Parallel Scattering Measurement Overlay Measurement

Abstract

The overlay metrology tool may include an illumination source 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, and an illumination subsystem for sequentially illuminating two or more cell pairs of an overlay target on a sample having orthogonally oriented grating-over-grating structures. A collection subsystem having two collection channels for capturing light collected from the illuminated cell pairs and filtering optics for directing light from different cells in the illuminated cell pairs to different collection channels for detection. The tool may further include a controller for generating separate overlay measurements for the orthogonally oriented grating-over-grating structures in the two or more cell pairs.

Description

Technical Field

[0001] The present disclosure generally relates to overlay measurement, and more particularly to overlay measurement based on scatterometry.

Background Art

[0002] Overlay measurement generally refers to the measurement of the relative alignment of layers on a sample such as a semiconductor device, although not limited thereto. Measurement of overlay or overlay error typically refers to the measurement of misalignment of features fabricated on one or more sample layers. In a general sense, proper alignment of features fabricated on multiple sample layers is required for the proper functioning of a device. The requirements to reduce feature size and increase feature density have led to a corresponding increased demand for accurate and efficient overlay measurement. Many existing overlay measurement techniques require separate measurement steps for multiple cells of an overlay target to generate overlay data in a particular direction, and further require an additional set of measurement steps to generate overlay data in additional directions.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, the time required to perform such measurement steps may limit the throughput of the overlay measurement system. Therefore, there is a need to develop systems and methods to overcome these drawbacks.

Means for Solving the Problem

[0005] An overlay measurement tool is disclosed in accordance with one or more exemplary embodiments of the present disclosure. In an exemplary embodiment, the tool includes an illumination source that generates a first illumination beam distribution having a first linearly polarized light and a second illumination beam distribution having a second linearly polarized light orthogonal to the first linearly polarized light. In another exemplary embodiment, the tool includes an illumination subsystem that sequentially illuminates two or more cell pairs of an overlay target on a sample. A particular one of the two or more cell pairs can include a first-direction cell having a lattice-over-lattice structure with periodicity along a first direction and a second-direction cell having a lattice-over-lattice structure with periodicity along a second direction orthogonal to the first direction. In another exemplary embodiment, the illumination subsystem simultaneously illuminates the first-direction cell with the first illumination beam distribution and the second-direction cell with the second illumination beam distribution. In another exemplary embodiment, the tool includes a first collection channel including one or more first channel detectors in a first channel detection plane, a second collection channel including one or more second channel detectors in a second channel detection plane, and an objective lens for collecting light from the sample as the collected light. And one or more filtering optics direct a portion of the collected light associated with the first-direction cells of the two or more cell pairs to the first collection channel and a portion of the collected light associated with the second-direction cells of the two or more cell pairs to the second collection channel. In another exemplary embodiment, the tool includes a controller that generates a first overlay measurement value along a first direction based on data associated with the first-direction cells of two or more cell pairs from one or more first channel detectors. And a second overlay measurement value along a second direction is generated based on data associated with the second-direction cells of two or more cell pairs from one or more second channel detectors.

[0006] An overlay measurement tool is disclosed in accordance with one or more exemplary embodiments of the present disclosure. In one exemplary embodiment, the tool includes an illumination source that generates 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 exemplary embodiment, the tool includes an illumination subsystem that sequentially illuminates two or more cell pairs of an overlay target on a sample. A particular one of the two or more cell pairs can include a first-direction cell having a grating-over-grating structure with periodicity along a first direction and a second-direction cell having a grating-over-grating structure with periodicity along a second direction orthogonal to the first direction. In another exemplary embodiment, the illumination system includes a first illumination channel that directs the first illumination beam distribution to a first-direction cell of one of the two or more cell pairs, and the first illumination beam distribution includes one or more first illumination beams having a first linear polarization. In another exemplary embodiment, the illumination subsystem further includes a second illumination channel for directing a second illumination beam distribution to a second-direction cell of one of the two or more cell pairs simultaneously with the first illumination beam distribution, and the second illumination beam distribution includes one or more second illumination beams having a second linear polarization. In another exemplary embodiment, the tool includes a first collection channel that includes one or more first channel detectors on a first channel detection surface, a second collection channel that includes one or more second channel detectors on a second channel detection surface, and an objective lens that collects light from the sample as collected light. A polarization beam splitter directs a portion of the collected light associated with a first-direction cell having a first linear polarization to the first collection channel and a portion of the collected light associated with a second-direction cell of two or more cell pairs having a second linear polarization to the second collection channel. In another exemplary embodiment, the tool includes a controller that generates a first overlay measurement value along a first direction based on data associated with a first-direction cell of two or more cell pairs from one or more first channel detectors. And a second overlay measurement value along a second direction is generated based on data associated with a second-direction cell of two or more cell pairs from one or more second channel detectors.

[0007] An overlay measurement method is disclosed in accordance with one or more exemplary embodiments of the present disclosure. In an exemplary embodiment, the method includes generating a first illumination beam distribution having a first linearly polarized light and a second illumination beam distribution having a second linearly polarized light orthogonal to the first linearly polarized light. In another exemplary embodiment, the method includes sequentially irradiating two or more cell pairs of an overlay target on a sample, each of the two or more cell pairs including a first direction cell having a lattice-over-lattice structure having periodicity along a first direction and a second direction cell having a lattice-over-lattice structure having periodicity along a second direction orthogonal to the first direction. Here, the illumination subsystem simultaneously illuminates the first direction cell with the first illumination beam distribution and the second direction cell with the second illumination beam distribution. In another exemplary embodiment, the method includes collecting light from the sample as collected light. In another exemplary embodiment, the method includes directing a portion of the collected light associated with the first direction cells of the two or more cell pairs to a first collection channel. In another exemplary embodiment, the method includes directing a portion of the collected light associated with the second direction cells of the two or more cell pairs to a second collection channel. In another exemplary embodiment, the method includes generating a first overlay measurement value along a first direction based on data associated with the first direction cells of the two or more cell pairs. In another exemplary embodiment, the method includes generating a second overlay measurement along a second direction based on data associated with the second direction cells of the two or more cell pairs.

[0008] It should be understood that both the foregoing summary and the following detailed description are exemplary and explanatory only and are not necessarily limiting of the claimed invention. 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

[0009] Many advantages of the present disclosure can be better understood by those skilled in the art by referring to the accompanying drawings.

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DETAILED DESCRIPTION OF THE INVENTION

[0010] Here, reference is made in detail to the disclosed subject matter shown in the accompanying drawings. The present disclosure has been specifically shown and described with respect to particular embodiments and specific features thereof. The embodiments described herein are to be construed as illustrative rather than limiting. 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.

[0011] Embodiments of the present disclosure relate to systems and methods for generating parallel measurements of cells of an overlay target using scattering measurement techniques. In some embodiments, two cells of an overlay target associated with overlay 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 into separate detection paths. In this way, measurements of multiple cells of an overlay target can be performed in parallel, which can advantageously increase the measurement throughput relative to sequential measurements of each cell on the target. Further embodiments are directed to a multi-channel scattering measurement overlay tool suitable for simultaneously illuminating multiple cells of an overlay target and separating light from the multiple cells into separate detection channels.

