Scanning scatterometry overlay measurement

Abstract

To solve the problem that a time required for setting before measurement of a translational movement stage may apply a harmful effect to throughput.SOLUTION: An illumination sub-system of an overlay measurement system sequentially illuminates an overlay target with a first illumination lobe and a second illumination beam opposite to a first illumination beam thereof. The overlay target includes grating-over-grating features formed of periodic structures on a first sample layer and a second sample layer. The system further includes an imaging sub-system to generate a first image and a second image of the overlay target. The first image includes an unresolved image of the grating-over-grating structures formed of a single non-zero diffraction order of the first illumination beam. The second image includes an unresolved image of the one or more grating-over-grating structures formed of a single non-zero diffraction order of the second illumination beam. The system determines an overlay error between the first layer and the second layer based on the first image and the second image.SELECTED DRAWING: Figure 1A

Description

Technical Field

[0001] The present disclosure generally relates to scatterometry overlay measurement, and more particularly to scanning overlay measurement on a moving sample.

Background Art

[0002] With the increasing demand for smaller semiconductor devices, there is a corresponding increase in the demand for accurate and efficient measurement. One way to improve the efficiency and throughput of measurement tools is to generate measurement data for a sample while the sample is moving rather than when it is in a stationary position within the measurement field of view. This can eliminate the time delay associated with the pre-measurement alignment of the translation stage. A measurement system typically generates sample-related measurement data by measuring or otherwise examining dedicated measurement targets distributed throughout the sample. Thus, the sample is generally placed on a translation stage and translated so that the measurement targets sequentially move into the measurement field of view. In a typical measurement system employing the move-and-measure (MAM) method, the sample is stationary during each measurement. However, the time required for pre-measurement alignment of the translation stage can have an adverse effect on throughput.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] Therefore, it is desirable to provide a system and method for solving the above drawbacks.

Means for Solving the Problem

[0005] Disclosed is an overlay measurement system according to one or more exemplary embodiments of the present disclosure. In an exemplary embodiment, the system includes an illumination subsystem configured to sequentially illuminate an overlay target on a sample with a first illumination lobe and a second illumination lobe opposite the first illumination lobe. The overlay target includes one or more grating-over-grating features formed of periodic structures on a first sample layer and a second sample layer, and the one or more grating-over-grating structures have periodicity along one measurement direction. In another exemplary embodiment, the system includes an imaging subsystem having an objective lens and a detector, the detector being configured to generate a first image and a second image of the overlay target. The first image includes a non-resolved image of the one or more grating-over-grating structures formed by a single diffraction order other than the zero order of the first illumination lobe generated along the measurement direction by the one or more grating-over-grating structures, and the second image includes a non-resolved image of the one or more grating-over-grating structures formed by a single diffraction order other than the zero order of the second illumination lobe generated along the measurement direction by the one or more grating-over-grating structures. In another exemplary embodiment, the system includes a controller coupled to communicate with the detector, the controller being configured to determine an overlay error between the first layer and the second layer of the sample along the measurement direction based on the first image and the second image.

[0006] Disclosed is an overlay measurement system according to one or more exemplary embodiments of the present disclosure. In an exemplary embodiment, the system includes an illumination subsystem, and the illumination subsystem provides a first pair of opposing illumination lobes along a first measurement direction and a second pair of illumination lobes along a second measurement direction orthogonal to the first measurement direction. In another exemplary embodiment, the illumination subsystem sequentially illuminates an overlay target on a sample with a first illumination configuration and a second illumination configuration, the first illumination configuration including one illumination lobe from each of the first pair and the second pair of opposing illumination lobes, and the second illumination configuration including the remaining other illumination lobe from each of the first illumination lobe pair and the second illumination lobe pair. In another exemplary embodiment, the overlay target is a first set of one or more cells including one or more overlay grating features formed of a periodic structure on a first sample layer and a second sample layer, the one or more overlay grating structures having periodicity along the first direction, and including the first set. In another exemplary embodiment, the overlay target is a second set of one or more cells including one or more overlay grating features formed of a periodic structure on the first sample layer and the second sample layer, the one or more overlay grating structures having periodicity along the second measurement direction, and including the second set. In another exemplary embodiment, the system includes an imaging subsystem, the imaging subsystem including an objective lens and a detector, the detector being configured to generate a first image and a second image of the overlay target, the first image being formed by a single diffraction order other than the zero order from each illumination lobe in the first illumination configuration, and the second image being formed by a single diffraction order other than the zero order from each illumination lobe in the second illumination configuration. In another exemplary embodiment, the system includes a controller coupled to communicate with the detector, the controller being for determining an overlay error between the first layer and the second layer of the sample along the first measurement direction and the second measurement direction based on the first image and the second image.

[0007] Disclosed is an overlay measurement method according to one or more exemplary embodiments of the present disclosure. In an exemplary embodiment, the method includes sequentially illuminating an overlay target on a sample with a first illumination lobe and a second illumination lobe, the overlay target including one or more stacked grating features formed of structures having periodicity on a first sample layer and a second sample layer, the one or more stacked grating structures having periodicity along one measurement direction. In another exemplary embodiment, the method includes generating a first image and a second image of the overlay target using a detector, the first image including a non-resolved image of the one or more stacked grating structures formed with a single diffraction order other than the zero order of the first illumination lobe along the measurement direction from the one or more stacked grating structures, the second image including a non-resolved image of the one or more stacked grating structures formed with a single diffraction order other than the zero order of the second illumination lobe generated along the measurement direction by the one or more stacked grating structures. In another exemplary embodiment, the method includes determining an overlay error between the first layer and the second layer of the sample along the measurement direction based on the first image and the second image.

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

[0009] Those skilled in the art will gain a deeper understanding of many advantages of the present disclosure by referring to the accompanying drawings.

Brief Description of the Drawings

[0010]

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Best Mode for Carrying Out the Invention

[0011] Next, reference will be made in detail to the subject matter of the present disclosure shown in the accompanying drawings. In particular, specific embodiments and features specific to those embodiments are shown and described herein. The embodiments described herein are to be construed as illustrative and not restrictive. It should be readily apparent to those skilled in the art that various changes / modifications of the form and details can be made without departing from the spirit and scope of the present disclosure.

[0012] Each embodiment of the present disclosure is directed to a system and method for scanning scatterometry measurement using a field-of-view imaging of a stacked grating overlay target, wherein the measurement is based on sequentially imaging the target with a symmetrically opposed illumination beam while the overlay target is moving, and the field-of-view image is formed using a single diffraction order along one measurement direction. From this point, the field-of-view image corresponds to a dark-field image, and although the features of the overlay target are not resolved, the intensity difference between the two sequentially obtained images of this overlay target feature is proportional to the overlay.

[0013] In the present disclosure, the term "scatterometry measurement" is used to broadly encompass the terms "scatterometry-based measurement" and "diffraction-based measurement". Also, the term "scanning measurement" is used to describe measurement values generated during the movement of a sample. In a general sense, a scanning measurement can be performed by scanning a sample along a measurement path (such as a swath) such that the target area of the sample (e.g., a measurement target or device area on the sample) translates through the measurement field of view of the measurement system. Further, this process can be repeated any number of times for any number of measurement paths or for repeated measurements of a specific measurement path to provide any selected number of measurement values of the sample. Regarding scatterometry measurements based on dark-field imaging of an overlay target having periodicity, an overview is described in U.S. Patent Application No. 16 / 996,328, filed on August 18, 2020, the entire contents of which are incorporated herein by reference.

