Continuous degenerate elliptical retarder for detecting sensitive particles

Abstract

Surface haze is eliminated.SOLUTION: The inspection system 100 may include an illumination source for generating an illumination beam, illumination optics for directing the illumination beam to a sample along an illumination direction at a non-axial angle, and collection optics for collecting scattered light emanating from the sample in a dark-field mode. Light scattered from specimen 106 includes surface haze associated with light scattered from the surface of the specimen and at least a portion of the surface haze having elliptical polarization. The system may further comprise pupil plane optics for converting the polarization of the surface haze passing through the pupil to a linear polarization aligned parallel to the selected haze orientation direction. The system may include a linear polarizer 108 that rejects surface haze aligned parallel to the haze orientation direction, and a detector 110 that generates a dark field image of the sample based on light passed through the linear polarizer 108.SELECTED DRAWING: Figure 7

Description

[Technical Field]

[0001] REFERENCE TO RELATED APPLICATIONS As used herein, the inventors of Xue Feng, U.S. Patent Act § 119(e) Liu, U.S. Provisional Application No. 63 / 151,068 (filed February 19, 2021, ADVANCED METHOD TO IMPROVE PARTICLE DETECTION SENSITIVITY IN WAFER INSPECTION), Jenn Kuen, named Leong, and John Alex Fielden Chuang are all named as the inventors. "Technical Field" FIELD OF THE DISCLOSURE The present disclosure relates generally to particle inspection, and more particularly to particle inspection using dark-field imaging based on scattered or diffracted light. [Background technology]

[0002] Particle detection systems are typically utilized in semiconductor processing lines to identify defects or particles on wafers, such as, but not limited to, unpatterned wafers. As semiconductor devices continue to shrink, particle detection systems require a corresponding increase in sensitivity and resolution. A significant source of noise that can limit measurement sensitivity is surface scattering (e.g., surface haze) on the wafer, which can be present even on optically polished surfaces. While various methods have been proposed to suppress surface scattering relative to scattering from particles, such methods may not achieve the desired sensitivity level and / or may achieve sensitivity at the expense of reduced image quality. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] U.S. Patent No. 9,874,526 [Patent Document 2] U.S. Patent No. 9,291,575 [Patent Document 3] U.S. Patent No. 8,891,079 [Patent Document 4] U.S. Patent No. 9,891,177 [Patent Document 5] U.S. Patent No. 10,942,135 [Patent Document 6] U.S. Patent No. 10,948,423 Summary of the Invention [Problem to be solved by the invention]

[0004] Therefore, there is a need to develop a system and method that alleviates the above-mentioned drawbacks. [Means for solving the problem]

[0005] An inspection system is disclosed according to one or more embodiments of the present disclosure. In one exemplary embodiment, the system includes an illumination source for generating an illumination beam. In another exemplary embodiment, the system includes one or more illumination optics for directing the illumination beam toward the sample at an off-axis angle along the illumination direction. In another exemplary embodiment, the system includes one or more collection optics for collecting scattered light from the sample in response to the illumination beam in a dark-field mode, wherein the scattered light from the sample collected by the one or more collection optics includes surface haze associated with light scattered from the surface of the sample, and at least a portion of the surface haze has an elliptically polarized light. In another exemplary embodiment, the system includes one or more optical elements disposed in one or more pupil planes of the one or more collection optics (e.g., pupil plane optics). In another exemplary embodiment, the pupil plane optics includes a first polarization rotator providing a first spatially varying polarization rotation distribution for rotating the surface haze with respect to a selected orientation direction, wherein the rotation of the surface haze includes a rotation of a major axis of the elliptically polarized light with respect to the selected orientation direction. In another exemplary embodiment, the pupil plane optics includes a quarter-wave plate for converting the surface haze from the first polarization rotator to linearly polarized light. In another exemplary embodiment, the pupil plane optics includes a second polarization rotator providing a second spatially varying polarization rotation distribution to rotate the linear polarization of the surface haze from the quarter-wave plate to a selected haze orientation direction. In another exemplary embodiment, the system includes a linear polarizer aligned to reject light polarized parallel to the selected haze orientation direction to reject the surface haze from the second polarization rotator. In another exemplary embodiment, the system includes a detector that generates a dark-field image of the sample based on scattered light from the sample that has passed through the linear polarizer, the scattered light from the sample that has passed through the linear polarizer comprising at least a portion of light scattered by one or more particles on the surface of the sample.

[0006] An inspection system is disclosed according to one or more embodiments of the present disclosure. In one exemplary embodiment, the system includes an illumination source for generating an illumination beam. In another exemplary embodiment, the system includes one or more illumination optics for directing the illumination beam toward the sample at an off-axis angle along the illumination direction. In another exemplary embodiment, the system includes one or more collection optics for collecting scattered light from the sample in response to the illumination beam in a dark-field mode, where the scattered light from the sample collected by the one or more collection optics includes surface haze associated with the light scattered from the surface of the sample, and at least a portion of the surface haze has an elliptically polarized light. In another exemplary embodiment, the system includes one or more optical elements disposed in one or more pupil planes of the one or more collection optics (e.g., pupil plane optics). In another exemplary embodiment, the pupil plane optics includes a polarization rotator that provides a spatially varying polarization rotation distribution to rotate the surface haze into a selected orientation distribution, where the rotation of the surface haze includes rotating a major axis of the elliptical polarization into the selected orientation distribution. In another exemplary embodiment, the pupil plane optics includes a segmented quarter-wave plate for converting the surface haze from the polarization rotator into linearly polarized light, where the linearly polarized light is aligned along the selected haze orientation direction. In another exemplary embodiment, the system includes a linear polarizer aligned to reject light polarized parallel to a selected haze orientation direction to reject surface haze from a segmented quarter-wave plate. In another exemplary embodiment, the system includes a detector that generates a dark-field image of the sample based on scattered light from the sample that has passed through the linear polarizer, the scattered light from the sample that has passed through the linear polarizer comprising at least a portion of light scattered by one or more particles on the surface of the sample.

[0007] An inspection system is disclosed according to one or more embodiments of the present disclosure. In one exemplary embodiment, the system includes an illumination source for generating an illumination beam. In another exemplary embodiment, the system includes one or more illumination optics for directing the illumination beam toward the sample at an off-axis angle along the illumination direction. In another exemplary embodiment, the system includes one or more collection optics for collecting scattered light from the sample in response to the illumination beam in a dark-field mode, where the scattered light from the sample collected by the one or more collection optics includes surface haze associated with the light scattered from the surface of the sample, and at least a portion of the surface haze has elliptically polarized light. In another exemplary embodiment, the system includes one or more optical elements disposed in one or more pupil planes of the one or more collection optics (e.g., pupil plane optics). In another exemplary embodiment, the pupil plane optics includes a spatially varying waveplate positioned in the pupil plane of the one or more collection optics to convert the surface haze to linearly polarized light. In another exemplary embodiment, the pupil plane optics includes a polarization rotator that provides a spatially varying polarization rotation distribution, where the surface haze is rotated to a selected orientation distribution from the spatially varying waveplate, and the rotation of the surface haze includes rotating a major axis of the elliptically polarized light toward a selected haze orientation direction. In another exemplary embodiment, the system includes a linear polarizer aligned to reject light polarized parallel to a selected haze orientation direction to reject surface haze from the polarization rotator. In another exemplary embodiment, the system includes a detector that generates a dark-field image of the sample based on scattered light from the sample that has passed through the linear polarizer, the scattered light from the sample that has passed through the linear polarizer comprising at least a portion of light scattered by one or more particles on the surface of the sample.

[0008] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not necessarily restrictive of the invention as claimed. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the general description, serve to explain the principles of the invention. [Brief explanation of the drawings]

