WO2025045510A1 - Inspection apparatus, wedge system for reducing aberrations, and method of fabrication thereof - Google Patents

Abstract

A method of reducing optical aberrations in an optical system includes determining an aberration induced by the optical system. The determining includes transmitting a first beam of radiation having a first wavelength through a wedge system of the optical system. The wedge system includes radiation-curable adhesive disposed between first and second wedges. The determining also includes analyzing the first beam using a detector disposed downstream of the wedge system to determine the aberration. The method also includes curing the radiation-curable adhesive based on the analyzing of the first beam. The curing includes adjusting, using a spatial light modulator, an intensity distribution of a second beam of radiation, having a second wavelength different than the first wavelength, based on the analyzing of the first beam. The curing also includes directing the second beam to the radiation-curable adhesive to induce a position-dependent optical property at the radiation-curable adhesive.

Description

INSPECTION APPARATUS, WEDGE SYSTEM FOR REDUCING ABERRATIONS, AND METHOD OF FABRICATION THEREOF

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 63/535,149 which was filed on August 29, 2023, and which is incorporated herein in its entirety by reference.

FIELD

[0002] The present disclosure relates to metrology systems, for example, optical sensors for inspection measurements used in lithographic systems and processes.

BACKGROUND

[0003] A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which can be a mask or a reticle, can be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (photoresist or simply “resist”) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatuses include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the target portions parallel or anti-parallel to this scanning direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.

[0004] During lithographic operation, different processing steps can entail different layers to be sequentially formed on the substrate. Accordingly, it can be necessary to position the substrate relative to prior patterns formed thereon with a high degree of accuracy. Generally, alignment marks are placed on the substrate to be aligned and are located with reference to a second object. A lithographic apparatus can use an alignment apparatus for detecting positions of the alignment marks and for aligning the substrate using the alignment marks to ensure accurate exposure from a mask. Misalignment between the alignment marks at two different layers is measured as overlay error.

[0005] In order to monitor the lithographic process, parameters of the patterned substrate are measured. Parameters can include, for example, the overlay error between successive layers formed in or on the patterned substrate and critical linewidth of developed photosensitive resist. This measurement can be performed on a product substrate and/or on a dedicated metrology target. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. A fast and non-invasive form of a specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. By contrast, angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.

[0006] Such optical scatterometers can be used to measure parameters, such as critical dimensions (CD) of developed photosensitive resist or overlay error (OV) between two layers formed in or on the patterned substrate. Properties of the substrate can be determined by comparing the properties of an illumination beam before and after the beam has been reflected or scattered by the substrate.

[0007] Optical aberrations have been one of the major challenges for metrology tools used in the service of lithographic fabrication of nanoscale devices. Many of the options for treating aberrations are typically accompanied by high cost and incompatibility due to size requirements.

SUMMARY

[0008] Accordingly, it is desirable to reduce aberrations in high precision inspection tools. Features disclosed herein can be used to mitigate aberration using optical parts that fit within a small volume constraint and in a cost-effective manner.

[0009] In some aspects, an inspection apparatus comprises a radiation source, a wedge system, an optical device, and a detector. The radiation source is configured to generate radiation. The wedge system comprises a first wedge, a second wedge, and a radiation-cured adhesive disposed between the first and second wedges. The first wedge is configured to receive the radiation. The second wedge is configured to output the radiation. The radiation-cured adhesive comprises a region-dependent optical property configured to adjust a wavefront of the radiation. The optical device is configured to direct the radiation toward a target to generate scattered radiation from the target. The detector is configured to receive the scattered radiation from the target and to generate a measurement signal based on the scattered radiation and the adjusting of the wavefront.

[0010] In some aspects, a wedge system comprises a first wedge, a second wedge, and a radiation- cured adhesive disposed between the first wedge and the second wedge. The first wedge is configured to receive radiation. The second wedge is configured to output the radiation. The radiation-cured adhesive comprises a region-dependent optical property configured to adjust a wavefront of the radiation.

[0011] In some aspects, a method for reducing optical aberrations in an optical system can comprise one or more of the following operations. The method can comprise determining an aberration induced by the optical system. The determining of the aberration can comprise transmitting a first beam of radiation having a first wavelength through a wedge system of the optical system. The wedge system can comprise first and second wedges and radiation-curable adhesive disposed between the first and the second wedges. The determining of the aberration can further comprise analyzing the first beam using a detector disposed downstream of the wedge system to determine the aberration. The method can further comprise curing the radiation-curable adhesive based on the analyzing of the first beam. The curing of the radiation-curable adhesive can comprise adjusting, using a spatial light modulator, an intensity distribution of a second beam of radiation based on the analyzing of the first beam, the second beam having a second wavelength different than the first wavelength. The curing of the radiation-curable adhesive can further comprise directing the second beam to the radiation-curable adhesive to induce a position-dependent optical property at the radiation-curable adhesive to thereby reduce the aberration. The radiation-curable adhesive is reactive to the second wavelength.

[0012] Further features of various aspects of the present disclosure are described in detail below with reference to the accompanying drawings. It is noted that the present disclosure is not limited to the specific aspects described herein. Such aspects are presented herein for illustrative purposes only. Additional aspects will be apparent to those skilled in the relevant art(s) based on the teachings contained herein.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

[0013] The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the present disclosure and, together with the description, further serve to explain the principles of the present disclosure and to enable those skilled in the relevant art(s) to make and use aspects described herein.

[0014] FIG. 1A shows a reflective lithographic apparatus, according to some aspects of the disclosure.

[0015] FIG. IB shows a transmissive lithographic apparatus, according to some aspects of the disclosure.

[0016] FIG. 2 shows more details of a reflective lithographic apparatus, according to some aspects of the disclosure.

[0017] FIG. 3 shows a lithographic cell, according to some aspects of the disclosure.

[0018] FIGS. 4A, 4B, and 5 show inspection apparatuses, according to some aspects of the disclosure. [0019] FIG. 6 shows a flowchart of a method to reduce optical aberrations in an optical system, according to some aspects of the disclosure.

[0020] The features of the present disclosure will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. Additionally, generally, the left-most digit(s) of a reference number identifies the drawing in which the reference number first appears. Unless otherwise indicated, the drawings provided throughout the disclosure should not be interpreted as to-scale drawings.

DETAILED DESCRIPTION

[0021] The aspects described herein, and references in the specification to “one aspect,” “an aspect,” “an exemplary aspect,” “an example aspect,” etc., indicate that the aspects described can include a particular feature, structure, or characteristic, but every aspect may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same aspect. Further, when a particular feature, structure, or characteristic is described in connection with an aspect, it is understood that it is within the knowledge of those skilled in the art to effect such feature, structure, or characteristic in connection with other aspects whether or not explicitly described.

[0022] Spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “on,” “upper” and the like, can be used herein for ease of description to describe one element or feature’s relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus can be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein can likewise be interpreted accordingly. [0023] The terms “about,” “approximately,” or the like can be used herein to indicate the value of a given quantity that can vary based on a particular technology. Based on the particular technology, the terms “about,” “approximately,” or the like can indicate a value of a given quantity that varies within, for example, 10-30% of the value (e.g., ±10%, ±20%, or ±30% of the value).

[0024] Enumerative adjectives (e.g., “first,” “second,” “third,” or the like) can be used to distinguishing like elements without establishing an order, hierarchy, quantity, or permanent numeric assignment (unless otherwise noted). For example, the terms “first target” and “second target” can be used in a manner analogous to “i^th target” and “j* target” so as to facilitate the distinguishing of two targets without specifying a particular order, hierarchy, quantity, or immutable numeric correspondence.

[0025] Aspects of the present disclosure can be implemented in hardware, firmware, software, or any combination thereof. Aspects of the disclosure can also be implemented as instructions stored on a computer-readable medium, which can be read and executed by one or more processors. A machine- readable medium can include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine -readable medium can include read only memory (ROM); random access memory (RAM); magnetic disk storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.), and others. Furthermore, firmware, software, routines, and/or instructions can be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. The term “machine -readable medium” can be interchangeable with similar terms, for example, “computer program product,” “computer-readable medium,” “non-transitory computer-readable medium,” or the like. The term “non-transitory” can be used herein to characterize one or more forms of computer readable media except for a transitory, propagating signal.

