High target single grab stack measurement

Abstract

An overlay metrology system can include an objective lens, illumination optics to illuminate an overlay target including a first grating having a first pitch on a first sample layer and a second grating having a second pitch on a second sample layer, where the first sample layer is separated from the second sample layer by a layer separation distance that is greater than a depth of field of the objective lens. The system can further include a collection optic having a radially varying defocus profile to compensate for the layer separation distance such that the first grating and the second grating are simultaneously in focus on a detector.

Description

TECHNICAL FIELD

[0001] The present disclosure relates generally to overlay metrology, and more particularly to overlay metrology on overlay targets having features with relatively large axial separation. BACKGROUND

[0002] Advanced packaging schemes for semiconductor devices increasingly utilize three-dimensional (3D) manufacturing techniques, such as die-to-wafer (D2W) or wafer-to-wafer (W2W) processing, in order to support aggressive roadmaps for connection density and data transfer speed. Such techniques can dictate reduced bond sizes, which can shrink the allowable space on a sample dedicated for overlay measurement, despite the need for tighter overlay tolerances and high measurement throughput at the same time. Furthermore, 3D manufacturing techniques can require registration measurements between features with relatively large axial separation, such as on the order of 100 microns or more. Accordingly, there is a need to develop systems and methods that address these challenges. SUMMARY

[0003] According to one or more illustrative embodiments of the present disclosure, an overlay metrology system is disclosed. In one illustrative embodiment, the system includes an objective lens. In another illustrative embodiment, the system includes one or more illumination optics configured to direct illumination from an illumination source through the objective lens to an overlay target on a sample when implementing a metrology recipe, wherein the overlay target according to the metrology recipe includes a first grating with a first pitch on a first sample layer and a second grating with a second pitch on a second sample layer, and wherein the second pitch is less than the first pitch, wherein the first sample layer is separated from the second sample layer by a layer separation distance that is greater than a depth of field of the objective lens. In another illustrative embodiment, the system includes a detector. In another illustrative embodiment, the system includes one or more collection optics configured to direct at least a portion of light collected through the objective lens to the detector, wherein the metrology recipe dictates that diffraction orders of the illumination by the first grating that are positioned below a threshold radius in a pupil plane collected through the objective lens and diffraction orders of the illumination by the second grating that are positioned above the threshold radius in the pupil plane collected through the objective lens. In another illustrative embodiment, the one or more collection optics provide a radially varying defocus profile to compensate for the layer separation distance, wherein the first grating and the second grating are simultaneously in focus on the detector. In another illustrative embodiment, the system includes a controller. In another illustrative embodiment, the controller receives one or more images of the overlay target from the detector, wherein the first grating and the second grating are simultaneously in focus on the detector. In another illustrative embodiment, the controller determines an overlay measurement between the first layer and the second layer of the sample based on the one or more images.

[0004] According to one or more illustrative embodiments of the disclosure, a overlay metrology system is disclosed. In one illustrative embodiment, the system includes a controller communicatively coupled with an overlay metrology subsystem. In another illustrative embodiment, the controller receives one or more images of an overlay target from a detector of the overlay metrology subsystem, where a first grating and a second grating are simultaneously in focus on the detector. In another illustrative embodiment, the overlay metrology subsystem includes an objective lens. In another illustrative embodiment, the overlay metrology subsystem includes one or more illumination optics configured to direct illumination from an illumination source to the overlay target on a sample through the objective lens when implementing the metrology recipe, where the overlay target according to the metrology recipe includes a first grating having a first pitch on a first sample layer and a second grating having a second pitch on a second sample layer, and where the second pitch is less than the first pitch, where the first sample layer is separated from the second sample layer by a layer separation distance that is greater than a depth of field of the objective lens. In another illustrative embodiment, the overlay metrology subsystem includes a detector. In another illustrative embodiment, the overlay metrology subsystem includes one or more collection optics configured to direct at least a portion of light collected through the objective lens to the detector, where the metrology recipe specifies that diffraction orders of the illumination by the first grating that are positioned below a threshold radius in a pupil plane collected through the objective lens and diffraction orders of the illumination by the second grating that are positioned above the threshold radius in the pupil plane collected through the objective lens. In another illustrative embodiment, the one or more collection optics provide a radially varying defocus profile to compensate for the layer separation distance, where the first grating and the second grating are simultaneously in focus on the detector. In another illustrative embodiment, the controller determines an overlay measurement between the first layer and the second layer of the sample based on the one or more images.

[0005] According to one or more illustrative embodiments of the present disclosure, a superposition metrology method is disclosed. In one illustrative embodiment, the method includes receiving one or more images of a superposition target on a sample from a detector of a superposition metrology subsystem, wherein the superposition target according to a metrology recipe includes a first grating having a first pitch on a first sample layer and a second grating having a second pitch on a second sample layer, and wherein the second pitch is less than the first pitch. In another illustrative embodiment, the first grating and the second grating are simultaneously in focus on the detector. In another illustrative embodiment, the superposition metrology subsystem includes an objective lens. In another illustrative embodiment, the superposition metrology subsystem includes one or more illumination optics configured to direct illumination from an illumination source to the superposition target on the sample through the objective lens when implementing the metrology recipe, wherein the first sample layer is separated from the second sample layer by a layer separation distance that is greater than a depth of field of the objective lens. In another illustrative embodiment, the superposition metrology subsystem includes a detector. In another illustrative embodiment, the superposition metrology subsystem includes one or more collection optics configured to direct at least a portion of light collected through the objective lens to the detector, wherein the metrology recipe specifies that diffraction orders of the illumination by the first grating that are positioned below a threshold radius in a pupil plane collected through the objective lens and diffraction orders of the illumination by the second grating that are positioned above the threshold radius in the pupil plane collected through the objective lens. In another illustrative embodiment, the one or more collection optics provide a radially varying defocus profile to compensate for the layer separation distance, wherein the first grating and the second grating are simultaneously in focus on the detector. In another illustrative embodiment, the method includes determining a superposition measurement between the first layer and the second layer of the sample based on the one or more images.

[0006] 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 application as claimed. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the application and together with the general description, serve to explain the principles of the application. BRIEF DESCRIPTION OF DRAWINGS

[0007] Those skilled in the art will better appreciate the many advantages of the present disclosure upon consideration of the detailed description in conjunction with the accompanying drawings.

[0008] Figure 1A is a block diagram of a superposition metrology system according to one or more embodiments of the present disclosure.

[0009] Figure 1B is a conceptual diagram of a superposition metrology subsystem according to one or more embodiments of the present disclosure.

[0010] Figure 1C is a conceptual diagram of a portion of an overlay metrology system according to one or more embodiments of the disclosure, illustrating a collection field stop for blocking zeroth order reflections from a top surface of a sample.

[0011] Figure 1D is a conceptual diagram of an overlay metrology system including a Linnik interferometer according to one or more embodiments of the disclosure.

[0012] Figure 2A is a side view of a cell of an overlay target according to one or more embodiments of the disclosure.

[0013] Figure 2B is a top view of a cell of Figure 2A according to one or more embodiments of the disclosure.

[0014] Figure 2C is a top view of an overlay target including four cells according to one or more embodiments of the disclosure.

[0015] Figure 2D is a top view of a portion of a sample according to one or more embodiments of the disclosure, depicting two overlay targets having cells with features split between a die and a scribe lane.

[0016] Figure 3A is a top view of a collection pupil plane including a phase plate having a radius that is less than a boundary of the collection pupil plane according to one or more embodiments of the disclosure.

[0017] Figure 3B is a top view of a collection pupil plane including a ring phase plate according to one or more embodiments of the disclosure.

[0018] Figure 4A is a top view of an illumination pupil plane providing a single illumination beam at normal incidence according to one or more embodiments of the disclosure.

[0019] Figure 4B is a top view of a collection pupil plane having a phase plate configured as illustrated in Figure 3A according to one or more embodiments of the disclosure, configured for mixed bright field and dark field imaging.

[0020] Figure 4C is a top view of a collection pupil plane having a phase plate configured as illustrated in Figure 3B according to one or more embodiments of the disclosure, configured for mixed bright field and dark field imaging.

[0021] Figure 4D is a top view of a collection pupil plane having a phase plate configured as illustrated inFigure 3A Top view of a collection pupil plane of a phase plate configured as described in the

[0022] Figure 4E configured for dark-field imaging according to one or more embodiments of the present disclosure having a phase plate as described in the Figure 3B Top view of a collection pupil plane of a phase plate configured as described in the

[0023] Figure 5A configured for imaging based on oblique illumination and having a phase plate as described in the

[0024] Figure 5B configured for imaging based on oblique illumination and having a phase plate as described in the Figure 3A Top view of a collection pupil plane of a phase plate configured as described in the

[0025] Figure 5C configured for imaging based on oblique illumination and having a phase plate as described in the Figure 3B Top view of a collection pupil plane of a phase plate configured as described in the

[0026] Figure 6A configured for imaging based on oblique illumination and having a phase plate as described in the

[0027] Figure 6B configured for imaging based on oblique illumination and having a phase plate as described in the Figure 6A configured for imaging based on oblique illumination and having a phase plate as described in the Figure 3A Top view of a collection pupil plane of a phase plate configured as described in the

[0028] Figure 6C configured for imaging based on oblique illumination and having a phase plate as described in the Figure 6A configured for imaging based on oblique illumination and having a phase plate as described in the Figure 3A Top view of a collection pupil plane of a phase plate configured as described in the DETAILED DESCRIPTION

[0029] The disclosed subject matter will now be described in detail with reference to the drawings, which are intended to be read in conjunction with the written description. The present disclosure has been presented with reference to particular embodiments and specific features thereof, which are described in detail in the written description. The embodiments described herein are to be considered illustrative and not restrictive, and the various modifications can be conceived without departing from the spirit and scope of the present disclosure.

