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WO2024260846A1 - Marque de mesure optique, système de mesure optique et appareil lithographique - Google Patents

Marque de mesure optique, système de mesure optique et appareil lithographique Download PDF

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Publication number
WO2024260846A1
WO2024260846A1 PCT/EP2024/066450 EP2024066450W WO2024260846A1 WO 2024260846 A1 WO2024260846 A1 WO 2024260846A1 EP 2024066450 W EP2024066450 W EP 2024066450W WO 2024260846 A1 WO2024260846 A1 WO 2024260846A1
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WO
WIPO (PCT)
Prior art keywords
optical measurement
wavefront
measurement mark
radiation
substructure
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
PCT/EP2024/066450
Other languages
English (en)
Inventor
Derick Yun Chek CHONG
Li GUI
Frank Staals
Manfred Petrus Johannes Maria DIKKERS
Marie-Claire VAN LARE
Johannes Jacobus Matheus Baselmans
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ASML Netherlands BV
Original Assignee
ASML Netherlands BV
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Filing date
Publication date
Application filed by ASML Netherlands BV filed Critical ASML Netherlands BV
Publication of WO2024260846A1 publication Critical patent/WO2024260846A1/fr
Anticipated expiration legal-status Critical
Pending legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
    • G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
    • G03F7/70—Microphotolithographic exposure; Apparatus therefor
    • G03F7/70483—Information management; Active and passive control; Testing; Wafer monitoring, e.g. pattern monitoring
    • G03F7/70591—Testing optical components
    • G03F7/706—Aberration measurement
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
    • G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
    • G03F7/70—Microphotolithographic exposure; Apparatus therefor
    • G03F7/70483—Information management; Active and passive control; Testing; Wafer monitoring, e.g. pattern monitoring
    • G03F7/70605—Workpiece metrology
    • G03F7/70681—Metrology strategies
    • G03F7/70683—Mark designs
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
    • G03F9/00—Registration or positioning of originals, masks, frames, photographic sheets or textured or patterned surfaces, e.g. automatically
    • G03F9/70—Registration or positioning of originals, masks, frames, photographic sheets or textured or patterned surfaces, e.g. automatically for microlithography
    • G03F9/7049—Technique, e.g. interferometric
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
    • G03F9/00—Registration or positioning of originals, masks, frames, photographic sheets or textured or patterned surfaces, e.g. automatically
    • G03F9/70—Registration or positioning of originals, masks, frames, photographic sheets or textured or patterned surfaces, e.g. automatically for microlithography
    • G03F9/7073—Alignment marks and their environment
    • G03F9/7076—Mark details, e.g. phase grating mark, temporary mark
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
    • G03F9/00—Registration or positioning of originals, masks, frames, photographic sheets or textured or patterned surfaces, e.g. automatically
    • G03F9/70—Registration or positioning of originals, masks, frames, photographic sheets or textured or patterned surfaces, e.g. automatically for microlithography
    • G03F9/7088—Alignment mark detection, e.g. TTR, TTL, off-axis detection, array detector, video detection

Definitions

  • the present invention relates to an optical measurement mark and method for an imaging system.
  • the optical measurement mark and method may be suitable for use in a lithographic apparatus or process.
  • the present disclosure has particular use in connection with EUV lithographic apparatus, EUV lithographic tools and EUV lithographic processes.
  • a lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate.
  • a lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs).
  • a lithographic apparatus may, for example, project a pattern at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate.
  • a patterning device e.g., a mask
  • resist radiation-sensitive material
  • a lithographic apparatus may use electromagnetic radiation.
  • the wavelength of this radiation at least partially determines the minimum size of features which can be formed on the substrate.
  • a lithographic apparatus which uses extreme ultraviolet (EUV) radiation, having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.
  • EUV extreme ultraviolet
  • a lithographic apparatus may comprise an optical alignment system which may be used to determine and improve an alignment between the patterning device and the substrate.
  • the patterning device may include an optical measurement mark that may be imaged by a projection system of the lithographic apparatus.
  • the optical measurement mark may impart a radiation beam with a mark which may subsequently be measured after imaging by the projection system in order to derive one or more properties of the lithographic apparatus.
  • the optical alignment system may comprise a sensor apparatus configured to detect the image of the optical measurement mark and thereby determine information such as, for example, a position of the substrate relative to the patterning device and/or one or more optical aberrations associated with the projection system.
  • Embodiments of the invention which are described herein may have use in an EUV lithographic apparatus.
  • an optical measurement mark for an imaging system.
  • the optical measurement mark comprises a reflective region configured to reflect incident radiation for the imaging system to form an image of the optical measurement mark.
  • the optical measurement mark comprises a substructure configured to scatter the incident radiation for increasing an illumination of a numerical aperture of the imaging system.
  • the substructure may comprise a diffusive substructure.
  • Imaging systems suffer from optical aberrations.
  • Optical measurement marks may be used in measurements to determine one or more optical aberrations of imaging systems.
  • the diffusive substructure scatters incident radiation, thereby advantageously suppressing specular reflection and/or a zeroth diffraction order, and instead distributes scattered radiation substantially evenly over the numerical aperture of the imaging system. That is, the diffusive substructure advantageously scatters the incident radiation across substantially the entire angular space accepted by the imaging system.
  • the radiation may be substantially uniformly diffused across substantially the entirety of the numerical aperture of the imaging system after interaction with the diffusive substructure.
  • the diffusive substructure may be implemented directly on the optical measurement mark (e.g., which may form part of a lithographic reticle) without requiring any additional fabrication steps. This advantageously avoids complex fabrication steps such as, for example, forming rough layers using additional materials. This also advantageously reduces the risk of defectivity that may otherwise be caused by the addition of additional layers of material (e.g. on a lithographic reticle).
  • the reflective region may correspond to and/or be referred to as a bright field region of the optical measurement mark.
  • the bright field region of the optical measurement mark may correspond to bright portions of an image of the optical measurement mark that is formed by an imaging system.
  • the optical measurement mark may comprise a plurality of bright field regions.
  • the optical measurement mark may comprise a plurality of reflective regions configured to preferentially reflect a radiation beam.
  • the optical measurement mark may comprise a dark field region.
  • the dark field region of the optical measurement mark may correspond to dark portions of an image of the optical measurement mark that is formed by an imaging system.
  • the optical measurement mark may comprise a plurality of dark field regions.
  • the dark field regions may comprise an absorptive material configured to absorb incident radiation.
  • the optical measurement mark may comprise a plurality of absorptive regions configured to preferentially absorb the radiation beam.
  • the dark field regions may comprise a reflective phase shift material configured to introduce destructive interference to incident radiation.
  • the optical measurement mark may comprise a plurality of reflective phase shift regions configured to introduce destructive interference to the reflected radiation beam.
  • the bright field regions and the dark field regions may be arranged to form a periodic structure, e.g. a line-and- space pattern.
  • the absorptive regions and/or the reflective phase shift regions may correspond to and/or may be referred to as dark field regions.
  • the dark field region may comprise sub-resolution features that are formed in the reflective phase shift material such as, for example, a sub-resolution phase grating.
  • the sub-resolution features may be configured to reduce a transmission of incident radiation (e.g. to 1% or less) such that the dark field region effectively acts as an absorber.
  • the sub-resolution features may have a critical dimension of about 35 nm or more.
  • the sub-resolution features may have a critical dimension of about 39 nm or less.
  • the sub-resolution features may have a pitch of about 80 nm or more.
  • the sub-resolution features may have a pitch of about 130 nm or less.
  • the sub-resolution features may be manufactured or printed using, for example, lithography.
  • a pitch of the sub-resolution features may be selected in at least partial dependence upon, for example, a size of the printable features of a reticle and/or a desired minimum signal strength of an optical measurement such as, for example, an ILIAS or PARIS measurement.
  • references herein to a reflective region being configured to preferentially reflect radiation of a given wavelength should be interpreted to mean that the reflective region is configured such that the reflectivity of the reflective region is higher at the given wavelength than at other wavelengths.
  • the reflective region may additionally reflect radiation having wavelengths other than the given wavelength.
  • a bright field region (or “reflective region”) may, for example, comprise a multilayer structure comprising layers of two or more materials having different refractive indices. Radiation may be reflected from interfaces between different layers. The layers may be arranged to provide a separation between interfaces which causes constructive interference between radiation reflected at different interfaces. The separation between interfaces which causes constructive interference between radiation reflected at different interfaces depends on the wavelength of the radiation.
  • a multilayer reflective region may therefore be configured to preferentially reflect radiation of a given wavelength by providing a separation between layer interfaces which causes constructive interference between radiation of the given wavelength reflected from different interfaces.
  • a bright field material e.g. a reflective layer such as the multilayer structure
  • a dark field material e.g. an absorptive layer or a reflective phase shift layer
  • the dark field material e.g. the absorptive layer or the reflective phase shift layer
  • the radiation beam may comprise extreme-ultraviolet (EUV) radiation.
  • EUV radiation may have a wavelength of about 4 nm or more.
  • the EUV radiation may have a wavelength of about 20 nm or less.
  • the EUV radiation may have a wavelength of about 6.7 nm or 13.5 nm
  • the diffusive substructure may comprise a sub-resolution feature.
  • Sub-resolution features may be considered to be features that are small enough such that they do not result in a significant feature of their own after the lithographic steps of develop and etch, whilst still being large enough to influence imaging of neighboring features.
  • the size and form of sub-resolution features may at least partially depend upon the processes used and may be substrate layer-specific. In general, features that are small enough to not lead to image formation at an image plane of the imaging system (e.g. a substrate W level of a lithographic apparatus) may be considered to be “sub-resolution”.
  • the reflective region may be a pin hole.
  • the reflective region may be a rectangle.
  • the reflective region may be a square.
  • the reflective region may form part of a periodic structure.
  • the periodic structure may be periodic in one dimension.
  • the periodic structure may comprise a line grating.
  • the periodic structure may comprise a line-and-space pattern.
  • the periodic structure may be periodic in two dimensions.
  • the periodic structure may comprise a checkerboard pattern.
  • the periodic structure may comprise a binary Gingham pattern.
  • the reflective region may be configured to reflect extreme ultraviolet radiation.
  • the reflective region may comprise a multilayer configured to introduce constructive interference of reflected EUV radiation.
  • the diffusive substructure may be configured to introduce destructive interference to incident radiation.
  • the diffusive substructure may be a controlled or organized structure. That is, the diffusive substructure may have some degree of order in its arrangement rather than simply being a rough surface having an uncontrolled or disorganized arrangement of peaks and troughs.
  • a pupil plane may be defined by the numerical aperture (NA) of the imaging system (e.g. a projection system of a lithographic apparatus). That is, the pupil plane may be defined by the maximum angular distribution of radiation accepted by the imaging system.
  • the numerical aperture may be represented as a radius of the pupil plane.
  • the pupil plane may be a Fourier transform plane of the plane in which the substrate is disposed (which may be referred to as an image plane) or in which the patterning device is disposed (which may be referred to as an object plane). Therefore, the distribution of electric field strength of the radiation in the pupil plane may be related to a Fourier transform of an object (e.g., the optical measurement mark) disposed in the object plane.
  • the distribution of electric field strength of the radiation in the pupil plane may be given by a convolution of: (a) the distribution of electric field strength of the radiation in an illumination pupil plane (i.e. the angular distribution of radiation that illuminates the object, e.g. the optical measurement mark) and (b) a Fourier transform of the object.
  • the imaging system may be configured to receive EUV radiation, having a wavelength of about 4 nm or more.
  • the imaging system may be configured to receive EUV radiation having a wavelength about 20 nm or less.
  • the imaging system may be configured to receive EUV radiation having a wavelength of about 13.5 nm.
