Scanning scatterometry overlay measurement

Abstract

An overlay metrology system may include an illumination subsystem for sequentially illuminating an overlay target with a first illumination lobe and a second illumination lobe opposite the first illumination lobe, wherein the overlay target includes grating-on-grating features formed from periodic structures on first and second sample layers. The system may further include an imaging subsystem for generating a first image and a second image of the overlay target. The first image includes an unresolved image of a single non-zero diffraction order of the grating-on-grating structure formed from the first illumination lobe. The second image includes an unresolved image of a single non-zero diffraction order of the one or more grating-on-grating structures formed from the second illumination lobe. The system may further include a controller for determining an overlay error between the first layer and the second layer based on the first image and the second image.

Description

Technical Field

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

[0002] The ever-increasing demand for smaller semiconductor devices has led to a corresponding increase in the demand for accurate and efficient metrology. One way to increase the efficiency and throughput of metrology tools is to generate metrology data on the sample when the sample is in motion rather than in a static position in the measurement field of view. In this way, the time delay associated with the placement of the translation stage before measurement can be eliminated. The metrology system typically generates metrology data associated with the sample by measuring or otherwise verifying a dedicated metrology target distributed across the sample. Therefore, the sample is typically mounted on a translation stage and translated so that the metrology target is moved sequentially into the measurement field of view. In a typical metrology system that adopts the move and measure (MAM) method, the sample is static during each measurement. However, the time required to place the translation stage before measurement can negatively affect throughput. Therefore, it is desirable to provide a system and method for solving the above defects. Summary of the invention

[0003] According to one or more illustrative embodiments of the present disclosure, an overlay metrology system is disclosed. In one illustrative embodiment, the system includes an illumination subsystem for sequentially illuminating an overlay target on a sample with a first illumination lobe and a second illumination lobe opposite the first illumination lobe, wherein the overlay target includes one or more grating-over-grating features formed from periodic structures on a first sample layer and a second sample layer, and wherein the one or more grating-over-grating structures are periodic along a measurement direction. In another illustrative embodiment, the system includes an imaging subsystem having an objective lens and a detector to generate a first image and a second image of the overlay target, wherein the first image includes an unresolved image of the one or more grating-on-grating structures formed from a single non-zero diffraction order of the first illumination lobe, the single non-zero diffraction order being produced by the one or more grating-on-grating structures along the measurement direction, and wherein the second image includes an unresolved image of the one or more grating-on-grating structures formed from a single non-zero diffraction order of the second illumination lobe, the single non-zero diffraction order being produced by the one or more grating-on-grating structures along the measurement direction. In another illustrative embodiment, the system includes a controller communicatively coupled to the detector 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.

[0004] According to one or more illustrative embodiments of the present disclosure, an overlay metrology system is disclosed. In one illustrative embodiment, the system includes an illumination subsystem, the illumination subsystem being used to provide a first pair of relative 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 illustrative embodiment, the illumination subsystem sequentially illuminates an overlay target on a sample using a first illumination configuration and a second illumination configuration, wherein the first illumination configuration includes one illumination lobe from each of the first pair of relative illumination lobes and the second pair of relative illumination lobes, and the second illumination configuration includes the remaining illumination lobe from each of the first pair of illumination lobes and the second pair of illumination lobes. In another illustrative embodiment, the overlay target includes a first group of one or more cells, the first group of one or more cells including one or more grating-on-grating features formed from periodic structures on a first sample layer and a second sample layer, wherein the one or more grating-on-grating structures are periodic along the first direction. In another illustrative embodiment, the overlay target further includes a second set of one or more cells, the second set of one or more cells including one or more grating-on-grating features formed from periodic structures on the first sample layer and the second sample layer, wherein the one or more grating-on-grating structures are periodic along the second measurement direction. In another illustrative embodiment, the system includes an imaging subsystem having an objective lens and a detector, the detector to generate a first image and a second image of the overlay target, wherein the first image is formed using a single non-zero diffraction order from each illumination lobe in the first illumination configuration, and wherein the second image is formed using a single non-zero diffraction order from each illumination lobe in the second illumination configuration. In another illustrative embodiment, the system includes a controller, the controller communicatively coupled to the detector 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.

[0005] According to one or more illustrative embodiments of the present disclosure, an overlay metrology method is disclosed. In one illustrative embodiment, the method includes: sequentially illuminating an overlay target on a sample with a first illumination lobe and a second illumination lobe, wherein the overlay target includes one or more grating-on-grating features formed from periodic structures on a first sample layer and a second sample layer, and wherein the one or more grating-on-grating structures are periodic along a measurement direction. In another illustrative embodiment, the method includes: generating a first image and a second image of the overlay target with a detector, wherein the first image includes an unresolved image of the one or more grating-on-grating structures formed from a single non-zero diffraction order of the first illumination lobe, the single non-zero diffraction order being from the one or more grating-on-grating structures along the measurement direction, and wherein the second image includes an unresolved image of the one or more grating-on-grating structures formed from a single non-zero diffraction order of the second illumination lobe, the single non-zero diffraction order being generated by the one or more grating-on-grating structures along the measurement direction. In another illustrative 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.

[0006] It should be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and do not necessarily limit the invention as claimed. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the general description, serve to explain the principles of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Those skilled in the art may better understand the numerous advantages of the present disclosure by referring to the accompanying drawings, in which:

[0008] Figure 1A is a conceptual diagram of a system for performing overlay metrics according to one or more embodiments of the present disclosure.

[0009] Figure 1B is a conceptual diagram of an overlay metrology tool illustrating OTL (out-of-scope light) illumination, according to one or more embodiments of the present disclosure.

[0010] Figure 1C is a conceptual diagram of an overlay metrology tool illustrating tilted TTL (through-the-lens) darkfield imaging using a darkfield stop, in accordance with one or more embodiments of the present disclosure.

[0011] Figure 1D is a conceptual diagram of an overlay metrology tool illustrating tilted TTL dark field imaging using a dark field mirror, according to one or more embodiments of the present disclosure.

[0012] Figure 2Ais a top view of an overlaid target having overlaid grating features according to one or more embodiments of the present disclosure.

[0013] Figure 2B is a side view of a single unit of an overlay target according to one or more embodiments of the present disclosure.

[0014] Figure 3 is a schematic diagram of a pupil plane illustrating the distribution of an illumination beam providing suitable quadrupole illumination according to one or more embodiments of the present disclosure.

[0015] Figure 4A According to one or more embodiments of the present disclosure, Figure 3 Schematic diagram of imaging a cell overlapping a target by a first illumination beam directed along the X direction.

[0016] Figure 4B is a schematic diagram of a collection pupil illustrating the distribution of diffraction orders of a first illumination beam generated by a cell according to one or more embodiments of the present disclosure.

[0017] Figure 4C According to one or more embodiments of the present disclosure, Figure 3 Schematic diagram of imaging a unit of an overlapping target by a second illumination beam directed along the X direction at an azimuth angle opposite to the first illumination beam.

[0018] Figure 4D is a schematic diagram of a collection pupil illustrating the distribution of diffraction orders of a third illumination beam generated by a unit according to one or more embodiments of the present disclosure.

[0019] Figure 4E According to one or more embodiments of the present disclosure, Figure 3 Schematic diagram of imaging the cells of the overlapping target with a third illumination beam directed along the Y direction.

[0020] Figure 4F is a schematic diagram of a collection pupil illustrating the distribution of diffraction orders of a third illumination beam generated by a unit according to one or more embodiments of the present disclosure.

