Scanning overlay metrology with high signal-to-noise ratio

Abstract

The overlay metrology system can include illumination optics for splitting illumination from an illumination source into primary and secondary illumination and directing the primary illumination toward a sample including an overlay target having two or more layers of gratings and an objective lens for collecting positive and negative diffraction from the constituent gratings. The system can further include collection optics for overlaying auxiliary illumination with at least some of the collected diffraction lobes to generate a time-varying interference signal. The system can further include a controller for generating overlay measurements based on the time-varying interference signal.

Description

[Technical Field]

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

[0002] The increasing demand for smaller semiconductor devices leads to a corresponding increase in the demand for accurate and efficient metrology. One approach to improving the efficiency and throughput of metrology tools is to generate metrology data for a moving sample rather than a static position within the measurement field. In this way, the time delay associated with setting up a translation stage before measurement can be eliminated or reduced. However, increasing the sensitivity and throughput of such measurements remains a central challenge for such scanning metrology methods. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] U.S. Patent Application Publication No. 2022 / 0034652 Summary of the Invention [Problem to be solved by the invention]

[0004] It would therefore be desirable to provide a system and method for remedying the above deficiencies. [Means for solving the problem]

[0005] An overlay metrology system is disclosed in accordance with one or more exemplary embodiments. In one exemplary embodiment, the system includes an illumination source. In another exemplary embodiment, the system includes a first beam splitter configured to split illumination from the illumination source into primary illumination and supplemental illumination. In another exemplary embodiment, the system includes one or more illumination optics configured to direct the primary illumination toward an overlay target on the specimen when performing a metrology recipe, the overlay target according to the metrology recipe including gratings of two or more layers. In another exemplary embodiment, the system includes an objective lens for collecting at least one positive diffraction lobe and at least one negative diffraction lobe associated with diffraction of the primary illumination from the gratings of each of the two or more layers when performing the metrology recipe. In another exemplary embodiment, the system includes one or more photodetectors at one or more collection pupil planes. In another exemplary embodiment, the system includes one or more collection optics configured to implement a metrology recipe by overlapping at least one diffraction lobe of the primary illumination by the overlay target with a first portion of the supplemental illumination on at least one of the one or more photodetectors to generate a first interference pattern and overlapping at least one additional diffraction lobe of the primary illumination by the overlay target with a second portion of the supplemental illumination on at least one of the one or more photodetectors to generate a second interference pattern. In another exemplary embodiment, the system includes a scanning subsystem including at least one of a stage or one or more scanning optics for modulating the phase of the first and second interference patterns during scanning of the overlay target when implementing the metrology recipe. In another exemplary embodiment, the system includes a controller for receiving time-varying interference signals from the one or more photodetectors during scanning of the overlay target and generating one or more overlay measurements of the overlay target based on the time-varying interference signals.

[0006] An overlay metrology method is disclosed according to one or more exemplary embodiments. In one exemplary embodiment, the method includes generating illumination using an illumination source. In another exemplary embodiment, the method includes splitting illumination from the illumination source into primary illumination and supplemental illumination. In another exemplary embodiment, the method includes directing the primary illumination toward an overlay target on the sample, the overlay target including a grating of two or more layers. In another exemplary embodiment, the method includes overlapping a first portion of the supplemental illumination with at least one diffraction lobe associated with diffraction of the primary illumination from a grating of each of the two or more layers onto one or more first photodetectors in a collection pupil plane to generate a first interference pattern. In another exemplary embodiment, the method includes overlapping a second portion of the supplemental illumination with at least one additional diffraction lobe associated with diffraction of the primary illumination from a grating of each of the two or more layers onto one or more second photodetectors in the collection pupil plane to generate a second interference pattern. In another exemplary embodiment, the method includes modulating the phases of the first and second interference patterns during scanning of the overlay target using a scanning subsystem, the scanning subsystem including at least one of a translation stage that scans the sample relative to the primary illumination, beam scanning optics that scans the primary illumination relative to the sample, or a phase modulator that modulates the phase of the supplemental illumination. In another exemplary embodiment, the method includes generating time-varying interference signals with one or more first photodetectors and one or more second photodetectors based on the modulated phases of the first and second interference patterns. In another exemplary embodiment, the method includes generating one or more overlay measurements of the overlay target based on the time-varying interference signals.

[0007] An overlay metrology system is disclosed in accordance with one or more exemplary embodiments. In one exemplary embodiment, the system includes an illumination source configured to generate illumination. In another exemplary embodiment, the system includes one or more beam splitters configured to split illumination from the illumination source into primary illumination and auxiliary illumination. In another exemplary embodiment, the system includes one or more illumination optics configured to direct the primary illumination to an overlay target on a specimen with a grating of two or more layers when performing a metrology recipe, the overlay target according to the metrology recipe including a first set of one or more cells with a grating having a first grating orientation and a second set of one or more cells with a grating having a second grating orientation, the primary illumination having a rotated quadrupole distribution with respect to the first and second grating orientations. In another exemplary embodiment, the system includes an objective lens for collecting at least one positive diffraction lobe and at least one negative diffraction lobe associated with diffraction of the primary illumination from the grating of each of the two or more layers when performing the metrology recipe. In another exemplary embodiment, the system includes a first collection channel having a first set of one or more photodetectors at one or more collection pupil planes. In another exemplary embodiment, the first collection channel is configured to implement the metrology recipe by overlapping a first portion of the supplemental illumination and at least one diffraction lobe of the primary illumination along a first diagonal of the rotated quadrupole distribution by the overlay target on at least one of the first set of one or more photodetectors to generate a first interference pattern, and overlapping a second portion of the supplemental illumination and at least one additional diffraction lobe of the primary illumination along a first diagonal of the rotated quadrupole distribution by the overlay target on at least one of the first set of one or more photodetectors to generate a second interference pattern. In another exemplary embodiment, the system includes a second collection channel having a second set of one or more photodetectors at one or more collection pupil planes.In another exemplary embodiment, the second collection channel is configured to implement the metrology recipe by overlapping a first portion of the supplemental illumination with at least one diffraction lobe of the primary illumination along a second diagonal of the rotated quadrupole distribution by the overlay target on at least one of the second set of one or more photodetectors to generate a third interference pattern, and overlapping a second portion of the supplemental illumination with at least one additional diffraction lobe of the primary illumination along a second diagonal of the rotated quadrupole distribution by the overlay target on at least one of the second set of one or more photodetectors to generate a fourth interference pattern. In another exemplary embodiment, the system includes a scanning subsystem configured to modulate the phases of the first, second, third, and fourth interference patterns during scanning of the overlay target when implementing the metrology recipe, the scanning subsystem including at least one of a translation stage that scans the sample relative to the primary illumination, beam scanning optics that scans the primary illumination relative to the sample, or a phase modulator that modulates the phase of the supplemental illumination. In another exemplary embodiment, the system includes a controller for receiving time-varying interference signals from the first and second collection channels during scanning of the overlay target and generating one or more overlay measurements of the overlay target along the first and second grating directions based on the time-varying interference signals.

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

[0009] The many advantages of the present disclosure may be better understood by those skilled in the art by reference to the accompanying drawings. [Brief explanation of the drawings]

[0010] [Figure 1A]FIG. 1 is a conceptual diagram of an overlay metrology system for performing scatterometry overlay metrology on an overlay target suitable for overlay measurement along any particular measurement direction, in accordance with one or more embodiments of the present disclosure. [Figure 1B] FIG. 1 is a schematic diagram of an overlay metrology tool in accordance with one or more embodiments of the present disclosure. [Figure 2A] FIG. 1 is a top view of a cell of an overlay target having a moiré structure in accordance with one or more embodiments of the present disclosure. [Figure 2B] FIG. 2B is a side view of a single cell of the overlay target of FIG. 2A on a substrate in accordance with one or more embodiments of the present disclosure. [Figure 2C] 1 is a side view of an overlay target including two cells with different configurations of Moiré structures suitable for overlay measurements along a particular measurement direction, in accordance with one or more embodiments of the present disclosure. FIG. [Figure 3A] FIG. 1 is a top view of an illumination pupil plane of an overlay metrology tool in accordance with one or more embodiments of the present disclosure. [Figure 3B] FIG. 2C is a top view of a collection pupil boundary at a collection pupil plane of an overlay metrology tool, illustrating the overlap between the supplemental illumination and the + / -1st order diffraction lobes of the primary illumination due to the moiré structure of FIGS. 2A-2C, in accordance with one or more embodiments of the present disclosure. [Figure 4] FIG. 1 is a conceptual diagram of an overlay metrology tool that provides oblique primary illumination and associated supplemental illumination, in accordance with one or more embodiments of the present disclosure. [Figure 5] FIG. 1 is a conceptual diagram of an overlay metrology tool for simultaneously characterizing an overlay target in multiple lighting conditions, in accordance with one or more embodiments of the present disclosure. [Figure 6] 1 is a flow chart illustrating steps performed in an overlay metrology method in accordance with one or more embodiments of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

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

[0012] Embodiments of the present disclosure relate to systems and methods for scanning scatterometry overlays based on a time-varying interference signal generated by interfering one or more diffraction orders of a grating-over-grating overlay target with a reference light (e.g., auxiliary illumination herein). It is believed that the systems and methods disclosed herein can facilitate rapid overlay measurements of moving samples with high signal-to-noise ratios.

[0013] Some scatterometry-based overlay techniques involve illuminating an overlay target with periodic structures (e.g., grating structures) associated with different lithography exposures and determining the overlay error associated with those exposures based on asymmetry in the diffraction orders (e.g., +1 and -1 diffraction lobes). For example, phase differences between light associated with diffraction from different grating structures can result in intensity variations in the pupil plane, such as, but not limited to, interference fringes. In this way, asymmetry in the periodic structures, such as, but not limited to, overlay errors, can result in asymmetry in the interference fringes, which can be the basis for overlay measurements.

