Position deviation measurement using fringe moire and optical moire effects

Abstract

A metrology system and method are disclosed. The metrology system includes an illumination subsystem, a collection subsystem, a detector, and a controller. The controller is configured to receive an image of an overlay target on a sample, determine an apparent overlay between two working zones along a measurement direction based on the image, and calculate an overlay between two sample layers by dividing the apparent overlay by a Moiré gain to compensate for Moiré interference.

Description

[Technical field]

[0001] The present disclosure relates generally to imaging overlay metrology, and more particularly to imaging overlay targets having Moire elements. [Background technology]

[0002] Shrinking design rules and higher specification demands for overlay metrology have increased the demand for sensitivity and robustness of overlay metrology methods. Overlay metrology is typically performed by fabricating dedicated metrology targets with features fabricated in multiple sample layers of interest. Analysis of the fabricated metrology targets can thus provide a measure of the overlay error (e.g., relative alignment error) between the sample layers of interest. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] US Patent Application Publication No. 2019 / 0219930 [Patent Document 2] US Patent Application Publication No. 2007 / 0279630 Summary of the Invention [Problem to be solved by the invention]

[0004] Although a wide variety of overlay metrology target designs have been proposed, there is a continuing need to improve metrology target designs and measurement methods for accurately and efficiently analyzing fabricated metrology targets. [Means for solving the problem]

[0005] A metrology system is disclosed in accordance with one or more exemplary embodiments of the present disclosure. In one exemplary embodiment, the metrology system includes an imaging system. In another exemplary embodiment, the imaging system includes: (1) an illumination subsystem configured to illuminate an overlay target with at least one illumination lobe at an oblique illumination angle relative to the overlay target, the overlay target including at least a first working zone and a second working zone for overlay measurement along a measurement direction, the at least first working zone including a moiré pattern formed from overlapping periodic structures along the measurement direction on two sample layers, the periods of the periodic structures on the two sample layers being different; (2) a collection subsystem configured to collect a zeroth diffraction order and one moiré diffraction order associated with a diffraction of each of the at least one illumination lobe by the first working zone, the collection subsystem further configured to collect a zeroth diffraction order and one additional diffraction order associated with a diffraction of each of the at least one illumination lobe by the second working zone; and (3) a detector configured to generate an image of the overlay target using light collected by the collection subsystem. In another exemplary embodiment, the metrology system comprises a controller configured to be coupled to the imaging system, the controller including one or more processors configured to execute program instructions that cause the one or more processors to (a) receive an image of an overlay target on a sample, where an apparent periodicity of a first working zone in the image is based on Moiré interference, (b) determine an apparent overlay along a measurement direction between the first working zone and a second working zone based on the image, and (c) calculate an overlay between two sample layers by dividing the apparent overlay by a Moiré gain to compensate for the Moiré interference.

[0006] In another exemplary embodiment, the at least one illumination lobe includes two illumination lobes at two oblique illumination angles relative to the overlay target.

[0007] In another exemplary embodiment, the illumination subsystem simultaneously illuminates the overlay target with two illumination lobes.

[0008] In another exemplary embodiment, the illumination subsystem sequentially illuminates the overlay target with two illumination lobes.

[0009] In another exemplary embodiment, two illumination lobes are directed along the measurement direction.

[0010] In another exemplary embodiment, the two illumination lobes are oriented at a non-zero angle relative to the measurement direction.

[0011] In another exemplary embodiment, the collection subsystem generates a first image of the overlay target based on a zeroth order diffraction and one Moiré diffraction order associated with the diffraction of each of the at least one illumination lobe by the first working zone, where the collection subsystem generates a second image of the overlay target based on the zeroth order diffraction and one additional diffraction order associated with the diffraction of each of the at least one illumination lobe by the second working zone, where the controller is further configured to determine the apparent overlay by averaging apparent overlay associated with the first image and the second image.

[0012] In another exemplary embodiment, the at least one illumination lobe includes two illumination lobes at two oblique illumination angles relative to the overlay target.

[0013] In another exemplary embodiment, the illumination subsystem simultaneously illuminates the overlay target with two illumination lobes, where the collection subsystem simultaneously generates a first image and a second image by separating collected light associated with the first working zone from collected light associated with the second working zone.

[0014] In another exemplary embodiment, the illumination subsystem sequentially illuminates the overlay target with two illumination lobes.

[0015] In another exemplary embodiment, the moiré pattern of the first working zone is a first moiré pattern, where the second working zone includes a second moiré pattern formed from overlapping periodic structures along the measurement direction on the two sample layers, where the periods of the periodic structures on the two sample layers in the second working zone are different, and where the second moiré pattern is different from the first moiré pattern.

[0016] A metrology method is disclosed in accordance with one or more exemplary embodiments of the present disclosure. In one exemplary embodiment, the metrology method includes receiving an image of an overlay target on a sample based on illuminating the overlay target with at least one illumination lobe at an oblique illumination angle relative to the overlay target, the overlay target including at least a first working zone and a second working zone for overlay measurement along a measurement direction, where the at least the first working zone includes a Moiré pattern formed from overlapping periodic structures along the measurement direction on two sample layers, where the periodic structures on the two sample layers have different periods, the image is based on a zeroth diffraction order and one Moiré diffraction order associated with a diffraction of each of the at least one illumination lobe by the first working zone, the image is further based on a zeroth diffraction order and one additional diffraction order associated with a diffraction of each of the at least one illumination lobe by the second working zone, and the apparent period of the first working zone in the image is based on Moiré interference. In another exemplary embodiment, the metrology method includes determining an apparent overlay along a measurement direction between the first working zone and the second working zone based on the images. In another exemplary embodiment, the metrology method includes calculating the overlay between the two sample layers by dividing the apparent overlay by a Moiré gain to compensate for Moiré interference.

[0017] In another exemplary embodiment, the at least one illumination lobe includes two illumination lobes at two oblique illumination angles relative to the overlay target.

[0018] A metrology system is disclosed in accordance with one or more exemplary embodiments of the present disclosure. In one exemplary embodiment, the metrology system includes an imaging system. In another exemplary embodiment, the imaging system includes: (1) an illumination subsystem configured to illuminate an overlay target with at least one illumination lobe at an oblique illumination angle relative to the overlay target, the overlay target including at least a first working zone and a second working zone for overlay measurement along a measurement direction, the at least first working zone including a moiré pattern formed from overlapping periodic structures along the measurement direction on two sample layers, the two sample layers including the first sample layer and the second sample layer, the periods of the periodic structures on the two sample layers being different; (2) a collection subsystem configured to collect first order diffraction from each of the two sample layers associated with the first working zone, the collection subsystem further configured to collect first order diffraction from each of the two sample layers associated with the second working zone; and (3) a detector configured to generate an image of the overlay target using light collected by the collection subsystem. In another exemplary embodiment, a metrology system includes a controller configured to be coupled to the imaging system, the controller including one or more processors configured to execute program instructions that cause the one or more processors to (a) receive an image of an overlay target on the sample, where an apparent periodicity of a first working zone in the image is based on Moiré interference at a detector, (b) filter the image to isolate the Moiré interference, (c) determine an apparent overlay along a measurement direction between the first working zone and the second working zone based on the image, and (d) calculate an overlay between two sample layers by dividing the apparent overlay by a Moiré gain to compensate for the Moiré interference.

[0019] In another exemplary embodiment, the collection subsystem includes one or more blockers to block zero orders of diffraction from at least one of the first working zone or the second working zone.

[0020] In another exemplary embodiment, the numerical aperture of the collection subsystem is smaller than the numerical aperture of the illumination subsystem to prevent collection of zeroth diffraction orders from at least one of the first working zone or the second working zone.

[0021] In another exemplary embodiment, the at least one illumination lobe includes two illumination lobes at two oblique illumination angles relative to the overlay target.

[0022] In another exemplary embodiment, the illumination subsystem simultaneously illuminates the overlay target with two illumination lobes.

[0023] In another exemplary embodiment, the illumination subsystem sequentially illuminates the overlay target with two illumination lobes.

[0024] In another exemplary embodiment, two illumination lobes are directed along the measurement direction.

[0025] In another exemplary embodiment, the two illumination lobes are oriented at a non-zero angle relative to the measurement direction.

[0026] In another exemplary embodiment, the collection subsystem generates a first image of the overlay target based on first order diffraction from each of two sample layers associated with the first working zone, where the collection subsystem generates a second image of the overlay target based on first order diffraction from each of two sample layers associated with the second working zone, and where the controller is further configured to determine an apparent overlay by averaging the apparent overlay associated with the first image and the second image.

[0027] In another exemplary embodiment, the at least one illumination lobe includes two illumination lobes at two oblique illumination angles relative to the overlay target.

[0028] In another exemplary embodiment, the illumination subsystem simultaneously illuminates the overlay target with two illumination lobes, and the collection subsystem simultaneously generates a first image and a second image by separating the collected light associated with the first working zone from the collected light associated with the second working zone.

[0029] In another exemplary embodiment, the illumination subsystem sequentially illuminates the overlay target with two illumination lobes.

[0030] A metrology method is disclosed in accordance with one or more exemplary embodiments of the present disclosure. In one exemplary embodiment, the metrology method includes receiving an image of an overlay target on a sample based on illuminating the overlay target with at least one illumination lobe at an oblique illumination angle relative to the overlay target, the overlay target including at least a first working zone and a second working zone for overlay measurement along a measurement direction, where the at least the first working zone includes a Moiré pattern formed from overlapping periodic structures along the measurement direction on two sample layers, where the periodic structures on the two sample layers are different, the image is based on first order diffraction from each of the two sample layers associated with the first working zone, the image is further based on first order diffraction from each of the two sample layers associated with the second working zone, and an apparent period of the first working zone in the image is based on Moiré interference. In another exemplary embodiment, the metrology method includes determining an apparent overlay along the measurement direction between the first working zone and the second working zone based on the image. Another exemplary embodiment involves calculating the overlay between two sample layers by dividing the apparent overlay by the Moire gain to compensate for Moire interference.

[0031] In another exemplary embodiment, the at least one illumination lobe includes two illumination lobes at two oblique illumination angles relative to the overlay target.

[0032] 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 the 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]