[0009] The many advantages of the present disclosure may be better understood by those skilled in the art by reference to the accompanying drawings. [Figure 1A] FIG. 1A is a block diagram of a particle detection system in accordance with one or more embodiments of the present disclosure. [Figure 1B] FIG. 1B is a conceptual diagram of an inspection tool in accordance with one or more embodiments of the present disclosure. [Figure 2] FIG. 2 is a conceptual top view of a phase mask including two segments for dividing a pupil into two segments, in accordance with one or more embodiments of the present disclosure. [Figure 3A] FIG. 3A is a pupil plane scatter map of surface scattering in response to obliquely incident p-polarized light, in accordance with one or more embodiments of the present disclosure. [Figure 3B] FIG. 3B is a pupil plane scatter map of light scattered by a small particle in response to obliquely incident p-polarized light, in accordance with one or more embodiments of the present disclosure. [Figure 4A] FIG. 4A is a conceptual diagram of a first configuration of a collection path of an inspection tool including a continuous degenerate elliptical retarder (CDER), in accordance with one or more embodiments of the present disclosure. [Figure 4B] FIG. 4B is a plot of the surface haze electric field distribution shown in FIG. 3A after propagating through a first polarization rotator positioned to align the polarization ellipse of the surface haze along the illumination direction, in accordance with one or more embodiments of the present disclosure. [Figure 4C] FIG. 4C is a plot illustrating the spatially varying polarization rotation distribution of a first polarization rotator for producing the surface haze electric field distribution of FIG. 4B in accordance with one or more embodiments of the present disclosure. [Figure 4D] FIG. 4D is a plot of the surface haze electric field distribution shown in FIG. 4B after propagating through a quarter-wave plate, in accordance with one or more embodiments of the present disclosure. [Figure 4E] FIG. 4E is a plot of the surface haze electric field distribution shown in FIG. 4D after propagating through a second polarization rotator, in accordance with one or more embodiments of the present disclosure. [Figure 4F]FIG. 4F is a plot illustrating the spatially varying polarization rotation distribution of a second polarization rotator for producing the surface haze electric field distribution of FIG. 4E in accordance with one or more embodiments of the present disclosure. [Figure 4G] FIG. 4G is a plot of the surface haze electric field distribution shown in FIG. 4E after propagation through a linear polarizer, in accordance with one or more embodiments of the present disclosure. [Figure 4H] FIG. 4H is a block diagram of a polarization rotator formed as a segmented half-wave plate 406 in accordance with one or more embodiments of the present disclosure. [Figure 4I] FIG. 4I is a block diagram of a polarization rotator formed as a segmented half-wave plate in accordance with one or more embodiments of the present disclosure. [Figure 4J] FIG. 4J corresponds to a simulation of the efficiency of a CDER as illustrated in FIG. 4A with a polarization rotator formed from an optically active material with a 2D thickness profile, in accordance with one or more embodiments of the present disclosure. [Figure 4K] FIG. 4K is a conceptual cross-sectional view of the component illustrated in FIG. 4A in which the polarization rotator has a thickness profile that varies continuously along a single direction to provide a one-dimensionally varying polarization rotation distribution, in accordance with one or more embodiments of the present disclosure. [Figure 4L] 4L is a conceptual cross-sectional view of the component illustrated in FIG. 4A in which the polarization rotator has a thickness profile that varies linearly along a single direction to provide a linearly varying polarization rotation distribution, in accordance with one or more embodiments of the present disclosure. [Figure 4M] 4M is a conceptual cross-sectional view of the components illustrated in FIG. 4A in which the polarization rotator is formed as a stack of elements including two optically active materials and corresponding phase compensators, according to one or more embodiments of the present disclosure. [Figure 5A] FIG. 5A is a conceptual diagram of a second configuration of a collection path of an inspection tool including a CDER, in accordance with one or more embodiments of the present disclosure. [Figure 5B]FIG. 5B is a plot of the surface haze electric field distribution shown in FIG. 3A after propagating through the polarization rotator of FIG. 5A in accordance with one or more embodiments of the present disclosure. [Figure 5C] FIG. 5C is a plot of the surface haze electric field distribution shown in FIG. 5B after propagating through the quarter wave plate of FIG. 5A in accordance with one or more embodiments of the present disclosure. [Figure 5D] FIG. 5D is a plot of the surface haze electric field distribution illustrated in FIG. 5C after propagation through the linear polarizer of FIG. 5A in accordance with one or more embodiments of the present disclosure. [Figure 5E] FIG. 5E is a conceptual cross-sectional view of the components shown in FIG. 5A in which the polarization rotator is formed as a segmented half-wave plate and the quarter-wave plate is formed as a segmented quarter-wave plate, in accordance with one or more embodiments of the present disclosure. [Figure 5F] FIG. 5F is a conceptual cross-sectional view of the components shown in FIG. 5A, where the polarization rotator is formed as a continuous element and the quarter-wave plate is formed as a segmented quarter-wave plate, in accordance with one or more embodiments of the present disclosure. [Figure 6A] FIG. 6A is a conceptual cross-sectional view of a third configuration of a light collection path of an inspection tool including a CDER, in accordance with one or more embodiments of the present disclosure. [Figure 6B] FIG. 6B is a plot of the surface haze electric field distribution shown in FIG. 3A after propagating through the spatially varying waveplate of FIG. 6A in accordance with one or more embodiments of the present disclosure. [Figure 6C] FIG. 6C is a plot of the surface haze electric field distribution shown in FIG. 6B after propagating through the optical rotator of FIG. 6A in accordance with one or more embodiments of the present disclosure. [Figure 6D] FIG. 6D is a plot of the surface haze electric field distribution shown in FIG. 6C after propagation through the linear polarizer of FIG. 6A in accordance with one or more embodiments of the present disclosure. [Figure 7] FIG. 7 is a flow diagram illustrating steps performed in a method for particle detection in accordance with one or more embodiments of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0010] Reference will now be made in detail to the disclosed subject matter, which is illustrated in the accompanying drawings. The present disclosure has been particularly shown and described with reference to certain embodiments and certain features thereof. The embodiments described herein are to be construed as illustrative and not restrictive. It will be readily apparent to those skilled in the art that various changes and modifications in form and detail may be made therein without departing from the spirit and scope of the present disclosure.

[0011] Embodiments of the present disclosure are directed to systems and methods for particle detection based on dark-field imaging (imaging in dark-field mode) in which surface scattering (e.g., surface haze) is separated from light scattered by particles on the surface (e.g., particle scattering). Further embodiments of the present disclosure are directed to simultaneously generating separate images of a sample based on surface scattering and particle scattering.

[0012] Wafer inspection is generally described in the United States: U.S. Patent 9,874,526 issued January 1, 2018, U.S. Patent 9,291,575 issued March 22, 2016, U.S. Patent 8,891,079 issued November 18, 2014, and U.S. Patent 9,891,177 issued February 13, 2018, all of which are incorporated herein in their entirety. Additionally, for purposes of this disclosure, particles may include any surface defects on the specimen of interest, including, but not limited to, foreign particles, scratches, pits, holes, bumps, or the like.

[0013] It is recognized herein that light scattered from particles and light scattered from surfaces can exhibit different electric field distributions (e.g., polarization and electric field strength) as a function of scattering angle. Furthermore, the difference in the electric field distributions (e.g., scattering maps) of these scattering sources can be particularly significant for obliquely incident p-polarized light. For example, surface haze from obliquely incident p-polarized light can have an elliptically polarized light polarized approximately radially relative to the angle of specular reflection, while scattering from particles can be approximately radially polarized relative to the surface normal. In this manner, surface haze can be isolated from particle scattering based at least in part on these polarization differences. As an example, surface haze suppression using a continuous radial polarizer is generally described in U.S. Patent 10,942,135 (March 9, 2021), which is incorporated herein by reference in its entirety. As another example, surface haze suppression using a combination of segmented half-wave plates with a linear polarizer is generally described in U.S. Patent 10,948,423, which is incorporated herein by reference in its entirety. However, it is contemplated herein that existing techniques such as these may not be able to completely convert the elliptical polarization state of the surface haze across the pupil to linearly polarized light and / or may not be able to completely align the polarization of the surface haze along a common direction, which may result in leakage of the surface haze through the linear polarizer and a corresponding reduced signal-to-noise ratio.

[0014] In some embodiments, surface haze is isolated and separated using a continuous degenerate elliptical retarder, referred to herein for convenience as a CDER, in conjunction with a linear polarizer or polarizing beam splitter. Hereinafter, a continuous degenerate elliptical retarder is referred to as a CDER. For example, a CDER can be an optical element designed for placement in the pupil plane of an optical system that can functionally convert the polarization ellipse associated with surface haze across the pupil plane into linearly polarized light oriented along a common angle. In this way, the linear polarizer can separate light associated with surface haze from light associated with particle scattering. As a result, particle detection can be achieved with high sensitivity. In some embodiments, the system further includes a phase mask (e.g., a π-phase mask) that provides a central peak of the point spread function (PSF) to sharpen the PSF and further increase the signal-to-noise ratio (SNR) of light scattered by particles, thus increasing particle detection sensitivity.

[0015] In some embodiments, the CDER may be functionally decomposed into a first polarization rotator (e.g., an optical rotator, a segmented or continuous half-wave plate, etc.), a quarter-wave plate, and a second polarization rotator, at least some of which provide spatially varying transformations across the pupil to provide tailored separation of surface haze. In this configuration, the first polarization rotator may rotate elliptically polarized surface haze in a common orientation across the pupil plane. This first polarization rotator may be similar in function and / or physical configuration to the polarization rotation techniques disclosed in U.S. Patent 10,948,423, incorporated herein by reference, although it should be understood that this is merely an exemplary comparison and not a limitation of the present disclosure. However, it is further contemplated herein that simply rotating elliptically polarized surface haze in a common orientation may not provide adequate separation and / or suppression of surface haze by a subsequent linear polarizer. In particular, the polarization component orthogonal to the common alignment direction is not suppressed by the subsequent linear polarizer, thus resulting in a lower extinction ratio of surface haze and a lower signal-to-noise ratio associated with particle scattering.