[0014] Furthermore, scanning-based scatterometry overlay measurements can be performed by scanning an illumination beam over an overlay target containing such grating structures (or vice versa), with the illumination beam sized to be smaller than the overlay target. In this configuration, the interference pattern associated with diffraction from both grating structures can oscillate during scanning as a time-varying interference signal that can be captured using a photodetector with sufficient bandwidth. For example, such techniques can be implemented using relatively fast photodetectors, such as, but not limited to, photodiodes, avalanche photodiodes, etc., positioned in the collection pupil plane. Furthermore, scanning the illumination beam over the overlay target can generally be performed by movement of the overlay target, the illumination beam, or both.

[0015] Based on these general principles, various measurement techniques have been developed. In a general sense, the position of the time-varying interference signal within the collection pupil may generally depend on the layout and pitch of the grating structure of the overlay target. Scanning-based scatterometry overlay techniques are generally described in U.S. Patent No. 11,300,405, issued April 12, 2022, U.S. Patent No. 11,378,394, issued July 5, 2022, U.S. Patent No. 10,197,389, issued February 5, 2019, U.S. Patent Application No. 17 / 708,958, filed March 30, 2022, U.S. Patent Application No. 17 / 709,104, filed March 30, 2022, and U.S. Patent Application No. 18 / 099,798, all of which are incorporated herein by reference in their entireties.

[0016] It is contemplated herein that it may be advantageous to capture such time-varying interference signals at the overlap of the zero-order illumination with the diffraction lobes from the constituent gratings. In this way, the zero-order illumination can act as a reference, facilitating the determination of asymmetries in the time-varying interference signals associated with positive and negative diffraction, which may indicate overlay errors. However, the requirement that the zero-order diffraction overlap with such higher diffraction orders within the collection pupil may impose limitations on various aspects of the overlay target or corresponding overlay metrology system, such as, but not limited to, the pitch of the constituent grating structures on the overlay target, the illumination beam wavelength, or the illumination beam shape. Furthermore, such limitations may conflict with other performance tradeoffs. For example, it may be desirable to provide an extended illumination beam shape to mitigate the effects of target nonuniformity. However, such an extended illumination beam shape may limit the overlap region of the zero-order diffraction with the higher diffraction order lobes of the target, resulting in a relatively low signal-to-noise ratio for the time-varying signal of the target.

[0017] Embodiments of the present disclosure relate to generating a time-varying interference signal indicative of overlay based on the overlap of an auxiliary illumination beam (e.g., a reference beam) with a diffraction lobe of interest. By way of example, coherent illumination can be split into primary and auxiliary illumination, with the primary illumination directed toward an overlay target in a scanning-based scatterometer overlay metrology system. The diffraction orders of interest from the overlay target can then be collected and overlapped with the auxiliary illumination (e.g., at a collection pupil plane). In this way, time-varying interference signals associated with positive and negative diffraction can share a common reference.

[0018] It is believed that the systems and methods disclosed herein can remove or relax constraints on overlay targets and / or overlay metrology tools related to zero-order diffraction overlapping with selected higher-order diffraction lobes. As a result, the systems and methods disclosed herein can allow substantial flexibility in adjusting the overlay targets and / or overlay metrology tools to provide measurements with high signal-to-noise ratios (SNRs) and high throughput.

[0019] For example, using supplemental illumination to generate a time-varying signal can eliminate the requirement that the zero-order illumination overlap with the higher diffraction orders of the target in the pupil plane. As a result, the shape of the primary illumination (and therefore the shape of the associated diffraction lobes) can be tailored to facilitate efficient sampling of the overlay target. By way of example, the primary illumination can be extended along a direction orthogonal to the scan to facilitate interaction with the extended portion of the overlay target, mitigating manufacturing non-uniformities without the requirement for zero-order illumination overlap with the higher diffraction orders in the collection pupil. Instead, the supplemental illumination can be shaped into any suitable distribution to overlap with selected diffraction orders. Furthermore, the pitch of the grating structure and / or the illumination wavelength can generally be selected more flexibly to facilitate smaller pitches and / or smaller targets.

[0020] As another example, generating a time-varying signal using supplemental illumination can allow for tuning of the relative intensity of the supplemental illumination for diffraction orders of interest to promote high contrast in the time-varying interference signal. This technique can generally increase the signal-to-noise ratio of the captured time-varying interference signal and therefore increase the sensitivity of the corresponding overlay measurement. More generally, any characteristic of the supplemental illumination can be tailored to different diffraction lobes of interest, such as, but not limited to, intensity, wavelength, or polarization. Such tailored supplemental illumination is referred to herein as structured supplemental illumination.

[0021] It is further contemplated herein that the systems and methods disclosed herein may be extended to apply to any scanning scatterometry overlay metrology techniques, including, but not limited to, those described in U.S. Patent No. 11,300,405, issued April 12, 2022, U.S. Patent No. 11,378,394, issued July 5, 2022, U.S. Patent No. 10,197,389, issued February 5, 2019, U.S. Patent Application No. 17 / 708,958, filed March 30, 2022, U.S. Patent Application No. 17 / 709,104, filed March 30, 2022, and U.S. Patent Application No. 18 / 099,798, referenced above and incorporated herein in their entireties.

[0022] Additional embodiments of the present disclosure relate to providing recipes for configuring an overlay metrology tool to facilitate overlay measurements based on selected diffraction orders. Overlay metrology tools are typically configurable according to recipes that include sets of parameters for controlling various aspects of overlay measurements, such as, but not limited to, the illumination of the sample, the collection of light from the sample, or the position of the sample during measurement. In this manner, the overlay metrology tool can be configured to provide selected types of measurements for one or more overlay target designs of interest. For example, a metrology recipe can include illumination parameters such as, but not limited to, illumination wavelength, illumination pupil distribution (e.g., the distribution of illumination angles and the associated intensities of the illumination at those angles), polarization of incident illumination, or spatial distribution of illumination. As another example, a metrology recipe can include collection parameters such as, but not limited to, collection pupil distribution (e.g., the desired distribution of angular light from the sample used in the measurement and the associated filtered intensities at those angles), collection field stop settings for selecting portions of the sample of interest, polarization of collected light, wavelength filters, or parameters for controlling one or more detectors. As a further example, the metrology recipe may include various parameters related to the sample position during the measurement, such as, but not limited to, the sample height, the sample orientation, whether the sample is stationary during the measurement, or whether the sample is moving during the measurement (along with related parameters describing the speed, scan pattern, etc.).

[0023] Furthermore, for purposes of this disclosure, the term overlay target is used generally to refer to a structure on a sample that is suitable for a particular overlay measurement. In some embodiments, an overlay target includes a dedicated structure designed to facilitate overlay measurements by a particular technique (e.g., based on a metrology recipe). For example, a dedicated overlay target may include one or more cells having a grating structure with a pitch and orientation selected according to a metrology recipe to provide a selected distribution of diffraction orders at the collection pupil plane. In some embodiments, the overlay target is formed from device features on the sample. For example, device features in some applications may be distributed such that overlay measurements can be generated directly on these device features. In this way, potential systematic measurement errors associated with dedicated overlay targets may be avoided.

[0024] 1A-6, systems and methods for scanning scatterometry overlay metrology with high signal-to-noise ratios according to one or more embodiments of the present disclosure will be described in more detail.

[0025] 1A is a conceptual diagram of an overlay metrology system 100 for performing scatterometry overlay metrology on an overlay target 102 suitable for overlay measurement along any particular measurement direction, in accordance with one or more embodiments of the present disclosure. In some embodiments, the overlay metrology system 100 includes an optical subsystem 104 for performing scatterometry overlay measurements on the overlay targets 102 distributed across a sample 106. FIG. 1B is a schematic diagram of the optical subsystem 104, in accordance with one or more embodiments of the present disclosure.

[0026] For purposes of this disclosure, the term overlay is generally used to describe the relative positions of features on a sample fabricated by two or more lithographic patterning steps, while the term overlay error describes the deviation of features from their nominal placement. In this context, overlay measurements can be expressed as measurements of relative positions or measurements of overlay error associated with these relative positions. For example, multilayer devices can include features patterned on multiple sample layers using different lithographic steps for each layer, and alignment of features between layers must typically be tightly controlled to ensure proper performance of the resulting device. Thus, overlay measurements can characterize the relative positions of features on two or more sample layers. As another example, features can be fabricated on a single sample layer using multiple lithographic steps. Such techniques, commonly referred to as double or multiple patterning techniques, can facilitate the fabrication of very high densities of features near the resolution of the lithography system. Overlay measurements in this context can characterize the relative positions of features from different lithographic steps on this single layer. It should be understood that the 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 the disclosure.

[0027] Additionally, the term scatterometry metrology is used herein to broadly encompass the terms scatterometry-based metrology and diffraction-based metrology, in which a sample having periodic features on one or more sample layers is illuminated with an illumination beam having a limited angular range, and one or more distinct diffraction orders are collected for measurement. Furthermore, the term scanning metrology is used to describe metrology measurements generated when the sample is moving relative to the illumination used in the measurement. In a general sense, scanning metrology can be performed by moving the sample, the illumination, or both. As such, specific descriptions herein of particular techniques for performing scanning metrology are merely exemplary and should not be construed as limiting.