[0033] Many advantages of the present disclosure may be better understood by those skilled in the art by reference to the following drawings. [Figure 1] FIG. 1 is a block diagram of a metrology system in accordance with one or more embodiments of the present disclosure. [Figure 2A] FIG. 13 is a top view block diagram of a metrology target having a moiré pattern based on grating-over-grating structures in multiple rotationally symmetric working zones in accordance with one or more embodiments of the present disclosure. [Figure 2B] FIG. 2B is a conceptual diagram of a multi-layer grating structure in the Moiré pattern of FIG. 2A in accordance with one or more embodiments of the present disclosure. [Figure 2C] FIG. 2C is a top view of a metrology target based on FIGS. 2A and 2B for measurements along two orthogonal directions in accordance with one or more embodiments of the present disclosure. [Diagram 3] 1 is a flow diagram illustrating a method for calculating overlay using double scattering Moire effects in accordance with one or more embodiments of the present disclosure. [Figure 4] FIG. 1 is a conceptual diagram illustrating a double scattering Moire effect, in accordance with one or more embodiments of the present disclosure. [Diagram 5] 1 illustrates a one-step, off-center configuration for sample illumination and collection using the double scattering Moiré effect, in accordance with one or more embodiments of the present disclosure. [Figure 6]1 illustrates a one-step, off-center configuration for sample illumination and collection using the double scattering Moiré effect, in accordance with one or more embodiments of the present disclosure. [Figure 7] 1 illustrates a one-step, off-center configuration for sample illumination and collection using the double scattering Moiré effect, in accordance with one or more embodiments of the present disclosure. [Figure 8A] 1 illustrates a two-step off-center configuration for sample illumination and collection using the double scattering Moiré effect, in accordance with one or more embodiments of the present disclosure. [Figure 8B] 1 illustrates a two-step off-center configuration for sample illumination and collection using the double scattering Moiré effect, in accordance with one or more embodiments of the present disclosure. [Figure 9A] 1 illustrates a two-step off-center configuration for sample illumination and collection using the double scattering Moiré effect, in accordance with one or more embodiments of the present disclosure. [Figure 9B] 1 illustrates a two-step off-center configuration for sample illumination and collection using the double scattering Moiré effect, in accordance with one or more embodiments of the present disclosure. [Figure 10] 1 is a flow diagram illustrating a method for calculating overlay using single-scattering light Moiré effects in accordance with one or more embodiments of the present disclosure. [Figure 11] FIG. 1 is a conceptual diagram illustrating a single-scattering light Moire effect, in accordance with one or more embodiments of the present disclosure. [Figure 12] 1 illustrates a one-step central setup for sample illumination and collection using the single-scattering light Moiré effect, in accordance with one or more embodiments of the present disclosure. [Figure 13A] 1 illustrates a two-step off-center configuration for sample illumination and collection using a single-scattering light Moiré effect, in accordance with one or more embodiments of the present disclosure. [Figure 13B] 1 illustrates a two-step off-center configuration for sample illumination and collection using a single-scattering light Moiré effect, in accordance with one or more embodiments of the present disclosure. [Figure 14A]1 illustrates a two-step off-center configuration for sample illumination and collection using a single-scattering light Moiré effect, in accordance with one or more embodiments of the present disclosure. [Figure 14B] 1 illustrates a two-step off-center configuration for sample illumination and collection using a single-scattering light Moiré effect, in accordance with one or more embodiments of the present disclosure. [Figure 15A] 1 illustrates a two-step off-center configuration for sample illumination and collection using a single-scattering light Moiré effect, in accordance with one or more embodiments of the present disclosure. [Figure 15B] 1 illustrates a two-step off-center configuration for sample illumination and collection using a single-scattering light Moiré effect, in accordance with one or more embodiments of the present disclosure. [Figure 16A] 1 illustrates a two-step off-center configuration for sample illumination and collection using a single-scattering light Moiré effect, in accordance with one or more embodiments of the present disclosure. [Figure 16B] 1 illustrates a two-step off-center configuration for sample illumination and collection using a single-scattering light Moiré effect, in accordance with one or more embodiments of the present disclosure. [Figure 17A] 1 illustrates a two-step off-center configuration for sample illumination and collection using a single-scattering light Moiré effect, in accordance with one or more embodiments of the present disclosure. [Figure 17B] 1 illustrates a two-step off-center configuration for sample illumination and collection using a single-scattering light Moiré effect, in accordance with one or more embodiments of the present disclosure. [Figure 18A] 1 illustrates a two-step off-center configuration for sample illumination and collection using a single-scattering light Moiré effect, in accordance with one or more embodiments of the present disclosure. [Figure 18B] 1 illustrates a two-step off-center configuration for sample illumination and collection using a single-scattering light Moiré effect, in accordance with one or more embodiments of the present disclosure. [Figure 19] 1 illustrates a one-step, off-center configuration for sample illumination and collection using a single-scattering light Moiré effect, in accordance with one or more embodiments of the present disclosure. [Figure 20]1 illustrates a one-step, off-center configuration for sample illumination and collection using a single-scattering light Moiré effect, in accordance with one or more embodiments of the present disclosure. [Figure 21] 1 illustrates a one-step, off-center configuration for sample illumination and collection using a single-scattering light Moiré effect, in accordance with one or more embodiments of the present disclosure. [Figure 22] 1 illustrates a one-step central setup for sample illumination and collection using the single-scattering light Moiré effect, in accordance with one or more embodiments of the present disclosure. [Figure 23A] 1 is a flow diagram illustrating steps performed in a method for measuring overlay in accordance with one or more embodiments of the present disclosure. [Figure 23B] 1 is a flow diagram illustrating a method for determining an axis of symmetry for a working group in accordance with one or more embodiments of the present disclosure. [Figure 24A] FIG. 2D is a top view of the metrology target of FIG. 2C with additional markings indicating application of a method in accordance with one or more embodiments of the present disclosure. [Figure 24B] FIG. 24B is a top view of FIG. 24A illustrating a case where the symmetry axes do not overlap and an overlay error exists, in accordance with one or more embodiments of the present disclosure. [Diagram 25] FIG. 13 is a top view of a metrology target including a first working zone including two instances of a first moiré pattern and a second working zone including two instances of a second moiré pattern, in accordance with one or more embodiments of the present disclosure. [Figure 26A] FIG. 1C is a top view of a metrology target including a first working zone that includes two instances of a first moiré pattern and a second working zone that includes a single instance of a second moiré pattern, in accordance with one or more embodiments of the present disclosure. [Figure 26B] FIG. 26B is a top view of the metrology target of FIG. 26A with additional markings indicating application of a method in accordance with one or more embodiments of the present disclosure. [Figure 27]FIG. 2 is a top view of a moiré rain box metrology target in accordance with one or more embodiments of the present disclosure, where a first working zone includes a single instance of a moiré pattern and a second working zone includes a non-periodic single layer structure surrounding the moiré pattern. [Figure 28A] FIG. 1 is a top view block diagram of a three-layer metrology target having a Moiré pattern based on a grating-over-grating structure across multiple layers in accordance with one or more embodiments of the present disclosure. [Figure 28B] FIG. 28B is a conceptual diagram of a multi-layer grating structure in the Moiré pattern of FIG. 28A in accordance with one or more embodiments of the present disclosure. [Figure 28C] FIG. 28C is a plan view of a metrology target based on FIGS. 28A and 28B for measurements along two orthogonal directions in accordance with one or more embodiments of the present disclosure. [Figure 29A] FIG. 2 is a top view block diagram of a triple moire metrology target in accordance with one or more embodiments of the present disclosure. [Figure 29B] FIG. 29B is a conceptual diagram of a multi-layer grating structure in the Moiré pattern of FIG. 29A in accordance with one or more embodiments of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0034] Reference will now be made in detail to the disclosed subject matter, which is illustrated in the accompanying drawings. The present disclosure has been particularly shown and described with respect to certain embodiments and certain features thereof. The embodiments described herein are considered to be illustrative and not limiting. As used herein, directional terms such as "left", "right", "top", "bottom", "up", "down", "upward", "downward", "downwards" and "downward" are intended to provide relative positions for illustrative purposes and are not intended to indicate an absolute frame of reference. It will be readily apparent to those skilled in the art that various changes and modifications in form and detail may be made without departing from the spirit and scope of the present disclosure.

[0035] Currently, various major techniques are used for overlay metrology, including imaging, scatterometry, and scanning electron microscopy (SEM). The main optical architecture used for imaging is centrally illuminated conventional imaging. There are several drawbacks to using conventional imaging techniques for overlay metrology, including (1) limited sensitivity due to the focus dependence of the imaging system, (2) amplitude inaccuracy depending on focus selection, and (3) numerical aperture (NA) of the objective lens, which limits the pitch of the grating in the measurement map.

[0036] The present disclosure relates to imaging overlay measurement techniques, which require placing a collection camera or another collection device in the field plane to capture an image of an overlay target on a sample. In particular, the present disclosure relates to measuring overlay using a moire grating overgrating effect. The present disclosure provides methods to measure overlay using a double scattering moire effect (e.g., collecting a zeroth diffraction order and at least one moire diffraction order) and a single scattering light moire effect (a moire effect that appears to the camera due to interference between single scattering effects, e.g., collecting a first diffraction order and filtering the image to separate the moire interference). The illumination and collection system of the present disclosure is advantageous in that (a) the moire effect is measurable, (b) other effects do not interfere with the measurement, (c) precision is maximized, (d) measurability is maximized, and (e) simplicity of design is emphasized.

[0037] For double scattering Moiré effects, designing the optical scheme so that only two diffraction orders interfere at any given time improves contrast and eliminates amplitude inaccuracies. Whenever the zeroth diffraction order is used, it can be replaced with a reference beam and an interferometric solution can be generated (with its problems, such as instability, and its benefits, such as intensity control) instead of regular imaging. For single scattering Moiré effects, eliminating the zeroth order and Moiré fringes improves contrast and eliminates amplitude inaccuracies.

[0038] FIG. 1 illustrates a metrology system configured to generate an image of a sample. FIGS. 2A-2C illustrate metrology of an overlay target having a moiré pattern based on a grating-over-grating structure in one or more rotationally symmetric working zones. FIGS. 3-9B illustrate calculating the overlay of a sample using a double scattering moiré effect. FIGS. 10-22 illustrate calculating the overlay of a sample using a single scattering light moiré effect. FIGS. 23A-23B are flow diagrams illustrating a method of calculating overlay by exploiting the symmetry of the working zones. FIGS. 24A-26B illustrate an overlay using a metrology target having a moiré pattern in two working zones. FIGS. 27-28C illustrate an overlay using a metrology target having a moiré pattern in one working zone and a single layer structure (e.g., not a grating-over-grating structure) in a second working zone. FIGS. 29A and 29B illustrate an overlay using a metrology target comprising three or more layers having a moiré pattern in two working zones, with one layer serving as a reference layer.

[0039] A moiré pattern on a metrology target may include a grating-over-grating structure formed from grating structures on different sample layers having different periodicities. For purposes of this disclosure, a "grating" or "grating structure" may describe any periodic structure or series of elements, including but not limited to line / space patterns. An image of a moiré pattern includes moiré fringes at a moiré pitch, which is related to the spatially varying overlap between the gratings on the different sample layers. Typically, the moiré pitch is a longer pitch than any of the grating structures and is related to the difference between the pitches of the grating structures. For example, the moiré pitch (p M )teeth, p M =(p1 p2) / (p1-p2) (1) where p1 is the period of the first grating structure on the first layer and p2 is the period of the second grating structure on the second layer.

[0040] It is believed herein that metrology targets including moiré patterns can facilitate sensitive overlay measurements. In particular, a physical shift of one grating relative to another grating in the moiré pattern along a periodic direction (e.g., an overlay error associated with a relative shift of two sample layers on a sample) results in a corresponding lateral shift of the moiré fringes along the measurement direction. Moreover, the magnitude of the shift of the moiré fringes is typically larger than the magnitude of the physical shift. In particular, the magnitude of the shift of the moiré fringes is proportional to the physical shift (e.g., overlay error) by a conditional moiré factor that depends on the reference system. Continuing with the above example, a shift of the moiré fringes from a nominal position (e.g., no overlay error) due to a shift of the second layer relative to the first layer is: M = p1 / (p1-p2) (2) However, in the context of this disclosure, references to a "first layer," "second layer," "third layer," etc. are intended merely to distinguish between various sample layers and do not indicate a physical order of layers on the sample. Thus, a "first layer" may be above or below a "second layer" on the sample.

[0041] In this regard, overlay measurements may be performed by measuring the shift of the moiré fringes along the periodic direction of the associated grating structure on the metrology target and adjusting the value by the moiré gain, which depends on the particular design of the metrology target and the particular measurement being performed. For example, it may be desirable to measure the shift of the moiré fringes relative to another structure or set of moiré fringes, since absolute measurement of the shift of a single moiré fringe may be difficult, impractical, or impossible for some target designs. The use of moiré patterns in overlay metrology is described in U.S. Patent No. 7,440,105, issued October 21, 2008, U.S. Patent No. 7,349,105, issued March 25, 2008, and U.S. Patent Application Publication No. 2018 / 0188663, published July 5, 2018, all of which are incorporated herein by reference in their entirety.

[0042] It is recognized herein that arranging the elements of a metrology target in a symmetrical configuration may facilitate determining the relative positions of the elements based on the use of symmetry. For example, an overlay metrology target may be formed such that the center of symmetry of the elements on a first sample layer overlaps with the center of symmetry of the elements on a second sample layer. Thus, differences between the centers of symmetry of the elements of a first layer relative to a second layer of a fabricated metrology target may be due to overlay error. The use of symmetry in overlay metrology targets (e.g., Advanced Imaging Metrology (AIM) targets) is described in U.S. Patent No. 7,068,833, issued June 27, 2006, U.S. Patent No. 6,921,916, issued July 26, 2005, and U.S. Patent No. 7,177,457, issued February 13, 2007, all of which are incorporated herein by reference in their entireties.

