IL298613A

IL298613A - Precise vacuum window viewports and pellicles for rapid metrology recovery

Info

Publication number: IL298613A
Application number: IL298613A
Authority: IL
Original assignee: Asml Netherlands Bv
Priority date: 2020-07-01
Filing date: 2021-06-10
Publication date: 2023-01-01
Also published as: CN115735163A; WO2022002560A1; KR20230031830A; JP2023530849A

Description

WO 2022/002560 PCT/EP2021/065676 1 PRECISE VACUUM WINDOW VIEWPORTS AND PELLICLES FOR RAPID METROLOGY RECOVERY CROSS REFERENCE TO RELATED APPLICATIONS [0001]This application claims priority to U.S. Application No. 63/046,984, filed July 01, 2020 and titled PRECISE VACUUM WINDOW VIEWPORTS AND PELLICLES FOR RAPID METROLOGY RECOVERY and which is incorporated herein in its entirety by reference.

TECHNICAL FIELD [0002]The present disclosure relates to metrology systems and windows for extreme ultraviolet (EUV) radiation systems.

BACKGROUND [0003]A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is interchangeably referred to as a mask or a reticle, can be used to generate a circuit pattern to be formed on an individual layer of the IC being formed. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Traditional lithographic apparatuses include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the "scanning"-direction) while synchronously scanning the target portions parallel or anti-parallel (e.g., opposite) to this scanning direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate. [0004]Extreme ultraviolet (EUV) light, for example, electromagnetic radiation having wavelengths of around 50 nanometers (nm) or less (also sometimes referred to as soft x-rays), and including light at a wavelength of about 13 nm, can be used in or with a lithographic apparatus to produce extremely small features in or on substrates, for example, silicon wafers. Methods to produce EUV light include, but are not necessarily limited to, converting a material that has an element, for example, xenon (Xe), lithium (Li), or tin (Sn), with an emission line in the EUV range to a plasma state. For example, in one such method called laser produced plasma (EPP), the plasma can be produced by irradiating a target material, which is interchangeably referred to as fuel in the context of EPP sources, for example, in the form of a droplet, plate, tape, stream, or cluster of material, with an amplified light beam that can be referred to WO 2022/002560 PCT/EP2021/065676 2as a drive laser. For this process, the plasma is typically produced in a sealed vessel, for example, a vacuum chamber, and monitored using various types of metrology equipment.

SUMMARY [0005]The present disclosure describes various aspects of systems, apparatuses, and methods for optical metrology and various other aspects in an extreme ultraviolet (EUV) radiation system. [0006]In some aspects, the present disclosure describes a system for optical metrology in a radiation system, such as an EUV radiation system. The system can include a metrology system configured to be disposed in a first environment. The metrology system can be further configured to perform one or more measurements of a region in a second environment along an optical axis of the metrology system. The second environment can be different from the first environment. The window can be configured to be disposed intersecting the optical axis. The window can be further configured to isolate the metrology system from the second environment. The window can be further configured to limit a transverse displacement from the optical axis to less than about + 50 microns from a nominal transverse displacement from the optical axis at a primary focus of a radiation collector. In some aspects, the primary focus can be located at a distance of about 1 meter from a surface of the window. [0007]In some aspects, the window can be configured to limit the transverse displacement to less than about + 33 microns. In some aspects, the window can be configured to limit an angular deviation along the optical axis to less than about + 0.5 arcmin from a nominal angular deviation along the optical axis. In some aspects, the window can be configured to limit the angular deviation to less than about ±0.arcmin. In some aspects, the window can be configured to limit a longitudinal displacement to less than about + 330 microns from a nominal longitudinal displacement from the primary focus along the optical axis. In some aspects, the window can be configured to limit the longitudinal displacement to less than about + 200 microns. [0008]In some aspects, the window can include a first part (e.g., a viewport) configured to be disposed intersecting the optical axis. In some aspects, the window can further include a second part (e.g., a pellicle) configured to be disposed intersecting the optical axis and opposite the first part. In some aspects, the window can include a wedge angle of less than about + 0.1 arcmin from a nominal wedge angle. In some aspects, the nominal wedge angle can be about zero degrees. In other aspects, the nominal wedge angle can be greater than about zero degrees. [0009]In some aspects, the metrology system can be modular. In some aspects, the window can be configured to limit the displacement to less than about + 50 microns at a time at which the metrology system is installed in the system. In some aspects, the window can be configured to limit the displacement to less than about + 50 microns without a calibration action. [0010]In some aspects, the present disclosure describes an apparatus for optical metrology in a radiation system, such as an EUV radiation system. The apparatus can include a first part (e.g., a viewport) configured to be disposed intersecting an optical axis. The apparatus can further include a WO 2022/002560 PCT/EP2021/065676 3second part (e.g., a pellicle) configured to be disposed intersecting the optical axis and opposite the first part. The apparatus can be configured to transmit radiation along the optical axis through the first part and the second part. The apparatus can be further configured to limit a transverse displacement from the optical axis to less than about ± 50 microns from a nominal transverse displacement from the optical axis at a primary focus of a radiation collector.[0011] In some aspects, the primary focus can be located at a distance of about 1 meter from a surface of the apparatus. In some aspects, the first part can include a viewport. In some aspects, the second part can include a pellicle. In some aspects, the apparatus can include a wedge angle of less than about ±0.arcmin from a nominal wedge angle. In some aspects, the apparatus can be, or include, a window as described herein.[0012] In some aspects, the present disclosure describes a method for optical metrology in a radiation system, such as an EUV radiation system. The method can include disposing a metrology system in a first environment. The metrology system performs one or more measurements of a region in a second environment along an optical axis of the metrology system, the second environment being different from the first environment. The method can further include isolating the metrology system from the second environment using a window that is disposed to intersect the optical axis. The method can further include limiting, based on a disposition of the window, a transverse displacement from the optical axis to less than about + 50 microns from a nominal transverse displacement from the optical axis at a primary focus of a radiation collector.[0013] Further features and advantages, as well as the structure and operation of various aspects, are described in detail below with reference to the accompanying drawings. It is noted that the disclosure is not limited to the specific aspects described herein. Such aspects are presented herein for illustrative purposes only. Additional aspects will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein.

BRIEF DESCRIPTION OF THE DRAWINGS[0014] The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the present disclosure and, together with the description, further serve to explain the principles of the aspects of this disclosure and to enable a person skilled in the relevant art(s) to make and use the aspects of this disclosure.[0015] FIG. 1A is a schematic illustration of an example reflective lithographic apparatus according to some aspects of the present disclosure.[0016] FIG. IB is a schematic illustration of an example transmissive lithographic apparatus according to some aspects of the present disclosure.[0017] FIG. 2 is a more detailed schematic illustration of the reflective lithographic apparatus shown in FIG. 1A according to some aspects of the present disclosure.

WO 2022/002560 PCT/EP2021/065676 4 [0018]FIG. 3 is a schematic illustration of an example lithographic cell according to some aspects of the present disclosure. [0019]FIG. 4 is a schematic illustration of an example radiation source for an example reflective lithographic apparatus according to some aspects of the present disclosure. [0020]FIG. 5 is a schematic illustration of a portion an example EUV radiation system according to some aspects of the present disclosure. [0021]FIGS. 6A, 6B, 6C, and 6D are schematic illustrations of portions an example EUV radiation system according to some aspects of the present disclosure. [0022]FIGS. 7A, 7B, and 7C are schematic illustrations of a fast swap window assembly according to some aspects of the present disclosure. [0023]FIG. 8 is an example method according to some aspects of the present disclosure or portion(s) thereof. [0024]The features and advantages of the present disclosure will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, unless otherwise indicated, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. Additionally, generally, the left-most digit(s) of a reference number identifies the drawing in which the reference number first appears. Unless otherwise indicated, the drawings provided throughout the disclosure should not be interpreted as to-scale drawings.