[0016] To address this, the CDER may further include a quarter-wave plate to convert the elliptically polarized light into linearly polarized light. However, this linearly polarized surface haze may no longer be aligned (e.g., along the common orientation direction). In particular, the orientation of the linearly polarized surface haze after the quarter-wave plate may depend on the eccentricity of the elliptically polarized light. The CDER may then include a second polarization rotator to rotate the linearly polarized surface haze into the common orientation direction (or any other selected orientation direction). It is contemplated herein that this technique may convert the varying polarization ellipse of the surface haze across the pupil plane into linearly polarized light oriented along any selected (but arbitrary) direction, enabling highly efficient separation and / or suppression using a subsequent linear polarizer or polarizing beam splitter.

[0017] However, it is contemplated herein that the CDER may be implemented in various ways. In some embodiments, the CDER includes a single polarization rotator and a quarter-wave plate. In this configuration, the polarization rotator does not need to rotate the elliptically polarized light to a common orientation across the pupil. Rather, the polarization rotator may compensate for the subsequent effect of the quarter-wave plate so that the polarization state of the surface haze across the pupil is aligned only after passing through the quarter-wave plate. In particular, because the quarter-wave plate generates linearly polarized light in different directions depending on the eccentricity of the input light, the polarization rotator may compensate for this variation by rotating the surface haze by different amounts across the pupil plane based on the eccentricity distribution. In this way, the second polarization rotator described above is unnecessary. Furthermore, in some embodiments, the quarter-wave plate is provided before the polarization rotator. In this configuration, the surface haze is first converted to linear polarization and then rotated to a common orientation for subsequent separation and / or suppression by the linear polarizer.

[0018] It is contemplated herein that CDER for the isolation and suppression of surface haze for particle detection purposes can provide numerous benefits. For example, CDER may provide both the conversion of elliptically polarized surface haze across the pupil to linearly polarized light and the rotation of linearly polarized light to a common angle (e.g., a rejection angle) for rejection using a linear polarizer. As a result, surface haze can be isolated and suppressed with a high extinction ratio for high-sensitivity particle detection.

[0019] In some embodiments, the CDER is provided as a single optical element. In this manner, the CDER can be easily positioned within the pupil plane of the imaging system. The CDER may be provided as a single optical element or as multiple optical elements. It is contemplated herein that the design of a particular optical system may limit the thickness or number of optical elements that can be positioned at or sufficiently close to a particular pupil plane to provide a desired effect within a selected tolerance. For purposes of this disclosure, a description indicating the placement of an element such as a CDER within a pupil plane may be understood as the placement of the CDER, or a component thereof, within a range of distances from the pupil plane that provides a desired level of performance according to a selected metric (e.g., the extinction ratio of surface haze when combined with a linear polarizer). Furthermore, it should be understood that an optical system may include any number of optical relays to provide any number of conjugate pupil planes at which the CDER, or a component thereof, may be positioned. Thus, a description herein indicating the placement of a CDER within a pupil plane may be understood to include any placement of CDER components within any number of conjugate pupil planes.

[0020] 1-7, systems and methods for highly sensitive particle detection according to one or more embodiments of the present disclosure will be described in more detail.

[0021] FIG. 1A is a block diagram of a particle detection system 100 in accordance with one or more embodiments of the present disclosure.

[0022] In some embodiments, the particle detection system 100 includes an inspection tool 102 having a CDER 104 for providing inspection data related to particles on the sample 106 while suppressing and / or isolating surface haze. For example, the inspection tool 102 may include a CDER 104 in a collection pupil plane to manipulate the surface haze to be linearly polarized along a selected haze orientation direction. The inspection tool 102 may further include a linear polarizer 108 oriented to suppress surface haze along the haze orientation direction from the sample light 122, and at least one detector 110 that generates inspection data based on the sample light 122 after surface haze suppression.

[0023] The linear polarizer 108 can be formed as any type of polarizer known in the art. In some embodiments, the linear polarizer 108 absorbs surface haze along the haze orientation direction. In some embodiments, the linear polarizer 108 is formed as a polarizing beam splitter. In this manner, the linear polarizer 108 can direct surface haze along one optical path (e.g., to separate the surface haze) and direct the remaining light along an additional optical path. The inspection tool 102 may then optionally include an additional detector 110 for measuring a portion of the sample light 122 associated with the surface haze.

[0024] FIG. 1B is a conceptual diagram of an inspection tool 102 in accordance with one or more embodiments of the present disclosure.

[0025] In some embodiments, the inspection tool 102 includes an illumination source 112 for generating an illumination beam 114 and an illumination path 116 that includes one or more illumination optics for directing the illumination beam 114 toward the sample 106 .

[0026] The illumination source 112 can include any type of light source known in the art. Furthermore, the illumination source 112 can provide the illumination beam 114 with any selected spatial or temporal coherence characteristics. In some embodiments, the illumination source 112 includes one or more laser sources, such as, but not limited to, one or more narrowband laser sources, one or more broadband laser sources, one or more supercontinuum laser sources, or one or more white-light laser sources. In some embodiments, the illumination source 112 includes a laser-driven light source (LDLS), such as, but not limited to, a laser-sustained plasma (LSP) source. For example, the illumination source 112 can include, but is not limited to, an LSP lamp, an LSP bulb, or an LSP chamber suitable for housing one or more elements capable of emitting broadband illumination when excited into a plasma state by a laser source. In some embodiments, the illumination source 112 includes a lamp source, such as, but not limited to, an arc lamp, a discharge lamp, or an electrodeless lamp.

[0027] The illumination beam 114 may include one or more selected wavelengths of light, including, but not limited to, ultraviolet (UV) radiation, visible light, or infrared (IR) radiation. For example, the illumination source 112 may, but need not, provide an illumination beam 114 having a wavelength shorter than approximately 350 nm. As another example, the illumination beam 114 may provide a wavelength of approximately 266 nm. As another example, the illumination beam 114 may provide a wavelength of approximately 213 nm. As another example, the illumination beam 114 may provide a wavelength of approximately 193 nm. It is recognized herein that both imaging resolution and light scattering by small particles (e.g., relative to the wavelength of the illumination beam 114) generally scale with wavelength, such that decreasing the wavelength of the illumination beam 114 may generally increase the imaging resolution and scattering signal from small particles. Thus, the illumination beam 114 may include short-wavelength light, including, but not limited to, extreme ultraviolet (EUV) light, deep ultraviolet (DUV) light, or vacuum ultraviolet (VUV) light.

[0028] In some embodiments, the illumination source 112 provides an adjustable illumination beam 114. For example, the illumination source 112 may include an adjustable illumination source (e.g., one or more tunable lasers, etc.). As another example, the illumination source 112 may include a broadband illumination source coupled to any combination of fixed or tunable filters.

[0029] The illumination source 112 can further provide an illumination beam 114 having any temporal profile. For example, the illumination beam 114 can have a continuous temporal profile, a modulated temporal profile, a pulsed temporal profile, etc.

[0030] It is recognized herein that the intensity of surface haze can depend on multiple factors, including, but not limited to, the angle of incidence or polarization of the illumination beam 114. For example, the intensity of surface haze can be relatively high at near-normal angles of incidence and decrease at higher angles of incidence. In some embodiments, the illumination path 116 can include one or more illumination optics, such as, but not limited to, a lens 118, a mirror, etc., to direct the illumination beam 114 at an oblique angle of incidence (off-axis angle) to the sample 106 to reduce the occurrence of surface haze. An oblique angle of incidence can generally include any selected angle of incidence. For example, the angle of incidence can be, but need not be, greater than 60 degrees relative to the surface normal.

[0031] In some embodiments, the illumination path 116 includes one or more illumination beam conditioning components 120 suitable for modifying and / or conditioning the illumination beam 114. For example, the one or more illumination beam conditioning components 120 may include, but are not limited to, one or more polarizers, one or more wave plates, 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. In some embodiments, the one or more illumination beam conditioning components 120 include a polarizer or wave plate oriented to provide a p-polarized illumination beam 114 on the sample 106.

[0032] Illumination of the sample 106 with the illumination beam 114 may result in light emanating from the sample 106 (e.g., sample light 122) based on any combination of reflection, scattering, diffraction, or emission from the sample 106. In this manner, the sample light 122 may include a combination of surface haze from the sample 106 (e.g., resulting from a bare semiconductor wafer, etc.) and light scattered from any particles on the sample 106 (e.g., particle scattering).

[0033] In some embodiments, the inspection tool 102 includes a collection path 124 (collection optics) having an objective lens 126 for collecting at least a portion of the sample light 122. The sample light 122 may include any type of light that emanates from the sample 106 in response to the illumination beam 114, including, but not limited to, scattered light, reflected light, diffracted light, or luminescence.

