Patent Publication Number: US-2023133640-A1

Title: Moiré scatterometry overlay

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Application Serial No. 63/273,322, filed Oct. 29, 2021 entitled MOIRE SCATTEROMETRY OVERLAY, naming Andrew V. Hill, Vladimir Levinsky, Amnon Manassesn, and Yuri Paskover as inventors, which is incorporated herein by reference in the entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates generally to scatterometry overlay metrology and, more particularly, to scatterometry overlay metrology with Moire targets. 
     BACKGROUND 
     Overlay metrology generally refers to measurements of the relative alignment of layers on a sample such as, but not limited to, semiconductor devices. An overlay measurement, or a measurement of overlay error, typically refers to a measurement of the misalignment of fabricated features two or more sample layers. In a general sense, proper alignment of fabricated features on multiple sample layers is necessary for proper functioning of the device. 
     Demands to decrease feature size and increase feature density are resulting in correspondingly increased demand for accurate and efficient overlay metrology. One approach to increasing the efficiency and throughput of an overlay metrology tool is based on scanning overlay metrology in which metrology data is generated on a sample as it is in motion. In this way, time delays associated with settling of the translation stage prior to a measurement may be eliminated. Metrology systems typically generate metrology data associated with a sample by measuring or otherwise inspecting dedicated overlay targets distributed across the sample. However, existing scanning overlay metrology techniques are unsuitable for use on advanced overlay target designs. Therefore, it is desirable to provide systems and methods for curing the above deficiencies. 
     SUMMARY 
     An overlay metrology system is disclosed in accordance with one or more illustrative embodiments of the present disclosure. In one illustrative embodiment, the system includes a translation stage to scan a sample including a Moire overlay target along a scan direction, where the Moire overlay target includes one or more inverted Moire structure pairs, and where a particular one of the one or more inverted Moire structure pairs includes a first Moire structure and a second Moire structure, each including an upper grating on a first layer of the sample and a lower grating on a second layer of the sample that overlaps the upper grating. In another illustrative embodiment, the upper grating of the first Moire structure and the lower grating of the second Moire structure have a first pitch along a particular measurement direction, where the lower grating of the first Moire structure and the upper grating of the second Moire structure have a second pitch different than the first pitch. In another illustrative embodiment, the system includes an illumination sub-system to illuminate the first and second Moire structures of one of the one or more inverted Moire structure pairs with common mutually coherent illumination beam distributions. In another illustrative embodiment, the system includes an objective lens to capture at least +/-1 diffraction orders from the upper and lower gratings of the illuminated one of the one or more inverted Moire structure pairs as collected light, where a first pupil plane includes overlapping pupil-plane distributions of the collected light from the first and second Moire structures with an interference pattern associated with a relative wavefront tilt between the collected light from the first and second Moire structures. In another illustrative embodiment, the system includes a diffractive element located in the first pupil plane, where one diffraction order of the collected light associated with the first Moire structure and one diffraction order of the collected light associated with the second Moiré structure overlap at a common overlap region in a field plane. In another illustrative embodiment, the system includes a collection field stop located in the field plane to pass light in the common overlap region and block remaining light. In another illustrative embodiment, the system includes one or more detectors located at a second pupil plane, where the second pupil plane includes the overlapping pupil-plane distributions of the collected light from the first and second Moire structures without the interference pattern associated with the relative wavefront tilt between the collected light from the first and second Moire structures. In another illustrative embodiment, the system includes a controller to determine an overlay measurement between the first sample layer and the second sample layer along the particular measurement direction based on data from the one or more detectors. 
     An overlay metrology method is disclosed in accordance with one or more illustrative embodiments of the present disclosure. In one illustrative embodiment, the method includes translating a sample including a Moire overlay target along a scan direction, where the Moire overlay target includes one or more inverted Moire structure pairs, and where a particular one of the one or more inverted Moire structure pairs includes a first Moire structure and a second Moire structure, each including an upper grating on a first layer of the sample and a lower grating on a second layer of the sample that overlaps the upper grating. In another illustrative embodiment, the upper grating of the first Moire structure and the lower grating of the second Moire structure have a first pitch along a particular measurement direction, where the lower grating of the first Moire structure and the upper grating of the second Moire structure have a second pitch different than the first pitch. In another illustrative embodiment, the method includes illuminating the first and second Moire structures of one of the one or more inverted Moire structure pairs with common mutually coherent illumination beam distributions. In another illustrative embodiment, the method includes capturing at least +/-1 diffraction orders from the upper and lower gratings of the illuminated one of the one or more inverted Moire structure pairs as collected light, where a first pupil plane includes overlapping pupil-plane distributions of the collected light from the first and second Moire structures with an interference pattern associated with a relative wavefront tilt between the collected light from the first and second Moire structures. In another illustrative embodiment, the method includes diffracting, with a diffractive element located in the first pupil plane, the collected light from the first and second Moire structures, where one diffraction order of the collected light associated with the first Moire structure and one diffraction order of the collected light associated with the second Moiré structure overlap at a common overlap region in a field plane. In another illustrative embodiment, the method includes selectively passing light in a common overlap region and blocking remaining light. In another illustrative embodiment, the method includes detecting at least a portion of a distribution of light at a second pupil plane with one or more detectors, where the second pupil plane includes the overlapping pupil-plane distributions of the collected light from the first and second Moire structures without the interference pattern associated with the relative wavefront tilt between the collected light from the first and second Moire structures. In another illustrative embodiment, the method includes determining an overlay measurement between the first sample layer and the second sample layer along the particular measurement direction based on data from the one or more detectors. 
     An overlay metrology system is disclosed in accordance with one or more illustrative embodiments of the present disclosure. In one illustrative embodiment, the system includes an illumination sub-system to illuminate an inverted Moire structure pair on a sample, where the inverted Moire structure pair includes a first Moire structure and a second Moire structure, each including an upper grating on a first layer of the sample and a lower grating on a second layer of the sample that overlaps the upper grating. In another illustrative embodiment, the upper grating of the first Moire structure and the lower grating of the second Moire structure have a first pitch along a particular measurement direction, where the lower grating of the first Moire structure and the upper grating of the second Moire structure have a second pitch along the particular measurement direction different than the first pitch. In another illustrative embodiment, the illumination sub-system illuminates the first and second Moire structures with common mutually coherent illumination beam distributions. In another illustrative embodiment, the system includes an objective lens to capture at least +/-1 diffraction orders from the upper and lower gratings of the illuminated one of the one or more inverted Moire structure pairs as collected light, where a first pupil plane includes overlapping pupil-plane distributions of the collected light from the first and second Moire structures with an interference pattern associated with a relative wavefront tilt between the collected light from the first and second Moire structures. In another illustrative embodiment, the system includes a diffractive element located in the first pupil plane, where one diffraction order of the collected light associated with the first Moire structure and one diffraction order of the collected light associated with the second Moiré structure overlap at a common overlap region in a field plane. In another illustrative embodiment, the system includes a collection field stop located in the field plane to pass light in the common overlap region and block remaining light. In another illustrative embodiment, the system includes one or more detectors to capture the one diffraction order of the collected light associated with the first Moire structure and one diffraction order of the collected light associated with the second Moire structure passed by the collection field stop. 