[0012] For purposes of the present disclosure, the term overlay is generally used to describe the relative position of features on a sample fabricated by two or more lithographic patterning steps, and the term overlay error describes the deviation of a feature from its nominal placement. For example, a multi-layer device may include features patterned on multiple sample layers using different lithographic steps for each layer, and the alignment of features between layers typically must be tightly controlled to ensure proper performance of the resulting device. Thus, overlay measurements can characterize the relative position of features on two or more sample layers. As another example, multiple lithographic steps can be used to fabricate features on a single sample layer. Such techniques are generally referred to as double patterning or multi-patterning techniques and can facilitate the fabrication of high-density features approaching the resolution of the lithography system. Overlay measurements in this context can characterize the relative position of features from different lithographic steps on this single layer. It should be understood that the examples and illustrations throughout this disclosure regarding specific applications of overlay metrology are provided for illustrative purposes only and should not be construed as limiting the present disclosure.

[0013] In some applications, the overlay measurement may be performed directly on a feature of the manufactured device (e.g., a device feature), but the overlay measurement is typically performed on a dedicated overlay target printed using the same lithography step as the device feature. In this way, the features of the overlay target (e.g., target features) can be specially designed to facilitate the overlay measurement. Further, the overlay measured at one manufacturing step (e.g., after the manufacture of one or more sample layers) can be used to generate a correctable for accurately aligning a process tool (e.g., a lithography tool or the like) for the manufacture of additional sample layers in subsequent manufacturing steps.

[0014] In some embodiments, an overlay target suitable for scatterometry measurements as disclosed herein may include one or more cells having a grid-over-grid structure, the grid-over-grid structure including periodic features (e.g., grid features) over an overlapping region of two or more target layers. In this way, the various grid features on the layer of interest can contribute to the diffraction of the incident illumination and an overlay measurement value can be generated based on the analysis of the diffracted light. For example, the overlay measurement may be generated based on pupil plane data (e.g., related to the relative intensity difference between selected diffraction orders within the pupil plane). As another example, the overlay measurement may be generated based on field plane data (e.g., related to the relative intensity of an image of a cell of the target generated using a selected diffraction order).

[0015] As used throughout this disclosure, the term "sample" generally refers to a substrate formed from a semiconductor or non-semiconductor material (e.g., a wafer, etc.). For example, the semiconductor or non-semiconductor material can 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, resist, dielectric material, conductive material, and semiconductor material. Many different types of such layers are known in the art, and the term "sample" as used herein is intended to encompass samples on which all types of such layers can be formed. One or more layers formed on the sample may or may not be patterned. For example, the sample may include a plurality of dies, each die having repeatable patterning features. The formation and processing of such layers of materials can ultimately result in a completed device. Many different types of devices can be formed on the sample, and the term "sample" as used herein is intended to encompass samples on which any type of device known in the art is fabricated. Further, for the purposes of this disclosure, the terms "sample" and "wafer" should be construed as interchangeable. Additionally, for the purposes of this disclosure, the terms "patterning device", "mask", and "reticle" should be construed as interchangeable.

[0016] In some embodiments, the overlay measurement tool simultaneously illuminates two cells of an overlay target with two illumination beams, and the two cells include a grating-over-grating structure where the orientation of the grating in one cell is orthogonal to the orientation of the grating in the other cell. For example, the overlay measurement tool can simultaneously illuminate an X-direction cell including a grating-over-grating structure having periodicity along the X direction with a first illumination beam, and simultaneously illuminate a Y-direction cell including a grating-over-grating structure having periodicity along the Y direction with a second illumination beam. It should be understood that the descriptions of the X and Y directions are used herein for illustrative purposes only to refer to any orthogonal directions on the sample. Further, the two cells can be illuminated with light having orthogonal linear polarizations (e.g., one cell is illuminated with linearly polarized light along the X direction and one cell is illuminated with linearly polarized light along the Y direction). In particular, the overlay measurement tool can illuminate each of the two cells with a distribution of one or more illumination beams (e.g., an illumination beam distribution), and the illumination beam distribution on each cell is smaller than the cell so that the cell is underfilled. Additionally, various aspects of the illumination light on each cell, such as but not limited to the spectrum (e.g., spectral bandwidth and / or central wavelength), intensity, illumination angle, angular distribution of illumination, or focus position, can be individually controlled or adjusted.

[0017] In some embodiments, the overlay measurement tool simultaneously separates the light collected from the illuminated cells into separate detection channels for each cell. In this way, the measurement data associated with the illuminated cells can be isolated from each other despite the simultaneous collection. Various techniques for separating the collected light into separate detection channels may be used within the spirit and scope of the present disclosure. For example, the light collected from the illuminated cells can be separated based on parameters such as but not limited to polarization, pupil aperture filter (e.g., a spatial filter in the pupil plane that passes selected diffraction orders), or field stop filter (e.g., a spatial filter in the field plane that passes light from a selected cell).

[0018] In some embodiments, the overlay measurement tool includes beam control optics in the illumination and / or collection paths to various aspects of the illumination light or the collection light. For example, the overlay measurement tool may include one or more scanning mirrors that scan the illumination beam across respective cells during measurement, which may reduce the noise associated with the target defect. As another example, the overlay measurement tool may include beam control optics that control or adjust the separation between two illumination beams, which may be used to adapt the illumination beam separation to the layout of the cells within a particular overlay target. Further, the overlay measurement tool may adjust the illumination and / or collection field stop based on a selected illumination beam separation.

[0019] Further embodiments of the present disclosure relate to overlay targets suitable for parallel measurement by an overlay measurement tool. In some embodiments, the overlay target includes X-direction cells distributed along a first row and Y-direction cells distributed along a second row. In this way, the cells associated with a particular direction can be sequentially illuminated under the same or similar illumination conditions (e.g., polarization, spectrum, intensity, angular distribution of the illumination, focal position of the sample during illumination, etc.) simply by scanning the overlay target along the row. For example, some overlay measurement techniques may require data capture from multiple cells with different intended offsets of a grid-over-grid structure. Thus, arranging cells with a common direction of periodicity along a row can reduce the burden on the overlay measurement tool such that the illumination conditions can be the same or substantially the same as when the target is scanned along the row direction.

[0020] Referring now to FIGS. 1A-4, a system and method for generating parallel measurements of cells of an overlay target using scattering measurement techniques are shown.

[0021] FIG. 1A is a block diagram of an overlay measurement system 100 according to one or more embodiments of the present disclosure.

[0022] In some embodiments, the overlay measurement system 100 includes a dual-channel illumination subsystem 104 that separately illuminates two cells 106 of an overlay target 108 on a sample 110 with a spatially separated distribution (e.g., an illumination beam distribution) of one or more illumination beams 112 having orthogonal linear polarization, and a dual-channel collection subsystem 114 that separately detects light or other radiation emitted from the sample 110 (e.g., collected light 116) associated with the two cells 106.

[0023] In some embodiments, the sample 110 is disposed on a sample stage 118 suitable for fixing the sample 110 and is further configured to position the sample 110 relative to the overlay measurement tool 102. For example, the sample stage 118 can include any combination of linear, rotational, or angular (e.g., tip / tilt) actuators suitable for positioning the sample 110 in any selected orientation.