[0014] An overlay target can typically include a well-defined transfer element. This transfer element is designed to accurately represent the relative arrangement of features associated with one or more lithography steps for one or more sample layers. From this point, the characteristics of each transfer element of the overlay target measured by actual measurement (e.g., by an overlay measurement tool) can represent the transfer device elements related to the device being manufactured. Further, the overlay target can include one or more measurement cells each including a transfer element in one or more layers on the sample. Thus, one overlay measurement value can be based on any combination of measurement values of such various cells of the overlay target.

[0015] In some embodiments, one overlay target suitable for the scanning scatterometry measurements disclosed herein can include one or more cells including one overlapping grating structure, where the overlapping grating structure includes features (e.g., grating features) having periodicity over an overlapping region of two or more target layers. Thus, various grating features on each target layer can contribute to the diffraction of the incident illumination, and overlay measurements can be generated based on the analysis of the diffracted light.

[0016] In some embodiments, a scanning scatterometry overlay tool generates one overlay measurement along one measurement direction. This is done by sequentially generating two field-of-view images of the scatterometry overlay target using opposing illumination beams (e.g., two beams having opposing incident azimuth angles along the measurement direction). Each field-of-view image is formed using a single diffraction order other than the zero order from the scatterometry overlay target. Thus, the overlapping grating structure appears as a gray-scale feature that is not resolved in the field-of-view images, and the intensity difference between the field-of-view images of this gray-scale feature is proportional to the overlay.

[0017] It is contemplated herein that the distribution of diffraction orders associated with the diffraction of the incident illumination beam can be determined by various illumination / collection conditions. The conditions include, but are not limited to, the incident angles in the azimuth and elevation directions of the illumination beam, the wavelength of the illumination beam, the pitch of the periodic features on the sample, or the numerical aperture (NA) of the collection optics. Further, the overlay measurement tool can include one or more elements for blocking one or more of the collected diffraction orders so that the selected diffraction order serves as the basis of the image. From this point, the illumination beam can be directed to the overlay target at an incident angle outside the angle of the NA of the collection optics (e.g., outside the lens (OTL) configuration), or through a lens common to the collection optics (e.g., through the lens (TTL) configuration).

[0018] Yet another embodiment of the present disclosure is directed to overlay measurements in two directions (e.g., two orthogonal directions). For example, an overlay target may include a first set of one or more cells having periodicity along a first measurement direction and a second set of one or more cells having periodicity along a second measurement direction different from the first direction (e.g., a direction orthogonal to the first direction). In this configuration, illumination of the overlay target by an illumination beam along the first direction (e.g., having an azimuth angle along the first direction) can achieve focusing of a single diffraction order along the first direction by the first cell set without including diffraction orders from the second cell set. As a result, only the first cell set may appear in the image. Similarly, only the second cell set may appear in the image generated using an illumination beam along the second direction. In some embodiments, the two-direction overlay measurement values are generated based on four images based on sequential quadrupole illumination. In this case, the directions of the illumination beams are such that two have azimuth angles facing each other along the first direction and two have azimuth angles facing each other along the second direction. In some embodiments, the overlay measurement values are based on two images based on sequentially illuminating with two pairs of illumination beams of a quadrupole distribution. In this case, the first pair of illumination beams includes one illumination beam along the first direction and one illumination beam along the second direction, and the second pair of illumination beams includes a beam along the first direction and a beam along the second direction that are opposite to the respective beams. Thus, all cells within the overlay target appear in their respective images, but the image of each cell is formed by a single diffraction order from a single illumination beam.

[0019] This specification further contemplates that scanning measurements based on sequentially illuminating opposing illumination beams can be implemented in various forms. In some embodiments, images of the overlay target by the opposing illumination beams are sequentially generated by sequential scanning of the overlay target. In some embodiments, the above images are generated during a single scan based on alternating the illumination and / or light collection conditions during scanning. For example, one interleaved image can be generated based on alternately illuminating the overlay target with opposing illumination beams. Two field-of-view images can be extracted from this interleaved image. In one example, a scanning scatterometry tool is configured to illuminate the alternately arranged columns on the TDI sensor and to synchronize the alternating illumination conditions of the scatterometry overlay target with the charge transfer speed of the TDI sensor to generate an interleaved image. The generation of interleaved images in scanning measurements using a TDI sensor is outlined in U.S. Patent Application No. 16 / 586,504, filed on September 27, 2019, which is hereby incorporated by reference in its entirety.

[0020] Furthermore, while many embodiments in this disclosure relate to scanning measurements, it should be understood that the systems and methods disclosed herein are not limited to scanning measurements. Rather, this specification contemplates that the systems and methods disclosed herein can also be used in a stationary measurement mode where the sample is stationary during measurement.

[0021] Next, with reference to FIGS. 1A-9, systems and methods for scatterometry overlay measurement according to one or more embodiments of the present disclosure will be described in detail.

[0022] FIG. 1A is a conceptual diagram of a system 100 for performing overlay measurement according to one or more embodiments of the present disclosure.

[0023] In one embodiment, system 100 includes an overlay measurement tool 102. The overlay measurement tool 102 generates one or more field-of-view images of a sample 104 or a portion thereof based on a single diffraction order other than zero along an arbitrary given measurement direction from the sample 104. For example, system 100 may generate a dark-field image of an overlay target that includes features in two or more layers of sample 104.

[0024] In one embodiment, the overlay measurement tool 102 includes an illumination subsystem 106 that generates illumination in the form of at least one illumination beam 108 for each illumination of the sample 104, and an imaging subsystem 110 that images the illuminated sample 104. In another embodiment, the overlay measurement tool 102 includes a scanning subsystem 112 that positions the sample 104 within a measurement field of view for imaging. For example, the scanning subsystem 112 can include one or more translational stages that hold the sample 104, and the translational stages are configured to position an overlay target within the measurement field of view for imaging. Further, the overlay measurement tool 102 can illuminate and image the sample 104 in a scanning mode and / or a stationary mode. For example, the implementation of overlay scanning measurement in the overlay measurement tool 102 can be performed by coordinating the illumination by the illumination subsystem 106 and the image capture by the imaging subsystem 110 with the movement of the sample 104 along a measurement path (e.g., a swath) by the scanning subsystem 112 to capture an image of the overlay target on the measurement path. As another example, the implementation of overlay stationary measurement in the overlay measurement tool 102 can be performed by sequentially translating the sample 104 to position an overlay target within the measurement field of view and imaging the overlay target while the sample 104 is stationary.

[0025] Overlay measurement tool 102 may be suitable for generating perspective images of various overlay target designs in various sample configurations. In one embodiment, sample 104 is formed as two or more pattern layers on a single wafer. Thus, one overlay target including target features on two or more sample layers of a single wafer may provide an overlay measurement between the two or more sample layers of that single wafer. In another embodiment, sample 104 is formed as two wafers bonded at an interface, and each wafer may include one or more pattern layers near the interface (e.g., the inward-facing surface of the wafer). Thus, one overlay target including target features on at least one sample layer of each wafer may provide an overlay measurement for aligning the two wafers during the bonding process.

