High contrast imaging in bonding sample metrology using oblique illumination

Abstract

An illumination system for dark-field imaging can include one or more illumination sources providing temporally incoherent light having a first spatial profile, and two or more lens assemblies configured to direct the light from the one or more illumination sources as two or more illumination beams along an optical path external to an objective of an imaging subsystem to a buried target. The two or more illumination beams can fully overlap at the buried target to provide a common illumination field. The lens assemblies limit a numerical aperture of the illumination beams to prevent specular reflection of the illumination beams into the objective. A respective lens assembly of the two or more lens assemblies includes a homogenizer to cause a respective illumination beam of the two or more illumination beams to have a second spatial profile having a higher spatial uniformity than the first spatial profile according to a selected metric.

Description

Technical Field

[0001] This disclosure generally relates to the metrology of bonded samples, and more specifically, to high-contrast imaging of features in bonded samples. Background Technology

[0002] Many modern semiconductor devices are formed from stacked samples, also known as bonded samples (e.g., wafers, dies, or the like). Providing accurate imaging of the embedded features is crucial during the fabrication of these devices. However, current techniques (e.g., but not limited to standard bright-field imaging) suffer from low contrast when imaging embedded features due to significant reflections from the top surface of the bonded sample and / or absorption within the sample. Therefore, there is a need to develop systems and methods to overcome these shortcomings. Summary of the Invention

[0003] In embodiments, the technology described herein relates to a dark-field imaging system comprising: an imaging subsystem including an objective lens and a detector, the imaging subsystem being configured to image an embedded target within a volume of a sample; an illumination subsystem including one or more illumination sources providing light having a first spatial profile; and two or more lens assemblies configured to guide the light from the one or more illumination sources as two or more illumination beams along an optical path outside the objective lens to the embedded target, wherein the two or more illumination beams completely overlap at the embedded target to provide a common illumination field, and wherein the two or more lens assemblies constrain the two or more illumination beams. The numerical aperture prevents specular reflections of the two or more illumination beams into the objective lens, wherein a respective lens assembly in the two or more lens assemblies includes an equalizer to give a second spatial profile to the respective illumination beam in the two or more illumination beams, the second spatial profile having a higher spatial uniformity than the first spatial profile according to a selected metric; and a controller communicatively coupled to the detector, the controller including one or more processors configured to execute program instructions that cause the one or more processors to receive one or more images of the embedded target from the detector; and to generate one or more measurements of the sample based on the one or more images.

[0004] In an embodiment, the technology described herein relates to a dark-field imaging system, wherein a corresponding illumination source among the one or more illumination sources comprises a bare light-emitting diode (LED), a corresponding lens assembly among the two or more lens assemblies comprises a field stop, wherein the equalizer in the corresponding lens assembly provides a second spatial profile at the field stop; and a projection lens for imaging the field stop onto the embedded target to provide a corresponding illumination beam among the two or more illumination beams.

[0005] In an embodiment, the technology described herein relates to a dark-field imaging system, wherein the homogenizer in the respective lens assembly comprises a waveguide.

[0006] In the embodiments described herein, the technology relates to a dark-field imaging system, wherein the waveguide comprises a sealed waveguide.

[0007] In the embodiments described herein, the technology relates to a dark-field imaging system, wherein the waveguide comprises a multimode waveguide.

[0008] In an embodiment, the technology described herein relates to a dark-field imaging system, wherein the field stop includes an aperture.

[0009] In an embodiment, the technology described herein relates to a dark-field imaging system, wherein the field stop is positioned at the output surface of the homogenizer.

[0010] In an embodiment, the technology described herein relates to a dark-field imaging system, wherein the output surface of the field stop and the homogenizer is tilted to satisfy the Scheimpflug condition, wherein the projection lens images the field stop onto the embedded target to have an image plane coplanar with the embedded target.

[0011] In an embodiment, the technology described herein relates to a dark-field imaging system, wherein the respective lens assembly further includes an aperture stop located at the back focal plane of the projection lens, wherein the respective illumination beam is telecentric to provide a uniform angle of incidence across the common illumination field.

[0012] In an embodiment, the technology described herein relates to a dark-field imaging system, wherein one or more illumination sources provide spatially coherent light, and wherein the illumination subsystem includes two or more optical fibers to couple the spatially coherent light to the two or more lens assemblies.

[0013] In the embodiments described herein, the technology relates to a dark-field imaging system, wherein at least one of the one or more illumination sources comprises a superluminescent diode.

[0014] In the embodiments described herein, the technology relates to a dark-field imaging system, wherein at least one of the two or more optical fibers is a multimode optical fiber.

[0015] In the embodiments described herein, the technology relates to a dark-field imaging system, wherein the multimode fiber has at least one of a rectangular, square, or hexagonal core.

[0016] In an embodiment, the technology described herein relates to a dark-field imaging system, wherein a respective lens assembly of the two or more lens assemblies comprises: a collimating lens configured to collect light from an associated fiber of the two or more optical fibers; and two Powell lenses, wherein the vertices are rotated 90 degrees relative to each other.

[0017] In an embodiment, the technology described herein relates to a dark-field imaging system, wherein the respective lens assembly further includes an additional collimating lens following the two Powell lenses.

[0018] In an embodiment, the technology described herein relates to a dark-field imaging system, wherein the respective lens assembly is further included in a field stop behind the two Powell lenses.

[0019] In an embodiment, the technology described herein relates to a dark-field imaging system, wherein one or more processors are further configured to execute program instructions that cause the one or more processors to receive a dark reference image in the absence of the sample; receive a bright reference image of a plane of the sample at the depth of the embedded target, wherein the embedded target is outside the frame of the bright reference image; and generate a natural uniformity calibration (NUC) reference image based on the bright reference image and the dark reference image.

[0020] In an embodiment, the technology described herein relates to a dark-field imaging system, wherein generating one or more measurements of the sample based on the one or more images comprises subtracting the NUC reference image from the one or more images to form the one or more calibrated images; and generating the one or more measurements based on the one or more calibrated images.

[0021] In an embodiment, the technology described herein relates to a dark-field imaging system, wherein the selected metric includes at least one of the standard deviation or variance of the intensity across the common illumination field.

[0022] In the embodiments described herein, the technology relates to a dark-field imaging system, wherein the one or more measurements include at least one of metrological measurements, inspection measurements, or alignment measurements.

[0023] In embodiments, the technology described herein relates to an illumination system comprising: one or more illumination sources providing temporally incoherent light having a first spatial profile; and two or more lens assemblies configured to guide the light from the one or more illumination sources as two or more illumination beams along an optical path outside an objective of an imaging subsystem to an embedded target, wherein the two or more illumination beams completely overlap at the embedded target to provide a common illumination field, wherein the two or more lens assemblies limit the numerical aperture of the two or more illumination beams to prevent specular reflection of the two or more illumination beams into the objective, wherein a respective lens assembly of the two or more lens assemblies includes a homogenizer such that a respective illumination beam of the two or more illumination beams has a second spatial profile, the second spatial profile having a higher spatial uniformity than the first spatial profile, according to a selected metric.

