Imaging overlay using mutually coherent oblique illumination

Abstract

The overlay metrology system may include an illumination source configured to generate one or more pairs of mutually coherent illumination beams and illumination optics that direct the pair of illumination beams at a common elevation angle of incidence and symmetrically opposed azimuthal angles of incidence to an overlay target, the overlay target including two or more grating structures arranged along one or more measurement directions. The system may further include imaging optics for imaging the overlay target onto a detector when executing a metrology recipe, the image of a particular one of the two or more grating structures being generated using only a single non-zero diffraction order of light from each illumination beam in a particular one of the pairs of illumination beams. The system may further include a controller that determines an overlay metrology based on the image of the overlay target.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent Application No. 63 / 309,715, filed February 14, 2022, entitled IMAGING OVERLAY METROLOGY WITH MUTUALLY COHERENT OBLIQUE ILLUMINATION, the entire contents of which are incorporated herein by reference.

[0002] The present disclosure relates generally to overlay metrology, and more particularly to dark-field imaging overlay metrology with mutually coherent oblique illumination. [Background technology]

[0003] Overlay metrology systems typically characterize the overlay alignment of multiple sample layers by measuring the relative positions of overlay target features located on a target layer. As the size of manufactured features decreases and the density of features increases, the demands on the overlay metrology systems needed to characterize these features increase. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] U.S. Patent Application Publication No. 2021 / 0072021 Summary of the Invention [Problem to be solved by the invention]

[0005] It is therefore desirable to develop systems and methods that address such demands. [Means for solving the problem]

[0006] An overlay metrology system is disclosed according to one or more exemplary embodiments of the present disclosure. In one exemplary embodiment, the system includes one or more illumination sources for generating one or more pairs of illumination beams when executing a metrology recipe, the illumination beams in each pair of illumination beams being mutually coherent. In another exemplary embodiment, the system includes one or more illumination optics for directing the pair of illumination beams to an overlay target on a specimen when executing a metrology recipe, the illumination beams in a particular pair of the pairs of illumination beams are directed to the overlay target at a common elevation angle of incidence and symmetrically opposed azimuthal angles of incidence along a selected illumination direction. In another exemplary embodiment, an overlay target according to a metrology recipe includes two or more grating structures arranged along one or more measurement directions. In another exemplary embodiment, the system includes one or more imaging optics for imaging the overlay target to one or more detectors when executing a metrology recipe, the image of a particular one of the two or more grating structures is generated using only a single non-zero diffraction order of light from each illumination beam in a particular pair of the pairs of illumination beams. In another exemplary embodiment, the system includes a controller that determines overlay metrology along one or more measurement directions based on one or more images of the overlay target received from one or more detectors.

[0007] An overlay metrology method is disclosed according to one or more exemplary embodiments of the present disclosure. In one exemplary embodiment, the method includes illuminating an overlay target on a specimen with one or more pairs of illumination beams, where the illumination beams in each pair of illumination beams are mutually coherent. In another exemplary embodiment, the illumination beams in a particular one of the pairs of illumination beams are directed at the overlay target at a common elevation angle of incidence and symmetrically opposed azimuthal angles of incidence along a selected illumination direction. In another exemplary embodiment, the overlay target according to the metrology recipe includes two or more grating structures arranged along one or more measurement directions. In another exemplary embodiment, the method includes imaging the overlay target onto one or more detectors, where an image of a particular one of the two or more grating structures is generated using only a single non-zero diffraction order of light from each illumination beam in the particular one of the pairs of illumination beams. In another exemplary embodiment, the method includes determining an overlay metrology along one or more measurement directions based on one or more images of the overlay target received from the one or more detectors.

[0008] An overlay metrology system is disclosed according to one or more exemplary embodiments of the present disclosure. In one exemplary embodiment, the system includes a controller. In another exemplary embodiment, the controller determines overlay metrology along one or more measurement directions based on one or more images of the one or more overlay targets received from one or more detectors. In another exemplary embodiment, the overlay target according to the metrology recipe includes two or more grating structures arranged along one or more measurement directions. In another exemplary embodiment, the one or more images of the one or more overlay targets are generated based on illumination with one or more pairs of illumination beams. In another exemplary embodiment, the illumination beams within each illumination beam pair are mutually coherent. In another exemplary embodiment, the illumination beams within a particular one of the pairs of illumination beams are directed at the overlay target at a common elevation angle of incidence and symmetrically opposed azimuthal angles of incidence along a selected illumination direction. In another exemplary embodiment, the image of a particular one of the two or more grating structures is generated using only a single non-zero diffraction order of light from each illumination beam within a particular one of the pairs of illumination beams.