[0021] Figure 5A is a conceptual diagram of an image of an overlaid target based on illumination using a first illumination beam or a second illumination beam according to one or more embodiments of the present disclosure.

[0022] Figure 5B is a conceptual diagram of an image of an overlaid target based on illumination using a third illumination beam or a fourth illumination beam according to one or more embodiments of the present disclosure.

[0023] Figure 6is a conceptual diagram of an image of an overlaid target based on illumination using a first illumination beam and a third illumination beam or a second illumination beam and a fourth illumination beam according to one or more embodiments of the present disclosure.

[0024] Fig. 7A is a conceptual diagram of a split pupil plane according to one or more embodiments of the present disclosure.

[0025] Figure 7B is a conceptual diagram of a pupil plane according to one or more embodiments of the present disclosure, illustrating distribution of diffraction orders of a first illumination light beam by a unit having periodicity in the Y direction.

[0026] Figure 7C is a conceptual diagram of a pupil plane according to one or more embodiments of the present disclosure, illustrating distribution of diffraction orders of a first illumination light beam by a unit having periodicity in the X direction.

[0027] Fig.7D is a conceptual diagram of a pupil plane according to one or more embodiments of the present disclosure, illustrating distribution of diffraction orders of a second illumination light beam by a unit having periodicity in the Y direction.

[0028] Fig. 7E is a conceptual diagram of a pupil plane according to one or more embodiments of the present disclosure, illustrating distribution of diffraction orders of a second illumination light beam by a unit having periodicity in the X direction.

[0029] Figure 8 is a conceptual diagram of an imaging subsystem configured to produce interlaced images from two alternating illumination conditions using a detector including a TDI sensor, according to one or more embodiments of the present disclosure.

[0030] Fig. 9 is a flow chart illustrating steps performed in an overlay method according to one or more embodiments of the present disclosure. DETAILED DESCRIPTION

[0031] Reference will now be made in detail to the disclosed subject matter illustrated in the accompanying drawings. The present disclosure has been shown and described with particular reference to specific embodiments and specific features of the present disclosure. The embodiments set forth herein are to be considered illustrative rather than restrictive. It should be readily apparent to those skilled in the art that various changes and modifications in form and detail may be made without departing from the spirit and scope of the present disclosure.

[0032] Embodiments of the present disclosure are directed to systems and methods for scanning scatterometry metrics using field plane imaging of a grating-on-grating overlay target while the target is in motion based on sequential imaging of the overlay target using symmetrically opposed illumination beams, wherein the field plane image is formed using a single diffraction order along the measurement direction. In this regard, the field plane image corresponds to a dark field image, wherein features of the overlay target are unresolved and intensity differences of the overlay target features in the sequential images are proportional to the overlap.

[0033] For the purposes of this disclosure, the term scatterometry metrics is used to broadly encompass the terms scatterometry-based metrics and diffraction-based metrics. In addition, the term scanning metrics is used to describe metric measurements generated when the sample is in motion. In a general sense, scanning metrics can be implemented by scanning the sample along a measurement path (e.g., a scanning belt, etc.) so that the area of ​​interest on the sample (e.g., a metrology target, a device area, etc.) is translated through the measurement field of view of the metrology system. In addition, the process can be repeated for any number of measurement paths or repeated measurements of a particular measurement path to provide any selected number of measurements of the sample. Scatterometry metrics based on dark field imaging of periodic overlay targets are generally described in U.S. patent application No. 16 / 996,328, filed on August 18, 2020, which is incorporated herein by reference in its entirety.

[0034] The overlay target may typically include well-defined printed elements that are designed to provide an accurate representation of the relative placement of features associated with one or more lithography steps on one or more sample layers. In this regard, the measured properties of the printed elements of the overlay target (e.g., by an overlay metrology tool) may represent printed device elements associated with the device being fabricated. Furthermore, the overlay target may include one or more measurement cells, where each cell includes a printed element in one or more layers on the sample. Overlay measurements may then be based on any combination of measurements of the various cells of the overlay target.

[0035] In some embodiments, an overlay target suitable for a scanning scatterometry metric as disclosed herein may include one or more cells having a grating-on-grating structure, wherein the grating-on-grating structure includes periodic features (e.g., grating features) located over overlapping regions of two or more layers of interest. In this way, the various grating features on the layers of interest may contribute to diffraction of incident illumination and an overlay measurement may be generated based on analysis of the diffracted light.

[0036] In some embodiments, a scanning scatterometry overlay tool generates an overlay measurement along a measurement direction by sequentially generating two field plane images of a scatterometry overlay target using opposing illumination beams (e.g., beams having opposing incident azimuths along the measurement direction), wherein each of the field plane images is formed using a single non-zero diffraction order from the scatterometry overlay target. In this manner, grating-on-grating structures are unresolved in the field plane images and appear as grayscale features, wherein the intensity difference of the grayscale features between the field plane images is proportional to the overlay.

[0037] Consider herein that the distribution of diffraction orders associated with the diffraction of an incident illumination beam may depend on a variety of illumination and collection conditions, including, but not limited to: the angle of incidence of the illumination beam in both azimuth and elevation, the wavelength of the illumination beam, the spacing of periodic features on the sample, or the numerical aperture (NA) of the collection optics. Furthermore, the overlay metrology tool may include one or more elements that block one or more collected diffraction orders such that the image is based on selected diffraction orders. In this regard, the illumination beam may be directed to an overlay target through a common lens with the collection optics (e.g., in a through-the-lens (TTL) configuration) at an angle of incidence outside the NA of the collection optics (e.g., in an out-of-the-lens (OTL) configuration).

[0038] Additional embodiments of the present disclosure are directed to overlay measurements in two directions (e.g., two orthogonal directions). For example, an overlay target may include a first group of one or more cells having a periodicity along a first measurement direction and a second group of one or more cells having a periodicity along a second measurement direction different from the first direction (e.g., orthogonal to the first direction). In this configuration, illuminating the overlay target with an illumination beam along a first direction (e.g., having an azimuth along the first direction) may provide collection of a single diffraction order along the first direction by the first group of cells and collection of undiffracted orders along the second group of cells. Thus, only the first group of cells may be visible in an image. Similarly, only the second group of cells may be visible in an image generated with an illumination beam along the second direction. In some embodiments, overlay measurements along two directions are generated based on sequential quadrupole illumination based on four images, wherein two illumination beams are oriented at relative azimuths along the first direction and two illumination beams are oriented at relative azimuths along the second direction. In some embodiments, the overlay measurement is based on two images, based on sequential illumination with two pairs of illumination beams from a quadrupole distribution, wherein a first pair of illumination beams comprises one illumination beam along a first direction and one illumination beam along a second direction, and a second pair of illumination beams comprises opposing beams along the first direction and the second direction. In this way, all cells in the overlay target are visible in each image, but the image of any particular cell is formed from a single diffraction order from a single illumination beam.

[0039] Further consider herein that scanning metrology based on sequential illumination with relative illumination beams can be implemented in a variety of ways. In some embodiments, images of overlapping targets using relative illumination beams are sequentially generated in a sequential scan of the overlapping targets. In some embodiments, images are generated during the scan based on alternating illumination and / or collection conditions during a single scan. For example, an interlaced image can be generated based on alternating illumination of the overlapping targets with relative illumination beams, wherein two field plane images can be extracted from the interlaced image. In one example, a measurement tool for scanning scatterometry is configured to illuminate alternating rows on a TDI sensor and synchronize the alternating illumination conditions of the scatterometry overlapping targets with the charge transfer rate of the TDI sensor to generate an interlaced image. The use of a TDI sensor in scanning metrology to generate an interlaced image is generally described in U.S. Patent Application No. 16 / 586,504 filed on September 27, 2019, which is incorporated by reference in its entirety.

