A continuous degenerate elliptic retarder for detecting sensitive particles

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” As referred to in the claims of this application under 35 U.S.C. § 119(e), U.S. Provisional Application No. 63 / 151,068 (filed on February 19, 2021, ADVANCED METHOD TO IMPROVE PARTICLE DETECTION SENSITIVITY IN WAFER INSPECTION), named by Jenn-Kuen Leong, John Alex Fielden Chuang, the inventors of the present invention as a whole. “Technical Field” The present disclosure generally relates to particle inspection, and more specifically to particle inspection using darkfield imaging based on scattered or diffracted light.

Background Art

[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 corresponding increases in sensitivity and resolution. An important cause of noise that can limit measurement sensitivity is surface scattering on the wafer (e.g., surface haze), which can exist even for optically polished surfaces. Various methods have been proposed to suppress surface scattering relative to scattering from particles, but such methods may not achieve the desired sensitivity level and / or may achieve sensitivity at the expense of image quality degradation.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Patent Document 3

[0004] Therefore, it is necessary to develop systems and methods to mitigate the above-mentioned shortcomings. [Means for solving the problem]

[0005] Inspection systems according to one or more embodiments of the present disclosure are disclosed. 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 onto a sample at an off-axis angle along the illumination direction. In another exemplary embodiment, the system includes one or more collection optics that collect scattered light from a sample in response to the illumination beam in dark-field mode, wherein the scattered light from the sample collected by one or more collection optics includes a surface haze associated with light scattered from the surface of the sample, and at least a portion of the surface haze has elliptical polarization. In another exemplary embodiment, the system includes one or more optical elements arranged in the pupil plane of one or more collection optics (e.g., pupil plane optics). In another exemplary embodiment, the pupil plane optics includes a first polarization rotor that provides a first spatially varying polarization rotation distribution for rotating the surface haze with respect to a selected orientation direction, wherein the rotation of the surface haze includes a rotation of the major axis of elliptical polarization with respect to the selected orientation direction. In another exemplary embodiment, the pupil plane optical system includes a quarter-wave plate for converting surface haze from a first polarizing rotor into linearly polarized light. In another exemplary embodiment, the pupil plane optical system includes a second polarizing rotor that provides a second spatially varying polarization rotation distribution to rotate the linearly polarized surface haze from the quarter-wave plate in a selected haze orientation direction. In another exemplary embodiment, the system includes a linear polarizer aligned to reject surface haze from the second polarizing rotor by rejecting light polarized parallel to the selected haze orientation direction. In another exemplary embodiment, the system includes a detector that generates a dark-field image of a 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] Inspection systems according to one or more embodiments of the present disclosure are disclosed. 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 onto a sample at an off-axis angle along the illumination direction. In another exemplary embodiment, the system includes one or more focusing optics for focusing scattered light from a sample in response to the illumination beam in dark-field mode, wherein the scattered light from the sample focused by one or more focusing optics includes surface haze associated with light scattered from the surface of the sample, and at least a portion of the surface haze has elliptical polarization. In another exemplary embodiment, the system includes one or more optical elements arranged in the pupil plane of one or more focusing optics (e.g., pupil plane optics). In another exemplary embodiment, the pupil plane optics includes a polarization rotor that provides a spatially varied polarization rotation distribution to rotate the surface haze to a selected orientation distribution, wherein the rotation of the surface haze includes rotating the 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 rotor to linear polarization, wherein the linear polarization is aligned along a selected haze orientation direction. In another exemplary embodiment, the system includes a linear polarizer aligned to reject surface haze from a segmented 4-wavelength plate by rejecting light polarized parallel to a selected haze orientation direction. In another exemplary embodiment, the system includes a detector that generates a dark-field image of a 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.

[0007] Inspection systems according to one or more embodiments of the present disclosure are disclosed. 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 onto a sample at an off-axis angle along the illumination direction. In another exemplary embodiment, the system includes one or more focusing optics for focusing scattered light from a sample in response to the illumination beam in dark-field mode, wherein the scattered light from the sample focused by one or more focusing optics includes surface haze associated with light scattered from the surface of the sample, and at least a portion of the surface haze has elliptical polarization. In another exemplary embodiment, the system includes one or more optical elements positioned in the pupil plane of one or more focusing optics (e.g., pupil plane optics). In another exemplary embodiment, the pupil plane optics includes a spatially variable waveplate located in the pupil plane of one or more focusing optics for converting surface haze into linearly polarized light. In another exemplary embodiment, the pupil plane optics includes a polarization rotor that provides a spatially variable polarization rotation distribution, rotating the surface haze from the spatially variable waveplate to a selected orientation distribution, wherein the rotation of the surface haze includes rotation of the major axis of elliptical polarization to a selected haze orientation direction. In another exemplary embodiment, the system includes a linear polarizer aligned to reject surface haze from a polarizing rotor by rejecting light polarized parallel to a selected haze orientation direction. In another exemplary embodiment, the system includes a detector that generates a dark-field image of a 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.