[0030] Embodiments of the present disclosure relate to systems and methods for image-based overlay metrology of overlay targets having features separated axially by a distance greater than the focal depth of a collection objective, where the features are simultaneously in focus in a single image capture. In this way, features separated by a large axial distance (e.g., relative to the objective focal depth) can be simultaneously in focus in a single image of an overlay target. For the purposes of the present disclosure, an overlay target having features on layers separated axially is referred to as a high overlay target.

[0031] It is contemplated herein that an overlay metrology system (or subsystem) can be configured according to various metrology recipes that are suitable for generating overlay measurements on overlay targets having various designs. For example, a metrology recipe can include various design parameters associated with the design of a particular overlay target as well as various parameters of an overlay metrology subsystem suitable for measuring the overlay target. In this way, an overlay target and various aspects of an overlay metrology subsystem suitable for characterizing the overlay target can be designed together to provide measurements according to a metrology recipe.

[0032] In some embodiments, a metrology recipe specifies that an overlay target includes a first grating feature having a first pitch in a first layer of a sample and a second grating having a second pitch different from the first pitch in a second layer of the sample, where an axial distance (e.g., layer separation distance) between the first layer and the second layer of the sample is greater than the depth of focus (DOF) of an overlay metrology subsystem used to characterize the target. In this way, without further consideration, the gratings in the first and second layers will not be simultaneously in focus. For the purposes of the present disclosure, a grating of an overlay target having a relatively large pitch is referred to as a large pitch grating and a grating of the overlay target having a relatively small pitch is referred to as a small pitch grating.

[0033] In some embodiments, the metrology further specifies that the diffraction orders of interest of a particular measurement scheme are fully separated in different radial regions of a collection pupil of an overlay metrology subsystem. For example, a grating having a relatively large pitch (e.g., a large pitch grating) can provide diffraction orders 0 < r < r t while a grating having a relatively small pitch (e.g., a small pitch grating) can provide diffraction orders r t < r < r max Here, r max corresponds to the boundary of the collection pupil (e.g., the maximum collection numerical aperture (NA)) and r t corresponds to a threshold radius separating the diffraction orders of the first and second gratings.

[0034] In some embodiments, an overlay metrology sub-system configured according to an overlay recipe to measure an overlay of a sample based on an overlay target includes a collection optic to provide a radially varying defocus profile to compensate for a layer separation distance. Orders of illumination diffracted by a first and a second illumination grating can experience different portions of the radially varying defocus profile. Accordingly, the first and second gratings can be focused on a detector simultaneously.

[0035] The overlay metrology sub-system can generally include any type or combination of elements to provide a radially varying defocus.

[0036] In some embodiments, the overlay metrology sub-system includes a lens in a pupil plane (e.g., a relay pupil plane), where the lens has a radius equal to a threshold radius. In this way, the lens can operate on orders from a large pitch grating, but not on orders from a small pitch grating. Further, the lens can optionally be a positive or negative lens to adjust a focal plane associated with the large pitch grating to match a focal plane of the small pitch grating. For example, when the large pitch grating is closer to the objective lens than the small pitch grating, the lens can be a positive lens, and when the large pitch grating is farther from the objective lens than the small pitch grating, it can be a negative lens.

[0037] In some embodiments, the overlay metrology sub-system includes an objective lens having a correction ring that can be adjusted to provide a selected amount of spherical aberration. It is contemplated herein that spherical aberration corresponds to a radially dependent defocus. In this way, increasing the spherical aberration in a lens increases the amount of defocus difference between light incident on the center of the objective lens relative to light incident near the edge of the lens. It is further contemplated herein that this correction ring can be used to compensate for a layer separation distance and that the large pitch and small pitch gratings are focused simultaneously.

[0038] Additional embodiments of the present disclosure relate to high overlay target designs suitable for measurement using the systems and methods disclosed herein.

[0039] Reference is now made to Figures 1A to 6C , systems and methods for image-based overlay metrology on high overlay targets are described in greater detail in accordance with one or more embodiments of the present disclosure.

[0040] Figure 1A is a block diagram of an overlay metrology system 100 in accordance with one or more embodiments of the present disclosure. In some embodiments, the overlay metrology system 100 includes an overlay metrology sub-system 102 to generate an overlay measurement associated with a sample 104 based on characterization of an overlay target 106 on the sample 104. For example, the overlay target 106 can include features on multiple layers of the sample 104. In this way, an overlay measurement of the sample 104 can be generated based on one or more images of the overlay target 106.

[0041] In some embodiments, the overlay metrology system 102 generates at least one image of the overlay target 106. By way of illustration, the overlay metrology system 102 can illuminate the overlay target 106 with illumination including one or more illumination beams 108, collect light from the overlay target 106 (which is referred to herein as sample light 110), and generate an image of the overlay target 106 based on at least a portion of the sample light 110.

[0042] Furthermore, the overlay metrology system 102 can be configurable to generate measurements based on any number of metrology recipes that define various aspects of the overlay target 106 (e.g., target design) or measurement parameters of the overlay metrology system 102 that are suitable for generating overlay measurements for a particular overlay target 106 having a particular target design.

[0043] In other words, the overlay metrology system 102 can be configured to provide a selected measurement type for a selected overlay target design. For example, a metrology recipe can include various parameters associated with the design of the overlay target 106, such as but not limited to the location and orientation of sample features (e.g., pitch of grating features along a particular direction). By way of another example, a metrology recipe can include various parameters associated with the location of the sample 104 during measurement, such as but not limited to height, orientation, whether the sample 104 is static during measurement, or whether the sample 104 is in motion during measurement (and associated parameters describing velocity, scan pattern, or the like). By way of another example, a metrology recipe can include parameters of the illumination beams 108, such as but not limited to illumination wavelength, illumination pupil distribution (e.g., distribution of illumination angles and associated illumination intensity at those angles), polarization of the incident illumination, or spatial distribution of the illumination. By way of another example, a metrology recipe can include collection parameters associated with collection or filtering of the sample light 110, such as but not limited to collection pupil distribution, collection field stop settings to select portions of the overlay target 106 of interest for imaging, polarization of the sample light 110, wavelength filtering, or parameters for controlling one or more detectors.

[0044] In some embodiments, the overlay metrology sub-system 102 and / or the overlay target 106 is configured (e.g., according to a metrology recipe) to be capable of capturing images in which features of the overlay target 106 located on layers that are axially separated by a distance greater than the DOF (e.g., a layer separation distance) of the sample 104 are simultaneously in focus in a single image. Thus, such an overlay metrology sub-system 102 can be suitable for, but is not limited to, overlay metrology of samples fabricated by advanced packaging techniques such as, but not limited to, die-to-wafer (D2W) techniques, wafer-to-wafer (W2W) techniques, or other three-dimensional (3D) processing techniques. For example, D2W or W2W techniques can require overlay measurements (e.g., misregistration measurements more generally) between features on constituent substrates or these constituent substrates. By way of illustration, D2W or W2W techniques can require overlay measurements of features having layer separation distances on the order of hundreds of microns or more. It is further contemplated herein that, relative to alternative tools or techniques that produce separate images of features on axially separated layers, simultaneous imaging of such disparate features in a single image capture can substantially increase the throughput of the measurement process. In some embodiments, features of the overlay target 106 located on layers that are axially separated by a distance greater than the DOF (e.g., a layer separation distance) of the sample 104 are simultaneously in focus in a single image plane, but multiple detectors located at different instances of this image plane (e.g., in multiple collection channels) are used to capture different portions of the overlay target 106. In this manner, sample light 110 associated with features on different layers of the overlay target 106 can be isolated to improve image contrast, signal-to-noise ratio, or another quality metric.

[0045] In some embodiments, the overlay metrology system 100 includes a sample positioning sub-system 112 configured to adjust the sample 104 and / or the illumination beam 108 before, during, and / or after measurement. For example, the overlay metrology sub-system 102 can operate in a move-and-measure (MAM) mode of operation in which the sample 104 is stationary during measurement or in a scanning mode of operation in which the sample 104 is in motion during measurement.

[0046] Referring now to Figures 2A to 2D , an overlay target 106 having features separated by a relatively large axial distance is described in accordance with one or more embodiments of the present disclosure.

[0047] Figure 2A is a side view of a cell 202 of an overlay target 106 in accordance with one or more embodiments of the present disclosure. Figure 2B is a top view of the cell 202 of Figure 2A in accordance with one or more embodiments of the present disclosure.

[0048] In some embodiments, the cell 202 includes a large-pitch grating 204 (e.g., a first layer grating) positioned on a first layer 206 of the specimen 104 and a small-pitch grating 208 (e.g., a second layer grating) positioned on a second layer 210 of the specimen 104. The first layer 206 and the second layer 210 can be further separated by any number of intermediate layers 214. Figure 2A The layer separation distance 212 is denoted in as an axial separation distance along the Z-direction. For example, the first layer 206 and the second layer 210 can be separated by any number of intermediate layers 214.

[0049] The overlay target 106 (or, more generally, the specimen 104) can further include one or more substrates. As an illustration, Figure 2A A first substrate 216 proximate to the large-pitch grating 204 and a second substrate 218 proximate to the small-pitch grating 208 are depicted. In this way, the overlay target 106 can be formed on a wide range of specimen types. For example, the specimen 104 can be a D2W bonded specimen in which the first substrate 216 is a die and the second substrate 218 is a wafer. In this configuration, at least some of the large-pitch grating 204 and, optionally, the intermediate layers 214 are fabricated as part of the die prior to bonding, while at least some of the small-pitch grating 208 and, optionally, the intermediate layers 214 are fabricated as part of the wafer. As another example, the specimen 104 can be a W2W bonded specimen in which the first substrate 216 is a first wafer and the second substrate 218 is a second wafer. In this configuration, at least some of the large-pitch grating 204 and, optionally, the intermediate layers 214 are fabricated as part of the first wafer prior to bonding, while at least some of the small-pitch grating 208 and, optionally, the intermediate layers 214 are fabricated as part of the second wafer.