[0043] Embodiments of the present disclosure may be directed to overlay metrology targets having at least one working zone with one or more moiré patterns arranged in a symmetric distribution. For example, the moiré patterns within the working zone may be distributed with a mirror symmetric distribution or a rotationally symmetric distribution (e.g., a 90° rotationally symmetric distribution, a 180° rotationally symmetric distribution, etc.). In this regard, the advantages of the moiré patterns and the advantages of symmetry in the metrology target may be combined and utilized together to facilitate accurate and robust overlay metrology.

[0044] It is recognized herein that the effect of physical overlay errors on a working group that includes one or more instances of a Moiré pattern may shift the center of symmetry based on a shift in the Moiré fringes, as previously described herein. Thus, any measurement of overlay based on the center of symmetry may need to be adjusted based on a Moiré gain factor. It is further recognized herein that the Moiré gain factor may depend on the particular arrangement of elements within the overlay metrology target.

[0045] 1 is a block diagram of a metrology system 100 in accordance with one or more embodiments of the present disclosure. The metrology system 100 may generate one or more images of a sample 102 on at least one detector 104 using any method known in the art.

[0046] In one embodiment, the metrology system 100 includes an illumination source 106 and generates an illumination beam 108. The illumination beam 108 may include light of one or more selected wavelengths, including, but not limited to, vacuum ultraviolet radiation (VUV), deep ultraviolet radiation (DUV), ultraviolet (UV) radiation, visible radiation, or infrared (IR) radiation. The illumination source 106 may further generate an illumination beam 108 that includes any range of selected wavelengths. In another embodiment, the illumination source 106 may include a spectrally tunable illumination source and generate an illumination beam 108 having a tunable spectrum.

[0047] The illumination source 106 may further generate an illumination beam 108 having any temporal profile. For example, the illumination source 106 may generate a continuous illumination beam 108, a pulsed illumination beam 108, or a modulated illumination beam 108. Additionally, the illumination beam 108 may be transmitted from the illumination source 106 via free space propagation or waveguiding light (e.g., optical fiber, light pipe, etc.). In some embodiments, the illumination beam 108 may include incoherent light. In other embodiments, the illumination beam 108 may include coherent light.

[0048] In another embodiment, the illumination source 106 directs the illumination beam 108 to the sample 102 via an illumination path 110. The illumination path 110 may include one or more lenses 112 or additional illumination optics 114 suitable for modifying and / or conditioning the illumination beam 108. For example, the one or more illumination optics 114 may include, without limitation, one or more polarizers, 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 shutters (e.g., mechanical shutters, electro-optical shutters, acousto-optical shutters, etc.). As another example, the one or more illumination optics 114 may include an aperture stop for controlling the angle of illumination on the sample 102 and / or a field stop for controlling the spatial extent of illumination on the sample 102. In one example, the illumination path 110 may include an aperture stop located at a plane conjugate to the back focal plane of the objective lens 116 to provide telecentric illumination of the sample. In another embodiment, the metrology system 100 includes the objective lens 116 to focus the illumination beam 108 onto the sample 102.

[0049] In another embodiment, the sample 102 is positioned on a sample stage 118. The sample stage 118 may include any device suitable for positioning the sample 102 within the metrology system 100. For example, the sample stage 118 may include any combination of linear translation stages, rotational stages, tip / tilt stages, etc.

[0050] In another embodiment, the detector 104 is configured to capture radiation emanating from the sample 102 (e.g., sample light 120) via a collection path 122. For example, the collection path 122 may include, without limitation, a collection lens (e.g., the objective lens 116 shown in FIG. 1 ) or one or more additional collection path lenses 124. In this regard, the detector 104 may receive radiation reflected or scattered from the sample 102 (e.g., via specular reflection, diffuse reflection, etc.) or radiation generated by the sample 102 (e.g., luminescence associated with absorption of the illumination beam 108, etc.).

[0051] The collection path 122 may further include any number of collection optics 126 to direct and / or modify the illumination collected by the objective 116, including, but not limited to, one or more collection path lenses 124, one or more filters, one or more polarizers, and one or more beam blocks. In addition, the collection path 122 may further include a field stop to control the spatial extent of the sample imaged onto the detector 104, or an aperture stop to control the angular extent of the illumination from the sample, and may be used to generate an image on the detector 104. In another embodiment, the collection path 122 includes an aperture stop located at a plane conjugate to the back focal plane of the objective 116 optical element to provide telecentric imaging of the sample.

[0052] The detector 104 may include any type of optical detector known in the art suitable for measuring illumination received from the sample 102. For example, the detector 104 may include a sensor suitable for generating one or more images of the stationary sample 102 (e.g., in a static mode of operation), including, but not limited to, a charge-coupled device (CCD), a complementary metal-oxide semiconductor (CMOS) sensor, a photomultiplier tube (PMT) array, or an avalanche photodiode (APD) array. Additionally, the detector 104 may include a multi-tap sensor having two or more taps per pixel, including, but not limited to, a multi-tap CMOS sensor. In this case, charge in a multi-tap pixel may be directed to any selected tap during an exposure window based on one or more drive signals for that pixel. Thus, a multi-tap sensor including an array of multi-tap pixels may generate multiple images during a single readout phase, each associated with a different tap of an associated pixel. Additionally, for purposes of this disclosure, a tap of a multi-tap sensor may refer to an output tap connected to an associated pixel. In this case, each tap of the multi-tap sensor may be read out (eg, in a readout phase) to generate a separate image.

[0053] As another example, the detector 104 may include a sensor suitable for generating one or more images of the sample 102 during operation (e.g., a scanning mode of operation). For example, the detector 104 may include a line sensor including rows of pixels, where the metrology system 100 may generate successive images (e.g., strip images), one row at a time, by translating the sample 102 through a measurement field of view in a scan direction perpendicular to the rows of pixels, and by successively clocking the line sensor during successive exposure windows.

[0054] According to another example, the detector 104 may include a TDI sensor having multiple pixel rows and readout rows. A TDI sensor may operate similarly to a line sensor, except that a clocking signal sequentially moves charge from pixel row to next pixel row until the charge reaches a readout row, where a row of an image is generated. By synchronizing the charge transfer (e.g., based on a clocking signal) with the movement of the sample along the scan direction, charge continues to accumulate across the pixel rows, which may provide a relatively high signal-to-noise ratio compared to a line sensor.

[0055] In another embodiment, the detector 104 may include a spectroscopic detector suitable for identifying wavelengths of radiation emanating from the sample 102. In another embodiment, the metrology system 100 may include multiple detectors 104 (e.g., associated with multiple beam paths generated by one or more beam splitters) to facilitate multiple metrology measurements by the metrology system 100. For example, the metrology system 100 may include one or more detectors 104 suitable for static mode imaging and one or more detectors 104 suitable for scanning mode imaging. In another embodiment, the metrology system 100 may include one or more detectors 104 suitable for both static and scanning imaging modes.

[0056] In one embodiment, as shown in FIG. 1, the measurement system 100 includes a beam splitter 128 oriented such that the objective lens 116 can direct the illumination beam 108 to the sample 102 and simultaneously collect radiation emitted from the sample 102.

[0057] In another embodiment, the angle of incidence of the illumination beam 108 on the sample 102 is adjustable. For example, the path of the illumination beam 108 through the beam splitter 128 and the objective lens 116 may be adjusted to control the angle of incidence of the illumination beam 108 on the sample 102. In this regard, the illumination beam 108 has a nominal path through the beam splitter 128 and the objective lens 116 such that the illumination beam 108 has an orthogonal angle of incidence with respect to the sample 102. As another example, the angle of incidence of the illumination beam 108 on the sample 102 may be controlled by modifying the position and / or angle of the illumination beam 108 on the beam splitter 128 (e.g., by a rotating mirror, a spatial light modulator, a freeform illumination source, etc.). In another embodiment, the illumination source 106 directs one or more illumination beams 108 to the sample 102 at an angle (e.g., a glancing angle, 45°, etc.).

[0058] In another embodiment, the metrology system 100 includes a controller 130. In another embodiment, the controller 130 includes one or more processors 132 that are configured to execute program instructions maintained on a storage medium 134. In this case, the one or more processors 132 of the controller 130 may perform any of the various process steps described throughout this disclosure. Additionally, the controller 130 may be configured to receive data, including, but not limited to, an image of the sample 102 from the detector 104.

[0059] The one or more processors 132 of the controller 130 may include any processing element known in the art. In this sense, the one or more processors 132 may include any microprocessor-type device configured to execute algorithms and / or instructions. In one embodiment, the one or more processors 132 may be comprised of a desktop computer, a mainframe computer system, a workstation, an image computer, a parallel processor, or any other computer system (e.g., a networked computer) configured to execute programs, which programs are configured to operate the metrology system 100 as described throughout this disclosure. It is further recognized that the term "processor" may be broadly defined to encompass any device having one or more processing elements that execute program instructions from a non-transitory storage medium 134. Furthermore, the steps described throughout this disclosure may be performed by a single controller 130 or, alternatively, by multiple controllers. In addition, the controller 130 may include one or more controllers housed in a common housing or multiple housings. In this manner, any controller or combination of controllers may be individually packaged as a module suitable for integration into the metrology system 100. Additionally, the controller 130 may analyze data received from the detector 104 and communicate the data to additional components within the measurement system 100 or external to the measurement system 100 .

[0060] The storage medium 134 may include any storage medium known in the art suitable for storing program instructions executable by the associated one or more processors 132. For example, the storage medium 134 may include a non-transitory storage medium. As another example, the storage medium 134 may include, but is not limited to, a read-only memory, a random access memory, a magnetic or optical memory device (e.g., disk), a magnetic tape, a solid-state drive, and the like. It is further noted that the storage medium 134 may be contained in a common controller housing with the one or more processors 132. In one embodiment, the storage medium 134 may be located remotely with respect to the physical location of the one or more processors 132 and the controller 130. For example, the one or more processors 132 of the controller 130 may access a remote memory (e.g., a server) accessible via a network (e.g., the Internet, an intranet, etc.). Thus, the above description should be construed as merely illustrative and not as a limitation on the present invention.

[0061] In another embodiment, the controller 130 is communicatively coupled to one or more elements of the metrology system 100. In this regard, the controller 130 may transmit and / or receive data from any component of the metrology system 100. Additionally, the controller 130 may direct or control any component of the metrology system 100 by generating one or more drive signals to the associated component. For example, the controller 130 may be communicatively coupled to the detector 104 to receive one or more images from the detector 104.

[0062] Additionally, the controller 130 may provide one or more drive signals to the detector 104 to implement any of the detection techniques described herein. According to another example, the controller 130 may be communicatively coupled to any combination of components to control the optical configuration for the image, including but not limited to the illumination source 106, the illumination optics 114, the collection optics 126, the detector 104, etc.

[0063] Figure 2A is a top view block diagram of a metrology target 202 having a moiré pattern based on a grating-over-grating structure in multiple rotationally symmetric working zones in accordance with one or more embodiments of the present disclosure, and Figure 2B is a conceptual diagram of a multi-layer grating structure in the moiré pattern of Figure 2A in accordance with one or more embodiments of the present disclosure.

[0064] In one embodiment, the metrology target 202 includes a first working zone 204 that is formed from two instances of a first moiré pattern 206 arranged such that the first working zone 204 is rotationally symmetric. In another embodiment, the metrology target 202 includes a second working zone 208 that is formed from two instances of a second moiré pattern 210 arranged such that the second working zone 208 is rotationally symmetric. In the context of the present disclosure, a working zone is a group of features that are intended to be analyzed together. For example, overlay measurements may be performed by comparing the characteristics of one working zone to another working zone.

[0065] In a general sense, the centers of rotational symmetry of the various working zones of the metrology target 202 may be independent. However, it may be desirable to design the metrology target 202 such that the centers of symmetry of some or all of the working zones (within a selected tolerance) coincide in the absence of overlay errors. This target design may minimize errors that may be caused by rotational misalignment between the measurement system (e.g., metrology system 100) and the wafer axis. In one embodiment, working zones in the same direction (e.g., X-direction overlay measurements) have the same center of symmetry. Additionally, the centers of symmetry of working zones associated with overlay measurements along different directions may, but need not, coincide.

