Scanning scatterometry overlay measurement system and method

Abstract

To solve a problem in which a time required for settling a translational movement stage prior to measurement may adversely affect 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 a first illumination beam thereof. Therein the overlay target includes grating-over-grating features formed from periodic structures on a first sample layer and a second sample layer. The system further includes an imaging sub-system for generating 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 from 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 from a single non-zero diffraction order of the second illumination beam. Based on the first image and the second image, the system determines an overlay error between the first layer and the second layer.SELECTED DRAWING: Figure 1A

Description

[Technical Field]

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

[0002] The increasing demand for smaller semiconductor devices has led to a corresponding increase in the demand for accurate and efficient metrology. One approach to improving the efficiency and throughput of metrology tools is to generate metrology data for a sample while it is moving, rather than while it is stationary within the measurement field of view. This eliminates the time delay associated with the translation stage settling before a measurement. Metrology systems typically generate sample-related metrology data by measuring or otherwise interrogating dedicated metrology targets distributed throughout the sample. Therefore, the sample is typically mounted on a translation stage and translated so that the metrology targets sequentially move into the measurement field of view. In typical metrology systems employing a move-and-measure (MAM) approach, the sample remains stationary between measurements. However, the time required for the translation stage to settle before a measurement can negatively impact throughput. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] U.S. Patent Application Publication No. 2013 / 0278942 [Patent Document 2] U.S. Patent Application Publication No. 2016 / 0097727 Summary of the Invention [Problem to be solved by the invention]

[0004] Therefore, it would be desirable to provide a system and method to address the above-mentioned shortcomings. [Means for solving the problem]

[0005] According to one or more exemplary embodiments of the present disclosure, an overlay metrology system is disclosed, which includes an illumination subsystem for sequentially illuminating an overlay target on a sample with a first illumination lobe and a second illumination lobe relative to the first illumination lobe, the overlay target including one or more grating-over-grating features formed of periodic structures on a first sample layer and a second sample layer, the one or more grating-over-grating structures having periodicity along a measurement direction. In another exemplary embodiment, the system includes an imaging subsystem comprising an objective lens and a detector for generating first and second images of the overlay target, the first image including a non-resolved image of the one or more superimposed grating structures formed in a single diffraction order other than the zeroth order of the first illumination lobe generated by the one or more superimposed grating structures along the measurement direction, and the second image including a non-resolved image of the one or more superimposed grating structures formed in a single diffraction order other than the zeroth order of the second illumination lobe generated by the one or more superimposed grating structures along the measurement direction. In another exemplary embodiment, the system includes a controller coupled in communication with the detector, the controller for determining an overlay error between the first layer and the second layer of the sample along the measurement direction based on the first and second images.

[0006] An overlay metrology system is disclosed in accordance with one or more exemplary embodiments of the present disclosure. In one exemplary embodiment, the system includes an illumination subsystem for providing 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 and second pairs of opposing illumination lobes, and the second illumination configuration including the other illumination lobe from each of the first and second pairs of illumination lobes. In another exemplary embodiment, the overlay target includes a first set of one or more cells including one or more superimposed grating features formed of periodic structures on a first sample layer and a second sample layer, the one or more superimposed grating structures having periodicity along the first direction. In another exemplary embodiment, the overlay target includes 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. In another exemplary embodiment, the system includes an imaging subsystem including an objective lens and a detector configured to generate a first image and a second image of the overlay target, the first image being formed with a single diffraction order other than the zeroth order from each illumination lobe in the first illumination configuration, and the second image being formed with a single diffraction order other than the zeroth order from each illumination lobe in the second illumination configuration. In another exemplary embodiment, the system includes a controller coupled in communication with the detector, the controller configured to determine 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] An overlay metrology method according to one or more exemplary embodiments of the present disclosure is disclosed. In one 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 superimposed grating features formed of periodic structures on a first sample layer and a second sample layer, the one or more superimposed grating structures having periodicity along a 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 superimposed grating structures formed with a single diffraction order other than the zeroth order of the first illumination lobe from the one or more superimposed grating structures along the measurement direction, and the second image including a non-resolved image of the one or more superimposed grating structures formed with a single diffraction order other than the zeroth order of the second illumination lobe generated by the one or more superimposed grating structures along the measurement direction. In another exemplary embodiment, the method includes determining an overlay error between the first layer and the second layer of the specimen along the measurement direction based on the first image and the second image.

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

[0009] Those skilled in the art will better appreciate the many advantages of the present disclosure by reviewing the accompanying drawings. [Brief explanation of the drawings]

