focal point 6 to form an image of the plasma present in the plasma formation region 4 at the intermediate focal point 6. The image at the intermediate focal point 6 serves as a virtual radiation source for the irradiation system IL. The radiation source SO is arranged such that the intermediate focal point 6 is located at or near the opening 8 in the enclosed structure 9 of the radiation source SO.

[0081] although Figure 1 The radiation source is described as a laser-generated plasma (LPP) source, but any suitable source, such as a discharge-generated plasma (DPP) source or a free-electron laser (FEL), can be used to generate EUV radiation.

[0082] 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 the pre-pulse laser beam and the main pulse laser beam. The amplification chain may be operable to receive the pre-pulse laser beam and the main pulse laser beam from the seed module and amplify the power of each of the pre-pulse laser beam and the main pulse laser beam. The amplification chain may include a resonator chain.

[0083] The seed module may include two seed lasers: a pre-pulse seed laser and a main-pulse seed laser. The seed module may also include an amplification cavity arranged to amplify each of the pre-pulse laser beam and the main-pulse laser beam.

[0084] Figure 2 A seed module 100 is schematically shown, which may form Figure 1 This is a portion of the laser system 1 shown. The seed module includes a main pulse seed laser 102, a pre pulse seed laser 104, and an amplification cavity 106.

[0085] The amplification cavity 106 is arranged to amplify the output of each of the main pulse seed laser 102 and the pre-pulse seed laser 104. In the example shown, the output of the main pulse seed laser 102 is passed through the amplification cavity 106 twice: first in one direction, and then in the opposite direction after reflection, before being output as the main pulse laser beam 110. The output of the pre-pulse seed laser 104 is passed through the amplification cavity 106 once before being output as the pre-pulse laser beam 112.

[0086] Note that although the laser beams propagating through the amplification cavity 106 are shown to be spatially separated, this is merely for the purpose of distinguishing them more clearly. As explained further below, in practice, each laser beam travels through the amplification cavity 106 along substantially the same trajectory.

[0087] The seed module 100 also includes an optical diode system 108 arranged to prevent or at least substantially reduce back reflections incident on the main pulse seed laser 102 or the pre-pulse seed laser 104. Such back reflections incident on either the main pulse seed laser 102 or the pre-pulse seed laser 104 would adversely affect the stability of their output.

[0088] The amplification cavity 106 is of the type comprising two coaxial, generally cylindrical electrodes. Between the two electrodes, a gain medium (e.g., CO2) is disposed within the generally tubular cavity. Two generally annular mirrors are disposed at each end of the cavity. Each mirror is provided with an aperture to allow the laser beam to pass into and out of the cavity.

[0089] An input laser beam is transmitted through the aperture in the mirror at the first end of the amplifying cavity 106. The laser beam then propagates back and forth through the generally tubular cavity, reflected by the two mirrors. The shapes of the mirrors, as well as the initial position and orientation of the input laser beam, are arranged such that as the laser beam propagates back and forth between the two mirrors, its trajectory is also azimuthally processed around the axis of the amplifying cavity 106. For example, one of the mirrors may be generally conical in shape, and the other may be generally helical. Finally, the laser beam aligns (in azimuth) with the aperture in the mirror at the second end of the amplifying cavity 106, and the laser beam (now amplified) exits the amplifying cavity 106.

[0090] This type of amplifying cavity 106 is particularly suitable for high-power applications. However, the quality or performance of the output laser beam from such an amplifying cavity 106 is highly dependent on the initial input trajectory (both position and orientation) of the input laser beam. Therefore, it is important to align or realign the master pulse seed laser 102 and the pre-pulse seed laser 104 with the amplifying cavity 106 during installation or after maintenance.

[0091] Some embodiments of the present invention provide a novel measurement system for achieving accurate alignment of a seed laser (e.g., the main pulse seed laser 102 or the pre pulse seed laser 104) with an amplification cavity 106.

