Patent Description:
Image-based overlay metrology may typically include determining relative offsets between two or more layers on a sample based on relative imaged positions of features of an overlay target in the different layers of interest. Accordingly, the accuracy of the overlay measurement may be sensitive to alignment errors of the overlay target in the metrology tool. Typical overlay metrology systems may align a sample once per wafer batch or measurement recipe. However, overlay measurements may be performed at various overlay targets distributed across a sample and the optimal alignment of each overlay target may not be the same due to sample variations, differences in target design, or the like. Accordingly, a single alignment of a sample may result in decreased overlay measurement precision due to localized variations of overlay targets. Therefore, it may be desirable to have systems and methods to efficiently align an overlay metrology system to any selected overlay target on a sample.

<CIT> discloses a lithographic apparatus, device manufacturing method and associated data processing apparatus and computer program product.

<CIT> describes focus finding and alignment using a split linear mask.

<CIT> describes a lithographic apparatus and device manufacturing method.

<CIT> discloses an alternative target design for metrology using modulation techniques.

<CIT> describes a metrology method and inspection apparatus, lithographic system and device manufacturing method.

The present invention provides an overlay metrology system as recited in claim <NUM>.

The present invention further provides an overlay metrology method as recited in claim <NUM>.

Embodiments of the present disclosure are directed to systems and methods for site-by-site alignment of an overlay metrology tool. Alignment errors such as, but not limited to, focus errors, telecentricity errors, or centering errors may negatively impact the measurement accuracy of image-based overlay metrology. For example, defocus of a sample may decrease measurement precision due a loss of image contrast. Further, asymmetries of an overlay target may induce focus-dependent overlay errors during measurement. By way of another example, telecentricity errors may induce tool-induced shift (TIS), which may directly negatively impact overlay determinations.

It is recognized herein that optimal alignment of an overlay metrology tool may differ from one overlay target to the next due to a variety of factors such as, but not limited to, sample variations or stage errors. Further, some TIS components such as, but not limited to, sample chuck tilt or rotation stage wobble may rotate with the sample and may thus contribute to measurement inaccuracy when telecentricity is corrected only to minimize non-rotating TIS. Accordingly, embodiments of the present disclosure are directed to alignment of any number of selected overlay targets within an overlay metrology tool prior to or as part of an overlay measurement to provide highly-precise overlay measurements.

It is further recognized herein that metrology tool alignment operations may negatively impact the measurement throughput. Additional embodiments of the present disclosure are directed to efficient alignment of an overlay metrology tool at a selected measurement site (e.g., at a selected overlay target).

Some embodiments of the present disclosure are directed to capturing two or more alignment images of an overlay target with an imaging system at different focal positions and generating alignment data (e.g., focus data, telecentricity data, centering data, or the like) based on the alignment images. For example, alignment data may include data indicative of a difference between the focal positions at which the alignment images were captured and a nominal or target focal position (e.g., a focus error). In one instance, such focus data may include image contrast metrics of the alignment images indicative of focus errors in the alignment images. By way of another example, the alignment data may include lateral shifts of imaged features and/or magnification variations as a function of focal position. Accordingly, this alignment data may be indicative of telecentricity errors of the sample within the imaging system. Further, the alignment data may be used to accurately position (e.g., center) selected features of the overlay target within a field of view of the imaging system.

Additional embodiments of the present disclosure are directed to aligning the sample in the imaging system within selected alignment tolerances (e.g., focus tolerances, telecentricity tolerances, centering tolerances, or the like) based on the alignment data. Accordingly, an overlay metrology tool may be aligned to any number of selected overlay targets on a sample prior to generating overlay measurements on the selected overlay targets to facilitate robust and accurate overlay measurements for each target.

For example, the sample (or one or more features one a selected layer of the sample) may be aligned to be in focus on a selected camera of the imaging system based on the focus data. In this regard, one or more elements of the imaging system such as, but not limited to a sample stage, a position of an objective lens, or the like may be adjusted to focus the sample within the selected focus tolerances (e.g., a range of focus positions providing a desired image quality). By way of another example, one or more components of the imaging system such as, but not limited to, an aperture stop may be adjusted to provide telecentric imaging of the sample within the selected telecentricity tolerances (e.g., allowable deviations of the lateral position, magnification, or the like of imaged features).

Additional embodiments are directed to capturing a measurement of the sample after aligning the sample based on the alignment data. Further embodiments are directed to determining overlay between two or more layers of the sample based on the measurement image. In this regard, a highly precise overlay measurement may be generated for each overlay target.

<FIG> is a conceptual view illustrating an image-based overlay metrology system <NUM> suitable for site-by-site alignment, in accordance with one or more embodiments of the present disclosure. For example, the overlay metrology system <NUM> may determine overlay (e.g., overlay errors) between two or more layers of a sample <NUM> based on images of overlay targets distributed across the sample <NUM>.

In one embodiment, the overlay metrology system <NUM> includes a telecentric imaging system <NUM> to generate one or more images of the sample <NUM> and a controller <NUM> to determine overlay of two or more layers of the sample <NUM> based on images from the imaging system <NUM>. Further, overlay metrology system <NUM> may include a sample stage <NUM> for positioning selected portions of the sample <NUM> (e.g., selected overlay targets) within a field of view of the imaging system <NUM> for the determination of overlay. The sample stage <NUM> may include any device suitable for positioning the sample <NUM> within the overlay metrology system <NUM>. For example, the sample stage <NUM> may include any combination of linear translation stages, rotational stages, tip/tilt stages or the like.