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

[0010] The numerous advantages of the present disclosure may be better understood by those skilled in the art by reference to the following drawings. [Figure 1A] FIG. 1 is a conceptual diagram of an overlay metrology system in accordance with one or more embodiments of the present disclosure. [Figure 1B]FIG. 1 is a simplified schematic diagram of an overlay metrology subsystem suitable for illuminating an overlay target with one or more pairs of mutually coherent illumination beams and imaging the overlay target based on a single non-zero diffraction order from each illumination beam, in accordance with one or more embodiments of the present disclosure. [Figure 1C] FIG. 1 is a simplified schematic diagram of a portion of an overlay metrology subsystem that provides mutually coherent illumination beams in accordance with one or more embodiments of the present disclosure. [Figure 2A] FIG. 1 is a conceptual diagram of illuminating an overlay target with a pair of mutually coherent illumination beams in accordance with one or more embodiments of the present disclosure. [Figure 2B] FIG. 2B is a top view of an illumination pupil illustrating the illumination beam of FIG. 2A in accordance with one or more embodiments of the present disclosure. [Figure 2C] FIG. 1 is a top view of an illumination pupil illustrating two pairs of mutually coherent illumination beams in accordance with one or more embodiments of the present disclosure. [Diagram 3] FIG. 2 is a top view of an AIM overlay target in accordance with one or more embodiments of the present disclosure. [Figure 4] FIG. 1 is a conceptual schematic diagram illustrating the collection of a single non-zero diffraction order from an AIM overlay target in accordance with one or more embodiments of the present disclosure. [Diagram 5] FIG. 13 is a plot illustrating viable values ​​of grating pitch per wavelength assuming a 60 degree illumination incidence angle and 0.5 NA collection on an AIM overlay target in accordance with one or more embodiments of the present disclosure. [Figure 6A] FIG. 2 is a top view of an r-AIM overlay target in accordance with one or more embodiments of the present disclosure. [Figure 6B] FIG. 6B is a side view of two adjacent cells including a moire structure that may form a quarter of an r-AIM overlay target as shown in FIG. 6A in accordance with one or more embodiments of the present disclosure. [Figure 7]FIG. 1 is a conceptual schematic diagram illustrating collection of a single non-zero diffraction order from an r-AIM overlay target in accordance with one or more embodiments of the present disclosure. [Figure 8] FIG. 13 is a plot illustrating viable values ​​of grating pitch per wavelength assuming a 60 degree illumination incidence angle and 0.5 NA collection for an r-AIM overlay target in accordance with one or more embodiments of the present disclosure. [Figure 9] FIG. 1 is a flow diagram illustrating steps performed in a method for overlay metrology using mutually coherent illumination beams in accordance with one or more embodiments of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

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

[0012] Embodiments of the present disclosure relate to systems and methods for overlay metrology using pairs of mutually coherent illumination beams (e.g., mutually coherent illumination beam pairs), where each illumination beam fully illuminates the overlay target. In particular, embodiments of the present disclosure relate to illuminating an overlay target with pairs of mutually coherent illumination beams at symmetric azimuth angles for each measurement direction of the target. For example, overlay metrology for a single measurement direction may utilize one pair of mutually coherent illumination beams, while overlay metrology for two measurement directions (e.g., two orthogonal measurement directions) may utilize two pairs of mutually coherent illumination beams. Furthermore, while the illumination beams within each pair are mutually coherent, the beams of different pairs need not be mutually coherent. Rather, it may be beneficial, but not essential, for the pairs of illumination beams to be incoherent with respect to each other.

[0013] Overlay targets and / or overlay metrology tools suitable for characterizing overlay targets may be configured according to a metrology recipe suitable for producing overlay metrology based on a desired technology. More generally, overlay metrology tools may be configurable according to different metrology recipes to perform overlay metrology using different technologies and / or on different overlay targets with different designs.

[0014] For example, a metrology recipe can include various aspects of an overlay target or an overlay target design, including, but not limited to, the layout, feature size, or feature pitch of target features on one or more specimen layers. As another example, a metrology recipe can include illumination parameters such as, but not limited to, wavelength of illumination, 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 specimen height. By way of another example, a recipe for an overlay metrology tool can include collection parameters such as, but not limited to, collection pupil distribution (e.g., desired distribution of angular light from the specimen used for metrology and associated filtered intensity at those angles), collection field aperture settings for selecting a portion of the specimen of interest, polarization of collected light, or wavelength filters.

[0015] In some embodiments, the overlay metrology tool is configured (e.g., according to a metrology recipe) to image an overlay target having a periodic structure using a single non-zero order diffraction lobe from each illumination beam of a mutually coherent illumination beam pair. In this configuration, the periodic structure from the overlay target can be imaged as a sinusoidal interference pattern in a dark-field imaging mode. Furthermore, the overlay can be determined by comparing the relative phase of the various imaged sinusoidal patterns of the overlay target. The overlay metrology system can be configured in multiple ways according to the systems and methods disclosed herein. In some embodiments, the overlay metrology system is configured to direct a pair of mutually coherent illumination beams to an overlay target that is outside the numerical aperture (NA) of an objective lens used to collect light from the overlay target for imaging, referred to herein as outside-the-lens (OTL) illumination. In this configuration, the zero order diffraction naturally falls outside the NA of the objective lens and is not collected. In some embodiments, the overlay metrology system is configured to direct a pair of mutually coherent illumination beams to the overlay target within the numerical aperture (NA) of an objective lens used to collect light from the overlay target for imaging, referred to herein as through-the-lens (TTL) illumination. In this configuration, the overlay metrology system may include one or more elements that block the zeroth diffraction order so that it does not contribute to image formation.

[0016] As disclosed herein, various overlay target designs are contemplated as suitable for overlay metrology using a mutually coherent illumination beam pair. In some embodiments, the overlay target is an advanced imaging metrology (AIM) target. In this configuration, each cell of the overlay target can include grating structures from different lithographic exposures in non-overlapping regions of one or more layers, where the grating structures from the different lithographic exposures have the same pitch. In some embodiments, the overlay target is a moiré target. In this configuration, each cell can include grating structures from different lithographic exposures in overlapping regions of two layers, forming grating-over-grating structures or moiré structures, where the grating structures from the different lithographic exposures have different pitches. Additionally, the cells may include pairs of moiré structures where the pitches of the constituent layers are inverted relative to each other. For example, a first moiré structure can have a first pitch (P) in a first layer and a second pitch (Q) in a second layer, and a second moiré structure can have a first pitch (P) in a second layer and a second pitch (Q) in the first layer. Such overlay targets are sometimes referred to as robust-AIM (r-AIM) overlay targets, providing that overlay metrology can be determined based on the relative phase between the two moiré structures.

[0017] It is contemplated herein that overlay metrology based on mutually coherent illumination beam pairs as disclosed herein may offer multiple advantages compared to existing image-based overlay metrology techniques based on spatially incoherent illumination, including, but not limited to, support for fine grating pitches, high image contrast, high image brightness, matched brightness between cells of the overlay target, insensitivity to monochromatic aberrations (e.g., defocus, etc.), minimal encroachment on cell edges, and / or minimal stray light.

