Multi-directional overlay measurement and separate imaging using multiple illumination parameters

Abstract

The optical metrology system can include an overlay metrology tool 102 for evaluating properties of an overlay target 104 on a sample 110, the overlay target 104 including periodic features in a first direction in a first set of layers of the sample 110 and periodic features in a second direction in a second set of layers of the sample 110. The overlay metrology tool 102 can simultaneously illuminate the overlay target 104 with a first illumination beam 106a and a second illumination beam 106b and can further generate an image of the overlay target 104 based on diffraction of the first illumination beam 106a and the second illumination beam 106b by the overlay target 104, where the diffraction orders of the first illumination beam 106a contribute to resolving images of only the periodic features in the first direction and the diffraction orders of the second illumination beam 106b contribute to resolving images of only the periodic features in the second direction. The system may further generate overlay measurements along the first and second measurement directions based on the images.

Description

[Technical field]

[0001] FIELD OF THE DISCLOSURE This disclosure relates generally to imaging overlay metrology, and more particularly to simultaneous, separated imaging overlay metrology measurements along multiple directions. [Background technology]

[0002] Image-based overlay metrology typically generates overlay measurements by imaging multiple features on a sample fabricated using different lithography exposures and determining the relative positions of the imaged features. In this manner, overlay measurements can provide an indication of registration errors between lithography exposures. However, as smaller feature sizes are required to be manufactured, overlay tolerances become more stringent. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] US Patent Application Publication No. 2018 / 0224753 [Patent Document 2] US Patent Application Publication No. 2022 / 0057192 Summary of the Invention [Problem to be solved by the invention]

[0004] Therefore, there is a need to develop a system and method that provides overlay metrology with high accuracy and throughput. [Means for solving the problem]

[0005] An optical metrology system is disclosed according to one or more exemplary embodiments of the present disclosure. In one exemplary embodiment, the system includes an overlay metrology tool for performing a metrology recipe for evaluating a property of an overlay target on a specimen. According to one or more exemplary embodiments of the present disclosure, the overlay target includes periodic features in a first direction distributed along a first measurement direction and periodic features in a second direction in a second set of layers of the specimen. In another exemplary embodiment, the overlay metrology tool includes an illumination optical element for illuminating the overlay target with one or more first illumination beams and one or more second illumination beams, where at least one of the first illumination beams and at least one of the second illumination beams are simultaneously incident on the overlay target. In another exemplary embodiment, the overlay metrology tool further includes a focusing optical element configurable to generate an image of the overlay target on one or more detectors based on diffraction of the first illumination beam and the second illumination beam by the overlay target. In another exemplary embodiment, the diffraction orders of the first illumination beam contribute to resolving images of only the periodic features in the first direction and the diffraction orders of the second illumination beam contribute to resolving images of only the periodic features in the second direction. In another exemplary embodiment, the system further includes a controller for generating first and second overlay measurements along the first and second measurement directions based on the images.

[0006] An optical metrology system is disclosed according to one or more exemplary embodiments of the present disclosure. In one exemplary embodiment, the system includes an illumination optical element for illuminating an overlay target on the specimen with a first illumination beam and a second illumination beam when performing a metrology recipe, where at least one of the first illumination beam and at least one of the second illumination beam are simultaneously incident on the overlay target. In another exemplary embodiment, the overlay target according to the metrology recipe includes periodic features in a first direction distributed along a first measurement direction and periodic features in a second direction distributed along a second measurement direction different from the first measurement direction. In another exemplary embodiment, the system further includes a focusing optical element for generating an image of the overlay target on one or more detectors based on diffraction of the first illumination beam and the second illumination beam by the overlay target. In another exemplary embodiment, the diffraction orders of the first illumination beam contribute to resolving images of only the periodic features in the first direction, and the diffraction orders of the second illumination beam contribute to resolving images of only the periodic features in the second direction. In another exemplary embodiment, the system further includes a controller for generating first and second overlay measurements along first and second measurement directions based on the images.

[0007] An optical metrology method is disclosed according to one or more exemplary embodiments of the present disclosure. In one exemplary embodiment, the method includes illuminating an overlay target on a sample with a first illumination beam and a second illumination beam, where at least one of the first illumination beam and at least one of the second illumination beam are simultaneously incident on the overlay target. In another exemplary embodiment, the overlay target includes periodic features in a first direction distributed along a first measurement direction and periodic features in a second direction distributed along a second measurement direction different from the first measurement direction. In another exemplary embodiment, the method includes generating an image of the overlay target on one or more detectors based on diffraction of the first illumination beam and the second illumination beam by the overlay target. In another exemplary embodiment, diffraction orders of the first illumination beam contribute to resolving images of only the periodic features in the first direction and diffraction orders of the second illumination beam contribute to resolving images of only the periodic features in the second direction. In another exemplary embodiment, the method includes generating a first overlay measurement along the first measurement direction based on the one or more images. In another exemplary embodiment, the method includes generating a second overlay measurement along a second measurement direction based on the one or more images.

[0008] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not necessarily restrictive of the invention as claimed. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with this general description, serve to explain the principles of the invention.

[0009] The many advantages of the present disclosure may be better understood by those skilled in the art by reference to the following drawings. [Brief description of the drawings]

[0010] [Figure 1A] FIG. 1 is a conceptual diagram illustrating an overlay metrology system in accordance with one or more embodiments of the present disclosure. [Figure 1B]FIG. 1 is a conceptual diagram illustrating an overlay metrology tool in accordance with one or more embodiments of the present disclosure. [Figure 1C] FIG. 1 is a conceptual diagram of an overlay metrology tool including two collection channels in accordance with one or more embodiments of the present disclosure. [Figure 2A] FIG. 2 is a top view of an overlay target having non-overlapping features in accordance with one or more embodiments of the present disclosure. [Figure 2B] FIG. 2 is a top view of a cell of an overlay target having a moire structure in accordance with one or more embodiments of the present disclosure. [Figure 2C] FIG. 2C is a side view of the cell of FIG. 2B on a substrate in accordance with one or more embodiments of the present disclosure. [Figure 3A] FIG. 1 is a conceptual diagram of collection of diffraction orders from a normally incident illumination beam in accordance with one or more embodiments of the present disclosure. [Figure 3B] FIG. 1 is a conceptual diagram of collection of diffraction orders from an obliquely incident illumination beam in accordance with one or more embodiments of the present disclosure. [Figure 4A] A conceptual diagram of an illumination pupil plane including two illumination beams positioned to provide oblique illumination along orthogonal azimuthal directions within an illumination pupil boundary, in accordance with one or more embodiments of the present disclosure. [Figure 4B] FIG. 4B is a conceptual diagram of a collection pupil plane illustrating direction-separated imaging based on the illumination profile of FIG. 4A in accordance with one or more embodiments of the present disclosure. [Figure 5A] 1 is a conceptual diagram of an illumination pupil plane illustrating a directionally dependent illumination beam having a rotated configuration in accordance with one or more embodiments of the present disclosure. [Figure 5B] 1 is a conceptual diagram of an illumination pupil plane illustrating directionally dependent illumination beams differentiated based on NA in accordance with one or more embodiments of the present disclosure. [Figure 5C] 1 is a conceptual diagram of an illumination pupil plane illustrating directionally dependent illumination beams differentiated based on tilt in accordance with one or more embodiments of the present disclosure. [Figure 5D] 1 is a conceptual diagram of an illumination pupil plane illustrating directionally dependent illumination beams differentiated based on their spectral characteristics in accordance with one or more embodiments of the present disclosure. [Figure 5E] 1 is a conceptual diagram of an illumination pupil plane illustrating directionally dependent illumination beams differentiated based on intensity in accordance with one or more embodiments of the present disclosure. [Figure 5F] 1 is a conceptual diagram of an illumination pupil plane illustrating a direction-dependent illumination beam differentiated based on polarization, in accordance with one or more embodiments of the present disclosure. [Figure 6] FIG. 4B is a conceptual diagram of a collection pupil plane illustrating direction-separated imaging in an oblique brightfield imaging mode based on the illumination distribution shown in FIG. 4A in accordance with one or more embodiments of the present disclosure. [Figure 7A] 1 is a conceptual diagram of a collection pupil plane illustrating a configuration of an overlay metrology tool focused by an objective lens in accordance with one or more embodiments of the present disclosure. [Figure 7B] FIG. 7B is a conceptual diagram of the collection pupil plane of FIG. 7A in which the zeroth order diffraction lobe is blocked to provide dark field imaging, in accordance with one or more embodiments of the present disclosure. [Figure 8A] FIG. 2 is a conceptual diagram of the generation of various diffraction lobes in accordance with one or more embodiments of the present disclosure. [Figure 8B] FIG. 8B is a conceptual diagram of a collection pupil plane illustrating direction-separated imaging in optical moiré mode based on the diffraction lobes shown in FIG. 8A in accordance with one or more embodiments of the present disclosure. [Figure 8C] FIG. 8C is a conceptual diagram of the collection pupil plane of FIG. 8B including a blockage for selectively blocking the zeroth order diffraction lobe in the collection pupil plane in accordance with one or more embodiments of the present disclosure. [Figure 9] FIG. 1 is a conceptual diagram of an illumination pupil plane including four illumination beams positioned to provide symmetric oblique illumination along orthogonal azimuthal directions, in accordance with one or more embodiments of the present disclosure. [Figure 10] FIG. 1 is a flow diagram illustrating steps performed in a method for direction-separated imaging, in accordance with one or more embodiments of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Reference will now be made in detail to the disclosed subject matter, as illustrated in the accompanying drawings. The present disclosure has been particularly shown and described with reference to certain embodiments and detailed features thereof. The embodiments set forth herein are to be construed as illustrative and not limiting. It will be readily apparent to those skilled in the art that various changes and modifications in form and details may be made therein without departing from the spirit and scope of the present disclosure.