[0024] The overlay measurement tool 102 can be any type of overlay measurement tool known in the art suitable for generating an overlay signal suitable for determining an overlay associated with an overlay target on the sample 110. For example, the overlay measurement tool 102 can collect pupil plane data (e.g., one or more pupil plane images or portions thereof) in which the collected light 116 is analyzed at the pupil plane to characterize the angular distribution of the radiation from the cells 106 (e.g., associated with the scattering and / or diffraction of radiation by the cells 106). As another example, the overlay measurement tool 102 can 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 measurement tool 102 can characterize the cells 106 while the sample 110 is stationary (e.g., in a static or moving measurement (MAM) mode) or while the sample 110 is moving (e.g., in a scanning mode).

[0025] Referring now to FIGS. 2A-2B, various configurations of the overlay target 108 suitable for evaluating the parallel characteristics of the constituent cells 106 are described in more detail in accordance with one or more embodiments of the present disclosure.

[0026] FIG. 2A is a side view of a grid-over-grid structure within a single cell 106 of an overlay target 108 according to one or more embodiments of the present disclosure. In some embodiments, the grid-over-grid structure within each cell 106 includes a first layer grid feature 202 located on a first layer 204 of the sample 110 and a second layer grid feature 206 located on a second layer 208 of the sample 110, oriented such that the regions including the first layer grid feature 202 and the second layer grid feature 206 overlap. In this way, the first layer grid feature 202 and the second layer grid feature 206 can each diffract the incident illumination beam 112 into discrete diffraction orders. Generally, any number of additional layers (not shown) can be located above, below, or between the first layer 204, the second layer 208, and the substrate 210.

[0027] In a general sense, the first layer grating feature 202 and the second layer grating feature 206 may have the same or different distributions and may be intentionally offset from each other by any selected intended offset 212. In some embodiments, the first layer grating feature 202 and the second layer grating feature 206 have the same pitch along the same direction such that the associated diffraction orders overlap within the pupil plane. In some embodiments, the first layer grating feature 202 and the second layer grating feature 206 have different pitches along a particular direction, which can result in moiré fringes associated with a moiré pitch that is larger than the pitches of the first layer grating feature 202 and the second layer grating feature 206. Overlay measurement techniques that utilize the moiré effect are generally described in U.S. Patent No. 9,182,219, published March 11, 2011 (November 10, 2015), U.S. Patent No. 7,440,105 (October 21, 2008), U.S. Patent No. 7,349,105 (March 25, 2008), U.S. Patent No. 10,551,749 (February 4, 2020), U.S. Patent Application Publication No. 2021 / 0072650, U.S. Patent Application No. 16 / 935,117 (July 21, 2020), and U.S. Patent Application No. 16 / 931,078 (July 16, 2020), all of which are hereby incorporated by reference in their entirety.

[0028] FIG. 2B is a top view of an overlay target 108 according to one or more embodiments of the present disclosure. In some embodiments, the overlay 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 a grating-over-grating structure having different intended offsets 212.

[0029] FIG. 2B shows an overlay target 108 having two cells 106 (e.g., X-direction cells 106) having periodicity along the X direction for overlay measurement along the X direction and two cells 106 (e.g., Y-direction cells 106) having periodicity along the Y direction for overlay measurement along the Y direction. The various cells 106 may generally be arranged in any suitable distribution. In some embodiments, as shown in FIG. 2B, the cells 106 having the same periodic direction are dispersed in rows. For example, FIG. 2B shows 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.

[0030] However, it should be understood that the overlay target 108 of FIGS. 2A-2B and the related description are provided for illustrative purposes only and should not be construed as limiting. Rather, the overlay target 108 may include any suitable grid-overlay-overlay-overlay target design. For example, the overlay target 108 may generally include any number of cells 106 suitable for measurement along two directions. Further, the cells 106 may be dispersed in any pattern or arrangement. For example, a measurement target design suitable for scanning measurement is generally described in U.S. Patent Application No. 16 / 598,146 (October 10, 2019). This is hereby incorporated by reference in its entirety.

[0031] Referring now generally to FIGS. 1A-3B, the parallel characterization of the plurality of cells 106 of the overlay target 108 according to one or more embodiments of the present disclosure will be described in more detail.

[0032] Figure 1B is a schematic diagram of an overlay measurement tool 102 showing two illumination channels 120 and two collection channels 122, according to one or more embodiments of the present disclosure. In this way, the overlay measurement tool 102 can simultaneously characterize two cells 106 of an overlay target 108, such as, but not limited to, the overlay target 108 shown in FIGS. 2A-2B. In some embodiments, the overlay measurement tool 102 simultaneously characterizes two cells 106 having periodic orthogonal directions (e.g., one X-direction cell 106 and one Y-direction cell 106).

[0033] In some embodiments, the overlay measurement tool 102 includes at least one illumination source 124 configured to generate illumination 126 suitable for forming an illumination beam distribution directed at the cell 106.

[0034] The illumination 126 from the illumination source 124 can include light of one or more selected wavelengths, including, but not limited to, ultraviolet (UV) radiation, visible light, or infrared (IR) radiation.

[0035] The illumination source 124 can 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 can include, without limitation, one or more narrow-band laser sources, broadband laser sources, supercontinuum laser sources, white light laser sources, etc. In this regard, the illumination source 124 can provide illumination having 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 can include, without limitation, an LSP lamp, an LSP bulb, or an LSP chamber suitable for housing one or more elements that can emit broadband illumination when excited to a plasma state by a laser source. In some embodiments, the illumination source 124 includes a lamp source. For example, the illumination source 124 can include, without limitation, arc lamps, discharge lamps, electrodeless lamps, etc. In this regard, the illumination source 124 can provide illumination having low coherence (e.g., low spatial coherence and / or temporal coherence).

[0036] Each illumination channel 120 can direct the 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 generally includes, without limitation, one or more illumination beams 112 or illumination lobes directed to a specific cell 106 having a selected distribution of illumination parameters such as the angle of incidence (e.g., azimuth and polar angle of incidence), spectrum, or polarization. For example, the illumination beam distribution can include, but is 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 way, the illumination channel 120 can simultaneously provide different illumination conditions for different cells 106 on the overlay target 108. Further, when the illumination beam distribution includes a plurality of illumination beams 112, these illumination beams 112 can be provided simultaneously or sequentially in a given measurement of the cell 106.

[0037] In some embodiments, the illumination channel 120 provides an illumination beam 112 having orthogonally linearly polarized light. For example, one illumination channel 120 can provide a distribution of one or more illumination beams 112 having linearly polarized light along a first direction (e.g., the X direction), and another illumination channel 120 can provide a distribution of one or more illumination beams 112 having linearly polarized light along a second direction (e.g., the Y direction) orthogonal to the first direction. As used herein, providing orthogonally linearly polarized light within the illumination channel 120 is contemplated to have various benefits, including but not limited to efficiently utilizing the available power from the illumination source 124, providing diffraction that is well controlled by the grating-over-grating structure within each cell 106, and / or facilitating separating the collected light 146 of the illuminated cell 106 into different collection channels 122 having a high extinction ratio to prevent signal cross-contamination.