     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 the specification, illustrate embodiments of the invention and together with the general description, serve to explain the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The numerous advantages of the disclosure may be better understood by those skilled in the art by reference to the accompanying figures in which: 
         FIG.  1 A  is a block diagram view of a scanning overlay system, in accordance with one or more embodiments of the present disclosure. 
         FIG.  1 B  is a schematic view of the overlay metrology tool, in accordance with one or more embodiments of the present disclosure. 
         FIG.  1 C  is a schematic view of optical components suitable for generating two illumination beams through diffraction, in accordance with one or more embodiments of the present disclosure. 
         FIG.  1 D  is a schematic view of the wavefront-tilt mitigation optics, in accordance with one or more embodiments of the present disclosure. 
         FIG.  1 E  is a schematic view of the overlay metrology tool in which the wavefront-tilt mitigation optics are located on an optical path common to the illumination sub-system and the collection sub-system, in accordance with one or more embodiments of the present disclosure. 
         FIG.  2 A  is a side view of an inverted Moire structure pair, in accordance with one or more embodiments of the present disclosure. 
         FIG.  2 B  is a top view of a scanning Moire overlay target including two pairs of inverted Moire structures having periodicities along two orthogonal directions, in accordance with one or more embodiments of the present disclosure. 
         FIG.  3    includes conceptual views of non-limiting example illumination distributions of illumination beams in an illumination pupil, in accordance with one or more embodiments of the present disclosure. 
         FIG.  4 A  is a conceptual view of a non-overlapping pupil plane distribution of collected light from a Moire structure based on illumination at a normal incidence angle, in accordance with one or more embodiments of the present disclosure. 
         FIG.  4 B  is a conceptual view of a partially-overlapping pupil plane distribution of collected light from a Moire structure based on illumination at a normal incidence angle, in accordance with one or more embodiments of the present disclosure. 
         FIG.  5    is a flow diagram illustrating steps performed in a method for scanning overlay metrology, in accordance with one or more embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the subject matter disclosed, which is illustrated in the accompanying drawings. The present disclosure has been particularly shown and described with respect to certain embodiments and specific features thereof. The embodiments set forth herein are taken to be illustrative rather than limiting. It should be readily apparent to those of ordinary skill in the art that various changes and modifications in form and detail may be made without departing from the spirit and scope of the disclosure. 
     Embodiments of the present disclosure are directed to systems and methods for scatterometry overlay metrology using Moire targets. For the purposes of the present disclosure, the term scatterometry metrology is used to broadly encompass the terms scatterometry-based metrology and diffraction-based metrology in which a sample having periodic features on one or more sample layers is illuminated with an illumination beam having a limited angular extent, and one or more distinct diffraction orders are collected for measurement. Further, the term scanning metrology is used to describe metrology measurements generated when samples are in motion. In a general sense, scanning metrology may be implemented by scanning a sample along a measurement path (e.g., a swath, or the like) such that regions of interest on the sample (e.g., metrology targets, device areas, or the like) are translated through a measurement field of view of a metrology system. Further, the process may be repeated for any number of measurement paths or repeated measurements of particular measurement paths to provide any desired number of measurements of the sample. 
     It is contemplated herein that scatterometry overlay metrology is commonly performed using a scatterometry overlay target including one or more grating-over-grating structures formed as diffraction gratings with common pitches (e.g., periods) and directions of periodicity on two sample layers in an overlapping region. In particular, a scatterometry overlay target may commonly include multiple cells, where each cell has a grating-over-grating structure with a different intended overlay offset (e.g., a programmed overlay offset). In this configuration, an overlay measurement of actual overlay errors between the associated sample layers may be determined based on analysis of the multiple cells. Such targets may be characterized through analysis of pupil-plane images and/or field-plane images of the grating-over-grating structures and may further be characterized in static (e.g., Move-and-Measure) or scanning measurement modes. Various techniques for determining an overlay measurement with such scatterometry overlay targets are generally described in U.S. Pat. Application No. 17/068,328 filed on Oct. 12, 2020, which is incorporated herein by reference in their entirety. 
     It is further contemplated herein that overlay targets including grating-over-grating structures in which the overlapping gratings have different pitches (herein referred to as Moire structures) may take advantage of the Moire effect in which a Moire diffraction pattern is generated. This Moire diffraction pattern may be related to overlay by a gain factor associated with a difference between the pitches of the constituent gratings. As a result, such overlay targets may provide highly sensitive overlay measurements. Overlay metrology using overlay targets with Moire grating structures is generally described in U.S. Pat. Application 16/935,117 filed on Jul. 21, 2020 and U.S. Pat. Publication No. 2021/0072650 published on Mar. 11, 2021, both of which are incorporated herein by reference in their entirety. 
     However, overlay targets including Moire structure targets are commonly characterized using static measurement modes. In particular, because the pitches of the constituent gratings of a Moire structure are different, the offsets between individual lines of the constituent gratings vary across a cell. As a result, the apparent overlay (or apparent overlay error) may vary based on the particular location of an illumination beam used to characterize the target. Existing scanning scatterometry overlay techniques developed for traditional grating-over-grating structures with equal pitches may thus be unsuitable for characterization of Moire targets. 
     Embodiments of the present disclosure are directed to scanning scatterometry techniques suitable for characterization of Moire targets (e.g., targets including at least one Moire structure). In particular, some embodiments are directed to characterizing an overlay target including at least one pair of cells including Moire structures with inverted grating pitch configurations (e.g., a pair of inverted Moire structures or an inverted Moire structure pair). For example, a first Moire structure in a pair may include upper-layer grating features having a first pitch and lower-layer grating features having a second pitch, whereas a second Moire structure in the pair may include upper-layer grating structures with the second pitch and lower-layer grating features with the first pitch. 
     In some embodiments, a pair of inverted Moire structures is simultaneously illuminated with two spatially and temporally mutually coherent illumination beams, and at least first-order diffraction from both the first-layer and second-layer gratings is collected by an objective lens. Further, various measurement conditions such as, but not limited to, the wavelength of the illumination beams, incidence angles of the illumination beams, or the pitches of constituent gratings in the inverted Moire structures may be selected to provide that first-order diffraction from gratings with both the first and second pitches is collected and at least partially non-overlapping in a pupil plane. 