[0040] In addition, it should be understood that although many examples in the present disclosure involve scanning metrology, the systems and methods disclosed herein are not limited to scanning metrology. Instead, please consider that the systems and methods disclosed herein can also be utilized in a static measurement mode, where the sample is static during measurement.

[0041] Reference now Figures 1A to 9 , systems and methods for scatterometry overlay metrics are described in more detail according to one or more embodiments of the present disclosure.

[0042] Figure 1A is a conceptual diagram of a system 100 for performing overlay metrology according to one or more embodiments of the present disclosure.

[0043] In one embodiment, the system 100 includes an overlay metrology tool 102 to generate one or more field plane images of the sample 104 or a portion thereof based on a single non-zero diffraction order along any given measurement direction from the sample 104. For example, the system 100 may generate a dark field image of an overlay target including features in two or more layers of the sample 104.

[0044] In one embodiment, the overlay metrology tool 102 includes an illumination subsystem 106 and an imaging subsystem 110, the illumination subsystem being used to generate illumination in the form of at least one illumination beam 108 at a time to illuminate the sample 104, and the imaging subsystem being used to image the illuminated sample 104. In another embodiment, the overlay metrology tool 102 includes a scanning subsystem 112, the scanning subsystem being used to position the sample 104 in a measurement field of view for imaging. For example, the scanning subsystem 112 may include one or more translation stages that hold the sample 104, the one or more translation stages being configured to position the overlay target within the measurement field of view to be imaged. In addition, the overlay metrology tool 102 may illuminate and image the sample 104 in scanning and / or static modes. For example, the overlay metrology tool 102 may implement scanning overlay measurement by coordinating illumination by the illumination subsystem 106 and image capture by the imaging subsystem 110 with motion of the sample 104 along a measurement path (e.g., a scanning band) by the scanning subsystem 112 to capture images of overlay targets on the measurement path. By way of another example, the overlay metrology tool 102 may implement static overlay measurement by sequentially translating the sample 104 while the sample 104 is stationary to position the overlay targets in the measurement field of view and to image the overlay targets.

[0045] The overlay metrology tool 102 may be adapted to generate field plane images of a variety of overlay target designs on a variety of sample configurations. In one embodiment, the sample 104 is formed as two or more patterned layers on a single wafer. An overlay target comprising target features located on two or more sample layers of a single wafer may thus provide an overlay measurement between the two or more sample layers of a single wafer. In another embodiment, the sample 104 is formed as two wafers bonded together at an interface, wherein each wafer may include one or more patterned layers proximate the interface (e.g., the inward facing side of the wafer). An overlay target comprising target features located on at least one sample layer of each wafer may thus provide an overlay measurement of the alignment of the two wafers during the bonding process.

[0046] The overlay metrology tool 102 may be configured to generate an image based on any number of prescriptions that define measurement parameters for determining the overlay of an overlay target. For example, the prescription of the overlay metrology tool 102 may include, but is not limited to: the wavelength of illumination, the detected wavelength of radiation emitted from the sample, the spot size of the illumination on the sample, the incident illumination angle in azimuth or altitude, the polarization of the incident illumination, the position of the beam of incident illumination on the overlay target, the position of the overlay target in the focal volume of the overlay metrology tool, and the like.

[0047] In one embodiment, an overlay measurement along a particular measurement direction is generated by generating two images of the sample 104 or a portion thereof (e.g., an overlay target) based on sequential illumination using two opposing illumination beams (e.g., beams having opposing azimuth angles), wherein a periodic feature along a given direction on the sample 104 is imaged at a time based on a single diffraction order from a single illumination beam. In this regard, the periodic feature is unresolved, but the difference in intensity of the unresolved image of the periodic feature between the two images generated using the opposing illumination beams is proportional to the overlap along the periodic direction of the feature.

[0048] In another embodiment, the system 100 includes a controller 114 that includes one or more processors 116 that are 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 or 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. In addition, the memory device 118 may include any storage medium known in the art that is suitable for storing program instructions that can be executed by the associated one or more processors 116. For example, the memory device 118 may include a non-transitory memory medium. As additional examples, the memory device 118 may include, but is not limited to, a read-only memory, a random access memory, a magnetic or optical memory device (e.g., a disk), a tape, a solid-state drive, and the like. Further note that the memory device 118 may be housed in a common controller housing with the one or more processors 116.

[0049] In this regard, the controller 114 may perform any of the various processing steps associated with 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 component thereof. For example, the controller 114 may be configured to direct the overlay metrology tool 102, or any component thereof, to generate one or more images based on dark field imaging, based on one or more selected prescriptions that define measurement parameters for determining overlay of an overlay target. The prescription of the overlay metrology tool may include, but is not limited to: the wavelength of illumination, the detected wavelength of radiation emitted from the sample, the spot size of the illumination on the sample, the incident illumination angle, the polarization of the incident illumination, the position of the beam of incident illumination on the overlay target, the position of the overlay target in the focal volume of the overlay metrology tool, and the like.

[0050] By way of another example, the controller 114 can be further configured to receive data including, but not limited to, images from the overlay metrology tool 102. By way of another example, the controller 114 can be configured to determine an overlay associated with an overlay target based on the acquired images. By way of another example, the controller 114 can generate correctables for one or more additional fabrication tools based on the overlay measurements from the overlay metrology tool 102 as feedback and / or feed-forward control to the one or more additional fabrication tools.

[0051] Reference now Figures 1B to 1D , various non-limiting configurations of the overlay metrology tool 102 are described according to one or more embodiments of the present disclosure. Consider herein that the overlay metrology 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 a variety of configurations (including but not limited to OTL and TTL illumination).

[0052] Figure 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. Consider herein that OTL illumination for darkfield imaging as disclosed herein can advantageously provide relatively small target sizes, high light efficiency associated with direct illumination of the sample 104, and low stray light from physically separated illumination and collection paths without the need for blockers or masks. For example, OTL illumination can provide a small target size by illuminating the sample at a relatively high incident angle (e.g., an elevation angle) than TTL illumination, which in turn can require that periodic features on the sample 104 diffract a single diffraction order into the collection path of the imaging subsystem 110 based on the grating equation.

[0053] 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 light of one or more selected wavelengths, 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 not be affected by speckle.

[0054] 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-busting element, which may include any type of speckle-busting element known in the art (e.g., a rotating diffuser, a beam scanner for scanning the illumination beam 108 on the input face of the 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 housing one or more elements that can emit 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 having low coherence (eg, low spatial coherence and / or temporal coherence).

[0055] Furthermore, the sample 104 may be disposed on a sample stage 122 (eg, a portion of the scanning subsystem 112 ) that is suitable for securing the sample 104 and further configured to position the sample 104 relative to the illumination beam 108 .

[0056] In another embodiment, the overlay metrology tool 102 directs the illumination beam 108 to the sample 104 through one or more illumination channels 124 to illuminate the sample 104 to obtain an image. Each of the illumination channels 124 may include one or more optical components suitable for modifying and / or adjusting the illumination beam 108 and directing the illumination beam 108 to the sample 104. For example, each illumination channel 124 may include (but need not include) one or more illumination lenses 126 (e.g., to control the spot size, relay pupil and / or field plane of the illumination beam 108 on the sample 104, etc.) or one or more illumination control optical devices 128. For example, the illumination control optics 128 may include, but are not limited to, one or more polarizers, one or more filters, one or more beam splitters, one or more diffusers, one or more homogenizers, one or more apodizers, one or more beam shapers, or one or more mirrors (e.g., static mirrors, translatable mirrors, scanning mirrors, etc.) to adjust the polarization of the illumination beam 108.

