WO2025201816A1

WO2025201816A1 - Inspection apparatus with time domain multiplexing for multiple wavelengths and inspection system with multiplexed parallel sensors

Info

Publication number: WO2025201816A1
Application number: PCT/EP2025/055891
Authority: WO
Inventors: Henricus Petrus Maria Pellemans; Sven Jansen; Padmakumar RAMACHANDRA RAO; Joshua Adams; Aniruddha Ramakrishna SONDE
Original assignee: ASML Netherlands BV
Current assignee: ASML Netherlands BV
Priority date: 2024-03-29
Filing date: 2025-03-04
Publication date: 2025-10-02
Anticipated expiration: 2026-09-29

Abstract

A lithographic apparatus (100) includes a projection system and an inspection apparatus (400). The projection system projects an image of structures of a patterning device onto a substrate (420). The inspection apparatus (400) includes a photodetector (426), a demultiplexer (428), and an illuminator section with a multiplexer (506). The illuminator section directs a beam of illumination toward a target on the substrate. The multiplexer (506) transmits first and second portions of the beam via first and second pulses (510', 512', 514'), respectively. The first portion has a first illumination parameter and the second portion has a second illumination parameter. The multiplexer (506) interleaves, in time, the first and second pulses (5101, 512', 514'). The photodetector (426) receives pulses of scattered illumination from the target to generate a measurement signal based on the pulses of the scattered illumination. The demultiplexer (428) analyses the measurement signal to discriminate portions of the measurement signal according to illumination parameter based on timing of the pulses of the scattered illumination.

Description

INSPECTION APPARATUS WITH TIME DOMAIN MULTIPLEXING FOR MULTIPLE WAVELENGTHS AND INSPECTION SYSTEM WITH MULTIPLEXED PARALLEL SENSORS

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 63/571,775 which was filed on March 29, 2024 and which is incorporated herein in its entirety by reference.

FIELD

[0002] The present disclosure relates to metrology systems, for example, alignment sensors for positioning wafers in lithographic apparatuses and systems.

BACKGROUND

[0003] A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which 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 radiationsensitive 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 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] Inspection tools specialized in sub-nanometer precision can be complex and very costly. For example, implementing multiple wavelengths of illumination in an inspection apparatus can progressively increase cost and optical complexity for every additional wavelength.

SUMMARY

[0008] Accordingly, it is desirable to implement an inspection apparatus that can scale up the number of illumination wavelengths while mitigating a proportional scaling of the optical hardware. For example, aspects described herein can reduce part count and cost in implementations of inspection apparatuses with multiple illumination wavelengths.

[0009] In some aspects, a lithographic apparatus can comprise a projection system and an inspection apparatus. The projection system can project an image of structures of a patterning device onto a substrate. The inspection apparatus can comprise an illuminator section, a photodetector, and a demultiplexer. The illuminator section can comprise a multiplexer. The illuminator section can direct a beam of illumination toward a target on the substrate. The multiplexer can transmit a first portion of the beam via first pulses. The first portion can have a first illumination parameter. The multiplexer can also transmit a second portion of the beam via second pulses. The second portion can have a second illumination parameter different from the first illumination parameter. The multiplexer can also interleave, in time, the first pulses and the second pulses. The photodetector can receive pulses of scattered illumination from the target. The photodetector can also generate a measurement signal based on the pulses of the scattered illumination. The demultiplexer can analyze the measurement signal to discriminate portions of the measurement signal according to illumination parameter based on timing of the pulses of the scattered illumination. [0010] In some aspects, an inspection system can comprise an illuminator section, a photodetector, and a demultiplexer. The illuminator section can comprise a multiplexer. The multiplexer can transmit a first portion of the beam via first pulses. The first portion can have a first illumination parameter. The multiplexer can also transmit a second portion of the beam via second pulses. The second portion can have a second illumination parameter different from the first illumination parameter. The multiplexer can also interleave, in time, the first pulses and the second pulses. The photodetector can receive pulses of scattered illumination from the inspection target. The photodetector can also generate a measurement signal based on the pulses of the scattered illumination. The demultiplexer can analyze the measurement signal to discriminate portions of the measurement signal according to wavelength based on timing of the pulses of the scattered illumination.

[0011] In some aspects, a method can implement operations for performing time-division multiplexing using an inspection apparatus. The method can comprise multiplexing wavelengths of a beam of illumination directed toward an inspection target. The multiplexing can comprise transmitting a first portion of the beam via first pulses. The first portion can have a first illumination parameter. The multiplexing can also comprise transmitting a second portion of the beam via second pulses. The second portion can have a second illumination parameter different from the first illumination parameter. The multiplexing can also comprise interleaving, in time, the first pulses and the second pulses. The method can also comprise generating a measurement signal based on pulses of scattered illumination, from the inspection target, received at a photodetector. The method can also comprise discriminating portions of the measurement signal according to wavelength based on analysis of the measurement signal and timing of the pulses of the scattered illumination.

[0012] In some aspects, a lithographic apparatus can comprise a projection system and an inspection system. The projection system can project an image of structures of a patterning device onto a substrate to form targets on the substrate. The inspection system can comprise a modulator system, a first objective, a second objective, a multiplexer, and a demultiplexer. The modulator system can modulate a first beam of illumination directed toward a first target on the substrate using a first modulation parameter. The modulator system can also modulate a second beam of illumination directed to a second target on the substrate using a second modulation parameter different from the first modulation parameter. The first objective can collect first scattered illumination from the first target. The second objective can collect second scattered illumination from the second target. The multiplexer can form a combined signal using the first scattered illumination and the second scattered illumination. The demultiplexer can discriminate a first portion of the combined signal based on the first modulation parameter. The demultiplexer can also discriminate a second portion of the combined signal based on the second modulation parameters. The first and second portions comprise information about the first and second targets, respectively.

[0013] In some aspects, an inspection system comprises a modulator system, a first objective, a second objective, a multiplexer, and a demultiplexer. The modulator system can modulate, using a first modulation parameter, a first beam of illumination directed toward a first inspection target. The modulator system can also modulate, using a second modulation parameter different from the first modulation parameter, a second beam of illumination directed to a second inspection target. The first objective can collect first scattered illumination from the first inspection target. The second objective can collect second scattered illumination from the second inspection target. The multiplexer can form a combined signal based on the first scattered illumination and the second scattered illumination. The demultiplexer can separate a first portion and a second portion of the combined signal based on the first and second modulation parameters. The first and second portions comprise information about the first and second targets, respectively.

[0014] In some aspects, a non-statutory computer-readable medium can have instructions stored thereon. The instructions, when executed on a computing system of an inspection system, can cause the computing system to perform operations. The operations can comprise demultiplexing a combined signal to discriminate a first portion of the combined signal based on a first modulation parameter and a second portion of the combined signal based on a second modulation parameter. The first modulation parameter can be associated with first scattered illumination from a first inspection target illuminated by a first beam of illumination modulated with the first modulation parameter. The second modulation parameter can be associated with second scattered illumination from a second inspection target illuminated by a second beam of illumination modulated with the second modulation parameter. The combined signal can be based on the first scattered illumination and the second scattered illumination. The first and second portions can comprise information about the first and second inspection targets. The operations can also comprise determining a property of the first inspection target based on analysis of the first portion and the first modulation parameter. The operations can also comprise determining a property of the second inspection target based on analysis of the second portion and the second modulation parameter.

[0015] In some aspects, a lithographic apparatus can comprise a projection system and an inspection system. The projection system can receive illumination to project an image of a pattern of a patterning device onto a substrate to form targets on the substrate. The inspection system can comprise a first illuminator section, a first objective, a second illuminator section, a second objective, a multiplexer, and a demultiplexer. The first illuminator section can direct a first beam of illumination toward a first target. The first beam can comprise first pulses. The first objective can collect first scattered illumination from the first target. The second illuminator section can direct a second beam toward a second target. The second beam can comprise second pulses. The second objective can collect second scattered illumination from the second target. The multiplexer can interleave, in time, time slots corresponding to pulses of the first scattered illumination and time slots corresponding to pulses of the second scattered illumination to form a combined signal stream. The demultiplexer can discriminate portions of the combined signal stream based on pulse timings. A first portion discriminated from the combined signal stream can comprise information about the first target. The first portion can be discriminated based on timing of the pulses of the first scattered illumination. A second portion discriminated from the combined signal stream can comprise information about the second target. The second portion can be discriminated based on timing of the pulses of the second scattered illumination.

[0016] In some aspects, an inspection system can comprise a first illuminator section, a first objective, a second illuminator section, a second objective, a multiplexer, and a demultiplexer. The first illuminator section can direct a first beam of illumination toward a first target. The first beam can comprise first pulses. The first objective can collect first scattered illumination from the first target. The second illuminator section can direct a second beam toward a second target. The second beam can comprise second pulses. The second objective can collect second scattered illumination from the second target. The multiplexer can interleave, in time, time slots corresponding to pulses of the first scattered illumination and time slots corresponding to pulses of the second scattered illumination to form a combined signal stream. The demultiplexer can discriminate portions of the combined signal stream based on pulse timings. A first portion discriminated from the combined signal stream can comprise information about the first target. The first portion can be discriminated based on timing of the pulses of the first scattered illumination. A second portion discriminated from the combined signal stream can comprise information about the second target. The second portion can be discriminated based on timing of the pulses of the second scattered illumination.

[0017] In some aspects, a method can implement operations for performing time-division multiplexing for multiple sensor heads. The method can comprise directing a first beam of illumination toward a first target on a substrate. The first beam can comprise first pulses. The method can also comprise generating first scattered illumination from the first target using the first beam. The method can also comprise directing a second beam of illumination toward a second target on a substrate. The first beam comprises second pulses. The method can also comprise generating second scattered illumination from the second target using the second beam. The method can also comprise interleaving, in time, time slots corresponding to pulses of the first scattered illumination and time slots corresponding to pulses of the second scattered illumination to form a combined signal stream. The method can also comprise discriminating portions of the combined signal stream based on pulse timings. A first portion discriminated from the combined signal stream can comprise information about the first target. The first portion can be discriminated based on timing of the pulses of the first scattered illumination. A second portion discriminated from the combined signal stream can comprise information about the second target. The second portion can be discriminated based on timing of the pulses of the second scattered illumination.

[0018] 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

[0019] 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.

[0020] FIG. 1 A shows a reflective lithographic apparatus, according to some aspects.

[0021] FIG. IB shows a transmissive lithographic apparatus, according to some aspects.

[0022] FIG. 2 shows more details of a reflective lithographic apparatus, according to some aspects.

[0023] FIG. 3 shows a lithographic cell, according to some aspects.

[0024] FIGS. 4 A and 4B show inspection apparatuses, according to some aspects.

[0025] FIG. 5 shows a multiplexer, according to some aspects.

[0026] FIG. 6 shows a detector section of an inspection system, according to some aspects.

[0027] FIG. 7 shows a flowchart of a method for performing time-division multiplexing using an inspection system, according to some aspects.

[0028] FIG. 8 shows an illumination system of an inspection apparatus, according to some aspects.

[0029] FIG. 9 shows a portion of an inspection system, according to some aspects.

[0030] FIG. 10 shows a multiplexer, according to some aspects.