[0092] Generally, the novel measurement system can be positioned at one end of the amplification cavity 106, allowing for accurate alignment of the input seed laser beam with that end of the amplification cavity 106. For an arrangement where the amplification cavity 106 receives seed laser beams propagating in opposite directions, two novel measurement systems can be provided: one at each end of the amplification cavity 106.

[0093] Figure 3 The diagram shows a system 120 including the amplification cavity 106; a first measurement system 122; and a second measurement system 124.

[0094] The first measurement system 122 is located at one end of the amplification cavity 106 that receives the output of the main pulse seed laser 102 (note that this end is at...). Figure 3 The middle is located on the left, although in Figure 2 (The middle is located on the right).

[0095] The first measurement system 122 includes a beam splitter 126. The beam splitter 126 can be considered an input optical element for receiving an input radiation beam 127 and 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. Specifically, the beam splitter 126 is arranged to receive the output of the master pulse seed laser 102 and guide a first portion 128 of the input radiation beam 127 along the first optical path; and guide a second portion 130 of the input radiation beam 127 (into the amplification cavity 106) along the second optical path.

[0096] 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. Specifically, as will be further described below (see reference). Figure 4 The first lens 132 and the second lens 134 form an imaging system configured to form, within the plane of the detector 136, a first image and a second image of the first portion 128 of the input radiation beam 127, the first and second images belonging to two different planes along the first optical path.

[0097] Optionally, two reflectors 138 and 140 and a transparent window 142 are disposed between the second lens 134 and the detector 140.

[0098] The detector 136 may be in the form of a camera and may include a two-dimensional array of sensing elements. Each sensing element may be operable to detect different pixels in an image.

[0099] Optionally, the first measurement system 122 further includes a polarizer 144 (e.g., including one or more thin-film polarizers). The polarizer 144 may be arranged to transmit the first portion 128 of the output of the main pulse seed laser 102, and may be arranged to block any back reflection portions of the main pulse laser beam 110 and the pre-pulse laser beam 112.

[0100] like Figure 4 As shown, the second lens 134 includes 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 radiation from the pre-pulse and main pulse laser beams. Furthermore, the second portion 128 of the input radiation beam 127 is incident off-axis onto the second lens 134 (i.e., at a position at a non-zero distance from the axis of the second lens 134).

[0101] The first transmission portion 150 of the incident radiation 128 is transmitted at the first concave surface 146 and the second convex surface 148. The fraction of the incident radiation 128 forming this first transmission portion 150 is determined by (1-R). 2 The first transmission portion 150 can be referred to as the zero-order beam.

[0102] The second transmission portion 152 of the incident radiation 128 is transmitted at the first concave surface 146, transmitted at the second convex surface 148, reflected at the first concave surface 146, and then transmitted again at the second convex surface 148. The fraction of the incident radiation 128 forming this second transmission portion 152 is determined by R. 2 (1-R) 2 The second transmission portion 152 can be referred to as a first-order beam.

[0103] Because the second portion 128 of the input radiation beam 127 is incident off-axis onto the second lens 134 (i.e., at a non-zero distance from the axis of the second lens 134), the first transmission portion 150 and the second transmission portion 152 are spatially separated and therefore incident on different portions of the detector 136. The first transmission portion 150 and the second transmission portion 152 are, as... Figure 4 The middle is indicated as the y-direction (and enters) Figure 3 Separated in space in the direction of the plane.

[0104] Furthermore, the first transmission portion 150 and the second transmission portion 152 are transmitted through different paths in the second lens 134. Specifically, the second transmission portion 152 experiences two additional reflections from the two curved surfaces 146 and 148 of the second lens 134. In effect, the first transmission portion 150 and the second transmission portion 152 experience different amounts of optical power. Equivalently, the focal length of the second lens 134 is different for the first transmission portion 150 and the second transmission portion 152. As a result, in the plane of the detector 136, the first transmission portion 150 and the second transmission portion 152 are images of the first portion 128 of the input radiation beam 127 from two different planes along the first optical path.

