Superposition metrology of parallel scattering measurements

Abstract

An overlay metrology tool may include an illumination source for generating a first illumination beam profile having a first linear polarization and a second illumination beam profile having a second linear polarization orthogonal to the first linear polarization, an illumination subsystem for sequentially illuminating two or more unit pairs of an overlay target on a sample having an orthogonally oriented grating stack structure, a collection subsystem having two collection channels for capturing collected light from the illuminated unit pairs, and filtering optics for directing light from different units in the illuminated unit pairs to different collection channels for detection. The tool may further include a controller for generating separate overlay measurements of the orthogonally oriented grating stack structures in the two or more unit pairs.

Description

Technical Field

[0001] The present disclosure relates generally to overlay metrology and more particularly to scatterometry-based overlay metrology. Background Art

[0002] Overlay metrology generally refers to the measurement of the relative alignment of layers on a sample, such as (but not limited to) a semiconductor device. Overlay measurement or measurement of overlay error generally refers to the measurement of the misalignment of fabricated features on one or more sample layers. In a general sense, proper alignment of fabricated features on multiple sample layers is necessary for proper functioning of the device. The demand for decreasing feature size and increasing feature density has led to a correspondingly increased demand for accurate and efficient overlay metrology. Many existing overlay measurement techniques require separate measurement steps of multiple cells of an overlay target to produce overlay data for a particular direction and further require additional sets of measurement steps to produce overlay data for additional directions. However, the time required to perform such measurement steps may provide limitations on the processing capabilities of an overlay metrology system. Therefore, there is a need to develop systems and methods for overcoming these deficiencies. Summary of the invention

[0003] According to one or more illustrative embodiments of the present disclosure, a superposition metrology tool is disclosed. In one illustrative embodiment, the tool includes an illumination source for generating a first illumination beam distribution having a first linear polarization and a second illumination beam distribution having a second linear polarization orthogonal to the first linear polarization. In another illustrative embodiment, the tool includes an illumination subsystem for sequentially illuminating two or more unit pairs of a superposition target on a sample. A specific one of the two or more unit pairs may include a first direction unit having a grating stacking structure with a periodicity along a first direction and a second direction unit having a grating stacking structure with a periodicity along a second direction orthogonal to the first direction. In another illustrative embodiment, the illumination subsystem simultaneously illuminates the first direction unit using the first illumination beam distribution and illuminates the second direction unit using the second illumination beam distribution. In another illustrative embodiment, the tool includes a collection subsystem having: a first collection channel including one or more first channel detectors at a first channel detection plane; a second collection channel including one or more second channel detectors at a second channel detection plane; an objective lens for collecting light from the sample as collected light; and one or more filtering optical devices for directing a portion of the received light associated with a first directional unit of the two or more unit pairs to the first collection channel, and directing a portion of the received light associated with a second directional unit of the two or more unit pairs to the second collection channel. In another illustrative embodiment, the tool includes a controller to generate a first superposition measurement along the first direction based on data from the one or more first channel detectors associated with the first directional unit of the two or more unit pairs, and to generate a second superposition measurement along the second direction based on data from the one or more second channel detectors associated with the second directional unit of the two or more unit pairs.

[0004] According to one or more illustrative embodiments of the present disclosure, a superposition metrology tool is disclosed. In one illustrative embodiment, the tool includes an illumination source for generating a first illumination beam distribution having a first linear polarization and a second illumination beam distribution having a second linear polarization orthogonal to the first linear polarization. In another illustrative embodiment, the tool includes an illumination subsystem for sequentially illuminating two or more unit pairs of a superposition target on a sample. A specific one of the two or more unit pairs may include a first direction unit having a grating stacking structure with a periodicity along a first direction and a second direction unit having a grating stacking structure with a periodicity along a second direction orthogonal to the first direction. In another illustrative embodiment, the illumination system includes a first illumination channel to direct a first illumination beam distribution to the first direction unit of one of the two or more unit pairs, wherein the first illumination beam distribution includes one or more first illumination beams having a first linear polarization. In another illustrative embodiment, the illumination subsystem further includes a second illumination channel to direct a second illumination beam distribution to the second direction unit of one of the two or more unit pairs simultaneously with the first illumination beam distribution, wherein the second illumination beam distribution includes one or more second illumination beams having a first linear polarization. In another illustrative embodiment, the tool includes a collection subsystem having: a first collection channel including one or more first channel detectors at a first channel detection plane; a second collection channel including one or more second channel detectors at a second channel detection plane; an objective lens for collecting light from the sample as collected light; and a polarization beam splitter for directing a portion of the received light associated with a first direction element having the first linear polarization to the first collection channel, and directing a portion of the received light associated with a second direction element of the two or more element pairs having the second linear polarization to the second collection channel. In another illustrative embodiment, the tool includes a controller to generate a first superposition measurement along the first direction based on data from the one or more first channel detectors associated with the first direction element of the two or more element pairs, and to generate a second superposition measurement along the second direction based on data from the one or more second channel detectors associated with the second direction element of the two or more element pairs.

[0005] According to one or more illustrative embodiments of the present disclosure, a superposition metrology method is disclosed. In one illustrative embodiment, the method includes generating a first illumination beam distribution having a first linear polarization and a second illumination beam distribution having a second linear polarization orthogonal to the first linear polarization. In another illustrative embodiment, the method includes sequentially illuminating two or more unit pairs of a superposition target on a sample, wherein each of the two or more unit pairs includes a first direction unit having a grating stacking structure with periodicity along a first direction and a second direction unit having a grating stacking structure with periodicity along a second direction orthogonal to the first direction, and wherein the illumination subsystem simultaneously illuminates the first direction unit using the first illumination beam distribution and illuminates the second direction unit using the second illumination beam distribution. In another illustrative embodiment, the method includes collecting light from the sample as collected light. In another illustrative embodiment, the method includes directing a portion of the collected light associated with the first direction unit of the two or more unit pairs to a first light collection channel. In another illustrative embodiment, the method includes directing a portion of the collected light associated with the second direction unit of the two or more unit pairs to a second light collection channel. In another illustrative embodiment, the method includes generating a first superposition measurement along the first direction based on data associated with the first direction cells of the two or more cell pairs. In another illustrative embodiment, the method includes generating a second superposition measurement along the second direction based on data associated with the second direction cells of the two or more cell pairs.

[0006] It should be understood that both the foregoing summary 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 summary, serve to explain the principles of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Those skilled in the art may better appreciate the numerous advantages of the present disclosure by referring to the accompanying drawings.

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

[0009] Figure 1B is a schematic diagram illustrating an overlay metrology tool for two illumination channels and two light collection channels according to one or more embodiments of the present disclosure.

[0010] Figure 1C is a schematic diagram illustrating a portion of an overlay metrology tool having two illumination channels with separately configurable illumination conditions based on linearly varying filters in accordance with one or more embodiments of the present disclosure.

[0011] Figure 2A is a side view of a grating stack structure in a single unit of a stacking target according to one or more embodiments of the present disclosure.

[0012] Figure 2B is a top view of a superimposed target according to one or more embodiments of the present disclosure.

[0013] Figure 3A The description of one or more embodiments of the present disclosure is from Figure 2B Top view of the pupil plane showing the distribution of the diffraction orders of the X-direction units of the superimposed target.

[0014] Figure 3B The description of one or more embodiments of the present disclosure is from Figure 2B FIG. 1 is a top view of the pupil plane showing the distribution of the diffraction orders of the Y-direction units of the superimposed target 108. FIG.

