Overlay metrology system, target, and method

Abstract

This invention discloses an overlay measurement target, which may include: a grating-overlay grating structure formed by a lower grating structure having a first coarse pitch in a first sample layer and an upper grating structure having a second coarse pitch in a second sample layer, wherein the upper and lower grating structures overlap on the sample. At least one of the upper or lower grating structure may include a fine pitch characteristic of having a pitch smaller than a wavelength of an illumination beam and is configured to rotate a first-order diffraction of the illumination beam associated with at least one of the first or second coarse pitch relative to at least one of specular reflection from a top surface of the sample or zero-order diffraction from one or more grating structures. The overlay between the first and second layers of the sample can be determined based on the image of the first-order diffraction from one of the grating structures.

Description

Technical Field

[0001] The present invention generally relates to scatter overlay metrology, and more particularly, to scatter overlay metrology of targets having orthogonal fine pitch. Prior Art

[0002] Overlay metrology techniques utilizing grating-on-grating structures, such as (but not limited to) scattered-light overlay (SCOL) or moiré techniques, offer relatively compact overlay targets. However, these techniques can suffer from reduced contrast or signal-to-noise ratio due to the presence of unwanted signals, such as (but not limited to) specular reflections (e.g., DC signals). Therefore, there is a need to develop systems and methods that address these deficiencies. Summary of the Invention

[0003] According to one or more illustrative embodiments, an overlay metrology system is disclosed. In one illustrative embodiment, the system includes an illumination source configured to generate an illumination beam. In another illustrative embodiment, the system includes one or more illumination optics for directing the illumination beam toward a sample having a field of view including one or more overlay targets when implementing a metrology recipe. In another illustrative embodiment, a particular overlay target of the one or more overlay targets according to the metrology recipe includes one or more grating-overlying grating structures formed from a lower grating structure having a first coarse pitch and an upper grating structure having a second coarse pitch, wherein the upper and lower grating structures overlap on the sample. In another illustrative embodiment, at least one of the upper grating structure or the lower grating structure further includes features having a fine pitch smaller than a wavelength of the illumination beam and is configured to rotate a first-order diffraction of the illumination beam associated with at least one of the first or second coarse pitches relative to at least one of specular reflection from a top surface of the sample or zero-order diffraction from the one or more grating overlying grating structures. In another illustrative embodiment, the system includes an objective lens to collect light from the sample within the field of view when implementing the metrology recipe. In another illustrative embodiment, the system includes a polarizer configured to pass a portion of the light from the sample associated with the first-order diffraction. In another illustrative embodiment, the system includes a detector to generate an image of the sample including the one or more overlay targets. In another illustrative embodiment, the system includes a controller that executes program instructions that cause the one or more processors to determine overlay measurements of the sample based on the image of the one or more overlay targets.

[0004] According to one or more illustrative embodiments, an overlay metrology target is disclosed. In one illustrative embodiment, the target includes one or more grating-overlying grating structures formed from a lower grating structure having a first coarse pitch in a first layer of a sample and an upper grating structure having a second coarse pitch in a second layer of the sample, wherein the upper and lower grating structures overlap on the sample. In another illustrative embodiment, at least one of the upper grating structure or the lower grating structure further includes features having a fine pitch less than a wavelength of an illumination beam and is configured to rotate a first-order diffraction of the illumination beam associated with at least one of the first or second coarse pitch relative to at least one of a specular reflection from a top surface of the sample or a zero-order diffraction from the one or more grating structures. In another illustrative embodiment, overlay between the first and second layers of the sample can be determined based on an image of the first-order diffraction from the one or more grating structures.

[0005] According to one or more illustrative embodiments, an overlay metrology method is disclosed. In one illustrative embodiment, the method includes fabricating one or more overlay targets on a sample. In another illustrative embodiment, a particular overlay target of the one or more overlay targets includes one or more grating-overlying grating structures formed from a lower grating structure having a first coarse pitch and an upper grating structure having a second coarse pitch, wherein the upper and lower grating structures overlap on the sample. In another illustrative embodiment, at least one of the upper grating structure or the lower grating structure further includes features having a fine pitch less than a wavelength of an illumination beam and is configured to rotate a first-order diffraction of the illumination beam associated with at least one of the first or second coarse pitch relative to at least one of specular reflection from a top surface of the sample or a zero-order diffraction from the one or more grating-overlying grating structures. In another illustrative embodiment, the method includes illuminating one or more of the overlay targets with the illumination beam. In another illustrative embodiment, the method includes generating an image of the sample by aligning a polarizer to pass through the first-order diffraction. In another illustrative embodiment, the method includes determining an overlay metric based on the image of the sample.

[0006] According to one or more illustrative embodiments, an overlay metrology system is disclosed. In one illustrative embodiment, the system includes an illumination source configured to generate at least one illumination beam. In another illustrative embodiment, the system includes one or more illumination optics to direct the illumination beam at an oblique angle toward a sample having a field of view including one or more overlay targets when implementing a metrology recipe. In another illustrative embodiment, a particular overlay target of the one or more overlay targets includes one or more corrugated grating overlying grating structures formed from a lower grating structure having a first coarse pitch and an upper grating structure having a second coarse pitch different from the first pitch, wherein the upper and lower grating structures overlap on the sample. In another illustrative embodiment, the system includes an objective lens to collect light diffraction from the sample within the field of view when implementing the metrology recipe, wherein at least one of specular reflection from the sample or zeroth-order diffraction from the one or more overlay targets is outside a collection numerical aperture of the objective lens. In another illustrative embodiment, the system includes a detector to generate an image of the sample including the one or more overlay targets. In another illustrative embodiment, the system includes a controller to determine an overlay measurement of the sample based on the image of the one or more overlay targets.

[0007] It should be understood that both the foregoing general description and the following detailed description are for illustration and explanation only and are not necessarily limiting of the present invention. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the present invention and, together with the general description, serve to explain the principles of the present invention. Simple diagram description

[0008] Those skilled in the art may better understand the advantages of the present invention by referring to the accompanying drawings.

[0009] FIG. 1A is a conceptual diagram illustrating an overlay metrology system according to one or more embodiments of the present invention.

[0010] FIG. 1B is a conceptual diagram illustrating an overlay metrology subsystem according to one or more embodiments of the present invention.

[0011] 2 is a top view of a polarization rotation feature on a first layer of a stacked target in accordance with one or more embodiments of the present invention.

[0012] 3A depicts an effective medium representation of the structure within a period associated with a coarse pitch of light polarized in the X direction as defined in FIG. 2 , in accordance with one or more embodiments of the present invention.

[0013] 3B depicts an effective medium representation of the structure within a coarse pitch for light polarized in the Y direction as defined in FIG. 2 , in accordance with one or more embodiments of the present invention.

[0014] 4 is a conceptual illustration of an electric field direction of various diffraction orders produced by the polarization rotation feature depicted in FIG. 2 after illumination with an illumination beam in accordance with one or more embodiments of the present invention.

[0015] 5 is a top view of a 2D polarization rotation feature suitable for overlay measurement along two measurement directions according to one or more embodiments of the present invention.

[0016] 6 is a top view of polarization rotating features configured with fine pitch periodicity at a 45 degree angle relative to a coarse pitch in accordance with one or more embodiments of the present invention.

[0017] 7 is a top view of an unsegmented 2D polarization rotation feature in accordance with one or more embodiments of the present invention.

[0018] 8A is a top view of an overlay target suitable for polarization-based isolation of first-order diffraction along a single measurement direction, according to one or more embodiments of the present invention.

[0019] 8B is a top view of a 2D overlay target suitable for polarization-based isolation of first-order diffraction along two measurement directions according to one or more embodiments of the present invention.

[0020] 9 depicts the use of an overlay target including polarization rotation features in a grating-on-grating structure for a zero-order SCOL technique in accordance with one or more embodiments of the present invention.

[0021] 10 is a conceptual side view of a grating-on-grating structure illustrating one or more embodiments of non-limiting components of a ripple signal according to the present invention.

[0022] 11 is a flow chart illustrating steps performed in a method for high-throughput overlay metrology according to one or more embodiments of the present invention.

[0023] 12 is a series of high-level graphs illustrating high-throughput imaging of multiple overlaid targets in a single image capture in accordance with one or more embodiments of the present invention.

[0024] 13A is a top view of a collection pupil of an overlay metrology subsystem according to one or more embodiments of the present invention.

[0025] 13B is a conceptual diagram of generating two illumination beams from an incoherent light source according to one or more embodiments of the present invention. Implementation Method

[0026] Reference will now be made in detail to the disclosed subject matter illustrated in the accompanying drawings. The invention has been particularly shown and described with respect to specific embodiments and features thereof. The embodiments described herein are to be considered illustrative rather than restrictive. Those skilled in the art will readily appreciate that various changes and modifications in form and detail may be made without departing from the spirit and scope of the invention.

