Continuous degenerate elliptical retarder for sensitive particle detection

Abstract

To reject surface haze.SOLUTION: An inspection system 100 may include an illumination source to generate an illumination beam, illumination optics to direct the illumination beam to a sample 106 at an off-axis angle along an illumination direction, and collection optics to collect scattered light from the sample in a dark-field mode. The scattered light from the sample includes surface haze associated with light scattered from the surface of the sample, and at least a portion of the surface haze having elliptical polarizations. The system may further include pupil-plane optics to convert the polarizations of the surface haze across the pupil to linear polarization that is aligned parallel to a selected haze orientation direction. The system may include a linear polarizer 108 to reject the surface haze aligned parallel to the haze orientation direction and a detector 110 to generate a dark-field image of the sample on the basis of light passed by the linear polarizer 108.SELECTED DRAWING: Figure 7

Description

[Technical field]

[0001] REFERENCE TO RELATED APPLICATIONS The inventors in the claims of this application, including Xue Feng, U.S. Patent Act § 119(e) Liu, U.S. Provisional Application No. 63 / 151,068 (February 19, 2021, ADVANCED METHOD TO IMPROVE PARTICLE DETECTION SENSITIVITY IN WAFER INSPECTION), Jenn Kuen, Leong, and John Alex Fielden Chuang, are all named as the inventors. "Technology field" 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 for optically polished surfaces. Although 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 mitigates the above-mentioned shortcomings. [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 to 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, the scattered light from the sample collected by the one or more collection optics includes a surface haze associated with the light scattered from the surface of the sample, at least a portion of the surface haze having an elliptical polarization. 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, the rotation of the surface haze includes a rotation of a major axis of the elliptical polarization 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 linear polarization. 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 to the selected haze orientation direction from the quarter wave plate. 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 includes at least a portion of the 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 to 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, 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 to a selected orientation distribution, and the rotation of the surface haze includes rotating a major axis of the elliptical polarization to 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 to linearly polarized light, and 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 includes at least a portion of light scattered by one or more particles on a 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, and in another exemplary embodiment, the system includes one or more illumination optics for directing the illumination beam to 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, 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 spatially varying waveplate located 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, and rotates the surface haze to a selected orientation distribution from the spatially varying waveplate, and the rotation of the surface haze includes a rotation of a major axis of the elliptical polarization to 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 includes at least a portion of light scattered by one or more particles on a 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 description of the drawings]

[0009] Many advantages of the present disclosure may be better understood by those skilled in the art by reference to the following 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. [Diagram 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 generating 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 illustrated 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 generating 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 illustrated 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 polarization rotation distribution that varies in one dimension, in accordance with one or more embodiments of the present disclosure. [Figure 4L] FIG. 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] FIG. 4M is a conceptual cross-sectional view of the components illustrated in FIG. 4A in which a polarization rotator is formed as a stack of elements including two optically active materials and corresponding phase compensators, in accordance with one or more embodiments of the present disclosure. [Figure 5A] FIG. 5A is a conceptual diagram of a second 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 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 where 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 illustrated 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 PREFERRED EMBODIMENTS