[0050] Referring now to Figure 2B and 2C various non-limiting designs of the overlay target 106 are described in greater detail in accordance with one or more embodiments of the present disclosure.

[0051] The overlay target 106 can generally be formed from one or more cells 202, where any particular cell 202 can include grating structures (e.g., the large-pitch grating 204 and the small-pitch grating 208) having any period and periodicity along any direction (e.g., the X-direction illustrated in Figure 2A Further, constituent gratings within any particular cell 202 can be positioned in overlapping regions, partially overlapping regions, or non-overlapping regions. In this way, the overlay target 106 can have any suitable design, including but not limited to an advanced imaging metrology (AIM) target, a robust AIM (rAIM) target, a triple AIM target, or a Moiré target (e.g., a grating-on-grating target).

[0052] As an illustration, Figure 2Bis a top view of an overlay target 106 including a single cell 202 suitable for measurement along one direction, e.g., the X direction here. As another illustration, Figure 2C is a top view of an overlay target 106 including four cells 202 according to one or more embodiments of the present disclosure. For example, Figure 2B The overlay target 106 in can be characterized as having an AIM target design. In particular, Figure 2B The overlay target 106 in includes cells 202a, 202c having constituent gratings oriented along the X direction (e.g., a first measurement direction) and cells 202b, 202d oriented along the Y direction (e.g., a second measurement direction).

[0053] The overlay target 106 and / or any particular cell 202 can generally be located at any suitable location on the sample 104, such as but not limited to, within a scribe lane between dies or within a die.

[0054] In some embodiments, different portions of the overlay target 106 and / or any particular cell 202 are split between a scribe lane and a die. Figure 2D is a top view of a portion of a sample 104 depicting two overlay targets 106 having cells 202 with features split between a die 220 and a scribe lane 222 according to one or more embodiments of the present disclosure. For example, the sample 104 can be a D2W sample in which the die 220 can correspond to a first substrate 216 and the scribe lane 222 is on a second substrate 218. In particular, Figure 2D depicts a first overlay target 106a and a second overlay target 106b, where each of the overlay targets 106a, 106b is configured with a single cell 202 as shown in Figure 2B For example, the first overlay target 106a has features distributed along the X direction for measurement along the X direction, while the second overlay target 106b has features distributed along the Y direction for measurement along the Y direction. Further, each of the overlay targets 106a, 106b has a large pitch grating 204 located in the die 220 and a small pitch grating 208 located in the scribe lane 222. As described with reference to Figure 2A The in-die features (e.g., the large pitch grating 204 here) can be located on the surface of the die 220 or can be buried within the sample 104. In this way, this design can be suitable for, but is not limited to, applications including two or more stacked dies 220. Further, it should be understood that the particular configuration of the large pitch grating 204 in the die 220 and the small pitch grating 208 in the scribe lane 222 is merely illustrative and does not limit the present disclosure. Rather, any suitable arrangement of the large pitch grating 204 and the small pitch grating 208 is within the spirit and scope of the present disclosure.

[0055] It is contemplated herein that the design of the overlay targets 106a, 106b can provide several benefits. For example, the single cell ID design can enable such overlay targets 106 to be relatively small and thus limit the space on the sample 104 dedicated to overlay measurements. As another example, using separate overlay targets 106a, 106b to make separate measurements along different directions (e.g., X and Y directions) located at different positions on the sample 104 can facilitate compensation for rotation of the sample 104 relative to the overlay metrology sub-system 102. As another example, placement of such overlay targets 106 near intersections between scribe lanes 222 (e.g., intersections between adjacent dies 220) can enable efficient measurement sampling. For example, both overlay targets 106a, 106b can be simultaneously within a single field of view 224 of the overlay metrology sub-system 102 and thus simultaneously characterized. Moreover, such overlay targets 106a, 106b can provide accurate measurements for all four adjacent dies 220 (or at least portions thereof).

[0056] Reference is now made to Figure 1B , Figure 1B is a conceptual diagram of an overlay metrology sub-system 102 in accordance with one or more embodiments of the present disclosure.

[0057] In some embodiments, the overlay metrology sub-system 102 includes an illumination source 114 configured to generate illumination to be directed to the sample 104 in the form of one or more illumination beams 108. In some embodiments, the overlay metrology system 100 includes an illumination path 116 (e.g., an illumination sub-system) containing one or more components to direct the one or more illumination beams 108 to the sample 104. For example, the illumination path 116 can include one or more illumination lenses 118 to direct the one or more illumination beams 108 from the illumination source 114 to the sample 104. Moreover, the illumination lenses 118 can be arranged to provide one or more conjugate or relayed illumination pupil planes 120 and / or illumination field planes 122. The illumination path 116 can further include one or more illumination conditioning components 124 suitable to modify and / or adjust the one or more illumination beams 108. The illumination conditioning components 124 can, but are not required to, be located in the illumination pupil planes 120 and / or the illumination field planes in the illumination path 116. For example, the one or more illumination conditioning components 124 can include, but are not limited to, an illumination aperture stop, an illumination field stop, one or more polarizers, one or more compensators, 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, one or more mirrors, or one or more lenses.

[0058] In some embodiments, the overlay metrology system 102 includes a collection path 126 (e.g., an imaging subsystem) that includes one or more components to collect sample light 110 (e.g., light emanating from the sample 104 in response to one or more illumination beams 108) and direct at least a portion of this sample light 110 to a detector 128. The sample light 110 can include any type of light emanating from the sample 104, including but not limited to diffracted, scattered, or reflected light. In some embodiments, the collection path 126 includes one or more collection lenses 130 to direct the sample light 110 from the sample 104 to the detector 128. Further, the collection lenses 130 can be arranged to provide one or more conjugate or relayed collection pupil planes 132 and / or collection field planes 134. In some embodiments, the collection path 126 includes one or more collection conditioning components 136 suitable to modify and / or condition the sample light 110. For example, the one or more collection conditioning components 136 can include, but are not limited to, a collection aperture stop, a collection field stop, one or more polarizers, one or more compensators, 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, one or more mirrors, or one or more lenses.

[0059] The detector 128 can include any optical detector known in the art suitable to capture sample light 110 received from the sample 104. Further, the detector 128 can be suitable to capture images of stationary or moving samples 104. For example, the detector 128 can include, but is not limited to, a photodiode array (PDA), a charge-coupled device (CCD), a complementary metal-oxide-semiconductor (CMOS) device, a time delay integration (TDI) detector, a line-scan detector, a photomultiplier tube (PMT), an avalanche photodiode (APD), or the like. In some embodiments, the detector 128 can include a spectral detector suitable to identify wavelengths of radiation emanating from the sample 104 and dispersed onto a sensor using a dispersive element.

[0060] In some embodiments, although not explicitly shown, the overlay metrology system 102 includes two or more detectors 128. For example, the collection path 126 can include one or more additional beam splitters to separate the sample light 110 into two or more collection channels. Further, the collection path 126 can separate the sample light 110 into various collection channels using any technique known in the art. For example, the collection path 126 can include one or more optical elements to separate the sample light 110 based on polarization, wavelength, or any other suitable optical property (e.g., using additional beam splitters, polarizers, dichroic mirrors, or other elements). This configuration can be suitable, without limitation, in cases where the sample light 110 associated with features on different layers of the overlay target 106 are optically distinguishable. As another example, the collection path 126 can include one or more spatial filters or beam blockers to spatially isolate features on different layers of the overlay target 106 in various collection channels.

[0061] In some embodiments, the overlay metrology system 102 includes an objective lens 138 to direct the one or more illumination beams 108 to the sample 104 and capture the sample light 110 from the sample 104 simultaneously. For example, as described in Figure 1B The overlay metrology system 102 can include a beam splitter 140 that is common to both the illumination path 116 and the collection path 126, as described in

[0062] Referring now to Figures 3A to 3B , overlay metrology on a high overlay target 106 is described in greater detail in accordance with one or more embodiments of the present disclosure. For example, the high overlay target 106 can have a large pitch grating 204 and a small pitch grating 208 with a layer separation distance that is greater than the DOF of the objective lens 138 or, more generally, the collection path 126.

[0063] In some embodiments, the overlay metrology system 102 (e.g., the collection path 126 of the overlay metrology system 102) can be configured in accordance with a metrology recipe to a radially varying defocus profile to compensate for the layer separation distance 212. In this way, the diffracted orders of the one or more illumination beams 108 by the large pitch grating 204 and the small pitch grating 208 can be subjected to different portions of the radially varying defocus profile. As a result, the large pitch grating 204 and the small pitch grating 208 can be focused on the detector 128 simultaneously.

[0064] The overlay metrology subsystem 102 can provide any radially varying defocus profile suitable to compensate for layer separation distances 212 between features of the overlay target 106 such that the features within the respective layers are simultaneously in focus on the detector 128. Further, the radially varying defocus profile can be implemented using any combination of components in the overlay metrology subsystem 102, including but not limited to components of the collection path 126 (e.g., the collection lens 130, the collection conditioning components 136, or the like).

[0065] In some embodiments, the overlay metrology subsystem 102 includes a phase plate in the collection pupil plane 132 to provide the radially varying defocus profile. The phase plate can have any size or spatially varying phase profile suitable to provide a selected radially varying defocus profile.

[0066] Figure 3A is a top view of the collection pupil plane 132 including a phase plate 302 having a radius less than a boundary 304 (r b ) of the collection pupil plane 132 in accordance with one or more embodiments of the present disclosure. In particular, Figure 3A The phase plate 302 illustrated in FIG. 3 has a radius r t In this way, the phase plate 302 can introduce defocus to portions of light in the collection pupil plane 132 at locations up to the radius r t of the phase plate 302 (e.g., 0 < r < r t of the phase plate 302 (e.g., r t < r < r t of the phase plate 302 (e.g., r b Thus, the phase plate 302 can compensate for layer separation distances 212 between layers of the overlay target 106 such that features (e.g., the large pitch grating 204 and the small pitch grating 208 in FIG. 2) on the layers can be simultaneously in focus on the detector 128.

