JP7750876B2 - Precision vacuum window viewports and pellicles for rapid metrology recovery - Google Patents

Description

CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Patent Application No. 63/046,984, entitled "PRECISE VACUUM WINDOW VIEWPORTS AND PELLICLES FOR RAPID METROLOGY RECOVERY," filed July 1, 2020, which is incorporated herein by reference in its entirety.

[0002] This disclosure relates to metrology systems and windows for extreme ultraviolet (EUV) radiation systems.

[0003] A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually a target portion of the substrate. Lithographic apparatus may be used, for example, in the manufacture of integrated circuits (ICs). Patterning devices, referred to interchangeably as masks or reticles, may be used to generate the circuit pattern that will be formed on an individual layer of the IC being formed. This pattern may be transferred onto a target portion (e.g., comprising part of, one, or several dies) on the 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. Typically, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers and scanners: in a stepper, each target portion is irradiated by exposing the entire pattern onto the target portion at once, and in a scanner, each target portion is irradiated by scanning the radiation beam in a given direction (the "scan" direction) while simultaneously scanning the target portion parallel or anti-parallel (e.g., opposite) to the scan direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.

[0004] Electromagnetic radiation with wavelengths of about 50 nanometers (nm) or less (sometimes referred to as soft x-rays), including extreme ultraviolet (EUV) light, e.g., light with a wavelength of about 13 nm, can be used in or with lithography apparatus to create very small features in or on substrates, e.g., silicon wafers. Methods for producing EUV light include, but are not necessarily limited to, converting materials having elements with emission lines in the EUV range, such as xenon (Xe), lithium (Li), or tin (Sn), into a plasma state. For example, in one such method, called laser-produced plasma (LPP), plasma can be generated by irradiating a target material (interchangeably referred to as a fuel in the context of an LPP source), e.g., in the form of droplets, plates, tapes, streams, or clusters of material, with an amplified light beam, which may be referred to as a drive laser. For this process, the plasma is typically generated in a sealed vessel, e.g., a vacuum chamber, and monitored using various types of metrology instruments.

[0005] This disclosure describes various aspects of systems, apparatus, and methods for optical metrology and various other aspects in extreme ultraviolet (EUV) radiation systems.

[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 may include a metrology system configured to be disposed in a first environment. The metrology system may be further configured to perform one or more measurements of an area in a second environment along an optical axis of the metrology system. The second environment may be different from the first environment. A window may be configured to be disposed across the optical axis. The window may be further configured to isolate the metrology system from the second environment. The window may be further configured to limit lateral displacement from the optical axis to less than about ±50 microns from a nominal lateral displacement from the optical axis at a first focal point of the radiation collector. In some aspects, the first focal point may 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 lateral displacement to less than approximately ±33 microns. In some aspects, the window can be configured to limit angular deviation along the optical axis to less than approximately ±0.5 arc minutes from a nominal angular deviation along the optical axis. In some aspects, the window can be configured to limit angular deviation to less than approximately ±0.1 arc minutes. In some aspects, the window can be configured to limit longitudinal displacement to less than approximately ±330 microns from a nominal longitudinal displacement from the first focal point along the optical axis. In some aspects, the window can be configured to limit longitudinal displacement to less than approximately ±200 microns.

[0008] In some aspects, the window may include a first portion (e.g., a viewport) configured to be positioned across the optical axis. In some aspects, the window may further include a second portion (e.g., a pellicle) configured to be positioned across the optical axis and opposite the first portion. In some aspects, the window may include a wedge angle less than about ±0.1 arc minutes from the nominal wedge angle. In some aspects, the nominal wedge angle may be about 0 degrees. In other aspects, the nominal wedge angle may be greater than about 0 degrees.

[0009] In some aspects, the metrology system may be modular. In some aspects, the window may be configured to limit displacement to less than about ±50 microns once the metrology system is installed in the system. In some aspects, the window may be configured to limit displacement to less than about ±50 microns without a calibration operation.

[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 may include a first portion (e.g., a viewport) configured to be positioned across an optical axis. The apparatus may further include a second portion (e.g., a pellicle) configured to be positioned across the optical axis and opposite the first portion. The apparatus may be configured to transmit radiation through the first portion and the second portion along the optical axis. The apparatus may be further configured to limit lateral displacement from the optical axis to less than about ±50 microns from a nominal lateral displacement from the optical axis at a first focus of the radiation collector.

[0011] In some embodiments, the first focal point can be located at a distance of about 1 meter from the surface of the device. In some embodiments, the first portion can include a viewport. In some embodiments, the second portion can include a pellicle. In some embodiments, the device can include a wedge angle that is less than about ±0.1 arc minutes from the nominal wedge angle. In some embodiments, the device 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 may include positioning a metrology system in a first environment. The metrology system performs one or more measurements of an area in a second environment along an optical axis of the metrology system, the second environment being different from the first environment. The method may further include isolating the metrology system from the second environment using a window positioned across the optical axis. The method may further include limiting lateral displacement from the optical axis to less than about ±50 microns from a nominal lateral displacement from the optical axis at a first focal point of the radiation collector based on the positioning of the window.

[0013] Further features and advantages, as well as the structure and operation of various embodiments, are described in detail below with reference to the accompanying drawings. It should be noted that the present disclosure is not limited to the particular embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Further embodiments will be apparent to those skilled in the art based on the teachings contained herein.

[0014] The accompanying drawings, which are incorporated in and form a part of this specification, illustrate the present disclosure and, together with the description, further serve to explain the principles of aspects of the present disclosure and to enable those skilled in the art to make and use aspects of the present disclosure.

[0015] FIG. 1 is a schematic diagram of an exemplary reflective lithographic apparatus according to some aspects of the present disclosure. [0016] FIG. 1 is a schematic diagram of an exemplary transmissive lithographic apparatus according to some aspects of the present disclosure. [0017] FIG. 1B is a more detailed schematic diagram of the reflective lithographic apparatus shown in FIG. 1A, according to some aspects of the present disclosure. [0018] FIG. 1 is a schematic diagram of an exemplary lithographic cell according to some aspects of the present disclosure. [0019] FIG. 2 is a schematic diagram of an exemplary radiation source for an exemplary reflective lithographic apparatus according to some aspects of the present disclosure. [0020] FIG. 1 is a schematic diagram of a portion of an exemplary EUV radiation system, in accordance with some aspects of the present disclosure. [0021] FIG. 1 is a schematic diagram of a portion of an exemplary EUV radiation system, in accordance with some aspects of the present disclosure. [0021] FIG. 1 is a schematic diagram of a portion of an exemplary EUV radiation system, in accordance with some aspects of the present disclosure. [0021] FIG. 1 is a schematic diagram of a portion of an exemplary EUV radiation system, in accordance with some aspects of the present disclosure. [0021] FIG. 1 is a schematic diagram of a portion of an exemplary EUV radiation system, in accordance with some aspects of the present disclosure. [0022] FIG. 1 is a schematic diagram of a rapid replacement window assembly according to some aspects of the present disclosure. [0022] FIG. 1 is a schematic diagram of a rapid replacement window assembly according to some aspects of the present disclosure. [0022] FIG. 1 is a schematic diagram of a rapid replacement window assembly according to some aspects of the present disclosure. [0023] An exemplary method according to some aspects or portions of the present disclosure.

[0024] The features and advantages of the present disclosure will become more apparent from the following detailed description taken in conjunction with the drawings. In the drawings, unless otherwise noted, like reference numbers identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. Furthermore, the left-most digit(s) of a reference number generally identifies the drawing in which that reference number first appears. Unless otherwise noted, the drawings provided throughout this disclosure should not be construed as drawings to scale.

This specification discloses one or more embodiments that incorporate features of the present disclosure. The disclosed embodiments are merely illustrative of the present disclosure. The scope of the present disclosure is not limited to the disclosed embodiments. The breadth and scope of the present disclosure are defined by the claims appended hereto and their equivalents.

[0025] References herein to a described embodiment and to "one embodiment," "an embodiment," "an exemplary embodiment," "example embodiment," etc., indicate that the described embodiment may include a particular feature, structure, or characteristic, but not all embodiments may necessarily include that particular feature, structure, or characteristic. Moreover, such phrases do not necessarily refer to the same embodiment. Furthermore, when a particular feature, structure, or characteristic is described in connection with one embodiment, it is understood that it is within the knowledge of one skilled in the art to provide such feature, structure, or characteristic in connection with other embodiments, whether or not explicitly described.

[0026] Spatially relative terms such as "below," "below," "downward," "above," "above," and "above" may be used herein for ease of description to describe the relationship of one element or feature to another element or feature as shown in the figures. Spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation shown in the figures. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may similarly be interpreted accordingly.