[0105] Figure 3A An example is shown of a first image 180 and a second image 182 formed in an image plane 184 in which the detector 136 of the first measurement system 122 is disposed. The first image 180 is formed by the first transmission portion 150 of the second lens 134 of the first measurement system 122. The second image 182 is formed by the second transmission portion 152 of the second lens 134 of the first measurement system 122. The first image 180 and the second image 182 are spatially separated in the y-direction.

[0106] It will be understood that the two different planes imaged onto the detector 136 along the first optical path are axially spaced apart along the first optical path. Here, the axial direction should be understood as the direction along the optical path followed by radiation or light propagation. Since the first image 180 and the second image 182 belong to two different planes along the first optical path, combining the first image 180 and the second image 182 can provide information related to the direction of the input radiation beam 127.

[0107] The first measurement system 122 is advantageous because it allows the determination of the position and orientation of the input radiation beam 127 (a portion of which is to be input into the amplification cavity 106). It should be understood that the position and / or orientation of the first portion 128 of the input radiation beam 127 indicates, or is related to, the position and / or orientation of, the second portion 130 of the input radiation beam 127 (into the amplification cavity 106).

[0108] The first measurement system 122 allows for the measurement of the position and orientation of the input radiation beam 127, which is to be used as part of the feedback alignment process for aligning the input radiation beam 127 with the amplification cavity 106 (as now referred to). Figure 5 (As discussed). Figure 5 The first measurement system 122 and the amplification cavity 106 are shown (some parts of the first measurement system 122 are not shown for ease of understanding).

[0109] The first measurement system 122 also includes two movable mirrors 154, 156 arranged to receive the input radiation beam 127 sequentially. In this embodiment, each movable mirror is rotatable about two mutually orthogonal axes. The two movable mirrors 154, 156 can be considered as providing manipulation optics arranged to receive the input radiation beam 127 and guide it to the beam splitter 126. Since each of the two movable mirrors 154, 156 is rotatable about two mutually orthogonal axes, the manipulation optics can be considered as including an adjustment mechanism operable to control the direction and / or position of the input radiation beam 127 at the beam splitter 126. It should be understood that the manipulation optics can include any optical system that allows control of the direction and / or position of the input radiation beam.

[0110] Advantageously, this arrangement allows for optimization of the position and / or orientation of the input radiation beam 127 fed into the optical amplification cavity. The adjustment mechanism of the manipulating optics can be used (e.g., by rotating one or both of the two rotatable mirrors 154, 156) to control the orientation and / or position of the input radiation beam 127 at the beam splitter 126. Simultaneously, the position and / or orientation of the radiation beam 127 can be monitored using the first image 180 and the second image 182 formed on the detector 136. Therefore, a user can use the adjustment mechanism of the manipulating optics (i.e., by rotating one or both of the two rotatable 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 rotatable mirrors 154, 156 to control the position and / or orientation of the input radiation beam until the first image 180 and the second image 182 have the desired position and / or shape.

[0111] In some embodiments, the first measurement system 122 may further include a display (e.g., a screen or monitor) for displaying an image formed on the detector 136 (of the input radiation beam 127 in planes spaced apart along two different axes of the first optical path). This can provide useful visual guidance to a user performing the calibration process.

[0112] In some embodiments, the first measurement system 122 may further include a memory operable to store information about the nominal direction and / or nominal position of the input radiation beam 127. For example, the memory may store information about the first and second images that will be formed when the input radiation beam 127 is pointed to the nominal direction and / or at the nominal position. The memory may store the first and second images that will be formed when the input radiation beam 127 is pointed to the nominal direction and / or at the nominal position. Additionally or alternatively, the memory may store information related to these first and second images, such as, for example, their center position.

[0113] In some embodiments, the first measurement system 122 may further include a feedback loop operable to control the position and / or orientation of the input radiation beam 127 using the adjustment mechanism of the manipulating optics (i.e., the positions of the two rotatable mirrors 154, 156) until the position and / or orientation of the input radiation beam 127 substantially coincides with or corresponds to the nominal orientation and / or nominal position of the input radiation beam. Such an arrangement advantageously allows the alignment process of the optical amplification cavity to be substantially automated.