  • the imaging system may have a refractive index of about 0.87 or more.
  • the imaging system may have a refractive index of about 1 or less.
  • the imaging system may have a refractive index of about 0.93 or less.
  • the imaging system may have a numerical aperture of about 0.33 or more.
  • the imaging system may have a numerical aperture of about 0.55 or more.
  • the imaging system may have a numerical aperture in the inclusive range of between about 0.7 and about 0.75.
  • the diffusive substructure may scatter the incident radiation without having to modify a macroscopic geometry of the optical measurement mark relative to known optical measurement marks. That is, for example, the pitch and line width of the periodic structure formed by the bright field (e.g. reflective) regions and the dark field (e.g. absorptive or reflective phase shift) regions of the optical measurement mark may be equivalent to those of a known optical measurement mark.
  • the measurement performance of the optical measurement mark matches the measurement performance of the known optical measurement mark whilst ensuring increased illumination of the numerical aperture of the imaging system (e.g. a projection system of a lithographic apparatus).
  • the diffusive substructure may comprise a reflective phase shift substructure configured to introduce destructive interference to the incident radiation.
  • the reflective phase shift substructure may be configured to modulate a phase of the incident radiation between about zero radians and about 2 it radians.
  • the reflective phase shift structure may be configured to modulate a phase of the incident radiation between about zero radians and about 1.2 it radians.
  • the reflective phase shift structure may be configured to modulate a phase of the incident radiation between about zero radians and less than it radians.
  • the diffusive substructure may comprise a plurality of plateaus and troughs.
  • a height of the plateaus relative to the troughs may be selected to introduce a phase difference between the incident radiation reflected by the plateaus and the incident radiation reflected by the troughs.
  • the height of the plateaus relative to the troughs may correspond to a thickness of a layer from which the plateaus and troughs are formed.
  • the layer may comprise a reflective phase shift material configured to introduce destructive interference to the incident radiation.
  • the layer may comprise a low-n reflective phase shift material configured to introduce destructive interference to incident EUV radiation.
  • the reflective phase shift material may be configured to reflect a portion of the incident radiation whilst transmitting another portion of the incident radiation.
  • the transmitted portion of the radiation may be reflected by the reflective layer (i.e. the multilayer structure) before propagating back through the reflective phase shift material and exiting the optical measurement mark.
  • a portion of the radiation that is transmitted through the plateaus is phase retarded twice by the reflective phase shift material.
  • the plateaus may correspond to portions of the reflective phase shift material.
  • the troughs may correspond to uncovered portions of the multilayer structure.
  • the troughs may correspond to an open space on a dark-field reticle or dark-field mask.
  • a thickness of a layer from which the plateaus and troughs are formed and/or a duty cycle (i.e. a ratio of plateaus area to trough area) may be tuned to suppress a specular component and/or a zeroth diffraction order of the incident radiation.
  • the height of the plateaus relative to the troughs may be configured to introduce a relative phase difference of about it radians to the incident radiation.
  • the height of the plateaus relative to the troughs may be selected to introduce a phase difference of between about zero radians and about 2 it radians.
  • the height of the plateaus relative to the troughs may be selected to introduce a phase difference of between about zero radians and about 1.2 it radians.
  • the height of the plateaus relative to the troughs may be selected in at least partial dependence upon a refractive index of the reflective phase shift material.
  • a relative phase difference of greater than zero radians and less than about 2 it radians (e.g. about 1.2 it radians or less) advantageously introduces destructive interference to the specular component of the incident radiation and has been found to improve scattering of the incident radiation.
  • the height of the plateaus relative to the troughs may be about 40 nm or more.
  • a relative height of about 40 nm or more has been found to be particularly advantageous in introducing destructive interference to the specular component of, and improving scattering of, EUV radiation.
  • the height of the plateaus relative to the troughs may be about 70 nm or less.
  • a relative height of about 70 nm or less has been found to be particularly advantageous in introducing destructive interference to the specular component of, and improving scattering of, EUV radiation.
  • a critical dimension of the diffusive substructure may be about 20 nm or more.
  • a critical dimension of about 20 nm or more has been found to provide improved specular component suppression across a large range of angles of incident light.
  • features having a critical dimension of about 20 nm or more may be manufactured using known EUV mask processes.
  • manufacturing of the diffusive substructure may be performed as part of the same step used to form product features on a lithographic reticle.
  • the critical dimension of the plateaus may be about 20 nm or more.
  • the critical dimension of the troughs may be about 20 nm or more.
  • the critical dimension may be in a direction that is orthogonal to the height of the plateaus relative to the troughs.
  • the critical dimension may correspond to a length or width of the features that form the diffusive substructure, rather than a height of said features.
  • the plurality of plateaus and troughs may be arranged to form a pattern configured to introduce a plurality of different phase differences to the incident radiation.
  • a plurality of different diffusive patterns that provide different desired scattering properties may be used.
  • the pattern may comprise a random phase pattern configured to introduce randomized relative phase differences to the incident radiation.
  • the pattern may comprise a plurality of features.
  • the features may be formed of different shapes such as, for example, squares, circles, triangles, rectangles, etc.
  • the shape of the features may be defined in a plane that is parallel to the plane of the optical measurement mark.
  • the shape of features may correspond to the shape of the troughs. For example, if the troughs are formed of square columns, then the features may be defined as being squares. As another example, if the troughs are formed of circular columns, then the features may be defined as being circles.
  • Randomized relative phase differences have been found to advantageously improve a scattering of the incident radiation across a greater range of angles of incidence.
  • the random phase pattern may comprise a QR code pattern.
  • the QR code pattern may be formed of square features.
  • a feature size of the QR code pattern (e.g. a critical dimension of unit features, e.g. squares or cells, used to form the QR code pattern) may be selected to achieve a desired angular extent of scattering.
  • the random phase pattern may comprise a random features pattern.
  • the features may be formed of different shapes such as, for example, squares, circles, triangles, rectangles, etc.
  • a random features pattern has been found to advantageously provide a more controlled distribution of different feature sizes (e.g. different square sizes) to achieve a desired scattering performance.
  • the random features pattern may comprise differently shaped polyominoes.
  • the polyominoes may have greater than two cells.
  • the polyominoes may comprise a tromino (i.e. three cells), a tetromino (i.e. four cells), a pentomino (i.e. five cells), etc.
  • the features may not coalesce with one another.
  • the features may coalesce with one another in accordance with a distribution function.
  • the distribution function may specify a maximum and minimum size (e.g. critical dimensions) of the structures formed by the coalesced features.
  • the QR code pattern and the polyominoes pattern may be thought of as being constructed using coalesced squares.
  • the random features pattern may comprise features (e.g. squares) of different sizes.
  • the word “size” may correspond to a critical dimension of the features.
  • the critical dimension of a feature may be orthogonal to the height of the plateaus relative to the troughs.
  • the word “size” may correspond to the length/width of a square feature.
  • a distribution of the sizes of the features may be unequal.
  • a distribution of the sizes of the features may be selected.
  • the pattern of features may, for example, comprise features having sizes of about 50 nm and features having sizes of about 100 nm.
  • the 50 nm sized features may account for 30% of the area that is formed of features and the 100 nm sized features may account for 70% of the area that is formed of features.
  • a first feature size of the random feature pattern may be about 20 nm or more.
  • the first feature size of the random feature pattern may be about 60 nm or less.
  • a second feature size of the random feature pattern may be about 90 nm or more.
  • the second feature size of the random feature pattern may be about 120 nm or less.
  • a duty cycle of the random phase pattern may satisfy the following relationship:
  • CD sub is a critical dimension of the diffusive substructure and pitch sub is a pitch of the diffusive substructure.
  • the critical dimension of the diffusive substructure may correspond to the feature size.
  • the critical dimension of the diffusive substructure may correspond to a size of an open space on a dark-field reticle or dark-field mask.
  • the duty cycle may be selected such that about 50% of the diffusive substructure provides a zero radians phase difference to the incident radiation and about 50% of the diffusive substructure provides a 2 it radians phase delay or less to the incident radiation.
  • the duty cycle may be selected such that about 50% of the diffusive substructure provides a zero radians phase difference to the incident radiation and about 50% of the diffusive substructure provides a phase difference of about 1.2 it radians or less to the incident radiation.
  • a duty cycle of the random phase pattern may be about 0.385 or less, or about 0.32 or less, or about 0.28 or less.
  • a selected duty cycle of the random phase pattern may be selected in at least partial dependence upon the reflective phase shift material used to form the diffusive substructure.
  • the diffusive substructure may form part of the bright field (e.g. a reflective material or structure) region of the optical measurement mark.
  • the diffusive substructure may form part of the dark field (e.g. an absorptive material or structure) region of the optical measurement mark.
  • the diffusive substructure may form part of the dark field (e.g. a reflective phase shift material or structure) region of the optical measurement mark.
  • the reflective phase shift region may comprise a reflective phase shift structure configured to introduce destructive interference to the incident radiation.
  • the dark field region may comprise sub-resolution features that are formed in the reflective phase shift material such as, for example, a sub-resolution phase grating.
  • the sub-resolution features may be configured to reduce a transmission of incident radiation (e.g. to 1% or less) such that the dark field region effectively acts as an absorber.
  • the optical measurement mark may comprise a reflective phase shift material configured to introduce destructive interference to incident EUV radiation.
  • the reflective phase shift material may be configured to introduce destructive interference to the specular components and/or zeroth diffraction order of the incident radiation.
  • the reflective phase shift material may have a combination of a low value of the real part of refractive index (e.g. an n value of about 0.93 or less) and a lower value of the imaginary part of refractive index (e.g. absorption coefficient or k value of about 0.025 or less).
  • a low value of the real part of refractive index e.g. an n value of about 0.93 or less
  • a lower value of the imaginary part of refractive index e.g. absorption coefficient or k value of about 0.025 or less.
  • the reflective phase shift material may comprise Ruthenium.
  • the reflective phase shift material may comprise an alloy such as, for example, a Ruthenium-based alloy.
  • the reflective phase shift material may comprise an alloy having a refractive index of about 0.87 or more.
  • the reflective phase shift material may comprise an alloy having a refractive index of about 0.93 or less.
  • the substructure may comprise a diffractive substructure.
  • the reflective region may be one of a plurality of reflective regions that are arranged in a periodic pattern to form a reflective diffraction grating.
  • the diffractive substructure may comprise a segmentation of the periodic pattern configured to broaden a diffraction peak of the reflective diffraction grating.
  • a periodicity of the segmentation of the periodic pattern may be greater than a periodicity of the periodic pattern.
  • the periodicity of the segmentation of the periodic pattern may be about two times greater than the periodicity of the periodic pattern or more.
  • the segmentation of the periodic pattern may be present in a plurality of directions.
  • Two of the plurality of directions may be substantially perpendicular to each other.
  • an optical measurement system comprises the optical measurement mark of the first aspect.
  • the optical measurement system comprises the imaging system that is configured to collect at least some radiation reflected by the reflective region and form the image of the optical measurement mark.
  • the optical measurement system comprises a sensor apparatus configured to detect the image of the optical measurement mark.
  • the optical measurement system may be configured to determine an alignment between the optical measurement mark and the sensor apparatus.
  • the optical measurement system may be configured to determine optical aberrations of the imaging system.
  • the imaging system may comprise reflective optics configured to form the image of the optical measurement mark.
  • the optical measurement system may be suitable for use in a lithographic apparatus. Radiation which is reflected from the optical measurement mark may enter a projection system of a lithographic apparatus. Radiation which is reflected from the optical measurement mark may be projected by a projection system onto a sensor placed at or near to an image plane of the projection system.