DETAILED DESCRIPTIONThis specification discloses one or more embodiments that incorporate the features of the present disclosure. The disclosed embodiment(s) merely describe the present disclosure. The scope of the disclosure is not limited to the disclosed embodiment(s). The breadth and scope of the disclosure are defined by the claims appended hereto and their equivalents. [0025]The embodiment(s) described, and references in the specification to "one embodiment," "an embodiment," "an exemplary embodiment," "an example embodiment," etc., indicate that the embodiment(s) described can include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is understood that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. [0026]Spatially relative terms, such as "beneath," "below," "lower," "above," "on," "upper" and the like, may be used herein for ease of description to describe one element or feature’s relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted WO 2022/002560 PCT/EP2021/065676 5in the figures. The apparatus can be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly. [0027]The term "about" as used herein indicates the value of a given quantity that can vary based on a particular technology. Based on the particular technology, the term "about" can indicate a value of a given quantity that varies within, for example, 10-30% of the value (e.g., ±10%, ±20%, or ±30% of the value).Overview [0028]In one example, windows for EUV radiation systems include a vacuum window, also referred to as a viewport, assembled with a second window, also referred to as a pellicle. Both the vacuum window and the pellicle are made of a non-optical quality glass, e.g., soda glass. The vacuum window provides the vacuum seal to the EUV source vessel and enables the vessel metrology to see into the vessel. The pellicle is located inside the vacuum vessel and provides a barrier for tin debris from getting to the vacuum window. The pellicle becomes contaminated with tin over time and must be swapped periodically. When the pellicle is swapped, the entire viewport-pellicle assembly is swapped. [0029]However, because the viewport and pellicle are optical windows, they can cause both optical axis deviations (offsets and angular pointing errors) as well as impart aberration into the wavefront. As a result, the viewport and pellicle can cause misalignment of the metrology optical axis. This misalignment can necessitate realignment of the metrology when the viewport and pellicle are swapped. This realignment process can add additional recovery time (MTTR) on the order of about 1 to 10 hours depending on the metrology module and the risk of B-time (e.g., recovery time) during the realignment. Further, the existing viewports and pellicles may not have sufficiently well controlled manufacturing tolerances to avoid disturbing the optical axis of the metrology. Further, since the current vacuum window viewports may not be optical quality, many specifications that are critical to metrology performance may be unknown (e.g., refractive index vs. wavelength, transmitted wavefront error, wedge angle, etc.). [0030]In contrast, some embodiments of the present disclosure can provide a window having improved structures and tolerances to reduce substantially the effect of the window on the alignment of the metrology system coupled to the window. [0031]In some aspects, the present disclosure provides a window that replaces soda glass with an improved material structure (e.g., optical glass) for the viewport and pellicle to reduce transmitted wavefront aberrations due to (a) inhomogeneity of refractive index and (b) presence of non-controlled bubbles and striae. For example, the material of the viewport and pellicle can be optical glass, e.g., borosilicate crown glass, having a transmission range of about 350 nanometers to about 2.5 microns and a refractive index of about 1.51680 at 587.5618 nanometers (e.g., yellow helium line). In some aspects, the viewport can be coated with an anti-reflection (AR) coating. [0032]In some aspects, the present disclosure further provides a window that improves the tolerances on: (i) the wedge angles of the viewport and pellicle to reduce pointing error; (ii) the thickness of the WO 2022/002560 PCT/EP2021/065676 6viewport and pellicle to reduce decenter and focus error; (iii) the refractive index of the viewport and pellicle to reduce focus error; (iv) the transmitted wavefront power of the viewport and pellicle to reduce focus error; and (v) if necessary, use compensation between elements (e.g., balancing negative & positive errors) to further reduce the overall alignment error. In some aspects, the present disclosure improves the alignment tolerances of an example window disclosed herein compared to a traditional window as shown in Table 1 below.

Table 1: Alignment tolerances of an example window disclosed herein compared to a traditional window.

Error Source Effect Sensitivity1 Traditional Window Example Window Disclosed Herein Tolerance Error @ PF Tolerance Error @ PF WedgeError in Measured Droplet Position 140 microns/ arcmin-4.arcmin5microns0.1 arcminmicrons Thickness Consumes Module Focus Range 0.34 mm/mm - 0.2 mmmicrons0.1 mmmicronsWavefront Error (power)mm/wave - 1 wave 2 mm 0.14 waves2micronsFine Droplet Steering Camera (FDSC). id="p-33" id="p-33" id="p-33" id="p-33" id="p-33" id="p-33"

[0033]In some aspects, the improvements provided herein can reduce the metrology alignment error from about 600 microns transverse and 2 millimeters axial focus error to less than about 30 microns transverse and less than about 200 microns axial focus error. With this reduction in metrology alignment error, metrology systems will not need to be realigned after a window swap (e.g., a viewport and pellicle swap). [0034]In some aspects, the present disclosure improves the availability of the EUV radiation system by shortening the "green to green" time (also referred to as A-time) for a viewport swap. In addition, by eliminating metrology recovery actions, the present disclosure eliminates the risk of something going wrong and taking longer than planned to recover (also referred to as B-time), which also improves availability. [0035]In some aspects, the present disclosure provides for a technique whereby the viewport can be selected to cancel the errors of the pellicle, and vice versa. This technique provides for looser manufacturing tolerances in exchange for a more complicated pairing and build process. [0036]There are many benefits to the window disclosed herein. For example, the present disclosure provides for precisely controlled manufacturing tolerances of the viewports and pellicles including: less wedge tolerance; tighter angular mounting tolerances; lower transmitted wavefront power tolerance; less stress on the vacuum window due to using an optical-quality designed vacuum interface; optical WO 2022/002560 PCT/EP2021/065676 7quality glass instead of the borosilicate glass used in the existing viewports; and decreased recovery time for swapping vacuum windows. In another example, the optical and mechanical tolerances of the windows, viewports, and pellicles disclosed herein are greatly improved, which, in some aspects, can eliminate the need for metrology recovery steps following a viewport and pellicle swap. [0037]In some aspects, the optical and mechanical tolerances of the windows, viewports, and pellicles disclosed herein also simplify the EUV source manufacturing process by eliminating setup and alignment steps. For example, a radiation source can have nine metrology systems that all need to point at specific locations within the vessel. In some aspects, radiation sources required that technicians set up complex targets inside the vessel and align the metrology systems to those targets once installed on the vessel. This was a time consuming process that could be done incorrectly as a result of technician error. In contrast, in other aspects the alignment tolerances of all related hardware (e.g., metrology systems, windows, vessel frame) can be small enough that those setup steps may no longer be necessary. Accordingly, once all related hardware have been assembled, they already should be aligned sufficiently well to not require alignment actions. The high precision windows disclosed herein can be critical to achieving this. [0038]As discussed above, pointing errors can have a critical impact on the total alignment error budget for the metrology systems and, in turn, the performance of the EUV radiation source. In one illustrative example, the optical distance from the viewport to the measurement location, the primary focus PF of the radiation collector, is about 1 meter. The wedge in the viewport induces a pointing error proportional to the refractive index as shown by the equation D = L*A*(n-l), where D = distance shifted at the primary focus PF, L = distance from the primary focus PF, A = wedge angle, and n = refractive index. The existing wedge tolerance is ± 3 arcmin, or about ± 870 microradians (urad). With a refractive index n of around 1.5 and a distance D from the primary focus PF of around 1 meter, the tolerance at the primary PF could be about 435 microns (e.g., 0.5 * 870) from the viewport alone. When also considering the pellicle, the tolerance could be between about 600 microns and 870 microns. [0039]Continuing the example above, the droplet detection module (DDM) has a field of view (FOV) of about 540 microns. If the droplet illumination module (DIM) viewport and pellicle are swapped, and an alignment error of between about 600 microns and 870 microns is realized, the DIM and DDM would require realignment, which can take up to 20 hours. With the window disclosed herein, the wedge tolerance is about ± 5 arcsec for both the viewport and pellicle, resulting in less than about 30 microns of deviation at primary focus PF, well within the field of view of the DDM. [0040]In some aspects, as a result of the techniques described in the present disclosure, the viewport and pellicle disclosed herein can reduce uncertainty in optical modeling for the metrology system. Further, because the viewports and pellicles disclosed herein are optical quality, many specifications that are critical to metrology performance can be known, such as refractive index vs. wavelength, transmitted wavefront error, wedge angle, and other suitable characteristics. In addition, the use of a toleranced viewport assembly can allow for: (i) pre-alignment of metrology modules on optical bench WO 2022/002560 PCT/EP2021/065676 8test station; and (ii) direct swap of metrology module (e.g., upon failure) without the need to re-align on-vessel, saving time (e.g., up to 10 hours per swap). [0041]Before describing such aspects in more detail, however, it is instructive to present an example environment in which aspects of the present disclosure can be implemented.

Example Lithographic Systems [0042]FIGS. 1A and IB are schematic illustrations of a lithographic apparatus 100 and lithographic apparatus 100’, respectively, in which aspects of the present disclosure can be implemented. As shown in FIGS. 1A and IB, the lithographic apparatuses 100 and 100’ are illustrated from a point of view (e.g., a side view) that is normal to the XZ plane (e.g., the X-axis points to the right and the Z-axis points upward), while the patterning device MA and the substrate W are presented from additional points of view (e.g., a top view) that are normal to the XY plane (e.g., the X-axis points to the right and the Y- axis points upward). [0043]Lithographic apparatus 100 and lithographic apparatus 100’ each include the following: an illumination system IL (e.g., an illuminator) configured to condition a radiation beam B (e.g., a deep ultra violet (DUV) radiation beam or an extreme ultra violet (EUV) radiation beam); a support structure MT (e.g., a mask table) configured to support a patterning device MA (e.g., a mask, a reticle, or a dynamic patterning device) and connected to a first positioner PM configured to accurately position the patterning device MA; and, a substrate holder such as a substrate table WT (e.g., a wafer table) configured to hold a substrate W (e.g., a resist-coated wafer) and connected to a second positioner PW configured to accurately position the substrate W. Lithographic apparatuses 100 and 100’ also have a projection system PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., a portion including one or more dies) of the substrate W. In lithographic apparatus 100, the patterning device MA and the projection system PS are reflective. In lithographic apparatus 100’, the patterning device MA and the projection system PS are transmissive. [0044]The illumination system IL can include various types of optical components, such as refractive, reflective, catadioptric, magnetic, electromagnetic, electrostatic, or other types of optical components, or any combination thereof, for directing, shaping, or controlling the radiation beam B. [0045]The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device MA with respect to a reference frame, the design of at least one of the lithographic apparatuses 100 and 100’, and other conditions, such as whether or not the patterning device MA is held in a vacuum environment. The support structure MT can use mechanical, vacuum, electrostatic, or other clamping techniques to hold the patterning device MA. The support structure MT can be a frame or a table, for example, which can be fixed or movable, as required. By using sensors, the support structure MT can ensure that the patterning device MA is at a desired position, for example, with respect to the projection system PS.