[0067] Figure 3B is a top view of the collection pupil plane 132 including a ring-shaped phase plate 302 in accordance with one or more embodiments of the present disclosure. In particular, Figure 3B The phase plate 302 illustrated in FIG. 4 spans a radius value of r t < r < r b In this way, the phase plate 302 can introduce defocus to portions of light in the collection pupil plane 132 at locations beyond the radius r t of the phase plate 302 (e.g., r t < r < r b of the phase plate 302 (e.g., r tThe portion of the light at the location of the phase plate 302 can be unaffected by the phase plate 302 (e.g., 0 < r < r t The radius value of the phase plate 302 can be less than the radius value of the boundary 304 of the relayed pupil plane 132.

[0068] Referring generally to Figure 3A and 3B The phase plate 302 can be positioned at any suitable location. For example, the phase plate 302 can be positioned at a relayed collection pupil plane 132 that corresponds to a relayed version (e.g., a conjugate version) of the back focal plane of the objective lens 138.

[0069] Further, the phase plate 302 can provide a phase delay having any spatial distribution. In some embodiments, the phase plate 302 provides a uniform phase delay. Such a phase plate 302 can be suitable for configurations in which the phase plate 302 is less than the boundary 304 of the collection pupil plane 132 (e.g., as illustrated in Figure 3A and 3B In this way, since the phase plate 302 interacts with only a portion of the light within the collection pupil plane 132, a radially varying defocus distribution can be generated across the collection pupil plane 132. Further, the overlay metrology system 102 can include multiple radially symmetric phase plates 302 to provide different phase delays within multiple radial regions of the illumination pupil plane 120. These phase plates 302 can be positioned in a single relayed pupil plane 132 or in different relayed planes (e.g., conjugate planes). Such a configuration can be suitable, but not limited to, applications in which features on three or more layers of the overlay target 106 (e.g., a triple AIM target or the like) are simultaneously focused on the detector 128.

[0070] In some embodiments, the phase plate 302 provides a non-uniform phase delay across the collection pupil plane 132. For example, the phase plate 302 can provide different uniform phase delays within two or more radial regions within the collection pupil plane 132. As an illustration, a single phase plate 302 can provide a first phase delay for light below a threshold radius 0 < r < r t and a second phase delay for light above the threshold radius r t < r < r b In this way, a single phase plate 302 across the collection pupil plane 132 can operate in a manner similar to the configuration illustrated in Figure 3A

[0071] As another example, the phase plate 302 can provide a radially symmetric, smoothly varying phase delay distribution across the collection pupil plane 132. For example, the phase plate 302 can be formed as a lens positioned in the collection pupil plane 132. As an illustration, a lens positioned in the collection pupil plane 132 having a radius less than the boundary 304 of the collection pupil plane 132 can operate in a manner similar to the configuration illustrated in Figure 3A ​In another illustrative example, a lens positioned in the collection pupil plane 132 having a radius equal to or greater than the boundary 304 of the collection pupil plane 132 can have a phase retardation profile across the collection pupil plane 132 sufficient to directly compensate for the layer separation distance 212.

[0072] It is further contemplated herein that spherical aberration in one or more components (e.g., one or more lenses) can provide a radially varying defocus profile. Accordingly, the overlay metrology system 102 can include one or more lenses having spherical aberration sufficient to compensate for the layer separation distance of the overlay target 106 such that features on multiple layers of the overlay target 106 can be simultaneously focused on the detector 128.

[0073] In some embodiments, the overlay metrology system 102 includes an objective lens 138 having spherical aberration to provide a radially varying defocus profile sufficient for overlay measurement on the high cell 202. For example, the objective lens 138 can include one or more components to provide an adjustable amount of spherical aberration. Such an objective lens 138 can include, but is not required to include, one or more optical elements (e.g., lenses) having a position that is adjustable with an adjustment device (e.g., a correction ring, an adjustment ring, or the like).

[0074] An objective lens 138 having adjustable spherical aberration can generally be used to compensate for spherical aberration induced by a microscopic cover glass or other components in the optical path before the sample of interest. However, it is contemplated herein that the overlay metrology system 102 can utilize an objective lens 138 having adjustable spherical aberration to intentionally introduce a radially varying defocus during measurement. Further, the amount of spherical aberration, and thus the amount of radially varying defocus, can be selected (e.g., based on a metrology recipe) to compensate for the layer separation distance 212 between layers of the overlay target 106 such that features on the layers can be simultaneously focused on the detector 128. Further, it is contemplated herein that an objective lens 138 having adjustable spherical aberration can directly compensate for the layer separation distance 212 without the need for additional components in the relayed collection pupil plane 132, which can advantageously provide a relatively simple and robust design of the overlay metrology system 102.

[0075] Referring now to Figures 4A to 6C , overlay metrology on a high overlay target 106 using different imaging modes is described in greater detail in accordance with one or more embodiments of the present disclosure. The overlay metrology system 102 can be configured to operate in various imaging modes, including but not limited to, a brightfield imaging mode or a darkfield imaging mode.

[0076] Figure 4A is a top view of an illumination pupil plane 120 providing a single illumination beam 108 at normal incidence in accordance with one or more embodiments of the present disclosure. In particular, Figure 4AA single illumination beam 108 is illustrated centered within the boundary 402 of the illumination pupil plane 120.

[0077] Figures 4B to 4E Various non-limiting imaging configurations based on illumination with a profile as illustrated in Figure 4A Figure 4B is configured for imaging according to one or more embodiments of the present disclosure with a phase plate 302 as configured in Figure 3A is configured for imaging according to one or more embodiments of the present disclosure with a phase plate 302 as configured in Figure 4C is configured for imaging according to one or more embodiments of the present disclosure with a phase plate 302 as configured in Figure 3B is configured for imaging according to one or more embodiments of the present disclosure with a phase plate 302 as configured in

[0078] Figure 4B and 4C A 0thorder diffraction lobe 404 (e.g., a specular reflection of the illumination beam 108), two large pitch diffraction lobes 406 associated with diffraction of the illumination beam 108 by the large pitch grating 204, and two small pitch diffraction lobes 408 associated with diffraction of the illumination beam 108 by the small pitch grating 208 are illustrated. For example, the large pitch diffraction lobes 406 and the small pitch diffraction lobes 408 can have any diffraction order. For example, the large pitch diffraction lobes 406 and / or the small pitch diffraction lobes 408 can include, but are not limited to, + / - 1storder (first order) diffraction lobes.

[0079] Because the large pitch grating 204 and the small pitch grating 208 have different pitches, the large pitch diffraction lobes 406 and the small pitch diffraction lobes 408 can be located at different radial positions in the collection pupil plane 132. Accordingly, the overlay metrology system 102 can provide a radially varying defocus profile to introduce different amounts of defocus to the large pitch diffraction lobes 406 and the small pitch diffraction lobes 408 to compensate for the layer separation distance 212 so that the large pitch grating 204 and the small pitch grating 208 can be simultaneously focused on the detector 128.

[0080] As an illustrative example, Figure 4B A phase plate 302 is depicted having a radius (r t ​) to introduce defocus into the large pitch diffraction lobes 406 but not into the small pitch diffraction lobes 408. In this configuration, the detector 128 can be positioned at a field plane conjugate to the small pitch grating 208 so that the small pitch grating 208 is in focus and the phase plate 302 can introduce enough defocus into the large pitch diffraction lobes 406 to specify that the large pitch grating 204 is also in focus on the detector (despite the layer separation distance 212). Since the phase plate 302 also introduces at least some defocus into the 0thorder diffraction lobe 404, the portion of the image that includes the large pitch grating 204 can be a bright field image. However, the 0thorder diffraction lobe 404 can contribute less to the image of the small pitch grating 208.

[0081] As another illustration, Figure 4C depicts an annular phase plate 302 having an inner radius (r t ) and an outer radius to introduce defocus into the small pitch diffraction lobes 408 but not into the large pitch diffraction lobes 406. In this configuration, the detector 128 can be positioned at a field plane conjugate to the large pitch grating 204 so that the large pitch grating 204 is in focus and the phase plate 302 can introduce enough defocus into the small pitch diffraction lobes 408 to specify that the small pitch grating 208 is also in focus on the detector (despite the layer separation distance 212).

[0082] It should be understood that although Figure 4B and 4C depicts the use of a phase plate 302 that can be positioned in any of the collection pupil planes 132, this is for illustrative purposes only and is not limiting of the disclosure. Rather, the overlay metrology system 102 can include any suitable phase plate 302 at any location suitable to provide radially varying defocus selected to compensate for the layer separation distance 212 so that the large pitch grating 204 and the small pitch grating 208 are simultaneously in focus on the detector 128. In some embodiments, the overlay metrology system 102 includes an objective lens 138 that provides spherical aberration that introduces different amounts of defocus into the large pitch diffraction lobes 406 and the small pitch diffraction lobes 408 to specify that the large pitch grating 204 and the small pitch grating 208 are simultaneously in focus on the detector 128.

[0083] Figure 4D and 4E depicts dark field imaging of a high overlay target 106 according to one or more embodiments of the disclosure. In particular, Figure 4D is a top view of a collection pupil plane 132 configured for dark field imaging having a phase plate 302 as explained in Figure 3A is a top view of a collection pupil plane 132 configured for dark field imaging having a phase plate 302 as explained in Figure 4E is a top view of a collection pupil plane 132 configured for dark field imaging having a phase plate 302 as explained in Figure 3Ba top view of the collection pupil plane 132 of the phase plate 302 illustrated in Figure 4D and 4E are similar to Figure 4B and 4C except that Figure 4D and 4E depict a blocker 410 positioned to block the 0th order diffraction lobe 404 and thus provide dark-field imaging for both the large pitch grating 204 and the small pitch grating 208. The blocker 410 can be positioned in any collection pupil plane 132 (e.g., a relayed collection pupil plane 132). In some embodiments, the blocker 410 is positioned in a plane common to the phase plate 302. In some embodiments, the blocker 410 is positioned in a collection pupil plane 132 different (e.g., relayed) from the phase plate 302.