  • the sensor apparatus may comprise a periodic structure that is similar to or substantially matches the periodic structure of the optical measurement mark.
  • the sensor apparatus may comprise a photodetector.
  • the photodetector may be configured to detect the radiation after interaction with the periodic structure of the sensor apparatus.
  • the sensor apparatus may comprise a periodic structure that is similar to or substantially matches the periodic structure of the optical measurement mark.
  • the sensor apparatus may comprise a photodetector.
  • the photodetector may be configured to detect the radiation after interaction with the periodic structure of the sensor apparatus.
  • the sensor apparatus may form part of an interferometric sensing system.
  • the optical measurement mark may be configured to act as a reference diffractive structure for the interferometric sensing system.
  • the interferometric sensing system may be configured to detect radiation that has interacted with the substructure.
  • Increasing illumination of the numerical aperture (or “pupil plane”) of the imaging system using the substructure advantageously allows the interferometric sensing system to be used to perform alignment measurements (rather than just optical aberration measurements). This advantageously removes the need for separate alignment systems and processes, thereby reducing alignment measurement time and complexity. When used as part of a lithographic apparatus and method, this advantageously leads to increased availability and throughput of the lithographic apparatus, as well as more accurate measurements leading to an overlay performance gain.
  • the optical measurement system may be configured to use the radiation that has interacted with the substructure to determine an alignment between the optical measurement mark and the sensor apparatus.
  • the optical measurement system may be configured to use the radiation that has interacted with the substructure to determine a portion of a wavefront of the radiation.
  • the optical measurement system may comprise a processor configmed to determine an expected wavefront of the radiation at least partially based on a previously measured wavefront.
  • the processor may be configured to determine an estimated wavefront of the radiation at least partially based on the expected wavefront and the portion of the wavefront.
  • the processor may be configmed to interpolate at least part of the estimated wavefront.
  • the processor may be configmed to extrapolate at least part of the estimated wavefront.
  • the processor may be configured to determine a measmement uncertainty at least partially based on the portion of the wavefront.
  • the processor may be configured to determine a polynomial order of a wavefront fitting process at least partially based on the measmement uncertainty.
  • the processor may be configured to perform the wavefront fitting process to determine a modified expected wavefront that forms at least part of the estimated wavefront.
  • the wavefront fitting process may comprise a Zernike fitting process.
  • a basis function of the fitting process may comprise a Zernike polynomial.
  • Other basis functions and fitting processes may be used.
  • the basis function may comprise a sine function and/or a cosine function.
  • Runge it may not always be desirable to rely upon higher order basis functions to improve an accuracy of the estimated wavefront, and to reduce edge ringing artefacts it may sometimes be desirable to use lower order basis functions.
  • the processor may be configmed to replace at least some of the modified expected wavefront with at least some of the portion of the wavefront to form at least part of the estimated wavefront.
  • the processor may be configured to apply an intensity threshold such that a component of the portion of the wavefront that does not satisfy the intensity threshold is excluded in determining the estimated wavefront.
  • the processor may be configured to apply an image filter to the portion of the wavefront.
  • a lithographic apparatus arranged to project a pattern from a patterning device onto a substrate.
  • the lithographic apparatus comprises a support structure constructed to support the patterning device.
  • the patterning device is capable of imparting incident radiation with a pattern in its cross-section to form a patterned radiation beam.
  • the lithographic apparatus comprises a substrate table constructed to hold the substrate.
  • the lithographic apparatus comprises the optical measurement system of the second aspect.
  • the optical measmement mark may form part of the support structure.
  • the optical measmement mark may form part of the patterning device.
  • the substructure may be formed directly on a material layer that is already present on known reticles and in the same process step as the product features of said reticles. As such, extra process steps are advantageously avoided.
  • the manufacture of the substructure may involve well-established lithographic techniques used in the formation of known reticles, thereby advantageously avoiding the need for additional complex fabrication steps that may risk introducing defects to the reticle. For example, the complex fabrication step of forming a rough layer in or on the substrate (i.e. beneath the multilayer structure) is advantageously avoided.
  • the patterning device may be a low-n attenuated phase shift patterning device.
  • the sensor apparatus may form part of the substrate table.
  • the imaging system may be configured to project the patterned radiation beam onto the substrate.
  • a patterning device for a lithographic apparatus comprising the optical measurement mark of the first aspect.
  • the patterning device may be a low-n attenuated phase shift patterning device.
  • a method of forming an image of optical measurement mark comprises using a reflective region of the optical measurement mark to reflect incident radiation.
  • the method of forming an image of optical measurement mark comprises using a substructure of the optical measurement mark to scatter the incident radiation.
  • the reflective region may correspond to and/or be referred to as a bright field region of the optical measurement mark.
  • the optical measurement mark may comprise one or more dark field regions formed from, for example, an absorptive material or a reflective phase shift material.
  • the dark field region may comprise sub-resolution features that are formed in the reflective phase shift material such as, for example, a sub-resolution phase grating.
  • the sub-resolution features may be configured to reduce a transmission of incident radiation (e.g. to 1% or less) such that the dark field region effectively acts as an absorber.
  • the substructure may comprise a diffusive substructure.
  • the substructure may comprise a diffractive substructure.
  • the reflective region may be one of a plurality of reflective regions that are arranged in a periodic pattern to form a reflective diffraction grating.
  • the diffractive substructure may comprise a segmentation of the periodic pattern configured to broaden a diffraction peak of the reflective diffraction grating.
  • a method of projecting a patterned beam of radiation onto a substrate comprising the method of the fifth aspect.
  • a method of using an interferometric sensing system to determine an alignment between an optical measurement mark and a sensor apparatus that forms part of the interferometric sensing system comprises using the optical measurement mark as a reference diffractive structure for the interferometric sensing system.
  • the method comprises using the interferometric sensing system to detect radiation that has scattered from a substructure of the optical measurement mark.
  • the substructure may comprise a diffusive substructure.
  • the substructure may comprise a diffractive substructure.
  • a method of designing an optical measurement mark comprising a diffusive substructure having a random phase pattern for a lithographic patterning device.
  • the method comprises selecting a mask absorber type of the lithographic patterning device.
  • the method comprises selecting an illumination mode for illuminating the lithographic patterning device.
  • the method comprises using the selected mask absorber type and illumination mode to determine a critical dimension and a duty cycle of the random phase pattern.
  • the method advantageously provides a diffusive substructure having desired scattering characteristics for different illumination modes (e.g. dipole, quadrupole, etc.).
  • the duty cycle and/or critical dimension may be tuned to compensate for changes in a thickness of a material layer from which the diffusive substructure is formed such that the specular component of the incident radiation is suppressed sufficiently.
  • a method of determining an estimated wavefront of radiation comprising detecting the radiation after the radiation has interacted with an optical measurement mark.
  • the method comprises determining a portion of the wavefront of the radiation.
  • the method comprises determining an expected wavefront of the radiation at least partially based on a previously measured wavefront.
  • the method comprises determining the estimated wavefront of the radiation at least partially based on the expected wavefront and the portion of the wavefront.
  • Determining the estimated wavefront may comprise interpolation.
  • Determining the estimated wavefront may comprise extrapolation.
  • the method may comprise determining a measurement uncertainty at least partially based on the portion of the wavefront.
  • the method may comprise determining a polynomial order of a wavefront fitting process at least partially based on the measurement uncertainty.
  • the method may comprise performing the wavefront fitting process to determine a modified expected wavefront that forms at least part of the estimated wavefront.
  • the method may comprise replacing at least some of the modified expected wavefront with at least some of the portion of the wavefront to form at least part of the estimated wavefront.
  • the method may comprise applying an intensity threshold such that a component of the portion of the wavefront that does not satisfy the intensity threshold is excluded in determining the estimated wavefront.
  • the method may comprise applying an image filter to the measured portion of the wavefront.
  • Any of the aspects of the present disclosure may be combined in any way. Any of aspects, and any combination thereof, may further include one or more of the optional features listed above. It will be appreciated that different features of different aspects of the present disclosure may be combined in different ways.
  • FIG. 1 schematically depicts a lithographic system comprising a lithographic apparatus, a radiation source and an optical measurement system comprising an optical measurement mark having a diffusive substructure in accordance with the present disclosure.
  • FIG. 2A schematically depicts a portion of an optical measurement mark comprising a diffusive substructure having a QR code pattern in accordance with the present disclosure.
  • FIG. 2B schematically depicts a cross-sectional view from the side of a portion of the optical measurement mark of Fig. 2A.
  • FIG. 3 schematically depicts a cross-sectional view from the side of a portion of a reflective phase shift region of an optical measurement mark.
  • Fig. 4 schematically depicts a diffusive substructure having a random squares pattern comprising different sized squares in accordance with the present disclosure.
  • Fig. 5 schematically depicts a diffusive substructure having a random squares pattern comprising differently shaped polyominoes in accordance with the present disclosure.
  • - Fig. 6 shows a graph depicting a sheared wavefront of radiation measured across an entire numerical aperture of an imaging system.
  • Figs. 7A-C schematically depict a known optical measurement mark, a diffraction spectrum of radiation after interacting with the known optical measurement mark, and a portion of a wavefront of the radiation after interacting with the known optical measurement mark.
  • FIG. 8A-C schematically depict an optical measurement mark comprising a diffractive substructure in accordance with the present disclosure, a diffraction spectrum of radiation after interacting with the optical measurement mark, and a portion of a wavefront of the radiation after interacting with the optical measurement mark.
  • Figs. 9A-C shows a portion of a wavefront measured using the optical measurement mark of Fig. 7A when illuminated using a dipole illumination mode, a previously measured wavefront, and an estimated wavefront determined in accordance with the present disclosure.
  • FIG. 10 shows a flowchart of a method of determining an estimated wavefront of radiation in accordance with the present disclosure.
  • Fig. 1 shows a lithographic system comprising a radiation source SO and a lithographic apparatus LA.
  • the radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA.
  • the lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS and a substrate table WT configured to support a substrate W.
  • the illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA.
  • the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11.
  • the faceted field mirror device 10 and faceted pupil mirror device 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution.
  • the illumination system IL may include other mirrors or devices in addition to, or instead of, the faceted field mirror device 10 and faceted pupil mirror device 11.
  • the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B’ is generated.
  • the projection system PS is configured to project the patterned EUV radiation beam B’ onto the substrate W.
  • the projection system PS may comprise a plurality of mirrors 13,14 which are configured to project the patterned EUV radiation beam B’ onto the substrate W held by the substrate table WT.
  • the projection system PS may apply a reduction factor to the patterned EUV radiation beam B’, thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied.
  • the projection system PS is illustrated as having only two mirrors 13, 14 in Fig. 1, the projection system PS may include a different number of mirrors (e.g. six or eight mirrors).
  • the substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV radiation beam B’, with a pattern previously formed on the substrate W.
  • a relative vacuum i.e. a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, and/or in the projection system PS.
  • gas e.g. hydrogen
  • the radiation source SO may be a laser produced plasma (LPP) source, a discharge produced plasma (DPP) source, a free electron laser (FEL) or any other radiation source that is capable of generating EUV radiation.
  • LPP laser produced plasma
  • DPP discharge produced plasma
  • FEL free electron laser
  • the lithographic apparatus LA may be used to expose portions of a substrate W in order to form a pattern in the substrate W.
  • one or more properties of the lithographic apparatus LA may be measured. Such properties may be measured on a regular basis, for example before and/or after exposure of each substrate W, or may be measured more infrequently, for example, as part of a calibration process. Examples of properties of the lithographic apparatus LA which may be measured include a relative alignment of components of the lithographic apparatus LA and/or optical aberrations associated with components of the lithographic apparatus (e.g. the projection system PS).