[0018] For example, image contrast of periodic features is high (possibly maximized) by interfering only two diffraction orders with equal amplitude. As another example, image brightness is high (possibly maximized) based on the use of spatially coherent laser illumination, which avoids light losses associated with removing coherence for incoherent imaging with high-brightness laser sources. As another example, for r-AIM targets, image brightness is consistent between target cells because only the grating pitch differs between cells. As another example, overlay metrology is insensitive to monochromatic aberrations and defocus because only two points in the pupil are sampled by the focused diffraction orders, and the same points are sampled by cells from different layers. As another example, the use of oblique illumination (possibly OTL illumination) reduces cell edge ringing in the resulting image. As another example, especially with the OTL configuration, light losses of mutually coherent illumination beams may be further reduced because they do not propagate through the objective lens. Such configurations also limit the number of ghost reflections or scattering sites due to the relatively small number of optical surfaces used, and also reduce backscattered illumination to the image detector.

[0019] 1A-9, systems and methods for overlay metrology based on mutually coherent illumination beam pairs, according to one or more embodiments of the present disclosure, are described in more detail.

[0020] FIG. 1A is a conceptual diagram of an overlay metrology system 100 in accordance with one or more embodiments of the present disclosure.

[0021] In some embodiments, the overlay metrology system 100 includes an overlay metrology subsystem 102 configured to illuminate an overlay target 104 of a specimen 106 with a pair of mutually coherent illumination beams 108 for each measurement direction of interest. In particular, each of the illumination beams 108a,b can fully illuminate the entire overlay target 104. In this manner, each cell of the overlay target 104 is subjected to common illumination conditions, facilitating consistent image brightness for all cells.

[0022] FIG. 2A is a conceptual diagram of illuminating an overlay target 104 with a mutually coherent illumination beam pair 108, according to one or more embodiments of the present disclosure. FIG. 2A illustrates two illumination beams 108a,b as part of the mutually coherent illumination beam pair 108, with each illumination beam 108 shown as providing a planar wavefront. In some embodiments, the overlay metrology subsystem 102 directs the mutually coherent illumination beam pair 108 at symmetric angles of incidence. For example, the illumination beams 108a,b have symmetric polar angles of incidence (±α) and symmetric (e.g., opposing) azimuthal angles of incidence. This is illustrated in FIG. 2A by the two illumination beams 108a,b propagating in opposing azimuthal directions in the plane of the figure.

[0023] FIG. 2B is a top view of the illumination pupil 202 showing the illumination beams 108a,b of FIG. 2A in accordance with one or more embodiments of the present disclosure. Symmetric elevation and azimuth angles are also evident in FIG. 2B. For example, FIG. 2B shows the mutually coherent illumination beam pair 108a,b as a dipole illumination. In some embodiments, such an overlay target 104 includes a lattice structure along the X direction. In some embodiments, such an overlay target 104 includes a lattice structure along a different direction. In this case, the distribution of FIG. 2B may be a rotated dipole configuration.

[0024] FIG. 2C is a top view of an illumination pupil 202 showing two pairs of mutually coherent illumination beams 108 according to one or more embodiments of the present disclosure. In particular, FIG. 2C shows a first pair of mutually coherent illumination beams 108a, b oriented along the X direction in the figure, and a second pair of mutually coherent illumination beams 108c, d oriented along the Y direction in the figure. In this manner, FIG. 2C shows the two pairs of mutually coherent illumination beams 108 as a quadrupole illumination. It is contemplated herein that the quadrupole illumination distribution of FIG. 2C may be suitable for imaging a two-dimensional (2D) overlay target 104 having a grating structure oriented along two different (e.g., orthogonal) directions. In some embodiments, such a 2D overlay target 104 includes grating structures along the X and Y directions. In some embodiments, such a 2D overlay target 104 includes grating structures along two different orthogonal directions. In this case, the distribution of FIG. 2C may be in a rotated quadrupole configuration.

[0025] In some embodiments, all four illumination beams 108a-d are mutually coherent. In some embodiments, different pairs of mutually coherent illumination beams 108 are incoherent with respect to each other. For example, a first pair of mutually coherent illumination beams 108a,b may be mutually coherent with respect to each other, but incoherent with respect to a second pair of mutually coherent illumination beams 108c,d.

[0026] 1A , the overlay metrology subsystem 102 may image the overlay target 104 based on a single non-zero order diffraction lobe associated with each illumination beam 108 of each mutually coherent illumination beam pair 108. In this manner, the overlay metrology subsystem 102 may provide a dark field image because the zero order diffraction of the illumination beam 108 does not contribute to the image formation. Furthermore, because each illumination beam pair 108 is mutually coherent, the single diffraction lobe associated with each illumination beam 108 of the pair interferes to form a sinusoidal interference pattern in the image. As a result, the various grating structures of the overlay target 104 may be imaged with high contrast as pure sinusoids, and overlay metrology may be generated based on a comparison of the relative phase of adjacent cell images according to a metrology recipe.

[0027] In some embodiments, the overlay metrology system 100 includes a controller 110 communicatively coupled to the overlay metrology subsystem 102. The controller 110 may be configured to instruct the overlay metrology subsystem 102 to generate darkfield images based on one or more selected metrology recipes. The controller 110 may be further configured to receive data, including but not limited to darkfield images, from the overlay metrology subsystem 102. Additionally, the controller 110 may be configured to determine an overlay associated with the overlay target 104 based on the acquired darkfield images.

[0028] In some embodiments, the controller 110 includes one or more processors 112. For example, the one or more processors 112 can be configured to execute a set of program instructions held in the memory 114, or a memory device. The one or more processors 112 of the controller 110 can include any processing element known in the art. In this sense, the one or more processors 112 can include any microprocessor-type device configured to execute algorithms and / or instructions. Furthermore, the memory 114 can include any storage medium known in the art suitable for storing program instructions executable by the associated one or more processors 112. For example, the memory 114 can include a non-transitory memory medium. As additional examples, the memory 114 can 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, and the like. Furthermore, it should be noted that the memory 114 can be housed in a common controller housing with the one or more processors 112.