[0024] In embodiments, the techniques described herein relate to a lighting system in which the selected metric includes at least one of the standard deviation or variance of the intensity across the common lighting field.

[0025] In an embodiment, the technology described herein relates to a dark-field imaging method comprising illuminating an embedded target within a volume of a sample with two or more illumination beams using an illumination subsystem, wherein the illumination subsystem comprises: one or more illumination sources providing temporally incoherent light having a first spatial profile; and two or more lens assemblies configured to guide the light from the one or more illumination sources as two or more illumination beams along an optical path outside an objective of an imaging subsystem comprising an objective and a detector to the embedded target, wherein the two or more illumination beams completely overlap at the embedded target to provide a common illumination field, wherein the two or more lens assemblies limit the numerical aperture of the two or more illumination beams to prevent specular reflection of the two or more illumination beams into the objective, wherein a respective lens assembly of the two or more lens assemblies includes an equalizer such that a respective illumination beam of the two or more illumination beams has a second spatial profile, the second spatial profile having a higher spatial uniformity than the first spatial profile according to a selected metric; generating one or more images of the embedded target from the detector; and generating one or more measurements of the sample based on the one or more images.

[0026] In an embodiment, the technique described herein relates to a dark-field imaging method, which further includes generating a dark reference image in the absence of the sample; generating a bright reference image of a plane of the sample at the depth of the embedded target, wherein the embedded target is outside the frame of the bright reference image; and generating a natural uniformity calibration (NUC) reference image based on the bright reference image and the dark reference image.

[0027] In an embodiment, the technology described herein relates to a dark-field imaging method, wherein generating one or more measurements of the sample based on the one or more images comprises subtracting the NUC reference image from the one or more images to form one or more calibrated images; and generating the one or more measurements based on the one or more calibrated images.

[0028] In the embodiments described herein, the technology relates to a dark-field imaging method, wherein the one or more measurements include at least one of metrological measurements, inspection measurements, or alignment measurements.

[0029] It should be understood that both the foregoing general description and the following detailed description are exemplary and illustrative only and do not necessarily limit the claimed invention. 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. Attached Figure Description

[0030] Those skilled in the art can better understand the many advantages of this disclosure by referring to the accompanying drawings.

[0031] Figure 1A This is a conceptual diagram of a measurement system according to one or more embodiments of the present disclosure.

[0032] Figure 1B This is a simplified schematic diagram of a measurement system according to one or more embodiments of the present disclosure.

[0033] Figure 2A This is a side view of a mating sample according to one or more embodiments of the present disclosure.

[0034] Figure 2B This is a top view of a first design of an embedded target according to one or more embodiments of the present disclosure.

[0035] Figure 2C This is a top view of a second design of an embedded target according to one or more embodiments of the present disclosure.

[0036] Figure 3 This is a simplified schematic diagram of conventional ring lighting according to one or more embodiments of the present disclosure.

[0037] Figure 4AThis is a conceptual diagram of a first design of a lighting subsystem according to one or more embodiments of the present disclosure.

[0038] Figure 4B This is a conceptual diagram of a second design of a lighting subsystem according to one or more embodiments of the present disclosure.

[0039] Figure 4C This is a conceptual diagram of a third design of a lighting subsystem according to one or more embodiments of the present disclosure.

[0040] Figure 5 This is a conceptual diagram of a fourth design of a lighting subsystem according to one or more embodiments of the present disclosure.

[0041] Figure 6 This is a flowchart illustrating the steps performed in a method 600 for imaging an embedded target 106 according to one or more embodiments of the present disclosure.

[0042] Figure 7 This is a flowchart illustrating the steps performed in a method for calibrating an image for embedding a target according to one or more embodiments of the present disclosure.

[0043] Figure 8 This is a simplified schematic diagram of a sample of an embedded target used as a high-resolution reference point in a high-resolution alignment process, according to one or more embodiments of the present disclosure. Detailed Implementation

[0044] The disclosed subject matter will now be described in detail with reference to the accompanying drawings. This disclosure has been specifically shown and described with respect to certain embodiments and particular features. The embodiments set forth herein are to be considered illustrative rather than restrictive. Those skilled in the art will readily understand that various changes and modifications in form and detail may be made without departing from the spirit and scope of this disclosure.

[0045] Embodiments of this disclosure relate to systems and methods for providing high-contrast imaging of embedded features in a sample. With careful consideration, high-contrast imaging of embedded features can be challenging due to specular reflections from the top surface of the sample and / or light absorption by material above the feature of interest (e.g., between the feature of interest and the imaging device).

[0046] In one embodiment, the measuring tool includes an imaging device configured to image embedded features in a sample and an illumination device providing multi-directional tilted illumination (e.g., ring illumination, orthogonal illumination, dipole illumination, or the like), having a controlled angular profile adjusted to prevent specular reflections collected by the imaging device. In this way, high-contrast imaging of the embedded features can be achieved.

[0047] There is a growing desire to image features within a sample volume with the top surface of the sample polished and / or the features of interest embedded beneath an absorbing material layer. By way of non-limiting illustration, many modern semiconductor manufacturing systems utilize bonding sample techniques, in which multiple components (e.g., wafers and / or dies) are bonded together. For example, a sample may be formed from two bonded wafers (wafer-to-wafer (W2W)), two bonded dies (die-to-die (D2D)), or a die bonded to a wafer (D2W).

[0048] As an illustration, it is generally desirable to image targets located at or near the interface between these bonded components. For example, the bonded sample may contain metrological targets (e.g., but not limited to stacking targets) to facilitate registration measurements between the bonded components. As another example, the bonded sample may contain alignment targets to facilitate spatial navigation across the sample (e.g., to facilitate precise navigation to a metrological target used for measurement). In either case, these targets are typically embedded beneath a large amount of semiconductor material (e.g., up to hundreds of micrometers thick), typically doped and polished on one side. For example, the top surface of the bonded sample may correspond to the back surface of a bare die or wafer. Furthermore, the doped semiconductor typically absorbs light used in the imaging system.

[0049] Therefore, the use of typical imaging systems can severely limit image contrast. For example, typical through-lens (TTL) bright-field imaging (where a common objective is used for both illumination and collection) suffers from low contrast due to significant specular reflections from the top surface of the sample, which can overwhelm the light from the embedded feature of interest. As another example, dark-field TTL imaging avoids specular reflections but suffers from a limited collection numerical aperture (NA) because most of the NA is used by the illumination beam and therefore unavailable for collection. As yet another example, out-of-lens (OTL) illumination using typical LED illumination (e.g., a typical LED ring illuminator) (where different optics are used for both illumination and collection) suffers from poor angular uniformity and / or some specular reflections, especially when used with non-telecentric imaging systems.

