Metrology target, and metrology system

Abstract

A measurement target may include a first rotationally symmetric working area having one or more examples of a first pattern and a second rotationally symmetric working area having one or more examples of a second pattern, wherein at least one of the first pattern or the second pattern is a mottled pattern formed by a first grating structure having a first pitch along a measurement direction on a first sample layer and a second grating structure having a second pitch different from the first pitch along the measurement direction on a second sample layer. When the overlap error between the first sample layer and the second layer is zero, the rotational symmetry centers of the first working area and the second working area can be overlapped by design. A difference between the rotational symmetry centers of the first working area and the second working area can indicate the overlap error between the first sample layer and the second sample layer.

Description

Technical Field

[0001] This invention relates generally to imaging overlay measurement, and more specifically to imaging overlay targets having moiré elements. Prior Technology

[0002] The shrinking design rules and more stringent specifications of pairwise metrology are driving an increasing demand for the sensitivity and robustness of pairwise metrology methods. Pairwise metrology is typically performed by fabricating specialized metrological targets with manufactured characteristics across multiple layers of interest. Therefore, analysis of the fabricated metrological targets provides a measure of the pairwise error (e.g., relative alignment error) between the layers of interest. While a wide variety of pairwise metrological target designs have been proposed, there remains a continued need to improve metrological target designs and measurement methods for accurately and effectively analyzing fabricated metrological targets. Summary of the Invention

[0003] This invention discloses a metrological target according to one or more illustrative embodiments. In one illustrative embodiment, the target includes a first working area comprising one or more examples of a first pattern, wherein the first working area is rotationally symmetrical. In another illustrative embodiment, the target includes a second working area comprising one or more examples of a second pattern, wherein the second working area is rotationally symmetrical. In another illustrative embodiment, at least one of the first pattern or the second pattern is a moiré pattern formed by a first grating structure having a first pitch along a measurement direction on a first sample layer and a second grating structure having a second pitch along the measurement direction on a second sample layer, wherein the second pitch is different from the first pitch. In another illustrative embodiment, when the overlap error between the first sample layer and the second layer is zero, one rotational symmetry center of the first working area overlaps with one rotational symmetry center of the second working area. In another illustrative embodiment, a difference between the rotational symmetry center of the first working area and the rotational symmetry center of the second working area along the measurement direction indicates a stacking error between the first sample layer and the second sample layer along the measurement direction.

[0004] A metrology system according to one or more illustrative embodiments of the present invention is disclosed. In one illustrative embodiment, the system includes a controller configured to be coupled to an imaging system. In another illustrative embodiment, the controller receives an image of a metrological target on a sample. In one illustrative embodiment, the target includes a first working area comprising one or more examples of a first pattern, wherein the first working area is rotationally symmetric. In another illustrative embodiment, the target includes a second working area comprising one or more examples of a second pattern, wherein the second working area is rotationally symmetric. In another illustrative embodiment, at least one of the first pattern or the second pattern is a moiré pattern formed by a first grating structure having a first pitch along a measurement direction on a first sample layer and a second grating structure having a second pitch along the measurement direction on a second sample layer, wherein the second pitch is different from the first pitch. In another illustrative embodiment, the controller determines a difference between the axes of symmetry of the first working area and the second working area along the measurement direction in the image. In another illustrative embodiment, the controller calculates an overlay error along the measurement direction associated with the first and second sample layers by adjusting the difference between the axes of symmetry of the first and second working areas along the measurement direction using a moiré gain adjustment based on one of the pitches of the moiré pattern.

[0005] This invention discloses a metrological target according to one or more illustrative embodiments. In one illustrative embodiment, the target includes a working area comprising one or more examples of a moiré pattern, wherein the working area is rotationally symmetric. In another illustrative embodiment, the moiré pattern includes a grating structure on a first sample layer having a first pitch (p1) along a measurement direction and a grating structure on a second sample layer having a second pitch (p2) along the measurement direction, wherein the second pitch is different from the first pitch. In another illustrative embodiment, an overlay error associated with the first sample layer and the second sample layer along the measurement direction corresponds to a difference in moiré gain adjustment between the axes of symmetry of the first working area imaged using a first optical configuration of an imaging system and a second optical configuration of the imaging system, based on the pitch value of the first moiré pattern. In another illustrative embodiment, moiré patterns can be detected by the first optical configuration, wherein at least one of the grating structure having the first pitch or the grating structure having the second pitch can be detected by the second optical configuration.

[0006] A metrology system according to one or more illustrative embodiments of the present invention is disclosed. In one illustrative embodiment, the system includes an imaging system. In another illustrative embodiment, the imaging system includes an illumination source for generating illumination. In another illustrative embodiment, the imaging system includes illumination optics for directing illumination from the illumination source to one or more illumination targets. In another illustrative embodiment, the target includes a working area comprising one or more examples of a moiré pattern, wherein the working area is rotationally symmetric. In another illustrative embodiment, the moiré pattern includes a grating structure on a first sample layer having a first pitch (p1) along a measurement direction and a grating structure on a second sample layer having a second pitch (p2) along the measurement direction, wherein the second pitch is different from the first pitch. In another illustrative embodiment, the imaging system includes a detector for generating an image of the metrology target based on the illumination from the illumination source, wherein an optical configuration of the imaging system is configurable. In another illustrative embodiment, the optical configuration includes a wavelength of the illumination, a polarized beam of the illumination incident on the metrological target, an angle of the illumination incident on the metrological target, or a focal position of the metrological target relative to the detector. In another illustrative embodiment, the system includes a controller configured to be coupled to the imaging system. In another illustrative embodiment, the controller uses a first optical configuration to receive a first image of the metrological target on the sample. In another illustrative embodiment, the controller determines a first axis of symmetry of the working area along the measurement direction based on the first image. In another illustrative embodiment, the controller uses a second optical configuration different from the first optical configuration to receive a second image of the metrological target on the sample. In another illustrative embodiment, the controller determines a second axis of symmetry of the working area along the measurement direction based on the second image. In another illustrative embodiment, the controller determines an optical overlay measurement based on a difference between the first and second axes of symmetry. In another illustrative embodiment, the controller calculates an overlay error along the measurement direction associated with the first and second sample layers by adjusting the difference between the axes of symmetry of the first and second images, which are adjusted by a moiré gain based on the pitch of the moiré pattern.

[0007] A method for designing a moiré metrological target according to one or more illustrative embodiments of the present invention is disclosed. In one illustrative embodiment, the method includes receiving a design of a metrological target. In another illustrative embodiment, the target includes a working area comprising one or more examples of a moiré pattern, the moiré pattern comprising a first grating structure on a first sample layer having a first pitch (p1) along a measurement direction and a second grating structure on a second sample layer having a second pitch (p2) along the measurement direction, wherein the second pitch is different from the first pitch. In another illustrative embodiment, the method includes receiving an optical configuration of an imaging system for imaging the metrological target. In another illustrative embodiment, the method includes performing an experimental design to select a target design parameter set for the metrological target that provides a moiré stripe pattern satisfying a selected design criterion when imaged by the imaging system, wherein the target design parameter set includes the first pitch and the second pitch.