[0028] In some embodiments, the optical subsystem 104 includes an illumination source 108 for generating illumination 110-1. The illumination source 108 may include any type of illumination source suitable for providing illumination 110-1 suitable for overlay metrology as disclosed herein. In some embodiments, the illumination source 108 is a laser source. For example, the illumination source 108 may include, but is not limited to, one or more narrowband laser sources, broadband laser sources, supercontinuum laser sources, white light laser sources, etc. In this regard, the illumination source 108 may provide illumination with high coherence (e.g., high spatial coherence and / or temporal coherence). In some embodiments, the illumination source 108 includes a laser-sustained plasma (LSP) source. For example, the illumination source 108 may include, but is not limited to, an LSP lamp, an LSP bulb, or an LSP chamber suitable for housing one or more elements capable of emitting broadband illumination when excited into a plasma state by a laser source.

[0029] Additionally, the illumination 110-1 may include light of one or more selected wavelengths, including, but not limited to, ultraviolet (UV) radiation, visible radiation, or infrared (IR) radiation.

[0030] The optical subsystem 104 may then include an interferometer 112 for splitting the illumination 110-1 into two portions, referred to herein as primary illumination 110-2 and auxiliary illumination 110-3. In this configuration, the illumination subsystem 124 may direct the primary illumination 110-2 toward the overlay target 102 (e.g., in the form of one or more illumination beams) and collect at least a portion of the primary illumination 110-2 emanating from the sample 106 (e.g., one or more diffraction orders of the primary illumination 110-2). The collected light from the sample 106, which may include the diffraction orders of the primary illumination 110-2, is referred to herein as measurement light 114. The interferometer 112 may then combine the auxiliary illumination 110-3 with one or more selected diffraction orders of the primary illumination 110-2, which may generate a time-varying signal indicative of the overlay of the sample 106. The optical subsystem 104, in turn, may include one or more photodetectors 116 for capturing time-varying signals indicative of overlay, and a controller 118 having one or more processors 120 and a memory 122 (e.g., a memory device). The one or more processors 120 may be configured to execute a set of program instructions retained in the memory 122. In this manner, the controller 118 may perform any of the various process steps described throughout this disclosure, such as, but not limited to, receiving the time-varying interference signals from the photodetectors 116, processing or filtering the time-varying interference signals, or generating overlay measurements related to the sample 106 based on the time-varying interference signals.

[0031] 2A-2C, various aspects of an overlay target 102 suitable for scanning overlay metrology, according to one or more embodiments of the present disclosure, are described in more detail. In particular, FIGS. 2A-2C show an overlay target 102 including at least one Moiré structure, according to one or more embodiments of the present disclosure. Such an overlay target 102 may be suitable for measurement by an optical subsystem 104, but is not limited to such.

[0032] FIG. 2A is a top view of a cell 202 of an overlay target 102 having a moiré structure 204, in accordance with one or more embodiments of the present disclosure. FIG. 2B is a side view of a single cell 202 of the overlay target 102 of FIG. 2A on a substrate 206, in accordance with one or more embodiments of the present disclosure. In some embodiments, the moiré structure 204 includes a first layer grating 208 (e.g., a top grating) located on a first layer 210 of the sample 106 and a second layer grating 212 (e.g., a bottom grating) located on a second layer 214 of the sample 106, and are oriented such that regions including the first layer grating 208 and the second layer grating 212 overlap to form a grating-over-grating structure. Additionally, the first layer grating 208 and the second layer grating 212 have different pitches. For example, FIG. 2B illustrates the pitch of the first layer grating 208 as P and the pitch of the second layer grating 212 as Q.

[0033] The overlay target 102 may generally be formed from any number of cells 202, and any particular cell 202 may include a moiré structure 204 with periodicity along any direction. Furthermore, in some embodiments, the overlay target 102 includes multiple cells 202 having moiré structures 204 with periodicity along a common direction, with different cells 202 having different configurations of the periodicity of the associated grating.

[0034] 2C is a side view of an overlay target 102 including two cells 202a, b having different configurations of Moiré structures 204 suitable for overlay measurement along a particular measurement direction (e.g., here, the X direction) in accordance with one or more embodiments of the present disclosure. In particular, FIG. 2C illustrates an inverted Moiré structure pair in which a first cell 202a includes a first layer grating 208 having a first pitch (P) and a second layer grating 212 having a second pitch (Q), while a second cell 202b includes a first layer grating 208 having a second pitch (Q) and a second layer grating 212 having the first pitch (P). It is contemplated herein that such an inverted Moiré structure pair can facilitate overlay determination based on a time-varying interference signal generated when both cells 202 are scanned relative to an illumination beam during measurement.

[0035] It is contemplated herein that the different cells 202a, b of an inverted moiré structure pair may be oriented in various configurations within the overlay target 102. In some embodiments, the cells 202a, b are oriented side-by-side along the direction of periodicity (e.g., the X direction in FIG. 2C ). Furthermore, as illustrated in FIG. 2C , the cells 202 may be arranged to provide a continuous structure such that phase transitions associated with time-varying interference signals are known and can be accounted for. Such an arrangement of an inverted moiré structure pair may be, but is not necessarily, referred to as a perpendicular overlay target. In particular, FIG. 2C depicts a specific, non-limiting configuration in which the central target features 216 on the first and second layers 210, 214 overlap. In this manner, the center of the combined inverted moiré structure pair can provide a reference point for phase transitions (e.g., φ), as described in more detail below.

[0036] However, it should be understood that the overlay target 102 in Figures 2A-2C and the associated description are provided for illustrative purposes only and should not be construed as limiting. Rather, the overlay target 102 can include any suitable overlay target design. For example, the overlay target 102 can include a grating-over-grating structure in which the first layer grating 208 and the second layer grating 212 have the same period (e.g., P). As another example, the overlay target 102 can include any number of cells 202 suitable for measurements along two directions. Furthermore, the cells 202 can be distributed in any pattern or arrangement. For example, metrology target designs suitable for scanning metrology are generally described in U.S. Patent Application No. 16 / 598,146, filed October 10, 2019, which is incorporated herein by reference in its entirety. In some embodiments, the overlay target 102 includes one or more cell groups distributed along a scan direction (e.g., the direction of motion of the sample 106), with the cells 202 within each particular cell group oriented to have a periodic Moiré structure along a common direction. For example, a first cell group may include one or more cells 202 with periodicity along the X direction, and a second cell group may include one or more cells 202 with periodicity along the Y direction. In this manner, all cells 202 within a particular cell group may be simultaneously imaged while scanning the sample 106 through the measurement field of view of the collection subsystem 136. As another example, diagonal targets suitable for metrology measurements in orthogonal directions in a single scan are generally described in U.S. Patent Application No. 16 / 964,734, filed July 24, 2020, which is incorporated herein by reference in its entirety.

[0037] 1A and 1B, various aspects of the optical subsystem 104 will be described in more detail, in accordance with one or more embodiments of the present disclosure.

[0038] In some embodiments, the optical subsystem 104 includes an illumination subsystem 124 for steering light directed toward the sample 106 and may include any suitable combination of one or more components (e.g., optical elements, etc.). For example, the illumination subsystem 124 may include one or more components for steering the illumination 110-1 before the interferometer 112. As another example, the illumination subsystem 124 may include one or more components for steering the primary illumination 110-2 and / or directing the primary illumination 110-2 toward the sample 106.

[0039] 1B , in some embodiments, the illumination subsystem 124 may include one or more illumination lenses 126 (e.g., to collimate the primary illumination 110-2, relay the illumination pupil plane 128 and / or the illumination field plane 130, etc.). In some embodiments, the illumination subsystem 124 includes one or more illumination control optics 132 for shaping or controlling the primary illumination 110-2. For example, the illumination control optics 132 may include, but are not limited to, one or more field stops, one or more pupil stops, one or more polarizers, one or more filters, one or more beam splitters, one or more diffusers, one or more homogenizers, one or more apodizers, one or more beam shapers, or one or more mirrors (e.g., static mirrors, translatable mirrors, scanning mirrors, etc.).

[0040] In some embodiments, the optical subsystem 104 includes an objective lens 134 for focusing the primary illumination 110-2 onto the sample 106 (eg, the overlay target 102 on the sample 106).

[0041] The primary illumination 110-2 may be directed at the overlay target 102 in any suitable spatial or angular distribution according to the particular metrology recipe being implemented.

[0042] In some embodiments, the illumination subsystem 124 illuminates the overlay target 102 with a single illumination beam from the primary illumination 110-2 during scanning. For example, the overlay target 102 can be designed according to a metrology recipe to include one or more cells distributed along the scan direction that can be sequentially illuminated with a single illumination beam.

[0043] In some embodiments, the illumination subsystem 124 illuminates the overlay target 102 with two or more illumination beams from the primary illumination 110-2, where the two or more illumination beams can be incident on the same or different portions of the overlay target 102. For example, the illumination subsystem 124 can illuminate different cells of the overlay target 102 with different illumination beams. For example, simultaneous illumination of multiple cells 202 of the overlay target 102 can facilitate simultaneous measurement along multiple measurement directions and / or measurement of multiple cells 202 necessary to complete a single measurement along a particular measurement direction. As another example, the illumination subsystem 124 can illuminate specific cells of the overlay target 102 with illumination beams having different angles of incidence, which can be useful, without limitation, for controlling the distribution of diffraction lobes at the collection pupil plane.

[0044] The primary illumination 110-2 may be provided as two or more illumination beams using any technique known in the art. In some embodiments, the optical subsystem 104 includes various optical elements (e.g., apertures, beam splitters, diffractive elements, etc.) for splitting the illumination 110-1 into two or more beams before the interferometer 112. In this manner, the primary illumination 110-2 and the auxiliary illumination 110-3 can have the same number of beams. For example, the optical subsystem 104 may include two or more apertures in the illumination pupil plane 128 to define two or more illumination beams based on the azimuth and / or elevation angles of incidence. As another example, the optical subsystem 104 may include one or more beam splitters or diffractive elements for splitting the illumination 110-1 into two or more illumination beams. As another example, the illumination source 108 directly generates the illumination 110-1 in the form of two or more illumination beams. In some embodiments, the optical subsystem 104 includes various optical elements in the paths of the primary illumination 110-2 and / or the auxiliary illumination 110-3 to separately modify the number and / or distribution of the associated illumination beams.