  • measurements may be made in order to determine the relative alignment of the support structure MT for supporting a patterning device MA and the substrate table WT for supporting a substrate W. Determining the relative alignment of the support structure MT and the substrate table WT assists in projecting the patterned radiation beam B’ onto a desired portion of the substrate W. This may be particularly important when projecting patterned radiation B’ onto a substrate W which includes portions which have already been exposed to radiation, so as to improve alignment of the patterned radiation B’ with the previously exposed regions.
  • Measurements such as the alignment measurement described above may be performed by illuminating an optical measurement mark 17 (as schematically shown in Fig. 1) with radiation.
  • An optical measurement mark 17 is a reflective feature which when placed in the field of view of an imaging system (such as the projection system PS of the lithographic apparatus LA of Fig. 1) appears in an image produced by the imaging system.
  • Reflective optical measurement marks described herein are suitable for use as a point of reference and/or for use as a measure of properties of the image formed by the imaging system.
  • radiation reflected from a reflective optical measurement mark 17 may be used to determine an alignment of one or more components of the imaging system and/or optical aberrations associated with one or more components of the imaging system.
  • the optical measurement mark 17 forms part of a patterning device MA.
  • the patterning device MA may be referred to as a reticle.
  • One or more optical measurement marks 17 may be provided on patterning devices MA used to perform lithographic exposures.
  • An optical measurement mark 17 may be positioned outside of a patterned region of the patterning device MA, which is illuminated with radiation during a lithographic exposure.
  • one or more optical measurement marks 17 may additionally or alternatively be provided on the support structure MT.
  • a dedicated piece of hardware often referred to as a fiducial, may be provided on the support structure MT.
  • a fiducial may include one or more optical measurement marks 17.
  • a fiducial is considered to be an example of a patterning device MA.
  • a patterning device MA specifically designed for measuring one or more properties of the lithographic apparatus LA may be placed on the support structure MT in order to perform a measurement process.
  • the patterning device MA may include one or more optical measurement marks 17 for illumination as part of a measurement process.
  • the lithographic apparatus LA is an EUV lithographic apparatus and therefore uses a reflective patterning device MA.
  • the optical measurement mark 17 is thus a reflective marker 17.
  • the configuration of an optical measurement mark 17 may depend on the nature of the measurement which is to be made using the optical measurement mark 17.
  • Known optical measurement marks comprise one or more reflective pin-hole features comprising a reflective region surrounded by an absorbing region, a reflective line feature, an arrangement of a plurality of reflective line features and absorptive features, a reflective square feature, etc.
  • a sensor apparatus 19 (as shown schematically in Fig. 1) is provided to measure radiation which is output from the projection system PS.
  • the sensor apparatus 19 may, for example, be provided on the substrate table WT as shown in Fig 1.
  • the support structure MT may be positioned such that the optical measurement mark 17 on the patterning device MA is illuminated with radiation.
  • the substrate table WT may be positioned such that radiation which is reflected from reflective regions of the optical measurement mark 17 is projected, by the projection system PS, onto the sensor apparatus 19.
  • the sensor apparatus 19 is in communication with a controller CN which may determine one or more properties of the lithographic apparatus LA from the measurements made by the sensor apparatus 19.
  • a plurality of optical measurement marks 17 and/or sensor apparatuses 19 may be provided and properties of the lithographic apparatus LA may be measured at a plurality of different field points (i.e. locations in a field or object plane of the projections system PS).
  • radiation reflected from a reflective region of an optical measurement mark 17 may be used to determine a relative alignment of components of the lithographic apparatus LA.
  • the reflective region may act as an alignment feature, or may form part of a periodic structure which acts as an alignment feature.
  • the position of the alignment feature in the radiation beam B may be measured by a sensor apparatus 19 positioned at a substrate W level (e.g. on the substrate table WT as shown in Fig. 1).
  • the sensor apparatus 19 may be operable to detect the position of an alignment feature in the radiation incident upon it. This may allow the alignment of the substrate table WT relative to the optical measurement mark 17 on the pattering device MA to be determined.
  • the patterning device MA and the substrate table WT may be moved relative to each other so as to form a pattern (using the patterned radiation beam B’ reflected from the patterning device MA) at a desired location on the substrate W.
  • the position of the substrate W on the substrate table WT may be determined using a separate measurement process.
  • the sensor apparatus 19 may form part of an interferometric sensing system that is typically configured to determine one or more optical aberrations associated with the imaging system (e.g. the projection system PS of Fig. 1).
  • the sensor apparatus 19 may be an Integrated Lens Interferometry At Scanner (“ILIAS”) sensor or a Parallel ILIAS sensor (“PARIS”).
  • ILIAS Integrated Lens Interferometry At Scanner
  • PARIS Parallel ILIAS sensor
  • the optical measurement mark 17 of the present disclosure may alternatively or additionally be used to determine an alignment between components of the lithographic apparatus LA due to the scattering effects of a diffusive substructure of the optical measurement mark 17.
  • the illumination system IS, optical measurement mark 17, projection system PS and sensor apparatus 19 of Fig. 1 may together act as an optical measurement system.
  • the optical measurement system may be configured to determine and improve an alignment between the patterning device MA and the substrate W before the dual lithographic exposure is performed.
  • the optical measurement system may be configured to determine and improve an image quality at the substrate W level.
  • FIG. 2 A schematically depicts a view from above a portion of an optical measurement mark 800 comprising a diffusive substructure 880 in accordance with the present disclosure.
  • the optical measurement mark 800 comprises a plurality of bright field (or “reflective”) regions 110 (of which, only a portion of one reflective region 110 is shown in Fig. 2A) and a dark field region 120 that surrounds the bright field regions 110 and forms a periodic structure comprising the bright field 110 and dark field regions 120.
  • the dark field region 120 may comprise an absorptive material configured to absorb incident radiation or a reflective phase shift material configured to introduce destructive interference to incident radiation.
  • a reflective phase shift material configured to introduce destructive interference to incident radiation.
  • the dark field region 120 comprises a reflective phase shift material, and may be referred to as a “reflective phase shift region”.
  • the periodic structure is a line-and-space pattern. Other periodic structures may be formed depending on the shape and arrangement of the reflective regions 110 (e.g. checkerboard patterns, binary Gingham patterns, etc.).
  • the optical measurement mark 800 may comprise hundreds of reflective regions 110.
  • the reflective region 110 is configured to reflect incident radiation for the imaging system to form an image of the optical measurement mark 800.
  • the reflective region 110 may comprise a multilayer (visible in cross-section in Fig. 2B) configured to introduce constructive interference to radiation (e.g.
  • the reflective phase shift region 120 may comprise a reflective phase shift structure configured to introduce destructive interference to incident radiation.
  • the reflective phase shift region 120 may be configured to reflect a portion of the incident radiation whilst transmitting another portion of the incident radiation.
  • the transmitted portion of the radiation may be reflected by the reflective layer 110 underneath the reflective phase shift region 120 (i.e. the multilayer structure) before propagating back through the reflective phase shift region 120 and exiting the optical measurement mark 800.
  • the portion of the radiation that is transmitted through the reflective phase shift region 120 before being reflected by the reflective layer 110 is phase retarded twice by the reflective phase shift material that forms the reflective phase shift region 120.
  • the reflective phase shift region 120 may be of the type used in low-n attenuated phase shift patterning devices.
  • the reflective phase shift region 120 may comprise a stepped arrangement of plateaus and troughs as shown in Fig. 3.
  • the reflective phase shift region 120 may comprise sub-resolution features such as, for example, a subresolution phase grating (not shown).
  • the sub-resolution features may be configured to reduce a transmission of incident radiation (e.g. to 1% or less) such that the reflective phase shift region 120 effectively acts as an absorber.
  • the sub-resolution features may have a critical dimension of about 35 nm or more.
  • the sub-resolution features may have a critical dimension of about 39 nm or less.
  • the sub-resolution features may have a pitch of about 80 nm or more.
  • the sub-resolution features may have a pitch of about 130 nm or less.
  • the optical measurement mark 800 is configured to be used in an optical alignment measurement.
  • An orientation of the reflective regions 110 relative to an orientation of an optical measurement system configured to use the optical measurement mark 800 may determine a direction along which an alignment is determined by the optical alignment measurement.
  • the reflective regions 110 of Figs. 2A and 2B may extend along a Y axis and are configured to determine an alignment along an orthogonal X direction, whereas reflective regions arranged substantially orthogonally to those shown in Figs. 2A and 2B (i.e. along the X axis) may be used to determine an alignment along the orthogonal Y direction.
  • the optical measurement mark 800 is configured for use with EUV radiation.
  • the reflective regions 110 may have a line width 130 of about 200 nm and the periodic structure formed by the reflective and reflective phase shift regions 110, 120 may have a pitch 140 (i.e. periodicity) of about 4 pm.
  • a reflective (or “bright field”) region 110 of an optical measurement mark 800 in accordance with the present disclosure may, for example, comprise a multilayer structure comprising layers of two or more materials having different refractive indices.
  • the multilayer structure may comprise Mo and Si, or Mo and Ruthenium. Radiation may be reflected from interfaces between different layers of the multilayer structure. The layers may be arranged to provide a separation between interfaces which causes constructive interference between radiation reflected at different interfaces of the multilayer structure. The separation between interfaces which causes constructive interference between radiation reflected at different interfaces depends on the wavelength of the radiation.
  • a multilayer reflective region 110 may therefore be configured to preferentially reflect radiation of a given wavelength (e.g.
  • the reflective regions 110 may be disposed on a dark field material such as a reflective phase shift layer 120 or an absorbing layer.
  • the multilayer structure may be disposed on a substrate (which may be referred to in the art as a “blank”) and a dark field material such as a reflective phase shift layer or an absorber layer may then be disposed on the multilayer structure.
  • the reflective phase shift layer 120 or the absorber layer may then be etched to reveal the reflective region 110 (i.e. the multilayer structure visible under the etched portions of the reflective phase shift layer or the absorber layer).
  • the absorber layer may comprise one or more of Tantalum, Ruthenium -based alloys, and other alloys having a refractive index within the inclusive range of about 0.87 and about 0.93.
  • the alloys may, for example, comprise one or more of Mo, Cr, N and/or other elements.
  • the absorber layer may, for example, comprise one or more of Ni, Ta and Co.
  • Fig. 3 schematically depicts a cross-sectional view from the side of a portion of the reflective phase shift region 120 of Figs. 2A and 2B.
  • the reflective phase shift region 120 comprises a reflective phase shift structure configured to introduce destructive interference to incident radiation.
  • the reflective phase shift structure comprises a stepped arrangement of a multilayer reflector 620 that forms a series of plateaus 622 and troughs 624 resembling a square wave pattern. Incident radiation reflects from both the plateaus 625 and the troughs 624.
  • a height 625 of the plateaus 622 relative to the troughs 624 is selected to introduce a phase difference of between about zero radians and about 2 it radians between radiation reflected by the plateaus 622 and radiation reflected by the troughs 624.
  • the height 625 of the plateaus 622 may be selected in accordance with the wavelength of the radiation that is to be reflected by the reflective phase shift region 120. In the case of EUV radiation, the height 625 of the plateaus 622 may be about 40 nm or more. In the case of EUV radiation, the height 625 of the plateaus 622 may be about 70 nm or less.
  • the phase difference introduced to the reflected radiation by the reflective phase shift structure 622, 624 results in destructive interference of the radiation reflected by the reflective phase shift region 120.
  • the radiation reflected by the reflective phase shift region 120 forms “dark” regions of the image of the optical measurement mark 800 formed by the imaging system (e.g. the projection system PS of Fig. 1) compared to the “bright” regions of the image that are formed by the reflective regions 110.