[0027] Embodiments of the present invention are directed to systems and methods for performing overlay metrology using an overlay target having a grating-on-grating structure, wherein selected diffraction orders used for a measurement are optically isolated using polarization control. In other words, some embodiments of the present invention are directed to systems and methods for performing overlay metrology using an overlay target having a grating-on-grating structure, wherein polarization control is used to block at least a portion of an unwanted optical signal emanating from a sample (e.g., light from the sample that may not contribute to an overlay measurement). As a non-limiting illustration, in some overlay metrology techniques, it may be desirable to block pure reflections from a sample that do not interact with the two gratings of a grating-on-grating structure (e.g., reflections from a top surface of the sample), referred to herein as a DC signal, to distinguish this signal from zeroth-order diffraction from the two gratings. It is contemplated herein that these DC signals can be relatively strong compared to the various diffraction orders from a grating-on-grating structure and, therefore, can reduce the contrast of a measurement.

[0028] For the purposes of this disclosure, the term "grating-on-grating structure" is used to refer to a set of features in the overlapping region of two or more layers of a sample, where the features in each layer have a periodicity that can be characterized by at least one pitch or spatial frequency. In this manner, the features in the overlapping region are described as a grating structure. However, it should be understood that the grating structure on a particular sample layer can be characterized by multiple pitches along a particular direction and / or multiple pitches along different directions. Thus, the constituent features of the grating-on-grating structure can be segmented to provide multiple pitches in any combination of directions or otherwise distributed to provide a spatial frequency suitable for producing diffraction of incident light.

[0029] Furthermore, optical isolation of selected diffraction orders from an overlay target comprising a grating-on-grating structure can also block signals from other features on a sample (e.g., device features, dummy features, or the like), enabling simultaneous characterization of multiple overlay targets within a field of view. This metrology approach enables extremely high-throughput metrology.

[0030] An overlay target and / or an overlay metrology tool suitable for characterizing the overlay target can be configured according to a metrology recipe suitable for generating overlay measurements based on a desired technology. More generally, an overlay metrology tool can be configured according to multiple metrology recipes to perform overlay metrology using multiple technologies and / or to perform overlay metrology on multiple overlay targets having different designs.

[0031] For example, a metrology recipe may include aspects of an overlay target or a design for an overlay target, including, but not limited to, a layout of target features, feature size, or feature spacing on one or more sample layers. As another example, a metrology recipe may include illumination parameters such as, but not limited to, an illumination wavelength, an illumination pupil distribution (e.g., a distribution of illumination angles and associated illumination intensities at those angles), a polarization of the incident illumination, a spatial distribution of illumination, or a sample height. As another example, a recipe for an overlay metrology tool may include collection parameters such as, but not limited to, a collection pupil distribution (e.g., a desired distribution of angular light from the sample to be used for a measurement and associated filtering intensities at those angles), a collection field stop setting that selects portions of the sample of interest, and polarization or wavelength filters for the collected light.

[0032] Some embodiments of the present invention are directed to an overlay target suitable for providing polarization-based optical isolation of a selected diffraction order and a grating-on-grating structure (such as, but not limited to, the first-order diffraction). Some embodiments of the present invention are directed to an overlay metrology technique (e.g., which may be implemented using a measurement recipe) suitable for measuring overlay on an overlay target based on polarization-based optical isolation of a selected diffraction order. Some embodiments of the present invention are directed to an overlay metrology tool that can be configured (e.g., based on a measurement recipe) to perform overlay measurement based on polarization-based optical isolation of a selected diffraction order.

[0033] In some embodiments, an overlay target suitable for polarization-isolated overlay metrology based on first-order diffracted light comprises at least one grating-on-grating structure, wherein a grating on a first layer comprises features that impart different phase variations on orthogonal polarization components of the first-order diffracted light. The use of polarization target features to provide amplitude and phase control of diffraction orders is generally described in U.S. Patent No. 10,337,991, issued on July 2, 2019, the entire contents of which are incorporated herein by reference. It is contemplated that some embodiments of the present invention extend the use of the polarization target features described in U.S. Patent No. 10,337,991 to various overlay metrology techniques utilizing grating-on-grating structures, and further extend the use of the polarization target features described in U.S. Patent No. 10,337,991 to high-throughput overlay metrology based on simultaneous imaging of multiple overlay targets in a single image capture.

[0034] A grating-on-grating structure can be formed from periodic structures (e.g., gratings, or more generally, grating structures) derived from different lithographic exposures on two layers in the overlapping region of an overlay target (e.g., an upper layer and a lower layer). Consequently, light incident on a grating-on-grating structure can interact with the gratings on each layer, such that the diffraction orders emanating from the overlay target can be influenced, among other things, by the relative position of the grating structures. Consequently, overlay measurements associated with the relative registration (or offset) of sample layers can be based on one or more diffraction orders of light interacting with the grating structure in each of the layers of interest.

[0035] For example, a grating on a first layer of an overlay target can include features configured to provide a phase shift of π for a first-order diffraction along a Y direction relative to a first-order diffraction along an X direction. In other words, the grating on the first layer can have a different effective dielectric constant along the X direction relative to an orthogonal Y direction. In this way, the grating on the first layer can operate as a wave plate for first-order diffraction but not for other light from the sample, such as (but not limited to) zeroth-order diffraction, a DC signal, or higher-order diffraction. Thus, the grating on the first layer can rotate the polarization of first-order diffraction light relative to other light. Furthermore, the grating on the second layer of the overlay target can have any design and need not affect the polarization of light. In this way, the overlay target, along with the first layer and the second layer of a photoresist layer, can be formed on a process layer with relatively high contrast between the grating features and the surrounding material necessary to modify the polarization of the first-order diffraction. For the purposes of this disclosure, the term process layer refers to a layer having one or more features that is fabricated using various processing steps, including, but not limited to, a lithographic step (e.g., in a photomask deposited on the layer) followed by an etch step. In this manner, features in a process layer can be a material different from the surrounding material, which can be air (e.g., unfilled) or an additional material such as, but not limited to, an oxide. For the purposes of this disclosure, features in a photoresist layer refer to features created in the photoresist by a lithographic exposure prior to an etch step.

[0036] An overlay metrology tool configured (e.g., according to a metrology recipe) to perform overlay metrology using such an overlay target can illuminate the overlay target using linearly polarized light and include a polarizer in a collection path with an orthogonal polarization. In this manner, the collection path can selectively pass first-order diffraction that has been rotated by the overlay target's first polarization layer and block non-rotated light, including (but not limited to) a DC signal or diffraction other than the first-order diffraction. Consequently, the signal associated with the first-order diffraction can have high contrast and / or a high signal-to-noise ratio.

[0037] It is contemplated herein that polarization-based isolation overlay measurement based on first-order diffraction can be implemented with any first-order diffraction-based overlay technique (and associated overlay target design). It is contemplated herein that this technique can be particularly beneficial for, but not limited to, mitigating DC reflections that can degrade the contrast and / or signal-to-noise ratio of an overlay measurement.

[0038] Some embodiments of the present invention are directed to scattered light overlay (SCOL) techniques and associated overlay targets. SCOL overlay metrology techniques can utilize overlay targets comprising grating-on-grating structures, where the constituent grating features have a common pitch. These targets are referred to herein as SCOL targets.

[0039] This paper recognizes that a conventional zero-order SCOL measurement technique is based on a grating-on-grating target design, where an upper grating and a lower grating have the same pitch. Furthermore, each photon emitted from any point in the illumination pupil is scattered by both the upper and lower gratings, and vice versa, so that the measured zero-order signal contains overlay information attributable to the combination of ±1, ±2, ... diffraction orders from the upper and lower gratings.

[0040] One problem with this approach is a large DC signal due to pure zero-order reflections, combined with algorithmic inaccuracies stemming from the various combinations of diffraction orders contributing to the measurement signal. Specifically, a model based on a relatively small number of cells with a predetermined offset between the upper and lower gratings cannot realistically account for all possible combinations of diffraction orders contributing to the measurement signal. For example, the number of cells associated with a measurement, and therefore reflected in such a model, may be limited by considerations such as (but not limited to) target size and mobile acquisition measurement (MAM) throughput.

[0041] It is further recognized that similar problems arise when using a corrugated overlay target with a relatively large gain (e.g., >300). Moreover, a corrugated target with a gain of about 10 to about 300 to about 400 may be impractical due to the inherent ambiguity in overlay measurements with such values. For example, using techniques such as darkfield illumination to address DC signals may be impractical or impossible.

[0042] Some embodiments of the present invention are directed to a SCOL target in which a grating in one layer can rotate the polarization of a first-order diffraction (e.g., a DC signal, zeroth-order diffraction, or higher-order diffraction) relative to other light from the sample. In this way, an overlay metrology tool can provide polarization-based isolation of first-order diffraction. This target can be used with any SCOL overlay technique that utilizes first-order diffraction.

[0043] Some embodiments of the present invention are directed to corrugation overlay techniques and associated overlay targets. Corrugation overlay metrology can utilize overlay targets comprising one or more grating-on-grating structures, where the constituent gratings have different pitches. These targets are referred to herein as corrugated targets and provide diffraction from both gratings (e.g., double diffraction) in addition to the typical diffraction orders from each grating. For example, based on a gain factor associated with the difference in pitch between the constituent gratings, a corrugated target can produce corrugation diffraction orders with relatively small diffraction angles. Corrugated targets can be used in a variety of measurement modes. Thus, even if the pitch between the constituent gratings is unresolvable, an image of a corrugated target can include a periodic signal based on the corrugation diffraction orders (e.g., a signal with a corrugation pitch). Furthermore, because the corrugation pitch is typically larger than the pitch associated with the constituent gratings (e.g., by the gain factor), corrugation overlay techniques can provide overlay measurements with sensitivity amplified by the gain factor. Corrugated targets can also be characterized using pupil-based metrology techniques.