[0045] Additionally, the primary illumination 110-2 may be angularly limited on the sample 106 so that the grating structure within one or more cells of the overlay target 102 can generate discrete diffraction orders. Additionally, the primary illumination 110-2 may be spatially limited (e.g., as one or more illumination beams or lobes) to illuminate selected portions of the overlay target 102. For example, the primary illumination 110-2 may be split into two or more illumination beams, each of which may be spatially limited to illuminate a specific cell of the overlay target 102. In some embodiments, the primary illumination 110-2 underfills a specific cell of the overlay target 102.

[0046] In some embodiments, the optical subsystem 104 includes a collection subsystem 136 for collecting measurement light 114 from the sample 106. For example, the measurement light 114 can include selected diffraction orders of the primary illumination 110-2 by the overlay target 102, as determined by a metrology recipe.

[0047] In some embodiments, the illumination subsystem 124 and the collection subsystem 136 can utilize or include an objective lens 134. For example, Figure 1B shows a beam splitter 138 used to direct the illumination beam to the objective lens 134 to illuminate the sample 106, and also to direct the measurement light 114 from the objective lens 134 to the photodetector 116.

[0048] The collection subsystem 136 may further include one or more collection optics suitable for modifying and / or conditioning the measurement light 114 from the sample 106. In some embodiments, the collection subsystem 136 includes one or more collection lenses 140 (e.g., to collimate the illumination beam, relay a collection pupil plane 142 and / or a collection field plane 144, etc.). Additionally, the collection lenses 140 may, but need not, include the objective lens 134. In some embodiments, the collection subsystem 136 includes one or more collection control optics 146 for shaping or controlling the measurement light 114. For example, the collection control optics 146 may include, but are not limited to, one or more field diaphragms, one or more pupil diaphragms, one or more polarizers, one or more filters, one or more beam splitters, one or more diffusers, one or more homogenizers, one or more apodizers, one or more beam shapers, or one or more mirrors 148 (e.g., static mirrors, translatable mirrors, scanning mirrors, etc.).

[0049] The interferometer 112 can then combine the auxiliary illumination 110-3 with selected diffraction orders of the primary illumination 110-2 (e.g., selected portions of the measurement light 114) for detection. The primary illumination 110-2 and auxiliary illumination 110-3 through the interferometer 112 can generally have any optical path length as long as coherence is maintained for generation of a time-varying interference signal. In some embodiments, the optical path lengths of the primary illumination 110-2 and auxiliary illumination 110-3 through the interferometer 112 are designed to be approximately equal.

[0050] The interferometer 112 may include any component or combination of components suitable for combining a portion of the primary illumination 110-2 from the sample 106 (e.g., a selected portion of the measurement light 114) with the supplemental illumination 110-3 to generate interference.

[0051] Interferometer 112 may generally include any combination of optical elements suitable for interfering auxiliary illumination 110-3 with selected diffraction orders of primary illumination 110-2. For example, as shown in FIG. 1B , interferometer 112 may include one or more beam splitters 150 for splitting illumination 110-1 from illumination source 108 into primary illumination 110-2 and auxiliary illumination 110-3. Interferometer 112 may then further include one or more beam combiners 152 (e.g., beam splitters or other combining elements) for combining auxiliary illumination 110-3 with selected diffraction lobes of primary illumination 110-2 via overlay target 102.

[0052] The collection subsystem 136 may further include at least two photodetectors 116a,b positioned in at least one collection pupil plane 142 at positions relative to the time-varying interference signal indicative of the overlay (e.g., positions within the collection pupil plane 142 corresponding to overlap between the supplemental illumination 110-3 and selected diffraction orders of the primary illumination 110-2 by the overlay target 102). For example, as described in more detail below, suitable positions for the photodetectors 116 may include, but are not limited to, positions in the collection pupil plane 142 that include selected diffraction orders of the primary illumination 110-2 by the overlay target 102.

[0053] The photodetectors 116 (e.g., photodetectors 116a, b) can generally include any type of photodetector known in the art suitable for capturing interference signals generated when the sample 106 is translated by the translation stage 154 and / or when one or more illumination beams are scanned by the beam scanning subsystem 156. In some embodiments, the photodetectors 116a, b include single-pixel photodiodes, such as, but not limited to, photodiodes (e.g., high-speed photodiodes), photomultiplier tubes, or avalanche photodiodes (APDs). Such single-pixel photodetectors 116 can be individually positioned at selected locations within the collection pupil plane 142, such as locations associated with diffraction orders from features (e.g., measurement light 114) of the overlay target 102, according to a selected metrology recipe. In some embodiments, the photodetectors 116a, b are part of a multi-pixel sensor, such as, but not limited to, a line sensor, a complementary metal-oxide semiconductor (CMOS) sensor, or a charge-coupled device (CCD). In this manner, the photodetectors 116a,b may correspond to pixels or groups of pixels of such a multi-pixel sensor.

[0054] In general terms, the bandwidth or response time of the photodetector 116 should be sufficient to resolve the temporal frequency of the interference fringes related to the pitch of the component grating structures and the scanning speed along the scan. For example, for a scanning speed of 10 centimeters per second and a target pitch of 1 micrometer, the interference signal oscillates at a rate of approximately 100 kHz. In some embodiments, the photodetector 116 includes a photodetector having a bandwidth of at least 1 GHz. However, it should be understood that this value is not a requirement. Rather, the bandwidth of the photodetector 116, the translation speed along the measurement direction, and the pitch of the Moiré structures can be selected together to provide a desired sampling rate of the interference signal.

[0055] In some embodiments, the optical subsystem 104 includes a translation stage 154 for scanning the sample 106 through a measurement field of view of the optical subsystem 104 during measurement to perform scanning metrology.

[0056] In some embodiments, the optical subsystem 104 includes a beam scanning subsystem 156 configured to modify or control the position of the primary illuminator 110-2 (e.g., in the form of at least one illumination beam) on the sample 106. For example, the beam scanning subsystem 156 can scan the primary illuminator 110-2 in a direction orthogonal to the scan direction (e.g., the direction in which the translation stage 154 scans the sample 106) during measurement. In this manner, scanning measurements can be performed with any relative motion between the sample 106 and the primary illuminator 110-2.

[0057] The translation stage 154 and / or beam scanning subsystem 156 can be synchronized to the photodetector 116 during scanning using any suitable technique so that the signal generated by the photodetector 116 can be correlated to the position of the primary illuminator 110-2 on the overlay target 102 and / or the relative velocity of the overlay target 102 with respect to the primary illuminator 110-2 during scanning. Such information can facilitate correlation of the signal generated by the photodetector 116 with features of the overlay target 102 to determine overlay measurements.

[0058] In some embodiments, the optical subsystem 104 includes a phase modulator (not shown) for modulating the phase of the supplemental illumination 110-3. In this manner, both the primary illumination 110-2 and the sample 106 may remain stationary during scanning.

[0059] The optical subsystem 104 may include one or more collection channels 158. For example, FIG. 1B illustrates a configuration of the optical subsystem 104 having two collection channels 158. It is contemplated herein that the photodetectors 116 may be distributed in any desired distribution among the one or more collection channels 158. In some embodiments, multiple photodetectors 116 are disposed in at least one collection channel 158. In this configuration, the multiple collection channels 158 can facilitate simultaneous measurements of different cells (e.g., associated with different measurement directions, different cells required for a particular measurement based on a particular metrology recipe, etc.). In some embodiments, each collection channel 158 includes a single photodetector 116 (e.g., photodetector 116a or photodetector 116b). In this manner, each photodetector 116 can be positioned anywhere within the collection pupil plane 142, regardless of its physical size or any associated components.

[0060] The optical subsystem 104 may include one or more channel beam splitters 160 positioned to split the light in the collection subsystem 136 into different collection channels 158. In some embodiments, the channel beam splitters 160 are positioned after the interferometer 112, as shown in FIG. 1B. In this manner, the light in the collection channels 158 includes time-varying interference patterns that exhibit overlay as described herein.

[0061] The channel beam splitter 160 may include any component suitable for splitting light using any technique, including, but not limited to, a polarizing beam splitter or a dichroic beam splitter (e.g., a dichroic mirror). In some embodiments, the channel beam splitter 160 splits light into different collection channels 158 based on a property of the light, such as, but not limited to, polarization or wavelength. In this manner, the channel beam splitter 160 may separate light from different beams of the primary illumination 110-2 and / or different cells 202 of the overlay target 102 into different collection channels 158. Illustratively, the optical subsystem 104 may direct different beams of the primary illumination 110-2 to different cells 202 of the overlay target 102, where the different beams of the primary illumination 110-2 have different properties (e.g., polarization, wavelength, etc.). One or more channel beam splitters 160 can then separate different beams of the main illumination 110-2 and / or measurement light 114 associated with different cells 202 of the overlay target 102 into different collection channels 158 to provide simultaneous measurements.

