Reference Image Grouping in Overlay Metrology

Abstract

The overlay metrology system can include a controller that receives metrology data for a plurality of overlay targets on one or more specimens and generates reference indicia for at least some of the plurality of overlay targets based on the metrology data; the reference indicia being associated with one or more characteristics of the individual overlay targets that contribute to overlay error; and classifying the plurality of overlay targets into one or more groups based on the calculated reference indicia for the plurality of overlay targets, generating reference images for at least some of the one or more groups, generating corrected metrology data for at least some of the one or more groups using the associated reference images, and generating overlay measurements for the plurality of overlay targets based on the corrected metrology data.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent Application No. 63 / 306103, filed February 3, 2022, named as inventors Einat Peled, Naama Cohen and Yuval Lamhot, and is incorporated herein by reference in its entirety.

[0002] This disclosure relates generally to overlay metrology, and more specifically to mitigating tool induced errors in overlay metrology. [Background technology]

[0003] Overlay metrology provides measurements of the relative alignment (i.e., misregistration) between features created using separate patterning processes. In some overlay metrology techniques, overlay metrology is determined by illuminating a specimen component with overlapping features associated with separate patterning processes of interest and collecting data on the symmetry of the overlapping features. For example, some scatterometry overlay metrology (SCOL) techniques utilize overlay targets with periodic features associated with different patterning processes on different layers within the overlapping region of the specimen. In this configuration, diffracted waves of opposite sign orders (e.g., ±1st, ±2nd, etc.) can have a common amplitude when the overlay target features are symmetric and different amplitudes when the overlay target features are offset. As a result, overlay metrology for the patterning processes can be determined based on such symmetry data. However, these techniques are sensitive to metrology non-uniformities, which can manifest as overlay metrology errors commonly referred to as tool induced shift (TIS) errors. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] U.S. Patent No. 7,317,531 [Patent Document 2] U.S. Pat. No. 10,197,389 [Patent Document 3] International Publication No. WO2017 / 044283 Brochure [Patent Document 4] U.S. Patent No. 9,909,982 [Patent Document 5] U.S. Pat. No. 9,164,397 [Patent Document 6] U.S. Patent No. 9,390,492 [Patent Document 7] U.S. Pat. No. 1,069,9969 [Patent Document 8] International Publication No. WO2014 / 138741 Brochure [Non-patent literature]

[0005] [Non-Patent Document 1] Adel, et al., “Diffraction order control in overlay metrology - a review of the roadmap options,” Proc. Of SPIE Vol. 6922, 692202-1 (2008) Summary of the Invention [Problem to be solved by the invention]

[0006] Therefore, there is a need to develop a system and method that remedies the above-mentioned shortcomings. [Means for solving the problem]

[0007] An overlay metrology system is disclosed in accordance with one or more exemplary embodiments. In an exemplary embodiment, the system includes a controller that executes a metrology recipe. In an exemplary embodiment, the controller receives metrology data for a plurality of overlay targets on one or more specimens. In an exemplary embodiment, the controller generates reference indices for at least some of the plurality of overlay targets based on the metrology data, the reference indices being associated with one or more characteristics of the individual overlay targets that contribute to an overlay error. In an exemplary embodiment, the controller classifies the plurality of overlay targets into one or more groups based on the calculated reference indices for the plurality of overlay targets. In an exemplary embodiment, the controller generates reference images for at least some of the one or more groups. In an exemplary embodiment, the controller generates corrected metrology data for at least some of the one or more groups using associated reference images. Additionally, in one illustrative embodiment, the controller generates overlay measurements for the plurality of overlay targets based on the corrected metrology data.

[0008] An overlay metrology system is disclosed in accordance with one or more exemplary embodiments. The system of an exemplary embodiment includes an optical subsystem. In an exemplary embodiment, the optical subsystem includes one or more optical elements configured to illuminate a specimen with light from an illumination source according to a metrology recipe. In an exemplary embodiment, the optical subsystem includes one or more optical elements configured to direct at least a portion of the light from the specimen to a detector at a pupil plane. In an exemplary embodiment, the system includes a controller that implements the metrology recipe. In an exemplary embodiment, the controller receives metrology data for a plurality of overlay targets on one or more specimens. In an exemplary embodiment, the controller generates reference indicia for at least some of the plurality of overlay targets based on the metrology data; the reference indicia being associated with one or more characteristics of the individual overlay targets that contribute to an overlay error. In some exemplary embodiments, the controller classifies the plurality of overlay targets into one or more groups based on the calculated reference indices for the plurality of overlay targets. In some exemplary embodiments, the controller generates reference images for at least some of the one or more groups. In some exemplary embodiments, the controller generates corrected metrology data for at least some of the one or more groups using associated reference images. In some exemplary embodiments, the controller generates overlay measurements for the plurality of overlay targets based on the corrected metrology data.

[0009] An overlay method is disclosed in accordance with one or more exemplary embodiments. In an exemplary embodiment, the method receives metrology data for a plurality of overlay targets on one or more specimens. In an exemplary embodiment, the method generates reference indicia for at least some of the plurality of overlay targets based on the metrology data, where the reference indicia are associated with one or more characteristics of the individual overlay targets that contribute to an overlay error. In an exemplary embodiment, the method classifies the plurality of overlay targets into one or more groups based on the calculated reference indicia for the plurality of overlay targets. In an exemplary embodiment, the method generates reference images for at least some of the one or more groups. In an exemplary embodiment, the method generates compensated metrology data for at least some of the one or more groups using associated reference images. In an exemplary embodiment, the method generates overlay measurements for the plurality of overlay targets based on the compensated metrology data.

[0010] 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.

[0011] Those skilled in the art will be able to better appreciate the numerous advantages of the present disclosure by referring to the accompanying drawings. [Brief description of the drawings]

[0012] [Figure 1A] FIG. 1 is a block diagram of an overlay metrology system in accordance with one or more embodiments of the present disclosure. [Figure 1B] FIG. 1 is a simplified schematic diagram of an optical subsystem according to one or more embodiments of the present disclosure. [Figure 2A] FIG. 2 is a top view of one cell of an overlay target having periodic features in accordance with one or more embodiments of the present disclosure. [Figure 2B] FIG. 2B is a side view of one of the cells in FIG. 2A in accordance with one or more embodiments of the present disclosure. [Diagram 3] FIG. 1 is a conceptual pupil image diagram illustrating differential signal generation in accordance with one or more embodiments of the present disclosure. [Figure 4] FIG. 1 is a flow diagram illustrating steps performed in a method for generating separate reference images for metrology data associated with separate groups of overlay targets in accordance with one or more embodiments of the present disclosure. [Diagram 5] 4 is a conceptual pupil image diagram illustrating differential signal generation at a different pupil center position than in FIG. 3 in accordance with one or more embodiments of the present disclosure. [Figure 6] FIG. 1 is a pupil center shift vs. TIS relationship plot for several overlay targets illustrating the relationship between pupil center position, TIS, and pupil center gradient for separate overlay targets in accordance with one or more embodiments of the present disclosure. [Figure 7A] FIG. 13 is a plot of pupil center shift vs. TIS based on metrology data from multiple overlay targets, uncorrected with a reference image, in accordance with one or more embodiments of the present disclosure. [Figure 7B] FIG. 13 is a pupil center shift vs. TIS relationship plot based on metrology data from multiple overlay targets corrected with a reference image generated based on metrology data from a specific one of the overlay targets in accordance with one or more embodiments of the present disclosure. [Figure 8A] FIG. 13 is a plot illustrating a map of pupil center gradient with respect to overlay targets throughout a specimen in accordance with one or more embodiments of the present disclosure. [Figure 8B] FIG. 1 is a plot illustrating a map of groupings into which overlay targets are classified in accordance with one or more embodiments of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Reference will now be made in detail to the disclosed subject matter, which is illustrated in the accompanying drawings. The present disclosure will be specifically shown and described with reference to certain embodiments and their individual features. The embodiments described herein are to be understood as illustrative and not limiting. As will be readily apparent to those skilled in the art, various changes and modifications in form and detail can be made without departing from the spirit and scope of the present disclosure.

