Parallel scattering system overlay measurement

Abstract

An overlay measurement tool may include 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, an illumination subsystem for sequentially illuminating two or more pairs of cells of an overlay target on a sample having an orthogonally oriented grating-over-gripping structure, a collection subsystem having two collection channels for capturing light collected from the illuminated pairs of cells, and a filtering optical system for directing light from different cells of the illuminated pairs of cells to different collection channels for detection. The tool may further include a controller for generating individual overlay measurements for the orthogonally oriented grating-over-gripping structure in two or more pairs of cells.

Description

Technology Field

[0001] The present disclosure generally relates to overlay metrology, and more specifically to scatterometry-based overlay metrology. Background Technology

[0002] Overlay metrology generally refers to measuring the relative alignment of layers on a sample, for example, in semiconductor devices (but not limited thereto). Overlay metrology, or the measurement of overlay error, generally 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 operation of a device. There is a growing demand for accurate and efficient overlay metrology driven by the need to reduce feature size and increase feature density. Many existing overlay metrology techniques require separate measurement steps for multiple cells of an overlay target to generate overlay data for a specific orientation, and require an additional set of measurement steps to generate overlay data for additional orientations. However, the time required to perform these measurement steps can limit the throughput of the overlay metrology system. Therefore, there is a need to develop systems and methods to overcome these limitations.

[0003] An overlay measurement tool according to one or more exemplary embodiments of the present disclosure is disclosed. In one exemplary embodiment, the tool comprises a light source for generating a first light beam distribution having a first linear polarization and a second light beam distribution having a second linear polarization orthogonal to the first linear polarization. In another exemplary embodiment, the tool comprises a light subsystem for sequentially illuminating two or more pairs of cells of an overlay target on a sample. Among the two or more pairs of cells, a specific pair of cells may include 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. In another exemplary embodiment, the light subsystem illuminates the first-direction cell with the first light beam distribution and simultaneously illuminates the second-direction cell with the second light beam distribution. In another exemplary embodiment, the tool comprises a collection subsystem, wherein the collection subsystem comprises a first collection channel comprising one or more first channel detectors in a first channel detection plane, a second collection channel comprising one or more second channel detectors in a second channel detection plane, an objective lens for collecting light from a sample as collected light, and one or more filtering optical systems for directing a portion of the collected light associated with a first direction cell of two or more cell pairs to the first collection channel and directing a portion of the collected light associated with a second direction cell of two or more cell pairs to the second collection channel.In another exemplary embodiment, the tool includes a controller for generating a first overlay measurement according to 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 generating a second overlay measurement according to a second direction based on data associated with a second direction cell of two or more cell pairs from one or more second channel detectors.

[0004] An overlay measurement tool according to one or more exemplary embodiments of the present disclosure is disclosed. In one exemplary embodiment, the tool comprises a light 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 exemplary embodiment, the tool comprises a light subsystem for sequentially illuminating two or more pairs of cells of an overlay target on a sample. Among the two or more pairs of cells, a particular pair of cells may comprise a first direction cell having a grid-over-grid structure having periodicity along a first direction and a second direction cell having a grid-over-grid structure having periodicity along a second direction orthogonal to the first direction. In another exemplary embodiment, the light system comprises a first light channel for directing the first illumination beam distribution to the first direction cell of one of the two or more pairs of cells, wherein the first illumination beam distribution comprises one or more first illumination beams having a first linear polarization. In another exemplary embodiment, the illumination subsystem further comprises a second illumination channel for directing a second illumination beam distribution simultaneously with a first illumination beam distribution to a second direction cell of one of two or more cell pairs, wherein the second illumination beam distribution comprises one or more second illumination beams having a first linear polarization. In another exemplary embodiment, the tool comprises a collection subsystem, wherein the collection subsystem comprises a first collection channel comprising one or more first channel detectors in a first channel detection plane, a second collection channel comprising one or more second channel detectors in a second channel detection plane, an objective lens for collecting light from a sample as collected light, and a polarization beam splitter for directing a portion of the collected light associated with a first direction cell having a first linear polarization to the first collection channel and directing 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 for generating a first overlay measurement according to 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 generating a second overlay measurement according to a second direction based on data associated with a second direction cell of two or more cell pairs from one or more second channel detectors.

[0005] An overlay measurement method according to one or more exemplary embodiments of the present disclosure is disclosed. In one exemplary embodiment, the method comprises the step of 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 exemplary embodiment, the method comprises the step of sequentially illuminating two or more pairs of cells of an overlay target on a sample, wherein each of the two or more pairs of cells comprises a first direction cell having a grating-over-greasing structure having periodicity along a first direction and a second direction cell having a grating-over-greasing structure having periodicity along a second direction orthogonal to the first direction, and the illumination subsystem illuminates the first direction cell with the first illumination beam distribution while simultaneously illuminating the second direction cell with the second illumination beam distribution. In another exemplary embodiment, the method comprises the step of collecting light from a sample as collected light. In another exemplary embodiment, the method comprises the step of directing a portion of the collected light associated with the first direction cell of the two or more pairs of cells to a first collection channel. In another exemplary embodiment, the method includes the step of directing a portion of the collected light associated with a second direction cell of two or more cell pairs to a second collection channel. In another exemplary embodiment, the method includes the step of generating a first overlay measurement according to a first direction based on data associated with a first direction cell of two or more cell pairs. In another exemplary embodiment, the method includes the step of generating a second overlay measurement according to a second direction based on data associated with a second direction cell of two or more cell pairs.

[0006] It should be understood that the foregoing general description and the following detailed description are all illustrative and for illustrative purposes only, and are not to limit the invention as claimed. The accompanying drawings, incorporated herein and constituting part of the specification, serve to illustrate embodiments of the invention and, together with the general description, explain the principles of the invention. Brief explanation of the drawing

[0007] Many of the advantages of the present disclosure may be better understood by those skilled in the art by referring to the accompanying drawings. FIG. 1a is a block diagram of an overlay measurement system according to one or more embodiments of the present disclosure. FIG. 1b is a schematic diagram of an overlay measurement tool illustrating two illumination channels and two collection channels according to one or more embodiments of the present disclosure. FIG. 1c is a schematic diagram of a part of an overlay measurement tool illustrating two lighting channels having individually configurable lighting conditions based on a linearly varying filter according to one or more embodiments of the present disclosure. FIG. 2a is a side view of a grid-over-grid structure in a single cell of an overlay target according to one or more embodiments of the present disclosure. FIG. 2b is a plan view of an overlay target according to one or more embodiments of the present disclosure. FIG. 3a is a plan view of a pupil plane illustrating the diffraction order distribution from the X-direction cell of the overlay target of FIG. 2b according to one or more embodiments of the present disclosure. FIG. 3b is a plan view of a pupil plane illustrating the diffraction order distribution from the Y-direction cell of the overlay target of FIG. 2b according to one or more embodiments of the present disclosure. FIG. 4 is a flowchart illustrating steps performed in a method for overlay measurement according to one or more embodiments of the present disclosure. Specific details for implementing the invention

[0008] Now, detailed reference will be made to the disclosed subject matter illustrated in the attached drawings. The present disclosure has been specifically illustrated and described in relation to specific embodiments and specific features thereof. The embodiments described herein are to be considered illustrative rather than restrictive. It will be apparent to those skilled in the art that various changes and modifications to the form and details may be made without departing from the spirit and scope of the present disclosure.