[0084] Referring now to Figures 5A to 5C various non-limiting imaging configurations associated with oblique illumination are described in greater detail in accordance with one or more embodiments of the present disclosure.

[0085] Figure 5A is a top view of an illumination pupil plane 120 providing an illumination beam 108 at an oblique angle of incidence in accordance with one or more embodiments of the present disclosure.

[0086] Figure 5B is a collection pupil plane 132 of a phase plate 302 configured for imaging based on oblique illumination and having a blocker 410 positioned to block the 0th order diffraction lobe 404 in accordance with one or more embodiments of the present disclosure. Figure 3A is a top view of the collection pupil plane 132 of the phase plate 302 illustrated in Figure 5C is a collection pupil plane 132 of a phase plate 302 configured for imaging based on oblique illumination and having a blocker 410 positioned to block the 0th order diffraction lobe 404 in accordance with one or more embodiments of the present disclosure. Figure 3B is a top view of the collection pupil plane 132 of the phase plate 302 illustrated in

[0087] Figure 5B and 5C illustrate the 0th order diffraction lobe 404 (e.g., a specular reflection of the illumination beam 108), a single small pitch diffraction lobe 408 associated with the diffraction of the illumination beam 108 by the small pitch grating 208, and two large pitch diffraction lobes 406 associated with the diffraction of the illumination beam 108 by the large pitch grating 204. Further, Figure 5B and 5C depict a Littrow configuration (e.g., in accordance with a metrology recipe) of the overlay metrology sub-system 102 and the small pitch grating 208, where the single small pitch diffraction lobe 408 is along the Figure 5AThe illumination beam 108 shown in FIG. 1 is retro-reflected back. In addition, the overlay metrology system 102 and the overlay target 106 are further configured (e.g., according to a metrology recipe) to specify that both the large pitch diffraction lobes 406 (e.g., the first order diffraction lobe and the second order diffraction lobe) are positioned at a smaller radial location in the collection pupil plane 132, such that light from the large pitch grating 204 and the small pitch grating 208 can be subjected to different amounts of defocus based on the radially varying defocus profile as disclosed herein.

[0088] For example, Figure 5B A configuration is described in which the phase plate 302 introduces defocus to the large pitch diffraction lobes 406 but not to the 0thorder diffraction lobe 404 or the small pitch diffraction lobes 408. In this configuration, the detector 128 can be positioned at a field plane conjugate to the small pitch grating 208, such that a bright field image is formed based on the 0thorder diffraction lobe 404 and the small pitch diffraction lobes 408. In addition, the defocus introduced by the phase plate 302 simultaneously provides a focused image of the large pitch grating 204 on the detector 128. Similarly, Figure 5C An opposite configuration is described in which the detector 128 is positioned at a field plane conjugate to the large pitch grating 204, while the phase plate 302 introduces defocus to the 0thorder diffraction lobe 404 and the small pitch diffraction lobes 408 to simultaneously provide focused images of the large pitch grating 204 and the small pitch grating 208.

[0089] In some embodiments, the overlay metrology system 102 includes two detectors 128 (e.g., in two collection channels) that image the large pitch grating 204 and the small pitch grating 208, respectively. For example, a first collection channel can include one or more blockers (e.g., in the collection pupil plane 132) to block the 0thorder diffraction lobe 404 and the small pitch diffraction lobes 408. In this way, the detector 128 in this channel can produce a dark field image of the large pitch grating 204 based on only the large pitch diffraction lobes 406. Similarly, a second collection channel can include one or more blockers (e.g., in the collection pupil plane 132) to block the large pitch diffraction lobes 406 to produce an image of the small pitch grating 208 based on only the 0thorder diffraction lobe 404 and the small pitch diffraction lobes 408.

[0090] Referring now to Figures 6A to 6C In some embodiments, the overlay metrology system 102 produces an overlay measurement based on illumination of the overlay target 106 with two or more illumination beams 108. The two or more illumination beams 108 can be provided simultaneously or sequentially. In addition, multiple illumination beams 108 can be used for measurements along a particular direction (e.g., along the X direction in FIG. 1) or can be used for measurements along multiple directions. Figure 2B

[0091] Figure 6A ​is a top-down view of an illumination pupil plane 120 providing two illumination beams 108a, 108b at symmetrically opposite azimuthal angles of incidence according to one or more embodiments of the present disclosure. Illumination with symmetrically opposite azimuthal angles of incidence can mitigate the effects of asymmetry in the profile of the illumination beams 108 and / or the features of the overlay target 106. Further, the two illumination beams 108 can be directed to the overlay target 106 simultaneously or sequentially to facilitate simultaneous or sequential imaging. However, it should be understood that the overlay metrology sub-system 102 can illuminate the overlay target 106 with any number of illumination beams 108 having any azimuthal angles of incidence.

[0092] Figure 6B is a top-down view of a collection pupil plane 132 of a phase plate 302 configured for imaging under Littrow conditions with two illumination beams 108 as Figure 6A described in FIG. 1. For example, the phase plate 302 is configured to direct the illumination beams 108a, 108b to the collection pupil plane 132 in a symmetrically opposite azimuthal pattern as Figure 3A described in FIG. 1. For example, the phase plate 302 is configured to direct the illumination beams 108a, 108b to the collection pupil plane 132 in a symmetrically opposite azimuthal pattern as Figure 6B depicts all of the diffraction lobes associated with the diffraction of the illumination beam 108a corresponding to Figure 5B depicted in FIG. 1. For example, the phase plate 302 is configured to direct the illumination beams 108a, 108b to the collection pupil plane 132 in a symmetrically opposite azimuthal pattern as Figure 6B depicts all of the diffraction lobes associated with the diffraction of the illumination beam 108b corresponding to Figure 5B depicted in FIG. 1. For example, the phase plate 302 is configured to direct the illumination beams 108a, 108b to the collection pupil plane 132 in a symmetrically opposite azimuthal pattern as

[0093] depicts all of the diffraction lobes associated with the diffraction of the illumination beam 108b corresponding to Figures 5A to 5CIn some embodiments, in the manner described, the overlay metrology system 102 includes two detectors 128 (e.g., in two collection channels) that image the large pitch grating 204 and the small pitch grating 208, respectively. For example, a first collection channel can include one or more blockers (e.g., in the collection pupil plane 132) to block the 0thorder diffraction lobes 404a-b and the small pitch diffraction lobes 408a-b. In this manner, the detector 128 in this channel can produce a dark field image of the large pitch grating 204 based only on the large pitch diffraction lobes 406a-b. Similarly, a second collection channel can include one or more blockers (e.g., in the collection pupil plane 132) to block the large pitch diffraction lobes 406a-b to produce an image of the small pitch grating 208 based only on the 0thorder diffraction lobes 404a-b and the small pitch diffraction lobes 408a-b.

[0094] Referring generally to Figures 3A to 6B , it should be understood that Figures 3A to 6B and the associated description is provided for illustrative purposes only and should not be construed as limiting the disclosure. For example, the overlay metrology system 102 can illuminate the overlay target 106 with any number of illumination beams 108 at any combination of amplitude or azimuthal angle of incidence.

[0095] As an illustration, Figure 6C configured for imaging based on oblique illumination with two illumination beams 108 as explained in Figure 6A with a phase plate 302 configured as explained in Figure 3A is a top view of the collection pupil plane 132 with a phase plate 302 configured as explained in Figure 6C corresponding to Figure 6B variations of, where the illumination beams 108a, 108b and the small pitch grating 208 are not in a Littrow configuration, but still provide that the diffraction orders from the large pitch grating 204 and the small pitch grating 208 are located in different radial bands in the collection pupil plane 132 to facilitate simultaneous imaging based on radially varying defocus distributions as disclosed herein.

[0096] As another illustration, although not shown, the illumination using the illumination beams 108a, 108b can also be used with additional configurations of the overlay metrology system 102 including, but not limited to, a ring phase plate 302 as depicted in Figure 3B or an objective lens 138 with adjustable spherical aberration as previously disclosed herein.

[0097] As another illustration, although not shown, the overlay metrology system 102 can illuminate the overlay target 106 with an illumination beam 108 oriented along different azimuthal directions (e.g., X and Y directions) to facilitate simultaneous imaging of target features having periodicity along different directions (e.g., as Figure 2B depicted in FIG. 1C).

[0098] Referring now to Figure 1C and 1D it is contemplated herein that reflections from the top surface of the sample 104 can reduce the contrast of the image of the overlay target 106, particularly in the case of oblique illumination.

[0099] In some embodiments, the overlay metrology system 102 includes an illumination collection field stop sized to match the field of view of the overlay target 106 (e.g., the cells 202 thereon) or the detector 128. For example, such a field stop can be configured as a confocal field stop sized to match the field of view of the overlay target 106 and / or the detector 128. Figure 1C is a conceptual diagram of a portion of the overlay metrology system 102 in accordance with one or more embodiments of the present disclosure, illustrating a field stop for blocking zero-order reflections from the top surface of the sample 104. In Figure 1C , the image of the field stop 144 is at the plane of the detector 128.

[0100] In particular, Figure 1C illustrates the objective lens 138 and a pupil plane 142 (e.g., the back focal plane of the objective lens 138) common to both the illumination path 116 (not shown) and the collection path 126. For clarity, Figure 1C further depicts illumination using a single oblique illumination beam 108 emanating from the pupil plane 142, but it is understood that the illumination beam 108 can be provided to the pupil plane 142 via the illumination path 116 as Figure 1B depicted in FIG. 1C.