[0034] In some embodiments, the inspection tool 102 is a dark-field imaging system to reject specularly reflected light 128. In this regard, the inspection tool 102 can image the sample 106 primarily based on scattered light. Dark-field imaging may also be implemented using any technique known in the art. In some embodiments, the orientation and / or numerical aperture (NA) of the objective lens 126 may be selected so as not to collect specularly reflected light. For example, as shown in FIG. 1B , the objective lens 126 is oriented approximately perpendicular to the sample 106 and has an NA that does not include specularly reflected light 128 associated with the illumination beam 114. Furthermore, the objective lens 126 may, but need not, have an NA of about 0.9 or greater. In some embodiments, the inspection tool 102 may include one or more components that block specularly reflected light 128 from reaching the detector 110.

[0035] In some embodiments, the inspection tool 102 includes at least one detector 110 configured to capture at least a portion of the sample light 122 collected by the collection path 124. The detector 110 may include any type of photodetector known in the art suitable for measuring illumination received from the sample 106. For example, the detector 110 may include a multi-pixel detector suitable for capturing an image of the sample 106, such as, but not limited to, a charge-coupled device (CCD) detector, a complementary metal-oxide semiconductor (CMOS) detector, a time-delay integration (TDI) detector, a photomultiplier tube (PMT) array, an avalanche photodiode (APD) array, or the like. In some embodiments, the detector 110 includes a spectroscopic detector suitable for identifying the wavelength of the sample light 122.

[0036] In some embodiments, the collection path 124 includes a CDER 104 located at or near the pupil plane 130. In this manner, the CDER 104 can manipulate surface haze from the sample 106 (e.g., that is part of the sample light 122) to be linearly polarized and aligned along a haze orientation direction across the pupil plane 130.

[0037] In some embodiments, a linear polarizer 108 is disposed between the CDER 104 and the detectors 110 to suppress surface haze along the haze orientation direction along the path of at least one of the detectors 110. The linear polarizer 108 can be formed as any type of polarizer known in the art. For example, the linear polarizer 108 can be formed as an absorbing polarizer that absorbs light along the rejection direction and passes the remaining light through either transmission or reflection. In this configuration, the rejection direction can be aligned with the haze orientation direction so that the linear polarizer 108 can absorb the surface haze and pass the remaining light (e.g., the portion of the sample light 122 associated with particle scattering). As another example, as shown in FIG. 1B, the linear polarizer 108 can be formed as a polarizing beam splitter that can split light with orthogonal polarizations along two separate optical paths. In this configuration, the linear polarizer 108 can direct the surface haze aligned along the haze orientation direction along a first optical path and the remaining light along a second optical path. 1B, the inspection tool 102 may include a detector 110 and any associated optical elements (e.g., lenses, apertures, filters, etc.) along each of the two optical paths. Thus, the inspection tool 102 may generate data related to surface haze in addition to the remaining light, which may be useful for, but not limited to, diagnostic or evaluation purposes.

[0038] The collection path 124 may further include any number of beam conditioning elements for directing and / or modifying the sample light 122, including, but not limited to, one or more lenses (e.g., lens 132), one or more filters, one or more apertures, one or more polarizers, or one or more phase plates. In this regard, the inspection tool 102 can control and / or adjust selected aspects of the sample light 122 used to generate an image on the detector 110, including, but not limited to, the intensity, phase, and polarization of the sample light 122 as a function of scattering angle and / or position on the sample 106. For example, as described in more detail below, the beam conditioning element may include, but is not limited to, a phase mask 202.

[0039] It is recognized herein that a limited number of components and / or components having a limited thickness may be positioned at or sufficiently near a particular pupil plane 130 to provide a desired effect. Thus, for purposes of this disclosure, reference to one or more elements at pupil plane 130 may generally describe one or more elements at or sufficiently near pupil plane 130 to produce a desired effect. In some embodiments, although not shown, collection path 124 may include additional lenses to generate one or more additional pupil planes 130 such that any number of elements, including but not limited to elements related to CDER 104, phase mask 202 (see, e.g., FIG. 2 below), or linear polarizer 108, may be positioned at or near pupil plane 130.

[0040] Referring now to FIG. 2 , in some embodiments, particle detection system 100 includes one or more components located at or near the pupil plane to reshape (shape) the point spread function (PSF) of p-polarized light scattered by sub-resolution particles. It is recognized herein that images of particles smaller than the system's imaging resolution are generally limited by the system PSF, which is typically an Airy function when the image includes specularly reflected light. However, the actual PSF associated with a particle (e.g., particle PSF), and therefore the actual image of the particle generated by the system, is related to the particular electric field distribution of light from the particle in the pupil plane and may have a different size or shape than the system PSF, especially when the image is formed from scattered light.

[0041] In particular, a dark-field image of a particle smaller than the imaging resolution when illuminated with oblique p-polarized light (e.g., an image of the particle formed with scattered or diffracted light) may be a ring that spans an area larger than the system PSF, which adversely affects particle detection sensitivity. This ring shape and the increase in size of the particle's PSF or imaging spot may be associated with destructive interference of the collected light at the center of the particle's imaging spot on the detector 110.

[0042] Thus, in some embodiments, particle detection system 100 includes one or more components, such as, but not limited to, one or more phase plates or one or more phase compensators, for modifying the phase of sample light 122 traversing pupil plane 130 to promote constructive interference of light at the center of the particle's imaging spot on detector 110.

[0043] For example, the phase mask may have various configurations suitable for reshaping the PSF of an imaged particle. Phase masks for reshaping the PSF of an imaged particle based on scattered light are generally described in U.S. Patent No. 10,942,135, issued March 9, 2021, which is incorporated herein by reference in its entirety. In some embodiments, the phase mask may include one or more half-wave plates covering selected portions of the pupil plane 130. In this regard, the phase mask may be formed as a segmented optical component, with at least one of the segments including a half-wave plate.

[0044] 2 is a conceptual top view of a phase mask 202 including two segments for dividing a pupil into two segments (e.g., halves), in accordance with one or more embodiments of the present disclosure. For example, as shown in FIG. 2, the phase mask 202 may be configured to transmit light with orthogonal polarizations (E x , e iπ E yThe phase mask 202 may include a segment 204 formed from a half-wave plate having an optical axis along the X direction to introduce a phase shift of p for light polarized along the Y direction relative to light polarized along the Y direction (denoted as ). Additionally, the phase mask 202 may include a segment 206 that does not rotate the polarization of light. For example, the segment 206 may include a compensating plate formed from a material that is optically homogeneous along the propagation direction such that light passing through the segment 206 propagates along the same (or substantially the same) optical path length as the light in the segment 204. In one embodiment, the compensating plate has approximately the same thickness and refractive index as the half-wave plate in the segment 204 but is formed from a material that has no birefringence along the propagation direction. In another embodiment, the compensating plate is formed from the same material as the half-wave plate in the segment 204 but is cut along a different axis such that light propagating through the compensating plate does not experience birefringence. For example, light propagating along the optic axis of a uniaxial crystal may not experience birefringence, and as a result, the crystal may be optically homogeneous for light propagating along the optic axis. As another example, the segment 206 may include an aperture.

[0045] Additionally, in some embodiments, the phase mask 202 may be tilted out of the pupil plane to at least partially compensate for optical path length differences across the pupil plane 130 .

[0046] The segmented phase mask 202 may be formed using any technique known in the art, and in one embodiment, the various segments (e.g., segments 204-206 in FIG. 2) are formed as a single component, with the various segments arranged in a single plane.

[0047] However, it should be understood that FIG. 2 and the associated description are provided for illustrative purposes only and should not be construed as limiting. For example, a phase mask 202 having two segments may include a half-wave plate positioned at the bottom of the collection region 306 rather than at the top, as shown in FIG. 4 . Furthermore, the phase mask 202 may include any number of segments formed from any combination of materials in any pattern across the pupil plane 130 to reshape the PSF of light scattered from particles. For example, given a known electric field distribution of light in the pupil plane associated with an object of interest (e.g., measured, simulated, etc.), a segmented phase mask 202 as described herein may be formed to selectively adjust the phase of various regions of light in the pupil plane to reshape the PSF of an image of the object of interest. In particular, the various segments of the phase mask 202 may be selected to promote constructive interference at the detector 110 and provide a PSF that closely approximates (e.g., within a selected tolerance) the system PSF.

[0048] It is further recognized herein that the design of the phase mask 202 may represent a trade-off between an “ideal” phase mask based on a known electric field distribution associated with particles of interest (e.g., as shown in FIG. 3B ) and practical design and / or manufacturing considerations. For example, an ideal or other desired phase mask 202 may be unreasonably expensive or difficult to manufacture. However, a particular design of the phase mask 202 may meet both manufacturing and performance specifications (e.g., particle PSF for a selected shape, etc.). Thus, the design of the phase mask 202 shown in FIG. 2 may represent a non-limiting example that offers a particular trade-off between performance and manufacturability.

[0049] In another embodiment, as described in more detail below, the particle detection system 100 may include a phase compensator formed from an optically homogeneous material having a thickness that varies spatially across the pupil plane to promote constructive interference of the sample light 122 associated with particle scattering in the center of the particle's image on the detector 110.