[0009] Embodiments of the present disclosure relate to a system and method for generating parallel measurements of cells of an overlay target using scatterometry techniques. In some embodiments, two cells of an overlay target associated with overlay measurements according to two different (e.g., orthogonal) directions are simultaneously illuminated, light from the two illuminated cells is simultaneously collected, and the collected light associated with the illuminated cells is directed to individual detection paths. In this way, measurements of multiple cells of an overlay target can be performed in parallel, which can advantageously increase measurement throughput compared to sequential measurements of each cell on the target. Further embodiments relate to a multi-channel scatterometry overlay tool suitable for simultaneously illuminating multiple cells of an overlay target and separating light from multiple cells into individual detection channels.

[0010] For the purposes of this disclosure, the term overlay is generally used to describe the relative position of features on a sample produced by two or more lithographic patterning steps, wherein the term overlay error indicates a deviation of the features from the nominal alignment. 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 strictly controlled to ensure proper performance of the resulting device. Accordingly, overlay measurements can characterize the relative position of features on two or more sample layers. As another example, multiple lithographic steps may be used to produce features on a single sample layer. This technique, generally referred to as dual patterning or multi-patterning techniques, can enable the production of high-density features close to the resolution of the lithographic system. In this context, overlay measurements can characterize the relative position of features from various lithographic steps on this single layer. It should be understood that examples and illustrations throughout this disclosure relating to specific applications of overlay metrology are provided for illustrative purposes only and should not be construed as limiting this disclosure.

[0011] In some applications, overlay measurements may be performed directly on features of the manufactured device (e.g., device features), but overlay measurements are typically performed on a dedicated overlay target printed using the same lithography step as the device features. In this way, features of the overlay target (e.g., target features) can be specifically designed to enable overlay measurements. Additionally, the overlay measured in one manufacturing step (e.g., after manufacturing one or more sample layers) can be used to create a modifiable item for precisely aligning process tools (e.g., lithography tools, etc.) for manufacturing additional sample layers in a subsequent manufacturing step.

[0012] In some embodiments, an overlay target suitable for scatterometer measurements as disclosed herein may comprise one or more cells having a grating-over-gripping structure, wherein the grating-over-gripping structure comprises periodic features (e.g., grating features) on the overlapping region of two or more layers of interest. In this way, various grating features of the layers of interest may contribute to the diffraction of incident illumination, and an overlay measurement may 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., associated with the relative intensity difference between selected diffraction orders in the pupil plane). As another example, the overlay measurement may be generated based on field plane data (e.g., associated with the relative intensity of a target cell image generated by a selected diffraction order).

[0013] As used throughout this disclosure, the term “sample” generally refers to a substrate formed of a semiconductor or non-semiconductor material (e.g., a wafer, etc.). For example, semiconductor or non-semiconductor materials may include, but are 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, resists, dielectric materials, conductive materials, and semiconducting materials. Many different types of such layers are known in the art, and as used herein, the term “sample” is intended to encompass a sample on which all types of such layers may 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 having repeatable patterned features. A device ultimately completed by the formation and processing of these material layers may be produced. Many different types of such devices may be formed on a sample, and as used herein, the term "sample" is intended to encompass a sample on which any type of device known in the art is manufactured. Additionally, for the purposes of this disclosure, the terms sample and wafer should be interpreted as interchangeable. Additionally, for the purposes of this disclosure, the terms patterning device, mask, and reticle should be interpreted as interchangeable.

[0014] In some embodiments, the overlay measurement tool simultaneously illuminates two cells of an overlay target with two illumination beams, wherein the two cells comprise a grating-over-grid structure in which the direction of the grating in one cell is orthogonal to the direction of the grating in the other cell. For example, the overlay measurement tool may illuminate an X-direction cell comprising a grating-over-grid structure having periodicity along the X direction with a first illumination beam, while simultaneously illuminating a Y-direction cell comprising a grating-over-grid structure having periodicity along the Y direction with a second illumination beam. It should be understood that the descriptions of X and Y directions are used herein merely for illustrative purposes to refer to any orthogonal direction on the sample. Additionally, the two cells may be illuminated with light having orthogonal linear polarization (e.g., one cell is illuminated with light linearly polarized along the X direction, and one cell is illuminated with light linearly polarized 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., illumination beam distribution), wherein the illumination beam distribution on each cell is smaller than the cell and the cell is underfilled. Additionally, various aspects of the illumination light on each cell, such as but not limited to spectrum (e.g., spectral bandwidth and / or center wavelength), intensity, illumination angle, illumination angle distribution, or focal position, can be individually controlled or adjusted.

[0015] In some embodiments, the overlay measurement tool simultaneously separates light collected from an illuminated cell into individual detection channels for each cell. In this way, measurement data associated with the illuminated cell can be separated from one another despite simultaneous collection. Within the spirit and scope of the present disclosure, various techniques may be used to separate the collected light into individual detection channels. For example, light collected from an illuminated cell may be separated based on parameters such, but not limited to, polarization, a pupil aperture filter (e.g., a spatial filter in the pupil plane to pass a selected diffraction order), or a field stop filter (e.g., a spatial filter in the field plane to pass light from a selected cell).

[0016] In some embodiments, the overlay metrology tool includes beam control optics in the illumination and / or collection path for various aspects of the illumination light or collected light. For example, the overlay metrology tool may include one or more scan mirrors for scanning the illumination beam across each cell during measurement, which can reduce noise associated with target defects. As another example, the overlay metrology tool may include beam control optics for controlling or adjusting the separation between two illumination beams, which can be used to match the illumination beam separation to the layout of the cells of a specific overlay target. Additionally, the overlay metrology tool may adjust the illumination and / or collection field aperture based on the selected illumination beam separation.

[0017] Further embodiments of the present disclosure relate to an overlay target suitable for parallel measurement by an overlay measurement tool. In some embodiments, the overlay target comprises X-direction cells distributed along a first row and Y-direction cells distributed along a second row. In this way, cells associated with a specific direction can be sequentially illuminated under the same or similar illumination conditions (e.g., polarization, spectrum, intensity, illumination angle distribution, 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 using different intended offsets of a grid-over-grid structure. Thus, by arranging the cells in a common direction of periodicity along the row, the burden on the overlay measurement tool can be reduced so that the illumination conditions are the same or substantially the same as when the target is scanned along the row direction.

[0018] Now, referring to FIGS. 1a through 4, a system and method for generating parallel measurements of cells of an overlay target using scatterometer technology are disclosed.

[0019] FIG. 1a is a block diagram of an overlay measurement system according to one or more embodiments of the present disclosure.