[0044] Some embodiments of the present invention are directed to high-throughput overlay metrology based on the simultaneous capture of signals from multiple overlay targets (sometimes hundreds or more). Specifically contemplated herein are overlay targets comprising a grating-on-grating structure, wherein one grating rotates the polarization of a first-order diffraction relative to other light from the overlay target, enabling isolated imaging of the target based on this first-order diffraction. For example, an overlay measurement based on illumination using linear polarization and collection using a crossed polarizer (e.g., a polarizer in a collection path oriented to exclude non-rotated polarization) can optionally optically isolate the first-order diffraction from the overlay target and block other light, including but not limited to extraneous light from the overlay target and light from device features. Consequently, an image of a field of view comprising multiple overlay targets can display the overlay targets with high contrast.

[0045] It should be noted that the use of a polarization-rotating target (e.g., within a process layer) provides for the first-order diffraction from two layers with perpendicular polarizations. This can be generally beneficial for various overlay metrology techniques. For example, this can be beneficial for optical SCOL techniques, where a ripple signal is obtained as interference between the fields of the +1st order of a grating structure in two layers in one detector and the -1st order of a grating structure in two layers in a second detector. A polarizer aligned at a suitable angle can then be used to equalize the amplitudes of the diffraction orders from the grating structure and thereby improve the signal-to-noise ratio. Similar techniques can be applied in typical imaging overlay metrology techniques to equalize the contrast of signals from layers formed of different materials (e.g., from grating structures in photoresist and process layers).

[0046] 1A to 13B , a system and method for performing overlay measurement using polarization-based isolation of light of selected diffraction orders according to one or more embodiments of the present invention are described in more detail.

[0047] FIG. 1A is a conceptual diagram illustrating an overlay metrology system 100 according to one or more embodiments of the present invention.

[0048] In some embodiments, the overlay metrology system 100 includes an overlay metrology subsystem 102 for acquiring overlay signals from an overlay target based on any number of overlay recipes. For example, the overlay metrology subsystem 102 may direct at least one illumination beam 104 toward an overlay target 106 on a sample 108 and may further collect light or other electromagnetic radiation emanating from the overlay target 106 (referred to herein as sample light 110), wherein at least a portion of the sample light 110 is used to generate one or more images of the overlay target 106. In this manner, one or more overlay measurements of the sample 108 (e.g., overlay measurements associated with a registration or offset of two or more layers of the sample 108) may be generated based on the at least one image of the overlay target 106.

[0049] Furthermore, the overlay metrology subsystem 102 can be configured to generate images based on any number of metrology recipes that define metrology parameters for image acquisition and / or design parameters of an overlay target 106 to be imaged. For example, a metrology recipe can include various aspects of an overlay target 106 or a design of an overlay target 106, including, but not limited to, a layout, feature size, or feature spacing of target features on one or more sample layers. As another example, a metrology recipe can include illumination parameters such as, but not limited to, an illumination wavelength, an illumination pupil distribution (e.g., a distribution of illumination angles and associated illumination intensities at those angles), a polarization of the incident illumination, a spatial distribution of illumination, or a sample height. As another example, a metrology recipe can include collection parameters such as, but not limited to, a collection pupil distribution (e.g., a desired distribution of angular light from the sample 108 to be measured and associated filtering intensities at those angles), collection field stop settings for selecting portions of the sample of interest, and polarization or wavelength filters for the collection light.

[0050] The overlay metrology subsystem 102 may be any type of overlay metrology tool known in the art suitable for generating an overlay signal suitable for determining an overlay associated with an overlay target 106 on a sample 108 .

[0051] In some embodiments, the overlay metrology subsystem 102 generates an image of the overlay target 106 in which one or more features of the overlay target 106 are unresolved. For example, the overlay metrology subsystem 102 can be configured (e.g., based on a metrology recipe) to form an image of the overlay target 106 based on light from a single diffraction order or light from multiple diffraction orders that are not combined in a manner that provides a resolved image of the overlay target 106 or selected features therein. As an illustration, an image of the overlay target 106 associated with a zero-order SCOL technique can include an unresolved grating-on-grating structure, where the unresolved grating-on-grating structure is represented as grayscale regions. In this configuration, the intensity of a particular grayscale region is affected by the physical overlay of the grating-on-grating structure (e.g., a physical offset associated with an expected offset (if applicable) and an unintended overlay error). In this manner, the overlay metrology subsystem 102 can generate an overlay measurement (e.g., a measurement of unintended overlay error) of the sample 108 based on the image intensity of unresolved grating-on-grating features in multiple cells of the overlay target 106. As another illustration, an image of the overlay target 106 associated with a moiré technique may include unresolved grating-on-grating structure but may include a resolved periodic signal associated with diffraction (e.g., double diffraction) from the moiré corrugations that constitute the grating.

[0052] FIG1B is a conceptual diagram illustrating an overlay metrology subsystem 102 according to one or more embodiments of the present invention. In some embodiments, the overlay metrology subsystem 102 includes an illumination source 112 configured to generate an illumination beam 104. The illumination beam 104 may include light of one or more selected wavelengths, including but not limited to ultraviolet (UV) radiation, visible radiation, or infrared (IR) radiation.

[0053] In some embodiments, the overlay metrology subsystem 102 directs the illumination beam 104 toward the sample 108 or a portion of the sample 108 including at least one overlay target 106 via an illumination path 114. The illumination path 114 may include one or more optical components suitable for modifying and / or conditioning the illumination beam 104 and directing the illumination beam 104 toward the sample 108. For example, the illumination path 114 may include (but need not include) one or more lenses 116 (e.g., for collimating the illumination beam 104, relaying a pupil and / or field plane, or the like), one or more illumination polarizers 118 for adjusting the polarization of the illumination beam 104, 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, or the like). In some embodiments, the overlay metrology subsystem 102 includes an objective lens 120 to focus the illumination beam 104 onto the sample 108 (e.g., an overlay target 106 having overlay target elements located on two or more layers of the sample 108). In some embodiments, the sample 108 is disposed on a sample stage 122 suitable for securing the sample 108 and further configured to position the sample 108 relative to the illumination beam 104.

[0054] In some embodiments, the overlay metrology subsystem 102 includes one or more detectors 124 configured to capture sample light 110 extracted from the sample 108 through a collection path 126 and generate one or more overlay signals indicative of an overlay of two or more layers of the sample 108. The collection path 126 may include a plurality of optical elements to direct and / or modify the illumination collected by the objective 120, including, but not limited to, one or more lenses 128, one or more filters, one or more collection polarizers 130, one or more beam blocks, or one or more beam splitters. For example, a detector 124 may receive an optical image of the sample 108 provided by an element in the collection path 126 (e.g., the objective 120, one or more lenses 128, or the like). As another example, the detector 124 may receive radiation reflected or scattered (e.g., via specular reflection, diffuse reflection, and the like) from the sample 108. As another example, a detector 124 may receive radiation from one or more diffraction orders of the overlay target 106 (eg, 0th order diffraction, ±1st order diffraction, ±2nd order diffraction, and the like).

[0055] The illumination path 114 and the collection path 126 of the overlay metrology subsystem 102 can be oriented in a wide range of configurations suitable for illuminating the overlay target 106 with the illumination beam 104 and collecting sample light 110 in response to the incident illumination beam 104. For example, as shown in FIG1B , the overlay metrology subsystem 102 can include a beam splitter 132 oriented so that the objective lens 120 can simultaneously direct the illumination beam 104 toward the overlay target 106 and collect radiation extracted from the sample 108. As another example, the illumination path 114 and the collection path 126 can contain non-overlapping optical paths.

[0056] Referring now to Figures 2 through 10 , the design of an overlay target 106 suitable for polarization-based isolation of first-order diffraction according to one or more embodiments of the present invention is described in greater detail. Figures 2 through 7 illustrate various non-limiting examples of features suitable for rotating a polarization of first-order diffraction relative to other sample light 110, such as, but not limited to, a DC signal, zeroth-order diffraction, or higher-order diffraction. These features are referred to herein as polarization rotation features. It is contemplated that an overlay target 106 according to the systems and methods disclosed herein may be formed from or otherwise include a grating-on-grating structure in which at least one of the layers includes a polarization rotation feature. Next, Figures 8A through 10 depict various non-limiting examples of overlay targets 106 including polarization rotation features.

[0057] In some embodiments, the stack of targets 106 includes at least one grating-on-grating structure, wherein features on at least one of the layers of the sample 108 rotate a polarization of first-order diffraction, such as (but not limited to) DC reflection, zero-order diffraction, or higher-order diffraction, relative to other light extracted from the grating-on-grating structure.

[0058] A grating-on-grating structure can have any combination or orientation of features suitable for providing rotation of the first-order diffraction. In some embodiments, a grating-on-grating structure comprises features having a coarse pitch on at least one sample layer, which are further segmented in an orthogonal direction (e.g., by a fine pitch). Furthermore, the fine pitch can be smaller than a wavelength of light such that the orthogonal segmentation at the fine pitch provides different effective dielectric constants for light polarized in orthogonal directions.