[0015] Figure 4 is a flow chart illustrating steps performed in a method for overlay metrology according to one or more embodiments of the present disclosure. DETAILED DESCRIPTION

[0016] Reference will now be made in detail to the disclosed subject matter illustrated in the accompanying drawings. The present disclosure has been particularly shown and described with respect to certain embodiments and specific features thereof. The embodiments set forth herein are to be considered illustrative rather than restrictive. It will be readily appreciated by those skilled in the art that various changes and modifications in form and details may be made without departing from the spirit and scope of the present disclosure.

[0017] Embodiments of the present disclosure relate to systems and methods for producing parallel measurements of cells of a stacked target using scatterometry techniques. In some embodiments, two cells of a stacked target associated with stacked measurements along two different (e.g., orthogonal) directions are illuminated simultaneously, light from the two illuminated cells is collected simultaneously, and the collected light associated with the illuminated cells is directed to separate detection paths. In this way, measurements of multiple cells of a stacked target can be performed in parallel, which can beneficially increase measurement processing capabilities for sequential measurements of each cell on the target. Additional embodiments relate to multi-channel scatterometry stacking tools suitable for simultaneously illuminating multiple cells of a stacked target and separating light from the multiple cells into separate detection channels.

[0018] For the purposes of this disclosure, the term overlay is generally used to describe the relative position of features on a sample manufactured by two or more lithographic patterning steps, where the term overlay error describes the deviation of features from the nominal arrangement. For example, a multilayer device may include features patterned on multiple sample layers using different lithographic steps for each layer, where the alignment of features between layers must generally be tightly controlled to ensure proper performance of the resulting device. Therefore, overlay measurements can characterize the relative position of features on two or more sample layers. By way of another example, multiple lithographic steps can be used to manufacture features on a single sample layer. Such techniques (commonly referred to as double patterning or multi-patterning techniques) can facilitate the manufacture of highly dense features approaching the resolution of the lithographic system. In this context, overlay measurements can characterize the relative position of features from different lithographic steps on this single layer. It should be understood that the examples and illustrations related to specific applications of overlay metrology throughout this disclosure are provided for illustrative purposes only and should not be construed as limiting the present disclosure.

[0019] While in some applications, overlay measurements can be performed directly on features of a fabricated device (e.g., device features), overlay measurements are typically performed on dedicated overlay targets that are printed using the same lithography steps as the device features. In this way, features of the overlay target (e.g., target features) can be specifically designed to facilitate overlay measurements. Furthermore, overlays measured at one fabrication step (e.g., after fabrication of one or more sample layers) can be used to generate correctable terms for accurately aligning process tools (e.g., lithography tools or the like) used to fabricate additional sample layers in subsequent fabrication steps.

[0020] In some embodiments, a superposition target suitable for scatterometry metrology as disclosed herein may include one or more cells having a grating superposition structure, wherein the grating superposition structure includes periodic features (e.g., grating features) over overlapping regions of two or more layers of interest. In this manner, various grating features on the layers of interest may contribute to diffraction of incident illumination and superposition measurements may be generated based on analysis of the diffracted light. For example, superposition measurements may be generated based on pupil plane data (e.g., associated with relative intensity differences between selected diffraction orders in the pupil plane). By way of another example, superposition measurements may be generated based on field plane data (e.g., associated with relative intensities of images of cells of the target generated using selected diffraction orders).

[0021] As used throughout this disclosure, the term "sample" generally refers to a substrate (e.g., a wafer or the like) formed of a semiconductor or non-semiconductor material. For example, the semiconductor or non-semiconductor material may include, but is not limited to, single crystal silicon, gallium arsenide, and indium phosphide. The sample may include one or more layers. For example, such layers may include, but are not limited to, photoresists, dielectric materials, conductive materials, and semiconductive materials. Many different types of such layers are known in the art, and the term sample as used herein is intended to cover samples on which all types of such layers may be formed. One or more layers formed on the sample may be patterned or unpatterned. For example, the sample may include multiple dies each having repeatable patterned features. The formation and processing of such material layers may ultimately produce a finished device. Many different types of devices may be formed on the sample, and the term sample as used herein is intended to cover samples on which any type of device known in the art is manufactured. In addition, for the purposes of this disclosure, the terms sample and wafer should be interpreted as interchangeable. In addition, for the purposes of this disclosure, the terms patterned device, mask, and photomask should be interpreted as interchangeable.

[0022] In some embodiments, the overlay metrology tool simultaneously illuminates two cells of the overlay target using two illumination beams, wherein the two cells include a grating stack structure in which the orientation of the grating in one cell is orthogonal to the orientation of the grating in the other cell. For example, the overlay metrology tool may simultaneously illuminate an X-direction cell including a grating stack structure having periodicity along the X direction using a first illumination beam and illuminate a Y-direction cell including a grating stack structure having periodicity along the Y direction using a second illumination beam. It should be understood that the description of the X and Y directions is used herein to refer to any orthogonal directions on the sample for illustrative purposes only. In addition, the two cells may be illuminated using light having orthogonal linear polarizations (e.g., one cell is illuminated using linear polarization light along the X direction and one cell is illuminated using linear polarization light along the Y direction). In particular, the overlay metrology tool may illuminate each of the two cells using a distribution of one or more illumination beams (e.g., an illumination beam distribution), wherein the illumination beam distribution on each cell is smaller than the cell such that the cell is not filled. Additionally, various aspects of the illumination light on each unit may be individually controlled or adjusted, such as, but not limited to, spectrum (eg, spectral bandwidth and / or central wavelength), intensity, illumination angle, angular distribution of illumination, or focal position.

[0023] In some embodiments, the superposition metrology tool simultaneously separates the collected light from the illuminated units into separate detection channels for each unit. In this way, the measurement data associated with the illuminated units can be isolated from each other, although collected simultaneously. Various techniques for separating the collected light into separate detection channels can be used within the spirit and scope of the present disclosure. For example, the collected light from the illuminated units can be separated based on parameters such as (but not limited to) polarization, a pupil stop filter (e.g., a spatial filter in the pupil plane for passing selected diffraction orders), or a field stop filter (e.g., a spatial filter in the field plane for passing light from selected units).

[0024] In some embodiments, the overlay metrology tool includes beam control optics in the illumination and / or collection paths according to various aspects of the illumination light or collected light. For example, the overlay metrology tool may include one or more scanning mirrors to scan the illumination beam across the respective cells during measurement, which may reduce noise associated with target defects. By way of another example, the overlay metrology tool may include beam control optics to control or adjust the separation between two illumination beams, which may be used to match the illumination beam separation to the layout of cells in a particular overlay target. Furthermore, the overlay metrology tool may adjust the illumination and / or collection field stops based on the selected illumination beam separation.

[0025] Additional embodiments of the present disclosure relate to a stacking target suitable for parallel measurement by a stacking metrology tool. In some embodiments, the stacking target includes X-direction cells distributed along a first row and Y-direction cells distributed along a second row. In this way, cells associated with a particular direction can be sequentially illuminated with the same or similar illumination conditions (e.g., polarization, spectrum, intensity, angular distribution of illumination, focal position of the sample during illumination, or the like) simply by scanning the stacking target along the columns. For example, some stacking measurement techniques may require data capture from multiple cells with different expected offsets of the grating stacking structure. Therefore, arranging cells with a common direction with periodicity along the columns can reduce the burden on the stacking metrology tool so that the illumination conditions can be the same or substantially the same as scanning the target along the column direction.

[0026] Reference Figures 1A to 4 , systems and methods for producing parallel measurements of cells of a superimposed target using scatterometry techniques.

[0027] Figure 1A is a block diagram of an overlay metrology system 100 in accordance with one or more embodiments of the present disclosure.