[0050] This paper proposes using multi-directional out-of-lens (OTL) illumination for high-contrast imaging of embedded features, where the angular distribution is selected to prevent specular reflections from the top surface of the sample. Furthermore, high image uniformity is achieved when the illumination has a consistent angular distribution across the illumination field, ensuring that the relative contributions of illumination from different directions are consistent across the field. In this way, the illumination density across the illumination field can be increased to improve contrast (e.g., by improving the signal-to-noise ratio associated with the feature of interest) while avoiding the negative effects of specular reflections. Even with minor process variations (e.g., variations in sample height during imaging and / or variations in sample properties attributable to manufacturing process variations), a uniform angular distribution of light across the illumination field can beneficially promote robust, high-contrast imaging of embedded features without artifacts.

[0051] For reference Figures 1A to 8 The present disclosure provides in more detail a system and method for providing high-contrast imaging of embedded features in a sample based on multidirectional illumination with a consistent angular light distribution across an illumination field, according to one or more embodiments of the present disclosure.

[0052] Figure 1A This is a conceptual diagram of a measurement system 100 according to one or more embodiments of the present disclosure.

[0053] In one embodiment, the measurement system 100 includes an imaging subsystem 102 having a detector 104 configured to image at least one embedded target 106 within the volume of a sample 108, and further includes an illumination subsystem 110 configured to illuminate the embedded target 106 using a plurality of illumination beams 112 having a controlled angular distribution adjusted to prevent specular reflections from entering the imaging subsystem 102. In this manner, the measurement system 100 can provide a dark-field image of the embedded target 106 based on diffracted or scattered light from the embedded target 106, referred herein as collection light 114.

[0054] This paper, after careful consideration, suggests that the image contrast of the embedded target 106 can be achieved through strict control of the properties of the illumination beam 112, including but not limited to the size of the illumination field, the location of the illumination field, the numerical aperture (NA) of the illumination field, the spatial uniformity of all illumination beams 112 across the illumination field, and the angular uniformity of all illumination beams 112 across the illumination field.

[0055] Specifically, strict control over the illumination NA enables illumination at an angle that prevents the imaging subsystem 102 from capturing specular reflections. Strict control over the size and location of the illumination field further provides high illumination density and efficient utilization of the light provided by the illumination subsystem 110. Therefore, in this embodiment, the illumination subsystem 110 can guide the illumination beam 112 into a common illumination field within the volume of the sample 108 to provide high illumination density.

[0056] Upon further careful consideration, this paper proposes that uniform image contrast across the image can be achieved by providing multi-directional illumination with a consistent angular distribution along the illumination field, such that the ratio of the intensities from the various illumination beams 112 remains constant across the illumination field. Specifically, this configuration provides a consistent response from the embedded target 106, regardless of the orientation or design of the embedded target 106. In this embodiment, the consistent angular distribution across the illumination field is achieved by providing a uniform illumination distribution from each illumination beam 112 across the illumination field.

[0057] In another embodiment, the measurement system 100 includes a controller 116 communicatively coupled to the measurement system 100. The controller 116 may be configured to guide the imaging subsystem 102 to generate dark-field images based on one or more selected schemes. The controller 116 may be further configured to receive data from the imaging subsystem 102, including, but not limited to, dark-field images. Additionally, the controller 116 may be configured to determine the overlay associated with the overlay target based on the acquired dark-field images.

[0058] In another embodiment, controller 116 includes one or more processors 118. For example, one or more processors 118 may be configured to execute a set of program instructions held in memory 120 or a memory device. The one or more processors 118 of controller 116 may include any processing element known in the art. In this sense, one or more processors 118 may include any microprocessor type device configured to execute algorithms and / or instructions. Furthermore, memory 120 may include any storage medium known in the art suitable for storing program instructions executable by the associated one or more processors 118. For example, memory 120 may include non-transitory memory media. As additional examples, memory 120 may include, but is not limited to, read-only memory, random access memory, magnetic or optical memory devices (e.g., magnetic disks), magnetic tape, solid-state drives, and the like. It should be further noted that memory 120 may be housed together with one or more processors 118 in a common controller housing.

[0059] Figures 2A to 2C Various non-limiting examples of embedded targets 106 suitable for imaging by measurement system 100 are described.

[0060] Sample 108 may comprise any type of object or material having embedded target 106. In some embodiments, sample 108 comprises one or more substrates formed of semiconductor or non-semiconductor materials (e.g., wafers or the like). For example, semiconductor or non-semiconductor materials may include, but are not limited to, single-crystal silicon, gallium arsenide, and indium phosphide. Sample 108 may comprise one or more layers. For example, these layers may include, but are not limited to, photoresists, dielectric materials, conductive materials, and semiconductor materials. Many different types of these layers are known in the art, and the term sample 108 (as used herein) is intended to cover any object on which all types of these layers may be formed. The one or more layers formed on sample 108 may be patterned or unpatterned. For example, sample 108 may comprise multiple dies, each having repeatable patterned features. The formation and processing of these material layers may ultimately form a finished device. Many different types of devices may be formed on sample 108, and the term sample 108 (as used herein) is intended to cover any object on which any type of device known in the art may be manufactured.

[0061] The embedded target 106 can be located at any depth in the sample 108. In some embodiments, the embedded target 106 is located beneath one or more sample layers disposed on a substrate. In some embodiments, the sample 108 is a bonded sample formed from two or more bonded substrates. In this configuration, the embedded target 106 may include features on a layer associated with any of the substrates.

[0062] Figure 2A This is a side view of a mating sample 108 according to one or more embodiments of this disclosure. Specifically, Figure 2A A first substrate 202 having one or more first substrate layers 204 and a second substrate 206 having one or more second substrate layers 208 are depicted, as well as an interface 210 between the bonding portions. Figure 2A The patterned first substrate feature 212 in the first substrate layer 204 and the second substrate feature 214 in the second substrate layer 208 are further depicted. Figure 2A The bonding sample 108 in the text can typically correspond to a W2W sample 108, a D2D sample 108, or a D2W sample 108. However, it should be understood that... Figure 2A Provided for illustrative purposes only and should not be construed as limiting. Specifically, the embedded target 106 may include any number of features on any layer of the sample 108 or more generally at any location within the volume of the sample 108. Furthermore, the sample 108 may contain any number of intermediate layers between any of the first substrate feature 212 and the second substrate feature 214 and / or between these features and the respective substrates.

[0063] In some embodiments, the embedded target 106 includes device features (e.g., patterned features that may be part of a device when fully formed). In some embodiments, the embedded target 106 includes dedicated features designed for imaging by the measurement system 100 for a specific purpose. For example, the embedded target 106 may include features forming an alignment target, which may be adapted to calibrate sample coordinates to chuck coordinates after bonding, to align the first substrate 202 and the second substrate 206 before bonding, or for any other suitable purpose. As another example, the embedded target 106 may include features forming a superposition target, which may be adapted for registration measurements between the first substrate feature 212 and the second substrate feature 214. For example, the superposition target can be used to determine the registration error between the bonded substrates. Figure 2A As an illustration, registration measurements (e.g., superposition measurements between a first substrate feature 212 on a first substrate layer 204 and a second substrate feature 214 on a second substrate layer 208) can be used to evaluate the bonding process. In another example, superposition targets can be used to determine the registration error between features on different first substrate layers 204 and / or between features on different second substrate layers 208.