[0012] Embodiments of the present disclosure are directed to systems and methods for image-based overlay metrology that provide directionally separated imaging of sample features along different directions using simultaneous illumination with illumination beams having different illumination conditions. For example, an image (or a portion thereof) of a sample feature along a first direction (e.g., a first measurement direction) may be formed from illumination associated with a first illumination condition, and an image (or a portion thereof) of a sample feature along a second direction (e.g., a second measurement direction) may be formed from illumination associated with a second illumination condition. For purposes of this disclosure, an illumination condition may include a set of parameters of an illumination beam used to generate an image, including, but not limited to, numerical aperture (NA), beam shape, beam tilt, azimuth angle of incidence, elevation angle of incidence, wavelength, spectral bandwidth, polarization, or intensity.

[0013] In some embodiments, overlay metrology is performed by illuminating an overlay target having at least two sets of periodic target features arranged to facilitate overlay measurements along at least two different (e.g., orthogonal) measurement directions. For example, the set of periodic target features may include a periodic distribution of features along a particular measurement direction. By way of non-limiting example, such a periodic distribution of features may include a diffraction grating (e.g., a grating structure) characterized by a grating pitch along a grating direction. In this manner, an overlay target designed for overlay measurements along two different directions may include one set of periodic features having a first grating direction and a second set of periodic features having a second grating direction.

[0014] It is contemplated herein that various image quality metrics associated with an image of a periodic target feature may be affected by the number and configuration of diffraction orders from the periodic target feature used to generate the image. Furthermore, the impact of the number and configuration of the particular diffraction orders used to generate the image may be more pronounced in challenging applications, such as, but not limited to, imaging sample features at or near the optical resolution of the imaging system. As an example involving dark-field imaging techniques, zero order diffraction (e.g., specular reflection) may introduce a DC bias that may reduce image contrast. As another example that is particularly relevant to overlay metrology, diffraction from features that are not of interest for a particular measurement may similarly reduce image contrast and / or introduce other artifacts that may affect the sensitivity of the resulting measurement. For example, diffraction from a Y-direction grating may adversely affect (e.g., reduce contrast) an image of an X-direction grating, which may in turn adversely affect an overlay measurement based on the image.

[0015] Embodiments of the present disclosure are directed to systems and methods for providing illumination-separated imaging of features having different grating directions. In this manner, image quality (e.g., contrast, or any other suitable image quality metric) may be improved compared to conventional techniques. In some embodiments, an overlay metrology system simultaneously illuminates an overlay target with multiple illumination beams having different direction-dependent illumination conditions, the overlay target including features having different (e.g., orthogonal) grating directions. In this manner, each illumination beam may be diffracted along each of the grating directions. However, in embodiments of the present disclosure, the overlay metrology tool is configured (e.g., using a metrology recipe) such that the diffracted lobes from any particular illumination beam only form an image of the periodic features along one particular direction. In this configuration, diffraction of the particular illumination beam along other directions is insufficient to form a resolved image of the periodic structure.

[0016] It is recognized herein that forming an image of a periodic structure requires that at least two diffraction lobes are collected and passed to a detector to contribute to image formation. For purposes of this disclosure, directionally separated imaging refers to a configuration in which for any particular illumination beam, there is only one direction in which two or more diffraction lobes contribute to image formation. In other directions, at most one diffraction lobe is collected and passed to the detector. Under such conditions, periodic features along other directions are not resolved. In some embodiments, diffraction orders associated with other directions are not collected to avoid or reduce residual effects of these other diffraction orders (e.g., DC bias associated with the zeroth reflected order along the other direction) that may adversely affect image quality. In some embodiments, a single diffraction order along the other direction is allowed to be passed to the detector and contribute to image formation (e.g., the zeroth reflected order). In such cases, the residual effects of a single diffraction lobe may be within the tolerance of a particular application.

[0017] For example, an overlay target including orthogonal grating features (e.g., X-direction and Y-direction gratings) may be simultaneously illuminated by a first illumination beam having a first set of illumination conditions and a second illumination beam having a second set of illumination conditions. However, only the diffraction of the first illumination beam by the X-direction grating contributes to the formation of the portion of the image including the X-direction grating. Similarly, only the diffraction of the second illumination beam by the Y-direction grating contributes to the formation of the portion of the image including the Y-direction grating. As a result, the X-direction and Y-direction gratings are imaged simultaneously, but each image (or portion thereof) is formed based on different illumination conditions provided by the first and second illumination beams.

[0018] 1A-10, systems and methods for optically separated imaging with directionally dependent illumination conditions in accordance with one or more embodiments of the present disclosure are described in more detail.

[0019] FIG. 1A is a conceptual diagram illustrating an overlay metrology system 100 in accordance with one or more embodiments of the present disclosure.

[0020] In some embodiments, the overlay metrology system 100 includes an overlay metrology tool 102 that can be configured to generate overlay measurements according to a metrology recipe (e.g., an overlay recipe) and associated with an overlay target 104 having a design based on the metrology recipe. For example, the overlay metrology tool 102 can direct two or more illumination beams 106 (e.g., directionally dependent illumination beams 106 having different illumination conditions) from at least one illumination source 108 to the overlay target 104 on a sample 110, collect light or other radiation (referred to herein as measurement light 112) emanating from the overlay target 104 in response to the illumination beams 106, and generate a detection signal from at least one detector 114 based on the measurement light 112.

[0021] The overlay metrology tool 102 may include any type of tool known in the art suitable for generating an overlay signal suitable for determining an overlay associated with an overlay target on the specimen 110. In some embodiments, the overlay metrology tool 102 is an imaging metrology tool that generates overlay measurements based on one or more images of the overlay target 104. The overlay metrology tool 102 may generate the images using any imaging technique or combination of imaging techniques known in the art, including, but not limited to, bright-field or dark-field imaging techniques. Additionally, as disclosed herein, the overlay metrology tool 102 may be configured to provide directionally dependent illumination conditions (e.g., based on a metrology recipe) to provide optically separated imaging of features oriented along different directions.

[0022] Some embodiments of the present disclosure are directed to providing recipes for configuring the overlay metrology tool 102 to facilitate overlay measurements based on selected diffraction orders. The metrology recipe may include a set of parameters for controlling various aspects of the overlay measurement, such as, but not limited to, the illumination of the sample, the collection of light from the sample, or the position of the sample during the measurement. In this manner, the overlay metrology tool 102 may be configured to achieve a selected type of measurement for a selected overlay target design. For example, the metrology recipe may include parameters of the illumination beam 106, such as, but not limited to, the illumination wavelength, the illumination pupil distribution (e.g., the distribution of illumination angles and the associated intensities of the illumination at those angles), the polarization of the incident illumination, or the spatial distribution of the illumination. As another example, the metrology recipe may include collection parameters, such as, but not limited to, the collection pupil distribution (e.g., the desired distribution of angled light from the overlay target 104 used for the measurement and the associated filtered intensities at those angles), the collection field stop setting for selecting the portion of interest of the overlay target 104, the polarization of the collected light, wavelength filters, or parameters for controlling one or more detectors. As another example, the metrology recipe may include various parameters associated with the design of the overlay target 104, such as, but not limited to, the position and orientation of the sample features (e.g., the pitch of the grating features along a particular direction). As a further example, the metrology recipe may include various parameters associated with the position of the sample 110 during the measurement, such as, but not limited to, the height of the sample, the orientation of the sample, whether the sample is stationary during the measurement or whether the sample is moving during the measurement (in addition to associated parameters describing the speed, scan pattern, etc.).

[0023] In this manner, the overlay metrology tool 102 may be configurable to provide a selected distribution of light in the collection pupil when analyzing a particular overlay target 104 having a known design according to a metrology recipe, and this distribution of light in the collection pupil may then be further modified by various optical elements as desired to perform the desired overlay measurement.

[0024] In some embodiments, overlay metrology system 100 further includes a controller 116 having one or more processors 118 configured to execute program instructions retained in memory 120 (e.g., a memory medium). Controller 116 may be communicatively coupled to any of the components of overlay metrology system 100, such as, but not limited to, detector 114. In this manner, controller 116 may follow a metrology recipe and generate overlay measurements based on the detection signals.

[0025] The one or more processors 118 of the controller 116 may include any processor or processing element known in the art. For purposes of this disclosure, the term "processor" or "processing element" may be broadly defined to encompass any device having one or more processing or logic elements (e.g., one or more microprocessor devices, one or more application specific integrated circuit (ASIC) devices, one or more field programmable gate arrays (FPGAs), or one or more digital signal processors (DSPs)). In this sense, the one or more processors 118 may include any device configured to execute algorithms and / or instructions (e.g., program instructions stored in a memory). In an embodiment, the one or more processors 118 may be embodied as a desktop computer, a mainframe computer system, a workstation, an image computer, a parallel processor, a networked computer, or any other computer system configured to execute programs configured to operate or in conjunction with the overlay metrology system 100 as described throughout this disclosure.