[0020] In some embodiments, the overlay measurement system (100) comprises an overlay measurement tool (102) having a dual-channel illumination subsystem (104) for individually illuminating two cells (106) of an overlay target (108) on a sample (110) with a spatially separated distribution (e.g., illumination beam distribution) of one or more illumination beams (112) having orthogonal linear polarization, and a dual-channel collection subsystem (114) for individually detecting light or other radiation (e.g., collected light (116)) coming from the sample (110) associated with the two cells (106).

[0021] In some embodiments, the sample (110) is placed on a sample stage (118) suitable for fixing the sample (110) and is further configured to position the sample (110) relative to an overlay measuring tool (102). For example, the sample stage (118) may include any combination of linear, rotary, or angular (e.g., tip / tilt) actuators suitable for positioning the sample (110) in any selected direction.

[0022] The overlay measurement tool (102) may be any type of overlay measurement tool known in the technical field 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) may collect pupil plane data (e.g., one or more pupil plane images or parts thereof) for analyzing light (116) collected in the pupil plane to characterize the angular distribution of radiation from the cell (106) (e.g., associated with scattering and / or diffraction of radiation by the cell (106)). As another example, the overlay measurement tool (102) may collect field plane data (e.g., one or more field plane images or parts thereof) based on a selected diffraction order from the cell (106). Additionally, the overlay measurement tool (102) can characterize the cell (106) while the sample (110) is stationary (e.g., while in static or moving and measuring (MAM) mode) or while the sample (110) is moving (e.g., while in scanning mode).

[0023] Now, referring to FIGS. 2a and 2b, various configurations of an overlay target (108) suitable for parallel characterization of a configuration cell (106) according to one or more embodiments of the present disclosure are described in more detail.

[0024] FIG. 2a is a side view of a grating-over-grating structure in a single cell (106) of an overlay target (108) according to one or more embodiments of the present disclosure. In some embodiments, the grating-over-grating structure in each cell (106) comprises a first layer grating feature (202) located in a first layer (204) of a sample (110) and a second layer grating feature (206) located in a second layer (208) of a sample (110), wherein these features are oriented such that regions comprising the first layer grating feature (202) and the second layer grating feature (206) overlap. In this manner, the first layer grating feature (202) and the second layer grating feature (206) can each diffract an incident illumination beam (112) by an individual diffraction order. Generally, any number of additional layers not shown may be located above, below, or between the first layer (204), the second layer (208), and the substrate (210).

[0025] 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 so that the associated diffraction order overlaps in the pupil plane. In some embodiments, the first layer grating feature (202) and the second layer grating feature (206) have different pitches along a specific direction, which may result in a Moire fringe associated with a Moire pitch greater than the pitch of the first layer grating feature (202) and the second layer grating feature (206). Overlay metrology techniques utilizing the moiré effect are generally described in U.S. Patent No. 9,182,219, registered November 10, 2015; U.S. Patent No. 7,440,105, registered October 21, 2008; U.S. Patent No. 7,349,105, registered March 25, 2008; U.S. Patent No. 10,551,749, registered February 4, 2020; U.S. Patent Publication No. 2021 / 0072650, published March 11, 2021; U.S. Patent Application No. 16 / 935,117, filed July 21, 2020; and U.S. Patent Application No. 16 / 931,078, filed July 16, 2020, all of which are incorporated herein by reference in their entirety.

[0026] FIG. 2b is a plan 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 specific measurement direction may include a grid-over-grid structure having different intended offsets (212).

[0027] FIG. 2b illustrates an overlay target (108) having two cells (106) having periodicity along the X direction (e.g., X direction cells (106)) for overlay measurement along the X direction and two cells (106) having periodicity along the Y direction (e.g., Y direction cells (106)) for overlay measurement along the Y direction. The various cells (106) can generally be arranged in any suitable distribution. In some embodiments, as illustrated in FIG. 2b, cells (106) having the same periodicity direction are distributed in rows. For example, FIG. 2b illustrates X direction cells (106) along a first row (214) and Y direction cells along a second row (216). As described in more detail below, this configuration can enable efficient sequential illumination of cells having the same periodicity direction under the same or similar lighting conditions.

[0028] However, it should be understood that in FIGS. 2a and 2b and the associated description, the overlay target (108) is provided for exemplary purposes only and should not be interpreted as limiting. Rather, the overlay target (108) may include any suitable grid-over-grid overlay target design. For example, the overlay target (108) may generally include any number of cells (106) suitable for measurements along two directions. Additionally, the cells (106) may be distributed in any pattern or arrangement. For example, a measurement target design suitable for scanning measurements is generally described in U.S. Patent Application No. 16 / 598,146 filed on October 10, 2019, the entirety of which is incorporated herein by reference.

[0029] Now, generally referring to FIGS. 1a to 3b, the parallel characterization of a plurality of cells (106) of an overlay target (108) according to one or more embodiments of the present disclosure is described in more detail.

[0030] FIG. 1b is a schematic diagram of an overlay measurement tool (102) illustrating 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) illustrated in FIG. 2a and FIG. 2b. In some embodiments, the overlay measurement tool (102) simultaneously characterizes two cells (106) in orthogonal directions of periodicity (e.g., one X-direction cell (106) and one Y-direction cell (106)).

[0031] In some embodiments, the overlay measurement tool (102) includes at least one light source (124) configured to generate light (126) suitable for forming a light beam distribution to be directed toward a cell (106).

[0032] The light (126) from the light source (124) may include one or more selected wavelengths of light, including but not limited to ultraviolet (UV) radiation, visible light, or infrared (IR) radiation.

[0033] The light source (124) may include any type of light source known in the art. In some embodiments, the light source (124) is a laser source. For example, the light source (124) may include, but is not limited to, one or more narrowband laser sources, broadband laser sources, supercontinuous laser sources, white light laser sources, etc. In this regard, the light source (124) may provide light having high coherence (e.g., high spatial coherence and / or temporal coherence). In some embodiments, the light source (124) includes a laser-sustained plasma (LSP) source. For example, the light source (124) may include, but is not limited to, an LSP lamp, an LSP bulb, or an LSP chamber suitable for including one or more elements capable of emitting broadband light when excited to a plasma state by a laser source. In some embodiments, the light source (124) includes a lamp source. For example, the light source (124) may include, but is not limited to, an arc lamp, a discharge lamp, an electrodeless lamp, etc. In this regard, the light source (124) may provide light having low coherence (e.g., low spatial coherence and / or temporal coherence).

[0034] Each lighting channel (120) can direct a distribution of one or more lighting beams (112) (e.g., lighting beam distribution) to a specific location on the sample (110). The lighting beam distribution may generally include one or more lighting beams (112) or lighting lobes directed to a specific cell (106) with a selected distribution of lighting parameters such as, but not limited to, an angle of incidence (e.g., azimuth and polar angle of incidence), a spectrum, or polarization. For example, the lighting beam distribution may include, but is not limited to, a single lighting beam (112) at a selected angle of incidence, a dipole distribution of the lighting beam (112), or a quadrupole distribution of the lighting beam (112). In this way, the lighting channel (120) can simultaneously provide different lighting conditions for different cells (106) on the overlay target (108). Additionally, if the illumination beam distribution includes a plurality of illumination beams (112), these illumination beams (112) may be provided simultaneously or sequentially in a given measurement of the cell (106).