[0064] Figures 2B to 2C A non-limiting description of the design of the embedded target 106 according to one or more embodiments of the present disclosure is provided.

[0065] Figure 2B This is a top view of a first design for an embedded target 106 according to one or more embodiments of this disclosure. Figure 2B In this design, the first substrate feature 212 includes a cross-shaped feature and the second substrate feature 214 includes a box-shaped feature. This design is particularly suitable, but not limited to, for use as an alignment target.

[0066] Figure 2C This is a top view of a second design for an embedded target 106 according to one or more embodiments of this disclosure. Figure 2B In this context, the first substrate feature 212 and the second substrate feature 214 each contain periodic features. It should be noted that although the first substrate feature 212 and the second substrate feature 214 are... Figure 2C The elements are depicted as having different periods, but this is illustrative rather than limiting. Furthermore, the number and orientation of any of the features are also illustrative rather than limiting. This design is particularly suitable, but not limited to, use as a superposition target. Additionally, Figure 2C The design described herein can be characterized as an advanced imaging metrology (AIM) target.

[0067] For reference Figure 1B and 3 Section 5 describes in more detail imaging using a highly controlled illumination beam 112 according to one or more embodiments of the present disclosure.

[0068] Figure 1B This is a simplified schematic diagram of a measurement system 100 according to one or more embodiments of the present disclosure.

[0069] In an embodiment, the imaging subsystem 102 includes various components configured to generate an image (e.g., a dark-field image) of the embedded target 106 on the detector 104. For example, the imaging subsystem 102 may include an objective lens 122 to collect collected light 114. The imaging subsystem 102 may further include one or more imaging lenses 124 to manipulate the collected light 114 and provide an image of the embedded target 106 on the detector 104. As an illustration, although not explicitly shown, the imaging lens 124 may relay one or more field planes and / or pupil planes. The imaging subsystem 102 may further include one or more imaging optics 126 to manipulate the collected light 114, such as, but not limited to, one or more filters, one or more polarizers, one or more beam blockers, or one or more beam splitters. The imaging optics 126 may be positioned at any suitable location, including, but not limited to, the field plane and / or pupil plane. Imaging lens 124 and imaging optics 126 can work together to manipulate the collected light 114 to provide the desired dark field image of the embedded target 106.

[0070] The illumination subsystem 110 may include any number or type of illumination sources 128 to generate light associated with one or more illumination beams 112. In embodiments, illumination sources 128 may include one or more LEDs that provide time- and spatially incoherent light. In embodiments, illumination sources 128 may include superluminescent diodes (SLDs) that provide time-incoherent but spatially coherent light. In embodiments, illumination sources 128 may include a laser-supplied plasma (LSP) source. For example, illumination source 128 may include, but is not limited to, LSP lamps, LSP bulbs, or LSP chambers suitable for housing one or more elements that can emit broadband illumination when excited into a plasma state by a laser source. In embodiments, illumination source 128 may include a lamp source. For example, illumination source 128 may include, but is not limited to, arc lamps, discharge lamps, electrodeless lamps, or the like.

[0071] The illumination beam 112 may contain any desired spectral content. For example, the illumination beam 112 may contain wavelengths within any spectral range, such as, but not limited to, the ultraviolet spectral range, the visible spectral range, the infrared (IR) spectral range, the short-wave infrared (SWIR) spectral range, the near-IR spectral range, the mid-IR spectral range, or the far-IR spectral range. Furthermore, the illumination beam 112 may have any bandwidth. In this way, the illumination beam 112 may be characterized as white light, broadband light, supercontinuum spectral light, or narrowband light.

[0072] In some embodiments, the illumination beam 112 provided by the illumination subsystem 110 is temporally and / or spatially incoherent, which avoids the appearance of undesired speckle patterns in the resulting image. It should be appreciated herein that the coherence of light can be characterized by properties at temporal or spatial scales beyond which interference is generally considered negligible. In this way, the term incoherent light may encompass properties at temporal and / or spatial scales selected to prevent interference with the sample 108 and / or detector 104 (at least within selected tolerances).

[0073] In one embodiment, the measurement system 100 includes a translation stage 130 for fixing and positioning the embedded target 106 relative to the illumination subsystem 110 and / or the imaging subsystem 102. The translation stage 130 may include any number or type of actuators, including, but not limited to, linear actuators (e.g., triaxial linear actuators), rotary actuators, or skew / tilt actuators.

[0074] In one embodiment, the illumination subsystem 110 includes two or more lens assemblies 132 to form two or more illumination beams 112 and to direct the two or more illumination beams 112 into a common illumination field. In other words, the lens assemblies 132 allow the illumination beams 112 to completely overlap at the location of the embedded target 106 within the volume of the sample 108.

[0075] In an embodiment, each of the lens assemblies 132 includes a homogenizer 134 to homogenize the spatial distribution of the corresponding illumination beam 112. For example, the homogenizer 134 may receive light from an illumination source 128 having a first spatial distribution and manipulate the light to provide an illumination beam 112 having a second spatial distribution different from the first spatial distribution, wherein the second spatial distribution has a higher spatial uniformity than the first spatial distribution according to a selected metric. Any suitable metric may be used, including but not limited to standard deviation, variance, or the like.

[0076] In an embodiment, the imaging subsystem 102 provides two or more illumination beams 112 that have a common polarity incident angle (e.g., measured from the surface normal of the sample 108) but different azimuth incident angles. For example, the illumination subsystem 110 can provide illumination beams 112 with a dipole, quadrupole, or annular distribution, and has any number of illumination beams 112. As an illustration, Figure 1B Two illumination sources 128 are depicted providing symmetrical illumination beams 112 with opposite incident angles and common polarity incident angles; however, this does not limit the number or orientation of the illumination beams 112.

[0077] Furthermore, each of the lens assemblies 132 can limit the numerical aperture of the corresponding illumination beam 112. For example, each of the lens assemblies 132 can limit the numerical aperture of the corresponding illumination beam 112 such that specular reflections of the corresponding illumination beam 112 do not enter the objective lens 122 of the imaging subsystem 102. In this way, specular reflections from the top surface of the sample 108 and / or from the embedded target 106 can be prevented. In some embodiments, the lens assemblies 132 are configured such that the angular illumination profile is constant across the illumination field within a selected tolerance (e.g., standard deviation, variance, or the like). For example, the lens assemblies 132 can make the relative contribution of light, which varies with angle, uniform across the illumination field. In other words, for each location in the illumination field, the ratio of the intensities of two or more illumination beams 112 is constant. This uniform angular distribution of light across the illumination field can mitigate blurring or angle-induced variations in the image of the embedded target 106 produced by the imaging subsystem 102, as carefully considered herein.

[0078] Figures 3 to 5 Various dark-field imaging configurations according to one or more embodiments of the present disclosure are described.