[0066] As in FIG. 2A , in a situation where there is no overlay error, the first working zone 204 and the second working zone 208 are rotationally symmetrical about a center of symmetry 212 by 180 degrees.

[0067] The metrology target 202 of FIG. 2A may have rotationally symmetric properties similar to advanced imaging metrology (AIM) targets, which are generally described in U.S. Patent No. 7,068,833 issued June 27, 2006, U.S. Patent No. 6,921,916 issued July 26, 2005, and U.S. Patent No. 7,177,457 issued February 13, 2007. However, whereas each working zone in a typical AIM target contains features in a single layer, at least one working zone (e.g., first working zone 204 and second working zone 208) of metrology target 202 contains a moiré pattern formed from grating structures in multiple layers of interest. As shown in FIG. 2B, the first moiré pattern 206 is formed from a first grating structure 214 having a pitch Q in a first layer (layer A) and a second grating structure 216 having a pitch P in a second layer (layer B), where P ≠ Q. Furthermore, a second Moire pattern 210 is formed from a third grating structure 218 having a pitch T in the first layer (Layer A) and a fourth grating structure 220 having a pitch S in the second layer (Layer B), where S ≠ T. In a general sense, the values ​​of P, Q, S, and T can be independently varied, and Figures 2A and 2B represent a general Moire AIM target.

[0068] As previously described herein with respect to equations (1) and (2), the first working zone 204 has a moiré pitch (p M1 ) p M1 =QP / (QP) (3) And the second working zone 208 has the next moiré pitch (p M2 ). p M2 =ST / (ST) (4)

[0069] The first working zone 204 has the following conditional moiré factor (M1): M1=P / (PQ) (5) associated with the misalignment of layer A relative to layer B, and the second working zone 208 has a conditional moiré factor (M2) of: M2 = S / (ST) (6) is associated with a shift of layer A relative to layer B. Similarly, with a shift of layer B relative to layer A, M1=Q / (QP) and M2=T / (TS).

[0070] In one embodiment, the first moiré pattern 206 and / or the second moiré pattern 210 include grating-over-grating structures where the respective gratings completely overlap. For example, the first grating structure 214 and the second grating structure 216 of the first moiré pattern 206 can completely overlap on the sample 102. Similarly, the third grating structure 218 and the fourth grating structure 220 of the second moiré pattern 210 can completely overlap on the sample 102. In this case, the size of the metrology target 202 may be minimized to efficiently utilize the space on the sample 102.

[0071] As used herein, metrology targets 202 that include adjacent moiré patterns are considered susceptible to crosstalk. Furthermore, the severity of the crosstalk may depend on several factors, including, but not limited to, stack thickness, material properties, grating design, or conditions of the measurement system (e.g., metrology system 100). In some embodiments, metrology targets 202 may include exclusion zones between working zones, or moiré patterns associated with different working zones, to reduce crosstalk within a selected tolerance. For example, instances of first moiré pattern 206 and second moiré pattern 210 may be separated by an exclusion zone 222 large enough to reduce crosstalk between corresponding moiré fringes within a specified tolerance. In some embodiments, the exclusion zone 222 is in the range of 0.25-0.5 microns or greater.

[0072] The exclusion zones 222 may be empty or filled with sub-resolution assist features as may be required by some design rules. In one embodiment, the exclusion zones 222 are filled with sub-resolution assist features oriented orthogonal to the measurement direction (here the X-direction). In a general sense, as long as each working zone is rotationally symmetric, the elements within each working zone (including the moiré pattern) may be arranged in any 2D distribution suitable to mitigate crosstalk to within a selected tolerance.

[0073] Additionally, while FIG. 2A illustrates working zones associated with measurements along a single direction (e.g., the X direction), metrology target 202 targets and methods associated with measurements along additional directions (e.g., the Y direction) are within the spirit and scope of the present disclosure. In one embodiment, metrology target 202 includes a third working zone 224 and a fourth working zone 226 that are suitable for measurements along the Y direction. Additionally, features within the third working zone 224 and / or the fourth working zone 226 may have any features suitable for overlay measurements and may, but need not, include one or more moiré patterns. In another embodiment, not shown, metrology target 202 includes working zones having a 2D moiré pattern that includes periodic features in multiple directions (e.g., the X direction and the Y direction). In this case, the 2D moiré pattern may exhibit moiré fringes along multiple dimensions simultaneously and therefore may be suitable for simultaneous overlay measurements in multiple directions.

[0074] FIG 2C is a top view of the metrology target 202 based on FIG 2A and FIG 2B for measurements along two orthogonal directions in accordance with one or more embodiments of the present disclosure. In particular, the metrology target 202 shown in FIG 2C represents an example where P=T and Q=S. Additionally, FIG 2C illustrates a situation where there is no overlay error in either the X or Y direction. In this case, the axis of symmetry of the first working zone 204 and the axis of symmetry of the second working zone 208 overlap along both the X and Y directions (e.g., the axis of symmetry of the first working zone 204 and the axis of symmetry of the second working zone 208 overlap).

[0075] However, it should be understood that FIG. 2C and the associated description are provided for illustrative purposes only and should not be construed as limiting.

[0076] In one embodiment, the metrology target 202 includes a first working zone 204 with two instances of the first moiré pattern 206 for measurements along the X direction, a second working zone 208 with two instances of the second moiré pattern 210, a third working zone 224 with two instances of the first moiré pattern 206 for measurements along the Y direction, and a fourth working zone 226 with two instances of the second moiré pattern 210 for measurements along the vertical direction, where the third working zone 224 and the fourth working zone 226 are rotated versions of the first working zone 204 and the second working zone 208, respectively.

[0077] 3 is a flow diagram illustrating a method 300 for measuring overlay using the double scattering Moiré effect, according to one or more embodiments of the present disclosure. For example, the method 300 may be applied to measure overlay based on an overlay target, such as, but not limited to, the metrology targets and their associated working zones (e.g., metrology target 202 and working zones 204 and 208) shown in FIGS. 2A-2C and 24A-29B. Applicant notes that the embodiments and enabling technologies described herein above in the context of metrology system 100 should be construed to extend to method 300. However, it is further noted that method 300 is not limited to the architecture of metrology system 100. Additionally, method 300 may be applied to measure overlay along one or more measurement directions.

[0078] In one embodiment, the method 300 includes illuminating 310 an overlay target on a sample (e.g., metrology target 202) with two illumination lobes at two oblique illumination angles. The overlay target may include at least a first working zone and a second working zone (e.g., working zones 204 and 208) for overlay measurements along a measurement direction. The first working zone may include a moiré pattern (e.g., moiré pattern 206) formed from overlapping periodic structures (e.g., grating structures 214 and 216) along the measurement direction on the two sample layers, where the periods of the periodic structures on the two sample layers are different.

[0079] In another embodiment, the method 300 includes receiving 320 an image based on the illumination. The image may be based on a zeroth diffraction order and one Moiré diffraction order associated with the diffraction of each of the two illumination lobes by a first working zone (e.g., working zone 204) and may be based on a zeroth diffraction order and one additional diffraction order associated with the diffraction of each of the two illumination lobes by a second working zone (e.g., working zone 208). The apparent periodicity of the first working zone in the image may be based on Moiré interference.

[0080] In another embodiment, the method 300 includes a step 330 of determining an apparent overlay along the measurement direction between the first working zone and the second working zone based on the images. This determination may be performed, for example, by the methods 2300 and 2312 described with respect to FIG. 23A and FIG. 23B (e.g., finding the difference between the centers of symmetry), but step 330 is not limited thereto and any method of calculating the misregistration between the working zones may be used. It should be noted that the apparent overlay is not equal to the physical overlay error between the two sample layers. Rather, the apparent overlay corresponds to the relative shift of the Moiré fringes between the first working zone and the second working zone. Thus, the physical overlay error between the sample layers along the measurement direction may be determined by dividing the difference between the symmetry axes orthogonal to the measurement direction by the Moiré gain.

[0081] In another embodiment, method 300 includes a step 340 of calculating the overlay between the two sample layers by dividing the apparent overlay by the Moiré gain to compensate for Moiré interference, which may be done, for example, by step 2322 of method 2300 described with respect to FIG.

[0082] FIG. 4 is a conceptual diagram illustrating a double scattering Moiré effect between two sample layers 214 and 216 in the working zone 204 (shown in FIGS. 2A-2C). A light beam (e.g., illumination beam 108) may illuminate the sample layers 214 and 216 such that one Moiré diffraction order (+M) may be collected by a collection system (e.g., detector 104). The light beam may include incoherent light. The angle of incidence of the light beam may be configured such that at least one of the diffraction orders (either 1st or -1st order if light is diffracted from the top grating, or 1st or -1st order if light is diffracted from the bottom grating) propagates within the stack of two sample layers 214 and 216. It should be noted that this effect is not limited to sample layers 214 and 216, but may occur, for example, between sample layers 218 and 220 in FIG. 2B, between sample layers 806 and 808 in FIG. 28B, between sample layers 914 and 916 in FIG. 29B, etc.

[0083] 5-9B show various configurations for sample illumination (e.g., at the illumination or entrance pupil) and collection (e.g., at the collection or exit pupil) using the double scattering Moiré effect. The illuminated sample can be, for example, sample layers 214 and 216 in working zone 204. Additional sample layers (e.g., sample layers 218 and 220 in working zone 208) can be illuminated simultaneously or separately. One measurement direction (e.g., X direction) can be measured at a time, or two measurement directions (e.g., X and Y directions) can be measured. By configuring the pitch of the metrology target, the numerical aperture of the illumination and collection paths, the illumination arrangement, and the wavelength of the sample light, the appropriate diffraction orders can be collected by the collection path to generate an image for overlay measurement.

[0084] FIG. 5 illustrates a one-step off-center configuration for sample illumination and collection using the double scattering Moiré effect. Illumination lobes 501 and 502 can illuminate a sample (e.g., sample layers 214 and 216, and / or sample layers 218 and 220, etc.), and Moiré diffraction orders (+M and -M) and the zeroth diffraction order (0) can be collected by a collection system. In this embodiment, the Moiré diffraction orders and the zeroth diffraction order can be a Littrow illumination configuration. Littrow illumination is described in U.S. Patent Application Publication No. 2020 / 0132446 (see, e.g., FIG. 1). In a Littrow illumination configuration, one of the Moiré diffraction orders (e.g., +M) can be diffracted at 180° relative to the direction of the illumination lobe (e.g., illumination lobe 501), and another Moiré diffraction order (e.g., -M) can be diffracted at 180° relative to the direction of the other illumination lobe (e.g., illumination lobe 502). In addition, each of the Moiré diffraction orders may be mirror symmetric with respect to the other of the Moiré diffraction orders, and each of the zero diffraction orders may be mirror symmetric with respect to the other of the zero diffraction orders. Advantages of a Littrow illumination configuration may include focus independence and improved accuracy. It should be noted that this disclosure is not limited thereto, and other illumination configurations (e.g., asymmetric placement of the diffraction orders) are possible. If a Littrow illumination configuration is used, then both the overlay measurement and the contrast may be insensitive to focus position. If a Littrow illumination configuration is not used, then only the contrast may be insensitive to focus position.

[0085] FIG. 6 shows an alternative one-step off-center configuration for sample illumination and collection using the double scattering Moiré effect. Illumination lobes 501 and 502 can illuminate a sample (e.g., sample layers 214 and 216, and / or sample layers 218 and 220, etc.), and Moiré diffraction orders (+M and -M) and zeroth diffraction order (0) can be collected by the collection system. By moving the position of illumination lobes 501 and 502, the 1st and -1st diffraction orders of the top and bottom gratings can be blocked from collection, while Moiré orders (+M and -M) and zeroth diffraction order (0) can still be collected, thus extending the design and measurement modes of Moiré targets. Additionally, the same configuration may be used simultaneously for X and Y directions (e.g., when X and Y directions have the same design).