[0029] 1B and 1C, an overlay metrology subsystem 102 suitable for illuminating an overlay target 104 with one or more pairs of mutually coherent illumination beams 108 and imaging the overlay target 104 based on a single non-zero diffraction order from each illumination beam 108 is described in more detail, in accordance with one or more embodiments of the present disclosure.

[0030] FIG. 1B is a simplified schematic diagram of an overlay metrology subsystem 102 suitable for illuminating an overlay target 104 with one or more pairs of mutually coherent illumination beams 108 and imaging the overlay target 104 based on a single non-zero diffraction order from each illumination beam 108 in accordance with one or more embodiments of the present disclosure.

[0031] In some embodiments, the overlay metrology subsystem 102 includes at least one illumination source 116 configured to generate one or more pairs of illumination beams 108. Each illumination beam 108 may include one or more selected wavelengths of light, including, but not limited to, ultraviolet (UV) radiation, visible radiation, or infrared (IR) radiation.

[0032] The illumination source 116 may include any type of illumination source suitable for providing at least one pair of mutually coherent illumination beams 108. In some embodiments, the illumination source 116 includes at least one laser light source. For example, the illumination source 116 may include, but is not limited to, one or more narrowband laser light sources, broadband laser light sources, supercontinuum laser light sources, white laser light sources, etc. In this regard, the illumination source 116 may provide an illumination beam 108 having high coherence (e.g., high spatial coherence and / or temporal coherence).

[0033] In some embodiments, the overlay metrology subsystem 102 includes illumination optics that direct various illumination beams 108 to the overlay targets 104 of the specimen 106 via one or more illumination channels 118. Additionally, the specimen 106 can be placed on a specimen stage (not shown) suitable for fixing the specimen 106 and further configured to position the overlay targets 104 relative to the illumination beams 108.

[0034] Note that Figure 1B illustrates illumination of the overlay target 104 with a single pair of mutually coherent illumination beams 108. For example, the illumination beam 108 illustrated in Figure 1B may correspond to the illumination beams 108a, b illustrated in Figures 2A and 2B. In some embodiments, the overlay metrology subsystem 102 illuminates the overlay target 104 with two pairs of mutually coherent illumination beams 108 (e.g., as shown in Figure 2C). In this configuration, the overlay metrology subsystem 102 may include an additional pair of illumination channels 118 oriented in a plane orthogonal to Figure 1B.

[0035] Each of the illumination channels 118 may include one or more optical elements suitable for modifying and / or dimming the illumination beam 108 and for directing the illumination beam 108 to the overlay target 104. For example, each of the illumination channels 118 may, but is not required to, include one or more illumination lenses 120 (e.g., to control the spot size of the illumination beam 108 on the overlay target 104, to relay a pupil plane and / or a field plane, etc.), one or more polarizers to adjust the polarization of the illumination beam 108 in the channel, 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., stationary mirrors, translating mirrors, scanning mirrors, etc.).

[0036] Reference is now made to FIG. 1C, which is a simplified schematic diagram of a portion of an overlay metrology subsystem 102 that provides mutually coherent illumination beams 108, in accordance with one or more embodiments of the present disclosure.

[0037] In some embodiments, the light 122 from the coherent illumination source 116 is collimated (e.g., using a collimating lens 124) and split using a beam splitter 126 or other optical elements (e.g., diffractive elements, etc.) into two illumination beams 108 that propagate along two illumination channels 118a,b. The illumination beam 108 may then be directed to the overlay target 104 using any technique known in the art. For example, FIG. 1C illustrates focusing the illumination beam 108 into optical fibers 128a,b, also shown in FIG. 1B. The optical fibers 128a,b may be any type of optical fiber known in the art, including, but not limited to, single mode fiber, polarization maintaining fiber, single mode polarization maintaining fiber, multimode fiber, etc. However, it should be understood that the illumination beam 108 may be illuminated to the overlay target 104 using any suitable optical elements, including, but not limited to, free space optical elements.

[0038] In some embodiments, the overlay metrology subsystem 102 includes one or more elements for providing that the optical path lengths of the two illumination channels 118 are identical so that the two wavefronts at the overlay target 104 are mutually coherent and therefore interfere coherently. The optical path lengths of the two illumination channels 118 can be matched by any technique known in the art. For example, the thickness of all optical elements, the refractive index and dispersion of the optical materials, and the optical path length in air can all be set to be identical. This includes matching the length of the two optical fibers 128a. As another example, one or more optical path lengths of the illumination channel 118 can be made adjustable to compensate for any deviations. As an example, FIG. 1C shows one optical fiber 128a and associated focusing lens 130a of the illumination channel 118 on a translation stage 132 so that the optical path lengths can be precisely matched. For example, the translation stage 132 can be adjusted to maximize the contrast of the interference fringes between the wavefronts of the two illumination channels 118 at the overlay target 104.

[0039] FIG. 1C further illustrates various filters suitable for manipulating the illumination beams 108 together or individually. For example, FIG. 1C illustrates a first filter 134a before the beam splitter 126 suitable for manipulating both illumination beams 108. As another example, FIG. 1C illustrates a second filter 134b for manipulating the illumination beam 108a individually and a third filter 134c for manipulating the illumination beam 108b individually. The filters 134a-c may be any type of filter known in the art, such as, but not limited to, spectral filters or neutral density filters. Additionally, although not illustrated, the overlay metrology subsystem 102 may include a polarizer for adjusting the polarization of the illumination beams 108 together or individually.