[0026] Before describing such aspects in more detail, however, it is instructive to present an example environment in which aspects of the present disclosure can be implemented.

[0027] Example Lithographic Systems

[0028] FIGS. 1A and IB show a lithographic apparatus 100 and a lithographic apparatus 100’, respectively, in which aspects of the present disclosure can be implemented. Lithographic apparatus 100 and lithographic apparatus 100’ each include the following: an illumination system (illuminator) IL configured to condition a radiation beam B (for example, deep ultra violet or extreme ultra violet radiation); a support structure (for example, a mask table) MT configured to support a patterning device (for example, a mask, a reticle, or a dynamic patterning device) MA and connected to a first positioner PM configured to accurately position the patterning device MA; and, a substrate table (for example, a wafer table) WT configured to hold a substrate (for example, a resist coated wafer) W and connected to a second positioner PW configured to accurately position the substrate W. Lithographic apparatus 100 and 100’ also have a projection system PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion (for example, comprising one or more dies) C of the substrate W. In lithographic apparatus 100, the patterning device MA and the projection system PS are reflective. In lithographic apparatus 100’, the patterning device MA and the projection system PS are transmissive.

[0029] The illumination system IL can include various types of optical components, such as refractive, reflective, catadioptric, magnetic, electromagnetic, electrostatic, or other types of optical components, or any combination thereof, for directing, shaping, or controlling the radiation beam B. [0030] The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device MA with respect to a reference frame, the design of at least one of the lithographic apparatus 100 and 100’, and other conditions, such as whether or not the patterning device MA is held in a vacuum environment. The support structure MT can use mechanical, vacuum, electrostatic, or other clamping techniques to hold the patterning device MA. The support structure MT can be a frame or a table, for example, which can be fixed or movable. By using sensors, the support structure MT can ensure that the patterning device MA is at a desired position, for example, with respect to the projection system PS.

[0031] The term “patterning device” MA should be broadly interpreted as referring to any device that can be used to impart a radiation beam B with a pattern in its cross-section, such as to create a pattern in the target portion C of the substrate W. The pattern imparted to the radiation beam B can correspond to a particular functional layer in a device being created in the target portion C to form an integrated circuit.

[0032] The patterning device MA can be transmissive (as in lithographic apparatus 100’ of FIG. IB) or reflective (as in lithographic apparatus 100 of FIG. 1A). Examples of patterning devices MA include reticles, masks, programmable mirror arrays, or programmable LCD panels. Masks are well known in lithography, and 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 employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in the radiation beam B, which is reflected by a matrix of small mirrors.

[0033] The term “projection system” PS can encompass any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors, such as the use of an immersion liquid on the substrate W or the use of a vacuum. A vacuum environment can be used for EUV or electron beam radiation since other gases can absorb too much radiation or electrons. A vacuum environment can therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps.

[0034] Lithographic apparatus 100 and/or lithographic apparatus 100’ can be of a type having two (dual stage) or more substrate tables WT (and/or two or more mask tables). In such “multiple stage” machines, the additional substrate tables WT can be used in parallel, or preparatory steps can be carried out on one or more tables while one or more other substrate tables WT are being used for exposure. In some situations, the additional table may not be a substrate table WT.

[0035] The lithographic apparatus can also be of a type wherein at least a portion of the substrate can be covered by a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the projection system and the substrate. An immersion liquid can also be applied to other spaces in the lithographic apparatus, for example, between the mask and the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems. The term “immersion” as used herein does not mean that a structure, such as a substrate, must be submerged in liquid. For example, a liquid can be located between the projection system and the substrate during exposure.

[0036] Referring to FIGS. 1A and IB, the illuminator IL receives a radiation beam from a radiation source SO. The source SO and the lithographic apparatus 100, 100’ can be separate physical entities, for example, when the source SO is an excimer laser. In such cases, the source SO is not considered to form part of the lithographic apparatus 100 or 100’, and the radiation beam B passes from the source SO to the illuminator IL with the aid of a beam delivery system BD (in FIG. IB) including, for example, suitable directing mirrors and/or a beam expander. In other cases, the source SO can be an integral part of the lithographic apparatus 100, 100’, for example, when the source SO is a mercury lamp. A radiation system can comprise the source SO, the illuminator IL, and/or the beam delivery system BD.

[0037] The illuminator IL can include an adjuster AD (in FIG. IB) for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as “o-outer” and “o-inner,” respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL can comprise various other components (in FIG. IB), such as an integrator IN and a condenser CO. The illuminator IL can be used to condition the radiation beam B to have a desired uniformity and intensity distribution in its cross section.

[0038] Referring to FIG. 1A, the radiation beam B is incident on the patterning device (for example, mask) MA, which is held on the support structure (for example, mask table) MT, and is patterned by the patterning device MA. In lithographic apparatus 100, the radiation beam B is reflected from the patterning device (for example, mask) MA. After being reflected from the patterning device (for example, mask) MA, the radiation beam B passes through the projection system PS, which focuses the radiation beam B onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF2 (for example, an interferometric device, linear encoder, or capacitive sensor), the substrate table WT can be moved accurately (for example, so as to position different target portions C in the path of the radiation beam B). Similarly, the first positioner PM and another position sensor IF1 can be used to accurately position the patterning device (for example, mask) MA with respect to the path of the radiation beam B. Patterning device (for example, mask) MA and substrate W can be aligned using mask alignment marks Ml, M2 and substrate alignment marks Pl, P2.

[0039] Referring to FIG. IB, the radiation beam B is incident on the patterning device (for example, mask MA), which is held on the support structure (for example, mask table MT), and is patterned by the patterning device. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. The projection system has a pupil conjugate PPU to an illumination system pupil IPU. Portions of radiation emanate from the intensity distribution at the illumination system pupil IPU and traverse a mask pattern without being affected by diffraction at the mask pattern and create an image of the intensity distribution at the illumination system pupil IPU.

[0040] The projection system PS projects an image of the mask pattern MP, where the image is formed by diffracted beams produced from the mark pattern MP by radiation from the intensity distribution, onto a photoresist layer coated on the substrate W. For example, the mask pattern MP can include an array of lines and spaces. A diffraction of radiation at the array and different from zeroth order diffraction generates diverted diffracted beams with a change of direction in a direction perpendicular to the lines. Undiffracted beams (i.e., so-called zeroth order diffracted beams) traverse the pattern without any change in propagation direction. The zeroth order diffracted beams traverse an upper lens or upper lens group of the projection system PS, upstream of the pupil conjugate PPU of the projection system PS, to reach the pupil conjugate PPU. The portion of the intensity distribution in the plane of the pupil conjugate PPU and associated with the zeroth order diffracted beams is an image of the intensity distribution in the illumination system pupil IPU of the illumination system IL. The aperture device PD, for example, is disposed at or substantially at a plane that includes the pupil conjugate PPU of the projection system PS.

[0041] The projection system PS is arranged to capture (e.g., using a lens or lens group L) the zeroth order diffracted beams, first order diffracted beams, and/or higher order diffracted beams (not shown). In some aspects, dipole illumination for imaging line patterns extending in a direction perpendicular to a line can be used to utilize the resolution enhancement effect of dipole illumination. For example, first-order diffracted beams interfere with corresponding zeroth-order diffracted beams at the level of the wafer W to create an image of the line pattern MP at highest possible resolution and process window (i.e., usable depth of focus in combination with tolerable exposure dose deviations). In some aspects, astigmatism aberration can be reduced by providing radiation poles (not shown) in opposite quadrants of the illumination system pupil IPU. Further, in some aspects, astigmatism aberration can be reduced by blocking the zeroth order beams in the pupil conjugate PPU of the projection system associated with radiation poles in opposite quadrants. This is described in more detail in US 7,511,799 B2, issued Mar. 31, 2009, which is incorporated by reference herein in its entirety.

[0042] With the aid of the second positioner PW and position sensor IFD (for example, an interferometric device, linear encoder, or capacitive sensor), the substrate table WT can be moved accurately (for example, so as to position different target portions C in the path of the radiation beam B). Similarly, the first positioner PM and another position sensor (not shown in FIG. IB) can be used to accurately position the mask MA with respect to the path of the radiation beam B (for example, after mechanical retrieval from a mask library or during a scan).