[0028] In some embodiments, the overlay metrology system 100 includes an overlay metrology tool 102 having a dual-channel illumination subsystem 104 for separately illuminating two cells 106 of an overlay target 108 on a sample 110 using spatially separated distributions (e.g., illumination beam distributions) of one or more illumination light beams 112 having orthogonal linear polarizations, and a dual-channel light collection subsystem 114 for separately detecting light or other radiation (e.g., collected light 116) emitted from the sample 110 and associated with the two cells 106.

[0029] In some embodiments, the sample 110 is disposed on a sample stage 118 adapted to secure the sample 110 and further configured to position the sample 110 relative to the overlay metrology tool 102. For example, the sample stage 118 may include any combination of linear, rotational, or angular (e.g., flip / tilt) actuators adapted to position the sample 110 in any selected orientation.

[0030] The overlay metrology tool 102 may be any type of overlay metrology tool known in the art suitable for generating an overlay signal suitable for determining an overlay target associated with the sample 110. For example, the overlay metrology tool 102 may collect pupil plane data (e.g., one or more pupil plane images or portions thereof), wherein the collected light 116 is analyzed at the pupil plane to characterize the angular distribution of radiation from the cells 106 (e.g., associated with scattering and / or diffraction of the radiation by the cells 106). By way of another example, the overlay metrology tool 102 may collect field plane data (e.g., one or more field plane images or portions thereof) based on selected diffraction orders from the cells 106. Additionally, the overlay metrology tool 102 may characterize the cells 106 when the sample 110 is static (e.g., in static or move and measure (MAM) modes) or when the sample 110 is in motion (e.g., in scanning mode).

[0031] Reference Figure 2A to 2B , various configurations of overlay targets 108 suitable for parallel characterization of component units 106 are described in more detail according to one or more embodiments of the present disclosure.

[0032] Figure 2A1 is a side view of a grating stack structure in a single cell 106 of a stack target 108 according to one or more embodiments of the present disclosure. In some embodiments, the grating stack structure in each cell 106 includes a first layer of grating features 202 positioned on a first layer 204 of a sample 110 and a second layer of grating features 206 positioned on a second layer 208 of the sample 110, which are oriented such that the regions including the first layer of grating features 202 and the second layer of grating features 206 overlap. In this manner, the first layer of grating features 202 and the second layer of grating features 206 can each diffract an incident illumination beam 112 into discrete diffraction orders. In general, any number of additional layers (not shown) can be positioned above, below, or between the first layer 204, the second layer 208, and the substrate 210.

[0033] In a general sense, the first layer of grating features 202 and the second layer of grating features 206 may have the same or different distributions and may be intentionally offset from each other by any selected desired offset 212. In some embodiments, the first layer of grating features 202 and the second layer of grating features 206 have the same pitch along the same direction so that the associated diffraction orders overlap in the pupil plane. In some embodiments, the first layer of grating features 202 and the second layer of grating features 206 have different pitches along a particular direction, which may result in moiré fringes associated with a Moiré pitch that is greater than the pitch of the first layer of grating features 202 and the second layer of grating features 206. The superposition metering technology utilizing the molar effect is generally described in the following patents: U.S. Patent 9,182,219 issued on November 10, 2015; U.S. Patent 7,440,105 issued on October 21, 2008; U.S. Patent 7,349,105 issued on March 25, 2008; U.S. Patent 10,551,749 issued on February 4, 2020; U.S. Patent Publication 2021 / 0072650 published on March 11, 2021; U.S. Patent Application 16 / 935,117 filed on July 21, 2020; and U.S. Patent Application 16 / 931,078 filed on July 16, 2020, the entire contents of all of the said patents are incorporated herein by reference.

[0034] Figure 2B 2 is a top view of a stacking target 108 according to one or more embodiments of the present disclosure. In some embodiments, the stacking target includes two or more cells 106 associated with each measurement direction of interest. For example, different cells 106 associated with a particular measurement direction may include grating stacks with different expected offsets 212.

[0035] Figure 2BA stacking target 108 is illustrated having two units 106 (e.g., X-direction units 106) with periodicity along the X direction for stacking measurements along the X direction and two units 106 (e.g., Y-direction units 106) with periodicity along the Y direction for stacking measurements along the Y direction. The various units 106 may generally be arranged in any suitable distribution. In some embodiments, as Figure 2B As shown in FIG. 1 , the units 106 having the same periodic direction are arranged in columns. For example, Figure 2B Illustrated are X-direction cells 106 along a first row 214 and Y-direction cells along a second row 216. As will be described in more detail below, this configuration can facilitate efficient sequential illumination of cells having the same periodic direction under the same or similar illumination conditions.

[0036] However, it should be understood that Figure 2A to 2B The overlay target 108 and associated description in are provided for illustrative purposes only and should not be construed as limiting. In practice, the overlay target 108 may include any suitable grating overlay overlay target design. For example, the overlay target 108 may generally include any number of cells 106 suitable for measuring along two directions. In addition, the cells 106 may be distributed in any pattern or arrangement. For example, metrology target designs suitable for scanning metrology are generally described in U.S. Patent Application No. 16 / 598,146 filed on October 10, 2019, the entire contents of which are incorporated herein by reference.

[0037] Now generally refer to Figures 1A to 3B , the parallel characterization of multiple cells 106 of the superposition target 108 is described in more detail according to one or more embodiments of the present disclosure.

[0038] Figure 1B 1 is a schematic diagram of an overlay metrology tool 102 illustrating two illumination channels 120 and two light collection channels 122 according to one or more embodiments of the present disclosure. In this manner, the overlay metrology tool 102 can simultaneously characterize an overlay target 108 (for example, but not limited to, Figure 2A to 2B In some embodiments, the overlay metrology tool 102 simultaneously characterizes two cells 106 having orthogonal periodic directions (eg, one X-direction cell 106 and one Y-direction cell 106).

[0039] In some embodiments, the overlay metrology tool 102 includes at least one illumination source 124 configured to produce illumination 126 suitable for forming an illumination beam distribution to be directed to the cell 106 .

[0040] Illumination 126 from illumination source 124 may include one or more selected wavelengths of light including, but not limited to, ultraviolet (UV) radiation, visible radiation, or infrared (IR) radiation.

[0041] The illumination source 124 may include any type of illumination source known in the art. In some embodiments, the illumination source 124 is a laser source. For example, the illumination source 124 may include, but is not limited to, one or more narrowband laser sources, broadband laser sources, supercontinuum laser sources, white light laser sources, or the like. In this regard, the illumination source 124 may provide illumination with high coherence (e.g., high spatial coherence and / or temporal coherence). In some embodiments, the illumination source 124 includes a laser sustained plasma (LSP) source. For example, the illumination source 124 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 some embodiments, the illumination source 124 includes a lamp source. For example, the illumination source 124 may include, but is not limited to, an arc lamp, a discharge lamp, an electrodeless lamp, or the like. In this regard, the illumination source 124 may provide illumination with low coherence (e.g., low spatial coherence and / or temporal coherence).

[0042] Each illumination channel 120 may direct a distribution of one or more illumination beams 112 (e.g., an illumination beam distribution) to a specific location on the sample 110. The illumination beam distribution may generally include one or more illumination beams 112 or illumination lobes having a selected distribution of illumination parameters, such as, but not limited to, angles of incidence (e.g., azimuthal and polar angles of incidence), spectrum, or polarization, directed to a specific cell 106. For example, the illumination beam distribution may include, but not limited to, a single illumination beam 112 at a selected angle of incidence, a dipole distribution of illumination beams 112, or a quadrupole distribution of illumination beams 112. In this manner, the illumination channels 120 may simultaneously provide different illumination conditions for different cells 106 on the overlay target 108. Furthermore, in instances where the illumination beam distribution includes multiple illumination beams 112, these illumination beams 112 may be provided simultaneously or sequentially in a given measurement of a cell 106.