[0027] As used herein, the term "about" refers to a given quantity value that can vary based on a particular technique. Based on a particular technique, the term "about" can refer to a given quantity value that varies, for example, within 10-30% of the value (e.g., ±10%, ±20%, or ±30% of the value).

Overview
In one example, a window for an EUV radiation system includes a vacuum window (also called a viewport) that is assembled with a second window (also called a pellicle). Both the vacuum window and the pellicle are made of non-optical quality glass, for example soda glass. The vacuum window provides a vacuum seal to the EUV radiation source vessel, allowing instruments in the vessel to see into the vessel. The pellicle is located inside the vacuum vessel and prevents tin debris from reaching the vacuum window. The pellicle becomes contaminated with tin over time and needs to be replaced periodically. When the pellicle is replaced, the entire viewport and pellicle assembly is replaced.

[0029] However, because viewports and pellicles are optical windows, they can cause deviations in the optical axis (offset and angular pointing errors) and can impart aberrations to the wavefront. As a result, viewports and pellicles can cause misalignment of the measurement optical axis. This misalignment can require realignment of the measurement instrument when the viewport or pellicle is replaced. This realignment process can add approximately 1 to 10 hours of additional recovery time (MTTR), depending on the measurement module and the potential for B-time (e.g., recovery time) during realignment. Furthermore, existing viewports and pellicles may not have manufacturing tolerances well-controlled enough to avoid disrupting the measurement optical axis. Furthermore, current vacuum window viewports may not be of optical quality, so many specifications important to measurement performance (e.g., refractive index vs. wavelength, transmitted wavefront error, wedge angle, etc.) may be unknown.

[0030] In contrast, some embodiments of the present disclosure may provide windows with improved construction and tolerances that substantially reduce the effect of the window on the alignment of a metrology system coupled to the window.

[0031] In some aspects, the present disclosure replaces soda glass with an improved material structure (e.g., optical glass) for viewports and pellicles to provide a window that reduces aberrations in the transmitted wavefront due to (a) refractive index inhomogeneities and (b) the presence of uncontrolled bubbles and striae. For example, the viewport and pellicle material can be 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., the yellow helium line). In some aspects, the viewport can be coated with an anti-reflective (AR) coating.

[0032] In some aspects, the present disclosure further provides windows that improve tolerances in: (i) viewport and pellicle wedge angles to reduce pointing errors; (ii) viewport and pellicle thickness to reduce decentering and focus errors; (iii) viewport and pellicle refractive index to reduce focus errors; (iv) viewport and pellicle transmitted wavefront power to reduce focus errors; and (v) optionally, using compensation between elements to further reduce overall alignment errors (e.g., balancing negative and positive errors). In some aspects, the present disclosure improves alignment tolerances of exemplary windows disclosed herein compared to conventional windows, as shown in Table 1 below.

[0033] In some aspects, the improvements provided herein can reduce metrology alignment errors from approximately 600 microns lateral and 2 millimeters axial focus errors to less than approximately 30 microns lateral and less than approximately 200 microns axial focus errors. This reduction in metrology alignment errors eliminates the need to realign the metrology system after window replacement (e.g., viewport and pellicle replacement).

[0034] In some aspects, the present disclosure improves the availability of EUV radiation systems by reducing the "green to green" time (also referred to as A time) for viewport replacement. Furthermore, by eliminating metrology recovery operations, the present disclosure eliminates the risk of something going wrong and taking longer than expected to recover (also referred to as B time), which also improves availability.

[0035] In some aspects, the present disclosure provides a technique whereby the viewport can be selected to offset the tolerances of the pellicle, and vice versa. This technique offers looser manufacturing tolerances at the expense of a more complex pairing and construction process.

[0036] The windows disclosed herein have numerous advantages. For example, the present disclosure provides precisely controlled manufacturing tolerances for viewports and pellicles, including tighter wedge tolerances, tighter angular mounting tolerances, lower transmitted wavefront power tolerances, less stress on the vacuum window due to the use of an optically engineered vacuum interface, optically-quality glass instead of the borosilicate glass used in existing viewports, and reduced recovery time for replacing vacuum windows. In another example, the optical and mechanical tolerances of the windows, viewports, and pellicles disclosed herein are significantly improved, which in some aspects can eliminate the need for a metrology recovery step after viewport and pellicle replacement.

[0037] In some aspects, the optical and mechanical tolerances of the windows, viewports, and pellicles disclosed herein also simplify the EUV radiation source manufacturing process by eliminating setup and alignment steps. For example, a radiation source may have nine metrology systems, all of which need to point to specific locations inside the vessel. In some aspects, the radiation source required a technician to set up complex targets inside the vessel and align the metrology systems with those targets once they were installed in the vessel. This was a time-consuming process that could be performed inaccurately as a result of technician error. In contrast, in other aspects, the alignment tolerances of all associated hardware (e.g., metrology systems, windows, vessel frame) can be small enough that those setup steps may no longer be required. Thus, once all associated hardware is assembled, it should already be sufficiently aligned so that no alignment operations are required. The high-precision windows disclosed herein can be important to achieving this.

[0038] As mentioned above, pointing error can have a significant impact on the total alignment error budget of a metrology system and therefore the performance of an EUV radiation source. In one illustrative example, the optical distance from the viewport to the measurement location, the first focal point PF of the radiation collector, is about 1 meter. A wedge in the viewport induces a pointing error proportional to the refractive index, as shown by the equation D = L * A * (n-1), where D = the shifted distance at the first focal point PF, L = the distance from the first focal point PF, A = the wedge angle, and n = the refractive index. Existing wedge tolerances are ±3 arc minutes or about ±870 microradians (urad). If the refractive index, n, is about 1.5 and the distance, D, from the first focal point PF is about 1 meter, the tolerance at the first focal point PF may be about 435 microns (e.g., 0.5 * 870) for the viewport alone. Taking into account the pellicle, the tolerance can be approximately 600 microns to 870 microns.

[0039] Continuing with the example above, the droplet detection module (DDM) has a field of view (FOV) of approximately 540 microns. If the droplet illumination module (DIM) viewport and pellicle are replaced and an alignment error of approximately 600 microns to 870 microns is realized, the DIM and DDM will require realignment, which can take up to 20 hours. In the window disclosed herein, the wedge tolerance for both the viewport and pellicle is approximately ±5 arc seconds, resulting in a deviation of less than approximately 30 microns at the first focal point PF, which is well within the field of view of the DDM.

[0040] In some aspects, as a result of the techniques described in this disclosure, the viewports and pellicles disclosed herein can reduce uncertainty in the optical modeling of metrology systems. Furthermore, because the viewports and pellicles disclosed herein are optical quality, many specifications important to metrology performance can be known, such as refractive index versus wavelength, transmitted wavefront error, wedge angle, and other suitable characteristics. Furthermore, the use of a toleranced viewport assembly enables: (i) pre-alignment of the metrology module on an optical bench test station, and (ii) direct replacement of the metrology module (e.g., in the event of a failure) without the need for realignment on the vessel. This saves time (e.g., up to 10 hours per replacement).

[0041] However, before describing such aspects in more detail, it is useful to present an exemplary environment in which aspects of the present disclosure can be implemented.

Lithography System Example
1A and 1B are schematic illustrations of lithographic apparatus 100 and lithographic apparatus 100', respectively, in which aspects of the present disclosure may be implemented. As shown in Figures 1A and 1B, lithographic apparatus 100 and 100' are illustrated from a perspective perpendicular to the XZ plane (e.g., a side view) (e.g., the X axis points to the right and the Z axis points up), while patterning device MA and substrate W are illustrated from an additional perspective perpendicular to the XY plane (e.g., a top view) (e.g., the X axis points to the right and the Y axis points up).

[0043] Lithographic apparatus 100 and lithographic apparatus 100' each include: an illumination system IL (e.g., an illuminator) configured to condition a radiation beam B (e.g., a deep ultraviolet (DUV) radiation beam or an extreme ultraviolet (EUV) radiation beam); a support structure MT (e.g., a mask table) configured to support a patterning device MA (e.g., a mask, reticle, or 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 apparatus 100 and 100' also include a projection system PS configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C of the substrate W (e.g., a portion comprising one or more dies). In lithographic apparatus 100, patterning device MA and projection system PS are reflective. In lithographic apparatus 100', patterning device MA and projection system PS are transmissive.

[0044] The illumination system IL may 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 relative to a reference coordinate system, the design of at least one of 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 may use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device MA. The support structure MT may, for example, be a frame or a table, which may be fixed or movable as required. By using sensors, the support structure MT may ensure that the patterning device MA is at a desired position, for example with respect to the projection system PS.

[0046] The term "patterning device" MA should be interpreted broadly as referring to any device that can be used to impart a radiation beam B with a pattern in its cross-section so as to create a pattern in a target portion C of a substrate W. The pattern imparted to the radiation beam B may correspond to a particular functional layer in a device being created in the target portion C to form an integrated circuit.