[0079] Dark-field imaging relies on light deflected by mechanisms such as, but not limited to, scattering or diffraction by the features of interest. In an ideal dark-field imaging system, contrast can typically be improved by increasing the illumination intensity or flux across the illumination field. For example, the illumination intensity or flux across the illumination field, and thus the image contrast, can be increased by increasing the output power of the illumination beam 112, reducing the size of the illumination field for a given power level, and / or ensuring high overlap between the illumination fields of the various illumination beams 112. In some applications, it may be further desirable to provide illumination with a consistent angular distribution across the illumination field, where the relative contributions of the various illumination beams 112 are consistent across the illumination field. This configuration can advantageously avoid image inhomogeneities between features of the embedded target 106 with different orientations.

[0080] However, practical systems can suffer from unwanted signals that may reduce image contrast and / or introduce image inhomogeneities. As an illustration, Figure 3 This is a simplified schematic diagram of conventional ring lighting according to one or more embodiments of the present disclosure. Figure 3 In this configuration, the ring of LED 302 is placed around the objective lens 304 and projects light 306 onto the sample 308. For example... Figure 3 As depicted, a typical LED illuminator can provide highly divergent light 306 with a relatively large angular distribution, which can result in a relatively large illumination field 310 on sample 308. However, this configuration may cause some specular reflections to be collected from the top surface 312 of sample 108. For example, Figure 3 Rays 314 depicting light 306 collected by objective lens 304 and associated with specular reflection from the top surface 312 of sample 308 can negatively impact the image contrast of embedded features, especially when using a non-telecentric imaging configuration.

[0081] Despite Figure 3 Not explicitly shown, but a typical LED 302 within a typical ring illuminator can also provide a highly non-uniform intensity (e.g., a non-uniform illumination field) of light 306 across sample 308. For example, light from a typical LED 302 may have a Gaussian profile on sample 308. In some cases, a typical ring illuminator may attempt to mitigate this field non-uniformity from each LED 302 by arranging the illumination fields of each LED 302 to only partially overlap in a pattern providing a relatively uniform total intensity over at least a portion of sample 308. However, for certain applications (e.g., but not limited to alignment or metrology), this configuration may be undesirable. Specifically, poor angular uniformity of illumination across sample 308 (e.g., variations in the relative intensity of individual LEDs 302 from different angles) can lead to spatial variations in image contrast characteristics with different orientations.

[0082] Figures 4A to 5Non-limiting examples of an illumination subsystem 110 suitable for high-contrast imaging of an embedded target 106, according to one or more embodiments of the present disclosure, are described. Specifically, Figures 4A to 4C The image depicts a configuration incorporating a bare LED light source 128, wherein a lens assembly 132 is arranged to provide an illumination beam 112 with high spatial and angular uniformity. Figure 5 The configuration of the SLD, which is transmitted by optical fiber, and the lens assembly 132 arranged to provide a collimated illumination beam 112 is depicted.

[0083] Figure 4A This is a conceptual diagram of a first design of a lighting subsystem 110 according to one or more embodiments of the present disclosure.

[0084] In one embodiment, the measurement system 100 includes two or more illumination sources 128 configured to couple to a homogenizer 134 and a field stop 402 formed as an aperture. In this configuration, the homogenizer 134 in the lens assembly 132 provides a spatially uniform distribution of light within the aperture of the field stop 402 in the lens assembly 132. More specifically, the homogenizer 134 may receive a first spatial distribution of light and provide a second spatial distribution of light at the aperture of the field stop 402 that is more uniform than the first spatial distribution (e.g., based on a standard deviation metric, a variance metric, or any other suitable metric). The lens assembly 132 may then include a projection lens 404 configured to project (e.g., image) the light from the field stop 402 onto a plane embedded in the target 106 within the volume of the sample 108. This configuration, after careful consideration, limits the angular distribution of light within each illumination beam 112 (e.g., producing a controlled angular profile), which can be adjusted to prevent specular reflections into the imaging subsystem 102 and, more generally, promote high-contrast imaging.

[0085] Each equalizer 134 may include any component or combination of components suitable for equalizing the spatial distribution of light from the corresponding illumination source 128. In some embodiments, such as Figure 4AAs depicted herein, the homogenizer 134 includes a sealed waveguide. For example, the homogenizer 134 may include a glass structure (e.g., a glass cover) configured to receive light from the illumination source 128 and guide said light to a top surface 312. Furthermore, this sealed waveguide may include one or more coatings on the input and / or output surfaces to provide a desired level of power efficiency. In some embodiments, the homogenizer 134 includes a multimode optical fiber. This multimode optical fiber may have any core shape. For example, the multimode optical fiber may have a square, rectangular, hexagonal, or similar shape configured to facilitate spatial homogenization of the guided light. In some embodiments, the homogenizer 134 is provided by placing the field stop 402 at a selected distance from the illumination source 128. With careful consideration, this configuration provides some spatial homogenization of the light from the illumination source 128, but at the cost of some light loss. However, in some applications, this loss is acceptable.

[0086] The field stop 402 can have any size or configuration. In a general sense, the size of the field stop 402 (e.g., the size of the aperture forming the field stop 402) controls the size of the illumination field. Therefore, the field stops 402 associated with different lens assemblies 132 can all have a common size so that the corresponding illumination beams 112 all have a common illumination field.

[0087] In this embodiment, the field stop 402 is positioned at the output of the equalizer 134. For example, Figure 4A A configuration is depicted in which a field stop 402 is positioned at the output of a sealed waveguide homogenizer 134. Similarly, a field stop 402 may be positioned at the output of a homogenizer 134 formed as a multimode fiber.

[0088] The projection lens 404 may include one or more lenses (e.g., a single lens or a combination of lenses) or other focusing elements suitable for projecting (e.g., imaging) the field stop 402 as the illumination beam 112 onto the plane of the embedded target 106 within the volume of the sample 108. In this way, the light distribution associated with the illumination beam 112 at the illumination field (e.g., at the plane of the embedded target 106) may correspond to the light distribution at the field stop 402.

[0089] In one embodiment, the lens assembly 132 includes an aperture stop 406 to control the numerical aperture (e.g., the range of incident angles) of the associated illumination beam 112. For example, the aperture stop 406 may be formed as an aperture whose size controls the numerical aperture of the illumination beam 112. In some applications, the opening size of the aperture stop 406 may be selectively controlled to minimize specular reflections and / or unwanted scattering into the objective lens 122 of the imaging subsystem 102 to facilitate high-contrast imaging.

[0090] In some embodiments, the incident angle of the illumination beam 112 is adjustable. For example, the illumination subsystem 110 may include an angle control platform 136 to position the lens assembly 132 or a portion thereof. This angle control platform 136 may include a rotary actuator, a skew / tilt actuator, a linear actuator, or any combination thereof. As an illustration, Figure 4A An angle control platform 136 is depicted that is associated with each illumination source 128 and the associated lens assembly 132.

