Scatterometry overlay measurement with orthogonal fine pitch division

Abstract

The overlay metrology target may include a grating-on-grating structure formed from 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, the upper and lower grating structures overlapping on the sample. At least one of the upper or lower grating structures may include features having a fine pitch smaller than the wavelength of the illumination beam, the features arranged to rotate a first diffraction order of the illumination beam associated with at least one of the first coarse pitch or the second coarse pitch relative to at least one of specular reflection from the top surface of the sample or a zeroth diffraction order from the one or more grating structures. Overlay between the first and second layers of the sample can be determined from an image of the grating structure based on the first diffraction order.

Description

[Technical Field]

[0001] The present disclosure relates generally to scatterometry overlay measurements, and more particularly to scatterometry overlay measurements on targets having orthogonal fine pitch. [Background technology]

[0002] Overlay metrology techniques that utilize grating-on-grating structures, such as, but not limited to, scatterometry overlay (SCOL) or Moire techniques, provide relatively compact overlay targets. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] U.S. Patent Application Publication No. 2018 / 0031470 [Patent Document 2] U.S. Patent Application Publication No. 2020 / 0103772 [Patent Document 3] U.S. Patent Application Publication No. 2015 / 0177135 Summary of the Invention [Problem to be solved by the invention]

[0004] However, such techniques may 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 to overcome the above deficiencies. [Means for solving the problem]

[0005] An overlay metrology system is disclosed in accordance with one or more exemplary embodiments. In one exemplary embodiment, the system includes an illumination source configured to generate an illumination beam. In another exemplary embodiment, the system includes one or more illumination optics for directing the illumination beam to a specimen with a field of view including one or more overlay targets when performing a metrology recipe. In another exemplary embodiment, a specific one of the one or more overlay targets according to the metrology recipe includes one or more grating-on-grating structures formed from a lower grating structure having a first coarse pitch and an upper grating structure having a second coarse pitch, the upper and lower grating structures overlapping on the specimen. In another exemplary embodiment, at least one of the upper or lower grating structures further includes features having a fine pitch smaller than a wavelength of the illumination beam, the features arranged to rotate a first diffraction order of the illumination beam associated with at least one of the first coarse pitch or the second coarse pitch relative to at least one of a specular reflection from a top surface of the specimen or a zeroth diffraction order from the one or more grating-on-grating structures. In another exemplary embodiment, the system includes an objective lens for collecting light from the specimen within a field of view when performing a metrology recipe. In another exemplary embodiment, the system includes a polarizer arranged to pass a portion of light from the sample associated with a first diffraction order. In another exemplary embodiment, the system includes a detector for generating an image of the sample including one or more overlay targets. In another exemplary embodiment, the system includes a controller for executing program instructions that cause one or more processors to determine overlay measurements of the sample based on the images of the one or more overlay targets.

[0006] An overlay metrology target is disclosed in accordance with one or more exemplary embodiments. In one exemplary embodiment, the target includes one or more grating-on-grating structures formed from a lower grating structure having a first coarse pitch in a first layer of the sample and an upper grating structure having a second coarse pitch in a second layer of the sample, the upper and lower grating structures overlapping on the sample. In another exemplary embodiment, at least one of the upper or lower grating structures further includes features having a fine pitch smaller than the wavelength of the illumination beam, the features arranged to rotate a first diffraction order of the illumination beam associated with at least one of the first coarse pitch or the second coarse pitch relative to at least one of a specular reflection from the top surface of the sample or a zeroth diffraction order from the one or more grating structures. In another exemplary embodiment, overlay between the first and second layers of the sample can be determined from images of the one or more grating structures based on the first diffraction order.

[0007] An overlay metrology method is disclosed in accordance with one or more exemplary embodiments. In one exemplary embodiment, the method includes fabricating one or more overlay targets on a sample. In another exemplary embodiment, a particular one of the one or more overlay targets includes one or more grating-on-grating structures formed from a lower grating structure having a first coarse pitch and an upper grating structure having a second coarse pitch, the upper and lower grating structures overlapping on the sample. In another exemplary embodiment, at least one of the upper or lower grating structures further includes features having a fine pitch smaller than a wavelength of the illumination beam, the features being arranged to rotate a first diffraction order of the illumination beam associated with at least one of the first coarse pitch or the second coarse pitch relative to at least one of a specular reflection from a top surface of the sample or a zeroth diffraction order from the one or more grating-on-grating structures. In another exemplary embodiment, the method includes illuminating the one or more overlay targets with the illumination beam. In another exemplary embodiment, the method includes generating an image of the sample through a polarizer aligned to pass first diffraction orders. In another exemplary embodiment, a method includes determining overlay measurements based on images of the sample.

[0008] An overlay metrology system is disclosed in accordance with one or more exemplary embodiments. In one exemplary embodiment, the system includes an illumination source configured to generate at least one illumination beam. In another exemplary embodiment, the system includes one or more illumination optics for directing the illumination beam to a sample at an oblique angle with a field of view including one or more overlay targets when performing a metrology recipe. In another exemplary embodiment, a specific one of the one or more overlay targets includes one or more Moiré grating-on-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, the upper and lower grating structures overlapping on the sample. In another exemplary embodiment, the system includes an objective lens for collecting optical diffraction from the sample within a field of view when performing a metrology recipe, wherein at least one of a specular reflection from the sample or a zeroth order diffraction from the one or more overlay targets is outside the collection numerical aperture of the objective lens. In another exemplary embodiment, the system includes a detector for generating an image of the sample including the one or more overlay targets. In another exemplary embodiment, the system includes a controller for determining overlay measurements of the sample based on images of one or more overlay targets.

[0009] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and do not necessarily restrict the invention as claimed. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the general description, serve to explain the principles of the invention. [Brief explanation of the drawings]

[0010] The many advantages of the present disclosure may be better understood by those skilled in the art by reference to the accompanying drawings, which are set forth below.

[0011] [Figure 1A] FIG. 1 is a conceptual diagram illustrating an overlay metrology system in accordance with one or more embodiments of the present disclosure. [Figure 1B] FIG. 1 is a conceptual diagram illustrating an overlay metrology subsystem in accordance with one or more embodiments of the present disclosure. [Figure 2] FIG. 1 is a top view of polarization rotating features disposed on a first layer of an overlay target in accordance with one or more embodiments of the present disclosure. [Figure 3A] FIG. 3 illustrates an effective medium representation of structures within a period range associated with a coarse pitch for light polarized along the X direction as defined in FIG. 2 , in accordance with one or more embodiments of the present disclosure. [Figure 3B] FIG. 3 illustrates an effective medium representation of structures within the coarse pitch range for light polarized along the Y direction as defined in FIG. 2, in accordance with one or more embodiments of the present disclosure. [Figure 4] 3 is a conceptual diagram of the electric field directions of various diffraction orders produced by the polarization rotation feature shown in FIG. 2 upon illumination with an illumination beam, in accordance with one or more embodiments of the present disclosure. [Figure 5] FIG. 10 is a top view of a 2D polarization rotation feature suitable for overlay measurements along two measurement directions, in accordance with one or more embodiments of the present disclosure. [Figure 6] FIG. 1B is a top view of polarization-rotating features arranged with a fine pitch periodicity at an angle of 45 degrees relative to the coarse pitch, in accordance with one or more embodiments of the present disclosure. [Figure 7] FIG. 10 is a top view of a 2D polarization rotating feature without splitting, in accordance with one or more embodiments of the present disclosure. [Figure 8A] FIG. 10 is a top view of an overlay target suitable for polarization-based isolation of first diffraction orders along a single measurement direction, in accordance with one or more embodiments of the present disclosure. [Figure 8B] FIG. 10 is a top view of a 2D overlay target suitable for polarization-based isolation of first diffraction orders along two measurement directions, in accordance with one or more embodiments of the present disclosure. [Figure 9] 10A-10C illustrate the use of an overlay target including polarization rotation features within a grating-on-grating structure for a zeroth-order SCOL technique in accordance with one or more embodiments of the present disclosure. [Figure 10] FIG. 1 is a conceptual side view of a grating-on-grid structure illustrating non-limiting components of a Moiré signal, in accordance with one or more embodiments of the present disclosure. [Figure 11] FIG. 1 is a flow diagram illustrating steps performed in a method for high throughput overlay metrology in accordance with one or more embodiments of the present disclosure. [Figure 12] 1A-1C are a series of high-level diagrams illustrating high-throughput imaging of multiple overlay targets in a single image grab, in accordance with one or more embodiments of the present disclosure. [Figure 13A] FIG. 10 is a top view of a collection pupil of an overlay metrology subsystem, in accordance with one or more embodiments of the present disclosure. [Figure 13B] FIG. 1 is a conceptual diagram of generating two illumination beams from a non-coherent light source, in accordance with one or more embodiments of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0012] Reference will now be made in detail to the disclosed subject matter, which is illustrated in the accompanying drawings. The present disclosure has been particularly shown and described with respect to certain embodiments and their distinctive features. The embodiments described herein are to be considered illustrative and not restrictive. It will be readily apparent to those skilled in the art that various changes and modifications in form and detail may be made therein without departing from the spirit and scope of the present disclosure.