[0059] FIG. 2 is a top view of a polarization rotation feature 202 on a first layer of a stacked target 106 in accordance with one or more embodiments of the present invention.

[0060] With specific reference now to FIG2 , polarization-rotating features 202 include grating structures 204 of a first material type separated by regions of a second material type, referred to herein as surrounding material 206. For the purposes of this disclosure, the term grating structure 204 is used herein to refer to a set of features patterned in a sample layer that have a periodicity that can be characterized by at least one pitch or spatial frequency. In a general sense, the grating structure 204 on any particular layer can have multiple periodicities (and associated pitches) in any particular direction and / or can have periodicity in multiple directions.

[0061] Polarization rotation features 202 can be characterized by a coarse pitch 208 (e.g., a coarse period) and a fine pitch 210 (e.g., a fine period, a segmented pitch, a segmented period, or the like). Specifically, features within a coarse pitch 208 are further arranged into two regions 212a and 212b having orthogonal periodicity relative to the fine pitch 210. For example, box 214 in FIG. 2 depicts features within a period associated with the coarse pitch 208, which are divided into a first region 212a characterized by segmentation along the X direction using the fine pitch 210, and a second region 212b characterized by segmentation along the Y direction using the fine pitch 210.

[0062] It is contemplated herein that the polarization rotation feature 202 depicted in FIG. 2 operates as a wave plate to impart different phases to light having orthogonal polarizations along the fine pitch 210. In this manner, the polarization rotation feature 202 can modify or rotate the polarization of incident light (e.g., an illumination beam 104). For example, the polarization rotation feature 202 can operate as a half-wave plate for first-order diffraction, because the polarization rotation feature 202 shifts the phase of the first-order diffraction by π along the Y direction while maintaining the phase of the first-order diffraction along the X direction.

[0063] FIG3A depicts an effective medium representation of structures within a period associated with coarse pitch 208 (e.g., features within box 214 in FIG2 ) for light polarized in the X direction, as defined in FIG2 , in accordance with one or more embodiments of the present invention. FIG3B depicts an effective medium representation of structures within coarse pitch 208 (e.g., features within box 214 in FIG2 ) for light polarized in the Y direction, as defined in FIG2 , in accordance with one or more embodiments of the present invention.

[0064] In FIG3A and FIG3B , the effective dielectric constant is defined by the following formula: (1) (2) in is a dielectric constant of the grating structure 204 of the first material type, is the dielectric constant of the surrounding material 206 of the second material type, and η is a fraction of the coarse pitch 208 associated with the segmentation having fine pitch 210 along the X direction (e.g., the width of the first region 212a in FIG. 2 relative to the coarse pitch 206). In other words, η corresponds to a duty cycle of the orthogonal fine pitch segmentation.

[0065] As shown in equations (1) and (2) in conjunction with Figures 3A-3B, configuring features on a layer of a stacked target 106 to provide regions in a coarse pitch 208 and orthogonal fine pitch 210 direction effectively operates as a wave plate and provides different phase shifts for light polarized in orthogonal directions. Figure 4 is a conceptual illustration of the electric field directions of various diffraction orders generated by the polarization rotating feature 202 shown in Figure 2 after illumination with an illumination beam 104, according to one or more embodiments of the present invention. As shown in Figure 4, the zeroth order diffraction 402 can have the same electric field direction as the incident illumination beam 104, while the first order diffraction 404 can be polarized orthogonally to the zeroth order diffraction 402.

[0066] In some embodiments, as shown in FIG4 , an overlay metrology subsystem 102 configured according to a measurement recipe provides an illumination beam 104 polarized at a 45-degree angle relative to directions orthogonal to the fine-pitch periodicity 210 (e.g., the X and Y directions in FIG2 ). In this manner, the illumination beam 104 will have polarization components along two directions of fine-pitch periodicity (e.g., the periodicity of fine pitch 210), allowing the polarization rotation feature 202 to modify or rotate the polarization of a first-order diffraction of the illumination beam 104 relative to other sample light 110, such as (but not limited to) a DC signal, zeroth-order diffraction, or higher-order diffraction. In some embodiments, the overlay metrology subsystem 102 is further configured according to the measurement recipe to provide a crossed collection polarizer 130 in the collection path 126 oriented to isolate the first-order diffraction 404.

[0067] Referring now to FIG. 5 , FIG. 5 illustrates a top view of a 2D polarization rotation feature 202 suitable for overlay measurement along two measurement directions, according to one or more embodiments of the present invention. The 2D polarization rotation feature 202 may be suitable for overlay measurement along two dimensions within an overlay target 106 (e.g., a 2D overlay target 106). As an illustration, the 2D polarization rotation feature 202 depicted in FIG. 5 may correspond to the polarization rotation feature depicted in FIG. 2 , extending along a Y-direction. For example, the 2D polarization rotation feature 202 in FIG. 5 may be formed as a shifted version of the polarization rotation feature from FIG. 2 in alternating rows and the polarization rotation feature 202 in staggered rows. Specifically, the polarization rotation features 202 in the staggered rows are shifted along the X-direction by half the coarse pitch 208. In this manner, the features within a coarse pitch 208 along the Y-direction are also segmented into regions 212 a and 212 c having orthogonal fine pitch periodicity in a manner similar to that in the X-direction.

[0068] 2-5 , which are provided for illustration only and should not be construed as limiting, the polarization rotation feature 202 may have any configuration suitable for rotating the first-order diffraction 404 , such as but not limited to the zeroth-order diffraction 402 , relative to the rest of the sample light 110 .

[0069] For example, the fine pitch 210 need not be the same for two orthogonal directions (e.g., the fine pitch 210 need not be the same for the first region 212a and the second region 212b). As an illustration based on FIG2 but not explicitly shown, the first region 212a may have a first fine pitch 210 and the second region 212b may have a second fine pitch 210 of a value different from the first fine pitch 210. However, the fine pitch 210 in any direction should be sufficiently smaller than a wavelength of the illumination beam 104 (e.g., according to a measurement recipe) to satisfy the effective medium condition so that equations (1) and (2) are valid. In addition, the duty cycle (e.g., associated with the value of η) should be the same for different measurement directions. Considering FIG5 as an illustration, the duty cycle along the X direction associated with regions 212a and 212b should be the same as the duty cycle along the Y direction associated with regions 212a and 212c.

[0070] As another example, the polarization rotation feature 202 can have any duty cycle (e.g., any value of n). However, in some designs, a duty cycle that deviates from 1:1 (e.g., a value of n = 0.5) can reduce the effectiveness of rotating the first-order diffraction by the polarization rotation feature 202.

[0071] For example, the polarization rotating features 202 can be configured to provide fine pitch periodicity in any two orthogonal directions. As an illustration, FIG6 shows a top view of the polarization rotating features 202 configured with a fine pitch periodicity at a 45-degree angle relative to a coarse pitch 208, according to one or more embodiments of the present invention. Specifically, the grating structure 204 is oriented at angles of 45 and 135 degrees relative to the coarse pitch 208 (here, in the X direction).

[0072] As previously described herein, the illumination beam 104 should be polarized at 45 degrees relative to both directions of the fine-pitch periodicity. Therefore, for the polarization rotation feature 202 in FIG6 , the illumination beam 104 should be polarized in either the X or Y direction. Similarly, the overlay measurement subsystem 102 should include a collection polarizer 130 in a crossed polarization to capture the rotational first-order diffraction.

[0073] As a further example, the polarization rotation features 202 need not be segmented into separate regions (e.g., regions 212a and 212b in Figures 2-3B and 5), with different fine-pitch segmentation within each region 212. Specifically, the polarization rotation features 202 imparting different phases to the orthogonal polarizations may be sufficient to rotate the first-order diffraction relative to other sample light 110, such as (but not limited to) a DC signal, zero-order diffraction, or higher-order diffraction. In other words, the polarization rotation features 202 may have any size, orientation, or distribution, as long as the effective dielectric constant distributions of the X and Y polarizations impart different phases to the orthogonal polarizations in a manner similar to (but not limited to) Figures 3A-3B and equations (1)-(2).

[0074] As an illustration, FIG7 is a top view of a portion of an unsegmented 2D polarization rotation feature 202, according to one or more embodiments of the present invention. For example, the 2D polarization rotation feature 202 includes features with different orientations and spacings along the X and Y directions, allowing the 2D polarization rotation feature 202 to operate in a manner similar to the 2D polarization rotation feature 202 in FIG5 . Furthermore, the frame 702 in FIG7 depicts a period (e.g., a coarse spacing 208) of the 2D polarization rotation feature 202, with the ellipses in FIG7 indicating that the pattern depicted therein repeats in two dimensions.

[0075] 8-10 , various non-limiting examples of overlay targets 106 incorporating polarization rotation features 202 according to one or more embodiments of the present invention are described in greater detail.

[0076] It is contemplated herein that the effectiveness of the polarization rotation feature 202 in achieving polarization-based isolation of the first-order diffraction when rotating the first-order diffraction may depend on the specific layout and composition of the polarization rotation feature 202 .