[0014] Embodiments of the present disclosure are directed to systems and methods for mitigating tool-induced shift (TIS) errors associated with metrology non-uniformity by generating reference images for scatterometry overlay (SCOL) metrology measurements. The reference images can characterize metrology non-uniformity and can be used to at least partially remove the effects of such metrology non-uniformity and associated TIS errors during measurement. In embodiments, multiple reference images are generated that may be suitable for separate groups of metrology data (e.g., metrology data from separate groups of overlay targets). For example, metrology data from various overlay targets across a specimen can be grouped based on a metric associated with metrology non-uniformity, and separate reference images can be generated for each group. Metrology data for subsequent measurements of overlay targets on the same or different specimens can then be grouped based on the metric for selection of appropriate reference images. As an example, one possible source of metrology non-uniformity is non-uniformity of printed features, which may vary across the specimen. Ultimately, a single reference image cannot mitigate the associated TIS errors with the same effectiveness. In certain embodiments, multiple reference images are generated and applied to different groups of features.

[0015] The metrology data on which the SCOL measurements are based is generated by illuminating a specimen component having features associated with two different patterning processes (e.g., lithography exposure, etching process, etc.) and collecting light from the specimen, where the features associated with the different patterning processes may be located on the same or different layers of the specimen. Overlay targets commonly available for SCOL technology have one or more cells, but this is not a requirement. Overlay targets, as used herein, generally refer to any specimen component suitable for overlay metrology, including but not limited to dedicated features designed for overlay metrology purposes and device features associated with the device being fabricated. Additionally, the features of an overlay target (referred to herein as target features) may be located on any layer or combination of layers of the specimen. For example, the target features may be located on a photoresist layer, where they can be characterized by refractive index variations in the photoresist layer caused by lithography exposure. For example, the target features may be located on a process layer. In this case, the features can be characterized by material composition variations induced by etching the specimen after patterning with photoresist, or by some other patterning process (e.g., direct etching, etc.).

[0016] As contemplated herein, TIS errors in SCOL metrology can be attributed to a variety of sources, including illumination non-uniformity in the overlay metrology system, the specific physical and optical properties of the specimen features being measured, etc. Because process variations can introduce slight deviations between the properties of overlay targets across the specimen, a single reference image may not be sufficient to mitigate TIS errors for metrology targets across the specimen.

[0017] Embodiments of the present disclosure are directed to systems and methods that classify metrology data from various metrology targets throughout a specimen into groups (referred to herein as reference groups) and provide separate reference images for the various reference groups.

[0018] The metrology data from the various overlay targets can be separated into various reference groups using any suitable technique. In certain embodiments, the indices on which the reference groups are based are derived from measured or expected process variations across the specimen. In certain embodiments, the indices on which the reference groups are based are derived from the metrology data itself. For example, in a pupil plane image-based SCOL measurement, the indices can be pupil center gradient, which relates to the variation of overlay determinations given the pupil center location selection. For example, certain pupil plane SCOL techniques can determine overlay based on difference signals related to the amplitude difference between diffracted waves of opposite sign orders (e.g. +1 and -1 diffracted waves), or more generally between opposite pupil coordinates. Since the difference signals are defined according to the pupil center location, different pupil center locations selected can lead to variations in the determined overlay measurements. It is contemplated herein that pupil center gradient can be a suitable indices on which metrology data from overlay targets across the specimen can be grouped for the purpose of generating a reference image. In other words, the reference image can effectively mitigate the TIS error for overlay targets with similar pupil-center gradient values, i.e., the pupil-center gradient can be an indirect indicator of the sampling variation that may contribute to the TIS error.

[0019] According to certain embodiments, the groupings derived for one specimen can be applied to additional specimens. Following the example above, the pupil center gradients can be determined for the metric data associated with the overlay targets on one or more additional specimens. The data can then be grouped based on predefined bins representing ranges of pupil gradients, and overlay determinations can be made using the corresponding reference images.

[0020] Additional embodiments of the present disclosure are directed to monitoring the value of the pupil center gradient (or some other selected metric) over time across multiple samples as a function of sample location. As contemplated herein, the variation of the pupil center gradient (or some other selected metric) over time across samples at a particular sample location can indicate process variation. That information can then be used directly to control the process (e.g., generate corrections for a process tool to compensate for those process variations) and / or adjust the reference group definitions. For example, a reference group could generally be associated with a spatial domain of samples used to define the reference group (e.g., a radial spatial domain for radial process variations). However, if metrology data for a particular sample location begins to be sorted over time into different reference groups, it may be necessary to update the definitions of those reference groups (e.g., bin definitions for ranges of pupil center gradient).

[0021] 1A-8, a system and method for generating grouped reference images for overlay metrics will now be described in detail in accordance with one or more embodiments of the present disclosure.

[0022] FIG. 1A is a block diagram of an overlay metrology system 100 in accordance with one or more embodiments of the present disclosure.

[0023] Certain embodiments of the overlay metrology system 100 include an optical subsystem 102 that generates metrology data derived from an overlay target 104 on a specimen 106. Certain embodiments of the overlay metrology system 100 further include a controller 108 including one or more processors 110 configured to execute program instructions stored on a memory 112 (e.g., a storage medium). The controller 108 may be communicatively coupled to any of the components of the overlay metrology system 100, such as, but not limited to, a detector 124. The controller 108 may then generate overlay measurements based on the metrology data.

[0024] Generally, the overlay metrology system 100 may be configurable according to one or more metrology recipes. A metrology recipe may include a set of parameters for controlling various aspects of the overlay metrology, such as, but not limited to, the illumination of the specimen, the collection of light in the form of metrology data from the specimen, the position of the specimen during the measurement, or the processing steps used to generate measurements based on the metrology data collected. Thus, the optical subsystem 102 may be configured to provide a specified type of measurement for a selected overlay target design.