[0101] An illumination beam 108 incident on the sample 104 at an oblique angle (Θ inc ) can result in various zero-order reflection beams, including but not limited to a top surface reflection beam 146 associated with reflection of the illumination beam 108 by the top surface 148 of the sample 104, and a buried feature reflection beam 150 associated with reflection of the illumination beam 108 by a buried feature of the overlay target 106. Each of these beams can have a width (w) that can be controlled by an illumination field stop or by any other suitable technique. As Figure 1C illustrated in FIG. 1C, these beams can be at an angle (Θ inc), the width (w) of the illumination beam 108, and the depth (h) of the buried feature within the sample 104. In particular, the reflected beams overlap (e.g., begin to flare) under the following condition:

[0102]

[0103] This condition can also be written based on Snell’s law in the sample 104 as:

[0104]

[0105] where n s is the refractive index of the sample 104 and NA min，obsc is the minimum numerical aperture (NA) that the illumination beam 108 can have to avoid flaring.

[0106] If the reflected beams do not overlap (e.g., flaring does not occur), then the top surface reflected beam 146 can be filtered using a collection field stop. In Figure 1C , this is illustrated as the top surface reflected beam 146 falling outside the field of the detector 128. However, it should be understood that the overlay metrology system 102 can also include a collection field stop in a relayed field plane in the collection path 126 to filter the top surface reflected beam 146 before the detector 128.

[0107] As an illustrative example considering a target feature (e.g., n s = 3.5) buried 270 microns below the surface of a silicon sample 104 and an illumination beam 108 that is 50 microns wide (e.g., w = 50 pm), the minimum NA (NA min，obsc ) of the illumination beam 108 is 0.323, which can be achieved with oblique illumination as disclosed herein.

[0108] The overlay metrology system 102 can be further configured (e.g., according to a metrology recipe) to provide customized illumination and / or feature sets on each layer of the overlay target 106. For example, the overlay metrology system 102 can illuminate the overlay target 106 with multiple illumination beams 108, where different illumination beams 108 and / or associated components of the collection path 126 are customized for each layer.

[0109] In some embodiments, the overlay metrology system 102 provides two illumination beams 108 with a common angle of incidence (e.g., having overlapping positions in the illumination pupil plane 120), but where the two illumination beams 108 have different beam widths (w) selected to prevent glare between the corresponding top surface reflected beam 146 and the buried feature reflected beam 150 for features at different depths. For example, the first illumination beam 108 can have a first width (w) selected to prevent glare between the top surface reflected beam 146 and the buried feature reflected beam 150 at a first depth (h) associated with the first layer 206 of the overlay target 106, while the second illumination beam 108 can have a second width (w) selected to prevent glare between the top surface reflected beam 146 and the buried feature reflected beam 150 at a second depth (not illustrated). Further, the collection path 126 can include one or more field stops tailored to block the top surface reflected beam 146 associated with each of the illumination beams 108.

[0110] The multiple illumination beams 108 having overlapping angles of incidence but different beam widths (w) can be produced using any technique known in the art. For example, the illumination path 116 can provide two illumination beams 108 in two arms including illumination field stops having different widths (w), and can further include a beam combiner to overlap the two illumination beams 108 at the common illumination pupil plane 120.

[0111] It is contemplated herein that the overlay target 106 can have features (e.g., the large pitch grating 204 or the small pitch grating 208) at any layer or any depth (h). In configurations where the overlay target 106 includes features (e.g., the large pitch grating 204 or the small pitch grating 208) on the top surface 148 of the sample 104 (e.g., h = 0), an interferometer (e.g., but not limited to, a Linnik interferometer having a reference beam tuned to be opposite in phase to the unwanted top surface reflected beam 146) can be used to mitigate (e.g., gray out) the unwanted zeroth order reflection.

[0112] Figure 1D is a conceptual diagram of an overlay metrology system 102 including a Linnik interferometer in accordance with one or more embodiments of the present disclosure.

[0113] In one embodiment, the overlay metrology system 102 includes a reference objective 152 configured to receive a portion of the illumination beam 108 through the beamsplitter 140, direct this portion of the illumination beam 108 to a reference sample 154, and collect light reflected from this reference sample 154. In this regard, the objective 138 and the sample 104 can form a measurement arm 156 of a Linnik interferometer, while the reference objective 152 and the reference sample 154 can form a reference arm 158 of the Linnik interferometer.

[0114] Reference sample 154 can include any sample suitable for providing a reference light in a Linnik interferometer. For example, reference sample 154 can be designed to at least partially replicate sample 104. In this regard, the Linnik interferometer can be balanced and the optical properties of light propagating through reference sample 154 can be the same or substantially similar to the optical properties of light propagating through sample 104.

[0115] In some embodiments, the Linnik interferometer further includes a phase adjuster 160 configured to provide that the phase of top-surface reflected beam 146 from sample 104 can have an opposite phase from a corresponding top-surface reflected beam 162 from reference sample 154. In this manner, top-surface reflected beam 146 from sample 104 and top-surface reflected beam 162 from reference sample 154 can negatively interfere on detector 128. Moreover, phase adjuster 160 can include any combination of one or more optical components suitable for adjusting the optical phase of light, such as but not limited to a wedge or a waveplate.

[0116] It should be appreciated that, Figure 1D and the associated description of the Linnik interferometer is provided for illustrative purposes only and should not be construed as limiting. Rather, overlay metrology system 102 can include any type of interferometer known in the art to mitigate unwanted top-surface reflected beam 146 from a target feature on top surface 148 of sample 104.

[0117] Again generally referring to Figures 1A to 1D In accordance with one or more embodiments of the present disclosure, various additional aspects of overlay metrology system 102 are described in greater detail.

[0118] Illumination source 114 can include any type of light source known in the art. In one embodiment, illumination source 114 comprises one or more coherent sources, such as but not limited to one or more laser sources. In this regard, illumination source 114 can generate illumination beam 108 having high coherence (e.g., high spatial coherence and / or temporal coherence). For example, illumination source 114 can include one or more broadband lasers, such as but not limited to one or more supercontinuum lasers or white light lasers. By way of another example, illumination source 114 can include one or more narrowband lasers. By way of another example, illumination source 114 can include one or more tunable lasers that provide illumination beam 108 having a tunable spectral intensity. Moreover, coherent illumination source 114 can be based on any type of technology or product design. For example, illumination source 114 can include, but is not limited to, any combination of one or more fiber lasers, one or more diode lasers, or one or more gas lasers.

[0119] In another embodiment, the illumination source 114 includes one or more sources that provide the illumination beam 108 with low or partial coherence (e.g., spatial and / or temporal coherence). For example, the illumination source 114 can include one or more light emitting diodes (LEDs) or superluminescent LEDs. By way of another example, the illumination source 114 can include a laser-sustained plasma (LSP) source such as, but not limited to, an LSP lamp, LSP bulb, or LSP chamber adapted to house one or more elements that can emit broadband illumination when excited into a plasma state by a laser source. By way of another example, the illumination source 114 can include a lamp source such as, but not limited to, an arc lamp, a discharge lamp, an electrodeless lamp, or the like.

[0120] The illumination source 114 can provide the illumination beam 108 with any selected wavelength or range of wavelengths (e.g., spectrum). It is contemplated herein that the spectrum of the illumination beam 108 can be selected to be transmitted through at least a portion of the sample 104 to reach a feature on the overlay target 106 in a subsurface layer with minimal or at least acceptable absorption. For example, in the case of a sample 104 formed of two bonded semiconductor substrates, the spectrum of the illumination beam 108 can be selected to include wavelengths in the infrared spectral range. However, it should be understood that the systems and methods disclosed herein can be widely applicable to a broad range of samples such that the illumination beam 108 can have any selected spectrum based on the composition of the sample 104.

[0121] The illumination source 114 can further provide light with any selected temporal characteristic. In one embodiment, the illumination source 114 includes one or more continuous wave sources that provide a continuous wave illumination beam 108. In another embodiment, the illumination source 114 includes one or more pulsed sources that provide a pulsed or otherwise modulated illumination beam 108. For example, the illumination source 114 can include one or more mode-locked lasers, one or more Q-switched lasers, or the like. It is contemplated herein that the use of pulsed laser sources for illumination can provide high throughput measurements in either static or scanning modes of operation.

[0122] It is contemplated herein that an illumination source 114 that provides a pulsed laser illumination beam 108 can provide various benefits to the measurement of high overlay targets 106 as disclosed herein.

[0123] For example, the pulsed laser illumination beam 108 can provide a relatively high peak power suitable for overlay measurements coupled with a relatively low NA. It is contemplated herein that reducing the NA of the illumination beam 108 generally can alleviate the requirement for a radially varying defocus profile as disclosed herein. In particular, the size of the diffraction lobes in the collection pupil plane 132 is based on the NA of the illumination beam 108. Thus, reducing the NA of the illumination beam 108 relaxes the requirement to separate the associated diffraction lobes into different radial bands of the collection pupil plane 132 (e.g., a required pitch difference between the large pitch grating 204 and the small pitch grating 208, or the like). In the case of a stepped radially varying defocus profile, reducing the NA of the illumination beam 108 can relax the width and / or radius requirements of the phase plate 302 providing the radially varying defocus profile. In the case of a continuously varying defocus profile (e.g., as provided by the objective lens 138 with adjustable spherical aberration, or the like), reducing the NA of the illumination beam 108 can reduce the impact of the spatially varying defocus across the beam profile of the collected diffraction orders.

[0124] As another example, the pulsed laser illumination beam 108 can facilitate high throughput measurements in a scanning mode of operation. As a non-limiting illustration, the illumination beam 108 having pulses on the order of 1 nanosecond can be suitable for sampling at approximately 250 measurements per second. However, in a general sense, the illumination beam 108 can have any suitable pulse duration and / or repetition rate. In this manner, the illumination beam 108 can have, without limitation, a pulse duration on the order of microseconds, nanoseconds, picoseconds, or femtoseconds. Further, the illumination beam 108 can have, without limitation, a repetition rate on the order of kHz, MHz, or GHz.