[0031] FIG. 11 shows a flowchart of a method for performing demultiplexing of measurement signals acquired by multiple sensor heads, according to some aspects.

[0032] FIG. 12 shows a multiplexer, according to some aspects.

[0033] FIG. 13 shows a flowchart of a method for performing demultiplexing of measurement signals acquired by multiple sensor heads, according to some aspects.

[0034] FIG. 14 shows a computer system for implementing operations in connection with inspection systems, according to some aspects.

[0035] 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

[0036] 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.

[0037] 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.

[0038] 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).

[0039] Enumerative adjectives (e.g., “first,” “second,” “third,” or the like) can be used as labels to distinguish like elements without establishing an order, hierarchy, quantity, or permanent numeric assignment (unless otherwise noted). For example, a given element can be referred to as a “second target” in one labeling scheme while the same element can be referred to as a “first target” in another labeling scheme.

[0040] 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.

[0041] Before describing such aspects in more detail, however, it is instructive to present an example environment in which aspects of the present disclosure can be implemented.

[0042] Example Lithographic Systems [0043] 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.

[0044] The illumination system IL can include various types of optical components, such as refractive, reflective, catadioptric, magnetic, electromagnetic, electrostatic, or other types of optical components, or any combination thereof, for directing, shaping, or controlling the radiation beam B.

[0045] The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device MA with respect to a reference frame, the design of at least one of the lithographic 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.

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

[0047] The patterning device MA can be transmissive (as in lithographic apparatus 100’ of FIG. IB) or reflective (as in lithographic apparatus 100 of FIG. 1 A). 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.

[0048] The term “projection system” PS can encompass any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors, such as the use of an immersion liquid on the substrate W or the use of a vacuum. A vacuum environment can be used for EUV or electron beam radiation since other gases can absorb too much radiation or electrons. A vacuum environment can therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps.

[0049] Lithographic apparatus 100 and/or lithographic apparatus 100’ can be of a type having two (dual stage) or more substrate tables WT (and/or two or more mask tables). In such “multiple stage” machines, the additional substrate tables WT can be used in parallel, or preparatory steps can be carried out on one or more tables while one or more other substrate tables WT are being used for exposure. In some situations, the additional table may not be a substrate table WT.

[0050] The lithographic apparatus can also be of a type wherein at least a portion of the substrate can be covered by a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the projection system and the substrate. An immersion liquid can also be applied to other spaces in the lithographic apparatus, for example, between the mask and the projection system. Immersion techniques 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.

[0051] 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.

[0052] 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-outcr” 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. [0053] 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.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] 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).

[0058] 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.

[0059] Mask table MT and patterning device MA can be in a vacuum chamber V, where an in-vacuum robot IVR can be used to move patterning devices such as a mask in and out of vacuum 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 in- vacuum 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.

[0060] The lithographic apparatus 100 and 100’ can be used in at least one of the following modes:

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.

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-)magnification and image reversal characteristics of the projection system PS.

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.

[0061] Combinations and/or variations on the described modes of use or entirely different modes of use can also be employed.

[0062] 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.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] 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.

[0070] Example Lithographic Cell

[0071] 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.

[0072] Example Inspection Apparatus

[0073] 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 self-referencing 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.

[0074] 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.

[0075] 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.

[0076] 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. [0077] 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.

[0078] 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.

[0079] 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. [0080] 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.

[0081] 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.

[0082] 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:

1. measuring position variations for various wavelengths (position shift between colors);

2. measuring position variations for various orders (position shift between diffraction orders);

3. measuring position variations for various polarizations (position shift between polarizations); and

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

[0083] 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.

[0084] 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. [0085] 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.

[0086] 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. [0087] 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 read-out at high speed and are especially of interest if phase-stepping detection is used.

[0088] 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.

[0089] 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.

[0090] 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.

[0091] 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.

[0092] 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.

[0093] 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.

[0094] Example Optical Signal Multiplexing Systems for Inspection Apparatuses

[0095] The size of a lithographic apparatus is a parameter that can determine various aspects of a nanofabrication facility, including equipment cost, space requirements, wafer travel distance, wafer throughput, and more. The space inside a lithographic apparatus can be even more constrained, allowing only a limited number of mechatronics and inspection tools. One such inspection tool that can be used inside a lithographic apparatus is inspection apparatus 400 of FIGS. 4A and 4B (e.g., alignment sensor). Wafer alignment sensing is important because it allows overlay (stacking of different device layers) to be performed with sub-nanometer precision. To extract the most accurate alignment information from a measurement, an optical measurement can perform measurements of targets using two or more wavelengths, as different wavelengths can provide independent information about the positioning of targets 418. The collective measurement information across different wavelengths can increase the accuracy of measured values for properties of targets (e.g., x/y position values). Measurement information and accuracy can be enhanced by implementing variations to other illumination parameters, for example, polarization and/or angle of incidence.

[0096] Terms such as “multiple wavelengths,” “multiple photon frequencies,” “multiple colors,” or the like, can be used to refer to a set of discrete narrow bands within a broader range (a broadband). For example, a first wavelength can be characterized as comprising a narrow wavelength band centered at a first central wavelength. A second wavelength can be similarly characterized as comprising a narrow wavelength band centered at a second central wavelength. A characterization of the first wavelength as being different from the second wavelength can be interpreted as the first central wavelength being different from the second central wavelength.

[0097] A problem in implementing multiple illumination parameters (e.g., multiple wavelengths) in inspection apparatus 400 is the amount of costly optical hardware that scales for every additional illumination parameter. In an example scenario, it is desired to perform an inspection of target 418 using two different polarizations and two types of measurements (one for alignment position and one for diffraction order asymmetry (correction term to alignment position)). Two types of measurements for two polarizations can be implemented as 2x2 illumination paths for a single wavelength — that is 4x optical fibers, 4x lenses, 4x prisms, 4x of other optical parts. Increasing the number of wavelengths proportionally increases the amount of optical hardware to accommodate additional optical paths (e.g., two wavelengths has eight optical paths, three wavelengths has twelve optical paths, and so on). It is desirable to implement an inspection apparatus that can scale number of different illumination parameters while mitigating a proportional scaling of the optical hardware.

[0098] Some aspects described herein allow for merging of optical signals with different illumination parameters into a combined optical path, reducing the number and space required by optical hardware, while also allowing for discriminating the different wavelength signals at detection. It is also envisaged that techniques described herein (e.g., time-division multiplexing, signal modulation) can be used to merge optical signals of two or more sensors and subsequently, with a signal discrimination method, identify which of the sensors the optical signals are from.

[0099] FIG. 5 shows a multiplexer 500, according to some aspects. For example, multiplexer 500 shall be described with reference to inspection apparatus 400 in FIGS. 4A and 4B. However, multiplexer 500 is not limited to the aspects referencing FIGS. 4A and 4B and can be implemented with any suitable inspection apparatus.

[0100] In some aspects, multiplexer 500 can comprise a wavelength separator 502 (e.g., an optical element capable of separating wavelengths, such as a prism, a grating, or an integrated optical device), a timing system 504, and a wavelength combiner 506 (e.g., an optical element capable of combining wavelengths, such as a prism). Other implementations of wavelength selection or separation are envisaged, for example, using color filters to select each wavelength. In the context of FIGS. 4A and 4B, multiplexer 500 can be implemented in an illuminator section of inspection apparatus 400. The illuminator section can comprise the components that are responsible for delivering illumination to substrate 420 and target(s) 418 thereon. For example, multiplexer 500 can be implemented at any part of the optical path between illumination system 412 and target 418, including within illumination system 412. The illumination illuminator can also be referred to as the illumination branch or illumination side of inspection apparatus 400. Similarly, a detector section (or detection branch/side) can include components that are downstream of target 418 — components that are responsible for the collection of illumination scattered by target 418 and subsequent detection, which will be described further below in reference to further figures.

[0101] In some aspects, a beam of illumination, represented as pulses 508, can be generated by an illumination source (not shown in Figure 5). The illumination source can be a multi-wavelength source having two or more narrow bands across a wide spectrum, also described as one or more passbands in the above example of illumination system 412. The illumination source can be referred to as a “white light source” for its capability to provide multiple colors, and pulses 508 can be similarly described as white light pulses. The illumination source can generate “coherent” white light in the sense that individual wavelengths can be spatially and/or temporally coherent. As the illumination source is capable of generating all wavelengths simultaneously, each pulse of pulses 508 can have multiple wavelengths (e.g., a near infrared (NIR) band, a red band, a green band, and more, which can be spread over a range of wavelengths covering about 500 nm to about 900 nm). Described differently, a single pulse of pulses 508 can be made up of a superposition of multiple pulses having different wavelengths. [0102] In some aspects, wavelength separator 502 can separate the different wavelengths into different channels or paths. As an example, three wavelengths are represented by pulses 510, pulses 512, and pulses 514 (e.g., first pulses, second pulses, and third pulses). Pulses 510 can be considered a first portion of the beam of illumination that has been separated out by wavelength separator 502, pulses 512 can be a second portion, and so on. It is to be appreciated that the components of multiplexer 500 can be scaled to transmit any number of wavelengths via corresponding pulses and optical paths (e.g., two or more) in accordance with aspects disclosed herein. Referring to time instance 516, one or more pulses of pulses 510, 512, and 514 can overlap in time (a horizontal time axis is provided for reference) or may be too close together in time, hindering time-based signal analysis performed downstream. In an example using a prism as wavelength separator 502, the different timings of pulses 510, 512, and 514 emerging from wavelength separator 502 can be attributed to wavelength-dependent refractive index and path length, among other physical phenomena. Other suitable implementations of wavelength separation are also envisaged.

[0103] In some aspects, timing system 504 can comprise a delay device 518, a delay device 520, and a delay device 522 (e.g., first delay device, second delay device, third delay device). It is to be appreciated that wavelengths are scalable and any suitable number delay devices (e.g., two or more) can be implemented in accordance with aspects disclosed herein. Delay device 518 can receive pulses 510. Delay device 518 can time shift pulses 510 by a suitable delay offset. Delay device 520 can receive pulses 512. Delay device 520 can time shift pulses 512 by a suitable delay offset. Delay device 522 can receive pulses 514. Delay device 522 can time shift pulses 514 by a suitable delay offset. For any given wavelength, a delay offset can be zero in the event that no delay is needed. In this manner, timing system can offset a timing of one set of pulses respect to another set of pulses. Any suitable device or medium can be used to provide delay offsets in timing system 504, for example, a path of free space for no delay offset, one or more mirrors, optical fibers, or the like.

[0104] The output of timing system 504 can be pulses for different wavelengths. In the example of FIG. 5, pulses 510’ is the output corresponding to pulses 510. Pulses 510’ can have a zero delay offset. Pulses 512’ is the output corresponding to pulses 512. Pulses 512’ are essentially pulses 512 with a timing offset relative to pulses 510’. Pulses 514’ is the output corresponding to pulses 514. Pulses 514’ are essentially pulses 514 with a timing offset relative to pulses 510’ and/or relative to pulses 512’. Pulses 510’, 512’, and 514’ are provided with a horizontal time axis to illustrate relative offsets/delays. [0105] In some aspects, pulses 510’, 512’, and 514’ can be combined using wavelength combiner 506 such that the timing of the pulses of different wavelengths are interleaved (or interlaced). The recombined pulses are represented as interleaved pulses 524. The timing of pulses 510’, 512’, and 514’ are staggered such that interleaved pulses 524 have non overlapping pulses, with one pulse having the wavelength of pulses 510, a next pulse having the wavelength of pulses 512, and a yet next pulse having the wavelength of pulses 514, and so on, in a repeating cycle. In other words, multiplexer 500 can separate pulses of different wavelengths from a white light source, adjust the timing of the pulses of different wavelengths, and recombine the pulses such that the pulses of different wavelengths are sequentially arranged along the time axis.