[0077] For example, in a case where the surrounding material 206 ( = 1.45) and a 1:1 duty cycle (η = 0.5) silicon grating structure 204 for orthogonal directional segmentation ( = 3.8) provides effective dielectric constant values ​​of ε1 = 2.63 and ε2 = 2.1. This is similar to the contrast obtained using a typical photoresist target. However, the polarization rotating features 202 with the same 1:1 duty cycle are fabricated in a photoresist layer (e.g. = 1.4 and = 1) provides effective dielectric constant values ​​of ε1 = 1.2 and ε2 = 1.17, which provides minimal contrast and thus relatively poor rotation of the first-order diffraction 404. Thus, the polarization rotation feature 202 may be suitable for use in a process layer, but may be impractical for use in a photoresist layer (or may be effective at polarization rotation below an application tolerance).

[0078] Therefore, it may be impractical to utilize polarization rotation features 202 in a stack of targets 106 where features on different sample layers (or more generally, features associated with different lithographic exposures) are in non-overlapping or side-by-side regions when one of the sample layers is a photoresist layer. In such a configuration, the amplitude of the first-order diffraction from the feature in the photoresist layer may have a relatively small amplitude due to the relatively small refractive index difference between the feature and the surrounding medium. Consequently, the signal-to-noise ratio may be low in such a configuration.

[0079] However, it is contemplated herein that the polarization rotation features 202 are well-suited for, but not limited to, an overlay target 106 having a grating-on-grating structure. In this configuration, it may be sufficient to provide polarization rotation for first-order diffraction on one layer of the overlay target 106 (e.g., a process layer). Thus, the features on the second layer (e.g., a photoresist layer or a process layer) may have any suitable pattern or orientation without any polarization rotation requirements.

[0080] It is further contemplated herein that any overlay target 106 design comprising at least one grating-on-grating structure may incorporate polarization rotation features 202 in at least one layer.

[0081] FIG8A is a top view of an overlay target 106 suitable for polarization-based isolation of first-order diffraction along a single measurement direction, according to one or more embodiments of the present invention. Features in a first sample layer are depicted in a first panel 802, and features in a second sample layer are depicted in a second panel 804. In this manner, the features in the first panel 802 constitute a lower grating structure 806, while the features in the second panel 804 constitute an upper grating structure 808. Combined, the lower grating structure 806 and the upper grating structure 808 form a grating-on-grating structure.

[0082] In some embodiments, the overlay target 106 includes polarization-rotating features 202 in a first layer, which may be a process layer that provides high-contrast features. For example, the first panel 802 includes a replica of the polarization-rotating features 202 depicted in FIG. The overlay target 106 can then include any suitable grating structure in a second layer, which can be any type of layer, including, but not limited to, a photoresist layer or a process layer. In this manner, the polarization-rotating features 202 can be located in both the first and second layers (e.g., the lower and upper grating structures 806 and 808), or only in the first layer (e.g., the lower grating structure 806). For example, the second panel 804 depicts undivided features characterized by a spacing 810. Depending on the specific technology, this spacing 810 can have different values. For example, an overlay target 106 suitable for SCOL technology can provide (e.g., according to a metrology recipe) a spacing 810 between the upper grating structures 808 that is equal to the coarse pitch 208 of the polarization rotation features 202 in the lower grating structure 806. In another example, an overlay target 106 suitable for moiré technology can provide (e.g., according to a metrology recipe) a spacing 810 between the upper grating structures 808 that is different from the coarse pitch 208 of the polarization rotation features 202 in the lower grating structure 806 to provide a moiré gain.

[0083] FIG8B is a top view of a 2D overlay target 106 suitable for polarization-based isolation of first-order diffraction along two measurement directions, according to one or more embodiments of the present invention. For example, the third panel 812 includes a replica of the polarization rotation feature 202 depicted in FIG5 . The overlay target 106 can then include any suitable grating structure on a second layer, which can be any type of layer, including, but not limited to, a photoresist layer or a process layer. For example, the fourth panel 814 depicts undivided hatch features characterized by the spacing 810.

[0084] Although not explicitly illustrated, it should be understood that an overlay target 106 suitable for overlay metrology in one or two dimensions can be produced based on any design of the polarization rotation features 202 in at least one layer. In the case where the polarization rotation features 202 are located on more than one layer, the polarization rotation features 202 can be adjusted to provide a desired polarization rotation of the first order diffraction as a whole.

[0085] 9-10 , various non-limiting overlay techniques using an overlay target 106 including polarization rotation features 202 in a grating-on-grating structure are described. It is contemplated that the overlay target 106 including polarization rotation features 202 in a grating-on-grating structure may be used in any overlay technique that utilizes first-order diffraction from the grating-on-grating structure.

[0086] FIG9 illustrates the use of an overlay target 106 including polarization rotation features 202 in a grating-on-grating structure for a zero-order SCOL technique according to one or more embodiments of the present invention. Specifically, FIG9 illustrates a conceptual side view of a grating-on-grating structure illustrating a non-limiting component of a zero-order SCOL signal according to one or more embodiments of the present invention.

[0087] In a zero-order SCOL technique, an overlay measurement can be generated based on measurements of zero-order signals from multiple grating-on-grating structures in different cells of an overlay target 106, where the grating-on-grating structures in the different cells have a common pitch but different predetermined offsets between the constituent grating features. For example, a zero-order SCOL technique can utilize measurements from four or more cells of grating-on-grating structures having different predetermined offsets.

[0088] It should be noted herein that the term zero-order signal from a grating structure is distinct from zero-order diffraction. Specifically, a zero-order signal in the context of a SCOL measurement includes multiple signals extracted at a common angle from a grating-on-grating structure. For example, a zero-order signal from a grating-on-grating structure in response to an illumination beam 104 at a normal angle of incidence may include multiple light signals extracted from a sample 108 at a normal angle of incidence.

[0089] As shown in FIG9 , the illumination beam 104 can interact with various features of a sample 108 to generate a zero-order signal. For example, a zero-order signal can include specular reflections (e.g., a DC signal 904) from a top surface 902 of the sample 108 or other interfaces between sample layers not explicitly shown. As another example, a zero-order signal can include zero-order diffraction 906 from an upper grating structure 808, which can propagate along the same path as the DC signal 904.

[0090] A zero-order signal can further include various signals associated with the combined interaction between the upper grating structure 808 and the lower grating structure 806. As shown in FIG9 , a portion of the illumination beam 104 can pass through the upper grating structure 808 (e.g., as transmitted zero-order diffraction) to reach the lower grating structure 806. The lower grating structure 806 can then generate various reflected diffraction orders from this portion of the illumination beam 104, which can then interact with the upper grating structure 808 again before leaving the sample 108.

[0091] For example, a zero-order signal from a grating-overlying grating structure can include zero-order diffraction 908 from the lower grating structure 806, which passes through the upper grating structure 808 (e.g., as zero-order diffraction). As another example, a zero-order signal from a grating-overlying grating structure can include double diffraction of a common diffraction order and opposite sign from the lower grating structure 806 and the upper grating structure 808, which exit the sample 108 at a normal exit angle. As an illustration, FIG9 depicts light + / -1 diffraction 910 associated with a +1 diffraction order 912 from the lower grating structure 806 and a -1 diffraction order 914 from the upper grating structure 808. Although not shown in the figure, the light associated with the -1 diffraction order from the lower grating structure 806 and the +1 diffraction order from the upper grating structure 808 can follow a symmetrical path. As another illustration, FIG9 depicts + / -2nd order diffraction 916 associated with a -2nd diffraction order 918 from the lower grating structure 806 and a +2nd diffraction order 920 from the upper grating structure 808. Although not shown, the light associated with the +2nd diffraction order from the lower grating structure 806 and the -2nd diffraction order from the upper grating structure 808 can follow a symmetrical path. Thus, the zeroth-order SCOL signal can include + / -N double-diffraction signals that follow similar patterns for the additional diffraction orders.

[0092] It should be understood that the grating overlying grating structure can generate various additional diffraction orders from the lower grating structure 806 and the upper grating structure 808 alone or in combination. For the sake of clarity, these diffraction orders are not explicitly shown in FIG. 9 .

[0093] It is contemplated herein that the zero-order SCOL technique can provide relatively high accuracy and relatively low tool-induced errors (e.g., tool-induced displacement (TIS) error, TIS 3σ error, etc.). However, this approach can suffer from a relatively strong unwanted signal associated with the DC signal 904 and the like, as well as algorithmic inaccuracies associated with multi-order diffraction signals, which can be difficult, impractical, or sometimes impossible to account for under typical measurement constraints, such as, but not limited to, target size or the number of cells within a target required for a measurement.

[0094] It is further contemplated herein that a grating-on-grating structure having a polarization rotation feature 202 to rotate a polarization of a first-order diffraction from the lower grating structure 806 and / or upper grating structure 808 can be used to optically isolate the first-order diffraction using a collecting polarizer 130 in an overlay measurement subsystem 102 as disclosed herein. For example, using a grating-on-grating structure having a polarization rotation feature 202 to rotate a polarization of a first-order diffraction from the lower grating structure 806 and / or upper grating structure 808 coupled to a collecting polarizer 130 can achieve optical isolation of the + / -1 diffraction 910 from other zero-order SCOL signals, which can provide enhanced contrast or signal-to-noise ratio for associated overlay measurements. As an illustration, an overlay target 106 configured as depicted in FIG8A can include a grating-on-grating structure wherein the spacing 810 between the upper grating structures 808 is equal to the coarse spacing 208 of the lower grating structure 806.