[0043] In general, movement of the mask table MT can be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the first positioner PM. Similarly, movement of the substrate table WT can be realized using a long-stroke module and a short-stroke module, which form part of the second positioner PW. In the case of a stepper (as opposed to a scanner), the mask table MT can be connected to a short-stroke actuator or can be fixed. Mask MA and substrate W can be aligned using mask alignment marks Ml, M2, and substrate alignment marks Pl, P2. Although the substrate alignment marks (as illustrated) occupy dedicated target portions, they can be located in spaces between target portions (known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the mask MA, the mask alignment marks can be located between the dies.

[0044] Mask table MT and patterning device MA can be in a vacuum chamber V, where an invacuum robot IVR can be used to move patterning devices such as a mask in and out of vacuum chamber. Alternatively, when mask table MT and patterning device MA are outside of the vacuum chamber, an out-of- vacuum robot can be used for various transportation operations, similar to the invacuum robot IVR. Both the in-vacuum and out-of-vacuum robots can be calibrated for a smooth transfer of any payload (e.g., mask) to a fixed kinematic mount of a transfer station.

[0045] The lithographic apparatus 100 and 100’ can be used in at least one of the following modes: [0046] 1. In step mode, the support structure (for example, mask table) MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam B is projected onto a target portion C at one time (i.e., 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.

[0047] 2. In scan mode, the support structure (for example, mask table) 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 (i.e., a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure (for example, mask table) MT can be determined by the (de- jmagnification and image reversal characteristics of the projection system PS.

[0048] 3. In another mode, the support structure (for example, mask table) MT is kept substantially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam B is projected onto a target portion C. A pulsed radiation source SO can be employed and the programmable patterning device is updated as needed after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes a programmable patterning device, such as a programmable mirror array.

[0049] Combinations and/or variations on the described modes of use or entirely different modes of use can also be employed.

[0050] In some aspects, lithographic apparatus 100 includes an extreme ultraviolet (EUV) source, which is configured to generate a beam of EUV radiation for EUV lithography. In general, the EUV source is configured in a radiation system, and a corresponding illumination system is configured to condition the EUV radiation beam of the EUV source.

[0051] In some aspects, lithographic apparatus 100’ includes a deep ultraviolet (DUV) source, which is configured to generate a beam of DUV radiation for DUV lithography. In general, the DUV source is configured in a radiation system, and a corresponding illumination system is configured to condition the DUV radiation beam of the DUV source.

[0052] FIG. 2 shows the lithographic apparatus 100 in more detail, including the source collector apparatus SO, the illumination system IL, and the projection system PS. The source collector apparatus SO is constructed and arranged such that a vacuum environment can be maintained in an enclosing structure 220 of the source collector apparatus SO. An EUV radiation emitting plasma 210 can be formed by a discharge produced plasma source. EUV radiation can be produced by a gas or vapor, for example Xe gas, Li vapor, or Sn vapor in which EUV radiation emitting plasma 210 is created to emit radiation in the EUV range of the electromagnetic spectrum. The EUV radiation emitting plasma 210 is created by, for example, an electrical discharge causing at least a partially ionized plasma. Partial pressures of, for example, 10 Pa of Xe, Li, Sn vapor, or any other suitable gas or vapor can be used for efficient generation of the radiation. In some aspects, a plasma of excited tin (Sn) (e.g., excited via a laser) is provided to produce EUV radiation.

[0053] The radiation emitted by the EUV radiation emitting plasma 210 is passed from a source chamber 211 into a collector chamber 212 via an optional gas barrier or contaminant trap 230 (in some cases also referred to as contaminant barrier or foil trap), which is positioned in or behind an opening in source chamber 211. The contaminant trap 230 can include a channel structure. Contamination trap 230 can also include a gas barrier or a combination of a gas barrier and a channel structure. The contaminant trap or contaminant barrier 230 further indicated herein at least includes a channel structure.

[0054] The collector chamber 212 can include a radiation collector CO, which can be a so-called grazing incidence collector. Radiation collector CO has an upstream radiation collector side 251 and a downstream radiation collector side 252. Radiation that traverses collector CO can be reflected off a grating spectral filter 240 to be focused in a virtual source point INTF. The virtual source point INTF is commonly referred to as the intermediate focus, and the source collector apparatus is arranged such that the intermediate focus INTF is located at or near an opening 219 in the enclosing structure 220. The virtual source point INTF is an image of the EUV radiation emitting plasma 210. Grating spectral filter 240 is used in particular for suppressing infra-red (IR) radiation.

[0055] Subsequently the radiation traverses the illumination system IL, which can include a faceted field mirror device 222 and a faceted pupil mirror device 224 arranged to provide a desired angular distribution of the radiation beam 221, at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA. Upon reflection of the beam of radiation 221 at the patterning device MA, held by the support structure MT, a patterned beam 226 is formed and the patterned beam 226 is imaged by the projection system PS via reflective elements 228, 229 onto a substrate W held by the wafer stage or substrate table WT.

[0056] More elements than shown can generally be present in illumination optics unit IL and projection system PS. The grating spectral filter 240 can optionally be present, depending upon the type of lithographic apparatus. Further, there can be more mirrors present than those shown in the FIG. 2, for example there can be one to six additional reflective elements present in the projection system PS than shown in FIG. 2.

[0057] Collector optic CO, as illustrated in FIG. 2, is depicted as a nested collector with grazing incidence reflectors 253, 254, and 255, just as an example of a collector (or collector mirror). The grazing incidence reflectors 253, 254, and 255 are disposed axially symmetric around an optical axis O and a collector optic CO of this type is preferably used in combination with a discharge produced plasma source, often called a DPP source.

[0058] Example Lithographic Cell

[0059] FIG. 3 shows a lithographic cell 300, also sometimes referred to a lithocell or cluster, according to some aspects. Lithographic apparatus 100 or 100’ can form part of lithographic cell 300. Lithographic cell 300 can also include one or more apparatuses to perform pre- and post-exposure processes on a substrate. Conventionally these include spin coaters SC to deposit resist layers, developers DE to develop exposed resist, chill plates CH, and bake plates BK. A substrate handler, or robot, RO picks up substrates from input/output ports I/Ol, I/O2, moves them between the different process apparatuses and delivers them to the loading bay LB of the lithographic apparatus 100 or 100’. These devices, which are often collectively referred to as the track, are under the control of a track control unit TCU, which is itself controlled by a supervisory control system SCS, which also controls the lithographic apparatus via lithography control unit LACU. Thus, the different apparatuses can be operated to maximize throughput and processing efficiency.

[0060] Example Inspection Apparatus

[0061] In order to control the lithographic process to place device features accurately on the substrate, alignment marks are generally provided on the substrate, and the lithographic apparatus includes one or more inspection apparatuses for accurate positioning of marks on a substrate. These alignment apparatuses are effectively position measuring apparatuses. Different types of marks and different types of alignment apparatuses and/or systems are known from different times and different manufacturers. A type of system widely used in current lithographic apparatus is based on a selfreferencing interferometer as described in U.S. Patent No. 6,961,116 (den Boef et al.). Generally marks are measured separately to obtain X- and Y-positions. A combined X- and Y-measurement can be performed using the techniques described in U.S. Publication No. 2009/195768 A (Bijnen et al.), however. The full contents of both of these disclosures are incorporated herein by reference.

[0062] FIG. 4A shows a cross-sectional view of an inspection apparatus 400 that can be implemented as a part of lithographic apparatus 100 or 100’, according to some aspects. In some aspects, inspection apparatus 400 can be configured to align a substrate (e.g., substrate W) with respect to a patterning device (e.g., patterning device MA). Inspection apparatus 400 can be further configured to detect positions of alignment marks on the substrate and to align the substrate with respect to the patterning device or other components of lithographic apparatus 100 or 100’ using the detected positions of the alignment marks. Such alignment of the substrate can ensure accurate exposure of one or more patterns on the substrate.

[0063] The terms “inspection apparatus,” “metrology system,” or the like can be used herein to refer to, e.g., a device used for measuring a property of a structure (e.g., overlay sensor, critical dimension sensor, or the like), a device or system used in a lithographic apparatus to inspect an alignment of a wafer (e.g., alignment sensor), or the like.