In another embodiment, the imaging system <NUM> includes one or more adjustable components suitable for aligning a portion of the sample <NUM> within the selected alignment tolerances. For example, the adjustable components may include, but are not limited to, the sample stage <NUM>, one or more aperture stops, or one or more additional translation stages suitable for adjusting optical components.

In another embodiment, the controller <NUM> includes one or more processors <NUM> configured to execute program instructions maintained on a memory medium <NUM>. In this regard, the one or more processors <NUM> of controller <NUM> may execute any of the various process steps described throughout the present disclosure. Further, the controller <NUM> may be communicatively coupled to any component of the overlay metrology system <NUM>. For example, the controller <NUM> may be communicatively coupled to the imaging system <NUM> to receive images from the imaging system <NUM> and/or control the adjustable components of the imaging system <NUM> to align the sample <NUM> within selected alignment tolerances. Further, the controller <NUM> may determine overlay associated with two or more layers of the sample <NUM> based on images received from the imaging system <NUM>.

The one or more processors <NUM> of a controller <NUM> may include any processing element known in the art. In this sense, the one or more processors <NUM> may include any microprocessor-type device configured to execute algorithms and/or instructions. In one embodiment, the one or more processors <NUM> may consist of a desktop computer, mainframe computer system, workstation, image computer, parallel processor, or any other computer system (e.g., networked computer) configured to execute a program configured to operate the overlay metrology system <NUM>, as described throughout the present disclosure. It is further recognized that the term "processor" may be broadly defined to encompass any device having one or more processing elements, which execute program instructions from a non-transitory memory medium <NUM>. Further, the steps described throughout the present disclosure may be carried out by a single controller <NUM> or, alternatively, multiple controllers. Additionally, the controller <NUM> may include one or more controllers housed in a common housing or within multiple housings. In this way, any controller or combination of controllers may be separately packaged as a module suitable for integration into overlay metrology system <NUM>. Further, the controller <NUM> may analyze data received from the detector assembly <NUM> and feed the data to additional components within the overlay metrology system <NUM> or external to the overlay metrology system <NUM>.

The memory medium <NUM> may include any storage medium known in the art suitable for storing program instructions executable by the associated one or more processors <NUM>. For example, the memory medium <NUM> may include a non-transitory memory medium. By way of another example, the memory medium <NUM> 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 memory medium <NUM> may be housed in a common controller housing with the one or more processors <NUM>. In one embodiment, the memory medium <NUM> may be located remotely with respect to the physical location of the one or more processors <NUM> and controller <NUM>. For instance, the one or more processors <NUM> of controller <NUM> may access a remote memory (e.g., server), accessible through a network (e.g., internet, intranet and the like). Therefore, the above description should not be interpreted as a limitation on the present invention but merely an illustration.

<FIG> is a conceptual view of an imaging overlay metrology system <NUM>, in accordance with one or more embodiments of the present disclosure.

In another embodiment, the overlay metrology system <NUM> includes an illumination source <NUM> to generate an illumination beam <NUM>, an illumination pathway <NUM> to direct the illumination beam <NUM> to the sample <NUM> mounted on the sample stage <NUM>, a collection pathway <NUM> to direct radiation emanating from the sample <NUM> to a detector assembly <NUM>. For example, the detector assembly <NUM> may include at least one imaging detector suitable for capturing an image of the sample <NUM>.

The illumination beam <NUM> may include one or more selected wavelengths of light 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 <NUM> may further generate an illumination beam <NUM> including any range of selected wavelengths. In another embodiment, the illumination source <NUM> may include a spectrally-tunable illumination source to generate an illumination beam <NUM> having a tunable spectrum. The illumination source <NUM> may further produce an illumination beam <NUM> having any temporal profile. For example, the illumination source <NUM> may produce a continuous illumination beam <NUM>, a pulsed illumination beam <NUM>, or a modulated illumination beam <NUM>. Additionally, the illumination beam <NUM> may be delivered from the illumination source <NUM> via free-space propagation or guided light (e.g. an optical fiber, a light pipe, or the like).

The illumination source <NUM> may include any type of illumination source suitable for providing an illumination beam <NUM>. In one embodiment, the illumination source <NUM> is a laser source. For example, the illumination source <NUM> may include, but is not limited to, one or more narrowband laser sources, a broadband laser source, a supercontinuum laser source, a white light laser source, or the like. In this regard, the illumination source <NUM> may provide an illumination beam <NUM> having high coherence (e.g., high spatial coherence and/or temporal coherence). In another embodiment, the illumination source <NUM> includes a laser-sustained plasma (LSP) source. For example, the illumination source <NUM> may include, but is not limited to, a LSP lamp, a LSP bulb, or a LSP chamber suitable for containing one or more elements that, when excited by a laser source into a plasma state, may emit broadband illumination. In another embodiment, the illumination source <NUM> includes a lamp source. For example, the illumination source <NUM> may include, but is not limited to, an arc lamp, a discharge lamp, an electrode-less lamp, or the like. In this regard, the illumination source <NUM> may provide an illumination beam <NUM> having low coherence (e.g., low spatial coherence and/or temporal coherence).