[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 specifically shown and described with respect 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 U.S. Patent 9,874,526, issued January 1, 2018, U.S. Patent 9,291,575 (March 22, 2016), U.S. Patent 8,891,079 (November 18, 2014), and U.S. Patent 9,891,177 (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 may 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 may be particularly significant for obliquely incident p-polarized light. For example, surface haze from obliquely incident p-polarized light may have an elliptically polarized light and may be polarized approximately radially relative to the angle of specular reflection, while scattering from particles may be polarized approximately radially relative to the surface normal. In this manner, surface haze may 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 linear polarizers 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 perfectly 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, the surface haze is isolated and separated using a continuous degenerate elliptical retarder, referred to herein for convenience as a CDER, together with a linear polarizer or polarizing beam splitter. Hereinafter, the continuous degenerate elliptical retarder is referred to as a CDER. For example, the CDER may be an optical element designed for placement in the pupil plane of an optical system that can functionally convert the polarization ellipse associated with the surface haze across the pupil plane into linearly polarized light oriented along a common angle. In this way, the linear polarizer can separate the light associated with the surface haze from the 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 the light scattered by the 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, with at least some of the components providing spatially varying transformation across the pupil to provide tailored separation of surface haze. In this configuration, the first polarization rotator may rotate the elliptically polarized surface haze in a common orientation direction 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, which is incorporated herein by reference above, but 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 the elliptically polarized surface haze in a common orientation direction 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, allowing for 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 may not rotate the elliptically polarized light to a common orientation across the pupil. Rather, the polarization rotator may compensate for the subsequent action of the quarter wave plate such that the polarization state of the surface haze across the pupil is aligned only after passing through the quarter wave plate. In particular, since the quarter wave plate produces 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 not necessary. 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 the CDER for the isolation and suppression of surface haze for particle detection purposes can provide a number of benefits.For example, the 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., rejection angle) for rejection using a linear polarizer.As a result, the 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 way, the CDER can be easily positioned in 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 showing the placement of an element such as a CDER in 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 by a selected metric (e.g., extinction ratio of surface haze when combined with a linear polarizer, etc.). Furthermore, it should be understood that an optical system may include any number of optical relays to provide any number of conjugate pupil planes in which the CDER or a component thereof may be positioned. Thus, a description showing the placement of the CDER in a pupil plane herein may be understood to include any placement of the CDER component in any number of conjugate pupil planes.

[0020] 1-7, systems and methods for highly sensitive particle detection in accordance with one or more embodiments of the present disclosure will be described in greater 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 to generate inspection data based on the sample light 122 after suppression of the surface haze.

[0023] The linear polarizer 108 may 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 the 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 to measure 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 towards the specimen 106 .

[0026] The illumination source 112 may include any type of light source known in the art. Furthermore, the illumination source 112 may 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 may 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 about 350 nm. As another example, the illumination beam 114 may provide a wavelength of about 266 nm. As another example, the illumination beam 114 may provide a wavelength of about 213 nm. As another example, the illumination beam 114 may provide a wavelength of about 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 can include an adjustable illumination source (e.g., one or more tunable lasers, etc.). As another example, the illumination source 112 can 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, and the like.

[0030] It is recognized herein that the intensity of surface haze may 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 may be relatively high at near-normal angles of incidence and decrease at higher angles of incidence. In some embodiments, the illumination path 116 may include one or more illumination optics, such as, but not limited to, lenses 118, mirrors, 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. Oblique angles of incidence may generally include any selected angle of incidence. For example, the angle of incidence may 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 (e.g., sample light 122) emanating from the sample 106 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 emanating 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 may image the sample 106 based primarily on scattered light. Dark-field imaging may further 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 the 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 the 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 light detector 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, the linear polarizer 108 is disposed between the CDER 104 and the detector 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 may be formed as any type of polarizer known in the art. For example, the linear polarizer 108 may 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 may be aligned with the haze orientation direction such that the linear polarizer 108 may 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 may be formed as a polarizing beam splitter that may split light having orthogonal polarizations along two separate optical paths. In this configuration, the linear polarizer 108 may direct the surface haze aligned along the haze orientation direction along a first optical path and direct the remanding 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 is 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 elements can include, but are 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 disposed 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 the pupil plane 130 may generally describe one or more elements at or sufficiently near the pupil plane 130 to produce a desired effect. In some embodiments, although not shown, the collection path 124 may include additional lenses to generate one or more additional pupil planes 130 such that any number of elements may be disposed at or near the pupil plane 130, including, but not limited to, elements associated with the CDER 104, the phase mask 202 (see, e.g., FIG. 2 below), or the linear polarizer 108.

[0040] 2, in some embodiments, the 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 a sub-resolution particle. It is recognized herein that images of particles smaller than the imaging resolution of the system are generally limited by the system PSF, which is typically an Airy function when the image includes specular 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 imaging spot of the particle 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. Pat. No. 10,942,135, issued Mar. 9, 2021, which is incorporated 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 splitting 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 the light. For example, the segment 206 may include a compensation 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 compensation plate is formed from a material that has approximately the same thickness and refractive index as the half-wave plate in the segment 204, but has no birefringence along the propagation direction. In another embodiment, the compensation 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 compensation plate does not experience birefringence. For example, light propagating along the optical axis of a uniaxial crystal may not experience birefringence, and as a result, the crystal may be optically homogeneous for light propagating along the optical axis. As another example, the segment 206 may include an aperture.