[0064] In some aspects, inspection apparatus 400 can include an illumination system 412, a beam splitter 414, an interferometer 426, a detector 428, a beam analyzer 430, and a processor 432. Illumination system 412 can be configured to provide an electromagnetic narrow band radiation beam 413 having one or more passbands. In an example, the one or more passbands can be within a spectrum of wavelengths between about 500 nm to about 900 nm. In another example, the one or more passbands can be discrete narrow passbands within a spectrum of wavelengths between about 500 nm to about 900 nm. Illumination system 412 can be further configured to provide one or more passbands having substantially constant center wavelength (CWL) values over a long period of time (e.g., over a lifetime of illumination system 412). Such configuration of illumination system 412 can help to prevent the shift of the actual CWL values from the desired CWL values, as discussed above, in current alignment systems. And, as a result, the use of constant CWL values can improve long-term stability and accuracy of alignment systems (e.g., inspection apparatus 400) compared to the current alignment apparatuses.

[0065] In some aspects, beam splitter 414 can be configured to receive radiation beam 413 and split radiation beam 413 into at least two radiation sub-beams. For example, radiation beam 413 can be split into radiation sub-beams 415 and 417, as shown in FIG. 4A. Beam splitter 414 can be further configured to direct radiation sub-beam 415 onto a substrate 420 placed on a stage 422. In one example, the stage 422 is movable along direction 424. Radiation sub-beam 415 can be configured to illuminate an alignment mark or a target 418 located on substrate 420. Alignment mark or target 418 can be coated with a radiation sensitive film. In some aspects, alignment mark or target 418 can have one hundred and eighty degrees (i.e., 180°) symmetry. That is, when alignment mark or target 418 is rotated 180° about an axis of symmetry perpendicular to a plane of alignment mark or target 418, rotated alignment mark or target 418 can be substantially identical to an unrotated alignment mark or target 418. The target 418 on substrate 420 can be (a) a resist layer grating comprising bars that are formed of solid resist lines, or (b) a product layer grating, or (c) a composite grating stack in an overlay target structure comprising a resist grating overlaid or interleaved on a product layer grating. The bars can alternatively be etched into the substrate. This pattern is sensitive to chromatic aberrations in the lithographic projection apparatus, particularly the projection system PL, and illumination symmetry and the presence of such aberrations will manifest themselves in a variation in the printed grating. One in-line method used in device manufacturing for measurements of line width, pitch, and critical dimension makes use of a technique known as “scatterometry”. Methods of scatterometry are described in Raymond et al., “Multiparameter Grating Metrology Using Optical Scatterometry”, J. Vac. Sci. Tech. B, Vol. 15, no. 2, pp. 361-368 (1997) and Niu et al., “Specular Spectroscopic Scatterometry in DUV Lithography”, SPIE, Vol. 3677 (1999), which are both incorporated by reference herein in their entireties. In scatterometry, light is reflected by periodic structures in the target, and the resulting reflection spectrum at a given angle is detected. The structure giving rise to the reflection spectrum is reconstructed, e.g. using Rigorous Coupled-Wave Analysis (RCWA) or by comparison to a library of patterns derived by simulation. Accordingly, the scatterometry data of the printed gratings is used to reconstruct the gratings. The parameters of the grating, such as line widths and shapes, can be input to the reconstruction process, performed by processing unit PU, from knowledge of the printing step and/or other scatterometry processes.

[0066] In some aspects, beam splitter 414 can be further configured to receive diffraction radiation beam 419 and split diffraction radiation beam 419 into at least two radiation sub-beams, according to an aspect. Diffraction radiation beam 419 can be split into diffraction radiation sub-beams 429 and 439, as shown in FIG. 4A.

[0067] It should be noted that even though beam splitter 414 is shown to direct radiation sub-beam 415 towards alignment mark or target 418 and to direct diffracted radiation sub-beam 429 towards interferometer 426, the disclosure is not so limiting. Other optical arrangements can be used to obtain the similar result of illuminating alignment mark or target 418 on substrate 420 and detecting an image of alignment mark or target 418.

[0068] As illustrated in FIG. 4A, interferometer 426 can be configured to receive radiation sub-beam 417 and diffracted radiation sub-beam 429 through beam splitter 414. In an example aspect, diffracted radiation sub-beam 429 can be at least a portion of radiation sub-beam 415 that can be reflected from alignment mark or target 418. In an example of this aspect, interferometer 426 comprises any appropriate set of optical-elements, for example, a combination of prisms that can be configured to form two images of alignment mark or target 418 based on the received diffracted radiation sub-beam 429. It should be appreciated that a good quality image need not be formed. It can be enough to have the features of alignment mark 418 resolved. Interferometer 426 can be further configured to rotate one of the two images with respect to the other of the two images 180° and recombine the rotated and unrotated images interferometrically.

[0069] In some aspects, detector 428 can be configured to receive the recombined image via interferometer signal 427 and detect interference as a result of the recombined image when alignment axis 421 of inspection apparatus 400 passes through a center of symmetry (not shown) of alignment mark or target 418. Such interference can be due to alignment mark or target 418 being 180° symmetrical, and the recombined image interfering constructively or destructively, according to an example aspect. Based on the detected interference, detector 428 can be further configured to determine a position of the center of symmetry of alignment mark or target 418 and consequently, detect a position of substrate 420. According to an example, alignment axis 421 can be aligned with an optical beam perpendicular to substrate 420 and passing through a center of image rotation interferometer 426. Detector 428 can be further configured to estimate the positions of alignment mark or target 418 by implementing sensor characteristics and interacting with wafer mark process variations.

[0070] In a further aspect, detector 428 determines the position of the center of symmetry of alignment mark or target 418 by performing one or more of the following measurements:

[0071] 1. measuring position variations for various wavelengths (position shift between colors); [0072] 2. measuring position variations for various orders (position shift between diffraction orders); [0073] 3. measuring position variations for various polarizations (position shift between polarizations); and

[0074] 4. measuring intensity difference between opposite orders of a diffraction order pair (e.g., to characterize and correct for asymmetry).

[0075] This data can be obtained using any type of alignment sensor, for example, a SMASH (SMart Alignment Sensor Hybrid) sensor, as described in U.S. Patent No. 6,961,116 that employs a selfreferencing interferometer with a single detector and four different wavelengths, and extracts the alignment signal in software, or Athena (Advanced Technology using High order ENhancement of Alignment), as described in U.S. Patent No. 6,297,876, which directs each of seven diffraction orders to a dedicated detector, which are both incorporated by reference herein in their entireties.

[0076] In some aspects, beam analyzer 430 can be configured to receive and determine an optical state of diffracted radiation sub-beam 439. The optical state can be a measure of beam wavelength, polarization, or beam profile. Beam analyzer 430 can be further configured to determine a position of stage 422 and correlate the position of stage 422 with the position of the center of symmetry of alignment mark or target 418. As such, the position of alignment mark or target 418 and, consequently, the position of substrate 420 can be accurately known with reference to stage 422. Alternatively, beam analyzer 430 can be configured to determine a position of inspection apparatus 400 or any other reference element such that the center of symmetry of alignment mark or target 418 can be known with reference to inspection apparatus 400 or any other reference element. Beam analyzer 430 can be a point or an imaging polarimeter with some form of wavelength-band selectivity. In some aspects, beam analyzer 430 can be directly integrated into inspection apparatus 400, or connected via fiber optics of several types: polarization preserving single mode, multimode, or imaging, according to other aspects.

[0077] In some aspects, beam analyzer 430 can be further configured to determine the overlay data between two patterns on substrate 420. One of these patterns can be a reference pattern on a reference layer. The other pattern can be an exposed pattern on an exposed layer. The reference layer can be an etched layer already present on substrate 420. The reference layer can be generated by a reference pattern exposed on the substrate by lithographic apparatus 100 and/or 100’. The exposed layer can be a resist layer exposed adjacent to the reference layer. The exposed layer can be generated by an exposure pattern exposed on substrate 420 by lithographic apparatus 100 or 100’. The exposed pattern on substrate 420 can correspond to a movement of substrate 420 by stage 422. In some aspects, the measured overlay data can also indicate an offset between the reference pattern and the exposure pattern. The measured overlay data can be used as calibration data to calibrate the exposure pattern exposed by lithographic apparatus 100 or 100’, such that after the calibration, the offset between the exposed layer and the reference layer can be minimized.