[0013] Embodiments of the present disclosure relate to systems and methods for overlay metrology using an overlay target comprising a grating-on-grating structure in which selected diffraction orders used in the measurement are optically isolated using polarization control. In other words, some embodiments of the present disclosure relate to systems and methods for overlay metrology using an overlay target comprising a grating-on-grating structure in which at least a portion of unwanted optical signals emanating from the sample (e.g., light from the sample that may not contribute to the overlay measurement) are blocked using polarization control. By way of non-limiting example, in some overlay measurement techniques, it may be desirable to block pure reflections from the sample that do not interact with both gratings of the grating-on-grating structure (e.g., reflections from the top surface of the sample); such reflections are referred to herein as DC signals to distinguish such signals from the zeroth-order diffraction from both gratings. It is considered herein that such DC signals may be relatively strong compared to the various diffraction orders from the grating-on-grating structure, potentially reducing the contrast of the measurement.

[0014] For purposes of this disclosure, the term "grating-on-grating structure" is used to refer to a set of features within an overlapping region of two or more layers of a sample, where the features in each of the layers have a periodicity that may be characterized by at least one pitch or spatial frequency. As such, the features within the overlapping region are described as a grating structure. However, it should be understood that the grating structure on a particular sample layer may be characterized by multiple pitches along a particular direction and / or by multiple pitches along different directions. Thus, the constituent features of a grating-on-grating structure may be divided to provide multiple pitches in any combination of directions or otherwise distributed to provide spatial frequencies suitable for generating diffraction of incident light.

[0015] Additionally, optically isolating selected diffraction orders from an overlay target containing a grating-on-grating structure may also block signals from other features on the sample (e.g., device features, dummy features, etc.), thereby allowing multiple overlay targets within the field of view to be characterized simultaneously. Such measurement techniques may enable very high throughput measurements.

[0016] Overlay targets and / or overlay metrology tools suitable for characterizing overlay targets may be configured according to a metrology recipe suitable for generating overlay measurements based on a desired technique. More generally, an overlay metrology tool may be configurable according to different metrology recipes to perform overlay measurements using different techniques and / or for different overlay targets having different designs.

[0017] For example, a metrology recipe may include various aspects of an overlay target or the design of the overlay target, including, but not limited to, the layout, feature size, or feature pitch of target features on one or more sample layers. As another example, a metrology recipe may include illumination parameters such as, but not limited to, illumination wavelength, illumination pupil distribution (e.g., distribution of illumination angles and associated illumination intensities at those angles), polarization of incident illumination, spatial distribution of illumination, or height of the sample. As another example, a recipe for an overlay metrology tool may include collection parameters such as, but not limited to, collection pupil distribution (e.g., desired distribution of angular light from the sample used for measurement and associated filtered intensities at those angles), collection field aperture settings for selecting portions of the sample of interest, polarization of collected light, or wavelength filters.

[0018] Some embodiments of the present disclosure relate to overlay targets suitable for providing polarization-based optical isolation of selected diffraction orders from a grating-on-grating structure, such as, but not limited to, first diffraction order. Some embodiments of the present disclosure relate to overlay metrology techniques (e.g., implementable using a metrology recipe) suitable for measuring overlay on an overlay target based on polarization-based optical isolation of selected diffraction orders. Some embodiments of the present disclosure relate to overlay metrology tools configurable (e.g., based on a metrology recipe) to perform overlay metrology based on polarization-based optical isolation of selected diffraction orders.

[0019] In some embodiments, an overlay target suitable for overlay metrology based on polarization-based isolation of first-order diffracted light includes at least one grating-on-grating structure, wherein a grating on a first layer includes features that impart different phase changes to orthogonally polarized components of the first-order diffracted light. The use of polarization target features to control the amplitude and phase of diffraction orders is outlined in U.S. Pat. No. 10,337,991, issued July 2, 2019, and is incorporated herein by reference in its entirety. It is contemplated herein that some embodiments of the present disclosure may extend the use of polarization target features described in U.S. Pat. No. 10,337,991 to multiple overlay metrology techniques utilizing grating-on-grating structures, and further extend the use of polarization target features described in U.S. Pat. No. 10,337,991 to high-throughput overlay measurements based on simultaneously imaging multiple overlay targets within a single image grab.

[0020] A grating-on-grating structure may be formed from periodic structures (e.g., gratings or grating structures, more generally) derived from different lithographic exposures on two layers within the overlap region of an overlay target (e.g., an upper and lower layer). Light incident on the grating-on-grating structure may thus interact with gratings on each layer, such that the diffraction orders emanating from the overlay target may be affected, among other things, by the relative positions of the grating structures. Consequently, overlay measurements associated with the relative alignment (or misalignment) of sample layers may be based on one or more diffraction orders of light interacting with the respective grating structures in the layers of interest.

[0021] For example, such a grating on the first layer of the overlay target may include features arranged such that the phase of first-order diffraction along the Y direction is shifted by π relative to the phase of first-order diffraction along the X direction. In other words, the grating on the first layer may have a different effective dielectric constant along the X direction relative to the orthogonal Y direction. In this way, the grating on the first layer may act as a waveplate for first-order diffraction but not for other light from the sample, such as, but not limited to, zeroth-order diffraction, DC signals, or higher diffraction orders. Thus, the grating on the first layer may rotate the polarization of first-order diffracted light relative to other light. Furthermore, the grating on the second layer of the overlay target may have any design and need not affect the polarization of the light. In this way, such an overlay target may be formed by a first layer on the process layer with a relatively high contrast between the grating features and the surrounding material necessary to modify the polarization of first-order diffraction, and a second layer in the resist layer. For purposes of this disclosure, the term process layer refers to a layer having one or more features fabricated using various processing steps, including, but not limited to, a lithography step (e.g., in a photomask deposited on the layer) followed by an etching step. As such, features in a process layer may be of a material different from the surrounding material, such as, but not limited to, air (e.g., unfilled) or an additional material, such as an oxide. For purposes of this disclosure, features in a resist layer refer to features in photoresist created by lithography exposure prior to an etching step.

[0022] An overlay metrology tool configured to perform overlay metrology using such an overlay target (e.g., according to a metrology recipe) may illuminate the overlay target with linearly polarized light and include a polarizer in the collection path with an orthogonal polarization. In this manner, the collection path may selectively pass first-order diffraction, which is polarization rotated by the first layer of the overlay target, and may block non-rotated light, including, but not limited to, DC signals or diffractions other than the first-order diffraction. As a result, signals associated with the first-order diffraction may have high contrast and / or a high signal-to-noise ratio.

[0023] It is contemplated herein that overlay metrology based on polarization-based isolation of first-order diffraction may be implemented with any overlay technique (and associated overlay target design) that is based on first-order diffraction. It is contemplated herein that such techniques may be particularly useful, but not limited to, for mitigating DC reflections that can reduce the contrast and / or signal-to-noise ratio of overlay measurements.

[0024] Some embodiments of the present disclosure relate to scatterometry overlay (SCOL) techniques and related overlay targets. SCOL overlay metrology techniques may utilize overlay targets that include grating-on-grating structures in which constituent grating features have a common pitch. Such targets are referred to herein as SCOL targets.

[0025] It is recognized herein that conventional zeroth-order SCOL metrology techniques are based on a grating-on-grating target design, where the top and bottom gratings have the same pitch. Furthermore, each photon emitted from any point within the illumination pupil is scattered on both the top and bottom gratings (and vice versa), resulting in the measured zeroth-order signal containing overlaid information from a combination of ±1, ±2... diffraction orders from the top and bottom gratings.

[0026] One problem with this approach is the large DC signal based on pure zero-order reflection, in addition to algorithm inaccuracies arising from various combinations of diffraction orders contributing to the measured signal. In particular, all of these possible combinations of diffraction orders contributing to the measured signal cannot be practically accounted for by a model based on a relatively small number of cells with a predetermined offset between the upper and lower gratings. For example, the number of cells associated with a measurement, and therefore reflected by such a model, may be limited by considerations such as, but not limited to, target size and moving acquisition measurement (MAM) throughput considerations.