[0026] Additionally, various subsystems of the overlay metrology system 100 may include processors or logic elements suitable for performing at least some of the steps described in the present disclosure. Thus, the above description should not be construed as a limitation on the embodiments of the present disclosure, but merely as an example. Additionally, the steps described throughout the present disclosure may be performed by a single controller 116, or alternatively, multiple controllers. Additionally, the controller 116 may include one or more controllers housed within a common housing or within multiple housings. In this manner, any controller or combination of controllers may be individually packaged as a module suitable for incorporation into the overlay metrology system 100.

[0027] The memory 120 may include any storage medium known in the art suitable for storing program instructions executable by the associated one or more processors 118. For example, the memory 120 may include a non-transitory memory medium. As another example, the memory 120 may include, but is not limited to, a read-only memory (ROM), a random access memory (RAM), a magnetic or optical memory device (e.g., disk), a magnetic tape, a solid-state device, and the like. It is further noted that the memory 120 may be housed together with the one or more processors 118 in a common controller housing. In one embodiment, the memory 120 may be located remotely relative to the physical location of the one or more processors 118 and the controller 116. For example, the one or more processors 118 of the controller 116 may access a remote memory (e.g., a server) accessible via a network (e.g., the Internet, an intranet, etc.).

[0028] 2A-2C, various non-limiting examples of overlay targets 104 in accordance with one or more embodiments of the present disclosure are described in greater detail.

[0029] For the purposes of this disclosure, the term overlay is generally used to describe the relative positions of features on the sample 110 fabricated by two or more lithographic patterning steps, where the term overlay error describes the deviation of the features from their nominal placement. In this context, overlay measurements can be expressed as measurements of relative positions, or measurements of overlay errors associated with these relative positions. For example, a multi-layer device may include features patterned on multiple sample layers using different lithographic steps for each layer, where alignment of features between layers must typically be tightly controlled to ensure proper performance of the resulting device. As a result, overlay measurements can characterize the relative positions of features on two or more of the sample layers. As another example, multiple lithographic steps may be used to fabricate features on a single sample layer. Such techniques, commonly referred to as double or multiple patterning techniques, can facilitate fabrication of high density features approaching the resolution of the lithography system. Overlay measurements in this context can characterize the relative positions of features from different lithographic steps on this single layer. It should be understood that throughout this disclosure, examples and illustrations relating to specific applications of overlay metrology are provided for illustrative purposes only and should not be construed as limiting the disclosure.

[0030] Moreover, the term overlay target 104 is used broadly herein to refer to any set of features on a sample that have a known periodicity suitable for overlay measurements (e.g., corresponding to grating features). In some embodiments, the overlay target 104 includes a dedicated target having features fabricated on one or more sample layers for the purpose of providing overlay measurements that are typically representative of the overlay of device features on the same layers. For example, a dedicated overlay target may include one or more cells having grating features with characteristics (e.g., grating pitch, feature size, grating orientation, etc.) designed to achieve overlay measurements according to a particular overlay technology. Such dedicated overlay targets may be printed in various locations on the sample, including, but not limited to, within a scribe line or within a die. In some embodiments, the overlay target includes device features with a known periodicity suitable for direct overlay measurements.

[0031] FIG. 2A is a top view of an overlay target 104 having non-overlapping features, according to one or more embodiments of the present disclosure. In particular, FIG. 2A illustrates a non-limiting configuration of the overlay target 104 including four cells 202a-d, depicted here as quadrants of the overlay target 104. In this configuration, each cell 202a-d may include a first set of periodic features 204 associated with a first lithographic exposure and a second set of periodic features 206 associated with a second lithographic exposure. Additionally, cells 202b and 202d may be configured to provide overlay measurements along the X-direction shown in FIG. 2A. For example, overlay measurements along the X-direction may be performed by directly comparing the relative positions of the first set of periodic features 204 and the second set of periodic features 206 within each cell or between cells 202b and 202d. In another example, an overlay measurement along the X direction can be performed by comparing a point of rotational symmetry (e.g., rotational symmetry, reflection symmetry, mirror symmetry, etc.) between a first set of periodic features 204 distributed across cells 202b and 202d with a point of symmetry between a second set of periodic features 206 distributed across cells 202b and 202d. Similarly, cells 202a and 202c can be configured to achieve an overlay measurement along the Y direction shown in FIG. 2A. In this case, the first set of periodic features 204 and the second set of periodic features 206 in cells 202b and 202d having periodicity along a first measurement direction (e.g., X direction) may be referred to as periodic features in a first direction, while the first set of periodic features 204 and the second set of periodic features 206 in cells 202a and 202c having periodicity along a second measurement direction (e.g., Y direction) may be referred to as periodic features in a second direction.

[0032] The first set of periodic features 204 and the second set of periodic features 206 in any particular cell 202 may be disposed on any layer of the sample 110. For example, the first set of periodic features 204 and the second set of periodic features 206 may be located on a common layer of the sample 110 to facilitate overlay measurements between a first and a second lithography exposure in a multiple exposure process flow. As another example, the first set of periodic features 204 may be disposed on a first layer of the sample 110 and the second set of periodic features 206 may be disposed on a second layer of the sample 110 to facilitate overlay measurements between the first and second layers. Further, the first set of periodic features 204 and the second set of periodic features 206 associated with a first measurement direction (e.g., periodic features in a first direction) may be arranged in a first set of layers of the sample 110, while the first set of periodic features 204 and the second set of periodic features 206 associated with a second measurement direction (e.g., periodic features in a second direction) may be arranged in a second set of layers of the sample 110, which may be the same or different as the periodic features in the first direction. As an example, cells 202b and 202d may include a first set of periodic features 204 in a first layer 210 of the sample 110 and a second set of periodic features 206 in a second layer 212 of the sample 110, as described above, while cells 202a and 202c may include a first set of periodic features 204 in a first layer 210 of the sample 110 and a second set of periodic features 206 in a third layer (not explicitly shown) of the sample 110.

[0033] In some embodiments, the overlay target 104 includes various configurations of print elements oriented along different directions. For example, cells 202b and 202d may include a first set of periodic features 204 in a first layer 210 and a second set of periodic features 206 in a second layer 212, while cells 202a and 202c may include a first set of periodic features 204 in a first layer 210 and a second set of periodic features 206 in a third layer of the sample 110. In this manner, the overlay target 104 may facilitate simultaneous overlay measurements between multiple pairs of different layers.

[0034] Additionally, the pitch of the first set of periodic features 204 and the second set of periodic features 206 within any particular cell 202 may be the same or different. For example, Figure 2A shows a configuration of overlay target 104 in which the first set of periodic features 204 and the second set of periodic features 206 within each of cells 202a-d have a common pitch. However, the first set of periodic features 204 and the second set of periodic features 206 may have different pitches.

[0035] In some embodiments, the first set of periodic features 204 and the second set of periodic features 206 in any particular cell 202 are formed in different layers of the sample 110. For example, the first set of periodic features 204 and the second set of periodic features 206 in any particular cell 202 may be formed in at least partially overlapping regions of the sample to generate a grating-over-grating structure, or may be formed in non-overlapping regions. In this configuration, the pitch of the first set of periodic features 204 and the second set of periodic features 206 may be the same or different. Illustratively, a configuration in which the first set of periodic features 204 and the second set of periodic features 206 have different pitches may form a Moiré structure. Such a Moiré structure may generate Moiré diffraction (e.g., double diffraction, compound diffraction, etc.) associated with diffraction from both the first set of periodic features 204 and the second set of periodic features 206. It is contemplated herein that various overlay techniques (e.g., associated with various metrology recipes) may utilize any combination of diffraction lobes from the constituent features (e.g., first set of periodic features 204 and second set of periodic features 206), alone or in combination with Moiré diffraction lobes. For example, a first metrology recipe may utilize first order diffraction lobes from each of the constituent features, while a second metrology recipe may utilize Moiré diffraction lobes, alone or in combination with diffraction lobes from one or more of the constituent features.

[0036] FIG. 2B is a top view of a cell 202 of the overlay target 104 having a moiré structure 208, according to one or more embodiments of the present disclosure. FIG. 2C is a side view of the cell 202 of FIG. 2B, according to one or more embodiments of the present disclosure. In particular, FIGS. 2B and 2C show the pitch of a first set of periodic features 204 (e.g., a grating of a first layer) in a first layer 210 of the sample 110 and the pitch of a second set of periodic features 206 (e.g., a grating of a second layer) on a second layer 212 of the sample 110 as P and Q, respectively. FIG. 2B further shows a substrate 214 on which the various layers are deposited. In some embodiments, although not explicitly shown, the overlay target 104 includes two cells 202 for each measurement direction, providing an inverted pair of moiré structures. For example, a first cell 202 may include a first set 204 of periodic features having a pitch P and a second set 206 of periodic features having a pitch Q, while a second cell 202 may include a first set 204 of periodic features having a pitch Q and a second set 206 of periodic features having a pitch P.

[0037] 2A-2C, the overlay target 104 may generally include any known overlay target design suitable for image-based diffraction. For example, the overlay target 104 may include, but is not limited to, an advanced imaging and metrology (AIM) target, a triple AIM target having features on three layers, a robust AIM (rAIM) target including one or more moiré structures, etc.

[0038] Referring now to FIG. 1B, various aspects of an overlay metrology tool 102 are illustrated in accordance with one or more embodiments of the present disclosure.

[0039] 1B is a conceptual diagram illustrating an overlay metrology tool 102 in accordance with one or more embodiments of the present disclosure. In one embodiment, the overlay metrology tool 102 includes an illumination source 108 configured to generate at least one illumination beam 106. The illumination from the illumination source 108 may include one or more selected wavelengths of light, including, but not limited to, ultraviolet (UV) radiation, visible radiation, or infrared (IR) radiation.