[0086] FIG. 7 shows a one-step off-center configuration for sample illumination and collection using the double scattering moiré effect. Illumination lobes 501 and 502 can illuminate the sample (e.g., sample layers 214 and 216 and / or sample layers 218 and 220, etc.), and the moiré diffraction orders (+M and -M) and the zeroth diffraction order (0) can be collected by the collection system. In this configuration, the collection can be split into multiple regions (e.g., collection regions 513 and 517) using a prism. Since two beams are used for imaging, the asymmetry in the amplitudes of the 1st and -1st moiré orders does not indicate inaccuracy, and the measurement contrast can be less sensitive to the focus position (e.g., lower requirements on the focusing system). By using two different images for overlay calculation (one from collection region 513 and the other from collection region 517), the difference between the images can be used as an additional metric during the measurement, thus allowing for improved accuracy and stability in future measurements.

[0087] 8A-8B show a two-step off-center configuration for sample illumination and collection using the double scattering Moiré effect. In this embodiment, the sample (e.g., sample layers 214 and 216, and / or sample layers 218 and 220, etc.) may be illuminated sequentially (first by illumination lobe 501, then by illumination lobe 502). In FIG. 8A, illumination lobe 501 may illuminate the sample, and Moiré diffraction orders (-M) and zeroth diffraction orders (0) may be collected by the collection system. In FIG. 8B, illumination lobe 502 may illuminate the sample, and Moiré diffraction orders (+M) and zeroth diffraction orders (0) may be collected by the collection system. Because two beams are used for imaging, the asymmetry in the amplitude between the 1st and -1st orders does not indicate inaccuracy, and the overlay measurement may be less sensitive to focus position (e.g., lower requirements on the focusing system). In this embodiment, the moiré diffraction orders and the zeroth diffraction order may be a Littrow illumination configuration. Littrow illumination is described in US Patent Application Publication No. 2020 / 0132446 (see, e.g., FIG. 1). In a Littrow illumination configuration, one of the moiré diffraction orders (e.g., +M) may be diffracted at 180° to the direction of the illumination lobe (e.g., illumination lobe 501), and another moiré diffraction order (e.g., -M) may be diffracted at 180° to the direction of the other illumination lobe (e.g., illumination lobe 502). In addition, each of the moiré diffraction orders may be mirror symmetric to the other of the moiré diffraction orders, and each of the zeroth diffraction orders may be mirror symmetric to the other of the zeroth diffraction orders. Advantages of a Littrow illumination configuration may include focus independence and improved accuracy. When a Littrow illumination configuration is used, both the overlay measurement and the contrast may be insensitive to the focus position. If a Littrow illumination configuration is not used, the contrast alone may be insensitive to focus position.

[0088] In addition, compared to the configuration described with respect to FIG. 7, the configuration of FIGS. 8A-8B may be configured to use a smaller moire pitch. For example, if the objective has a numerical aperture of 0.7, the illumination has a numerical aperture of 0.2, and the wavelength is 500 nm, then the pitch is (wavelength) / (NA distance)=500 / 1=500 nm. This pitch may allow for smaller metrology targets (e.g., a pitch of about 5-6 may be required for the algorithm to function robustly, so that the effective target size may be about 6 microns by 6 microns for two directional metrology targets). In some embodiments, the effective target size may be 5-50 microns by 5-50 microns (e.g., 20 microns by 20 microns, 25 microns by 25 microns, etc.).

[0089] 9A-9B show an alternative two-step off-center configuration for sample illumination and collection using the double scattering Moiré effect. In this embodiment, a sample (e.g., sample layers 214 and 216, and / or sample layers 218 and 220, etc.) may be sequentially illuminated (first by illumination lobe 501, then by illumination lobe 502). In FIG. 9A, illumination lobe 501 may illuminate the sample, and Moiré diffraction orders (-M) and zeroth diffraction orders (0) may be collected by the collection system. In FIG. 9B, illumination lobe 502 may illuminate the sample, and Moiré diffraction orders (+M) and zeroth diffraction orders (0) may be collected by the collection system. By moving the positions of the illumination lobes 501 and 502, the 1st and -1st diffraction orders of the top and bottom gratings can be blocked from collection, while the moiré orders (+M and -M) and the zeroth diffraction order (0) can still be collected, thus extending the moiré target design and measurement modes. In addition, the same configuration may be used simultaneously in the X and Y directions (e.g., when the X and Y directions have the same design). Since two beams are used for imaging, the asymmetry in amplitude between the 1st and -1st orders does not indicate inaccuracies, and the image contrast may be less sensitive to the focus position (e.g., lower requirements on the focusing system).

[0090] In some embodiments, the present disclosure may be directed to a one-step central configuration for sample illumination and collection as shown in FIG. 17A of U.S. Pat. No. 7,528,941 (incorporated herein by reference in its entirety). As shown in FIG. 17A, a spatial modulation device 406 (e.g., blocker 406) may block the zeroth diffraction order 404b from being collected while allowing diffraction orders 404a and 404c (e.g., +M and -M Moire diffraction orders) to be collected. In these embodiments, the signal DC is minimized (e.g., DC is the same magnitude as the non-zero harmonic collected). Since two beams are used for imaging, the asymmetry of the amplitude between the 1st and -1st orders does not indicate inaccuracy, and furthermore, contrast and overlay measurements may be less dependent on focus (overlay focus independence requires that the target sample layer is of the PQQP type). The optical paths of the +M and -M Moire diffraction orders are similar, resulting in high sensitivity to focus position and minimizing tool-induced shifts.

[0091] FIG. 10 is a flow diagram illustrating a method 1000 for measuring overlay using single-scattering light Moiré effect, according to one or more embodiments of the present disclosure. For example, the method 1000 may be applied to measure overlay based on an overlay target, such as, but not limited to, the metrology targets and their associated working zones (e.g., metrology target 202 and working zones 204 and 208) shown in FIGS. 2A-2C and 24A-29B. Applicant notes that the embodiments and enabling technologies described herein above in the context of metrology system 100 should be construed to extend to the method 1000. However, it is further noted that the method 1000 is not limited to the architecture of metrology system 100. Additionally, the method 1000 may be applied to measure overlay along one or more measurement directions.

[0092] In one embodiment, method 1000 includes illuminating 1010 an overlay target (e.g., metrology target 202) on a sample with two illumination lobes at two oblique illumination angles. The overlay target may include at least a first working zone and a second working zone (e.g., working zones 204 and 208) for overlay measurements along a measurement direction. The first working zone may include a moiré pattern (e.g., moiré pattern 206) formed from overlapping periodic structures (e.g., grating structures 214 and 216) along the measurement direction on the two sample layers, where the periods of the periodic structures on the two sample layers are different.

[0093] In another embodiment, the method 1000 includes receiving 1020 an image based on the illumination. The image may be based on first order diffraction from each of two sample layers associated with a first working zone (e.g., working zone 204) and may also be based on first order diffraction from each of two sample layers associated with a second working zone (e.g., working zone 208). The apparent periodicity of the first working zones in the image may be based on Moiré interference.

[0094] In another embodiment, the method 1000 includes a step 1030 of determining an apparent overlay along the measurement direction between the first working zone and the second working zone based on the images. This determination may be performed, for example, by the methods 2300 and 2312 described with respect to FIG. 23A and FIG. 23B (e.g., finding the difference between the symmetry centers), but step 1030 is not limited thereto and any method of calculating the misregistration between the two working zones may be used. It should be noted that the apparent overlay is not equal to the physical overlay error between the two sample layers. Rather, the apparent overlay corresponds to the relative shift of the Moiré fringes between the first working zone and the second working zone. Thus, the physical overlay error between the two sample layers along the measurement direction may be determined by dividing the difference between the symmetry axes orthogonal to the measurement direction by the Moiré gain.

[0095] In another embodiment, method 1000 includes step 1040 of calculating the overlay between the two sample layers by dividing the apparent overlay by the Moiré gain to compensate for Moiré interference, which may be done, for example, by step 2322 of method 2300 described with respect to FIG.

[0096] FIG. 11 is a conceptual diagram illustrating a single-scattered light Moiré effect between two sample layers 214 and 216 in the working zone 204 (shown in FIGS. 2A-2C). A light beam (e.g., sample light 120) may illuminate the sample layers 214 and 216 such that first diffraction orders (e.g., +1 and +1') may be collected by a collection system (e.g., detector 104) from each of the sample layers 214 and 216. The light beam may include incoherent light. Note that this effect is not limited to the sample layers 214 and 216, but may occur, for example, between sample layers 218 and 220 in FIG. 2B, between sample layers 806 and 808 in FIG. 28B, between sample layers 914 and 916 in FIG. 29B, etc. Using the single-scattered light Moiré effect may require less light, making the design of the metrology target and the selection of the measurement mode easier.

[0097] 12-22 show various configurations for sample illumination (e.g., at the illumination or entrance pupil) and collection (e.g., at the collection or exit pupil) using the single-scattered light Moiré effect. The illuminated samples can be, for example, sample layers 214 and 216 in working zone 204. Additional sample layers (e.g., sample layers 218 and 220 in working zone 208) can be illuminated simultaneously or individually. One measurement direction (e.g., X direction) can be measured at a time, or two measurement directions (e.g., X and Y directions) can be measured. By configuring the pitch of the metrology target, the numerical aperture of the illumination and collection paths, the illumination arrangement, and the wavelength of the sample light, the appropriate diffraction orders can be collected by the collection path to generate an image for overlay measurement.

[0098] FIG. 12 shows a one-step central configuration for sample illumination and collection using single-scattering light Moiré effect including a blocker 523. The blocker 523 may block the zeroth diffraction order and possibly Moiré diffraction orders. The illumination lobe 501 may illuminate the sample (e.g., sample layers 214 and 216, and / or sample layers 218 and 220, etc.) and the first diffraction orders (+1, +1', -1, -1') may be collected by the collection system. The illumination lobe 501 may illuminate the sample at an angle of about 90° to the sample and may impinge on the sample at or near the center of the first or second working zone (e.g., working zones 204 or 208). In this embodiment, the signal DC is minimized (e.g., DC is the same magnitude as the collected non-zero harmonics). Images produced by this configuration may include the Moiré pitch harmonic and at least three additional harmonics of much smaller pitch. Therefore, once an image is generated, the non-moire harmonics may be algorithmically removed (eg, using an analogue of a Fourier low-pass filter).

[0099] 13A-13B show a two-step off-center configuration for sample illumination and collection using a single scattering light moiré effect including a blocker 523. The blocker 523 may block the zeroth diffraction order and possibly the moiré diffraction orders. In this embodiment, the sample (e.g., sample layers 214 and 216, and / or sample layers 218 and 220, etc.) may be sequentially illuminated (first by illumination lobe 501, then by illumination lobe 502). In FIG. 13A, illumination lobe 501 may illuminate the sample and the first diffraction orders (-1 and -1') may be collected by the collection system. In FIG. 13B, illumination lobe 502 may illuminate the sample and the first diffraction orders (+1 and +1') may be collected by the collection system. In this embodiment, the signal DC is minimized (e.g., DC is the same magnitude as the non-zero harmonics collected). Since two beams are used for imaging, an asymmetry in the amplitude between 1st and -1st orders does not indicate an inaccuracy, and furthermore contrast and overlay measurements may be less sensitive to focus position (overlay focus independence requires that the target sample layer is of the PQQP type).

[0100] 14A-14B show an alternative two-step off-center configuration for sample illumination and collection using the single-scattered light Moiré effect, including an objective lens 537 with a non-central working area. The objective lens 537 with a non-central working area may block the zeroth diffraction order and possibly Moiré diffraction orders. This configuration is simpler because it does not require access to the collection pupil. In this embodiment, the sample (e.g., sample layers 214 and 216, and / or sample layers 218 and 220, etc.) may be illuminated sequentially (first by illumination lobe 501, then by illumination lobe 502). In FIG. 14A, illumination lobe 501 may illuminate the sample, and first diffraction orders (-1 and -1') may be collected by the collection system. In FIG. 14B, illumination lobe 502 may illuminate the sample, and first diffraction orders (+1 and +1') may be collected by the collection system. In this embodiment, the signal DC is minimized (e.g., DC is the same magnitude as the collected non-zero harmonic). Since two beams are used for imaging, an asymmetry in the amplitude between 1st and -1st orders does not indicate inaccuracy, and furthermore contrast and overlay measurements may be less sensitive to focus position (overlay focus independence requires that the target sample layer is of the PQQP type).