[0078] In some aspects, beam analyzer 430 can be further configured to determine a model of the product stack profile of substrate 420, and can be configured to measure overlay, critical dimension, and focus of target 418 in a single measurement. The product stack profile contains information on the stacked product such as alignment mark, target 418, or substrate 420, and can include mark process variation-induced optical signature metrology that is a function of illumination variation. The product stack profile can also include product grating profile, mark stack profile, and mark asymmetry information. An example of beam analyzer 430 is Yieldstar™, manufactured by ASML, Veldhoven, The Netherlands, as described in U.S. Patent No. 8,706,442, which is incorporated by reference herein in its entirety. Beam analyzer 430 can be further configured to process information related to a particular property of an exposed pattern in that layer. For example, beam analyzer 430 can process an overlay parameter (an indication of the positioning accuracy of the layer with respect to a previous layer on the substrate or the positioning accuracy of the first layer with respective to marks on the substrate), a focus parameter, and/or a critical dimension parameter (e.g., line width and its variations) of the depicted image in the layer. Other parameters are image parameters relating to the quality of the depicted image of the exposed pattern.

[0079] In some aspects, an array of detectors (not shown) can be connected to beam analyzer 430, and allows the possibility of accurate stack profile detection as discussed below. For example, detector 428 can be an array of detectors. For the detector array, a number of options are possible: a bundle of multimode fibers, discrete pin detectors per channel, or CCD or CMOS (linear) arrays. The use of a bundle of multimode fibers enables any dissipating elements to be remotely located for stability reasons. Discrete PIN detectors offer a large dynamic range but each need separate pre-amps. The number of elements is therefore limited. CCD linear arrays offer many elements that can be readout at high speed and are especially of interest if phase-stepping detection is used.

[0080] In some aspects, a second beam analyzer 430’ can be configured to receive and determine an optical state of diffracted radiation sub-beam 429, as shown in FIG. 4B. The optical state can be a measure of beam wavelength, polarization, or beam profile. Second beam analyzer 430’ can be identical to beam analyzer 430. Alternatively, second beam analyzer 430’ can be configured to perform one or more of the functions of beam analyzer 430, such as determining a position of stage 422 and correlating the position of stage 422 with the position of the center of symmetry of alignment mark or target 418. As such, the position of alignment mark or target 418 and, consequently, the position of substrate 420, can be accurately known with reference to stage 422. Second beam analyzer 430’ can also be configured to determine a position of inspection apparatus 400, or any other reference element, such that the center of symmetry of alignment mark or target 418 can be known with reference to inspection apparatus 400, or any other reference element. Second beam analyzer 430’ can be further configured to determine the overlay data between two patterns and a model of the product stack profile of substrate 420. Second beam analyzer 430’ can also be configured to measure overlay, critical dimension, and focus of target 418 in a single measurement.

[0081] In some aspects, second beam analyzer 430’ can be directly integrated into inspection apparatus 400, or it can be connected via fiber optics of several types: polarization preserving single mode, multimode, or imaging, according to other aspects. Alternatively, second beam analyzer 430’ and beam analyzer 430 can be combined to form a single analyzer (not shown) configured to receive and determine the optical states of both diffracted radiation sub-beams 429 and 439.

[0082] In some aspects, processor 432 receives information from detector 428 and beam analyzer 430. For example, processor 432 can be an overlay calculation processor. The information can comprise a model of the product stack profile constructed by beam analyzer 430. Alternatively, processor 432 can construct a model of the product mark profile using the received information about the product mark. In either case, processor 432 constructs a model of the stacked product and overlay mark profile using or incorporating a model of the product mark profile. The stack model is then used to determine the overlay offset and minimizes the spectral effect on the overlay offset measurement. Processor 432 can create a basic correction algorithm based on the information received from detector 428 and beam analyzer 430, including but not limited to the optical state of the illumination beam, the alignment signals, associated position estimates, and the optical state in the pupil, image, and additional planes. The pupil plane is the plane in which the radial position of radiation defines the angle of incidence and the angular position defines the azimuth angle of the radiation. Processor 432 can utilize the basic correction algorithm to characterize the inspection apparatus 400 with reference to wafer marks and/or alignment marks 418.

[0083] In some aspects, processor 432 can be further configured to determine printed pattern position offset error with respect to the sensor estimate for each mark based on the information received from detector 428 and beam analyzer 430. The information includes but is not limited to the product stack profile, measurements of overlay, critical dimension, and focus of each alignment marks or target 418 on substrate 420. Processor 432 can utilize a clustering algorithm to group the marks into sets of similar constant offset error, and create an alignment error offset correction table based on the information. The clustering algorithm can be based on overlay measurement, the position estimates, and additional optical stack process information associated with each set of offset errors. The overlay is calculated for a number of different marks, for example, overlay targets having a positive and a negative bias around a programmed overlay offset. The target that measures the smallest overlay is taken as reference (as it is measured with the best accuracy). From this measured small overlay, and the known programmed overlay of its corresponding target, the overlay error can be deduced. Table 1 illustrates how this can be performed. The smallest measured overlay in the example shown is -1 nm. However this is in relation to a target with a programmed overlay of -30 nm. The process may have introduced an overlay error of 29 nm.

[0084] The smallest value can be taken to be the reference point and, relative to this, the offset can be calculated between measured overlay and that expected due to the programmed overlay. This offset determines the overlay error for each mark or the sets of marks with similar offsets. Therefore, in the Table 1 example, the smallest measured overlay was -1 nm, at the target position with programmed overlay of 30 nm. The difference between the expected and measured overlay at the other targets is compared to this reference. A table such as Table 1 can also be obtained from marks and target 418 under different illumination settings, the illumination setting, which results in the smallest overlay error, and its corresponding calibration factor, can be determined and selected. Following this, processor 432 can group marks into sets of similar overlay error. The criteria for grouping marks can be adjusted based on different process controls, for example, different error tolerances for different processes.

[0085] In some aspects, processor 432 can confirm that all or most members of the group have similar offset errors, and apply an individual offset correction from the clustering algorithm to each mark, based on its additional optical stack metrology. Processor 432 can determine corrections for each mark and feed the corrections back to lithographic apparatus 100 or 100’ for correcting errors in the overlay, for example, by feeding corrections into the inspection apparatus 400.

[0086] Example Aberration Correction for Optical Systems

[0087] Optical aberrations (e.g., wavefront error (WFE)) have been one of the major challenges for metrology tools used in the service of lithographic fabrication of nanoscale devices. Machine-to- machine (M2M) matching is difficult to achieve when it comes to sensors, such as inspection apparatus 400. Ideally, each sensor is manufactured identically. However, due to uncertainties in manufacture of parts and assembly, each sensor build can vary slightly. In other words, M2M performance of a sensor design can be matched, but to within some tolerance. Consider using the same target 418 for measurement under two separate constructions of inspection apparatus 400, the constructions differing by a small amount due to manufacturing uncertainties. The two versions of inspection apparatus 400 would produce very similar measurement results, but with some systematic variation due to the minute differences in construction. M2M mismatch can be considered as performance (e.g., sensor performance) that is outside of a prescribed tolerance.

[0088] In some aspects, aberrations contribute to M2M mismatch. There are various types of aberrations that can occur throughout the optical path of inspection system 400, which can include spherical aberrations (e.g., Zernikes Z9/Z16, correlated to certain user-designed alignment marks), astigmatism (e.g., Zernikes Z5/Z6), coma (e.g., Zernikes Z7/8), trefoils, and higher order Zernike terms, all of which can have different sensitivities to OV measurement depending on the design of overlay targets fashioned by a user of the inspection apparatus. The options for aberration compensators is limited and often costly.

[0089] In some embodiments, many of the optical components within the sensor contribute to the overall sensor aberrations (e.g., lenses, high numerical aperture (NA) objective, or the like) and they are very difficult to compensate without the use of a very expensive approach, such as a deformable mirror, to actively control the aberrations. The deformable mirror is a very expensive and bulky device. In order to implement a deformable mirror in a sensor like Yieldstar™, optical arrangements would be significantly changed to accommodate the shape and size of a deformable mirror in the existing volume constraints. Some approaches described herein are able to solve the issues of aberration within a small volume constraint and in a cost-effective manner.