[0038] Further, the various cells 106 of the overlay target 108 may be illuminated in any selected polarization direction. For example, the overlay measurement tool 102 can illuminate the X-direction cells 106 with linearly polarized light along the X direction and illuminate the Y-direction cells 106 with linearly polarized light along the Y direction, and vice versa.

[0039] The overlay measurement tool 102 can generally include any combination of optical components for simultaneously generating a plurality of illumination beam distributions.

[0040] In some embodiments, the overlay measurement tool 102 can include a single illumination source 124 that generates illumination 126 for each illumination channel 120. For example, the overlay measurement tool 102 can include a polarization beam splitter that splits the illumination 126 into orthogonally linearly polarized light directed to different illumination channels 120. In some embodiments, the overlay measurement tool 102 includes separate illumination sources 124 for one or more of the illumination channels 120.

[0041] The overlay measurement tool 102 may further include various optical components for generating the selected illumination beam distribution for each illumination channel 120. In some embodiments, as illustrated in FIG. 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, laterally movable mirrors, scanning mirrors, etc.). By way of example, each illumination channel 120 may include an aperture stop for generating the distribution of one or more illumination beams 112 and / or a field stop for restricting the spatial spread of the illumination beams 112 within that channel to an area below the cell 106 so that the cell is underfilled with illumination.

[0042] In a general sense, each illumination channel 120 can provide independent control of the illumination parameters of one or more illumination beams 112. In some embodiments, although not explicitly shown in FIG. 1B, the overlay measurement tool 102 may include an illumination lens 128 and / or illumination control optics 130 for manipulating the illumination 126 before it enters the illumination channel 120. In this way, the selected aspects of the illumination beam distribution can be made consistent between the illumination channels 120.

[0043] Furthermore, the illumination parameters within each illumination channel 120 may be static or adjustable. The tunable illumination parameters may 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 (August 6, 2019) and U.S. Patent Application No. 17,076,312 (October 21, 2020). Both of these are hereby incorporated by reference in their entirety.

[0044] In some embodiments, the adjustable lighting parameter is provided by selectively directing the illumination 126 to one of a plurality of available optical paths having one or more static filters (such as a spectral filter, a dimming filter, etc.). In some embodiments, the tunable lighting parameter is provided by adjusting the position of one or more tunable filters (such as a spectral filter, a dimming filter, etc.) within the illumination channel 120. In some embodiments, the adjustable lighting parameter is provided by selectively controlling the position of the illumination 126 from the illumination source 124 on one or more spatially varying filters (such as a spectral filter, a dimming filter, etc.).

[0045] FIG. 1C is a schematic diagram of a portion of an overlay metrology tool 102 showing two illumination channels 120 having separately configurable illumination conditions based on a linearly varying filter, according to one or more embodiments of the present disclosure. The linearly varying filter can include a filter having filtering characteristics that vary along a linear filtering direction. For example, a linearly varying neutral density filter can provide various amounts of broadband intensity reduction based on the spatial position of the input beam along the linear axis. As another example, a linearly varying low-pass (or high-pass) filter can provide low-pass filtering with a cut-off wavelength that varies based on the spatial position of the input beam along the linear filtering direction. As another example, the linearly varying filter may be formed as a polarizer, and the direction of polarization passed by the linearly varying filter may vary in different directions along the linear filtering direction. It is contemplated herein that the systems and methods disclosed herein may utilize a linearly varying filter that modifies any selected characteristic of the input beam.

[0046] In some embodiments, the overlay measurement tool 102 includes an illumination source 124 having a spectral bandwidth that covers a range of wavelengths of interest, and a polarizer 132 that divides illumination 126 from the illumination source 124 along two paths associated with two illumination channels 120. For example, the polarizer 132 may be a polarizing beam splitter that provides orthogonally linearly polarized light within the two illumination channels 120.

[0047] Each illumination channel 120 can include at least one adjustable filter 134 that provides adjustable 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 linear variable filter 138 located in the pupil plane (e.g., the focal plane common to the focusing optics 136), and an angle scanner 140 located in the other focal plane of the focusing optics 136 as an input and / or output coupler to the tunable filter 134. In this way, the position of the illumination 126 on the linearly varying filter 138, and thus the resulting filtering effect on the illumination 126, can be controlled by adjusting the angle of the input angle scanner 140 that receives the illumination 126. Further, regardless of the angle of the input angle scanner 140 and the associated position on the linearly varying filter 138, the illumination 126 is 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 can be adjusted simultaneously to provide adjustable filtering without modifying the output beam path and thus without affecting the alignment of additional optics of the overlay measurement system 100.

[0048] Furthermore, the plurality of tunable filters 134 may be arranged in series to provide tuning of the plurality of illumination parameters. For example, FIG. 1C shows three tunable filters 134 within each illumination channel 120. Such a configuration may be suitable, but not limited to, adjusting the intensity and spectrum of the illumination 126 within each illumination channel 120. For example, one linearly varying filter 138 may be a linearly varying intensity filter, one linearly varying filter 138 may be a linearly varying low-pass spectrum filter, and one linearly varying filter 138 may be a linearly varying high-pass spectrum filter.

[0049] In some embodiments, the overlay measurement tool 102 includes one or more focus control optics 142 for adjusting or controlling the focus position of one or more illumination beams 112 within one or more illumination channels 120. It is contemplated herein that focus position adjustment may be particularly useful, but not limited to, measurements having different wavelengths or measurements of features at different depths. For example, the illumination beams 112 within different illumination channels 120 may have different spectral ranges. As another example, measurement robustness may be improved by capturing data from each cell 106 at a plurality of different wavelengths. In either case, chromatic aberration in the sample 110 and / or the overlay measurement 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.

[0050] As an example, FIG. 1B includes focus control optics 142 in each of the illumination channels 120 to provide independent focus control for the associated illumination beam 112. The focus control optics 142 may include, but is not limited to, one or more deformable mirrors, one or more acousto-optic lenses, one or more voice coils, or one or more parallel movable lenses (e.g., any of the illumination lenses 128), and may include any type or combination of optical elements suitable for modifying the focus position of at least one illumination beam 112. In some embodiments, the focus control optics 142 can be designed to provide focus switching within the time required to adjust one or more additional illumination parameters (e.g., the spectrum, intensity, angular distribution, etc. of the illumination) to facilitate high measurement throughput. In this way, the focus control optics 142 can operate as a high-speed focus controller.

[0051] Referring again to FIGS. 1A - 1C, the overlay measurement tool 102 can generate or control the illumination beam distribution (e.g., the number and incident angles of the structured illumination beams 112) using any technique known in the art.

[0052] In some embodiments, the overlay measurement system 100 includes one or more apertures (e.g., illumination control optics 130) in the illumination pupil plane to define one or more illumination beams 112. In some embodiments, the overlay measurement system 100 generates the illumination beam 112 by providing light to two or more optical fibers, and the light output from each optical fiber is provided to or directed towards the illumination pupil to provide the illumination beam 112. In some embodiments, the overlay measurement system 100 generates one or more illumination beams 112 by diffracting the illumination 126 from the illumination source 124 into two or more diffraction orders, and at least one of the diffraction orders forms at least one illumination beam 112. The efficient generation of multiple illumination beams by controlled diffraction is generally described in U.S. Patent Application Publication No. US2020 / 0124408, published on April 23, 2020, which is hereby incorporated by reference in its entirety.