[0050] 3A-6D, the CDER 104 according to one or more embodiments of the present disclosure will be described in more detail. Figures 3A and 3B show pupil plane scatter maps corresponding to the electric field distribution (e.g., polarization state) of the sample light 122 associated with light scattered from the bare sample 106 (e.g., surface haze) and light scattered from particles. Figures 4A-6D then show various non-limiting configurations of the CDER 104 and the associated evolution of the polarization state of the surface haze.

[0051] It is recognized herein that light scattered from the surface of the sample 106 (e.g., surface haze, surface scattering, etc.) may be considered noise in particle detection applications. Therefore, it may be desirable to filter the portion of the sample light 122 associated with surface haze from the portion of the sample light 122 associated with light scattered by particles of interest.

[0052] 3A is a pupil plane scatter map 302 of surface scattering (e.g., surface haze) in response to obliquely incident p-polarized light, and FIG. 3B is a pupil plane scatter map 304 of light scattered by small particles (e.g., small relative to the imaging resolution of particle detection system 100 or the wavelength of illumination beam 114) in response to obliquely incident p-polarized light, according to one or more embodiments of the present disclosure.

[0053] In particular, the scattering maps 302, 304 include electric field intensities indicated by shading, with white as the highest intensity and black as the lowest intensity. Additionally, the scattering maps 302, 304 include the polarization orientation of light as a function of collection angle (e.g., scattering angle) in the pupil plane 130, indicated by a superimposed ellipse. The scattering maps 302, 304 are bounded by a collection region 306 in the pupil plane, which is associated with the range of angles over which the sample light 122 is collected by the inspection tool 102. For example, the collection region 306 may correspond to the numerical aperture (NA) of the objective lens in the collection path 124.

[0054] Scattering maps 302, 304 are based on the configuration of particle detection system 100 shown in Figures 1A and 1B. In Figures 3A and 3B, specular angle 308 is located outside collection area 306 along illumination direction 310 (e.g., outside collection area 306 to the right of circular collection area 306 in Figure 3A), indicating that inspection tool 102 does not capture specularly reflected light. However, alternative configurations are within the scope of this disclosure. For example, if specular angle 308 is in the pupil plane, the specularly reflected light can be blocked before detector 110 to generate a dark-field image.

[0055] Additionally, the scattering maps 302, 304 may represent scattering from a wide variety of materials, including, but not limited to, silicon, epitaxial silicon, and polycrystalline silicon wafers. However, it should be understood that the scattering maps 302, 304 are provided for illustrative purposes only and should not be construed as limiting the present disclosure.

[0056] As illustrated in Figures 3A and 3B, the electric field distribution (e.g., electric field strength and polarization orientation) of light scattered by particles can be substantially different from the electric field distribution of light scattered by surfaces, particularly when the illumination beam 114 is p-polarized. For example, sample light 122 associated with surface haze generally exhibits a nearly radial polarization distribution with respect to the specular reflection angle 308 within the collection region 306, as illustrated in Figure 3A. In contrast, sample light 122 associated with particle scattering generally exhibits a radial polarization distribution with respect to the surface normal, as illustrated in Figure 3B. Furthermore, the polarization of the scattered sample light 122 light is generally elliptical. As can be seen from Figures 3A and 3B, at most positions within the pupil plane 130, the ellipse is very elongated, meaning that one linearly polarized component is much stronger than the other linearly polarized component. For sample light 122 scattered from a small particle (e.g., Figure 3B), the polarization may be more elliptical near the center of the pupil, meaning that the magnitudes of the two linearly polarized components may be roughly equivalent. However, the intensity of the light in this region of the pupil is relatively low and contributes little to the total scattering signal from the small particle.

[0057] FIG. 4A is a conceptual diagram of a first configuration of a collection path 124 of an inspection tool 102 including a CDER 104, in accordance with one or more embodiments of the present disclosure. It is contemplated herein that each of the components illustrated in FIG. 4A may be formed as a single optical element or may be distributed among any number of optical elements. In this manner, adjacent components may be in physical contact, but need not be in physical contact. Furthermore, the inspection tool 102 may include any number of pupil planes, such that the components illustrated in FIG. 4A may be located at or near a single pupil plane or distributed among multiple pupil planes.

[0058] In some embodiments, the collection path 124 includes a CDER 104 formed from two polarization rotators 402 (e.g., optical rotators) on either side of a quarter-wave plate 404, followed by a linear polarizer 108. In this manner, the CDER 104 can manipulate surface haze to be linearly polarized along a selected haze orientation direction, and the linear polarizer 108 can be oriented to reject light along this haze orientation direction and thus suppress surface haze. In some embodiments, the collection path 124 further includes a phase mask 202 to reshape the PSF of the light through the CDER 104 and linear polarizer 108 to provide sharp imaging on a detector 110 (not shown) of the inspection tool 102.

[0059] Polarization rotator 402 may include any combination of optical elements known in the art that provide a spatially varying amount of polarization rotation (e.g., a spatially varying polarization rotation angle) across the pupil plane. In this manner, polarization rotator 402 can selectively rotate the polarization of light at any location in the pupil plane by any selected amount.

[0060] Furthermore, the selected polarization direction (e.g., haze orientation direction) for eliminating surface haze may be any suitable direction. For example, the selected polarization direction may be selected based on the expected distribution of particle-scattered sample light 122 (e.g., as shown in FIG. 3B) to minimize the intensity of rejected particle-scattered sample light 122.

[0061] 4B-4G show the evolution of the electric field distribution of surface haze through the components shown in FIG. 4A, in accordance with one or more embodiments of the present disclosure. In FIG. 4B-4G, grayscale intensity maps correspond to the intensity of the surface haze. Additionally, polarization states at selected locations across the pupil plane are shown as overlays to illustrate the distribution of polarization states across the pupil plane.

[0062] In some embodiments, the first polarization rotator 402a provides a spatially varying polarization rotation distribution to rotate the polarization of surface haze (e.g., as shown in FIG. 3A) to a common haze orientation direction. The common direction can include any selected direction in the pupil plane. For example, the haze orientation direction can be selected to correspond to a direction in the pupil plane that is different from the polarization direction of particle scattering (e.g., as shown in FIG. 3B) so that the surface haze can be distinguished from particle scattering. In another example, the haze orientation direction can be selected to correspond to a vertical direction (e.g., Y direction) or a horizontal direction (e.g., X direction) in the orientation shown in FIG. 4B (e.g., the haze orientation direction can be selected to be perpendicular or parallel to the plane of incidence of the illumination beam 114 on the sample 106).

[0063] 4B is a plot of the surface haze electric field distribution shown in FIG. 3A after propagation through a first polarization rotator 402a positioned to align the polarization ellipse of the surface haze (e.g., the major axis of the polarization ellipse) along the direction of illumination (e.g., the X direction), in accordance with one or more embodiments of the present disclosure. As shown in FIG. 4B, the shape of the polarization ellipse remains substantially unchanged as the polarization is rotated. FIG. 4C is a plot showing the spatially varying polarization rotation distribution of the first polarization rotator 402a for producing the surface haze electric field distribution of FIG. 4B, in accordance with one or more embodiments of the present disclosure.

[0064] In some embodiments, the quarter-wave plate 404 converts elliptically polarized light across the pupil plane into linearly polarized light. Figure 4D is a plot of the surface haze electric field distribution illustrated in Figure 4B after propagation through the quarter-wave plate 404, in accordance with one or more embodiments of the present disclosure. As shown in Figure 4D, the resulting orientation of the linearly polarized light from the quarter-wave plate 404 depends on the eccentricity of the incident light. For example, the greater the eccentricity of the incident light, the greater the degree of rotation relative to the haze orientation direction.

[0065] In some embodiments, the second polarization rotator 402b rotates the polarization of the surface haze back to the common haze orientation direction. Figure 4E is a plot of the surface haze electric field distribution shown in Figure 4D after propagating through the second polarization rotator 402b, in accordance with one or more embodiments of the present disclosure. In this manner, the second polarization rotator 402b can compensate for or otherwise correct any deviation of the polarization state from the haze orientation direction induced by the quarter-wave plate 404. For example, Figure 4E illustrates surface haze that is linearly polarized across the pupil plane along the haze orientation direction (e.g., the X direction). However, it should be understood that the second polarization rotator 402b can generally align the polarization of the surface haze across the pupil plane with any arbitrarily selected haze orientation direction that is not necessarily the same as the haze orientation direction provided by the first polarization rotator 402a. FIG. 4F is a plot illustrating the spatially varying polarization rotation distribution of the second polarization rotator 402b for generating the surface haze electric field distribution of FIG. 4E in accordance with one or more embodiments of the present disclosure.

[0066] 4G is a plot of the surface haze electric field distribution illustrated in FIG. 4E after propagation through a linear polarizer 108, in accordance with one or more embodiments of the present disclosure. As illustrated by the magnitude scale in FIG. 4G, precise manipulation of the surface haze to linear polarization along the haze orientation direction facilitates sensitive suppression of the surface haze across the entire pupil plane.