[0106] In some aspects, the delay devices in timing system 504 can be configured taking into account any expected offsets resulting from the material and path lengths of the illumination side optics and the detection side optics. The time discretization of the different wavelengths in interleaved pulses 524 allows for time-gated discrimination and analysis at the detector branch of inspection apparatus 400. To aid in analysis functions downstream, an oscillator device 526 (e.g., a master clock) can be used to inform the timing of pulses 508. For example, oscillator device 526 can send a timing signal to the illumination source that generates the initial beam of illumination with the multiple wavelengths. The timing signal can be used by the illumination system to set the timing of pulses 508. In the absence of a dedicated oscillator device, the timing and phase of the laser pulses themselves can provide master clock timing information.

[0107] In some aspects, at the output of multiplexer 500, interleaved pulses 524 can take the form of a beam of illumination that is incident on target 418. That is, the wavelengths of illumination that reach target 418 can travel along a coincident beam path, but have a non-coincident sequential timing.

[0108] In some aspects, the interleaving of pulses is not limited to wavelength discrimination. Other illumination parameters can be discriminated via pulse-interleaving. An example measurement can implement first through fourth illumination parameters that are different from one another in terms of value and/or type (e.g., first wavelength, second wavelength, first polarization, and second polarization) along with first and second sensors. Pulses 510’ can be associated with illumination having the first and third illumination parameters (first wavelength and first polarization), as well as the first sensor. Pulses 512’ can be associated with illumination having the first and fourth illumination parameters (first wavelength and second polarization), as well as the first sensor. Pulses 514’ can be associated with illumination having the first and third illumination parameters (first wavelength and first polarization), as well as the second sensor. Additional pulses can be implemented to accommodate the remaining permutations of the different parameters. In this manner, the number of interleaved pulses can be chosen so as to accommodate any desired combination of illumination parameters and/or number of sensors.

[0109] FIG. 6 shows a detector section 600 of an inspection system, according to some aspects. For example, detector section 600 shall be described with reference to inspection apparatus 400 in FIGS. 4 A and 4B and multiplexer 500 of FIG. 5. However, detector section 600 is not limited to the aspects referencing FIGS. 4A and 4B and can be implemented with any suitable inspection apparatus. Though not shown in FIG. 6, it is to be appreciated that detector section 600 can include components that are downstream of target 418.

[0110] In some aspects, detector section 600 can comprise a detector element 602, a demultiplexer 604, channel system 606, aggregator 608, and analog-to-digital converter system 610. When inspection apparatus 400 is used to perform a multi-wavelength optical measurement, a beam of illumination with pulsed wavelength timings (e.g., interleaved pulses 524) can be sent to target 418. As pulsed illumination at various wavelengths is scattered by target 418, an optical objective (e.g., lens) and other hardware of inspection apparatus 400 can be used to collect illumination and routed to detector element 602 for conversion to an electrical measurement signal 612. Scattered illumination from target 418 can be characterized by interleaved pulses 614, which corresponds to pulsed wavelength timings of interleaved pulses 524. Interleaved pulses 614 can comprise pulses 616, pulses 618, and pulses 620. Wavelengths are scalable as described above in reference to FIG. 5. The timing of pulses 616 can correspond to pulses 510’. The timing of pulses 618 can correspond to pulses 512’. The timing of pulses 620 can correspond to pulses 514’. Detector element 602 can correspond to the photosensitive element of detector 428. Various suitable photosensitive elements are envisaged for the role of detector element 602, such as an array of detector elements (e.g., camera) or a single pixel photodetector such as a photodiode. In an example of a fast readout detector implementation, detector element 602 can comprise a read and reset device to quickly decay the measurement signal to prepare for a next read out.

[0111] In some aspects, the number of output channels of a color camera corresponds to the number of pixels on the camera, with each pixel being dedicated to a single color via color filter disposed in front of the pixel. Successful detection of each photon enhances signal-to-noise ratio (SNR). Photons are essentially discarded (not detected) when there is a mismatch between photon wavelength and color filter, reducing SNR. Furthermore, as each pixel is an independent channel, readout of pixels is typically performed in sequence (e.g., raster), increasing the amount of time to perform a measurement when a large number of pixels are involved. In contrast, a single pixel (single channel) photodetector can present various features that fully exploit the interleaved pulses 524 for greatest effect. A single pixel photodetector can be essentially colorblind — generating a single channel intensity value based on the intensity of illumination (e.g., number of photons) regardless of photon wavelength. With photon detection maximized (e.g., photons not being rejected by a filter), SNR can be optimized. And while the in the electrical measurement signal the devices and functions described in reference to FIG. 6 can be used to perform wavelength discrimination using single channel detection, thereby allowing for faster measurements in comparison to multichannel raster of a camera.

[0112] In some aspects, detector element 602 can generate measurement signal 612 in response to receiving interleaved pulses 614. Measurement signal 612 can comprise electrical pulses that correspond to pulses 616, 618, and 620. Pulses 616, 618, and 620, having interacted with target 418, carry information about one or more properties of target 418 (e.g., an alignment position). The one or more properties can be more accurately resolved by increasing the number of wavelengths of interrogation.

[0113] In some aspects, measurement signal 612 can be processed via demultiplexer 604 so as to discriminate portions of measurement signal 612 according to wavelength based on timing of interleaved pulses 614 of the scattered illumination. Demultiplexer 604 can perform time-division demultiplexing. Using the master clock (e.g., oscillator device 526) that is responsible for supplying the timing reference of pulses 510’, 512’, and 514’, the timing reference can be received at demultiplexer 604. Demultiplexer 604 can discriminate the alternating wavelength signals to corresponding channels of channel system 606, a channel 622, a channel 624, and a channel 626. Channels 622, 624, and 626 can be signal paths for transmitting electrical pulses (e.g., wires, traces of an integrated circuit, or the like).

[0114] In some aspects, aggregator 608 can comprise a summation element 628, a summation element 630, and a summation element 632. Analog-to-digital converter (ADC) system 610 can comprise an ADC element 634, an ADC element 636, and an ADC element 638. After separation, the portion of the measurement signal associated with pulses 616 can be routed through channel 622, summation element 628, and ADC element 634. The portion of the measurement signal associated with pulses 618 can be routed through channel 624, summation element 630, and ADC element 636. The portion of the measurement signal associated with pulses 616 can be routed through channel 626, summation element 632, and ADC element 638. Summation elements receive fast signals and aggregate the fast signals together into a much slower summed signal. Hence, some aspects can combine the summed signals from multiple summation elements into a single ADC element (with appropriate digital identifiers to track which summed signal corresponds to which summation element).

[0115] In some aspects, for each wavelength, aggregator 608 can integrate, sum, or otherwise accumulate the measured intensity. As a single pulse of one wavelength may not provide sufficient SNR, a plurality of pulses for a given wavelength can be summed in order to increase measurement accuracy. The summation refresh frequency can be different from the sample/refresh rate or readout rate of detector element 602.

[0116] In some aspects, ADC system 610 can convert any analog measurement information into digital information. The digital information can be processed by a suitable computing system. The computing system can analyze the portions of the measurement signal according to wavelength and in the digital domain. The computing system can determine a value for a property of target 418 based on the analyzed portions of the measurement signal according to wavelength (e.g., determining a value of a position of target 418 averaged across the measured wavelengths). The functions of aggregator 608 can also be performed in the digital domain by the computing system (e.g., by swapping the order of aggregator 608 and ADC system 610).

[0117] In some aspects, the functions of detector element 602 and demultiplexer 604 can be combined as a single device. For example, a multi-tap pixel device 640 can be used (or a system of two or more multi-tap pixel devices) for performing both photodetection and demultiplexing. Multi-tap pixel device 640 can comprise a detector face 642 and a plurality of taps (e.g., gates). For example purposes, eight taps are illustrated, with three taps expressly labeled as taps 644, 646, and 648. More or fewer taps may be implemented according to the number of permutations of illumination parameters and/or number of sensors (e.g., two or more taps).

[0118] In some aspects, pulses 616, 618, and 620 can be incident on detector face 642. Detector face 642, comprising photosensitive material, can generate charge carriers 650 (e.g., electrons, holes) in response to receiving pulsed illumination. For example purposes, let the illustrated charge carriers 650 correspond to pulses 620. By applying a bias voltage to tap 648, the photocurrent of charge carriers 650 can be biased toward to tap 648 (illustrated by the solid-line arrow). Tap 648 can be coupled to channel 626. Tap 646 can be coupled to channel 624. Tap 644 can be coupled to channel 622. Hence, photocurrent signals associated with pulses 620 can be routed to channel 626, thereby achieving demultiplexing. Similarly, by applying a bias voltage to tap 646 when pulses 618 are received, photocurrent signals associated with pulses 618 can be demultiplexed to channel 624. And by applying a bias voltage to tap 644 when pulses 616 are received, photocurrent signals associated with pulses 616 can be demultiplexed to channel 622. This implementation can be extrapolated to more pulse sets and taps.

[0119] In some aspects, the timing of bias voltages applied to the taps can be a periodic sequence according to the timing information of the pulses (e.g., from oscillator device 526). Timing information is used to sequentially apply bias voltages to taps 644, 646, 648, and so on, in order to identifiably route the signals of the different pulses to their respective channels. In post processing, distinguishing between the different pulses can be achieved by identifying the channel from whence the signal is received.

[0120] In some aspects, a tap can be used in degenerate mode. Degeneracy at a tap can occur when a single tap is configured to receive two or more sets of pulses that correspond to two or more sets of illumination parameters. Degeneracy can be lifted by sending the timing information of the pulses to the analyzer (e.g., a computer system) for post-processing analysis. In this scenario, summation provided by aggregator 608 can be performed in the digital domain (e.g., by the computer system) so as to lift the degeneracy prior to aggregation.

[0121] In some aspects, illumination scattered from a measurement target can generate an illumination spot that is much larger than the area of detector face 642. It is wasteful to have photons fall outside of detector face 642. Therefore, an array of multi-tap pixel devices can be implemented so as to cover the full area of the illumination spot and maximize photon-to- signal conversion.