[0008] It should be understood that both the foregoing overview and the following detailed description are illustrative and exemplary only and do not necessarily limit the invention as claimed. The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments of the invention and, together with the overview, serve to explain the principles of the invention. Simple Explanation of the Diagram

[0009] Those skilled in this art can better understand the many advantages of this invention by referring to the accompanying drawings. Figure 1 is a block diagram of a metering system according to one or more embodiments of the present invention. Figure 2A is a top block diagram of a metrological target having a cloud-like pattern based on a grating-stacked grating structure in multiple rotationally symmetric working areas according to one or more embodiments of the present invention. Figure 2B is a conceptual view of a multilayer grating structure in the cloud pattern of Figure 2A according to one or more embodiments of the present invention. Figure 2C is a top view of a measurement target based on one of Figures 2A and 2B for measurement along two or more orthogonal directions according to one or more embodiments of the present invention. Figure 3A is a flowchart illustrating the steps performed in a method for measuring overlapping according to one or more embodiments of the present invention. Figure 3B shows a flowchart of a method for determining one of the axes of symmetry of a working group. Figure 4 is a top view of the measurement target of Figure 2C with additional markings according to one or more embodiments of the present invention, which has the application of the drawing method. Figure 5 is a top view of a measurement target according to one or more embodiments of the present invention, comprising a first working area including a first cloud pattern and a second working area including a second cloud pattern. Figure 6 is a top view of a measurement target according to one or more embodiments of the present invention, comprising a first working area including one of two examples of a first cloud pattern and a second working area including a single example of a second cloud pattern. Figure 7 is a top view of a hybrid measurement target for measurement along two or more orthogonal directions according to one or more embodiments of the present invention, wherein one measurement in each direction is based on a working area having one or more cloud patterns and another working area having one or more single-layer structures. Figure 8 is a top view of a measurement target for measurement along a single direction according to one or more embodiments of the present invention. The measurement target includes a first working area comprising one of two examples of a first mottled pattern and a second working area comprising one of two examples of a single-layer grating structure. Figure 9 is a top view of a measurement target for measuring along a single direction according to one or more embodiments of the present invention. Figure 10 is a top view of a cloud-pattern measurement target in a frame according to one or more embodiments of the present invention, wherein a first working area includes a single example of a cloud pattern and a second working area includes a non-periodic single-layer structure surrounding the cloud pattern. Figure 11A is a top block diagram of a three-layer metrological target having a cloud pattern based on a grating stacked grating structure across multiple layers according to one or more embodiments of the present invention. Figure 11B is a conceptual view of a multilayer grating structure in the cloud pattern of Figure 11A according to one or more embodiments of the present invention. Figure 11C is a top view of a measurement target based on one of Figures 11A and 11B for measurement along two or more orthogonal directions according to one or more embodiments of the present invention. Figure 12A is a top block diagram of one of the triple cloud-pattern measurement targets according to one or more embodiments of the present invention. Figure 12B is a conceptual view of a multilayer grating structure in the cloud pattern of Figure 12A according to one or more embodiments of the present invention. Figure 13A is a top view of a metrological target comprising a single mottled pattern adapted to provide superimposed measurement along a single direction based on an image generated by multiple optical configurations according to one or more embodiments of the present invention. Figure 13B is a top view of a measurement target comprising one of two example work areas having a cloud-like pattern according to one or more embodiments of the present invention, the cloud-like pattern being adapted to provide superimposed measurements along a single direction based on images generated by multiple optical configurations. Figure 14 is a flowchart illustrating the steps performed in a method for measuring overlapping according to one or more embodiments of the present invention. Implementation

[0010] [Cross-reference to related applications] [] This application claims the right to U.S. Provisional Application No. 62 / 898,980, filed September 11, 2019, entitled "IMAGING OVERLAY TARGETS INCLUDING MOIRÉ ELEMENTS AND ROTATIONAL SYMMETRY ARRANGEMENTS," which designates Yoel Feler, Mark Ghinovker, Diana Shaphirov, Evgeni Gurevich, and Vladimir Levinski as inventors, the entire contents of which are incorporated herein by reference.

[0011] This application also claims the right under 35 USC § 119(e) to U.S. Provisional Application No. 62 / 954,256 entitled “IMAGING OVERLAY TARGETS INCLUDING MOIRÉ ELEMENTS AND ROTATIONAL SYMMETRY ARRANGEMENTS”, filed December 27, 2019, entitled “IMAGING OVERLAY TARGETS INCLUDING MOIRÉ ELEMENTS AND ROTATIONAL SYMMETRY ARRANGEMENTS”, the entire contents of which are incorporated herein by reference.

[0012] The subject matter will now be shown in detail with reference to the accompanying drawings. The invention has been illustrated and described with particular reference to certain embodiments and specific features thereof. The embodiments set forth herein should be considered illustrative rather than restrictive. It will be readily apparent to those skilled in the art that various changes and modifications in form and detail can be made without departing from the spirit and scope of the invention.

[0013] Embodiments of this invention relate to overlay metrological targets having mottled patterns and methods for measuring overlays based on such targets. For example, a mottled pattern on a metrological target may comprise a grating-to-grating structure formed by grating structures with different periods on different sample layers. Furthermore, for the purposes of this invention, a "grating" or a "grating structure" can describe any periodic structure or series of elements that may include (but is not limited to) line / space patterns. An image of the mottled pattern will include mottled stripes at a mottled pitch that is correlated with the spatial variation overlap between gratings on different sample layers. The mottled pitch is typically a pitch longer than any of the grating structures and is related to the difference between the pitches of the grating structures. For example, the mottled pitch... It can be characterized as: in It is the period of the first grating structure on the first layer. It refers to the period of the second grating structure on the second layer.

[0014] In this paper, it is considered that metrological targets incorporating moiré patterns can facilitate sensitive overlay measurements. Specifically, a physical offset of one grating in a moiré pattern relative to another grating along a periodic direction (e.g., an overlay error associated with a relative offset of two sample layers on a sample) will cause a corresponding lateral offset of the moiré fringes along a measurement direction. Furthermore, the magnitude of this offset of the moiré fringes is typically greater than the magnitude of the physical offset. Specifically, the magnitude of the moiré fringes offset is proportional to the physical offset (e.g., overlay error) by a conditional moiré factor dependent on the reference frame. Continuing the example above, an offset of the second layer relative to the first layer will cause the moiré fringes from a nominal position (e.g., without overlay error) according to the following conditional moiré factor. Offset: . However, in the background of this invention, references to "first layer," "second layer," "third layer," or the like are intended only to distinguish various sample layers and do not indicate the order of entities of one layer on the sample. Thus, on a sample, a "first layer" may be above or below a "second layer."

[0015] In this regard, a pairwise measurement can be performed by measuring the offset of moiré fringes along one of the periodic directions of an associated grating structure on a metrological target and by adjusting this value by a moiré gain that depends on the specific design of the metrological target and the specific measurement being performed. For example, it may be desirable to measure the offset of a moiré fringes relative to another structure or another set of moiré fringes, because for some target designs, an absolute measurement of the offset of a single moiré fringe may be difficult, impractical, or impossible. The use of moiré patterns in pairwise metrology is generally described in U.S. Patent No. 7,440,105, published October 21, 2008; U.S. Patent No. 7,349,105, published March 25, 2008; and U.S. Patent Application No. 2018 / 0188663, published July 5, 2018, all of which are incorporated herein by reference.

[0016] It should be further understood here that configuring the elements of a metrology target in a symmetrical configuration facilitates the determination of the relative positions of the elements based on the utilization of symmetry. For example, a stacked metrology target can be formed such that one center of symmetry of an element on a first sample layer overlaps with one center of symmetry of an element on a second sample layer. Therefore, any difference between the centers of symmetry of the elements in the first layer of a manufactured metrology target relative to the second layer can be attributed to a stacking error. The utilization of symmetry in stacked metrology targets (e.g., advanced imaging metrology (AIM) targets) is 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, all of which are incorporated herein by reference in their entirety.

[0017] Embodiments of the present invention relate to a superimposed metrology target having at least one working area with one or more mottle patterns arranged in a symmetrical distribution. For example, the mottle patterns within the working area can be distributed in a mirror-symmetric or rotationally symmetric manner (e.g., a 90° rotational symmetry distribution, a 180° rotational symmetry distribution, or the like). In this respect, the advantages of the mottle patterns and the advantages of symmetry in the metrology target can be combined and used together to promote accurate and robust superimposed metrology.

[0018] It should be understood in this paper that the effect of a physical overlay error on a working group containing one or more examples of a moiré pattern can be due to a shift in a center of symmetry caused by an offset of one of the moiré fringes, as previously described herein. Therefore, any measurement of an overlay based on a center of symmetry must be adjusted based on a moiré gain factor. It should be further understood in this paper that this moiré gain factor can depend on the specific configuration of the elements in an overlay metrology target.

[0019] Figure 1 is a block diagram of a metering system 100 according to one or more embodiments of the present invention. The metering system 100 may use any method known in the art to generate one or more images of a sample 102 on at least one detector 104.