WO 2022/002560 PCT/EP2021/065676 9 [0046]The term "patterning device" MA should be broadly interpreted as referring to any device that can be used to impart a radiation beam B with a pattern in its cross-section, such as to create a pattern in the target portion C of the substrate W. The pattern imparted to the radiation beam B can correspond to a particular functional layer in a device being created in the target portion C to form an integrated circuit. [0047]The patterning device MA can be transmissive (as in lithographic apparatus 100’ of FIG.IB) or reflective (as in lithographic apparatus 100 of FIG. 1A). Examples of patterning devices MA include reticles, masks, programmable minor arrays, or programmable LCD panels. Masks include mask types such as binary, alternating phase shift, or attenuated phase shift, as well as various hybrid mask types. An example of a programmable minor array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted minors impart a pattern in the radiation beam B, which is reflected by a matrix of small minors. [0048]The term "projection system" PS can encompass any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors, such as the use of an immersion liquid on the substrate W or the use of a vacuum. A vacuum environment can be used for EUV or electron beam radiation since other gases can absorb too much radiation or electrons. A vacuum environment can therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps. [0049]Lithographic apparatus 100 and/or lithographic apparatus 100’ can be of a type having two (dual stage) or more substrate tables WT (and/or two or more mask tables). In such "multiple stage" machines, the additional substrate tables WT can be used in parallel, or preparatory steps can be carried out on one or more tables while one or more other substrate tables WT are being used for exposure. In some situations, the additional table may not be a substrate table WT. [0050]The lithographic apparatus can also be of a type wherein at least a portion of the substrate can be covered by a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the projection system and the substrate. An immersion liquid can also be applied to other spaces in the lithographic apparatus, for example, between the mask and the projection system. Immersion techniques provide for increasing the numerical aperture of projection systems. The term "immersion" as used herein does not mean that a structure, such as a substrate, must be submerged in liquid, but rather only means that liquid is located between the projection system and the substrate during exposure. [0051]Referring to FIGS. 1A and IB, the illumination system IL receives a radiation beam B from a radiation source SO. The radiation source SO and the lithographic apparatus 100 or 100’ can be separate physical entities, for example, when the radiation source SO is an excimer laser. In such cases, the radiation source SO is not considered to form part of the lithographic apparatus 100 or 100’, and the radiation beam B passes from the radiation source SO to the illumination system IL with the aid of a beam delivery system BD (e.g., shown in FIG. IB) including, for example, suitable directing mirrors WO 2022/002560 PCT/EP2021/065676 10and/or a beam expander. In other cases, the radiation source SO can be an integral part of the lithographic apparatus 100 or 100’, for example, when the radiation source SO is a mercury lamp. The radiation source SO and the illuminator IL, together with the beam delivery system BD, if required, can be referred to as a radiation system. [0052]The illumination system IL can include an adjuster AD (e.g., shown in FIG. IB) for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as "o-outer" and "o-inner," respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illumination system IL can include various other components (e.g., shown in FIG. IB), such as an integrator IN and a radiation collector CO (e.g., a condenser or collector optic). The illumination system IL can be used to condition the radiation beam B to have a desired uniformity and intensity distribution in its cross section. [0053]Referring to FIG. 1A, the radiation beam B is incident on the patterning device MA (e.g., a mask), which is held on the support structure MT (e.g., a mask table), and is patterned by the patterning device MA. In lithographic apparatus 100, the radiation beam B is reflected from the patterning device MA. After being reflected from the patterning device MA, the radiation beam B passes through the projection system PS, which focuses the radiation beam B onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IFD2 (e.g., an interferometric device, linear encoder, or capacitive sensor), the substrate table WT can be moved accurately (e.g., so as to position different target portions C in the path of the radiation beam B). Similarly, the first positioner PM and another position sensor IFD1 (e.g., an interferometric device, linear encoder, or capacitive sensor) can be used to accurately position the patterning device MA with respect to the path of the radiation beam B. Patterning device MA and substrate W can be aligned using mask alignment marks Ml and M2 and substrate alignment marks Pl and P2. [0054]Referring to FIG. IB, the radiation beam B is incident on the patterning device MA, which is held on the support structure MT, and is patterned by the patterning device MA. Having traversed the patterning device MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. The projection system has a pupil conjugate PPU to an illumination system pupil IPU. Portions of radiation emanate from the intensity distribution at the illumination system pupil IPU and traverse a mask pattern without being affected by diffraction at the mask pattern and create an image of the intensity distribution at the illumination system pupil IPU.[0055] The projection system PS projects an image MP’ of the mask pattern MP, where image MP’ is formed by diffracted beams produced from the mask pattern MP by radiation from the intensity distribution, onto a resist layer coated on the substrate W. For example, the mask pattern MP can include an array of lines and spaces. A diffraction of radiation at the array and different from zeroth-order diffraction generates diverted diffracted beams with a change of direction in a direction perpendicular to the lines. Undiffracted beams (e.g., so-called zeroth-order diffracted beams) traverse the pattern without any change in propagation direction. The zeroth-order diffracted beams traverse an upper lens WO 2022/002560 PCT/EP2021/065676 11or upper lens group of the projection system PS, upstream of the pupil conjugate PPU of the projection system PS, to reach the pupil conjugate PPU. The portion of the intensity distribution in the plane of the pupil conjugate PPU and associated with the zeroth-order diffracted beams is an image of the intensity distribution in the illumination system pupil IPU of the illumination system IL. The aperture device PD, for example, is disposed at or substantially at a plane that includes the pupil conjugate PPU of the projection system PS. [0056]The projection system PS is arranged to capture, by means of a lens or lens group L, not only the zeroth-order diffracted beams, but also first-order or first- and higher-order diffracted beams (not shown). In some aspects, dipole illumination for imaging line patterns extending in a direction perpendicular to a line can be used to utilize the resolution enhancement effect of dipole illumination. For example, first-order diffracted beams interfere with corresponding zeroth-order diffracted beams at the level of the substrate W to create an image of the mask pattern MP at highest possible resolution and process window (e.g., usable depth of focus in combination with tolerable exposure dose deviations). In some aspects, astigmatism aberration can be reduced by providing radiation poles (not shown) in opposite quadrants of the illumination system pupil IPU. Further, in some aspects, astigmatism aberration can be reduced by blocking the zeroth-order beams in the pupil conjugate PPU of the projection system associated with radiation poles in opposite quadrants. This is described in more detail in U.S. Patent No. 7,511,799, issued March 31, 2009, and titled "Lithographic projection apparatus and a device manufacturing method," which is incorporated by reference herein in its entirety. [0057]With the aid of the second positioner PW and position sensor IFD (e.g., an interferometric device, linear encoder, or capacitive sensor), the substrate table WT can be moved accurately (e.g., so as to position different target portions C in the path of the radiation beam B). Similarly, the first positioner PM and another position sensor (not shown in FIG. IB) can be used to accurately position the patterning device MA with respect to the path of the radiation beam B (e.g., after mechanical retrieval from a mask library or during a scan). [0058]In general, movement of the support structure MT can be realized with the aid of a long-stroke positioner (coarse positioning) and a short-stroke positioner (fine positioning), which form part of the first positioner PM. Similarly, movement of the substrate table WT can be realized using a long-stroke positioner and a short-stroke positioner, which form part of the second positioner PW. In the case of a stepper (as opposed to a scanner), the support structure MT can be connected to a short-stroke actuator only or can be fixed. Patterning device MA and substrate W can be aligned using mask alignment marks Ml, M2, and substrate alignment marks Pl, P2. Although the substrate alignment marks (as illustrated) occupy dedicated target portions, they can be located in spaces between target portions (e.g., scribe- lane alignment marks). Similarly, in situations in which more than one die is provided on the patterning device MA, the mask alignment marks can be located between the dies. [0059]Support structure MT and patterning device MA can be in a vacuum chamber V, where an in- vacuum robot IVR can be used to move patterning devices such as a mask in and out of vacuum WO 2022/002560 PCT/EP2021/065676 12chamber. Alternatively, when support structure MT and patterning device MA are outside of the vacuum chamber, an out-of-vacuum robot can be used for various transportation operations, similar to the in-vacuum robot IVR. In some instances, both the in-vacuum and out-of-vacuum robots need to be calibrated for a smooth transfer of any payload (e.g., a mask) to a fixed kinematic mount of a transfer station. [0060]The lithographic apparatuses 100 and 100’ can be used in at least one of the following modes: [0061] 1. In step mode, the support structure MT and the substrate table WT are kept essentiallystationary, while an entire pattern imparted to the radiation beam B is projected onto a target portion C at one time (e.g., a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed. [0062] 2. In scan mode, the support structure MT and the substrate table WT are scannedsynchronously while a pattern imparted to the radiation beam B is projected onto a target portion C (e.g., a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure MT (e.g., mask table) can be determined by the (de-)magnification and image reversal characteristics of the projection system PS. [0063] 3. In another mode, the support structure MT is kept substantially stationary holding aprogrammable patterning device MA, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam B is projected onto a target portion C. A pulsed radiation source SO can be employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes a programmable patterning device MA, such as a programmable minor array. [0064]Combinations and/or variations on the described modes of use or entirely different modes of use can also be employed. [0065]In a further aspect, lithographic apparatus 100 includes an EUV source, which is configured to generate a beam of EUV radiation for EUV lithography. In general, the EUV source is configured in a radiation system, and a corresponding illumination system is configured to condition the EUV radiation beam of the EUV source. [0066] FIG.2 shows the lithographic apparatus 100 in more detail, including the radiation source SO (e.g., a source collector apparatus), the illumination system IL, and the projection system PS. As shown in FIG. 2, the lithographic apparatus 100 is illustrated from a point of view (e.g., a side view) that is normal to the XZ plane (e.g., the X-axis points to the right and the Z-axis points upward). [0067]The radiation source SO is constructed and arranged such that a vacuum environment can be maintained in an enclosing structure 220. The radiation source SO includes a source chamber 211 and a collector chamber 212 and is configured to produce and transmit EUV radiation. EUV radiation can be produced by a gas or vapor, for example xenon (Xe) gas, lithium (Li) vapor, or tin (Sn) vapor in which an EUV radiation emitting plasma 210 is created to emit radiation in the EUV range of the WO 2022/002560 PCT/EP2021/065676 13electromagnetic spectrum. The EUV radiation emitting plasma 210, at least partially ionized, can be created by, for example, an electrical discharge or a laser beam. Partial pressures of, for example, about 10.0 pascals (Pa) of Xe gas, Li vapor, Sn vapor, or any other suitable gas or vapor can be used for efficient generation of the radiation. In some aspects, a plasma of excited tin is provided to produce EUV radiation. [0068]The radiation emitted by the EUV radiation emitting plasma 210 is passed from the source chamber 211 into the collector chamber 212 via an optional gas barrier or contaminant trap 230 (e.g., in some cases also referred to as contaminant barrier or foil trap), which is positioned in or behind an opening in source chamber 211. The contaminant trap 230 can include a channel structure. Contamination trap 230 can also include a gas barrier or a combination of a gas barrier and a channel structure. The contaminant trap 230 further indicated herein at least includes a channel structure. [0069]The collector chamber 212 can include a radiation collector CO (e.g., a condenser or collector optic), which can be a so-called grazing incidence collector. Radiation collector CO has an upstream radiation collector side 251 and a downstream radiation collector side 252. Radiation that traverses radiation collector CO can be reflected off a grating spectral filter 240 to be focused in a virtual source point IF. The virtual source point IF is commonly referred to as the intermediate focus, and the source collector apparatus is arranged such that the virtual source point IF is located at or near an opening 2in the enclosing structure 220. The virtual source point IF is an image of the EUV radiation emitting plasma 210. Grating spectral filter 240 is used in particular for suppressing infrared (IR) radiation. [0070]Subsequently the radiation traverses the illumination system IL, which can include a faceted field minor device 222 and a faceted pupil minor device 224 arranged to provide a desired angular distribution of the radiation beam 221, at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA. Upon reflection of the radiation beam 221 at the patterning device MA, held by the support structure MT, a patterned beam 226 is formed and the patterned beam 226 is imaged by the projection system PS via reflective elements 228, 229 onto a substrate W held by the wafer stage or substrate table WT. [0071]More elements than shown can generally be present in illumination system IL and projection system PS. Optionally, the grating spectral filter 240 can be present depending upon the type of lithographic apparatus. Further, there can be more minors present than those shown in the FIG. 2. For example, there can be one to six additional reflective elements present in the projection system PS than shown in FIG. 2. [0072]Radiation collector CO, as illustrated in FIG. 2, is depicted as a nested collector with grazing incidence reflectors 253, 254, and 255, just as an example of a collector (or collector minor). The grazing incidence reflectors 253, 254, and 255 are disposed axially symmetric around an optical axis O and a radiation collector CO of this type is preferably used in combination with a discharge produced plasma (DPP) source.