[0047] Patterning device MA can be transmissive (such as lithographic apparatus 100' in FIG. 1B) or reflective (such as lithographic apparatus 100 in FIG. 1A). Examples of patterning device MA include a reticle, a mask, a programmable mirror array, or a programmable LCD panel. 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 mirror array uses 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 mirrors impart a pattern to a radiation beam B that is reflected by the matrix of small mirrors.

[0048] The term "projection system" PS can encompass any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optics, or any combination thereof, appropriate to the exposure radiation used or other factors such as the use of an immersion liquid or a vacuum on the substrate W. A vacuum environment can be used for EUV or electron beam radiation, as other gases may absorb too much radiation or electrons. Therefore, a vacuum wall and vacuum pumps can be used to provide a vacuum environment throughout the beam path.

[0049] Lithographic apparatus 100 and/or lithographic apparatus 100' may be of a type having two (dual stage) or more substrate tables WT (and/or two or more mask tables). In such a "multi-stage" machine, the additional substrate tables WT may be used in parallel, or one or more tables may be performing preparatory steps while one or more other substrate tables WT are used for exposure. In some cases, the additional tables may not be substrate tables WT.

[0050] The lithographic apparatus may be of a type in which at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, for example water, so as to fill a space between the projection system and the substrate. Immersion liquids may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the projection system. Immersion techniques can increase the numerical aperture of projection systems. As used herein, the term "immersion" does not imply that a structure such as the substrate must be submerged in liquid, but rather only that a liquid is located between the projection system and the substrate during exposure.

1A and 1B, the illumination system IL receives a radiation beam B from a radiation source SO. The source SO and the lithographic apparatus 100 or 100' may be separate physical entities, for example if 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 travels from the radiation source SO to the illumination system IL using a beam delivery system BD (as shown, for example, in FIG. 1B) that may include, for example, appropriate directing mirrors and/or beam expanders. In other cases, the radiation source SO may be an integral part of the lithographic apparatus 100 or 100', for example if the radiation source SO is a mercury lamp. The radiation source SO and the illuminator IL, together with the beam delivery system BD, may be referred to as a radiation system, if desired.

[0052] The illumination system IL may include an adjuster AD (e.g. as shown in FIG. 1B ) for adjusting the angular intensity distribution of the radiation beam. Typically, at least the outer and/or inner radial extent (commonly referred to as "σ-outer" and "σ-inner", respectively) of the intensity distribution in a pupil plane of the illuminator may be adjusted. In addition, the illumination system IL may include various other components (e.g. as shown in FIG. 1B ), such as an integrator IN and a radiation collector CO (e.g., condenser or collector optics). The illumination system IL may be used to adjust the radiation beam B to have a desired uniformity and intensity distribution in its cross-section.

1A , a radiation beam B is incident on a patterning device MA (e.g., a mask), which is held on a 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 reflecting from the patterning device MA, the radiation beam B passes through a projection system PS, which focuses the radiation beam B onto a target portion C of a substrate W. A second positioner PW and a position sensor IFD2 (e.g., an interferometric device, a linear encoder, or a capacitance sensor) can be used to accurately move the substrate table WT (e.g., to position different target portions C in the path of the radiation beam B). Similarly, a first positioner PM and another position sensor IFD1 (e.g., an interferometric device, a linear encoder, or a capacitance sensor) can be used to accurately position the patterning device MA relative to the path of the radiation beam B. Patterning device MA and substrate W may be aligned using mask alignment marks M1 and M2 and substrate alignment marks P1 and P2.

[0054] Referring to Figure 1B, a radiation beam B is incident on a patterning device MA held on a support structure MT and is patterned by the patterning device MA. After traversing the patterning device MA, the radiation beam B passes through a projection system PS, which focuses the beam onto a target portion C of a substrate W. The projection system has a pupil conjugate PPU to an illumination system pupil IPU. A portion of the radiation emerges from the intensity distribution at the illumination system pupil IPU and passes through the mask pattern without being affected by diffraction at the mask pattern, producing 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 onto a resist layer coated on the substrate W, the image MP' being formed by diffracted beams generated from the mask pattern MP by radiation from the intensity distribution. For example, the mask pattern MP may include an array of lines and spaces. Diffraction of radiation at the array, other than the zeroth order, produces divergent diffracted beams with a change in direction in a direction perpendicular to the lines. The undiffracted beams (e.g., so-called zeroth order diffracted beams) traverse the pattern without changing their direction of propagation. The zeroth order diffracted beams traverse an upper lens or group of upper lenses 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 associated with the zeroth order diffracted beam in the plane of the pupil conjugate PPU is an image of the intensity distribution in the illumination system pupil IPU of the illumination system IL. The aperture device PD is, for example, positioned in or substantially in a plane containing the pupil conjugate PPU of the projection system PS.

[0056] The projection system PS is configured to capture not only the zeroth-order diffracted beam but also first-order or first- and higher-order diffracted beams (not shown) with one lens or group of lenses L. In some aspects, the resolution-enhancing effect of dipole illumination can be exploited by using dipole illumination to image a line pattern extending in a direction perpendicular to the lines. For example, a first-order diffracted beam interferes with a corresponding zeroth-order diffracted beam at the level of the substrate W to produce an image of the mask pattern MP at the highest achievable resolution and process window (e.g., usable depth of focus combined with an acceptable exposure dose deviation). In some aspects, astigmatism can be reduced by providing a radiation pole (not shown) in the opposite quadrant of the illumination system pupil IPU. Furthermore, in some aspects, astigmatism can be reduced by blocking the zeroth-order beam at the projection system pupil conjugate PPU associated with the radiation pole in the opposite quadrant. This is described in more detail in U.S. Patent No. 7,511,799, entitled "Lithographic projection apparatus and a device manufacturing method," issued March 31, 2009, which is incorporated herein by reference in its entirety.

[0057] The second positioner PW and a position sensor IFD (e.g. an interferometric device, linear encoder or capacitive sensor) can be used to accurately move the substrate table WT (e.g. to position different target portions C in the path of the radiation beam B). Similarly, the first positioner PM and a further position sensor (not shown in FIG. 1B ) can be used to accurately position the patterning device MA with respect to the path of the radiation beam B (e.g. after a mechanical search of a mask library or during a scan).

[0058] In general, movement of the support structure MT may be realized using 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 may 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 may be connected to a short-stroke actuator only, or may be fixed. The patterning device MA and the substrate W may be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. The substrate alignment marks occupy dedicated target portions (as shown), but may also 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 may be located between the dies.

[0059] The support structure MT and patterning device MA may be within a vacuum chamber V, where an in-vacuum robot IVR may be used to move a patterning device, such as a mask, in and out of the vacuum chamber. Alternatively, if the support structure MT and patterning device MA are outside the vacuum chamber, an out-of-vacuum robot may be used for various transfer operations, similar to the in-vacuum robot IVR. In some cases, both the in-vacuum and out-of-vacuum robots may need to be calibrated to smoothly transfer any payload (e.g., a mask) to the fixed kinematic mount of the transfer station.

[0060] Lithographic apparatus 100 and 100' can be used in at least one of the following modes:

[0061] 1. In step mode, the support structure MT and substrate table WT are kept essentially stationary, while the entire pattern imparted to the radiation beam B is projected onto a target portion C in one go (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 scanned synchronously 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. a mask table) may 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 whilst holding the programmable patterning device MA, and the substrate table WT is moved or scanned whilst a pattern imparted to the radiation beam B is projected onto a target portion C. A pulsed radiation source SO may be used, and the programmable patterning device is updated as required after each movement of the substrate table WT or between successive radiation pulses during a scan. This mode of operation is readily adaptable to maskless lithography using a programmable patterning device MA, such as a programmable mirror array.

[0064] Combinations and/or variations on the described modes of use or entirely different modes of use may also be employed.

[0065] In a further aspect, lithographic apparatus 100 includes an EUV radiation source configured to generate a beam of EUV radiation for EUV lithography. Typically, the EUV radiation source is arranged in a radiation system, and a corresponding illumination system is configured to condition the EUV radiation beam of the EUV radiation source.

[0066] Figure 2 shows 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 Figure 2, lithographic apparatus 100 is shown from a perspective (e.g., a side view) perpendicular to the XZ plane (e.g., the X axis points to the right and the Z axis points up).