[0008] It should be understood that both the above summary and the following detailed description are illustrative and descriptive only and do not necessarily limit the claimed invention. The accompanying drawings incorporated into and constituting part of the specification illustrate embodiments of the invention and, together with the general description, help to illustrate the principles of the invention. [Brief explanation of the drawing]

[0009] Many of the advantages of this disclosure can be better understood by those skilled in the art by referring to the accompanying drawings. [Figure 1A] Figure 1A is a block diagram of a particle detection system according to one or more embodiments of the present disclosure. [Figure 1B] Figure 1B is a conceptual diagram of an inspection tool according to one or more embodiments of the present disclosure. [Figure 2] Figure 2 is a conceptual top view of a phase mask including two segments for dividing the pupil into two segments, according to one or more embodiments of the present disclosure. [Figure 3A] Figure 3A shows pupil-plane scattering maps of surface scattering in response to obliquely incident p-polarized light according to one or more embodiments of the present disclosure. [Figure 3B] Figure 3B is a pupil plane scattering map of light scattered by small particles in response to obliquely incident p-polarized light, according to one or more embodiments of the present disclosure. [Figure 4A] Figure 4A is a conceptual diagram of a first configuration of a focusing path for an inspection tool including a continuous degenerate elliptic retarder (CDER) according to one or more embodiments of the present disclosure. [Figure 4B] Figure 4B is a plot of the surface haze electric field distribution shown in Figure 3A after propagation through a first polarizing rotor positioned to align the polarization ellipses of the surface haze along the illumination direction, according to one or more embodiments of the present disclosure. [Figure 4C] Figure 4C is a plot showing the spatially varying polarization rotation distribution of a first polarization rotor for generating the surface haze electric field distribution of Figure 4B, according to one or more embodiments of the present disclosure. [Figure 4D] Figure 4D is a plot of the surface haze electric field distribution shown in Figure 4B after propagation through a quarter-wave plate, according to one or more embodiments of the present disclosure. [Figure 4E] Figure 4E is a plot of the surface haze electric field distribution shown in Figure 4D after propagation through a second polarizing rotor, according to one or more embodiments of the present disclosure. [Figure 4F]FIG. 4F is a plot showing the spatially varying polarization rotation distribution of a second polarization rotator for generating the surface haze electric field distribution of FIG. 4E, according to 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, according to 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, according to 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, according to 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, according to one or more embodiments of the present disclosure. [Figure 4K] FIG. 4K is a conceptual cross-sectional view of a component illustrated in FIG. 4A, having a thickness profile that varies continuously along a single direction such that the polarization rotator provides a one-dimensionally varying polarization rotation distribution, according to one or more embodiments of the present disclosure. [Figure 4L] FIG. 4L is a conceptual cross-sectional view of a component illustrated in FIG. 4A, having a thickness profile that varies linearly along a single direction such that the polarization rotator provides a linearly varying polarization rotation distribution, according to one or more embodiments of the present disclosure. [Figure 4M] FIG. 4M is a conceptual cross-sectional view of a component illustrated in FIG. 4A, where the polarization rotator is formed as a stack of elements including two optically active materials and corresponding phase compensators, according to one or more embodiments of the present disclosure. [Figure 5A] FIG. 5A is a conceptual diagram of a second configuration of a collection path of an inspection tool including a CDER, according to one or more embodiments of the present disclosure. [Figure 5B]Figure 5B is a plot of the surface haze electric field distribution shown in Figure 3A after propagation through the polarizing rotor shown in Figure 5A, according to one or more embodiments of the present disclosure. [Figure 5C] Figure 5C is a plot of the surface haze electric field distribution shown in Figure 5B after propagation through the quarter-wave plate shown in Figure 5A, according to one or more embodiments of the present disclosure. [Figure 5D] Figure 5D is a plot of the surface haze electric field distribution shown in Figure 5C after propagation through the linear polarizer shown in Figure 5A, according to one or more embodiments of the present disclosure. [Figure 5E] Figure 5E is a conceptual cross-sectional view of the components shown in Figure 5A, according to one or more embodiments of the present disclosure, in which the polarizing rotor is formed as a segmented half-wave plate and the quarter-wave plate is formed as a segmented quarter-wave plate. [Figure 5F] Figure 5F is a conceptual cross-sectional view of the components shown in Figure 5A, according to one or more embodiments of the present disclosure, in which the polarizing rotor is formed as a continuous element and the quarter-wave plate is formed as a segmented quarter-wave plate. [Figure 6A] Figure 6A is a conceptual cross-sectional view of a third configuration of the focusing path of an inspection tool including a CDER, according to one or more embodiments of the present disclosure. [Figure 6B] Figure 6B is a plot of the surface haze electric field distribution shown in Figure 3A after propagation through the spatially varying waveplate shown in Figure 6A, according to one or more embodiments of the present disclosure. [Figure 6C] Figure 6C is a plot of the surface haze electric field distribution shown in Figure 6B after propagation through the optical rotor shown in Figure 6A, according to one or more embodiments of the present disclosure. [Figure 6D] Figure 6D is a plot of the surface haze electric field distribution shown in Figure 6C after propagation through the linear polarizer shown in Figure 6A, according to one or more embodiments of the present disclosure. [Figure 7] Figure 7 is a flowchart illustrating the steps performed in a method for particle detection according to one or more embodiments of the present disclosure. [Modes for carrying out the invention]

[0010] Herein, we refer in detail to the disclosed subject matter shown in the accompanying drawings. This disclosure has been specifically shown and described with respect to particular embodiments and their particular features. The embodiments described herein are to be construed as illustrative rather than restrictive. It should be readily apparent to those skilled in the art that various changes and modifications in form and detail can be made without departing from the spirit and scope of this disclosure.

[0011] Embodiments of this disclosure relate 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 a surface (e.g., particle scattering). Further embodiments of this disclosure relate to simultaneously generating separate images of a sample based on surface scattering and particle scattering.

[0012] Wafer inspection is generally described in the United States. U.S. Patent 9,874,526 was issued on January 1, 2018, and includes 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. Furthermore, for the purposes of this disclosure, particles may include, but are not limited to, any surface defects on the sample in question, including foreign particles, scratches, pits, holes, bumps, or equivalents.

[0013] It is recognized herein that light scattered from particles and light scattered from a surface may exhibit different electric field distributions (e.g., polarization and electric field intensity) as a function of the 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 elliptical polarization and be polarized approximately radially with respect to the angle of specular reflection, while scattering from particles may be polarized approximately radially with respect to the surface normal. In this way, surface haze can be isolated from particle scattering, at least partially based 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 a linear polarizer and segmented half-wave plates is generally described in U.S. Patent 10,948,423, which is incorporated herein by reference in its entirety. However, as described herein, existing techniques such as these may not be able to completely convert the elliptic polarization state of surface haze across the pupil to linear polarization, and / or not be able to completely align the polarization of the surface haze along a common direction, which may result in leakage of surface haze through a linear polarizer and a correspondingly reduced signal-to-noise ratio.