[0114] The display is operable to display at least one visual identifier for each of the first image 180 and the second image 182. The at least one visual identifier can indicate the position and / or shape of one of the first image 180 and the second image 182 when the position and / or orientation of the input radiation beam 127 is substantially aligned with or corresponds to the nominal orientation and / or nominal position of the input radiation beam 127. For example, an identifier (e.g., a cross) can be displayed on the display to indicate the desired or nominal position of the center of each of the first image 180 and the second image 182. A user can use the adjustment mechanism of the manipulating optics (i.e., the orientation of the two rotatable mirrors 154, 156 can be controlled) to control the position and / or orientation of the input radiation beam 127 until the center of each of the first image 180 and the second image 182 coincides with one of the identifiers. Alternatively, an identifier (e.g., a circle) can be displayed on the display to indicate the desired or nominal position of the edge portion of each of the first image 180 and the second image 182. The user can use the adjustment mechanism of the manipulating optics (i.e., the orientation of the two rotatable mirrors 154, 156) to control the position and / or orientation of the input radiation beam 127 until the edge portion of each of the first image 180 and the second image 182 coincides with one of the identifiers.

[0115] Although the optical system forming the two images 180, 182 of the input radiation beam 127 in two different planes includes the first lens 132 and the second lens 134 as described above, it should be understood that, in alternative embodiments, this imaging system may include any optical system operable to split the radiation beam into two parts and form the first image 180 and the second image 182 from said two parts. However, as now discussed, the use of the second lens 134 provides a particularly advantageous arrangement for many reasons.

[0116] First, the second lens 134 is a single component operable to perform the following two operations: (a) splitting the incident radiation into two parts (a first transmission portion 150 and a second transmission portion 152); and (b) focusing the portions at different focal lengths (in order to image the two different images 180, 182).

[0117] As described above, the reflectivity R of the reflective coatings on the first surface 146 and the second surface 148 determines the relative intensity of the first transmissive portion 150 and the second transmissive portion 152. Furthermore, the radii of curvature of the first surface 146 and the second surface 148 determine the focal length of the imaging system traversed by the first transmissive portion 150 and the second transmissive portion 152 (together with the focal length of the first lens 132). Equivalently, the radii of curvature of the first surface 146 and the second surface 148 (together with the focal length of the first lens 132) determine the size 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 transmissive portion 150 and the second transmissive portion 152. Conversely, the focal length of the second lens 134 is different for both the first transmissive portion 150 and the second transmissive portion 152.

[0118] In some embodiments, the reflectivity R of the reflective coatings on the first surface 146 and the second surface 148, as well as the radii of curvature of the first surface 146 and the second surface 148, are selected to ensure that the intensity densities of the two images 180 and 182 are substantially matched. For example, the intensity of the first transmissive portion 150 is related to (1-R). 2 Proportional, and the intensity of the second transmission portion 152 is proportional to R. 2 (1-R) 2 Proportional, such that the ratio of the intensity of the second transmission portion 152 to the intensity of the first transmission portion 150 is R. 2For R = 1 / 3, the intensity of the second transmission portion 152 will be less than the intensity of the first transmission portion 150, i.e., 1 / 9. Therefore, the radii of curvature of the first surface 146 and the second surface 148 can be selected such that the area of ​​the first image 180 is nine times larger than the area of ​​the second image 182. Equivalently, the radii of curvature of the first surface 146 and the second surface 148 can be selected such that the diameter of the first image 180 is three times larger than the diameter of the second image 182. Typically, 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 image 180 and the second image 182 are matched to the dynamic range of the detector 136.

[0119] Now will describe Figure 3 The second measurement system 124 is described above. The second measurement system 124 shares several common features with the first measurement system 122 described above. Where the features are substantially equivalent, they share common reference numerals. The differences between the second measurement system 124 and the first measurement system 122 will be described in detail below.

[0120] The second measurement system 124 is located at one end of the amplification cavity 106, and receives the output of the pre-pulse seed laser 104 and the second pass of the output of the main pulse seed laser 102.