[0057] Additionally, the illumination channel 124 may be provided in a variety of configurations to direct one or more illumination beams 108 to the sample 104. In one embodiment, Figure 1B, each illumination channel 124 may include a separate set of components (e.g., illumination lenses 126 and / or illumination control optics 128). By way of another example, the illumination subsystem 106 may include a common set of illumination lenses 126 and / or illumination control optics 128 for generating and directing the one or more illumination beams 108 to the sample 104. Furthermore, the illumination subsystem 106 may direct the one or more illumination beams 108 to the sample 104 using any combination of free space or fiber optics.

[0058] In another embodiment, the imaging subsystem 110 of the overlay metrology tool 102 includes an objective lens 130 to collect diffracted or scattered light (e.g., sample light 132) from the sample 104 and at least one detector 134 located at a field plane conjugate to the sample 104, the at least one detector being configured to generate an image of the sample 104 based on at least some of the sample light 132. The imaging subsystem 110 may further include a plurality of optical elements to direct and / or modify the illumination collected by the objective lens 130, including but 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 stops, or one or more beam splitters.

[0059] Figure 1C is a conceptual diagram of an overlay metrology tool 102 illustrating tilted 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 merge the illumination subsystem 106 with the imaging subsystem 110 so that the objective lens 130 can simultaneously direct one or more illumination beams 108 to the sample 104 and collect sample light 132 from the sample 104. For example, Figure 1C The diagram illustrates multiple illumination channels 124 providing illumination beams 108 to a common illumination field stop 144 of the illumination subsystem 106. In another embodiment, the darkfield stop 140 is located at a pupil plane 146 that is unique to the imaging subsystem 110 and includes one or more opaque elements that are used to block specular reflections (e.g., 0th diffraction order) collected by the objective 130 and pass a single non-zero diffraction order for darkfield imaging.

[0060] Figure 1D is a conceptual diagram of an overlay metrology tool 102 illustrating tilted TTL dark field imaging using a dark field mirror 148 according to one or more embodiments of the present disclosure. Figure 1C Same as in Figure 1DThe diagram illustrates multiple illumination channels 124 that provide illumination beams 108 to a common illumination field aperture 144 of the illumination subsystem 106. Figure 1D As illustrated in , the overlay metrology tool 102 may include, but need not include, an optical relay 150 located between the darkfield mirror 148 and the objective lens 130 .

[0061] In one embodiment, the darkfield mirror 148 includes one or more reflective regions 152 and an open central region 154 to direct one or more illumination beams 108 to the objective 130 for illuminating the sample 104. For example, the darkfield mirror 148 may include, but is not limited to, a toroidal mirror with an open central aperture. In this manner, the darkfield mirror 148 may simultaneously direct the one or more illumination beams 108 along a range of oblique solid angles, block sample light 132 associated with specular reflections (e.g., 0th order diffraction) collected by the objective 130, and pass a single non-zero diffraction order for darkfield imaging. The darkfield mirror 148 may (but need not) be located at or near a pupil plane common to the illumination channel 124 and the imaging subsystem 110. In this regard, the spatial dimensions of the darkfield mirror 148 in the pupil plane may directly correspond to the solid angle range over which the illumination beam 108 is directed to the sample 104, and the open aperture may correspond to the solid angle range of the collected sample light 132 that is permitted to propagate to the detector 134 to form a darkfield image. In instances where the darkfield mirror 148 is located near the pupil plane or the darkfield mirror 148 is tilted relative to the pupil plane, at least a portion of the pupil plane may be defocused on the darkfield mirror 148, resulting in pupil blur at the darkfield mirror 148. However, the effects of this pupil blur may be mitigated by providing sufficient spacing between the illumination beam 108 and the non-zero diffraction orders desired to be delivered to the detector 134.

[0062] It should be understood that Figures 1B to 1D and the associated description are provided for illustrative purposes only and should not be construed as limiting. Rather, it is contemplated herein that the overlay metrology tool 102 may be configured in various ways to provide imaging using a single non-zero diffraction order. For example, Figures 1B to 1D A separate illumination source 120 is illustrated for each illumination channel 124. However, any number of illumination beams 108 may be generated by any number or combination of illumination sources 120. By way of another example, the overlay metrology tool 102 may include any combination of free space optics and fiber optics to provide a suitable physical layout.

[0063] Reference now Figures 2A to 7E , the generation of a field plane image of a grating-on-grating overlay metrology target along a given measurement direction formed from a single non-zero diffraction order is described in more detail according to one or more embodiments of the present disclosure.

[0064] Figure 2Ais a top view of an overlaid target 202 having overlaid grating features on overlaid gratings according to one or more embodiments of the present disclosure. Figure 2B 2 is a side view of a single cell 206 of an overlay target 202 located on a substrate 204 in accordance with one or more embodiments of the present disclosure. In one embodiment, the overlay target 202 includes a plurality of cells 206, wherein each cell 206 includes a first layer of printed elements 208 located on a first layer 210 of a sample 104 and a second layer of printed elements 212 located on a second layer 214 of the sample 104, the sample being oriented such that regions including the first layer of printed elements 208 and the second layer of printed elements 212 overlap to form a grating-on-grating structure. In this regard, the grating-on-grating features in each cell 206 can diffract an incident illumination beam 108 into discrete diffraction orders.

[0065] In another embodiment, the overlay target 202 includes a first set of one or more cells 206 where the grating-overlying grating structure is periodic along a first direction and a second set of one or more cells 206 where the grating-overlying grating structure is periodic along a second direction different from the first direction (e.g., orthogonal).

[0066] For example, Figure 2A to 2B An overlay target 202 having four cells 206a-206d is illustrated, where cells 206a and 206d form a first group of cells and cells 206b and 206c form a second group of cells. Specifically, the grating-on-grating structure in cells 206a and 206d is periodic in the X direction and the grating-on-grating structure in cells 206b and 206c is periodic in the Y direction. In this regard, cells 206a, 206d may be suitable for overlay measurements along the X direction and cells 206b, 206c may be suitable for overlay measurements along the Y direction, as will be described in more detail below.

[0067] However, it should be understood that Figure 2A and 2BThe overlay target 202 and associated description in are provided for illustrative purposes only and should not be construed as limiting. Rather, the overlay target 202 may include any suitable grating-on-grating overlay target design. For example, the overlay target 202 may include any number of cells 206 suitable for measurement along two directions. In addition, the cells 206 may be distributed in any pattern or arrangement. For example, a metrology target design suitable for scanning metrology is generally described in U.S. Patent Application No. 16 / 598,146 filed on October 10, 2019, which is incorporated herein by reference in its entirety. In one embodiment, the overlay target 202 includes one or more cell groups distributed along a scanning direction (e.g., a direction of motion of the sample 104), wherein the cells 206 within each particular cell group are oriented to have a grating-on-grating structure that is periodic along a common direction. For example, a first cell group may include one or more cells 206 having periodicity along the X direction and a second cell group may include one or more cells 206 having periodicity along the Y direction. In this way, all cells 206 within a particular cell group may be imaged while scanning the sample 104 across the measurement field of view of the imaging subsystem 110.

[0068] Reference now Figures 3 to 6 , the illumination and collection configurations for scatterometry overlay metrology measurements based on an overlay target 202 with a single non-zero diffraction order are described in more detail herein.