[0014] In some embodiments, surface haze is isolated and separated using a continuous degenerate elliptic retarder, referred to herein for convenience, along with a linear polarizer or a polarizing beam splitter. Hereinafter, the continuous degenerate elliptic retarder will be referred to as a CDER. For example, a CDER may be an optical element designed for placement in the pupil plane of an optical system that can functionally convert the polarization ellipses associated with surface haze across the pupil plane into linearly polarized light oriented along a common angle. In this way, the linear polarizer can separate the light associated with 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 in 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, and thus increase the particle detection sensitivity.

[0015] In some embodiments, the CDER may be functionally decomposed into a first polarization rotor (e.g., an optical rotor, segmented or continuous half-wave plate), a quarter-wave plate, and a second polarization rotor, where at least some of the components provide a spatially variable transformation across the pupil and provide tuned separation of surface haze. In this configuration, the first polarization rotor can rotate the elliptic polarized surface haze in a common orientation direction across the pupil plane. This first polarization rotor may, in function and / or physical configuration, be analogous to the polarization rotation technique disclosed in U.S. Patent 10,948,423, incorporated herein by reference above, but this is merely an illustrative comparison and not an limitation of this disclosure. However, it is further intended herein that simply rotating the elliptic polarized surface haze in a common orientation direction may not provide adequate separation and / or suppression of surface haze by subsequent linear polarizers. In particular, polarization components orthogonal to the common orientation direction are not suppressed by subsequent linear polarizers, 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 for converting elliptical polarization to linear polarization. However, this linearly polarized surface haze can no longer be aligned (e.g., along a 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 elliptical polarization. The CDER may then include a second polarization rotor for rotating the linearly polarized surface haze along a common orientation direction (or any other selected orientation direction). It is intended herein that this technique can convert the changing polarization ellipses of the surface haze across the pupil plane into linearly polarized light oriented along any selected (but arbitrary) direction, enabling highly efficient separation and / or suppression using a subsequent linear polarizer or polarization beam splitter.

[0017] However, in this specification, it is intended that CDERs can be implemented in various ways. In some embodiments, the CDER includes a single polarizing rotator and a quarter-wave plate. In this configuration, the polarizing rotator does not need to rotate the elliptical polarization in a common orientation direction across the pupil. Rather, the polarizing 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 matched only after it has passed through the quarter-wave plate. In particular, since the quarter-wave plate generates linearly polarized light in different directions depending on the eccentricity of the input light, the polarizing rotator may compensate for this variation by rotating the surface haze by different amounts across the pupil plane based on the eccentricity distribution. Thus, the second polarizing rotator described above is unnecessary. Furthermore, in some embodiments, the quarter-wave plate is provided before the polarizing rotator. In this configuration, the surface haze is first converted to linear polarization and then rotated in a common orientation direction for separation and / or suppression by the subsequent linear polarizer.

[0018] CDERs for the isolation and suppression of surface haze for the purpose of particle detection are intended herein to offer numerous benefits. For example, a CDER may provide both the conversion of elliptically polarized surface haze across the pupil to linearly polarized light and the rotation of the linearly polarized light to a common angle (e.g., exclusion angle) for exclusion using a linear polarizer. As a result, surface haze can be isolated and suppressed with a high extinction ratio for highly sensitive particle detection.

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

[0020] Referring here to Figures 1 to 7, a system and method for high-sensitivity particle detection according to one or more embodiments of the present disclosure will be described in more detail.

[0021] Figure 1A is a block diagram of a particle detection system 100 according to 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 a sample 106 while suppressing and / or isolating surface haze. For example, the inspection tool 102 may include the CDER 104 in the focusing pupil plane to manipulate the surface haze so that it is linearly polarized along a selected haze orientation direction. The inspection tool 102 may further include a linear polarizer 108 oriented to suppress surface haze along the haze orientation direction from the sample light 122, and at least one detector 110 that generates inspection data based on the sample light 122 after the suppression of surface haze.

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

[0024] Figure 1B is a conceptual diagram of an inspection tool 102 according to 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 including one or more illumination optics for directing the illumination beam 114 onto the sample 106.

[0026] The illumination source 112 may include any type of light source known in the art. Furthermore, the illumination source 112 can provide an illumination beam 114 having 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, but not limited to, a laser-driven light source (LDLS), such as a laser-sustained plasma (LSP) source. For example, the illumination source 112 may include, but not limited to, an LSP lamp, an LSP bulb, or an LSP chamber suitable for housing one or more elements that can emit broadband illumination when excited to a plasma state by a laser source. In some embodiments, the illumination source 112 includes, but not limited to, a lamp source, such as an arc lamp, a discharge lamp, or an electrodeless lamp.

[0027] The illumination beam 114 may include, but is not limited to, light of one or more selected wavelengths, including ultraviolet (UV) radiation, visible light, or infrared (IR) radiation. For example, the illumination source 112 may, but is not required to, 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 yet another example, the illumination beam 114 may provide a wavelength of about 213 nm. As yet another example, the illumination beam 114 may provide a wavelength of about 193 nm. It is recognized herein that both the imaging resolution and light scattering (e.g., with respect to the wavelength of the illumination beam 114) from small particles generally scale with wavelength, so that reducing the wavelength of the illumination beam 114 can generally increase the imaging resolution and scattering signal from small particles. Therefore, the illumination beam 114 may include, but is not limited to, short-wavelength light including extreme ultraviolet (EUV) light, deep ultraviolet (DUV) light, or vacuum ultraviolet (VUV) light.