[0125] The sample positioning subsystem 112 can include any components suitable for translating the sample 104 relative to the illumination beam 108 and / or translating the illumination beam 108 relative to the sample 104. For example, the sample positioning subsystem 112 can include at least one translation stage 164 adjusting the position of the sample 104 along any dimension (e.g., without limitation, lateral position within an X-Y plane, axially along a Z-axis (e.g., an optical axis of the objective lens 138), flipping, tilting, or the like). As another example, although not shown, the sample positioning subsystem 112 can include one or more beam scanning optical elements (not shown) suitable for scanning the illumination beam 108 across the sample 104 (e.g., without limitation, a galvanometer or rotatable mirror).

[0126] In some embodiments, the overlay metrology system 100 includes a controller 166. The controller 166 can include one or more processors 168 configured to execute program instructions maintained on a memory 170 or memory medium. In this regard, the one or more processors 168 of the controller 166 can perform any of the various program steps described throughout the present disclosure. Further, the controller 166 can be communicatively coupled to the overlay metrology sub-system 102 or any component therein.

[0127] The one or more processors 168 of the controller 166 can include any processor or processing element known in the art. For the purposes of the present disclosure, the term "processor" or "processing element" can be broadly defined to encompass any device with 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 168 can include any device configured to execute algorithms and / or instructions (e.g., program instructions stored in memory). In one embodiment, the one or more processors 168 can be embodied as a desktop computer, a mainframe computer system, a workstation, a graphics computer, a parallel processor, a networked computer, or any other computer system configured to execute a program configured to operate the overlay metrology system 100 or in conjunction with the overlay metrology system 100, as described throughout the present disclosure.

[0128] Further, different sub-systems of the overlay metrology system 100 can include processors or logic elements suitable for carrying out at least a portion of the steps described in the present disclosure. Accordingly, the above description should not be interpreted as a limitation on the embodiments of the present disclosure but merely an illustration. Further, the steps described throughout the present disclosure can be carried out by a single controller 166 or, alternatively, multiple controllers. Further, the controller 166 can include one or more controllers housed in a common housing or within multiple housings. In this manner, any controller or combination of controllers can be individually packaged as a module suitable for integration into the overlay metrology system 100.

[0129] The memory 170 can include any storage media suitable for storing program instructions executable by the associated one or more processors 168. For example, the memory 170 can include a non-transitory memory medium. By way of another example, the memory 170 can include, but is not limited to, read-only memory (ROM), random access memory (RAM), magnetic or optical memory devices (e.g., magnetic diskettes, magnetic tapes, solid state drives, and the like). It is further noted that the memory 170 can be housed in a common controller housing with the one or more processors 168. In one embodiment, the memory 170 can be remotely located relative to the physical location of the one or more processors 168 and the controller 166. For example, the one or more processors 168 of the controller 166 can access a remote memory (e.g., a server) that can be accessed over a network (e.g., the Internet, an intranet, and the like).

[0130] All of the methods described herein can include storing results of one or more steps of the method embodiments in a memory. The results can include any of the results described herein and can be stored in any manner known in the art. The memory can include any memory described herein or any other suitable storage medium known in the art. After the results have been stored, they can be accessed in the memory and used by any of the method or system embodiments described herein; formatted for display to a user; used by another software module, method, or system, and the like. Further, the results can be stored "permanently," "semi-permanently," "temporarily," or for a period of time. For example, the memory can be random access memory (RAM) and the results can not necessarily remain in the memory indefinitely.

[0131] It should be further appreciated that each of the embodiments of the methods described above can include any other step of any other method described herein. Additionally, each of the embodiments of the methods described above can be performed by any of the systems described herein.

[0132] Those skilled in the art will recognize that, for clarity and instructional purposes, the component operations, devices, objects and attendant discussions herein are set forth as examples and with the intent that various configurations modifications are contemplated. Accordingly, as is used herein the specific exemplars set forth and the accompanying discussion are intended to be representative of their more general classes. In general, the use of any specific exemplar is intended to be representative of its class, and the specific components, operations, devices and objects should not be construed as limiting.

[0133] As used herein, directional terms such as "top," "bottom," "upper," "lower," "up," "down," "over," "under," "above," and "below" are intended to provide relative positions for purposes of description, and are not intended to designate absolute reference frames. Various modifications to the implementations described in this document will be readily apparent to those skilled in the art, and the generic principles defined herein can be applied to other implementations without departing from the spirit or scope of the disclosure. Similarly, the generic principles defined herein can be applied to other implementations without departing from the spirit or scope of the disclosure.

[0134] With respect to the use of any plural and / or singular term herein, those having skill in the art can translate from the plural to the singular and / or from the singular to the plural as is appropriate to the context and / or application. For the sake of clarity, various singular / plural permutations are not explicitly addressed herein.

[0135] Objects described herein sometimes illustrate different components contained within other components or connected to 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 intermediate 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 mateable and / or physically interacting components and / or wirelessly interactable and / or wirelessly interacting components and / or logically interacting and / or logically interactable components.

[0136] Further, it is to be understood that the invention is defined by the appended claims. Those skilled in the art will appreciate that, in general, the terms used in the description herein, and particularly in the appended claims (e.g., the body of the appended claims), are intended to be interpreted broadly. For example, the terms "including" and "having" should be interpreted as "including but not limited to," the term "has" and its derivatives should be interpreted as "has at least" and the term "including" should be interpreted as "including but not limited to," and so on. It will be further understood that, in general, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to "a compound" includes a single compound as well as two or more compounds, and the like. In describing and claiming the present invention, the following terminology will be used in accordance with the definitions set out below. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting, unless the context clearly indicates otherwise. Thus, for example, terms such as "about," "substantially," "approximately," and the like are intended to mean that quantities, dimensions, and other parameters are within 30%, 20%, 10%, 5%, or 1% of the stated value, unless otherwise stated.For example, the phrase "A or B" will be understood to include the possibilities of "A" or "B" or "A and B."

[0137] It is believed that, through the description above, one will understand the disclosure and its many attendant features and advantages, and it will be apparent that various changes can be made in the form, construction and arrangement of the components without departing from the scope of the disclosure or sacrificing all of its material advantages. The form described is merely exemplary and is intended to be within the scope of the appended claims as to be interpreted in the broadest sense allowable. Additionally, it is intended that the application be defined by the claims appended hereto and their equivalents.

Claims

1. An overlay metrology system comprising: an objective lens; one or more illumination optics configured to direct illumination from an illumination source through the objective lens onto an overlay target on a sample when implementing a metrology recipe, wherein the overlay target according to the metrology recipe includes a first grating with a first pitch on a first sample layer and a second grating with a second pitch on a second sample layer, and wherein the second pitch is less than the first pitch, wherein the first sample layer is separated from the second sample layer by a layer separation distance that is greater than a depth of field of the objective lens; a detector; and one or more collection optics configured to direct at least a portion of light collected through the objective lens to the detector, wherein the metrology recipe specifies that diffraction orders of the illumination by the first grating that are positioned below a threshold radius in a pupil plane collected through the objective lens and diffraction orders of the illumination by the second grating that are positioned above the threshold radius in the pupil plane collected through the objective lens, wherein the one or more collection optics provide a radially varying defocus profile to compensate for the layer separation distance, wherein the first grating and the second grating are simultaneously focused on the detector; and a controller including one or more processors configured to execute program instructions that cause the one or more processors to perform the metrology recipe by: receiving one or more images of the overlay target from the detector, wherein the first grating and the second grating are simultaneously focused on the detector; and determining an overlay measurement between the first sample layer and the second sample layer of the sample based on the one or more images.

2. The overlay metrology system of claim 1, wherein the one or more collection optics include a phase plate in the pupil plane having a radius equal to the threshold radius to provide the radially varying defocus profile, wherein the phase plate introduces a phase delay to the diffraction orders of the illumination by the first grating to compensate for the layer separation distance.

3. The overlay metrology system of claim 1, wherein the one or more collection optics include a lens in the pupil plane having a radius equal to the threshold radius to provide the radially varying defocus profile, wherein a power of the lens compensates for the layer separation distance by adjusting a focal position of the diffraction orders of the illumination by the first grating to match a focal position of the diffraction orders of the illumination by the second grating outside of the lens.

4. The overlay metrology system of claim 1, wherein the one or more collection optics include an objective lens having one or more adjustable optics to provide an adjustable spherical aberration as the radially varying defocus profile.

5. The overlay metrology system of claim 4, wherein the one or more adjustable optics of the objective lens to provide the adjustable spherical aberration as the radially varying defocus profile comprise: a corrector ring. ​ 6. The overlay metrology system of claim 1, wherein the illumination from the illumination source is pulsed.

7. The overlay metrology system of claim 1, further comprising: at least one of a translation stage to translate the relative position of the sample and the illumination during measurement or one or more beam scanning optics.

8. The overlay metrology system of claim 1, wherein the first grating and the second grating are located in an overlapping region of the sample according to the metrology recipe.

9. The overlay metrology system of claim 1, wherein the first grating and the second grating are located in a non-overlapping region of the sample according to the metrology recipe.

10. The overlay metrology system of claim 1, wherein the layer separation distance is greater than 10 microns.

11. The overlay metrology system of claim 1, wherein the layer separation distance is greater than 100 microns.

12. The overlay metrology system of claim 1, wherein the first sample layer is closer to the objective lens than the second sample layer according to the metrology recipe.

13. The overlay metrology system of claim 1, wherein the second sample layer is closer to the objective lens than the first sample layer according to the metrology recipe.

14. The overlay metrology system of claim 1, wherein the sample includes a first substrate bonded to a second substrate according to the metrology recipe, wherein the first grating is located on the first substrate, wherein the second grating is located on the second substrate.