[0040] In another embodiment, the overlay metrology tool 102 directs illumination from the illumination source 108 to the overlay target 104 via an illumination path 122. The illumination path 122 may include one or more optical components suitable for transforming and / or conditioning the illumination beam 106 and further directing the illumination beam 106 to the sample 110. In one embodiment, the illumination path 122 includes one or more illumination path lenses 124 (e.g., for collimating the illumination, relaying a pupil plane and / or a field plane, etc.). In another embodiment, the illumination path 122 includes one or more illumination path optical elements 126 for shaping or otherwise controlling the illumination. For example, the illumination path optical elements 126 may include, but are not limited to, one or more field stops, one or more pupil stops, one or more polarizers, one or more filters, one or more beam splitters, one or more diffusers, one or more homogenizers, one or more apodizers, one or more beam shapers, or one or more mirrors (e.g., stationary mirrors, translating mirrors, scanning mirrors, etc.). Furthermore, the illumination path optical element 126 may be positioned at any suitable location, including but not limited to, an illumination pupil plane 128 or an illumination field plane.

[0041] In another embodiment, the overlay metrology tool 102 includes an objective lens 130 that focuses the illumination onto the overlay target 104. In another embodiment, the sample 110 is disposed on a sample stage 132 that is suitable for mounting the sample 110 and is further configured to position the overlay target 104 relative to the illumination beam 106.

[0042] In some embodiments, the overlay metrology tool 102 includes at least one detector 114 for capturing light or other illumination (e.g., measurement light 112) emanating from the overlay target 104 via a collection path 134. The collection path 134 may include one or more optical elements suitable for transforming and / or conditioning the measurement light 112 from the overlay target 104. In one embodiment, the collection path 134 includes one or more collection path lenses 136 (e.g., for collimating the illumination beam 106, relaying a pupil plane and / or a field plane, etc.), which may, but need not, include an objective lens 130. In another embodiment, the collection path 134 includes one or more collection path optical elements 138 for shaping or otherwise controlling the measurement light 112. For example, collection path optical elements 138 may include, but are not limited to, one or more field stops, one or more pupil stops, one or more polarizers, one or more filters, one or more beam splitters, one or more diffusers, one or more homogenizers, one or more apodizers, one or more beam shapers, or one or more mirrors (e.g., stationary mirrors, translating mirrors, scanning mirrors, etc.). Additionally, collection path optical elements 138 may be positioned in any suitable location, including, but not limited to, a collection pupil plane 140 or a collection field plane (not shown).

[0043] The detector 114 may be positioned at any selected location within the collection path 134. In some embodiments, the overlay metrology tool 102 includes a detector 114 in a field plane (e.g., a plane conjugate with the sample 110) to generate an image of the overlay target 104.

[0044] The illumination path 122 and the collection path 134 of the overlay metrology tool 102 may be oriented in a wide variety of configurations suitable for illuminating the sample 110 with the illumination beam 106 and collecting light emanating from the sample 110 in response to the incident illumination beam 106. For example, as shown in FIG. 1B , the overlay metrology tool 102 may include a beam splitter 142 oriented such that the common objective lens 130 may simultaneously direct the illumination beam 106 to the sample 110 and collect light from the sample 110. As another example, the illumination path 122 and the collection path 134 may include non-overlapping optical paths.

[0045] Additionally, the illumination path 122 and / or the collection path 134 may include one or more channels. For example, the illumination path 122 may provide two or more illumination beams 106 from a single illumination channel or via dedicated illumination channels. As another example, the collection path 134 may include a single detector 114 in a single collection channel or multiple detectors 114 distributed across multiple collection channels.

[0046] 1B illustrates an overlay metrology tool 102 having a single collection channel 144. FIG. 1C is a conceptual diagram of an overlay metrology tool 102 including two collection channels 144, according to one or more embodiments of the present disclosure. For example, the overlay metrology tool 102 may include one or more beam splitters 146 for splitting the measurement light 112 into at least two collection channels 144. Additionally, each collection channel 144 may include individual collection path optical elements 138 (e.g., polarizers, spectral filters, neutral density filters, blockers, apertures, etc.) for individually modifying the measurement light 112 in each collection channel 144 to provide directionally separated imaging. It is contemplated herein that the use of multiple collection channels 144 may provide greater versatility for generating directionally separated images than a single collection channel 144, although this may add additional complexity. As such, a particular implementation of the systems and methods disclosed herein may be selected or adapted to the tolerances of a particular application.

[0047] 3A-9, overlay measurements based on direction-separated imaging of features oriented along different directions, in accordance with one or more embodiments of the present disclosure, are described in more detail.

[0048] It is contemplated herein that periodic structures on the overlay target 104 (e.g., first set of periodic features 204 and second set of periodic features 206 shown in FIGS. 2A-2C ) can generate discrete diffraction orders when illuminated with an illumination beam 106 having a narrow spectral range and limited angular range. Furthermore, the particular arrangement of the diffraction orders of the illumination beam 106 at the collection pupil plane 140 can be affected by various characteristics of the illumination beam 106 (e.g., spectrum, polarization, azimuthal and / or elevational incidence angles, numerical aperture (NA), etc.), various characteristics of the collection path 134 (e.g., NA of the objective lens 130, etc.), or various characteristics of the overlay target 104 (e.g., pitch of the printed features, orientation of the printed features with respect to the incident illumination beam 106, etc.).

[0049] 3A and 3B, by way of illustration, depict the effect of high incidence angle on the distribution of diffraction orders from a grating (e.g., on overlay target 104). FIG. 3A is a conceptual diagram of collection of diffraction orders from a normally incident illumination beam 106, in accordance with one or more embodiments of the present disclosure. FIG. 3B is a conceptual diagram of collection of diffraction orders from an obliquely incident illumination beam 106, in accordance with one or more embodiments of the present disclosure. In particular, FIG. 3A and 3B illustrate control of the particular diffraction orders collected by objective lens 130 based on the incidence angle.

[0050] Thus, in some embodiments, the overlay metrology tool 102 and / or the sample are configured via a metrology recipe to provide collection of selected diffraction orders to facilitate directionally resolved imaging.

[0051] In some embodiments, the overlay metrology tool 102 simultaneously illuminates the overlay target 104 with two or more illumination beams 106, where various parameters of each illumination beam 106 are configured to facilitate separation of diffraction orders of the illumination beams 106 along different directions (e.g., at the collection pupil plane 140). Various parameters of the objective lens 130 and / or collection paths 134 can then be configured to provide directionally separated imaging on one or more detectors 114, where gratings having a particular grating direction are imaged exclusively with light from a particular illumination beam 106.

[0052] It is contemplated herein that various configurations of the overlay metrology tool 102 may be used to perform direction-separated imaging.

[0053] In some embodiments, the overlay metrology tool 102 is configured such that the objective lens 130 collects only selected diffraction orders associated with each illumination beam 106 providing direction-separated imaging as disclosed herein. In such a configuration, the objective lens 130 can provide direction-separated imaging using a single detector 114, where portions of the generated image associated with grating structures having different grating directions are imaged based on the different illumination beams 106.

[0054] 4A and 4B show directionally separated imaging of a moiré structure (e.g., as shown in FIGS. 2B and 2C) based on illumination beams 106 having different azimuthal angles of incidence, in accordance with one or more embodiments of the present disclosure. In particular, FIG. 4A shows a configuration in which an objective lens 130 collects only selected diffraction orders associated with each illumination beam 106, providing directionally separated imaging.

[0055] Figure 4A is a conceptual diagram of an illumination pupil plane 128 including two illumination beams 106a,b positioned to provide oblique illumination along orthogonal azimuthal directions within an illumination pupil boundary 402, in accordance with one or more embodiments of the present disclosure. Figure 4B is a conceptual diagram of a collection pupil plane 140 illustrating direction-separated imaging based on the illumination profile of Figure 4A, in accordance with one or more embodiments of the present disclosure. For example, Figure 4B illustrates the diffraction lobes produced in response to illumination of the overlay target 104 shown in Figure 2A.

[0056] As shown in FIG. 4B, illumination of overlay target 104 with illumination beam 106a results in diffraction lobes 404 distributed along both the X and Y directions. However, only selected diffraction orders along the X direction are aligned with collection pupil boundary 406 and therefore available to contribute to forming an image of the corresponding structure. Similarly, illumination of overlay target 104 with illumination beam 106b results in diffraction lobes 408 distributed along both the X and Y directions, but only selected diffraction orders along the Y direction are aligned with collection pupil boundary 406 and therefore available to contribute to forming an image of the corresponding structure.

[0057] In this way, portions of the image of the feature distributed along the X direction are optically separated from portions of the image of the feature distributed along the Y direction. In other words, the grating information is encoded for each illumination condition provided by the selection of the diffraction orders that contribute to image formation. As a result, various image quality metrics associated with features oriented along the X direction may depend entirely (or at least substantially) on the selected diffraction orders from illumination beam 106a, and image quality metrics associated with features oriented along the Y direction may depend entirely (or at least substantially) on the selected diffraction orders from illumination beam 106b.

[0058] With general reference to Figures 3A-4B, it should be understood that Figures 3A-4B are provided for illustrative purposes only and should not be construed as limiting. For example, in Figures 4A and 4B, the primary difference between the illumination conditions associated with illumination beams 106a, b is the azimuthal incidence angle. However, it is contemplated herein that directionally dependent illumination separation imaging can be achieved using multiple illumination beams 106 having illumination conditions that are differentiated based on any combination of parameters, including but not limited to NA, beam shape, wavelength, spectral bandwidth, polarization, or intensity. Furthermore, such differentiation of illumination conditions can be achieved using any technique known in the art, including but not limited to illumination path optical elements 126 or direct generation of illumination beams 106 having different illumination conditions.