[0035] In some embodiments, the lighting channel (120) provides a lighting beam (112) having orthogonal linear polarization. For example, one lighting channel (120) may provide a distribution of one or more lighting beams (112) having linear polarization along a first direction (e.g., X direction), and another lighting channel (120) may provide a distribution of one or more lighting beams (112) having linear polarization along a second direction (e.g., Y direction) orthogonal to the first direction. It is expected that providing orthogonal linear polarization to the illumination channel (120) in the present invention may have various advantages, such as, but not limited to, efficiently utilizing available power from the illumination source (124), providing well-controlled diffraction by the grating-over-greasing structure in each cell (106), and / or enabling the separation of the collected light (146) of the illuminated cell (106) into different collection channels (122) with a high extinction ratio to prevent mutual contamination of signals.

[0036] Additionally, various cells (106) of the overlay target (108) may be illuminated in any selected polarization direction. For example, the overlay measurement tool (102) may illuminate the X-direction cells (106) with light linearly polarized along the X direction and the Y-direction cells (106) with light linearly polarized along the Y direction, or vice versa.

[0037] The overlay measurement tool (102) may generally include any combination of optical components for simultaneously generating a plurality of illumination beam distributions.

[0038] In some embodiments, the overlay measurement tool (102) may include a single light source (124) for generating light (126) for each light channel (120). For example, the overlay measurement tool (102) may include a polarizing beam splitter that splits the light (126) into orthogonal linear polarizations directed toward different light channels (120). In some embodiments, the overlay measurement tool (102) includes individual light sources (124) for one or more light channels (120).

[0039] The overlay measurement tool (102) may further include various optical components for generating a 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 optical systems (130) for controlling various illumination parameters. For example, the illumination control optical system (130) may include, but is not limited to, one or more field apertures or filters, one or more pupil apertures 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, translational mirrors, scanning mirrors, etc.). As an example, each lighting channel (120) may include an aperture for creating a distribution of one or more lighting beams (112), and / or a field aperture for limiting the spatial range of the lighting beams (112) within the channel to an area smaller than or equal to the cell (106) so that the cell is underfilled with lighting.

[0040] In a general sense, each lighting channel (120) may provide independent control over the lighting parameters of one or more lighting beams (112). In some embodiments, although not explicitly shown in FIG. 1b, the overlay measurement tool (102) may include a lighting lens (128) and / or a lighting control optical system (130) for manipulating the lighting (126) before entering the lighting channel (120). In this way, a selected mode of lighting beam distribution may be consistent between lighting channels (120).

[0041] Additionally, the lighting parameters of each lighting channel (120) may be static or adjustable. Adjustable lighting parameters may be provided using any technique known in the art. The creation of adjustable lighting parameters is generally described in U.S. Patent No. 10,371,626, registered August 6, 2019, and U.S. Patent Application No. 17 / 076,312, filed October 21, 2020, both of which are incorporated herein by reference in their entirety.

[0042] In some embodiments, adjustable lighting parameters are provided by selectively directing the light (126) to one of a plurality of available light paths having one or more static filters (e.g., spectral filters, neutral density filters, etc.). In some embodiments, adjustable lighting parameters are provided by adjusting the position of one or more adjustable filters (e.g., spectral filters, neutral density filters, etc.) in the lighting channel (120). In some embodiments, adjustable lighting parameters are provided by selectively controlling the position of the light (126) from the light source (124) on one or more spatially varying filters (e.g., spectral filters, neutral density filters, etc.).

[0043] FIG. 1c is a schematic diagram of a portion of an overlay measurement tool (102) illustrating two lighting channels having individually configurable lighting conditions based on a linearly varying filter according to one or more embodiments of the present disclosure. The linearly varying filter may include a filter having filtering characteristics that vary along a linear filtering direction. For example, a linearly varying neutral density filter may provide varying amounts of broadband intensity reduction based on the spatial position of the input beam along a linear axis. As another example, a linearly varying low-pass (or high-pass) filter may provide low-pass filtering having a cutoff wavelength that varies based on the spatial position of the input beam along a linear filtering direction. As another example, the linearly varying filter may be formed as a polarizer, wherein the polarization direction passed by the linearly varying filter may differ in different directions along the linear filtering direction. It is expected that the system and method disclosed herein may utilize a linearly varying filter that modifies any selected characteristic of the input beam.

[0044] In some embodiments, the overlay measurement tool (102) includes a light source (124) having a spectral bandwidth covering a range of wavelengths of interest, and a polarizer (132) for splitting light (126) from the light source (124) along two paths associated with two light channels (120). For example, the polarizer (132) may be a polarizing beam splitter for providing orthogonal linear polarization to the two light channels (120).

[0045] Each lighting channel (120) may include at least one adjustable filter (134) that provides adjustable control of one or more lighting parameters. In some embodiments, the adjustable filter (134) includes a pair of focusing optical systems (136) in a 4-f configuration, a linearly varying filter (138) located in the pupil plane (e.g., a focal plane common to the focusing optical system (136), and an angle scanner (140) located in the other focal plane of the focusing optical system (136) as an input and / or output coupler to the adjustable filter (134). In this way, the position of the lighting (126) on the linearly varying filter (138), and the resulting filtering effect on the lighting (126) can be controlled by adjusting the angle of the input angle scanner (140) for receiving the lighting (126). Additionally, 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 adjustable filter (134), and this output adjustable filter (134) can direct the filtered illumination (126) along any desired optical path. For example, the input and output angle scanners (140) can be simultaneously adjusted to provide adjustable filtering without modifying the output beam path and, accordingly, without affecting the alignment of additional optical systems of the overlay measurement system (100).

[0046] Additionally, a plurality of adjustable filters (134) may be arranged in series to provide adjustment of a plurality of lighting parameters. For example, FIG. 1c illustrates three adjustable filters (134) in each lighting channel (120). This configuration may be suitable for adjusting the intensity and spectrum of the lighting (126) within each lighting channel (120), but is not limited thereto. 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.

[0047] In some embodiments, the overlay measurement tool (102) includes one or more focus control optical systems (142) for adjusting or controlling the focus position of one or more illumination beams (112) in one or more illumination channels (120). It is expected that, as provided herein, adjusting the focus position may be particularly useful for measurements using different wavelengths or for measuring features of different depths, but is not limited thereto. For example, the illumination beams (112) of different illumination channels (120) may have different spectral ranges. As another example, measurement robustness may be enhanced by capturing data from each cell (106) at a number of different wavelengths. In either case, chromatic aberration of the sample (110) and / or the overlay measurement system (100) may result in different focusing or imaging conditions at different wavelengths, and accordingly, focus correction in one or more illumination channels (120) may be required.