[0122] In some aspects, detector element 602 can have a sample/refresh rate about 1 kHz or more, about 10 kHz or more, about 100 kHz or more, about 1 MHz or more, about 10 MHz or more, about 100 MHz or more, or about 1 GHz or more. High sample/refresh rates are suitable in connection with illumination sources that have a high pulse repetition rate. For example, a white light source can operate with a pulse repetition rate of about 1 MHz. In an example inspection system with two wavelengths, detector element 602 can detect the interleaved pulses of the two wavelengths if its sample/refresh rate is about 2 MHz or more. In the case of ten wavelengths, then detector element 602 should operate at about 10 MHz or more. Other example scenarios can include illumination sources that operate at 10 MHz or more with ten or more wavelengths, and detector element 602 can operate at a suitable sample/refresh rate to accommodate such illumination sources. [0123] Consider alignment procedures for ascertaining a wafer position to sub-nanometer precision. It is desirable to perform this type of alignment measurement when a finished wafer exits the lithographic processing area and a new wafer is loaded into the lithographic processing area (the newly loaded wafer’s precise position is unknown). In the absence of the time-domain multiplexing devices disclosed herein, discriminating such multiple wavelengths in an alignment measurement can be accomplished by acquiescing to certain drawbacks. For example, since a white light source produces all wavelengths simultaneously, a system of filters can be used. Each wavelength is essentially an independent measurement, multiplying the amount of optical hardware in proportion to the number of wavelengths (e.g., additional optical path and detector element for each additional wavelength). Such a workaround increases cost, optical parts, and volume requirements.

[0124] In some aspects, multiplexer 500 allows all wavelengths of pulses 508 to be used without needing to scale the optical hardware. Since detector element 602 can be a common detector for multiple wavelength channels, a common optical path, such as an optical fiber or a waveguide, can be used as a merging point for the collected scattered illumination of multiple wavelengths from target 418 and also for subsequent routing to detector element 602. To further shrink hardware to save space (inside a lithographic apparatus, space can be highly constrained), the illumination collection optics can be implemented as an integrated optical device.

[0125] In some aspects, terms such as “integrated optics,” “integrated optical system,” “integrated optical circuit,” “photonic integrated circuits,” “integrated photonics,” “planar lightwave circuits,” “on- chip,” or the like, can be used to refer to on-chip integrated devices that can propagate radiation signals without relying on traditional free space optical hardware. An integrated optical device can comprise, for example, waveguides disposed on a substrate (on-chip) such that radiation signals can propagate through the waveguide/substrate device. It should be understood that free space radiation can still interact with an integrated optical device (e.g., launching radiation, from free space, into the integrated optical device and vice versa). The waveguides can guide optical signals to other areas of the substrate, where the optical signals can be received at a detector and analyzed for measurement information. Integrated optics can be made extremely small compared to bulky free space optics and at a fraction of the cost.

[0126] In some aspects, some of the free space optics in the detection section of inspection apparatus 400 can be replaced with an on-chip integrated optical system to collect the scattered radiation from target 418. On-chip integrated optical implementation can also be the case for elements of illumination side optics (e.g., multiplexer 500 can be implemented on-chip). In aspects in which target 418 is a diffractive structure (e.g., a grating), the scattering can be that of a diffraction pattern, with scattering angles depending on the wavelength of illumination and pitch of grating. The on-chip integrated optical system can comprise multiple optical couplers disposed at the different positions corresponding to the different diffraction angles. Optical couplers can receive and inject the scattered illumination into waveguides of the on-chip integrated optical system. Detector element 602 can be an on-chip photodetector on the on-chip integrated optical system (e.g., photodiode, phototransistor, single photon detector, single photon avalanche detector, multi-tap pixel device, or the like). Additional examples of integrated optical devices for inspection systems are described in more detail in U.S. Published Application No. 2021/0095957 Al (published April 1, 2021) and U.S. Published Application No. 2023/0266681 Al (published August 24, 2023), both of which are incorporated by reference herein in their entirety.

[0127] FIG. 7 shows a flowchart of a method 700, according to some aspects. For example, method 700 can be used for performing time-division multiplexing using an inspection apparatus. Method 700 shall be described with reference to devices in FIGS. 4A, 4B, 5, and 6. However, method 700 is not limited to the aspects referencing FIGS. 4A, 4B, 5, and 6 and can be implemented with any suitable multiplexer, demultiplexer, and inspection apparatus.

[0128] In some aspects, at operation 702, a beam of illumination can be directed toward an inspection target (e.g., target 418) using an illumination section of inspection apparatus 400. The ordering of pulses and wavelengths of the beam can be implemented via multiplexer 500 according to operations 704, 706, and 708.

[0129] In some aspects, at operation 704, a first wavelength of the beam can be transmitted via first pulses (e.g., pulses 510).

[0130] In some aspects, at operation 706, a second wavelength of the beam can be transmitted via second pulses (e.g., pulses 512).

[0131] In some aspects, at operation 708, the first pulses and the second pulses can be interleaved in time. The interleaving of pulses can have an alternating pattern, such as a pulse of the first wavelength followed by a pulse of the second wavelength, followed by another pulse of the first wavelength, followed by another pulse of the second wavelength, and so on.

[0132] In some aspects, at operation 710, illumination that is incident on target 418 is scattered, generating scattered illumination. The scattered illumination is associated with interleaved ones of the first pulses and the second pulses. The scattered illumination is collected and routed to a photodetector (e.g., detector element 602).

[0133] In some aspects, at operation 712, measurement signal 612 is generated based on the scattered illumination received at the photodetector.

[0134] In some aspects, at operation 714, demultiplexer 604 is used to discriminate portions of the measurement signal according to wavelength based on timing of the pulses of the scattered illumination. [0135] The operations of FIG. 7 can be performed in any conceivable order and it is not required that all operations be performed. Moreover, the operations of FIG. 7 described above merely reflect an example of operations and are not limiting. That is, further operations are envisaged based aspects described above in reference to FIGS. 1-6. Aspects of time domain multiplexing can also be implemented with aspects involving multiple sensor heads. Aspects with multiple sensor heads are described below. [0136] Aspects described above use signal merging techniques (e.g., time-domain multiplexing) to reduce cost, part count, and space usage for a single sensor. A plurality of targets can be printed on wafer undergoing lithographic processing. A single sensor is limited to measuring to a target that is in the current line-of-sight of the single sensor. To measure another target, the other target can be moved, the single sensor can be moved, or both the other target and the single sensor can be moved so that the other target enters the line-of-sight of the single sensor. Measuring multiple targets using the single sensor can be time consuming, resulting in slow throughput.

[0137] In some aspects, the term “throughput” can be used herein to refer to a speed at which an amount of material or items pass through a system or process. The term “throughput” can characterize a speed of overall lithographic fabrication, a rate at which a wafer passes through a lithographic apparatus, a rate at which a wafer clears a particular fabrication step and moves on to the next step, or the like. Hence, “throughput” can be a performance marker of a lithographic apparatus.

[0138] It is desirable for lithographic systems to output as many products as possible in as little time as possible. Lithographic fabrication can comprise several complex processes. In some aspects, each process can comprise tradeoffs that balance desired qualities and drawbacks (e.g., sub-nanometer accuracy, high yield/throughput, slower fabrication, increased cost). For example, a lithographic apparatus can implement a single instance of inspection apparatus 400 to measure positions of multiple target marks on a substrate. Since there is only one inspection apparatus, multiple marks are measured in sequence one after another (each target consuming a finite amount of measurement time), resulting in time loss and slower throughput. As a solution, a method to scan multiple marks in a relatively short time can include implementing a system of multiple sensors instead of a single sensor (e.g., multiple instances of inspection apparatus 400).

[0139] However, as explained previously, scaling of parts of an inspection apparatus (let alone an entire inspection apparatus) can increase cost, complexity, and space usage to exacerbate volume constraints. As a solution to this issue, some aspects described herein allow for merging of different optical signals from multiple sensors into a combined optical path, reducing the number of parts and space occupied by optical hardware, while also allowing for discriminating the different optical signals at detection. Recalling that time domain multiplexing described above is useful for discriminating single wavelengths from among multiple wavelengths, time domain multiplexing can also be implemented for a system with multiple sensors.

[0140] FIG. 8 shows an illumination system 800, according to some aspects. In some aspects, illumination system 800 comprises an illumination source 802, an optical system 804, a beam splitter system 806, and a modulator system 808.

[0141] In some aspects, illumination source 802 can generate source illumination 810 having a plurality of bands of wavelengths spread across a broad spectrum (e.g., white light from a white light source). [0142] In some aspects, optical system 804 can condition source illumination 810. Examples of illumination conditioning can include adjusting a polarization, filtering, adjusting intensity, adjusting wavelengths, and beam focusing.

[0143] In some aspects, beam splitter system 806 can comprise beam splitters 812-z to generate beams of illumination 814-z (e.g., beam splitters 812-1, 812-2, 812-3, 812-4, 812-5, and 812-6 (first, second, third, fourth, fifth, and sixth beam splitters); beams of illumination 814-1, 814-2, 814-3, 814-4, 814-5, and 814-6 (first, second, third, fourth, fifth, and sixth modulators)). In an alternative description, first, second, third, fourth, fifth, and sixth beam splitters can be used to generate first, second, third, fourth, fifth, and sixth beams of illumination, respectively.

[0144] In some aspects, modulator system 808 can comprise modulators 816-i (e.g., modulators 816- 1, 816-2, 816-3, 816-4, 816-5, and 816-6 — first, second, third, fourth, fifth, and sixth modulators). A modulator can be disposed at a pupil plane of its respective sensor head (e.g., disposed at the illumination side upstream of the targets on substrate 820).

[0145] In some aspects, the index i can denote an arbitrary multiplicity of an element (e.g., two or more beams of radiation, two or more modulators, two or targets, and the like). For example, the 6x multiplicity of elements in the drawings is not limiting and that a different multiplicity (e.g., 2x or more) can be implemented according to aspects described herein. Beam splitter 812-1, being the most upstream, can direct a small portion of illumination energy toward modulator system 808 while allowing most of the illumination energy onto beam splitter 812-2 (e.g., about a 16% going to beam of illumination 814-1 with the remaining 84% passed on to beam splitter 812-2). The ratio of split illumination energy at downstream beam splitters can be such that a larger percent of illumination is directed toward modulation system 808 with each successive beam splitter. In this manner, beams of illumination 814-1, 814-2, 814-3, 814-4, 814-5, and 814-6 (e.g., first, second, third, fourth, fifth, and sixth beams of illumination) can be substantially identical beams in terms of same energy or power, wavelength content, polarization, and the like.

[0146] In some aspects, since beams of illumination 814-1, 814-2, 814-3, 814-4, 814-5, and 814-6 can be substantially identical, a distinguishing characteristic can be added to each of the beams that would allow a downstream detector to identify the optical path of detected illumination. Modulator system 808 can encode the beams using distinguishable modulation parameters. For example, amplitude modulation can be used and the distinguishable modulation parameter can be unique frequencies of the different carrier signals. Other forms of modulation can be implemented (e.g., phase modulation, polarization modulation, time division modulation). A first beam of illumination can be modulated using a first modulation parameter (e.g., modulation frequency /;), a second beam of illumination can be modulated using a second modulation parameter (e.g., modulation frequency ?), and so on. Examples of modulators 816-1, 816-2, 816-3, 816-4, 816-5, and 816-6 can be choppers, micro-electro-mechanical system (MEMS) gratings or other MEMS-based devices, polarization-based (e.g., liquid crystal), variable retarders, and acousto-optic modulators. [0147] In some aspects, each of beams of illumination 814-1, 814-2, 814-3, 814-4, 814-5, and 814-6 can be used in the service of a matching number of inspection apparatuses (e.g., in this non-limiting example, six sensor heads). Multiple inspection apparatuses can be implemented by scaling multiple copies of inspection apparatus 400 (FIGS. 4A and 4B). Not all parts need to be scaled. For example, a single illumination system 800 can be used in lieu of multiple instances of illumination system 412 (FIGS. 4A and 4B). This arrangement allows for reduction of parts, cost, and space usage.