[0027] It is further recognized that similar problems arise when using moiré overlay targets with relatively large gains (e.g., >300). Furthermore, moiré targets with gains on the order of 10 to 300-400 may be impractical due to the inherent ambiguity in overlay measurements with these values. For example, addressing DC signals using techniques such as dark-field illumination may be impractical or impossible.

[0028] Some embodiments of the present disclosure relate to SCOL targets in which a grating in one layer may rotate the polarization of the first diffraction order relative to other light from the sample (e.g., a DC signal, a zeroth diffraction order, or higher diffraction orders). In this way, the overlay metrology tool may provide polarization-based isolation of the first diffraction order. Such targets may be used with any SCOL overlay technique that utilizes first diffraction orders.

[0029] Some embodiments of the present disclosure relate to moiré overlay techniques and related overlay targets. Moiré overlay metrology techniques may utilize overlay targets including one or more grating-on-grating structures with different constituent grating pitches. Such targets, referred to herein as moiré targets, provide diffraction from both gratings (e.g., double diffraction) in addition to typical diffraction orders from each of the gratings. For example, a moiré target may generate moiré diffraction orders with relatively small diffraction angles based on a gain factor associated with the difference between the pitches of the constituent gratings. Moiré targets may be used in various measurement modes. Thus, an image of a moiré target may include a periodic signal (e.g., a signal with moiré pitch) based on the moiré diffraction orders, even if the pitch of the constituent gratings cannot be resolved. Furthermore, because the moiré pitch is typically larger than the pitch associated (e.g., by a gain factor) with the constituent gratings, moiré overlay techniques may provide overlay measurements with amplified sensitivity due to the gain factor. Moiré targets may also be characterized using pupil-based measurement techniques.

[0030] Some embodiments of the present disclosure relate to high-throughput overlay measurements based on simultaneously capturing signals from multiple overlay targets (potentially hundreds or more overlay targets). It is contemplated herein that an overlay target including a grating-on-grating structure in which one grating rotates the polarization of a first diffraction order relative to other light from the overlay target allows for separated imaging of the target based on this first diffraction order. For example, an overlay measurement based on illumination with linearly polarized light and collection with crossed polarizers (e.g., polarizers in the collection path oriented to reject non-rotatingly polarized light) may optically isolate the first diffraction order from the overlay target and block other light, including, but not limited to, additional light from the overlay target and light from device features. As a result, an image having a field of view including multiple overlay targets may reveal the overlay targets with high contrast.

[0031] It is noted that the use of a polarization rotation target (e.g., in the process layer) provides perpendicular polarization to the first diffraction orders from the two layers. This can be beneficial for multiple overlay metrology techniques in general. For example, this can be beneficial for optical SCOL techniques, in which a Moiré signal is obtained as the interference within the field of view of the +1 order from the grating structure in the two layers on one detector and the −1 order from the grating structure in the two layers on the second detector. A polarizer aligned at an appropriate angle can then be used to equalize the amplitude of the diffraction orders from the grating structure, thus improving the signal-to-noise ratio. Similar techniques can be applied to typical imaging overlay metrology techniques to equalize the contrast of signals from layers formed of different materials (e.g., from the grating structure in the resist layer and the process layer).

[0032] 1A-13B, a system and method for overlay metrology using polarization-based isolation of selected diffraction orders of light, in accordance with one or more embodiments of the present disclosure, will now be described in more detail.

[0033] FIG. 1A is a conceptual diagram illustrating an overlay metrology system 100 in accordance with one or more embodiments of the present disclosure.

[0034] 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 to an overlay target 106 on a specimen 108 and may further collect light or other electromagnetic radiation (referred to herein as specimen light 110) emanating from the overlay target 106, at least a portion of which is used to generate one or more images of the overlay target 106. In this manner, one or more overlay measurements of the specimen 108 (e.g., overlay measurements associated with the positioning or misalignment of two or more layers of the specimen 108) may be generated based on at least one image of the overlay target 106.

[0035] Additionally, the overlay metrology subsystem 102 may be configurable to generate images based on any number of metrology recipes that define measurement parameters for image acquisition and / or design parameters of the overlay target 106 being imaged. For example, a metrology recipe may include various aspects of the overlay target 106 or the design of the overlay target 106, including, but not limited to, the layout, feature size, or feature pitch of target features on one or more sample layers. As another example, a metrology recipe may include illumination parameters such as, but not limited to, illumination wavelength, illumination pupil distribution (e.g., distribution of illumination angles and associated intensity of illumination at those angles), polarization of incident illumination, spatial distribution of illumination, or sample height. As another example, a metrology recipe may include collection parameters such as, but not limited to, collection pupil distribution (e.g., desired distribution of angular light from the sample 108 used for measurement and associated filtered intensity at those angles), collection field stop settings to select portions of the sample of interest, polarization of collected light, or wavelength filters.

[0036] 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 the overlay associated with the overlay target 106 on the specimen 108.

[0037] 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 not resolved. For example, the overlay metrology subsystem 102 may 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 do not combine in a manner to provide a resolved image of the overlay target 106 or selected features therein. As an example, an image of the overlay target 106 associated with a zero-order SCOL technique may include an unresolved grating-on-grating structure, which is represented as a grayscale region. In this configuration, the intensity of a particular grayscale region is affected by the physical overlay of the grating-on-grating structure (e.g., physical misalignment and unintentional overlay error associated with an intended offset, if applicable). In this manner, the overlay metrology subsystem 102 may generate overlay measurements (e.g., measurements of unintended overlay errors) of the sample 108 based on the imaged intensities of unresolved grating-on-grating features within multiple cells of the overlay target 106. As another example, an image of the overlay target 106 associated with a Moiré technique may include unresolved grating-on-grating structures, but may also include resolved periodic signals associated with Moiré diffraction (e.g., double diffraction) from the constituent gratings.

[0038] 1B is a conceptual diagram illustrating an overlay metrology subsystem 102, in accordance with one or more embodiments of the present disclosure. 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.

[0039] In some embodiments, the overlay metrology subsystem 102 directs the illumination beam 104 to the sample 108, or a portion thereof, 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 for directing the illumination beam 104 to the sample 108. For example, the illumination path 114 may, but is not necessarily required to, include one or more lenses 116 (e.g., to collimate the illumination beam 104, to relay a pupil and / or a field plane, etc.), 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, movable mirrors, scanning mirrors, etc.). In some embodiments, the overlay metrology subsystem 102 includes an objective lens 120 for focusing the illumination beam 104 onto the sample 108 (e.g., an overlay target 106 with overlay target elements disposed on two or more layers of the sample 108). In some embodiments, the sample 108 is disposed on a sample stage 122 suitable for fixing the sample 108 and further configured to properly position the sample 108 relative to the illumination beam 104.

[0040] In some embodiments, the overlay metrology subsystem 102 includes one or more detectors 124 configured to capture sample light 110 emanating from the sample 108 and passing through a collection path 126, and to generate one or more overlay signals indicative of the overlay of two or more layers of the sample 108. The collection path 126 may include multiple optical elements for directing and / or modifying the illumination collected by the objective lens 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, the detector 124 may receive an optical image of the sample 108 provided by elements in the collection path 126 (e.g., the objective lens 120, the one or more lenses 128, etc.). As another example, the detector 124 may receive radiation reflected or scattered from the sample 108 (e.g., via specular reflection, diffuse reflection, etc.). As another example, detector 124 may receive one or more diffraction orders of radiation from overlay target 106 (eg, 0th, ±1st, ±2nd diffraction orders, etc.).

[0041] The illumination path 114 and collection path 126 of the overlay metrology subsystem 102 may be oriented in a wide variety of configurations suitable for illuminating the overlay target 106 with the illumination beam 104 and for collecting the sample light 110 in response to the incident illumination beam 104. For example, as shown in FIG. 1B , the overlay metrology subsystem 102 may include a beam splitter 132 oriented such that the objective lens 120 can simultaneously direct the illumination beam 104 onto the overlay target 106 and collect radiation emanating from the sample 108. As another example, the illumination path 114 and the collection path 126 may comprise non-overlapping optical paths.