[0040] It should be noted that FIG. 1C illustrates the generation of a single pair of mutually coherent illumination beams 108a, b. Additional pairs of mutually coherent illumination beams 108 may be generated using any suitable technique. In some embodiments, the overlay metrology subsystem 102 includes a separate illumination source 116 for each mutually coherent illumination beam pair 108 and a separate set of elements shown in FIG. 1C for each light source. In some embodiments, the overlay metrology subsystem 102 includes a single illumination source 116 for generating all illumination beams 108. In embodiments in which different pairs of illumination beams 108 are not mutually coherent with respect to each other, the different pairs of illumination beams 108 from the common illumination source 116 can be made incoherent with respect to each other using any technique, including, but not limited to, using different lengths of optical fibers 128a, b for each pair. It is contemplated herein that by providing different pairs of mutually incoherent illumination beams 108 relative to one another, stray light on the generated image can be minimized and a particular grating structure can be ensured to be imaged only with a single pair of mutually coherent illumination beams 108.

[0041] In some embodiments, overlay metrology subsystem 102 includes imaging optics in a collection path 136 for collecting light (e.g., sample light 138) from overlay target 104. In some embodiments, collection path 136 includes an objective lens 140 for collecting diffracted or scattered light from overlay target 104. For example, objective lens 140 can collect one or more diffraction orders of light from overlay target 104 in response to illumination beam 108.

[0042] The collection path 136 may further include a number of optical elements for directing and / or modifying the illumination collected by the objective lens 140, including, but not limited to, one or more lenses 142, one or more filters, one or more polarizers, one or more beam blocks, or one or more beam splitters. Such elements may be located at any suitable location in the collection path 136, including, but not limited to, the collection pupil 144.

[0043] In some embodiments, the light collection path 136 includes a detector 146 configured to generate an image (e.g., a dark field image) of the overlay target 104. For example, the detector 146 may receive an image of the specimen 106 provided by elements in the light collection path 136 (e.g., the objective lens 140, one or more lenses 142, etc.).

[0044] For example, Figure 1B illustrates generating a darkfield image of the overlay target 104 with a single non-zero diffraction order from each illumination beam 108 of a pair of mutually coherent illumination beams 108. In particular, Figure 1B illustrates the collection of a first non-zero diffraction order 148a associated with a first illumination beam 108a and a second non-zero diffraction order 148b associated with a second illumination beam 108b, where the illumination beams 108a,b are mutually coherent.

[0045] It is contemplated herein that the illumination channel 118 and collection path 136 of the overlay metrology subsystem 102 may be oriented in a wide range of configurations suitable for generating a darkfield image of the overlay target 104. For example, FIG. 1B illustrates an OTL configuration in which the various illumination beams 108 are directed to the overlay target 104 outside the NA of the objective lens 140. In some embodiments, the overlay metrology subsystem 102 directs the illumination beams 108 to the overlay target 104 within the NA of the objective lens 140 in a TTL configuration. For example, the overlay metrology subsystem 102 may include one or more elements common to the collection path 136 and the illumination channel 118 to simultaneously provide the illumination beams 108 to the objective lens 140 for illumination of the overlay target 104 and direct a single non-zero diffraction order to the detector 146 to contribute to the image of the overlay target 104. As a non-limiting example, the overlay metrology subsystem 102 may include an annular mirror located at or near a pupil plane common to both the collection path 136 (e.g., conjugated to the collection pupil 144) and the illumination channel 118 (e.g., conjugated to the illumination pupil 202). Such an annular mirror may direct the various illumination beams 108 to the objective lens 140 (e.g., as shown in FIGS. 2B and 2C). Such an annular mirror may further block the zeroth diffraction order of the illumination beam 108 while passing the non-zero diffraction orders through a central aperture to provide high contrast darkfield imaging as described herein.

[0046] 3-8, overlay metrology using a pair of mutually coherent illumination beams 108 and imaging with a single non-zero diffraction order from each illumination beam 108 is described for various non-limiting examples of target types. In particular, Figures 3-5 show overlay metrology for an AIM target, and Figures 6A-8 show overlay metrology for an r-AIM target.

[0047] 3 is a top view of an AIM overlay target 104 in accordance with one or more embodiments of the present disclosure. In one embodiment, the overlay target 104 includes various cells 302 each including a single grating located on a single layer of the specimen 106. For example, the overlay target 104 may include one or more cells 302a having a first layer grating 304 on a first layer of the specimen 106 and one or more cells 302b having a second layer grating 306 on a second layer of the specimen 106. Additionally, each of the first layer grating 304 and the second layer grating 306 are formed from features having a common pitch 308. In this regard, the diffraction orders from the first layer grating 304 and the second layer grating 306 may be positioned within the collection pupil 144.

[0048] FIG. 4 is a conceptual schematic diagram illustrating collection of a single non-zero diffraction order from an AIM overlay target 104 in accordance with one or more embodiments of the present disclosure. FIG. 4 illustrates an illumination beam 108 being directed to the overlay target 104 using an OTL configuration as shown in FIG. 1B. In this configuration, a zero order signal 402, such as a zero order diffraction or specular reflection from the specimen 106, is naturally outside the collection NA 404 (e.g., the NA of the objective lens 140). FIG. 4 further illustrates a first diffraction order lobe 406 within the collection NA 404, while a second diffraction order lobe 408 is outside the collection NA 404 and therefore does not contribute to image formation.

[0049] It should be noted that for clarity, Figure 4 illustrates illumination and collection based on only one illumination beam 108 of the mutually coherent illumination beam pair 108. It should be understood that illumination with another illumination beam 108 of the mutually coherent illumination beam pair 108 would achieve similar results based on illumination symmetry.

[0050] Although FIG. 4 shows an OTL configuration, it should be understood that a similar result can be achieved in a TTL configuration if the zeroth order signal 402 (e.g., specular reflection and zeroth order diffraction) is blocked in the collection path 136 (e.g., by an element in the collection pupil 144).