[0148] In some aspects, beams of illumination 814-1, 814-2, 814-3, 814-4, 814-5, and 814-6 can be used to illuminate targets 818-z (targets 818-1, 818-2, 818-3, 814-8, 814-8, and 818-6 (first, second, third, fourth, fifth, and sixth targets)) on a substrate 820, which correspond to multiple targets 418 on substrate 420 (FIGS. 4A and 4B) (for clarity, some intervening optics between the targets and illumination system 800 are not shown).

[0149] FIG. 9 shows a portion of an inspection system 900, according to some aspects. In some aspects, inspection system 900 can comprise sensor heads, a multiplexer 906, a demultiplexer 908, and a channel system 910.

[0150] In some aspects, an optical objective can represent a part of a sensor head. In FIG. 9 sensor heads are partly represented by optical objectives 902-/ (e.g., optical objectives 902-1, 902-2, 902-3, 902-4, 902-5, and 902-6 (first, second, third, fourth, fifth, and sixth optical objectives)) and optical systems 904-/ (e.g., optical systems 904-1, 904-2, 904-3, 904-4, 904-5, and 904-6 (first, second, third, fourth, fifth, and sixth optical systems)).

[0151] While FIGS. 4A and 4B did not expressly show an optical objective (for simplifying the drawings), it is to be understood that an optical objective (e.g., a lens, a system of lenses, mirrors, and the like) can be used in inspection system 400 in order to correctly image the illumination scattered by target 418. A sensor head can comprise a set of one optical objective and its associated optical system. A sensor head can represent one instance of inspection apparatus 400 (FIGS. 4A and 4B).

[0152] In some aspects, each of optical systems 904-z can represent one or more optical components of inspection apparatus 400.

[0153] Modifications to multiple inspection apparatuses are envisaged according to embodiments described herein. For example, rather than using multiple illumination sources, multiple inspection apparatuses can have a common illumination source, such as illumination system 800 (FIG. 8). Furthermore, in order to better organize different wavelengths sent to different sensor heads, multiplexer 500 (FIG. 5) can be implemented to stagger and interleave pulses of different wavelengths. In this manner, a single detection element, such as a photodetector, can be color-agnostic yet receive signals of different wavelengths for subsequent discrimination in post processing.

[0154] In some aspects, targets 918-z (e.g., targets 918-1, 918-2, 918-3, 918-4, 918-5, and 918-6 (first, second, third, fourth, fifth, and sixth targets)) on substrate 920 are irradiated with modulated beams of illumination as described above in reference to targets 818-z of FIG. 8. Each of the targets can scatter the incident illumination, sending scattered illumination toward each of optical objectives 902-1, 902- 2, 902-3, 902-4, 902-5, and 902-6, respectively. The scattered illumination is represented by scattered illumination 912-z (e.g., scattered illumination 912-1, 912-2, 912-3, 912-4, 912-5, and 912-6 (first, second, third, fourth, fifth, and sixth scattered illumination)).

[0155] In some aspects, the collected scattered illumination 912-z is guided via illumination guide systems 914-z (e.g., illumination guide systems 914-1, 914-2, 914-3, 914-4, 914-5, and 914-6 (first, second, third, fourth, fifth, and sixth illumination guide systems)). Since diffraction angles are wavelength dependent, each of illumination guide systems 914-z can comprise a number of optical fibers and/or waveguides to accommodate the different paths defined by the different wavelengths. In some aspects, space constraints can limit the number of optical paths and fibers/waveguides that can be accommodated. In an example implementation of six sensor heads with ten wavelengths, there can be sixty different optical paths (fibers/waveguides) across all of illumination guide systems 914-z. Each optical path can also comprise associated mirrors, polarizers, prisms, and the like, increasing space usage and costs.

[0156] Therefore, in some aspects, multiplexer 906 can be used to combine optical signals (scattered illumination 912-z) from different sensor heads to form combined signal 916.

[0157] For clarity, a single wavelength example is described first, which can be extrapolated to implementations with multiple wavelengths. The non-limiting examples shall make reference to six sensor heads (more or fewer sensor heads can be used). In the single wavelength example, illumination guide systems 914-1, 914-2, 914-3, 914-4, 914-5, and 914-6 can be thought of as six distinct optical paths, which are combined at multiplexer 906 into a single optical path (e.g., six sensor head signals combined into one channel). In this manner, scattered illumination 912-1, 912-2, 912-3, 912-4, 912-5, and 912-6 are combined to form combined signal 916. The nature of combined signal 916 can be analog or digital (e.g., a photodetector upstream of demultiplexer 908 can be used to convert received photons to electrical signals, which can be digitized). The multi-tap pixel device of FIG. 6 can also be used for combining detection and demultiplexing functionalities.

[0158] In some aspects, bulk space usage and cost of parts can be reduced by positioning the multiplexing to occur proximal to optical objectives 902-z and allowing most of the optical signal to propagate in the combined state as combined signal 916.

[0159] Depending on the limited space availability in a lithographic apparatus, demultiplexer 908 and/or channel system 910 can be disposed proximal or distal to the sensor heads. The combined signal 916 allows cost-efficient and space-efficient routing of multiple optical signals over both short and long distances.

[0160] In some aspects, combined signal 916 can be routed to demultiplexer 908. Demultiplexer 908 can discriminate the optical signals from the different sensor heads by analyzing the modulation content encoded in combined signal 916 with the known modulation parameters that were used to source the illumination (source modulation is described in reference to FIG. 8). Demultiplexer 908 can use any suitable signal discrimination method (e.g., lock-in amplifier detection, demodulation, phase-sensitive detection, or the like) to separate optical signals by examining the signals’ modulation content/parameters .

[0161] In some aspects, corresponding signal channels are illustrated in FIG. 9 as signal channels 910- i (e.g., signal channels 910-1, 910-2, 910-3, 910-4, 910-5, and 910-6 (first, second, third, fourth, fifth, and sixth signal channels). Via the uniquely identifiable modulation parameters, each of the signal channels can be mapped to a respective one of the sensor heads.

[0162] In some aspects, combined signal 916 can be any type of suitable signal, such as an optical signal or an electrical signal. Signal types can be further divided into other categories, such as an analog signal or a digital signal (e.g., a digital optical signal). Inspection apparatus 900 can be implemented using any suitable type of combined signal 916 by suitable placement of converter and/or detector devices. For example, let multiplexer 916 comprise photodetectors to convert scattered illumination 912-1, 912-2, 912-3, 912-4, 912-5, and 912-6 into corresponding analog electrical signals. The analog electrical signals can be added together to form an analog electrical version of combined signal 916. Then, further downstream, demodulator 908 can deconstruct the electrical combined signal 916 as described above.

[0163] In some aspects, the analog domain examples described herein can be implemented in digital domain by using analog-to-digital converter devices strategically disposed in modulator 906 or demodulator 908. Signal channels 910-/ can be physical channels and/or digital channels (e.g., digital data with channel identifier information).

[0164] In some aspects, the implementation in FIG. 9 can be scaled up according to number of wavelengths. In an example involving ten wavelengths, a single one of illumination guide systems 914- i can have ten optical paths therein (one for each wavelength). The number of optical paths can be implemented to accommodate the different diffraction angles resulting from the different wavelengths. For example, illumination guide system 914-1 can comprise a bundle of ten optical fibers. With six sensor heads, the hardware implementation can scale up to sixty optical fibers. This can be further increased to 120 optical fibers if two polarizations are desired. The cost and volume usage due to optical fibers alone can increase dramatically, in particular if the 120 optical fibers are intended to cover a long distance (e.g., from objectives to a computer system). To reduce cost and space burdens, the hardware in multiplexer 906 and demultiplexer 908 can be scaled up by lOx to operate with ten distinct combined signals 916 — one for each wavelength. In this manner, cost and volume usage in a lithographic apparatus is reduced by allowing signals from multiple sensor heads to travel via combined channels for at least a part of the distance.

[0165] FIG. 10 shows a multiplexer 1000, according to some aspects. In some aspects, multiplexer 1000 can be multiplexer 906 (FIG. 9). In some aspects, multiplexer 1000 can be an optical multiplexer that takes, as input, two or more optical signals (e.g., optical signals resulting from illumination scattered by different targets) to produce a single combined signal as output. In some aspects, multiplexer 1000 comprises an adjustable mirror 1002 and an adjustable mirror 1004. Adjustable mirrors 1002 and 1004 can be MEMS mirrors. Adjustable mirrors 1002 and/or 1004 can rotate about two axes (represented by two double arrows at each mirror).

[0166] In some aspects, input optical signals 1006-z (e.g., optical signals 1006-1, 1006-2, 1006-3, 1006-4, 1006-5, and 1006-6 (first, second, third, fourth, fifth, and sixth optical signals) can correspond to scattered illumination 912-1, 912-2, 912-3, 912-4, 912-5, and 912-6 (FIG. 9). Output optical signal 1008 can correspond to combined signal 916 (FIG. 9). The adjustability aspect of adjustable mirrors 1002 and 1004 allows incoming signals to be optically aligned to a common optical fiber or common waveguide via which optical signal 1008 is transmitted.

[0167] Multiplexer 1000 is disclosed as an example, as other multiplexer aspects are also envisaged. In some aspects, multiple input optical fibers (e.g., small-core multimode fibers) carrying optical signals 1006-i can be coupled into an optical coupler of a single larger multimode output optical fiber. Alternatively, a series of tapered core optical fibers can be coupled via 2-to-l reduction of input- to- output until all inputs are combined into a single optical fiber. In yet another alternative, a setup similar to multiplexer 1000 can be used, but using prisms and/or waveplates to merge optical signals (instead of, or in addition to mirrors). One or more optical fibers implemented in aspects herein can replaced with a suitable waveguide.

[0168] In some aspects, a demultiplexer can be a digital device (e.g., a computing system). Digital domain demultiplexing can use advanced frequency fitting techniques, such as Fast Fourier Transforms (FFT) and single frequency signal extraction with patterns of frequency from known modulation inputs. Digital domain operations can also comprise post-processing to analyze the information from illumination scattered by the multiple targets to determine a property of the targets (e.g., determine alignment positions of the targets). In aspects in which time domain multiplexing is also implemented, the computing system can process both the analysis of the modulation as well as the time domain information.

[0169] In some aspects, inspection of multiple targets need not be performed all at the same time. This is because, while perfect line-of-sight acquisition of multiple targets is desirable, positioning of targets on a wafer can be arbitrary. In a single sensor implementation, much time waste is incurred when the single sensor shuttles from one target to the next target. The time used in shifting from target to target can be in the order of milliseconds. There can be hundreds of targets on a single wafer, and a lithographic apparatus can process hundreds of wafers in the span of an hour. The time burden can add up significantly.

[0170] Now referring to the 6-target scattering example in FIGS. 8 and 9, even if only one target is perfectly in line-of-sight for measurement acquisition, it is convenient if any of the other sensor heads are closer to the next target. This reduces sensor-head travel time. Even in this asynchronous example of target inspection, the multichannel to single channel multiplexing of FIGS. 8-10 are still useful and relevant for reducing cost and volume usage. [0171] FIG. 11 shows a flowchart of a method 1100, according to some aspects. For example, method 1100 can be for performing demultiplexing of measurement signals acquired by multiple sensor heads. Method 1100 shall be described with reference to devices in FIGS. 8-10. However, method 1100 is not limited to the aspects referencing FIGS. 8-10 and can be implemented with any suitable inspection system.