[0091] Figure 4B This is a conceptual diagram of a second design of an illumination subsystem 110 according to one or more embodiments of this disclosure. Except that the aperture stop 406 is positioned at the back focal plane of the projection lens 404 (e.g., at a distance f from the projection lens 404, where f is the focal length), Figure 4B Generally similar to Figure 4A This configuration provides telecentric illumination, in which associated illumination beams 112 illuminate all points of the illumination field at the same angle of incidence (e.g., the same tilt).

[0092] Figure 4C This is a conceptual diagram of a third design of the lighting subsystem 110 according to one or more embodiments of this disclosure. Except for the field stop 402, which is tilted according to the Schiemfleur condition to provide a corresponding tilted lighting field, Figure 4C Generally similar to Figure 4B .

[0093] For example, Figure 4A and 4B The illumination subsystem 110 described herein is configured such that both the field stop 402 and the projection lens 404 are oriented normally to the optical axis 138. In these configurations, the light distribution at the field stop 402 is projected onto the image plane in the sample 108 by the projection lens 404, which is also normal to the optical axis 138. However, since the optical axis 138 is tilted relative to the embedded target 106, the image of the field stop 402 may not be in focus across the entire illumination field (e.g., across the embedded target 106). Therefore, the light distribution across the embedded target 106 may exhibit non-uniformity, the degree of which may depend on factors such as, but not limited to, the depth of field provided by the projection lens 404 or the tilt angle of the optical axis 138 relative to the embedded target 106 (e.g., relative to the surface normal of the sample 108).

[0094] on the contrary, Figure 4C A configuration is depicted in which the image plane provided by the projection lens 404 is tilted to be coplanar with the embedded target 106. Therefore, the image of the field stop 402 is at focus across the entire illumination field (e.g., across the embedded target 106).

[0095] It is worth noting that when the field stop 402 is tilted at an angle θ iWhen the magnification M is imaged onto the embedded target 106, the tilt angle θ of the field stop 402... o The Schiemfle condition is satisfied when the following conditions are met:

[0096] (1)

[0097] For reference Figure 5 In some embodiments, the illumination subsystem 110 provides a collimated illumination beam 112. For example, an illumination source 128 providing spatially coherent (but possibly temporally incoherent) light can be collimated to provide an illumination beam 112 with extremely high spatial and angular uniformity. Any suitable spatially coherent illumination source 128 can be used, including but not limited to SLDs.

[0098] Figure 5 This is a conceptual diagram of a fourth design of a lighting subsystem 110 according to one or more embodiments of the present disclosure.

[0099] exist Figure 5 In this embodiment, the SLD high-power illumination source 128 is used in conjunction with a driver and a thermal controller to provide a stable short-wave infrared (SWIR) illumination beam 112. For example, the illumination beam 112 may have a center wavelength of approximately 1310 nm and a bandwidth of approximately 100 nm full width at half maximum (FWHM). Furthermore, the light from this illumination source 128 can be transmitted using optical fibers or free-space optics. As an illustration, Figure 5 A configuration is depicted in which a single SLD illumination source 128 is coupled to an optical fiber 502, wherein one or more beam splitters 504 split the light into multiple paths associated with a desired number of illumination beams 112. Any type of optical fiber 502 can be used, including, but not limited to, multimode or single-mode optical fibers 502. As another illustration, although not shown in the figures, the illumination subsystem 110 may include a separate illumination source 128 for each illumination beam 112.

[0100] In this embodiment, the lens assembly 132 operates as both a homogenizer 134 and a collimator to provide an illumination beam 112 with high spatial and angular uniformity. For example, Figure 5 A configuration is depicted in which each lens assembly 132 includes a collimating lens 506 (e.g., a fiber collimator) for collimating light from fiber 502, followed by a homogenizer 134 formed by two Powell lenses 508 whose vertices are rotated 90 degrees relative to each other. It has been carefully considered that the spatial profile of light exiting fiber 502 may depend on the pattern profile of the guided light. In some fiber designs, the spatial distribution may be Gaussian. Therefore, the homogenizer 134 (here depicted as a Powell lens) increases the spatial uniformity of the collimated light. Figure 5Further depiction is provided of an additional lens 510 to ensure collimation after the equalizer 134 and a field stop 512 to control the diameter of the illumination beam 112 and thus the size of the spot on the embedded target 106. This configuration, carefully considered, provides exceptionally high spatial and angular inhomogeneities to facilitate high-contrast imaging of the embedded target 106, as disclosed herein.

[0101] However, it should be understood that Figures 4A to 5 Provided for illustrative purposes only and should not be construed as limiting the scope of this disclosure. For example, the lighting subsystem 110 can produce any number of illumination beams 112 at any height or azimuth angle of incidence. In this way, Figures 4A to 5 The description of the two illumination beams 112 is merely illustrative. As another example, the components of the lens assembly 132 (e.g., but not limited to...) Figures 4A to 5 The homogenizer 134 depicted is also illustrative only. Any number or type of components can be used to provide an illumination beam 112 with spatial and angular uniformity that meets or exceeds selected metrics (e.g., standard deviation metric, variance, metric or similar).

[0102] For reference Figure 6 , Figure 6 This is a flowchart illustrating the steps performed in a method 600 for imaging an embedded target 106 according to one or more embodiments of this disclosure. The embodiments and enabling techniques previously described herein in the context of the measurement system 100 should be interpreted as extending to method 600. For example, the processor 118 of the controller 116 may execute program instructions that cause the processor 118 to perform any or all of the steps of method 600 directly or indirectly (e.g., via control signals to additional components of the measurement system 100). Method 600 is not limited to the architecture of the measurement system 100.

[0103] In one embodiment, method 600 includes step 602 of illuminating the embedded target 106 with an illumination subsystem 110 to provide two or more spatially uniform illumination beams 112 along an optical path outside the objective lens 122 of the imaging subsystem 102, wherein the illumination beams 112 completely overlap at the embedded target 106 to provide a common illumination field, and wherein the illumination subsystem 110 includes a lens assembly 132 to limit the numerical aperture of the illumination beams 112 to prevent specular reflections into the objective lens. In another embodiment, method 600 includes step 604 of generating one or more images of the embedded target 106 using the imaging subsystem 102. In yet another embodiment, method 600 includes step 606 of generating one or more measurements of the sample 108 based on one or more images.

[0104] For example, steps 602 and 604 of method 600 may (but are not required to) use references. Figures 1A to 5The measurement system 100 is implemented with an illumination subsystem 110 and an imaging subsystem 102. In this manner, it is connected with... Figures 1A to 5 The associated description can be extended to apply to method 600.

[0105] The measurement generated in step 606 can be of any type. In some embodiments, the measurement includes metrological measurements, such as, but not limited to, superposition measurements or critical dimension (CD) measurements. In some embodiments, the measurement includes inspection measurements, such as, but not limited to, defect identification and / or characterization. In some embodiments, the measurement includes alignment measurements. For example, in the case of the mating sample 108, the embedded target 106 may correspond to an alignment target at the interface of the mating components. In this configuration, the measurement of the embedded target 106 may indicate the alignment accuracy between the mating components before or after the mating process.