[0026] Overlay measurement tool 102 may be configured to generate images based on any number of recipes. A recipe defines measurement parameters for determining the overlay of an overlay target. For example, the recipe of overlay measurement tool 102 may include, but is not limited to, the wavelength of illumination, the detection wavelength of the radiation emitted from the sample, the illumination spot size on the sample, the incident illumination angle in the azimuthal or elevation direction, the polarization of the incident illumination, the position of the beam of the incident illumination on the overlay target, the position of the overlay target within the focus volume of the overlay measurement tool, and the like.

[0027] In one embodiment, the generation of overlay measurements along a particular measurement direction is based on sequentially illuminating with two opposing illumination beams (e.g., beams having opposing azimuthal angles) to generate two images of sample 104, or two images of a portion of sample 104 such as an overlay target. This imaging is performed based on a single diffraction order of a single illumination beam per pass for features having periodicity along a given direction on sample 104. From this point, although the periodic features are not resolved, the difference in intensity between the two non-resolved images generated with the opposing illumination beams is proportional to the overlay along the direction of periodicity of the features.

[0028] In another embodiment, system 100 includes a controller 114, and controller 114 includes one or more processors 116 communicatively coupled to overlay measurement tool 102. For example, the one or more processors 116 may be configured to execute a set of program instructions held in a memory device 118, i.e., memory. The one or more processors 116 of controller 114 may include any processing element known in the art. In this sense, the one or more processors 116 may include any microprocessor-type device configured to execute algorithms and / or instructions. Further, memory device 118 may include any storage medium known in the art suitable for storing program instructions executable by the one or more processors 116 associated therewith. For example, memory device 118 may include a non-transitory storage medium. As yet another example, memory device 118 may include, but is not limited to, read-only memory, random access memory, magnetic or optical memory devices (e.g., disks), magnetic tape, solid state drives, and the like. Note that memory device 118 may be housed in a common controller housing with the one or more processors 116.

[0029] From this point, the controller 114 can perform various optional processing steps related to overlay measurement. For example, the controller 114 can be configured to generate control signals for instructing the overlay measurement tool 102 or any of its components, or for controlling it in other ways. For example, the controller 114 can be configured to instruct the overlay measurement tool 102 or any of its components to generate one or more images based on one or more selected recipes. A recipe defines measurement parameters for determining the overlay of an overlay target based on dark-field imaging. The recipes of the overlay measurement tool can include, but are not limited to, the illumination wavelength, the detection wavelength of the radiation emitted from the sample, the illumination spot size on the sample, the angle of incident illumination, the polarization of the incident illumination, the position of the beam of incident illumination on the overlay target, the position of the overlay target within the focus volume of the overlay measurement tool, and the like.

[0030] As another example, the controller 114 can further be configured to receive data from the overlay measurement tool 102. This data includes, but is not limited to, images. As another example, the controller 114 can be configured to determine an overlay associated with the overlay target based on the acquired images. As another example, the controller 114 can generate correctable values (collectables) for one or more other manufacturing tools as feedback and / or feed-forward control means for the manufacturing tools based on the overlay measurement values from the overlay measurement tool 102.

[0031] Next, with reference to FIGS. 1B - 1D, various non - limiting configurations of the overlay measurement tool 102 according to one or more embodiments of the present disclosure will be described. In this specification, it is contemplated that the overlay measurement tool 102 can generate a dark - field image of the sample 104 (e.g., an image of an overlay target on the sample 104) based on a single diffraction order using various configurations (including but not limited to OTL and TTL illumination).

[0032] FIG. 1B is a conceptual diagram of an overlay measurement tool 102 showing OTL illumination according to one or more embodiments of the present disclosure. In this specification, the OTL illumination for dark - field imaging disclosed herein physically separates the illumination path and the light - collecting path, thereby realizing in an advantageous manner a reduction in the relative size of the target, an improvement in light efficiency associated with directly illuminating the sample 104, and a reduction in stray light without the need for a blocker or mask. For example, in each illumination, it may be required that a single diffraction order be diffracted by a periodic feature on the sample 104 into the light - collecting path of the imaging subsystem 110 based on the diffraction grating equation. However, with OTL illumination, it may be possible to reduce the target size by illuminating the sample at a relatively larger incident angle (e.g., elevation angle) than TTL illumination.

[0033] In one embodiment, the illumination subsystem 106 of the overlay measurement tool 102 includes at least one illumination source 120 configured to generate at least one illumination beam 108. The illumination beam 108 may include one or more selected wavelengths of light. The wavelengths of this light include, but are not limited to, ultraviolet (UV) radiation, visible radiation, or infrared (IR) radiation. Further, the illumination beam 108 may be an incoherent illumination beam. From this point, the image of the sample 104 generated based on the illumination beam 108 may not be affected by speckle.

[0034] The illumination source 120 can include any type of illumination source suitable for providing the illumination beam 108. In one embodiment, the illumination source 120 is a laser source combined with a speckle disruption element. The speckle disruption element can include any type of speckle disruption element known in the art (e.g., a rotating diffuser, a beam scanner for scanning the illumination beam 108 on the input surface of an optical fiber, a fiber agitation mechanism, etc.). For example, the illumination source 120 can include, but is not limited to, one or more narrowband laser sources, broadband laser sources, supercontinuum laser sources, white light laser sources, etc. In another embodiment, the illumination source 120 includes a laser sustained plasma (LSP) source. For example, the illumination source 120 can include, but is not limited to, an LSP lamp, an LSP bulb, or an LSP chamber suitable for containing one or more elements that can emit broadband illumination when excited to a plasma state by a laser source. In another embodiment, the illumination source 120 includes a lamp source. For example, the illumination source 120 can include, but is not limited to, an arc lamp, a discharge lamp, an electrodeless lamp, etc. From this point, the illumination source 120 can provide an illumination beam 108 with low coherence (e.g., low spatial coherence and / or low temporal coherence).

[0035] Furthermore, the sample 104 can be disposed on a sample stage 122 suitable for its fixation (e.g., a part of the scanning subsystem 112). The sample stage 122 is also configured to position the sample 104 relative to the illumination beam 108.

[0036] In another embodiment, the overlay measurement tool 102 illuminates a sample 104 by directing one illumination beam 108 for one image thereto via one or more illumination paths 124. Each of the illumination paths 124 may include one or more optical components suitable for modifying and / or adjusting the illumination beam 108 in addition to directing the illumination beam 108 to the sample 104. For example, each of the illumination paths 124 may include one or more illumination lenses 126 (e.g., for the purpose of controlling the spot size of the illumination beam 108 on the sample 104, relaying the pupil plane and / or the field plane, etc.), or one or more illumination control optical systems 128, but not necessarily including these. For example, the illumination control optical system 128 may include one or more polarizers for adjusting the polarization of the illumination beam 108, one or more filters, one or more beam splitters, one or more diffusers, one or more homogenizers, one or more apodizers, one or more beam shapers, or one or more mirrors (e.g., stationary mirrors, translationally movable mirrors, scanning mirrors, etc.), but is not limited thereto.