[0172] In some aspects, at operation 1102, combined signal 916 can be demultiplexed so as to discriminate a first portion of combined signal 916 based on a first modulation parameter and a second portion of combined signal 916 based on a second modulation parameter. The first modulation parameter is associated with scattered illumination 912-1 from inspection target 918-1 illuminated by a first beam of illumination 814-1 modulated with the first modulation parameter. The second modulation parameter is associated with scattered illumination 912-2 from inspection target 918-2 illuminated by a first beam of illumination 814-2 modulated with the first modulation parameter. Combined signal 916 is based on scattered illumination 912-7 as described above. The first and second portions of combined signal 916 comprise information about inspection targets 918-t.

[0173] In some aspects, at operation 1104, a property of inspection target 918-1 (e.g., an alignment position) can be determined based on analysis of the first portion and the first modulation parameter.

[0174] In some aspects, at operation 1106, a property of inspection target 918-2 (e.g., an alignment position) can be determined based on analysis of the second portion and the second modulation parameter.

[0175] In some aspects, method 1100 can be extrapolated to two or more targets.

[0176] The operations of FIG. 11 can be performed in any conceivable order and it is not required that all operations be performed. Moreover, the operations of FIG. 11 described above merely reflect an example of operations and are not limiting. That is, further operations are envisaged based aspects described above in reference to FIGS. 1-3, 4A, 4B, and 8-10.

[0177] While FIGS. 8-11 are directed to modulation techniques for multiple sensors, the modulation technique can be interchanged or combined with time-division multiplexing features in reference to FIGS. 5-7. Aspects directed to FIG. 12 serve as an example of such interchangeability.

[0178] FIG. 12 shows a multiplexer 1200, according to some aspects. In some aspects, multiplexer 1200 is essentially a modification of multiplexer 500 (FIG. 5) for implementation with multiple sensor heads described in reference to FIGS. 8-11. Hence, some elements in FIG. 12 can have structures and functions that are substantially similar to corresponding elements of FIG. 5 (e.g., corresponding elements can have reference numbers sharing the two right-most numeric digits). For example, unless otherwise stated, implementations of a separator 1202, timing system 1204, combiner 1206, pulses 1208, pulses 1210, pulses 1212, pulses 1214, time instance 1216, delay device 1218, delay device 1220, delay device 1222, interleaved pulses 1224 and oscillator device 1226 can be substantially similar to implementations described above in reference to wavelength separator 502, timing system 504, wavelength combiner 506, pulses 508, pulses 510, pulses 512, pulses 514, time instance 516, delay device 518, delay device 520, delay device 522, interleaved pulses 524 and oscillator device 526, respectively (FIG. 5).

[0179] In some aspects, pulses 1208 can correspond to a beam of source illumination. Separator 1202 can comprise a suitable device to separate the beam of source illumination (e.g., a beam splitter). The separated beams can be substantially identical (e.g., have the same wavelength(s)). If it is desired to separate beams according to different wavelengths, then wavelengths separator 502 can be used as separator 1202. Beams of radiation associated with pulses 1210, 1212, and 1214 (e.g., first, second, and third beams of radiation) can be generated from the beam of source illumination and pulses 1208. More or fewer beams of radiation can be generated by separator 1202 (e.g., two or more beams of radiation). The beam of radiation having pulses 1210 can be associated with a given sensor head, the beam of radiation having pulses 1212 can be associated with a different sensor head, and the beam of radiation having pulses 1214 can be associated with yet another sensor head (substantially similar to the beams and targets arrangement in FIG. 8). More than one illumination source can be used and a timing reference can be used (e.g., oscillator device 1226) so as to mitigate desync of two or more illumination sources.

[0180] Whereas aspects referencing FIG. 5 use interleaved pulses 524 for irradiating a target (pulseinterleaving is performed at illumination side), aspects in reference to FIG. 12 perform pulseinterleaving using the scattered radiation from multiple targets (pulse-interleaving is performed at detection side). For example purposes, targets 1228, 1230, and 1232 (e.g., first, second, and third targets) are disposed in, or near, the optical path of pulses 1210, 1212, and 1214. It is to be appreciated that elements of multiplexer 1200 can be distributed across both illumination and detection sides of a sensor (e.g., elements disposed upstream and downstream of targets). As will be explained further below, pulse-interleaving in FIG. 12 need not refer to interleaving of actual pulses of illumination as was the case in FIG. 5. Rather, pulse-interleaving in FIG. 12 can refer to the interleaving of identifiable time slots. A time slot is a finite period in which one can expect to find an illumination pulse, but an illumination pulse need not be present in that time slot for one or more reasons, as explained further below. Terms such as “time slot,” “time frame,” “time period,” or the like, can be used to refer to a finite range of time, the range being bounded by a beginning time and an ending time.

[0181] In some aspects, targets 1228, 1230, and 1232 can correspond to any three targets in FIGS. 8 or 9. More or fewer targets may be implemented (e.g., two or more targets). For example purposes, the following scenario will assume that the sensor heads associated with beams of illumination with pulses 1210, 1212, and 1214 are perfectly aligned with targets 1228, 1230, and 1232, respectively (in practice, this need not be the case, yet the arrangement is still useful for combining what would otherwise be separate optical paths, thereby reducing cost). Target 1228 can scatter illumination associated with pulses 1210 to generate scattered illumination associated with pulses 1234 (e.g., first scattered illumination). Target 1230 can scatter illumination associated with pulses 1212 to generate scattered illumination associated with pulses 1236 (e.g., second scattered illumination). Target 1232 can scatter illumination associated with pulses 1214 to generate scattered illumination associated with pulses 1238 (e.g., third scattered illumination).

[0182] In some aspects, delay device 1218 can receive pulses 1234. Delay device 1218 can time shift pulses 1234 by a suitable delay offset. Delay device 1220 can receive pulses 1236. Delay device 1220 can time shift pulses 1236 by a suitable delay offset. Delay device 1222 can receive pulses 1238. Delay device 1222 can time shift pulses 1238 by a suitable delay offset. For any given sensor head, a delay offset can be zero in the event that no delay is needed. Furthermore, though not expressly shown in FIG. 12, timing system 1204 can be disposed upstream of targets 1228, 1230, and 1232 so as to interact with pulses 1210, 1212, and 1214 (illumination side) rather than with pulses 1234, 1236, and 1238 (detection side scattered illumination). Pulses 1234’, 1236’, and 1238’ incorporate time offsets provided by timing system 1204 and respectively correspond to pulses 1234, 1236, and 1238.

[0183] In some aspects, pulses 1234’, 1236’, and 1238’ can be combined using combiner 1206 such that the timing of the pulses of different sensors are interleaved. The combined pulses are represented as interleaved pulses 1224. The timing of pulses 1234’, 1232’, and 1238’ (or the scheduled time slots of the pulses) are staggered such that interleaved pulses 1224 have non overlapping pulses or time slots, with one pulse or time slot being associated with pulses 1210 for a given sensor head, a next pulse or time slot being associated with pulses 1212 for a different sensor head, and a yet next pulse or time slot being associated with pulses 510 for yet another sensor head. Combiner 1206 can be a suitable beam combiner (e.g., a beam splitter, a refractive device, plural-to-1 optical fiber couple, or the like).

[0184] In some aspects, detection side functions and post processing can be performed as described in reference to FIG. 6. Scattered illumination from targets 1228, 1230, and 1232 can be characterized by interleaved pulses 614 (FIG. 6), which corresponds to pulsed timings of interleaved pulses 1224. A combined signal stream, whether optical, electrical, or otherwise, is associated with interleaved pulses 1224. The discriminated portions of the combined signal stream are analyzed in order to determine one or more properties of targets 1228, 1230, and 1232 (e.g., alignment positions). All of the implementations described above in reference to FIG. 6 can be implemented along with multiplexer 1200 in FIG. 12, with the discrimination being with respect to a first sensor head, a second sensor head, and so on as the case may be.

[0185] As mentioned above, interleaving of pulses in FIG. 12 can be described as interleaving of identifiable time slots of discriminable pulses. For example purposes, the following scenario will assume that a sensor head associated with beam of illumination with pulse 1210 is aligned with target 1228, and sensor heads associated with beams of illumination with pulses 1212 and 1214 are not aligned with their respective targets 1230 and 1232. This scenario is plausible since it is not requisite for a patterning device to print alignment marks (targets) spaced so as to exactly match the spacing of the sensor heads. In this scenario, pulses 1234’ are generated because target 1228 is aligned with its sensor head. However, pulses 1236’ and 1238’ are not generated for lack of alignment between targets 1230 and 1232 and respective sensor heads. However, even in this scenario, the pulse-interleaving works as time- slot-interleaving.

[0186] In some aspects, even though only pulses 1234’ are generated, time-division multiplexing as described in FIG. 12 allows pulses 1234’ to be passed via a combined signal stream (e.g., via a single optical fiber shared among many different pulses and sensor heads). The ordered scheduling of pulse time slots in the combined signal stream allows identification of which pulse corresponds to which sensor head, thereby allowing multiple sensor heads to work with combined hardware (reduced cost).

[0187] FIG. 13 shows a flowchart of a method 1300, according to some aspects. For example, method 1300 can be for performing demultiplexing of measurement signals acquired by multiple sensor heads. Method 1300 shall be described with reference to devices in FIGS. 12. However, method 1300 is not limited to the aspects referencing FIGS. 12 and can be implemented with any suitable inspection system. [0188] In some aspects, at operation 1302, a first beam of illumination can be directed toward first target 1228. The first beam can comprise first pulses 1210.

[0189] In some aspects, at operation 1304, first scattered illumination can be generated from first target 1228 using the first beam.

[0190] In some aspects, at operation 1306, a second beam of illumination can be directed toward second target 1230. The second beam can comprise second pulses 1212.

[0191] In some aspects, at operation 1308, second scattered illumination can be generated from second target 1230 using the second beam.

[0192] In some aspects, at operation 1310, time slots corresponding to pulses 1234 of the first scattered illumination and time slots corresponding to pulses 1236 of the second scattered illumination can be interleaved to form a combined signal stream.

[0193] In some aspects, at operation 1312, portions of the combined signal stream can be discriminated based on pulse timings. A first portion discriminated from the combined signal stream can comprise information about first target 1228. The first portion can be discriminated based on timing of pulses 1234 of the first scattered illumination. A second portion discriminated from the combined signal stream can comprise information about the second target. The second portion can be discriminated based on timing of pulses 1236 of the second scattered radiation.

[0194] The operations of FIG. 13 can be performed in any conceivable order and it is not required that all operations be performed. Moreover, the operations of FIG. 13 described above merely reflect an example of operations and are not limiting. That is, further operations are envisaged based aspects described above in reference to FIGS. 1-3, 4A, 4B, and 5-12.