[0020] In one embodiment, the metrology system 100 includes an illumination source 106 for generating an illumination beam 108. The illumination beam 108 may include one or more selected wavelengths of light, including (but not limited to) vacuum ultraviolet (VUV), deep ultraviolet (DUV), ultraviolet (UV) radiation, visible light radiation, or infrared (IR) radiation. The illumination source 106 may further generate an illumination beam 108 comprising any range of selected wavelengths. In another embodiment, the illumination source 106 may include a spectrally tunable illumination source for generating an illumination beam 108 having a tunable spectrum.

[0021] The illumination source 106 may further generate an illumination beam 108 having any time 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. Furthermore, the illumination beam 108 may be delivered from the illumination source 106 via free-space propagation or guiding light (e.g., an optical fiber, a light guide, or the like).

[0022] In another embodiment, illumination source 106 guides illumination beam 108 to sample 102 via illumination path 110. Illumination path 110 may include one or more lenses 112 or additional illumination optics 114 adapted to modify and / or adjust illumination beam 108. For example, one or more illumination optics 114 may include (but are not limited to): 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-optic shutters, acousto-optic shutters, or the like). By another example, one or more illumination optics 114 may include an aperture stop for controlling the angle of illumination on sample 102 and / or a field stop for controlling the spatial range of illumination on sample 102. In one example, illumination path 110 includes an aperture stop positioned in a plane conjugate to the focal plane behind objective lens 116 to provide telecentric illumination of the sample. In another embodiment, metrology system 100 includes an objective lens 116 for focusing illumination beam 108 onto sample 102.

[0023] In another embodiment, sample 102 is placed on a sample stage 118. Sample stage 118 may include any means suitable for positioning sample 102 within metrology system 100. For example, sample stage 118 may include a linear translation stage, a rotation stage, a tilting / flipping stage, or any combination thereof.

[0024] In another embodiment, a detector 104 is configured to capture radiation (e.g., sample light 120) emitted from sample 102 via a light-collecting path 122. For example, the light-collecting path 122 may include (but is not required to include) a light-collecting lens (e.g., objective lens 116 as illustrated in FIG. 1) or one or more additional light-collecting path lenses 124. In this respect, a detector 104 may receive radiation reflected or scattered from sample 102 (e.g., via specular reflection, diffuse reflection, and the like) or generated by sample 102 (e.g., luminescence associated with absorption of illumination beam 108, or the like).

[0025] The light-collecting path 122 may further include any number of light-collecting optics 126 for guiding and / or modifying the illumination collected by the objective lens 116, including (but not limited to) one or more light-collecting path lenses 124, one or more filters, one or more polarizers, or one or more beam stops. Additionally, the light-collecting path 122 may include a field stop for controlling the spatial extent of the sample imaged onto the detector 104, or an aperture stop for controlling the angular extent of the illumination from the sample used to generate an image on the detector 104. In another embodiment, the light-collecting path 122 includes an aperture stop positioned in a plane conjugate to the focal plane behind an optical element (objective lens 116) to provide telecentric imaging of the sample.

[0026] Detector 104 may include any type of optical detector known in the art suitable for measuring illumination received from sample 102. For example, detector 104 may include a sensor suitable for generating (e.g., in a static operating mode) one or more images of a static sample 102, such as (but not limited to) a charge-coupled device (CCD), a complementary metal-oxide-semiconductor (CMOS) sensor, a photomultiplier tube (PMT) array, or a burst photodiode (APD) array. Furthermore, detector 104 may include one of a multi-tap sensor having two or more taps per pixel, including (but not limited to) a multi-tap CMOS sensor. In this respect, based on one or more drive signals to the pixel, charge in a multi-tap pixel can be directed to any selected tap during an exposure window. Thus, a multi-tap sensor including an array of multi-tap pixels can generate multiple images associated with different taps of the associated pixel during a single readout phase. Furthermore, for the purposes of this invention, one of the taps of a multi-tap sensor may refer to an output tap connected to an associated pixel. In this respect, reading out each tap of a multi-tap sensor (e.g., during a readout phase) can produce a separate image.

[0027] By another example, a detector 104 may include a sensor adapted to generate one or more images of one sample 102 in motion (e.g., a scanning operation mode). For example, detector 104 may include a line sensor comprising a column of pixels. In this respect, metrology system 100 may generate a continuous image (e.g., a strip image) one column at a time by translating sample 102 through a measurement field of view in a scanning direction perpendicular to one of the pixel columns and continuously timing the line sensor during a continuous exposure window.

[0028] In another example, detector 104 may include a TDI sensor comprising multiple pixel columns and a readout column. The TDI sensor may operate similarly to a line sensor, except that a timing signal continuously moves charge from one pixel column to the next until the charge reaches the readout column where an image column is generated. By synchronizing charge transfer (e.g., based on a timing signal) to the movement of the sample along the scan direction, charge can continue to accumulate across pixel columns to provide a relatively higher signal-to-noise ratio compared to a line sensor.

[0029] In another embodiment, a detector 104 may include a spectral detector adapted to identify one wavelength of radiation emitted from sample 102. In another embodiment, the metrology system 100 may include a plurality of detectors 104 (e.g., associated with a plurality of beam paths generated by one or more beam splitters to facilitate a plurality of metrological measurements by the metrology system 100). For example, the metrology system 100 may include one or more detectors 104 adapted for static mode imaging and one or more detectors 104 adapted for scanning mode imaging. In another embodiment, the metrology system 100 may include one or more detectors 104 adapted for both static imaging mode and scanning imaging mode.

[0030] In one embodiment, as illustrated in FIG1, the metrology system 100 includes a beam splitter 128 oriented such that an objective lens 116 can simultaneously guide an illumination beam 108 to a sample 102 and collect radiation emitted from the sample 102.

[0031] 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 can be adjusted to control the angle of incidence of the illumination beam 108 on the sample 102. In this respect, the illumination beam 108 may have a nominal path through the beam splitter 128 and the objective lens 116 such that the illumination beam 108 has a normal angle of incidence on the sample 102. By another example, the angle of incidence of the illumination beam 108 on the sample 102 can be controlled by modifying the position and / or angle of the illumination beam 108 on the beam splitter 128 (e.g., by a rotatable mirror, a spatial light modulator, a freeform illumination source, or the like). In another embodiment, the illumination source 106 guides one or more illumination beams 108 to the sample 102 at an angle (e.g., a grazing angle, a 45-degree angle, or the like).

[0032] In another embodiment, the metering system 100 includes a controller 130. In another embodiment, the controller 130 includes one or more processors 132 configured to execute program instructions held on a memory medium 134. In this respect, the controller 130 and the processors 132 may execute any of the various program steps described herein. Furthermore, the controller 130 may be configured to receive data including (but not limited to) images of samples 102 from the detector 104.

[0033] One or more processors 132 of a controller 130 may comprise any processing element known in the art. In this sense, one or more processors 132 may comprise any microprocessor type device configured to execute algorithms and / or instructions. In one embodiment, one or more processors 132 may comprise a desktop computer, mainframe computer system, workstation, video computer, parallel processor, or any other computer system (e.g., a networked computer) configured to execute a program configured to operate the metering system 100, as described throughout the invention. It should be further appreciated that the term "processor" may be broadly defined to cover any device having one or more processing elements that execute program instructions from a non-transitory memory medium 134. Furthermore, the steps described throughout the invention may be performed by a single controller 130 or alternatively by multiple controllers. Additionally, controller 130 may comprise one or more controllers housed in a common housing or multiple housings. In this way, any controller or combination of controllers can be separately packaged as a module suitable for integration into the metering system 100. In addition, the controller 130 can analyze the data received from the detector 104 and feed the data to additional components inside or outside the metering system 100.

[0034] Memory media 134 may comprise any storage medium known in the art suitable for storing program instructions executable by one or more associated processors 132. For example, memory media 134 may comprise a non-transitory memory medium. By another example, memory media 134 may comprise (but is not limited to) a read-only memory, a random access memory, a magnetic or optical memory device (e.g., a magnetic disk), a magnetic tape, a solid-state drive, and the like. It should be further noted that memory media 134 may be housed in a common controller housing with one or more processors 132. In one embodiment, memory media 134 may be located remotely relative to the physical location of one or more processors 132 and controller 130. For example, one or more processors 132 of controller 130 may access a remote memory (e.g., a server) accessible via a network (e.g., the Internet, an intranet, and the like). Therefore, the above description should not be construed as limiting the invention but is merely an illustrative illustration.