In another embodiment, the illumination source <NUM> directs the illumination beam <NUM> to a sample <NUM> via the illumination pathway <NUM>. For example, the illumination pathway <NUM> may include an objective lens <NUM> to focus the illumination beam <NUM> onto the sample <NUM>. The illumination pathway <NUM> may include one or more illumination pathway lenses <NUM> or illumination conditioning components <NUM> suitable for modifying and/or conditioning the illumination beam <NUM>. For example, the one or more illumination conditioning components <NUM> 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, or one or more beam shapers. By way of another example, the illumination pathway <NUM> may include aperture stops to control the angle of illumination on the sample <NUM> and/or field stops to control the spatial extent of illumination on the sample <NUM>.

<FIG> is a conceptual view of an illumination pathway <NUM> with an adjustable illumination aperture stop <NUM>, in accordance with one or more embodiments of the present disclosure.

In one embodiment, the illumination pathway <NUM> includes an illumination aperture stop <NUM> to provide telecentric illumination of the sample. For example, the illumination aperture stop <NUM> may be located at a plane conjugate to the back focal plane of the objective lens <NUM> and/or a tube lens (not shown). In another embodiment, the illumination pathway <NUM> includes an illumination field stop <NUM> to control the spatial extent of illumination on the sample <NUM> to be directed to the sample <NUM>. For example, the illumination field stop <NUM> may be located at a plane conjugate to the sample <NUM>. Further, the illumination pathway <NUM> may include any number of illumination pathway lenses <NUM> that facilitate placement of the illumination aperture stop <NUM> and the illumination field stop <NUM> at convenient locations.

Referring again to <FIG>, the collection pathway <NUM> may include any number of optical elements to collect radiation emanating from the sample (e.g., in response to the illumination beam <NUM>) and direct the collected radiation to the detector assembly <NUM>. In one embodiment, the collection pathway <NUM> includes a beamsplitter <NUM> oriented such that the objective lens <NUM> may simultaneously direct the illumination beam <NUM> to the sample <NUM> and collect radiation from the sample <NUM>. The collection pathway <NUM> may further include one or more collection pathway lenses <NUM> and/or conditioning collection components <NUM> suitable for modifying and/or conditioning the radiation from the sample <NUM> such as, but not limited to, one or more filters, one or more polarizers, or one or more beam blocks. Additionally, the collection pathway <NUM> may include aperture stops to control the angular extent of radiation collected from the sample <NUM> and/or field stops to control the spatial extent of an image of the sample <NUM>. In another embodiment, the collection pathway <NUM> includes a stop <NUM>. For example, the stop <NUM> may include a collection aperture stop located in a plane conjugate to the back focal plane of the objective lens <NUM> and/or a tube lens (not shown) to provide image-space telecentricity when imaging the sample <NUM>. By way of another example, the stop <NUM> may include a collection field stop located at a plane conjugate to the sample <NUM> to control the spatial extent of the image on the detector assembly <NUM>. Further, the collection pathway <NUM> may include any number of illumination pathway lenses <NUM> that facilitate placement of the stop <NUM> and the collection field stop at convenient locations.

The detector assembly <NUM> may include any number of detectors suitable for capturing radiation emanating from the sample <NUM>. For example, the detector assembly <NUM> may include one or more imaging detectors suitable for generating images at selected focal positions. For instance, an imaging detector may include, but is not limited to, a charge-coupled device (CCD), a complementary metal-oxide-semiconductor (CMOS) device, a time delay imaging (TDI) detector, one or more photomultiplier tubes (PMT), one or more avalanche photodiodes (APD), or the like. In another embodiment, the detector assembly <NUM> may include a spectroscopic detector suitable for identifying wavelengths of radiation emanating from the sample <NUM>.

Referring now to <FIG> though 8B, site by site alignment is generally described.

In one embodiment, the overlay metrology system <NUM> performs site-by-site alignment of the imaging system <NUM>. For example, the overlay metrology system <NUM> may perform overlay measurements at a multitude of overlay targets distributed across the sample <NUM> and may further individually align the imaging system <NUM> for any selected number the overlay targets. For example, an overlay measurement of an overlay target may typically begin with translating the overlay target to the field of view of the imaging system <NUM> (e.g., with the sample stage <NUM>). However, physical variations of the sample <NUM> and/or errors of the sample stage <NUM> may lead to alignment errors that may negatively impact the accuracy of an overlay measurement if not corrected.

<FIG> is a top view of an overlay target <NUM> suitable for image-based overlay metrology, in accordance with one or more embodiments of the present disclosure. In one embodiment, the overlay target <NUM> includes an advanced imaging metrology (AIM) overlay target. For example, an overlay target <NUM> may include one or more first-layer features <NUM> on a first layer of the sample <NUM> and one or more second-layer features <NUM> on a second layer of the sample <NUM>. In this regard, the overlay between the first layer and the second layer may be determined based on the relative positions of the first-layer features <NUM> and the second-layer features <NUM>.

Further, the first-layer features <NUM> and/or the second-layer features <NUM> may be oriented to facilitate overlay measurements in orthogonal directions. For example, as illustrated in <FIG>, the overlay target <NUM> may include four quadrants in which a portion first-layer features <NUM> and the second-layer features <NUM> are aligned along a first direction and a portion of the first-layer features <NUM> and the second-layer features <NUM> are oriented along a second direction orthogonal to the first direction. Further, the first-layer features <NUM> and the second-layer features <NUM> may be, but are not required to be, segmented.