[0010] [Figure 1A] FIG. 1 is a conceptual diagram of a system for performing overlay metrology in accordance with one or more embodiments of the present disclosure. [Figure 1B]FIG. 1 is a conceptual diagram of an overlay metrology tool showing outside-the-lens (OTL) illumination in accordance with one or more embodiments of the present disclosure. [Figure 1C] FIG. 1 is a conceptual diagram of an overlay metrology tool showing oblique through-the-lens (TTL) dark field imaging using a dark field aperture in accordance with one or more embodiments of the present disclosure. [Figure 1D] FIG. 1 is a conceptual diagram of an overlay metrology tool showing oblique TTL dark field imaging using a dark field mirror in accordance with one or more embodiments of the present disclosure. [Figure 2A] FIG. 1 is a top view of an overlay target including overlapping grating features according to one or more embodiments of the present disclosure. [Figure 2B] FIG. 10 is a side view of a single cell of the overlay target, in accordance with one or more embodiments of the present disclosure. [Figure 3] FIG. 1 is a schematic diagram of a pupil plane illustrating a distribution of illumination beams that provides suitable quadrupole illumination in accordance with one or more embodiments of the present disclosure. [Figure 4A] FIG. 4 is a schematic diagram illustrating imaging of a cell of the overlay target with the first illumination beam of FIG. 3 oriented along the X direction, in accordance with one or more embodiments of the present disclosure. [Figure 4B] FIG. 10 is a schematic diagram of a collection pupil illustrating a distribution of diffraction orders of a first illumination beam produced by one cell, in accordance with one or more embodiments of the present disclosure. [Figure 4C] FIG. 4 is a schematic diagram illustrating imaging of a cell of an overlay target with a second illumination beam oriented along the X direction and along an opposite azimuth angle to the first illumination beam of FIG. 3, in accordance with one or more embodiments of the present disclosure. [Figure 4D] FIG. 10 is a schematic diagram of a collection pupil illustrating the distribution of diffraction orders of a second illumination beam produced by one cell, in accordance with one or more embodiments of the present disclosure. [Figure 4E] FIG. 4 is a schematic diagram illustrating imaging of a cell of the overlay target using the third illumination beam of FIG. 3 oriented along the Y direction, in accordance with one or more embodiments of the present disclosure. [Figure 4F]FIG. 10 is a schematic diagram of a collection pupil illustrating the distribution of diffraction orders of a third illumination beam produced by one cell, in accordance with one or more embodiments of the present disclosure. [Figure 5A] 1 is a conceptual diagram of an image of an overlay target based on illumination by a first illumination beam or a second illumination beam, in accordance with one or more embodiments of the present disclosure. [Figure 5B] 10 is a conceptual diagram of an image of an overlay target based on illumination with a third illumination beam or a fourth illumination beam, in accordance with one or more embodiments of the present disclosure. [Figure 6] 10A-10C are conceptual diagrams of images of an overlay target based on illumination by either a first illumination beam and a third illumination beam, or a second illumination beam and a fourth illumination beam, in accordance with one or more embodiments of the present disclosure. [Figure 7A] FIG. 1 is a conceptual diagram of a split-pupil plane, in accordance with one or more embodiments of the present disclosure. [Figure 7B] FIG. 10 is a conceptual diagram of a pupil plane illustrating the distribution of diffraction orders of a first illumination beam by cells having periodicity in the Y direction, in accordance with one or more embodiments of the present disclosure. [Figure 7C] FIG. 10 is a conceptual diagram of a pupil plane illustrating the distribution of diffraction orders of a first illumination beam by cells having periodicity in the X direction, in accordance with one or more embodiments of the present disclosure. [Figure 7D] FIG. 10 is a conceptual diagram of a pupil plane illustrating the distribution of diffraction orders of a second illumination beam by cells having periodicity in the Y direction, in accordance with one or more embodiments of the present disclosure. [Figure 7E] FIG. 10 is a conceptual diagram of a pupil plane illustrating the distribution of diffraction orders of a second illumination beam by cells having periodicity in the X direction, in accordance with one or more embodiments of the present disclosure. [Figure 8] FIG. 1 is a conceptual diagram of an imaging subsystem configured to generate interleaved images from two alternating illumination conditions using a detector including a TDI sensor, in accordance with one or more embodiments of the present disclosure. [Figure 9] FIG. 1 is a flow diagram illustrating steps performed in an overlay method in accordance with one or more embodiments of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0011] Reference will now be made in detail to the subject matter of the present disclosure, which is illustrated in the accompanying drawings. This disclosure particularly shows and describes certain embodiments and their particular features. The embodiments described herein are to be construed as illustrative and not limiting. It will be readily apparent to those skilled in the art that various changes / modifications in form and detail can be made therein without departing from the spirit and scope of the present disclosure.

[0012] Embodiments of the present disclosure are directed to systems and methods for scanning scatterometry metrology using field plane imaging of a superimposed grating overlay target, the metrology being based on sequentially imaging the overlay target with symmetrically opposed illumination beams while the target is moving, with the field plane images formed using a single diffraction order along one measurement direction. In this respect, the field plane images correspond to dark field images, where features of the overlay target are not resolved, but the intensity difference between the sequential images of the overlay target features is proportional to the overlay.

[0013] In this disclosure, the term scatterometry metrology is used broadly to encompass scatterometry-based metrology as well as diffraction-based metrology. The term scanning metrology is also used to describe metrology measurements generated while the sample is moving. In a general sense, scanning metrology may be performed by scanning the sample along a measurement path (e.g., a swath) such that an area of ​​interest (e.g., a measurement target or device area on the sample) is translated through the measurement field of view of the metrology system. Furthermore, this process may be repeated for any number of measurement paths or for repeated measurements of a particular measurement path to provide any selected number of measurements of the sample. Scatterometry metrology based on dark-field imaging of periodic overlay targets is outlined in U.S. patent application Ser. No. 16 / 996,328, filed August 18, 2020, which is incorporated herein by reference in its entirety.

[0014] Overlay targets typically include well-defined transfer elements designed to accurately represent the relative placement of features associated with one or more lithography steps on one or more sample layers. In this regard, measured (e.g., by an overlay metrology tool) characteristics of each transfer element on an overlay target can be representative of the transfer device elements associated with the device being fabricated. Additionally, an overlay target can include one or more measurement cells, each containing a transfer element on one or more layers on the sample. Thus, a single overlay measurement can be based on any combination of measurements from various cells of such an overlay target.

[0015] In some embodiments, an overlay target suitable for scanning scatterometry metrology as disclosed herein can include one or more cells containing an overlapping grating structure, the overlapping grating structure including periodic features (e.g., grating features) on the overlapping regions of two or more layers of interest. In this manner, various grating features on each layer of interest can contribute to the diffraction of incident illumination, and overlay measurements can be generated based on analysis of the diffracted light.

[0016] In some embodiments, a scanning scatterometry overlay tool generates a single overlay measurement along a single measurement direction by sequentially generating two field plane images of a scatterometry overlay target using opposing illumination beams (e.g., two beams with opposing azimuthal angles of incidence along the measurement direction). Each field plane image is formed using a single diffraction order other than the zeroth order from the scatterometry overlay target. Thus, overlaid grating structures appear as unresolved grayscale features in the field plane image, and the intensity difference between the field plane images of this grayscale feature is proportional to the overlay.