[0025] For example, a metrology recipe can include parameters of the illumination beam 114, such as, but not limited to, illumination wavelength, illumination pupil distribution (e.g., illumination angles and associated illumination intensity distributions at those angles), polarization of the incident illumination, or spatial distribution of illumination. For 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 overlay target 104 used for metrology and associated filtered intensity at those angles), collection field stop settings for selecting portions of the overlay target 104 of interest, polarization of collected light, wavelength filters, or parameters to control one or more detectors. For example, a metrology recipe can include various parameters of the design of the overlay target 104, such as, but not limited to, the location and orientation of sample features (e.g., the pitch of grating features along a particular direction). Also for example, a metrology recipe may include various parameters related to the position of the specimen 106 during measurement, such as, but not limited to, the specimen height, the specimen orientation, whether the specimen is stationary during the measurement or whether the specimen is moving during the measurement (and associated parameters describing the velocity, scan pattern, etc.) Also for example, a metrology recipe may include various processing steps that are used to generate determined overlay measurements (e.g., specific values ​​of overlay that will be output) based on the metrology data.

[0026] In certain embodiments, the overlay metrology system 100 operates as a SCOL metrology tool such that its metrology recipes relate to SCOL metrology techniques.

[0027] In general, an overlay target 104 for SCOL metrology (e.g., SCOL overlay target 104) may have target features for patterning processes of interest that are within an overlap region of the specimen 106 and are rotationally symmetric by 180 degrees. According to certain embodiments, the target features for any of the patterning processes may be periodic such that discrete orders of diffraction waves are generated from the incident illumination, although this is not required and the target features for any of the patterning processes do not have to be periodic.

[0028] 2A and 2B, a non-limiting example of a SCOL overlay target 104 is described. In general, the overlay target 104 may have one or more cells 202, each of which may have overlapping target features associated with two or more patterning processes of interest. In certain embodiments, the overlay target 104 may have target features dedicated to overlay metrology. In certain embodiments, the overlay target 104 may have an area of ​​the specimen 106 that contains device features (e.g., features associated with a device to be fabricated on the specimen 106).

[0029] Figure 2A is a top view of a single cell 202 of an overlay target 104 having periodic features in accordance with one or more embodiments of the present disclosure, and Figure 2B is a side view of the single cell 202 in Figure 2A in accordance with one or more embodiments of the present disclosure.

[0030] In some embodiments, a cell 202 may have a first layer print element 204 on a first layer 206 of a specimen 106 and a second layer print element 208 on a second layer 210 of the specimen 106, and may be oriented such that the areas containing the first layer print element 204 overlap the areas containing the second layer print element 208 to form a lattice-over-lattice structure. Also depicted in FIG. 2B is a substrate 212 below the first layer print element 204 and the second layer print element 208.

[0031] The first layer print element 204 and the second layer print element 208 in any particular cell 202 can be designed to have some desired offset (f0) along some direction (e.g., the X direction in FIG. 2B, which corresponds to the measurement direction). For example, the desired offset can be set to zero (f0=0) so that the first layer print element 204 and the second layer print element 208 completely overlap when the physical overlay is also zero (e.g., there is no overlay error). In this configuration, the relative shift between the first layer print element 204 and the second layer print element 208 can represent the overlay error being created. Also, for example, the desired offset can be set to a non-zero value (f0≠0) so that the first layer print element 204 and the second layer print element 208 have their desired offset when the physical overlay is zero (e.g., there is no overlay error). Overlay target 104 may generally be formed of any number of cells 202 and may have any combination of intended offsets (eg, f0 values).

[0032] Although not explicitly depicted, the overlay target 104 may be suitable for overlay metrology along multiple directions (e.g., orthogonal directions). In general, a measurement direction may correspond to a direction of periodicity of the first layer printed elements 204 and the second layer printed elements 208 (e.g., a direction of periodicity of a grating-over-grating structure). As an example, the cell 202 depicted in Figures 2A and 2B exhibits periodicity along the X direction and is suitable for overlay metrology along the X direction.

[0033] In some embodiments, the overlay target 104 has one or more cells 202 that are periodic along a first direction (e.g., the X direction as depicted in FIGS. 2A and 2B) and one or more cells that are periodic along a second direction (e.g., the Y direction as depicted in FIGS. 2A and 2B). In some embodiments, the overlay target 104 has one or more cells 202 that are periodic along two directions simultaneously. For example, the first layer print elements 204 and the second layer print elements 208 can have a structure that is periodic in both the X and Y directions (e.g., a hatch pattern, a grid of square or rectangular structures, etc.).

[0034] 2A and 2B in general, it should be understood that FIG. 2A and 2B and the accompanying description are merely illustrative and should not be construed as limitations on the present disclosure. For example, the first layer 206 and the second layer 210 may generally have any thickness. Also, for example, the specimen 106 may have any number of layers between the first layer print element 204 and the second layer print element 208 and / or between the first layer print element 204 and the substrate 212. Also, for example, the first layer print element 204 and the second layer print element 208 may generally have any 180 degree rotationally symmetric feature and need not have a grating over a grating or even a periodic feature.

[0035] Various SCOL techniques have been developed to generate overlay measurements based on metrology data from the overlay target 104. Metrology data for SCOL techniques can generally be generated in the pupil plane and / or field plane. For example, a pupil plane image can capture different orders of diffracted waves from target features associated with each patterning process. In this configuration, the intensity at any point in the pupil plane can be related to the interference of diffractions from target features associated with different patterning processes. Also, for example, various field plane images can be generated based on selected combinations of diffraction orders, or more generally, on light from different portions of the pupil plane. For example, the overlay metrology system 100 can incorporate various elements that separate out desired orders of diffracted waves for a particular field plane image.

[0036] As an example, a 1st order SCOL technique can be based on metrology data from 1st diffraction orders from target features from different patterning processes. Such techniques would typically require metrology data from two cells 202 with different intended offsets (e.g., ±f0). In this configuration, overlay measurements can be determined based on difference signals between the positive and negative 1st diffraction orders (e.g., more generally, opposite pupil coordinates) from each cell. These difference signals can be generated directly in the pupil plane image or based on separate field plane images of each cell (e.g., one image based on the positive 1st diffraction order and one image based on the negative 1st diffraction order).

[0037] As another example, the zeroth order SCOL technique can be based on metrology data relating to zeroth order light from overlay targets, such as, but not limited to, zeroth order diffraction or inverse order diffraction from overlapping target features (e.g., positive first order diffraction from a target feature in one layer and negative first order diffraction from a target feature in another layer).

[0038] Various non-limiting examples of SCOL techniques are outlined in Non-Patent Document 1, U.S. Patent Publication No. 2008 / 0133636, entitled “Apparatus and methods for detecting overlay errors using scatterometry,” issued on January 8, 2008, U.S. Patent Publication No. 2009 / 0133637, entitled “Approaches in first order scatterometry overlay based on introduction of auxiliary electromagnetic fields,” issued on February 5, 2019, and U.S. Patent Publication No. 2009 / 0133637, issued on March 16, 2017, all of which are incorporated herein by reference in their entireties.