[0043] In some embodiments, the illumination channels 120 provide illumination beams 112 having orthogonal linear polarizations. For example, one illumination channel 120 may provide a distribution of one or more illumination beams 112 having a linear polarization along a first direction (e.g., the X direction) and another illumination channel 120 may provide a distribution of one or more illumination beams 112 having a linear polarization along a second direction (e.g., the Y direction) orthogonal to the first direction. It is contemplated herein that providing orthogonal linear polarizations in the illumination channels 120 may have various benefits, such as, but not limited to, efficiently utilizing available power from the illumination source 124, providing well-controlled diffraction through the grating stack structure in each unit 106, and / or facilitating separation of the collected light 146 through the illumination unit 106 into different light collection channels 122 having high extinction ratios to prevent cross-contamination of signals.

[0044] Furthermore, any selected polarization direction may be used to illuminate the various elements 106 of the overlay target 108. For example, the overlay metrology tool 102 may illuminate the X-direction elements 106 using linear polarized light along the X direction and illuminate the Y-direction elements 106 using linear polarized light along the Y direction or vice versa.

[0045] The overlay metrology tool 102 may generally include any combination of optical components for simultaneously generating multiple illumination beam distributions.

[0046] In some embodiments, the overlay metrology tool 102 may include a single illumination source 124 for generating illumination 126 for each illumination channel 120. For example, the overlay metrology tool 102 may include a polarization beam splitter for separating the illumination 126 into orthogonal linear polarizations directed to different illumination channels 120. In some embodiments, the overlay metrology tool 102 includes separate illumination sources 124 for one or more illumination channels 120.

[0047] The overlay metrology tool 102 may further include various optical components for generating a selected illumination beam distribution for each illumination channel 120. In some embodiments, Figure 1B, each illumination channel 120 may include one or more illumination lenses 128 or one or more illumination control optics 130 to control various illumination parameters. For example, the illumination control optics 130 may include, but are not limited to, one or more field stops or filters, one or more pupil stops or filters, one or more polarizers, one or more spectral filters, one or more spatial filters, one or more beam splitters, one or more diffusers, one or more homogenizers, one or more apodizers, one or more beam shapers, or one or more mirrors (e.g., static mirrors, translatable mirrors, scanning mirrors, or the like). As an illustration, each illumination channel 120 may include an aperture stop for generating a distribution of one or more illumination beams 112 and / or a field stop for limiting the spatial extent of the illumination beams 112 in the channel to an area equal to or smaller than the cell 106 so that the cell is not filled with illumination.

[0048] In a general sense, each illumination channel 120 can provide independent control of illumination parameters of one or more illumination beams 112. Figure 1B 1, but the overlay metrology tool 102 may include an illumination lens 128 and / or illumination control optics 130 to manipulate the illumination 126 before the illumination 126 enters the illumination channel 120. In this way, selected aspects of the illumination beam distribution may be consistent between the illumination channels 120.

[0049] In addition, the illumination parameters in each illumination channel 120 can be static or tunable. The tunable illumination parameters can be provided using any technique known in the art. The generation of tunable illumination parameters is generally described in U.S. Patent No. 10,371,626, issued on August 6, 2019, and U.S. Patent Application No. 17,076,312, filed on October 21, 2020, the entire contents of which are incorporated herein by reference.

[0050] In some embodiments, tunable illumination parameters are provided by selectively directing illumination 126 to one of a plurality of available optical paths having one or more static filters (e.g., spectral filters, neutral density filters, or the like). In some embodiments, tunable illumination parameters are provided by adjusting the position of one or more tunable filters (e.g., spectral filters, neutral density filters, or the like) in illumination channel 120. In some embodiments, tunable illumination parameters are provided by selectively controlling the position of illumination 126 from illumination source 124 on one or more spatially varying filters (e.g., spectral filters, neutral density filters, or the like).

[0051] Figure 1C1 is a schematic diagram of a portion of a superimposed metrology tool 102 illustrating two illumination channels 120 having separately configurable illumination conditions based on linearly varying filters, according to one or more embodiments of the present disclosure. The linearly varying filters may include filters having filtering properties that vary along a linear filtering direction. For example, a linearly varying neutral density filter may provide different amounts of broadband intensity reduction based on the spatial position of the input beam along a linear axis. By another example, a linearly varying low pass (or high pass) filter may provide low pass filtering with a cutoff wavelength that varies based on the spatial position of the input beam along the linear filtering direction. By another example, the linearly varying filter may be formed as a polarizer, wherein the direction of polarization delivered by the linearly varying filter may be different in different directions along the linear filtering direction. It is contemplated herein that the systems and methods disclosed herein may utilize linearly varying filters that modify any selected property of the input beam.

[0052] In some embodiments, the overlay metrology tool 102 includes an illumination source 124 having a spectral bandwidth that overlays the wavelength range of interest and a polarizer 132 for splitting illumination 126 from the illumination source 124 along two paths associated with two illumination channels 120. For example, the polarizer 132 can be a polarization beam splitter for providing orthogonal linear polarizations in the two illumination channels 120.

[0053] Each illumination channel 120 may include at least one tunable filter 134 that provides tunable control of one or more illumination parameters. In some embodiments, the tunable filter 134 includes a pair of focusing optics 136 in a 4-f configuration, a linearly varying filter 138 positioned at a pupil plane (e.g., a focal plane common to the focusing optics 136), and an angle scanner 140 positioned at other focal planes of the focusing optics 136 as an input and / or output coupler to the tunable filter 134. In this manner, the position of the illumination 126 on the linearly varying filter 138, and thus, the resulting filtering effect on the illumination 126, may be controlled by adjusting the angle of the input angle scanner 140 for receiving the illumination 126. Furthermore, regardless of the angle of the input angle scanner 140 and the associated position on the linearly varying filter 138, the illumination 126 will be redirected to a common position on the output tunable filter 134, which can direct the filtered illumination 126 along any desired optical path. For example, the input and output angle scanners 140 may be adjusted simultaneously to provide tunable filtering without modifying the output beam path and thus without affecting the alignment of additional optics of the overlay metrology system 100 .

[0054] Furthermore, multiple tunable filters 134 may be arranged in series to provide tuning of multiple lighting parameters. Figure 1CThree tunable filters 134 are illustrated in each illumination channel 120. This configuration may be suitable for, but is not limited to, adjusting the intensity and spectrum of the illumination 126 within each illumination channel 120. For example, one linearly variable filter 138 may be a linearly variable intensity filter, one linearly variable filter 138 may be a linearly variable low pass spectral filter, and one linearly variable filter 138 may be a linearly variable high pass spectral filter.

[0055] In some embodiments, the overlay metrology tool 102 includes one or more focus control optics 142 for adjusting or controlling the focus position of one or more illumination beams 112 in one or more illumination channels 120. It is contemplated herein that focus position adjustment may be particularly useful for, but not limited to, measurements using different wavelengths or measurements of features at different depths. For example, the illumination beams 112 in different illumination channels 120 may have different spectral ranges. By way of another example, measurement robustness may be improved by capturing data from each unit 106 at multiple different wavelengths. In either case, chromatic aberrations in the sample 110 and / or the overlay metrology system 100 may result in different focusing or imaging conditions at different wavelengths, such that focus correction in one or more illumination channels 120 may be required.