[0195] FIG. 14 shows a computer system 1400, according to some aspects. Various aspects and components therein can be implemented, for example, using computer system 1400 or any other well- known computer systems. For example, one or more of the operations of FIGS. 7, 11, and 13 can be implemented via computer system 1400. [0196] In some aspects, computer system 1400 can comprise one or more processors (also called central processing units, or CPUs), such as a processor 1404. Processor 1404 can be connected to a communication infrastructure or bus 1406.

[0197] In some aspects, one or more processors 1404 can each be a graphics processing unit (GPU). In some aspects, a GPU is a processor that is a specialized electronic circuit designed to process mathematically intensive applications. The GPU can have a parallel structure that is efficient for parallel processing of large blocks of data, such as mathematically intensive data common to computer graphics applications, images, videos, etc.

[0198] In some aspects, computer system 1400 can further comprise user input/output device(s) 1403, such as monitors, keyboards, pointing devices, etc., that communicate with communication infrastructure 1406 through user input/output interface(s) 1402. Computer system 1400 can further comprise a main or primary memory 1408, such as random access memory (RAM). Main memory 1408 can comprise one or more levels of cache. Main memory 1408 has stored therein control logic (z.e., computer software) and/or data.

[0199] In some aspects, computer system 1400 can further comprise one or more secondary storage devices or memory 1410. Secondary memory 1410 can comprise, for example, a hard disk drive 1412 and/or a removable storage device or drive 1414. Removable storage drive 1414 can be a floppy disk drive, a magnetic tape drive, a compact disk drive, an optical storage device, tape backup device, and/or any other storage device/drive. Removable storage drive 1214 can interact with a removable storage unit 1418. Removable storage unit 1418 can comprise a computer usable or readable storage device having stored thereon computer software (control logic) and/or data. Removable storage unit 1418 can be a floppy disk, magnetic tape, compact disk, DVD, optical storage disk, and/ any other computer data storage device. Removable storage drive 1414 reads from and/or writes to removable storage unit 1418 in a well-known manner.

[0200] In some aspects, secondary memory 1410 can comprise other means, instrumentalities or other approaches for allowing computer programs and/or other instructions and/or data to be accessed by computer system 1400. Such means, instrumentalities or other approaches can comprise, for example, a removable storage unit 1422 and an interface 1420. Examples of the removable storage unit 1422 and the interface 1420 can comprise a program cartridge and cartridge interface (such as that found in video game devices), a removable memory chip (such as an EPROM or PROM) and associated socket, a memory stick and USB port, a memory card and associated memory card slot, and/or any other removable storage unit and associated interface.

[0201] In some aspects, computer system 1400 can further comprise a communication or network interface 1424. Communication interface 1424 enables computer system 1400 to communicate and interact with any combination of remote devices, remote networks, remote entities, etc. (individually and collectively referenced by reference number 1428). For example, communication interface 1424 can allow computer system 1400 to communicate with remote devices 1428 over communications path 1426, which can be wired and/or wireless, and which can comprise any combination of LANs, WANs, the Internet, etc. Control logic and/or data can be transmitted to and from computer system 1400 via communications path 1426.

[0202] In some aspects, a non-transitory, tangible apparatus or article of manufacture comprising a non-transitory, tangible computer useable or readable medium having control logic (software) stored thereon is also referred to herein as a computer program product or program storage device. This includes, but is not limited to, computer system 1400, main memory 1408, secondary memory 1410, and removable storage units 1418 and 1422, as well as tangible articles of manufacture embodying any combination of the foregoing. Such control logic, when executed by one or more data processing devices (such as computer system 1400), causes such data processing devices to operate as described herein.

[0203] Based on the teachings contained in this disclosure, it will be apparent to those skilled in the relevant art(s) how to make and use aspects of this disclosure using data processing devices, computer systems and/or computer architectures other than that shown in FIG. 14. In particular, aspects described herein can operate with software, hardware, and/or operating system implementations other than those described herein.

[0204] The embodiments may further be described using the following clauses:

1. A lithographic apparatus comprising: a projection system configured to project an image of structures of a patterning device onto a substrate; and an inspection apparatus comprising: an illuminator section configured to direct a beam of illumination toward a target on the substrate, the illuminator section comprising a multiplexer configured to: transmit a first portion of the beam via first pulses, the first portion having a first illumination parameter; transmit a second portion of the beam via second pulses, the second portion having a second illumination parameter different from the first illumination parameter; and interleave, in time, the first pulses and the second pulses; a photodetector configured to receive pulses of scattered illumination from the target and to generate a measurement signal based on the pulses of the scattered illumination; and a demultiplexer configured to analyze the measurement signal to discriminate portions of the measurement signal according to illumination parameter based on timing of the pulses of the scattered illumination.

2. The lithographic apparatus of clause 1, wherein: the inspection apparatus further comprises an illumination source configured to generate the beam at multiple wavelengths simultaneously; the first illumination parameter is a first wavelength; the second illumination parameter is a second wavelength; and the multiplexer comprises a separator configured to separate a propagation path of the first pulses from a propagation path of the second pulses.

3. The lithographic apparatus of clause 1, wherein the multiplexer further comprises a timing system configured to offset timings of the first pulses with respect to the second pulses.

4. The lithographic apparatus of clause 3, wherein the timing system comprises delay devices configured to compensate delay, per illumination parameter, across the inspection apparatus.

5. The lithographic apparatus of clause 1, wherein the multiplexer further comprises a combiner configured to interleave the first pulses and the second pulses.

6. The lithographic apparatus of clause 1 , wherein the inspection apparatus further comprises an oscillator device configured to provide a timing reference for the first pulses and the second pulses.

7. The lithographic apparatus of clause 1, wherein: the inspection apparatus further comprises a computing system; and the demultiplexer is a digital demultiplexer of the computing system.

8. The lithographic apparatus of clause 1, wherein the inspection apparatus further comprises a computing system configured to analyze the portions of the measurement signal according to illumination parameter and to determine illumination-parameter-dependent values of one or more properties of the target based on analyzed portions of the measurement signal according to illumination parameter.

9. The lithographic apparatus of clause 1, wherein the inspection apparatus further comprises an optical fiber configured to guide the scattered illumination at multiple wavelengths to the photodetector.

10. The lithographic apparatus of clause 1, wherein the inspection apparatus further comprises an on-chip integrated optical system configured to collect the scattered radiation at multiple wavelengths and at multiple diffraction angles corresponding to the multiple wavelengths.

11. The lithographic apparatus of clause 9, wherein the photodetector is an on-chip photodetector of the on-chip integrated optical system.

12. The lithographic apparatus of clause 1, wherein the inspection apparatus further comprises a multi-tap pixel device comprising the photodetector and the demultiplexer.

13. An inspection apparatus comprising: an illuminator section configured to direct a beam of illumination toward an inspection target, the illuminator section comprising a multiplexer configured to: transmit a first portion of the beam via first pulses, the first portion having a first illumination parameter; transmit a second portion of the beam via second pulses, the second portion having a second illumination parameter different from the first illumination parameter; and interleave, in time, the first pulses and the second pulses; a photodetector configured to receive pulses of scattered illumination from the inspection target and to generate a measurement signal based on the pulses of the scattered illumination; and a demultiplexer configured to analyze the measurement signal to discriminate portions of the measurement signal according to illumination parameter based on timing of the pulses of the scattered illumination.

14. The inspection apparatus of clause 13, further comprising: an illumination source configured to generate the beam at multiple wavelengths simultaneously, wherein the first illumination parameter is a first wavelength; wherein the second illumination parameter is a second wavelength; and wherein the multiplexer comprises a separator configured to separate a propagation path of the first pulses from a propagation path of the second pulses.

15. The inspection apparatus of clause 13, wherein the multiplexer further comprises a timing system configured to offset timings of the first pulses with respect to the second pulses.

16. The inspection apparatus of clause 15, wherein the multiplexer further comprises a combiner configured to interleave the first pulses and the second pulses.

17. The inspection apparatus of clause 13, further comprising an oscillator device configured to provide a timing reference for the first pulses and the second pulses.

18. The inspection apparatus of clause 13, further comprising a computing system configured to analyze the portions of the measurement signal according to illumination parameter and to determine illumination-parameter-dependent values of one or more properties of the inspection target based on analyzed portions of the measurement signal according to illumination parameter.

19. The inspection apparatus of clause 13, further comprising an on-chip integrated optical system configured to collect the scattered radiation at multiple wavelengths and at multiple diffraction angles corresponding to the multiple wavelengths.

20. A method comprising: multiplexing wavelengths of a beam of illumination directed toward an inspection target, the multiplexing comprising; transmitting a first portion of the beam via first pulses, the first portion having a first illumination parameter; transmitting a second portion of the beam via second pulses, the second portion having a second illumination parameter different from the first illumination parameter; and interleaving, in time, the first pulses and the second pulses; generating a measurement signal based on pulses of scattered illumination, from the inspection target, received at a photodetector; and discriminating portions of the measurement signal according to illumination parameter based on analysis of the measurement signal and timing of the pulses of the scattered illumination.

21. A lithographic apparatus comprising: a projection system configured to receive illumination to project an image of a pattern of a patterning device onto a substrate to form targets on the substrate; and an inspection system comprising: a modulator system configured to modulate a first beam of illumination directed toward a first target on the substrate using a first modulation parameter and to modulate a second beam of illumination directed to a second target on the substrate using a second modulation parameter different from the first modulation parameter; a first objective configured to collect first scattered illumination from the first target; a second objective configured to collect second scattered illumination from the second target; a multiplexer configured to form a combined signal using the first scattered illumination and the second scattered illumination; and a demultiplexer configured to discriminate a first portion of the combined signal based on the first modulation parameter and to discriminate a second portion of the combined signal based on the second modulation parameters, wherein the first and second portions comprise information about the first and second targets, respectively.

22. The lithographic apparatus of clause 21, wherein: the inspection system further comprises a computing system; the demultiplexer is a digital demultiplexer of the computing system; and the computing system is configured to analyze the combined signal to determine: a property of the first target based on the first modulation parameter; and a property of the second target based on the second modulation parameter.

23. The lithographic apparatus of clause 22, wherein the computing system is further configured to use a frequency fitting algorithm to analyze the combined signal.

24. The lithographic apparatus of clause 21, wherein the inspection system further comprises: an illumination source configured to generate source illumination; and a beam splitter configured to split the source illumination to generate the first beam and the second beam, wherein a modulator of the modulator system is disposed upstream of the beam splitter.

25. The lithographic apparatus of clause 21, wherein a modulator of the modulator system is disposed at a pupil plane of the inspection system and upstream of the substrate.

26. The lithographic apparatus of clause 21, wherein the modulator system comprises an amplitude modulator, a phase modulator, or a polarization modulator.

27. The lithographic apparatus of clause 21, wherein the inspection system further comprises: a first waveguide or first optical fiber configured to guide the first scattered illumination from the first objective to the multiplexer; and a second waveguide or first optical fiber configured to guide the second scattered illumination from the second objective to the multiplexer.

28. The lithographic apparatus of clause 27, wherein: the multiplexer comprises an optical coupler; and the first waveguide or the first optical fiber and the second waveguide or the second optical fiber are further configured to couple to the optical coupler to merge optical signals.

29. The lithographic apparatus of clause 27, wherein the multiplexer comprises a micromirror system configured to: receive the first beam from the first waveguide or the first optical fiber and the second beam from the second waveguide or the second optical fiber; and direct the first beam and the second beam to a common waveguide or common optical fiber.