[0039] With reference now to equations (1)-(8), overlay determination and TIS error mitigation using reference images will now be described in detail in accordance with one or more embodiments of the present disclosure. For illustrative purposes, a first order SCOL technique based on pupil plane images is described. However, it should be understood that this is not limiting and that one of ordinary skill in the art can extend the concepts described herein to any SCOL technique that uses any combination of pupil plane and / or field plane metrics.

[0040] According to certain embodiments, a differential signal, for example:

number

number

number

[0041] It is noted that the overlay target 104 need not have periodic features and thus need not result in diffracted waves of discrete orders, so equations (1) and (2) can be generalized for all pupil coordinates.

[0042] FIG. 3 is a conceptual diagram of a pupil image (eg, a focused pupil plane image) illustrating differential signal generation in accordance with one or more embodiments of the present disclosure.

[0043] 3 depicts a zeroth diffraction order 302, a positive first diffraction order 304, and a negative first diffraction order 306. This distribution can be generated by illuminating a cell 202 having periodic first layer print elements 204 and second layer print elements 208 with a single illumination beam at an orthogonal angle of incidence.

[0044] 3, a difference signal D(k) can be associated with the difference between the intensity at pupil coordinate k and the intensity at pupil coordinate −k, where coordinates k and −k are rotationally symmetric by 180 degrees about the pupil center position 308. Furthermore, the difference signal can be formed as the average (or weighted average) of all the pupil coordinates.

[0045] Then, these differential signals are combined to produce the so-called K and G signals.

number

[0046] Then, the overlay measurement (OVL) is calculated as follows, as long as G ≠ 0: OVL=(P / 2π)tan -1 ((K / G)tan(2πf0 / P)) (5) It can be generated as:

[0047] Assuming that OVL<<P and f0<<P, the K signal depends on the actual value of the overlay OVL measured at the location of the overlay target 104, as shown by equations (3) and (4), while the G signal does not. Rather, the G signal is (OVL / P) 2 which can be neglected under these conditions. In this case, Equation 5 becomes OVL = (K / G) · f0 (6) It is organized into.

[0048] As described herein, a reference image can be used to at least partially mitigate TIS errors in overlay measurements. TIS errors can be attributed to a number of sources, including, but not limited to, non-uniformities in the measurements performed by the optical subsystem 102 (e.g., illumination beam non-uniformities, illumination path non-uniformities, collection path non-uniformities, etc.) and / or non-uniformities in the overlay target 104. Such TIS errors can generally be observed from overlay measurements made with the overlay target 104 at two different orientations relative to the optical subsystem 102. Specifically, the TIS can be calculated by: TIS=(OVL0-OVL 180 ) / 2 (7) where OVL0 corresponds to an overlay measurement of the overlay target 104 with the sample in a first orientation relative to the optical subsystem 102, and OVL1 corresponds to an overlay measurement of the overlay target 104 with the sample in a first orientation relative to the optical subsystem 102. 180 corresponds to the overlay measurement of the same overlay target 104 rotated 180 degrees relative to the optical subsystem 102.

[0049] In this characterization, the TIS-corrected overlay measurement (OVL) is independent of the TIS. OVL=(OVL0+OVL 180 ) / 2 (8) It can be defined as:

[0050] It is noted that to obtain a TIS-corrected overlay measurement as defined by equation (8), typically two measurements of the same overlay target 104 at 180 degree rotations would be required. However, performing such two measurements for each overlay target 104 of interest throughout the specimen can be time consuming and may result in a relatively low measurement throughput.

[0051] To alleviate the need to measure each overlay target 104 twice at different rotation angles, a reference image may be generated that captures the relevant non-uniformity. For example, the reference image may be generated based on metrology data captured at two orientations (e.g., 0 degrees and 180 degrees) of the overlay target 104. As an example, if the overlay metrology system 100 is configured to determine overlay based on pupil images (e.g., images generated by the detector 124 at the pupil plane 138), the reference image may be generated by adding the pupil images (e.g., metrology data) generated at the 180 degree rotation. Additionally, the reference image may be symmetricalized to prevent noise amplification. For example, the reference image may be symmetricalized by dividing the reference image by the average of the reference image and its 180 degree rotated version. The effects of asymmetry on SCOL measurements and their calibration are generally described in U.S. Pat. No. 6,399,433, issued March 6, 2018, U.S. Pat. No. 6,499,433, issued October 20, 2015, and U.S. Pat. No. 6,499,433, issued July 12, 2016, all of which are incorporated herein by reference in their entireties.

[0052] Regardless of how the reference image is generated, the reference image can be used to correct metrology data associated with a single measurement of the overlay target 104 in a single orientation. For example, the effect of TIS errors can be reduced by dividing the pupil image (e.g., metrology data) by the reference image.

[0053] However, the effectiveness of a reference image in mitigating TIS errors is related to how well the reference image captures the measurement non-uniformity that contributes to the TIS error. This application recognizes that many prior SCOL techniques have utilized a single reference image for all overlay targets 104 on the specimen 106, assuming that the measurement non-uniformity that contributes to TIS errors is primarily due to illumination and / or collection non-uniformity by the optical subsystem 102. However, this application contemplates that a single reference image cannot accurately mitigate TIS errors for all overlay targets 104, since specimen variations between overlay targets 104 can also contribute to TIS errors.

[0054] 4 is a flow diagram illustrating steps performed in a method 400 for generating separate reference images for metrology data associated with separate groups of overlay targets 104 in accordance with one or more embodiments of the present disclosure. Applicants note that the embodiments and enabling technologies described herein above in the context of overlay metrology system 100 should be understood to extend to method 400. However, it is further noted that method 400 is not limited to the architecture of overlay metrology system 100.

[0055] In certain embodiments, method 400 receives metrology data for overlay targets 104 on one or more specimens 106 at step 402. For example, the metrology data may include any combination of pupil plane or field plane images suitable for overlay metrology using SCOL techniques (e.g., based on a selected metrology recipe). The metrology data generated at step 402 may be for all overlay targets 104 on the one or more specimens 106 or for a subset of the overlay targets 104. Additionally, the metrology data may be for overlay metrology along one or more measurement directions.

[0056] In certain embodiments, method 400 generates, at step 404, reference indicia for each of the overlay targets 104 based on the metrology data; the reference indicia being associated with one or more characteristics of the individual overlay targets 104 that contribute to overlay error. In certain embodiments, method 400 classifies the overlay targets 104 into two or more groups based on the reference indicia in step 406. In certain embodiments, method 400 generates, at step 408, reference images for at least some of the one or more groups.

[0057] Generally, steps 406 and 408 may provide a number of groups based on the reference indicia. It is contemplated herein that the number of groups will depend on the specific characteristics of each particular specimen or specimens 106 within a particular lot, and the desired number of groups may not be known a priori. Thus, method 400 facilitates both analyzing the overlay targets 104 on each specimen 106 based on the reference indicia (e.g., step 404), determining a preferred number of groups that would benefit from individual reference images (e.g., step 406), and generating associated reference images (e.g., step 408).

[0058] As discussed herein, the reference images may be generated using any suitable technique known in the art. In certain embodiments, a reference image for a particular group is generated based on metric data from a single representative overlay target 104 in that group (e.g., metric data at multiple orientations of that overlay target 104). In certain embodiments, a reference image for a particular group is generated based on metric data from multiple representative overlay targets 104.