[0095] FIG10 illustrates the use of an overlay target 106 including a polarization rotation feature 202 in a grating-on-grating structure for a moire technique. Specifically, FIG10 illustrates a conceptual side view of a grating-on-grating structure as a non-limiting component of a moire signal according to one or more embodiments of the present invention. For example, a grating-on-grating structure suitable for a moire technique (e.g., a moire grating-on-grating structure) may provide a coarse pitch 208 where the spacing 810 between the upper grating structure 808 differs from the coarse pitch 208 of the lower grating structure 806.

[0096] 9 , illuminating a corrugated grating overlying grating structure with an illumination beam 104 can generate various zero-order signals at a normal exit angle, including, but not limited to, a DC signal 904 associated with the top surface 902 of the sample 108, zero-order diffraction 906 from the upper grating structure 808, and zero-order diffraction 908 from the lower grating structure 806.

[0097] Additionally, a corrugated grating overlying grating structure can generate corrugated diffraction associated with double diffraction from both the lower grating structure 806 and the upper grating structure 808. For example, FIG10 depicts a corrugated diffraction order 1002 associated with a +1-order diffraction 1004 from the lower grating structure 806 and a -1-order diffraction 1006 from the upper grating structure 808. Although not explicitly shown in the figure, an additional corrugated diffraction order based on the -1-order diffraction and the +1-order diffraction from the lower grating structure 806 can be generated using a symmetric path. Because the spacing 810 between the upper grating structure 808 differs from the coarse spacing 208 of the lower grating structure 806, the corrugated diffraction order 1002 emerges from the grating overlying grating structure at a non-normal angle. Specifically, the exit angle of the corrugated diffraction order 1002 can be based on a difference between these equal spacings, where the exit angle deviates further from a normal exit angle as the difference between the spacings increases.

[0098] It is contemplated herein that for some applications, it may be desirable to provide a corrugated grating overlying grating structure in which the upper grating structure 808 and the lower grating structure 806 are spaced closely together to provide a high gain and corrugation diffraction with a low corrugation pitch that is easily resolvable even if the underlying pitch of the upper grating structure 808 and the lower grating structure 806 is unresolvable.

[0099] However, at high gain values, a moire diffraction order 1002 may overlap with various zero-order signals (e.g., a DC signal 904, zero-order diffraction 906 from the upper grating structure 808, and zero-order diffraction 908 from the lower grating structure 806), which may make it impractical to optically isolate the moire diffraction order 1002 from the zero-order signal. Furthermore, moire techniques with gains ranging from approximately 10 to approximately 300 may exhibit ambiguity in overlay measurements, which may also make these gain factors impractical.

[0100] In some embodiments, an overlay target 106 includes a grating-overlay grating structure having polarization rotation features 202 in at least one layer to rotate the polarization of a first-order diffraction from the lower grating structure 806 and / or the upper grating structure 808. In this manner, a corrugated diffraction order 1002 formed by a first-order diffraction from both the lower grating structure 806 and the upper grating structure 808 can be optically isolated using a collecting polarizer 130, which can provide an enhanced contrast or signal-to-noise ratio for the associated overlay measurement. It is further contemplated that this technique is suitable for, but not limited to, a corrugated grating-overlay grating structure having a relatively high gain (e.g., 300 or higher).

[0101] In some embodiments, the corrugation gain can be high enough so that the corrugation pitch increases to a point where the change in the signal associated with overlay becomes negligible for a given cell size of the overlay target 106. In this case, an image of the grating-on-grating structure can be represented as a grayscale intensity value. Thus, an overlay measurement can be generated based on the signals generated from multiple (e.g., two or more) grating-on-grating structures (e.g., multiple cells of an overlay target 106) with different predetermined offsets. For example, these grayscale measurements can be fitted to a sinusoidal function with the corrugation pitch to determine overlay.

[0102] With general reference to FIG9 and FIG10, it should be understood that FIG9 and FIG10 and the associated description are for illustrative purposes only and should not be construed as limiting. Specifically, the overlay target 106 comprising a grating-on-grating structure having polarization rotation features 202 on at least one sample layer can be used in any overlay technique that utilizes first-order diffraction.

[0103] 11-12 , techniques for characterizing an overlay target 106 that provides a polarization rotation of a first order diffraction relative to other sample light 110 are described in greater detail according to one or more embodiments of the present invention.

[0104] It is contemplated that, as disclosed herein, the selective rotation of the first-order diffraction of an overlay target 106 and the optical isolation of the first-order diffraction of a collection polarizer 130 of an overlay metrology subsystem 102 enable high-throughput imaging of multiple overlay targets 106 within a single field of view. For example, as disclosed herein, the selective rotation of the first-order diffraction of an overlay target 106 and the optical isolation of the first-order diffraction of a collection polarizer 130 of an overlay metrology subsystem 102 enable the first-order diffraction to be isolated not only from additional sample light 110 from the overlay target 106 but also from sample light 110 associated with device features. Consequently, the overlay targets 106 can be imaged with high contrast regardless of their placement on the sample 108.

[0105] FIG11 is a flow chart illustrating the steps performed in a method 1100 for high-throughput overlay metrology according to one or more embodiments of the present invention. The applicants state that the embodiments and enabling techniques previously described herein in the context of the overlay metrology system 100 should be interpreted as extending to the method 1100. However, it should also be noted that the method 1100 is not limited to the architecture of the overlay metrology system 100.

[0106] In some embodiments, the method 1100 includes a step 1102 of fabricating a plurality of overlay targets 106 on a sample 108, wherein the overlay target 106 includes a grating-on-grating structure having polarization rotation features 202 on at least one layer to rotate a first-order diffraction relative to at least one of specular reflection or a zero-order diffraction.

[0107] The overlay targets 106 or portions thereof can be dispersed across the sample 108 in any suitable location. Different elements (e.g., different gratings overlying grating structures) need not be adjacent to each other. In some embodiments, different elements (e.g., different gratings overlying grating structures) of one or more overlay targets 106 are distributed across the sample 108. In this configuration, various elements of various overlay targets 106 can be imaged simultaneously across a large field of view, and overlay measurements can be reconstructed based on data from one of the various elements.

[0108] It is further contemplated herein that a particular cell of an overlay target 106 (e.g., as depicted in FIG8A and FIG8B ) can be fabricated at relatively small sizes because the constituent grating structures need not be individually resolvable. For example, as previously described herein, a grating-overlay grating structure suitable for a SCOL technique may exhibit a single grayscale intensity associated with a true registration between the upper grating structure 808 and the lower grating structure 806 (e.g., due to an intentional predetermined offset and / or unintentional overlay error). As another example, a periodic signal associated with a moiré diffraction order 1002 (e.g., at a specific moiré pitch) may be visible even if the constituent grating structures are unresolvable.

[0109] As an illustration, a cell of an overlay target 106 comprising a grating-on-grating structure may, but need not, have a size of approximately 5 microns or less on a side. Thus, the cell is not limited to placement in a scribe line but can be placed within a die on the sample 108, which can beneficially increase metrology accuracy due in part to a reduced distance between the cell and the device feature of interest.

[0110] In some embodiments, method 1100 includes a step 1104 of illuminating a plurality of overlay targets 106 with an illumination beam 104. In some embodiments, method 1100 includes a step 1106 of generating an image of sample 108 through a polarizer aligned with the first-order diffraction of illumination beam 104 at coarse pitch 208. For example, an overlay metrology subsystem 102 can be configured (e.g., according to a metrology recipe) to image a relatively large field of view of sample 108 comprising multiple overlay targets 106 (or portions thereof) through a collecting polarizer 130 aligned to pass through first-order diffraction when rotated by overlay targets 106. A resulting image can then reveal the overlay targets 106 with high contrast. In some embodiments, method 1100 includes a step 1108 of determining overlay metrology based on the image of sample 108.

[0111] FIG. 12 is a series of high-level diagrams illustrating high-throughput imaging of multiple overlaid targets 106 in a single image capture in accordance with one or more embodiments of the present invention.

[0112] Panel 1202 depicts a field of view 1204 of an overlay metrology subsystem 102 that can (but need not) be used to perform step 1104 of illuminating the sample 108 and / or step 1106 of imaging the sample 108. Panel 1206 depicts a die 1208 within the field, which can include various overlay targets 106 or portions thereof (e.g., individual cells having a grating-on-grating structure). In some embodiments, the various overlay targets 106 or portions thereof are arranged in subsections 1210 of the die 1208 so that separate overlay measurements can be generated based on the overlay targets 106 or portions thereof in each subsection 1210. Panel 1212 depicts different overlay measurements in each of the subsections 1210.

[0113] It is contemplated herein that the overlay metrology techniques illustrated in FIG12 and described in FIG11 can achieve high throughput and high accuracy overlay metrology. In general, any number of overlay targets 106 can be imaged simultaneously in a single capture. For example, the field size can be on the order of several millimeters or larger, while the cell size (e.g., the size of the grating-on-grating structure) can be (but need not be) on the order of 5 microns or smaller. In some applications, the cell size can be on the order of 3 microns or smaller. In some applications, the cell size can be greater than 5 microns. Thus, the number of overlay targets 106 that can be imaged simultaneously can be on the order of tens, hundreds, or even larger. Furthermore, because the collection polarizer 130 can pass first-order diffraction from the overlay target 106 that is rotated by the overlay target 106 and block other light sources (e.g., a DC signal 904, zero-order diffraction 908 from the lower grating structure 806, zero-order diffraction 906 from the upper grating structure 808, light from device features, or the like), a large field of view image can display these overlay targets 106 with a high contrast, which can enable overlay measurement with a high sensitivity and / or signal-to-noise ratio.