[0048] As an example, FIG. 1b includes a focus control optical system (142) in each illumination channel (120) to provide independent focus control for an associated illumination beam (112). The focus control optical system (142) may include, but is not limited to, any type or combination of optical elements suitable for modifying the focus position of at least one illumination beam (112), such as one or more deformable mirrors, one or more acousto-optic lenses, one or more voice coils, or one or more translational lenses (e.g., any illumination lens (128)). In some embodiments, the focus control optical system (142) may be designed to provide focus switching within the time required to adjust one or more additional illumination parameters (e.g., spectrum, intensity, illumination angle distribution, etc.) to facilitate high measurement throughput. In this way, the focus control optical system (142) may operate as a fast focus controller.

[0049] Referring again to FIGS. 1a through 1c, the overlay measurement tool (102) can generate or control an illumination beam distribution (e.g., the number and angle of incidence of the configuration illumination beams (112)) using any technique known in the art.

[0050] In some embodiments, the overlay measurement system (100) includes one or more openings (e.g., a light control optical system (130)) in the light pupil plane to define one or more light beams (112). In some embodiments, the overlay measurement system (100) generates a light beam (112) by providing light to two or more optical fibers, wherein the light output from each optical fiber is provided to or directed toward the light pupil to provide a light beam (112). In some embodiments, the overlay measurement system (100) generates one or more light beams (112) by diffracting light (126) from a light source (124) by two or more diffraction orders, wherein at least one of the diffraction orders forms at least one light beam (112). The efficient generation of multiple illumination beams through controlled diffraction is generally described in U.S. Patent Publication No. 2020 / 0124408, published on April 23, 2020, the entirety of which is incorporated herein by reference.

[0051] In some embodiments, the overlay measurement tool (102) includes an objective lens (144) for capturing light or other radiation (e.g., collected light (146)) coming from all of the illuminated cells (106). Additionally, an illumination beam (112) from any illumination channel (120) can generally be directed toward a sample (110) through the objective lens (144) (e.g., in a through-the-lens (TTL) configuration) or directed toward a sample (110) outside the numerical aperture of the objective lens (144) (e.g., in an outside-the-lens (OTL) configuration). For example, FIG. 1b illustrates a TTL configuration.

[0052] In some embodiments, the overlay measurement tool (102) includes one or more filtering optical systems for separating light (146) collected from two illuminated cells (106) into different collection channels (122). For example, the filtering optical system may selectively direct a portion of the collected light (116) associated with the X-direction cell (106) to one collection channel (122) and a portion of the collected light (116) associated with the Y-direction cell (106) to another collection channel (122).

[0053] One or more filtering optical systems can separate collected light (116) associated with two illuminated cells (106) using any technique or combination of techniques including, but not limited to, polarization filtering, pupil plane filtering, or field plane filtering.

[0054] In some embodiments, the filtering optical system includes, but is not limited to, one or more polarizing filters, such as one or more linear polarizers or one or more polarizing beam splitters, to implement polarizing filtering. In the present invention, it is expected that polarizing filtering may be particularly effective when the illumination channel (120) provides an illumination beam distribution having orthogonal linear polarization, because the collected light (116) can generally maintain the polarization direction of the illumination.

[0055] For example, as illustrated in FIG. 1b, the overlay measurement tool (102) includes a polarizing beam splitter (148) at a location common to the illumination (126) and the collected light (116). In this configuration, the polarizing beam splitter (148) receives orthogonal linear polarized illumination (126) from each illumination channel (120) and can split this light with an objective lens (144). Then, the polarizing beam splitter (148) receives collected light (116) from two illuminated cells (106) and can split this collected light (116) into two collection channels (122) based on polarization. FIG. 1b further illustrates two non-polarizing beam splitters (150) associated with each polarization direction to separate the illumination (126) and the collected light (116) for their respective polarization directions.

[0056] As another example, although not explicitly illustrated, the overlay measurement tool (102) may include one or more linear polarizers in one or both of the collection channels (122). For example, one collection channel (122) may include a linear polarizer aligned along the X direction, and one collection channel (122) may include a linear polarizer aligned along the Y direction. In this configuration, the linear polarizers can implement polarization filtering that can be achieved even when the illumination beam distribution does not have orthogonal linear polarization. Additionally, the polarizers in the collection channels (122) may provide a higher extinction ratio than that provided by the polarization beam splitter (148), and thus further reduce crosstalk between the collected light (116) associated with different cells (106).

[0057] In some embodiments, the filtering optical system includes one or more spatial filters (e.g., pupil plane filters) in one or more pupil planes. In the invention, it is expected that a grating-over-greasing structure will produce diffraction orders distributed in a periodic direction. Accordingly, diffraction orders from a cell (106) having an orthogonal periodic direction will be distributed in an orthogonal direction in the pupil plane and can therefore be filtered using a pupil plane filter.

[0058] FIGS. 3a and 3b illustrate the spatial diffraction order distribution from an illuminated cell (106) of an overlay target (108) of FIG. 2b according to one or more embodiments of the present disclosure. FIG. 3a is a plan view of the pupil plane illustrating the diffraction order distribution from an X-direction cell (106) of an overlay target (108) of FIG. 2b according to one or more embodiments of the present disclosure. In particular, FIG. 3a illustrates 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 vertical angle of incidence. FIG. 3b is a plan view of the pupil plane illustrating the diffraction order distribution from a Y-direction cell (106) of an overlay target (108) of FIG. 2b according to one or more embodiments of the present disclosure. In particular, FIG. 3b illustrates 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 vertical angle of incidence. As shown in FIG. 3a and FIG. 3b, the non-zero diffraction orders from the X-direction cell (106) and the Y-direction cell (106) do not overlap in 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 opening oriented to pass at least X-direction -1st-order diffraction (304) and X-direction +1st-order diffraction (306) to separate light (116) collected from an illuminated X-direction cell (106), and one collection channel (122) may include a pupil plane filter having an opening oriented to pass at least Y-direction -1st-order diffraction (310) and Y-direction +1st-order diffraction (312) to separate light (116) collected from an illuminated Y-direction cell (106).

[0059] In some embodiments, the filtering optical system includes one or more spatial filters (e.g., field plane filters) in one or more field planes. For example, FIG. 1b illustrates a field aperture (152) within each collection channel (122). In particular, the field aperture (152) within the collection channel (122) may limit the collection area within the channel to a specific cell (106).

[0060] In some embodiments, each collection channel (122) includes one or more detectors (154) configured to capture light (116) collected from the channel. Each collection channel (122) may additionally include one or more optical elements suitable for modifying and / or controlling light (146) collected from a sample (110). In some embodiments, the collection channel (122) includes one or more collection path lenses (156) (e.g., for collimating illumination or relaying the pupil and / or field plane), and such collection path lenses (156) may include an objective lens (144), but are not required to include an objective lens (144). In some embodiments, the collection channel (122) includes one or more collection path optical systems (158) for shaping or controlling the collected light (146). For example, the collection path optical system (158) may include, but is not limited to, one or more field apertures, one or more aperture apertures, 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, translational mirrors, scanning mirrors, etc.).