[0059] Additionally, the overlay metrology tool 102 may include collection paths 134 in various configurations to further separate selected diffraction lobes to provide directionally separated imaging.

[0060] In some embodiments, collection path 134 includes one or more collection path optical elements 138 for filtering or blocking selected diffraction lobes (e.g., collected by objective lens 130 but nevertheless undesirable for a particular imaging technique) at collection pupil plane 140. For example, it may be desirable to block zero order diffracted light (e.g., specular reflection) along one or more directions to ensure optical isolation and / or perform dark-field imaging, as described in more detail below.

[0061] In some embodiments (e.g., as shown in FIG. 1C ), the overlay metrology tool 102 includes multiple collection channels 144. In this manner, each collection channel 144 may include different collection path optical elements 138 for filtering or blocking different diffraction lobes. For example, each collection channel 144 may provide an image of the grating along a different selected direction based on diffracted light from a different illumination beam 106.

[0062] 5A-5F, various techniques for providing directionally dependent illumination separated imaging are described in more detail, in accordance with one or more embodiments of the present disclosure. It is noted that Figures 5A-5F depict distinctions between illumination conditions based on both azimuthal incidence angle and additional characteristics, although it should be understood that distinctions between azimuthal incidence angles are not required.

[0063] 5A-5F illustrate various non-limiting techniques for distinguishing the illumination beam 106 based on the location, size, shape, or orientation of the illumination beam 106 at the illumination pupil plane 128. FIG.

[0064] 5A is a conceptual diagram of an illumination pupil plane 128 showing a directionally dependent illumination beam 106 having a rotated configuration, in accordance with one or more embodiments of the present disclosure. In particular, FIG. 5A shows a distribution of illumination lobes similar to FIG. 4A, but in this case, the illumination beams 106a,b are rotated with respect to the grating directions of the overlay target 104 (e.g., the X and Y directions shown in FIG. 2A). It is noted that although the illumination beams 106a,b are rotated with respect to the grating directions of the overlay target 104, the associated diffraction lobes are still distributed along the grating directions. However, the use of rotated lobes may allow for further separation of diffraction orders at the collection pupil plane 140, which may be desirable in some applications.

[0065] 5B is a conceptual diagram of an illumination pupil plane 128 illustrating directionally dependent illumination beams 106 differentiated based on NA, in accordance with one or more embodiments of the present disclosure. In particular, FIG. 5B illustrates apertures 502a,b having different sizes (labeled NA1, NA2) for providing illumination beams 106a,b. Furthermore, apertures 502a,b may be implemented as separate elements or by spatially selective elements.

[0066] Further, although not shown, the illumination beams 106a,b may generally have any selected shape at the illumination pupil plane 128 that may correspond to the shape of the associated diffraction order. For example, the shapes of the illumination beams 106a,b at the illumination pupil plane 128 may include, but are not limited to, a circle, an ellipse, or a cat's eye (e.g., a geometric lens). Further, the illumination beams 106a,b may have the same shape or different shapes. In some embodiments, the shapes of the illumination beams 106a,b at the illumination pupil plane 128 are controlled by apertures 502a,b at the illumination pupil plane 128.

[0067] 5C is a conceptual diagram of an illumination pupil plane 128 illustrating a directionally dependent illumination beam 106 differentiated based on tilt, in accordance with one or more embodiments of the present disclosure. In particular, FIG. 5C illustrates apertures 504a,b at different radial positions (labeled θ1, θ2) within the illumination pupil plane 128 to provide illumination beams 106a,b with different elevation angles of incidence. Additionally, apertures 502a,b may be implemented as separate elements or by spatially selective elements.

[0068] FIG. 5D is a conceptual diagram of an illumination pupil plane 128 illustrating directionally dependent illumination beams 106 differentiated based on spectral characteristics, according to one or more embodiments of the present disclosure. In particular, FIG. 5D illustrates spectral filters 506a,b (labeled BW1, BW2) for imparting different spectral characteristics to illumination beams 106a,b. The spectral filters 506a,b can be any type of spectral filter, including but not limited to high-pass filters, low-pass filters, band-pass filters, or band-reject filters. In this manner, the spectral filters 506a,b can differentiate illumination beams 106a,b based on spectral characteristics, such as but not limited to center wavelength or bandwidth. Furthermore, the spectral filters 506a,b can be implemented as separate elements or by spatially selective elements.

[0069] 5E is a conceptual diagram of an illumination pupil plane 128 illustrating directionally dependent illumination beams 106 differentiated based on intensity, in accordance with one or more embodiments of the present disclosure. In particular, FIG. 5E illustrates neutral density filters 508a,b (labeled ND1, ND2) for imparting different intensities to illumination beams 106a,b. Additionally, neutral density filters 508a,b may be implemented as separate elements or by spatially selective elements.

[0070] 5F is a conceptual diagram of an illumination pupil plane 128 illustrating a polarization-based differentiated direction-dependent illumination beam 106, in accordance with one or more embodiments of the present disclosure. In particular, FIG. 5F illustrates polarizers 510a,b (labeled S, P) for imparting different polarizations (e.g., orthogonal polarizations) to illumination beams 106a,b. Additionally, polarizers 510a,b may be implemented as separate elements or by spatially selective elements.

[0071] With general reference to Figures 4A-5F, it is contemplated herein that the overlay metrology tool 102 may generally implement any combination of such techniques. For example, Figures 4A-5C may generally be suitable for, but are not limited to, manipulating the distribution of diffraction lobes collected by the objective lens 130 and therefore available to contribute to imaging. However, as described in more detail below, it may be desirable to provide additional filtering and / or blocking of the collected diffraction lobes, either to perform fully separate directional imaging or to perform a specific imaging technique. Figures 5D-5F may generally be suitable for, but are not limited to, manipulating additional characteristics of the illumination beams 106a,b such that associated diffraction lobes can be separated and / or filtered by additional elements in any collection channel 144.

[0072] 6-8C, various non-limiting imaging modes based on direction-separated imaging, according to one or more embodiments of the present disclosure, are described in more detail. FIGs. 6-8C are based on illumination having a profile as shown in FIG. 4A. Additionally, FIGs. 6-7B show imaging of overlay targets 104 having periodic features of a common pitch (e.g., as shown in FIG. 2A), while FIGs. 8A-8C show imaging of overlay targets 104 having periodic features of different pitches (e.g., moiré structures as shown in FIGs. 2B-2C).

[0073] 6-8C, the overlay target 104 being imaged includes periodic structures oriented along orthogonal lattice directions (eg, the X and Y directions as shown in FIGS. 2A-2C).

[0074] It is contemplated herein that the principles disclosed herein for direction-separated imaging may be generally extended to a wide range of imaging techniques, and it should therefore be understood that Figures 6-8C are provided for illustrative purposes only and should not be construed as limiting.

[0075] Figure 6 is a conceptual diagram of a collection pupil plane 140 illustrating direction-separated imaging in an oblique bright-field imaging mode based on the illumination distribution shown in Figure 4A, in accordance with one or more embodiments of the present disclosure. In particular, Figure 6 illustrates a configuration of an overlay metrology tool 102 (e.g., based on a metrology recipe) that exclusively provides a zeroth order diffraction lobe 602 (e.g., specular reflection) and a first order diffraction lobe 604 of a first illumination beam 106a due to a periodic structure in the X direction (e.g., cells 202b, d in Figure 2A) and a zeroth order diffraction lobe 606 and a first order diffraction lobe 608 of a second illumination beam 106b due to a periodic structure in the Y direction (e.g., cells 202a, c in Figure 2A).

[0076] In this way, the first illumination beam 106a forms an image (or a portion thereof) of a periodic structure in the X direction (e.g., cells 202b, d in FIG. 2A), while the second illumination beam 106b forms an image (or a portion thereof) of a periodic structure in the Y direction (e.g., cells 202a, c in FIG. 2A).

[0077] However, it is noted that a single image based on the collection pupil plane 140 of Figure 6 may exhibit a residual DC bias from the zeroth order diffraction lobes 602, 606 from orthogonal directions. For example, an image of a periodic structure in the X direction may be affected by a residual DC bias associated with the zeroth order diffraction lobe 606. Similarly, an image of a periodic structure in the Y direction may be affected by a residual DC bias associated with the zeroth order diffraction lobe 602. Depending on the application, such a residual DC bias may be within an operational tolerance and may be acceptable.

[0078] However, depending on the application, the overlay metrology tool 102 may include one or more additional elements to remove or mitigate this residual DC bias. In some embodiments, the overlay metrology tool 102 imparts orthogonal polarizations to the illumination beams 106a,b (e.g., as shown in FIG. 5F) to provide polarization separation. Additionally, the collection path 134 may, but need not, include one or more polarizers to separate the diffraction lobes from the illumination beams 106a,b. For example, the overlay metrology tool 102 may include two collection channels and one or more polarizers (such as, for example, beam splitters 146 or other collection path optical elements 138 in any suitable location) such that the detectors 114 in the two collection channels 144 individually image the overlay target 104 with orthogonal polarizations. As another example, the collection path 134 may include a structured image plane (e.g., field plane) polarizer that provides different polarization directions that correspond to the locations of associated periodic features in the image plane.