     It is noted that diffraction from gratings in the inverted Moire structures with equal pitch will overlap in the pupil plane. For example, diffraction from the first-layer grating of the first Moire structure in a pair and diffraction from the second-layer grating of the second Moire structure in the pair will overlap, and so on. Since the illumination beam’s incident on the first and second Moire structures are mutually coherent, the overlapping diffraction orders will interfere. However, the spatial separation of the first and second Moire structures will result in a relative phase tilt in the pupil plane associated with the wavefronts of the first and second Moire structures that may be present in the associated interference patterns. 
     In some embodiments, a diffractive element is positioned in a collection pupil plane to diffract the wavefronts from the first and second Moire structures of a pair to a common field region at a collection field plane to remove the relative phase tilts between the associated wavefronts. For example, the pupil-plane diffractive element may provide that a +1 diffraction order associated with collected light from one Moire structure in the inverted pair and a -1 diffraction order associated with collected light from the other Moire structure in the inverted pair may overlap in the common field region in the collection field plane. Further, a field stop at the collection field plane may selectively pass light in a common field region in which the relative phase tilts have been corrected and block remaining light to isolate the light in this common field region. 
     In some embodiments, a detector is placed at a pupil plane (e.g., a plane conjugate to the pupil plane including the diffraction element). In this way, the detector may generate a pupil image based on light from the common field region in which the relative phase tilts between the wavefronts from the first and second Moire structures have been corrected. As a result, the distribution of light at the plane of the detector may be similar to the distribution at the plane of the diffractive element, but without the effects of the relative wavefront tilt associated with the spatial separation of the two inverted Moire structures. For example, the distribution of light at the plane of the detector may include at least first-order diffraction from the gratings with the first and second pitches in the inverted pair of Moire structures, where diffraction from gratings with equal periods overlap, but diffraction from gratings with different periods is at least partially non-overlapping. 
     It is contemplated herein that the phase and intensity of the diffraction orders at the plane of the detector may vary based on the particular locations of the illumination beams due to the varying offsets between individual grating elements. However, in the measurement configurations disclosed herein, the phases of the wavefronts from the first and second Moire structures will vary synchronously as the sample is scanned along the scan direction. In this way, although the phases and intensities of the collected overlapping diffraction orders may vary as the sample is scanned, differences between the phase and/or intensity between various diffraction orders may remain constant as the sample is scanned. 
     Various techniques may be utilized to determine overlay measurements based on one or more pupil plane images from the detector. In some embodiments, overlay measurements are generated by comparing relative intensities in opposing lobes from one grating pitch to the relative intensities in the opposing lobes from the other grating pitch. In some embodiments, the position of one illumination beam is varied along a direction of periodicity for one of the Moire structures in an inverted Moire structure pair relative to the other Moire structure during a scan. In this way, the phase of interference between wavefronts of the first and second Moire structures may modulate during a scan, and an overlay measurement may be generated based on measurements of this phase modulation. For example, multiple pupil images may be generated during a scan to capture the interference between the wavefronts of the first and second Moire structures at different relative positions of the two illumination beams to capture the phase modulation for the determination of overlay. 
     Referring now to  FIGS.  1 A- 5   , systems and methods for scanning overlay metrology using inverted pairs of Moire structures are described in greater detail, in accordance with one or more embodiments of the present disclosure. 
       FIG.  1 A  is a block diagram view of a scanning overlay system  100 , in accordance with one or more embodiments of the present disclosure. 
     In some embodiments, the scanning overlay system  100  includes an overlay metrology tool  102  to illuminate a pair of inverted Moire structures (e.g., an inverted Moire structure pair  104 ) on a sample  106  with mutually-coherent illumination beams  108  (or distributions of illumination beams  108 ) and one or more detectors  110  in a collection pupil to capture light (e.g., collected light  112 ) associated with diffraction from the inverted Moire structure pair  104 . The overlay metrology tool  102  may further include one or more optical elements to remove a relative phase tilt between collected light  112  from the constituent Moire structures of the inverted Moire structure pair  104 , which are referred to herein as wavefront-tilt mitigation optics  114 . 
     In some embodiments, the overlay metrology tool  102  includes a scanning sub-system  116  to scan the sample  106  with respect to a measurement field of the scanning Moire overlay target  214  during an overlay measurement. For example, the scanning sub-system  116  may include a translation stage to position and orient the sample  106  within a focal volume of the objective lens  142 , where the translation stage may include any number of actuators (e.g., linear, rotational, and/or angular tip/tilt actuators). By way of another example, the scanning sub-system  116  may include one or more beam-scanning optics (e.g., piezo-electric mirrors, micro-electro-mechanical system (MEMS) mirrors, rotatable mirrors, galvanometers, acousto-optic deflectors (AODs), or the like) to scan the illumination beams  108  with respect to the sample  106 ). In a general sense, the scanning sub-system  116  may thus actuate any combination of the sample  106  or the illumination beams  108  to provide a scan. 
       FIG.  2 A  is a side view of an inverted Moire structure pair  104 , in accordance with one or more embodiments of the present disclosure. In some embodiments, an inverted Moire structure pair  104  includes a first Moire structure  202   a  and a second Moire structure  202   b , each formed as a lower grating  204  on a first sample layer  206  and an upper grating  208  on a second sample layer  210 . The first sample layer  206  and the second sample layer  210  may be disposed on a substrate  212  with any number of additional sample layers above, below, or between them. Further, the upper grating  208  of the first Moire structure  202   a  and the lower grating  204  of the second Moire structure  202   b  may have a first pitch P1, whereas the lower grating  204  of the first Moire structure  202   a  and the upper grating  208  of the second Moire structure  202   b  may have a second pitch P2 that is different from the first pitch P1. In  FIG.  2 A , the first pitch P1 is greater than the second pitch P2, though it is to be understood that this configuration is illustrative and should not be interpreted as limiting. 