[0037] Furthermore, the illumination path 124 may be provided in various configurations for directing one or more illumination beams 108 to the sample 104. In one embodiment, as shown in FIG. 1B, each illumination path 124 may include an individual set of components (e.g., illumination lenses 126 and / or illumination control optical systems 128). As another example, the illumination subsystem 106 may include a common set of illumination lenses 126 and / or illumination control optical systems 128. This set is for generating one or more illumination beams 108 and directing those illumination beams 108 to the sample 104. Further, the illumination subsystem 106 may direct one or more illumination beams 108 to the sample 104 using any combination of free space optics or fiber optics.

[0038] In another embodiment, the imaging subsystem 110 of the overlay measurement tool 102 includes an objective lens 130 for collecting diffracted or scattered light (e.g., sample light 132) from the sample 104, and at least one detector 134 located on a field plane conjugate with the sample 104. The detector 134 is configured to generate an image of the sample 104 based on at least a portion of the sample light 132. The imaging subsystem 110 may further include a plurality of optical elements for directing and / or modifying the illumination collected by the objective lens 130. The optical elements include, but are not limited to, one or more imaging lenses 136 or imaging control optics 138. For example, the imaging control optics 138 may include, but are not limited to, one or more filters, one or more polarizers, one or more beam blocks, or one or more beam splitters.

[0039] FIG. 1C is a conceptual diagram of an overlay measurement tool 102 showing off-axis TTL dark-field imaging using a dark-field aperture 140, according to one or more embodiments of the present disclosure. In one embodiment, the overlay measurement tool 102 includes a beam splitter 142. The beam splitter 142 is configured to couple the illumination subsystem 106 and the imaging subsystem 110 such that the objective lens 130 can direct one or more illumination beams 108 simultaneously onto the sample 104 and collect the sample light 132 from the sample 104. For example, FIG. 1C shows a plurality of illumination paths 124 that provide the illumination beam 108 to a common illumination field aperture 144 of the illumination subsystem 106. In another embodiment, the dark-field aperture 140 is located at the pupil plane 146 of the imaging subsystem 110 alone and includes one or more opaque elements. The opaque elements block specular reflections (e.g., zero-order diffraction) collected by the objective lens 130 and allow only non-zero diffraction orders for dark-field imaging to pass through.

[0040] FIG. 1D is a conceptual diagram of an overlay measurement tool 102 showing dark-field TTL dark-field imaging using a dark-field mirror 148 according to one or more embodiments of the present disclosure. Similar to FIG. 1C, a plurality of illumination paths 124 are shown in FIG. 1D, and the plurality of illumination paths 124 provide an illumination beam 108 to a common illumination field stop 144 of the illumination subsystem 106. Further, as shown in FIG. 1D, the overlay measurement tool 102 may include, but does not necessarily include, an optical relay 150 between the dark-field mirror 148 and the objective lens 130.

[0041] In one embodiment, the dark-field mirror 148 includes one or more reflective regions 152 and a central aperture region 154. The reflective region 152 directs one or more illumination beams 108 to the objective lens 130 to illuminate the sample 104. For example, the dark-field mirror 148 may include, but is not limited to, an annular mirror with a central aperture. Thus, the dark-field mirror 148 directs one or more illumination beams 108 simultaneously along a range of inclined solid angles, blocks sample light 132 associated with specular reflection (e.g., zero-order diffraction) focused by the objective lens 130, and allows a single diffraction order other than zero-order for dark-field imaging to pass through. The dark-field mirror 148 may be located in or near a common pupil plane with the illumination path 124 and the imaging subsystem 110, but this is not essential. From this point, the spatial size of the dark-field mirror 148 at the pupil plane can directly correspond to the range of the solid angle at which the illumination beam 108 is directed to the sample 104, and its open aperture can correspond to the range of the solid angle at which the focused sample light 132 can propagate to the detector 134 to form a dark-field image. When the dark-field mirror 148 is located near the pupil plane or is tilted with respect to the pupil plane, at least a portion of the pupil plane may be defocused on the dark-field mirror 148, resulting in pupil blur on the dark-field mirror 148. However, the effect of this pupil blur can be reduced by providing a sufficient separation distance between the illumination beam 108 and the desired diffraction order other than zero-order sent to the detector 134.

[0042] It should be understood that FIGS. 1B-1D and the associated descriptions are provided for illustrative purposes only and should not be construed as limiting. Rather, this specification contemplates that the overlay measurement tool 102 can be configured in various ways to provide imaging with a single diffraction order other than the zero order. For example, FIGS. 1B-1D show individual illumination sources 120 for each illumination path 124. However, any number of illumination beams 108 may be generated by any number of illumination sources 120 or by any combination of illumination sources 120. As another example, the overlay measurement tool 102 may include any combination that provides a suitable physical layout consisting of free-space optics and fiber optics.

[0043] Next, with reference to FIGS. 2A-7E, the generation of a field-of-view image of an overlay grating measurement target formed with a single diffraction order other than the zero order along a given measurement direction according to one or more embodiments of the present disclosure will be described in detail.

[0044] FIG. 2A is a top view of an overlay target 202 including overlapping grating features according to one or more embodiments of the present disclosure. FIG. 2B is a side view of a single cell 206 of the overlay target 202 on a substrate 204 according to one or more embodiments of the present disclosure. In one embodiment, the overlay target 202 includes a plurality of cells 206. Each cell 206 includes an element 208 transferred to a first layer located on a first layer 210 of the sample 104 and an element 212 transferred to a second layer located on a second layer 214 of the sample 104. The region including the element 208 transferred to the first layer and the region including the element 212 transferred to the second layer overlap to form one overlay grating structure. From this point, the overlay grating features within each cell 206 can diffract the incident illumination beam 108 into discrete diffraction orders.

[0045] In another embodiment, the overlay target 202 includes a first set of one or more cells 206 including an overlay grating structure having periodicity along a first direction, and a second set of one or more cells 206 including an overlay grating structure having periodicity along a second direction different from (e.g., orthogonal to) the first direction.

[0046] For example, FIGS. 2A-2B show an overlay target 202 including four cells 206a-d, where cells 206a and 206d form a first cell set, and cells 206b and 206c form a second cell set. Specifically, the overlay grating structures of cells 206a and 206d have periodicity in the X direction, and the overlay grating structures of cells 206b and 206c have periodicity in the Y direction. From this, as will be described in detail below, cells 206a, d can be suitable for overlay measurements along the X direction, and cells 206b, c may be suitable for overlay measurements along the Y direction.