[0053] In some embodiments, the overlay measurement tool 102 includes an objective lens 144 for capturing light or other radiation (e.g., collected light 146) emitted from both illuminated cells 106. Further, an illumination beam 112 from any illumination channel 120 may 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 outside-the-lens (OTL) configuration). For example, FIG. 1B shows a TTL configuration.

[0054] In some embodiments, the overlay measurement tool 102 includes one or more filtering optics for separating the collected light 146 from the two illuminated cells 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 cell 106 to one collection channel 122 and the collected light 116 associated with the Y-direction cell 106 to another collection channel 122.

[0055] The one or more filtering optics may use any technique or combination of techniques, including but not limited to polarization filtering, pupil-plane filtering, or field-plane filtering, to separate the collected light 116 associated with the two illuminated cells 106.

[0056] In some embodiments, the filtering optics includes one or more polarization filters, such as one or more linear polarizers or one or more polarization beam splitters, etc., and implements polarization filtering. It is contemplated herein that polarization filtering may be particularly effective when the illumination channels 120 provide an illumination beam distribution having orthogonally linearly polarized light, since the collected light 116 may generally retain the polarization direction of the illumination.

[0057] For example, as shown in FIG. 1B, the overlay measurement tool 102 includes a polarization beam splitter 148 at a position common to the illumination 126 and the collected light 116. In this configuration, the polarization beam splitter 148 can receive orthogonally linearly polarized illumination 126 from each of the illumination channels 120 and send this light to the objective lens 144. The polarization beam splitter 148 can then receive the light 116 collected from both illuminated cells 106 and split this collected light 116 into two collection channels 122 based on polarization. FIG. 1B further shows two non-polarizing beam splitters 150, one associated with each polarization direction, separating the illumination 126 and the collected light 116 for that polarization direction.

[0058] As another example, although not explicitly shown, the overlay measurement tool 102 can include one or more linear polarizers in either or both of the collection channels 122. For example, one collection channel 122 can include a linear polarizer aligned along the X direction, and one collection channel 122 can include a linear polarizer aligned along the Y direction. In this configuration, the linear polarizer can perform polarization filtering, which can be achieved even when the illumination beam distribution does not have orthogonally linearly polarized light. Further, the polarizers within the collection channels 122 can provide a higher extinction ratio than that provided by the polarization beam splitter 148, and thus can further reduce crosstalk between the collected light 116 associated with different cells 106.

[0059] In some embodiments, the filtering optics includes one or more spatial filters (e.g., pupil plane filters) in one or more pupil planes. As used herein, a grating over grating structure is contemplated to generate diffraction orders distributed in the direction of periodicity. Thus, the diffraction orders from cells 106 having orthogonal directions of periodicity are distributed in orthogonal directions within the pupil plane and can thus be filtered using a pupil plane filter.

[0060] Figures 3A-3B show the spatial distribution of diffraction orders from the illuminated cells 106 of the overlay target 108 of FIG. 2B, according to one or more embodiments of the present disclosure. FIG. 3A is a top view of the pupil plane showing the distribution of diffraction orders from the X-direction cells 106 of the overlay target 108 of FIG. 2B, according to one or more embodiments of the present disclosure. In particular, FIG. 3A shows the zero-order diffraction 302 (e.g., specular reflection), X-direction -1st order diffraction 304, and X-direction +1st order diffraction 306 within the pupil boundary 308 in response to a single illumination beam 112 at a normal angle of incidence. FIG. 3B is a top view of the pupil plane showing the distribution of diffraction orders from the Y-direction cells 106 of the overlay target 108 of FIG. 2B, according to one or more embodiments of the present disclosure. In particular, FIG. 3B shows the zero-order diffraction 302, Y-direction -1st order diffraction 310, and Y-direction +1st order diffraction 312 within the pupil boundary 308 in response to a single illumination beam 112 at a normal angle of incidence. As shown in FIGS. 3A and 3B, the non-zero diffraction orders from the X-direction cells 106 and Y-direction cells 106 do not overlap within the pupil plane and can be separated using a pupil plane filter. For example, one collection channel 122 may include a pupil plane filter having an aperture oriented to pass at least the X-direction -1st order diffraction 304 and X-direction +1st order diffraction 306 to separate the light 116 collected from the illuminated X-direction cells 106. And one collection channel 122 may include a pupil plane filter having an aperture oriented to pass at least the Y-direction -1st order diffraction 310 and Y-direction +1st order diffraction 312, and can separate the collected light 116 from the illuminated Y-direction cells 106.

[0061] In some embodiments, the filtering optics includes one or more spatial filters (e.g., field plane filters) in one or more field planes. For example, FIG. 1B shows the field stop 152 in each of the collection channels 122. In particular, the field stop 152 within the collection channel 122 can limit the collection region within that channel to a particular cell 106.

[0062] In some embodiments, each collection channel 122 includes one or more detectors 154 configured to capture the light 116 collected within the channel. Each collection channel 122 can further include one or more optical elements suitable for modifying and / or conditioning the light 146 collected from the sample 110. In some embodiments, the collection channel 122 includes one or more collection path lenses 156 (e.g., for collimating illumination, relaying the pupil plane and / or field plane, etc.), which may include the objective lens 144, but need not include it. In some embodiments, the collection channel 122 includes one or more collection path optics 158 that shape or otherwise control the collected light 146. For example, the collection path optics 158 may include, but are not limited to, one or more field stops, 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, laterally movable mirrors, scanning mirrors, etc.).

[0063] The detector 154 can be placed at any selected position within the collection channel 122. In some embodiments, the overlay measurement tool 102 includes the detector 154 in the pupil plane (e.g., the 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, the diffraction orders associated with the grating-over-grating structure within the cell 106 may be imaged or otherwise observed within the pupil plane. In a general sense, the detector 154 can capture any combination of reflected (or transmitted), scattered, or diffracted light from the sample 110. In some embodiments, the overlay measurement tool 102 includes the detector 154 in the field plane (e.g., a plane conjugate to the sample 110) to generate an image of the sample 110 based on a selected diffraction order from the cell 106.

[0064] The overlay measurement tool 102 can generally include any number or type of detector 154 suitable for capturing light from the sample 110 that shows the overlay. In some embodiments, the detector 154 includes one or more detectors 154 suitable for characterizing a static sample. In this regard, the overlay measurement tool 102 can operate in a static mode where the sample 110 is static during measurement. For example, the detector 154 can include a two-dimensional pixel array such as, but not limited to, a charge-coupled device (CCD) or a complementary metal-oxide semiconductor (CMOS) device. In this regard, the detector 154 can generate a two-dimensional image (e.g., a field plane image or a pupil plane image) in a single measurement.