[0059] The reference indicia may include any indicia of a characteristic of the overlay target 104 that contributes to overlay error (e.g., TIS error). Such overlay error may be associated with non-uniformity of the overlay target 104 and may vary from location to location on the specimen 106. For example, the specimen may exhibit process variations with radial distribution. By way of example, but not by way of limitation, characteristics such as film thickness, sidewall angle of a created feature, or asymmetry of a created feature may vary with radial distribution across the specimen. Furthermore, one or more such variations may affect the overlay metrology and thus a single reference image may not be adequate to mitigate TIS error across the specimen. Therefore, metrology data may be sorted into reference groups, each with a separate reference image, based on measurements of one or more such variations. Process variation detection is generally described in U.S. Patent Application Publication No. 2020 / 0133933, issued Jun. 30, 2020, which is incorporated herein by reference in its entirety.

[0060] In some embodiments, the reference indices are based on physical measurements, predicted measurements, and / or simulations of one or more characteristics of the overlay target 104. Such measurements may be performed at locations on the overlay target 104 or may be performed at other locations on the specimen 106 and extrapolated to locations on the overlay target 104. For example, the reference indices may be based on measurements such as, but not limited to, asymmetry, sidewall angle along any direction, critical dimension (CD), or layer thickness (e.g., thickness of the first layer 206 and / or second layer 210 as depicted in FIGS. 2A and 2B) of the target features (e.g., first layer 204 and / or second layer 208 as depicted in FIGS. 2A and 2B).

[0061] In certain embodiments, the reference indices are derived from the metric data itself, which may eliminate the need for additional measurements, simulations or assumptions.

[0062] Reference landmark generation based on pupil plane SCOL technology will now be described in accordance with one or more embodiments of the present disclosure. However, it should be understood that this description is provided purely for illustrative purposes and should not be taken as limiting. Rather, one skilled in the art can extend the concepts of the present disclosure to field plane SCOL technology.

[0063] In certain embodiments, the reference indicia include or are based on a pupil center gradient measurement generated based on metrology data prior to correction with the reference image; the pupil center gradient can be associated with a variation in the overlay measurement determined in light of the selection of the pupil center position 308.

[0064] 3, overlay measurements can be determined based on difference signals generated from pupil plane images of light striking the overlay target 104. However, such techniques can be sensitive to the selection of the pupil center location 308.

[0065] 5 is a schematic diagram of a pupil image (e.g., a focused pupil plane image) illustrating generation of a difference signal at a different pupil center location 308 than that of FIG. 3 in accordance with one or more embodiments of the present disclosure. As shown in FIG. 5, opposite pupil coordinates are determined by the pupil center location 308. As a result, the difference signal and resulting overlay determinations will be affected by the selection of the pupil center location 308. In addition, the selection of the pupil center location 308 may affect the TIS error associated with the measurements, which will be described in more detail below.

[0066] Pupil Center Gradient (PC slope ), the relationships between the resulting overlay measurements and the TIS can be expressed as Equations (9)-(11), which share common variables and definitions with Equations (7) and (8). OVL0=OVL+Δp·PC slope (9) OVL 180 =OVL-Δp·PC slope (10) TIS = Δp PC slope (11) where Δp corresponds to the pupil center shift from a reference position (eg, an arbitrary position), which may be in pixels as measured by the pupil plane image (eg, metric data).

[0067] FIG. 6 illustrates the pupil center position 308, TIS, and pupil center gradient (PC) for various overlay targets 104 in accordance with one or more embodiments of the present disclosure. slope ) is a plot of pupil center shift (Δp) vs. TIS for several overlay targets 104. For example, TIS can generally be varied approximately linearly with respect to the selection of pupil center position 308 (e.g., with respect to pupil center shift), and pupil center slope (PC slope ) corresponds to the slope of that linear relationship. However, the metric data for different overlay targets 104 may have different values ​​of pupil center gradient (PC slope ) and furthermore the linear constants that affect the value at which TIS is minimal (e.g., 0) may be different.

[0068] Although not explicitly stated, the determined overlay value at a given orientation (e.g. OVL0, OVL 180 6, etc.) to pupil center position 308 (or pupil center shift as shown in FIG. 6).

[0069] It is noted that in a typical pupil-plane based SCOL technique, the pupil center location 308 can be selected in a variety of ways. For example, the pupil center location 308 can be selected to minimize TIS for all measurement samples. According to this technique, the pupil center shift (and associated pupil center location 308) can be selected to optimize both the minimum average TIS and 3σ variation across the measurement overlay targets 104, as shown by the dashed line 602 in FIG. 6. The pupil center optimization technique is also generally described in U.S. Pat. No. 6,399,433, published Sep. 12, 2014, which is incorporated herein by reference in its entirety.

[0070] As contemplated in the present application, the pupil center gradient (PC slope) is a metric that characterizes the properties of the specimen 106 at the location of the overlay target 104, independent of the particular optical subsystem 102 used to generate the metrology data associated therewith. In particular, simulations and SCOL theory have shown that the pupil center gradient (e.g., as depicted in FIG. 7A) is related to the thickness of the measured layer. As a result, the pupil center gradient is well suited to serve as a reference metric to group overlay targets having similar physical properties based solely on metrology data (e.g., pupil plane image of the overlay target 104) without additional metrology.

[0071] Furthermore, when a reference image is generated based on metrology data from a particular overlay target 104 (e.g., based on metrology data from a particular overlay target 104 at two orientations that are rotated 180 degrees from each other), a pupil center gradient (PCG) calculated based on the corrected metrology data is generated. slope ) can be approximately zero (at least over a preferred range of locations for the pupil center location 308). As a result, TIS at that location also approaches zero or a constant (e.g., as illustrated in equations (9)-(11) and FIGS. 3, 5, and 6).

[0072] 7A and 7B depict simulation results of the effect of reference image selection on the TIS of multiple overlay targets 104, in accordance with one or more embodiments of the present disclosure. Specifically, FIGS. 7A and 7B are based on simulations in which the overlay targets 104 are at different thickness locations. FIG. 7A is a pupil center shift vs. TIS relationship plot based on simulation-derived metric data from multiple overlay targets 104 that is not corrected with a reference image, in accordance with one or more embodiments of the present disclosure. In FIG. 7A, each overlay target 104 (represented by a site on the specimen 106) has a separate pupil center gradient (PC slope) indicates that the physical properties of the associated overlay targets 104 vary. FIG. 7B is a plot of pupil center shift vs. TIS based on simulation-derived metric data from a number of overlay targets 104 corrected with a reference image generated based on simulation-derived metric data from a particular one of the overlay targets 104, in accordance with one or more embodiments of the present disclosure. Specifically, the reference image used to generate FIG. 7B was generated based on metric data from an overlay target 104 at site 3. In FIG. 7B, the corrected pupil center gradient (and TIS) is near zero over the considered range of pupil center shifts. Furthermore, overlay targets 104 that exhibit similar metric data with pre-correction pupil center gradient values ​​with the reference image (as depicted in FIG. 7A) are also successfully corrected with the same reference image, as indicated by similarly low values ​​of post-correction pupil center gradient and TIS. However, overlay targets 104 that exhibit substantially different metric data (as depicted in FIG. 7A) in terms of pupil center gradient values ​​before correction by the reference image are not well corrected by the same reference image, as shown in FIG. 7B by the relatively high pupil center gradient and TIS values ​​after correction.