[0067] The radiation source SO is constructed and arranged to maintain a vacuum environment within the sealing structure 220. The radiation source SO includes a source chamber 211 and a collector chamber 212 and is configured to generate and transmit EUV radiation. The EUV radiation can be generated by a gas or vapor, such as xenon (Xe) gas, lithium (Li) vapor, or tin (Sn) vapor, that generates an EUV radiation-emitting plasma 210 to emit radiation in the EUV range of the electromagnetic spectrum. The at least partially ionized EUV radiation-emitting plasma 210 can be generated by, for example, an electric discharge or a laser beam. For efficient generation of radiation, for example, a partial pressure of about 10.0 Pascals (Pa) of Xe gas, Li vapor, Sn vapor, or any other suitable gas or vapor can be used. In some aspects, an excited tin plasma is provided to generate the EUV radiation.

[0068] Radiation emitted by the EUV radiation-emitting plasma 210 is transmitted from the source chamber 211 to the collector chamber 212 via an optional gas barrier or contaminant trap 230 (e.g., sometimes referred to as a contaminant barrier or foil trap) located in or behind an opening in the source chamber 211. The contaminant trap 230 may include a channel structure. The contaminant trap 230 may also include a gas barrier or a combination of a gas barrier and a channel structure. The contaminant trap 230 illustrated further herein includes at least a channel structure.

[0069] Collector chamber 212 may include a radiation collector CO (e.g. a condenser or collector optic), which may be a so-called grazing incidence collector. The radiation collector CO has an upstream radiation collector side 251 and a downstream radiation collector side 252. Radiation passing through the radiation collector CO may be reflected from a grating spectral filter 240 and focused into a virtual source point IF. The virtual source point IF is commonly referred to as the intermediate focus, and the source collector arrangement is configured such that this virtual source point IF is located at or near opening 219 in the sealing structure 220. The virtual source point IF is an image of the EUV radiation emitting plasma 210. The grating spectral filter 240 is used to suppress, in particular, infrared (IR) radiation.

[0070] The radiation then passes through an illumination system IL, which may include a faceted field mirror device 222 and a faceted pupil mirror device 224 configured to provide a desired angular distribution of the radiation beam 221 at the patterning device MA, and to provide a desired uniformity of radiation intensity at the patterning device MA. Upon reflection of the radiation beam 221 off the patterning device MA, which is held by the support structure MT, a patterned beam 226 is formed, which is imaged by the projection system PS via reflective elements 228, 229 onto a substrate W held by a wafer stage or substrate table WT.

[0071] In general, more elements than shown may be present in the illumination system IL and projection system PS. Optionally, a grating spectral filter 240 may be present, depending on the type of lithographic apparatus. Furthermore, more mirrors may be present than shown in FIG. 2. For example, there may be one to six additional reflective elements in the projection system PS beyond those shown in FIG. 2.

[0072] As shown in Figure 2, radiation collector CO is shown as a nested collector with grazing incidence reflectors 253, 254 and 255, just as an example of a collector (or collecting mirror). Grazing incidence reflectors 253, 254 and 255 are arranged axisymmetrically about optical axis O, and this type of radiation collector CO is preferably used in combination with a discharge produced plasma (DPP) source.

Lithography Cell Example
[0073] Figure 3 shows a lithography cell 300, which is sometimes also referred to as a lithocell or cluster. As shown in Figure 3, the lithography cell 300 is shown from a perspective (e.g., a top view) perpendicular to the XY plane (e.g., the X axis points to the right and the Y axis points up).

[0074] Lithographic apparatuses 100 and 100' may form part of a lithographic cell 300. Lithographic cell 300 may also include one or more apparatus for performing pre-exposure and post-exposure processes on a substrate. For example, these apparatus may include a spin coater SC for depositing a resist layer, a developer DE for developing exposed resist, a chill plate CH, and a bake plate BK. A substrate handler RO (e.g., a robot) picks up substrates from input/output ports I/O1, I/O2, moves them between different processing equipment, and delivers them to a loading bay LB of lithographic apparatus 100 or 100'. These devices, often collectively referred to as a 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 a lithography control unit LACU. Therefore, different devices can be operated to maximize throughput and processing efficiency.

Examples of radiation sources
[0075] An example of a radiation source SO for an exemplary reflective lithographic apparatus (e.g., lithographic apparatus 100 of Figure 1A) is shown in Figure 4. As shown in Figure 4, the radiation source SO is shown from a perspective perpendicular to the XY plane (e.g., a top view), as will be described below.

The radiation source SO shown in FIG. 4 is of a type that may be referred to as a laser-produced plasma (LPP) radiation source. A laser system 401, which may include, for example, a carbon dioxide (CO ₂ ) laser, is configured to deposit energy via one or more laser beams 402 in a fuel target 403′, such as one or more individual tin (Sn) droplets, provided by a fuel target generator 403 (e.g., a fuel emitter, a droplet generator). According to some aspects, the laser system 401 may be or operate in a pulsed continuous wave or quasi-continuous wave laser. The trajectory of the fuel target 403′ (e.g., droplets) emitted from the fuel target generator 403 may be parallel to the 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. The Z-axis is perpendicular to both the X-axis and the Y-axis and generally extends into (or out of) the plane of the page, although other configurations are used in other aspects. In some embodiments, the laser beam 402 may propagate in a direction other than parallel to the Y-axis (e.g., a direction other than perpendicular to the X-axis direction of the trajectory of the fuel target 403′).

[0077] While the following description refers to tin, any suitable target material can be used. The target material can be, for example, in liquid form and can be, for example, a metal or alloy. The fuel target generator 403 can include, for example, a nozzle configured to direct tin in the form of fuel targets 403' (e.g., individual droplets) along a trajectory toward the plasma formation region 404. Throughout the remainder of this specification, references to the terms "fuel," "fuel target," or "fuel droplets" should be understood to refer to the target material (e.g., droplets) emitted by the fuel target generator 403. The fuel target generator 403 can include a fuel emitter. One or more laser beams 402 are incident on the target material (e.g., tin) in the plasma formation region 404. Depositing laser energy in the target material generates a plasma 407 in the plasma formation region 404. Radiation, including EUV radiation, is emitted from the plasma 407 during de-excitation and recombination of the plasma's ions and electrons.

[0078] EUV radiation is collected and focused by radiation collector 405 (e.g., radiation collector CO). In some aspects, radiation collector 405 may include a near-normal incidence radiation collector (sometimes more commonly referred to as a normal incidence radiation collector). Radiation collector 405 may be a multi-layer structure configured to reflect EUV radiation (e.g., EUV radiation of a desired wavelength, such as about 13.5 nm). According to some aspects, radiation collector 405 may have an ellipsoidal configuration with two foci. As discussed herein, the first focus may be at plasma formation region 404 and the second focus may be at intermediate focus 406.

[0079] In some aspects, the laser system 401 may be located a relatively long distance from the radiation source SO. In that case, one or more laser beams 402 may travel from the laser system 401 to the radiation source SO using a beam delivery system (not shown) including, for example, appropriate directing mirrors, and/or beam expanders, and/or other optics. The laser system 401 and the radiation source SO may collectively be considered a radiation system.

[0080] The radiation reflected by the radiation collector 405 forms a radiation beam B. The radiation beam B focuses at a point (e.g., intermediate focus 406) to form an image of the plasma formation region 404, which acts as a virtual radiation source for the illumination system IL. The point at which the radiation beam B focuses may be referred to as the intermediate focus (IF) (e.g., intermediate focus 406). The radiation source SO is positioned such that the intermediate focus 406 is located at or near an opening 408 in a sealing structure 409 of the radiation source SO.

[0081] Radiation beam B travels from radiation source SO to illumination system IL, which is configured to condition radiation beam B. Radiation beam B travels from illumination system IL and is incident on patterning device MA, which is held by support structure MT. Patterning device MA reflects and patterns radiation beam B. After reflecting from patterning device MA, patterned radiation beam B enters projection system PS. The projection system includes a number of mirrors, which are configured to project radiation beam B onto substrate W, which is held by substrate table WT. Projection system PS may apply a demagnification factor to the radiation beam to form an image having smaller features than corresponding features on patterning device MA. For example, a demagnification factor of four may be applied. Although projection system PS is shown in Figure 2 as having two mirrors, the projection system may include any number of mirrors (for example, six mirrors).

[0082] The radiation source SO may also include components not shown in FIG. 4 . For example, a spectral filter may be provided in the radiation source SO. The spectral filter may be substantially transparent to EUV radiation but substantially blocking radiation of other wavelengths, such as infrared radiation.

[0083] The radiation source SO (or radiation system) may further include a fuel target imaging system for acquiring an image of the fuel target (e.g., droplets) in the plasma formation region 404, or more specifically, for acquiring a shadow image of the fuel target. The fuel target imaging system is capable of detecting light diffracted from the edge of the fuel target. References in the following text to an image of the fuel target should be understood to also refer to a shadow image of the fuel target or a diffraction pattern caused by the fuel target.