[0056] As a graphic illustration, Figure 1B Focus control optics 142 are included in each illumination channel 120 to provide independent focus control for the associated illumination beam 112. The focus control optics 142 may include any type or combination of optical elements suitable for modifying the focus position of at least one illumination beam 112, such as, but not limited to, one or more deformable mirrors, one or more acousto-optic lenses, one or more voice coils, or one or more translatable lenses (e.g., any illumination lens 128). In some embodiments, the focus control optics 142 may be designed to provide focus switching within the time required to tune one or more additional illumination parameters (e.g., spectrum, intensity, angular distribution of illumination, or the like) to facilitate high measurement throughput. In this way, the focus control optics 142 may operate as a fast focus controller.

[0057] Reference again Figures 1A to 1C , the overlay metrology tool 102 may generate or otherwise control the illumination beam distribution (eg, the number and angles of incidence of the constituent illumination beams 112) using any technique known in the art.

[0058] In some embodiments, the overlay metrology system 100 includes one or more apertures (e.g., illumination control optics 130) at an illumination pupil plane to define one or more illumination beams 112. In some embodiments, the overlay metrology system 100 generates the illumination beams 112 by providing light in two or more optical fibers, wherein light output from each optical fiber is provided at or directed to the illumination pupil to provide the illumination beams 112. In some embodiments, the overlay metrology system 100 generates the one or more illumination beams 112 by diffracting illumination 126 from an illumination source 124 into two or more diffraction orders, wherein at least one diffraction order forms at least one illumination beam 112. Efficient generation of multiple illumination beams by controlled diffraction is generally described in U.S. Patent Publication US2020 / 0124408, published on April 23, 2020, which is incorporated herein by reference in its entirety.

[0059] In some embodiments, the overlay metrology tool 102 includes an objective lens 144 for capturing light or other radiation (e.g., collected light 146) emitted from the two illumination units 106. In addition, the illumination beam 112 from any illumination channel 120 can generally be directed to the sample 110 through the objective lens 144 (e.g., in a through-the-lens (TTL) configuration) or outside the numerical aperture of the objective lens 144 (e.g., in an out-of-the-lens (OTL) configuration). For example, Figure 1B Describes TTL configuration.

[0060] In some embodiments, the overlay metrology tool 102 includes one or more filtering optics for separating the collected light 146 from the two illuminated units 106 into different collection channels 122. For example, the filtering optics may selectively direct a portion of the collected light 116 associated with the X-direction unit 106 to one collection channel 122 and direct the collected light 116 associated with the Y-direction unit 106 to another collection channel 122.

[0061] The one or more filtering optics may separate the collected light 116 associated with the two illuminated units 106 using any technique or combination of techniques including, but not limited to, polarization filtering, pupil plane filtering, or field plane filtering.

[0062] In some embodiments, the filtering optics include one or more polarization filters for implementing polarization filtering, such as, but not limited to, one or more linear polarizers or one or more polarization beam splitters. It is contemplated herein that polarization filtering can be particularly effective when the illumination channels 120 provide illumination beam profiles having orthogonal linear polarizations, because the collected light 116 can generally maintain the polarization direction of the illumination.

[0063] For example, Figure 1B, the overlay metrology tool 102 includes a polarization beam splitter 148 in a location common to the illumination 126 and the collected light 116. In this configuration, the polarization beam splitter 148 can receive the orthogonal linearly polarized illumination 126 from each illumination channel 120 and receive this light to the objective lens 144. This polarization beam splitter 148 can then receive the collected light 116 from the two illumination units 106 and split this collected light 116 into two light collection channels 122 based on polarization. Figure 1B Further illustrated are two non-polarizing beam splitters 150, one associated with each polarization direction to separate the illumination 126 and the collected light 116 for that polarization direction.

[0064] By way of another example, although not explicitly shown, the overlay metrology tool 102 may include one or more linear polarizers in one or both of the light collection channels 122. For example, one light collection channel 122 may include a linear polarizer aligned along the X direction and one light collection channel 122 may include a linear polarizer aligned along the Y direction. In this configuration, the linear polarizers may implement polarization filtering, which may be achieved even if the illumination beam profiles do not have orthogonal linear polarizations. Furthermore, the polarizers in the light collection channels 122 may provide an extinction ratio that is higher than that provided by the polarization beam splitter 148 and thus further reduce crosstalk between the collected light 116 associated with different cells 106.

[0065] In some embodiments, the filtering optics include one or more spatial filters in one or more pupil planes (e.g., pupil plane filters). It is contemplated herein that the grating stack structure will produce diffraction orders distributed in periodic directions. Therefore, the diffraction orders from the elements 106 having orthogonal periodic directions will be distributed in orthogonal directions in the pupil plane and, therefore, can be filtered using pupil plane filters.

[0066] Figures 3A to 3B Describe one or more embodiments of the present disclosure from Figure 2B The spatial distribution of the diffraction orders of the superimposed target 108 by the illumination unit 106. Figure 3A The description of one or more embodiments of the present disclosure is from Figure 2B 108 of the superimposed target 108 of the X-direction unit 106 of the pupil plane distribution of the top view. Specifically, Figure 3A Illustrated are 0th order diffraction 302 (eg, specular reflection), X-direction -1st order diffraction 304, and X-direction +1st order diffraction 306 within pupil boundary 308 in response to a single illumination beam 112 at a normal angle of incidence. Figure 3B The description of one or more embodiments of the present disclosure is from Figure 2B108 of the superimposed target 108 of the Y direction unit 106 of the top view of the pupil plane distribution. Specifically, Figure 3B 0th order diffraction 302, Y-direction -1st order diffraction 310, and Y-direction +1st order diffraction 312 within pupil boundary 308 in response to a single illumination beam 112 at normal incidence are illustrated. Figure 3A and 3B , the non-zero diffraction orders from the X-direction unit 106 and the Y-direction unit 106 do not overlap in the pupil plane and can be isolated using a pupil plane filter. For example, one light collection channel 122 may include a pupil plane filter having an aperture oriented to pass at least the X-direction -1 diffraction order 304 and the X-direction +1 diffraction order 306 to isolate the collected light 116 from the illuminated X-direction unit 106, and one light collection channel 122 may include a pupil plane filter having an aperture oriented to pass at least the Y-direction -1 diffraction order 310 and the Y-direction +1 diffraction order 312 to isolate the collected light 116 from the illuminated Y-direction unit 106.

[0067] In some embodiments, the filtering optics include one or more spatial filters (e.g., field plane filters) in one or more field planes. Figure 1B A field stop 152 is illustrated in each collection channel 122. In particular, a field stop 152 in a collection channel 122 may limit the collection area to a particular cell 106 in that channel.

[0068] In some embodiments, each light collection channel 122 includes one or more detectors 154 configured to capture the collected light 116 in the channel. Each light collection channel 122 may further include one or more optical elements suitable for modifying and / or conditioning the collected light 146 from the sample 110. In some embodiments, the light collection channel 122 includes one or more light collection path lenses 156 (e.g., for collimating illumination, for relaying a pupil and / or field plane, or the like), which may include (but need not include) an objective lens 144. In some embodiments, the light collection channel 122 includes one or more light collection path optics 158 for shaping or otherwise controlling the collected light 146. For example, the collection path optical device 158 may include (but is not limited to) one or more field apertures, one or more aperture stops, one or more polarizers, one or more spectral filters, one or more intensity filters, one or more beam splitters, one or more diffusers, one or more homogenizers, one or more apodizers, one or more beam shapers, or one or more mirrors (e.g., static mirrors, translatable mirrors, scanning mirrors, or the like).