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

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

[0030] In this specification, it is recognized that the intensity of surface haze may depend on several 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-perpendicular angles of incidence and decrease at higher angles of incidence. In some embodiments, the illumination path 116 may include, but not limited to, one or more illumination optical systems such as lenses 118 and mirrors, to direct the illumination beam 114 onto the sample 106 at an oblique angle of incidence (off-axis angle) to reduce the occurrence of surface haze. The oblique angle of incidence may generally include any selected angle of incidence. For example, the angle of incidence may be, but not limited to, greater than 60 degrees relative to the surface normal.

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

[0032] Illumination of the sample 106 by the illumination beam 114 can result in light emitted from the sample 106 (e.g., sample light 122) based on any combination of reflection, scattering, diffraction, or emission from the sample 106. In this way, the sample light 122 may include a combination of surface haze from the sample 106 (e.g., obtained 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 focusing path 124 (focusing optical system) having an objective lens 126 for focusing at least a portion of the sample light 122. The sample light 122 may include, but is not limited to, any type of light emitted from the sample 106 in response to the illumination beam 114, including scattered light, reflected light, diffracted light, or luminescence.

[0034] In some embodiments, the inspection tool 102 is a dark-field imaging system for eliminating specular reflection 128. In this regard, the inspection tool 102 can image the sample 106 primarily based 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 focus specular reflection. For example, as shown in Figure 1B, the objective lens 126 is oriented substantially perpendicular to the sample 106 and has an NA that does not include specular reflection 128 associated with the illumination beam 114. Furthermore, the objective lens 126 may have an NA of about 0.9 or greater, but is not required. In some embodiments, the inspection tool 102 may include one or more components that prevent specular reflection 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 focused by the focusing path 124. The detector 110 may include any type of photodetector known in the art that is suitable for measuring the illumination received from the sample 106. For example, the detector 110 may include, but is not limited to, a charge-coupled device (CCD) detector, a complementary metal-oxide-semiconductor (CMOS) detector, a time-delay integral (TDI) detector, a photomultiplier tube (PMT) array, an avalanche photodiode (APD) array, or a similar multipixel detector suitable for capturing an image of the sample 106. In some embodiments, the detector 110 may include a spectroscopic detector suitable for identifying the wavelength of the sample light 122.

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

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

[0038] The focusing path 124 may further include any number of beam adjustment elements for directing and / or correcting 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, the beam adjustment elements may include, but are not limited to, a phase mask 202, as will be described in more detail below.

[0039] It is recognized herein that a limited number of components and / or components having a limited thickness may be positioned on or sufficiently close to a particular pupil plane 130 to provide a desired effect. Therefore, for the purposes of this disclosure, references to one or more elements in the pupil plane 130 may generally describe one or more elements on or sufficiently close to the pupil plane 130 to produce a desired effect. In some embodiments, although not shown, the focusing path 124 may include additional lenses for generating one or more additional pupil planes 130, so that any number of elements, including but not limited to elements related to the CDER 104, phase mask 202 (see, for example, Figure 2 below), or linear polarizer 108, can be positioned on or near the pupil plane 130.

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

[0041] In particular, dark-field images of particles smaller than the imaging resolution when illuminated with oblique p-polarization (e.g., images of particles formed by scattered or diffracted light) can be rings extending over an area larger than the system PSF, which negatively impacts particle detection sensitivity. This ring shape and the increase in the size of the particle's PSF or imaging spot can be associated with canceling interference of focused light at the center of the particle's imaging spot on the detector 110.

[0042] Therefore, in some embodiments, the particle detection system 100 includes one or more components for correcting the phase of the sample light 122 crossing the pupil plane 130 to facilitate constructive interference of light at the center of the imaging spot of the particle on the detector 110, such as one or more phase plates or one or more phase compensators, etc.

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

[0044] Figure 2 is a conceptual top view of a phase mask 202 comprising two segments for dividing the pupil into two segments (e.g., half) according to one or more embodiments of the present disclosure. For example, as shown in Figure 2, the phase mask 202 comprises orthogonal polarization (E x , e iπ E yTo introduce a phase shift of p for light polarized along the Y direction (represented as ), the phase mask 202 may include a segment 204 formed from a half-wave plate having an optical axis along the X direction. Furthermore, the phase mask 202 may include a segment 206 that does not rotate the polarization of the light. For example, segment 206 may include a compensation plate formed from a material that is optically homogeneous along the propagation direction so that light passing through segment 206 propagates along the same (or substantially the same) optical path length as light in 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 segment 204, but does not exhibit birefringence along the propagation direction. In another embodiment, the compensation plate is formed from the same material as the half-wave plate in segment 204, but is cut along a different axis so that light propagating through the compensation plate does not experience birefringence. For example, light propagating along the optical axis of a uniaxial crystal cannot experience birefringence, and as a result, the crystal can be optically homogeneous with respect to light propagating along the optical axis. As another example, segment 206 may include an aperture.

[0045] Furthermore, in some embodiments, the phase mask 202 may be tilted outward from 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 can be formed using any technique known in the art. In one embodiment, the various segments (e.g., segments 204-206 in Figure 2) are formed as a single component in which the various segments are arranged in a single plane.

[0047] However, it should be understood that Figure 2 and the related description are provided for illustrative purposes only and should not be construed as limiting. For example, a phase mask 202 having two segments may include a half-wave plate positioned at the bottom of the focusing region 306 rather than at the top, as shown in Figure 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 particles. For example, assuming a known electric field distribution of light in the pupil plane associated with an object of interest (e.g., measurement, simulation, etc.), a segmented phase mask 202 as described herein may be formed to selectively adjust the phase of various regions of light in the pupil plane to reshape the PSF of an image of the object of interest. In particular, the various segments of the phase mask 202 may be selected to facilitate constructive interference in the detector 110 and provide a tight PSF (e.g., within selected tolerances) that approaches 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 particle in question (e.g., as shown in Figure 3B, etc.) and actual 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 satisfy both manufacturing specifications and performance specifications (e.g., particle PSF of a selected shape, etc.). Thus, the design of the phase mask 202 shown in Figure 2 may represent a non-limiting example that provides a particular trade-off between performance and manufacturability.