It is to be understood that the overlay target <NUM> depicted in <FIG> and the associated description are provided solely for illustrative purposes and should not be interpreted as limiting. For example, an overlay target may include features on any number of layers of the sample <NUM> (e.g., <NUM> layers, <NUM>, layers, or the like). By way of another example, features of an overlay target may be oriented in any selected pattern suitable for image-based overlay measurements such as, but not limited to, a box-in-box pattern or a grating-over-grating pattern.

Alignment errors may be associated with any type of alignment metric that may impact an overlay measurement from an overlay target (e.g., overlay target <NUM>) such as, but not limited to, focus errors and telecentricity errors. For example, focus errors may be associated with a deviation of the height of the overlay target with respect to a nominal focal position such that an image of the overlay target is out of focus. Defocus in a measurement image may negatively impact overlay measurements in a variety of ways such as, but not limited to, reducing the image contrast and the measurement precision of feature edges. By way of another example, telecentricity errors may give rise to TIS. In this regard, the apparent positions of features may appear to laterally shift as a function of focal position in the imaging system <NUM>, which may induce overlay offset errors.

In accordance with the invention, the imaging system <NUM> generates two or more alignment images from the imaging system <NUM> at different focal positions for each selected overlay measurement site (e.g., each selected overlay target). It is recognized herein that alignment images at two or more focal positions may provide sufficient information to accurately determine alignment errors (e.g., focus errors, telecentricity errors, centering errors, or the like) and thus facilitate mitigation of the alignment errors on a site-by-site basis. The controller <NUM> may then generate alignment data based on the alignment images and then direct the imaging system <NUM> to align the sample <NUM> within selected alignment tolerances in the case that the sample <NUM> is outside of the alignment tolerances at the selected site. Once the imaging system <NUM> is aligned, the imaging system <NUM> may generate a measurement image and the controller <NUM> may provide overlay measurements for two or more layers of the sample <NUM> based on the measurement image.

<FIG> is a conceptual view of an imaging system <NUM> suitable for capturing sequential alignment images, in accordance with one or more embodiments of the present disclosure. In one embodiment, the detector assembly <NUM> of the imaging system <NUM> includes a single imaging detector <NUM>. Accordingly, the imaging system <NUM> may capture alignment images by sequentially modifying the focal position of the sample <NUM> and capturing images with the imaging detector <NUM>.

The focal position of sample <NUM> within the imaging system <NUM> may be controlled using any combination of elements. For example, the focal position of the sample <NUM> may be adjusted via the sample stage <NUM>. By way of another example, the focal position of the sample <NUM> may be controlled by adjusting a position of one or more elements of the collection pathway <NUM> such as, but not limited to, the objective lens <NUM> or the imaging detector <NUM>.

<FIG> is a conceptual view of an imaging system <NUM> suitable for capturing simultaneous alignment images, in accordance with the invention. In one embodiment, the detector assembly <NUM> includes two or more imaging detectors configured to simultaneously capture images at selected focal positions. For example, as illustrated in <FIG>, the detector assembly <NUM> may include three imaging detectors 302a-c to simultaneously capture three images of the sample <NUM> at three selected focal positions. The detector assembly <NUM> may further include detector optical elements to split and direct the radiation emanating from the sample <NUM> along multiple paths to be captured by the imaging detectors <NUM> such as, but not limited to, one or more beamsplitters <NUM>, one or more prisms <NUM>, one or more mirrors (not shown), or one or more detector lenses (not shown).

The detector assembly <NUM> may be configured to capture images at any selected focal positions. In one embodiment, the detector assembly <NUM> includes one imaging detector configured to capture images at a nominal (e.g., ideal) focal position at which the sample <NUM> is expected to be aligned and one or more additional imaging detectors configured to capture images at selected offsets from the nominal focal position. For example, as illustrated in <FIG>, imaging detector 302b may be configured to capture images at the nominal focal position, imaging detector 302a may be configured to capture images at a selected positive offset from the nominal focal position, and imaging detector 302c may be configured to capture images at a selected negative offset from the nominal focal position. The nominal focal position may additionally correspond to an aberrationcorrecting imaging configuration based on the specifications of the objective lens <NUM>.

The alignment data extracted from the alignment images may include any type of data suitable for providing correctables to align a selected overlay target within the imaging system <NUM>. In one embodiment, the alignment data includes focus data indicative of the relative defocus of the alignment images such as, but not limited to the image contrast. Such data may then be used (e.g., by the controller <NUM>) to accurately adjust the focal position of the sample such that the selected overlay target is in focus on a selected camera of the detector assembly <NUM> (e.g., the single imaging detector <NUM> of <FIG>, the nominal imaging detector 302b of <FIG>, or the like).

For example, images of an overlay target (e.g., overlay target <NUM>) may typically include a high image contrast including the first-layer features <NUM> and the second-layer features <NUM> visible against a background. Further, the image contrast may typically be highest when the overlay target is in focus and may typically decrease with increasing defocus. Image contrast data associated with a difference in image intensity (e.g., pixel value) between the features of the overlay target and the background for each alignment image may thus be indicative of the relative defocus associated with each alignment image. Accordingly, focus data associated with two or more alignment images may be utilized (e.g., by the controller <NUM>) to adjust a focal position for the overlay target such that the selected overlay target is in focus within selected focus tolerances (e.g., a selected range of focal positions providing a desired image quality, or the like) on a selected camera of the detector assembly <NUM> (e.g., the single imaging detector <NUM> of <FIG>, the nominal imaging detector 302b of <FIG>, or the like).