[0084] The fuel target imaging system may include a photodetector, such as a CCD array or a CMOS sensor, although it should be understood that any imaging device suitable for acquiring an image of the fuel target may be used. It should be understood that the fuel target imaging system may include optical components, such as one or more lenses, in addition to the photodetector. For example, the fuel target imaging system may include a camera 410, e.g., a combination of a photosensor (or photodetector) and one or more lenses. The optical components may be selected such that the photosensor or camera 410 acquires near-field and/or far-field images. The camera 410 may be positioned within the radiation source SO at any suitable location where the camera has 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. However, in some aspects, it may be necessary to position the camera 410 away from the propagation path of the one or more laser beams 402 and the trajectory of the fuel target emitted from the fuel target generator 403 to avoid damage to the camera 410. According to some aspects, camera 410 is configured to provide an image of the fuel target to controller 411 via connection 412. While connection 412 is shown as a wired connection, it should be understood that connection 412 (and other connections mentioned herein) may be implemented as a wired connection, a wireless connection, or a combination thereof.

[0085] As shown in FIG. 4, the radiation source SO may include a fuel target generator 403 configured to generate and emit fuel targets 403' (e.g., discrete tin droplets) towards the plasma formation region 404. The radiation source SO may further include a laser system 401 configured to direct one or more laser beams 402 at one or more of the fuel targets 403' to generate a plasma 407 in the plasma formation region 404. The radiation source SO may further include a radiation collector 405 (e.g., radiation collector CO) configured to collect radiation emitted by the plasma 407.

[0086] Examples of metrology systems and windows located within the radiation source SO of an exemplary reflective lithographic apparatus are shown in Figures 5-7.

Measurement system and window examples
5 shows an isometric view 500 of an exemplary sealing structure 502 (e.g. sealing structure 220, sealing structure 409) configured to maintain a vacuum environment as part of the exemplary radiation source SO of an exemplary reflective lithographic apparatus. The exemplary sealing structure 502 may 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 first focal point 504 of the radiation collector 506 is shown with a Cartesian coordinate system including X, Y and Z axes, although any suitable relative or universal coordinate system may be used. In some aspects, the exemplary sealing structure 502 includes an opening 508 associated with a fuel target generator (e.g. fuel target generator 403, droplet generator DG) and an opening 509 associated with a fuel target receiver (e.g. tin catch TC).

5, one or more exemplary components may be mechanically coupled (e.g., secured or otherwise attached by one or more fasteners, clamps, adhesives, or combinations thereof) to the exemplary sealing structure 502 in accordance with certain aspects of the present disclosure. Examples of components that may be mechanically coupled to the exemplary sealing structure 502 of the radiation source SO include, but are not limited to, a metrology system 510 and a window 511, a metrology system 512 and a window 513, a metrology system 514 and a window 515, a metrology system 516 and a window 517, a metrology system 518 and a window 519, a metrology system 520 and a window 521, a metrology system 522 and a window 523, a metrology system 524 and a window 525, a metrology system 526 and a window 527, any other suitable component, or any combination thereof. In some aspects, the first focal point 504 may be located at a distance of about 1 meter from one or more surfaces of the windows 511, 513, 515, 517, 519, 521, 523, 525, and 527.

[0089] In some aspects, metrology system 510 may include a coarse droplet steering camera (CDSC), and metrology system 522 may include a precision droplet steering camera (FDSC). In some aspects, metrology system 512 may include a first droplet formation camera (DFC), and metrology system 520 may include a second DFC. In some aspects, metrology system 514 may include a droplet detection module (DDM). In some aspects, metrology system 516 may include a line laser module (LLM). In some aspects, metrology system 518 may include a droplet illumination module (DIM). In some aspects, metrology system 524 may include a first illumination module, such as a first backlight laser module (BLM), and metrology system 526 may include a second illumination module, such as a second BLM. In some aspects, metrology systems 524 and 526 (e.g., first and second BLMs) may be connected to metrology systems 512 and 520 (e.g., a pair of DFCs).

[0090] In some aspects, one or more of windows 511, 513, 515, 517, 519, 521, 523, 525, and 527 may be constructed and arranged as described with reference to window 640 shown in Figures 6A and 6B, window 740 shown in Figure 7, the exemplary rapid replacement window assembly 700 shown in Figure 7, any other suitable window or window assembly, any structure or feature included therein, or any combination thereof.

[0091] In some aspects, one or more of measurement systems 510, 512, 514, 516, 518, 520, 522, 524, and 526 may be configured to be disposed in a first environment, such as an atmospheric environment located outside a sealed vessel, such as exemplary sealing structure 502. In some aspects, one or more of measurement systems 510, 512, 514, 516, 518, 520, 522, 524, and 526 may be configured to take one or more measurements of a region in a second environment along an optical axis of the measurement system. In some aspects, this region may partially or entirely encompass any suitable geometric region, such as a region within the exemplary sealing structure including first focal point 504 of radiation collector 506, plasma formation region 404 shown in FIG. 4, 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 or partial vacuum environment located inside a sealed enclosure such as exemplary sealing structure 502. In some aspects, one or more of windows 511, 513, 515, 517, 519, 521, 523, 525, and 527 can be configured to be positioned intersecting the optical axis of the respective metrology system. In some aspects, one or more of windows 511, 513, 515, 517, 519, 521, 523, 525, and 527 can be configured to isolate the respective metrology system from the second environment.

[0092] In some aspects, one or more of windows 511, 513, 515, 517, 519, 521, 523, 525, and 527 may be configured to limit lateral displacement (e.g., lateral focus error) to a lateral displacement tolerance of less than about ±50 microns from a nominal lateral displacement from the optical axis (e.g., of the respective metrology system for the particular window) at first focal point 504 of radiation collector 506. [0092] In some aspects, one or more of windows 511, 513, 515, 517, 519, 521, 523, 525, and 527 may be configured to limit lateral displacement to a lateral displacement tolerance of less than about ±33 microns from a nominal lateral displacement from the optical axis at first focal point 504 of radiation collector 506.

[0093] In some aspects, one or more of windows 511, 513, 515, 517, 519, 521, 523, 525, and 527 can be configured to limit angular deviations along the optical axis (e.g., of the measurement system for each particular window) to an angular deviation tolerance of less than about ±0.5 arc minutes from a nominal angular deviation along the optical axis. [0093] In some aspects, one or more of windows 511, 513, 515, 517, 519, 521, 523, 525, and 527 can be configured to limit angular deviations to an angular deviation tolerance of less than about ±0.1 arc minutes from a nominal angular deviation along the optical axis.

[0094] In some aspects, one or more of windows 511, 513, 515, 517, 519, 521, 523, 525, and 527 can be configured to limit the longitudinal displacement (e.g., axial focus error) to a longitudinal displacement tolerance of less than about ±330 microns from a nominal longitudinal displacement from first focal point 504 along the optical axis (e.g., of the measurement system for each particular window). In some aspects, one or more of windows 511, 513, 515, 517, 519, 521, 523, 525, and 527 can be configured to limit the longitudinal displacement to a longitudinal displacement tolerance of less than about ±200 microns from a nominal longitudinal displacement from first focal point 504 along the optical axis.

[0095] In some aspects, one or more of metrology systems 510, 512, 514, 516, 518, 520, 522, 524, and 526 may be modular metrology systems. In some aspects, one or more of windows 511, 513, 515, 517, 519, 521, 523, 525, and 527 may be configured to limit lateral displacement to less than about ±50 microns when the respective metrology system is installed within the radiation source SO. In some aspects, one or more of windows 511, 513, 515, 517, 519, 521, 523, 525, and 527 may be configured to limit lateral displacement to less than about ±50 microns without a calibration operation (e.g., without performing a separate calibration operation).

6A, 6B, 6C, and 6D are schematic diagrams of portions of an exemplary EUV radiation system according to some aspects of the present disclosure. FIG. 6A shows a schematic diagram of an exemplary system 600 according to some aspects of the present disclosure. As shown in FIG. 6A, exemplary system 600 includes a metrology system 630 and a window 640. In some aspects, metrology system 630 may be or include one or more of metrology systems 510, 512, 514, 516, 518, 520, 522, 524, and 526 shown in FIG. 5. In some aspects, window 640 may be or include one or more of windows 511, 513, 515, 517, 519, 521, 523, 525, and 527 shown in FIG. 5.

[0097] In some aspects, metrology system 630 may be disposed in a first environment 680 (e.g., an atmospheric environment) located outside a sealed enclosure (e.g., sealing structure 220 shown in FIG. 2, sealing structure 409 shown in FIG. 4, sealing structure 502 shown in FIG. 5) and may be removably attached to window 640 (e.g., mechanically connected, fastened, or otherwise attached by one or more fasteners, clamps, adhesives, or combinations thereof). In some aspects, window 640 may be removably attached to a sealed enclosure (e.g., sealing structure 220 shown in FIG. 2, sealing structure 409 shown in FIG. 4, sealing 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 exemplary radiation source SO of an exemplary reflective lithographic apparatus.