15. The overlay metrology system of claim 1, wherein the sample includes a die bonded to a substrate according to the metrology recipe, wherein the first grating is located on the die, wherein the second grating is located on the substrate.

16. The overlay metrology system of claim 1, wherein the sample includes a die bonded to a substrate according to the metrology recipe, wherein the first grating is located on the die, wherein the second grating is located on a scribe lane.

17. The overlay metrology system of claim 1, wherein the metrology recipe specifies that the one or more illumination optics direct the illumination from the illumination source at a normal incidence angle, wherein + / - 1storder diffraction lobes from the first grating are located below the threshold radius in the pupil plane, wherein + / - 1storder diffraction lobes from the second grating are located above the threshold radius in the pupil plane.

18. The overlay metrology system of claim 17, wherein the one or more collection optics further include one or more elements to block zeroth order diffraction of the illumination from reaching the detector.

19. The overlay metrology system of claim 1, wherein the metrology recipe specifies that the one or more illumination optics direct a first illumination beam of the illumination from the illumination source at a first azimuthal angle of incidence, wherein first and second order diffracted lobes of the first illumination beam from the first grating are located below the threshold radius in the pupil plane, wherein a first order diffracted lobe and a zero order diffracted lobe of the first illumination beam from the second grating are located above the threshold radius in the pupil plane.

20. The overlay metrology system of claim 19, further comprising: an additional detector, wherein the detector receives the first and second order diffracted lobes of the first illumination beam from the first grating, wherein the additional detector receives the first order diffracted lobe and the zero order diffracted lobe of the first illumination beam from the second grating.

21. The overlay metrology system of claim 19, wherein the metrology recipe specifies that the one or more illumination optics direct a second illumination beam of the illumination from the illumination source at a second azimuthal angle of incidence, wherein first and second order diffracted lobes of the second illumination beam from the first grating are located below the threshold radius in the pupil plane, wherein a first order diffracted lobe and a zero order diffracted lobe of the second illumination beam from the second grating are located above the threshold radius in the pupil plane.

22. The overlay metrology system of claim 21, further comprising: an additional detector, wherein the detector receives: the first and second order diffracted lobes of the first illumination beam from the first grating; and the first and second order diffracted lobes of the second illumination beam from the first grating; wherein the additional detector receives: the first order diffracted lobe and the zero order diffracted lobe of the first illumination beam from the second grating; and the first order diffracted lobe and the zero order diffracted lobe of the second illumination beam from the second grating.

23. The overlay metrology system of claim 21, wherein the first azimuthal angle of incidence and the second azimuthal angle of incidence satisfy the Littrow condition.

24. The overlay metrology system of claim 21, wherein the first azimuthal angle of incidence and the second azimuthal angle of incidence are symmetric along a measurement direction.

25. The overlay metrology system of claim 21, wherein the first azimuthal angle of incidence and the second azimuthal angle of incidence are orthogonal.

26. The overlay metrology system of claim 19, wherein the one or more illumination optics further include a field stop to prevent reflections of the first illumination beam from a surface of the sample from reaching the detector. ​ ​ 27. The overlay metrology system of claim 19, wherein one of the first grating or the second grating is on a surface of the sample, wherein the one or more collection optics include an interferometer that provides a reference beam having an opposite phase to the zeroth order diffraction lobe to mitigate the zeroth order diffraction lobe.

28. An overlay metrology system, comprising: a controller communicatively coupled to an overlay metrology subsystem, the controller including one or more processors configured to execute program instructions that cause the one or more processors to perform a metrology recipe by: receiving one or more images of an overlay target from a detector of the overlay metrology subsystem, wherein a first grating and a second grating are simultaneously in focus on the detector, wherein the overlay metrology subsystem comprises: an objective; one or more illumination optics configured to direct illumination from an illumination source through the objective onto the overlay target on a sample when performing the metrology recipe, wherein the overlay target according to the metrology recipe includes the first grating having a first pitch on a first sample layer and the second grating having a second pitch on a second sample layer, wherein the second pitch is less than the first pitch, wherein the first sample layer is separated from the second sample layer by a layer separation distance that is greater than a depth of field of the objective; a detector; and one or more collection optics configured to direct at least a portion of light collected through the objective to the detector, wherein the metrology recipe prescribes that diffraction orders of the illumination by the first grating that are positioned below a threshold radius in a pupil plane collected through the objective and diffraction orders of the illumination by the second grating that are positioned above the threshold radius in the pupil plane collected through the objective, wherein the one or more collection optics provide a radially varying defocus profile to compensate for the layer separation distance, wherein the first grating and the second grating are simultaneously in focus on the detector; and determining an overlay measurement between the first sample layer and the second sample layer of the sample based on the one or more images.

29. The overlay metrology system of claim 28, wherein the metrology recipe prescribes that the one or more illumination optics direct the illumination from the illumination source at a normal angle of incidence, wherein + / - 1storder diffraction lobes from the first grating are positioned below the threshold radius in the pupil plane, wherein + / - 1storder diffraction lobes from the second grating are positioned above the threshold radius in the pupil plane.

30. The overlay metrology system of claim 29, wherein the one or more collection optics further include one or more elements to block a zeroth order diffraction of the illumination from reaching the detector.

31. The overlay metrology system of claim 28, wherein the metrology recipe specifies that the one or more illumination optics direct a first illumination beam of the illumination from the illumination source at a first azimuthal angle of incidence, wherein first and second order diffracted lobes of the first illumination beam from the first grating are located below the threshold radius in the pupil plane, wherein a first order diffracted lobe and a zeroth order diffracted lobe of the first illumination beam from the second grating are located above the threshold radius in the pupil plane.

32. The overlay metrology system of claim 31, further comprising: an additional detector, wherein the detector receives the first and second order diffracted lobes of the first illumination beam from the first grating, wherein the additional detector receives the first and zeroth order diffracted lobes of the first illumination beam from the second grating.

33. The overlay metrology system of claim 31, wherein metrology recipe specifies that the one or more illumination optics direct a second illumination beam of the illumination from the illumination source at a second azimuthal angle of incidence, wherein first and second order diffracted lobes of the second illumination beam from the first grating are located below the threshold radius in the pupil plane, wherein a first order diffracted lobe and a zeroth order diffracted lobe of the second illumination beam from the second grating are located above the threshold radius in the pupil plane.

34. The overlay metrology system of claim 33, further comprising: an additional detector, wherein the detector receives: the first and second order diffracted lobes of the first illumination beam from the first grating; and the first and second order diffracted lobes of the second illumination beam from the first grating; wherein the additional detector receives: the first and zeroth order diffracted lobes of the first illumination beam from the second grating; and the first and zeroth order diffracted lobes of the second illumination beam from the second grating.

35. The overlay metrology system of claim 33, wherein the first azimuthal angle of incidence and the second azimuthal angle of incidence satisfy the Littrow condition.

36. The overlay metrology system of claim 28, wherein the illumination from the illumination source is pulsed.

37. The overlay metrology system of claim 28, wherein the overlay metrology sub-system further comprises: at least one of a translation stage to translate the relative position of the sample and the illumination during measurement or one or more beam scanning optics.

38. A method of overlay metrology, comprising: receiving one or more images of an overlay target on a sample from a detector of an overlay metrology sub-system, wherein the overlay target according to a metrology recipe includes a first grating having a first pitch on a first sample layer and a second grating having a second pitch on a second sample layer, wherein the second pitch is less than the first pitch, wherein the first grating and the second grating are simultaneously in focus on the detector, wherein the overlay metrology sub-system comprises: ​ ​ an objective lens; one or more illumination optics configured to direct illumination from an illumination source onto the overlay target on the sample through the objective lens when implementing the metrology recipe, wherein the first sample layer is separated from the second sample layer by a layer separation distance greater than a depth of field of the objective lens; a detector; and one or more collection optics configured to direct at least a portion of light collected through the objective lens to the detector, wherein the metrology recipe specifies that diffraction orders of the illumination by the first grating located below a threshold radius in a pupil plane collected through the objective lens and diffraction orders of the illumination by the second grating located above the threshold radius in the pupil plane collected through the objective lens, wherein the one or more collection optics provide a radially varying defocus profile to compensate for the layer separation distance, wherein the first grating and the second grating are simultaneously focused on the detector; and determine an overlay measurement between the first sample layer and the second sample layer of the sample based on the one or more images.

39. An overlay metrology target, comprising: a first substrate; a second substrate bonded to the first substrate; a first overlay target, comprising: a first grating having a first pitch on one of a first sample layer or a second sample layer; a second grating having a second pitch on a different sample layer of the first sample layer or the second sample layer than the first grating target, wherein features of the first and second gratings are distributed in a first direction, wherein one of the first or second sample layers corresponds to a scribe lane on the second substrate and the other of the first or second sample layers corresponds to a layer in a die of the first substrate; a second overlay target, comprising: a third grating having the first pitch on one of the first sample layer or the second sample layer; a fourth grating having the second pitch on a different sample layer of the first sample layer or the second sample layer than the third grating target, wherein features of the third and fourth gratings are distributed in a first direction.

40. The overlay metrology target of claim 39, wherein the first sample layer and the second sample layer are designed to be separated by a layer separation distance greater than a depth of field of an objective lens of a metrology system according to a metrology recipe, wherein the first and second pitches are designed according to the metrology recipe to specify that diffraction orders of illumination by the first and third gratings located below a threshold radius in a pupil plane collected through the objective lens and diffraction orders of the illumination by the second and fourth gratings located above the threshold radius in the pupil plane collected through the objective lens, wherein overlay measurements associated with the first sample layer and the second sample layer are determinable based on images of the first and second overlay targets, wherein the metrology system provides a radially varying defocus profile to compensate for the layer separation distance such that the first, second, third, and fourth gratings are simultaneously focused on one or more collection optics of the images.

Images