[0067] 4H-4M, various implementations of the components illustrated in FIG. 4A are described in more detail, in accordance with one or more embodiments of the present disclosure. Polarization rotators 402 that provide spatially varying amounts of polarization rotation are generally described in U.S. Patent 10,948,423 (March 16, 2021), which is incorporated herein by reference in its entirety.

[0068] In some embodiments, the polarization rotator 402 is formed from segmented half-wave plates 406. FIG. 4H is a block diagram of a polarization rotator 402 formed as a segmented half-wave plate 406 in accordance with one or more embodiments of the present disclosure. For example, the polarization rotator 402 may include two or more half-wave plates distributed across the pupil plane, each having an optical axis (e.g., fast axis or slow axis) oriented in a direction selected to provide a selected spatial distribution of polarization rotation angles. In this manner, portions of the sample light 122 in different regions of the pupil plane may be rotated by different amounts. For example, the orientation of the optical axis in each region of the pupil plane may be selected based on the polarization state of the surface haze in each corresponding region of the pupil plane (e.g., as shown in FIG. 3A ). Such a polarization rotator 402 may generally include any number of half-wave plates (e.g., segments) in any distribution across the pupil plane to provide a selected, spatially varying distribution of polarization rotation. For example, the polarization rotator 402 may include a linear piecewise half-wave plate 406 having a series of half-wave plates distributed along a linear direction in the pupil plane (e.g., the Y direction in FIG. 3A ). In another case, the polarization rotator 402 may include an angle-splitting half-wave plate 406 having a series of wedge-shaped half-wave plates distributed radially around a vertex location, such as, but not limited to, the portion of the pupil plane corresponding to the specular reflection of the illumination beam 114.

[0069] In some embodiments, the polarization rotator 402 includes an optically active material 408 (e.g., a material that exhibits circular birefringence or circular dichroism, a chiral material, etc.) having a thickness that varies spatially to provide a selected spatial distribution of polarization rotation angles. Figure 4I is a block diagram of a polarization rotator 402 formed as a segmented half-wave plate 406 in accordance with one or more embodiments of the present disclosure.

[0070] It is recognized herein that polarization rotation using the optically active material 408 may be based on a different mechanism than polarization rotation using a half-wave plate (e.g., a segmented half-wave plate). In particular, the half-wave plate may be formed by a material that provides different refractive index values ​​for orthogonal directions (e.g., fast and slow axes) in the pupil plane and may have a constant thickness configured to provide a π phase delay (e.g., half-wave) between light along these orthogonal directions. In this manner, the amount of polarization rotation induced by the half-wave plate is controlled by the rotation of the half-wave plate in the pupil plane. In contrast, the optically active material 408 may exhibit chirality and provide a constant polarization rotation rate as a function of thickness. In this manner, the amount of polarization rotation induced by the optically active material 408 is controlled by the thickness of the optically active material. Furthermore, it is recognized herein that some materials, such as, but not limited to, quartz, can operate as either a wave plate or the optically active material 408 based on their orientation relative to the pupil plane. For example, quartz oriented with its optical axis in the pupil plane may act as a waveplate, while quartz oriented with its optical axis orthogonal to the pupil plane (e.g., along the propagation direction of the sample light 122) may act as the optically active material 408. However, it should be understood that this is merely an example and that the optically active polarization rotator 402 may generally be formed from any optically active material.

[0071] In some embodiments, the polarization rotator 402 formed from the optically active material 408 may further include a phase compensator 410 to promote constructive interference of the sample light 122 associated with particle scattering at the center of the particle's image on the detector 110. For example, the phase compensator 410 may include an optically homogeneous material having a spatially varying thickness across the pupil plane 130 that is complementary to the spatially varying thickness of the optically active material 408. In this manner, the total optical path of the sample light 122 through the polarization rotator 402 may be constant across the pupil plane. As another example, the phase compensator 410 may be formed from an optically active material having an opposite handedness (polarization orientation (right or left)) to the optically active material 408 that comprises the polarization rotator 402. In one example, the optically active material 408 includes right-handed quartz and the phase compensator 410 includes left-handed quartz, each with a thickness profile selected to achieve the desired polarization rotation and phase correction.

[0072] It is contemplated herein that a polarization rotator 402 formed from an optically active material 408 may therefore provide continuously varying polarization rotation across the pupil plane by fabricating the polarization rotator 402 from an optically active material 408 with a 2D continuously spatially varying thickness. In some embodiments, the first polarization rotator 402a and / or the second polarization rotator 402b are formed from an optically active material 408 with a 2D thickness profile that provides precise polarization rotation across the pupil plane (e.g., according to the distributions shown in Figures 4C and 4F).

[0073] 4J is a plot illustrating particle inspection sensitivity and efficiency of the CDER 104 in suppressing surface haze as a function of pixel size of the detector 110 in the inspection tool 102, in accordance with one or more embodiments of the present disclosure. It is contemplated herein that the particle detection sensitivity of the inspection tool 102 having the CDER 104 for suppressing surface haze may depend on the particular electric field distribution of particles on the sample 106 relative to the surface haze. For example, the electric field distribution in 3B may correspond to a particular particle. Generally, this distribution will vary based on various parameters, such as, but not limited to, particle size or composition.

[0074] For example, Figure 4J corresponds to a simulation of the efficiency of a CDER 104 as illustrated in Figure 4A with polarization rotators 402a,b formed from an optically active material 408 having a 2D thickness profile (e.g., as shown in Figures 4C and 4F), in accordance with one or more embodiments of the present disclosure. Additionally, the legend on the contours in Figure 4J corresponds to particle size in nanometers. As shown in Figure 4J, this non-limiting example of a CDER 104 provides high efficiency (e.g., suppression of surface haze for light scattered by particles) over a wide range of pixel sizes.

[0075] However, it is contemplated herein that in some cases it may be impractical or undesirable to fabricate an optically active material 408 with a spatially varying thickness profile that precisely provides a spatially varying distribution of polarization rotation appropriate for a particular step (e.g., the distributions shown in Figures 4C and 4F). Thus, in some embodiments, a particular polarization rotator 402 (e.g., first polarization rotator 402a or second polarization rotator 402b) can provide a spatial polarization rotation distribution that approximates the ideal polarization rotation distribution within a selected tolerance. In this manner, performance and manufacturability can be balanced.

[0076] In some embodiments, the polarization rotator 402 (e.g., the first polarization rotator 402a or the second polarization rotator 402b) has an optically active material 408 with a thickness profile that varies along a single direction (e.g., a 1D thickness profile), providing a 1D polarization rotation distribution. It is contemplated herein that it may be easier and / or more cost-effective to fabricate an optically active material 408 with a 1D thickness profile than a 2D thickness profile. Also, as shown in FIG. 3(A), the polarization ellipse of surface haze varies significantly along the Y direction and relatively weakly along the X direction. Therefore, a 1D polarization rotation distribution that varies along the Y direction may reasonably approximate an ideal polarization rotation distribution.

[0077] 4K is a conceptual cross-sectional view of the component shown in FIG. 4A where the polarization rotators 402a,b have a thickness profile that varies continuously along a single direction (e.g., the Y direction) to provide a one-dimensionally varying polarization rotation distribution, in accordance with one or more embodiments of the present disclosure. FIG. 4L is a conceptual cross-sectional view of the component shown in FIG. 4A where the polarization rotators 402a,b have a thickness profile that varies linearly along a single direction (e.g., the Y direction) to provide a linearly varying polarization rotation distribution, in accordance with one or more embodiments of the present disclosure.

[0078] In some embodiments, the polarization rotator 402 (e.g., the first polarization rotator 402a and / or the second polarization rotator 402b) may be formed as a stack of components, with the desired properties (e.g., spatial polarization rotation distribution) achieved through propagation of light through the stack. For example, it may be straightforward to create (e.g., based on cost, manufacturability, etc.) the optically active material 408 (or corresponding phase compensator 410) with the desired complex spatially-varying thickness profile (e.g., 1D or 2D). However, it may be possible to construct the spatially-varying thickness profile (e.g., 1D or 2D) or an approximation thereof from a series of optically active materials 408 having relatively easy-to-manufacture profiles (e.g., spherical, linear, etc.), the combination of which forms the desired complex spatially-varying thickness profile or a reasonable approximation thereof.

[0079] 4M is a conceptual cross-sectional view of the components illustrated in FIG. 4A in which the polarization rotator 402a,b is formed as a stack of elements including two optically active materials 408 and corresponding phase compensators 410, according to one or more embodiments of the present disclosure. For example, the optically active materials 408 and corresponding phase compensators 410 in FIG. 4M are formed with spherical and linear profiles.

[0080] It is further contemplated herein that the phase mask 202 or other beam conditioning elements may be incorporated into the CDER 104. Figure 4M further illustrates the phase mask 202 integrated with the first polarization rotator 402a.

[0081] 5A-5F, it is contemplated herein that the functional steps of converting the elliptically polarized light of the surface haze across the pupil plane into linearly polarized light and aligning the linearly polarized light along a common haze orientation direction for subsequent suppression and / or separation by the linear polarizer 108 may be performed using a variety of techniques.