[0051] 5, various conditions that ensure that only a single non-zero diffraction order of each illumination beam 108 is within the collection NA 404 for image formation are described in more detail, in accordance with one or more embodiments of the present disclosure. It is contemplated herein that the angular distribution of the diffraction orders from the grating (and thus the distribution within the collection pupil 144) may be affected by various factors, including, but not limited to, the wavelength (λ) of the illumination beam 108, the angle of incidence of the illumination beam 108, the pitch 308 (D) of the grating, or the size of the collection NA 404. It is further contemplated herein that all of these factors may be incorporated into or otherwise accounted for in a metrology recipe. Thus, one or more metrology recipes may be generated that specify aspects of the overlay target 104 and / or the overlay metrology subsystem 102 such that a single non-zero diffraction order per illumination beam 108 may contribute to image formation.

[0052] In order to collect the first order diffraction lobe 406, the grating pitch 308 (D) of the overlay target 104 must satisfy the following condition:

number

number

[0053] To avoid focusing of the second order diffraction lobes 408, the grating pitch 308 (D) must also satisfy a condition.

number

[0054] The pitch 308 (Dimg ) is shown below.

number

[0055] 5 is a plot illustrating feasible values ​​of grating pitch 308 per wavelength, assuming an illumination incidence angle of 60 degrees (0.866 NA) and 0.5 NA collection for the AIM overlay target 104, in accordance with one or more embodiments of the present disclosure. As illustrated by equations (1)-(4) and FIG. 5, a metrology recipe can be configured to provide a combination of grating pitch 308 and illumination beam 108 wavelength that satisfies the requirement to collect only a single non-zero diffraction order (e.g., 1 st diffraction lobe 406) for each illumination beam 108 using the AIM overlay target 104.

[0056] 6A and 6B show an r-AIM overlay target 104 according to one or more embodiments of the present disclosure. Fig. 6A is a top view of the r-AIM overlay target 104 according to one or more embodiments of the present disclosure. Fig. 6B is a side view of two adjacent cells 602 including a moiré structure 604 that may form a quarter of the r-AIM overlay target 104 as shown in Fig. 6A according to one or more embodiments of the present disclosure.

[0057] The moiré structure 604 can include two gratings in an overlapping region of the specimen 106, with the two gratings having different pitches. In some embodiments, the r-AIM overlay target 104 includes adjacent moiré structures 604 with opposite pitches in corresponding layers. In this way, an overlay error on the specimen 106 can cause the moiré diffraction orders associated with the moiré pitch to shift in opposite directions for the two adjacent moiré structures 604.

[0058] For example, Figure 6B shows a first moiré structure 604a having an upper grating 606a with a first pitch (P) on a first layer 608 of the specimen 106 and a lower grating 610a with a second pitch (Q) on a second layer 612 of the specimen 106. Figure 6B also shows a second moiré structure 604b having an upper grating 606b with a second pitch (Q) on the first layer 608 of the specimen 106 and a lower grating 610b with a first pitch (P) on the second layer 612 of the specimen 106.

[0059] Figure 7 is a conceptual schematic diagram illustrating collection of a single non-zero diffraction order from an r-AIM overlay target 104 in accordance with one or more embodiments of the present disclosure. Figure 7 shows an illumination beam 108 being directed at the overlay target 104 using an OTL configuration as shown in Figure 1B. In this configuration, a zero order signal 402 (not shown in Figure 7 for clarity), such as the zero order diffraction or specular reflection from the specimen 106, is naturally outside the collection NA 404 (e.g., the NA of the objective lens 140).

[0060] 7 further shows a Moiré diffraction lobe 702 associated with the first order diffraction from both gratings entering the collection NA 404. For example, the Moiré diffraction lobe 702 is formed from a first order diffraction lobe 704 from the upper grating 606 underlying a first order diffraction lobe 706 from the lower grating 610.

[0061] It should be noted that for clarity, Figure 7 illustrates illumination and collection based on only one illumination beam 108 of the mutually coherent illumination beam pair 108. It should be understood that illumination with another illumination beam 108 of the mutually coherent illumination beam pair 108 would achieve similar results based on illumination symmetry.

[0062] Although FIG. 7 shows an OTL configuration, it should be understood that a similar result can be achieved in a TTL configuration if the zeroth order signal 402 (e.g., specular reflection and zeroth order diffraction) is blocked in the collection path 136 (e.g., by an element in the collection pupil 144).

[0063] In some embodiments, the metrology recipe provides conditions such that all light diffracted from one of the gratings of the moiré structure 604 falls outside the collection NA404, and only one moiré diffraction order associated with the diffraction of light by each of the gratings of the moiré structure 604 falls within the collection NA404.

[0064] In order to avoid focusing the first order diffraction from only one of the gratings of the moiré structure 604, the following condition needs to be met for both pitches of the moiré structure 604 (eg, pitches P and Q):

number

[0065] For the first diffraction order from the upper grating 606 to propagate to the lower grating 610, the following condition must be met:

number

[0066] 8 is a plot illustrating feasible values ​​of grating pitch 308 per wavelength (e.g., values ​​of P and / or Q) assuming an illumination incidence angle of 60 degrees (0.866 NA) and 0.5 NA collection for the r-AIM overlay target 104 in accordance with one or more embodiments of the present disclosure. As illustrated by equations (5)-(6) and FIG. 8, a metrology recipe can be configured to provide a combination of grating pitch 308 and illumination beam 108 wavelength that satisfies the requirement to collect only a single non-zero diffraction order (e.g., moiré diffraction lobe 702) for each illumination beam 108 using the r-AIM overlay target 104.

[0067] Reference is now made to Figure 9, which is a flow diagram illustrating steps performed in a method 900 for overlay metrology using mutually coherent illumination beams 108, in accordance with one or more embodiments of the present disclosure. Applicant notes that the embodiments and enabling techniques described herein above in the context of overlay metrology system 100 should be construed to extend to method 900. However, it is further noted that method 900 is not limited to the architecture of overlay metrology system 100.