[0049] In another embodiment, as will be 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, which facilitates constructive interference of the sample light 122 associated with particle scattering at the center of the image of the particles on the detector 110.

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

[0051] This specification recognizes that light scattered from the surface of sample 106 (e.g., surface haze, surface scattering, etc.) may be considered noise in particle detection applications. Therefore, it may be desirable to filter out the portion of sample light 122 related to surface haze from the portion related to light scattered by the target particles.

[0052] Figure 3A is a pupil plane scattering 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. Figure 3B is a pupil plane scattering map 304 of light scattered by small particles (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 and 304 include electric field intensity, indicated by shading with white as the highest intensity and black as the lowest intensity. Furthermore, the scattering maps 302 and 304 include the polarization orientation of light as a function of the focusing angle (e.g., scattering angle) in the pupil plane 130, indicated by superimposed ellipses. The scattering maps 302 and 304 are bounded by focusing regions 306 in the pupil plane, which are associated with the range of angles at which the sample light 122 is focused by the inspection tool 102. For example, the focusing regions 306 may correspond to the numerical aperture (NA) of the objective lens in the focusing path 124.

[0054] Scatter maps 302 and 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 reflection angle 308 is located outside the focusing area 306 along the illumination direction 310 (e.g., outside the focusing area 306 to the right of the circular focusing area 306 in Figure 3A), indicating that the inspection tool 102 does not capture the specular reflection. However, alternative configurations are within the scope of this disclosure. For example, if the specular reflection angle 308 is in the pupil plane, the specular reflection can be blocked in front of the detector 110 to produce a dark-field image.

[0055] In addition, scattering maps 302 and 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 scattering maps 302 and 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 that of light scattered by a surface, especially when the illumination beam 114 is p-polarized. For example, the sample light 122 associated with surface haze generally exhibits a substantially radial polarization distribution with respect to the specular reflection angle 308 within the focusing 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 is generally elliptical. As can be seen from Figures 3A and 3B, at most positions within the pupil plane 130, the ellipse is very elongated, meaning that one linear polarization component is much stronger than the other linear polarization component. In the case of sample light 122 scattered from small particles (e.g., Figure 3B), the polarization may be more elliptical near the center of the pupil, meaning that the magnitudes of the two linearly polarized components may be nearly equal. However, the intensity of light in this region of the pupil is relatively low and contributes little to the total scattering signal from the small particles.

[0057] Figure 4A is a conceptual diagram of a first configuration of a focusing path 124 of an inspection tool 102 including a CDER 104, according to one or more embodiments of the present disclosure. Within this specification, it is considered that each of the components illustrated in Figure 4A may be formed as a single optical element or dispersed among any number of optical elements. In this way, adjacent components may or may not be in physical contact. Furthermore, the inspection tool 102 may include any number of pupil planes such that the components shown in Figure 4A may be located in or near a single pupil plane or dispersed among multiple pupil planes.

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

[0059] The polarization rotor 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 way, the polarization rotor 402 can selectively rotate the polarization of light at any position in the pupil plane by any selected amount.

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

[0061] Figures 4B–4G show the evolution of the electric field distribution of surface haze with the components shown in Figure 4A, according to one or more embodiments of the present disclosure. In Figures 4B–4G, the grayscale intensity maps correspond to the intensity of the surface haze. Furthermore, the polarization state at selected locations across the pupil plane is shown as an overlay to indicate the distribution of polarization states across the pupil plane.

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

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

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

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

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

[0067] Referring here to Figures 4H-4M, various implementations of the components illustrated in Figure 4A will be described in more detail according to one or more embodiments of this disclosure. The polarization rotor 402, which 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 polarizing rotor 402 is formed from segmented half-wave plates 406. Figure 4H is a block diagram of a polarizing rotor 402 formed as segmented half-wave plates 406 according to one or more embodiments of the present disclosure. For example, the polarizing rotor 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 way, portions of the sample light 122 in different regions of the pupil plane can 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 Figure 3A). Such a polarizing rotor 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 varied distribution of polarization rotation. For example, the polarizing rotor 402 may include a linear segmented half-wave plate 406 having a series of half-wave plates distributed along a linear direction within the pupil plane (e.g., the Y direction in Figure 3A). In another example, the polarizing rotor 402 may include an angular segmented half-wave plate 406 having a series of wedge-shaped half-wave plates distributed radially around vertex locations such as portions of the pupil plane corresponding to specular reflections of the illumination beam 114.

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

[0070] This specification recognizes that polarization rotation using the optically active material 408 can be achieved 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 of a material that provides different refractive index values ​​for orthogonal directions (e.g., fast and slow axes) in the pupil plane, and may have a constant thickness set to produce a phase delay of π (e.g., half-wave) between light along these orthogonal directions. Thus, 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 velocity as a function of thickness. Thus, the amount of polarization rotation induced by the optically active material 408 is controlled by the thickness of the optically active material. Furthermore, this specification recognizes, but is not limited to, that several materials, such as quartz, can act as a wave plate or optically active material 408 based on their orientation relative to the pupil plane. For example, quartz oriented along an optical axis in the pupil plane can act as a waveplate, while quartz oriented along an optical axis perpendicular to the pupil plane (e.g., along the propagation direction of the sample light 122) can act as an optically active material 408. However, this is merely illustrative, and it should be understood that the optically active polarizer rotator 402 can generally be formed from any optically active material.