[0035] In another embodiment, controller 130 is communicatively coupled to one or more elements of metering system 100. In this respect, controller 130 can transmit and / or receive data from any component of metering system 100. Furthermore, controller 130 can direct or otherwise control any component of metering system 100 by generating drive signals for one or more associated components. For example, controller 130 can be communicatively coupled to detector 104 to receive one or more images from detector 104.

[0036] Furthermore, the controller 130 may provide one or more drive signals to the detector 104 to implement any of the detection techniques described herein. By another example, the controller 130 may be communicatively coupled to any combination of components to control an optical configuration associated with an image, including (but not limited to) an illumination source 106, an illumination optics assembly 114, a light-collecting optics assembly 126, a detector 104, or the like.

[0037] Referring now to Figures 2A through 14, metrology of stacked targets with moiré patterns based on grating-to-grating structures in one or more rotationally symmetric working areas is described in more detail. Figures 2A through 6 illustrate stacked targets using metrology targets with moiré patterns in two working areas. Figures 7 through 10 illustrate stacked targets using a moiré pattern in one working area and a single-layer structure (e.g., not a grating-to-grating structure) in a second working area. Figures 11A and 12 illustrate stacked targets with three or more layers using moiré patterns in two working areas, one of which serves as a reference layer. Figures 13A and 14 illustrate stacked targets using a single working area with one or more moiré patterns, wherein the stacking can be determined based on imaging with different optical conditions.

[0038] Figure 2A is a top block diagram of a measurement target 202 having a mottled pattern based on a grating-stacked grating structure in multiple rotationally symmetric working areas according to one or more embodiments of the present invention. Figure 2B is a conceptual view of the multilayer grating structure in the mottled pattern of Figure 2A according to one or more embodiments of the present invention.

[0039] In one embodiment, a measurement target 202 includes a first working area 204 formed by two instances of a first mottled pattern 206, the two instances being configured such that the first working area 204 is rotationally symmetrical. In another embodiment, the measurement target 202 includes a second working area 208 formed by two instances of a second mottled pattern 210, the two instances being configured such that the second working area 208 is rotationally symmetrical. In the background of the invention, a working area is a group of features intended to be analyzed together. For example, a pairwise measurement can be performed by comparing the characteristics of one working area with the characteristics of another working area.

[0040] In a general sense, the rotational symmetry centers of the various working areas in a metrology target 202 can be independent. However, it is desirable to design metrology target 202 such that, in the absence of overlap errors, the symmetry centers of some or all working areas coincide (within a selected tolerance). This target design minimizes errors that may be attributable to rotational misalignment between a measurement system (e.g., metrology system 100) and the wafer axis. In one embodiment, working areas in the same direction (e.g., X-direction overlap measurement) have the same symmetry center. Furthermore, the symmetry centers of working areas associated with overlap measurements along different directions may, but are not required to, coincide.

[0041] As shown in Figure 2A, which depicts a scenario of non-overlapping error, the first working area 204 and the second working area 208 are rotationally symmetrical about a center of symmetry 212 180⁰.

[0042] The metrological target 202 in Figure 2A may have rotational symmetry characteristics similar to an advanced imaging metrology (AIM) target, typically 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, while each working area in a typical AIM target contains features in a single layer, at least one working area of ​​metrological target 202 (e.g., first working area 204 and second working area 208) contains a moiré pattern formed by grating structures in multiple layers of interest. As shown in Figure 2B, the first mottled pattern 206 is formed by a first grating structure 214 with a pitch Q in a first layer (layer A) and a second grating structure 216 with a pitch P in a second layer (layer B), where P ≠ Q. Furthermore, the second mottled pattern 210 is formed by a third grating structure 218 with a pitch T in the first layer (layer A) and a fourth grating structure 220 with 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 changed independently, such that Figures 2A and 2B represent a general mottled AIM target.

[0043] As previously described with reference to equations (1) and (2) in this document, the first working area 204 will then have the following moiré pitch. Furthermore, the second working area 208 will have the following cloud-patterned pitch. (4)

[0044] The first working area 204 will then have the following conditional moiré factor associated with a misalignment of layer A relative to layer B. Furthermore, the second working area 208 will have a conditional moiré factor associated with the offset of layer A relative to layer B. (6) Similarly, the offset of layer B relative to one of layers A will provide and .

[0045] In one embodiment, the first mottled pattern 206 and / or the second mottled pattern 210 include a grating-on-grating structure in which the respective gratings completely overlap. For example, the first grating structure 214 and the second grating structure 216 of the first mottled pattern 206 may completely overlap on the sample 102. Similarly, the third grating structure 218 and the fourth grating structure 220 of the second mottled pattern 210 may completely overlap on the sample 102. In this respect, the size of the metrology target 202 can be minimized to effectively utilize the space on the sample 102.

[0046] As considered herein, a metrological target 202 containing adjacent moiré patterns may be susceptible to crosstalk. Furthermore, the severity of crosstalk can depend on several factors, including (but not limited to) stack thickness, material properties, grating design, or the conditions of a measurement system (e.g., metrology system 100). In some embodiments, the metrological target 202 includes exclusion regions between working areas or moiré patterns associated with different working areas to reduce crosstalk to a selected tolerance. For example, examples of first moiré patterns 206 and second moiré patterns 210 can be separated by a sufficiently large exclusion region 222 to reduce crosstalk between corresponding moiré stripes to a specified tolerance. In some embodiments, the exclusion region 222 is in the range of 0.25 micrometers to 0.5 micrometers or greater.

[0047] Exclusion region 222 may be empty or may be filled with secondary resolution auxiliary features that some design rules may require. In one embodiment, exclusion region 222 is filled with secondary resolution auxiliary features orthogonal to the measurement direction (here, the X direction). In a general sense, as long as the working regions are rotationally symmetric, the elements (including moiré patterns) in each working region can be adapted to any 2D distribution that reduces crosstalk to a selected tolerance.

[0048] Furthermore, although Figure 2A depicts a working area associated with measurement along a single direction (e.g., the X direction), the measurement target 202 and the method for measurement along an additional direction (e.g., the Y direction) are within the spirit and scope of the invention. In one embodiment, the measurement target 202 includes a third working area 224 and a fourth working area 226 suitable for measurement along the Y direction. Furthermore, the features in the third working area 224 and / or the fourth working area 226 may have any features suitable for overlay measurement and may, but not necessarily, include one or more moiré patterns. In another embodiment, although not shown, the measurement target 202 includes a working area having a 2D moiré pattern containing periodic features in multiple directions (e.g., the X and Y directions). In this respect, the 2D moiré pattern may exhibit moiré stripes simultaneously along multiple dimensions and is therefore suitable for simultaneous overlay measurement in multiple directions.

[0049] Figure 2C is a top view of a measurement target 202 based on Figures 2A and 2B for measurement along two or more orthogonal directions according to one or more embodiments of the present invention. Specifically, the measurement target 202 shown in Figure 2C represents an instance where P=T and Q=S. Furthermore, Figure 2C illustrates a scenario where there is no overlap error in the X or Y direction. In this respect, the axes of symmetry of the first working area 204 and the second working area 208 overlap along both the X and Y directions (e.g., the axes of symmetry of the first working area 204 and the second working area 208 overlap).

[0050] However, it should be understood that Figure 2C and the associated descriptions are provided for illustrative purposes only and should not be construed as restrictive.

[0051] In one embodiment, the measurement target 202 includes a first working area 204 having a first mottled pattern 206 for measurement along the X direction, a second working area 208 having a second mottled pattern 210 for measurement along the Y direction, a third working area 224 having the first mottled pattern 206 for measurement along the Y direction, and a fourth working area 226 having the second mottled pattern 210 for measurement along the vertical direction. In this respect, the third working area 224 and the fourth working area 226 are respectively rotated versions of the first working area 204 and the second working area 208.