[0090] FIG. 5 shows an inspection apparatus 500, according to some aspects. In some embodiments, inspection apparatus 500 can be another version of inspection apparatus 400 (FIGS. 4A and 4B) with additional features. Therefore, it is to be understood that the description above in reference to FIGS. 4A and 4B is also applicable to inspection apparatus 500, unless otherwise noted. As examples, one or more of the labeled elements in FIG. 5 have reference numbers that match with elements illustrated in FIGS. 4A and 4B (e.g., reference numbers sharing the two right-most numeric digits), for example, beam of radiation 513, beam splitter 514, a portion 515 (e.g., a first portion) of beam of radiation 513, and substrate 520.

[0091] In some aspects, inspection apparatus 500 can comprise beam directing devices (e.g., beam splitters 514, 540, and 542), an optical objective 544, a wedge system 546, a spatial light modulator (SLM) 548, and a detector 550 (e.g., a first detector). Inspection apparatus 500 can also comprise a detector 552 (e.g., a second detector). Wedge system 546 can comprise a wedge 554 (e.g., a first wedge), a wedge 556 (e.g., a second wedge), and radiation-curable adhesive 558 disposed therebetween. Though SLM 548 is illustrated as a reflective type (e.g., a digital micromirror), SLM 548 can comprise any spatial light modulator capable of adjusting a spatial distribution of illumination intensity (e.g., a liquid crystal).

[0092] In some aspects, beam splitter 514 can split beam of radiation 513 into portions 515 and 541 (e.g., first and second portions). Though the term “beam splitter” is indicative of a beam splitting function, it is to be appreciated that beam splitters have other functions as well (e.g., directing/re- directing beams of radiation, combining beams of radiation, or the like). For example, beam splitter 542 can direct/re-direct beam of radiation 513 via reflection. In another example, beam splitter 540 can combine the optical paths of beam of radiation 513 and beam of radiation 570’.

[0093] In some aspects, beam of radiation 513 (e.g., generated by illumination system 412 (FIGS. 4A and 4B)) initially has a wavefront 560. Wavefront 560 can be a wavefront of coherent illumination or partially coherent illumination (e.g., spatially coherent but not temporally coherent, or vice versa). It is desirable that, by the time beam of radiation 513 (or portion 515 thereof) impinges on a target, its wavefront 562 is uniform (e.g., no aberrations). A target can be a diffraction target, which is sensitive to inconsistent wavefronts, which in turn can cause a wider error margin when inspection apparatus 500 is used for an optical measurement. Each piece of optical hardware can introduce its own, distinctive aberration (e.g., any combination of the Zernikes described above). This can apply to the source of beam of radiation 513, beam splitters 514, 540, and/or 542, optical objective 544, and wedge system 546. Additional wavefronts 564, 566, and 568 are noted at various parts of the optical path of beam of radiation 513, each wavefront slightly changed based on the optical element that preceded it. Optical elements used in connection with redirection of beam of radiation 570’ (e.g., beam splitter 540) can be a tooling that is removed after wavefront optimization or can remain as part of the sensor product.

[0094] In some aspects, beam of radiation 513 (or portion 515 thereof) can be directed toward substrate 520 (e.g., via beam splitter 514). Substrate 520 can scatter the received radiation as scattered radiation 543.

[0095] In some aspects, wedge system 546 can be used for slightly offsetting or redirecting the optical path of radiation beams (e.g., for reducing ghost signals that disrupt measurements and reduce measurement accuracy). Ghost signals can be generated by undesirable reflections at one or more optical components. Wedge system 546 can also be used to reduce aberrations. To achieve this, radiation-curable adhesive 558 is leveraged.

[0096] In some aspects, radiation-curable adhesive 558 can be cured in such a way it has a regiondependent optical property (e.g., refractive index or dispersion that varies based on location). Radiation-curable adhesive 558 can be cured using beam of radiation 570’. A beam of radiation 570 can be incident on SLM 548. SLM 548 can control the intensity distribution of beam of radiation 570 to produce beam of radiation 570’. Depending on the dose of radiation, local regions of radiation- curable adhesive 558 can be cured by different amounts. When performing the curing of radiation- curable adhesive 558, wavefront aberrations can be quantified using detectors 550 and/or 552. The measurement results can be used as a feedback loop to adjust the intensity distribution of beam of radiation 570’, thereby adjusting the localized exposure dose at radiation-curable adhesive 558, thereby adjusting the localized optical properties of radiation-curable adhesive 558. [0097] In some aspects, though a target is not illustrated in FIG. 5, it is to be understood that inspection apparatus 500 can measure a target (e.g., 418 (FIGS. 4A and 4B). A reason is that the setup in FIG. 5 is for characterizing and reducing aberrations by having beam of radiation 513 (or portion 515 thereof) be incident upon substrate 520. Once aberration treatment is completed, inspection apparatus 500 can be used for measurements of targets as described above in reference to FIGS. 4A and 4B.

[0098] FIG. 6 shows a flow chart of a method 600 for reducing optical aberrations in an optical system, according to some aspects. Some aspects of method 600 will be described with reference to one or more structures introduced in FIG. 5.

[0099] In some aspects, at operation 602, an aberration induced by an optical system (e.g., inspection apparatus 500) is determined. Operation 602 can comprise operations 604 and 606.

[0100] In some aspects, at operation 604, beam of radiation 513 (e.g., first beam of radiation), having a first wavelength, is transmitted through wedge system 546 of the optical system. At this stage, radiation-curable adhesive 558 can be in a partially cured state or uncured state. Radiation-curable adhesive 558 can be insensitive or non-reactive to the first wavelength (e.g., radiation having the first wavelength does not cure radiation-curable adhesive 558). A reason is that beam of radiation 513 can have the wavelengths that will be consistently used during actual measurement operations on targets (e.g., target 418 (FIGS. 4A and 4B)). It can be problematic if the optical characteristics of wedge system 546 were to change during measurements or over repeated measurements. The first wavelength can comprise wavelengths in the 400 nm to 900 nm (e.g., non-curing ultraviolet, visible range, and infrared). The non-curing ultraviolet can be different from the curing ultraviolet used for beam of radiation 570/570’.

[0101] In some aspects, at operation 606, the first beam is analyzed using a detector (e.g., 550, 552, or both) disposed downstream of wedge system 546 to determine the aberration. It is to be understood that analyzing the first beam can comprise analyzing beam of radiation 513 (or portion 541 thereof) at detector 550, analyzing beam of radiation 513 (or portion 515 thereof) at detector 552, or both. Analyzing beam of radiation 513 (or portion 515 thereof) at detector 552 can be achieved by receiving scattered radiation 543 at detector 552. Scattered radiation 543 carries information about beam of radiation 513 (or portion 515 thereof). Scattered radiation 543 can be generated by reflection at substrate 520 since no target is present in the optical path during this operation. Furthermore, scattered radiation 543 carries information about the aberration resulting from optical objective 544. For a high NA optical objective 544 (e.g., NA approximately 0.5 or greater), the collective aberration is expected to be primarily due to optical objective 544. A technical effect of having beam of radiation 513 (or portion 515 thereof) pass through optical objective 544 for a measurement is to characterize elements that contribute the most to aberration.

[0102] In some aspects, inspection system 500 can be thought of as comprising two branches: an illumination branch and a detection branch. The illumination branch can encompass those elements responsible for delivering radiation to substrate 520 (or to a target when performing measurements). That is, the illumination branch can encompass elements upstream of substrate 520. For example, FIG. 5 shows wedge system 546 disposed in the illumination branch. The detection branch can encompass detector(s) and those elements that are responsible for collecting and directing scattered radiation from substrate 520 (or from a target when performing measurements) to the detector(s). That is, the detection branch can encompass elements downstream of substrate 520. For example, wedge system 546 can be removed from the illumination branch and be replaced by a wedge system 572 of the same construction at the detection branch (or wedge systems can be implemented in both the illumination and detection branches).