[0114] It is further contemplated herein that the overlay metrology technique depicted in FIG12 is not necessarily limited to the overlay targets 106 previously described herein having the polarization rotation feature 202. Rather, this technique may be applicable to any overlay target 106 that provides diffraction orders of interest in a collection aperture of an overlay metrology subsystem 102.

[0115] As an illustration, FIG13A and FIG13B illustrate a configuration of an overlay metrology subsystem 102 for measuring a corrugated overlay target 106 having a relatively small gain (e.g., a gain of approximately 10 or less) that does not necessarily include a polarization rotation feature 202. For example, FIG13A and FIG13B illustrate a configuration of an overlay metrology subsystem 102 for measuring a corrugated overlay target 106 having only a process pitch.

[0116] FIG13A is a top view of a collection pupil 1302 of an overlay metrology subsystem 102 according to one or more embodiments of the present invention. In FIG13A , a field of view of a sample 108 including one or more overlay targets 106 (e.g., as depicted in FIG12 ) is illuminated using two oblique illumination beams 104 such that the zeroth-order diffraction lobe 1304 from the upper and lower grating structures and a DC signal 904 are located outside a boundary 1306 of the collection pupil 1302, but wherein the first-order diffraction 1308 from the upper and lower grating structures (e.g., upper and lower grating structures 806 and 808) are within the boundary 1306 of the collection pupil 1302. For example, FIG13A illustrates a configuration in which a +1st-order diffraction lobe from one of the illumination beams 104 and a -1st-order diffraction lobe from the other illumination beam 104 overlap in the center of the boundary 1306 of the collection pupil 1302. In this configuration, a collection polarizer 130 is not necessary because the first order of diffraction of interest is isolated using optical and / or mechanical techniques.

[0117] It is contemplated herein that the configuration depicted in FIG. 13A is suitable for large-field imaging of many overlay targets 106 (e.g., up to 100 or more overlay targets 106 simultaneously) or portions thereof (e.g., cells having any type of corrugated grating-on-grating structure), wherein the cells can be represented as grayscale intensity values ​​based on the actual registration between associated layers of the sample 108. Thus, overlay measurements can be generated based on multiple cells (e.g., two or more cells) of a grating-on-grating structure having a predetermined offset. Specifically, the intensity values ​​obtained for the cells in each overlay target 106 can be fit to a sinusoidal function having a pitch corresponding to the corrugation pitch of the overlay target 106.

[0118] FIG13B is a conceptual diagram of generating two illumination beams 104 from an incoherent light source according to one or more embodiments of the present invention. For example, light 1310 from an incoherent light source may be incident on a diffraction grating 1312 or other dispersive optical device, such that the two illumination beams 104 are formed as + / - 1st-order diffracted beams of the incoherent light 1310. FIG13B further illustrates a beam block 1314 for blocking zeroth-order light 1316 and a lens 1318 for collimating the two illumination beams 104.

[0119] However, it should be understood that FIG13A and FIG13B are for illustration only and should not be construed as limiting. Rather, the technique depicted in FIG12 can be used with any type of overlay target 106 and any configuration of the overlay metrology subsystem 102 suitable for providing imaging of a large field of view on the sample 108 using light isolated from the overlay target 106 to the diffraction orders of interest.

[0120] 1A and 1B , additional aspects of the overlay metrology subsystem 102 according to one or more embodiments of the present invention are described in more detail.

[0121] The illumination source 112 may include any type of illumination source suitable for providing an illumination beam 104. In some embodiments, the illumination source 112 is a laser source. For example, the illumination source 112 may include, but is not limited to, one or more narrowband laser sources, a broadband laser source, a supercontinuum laser source, a white light laser source, or the like. Accordingly, the illumination source 112 may provide an illumination beam 104 with high coherence (e.g., high spatial and / or temporal coherence). In some embodiments, the illumination source 112 includes a laser sustained plasma (LSP) source. For example, the illumination source 112 may include, but is not limited to, an LSP lamp, an LSP bulb, or an LSP chamber suitable for housing one or more components that, when excited into a plasma state by a laser source, can emit broadband illumination. In some embodiments, the illumination source 112 includes a lamp source. For example, the illumination source 112 may include, but is not limited to, an arc lamp, a discharge lamp, an electrodeless lamp, or the like. In this regard, the illumination source 112 may provide an illumination beam 104 having low coherence (eg, low spatial coherence and / or temporal coherence).

[0122] In some embodiments, the overlay metrology system 100 includes a controller 134 communicatively coupled to the overlay metrology subsystem 102. The controller 134 can be configured to direct the overlay metrology subsystem 102 to generate overlay signals based on one or more selected recipes. The controller 134 can be further configured to receive data including, but not limited to, the overlay signals from the overlay metrology subsystem 102. Additionally, the controller 134 can be configured to determine an overlay associated with the overlay target 106 based on the acquired overlay signals.

[0123] In some embodiments, controller 134 includes one or more processors 136. For example, one or more processors 136 may be configured to execute a set of program instructions maintained in a memory device 138 or memory. The one or more processors 136 of controller 134 may include any processing element known in the art. In this sense, the one or more processors 136 may include any microprocessor-type device configured to execute algorithms and / or instructions. Furthermore, the memory device 138 may include any storage medium known in the art suitable for storing program instructions executable by the associated one or more processors 136. For example, the memory device 138 may include a non-transitory memory medium. As an additional example, the memory device 138 may include, but is not limited to, a read-only memory, a random access memory, a magnetic or optical memory device (e.g., a disk), a magnetic tape, a solid-state drive, and the like. It should be further noted that the memory device 138 may be housed together with the one or more processors 136 in a common controller housing.

[0124] The subject matter described herein sometimes depicts various components contained within or connected to other components. It should be understood that these depicted architectures are illustrative only, and that many other architectures can be implemented to achieve the same functionality. Conceptually, any configuration of components that achieve the same functionality is effectively "associated" so as to achieve the desired functionality. Thus, any two components herein combined to achieve a particular functionality can be considered "associated" with each other so as to achieve the desired functionality, regardless of architecture or intermediary components. Similarly, any two components so associated can also be considered "connected" or "coupled" to each other so as to achieve the desired functionality, and any two components capable of being so associated can also be considered "coupleable" to each other so as to achieve the desired functionality. Specific examples of "coupleable" include, but are not limited to, components that can physically interact and / or physically interact, components that can wirelessly interact and / or wirelessly interact, and / or components that can logically interact and / or logically interact.

[0125] It is believed that the present invention and its many attendant advantages will be understood from the foregoing description, and it will be appreciated that various changes may be made in the form, construction, and arrangement of the components without departing from the present invention or sacrificing all of its material advantages. The forms described are for illustration only, and the following claims are intended to encompass and include such changes. Furthermore, it should be understood that the present invention is defined by the appended claims.

[0126] 100: Overlay measurement system 104: Illumination beam 106: Overlap Target 108: Sample 110: Sample light 112: Lighting Source 114: Lighting Path 116: Lens 118:Lighting polarizer 120:Objective lens 122: Sample carrier 124: Detector 126: Collection Path 128: Lens 130:Collecting polarizer 132: Beam splitter 134:Controller 136: Processor 138:Memory device 202: Polarization rotation feature 204: Grating structure 206:Surrounding materials 208: coarse spacing 210: Fine pitch 212a: District 1 212b: Second District 214: Frame 402: Zero-order diffraction 404: First-order diffraction 702: frame 802: First panel 804: Second panel 806: Lower grating structure 808: Upper grating structure 810: Spacing 812: Third Panel 814:Fourth Panel 902: Top surface 904: DC signal 906: Zero-order diffraction 908: Zero-order diffraction 910: Light + / -1 diffraction 912: +1 diffraction order 914:-1 diffraction order 916: + / - 2nd order diffraction 918:-2 diffraction order 920: +2 diffraction order 1002: Corrugation diffraction order 1004: +1 order diffraction 1006: -1 order diffraction 1100: Method 1102: Steps 1104: Steps 1106: Steps 1108: Steps 1202: Panel 1204: Field of view 1206: Panel 1208: Grain 1210: Subsection 1212: Panel 1302:Collecting pupils 1304: Zero-order diffraction lobe 1306: Border 1308: First-order diffraction 1310: Incoherent Light 1312: Diffraction Grating 1314: Beam Block 1316: Zero-order light 1318: Lens

Claims

1. An overlay measurement system, comprising: A light source configured to produce a beam of light; One or more illumination optics are used to guide an illumination beam to a sample having a field of view containing one or more overlapping targets when a measurement formulation is implemented, wherein a particular overlapping target of the one or more overlapping targets according to the measurement formulation includes: one or more grating-overlay grating structures formed by a lower grating structure having a first coarse pitch and an upper grating structure having a second coarse pitch, wherein the upper grating structure and the lower grating structure overlap on the sample, wherein at least one of the upper grating structure or the lower grating structure further includes a feature having a fine pitch smaller than a wavelength of the illumination beam and configured to rotate a first-order diffraction of the illumination beam relative to at least one of specular reflection from a top surface of the sample or zero-order diffraction from the one or more grating-overlay grating structures, wherein the first-order diffraction corresponds to a +1-order diffraction from one of the upper grating structure or the lower grating structure and a -1-order diffraction from the other of the upper grating structure or the lower grating structure; An objective lens for collecting light from the sample within the field of view when the measurement formulation is implemented; a polarizer configured to allow a portion of the light from the sample associated with the first-order diffraction to pass through; one or more detectors for generating one or more images of the sample including the one or more superimposed targets; and a controller communicatively coupled to the one or more detectors, the controller including one or more processors configured to execute program instructions that cause the one or more processors to determine superimposed measurements of the sample based on the one or more images of the one or more superimposed targets.