[0082] 5A is a conceptual diagram of a second configuration of the collection path 124 of the inspection tool 102 including the CDER 104, in accordance with one or more embodiments of the present disclosure. As described with respect to FIG. 4A, each of the components illustrated in FIG. 5A may be formed as a single optical element or may be distributed among any number of optical elements in any number of pupil planes.

[0083] In particular, FIG. 5A illustrates a configuration of the CDER 104 including a single polarization rotator 402 and a segmented quarter-wave plate 502, both of which have spatially varying behavior across the pupil plane. It is contemplated herein that the effect of the segmented quarter-wave plate 502 on the orientation of linearly polarized light may be considered when designing the polarization rotator 402 so that only a single polarization rotation step is required. In this configuration, the polarization rotator 402 does not align the major axis of the elliptical polarization, as shown in FIG. 4B . Rather, the polarization rotator 402 provides a spatially varying polarization rotation profile based at least in part on the eccentricity of the surface haze, such that the polarization of the surface haze across the pupil plane is aligned only after propagating through the segmented quarter-wave plate 502 and being converted to linear polarization. For example, the polarization rotator 402 may align the elliptical polarization so that the diagonals of the polarization ellipse are aligned along a common direction (e.g., the illumination direction or the horizontal direction, as depicted in FIG. 5B below). A segmented quarter-wave plate 502 with its optical axis oriented parallel to the minor axis of each polarization ellipse can then convert the elliptically polarized light into linearly polarized light, providing that the linearly polarized light is aligned along a common haze orientation direction (e.g., the illumination direction or X direction as depicted in Figure 5C below).

[0084] FIG. 5B is a plot of the surface haze electric field distribution shown in FIG. 3A after propagating through the polarization rotator 402 of FIG. 5A in accordance with one or more embodiments of the present disclosure. As shown in FIG. 5B, the major axes of the polarization ellipses of the surface haze are not parallel to each other, but are varied by eccentricity so that the diagonals of the ellipses are parallel to each other. FIG. 5C is a plot of the surface haze electric field distribution shown in FIG. 5B after propagating through the segmented quarter-wave plate 502 of FIG. 5A in accordance with one or more embodiments of the present disclosure. FIG. 5D is a plot of the surface haze electric field distribution shown in FIG. 5C after propagating through the linear polarizer 108 of FIG. 5A in accordance with one or more embodiments of the present disclosure. As shown in FIG. 5D, most of the surface haze is attenuated or redirected by the linear polarizer 108, so the intensity is very low.

[0085] As described with respect to Figure 4A, the polarization rotator 402 and segmented quarter-wave plate 502 of Figure 5A can be formed using any technique known in the art. Figure 5E is a conceptual cross-sectional view of the components shown in Figure 5A where the polarization rotator 402 is formed as a segmented half-wave plate (e.g., the segments are distributed along the horizontal direction of the pupil plane 130, as shown in Figures 5B, 5C, and 5D) and the segmented quarter-wave plate 502 is formed with segments distributed along the horizontal direction (e.g., the X direction), in accordance with one or more embodiments of the present disclosure. Figure 5F is a conceptual cross-sectional view of the components shown in Figure 5A where the polarization rotator 402 is formed as a continuous element (e.g., optically active material 408 having a continuously varying thickness profile) and the segmented quarter-wave plate 502 is formed with segments distributed along the illumination direction (e.g., the horizontal direction of the pupil plane as depicted in Figures 5B, 5C, and 5D), in accordance with one or more embodiments of the present disclosure.

[0086] 6A is a conceptual cross-sectional view of a third configuration of the collection path 124 of the inspection tool 102 including the CDER 104, in accordance with one or more embodiments of the present disclosure. As described with respect to FIGS. 4A and 5A, each of the components illustrated in FIG. 6A may be formed as a single optical element or may be distributed among any number of optical elements in any number of pupil planes.

[0087] In particular, FIG. 6A illustrates a configuration of a CDER 104 that includes a spatially varying waveplate 602 that converts the polarization ellipse of the surface haze into linear polarization across the pupil such that the linear polarization is aligned along a common haze orientation direction. The CDER 104 may further include a polarization rotator 402 that rotates the linear polarization of the surface haze into the common haze orientation direction. The polarization rotator 402 may have any suitable design, such as, but not limited to, the designs illustrated in FIGS. 4H and 4I. In some embodiments, the polarization rotator 402 includes an optically active material 408 and, optionally, a phase compensator 410. In some embodiments, the polarization rotator 402 includes a segmented half-waveplate 406. For example, FIG. 6A illustrates a non-limiting configuration of a CDER 104 in which the polarization rotator 402 includes an optically active material 408 and a phase compensator 410. FIG. 6A further illustrates a linear polarizer 108 and a phase mask 202 in the collection path 124 as previously described herein to isolate or suppress surface haze and reshape the PSF of the light passing through the CDER 104 (e.g., related to the sample light 122 of interest from particles on the sample 106).

[0088] In some embodiments, one or more components of the CDER 104 have a spatially varying thickness to compensate (at least within a selected tolerance) for spatial variations in the thickness of the spatially varying waveplate 602. For example, FIG. 6A illustrates a non-limiting configuration in which the CDER 104 includes a phase mask 202 having a segment 204 formed from a half-waveplate having an optical axis along the X direction to introduce a phase shift of p for light polarized along the Y direction (e.g., as described above with reference to FIG. 2 ), and a segment 206 including a compensator formed from an optically homogeneous material along the propagation direction, where all light propagating along the collection path 124 propagates along the same optical path length (e.g., within at least a selected tolerance). In particular, FIG. 6A illustrates the segment 206 having a spatially varying thickness with a thickness profile designed to compensate for the spatially varying thickness profile of the spatially varying waveplate 602. As another example, the CDER 104 can include an additional compensating element formed from an optically homogeneous material with a thickness profile designed to compensate for the spatially varying thickness profile of the spatially varying waveplate 602.

[0089] This spatially varying waveplate 602 may be formed as a continuously varying waveplate or a segmented waveplate as a birefringent material, where the thickness and orientation of the optic axis vary (e.g., continuously or between segments) to provide both surface haze and conversion of elliptical polarization to linear polarization.

[0090] FIG. 6B is a plot of the surface haze electric field distribution shown in FIG. 3A after propagating through the spatially varying waveplate 602 of FIG. 6A in accordance with one or more embodiments of the present disclosure. As shown in FIG. 6B, the elliptically polarized light of FIG. 3A is converted to linearly polarized light with an orientation that varies across the pupil plane. FIG. 6C is a plot of the surface haze electric field distribution shown in FIG. 6B after propagating through the optical rotator 402 of FIG. 6A in accordance with one or more embodiments of the present disclosure. In FIG. 6C, the orientation of the linearly polarized light is aligned along the common haze orientation direction (e.g., horizontal in FIG. 6C). FIG. 6D is a plot of the surface haze electric field distribution shown in FIG. 6C after propagating through the linear polarizer 108 of FIG. 6A in accordance with one or more embodiments of the present disclosure. As shown in FIG. 6D, most of the surface haze is attenuated or redirected by the linear polarizer 108, so the intensity is very low.

[0091] Referring now generally to Figures 4A-6D, it should be noted that substantially similar performance can be achieved with the different designs shown in Figures 4A, 5A, and 6A. In particular, Figures 4E, 5C, and 6C illustrate both the conversion of elliptically polarized surface haze to linearly polarized light and the alignment of the surface haze along the haze orientation direction (here, the horizontal direction in each figure). Similarly, Figures 4G, 5D, and 6D show substantially similar suppression of surface haze by a linear polarizer. Therefore, it should be understood that the teachings related to the performance of CDER 104 based on Figure 4J can be applied or extended to all variations of CDER 104, including, but not limited to, those shown in Figures 5A and 6A.

[0092] 4A-6D and the associated description are provided for illustrative purposes only and should not be construed as limiting. In particular, FIGS. 4A-6C represent non-limiting examples of CDER 104.

[0093] Referring again to FIG. 1A, various additional components of particle detection system 100, in accordance with one or more embodiments of the present disclosure, will now be described in greater detail.

[0094] In some embodiments, particle detection system 100 includes a controller 134 including one or more processors 136 configured to execute program instructions maintained on a storage medium 138 (e.g., memory). Additionally, controller 134 may be communicatively coupled to any component of particle detection system 100. In this regard, one or more processors 136 of controller 134 may perform any of the various process steps described throughout this disclosure. For example, controller 134 may receive, analyze, and / or process data from detector 110 (e.g., associated with an image of sample 106). As another example, controller 134 may control or otherwise direct any component of particle detection system 100 using control signals.