[0098] For reference, Figure 6A shows the first focal point 604 of a radiation collector (e.g., radiation collector CO shown in Figure 2, radiation collector 405 shown in Figure 4, or radiation collector 506 shown in Figure 5) along with the optical axis 602 of measurement system 630. In some aspects, first focal point 604 may be located at a distance of about 1 meter from the surface of window 640. For example, first focal point 604 may be located at a distance of about 1 meter from surface 648b of viewport 648 (shown in Figure 6B).

[0099] In some aspects, metrology system 630 may be configured to perform one or more measurements of region 601 within second environment 682 along optical axis 602 of metrology system 630. In some aspects, region 601 may partially or entirely encompass any suitable geometric region, such as a region within the exemplary sealing structure including first focal point 604 of the radiation collector, plasma formation region 404 shown in FIG. 4, any other suitable region, or any combination thereof. In some aspects, second environment 682 may be a vacuum or partial vacuum environment located within the sealed vessel. In some aspects, window 640 may be configured to be positioned intersecting optical axis 602 of metrology system 630.

[0100] In some aspects, window 640 may include a base structure 642, a viewport mounting structure 644, a viewport 648, a pellicle mounting structure 652, a pellicle 650, a radiation blocking structure 646 (e.g., a light shield), any other suitable components or structures, or any combination thereof. In some aspects, 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 may include an O-ring 664 configured to be removably attached to the outer surface of the sealed vessel. The base structure 642 may include an O-ring 660 configured to be removably attached to the surface 648b (e.g., the inner surface) of the viewport 648. The viewport mounting structure 644 may include an O-ring 661 configured to be removably attached to the surface 648a (e.g., the outer surface) of the viewport 648. In some aspects, the radiation shielding structure 646 and the viewport mounting structure 644 may be configured to be attached to the base structure 642 using fasteners (e.g., eight hexalobular socket flat head machine screws).

[0102] In some embodiments, viewport 648 may include anti-reflective (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., the yellow helium line). In some embodiments, pellicle 650 may include the same or different optical glass as the optical glass included in viewport 648.

[0103] In some aspects, window 640 can be configured to isolate measurement system 630 from second environment 682. For example, O-ring 661, O-ring 660, and O-ring 664 can separate first environment 680 from second environment 682. In some aspects, window 640 can include flow channel 668 configured to extend second environment 682 into a volume disposed between surface 650a and surface 648b.

[0104] In some embodiments, window 640 can include a wedge angle that is ±5.0 arc seconds or less than about ±0.1 arc minutes from the nominal wedge angle. In some embodiments, the nominal wedge angle can be about 0 degrees. For example, viewport 648, pellicle 650, or both can have a wedge angle that is ±5.0 arc seconds or less from a nominal wedge angle of 0 degrees. In one illustrative example, the nominal wedge angle between surfaces 648a and 648b can be about 0 degrees, and the wedge angle between surfaces 648a and 648b can be about -5.0 arc seconds and less than about 5.0 arc seconds. In another illustrative example, the nominal wedge angle between surfaces 650a and 650b can be about 0 degrees, and the wedge angle between surfaces 650a and 650b can be about -5.0 arc seconds to about 5.0 arc seconds.

[0105] In other aspects, the nominal wedge angle may be greater than about 0 degrees. For example, viewport 648, pellicle 650, or both, may have a wedge angle that is less than ±5.0 arc seconds from a nominal wedge angle greater than about 0 degrees (e.g., about 58 arc minutes, 1 degree 56 arc minutes, 3 degrees 52 arc minutes, or any other suitable wedge angle). In one illustrative example, the nominal wedge angle between surfaces 648a and 648b may be about 3,480 arc seconds, and the wedge angle between surfaces 648a and 648b may be about 3,475 arc seconds to about 3,485 arc seconds. In another illustrative example, the nominal wedge angle between surfaces 650a and 650b may be approximately 6,960 arc seconds, and the wedge angle between surfaces 650a and 650b may be between approximately 6,955 arc seconds and approximately 6,965 arc seconds.

[0106] Figure 6C shows region 601 in more detail. It should be understood that region 601 is not necessarily drawn to scale, and further, the linear, two-dimensional depiction shown in Figure 6C may actually refer to non-linear embodiments, three-dimensional embodiments, any other suitable embodiments, or combinations thereof.

[0107] As shown in Figure 6C, region 601 may include a first focal point 604 of the radiation collector. First focal point 604 may be disposed along an optical axis 602 of metrology system 630. Figure 6C also shows an axis 603 that intersects first focal point 604 and is orthogonal (e.g., perpendicular) to optical axis 602.

[0108] As further shown in FIG. 6C , region 601 may include a nominal displacement focus 606 (e.g., a predicted, estimated, planned, or intended focus error) of window 640 (e.g., if window 640 is not a perfect window). Nominal displacement focus 606 may be located along a nominal displacement optical axis 605 (e.g., a predicted, estimated, planned, or intended optical axis) of window 640. As used herein, the term "nominal" may refer to a predicted, estimated, planned, or intended value, measurement, location, geometry, or other suitable characteristic.

[0109] In some aspects, the nominal displaced focus 606 may have a nominal lateral displacement 610 (e.g., a predicted, estimated, planned, or intended lateral focus error) from the optical axis 602 at the first focal point 604 of the radiation collector. In one illustrative example, the nominal lateral displacement 610 may be about 1 millimeter. In some aspects, the nominal displaced focus 606 may have a nominal longitudinal displacement 611 (e.g., a predicted, estimated, planned, or intended axial focus error) from the first focal point 604 along the optical axis 602. In some aspects, the nominal displaced focus 606 may have a nominal angular deviation 618 (e.g., a predicted, estimated, planned, or intended nominal angular deviation) from the optical axis 602.

[0110] In some aspects, the nominal displacement focus 606 may be corrected by an initial metrology module alignment process, such that the nominal displacement focus 606 coincides with the first focus 604. As a result of the initial metrology module alignment process, the nominal lateral displacement 610 may be approximately 0 microns, the nominal vertical displacement 611 may be approximately 0 microns, and the nominal angular deviation 618 may be approximately 0 degrees.

[0111] As further shown in FIG. 6C, region 601 may include a displacement focus 608 (e.g., actual focus error) of window 640. Displacement focus 608 may be located along a displacement optical axis 607 (e.g., actual optical axis) of window 640.

[0112] In some aspects, the displaced focus 608 may have a lateral displacement 612 (e.g., actual lateral focus error) from the optical axis 602 at the first focal point 604 of the radiation collector. In some aspects, the displaced focus 608 may have a longitudinal displacement 614 (e.g., actual axial focus error) from the first focal point 604 along the optical axis 602. In some aspects, the displaced focus 608 may have an angular deviation 619 (e.g., actual angular deviation) from the optical axis 602.

[0113] In some aspects, the displacement focus 608 may have a nominal-to-actual lateral displacement 613 from the nominal displacement focus 606 that is located within a lateral displacement tolerance 616 from the nominal displacement focus 606. In some aspects, the lateral displacement tolerance 616 may be less than about ±50 microns, less than ±33 microns, or less than any other suitable tolerance.

[0114] In some aspects, the displacement focus 608 may have a nominal-to-actual longitudinal displacement 615 from the nominal displacement focus 606 that is located within a longitudinal displacement tolerance 617 from the nominal displacement focus 606. In some aspects, the longitudinal displacement tolerance 617 may be less than about ±330 microns, less than ±200 microns, or less than any other suitable tolerance.

[0115] In some aspects, the displacement focus 608 may have a nominal-to-actual angular deviation 620 from the nominal displacement focus 606 that is located within an angular deviation tolerance 621 from the nominal displacement focus 606. In some aspects, the angular deviation tolerance 621 may be less than about ±0.5 arc minutes, less than ±0.1 arc minutes, less than ±5 arc seconds, or less than any other suitable tolerance.

[0116] In some aspects, window 640 may be configured to limit lateral displacement 612 to a lateral displacement tolerance 616 of less than about ±50 microns from a nominal lateral displacement 610 from the optical axis 602 at the first focal point 604 of the radiation collector. In some aspects, window 640 may be configured to limit lateral displacement 612 to a lateral displacement tolerance 616 of less than about ±33 microns from the nominal lateral displacement 610 from the optical axis 602 at the first focal point 604 of the radiation collector. In other words, window 640 may be configured to limit nominal to actual lateral displacement 613 to less than about ±50 microns, less than ±33 microns, or less than any other suitable tolerance.