[0042] 2-10, designs of overlay targets 106 suitable for polarization-based isolation of first diffraction orders in accordance with one or more embodiments of the present disclosure will now be described in more detail. FIGS. 2-7 illustrate various non-limiting examples of features suitable for rotating the polarization of first diffraction orders relative to other sample light 110, such as, but not limited to, a DC signal, a zeroth diffraction order, or higher diffraction orders. Such features are referred to herein as polarization rotation features. Overlay targets 106 in accordance with the systems and methods disclosed herein may be formed from or otherwise include a grating-on-a-grating structure, where at least one of the layers within the grating-on-a-grating structure is contemplated herein to include a polarization rotation feature. Next, FIGS. 8A-10 illustrate various non-limiting examples of overlay targets 106 including polarization rotation features.

[0043] In some embodiments, the overlay target 106 includes at least one grating-on-a-grating structure in which features on at least one of the layers of the sample 108 rotate the polarization of the first diffraction order relative to other light emanating from the grating-on-a-grating structure, such as, but not limited to, DC reflection, zeroth diffraction order, or higher diffraction orders.

[0044] The grating-on-a-grating structure may have any combination or orientation of features suitable for providing a rotation of the first diffraction order. In some embodiments, the grating-on-a-grating structure includes features on at least one sample layer having a coarse pitch, the coarse pitch being further separated (e.g., by a fine pitch) in an orthogonal direction. Furthermore, the fine pitch may be smaller than the wavelength of light, such that the orthogonal separation at the fine pitch provides different effective dielectric constants for light polarized along the orthogonal directions.

[0045] FIG. 2 is a top view of polarization-rotating features 202 disposed on a first layer of an overlay target 106 in accordance with one or more embodiments of the present disclosure.

[0046] 2, 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 purposes of this disclosure, the term grating structure 204 is used herein to refer to a set of features patterned in a sample layer having a periodicity that may be characterized by at least one pitch or spatial frequency. In a general sense, grating structures 204 on any particular layer may have multiple periodicities (and associated pitches) in any particular direction and / or may have periodicity in multiple directions.

[0047] The polarization rotation features 202 may be characterized by a coarse pitch 208 (e.g., a coarse period) and a fine pitch 210 (e.g., a fine period, a divided pitch, a divided period, etc.). In particular, features within the coarse pitch 208 are further arranged into two sections 212a,b having orthogonal periodicity with the fine pitch 210. For example, box 214 in FIG. 2 shows features within the period associated with the coarse pitch 208, which are divided into a first section 212a characterized by division by the fine pitch 210 along the X direction and a second section 212b characterized by division by the fine pitch 210 along the Y direction.

[0048] 2 is contemplated herein to operate as a wave plate, thereby imparting different phases to light having orthogonal polarizations along the fine pitch 210 direction. In this manner, the polarization rotation feature 202 may change or rotate the polarization of incident light (e.g., illumination beam 104). For example, the polarization rotation feature 202 may operate as a half-wave plate for first diffraction orders because the polarization rotation feature 202 changes the phase of first diffraction orders along the Y direction by π without changing the phase of first diffraction orders along the X direction.

[0049] Figure 3A shows an effective medium representation of structures (e.g., features within box 214 in Figure 2) within a periodic range associated with coarse pitch 208 for light polarized along the X direction as defined in Figure 2, according to one or more embodiments of the present disclosure. Figure 3B shows an effective medium representation of structures (e.g., features within box 214 in Figure 2) within coarse pitch 208 for light polarized along the Y direction as defined in Figure 2, according to one or more embodiments of the present disclosure.

[0050] In Figures 3A and 3B, the effective medium dielectric constant is defined by the following equation:

number

[0051] As shown in equations (1) and (2) and Figures 3A-3B, arranging features on a layer of the overlay target 106 to provide both a coarse pitch 208 and an orthogonal fine pitch 210 direction segment effectively acts as a wave plate, providing different phase shifts to light polarized along the orthogonal directions. Figure 4 is a conceptual diagram of the electric field directions of various diffraction orders produced by the polarization rotation feature 202 shown in Figure 2 based on illumination with the illumination beam 104, in accordance with one or more embodiments of the present disclosure. As shown in Figure 4, the zeroth diffraction order 402 may have the same electric field direction as the incident illumination beam 104, while the first diffraction order 404 may be polarized in an orthogonal direction relative to the zeroth diffraction order 402.

[0052] 4, the overlay metrology subsystem 102 configured according to a metrology recipe provides an illumination beam 104 polarized at a 45-degree angle relative to the directions of the orthogonal fine pitch 210 periodicity (e.g., the X and Y directions in FIG. 2). With the illumination beam 104 thus having polarization components along both directions of the fine pitch periodicity (e.g., the periodicity due to the fine pitch 210), the polarization rotation feature 202 may change or rotate the polarization of the first diffraction order of the illumination beam 104 relative to other sample light 110, such as, but not limited to, a DC signal, a zeroth diffraction order, or higher diffraction orders. In some embodiments, the overlay metrology subsystem 102 is further configured according to a metrology recipe to provide a cross-collection polarizer 130 in the collection path 126 oriented to isolate the first diffraction order 404.

[0053] Referring now to FIG. 5 , FIG. 5 is a top view of a 2D polarization rotation feature 202 suitable for overlay measurements along two measurement directions, in accordance with one or more embodiments of the present disclosure. The 2D polarization rotation feature 202 may be suitable for use in an overlay target 106 (e.g., a 2D overlay target 106) for overlay measurements along two dimensions. As an example, the 2D polarization rotation feature 202 shown in FIG. 5 may correspond to an extension along the Y direction of the polarization rotation feature 202 shown in FIG. 2 . For example, the 2D polarization rotation feature 202 in FIG. 5 is formed as a shifted version of the alternating and interleaved rows of polarization rotation features 202 of FIG. 2 . In particular, the interleaved rows of polarization rotation features 202 are shifted along the X direction by only half of the coarse pitch 208. In this manner, features within the coarse pitch 208 along the Y direction are also divided into sections 212 a, c having orthogonal fine pitch periodicities, similar to those in the X direction.

[0054] 2-5, it should be understood that the latter are provided for illustrative purposes only and should not be construed as limiting. The polarization rotation features 202 may have any arrangement suitable for rotating the first diffraction order 404 relative to the other sample light 110, such as, but not limited to, the zeroth diffraction order 402.

[0055] For example, the fine pitch 210 need not be the same in two orthogonal directions (e.g., the fine pitch 210 need not be the same for the first section 212a and the second section 212b). While not explicitly shown in FIG. 2 , but based on it, the first section 212a may have a first fine pitch 210, and the second section 212b may have a second fine pitch 210 having a value different from the first fine pitch 210. However, the fine pitch 210 in any direction must be sufficiently smaller than the wavelength of the illumination beam 104 to satisfy the effective medium condition (e.g., according to a metrology recipe) so that equations (1) and (2) are valid. Furthermore, the duty cycle (e.g., related to the value of η) must be the same for different measurement directions. Considering FIG. 5 as an example, the duty cycle along the X direction associated with sections 212a,b must be the same as the duty cycle along the Y direction associated with sections 212a,c.

[0056] As another example, polarization rotation features 202 may have any duty cycle (e.g., any value of η), but deviating from a duty cycle of 1:1 (e.g., η=0.5) may reduce the effectiveness of rotation of first diffraction orders by polarization rotation features 202 in some designs.

[0057] For example, the polarization rotating features 202 may be arranged to provide fine pitch periodicity in any two orthogonal directions. As an example, Figure 6 shows a top view of polarization rotating features 202 arranged with fine pitch periodicity at a 45 degree angle relative to the coarse pitch 208, in accordance with one or more embodiments of the present disclosure. In particular, the grating structures 204 are oriented at angles of 45 degrees and 135 degrees relative to the coarse pitch 208 (here, the X direction).

[0058] As previously described herein, the illumination beam 104 should be polarized at 45 degrees to both directions for the fine pitch periodicity. Therefore, the illumination beam 104 should be polarized along either the X or Y direction for the polarization rotation feature 202 in Figure 6. Similarly, the overlay metrology subsystem 102 should include an orthogonally polarized collecting polarizer 130 to capture the rotated first-order diffraction.

[0059] As a further example, polarization rotation features 202 do not necessarily need to be divided into separate sections (e.g., sections 212a, b in FIGS. 2-3B and 5 ) with distinct fine-pitch divisions within each section 212. Rather, it may be sufficient that polarization rotation features 202 impart different phases to orthogonal polarizations such that the first diffraction order is rotated relative to other sample light 110, such as, but not limited to, a DC signal, a zeroth diffraction order, or higher diffraction orders. In other words, polarization rotation features 202 may have any size, orientation, or distribution that imparts different phases to orthogonal polarizations in a manner similar to, but not limited to, FIGS. 3A-3B and Equations (1)-(2).