WO 2022/002560 PCT/EP2021/065676 14Example Lithographic Cell [0073] FIG.3 shows a lithographic cell 300, also sometimes referred to a lithocell or cluster. As shown in FIG. 3, the lithographic cell 300 is illustrated from a point of view (e.g., a top view) that is normal to the XY plane (e.g., the X-axis points to the right and the Y-axis points upward). [0074]Lithographic apparatus 100 or 100’ can form part of lithographic cell 300. Lithographic cell 300 can also include one or more apparatuses to perform pre- and post-exposure processes on a substrate. For example, these apparatuses can include spin coaters SC to deposit resist layers, developers DE to develop exposed resist, chill plates CH, and bake plates BK. A substrate handler RO (e.g., a robot) picks up substrates from input/output ports I/O I and I/O2, moves them between the different process apparatuses and delivers them to the loading bay LB of the lithographic apparatus 100 or 100’. These devices, which are often collectively referred to as the track, are under the control of a track control unit TCU, which is itself controlled by a supervisory control system SCS, which also controls the lithographic apparatus via lithography control unit LACU. Thus, the different apparatuses can be operated to maximize throughput and processing efficiency.

Example Radiation Source [0075]An example of the radiation source SO for an example reflective lithographic apparatus (e.g., lithographic apparatus 100 of FIG. 1A) is shown in FIG. 4. As shown in FIG. 4, the radiation source SO is illustrated from a point of view (e.g., a top view) that is normal to the XY plane as described below. [0076]The radiation source SO shown in FIG. 4 is of a type which can be referred to as a laser produced plasma (EPP) source. A laser system 401, which can for example include a carbon dioxide (CO2) laser, is arranged to deposit energy via one or more laser beams 402 into fuel targets 403’, such as one or more discrete tin (Sn) droplets, which are provided from a fuel target generator 403 (e.g., example, fuel emitter, droplet generator). According to some aspects, laser system 401 can be, or can operate in the fashion of, a pulsed, continuous wave or quasi-continuous wave laser. The trajectory of fuel targets 403’ (e.g., example, droplets) emitted from the fuel target generator 403 can be parallel to an X-axis. According to some aspects, the one or more laser beams 402 propagate in a direction parallel to a Y-axis, which is perpendicular to the X-axis. A Z-axis is perpendicular to both the X-axis and the Y-axis and extends generally into (or out of) the plane of the page, but in other aspects, other configurations are used. In some embodiments, the laser beams 402 can propagate in a direction other than parallel to the Y-axis (e.g., in a direction other than orthogonal to the X-axis direction of the trajectory of the fuel targets 403’). [0077]Although tin is referred to in the following description, any suitable target material can be used. The target material can for example be in liquid form, and can for example be a metal or alloy. Fuel target generator 403 can include a nozzle configured to direct tin, e.g., in the form of fuel targets 403’ (e.g., discrete droplets) along a trajectory towards a plasma formation region 404. Throughout the WO 2022/002560 PCT/EP2021/065676 15remainder of the description, references to "fuel", "fuel target" or "fuel droplet" are to be understood as referring to the target material (e.g., droplets) emitted by fuel target generator 403. Fuel target generator 403 can include a fuel emitter. The one or more laser beams 402 are incident upon the target material (e.g., tin) at the plasma formation region 404. The deposition of laser energy into the target material creates a plasma 407 at the plasma formation region 404. Radiation, including EUV radiation, is emitted from the plasma 407 during de-excitation and recombination of ions and electrons of the plasma. [0078]The EUV radiation is collected and focused by a radiation collector 405 (e.g., radiation collector CO). In some aspects, radiation collector 405 can include a near normal-incidence radiation collector (sometimes referred to more generally as a normal-incidence radiation collector). The radiation collector 405 can be a multilayer structure, which is arranged to reflect EUV radiation (e.g., EUV radiation having a desired wavelength such as about 13.5 nm). According to some aspects, radiation collector 405 can have an ellipsoidal configuration, having two focal points. A first focal point can be at the plasma formation region 404, and a second focal point can be at an intermediate focus 406, as discussed herein. [0079]In some aspects, laser system 401 can be located at a relatively long distance from the radiation source SO. Where this is the case, the one or more laser beams 402 can be passed from laser system 401 to the radiation source SO with the aid of a beam delivery system (not shown) including, for example, suitable directing mirrors and/or a beam expander, and/or other optics. Laser system 401 and the radiation source SO can together be considered to be a radiation system. [0080]Radiation that is reflected by radiation collector 405 forms a radiation beam B. The radiation beam B is focused at a point (e.g., the intermediate focus 406) to form an image of plasma formation region 404, which acts as a virtual radiation source for the illumination system IL. The point at which the radiation beam B is focused can be referred to as the intermediate focus (IF) (e.g., intermediate focus 406). The radiation source SO is arranged such that the intermediate focus 406 is located at or near to an opening 408 in an enclosing structure 409 of the radiation source SO. [0081]The radiation beam B passes from the radiation source SO into the illumination system IL, which is configured to condition the radiation beam B. The radiation beam B passes from the illumination system IL and is incident upon the patterning device MA held by the support structure MT. The patterning device MA reflects and patterns the radiation beam B. Following reflection from the patterning device MA the patterned radiation beam B enters the projection system PS. The projection system includes a plurality of minors, which are configured to project the radiation beam B onto a substrate W held by the substrate table WT. The projection system PS can apply a reduction factor to the radiation beam, forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of four can be applied. Although the projection system PS is shown as having two minors in FIG. 2, the projection system can include any number of mirrors (e.g., six minors).