[0065] 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 measurement tool 102 can operate in a scanning mode where the sample 110 is scanned with respect to the measurement field of view during measurement. For example, the detector 154 can include a 2D pixel array having a capture time and / or a refresh rate sufficient to capture one or more images during scanning within a selected image tolerance (e.g., image blur, contrast, sharpness, etc.). As another example, the detector 154 can include a line scan detector that continuously generates an image of one line of pixels at a time. As another example, the detector 154 can include a time delay integration (TDI) detector. The TDI detector can generate a continuous image of the sample 110 when the movement of the sample 110 is synchronized with a charge transfer clock signal within the TDI detector.

[0066] In some embodiments, the overlay measurement 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. By way of example, FIG. 1B includes beam scanning optics 160 within each illumination channel 120.

[0067] In this specification, it is contemplated that the beam scanning optical system 160 can be utilized in various ways in accordance with one or more embodiments of the present disclosure.

[0068] In some embodiments, the beam scanning optical systems 160 disposed in both illumination channels 120 can scan in a synchronous pattern to scan their respective illumination distributions across the cell 106. This synchronous scanning may advantageously reduce noise associated with imperfections in the grid-over-grid structure and / or reduce the effects of speckle associated with coherent illumination 126. However, it is contemplated herein that such speckle may also be reduced, additionally or alternatively, using other techniques. For example, the coherent illumination 126 can be scanned on the input surface of a multimode fiber, and the output of the multimode fiber is imaged onto the sample 110. Such a multimode fiber may be located either before or within any of the illumination channels 120.

[0069] In some embodiments, the beam scanning optical systems 160 within one or more illumination channels 120 are used to adjust or otherwise control the separation of the associated illumination beam distributions. In this way, the illumination beam distributions from each illumination channel 120 can be centered on different cells 106. Further, this configuration can provide flexibility in measuring targets having any selected cell separation. For example, in FIG. 1B, the beam scanning optical system 160 can be tilted within the plane of the figure to deviate the associated illumination beam distribution on the sample 110 within the plane of the figure.

[0070] It may also be desirable, and is further contemplated herein, for the illumination 126 and / or the collected light 116 to be centered in any field stop to avoid asymmetric diffraction during either illumination or collection. In some embodiments, the overlay measurement system 100 includes one or more adjustable field stops (e.g., a field stop attached to an actuator, etc.), which may or may not be synchronized with other components. For example, FIG. 1B shows adjustable field stops 152 in both the illumination channel 120 and the collection channel 122 that can optionally be synchronized with the beam scanning optics 160. In this way, the illumination 126 and the collected light 116 can remain centered in their respective stops even if the position of the illumination beam distribution on the sample 110 is adjusted.

[0071] Referring again to FIG. 1A, various additional components of the overlay measurement system 100 are described in more detail according to one or more embodiments of the present disclosure.

[0072] In some embodiments, the overlay measurement system 100 includes a controller 162 communicatively coupled to the overlay measurement tool 102 and / or any component 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 in memory. The one or more processors 164 of the controller 162 can include any processing element known in the art. In this sense, the one or more processors 164 can include any microprocessor-type device configured to execute algorithms and / or instructions.

[0073] One or more processors 164 of the controller 162 can include any processor or processing element known in the art. For purposes of the present disclosure, the terms "processor" or "processing element" can be broadly defined to include any device having one or more processing or logic elements (e.g., one or more microprocessor devices, one or more application specific integrated circuit (ASIC) devices, one or more field programmable gate arrays (FPGA), or one or more digital signal processors (DSP)). In this sense, one or more processors 164 can include any device configured to execute algorithms and / or instructions (e.g., program instructions stored in memory). In some embodiments, one or more processors 164 can be embodied as a desktop computer, a mainframe computer system, a workstation, an image computer, a parallel processor, a network-connected computer, or any other computer system configured to execute a program that operates or is configured to operate with the overlay measurement system 100 as described throughout the present disclosure. Further, different subsystems of the overlay measurement system 100 can include processors or logic elements suitable for performing at least some of the steps described in the present disclosure. Accordingly, the foregoing description should not be construed as a limitation on embodiments of the present disclosure, but should be construed as merely illustrative. Further, the steps described throughout the present disclosure can be performed by a single controller or, alternatively, by a plurality of controllers. Further, the controller 162 can include one or more controllers housed within a common housing or a plurality of housings. In this way, any controller or combination of controllers can be separately packaged as a module suitable for integration into the overlay measurement system 100.

[0074] The memory device 166 can include any storage medium known in the art suitable for storing program instructions executable by one or more associated processors 164. For example, the memory device 166 may include a non-transitory memory medium, and as another example, the memory device 166 may include, without limitation, read-only memory (ROM), random access memory (RAM), magnetic or optical memory devices (e.g., disks), magnetic tapes, such as solid state drives. Further, it should be noted that the memory device 166 can be housed within a common controller housing with one or more processors 164. In some embodiments, the memory device 166 may be located remotely with respect to the physical location of one or more processors 164 and the controller 162. For example, one or more processors 164 of the controller 162 can access a remote memory (e.g., a server) accessible via a network (e.g., the Internet, an intranet, etc.).

[0075] The controller 162 can direct or receive data from the overlay measurement tool 102 or any component therein (e.g., via a control signal). The controller 162 can be further configured to perform any of the various process steps described throughout the present disclosure.

[0076] In some embodiments, the overlay measurement 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 desktops, laptops, tablets, and the like. In some embodiments, the user interface 168 includes a display used to display data of the overlay measurement system 100 to the 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. One of ordinary skill in the art should recognize that any display device that can be integrated with the user interface 168 is suitable for implementation in the present disclosure. In some embodiments, the user may input selections and / or commands in response to the data displayed to the user via the user input device of the user interface 168.

[0077] Referring now to FIG. 4, FIG. 4 is a flowchart showing steps performed in a method 400 for overlay measurement 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 the overlay measurement system 100 should be construed as being extended to the method 400. However, it should be further noted that the method 400 is not limited to the architecture of the overlay measurement system 100.

[0078] In some embodiments, method 400 includes step 402 of simultaneously illuminating the first-direction cells of a cell pair on an overlay target with a first illumination beam distribution having a first linear polarization, and simultaneously illuminating the second-direction cells of the cell pair with a second illumination beam distribution having a second linear polarization orthogonal to the first linear polarization. For example, the overlay target may include two or more cell pairs having a grating-over-grating structure oriented in orthogonal directions as shown in FIGS. 2A-2B.

[0079] Step 402 may include illuminating the first-direction cells and the second-direction cells with any number or configuration of illumination beams, and the first illumination beam distribution and the second illumination beam distribution may be controlled separately. In some embodiments, step 404 may include underfilling both the first-direction cells and the second-direction cells with the first and second illumination beam distributions. Further, step 402 can include synchronously scanning the first illumination beam distribution and the second illumination beam distribution across the first-direction cells and the second-direction cells, thereby reducing target noise and / or speckle.