[0062] 3A-5, various configurations for overlapping the supplemental illumination 110-3 with selected diffraction lobes of the primary illumination 110-2 are described in more detail in accordance with one or more embodiments of the present disclosure. In particular, FIGS. 3A-5 provide a non-limiting illustration of the specific case of overlay measurement on the overlay target 102 shown in FIGS. 2A-2C, which includes at least one Moiré structure formed from gratings within specific cells having different pitches (e.g., pitches P and Q). Scanning-based scatterometry overlay measurements based on Moiré structures are generally described in U.S. Patent Application No. 17 / 708,958, filed March 30, 2022, which is incorporated herein by reference in its entirety. However, it should be understood that FIGS. 3A-5 are provided solely for illustrative purposes and should not be construed as limiting. Rather, the systems and methods disclosed herein can be extended for use with any suitable design of the overlay target 102. In other words, the systems and methods disclosed herein can be extended to provide overlap between supplemental illumination 110-3 and any selected diffraction lobe from any selected target design of overlay target 102 suitable for overlay metrology based on any selected metrology recipe. For example, the systems and methods disclosed herein can be extended to overlay targets 102 and / or associated metrology recipes described in, but not limited to, U.S. Patent No. 11,300,405, issued April 12, 2022, U.S. Patent No. 11,378,394, issued July 5, 2022, and U.S. Patent Application No. 17 / 709,104, filed March 30, 2022, all of which are incorporated herein by reference in their entireties.

[0063]

[0031] Figures 3A and 3B show a first non-limiting example of illumination and collection pupil distributions for scanning overlay metrology based on normal-incidence primary illumination 110-2, in accordance with one or more embodiments of the present disclosure. Figure 4 shows a second non-limiting example of illumination and collection pupil distributions for scanning overlay metrology based on oblique-incidence primary illumination 110-2, in accordance with one or more embodiments of the present disclosure. Figure 5 shows a third non-limiting example of illumination and collection pupil distributions for scanning overlay metrology based on oblique-incidence primary illumination 110-2, in accordance with one or more embodiments of the present disclosure.

[0064] 3A is a top view of the illumination pupil plane 128 of the optical subsystem 104 (e.g., shown in FIG. 1B ) in accordance with one or more embodiments of the present disclosure. In some embodiments, the illumination subsystem 124 illuminates the overlay target 102 with one or more illumination beams of primary illumination 110-2 at normal incidence (or near-normal incidence), as shown in FIG. 3A . For example, FIG. 3A shows a single illumination beam of primary illumination 110-2 at the center of the pupil boundary 302 of the illumination pupil plane 128. Furthermore, the one or more illumination beams can illuminate the overlay target 102 at a limited range of angles of incidence, as indicated by the limited size of the collection pupil plane 142. In this regard, the overlay target 102 can diffract the primary illumination 110-2 into discrete diffraction orders.

[0065] It is recognized herein that the distribution of diffraction orders of an illumination beam due to periodic structures in the overlay target 102 can be affected by various parameters, such as, but not limited to, the wavelength of the illumination beam, the angle of incidence of the illumination beam in both elevation and azimuth directions, the pitch of the grating of the overlay target 102, or the numerical aperture (NA) of the collection lens. Accordingly, in embodiments of the present disclosure, the illumination subsystem 124, the collection subsystem 136, and the overlay target 102 may be configured to provide a desired distribution of diffraction orders in the collection pupil suitable for generating a time-varying interference pattern indicative of overlay. For example, the illumination subsystem 124 and / or the collection subsystem 136 may be configured (e.g., using a measurement recipe) to generate measurements on an overlay target 102 having grating features with a range of pitches selected to provide the desired collection pupil distribution. Additionally, various components (e.g., stops, pupil, etc.) of the illumination subsystem 124 and / or the collection subsystem 136 may be adjustable (e.g., using a measurement recipe) to provide the desired collection pupil distribution.

[0066] Furthermore, the size and shape of the diffraction orders at the collection pupil plane 142 may generally be related to the size and shape of the illumination beam on the sample 106. For example, although not shown, if the illumination beam is elongated, the associated diffraction orders may be similarly elongated.

[0067] Referring now to FIG. 3B, the tailored overlap between supplemental illumination 110-3 and selected diffraction lobes to promote a high signal-to-noise ratio, according to one or more embodiments of the present disclosure, is described in more detail.

[0068] It is contemplated herein that the signal-to-noise ratio of the time-varying interference signals, and therefore the sensitivity of overlay measurements based on these signals, can be affected by various factors, including, but not limited to, the strength and contrast of the time-varying interference signals generated by the photodetector 116. For example, it may generally be desirable to increase the overlap area between the diffraction lobes of interest to increase the total number of photons contributing to the time-varying interference signal. It may also generally be desirable to limit the collection of light from other portions of the collection pupil plane 142 (e.g., outside the overlap region of interest) to limit the constant (DC) bias signal captured by the photodetector 116 along with the time-varying interference signal of interest. Furthermore, it may generally be desirable to match the relative intensities of the diffraction orders to promote a high-contrast interference pattern so that the interference fringes have high visibility.

[0069] It is further contemplated herein that the use of an interferometer 112 to split illumination 110-1 from illumination source 108 into primary illumination 110-2 and separate auxiliary illumination 110-3 incident on overlay target 102 allows for adjustment of various conditions to promote a high signal-to-noise ratio.

[0070] For example, the amount of overlap between the diffraction lobes of the primary illumination 110-2 due to the overlay target 102 (and therefore the number of photons contributing to the time-varying interference signal) may be limited by parameters such as, but not limited to, the wavelength of the primary illumination 110-2, the pitch of the grating in the overlay target 102, and the beam shape of the incident primary illumination 110-2, and therefore the resulting number of diffraction orders. In some cases, it may be impractical or impossible to directly provide a distribution of diffraction lobes with a reasonable overlap area to provide an adequate signal-to-noise ratio. However, the use of a separate but coherent auxiliary illumination 110-3 eliminates or mitigates the requirement to directly generate overlapping diffraction lobes, as the auxiliary illumination 110-3 can be flexibly positioned as desired within the collection pupil plane 142. More generally, various parameters of the auxiliary illumination 110-3 can be adjusted (e.g., using collection control optics 146, etc.), including, but not limited to, the number of lobes in the collection pupil plane 142, lobe size, lobe shape, lobe intensity, lobe polarization, or lobe position.

[0071] 3B is a top view of the collection pupil boundary 304 in the collection pupil plane 142 of the optical subsystem 104, illustrating the overlap between the supplemental illumination 110-3 of the primary illumination 110-2 and the + / -1 order diffraction lobes from the moiré structure 204 of FIGS. 2A-2B, in accordance with one or more embodiments of the present disclosure. In particular, FIG. 3B illustrates the overlap between the -1 order diffraction lobe 306 (-1 P ) and the −1st order diffraction lobe 308 (−1 Q ) from the first layer grating 208. P ) and the +1st order diffraction lobe 312 (+1 Q 3B further shows the reflected zeroth order diffraction lobe 314 of the primary illuminator 110-2.

[0072] As used herein, the number or shape of the lobes of the auxiliary illumination 110-3 and / or the particular arrangement of the number or shape of the diffraction lobes relative to the diffraction of the primary illumination 110-2 shown in FIG. 3B are merely exemplary and should not be construed as limiting. Rather, any arrangement of such lobes that provides a time-varying signal is within the spirit and scope of the present disclosure. By way of example, the −1st order diffraction lobe 306 does not necessarily overlap with the −1st order diffraction lobe 308, and the +1st order diffraction lobe 310 does not necessarily overlap with the +1st order diffraction lobe 312. Rather, a time-varying interference signal (possibly having different frequencies) can be generated based on the overlap of these lobes with the auxiliary illumination 110-3. Furthermore, the auxiliary illumination 110-3 may be distributed into any number of lobes having any shape. For example, although not shown in FIG. 3B, the auxiliary illumination 110-3 may be divided into four lobes that are separately positioned to overlap with the −1st order diffraction lobe 306, the −1st order diffraction lobe 308, the +1st order diffraction lobe 310, and the +1st order diffraction lobe 312.

[0073] It is further contemplated herein that the time-varying signal indicative of overlay may be generated by various configurations of one or more photodetectors 116 in the collection pupil plane 142. In some embodiments, a first photodetector 116 (not shown) detects a first lobe 110-3a of supplemental illumination and a −1 order diffraction lobe 306 (−1 P ) and the −1st order diffraction lobe 308 (−1 Q ) from the first layer grating 208. Similarly, the second photodetector 116 (not shown) captures a portion of the pupil plane including the overlap between the second lobe 110-3b of the supplemental illumination and the +1st order diffraction lobe 310 (+1 P ) and the +1st order diffraction lobe 312 (+1 Q ) can capture a portion of the pupil plane including the overlap between the first layer grating 208 (-1). In some embodiments, the first photodetector 116 (not shown) can capture a portion of the pupil plane including the overlap between the first layer grating 208 (-1 P) can capture the overlap between the first lobe 110-3a of the supplemental illumination with the −1 order diffraction lobe 306 from the second layer grating 212 (−1 Q ) and a third photodetector 116 (not shown) can capture the overlap between the first lobe 110-3a of the supplemental illumination with the −1st order diffraction lobe 308 from the first layer grating 208 (+1 P ) and the +1st order diffraction lobe 310, and a fourth photodetector 116 (not shown) captures the overlap between the second lobe 110-3b of the supplemental illumination from the second layer grating 212 (+1 Q ) and the +1st order diffraction lobe 312.

[0074] Thus, in a general sense, any number of photodetectors 116 may be used to capture time-varying interference signals associated with overlap between various diffraction lobes. Furthermore, as previously described herein, the photodetectors 116 may be formed as single pixel detectors or may correspond to pixels of a detector array and / or multi-pixel sensor.