WO 2022/002560 PCT/EP2021/065676 16 [0082]The radiation source SO can also include components which are not illustrated in FIG. 4. For example, a spectral filter can be provided in the radiation source SO. The spectral filter can be substantially transmissive for EUV radiation but substantially blocking for other wavelengths of radiation such as infrared radiation. [0083]The radiation source SO (or radiation system) can further include a fuel target imaging system to obtain images of fuel targets (e.g., droplets) in the plasma formation region 404 or, more particularly, to obtain images of shadows of the fuel targets. The fuel target imaging system can detect light diffracted from the edges of the fuel targets. References to images of the fuel targets in the following text should be understood also to refer to images of shadows of the fuel targets or diffraction patterns caused by the fuel targets. [0084]The fuel target imaging system can include a photodetector such as a CCD array or a CMOS sensor, but it will be appreciated that any imaging device suitable for obtaining images of the fuel targets can be used. It will be appreciated that the fuel target imaging system can include optical components, such as one or more lenses, in addition to a photodetector. For example, the fuel target imaging system can include a camera 410, e.g., a combination of a photosensor (or: photodetector) and one or more lenses. The optical components can be selected so that the photosensor or camera 410 obtains near-Held images and/or far-field images. The camera 410 can be positioned within the radiation source SO at any appropriate location from which the camera has a line of sight to the plasma formation region 404 and one or more markers (not shown in Fig. 4) provided on the radiation collector 405. In some aspects, however, it can be necessary to position the camera 410 away from the propagation path of the one or more laser beams 402 and from the trajectory of the fuel targets emitted from fuel target generator 4so as to avoid damage to the camera 410. According to some aspects, the camera 410 is configured to provide images of the fuel targets to a controller 411 via a connection 412. The connection 412 is shown as a wired connection, though it will be appreciated that the connection 412 (and other connections referred to herein) can be implemented as either a wired connection or a wireless connection or a combination thereof. [0085]As shown in FIG. 4, the radiation source SO can include a fuel target generator 403 configured to generate and emit fuel targets 403’ (e.g., discrete tin droplets) towards a plasma formation region 404. The radiation source SO can further include a laser system 401 configured to hit one or more of the fuel targets 403’ with one or more laser beams 402 for generating a plasma 407 at the plasma formation region 404. The radiation source SO can further include a radiation collector 405 (e.g., a radiation collector CO) configured to collect radiation emitted by the plasma 407. [0086]An example of the metrology systems and windows disposed in the radiation source SO for an example reflective lithographic apparatus is shown in FIGS. 5-7.

WO 2022/002560 PCT/EP2021/065676 17Example Metrology System and Window [0087]FIG. 5 illustrates an isometric view 500 of an example enclosing structure 502 (e.g., enclosing structure 220, enclosing structure 409) configured to maintain a vacuum environment as part of an example radiation source SO of an example reflective lithographic apparatus. The example enclosing structure 502 can be disposed adjacent to a radiation collector 506 (e.g., radiation collector CO shown in FIG. 2, radiation collector 405 shown in FIG. 4). For reference, the primary focus 504 of the radiation collector 506 is illustrated together with a Cartesian coordinate system that includes an X-axis, Y-axis, and Z-axis, although any suitable relative or universal coordinate system can be used. In some aspects, the example enclosing structure 502 includes an opening 508 that is associated with a fuel target generator (e.g., fuel target generator 403, a droplet generator DG) and an opening 509 that is associated with a fuel target receiver (e.g., a tin catch TC). [0088]As shown in FIG. 5, one or more example components can be mechanically connected (e.g., anchored or otherwise attached by one or more fasteners, clamps, adhesives, or a combination thereof) to an example enclosing structure 502 according to some aspects of the present disclosure. Example components that can be mechanically connected to the example enclosing structure 502 of the radiation source SO can include, but are not limited to: metrology system 510 and window 511; metrology system 512 and window 513; metrology system 514 and window 515; metrology system 516 and window 517; metrology system 518 and window 519; metrology system 520 and window 521; metrology system 5and window 523; metrology system 524 and window 525; metrology system 526 and window 527; any other suitable component, or any combination thereof. In some aspects, the primary focus 504 can be located at a distance of about 1 meter from a surface of one or more of the windows 511, 513, 515, 517, 519, 521, 523, 525, and 527. [0089]In some aspects, metrology system 510 can include a coarse droplet steering camera (CDSC), and metrology system 522 can include a fine droplet steering camera (FDSC). In some aspects, metrology system 512 can include a first droplet formation camera (DEC), and metrology system 5can include a second DEC. In some aspects, metrology system 514 can include a droplet detection module (DDM). In some aspects, metrology system 516 can include a line laser module (ELM). In some aspects, metrology system 518 can include a droplet illumination module (DIM). In some aspects, metrology system 524 can include a first illumination module such as a first backlight laser module (BEM), and metrology system 526 can include a second illumination module such as a second BEM. In some aspects, the metrology systems 524 and 526 (e.g., first and second BLMs) can be connected to the metrology systems 512 and 520 (e.g., a pair of DFCs). [0090]In some aspects, one or more of the windows 511, 513, 515, 517, 519, 521, 523, 525, and 5can be constructed and arranged as described with reference to window 640 shown in FIGS. 6A and 6B, window 740 shown in FIG. 7, the example fast swap window assembly 700 shown in FIG. 7, any other suitable window or window assembly, any structure or feature included therein, or any combination thereof.