[0073] That is, as contemplated herein, the reference image may characterize the interaction of the optical subsystem 102 with the measured overlay target 104 (or the location of the measured overlay target 104), in addition to non-uniformities in the optical subsystem 102 alone. Because the characteristics of the overlay target 104 may vary across different locations on the specimen 106 due to factors such as, but not limited to, reflectance variations (which may require site-to-site illumination intensity changes) and other imperfections in the measurements or related calculations, the effectiveness of a particular reference image in mitigating TIS may vary across specimens 106, and more generally with process variations.

[0074] However, TIS can also be reduced for multiple overlay targets 104 with varying characteristics by first grouping (e.g., at step 406) the metric data from the overlay targets 104 into one or more groups based on pupil center gradient values ​​(e.g., measured prior to correction with any reference image), and then generating (e.g., at step 408) a reference image for each group based on the metric data for the overlay targets 104 in the individual groups. As illustrated by equations (9)-(11) and Figures 3, 5, and 6, the pupil center gradient values ​​may be calculated based on a determined overlay measurement (e.g., an OVL0 measurement) or a calculated TIS (e.g., an OVL0 and OVL0 measurement). 180 However, using overlay measurements from a single set of metrology data (e.g. OVL0 measurements) can be quicker and may be preferable in some applications.

[0075] Also, more generally with reference to FIG. 4, step 406 of classifying metrology data from overlay target 104 into groups based on a reference metric (e.g., pupil center gradient or some other suitable metric) can be performed in a variety of ways within the spirit and scope of the present disclosure.

[0076] In certain embodiments, the range of values ​​of the reference indices observed in the metric data from the overlay targets 104 can be divided into any number of bins representing one or more groups. In this manner, the metric data for a particular overlay target 104 can be classified according to the appropriate bin into which the calculated value of the reference indices for that overlay target 104 falls.

[0077] In general, any number of groups and associated reference images may be generated. However, because reference image generation requires metric data at two sample orientations (e.g., 0° and 180°), a larger number of groups and associated reference images requires more data collection at multiple sample orientations. In this case, there is a tradeoff between achieving minimum TIS (by having more groups) and speeding up measurements across the sample (by having fewer groups).

[0078] 8A and 8B depict three groupings of metrology data from overlay targets 104 across a specimen. FIG. 8A is a plot depicting a map of pupil center gradient (e.g., a selected reference indicia) with respect to overlay targets 104 across a specimen 106 in accordance with one or more embodiments of the present disclosure. In FIG. 8A, the reference indicia exhibit a distinct radial pattern. As discussed herein, such a pattern can be correlated to a radial layer thickness pattern across the specimen 106. However, it should be understood that FIG. 8A is merely illustrative and the reference indicia may exhibit any distribution across the specimen 106.

[0079] 8B is a plot illustrating a map of groups into which the overlay target 104 may be classified (e.g., following step 406) in accordance with one or more embodiments of the present disclosure. As shown in FIG. 8B, the groups may encompass spatial regions of the specimen 106 that have similar reference index values. In this example, a first group 802a includes a central region of the specimen, a second group 802b includes a mid-diameter region, and a third group 802c includes an edge region of the specimen 106. However, it should be understood that the groupings need not be contiguous on the specimen 106. For example, several disparate regions of the specimen 106 may fall within a common group.

[0080] Referring again to FIG. 4, additional steps of method 400 will now be described in accordance with one or more embodiments of the present disclosure.

[0081] In some embodiments, method 400 generates, at step 410, corrected metrology data for the overlay targets 104 (e.g., at least the overlay targets 104 in the group for which the reference image was generated) using the associated reference image for one or more of the groups. In some cases, all of the measured overlay targets 104 are sorted into groups (e.g., at step 406) and a reference image is generated for each of the groups (e.g., at step 408). In some embodiments, method 400 generates overlay measurements for the plurality of overlay targets based on the corrected set of metrology data (e.g., at step 412). In this manner, overlay measurements associated with each overlay target 104 can be better corrected for errors, such as, but not limited to, TIS, based on appropriate selection of the reference image.

[0082] As further contemplated herein, the reference groups and their associated reference images may be applied to additional overlay targets 104 on the same or different specimens 106 .

[0083] In some embodiments, the set of overlay targets 104 used to classify the reference groups and generate the associated reference images is only a subset of the overlay targets 104 on the specimen 106 (e.g., as defined by a metrology recipe). In this case, metrology data generated for overlay targets 104 that are not in the original set can simply be classified according to the groups that exist. As an example, in the case of classification according to bins or ranges of reference indices, reference indicia values ​​can be generated for the additional overlay targets 104 (e.g., as a result of step 404), which in turn can be classified into groups according to the bins or ranges associated with the reference indices (e.g., as a result of step 406). Corrected overlay measurements can then be generated for each additional overlay target 104 using the reference images associated with the group (e.g., as a result of steps 410 and 412).

[0084] Similarly, the reference groups and their associated reference images are applied to the additional overlay targets 104 on the additional specimen 106. In this case, reference indicia values ​​can be determined for the additional overlay targets 104 on the additional specimen 106 and sorted according to the existing group. The reference images for the groups can then be used to generate corrected overlay measurements for each of the additional overlay targets 104. It is noted that the metrology data for the additional overlay targets 104 can be sorted based on the reference indicia and not necessarily sorted into the same group as adjacent overlay targets 104 or overlay targets 104 that were in the same location on the previous specimen 106. In this manner, the most suitable reference image can be used for each overlay target 104.

[0085] As further contemplated herein, at some point it may become necessary to update the defined reference groups, for example, one or more manufacturing processes may drift significantly over time, reducing the usefulness of one or more reference images for the associated group.

[0086] In general, method 400 or portions thereof (e.g., step 406 involving defining a reference indicia range corresponding to one of the groups and / or step 408 involving generating a reference image for one of the groups) may be repeated or updated using any technique.

[0087] In certain embodiments, the method 400 or portions thereof may be performed at periodic intervals. In certain embodiments, the method 400 or portions thereof may be performed in response to a trigger condition. Any suitable trigger condition may be used. For example, a non-limiting example of a trigger condition may be the number of times (either absolute or within a specified time window) that an overlay target 104 at a particular location on the specimen 106 is classified into a different group than that for the original specimen 106. For example, it may be expected that an overlay target 104 at a particular location on the specimen 106 should have relatively consistent physical parameters and therefore relatively consistent reference indices. Under this assumption, variation in the reference indices for a particular location over time may represent sample-to-sample process variation. At some point, such sample-to-sample variation may cause the reference image to be invalid for one or more groups (e.g., a TIS error above a selected threshold may be present). However, by updating the classification of the groups (e.g., the range of reference indices for any of the groups) and / or the reference images for any of the groups, it is possible to again provide effective TIS error correction for all overlay targets 104.