In another embodiment, focus data from the alignment images is compared against calibrated focus data to adjust the focal position for the overlay target <NUM> within selected focus tolerances. For example, calibrated focus data may include, but is not limited to, image contrast data as a function of defocus on either side of a nominal focal position (e.g., a through-focus curve). Further, the calibrated focus data may be generated with a finer resolution of focal positions than provided by the two or more alignment images. In this regard, the focus data of the alignment images may be mapped to points of the calibrated focus data to provide efficient determination of the nominal focus for an overlay measurement.

The calibrated focus data may be generated in any manner. For instance, the calibrated focus data may be generated through a series of training images of an overlay target (e.g., overlay target <NUM>) at prior to runtime. In another instance, the calibrated focus data may be generated through a series of simulations.

In another embodiment, the alignment data includes telecentricity data indicative of a telecentricity error of the selected overlay target (e.g., overlay target <NUM>). For example, telecentricity error may manifest as a variation of feature size (e.g., variation of image magnification) as a function of focal position. By way of another example, telecentricity error may manifest as a lateral shift of features when imaged at different focal positions. Accordingly, telecentricity data may include relative location information of features in the overlay target. Further, the magnitude and/or the direction of the lateral shifts may then be used to correct the telecentricity errors.

Telecentricity errors may be determined based on two or more alignment images captured at different focal positions either sequentially or simultaneously. For example, in the case that the alignment images are captured on a single imaging detector <NUM> (e.g., in the configuration of or similar to <FIG>), the telecentricity data may include position and/or orientation information of selected features of the overlay target in each of the alignment images. In one instance, the position and/or orientation information may be represented in terms of the pixel locations of the features.

By way of another example, in the case that the alignment images are generated with multiple imaging detectors <NUM> (e.g., in the configuration of or similar to <FIG>), the position and/or orientation information of the selected features of the overlay target may be referenced to calibrated positions on each camera. For example, relative variations and/or instabilities in the positions of the multiple imaging detectors <NUM> may negatively impact the ability to accurately measure positional shifts of features across different focal positions.

In one embodiment, the imaging system <NUM> includes a pattern projector to project one or more reference patterns directly onto the two or more imaging detectors <NUM>. In this regard, the projected reference patterns may be used to calibrate the imaging detectors <NUM> with respect to each other and further provide a reference from which to measure the positions of selected features of the overlay target for each alignment image.

<FIG> is a conceptual view of a pattern projector <NUM> integrated into an imaging system <NUM>, in accordance with one or more embodiments of the present disclosure. In one embodiment, the pattern projector <NUM> includes a projector illumination source <NUM> configured to generate a beam of illumination, a pattern mask <NUM>, and projection optics <NUM> to generate an image of the pattern mask <NUM> onto the imaging detectors <NUM>. For example, illumination passed by the pattern mask <NUM> may be directed by the beamsplitter <NUM> directly to the imaging detectors <NUM>.

<FIG> is a top view of a pattern mask <NUM>, in accordance with one or more embodiments of the present disclosure. The pattern mask <NUM> includes one or more patterns <NUM> to be projected onto the imaging detectors <NUM>. Further, the patterns <NUM> may include any distribution of pattern elements suitable for monitoring the relative lateral positions of the overlay target on the detector assembly 122detector assembly <NUM>.

In one embodiment, the pattern mask <NUM> includes grating structures oriented along orthogonal directions to facilitate monitoring of the imaging detectors <NUM> along the orthogonal directions. For example, as illustrated in <FIG>, a pattern mask <NUM> may include at least one grating pattern with elements distributed along a first direction and at least one grating pattern with elements distributed along a second direction orthogonal to the first direction.

The patterns <NUM> may further be configured to be projected onto any portion of the imaging detectors <NUM>. In one embodiment, as illustrated in <FIG>, the patterns <NUM> may be placed along outer regions of the pattern mask <NUM>, leaving a central open area <NUM> for the imaging of the selected overlay target. <FIG> is a conceptual image <NUM> of an overlay target <NUM> superimposed with projected images of patterns <NUM> from the pattern mask <NUM> of <FIG>, in accordance with one or more embodiments of the present disclosure. For example, the image <NUM> of <FIG> may be captured by any imaging detector of the detector assembly <NUM> (e.g., imaging detector <NUM> of <FIG>, or any of imaging detectors 302a-c) of <FIG>).

In one embodiment, the overlay target <NUM> is visible in the central portion of the image <NUM>, while the patterns <NUM> are visible on outer regions of the image <NUM>. Further, the patterns <NUM> may be projected to portions of the imaging detectors of the detector assembly <NUM> not associated with the image of the overlay target <NUM>. For example, the overlay target <NUM> is visible as black features on a white background, where the boundaries <NUM> of the white background (e.g., a square in <FIG>) may be controlled using the collection field stop of the imaging system <NUM>.