  • the reflective phase shift region 120 may comprise one or more reflective phase shift materials configured to introduce destructive interference to incident EUV radiation.
  • the reflective phase shift region 120 may comprise one or more of Ruthenium, Ru-based alloys and/or other alloys having a refractive index of between about 0.87 and about 0.93.
  • the alloys may, for example, comprise one or more of Mo, Cr, N or other elements.
  • a patterning device MA comprising one or more reflective phase shift regions may be referred to as a low-n (i.e. a low refractive index) attenuated phase shift patterning device.
  • a low refractive index may be considered to be a refractive index within the inclusive range of about 0.87 to about 0.97.
  • the diffusive substructure 880 is configured to scatter incident radiation for increasing an illumination of a numerical aperture of the imaging system (e.g. the projection system PS of Fig. 1). That is, the diffusive substructure 880 increases the number of angles of radiation that enter the imaging system.
  • the diffusive substructure 880 forms part of the reflective region 110.
  • the diffusive substructure 880 may form part of the reflective phase shift region 820.
  • the diffusive substructure 880 may also be applied to an absorptive region.
  • the diffusive substructure 880 may be applied to any type of optical measurement mark, e.g. the bright field regions and/or dark field regions of a binary Gingham pattern mark or a checkerboard pattern mark.
  • Fig. 2B schematically depicts a cross-sectional view from the side of a portion 890 of the optical measurement mark 800 of Fig. 2A.
  • the optical measurement mark 800 comprises a multilayer reflective region 110.
  • a cap layer 830 is formed on the reflective region 110.
  • the cap layer 830 may be configured to act as a passivation layer and protect the reflective region 110 from an external environment.
  • the cap layer 830 may, for example, comprise Ruthenium.
  • the cap layer 830 may, for example, have a thickness within the inclusive range of about 1 nm to about 5 nm, e.g. about 3 nm.
  • the diffusive substructure 880 may be formed on the cap layer 830.
  • the diffusive substructure 880 may be formed of an EUV phase shift material, e.g. Ruthenium, Ru-based alloys, other alloys having a refractive index within the inclusive range of about 0.87 to about 0.93.
  • the alloys may, for example, comprise materials such as Mo, Or and N.
  • the EUV phase shift material 840 may be formed (e.g. deposited) on top of the cap layer 830.
  • the diffusive substructure 880 comprises a reflective phase shift substructure configured to scatter incident radiation and introduce destructive interference to a specular component (or a zeroth diffraction order) of incident radiation.
  • the reflective phase shift structure comprises a stepped arrangement of the EUV reflective phase shift material 840 that forms a plurality of plateaus 822 and troughs 824 resembling a square wave pattern having a pseudo-random duty cycle. Incident radiation reflects from both the plateaus 822 and the troughs 824.
  • a height 825 of the plateaus 822 relative to the troughs 824 is selected to introduce a relative phase difference of between about zero radians and about 2 it radians between radiation reflected by the plateaus 822 and radiation reflected by the troughs 824.
  • the plateaus 822 and troughs 824 may form a binary phase shift structure, with reflection from either a plateau 822 or a trough 824 giving either a “0” phase shift or a “1” phase shift.
  • the plateaus 822 and troughs 824 may be configured to introduce a distribution of different phase shifts across a desired range, e.g. from about 0.7 it radians to about 1.5 it radians or about 1.27t radians.
  • a height difference between the plateaus 822 and the troughs 824 may vary across the diffusive substructure 880.
  • heights of the plateaus 822 relative to the troughs 824 may follow a distribution, a root mean square of which may correspond to a phase shift of it radians.
  • the reflective phase shift material 840 that forms the plateaus of the diffusive substructure 880 may be configured to introduce destructive interference to the specular components and/or zeroth diffraction order of the incident radiation.
  • the plateaus 822 may be configured to reflect a portion of the incident radiation whilst transmitting another portion of the incident radiation.
  • the transmitted portion of the radiation may be reflected by the reflective layer 110 underneath the reflective phase shift region 120 (i.e. the multilayer structure) before propagating back through the plateaus 822 and exiting the optical measurement mark 800.
  • the portion of the radiation that is transmitted through the plateaus 822 before being reflected by the reflective layer 110 is phase retarded twice by the reflective phase shift material that forms the plateaus 822.
  • suitable reflective phase shift materials 840 may have a combination of a low value of the real part of refractive index (e.g. n of about 0.93 or less) and a lower value of the imaginary part of refractive index (e.g. an absorption coefficient or k value of about 0.025 or less).
  • the reflective phase shift material 840 may comprise Ruthenium.
  • the reflective phase shift material 840 may comprise a Ruthenium-based alloy.
  • the reflective phase shift material 840 may comprise an alloy having a refractive index of about 0.87 or more.
  • the reflective phase shift material 840 may comprise an alloy having a refractive index of about 0.93 or less.
  • the pseudo-random duty cycle of the diffusive substructure 880 also contributes to the introduction of a distribution of phase shifts to incident radiation.
  • the duty cycle of the diffusive substructure 880 may correspond to the ratio of the total portion of the diffusive substructure 880 that is formed by plateaus to the total portion of the diffusive substructure 880 that is formed by troughs.
  • the duty cycle may be selected such that about 50% of the diffusive substructure 880 provides a zero radians phase difference to the incident radiation and about 50% of the diffusive substructure 880 provides a it radians phase delay to the incident radiation.
  • a duty cycle of the diffusive substructure 880 may be about 0.385 or less.
  • a duty cycle of the diffusive substructure 880 may be about 0.32 or less.
  • a duty cycle of the diffusive substructure 880 may be about 0.28 or less.
  • the selected phase shifts may at least partially depend upon a material of the optical measurement mark 800 and/or a numerical aperture of the imaging system, etc.
  • the height 825 of the plateaus 822 may be selected in accordance with the wavelength of the radiation that is to be reflected by the diffusive substructure 880.
  • the height 825 of the plateaus 822 may be about 40 nm or more.
  • the height 825 of the plateaus 822 may be about 70 nm or less.
  • a critical dimension of the diffusive substructure 880 may be about 20 nm or more.
  • the critical dimension of the plateaus may be about 20 nm or more, and/or the critical dimension of the troughs may be about 20 nm or more.
  • the critical dimension may be in a direction that is orthogonal to the height 825 of the plateaus 822 relative to the troughs 824.
  • the critical dimension may correspond to a length or width of the features (e.g. squares corresponding to columns formed in the reflective phase shift material 120) that form the diffusive substructure 880, rather than a height 825 of said features.
  • the phase differences (or phase shifts) introduced to the reflected radiation by the reflective phase shift structure 822, 824 results in destructive interference of the specular component of radiation reflected by the diffusive substructure 880.
  • the diffusive substructure 880 may be configured to introduce a plurality of different phase shifts to incident radiation.
  • the plurality of plateaus 822 and troughs 824 may be arranged to form a pattern (e.g. a pixel map) configured to introduce a plurality of different phase differences to the incident radiation.
  • the diffusive substructure 880 may comprise a random phase pattern. That is, different areas of the diffusive substructure 880 may impart reflected radiation with different phase shifts (or phase delays) so as to introduce destructive interference to a specular component of radiation reflected by the diffusive substructure 880.
  • the random phase pattern is a QR code pattern.
  • the QR code pattern comprises a plurality of squares (as viewed face on). As can be seen in Fig. 2B, the squares are actually columns formed in the EUV phase shift material 840. Shapes other than squares may be used such as, for example, circles, triangles, rectangles, etc. The squares are sized and arranged so as to introduce different, pseudo-randomly distributed phase delays to radiation reflected by the diffusive substructure 880. The squares may, for example, have lengths of about 30 nm or more. The squares may, for example, have lengths of about 200 nm or less. As can be seen from Fig. 2A, some of the squares coalesce with one another (i.e. join) to form different, larger shapes.
  • phase-tuning effect of the height 825 of the columns, and the pseudo-random structure of a pixel map formed by the random phase pattern leads in the long range to destructive interference of the combined radiation reflected by the diffusive substructure 880, and thus acts as an optical diffuser at the EUV wavelength.
  • Fig. 4 schematically depicts an alternative diffusive substructure 980 comprising a random features pattern in accordance with the present disclosure.
  • the features are squares.
  • Other shapes may be used.
  • the random squares pattern comprises a distribution of squares of different sizes 991, 992. Only two different squares sizes are shown in the example of Fig. 4 but a greater number of sizes may be used. For example, lengths of the squares may vary within the inclusive range of about 30 nm to about 200 nm.
  • a first square size of the random square pattern may be between about 20 nm and about 60 nm, and a second square size of the random square pattern may be between about 90 nm and about 120 nm.
  • Each square 991, 992 may correspond to a column formed in an EUV phase shift material 840 in a similar manner to Fig. 2B.
  • the squares of the random squares pattern of Fig. 4 do not coalesce with one another.
  • a distribution of the sizes of the squares may be unequal. In the example of Fig. 4, there are a greater number of smaller squares 992 than there are larger squares 991.
  • the pattern of squares 992, 991 may, for example, comprise squares having sizes of about 50 nm 992 and squares having sizes of about 200 nm 991.
  • the 50 nm sized squares 992 may account for 30% of the area of the diffusive substructure 980 that is formed of squares and the 100 nm sized squares 991 may account for 70% of the area of the diffusive substructure 980 that is formed of squares. It will be appreciated that other unequal distributions are possible. To provide improved scattering properties for a given combination of illumination mode and reticle, an unequal distribution of the sizes of the squares may be selected.
  • Fig. 5 schematically depicts another alternative diffusive substructure 300 in which the random features pattern comprises differently shaped polyominoes in accordance with the present disclosure.
  • the features are squares.
  • a polyomino may be understood as being a shape formed by joining one or more equally sized squares at the edges of the squares. Each square may correspond to a column formed in an EUV phase shift material 840 in a similar manner to Fig. 2B.
  • the diffusive substructure 300 comprises monominoes 310 (i.e. a single square), dominoes 320 (i.e. two squares), trominoes 330 (i.e.
  • the diffusive substructure 300 may comprise higher order polyominoes. As can be seen from Fig. 5, polyominoes of the same order having three or more squares may have different shapes. The distribution of differently shaped polyominoes contributes to the pseudo-random structure of the pixel map formed by the random phase pattern and leads to destructive interference of the combined radiation reflected by the diffusive substructure 300, and thus acts as an optical diffuser at the EUV wavelength.
  • the sensor apparatus 19 may form part of an interferometric sensing system that is typically configured to determine one or more optical aberrations associated with the imaging system (e.g. the projection system PS of Fig. 1).
  • the relative phase of the projection system PS in its pupil plane may be determined by projecting radiation, for example from a point-like source in an object plane of the projection system PS (i.e. the plane of the mask MA), through the projection system PS and using a shearing interferometer (of which the sensor apparatus 19 may form part) to measure a wavefront (i.e.
  • Shearing interferometers are common path interferometers and therefore, advantageously, no secondary reference beam is required to measure the wavefront.
  • the shearing interferometer may comprise a diffraction grating (which may form part of the optical measurement mark 17), for example a two dimensional grid, in an image plane of the projection system (i.e. the substrate table WT) and a detector (which may form part of sensor apparatus 19) arranged to detect an interference pattern in a plane that is conjugate to a pupil plane of the projection system PS.
  • the interference pattern is related to the derivative of the phase of the radiation with respect to a coordinate in the pupil plane in the shearing direction.
  • the detector 19 may comprise an array of sensing elements such as, for example, charge coupled devices (CCDs).