[0017] It is contemplated herein that the distribution of diffraction orders associated with the diffraction of an incident illumination beam can depend on various illumination / collection conditions, including, but not limited to, the azimuthal and elevational angles of incidence of the illumination beam, the wavelength of the illumination beam, the pitch of periodic features on the sample, or the numerical aperture (NA) of the collection optics. Additionally, an overlay metrology tool can include one or more elements to block one or more of the diffraction orders being collected so that selected diffraction orders form the basis of the image. In this regard, the illumination beam can be directed to the overlay target at an angle of incidence outside the NA of the collection optics (e.g., an out-of-lens (OTL) configuration) or through a lens common to the collection optics (e.g., a 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., orthogonal to the first direction). In this configuration, illumination of the overlay target with an illumination beam along the first direction (e.g., having an azimuthal angle along the first direction) may result in collection of a single diffraction order along the first direction from the first set of cells without including diffraction orders from the second set of cells. As a result, only the first set of cells may appear in an image. Similarly, an image generated using an illumination beam along the second direction may include only the second set of cells. In some embodiments, overlay measurements along two directions are generated based on four sequential quadrupole illumination-based images. In this case, two illumination beams are oriented at opposite azimuthal angles along a first direction and two at opposite azimuthal angles along a second direction. In some embodiments, the overlay measurement is based on two images sequentially illuminated with two pairs of quadrupole-distributed illumination beams. 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, respectively, opposite the first and second beams. In this way, all cells in the overlay target appear in each image, but each cell image is formed by a single diffraction order from a single illumination beam.

[0019] It is further contemplated herein that scanning metrology based on sequential illumination with opposing illumination beams can be implemented in various ways. In some embodiments, images of the overlay target with opposing illumination beams are generated sequentially by sequentially scanning the overlay target. In some embodiments, the images are generated during a single scan based on alternating illumination and / or collection conditions during the scan. For example, an interleaved image can be generated based on alternating illumination of the overlay target with opposing illumination beams. Two field images can be extracted from this interleaved image. In one example, a scanning scatterometry tool is configured to illuminate alternating rows on a TDI sensor and is configured to synchronize the alternating illumination conditions of the scatterometry overlay target with the charge transfer rate of the TDI sensor to generate the interleaved image. Interleaved image generation in scanning metrology using TDI sensors is outlined in U.S. patent application Ser. No. 16 / 586,504, filed Sep. 27, 2019, which is incorporated herein by reference in its entirety.

[0020] Furthermore, while many of the examples in this disclosure relate to scanning measurements, it should be understood that the systems and methods disclosed herein are not limited to scanning measurements, but rather it is contemplated herein that the systems and methods disclosed herein may also be utilized in a stationary measurement mode in which the sample is stationary during the measurement.

[0021] 1A-9, systems and methods for scatterometry overlay metrology in accordance with one or more embodiments of the present disclosure will now be described in detail.

[0022] FIG. 1A is a conceptual diagram of a system 100 for performing overlay metrology in accordance with one or more embodiments of the present disclosure.

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

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

[0025] The overlay metrology tool 102 may be suitable for generating field images of various overlay target designs for various specimen configurations. In one embodiment, the specimen 104 is formed as two or more patterned layers on a single wafer. Thus, a single overlay target containing target features on two or more specimen layers of a single wafer may provide overlay measurements between the two or more specimen layers of the single wafer. In another embodiment, the specimen 104 is formed as two wafers bonded at an interface, and each wafer may include one or more patterned layers near the interface (e.g., the inward-facing surface of the wafer). Thus, a single overlay target containing target features on at least one specimen layer of each wafer may provide overlay measurements of the alignment of the two wafers during the bonding process.

[0026] The overlay metrology tool 102 can be configured to generate images based on any number of recipes, which define measurement parameters for determining the overlay of the overlay targets. For example, a recipe for the overlay metrology tool 102 can include, but is not limited to, the wavelength of illumination, the wavelength of detection of radiation emitted from the sample, the illumination spot size on the sample, the incident illumination angle in the azimuth or elevation direction, the polarization of the incident illumination, the position of the incident illumination beam on the overlay target, and the position of the overlay target within the focal volume of the overlay metrology tool.

[0027] In one embodiment, overlay measurements along a particular measurement direction are generated by generating two images of the sample 104 or a portion of the sample 104, such as an overlay target, based on sequential illumination with two opposing illumination beams (e.g., beams with opposing azimuthal angles). The imaging is performed based on a single diffraction order of a single illumination beam at a time for periodic features along a given direction on the sample 104. From this perspective, the periodic features are not resolved, but the difference in intensity between the two unresolved images generated by the opposing illumination beams is proportional to the overlay along the direction of the feature periodicity.

[0028] In another embodiment, the system 100 includes a controller 114, which includes one or more processors 116 communicatively coupled to the overlay metrology tool 102. For example, the one or more processors 116 may be configured to execute a set of program instructions maintained in a memory device 118, i.e., memory. The one or more processors 116 of the 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. Furthermore, the 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, the memory device 118 may include a non-transitory storage medium. As yet another example, the 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, etc. It should be noted that the memory device 118 may be housed in a common controller housing with the one or more processors 116.

[0029] To this extent, the controller 114 may perform any of a variety of process steps related to overlay metrology. For example, the controller 114 may be configured to generate control signals to direct or otherwise control the overlay metrology tool 102 or any of its components. For example, the controller 114 may be configured to direct the overlay metrology 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 the overlay target based on dark-field imaging. An overlay metrology tool recipe may include, but is not limited to, illumination wavelength, detection wavelength of radiation emitted from the sample, illumination spot size on the sample, angle of incident illumination, polarization of incident illumination, position of the beam of incident illumination on the overlay target, position of the overlay target within the focal volume of the overlay metrology tool, etc.