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

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

[0047] However, it should be understood that FIG. 2 and the associated description are provided merely for illustrative purposes and should not be construed as limiting. For example, a phase mask 202 having two segments may include a half-wave plate disposed at the bottom of the collection region 306 instead of 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 the particle. For example, given a known electric field distribution of light in the pupil plane (e.g., measured, simulated, etc.) associated with an object of interest, 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 the particles of interest (e.g., as shown in FIG. 3B, etc.) 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 spatially varying thickness across the pupil plane to promote constructive interference of sample light 122 associated with particle scattering in the center of the image of the particle on the detector 110.

[0050] 3A-6D, the CDER 104 according to one or more embodiments of the present disclosure is described in more detail. Figures 3A and 3B show pupil plane scattering 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, according to one or more embodiments of the present disclosure. FIG 3B is a pupil plane scatter map 304 of light scattered by a small particle (e.g., small relative to the imaging resolution of the particle detection system 100 or the wavelength of the 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 intensity 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 that is associated with a 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 an objective lens in the collection path 124.

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

[0055] In addition, 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, especially when the illumination beam 114 is p-polarized. For example, the sample light 122 associated with surface haze generally exhibits a nearly radial polarization distribution with respect to the specular reflection angle 308 in the collection region 306, as illustrated in Figure 3A. In contrast, the 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 in the pupil plane 130, the ellipse is very elongated, which means 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., FIG. 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. Additionally, 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 of 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 the 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, thus suppressing the 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 the linear polarizer 108 to provide sharp imaging on a detector 110 (not shown) of the inspection tool 102.

[0059] The 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, the 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 for eliminating surface haze (e.g., haze orientation direction) may be any suitable direction. For example, the selected polarization direction may be selected based on the expected distribution of the particle-scattered sample light 122 (e.g., as shown in FIG. 3B) to minimize the intensity of the rejected particle-scattered sample light 122.

[0061] 4B-4G show the evolution of the electric field distribution of the surface haze due to the components shown in FIG. 4A, in accordance with one or more embodiments of the present disclosure. In FIG. 4B-4G, the grayscale intensity maps correspond to the intensity of the surface haze. Additionally, the polarization states at selected positions across the pupil plane are shown as an overlay to show the distribution of the 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] FIG. 4B is a plot of the surface haze electric field distribution shown in FIG. 3A after propagating through a first polarization rotator 402a arranged 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 generating 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 the 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 propagating through the quarter-wave plate 404, according to 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. FIG. 4E is a plot of the surface haze electric field distribution illustrated in FIG. 4D after propagating through the second polarization rotator 402b, according to one or more embodiments of the present disclosure. In this manner, the second polarization rotator 402b can compensate or otherwise correct any deviation of the polarization state from the haze orientation direction induced by the quarter-wave plate 404. For example, FIG. 4E illustrates a 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 to any arbitrarily selected haze orientation direction that does not necessarily have to be 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] Figure 4G is a plot of the surface haze electric field distribution illustrated in Figure 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 Figure 4G, precise manipulation of the surface haze to linear polarization along the haze orientation direction facilitates high sensitivity 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. A polarization rotator 402 that provides a spatially varying amount of polarization rotation is 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 a segmented half-wave plate 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-split 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, a portion of the pupil plane corresponding to a 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.) that has a thickness that varies spatially to provide a selected spatial distribution of polarization rotation angles. Figure 4I is a block diagram of the 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 be of a constant thickness set to provide a phase delay of π (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. Additionally, it is recognized herein that some materials, such as, but not limited to, quartz, may operate as a wave plate or optically active material 408 based on its 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 image of the particle 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 (right or left) to the optically active material 408 that includes 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 such that the desired polarization rotation and phase correction are achieved.

[0072] It is contemplated herein that a polarization rotator 402 formed from an optically active material 408 may thus 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., following 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 to suppress 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. In general, 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 a polarization rotator 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 the 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) for 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., the first polarization rotator 402a or the second polarization rotator 402b) may provide a spatial polarization rotation distribution that approximates the ideal polarization rotation distribution within a selected tolerance. In this manner, performance and manufacturability may be balanced.