       FIG.  2 B  is a top view of a scanning Moire overlay target  214  including two pairs of inverted Moire structures  104  having periodicities along two orthogonal directions, in accordance with one or more embodiments of the present disclosure. In some embodiments, a scanning Moire overlay target  214  includes one or more measurement groups  216  distributed along a scan direction  218  (e.g., a direction along which a sample  106  is scanned during a scanning measurement), where a measurement group includes inverted Moire structure pairs  104  distributed along a transverse direction  220  orthogonal to the scan direction  218 . For example, the scanning Moire overlay target  214  in  FIG.  2 B  includes a first measurement group  216   a  with a first inverted Moire structure pair  104   a  with periodicity along a first measurement direction and a second measurement group  216   b  with a second inverted Moire structure pair  104   b  with periodicity along a second measurement direction (e.g., orthogonal to the first measurement direction). In this way, the scanning Moire overlay target  214  may facilitate overlay measurements along any direction within a plane of the sample  106 . It is recognized herein that a scanning Moire overlay target  214  including measurement groups  216  distributed along the scan direction  218  and pairs of inverted Moire structures  104  distributed along the transverse direction  220  within the measurement groups  216  may be well-suited for scanning metrology. For example, time-varying measurement errors may be consistent for the pairs of cells in any measurement group  216  as the sample  106  is scanned along the scan direction  218 . Metrology target designs suitable for scanning metrology are generally described in U.S. 11,073,768 issued on Jul. 27, 2021. 
       FIG.  1 B  is a schematic view illustrating the overlay metrology tool  102 , in accordance with one or more embodiments of the present disclosure. In some embodiments, the overlay metrology tool  102  includes an illumination source  118  configured to generate at least two illumination beams  108  (e.g., illumination lobes, or the like) for illumination of an inverted Moire structure pair  104 . The illumination from the illumination source  118  may include one or more selected wavelengths of light including, but not limited to, ultraviolet (UV) radiation, visible radiation, or infrared (IR) radiation. 
     The illumination source  118  may include any type of illumination source suitable for providing at least one illumination beam  108 . In some embodiments, the illumination source  118  is a laser source. For example, the illumination source  118  may include, but is not limited to, one or more narrowband laser sources, a broadband laser source, a supercontinuum laser source, a white light laser source, or the like. In this regard, the illumination source  118  may provide an illumination beam  108  having high coherence (e.g., high spatial coherence and/or temporal coherence). In some embodiments, the illumination source  118  includes a laser-sustained plasma (LSP) source. For example, the illumination source  118  may include, but is not limited to, an LSP lamp, an LSP bulb, or an LSP chamber suitable for containing one or more elements that, when excited by a laser source into a plasma state, may emit broadband illumination. In some embodiments, the illumination source  118  includes a lamp source. For example, the illumination source  118  may include, but is not limited to, an arc lamp, a discharge lamp, an electrode-less lamp, or the like. In this regard, the illumination source  118  may provide an illumination beam  108  having low coherence (e.g., low spatial coherence and/or temporal coherence). 
     In some embodiments, the overlay metrology tool  102  directs the illumination beams  108  to the inverted Moire structure pair  104  via an illumination sub-system  120 . The illumination sub-system  120  may include one or more optical components suitable for modifying and/or conditioning the illumination beam  108  as well as directing the illumination beam  108  to the inverted Moire structure pair  104 . In some embodiments, the illumination beams  108  underfill the constituent Moire structures of the inverted Moire structure pair  104 . 
     In some embodiments, the illumination sub-system  120  includes one or more illumination sub-system lenses  122  (e.g., to collimate the illumination beam  108 , to relay pupil and/or field planes, or the like). In some embodiments, the illumination sub-system  120  includes one or more illumination sub-system optics  124  to shape or otherwise control the illumination beam  108 . For example, the illumination sub-system optics  124  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., static mirrors, translatable mirrors, scanning mirrors, or the like). 
     In some embodiments, as illustrated in  FIG.  1 B , the overlay metrology tool  102  (e.g., the scanning sub-system  116  of the overlay metrology tool  102 ) may include both a translation stage  126  and one or more beam-scanning optics  128 . In this way, the overlay metrology tool  102  may independently adjust positions of one or more illumination beams  108  on the sample  106  with the beam-scanning optics  128  as the sample  106  is translated by the translation stage  126 . Further, beam-scanning optics  128  may be located at any suitable location in the overlay metrology tool  102  including, but not limited to, a pupil plane common to both the illumination sub-system  120  and a collection sub-system  130 . Pupil-plane beam scanning is generally described in U.S. Pat. Application No. 17/142,783 filed on Jan. 6, 2021, which is incorporated herein by reference in its entirety. 
     The illumination sub-system  120  may provide any number of illumination beams  108  in any distribution. As an illustration,  FIG.  3    includes conceptual views of non-limiting example illumination distributions of illumination beams  108  in an illumination pupil, in accordance with one or more embodiments of the present disclosure. For example, each Moire structure  202  in an inverted Moire structure pair  104  may be illuminated with a common mutually-coherent distribution of illumination beams  108  such as, but not limited to, those illustrated in  FIG.  3   . In particular,  FIG.  3    illustrates a first distribution  302  with single illumination beam  108  at a normal incidence angle, a second distribution  304  with two opposing illumination beams  108  in a dipole arrangement, a third distribution  306  with two opposing illumination beams  108  in a rotated dipole arrangement, a fourth distribution  308  with four illumination beams  108  in a quadrupole arrangement, and a fifth distribution  310  with four illumination beams  108  in a rotated quadrupole arrangement. 
     In some embodiments, the overlay metrology tool  102  illuminates each Moire structure  202  in an inverted Moire structure pair  104  with mutually coherent illumination beams  108  having the same illumination parameters (e.g., the same illumination and field plane distributions). In this way, the collected light  112  from the Moire structures  202  may also be the same. Further, in some embodiments, a size of each illumination beam  108  is smaller than the associated Moire structure  202  such that the Moire structures  202  are underfilled. 
     The illumination sub-system  120  may provide illumination beams  108  using any technique known in the art. In some embodiments, the illumination sub-system  120  includes one or more apertures at an illumination pupil plane to divide illumination from the illumination source  118  into two or more illumination beams  108 . In some embodiments, the illumination sub-system  120  generates illumination beams  108  by providing light in two or more optical fibers, where light output from each optical fiber provided at or directed to an illumination pupil to provide an illumination beam  108 . In some embodiments, the illumination source  118  generates illumination beams  108  by diffracting illumination from the illumination source  118  into two or more diffraction orders, where at least one of the diffraction orders forms at least one illumination beam  108 . Efficient generation of multiple illumination beams through controlled diffraction is generally described in U.S. Patent Publication No. US2020/0124408 titled Efficient Illumination Shaping for Scatterometry Overlay, which is incorporated herein by reference in its entirety. 