[0079] In some embodiments, the overlay metrology tool 102 (e.g., as shown in FIG. 5D ) imparts different spectral content (e.g., center wavelength, bandwidth, etc.) to the illumination beams 106a,b. In this case, the overlay metrology tool 102 may include a spectral filter for separation based on spectral content. For example, in a configuration having two collection channels 144, a dichroic beam splitter 146 and / or different spectral filters positioned in the different collection channels 144 may provide separate images of orthogonal target features.

[0080] 7A and 7B illustrate direction-separated imaging in a dark-field imaging mode, according to one or more embodiments of the present disclosure. FIG. 7A is a conceptual diagram of a collection pupil plane 140 illustrating a configuration of an overlay metrology tool 102 focused by an objective lens 130, according to one or more embodiments of the present disclosure. In particular, FIG. 7A illustrates a configuration of an overlay metrology tool 102 (e.g., based on a metrology recipe) in which the collection pupil plane 140 includes a zeroth order diffraction lobe 702, a first order diffraction lobe 704, and a second order diffraction lobe 706 of a first illumination beam 106a due to a periodic structure in the X direction (e.g., cells 202b, d in FIG. 2A ), and a zeroth order diffraction lobe 708, a first order diffraction lobe 710, and a second order diffraction lobe 712 of a second illumination beam 106b due to a periodic structure in the Y direction (e.g., cells 202a, c in FIG. 2A ).

[0081] Figure 7B is a conceptual diagram of the collection pupil plane 140 of Figure 7A where the zeroth order diffraction lobe is blocked to provide dark field imaging, in accordance with one or more embodiments of the present disclosure. For example, Figure 7B illustrates a blocking portion 714 in the collection pupil plane 140 (e.g., a portion of the collection path optical element 138) for selectively blocking the zeroth order diffraction lobes 702, 708. This blocking portion 714 can generally have any shape and can be formed from any number of elements. As such, the particular configuration illustrated in Figure 7B is not intended to be limiting of the present disclosure.

[0082] In this configuration, the image (or a portion thereof) of the periodic structure in the X direction (e.g., cells 202b, d in FIG. 2A) is formed only by the first illumination beam 106a, while the image (or a portion thereof) of the periodic structure in the Y direction (e.g., cells 202a, c in FIG. 2A) is formed only by the second illumination beam 106b. In particular, by blocking the zeroth order diffraction lobes 702, 708, any residual effects of orthogonal structures are eliminated. Furthermore, the resulting dark field image will have perfect contrast (e.g., 100% contrast).

[0083] FIG. 8A is a conceptual diagram of the generation of various diffraction lobes according to one or more embodiments of the present disclosure. FIG. 8B is a conceptual diagram of a collection pupil plane 140 illustrating direction-separated imaging in an optical moiré mode based on the diffraction lobes shown in FIG. 8A according to one or more embodiments of the present disclosure. For example, the overlay target 104 may include grating superimposed features (e.g., moiré structures) having different pitches as shown in FIGS. 2B-2C, but includes different cells 202 with moiré structures oriented along orthogonal directions (e.g., X-direction and Y-direction). In particular, Figure 8B illustrates a configuration of the overlay metrology tool 102 (e.g., based on a metrology recipe) in which the collection pupil plane 140 includes a zeroth order diffraction lobe 802, a first order diffraction lobe 804 (labeled 1) of the first illumination beam 106a from the first set of periodic features 204 distributed along the X direction, and a first order diffraction lobe 806 (labeled 1') of the first illumination beam 106a from the second set of periodic features 206 distributed along the X direction. Figure 8B also illustrates a zeroth order diffraction lobe 808, a first order diffraction lobe 810 (labeled 1) of the second illumination beam 106b from the first set of periodic features 204 distributed along the Y direction, and a first order diffraction lobe 812 (labeled 1') of the second illumination beam 106b from the second set of periodic features 206 distributed along the Y direction.

[0084] FIG. 8C is a conceptual diagram of the collection pupil plane 140 of FIG. 8B including a blocking portion 814 for selectively blocking the zeroth diffraction order lobes 802, 808 in the collection pupil plane 140, according to one or more embodiments of the present disclosure. The blocking portion 814 may have any shape in general and may be formed from any number of elements. As such, the particular configuration shown in FIG. 8 is not intended to limit the present disclosure. In this configuration, the image (or a portion thereof) of the periodic structure in the X direction is formed only by the first illumination beam 106a, while the image (or a portion thereof) of the periodic structure in the Y direction is formed only by the second illumination beam 106b. In particular, blocking the zeroth diffraction order lobes 802, 808 eliminates any residual effects of orthogonal structures. Furthermore, as previously described herein, various discrimination parameters of the illumination beams 106a, b, such as but not limited to spectral content or polarization, can be used to further separate the images of the X and Y direction structures in one or two collection channels 144.

[0085] 4A-8C in general, it should be understood that the imaging configurations shown are provided for illustrative purposes only and should not be construed as limiting. For example, direction-separated imaging can be extended to other imaging modes that utilize different combinations of diffraction orders per direction. As another example, it may be desirable to provide images of symmetric oblique illumination conditions.

[0086] 9 is a conceptual diagram of an illumination pupil plane 128 including four illumination beams 106a,b,c,d positioned to provide symmetric oblique illumination along orthogonal azimuthal directions, in accordance with one or more embodiments of the present disclosure. For example, FIG. 9 may represent a symmetric variation of the illumination distribution of FIG. 4A.

[0087] In some embodiments, the overlay target 104 is illuminated with all four illumination beams 106a,b,c,d simultaneously, producing directionally separated images in one or more collection channels 144 as disclosed herein. For example, but not limited to, the oblique brightfield imaging mode shown in FIG. 6 may be suitable for simultaneous illumination by illumination beams 106a,b,c,d.

[0088] In some embodiments, the overlay target 104 is sequentially illuminated with symmetrically opposed illumination beams 106. For example, the overlay target 104 may be first illuminated with illumination beams 106a, b to form a first set of directionally separated images in one or more collection channels 144, and then illuminated with illumination beams 106c, d to form a second set of directionally separated images in one or more collection channels 144. The resulting images may then be analyzed together to provide an overlay measurement.

[0089] 1A and 1B, various additional components of the overlay metrology system 100, in accordance with one or more embodiments of the present disclosure, are described in greater detail.

[0090] The illumination source 108 may include any type of illumination source suitable for providing at least one illumination beam 106. In one embodiment, the illumination source 108 is a laser source. For example, the illumination source 108 may include, but is not limited to, one or more narrowband laser sources, broadband laser sources, ultra-broadband laser sources, white light laser sources, and the like. In this regard, the illumination source 108 may provide an illumination beam 106 having high coherence (e.g., high spatial coherence and / or temporal coherence). In another embodiment, the illumination source 108 includes a laser-sustained plasma (LSP) source. For example, the illumination source 108 may include, but is not limited to, an LSP lamp, an LSP bulb, or an LSP chamber suitable for housing one or more elements that may emit broadband illumination when excited into a plasma state by a laser source. In another embodiment, the illumination source 108 includes a lamp source. For example, the illumination source 108 may include, but is not limited to, an arc lamp, a discharge lamp, an electrodeless lamp, and the like. In this regard, the illumination source 108 may provide an illumination beam 106 having low coherence (eg, low spatial coherence and / or temporal coherence).

[0091] The overlay metrology tool 102 may generate the two or more illumination beams 106 using any technique known in the art. Furthermore, the illumination beams 106 may be arranged in any configuration, including but not limited to dipole illumination, orthogonal illumination, etc.

[0092] In some embodiments, the illumination source 108 directly generates the two or more illumination beams 106. In some embodiments, the overlay metrology tool 102 includes two or more apertures in an illumination pupil plane 128 (e.g., in the illumination path optical elements 126) to split the illumination from the illumination source 108 into two or more illumination beams 106 or illumination lobes. Furthermore, the spatial profile of the one or more illumination beams 106 on the sample 110 can be controlled to have any selected spatial profile by a field plane aperture.

[0093] In some embodiments, the illumination source 108 provides light to two or more optical fibers, where the light output from each optical fiber is an illumination lobe of the illumination beam 106. For example, the output faces of the optical fibers may be in or imaged to the illumination pupil plane 128. In some embodiments, the overlay metrology tool 102 diffracts illumination from the illumination source 108 into two or more diffraction orders, where the illumination beam 106 is formed from at least some of the diffraction orders of the light source. Efficient generation of multiple illumination lobes through controlled diffraction is generally described in U.S. Pat. No. 11,118,903, entitled “EFFICIENT ILLUMINATION SHAPING FOR SCATTEROMETRY OVERLAY,” issued Sep. 14, 2021, which is incorporated herein by reference in its entirety.

[0094] The overlay metrology tool 102 may generally include any number or type of detectors 114 suitable for capturing light from the sample 110 exhibiting overlay. In one embodiment, the detectors 114 include one or more detectors 114 suitable for evaluating properties of a stationary sample. In this regard, the overlay metrology tool 102 may operate in a stationary mode in which the sample 110 is stationary during the measurement. For example, the detectors 114 may include a two-dimensional pixel array, such as, but not limited to, a charge-coupled device (CCD) or a complementary metal-oxide semiconductor (CMOS) device. In this regard, the detectors 114 may generate a two-dimensional image in a single measurement. In one embodiment, the detectors 114 include one or more detectors 114 suitable for evaluating properties of a moving sample (e.g., a scanned sample), such as, but not limited to, a 2D pixel array, a time-delay integration (TDI) detector, etc. In this regard, the overlay metrology tool 102 may operate in a scanning mode in which the sample 110 is scanned with respect to a measurement field of view during the measurement.