[0121] 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 low transmittance for both the output of the main pulse seed laser 102 and the output of the pre-pulse seed laser 104. Specifically, the dichroic mirror 158 can be arranged to at least partially attenuate the main pulse beam, reducing its intensity to a level similar to that of the pre-pulse beam. Note that although the dichroic mirror 158 can partially attenuate the main pulse beam, it does not significantly attenuate the pre-pulse beam. This ensures that the images of both the pre-pulse beam and the main pulse beam can be matched with the dynamic range of the detector 136.

[0122] Furthermore, the first reflecting mirror 138 downstream of the second lens 134 in the first measurement system 122 has been replaced by a novel dihedral wedge 160. Now refer to... Figure 6 Describe the bidirectional color wedge 160.

[0123] The dichroic wedge 160 is an optical element comprising a first surface 162 and a second surface 164, which are opposite to each other. The first surface 162 and the second surface 164 are arranged at a non-zero angle relative to each other. The first surface 162 is provided with a coating that is reflective to radiation of a first wavelength (e.g., the main pulse laser) and transmissive to radiation of a second wavelength (e.g., the pre-pulse laser). The second surface 164 is reflective to radiation of the second wavelength (e.g., the pre-pulse laser). For example, this reflection may 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 may be provided with a coating that is reflective to radiation of the second wavelength (e.g., the pre-pulse laser). Optionally, the coating provided on the second surface 164 may also be transmissive to radiation of the first wavelength (e.g., the main pulse laser).

[0124] In use, the dichroic wedge 160 is configured to receive (from the second lens 134) an input radiation beam comprising a mixture of a first wavelength and a second wavelength (e.g., a mixture of the prepulse and the main pulse radiation). Typically, this mixture is incident on the first surface 162 at a non-zero angle of incidence. Because the opposite first surface 162 and the second surface 164 are arranged at a non-zero angle relative to each other, and because the opposite first surface 162 and the second surface 164 reflect and transmit the two wavelength components in different ways, the dichroic wedge 160 is operable to guide the first wavelength component and the second wavelength component to separate locations.

[0125] Specifically, when incident on the first surface 162, radiation of the first wavelength (e.g., the main pulse laser) is reflected to form a first output beam 166. Radiation of the second wavelength (e.g., the pre-pulse laser) is transmitted and propagates through the dichroic wedge 160. The second wavelength radiation is then incident on the second surface 164 and reflected to form a second output beam 168 transmitted from the first surface 162. Note that any portion of the first wavelength radiation transmitted from the first surface 162 will tend to be transmitted through the second surface 164, thus preventing the formation of a portion of the second output beam 168. Furthermore, any portion of the first wavelength radiation transmitted from the first surface 162 and then reflected by the second surface 164 will tend to be subsequently reflected by the first surface 162 (back into the dichroic wedge 160), also preventing the formation of a portion of the second output beam 168. Since the first surface 162 and the second surface 164 are arranged at a non-zero angle θ relative to each other, the first output beam 166 and the second output beam 168 diverge and will be spatially separated from each other in the far field.

[0126] The dichroic wedge is configured to split the 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 radiation of two different wavelengths. Both different wavelengths of radiation can be directed into the amplification cavity 106.

[0127] It should be understood that each of the at least two wavelength components may be provided with discrete manipulating optics of the type described above (and include discrete adjustment mechanisms operable to control the direction and / or position of each of the at least two wavelength components of the input radiation beam).

[0128] Figure 3B An example of four images 186, 188, 190, and 192 formed in the image plane 194, in which the detector 136 of the second measurement system 124 is arranged.

[0129] As explained above, the second lens 134 is used to split the incident 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 (a pre-pulse portion and a main pulse portion). It should be understood that the dichroic wedge 160 splits the radiation in a direction different from (e.g., orthogonal) to the splitting performed by the second lens 134. As explained above, the second lens 134 splits the incident radiation beam into two spatially separated portions (the first transmission portion 150 and the second transmission portion 152) in the y-direction. The dichroic wedge 160 further splits each of these into two portions (a pre-pulse portion and a main pulse portion), which are spatially separated in the x-direction.