       FIG.  1 C  is a schematic view of optical components suitable for generating two illumination beams  108  through diffraction, in accordance with one or more embodiments of the present disclosure. In particular,  FIG.  1 C  illustrates a configuration of an illumination sub-system  120  including a diffractive element  132  at a pupil plane  134  and an illumination field stop  136  at a field plane  138 . In this configuration, the diffractive element  132  may diffract illumination  140  from the illumination source  118  (e.g., the illumination beams  108 ) and the illumination field stop  136  may include one or more apertures designed to selectively pass two diffraction orders and block remaining diffraction orders. For example,  FIG.  1 C  illustrates a configuration in which the illumination field stop  136  selectively passes +/- 1 diffraction orders of the illumination  140 . Further, the spacing of the +/- 1 diffraction orders and the corresponding one or more apertures may be designed to correspond to the size and spacing of an inverted Moire structure pair  104  such that each diffraction order passed by the illumination field stop  136  may correspond to an illumination beam  108  to be directed to a Moire structure  202 . 
     The diffractive element  132  may include any static or tunable diffraction element suitable for generating at least two diffraction orders of the illumination  140  such as, but not limited to, one or more diffraction gratings or one or more prisms. In some embodiments, the diffractive element  132  includes an acousto-optic deflector (AOD) to provide tunable diffraction. Further, in some embodiments, the diffractive element  132  includes an AOD in a Raman-Nath configuration such that the +1 and -1 diffraction orders are generated with equal efficiency. Accordingly, an RF drive frequency for the diffractive element  132  may determine diffraction angles for a given wavelength of illumination  140  and an RF drive amplitude may determine the diffraction efficiency into the +/-1 diffraction orders. 
     It is to be understood, however, that  FIG.  1 C  and the associated description are provided solely for illustrative purposes and should not be interpreted as limiting. Rather, the illumination sub-system  120  may include any optical components arranged to provide mutually coherent illumination of Moire structures  202  in an inverted Moire structure pair  104 . 
     Referring again to  FIG.  1 B , in some embodiments, the overlay metrology tool  102  includes an objective lens  142  to focus the illumination beams  108  onto an inverted Moire structure pair  104 . The objective lens  142  may further capture light emanating from the sample  106  (e.g., collected light  112 ). 
     In some embodiments, the overlay metrology tool  102  includes a collection sub-system  130  to direct at least a portion of the collected light  112  from the objective lens  142  to one or more detectors  110 . The collection sub-system  130  may include one or more optical elements suitable for modifying and/or conditioning the collected light  112  from the sample  106 . In some embodiments, the collection sub-system  130  includes one or more collection sub-system lenses  144  (e.g., to collimate the illumination beam  108 , to relay pupil and/or field planes, or the like), which may include, but is not required to include, the objective lens  142 . In some embodiments, the collection sub-system  130  includes one or more collection sub-system optics  146  to shape or otherwise control the collected light  112 . For example, the collection sub-system optics  146  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., static mirrors, translatable mirrors, scanning mirrors, or the like). 
     In some embodiments, the overlay metrology tool  102  includes at least one detector  110  at a detection plane  148  to capture collected light  112  associated with diffraction of light from the inverted Moire structure pair  104 , where the detection plane  148  corresponds to a collection pupil plane. In this way, diffraction orders from an inverted Moire structure pair  104  may be imaged or otherwise observed in the detection plane  148 . 
     The overlay metrology tool  102  may generally include any number or type of detectors  110  suitable for capturing light from the sample  106  indicative of overlay. In some embodiments, a detector  110  may include a two-dimensional pixel array (e.g., a focal plane array) such as, but not limited to, a charge-coupled device (CCD) or a complementary metal-oxide semiconductor (CMOS) device. In this way, a detector  110  may capture a pupil-plane image or a portion thereof. Further, such a detector  110  may provide a capture time and/or a refresh rate sufficient to capture one or more pupil images during a scan. In some embodiments, a detector  110  includes a single-pixel sensor such as, but not limited to, a photodiode. For example, the overlay metrology tool  102  may include two or more single-pixel detectors  110  in a pupil plane to capture diffracted light at particular locations in the pupil plane and thus capture light at particular diffraction angles. 
     The illumination sub-system  120  and the collection sub-system  130  of the overlay metrology tool  102  may be oriented in a wide range of configurations suitable for illuminating the sample  106  with the illumination beam  108  and collecting light emanating from the sample  106  in response to the incident illumination beam  108 . For example, as illustrated in  FIG.  1 B , the overlay metrology tool  102  may include a beamsplitter  150  oriented such that a common objective lens  142  may simultaneously direct the illumination beam  108  to the sample  106  and collect light from the sample  106 . By way of another example, the illumination sub-system  120  and the collection sub-system  130  may contain non-overlapping optical paths such that the illumination beams  108  are directed to the sample  106  outside of the objective lens  142 . 
     Further, the overlay metrology tool  102  may be configurable to generate measurements based on any number of recipes defining measurement parameters. For example, a recipe of an overlay metrology tool  102  may include, but is not limited to, parameters associated with an incidence illumination beam  108  (e.g., wavelength, angle of incidence, polarization, spot size, focal depth, or the like), parameters associated with a portion of collected light  112  that reaches a detector  110  (e.g., wavelength, collection angle, polarization, imaging depth, or the like), or parameters associated with a detector  110  (e.g., integration time, gain, or the like). 
     Referring now generally to  FIGS.  4 A- 4 B , the use of wavefront-tilt mitigation optics  114  to generate a wavefront-tilt-corrected pupil-plane distribution from an inverted Moire structure pair  104  at a common detection plane  148  is described in greater detail, in accordance with one or more embodiments of the present disclosure. 
     It is contemplated herein that a pupil-plane distribution of collected light  112  from each Moire structure  202  of an inverted Moire structure pair  104  may be substantially the same when illuminated with identical illumination conditions (e.g., illumination wavelength, angle, polarization, and the like), with the primary difference being a relative phase tilt associated with the spatial separation on the sample  106 . In some embodiments, a measurement recipe of the overlay metrology tool  102  is configured to provide that the first-order diffraction associated with the first and second pitches (e.g., P1 and P2 of the inverted Moire structure pair  104  illustrated in  FIG.  2 A ) are collected and are at least partially non-overlapping in the pupil plane. 
     As an illustration,  FIGS.  4 A and  4 B  include exemplary pupil-plane distributions.  FIG.  4 A  is a conceptual view of a non-overlapping pupil plane distribution of collected light  112  from a Moire structure  202  based on illumination at a normal incidence angle, in accordance with one or more embodiments of the present disclosure.  FIG.  4 B  is a conceptual view of a partially-overlapping pupil plane distribution of collected light  112  from a Moire structure  202  based on illumination at a normal incidence angle, in accordance with one or more embodiments of the present disclosure. For example, the pupil plane distributions in  FIGS.  4 A and  4 B  may be generated based on illumination with the first distribution  302  of  FIG.  3   . 