  • CCDs charge coupled devices
  • the diffraction grating 17 is sequentially scanned in two perpendicular directions, which may coincide with axes of a co-ordinate system of the projection system PS (e.g. x and y) or may be at an angle such as, for example, 45 degrees to these axes. Scanning may be performed over an integer number of grating periods, for example one grating period. The scanning averages out phase variations in one direction, allowing phase variations in the other direction to be reconstructed. This allows the wavefront to be determined as a function of both directions.
  • the projection system PS of a state of the art lithographic apparatus LA may not produce visible fringes and therefore the accuracy of the determination of the wavefront can be enhanced using phase stepping techniques such as, for example, moving the diffraction grating 17.
  • Stepping may be performed in the plane of the diffraction grating 17 and in a direction perpendicular to the scanning direction of the measurement.
  • the stepping range may be one grating period, and at least three (uniformly distributed) phase steps may be used.
  • three scanning measurements may be performed in the y-direction, each scanning measurement being performed for a different position in the x-direction. This stepping of the diffraction grating effectively transforms phase variations into intensity variations, allowing phase information to be determined.
  • the optical measurement mark 17 may be configured to act as a reference diffractive structure for the interferometric sensing system.
  • the interferometric sensing system may be configured to detect radiation that has interacted with the diffusive substructure of the optical measurement mark 17.
  • Increasing illumination of the numerical aperture or pupil plane of the imaging system (e.g. the projection system PS) using the diffusive substructure advantageously allows the interferometric sensing system to be used to perform alignment measurements (rather than just optical aberration measurements). That is, the optical measurement system (e.g.
  • the interferometric sensing system comprising optical measurement mark 17 and sensor apparatus 19
  • the interferometric sensing system may be configured to use the radiation that has interacted with the diffusive substructure of the optical measurement mark 17 to determine an alignment between the optical measurement mark 17 and the sensor apparatus 19, and thus an alignment between the mask MA and the substrate W.
  • This advantageously removes the need for separate alignment systems and processes, thereby reducing alignment measurement time and complexity.
  • this advantageously leads to increased availability and throughput of the lithographic apparatus LA, as well as more accurate measurements leading to an overlay performance gain.
  • a method of using an interferometric sensing system to determine an alignment between the optical measurement mark 17 and the sensor apparatus 19 that forms part of the interferometric sensing system may comprise using the optical measurement mark 17 as a reference diffractive structure for the interferometric sensing system.
  • the interferometric sensing system may be used to detect radiation that has scattered from a diffusive substructure of the optical measurement mark 17.
  • the optical measurement mark 17 forms part of the patterning device MA
  • the sensor apparatus 19 forms part of the substrate table WT
  • the imaging system i.e. the projection system PS
  • the optical measurement mark 17 may alternatively form part of the support structure MT.
  • the patterning device MA may be a low-n attenuated phase shift patterning device. That is, the patterning device MA may comprise reflective phase shift regions comprising a stepped arrangement of plateaus and troughs (e.g. as shown in Fig. 3).
  • the patterning device MA may comprise reflective phase shift materials such as Ruthenium, Ru-based alloys, other alloys having a refractive index of between about 0.87 and about 0.93.
  • the alloys may, for example, comprise one or more of Mo, Cr, N or other elements.
  • a low refractive index may be considered to be a refractive index within the inclusive range of about 0.87 to about 0.97.
  • a method of designing an optical measurement mark 17 comprising a diffusive substructure having a random phase pattern for a lithographic patterning device MA may comprise selecting a mask absorber type of the lithographic patterning device MA. This may involve selecting the materials (e.g. materials having acceptable values of refractive index n and/or absorption coefficient k) and/or layer thicknesses and/or materials stacks of the lithographic patterning device MA.
  • the method may comprise selecting an illumination mode for illuminating the lithographic patterning device MA.
  • the illumination mode may be a monopole illumination mode, a dipole illumination mode, a quadrupole illumination mode, etc.
  • the illumination mode may be selected to provide an improved imaging quality (e.g.
  • the method may comprise using the selected mask absorber type and illumination mode to determine a critical dimension and a duty cycle of the random phase pattern of the diffusive substructure.
  • Software such as, for example, an optical simulation program may be used to vary the duty cycle and critical dimension of the diffusive substructure in an iterative process to improve suppression of a specular component or the zeroth diffraction order of incident EUV radiation and scatters the EUV radiation substantially evenly across the numerical aperture of a given imaging system (e.g. the projection system PS of Fig. 1), thereby acting as an effective diffuser.
  • the numerical aperture of the imaging system may be known, and the different angles of incidence associated with different illumination modes may be known.
  • Different critical dimensions and/or duty cycles of the diffusive substructure e.g. the different sizes, shapes and distribution of plateaus and troughs shown in Figs. 2, 4 and 5
  • Optimization of the duty cycle and/or the critical dimension of the diffusive substructure may involve minimizing an optimization metric in an iterative process.
  • Forming an optical measurement mark 17 in accordance with the present disclosure may comprise coating a substrate (which may be referred to in the art as a “blank”) with layers of a multilayer structure comprising one or more materials to form a reflective layer for reflecting EUV radiation.
  • a substrate which may be referred to in the art as a “blank”
  • layers of a multilayer structure comprising one or more materials to form a reflective layer for reflecting EUV radiation.
  • one or more materials configured to absorb EUV radiation or introduce destructive interference through the introduction of phase differences to reflected EUV radiation may be deposited on the reflective layer to form a “dark” region in a desired pattern.
  • the reflective regions of the periodic structure and of the substructure effectively correspond to the reflective regions of the reflective layer that are not covered by the absorptive or EUV phase shift materials.
  • Formation of the diffusive substructure may comprise deposition of EUV phase shift materials (e.g. Ruthenium) at controlled thicknesses to provide a desired phase shift effect.
  • a method of forming an image of an optical measurement mark in accordance with the present disclosure may comprise using a reflective region of the optical measurement mark to reflect incident radiation.
  • the reflective regions 110 of the optical measurement mark 800 of Figs. 2A and 2B may be illuminated with EUV radiation and reflect said EUV radiation.
  • the method may comprise using the diffusive substructure of the optical measurement mark to scatter the incident radiation.
  • the diffusive substructures 880, 980 300 of Figs. 2, 4 or 5 may use diffusion as previously described to scatter the incident radiation.
  • a method of projecting a patterned beam B’ of radiation onto a substrate W in accordance with the present disclosure may comprise the method of forming an image of an optical measurement mark described above. For example, with reference to Fig.
  • the optical measurement mark 17 may take the form of the optical measurement mark 800 of Figs. 2A and 2B, or comprise the diffusive substructures 980, 300 of Figs. 4 or 5.
  • the illumination system IL may illuminate the optical measurement mark 17 with EUV radiation in a given illumination mode (e.g. a dipole illumination mode).
  • the projection system PS may form an image of the optical measurement mark 17 and the sensor apparatus 19 may detect the image of the optical measurement mark 17 and thereby determine an alignment between the patterning device MA and the substrate W and/or an optical aberration of the lithographic apparatus LA (e.g. of the projection system PS).
  • the determined alignment information and/or optical aberration information may be used to at least partially control a lithographic exposure performed by the lithographic apparatus LA.
  • the substructure may comprise one or more alternative or additional types of substructure.
  • the substructure may comprise a diffractive substructure.
  • the diffractive substructure may be configured to scatter the incident radiation for increasing an illumination of a numerical aperture of the imaging system.
  • Fig. 6 shows a graph depicting a sheared wavefront of radiation measured across an entire numerical aperture of the imaging system.
  • the numerical aperture of the imaging system is represented as a circular pupil plane.
  • the axes of the graph indicate x and y position coordinates within the pupil plane (i.e. o x and o y ) with -1 and 1 representing extremities of angles of incidence accepted by the numerical aperture of the imaging system.
  • Fig. 6 represents variations in phase of the sheared wavefront across the pupil plane in nm. As can be seen, the phase of the sheared wavefront varies at different positions across the pupil plane due to the effect of optical aberrations present within the imaging system.
  • the sheared wavefront of Fig. 6 may be measured using an interferometric sensing system such as, for example, an Integrated Lens Interferometry At Scanner (“ILIAS”) sensor or a Parallel ILIAS sensor (“PARIS”).
  • Fig. 6 shows multiple diffracted copies of the wavefront of radiation (i.e. a sheared wavefront as measured by an interferometric sensing system), rather than the wavefront itself. Such a measurement may be performed at the beginning of a lithographic process (i.e.
  • the wavefront of Fig. 6 may be measurable using the previously described diffusive substructure to illuminate the numerical aperture of the imaging system.
  • Fig. 7A schematically depicts a known optical measurement mark 700.
  • the optical measurement mark 700 comprises a plurality of reflective regions 710 that are arranged in a periodic pattern to form a reflective diffraction grating.
  • the reflective regions 710 are rectangles and the periodic pattern is a line grating. Spaces between the reflective regions 710 may be occupied by an absorptive material.
  • interactions with the optical measurement mark 700 of Fig. 7A may be referred to as specular or non- diffusive. Incident radiation undergoes diffraction after interacting with the optical measurement mark 700.
  • FIG. 7B shows a diffraction spectrum of radiation after interacting with the optical measurement mark 700 of Fig. 7A.
  • the diffraction spectrum of Fig. 7B is shown across a magnified portion of the numerical aperture, with o x and o y ranging between -0.5 and 0.5 (rather than the full numerical aperture of -1 and 1).
  • the optical measurement mark 700 of Fig. 7A results in a small number of well-defined diffraction peaks along a single direction (i.e. the o x axis).
  • Fig. 7C shows a portion of a wavefront of the radiation after interacting with the optical measurement mark 700 of Fig. 7A.
  • an interferometric sensing system such as, for example, an Integrated Lens Interferometry At Scanner (“ILIAS”) sensor or a Parallel ILIAS sensor (“PARIS”) and may be referred to as a sheared wavefront.
  • ILIAS Integrated Lens Interferometry At Scanner
  • PARIS Parallel ILIAS sensor
  • a relatively small or limited illumination mode such as, for example, a dipole illumination mode
  • only a small number of well-defined diffraction peaks at specific angles along a single axis i.e. the o x axis
  • only a relatively small number of detection pixels are illuminated, meaning that a relatively small portion of the wavefront can be measured.
  • Fig. 8A schematically depicts an optical measurement mark comprising a diffractive substructure in accordance with the present disclosure.
  • the optical measurement mark 500 of Fig. 8A comprises a plurality of reflective regions 510 that are arranged in a periodic pattern to form a reflective diffraction grating.
  • the reflective regions 510 may comprise the diffusive substructure.
  • the diffusive substructure may comprise a reflective phase shift substructure configured to introduce destructive interference to the incident radiation.
  • the reflective phase shift substructure may comprise a plurality of plateaus and troughs, and a height of the plateaus relative to the troughs may be selected to introduce a phase difference between the incident radiation reflected by the plateaus and the incident radiation reflected by the troughs
  • the reflective regions 510 are rectangles and the periodic pattern is a line grating. Spaces between the reflective regions 510 may be occupied by an absorptive material or a reflective phase shift region comprising a reflective phase shift structure configured to introduce destructive interference to the incident radiation.
  • the optical measurement mark 500 of Fig. 8A comprises a diffractive substructure 520, 530. Compared to radiation interacting with the diffusive substructure, interactions with the diffractive substructure 520, 530 may be referred to as diffractive.
  • the magnified portion of the optical measurement mark 500 to the right of Fig. 8A shows the diffractive substructure 520, 530 in more detail.
  • the diffractive substructure 520, 530 comprises a segmentation of the periodic pattern configured to broaden a diffraction peak of the reflective diffraction grating.
  • the segmentation may be formed using lithography to print the segmented version of the periodic pattern.
  • the segmentation of the periodic pattern is present in two directions. If segmentation was introduced along a single direction only, then the resulting diffraction peaks may blur in a single direction only.