[0047] However, it should be understood that the overlay target 202 in FIGS. 2A and 2B and the related description are provided for illustrative purposes only and should not be construed as limiting. Rather, the overlay target 202 may include any suitable overlay target design for a moiré grating. For example, the overlay target 202 may include any number of cells 206 suitable for measurement along two directions. Further, the cells 206 may be distributed in any pattern or arrangement. For example, the outline of a measurement target design suitable for scanning measurement is described in U.S. Patent Application No. 16 / 598,146, filed on October 10, 2019, the entire disclosure of which is incorporated herein by reference. In one embodiment, the overlay target 202 includes one or more cell groups distributed along the scanning direction (e.g., the moving direction of the sample 104), and the orientation of the cells 206 included in each cell group has periodicity along a common direction and includes a moiré grating structure. For example, the first cell group may include one or more cells 206 having periodicity along the X direction, and the second cell group may include one or more cells 206 having periodicity along the Y direction. Thus, while the sample 104 is being scanned within one measurement field of view of the imaging subsystem 110, all of the cells 206 within one cell group can be imaged at once.

[0048] Next, with reference to FIGS. 3-6, the illumination / collection configuration for scatterometry overlay measurement of the overlay target 202 based on a single diffraction order other than the zero order will be described in detail.

[0049] FIG. 3 is a schematic view of a pupil plane showing the distribution of illumination beams 108a - d that provide appropriate quadrupole illumination according to one or more embodiments of the present disclosure. In one embodiment, the directions of the first illumination beam 108a and the second illumination beam 108b may be azimuth angles that symmetrically oppose each other along the X direction to illuminate the sample 104. In another embodiment, the directions of the third illumination beam 108c and the fourth illumination beam 108d may be azimuth angles that symmetrically oppose each other along the Y direction to illuminate the sample 104. Further, FIG. 3 shows the boundary of the condenser pupil 302 of the imaging subsystem 110. For example, the boundary of the condenser pupil 302 in FIG. 3 may correspond to the NA of the objective lens 130 or the diameter of the pupil aperture of the imaging subsystem 110.

[0050] In FIG. 3, for the purpose of explaining dark - field condensation that does not use 0 - order diffraction for imaging, the illumination beams 108a - d are shown as being located outside the condenser pupil 302. It is contemplated herein that this dark - field configuration of FIG. 3 can be created in various forms. For example, the configuration of FIG. 3 can be created using OTL illumination as shown in FIG. 1B. From this point, the illumination beams 108a - d can be directly sent to the sample 104 from outside the angle of the NA of the objective lens 130 through one or more illumination paths 124. As another example, the configuration of FIG. 3 can be created using TTL illumination as shown in FIGS. 1B and 1C. In this case, the overlay measurement tool 102 selectively blocks the 0 - order diffraction of the illumination beams 108a - 108d (e.g., using a dark - field aperture 140 or a dark - field mirror 148, etc.). In a general sense, FIG. 3 does not necessarily need to correspond to a specific pupil plane of the above - mentioned system. Rather, FIG. 3 shows dark - field illumination according to one or more embodiments of the present disclosure.

[0051] FIGS. 4A - 4D show the imaging of the cell 206a of the overlay target 202 as an example of the imaging of a feature having periodicity along the X direction according to one or more embodiments of the present disclosure.

[0052] FIG. 4A is a schematic diagram showing imaging of cell 206a of overlay target 202 using first illumination beam 108a of FIG. 3 in an orientation along the X direction, according to one or more embodiments of the present disclosure. FIG. 4B is a schematic diagram of condenser pupil 302 showing the diffraction order distribution of first illumination beam 108a generated by cell 206a, according to one or more embodiments of the present disclosure.

[0053] The grating feature of cell 206a having periodicity along the X direction can diffract the first illumination beam 108a into a plurality of diffraction orders distributed along the X direction. Specifically, FIGS. 4A and 4B show the generation of zero-order diffraction 402 (e.g., specular reflection), -1st order diffraction 404, and -2nd order diffraction 406. In one embodiment, the selection of various parameters of illumination subsystem 106, imaging subsystem 110, and overlay target 202 is performed such that a single diffraction order (e.g., -1st order diffraction 404 shown in FIGS. 4A and 4B) is focused by imaging subsystem 110. For example, the periodicity of the features within cell 206a and the various measurement recipe-related parameters can be selected such that a single diffraction order obtained from first illumination beam 108a passes through condenser pupil 302. The measurement recipe-related parameters include, but are not limited to, the wavelength of first illumination beam 108a, the incident angle of first illumination beam 108a, the size of condenser pupil 302, and the like.

[0054] FIG. 4C is a schematic view of imaging of cell 206a of overlay target 202 using the second illumination beam 108b of FIG. 3 in a direction along the X direction and at an azimuth angle facing the first illumination beam 108a, according to one or more embodiments of the present disclosure. FIG. 4D is a schematic view of the condenser pupil 302 showing the distribution of diffraction orders of the second illumination beam 108b generated by cell 206a, according to one or more embodiments of the present disclosure. FIGS. 4C and 4D also show the generation of a plurality of diffraction orders along the X direction, and these diffraction orders can be symmetric with the distribution of the diffraction orders of the first illumination beam 108a. Specifically, FIGS. 4C and 4D show the generation of the zero-order diffraction 408 (e.g., specular reflection), the -1st order diffraction 410, and the -2nd order diffraction 412. FIGS. 4C and 4D also show the focusing of a single diffraction order (here the -1st order diffraction 410).

[0055] FIG. 4E is a schematic view of imaging of cell 206a of overlay target 202 using the third illumination beam 108c of FIG. 3 in a direction along the Y direction, according to one or more embodiments of the present disclosure. FIG. 4F is a schematic view of the condenser pupil 302 showing the distribution of diffraction orders of the third illumination beam 108c generated by cell 206a, according to one or more embodiments of the present disclosure.

[0056] As shown in FIGS. 4E and 4F, cell 206a does not diffract the third illumination beam 108c along the Y direction and diffracts only in the X direction (e.g., +1st order diffraction 414, zero-order diffraction 416 (e.g., specular reflection), and -1st order diffraction 418). Further, in one embodiment, the selection of various parameters of the illumination subsystem 106, the imaging subsystem 110, and the overlay target 202 is performed such that the diffraction orders (including the zero-order diffraction) do not pass through the condenser pupil 302. As a result, the features of cell 206a do not appear in the image of overlay target 202 based on the third illumination beam 108c. Although not shown, it should be understood that the illumination of cell 206a by the fourth illumination beam 108d also results in the same result as the illumination by the third illumination beam 108c.

[0057] Furthermore, it should be understood that FIGS. 4A-4F can also be used to explain any cell 206 (e.g., cell 206d) that includes a structure having periodicity along the X direction. It should also be understood that the concepts shown in FIGS. 4A-4F can further explain the imaging of cells (e.g., cells 206b and 206c) that include a structure having periodicity along the Y direction. Specifically, when the overlay target 202 is imaged using the third illumination beam 108c and the fourth illumination beam 108d, a single diffraction order generated by cells 206b and 206c is focused, and the light from cells 206a and 206d is not focused, so that only cells 206b and 206c can appear.

[0058] Next, with reference to FIGS. 5A-6, the field-of-view images of the overlay target 202 based on various illumination conditions according to one or more embodiments of the present disclosure will be described in detail. In one embodiment, as shown in FIGS. 5A-6, the overlay target 202 is entirely illuminated by incident illumination. From this point, the illumination directed at the overlay target 202 is larger than the size of the overlay target 202.