[0101] 15A-15B show a two-step off-center configuration for sample illumination and collection using a single scattering light moiré effect including a blocker 523. The blocker 523 may block the zeroth diffraction order and possibly the moiré diffraction orders. In this embodiment, the sample (e.g., sample layers 214 and 216, and / or sample layers 218 and 220, etc.) may be sequentially illuminated (first by illumination lobe 501, then by illumination lobe 502). In FIG. 15A, illumination lobe 501 may illuminate the sample and the first diffraction orders (-1 and -1') may be collected by the collection system. In FIG. 15B, illumination lobe 502 may illuminate the sample and the first diffraction orders (+1 and +1') may be collected by the collection system. In this embodiment, the signal DC is minimized (e.g., DC is the same magnitude as the non-zero harmonics collected). Using this configuration, the moiré target designs and measurement modes are extended, and the same configuration can be used in X and Y directions simultaneously (e.g. when X and Y directions have the same design). Since two beams are used for imaging, an asymmetry in the amplitude between 1st and -1st orders does not indicate an inaccuracy, and furthermore contrast and overlay measurements may be less sensitive to the focus position (overlay focus independence requires that the target sample layer is of the PQQP type).

[0102] 16A-16B show an alternative two-step off-center configuration for sample illumination and collection using the single-scattered light Moire effect, including objective lens 543. By illuminating outside of objective lens 543, the zeroth diffraction order and possibly Moire diffraction orders may be blocked. In this embodiment, the sample (e.g., sample layers 214 and 216, and / or sample layers 218 and 220, etc.) may be illuminated sequentially (first by illumination lobe 501, then by illumination lobe 502). In FIG. 16A, illumination lobe 501 may illuminate the sample and the first diffraction orders (-1 and -1') may be collected by the collection system. In FIG. 16B, illumination lobe 502 may illuminate the sample and the first diffraction orders (+1 and +1') may be collected by the collection system. In this embodiment, the signal DC is minimized (e.g., DC is the same magnitude as the non-zero harmonics collected). Since two beams are used for imaging, an asymmetry in the amplitude between 1st and -1st orders does not indicate an inaccuracy, and furthermore contrast and overlay measurements may be less sensitive to focus position (overlay focus independence requires that the target sample layer is of the PQQP type).

[0103] 17A-17B show an alternative two-step off-center configuration for sample illumination and collection using the single-scattered light Moire effect, including objective lens 543. By illuminating outside of objective lens 543, the zeroth diffraction order and possibly Moire diffraction orders may be blocked. In this embodiment, the sample (e.g., sample layers 214 and 216, and / or sample layers 218 and 220, etc.) may be illuminated sequentially (first by illumination lobe 501, then by illumination lobe 502). In FIG. 17A, illumination lobe 501 may illuminate the sample and the first diffraction orders (-1 and -1') may be collected by the collection system. In FIG. 17B, illumination lobe 502 may illuminate the sample and the first diffraction orders (+1 and +1') may be collected by the collection system. In this embodiment, the signal DC is minimized (e.g., DC is the same magnitude as the non-zero harmonics collected). Using this configuration, the moiré target designs and measurement modes are extended, and the same configuration can be used in X and Y directions simultaneously (e.g. when X and Y directions have the same design). Since two beams are used for imaging, an asymmetry in the amplitude between 1st and -1st orders does not indicate an inaccuracy, and furthermore contrast and overlay measurements may be less sensitive to the focus position (overlay focus independence requires that the target sample layer is of the PQQP type).

[0104] 18A-18B show an alternative two-step off-center configuration for sample illumination and collection using the single-scattered light Moiré effect, including an objective lens 557 at the entrance pupil with a specially designed working area. The objective lens 557 with a specially designed working area may block the zeroth diffraction order and possibly the Moiré diffraction orders. This configuration is simple because it does not require access to the collection pupil. In this embodiment, the sample (e.g., sample layers 214 and 216, and / or sample layers 218 and 220, etc.) may be illuminated sequentially (first by illumination lobe 501, then by illumination lobe 502). In FIG. 18A, illumination lobe 501 may illuminate the sample and first diffraction orders (-1 and -1') may be collected by the collection system. In FIG. 18B, illumination lobe 502 may illuminate the sample and first diffraction orders (+1 and +1') may be collected by the collection system. In this embodiment, the signal DC is minimized (e.g., DC is the same magnitude as the collected non-zero harmonics). Using this configuration, the design and measurement modes of the moiré target are expanded, and the same configuration can be used in the X and Y directions simultaneously (e.g., when the X and Y directions have the same design). Since two beams are used for imaging, an asymmetry in the amplitude between the 1st and -1st orders does not indicate an inaccuracy, and furthermore, contrast and overlay measurements may be less sensitive to the focus position (overlay focus independence requires that the target sample layer is of the PQQP type).

[0105] FIG. 19 shows a one-step off-center configuration for sample illumination and collection using the single-scattered light Moiré effect, including an objective lens 543. By illuminating outside the objective lens 543, the zeroth diffraction order and possibly the Moiré diffraction orders may be blocked. This configuration is simple because it does not require access to the collection pupil. In this embodiment, illumination lobes 501 and 502 may illuminate the sample (e.g., sample layers 214 and 216, and / or sample layers 218 and 220, etc.) and the first diffraction orders (+1, +1', -1, -1') may be collected by the collection system. In this embodiment, the signal DC is minimized (e.g., DC is the same magnitude as the non-zero harmonics collected). Images generated by this configuration may include the Moiré pitch harmonics and additional harmonics of different pitches. Thus, once an image is generated, the non-Moiré harmonics may be algorithmically removed (e.g., using an analogue of a Fourier low-pass filter).

[0106] FIG. 20 shows a one-step off-center configuration for sample illumination and collection using single-scattering light Moiré effect including a blocker 561. The blocker 561 may block the zeroth diffraction order and possibly Moiré diffraction orders. In this embodiment, illumination lobes 501 and 502 may illuminate the sample (e.g., sample layers 214 and 216, and / or sample layers 218 and 220, etc.) and first diffraction orders (+1, +1', -1, -1') may be collected by the collection system. In this embodiment, the signal DC is minimized (e.g., DC is the same magnitude as the non-zero harmonics collected). Images generated by this configuration may include the Moiré pitch harmonics and additional harmonics of different pitches. Thus, once an image is generated, the non-Moiré harmonics may be algorithmically removed (e.g., using an analogue of a Fourier low-pass filter).

[0107] FIG. 21 shows a one-step off-center configuration for sample illumination and collection using the single-scattered light Moiré effect. In this embodiment, illumination lobes 501 and 502 can illuminate the sample (e.g., sample layers 214 and 216, and / or sample layers 218 and 220, etc.) and the first diffraction orders (+1, +1', -1, -1') can be collected by the collection system. In this configuration, the collection can be split into several regions (e.g., collection regions 563 and 567) using a prism. Since two beams are used for imaging, the asymmetry in the amplitude between the 1st and -1st orders does not indicate inaccuracy, and furthermore, contrast and overlay measurements may be less sensitive to focus position (overlay focus independence requires that the target sample layer is of the PQQP type). In this embodiment, the signal DC is minimized (e.g., DC is as large as the collected non-zero harmonics). Images produced by this arrangement may contain Moire pitch harmonics and additional harmonics of different pitches, and therefore, once the image is produced, the non-Moire harmonics may be algorithmically removed (e.g., using an analogue of a Fourier low-pass filter).

[0108] FIG. 22 shows a one-step central configuration for sample illumination and collection using single scattering light Moiré effect including a blocker 523. The blocker 523 may block the zeroth diffraction order and possibly Moiré diffraction orders. The illumination lobe 501 may illuminate the sample (e.g., sample layers 214 and 216, and / or sample layers 218 and 220, etc.) and the first diffraction orders (+1, +1', -1, -1') may be collected by the collection system. The illumination lobe 501 may illuminate the sample at an angle of about 90° to the sample and may impinge on the sample at or near the center of the first or second working zone (e.g., working zones 204 or 208). In this configuration, the collection may be split into several regions (e.g., collection regions 563 and 567) using a prism. Since two beams are used for imaging, the asymmetry in the amplitude between the 1st and -1st orders does not indicate inaccuracies, and furthermore the contrast and overlay measurements may be less sensitive to focus position (overlay focus independence requires that the target sample layer is of the PQQP type). In this embodiment, the signal DC is minimized (e.g., DC is as large as the collected non-zero harmonics). Images produced by this configuration may contain moiré pitch harmonics and additional harmonics of different pitches. Thus, once an image is produced, the non-moiré harmonics may be algorithmically removed (e.g., using an analogue of a Fourier low-pass filter).

[0109] Each of the configurations of Figures 21 and 22 includes a prism configured to split an image into several images based on the position of the collection pupil. By splitting the collection pupil, a blocker (e.g., blocker 523) is not needed because the splitting can move undesired signals (e.g., undesired diffraction orders) to a different optical path (and, as a result, there is no interference between the undesired signals and other signals). Thus, in Figures 21 and 22, the undesired zeroth diffraction order is eliminated by splitting the collection pupil. In general, any of the configurations described with respect to Figures 5-9B and 12-22 can include a pupil split instead of a blocker.

[0110] 23A-29B, various metrology targets and methods for measuring overlay will be described in detail. FIGS. 23A-23B are flow diagrams illustrating a method for calculating overlay by utilizing symmetry of the working zones. FIGS. 24A-26B illustrate overlay using a metrology target with a moiré pattern in two working zones. FIGS. 27-28C illustrate overlay using a metrology target with a moiré pattern in one working zone and a single layer structure (e.g., not a grating-over-grating structure) in a second working zone. FIGS. 29A and 29B illustrate overlay using a metrology target with three or more layers with a moiré pattern in two working zones, with one layer acting as a reference layer. It is noted herein that the samples and metrology targets described with respect to the configurations of FIGS. 3-22 may be similar to or substantially identical to any of the samples and metrology targets described with respect to FIGS. 23A-29B. For example, any of the samples described with respect to Figures 3-22 may include a measurement target having sample layers 806 and 808 of Figure 28B, or a measurement target having sample layers 914 and 916 of Figure 29B, etc.

[0111] 23A is a flow diagram illustrating steps performed in a method 2300 for measuring overlay, according to one or more embodiments of the present disclosure. For example, the method 2300 may be applied to measure overlay based on a metrology target, such as, but not limited to, the metrology target 202 shown in FIGS. 2A-2C. Applicant notes that the embodiments and enabling technologies described herein above in the context of the metrology system 100 should be construed to extend to the method 2300. However, it is further noted that the method 2300 is not limited to the architecture of the metrology system 100. Moreover, the method 2300 may be applied to measure overlay along one or more measurement directions.

[0112] In one embodiment, the method 2300 includes receiving 2302 an image of a first working zone of the metrology target, the first working zone including one or more instances of a first moiré pattern including a grating in two sample layers. In another embodiment, the method 2300 includes receiving 2304 an image of a second working zone of the metrology target, the second working zone including one or more instances of a second moiré pattern including a grating in two sample layers.

[0113] The images of the first moiré pattern 206 and the second moiré pattern 210 can be associated as separate images or a single common image. In one embodiment, an image of the entire metrology target 202 is acquired (e.g., using the metrology system 100) such that the first working zone 204 and the second working zone 208 can be analyzed based on the same image. In another embodiment, separate images of the metrology target 202 (or a portion thereof) are generated for analysis of the first working zone 204 and the second working zone 208. In this case, the images may be generated using different imaging conditions (e.g., different illumination wavelengths, illumination incidence angles, polarization, focus, etc.).

[0114] In another embodiment, the method 2300 includes a step 2306 of determining a first axis of symmetry that is orthogonal to the measurement direction associated with the first working zone. In another embodiment, the method 2300 includes a step 2308 of determining a second axis of symmetry that is orthogonal to the measurement direction associated with the second working zone. In another embodiment, the method 2300 includes a step 2310 of measuring a difference between the first axis of symmetry and the second axis of symmetry. Additionally, the center of symmetry 212 of one or more working zones can be found by determining orthogonal axes of symmetry in different directions (e.g., orthogonal directions) and finding the point of intersection.