  • the diffractive substructure of Fig. 8A may be understood as a segmentation of the periodic pattern of Fig. 7A along the x axis and the y axis (i.e. along perpendicular directions). Along the x axis, every other two reflective regions have been removed relative to the periodic pattern of Fig. 7A. Along the y axis, each reflective region has been segmented into a plurality of smaller reflective regions 510.
  • a periodicity of the segmentation of the periodic pattern may be greater than a periodicity of the periodic pattern.
  • a periodicity of the periodic pattern may be about 255 nm.
  • a first periodicity 520 of the diffractive substructure along the y axis is about two times greater than the periodicity 720 of the periodic pattern that forms the reflective diffraction grating of Fig. 7A (an equivalent distance being shown in the magnified portion of Fig. 8A for ease of viewing).
  • a second periodicity 530 of the diffractive substructure along the x axis is about four times greater than the periodicity 720 of the periodic pattern that forms the reflective diffraction grating of Fig. 7A.
  • the reflective regions 510 may be curved, e.g. sinusoidally shaped segments. It will be appreciated that segmentation may be present in directions other than the x and/or y axis. For example, segmentation may be introduced along a direction that is about 45° relative to the x and y axes. Other angles may be used. It will be appreciated that different relationships between the periodicity 720 of the periodic pattern that forms the reflective diffraction grating and the pitch 520, 530 of any segmentation of the periodic pattern may be used. By adjusting the pitch of the segmentation, the amount of blurring introduced to the diffraction peaks may be modified.
  • the diffractive substructure 520, 530 may be combined with the diffusive substructure (e.g. the diffusive substructure 880 of Fig. 2A) to further increase illumination of the numerical aperture of the imaging system.
  • the diffractive substructure 520, 530 may be used in isolation.
  • FIG. 8B shows a diffraction spectrum of radiation after interacting with the optical measurement mark 500 of Fig. 8A.
  • the diffraction spectrum of Fig. 8B is shown across a magnified portion of the numerical aperture, with o x and o y ranging between -0.5 and 0.5 (rather than the full numerical aperture of -1 and 1).
  • the optical measurement mark 500 of Fig. 8A results in a reduction in coherence and a greater number of blurred diffraction peaks along multiple directions (i.e.
  • the diffractive substructure 520, 530 introduces further diffraction which introduces additional diffraction peaks and effectively broadens the diffraction peaks of the reflective diffraction grating.
  • the diffractive substructure 520, 530 is configured to scatter the incident radiation and thereby increase an illumination of the numerical aperture of the imaging system. By increasing the illumination of the numerical aperture of the imaging system, a greater amount of phase information is available, and a greater portion of the wavefront may be determined.
  • Fig. 8C shows a portion of a wavefront of the radiation after interacting with the optical measurement mark 500 of Fig. 8A.
  • the wavefront of Fig. 8C may be measured using an interferometric sensing system such as, for example, an Integrated Lens Interferometry At Scanner (“ILIAS”) sensor or a Parallel ILIAS sensor (“PARIS”) and may be referred to as a sheared wavefront.
  • ILIAS Integrated Lens Interferometry At Scanner
  • PARIS Parallel ILIAS sensor
  • the optical measurement mark 500 of Fig. 8A increases illumination of the numerical aperture of the imaging system and therefore allows a greater portion of the wavefront to be measured.
  • the wavefronts of Fig. 7C and Fig. 8C may be understood as a convolution of the illumination mode used by the imaging system and the diffraction spectrum resulting from the optical measurement mark 700, 500.
  • a “starry sky” type illumination mode is used in which only small, specific parts of the numerical aperture of the imaging system are illuminated.
  • the scattering effects of the diffractive substructure 520, 530 increase illumination of the numerical aperture, thereby providing a greater amount of phase information and allowing a greater portion of the wavefront of the radiation to be measured.
  • any optical measurement mark comprising a diffractive substructure such as the optical measurement mark 500 of Fig. 8A, may take the place of the optical measurement mark 17 of the lithographic apparatus LA of Fig. 1.
  • the sensor apparatus 19 may form part of an interferometric sensing system that is typically configured to determine one or more optical aberrations associated with the imaging system (e.g. the projection system PS of Fig. 1).
  • the relative phase of the projection system PS in its pupil plane may be determined by projecting radiation, for example from a point-like source in an object plane of the projection system PS (i.e.
  • an optical measurement system may be configured to use the radiation that has interacted with the diffractive substructure 520, 530 to determine a portion of a wavefront of the radiation (e.g. the portions shown in Fig. 8C).
  • the optical measurement mark 500 may be configured to act as a reference diffractive structure for the interferometric sensing system.
  • the interferometric sensing system may be configured to detect radiation that has interacted with the diffractive substructure 520, 530 of the optical measurement mark 500.
  • Increasing illumination of the numerical aperture or pupil plane of the imaging system (e.g. the projection system PS) using the diffractive substructure advantageously allows the interferometric sensing system to be used to perform alignment measurements (rather than just optical aberration measurements). That is, the optical measurement system (e.g.
  • the interferometric sensing system comprising optical measurement mark 500 and sensor apparatus 19
  • the interferometric sensing system may be configured to use the radiation that has interacted with the diffractive substructure 520, 530 of the optical measurement mark 500 to determine an alignment between the optical measurement mark 500 and the sensor apparatus 19, and thus an alignment between the mask MA and the substrate W.
  • This advantageously removes the need for separate alignment systems and processes, thereby reducing alignment measurement time and complexity.
  • this advantageously leads to increased availability and throughput of the lithographic apparatus LA, as well as more accurate measurements leading to an overlay performance gain.
  • a method of using an interferometric sensing system to determine an alignment between the optical measurement mark 500 and the sensor apparatus 19 that forms part of the interferometric sensing system may comprise using the optical measurement mark 500 as a reference diffractive structure for the interferometric sensing system.
  • the interferometric sensing system may be used to detect radiation that has scattered from a diffractive substructure 520, 530 of the optical measurement mark 500.
  • the diffusive substructure and/or the diffractive substructure may be used to scatter incident radiation and thereby increase illumination of the numerical aperture or pupil plane as much as possible regardless of the illumination mode used.
  • very limited illumination mode such as, for example, a dipole illumination mode, may result in large portions of the numerical aperture remaining dark in spite of the scattering effects of the diffusive and/or diffractive substructures.
  • Such a method may be advantageously applied to well-illuminated numerical apertures (e.g. starry sky illumination modes that have also been scattered by the diffusive and/or diffractive substructures) to fdl in any missing phase information.
  • numerical apertures e.g. starry sky illumination modes that have also been scattered by the diffusive and/or diffractive substructures
  • Such a method may be very advantageous in estimating significant portions of the wavefront that are missing from measurements.
  • the illumination system IS, optical measurement mark 500, projection system PS and sensor apparatus 19 of Fig. 1 may together act as an optical measurement system.
  • the optical measurement system may comprise a processor (which may, for example, form part of controller CN), configured to determine an expected wavefront of the radiation at least partially based on a previously measured wavefront.
  • Fig. 9A shows a portion of a wavefront measured using the optical measurement mark 700 of Fig. 7A when illuminated using a dipole illumination mode.
  • the measured portion of the wavefront of Fig. 9 A may be understood as illumination pixels convoluted with a diffraction grating of the interferometric system used to measure the wavefront (e.g. PARIS). It will appreciated that any optical measurement mark (i.e.
  • the processor may be configured to use a previously measured wavefront to estimate the portion of the wavefront that is missing from Fig. 9A.
  • Fig. 9B shows a previously measured wavefront.
  • the previously measured wavefront may, for example, be measured by the sensor apparatus 19 (e.g. an Integrated Lens Interferometry At Scanner (“ILIAS”) sensor or a Parallel ILIAS sensor (“PARIS”)) at the start of a substrate lot as part of a preexposure procedure to determine optical aberrations of the lithographic apparatus LA.
  • the wavefront may be measured using a fiducial including one or more optical measurement marks as previously described.
  • the previously measured wavefront may be determined across substantially an entirety of the numerical aperture of the imaging system such as, for example, the wavefront shown in Fig. 6.
  • the optical aberrations may change over time due to, for example, thermal effects, and so it may be desirable to periodically measure the optical aberrations within or between substrate lots.
  • the previously measured wavefront may be updated using predictions from other measurement apparatus and methods such as, for example reticle and/or mirror heating thermal effect predictions.
  • such methods may be limited in their accuracy, and it may be desirable to directly measure at least a portion of the wavefront rather than use indirect predictions.
  • Using separate wavefront measurement apparatus and/or processes may take additional time and thereby reduce a throughput of the lithographic process.
  • the processor may be configured to determine an estimated wavefront of the radiation at least partially based on the expected wavefront and the measured portion of the wavefront. That is, the processor may act to fill in the missing portions of the wavefront missing from a measurement, such as that shown in Fig. 9A, and thereby produce an estimated wavefront.
  • Fig. 9C shows an estimated wavefront determined in accordance with the present disclosure.
  • the estimated wavefront is determined at least partially based on the measured portion of the wavefront of Fig. 9A and the expected wavefront of Fig. 9B.
  • the processor may be configured to use an algorithm to combine the new measured wavefront sampling at a limited number and distribution of pupil plane pixels with the expected wavefront. By doing so, the results of a single wavefront measurement may be used to align the reticle and the substrate and also estimate the wavefront of the radiation, thereby saving time and increasing throughput of the lithographic apparatus LA.
  • the processor may be configured to determine a measurement uncertainty 540 at least partially based on the measured portion of the wavefront.
  • the measurement uncertainty 540 may correspond to a size of a largest circle that can be formed between measured pixels of the wavefront.
  • the processor may be configured to determine a diameter 550 of the largest circle 540 that can fit between pixels of the measured portion of the wavefront.
  • Other shapes and/or distances and/or measures of size e.g. area, number of pixels, etc.
  • the missing pupil plane pixels may be estimated using a process comprising interpolation (e.g. by performing a wavefront fitting process to the expected wavefront and/or the measured portion of the wavefront) and/or extrapolation (e.g. by comparison with the expected wavefront and or the measured portion of the wavefront).
  • the processor may be configured to determine a polynomial order of a wavefront fitting process at least partially based on the measurement uncertainty 540.
  • the processor may be configured to apply a Nyquist criterion based on the measurement uncertainty 540 to select a suitable polynomial order of the wavefront fitting process.
  • the wavefront fitting process may comprise a Zemike fitting process.
  • the Nyquist criterion may apply to the diameter 550 of the largest circle 540 that can fit between pixels of the measured portion of the wavefront. For example, if the diameter 550 of the circle 540 is greater than about half the numerical aperture (i.e. > lo) then a zeroth order polynomial fit (e.g.
  • a Zernike piston offset fit may be used.
  • a first order polynomial or linear fit e.g. a Zemike tilt in x and/or y fit
  • a second order or quadratic fit e.g. a Zemike oblique astigmatism and/or defocus and/or vertical astigmatism fit
  • a third order fit e.g.
  • a Zemike vertical trefoil and/or vertical coma and/or horizontal coma and/or oblique trefoil fit may be used. It will be appreciated that the Nyquist criterion may apply higher order polynomial fits as the measurement uncertainty decreases. The Zernike polynomial order selected using the Nyquist criterion may be applied as Zemike polynomials per row in a Wyant layout (i.e. ordered according to Cartesian spatial frequency).
  • a first order polynomial corresponds to Zl
  • a second order polynomial corresponds to Z2/Z3
  • a third order polynomial corresponds to Z4/Z5/Z6
  • a fourth order polynomial corresponds to Z7/Z8/Z10/Z11, etc.