[0030] As another example, the controller 114 may be further configured to receive data from the overlay metrology tool 102, including, but not limited to, imagery. As another example, the controller 114 may be configured to determine an overlay associated with an overlay target based on the acquired imagery. As another example, the controller 114 may generate correctables for one or more other manufacturing tools based on the overlay measurements from the overlay metrology tool 102 as a means of feedback and / or feedforward control of the manufacturing tools.

[0031] 1B-1D, various non-limiting configurations of an overlay metrology tool 102 according to one or more embodiments of the present disclosure are described. It is contemplated herein that the overlay metrology tool 102 may 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] 1B is a conceptual diagram of an overlay metrology tool 102 illustrating OTL illumination, in accordance with one or more embodiments of the present disclosure. It is contemplated herein that the OTL illumination for dark-field imaging disclosed herein advantageously reduces the relative size of the target, improves the light efficiency associated with directly illuminating the sample 104, and reduces stray light by physically separating the illumination and collection paths without the need for obstructions or masks. For example, while each illumination may require a single diffraction order to be diffracted by a periodic feature on the sample 104 into the collection path of the imaging subsystem 110 based on the diffraction grating equation, OTL illumination may enable the target size to be reduced by illuminating the sample at a relatively larger angle of incidence (e.g., elevation) than TTL illumination.

[0033] In one embodiment, the illumination subsystem 106 of the overlay metrology 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, including, but not limited to, ultraviolet (UV) radiation, visible radiation, or infrared (IR) radiation. Furthermore, the illumination beam 108 may be an incoherent illumination beam. In this regard, an image of the sample 104 generated based on the illumination beam 108 may be free from speckle effects.

[0034] The illumination source 120 may 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 may 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 face of an optical fiber, a fiber stirring mechanism, etc.). For example, the illumination source 120 may 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 may include, but is not limited to, an LSP lamp, an LSP bulb, or an LSP chamber suitable for containing one or more elements capable of emitting broadband illumination when excited into a plasma state by a laser source. In another embodiment, the illumination source 120 includes a lamp source. For example, the illumination source 120 may include, but is not limited to, an arc lamp, a discharge lamp, an electrodeless lamp, etc. In this regard, the illumination source 120 may provide an illumination beam 108 that is less coherent (eg, less spatially and / or temporally coherent).

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

[0036] In another embodiment, the overlay metrology tool 102 directs an illumination beam 108 for an image to illuminate the sample 104 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 conditioning 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, but is not required to, include one or more illumination lenses 126 (e.g., for controlling the spot size of the illumination beam 108 on the sample 104, relaying pupil and / or field planes, etc.) or one or more illumination control optics 128. For example, the illumination control optics 128 may include, but are not limited to, 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, translatable mirrors, scanning mirrors, etc.).

[0037] Furthermore, the illumination paths 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 a separate set of components (e.g., illumination lenses 126 and / or illumination control optics 128). As another example, the illumination subsystem 106 may include a common set of illumination lenses 126 and / or illumination control optics 128 for generating one or more illumination beams 108 and directing the illumination beams 108 to the sample 104. Furthermore, 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 metrology 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 in 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 multiple 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] 1C is a conceptual diagram of an overlay metrology tool 102 illustrating oblique TTL dark-field imaging using a dark-field stop 140, in accordance with one or more embodiments of the present disclosure. In one embodiment, the overlay metrology tool 102 includes a beam splitter 142 configured to couple the illumination subsystem 106 and the imaging subsystem 110 such that the objective lens 130 can simultaneously direct one or more illumination beams 108 onto the sample 104 and collect sample light 132 from the sample 104. For example, FIG. 1C illustrates multiple illumination paths 124 that provide illumination beams 108 to a common illumination field stop 144 of the illumination subsystem 106. In another embodiment, the dark-field stop 140 is located at a unique pupil plane 146 of the imaging subsystem 110 and includes one or more opaque elements. The opaque element blocks the specular reflection (eg, the zeroth diffraction order) collected by the objective lens 130 and passes a single diffraction order other than the zeroth order for dark-field imaging.

[0040] Figure 1D is a conceptual diagram of an overlay metrology tool 102 illustrating oblique TTL dark-field imaging using a dark-field mirror 148, in accordance with one or more embodiments of the present disclosure. Similar to Figure 1C, Figure 1D illustrates multiple illumination paths 124 that provide illumination beams 108 to a common illumination field stop 144 of the illumination subsystem 106. Additionally, as shown in Figure 1D, the overlay metrology tool 102 may, but need not, 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 toward the objective lens 130 for illumination of the sample 104. For example, the dark-field mirror 148 may include, but is not limited to, an annular mirror with a central aperture. In this manner, the dark-field mirror 148 may simultaneously direct one or more illumination beams 108 along a range of oblique solid angles, blocking sample light 132 associated with specular reflections (e.g., the zeroth diffraction order) collected by the objective lens 130 and passing a single diffraction order other than the zeroth order for dark-field imaging. The dark-field mirror 148 may be, but is not required to be, located at or near a pupil plane common to the illumination path 124 and the imaging subsystem 110. In this regard, the spatial extent of dark field mirror 148 at the pupil plane can directly correspond to the range of solid angles over which illumination beam 108 is directed toward sample 104, and its open aperture can correspond to the range of solid angles over which collected sample light 132 is allowed to propagate to detector 134 and form a dark field image. If dark field mirror 148 is located near the pupil plane or if dark field mirror 148 is tilted relative to the pupil plane, at least a portion of the pupil plane may be defocused on dark field mirror 148, resulting in pupil blurring at dark field mirror 148. However, the effect of this pupil blurring can be mitigated by providing sufficient separation between illumination beam 108 and the desired diffraction orders other than the zeroth order that are transmitted to detector 134.

[0042] It should be understood that Figures 1B-1D and the associated description are provided for illustrative purposes only and should not be construed as limiting. Rather, it is contemplated herein that overlay metrology tool 102 may be configured in various ways to provide imaging with a single diffraction order other than the zeroth order. For example, Figures 1B-1D show a separate illumination source 120 for each illumination path 124. However, any number of illumination beams 108 may be generated by any number of illumination sources 120 or any combination of illumination sources 120. As another example, overlay metrology tool 102 may include any combination of free-space optics and fiber optics that provides a suitable physical layout.