[0115] The axes of symmetry for the first working zone 204 and the second working zone 208 may be determined in step 2302 or step 2304 using any technique known in the art. For example, any technique suitable for measuring the center of symmetry of a typical AIM target without a Moiré pattern may be used.

[0116] Figure 23B is a flow diagram illustrating a method 2312 for determining an axis of symmetry of a working group that is orthogonal to the measurement direction. In this regard, method 2312 may be associated with step 2306 and / or step 2308. Figure 24A is a top view of the metrology target 202 of Figure 2C with additional markings illustrating application of method 2312 in accordance with one or more embodiments of the present disclosure.

[0117] In one embodiment, step 2314 includes selecting a region of interest (ROI) that includes at least a portion of the elements in the working group. For example, the solid box in Figure 24A represents an ROI 402 that selects a portion of the first moiré pattern 206 in the first working zone 204, and the dashed box represents an ROI 404 that selects a portion of the second moiré pattern 210 in the second working zone 208.

[0118] In another embodiment, step 2316 includes generating a measurement signal based on the ROI, for example, each value of the measurement signal along the measurement direction may include an average (or cumulative) intensity value in the ROI along an orthogonal direction.

[0119] It is contemplated herein that moiré fringes associated with a moiré pattern (e.g., first moiré pattern 206 or second moiré pattern 210) may have relatively low contrast relative to the grating structures forming the moiré pattern that may be seen in an image of metrology target 202. In another embodiment, step 2318 includes isolating moiré fringes in the measurement signal. For example, step 2318 may include one or more orders of moiré fringes (e.g., first order, second order, etc.). Separating the moiré fringes may reduce noise and improve the accuracy and / or sensitivity of the determination of the axis of symmetry in steps 2306 and / or 2308, which may improve the accuracy and / or robustness of the determination of the axis of symmetry.

[0120] The Moiré fringes may be separated in step 2318 using any technique known in the art. In one embodiment, the measurement signal is filtered to separate the Moiré fringes. For example, the spatial frequency components of the measurement signal may be analyzed using 1D or 2D Fourier filtering or other frequency-based decomposition techniques (e.g., decomposition of the measurement signal into sine and / or cosine signals). Furthermore, frequency selective filters may be used to separate the spatial frequencies of the Moiré fringes (e.g., first order Moiré fringes, second order Moiré fringes, etc.).

[0121] In another embodiment, the method 2312 may include a step 2320 of determining an axis of symmetry 406 of the working zone orthogonal to the measurement direction based on the measurement signal. The axis of symmetry 406 of the working zone orthogonal to the measurement direction may be determined using any technique known in the art (e.g., in step 2306 or step 2308 of method 2300, or in step 2320 of method 2312). In one embodiment, a portion of the image of the first moiré pattern 206 (e.g., the ROI of step 314) is correlated with a symmetric version of itself to determine the axis of symmetry. For example, the axis of symmetry 406 of the first working zone 204 may be determined by generating a copy of the portion of the image that includes the first working zone 204, rotating the image copy by the associated rotational symmetry of the metrology target 202 (e.g., 90°, 180°, etc.), and performing a correlation between the original image and the image copy as a function of position along the measurement direction at various locations. In this case, the position along the measurement direction that provides the highest correlation between the original image and the rotated version may correspond to the axis of symmetry 406 of the first working zone 204. As another example, for a working zone with reflection symmetry (or mirror symmetry), the axis of symmetry 406 of the working zone may be determined by correlating a reflected copy of the working zone with itself.

[0122] As previously described herein, steps associated with method 2312 may be applied to determine the axis of symmetry 406 for any number of working zones. For example, both steps 2306 and 2308 may involve application of method 2312.

[0123] However, it should be understood that the method 2312 for determining the axis of symmetry 406 of the working zone along the measurement direction is provided merely for illustrative purposes and should not be construed as limiting. Rather, the axis of symmetry 406 may be determined using any technique known in the art. In one embodiment, the axis of symmetry 406 of the working group along the measurement direction may be applied by first determining a region of interest (e.g., ROI 402 of step 314), isolating one or more orders of Moiré fringes along its measurement direction by performing a spatial frequency filtering operation on the entire ROI (e.g., by treating the feature as having infinite dimensions along the orthogonal direction and applying a 1D filtering operation similar to step 318), and correlating the ROI with a rotated version of itself to determine the axis of symmetry 406.

[0124] In another embodiment, the method 2300 includes a step 2322 of calculating the overlay error between the two sample layers along the measurement direction by dividing the difference between the symmetry axes of the first and second working zones 204, 208 orthogonal to the measurement direction by a moiré gain factor. For example, in FIG. 24A , both the first working zone 204 and the second working zone 208 have overlapping symmetry axes 406 orthogonal to the measurement direction (e.g., X-direction) because FIG. 24A illustrates a case with no overlay error. However, in the case of a physical overlay error, the symmetry axes 406 do not overlap.

[0125] For example, in FIG. 24B the first working zone 204 and the second working zone 208 do not have overlapping axes of symmetry. In FIG. 24B the first working zone 204 has an axis of symmetry 406 with a center of rotational symmetry 212, while the second working zone 208 has an axis of symmetry 407 with a center of rotational symmetry 213. Axis of symmetry 406 does not overlap with axis of symmetry 407, and center of rotational symmetry 212 does not overlap with center of rotational symmetry 213. Thus, FIG. 24B illustrates the case where there is an overlay error.

[0126] Due to the presence of Moiré fringes, the difference between the symmetry axes determined in step 2310 is different from the physical overlay error between the first and second sample layers. Rather, the difference between the symmetry axes corresponds to a relative shift of the Moiré fringes. Thus, the physical overlay error along the measurement direction may be determined by dividing the difference between the symmetry axes orthogonal to the measurement direction by the Moiré gain associated with the particular layout of the measurement target 202.

[0127] It is recognized herein that the Moiré gain is dependent on the particular layout of the metrology target 202. For example, the Moiré gain coefficient (M g ) if the overlay is related to the shift of layer A relative to layer B, M g =M1-M2=P / (PQ)-S / (ST)=-(Q / (QP)-T / (TS)) (7) Similarly, the Moiré gain coefficient associated with the shift of layer A relative to layer B is −M g =M2-M1.

[0128] As evidenced by equation (7), the moiré gain is affected by conditional moiré factors associated with the first moiré pattern 206 and the second moiré pattern 210, which are functions of the particular values ​​of the grating pitches P, Q, S, and T. Thus, by selecting values ​​of the grating pitches P, Q, S, and T, the composite moiré gain, and thereby the sensitivity to physical overlay errors, may be increased or optimized. For example, the composite moiré gain may generally be increased by selecting values ​​of the grating pitches P, Q, S, and T such that the moiré gains of the first working zone 204 and the second working zone 208 have different signs.

[0129] 25 and 26A-26B, it is contemplated herein that the method 2300 is not limited to the particular shape of the metrology target 202 shown in Figure 2A. In a general sense, a working zone (e.g., the first working zone 204 or the second working zone 208) may include any number of instances of a moiré pattern (e.g., the first moiré pattern 206 or the second moiré pattern 210) so long as the working zone is rotationally and / or reflection symmetric.

[0130] FIG. 25 is a top view of a metrology target 202 in accordance with one or more embodiments of the present disclosure, including a first working zone 204 that includes two instances of a first moiré pattern 206 and a second working zone 208 that includes two instances of a second moiré pattern 210.

[0131] FIG. 26A is a top view of a metrology target 202 in accordance with one or more embodiments of the present disclosure, including a first working zone 204 that includes two instances of a first moiré pattern 206 and a second working zone 208 that includes a single instance of a second moiré pattern 210.

[0132] Figure 26B is a top view of the metrology target of Figure 26A with additional markings illustrating the application of method 2312 of Figure 23B. The solid box in Figure 26A represents a ROI 402 that selects a portion of the first moiré pattern 206 in the first working zone 204, and the dashed box represents a ROI 404 that selects a portion of the second moiré pattern 210 in the second working zone 208 (e.g., step 2314 of method 2312). ROI 402 and ROI 404 are used to generate the underlying measurement signal (e.g., step 2316 of method 2312).

[0133] FIG. 27 is a top view of a moiré rain box metrology target 202 in accordance with one or more embodiments of the present disclosure, where a first working zone 204 includes a single instance of a moiré pattern 702 and a second working zone 208 includes a non-periodic monolayer structure surrounding the moiré pattern 702. In FIG. 27, the moiré pattern 702 includes a grating-over-grating structure with periodicity in only the X direction, such that the metrology target 202 of FIG. 7 may be used to make overlay measurements in the X direction (e.g., in accordance with method 300). However, it should be understood that the example metrology target 202 of FIG. 7 is presented for illustrative purposes only and should not be construed as limiting. In one embodiment, not shown, the moiré rain box target includes a 2D moiré pattern with a grating-over-grating structure with periodicity along both the X and Y directions, such that overlay measurements are made along both the X and Y directions.

[0134] 28A-29B, in some embodiments, the metrology target 202 includes a reference layer. In this case, structures in the reference layer may optically interact with structures in the additional layer to provide a device-like metrology target (e.g., a target with features having a similar size, density, and / or pitch as the device features). Device-like metrology targets including a reference layer are generally described in U.S. Patent Application Publication No. 2018 / 0188663, published July 5, 2018, which is incorporated by reference in its entirety.

[0135] FIG 28A is a top block diagram of a three layer metrology target 202 having a moiré pattern based on a grating-over-grating structure across multiple layers in accordance with one or more embodiments of the present disclosure. FIG 28B is a conceptual diagram of a multi-layer grating structure in the moiré pattern of FIG 28A in accordance with one or more embodiments of the present disclosure. In one embodiment, the metrology target 202 includes a first working zone 204 that includes two instances of a first moiré pattern 802 and a second working zone 208 that includes two instances of a second moiré pattern 804.

[0136] As shown in FIG. 28B, a first moiré pattern 802 is formed from a grating structure 806 having a pitch P in a first layer (Layer A), a grating structure 808 having a pitch Q in a reference layer (Layer B), and no grating structure in a third layer (Layer C), where P≠Q. Furthermore, a second moiré pattern 804 is formed from a grating structure 810 having a pitch x·Q in a reference layer (Layer B), and a grating structure 812 having a pitch x·P in a third layer (Layer A), where x is a positive number (e.g., a rational number). In this case, both the first moiré pattern 802 and the second moiré pattern 804 incorporate the structure of the reference layer. Furthermore, the reference layer structures need not have the same pitch in the first moiré pattern 802 and the second moiré pattern 804.

[0137] As used herein, the metrology target 202 of FIG. 28A is suitable for making overlay measurements between layers A and C. In this metrology target 202, the moiré gain (M g ) are as follows: M g =Q / (PQ) (9)

[0138] Figure 28C is a top view of the metrology target 202 based on Figures 28A and 28B for measurements along two orthogonal directions, in accordance with one or more embodiments of the present disclosure. In particular, the metrology target 202 shown in Figure 28C represents an example where x=1.

[0139] It should be understood that Figures 28A-C and the associated description are provided for illustrative purposes only and should not be construed as limiting. In some embodiments, the metrology target 202 includes at least one working zone with a Moiré pattern having three overlapping gratings in three layers, with one layer being a reference layer. In this case, the overlay between the other two pairs of layers can be restored either algorithmically (e.g., by filtering one or more measurement signals to separate or suppress desired spatial frequencies and / or Moiré fringes) or physically (e.g., using additional working zones).

[0140] FIG. 29A is a top view block diagram of a triple moiré metrology target 202 in accordance with one or more embodiments of the present disclosure. FIG. 29B is a conceptual diagram of a multi-layer grating structure in the moiré pattern of FIG. 29A in accordance with one or more embodiments of the present disclosure. In one embodiment, the metrology target 202 includes a first working zone 902 including two instances of a first moiré pattern 904, a second working zone 906 including two instances of a second moiré pattern 908, and a third working zone 910 including two instances of a third moiré pattern 912. As shown in FIG. 29B, the first moiré pattern 904 is formed from a grating structure 914 having a pitch Q in a first layer (Layer A), a grating structure 916 having a pitch P in a second layer (Layer B), and no grating structure in a third layer (Layer C), where P≠Q. The second moiré pattern 908 is formed from a grating structure 918 having a pitch T in the first layer (layer A) and a grating structure 920 having a pitch S in the second layer (layer B), where T≠S. The third moiré pattern 912 is formed from a grating structure 922 having a pitch T in the third layer (layer C) and a grating structure 924 having a pitch S in the second layer (layer B).