[0068] In some embodiments, the method 900 includes illuminating 902 an overlay target 104 having a grating structure (e.g., a grating, etc.) with one or more pairs of mutually coherent, symmetrically opposed illumination beams 108. For example, the illumination beams 108 in each pair may be mutually coherent with respect to one another and may be directed at the overlay target 104 at opposing angles of incidence (e.g., as shown in FIG. 2). In some embodiments, the illumination beams 108 in different pairs of illumination beams 108 are mutually incoherent with respect to one another. This may facilitate separate imaging of portions of the overlay target 104 having different directions of periodicity based on the illumination beams 108 of the different pairs. In some embodiments, the illumination beams 108 in different pairs are mutually coherent with respect to one another.

[0069] The overlay target 104 may have any design known in the art having a grating structure, including, but not limited to, an AIM target or an r-AIM target. Additionally, the grating structure may be located in any combination of one or more specimen layers, which may be process layers (e.g., a layer in which features are formed from one material surrounded by one or more different materials) or resist layers (e.g., a layer of a single material in which features are formed as regions of a different refractive index than the surrounding regions).

[0070] In some embodiments, the method 900 includes imaging 904 the overlay target 104 onto one or more detectors, where an image of a particular one of the two or more grating structures is generated with only a single non-zero diffraction order of light from each illumination beam 108 in a particular one of the illumination beam pairs 108 (e.g., illumination beams 108a, b). In this manner, the image may be a dark field image, and the grating structure may appear in the image as a pure sine wave. As an example for an AIM overlay target 104, the image may be formed based on interference of a single first diffraction order beam from each illumination beam 108 in a mutually coherent illumination beam pair 108, and the grating structure appears as a sine wave with a grating pitch 308. As an example for an r-AIM overlay target 104, the image may be formed based on interference of a single Moiré diffraction order associated with diffraction from both constituent grating structures from each illumination beam 108 in a mutually coherent illumination beam pair 108, and the grating structure appears as a sine wave with a Moiré pitch.

[0071] In some embodiments, the method 900 includes determining 906 overlay metrology along one or more measurement directions based on one or more images of the overlay target 104 received from one or more detectors. For example, the overlay metrology can be determined based on a relative phase shift between sinusoidal images of adjacent grating structures.

[0072] The subject matter described herein may depict different components that are included 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" with one another such that the desired functionality is achieved. Thus, in this specification, any two components that combine to achieve a particular functionality may be considered to be "associated" with one another such that the desired functionality is achieved, regardless of architecture or intervening components. Similarly, any two components that are so associated may also be considered to be "connected" or "coupled" with one another to achieve the desired functionality, and any two components that may be so associated may also be considered to be "coupleable" with one another to achieve the desired functionality. Examples of what may 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.

[0073] It will be believed that the present disclosure and many of its attendant advantages will be understood from the foregoing description, and it will be apparent that various changes in form, construction and arrangement of the parts may be made without departing from the disclosed subject matter or sacrificing all of its important advantages. The described forms are merely illustrative, and it is the intent of the following claims to encompass and embrace all such modifications. It will be further understood that the invention is defined by the appended claims.

Claims

1. 1. An overlay metrology system, comprising: one or more illumination sources configured to generate one or more pairs of illumination beams when executing a metrology recipe, the illumination beams within each said pair being mutually coherent; one or more illumination optics configured to direct the pairs of illumination beams to an overlay target on a specimen when executing the metrology recipe, wherein the illumination beams within a particular one of the pairs of illumination beams are directed to the overlay target at a common elevation angle of incidence and symmetrically opposed azimuthal angles of incidence along a selected illumination direction, and wherein the overlay target according to the metrology recipe includes two or more grating structures arranged along one or more measurement directions; one or more detectors; one or more imaging optics configured to image the overlay target onto the one or more detectors during execution of the metrology recipe, wherein the image of a particular one of the two or more grating structures is generated using only a single non-zero diffraction order of light from each illumination beam in the particular one of the pairs of illumination beams; a controller communicatively coupled to the detectors, the controller including one or more processors, the controller being configured to execute program instructions such that when executing the metrology recipe, the one or more processors determine overlay metrology along the one or more measurement directions based on one or more images of the overlay target received from the one or more detectors; 1. An overlay metrology system comprising:

2. The two or more grating structures on the overlay target are a first grating on a first layer of the specimen; a second grating on a second layer of the specimen, the first and second gratings being in non-overlapping areas of the specimen, the first and second gratings having a common pitch, and the corresponding overlay metrology being based on a relative image shift between the first and second gratings; and The overlay metrology system of claim 1 , comprising:

3. The overlay target is The overlay metrology system of claim 2 comprising an Advanced Imaging Metrology (AIM) target.

4. The two or more grating structures on the overlay target are A first moire structure, a first grating having a first pitch on a first layer of the specimen; a second grating having a second pitch on a second layer of the specimen, the first and second gratings being formed in a first overlap region of the specimen; and a first moire structure comprising: A second moire structure, a third grating having the second pitch on the first layer of the specimen; a fourth grating having the first pitch on the second layer of the specimen, the third and fourth gratings being formed in a second overlap region of the specimen; and a second moire structure comprising: The overlay metrology system of claim 1 , comprising:

5. The overlay target comprises:

5. The overlay metrology system of claim 4, comprising a robust advanced imaging metrology (r-AIM) target.

6. The two or more lattice structures are a first set of grating structures oriented along a first measurement direction; a second set of grating structures oriented along a second measurement direction, the one or more pairs of illumination beams including two pairs of illumination beams; and The overlay metrology system of claim 1 , comprising:

7. 7. The overlay metrology system of claim 6, wherein the one or more illumination sources include a first illumination source for generating a first one of the two pairs of illumination beams and a second illumination source for generating a second one of the two pairs of illumination beams.