[0043] 2A-7E, the generation of a field plane image of a stacked grating overlay metrology target formed with a single diffraction order other than the zeroth order along a given measurement direction in accordance with one or more embodiments of the present disclosure will be described in detail.

[0044] 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 multiple cells 206. Each cell 206 includes elements 208 transferred to a first layer 210 of the sample 104 and elements 212 transferred to a second layer 214 of the sample 104, where the regions including the elements 208 transferred to the first layer and the regions including the elements 212 transferred to the second layer overlap to form a single overlapping grating structure. From this perspective, the overlapping 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 containing a superimposed grating structure with periodicity along a first direction and a second set of one or more cells 206 containing a superimposed grating structure with periodicity along a second direction different from (e.g., orthogonal to) the first direction.

[0046] 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 structure of cells 206a and 206d is periodic in the X direction, and the overlay structure of cells 206b and 206c is periodic in the Y direction. In this regard, cells 206a and 206d may be suitable for overlay measurements along the X direction, and cells 206b and 206c may be suitable for overlay measurements along the Y direction, as described in more detail below.

[0047] It should be understood, however, that the overlay target 202 in FIGS. 2A and 2B and the associated description are provided for illustrative purposes only and should not be construed as limiting. Rather, the overlay target 202 may include any suitable overlay grating overlay target design. For example, the overlay target 202 may include any number of cells 206 suitable for measurements along two directions. Furthermore, the cells 206 may be distributed in any pattern or arrangement. For example, metrology target designs suitable for scanning metrology are outlined in U.S. Patent Application No. 16 / 598,146, filed October 10, 2019, which is incorporated herein by reference in its entirety. In one embodiment, the overlay target 202 includes one or more groups of cells distributed along a scanning direction (e.g., the direction of movement of the sample 104), with the cells 206 within each group of cells oriented to include a overlay grating structure with periodicity along a common direction. For example, a first group of cells may include one or more cells 206 with periodicity along the X direction, and a second group of cells may include one or more cells 206 with periodicity along the Y direction. In this way, all of the cells 206 in a group of cells may be imaged at once while the sample 104 is scanned within a measurement field of view of the imaging subsystem 110.

[0048] 3-6, an illumination / collection configuration for scatterometry overlay metrology measurements of the overlay target 202 based on a single diffraction order other than the zeroth order will now be described in detail.

[0049] FIG. 3 is a schematic diagram of a pupil plane illustrating the distribution of illumination beams 108a-d that provide suitable quadrupole illumination, in accordance with one or more embodiments of the present disclosure. In one embodiment, the first illumination beam 108a and the second illumination beam 108b may be oriented to illuminate the sample 104 at symmetrically opposed azimuthal angles along the X direction. In another embodiment, the third illumination beam 108c and the fourth illumination beam 108d may be oriented to illuminate the sample 104 at symmetrically opposed azimuthal angles along the Y direction. Additionally, FIG. 3 illustrates the boundaries of the collection pupil 302 of the imaging subsystem 110. For example, the boundaries of the collection pupil 302 in FIG. 3 may correspond to the NA of the objective lens 130 or the diameter of the pupil diaphragm of the imaging subsystem 110.

[0050] In FIG. 3 , illumination beams 108a-d are shown as being outside collection pupil 302 to illustrate dark-field collection in which the zeroth diffraction order is not used for imaging. It is contemplated herein that the dark-field configuration of FIG. 3 can be created in various ways. For example, the configuration of FIG. 3 can be created using OTL illumination, as shown in FIG. 1B . From this point, illumination beams 108a-d can be delivered directly to sample 104 by one or more illumination paths 124 from angles outside the NA of objective lens 130. As another example, the configuration of FIG. 3 can be created using TTL illumination, as shown in FIGS. 1B and 1C . In this case, overlay metrology tool 102 selectively blocks the zeroth diffraction order of illumination beams 108a-d (e.g., using dark-field aperture 140, dark-field mirror 148, etc.). In a general sense, FIG. 3 need not necessarily correspond to a particular pupil plane of the system; rather, FIG. 3 illustrates dark-field illumination in accordance with one or more embodiments of the present disclosure.

[0051] 4A-4D illustrate imaging of cell 206a of overlay target 202 as an example of imaging a feature with periodicity along the X direction, in accordance with one or more embodiments of the present disclosure.

[0052] Figure 4A is a schematic diagram illustrating imaging of cell 206a of overlay target 202 with first illumination beam 108a of Figure 3 oriented along the X direction, in accordance with one or more embodiments of the present disclosure. Figure 4B is a schematic diagram of collection pupil 302 illustrating the distribution of diffraction orders of first illumination beam 108a generated by cell 206a, in accordance with one or more embodiments of the present disclosure.

[0053] The overlay grating features of cell 206a, which have periodicity along the X direction, can diffract first illumination beam 108a into multiple diffraction orders distributed along the X direction. Specifically, FIGS. 4A and 4B illustrate the generation of a zeroth diffraction order 402 (e.g., specular reflection), a −1st diffraction order 404, and a −2nd diffraction order 406. In one embodiment, various parameters of illumination subsystem 106, imaging subsystem 110, and overlay target 202 are selected to focus a single diffraction order (e.g., −1st diffraction order 404 shown in FIGS. 4A and 4B) on imaging subsystem 110. For example, the periodicity of features within cell 206a and measurement recipe-related parameters can be selected to focus a single diffraction order resulting from first illumination beam 108a through collection pupil 302. The measurement recipe-related parameters include, but are not limited to, the wavelength of the first illumination beam 108a, the angle of incidence of the first illumination beam 108a, the size of the collection pupil 302, and the like.