2. The overlay measurement system of claim 1, wherein at least one of the one or more overlay targets comprises two or more of the one or more grating-overlay grating structures located in different cells in a non-adjacent portion of a grain on the sample.

3. The overlay measurement system of claim 1, wherein at least one of the one or more images contains more than 100 overlay targets.

4. The overlay measurement system of claim 1, wherein the first coarse spacing and the second coarse spacing of at least one of the one or more overlay targets are equal to form at least one scattering overlay (SCOL) target.

5. The overlay measurement system of claim 4, wherein the at least one SCOL target comprises two or more grating overlay grating structures having a predetermined offset between the upper grating structure and the lower grating structure.

6. The overlay measurement system of claim 4, wherein the at least one SCOL target comprises four grating overlay grating structures having different predetermined offsets between the upper grating structure and the lower grating structure, wherein one overlay measurement of the at least one SCOL target is generated by a zero-order SCOL technique.

7. The overlay measurement system of claim 1, wherein the first coarse spacing and the second coarse spacing of at least one of the one or more overlay targets are different to form at least one wavy target.

8. The overlay measurement system of claim 7, wherein the at least one corrugated target comprises two or more grating overlay grating structures having a predetermined offset between the upper grating structure and the lower grating structure.

9. The overlay measurement system of claim 1, wherein the grating structure on top of the one or more gratings in the one or more overlay targets is not resolved in one of the associated images of the one or more overlay targets and appears with a gray level intensity.

10. The overlay measurement system of claim 1, wherein the feature having the fine pitch is located only in the lower grating structure of at least one of the one or more overlay targets.

11. The superimposed measurement system of request item 10, wherein the lower grating structure is in one of the program layers of the sample.

12. The superimposed measurement system of claim 11, wherein the lower grating structure is in one of the photoresist layers of the sample.

13. The superimposed measurement system of request item 11, wherein the lower grating structure is in an additional procedural layer of the sample.

14. The overlay measurement system of claim 1, wherein the features having the fine pitch are disposed in two regions within the first coarse pitch, wherein the direction of the fine pitch in the first region is orthogonal to the direction of the fine pitch in the second region.

15. The overlapping measurement system of request item 14, wherein the two zones have a 1:1 working cycle within the first coarse pitch.

16. The overlay measurement system of claim 14, wherein the fine pitch in the first region is equal to the fine pitch in the second region.

17. The overlay measurement system of claim 14, wherein the fine pitch in the first region has a value different from the fine pitch in the second region.

18. The overlay measurement system of claim 14, wherein one direction of the fine pitch in the first region is parallel to at least one of the directions of the first coarse pitch or the second coarse pitch.

19. The overlay measurement system of claim 14, wherein one direction of the fine pitch in the first region forms a 45-degree angle with the direction of at least one of the first coarse pitch or the second coarse pitch.

20. The overlay measurement system of claim 1, wherein the one or more images correspond to optical images of the sample, and one or more features of the one or more overlay targets in the optical images have been resolved.

21. The superimposed measurement system of claim 1, wherein the one or more images are associated with radiation of a diffraction order from the one or more superimposed targets.

22. A superimposed measurement target, comprising: One or more grating-over-grating structures are formed by a lower grating structure having a first coarse pitch in a first layer of one sample and an upper grating structure having a second coarse pitch in a second layer of one sample, wherein the upper grating structure and the lower grating structure overlap on the sample, wherein at least one of the upper grating structure or the lower grating structure further includes a fine pitch feature having a wavelength less than a wavelength of an illumination beam and is configured to rotate a first-order diffraction of the illumination beam relative to at least one of specular reflection from a top surface of one sample or zero-order diffraction from the one or more grating-over-grating structures, wherein the first-order diffraction corresponds to a +1-order diffraction from one of the upper grating structure or the lower grating structure and a -1-order diffraction from the other of the upper grating structure or the lower grating structure, wherein the overlap between the first layer and the second layer of the sample is determined from one or more images of the one or more grating-over-grating structures based on the first-order diffraction.

23. The superimposed measurement target as requested in claim 22, wherein the first coarse spacing and the second coarse spacing are equal to form a scattering superimposed (SCOL) target.

24. The superimposed measurement target as claimed in claim 23, wherein the SCOL target comprises two or more grating overlay grating structures having a predetermined offset between the upper grating structure and the lower grating structure.

25. The superimposed measurement target as requested in item 22, wherein the first coarse spacing and the second coarse spacing are different to form a wavy target.

26. The superimposed measurement target as claimed in claim 25, wherein the ripple target comprises two or more grating overlay grating structures having a predetermined offset between the upper grating structure and the lower grating structure.

27. The superimposed measurement target as requested in claim 22, wherein the features having the fine pitch are located only in the lower grating structure.

28. As requested in item 27, the superimposed measurement target, wherein the lower grating structure is in one of the process layers of the sample.

29. The superimposed measurement target as requested in claim 28, wherein the lower grating structure is in one of the photoresist layers of the sample.

30. As requested in item 28, the superimposed measurement target, wherein the lower grating structure is in an additional procedural layer of the sample.

31. The superimposed measurement target as claimed in claim 22, wherein the features having the fine pitch are arranged in two regions within each coarse pitch, wherein the direction of the fine pitch in the first region of the two regions is orthogonal to the direction of the fine pitch in the second region of the two regions.

32. As requested in item 31, the two regions have a 1:1 working cycle within the first coarse spacing.

33. As requested in item 31, the overlay measurement target, wherein the fine pitch in the first region is equal to the fine pitch in the second region.

34. The superimposed measurement target as requested in claim 31, wherein the fine pitch in the first region has a value different from the fine pitch in the second region.

35. The superimposed measurement target as requested in claim 31, wherein one direction of the fine pitch in the first region is parallel to at least one of the directions of the first coarse pitch or the second coarse pitch.

36. The superimposed measurement target as requested in claim 31, wherein one direction of the fine pitch in the first region forms a 45-degree angle with the direction of at least one of the first coarse pitch or the second coarse pitch.

37. A method for superimposed measurement, comprising: Fabricating one or more stacked targets on a sample, wherein a particular stacked target of the one or more stacked targets includes: one or more grating-overlay grating structures formed by a lower grating structure having a first coarse pitch and an upper grating structure having a second coarse pitch, wherein the upper grating structure and the lower grating structure overlap on the sample, wherein at least one of the upper grating structure or the lower grating structure further includes a fine pitch feature having a pitch less than a wavelength of an illumination beam and configured to rotate a first-order diffraction of the illumination beam relative to at least one of specular reflection from a top surface of the sample or zero-order diffraction from the one or more grating-overlay grating structures, wherein the first-order diffraction corresponds to a +1-order diffraction from one of the upper grating structure or the lower grating structure and a -1-order diffraction from the other of the upper grating structure or the lower grating structure; illuminating one or more of the stacked targets using the illumination beam; By aligning with a polarizer that diffracts in the first order, one or more images of the sample are generated; and based on the one or more images of the sample, an overlay measurement is determined.

38. The method of superposition measurement as claimed in claim 37, wherein at least one of the one or more superposition targets includes two or more of the one or more grating-over-grating structures located in different cells in a non-adjacent portion of a grain on the sample.

39. The overlay measurement method of claim 37, wherein at least one of the one or more images contains more than 100 overlay targets.

40. The method of measuring the overlay as claimed in claim 37, wherein the first coarse spacing and the second coarse spacing of at least one of the one or more overlay targets are equal to form at least one scattering overlay (SCOL) target.

41. The overlay measurement method of claim 40, wherein the at least one SCOL target comprises two or more grating overlay grating structures having a predetermined offset between the upper grating structure and the lower grating structure.

42. The overlay measurement method of claim 40, wherein the at least one SCOL target comprises four grating overlay grating structures having different predetermined offsets between the upper grating structure and the lower grating structure, wherein one overlay measurement of the at least one SCOL target is generated by a zero-order SCOL technique.

43. The overlay measurement method of claim 37, wherein the first coarse spacing and the second coarse spacing of at least one of the one or more overlay targets are different to form at least one wavy target.

44. The overlay measurement method of claim 37, wherein the grating overlay structure of the one or more gratings in the one or more overlay targets is not resolved in one of the associated images of the one or more overlay targets and appears with a gray level intensity.

45. The overlay measurement method of claim 37, wherein the feature having the fine pitch is located only in the lower grating structure of at least one of the one or more overlay targets.

Images