[0117] In some aspects, the window 640 can be configured to limit the longitudinal displacement 614 to a longitudinal displacement tolerance 617 of less than approximately ±330 microns from the nominal longitudinal displacement 611 from the first focal point 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 approximately ±200 microns from the nominal longitudinal displacement 611 from the first focal point 604 along the optical axis 602. In other words, the window 640 can be configured to limit the nominal-to-actual longitudinal displacement 615 to less than approximately ±330 microns, less than ±200 microns, or less than 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 arc minutes from the nominal angular deviation 618 along the optical axis 602. In some aspects, the window 640 can be configured to limit the angular deviation 619 to an angular deviation tolerance 621 of less than about ±0.1 arc minutes from the nominal angular deviation 618 along the optical axis 602.

[0119] In other words, the window 640 may be configured to limit the nominal-to-actual angular deviation 620 to less than approximately ±0.5 arc minutes, less than ±0.1 arc minutes, less than ±5 arc seconds, or less than any other suitable tolerance.

[0120] In some aspects, metrology system 630 may be a modular metrology system. In some aspects, window 640 may be configured to limit lateral displacement 612 to less than about ±50 microns from a nominal lateral displacement 610 from optical axis 602 at first focal point 604 of the radiation collector at the time metrology system 630 is attached to radiation source SO. In some aspects, window 640 may be configured to limit lateral displacement 612 to less than about ±50 microns from a nominal lateral displacement 610 from optical axis 602 at first focal point 604 of the radiation collector without a calibration operation (e.g., without performing a separate calibration operation other than an initial metrology module alignment process to adjust nominal displacement focus 606).

[0121] As shown in FIG. 6D, viewport 648 can be configured to be positioned intersecting optical axis 602. In some aspects, pellicle 650 can be configured to be positioned intersecting optical axis 602 and opposite viewport 648 (e.g., at an angle of about 0 degrees, greater than about 0 degrees, or less than about 0 degrees). For example, viewport 648 can have a viewport axis 690, pellicle 650 can have a pellicle axis 692, and angle 691 between viewport axis 690 and pellicle axis 692 can be greater than 0 degrees (e.g., about 4.5 degrees) to reduce or prevent back reflections.

[0122] Example of a Rapid Replacement Window Assembly
7A, 7B, and 7C are schematic diagrams of an exemplary quick-change window assembly 700 according to some aspects of the present disclosure. As shown in FIG. 7A, the exemplary quick-change window assembly 700 may include a quick-change window 740 and a quick-change window frame 770 (e.g., a fastening mechanism). In some aspects, as shown in FIG. 7A, the exemplary quick-change window assembly 700 may include multiple fasteners, such as catches, 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 rapid exchange window 740 may include a base structure 742, a viewport mounting structure 744, a viewport 748 (e.g., an "optically flat" quality substrate), a pellicle mounting structure (not shown), a pellicle (not shown, e.g., angled relative to the viewport 748 to prevent back reflections), a radiation shielding structure (not shown), any other suitable components or structures, or any combination thereof. In some aspects, the rapid exchange window 740 may include one or more structures described with reference to windows 511, 513, 515, 517, 519, 521, 523, 525, and 527 shown in FIG. 5 and window 640 shown in FIGS. 6A and 6B. In some aspects, one or more rapid exchange mounting structures, such as ball bearing 743a and ball bearing 743b, may be attached to the base structure 742 for use in installing, aligning, and removing the exemplary rapid exchange window assembly 700.

[0125] In some aspects, the quick-change window frame 770 may include a frame structure 772 that can be attached to the quick-change window 740 (e.g., base structure 742). In some aspects, one or more quick-change mounting structures, such as ball bearing 774a, ball bearing 774b, ball bearing 774c, and ball bearing 774d, may be attached to the frame structure 772 for use in installing, aligning, and removing the exemplary quick-change window assembly 700. In some aspects, the quick-change window frame 770 may include a receiving structure 776 configured to receive an installation and removal tool 790 (shown in FIG. 7B ). In some aspects, the quick-change window frame 770 may have a built-in locking mechanism that ensures consistent orientation of the quick-change window 740 relative to a reference surface on the radiation source container. In some aspects, the quick-change window frame 770 may use an "over-center" cam to provide positive engagement and vacuum tightness. In some aspects, the quick change window frame 770 may be configured to prevent rubbing of the vacuum seal O-ring when the vacuum seal is formed with the radiation source container.

[0126] As shown in FIG. 7B, the exemplary quick-change window assembly 700 can be installed and removed by movement 792 of an installation and removal tool 790. In some aspects, a fixed portion of the exemplary quick-change window assembly 700 (e.g., base structure 742 of the quick-change window 740) can include bearings (e.g., ball bearings 743a, 743b) that act as detents when the quick-change window 740 is inserted into a pocket in the radiation source container 702. In some aspects, the exemplary quick-change window assembly 700 can be configured to prevent further translational movement (e.g., orientation of the sealing O-ring relative to the sealing surface). In some aspects, actuation of the installation and removal mechanism results only in a movement that pushes out the sealing ring.

[0127] As shown in FIG. 7C, the exemplary rapid change window assembly 700 can be installed and removed from a pocket in the radiation source container 702 by movement 794. During installation and removal, a metrology system 796 (e.g., metrology system 522 shown in FIG. 5) can remain fixed to the radiation source container 702.

[0128] In some examples, the retention method can be: (i) a loose metal seal with multiple set screws; (ii) a loose elastomeric seal with multiple set screws; or (iii) a loose elastomeric seal with a secondary loose clamp ring. Furthermore, the vacuum viewport includes a glass-to-metal bond, which can stress and therefore deform the window. Furthermore, the pellicle has a finite lifespan. For example, tin debris from the EUV plasma accumulates on the pellicle (via both vapor and bombarding particles), reducing the metrology system's ability to see the region of interest (e.g., region 601 shown in Figures 6A and 6C). The pellicle exists to protect the viewport window from contamination and resulting thermal stress, which has historically caused window breakage and loss of vacuum (e.g., system down conditions). The orientation of the pellicle relative to the viewport is somewhat random.

[0129] In some aspects, the exemplary rapid change window assembly 700 couples the pellicle to a precision housing with optics secured via an O-ring, which may allow for controlled and minimized optical distortion. In some aspects, by including an "over-center" cam mechanism, installation and removal can be accomplished in seconds. Only one arm access may be required to actuate the installation and removal mechanism. Removal of the viewport may also require only one arm extension.

[0130] In some aspects, the exemplary rapid change window assembly 700 provides a viewport 748 with excellent optical properties (e.g., wavefront error) and optimized installation and removal capabilities that maximize availability of the EUV radiation source. In some aspects, the exemplary rapid change window assembly 700 provides a sacrificial window (called a pellicle) in the same optical path. Thus, the exemplary rapid change window assembly 700 provides an optimized viewport that encompasses both a vacuum window and a pellicle, along with rapid installation and removal capabilities.

[0131] The exemplary rapid replacement window assembly 700 can meet the stringent availability requirements of an EUV radiation system, allowing all maintenance actions to be performed quickly. In other words, rapid replacement times support the need for stringent availability requirements regarding system uptime and availability. In another example, the use of an elastomeric seal, as opposed to a glass and metal solder, reduces distortion of the optical glass, leading to lower residual stresses in the glass. As a result, there is less chance of breakage, loss of vacuum in the radiation source, and long downtimes (e.g., B-times) to restore.

Optical measurement process example
[0132] Figure 8 illustrates an exemplary method 800 for optical metrology in a radiation system (e.g. an EUV radiation system such as the exemplary radiation source SO shown in Figures 1A, 2 and 4), according to some aspects or portions of the present disclosure. The operations described with reference to exemplary method 800 may be performed by or in accordance with any of the systems, apparatus, components, techniques, or combinations thereof described herein, as described with reference to Figures 1 to 7 above.

[0133] In operation 802, the method may include disposing a measurement system (e.g., measurement systems 510, 512, 514, 516, 518, 520, 522, 524, or 526 shown in FIG. 5 , measurement 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 measurement 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 ), the second environment being different from the first environment. In some aspects, positioning the metrology system may be accomplished using suitable mechanical or other methods, and may include positioning the metrology system according to any aspect or combination of aspects described with reference to Figures 1-7 above.

[0134] In operation 804, the method may include isolating the metrology system from the second environment using a window positioned to intersect the optical axis (e.g., windows 511, 513, 515, 517, 519, 521, 523, 525, or 527 shown in FIG. 5 , window 640 shown in FIGS. 6A and 6B , or rapid exchange window 740 shown in FIG. 7A ). In some aspects, isolating the metrology system from the second environment may be performed based on a vacuum seal or partial vacuum seal provided by the window. In some aspects, isolating the metrology system may be achieved using suitable mechanical or other methods and may include isolating the metrology system according to any aspect or combination of aspects described with reference to FIGS. 1-7 above.