[0054] FIG. 4C is a schematic diagram of imaging a cell 206a of the overlay target 202 using the second illumination beam 108b of FIG. 3 oriented along the X direction and at an azimuth angle opposite to the first illumination beam 108a, in accordance with one or more embodiments of the present disclosure. FIG. 4D is a schematic diagram of the collection pupil 302 illustrating the distribution of diffraction orders of the second illumination beam 108b generated by the cell 206a, in accordance with one or more embodiments of the present disclosure. FIGS. 4C and 4D also illustrate the generation of multiple diffraction orders along the X direction, which may be symmetrical to the distribution of diffraction orders of the first illumination beam 108a. Specifically, FIGS. 4C and 4D illustrate the generation of a zeroth diffraction order 408 (e.g., specular reflection), a −1st diffraction order 410, and a −2nd diffraction order 412. FIGS. 4C and 4D also illustrate the collection of a single diffraction order (here, the −1st diffraction order 410).

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

[0056] As shown in FIGS. 4E and 4F , cell 206a does not diffract third illumination beam 108c along the Y direction, but only in the X direction (e.g., +1 diffraction order 414, 0 diffraction order 416 (e.g., specular reflection), and −1 diffraction order 418). Furthermore, in one embodiment, various parameters of illumination subsystem 106, imaging subsystem 110, and overlay target 202 are selected such that diffraction orders (including the 0 diffraction order) do not pass through collection pupil 302. As a result, features of cell 206a do not appear in an image of overlay target 202 based on third illumination beam 108c. Although not shown, it should be understood that illumination of cell 206a with fourth illumination beam 108d produces results similar to those of illumination with third illumination beam 108c.

[0057] 4A-4F may also be used to illustrate any cell 206 containing structures with periodicity along the X direction (e.g., cell 206d). It should also be understood that the concepts illustrated in Figures 4A-4F may also be used to illustrate imaging of cells containing structures with periodicity along the Y direction (e.g., cells 206b and 206c). Specifically, imaging overlay target 202 with third illumination beam 108c and fourth illumination beam 108d may reveal only cells 206b and 206c, as the single diffraction orders produced by cells 206b and 206c are collected and light from cells 206a and 206d is not collected.

[0058] 5A-6, field images of overlay target 202 under various lighting conditions will be described in detail, in accordance with one or more embodiments of the present disclosure. In one embodiment, as shown in FIGS. 5A-6, overlay target 202 is fully illuminated by incident illumination. In this regard, the illumination directed at overlay target 202 is larger than the size of overlay target 202.

[0059] 5A is a conceptual diagram of an image of the overlay target 202 based on illumination with the first illumination beam 108a or the second illumination beam 108b, in accordance with one or more embodiments of the present disclosure. As described with reference to FIGS. 4A-4F, illumination with the first illumination beam 108a or the second illumination beam 108b focuses a single diffraction order (e.g., −1 diffraction order 404 or −1 diffraction order 410) generated by structures with periodicity along the X direction (here, cells 206a and 206d), but not light from structures with periodicity along the Y direction. As a result, only cells 206a and 206d appear in the image, but the overlay features within these cells are not resolved and instead appear as monolithic features. It is further contemplated herein that the relative intensity of cells appearing in the image produced by the first illumination beam 108a relative to cells appearing in the image produced by the second illumination beam 108b may indicate an overlay error associated with a relative misalignment of the first layer 210 and second layer 214 of the sample 104 along the X-direction.

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

[0061] 5A and 5B may be used to generate overlay measurements related to the relative misalignment of the first and second layers 210, 214 of the sample 104. Specifically, the relative intensity of cells appearing in the image generated by the first illumination beam 108a relative to cells appearing in the image generated by the second illumination beam 108b may indicate an overlay error along the X direction, while the relative intensity of cells appearing in the image generated by the third illumination beam 108c relative to cells appearing in the image generated by the fourth illumination beam 108d may indicate an overlay error along the Y direction.

[0062] 6, in another embodiment, overlay measurements along the X and Y directions can be generated using two images, each formed by a pair of two orthogonal illumination beams 108.

[0063] 6 is a conceptual diagram of an image of an overlay target 202 based on illumination by either a first illumination beam 108a and a third illumination beam 108c, or a second illumination beam 108b and a fourth illumination beam 108d, in accordance with 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 azimuthal angles.

[0064] In Figure 6, all of the cells 206a-d in the overlay target 202 are visible. However, similar to those shown in Figures 4A-5D, the image of any cell 206 is formed with light associated with a single illumination beam 108. Thus, an image such as that shown in Figure 6 produced by an opposing pair of illumination beams 108 may contain the same information as four images, such as those shown in Figures 5A and 5B, in a pair. Thus, equivalent overlay measurements may be produced based on two images (and two corresponding measurement recipes) rather than four images (and four corresponding measurement recipes).

[0065] 7A-7E, dark-field imaging using a split-pupil according to one or more embodiments of the present disclosure will now be described in detail.

[0066] In one embodiment, the overlay metrology tool 102 includes pupil-splitting optics to selectively direct portions of light contained in each portion of the collection pupil 302 to different detectors 134. The pupil-splitting optics may include, but are not limited to, a prism located at the pupil plane 702. For example, although not shown, the overlay metrology tool 102 shown in FIG. 1D may include pupil-splitting optics at the pupil plane before the detectors 134, at or conjugate with the aperture of the dark field mirror 148.

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

[0068] In another embodiment, as shown in FIGS. 7B-7E, the illumination beam 108 is oriented obliquely relative to the measurement direction (eg, the periodic direction of the various cells 206 of the overlay target 202).

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

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

[0071] 1A-1D, a configuration of the overlay metrology tool 102 for scanning or stationary metrology mode will now be described in detail, in accordance with one or more embodiments of the present disclosure. Overlay metrology in scanning or stationary mode is generally described in U.S. patent application Ser. No. 16 / 586,504, filed Sep. 27, 2019, and U.S. patent application Ser. No. 16 / 598,146, filed Oct. 10, 2019, both of which are incorporated herein by reference in their entireties.