8. The overlay metrology system of claim 6 , wherein the one or more illumination sources comprises a single illumination source for generating the two pairs of illumination beams.

9. 9. The overlay metrology system of claim 8, wherein the illumination beam of a first one of the two pairs of illumination beams is incoherent with respect to the illumination beam of a second one of the two pairs of illumination beams.

10. The overlay metrology system of claim 8 , wherein all of the illumination beams of the two pairs of illumination beams are mutually coherent.

11. 2. The overlay metrology system of claim 1, wherein the two or more grating structures are oriented along a single measurement direction, and the one or more pairs of illumination beams comprise a single pair of illumination beams.

12. The overlay metrology system of claim 1 , wherein the one or more pairs of illumination beams are incident on a common spot on the specimen overlying the overlay target.

13. 2. The overlay metrology system of claim 1 , wherein the one or more imaging optics include an objective lens that collects light from the overlay target, and the one or more illumination optics direct the one or more pairs of illumination beams to the overlay target outside a numerical aperture (NA) of the objective lens.

14. 2. The overlay metrology system of claim 1 , wherein the one or more imaging optics include an objective lens that collects light from the overlay target, and the one or more illumination optics direct the one or more pairs of illumination beams through the objective lens onto the overlay target.

15. 1. A method of overlay metrology comprising: illuminating an overlay target on a specimen with one or more pairs of illumination beams, the illumination beams within each of the illumination beam pairs being mutually coherent, the illumination beams within a particular one of the illumination beam pairs being directed at the overlay target at a common elevation angle of incidence and symmetrically opposed azimuthal angles of incidence along a selected illumination direction, the overlay target according to a metrology recipe including two or more grating structures arranged along one or more measurement directions; imaging the overlay target onto one or more detectors, wherein the image of a particular one of the two or more grating structures is generated using only a single non-zero diffraction order of light from each illumination beam in a particular one of the pairs of illumination beams; determining overlay metrology along the one or more measurement directions based on one or more images of the overlay target received from the one or more detectors; A method comprising:

16. 1. An overlay metrology system, comprising:

11. An overlay metrology system comprising: a controller communicatively coupled to one or more detectors, the controller comprising: one or more processors configured to execute program instructions that, when executing a metrology recipe, the one or more processors determine overlay metrology along one or more measurement directions based on one or more images of one or more overlay targets received from one or more detectors, the overlay targets according to the metrology recipe include two or more grating structures positioned along the one or more measurement directions, the one or more images of the one or more overlay targets are generated based on illumination with one or more pairs of illumination beams, the illumination beams within each of the illumination beam pairs being mutually coherent, the illumination beams within the particular one of the illumination beam pairs are directed at the overlay target at a common elevation angle of incidence and symmetrically opposed azimuthal angles of incidence along a selected illumination direction, and the image of a particular one of the two or more grating structures is generated using only a single non-zero diffraction order of light from each illumination beam within the particular one of the illumination beam pairs.

17. The two or more grating structures on the overlay target are a first grating on a first layer of the specimen; a second grating on a second layer of the specimen, the first and second gratings being in non-overlapping areas of the specimen, the first and second gratings having a common pitch, and the corresponding overlay metrology being based on a relative image shift between the first and second gratings; and 17. The overlay metrology system of claim 16, comprising:

18. The overlay target comprises:

20. The overlay metrology system of claim 17, comprising an advanced imaging metrology (AIM) target.

19. The two or more grating structures on the overlay target are A first moire structure, a first grating having a first pitch on a first layer of the specimen; a second grating having a second pitch on a second layer of the specimen, the first and second gratings being formed in a first overlap region of the specimen; and a first moire structure comprising: A second moire structure, a third grating having the second pitch on the first layer of the specimen; a fourth grating having the first pitch on the second layer of the specimen, the third and fourth gratings being formed in a second overlap region of the specimen; and a second moire structure comprising:

17. The overlay metrology system of claim 16, comprising:

20. The overlay target comprises:

20. The overlay metrology system of claim 19, comprising a robust advanced imaging metrology (r-AIM) target.

21. The two or more lattice structures are a first set of grating structures oriented along a first measurement direction; a second set of grating structures oriented along a second measurement direction, the one or more pairs of illumination beams including two pairs of illumination beams; and 17. The overlay metrology system of claim 16, comprising:

22. 22. The overlay metrology system of claim 21 , wherein the one or more illumination sources include a first illumination source for generating a first one of the two pairs of illumination beams and a second illumination source for generating a second one of the two pairs of illumination beams.

23. 22. The overlay metrology system of claim 21, wherein the one or more illumination sources include a single illumination source for generating the two pairs of illumination beams.

24. 24. The overlay metrology system of claim 23, wherein the illumination beam of a first pair of the two pairs of illumination beams is incoherent with respect to the illumination beam of a second pair of the two pairs of illumination beams.

25. 24. The overlay metrology system of claim 23, wherein all of the illumination beams of the two pairs of illumination beams are mutually coherent.

26. 17. The overlay metrology system of claim 16, wherein the two or more grating structures are oriented along a single measurement direction, and the one or more pairs of illumination beams comprise a single pair of illumination beams.

27. 17. The overlay metrology system of claim 16, wherein the one or more pairs of illumination beams are incident on a common spot on the specimen overlying the overlay target.

28. 17. The overlay metrology system of claim 16, wherein the one or more imaging optics include an objective lens that collects light from the overlay target, and the one or more illumination optics direct the one or more pairs of illumination beams to the overlay target outside a numerical aperture (NA) of the objective lens.

29. 17. The overlay metrology system of claim 16, wherein the one or more imaging optics include an objective lens that collects light from the overlay target, and the one or more illumination optics direct the one or more pairs of illumination beams through the objective lens to the overlay target.

Images