[0069] Figure 3 is a schematic diagram of a pupil plane according to one or more embodiments of the present disclosure, illustrating the distribution of illumination beams 108a to 108d that provide suitable quadrupole illumination. In one embodiment, the first illumination beam 108a and the second illumination beam 108b may be directed to illuminate the sample 104 along the X direction at symmetrically opposite azimuth angles. In another embodiment, the third illumination beam 108c and the fourth illumination beam 108d may be directed to illuminate the sample 104 along the Y direction at symmetrically opposite azimuth angles. In addition, Figure 3 The diagram illustrates the boundaries of the collection pupil 302 of the imaging subsystem 110. For example, Figure 3 The boundary of the collection pupil 302 in may correspond to the NA of the objective lens 130 or the diameter of the pupil stop in the imaging subsystem 110 .

[0070] exist Figure 3 In FIG. 3 , the illumination beams 108a to 108d are illustrated as being outside the collection pupil 302 to illustrate dark field collection where the 0th order diffraction is not used for imaging. In this context, please consider that Figure 3 This dark field configuration in can be produced in various ways. For example, Figure 3 The configuration in can be used as Figure 1BIn this regard, one or more illumination channels 124 may deliver illumination beams 108a to 108d directly to sample 104 outside the NA of objective lens 130. By way of another example, Figure 3 The configuration in can be used as Figure 1B and 1C , where the overlay metrology tool 102 selectively blocks the 0th order diffraction of the illumination beams 108a to 108d (e.g., using a dark field stop 140, a dark field mirror 148, etc.). In a general sense, Figure 3 May not necessarily correspond to a specific pupil plane in a system, but rather illustrates dark field illumination in accordance with one or more embodiments of the present disclosure.

[0071] Figures 4A to 4D The diagram illustrates imaging of cell 206a of overlay target 202 as an example of imaging a feature having periodicity along the X direction in accordance with one or more embodiments of the present disclosure.

[0072] Figure 4A According to one or more embodiments of the present disclosure, Figure 3 Schematic diagram of a first illumination beam 108a directed along the X direction imaging a cell 206a of an overlapping target 202. Figure 4B is a schematic diagram of a collection pupil 302 illustrating the distribution of diffraction orders of a first illumination beam 108a generated by unit 206a, according to one or more embodiments of the present disclosure.

[0073] The grating features on the grating having periodic units 206a along the X direction can diffract the first illumination beam 108a into a plurality of diffraction orders distributed along the X direction. Specifically, Figure 4A and 4B The diagram illustrates the generation of a 0th order diffraction 402 (e.g., specular reflection), a -1st order diffraction 404, and a -2nd order diffraction 406. In one embodiment, various parameters of illumination subsystem 106, imaging subsystem 110, and overlay target 202 are selected to provide: a single diffraction order is collected by imaging subsystem 110 (e.g., as Figure 4A and 4B 4. For example, the periodicity of the features in cell 206a and parameters associated with the measurement recipe (such as, but not limited to, the wavelength of first illumination beam 108a, the angle of incidence of first illumination beam 108a, or the size of collection pupil 302) can be selected to provide: a single diffraction order from first illumination beam 108a passes through collection pupil 302.

[0074] Figure 4C According to one or more embodiments of the present disclosure, Figure 3Schematic diagram of imaging a cell 206a of an overlapping target 202 by a second illumination beam 108b oriented along the X direction at an azimuth angle opposite to the first illumination beam 108a. Figure 4D is a schematic diagram of a collection pupil 302 illustrating the distribution of diffraction orders of the second illumination beam 108b generated by the unit 206a, according to one or more embodiments of the present disclosure. Figure 4C and 4D Similarly, the generation of multiple diffraction orders along the X direction is illustrated, and the multiple diffraction orders can be symmetrical with the distribution of the diffraction orders of the first illumination beam 108a. In particular, Figure 4C and 4D The diagram illustrates the generation of 0th order diffraction 408 (eg, specular reflection), -1st order diffraction 410, and -2nd order diffraction 412. Similarly, Figure 4C and 4D The diagram illustrates the collection of a single diffraction order (here, the -1 order 410).

[0075] Figure 4E According to one or more embodiments of the present disclosure, Figure 3 FIG. 1 is a schematic diagram showing a third illumination light beam 108 c directed along the Y direction imaging a cell 206 a of an overlapping target 202 . Figure 4F is a schematic diagram of a collection pupil 302 illustrating the distribution of diffraction orders of the third illumination beam 108c generated by the unit 206a, according to one or more embodiments of the present disclosure.

[0076] like Figure 4E and 4F , cell 206a does not diffract third illumination beam 108c along the Y direction, but only along the X direction (e.g., +1 order diffraction 414, 0 order diffraction 416 (e.g., specular reflection), and -1 order diffraction 418). In addition, in one embodiment, various parameters of illumination subsystem 106, imaging subsystem 110, and overlay target 202 are selected to provide that no diffraction orders (including 0 order diffraction) pass through collection pupil 302. Therefore, features of cell 206a will not be visible in the image of overlay target 202 based on third illumination beam 108c. Although not shown, it should be understood that illuminating cell 206a with fourth illumination beam 108d will have similar results as illuminating with third illumination beam 108c.

[0077] Furthermore, it should be understood that Figures 4A to 4F Any unit 206 (e.g., unit 206d) having a structure that is periodic along the X direction may be illustrated. It should also be understood that Figures 4A to 4FThe concepts illustrated in can further illustrate the imaging of cells (e.g., cells 206b and 206c) having a structure that is periodic along the Y direction. In particular, imaging the overlay target 202 with the third illumination beam 108c and the fourth illumination beam 108d can result in collecting a single diffraction order produced by cells 206b and 206c and not collecting light from cells 206a and 206d, such that only cells 206b and 206c are visible.

[0078] Reference now Figures 5A to 6 , the field plane image of the overlay target 202 based on various lighting conditions is described in more detail according to one or more embodiments of the present disclosure. In one embodiment, as Figures 5A to 6 As illustrated in , the overlay target 202 is fully illuminated by the incident illumination. In this regard, the illumination directed to the overlay target 202 is greater than the size of the overlay target 202.

[0079] Figure 5A 1 is a conceptual diagram of an image of an overlaid target 202 based on illumination using the first illumination beam 108a or the second illumination beam 108b according to one or more embodiments of the present disclosure. Figures 4A to 4F As described, illumination with the first illumination beam 108a or the second illumination beam 108b results in the collection of a single diffraction order (e.g., -1 diffraction order 404 or -1 diffraction order 410) generated by a structure that is periodic along the X direction (here, cells 206a and 206d) and no collection of light from a structure that is periodic along the Y direction. Therefore, only cells 206a and 206d are visible in the image, but the grating-on-grating features in those cells are not resolved, but appear as integral features. Further consider herein that the relative intensity of the visible cells in the image generated with the first illumination beam 108a compared to the visible cells in the image generated with the second illumination beam 108b may indicate an overlay error associated with a relative displacement of the first layer 210 and the second layer 214 of the sample 104 along the X direction.

[0080] Figure 5B 1 is a conceptual diagram of an image of an overlaid target 202 based on illumination using the third illumination beam 108c or the fourth illumination beam 108d according to one or more embodiments of the present disclosure. In this configuration, only the cells 206b and 206c are visible in the image. In addition, an overlay measurement along the Y direction can be generated based on the relative intensity of the visible cells in the image generated using the third illumination beam 108c compared to the visible cells in the image generated using the fourth illumination beam 108d.