     In particular,  FIGS.  4 A and  4 B  illustrate zero-order diffraction  402  labeled as “D 1  0 &amp; D 2  0,” first-order diffraction  404  from gratings having the first pitch P1 (e.g., the upper grating  208  of the first Moire structure  202   a  and the lower grating  204  of the second Moire structure  202   b  in  FIG.  2 A ) labeled as “D 1  +1” and “D 1  -1,” and first-order diffraction  406  from gratings having the second pitch P2 (e.g., the lower grating  204  of the first Moire structure  202   a  and the upper grating  208  of the second Moire structure  202   b  in  FIG.  2 A ) labeled as “D 2  +1” and “D 2  -1.” Further,  FIG.  4 A  illustrates a configuration in which the first-order diffraction  404  from gratings having the first pitch P1 and the first-order diffraction  406  from gratings having the second pitch P2 are fully collected (e.g., not truncated) and fully non-overlapping. In contrast,  FIG.  4 B  illustrates a configuration in which the first-order diffraction  406  from gratings having the second pitch P2 is partially truncated (e.g., includes angles outside a boundary  408  of the collection pupil) and further partially overlap the first-order diffraction  404  from gratings having the first pitch P1. 
     Although not explicitly illustrated in  FIGS.  4 A and  4 B , diffraction orders from gratings with a common pitch emanating from different Moire structures  202  of an inverted Moire structure pair  104  may overlap, but may generate an interference pattern due to the differences in wavefront tilt associated with the spatial separation on the sample  106 . In some embodiments, the wavefront-tilt mitigation optics  114  remove or otherwise mitigate interference patterns associated with the relative wavefront tilt differences. 
     The wavefront-tilt mitigation optics  114  may generally include any combination of optical components suitable for removing or otherwise mitigating the interference patterns associated with the relative wavefront tilt differences. 
       FIG.  1 D  is a schematic view of the wavefront-tilt mitigation optics  114 , in accordance with one or more embodiments of the present disclosure. In some embodiments, the wavefront-tilt mitigation optics  114  include a diffractive element  152  in a pupil plane  154  and a collection field stop  156  at a field plane  158 . For example, the pupil plane  154  at which the diffractive element  152  is located may correspond to the pupil-plane distribution illustrated in  FIG.  4 A ,  FIG.  4 B , or a similar distribution in which an interference pattern associated with wavefront tilt is present. 
     In this configuration, the diffractive element  152  may diffract collected light  112  from each Moire structure  202  of an inverted Moire structure pair  104 . For example,  FIG.  1 D  illustrates collected light  112   a  associated with the first Moire structure  202   a  emanating from a field plane  160  (or the sample  106 ), which is diffracted into multiple diffraction orders labeled +1 A , 0 A , and -1 A . The collected light  112   b  associated with the second Moire structure  202   b  emanating from the field plane  160  (or the sample  106 ) is similarly diffracted into multiple diffraction orders labeled +1 B , 0 B , and -1 B . Although not explicitly shown for purposes of clarity, it is noted that the collected light  112  from each Moire structure  202  (e.g., collected light  112   a  and collected light  112   b ), along with each associated diffraction order (e.g., +1 A , 0 A , -1 A , +1 B , 0 8 , and -1 B , and so on) contains the full distribution of light diffracted by the Moire structures  202  that is collected by the objective lens  142 . 
     It is contemplated herein that the diffractive element  152  may be configured to provide that a positive diffraction order of collected light  112  from one Moire structure  202  overlaps with a corresponding negative diffraction order of collected light  112  from the other Moire structure  202  at a common overlap location at the field plane  158  and further contemplated herein that such a configuration may remove or otherwise compensate for the wavefront tilt associated with the spatial separation of the Moire structures  202 . In some embodiments, the collection field stop  156  at the field plane  158  includes one or more apertures configured to selectively pass light at the common overlap location and block remaining light. For example,  FIG.  1 D  illustrates a configuration in which the +1 A  and -1 B  diffraction orders overlap at a common overlap location corresponding to an optical axis (e.g., a center of the field plane  158 ) and are selectively passed by the collection field stop  156 . 
     As a result, the pupil-plane distribution at a detector  110  located at a subsequent pupil plane  162  (e.g., the detection plane  148 ) may be substantially the same as at the pupil plane  154  including the diffractive element  152  except that the interference pattern associated with the wavefront tilt is removed. 
     The diffractive element  152  may include any static or tunable diffraction element suitable for generating at least two diffraction orders of the collected light  112  such as, but not limited to, one or more diffraction gratings or one or more prisms. In some embodiments, the diffractive element  152  includes an AOD to provide tunable diffraction. Further, in some embodiments, the diffractive element  132  includes an AOD in a Raman-Nath configuration such that the +1 and -1 diffraction orders are generated with equal efficiency. Accordingly, an RF drive frequency for the diffractive element  132  may determine diffraction angles for a given wavelength of illumination  140  and an RF drive amplitude may determine the diffraction efficiency into the +/-1 diffraction orders. 
     Referring now to  FIG.  1 E , in some embodiments, the wavefront-tilt mitigation optics  114  are located on an optical path common to the illumination sub-system  120  and the collection sub-system  130  and are configured to simultaneously generate illumination beams  108  for illuminating an inverted Moire structure pair  104  and for mitigating wavefront tilt from the associated collected light  112 . 
     For example,  FIG.  1 E  is a schematic view of the overlay metrology tool  102  in which the wavefront-tilt mitigation optics  114  are located on an optical path common to the illumination sub-system  120  and the collection sub-system  130 , in accordance with one or more embodiments of the present disclosure. In particular,  FIG.  1 E  illustrates a configuration in which the wavefront-tilt mitigation optics  114  includes a diffraction grating  164  formed as an AOD in a Raman-Nath configuration positioned to simultaneously operate as the diffractive element  132  illustrated in  FIG.  1 C  and the diffractive element  152  illustrated in  FIG.  1 D . For example, the diffraction grating  164  may receive and diffract illumination  140  from the illumination source  118  (e.g., the illumination beams  108 ). The illumination field stop  136  may then pass +/- 1 diffraction orders of the illumination  140  as illustrated in  FIG.  1 C , which may be directed to an inverted Moire structure pair  104  on a sample  106  through the objective lens  142 . The collected light  112  from the objective lens  142  may then propagate back through the illumination field stop  136  to the diffraction grating  164  and be diffracted as illustrated in  FIG.  1 D . The collection field stop  156  may then pass overlapping light associated with one diffraction order from each Moire structure  202  to a detector  110  at the subsequent pupil plane  162  as further illustrated in  FIG.  1 D . 