WO 2022/002560 PCT/EP2021/065676 18 [0091]In some aspects, one or more of the metrology systems 510, 512, 514, 516, 518, 520, 522, 524, and 526 can be configured to be disposed in a first environment, such as an atmospheric environment located outside a sealed vessel such as example enclosing structure 502. In some aspects, one or more of the metrology systems 510, 512, 514, 516, 518, 520, 522, 524, and 526 can be configured to perform one or more measurements of a region in a second environment along an optical axis of the metrology system. In some aspects, the region can encompass, either partially or wholly, any suitable geometric region such as: a region inside the example enclosing structure that includes primary focus 504 of the radiation collector 506; the plasma formation region 404 shown in FIG. 4; the region 601 shown in FIGS. 6A and 6C; any other suitable region; or any combination thereof. In some aspects, the optical axis of the metrology system can be an optical axis such as optical axis 602 shown in FIGS. 6A and 6C. In some aspects, the second environment can be a vacuum environment, or a partial vacuum environment, located inside a sealed vessel such as example enclosing structure 502. In some aspects, one or more of the windows 511, 513, 515, 517, 519, 521, 523, 525, and 527 can be configured to be disposed intersecting the optical axis of a respective metrology system. In some aspects, one or more of the windows 511, 513, 515, 517,519,521,523,525, and 527 can be configured to isolate a respective metrology system from the second environment. [0092]In some aspects, one or more of the windows 511, 513, 515, 517, 519, 521, 523, 525, and 5can be configured to limit a transverse displacement (e.g., transverse focus error) to a transverse displacement tolerance of less than about ± 50 microns from a nominal transverse displacement from the optical axis (e.g., of the particular window’s respective metrology system) at the primary focus 5of the radiation collector 506. In some aspects, one or more of the windows 511, 513, 515, 517, 519, 521, 523, 525, and 527 can be configured to limit the transverse displacement to a transverse displacement tolerance of less than about ± 33 microns from the nominal transverse displacement from the optical axis at the primary focus 504 of the radiation collector 506. [0093]In some aspects, one or more of the windows 511, 513, 515, 517, 519, 521, 523, 525, and 5can be configured to limit an angular deviation along the optical axis (e.g., of the particular window’s respective metrology system) to an angular deviation tolerance of less than about ± 0.5 arcmin from a nominal angular deviation along the optical axis. In some aspects, one or more of the windows 511, 513, 515, 517, 519,521, 523, 525, and 527 can be configured to limit the angular deviation to an angular deviation tolerance of less than about ±0.1 arcmin from the nominal angular deviation along the optical axis. [0094]In some aspects, one or more of the windows 511, 513, 515, 517, 519, 521, 523, 525, and 5can be configured to limit a longitudinal displacement (e.g., axial focus error) to a longitudinal displacement tolerance of less than about + 330 microns from a nominal longitudinal displacement from the primary focus 504 along the optical axis (e.g., of the particular window’s respective metrology system). In some aspects, one or more of the windows 511, 513, 515, 517, 519, 521, 523, 525, and 5can be configured to limit the longitudinal displacement to a longitudinal displacement tolerance of less WO 2022/002560 PCT/EP2021/065676 19than about ± 200 microns from the nominal longitudinal displacement from the primary focus 504 along the optical axis. [0095]In some aspects, one or more of the metrology systems 510, 512, 514, 516, 518, 520, 522, 524, and 526 can be a modular metrology system. In some aspects, one or more of the windows 511, 513, 515, 517, 519, 521, 523, 525, and 527 can be configured to limit the transverse displacement to less than about ± 50 microns at a time at which the respective metrology system is installed in the radiation source SO. In some aspects, one or more of the windows 511, 513, 515, 517, 519, 521, 523, 525, and 527 can be configured to limit the transverse displacement to less than about ± 50 microns without a calibration action (e.g., without the performance of a separate calibration action). [0096]FIGS. 6A, 6B, 6C, and 6D are schematic illustrations of portions an example EUV radiation system according to some aspects of the present disclosure. FIG. 6A illustrates a schematic illustration of an example system 600 according to some aspects of the present disclosure. As shown in FIG. 6A, the example system 600 includes a metrology system 630 and a window 640. In some aspects, the metrology system 630 may be, or include, one or more of the metrology systems 510, 512, 514, 516, 518, 520, 522, 524, and 526 shown in FIG. 5. In some aspects, the window 640 may be, or include, one or more of the windows 511, 513, 515, 517, 519, 521, 523, 525, and 527 shown in FIG. 5. [0097]In some aspects, the metrology system 630 can be disposed in a first environment 680 (e.g., an atmospheric environment) located outside a sealed vessel (e.g., enclosing structure 220 shown in FIG. 2, enclosing structure 409 shown in FIG. 4, enclosing structure 502 shown in FIG. 5) and removably attached (e.g., mechanically connected, anchored or otherwise attached by one or more fasteners, clamps, adhesives, or a combination thereof) to the window 640. In some aspects, the window 640 can be removably attached to a sealed vessel (e.g., enclosing structure 220 shown in FIG. 2, enclosing structure 409 shown in FIG. 4, enclosing structure 502 shown in FIG. 5) configured to maintain a second environment 682 (e.g., a vacuum environment, a partial vacuum environment) as part of an example radiation source SO of an example reflective lithographic apparatus. [0098]For reference, FIG. 6A illustrates the primary focus 604 of a radiation collector (e.g., radiation collector CO shown in FIG. 2, radiation collector 405 shown in FIG. 4, radiation collector 506 shown in FIG. 5) together with an optical axis 602 of the metrology system 630. In some aspects, the primary focus 604 can be located at a distance of about 1 meter from a surface of the window 640. For example, the primary focus 604 can be located at a distance of about 1 meter from the surface 648b (shown in FIG. 6B) of the viewport 648. [0099]In some aspects, the metrology system 630 can be configured to perform one or more measurements of a region 601 in a second environment 682 along an optical axis 602 of the metrology system 630. In some aspects, the region 601 can encompass, either partially or wholly, any suitable geometric region such as: a region inside the example enclosing structure that includes primary focus 604 of the radiation collector; the plasma formation region 404 shown in FIG. 4; any other suitable region; or any combination thereof. In some aspects, the second environment 682 can be a vacuum WO 2022/002560 PCT/EP2021/065676 20environment, or a partial vacuum environment, located inside the sealed vessel. In some aspects, the window 640 can be configured to be disposed intersecting the optical axis 602 of the metrology system 630. [0100]In some aspects, the window 640 can include a base structure 642, a viewport mounting structure 644, a viewport 648, a pellicle mounting structure 652, a pellicle 650, a radiation shield structure 646 (e.g., a light shield), any other suitable component or structure, or any combination thereof. In some aspects, the window 640 is described in further detail with reference to FIG. 6B. [0101]As shown in FIG.6B, the base structure 642 of the window 640 can include an O-ring 6configured to be removably attached to an outer surface of the sealed vessel. The base structure 642 can include an O-ring 660 configured to be removably attached to a surface 648b (e.g., an inner surface) of the viewport 648. The viewport mounting structure 644 can include an O-ring 661 configured to be removably attached to a surface 648a (e.g., an outer surface) of the viewport 648. In some aspects, the radiation shield structure 646 and the viewport mounting structure 644 can be configured to be attached to the base structure 642 using fasteners (e.g., eight hexalobular socket flat head machine screws). [0102]In some aspects, the viewport 648 can include anti-reflection (AR) coated optical glass such as borosilicate crown glass having a transmission range of about 350 nanometers to about 2.5 microns and a refractive index of about 1.51680 at 587.5618 nanometers (e.g., yellow helium line). In some aspects, the pellicle 650 can include optical glass that is the same as, or different from, the optical glass included in the viewport 648. [0103]In some aspects, the window 640 can be configured to isolate the metrology system 630 from the second environment 682. For example, O-ring 661, O-ring 660, and O-ring 664 can separate the first environment 680 from the second environment 682. In some aspects, the window 640 can include a flow channel 668 configured to extend the second environment 682 to the volume disposed between the surface 650a and the surface 648b. [0104]In some aspects, the window 640 can include a wedge angle of less than ± 5.0 arcsec, or about ±0.1 arcmin, from a nominal wedge angle. In some aspects, the nominal wedge angle can be about zero degrees. For instance, the viewport 648, the pellicle 650, or both can have a wedge angle of less than + 5.0 arcsec from a nominal wedge angle of zero degrees. In one illustrative example, the nominal wedge angle between surface 648a and surface 648b can be about zero degrees, and the wedge angle between surface 648a and surface 648b can be less than about - 5.0 arcsec and about 5.0 arcsec. In another illustrative example, the nominal wedge angle between surface 650a and surface 650b can be about zero degrees, and the wedge angle between surface 650a and surface 650b can be between about - 5.0 arcsec and about 5.0 arcsec. [0105]In other aspects, the nominal wedge angle can be greater than about zero degrees. For instance, the viewport 648, the pellicle 650, or both can have a wedge angle of less than + 5.0 arcsec from a nominal wedge angle greater than about zero degrees (e.g., about 58 arcmin, 1 degree 56 arcmin, degrees 52 arcmin, or any other suitable wedge angle). In one illustrative example, the nominal wedge WO 2022/002560 PCT/EP2021/065676 21angle between surface 648a and surface 648b can be about 3,480 arcsec, and the wedge angle between surface 648a and surface 648b can be between about 3,475 arcsec and about 3,485 arcsec. In another illustrative example, the nominal wedge angle between surface 650a and surface 650b can be 6,9arcsec, and the wedge angle between surface 650a and surface 650b can be between about 6,955 arcsec and about 6,965 arcsec. [0106]FIG. 6C illustrates region 601 in greater detail. It is to be understood that region 601 is not necessarily drawn to scale and further that linear, two-dimensional depictions shown in FIG. 6C can in fact refer to non-linear aspects, three-dimensional aspects, any other suitable aspects, or a combination thereof. [0107]As shown in FIG. 6C, region 601 can include a primary focus 604 of a radiation collector. The primary focus 604 can be disposed along the optical axis 602 of the metrology system 630. FIG. 6C also illustrates an axis 603 transverse to the primary focus 604 and orthogonal (e.g., perpendicular) to the optical axis 602. [0108]As further shown in FIG. 6C, region 601 can include a nominal displaced focal point 606 (e.g., a predicted, estimated, planned, or designed focus error) of the window 640 (e.g., in an instance in which the window 640 is not a perfect window). The nominal displaced focal point 606 can be disposed along a nominal displaced optical axis 605 (e.g., a predicted, estimated, planned, or designed optical axis) of the window 640. As used herein, the term "nominal" can refer to a predicted, estimated, planned, or designed value, measurement, location, geometry, or other suitable characteristic. [0109]In some aspects, the nominal displaced focal point 606 can have a nominal transverse displacement 610 (e.g., a predicted, estimated, planned, or designed transverse focus error) from the optical axis 602 at the primary focus 604 of the radiation collector. In one illustrative example, the nominal transverse displacement 610 can be about 1 millimeter. In some aspects, the nominal displaced focal point 606 can have a nominal longitudinal displacement 611 (e.g., a predicted, estimated, planned, or designed axial focus error) from the primary focus 604 along the optical axis 602. In some aspects, the nominal displaced focal point 606 can have a nominal angular deviation 618 (e.g., a predicted, estimated, planned, or designed nominal angular deviation) from the optical axis 602. [0110]In some aspects, the nominal displaced focal point 606 can be corrected by an initial metrology module alignment process and, as a result, the nominal displaced focal point 606 can be co-incident with the primary focus 604. As a result of the initial metrology module alignment process, the nominal transverse displacement 610 can be about zero microns, the nominal longitudinal displacement 611 can be about zero microns, and the nominal angular deviation 618 can be about zero degrees. [0111]As further shown in FIG. 6C, region 601 can include a displaced focal point 608 (e.g., an actual focus error) of the window 640. The displaced focal point 608 can be disposed along a displaced optical axis 607 (e.g., an actual optical axis) of the window 640. [0112]In some aspects, the displaced focal point 608 can have a transverse displacement 612 (e.g., an actual transverse focus error) from the optical axis 602 at the primary focus 604 of the radiation WO 2022/002560 PCT/EP2021/065676 22collector. In some aspects, the displaced focal point 608 can have a longitudinal displacement 614 (e.g., an actual axial focus error) from the primary focus 604 along the optical axis 602. In some aspects, the displaced focal point 608 can have an angular deviation 619 (e.g., an actual angular deviation) from the optical axis 602. [0113]In some aspects, the displaced focal point 608 can have an actual-to-nominal transverse displacement 613 from the nominal displaced focal point 606 that is disposed within a transverse displacement tolerance 616 from the nominal displaced focal point 606. In some aspects, the transverse displacement tolerance 616 can be less than about ± 50 microns, ± 33 microns, or any other suitable tolerance. [0114]In some aspects, the displaced focal point 608 can have an actual-to-nominal longitudinal displacement 615 from the nominal displaced focal point 606 that is disposed within a longitudinal displacement tolerance 617 from the nominal displaced focal point 606. In some aspects, the longitudinal displacement tolerance 617 can be less than about ± 330 microns, ± 200 microns, or any other suitable tolerance. [0115]In some aspects, the displaced focal point 608 can have an actual-to-nominal angular deviation 620 from the nominal displaced focal point 606 that is disposed within an angular deviation tolerance 621 from the nominal displaced focal point 606. In some aspects, the angular deviation tolerance 6can be less than about ± 0.5 arcmin, ±0.1 arcmin, + 5 arcsec, or any other suitable tolerance. [0116]In some aspects, the window 640 can be configured to limit the transverse displacement 612 to a transverse displacement tolerance 616 of less than about + 50 microns from the nominal transverse displacement 610 from the optical axis 602 at the primary focus 604 of the radiation collector. In some aspects, the window 640 can be configured to limit the transverse displacement 612 to a transverse displacement tolerance 616 of less than about + 33 microns from the nominal transverse displacement 610 from the optical axis 602 at the primary focus 604 of the radiation collector. In other words, the window 640 can be configured to limit the actual-to-nominal transverse displacement 613 to less than about + 50 microns, + 33 microns, or any other suitable tolerance. [0117]In some aspects, the window 640 can be configured to limit the longitudinal displacement 6to a longitudinal displacement tolerance 617 of less than about + 330 microns from the nominal longitudinal displacement 611 from the primary focus 604 along the optical axis 602. In some aspects, the window 640 can be configured to limit the longitudinal displacement 614 to a longitudinal displacement tolerance 617 of less than about + 200 microns from the nominal longitudinal displacement 611 from the primary focus 604 along the optical axis 602. In other words, the window 640 can be configured to limit the actual-to-nominal longitudinal displacement 615 to less than about + 330 microns, + 200 microns, or any other suitable tolerance. [0118]In some aspects, the window 640 can be configured to limit the angular deviation 619 along the optical axis 602 to an angular deviation tolerance 621 of less than about + 0.5 arcmin from the nominal angular deviation 618 along the optical axis 602. In some aspects, the window 640 can be configured WO 2022/002560 PCT/EP2021/065676 23to limit the angular deviation 619 to an angular deviation tolerance 621 of less than about ±0.1 arcmin from the nominal angular deviation 618 along the optical axis 602. [0119]In other words, the window 640 can be configured to limit the actual-to-nominal angular deviation 620 to less than about ± 0.5 arcmin, ±0.1 arcmin, ± 5 arcsec, or any other suitable tolerance. [0120]In some aspects, the metrology system 630 can be a modular metrology system. In some aspects, the window 640 can be configured to limit the transverse displacement 612 to less than about ± 50 microns from the nominal transverse displacement 610 from the optical axis 602 at the primary focus 604 of the radiation collector at a time at which the metrology system 630 is installed in the radiation source SO. In some aspects, the window 640 can be configured to limit the transverse displacement 612 to less than about ± 50 microns from the nominal transverse displacement 610 from the optical axis 602 at the primary focus 604 of the radiation collector without a calibration action (e.g., without the performance of a separate calibration action beyond an initial metrology module alignment process to adjust the nominal displaced focal point 606). [0121]As shown in FIG. 6D, the viewport 648 can be configured to be disposed intersecting the optical axis 602. In some aspects, the pellicle 650 can be configured to be disposed intersecting the optical axis 602 and opposite the viewport 648 (e.g., at an angle of about zero degrees, greater than about zero degrees, or less than about zero degrees). For instance, the viewport 648 can have a viewport axis 690, the pellicle 650 can have a pellicle axis 692, and the angle 691 between the viewport axis 690 and the pellicle axis 692 can be greater than zero degrees (e.g., about 4.5 degrees) to decrease or prevent back reflection. id="p-122" id="p-122" id="p-122" id="p-122" id="p-122" id="p-122"