[0075] Various aspects of the configuration shown in FIG. 3B can contribute to a high signal-to-noise ratio. For the non-limiting cell 202 of FIGS. 2A-2B , a time-varying interference signal can be generated by the overlap of the diffraction lobes associated with both the first layer grating 208 and the second layer grating 212 with each other or with another reference, such as the zeroth diffraction order. Absent the separate auxiliary illumination 110-3 provided by the systems and methods disclosed herein, a time-varying interference signal can be generated at overlap locations between the + / -1 diffraction lobes of these constituent gratings (e.g., the overlap between the +1st diffraction order lobe 310 and the +1st diffraction order lobe 312, the overlap between the -1st diffraction order lobe 306 and the -1st diffraction order lobe 308, etc.), Moiré diffraction lobes (e.g., associated with double diffraction from both the first layer grating 208 and the second layer grating 212), or the overlap region between the Moiré diffraction lobe and the zeroth diffraction order lobe 314. However, the amount of overlap between the diffraction lobes in each of these configurations depends on the difference between the pitches of these gratings as well as the beam shape of the incident illumination.

[0076] In contrast, Figure 3B illustrates a configuration in which the supplemental illumination 110-3 overlaps with the + / -1 diffraction lobes, as described above. In this configuration, the + / -1 diffraction lobes from the first layer grating 208 and the second layer grating 212 do not need to overlap with each other or with the zeroth order diffraction lobe 314, as shown in Figure 3B. Rather, the lobes of the supplemental illumination 110-3a,b may be positioned and sized to overlap with the diffraction lobes from the first layer grating 208 and the second layer grating 212. In particular, Figure 3B illustrates a configuration in which there is minimal overlap between the diffraction lobes from the first layer grating 208 and the second layer grating 212 (e.g., minimal overlap between the +1st order diffraction lobe 310 and the +1st order diffraction lobe 312), and no overlap with the reflected zeroth order diffraction lobe 314. This allows flexibility in selecting the shape of the primary illumination 110-2, the wavelength of the primary illumination 110-2, and the pitch of the first layer grating 208 and the second layer grating 212. The size, position, shape, and intensity of the lobes of the supplemental illumination 110-3 can then be selected to provide time-varying interference fringes with a high signal-to-noise ratio.

[0077] Reference is now made to FIG. 4, which is a conceptual diagram of an optical subsystem 104 that provides oblique primary illumination 110-2 and associated supplemental illumination 110-3, in accordance with one or more embodiments of the present disclosure.

[0078] FIG. 4 conceptually illustrates the rotating dipole distribution of the illumination beam of primary illumination 110-2 (110-2a, 110-2b) at illumination pupil plane 128, directed toward overlay target 102 via beam splitter 138 and objective lens 134. FIG. 4 further illustrates collection pupil plane 142, including the diffraction lobes of primary illumination 110-2, oriented along overlay target 102 of FIGS. 2A-2B, but with periodic orientation along the Y direction. In particular, FIG. 4 illustrates −1st order diffraction lobe 306 and −1st order diffraction lobe 308 from first illumination lobe 110-2a, along with +1st order diffraction lobe 310 and +1st order diffraction lobe 312 from second illumination lobe 110-2b. Finally, Figure 4 shows the dipole distribution 402 of the supplemental illumination 110-3 rotated relative to the primary illumination 110-2 so that the lobes of the supplemental illumination 110-3a,b overlap with the first-order diffraction from the first layer grating 208 and the second layer grating 212. In this manner, Figure 4 can provide measurements similar to those shown in Figure 3B. However, the use of oblique illumination can enable an overlay target 102 with a smaller pitch and potentially a smaller overall size.

[0079] 4 illustrates both the operation of beam splitter 138 to facilitate illumination and collection by objective lens 134, and as beam combiner 152 of interferometer 112 to combine supplemental illumination 110-3 with the diffraction lobe of primary illumination 110-2. However, it should be understood that this is merely an example and that separate components may be used.

[0080] Reference is now made to FIG. 5, which is a conceptual diagram of an optical subsystem 104 for simultaneously characterizing an overlay target 102 in multiple lighting conditions, in accordance with one or more embodiments of the present disclosure.

[0081] In some embodiments, the optical subsystem 104 provides illumination 110-1 with two different illumination conditions having different characteristics, such as, but not limited to, wavelength or polarization. In this manner, the optical subsystem 104 can provide two distributions of primary illumination 110-2 and associated supplemental illumination 110-3 for different measurements in different collection channels 158.

[0082] For example, Figure 5 shows an illumination pupil plane 128 including a first rotated dipole distribution (110-2a, 110-2b) of the illumination beam of the primary illumination 110-2 (e.g., as shown in Figure 4) and a second rotated dipole distribution (110-2c, 110-2d) of the illumination beam of the primary illumination 110-2 at the anti-symmetry limit of the illumination pupil plane 128. Figure 5 further shows a corresponding rotated dipole distribution of the supplemental illumination 110-3 including lobes (110-3a, 110-3b) and a second rotated dipole distribution of the supplemental illumination 110-3 including lobes (110-3c, 110-3d).

[0083] 5 further illustrates a channel beam splitter 160 for separating the diffraction lobes of the primary illumination 110-2 and the corresponding auxiliary illumination 110-3 having different illumination distributions into different collection channels 158. For example, the channel beam splitter 160 may include a dichroic beam splitter, a polarizing beam splitter, or any other component suitable for separating the primary illumination 110-2 and the auxiliary illumination 110-3 under different illumination conditions.

[0084] By way of example, FIG. 5 shows a first collection channel 158a that can include lobes of supplemental illumination 110-3a,b that overlap selected diffraction lobes of primary illumination 110-2a,b, while a second collection channel 158b can include lobes of supplemental illumination 110-3c,d that overlap selected diffraction lobes of primary illumination 110-2c,d.

[0085] It is contemplated herein that the technique illustrated in FIG. 5 may be applicable to a wide variety of simultaneous measurement schemes. For example, as illustrated in FIG. 5, two illumination conditions may provide simultaneous measurements of the same cell 202 of the overlay target 102 having different illumination conditions. As another example, lobes of the primary illumination 110-2 having different illumination conditions may be directed at different cells 202 of the overlay target 102 to provide simultaneous measurements of the different cells 202. Furthermore, the different cells 202 may include grating structures having periodicities in the same or different directions. In this manner, any of the systems and methods disclosed herein may be extended to, but are not limited to, any of the multi-cell illumination measurement techniques shown in U.S. Patent No. 11,300,405, issued April 12, 2022, and U.S. Patent Application No. 17 / 708,958, filed March 30, 2022, both of which are incorporated herein by reference in their entireties.

[0086] 1A and 1B, additional details of the optical subsystem 104, in accordance with one or more embodiments of the present disclosure, will be described in greater detail.

[0087] The one or more processors 120 of the controller 118 may generally include any processor or processing element known in the art. For purposes of this disclosure, the term “processor” or “processing element” may be broadly defined to encompass any device having one or more processing or logic elements (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, the one or more processors 120 may include any apparatus configured to execute algorithms and / or instructions (e.g., program instructions stored in a memory). In some embodiments, as described throughout this disclosure, the one or more processors 120 may be embodied as a desktop computer, a mainframe computer system, a workstation, an image computer, a parallel processor, a networked computer, or any other computer system configured to execute programs configured to operate in conjunction with the overlay metrology system 100. Additionally, different subsystems of the overlay metrology system 100 may include processors or logic elements suitable for performing at least some of the steps described in this disclosure. Therefore, the above description should not be construed as a limitation on embodiments of the present disclosure, but merely as an example. Additionally, the steps described throughout this disclosure may be performed by a single controller or, alternatively, by multiple controllers. Furthermore, the controller 118 may include one or more controllers housed within a common housing or multiple housings. In this manner, any controller or combination of controllers may be packaged separately as a module suitable for integration into the metrology overlay metrology system 100.Additionally, the controller 118 may analyze or process the data received from the photodetector 116 and provide the data to additional components within the overlay metrology system 100 or external to the overlay metrology system 100 .

[0088] Additionally, memory 122 may include any storage medium known in the art suitable for storing program instructions executable by the associated one or more processors 120. For example, memory 122 may include a non-transitory memory medium. As additional examples, memory 122 may include, but is not limited to, read-only memory, random access memory, magnetic or optical memory devices (e.g., disks), magnetic tape, solid-state drives, etc. It is further noted that memory 122 may be housed within a common controller housing with one or more processors 120.

[0089] In this regard, the controller 118 can perform any of a variety of processing steps associated with overlay metrology. For example, the controller 118 can be configured to generate control signals to guide or control the optical subsystem 104, or any component thereof. For example, the controller 118 can be configured to direct the translation stage 154 to translate the sample 106 along one or more measurement paths or swaths to scan one or more overlay targets through the measurement field of the optical subsystem 104, and / or direct the beam scanning subsystem 156 to position or scan one or more illumination beams on the sample 106. As another example, the controller 118 can be configured to receive a signal corresponding to a time-varying interference signal from the photodetector 116. As another example, the controller 118 can generate correction tables for one or more additional build tools as feedback and / or feedforward control of the one or more additional build tools based on the overlay measurements from the optical subsystem 104.

[0090] In some embodiments, the controller 118 captures the interference signal detected by the photodetector 116. The controller 118 can generally capture data such as, but not limited to, the magnitude or phase of the time-varying interference signal (e.g., intensity and / or phase information) using any technique known in the art. For example, the controller 118 can capture data such as, but not limited to, the magnitude or phase of the time-varying interference signal (e.g., intensity and / or phase information) using a phase-locking technique (e.g., a phase-locked loop). For example, the controller 118 can capture data such as, but not limited to, the magnitude or phase of the time-varying interference signal (e.g., intensity and / or phase information) using a Fourier analysis technique (or any other suitable spectral decomposition technique). In a general sense, the controller 118 can capture the interference signal, or any data related to the interference signal, using any combination of hardware (e.g., circuitry) or software techniques.