[0072] The overlay metrology tool 102 may include any type of optical detector 134 known in the art that is suitable for measuring illumination received from the sample 104. Furthermore, the overlay metrology tool 102 may generally include any number of detectors 134.

[0073] In one embodiment, the overlay metrology tool 102 includes one or more sensors suitable for generating one or more images of the sample 104 while it is moving. For example, the detector 134 may include a line sensor including a single horizontal row of pixels. From this perspective, the system 100 may generate a series of images (e.g., a swath) of images, one horizontal row at a time. This is done by translating the sample 104 through a measurement field of view in a scanning direction perpendicular to the pixel rows and continuously clocking the line sensor during successive exposure periods. As another example, the detector 134 may include a time-domain integration (TDI) sensor including multiple pixel rows and a readout row. A TDI sensor operates similarly to a line sensor, except that a clock signal continuously transfers charge from one pixel row to the next until it reaches the readout row, where the image of the single horizontal row is generated. By synchronizing this charge transfer with the movement of the sample along the scanning direction (e.g., based on a clock signal), charge can be continuously accumulated across multiple pixel columns, making it possible to achieve a relatively higher signal-to-noise ratio than a line sensor.

[0074] In another embodiment, the overlay metrology 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, such as, but 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. Additionally, the detector 134 may include a multi-tap sensor with two or more taps per pixel, such as, but not limited to, a multi-tap CMOS sensor. In this regard, charge within a multi-tap pixel may be directed to any selected tap during an exposure period based on one or more drive signals to that pixel. Thus, a multi-tap sensor including a multi-tap pixel array can generate multiple images during a single readout phase, each associated with a different tap of an associated pixel. Additionally, in this disclosure, the tap of a multi-tap sensor may refer to the output tap connected to the associated pixel. From this point, a separate image can be generated by reading out each tap of the multi-tap sensor (eg, during a readout phase).

[0075] In another embodiment, the detector 134 may include a spectroscopic detector suitable for identifying wavelengths of radiation emitted from the sample 104. In another embodiment, the system 100 may include multiple detectors 134 (e.g., the detectors may be associated with multiple beam paths generated by one or more beam splitters, thus facilitating multiple metrology measurements by the system 100). For example, the system 100 may include one or more detectors 134 suitable for static imaging and one or more detectors 134 suitable for scanning imaging. In another embodiment, the system 100 may include one or more detectors 134 suitable for both static and scanning imaging modes. For example, during an exposure period, the TDI sensor may operate in a static mode by not clocking the TDI sensor to transfer charge between pixel columns. In this case, once the exposure period is interrupted (e.g., by a shutter or by turning off the illumination source 120) and no more light is incident on the pixels, the TDI sensor may be clocked to transfer charge, step by step, to the readout columns to generate a single image with a length equal to the number of pixel columns.

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

[0077] In one embodiment, separate images of the overlay target 202 are generated with the opposing illumination beams 108 by sequentially illuminating the sample 104 with both illumination beams 108 to generate successive images. For example, in a scanning configuration, successive images may be generated by sequentially illuminating the overlay target 202 with each illumination condition (e.g., with a different illumination beam 108 for each scan). As another example, in a stationary configuration, successive images may be generated by sequentially illuminating and imaging the overlay target 202 with different illumination conditions while the overlay target 202 remains stationary within the overlay metrology tool 102.

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

[0079] For example, in a scanning mode, an interleaved image may be generated using a detector 134 including a TDI sensor. This is done by selectively illuminating alternating pixel columns 802 of the TDI sensor with the sample light 132 (e.g., every other column in a 2-alternating illumination condition, every third column in a 4-alternating illumination condition, every N-1 columns in an N-alternating illumination condition, etc.). From this point, imaging of the overlay target 202 is distributed across these selected columns, with the intervening columns 804 not illuminated. This selective, alternating illumination of the alternating pixel columns 802 may be performed using various techniques, including, but not limited to, a cylindrical lens array or a slit array positioned to occlude the intervening columns 804. FIG. 8 is a conceptual diagram of an imaging subsystem 110 configured to generate an interleaved image from two alternating illumination conditions using a detector 134 including a TDI sensor, in accordance with one or more embodiments of the present disclosure. Selective imaging of the alternating columns may be achieved using any suitable combination of optical elements. Such optical elements include, but are not limited to, a cylindrical lens array 806 shown in FIG.

[0080] Additionally, the charge transfer rate of the TDI sensor along the scan direction 808 can be synchronized with the switching rate of the illumination subsystem 106. For example, in the case of two alternating illumination conditions (such as when the illumination is an illumination beam 108 suitable for forming the image shown in FIG. 6), the TDI sensor can be clocked to accumulate charge from a first illumination condition in pixel column 802, and then the intervening column 804 can be clocked to transfer the charge while the pixel column 802 is illuminated with a second illumination configuration. This procedure can then be repeated, resulting in alternating image sequences (e.g., interleaved images) corresponding to the alternating illumination conditions. The images associated with each illumination condition can then be separated by extracting the alternating image sequences as separate images.

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

[0082] 9 is a flow diagram illustrating steps performed in an overlay method 900 in accordance with one or more embodiments of the present disclosure. It should be understood that embodiments and implementations already described herein in the context of system 100 also apply to method 900. It should be further noted, however, that method 900 is not limited to the configuration of 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 overlay grating features formed of periodic structures on a first sample layer and a second sample layer, with the one or more overlay grating structures having periodicity along one measurement direction, as shown in FIG. 2 . Furthermore, the overlay target generally can include multiple cell sets, each of which includes a plurality of cells including the overlay grating features, for each measurement direction to facilitate overlay measurements along multiple measurement directions. In this case, step 902 can include sequentially illuminating the overlay target with two illumination conditions, where the first illumination condition includes a first pair of illumination lobes including illumination lobes along a first direction and a second direction, respectively, and the second illumination condition includes a second pair of illumination lobes opposite (e.g., azimuthally opposite) the first pair of illumination lobes.