[0122]Example Fast Swap Window Assembly [0123]FIGS. 7A, 7B, and 7C are schematic illustrations of an example fast swap window assembly 700 according to some aspects of the present disclosure. As shown in FIG. 7A, the example fast swap window assembly 700 can include a fast swap window 740 and a fast swap window frame 770 (e.g., a securing mechanism). In some aspects, as depicted in FIG. 7A, the example fast swap window assembly 700 can include a plurality of fasteners, such as fasteners, pins, clips, rotating arms, and other such structures, which are not labeled in FIG. 7A for the sake of brevity. [0124]In some aspects, the fast swap window 740 can include a base structure 742, a viewport mounting structure 744, a viewport 748 (e.g., an "optical flat" quality substrate), a pellicle mounting structure (not shown), a pellicle (not shown; e.g., angled with respect to the viewport 748 to prevent back reflection), a radiation shield structure (not shown) any other suitable component or structure, or any combination thereof. In some aspects, the fast swap window 740 can include one or more structures described with reference to windows 511, 513, 515, 517, 519, 521, 523, 525, and 527 shown in FIG. and window 640 shown in FIGS. 6A and 6B. In some aspects, one or more fast swap mounting structures, such as ball bearing 743a and ball bearing 743b, can be attached to the base structure 7for use in installation, alignment, and removal of the example fast swap window assembly 700.

WO 2022/002560 PCT/EP2021/065676 24 [0125]In some aspects, the fast swap window frame 770 can include a frame structure 772 that can be attached to the fast swap window 740 (e.g., to the base structure 742). In some aspects, one or more fast swap mounting structures, such as ball bearing 774a, ball bearing 774b, ball bearing 774c, and ball bearing 774d, can be attached to the frame structure 772 for use in installation, alignment, and removal of the example fast swap window assembly 700. In some aspects, the fast swap window frame 770 can include a receiving structure 776 configured to receive an installation and removal tool 790 (shown in FIG. 7B). In some aspects, the fast swap window frame 770 can be a built in securing mechanism that assures a consistent orientation of the fast swap window 740 with respect to a reference surface on the radiation source vessel. In some aspects, the fast swap window frame 770 can use an "over center" cam to provide positive engagement and vacuum tightness. In some aspects, the fast swap window frame 770 can be configured such that no scrubbing of the vacuum seal O-ring occurs when the vacuum seal is made with the radiation source vessel. [0126]As shown in FIG. 7B, the example fast swap window assembly 700 can be installed and removed by a motion 792 of an installation and removal tool 790. In some aspects, the fixed part of the example fast swap window assembly 700 (e.g., the base structure 742 of the fast swap window 740) can include bearings (e.g., ball bearing 743a, ball bearing 743b) that act as a travel stop when the fast swap window 740 is inserted into its pocket on the radiation source vessel 702. In some aspects, the example fast swap window assembly 700 can be configured such that no additional translation (e.g., orientation of sealing O-ring versus sealing surface) can occur. In some aspects, actuating the installation and removal mechanism provides only a motion that squeezes the sealing ring. [0127]As shown in FIG. 7C, the example fast swap window assembly 700 can be installed and removed from its pocket on the radiation source vessel 702 by a motion 794. During installation and removal, the metrology system 796 (e.g., metrology system 522 shown in FIG. 5) can remain secured to the radiation source vessel 702. [0128]In some examples, retention methodologies can be: (i) loose metal seal with multiple fixing screws; (ii) loose elastomer seal with multiple fixing screws; and (iii) loose elastomer seal with supplemental loose clamp ring. Furthermore, vacuum viewports may include a glass and metal bond that imparts stress and therefore deformations in the window. In addition, pellicles have a finite lifetime. For example, tin debris from EUV plasma accumulates (via both vapor and ballistic particles) on the pellicle, obscuring the ability of metrology systems to view regions of interest (e.g., region 601 shown in FIGS. 6A and 6C). The pellicle is present to protect viewport windows from contamination and resulting thermal stress that historically caused window breakage and loss of vacuum (e.g., a system down condition). Orientation of pellicle with respect to the viewport is somewhat random. [0129]In some aspects, the example fast swap window assembly 700 combines the pellicle into a precision housing with the optics secured via O-rings, which can allow for the control and minimization of optical distortions. In some aspects, by including an "over-center" camming WO 2022/002560 PCT/EP2021/065676 25mechanism, installation and removal can be accomplished in seconds. Only single arm access may be required to actuate the installation and removal mechanism. Viewport removal may also require single arm reach. [0130]In some aspects, the example fast swap window assembly 700 provides a viewport 748 having excellent optical properties (e.g., wavefront error) and optimized installation and removal features that maximize availability of the EUV radiation source. In some aspects, the example fast swap window assembly 700 provides a sacrificial window (referred to as a pellicle) in the same optical path. Accordingly, the example fast swap window assembly 700 provides an optimized viewport that encompasses both a vacuum window and pellicle in concert with a fast installation and removal capability. [0131]The example fast swap window assembly 700 can meet extreme requirements for availability of the EUV radiation system and enable all service actions to be accomplished quickly. In other words, the fast swap time supports the needs of extreme availability requirements with regard to system uptime and availability. In another example, the use of elastomer seals as opposed to glass and metal solder promotes lower optical glass distortion and lower residual stresses in the glass. As a result, there can be a lower chance of fracture, loss of vacuum in the radiation source, and long down time for recovery (e.g., B-time).

Example Processes for Optical Metrology [0132]FIG. 8 is an example method 800 for optical metrology in a radiation system (e.g., an EUV radiation system, such as an example radiation source SO shown in FIGS. 1A, 2, and 4) according to some aspects of the present disclosure or portion(s) thereof. The operations described with reference to example method 800 can be performed by, or according to, any of the systems, apparatuses, components, techniques, or combinations thereof described herein, such as those described with reference to FIGS. 1-7 above. [0133]At operation 802, the method can include disposing a metrology system (e.g., metrology system 510, 512, 514, 516, 518, 520, 522, 524, or 526 shown in FIG. 5; metrology system 630 shown in FIG. 6A) in a first environment (e.g., an atmospheric environment such as first environment 680 shown in FIGS. 6A and 6B). The metrology system performs one or more measurements of a region (e.g., plasma formation region 404 shown in FIG. 4; region 601 shown in FIGS. 6A and 6C) in a second environment (e.g., a vacuum or partial-vacuum environment such as second environment 682 shown in FIGS. 6A and 6B) along an optical axis (e.g., optical axis 602 shown in FIGS. 6A and 6C) of the metrology system, the second environment being different from the first environment. In some aspects, the disposing of the metrology system can be accomplished using suitable mechanical or other methods and include disposing the metrology system in accordance with any aspect or combination of aspects described with reference to FIGS. 1-7 above.