[0059] FIG. 5A is a conceptual diagram of an image of an overlay target 202 based on illumination by a first illumination beam 108a or a second illumination beam 108b, according to one or more embodiments of the present disclosure. As described with reference to FIGS. 4A-4F, illumination by the first illumination beam 108a or the second illumination beam 108b causes a single diffraction order (e.g., -1st order diffraction 404 or -1st order diffraction 410) generated by structures having periodicity along the X direction (here, cells 206a and 206d) to be focused, and light from structures having periodicity along the Y direction is not focused. As a result, only cells 206a and 206d appear in the image, but the overlay grating features within these cells are not resolved and instead appear as monolithic features. Further, in this specification, it is contemplated that the relative intensity of the cells appearing in the image generated by the first illumination beam 108a with respect to the cells appearing in the image generated by the second illumination beam 108b can indicate an overlay error associated with the relative shift along the X direction between the first layer 210 and the second layer 214 of the sample 104.

[0060] FIG. 5B is a conceptual diagram of an image of an overlay target 202 based on illumination by a third illumination beam 108c or a fourth illumination beam 108d, according to one or more embodiments of the present disclosure. In this configuration, only cells 206b and 206c appear in the image. Further, an overlay measurement along the Y direction can be generated based on the relative intensity of the cells appearing in the image generated by the third illumination beam 108c with respect to the cells appearing in the image generated by the fourth illumination beam 108d.

[0061] In this specification, it is contemplated that overlay measurement values associated with the relative displacement between the first layer 210 and the second layer 214 of the sample 104 can be generated using the images shown in FIGS. 5A and 5B. Specifically, the relative intensity of the cells appearing in the image generated by the first illumination beam 108a with respect to the cells appearing in the image generated by the second illumination beam 108b can indicate an overlay error along the X direction. On the other hand, the relative intensity of the cells appearing in the image generated by the third illumination beam 108c with respect to the cells appearing in the image generated by the fourth illumination beam 108d can indicate an overlay error along the Y direction.

[0062] Referring next to FIG. 6, in another embodiment, overlay measurement values along the X and Y directions can be generated using two images. In this case, each image is formed by a pair of two orthogonal illumination beams 108.

[0063] FIG. 6 is a conceptual diagram of an image of an overlay target 202 based on illumination by either the first illumination beam 108a and the third illumination beam 108c, or the second illumination beam 108b and the fourth illumination beam 108d, according to one or more embodiments of the present disclosure. In this configuration, the image in FIG. 6 is formed using a pair of illumination beams 108 having orthogonal azimuth angles.

[0064] In FIG. 6, all of the cells 206a - d within the overlay target 202 are shown. However, similar to those shown in FIGS. 4A - 5D, the image of any cell 206 is formed by light associated with a single illumination beam 108. Thus, an image such as that shown in FIG. 6 generated by a pair of opposing illumination beams 108 can include the same information as four images such as those shown in FIGS. 5A and 5B. Therefore, equivalent overlay measurement values can be generated based on two images (and two corresponding measurement recipes) instead of four images (and four corresponding measurement recipes).

[0065] Next, with reference to FIGS. 7A - 7E, the generation of a dark - field image using a split pupil according to one or more embodiments of the present disclosure will be described in detail.

[0066] In one embodiment, the overlay measurement tool 102 includes a pupil - splitting optical system to selectively direct a portion of the light included in each part of the condenser pupil 302 to separate detectors 134. This pupil - splitting optical system includes, but is not limited to, a prism located on the pupil plane 702. For example, although not shown, the overlay measurement tool 102 shown in FIG. 1D may include the pupil - splitting optical system in the aperture of the dark - field mirror 148 or in the pupil plane in front of the detector 134 conjugate thereto.

[0067] FIG. 7A is a conceptual diagram of a split pupil plane 702 according to one or more embodiments of the present disclosure. In one embodiment, the pupil plane 702 is divided into four quadrants, and two opposing illumination quadrants 704 provide illumination by two opposing illumination beams 108 (e.g., a first diagonal illumination beam 108e and a second diagonal illumination beam 108f), and two opposing condenser quadrants 706 provide the condensation of the sample light 132. Further, in the illumination quadrants 704, the zero - order diffraction from the illumination beam 108 is blocked (e.g., by a dark - field mirror 148, etc.) to provide dark - field imaging.

[0068] In another embodiment, as shown in FIGS. 7B - 7E, the direction of the illumination beam 108 is oblique with respect to the measurement direction (e.g., the periodic direction of various cells 206 of the overlay target 202).

[0069] FIG. 7B is a conceptual diagram of pupil plane 702 showing the diffraction order distribution of the first oblique illumination beam 108e by cells 206b and 206c having periodicity in the Y direction according to one or more embodiments of the present disclosure. Specifically, FIG. 7B shows a -1st order diffraction 708, a 0th order diffraction 710, a +1st order diffraction 712, and a +2nd order diffraction 714. FIG. 7C is a conceptual diagram of pupil plane 702 showing the diffraction order distribution of the first oblique illumination beam 108e by cells 206a and 206d having periodicity in the X direction according to one or more embodiments of the present disclosure. Specifically, FIG. 7C shows a -1st order diffraction 716, a 0th order diffraction 718, a +1st order diffraction 720, and a +2nd order diffraction 722. As shown in FIGS. 7B and 7C, a single diffraction order is collected from any given cell. As a result, this configuration can generate an image similar to that of FIG. 6.

[0070] FIG. 7D is a conceptual diagram of pupil plane 702 showing the diffraction order distribution of the second oblique illumination beam 108f by cells 206b and 206c having periodicity in the Y direction according to one or more embodiments of the present disclosure. Specifically, FIG. 7D shows a -1st order diffraction 724, a 0th order diffraction 726, a +1st order diffraction 728, and a +2nd order diffraction 730. FIG. 7E is a conceptual diagram of pupil plane 702 showing the diffraction order distribution of the second oblique illumination beam 108f by cells 206a and 206d having periodicity in the X direction according to one or more embodiments of the present disclosure. Specifically, FIG. 7E shows a -1st order diffraction 732, a 0th order diffraction 734, a +1st order diffraction 736, and a +2nd order diffraction 738. Similar to FIGS. 7B and 7C, FIGS. 7D and 7E show that the collection of a single diffraction order (from the opposing second illumination beam 108f) is collected from any given cell of overlay target 202. As a result, this configuration can also generate an image similar to that of FIG. 6, and the overlay can be determined using a technique similar to that described above with respect to FIG. 6.

[0071] Referring back to FIGS. 1A - 1D, the configuration of the overlay measurement tool 102 for the scanning mode or the stationary measurement mode according to one or more embodiments of the present disclosure will be described in detail. Regarding overlay measurement in the scanning mode or the stationary mode, an overview is described in U.S. Patent Application No. 16 / 586,504 filed on September 27, 2019 and U.S. Patent Application No. 16 / 598,146 filed on October 10, 2019, both of which are hereby incorporated by reference in their entirety.

[0072] The overlay measurement tool 102 may include any type of optical detector 134 known in the art suitable for measuring the irradiation received from the sample 104. Further, the overlay measurement tool 102 may generally include any number of detectors 134.