[0088] 1A and 1B, additional aspects of the overlay metrology system 100 are detailed in accordance with one or more embodiments of the present disclosure.

[0089] The one or more processors 110 of the controller 108 may 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 include any device having one or more processing or logic elements (e.g., one or more microprocessor devices, one or more application specific integrated circuit (ASIC) devices, one or more field programmable gate arrays (FPGAs), or one or more digital signal processors (DSPs)). In this sense, the one or more processors 110 may include any device configured to execute algorithms and / or instructions (e.g., program instructions stored in memory). In one embodiment, the one or more processors 110 may be implemented 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 or operate in conjunction with the overlay metrology system 100 as described elsewhere in this disclosure.

[0090] Further, the various subsystems of the overlay metrology system 100 may have processors or logic elements suitable for performing at least a portion of the steps described in the present disclosure. Thus, the above description should be taken as merely illustrative and not limiting on the embodiments of the present disclosure. Furthermore, the steps described throughout the present disclosure may be performed by a single controller 108 or, alternatively, by multiple controllers. Additionally, the controller 108 may have one or multiple controllers housed within a common housing or in multiple housings. In this manner, any controller or combination of controllers may be individually packaged as a module suitable for integration into the overlay metrology system 100.

[0091] The memory 112 may include any suitable storage medium known in the art for storing program instructions executable by the associated processor(s) 132. For example, the memory 112 may include a non-transitory storage medium. In another example, the memory 112 may include, but is not limited to, a read-only memory (ROM), a random access memory (RAM), a magnetic or optical storage device (e.g., disk), a magnetic tape, a solid state drive, and the like. It is further noted that the memory 112 may be housed in a common controller housing with the processor(s) 110. In some embodiments, the memory 112 may be located remotely relative to the physical location of the processor(s) 110 and the controller 108. For example, the processor(s) 110 of the controller 108 may access a remote memory (e.g., a server) accessible over a network (e.g., the Internet, an intranet, etc.).

[0092] FIG. 1B is a simplified schematic diagram of an optical subsystem 102 according to one or more embodiments of the present disclosure.

[0093] According to embodiments, the optical subsystem 102 may be configured according to a metrology recipe (e.g., an overlay recipe) to generate overlay measurements of a design of the overlay target 104. For example, the optical subsystem 102 may direct one or more illumination beams 114 (e.g., via an illumination path 118) from an illumination source 116 to the overlay target 104 on the specimen 106, collect light or other radiation (referred to herein as metrology light 120) (e.g., via a collection path 122) emitted from the overlay target 104, and generate metrology data from at least one detector 124 based on the metrology light 120. For example, the metrology data may be related to the diffraction of the illumination beam 114 at the overlay target 104. The illumination path 118 and collection path 122 may further include additional optical elements, such as, but not limited to, lenses 126 and beam steering elements 128 (e.g., polarizers, spectral filters, spatial filters, beam blockers, apertures, etc.) in any suitable location. According to certain embodiments, one or more beam splitters 130 may be provided in the optical subsystem 102 to allow for simultaneous illumination and collection through a common objective lens 132 or other focusing element, as depicted in FIG. 1B. According to certain embodiments, the specimen 106 may be fixed and / or positioned by a translation stage 134 in the optical subsystem 102 having a number of linear or rotary actuators.

[0094] The optical subsystem 102 can include one or more detectors 124 at any location suitable for collecting metrology data. For example, the optical subsystem 102 can include at least one detector 124 at a field plane 136 (e.g., a plane conjugate to the sample 106) as depicted in FIG. 1B. For example, although not depicted, the optical subsystem 102 can include at least one detector 124 at a pupil plane 138 (e.g., a diffraction plane corresponding to the angular distribution of light from the sample 106). Furthermore, although not depicted, the optical subsystem 102 can include multiple channels, each with a separate detector 124. In this manner, the optical subsystem 102 can perform multiple simultaneous measurements using multiple detectors 124 at any combination of the field plane 136 or pupil plane 138. For example, the optical subsystem 102 may include one or more beam splitters (e.g., non-polarizing beam splitters, polarizing beam splitters, dichroic mirrors that provide spectral selectivity, etc.) to split the measurement light 120 into various channels for detection. The optical subsystem 102 may further include optical elements that modify the characteristics of the measurement light 120 in each channel, such as, but not limited to, a polarizer, a polarization rotator, a spectral filter, a spatial filter, or a pupil filter (e.g., a beam block or aperture in a pupil plane to block or pass specified orders of diffraction).

[0095] The subject matter described herein is depicted in places with various components included or connected to other components. It should be understood that the illustrated configurations are merely exemplary, and that in fact many other configurations can be implemented to achieve the same function. Conceptually, any arrangement of components that achieves the same function achieves the desired function through an effective "cooperation." Thus, any two components in this application that are combined to achieve a particular function can be viewed as "cooperating" with each other to achieve the desired function, regardless of the configuration or intervening components. Similarly, any two components that are so coordinated can be viewed as "connected" or "coupled" to each other to achieve the desired function, and any two components that can be so coordinated can be viewed as "combinable" with each other to achieve the desired function. Examples of coupleable include, but are not limited to, components being capable of and / or physically interacting with each other, and / or components being capable of and / or wirelessly interacting with each other, and / or components being capable of and / or logically interacting with each other.

[0096] The present disclosure and many of its attendant advantages will be understood from the foregoing description, and it will also be apparent that various changes in the form, construction and arrangement of the parts may be made without departing from the disclosed subject matter or diminishing all of its essential advantages. The form described is merely illustrative, and it is the intent of the following claims to encompass and embrace all such modifications. It will be further understood that it is the appended claims which define the invention.

Claims

1. 1. An overlay metrology system, comprising: a controller having one or more processors configured to execute program instructions, the one or more processors: receiving metrology data for a plurality of overlay targets on one or more specimens; generating reference indicia for at least some of the plurality of overlay targets based on the metrology data, wherein the reference indicia for a particular one of the plurality of overlay targets is related to one or more characteristics of the particular one of the plurality of overlay targets that contribute to an overlay error; classifying the plurality of overlay targets into one or more groups based on the calculated reference indices for the plurality of overlay targets; generating reference images for at least some of the one or more groups; generating corrected metrology data for at least some of the one or more groups using the reference images associated therewith; and generating overlay measurements for the plurality of overlay targets based on the corrected metrology data; An overlay weighing system that executes a weighing recipe by

2. 2. The overlay metrology system of claim 1, wherein the one or more groups include: An overlay metrology system that includes two or more groups.

3. 2. The overlay metrology system of claim 1, wherein the metrology data comprises: Overlay metrology system including pupil plane image.

4. 4. The overlay metrology system of claim 3, wherein the reference indicia is: Overlay metrology system including pupil center gradient.