     It is contemplated herein that the use of a common diffraction grating  164  for both the generation of mutually coherent illumination beams  108  to be directed to Moire structures  202  of an inverted Moire structure pair  104  and to mitigate wavefront tilt in the associated collected light  112  (e.g., as illustrated in  FIG.  1 E ) may facilitate straightforward alignment of the associated components and may provide a robust overlay metrology tool  102 . 
     Referring again generally to  FIGS.  1 A- 1 E , various additional components of the scanning overlay system  100  are described in greater detail, in accordance with one or more embodiments of the present disclosure. 
     As described previously herein, the overlay metrology tool  102  may be configurable according to various metrology recipes including parameters associated with illumination, collection, and or detection. It is contemplated herein that the overlay metrology tool  102  may be configured to implement such metrology recipes using any of a variety of techniques within the spirit and scope of the present disclosure. 
     In some embodiments, the overlay metrology tool  102  includes two or more channels in the illumination sub-system  120  and/or the collection sub-system  130 . For example, each channel may provide a different set of illumination and/or collection conditions. The overlay metrology tool  102  may then select any combination of one or more channels at a time for a particular measurement using shutters or any suitable switching technique. 
     As an illustration,  FIG.  1 E  illustrates two illumination channels  166 . For example, the illumination channels  166  may provide illumination beams  108  with different (e.g., orthogonal) polarizations, which may be suitable for, but not limited to, adjusting the polarization of the illumination beams  108  based on direction of periodicity of the Moire structures  202 . 
     In some embodiments, the scanning overlay system  100  includes a controller  168  communicatively coupled to the overlay metrology tool  102  and/or any components therein. In some embodiments, the controller  168  includes one or more processors  170 . For example, the one or more processors  170  may be configured to execute a set of program instructions maintained in a memory device  172 , or memory. The one or more processors  170  of a controller  168  may include any processing element known in the art. In this sense, the one or more processors  170  may include any microprocessor-type device configured to execute algorithms and/or instructions. 
     The one or more processors  170  of a controller  168  may include any processor or processing element known in the art. For the purposes of the present 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  170  may include any device configured to execute algorithms and/or instructions (e.g., program instructions stored in memory). In some embodiments, the one or more processors  170  may be embodied as a desktop computer, mainframe computer system, workstation, image computer, parallel processor, networked computer, or any other computer system configured to execute a program configured to operate or operate in conjunction with the scanning overlay system  100 , as described throughout the present disclosure. Moreover, different subsystems of the scanning overlay system  100  may include a processor or logic elements suitable for carrying out at least a portion of the steps described in the present disclosure. Therefore, the above description should not be interpreted as a limitation on the embodiments of the present disclosure but merely as an illustration. Further, the steps described throughout the present disclosure may be carried out by a single controller or, alternatively, multiple controllers. Additionally, the controller  168  may include one or more controllers housed in a common housing or within multiple housings. In this way, any controller or combination of controllers may be separately packaged as a module suitable for integration into scanning overlay system  100 . 
     The memory device  172  may include any storage medium known in the art suitable for storing program instructions executable by the associated one or more processors  170 . For example, the memory device  172  may include a non-transitory memory medium. By way of another example, the memory device  172  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 drive and the like. It is further noted that the memory device  172  may be housed in a common controller housing with the one or more processors  170 . In some embodiments, the memory device  172  may be located remotely with respect to the physical location of the one or more processors  170  and the controller  168 . For instance, the one or more processors  170  of the controller  168  may access a remote memory (e.g., server), accessible through a network (e.g., internet, intranet and the like). 
     The controller  168  may direct (e.g., through control signals) or receive data from the overlay metrology tool  102  or any components therein. The controller  168  may further be configured to perform any of the various process steps described throughout the present disclosure. 
     In some embodiments, the scanning overlay system  100  includes a user interface  174  communicatively coupled to the controller  168 . In some embodiments, the user interface  174  may include, but is not limited to, one or more desktops, laptops, tablets, and the like. In some embodiments, the user interface  174  includes a display used to display data of the scanning overlay system  100  to a user. The display of the user interface  174  may include any display known in the art. For example, the display may include, but is not limited to, a liquid crystal display (LCD), an organic light-emitting diode (OLED) based display, or a CRT display. Those skilled in the art should recognize that any display device capable of integration with a user interface  174  is suitable for implementation in the present disclosure. In some embodiments, a user may input selections and/or instructions responsive to data displayed to the user via a user input device of the user interface  174 . 
       FIG.  5    is a flow diagram illustrating steps performed in a method  500  for scanning overlay metrology, in accordance with one or more embodiments of the present disclosure. Applicant notes that the embodiments and enabling technologies described previously herein in the context of the scanning overlay system  100  should be interpreted to extend to method  500 . It is further noted, however, that the method  500  is not limited to the architecture of the scanning overlay system  100 . 
     In some embodiments, the method  500  includes a step  502  of translating a sample including a Moire overlay target with one or more inverted Moire structure pairs along a scan direction. For example, a Moire overlay target may include an inverted Moire structure pair  104  as illustrated in  FIGS.  2 A and  2 B . 
     In some embodiments, the method  500  includes a step  504  of illuminating the first and second Moire structures of one of the one or more inverted Moire structure pairs with common mutually coherent illumination beam distributions. In some embodiments, the method  500  includes a step  506  of capturing at least +/-1 diffraction orders from the upper and lower gratings of one of the one or more inverted Moire structure pairs as collected light, where a first pupil plane includes overlapping pupil-plane distributions of the collected light from the first and second Moire structures with an interference pattern associated with a relative wavefront tilt between the collected light from the first and second Moire structures. 
     For example, each of the common mutually coherent illumination beam distributions may correspond to, but are not limited to, one of the distributions illustrated in  FIG.  3   . Further, the distributions may be varied or otherwise selected based on the direction of periodicity of a particular inverted Moire structure pair  104  being illuminated. For example, the illumination beam distributions may be selected, perhaps along with illumination or collection parameters, to facilitate the collection of the at least +/- 1 diffraction orders from the upper and lower gratings of the illuminated first and second Moire structures having the first and second pitches. 
     In some embodiments, the method  500  includes a step  508  of diffracting, with a diffractive element located in the first pupil plane, the collected light from the first and second Moire structures. In some embodiments, the method  500  includes a step  510  of selectively passing one diffraction order of the collected light associated with the first Moire structure and one diffraction order of the collected light associated with the second Moire structure overlapping in a common overlap region in a collection field plane and blocking remaining light. For example, a lens or other optical element may generate a field-plane distribution of light emanating from the first pupil plane, which may include multiple diffracted orders of field-plane distributions of the collected light from each of the first and second Moire structures (e.g., as illustrated in  FIG.  1 D ). Further, the spacing between the diffraction orders (e.g., governed by a periodicity of the diffractive element in the first pupil plane, the wavelengths of the collected light, or the like) may be selected to provide that one diffraction order of the collected light associated with the first Moire structure and one diffraction order of the collected light associated with the second Moiré structure overlap in a common overlap region in the collection field plane. Accordingly, this overlapping light in the common overlap region may be selectively passed by a field stop located at the collection field plane having an aperture sized and positioned to correspond to the common overlap region. 