[0084] In another embodiment, method 900 includes generating 904 a first image and a second image of the overlay target using a detector, where the first image includes a non-resolved image of the one or more superimposed grating structures formed in a single diffraction order other than the zeroth order of the first illumination lobe, and the second image includes a non-resolved image of the one or more superimposed grating structures formed in a single diffraction order other than the zeroth order of the second illumination lobe. If the overlay target includes multiple cell sets, each of which includes periodic superimposed grating features, for each measurement direction, the first and second images may include non-resolved images of all cells, but each cell image may be generated based on a single diffraction order from a single illumination lobe.

[0085] In another embodiment, the method 900 includes determining 906 an overlay error between the first and second layers of the sample along the measurement direction based on the first and second images. Specifically, overlay may be proportional to a difference in intensity of a single overlay grating structure in the first and second images.

[0086] The subject matter described herein may in some cases depict various components contained within or connected to other components. It should be understood that such configurations described herein are merely exemplary, and that in fact many other configurations that achieve the same functionality may be implemented. In a conceptual sense, any arrangement of components to achieve the same functionality is effectively “associated” such that the desired functionality is achieved. Thus, any two components herein that combine to achieve a particular function may be considered to be “associated” with each other such that the desired functionality is achieved, regardless of their respective configurations or intermediate components. Similarly, any two components so associated may also be considered to be “connected” or “coupled” to each other to achieve the desired functionality, and any two components so associated may also be considered to be “couplable” to each other to achieve the desired functionality. Examples of combinable include, but are not limited to, physically interactable and / or physically interacting components, wirelessly interactable and / or wirelessly interacting components, and / or logically interactable and / or logically interacting components.

[0087] The present disclosure and many of its attendant advantages will be understood from the foregoing description, and it will be apparent that various changes in form, construction, and arrangement of the elements described above may be made without departing from the subject matter herein disclosed or sacrificing all of its important advantages. The embodiments described herein are merely illustrative, and it is the intent of the following claims to cover such modifications. It will further be understood that the invention is defined by the appended claims.

Claims

1. 1. An overlay metrology system, comprising: an illumination subsystem configured to sequentially illuminate an overlay target on the sample with a first illumination beam and a second illumination beam relative to the first illumination beam, the overlay target including one or more overlay grating features formed of periodic structures on the first and second sample layers, the one or more overlay grating structures having periodicity along the measurement direction; a lighting subsystem; an imaging subsystem, Objective lens, and a detector configured to generate a first image and a second image of the overlay target, the first image including a non-resolved image of the one or more superimposed grating structures formed at a single diffraction order other than the zeroth order of the first illumination beam generated by the one or more superimposed grating structures along the measurement direction, and the second image including a non-resolved image of the one or more superimposed grating structures formed at a single diffraction order other than the zeroth order of the second illumination beam generated by the one or more superimposed grating structures along the measurement direction; 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 measurement direction based on the first image and the second image; A controller; Including, the imaging subsystem comprises a dark field mirror including one or more reflective regions and a central aperture region, the reflective regions directing the first illumination beam and the second illumination beam toward the objective lens to illuminate the overlay target and blocking a zeroth order diffracted light collected by the objective lens, and the central aperture region passing a single diffraction order other than the zeroth order diffracted light collected by the objective lens to the detector. Overlay measurement system.

2. determining an overlay error between the first sample layer and the second sample layer of the sample along the 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 measurement direction based on an intensity difference between the one or more superimposed grating structures in the first image and the second image. The overlay metrology system of claim 1 , comprising:

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 beam and the second illumination beam at an angle of incidence outside an angle of a numerical aperture of the objective lens.

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

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

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

7. The detector Time Domain Integration (TDI) Sensor The overlay metrology system of claim 6 , comprising:

8. a cylindrical lens array configured to direct light emitted from the sample to every N-1 pixel columns of the TDI sensor, and the overlay metrology system is configured to perform scanning mode measurements by: translating the sample at the charge transfer rate of the TDI sensor; alternatingly illuminating the sample with the first illumination beam and the second illumination beam, wherein a switching time between successive optical configurations of the imaging subsystem corresponds to a charge transfer rate of the TDI sensor; generating the interleaved image including the first image and the second image using the TDI sensor during an exposure period; and separating the interleaved image into the first image and the second image; The overlay metrology system of claim 7 , wherein the measurement is performed by:

9. a slit array arranged to block pixel rows of the TDI sensor that are not illuminated by the cylindrical lens array; The overlay metrology system of claim 8 further comprising:

10. The detector a stationary imaging detector configured to generate the first and second images 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 , comprising:

11. At least one of the first illumination beam or the second illumination beam: Incoherent illumination beam The overlay metrology system of claim 1 , comprising:

12. the incoherent illumination beam Speckle-destruction laser beam The overlay metrology system of claim 11 , comprising:

13. 1. An overlay metrology method, comprising: sequentially illuminating an overlay target on the sample with a first illumination beam and a second illumination beam, the overlay target including one or more overlaid grating features formed of periodic structures on the first and second sample layers, the one or more overlaid grating structures having periodicity along one measurement direction; generating a first image and a second image of the overlay target using a detector, wherein the first image includes a non-resolved image of the one or more superimposed grating structures formed at a single diffraction order other than the zeroth order of the first illumination beam along the measurement direction from the one or more superimposed grating structures, and the second image includes a non-resolved image of the one or more superimposed grating structures formed at a single diffraction order other than the zeroth order of the second illumination beam generated by the one or more superimposed grating structures along the measurement direction; determining an overlay error between the first sample layer and the second sample layer of the sample along the measurement direction based on the first image and the second image; Including, A method for detecting a diffraction order of a dark field mirror, the method comprising: a dark field mirror including one or more reflective areas and a central aperture area; the reflective areas directing the first illumination beam and the second illumination beam to an objective lens to illuminate the overlay target and blocking the zeroth order diffracted light collected by the objective lens; and the central aperture area passing a single diffraction order other than the zeroth order diffracted light collected by the objective lens to guide it to the detector.

Images