5. 4. The overlay metrology system of claim 3, wherein generating the reference indicia for at least some of the plurality of overlay targets based on the metrology data comprises: calculating preliminary overlay values ​​for at least some of the plurality of overlay targets using selected pupil center positions of different values ​​based on the metrology data; and calculating the reference index based on a relationship between the selected pupil center position and the preliminary overlay value; Overlay metrology system.

6. 2. The overlay metrology system of claim 1, wherein the metrology data comprises: An overlay metrology system including a field image.

7. 2. The overlay metrology system of claim 1, wherein generating the reference indicia for at least some of the plurality of overlay targets based on the metrology data comprises: performing at least one of measurements and simulations of one or more properties of at least one of the specimen and the associated overlay targets at at least some of the plurality of overlay targets; and generating the reference index based on at least one of the measurement and the simulation; Overlay metrology system.

8. 8. The overlay metrology system of claim 7, wherein the one or more characteristics are: Overlay metrology system including layer thickness.

9. 8. The overlay metrology system of claim 7, wherein the one or more characteristics are: An overlay metrology system that includes at least one of sidewall angle, asymmetry of one or more created features, and critical dimensions.

10. 2. The overlay metrology system of claim 1, wherein classifying the plurality of overlay targets into one or more groups based on the reference indices calculated for the plurality of overlay targets comprises: dividing the range of the calculated reference indices for the plurality of overlay targets into one or more bins; and classifying the plurality of overlay targets into one or more groups based on the one or more bins; Overlay metrology system.

11. 2. The overlay metrology system of claim 1, wherein the metrology data includes first metrology data, and generating a reference image for a particular group of the one or more groups comprises: selecting one or more representative overlay targets from the plurality of overlay targets for the particular group; receiving second metrology data for the representative overlay target, the second metrology data being generated by rotating the representative overlay target 180 degrees relative to corresponding data in the first metrology data; and generating the reference image for the particular group based on the second metric data and the corresponding data in the first metric data; Overlay metrology system.

12. 12. The overlay metrology system of claim 11, further comprising: generating the reference image for the specific group based on the second metrology data and corresponding data in the first metrology data; generating the reference image for the particular group by adding an image in the second metric data and the corresponding image in the first metric data; Overlay metrology system.

13. 12. The overlay metrology system of claim 11, further comprising: generating the reference image for the specific group based on the second metrology data and a corresponding portion of the first metrology data, the step of: generating a first reference image for the particular group by adding an image in the second metric data to a corresponding image in the first metric data; generating a second reference image for the particular group by rotating the first reference image for the particular group by 180 degrees; and generating said first reference image for said particular group by an average of said first and second reference images for said particular group; Overlay metrology system.

14. 2. The overlay metrology system of claim 1, wherein generating corrected metrology data for at least some of the one or more groups using the reference images associated therewith comprises: Separating the images in the first metric data according to the reference images corresponding to the one or more groups; Overlay metrology system.

15. 2. The overlay metrology system of claim 1, wherein the one or more program instructions further comprise: receiving additional metrology data relating to a plurality of additional overlay targets; generating the reference indicia for at least some of the plurality of additional overlay targets based on the additional metrology data; classifying the plurality of additional overlay targets into one or more groups based on the calculated reference indices for the plurality of additional overlay targets; generating additional corrected metrology data for at least some of the one or more groups using the reference images associated therewith; and generating overlay measurements for the plurality of additional overlay targets based on the additional corrected metrology data; and causing the one or more processors to execute the metrology recipe by

16. 16. The overlay metrology system of claim 15, wherein at least one of the one or more additional overlay targets is located on one or more additional of the specimens.

17. 16. The overlay metrology system of claim 15, wherein at least one of the one or more additional overlay targets is on the one or more specimens.

18. 20. The overlay metrology system of claim 17, wherein the one or more program instructions further comprise: monitoring the value of the reference index at one or more locations for one or more additional of the samples; and When a selected condition based on the monitored value of the reference index is satisfied, classifying the overlay targets of the current specimen into one or more groups of a new set based on the reference indices; and generating a reference image for one or more groups of said new set; and causing the one or more processors to execute the metrology recipe by

19. 20. The overlay metrology system of claim 17, wherein the one or more program instructions further comprise: monitoring the value of the reference index at one or more locations for one or more additional of the samples; and generating a new reference image for the one or more groups when a selected condition based on the monitored values ​​of the reference indices is satisfied; and causing the one or more processors to execute the metrology recipe by

20. 10. The overlay metrology system of claim 1, wherein at least some of the plurality of overlay targets have one or more cells with overlapping structures associated with two or more patterning processes.

21. 1. An overlay metrology system, comprising: an optical subsystem; one or more optical elements configured to illuminate the specimen with light from an illumination source according to a metrology recipe; one or more optical elements configured to direct at least a portion of the light from the sample to a detector at a pupil plane; an optical subsystem comprising: a controller communicatively coupled to the optical subsystem; the controller having one or more processors configured to execute program instructions, the one or more processors: receiving metrology data for a plurality of overlay targets on one or more specimens from the optical subsystem; generating reference indicia for at least some of the plurality of overlay targets based on the metrology data, the reference indicia being associated with one or more characteristics of the individual overlay targets that contribute to overlay error; classifying the plurality of overlay targets into one or more groups based on the calculated reference indices for the plurality of overlay targets; generating reference images for at least some of the one or more groups; generating corrected metrology data for at least some of the one or more groups using the reference images associated therewith; and generating overlay measurements for the plurality of overlay targets based on the corrected metrology data; an overlay metrology system that executes the metrology recipe by

22. 1. A method of overlay metrology comprising: receiving metrology data for a plurality of overlay targets on one or more specimens; generating reference indicia for at least some of the plurality of overlay targets based on the metrology data, the reference indicia being associated with one or more characteristics of the individual overlay targets that contribute to overlay error; classifying the plurality of overlay targets into one or more groups based on the calculated reference indices for the plurality of overlay targets; generating reference images for at least some of the one or more groups; generating corrected metrology data for at least some of the one or more groups using the reference images associated therewith; and generating overlay measurements for the plurality of overlay targets based on the corrected metrology data; Overlay metrology method.

23. 23. The method of claim 22 further comprising: receiving additional metrology data for a plurality of additional overlay targets; generating the reference indicia for at least some of the plurality of additional overlay targets based on the additional metrology data; classifying the plurality of additional overlay targets into one or more groups based on the calculated reference indices for the plurality of additional overlay targets; generating additional corrected metric data for at least some of the one or more groups using the reference images associated therewith; and generating overlay measurements for the plurality of additional overlay targets based on the additional corrected metrology data; method.

24. 24. The method of claim 23 further comprising: monitoring the value of the reference index at one or more locations for one or more additional samples; and When a selected condition based on the monitored value of the reference index is satisfied, classifying the overlay targets of the current specimen into one or more groups of a new set based on the reference indices; and generating a reference image for one or more groups of said new set; method.

25. 24. The method of claim 23 further comprising: monitoring the value of the reference index at one or more locations for one or more additional samples; and generating a new reference image for the one or more groups when a selected condition based on the monitored values ​​of the reference indices is satisfied. method.

Images