[0081] In this article please consider that you can use Figure 5A and 5B108a to generate overlay measurements associated with the relative displacement of the first layer 210 and the second layer 214 of the sample 104. In particular, the relative intensity of visible cells in the image generated using the first illumination beam 108a compared to the visible cells in the image generated using the second illumination beam 108b can indicate overlay error along the X direction, while the relative intensity of visible cells in the image generated using the third illumination beam 108c compared to the visible cells in the image generated using the fourth illumination beam 108d can indicate overlay error along the Y direction.

[0082] Reference now Figure 6 In another embodiment, two images may be used to generate overlay measurements along the X and Y directions, where each image is formed from a pair of two orthogonal illumination beams 108 .

[0083] Figure 6 1 is a conceptual diagram of an image of an overlaid target 202 based on illumination using 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, Figure 6 The image in is formed using a pair of illumination beams 108 having orthogonal azimuth angles.

[0084] exist Figure 6 In FIG. 2 , all cells 206 a to 206 d in the overlay object 202 are visible. Figures 4A to 5B As illustrated in FIG. 1 , the image of any particular cell 206 is formed from the light associated with a single illumination beam 108. In this manner, a pair of images (e.g., Figure 6 The image illustrated in FIG. 1 may include four images (eg Figure 5A and 5B ) so that an equivalent overlay measurement can be produced based on two images (and two corresponding measurement prescriptions) rather than four images (and four corresponding measurement prescriptions).

[0085] Reference now Figures 7A to 7E , generating a dark field image using a split pupil is described in more detail according to one or more embodiments of the present disclosure.

[0086] In one embodiment, the overlay metrology tool 102 includes pupil segmentation optics, such as but not limited to prisms located in the pupil plane 702, which are used to selectively direct portions of light in different portions of the collection pupil 302 to different detectors 134. For example, although not shown, Figure 1DThe overlay metrology tool 102 illustrated in may include pupil segmentation optics located within the aperture of the darkfield mirror 148 or in a conjugate pupil plane before the detector 134 .

[0087] Fig. 7A 7 is a conceptual diagram of a segmented pupil plane 702 according to one or more embodiments of the present disclosure. In one embodiment, the pupil plane 702 is segmented into four quadrants, wherein two opposing illumination quadrants 704 provide illumination with two opposing illumination beams 108 (e.g., a first diagonal illumination beam 108e and a second diagonal illumination beam 108f) and two opposing collection quadrants 706 provide collection of sample light 132. In addition, the 0th order diffraction from the illumination beam 108 is blocked in the illumination quadrant 704 (e.g., by a dark field mirror 148, etc.) to provide dark field imaging.

[0088] In another embodiment, if Figures 7B to 7E , the illumination beam 108 is oriented diagonally relative to the measurement direction (eg, the direction of the periodicity of the various cells 206 that overlap the target 202 ).

[0089] Figure 7B is a conceptual diagram of a pupil plane 702 illustrating the distribution of diffraction orders of a first diagonal illumination beam 108e through cells 206b and 206c having periodicity in the Y direction, according to one or more embodiments of the present disclosure. Specifically, Figure 7B The diagram illustrates -1 order diffraction 708, 0 order diffraction 710, +1 order diffraction 712, and +2 order diffraction 714. Figure 7C is a conceptual diagram of a pupil plane 702 illustrating the distribution of diffraction orders of a first diagonal illumination beam 108e through cells 206a and 206d having periodicity in the X direction, according to one or more embodiments of the present disclosure. Specifically, Figure 7C The diagram illustrates -1 order diffraction 716, 0 order diffraction 718, +1 order diffraction 720, and +2 order diffraction 722. Figure 7B and 7C As illustrated, a single diffraction order is collected from any given cell. Thus, this configuration can produce Figure 6 Similar images.

[0090] Fig.7D is a conceptual diagram of a pupil plane 702 illustrating the distribution of diffraction orders of a second diagonal illumination beam 108f through cells 206b and 206c having periodicity in the Y direction, according to one or more embodiments of the present disclosure. Specifically, Fig.7D The diagram illustrates -1 order diffraction 724, 0 order diffraction 726, +1 order diffraction 728, and +2 order diffraction 730. Fig. 7Eis a conceptual diagram of a pupil plane 702 illustrating the distribution of diffraction orders of a second diagonal illumination beam 108f through cells 206a and 206d having periodicity in the X direction, according to one or more embodiments of the present disclosure. Specifically, Fig. 7E The diagram illustrates -1 order diffraction 732, 0 order diffraction 734, +1 order diffraction 736 and +2 order diffraction 738. Similar to Figure 7B and 7C , Fig.7D and 7E The diagram illustrates that a single diffraction order is collected from any given cell of the overlapping target 202 but from the opposite second illumination beam 108f. Thus, this configuration can also produce the same Figure 6 Similar images and can be used as above about Figure 6 Similar techniques are described to determine overlap.

[0091] Reference again Figures 1A to 1D , configurations of the overlay metrology tool 102 for scanning or static metrology mode are described in more detail according to one or more embodiments of the present disclosure. Overlay metrology in scanning or static mode is generally 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 incorporated by reference in their entirety.

[0092] The overlay metrology tool 102 may include any type of optical detector 134 known in the art suitable for measuring illumination received from the sample 104. Furthermore, the overlay metrology tool 102 may generally include any number of detectors 134.

[0093] In one embodiment, the overlay metrology tool 102 includes one or more sensors adapted to generate one or more images of the sample 104 while the sample 104 is in motion. For example, the detector 134 may include a line sensor, which includes a row of pixels. In this regard, the system 100 may generate a continuous image (e.g., a stripe image) one row at a time by translating the sample 104 across a measurement field of view in a scan direction perpendicular to the rows of pixels and continuously clocking the line sensor during continuous exposure windows. By way of another example, the detector 134 may include a time domain integration (TDI) sensor, which includes a plurality of rows of pixels and a readout row. The TDI sensor may operate in a similar manner to a line sensor, except that a clock signal may continuously move charge from one row of pixels to the next until the charge reaches the readout row, where a row of the image is generated. By synchronizing the charge transfer (e.g., based on a clock signal) with the motion of the sample along the scan direction, the charge may continue to accumulate across the rows of pixels to provide a relatively high signal-to-noise ratio compared to a line sensor.

[0094] 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 static. For example, the detector 134 may include a sensor suitable for generating one or more images of the static 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. In addition, the detector 134 may include a multi-tap sensor having two or more taps per pixel, including, but not limited to, a multi-tap CMOS sensor. In this regard, the charge in the multi-tap pixel may be directed to any selected tap based on one or more drive signals to the pixel during an exposure window. Thus, a multi-tap sensor including an array of multi-tap pixels may generate multiple images during a single readout phase, each image being associated with a different tap of an associated pixel. In addition, for the purposes of this disclosure, a tap of a multi-tap sensor may refer to an output tap connected to an associated pixel. In this regard, reading out each tap of a multi-tap sensor (eg, in a readout phase) may produce a separate image.

[0095] In another embodiment, the detector 134 may include a spectral 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., associated with multiple beam paths generated by one or more beam splitters to facilitate multiple metric measurements made by the system 100). For example, the system 100 may include one or more detectors 134 suitable for static mode imaging and one or more detectors 134 suitable for scanning mode imaging. In another embodiment, the system 100 may include one or more detectors 134 suitable for both static and scanning imaging modes. For example, the TDI sensor may be operated in a static mode by not clocking the TDI sensor to transfer charge between pixel rows during an exposure window. Then, once the exposure window stops (e.g., via a shutter, turning off the illumination source 120, etc.) and additional light is not incident on the pixels, the TDI sensor may be clocked to transfer charge to the readout rows line by line to produce an image having a length equal to the number of pixel rows.