[0095] In another embodiment, the overlay metrology tool 102 includes a scanning subsystem for scanning the sample 110 relative to a measurement field of view during metrology measurements. For example, the sample stage 132 can position and orient the sample 110 within a focal volume of the objective lens 130. In another embodiment, the sample stage 132 includes one or more adjustable stages, such as, but not limited to, a linear translation stage, a rotational stage, or a tip / tilt stage. In another embodiment, not shown, the scanning subsystem includes one or more beam scanning optical elements (e.g., rotatable mirrors, galvanometers, etc.) for scanning the illumination beam 106 relative to the sample 110.

[0096] 10 is a flow diagram illustrating steps performed in a method 1000 for direction-separated imaging, in accordance with one or more embodiments of the present disclosure. Applicant notes that the embodiments and enabling techniques previously described herein in the context of overlay metrology system 100 are to be construed as extending to method 1000. However, it is further noted that method 1000 is not limited to the architecture of overlay metrology system 100.

[0097] In some embodiments, the method 1000 includes illuminating 1002 an overlay target on the specimen with one or more first illumination beams and one or more second illumination beams, the overlay target including periodic features in a first direction in a first set of layers of the specimen distributed along a first measurement direction and periodic features in a second direction in a second set of layers of the specimen distributed along a second measurement direction different from the first measurement direction. In this manner, the overlay target can generate diffraction lobes of the first and second illumination beams in both the X and Y directions. Step 1002 may further include simultaneously illuminating the overlay target with the first and second illumination beams.

[0098] The overlay target may generally include any known overlay target design suitable for image-based diffraction. For example, the overlay target may include, but is not limited to, an advanced imaging metrology (AIM) target, a triple AIM target having features on three layers, a robust AIM (rAIM) target including one or more moiré structures, etc. Additionally, the overlay target may include periodic features on any number of layers, where the periodic features in one measurement direction (e.g., X-direction) need not be on the same layer as the periodic features in another measurement direction (e.g., Y-direction).

[0099] In some embodiments, the method 1000 includes generating 1004, with one or more collection optical elements, one or more images of the overlay target on one or more detectors based on diffraction of the one or more first illumination beams and the one or more second illumination beams by the overlay target, where diffraction orders of the one or more first illumination beams contribute to resolving images of only a first set of periodic features and diffraction orders of the one or more second illumination beams contribute to resolving images of only a second set of periodic features. In some embodiments, the collection optical elements, the overlay target, and the first and second illumination beams may be configured (e.g., according to a metrology recipe) such that diffraction orders of the first illumination beam by the periodic features in a first direction are collected, but diffraction orders of the first illumination beam by the periodic features in a second direction are not collected (e.g., are outside the NA of the collection optical elements, outside a boundary of a collection pupil, etc.). Similarly, the collection optics, overlay target, and first and second illumination beams may be configured (e.g., according to a metrology recipe) such that diffraction orders of the second illumination beam by the periodic features in the first direction are not collected, but diffraction orders of the second illumination beam by the periodic features in the second direction are collected. In this way, periodic structures in the first direction are resolvable only based on the diffraction of the first illumination beam, while periodic structures in the second direction are resolvable only based on the diffraction of the second illumination beam.

[0100] In some embodiments, the method 1000 includes step 1006 of generating a first overlay measurement along a first measurement direction based on the one or more images. For example, step 1006 may include generating the overlay measurement based on periodic features resolved in a first direction based on the first illumination beam. In some embodiments, the method 1000 includes step 1008 of generating a second overlay measurement along a second measurement direction based on the one or more images. For example, step 1008 may include generating the overlay measurement based on periodic features resolved in a second direction based on the second illumination beam.

[0101] The subject matter described herein illustrates various components that are sometimes included within or connected to other components. It should be understood that such configurational manners depicted are merely exemplary, and that in fact many other configurational manners that achieve the same functionality can be implemented. In a conceptual sense, configurations of components that achieve the same functionality are effectively "associated" such that the desired functionality is achieved. Thus, any two components that are combined herein to achieve a particular functionality can be considered to be "associated with" each other such that the desired functionality is achieved, regardless of the configurational manner or intervening components. Similarly, any two components so associated can also be considered to be "connected" or "coupled" to each other such that the desired functionality is achieved, and any two components so associateable can also be considered to be "couplable" to each other such that the desired functionality is achieved. Specific examples of being couplable include, but are not limited to, physically interactable and / or physically interacting components, and / or wirelessly interactable and / or wirelessly interacting components, and / or logically interactable and / or logically interacting components.

[0102] It will be believed that the present disclosure and many of its attendant advantages will be understood from the foregoing description, and it will be apparent that various changes in form, construction and arrangement of the elements may be made without departing from the disclosed subject matter or sacrificing all of its important advantages. The described forms are merely illustrative, and it is intended that such modifications be embraced and included within the scope of the following claims. It will be further understood that the invention is defined by the appended claims.

Claims

1. 1. An overlay metrology tool configurable to perform a metrology recipe for evaluating a property of an overlay target on a specimen, the overlay target according to the metrology recipe comprising periodic features in a first direction in a first set of layers of the specimen, the periodic features in the first direction being distributed along a first measurement direction, the overlay target according to the metrology recipe further comprising periodic features in a second direction in a second set of layers of the specimen, the periodic features in the second direction being distributed along a second measurement direction different from the first measurement direction, and the overlay metrology tool according to the metrology recipe: one or more illumination optical elements configurable to illuminate the overlay target with one or more first illumination beams and one or more second illumination beams, where at least one of the first illumination beams and at least one of the second illumination beams are simultaneously incident on the overlay target; and one or more collection optical elements configurable to generate one or more images of the overlay target on one or more detectors based on diffraction of the one or more first illumination beams and the one or more second illumination beams by the overlay target, where diffraction orders of the one or more first illumination beams contribute to resolving images of only periodic features in the first direction and diffraction orders of the one or more second illumination beams contribute to resolving images of only periodic features in the second direction. an overlay metrology tool comprising: a controller communicatively coupled to the one or more detectors, the controller comprising one or more processors configured to execute program instructions that cause the one or more processors to: generating a first overlay measurement along the first measurement direction based on the one or more images; and generating second overlay measurements based on the one or more images along the second measurement direction; A control device that causes the An optical measurement system comprising:

2. 2. The optical metrology system of claim 1, wherein the first illumination beam and the second illumination beam are distinguished by at least one of an azimuthal angle of incidence on the overlay target and an elevational angle of incidence on the overlay target, wavelength, bandwidth, polarization, intensity, numerical aperture at an illumination pupil of the one or more illumination optical elements, lobe shape at the illumination pupil, or tilt.

3. 2. The optical metrology system of claim 1, wherein the first illumination beam comprises a single first illumination beam providing a first azimuthal angle of incidence on the overlay target, and the second illumination beam comprises a single second illumination beam providing a second azimuthal angle of incidence on the overlay target.

4. 4. The optical metrology system of claim 3, wherein the first and second azimuthal angles of incidence are orthogonal.

5. 2. The optical metrology system of claim 1, wherein the first illumination beam comprises a first set of two illumination beams providing a first set of symmetrically opposed azimuthal angles of incidence on the overlay target, and the second illumination beam comprises a second set of two illumination beams providing a second set of symmetrically opposed azimuthal angles of incidence on the overlay target.

6. 6. The optical metrology system of claim 5, wherein the first and second sets of azimuthal incidence angles are orthogonal.

7. 6. The optical metrology system of claim 5, wherein the one or more illumination optical elements direct the first set of two illumination beams and the second set of two illumination beams simultaneously to the overlay target when performing the metrology recipe.

8. 6. The optical metrology system of claim 5, wherein the one or more illumination optical elements direct one of the first set of two illumination beams and one of the second set of two illumination beams to the overlay target as a first exposure, and the one or more illumination optical elements direct the other of the first set of two illumination beams and the other of the second set of two illumination beams to the overlay target as a second exposure.

9. The optical metrology system of claim 1 , wherein the one or more collection optical elements include a single collection channel and the one or more detectors include a single detector.

10. 2. The optical metrology system of claim 1 , wherein the one or more collection optical elements include two collection channels, the one or more detectors include two detectors, each of the two collection channels includes one of the two detectors, the one or more collection optical elements separate diffraction orders from the first illumination beam into a first of the two collection channels and diffraction orders from the second illumination beam into a second of the two collection channels, the first of the two collection channels provides one or more images of the overlay target in which only periodic features in the first direction are resolved, and the second of the two collection channels provides one or more images of the overlay target in which only periodic features in the second direction are resolved.

11. 2. The optical metrology system of claim 1 , wherein in accordance with the metrology recipe, the overlay metrology tool comprises: two or more diffraction lobes along the first measurement direction generated by periodic features in the first direction from each of the one or more first illumination beams; two or more diffraction lobes along the second measurement direction generated by periodic features in the second direction from each of the one or more second illumination beams; 1. An optical measurement system comprising:

12. 2. The optical metrology system of claim 1 , wherein in accordance with the metrology recipe, the overlay metrology tool comprises: a zeroth order diffraction lobe and a first order diffraction lobe along the first measurement direction generated by a periodic feature in the first direction from each of the one or more first illumination beams; a zeroth order diffraction lobe and a first order diffraction lobe along the second measurement direction generated by a periodic feature in the second direction from each of the one or more second illumination beams; Exclusively focusing the 11. An optical metrology system, wherein one or more images of the overlay target are oblique bright field images.