[0061] The detector (154) may be located 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., 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 order associated with diffraction from the grating-over-grating structure of the cell (106) may be imaged or 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 measurement tool (102) includes a detector (154) in a field plane (e.g., a plane pair for the sample (110)) to generate an image of the sample (110) based on a diffraction order selected from the cell (106).

[0062] The overlay measurement tool (102) may generally include any number or type of detectors (154) suitable for capturing light from a sample (110) representing an overlay. In some embodiments, the detectors (154) include one or more detectors (154) suitable for characterizing a static sample. In this regard, the overlay measurement tool (102) may operate in a static mode where the sample (110) is static during measurement. For example, the detectors (154) may 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 detectors (154) may generate a two-dimensional image (e.g., a field plane image or a pupil plane image) in a single measurement.

[0063] 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) may operate in a scanning mode that scans the sample (110) against 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 during scanning within a selected image tolerance (e.g., image blur, contrast, sharpness, etc.). As another example, the detector (154) may include a line scan detector for continuously generating images one pixel line at a time. As another example, the detector (154) may 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 in the TDI detector.

[0064] In some embodiments, the overlay measurement system (100) includes one or more beam scanning optical systems (160) for controlling the position of one or more illumination beams (112) (or illumination beam distributions) on a sample (110). As an example, FIG. 1b includes beam scanning optical systems (160) in each illumination channel (120).

[0065] In the present invention, it is expected that the beam scanning optical system (160) can be used in various ways according to one or more embodiments of the present disclosure.

[0066] In some embodiments, a beam scanning optical system (160) located in both of the two illumination channels (120) may scan in a synchronous pattern to scan each illumination distribution across the cell (106). Such synchronous scanning may advantageously mitigate noise associated with imperfections in the grating-over-grating structure and / or mitigate the effects of speckle associated with coherent illumination (126). However, it is expected herein that such speckle may be further or alternatively mitigated using other techniques. For example, the coherent illumination (126) may be scanned across the input surface of a multimode fiber, whereby the output of the multimode fiber is imaged onto the sample (110). This multimode fiber may be located before or within any illumination channel (120).

[0067] In some embodiments, a beam scanning optical system (160) within one or more illumination channels (120) is used to adjust or control the separation of associated illumination beam distributions. In this way, the illumination beam distributions from each illumination channel (120) can be concentrated onto different cells (106). Additionally, this configuration can flexibly provide measurements on a target through any selected cell separation. For example, in FIG. 1b, the beam scanning optical system (160) can be tilted from the plane of example to cause the associated illumination beam distribution on the sample (110) to move away from the plane of example.

[0068] It may be additionally anticipated in the present invention that it may be desirable to ensure that the illumination (126) and / or collected light (116) are positioned at the center of any field aperture to avoid asymmetric diffraction during illumination or collection. In some embodiments, the overlay metrology system (100) includes one or more adjustable field apertures (e.g., field apertures mounted on actuators, etc.), and these field apertures may be synchronized with other components, but are not necessarily required to be. For example, FIG. 1b illustrates adjustable field apertures (152) in both the illumination channel (120) and the collection channel (122), and these field apertures may optionally be synchronized with the beam scanning optical system (160). In this way, the illumination (126) and the collected light (116) can be maintained at the center of each aperture even if the position of the illumination beam distribution on the sample (110) is adjusted.

[0069] Referring again to FIG. 1a, various additional components of an overlay measurement system (100) according to one or more embodiments of the present disclosure are described in more detail.

[0070] In some embodiments, the overlay measurement system (100) includes a controller (162) that is communicably connected to an overlay measurement tool (102) and / or any of its components. In one embodiment, the controller (162) includes one or more processors (164). For example, one or more processors (164) may be configured to execute a memory device (166) or a set of program instructions maintained in memory. One or more processors (164) of the controller (162) may include any processing element known in the art. In this sense, one or more processors (164) may include any microprocessor-type device configured to execute algorithms and / or instructions.

[0071] 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 terms “processor” or “processing element” may be broadly defined to encompass 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 (FPGAs), or one or more digital signal processors (DSPs). In this sense, one or more processors (164) may 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) may be implemented as a desktop computer, mainframe computer system, workstation, image computer, parallel processor, network computer, or any other computer system configured to execute a program configured to operate the overlay measurement system (100) or to operate in conjunction with it, as described throughout this disclosure. Furthermore, different subsystems of the overlay measurement system (100) may include processors or logic elements suitable for performing at least some of the steps described in this disclosure. Therefore, the foregoing description should not be interpreted as a limitation to the embodiments of this disclosure, but only as an example. Additionally, the steps described throughout this disclosure may be performed by a single controller or, alternatively, by multiple controllers. Additionally, the controller (162) may include one or more controllers housed in a single common housing or in multiple housings. In this way, any controller or combination of controllers may be individually packaged as modules suitable for integration into the overlay measurement system (100).

[0072] The memory device (166) may 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-transient memory medium. As other examples, the memory device (166) may include, but is not limited to, read-only memory (ROM), random-access memory (RAM), magnetic or optical memory devices (e.g., disks), magnetic tape, solid-state drive, etc. It is further noted that the memory device (166) may be housed within a common controller housing together with one or more processors (164). In some embodiments, the memory device (166) may be located remotely from 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 remote memory (e.g., a server) accessible via a network (e.g., the Internet, an intranet, etc.).

[0073] The controller (162) may send or receive data from the overlay measurement tool (102) or any component within it (e.g., via a control signal). The controller (162) may be further configured to perform any of the various process steps described throughout this disclosure.

[0074] In some embodiments, the overlay measurement system (100) includes a user interface (168) that is communicably connected to a controller (162). In some embodiments, the user interface (168) may include one or more desktops, laptops, tablets, etc., but is not limited thereto. 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 a liquid crystal display (LCD), an organic light-emitting diode (OLED) based display, or a CRT display, but is not limited thereto. Those skilled in the art should recognize that any display device that can be integrated with the user interface (168) is suitable for implementation of the present disclosure. In some embodiments, the user may input options and / or instructions in response to data displayed to the user through a user input device of the user interface (168).

[0075] Now, referring to FIG. 4, FIG. 4 is a flowchart illustrating steps performed in a method (400) for overlay measurement according to one or more embodiments of the present disclosure. It is noted by the applicant that the aforementioned embodiments and possible techniques should be interpreted to be extended to the method (400) in the context of an overlay measurement system (100). However, it is also noted that the method (400) is not limited to the architecture of the overlay measurement system (100).

[0076] In some embodiments, the method (400) includes the step (402) of illuminating a first-direction cell of a pair of cells on an overlay target with a first illumination beam distribution having a first linear polarization, and simultaneously illuminating a second-direction cell of the pair of cells 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 pairs of cells having a grid-over-grid structure oriented in orthogonal directions, as illustrated in FIGS. 2a and 2b.