[0080] In some embodiments, method 400 includes step 404 of collecting light (e.g., from the first-direction cells and the second-direction cells) from the sample as collected light. In some embodiments, method 400 includes step 406 of directing a portion of the collected light associated with the first-direction cells of two or more cell pairs to a first collection channel. In some embodiments, method 400 includes step 408 of directing a portion of the collected light associated with the second-direction cells of two or more cell pairs to a second collection channel. In this way, the collected light indicating the overlay of the first-direction cells and the second-direction cells can be collected in parallel. For example, step 406 and / or step 408 can be implemented using any combination of polarization filters, pupil plane filters, or field plane filters, such as those shown in the context of overlay measurement system 100, but not limited thereto.

[0081] In some embodiments, method 400 includes step 410 of generating a first overlay measurement along a first direction based on data associated with first-direction cells of two or more cell pairs (e.g., based on repeating steps 402-408 for two or more cell pairs). In some embodiments, method 400 includes step 412 of generating a second overlay measurement along a second direction based on data associated with second-direction cells of two or more cell pairs.

[0082] It is contemplated herein that method 400 can advantageously provide parallel measurements of target cells with minimal or negligible cross-talk of the collected light. In this way, method 400 can reduce by a factor of two the number of sequential measurements required for a given overlay target as compared to techniques where all cells are measured sequentially. Further, this increase in throughput may not come at the expense of measurement accuracy or sensitivity.

[0083] The subject matter described in this specification, in some cases, illustrates different components that are included within or connected to other components. It should be understood that such depicted architectures are merely exemplary, and in practice, many other architectures that achieve the same functionality can be implemented. In a conceptual sense, any arrangement of components for achieving the same function is effectively "associated" so that the desired function is achieved. Thus, any two components of this specification combined to achieve a particular function can be considered to be "associated" with each other regardless of the architecture or intermediate components, so that the desired function is achieved. Similarly, any two components so associated can also be considered to be "connected" or "coupled" to each other to achieve the desired functionality, and any two components that can be so associated can also be considered to be "couplable" to each other to achieve the desired functionality. Particular examples of couplable include, but are not limited to, components that can physically interact and / or physically interact and / or wirelessly interact and / or wirelessly interact and / or logically interact and / or logically interact.

[0084] It will be appreciated that many of the aspects of the present disclosure and its attendant advantages will be understood from the foregoing description, and that various changes can be made in the form, structure, and arrangement of the components without departing from the disclosed subject matter or sacrificing all of its material advantages. The described form is merely illustrative, and it is intended that the following claims encompass and include such changes. Further, it should be understood that the invention is defined by the appended claims.

Claims

1. An overlay measurement tool, comprising: a light source configured to generate a first illumination beam distribution having a first linearly polarized light and a second illumination beam distribution having a second linearly polarized light orthogonal to the first linearly polarized light; an illumination subsystem configured to sequentially illuminate two or more cell pairs of an overlay target on a sample, wherein a specific one of the two or more cell pairs includes a first-direction cell having a grating-over-grating structure with periodicity along a first direction and a second-direction cell having a grating-over-grating structure with periodicity along a second direction orthogonal to the first direction, and the first-direction cell is simultaneously illuminated with the first illumination beam distribution and the second-direction cell is simultaneously illuminated with the second illumination beam distribution; a collection subsystem, comprising: a first collection channel including one or more first channel detectors on a first channel detection plane; a second collection channel including one or more second channel detectors on a second channel detection plane; an objective lens configured to collect light from the sample as collected light; one or more filtering optical systems configured to direct a portion of the collected light related to the first-direction cell of two or more cell pairs to the first collection channel and a portion of the collected light related to the second-direction cell of two or more cell pairs to the second collection channel; the collection subsystem; a controller communicatively coupled to the first and second collection channels, the controller including one or more processors configured to execute program instructions to: generate a first overlay measurement value along the first direction based on data related to the first-direction cell of two or more cell pairs from the one or more first channel detectors; generate a second overlay measurement value along the second direction based on data related to the second-direction cell of two or more cell pairs from the one or more second channel detectors; an overlay measurement tool.

2. The overlay measurement tool according to claim 1, further comprising one or more beam scanning optical systems configured to adjust a separation between the first and second illumination beam distributions.

3. ​ ​ The one or more beam scanning optical systems adjust a separation between the first illumination beam distribution and the second illumination beam distribution to center the first illumination beam distribution and the second illumination beam distribution on the first direction cells and the second direction cells of the illuminated cells among two or more cell pairs. The overlay measurement tool according to claim 2, characterized in that.

4. A first illumination field stop that defines a spatial range of the first illumination beam distribution, A second illumination field stop that defines a spatial range of the second illumination beam distribution, Further comprising at least one of the first illumination field stop or the second illumination field stop being adjustable such that the first illumination beam distribution and the second illumination beam distribution are centered on the first illumination field stop and the second illumination field stop when the separation between the first illumination beam distribution and the second illumination beam distribution is adjusted and synchronized with the one or more beam scanning optical systems. The overlay measurement tool according to claim 2.

5. A first collection field stop for defining a collection field of view for the first collection channel, A second collection field stop for defining a collection field of view for the second collection channel, Further comprising at least one of the first collection field stop or the second collection field stop being adjustable such that the first collection field of view and the second collection field of view are aligned with the first illumination beam distribution and the second illumination beam distribution when the separation between the first illumination beam distribution and the second illumination beam distribution is adjusted and synchronized with the one or more beam scanning optical systems. The overlay measurement tool according to claim 2.

6. One or more beam scanning optical systems configured to modulate the first and second illumination beam distributions across the first direction cells and the second direction cells of the illuminated cells among two or more cell pairs during measurement. The overlay measurement tool according to claim 1, further comprising.

7. The lattice-over-lattice structure in the first direction cells of the two or more cell pairs is arranged along the first row of the overlay target, and the lattice-over-lattice structure in the second direction cells of the two or more cell pairs is arranged along the second row of the overlay target. The overlay measurement tool according to claim 1, characterized in that.

8. The grid-over-grid structure in the first-direction cells of the two or more cell pairs has different intended overlay offsets along the first direction, and the grid-over-grid structure in the second-direction cells of the two or more cell pairs has different intended overlay offsets along the second direction. The overlay measurement tool according to claim 1, characterized in that

9. The two or more cell pairs further include a first cell pair and a second cell pair. The first-direction cells of the first cell pair and the first-direction cells of the second cell pair have equal and opposite intended offsets along the first direction, and the second-direction cells of the first cell pair and the second-direction cells of the second cell pair have equal and opposite intended offsets along the second direction. The overlay measurement tool according to claim 1, characterized in that

10. The second illumination beam distribution is equal to the first illumination beam distribution. The overlay measurement tool according to claim 1, characterized in that

11. The second illumination beam distribution is different from the first illumination beam distribution. The overlay measurement tool according to claim 1, characterized in that

12. The second illumination beam distribution is different from the first illumination beam distribution based on at least one of a plurality of illumination beams, the wavelength of one or more illumination beams, or the incident angle of one or more illumination beams. The overlay measurement tool according to claim 1, characterized in that

13. At least one of the first or the second illumination beam distribution comprises a single illumination beam . The overlay measurement tool according to claim 1, characterized in that

14. The first illumination beam distribution comprises two illumination beams of a dipole distribution aligned along the first direction , and the second illumination beam distribution comprises two illumination beams of a dipole distribution aligned along the second direction . The overlay measurement tool according to claim 1, characterized in that

15. The one or more filtering optical systems comprise at least one of one or more polarization filtering optical systems, one or more pupil plane filters, or one or more field plane filters. The overlay measurement tool according to claim 1, characterized in that

16. The one or more polarization filtering optical systems ​ One or more polarization beam splitters that direct the portion of the collected light having the first linearly polarized light toward the first collection channel and the portion of the collected light having the second linearly polarized light toward the second collection channel. The overlay measurement tool according to claim 15, comprising the above.