[0073] In one embodiment, the overlay measurement tool 102 includes one or more sensors suitable for generating one or more images of the sample 104 while the sample 104 is moving. For example, the detector 134 may include a line sensor including a row of pixels in a horizontal row. From this point, the system 100 may generate a continuous row of images (e.g., strip - shaped images) in a horizontal row at a time. This image generation is performed by translating the sample 104 within the measurement field of view in a scanning direction perpendicular to the row of pixels and continuously sending a clock to the line sensor during a continuous exposure period. As another example, the detector 134 may include a time - domain integration (TDI) sensor including a plurality of pixel rows and one read - out row. The TDI sensor operates similarly to the line sensor, but differs in that, by a clock signal, the charge is continuously transferred from one pixel row to the next until it reaches the read - out row, and an image of a horizontal row is generated at the read - out row. By synchronizing this charge transfer with the movement of the sample along the scanning direction (e.g., based on the clock signal), the charge can be continuously accumulated over a plurality of pixel rows, making it possible to make the signal - to - noise ratio relatively higher than that of the line sensor.

[0074] In another embodiment, the overlay measurement tool 102 includes one or more sensors suitable for generating one or more images of the sample 104 while the sample 104 is stationary. For example, the detector 134 may include a sensor suitable for generating one or more images of the stationary sample 104. The sensor can be, but is not limited to, a charge-coupled device (CCD), a complementary metal-oxide semiconductor (CMOS) sensor, a photomultiplier tube (PMT) array, or an avalanche photodiode (APD) array. Further, the detector 134 may include a multi-tap sensor having two or more taps per pixel. The sensor includes, but is not limited to, a multi-tap CMOS sensor. From this point, the charge within one multi-tap pixel can be directed to any selected tap based on one or more drive signals to that pixel during the exposure period. Thus, a multi-tap sensor including a multi-tap pixel array can generate multiple images during a single readout step, and each of these images is associated with a separate tap of the associated pixel. Further, in the present disclosure, the taps of the multi-tap sensor can indicate output taps connected to the associated pixels. From this point, individual images can be generated by reading each tap of the multi-tap sensor (e.g., during the readout step).

[0075] In another embodiment, detector 134 may include a spectroscopic detector suitable for identifying the wavelength of the radiation emitted from sample 104. In another embodiment, system 100 may include a plurality of detectors 134 (e.g., these detectors are associated with a plurality of beam paths generated by one or more beam splitters, thus facilitating a plurality of measurement measurements by system 100). For example, system 100 may include one or more detectors 134 suitable for imaging in a stationary mode and one or more detectors 134 suitable for imaging in a scanning mode. In another embodiment, system 100 may include one or more detectors 134 suitable for both stationary imaging mode and scanning imaging mode. For example, during the exposure period, by not sending a clock for transferring charge between pixel columns to the TDI sensor, the TDI sensor can operate in a stationary mode. In that case, when the exposure period is interrupted (e.g., by a shutter or turning off illumination source 120, etc.) and no more light is incident on the pixels, a clock for transferring charge one by one to the readout column is sent to the TDI sensor, and one image of a length equal to the number of stages of the pixel column can be generated.

[0076] This specification further contemplates that various techniques may be used to generate individual images of sample 104 (e.g., overlay target 202 on sample 104) using opposing illumination beams 108 for the overlay measurements disclosed herein.

[0077] In one embodiment, generation of an individual image of overlay target 202 by opposing illumination beam 108 is performed by sequentially illuminating sample 104 with both illumination beams 108 to generate successive images. For example, in a scanning configuration, successive images may be generated by sequentially performing scans that illuminate overlay target 202 under respective illumination conditions (e.g., with different illumination beams 108 for each scan). As another example, in a stationary configuration, successive images may be generated by sequentially illuminating and imaging overlay target 202 under different illumination conditions while it is stationary within overlay measurement tool 102.

[0078] In another embodiment, while the overlay target 202 is scanned within the measurement field of view of the overlay measurement tool 102, it is alternately illuminated with different illumination conditions (e.g., different illumination beams 108), and based on the alternate illumination conditions, an interleaved image of the overlay target 202 is generated. Then, individual images associated with the separate illumination conditions can be extracted from the interleaved image. From this point, multiple individual images associated with multiple individual illumination conditions can be generated in a single scan, which can facilitate high-throughput scanning measurement.

[0079] For example, in a scanning mode, an interleaved image can be generated using a detector 134 including a TDI sensor. This is done by selectively irradiating the sample light 132 onto the alternately arranged pixel columns 802 of the TDI sensor (e.g., arranged every other row for 2-alternate illumination conditions, every third row for 4-alternate illumination conditions, every N - 1 row for N-alternate illumination conditions, etc.). From this point, imaging of the overlay target 202 is performed by dispersing it among these selected multiple columns and the intervening columns 804 are not irradiated. This selective alternate irradiation of the alternately arranged pixel columns 802 can be performed using various techniques. Such techniques include, but are not limited to, a cylindrical lens array or a slit array arranged to block the intervening columns 804. FIG. 8 is a conceptual diagram of an imaging subsystem 110 configured to generate an interleaved image from two alternate illumination conditions using a detector 134 including a TDI sensor according to one or more embodiments of the present disclosure. Selective imaging of the alternately arranged columns can be achieved using any suitable combination of optical elements. The optical elements include, but are not limited to, the cylindrical lens array 806 shown in FIG. 8.

[0080] Furthermore, the charge transfer speed of the TDI sensor along the scanning direction 808 can be synchronized with the switching speed of the illumination subsystem 106. For example, in the case of two alternating illumination conditions (such as when the illumination is the illumination beam 108 suitable for forming the image shown in FIG. 6), a clock for accumulating charges in the pixel column 802 under the first illumination condition can be sent to the TDI sensor, and then, while sending the clock for transferring the charges to the intervening column 804, irradiation under the second illumination configuration is performed on the pixel column 802. Thus, this procedure can be repeated, and accordingly, the columns of images arranged alternately (for example, interleaved images) correspond to the alternating illumination conditions. Then, by extracting these alternately arranged columns of images in the form of individual images, the images related to each illumination condition can be separated.

[0081] As another example, efficient capture of an image of the overlay target 202 using alternating illumination conditions in the stationary mode can be achieved using the multi-tap image sensor described above. In this regard, the charge within each pixel can be directed to any selected tap based on one or more drive signals to that pixel during the exposure period. A multi-tap sensor including a multi-tap pixel array can generate multiple images during a single readout step. These images are each associated with a separate tap of the associated pixel. Thus, the system 100 can perform stationary mode measurements by sequentially providing any selected number of optical configurations during the exposure period while the sample 104 is stationary.

[0082] FIG. 9 is a flowchart showing the steps performed in an overlay method 900 according to one or more embodiments of the present disclosure. It should be understood that the embodiments and implementation techniques already described in the context of the system 100 in this specification are also applicable to the method 900. However, it should be further noted that the method 900 is not limited to the configuration of the system 100.