30. An inspection system comprising: a modulator system configured to modulate, using a first modulation parameter, a first beam of illumination directed toward a first inspection target and to modulate, using a second modulation parameter different from the first modulation parameter, a second beam of illumination directed to a second inspection target; a first objective configured to collect first scattered illumination from the first inspection target; a second objective configured to collect second scattered illumination from the second inspection target; a multiplexer configured to form a combined signal based on the first scattered illumination and the second scattered illumination; and a demultiplexer configured to separate a first portion and a second portion of the combined signal based on the first and second modulation parameters, wherein the first and second portions comprise information about the first and second targets, respectively.

31. The inspection system of clause 30, further comprising a computing system wherein: the demultiplexer is a digital demultiplexer of the computing system; and the computing system is configured to analyze the combined signal to determine: a property of the first target based on the first modulation parameter; and a property of the second target based on the second modulation parameter.

32. The inspection system of clause 31 , wherein the computing system is further configured to use a frequency fitting algorithm to analyze the combined signal.

33. The inspection system of clause 30, further comprising: an illumination source configured to generate source illumination; and a beam splitter configured to split the source illumination to generate the first beam and the second beam, wherein the modulation system is disposed upstream of the beam splitter. 34. The inspection system of clause 30, wherein a modulator of the modulator system is disposed at a pupil plane of the inspection system and upstream of the first or second targets.

35. The inspection system of clause 30, wherein the modulation system comprises an amplitude modulator or a phase modulator.

36. The inspection system of clause 30, further comprising: a first waveguide or first optical fiber configured to guide the first scattered illumination from the first objective to the multiplexer; and a second waveguide or first optical fiber configured to guide the second scattered illumination from the second objective to the multiplexer.

37. The inspection system of clause 36, wherein: the multiplexer comprises an optical coupler; and the first waveguide or the first optical fiber and the second waveguide or the second optical fiber are further configured to couple to the optical coupler to merge optical signals.

38. The inspection system of clause 36, wherein the multiplexer comprises a micro-electro- mechanical system (MEMS) mirror configured to: receive the first beam from the first waveguide or the first optical fiber and the second beam from the second waveguide or the second optical fiber; and direct the first beam and the second beam to a common waveguide or common optical fiber.

39. A non-transitory computer-readable medium having instructions stored thereon, that, when executed on a computing system of an inspection system cause the computing system to perform operations, the operations comprising: demultiplexing a combined signal to discriminate a first portion of the combined signal based on a first modulation parameter and a second portion of the combined signal based on a second modulation parameter, wherein: the first modulation parameter is associated with first scattered illumination from a first inspection target illuminated by a first beam of illumination modulated with the first modulation parameter, the second modulation parameter is associated with second scattered illumination from a second inspection target illuminated by a second beam of illumination modulated with the second modulation parameter, the combined signal is based on the first scattered illumination and the second scattered illumination, and the first and second portions comprise information about the first and second inspection targets; determining a property of the first inspection target based on analysis of the first portion and the first modulation parameter; and determining a property of the second inspection target based on analysis of the second portion and the second modulation parameter.

40. The non-transitory computer-readable medium of clause 39, wherein: the first modulation parameter is an amplitude modulation at a first frequency; and the second modulation parameter is an amplitude modulation at a second frequency.

41. A lithographic apparatus comprising: a projection system configured to receive illumination to project an image of a pattern of a patterning device onto a substrate to form targets on the substrate; and an inspection system comprising: a first illuminator section configure to direct a first beam of illumination toward a first target, wherein the first beam comprises first pulses; a first objective configured to collect first scattered illumination from the first target; a second illuminator section configured to direct a second beam of illumination toward a second target, wherein the second beam comprises second pulses; a second objective configured to collect second scattered illumination from the second target; a multiplexer configured to interleave, in time, time slots corresponding to pulses of the first scattered illumination and time slots corresponding to pulses of the second scattered illumination to form a combined signal stream; a demultiplexer configured to discriminate portions of the combined signal stream based on pulse timings, wherein a first portion discriminated from the combined signal stream comprises information about the first target and is based on timing of the pulses of the first scattered illumination, and wherein a second portion discriminated from the combined signal stream comprises information about the second target and is based on timing of the pulses of the second scattered illumination.

42. The lithographic apparatus of clause 41, wherein: the inspection system further comprises an illumination source configured to generate source illumination; and the multiplexer comprises a separator configured to split the source illumination to generate the first beam and the second beams.

43. The lithographic apparatus of clause 41, wherein the multiplexer further comprises a timing system configured to offset timings of the first pulses with respect to the second pulses.

44. The lithographic apparatus of clause 43, wherein the timing system comprises delay devices configured to compensate delay caused by a difference between an optical path associated with the first beam and an optical path associated with the second beam.

45. The lithographic apparatus of clause 41, wherein the multiplexer further comprises a combiner configured to interleave the time slots corresponding to pulses of the first scattered illumination and time slots corresponding to pulses of the second scattered illumination.

46. The lithographic apparatus of clause 41, wherein the inspection system further comprises an oscillator device configured to provide a timing reference for the first pulses and the second pulses.

47. The lithographic apparatus of clause 41, wherein: the inspection system further comprises a computing system; and the demultiplexer is a digital demultiplexer of the computing system.

48. The lithographic apparatus of clause 41, wherein the inspection system further comprises a computing system configured to analyze the first portion and the second portion and to determine one or more values of one or more properties of the first target and one or more values of one or more properties of the second target based on analysis of the first portion and the second portion.

49. The lithographic apparatus of clause 41, wherein the inspection system further comprises a photodetector configured to receive pulses of the first scattered illumination and pulses the second scattered illumination according to interleaved time slots arranged by the multiplexer.

50. The lithographic apparatus of clause 49, wherein the inspection system further comprises an optical fiber configured to guide the interleaved pulses to the photodetector.

51. The lithographic apparatus of clause 49, wherein the photodetector is an on-chip photodetector of an on-chip integrated optical system.

52. The lithographic apparatus of clause 49, wherein the inspection system further comprises a multi-tap pixel device comprising the photodetector and the demultiplexer.

53. An inspection system comprising: a first illuminator section configure to direct a first beam of illumination toward a first target on the substrate via first pulses; a first objective configured to collect first scattered illumination from the first target; a second illuminator section configured to direct a second beam of illumination toward a second target on the substrate via second pulses; a second objective configured to collect second scattered illumination from the second target; a multiplexer configured interleave, in time, time slots corresponding to pulses of the first scattered illumination and time slots corresponding to pulses of the second scattered illumination to form a combined signal stream; a demultiplexer configured to discriminate portions of the combined signal stream based on pulse timings, wherein a first portion discriminated from the combined signal stream comprises information about the first target and is based on timing of the pulses of the first scattered illumination, and wherein a second portion discriminated from the combined signal stream comprises information about the second target and is based on timing of the pulses of the second scattered illumination.

54. The inspection system of clause 53, further comprising: an illumination source configured to generate source illumination, wherein the multiplexer comprises a separator configured to split the source illumination to generate the first beam and the second.

55. The inspection system of clause 53, wherein the multiplexer further comprises a timing system configured to offset timings of the first pulses with respect to the second pulses. 56. The inspection system of clause 55, wherein the multiplexer further comprises a combiner configured to interleave the time slots corresponding to pulses of the first scattered illumination and time slots corresponding to pulses of the second scattered illumination.

57. The inspection system of clause 53, further comprising an oscillator device configured to provide a timing reference for the first pulses and the second pulses.

58. The inspection system of clause 53, further comprising a computing system configured to analyze the first portion and the second portion and to determine one or more values of one or more properties of the first target and one or more values of one or more properties of the second target based on analysis of the first portion and the second portion.

59. The inspection system of clause 53, further comprising a multi-tap pixel device comprising: the demultiplexer; and a photodetector configured to receive pulses of the first scattered radiation and pulses the second scattered radiation according to interleaved time slots arranged by the multiplexer.

60. A method comprising: directing a first beam of illumination toward a first target on a substrate, wherein the first beam comprises first pulses; generating first scattered illumination from the first target using the first beam; directing a second beam of illumination toward a second target on a substrate, wherein the second beam comprises second pulses; generating second scattered illumination from the second target using the second beam; interleaving, in time, time slots corresponding to pulses of the first scattered illumination and time slots corresponding to pulses of the second scattered illumination to form a combined signal stream; discriminating portions of the combined signal stream based on pulse timings, wherein a first portion discriminated from the combined signal stream comprises information about the first target and is based on timing of the pulses of the first scattered illumination, and wherein a second portion discriminated from the combined signal stream comprises information about the second target and is based on timing of the pulses of the second scattered illumination.

[0205] 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 X 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.

[0206] 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.

[0207] 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.

[0208] 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.

[0209] 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.

Claims

2. The lithographic apparatus of claim 1, wherein: the inspection apparatus further comprises an illumination source configured to generate the beam at multiple wavelengths simultaneously; the first illumination parameter is a first wavelength; the second illumination parameter is a second wavelength; and the multiplexer comprises a separator configured to separate a propagation path of the first pulses from a propagation path of the second pulses.

3. The lithographic apparatus of claim 1, wherein the multiplexer further comprises a timing system configured to offset timings of the first pulses with respect to the second pulses.

4. The lithographic apparatus of claim 3, wherein the timing system comprises delay devices configured to compensate delay, per illumination parameter, across the inspection apparatus.

5. The lithographic apparatus of claim 1, wherein the multiplexer further comprises a combiner configured to interleave the first pulses and the second pulses.

6. The lithographic apparatus of claim 1, wherein the inspection apparatus further comprises an oscillator device configured to provide a timing reference for the first pulses and the second pulses.

7. The lithographic apparatus of claim 1, wherein: the inspection apparatus further comprises a computing system; and the demultiplexer is a digital demultiplexer of the computing system.

8. The lithographic apparatus of claim 1, wherein the inspection apparatus further comprises a computing system configured to analyze the portions of the measurement signal according to illumination parameter and to determine illumination-parameter-dependent values of one or more properties of the target based on analyzed portions of the measurement signal according to illumination parameter.

9. The lithographic apparatus of claim 1, wherein the inspection apparatus further comprises an optical fiber configured to guide the scattered illumination at multiple wavelengths to the photodetector.

10. The lithographic apparatus of claim 1, wherein the inspection apparatus further comprises an on-chip integrated optical system configured to collect the scattered radiation at multiple wavelengths and at multiple diffraction angles corresponding to the multiple wavelengths.

11. The lithographic apparatus of claim 9, wherein the photodetector is an on-chip photodetector of the on-chip integrated optical system.

12. The lithographic apparatus of claim 1, wherein the inspection apparatus further comprises a multi-tap pixel device comprising the photodetector and the demultiplexer.

14. The inspection apparatus of claim 13, further comprising: an illumination source configured to generate the beam at multiple wavelengths simultaneously, wherein the first illumination parameter is a first wavelength; wherein the second illumination parameter is a second wavelength; and wherein the multiplexer comprises a separator configured to separate a propagation path of the first pulses from a propagation path of the second pulses.

15. The inspection apparatus of claim 13, wherein the multiplexer further comprises a timing system configured to offset timings of the first pulses with respect to the second pulses.