[0071] In some embodiments, the polarizing rotor 402 formed from the optically active material 408 may further include a phase compensator 410 to facilitate constructive interference of the sample light 122 associated with particle scattering at the center of the image of particles 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, complementary to the spatially varying thickness of the optically active material 408. In this way, the entire optical path of the sample light 122 passing through the polarizing rotor 402 can be constant across the pupil plane. In another example, the phase compensator 410 may be formed from an optically active material having the opposite handedness (direction of polarization (right or left)) to the optically active material 408 containing the polarizing rotor 402. In one example, the optically active material 408 includes right-handed quartz and the phase compensator 410 includes left-handed quartz, each having a thickness profile selected to achieve the desired polarization rotation and phase correction.

[0072] This specification explores how a polarizing rotor 402 formed from an optically active material 408 can therefore provide polarization rotation that continuously varies across the pupil plane by fabricating the polarizing rotor 402 from the optically active material 408 with a 2D continuous spatial variation thickness. In some embodiments, a first polarizing rotor 402a and / or a second polarizing rotor 402b are formed from the optically active material 408 with a 2D thickness profile that provides precise polarization rotation across the pupil plane (e.g., according to the distribution shown in Figures 4C and 4F).

[0073] Figure 4J is a plot showing the particle inspection sensitivity and efficiency of the CDER 104 in suppressing surface haze as a function of the pixel size of the detector 110 in the inspection tool 102, according to one or more embodiments of the present disclosure. It is intended herein that the particle detection sensitivity of the inspection tool 102 having the CDER 104 for suppressing surface haze may depend on the specific electric field distribution of the 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 varies based on various parameters such as particle size or composition, but is not limited to these.

[0074] For example, Figure 4J corresponds to a simulation of the efficiency of a CDER104 as illustrated in Figure 4A, with polarizing rotors 402a, b formed from an optically active material 408 having a 2D thickness profile (as shown in Figures 4C and 4F), according to one or more embodiments of the present disclosure. Furthermore, the legend on the contour of Figure 4J corresponds to particle sizes in nanometer units. As shown in Figure 4J, this non-limiting example of a CDER104 provides high efficiency over a wide range of pixel sizes (e.g., suppression of surface haze for light scattered by particles).

[0075] However, this specification intends that in some cases it may be impractical or undesirable to manufacture an optically active material 408 having a spatially varying thickness profile that precisely provides a spatially varying distribution of polarization rotation suitable for a particular step (e.g., the distribution shown in Figures 4C and 4F). Therefore, in some embodiments, a particular polarizing rotor 402 (e.g., a first polarizing rotor 402a or a second polarizing rotor 402b) can provide a spatial polarization rotation distribution that approximates an ideal polarization rotation distribution within a selected tolerance. In this way, performance and manufacturability can be balanced.

[0076] In some embodiments, the polarizing rotor 402 (e.g., a first polarizing rotor 402a or a second polarizing rotor 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 intended herein that it may be easier and / or more cost-effective to fabricate an optically active material 408 having a 1D thickness profile than a 2D thickness profile. Furthermore, as shown in Figure 3(A), the polarization ellipse of the surface haze varies significantly along the Y direction and relatively weakly along the X direction. Therefore, a 1D polarization rotation distribution that varies along the Y direction can reasonably approximate an ideal polarization rotation distribution.

[0077] Figure 4K is a conceptual cross-sectional view of the component shown in Figure 4A, in which the polarizing rotors 402a,b, according to one or more embodiments of the present disclosure, 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. Figure 4L is a conceptual cross-sectional view of the component illustrated in Figure 4A, in which the polarizing rotors 402a,b, according to one or more embodiments of the present disclosure, have a thickness profile that varies linearly along a single direction (e.g., the Y direction) to provide a linearly varying polarization rotation distribution.

[0078] In some embodiments, the polarizing rotor 402 (e.g., a first polarizing rotor 402a and / or a second polarizing rotor 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, a desired complex spatially-varying thickness profile (e.g., 1D or 2(d)) can be directly fabricated from an optically active material 408 (or corresponding phase compensator 410) that is practical (e.g., based on cost, manufacturability, etc.). However, it may be possible to construct a 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 fabricate (e.g., spherical, linear surfaces, etc.), and the combination thereof forms a 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 polarizing rotors 402a,b according to one or more embodiments of the present disclosure are formed as a stack of elements comprising two optically active materials 408 and corresponding phase compensators 410. For example, the optically active materials 408 and corresponding phase compensators 410 in Figure 4M are formed in spherical and linear profiles.

[0080] This specification further intends that a phase mask 202 or other beam tuning elements may be incorporated into the CDER 104. Figure 4M further shows the phase mask 202 integrated with the first polarizing rotor 402a.

[0081] Referring here to Figures 5A to 5F, it is intended herein that a functional step can be performed using various techniques to convert the elliptical polarization of the surface haze across the pupil plane into linear polarization and align the linear polarization along a common haze orientation direction for suppression and / or separation by the subsequent linear polarizer 108.

[0082] Figure 5A is a conceptual diagram of a second configuration of the focusing path 124 of an inspection tool 102 including a CDER 104, according to 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 distributed among any number of optical elements in any number of pupil planes.