[0069] The detector 154 may be positioned at any selected location within the light collection channel 122. In some embodiments, the overlay metrology tool 102 includes a detector 154 at a pupil plane (e.g., a diffraction plane) to generate a pupil image. In this regard, the pupil image may correspond to the angular distribution of light from the sample 110 on the detector 154. For example, diffraction orders associated with diffraction from the grating stack structure in the unit 106 may be imaged or otherwise observed in the pupil plane. In a general sense, the detector 154 may capture any combination of reflected (or transmitted), scattered, or diffracted light from the sample 110. In some embodiments, the overlay metrology tool 102 includes a detector 154 at a field plane (e.g., a plane conjugate to the sample 110) to generate an image of the sample 110 based on selected diffraction orders from the unit 106.

[0070] The overlay metrology tool 102 may generally include any number or type of detectors 154 suitable for capturing light indicative of the overlay from the sample 110. In some embodiments, the detectors 154 include one or more detectors 154 suitable for characterizing a static sample. In this regard, the overlay metrology tool 102 may operate in a static mode in which the sample 110 is static during measurement. For example, the detectors 154 may include a two-dimensional array of pixels, such as, but not limited to, a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) device. In this regard, the detectors 154 may produce a two-dimensional image (e.g., a field plane image or a pupil plane image) in a single measurement.

[0071] In some embodiments, the detector 154 includes one or more detectors 154 suitable for characterizing a moving sample (e.g., a scanned sample). In this regard, the overlay metrology tool 102 may be operated in a scanning mode in which the sample 110 is scanned relative to the measurement field during measurement. For example, the detector 154 may include a 2D pixel array having a capture time and / or refresh rate sufficient to capture one or more images within a selected image tolerance (e.g., image blur, contrast, sharpness, or the like) during scanning. By another example, the detector 154 may include a line scan detector for continuously generating images one row of pixels at a time. By another example, the detector 154 may include a time delay integration (TDI) detector. The TDI detector may generate continuous images of the sample 110 when the motion of the sample 110 is synchronized with the charge transfer frequency signal in the TDI detector.

[0072] In some embodiments, the overlay metrology system 100 includes one or more beam scanning optics 160 for controlling the position of one or more illumination beams 112 (or illumination beam distributions) on the sample 110. As an illustration, Figure 1B Included in each illumination channel 120 is a beam scanning optic 160 .

[0073] It is contemplated herein that the beam scanning optics 160 may be utilized in a variety of ways in accordance with one or more embodiments of the present disclosure.

[0074] In some embodiments, beam scanning optics 160 positioned in two illumination channels 120 can synchronize pattern scanning to scan respective illumination profiles across the cell 106. This synchronized scanning can beneficially mitigate noise associated with imperfections in the grating stack structure and / or mitigate the effects of speckle associated with the coherent illumination 126. However, it is contemplated herein that other techniques may additionally or alternatively be used to mitigate such speckle. For example, the coherent illumination 126 may scan across the input face of a multimode fiber, where the output of the multimode fiber is imaged onto the sample 110. This multimode fiber may be positioned in front of or within any illumination channel 120.

[0075] In some embodiments, beam scanning optics 160 in one or more illumination channels 120 are used to adjust or otherwise control the separation of the associated illumination beam profiles. In this way, the illumination beam profiles from each illumination channel 120 can be centered on different cells 106. Furthermore, this configuration can flexibly provide measurements of targets with any selected cell separation. For example, in Figure 1B , the beam scanning optics 160 may be tilted in the plane of illustration to deviate the associated illumination beam distribution on the sample 110 in the plane of illustration.

[0076] As further contemplated herein, it may be desirable to ensure that the illumination 126 and / or the collected light 116 are centered within any field stop to avoid asymmetric diffraction during either illumination or collection. In some embodiments, the superposition metrology system 100 includes one or more adjustable field stops (e.g., field stops mounted to an actuator or the like) that may (but need not) be synchronized with other components. For example, Figure 1B Adjustable field apertures 152 are illustrated in both illumination channel 120 and collection channel 122, optionally synchronized with beam scanning optics 160. In this way, illumination 126 and collected light 116 can remain centered in respective apertures even as the position of the illumination beam profile on sample 110 is adjusted.

[0077] Reference again Figure 1A , various additional components of the overlay metrology system 100 are described in greater detail according to one or more embodiments of the present disclosure.

[0078] In some embodiments, the superposition metrology system 100 includes a controller 162 that is communicatively coupled to the superposition metrology tool 102 and / or any components therein. In some embodiments, the controller 162 includes one or more processors 164. For example, the one or more processors 164 may be configured to execute a set of program instructions maintained in a memory device 166 or memory. The one or more processors 164 of the controller 162 may include any processing element known in the art. In this sense, the one or more processors 164 may include any microprocessor type device configured to execute algorithms and / or instructions.

[0079] The one or more processors 164 of the controller 162 may include any processor or processing element known in the art. For the purposes of this disclosure, the term "processor" or "processing element" may be broadly defined to encompass any device having one or more processing or logic elements, such as one or more microprocessor devices, one or more application specific integrated circuit (ASIC) devices, one or more field programmable gate arrays (FPGAs), or one or more digital signal processors (DSPs). In this sense, the one or more processors 164 may include any device configured to execute algorithms and / or instructions, such as program instructions stored in a memory. In some embodiments, the one or more processors 164 may be embodied as a desktop computer, a mainframe computer system, a workstation, an image computer, a parallel processor, a networked computer, or any other computer system configured to execute a program that is configured to operate or operate in conjunction with the overlay metrology system 100, as described throughout this disclosure. In addition, different subsystems of the overlay metrology system 100 may include processors or logic elements suitable for implementing at least a portion of the steps described in this disclosure. Therefore, the above description should not be interpreted as limiting the embodiments of the present disclosure but is merely illustrative. In addition, the steps described throughout the present disclosure may be implemented by a single controller or alternatively multiple controllers. In addition, the controller 162 may include one or more controllers housed in a common housing or multiple housings. In this way, any controller or combination of controllers may be separately packaged as a module suitable for integration into the stacking metering system 100.

[0080] The memory device 166 may include any storage media known in the art suitable for storing program instructions that can be executed by the associated one or more processors 164. For example, the memory device 166 may include a non-transitory memory medium. By way of another example, the memory device 166 may include, but is not limited to, a read-only memory (ROM), a random access memory (RAM), a magnetic or optical storage device (e.g., a disk), a tape, a solid-state drive, and the like. It should be further noted that the memory device 166 may be housed in a common controller housing with the one or more processors 164. In some embodiments, the memory device 166 may be remotely located relative to the physical location of the one or more processors 164 and the controller 162. For example, the one or more processors 164 of the controller 162 may access a remote memory (e.g., a server) that is accessible via a network (e.g., the Internet, an intranet, and the like).

[0081] The controller 162 may direct or receive data from the overlay metrology tool 102 or any component therein (eg, via control signals). The controller 162 may further be configured to perform any of the various process steps described throughout the present disclosure.

[0082] In some embodiments, the superimposed metering system 100 includes a user interface 168 communicatively coupled to the controller 162. In some embodiments, the user interface 168 may include, but is not limited to, one or more desktop computers, laptop computers, tablet computers, and the like. In some embodiments, the user interface 168 includes a display for displaying data of the superimposed metering system 100 to a user. The display of the user interface 168 may include any display known in the art. For example, the display may include, but is not limited to, a liquid crystal display (LCD), an organic light emitting diode (OLED)-based display, or a CRT display. Those skilled in the art will recognize that any display device capable of being integrated with the user interface 168 is suitable for implementation in the present disclosure. In some embodiments, a user may enter selections and / or instructions in response to data displayed to the user via a user input device of the user interface 168.