     In some embodiments, the method  500  includes a step  512  of detecting at least a portion of a distribution of light at a second pupil plane with one or more detectors, where the second pupil plane includes the overlapping pupil-plane distributions of the collected light from the first and second Moire structures without the interference pattern associated with the relative wavefront tilt between the collected light from the first and second Moire structures. The one or more detectors may include any suitable number or type of detectors such as, but not limited to, a two-dimensional sensor array or two or more single-pixel sensors. 
     In some embodiments, the method  500  includes a step  514  of determining an overlay measurement between the first sample layer and the second sample layer along the particular measurement direction based on data from the one or more detectors. 
     It is contemplated herein that the step  514  of determining the overlay measurement may be implemented in a variety of ways within the spirit and scope of the present disclosure. 
     In some embodiments, the overlay is determined based on intensity asymmetries in the second pupil plane. The intensities of the diffraction orders in the second pupil plane will depend on the interference of the overlapping wavefronts from the first and second Moire structures. For example, there may be constructive or destructive interference in each of the first diffraction orders (e.g., the +/-1 diffraction orders from the gratings with the first and second pitches in the first and second Moire structures), where the phases of the wavefronts will depend on the positions of illumination beams on the first and second Moire structures. It is contemplated herein that a lateral shift between the first and second sample layers induced by overlay errors may induce phase shifts of opposing diffraction orders with respect to each other. For example, overlay errors may induce phase shifts between +1 diffraction and -1 diffraction from the gratings with the first pitch as well as phase shifts between +1 diffraction and -1 diffraction from the gratings with the second pitch. Accordingly, overlay error may be determined by comparing relative intensities of opposing diffraction orders from gratings with the first pitch with relative intensities of opposing diffraction orders from gratings with the second pitch. 
     In some embodiments, the overlay is determined based on phase asymmetries in the second pupil plane generated by translating the illumination distributions on the first and second Moire structures with respect to each other along the direction of periodicity. It is contemplated herein that translating the illumination distributions in a common pattern (e.g., in tandem) on the first and second Moire structures may result in simultaneous modulations of interference associated with overlapping light in each diffraction order in the second pupil plane. However, translating the illumination distributions on the first and second Moire structures with respect to each other along the direction of periodicity may induce a modulation of the interference patterns. This interference modulation enables the extraction of the relative spatial phases of the upper and lower gratings of the first and second Moire structures and thus extraction of the overlay error between the first and second sample layers. 
     For example, multiple measurements (e.g., pupil images or portions thereof) may be generated for multiple relative positions of the illumination distributions of the first and second Moire structures (e.g., for multiple induced phase asymmetries) to sample this induced interference modulation. The overlay may then be determined based on the multiple measurements. 
     The relative positions of the illumination distributions on the first and second Moire structures may be varied using any technique known in the art. Further, different techniques may be used to provide relative variations along different directions. For example, as illustrated in  FIG.  2 B , a scanning Moire overlay target  214  may include one inverted Moire structure pair  104  with periodicity along the scan direction and one inverted Moire structure pair  104  with periodicity along the transverse direction. 
     For example, the relative positions of illumination distributions on the first and second Moire structures along the transverse direction may be varied during a scan through techniques such as, but not limited to, optical zooming, wavelength modulation, or modulation of an RF drive frequency for an AOD used to generate one or more illumination beams (e.g., as illustrated in  FIG.  1 C ). By way of another example, the relative positions of illumination distributions on the first and second Moire structures along the scan direction may be varied during a scan by rotating a modulation direction of a diffraction grating used to generate one or more illumination beams. For instance, an AOD used to generate one or more illumination beams may include transducers along multiple axes. Acoustic waves from such multiple transducers may interfere in the acoustic medium of the AOD to generate a standing grating. Further, modulating the relative drive amplitudes of the transducers may rotate the grating axis. 
     Additionally, in some embodiments, the illumination distributions of the first and second Moire structures are modulated simultaneously in a common pattern (e.g., in tandem) during a particular data capture by the one or more detectors to mitigate noise (e.g., target noise, or the like) in an overlay measurement. Simultaneous modulation of the illumination distributions of the first and second Moire structures may be implemented using any technique known in the art. As an example, in the context of the scanning overlay system  100 , the beam-scanning optics  128  may provide simultaneous modulation of the illumination distributions of the first and second Moire structures. 
     Referring now generally to  FIG.  1 A  through 5, it is to be understood that  FIG.  1 A -5 are provided solely for illustrative purposes and should not be interpreted to be limiting. For example, it is contemplated herein that the scanning overlay system  100  and/or method  500  may be modified to provide overlay measurements from an inverted Moire structure pair  104  in a static (e.g., Move-and-Measure) mode of operation. In this way, references to a scanning system or to measurements of a sample in motion are merely illustrative. By way of another example, it is contemplated herein that the scanning overlay system  100  and/or method  500  may be modified to provide field-plane measurements of an inverted Moire structure pair  104 . For example, the one or more detectors  110  may be located at a field plane (e.g., the field plane  160  or a conjugate thereof). Further, overlay measurements may be generated at least in part based on this field-plane data. 
     The herein described subject matter sometimes illustrates different components contained within, or connected with, other components. It is to be understood that such depicted architectures are merely exemplary, and that in fact many other architectures can be implemented which achieve the same functionality. In a conceptual sense, any arrangement of components to achieve the same functionality is effectively “associated” such that the desired functionality is achieved. Hence, any two components herein combined to achieve a particular functionality can be seen as “associated with” each other such that the desired functionality is achieved, irrespective of architectures or intermedial components. Likewise, any two components so associated can also be viewed as being “connected” or “coupled” to each other to achieve the desired functionality, and any two components capable of being so associated can also be viewed as being “couplable” to each other to achieve the desired functionality. Specific examples of 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. 
     It is believed that the present disclosure and many of its attendant advantages will be understood by the foregoing description, and it will be apparent that various changes may be made in the form, construction, and arrangement of the components without departing from the disclosed subject matter or without sacrificing all of its material advantages. The form described is merely explanatory, and it is the intention of the following claims to encompass and include such changes. Furthermore, it is to be understood that the invention is defined by the appended claims.