[0060] As an example, Figure 7 is a top view of a portion of a 2D polarization rotation feature 202 without segmentation, in accordance with one or more embodiments of the present disclosure. For example, the 2D polarization rotation feature 202 may include features with different orientations and spacing along the X and Y directions, such that the 2D polarization rotation feature 202 operates similarly to the 2D polarization rotation feature 202 of Figure 5. Additionally, box 702 in Figure 7 indicates one period of the 2D polarization rotation feature 202 (e.g., coarse pitch 208), and the ellipse in Figure 7 indicates that the depicted pattern repeats in two dimensions.

[0061] 8-10, various non-limiting examples of overlay targets 106 incorporating polarization rotation functionality 202 in accordance with one or more embodiments of the present disclosure are described in more detail.

[0062] It is contemplated herein that the effectiveness of the polarization rotation feature 202 in rotating first order diffraction to enable polarization-based isolation of first order diffraction may depend on the particular layout and configuration of the polarization rotation feature 202.

[0063] For example, the oxide surrounding material 206 (ε sur =1.45) and a silicon lattice structure 204 with a 1:1 duty cycle (ε Gr Fabricating polarization-rotating features 202 in a process layer with a dielectric constant of ε = 3.8 provides effective dielectric constant values ​​of ε = 2.63 and ε = 2.1. This is similar contrast to that obtained with a typical resist layer target. However, fabric ... Gr = 1.4 and ε sur Fabricating polarization-rotating features 202 within ε1=1.2 and ε2=1.17 provides effective dielectric constant values ​​of ε1=1.2 and ε2=1.17, which results in minimal contrast and therefore relatively poor rotation of the first diffraction order 404. As a result, polarization-rotating features 202 may be suitable for use in process layers, but may not be practical for use in resist layers (or may have polarization rotation effectiveness below application tolerances).

[0064] As a result, it may be impractical to utilize polarization-rotating features 202 in overlay targets 106 where features on different sample layers (or, more generally, features associated with different lithographic exposures) are in non-overlapping or adjacent regions when one of the sample layers is a resist layer. In such a configuration, the amplitude of first-order diffraction from features in the resist layer may have a relatively small amplitude due to the relatively small refractive index difference between the feature and the surrounding medium. As a result, the signal-to-noise ratio may be low in this configuration.

[0065] However, it is contemplated herein that the polarization rotation features 202 may be well suited to, but not limited to, an overlay target 106 having a grating-on-grating structure. In this configuration, it may be sufficient to provide the polarization rotation features 202 for polarization rotation for the first diffraction order on one layer (e.g., a process layer) of the overlay target 106. Consequently, features on a second layer (e.g., a resist layer or a process layer) may have any suitable pattern or orientation without requiring any polarization rotation.

[0066] It is further contemplated herein that any overlay target 106 design that includes at least one grating-on-grating structure may incorporate polarization rotating features 202 within at least one layer.

[0067] 8A is a top view of an overlay target 106 suitable for polarization-based isolation of first diffraction orders along a single measurement direction, in accordance with one or more embodiments of the present disclosure. Features in a first sample layer are shown in a first panel 802, and features in a second sample layer are shown in a second panel 804. In this manner, the features in the first panel 802 may constitute a lower grating structure 806, and the features in the second panel 804 may constitute an upper grating structure 808. The lower grating structure 806 and the upper grating structure 808 may be combined to form a grating-on-a-grating structure.

[0068] In some embodiments, the overlay target 106 includes polarization rotation features 202 in a first layer, which may be a process layer such that the features have high contrast. For example, the first panel 802 includes a repeat of the polarization rotation features 202 shown in FIG. 2 . The overlay target 106 may further include any suitable grating structure on a second layer, which may be any type of layer, including, but not limited to, a resist layer or a process layer. Thus, the polarization rotation features 202 may be located in both the first and second layers (e.g., the lower grating structure 806 and the upper grating structure 808), or may be located only in the first layer (e.g., the lower grating structure 806). For example, the second panel 804 exhibits undivided features characterized by a pitch 810. This pitch 810 may have different values ​​depending on the particular technology. For example, an overlay target 106 suitable for SCOL technology may provide (e.g., according to a metrology recipe) that the pitch 810 of the upper grating structure 808 is equal to the coarse pitch 208 of the polarization-rotating features 202 in the lower grating structure 806. In another example, an overlay target 106 suitable for Moiré technology may provide (e.g., according to a metrology recipe) that the pitch 810 of the upper grating structure 808 is different from the coarse pitch 208 of the polarization-rotating features 202 in the lower grating structure 806 so as to provide Moiré gain.

[0069] 8B 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 disclosure. For example, the third panel 812 includes a repetition of the polarization rotation feature 202 shown in FIG. 5. The overlay target 106 may further include any suitable grating structure on a second layer, which may be any type of layer, including, but not limited to, a resist layer or a process layer. For example, the fourth panel 814 shows unbroken hatch features characterized by a pitch 810.

[0070] Although not explicitly shown, it should be understood that overlay targets 106 suitable for overlay measurements along one or two dimensions may be generated based on any design for polarization rotating features 202 in at least one layer. When polarization rotating features 202 are disposed on multiple layers, the polarization rotating features 202 may be adjusted to collectively provide the desired polarization rotation of the first diffraction order.

[0071] 9-10, the use of an overlay target 106 including polarization rotating features 202 within a grating-on-grating structure will be described with respect to various non-limiting overlay techniques. It is contemplated herein that an overlay target 106 including polarization rotating features 202 within a grating-on-grating structure may be used with any overlay technique that utilizes first order diffraction from the grating-on-grating structure.

[0072] 9 illustrates the use of an overlay target 106 including polarization rotation features 202 within a grating-on-grating structure for a zeroth-order SCOL technique, in accordance with one or more embodiments of the present disclosure. In particular, FIG. 9 is a conceptual side view of a grating-on-grating structure showing non-limiting components of a zeroth-order SCOL signal, in accordance with one or more embodiments of the present disclosure.

[0073] In a zero-order SCOL technique, overlay measurements may be generated based on measurements of zero-order signals from multiple grating-on-grating structures in different cells of the overlay target 106, where the grating-on-grating structures in different cells have a common pitch but different predetermined offsets between the constituent grating features. For example, a zero-order SCOL technique may utilize measurements from four or more cells having grating-on-grating structures with different predetermined offsets.

[0074] It should be noted herein that the term zeroth order signal from a grating structure is distinct from zeroth order diffraction. In particular, the zeroth order signal in the context of a SCOL measurement includes multiple signals emanating from the grating-on-a-grating structure at a common angle. For example, the zeroth order signal from a grating-on-a-grating structure in response to an illumination beam 104 at a normal incidence angle may include multiple optical signals emanating from the sample 108 at a normal exit angle.

[0075] 9, the illumination beam 104 may interact with various features of the sample 108 to generate a zeroth order signal. For example, the zeroth order signal may include a specular reflection from the top surface 902 of the sample 108 (e.g., DC signal 904) or other interfaces between sample layers not explicitly shown. As another example, the zeroth order signal may include a zeroth order diffraction 906 from the upper grating structure 808, which may propagate along the same path as the DC signal 904.

[0076] The zeroth order signal may further include various signals associated with the complex interaction between the upper grating structure 808 and the lower grating structure 806. As shown in Figure 9, a portion of the illumination beam 104 may pass through the upper grating structure 808 (e.g., as a transmitted zeroth order diffraction) and reach the lower grating structure 806. The lower grating structure 806 may then generate various reflected diffraction orders from this portion of the illumination beam 104, which may then interact again with the upper grating structure 808 before exiting the sample 108.

[0077] For example, the zeroth order signal from the grating-on-a-grating structure may include a zeroth order diffraction 908 from the lower grating structure 806, which passes through the upper grating structure 808 (e.g., as a zeroth order diffraction). As another example, the zeroth order signal from the grating-on-a-grating structure may include double diffraction from the lower grating structure 806 and the upper grating structure 808, which have a common diffraction order and opposite signs, exiting the sample 108 at a normal exit angle. As an example, FIG. 9 shows 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, 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 may follow symmetric paths. 9 shows + / -2 diffraction orders 916 associated with a -2 diffraction order 918 from the lower grating structure 806 and a +2 diffraction order 920 from the upper grating structure 808. Although not shown, the light associated with the +2 diffraction order from the lower grating structure 806 and the -2 diffraction order from the upper grating structure 808 may follow symmetric paths. Thus, the 0th order SCOL signal may include + / -N double diffraction signals that follow a similar pattern for the additional diffraction orders.