[0083] In particular, Figure 5A shows the configuration of the CDER104, which includes a single polarizing rotor 402 and a segmented quarter-wave plate 502, both of which have spatially variable behavior across the pupil plane. It is intended that the influence of the segmented quarter-wave plate 502 on the orientation of linearly polarized light may be considered when designing the polarizing rotor 402 such that only a single polarization rotation step is required. In this configuration, the polarizing rotor 402 does not align the major axes of elliptical polarization, as shown in Figure 4B. Rather, the polarizing rotor 402 provides a spatially variable polarization rotation profile based at least partially on the eccentricity of the surface haze, so that the polarization of the surface haze traversing the pupil plane propagates through the segmented quarter-wave plate 502 and is aligned only after being converted to linear polarization. For example, the polarizing rotor 402 can align elliptical polarization such that the diagonals of the polarization ellipses align along a common direction (e.g., the illumination direction or the horizontal direction, as depicted in Figure 5B below). A segmented quarter-wave plate 502 having optical axes oriented parallel to the minor axis of each polarization ellipse can then provide that the elliptical polarization is converted to linear polarization and that the linear polarization is aligned along a common haze orientation direction (e.g., the illumination direction or X direction as depicted in Figure 5C below).

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

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

[0086] Figure 6A is a conceptual cross-sectional view of a third configuration of a focusing path 124 of an inspection tool 102 including a CDER 104, according to 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 distributed among any number of optical elements in any number of pupil planes.

[0087] In particular, Figure 6A illustrates the configuration of CDER104, which includes a spatially variable waveplate 602 that converts the polarization ellipse of the surface haze into linear polarization across the pupil, so that the linear polarization is aligned along the common haze orientation direction. CDER104 may further include a polarization rotor 402 that rotates the linear polarization of the surface haze in the common haze orientation direction. The polarization rotor 402 may have any preferred design, such as the designs illustrated in Figures 4H and 4I, but is not limited. In some embodiments, the polarization rotor 402 includes an optically active material 408 and optionally a phase compensator 410. In some embodiments, the polarization rotor 402 includes a segmented half-waveplate 406. For example, Figure 6A shows a non-limiting configuration of CDER104 in which the polarization rotor 402 includes the optically active material 408 and the phase compensator 410. Figure 6A further illustrates a linear polarizer 108 and a phase mask 202 in a focusing path 124, as previously described herein, for isolating or suppressing surface haze and reshaping the PSF of light passing through the CDER 104 (e.g., related to the sample light of interest 122 from particles on the sample 106).

[0088] In some embodiments, one or more components of the CDER104 have a spatially varying thickness to compensate for (at least within selected tolerances) the spatial variation of the thickness of the spatially varying waveplate 602. For example, Figure 6A shows a phase mask 202 in which the CDER104 includes a segment 204 formed from a half-waveplate having an optical axis along the X direction to introduce a phase shift of p for light polarized along the Y direction (e.g., as described above with reference to Figure 2), and a segment 206 including a compensating plate formed from an optically homogeneous material along the propagation direction, and a focusing path 124 showing an unrestricted configuration through which all light propagates along the same optical path length (e.g., at least within selected tolerances). In particular, Figure 6A shows a 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 CDER104 may include additional compensating elements 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, and the thickness and orientation of the optical axis vary (e.g., continuously or between segments) to provide both the conversion of elliptical polarization of the surface haze to linear polarization.

[0090] Figure 6B is a plot of the surface haze electric field distribution shown in Figure 3A after propagation through the spatially varying waveplate 602 of Figure 6A, according to one or more embodiments of the present disclosure. As shown in Figure 6B, the elliptical polarization of Figure 3A is converted to linear polarization with orientation that varies across the pupil plane. Figure 6C is a plot of the surface haze electric field distribution shown in Figure 6B after propagation through the optical rotor 402 of Figure 6A, according to one or more embodiments of the present disclosure. In Figure 6C, the orientation of the linear polarization is oriented along a common haze orientation direction (e.g., the horizontal direction in Figure 6C). Figure 6D is a plot of the surface haze electric field distribution shown in Figure 6C after propagation through the linear polarizer 108 of Figure 6A, according to one or more embodiments of the present disclosure. As shown in Figure 6D, the intensity is very low because most of the surface haze is attenuated or redirected by the linear polarizer 108.

[0091] Referring here to Figures 4A to 6D in general, 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 elliptic surface haze to linear polarization and the alignment of 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 linear polarizers. Therefore, it should be understood that the teachings relating to the performance of CDER104 based on Figure 4J can be applied to or extended to all variations of CDER104, including, but not limited to, the variations shown in Figures 5A and 6A.

[0092] Furthermore, it should be understood that Figures 4A to 6D and their associated explanations are provided for illustrative purposes only and should not be interpreted as limiting. In particular, Figures 4A to 6C represent non-limiting examples of CDER104.

[0093] Referring again to Figure 1A, various additional components of the particle detection system 100 according to one or more embodiments of the present disclosure are described in more detail.

[0094] In some embodiments, the particle detection system 100 includes a controller 134 which includes one or more processors 136 configured to execute program instructions maintained on a storage medium 138 (e.g., memory). Furthermore, the controller 134 may be communicatively coupled to any component of the particle detection system 100. In this regard, 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 use control signals to control or otherwise instruct any component of the particle detection system 100.

[0095] One or more processors 136 of controller 134 may include any processing elements known in the art. In this sense, one or more processors 136 may include any microprocessor type device configured to execute algorithms and / or instructions. In some embodiments, one or more processors 136 may consist of a desktop computer, a mainframe computer system, a workstation, an imaging 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” can be broadly defined to include any device having one or more processing elements that execute program instructions from a non-temporary storage medium (memory) 138. Furthermore, the steps described throughout this disclosure may be performed by a single controller 134 or, alternatively, by multiple controllers. Furthermore, controller 134 may include one or more controllers housed in a common housing or multiple housings. Thus, any controller or combination of controllers may be separately packaged as modules suitable for integration into the particle detection system 100.

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

[0097] Referring here to Figure 7, which is a flowchart illustrating the steps performed in Method 700 for particle detection according to one or more embodiments of the present disclosure. The applicant notes that the embodiments and enabling techniques described herein in the context of the particle detection system 100 should be interpreted as extending to Method 700. However, it should be further noted that Method 700 is not limited to the architecture of the particle detection system 100.