[0083] In one embodiment, method 900 includes step 902 of sequentially illuminating an overlay target on a sample with a first illumination lobe and a second illumination lobe. For example, the overlay target can include one or more stacked grating features formed of structures having periodicity on a first sample layer and a second sample layer, and the one or more stacked grating structures have periodicity along one measurement direction as shown in FIG. 2. Further, the overlay target generally can include a plurality of cell sets each including stacked grating features for each measurement direction, in order to facilitate overlay measurements along a plurality of measurement directions. In this case, step 902 can include sequentially illuminating the overlay target under two illumination conditions. Here, the first illumination condition includes a first pair of illumination lobes including illumination lobes respectively along a first direction and a second direction, and the second illumination condition includes a second pair of illumination lobes opposite to the first pair of illumination lobes (for example, having opposite azimuth angles).

[0084] In another embodiment, method 900 includes step 904 of generating a first image and a second image of the overlay target using a detector. Here, the first image includes a non-resolved image of the one or more stacked grating structures formed at a diffraction order other than the zero order of the first illumination lobe, and the second image includes a non-resolved image of the one or more stacked grating structures formed at a diffraction order other than the zero order of the second illumination lobe. When the overlay target includes a plurality of cell sets each including stacked grating features having periodicity for each measurement direction, the first and second images can include non-resolved images of all cells, but the image of each cell can be generated based on a single diffraction order from a single illumination lobe.

[0085] In another embodiment, method 900 includes step 906 of determining an overlay error between the first layer and the second layer of the sample along the measurement direction based on the first image and the second image. Specifically, the overlay can be proportional to the difference in intensity of one stacked grating structure in the first and second images.

[0086] The subject matter described in this specification may in some cases show various components that are included within or connected to other components. It should be understood that such configurations described herein are merely exemplary, and in reality, many other configurations that achieve the same function may be implemented. In a conceptual sense, any arrangement of components for achieving the same function is effectively "associated" so that the intended function is achieved. Thus, any two components of this specification combined to achieve a particular function can be considered to be "associated" with each other such that the intended function is achieved, regardless of each configuration or intermediate components. Similarly, any two components so associated can also be considered to be "connected" or "coupled" to each other to achieve the intended function, and any two components that can be so associated can also be considered to be "couplable" to each other to achieve the intended function. Specific examples of couplable include, but are not limited to, components that can physically interact and / or physically interact with each other, and / or components that can wirelessly interact and / or wirelessly interact with each other, and / or components that can logically interact and / or logically interact with each other.

[0087] Many of the present disclosure and its attendant advantages are believed to be understood from the foregoing description, and it is apparent that various changes can be made to the form, structure, and arrangement of the above components without departing from the subject matter disclosed herein or sacrificing all of its important advantages. The form described herein is merely illustrative, and it is the intention of the following claims to embrace such changes. Further, it should be understood that the present invention is defined by the appended claims.

Claims

1. 1. An overlay metrology system, comprising: an illumination subsystem configured to provide a first pair of opposing illumination lobes along a first measurement direction and a second pair of illumination lobes along a second measurement direction orthogonal to the first measurement direction, and further configured to sequentially illuminate an overlay target on a specimen with a first illumination configuration and a second illumination configuration, the first illumination configuration including one illumination lobe from each of the first and second pairs of opposing illumination lobes and the second illumination configuration including the remaining illumination lobe from each of the first and second illumination lobe pairs, the overlay target being illuminated by: a first set of one or more cells including one or more stacked grating features formed of periodic structures on the first and second sample layers, the one or more stacked grating structures having a periodicity along the first measurement direction; a second set of one or more cells including one or more superimposed grating features formed with periodic structures on the first sample layer and the second sample layer, the one or more superimposed grating structures having periodicity along the second measurement direction; and a lighting subsystem comprising:

1. An imaging subsystem, comprising: Objective lens, and a detector configured to generate a first image and a second image of the overlay target, the first image being formed at a single diffraction order other than the zeroth order from each illumination lobe in the first illumination configuration, and the second image being formed at a single diffraction order other than the zeroth order from each illumination lobe in the second illumination configuration. an imaging subsystem; a controller coupled in communication with the detector, the controller including one or more processors configured to execute program instructions that cause the one or more processors to determine an overlay error between the first sample layer and the second sample layer of the sample along the first measurement direction and the second measurement direction based on the first image and the second image. A controller; an overlay metrology system,

2. determining an overlay error between the first sample layer and the second sample layer of the sample along the first measurement direction and the second measurement direction based on the first image and the second image; determining an overlay error between the first sample layer and the second sample layer of the sample along the first measurement direction based on an intensity difference in the first image and the second image of the first set of one or more cells; determining an overlay error between the first sample layer and the second sample layer of the sample along the second measurement direction based on an intensity difference in the first image and the second image of the second set of one or more cells; The overlay metrology system of claim 1 .

3. 2. The overlay metrology system of claim 1 , wherein the illumination subsystem includes one or more illumination lenses configured to illuminate the overlay target with the first illumination lobe and the second illumination lobe at a numerical aperture greater than a numerical aperture of the objective lens.

4. 2. The overlay metrology system of claim 1, wherein the illumination subsystem directs the first and second illumination configurations to the sample through the objective lens, and the imaging subsystem further comprises one or more beam blocks for blocking specular reflections associated with illumination lobes in the first and second illumination configurations.

5. the one or more beam blocks The imaging subsystem's dark field aperture The overlay metrology system of claim 4 .

6. 5. The overlay metrology system of claim 4, further comprising a dark field mirror common to the illumination subsystem and the imaging subsystem, the dark field mirror directing the first and second illumination configurations through the objective lens to the overlay target, the dark field mirror acting as the one or more beam blocks blocking specular reflections of the illumination lobes in the first and second illumination configurations, and the dark field mirror passing a single diffraction order associated with the illumination lobes in the first and second illumination configurations.

7. The dark field mirror is Annular mirror The overlay metrology system of claim 6 .

8. The detector comprises: a scanning detector configured to generate the first image and the second image while the sample is translated by the translation stage through a measurement field of the objective lens; The overlay metrology system of claim 1 .

9. The detector comprises: Time Domain Integration (TDI) Sensor The overlay metrology system of claim 8 , comprising:

10. a cylindrical lens array configured to direct light emitted from the sample to every N-1 pixel columns of the TDI sensor; The overlay metrology system may be configured to perform scanning mode measurements, translating the sample at a charge transfer rate of the TDI sensor; alternatingly illuminating the sample with the first illumination lobe and the second illumination lobe, wherein a switching time between successive optical configurations of the imaging subsystem corresponds to a charge transfer rate of the TDI sensor; generating an interleaved image including the first image and the second image during an exposure period using the TDI sensor; and separating the interleaved image into the first image and the second image; The overlay metrology system of claim 9 , wherein the overlay metrology system is configured to:

11. A slit array arranged to block pixel rows of the TDI sensor that are not irradiated by the cylindrical lens array. The overlay metrology system of claim 10 further comprising:

12. The detector comprises: a stationary imaging detector configured to generate the first image and the second image while the sample is held stationary within a measurement field of view of the objective lens by a translation stage; The overlay metrology system of claim 1 .

13. At least one of the first illumination lobe or the second illumination lobe is Incoherent illumination beams The overlay metrology system of claim 1 .

14. The incoherent illumination beam: Speckle destruction laser beam The overlay metrology system of claim 13 , comprising:

Images