[0096] It is further contemplated herein that various techniques may be used to generate separate images of the sample 104 (eg, the overlay target 202 on the sample 104 ) utilizing opposing illumination beams 108 for overlay measurements as disclosed herein.

[0097] In one embodiment, separate images of the overlay target 202 with opposing illumination beams 108 are generated by using sequential images generated by sequential illumination of the sample 104 with the illumination beams 108. For example, sequential images in a scanning configuration may be generated by sequentially scanning the overlay target 202, wherein the overlay target 202 is illuminated with different illumination conditions (e.g., a different illumination beam 108 in each scan). By way of another example, sequential images in a static configuration may be generated by sequentially illuminating and imaging the overlay target 202 with different illumination conditions while the overlay target 202 remains static in the overlay metrology tool 102.

[0098] In another embodiment, the overlay target 202 is alternately illuminated using different illumination conditions (e.g., different illumination beams 108) while the overlay target 202 is scanned across the measurement field of view of the overlay metrology tool 102 and an interleaved image of the overlay target 202 is generated based on the alternating illumination conditions. Individual images associated with the 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.

[0099] For example, an interlaced image may be produced using a detector 134 including a TDI sensor in a scan mode by selectively illuminating alternating pixel rows 802 of the TDI sensor with sample light 132 (e.g., every other pixel row for two alternating illumination conditions, every fourth row for four alternating illumination conditions, every Nth row for N alternating illumination conditions, etc.). In this regard, an image of the overlay target 202 is distributed across the selected rows, while intervening rows 804 are not illuminated. The selective alternating illumination of alternating pixel rows 802 may be performed using a variety of techniques, including, but not limited to, a cylindrical lens array or a slit array positioned to block the intervening rows 804. Figure 8 is a conceptual diagram of an imaging subsystem 110 configured to produce interlaced images from two alternating illumination conditions using a detector 134 including a TDI sensor, according to one or more embodiments of the present disclosure. Figure 8 Any suitable combination of optical elements of the cylindrical lens array 806 illustrated in can be used to accomplish selected imaging onto alternating rows.

[0100] Additionally, the charge transfer rate of the TDI sensor along the scan direction 808 may be synchronized with the rate at which the illumination subsystem 106 is switched. For example, between two alternating illumination conditions (e.g., suitable for forming, e.g., Figure 6 ), the TDI sensor may be clocked to allow charge to accumulate in pixel row 802 under a first illumination condition, and then clocked to transfer charge to intervening row 804 when pixel row 802 is illuminated with a second illumination configuration. This process may then be repeated so that alternating rows of an image (e.g., an interlaced image) correspond to alternating illumination conditions. Separate images associated with each of the illumination conditions may then be extracted into separate images.

[0101] By way of another example, a multi-tap imaging sensor as previously described herein may be used to obtain efficient capture of images of overlay target 202 in static mode using alternating illumination conditions. In this regard, the charge in each pixel may be directed to any selected tap during an exposure window based on one or more drive signals to the pixel. During a single readout phase, a multi-tap sensor including an array of multi-tap pixels may produce multiple images, each associated with a different tap of an associated pixel. Thus, system 100 may perform static mode measurements by sequentially providing any selected number of optical configurations during an exposure window while sample 104 is static.

[0102] Fig. 9is a flow chart illustrating steps performed in an overlay method 900 according to one or more embodiments of the present disclosure. Applicants note that the embodiments and enabling techniques previously described herein in the context of system 100 should be interpreted as extending to method 900. However, further note that method 900 is not limited to the architecture of system 100.

[0103] 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 may include one or more grating-on-grating features forming a periodic structure on a first sample layer and a second sample layer, wherein the one or more grating-on-grating structures are periodic along a measurement direction as illustrated in FIG. 2 . Furthermore, the overlay target may generally include multiple groups of cells including grating-on-grating features along multiple measurement directions to facilitate overlay measurements along multiple measurement directions. In this case, step 902 may include sequentially illuminating the overlay target with two illumination conditions, wherein the first illumination condition includes a first pair of illumination lobes including one illumination lobe each along a first direction and a second direction, and wherein the second illumination condition includes a second pair of illumination lobes opposite (e.g., at a relative azimuth angle) to the first pair of illumination lobes.

[0104] In another embodiment, method 900 includes step 904 of generating, using a detector, a first image and a second image of an overlay target, wherein the first image includes an unresolved image of one or more grating-on-grating structures formed from a single non-zero diffraction order of a first illumination lobe, and wherein the second image includes an unresolved image of one or more grating-on-grating structures formed from a single non-zero diffraction order of a second illumination lobe. In the case where the overlay target includes multiple groups of cells having a grating-on-grating structure that is periodic along multiple measurement directions, the first image and the second image may include unresolved images of all cells, but the image of each cell may be generated based on a single diffraction order from a single illumination lobe.

[0105] In another embodiment, method 900 includes determining an overlay error between a first layer and a second layer of the sample along a measurement direction based on the first image and the second image at step 906. Specifically, the overlay may be proportional to the intensity difference of a particular grating-on-grating structure in the first image and the second image.

[0106] The subject matter described herein sometimes illustrates different components contained in or connected to other components. It should be understood that such depicted architectures are only exemplary, and in fact many other architectures that realize the same functionality can be implemented. In a conceptual sense, any component arrangement that realizes the same functionality is effectively "associated" so that the desired functionality is realized. Therefore, any two components combined to realize a specific functionality herein can be considered to be "associated" with each other so that the desired functionality is realized, regardless of the architecture or intermediate components. Similarly, any two components so associated can also be considered to be "connected" or "coupled" to each other to realize the desired functionality, and any two components that can be so associated can also be considered to be "coupleable" to each other to realize the desired functionality. Specific examples that can be coupled include, but are not limited to, components that can physically interact and / or physically interact and / or components that can wirelessly interact and / or wirelessly interact and / or components that can logically interact and / or logically interact.

[0107] It is believed that many of the advantages of the present disclosure and its attendant advantages will be appreciated from the foregoing description, and it will be apparent that various changes may be made in the form, construction, and arrangement of components without departing from the disclosed subject matter or sacrificing all of its material advantages. The forms described are merely illustrative, and it is intended that such changes be encompassed and included by the appended claims. Furthermore, it should be understood that the invention is defined by the appended claims.

Claims

1. An overlay measurement system, comprising: an illumination subsystem configured to sequentially illuminate an overlying target on the sample with a first illumination lobe and a second illumination lobe opposite the first illumination lobe, wherein the overlying target comprises one or more grating-on-grating features formed from periodic structures on the first sample layer and the second sample layer, wherein the one or more grating-on-grating structures are periodic along a measurement direction; An imaging subsystem comprising: objective lens; and a detector configured to generate a first image and a second image of the overlaid target, wherein the first image comprises an unresolved image of the one or more grating-on-grating structures formed from a single non-zero diffraction order of the first illumination lobe, the single non-zero diffraction order being generated by the one or more grating-on-grating structures along the measurement direction, wherein the second image comprises an unresolved image of the one or more grating-on-grating structures formed from a single non-zero diffraction order of the second illumination lobe, the single non-zero diffraction order being generated by the one or more grating-on-grating structures along the measurement direction, wherein the detector comprises a scanning detector configured to generate the first image and the second image while the sample is translated through the measurement field of the objective lens by a translation stage; and A controller communicatively coupled to the detector, the controller comprising one or more processors configured to execute program instructions causing 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.