[0076] In some embodiments, the polarization rotator 402 (e.g., the first polarization rotator 402a or the second polarization rotator 402b, etc.) has an optically active material 408 with a thickness profile (e.g., a 1D thickness profile) that varies along a single direction, 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 the surface haze varies significantly along the Y direction and relatively weakly along the X direction. Thus, the 1D polarization rotation distribution that varies along the Y direction may reasonably approximate an ideal polarization rotation distribution.

[0077] Figure 4K is a conceptual cross-sectional view of the component shown in Figure 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. Figure 4L is a conceptual cross-sectional view of the component illustrated in Figure 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, and the desired properties (e.g., spatial polarization rotation distribution) are achieved through the propagation of light through the stack. For example, it may be directly practical (e.g., based on cost, manufacturability, etc.) to create (the optically active material 408 (or the corresponding phase compensator 410)) the desired complex spatially-varying thickness profile (e.g., 1D or 2(d)). 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 profiles that are relatively easy to manufacture (e.g., being spherical, linear surfaces, etc.), the combination of which forms the desired complex spatially-varying thickness profile or a reasonable approximation thereof.

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

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

[0082] Figure 5A is a conceptual diagram of a second configuration of collection path 124 of inspection tool 102 including CDER 104, in accordance with one or more embodiments of the present disclosure. As described with respect to Figure 4A, each of the components illustrated in Figure 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 such 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 such 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 to 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 FIG. 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, according to 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 vary with 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, according to 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, according to 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 the segmented quarter-wave plate 502 of Figure 5A may 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., X-direction), according to 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 with 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), according to one or more embodiments of the present disclosure.

[0086] Figure 6A is a conceptual cross-sectional view of a third 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. As described with respect to Figures 4A and 5A, each of the components illustrated in Figure 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 the 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-wave plate 406. For example, FIG. 6A illustrates a non-limiting configuration of the 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 sample light of interest 122 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 with 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 through 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, according to 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, according to one or more embodiments of the present disclosure. In FIG. 6C, the orientation of the linearly polarized light is oriented 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, according to 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 by 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. It should therefore be understood that the teachings related to the performance of CDER104 based on Figure 4J can be applied or extended to all variations of CDER104, including, but not limited to, the variations shown in Figures 5A and 6A.

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

[0093] Referring again to FIG. 1A, various additional components of the 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, the 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, the controller 134 may be communicatively coupled to any of the components of the particle detection system 100. In this regard, the one or more processors 136 of the controller 134 may perform any of the various process steps described throughout this disclosure. For example, the controller 134 may receive, analyze, and / or process data from the detector 110 (e.g., associated with an image of the sample 106). As another example, the controller 134 may control or otherwise direct any of the components of the particle detection system 100 using control signals.

[0095] The one or more processors 136 of the controller 134 may include any processing element 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 a program 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. Additionally, 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 packaged separately 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, a read-only memory (ROM), a random access memory (RAM), a magnetic or optical memory device (e.g., disk), a magnetic tape, a solid-state drive, and the like. Furthermore, it should be noted that the storage medium 138 may be housed in 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 the 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.). Thus, the above description should not be construed as limiting the present invention, but merely as illustrative.

[0097] Reference is now made to Figure 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 in response to an illumination beam having a known polarization at a known angle of incidence (e.g., surface haze), where 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 may 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, the method 700 includes a step 706 of designing one or more polarization control 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 a fully linearly polarized light across the pupil plane and to have the linearly polarized light across the pupil plane 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 the light in the second electric field distribution. In this manner, the first electric field distribution can be manipulated and differentiated from the second electric field distribution using polarization control optics.

[0101] It is contemplated herein that the polarization control optical element designed in 706 (e.g., forming the 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 polarizations across the pupil to linear polarization, and a second polarization rotator for rotating the linear polarization 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 the light across 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 a common orientation direction to the other segments.

[0102] In another embodiment, the method 700 includes generating 708 a darkfield image of the sample using an imaging system having polarization control optics in the pupil plane and a linear polarizer aligned to reject light polarized along a selected orientation direction, the darkfield 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 a surface of the sample within a selected tolerance, and surface haze is suppressed.