[0077] Step (402) may include illuminating the first direction cell and the second direction cell with any number or configuration of illumination beams, wherein the first and second illumination beam distributions may be controlled individually. In some embodiments, step (404) may include underfilling both the first direction cell and the second direction cell with the first and second illumination beam distributions. Additionally, step (402) may include synchronously scanning the first and second illumination beam distributions across the first direction cell and the second direction cell, which may mitigate target noise and / or speckle.

[0078] In some embodiments, the method (400) includes the step (404) of collecting light from a sample (e.g., a first direction cell and a second direction cell) as collected light. In some embodiments, the method (400) includes the step (406) of directing a portion of the collected light associated with the first direction cell of two or more cell pairs to a first collection channel. In some embodiments, the method (400) includes the step (408) of directing a portion of the collected light associated with the second direction cell of two or more cell pairs to a second collection channel. In this way, the collected light representing an overlay for the first direction cell and the second direction cell may be collected in parallel. For example, steps (406) and / or steps (408) may be implemented using any combination of polarizing filters, pupil plane filters, or field plane filters, such as but not limited to those exemplified in the context of the overlay measurement system (100).

[0079] In some embodiments, the method (400) includes the step (410) of generating a first overlay measurement according to a first direction based on data associated with a first direction cell of two or more cell pairs (e.g., based on iterative steps (402 to 408) for two or more cell pairs). In some embodiments, the method (400) includes the step (412) of generating a second overlay measurement according to a second direction based on data associated with a second direction cell of two or more cell pairs.

[0080] It is expected in the present invention that the method (400) can beneficially provide parallel measurements of target cells to a level where the crosstalk of the collected light is minimized or ignored. In this way, the method (400) can reduce the number of sequential measurements of a given overlay target required by a factor of two compared to a technique where all cells are measured sequentially. Furthermore, this increase in throughput may not result in a decrease in the accuracy or sensitivity of the measurements.

[0081] The subject matter of the invention described herein illustrates different components that are sometimes contained within or connected to other components. It should be understood that such illustrated architectures are merely exemplary and that many other architectures may actually be implemented to achieve the same function. In a conceptual sense, any arrangement of components to achieve the same function is effectively "associated" so that the desired function is achieved. Accordingly, any two components of the present invention combined to achieve a specific functionality may be considered "associated" to each other so that the desired functionality is achieved, regardless of the architecture or intermediate components. Similarly, any two components thus associated may also be considered "connected" or "connected" to each other to achieve the desired functionality, and any two components so associated may also be considered "connectable" to each other to achieve the desired functionality. Specific examples of connectable include, but are not limited to, physically interactable and / or physically interacting components and / or wirelessly interactable and / or wirelessly interacting components and / or logically interactable and / or logically interacting components.

[0082] The present disclosure and the numerous advantages thereof will be understood from the foregoing description, and it will be apparent that various modifications to the form, configuration, and arrangement of the components may be made without departing from the essence of the invention disclosed herein or without sacrificing any material advantages. The described forms are for illustrative purposes only, and it is the intent of the following claims to encompass and include such modifications. Furthermore, the present invention should be understood as defined by the appended claims.