[0052] Figure 3A is a flowchart illustrating the steps performed in a method 300 for measuring a pair according to one or more embodiments of the present invention. For example, method 300 may be applicable to measuring a pair based on a metrological target (such as, but not limited to, metrological target 202 illustrated in Figures 2A to 2C). The applicant mentions that the embodiments and enabling techniques previously described in the background of metrological system 100 herein should be interpreted as extensions to method 300. However, it should be further noted that method 300 is not limited to the architecture of metrological system 100. Furthermore, method 300 may be applicable to measuring a pair along one or more measurement directions.

[0053] In one embodiment, method 300 includes step 302 of receiving an image of a first working area of ​​a measurement target, the first working area including one or more examples of a first moiré pattern comprising gratings in two sample layers. In another embodiment, method 304 includes step 304 of receiving an image of a second working area of ​​a measurement target, the second working area including one or more examples of a second moiré pattern comprising gratings in the two sample layers.

[0054] Images of the first cloud pattern 206 and the second cloud pattern 210 can be associated with separate images or a single, shared image. In one embodiment (e.g., using metrology system 100), an image of the entire metrology target 202 is obtained, allowing analysis of the first working area 204 and the second working area 208 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 area 204 and the second working area 208. In this regard, different imaging conditions (e.g., different illumination wavelengths, illumination incident angles, polarization, focal points, or the like) can be used to generate images.

[0055] In another embodiment, method 300 includes a step 306 of determining a first axis of symmetry orthogonal to one of the measurement directions associated with the first working area. In another embodiment, method 300 includes a step 308 of determining a second axis of symmetry orthogonal to the measurement direction associated with the second working area. In another embodiment, method 300 includes a step 310 of measuring a difference between the first axis of symmetry and the second axis of symmetry. Furthermore, one or more centers of symmetry 212 of the working areas can be found by determining axes of symmetry orthogonal to different directions (e.g., orthogonal directions) and finding an intersection point.

[0056] In step 302 or step 304, any technique known in this art can be used to determine the axis of symmetry of the first working area 204 and the second working area 208. For example, a technique suitable for measuring the center of symmetry of a typical AIM target without cloud-like patterns can be used.

[0057] Figure 3B is a flowchart illustrating a method 312 for determining a work group orthogonal to an axis of symmetry of a measurement direction. In this respect, method 312 may be associated with steps 306 and / or 308. Figure 4 is a top view of the measurement target 202 of Figure 2C with additional markings illustrating the application of method 312 according to one or more embodiments of the present invention.

[0058] In one embodiment, step 314 includes selecting a region of interest (ROI) comprising at least a portion of the elements within a working group. For example, the solid box in FIG4 represents a portion of ROI 402 of the first moiré pattern 206 in the first working area 204, and the dashed box represents a portion of ROI 404 of the second moiré pattern 210 in the second working area 208.

[0059] In another embodiment, step 316 includes generating a measurement signal based on the ROI. For example, the values ​​of a measurement signal along the measurement direction may include the average (or cumulative) intensity values ​​of the ROI along an orthogonal direction.

[0060] As considered herein, moiré patterns associated with a moiré pattern (e.g., first moiré pattern 206 or second moiré pattern 210) may have relatively low contrast compared to a grating structure forming a moiré pattern visible in an image of the measurement target 202. In another embodiment, step 318 includes isolating moiré patterns in the measurement signal. For example, step 318 may include one or more orders of moiré patterns (e.g., first order, second order, or similar). Isolating moiré patterns can reduce noise and increase the accuracy and / or sensitivity of the determination of the axis of symmetry in steps 306 and / or 308, and thus improve the accuracy and / or robustness of the determination of the axis of symmetry.

[0061] In step 318, any technique known in this art can be used to isolate the moiré patterns. In one embodiment, the measurement signal is filtered to isolate the moiré patterns. For example, a 1D or 2D Fourier filter or other frequency-based decomposition technique (e.g., decomposing the measurement signal into sine and / or cosine signals) can be used to analyze the spatial frequency content of the measurement signal. Furthermore, a frequency-selective filter can be used to isolate the spatial frequency of the moiré patterns (e.g., first-order moiré patterns, second-order moiré patterns, or the like).

[0062] Referring again to FIG3A, in another embodiment, method 312 may include step 320 of determining one of the axes of symmetry 406 orthogonal to the measurement direction of the working area based on the measurement signal. Any technique known in this art can be used to determine one of the axes of symmetry 406 orthogonal to the measurement direction of a working area (e.g., in step 306 or step 308 of method 300 or step 320 of method 312). In one embodiment, a portion of the image of the first mottled pattern 206 (e.g., the ROI of step 314) is associated with one of its own symmetrical versions to determine the axis of symmetry. For example, one of the axes of symmetry 406 of the first working area 204 may be determined by: generating a copy of a portion of the image containing the first working area 204; rotating the image copy by the associated rotational symmetry of the measurement target 202 (e.g., 90⁰, 180⁰, or similar); and performing a correlation between the original image and the image copy at various positions varying along the measurement direction. In this regard, the position along the measurement direction that provides the highest correlation between the original image and the rotated version can correspond to the axis of symmetry 406 of the first working area 204. By another example, in the case of a working area with reflection symmetry (or mirror symmetry), the axis of symmetry 406 of a working area can be determined by correlating a reflection copy of the working area with the working area itself.

[0063] As previously described herein, the steps associated with method 312 can be applied to determine the axis of symmetry 406 of any number of work areas. For example, steps 306 and 308 may both incorporate the application of method 312.

[0064] However, it should be understood that the method 312 for determining a symmetry axis 406 along a measurement direction of a working area is provided for illustrative purposes only and should not be disclosed as limiting. In fact, any technique known in this art can be used to determine a symmetry axis 406. In one embodiment, a symmetry axis 406 along a measurement direction of a working group can be applied by: first determining the region of interest (e.g., ROI 402 in step 314); isolating one or more orders of moiré fringes along the measurement direction by performing a spatial frequency filtering operation on the entire ROI (e.g., by processing the features to have infinite dimensions along an orthogonal direction and applying a 1D filtering operation similar to step 318); and associating the ROI with one of its rotated versions to determine the symmetry axis 406.

[0065] In another embodiment, method 300 includes step 322 of calculating a superposition error between two sample layers along the measurement direction by dividing the difference between the axes of symmetry of the first working region 204 and the second working region 208 orthogonal to the measurement direction by a moiré gain factor. For example, in FIG4, the first working region 204 and the second working region 208 both have overlapping axes of symmetry 406 orthogonal to the measurement direction (e.g., the X direction), because FIG4 illustrates a case where there is no superposition error. However, in the case of a physical superposition error, the axes of symmetry 406 will not overlap.

[0066] Due to the presence of moiré patterns, the difference between the symmetry axes determined in step 310 is not equal to the entity overlap error between the first and second sample layers. In fact, the difference between the symmetry axes corresponds to the relative offset of the moiré patterns. Therefore, the entity overlap error along a measurement direction can be determined by dividing the difference between the symmetry axes orthogonal to the measurement direction by a moiré gain associated with a specific layout of the measurement target 202.

[0067] It should be understood in this paper that the moiré gain depends on the specific layout of the metrology target 202. For example, the moiré gain factor associated with the metrology target 202 in Figure 2A, where the stacking system is related to the offset of layer A relative to layer B. It can be characterized as: (7) Similarly, the moiré gain factor associated with the offset of layer A relative to layer B can be characterized as follows: .

[0068] As can be understood from 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 in turn are functions of specific values ​​of the grating pitches P, Q, S, and T. Therefore, the values ​​of the grating pitches P, Q, S, and T can be selected to increase or otherwise optimize the combined moiré gain and thus its sensitivity to physical overlap errors. For example, the combined moiré gain can typically be increased by selecting the values ​​of the grating pitches P, Q, S, and T such that the moiré gains of the first working area 204 and the second working area 208 have different signs.

[0069] Referring now to Figures 5 and 6, it is understood that method 300 is not limited to the specific geometry of the measurement target 202 shown in Figure 2A. In a general sense, a working area (e.g., a first working area 204 or a second working area 208) may contain any number of instances of a mottled pattern (e.g., a first mottled pattern 206 or a second mottled pattern 210), provided that the working area is rotationally and / or reflectively symmetrical.