2. The overlay metrology system of claim 1 , wherein 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 comprises: An overlay error between the first sample layer and the second sample layer of the sample along the measurement direction is determined based on a difference in intensities of the one or more grating overlying grating structures in the first image and the second image.

3. The overlay metrology system of claim 1 , wherein the illumination subsystem comprises 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 outside of a numerical aperture of the objective lens.

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

5. The overlay metrology system of claim 4, wherein the one or more beam stops comprise: A dark field stop in the imaging subsystem.

6. The overlay metrology system of claim 4, further comprising: a darkfield mirror shared by the illumination subsystem and the imaging subsystem, wherein the darkfield mirror directs the first illumination lobe and the second illumination lobe to the overlapping target through the objective lens, wherein the darkfield mirror operates as the one or more beam stops to block the specular reflections of the first illumination lobe and the second illumination lobe, wherein the darkfield mirror passes the single non-zero diffraction order associated with the first illumination lobe and the second illumination lobe.

7. The overlay metrology system of claim 6, wherein the dark field mirror comprises: Ring mirror.

8. The overlay metrology system of claim 1, wherein the detector comprises: Time domain integrating sensor.

9. The overlay metrology system of claim 8, further comprising: a cylindrical lens array configured to direct light emitted from the sample to every Nth pixel row of the time-domain integrating sensor, wherein the overlay metrology system is configured to perform scan mode measurements by: translating the sample at a charge transfer rate of the time-domain integrating sensor; illuminating the sample with the first illumination lobe and the second illumination lobe alternately, wherein a switching time between successive optical configurations of the imaging subsystem corresponds to the charge transfer rate of the time-domain integrating sensor; generating an interlaced image including the first image and the second image using the time-domain integration sensor during an exposure window; and The interlaced image is separated into the first image and the second image.

10. The overlay metrology system of claim 9, further comprising: A slit array is positioned to block rows of pixels of the time domain integrating sensor that are not illuminated by the cylindrical lens array.

11. The overlay metrology system of claim 1 , wherein the detector comprises: A static imaging detector is configured to generate the first image and the second image while the sample is static in the measurement field of the objective lens by the translation stage.

12. The overlay metrology system of claim 1 , wherein at least one of the first illumination lobe or the second illumination lobe illuminates: Incoherent illumination beam.

13. The overlay metrology system of claim 12, wherein the incoherent illumination beam comprises: Speckle-removing laser beam.

14. 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, wherein the illumination subsystem is further configured to sequentially illuminate an overlapping target on a sample using the first illumination configuration and the second illumination configuration, the first illumination configuration including one illumination lobe from each of the first pair of opposing illumination lobes and the second pair of opposing illumination lobes, the second illumination configuration including the remaining illumination lobe from each of the first pair of illumination lobes and the second pair of illumination lobes, wherein the overlapping target comprises: a first set of one or more cells comprising one or more grating-on-grating features formed from periodic structures on the first and second sample layers, wherein the one or more grating-on-grating structures are periodic along the first measurement direction; and a second set of one or more cells comprising one or more grating-on-grating features formed from periodic structures on the first sample layer and the second sample layer, wherein the one or more grating-on-grating structures are periodic along the second measurement direction; An imaging subsystem comprising: objective lens; and a detector configured to generate a first image and a second image of the overlaid target, wherein the first image is formed using a single non-zero diffraction order from each illumination lobe in the first illumination configuration, wherein the second image is formed using a single non-zero diffraction order from each illumination lobe in the second illumination configuration, wherein the detector comprises a scanning detector configured to generate the first image and the second image while the sample is translated through a measurement field of the objective lens by a translation stage; and A controller communicatively coupled to the detector, the controller comprising one or more processors configured to execute program instructions causing 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.

15. The overlay metrology system of claim 14, wherein 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 comprises: determining an overlay error between the first sample layer and the second sample layer of the sample along the first measurement direction based on a difference in intensities of the first group of one or more cells in the first image and the second image; and An overlay error between the first sample layer and the second sample layer of the sample along the second measurement direction is determined based on a difference in intensities of the second set of one or more cells in the first image and the second image.

16. The overlay metrology system of claim 14, wherein the illumination subsystem comprises 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 outside of a numerical aperture of the objective lens.

17. The overlay metrology system of claim 14, wherein the illumination subsystem directs the first and second illumination configurations to the sample through the objective lens, wherein the imaging subsystem further comprises one or more beam stops for blocking specular reflections associated with illumination lobes in the first and second illumination configurations.

18. The overlay metrology system of claim 17, wherein the one or more beam stops comprise: A dark field stop in the imaging subsystem.

19. The overlay metrology system of claim 17, further comprising: a darkfield mirror shared by the illumination subsystem and the imaging subsystem, wherein the darkfield mirror directs the first illumination configuration and the second illumination configuration to the overlay target through the objective lens, wherein the darkfield mirror operates as the one or more beam stops to block the specular reflections of the illumination lobes in the first illumination configuration and the second illumination configuration, wherein the darkfield mirror passes a single diffraction order associated with the illumination lobes in the first illumination configuration and the second illumination configuration.

20. The overlay metrology system of claim 19, wherein the dark field mirror comprises: Ring mirror.

21. The overlay metrology system of claim 14, wherein the detector comprises: Time domain integrating sensor.

22. The overlay metrology system of claim 21, further comprising: a cylindrical lens array configured to direct light emitted from the sample to each Nth pixel row of the time-domain integrating sensor, Wherein the overlay metrology system is configured to perform scanning mode measurement by: translating the sample at a charge transfer rate of the time-domain integrating sensor; illuminating the sample with the first illumination lobe and the second illumination lobe alternately, wherein a switching time between successive optical configurations of the imaging subsystem corresponds to the charge transfer rate of the time-domain integrating sensor; generating an interlaced image including the first image and the second image using the time-domain integration sensor during an exposure window; and The interlaced image is separated into the first image and the second image.

23. The overlay metrology system of claim 22, further comprising: A slit array is positioned to block rows of pixels of the time domain integrating sensor that are not illuminated by the cylindrical lens array.

24. The overlay metrology system of claim 14, wherein the detector comprises: A static imaging detector is configured to generate the first image and the second image while the sample is static in the measurement field of the objective lens by the translation stage.

25. The overlay metrology system of claim 14, wherein at least one of the first illumination lobe or the second illumination lobe illuminates: Incoherent illumination beam.

26. The overlay metrology system of claim 25, wherein the incoherent illumination beam comprises: Speckle-removing laser beam.

27. A method for measuring overlay, comprising: sequentially illuminating an overlying target on the sample with a first illumination lobe and a second illumination lobe, wherein the overlying target comprises one or more grating-on-grating features formed from periodic structures on the first sample layer and the second sample layer, wherein the one or more grating-on-grating structures are periodic along a measurement direction; generating a first image and a second image of the overlaid object using a detector, wherein the first image comprises an unresolved image of a single non-zero diffraction order of the one or more grating-on-grating structures formed from the first illumination lobe, the single non-zero diffraction order being from the one or more grating-on-grating structures along the measurement direction, wherein the second image comprises an unresolved image of the one or more grating-on-grating structures formed from a single non-zero diffraction order of the second illumination lobe, the single non-zero diffraction order being generated by the one or more grating-on-grating structures along the measurement direction, wherein the detector comprises a scanning detector configured to generate the first image and the second image while the sample is translated through a measurement field of an objective lens by a translation stage; and An overlay error between the first sample layer and the second sample layer of the sample along the measurement direction is determined based on the first image and the second image.

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