[0106] Conventional metrological instruments utilize low-NA bright-field epi-illumination microscopy configurations. However, this configuration suffers from low contrast due to strong specular reflection and absorption by dopants as previously described. With careful consideration, measurement system 100 is designed to be used as an AMS to provide high-contrast imaging, as described throughout this disclosure. Similarly, method 600 can be used for AMS calibration purposes of sample 108 and the chuck coordinate system (e.g., the coordinate system associated with translation stage 130). In this way, the high-contrast images associated with the systems and methods disclosed herein can advantageously achieve more accurate calibration than conventional systems.

[0107] In some embodiments, the image of the embedded target 106 is calibrated to further improve the contrast and / or visibility of the embedded target 106.

[0108] Figure 7 This is a flowchart illustrating the steps performed in a method 700 for calibrating an image of an embedded target 106 according to one or more embodiments of the present disclosure. For example, Figure 7 A Natural Uniformity Calibration (NUC) process can be described. The embodiments and enabling techniques previously described in the context of measurement system 100 should be interpreted as extending to method 700. For example, the processor 118 of controller 116 can execute program instructions that cause processor 118 to perform any or all steps of method 700 directly or indirectly (e.g., via control signals to additional components of measurement system 100). Method 700 is not limited to the architecture of measurement system 100.

[0109] In an embodiment, method 700 includes step 702 of generating a dark reference image without sample 108. For example, sample 108 may be removed from the measurement field of imaging subsystem 102 such that photons from sample 108 are not captured by objective lens 122.

[0110] In one embodiment, method 700 includes step 704 of generating a bright reference image of a portion of sample 108 at the depth of the embedded target 106, but wherein the embedded target 106 is not within the frame of the bright reference image. In this way, the bright reference image may correspond to a fixed parasitic pattern that may contain scattering or other parasitic signals from sample 108.

[0111] In one embodiment, method 700 includes step 706 of generating a NUC reference image based on bright and dark reference images. For example, the NUC reference image can be generated by subtracting the dark reference image from the bright reference image.

[0112] In an embodiment, method 700 includes a step 708 of calibrating an image of the embedded target 106 using a NUC reference image to form a calibrated image. For example, the NUC reference image can be subtracted from the image of the embedded target 106 (e.g., the image generated in step 604 of method 600). In this way, the contrast of the embedded target 106 can be further improved. Furthermore, this calibrated image can be used to generate measurements (e.g., alignment measurements, metrological measurements, inspection measurements, or the like). As an illustration, step 606 may include generating one or more measurements using the calibrated image of the embedded target 106.

[0113] As an illustration, markings forming the embedded target 106 can be printed between stacked wafers / dies at different locations on the bonding sample 108, for both stacking metrology and spatial navigation across wafers. To navigate between sites during runtime, an initial alignment process of the bonding sample 108 axis with the tool chuck axis can be performed. Subsequently, an alignment measurement system (AMS) can be used to image the embedded target 106 between the bonding components and further calibrate the sample and chuck coordinate systems. After calibration, a user-provided map of the sample 108 can be used to navigate to the desired location on the sample 108.

[0114] For example, method 600 may further include a step of calibrating sample coordinates associated with one or more images of one or more embedded targets 106 to chuck coordinates associated with a translation stage of a stationary sample 108 (not shown in the figure). In some cases, multiple embedded targets 106 may be visible within an image. In some cases, multiple embedded targets 106 across sample 108 are imaged individually.

[0115] In some embodiments, the embedded target 106 is used as a high-resolution reference point (HRRP) in a high-resolution alignment process that can be performed after AMS calibration.

[0116] For example, embedded targets 106 with a common design can be distributed across sample 108 (e.g., distributed in different fields on sample 108). The measurement system 100 then translates sample 108 to the intended locations of these embedded targets 106 (possibly after the first AMS calibration step described above) and captures images of the embedded targets 106 at these intended locations. Finally, a high-resolution calibration of sample coordinates and chuck coordinates can be generated based on the deviation of the imaging location of the embedded targets 106 from the intended location (e.g., the deviation from the center of the corresponding image).

[0117] Figure 8 This is a simplified schematic diagram of a sample 108 depicting an embedded target 106 used as a high-resolution reference point in a high-resolution alignment process, according to one or more embodiments of this disclosure. Specifically, Figure 8 A sample 108 is depicted having various fields 802 associated with the exposure field of the photolithography system. Furthermore, embedded targets 106 are distributed within selected fields 802, wherein the embedded targets 106 are marked as sites 0 to 5. In an embodiment, a high-resolution alignment process may include generating an image at the intended location of the embedded targets 106 and recording the deviation of the imaging position of the embedded targets 106 relative to the intended location (e.g., the image center). In this way, these deviations can be used to generate a high-resolution calibration for aligning the sample coordinates with the chuck coordinates. Furthermore, as... Figure 8 The various fields 802 depicted, including the embedded target 106, can be intentionally distributed to capture positioning errors associated with both large and small movements. For example, step 1 may correspond to a relatively short step in the vertical direction, step 2 may correspond to a medium step in the vertical direction, step 3 may correspond to a long step in the vertical direction, step 4 may correspond to a combination of horizontal and vertical movements, step 5 may correspond to a long step in the horizontal direction, and step 6 may correspond to a medium step in the horizontal direction. It should be understood that the pattern of the embedded target 106 is provided for illustrative purposes only and should not be construed as limiting. Specifically, the embedded target 106 can be distributed in any suitable pattern.

[0118] In some cases, the measurement system 100 includes at least two detectors 104 to perform both AMS and HRRP calibrations. For example, a first detector 104 with a first resolution can be used for AMS calibration, while a second detector 104 with a second resolution higher than the first resolution can be used for HRRP calibration. Furthermore, the embedded targets 106 used for AMS and HRRP calibrations may be the same or different. In some cases, the first measurement system 100 is configured to perform AMS calibration, while the second measurement system 100 is configured to perform HRRP calibration.

[0119] The topics described herein sometimes illustrate different components contained within or connected to other components. It should be understood that these depicted architectures are for illustrative purposes only, and many other architectures can in fact be implemented to achieve the same functionality. Conceptually, any arrangement of components that achieve the same functionality is effectively “associated” to achieve the desired functionality. Therefore, any two components combined in this document to achieve a particular functionality can be considered “associated” with each other to achieve the desired functionality, regardless of the architecture or intermediate components. Similarly, any two such associated components can also be considered “connected” or “coupled” with each other to achieve the desired functionality, and any two components that can be suchly associated can also be considered “coupleable” with each other to achieve the desired functionality. Specific instances of “coupleable” include, but are not limited to, physically interactive and / or physically interactive components and / or wirelessly interactive and / or logically interactive components.

[0120] It is believed that this disclosure and its many accompanying advantages will be understood from the foregoing description, and it should be understood that various changes can be made to the form, construction, and arrangement of the components without departing from this disclosure or sacrificing all its material advantages. The forms described are for illustrative purposes only, and the appended claims are intended to cover and encompass these changes. Furthermore, it should be understood that the invention is defined by the appended claims.