[0070] Figure 5 is a top view of a measurement target 202 comprising a first working area 204 including a first cloud pattern 206 and a second working area 208 including a second cloud pattern 210, according to one or more embodiments of the present invention.

[0071] Figure 6 is a top view of a measurement target 202 comprising a first working area 204 including a first cloud pattern 206 and a second working area 208 including a second cloud pattern 210, according to one or more embodiments of the present invention.

[0072] Referring now to Figures 7 to 10, the general moiré measurement target 202 and method 300 shown in Figure 2A can be further extended to a hybrid measurement target, wherein one working area contains one or more examples of a moiré pattern and a second working area contains one or more examples of a single-layer structure (e.g., a grating-free grating structure without a moiré pattern).

[0073] Figure 7 is a top view of a hybrid metrology target 202 for measurement along two or more orthogonal directions according to one or more embodiments of the present invention, wherein one measurement in each direction is based on a working area having one or more moiré patterns and another working area having one or more single-layer structures. Specifically, the metrology target 202 shown in Figure 7 represents a variant of Figure 2A where T=0 and no grating structure exists on layer A for the second working area 208. Therefore, the moiré gain for this metrology target 202 is related to an offset of layer A relative to layer B. Tie: (8) Furthermore, the third working area 224 and the fourth working area 226 of the measurement target 202 shown in Figure 7 are rotated versions of the first working area 204 and the second working area 208 to provide similar measurements along the Y direction. However, it should be understood that Figure 7 and the associated descriptions are provided for illustrative purposes only and should not be construed as limiting. Hybrid targets are not limited to the layout shown in Figure 7. For example, the working areas in the X or Y direction may have any rotationally symmetric distribution of features. By another example, the single-layer features in the second working area 208 may contain aperiodic features and are therefore not limited to the grating structure shown in Figure 7.

[0074] Figure 8 is a top view of a measurement target 202 for measurement along a single direction (e.g., the X direction) according to one or more embodiments of the present invention. The measurement target 202 includes a first working area 204 including a first mottled pattern 206 and a second working area 208 including (e.g., on layer A or layer B) a single-layer grating structure.

[0075] Figure 9 is a top view of a measurement target 202 for measurement along a single direction (e.g., the X direction) according to one or more embodiments of the present invention. Specifically, the first working area 204 includes a single example of a mottled pattern and the second working area 208 includes two examples of a single-layer grating structure (e.g., on layer A or layer B).

[0076] For consideration purposes, Figures 8 and 9 illustrate examples of two non-mottled patterns in one working area and one or two mottled patterns in another working area. However, it should be understood that additional variations are within the spirit and scope of this invention. In a general sense, the first working area 204 may contain any number of mottled patterns arranged in any rotational symmetry, and the second working area 208 may contain any number of non-mottled patterns arranged in any rotational symmetry.

[0077] As previously described herein, a working area containing one or more non-mosaic patterns (e.g., as a reference to one working area having one or more mosaic patterns) may have any rotationally symmetric distribution with periodic or aperiodic characteristics. Furthermore, a working area with non-mosaic characteristics may, but is not required to, partially or completely overlap with a working area having a mosaic pattern.

[0078] Figure 10 is a top view of a mottled metrological target 202 in a frame according to one or more embodiments of the present invention, wherein a first working area 204 includes a single example of a mottled pattern 1002 and a second working area 208 includes a non-periodic single-layer structure surrounding the mottled pattern 1002. In Figure 10, the mottled pattern 1002 includes a grating-on-grating structure that is periodic only in the X direction, such that the metrological target 202 in Figure 10 can be used to provide overlay measurements in the X direction (e.g., according to method 300). However, it should be understood that the exemplary metrological target 202 in Figure 10 is provided for illustrative purposes only and should not be construed as limiting. In one embodiment, but not shown, a mottled target in a frame includes a 2D mottled pattern having a grating-on-grating structure that is periodic along both the X and Y directions to provide overlay measurements along both the X and Y directions.

[0079] Referring now to Figures 11A through 12B, in some embodiments, the metrological target 202 includes a reference layer. In this regard, the structure in the reference layer can optically interact with the structure in an additional layer to provide a device-like metrological target (e.g., a target having characteristics similar to device features in terms of size, density, and / or pitch). Device-like metrological targets including a reference layer are generally described in U.S. Patent Application No. 2018 / 0188663, published July 5, 2018, the entire contents of which are incorporated herein by reference.

[0080] Figure 11A is a top block diagram of a three-layer metrological target 202 having a moiré pattern based on a grating stacked grating structure across multiple layers according to one or more embodiments of the present invention. Figure 11B is a conceptual view of a multi-layer grating structure in the moiré pattern of Figure 11A according to one or more embodiments of the present invention. In one embodiment, the metrological target 202 includes a first working area 204 including one of two examples of a first moiré pattern 1102 and a second working area 208 including one of two examples of a second moiré pattern 1104. As shown in Figure 11B, the first moiré pattern 1102 is formed by a grating structure 1106 with a pitch P in a first layer (layer A), a grating structure 1108 with a pitch Q in a reference layer (layer B), and a gratingless structure in a third layer (layer C), wherein P ≠ Q. Furthermore, the second mottled pattern 1104 is formed by a grating structure 1110 with a pitch x∙Q in the reference layer (layer B) and a grating structure 1112 with a pitch x∙P in the third layer (layer C), where x is a positive number (e.g., a rational number). In this respect, both the first mottled pattern 1102 and the second mottled pattern 1104 are incorporated into the structure of the reference layer. Moreover, the reference layer structure does not need to have the same pitch in the first mottled pattern 1102 and the second mottled pattern 1104.

[0081] In this paper, the metrological target 202 in Figure 11A is considered to be suitable for providing overlay measurements between layers A and C. The moiré gain associated with an offset of layer A relative to layer B is used for this metrological target 202. Tie: (9)

[0082] Figure 11C is a top view of a measurement target 202 based on Figures 11A and 11B for measurement along two or more orthogonal directions according to one or more embodiments of the present invention. Specifically, the measurement target 202 shown in Figure 11C represents an instance where x=1.

[0083] It should be understood that Figures 11A to 11C and their associated descriptions are provided for illustrative purposes only and should not be construed as limiting. In some embodiments, the metrological target 202 includes at least one working area having a moiré pattern of one of three overlapping gratings in three layers (one of which is a reference layer). In this regard, the overlap between the other two pairs of layers can be recovered by algorithm (e.g., by filtering one or more measurement signals to isolate or suppress desired spatial frequencies and / or moiré fringes) or physically (e.g., using an additional working area).

[0084] Figure 12A is a top block diagram of a triple-mottled metrological target 202 according to one or more embodiments of the present invention. Figure 12B is a conceptual view of a multilayer grating structure in the mottled pattern of Figure 12A according to one or more embodiments of the present invention. In one embodiment, the metrological target 202 includes a first working area 1202 including one of two examples of a first mottled pattern 1204, a second working area 1206 including one of two examples of a second mottled pattern 1208, and a third working area 1210 having one of two examples of a third mottled pattern 1212. As shown in Figure 12B, the first mottled pattern 1204 is formed by a grating structure 1214 with a pitch Q in a first layer (layer A), a grating structure 1216 with a pitch P in a second layer (layer B), and a grating-free structure in a third layer (layer C), wherein P≠Q. The second cloud pattern 1208 is formed by a grating structure 1218 with a pitch T in the first layer (layer A) and a grating structure 1220 with a pitch S in the second layer (layer B), wherein T≠S. The third cloud pattern 1212 is formed by a grating structure 1222 with a pitch T in the third layer (layer C) and a grating structure 1224 with a pitch S in the second layer (layer B).

[0085] Considering this paper, the metrological target 202 in Figure 12A is suitable for providing overlay measurements between layers A and C, or between layers A and B. The moiré gain associated with an offset of layer A relative to layer B is used for this metrological target 202. Tie: (10)

[0086] Moiré gain associated with the offset of layer A relative to layer C for this measurement target 202 Tie: (11)

[0087] It should be understood that Figures 12A and 12B and their associated descriptions are provided for illustrative purposes only and should not be construed as limiting. For example, a triple moiré metrology target 202 may typically include additional variations in the configuration of grating structures spanning three layers. In one embodiment, a triple moiré metrology target 202 includes a first working area 1202 having a moiré pattern based on grating structures on layers A and B, a second working area 1206 having grating structures on layers B and C, and a third working area 1210 including grating structures on layers A and C. This target may also be adapted for overlay measurements between layers A and B, and between layers B and C. Furthermore, a metrology target 202 may typically have any number of layers. For example, an N-tuple metrology target 202 may have N layers, wherein grating structures are provided on selected layers to provide overlay measurements between those selected layers.