  • the detection pixels are spread all over the pupil (e.g. like the measured wavefronts of Fig. 7C and Fig. 8C due to greater illumination by the illumination system and/or scattering introduced by a substructure) then the measurement uncertainty is reduced, and the Nyquist criterion may allow for a relatively high frequency (i.e. higher order Zemike polynomial) global fit to take place.
  • the processor is effectively fitting Zernike polynomials on a sampled subset of pixels, compared to fitting the same Zernike polynomials across all pixels of a complete wavefront measurement. If the detection pixel groups are spaced relatively far apart in the pupil plane (e.g. like the measured wavefront of Fig.
  • the measurement uncertainty is increased, and the Nyquist criterion may only allow for a relatively low frequency (i.e. a lower order Zemike polynomial) global fit to take place. It is therefore again advantageous to illuminate as much of the pupil as possible. However, the present method may still be applied to less illuminated pupils (e.g. the pupil of Fig. 9A) to advantageous effect.
  • the diameter 550 of the largest circle 540 between measured pixels is about half the size of the pupil (i.e. about Io). As such, in the example of Fig.
  • the Nyquist criterion may allow a first order polynomial or linear fit (e.g. a Zemike tilt in x and/or y fit) to be used.
  • the processor may be configured to perform the Zemike wavefront fitting process using the selected polynomial order to determine at least part of the estimated wavefront.
  • the Zernike wavefront fitting process may be used to perform a global fit (i.e. across the entire pupil) rather than a local fit (i.e. across a subset of pixels).
  • the phase of the wavefront changes from higher values at the bottom of the pupil to lower values at the top of the pupil, indicating the presence of a tilt optical aberration along the y axis.
  • the pupil center shape determined by the processor is effectively a version of the expected wavefront of Fig. 9B after a piston offset and tilt has been applied via the Zemike global wavefront fitting process.
  • the processor may be configured to apply an intensity threshold such that a component of the portion of the wavefront that does not satisfy the intensity threshold is excluded in determining the estimated wavefront.
  • an intensity threshold such that a component of the portion of the wavefront that does not satisfy the intensity threshold is excluded in determining the estimated wavefront.
  • the Zemike wavefront fitting process may not apply to pixels that have less than, for example 70% of a maximum intensity weight. It will be appreciated that other percentage thresholds may be used.
  • the processor may be configured to apply an image filter to the portion of the wavefront. That is, rather than performing the wavefront fitting process to pixels directly, the wavefront fitting process may be performed after a smoothing filter has been applied such that single pixels are not relied upon in isolation.
  • the image filter may be referred to in the art as a kernel. Applying the image filter may comprise applying a convolution matrix to the measured portion of the wavefront.
  • the image filter may be a smoothing filter configured to smear the measured portion of the wavefront. For example, a ring shaped convolution kernel having a diameter of about 0.2 o may be used to apply a smearing effect to the measured portion of the wavefront. Other diameters of convolution kernel may be used.
  • the processor may be configured to replace a fit of the expected wavefront with a fit of the measured portion of the wavefront. That is, in the expected wavefront, the pixels corresponding to lower order Zemike polynomials may be replaced by the Zernike fitting process performed by the processor.
  • the process may be represented via equations as follows. Firstly, a residual wavefront (Res) may be determined by deducting the measured portion of the wavefront (Meas) from the expected portion of the wavefront (Exp)'.
  • the original expected wavefront may be modified by replacing pixels corresponding to lower order Zemike polynomials with the Zemike fitting process performed by the processor to form a modified expected wavefront (Exp ”) :
  • the residual wavefront may be modified by removing pixels corresponding to lower order Zemike polynomials and retaining pixels corresponding to higher order Zemike polynomials to form a modified residual wavefront (Res ’) that has been updated globally (i.e. across the entire pupil):
  • Res’ Res — fit order (Res)
  • the processor may be configured to locally replace the higher spatial frequencies by the measured portion of the wavefront. That is, non-measured pixels in the estimated wavefront (Esf) are given by the modified expected wavefront:
  • ⁇ non -measured Exp' and measured pixels in the estimated wavefront are given by the measured portion of the wavefront after an imaging filter is applied (e.g. a low pass version such as a circular convolution kernel with diameter 0.2o):
  • a method of forming an image of an optical measurement mark in accordance with the present disclosure may comprise using a reflective region of the optical measurement mark to reflect incident radiation.
  • the reflective regions 510 of the optical measurement mark 500 of Fig.8A may be illuminated with EUV radiation and reflect said EUV radiation.
  • the method may comprise using the diffractive substructure of the optical measurement mark 500 to scatter the incident radiation.
  • the diffractive substructures 520, 530 of Fig. 8A may use diffraction as previously described to scatter the incident radiation.
  • a method of projecting a patterned beam B’ of radiation onto a substrate W in accordance with the present disclosure may comprise the method of forming an image of an optical measurement mark described above. For example, with reference to Fig.
  • the optical measurement mark 17 may take the form of the optical measurement mark 500 of Fig. 8A, and/or comprise the diffusive substructures previously described.
  • the illumination system IL may illuminate the optical measurement mark 17 with EUV radiation in a given illumination mode (e.g. a dipole illumination mode or a “starry sky” illumination mode).
  • the projection system PS may form an image of the optical measurement mark 17 and the sensor apparatus 19 may detect the image of the optical measurement mark 17 and thereby determine an alignment between the patterning device MA and the substrate W and/or an optical aberration of the lithographic apparatus LA (e.g. of the projection system PS).
  • the determined alignment information and/or optical aberration information may be used to at least partially control a lithographic exposure performed by the lithographic apparatus LA.
  • Fig. 10 shows a flowchart of a method of determining an estimated wavefront of radiation in accordance with the present disclosure.
  • a first step 910 of the method comprises detecting the radiation after the radiation has interacted with an optical measurement mark (e.g. using the sensor apparatus 19 to detect Radiation that has diffracted from the diffractive substructure 520, 530 of the optical measurement mark 500 of Fig. 8A).
  • a second step 920 of the method comprises determining a portion of the wavefront of the radiation (e.g. determining the wavefronts of Figs. 7C, 8C or 9A depending on the illumination mode used and/or the presence of a diffusive substructure and/or a diffractive substructure).
  • a third step 930 of the method comprises determining an expected wavefront of the radiation at least partially based on a previously measured wavefront (e.g. the expected wavefront of Fig. 9B).
  • a fourth step 940 of the method comprises determining the estimated wavefront of the radiation at least partially based on the expected wavefront and the portion of the wavefront (e.g. the estimated wavefront of Fig. 9C).
  • the method may comprise determining a measurement uncertainty at least partially based on the portion of the wavefront.
  • the method may comprise determining a polynomial order of a wavefront fitting process by comparing the measurement uncertainty to a standard deviation of the portion of the wavefront.
  • the method may comprise performing the wavefront fitting process (e.g.
  • the method may comprise applying an intensity threshold such that a component of the portion of the wavefront that does not satisfy the intensity threshold is excluded in determining the estimated wavefront.
  • the method may comprise applying an image filter to the measured portion of the wavefront.
  • lithographic apparatus may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquidcrystal displays (LCDs), thin-film magnetic heads, etc.
  • embodiments of the invention may be used in other apparatus.
  • Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.
  • embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented as instructions stored on a machine -readable medium, which may be read and executed by one or more processors.
  • a machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device).
  • a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic 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.
  • firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world.
  • An optical measurement mark for an imaging system comprising: a reflective region configured to reflect incident radiation for the imaging system to form an image of the optical measurement mark; and, a substructure configured to scatter the incident radiation for increasing an illumination of a numerical aperture of the imaging system.
  • Ruthenium a Ruthenium-based alloy
  • the reflective region is one of a plurality of reflective regions that are arranged in a periodic pattern to form a reflective diffraction grating, wherein the diffractive substructure comprises a segmentation of the periodic pattern configured to broaden a diffraction peak of the reflective diffraction grating.
  • An optical measurement system comprising: the optical measurement mark of any preceding clause; the imaging system configured to collect at least some radiation reflected by the reflective region and form the image of the optical measurement mark; and, a sensor apparatus configured to detect the image of the optical measurement mark.
  • a lithographic apparatus arranged to project a pattern from a patterning device onto a substrate comprising: a support structure constructed to support the patterning device, the patterning device being capable of imparting incident radiation with a pattern in its cross-section to form a patterned radiation beam; a substrate table constructed to hold the substrate; and, the optical measurement system of any of clauses 30 to 40.
  • a patterning device for a lithographic apparatus comprising the optical measurement mark of any of clauses 1 to 29.
  • a method of forming an image of optical measurement mark comprising: using a reflective region of the optical measurement mark to reflect incident radiation; and, using a substructure of the optical measurement mark to scatter the incident radiation.
  • the reflective region is one of a plurality of reflective regions that are arranged in a periodic pattern to form a reflective diffraction grating, wherein the diffractive substructure comprises a segmentation of the periodic pattern configured to broaden a diffraction peak of the reflective diffraction grating.
  • a method of projecting a patterned beam of radiation onto a substrate comprising the method of any of clauses 48 to 51.
  • a method of using an interferometric sensing system to determine an alignment between an optical measurement mark and a sensor apparatus that forms part of the interferometric sensing system comprising: using the optical measurement mark as a reference diffractive structure for the interferometric sensing system; and, using the interferometric sensing system to detect radiation that has scattered from a substructure of the optical measurement mark.
  • a method of designing an optical measurement mark comprising a diffusive substructure having a random phase pattern for a lithographic patterning device comprising: selecting a mask absorber type of the lithographic patterning device; selecting an illumination mode for illuminating the lithographic patterning device; and, using the selected mask absorber type and illumination mode to determine a critical dimension and a duty cycle of the random phase pattern.
  • a method of determining an estimated wavefront of radiation comprising: detecting the radiation after the radiation has interacted with an optical measurement mark; determining a portion of the wavefront of the radiation; determining an expected wavefront of the radiation at least partially based on a previously measured wavefront; and, determining the estimated wavefront of the radiation at least partially based on the expected wavefront and the portion of the wavefront.

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Abstract

L'invention concerne une marque de mesure optique pour un système d'imagerie comprenant une région réfléchissante conçue pour réfléchir un rayonnement incident pour le système d'imagerie afin de former une image de la marque de mesure optique. La marque de mesure optique comprend une sous-structure de diffusion conçue pour diffuser le rayonnement incident afin d'augmenter un éclairage d'une ouverture numérique du système d'imagerie.
PCT/EP2024/066450 2023-06-20 2024-06-13 Marque de mesure optique, système de mesure optique et appareil lithographique Pending WO2024260846A1 (fr)

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Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20050248743A1 (en) * 2003-01-15 2005-11-10 Asml Holding N.V. Diffuser plate and method of making same
US20060109533A1 (en) * 2003-04-11 2006-05-25 Carl Zeiss Smt Ag Diffuser, wavefront source, wavefront sensor and projection exposure apparatus
CN103488036A (zh) * 2013-09-24 2014-01-01 苏州苏大维格光电科技股份有限公司 全息立体投影屏及其投影方法
US20190324365A1 (en) * 2016-06-03 2019-10-24 Asml Netherlands B.V. Patterning Device

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20050248743A1 (en) * 2003-01-15 2005-11-10 Asml Holding N.V. Diffuser plate and method of making same
US20060109533A1 (en) * 2003-04-11 2006-05-25 Carl Zeiss Smt Ag Diffuser, wavefront source, wavefront sensor and projection exposure apparatus
CN103488036A (zh) * 2013-09-24 2014-01-01 苏州苏大维格光电科技股份有限公司 全息立体投影屏及其投影方法
US20190324365A1 (en) * 2016-06-03 2019-10-24 Asml Netherlands B.V. Patterning Device

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