[0141] As used herein, the metrology target 202 of FIG. 29A is suitable for making overlay measurements between layers A and C, or between layers A and B. In this metrology target 202, the moiré gain (M g ) are as follows: M g =P / (PQ)-S / (ST) (10)

[0142] In this measurement target 202, the moire gain (M g ) are as follows: M g =T / (ST) (11)

[0143] It should be understood that Figures 29A and 29B and the associated description are presented for illustrative purposes only and should not be construed as limiting. For example, the triple moiré metrology target 202 generally includes further versions of an arrangement of grating structures across three layers. In one embodiment, the triple moiré metrology target 202 includes a first working zone 902 having a moiré pattern based on grating structures on layers A and B, a second working zone 906 having grating structures on layers B and C, and a third working zone 910 having grating structures on layers A and C. Such a target may also be suitable for overlay measurements between layers A and B, as well as between layers B and C. Furthermore, the metrology target 202 may generally have any number of layers. For example, an N-fold metrology target 202 may have N layers with grating structures on selected layers to perform overlay measurements between selected layers.

[0144] The subject matter described herein sometimes shows different components contained within or connected to another component. It should be understood that such illustrated architectures are merely exemplary, and that in fact many other architectures that achieve the same functionality may be implemented. Conceptually, any arrangement of components to achieve the same functionality is effectively "associated" such that the desired functionality is achieved. That is, any two components that combine to achieve a particular functionality can be viewed as being "associated" with one another such that the desired functionality is achieved, regardless of architecture or intervening components. Similarly, any two components so associated can be viewed as being "connected" or "coupled" with one another such that the desired functionality is achieved, and any two components that can be so associated can also be viewed as being "couplable" with one another such that the desired functionality is achieved. Specific examples of couplable include, but are not limited to, physically interactable and / or physically interacting components, and / or wirelessly interactable and / or wirelessly interacting components, and / or logically interactable and / or logically interacting components.

[0145] 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 may be made in the form, construction and arrangement of the elements 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 include all such modifications. It will be further understood that the invention is defined by the appended claims.

Claims

1. 1. An imaging system comprising: an illumination subsystem configured to illuminate an overlay target with at least one illumination lobe, the overlay target including at least a first working zone and a second working zone for overlay measurement along a measurement direction, where at least the first working zone includes a moiré pattern formed from overlapping periodic structures on two sample layers along the measurement direction, where periods of the periodic structures on the two sample layers are different; a collection subsystem configured to collect a zeroth diffraction order and one Moiré diffraction order associated with diffraction of each of the at least one illumination lobe by the first working zone, the collection subsystem being further configured to collect a zeroth diffraction order and one additional Moiré diffraction order associated with diffraction of each of the at least one illumination lobe by the second working zone; and a detector configured to generate an image of the overlay target using light collected by the collection subsystem; an imaging system comprising: a controller configured to be coupled to the imaging system, the controller comprising one or more processors configured to execute program instructions, the program instructions causing the one or more processors to: receiving the image of the overlay target, where an apparent periodicity of the first working zones in the image is based on Moire interference; determining an apparent overlay along the measurement direction between the first working zone and the second working zone based on the images; and calculating an overlay between the two sample layers by dividing the apparent overlay by a Moire gain to compensate for Moire interference; A controller; Equipped with the first working zone and the second working zone are both rotationally symmetric and the respective rotational symmetries are coincident. Measurement system.

2. The metrology system of claim 1 , wherein the at least one illumination lobe is configured to illuminate the overlay target at an orthogonal angle of incidence to the overlay target.

3. 3. The metrology system of claim 2, wherein the at least one illumination lobe is configured to illuminate the overlay target at or near a central portion of at least one of the first working zone or the second working zone.

4. The metrology system of claim 1 , wherein the at least one illumination lobe comprises two illumination lobes at two oblique illumination angles relative to the overlay target.

5. The metrology system of claim 4 , wherein the illumination subsystem simultaneously illuminates the overlay target with the two illumination lobes.

6. The metrology system of claim 4 , wherein the illumination subsystem sequentially illuminates the overlay target with the two illumination lobes.

7. The metrology system of claim 4 , wherein the two illumination lobes are directed along the measurement direction.

8. The metrology system of claim 4 , wherein the two illumination lobes are oriented at a non-zero angle relative to the measurement direction.

9. 2. The metrology system of claim 1 , wherein the collection subsystem generates a first image of the overlay target based on the zeroth diffraction order and one Moire diffraction order associated with a diffraction of each of the at least one illumination lobe by the first working zone and generates a second image of the overlay target based on the zeroth diffraction order and one additional Moire diffraction order associated with a diffraction of each of the at least one illumination lobe by the second working zone, wherein the controller is further configured to determine the apparent overlay by averaging apparent overlay associated with the first image and the second image.

10. 10. The metrology system of claim 9, wherein the at least one illumination lobe comprises two illumination lobes at two oblique illumination angles relative to the overlay target.

11. 11. The metrology system of claim 10, wherein the illumination subsystem simultaneously illuminates the overlay target with the two illumination lobes, and wherein the collection subsystem simultaneously generates the first image and the second image by separating collected light associated with the first working zone from collected light associated with the second working zone.

12. The metrology system of claim 10 , wherein the illumination subsystem sequentially illuminates the overlay target with the two illumination lobes.

13. 2. The metrology system of claim 1 , wherein the moiré pattern in the first working zone is a first moiré pattern and the second working zone includes a second moiré pattern formed from overlapping periodic structures along the measurement direction on the two sample layers, wherein periods of the periodic structures on the two sample layers in the second working zone are different, and the second moiré pattern is different from the first moiré pattern.

14. A measurement method comprising: receiving an image of an overlay target on a sample based on illumination of the overlay target with at least one illumination lobe, the overlay target including at least a first working zone and a second working zone for overlay measurement along a measurement direction, where at least the first working zone includes a moiré pattern formed from overlapping periodic structures along the measurement direction on two sample layers, where periods of the periodic structures on the two sample layers are different; wherein the image is based on a zeroth diffraction order and one Moiré diffraction order associated with a diffraction of each of the at least one illumination lobe by the first working zone, the image is further based on a zeroth diffraction order and one additional Moiré diffraction order associated with a diffraction of each of the at least one illumination lobe by the second working zone, and an apparent periodicity of the first working zone in the image is based on Moiré interference; determining an apparent overlay along the measurement direction between the first working zone and the second working zone based on the images; and calculating an overlay between the two sample layers by dividing the apparent overlay by a Moire gain to compensate for the Moire interference; Including, the first working zone and the second working zone are both rotationally symmetric and the respective rotational symmetries are coincident. Measurement method.

15. The metrology method of claim 14 , wherein the at least one illumination lobe is configured to illuminate the overlay target at an orthogonal angle of incidence to the overlay target.

16. 16. The metrology method of claim 15, wherein the at least one illumination lobe is configured to illuminate the overlay target at or near a central portion of at least one of the first working zone or the second working zone.

17. The metrology method of claim 14 , wherein the at least one illumination lobe comprises two illumination lobes at two oblique illumination angles relative to the overlay target.

18. 1. A measurement system comprising: The imaging system an illumination subsystem configured to illuminate an overlay target with at least one illumination lobe, the overlay target including at least a first working zone and a second working zone for overlay measurement along a measurement direction, where at least the first working zone includes a Moiré pattern formed from overlapping periodic structures along the measurement direction on two sample layers, the two sample layers including a first sample layer and a second sample layer, and periods of the periodic structures on the two sample layers are different; a collection subsystem configured to collect first order diffraction from each of the two sample layers associated with the first working zone, the collection subsystem being further configured to collect first order diffraction from each of the two sample layers associated with the second working zone; and a detector configured to generate an image of the overlay target using light collected by the collection subsystem; an imaging system comprising: a controller configured to be coupled to the imaging system, the controller comprising one or more processors configured to execute program instructions, the program instructions causing the one or more processors to: receiving the image of the overlay target, where an apparent periodicity of the first working zones in the image is based on Moire interference at the detector; filtering the image to isolate the Moire interference; determining an apparent overlay along the measurement direction between the first working zone and the second working zone based on the images; and calculating an overlay between the two sample layers by dividing the apparent overlay by a Moire gain to compensate for the Moire interference; A controller; Equipped with the first working zone and the second working zone are both rotationally symmetric and the respective rotational symmetries are coincident. Measurement system.

19. 20. The metrology system of claim 18, wherein the at least one illumination lobe is configured to illuminate the overlay target at an orthogonal angle of incidence to the overlay target.

20. 20. The metrology system of claim 19, wherein the at least one illumination lobe is configured to illuminate the overlay target at or near a central portion of at least one of the first working zone or the second working zone.

21. 20. The metrology system of claim 18, wherein the collection subsystem includes one or more blockers to block a zero order diffraction from at least one of the first working zone or the second working zone.

22. 20. The metrology system of claim 18, wherein a numerical aperture of the collection subsystem is smaller than a numerical aperture of the illumination subsystem to prevent collection of zero orders of diffraction from at least one of the first working zone or the second working zone.

23. 20. The metrology system of claim 18, wherein the at least one illumination lobe comprises two illumination lobes at two oblique illumination angles relative to the overlay target.

24. 24. The metrology system of claim 23, wherein the illumination subsystem simultaneously illuminates the overlay target with the two illumination lobes.

25. 24. The metrology system of claim 23, wherein the illumination subsystem sequentially illuminates the overlay target with the two illumination lobes.

26. 24. The metrology system of claim 23, wherein the two illumination lobes are directed along the measurement direction.

27. 24. The metrology system of claim 23, wherein the two illumination lobes are oriented at a non-zero angle relative to the measurement direction.

28. 20. The metrology system of claim 18, wherein the collection subsystem generates a first image of the overlay target based on the first order diffraction from each of the two sample layers associated with the first working zone and generates a second image of the overlay target based on the first order diffraction from each of the two sample layers associated with the second working zone, and the controller is further configured to determine the apparent overlay by averaging apparent overlay associated with the first image and the second image.

29. 20. The metrology system of claim 18, wherein the at least one illumination lobe comprises two illumination lobes at two oblique illumination angles relative to the overlay target.

30. 30. The metrology system of claim 29, wherein the illumination subsystem simultaneously illuminates the overlay target with the two illumination lobes, and wherein the collection subsystem simultaneously generates a first image and a second image by separating collected light associated with the first working zone from collected light associated with the second working zone.

31. The metrology system of claim 30 , wherein the illumination subsystem sequentially illuminates the overlay target with the two illumination lobes.

32. A measurement method comprising: receiving an image of an overlay target on a sample based on illumination of the overlay target with at least one illumination lobe, the overlay target including at least a first working zone and a second working zone for overlay measurement along a measurement direction, where at least the first working zone includes a moiré pattern formed from overlapping periodic structures along the measurement direction on two sample layers, where periods of the periodic structures on the two sample layers are different; wherein the image is based on first order diffraction from each of the two sample layers associated with the first working zone, the image is further based on first order diffraction from each of the two sample layers associated with the second working zone, and an apparent periodicity of the first working zone in the image is based on Moiré interference; determining an apparent overlay along the measurement direction between the first working zone and the second working zone based on the images; and calculating an overlay between the two sample layers by dividing the apparent overlay by a Moire gain to compensate for the Moire interference; Including, the first working zone and the second working zone are both rotationally symmetric and the respective rotational symmetries are coincident. Measurement method.

33. 33. The metrology method of claim 32, wherein the at least one illumination lobe is configured to illuminate the overlay target at an orthogonal angle of incidence to the overlay target.

34. 34. The metrology method of claim 33, wherein the at least one illumination lobe is configured to illuminate the overlay target at or near a central portion of at least one of the first working zone or the second working zone.

35. 33. The metrology method of claim 32, wherein the at least one illumination lobe comprises two illumination lobes at two oblique illumination angles.

Images