[0091] In some embodiments, the controller 118 determines an overlay measurement between layers of the overlay target 102 along the measurement direction based on a comparison of the interference signals. For example, the controller 118 can compare the magnitude and / or phase of the interference signals to generate an overlay measurement. For example, the electric field of the diffraction orders within the collection pupil and further providing a specific relationship between overlay and measured intensity within the pupil plane are generally described in U.S. Patent No. 10,824,079, issued November 3, 2020, and incorporated herein by reference in its entirety. It is believed that the systems and methods disclosed herein can extend the teachings of U.S. Patent No. 10,824,079 to time-varying interference signals captured by the photodetector 116 disposed in the overlap region disclosed herein. In particular, it is contemplated herein that overlay on the sample 106 may be proportional to an asymmetry, such as, but not limited to, the relative phase shift between two time-varying interference signals. In another example, the relative intensity of the diffraction orders within the pupil plane may be extracted from the time-varying interference signal. In this manner, any overlay algorithm based on relative intensity differences of diffraction orders known in the art can be applied to generate overlay measurements.

[0092] Additionally, the controller 118 can calibrate or modify the overlay measurements based on known, assumed, or measured features of the sample that may also affect the time-varying interference signal, such as, but not limited to, sidewall angle or other sample asymmetries.

[0093] In some embodiments, the optical subsystem 104 includes a beam scanning subsystem 156 for positioning, scanning, or modulating the position of one or more illumination beams on the sample 106 during measurements. Additionally, the beam scanning subsystem 156 and the photodetector 116 can be synchronized to facilitate a link between the scanning speed and the pitch of the features of the overlay target 102.

[0094] The beam scanning subsystem 156 can include any type or combination of elements suitable for scanning the position of one or more illumination beams. In some embodiments, the beam scanning subsystem 156 includes one or more deflectors suitable for changing the direction of the illumination beams. For example, the deflectors can include, but are not limited to, rotatable mirrors (e.g., mirrors with adjustable tip and / or tilt). Furthermore, the rotatable mirrors can be actuated using any technique known in the art. For example, the deflectors can include, but are not limited to, galvanometers, piezoelectric mirrors, or microelectromechanical systems (MEMS) devices. As another example, the beam scanning subsystem 156 may include an electro-optic modulator, an acousto-optic modulator, or the like.

[0095] Deflectors may further be disposed at any suitable location within the optical subsystem 104. In some embodiments, one or more deflectors are disposed at one or more pupil planes common to both the illumination subsystem 124 and the collection subsystem 136. In this regard, the beam scanning subsystem 156 may be a pupil plane beam scanner, and an associated deflector may modify the position of one or more illumination beams on the sample 106 without affecting the position of the diffraction orders in the collection pupil plane 142. Furthermore, the distribution of one or more illumination beams in the illumination field plane 130 may be further stabilized as the beam scanning subsystem 156 changes the position of one or more illumination beams on the sample 106. Pupil plane beam scanning is generally described in U.S. Patent No. 11,300,524, issued April 12, 2022, which is incorporated herein by reference in its entirety.

[0096] 6, which is a flow diagram illustrating steps performed in an overlay metrology method 600, in accordance with one or more embodiments of the present disclosure. Applicant notes that the embodiments and enabling techniques described herein above in the context of overlay metrology system 100 should be construed to extend to method 600. However, it is further noted that method 600 is not limited to the architecture of overlay metrology system 100.

[0097] In some embodiments, method 600 includes generating illumination using an illumination source 602. In some embodiments, method 600 includes splitting illumination from the illumination source into primary illumination and supplemental illumination 604. In some embodiments, method 600 includes directing the primary illumination onto an overlay target on the specimen 606, where the overlay target includes a grating of two or more layers.

[0098] In some embodiments, method 600 includes step 608 of superimposing a first portion of the supplemental illumination and at least one diffraction lobe of the primary illumination from the overlay target onto a first photodetector in the collection pupil plane to generate a first interference pattern. In some embodiments, method 600 includes step 610 of superimposing a second portion of the supplemental illumination and at least one additional diffraction lobe of the primary illumination from the overlay target onto a second photodetector in the collection pupil plane to generate a second interference pattern. In some embodiments, method 600 includes step 612 of modulating the phase of the first and second interference patterns during scanning of the overlay target with a scanning subsystem. For example, the scanning subsystem can include at least one of a translation stage that scans the sample relative to the primary illumination, beam scanning optics that scans the primary illumination relative to the sample, or a phase modulator that modulates the phase of the supplemental illumination.

[0099] In some embodiments, method 600 includes generating 614 a time-varying interference signal at the first and second photodetectors based on the modulated phase of the first and second interference patterns. In some embodiments, method 600 includes generating 616 one or more overlay measurements of the overlay target based on the time-varying interference signal.

[0100] It is contemplated herein that method 600 can be used to generate overlay metrology measurements with any overlay target design and associated metrology recipe suitable for scanning scatterometry overlay metrology. For example, the overlay targets can include gratings with a common pitch. As another example, the overlay targets can include gratings with different pitches (e.g., forming a Moiré structure). In this manner, the particular diffraction lobes that overlap with the supplemental illumination can vary (e.g., in steps 608 and 610) based on the particular overlay target design and associated metrology recipe.

[0101] It is further contemplated herein that method 600 can be extended to provide simultaneous measurements. For example, the primary and auxiliary illumination can be split into multiple beams with different illumination conditions (e.g., variations in wavelength, polarization, or other suitable parameters) so that the diffraction lobes associated with each illumination condition can be separated into different channels. In this manner, method 600 can be extended to provide measurements of a single cell with different illumination conditions or measurements of different cells with different illumination conditions. For simultaneous measurements of different cells, the different cells can include grating structures with different periodicity directions (e.g., grating directions) for simultaneous overlay measurements along different directions, or grating structures with the same periodicity direction but different grating layouts (e.g., offsets between the upper and lower gratings, different pitch configurations between the upper and lower gratings, etc.) to facilitate multi-cell metrology recipes.

[0102] The subject matter described herein may depict different components contained within or connected to other components. It should be understood that any architectures depicted are merely exemplary, and that in fact, many other architectures that achieve the same functionality may be implemented. In a conceptual sense, any arrangement of components to achieve the same functionality is effectively “associated” such that the desired functionality is achieved. Thus, any two components herein that combine to achieve a particular functionality can be considered to be “associated” with each other such that the desired functionality is achieved, regardless of the architecture or intermediate components. Similarly, any two components so associated can also be considered to be “connected” or “coupled” to each other to achieve the desired functionality, and any two components so associated can also be considered to be “couplable” to each other to achieve the desired functionality. Specific examples of combinable components include, but are not limited to, components that are physically interactable and / or physically interacting, components that are wirelessly interactable and / or wirelessly interacting, and / or components that are logically interactable and / or logically interacting.

[0103] The present disclosure and many of its attendant advantages will be understood from the foregoing description, and it will be apparent that various changes in form, construction, and arrangement of the elements can be made without departing from the disclosed subject matter or sacrificing all of its material advantages. The described forms are merely illustrative, and it is the intent of the following claims to embrace and cover all such modifications. It will further be understood that the invention is defined by the appended claims.

Claims

1. 1. An overlay metrology system, comprising: an illumination source; a first beam splitter configured to split illumination from the illumination source into primary illumination and supplemental illumination; one or more illumination optics configured to direct the primary illumination to an overlay target on a specimen when performing a metrology recipe, the overlay target according to the metrology recipe including gratings in two or more layers; an objective lens for collecting at least one positive diffraction lobe and at least one negative diffraction lobe associated with diffraction of the primary illumination from the grating of each of the two or more layers when performing the metrology recipe; one or more photodetectors in one or more collection pupil planes; one or more collection optics, superimposing a first portion of the supplemental illumination and at least one diffraction lobe of the primary illumination by the overlay target on at least one of the one or more photodetectors to generate a first interference pattern; one or more collection optics configured to implement the metrology recipe by superimposing a second portion of the supplemental illumination and at least one additional diffraction lobe of the primary illumination by the overlay target onto at least one of the one or more photodetectors to generate a second interference pattern; a scanning subsystem including at least one of a translation stage that scans the specimen relative to the primary illumination, beam scanning optics that scans the primary illumination relative to the specimen, or a phase modulator that modulates the phase of the supplemental illumination during scanning of the overlay target when performing the metrology recipe; a controller communicatively coupled to the one or more light detectors, the controller comprising one or more processors configured to execute program instructions, the program instructions causing the one or more processors to: receiving a time-varying interference signal from the one or more photodetectors during the scanning of the overlay target; generating one or more overlay measurements of the overlay target based on the time-varying interference signal; and a controller that causes the metrology recipe to be performed.

2. 2. The overlay metrology system of claim 1, wherein the at least one diffraction lobe comprises at least one positive diffraction lobe of a selected diffraction order, and the at least one additional diffraction lobe comprises at least one negative diffraction lobe of the selected diffraction order.

3. The overlay metrology system of claim 1 , wherein the primary illumination has a rotating dipole distribution with respect to a grating direction of the grating structure on the overlay target.

4. The overlay target according to the metrology recipe is a first cell including a grating having a pitch P on a first layer and a grating having a pitch Q on a second layer to form a first moiré structure; a second cell including a grating having pitch Q on the first layer and a grating having pitch P on the second layer to form a second moiré structure; the one or more collection optics: superimposing, on at least one of the one or more photodetectors, the first portion of the supplemental illumination and both a +1 diffraction lobe and a positive Moire diffraction lobe of the primary illumination by the overlay target to generate the first interference pattern; 2. The overlay metrology system of claim 1, configured to perform the metrology recipe by superimposing, on at least one of the one or more photodetectors, the second portion of the supplemental illumination and both a −1 diffraction lobe and a negative Moire diffraction lobe of the primary illumination by the overlay target to produce the second interference pattern.