[0078] It should be understood that the grating-on-grating structure may generate various additional diffraction orders from the lower grating structure 806 and the upper grating structure 808, either alone or in combination, which are not explicitly shown in FIG. 9 for ease of understanding.

[0079] It is contemplated herein that the zeroth order SCOL technique may provide relatively high accuracy along with relatively low tool-induced errors (e.g., tool-induced shift (TIS) error, TIS 3σ error, etc.). However, this approach may suffer from relatively strong spurious signals, such as those associated with the DC signal 904, and algorithmic inaccuracies associated with multi-order diffraction signals, which may be difficult, impractical, or sometimes impossible to account for under typical measurement constraints, such as, but not limited to, the target size required for the measurement or the number of cells within the target range.

[0080] It is further contemplated herein that a grating-on-a-grating structure having polarization rotation features 202 for rotating the polarization of the first diffraction order from the lower grating structure 806 and / or upper grating structure 808 may enable optical isolation of this first diffraction order using the collecting polarizer 130 in the overlay metrology subsystem 102 as disclosed herein. For example, using a grating-on-a-grating structure with polarization rotation features 202 to rotate the polarization of the first diffraction order from the lower grating structure 806 and / or upper grating structure 808, coupled with the collecting polarizer 130, may enable optical isolation of the + / -1 diffraction order 910 from other zeroth order SCOL signals, which may improve the contrast or signal-to-noise ratio of the associated overlay measurements. As an example, the overlay target 106 configured as shown in FIG. 8A may include a grating-on-a-grating structure in which the pitch 810 of the upper grating structure 808 is equal to the coarse pitch 208 of the lower grating structure 806.

[0081] 10 illustrates the use of an overlay target 106 including polarization rotation features 202 within a grating-on-grating structure for Moiré techniques. In particular, FIG. 10 is a conceptual side view of a grating-on-grating structure illustrating non-limiting components of a Moiré signal, in accordance with one or more embodiments of the present disclosure. For example, a grating-on-grating structure suitable for Moiré techniques (e.g., a Moiré grating-on-grating structure) may provide that the pitch 810 of the upper grating structure 808 is different from the coarse pitch 208 of the lower grating structure 806.

[0082] Similar to the example of FIG. 9 , illuminating the moiré grating-on-grating structure with the illumination beam 104 may generate various zero-order signals at normal exit angles, including, but not limited to, a DC signal 904 associated with the top surface 902 of the sample 108, a zero-order diffraction 906 from the upper grating structure 808, and a zero-order diffraction 908 from the lower grating structure 806.

[0083] Additionally, the Moiré grating-on-a-grating structure may generate Moiré diffraction associated with double diffraction from both the lower grating structure 806 and the upper grating structure 808. For example, FIG. 10 shows Moiré diffraction order 1002 associated with +1 diffraction order 1004 from the lower grating structure 806 and −1 diffraction order 1006 from the upper grating structure 808. Although not explicitly shown, additional Moiré diffraction orders based on −1 and +1 diffraction orders from the lower grating structure 806 may be generated by symmetric paths. Because the pitch 810 of the upper grating structure 808 differs from the coarse pitch 208 of the lower grating structure 806, the Moiré diffraction orders 1002 emanate from the grating-on-a-grating structure at non-perpendicular angles. In particular, the exit angle of the Moiré diffraction order 1002 may be based on the difference between these pitches, where the exit angle deviates further from the perpendicular exit angle as the difference between the pitches increases.

[0084] It is contemplated herein that for some applications it may be desirable to provide a moiré grating-on-grating structure in which the pitch of the upper grating structure 808 and the pitch of the lower grating structure 806 are close to one another to provide moiré diffraction with high gain and a low moiré pitch that may be easily resolved even when the fundamental pitch of the upper grating structure 808 and the lower grating structure 806 is not resolvable.

[0085] However, at high gain values, the Moiré diffraction orders 1002 may overlap with various zeroth order signals (e.g., the DC signal 904, the zeroth order diffraction 906 from the upper grating structure 808, and the zeroth order diffraction 908 from the lower grating structure 806), which may make it impractical to optically isolate the Moiré diffraction orders 1002 from the zeroth order signals. Furthermore, Moiré techniques with gains in the range of about 10 to about 300 may exhibit ambiguity issues in overlay measurements that may make these gain factors impractical as well.

[0086] In some embodiments, the overlay target 106 includes a grating-on-grating structure with polarization-rotating features 202 in at least one layer to rotate the polarization of first diffraction orders from the lower grating structure 806 and / or the upper grating structure 808. In this manner, the Moiré diffraction orders 1002 formed from the first diffraction orders from both the lower grating structure 806 and the upper grating structure 808 may be optically isolated using the collection polarizer 130, which may improve the contrast or signal-to-noise ratio of the associated overlay measurement. It is further contemplated that such a technique may be suitable for, but is not limited to, Moiré grating-on-grating structures having a relatively high gain (e.g., 300 or greater).

[0087] In some embodiments, the moiré gain may be sufficiently high so that the moiré pitch increases to the point where signal changes associated with overlay are negligible for a given cell size of the overlay target 106. In this case, an image of such a grid-on-grid structure may be displayed as grayscale intensity values. Accordingly, overlay measurements may be generated based on signals generated from multiple (e.g., two or more) grid-on-grid structures (e.g., multiple cells of the overlay target 106) with different predetermined offsets. For example, these grayscale measurements may be fitted to a sine function using the moiré pitch to determine overlay.

[0088] 9 and 10, it should be understood that Figures 9 and 10, together with the associated description, are provided for illustrative purposes only and should not be construed as limiting. Rather, an overlay target 106 including a grating-on-grating structure having polarization-rotating features 202 on at least one sample layer may be utilized in any overlay technique that utilizes first-order diffraction.

[0089] 11-12, a technique for characterizing an overlay target 106 that provides polarization rotation of first-order diffraction relative to other sample light 110, according to one or more embodiments of the present disclosure, will be described in more detail.

[0090] It is contemplated herein that selective rotation of first diffraction orders using an overlay target 106 and optical isolation of first diffraction orders using a collection polarizer 130 of the overlay metrology subsystem 102, as disclosed herein, enables high-throughput imaging of multiple overlay targets 106 within a single field of view. For example, selective rotation of first diffraction orders using an overlay target 106 and optical isolation of first diffraction orders using a collection polarizer 130 of the overlay metrology subsystem 102, as disclosed herein, enables isolating first diffraction orders not only from the overlay target 106 but also from additional sample light 110 from sample light 110 associated with device features. As a result, such overlay targets 106 may be imaged with high contrast regardless of their placement on the sample 108.

[0091] 11 is a flow diagram illustrating steps performed in a method 1100 for high-throughput overlay metrology in accordance with one or more embodiments of the present disclosure. Applicant notes that the embodiments and enabling techniques previously described herein in the context of overlay metrology system 100 should be construed as extending to method 1100. However, it is further noted that method 1100 is not limited to the architecture of overlay metrology system 100.

[0092] In some embodiments, the method 1100 includes a step 1102 of fabricating a plurality of overlay targets 106 on the sample 108, where the overlay targets 106 include a grating-on-grating structure with polarization rotating features 202 on at least one layer to rotate the first diffraction order relative to at least one of the specular reflection or the zeroth diffraction order.

[0093] The overlay targets 106, or portions thereof, may be distributed throughout the sample 108 in any suitable location. Different cells (e.g., different lattice-on-lattice structures) need not be adjacent to one another. In some embodiments, different cells (e.g., different lattice-on-lattice structures) of one or more overlay targets 106 are distributed throughout the sample 108. In this configuration, different cells of different overlay targets 106 may be imaged simultaneously within a wide field of view, and overlay measurements may be based on reconstruction of data from the different cells.

[0094] It is further contemplated herein that particular cells of the overlay target 106 (e.g., as shown in FIGS. 8A and 8B ) may be fabricated to relatively small sizes, since the constituent grating structures need not be individually resolvable. For example, as previously described herein, a grating-on-grating structure suitable for SCOL technology may be displayed with a single grayscale intensity associated with the actual alignment 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 the Moiré diffraction orders 1002 (e.g., at the Moiré pitch) may be visible even if the constituent grating structures are not resolvable.

[0095] As an example, the cells of the overlay target 106, including the grating-on-grating structure, may, but need not, have a size of about 5 microns on a side or less. As a result, the cells are not limited to being located within a streak, but may also be located within a die on the sample 108, which may advantageously improve measurement accuracy due in part to the reduced distance between the cells and the device features of interest.