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

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

[0100] In another embodiment, method 700 includes step 706 of designing one or more polarization-controlling optical elements (e.g., CDER104) suitable for placement in the pupil plane of an imaging system such that the first electric field distribution is transformed to include perfectly linearly polarized light across the pupil plane and the linearly polarized light across the pupil plane is aligned along a common orientation direction (e.g., haze). In some embodiments, the common orientation direction is substantially different from the polarization direction of light in the second electric field distribution. Thus, the first electric field distribution can be manipulated and distinguished from the second electric field distribution using a polarization-controlling optical system.

[0101] Polarization control optical elements designed in 706 (e.g., forming CDER104) can be provided in various configurations within the spirit and scope of this disclosure, as intended herein. For example, a polarization control optical element may include a first polarization rotator for rotating the polarization of light associated with a first electric field distribution in a first common orientation direction, a quarter-wave plate for converting all polarizations crossing the pupil into linear polarization, and a second polarization rotator for rotating the linear polarization in a second common orientation direction, which may be the same as or different from the first common orientation direction. As another example, a polarization control optical element may include a single polarization rotator and a quarter-wave plate, the single polarization rotator rotating the polarization of light associated with a first electric field distribution to an intermediate distribution such that light crossing the pupil propagates through the quarter-wave plate, is linearly polarized, and aligned across the common orientation direction. As yet another example, a polarization control optical element may include a segmented wavelight including multiple segments distributed across the pupil plane. For example, each segment 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 into 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, method 700 includes step 708 of generating a dark-field image of a sample using an imaging system having a polarization-controlled optical system in the pupil plane and a linear polarizer aligned to reject light polarized along a selected orientation direction, the dark-field image being based on light passed through the linear polarizer. For example, the light passed through the polarizer can correspond to light scattered by one or more particles on the surface of the sample within a selected tolerance, and surface haze is suppressed.

[0103] The subject matter described herein illustrates different components that, in some cases, are contained within or connected to other components. It should be understood that such depicted architectures are merely illustrative, and in practice, many other architectures can be implemented to achieve the same functionality. Conceptually, any arrangement of components to achieve the same function is effectively “associated” in such a way that the desired function is achieved. Thus, any two components in this specification combined to achieve a particular function, whether in architecture or as intermediate components, can be considered “associated” with each other in such a way that the desired function is achieved. Similarly, any two such associated components can also be considered “connected” or “joined” with each other in such a way that the desired functionality is achieved, and any two components that can be associated in such a way can also be considered “joinable” with each other in such a way that the desired functionality is achieved. Specific examples of joinable components 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] Many of the present disclosure and its associated advantages will be understood from the foregoing description, and it will become clear that various modifications can be made to the form, structure, and arrangement of the components without departing from the disclosed subject matter or sacrificing any of its material advantages. The described forms are for illustrative purposes only, and it is the intent of the following claims to encompass and include such modifications. Furthermore, it should be understood that the present invention is defined by the appended claims.

Claims

1. It is a system, A lighting source configured to generate an illumination beam, One or more illumination optical systems that direct the illumination beam onto the sample at an off-axis angle along the illumination direction, One or more focusing optical systems for focusing scattered light from a sample in response to an illumination beam in dark-field mode, wherein the scattered light from the sample focused by the one or more focusing optical systems includes surface haze related to light scattered from the surface of the sample, and at least a portion of the surface haze is elliptically polarized. One or more optical elements located in the pupil plane of one or more of the one or more light-gathering optical systems, A polarization rotor that provides a spatially varying polarization rotation distribution and rotates the surface haze to a selected orientation distribution, wherein the rotation of the surface haze includes a rotation of the major axis of elliptical polarization to the selected orientation distribution, and the selected orientation distribution is an orientation distribution in which the orientation direction of the polarization of the surface haze is not common and compensates for the rotation of the polarization of the surface haze by a subsequent quarter-wave plate, A segmented quarter-wave plate for converting the surface haze from the polarizing rotor into linearly polarized light, wherein the segmented quarter-wave plate rotates the elliptically polarized light with the selected orientation distribution to align the linearly polarized light along the selected haze orientation direction, One or more optical elements, A linear polarizer positioned to reject light polarized parallel to the selected haze orientation direction, thereby rejecting the surface haze from the segmented quarter-wave plate, A detector that generates a dark-field image of a sample based on scattered light from the sample that has passed through the linear polarizer, wherein 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. including, system.

2. The system according to claim 1, wherein one or more illumination optical systems are configured to direct the illumination beam towards the sample with p-polarization.

3. The system according to claim 1, wherein the linear polarizer is a polarizing beam splitter that directs scattered light 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. The system according to 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 according to claim 1, which includes a segmented half-wave plate as the polarizing rotor.

6. The system according to claim 5, wherein the segmented half-wave plate includes a plurality of segments distributed in one direction within the pupil plane.

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

8. The system according to claim 1, wherein the polarization rotor has a spatially varying thickness and includes an optically active material that provides a spatially varying polarization rotation distribution in response to the change in thickness.

9. The system according to claim 8, wherein the optically active material is oriented such that its optical axis is perpendicular to the pupil plane.

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

11. The system according to claim 8, further comprising a phase compensator that equalizes the optical path length of the surface haze passing through the optically active material, wherein the polarization rotor is further a phase compensator.

12. The system according to claim 11, wherein the phase compensator is formed from an optically homogeneous material along the propagation direction passing through the phase compensator.

13. The system according to claim 11, wherein the phase compensator is formed from an optically active material having opposite characteristics to the optically active material.

14. The system according to claim 1, wherein one or more optical elements located in one or more pupil planes of the one or more focusing optical systems further include a phase mask that provides different phase shifts for light in two or more pupil regions of the focusing region in order to shape the point spreading function of light scattered from one or more particles on the surface of the sample.

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

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