5. The overlay target according to the metrology recipe is a cell including one or more gratings having a first and second pitch on a first layer and a grating having a third pitch on a second layer; the one or more collection optics: superimposing the first portion of the supplemental illumination and a +1 diffraction order from the first, second, and third pitches on at least one of the one or more photodetectors to generate the first interference pattern; 2. The overlay metrology system of claim 1, configured to implement the metrology recipe by superimposing the second portion of the supplemental illumination and −1 diffraction orders from the first, second, and third pitches on at least one of the one or more photodetectors to generate the second interference pattern.

6. The overlay target according to the metrology recipe is a cell including a grating having a first pitch on a first layer and a grating having a second pitch on a second layer; the one or more collection optics: superimposing a first portion of the supplemental illumination and +1 and +2 diffraction orders from the first pitch along with +1 diffraction from the second pitch on at least one of the one or more photodetectors to generate the first interference pattern; 2. The overlay metrology system of claim 1, configured to perform the metrology recipe by superimposing a second portion of the supplemental illumination and −1 and −2 diffraction orders from the first pitch along with −1 diffraction from the second pitch on at least one of the one or more photodetectors to generate the second interference pattern.

7. the one or more photodetectors: one or more phase-locked photodetectors locked to a frequency of the time-varying interference signal to generate the one or more overlay measurements of the overlay target based on the time-varying interference signal; extracting at least one of intensity or phase information associated with the time-varying interference signal using a phase-locking technique; and determining an overlay error between the two or more layers of the specimen based on the at least one of the intensity or the phase information.

8. generating the one or more overlay measurements of the overlay target based on the time-varying interference signal; extracting at least one of intensity or phase information associated with the time-varying interference signal using Fourier analysis techniques; and determining an overlay error between the two layers of the specimen based on the at least one of the intensity or the phase information.

9. The overlay metrology system of claim 1 , wherein the one or more illumination optics direct the primary illumination onto the overlay target at a normal angle of incidence.

10. The overlay metrology system of claim 1 , wherein the illumination from the illumination source is spatially coherent.

11. 1. An overlay metrology method, comprising: generating illumination using an illumination source; dividing the illumination from the illumination source into primary illumination and supplemental illumination; directing the primary illumination onto an overlay target on a specimen, the overlay target including a grating of two or more layers; superimposing a first portion of the supplemental illumination and at least one diffraction lobe associated with diffraction of the primary illumination from the grating of each of the two or more layers onto one or more first photodetectors in a collection pupil plane to generate a first interference pattern; superimposing a second portion of the supplemental illumination and at least one additional diffraction lobe associated with the diffraction of the primary illumination from the grating of each of the two or more layers onto one or more second photodetectors in the collection pupil plane to generate a second interference pattern; modulating the phase of the first and second interference patterns during scanning of the overlay target using a scanning subsystem, the scanning subsystem including at least one of a translation stage that scans the sample relative to the primary illumination, beam scanning optics that scans the primary illumination relative to the sample, or a phase modulator that modulates the phase of the supplemental illumination; generating a time-varying interference signal at the one or more first photodetectors and the one or more second photodetectors based on the modulated phases of the first and second interference patterns; generating one or more overlay measurements of the overlay target based on the time-varying interference signal.

12. superimposing, on the one or more first photodetectors in the collection pupil plane, the first portion of the supplemental illumination with the at least one diffraction lobe associated with diffraction of the primary illumination from the grating of each of the two or more layers to generate the first interference pattern; 12. The overlay metrology method of claim 11, comprising superimposing, on the one or more first photodetectors in the collection pupil plane, the first portion of the supplemental illumination and at least one positive diffraction lobe of a selected diffraction order associated with diffraction of the primary illumination from the grating of each of the two or more layers to produce the first interference pattern.

13. superimposing the second portion of the supplemental illumination and the at least one additional diffraction lobe associated with diffraction of the primary illumination from the grating of each of the two or more layers onto the one or more second photodetectors in the collection pupil plane to generate the second interference pattern; 12. The overlay metrology method of claim 11, comprising superimposing, on the one or more second photodetectors in the collection pupil plane, the second portion of the supplemental illumination and at least one negative diffraction lobe of the selected diffraction order associated with diffraction of the primary illumination from the grating of each of the two or more layers to produce the second interference pattern.

14. The overlay target is a first cell including a grating having a pitch P on a first layer and a grating having a pitch Q on a second layer to form a first moiré structure; a second cell including a grating having pitch Q on the first layer and a grating having pitch P on the second layer to form a second moiré structure; the one or more collection optics: superimposing, on at least one of the one or more photodetectors, the first portion of the supplemental illumination and both a +1 diffraction lobe and a positive Moire diffraction lobe of the primary illumination by the overlay target to generate the first interference pattern; and superimposing, on at least one of the one or more photodetectors, the second portion of the supplemental illumination and both a −1 diffraction lobe and a negative Moire diffraction lobe of the primary illumination by the overlay target to produce the second interference pattern.

15. The overlay target is a cell including one or more gratings having a first and second pitch on a first layer and a grating having a third pitch on a second layer; the one or more collection optics: superimposing the first portion of the supplemental illumination and a +1 diffraction order from the first, second, and third pitches on at least one of the one or more photodetectors to generate the first interference pattern; and overlapping the second portion of the supplemental illumination and −1 diffraction orders from the first, second, and third pitches on at least one of the one or more photodetectors to produce the second interference pattern.

16. The overlay target is a cell including a grating having a first pitch on a first layer and a grating having a second pitch on a second layer; the one or more collection optics: superimposing a first portion of the supplemental illumination and +1 and +2 diffraction orders from the first pitch along with +1 diffraction from the second pitch on at least one of the one or more photodetectors to generate the first interference pattern; 12. The overlay metrology method of claim 11, configured to perform the metrology recipe by superimposing a second portion of the supplemental illumination and −1 and −2 diffraction orders from the first pitch along with −1 diffraction from the second pitch on at least one of the one or more photodetectors to generate the second interference pattern.

17. 1. An overlay metrology system, comprising: an illumination source configured to generate illumination; one or more beam splitters configured to split the illumination from the illumination source into primary illumination and supplemental illumination; one or more illumination optics configured to direct the primary illumination to an overlay target on a specimen, the overlay target having gratings of two or more layers when performing a metrology recipe, the overlay target according to the metrology recipe including a first set of one or more cells having gratings with a first grating orientation and a second set of one or more cells having gratings with a second grating orientation, the primary illumination having a rotated quadrupole distribution with respect to the first and second grating orientations; an objective lens for collecting at least one positive diffraction lobe and at least one negative diffraction lobe associated with diffraction of the primary illumination from the grating of each of the two or more layers when performing the metrology recipe; a first collection channel, a first set of one or more photodetectors in one or more collection pupil planes; one or more first collection optics, overlaying a first portion of the supplemental illumination and at least one diffraction lobe of the primary illumination along a first diagonal of the rotated quadrupole distribution by the overlay target on at least one of the first set of one or more photodetectors to generate a first interference pattern; a first collection channel including one or more first collection optics configured to implement the metrology recipe by superimposing a second portion of the supplemental illumination and at least one additional diffraction lobe of the primary illumination along the first diagonal of the rotated quadrupole distribution by the overlay target on at least one of the first set of one or more photodetectors to generate a second interference pattern; a second collection channel, a second set of one or more photodetectors in one or more collection pupil planes; one or more second collection optics, superimposing a first portion of the supplemental illumination and at least one diffraction lobe of the primary illumination along a second diagonal of the rotated quadrupole distribution by the overlay target on the at least one of the second set of one or more photodetectors to generate a third interference pattern; a second collection channel including one or more second collection optics configured to implement the metrology recipe by superimposing a second portion of the supplemental illumination and at least one additional diffraction lobe of the primary illumination along the second diagonal of the rotated quadrupole distribution by the overlay target on at least one of the second set of one or more photodetectors to generate a fourth interference pattern; a scanning subsystem configured to modulate phases of the first, second, third, and fourth interference patterns during scanning of the overlay target when performing the metrology recipe, the scanning subsystem including at least one of a translation stage that scans the sample relative to the primary illumination, beam scanning optics that scans the primary illumination relative to the sample, or a phase modulator that modulates a phase of the supplemental illumination; a controller communicatively coupled to the first and second photodetectors, the controller including one or more processors configured to execute program instructions, the program instructions causing the one or more processors to: receiving time-varying interference signals from the first and second collection channels during the scanning of the overlay target; generating one or more overlay measurements of the overlay target along the first and second grating directions based on the time-varying interference signals; and a controller that causes the metrology recipe to be implemented.

18. 18. The overlay metrology system of claim 17, wherein the at least one diffraction lobe of the primary illumination along the first diagonal of the rotating quadrupole distribution comprises at least one positive diffraction lobe of a selected diffraction order, the at least one additional diffraction lobe of the primary illumination along the first diagonal of the rotating quadrupole distribution comprises at least one negative diffraction lobe of the selected diffraction order, the at least one diffraction lobe of the primary illumination along the second diagonal of the rotating quadrupole distribution comprises at least one positive diffraction lobe of the selected diffraction order, and the at least one additional diffraction lobe of the primary illumination along the second diagonal of the rotating quadrupole distribution comprises at least one negative diffraction lobe of the selected diffraction order.

19. 20. The overlay metrology system of claim 17, wherein the first diagonal illumination lobe of the rotating quadrupole distribution is distinguished from the second diagonal illumination lobe of the rotating quadrupole distribution by at least one of wavelength or polarization.

20. The overlay metrology system of claim 17 , wherein the one or more illumination optics direct the primary illumination onto the overlay target at an oblique angle of incidence.

21. The overlay metrology system of claim 17 , wherein the illumination from the illumination source is spatially coherent.

Images