[0083] Reference Figure 4 , Figure 4 is a flow chart illustrating steps performed in a method 400 for superposition metrology according to one or more embodiments of the present disclosure. Applicants should note that the embodiments and implementation techniques previously described herein in the context of the superposition metrology system 100 should be interpreted as extending to the method 400. However, it should be further noted that the method 400 is not limited to the architecture of the superposition metrology system 100.

[0084] In some embodiments, method 400 includes step 402 of simultaneously illuminating a first direction unit of a unit pair on a stacking target with a first illumination beam distribution having a first linear polarization and illuminating a second direction unit of the unit pair with a second illumination beam distribution having a second linear polarization orthogonal to the first linear polarization. For example, the stacking target may include two or more unit pairs having grating stack structures oriented in orthogonal directions, such as Figure 2A to 2B In the description.

[0085] Step 402 may include illuminating the first direction and the second direction unit using any number or configuration of illumination beams, wherein the first and second illumination beam distributions may be separately controlled. In some embodiments, step 404 may include underfilling both the first direction and the second direction unit using the first and second illumination beam distributions. Furthermore, step 402 may include scanning the first and second illumination beam distributions simultaneously across the first direction and the second direction unit, which may mitigate target noise and / or speckle.

[0086] In some embodiments, method 400 includes a step 404 of collecting light from a sample (e.g., first and second direction units) as collected light. In some embodiments, method 400 includes a step 406 of directing a portion of the collected light associated with a first direction unit of two or more unit pairs to a first light collection channel. In some embodiments, method 400 includes a step 408 of directing a portion of the collected light associated with a second direction unit of two or more unit pairs to a second light collection channel. In this manner, collected light indicative of a superposition of the first and second direction units may be collected in parallel. For example, steps 406 and / or 408 may be implemented using any combination of polarization filters, pupil plane filters, or field plane filters, such as, but not limited to, those described in the context of superposition metrology system 100.

[0087] In some embodiments, method 400 includes a step 410 of generating a first superposition measurement along a first direction based on data associated with a first direction unit of two or more unit pairs (e.g., based on repeating steps 402 to 408 for two or more unit pairs). In some embodiments, method 400 includes a step 412 of generating a second superposition measurement along a second direction based on data associated with a second direction unit of the two or more unit pairs.

[0088] As contemplated herein, method 400 can advantageously provide parallel measurement of target cells with minimal or negligible crosstalk of collected light. In this manner, method 400 can reduce the required number of sequential measurements of a given stacked target by a factor of two relative to techniques in which all cells are measured sequentially. Furthermore, this increase in processing power can be achieved without sacrificing accuracy or sensitivity of the measurement.

[0089] 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 exemplary only, and in fact many other architectures that realize the same functionality can be implemented. In a conceptual sense, any arrangement of components for realizing the same functionality is effectively "associated" so that the desired functionality is realized. Therefore, any two components combined to realize a specific functionality in this article can be regarded as "associated" with each other so that the desired functionality is realized without considering the architecture or intermediate components. Similarly, any two components so associated can also be regarded as "connected" or "coupled" to each other to realize the desired functionality, and any two components that can be so associated can also be regarded as "coupleable" to each other to realize the desired functionality. Specific examples that can be coupled include but are not limited to physically interactable and / or physically interactive components and / or wirelessly interactive and / or wirelessly interactive components and / or logically interactive and / or logically interactive components.

[0090] It is believed that the present disclosure and many of its attendant advantages will be understood from the foregoing description, and it will be appreciated that various changes may be made to the form, construction, and arrangement of components without departing from the disclosed subject matter or sacrificing all of its important advantages. The forms described are merely illustrative, and it is intended that such changes be covered and included by the appended claims. Furthermore, it should be understood that the invention is defined by the appended claims.

Claims

1. A superposition metrology tool comprising: an illumination source configured to generate a first illumination beam distribution having a first linear polarization and a second illumination beam distribution having a second linear polarization orthogonal to the first linear polarization; an illumination subsystem for sequentially illuminating two or more unit pairs of a superimposed target on a sample, wherein a specific one of the two or more unit pairs includes a first direction unit having a grating stacking structure with periodicity along a first direction and a second direction unit having a grating stacking structure with periodicity along a second direction orthogonal to the first direction, wherein the illumination subsystem simultaneously illuminates the first direction unit using the first illumination beam distribution and illuminates the second direction unit using the second illumination beam distribution; A photon collection system, comprising: a first light collection channel including one or more first channel detectors at a first channel detection plane; a second light collection channel including one or more second channel detectors at a second channel detection plane; an objective lens for collecting light from the sample as collected light; and one or more filtering optics for directing a portion of the collected light associated with a first directional element of the two or more element pairs to the first collection channel, and directing a portion of the collected light associated with a second directional element of the two or more element pairs to the second collection channel; and A controller communicatively coupled to the first and second light collection channels, the controller comprising one or more processors configured to execute program instructions, thereby causing the one or more processors to: generating a first superposition measurement along the first direction based on data from the one or more first channel detectors associated with the first direction cells of the two or more cell pairs; and A second superposition measurement is generated along the second direction based on data from the one or more second channel detectors associated with the second direction cells of the two or more cell pairs.

2. The stacking metrology tool according to claim 1, further comprising: One or more beam scanning optics configured to adjust the separation between the first and second illumination beam distributions.

3. The overlay metrology tool of claim 2, wherein the one or more beam scanning optics adjust the separation between the first and second illumination beam distributions to center the first and second illumination beam distributions on the first and second directional units of the illuminated ones of the two or more unit pairs.

4. The stacking metrology tool of claim 2, further comprising: A first illumination field aperture stop, which is used to define a spatial range of the first illumination light beam distribution; and a second illumination field stop for defining a spatial extent of said second illumination beam distribution, wherein at least one of said first or second illumination field stops is adjustable and synchronized with said one or more beam scanning optics to provide, when said separation between said first and second illumination beam distributions is adjusted, said first and second illumination beam distributions being centered in said first and second illumination field stops.

5. The stacking metrology tool of claim 2, further comprising: A first light collection field aperture, which is used to define the light collection field of the first light collection channel; and A second illumination field aperture stop for defining a collection field of the second collection channel, wherein at least one of the first or second collection field aperture stops is adjustable and synchronized with the one or more beam scanning optical devices to provide, when adjusting the separation between the first and second illumination beam distributions, the first and second collection fields are aligned with the first and second illumination beam distributions.

6. The stacking metrology tool of claim 1, further comprising: One or more beam scanning optics configured to modulate the first and second illumination beam distributions across the first and second directional units of the illuminator of the two or more unit pairs during measurement.

7. The overlay metrology tool of claim 1 , wherein the grating overlay structures in the first direction units of the two or more unit pairs are arranged along a first row of the overlay target, and wherein the grating overlay structures in the second direction units of the two or more unit pairs are arranged along a second row of the overlay target.

8. The overlay metrology tool of claim 1 , wherein the grating overlay structures in the first direction cells of the two or more cell pairs have different expected overlay offsets along the first direction, wherein the grating overlay structures in the second direction cells of the two or more cell pairs have different expected overlay offsets along the second direction.

9. The overlay metrology tool of claim 1 , wherein the two or more cell pairs comprise: A first unit pair and a second unit pair, wherein the first direction unit of the first unit pair and the first direction unit of the second unit pair have equal and opposite expected offsets along the first direction, wherein the second direction unit of the first unit pair and the second direction unit of the second unit pair have equal and opposite expected offsets along the second direction.