WO 2022/002560 PCT/EP2021/065676 26 [0134]At operation 804, the method can include isolating the metrology system from the second environment using a window (e.g., window 511, 513, 515, 517, 519, 521, 523, 525, or 527 shown in FIG. 5; window 640 shown in FIGS. 6A and 6B; fast swap window 740 shown in FIG. 7A) that is disposed to intersect the optical axis. In some aspects, the isolating of the metrology system from the second environment can be performed based on a vacuum seal or a partial vacuum seal provided by the window. In some aspects, the isolating of the metrology system can be accomplished using suitable mechanical or other methods and include isolating the metrology system in accordance with any aspect or combination of aspects described with reference to FIGS. 1-7 above. [0135]At operation 806, the method can include limiting, based on a disposition of the window, a transverse displacement (e.g., transverse displacement 612 shown in FIG. 6C) from the optical axis to less than about ± 50 microns from a nominal transverse displacement (e.g., nominal transverse displacement 610 shown in FIG. 6C) from the optical axis at a primary focus (e.g., primary focus 5shown in FIG. 5; primary focus 604 shown in FIGS. 6A and 6C) of a radiation collector (e.g., radiation collector CO shown in FIG. 2, radiation collector 405 shown in FIG. 4, radiation collector 506 shown in FIG. 5). In some aspects, the limiting of the transverse displacement can be accomplished using suitable mechanical or other methods and include limiting the transverse displacement in accordance with any aspect or combination of aspects described with reference to FIGS. 1-7 above. [0136]Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatuses described herein can have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, LCDs, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms "wafer" or "die" herein may be considered as synonymous with the more general terms "substrate" or "target portion", respectively. The substrate referred to herein can be processed, before or after exposure, in for example a track unit (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology unit and/or an inspection unit. Where applicable, the disclosure herein can be applied to such and other substrate processing tools. Further, the substrate can be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers. [0137]It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present specification is to be interpreted by those skilled in relevant art(s) in light of the teachings herein. [0138]The term "substrate" as used herein describes a material onto which material layers are added. In some aspects, the substrate itself can be patterned and materials added on top of it can also be patterned, or can remain without patterning. [0139]The examples disclosed herein are illustrative, but not limiting, of the embodiments of this disclosure. Other suitable modifications and adaptations of the variety of conditions and parameters WO 2022/002560 PCT/EP2021/065676 27normally encountered in the field, and which would be apparent to those skilled in the relevant art(s), are within the spirit and scope of the disclosure. [0140]While specific aspects of the disclosure have been described above, it will be appreciated that the aspects can be practiced otherwise than as described. The description is not intended to limit the embodiments of the disclosure. [0141]It is to be appreciated that the Detailed Description section, and not the Background, Summary, and Abstract sections, is intended to be used to interpret the claims. The Summary and Abstract sections may set forth one or more but not all example embodiments as contemplated by the inventor(s), and thus, are not intended to limit the present embodiments and the appended claims in any way. [0142]Some aspects of the disclosure have been described above with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed. [0143]The foregoing description of the specific aspects of the disclosure will so fully reveal the general nature of the aspects that others can, by applying knowledge within the skill of the art, readily modify and/or adapt for various applications such specific aspects, without undue experimentation, without departing from the general concept of the present disclosure. Therefore, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed aspects, based on the teaching and guidance presented herein. [0144]Other aspects of the invention are set out in the following numbered clauses.1. A system comprising:a metrology system configured to be disposed in a first environment and to perform one or more measurements of a region in a second environment along an optical axis of the metrology system, wherein the second environment is different from the first environment; anda window configured to be disposed intersecting the optical axis and to:isolate the metrology system from the second environment; andlimit a transverse displacement from the optical axis to less than about ± 50 microns from a nominal transverse displacement from the optical axis at a primary focus of a radiation collector.2. The system of clause 1, wherein the primary focus is located at a distance of about 1 meter fromthe window.3. The system of clause 1, wherein the window is configured to limit the transverse displacement to less than about ± 33 microns.4. The system of clause 1, wherein the window is configured to limit an angular deviation along the optical axis to less than about ± 0.5 arcmin from a nominal angular deviation along the optical axis. 5. The system of clause 4, wherein the window is configured to limit the angular deviation to less than about ±0.1 arcmin.

WO 2022/002560 PCT/EP2021/065676 286. The system of clause 1, wherein the window is configured to limit a longitudinal displacement to less than about ± 330 microns from a nominal longitudinal displacement from the primary focus along the optical axis.ר. The system of clause 6, wherein the window is configured to limit the longitudinal displacement to less than about ± 200 microns.8. The system of clause 1, wherein the window comprises:a first part configured to be disposed intersecting the optical axis; anda second part configured to be disposed intersecting the optical axis and opposite the first part.9. The system of clause 8, wherein:the first part comprises a viewport; andthe second part comprises a pellicle.10. The system of clause 1, wherein the window comprises a wedge angle of less than about ±0.arcmin from a nominal wedge angle.11. The system of clause 10, wherein the nominal wedge angle is about zero degrees.12. The system of clause 10, wherein the nominal wedge angle is greater than about zero degrees.13. The system of clause 1, wherein the metrology system is a modular metrology system.14. The system of clause 1, wherein the window is configured to limit the displacement to less than about + 50 microns at a time at which the metrology system is installed in the system.15. The system of clause 1, wherein the window is configured to limit the displacement to less than about + 50 microns without a calibration action.16. A window comprising:a first part configured to be disposed intersecting an optical axis; anda second part configured to be disposed intersecting the optical axis and opposite the first part, wherein the window is configured to:transmit radiation along the optical axis through the first part and the second part; andlimit a transverse displacement from the optical axis to less than about + 50 microns from a nominal transverse displacement from the optical axis at a primary focus of a radiation collector.17. The window of clause 16, wherein the primary focus is located at a distance of about 1 meter from the window.18. The window of clause 16, wherein the first part comprises a viewport, and wherein the second part comprises a pellicle.19. The window of clause 16, wherein the window comprises a wedge angle of less than about + 0.1 arcmin from a nominal wedge angle.20. A method comprising:disposing a metrology system in a first environment, wherein the metrology system performs one or more measurements of a region in a second environment along an optical axis of the metrology system, the second environment being different from the first environment;

Claims

1.Company Secret

2.CLAIMS: 1. A system comprising: a metrology system configured to be disposed in a first environment and to perform one or more measurements of a region in a second environment along an optical axis of the metrology system, wherein the second environment is different from the first environment; and a window configured to be disposed intersecting the optical axis and to: isolate the metrology system from the second environment; and limit a transverse displacement from the optical axis to less than about ± 50 microns from a nominal transverse displacement from the optical axis at a primary focus of a radiation collector. 2. The system of claim 1, wherein the primary focus is located at a distance of about 1 meter from the window.

3. The system of claim 1, wherein the window is configured to limit an angular deviation along the optical axis to less than about ± 0.5 arcmin from a nominal angular deviation along the optical axis.

4. The system of claim 1, wherein the window is configured to limit a longitudinal displacement to less than about ± 330 microns from a nominal longitudinal displacement from the primary focus along the optical axis.

5. The system of claim 1, wherein the window comprises: a first part configured to be disposed intersecting the optical axis; and a second part configured to be disposed intersecting the optical axis and opposite the first part.

6. The system of claim 5, wherein: the first part comprises a viewport; and the second part comprises a pellicle.

7. The system of claim 1, wherein the window comprises a wedge angle of less than about ± 0.arcmin from a nominal wedge angle.

8. The system of claim 7, wherein the nominal wedge angle is greater than about zero degrees. 2019P00398WO Company Secret

9. The system of claim 1, wherein the metrology system is a modular metrology system.

10. The system of claim 1, wherein the window is configured to limit the displacement to less than about ± 50 microns at a time at which the metrology system is installed in the system.

11. A window comprising: a first part configured to be disposed intersecting an optical axis; and a second part configured to be disposed intersecting the optical axis and opposite the first part, wherein the window is configured to: transmit radiation along the optical axis through the first part and the second part; and limit a transverse displacement from the optical axis to less than about ± 50 microns from a nominal transverse displacement from the optical axis at a primary focus of a radiation collector.

12. The window of claim 11, wherein the primary focus is located at a distance of about 1 meter from the window.

13. The window of claim 11, wherein the first part comprises a viewport, and wherein the second part comprises a pellicle.

14. The window of claim 11, wherein the window comprises a wedge angle of less than about ± 0.1 arcmin from a nominal wedge angle.

15. A method comprising: disposing a metrology system in a first environment, wherein the metrology system performs one or more measurements of a region in a second environment along an optical axis of the metrology system, the second environment being different from the first environment; isolating the metrology system from the second environment using a window that is disposed to intersect the optical axis; and limiting, based on a disposition of the window, a transverse displacement from the optical axis to less than about ± 50 microns from a nominal transverse displacement from the optical axis at a primary focus of a radiation collector.