[0088] Referring now to Figures 13A to 14, a more detailed description is given of a measurement of a superimposed target having a single working area with one or more examples of a mottled pattern for measurement in a single direction.

[0089] As considered herein, the information associated with an image of a mottled pattern 1302 can vary based on the optical conditions associated with the generation of that image, such optical conditions being (but not limited to): the wavelength of the illumination used to generate an image (e.g., the wavelength of the illumination beam 108), the angle of incidence of the illumination, or the focal position of a detector used to generate an image (e.g., detector 104). For example, mottled fringes may be observable in some optical configurations but not in others. Similarly, one or more of the underlying grating structures may be visible in some optical configurations but not in others. Therefore, the axis of symmetry of two images of a single mottled pattern acquired under different optical configurations can differ from that described in almost the same manner as in Figures 2A to 6.

[0090] Figure 13A is a top view of a metrological target 202 according to one or more embodiments of the present invention, comprising a single moiré pattern 1302 adapted to provide superimposed measurement along a single direction based on an image generated by multiple optical configurations. For example, the metrological target 202 of Figure 13A may be adapted to measure along a single direction (e.g., the X direction) based on an image generated under the condition that the moiré pattern stripes are detectable and an image generated under the condition that at least one of the gratings below the moiré pattern 1302 is detectable.

[0091] As considered in this paper, the measurement target 202 shown in Figure 13 can be about 3 to 4 times smaller than a target designed using typical methods, which can significantly reduce the space on a sample 102 that must be dedicated to measurement.

[0092] Figure 13B is a top view of a measurement target 202 comprising one of two examples of a working area having a mottled pattern 1302 according to one or more embodiments of the present invention. The mottled pattern 1302 is adapted to provide superimposed measurement along a single direction based on an image generated by multiple optical configurations. As with Figure 13A, the measurement target 202 illustrated in Figure 13B can be adapted to measure along a single direction (e.g., the X direction) based on an image generated under the condition that the mottled stripes are detectable and an image generated under the condition that at least one of the grating below the mottled pattern 1302 is detectable.

[0093] Furthermore, the use of multiple examples of the cloud pattern 1302 in the metrological target 202 illustrated in Figure 13B provides flexibility in distributing the metrological target 202 across the sample 102. That is, various elements including (but not limited to) device features can be positioned among various features of the metrological target 202.

[0094] Figure 14 is a flowchart illustrating the steps performed in a method 1400 for measurement overlay according to one or more embodiments of the present invention. The applicant notes that the embodiments and enabling techniques previously described in the background of the metrology system 100 herein should be interpreted as extensions of method 1400. However, it should be further noted that method 1400 is not limited to the architecture of the metrology system 100.

[0095] In one embodiment, method 1400 includes a step 1402 of receiving a first image of a metrological target on a sample using a first optical configuration, wherein the metrological target is a working area having one or more examples of a mottled pattern (e.g., a first mottled pattern 206). In another embodiment, method 1400 includes a step 1404 of determining a first axis of symmetry of the working area along the measurement direction based on the first image. In another embodiment, method 1400 includes a step 1406 of receiving a second image of a metrological target on a sample using a second optical configuration different from the first optical configuration. In another embodiment, method 1400 includes a step 1408 of determining a second axis of symmetry of the working area along the measurement direction based on the second image.

[0096] Furthermore, images generated in steps 1402 and 1406 based on different optical configurations can be generated sequentially or simultaneously. For example, associated optical conditions can be generated sequentially or simultaneously (e.g., via metrology system 100).

[0097] In another embodiment, method 1400 includes step 1410 of determining a difference between a first axis of symmetry and a second axis of symmetry.

[0098] As considered in this paper, it is desirable to filter the spatial frequencies of the first or second signal in order to isolate a desired signal. The technique for isolating moiré fringes described in step 318 of Figure 3B can generally be used to isolate any selected group of spatial frequencies, such as (but not limited to) spatial frequencies associated with one of the grating structures in a moiré fringe or spatial frequencies associated with any selected order of the moiré fringe.

[0099] For example, it is desirable to provide a first optical configuration in which only the moiré fringes are resolvable and none of the underlying grating structures are desired. However, if one or both of the underlying grating structures are visible, the moiré fringes can be isolated before determining the axis of symmetry. By another example, it is desirable to provide a second optical configuration in which only one of the underlying grating structures is resolvable (e.g., for use as one of the "reference layer" operations described earlier herein). However, if the moiré fringes or other grating structures are resolvable, the desired grating structure can be isolated.

[0100] In another embodiment, method 1400 includes a step 1412 of calculating an overlay error associated with a misalignment of the second sample layer relative to one of the first sample layers by dividing the difference between the first and second symmetry axes by the moiré gain.

[0101] It should be understood in this document that method 1400 can be substantially similar to method 300 described above. However, the moiré gain can differ depending on which structures are resolved in the two images (e.g., in steps 1402 and 1404). For example, the moiré gain may differ depending on whether a second optical configuration resolves moiré fringes in one optical configuration and if a receiver layer A is also resolved in another optical configuration. It can be characterized as: (10) The sign depends on which layer is used as the reference frame. Similarly, in the case of resolving moiré fringes in an optical configuration and if an optical configuration receiver layer B is used for resolving a second optical configuration, the moiré gain... It can be characterized as: (11)

[0102] Method 1400 is also applicable to any type of metrological target having a mottled pattern, including (but not limited to) any of the targets described herein. In this regard, different optical configurations providing different image content can be used to generate additional information from any work area containing a mottled pattern.

[0103] It should be understood that Figures 2A to 14 are provided for illustrative purposes only and should not be construed as limiting. In some embodiments, a metrology target 202 includes a working area having one or more examples of a moiré pattern comprising three or more layers of gratings, wherein each of the gratings may have a different pitch. In this respect, an image of one of the moiré patterns may contain a scattering signal that is two, three, or higher times greater than that associated with moiré fringes caused by interference between the three or more gratings. Further considered herein, a moiré pattern based on three or more overlapping gratings can be incorporated into various designs of a metrology target 202 and can be adapted for analysis using multiple algorithms. For example, method 1400 can be applied to extract a stack pair between two sample layers of this metrology target 202 based on multiple images of the moiré pattern, wherein different optical configurations are selected to detect moiré fringes between different sample layers and / or detect any of the underlying grating structures. In one example, different optical configurations in which different moiré patterns (e.g., interference between different sample layers) are visible can be generated. By another example, as previously described herein, any of the techniques for isolating moiré patterns described in step 318 of reference method 312 (e.g., Fourier filtering) can be applied to isolate any selected spatial frequencies, such as (but not limited to) those associated with moiré patterns between different sample layers and / or any of the underlying grating structure. By another example, method 300 can be applied to generate a stacked measurement between two sample layers based on a comparison of one axis of symmetry of a working group having a moiré pattern with one axis of symmetry of another working group along a measurement direction. In this respect, an additional layer of the moiré pattern can be used as a reference layer. Furthermore, the stacked measurement can be based on any combination of the above techniques.

[0104] It should be understood in this document that specific design parameters of the grating-on-grating moiré pattern described herein can affect various aspects of the associated moiré stripes, such as (but not limited to) the number of moiré stripes, the period of the moiré stripes, the order of the moiré stripes (e.g., first-order, second-order, or similar), the contrast of the moiré stripes, or crosstalk between adjacent moiré patterns. For example, moiré stripes can typically be affected by target design parameters (e.g., the pitch of the grating structure, the duty cycle of the grating structure, the feature segmentation that provides features with design-level dimensions, the size of the exclusion area, the overall target size, or similar), measurement workpiece parameters, stacking parameters (e.g., information containing printing / patterning procedures), and / or lithography tool parameters.