10. The overlay metrology tool of claim 1, wherein the second illumination beam distribution is equal to the first illumination beam distribution.

11. The overlay metrology tool of claim 1, wherein the second illumination beam distribution is different from the first illumination beam distribution.

12. The overlay metrology tool of claim 1, wherein the second illumination beam distribution differs from the first illumination beam distribution based on at least one of a number of illumination beams, a wavelength of one or more illumination beams, or an incident angle of one or more illumination beams.

13. The overlay metrology tool of claim 1 , wherein at least one of the first or second illumination beam profiles comprises: Single lighting beam.

14. The overlay metrology tool of claim 1 , wherein the first illumination beam distribution comprises: two illumination beams in a dipole distribution aligned along the first direction, wherein the second illumination beam distribution comprises: Two illumination light beams in a dipole distribution aligned along the second direction.

15. The overlay metrology tool of claim 1, wherein the one or more filtering optics comprise: At least one of one or more polarization filtering optics, one or more pupil plane filters, or one or more field plane filters.

16. The overlay metrology tool of claim 15, wherein the one or more polarization filtering optics comprise: One or more polarization beam splitters for directing a portion of the collected light having the first linear polarization to the first collection channel and directing a portion of the collected light having the second linear polarization to the second collection channel.

17. The overlay metrology tool of claim 15, wherein the one or more polarization filtering optics comprise: At least one of a first polarizer in the first collection channel oriented to pass the first linear polarization or a second polarizer in the second collection channel oriented to pass the second linear polarization.

18. The overlay metrology tool of claim 15, wherein the one or more pupil plane filters comprise: At least one of a first aperture in the pupil plane of the first collection channel for transmitting the portion of the collected light associated with the diffraction along the first direction, a second aperture in the pupil plane of the second collection channel for transmitting the portion of the collected light associated with the diffraction along the second direction, or a beam splitter in a pupil plane common to the first and second collection channels for directing the portion of the collected light associated with the diffraction along the first direction to the first collection channel and directing the portion of the collected light associated with the diffraction along the second direction to the second collection channel.

19. The overlay metrology tool of claim 15, wherein the one or more field plane filters comprise: At least one of an aperture in a field plane of the first collection channel for passing a portion of the collected light associated with the first direction unit or an aperture in a field plane of the second collection channel for passing a portion of the collected light associated with the second direction unit.

20. The overlay metrology tool of claim 1, further comprising: A translation stage is provided for sequentially positioning the two or more unit pairs for illumination using the illumination subsystem.

21. The overlay metrology tool of claim 1, further comprising: One or more focus control optics for adjusting a focus position of at least one of the first or second illumination beam distributions.

22. The overlay metrology tool of claim 1, wherein at least one of the first or second detection planes comprises: Pupil plane.

23. The overlay metrology tool of claim 1, wherein at least one of the first or second detection planes comprises: Field plane.

24. A superposition metrology tool comprising: an illumination source configured to generate a first illumination beam distribution having a first linear polarization and a second illumination beam distribution having a second linear polarization orthogonal to the first linear polarization; An illumination subsystem for sequentially illuminating two or more unit pairs of a superimposed target on a sample, wherein a specific one of the two or more unit pairs includes a first direction unit having a grating superposition structure with periodicity along a first direction and a second direction unit having a grating superposition structure with periodicity along a second direction orthogonal to the first direction, the illumination subsystem comprising: a first illumination channel for directing the first illumination beam distribution to the first direction unit of one of the two or more unit pairs, wherein the first illumination beam distribution includes one or more first illumination beams having the first linear polarization; and a second illumination channel for directing the second illumination beam distribution to the second direction unit of one of the two or more unit pairs simultaneously with the first illumination beam distribution, wherein the second illumination beam distribution includes one or more second illumination beams having the second linear polarization; A photon collection system, comprising: a first light collection channel including one or more first channel detectors at a first channel detection plane; a second light collection channel including one or more second channel detectors at a second channel detection plane; an objective lens for collecting light from the sample as collected light; and a polarization beam splitter for directing a portion of the collected light associated with a first directional element having the first linear polarization to the first light collection channel, and directing a portion of the collected light associated with a second directional element of the two or more element pairs having the second linear polarization to the second light collection channel; and A controller communicatively coupled to the first and second light collection channels, the controller comprising one or more processors configured to execute program instructions, thereby causing the one or more processors to: generating a first superposition measurement along the first direction based on data from the one or more first channel detectors associated with the first direction cells of the two or more cell pairs; and A second superposition measurement is generated along the second direction based on data from the one or more second channel detectors associated with the second direction cells of the two or more cell pairs.

25. The stacking metrology tool of claim 24, further comprising: At least one of a first aperture in the pupil plane of the first collection channel for transmitting the portion of the collected light associated with the diffraction along the first direction, a second aperture in the pupil plane of the second collection channel for transmitting the portion of the collected light associated with the diffraction along the second direction, or a beam splitter in a pupil plane common to the first and second collection channels for directing the portion of the collected light associated with the diffraction along the first direction to the first collection channel and directing the portion of the collected light associated with the diffraction along the second direction to the second collection channel.

26. The overlay metrology tool of claim 24, further comprising: At least one of an aperture in a field plane of the first collection channel for passing a portion of the collected light associated with the first direction unit or an aperture in a field plane of the second collection channel for passing a portion of the collected light associated with the second direction unit.

27. The overlay metrology tool of claim 24, further comprising: One or more beam scanning optics configured to adjust the separation between the first and second illumination beam distributions.

28. The overlay metrology tool of claim 27, wherein the one or more beam scanning optics adjust the separation between the first and second illumination beam distributions to center the first and second illumination beam distributions on the first and second directional units of the illuminated ones of the two or more unit pairs.

29. The overlay metrology tool of claim 27, further comprising: A first illumination field aperture stop, which is used to define a spatial range of the first illumination light beam distribution; and a second illumination field stop for defining a spatial extent of said second illumination beam distribution, wherein at least one of said first or second illumination field stops is adjustable and synchronized with said one or more beam scanning optics to provide, when said separation between said first and second illumination beam distributions is adjusted, said first and second illumination beam distributions being centered in said first and second illumination field stops.

30. The overlay metrology tool of claim 27, further comprising: A first light collection field aperture, which is used to define the light collection field of the first light collection channel; and A second illumination field aperture stop for defining a collection field of the second collection channel, wherein at least one of the first or second collection field aperture stops is adjustable and synchronized with the one or more beam scanning optical devices to provide, when adjusting the separation between the first and second illumination beam distributions, the first and second collection fields are aligned with the first and second illumination beam distributions.

31. A superposition measurement method, comprising: generating a first illumination beam distribution having a first linear polarization and a second illumination beam distribution having a second linear polarization orthogonal to the first linear polarization; sequentially illuminating two or more unit pairs of a superimposed target on a sample using an illumination system, wherein each of the two or more unit pairs includes a first direction unit having a grating stacking structure with a periodicity along a first direction and a second direction unit having a grating stacking structure with a periodicity along a second direction orthogonal to the first direction, wherein the illumination system simultaneously illuminates the first direction unit using the first illumination beam distribution and illuminates the second direction unit using the second illumination beam distribution; collecting light from the sample as collected light; directing a portion of the collected light associated with a first directional element of the two or more element pairs to a first light collection channel; directing a portion of the collected light associated with a second directional element of the two or more element pairs to a second light collection channel; generating a first superposition measurement along the first direction based on data associated with the first direction cells of the two or more cell pairs; and A second superposition measurement is generated along the second direction based on data associated with the second direction cells of the two or more cell pairs.

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