Claims

1. A dark-field imaging system, comprising: An imaging subsystem comprising an objective lens and a detector, the imaging subsystem being configured to image embedded targets within the volume of a sample; The lighting subsystem includes: One or more light sources that provide light with a first spatial profile; and Two or more lens assemblies configured to guide light from one or more illumination sources as two or more illumination beams along an optical path outside the objective lens to the embedded target, wherein the two or more illumination beams completely overlap at the embedded target to provide a common illumination field, wherein the two or more lens assemblies limit the numerical aperture of the two or more illumination beams to prevent specular reflection of the two or more illumination beams into the objective lens, wherein a respective lens assembly in the two or more lens assemblies includes a homogenizer such that a respective illumination beam in the two or more illumination beams has a second spatial profile, the second spatial profile having a higher spatial uniformity than a first spatial profile according to a selected metric; and A controller communicatively coupled to the detector, the controller comprising one or more processors configured to execute program instructions that cause the one or more processors to: Receive one or more images of the embedded target from the detector; and One or more measurements of the sample are generated based on the one or more images.

2. The dark-field imaging system of claim 1, wherein a corresponding illumination source among the one or more illumination sources comprises a bare light-emitting diode (LED), and a corresponding lens assembly among the two or more lens assemblies comprises: A field stop, wherein the homogenizer in the respective lens assembly provides the second spatial profile at the field stop; and A projection lens is used to image the field stop onto the embedded target to provide a corresponding illumination beam among the two or more illumination beams.

3. The dark-field imaging system of claim 2, wherein the homogenizer in the corresponding lens assembly comprises: waveguide.

4. The dark-field imaging system according to claim 3, wherein the waveguide comprises: Sealed waveguide.

5. The dark-field imaging system according to claim 3, wherein the waveguide comprises: Multimode waveguide.

6. The dark-field imaging system according to claim 2, wherein the field stop comprises: Aperture.

7. The dark field imaging system according to claim 2, wherein the field stop is positioned at the output surface of the homogenizer.

8. The dark field imaging system of claim 7, wherein the output surface of the field stop and the homogenizer is tilted to satisfy the Schiemfleur condition, wherein the projection lens images the field stop onto the embedded target to have an image plane coplanar with the embedded target.

9. The dark-field imaging system of claim 2, wherein the corresponding lens assembly further comprises: An aperture stop is located at the back focal plane of the projection lens, wherein the corresponding illumination beam is telecentric to provide a uniform angle of incidence across the common illumination field.

10. The dark-field imaging system of claim 1, wherein the one or more illumination sources provide spatially coherent light, wherein the illumination subsystem comprises two or more optical fibers to couple the spatially coherent light to the two or more lens assemblies.

11. The dark-field imaging system of claim 10, wherein at least one of the one or more illumination sources comprises: Superluminescent diode.

12. The dark-field imaging system of claim 10, wherein at least one of the two or more optical fibers is a multimode optical fiber.

13. The dark-field imaging system of claim 12, wherein the multimode fiber has at least one of a rectangular, square, or hexagonal core.

14. The dark-field imaging system of claim 10, wherein a respective lens assembly in the two or more lens assemblies comprises: A collimating lens configured to collect light from an associated fiber of the two or more optical fibers; and Two Powell lenses, with their vertices rotated 90 degrees relative to each other.

15. The dark-field imaging system of claim 14, wherein the respective lens assembly further includes an additional collimating lens following the two Powell lenses.

16. The dark-field imaging system of claim 14, wherein the respective lens assembly further includes a field stop following the two Powell lenses.

17. The dark-field imaging system of claim 1, wherein the one or more processors are further configured to execute program instructions that cause the one or more processors to: Receive a dark reference image in the absence of the sample; A bright reference image of the plane of the sample is received at the depth of the embedded target, wherein the embedded target is outside the frame of the bright reference image; and A natural uniformity calibration (NUC) reference image is generated based on the bright reference image and the dark reference image.

18. The dark-field imaging system of claim 17, wherein generating the one or more measurements of the sample based on the one or more images comprises: The NUC reference image is subtracted from the one or more images to form the one or more calibrated images; and The one or more measurements are generated based on the one or more calibrated images.

19. The dark-field imaging system of claim 1, wherein the selected metric includes: At least one of the standard deviation or variance of the intensity across the common illumination field.

20. The dark-field imaging system of claim 1, wherein the one or more measurements include: At least one of metrological measurement, inspection measurement, or alignment measurement.

21. A lighting system comprising: One or more lighting sources that provide temporally incoherent light with a first spatial profile; and Two or more lens assemblies configured to guide light from one or more illumination sources as two or more illumination beams along an optical path outside the objective of an imaging subsystem to an embedded target, wherein the two or more illumination beams completely overlap at the embedded target to provide a common illumination field, wherein the two or more lens assemblies limit the numerical aperture of the two or more illumination beams to prevent specular reflection of the two or more illumination beams into the objective, wherein a respective lens assembly in the two or more lens assemblies includes a homogenizer such that a respective illumination beam in the two or more illumination beams has a second spatial profile, which, according to a selected metric, has a higher spatial uniformity than a first spatial profile.

22. The lighting system of claim 21, wherein the selected metric includes: At least one of the standard deviation or variance of the intensity across the common illumination field.

23. A dark-field imaging method, comprising: An illumination subsystem is used to illuminate an embedded target within the volume of a sample using two or more illumination beams, wherein the illumination subsystem comprises: One or more illumination sources that provide temporally incoherent light with a first spatial profile; and Two or more lens assemblies configured to guide light from one or more illumination sources as two or more illumination beams along an optical path outside the objective of an imaging subsystem including an objective and a detector to the embedded target, wherein the two or more illumination beams completely overlap at the embedded target to provide a common illumination field, wherein the two or more lens assemblies limit the numerical aperture of the two or more illumination beams to prevent specular reflection of the two or more illumination beams into the objective, wherein a respective lens assembly in the two or more lens assemblies includes a homogenizer such that a respective illumination beam in the two or more illumination beams has a second spatial profile, the second spatial profile having a higher spatial uniformity than a first spatial profile according to a selected metric. One or more images of the embedded target are generated from the detector; and One or more measurements of the sample are generated based on the one or more images.

24. The dark-field imaging method according to claim 23, further comprising: A dark reference image is generated in the absence of the sample; A bright reference image of the plane of the sample is generated at the depth of the embedded target, wherein the embedded target is outside the frame of the bright reference image; and A natural uniformity calibration (NUC) reference image is generated based on the bright reference image and the dark reference image.

25. The dark-field imaging method of claim 24, wherein generating the one or more measurements of the sample based on the one or more images comprises: Subtract the NUC reference image from the one or more images to form one or more calibrated images; and The one or more measurements are generated based on the one or more calibrated images.

26. The dark-field imaging method of claim 23, wherein the one or more measurements include: At least one of metrological measurement, inspection measurement, or alignment measurement.

Images