The projection optics <NUM> may include any combination of optical elements suitable for projecting an image of the pattern mask <NUM> onto the imaging detectors <NUM> such as, but not limited to, lenses or stops. In one embodiment, the projection optics <NUM> includes a darkfield mask to control and/or select the diffraction orders used to generate images of the patterns <NUM> on the imaging detectors. For example, it is recognized herein that it may be desirable to project an in-focus image of the pattern mask <NUM> on each of the imaging detectors <NUM> of the detector assembly <NUM>. However, the optical path lengths between the pattern mask <NUM> and the individual imaging detectors <NUM> may not be the same (e.g., see <FIG>). It is further recognized herein that images of grating structures (e.g., the grating patterns <NUM> on the pattern mask <NUM> illustrated in <FIG>) formed by only two diffraction orders of equal amplitude may have a high contrast through a large depth of field.

<FIG> and <FIG> illustrate projection optics <NUM> with a darkfield filter configured to pass only first-order diffraction (e.g., +/- <NUM> diffraction orders) from the pattern mask <NUM>. <FIG> is a conceptual view of projection optics <NUM> including a darkfield stop <NUM>, in accordance with one or more embodiments of the present disclosure. <FIG> is a top view of a darkfield stop <NUM> configured to select first-order diffraction (e.g., +/- <NUM> diffraction orders), in accordance with one or more embodiments of the present disclosure.

In one embodiment, the projection optics <NUM> of the pattern projector <NUM> include a darkfield stop <NUM> located at a diffraction plane of a projection lens <NUM>. In this regard, the darkfield stop <NUM> may operate as a spatial filter to pass selected diffraction orders of illumination from the projector illumination source <NUM> diffracted by the pattern mask <NUM>. For example, as illustrated in <FIG> and <FIG>, the darkfield stop <NUM> may include an annular transmissive portion <NUM> configured to pass the first-order diffraction order (e.g., +<NUM> diffraction order 808a and -<NUM> diffraction order 808b) from grating patterns <NUM> oriented in any direction on the pattern mask <NUM>. The darkfield stop <NUM> may thus include reflective and/or absorbing regions to block remaining diffraction orders such as, but not limited to zero-order diffraction <NUM> and second-order diffraction (e.g., +<NUM> diffraction order 812a and -<NUM> diffraction order 812b).

The projector illumination source <NUM> may provide any spatial profile of illumination. For example, as illustrated in <FIG>, the projector illumination source <NUM> may include a square-core fiber-based illumination source such that the diffracted orders 808a-<NUM> may have square profiles. By way of another example, though not shown, the projector illumination source <NUM> may generate illumination with a circular distribution such that the diffracted orders 808a-<NUM> may have circular profiles.

Further, the projection optics <NUM> may include one or more additional projection lenses <NUM> configured to relay the illumination passed by the darkfield stop <NUM> to the imaging detectors <NUM>. For example, as illustrated in <FIG>, the additional projection lenses <NUM> may collimate the light passed by the darkfield stop <NUM>. Accordingly, one or more additional lenses (e.g., the collection pathway lenses <NUM> and/or additional lenses within the detector assembly <NUM> (not shown) may receive the collimated light and generate images of the patterns <NUM> on the imaging detectors <NUM>.

The pattern mask <NUM> may be configured as a reflective or a transmissive mask. For example, as illustrated in <FIG>, patterns <NUM> on a transmissive pattern mask <NUM> may be formed as transparent regions of the pattern mask <NUM> surrounded by opaque regions. In contrast, patterns <NUM> on a reflective pattern mask <NUM> (not shown) may be formed as reflective regions of the pattern mask <NUM> surrounded by transparent and/or absorbing regions.

It is to be understood that the pattern projector <NUM> illustrated in <FIG>, along with the associated descriptions are provided solely for illustrative purposes and should not be interpreted as limiting. For example, the pattern mask <NUM> may have any distribution of pattern elements suitable for monitoring relative positions of the imaging detectors <NUM>. By way of another example, the darkfield stop <NUM> may include any distribution of transmissive, reflecting, or absorbing portions to pass any selected diffraction orders.

As described previously herein, images of patterns <NUM> from a pattern mask <NUM> generated on multiple imaging detectors <NUM> may facilitate monitoring any lateral displacements between the imaging detectors <NUM>. For example, a relative shift (e.g., due to vibrations, misalignment, or the like) of one imaging detector <NUM> may be observed as a shift of the locations of projected patterns on the shifted imaging detector <NUM>.

In addition, images of patterns <NUM> from a pattern mask <NUM> generated on multiple imaging detectors <NUM> may facilitate monitoring of the telecentricity of an overlay target in the imaging system <NUM>. For example, the positions of features of the overlay target may be measured with respect to the imaged patterns <NUM> on each imaging detector <NUM>. Further, the imaging detectors <NUM> may be calibrated with respect to each other such that a feature observed at a given location on one imaging detector <NUM> (e.g., measured relative to the projected patterns <NUM>) may be expected at known locations on the other imaging detectors <NUM> when the imaging system <NUM> is aligned within selected tolerances. In this regard, telecentricity error may be determined based on deviations of the positions of features of the overlay target from expected positions based on the calibration.