[0105] Therefore, in some embodiments, one or more of a metrology target design, metrology tool metrology formulation, lithography tool formulation, or the like are selected based on the simulation and / or measurement of the test target. For example, an experimental design in which a series of candidate parameters (e.g., target design parameters, metrology tool parameters, lithography tool parameters, or the like) are systematically varied can be generated. The performance of each candidate parameter set can be evaluated through any combination of simulation and fabrication / measurement of the test target. Then, a parameter set can be selected from the candidate parameter set based on any selected metric or combination of metrics, such as (but not limited to): contrast of a moiré pattern, a moiré pattern period, generation of a selected moiré pattern fringe, overlay accuracy, measurement repeatability, robustness to variations in the optical configuration of the imaging system, or robustness to variations in the moiré pattern.

[0106] In some embodiments, Maxwell-based simulations are used to optimize parameters associated with grating structures on one or more working areas containing at least one moiré pattern. Maxwell-based simulations may include any type of simulation incorporating Maxwell's equations, such as (but not limited to) a strictly coupled-wave analysis (RCWA) simulation. For example, Maxwell-based simulations may, but are not required to, be performed using AcuRate software from KLA. Furthermore, Maxwell-based approximations or simpler approximations (such as the Kirchoff approximation assuming Fourier optical effects) may be used to simulate optical crosstalk between adjacent moiré patterns.

[0107] In some embodiments, the tool parameters (e.g., metrological tool parameters, lithography tool parameters, or the like) are optimized by first identifying a selected set of candidate target design parameters based on simulation, manufacturing only these selected candidate sets, and measuring performance using various tool parameters. Then, the optimal combination of design parameters and tool parameters can be selected based on a desired performance metric. In this respect, using simulation to reduce the number of manufactured test samples can effectively provide the selection of optimal parameters.

[0108] The objects described herein sometimes depict different components contained within or connected to other components. It should be understood that the architectures depicted are merely illustrative, and many other architectures can actually be implemented to achieve the same functionality. In a conceptual sense, any configuration of components achieving the same function is effectively “associated” to achieve the desired function. Therefore, any two components combined herein to achieve a particular function can be considered “associated” with each other to achieve the desired function, regardless of the architecture or intermediate components. Similarly, any two such associated components can also be considered “connected” or “coupled” to each other to achieve the desired function, and any two components that can be suchly associated can also be considered “coupled” to each other to achieve the desired function. Specific examples of couplerability include (but are not limited to) physically interactive and / or physically interactive components and / or wirelessly interactive and / or logically interactive and / or logically interactive components.

[0109] It is believed that the invention and many of its accompanying advantages will be understood from the foregoing description, and it will be apparent that various changes can be made to the form, construction, and configuration of the components without departing from the disclosed subject matter or without sacrificing all its important advantages. The described forms are merely illustrative, and the following claims are intended to cover and include such changes. Furthermore, it should be understood that the invention is defined by the appended claims.

[0110] 100: Metering System 102: Sample / Static Sample 104: Detector 106: Lighting source 108: Illumination Beam / Continuous Illumination Beam / Pulsed Illumination Beam / Modulated Illumination Beam 110: Lighting Path 112: Lens 114: Illumination Optical Components 116: Objective lens 118: Sample stage 120: Sample light 122: Light collection path 124: Light-collecting path lens 126: Light-collecting optical components 128: Beam splitter 130: Controller 132: Processor 134: Memory Media / Non-Temporary Memory Media 202: Metrological Targets / General Moiré Metrological Targets / Hybrid Metrological Targets / Moiré Metrological Targets within a Frame / Three-Layer Metrological Targets / Triple Moiré Metrological Targets / N-Tuple Metrological Targets 204: First Working Area 206: First cloud pattern 208: Second Working Area 210: Second cloud pattern 212: Center of Symmetry 214: First grating structure 216: Second grating structure 218: Third grating structure 220: Fourth grating structure 222: Exclusion Zone 224: Third Work Area 226: Fourth Working Area 300: Method 302: Steps 304: Steps 306: Steps 308: Steps 310: Steps 312: Method 314: Steps 316: Steps 318: Steps 320: Steps 322: Steps 402: Region of Interest (ROI) 404: Region of Interest (ROI) 406: Axis of symmetry 1002: Cloud pattern 1102: First cloud pattern 1104: Second cloud pattern 1106: Grating Structure 1110: Grating Structure 1112: Grating Structure 1202: First Working Area 1204: First cloud pattern 1206: Second Work Area 1208: Second cloud pattern 1210: Third Work Area 1212: Third cloud pattern 1214: Grating Structure 1216: Grating Structure 1218: Grating Structure 1220: Grating Structure 1222: Grating Structure 1224: Grating Structure 1302: Cloud Pattern 1400: Method 1402: Steps 1404: Steps 1406: Steps 1408: Steps 1410: Steps 1412: Steps

Claims

1. A method for designing a moiré measurement target, comprising: The method receives a design for a metrological target, the metrological target comprising: a working area including one or more non-overlapping examples of a moiré pattern, wherein the moiré pattern includes a first grating structure on a first sample layer having a first pitch (p1) along a measurement direction and a second grating structure on a second sample layer having a second pitch (p2) along the measurement direction, wherein the second pitch is different from the first pitch; receives an optical configuration of an imaging system for imaging the metrological target; and performs an experimental design to select a set of target design parameters for the metrological target that provides a moiré pattern satisfying a selected design criterion when imaged by the imaging system, wherein the target design parameter set includes the first pitch and the second pitch, wherein the experimental design includes: manufacturing test metrological targets having N candidate target design parameter sets, wherein each of the test metrological targets includes a working area as described in the metrological target; and imaging the test metrological targets; The images of the test and measurement targets are analyzed using the selected design criteria, wherein one of the superposition errors of the test and measurement targets along the measurement direction corresponds to a difference between the axis of symmetry of the working area of ​​the test and measurement target imaged using a first optical configuration of the imaging system and a second optical configuration of the imaging system, the superposition error being associated with the first sample layer and the second sample layer of the test and measurement target, wherein moiré patterns can be detected by the first optical configuration, and at least one of the grating structure having the first pitch or the grating structure having the second pitch can be detected by the second optical configuration; and based on the analysis of the images of the test and measurement targets, a target design parameter set is selected from the N candidate target design parameter sets.

2. The method of claim 1, wherein the selected design criteria for the cloud-pattern stripe pattern include: At least one of the following: contrast of a cloud-like stripe pattern, overlay accuracy, measurement repeatability, generation of a cloud-like stripe period, or generation of a selected cloud-like stripe order.

3. The method of claim 1, wherein the target design parameter set further includes at least one of a critical size of the first grating structure, a critical size of the second grating structure, a segmentation period of the first grating structure, a segmentation period of the second grating structure, a size of the first grating structure, or a size of the second grating structure.

4. The method of claim 1, wherein crosstalk is simulated using at least one of a Maxwell-based simulation or a Kirchhoff approximation.

5. The method of claim 1, wherein the imaging system comprises: A light source for generating illumination; one or more illumination optics, or the like, for directing illumination from the light source to the measurement target; and a detector for generating an image of the measurement target based on the illumination from the light source, wherein an optical configuration of the imaging system is configurable, wherein the optical configuration includes a wavelength of the illumination, a polarized light of the illumination incident on the measurement target, an angle of the illumination incident on the measurement target, or a focal position of the measurement target relative to the detector, wherein the target design parameter set further includes the optical configuration of the imaging system.

6. The method of request item 1, wherein the experimental design includes: Simulate the image of the measurement target with a plurality of candidate target design parameter sets; And based on these selected design criteria, select a target design parameter set from the plurality of candidate target design parameter sets.

7. The method of claim 6, wherein simulating the image of the measurement target having the plurality of candidate target design parameter sets includes: The image of the measurement target with multiple candidate target design parameter sets is simulated using Maxwell-based simulation.

8. The method of request item 1, wherein the experimental design includes: Simulate the image of the measurement target with a plurality of candidate target design parameter sets; Based on the selected design criteria, the plurality of candidate target design parameter sets are narrowed down to the N target design parameter sets; and based on the analysis of the images of the test and measurement targets, a measurement tool parameter set is selected.

Images