17. The one or more polarization filtering optical systems At least one of a first polarizer in the first collection channel oriented to pass the first linearly polarized light or a second polarizer in the second collection channel oriented to pass the second linearly polarized light. The overlay measurement tool according to claim 15, comprising the above.

18. The one or more pupil plane filters A first aperture in the pupil plane of the first collection channel that passes the portion of the collected light related to diffraction along the first direction, a second aperture in the pupil plane of the second collection channel that passes the portion of the collected light related to diffraction along the second direction, or a beam splitter in a common pupil plane of the first and second collection channels that directs the portion of the collected light related to diffraction toward the first collection channel along the first direction and the portion of the collected light related to diffraction toward the second collection channel along the second direction. At least one of them. The overlay measurement tool according to claim 15, comprising the above.

19. The one or more field plane filters At least one of an aperture in the field plane of the first collection channel for passing the portion of the collected light related to the first direction cell or an aperture in the field plane of the second collection channel for passing the portion of the collected light related to the second direction cell. The overlay measurement tool according to claim 15, comprising the above.

20. A translation stage that sequentially arranges two or more cell pairs for illumination by an illumination subsystem. The overlay measurement tool according to claim 1, further comprising the above.

21. One or more focus control optical systems for adjusting at least one focal position of the first or second illumination beam distribution. The overlay measurement tool according to claim 1, further comprising the above.

22. At least one of the first or second detection planes Pupil plane The overlay measurement tool according to claim 1, comprising the above.

23. At least one of the first or second detection planes is a field plane, The overlay measurement tool according to claim 1, characterized in that it comprises

24. An overlay measurement tool, An illumination source configured to generate a first illumination beam distribution having a first linearly polarized light and a second illumination beam distribution having a second linearly polarized light orthogonal to the first linearly polarized light, An illumination subsystem for sequentially illuminating two or more cell pairs of an overlay target on a sample, wherein a specific one of the two or more cell pairs has a lattice-over-lattice structure having periodicity along a first direction A first-direction cell and a second-direction cell having a lattice-over-lattice structure having periodicity along a second direction orthogonal to the first direction, an illumination subsystem, comprising The illumination subsystem is A first illumination channel that directs the first illumination beam distribution to a first-direction cell of one of the two or more cell pairs, wherein the first illumination beam distribution includes one or more first illumination beams having the first linearly polarized light, the first illumination channel, A second illumination channel that directs the second illumination beam distribution to a second-direction cell of one of the two or more cell pairs simultaneously with the first illumination beam distribution, wherein the second illumination beam distribution includes one or more second illumination beams having the second linearly polarized light, the second illumination channel, comprising A collection subsystem, A first collection channel including one or more first channel detectors on a first channel detection plane, A second collection channel including one or more second channel detectors on a second channel detection plane, An objective lens that condenses the light from the sample as collected light, A polarization beam splitter that directs the portion of the collected light related to the first-direction cell having the first linearly polarized light to the first collection channel and directs the portion of the collected light related to the second-direction cells of two or more cell pairs having the second linearly polarized light to the second collection channel, A collection subsystem comprising A controller communicably coupled to the first and second collection channels, Generating a first overlay measurement value along the first direction based on the data related to the first-direction cells of the two or more cell pairs from the one or more first channel detectors, Generating a second overlay measurement value along the second direction based on data related to the second direction cells of the two or more cell pairs from the one or more second channel detectors; A controller including one or more processors configured to execute program instructions to cause the execution; An overlay measurement tool comprising the same. **Claim 25** The overlay measurement tool according to claim 24, further comprising at least one of: a first aperture within the pupil plane of the first collection channel that passes a portion of the collected light related to diffraction along the first direction; a second aperture within the pupil plane of the second collection channel that passes a portion of the collected light related to diffraction along the second direction; or a beam splitter within a common pupil plane of the first and second collection channels that directs a portion of the collected light related to diffraction along the first direction to the first collection channel and a portion of the collected light related to diffraction along the second direction to the second collection channel. **Claim 26** The overlay measurement tool according to claim 24, further comprising at least one of: an aperture within the field plane of the first collection channel for passing a portion of the collected light related to the first direction cell; or an aperture within the field plane of the second collection channel for passing a portion of the collected light related to the second direction cell. **Claim 27** One or more beam scanning optical systems configured to adjust the separation between the first and second illumination beam distributions; The overlay measurement tool according to claim 24, further comprising the same. **Claim 28** The overlay measurement tool according to claim 27, wherein the one or more beam scanning optical systems adjust the separation between the first and second illumination beam distributions to center the first and second illumination beam distributions on the first direction cell and the second direction cell of the illuminated cells among the two or more cell pairs. **Claim 29** A first illumination field stop that defines the spatial extent of the first illumination beam distribution; A second illumination field stop that defines the spatial extent of the second illumination beam distribution; further comprising, at least one of the first illumination field stop or the second illumination field stop being adjustable such that the first illumination beam distribution and the second illumination beam distribution are centered on the first illumination field stop and the second illumination field stop when the separation between the first illumination beam distribution and the second illumination beam distribution is adjusted, and being synchronized with the one or more beam scanning optical systems The overlay measurement tool according to claim 27.

30. a first collection field stop defining a collection field for the first collection channel a second collection field stop defining a collection field for the second collection channel further comprising, at least one of the first collection field stop or the second collection field stop being adjustable such that the first collection field and the second collection field are aligned with the first illumination beam distribution and the second illumination beam distribution when the separation between the first illumination beam distribution and the second illumination beam distribution is adjusted, and being synchronized with the one or more beam scanning optical systems The overlay measurement tool according to claim 27.

31. An overlay measurement method, comprising generating a first illumination beam distribution having a first linearly polarized light and a second illumination beam distribution having a second linearly polarized light orthogonal to the first linearly polarized light sequentially illuminating two or more cell pairs of an overlay target on a sample using an illumination system, each of the two or more cell pairs including a first-direction cell having a grating-over-grating structure having periodicity along a first direction and a second-direction cell having a grating-over-grating structure having periodicity along a second direction orthogonal to the first direction, the illumination system simultaneously illuminating the first-direction cell with the first illumination beam distribution and the second-direction cell with the second illumination beam distribution collecting light from the sample as collected light directing a portion of the collected light related to the first-direction cell of the two or more cell pairs to a first collection channel directing a portion of the collected light related to the second-direction cell of the two or more cell pairs to a second collection channel generating a first overlay measurement value along the first direction based on data related to the first-direction cell of the two or more cell pairs generating a second overlay measurement value along the second direction based on data associated with the second direction cells of the two or more cell pairs; An overlay measurement method comprising the above.

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