[0103] In some embodiments, the analyzed beam of radiation 513 (or portion 541 thereof) via detector 550 effectively measures the aberrations introduced by the illumination branch. This is because radiation received at detector 550 bypasses detection branch elements (e.g., bypasses substrate 520 and optical objective 544). In addition to the analysis via detector 550, or in the alternative, the analyzed beam of radiation 513 (or portion 515 thereof) via detector 552 measures the aberrations introduced by the illumination branch and the detection branch. This is because detector 552 is disposed downstream of wedge system 546, optical objective 544, and substrate 520. The aberration introduced by the detection branch can be inferred from a comparison of the analysis via detector 550 and the analysis via detector 552. Aberrations can be characterized using wavefrontsensitive cameras. Detectors 550 and 552 can be, for example, Shack-Hartmann wavefront sensors.

[0104] In some aspects, at operation 608, radiation-curable adhesive 558 is cured based on the analyzing of the first beam. Operation 608 can comprise operations 610 and 612.

[0105] In some aspects, with the aberrations quantified based on the analyzing of the first beam, the method can proceed to spatially tailoring the curing dose. At operation 610, using SLM 548, an intensity distribution of beam of radiation 570’ (e.g., a second beam of radiation) is adjusted based on the analyzing of the first beam. Beam of radiation 570, which sources beam of radiation 570’, can be generated by a source that generates a second wavelength. Radiation-curable adhesive 558 can be sensitive or reactive to the second wavelength (e.g., ultraviolet wavelength smaller than 400 nm) while remaining non-reactive to the first wavelength. Based on the tailoring of the intensity distribution of beam of radiation 570’, different regions of radiation-curable adhesive 558 can be selectively cured such that a localized position-dependent optical property is induced. For example, one region can have a prescribed refractive index while another region can have a different prescribed refractive index. The induced position-dependent optical property can be such that aberrations are reduced in inspection apparatus 500. The aberration reduction can be ascertained in a subsequent measurement.

[0106] In some aspects, at operation 614, an efficacy of the curing in reducing the aberration can be determined via a measurement using the detector (e.g., 550, 552, or both). Operation 614 can be performed as described in reference to operation 606 and its sub-operations. After the curing, beam of radiation 513 can be transmitted through wedge system 546. Since beam of radiation 513 is being reused, the reuse of beam of radiation 513 can be referred to as a “third beam of radiation having the first wavelength,” (e.g., to distinguish from the previous first and second beams). In other words, at operation 614, beam of radiation 513 is reapplied to wedge system 546. Then, the transmitted third beam (e.g., beam of radiation 513 or portion 515 thereof) can be analyzed via the detector to determine a change of the aberration that has resulted from the spatially tailored curing. The determining of the efficacy can comprise determining whether an extent of the aberration has been meets a prescribed threshold (e.g., reduced to, or crosses, the threshold).

[0107] In some aspects, if the aberration reduction is found to be unsatisfactory, the curing can be repeated. At operation 616, radiation-curable adhesive 558 is further cured based on the determining of the efficacy. Operation 616 can be performed as described in reference to operation 608 and its sub-operations. After the determining of the efficacy, using SLM 548, an intensity distribution of beam of radiation 570’ can be adjusted based on the determining of the efficacy. Since beam of radiation 570’ is being reused, the reuse of beam of radiation 570’ can be referred to as a “fourth beam of radiation having the second wavelength” (e.g., to distinguish from the previous first, second, and third beams). In other words, at operation 616, beam of radiation 570’ is reapplied to wedge system 546. Then, the fourth beam can be directed to the radiation-curable adhesive to adjust the position-dependent optical property at the radiation curable adhesive. Terms such as “first wavelength” and “second wavelength” can be interpreted as wavelength bands (e.g., narrowband). For example, terms such as “a wavelength” can include a peak or central wavelength along with a range of wavelengths centered around the peak wavelength.

[0108] In some aspects, the determining of the efficacy and the further curing can be repeated until the aberration is reduced so as to meet the prescribed threshold. For example, method 600 can comprise determining an efficacy of the further curing of operation 616 via measurement using the detector. If the aberration is not satisfactory, yet additional curing can be spatially tailored to further adjust the position-dependent optical property.

[0109] In some aspects that implement wedge system 572 (whether alone or in addition to wedge system 546), it is to be appreciated that SLM 548 can be positioned so as to direct beam of 570’ toward wedge system 572, or an additional SLM can be used. The operations described above in reference to wedge system 546 can also be performed with respect to wedge system 572.

[0110] The method steps of FIG. 6 can be performed in any conceivable order and it is not required that all steps be performed. Moreover, the method steps of FIG. 6 described above merely reflect an example of steps and are not limiting. For example, in some aspects, wedge systems 546 and/or 572 can be assembled. The assembly operations can comprise disposing radiation-curable adhesive 558 on a facet of either of wedges 554 and 556 and then bringing wedges 554 and 556 together. The assembling of wedges 554 and 556 with radiation-curable adhesive 558 can be performed such that radiation-curable adhesive 558 has a substantially constant thickness (e.g., approximately 50 pm or less, approximately 30 pm or less, approximately 20 pm or less, approximately 10 pm or less, or the like. All of the functions described herein form the basis for additional or alternative operations.

[0111] The embodiments may further be described using the following clauses:

1. A method of reducing optical aberrations in an optical system, the method comprising: determining an aberration induced by the optical system, comprising: transmitting a first beam of radiation having a first wavelength through a wedge system of the optical system, the wedge system comprising first and second wedges and radiation-curable adhesive disposed between the first and the second wedges; and analyzing the first beam using a detector disposed downstream of the wedge system to determine the aberration; and curing the radiation-curable adhesive based on the analyzing of the first beam, comprising: adjusting, using a spatial light modulator, an intensity distribution of a second beam of radiation based on the analyzing of the first beam, wherein the second beam has a second wavelength different than the first wavelength; and directing the second beam to the radiation-curable adhesive to induce a position-dependent optical property at the radiation-curable adhesive to thereby reduce the aberration, wherein the radiation- curable adhesive is reactive to the second wavelength.

2. The method of clause 1 , further comprising: determining an efficacy of the curing in reducing the aberration via a measurement using the detector, comprising: transmitting a third beam of radiation having the first wavelength through the wedge system; and analyzing the transmitted third beam via the detector to determine a change of the aberration.

3. The method of clause 2, wherein the determining of the efficacy comprises determining whether an extent of the aberration meets a threshold.

4. The method of clause 2, further comprising: further curing the radiation-curable adhesive based on the determining of the efficacy, comprising: adjusting, using the spatial light modulator, an intensity distribution of a fourth beam of radiation based on the determining of the efficacy, wherein the fourth beam has the second wavelength; and directing the fourth beam to the radiation-curable adhesive to adjust the position-dependent optical property at the radiation-curable adhesive.

5. The method of clause 4, further comprising: determining an efficacy of the further curing via a measurement using the detector.

6. The method of clause 1 , further comprising: directing the first beam or a portion of the first beam through an optical objective of the optical system; and reflecting the first beam or the portion of the first beam at a substrate. 7. The method of clause 6, wherein the analyzing of the first beam to determine the aberration comprises determining a contribution to the aberration from the optical objective having a numerical aperture that is approximately 0.5 or greater.

8. The method of clause 6, wherein the analyzing of the first beam comprises analyzing the first beam or the portion of the first beam using the detector disposed downstream of the wedge system, the optical objective, and the substrate.

9. The method of clause 6, wherein: the detector is a first detector; the portion is a first portion; sending a second portion the first beam to a second detector along a path that does not include the optical objective and the substrate; and the analyzing of the first beam further comprises analyzing the second portion of the first beam using the second detector.

10. The method of clause 6, wherein the wedge system is disposed downstream of the optical objective and the substrate.

11. The method of clause 6, wherein: the wedge system is a first wedge system disposed upstream of the substrate; the optical system comprises a second wedge system comprising radiation-curable adhesive between two wedges disposed downstream of the substrate; and the transmitting of the first beam through the wedge system comprises transmitting the first beam through the first and second wedge systems.

12. The method of clause 1, further comprising assembling the first and second wedges such that the radiation-curable adhesive has a thickness that is substantially constant.

13. The method of clause 12, wherein the thickness is about 50 microns or less.

14. The method of clause 1, wherein: the radiation-curable adhesive is reactive to ultraviolet wavelength; and the second wavelength is within a range of wavelengths that includes ultraviolet range.

15. The method of clause 1, wherein the first wavelength is within a range of wavelengths that includes infrared range and visible range.