[0095] The one or more processors 136 of the controller 134 may include any processing elements known in the art. In this sense, the one or more processors 136 may include any microprocessor-type device configured to execute algorithms and / or instructions. In some embodiments, the one or more processors 136 may comprise a desktop computer, a mainframe computer system, a workstation, an image computer, a parallel processor, or any other computer system (e.g., a networked computer) configured to execute programs configured to operate the particle detection system 100 as described throughout this disclosure. Furthermore, it should be recognized that the term “processor” may be broadly defined to encompass any device having one or more processing elements that execute program instructions from a non-transitory storage medium (memory) 138. Furthermore, the steps described throughout this disclosure may be performed by a single controller 134 or, alternatively, by multiple controllers. Furthermore, the controller 134 may include one or more controllers housed within a common housing or multiple housings. In this manner, any controller or combination of controllers may be separately packaged as a module suitable for integration into the particle detection system 100.

[0096] The storage medium 138 may include any storage medium known in the art suitable for storing program instructions executable by the associated one or more processors 136. For example, the storage medium 138 may include a non-transitory storage medium. As another example, the storage medium 138 may include, but is not limited to, read-only memory (ROM), random access memory (RAM), magnetic or optical memory devices (e.g., disks), magnetic tape, solid-state drives, etc. Furthermore, it should be noted that the storage medium 138 may be housed within a common controller housing along with one or more processors 136. In some embodiments, the storage medium 138 may be located remotely relative to the physical location of one or more processors 136 and the controller 134. For example, one or more processors 136 of the controller 134 may access a remote memory (e.g., a server) accessible via a network (e.g., the Internet, an intranet, etc.). Therefore, the above description should not be construed as limiting the present invention, but merely as illustrative.

[0097] 7, which is a flow diagram illustrating steps performed in a method 700 for particle detection, in accordance with one or more embodiments of the present disclosure. Applicant notes that the embodiments and enabling techniques previously described herein in the context of particle detection system 100 should be construed to extend to method 700. However, it is further noted that method 700 is not limited to the architecture of particle detection system 100.

[0098] In one embodiment, the method 700 includes receiving 702 a first electric field distribution of light scattered from a surface of the specimen (e.g., surface haze) in response to an illumination beam having a known polarization at a known angle of incidence, wherein at least a portion of the first electric field distribution includes elliptically polarized light. In another embodiment, the method 700 includes receiving 704 a second electric field distribution of light scattered from particles on the surface of the specimen in response to the illumination beam.

[0099] For example, it may be the case that surface haze can have a different electric field distribution in the pupil plane of an imaging system than light scattered by particles on the surface. In particular, it is recognized herein that surface haze and particle scattering have substantially different electric field distributions when scattered by obliquely incident p-polarized light.

[0100] In another embodiment, method 700 includes step 706 of designing one or more polarization-controlling optical elements (e.g., CDER 104) suitable for placement at a pupil plane of the imaging system to both transform the first electric field distribution to include fully linearly polarized light across the pupil plane and to cause the linear polarization across the pupil plane to be aligned along a common orientation direction (e.g., haze orientation direction). In some embodiments, the common orientation direction is substantially different from the polarization direction of light in the second electric field distribution. In this way, the first electric field distribution can be manipulated and distinguished from the second electric field distribution using polarization-controlling optics.

[0101] It is contemplated herein that the polarization control optical element designed in 706 (e.g., forming CDER 104) may be provided in various configurations within the spirit and scope of the present disclosure. For example, the polarization control optical element may include a first polarization rotator for rotating the polarization of light associated with the first electric field distribution to a first common orientation direction, a quarter-wave plate for converting all polarized light traversing the pupil to linearly polarized light, and a second polarization rotator for rotating the linearly polarized light to a second common orientation direction, which may be the same or different from the first common orientation direction. As another example, the polarization control optical element may include a single polarization rotator and a quarter-wave plate, where the single polarization rotator rotates the polarization of light associated with the first electric field distribution to an intermediate distribution such that light traversing the pupil is linearly polarized and aligned across the common orientation direction after propagating through the quarter-wave plate. As a further example, the polarization control optical element may include a segmented wave light including multiple segments distributed across the pupil plane. For example, each of the segments may be formed from a birefringent material having an optical axis in the pupil plane and a thickness configured to both convert light incident on the segment to linearly polarized light and further provide that the linearly polarized light is aligned along an orientation direction common to the other segments.

[0102] In another embodiment, the method 700 includes step 708 of generating a dark-field image of the sample using an imaging system having polarization-controlled optics in a pupil plane and a linear polarizer aligned to reject light polarized along a selected orientation direction, the dark-field image being based on light passed by the linear polarizer. For example, the light passed through the polarizer can correspond to light scattered by one or more particles on the surface of the sample within a selected tolerance, and surface haze is suppressed.

[0103] The subject matter described herein illustrates different components that are, in some cases, included within or connected to other components. It should be understood that such depicted architectures are merely exemplary, and that in fact many other architectures that achieve the same functionality may be implemented. In a conceptual sense, any arrangement of components to achieve the same functionality is effectively “associated” such that the desired functionality is achieved. Thus, any two components herein that combine to achieve a particular function can be considered to be “associated” with each other such that the desired functionality is achieved, regardless of the architecture or intermediate components. Similarly, any two components so associated can also be considered to be “connected” or “coupled” to each other to achieve the desired functionality, and any two components that can be so associated can also be considered to be “couplable” with each other to achieve the desired functionality. Specific examples of what can be coupled 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.

[0104] It is believed that the present disclosure and many of its attendant advantages will be understood from the foregoing description, and it will be apparent that various changes can be made in the form, construction, and arrangement of the elements without departing from the disclosed subject matter or sacrificing all of its material advantages. The described forms are merely illustrative, and it is the intent of the following claims to embrace and include such modifications. It is further understood that the invention is defined by the appended claims.

Claims

1. 1. A system comprising: an illumination source configured to generate an illumination beam; one or more illumination optics that direct the illumination beam onto the sample at an off-axis angle along the illumination direction; one or more collection optics that collect light scattered from the sample in response to the illumination beam in a dark field mode, wherein the scattered light from the sample collected by the one or more collection optics includes surface haze associated with light scattered from a surface of the sample, at least a portion of the surface haze having an elliptically polarized light; one or more optical elements located at one or more pupil planes of the one or more collection optics, a spatially varying waveplate disposed at a pupil plane of the one or more collection optics to convert the surface haze into linearly polarized light; a polarization rotator that provides a spatially varying polarization rotation distribution that rotates the surface haze from the spatially varying waveplate to a selected orientation distribution, the surface haze rotation comprising a rotation of a major axis of elliptically polarized light to the selected haze orientation direction; one or more optical elements including a linear polarizer aligned to reject light polarized parallel to the selected haze orientation direction to reject the surface haze from the polarization rotator; a detector that generates a dark-field image of the sample based on scattered light from the sample that has passed through the linear polarizer, the scattered light from the sample that has passed through the linear polarizer comprising at least a portion of light scattered by one or more particles on a surface of the sample; and include, system.

2. The system of claim 1 , wherein the spatially varying waveplate is a continuously varying waveplate.

3. The system of claim 1 , wherein the spatially varying waveplate is a segmented waveplate.

4. The system of claim 1 , wherein the one or more illumination optics are configured to direct the illumination beam toward the sample with p-polarized light.

5. 10. The system of claim 1, wherein the linear polarizer is a polarizing beam splitter that directs light scattered from the sample along a first optical path and directs the surface haze along a second optical path different from the first optical path.

6. The system of claim 5 , further comprising an additional detector configured to generate a dark-field image of the sample based on the surface haze along the second optical path.

7. The system of claim 1 , wherein the segmented half-wave plate comprises a linearly segmented wave plate including a plurality of segments distributed in a pupil plane.

8. The system of claim 7 , wherein the plurality of segments are distributed along a direction orthogonal to the illumination direction in a pupil plane.

9. 10. The system of claim 1, wherein the polarization rotator comprises an optically active material having a spatially varying thickness to provide a corresponding first or second spatially varying polarization rotation distribution.

10. The system of claim 9 , wherein the optically active material is oriented such that an optical axis is orthogonal to a corresponding one of the one or more pupil planes.

11. The system of claim 9 , wherein the optically active material is quartz.

12. 10. The system of claim 9, wherein the polarization rotator further comprises a phase compensator that equalizes the optical path length of the surface haze through the optically active material.

13. The system of claim 12 , wherein the phase compensator is formed from a material that is optically homogeneous along a direction of propagation through the phase compensator.

14. The system of claim 12 , wherein the phase compensator is formed from optically active material having opposite handedness of the optically active material along a direction of propagation through the phase compensator.

15. 10. The system of claim 1, wherein one or more optical elements located at one or more pupil planes of the one or more collection optical systems further comprise a phase mask that provides different phase shifts to light in two or more pupil regions of the collection region to shape a point spread function of light scattered from one or more particles on the surface of the sample.

16. 16. The system of claim 15, wherein the phase mask shapes a point spread function of light scattered from one or more particles on the surface of the specimen to provide a central peak of the point spread function.

17. The system of claim 1 , wherein the selected haze orientation direction is parallel to the selected alignment direction.

18. The system of claim 1 , wherein the selected haze orientation direction is orthogonal to the selected orientation direction.

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