Claims

Claim 1 As an overlay measurement tool, 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; focus control optical systems for independently adjusting the respective focal positions of the first illumination beam distribution and the second illumination beam distribution; an illumination subsystem for sequentially illuminating two or more pairs of cells of an overlay target on a sample ― wherein a specific pair of cells among the two or more pairs of cells comprises 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, and the illumination subsystem illuminates the first direction cell with the first illumination beam distribution while simultaneously illuminating the second direction cell with the second illumination beam distribution ―; a collection subsystem ― wherein the collection subsystem comprises: a first collection channel comprising one or more first channel detectors in a first channel detection plane; A second collection channel comprising one or more second channel detectors in 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 cell of the two or more cell pairs to the first collection channel and directing a portion of the collected light associated with a second directional cell of the two or more cell pairs to the second collection channel.An overlay measurement tool comprising: a controller communicatably connected to the first collection channel and the second collection channel — the controller comprising one or more processors configured to execute program instructions — wherein the program instructions cause the one or more processors to: generate a first overlay measurement according to the first direction based on data associated with a first direction cell of the two or more cell pairs from the one or more first channel detectors; and generate a second overlay measurement according to the second direction based on data associated with a second direction cell of the two or more cell pairs from the one or more second channel detectors.; Claim 2 An overlay measurement tool according to claim 1, further comprising one or more beam scanning optical systems configured to adjust the separation between the first illumination beam distribution and the second illumination beam distribution. Claim 3 In paragraph 2, the one or more beam scanning optical systems are an overlay measurement tool that adjusts the 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 cell and the second direction cell of the illuminated cell pair among the two or more cell pairs. Claim 4 An overlay measurement tool according to claim 2, further comprising: a first illumination field stop for defining the spatial range of the first illumination beam distribution; and a second illumination field stop for defining the spatial range of the second illumination beam distribution, wherein at least one of the first illumination field stop and the second illumination field stop is adjustable and synchronized with respect to the one or more beam scanning optical systems as the separation between the first illumination beam distribution and the second illumination beam distribution is adjusted, so that the first illumination beam distribution and the second illumination beam distribution are positioned at the center of the first illumination field stop and the second illumination field stop. Claim 5 An overlay measurement tool according to claim 2, further comprising: a first collection field aperture for defining a collection field for the first collection channel; and a second collection field aperture for defining a collection field for the second collection channel, wherein at least one of the first collection field aperture and the second collection field aperture is adjustable and synchronized with respect to the one or more beam scanning optical systems as the separation between the first illumination beam distribution and the second illumination beam distribution is adjusted, so as to align the first collection field and the second collection field with the first illumination beam distribution and the second illumination beam distribution. Claim 6 An overlay measurement tool according to claim 1, further comprising one or more beam scanning optical systems configured to control the first illumination beam distribution and the second illumination beam distribution across the first direction cell and the second direction cell of the illuminated cell pair among the two or more cell pairs during measurement. Claim 7 An overlay measuring tool according to claim 1, wherein the grid-over-grid structure of the first direction cell of the two or more cell pairs is arranged along the first row of the overlay target, and the grid-over-grid structure of the second direction cell of the two or more cell pairs is arranged along the second row of the overlay target. Claim 8 An overlay measurement tool according to claim 1, wherein the grid-over-grid structure of the first direction cell of the two or more cell pairs has different intended overlay offsets along the first direction, and the grid-over-grid structure of the second direction cell of the two or more cell pairs has different intended overlay offsets along the second direction. Claim 9 An overlay measurement tool according to claim 1, wherein the two or more cell pairs include a first cell pair and a second cell pair, wherein the first direction cell of the first cell pair and the first direction cell of the second cell pair have the same and opposite intended offset along the first direction, and the second direction cell of the first cell pair and the second direction cell of the second cell pair have the same and opposite intended offset along the second direction. Claim 10 In claim 1, the second illumination beam distribution is the same as the first illumination beam distribution, an overlay measurement tool. Claim 11 In claim 1, the second illumination beam distribution is different from the first illumination beam distribution, an overlay measurement tool. Claim 12 An overlay measurement tool according to claim 1, wherein 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, and the angle of incidence of one illumination beam. Claim 13 An overlay measurement tool according to claim 1, wherein at least one of the first illumination beam distribution and the second illumination beam distribution comprises a single illumination beam. Claim 14 An overlay measurement tool according to claim 1, wherein 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. Claim 15 An overlay measurement tool according to claim 1, wherein 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, and one or more field plane filters. Claim 16 In claim 15, the one or more polarization filtering optical systems comprises: 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 a portion of the collected light having the second linear polarization to the second collection channel, an overlay measurement tool. Claim 17 In claim 15, the one or more polarization filtering optical systems comprises: at least one of a first polarizer of the first collection channel oriented to pass the first linear polarization, and a second polarizer of the second collection channel oriented to pass the second linear polarization, an overlay measurement tool. Claim 18 In claim 15, the one or more pupil plane filters comprises: a first opening in the pupil plane of the first collection channel for passing a portion of collected light associated with diffraction along the first direction; a second opening in the pupil plane of the second collection channel for passing a portion of collected light associated with diffraction along the second direction; and a beam splitter in the pupil plane common to the first collection channel and the second collection channel for directing a portion of collected light associated with diffraction along the first direction to the first collection channel and a portion of collected light associated with diffraction along the second direction to the second collection channel, an overlay measurement tool. Claim 19 An overlay measurement tool according to claim 15, wherein the one or more field plane filters comprise: at least one of an opening in the field plane of the first collection channel for passing a portion of the collected light associated with the first direction cell, and an opening in the field plane of the second collection channel for passing a portion of the collected light associated with the second direction cell. Claim 20 An overlay measurement tool according to claim 1, further comprising a translational stage for sequentially arranging two or more pairs of cells to illuminate the lighting subsystem. Claim 21 delete Claim 22 An overlay measurement tool according to claim 1, wherein at least one of the first channel detection plane and the second channel detection plane comprises a pupil plane. Claim 23 An overlay measurement tool according to claim 1, wherein at least one of the first channel detection plane and the second channel detection plane comprises a field plane. Claim 24 As an overlay measurement tool, 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; focus control optical systems for independently adjusting the respective focal positions of the first illumination beam distribution and the second illumination beam distribution; an illumination subsystem for sequentially illuminating two or more pairs of cells of an overlay target on a sample ― wherein a specific pair of cells among the two or more pairs of cells comprises a first direction cell having a grating-over-greasing structure having periodicity along a first direction and a second direction cell having a grating-over-greasing structure having periodicity along a second direction orthogonal to the first direction, and the illumination subsystem comprises: a first illumination channel for directing the first illumination beam distribution toward the first direction cell of one of the two or more pairs of cells ― wherein the first illumination beam distribution comprises one or more first illumination beams having the first linear polarization ―; and includes a second illumination channel for directing 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 linear polarization —; a collection subsystem — the collection subsystem comprises: 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; 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 the first direction cell having the first linear polarization to the first collection channel and directing a portion of the collected light associated with the second direction cell of the two or more cell pairs having the second linear polarization to the second collection channel —;An overlay measurement tool comprising: a controller communicatably connected to the first collection channel and the second collection channel — the controller comprising one or more processors configured to execute program instructions — wherein the program instructions cause the one or more processors to: generate a first overlay measurement according to the first direction based on data associated with a first direction cell of the two or more cell pairs from the one or more first channel detectors; and generate a second overlay measurement according to the second direction based on data associated with a second direction cell of the two or more cell pairs from the one or more second channel detectors.; Claim 25 An overlay measurement tool according to claim 24, further comprising at least one of a first opening in the pupil plane of the first collection channel for passing a portion of the collected light associated with diffraction along the first direction, a second opening in the pupil plane of the second collection channel for passing a portion of the collected light associated with diffraction along the second direction, and a beam splitter in the pupil plane common to the first collection channel and the second collection channel for directing a portion of the collected light associated with diffraction along the first direction to the first collection channel and a portion of the collected light associated with diffraction along the second direction to the second collection channel. Claim 26 An overlay measurement tool according to claim 24, further comprising at least one of an opening in the field plane of the first collection channel for passing a portion of the collected light associated with the first direction cell, and an opening in the field plane of the second collection channel for passing a portion of the collected light associated with the second direction cell. Claim 27 An overlay measurement tool according to claim 24, further comprising one or more beam scanning optical systems configured to adjust the separation between the first illumination beam distribution and the second illumination beam distribution. Claim 28 In claim 27, the one or more beam scanning optical systems are an overlay measurement tool that adjusts the 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 cell and the second direction cell of the illuminated cell pair among the two or more cell pairs. Claim 29 An overlay measurement tool according to claim 27, further comprising: a first illumination field aperture for defining the spatial range of the first illumination beam distribution; and a second illumination field aperture for defining the spatial range of the second illumination beam distribution, wherein at least one of the first illumination field aperture and the second illumination field aperture is adjustable and synchronized with respect to the one or more beam scanning optical systems as the separation between the first illumination beam distribution and the second illumination beam distribution is adjusted, so as to position the first illumination beam distribution and the second illumination beam distribution at the center of the first illumination field aperture and the second illumination field aperture. Claim 30 An overlay measurement tool according to claim 27, further comprising: a first collection field aperture for defining a collection field for the first collection channel; and a second collection field aperture for defining a collection field for the second collection channel, wherein at least one of the first collection field aperture and the second collection field aperture is adjustable and synchronized with respect to the one or more beam scanning optical systems as the separation between the first illumination beam distribution and the second illumination beam distribution is adjusted, so as to align the first collection field and the second collection field with the first illumination beam distribution and the second illumination beam distribution. Claim 31 As an overlay measurement method, the method comprises the steps of: 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; independently adjusting the respective focal positions of the first illumination beam distribution and the second illumination beam distribution using focus control optical systems; sequentially illuminating two or more pairs of cells of an overlay target on a sample using an illumination system ― wherein each of the two or more pairs of cells includes a first direction cell having a grating-over-greasing structure having periodicity along a first direction and a second direction cell having a grating-over-greasing structure having periodicity along a second direction orthogonal to the first direction, and the illumination system illuminating the first direction cell with the first illumination beam distribution while simultaneously illuminating the second direction cell with the second illumination beam distribution ―; collecting light from the sample as collected light; and directing a portion of the collected light associated with the first direction cell of the two or more pairs of cells toward a first collection channel. An overlay measurement method comprising: a step of directing a portion of collected light associated with a second direction cell of the two or more cell pairs to a second collection channel; a step of generating a first overlay measurement according to the first direction based on data associated with a first direction cell of the two or more cell pairs; and a step of generating a second overlay measurement according to the second direction based on data associated with a second direction cell of the two or more cell pairs.

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