[0130] As a result, four images 186, 188, 190, and 192 are formed in the image plane 194 of the detector 136: a first image 186 and a second image 188 for the prepulse radiation, and a first image 190 and a second image 192 for the main pulse radiation.

[0131] The dihedral color wedge 160 is particularly advantageous because it can perform multiple functions and thus saves space. For example... Figure 7As shown, the alternative dichroic optics 170 may include, for example, two dichroic mirrors 172 and 174, a beam splitter 176, and an optical component 178 to ensure that the two wavelength components travel equal optical path lengths (so that the same two planes are imaged for the main pulse radiation and the prepulse radiation). The dichroic wedge 160 can perform these multiple functions and thus save space.

[0132] While this article provides specific references to the use of lithography equipment in IC manufacturing, it should be understood that the lithography equipment described herein can have other applications. Other possible applications include manufacturing integrated optical systems, guiding and detecting patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin-film magnetic heads, etc.

[0133] Although embodiments of the invention are specifically referred to herein in the context of a lithography apparatus, these embodiments can be used in other apparatuses. Embodiments of the invention can form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes objects such as wafers (or other substrates) or masks (or other patterning apparatuses). These apparatuses are commonly referred to as lithography tools. Such lithography tools can use vacuum conditions or ambient (non-vacuum) conditions.

[0134] Where circumstances permit, embodiments of the present invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the present invention may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a machine-readable (e.g., computing device) form. For example, a machine-readable medium may include read-only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash storage devices; electrical, optical, acoustic, or other forms of propagation signals (e.g., carrier waves, infrared signals, digital signals, etc.) and others. Furthermore, firmware, software, routines, and instructions may be described herein as performing certain actions. However, it should be understood that such description is merely for convenience, and such actions are actually produced by the computing device, processor, controller, or other device executing the firmware, software, routines, instructions, etc., and by operations that may cause the actuator or other device to interact with the physical world.

[0135] While specific embodiments of the invention have been described above, it should be understood that the invention can be practiced in ways different from those described. The above description is intended to illustrate rather than limit. Therefore, those skilled in the art will understand that the invention can be modified as described without departing from the scope of the following claims.

Claims

1. A measurement system for an optical amplification cavity, the measurement system comprising: An input optical element is provided for receiving an input radiation beam, and the input optical element guides a first portion of the input radiation beam along a first optical path. And simultaneously guide the second part of the input radiation beam into the optical amplification cavity along the second optical path; An imaging system arranged along the first optical path and configured to form in an image plane a first image and a second image of the first portion of the input radiation beam, the first image and the second image belonging to two different planes along the first optical path; and A detector, which is disposed in the image plane and is operable to detect the first image and the second image.

2. The measurement system of claim 1, further comprising a manipulation optics device arranged to receive the input radiation beam and guide the input radiation beam to the input optical element, wherein, The manipulation optics include an adjustment mechanism operable to control the direction and / or position of the input radiation beam at the input optics.

3. The measurement system of claim 1 further includes a display for displaying an image of the input radiation beam in two different planes along the first optical path.

4. The measurement system of claim 2 further includes a display for displaying an image of the input radiation beam in two different planes along the first optical path.

5. The measurement system of claim 1 further includes a memory operable to store information relating to the nominal direction and / or nominal position of the input radiation beam.

6. The measurement system of claim 2 further includes a memory operable to store information relating to the nominal direction and / or nominal position of the input radiation beam.

7. The measurement system of claim 4 further includes a memory operable to store information relating to the nominal direction and / or nominal position of the input radiation beam.

8. The measurement system of claim 6 further includes a feedback loop operable to control the position and / or direction of the input radiation beam using the adjustment mechanism of the manipulating optics until the position and / or direction of the input radiation beam is substantially aligned with the nominal direction and / or nominal position of the input radiation beam.

9. The measurement system of claim 7 further includes a feedback loop operable to control the position and / or direction of the input radiation beam using the adjustment mechanism of the manipulating optics until the position and / or direction of the input radiation beam is substantially aligned with the nominal direction and / or nominal position of the input radiation beam.