[0103] The subject matter described herein illustrates different components that are sometimes 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 are combined to achieve a particular functionality 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" with each other to achieve the desired functionality, and any two components capable of being so associated can also be considered to be "couplable" with each other to achieve the desired functionality. Specific examples of components that 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 forms described are merely illustrative, and it is the intent of the following claims to embrace and include such modifications. It is to be understood, further, 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 an illumination direction; one or more collection optics for collecting 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 elliptical polarization; one or more optical elements located at one or more pupil planes of the one or more collection optics, a polarization rotator providing a spatially varying polarization rotation distribution to rotate the surface haze to a selected orientation distribution, the rotation of the surface haze comprising a rotation of a major axis of an elliptically polarized light to a selected orientation distribution; a segmented quarter wave plate for converting the surface haze from the polarization rotator into linearly polarized light, the linearly polarized light being aligned along the selected haze orientation direction; one or more optical elements, a linear polarizer aligned to reject light polarized parallel to the selected haze orientation direction to reject the surface haze from the segmented quarter wave plate; a detector for generating a dark-field image of the sample based on light scattered from the sample that has passed through the linear polarizer, the light scattered from the sample that has passed through the linear polarizer including at least a portion of light scattered by one or more particles on a surface of the sample; Including, system.

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

3. 2. 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.

4. 4. The system of claim 3, 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.

5. The system of claim 1 , wherein the polarization rotator comprises a segmented half-wave plate.

6. The system of claim 5 , wherein the segmented half-wave plate is a linearly segmented half-wave plate including a plurality of segments distributed across a pupil plane.

7. The system of claim 6 , wherein the plurality of segments are distributed along a direction orthogonal to an illumination direction in a pupil plane.

8. 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.

9. The system of claim 8 , 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.

10. The system of claim 8 , wherein the optically active material is quartz.

11. The system of claim 8 , wherein the polarization rotator further comprises a phase compensator that equalizes the optical path length of the surface haze through the optically active material.

12. The system of claim 11 , wherein the phase compensator is formed from a material that is optically homogeneous along a direction of propagation through the phase compensator.

13. The system of claim 11 , 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.

14. 2. The system of claim 1, wherein the one or more optical elements located at one or more pupil planes of the one or more collection optics 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 a surface of the sample.

15. 15. The system of claim 14, wherein the phase mask shapes a point spread function of light scattered from one or more particles on a surface of the specimen to provide a central peak of the point spread function.

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

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

18. 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 an illumination direction; one or more collection optics for collecting light scattered from the sample in response to the illumination beam in a dark field mode, wherein the light scattered 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 elliptical polarization; one or more optical elements located at one or more pupil planes of the one or more collection optics, a spatially varying waveplate disposed in 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 elliptical polarization to the selected haze orientation direction; and one or more optical elements comprising: 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 for generating a dark-field image of the sample based on light scattered from the sample that has passed through the linear polarizer, the light scattered from the sample that has passed through the linear polarizer including at least a portion of light scattered by one or more particles on a surface of the sample; include, system.

19. 20. The system of claim 18, wherein the spatially varying waveplate is a continuously varying waveplate.

20. 20. The system of claim 18, wherein the spatially varying waveplate is a segmented waveplate.

21. 20. The system of claim 18, wherein the one or more illumination optics are configured to direct the illumination beam to the sample with p-polarized light.

22. 20. The system of claim 18, 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.

23. 20. The system of claim 18, 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.

24. 20. The system of claim 18, wherein the segmented half-wave plate comprises a linearly segmented wave plate including a plurality of segments distributed in a pupil plane.

25. The system of claim 24 , wherein the plurality of segments are distributed along a direction orthogonal to an illumination direction in a pupil plane.

26. 20. The system of claim 18, 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.

27. 27. The system of claim 26, 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.

28. 27. The system of claim 26, wherein the optically active material is quartz.

29. 27. The system of claim 26, wherein the polarization rotator further comprises a phase compensator that equalizes the optical path length of the surface haze through the optically active material.

30. 30. The system of claim 29, wherein the phase compensator is formed from a material that is optically homogeneous along a direction of propagation through the phase compensator.

31. 30. The system of claim 29, 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.

32. 20. The system of claim 18, wherein the one or more optical elements located at one or more pupil planes of the one or more collection optics 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 a surface of the sample.

33. 33. The system of claim 32, wherein the phase mask shapes a point spread function of light scattered from one or more particles on a surface of the specimen to provide a central peak of the point spread function.

34. 20. The system of claim 18, wherein the selected haze orientation direction is parallel to a selected alignment direction.

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

Images