13. 2. The optical metrology system of claim 1 , wherein in accordance with the metrology recipe, the overlay metrology tool comprises: a zeroth order diffraction lobe, a first order diffraction lobe, and a second order diffraction lobe along the first measurement direction generated by a periodic feature in the first direction from each of the one or more first illumination beams; a zeroth order diffraction lobe, a first order diffraction lobe, and a second order diffraction lobe along the second measurement direction generated by a periodic feature in the second direction from each of the one or more second illumination beams; Exclusively focusing the the one or more collection optical elements include one or more elements for blocking the zero order diffraction lobe along the first and second measurement directions; 11. An optical metrology system, comprising: one or more images of the overlay target that are dark field images.

14. 2. The optical metrology system of claim 1 , wherein the periodic feature in a first direction comprises a grating having a first pitch and a second pitch distributed along the first measurement direction such that a moire structure in a first direction is formed, and the periodic feature in a second direction comprises a grating having the first pitch and the second pitch distributed along the second measurement direction such that a moire structure in a second direction is formed, and according to the metrology recipe, the overlay metrology tool: a zero order diffraction lobe, a first order diffraction lobe from the grating having the first pitch in a Moire structure in the first direction, and a first order diffraction lobe from the grating having the second pitch in a Moire structure in the first direction, associated with each of the one or more first illumination beams; a zeroth order diffraction lobe associated with each of the one or more second illumination beams, a first order diffraction lobe from the grating having the first pitch in a Moire structure in the second direction, and a first order diffraction lobe from the grating having the second pitch in a Moire structure in the second direction; 1. An optical measurement system comprising:

15. 15. The optical metrology system of claim 14, wherein the one or more collection optical elements further include one or more elements for blocking the zero order diffraction lobe along the first and second measurement directions.

16. 2. The optical metrology system of claim 1, wherein the overlay target comprises: An optical metrology system comprising at least one of an advanced imaging metrology (AIM) target, a triple AIM target, or a robust AIM (rAIM) target.

17. 2. The optical metrology system of claim 1, wherein the first and second sets of sample layers are a common sample layer set.

18. 10. The optical metrology system of claim 1, wherein the first set of sample layers is different from the second set of sample layers.

19. 20. The optical metrology system of claim 18, wherein the first set of sample layers includes a first layer of the sample and a second layer of the sample, and the second set of sample layers includes the first layer of the sample and a third layer of the sample.

20. 2. The optical metrology system of claim 1 , wherein at least one of the first or second direction periodic features includes features on a single layer that are associated with different lithographic exposures.

21. one or more illumination optical elements configurable to illuminate an overlay target on a specimen with one or more first illumination beams and one or more second illumination beams when performing a metrology recipe, where at least one of the first illumination beams and at least one of the second illumination beams are simultaneously incident on the overlay target, where the overlay target according to the metrology recipe includes periodic features in a first direction in a first set of layers of the specimen, the periodic features in the first direction being distributed along a first measurement direction, and where the overlay target according to the metrology recipe further includes periodic features in a second direction in a second set of layers of the specimen, the periodic features in the second direction being distributed along a second measurement direction different from the first measurement direction; one or more collection optical elements configurable to generate one or more images of the overlay target on one or more detectors based on diffraction of the one or more first illumination beams and the one or more second illumination beams by the overlay target when performing the metrology recipe, where diffraction orders of the one or more first illumination beams contribute to forming a resolved image of only periodic features in the first direction and diffraction orders of the one or more second illumination beams contribute to forming a resolved image of only periodic features in the second direction; a controller communicatively coupled to the one or more detectors, the controller comprising one or more processors configured to execute program instructions that cause the one or more processors to: generating a first overlay measurement along the first measurement direction based on the one or more images; and generating second overlay measurements based on the one or more images along the second measurement direction; A control device that causes the An optical measurement system comprising:

22. illuminating an overlay target on a specimen with one or more first illumination beams and one or more second illumination beams, where at least one of the first illumination beams and at least one of the second illumination beams are simultaneously incident on the overlay target, the overlay target according to a metrology recipe comprising periodic features in a first direction in a first set of layers of the specimen, the periodic features in the first direction being distributed along a first measurement direction, and the overlay target according to the metrology recipe further comprising periodic features in a second direction in a second set of layers of the specimen, the periodic features in the second direction being distributed along a second measurement direction different from the first measurement direction; generating one or more images of the overlay target on one or more detectors based on diffraction of the one or more first illumination beams and the one or more second illumination beams by the overlay target, where diffraction orders of the one or more first illumination beams contribute to resolving images of only periodic features in the first direction and diffraction orders of the one or more second illumination beams contribute to resolving images of only periodic features in the second direction; generating a first overlay measurement along the first measurement direction based on the one or more images; generating a second overlay measurement based on the one or more images along the second measurement direction; and An optical measurement method comprising:

23. 23. The optical metrology method of claim 22, wherein the first illumination beam and the second illumination beam are distinguished by at least one of an azimuthal angle of incidence on the overlay target and an elevational angle of incidence on the overlay target, wavelength, bandwidth, polarization, intensity, numerical aperture at an illumination pupil of one or more illumination optical elements, lobe shape at the illumination pupil, or tilt.

24. 23. The optical metrology method of claim 22, wherein the first illumination beam comprises a single first illumination beam providing a first azimuthal angle of incidence on the overlay target, and the second illumination beam comprises a single second illumination beam providing a second azimuthal angle of incidence on the overlay target.

25. 23. The optical metrology method of claim 22, wherein the first illumination beam comprises a first set of two illumination beams providing a first set of symmetrically opposed azimuthal angles of incidence on the overlay target, and the second illumination beam comprises a second set of two illumination beams providing a second set of symmetrically opposed azimuthal angles of incidence on the overlay target.

26. 26. The optical metrology method of claim 25, wherein illuminating the overlay target on the specimen with the one or more first illumination beams and the one or more second illumination beams comprises:

13. An optical metrology method comprising: simultaneously directing the first set of two illumination beams and the second set of two illumination beams onto the overlay target.

27. 26. The optical metrology method of claim 25, wherein illuminating the overlay target on the specimen with one or more first illumination beams and one or more second illumination beams comprises: directing one of the first set of two illumination beams and one of the second set of two illumination beams toward the overlay target as a first exposure; directing another of the first set of two illumination beams and another of the second set of two illumination beams toward the overlay target as a second exposure; An optical measurement method comprising:

28. 24. The optical metrology method of claim 23, wherein generating one or more images of the overlay target on the one or more detectors based on the diffraction of the one or more first illumination beams and the one or more second illumination beams by the overlay target using one or more focusing optical elements comprises: the one or more images of the overlay target; two or more diffraction lobes along the first measurement direction generated by periodic features in the first direction from each of the one or more first illumination beams; two or more diffraction lobes along the second measurement direction generated by periodic features in the second direction from each of the one or more second illumination beams; 4. An optical metrology method comprising: generating an optical signal using exclusively

29. 24. The optical metrology method of claim 23, wherein generating one or more images of the overlay target on one or more detectors based on diffraction of the one or more first illumination beams and the one or more second illumination beams by the overlay target using one or more focusing optical elements comprises: the one or more images of the overlay target; a zeroth order diffraction lobe and a first order diffraction lobe along the first measurement direction generated by a periodic feature in the first direction from each of the one or more first illumination beams; a zeroth order diffraction lobe and a first order diffraction lobe along the second measurement direction generated by a periodic feature in the second direction from each of the one or more second illumination beams; Includes exclusively generating using 11. A method of optical metrology, comprising: detecting a position of said overlay target relative to said one or more images;

30. 24. The optical metrology method of claim 23, wherein generating one or more images of the overlay target on one or more detectors based on diffraction of the one or more first illumination beams and the one or more second illumination beams by the overlay target using one or more focusing optical elements comprises: the one or more images of the overlay target; a zeroth order diffraction lobe, a first order diffraction lobe, and a second order diffraction lobe along the first measurement direction generated by a periodic feature in the first direction from each of the one or more first illumination beams; a zeroth order diffraction lobe, a first order diffraction lobe, and a second order diffraction lobe along the second measurement direction generated by a periodic feature in the second direction from each of the one or more second illumination beams; Includes exclusively generating using the one or more collection optical elements include one or more elements for blocking the zero order diffraction lobe along the first and second measurement directions; 11. A method of optical metrology, comprising: detecting a position of said overlay target relative to said optical axis; ...

31. 24. The optical metrology method of claim 23, wherein the first direction periodic feature comprises a grating having a first pitch and a second pitch distributed along the first measurement direction such that a Moire structure in a first direction is formed, and the second direction periodic feature comprises a grating having the first pitch and the second pitch distributed along the second measurement direction such that a Moire structure in a second direction is formed, and generating, using one or more collection optical elements, one or more images of the overlay target on one or more detectors based on diffraction of the one or more first illumination beams and the one or more second illumination beams by the overlay target, further comprises: the one or more images of the overlay target; a zero order diffraction lobe, a first order diffraction lobe from the grating having the first pitch in a Moire structure in the first direction, and a first order diffraction lobe from the grating having the second pitch in a Moire structure in the first direction, associated with each of the one or more first illumination beams; a zeroth order diffraction lobe associated with each of the one or more second illumination beams, a first order diffraction lobe from the grating having the first pitch in a Moire structure in the second direction, and a first order diffraction lobe from the grating having the second pitch in a Moire structure in the second direction; 4. An optical metrology method comprising: generating an optical signal using exclusively

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