10. The measurement system according to claim 7 or 9, wherein, The display is also operable to display at least one visual identifier for each of the first and second images, the at least one visual identifier indicating the position and / or shape of each of the first and second images when the position and / or direction of the input radiation beam is substantially aligned with the nominal direction and / or nominal position of the input radiation beam.

11. The measurement system according to claim 1, wherein, The imaging system includes a lens having two surfaces, each having a reflective coating, at least one of the two surfaces being curved, wherein a second portion of an input radiation beam is incident off-axis onto the lens, and wherein a first image and a second image are formed by discrete portions of the second portion of the input radiation beam, the discrete portions undergoing different numbers of reflections from the two surfaces.

12. The measurement system according to claim 11, wherein, The first image is formed by a first transmissive portion of radiation incident on the lens, the first transmissive portion being transmitted at both a first surface and a second surface of the lens; and wherein the second image is formed by a second transmissive portion of the radiation incident on the lens, the second transmissive portion being transmitted at the first surface, internally reflected at the second surface, internally reflected at the first surface, and then transmitted at the second surface.

13. The measurement system according to claim 12, wherein, The radii of curvature of the first surface and the second surface are formed 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.

14. The measurement system according to claim 13, wherein, The first surface and the second surface have a reflectivity of R, and the radii of curvature of the first surface and the second surface are formed 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.

15. The measurement system according to claim 12 or 13, wherein, The reflectivity of the reflective coatings on the first and second surfaces of the lens, as well as the radii of curvature of the first and second surfaces of the lens, are configured such that the intensity densities of the two images are substantially matched.

16. The measurement system of claim 1, further comprising a dichroic optics arranged in the first optical path, wherein the dichroic optics is 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.

17. The measurement system according to claim 16, wherein, The dichroic optical device includes an optical device having opposite first and second surfaces, the opposite first and second surfaces being arranged at a non-zero angle relative to each other; The first surface has a coating that is reflective to radiation of a first wavelength and transmissive to radiation of a second wavelength; and The second surface is reflective to radiation of the second wavelength.

18. The measurement system according to claim 17, wherein, The second surface of the optical element has a coating that is reflective to radiation of the second wavelength.

19. A system comprising: Amplifying cavity; The measurement system according to any of the preceding claims, wherein the amplification cavity is arranged along the second optical path.

20. The system according to claim 19, wherein, The amplification cavity includes: Two coaxial, roughly cylindrical electrodes; A gain medium disposed within a generally tubular cavity defined between the two coaxial generally cylindrical electrodes; and A generally annular reflector is disposed at each end of the cavity.

21. The system according to claim 19 or 20, further comprising: A seed laser, operable to output a seed laser beam, wherein the input optics are arranged to receive the seed laser beam as the input radiation beam.

22. A laser system comprising the system according to any one of claims 19 to 21.

23. A laser-generated plasma radiation source, comprising: A fuel emitter, operable to generate a fuel target at a plasma formation region; and The laser system of claim 22 is arranged to irradiate the fuel target at the plasma formation region to generate plasma.

24. A photolithography system, comprising: The laser-generated plasma radiation source according to claim 23; and Photolithography equipment.

25. A method for aligning an input radiation beam with an amplifying cavity, the method comprising: Receive input radiation beam; The first portion of the input radiation beam is guided along the first optical path; And simultaneously guides the second part of the input radiation beam into the amplification cavity along the second optical path; Formed in the image plane: a first image and a second image of the first portion of the input radiation beam, the first image and the second image belonging to two different planes along the first optical path; and Detect the first image and the second image in the image plane.

26. The method of claim 25, further comprising controlling the direction and / or position of the input radiation beam.

27. The method of claim 25 or 26, further comprising comparing at least one characteristic of each of the first and second images with a nominal value of the at least one characteristic, the nominal value indicating the nominal direction and / or nominal position of the input radiation beam.

28. The method of claim 27, which is dependent on claim 26, further comprising: The position and / or orientation of the input radiation beam are controlled until at least one characteristic of each of the first and second images substantially matches the nominal value of the at least one characteristic.

Images