16. An inspection apparatus, comprising: a radiation source configured to generate radiation; a wedge system comprising: a first wedge configured to receive the radiation; a second wedge configured to output the radiation; and a radiation-cured adhesive disposed between the first wedge and the second wedge, wherein the radiation-cured adhesive comprises a region-dependent optical property configured to adjust a wavefront of the radiation; an optical device configured to direct the radiation toward a target to generate scattered radiation from the target; a detector configured to receive the scattered radiation from the target and to generate a measurement signal based on the scattered radiation and the adjusting of the wavefront.

17. The inspection apparatus of clause 16, further comprising an optical objective disposed downstream of the optical device, wherein a numerical aperture of the optical objective is approximately 0.5 or greater.

18. The inspection apparatus of clause 16, the radiation-cured adhesive has a thickness that is substantially constant.

19. The inspection apparatus of clause 16, wherein: the wedge system is disposed at an illumination branch of the inspection apparatus; the wedge system is disposed at a detection branch of the inspection apparatus; or the wedge system is disposed at the illumination branch and a further wedge system is disposed at the detection branch, the further wedge system comprising radiation-cured adhesive between two wedges.

20. A wedge system, comprising: a first wedge configured to receive radiation; a second wedge configured to output the radiation; and radiation-cured adhesive disposed between the first wedge and the second wedge, wherein the radiation-cured adhesive comprises a regiondependent optical property configured to adjust a wavefront of the radiation.

[0112] A technical significance of the features disclosed herein is that aberrations in optical systems, such as optical sensors used with lithographic processes, can be mitigated using optical parts that fit within a small volume constraint and in a cost-effective manner (e.g., a wedge system can be considerably smaller and more cost-effective than a deformable mirror).

[0113] The terms “radiation,” “beam,” “light,” “illumination,” or the like can be used herein to refer to one or more types of electromagnetic radiation, for example, ultraviolet (UV) radiation (for example, having a wavelength I of 365, 248, 193, 157 or 126 nm), extreme ultraviolet (EUV or soft X-ray) radiation (for example, having a wavelength in the range of 5-100 nm such as, for example, 13.5 nm), or hard X-ray working at less than 5 nm, as well as particle beams, such as ion beams or electron beams. Generally, radiation having wavelengths between about 400 to about 700 nm is considered visible radiation; radiation having wavelengths between about 780-3000 nm (or larger) is considered IR radiation. UV refers to radiation with wavelengths of approximately 100-400 nm. Within lithography, the term “UV” also applies to the wavelengths that can be produced by a mercury discharge lamp: G-line 436 nm; H-line 405 nm; and/or, I-line 365 nm. Vacuum UV, or VUV (i.e., UV absorbed by gas), refers to radiation having a wavelength of approximately 100-200 nm. Deep UV (DUV) generally refers to radiation having wavelengths ranging from 126 nm to 428 nm, and in some aspects, an excimer laser can generate DUV radiation used within a lithographic apparatus. It should be appreciated that radiation having a wavelength in the range of, for example, 5-20 nm relates to radiation with a certain wavelength band, of which at least part is in the range of 5-20 nm.

[0114] Although some aspects of the present disclosure are described in the context of lithographic apparatuses in the manufacture of ICs, it should be understood that lithographic apparatuses described herein can be used in other applications, for example, in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, LCDs, thin-film magnetic heads, etc. Those skilled in the art will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein can be considered as specific examples of the more general terms “substrate” or “target portion”, respectively. A substrate can be processed before or after exposure in, for example, a track unit (a tool that typically applies a layer of resist to a substrate and develops the exposed resist) and/or a metrology unit. Where applicable, aspects disclosed herein can be applied to such and other substrate processing tools. Furthermore, a substrate can be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein can also refer to a substrate that already contains multiple processed layers. [0115] Furthermore, although some aspects of the present disclosure are described in the context of optical lithography, it should be understood that aspects of the present disclosure are not limited to optical lithography. For example, in imprint lithography, a topography in a patterning device defines the pattern created on a substrate. The topography of the patterning device can be pressed into a layer of resist supplied to the substrate whereupon the resist is cured by applying electromagnetic radiation, heat, pressure or a combination thereof. The patterning device is moved out of the resist leaving a pattern in it after the resist is cured.

[0116] It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present specification is to be interpreted by those skilled in relevant art(s) in light of the teachings herein.

[0117] The present disclosure has been described above with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed. The foregoing description of specific aspects will so fully reveal the general nature of the present disclosure that others can, by applying knowledge within the skill of the art, readily modify and/or adapt for various applications such specific aspects, without undue experimentation and without departing from the general concept of the present disclosure. Therefore, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed aspects, based on the teaching and guidance presented herein.

[0118] It is to be understood that the Detailed Description section, and not the Summary and Abstract sections, is intended to be used to interpret the claims. The Summary and Abstract sections can set forth one or more, but not necessarily all, aspects of the present disclosure as contemplated by the inventor(s), and thus, are not intended to limit the present disclosure and the appended claims in any way. The breadth and scope of the protected subject matter should not be limited by any of the abovedescribed aspects, but should be defined in accordance with the following claims and their equivalents.

Claims

1. A method of reducing optical aberrations in an optical system, the method comprising: determining an aberration induced by the optical system, comprising: transmitting a first beam of radiation having a first wavelength through a wedge system of the optical system, the wedge system comprising first and second wedges and radiation- curable adhesive disposed between the first and the second wedges; and analyzing the first beam using a detector disposed downstream of the wedge system to determine the aberration; and curing the radiation-curable adhesive based on the analyzing of the first beam, comprising: adjusting, using a spatial light modulator, an intensity distribution of a second beam of radiation based on the analyzing of the first beam, wherein the second beam has a second wavelength different than the first wavelength; and directing the second beam to the radiation-curable adhesive to induce a positiondependent optical property at the radiation-curable adhesive to thereby reduce the aberration, wherein the radiation-curable adhesive is reactive to the second wavelength.

2. The method of claim 1, further comprising: determining an efficacy of the curing in reducing the aberration via a measurement using the detector, comprising: transmitting a third beam of radiation having the first wavelength through the wedge system; and analyzing the transmitted third beam via the detector to determine a change of the aberration.

3. The method of claim 2, wherein the determining of the efficacy comprises determining whether an extent of the aberration meets a threshold.

4. The method of claim 2, further comprising: further curing the radiation-curable adhesive based on the determining of the efficacy, comprising: adjusting, using the spatial light modulator, an intensity distribution of a fourth beam of radiation based on the determining of the efficacy, wherein the fourth beam has the second wavelength; and directing the fourth beam to the radiation-curable adhesive to adjust the positiondependent optical property at the radiation-curable adhesive.

5. The method of claim 4, further comprising: determining an efficacy of the further curing via a measurement using the detector.

6. The method of claim 1, further comprising: directing the first beam or a portion of the first beam through an optical objective of the optical system; and reflecting the first beam or the portion of the first beam at a substrate.

7. The method of claim 6, wherein the analyzing of the first beam to determine the aberration comprises determining a contribution to the aberration from the optical objective having a numerical aperture that is approximately 0.5 or greater.

8. The method of claim 6, wherein the analyzing of the first beam comprises analyzing the first beam or the portion of the first beam using the detector disposed downstream of the wedge system, the optical objective, and the substrate.

9. The method of claim 6, wherein: the detector is a first detector; the portion is a first portion; sending a second portion the first beam to a second detector along a path that does not include the optical objective and the substrate; and the analyzing of the first beam further comprises analyzing the second portion of the first beam using the second detector.

10. The method of claim 6, wherein the wedge system is disposed downstream of the optical objective and the substrate.

11. The method of claim 6, wherein: the wedge system is a first wedge system disposed upstream of the substrate; the optical system comprises a second wedge system comprising radiation-curable adhesive between two wedges disposed downstream of the substrate; and the transmitting of the first beam through the wedge system comprises transmitting the first beam through the first and second wedge systems.

12. The method of claim 1, further comprising assembling the first and second wedges such that the radiation-curable adhesive has a thickness that is substantially constant.

13. The method of claim 12, wherein the thickness is about 50 microns or less.

14. The method of claim 1, wherein: the radiation-curable adhesive is reactive to ultraviolet wavelength; and the second wavelength is within a range of wavelengths that includes ultraviolet range.

15. The method of claim 1, wherein the first wavelength is within a range of wavelengths that includes infrared range and visible range.