[0096] In some embodiments, the method 1100 includes illuminating 1104 a plurality of overlay targets 106 with the illumination beam 104. In some embodiments, the method 1100 includes generating 1106 an image of the sample 108 through a polarizer aligned to a first diffraction order of the illumination beam 104 at a coarse pitch 208. For example, the overlay metrology subsystem 102 may be configured (e.g., according to a metrology recipe) to image a relatively wide field of view of the sample 108 including a plurality of overlay targets 106 (or portions thereof) through a collection polarizer 130 aligned to pass first diffraction orders as rotated by the overlay targets 106. The resulting image may then reveal the overlay targets 106 in high contrast. In some embodiments, the method 1100 includes determining 1108 an overlay measurement value based on the image of the sample 108.

[0097] FIG. 12 is a series of high-level diagrams illustrating high-throughput imaging of multiple overlay targets 106 within a single image grab, in accordance with one or more embodiments of the present disclosure.

[0098] Panel 1202 shows a field of view 1204 of the overlay metrology subsystem 102, which may, but is not necessarily, used to perform step 1104 of illuminating the sample 108 and / or step 1106 of imaging the sample 108. Panel 1206 shows a die 1208 within the field of view, which may include various overlay targets 106 or portions thereof (e.g., individual cells having a grid-on-grid structure). In some embodiments, the various overlay targets 106 or portions thereof are arranged in subsections 1210 of the die 1208, such that individual overlay measurements may be generated based on the overlay targets 106 or portions thereof in each subsection 1210. Panel 1212 shows different overlay measurements in each of the subsections 1210.

[0099] The overlay measurement technique shown in FIG. 12 and depicted in FIG. 11 is contemplated herein to enable overlay measurements with high throughput and high accuracy. In a general sense, any number of overlay targets 106 may be imaged simultaneously in a single grab. For example, the field of view size may be on the order of millimeters or larger, while the cell size (e.g., the size of the grating-on-grid structure) may be, but is not necessarily, on the order of 5 micrometers or smaller. In some applications, the cell size may be on the order of 3 micrometers or smaller. In some applications, the cell size may be greater than 5 micrometers. As a result, the number of overlay targets 106 that may be imaged simultaneously may be on the order of tens, hundreds, or more. Furthermore, because the collecting polarizer 130 may pass first order diffraction from the overlay target 106 that is rotated by the overlay target 106 and may block other light sources (e.g., DC signal 904, zeroth order diffraction 908 from the lower grating structure 806, zeroth order diffraction 906 from the upper grating structure 808, light from device features, etc.), a wide field of view image may reveal these overlay targets 106 with high contrast, which may enable overlay measurements with high sensitivity and / or signal-to-noise ratio.

[0100] 12 is not necessarily limited to overlay targets 106 having polarization rotation features 202 as previously described herein. Rather, such techniques may be suitable for any overlay target 106 that provides diffraction orders of interest within the collection aperture of the overlay metrology subsystem 102.

[0101] 13A and 13B show a configuration of the overlay metrology subsystem 102 for measuring a moiré overlay target 106 having a relatively small gain (e.g., a gain on the order of 10 or less) that does not necessarily include polarization rotation features 202. For example, FIGS. 13A and 13B show a configuration of the overlay metrology subsystem 102 for measuring a moiré overlay target 106 having only a coarse pitch.

[0102] 13A is a top view of a collection pupil 1302 of an overlay metrology subsystem 102 in accordance with one or more embodiments of the present disclosure. In FIG. 13A , a field of view of a sample 108 including one or more overlay targets 106 (e.g., as shown in FIG. 12 ) is illuminated with two oblique illumination beams 104 such that the zeroth-order diffraction lobe 1304 from the constituent upper and lower grating structures and the DC signal 904 are outside the boundary 1306 of the collection pupil 1302, but the first-order diffraction 1308 from the upper and lower grating structures (e.g., upper and lower grating structures 806, 808) is within the boundary 1306 of the collection pupil 1302. For example, FIG. 13A illustrates a configuration in which a +1st-order diffraction lobe from one of the illumination beams 104 and a −1st-order diffraction lobe from another of the illumination beams 104 overlap at the center of the boundary 1306 of the collection pupil 1302. In this configuration, the collecting polarizer 130 is not required, as the first diffraction order of interest is isolated using optical and / or mechanical techniques.

[0103] It is contemplated herein that the configuration shown in FIG. 13A may be suitable for wide-field imaging of multiple overlay targets 106 (e.g., up to 100 or more overlay targets 106 simultaneously) or portions thereof (e.g., cells having any type of Moiré grid-on-grid structure), where the cells may be displayed as grayscale intensity values ​​based on the actual alignment between associated layers of the sample 108. Accordingly, overlay measurements may be generated based on multiple cells (e.g., two or more cells) having grid-on-grid structures with a predetermined offset. In particular, the intensity values ​​obtained for the cells in each overlay target 106 may be fitted to a sine function having a pitch corresponding to the Moiré pitch of the overlay target 106.

[0104] 13B is a conceptual diagram of generating two illumination beams 104 from a non-coherent light source, according to one or more embodiments of the present disclosure. For example, light 1310 from the non-coherent light source may be incident on a diffraction grating 1312 or other dispersive optic, such that the two illumination beams 104 are formed as + / 1 order diffracted beams of the non-coherent light 1310. FIG. 13B further shows a beam block 1314 that blocks zero order light 1316, and a lens 1318 that collimates the two illumination beams 104.

[0105] However, it should be understood that Figures 13A and 13B are provided for illustrative purposes only and should not be construed as limiting. Rather, the technique shown in Figure 12 may be utilized with any type of overlay target 106 and overlay metrology subsystem 102 configuration suitable for providing imaging of a wide field of view on a sample 108 with light isolated to diffraction orders of interest from the overlay target 106.

[0106] 1A and 1B, further aspects of the overlay metrology subsystem 102, in accordance with one or more embodiments of the present disclosure, will be described in more detail.

[0107] The illumination source 112 may include any type of illumination source suitable for providing the 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, broadband laser sources, supercontinuum laser sources, white light laser sources, etc. In this regard, the illumination source 112 may provide the illumination beam 104 with high coherence (e.g., high spatial coherence and / or high 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 containing one or more elements that may emit broadband illumination when excited into a plasma state by a laser source. 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, etc. In this regard, the illumination source 112 may provide an illumination beam 104 having low coherence (eg, low spatial coherence and / or low temporal coherence).

[0108] In some embodiments, the overlay metrology system 100 includes a controller 134 communicatively coupled to the overlay metrology subsystem 102. The controller 134 may be configured to instruct the overlay metrology subsystem 102 to generate an overlay signal based on one or more selected recipes. The controller 134 may be further configured to receive data including, but not limited to, the overlay signal from the overlay metrology subsystem 102. Furthermore, the controller 134 may be configured to determine an overlay associated with the overlay target 106 based on the acquired overlay signal.

[0109] In some embodiments, the controller 134 includes one or more processors 136. For example, the one or more processors 136 may be configured to execute a set of program instructions held in a memory device 138 or memory. The one or more processors 136 of the 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 additional examples, the memory device 138 may include, but is not limited to, read-only memory, random access memory, magnetic or optical memory devices (e.g., disks), magnetic tape, solid-state drives, etc. It is further noted that the memory device 138 may be housed within a common controller housing along with the one or more processors 136.

[0110] The subject matter described herein sometimes depicts different components contained within or connected to other components. It should be understood that such depicted architectures are merely exemplary, and that in fact many other architectures that achieve the same functionality may be implemented. In a conceptual sense, any arrangement of components to achieve the same functionality is effectively “associated” such that the desired functionality is achieved. Thus, any two components combined herein to achieve a particular function may be considered to be “associated” with each other such that the desired functionality is achieved, regardless of the architecture or intermediate components. Similarly, any two components so associated may also be considered to be “connected” or “coupled” with each other to achieve the desired functionality, and any two components capable of being so associated may also be considered to be “couplable” with each other to achieve the desired functionality. Specific examples of what can be coupled include, but are not limited to, physically interactable and / or physically interacting components, wirelessly interactable and / or wirelessly interacting components, and / or logically interactable and / or logically interacting components.

[0111] It will be apparent that the present disclosure and many of its attendant advantages will be understood from the foregoing description, and that various changes may be made in the form, construction, and arrangement of elements without departing from the disclosed subject matter or sacrificing all of its important advantages. The forms described are merely illustrative, and it is intended that the following claims encompass and include all such modifications. It is further understood that the invention is defined by the appended claims.