Referring again to <FIG>, it is recognized herein that the quality of an image of an overlay target may not be uniform due to a variety of factors such as, but not limited to edge-diffraction effects associated with feature shape or aberrations of the imaging system <NUM>. In another embodiment, alignment data (e.g., focus data, telecentricity data, centering data, or the like) is generated based on one or more selected portions of an overlay target (e.g., one or more selected features or selected portions of features). For example, images of overlay targets having periodic features may have a higher quality in the central regions <NUM> relative to the surrounding regions due to diffraction effects. Accordingly, alignment data may be generated based solely on selected regions of overlay targets such as, but not limited to, central regions <NUM> of overlay targets including periodic structures.

It is further recognized herein that the number and/or the distribution of the focal positions at which alignment images are captured may influence tradeoffs between accuracy of alignment corrections and throughput. For example, increasing the number of focal positions at which alignment images are measured may increase the accuracy of alignment data (e.g., focus data, telecentricity data, centering data or the like), but may decrease the throughput. Accordingly, the number of alignment images may be adjusted based on the needs and specifications of any given application.

In another embodiment, one or more components of the overlay metrology system <NUM> are adjustable to facilitate alignment adjustments (e.g., focus adjustments, telecentricity adjustments, or the like) for each selected overlay target on the sample <NUM>. Further, the controller <NUM> may be communicatively coupled to the one or more adjustable components of the overlay metrology system <NUM>. In this regard, the controller <NUM> may direct and/or control the adjustable components to align the sample within selected tolerances (e.g., focus tolerances, telecentricity tolerances, centering tolerances, or the like).

For example, focus errors may be controlled by adjusting (e.g., with the controller <NUM>) the focal position of the sample <NUM> using any component or components of the imaging system <NUM>. In one instance, the focal position of the sample <NUM> may be adjusted via the sample stage <NUM>. In another instance, the focal position of the sample <NUM> may be controlled by adjusting a position of one or more elements of the collection pathway <NUM> such as, but not limited to, the objective lens <NUM> or the imaging detector <NUM>.

By way of another example, telecentricity errors may be controlled by adjusting (e.g., with the controller <NUM>) one or more components of the imaging system <NUM>. In one instance, the telecentricity may be controlled by adjusting a position of the illumination aperture stop <NUM> to adjust the angle of illumination on the sample <NUM>. In another instance, the telecentricity may be controlled by adjusting a position of the stop <NUM> to adjust the angle of radiation from the sample <NUM> used to generate an image. In another instance, the telecentricity may be controlled by adjusting a tilt of the sample <NUM> (e.g., using the sample stage <NUM>).

By way of another example, centering errors may be controlled by adjusting (e.g., with the controller <NUM>) the sample stage <NUM> to adjust the position of selected features of an overlay target to a selected position within the field of view of the imaging system <NUM> such as, but not limited to, a center of the field of view.

<FIG> is a flow diagram illustrating steps performed in a method <NUM> for site-by-site overlay metrology, in accordance with one or more embodiments of the present disclosure. Applicant notes that the embodiments and enabling technologies described previously herein in the context of system <NUM> should be interpreted to extend to method <NUM>. It is further noted, however, that the method <NUM> is not limited to the architecture of system <NUM>.

It is recognized herein that site-by-site alignment of an overlay metrology tool for a selected number of overlay targets may correct for local variations of the sample to facilitate highly accurate overlay measurements.

In one embodiment, the method <NUM> includes a step <NUM> of receiving two or more alignment images of a sample captured at two or more focal positions by an imaging system (e.g., the imaging system <NUM>), where the two or more alignment images include one or more features of an overlay target. The alignment images may be generated sequentially or simultaneously. For example, the alignment images may be generated sequentially using a single camera by adjusting the focal position of the sample and sequentially capturing alignment images. By way of anther example, the alignment images may be generated sequentially using multiple cameras configured to generate images at pre-selected focal positions.

In another embodiment, the method <NUM> includes a step <NUM> of generating alignment data of the sample in the imaging system based on the two or more alignment images. For example, alignment data may include focus data extracted from the alignment images indicative of the focal positions at which the alignment images were taken. The focus data may include, but is not required to include, image contrast data of the alignment images. Further, the image contrast data may be generated from the all pixels of the alignment images or within selected portions of the alignment images. For instance, the image contrast data may be, but is not required to be, extracted from portions of the alignment images including selected features of the metrology target. By way of another example, alignment data may include telecentricity data of the sample in the imaging system based on the two or more alignment images. The telecentricity data may include, but is not required to include, telecentricity errors manifested as lateral shifts of imaged features of the overlay target in the alignment images as a function of the focal position. By way of a further example, alignment data may include centering data associated with the alignment of the overlay target within the field of view of the imaging system. Accordingly, centering data may include positions of one or more features of the overlay target within the alignment images. For instance, it may be desirable to center the entire overlay target within the field of view of the imaging system. In another instance, it may be desirable to center a selected portion of the overlay target (e.g., a selected quadrant of the overlay target, a selected group of features on a selected layer, or the like) within the field of view of the imaging system.

Positions of imaged features of the overlay target as well as lateral shifts of the imaged features across a set of alignment images taken at different focal positions may be measured using any technique known in the art. In one embodiment, position data suitable for centering and/or telecentricity monitoring are determined based on reference patterns projected onto imaging cameras used to generate the alignment images. In this regard, the lateral positions of multiple imaging cameras may be cross-referenced and calibrated. Accordingly, the lateral shifts of imaged features of the overlay target in the alignment images may be measured based on the positions of the imaged features relative to the reference patterns for each respective camera. Further, the reference patterns may be used to calibrate the cameras such that deviations and/or misalignments of the cameras may be monitored and mitigated.