[0135] In operation 806, the method may include limiting a lateral displacement from the optical axis (e.g., lateral displacement 612 shown in FIG. 6C ) at a first focal point (e.g., first focal point 504 shown in FIG. 5 , first focal point 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 ) based on the placement of the window to less than about ±50 microns from a nominal lateral displacement from the optical axis (e.g., nominal lateral displacement 610 shown in FIG. 6C ). In some aspects, limiting the lateral displacement may be achieved using suitable mechanical or other methods and may include limiting the lateral displacement according to any aspect or combination of aspects described with reference to FIGS. 1-7 above.

[0136] While specific reference may be made herein to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of guidance and detection patterns for magnetic domain memories, flat panel displays, LCDs, thin-film magnetic heads, and the like, integrated optical systems, and the like. Those skilled in the art will recognize that, in connection with such alternative applications, any use of the terms "wafer" or "die" herein can be considered synonymous with the more general terms "substrate" or "target portion," respectively. Substrates referred to herein may 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 may be applied to such other substrate processing tools. Furthermore, a substrate may be processed more than once, for example to produce a multi-layer IC, and as a result, the term substrate, as used herein, may also refer to a substrate that already includes multiple processed layers.

[0137] It is to be understood that the words and phrases used herein are for purposes of description and not limitation, and that they are to be interpreted in light of the teachings herein by one of ordinary skill in the relevant art.

[0138] As used herein, the term "substrate" refers to a material onto which a layer of material is added. In some aspects, the substrate itself may be patterned, and the material added on top of the substrate may also be patterned or may remain unpatterned.

[0139] The examples disclosed herein are intended to illustrate, but not limit, embodiments of the present disclosure. Other suitable modifications and adaptations of the variety of conditions and parameters normally encountered in the art will be apparent to those skilled in the art and are within the spirit and scope of the present disclosure.

[0140] While specific aspects of the present disclosure have been described above, it should be understood that these aspects may be practiced otherwise than as described. The descriptions are not intended to limit the embodiments of the present disclosure.

[0141] It should be understood that the Detailed Description section, and not the Background, Overview, and Abstract sections, is intended to be used to interpret the claims. The Overview and Abstract sections may describe one or more, but not all, of the exemplary embodiments contemplated by the inventors, and are therefore not intended to limit the scope of those embodiments and the appended claims in any way.

[0142] Some aspects of the present disclosure have been described above using functional building blocks that illustrate specific functional implementations and relationships thereof. The boundaries of these functional building blocks are arbitrarily defined herein for convenience of explanation. Alternate boundaries may be defined so long as the specified functions and relationships thereof are appropriately performed.

[0143] The foregoing description of specific embodiments of the present disclosure fully reveals the general nature of those embodiments so that others, applying knowledge within the skill of the art, may readily modify and/or adapt such specific embodiments for various uses without departing from the general concept of the disclosure and without undue experimentation. Accordingly, such adaptations and modifications are intended to be within the spirit and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein.

[0144] Other aspects of the invention are described in the following numbered clauses:
1. A metrology system disposed in a first environment and configured to perform one or more measurements of an area in a second environment along an optical axis of the metrology system, the second environment being different from the first environment;
a window configured to be positioned across the optical axis and isolating the measurement system from the second environment;
and limiting the lateral displacement from the optical axis to less than about ±50 microns from a nominal lateral displacement from the optical axis at a first focal point of the radiation collector.
2. The system of clause 1, wherein the first focal point is located at a distance of about 1 meter from the window.
3. The system of clause 1, wherein the window is configured to limit lateral displacement to less than about ±33 microns.
4. The system of clause 1, wherein the window is configured to limit angular deviation along the optical axis to less than about ±0.5 arc minutes from a nominal angular deviation along the optical axis.
5. The system of clause 4, wherein the window is configured to limit angular deviation to less than about ±0.1 arc minutes.
6. The system of clause 1, wherein the window is configured to limit the longitudinal displacement to less than about ±330 microns from a nominal longitudinal displacement from the first focal point along the optical axis.
7. The system of clause 6, wherein the window is configured to limit longitudinal displacement to less than about ±200 microns.
8. The window is
a first portion configured to be positioned across the optical axis;
and a second portion configured to be positioned across the optical axis and opposite the first portion.
9. The system of clause 8, wherein the first portion includes a viewport and the second portion includes a pellicle.
10. The system of clause 1, wherein the window comprises a wedge angle of less than about ±0.1 arc minutes from the nominal wedge angle.
11. The system of clause 10, wherein the nominal wedge angle is about 0 degrees.
12. The system of clause 10, wherein the nominal wedge angle is greater than about 0 degrees.
13. The system of clause 1, wherein the measurement system is a modular measurement system.
14. The system of clause 1, wherein the window is configured to limit displacement to less than about ±50 microns at the time the measurement system is installed within the system.
15. The system of clause 1, wherein the window is configured to limit displacement to less than about ±50 microns without calibration operation.
16. A first portion configured to be positioned across the optical axis;
a second portion configured to be positioned across the optical axis and opposite the first portion,
transmitting radiation through the first portion and the second portion along an optical axis;
and limiting the lateral displacement from the optical axis to less than about ±50 microns from a nominal lateral displacement from the optical axis at the first focal point of the radiation collector.
17. The window of clause 16, wherein the first focal point is located at a distance of about 1 meter from the window.
18. The window of clause 16, wherein the first portion includes a viewport and the second portion includes a pellicle.
19. The window of clause 16, comprising a wedge angle of less than about ±0.1 arc minutes from the nominal wedge angle.
20. Disposing a metrology system in a first environment, the metrology system performing one or more measurements of an area in a second environment along an optical axis of the metrology system, the second environment being different from the first environment;
isolating the measurement system from the second environment using a window positioned across the optical axis;
and limiting the lateral displacement from the optical axis based on the placement of the window to less than about ±50 microns from a nominal lateral displacement from the optical axis at the first focal point of the radiation collector.

[0145] The breadth and scope of the present disclosure should not be limited by any of the above-described exemplary aspects or embodiments, but should be defined only in accordance with the following claims and their equivalents.

Claims (15)

a metrology system disposed in a first environment and configured to perform one or more measurements of an area in a second environment along an optical axis of the metrology system, the second environment being a vacuum environment or a partial vacuum environment that is different from the first environment;
a window positioned across the optical axis and isolating the metrology system from the second environment; and
Including,
the window includes a viewport positioned across the optical axis;
the viewport comprises a distance from the first focal point and a wedge angle between one surface and another surface that limit lateral displacement from the optical axis to less than about ±50 microns from a nominal lateral displacement from the optical axis at the first focal point of the radiation collector;
system. The system of claim 1, wherein the first focal point is located at a distance of approximately 1 meter from the window. The system of claim 1, wherein the window limits angular deviation along the optical axis to less than approximately ±0.5 arc minutes from a nominal angular deviation along the optical axis. The system of claim 1, wherein the window limits longitudinal displacement to less than approximately ±330 microns from a nominal longitudinal displacement from the first focal point along the optical axis. The window is
a first portion disposed across the optical axis;
10. The system of claim 1, further comprising a second portion disposed across the optical axis and opposite the first portion. The system of claim 5 , wherein the first portion includes the viewport, and the second portion includes a pellicle. The system of claim 1, wherein the wedge angle is less than approximately ±0.1 arc minutes from the nominal wedge angle. The system of claim 7, wherein the nominal wedge angle is greater than approximately 0 degrees. The system of claim 1, wherein the measurement system is a modular measurement system. The system of claim 1, wherein the window limits the displacement to less than approximately ±50 microns when the measurement system is installed within the system. a first portion disposed across the optical axis;
a second portion disposed across the optical axis and opposite the first portion,
transmitting radiation through the first portion and the second portion along the optical axis;
a viewport disposed across the optical axis;
the viewport comprises a distance from the first focal point and a wedge angle between one surface and another surface that limit lateral displacement from the optical axis to less than about ±50 microns from a nominal lateral displacement from the optical axis at the first focal point of the radiation collector;
window. The window of claim 11, wherein the first focal point is located at a distance of approximately 1 meter from the window. The window of claim 11 , wherein the first portion includes the viewport and the second portion includes a pellicle. The window of claim 11, wherein the wedge angle is less than about ±0.1 arc minutes from the nominal wedge angle. disposing a metrology system in a first environment, the metrology system performing one or more measurements of an area in a second environment along an optical axis of the metrology system, the second environment being a vacuum environment or a partial vacuum environment and different from the first environment;
isolating the metrology system from the second environment using a window positioned across the optical axis;
Including,
the window includes a viewport positioned across the optical axis;
the viewport comprises a distance from the first focal point and a wedge angle between one surface and another surface that limit lateral displacement from the optical axis to less than about ±50 microns from a nominal lateral displacement from the optical axis at the first focal point of the radiation collector;
method.