Claims

1. A lighting source configured to generate an illumination beam, When performing a measurement recipe, one or more illumination optics for directing the illumination beam onto a sample along with a field of view including one or more overlay targets, wherein a specific one of the one or more overlay targets according to the measurement recipe is One or more lattice-on-lattice structures formed from a lower lattice structure having a first coarse pitch and an upper lattice structure having a second coarse pitch, wherein the upper lattice structure and the lower lattice structure overlap on the sample, and at least one of the upper lattice structure or the lower lattice structure further includes a feature having a fine pitch smaller than the wavelength of the illumination beam, wherein the feature is arranged to rotate the first-order diffraction of the illumination beam with respect to at least one of specular reflection from the upper surface of the sample or zero-order diffraction from the one or more lattice-on-lattice structures, and the first-order diffraction corresponds to a +1 diffraction from one of the upper lattice structure or the lower lattice structure and a -1 diffraction from the other of the upper lattice structure or the lower lattice structure, A one or more illumination optical system, When carrying out the above measurement recipe, an objective lens for collecting light from the sample within the field of view, A polarizer arranged to allow the portion of light from the sample related to the primary diffraction to pass through, One or more detectors for generating one or more images of the sample including one or more overlay targets, A controller connected to one or more detectors, the controller comprising one or more processors configured to execute program instructions, the program instructions causing the one or more processors to determine an overlay measurement of the sample based on one or more images of the one or more overlay targets, An overlay measurement system equipped with the following features.

2. The overlay measurement system according to claim 1, wherein at least one of the one or more overlay targets includes two or more of the one or more lattice structures arranged in different cells within non-adjacent portions of the die on the sample.

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

4. The overlay measurement system according to claim 1, wherein at least one optical wave scattering measurement overlay (SCOL) target is formed by the first coarse pitch and the second coarse pitch being equal for at least one of the one or more overlay targets.

5. The overlay measurement system according to claim 4, wherein the at least one SCOL target includes two or more grid-on-grid structures having different predetermined offsets between the upper grid structure and the lower grid structure.

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

7. The overlay measurement system according to claim 1, wherein at least one moiré target is formed by different first coarse pitches and second coarse pitches for at least one of the one or more overlay targets.

8. The overlay measurement system according to claim 7, wherein the at least one moiré target includes two or more grid-on-grid structures having different predetermined offsets between the upper grid structure and the lower grid structure.

9. The overlay measurement system according to claim 1, wherein the one or more grid structures on the grid within the one or more overlay targets are not resolved within one related image of the one or more images of the one or more overlay targets, and are displayed using grayscale intensity.

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

11. The overlay measurement system according to claim 10, wherein the lower lattice structure is located within the process layer of the sample.

12. The overlay measurement system according to claim 11, wherein the lower lattice structure is located within the resist layer of the sample.

13. The overlay measurement system according to claim 11, wherein the lower lattice structure is located within an additional process layer of the sample.

14. The overlay measurement system according to claim 1, wherein the features having the fine pitch are arranged within two sections within the first coarse pitch range, and the direction of the fine pitch within the first section of the two sections is perpendicular to the direction of the fine pitch within the second section of the two sections.

15. The overlay measurement system according to claim 14, wherein the two sections have a duty cycle of 1:1 within the first coarse pitch range.

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

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

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

19. The overlay measurement system according to claim 14, wherein the direction of the fine pitch within the first section is at an angle of 45 degrees with at least one of the directions of the first coarse pitch or the second coarse pitch.

20. The overlay measurement system according to claim 1, wherein the one or more images correspond to an optical image of the sample in which one or more features of the one or more overlay targets are resolved.

21. The overlay measurement system according to claim 1, wherein the one or more images are related to the diffraction order of radiation from the one or more overlay targets.

22. A lattice-top lattice structure comprising one or more lattice-top lattice structures formed from a lower lattice structure having a first coarse pitch in a first layer of the sample and an upper lattice structure having a second coarse pitch in a second layer of the sample, wherein the upper lattice structure and the lower lattice structure overlap on the sample, and at least one of the upper lattice structure or the lower lattice structure further includes features having a fine pitch smaller than the wavelength of the illumination beam, wherein the features are arranged to rotate the first-order diffraction of the illumination beam with respect to at least one of specular reflection from the upper surface of the sample or zero-order diffraction from the one or more lattice-top lattice structures, wherein the first-order diffraction corresponds to a +1 diffraction from one of the upper lattice structure or the lower lattice structure and a -1 diffraction from the other of the upper lattice structure or the lower lattice structure, and the overlay between the first layer and the second layer of the sample is an overlay measurement target that can be determined from one or more images of the one or more lattice-top lattice structures based on the first-order diffraction.

23. The overlay measurement target according to claim 22, wherein the first coarse pitch and the second coarse pitch are equal, thereby forming a light wave scattering measurement overlay (SCOL) target.

24. The overlay measurement target according to claim 23, wherein the SCOL target includes two or more grid-on-grid structures having different predetermined offsets between the upper grid structure and the lower grid structure.

25. The overlay measurement target according to claim 22, wherein the first coarse pitch and the second coarse pitch are different, thereby forming a moiré target.

26. The overlay measurement target according to claim 25, wherein the moiré target includes two or more grid-on-grid structures having different predetermined offsets between the upper grid structure and the lower grid structure comprising the moiré target.

27. The overlay measurement target according to claim 22, wherein the features having the fine pitch are arranged only within the lower grid structure.

28. The lower lattice structure is located within the process layer of the sample, as an overlay measurement target according to claim 26.

29. The lower lattice structure is located within the resist layer of the sample, as an overlay measurement target according to claim 28.

30. The overlay measurement target according to claim 28, wherein the lower lattice structure is located within an additional process layer of the sample.

31. The overlay measurement target according to claim 22, wherein the features having the fine pitch are arranged within two sections within each coarse pitch range, and the direction of the fine pitch in the first section of the two sections is perpendicular to the direction of the fine pitch in the second section of the two sections.

32. The overlay measurement target according to claim 31, wherein the two sections have a duty cycle of 1:1 within the first coarse pitch range.

33. The overlay measurement target according to claim 31, wherein the fine pitch in the first section is equal to the fine pitch in the second section.

34. The overlay measurement target according to claim 31, wherein the fine pitch in the first section has a different value from the fine pitch in the second section.

35. The overlay measurement target according to claim 31, wherein the direction of the fine pitch within the first section is parallel to the direction of at least one of the first coarse pitch or the second coarse pitch.

36. The overlay measurement target according to claim 31, wherein the direction of the fine pitch within the first section is at an angle of 45 degrees with at least one of the directions of the first coarse pitch or the second coarse pitch.

37. A step of fabricating one or more overlay targets on a sample, wherein a particular one of the one or more overlay targets is Steps include: one or more lattice-top lattice structures formed from a lower lattice structure having a first coarse pitch and an upper lattice structure having a second coarse pitch, wherein the upper lattice structure and the lower lattice structure overlap on the sample, and at least one of the upper lattice structure or the lower lattice structure further includes a feature having a fine pitch smaller than the wavelength of the illumination beam, wherein the feature is arranged to rotate the first-order diffraction of the illumination beam with respect to at least one of specular reflection from the upper surface of the sample or zero-order diffraction from the one or more lattice-top lattice structures, and the first-order diffraction corresponds to a +1 diffraction from one of the upper lattice structure or the lower lattice structure and a -1 diffraction from the other of the upper lattice structure or the lower lattice structure; The steps include illuminating one or more overlay targets with the illumination beam, The steps include generating one or more images of the sample through polarizers aligned to allow the first diffraction to pass through, The steps include determining the overlay measurement value based on one or more images of the sample, An overlay measurement method including

38. The overlay measurement method according to claim 37, wherein at least one of the one or more overlay targets includes two or more of the one or more lattice structures arranged in different cells within non-adjacent portions of the die on the sample.

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

40. The overlay measurement method according to claim 37, wherein at least one optical wave scattering measurement overlay (SCOL) target is formed by having the first coarse pitch and the second coarse pitch of at least one of the one or more overlay targets be equal.

41. The overlay measurement method according to claim 40, wherein the at least one SCOL target includes two or more grid-on-grid structures having different predetermined offsets between the upper grid structure and the lower grid structure.

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

43. The overlay measurement method according to claim 37, wherein at least one moiré target is formed by different first coarse pitches and second coarse pitches for at least one of the one or more overlay targets.

44. The overlay measurement method according to claim 37, wherein the one or more grid structures on the grid within the one or more overlay targets are not resolved within one related image of the one or more images of the one or more overlay targets, and are displayed using grayscale intensity.

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

Images