In another embodiment, the method <NUM> includes a step <NUM> of setting the two or more alignment images as measurement images when the alignment of the overlay target it within selected alignment tolerances (e.g., focus tolerances, telecentricity tolerances, centering tolerances, or the like). For example, it may be the case that an overlay target may be properly aligned within selected alignment tolerances when the alignment images are captured. Accordingly, the alignment images may serve as measurement images such that overlay between two or more sample layers may be extracted based on the alignment images.

In another embodiment, the method <NUM> includes a step <NUM> of directing the imaging system to adjust the alignment of the overlay target in the imaging system and further receiving one or more measurement images from the imaging system when the alignment of the overlay target is outside the selected alignment tolerances. For example, in the case where the alignment of the overlay target is outside the selected alignment tolerances, the overlay target may be realigned within the imaging system and additional images may be generated that are suitable for determining overlay.

For example, the step <NUM> may include directing (e.g., via the controller <NUM>) the adjustment of one or more components of the imaging system to realign the overlay target within selected focus tolerances. In one instance, focus data such as, but not limited to, image contrast of each alignment image may be mapped to a calibrated set of focus data to determine a current focal position of the sample. Accordingly, the focal position of the sample may be adjusted such that the overlay target is in focus on a detector of the imaging system. Further, the focal position may be adjusted by any means known in the art such as, but not limited to, adjusting a position of a sample stage securing the sample, adjusting a position of an objective lens of the imaging system, or adjusting a position of a detector of the imaging system.

By way of another example, the step <NUM> may include directing (e.g., via the controller <NUM>) the adjustment of one or more components of the imaging system to realign the overlay target within selected telecentricity tolerances. In one instance, the telecentricity may be controlled by adjusting an aperture stop in either an illumination arm or an imaging arm of the overlay metrology tool. In another instance, the telecentricity may be controlled by adjusting the tilt of the sample.

By way of another example, the step <NUM> may include directing (e.g., via the controller <NUM>) the adjustment of one or more components of the imaging system to position a portion of the overlay target at a selected location within a field of view of the imaging system. For instance, a portion of the overlay target may be, but is not required to be, centered within the field of view of the imaging system.

Once the overlay target is aligned within the selected alignment tolerances, the step <NUM> may include receiving one or more additional images (e.g., measurement images) from the imaging system suitable for determining overlay of two or more layers of the sample.

In another embodiment, the method <NUM> includes a step <NUM> of determining overlay between two or more layers of the sample based on at least one measurement image (e.g., a measurement image associated with an alignment image or an additional image captured after realignment).

Overlay between two or more sample layers may be determined using any number of measurement images. For example, overlay may be determined using a single measurement image using any overlay method known in the art (e.g., an image-based overlay method in which overlay is determined based on relative positions of imaged features in two or more layers, or a scatterometry-based method in which overlay is determined based on a model-based analysis of diffracted light from overlapping grating structures on two more layers). By way of another example, overlay may be determined using two or more measurement images at different focal positions. For instance, overlay may be determined based on a first image, and variations between additional measurement images at different focal positions may be used to provide corrections to the overlay measurement to increase the measurement accuracy. As an example, telecentricity errors are first determined based on relative positional shifts of imaged features across two more measurement images captured at different focal positions. Further, the telecentricity errors, once known, may be used to generate corrections to the overlay measurement. In this regard, it may be the case that some alignment inaccuracies within certain tolerances may be corrected using post-processing using multiple measurement images such that additional realignment and measurement procedures (e.g., step <NUM>) are not necessary.

The herein described subject matter sometimes illustrates different components contained within, or connected with, other components. It is to be understood that such depicted architectures are merely exemplary, and that in fact many other architectures can be implemented which achieve the same functionality. Likewise, any two components so associated can also be viewed as being "connected" or "coupled" to each other to achieve the desired functionality, and any two components capable of being so associated can also be viewed as being "couplable" to each other to achieve the desired functionality. 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.

Claim 1:
An overlay metrology system (<NUM>) comprising:
a controller (<NUM>) communicatively coupled to a telecentric imaging system (<NUM>) including two or more cameras configured to capture images through an objective lens (<NUM>) at two or more focal positions, the controller including one or more processors configured to execute program instructions causing the one or more processors to:
receive two or more alignment images of an overlay target (<NUM>) on a sample captured at two or more focal positions by the imaging system, the two or more alignment images including one or more features of the overlay target;
generate alignment data indicative of an alignment of the overlay target within the imaging system based on the two or more alignment images;
set the two or more alignment images as measurement images when the alignment of the overlay target is within selected alignment tolerances;
direct the imaging system to adjust the alignment of the overlay target in the imaging system and further receive one or more measurement images from the imaging system when the alignment of the overlay target is outside the selected alignment tolerances; and
determine overlay between two or more layers of the sample based on at least one of the measurement images;
wherein the
two or more cameras (<NUM>) are configured to image two or more different focal positions, wherein one of the cameras (302a) is configured to image at a nominal focal position at which the overlay target is expected to be aligned and the one or more other cameras (302b; 302c) are configured to image at selected offsets from the nominal focal position, wherein the two or more cameras simultaneously generate the two or more alignment images.